ICEPMU-2016 conference proceedings(2017) - Technology Letters [PDF]

Dr. Hukum Singh (Chairman). Dr. Ravindra Bisht. Dr. Tejpal Singh. Mr. Manoj Sharma (CSE) ... In memory switching materia

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ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

PROCEEDINGS OF International Conference on Engineering Physics, Materials and Ultrasonics (June 3-4, 2016)

ICEPMU-2016 Editors: Prof S K Jain, Convener Dr. Ambika Sharma, HoD Department of Applied Sciences The NorthCap University Gurgaon Email: [email protected] Website:www.ncuindia.edu

Sponsored by

Science and Engineering Research Board

Materials Research Society of India

Ultrasonic Society of India

Organized by Department of Applied Sciences

Sector 23 A, Gurgaon 122017 - Haryana Tel: 0124-2365811, Fax: 0124-2367488 Website: www.ncuindia.edu

1

Defence Research and Development Organization

ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

Advisory Committee Organizing committees

Prof. Vikram Kumar, I.I.T Delhi & Pres. USI (Chairman) Prof. Krishan Lal, NPL, New Delhi Prof. KL Chopra, NCU Gurgaon Prof. M.S. Sodha, NCU Gurgaon Prof. A.K. Ghatak, NCU Gurgaon Prof. Kehar Singh, NCU Gurgaon Prof. RC Budhani, I.I.T. Kanpur Dr. R.K. Sharma, Dir. SSPL, Delhi Prof. Anurag Kumar, Dir., IISc, Bangalore Dr. VN Bindal, Patron USI Prof. ESR Gopal Emer. Sc., IISC Bangalore Dr. Baldev Raj, Ex-Dir. IGCAR & Pres. Res, PSG Institutions, Coimbatore Prof. Yogesh K. Vohra, University of Alabama at Birmingham, U.S.A. Dr. Niloy Dutta, Univ. of Connecticut, U.S.A. Dr. Mekonnen Abebe, Def Univ., Ethiopia. Dr. Andrej Nowicki, IFTR, Warsaw Dr. Adam Shaw, NPL (UK), Teddington Dr. David Gilbert, BINDT, UK Dr. J. Szilard, Sydney, Australia Prof. BK Das, NCU Gurgaon Dr. VR Singh, Advisor, PDM, Bahadurgarh Prof. Karmeshu, JNU, Delhi Prof. Promila Goel, NCU Gurgaon Prof. S.B. Krupanidhi, IISc, Bangalore Prof. RR Yadav, AU Allahabad Dr. Chandra Prakash, S.S.P.L. Delhi Prof. Amitava Sen Gupta, NCU Gurgaon Prof. P.K. Bhatnagar, South Campus, DU Prof. S. K. Ray, IIT Kharagpur Prof. Amlan J. Pal, IACS, Kolkata Dr. Nitin Goel, Facebook, California Dr. D. Kanjilal, IUAC, New Delhi Dr. Avinashi Kapoor, DU, South Campus Dr. Reji Philip, RR Institute, Bangalore

Convener Prof. SK Jain, NCU [email protected]

Co-conveners Dr. Rashmi Tyagi Prof. AK Yadav Prof.Kallika Srivastava Dr Devraj Singh, ASET, N. Delhi Dr.Yudhisther Kumar, NPL, N. Delhi Technical Program Committee Dr. Ambika Devi (Chairman) [email protected]

Dr. Pranati Purohit Dr. Sangeet Srivastava Dr. Kamlesh Sharma Dr. Amita Bhagat Dr. Satwanti Devi Dr. Srijanani Hospitality Committee Dr. Hukum Singh (Chairman) Dr. Ravindra Bisht Dr. Tejpal Singh Mr. Manoj Sharma (CSE) Reception Committee Dr. Sunanda Vashistha (Chairman) Dr. Phool Singh Dr. Sunita Sharma Dr. Sandeep Mogha

Chief Patrons Sh. NK Dewan, Chancellor, NCU Sh. V Daulet Singh, GB Member, NCU Sh. Avdhesh Mishra, GB Member, NCU Sh. Shiv S Mehra, GB Member, NCU

Treasurer Committee Dr. Pranati Purohit Dr. Ashutosh Pandey Dr. Chetna Tyagi

Patrons Prof. Prem Vrat Pro-chancellor, NCU Brig. S.K. Sharma, Pro-VC, NCU Prof. R. Ojha, Director, SOET, NCU

Conference CD and photographs Dr. Gaurav Gupta (CD proceedings) Dr. Sangeet Srivastava

2

ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

Contents Sr No

Title

Author

Page No

1

Electrical switching in Cu doped As-Se glasses

K. Ramesh, Pumlianmunga, E.S.R. Gopal

4

2

Bilayer Lift-off Technique for Micromachining

Neha Yadav

9

3

Effect of change in titanium isopropoxide (TTIP) concentration on the preparation of TiO2 nanopowder

Mamta Arya, Shubhra Mathur*, Rohit Jain

12

4

Calculation Of Some Oscillating Parameters For Graphene

D. K. Das, K. V. V. Nagaraju, S. Roy and S. Sahoo

16

5

Study of doped graphene quantum dots by chlorine containing compounds: Electronic Spectroscopy

Poonam R. Kharangarh, Gurmeet Singh

and

19

6

Electromagnetic Wave Propagation in Photonic Structures: Dielectric and Metallo-Dielectric Waveguides

Triranjita Srivastava, Pushpa Bindal, Priyanka, Anuradha, Priyam and Priscilla

23

7

A Comparative Study of Numerical Methods for Analysing Planar Plasmonic Waveguides

Triranjita Srivastava, Pushpa Bindal, Asmita Deep and Ashima Sharda

29

8

Study Of Propagation Characteristics Of Optical Fibers: Experiment And Simulation

Pushpa Bindal, Srivastava, Sujata, Diksha Tandon

Triranjita Anju and

33

9

Experimental Study of Microbending Losses in Optical Fiber

Pushpa Bindal, Triranjita Srivastava, Ananya, Aastha Dhankhar

37

10

Growth of (001) oriented Cr and MgO thin films on Amorphous Substrate for Magnetic Tunnel Junctions

Sajid Husain, Chaudhary

41

11

Bio ceramics: Future implant material

Aruna Dani

45

12

Intelligent Transportation System

Shubham Sehgal, Akshat Mathur, Mona Aggrawal, Ram Sharma

47

3

and

Sujeet

ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

threshold switching device returns to the high resistive OFF state once the applied current is reduced below the holding current (I < Ih). In contrast, the memory device once switched retains the ON state even after the applied current is reduced to zero. The memory device can be brought back to its high resistive OFF state by the application of a suitable current pulse. In memory switching materials, a high conducting crystalline filament is formed due to the Joule heating at the time of switching. Threshold switching is reversible and is generally belie ved to be due to the electronic transitions. It is also proposed that the presence of cross-linking elements like Ge, Si, etc., make the structural reorganization difficult resulting in threshold switching. Memory switching is irreversible and requires a structural transition from glass phase to crystalline phase. So, structural reorganization is very important for memory switching to occur [2-3].

Electrical switching in Cu doped As-Se glasses K. Ramesh*, Pumlianmunga, E.S.R. Gopal Department of Physics, Indian Institute of Science, Bangalore 560012, India. *Corresponding Author: [email protected] Abstract: Bulk CuxAs30Se60-x glasses (0  x  34) prepared by melt quenching method exhibit interesting phase change properties when subjected to high electric fields. The glasses in the composition range 0  x  14 do not exhibit switching. Glasses in the composition range 15  x < 25 exhibit threshold switching. An unusual switching from low resistance to high resistance state has been observed for the glasses in the composition window 25  x  28. A memory switching is observed for the glasses with x ≥ 30. The observation of ‘no switching  threshold switching  low resistance to high resistance  memory switching’ is unique to Cu-As-Se glasses. With the thermal crystallization studies and thermal model, the unique switching behaviour in CuxAs30Se70-x glasses has been understood.

The addition of metal atoms significantly alters the network connectivity, network rigidity, local structure and consequently the electrical properties including the switching behaviour [4-7]. The structural studies show that the metal atoms in chalcogenide glass network are usually in 4- fold coordination [8]. As Cu is a monovalent atom, and for Cu to be in 4- fold coordination, the lone pair electrons of Se and As atoms are transferred to Cu. By donating its electrons, the chalcogen atom increases its local coordination. This transfer of lone-pair electrons and the changes in the local structure around each atom influences the optical Key words: Chalcogenide glasses, Electrical and electrical properties to a larger extent [5]. In the switching, Filament formation, Thermal model, present work, electrical switching in CuxAs30Se70-x glasses has been studied over a wide composition Thermal crystallization. range 0  x  35. The observed electrical switching behaviour of CuxAs30Se70-x glasses has been 1. Introduction understood with the help of thermal crystallization Chalcogenide glasses are known for their electrical studies. switching and memory effects and are popularly known as phase change memory materials (PCM) 2. Experimental [1-3]. The application of high current drives the system from a high resistive (OFF) state to a low Bulk CuxAs30Se70-x glasses (0  x  35) were resistive (ON) state. This electrical switching is of prepared by conventional melt quenching method. two types namely, threshold and memory [2-3]. The 4

ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

The melt quenched samples were subjected to XRD to confirm their amorphous nature. The thermal properties were measured by Differential Scanning Calorimeter (DSC) with a scan rate of 10 °C/min. The prepared CuxAs30Se70-x glasses were thermally crystallized in two ways in vacuum sealed quartz ampoules: (a) by annealing at their respective crystallization temperatures (Tc) for two hours (b) heated up to their respective melting temperatures (Tm) and then quenched in water at room temperature. These samples were subjected to XRD to identify the crystallized phases. I – V characteristics of these glasses were studied using a Keithley Source meter (Model: 2410c). Sample polished to a thickness of 0.3mm is mounted in a holder (made of brass), in between a flat-plate bottom electrode and a point-contact top electrode using a spring-loading mechanism. A constant current (0 – 2 mA) is applied and the corresponding switching voltage developed across the sample was measured.

heating. The threshold switching is generally understood based on the electronic transitions [9]. The defect states C3+ and C1- present in the mobility gap act as trap centres for charge carriers. When the traps are filled, a high conduction occurs. I-V characteristics of representative glasses in the

Fig. 2. Tg as a function of Cu concentration.

3. Results and Discussion

The phase change from glass (high resistance OFF CuxAs30Se70-x system shown in figure 1, indicates state) to crystal (low resistance ON state) at the time the glasses can be divided into 4 regions. (i) 0  x < 15 do not undergo switching; (ii) 15  x < 25, exhibit threshold switching; (iii) 25  x < 30, unusually switches from a low resistance state to a high resistance state (iv) x ≥ 30, memory switching is observed. The composition dependence of Tg also shows an interesting variation in these four regions as shown figure 2 indicating the glass network undergoes a change in these regions. The electronic and thermal models are usually used to explain the threshold and memory switching, respectively. The different kinds of switching observed in a single system are difficult to understand either by electronic or thermal models. By varying the concentration of Cu in the CuxAs30Se70-x glasses, a Fig. 1. I-V curves of representative CuxAs30Se70-x glasses ‘no switching  threshold  low resistance to showing the different types of switching. high resistance  memory’ is observed. To understand the threshold switching we need to use electronic model and to explain memory switching of switching is responsible for the memory we need to use thermal model. It may be difficult to switching. At sufficient voltage (threshold voltage), justify using different models to understand the a filamentary path is formed due to the Joule observed behaviour in a single system. 5

ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

been shown in the typical STAG glasses by With the help of thermal crystallization studies, the Nakashima and Kao [10]. The filament can have electrical switching exhibited by CuxAs30Se70-x permanent and temporary portions. The size of the permanent and temporary portions depends on the glasses can be understood in the context of thermal amount of current passing through the sample in between the electrodes. By allowing higher current, the size of the permanent portions will increase with a corresponding decrease in temporary portions. At sufficient higher current, the permanent portions can close together leading to memory switching. There are many experimental studies indicating that the increase in the temperature at the time of switching is as high as the melting temperature [1114]. In Ge-Te nano wires, melting of the nano wires and the formation of voids near the top contact are observed [15]. The voids are subsequently, filled by the formation of the conducting crystallites. The temperature rise in the filament of Ge30As20Se50 glass at the time of switching has been estimated to be about 650oC [14]. Simulation and experimental studies also show the temperature rise in the phase change memory material (Ge2Sb2Te5) can be as high as its melting temperature29. Microscopic studies on many of the semiconducting glasses show the liquid phase in between the electrodes at the time of switching [12]. In NiO thin films, the SET and RESET states shows the formation of conducting filaments [16]. In-situ transmission electron microscopy observations reveal that the conducting filaments are in nano size consisting of amorphous and crystalline phases. Hence, it is possible that in CuxAs30Se70-x glasses the material in the interelectrode region can melt and form the filament. This filament may have Cu3AsS3 and Cu3AsSe4 conducting phase (permanent regions) and some high resistive amorphous phase, probably As2Se3 (temporary region). When I ≥ Ih, the permanent portions are linked to have a conducting path. For I < Ih, the activation energy to have the conducting path may not be sufficient and the material reverts back to its high resistive OFF state.

model. The thermal model needs minimal modification to accommodate the different switching types observed in the CuxAs30Se70-x glasses. The samples annealed at Tc show only the ternary Cu3AsSe4 phase. In contrast, the samples melted and quenched in water show Cu3AsSe3 and Cu3AsSe4 phases with considerable amorphous background (fig. 3). The formation of Cu3AsSe4 and Cu3AsSe3 phases is possible only if the added Cu interacts with the parent matrix As-Se. The interaction of Cu with As-Se increases the crosslinking and the rigidity of the structural network, which is reflected as an increase in Tg. DSC, studies also show crystallization peak for all the compositions irrespective of the switching type (fig. 4) indicating all the glasses undergo a phase change in CuxAs30Se70-x glasses. Formation of filament has

6

ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

The unusual switching of low resistance state to a high resistance state observed for x = 25 and 28 is interesting. Similar kind of switching behaviour has been reported for CuxAs30Se70-x and As2Se3Cu glasses [17,18]. In this context, it is worth to mention the work of Bagley and Bair on As2Se33As2Te3 glasses. The surface of the glass was

surface of the Cu25As30Se45 and Cu28As30Se42 glasses may have crystallites as in the case of As2Se3-3As2Te3. The concentration of permanent portions Cu3AsSe4 and Cu3AsSe3 crystallites is high for glasses with x ≥ 30 consequently they exhibit memory switching. The present studies show that both the threshold and memory switching can be understood with the thermal model and filament formation. The filament is formed by glass  melt  crystal/amorphous transition and not by a direct glass  crystal transition. The ratio between the permanent and temporary portions determines the switching type. If the ratio is high, memory switching can be expected and if the ratio is low threshold, switching can be expected. 4. Conclusions Bulk CuxAs30Se70-x glasses showed interesting switching behaviour from ‘absence of switching  threshold switching  low resistance to high resistance  memory switching’. The observation different type of switching is unique to Cu-As-Se glasses. The thermal model with the filament formation very well explains the observed switching behaviour. At the time of switching, the material in the inter-electrode region may melt to form a filament. The melt solidified into permanent (crystalline) and temporary (amorphous) phases in the filament. The ratio between the permanent and the temporary portions dictates the switching type. If the ratio is high, a memory switching will occur and if the ratio is less, threshold switching can be expected. The present study paved a way to understand both the threshold and memory switching within the frame work of the thermal model.

Fig. 4. DSC thermograms of CuxAs30Se70-x glasses.

crystallized before making the contacts for switching measurements [19]. The samples were found to be in high conducting state (ON) before the application of the electric field. Upon the application of the electric field, the samples were found to switch to high resistance state (OFF) as in the present Cu25As30Se45 and Cu28As30Se42 glasses. In this composition range, the structural network may have conducting nano- crystallites, which are connected by weak link[16,18]. The current flowing through this weak conducting path induces Joule heating and ruptures the path. This results in the loss of connectivity and thus the system switches to a high resistive state. The sharp crystallization peak observed for x = 25 and 28 in the DSC spectra indicates that they are prone to crystallization. The

References

7

[1]

Ovshinsky, S.R. Phys. Rev. Lett. 1968, 21, 1450.

[2]

Hudgens, S. Phys. Stat. Solidi B 2012, 249, 1951.

ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

[3]

Bogoslovskiy, N.A.; Tsendin, Semiconductors 2012, 46, 559.

K.D.

[4]

Tohge, N.; Minami, T.; Yanamoto, Y.; Tanaka, M. J. Appl. Phys. 1980, 51, 1048.

[5]

Liu, J.Z.; Taylor, P.C. J. Non-Cryst. Solids 1989, 114, 25

[6]

Ramesh, K.; Asokan, S.; Gopal, E.S.R. J. Non-Cryst. Solids 2006, 352, 2905.

[7]

[8]

[9]

[13] Radaelli, A.; Pirovavo, A.; Benvenuti, A.; Lacaita, L. J. Appl. Phys. 2008, 103, 111101. [14] Weirauch, D.F. Appl. Phys. Lett. 1970, 16, 72. [15] Meister, S.; Schoen, T.; Topinka, M.A.; Minor, A.M.; Cui, Y. Nano Lett. 2008, 8, 4562. [16] Son, J.Y.; Shin, Y.H. Appl. Phys. Lett. 2008, 92, 222106.

Murugavel, S.; Asokan, S. Phys. Rev. B 1998, 58, 3022.

[17] Asahara, Y.; Izumitani, T. J. Non-Cryst. Solids 1972, 11, 97.

Xin, S.; Liu, J.; Salmon, P.S. Phys. Rev. B 2008, 78, 064207.

[18] Haifz, M. M.; Ibrahim, M.M.; Dongal, M. J. Appl. Phys. 1983, 54, 1950.

Adler, D.; Shur, M.S.; Silver, M.; Ovshinsky, S.R. Appl. Phys. Lett. 1980, 153, 289.

[19] Bagley, B.G.; Bair, H.E. J. Non-Cryst. Solids 1970, 2, 155.

[10] Nakashima, K.; Kao, K.C. J. Non-Cryst. Solids 1979, 33, 189.

Acknowledgements The authors thank the Department of Science & Technology (DST) for the financial support.

[11] Yang, T.Y.; Park, I.M.; Kim, B.J.; Joo, Y.C. Appl. Phys. Lett. 2009, 95, 032104. [12] Pearson, A.D.; Miller, C.E. Phys. Lett. 1969, 14, 280.

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ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

case of nobel metals or very small dimensions, lift-off is preferred. Following are the general steps involved in the lift-off process:

Bilayer Lift-off Technique for Micromachining

A thin layer of photoresist is spin coated on the substrate, dried off and exposed to UV radiation through a pattern and developed using a developer. After the development process, patterned photoresist is obtained. The wafer is then placed in vacuum chamber and thermal deposition of metallic thin film is done by ‘thermal evaporation’. The slide is placed in a solvent which seeps under and dissolves the photoresist and the film which is directly deposited is left behind on the substrate.

Neha Yadav Department of Physics, Keshav Mahavidyalay, University of Delhi *Corresponding Author: [email protected] Abstract: This paper discusses the application of bilayer lift off technique for micromachining applications. In micro-machined devices, patterning of metal films is required. The metals can be patterned either by etching or lift-off. In this paper, using two-layer photoresist for liftoff has been presented. This technique can be used for lift-off films having thickness upto 7-8 micron and is very effective in getting desired photoresist profile. The prerequisite for the lift-off is negative profile of the photoresist. The bilayer photoresist can be patterned using photo mask. The resultant pattern can be analysed in optical microscope and SEM. It can be seen that by varying the flood exposure time of the bottom layer, negative profile required for lift-off with desired under-cut could be achieved.

Following are the requirements for a metallic film to be lifted-off: 1. Temperature should not be very high otherwise the photoresist might get burnt. 2. The metal thickness is to be around or less 100nm to allow solvent seep under it and dissolve the photoresist. 3. The deposition of film on the substrate is to be very good. 4. The film is to be easily wetted by the solvent. 5. The film is not to be elastic but brittle, otherwise it will tear along adhesion lines. 6. The film quality is not absolutely critical. That means if requirements on film quality are stringent, then, lift-off is not to be used Photoresist will outgas very slightly in vacuum systems, which may adversely affect the quality of the deposited film.

Key words: lift-off, micromachining, negative profile, under-cut, photoresist 1. Introduction

2. Important parameters for desired Liftoff Result:

Micro-machined devices can be fabricated by either bulk or surface micromachining. Both the processes require patterning of metals at various stages of device fabrication. For patterning of metals, commonly used technique is etching. In etching the wafer is put in a chemical etchant, removing the metal from desired places. But in

1.

2.

9

It is important to create negative slope profile or undercut profile so that liftoff becomes easy. Prebake temperature has the greatest influence on negative slope rate. The parameters which have influence are

ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

prebake time, UV exposure intensity and time of photoresist, the developer, the mode of development and time of development. 3. Careful consideration should be given to the resist/developer system

Tantalum, Titanium and others, the etching chemicals may not be available. The substrate or layers may be sensitive to harsh chemicals. The harsh chemicals may degrade the quality of the substrate (semiconductor) and thereby affecting its quality of performance. Also, smaller the dimensions etch control becomes more difficult. Lift-off technique using Positive photoresists

3. Different technique

methods

for

lift-off

Depending on requirements methods are employed.

different

Positive photoresists are preferred in the IC industry or MEMS foundries due to their ease of removal and better resolution capabilities But for the lift-off applications , the positive photoresists have the limitation of lower softening points (around 120-130°C). This range of temperatures is reached even during the normal coating and hence the resist features rounding and makes it very difficult even impossible to lift-off. Another drawback is that, by using positive photoresists only positive profile or at the most vertical profile is obtained covering the sidewalls during coating and hence making lift-off difficult. If the desired pattern is such that positive photoresists is to be used then the positive resist used should have higher thermal stability and sidewalls of the photoresist should be very steep.

1. Single Layer Resist Processing a) Standard Photoresist Processing: Only one mask step and the standard photolithography procedure are involved. The main disadvantage of this method is that the film is deposited on the sidewall of the photoresist, and adheres to the substrate even after the resist removal. This sidewall may be peeled off in subsequent processing, resulting in particulates and shorts, or it may flop over and interfere with etches or depositions that follow. b) Single Layer lift off technique using negative photoresist. c) Very Thick Negative Photoresist Single Layer

4. Experimental Procedure In bilayer lift-off technique, as the name suggests two layers of photoresist is used with different flood exposure time.

2. Bi-Layer Resist Processing a) PR/LOL 2000 b) PR/ LOR Lift-Off Resist (or PMGI Resist) c) PMMA/PMMA d) PMMA/LOL2000 e) Composite Layers of Aluminum (Al) and Photoresist

A thin film of the assisting material is deposited over the substrate and it is exposed to UV light without masking. A layer of photoresist is spin coated on the substrate and again exposed to UV radiation through a pattern. The mask exposure time is less than the previous exposure time without masking and developed using developer. The underlying layer of the assisted 3. Tri-Layer Processing material is etched by the developer. Metallic thin film is deposited by ‘evaporation’ process. 4. Surface Modified Resist Processing The photoresist is removed and the layer of metal also gets removed along with it and The need for using lift-off technique instead of finally the underlying layer of assisted material etching by conventional methods is that for noble is also removed and well defined metal pattern elements such as Gold, Nickel, Platinum, alone is left. 10

ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

5. Results and Discussion The important point of bi-layer lift-off technique is that the underlying assisting layer is more sensitive to the exposure dose or has a higher dissolution rate in the developer as compared to upper photoresist layer and hence negative profile is obtained which makes it easier to lift-off.

If the exposure time is increased the θ i.e. the angle with the tangent also increases which signifies a steeper undercut and is very much desirable for the lift-off to take place. It can therefore be concluded that by varying the exposure time for bottom layer, desired resist sidewall can be achieved.

4.1. Details

The experimental work involves spin coating of photoresist like AZ9260 to achieve a uniform6. Conclusion: film of thickness 10 micron. After pre-bake at 1000C, the film is to be given flood exposure of For patterning of metal films at various stages i-line UV light using mask aligner. The film is of surface micro-machined devices, this to be post baked at 1200C and same photoresist technique of using double layer photoresist is is coated over it. The thickness of the second quite simple. This technique can be used for layer is to be taken to be 5 micron. lift-off films having thickness upto 7-8 micron and is very effective in getting desired photoresist profile.

References [1] Yifang, Chen, Peng Kaiwu and Cui Zheng. A lift-off process for high resolution patterns using PMMA/LOR resist stack. Microelectronic Engineering, 2004, 73-74, p. 278-281 [2] Shih-Chia Chang and Jeffrey M. Kempisty, 'Lift-off Methods for MEMS Devices’, Mat. Res. Soc. Symp. Proc. Vol. 729

Exposure through mask and development

[3] Photoresist AZ9260 from http://www.nfc.umn.edu/assets/pdf/az_9200.pdf

11

ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

Effect of change in titanium isopropoxide (TTIP) concentration on the preparation of TiO2 nanopowder Mamta Arya, Shubhra Mathur*, Rohit Jain Department of Physics, JaganNath Gupta Institute of Engineering & Technology, Jaipur, 303905, India *Corresponding author. E-mail: [email protected], Abstract TiO2 nano-powder is prepared by changing titanium isopropoxide (TTIP) concentration as 3.5 ml, 4.5 ml and 5.5 ml in 40 ml methanol and thus annealing at 6000 C. X-ray diffraction (XRD) pattern exhibits the presence of mixed phase anatase/rutile in various TiO2 nanopowder specimens prepared by different concentrations of TTIP. It was observed that the content of rutile phase is more in case of 5.5 ml TITP as compared to 4.5 ml and 3.5 ml TTIP of TiO2 nanopowder specimens. The average crystallite size was found to be 35±5 nm for TiO2 nanopowder specimens. UV studies show that indirect and direct band gap lies in the range of 2.95-2.76 eV for different TTIP concentrations 3.5 ml, 4.5 ml and 5.5 ml of TiO2 nanopowder specimens.

Keywords: nanopowder, band gap, XRD, TiO2

phases of TiO2 amongst which rutile is a high temperature stable phase. However, anatase and brookite are metastable phases and transform to rutile on heating. Anatase phase show an energy band gap of 3.2 eV whereas rutile phase exhibits an optical band gap of 3.0 eV [3]. Sol-gel is a versatile method used for the preparation of TiO2 nanopowder [4-5]. The change in concentration of titanium isopropoxide, which acts as a starting material in our investigation, may lead to change in structural and optical properties of TiO2. This motivated us to carry out the present study. 2. Experimental 2.1. Materials Titanium isopropoxide (TTIP) and methanol are used as starting materials. The chemicals used are of analytical research (AR) grade. 2.2. Methods TiO2 nanopowder is prepared by using sol gel method. Sol-gel process also known as a wetchemical technique is used for the fabrication of both glassy and ceramic materials. In this process, the sol (or solution) evolves gradually towards the formation of a gel-like network containing both a liquid phase and a solid phase [5]. 2.2.1. Preparation of Samples Titanium isopropoxide (TTIP) taken in different concentrations as 3.5 ml, 4.5 ml and 5.5 ml is mixed in 40 ml methanol. This results in a milky white solution and is vigorously stirred for 1:30 hours at a temperature 57±3⁰C. The gel thus produced is kept for drying at room temperature for 12 hrs. Hence the powder is obtained and annealed at 600⁰C for 1 hour in air [5].

1. Introduction

3. Results & Discussion

Titanium dioxide (TiO2) is considered as the most promising semiconductor metal oxide because it exhibits highly enhanced photo catalytic activity [1] and improvement in gas sensing properties [2]. Anatase, rutile and brookite are three well known

X-Ray diffraction pattern (XRD) of TiO2 specimens having different concentration of (TTIP) is recorded using Cu-Kα radiation as shown in Fig1. Diffraction peaks showing the presence of both anatase and rutile phase are in good 12

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Table 1: Average crystallite size and content of phases in TiO2 nanopowder specimens.

agreement with the JCPDS no. 21-1272 for anatase, 21-1276 for rutile and data reported in the literature [6-7].[0]›

301 R

204 A + 002 R

105 A + 211 R 211 A

110 R

101R 103 A

200 A

004 A 112 A 111 R

101 A

(a) TTIP 3.5 (b) TTIP 4.5 (c)TTIP 5.5 A-Anatase, R-Rutile

110 R

Intensity (arb. units)

TTIP (ml)

3.5 4.5 5.5

Intensity Ia (101 anatase)

Intensity Ir (110 rutile)

1496.63 1336.21 1497.34

30.79 59.43 105.54

XRD Average crystallite size (nm) 34 36 39

% Anatase

% Rutile

97.27 94.69 91.84

2.73 5.31 8.16

(c)

(b) TTIP 5.5

110 R

4.1

TTIP 4.5

4.0

(a)

TTIP 3.5

3.9

20

30

40

50

60

Absorbance

3.8

70



Fig.1. X-ray diffraction pattern (XRD) of TiO2 nanopowder prepared with different

3.7 3.6 3.5 3.4 3.3 3.2 3.1

concentrations of titanium isopropoxide as (a) TTIP-3.5 ml, (b) TTIP-4.5 ml and (c) TTIP-5.5 ml. Table 1 shows the average crystallite size calculated using Scherrer formula [6] and the content of anatase and rutile phase which is calculated using formula Xa = 100/ 1+ [3]›1.265 (Ir/Ia) where Xa is the weight fraction of anatase in the mixture, Ia and Ir are intensities of anatase (101) and rutile (110) diffraction peaks [6].

200

300

400

500

600

700

Wavelength (nm)

Fig. 2(a) TTIP 5.5 TTIP 4.5 TTIP 3.5

25 20

(h

15 10 5 0

1.6

2.0

2.4

2.8 (h)

Fig. 2(b)

13

3.2

3.6

4.0

4.4

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TTIP 5.5 TTIP 4.5 TTIP 3.5

2.2

Therefore, increase in content of rutile phase leads to increase in the crystallite size of TiO2 nanopowder [6]. The band gap energy of TiO2 specimens with different concentration of TTIP as formulated in Table 2 shows lower band gap values as compared to band gap energy 3.2 eV for pure anatase and 3.0 eV for pure rutile phase because in our investigation TiO2 nanopowder specimen is a mixture of both anatase and rutile phases [9]. Moreover it was observed that optical band gap energy increases with decrease in crystallite size, which leads to blue shift of the optical absorption edge [8]. Further it was reported that the specimens with mixed phase anatase/rutile TiO2 nanopowder show improved photo catalytic and gas sensing properties [1-2]. Hence, TiO2 nanopowder specimens prepared in our investigation by simple sol gel method may be use to study photocatalytic and gas sensing properties.

2.1

(h

2.0 1.9 1.8 1.7 1.6 1.5 1.4

1.6

2.0

2.4

2.8

3.2

(h

3.6

4.0

4.4

Fig. 2(c) Fig. 2. UV spectroscopy results (a) absorption spectra (b) Tauc plot for direct band gap energy (c) Tauc plot for indirect band gap energy.

Fig. 2 (a) represents UV spectra of TiO2 specimens with different concentration of TTIP. The band gap energy is determined by Tauc plot as shown in Fig 2 (b) and Fig. 2 (c) [8]. The band gap energies thus obtained are summarised in Table 2. Table 2: Energy band gap values of TiO2 nanopowder specimens TTIP(ml)

3.5 4.5 5.5

2. The mixed phase anatase/rutile TiO2 nanopowder exhibits lower band gap energy as compared to pure anatase and rutile phases. References

UV

Indirect Bandgap (eV) 2.95 2.87 2.79

4. Conclusion 1. The least concentration of TTIP (3.5 ml) leads to formation of TiO2 nanopowder having smallest average crystallite size.

Direct band gap (eV)

[1] Singh.J.; Mohapatra,S. Adv. Mater Lett. 2015, 6, 924.

2.93 2.84 2.76

[2] Enachi, M.; Lupan, O.; Braniste, T.; Sarua, A.; Chow, L.; Mishra, Y.K.; Gedamu, D.; Adelung, R.; Tiginyanu, I.; Phys. Status Solidi RRL 2015, 1

The X-ray diffraction pattern (XRD) revealed the [3] Hanaor,A.D.H.; Sorrell,C.C. J Mater Sci. 2011, presence of both anatase and rutile phase in TiO2 46,855. nanopowder specimens and the average crystallite size increases with increase in concentration of [4] Zainurul, A. Z.; M.; Abdullah. S. Achoi, M.F.; Rusop, Advanced Materials Research 2014, 832, TTIP.[0]› It is noteworthy here that the content of 649. the rutile phase also increases with increase in concentration of TTIP. [5] Pawar, S.; Chowgule, Patil S.; Raut, B.; Dalvi, D.; Sen, S.; Joshi, P.; Patil, V. Journal of Sensor Technology 2011, 1, 9.

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[6] Dai, S.; Wu, Y.; Sakai, T.; Du, Z.; Sakai, H.; [9] Paul, S.; Choudhury, A. Appl Nano Sci 2014, Abe, M. Nanoscale Research Letters, 2010, 5, 4, 839. 1829. Acknowledgment [7] Vijayalakshmi, K.; Rajendran, K.V. 2010, Authors thank Science & Engineering Research AZojomo 2010, 6, DOI: 10.2240/azojomo0298 Board (SERB) for providing financial grant vide no SERB/F/5303/2014-15 and MRC, MNIT, Jaipur [8] Tripathi, A.K.; Singh, M. K.; Mathpal, M. C. ; for XRD facility. Mishra, S. K. ; Agarwal, A. Journal of Alloys and Compounds, 2013, 549, 114.

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2]. Graphene has an electron mobility of 2.5 × 105 cm2 V-1 s-1 [1].

Calculation of Some Oscillating Parameters For Graphene

Frank et al. [3] have experimentally studied effective spring constant of stacks of suspended graphene sheets (less than 5) and found the value of spring constant lies in the range 1 to 5 N/m. In this paper, we intend to determine some oscillating parameters such as frequency, spring constant and damping coefficient of a graphene sheet under oscillation due to tensile force theoretically.

D. K. Das*1, K. V. V. Nagaraju2, S. Roy 3 and S. Sahoo4 1

Department of Metallurgical and Materials Engineering National Institute of Technology, Durgapur-713209, West Bengal, India. 2, 3, 4 Department of Physics, National Institute of Technology Durgapur-713209, West Bengal, India. *Corresponding Author: [email protected]

This paper is organized as follows: In Sec. 2, we calculate the frequency (ωnA), spring constant (K) and damping coefficient (Cc) of a graphene sheet under oscillation due to tensile force. In Sec. 3, we discuss our results. In Sec. 4, we present our conclusion.

Abstract In recent years graphene has become a hot topic of research in various sectors due to its many advanced properties such as high tensile strength, stiffness etc. It is a two-dimensional (2D) nanomaterial. Reduced dimensional structure makes graphene mechanically rigid and stiffest ever. Frank et al. have experimentally studied effective spring constant of stacks of suspended graphene sheets (less than 5) and found the value of spring constant lies in the range 1 to 5 N/m. In this paper, we calculate the frequency, spring constant and damping coefficient of graphene under oscillation due to tensile force theoretically.

Calculation graphene

of

oscillating

parameters

for

Let us consider a graphene sheet with dimension 800×300 nm in length and breadth respectively which is being fixed at one end. A force is applied at the other end and released. The sheet starts oscillating as shown in Fig. 1 below:

Keywords: Graphene; frequency; spring constant; damping coefficient.

1. Introduction Fig. 1: Graphene sheet fixed at one end, force is applied

Graphene is sp2 hybridized, single atomic layer on the other end and released (oscillation) hexagonally arranged network of carbon atoms. A single pi (π) bond and three sigma (σ) bonds joins The original length of the sample L = 800 nm. Now each carbon atom in graphene with its neighboring the frequency of oscillation for the graphene sheet carbon atoms. A loan pair of free motile electrons is given by [4] forms each pi bond. The soft, lustrous and n E , (1)  nA  lubricating nature of graphene is due to presence of l  e these free electrons. They also results in high electrical and thermal conductivity of graphene [1,

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where n = mode value =1 (for here), le = effective length of sheet = 750 nm (say),  = density of graphene = 2300 kg/m3 and E = Young’s modulus of graphene sheet = 1 TPa [5]. Putting these values in equation (1), we get ωnA = 8.7298×1010 rad/s or 1.39×1010 Hz. Again we know time period of oscillation for a vibrating body is given by [6]

,

(2)

where, ω is the frequency for oscillation (ωnA) for this case. The time period of oscillation for the said Fig.2. Single unit cell of graphene sheet [7] graphene sheet is found to be 7.1937×10-11 s. The relation between frequency and spring constant for where, T = Tension applied in one end of the sheet, oscillation motion is given by the relation [6] m = 3.4293 × 10-13  = mass per unit length in = l kg/m and l = length of sheet = 800 nm. Putting 1 K fn  , (3) these values in equation (4) we calculate the 2 m magnitude of tensile force (T) = 1.6939  10 4 N . where, m is the mass of the object and K is the We also know for oscillation [6], spring constant. , (5) For the considered graphene sheet (Fig. 2), the C-C where, x is the increment in length of the sheet due bond length = a = 1.42Å = 0.142nm [8], length of to application of force. So putting above obtained unit cell = 3 a = 0.426 nm, width of unit cell = 3a values of T and K in equation (5) we get, x = -8 = 0.246 nm, area of the unit cell of graphene = 8.1134×10 m. The generalized wave equation is 0.104796 nm2, total surface area of graphene sheet given by [6] = l × b = 240000 nm2. Hence, the total number of x  A0 cos nAt  B0 sin  nAt , (6) atoms (n) in the considered graphene sheet is 13740983. We know the mass of each carbon atom where, A0 and B0 are the amplitudes in x and y ( mc ) = 1.994 × 10-23 gm [9]. Hence, the total mass directions respectively. At t = 0 i.e. starting of of the graphene sheet is 13740983 × 1.994 × 10-26 = oscillation equation (6) is reduced to 2.7399 × 10-19 kg. x  A0 , (7) Now putting these values in eq.(3) we get K = 2087.7746 N/m. The relation between frequency of Here, we have obtained the values of A0 = oscillation and force applied on the material can be 8.1134×10-8 m and An = 8.1631×10-8 m at t = 7.1937×10-11 s. So it is a damped vibration. The written as [6] relation between decrement in amplitude with time can be stated as [10] 1 T fn  2l  ,… (8) An  A0 e nt , (4)

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where, ξ is the damping ratio. Putting the above We hope our results can be useful for the design of values in equation (8) we get ξ = -9.7245×10-6. The the next generation nanodevices and coefficient of critical damping (Cc) is given by [10] nanofabrication technologies that use the vibration properties of graphene. Our theoretical results would be verified theoretically as well as , (9) experimentally in future for confirmation. Using the values of K and m in equation (9), we get Acknowledgement Cc = 4.7834×10-8 kg/s. Further, we know that [10] Mr. K. V. V. Nagaraju thanks NIT Durgapur for providing fellowship during his M. Tech. study. ξ = C/ CC , (10) References where, C is the coefficient of damping. From here we calculate C = 4.5616×10-13 kg/s. [1] Novoselov, K. S; Fal′ko, V. I; Colombo, L; Gellert, P. R; Schwab, M. G; Kim, K; Nature, 2. Results 2012, 490, 192. We have found that the oscillation parameters for graphene are depending on the Young’s modulus and size of the material. Complete analytical work is carried out with a graphene sheet of dimensions (800nm×300nm). Our results show that mechanical stiffness of our graphene sheet (K= 1996 .20784 N/m) is much higher than previously reported values. Our calculated parameters are reported in tabular form below:

[2] Maity, S; Ganguly, M.; Elements of Chemistry1, Publishing Syndicate; Kolkata, 2003.

Table:1. Oscillating parameters for graphene

[5] Lee, C; Wei, X; Kysar, J. W; Hone, J; Science, 2008, 321(5887), 385.

Sl.

Oscillating Parameters

Our calculated values 8.7298×1010 rad/s

2.

Frequency of vibration (ωnA) Spring constant (K)

3.

Damping Coefficient

4.5616×10-13 kg/s.

No. 1.

[3] Frank, I. W; Tanenbaum, D. M: J Vac. Sci. Technol. B, 2007, 25(6), 2558. [4] Gupta, S. S; Batra, R. C; J. Comput. Theor. Nanosci., 2010, 7, 1.

[6] Datta, D; Pal, B; Chaudhuri, B; Elements of Higher Secondary Physics-1, Publishing Syndicate, Kolkata, 2002.

2087.7746 N/m

[7] Yamayose, Y; Kinoshita, Y; Doi, Y; Nakatani, A; Kitamura, T; Eur. Phys. Lett., 2007, 80, 40008. [8] Fujita, T. K. W; Oshima, C; Surface and Interface Analysis., 2005, 37(2), 120.

3. Conclusion

Analysis on these oscillating parameters of [9]http://chemistry.about.com/od/workedchemistry graphene is very useful to study its mechanical problems/a/avogadroexampl1.htm. properties. These are also useful to design the nanomechanical resonators and [10] Nag, D; Mechanical Vibrations, Wiley, Delhi, nanoelectromechanical resonator sensors because 2011. graphene shows ultra-high sensitivity of vibrations.

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water solubility, low cytotoxicity, excellent biocompatibility, and resistance to photo-bleaching [1-4].

Study of doped graphene quantum dots by chlorine containing compounds: Electronic Spectroscopy

Doping with different metals is the most realistic tool to tune the semiconducting properties in the conventional semiconductor community. Nevertheless, due to presence of low defects in undoped GQDs, weak optical properties can be seen. Doping heteroatoms including boron, nitrogen, chlorine, sulphur, fluorine can improve the electronic characteristics of GQDs to introduce more defects [5-8]. Nevertheless, bandgap is increased in GQDs after doping with different heteroatoms showing ideal p- and n-type semiconducting electronic properties for potential applications of GQDs in electronic devices.

Poonam R. Kharangarh*, and Gurmeet Singh

Department of Chemistry, University of Delhi, Delhi 110007, India *Corresponding Author: [email protected]

A lot of research has been declared that the doping of different atoms into GQDs alters the band gap between conduction band maximum and valence band minimum. Results were shown that a new energy level was introduced to tune the optical properties in order to make GQDs for solar cells applications. In order to fulfill the energy requirements and to generate the photo-current, we need to choose a appropriate material which can modify the energy band structure. Herein, we present a facile hydrothermal method to prepare doped GQDs with different transition metals having chlorine containing elements. When chlorine containing compounds are doped into GQDs, it usually has different absorption bands induced by edge effect in modified GQDs. Furthermore, the effect of metals on the electronic structure of GQDs still remains unclear. Hence, there is a need to investigate how these metals modifies the energyKey words: Graphene Quantum Dots, TEM, UV- level structure in case of doped-GQDs. Cyclic Visible, Transition Metals, Energy Gap, HOMO, voltammetry characterization technique [9-11] LUMO reveals that the different band gap is obtained upon the integration of chlorates into the GQDs. 1. Introduction Abstract: For the study of high quality doped graphene quantum dots, a series of chlorine containing compounds such as CoCl2, HCl, and NH4Cl were used. The morphology of the samples were done by Transmission Electron Microscope (TEM). The absorption of the doped material was found by U-V visible spectroscopy for optical study. The redox behaviour has been observed by using Cyclic Voltammetry tool. Different electronic structures for different doped graphene quantum dots were observed from UV- Visible Spectroscopy. Cyclic Voltammetry measurements show the oxidation and reduction of different metal doped GQDs to calculate the energy for the conduction band edges parameters (HOMO and LUMO).

2. Experimental Graphene Quantum Dots (GQDs), fragments of graphene has been brought tremendous attention 2.1. Materials due to their physical properties, including excellent

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In this work we have used commercially available different transition metals are estimated to be in the graphite powder, NaNO3, KMNO4, H2O2, NH4Cl, narrow range of 5-15 nm in diameter. CoCl2, HCl and H2SO4. Double distilled water was used for all the experiments during the preparation of graphene oxide (GO) and doped GQDs 2.2 Synthesis of Graphene Oxide/ Different Metals doped GQDs Graphite oxide was prepared in accordance with the procedure described by Hummers and Offemann [12]. The brief description of doped CoCl2-GQDs was explained in refs [13-14]. The same procedure was followed for 6.06 mg of NH4Cl doped GQDs and 6mg of HCl doped GQDs. The centrifugation was done at 4000 rpm for as prepared solution before to carry out the further characterizations.

(a)

(b) 50 nm

2.3 Characterization Techniques Transmission Electron Microscope (TEM) was recorded on samples using FEI Technai G2 20 electron microscope operating at 200 kV. Perkin Elmer Lambda 35 spectrophotometer was used to record the absorption spectra of dispersions with a slit width of 2 nm and scan speed of 240 nm/min. The electrochemical measurements were performed with the help of CHI-760C potentiostat galvanostat instrument by using a three electrode system where glassy carbon electrode (diameter ~ 3 mm) was used as a working electrode, Ag/AgCl as a reference electrode and Pt wire as a counter electrode in aqueous electrolyte. The electrolyte was chosen as 0.05M KCl in aqueous medium. The working electrode was prepared by dropwise casting on glassy carbon electrode. Cyclic voltammetry (CV) experiments were carried in the potential range of -0.8V to 0.2Vfor HCl doped GQDs and NH4Cl doped GQDs whereas the potential window was adjusted from -0.8 to 0.4 V for CoCl2 doped GQDs.

(c) 50 nm

Fig. 1. TEM images of the (a) CoCl2-GQDs, (c) HClGQDs [ref14] and (c) NH4Cl-GQDs

Fig. 2 shows that the UV-visible absorption spectrum of NH4Cl-GQDs, HCl-GQDs and CoCl2GQDs in aqueous solutions. As we know that the absorption peak for GO is at 232nm [15] and GQDs is characterized by a 323 nm band, which is redshifted from 232 nm of GO resulted from n-π* transitions of C=O. bond [16]. New energy levels are observed due to the presence of functional group state possibly or related to oxygen after doping in between valence band (π band) and conduction band (π* band). The new shifted peak is observed at 298nm (4.16 eV) after the treatment of CoCl2, 330nm (3.8eV) in case of HCl and 5.4eV for NH4Cl. A large band is observed in NH4Cl -GQDs as compare to all other different transition metals.

3. Results and Discussion The optical energy band gap, Eg, can be calculated Fig. 1(a, b, c) show the HRTEM images of the to find out the energy levels of the electronic states CoCl2-GQDs, HCl-GQDs and NH4Cl-GQDs by using equation [17] respectively. The majority of the doped GQDs with

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Eg = 1242/λonset

Ag/AgCl, but redox behavior is absent in NH4ClGQDs.

(1)

where λonset is the longest absorption wavelength.

In CoCl2-GQDs, anodic peak of redox pair is responsible for the oxidation of Co2+/Co4+ whereas cathodic peak corresponds to a reduction process following the Faradic reduction reactions from Co4+ to Co2+. It is noted that the cathodic peaks shifts more positively in CoCl2 doped GQDs in comparison to NH4Cl-GQDs and HCl-GQDs and the anodic peaks is more negatively in NH4ClGQDs and HCl-GQDs which is mainly due to the resistance of electrode. Table 1 Energy levels of CoCl2-GQDs, and HCl-GQDs

Materials

CoCl2GQDs, Eox (V) -0.65 HOMO level (eV) -5.05 Ered (V) 0.8 LUMO level (eV) -3.6 Eg [from CV (eV) 1.45 Optical Eg(eV) 4.16 [from UV]

Fig. 2. UV-Vis Spectroscopy for NH4Cl-GQDs, HClGQDs and CoCl2-GQDs.

The energy levels were calculated by using the following empirical Bredas et al. [18] equations: E (HOMO) = -e [Eox onset + 4.4] (2) E (LUMO) = -e [Eredonset + 4.4]

(3)

HClGQDs 0.2 -4.2 0.35 -4.05 0.15 3.8

4. Conclusions In this study, GQDs doped with different transition metals like CoCl2, NH4Cl and HCl were prepared by a facile hydrothermal method. Transition levels of GQDs doped with chlorine containing compounds were also studied by using U-V Visible spectroscopy. Cyclic voltammetry measurements were done for each of these elements to estimate their energy levels. The reversible redox behavior has been observed in CoCl2 doped GQDs and HCl doped GQDs. The presence of high electron affinity in CoCl2 related compounds suggests that they are high-quality candidates as acceptor elements for Fig. 3. Cyclic Voltammetry curve for, NH4Cl-GQDs, solar cells applications. HCl-GQDs, and CoCl2-GQDs.

References Fig. 3 shows that the cyclic voltammetry behavior of different doped graphene quantum dots. A [1] Zhou, X. ; Zhang, Y. ; Wang, C.; Wu, X.; Yang, reversible two electron reduction is observed in Y.; Zheng, B.; Wu, H.; Guo, S.; and Zhang, J.; ACS CoCl2-GQDs, and HCl-GQDs with respect to Nano, 2012, 6, 6592–6599

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[10] Wang, L.; Wang, Y.; Xu, T.; Liao, H.; Yao, C.; Liu, Y.; Li, Z.; Chen, Z.; Pan, D.; Sun, L. and Wu, M.; Nature Communication, 2014, 52, 1-9

[2] Pan, D. Y.; Zhang, J. C.; Li, Z.; and Wu, M. H.; Adv. Mater., 2010, 22, 734–738. [3] Tang, L.; Ji, R.; Cao, X.; Lin, J.; Jiang, H.; Li, X.; Teng, K. S.; Luk, C. M.; Zeng, S.; Hao, J.; and Lau, S. P.; ACS Nano, 2012, 6, 5102–5110.

[11] Mondal, S.; Rana, U.; Malik, S.; Chemical Communications, 2015, 51, 12365-12368

[4] Zhuo, S.; Shao, M.; and Lee, S. T.; ACS Nano, 2012, 6, 1059 – 1064.

[12] Hummers, W. S.; Offeman, R. E.; J. Am. Chem. Soc., 1958, 80, 1339-1339

[5] Yang, Z.; Yao; Z; Li, G.; Fang, G.; Nie, H.; Liu, Z.; Zhou, X.; Chen, X., Huang, S.; ACS Nano. , 2012 , 6(1), 205-11

[13] Poonam R. Kharangarh, Akshay Kumar, Siva Umapathy and Gurmeet Singh, Synthesis of CoCl2Doped Graphene Quantum Dots and its Photocatalysis, ISST Journal of Applied Physics, 2016, 7, pp. 42-46.

[6] Panchakarla, L. S.; Subrahmanyam, K. S.; Saha, S. K.; Govindaraj, A.; Krishnamurthy, H. R.; Waghmare, U. V.; Rao, C. N.; Adv. Mater., 2009, [14] Poonam R. Kharangarh, Akshay Kumar, Raj K. Sharma and Gurmeet Singh, “Thermal effects 21, 4726–4730 for the doped Graphene Quantum Dots: Cyclic Voltammetry, Advanced Materials Proceedings, [7] Li, Y.; Zhao, Y.; Cheng, H.; Hu, Y.; J. Am. 2017, 2(3), 171-175. Chem. Soc., 2012, 134 (1), 15–18 [15] Luo, Z.; Lu, Y.; Somers, L. A.; and Johnson, [8] Zhao, J.; Tang L.; Jinzhong, X. J.; Ji, R.; Yuan, A. T. C.; J. Am. Chem. Soc., 2009, 131, 898–899. J.; Zhao, J.; Yu R.; Tai, Y.; and Song, L.; Appl. Phys. Lett., 2014, 105, 111116 [16] Li, L.; Wu, G.; Yang, G.; Peng, J.; Zhao, J.; and J. Zhu, Nanoscale, 2013, 5, 4015 [9] Liu, W. W.; Feng, Y. Q.; Yan, X. B.; Chen, J. T.; Xue, Q. J.; Adv. Func. Mater., 2013, 23, 4111- [17] Mohamed, M.; Eichborn, A. H.; Eichborn, S. 4122 H.; ECS Transactions, 2010, 25, 1-10 [18] Bredas, J. L.; Silbey R.;, Boudreux, D. S.; Chance, R. R.; J. Am. Chem. Soc., 1983, 105 , 6555

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computers, etc. but, they prevent the processor speed above few Gb/s [1]. On the other hand, the photonic interconnects, such as optical fibers offer ultra-fast and large information carrying capacity (Tb/s). Unfortunately, the photonic devices are limited in size by the diffraction limit of about half the wavelength of light (~ submicron), and tend to be at least two orders larger than that of the electronic components [2]. This size-mismatch between the electronic and the photonic components has been overcome by the study of propagation of surface modes in the metallodielectric waveguides [3].

Electromagnetic Wave Propagation in Photonic Structures: Dielectric and Metallo-Dielectric Waveguides Triranjita Srivastava, Pushpa Bindal*, Priyanka, Anuradha, Priyam and Priscilla Department of Physics, Kalindi College (University of Delhi), Delhi, India, 110008

In this paper, we present the propagation characteristics of planar dielectric as well as few be metallo dielectric waveguides, which can be realized at subwavelength scale. The modal analysis for the evaluation of propagation constant and the modal fields for both TE and TM modes have been done for dielectric waveguides. Moreover, SPP modes have been studied for two types of basic metallo-dielectric waveguides, namely; dielectric layer between metal on either side (MDM) and metal layer between dielectric on either side (DMD) waveguides.

*Corresponding Author: [email protected] Abstract: The photonic waveguides are the vital elements of integrated optics. In this paper, we present the analysis of the electromagnetic wave propagation in dielectric and few metallo-dielectric waveguides. We present the universal V~b curves and the modal fields for both TE and TM modes for dielectric waveguide. The metallo-dielectric waveguides comprise of various combinations of metal and dielectric materials. The propagation characteristics of basic metallo-dielectric waveguides have been studied. We believe that present work will enhance physical understanding of the electromagnetic wave propagation through various photonic waveguides.

2. Mathematical Description

The analysis of dielectric planar waveguide (as shown in Fig.1) is done by solving the Maxwell’s equations. One obtains two sets of independent equations consisting of only transverse electric field (TE Modes) and transverse magnetic field (TM modes) respectively. It is well known that the symmetry in the structure results into symmetric Key words: Dielectric waveguides, Metallo- and antisymmetric modal field solutions as given Dielectric waveguides, Surface Plasmon Polaritons. below [3]: Symmetric mode: 1. Introduction  A cosx x  d /2     x (1a) The increasing demand of faster and huge data x  d /2 Ce transportation and processing has resulted into a tremendous surge in developmental activities of electronics and photonics. The electronic circuit elements are now a days realized as small sized functional devices such as mobiles, televisions,

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1  1  1   V 1  b  tan  V 1  b     V b  2  2  2 

Antisymmetric mode:

 A sin x     x Ce

x  d /2 (1b)

x  d /2

(3a)

Antisymmetric mode: where   k 02 n12   2 ,    2  k 02 n22 , A and C 1  1  1  are the constants to be determined. It is to be  V 1  b  cot V 1  b     V b   2  2  mentioned that, the non-vanishing field components  2 (3b) for TE are Hx, Ey and Hz, whereas for TM modes, (a) Metallo-dielectric Waveguides The metallo-dielectric waveguides comprise of metals and dielectric in different configurations. Such waveguides support SPP modes which are known to be TM polarized in nature and are highly z confined to the metal/dielectric interface. In d literature, several types of metallo-dielectric waveguides are reported, in which the two basic Fig. 1. Schematic of the planar dielectric waveguide. metallo-dielectric waveguides are MDM (Fig. 2a) they are Ex, Hy and Ez. Now applying the boundary and DMD (Fig. 2b) waveguides. conditions for the TE (continuity of  and d /dx) and TM mode ( and (1/n2) d /dx) gives the (i) Metal/dielectric/metal (MDM) Waveguide following eigen-value equations:

Symmetric mode:

  d  tan       2 

  d  Antisymmetric mode: cot      2 

(2a)

The SPP mode arising at the metal/dielectric interfaces forms two coupled SPP modes, having symmetric and antisymmetric field distributions with respect to the central axis, schematically shown in Fig. 2.

(2b)

The modal field for the symmetric and antisymmetric SPP mode can be written as follows: Symmetric SPP:

 A cosh   d y  E y ( y)   B exp    m  y  t  

where   1 for TE mode and   n12 / n22 for TM modes. To obtain the universal characteristics of planar dielectric waveguides, we rewrite the above eigen-value equations in the form of normalized





frequency V  (2 /  )d n12  n 22 and normalized

 





2  n22 / n12  n22 : propagation constant b  neff

Symmetric mode:

24

y t y t

(4a)

ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

3. Results and Discussion

MDM

(a) Dielectric planar waveguide Figure 3, illustrates the variation of b (normalized propagation constant) with V (normalized frequency) for three lower order TE and TM modes. It is observed that b-values for TE modes are slightly greater than that of TM modes. Also, the fundamental TE0 and TM0 modes have no cut-off VDMD values, whereas the higher order TE1 (TM1) and TE2 (TM2) modes have a finite cut off V-value corresponding to V= π and 2π. The b-value for fundamental TE0 mode is highest indicating maximum mode confinement within the core of the waveguide. In order to clarify this point, Fig. 4 (a) Fig. 2. The schematic of a MDM and DMD waveguides, and (b) illustrates the electric field of the first three lower order TE and TM modes respectively. It is showing the symmetric and antisymmetric SPP modes. observed that the modal power for the fundamental TE0 and TM0 mode is tightly confined within the core (i,e, d) of 4 µm. Whereas, the evanescent field Anti symmetric SPP: in the cladding region increases with the order of  A' sinh   d y  y t the mode, thereby reducing the field confinement E y ( y)   (4b) with increasing order. y t B' exp    m  y  t   where, A, B, A’ and B’ are the constants to be

1

determined,  d ,m   2  k 02 d ,m . After solving

TE

0

these equations as mentioned above, we obtain following two eigenvalue equations:

0.8

  Symmetric SPP: tanh ( d t )   d m  m d

0.6

  Antisymmetric SPP: coth ( d t )   d m  m d

TM

0

tanh ( m t )  

 m d  d m

Anti-Symmetric SPP: coth ( m t )  

 m d d m

TM

TE

2

1

0.4

TM

1

(5b)

0.2

(ii) Dielectric/metal/dielectric DMD waveguide Similarly, the eigenvalue equation for both the modes of DMD waveguide is given as: Symmetric SPP:

2

b

(5a)

TE

0 0

2

4

6

8

10

12

14

V

Fig. 3. Variation of b (normalized propagation constant) with V (normalized frequency)

(6a)

(6b)

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(a) 1 TE

Electric field(a.u)

0

TE

0.5

1

TE

2

0

-0.5

-1 -8

-6

-4

-2

0

2

4

6

8

x-coordinate( m)

(b) 1

TM

0

1

0.5

TM

2

3

(a)

0

2.5

-0.5 eff

)

Electric field(a.u)

TM

We have shown the variation of real part of effective indices neff and the propagation length [2] for the symmetric as well as the antisymmetric mode with respect to the waveguide thickness (at wavelength 633 nm) for MDM waveguide comprising of Au and Silica (RI = 1.45) in Fig. 5(a) and (b). It is observed that at a large value of '2t', the neff as well as the propagation lengths of both the SPP modes approaches to that of the SPP mode at a single interface (Si/Au). It is also observed that with decreasing '2t', neff for the symmetric SPP mode increases; whereas for the antisymmetric SPP mode, it decreases. It is to be noted that, higher the neff, more is the mode confinement and thereby, higher is the Ohmic loss inside the metal (i.e. smaller propagation lengths). Although the propagation

Re(n

Symmetric mode

-1 -6

-4

-2

0

2

4

2

6

x-coordinate( m)

Fig. 4. Electric Field distribution for 3 lowest order TE modes (V = 7.7) and TM modes (V=13.3), d = 4μm. (b)

It is to be mentioned here, that although the fundamental mode has zero cut-off V-value, still such dielectric waveguides cannot be realized at very smaller V-value, i.e. smaller (~ subwavelength) width. The reason is attributed to the fact that the smaller the V-value, smaller is b and hence, the mode confinement within the core region is lost, which is also understood the diffraction limit of light.

1.5 0 12

0.2

0.4

Anti Symmetric mode 0.6 0.8 2t (m)

1

Symmetric mode 10

L (m)

8 Anti-Symmetric mode 6 4 2

(b) Metallo-Dielectric Waveguides 0 0

0.2

0.4

0.6

0.8

1

2t (m)

(i) MDM waveguide:

Fig. 5. Variation of (a) real (neff) and (b) Propagation length with respect to the width of MDM waveguide.

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length of both the modes are ~ few μm, still it is sufficient for the nanoscale dimensions [1]. Also, the symmetric SPP mode does not have cut-off thickness, whereas the antisymmetric SPP mode has finite cut-off thickness, indicating that the symmetric SPP mode can be realized at a very small waveguide thickness ~ few 10 nm, thereby indicating the realization of highly miniaturized waveguides.

(b)

L (m)

10

10

(ii) DMD Waveguide: In contrast to MDM waveguides, the DMD structures possess a complementary behavior for the symmetric and the anti-symmetric modes. Fig. 6 (a) and (b) illustrates the variation of the real part of neff and propagation lengths of both the symmetric and antisymmetric SPP modes with respect to the metal stripe thickness '2t' (at wavelength 633 nm) for DMD waveguide comprising of Si/Au/Si. The figure shows that for both the SPP modes there is no cut off thickness and at larger values of '2t' the mode effective indices of both the modes approach to that of the SPP at the single metal/dielectric interface.

Symmetric mode

0

Anti-Symmetric mode

0.05

0.1

0.15 2t (m)

0.2

0.25

0.3

Fig. 6. Variation of (a) real part of neff and (b) Propagation length with respect to width of the DMD waveguide.

the antisymmetric mode it increases. Therefore, the anti-symmetric SPP mode is confined to the metal stripe of very small thickness. The propagation length of the symmetric mode is ~ few mm, which is several orders higher than that of MDM waveguides. It is to mention here that the DMD waveguides are highly useful in sensing applications, as the Ohmic loss inside the metal is very low and the modal field has large spatial extent in the dielectric region.

1.7

Thus the metallo-dielectric waveguides can support SPP modes, which are confined to the metal/dielectric interface at subwavelength. However, such modes suffer ohmic loss due to the presence of metal, but the propagation length ~ few 10 nm, which is sufficient for the miniature structures.

1.6

4. Conclusions

2

1.9

1.8

)

Anti-Symmetric mode

eff

Re(n

2

0

As the metal stripe thickness '2t' decreases, neff for the symmetric SPP mode decreases whereas, for

(a)

10

4

1.5 0

In this work we present the modal characteristics of the planar dielectric and the plasmonic waveguides. The plot of normalized propagation constant b Verses normalized frequency V, and the modal field distributions are shown for dielectric

Symmetric mode 0.05

0.1

0.15 2t (m)

0.2

0.25

0.3

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ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

waveguides, which indicates that such waveguides [4] A. Ghatak and K. Thyagarajan, “Introduction to have a constraint on the waveguide dimension Fiber Optics,” Cambridge University Press, being limited by the diffraction limit of light. In Cambridge (1998). Reprinted by Foundation contrast to this, the metallo-dielectric waveguides Books, New Delhi, 2008 based on SPP modes can be realized at the subwavelength dimensions. [5] E. D. Palik, “Handbook of Optical Constant of Solids”, New York: Academic, 1985. References Acknowledgements: We would like to thank the [1] M. L. Borngersma, R. Zia and J. A. Schuller, National Academy of Sciences India-Delhi Chapter “Plasmonics- the missing link between and Kalindi College for the financial support. nanoelectronics and microphotonics,” Appl. Phys. 2007, A89, 221 - 223 [2] W. L Barnes, “Surface plasmon-polaritons length scales: a route to sub-wavelength optics,” J. Opt. A: Pure Appl. Opt., 2006, 8, S87 - S93 [3] S. I. Bozhevolnyi, “Effective-index modeling of channel plasmon polaritons,” Opt. Express, 2006, 14, 9467-9476

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Key words: Numerical methods, waveguides, complex eigen-values.

A Comparative Study of Numerical Methods for Analysing Planar Plasmonic Waveguides

plasmonic

1. Introduction Numerical methods involve the analysis of algorithms which are based on certain numerical approximations for mathematical analysis of the real problems. Hence, the numerical methods are applied in all areas of science and engineering. In particular, the analytical methods are employed for the analysis of the planar photonic waveguides, because they are simple to implement and provide physical understanding of the electromagnetic wave propagation in such waveguides. These analytical methods comprise of solving the eigenvalue equations, which are well known in the literature [1]. However, the application of such method for the analysis of planar plasmonics waveguides is cumbersome, as the eigen-value equations become complex, due to complex dielectric constant of metals [2]. Therefore, in the absence of exact analytical methods, the modeling of the plasmonic waveguides is carried out by employing either numerical methods or semianalytical methods. The numerical techniques, such as finite difference method, finite element method, etc, are time consuming, rigorous and require high computational memory. Such methods are sometimes found to be unstable, because of the fine mesh at the vicinity of the edges of the metal. On the other hand, the approximate analytical methods, although are less accurate, but are simple to implement and give better physical understanding of the problem such as, the effect of the respective role of the various waveguide parameters like waveguide shape, size and operating wavelength.

Triranjita Srivastava, Pushpa Bindal*, Asmita Deep# and Ashima Sharda# Department of Physics, Kalindi College (University of Delhi), Delhi, India, 110008 # B.Sc. (H) Physics IIIrd year, Kalindi College *Corresponding Author: [email protected] Abstract: The analysis of planar dielectric waveguides have been widely done by employing analytical numerical methods for solving the eigenvalue equations derived from Maxwell’s equation. However, the analysis of planar plasmonic waveguides is cumbersome, as the eigen-value equations are complex and the dielectric constant of metals, in general, is complex in nature. Newton-Raphson method is a well-known method for solving the complex eigen-value equations. But, this method has certain limitations. It is a bit tedious as it needs function & its derivative evaluation. In this paper, we propose a modified bisection method to solve complex eigen-value equation, which is found to be simple and robust. This method iteratively, bisects an appropriate interval containing the root and then selects a subinterval within which the root exists. The comparison shows that the number of iteration required in bisection method is many times less than that of Newton Raphson method for the same initial approximation. However, the time elapsed in the executing the modified bisection method is slightly larger than that required in Newton Raphson Method; still the proposed method has certain advantages over Newton Raphson Method. Thus we employ the proposed method for the analysis of various plasmonics waveguides, such as metal/dielectric/metal waveguides.

Thus, in this paper we discuss the numerical methods to solve complex eigen-value equations. Newton-Raphson method is a well-known numerical method for solving the complex eigenvalue equations [3]. But, this method is a bit tedious as it needs function and its derivative evaluation and has certain limitations. In this paper, we propose a modified bisection method to solve complex eigen-value equation, which is found to be very simple & robust as it iteratively, bisects an interval & then determines a subinterval within which the root exists. The comparison shows that the number of iterations required in bisection 29

ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

method is many times less than that of Newton Raphson method for the same initial approximation. However, the time elapsed in the executing the modified bisection method is slightly larger than that required in Newton Raphson Method; still the proposed method has certain advantages over Newton Raphson Method. Further, the modified bisection method is employed for the analysis of metal/dielectric/metal plasmonics waveguides.

It is to be noted that the Newton Raphson method requires that the function and its derivative has to be evaluated at each point, which is not always possible. Moreover, the method fails when the tangent to the function is parallel to the x-axis. 2.2. Modified Bisection Method The general bisection method, based on mean value theorem for continuous functions, is a well-known root-finding method. It is implemented to solve real functions and achieve the real roots. This method repeatedly bisects an appropriate interval and selects a subinterval which encloses the root, for the next iteration.

2. Mathematical Description The detailed mathematical description of the Newton Raphson method and the proposed modified bisection method is given below:

The method is applied for numerically solving the equation f(x) = 0, where x is a real variable. Here f(x) is a continuous function within an interval [a, b] such that the values of f(a) and f(b) are opposite in signs. At each iteration, the method bisects the chosen interval [a, b] into two subintervals by calculating the midpoint c = (a+b)/2 of the interval.

2.1. Newton Raphson method The Newton Raphson Method is a widely used method for determining the roots of equations accurately. It requires an initial approximation, xₒ. A tangent to the function f(x) at x = xₒ is draw which intersects the x- axis at x1, as shown in Fig.1. The intersection point x1, is now the new approximation to the root. The entire procedure is repeated till the convergence for desired accuracy is achieved.

In this paper, we propose a modified bisection method which is applicable for the determination of complex roots. The method is discussed below: Let the exact root of the given eigenvalue equation be of the form x = xr + ixi

(2)

where, xr and xi are real and imaginary parts of the root respectively.  Fig. 1. Schematic representation of Newton Raphson method

The formula for the (i+1)th approximation is given by: xi+1 = xi –

Iteration 1: In the first approximation we choose xi(0) = 0, and apply the general bisection method to the real part of f(x) to obtain the real root xr(1). Now the root is x(0)=xr(1)+ ixi(0).

We again apply the general bisection method on the imaginary part of f(x), by taking the initial approximation of the root as x(0)=xr(1)+ ixi(0) and obtain xi(1). Therefore, after first iteration we get the approximate root as

(1)

where f and are the function and its derivative evaluated at the ith iteration i.e. . 30

ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

x(1)=xr(1)+ ixi(1). 

The electromagnetic wave propagation theory reveals following complex eigen-value equation for the waveguide:

(3)

Iteration 2: We apply the bisection method on the real part of f(x), by taking above equation as initial approximation and obtain xr(2). Now the approximation becomes x(1)=xr(2) + ixi(1), for the application of bisection method on the imaginary part f(x), to determine xi(2).

tanh(x) = -

where, we have chosen gold as a metal and air as a dielectric medium of refractive index , d= 1, m =-15.21+0.65i, x is the complex root to be determined, and V = d ; normalised

Thus, after second iteration the approximate root is x(2)=xr(2)+ ixi(2). 

frequency with d = 40 nm; width of dielectric layer, λ = 0.633 µm; operating wavelength. We first apply Newton Raphson method to the above complex eigen-value equation with different initial approximations, as shown in TABLE I. It is observed that the root of this equation is 0.2361334623751 + 0.0030586326129i which is achieved in 5 iterations, only if the initial approximation (xₒ = 0.23) is sufficiently close to the root. The number of required iterations increases as the chosen initial approximation is away from the root. Moreover, at the large value of xₒ = 1.0, the solution becomes negative. Thus, the efficiency of Newton Raphson Method is dependent on the selection of initial approximation; without knowing this, one cannot get accurate results.

The above process is repeated till the result converges to the desired accuracy.

In short, the modified bisection method solves the complex eigen-value equation by iteratively applying the general bisection method on the real and imaginary parts of the function f(x) separately. The advantage of this method is that it is simple to implement and robust, as it doesn’t require any derivative evaluation. Moreover, in absence of any information of root, it is the best method, as it gives definite convergence. 3. Results and Discussion

TABLE I: Newton Raphson Method: Variation of number of iterations with respect to the initial approximation

In order to compare both the methods we choose an example of a metal/dielectric/metal (MDM) waveguide. Such a waveguide comprises of a dielectric layer of thickness ‘d’ sandwiched between two metals as shown in Fig 2.

d

(4)

Metal Dielectric Metal

Fig.2. Schematic of a metal/dielectric/metal waveguide

Initial Guess

Iteration s

Root obtained

0.078

07

0.2361334623751 + 0.0030586326129i

0.23

05

0.2361334623751 + 0.0030586326129i

0.44

07

0.2361334623751 + 0.0030586326129i

1.0

08

-0.2361334623751 0.0030586326129i

Nature of result

X

Further, in TABLE II, the comparison of Newton Raphson Method (NRM) and modified bisection 31

ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

TABLE III: Evaluation of the root at different values of thickness ‘d’ for MDM waveguide by employing modified bisection method

method (MBM) is shown in terms of number of iterations and time elapsed in executing the method. The result obtained by both these methods is exactly same. It is found that the exact root obtained by modified bisection method converges in 3 iterations for accuracy of 10-13. TABLE II: Comparative study of the roots obtained & no. of iterations, for Newton-Raphson (NR) & modified bisection (MB) methods

Metho d

Iteratio n

Root obtained

Time elapsed (sec)

Waveguide thickness ‘d’ (nm)

Root obtained

20 40 60 80 100

0.16967124 + 0.00239925i 0.23613346 + 0.00305863i 0.28851321 + 0.00364233i 0.33349534 + 0.00417292i 0.37376839 + 0.00466773i

4. Conclusions NRM MBM

5

0.2361334623751 + 0.0030586326129i

0.0118

3

0.2361334623751 + 0.0030586326129i

0.0210

In this paper we proposed modified bisection method, which is simple and robust as compared to Newton Raphson Method. The metal-dielectricmetal (MDM) waveguide has been studied by solving its complex eigenvalue equation.

It is to be noted that, although the time elapsed in executing the modified bisection method is slightly larger than that of Newton Raphson Method, but the modified bisection method is found to be independent of the initial approximation. This method has a definite convergence, provided the root is lying within the interval. Moreover, the proposed method is simple to implement and doesn’t require the evaluation of the derivative of the function, which is not always possible for all complex modal functions. Therefore, in the case of unknown initial approximation, the modified bisection method is known to be more robust, in comparison to Newton Raphson Method. Further, we applied the modified bisection method for the analysis of MDM waveguides. TABLE III illustrates the exact value of the root at different values of waveguide thickness ‘d’.

References [1] Ajoy Ghatak and K. Thyagarajan, “Introduction to Fiber Optics,” Cambridge University Press, Cambridge (1998). Reprinted by Foundation Books, New Delhi, 2008 [2] E. D. Palik, “Handbook of Optical Constant of Solids”, NewYork: Academic, 1985. [3] S. S. Sastry, “Introduction to Numerical Methods,” PHI, 2005

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(~0.2 dB/km) in optical fibers [1, 2]. The optical fibers have found applications in data storing equipment, telecommunication, medical use, oil and gas industries, military, transport and also as decorative material. Since few decades, there has been a phenomenal growth in fiber optic industry, which gave rise to various applications such as: fiber optic sensors, integrated optic components (polarizers, directional couplers, fiber gratings, fiber amplifiers, optical switches, etc.) optical signal processing, etc [3]. In addition to its tremendous technological importance, fiber optics also offers a platform to present demonstration and understanding of various physical concepts. We study the modal properties of optical fiber. The variation of mode effective index has been obtained for GeO2 doped optical fibers. We also present the 3-D modal field distribution of two lowest order modes, along with their 2-D surface plots. It is to be mentioned here, that the modal characteristics of fibers are almost dictated by the refractive index variation in the core of the fiber. Therefore, in this work we experimentally employed near field scanning technique [4] to determine the diameter and the refractive index variation in the core of a given multimode optical fiber. It has been observed that our results are matching with the specifications of given optical fiber.

STUDY OF PROPAGATION CHARACTERISTICS OF OPTICAL FIBERS: EXPERIMENT AND SIMULATION Pushpa Bindal*, Triranjita Srivastava, Sujata#, Anju# and Diksha Tandon# Department of Physics, Kalindi College (University of Delhi), Delhi, India, 110008 # B.Sc. (H) Physics IIIrd year, Kalindi College *Corresponding Author: [email protected]

Abstract: In this work, we present propagation characteristics of fiber by employing simulations and experiment. The variation of mode effective index with respect to the wavelength is obtained analytically by solving scalar wave equations. The 3-D modal field distribution and surface plots for the first two lowest order LPlm modes are obtained. Moreover, we experimentally employed the near field scanning technique to obtain the diameter of the core and refractive index variation in a multimode optical fiber. It is observed that the obtained results are in consensus with the given specifications of optical fiber. We believe that the present study will enhance the understanding of the electromagnetic wave propagation in optical fibers.

2. Theory The refractive index of the step-index optical fiber (cross section shown in Fig. 1) is given by: 0ra n1 n(r)=  (1) ra n 2 where n1 and n2 are the refractive indices of core and cladding regions respectively, is the core radius. In order to obtain the propagation characteristics of the optical fiber, we numerically solved the Maxwell’s equations under weakly guiding approximation.

Key words: Optical fiber, Refractive index profile, Near field measurement technique. 1. Introduction The increasing demand of faster and huge data transportation, networking and processing has been achieved only due to the very low transmission loss 33

ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

center and and are the refractive indices at the center and cladding region, respectively. For small refractive index differences,

(3)

The refractive index variation for the multimode fibers is known to follow [1]:

Fig. 1. Schematic diagram of cross – section of optical fiber

In this work, we also used the near field scanning technique to determine refractive index profile (RIP) and the core diameter of the optical fiber. The experimental setup is shown in Fig. 2, in which light from tungsten halogen lamp is launched into the optical fiber with the help of 20X microscopic objective. The output from fiber is measured by a photo-detector.

(4)

where

is the relative core-cladding difference is the index exponent

depicting the shape of RIP in the core region. For example, corresponds to a triangular core RIP and ideally corresponds to a step index fiber. Solving the above equations, we get:

(5) Or (6) Fig.2. Experimental Setup for determining the RIP

Hence a log-log plot of

against

would result in a straight line of slope q and

The analysis of power emitted by an incoherent source and launched into multimode optical fiber yields [4]:

hence gives the shape of the profile. 3. Results and Discussion

(2)

We first present the propagation characteristics of a GeO2 doped optical fiber. Figure 3 illustrates the variation of mode effective index neff with respect to wavelength for fundamental mode of optical fiber comprising of core diameter 8.2µm. The core and cladding of the optical fiber is chosen as 6.3 % GeO2 doped silica and pure silica respectively [1]. It is observed that the neff decreases with

where and are the near field intensity and RIP obtained at r distance from the center of the fiber. is the maximum power at the

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increasing wavelength indicating that the modal confinement within the core region decreases [1]. 1.47

1.465

neff

1.46

1.455

1.45

1.445 0.6

0.8

1

1.2

1.4

1.6

 (m)

Fig. 4. Electric field distribution of LP01 mode (inset: respective surface plot).

Fig. 3. Variation of mode effective index neff with respect to wavelength.

It is well known in the literature that electromagnetic field propagates in the form of modes within the waveguides/optical fibers. Therefore, in order to understand the behavior of mode propagation, Fig. 4 and 5 illustrates the 3-D electric field variation of two lower order modes, LP01 and LP11 respectively. The corresponding surface plots are shown in the inset of the figures, which are important to study for the nomenclature of the LPlm mode. Here, (m - 1) is gives number of zeros in radial directions, and 2l represents the number of zeros in the azimuthal direction. Hence for fundamental mode, l=0, as it has no zero crossing in azimuthal direction. It can be seen that the LP01 mode exhibit no zero crossing respectively, along the radial direction, resulting in m = 1 respectively. A similar analysis is done for LP11 modes, which has two zero crossing in azimuthal direction and one zero crossing in radial directions, therefore l = 1, m = 1.

Fig. 5. Electric field distribution of LP11 mode (inset: respective surface plot).

As mentioned above, we employed near field scanning technique for obtaining the RIP of the optical fiber. Fig. 6, illustrates the variation of normalized near field intensity with respect to the radial distance.

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ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

1

2.2

0.6

2.15

n2 (r)

Normalized Intensity (a.u.)

2.25

0.8

0.4

2.1 2.05

0.2 a = 0.14 mm

2

0 0

0.05

0.1 r (mm)

0.15

1.95 0

0.2

Fig. 6. Experimentally observed near field pattern of a given fiber.

0.1 r (mm)

0.15

0.2

Fig. 8. Plot of RIP in the given fiber with radial distance.

It is observed that the intensity falls off with increasing distance. It is known that the distance over which the intensity of near field drops by 95% on x-axis, represents the core radius a = 0.14 mm. This value is approximately equal to the core radius 0.125 mm given by the manufacturer. In order to obtain the q value for the fiber, Fig. 7, shows the log-log plot of with respect to . As expected, the curve is a straight line, which has a slope of 11.2. It is worthy to note that such a high value to q leads to a step index profile, as shown in Fig. 8, which gives the variation of refractive index as given by Eq. (4) for q = 11.2 and a = 0.14 mm.

4. Conclusions In this work, we present the modal characteristics of step index fiber. Moreover, the diameter of the fiber core and its RIP are experimentally obtained by using near field scanning technique. References [1] A. Ghatak and K. Thyagarajan, “Introduction to Fiber Optics,” Cambridge University Press, Cambridge (1998). Reprinted by Foundation Books, New Delhi (2008). [2] A. Ghatak and K. Thyagarajan, “Optical Electronics,” Cambridge University Press, Cambridge (1989).

-2.5 -3

[3] B. P. Pal (Ed), “Fundamentals of Fiber Optics in Telecommunication and Sensor Systems,” Wiley Eastern, New Delhi (1992).

-3.5

log [1-P(r)/P(0)]

0.05

-4 -4.5

[4] M. R. Shenoy, Sunil K. Khijwania, Ajoy Ghatak and Bishnu P. Pal (Ed), “Fiber optics through experiments,” Viva Books, New Delhi.

-5 -5.5 -6 -6.5 -7 -5

-4

-3

-2

-1

0

log(r/a)

Fig. 7. log-log plot of

against

. 36

Acknowledgements We would like to thank the National Academy of Sciences India-Delhi Chapter and Kalindi College for the financial support.

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vibrations and aging of various structures like pipelines, oil wells, bridges, turbines, buildings etc. with integrated fiber-optic sensors, called “smart structures”. In short, the fibers are deployed in the harsh environment, where they are subjected to various stresses, which might affect the transmission through the fiber resulting in distortion in the optical signal. In general, lateral stress may be caused by the pressure induced due to manufacturing or installation faults. Moreover, it can also be generated by temperature induced dimensional changes in cabling materials. In particular, this lateral stress along the length of fiber is known as microbending loss, if the bend diameter is of the order of fiber diameter [3].

Experimental Study of Microbending Losses in Optical Fiber Pushpa Bindal*, Triranjita Srivastava, Ananya#, Aastha Dhankhar# Department of Physics, Kalindi College (University of Delhi), Delhi, India, 110008 #

B.Sc. (H) Physics IIIrd year, Kalindi College *Corresponding Author: [email protected]

In this work we study the application of optical fiber as a pressure sensor subject to microbends. In optical fiber pressure sensor, light is coupled to one end and detected at the other end, in terms of modulated intensity. Microbends are introduced in fiber using deformer elements, known as microbenders. In this paper, we experimentally present the microbending losses in optical fibers of different core sizes by employing two microbenders of different pitch at the wavelength 633 nm. The results are in good agreement with theoretical predictions and show that microbending losses are higher for (i) fibers of larger radius and (ii) smaller pitch of microbender. We believe that the study will help in understanding and eliminating sources of microbending losses and using optical fiber as a sensor.

Abstract: The real fibers are deployed under the sea/earth, where they experience various pressures which introduce the optical power loss, due to which the strength of the received output signal gets reduced. When the fibers bend slightly due to these pressures and this bend is of the order of fiber diameter, corresponding power loss is termed as the microbending loss in the optical fiber. In this work, we experimentally studied the microbending losses in optical fibers of different core sizes by employing two different types of microbenders with unequal pitch. The results are in good agreement with theoretical predictions and show that microbending losses are higher for (i) fibers of larger radius and (ii) smaller pitch of microbender. We believe that the study will help in understanding and eliminating sources of microbending losses and using optical fiber as a pressure sensor.

2. Experimental Setup Figure 1, illustrates the schematic of the experimental setup to study the microbending loss in the optical fiber. A Helium-Neon laser emitting light at wavelength 633 nm is used to launch power into the input end of the optical fiber and fed at the other end to photodetector. In between, the fiber is subjected to a microbender of pitch 2D, which has a periodic deformer element. When a portion of the fiber is sandwiched between the microbender, fiber undergoes periodic deformation in the form of microbends [4]. The resultant mechanical deformation is perpendicular to its axis, causing

Key words: Optical fiber, Pressure Sensor, Microbending Losses. 1. Introduction The optical fibers have ultra-high capacity of data transmission and processing over very large distances ~ 1000 km [1-3] with minimal loss ~ 0.2 dB/km. At present the optical fiber cables are running around the earth, being installed in the oceans and seas, where they experience various pressures. As is well known, optical fibers are widely used to monitor internal conditions, 37

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higher-order guided modes to radiate out of the fibers core through the cladding interface.

Fig. 1. Micro-bending losses in Optical Fiber due to pressure applied

Fig. 2. Geometry of the micro-bend

When the pressure is applied to the microbender, microbends are created which in turn modulate the intensity of transmitted light at the other end of the fiber. Higher order modes radiating out of fiber core through core cladding interface due to external pressure applied to the optical fiber perpendicular to its axis, cause this fall in intensity of transmitted light.

3. Results and Discussion Fig. 3- 4, illustrates the variation of transmitted intensity through a microbend modulated fiber optic sensor with respect to the applied weight over two types of microbenders namely; microbender 1 (pitch = 2.17 cm) and microbender 2 (pitch = 0.96cm). The chosen fiber is of core diameter 250 µm and 750 µm in fig. 3 and fig. 4 respectively. In both the figures, as expected, the intensity decreases parabolically with increasing weight. Also the intensity is more for microbender 1, which has larger pitch as compared to the microbender 2. The reason attributed to the fact that larger the pitch is, larger is the radius of curvature R (Eq. 2), which results in decrease in transmission loss , hence we observe more transmitted intensity in the case of microbender 1.

It is well known that the loss of guided power by radiation at the bend is given by [5]; (1) where ‘d’ represents the radius of core, ‘R’ is the radius of curvature of bend, and A is a constant. Thus, for a given fiber, the pressure applied to the bend radius, which is given by (2)

Figure 4, illustrates the similar variations in transmitted intensity for the two microbenders for fiber 2 of core diameter 750 µm. Clearly intensity losses are higher for both microbenders since the fiber has a larger diameter in comparison to fiber of

where, y is the displacement of fiber caused by pressure in micro-bender element and 2D is the distance between micro-bender element’s contact points (pitch), as shown in Fig. 2.

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and hence larger intensity. These findings can be easily understood from Eq. (1).

1

1

microbender 1 0.8 Fiber 1

0.5 Intensity (a.u.)

Intensity (a.u)

0.75

microbender 2 0.25

0 0

500

1000 1500 Weight (gm)

2000

0.6 Fiber 2 0.4

0.2

2500

0 0

Fig. 3. Variation of normalized intensity with respect to weight applied at microbender 1 and microbender 2, for fiber 1 of core diameter 250 µm.

500

1000 1500 Weight (gm)

2000

2500

Fig. 5. Variation of normalized intensity with respect to weight applied at microbender 1 for two fibers; fiber 1 of core diameter 250 µm and fiber 2 of core diameter 750 µm. 1

0.8

Intensity (a. u.)

Fiber 1 0.6 Fiber 2 0.4

0.2

0 0

Fig. 4. Variation of normalized intensity with respect to weight applied at microbender 1 and microbender 2, for fiber 2 of core diameter 750 µm.

500

1000 Weight (gm)

1500

2000

Fig. 6. Variation of normalized intensity with respect to weight applied at microbender 2 for two fibers; fiber 1of core diameter 250 µm and fiber 2 of core diameter 750 µm.

Fig. 1, as expected. Further, Fig. 5 and 6, illustrate the variation of normalized intensity with respect to weight applied at microbender 1 and microbender 2, for two fibers; first fiber termed as fiber 1 has core diameter of 250 µm and the second one called fiber 2 with core diameter 750 µm. In both the figures, it is observed that the transmitted intensity falls off parabolically with respect to applied weight. Moreover, fiber 1, which is of lesser diameter as compared to fiber 2, exhibits low loss

Again losses are more in case of mirobender 2 in Fig. 6 due to its smaller pitch. It is to be mentioned here, that in Fig. 3-6, we employed quadratic curve fitting of MATLAB®, which reveals parabolic fall of intensity with respect to increase in weight. In short, the results show good agreement with the theoretical predictions of micro-bending losses. 39

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The finding show that micro-bending losses are higher for (i) (ii)

[2] A. Ghatak and K. Thyagarajan, “Optical Electronics,” Cambridge University Press, Cambridge, 1989

fibers of larger radius smaller pitch of micro-bender.

[3] B. P. Pal (Ed), “Fundamentals of Fiber Optics in Telecommunication and Sensor Systems,” Wiley Eastern, New Delhi, 1992

4. Conclusions In this paper we study the optical fiber pressure sensor. The variation of transmitted intensity is studied for two different micro-benders and optical fibers. We believe that the study will help in understanding and eliminating sources of micro-bending losses and using optical fiber as a sensor.

[4] M. R. Shenoy, Sunil K. Khijwania, Ajoy Ghatak and Bishnu P. Pal (Ed), “Fiber optics through experiments,” Viva Books, New Delhi. [5] C. K. Kao, “Optical Fiber Systems: Technology, design and application,” Mcgraw – Hill, New York, 1982.

References Acknowledgements We would like to thank the National Academy of Sciences India-Delhi Chapter and Kalindi College for the financial support.

[1] A. Ghatak and K. Thyagarajan, “Introduction to Fiber Optics,” Cambridge University Press, Cambridge (1998). Reprinted by Foundation Books, New Delhi, 2008

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energetic ion-beam deposition process which is expedient in spintronic devices i.e., MTJs devices.

Growth of (001) oriented Cr and MgO thin films on Amorphous Substrate for Magnetic Tunnel Junctions

Key words: Cr, MgO, X-ray reflectivity 1. 1. Introduction The chromium (Cr) and magnesium oxide (MgO) thin films are pivotal choice of materials for development of spintronics devices. Cr is one of the inevitable choices of the buffer layer materials for the growth of epitaxial ferromagnetic (FM) layer particularly Co based Heusler alloy (Co2FeAl) layer. However the MgO thin films not only accommodating to induce perpendicular magnetic anisotropy (PMA) but also inevitably important as an insulator tunnel barrier for good band matching with Heusler alloys [1]. This suitable band matching among the FM Co2FeAl (CFA) having low damping [2] and MgO layers enhanced the electron tunnelling probability consequently larger percentage change in magnetoresistance ratio in magnetic tunnel junctions (MTJs) devices. The MgO, alike Cr layer, is indeed a choice of buffer layer materials to grow an epitaxial Heusler alloy thin films [3]. It is well known that the Cr thin films have either (110) or (002) crystallographic texture. However, it is difficult to grow (001) oriented Cr epitaxial layer even on single crystal substrate [4]. To prevent shunting to substrate in MTJs and spin transfer torque (STT) spintronics devices it is indeed needed to grow the epitaxial structures on insulating substrates; particularly on the technologically and industrially important thermally oxidized silicon (Si/SiOx) substrate. However the oriented growth on amorphous substrate is technologically critical. To overcome this issue we utilized energy enhanced ion beam sputtering process to grow the Cr and MgO thin films over (Si/SiOx) and glass substrates. In this paper we report the growth rate dependency of the orientation of Cr thin films, and oxygen ion assisted 200 oriented stoichiometric phase formation of MgO thin films.

Sajid Husain, and Sujeet Chaudhary* Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016 (INDIA) Ankit Kumar, Serkan Akansel, and Peter Svedlindh, Ångström Laboratory, Department of Engineering Sciences, Box 534, SE-751 21 Uppsala, Sweden *Corresponding Author: [email protected] Abstract: We have carried out a systematic study to optimize the growth parameters to obtain the oriented growth of Cr (002) and MgO (200) thin films using dual ion-beam sputtering technique on thermally oxidized silicon and glass substrates. It is found that the preferred crystallographic orientation of Cr depends on the film growth rate (sputtering rate) and its post annealing treatment. X-ray diffraction analysis has revealed that 110 W and 85 W grown and subsequently 500°C post annealed Cr thin films result in the (110) and (002) crystallographic orientations, respectively. The MgO thin film grown at room temperature using the oxygen ion assisted ion-beam sputter deposition, without requiring any pre/post substrate annealing treatment exhibits (200) orientation. The interface/surface qualities of all the samples have been investigated using X-ray reflectivity analysis. Extremely small surface roughness of 0.28 and 1.49nm are observed for Cr and MgO films, respectively. The oriented growth of MgO and Cr thin films is established in correlation with the

2. Experimental In this work the Cr thin films were deposited on (Si/SiOx) substrates at 100W and 85W powers at 41

ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

room temperature using ion beam sputtering deposition technique (NORDIKO-3450) and subsequently annealed at 500°C temperature. The HV chamber was evacuated down to ~2×10-7 Torr base pressure using a turbo and cryo-pump. The 6” dia. Cr and Mg target fixed on a remote controlled water cooled turret was sputtered by ~4.5 inch dia. high energy Ar-ion beam (500 eV) extracted from a RF ion-source. During the deposition, the chamber pressure was maintained at ~8.2×10-5 Torr by bleeding 4 sccm Ar gas directly into the ion-source operated at 100W and 85W for Cr. The MgO thin films were deposited on (Si/SiOx) as well as on glass substrate using ion assisted ion-beam sputtering deposition technique (see ref. 5 for details). The deposited Cr and MgO thin films were investigated by Bragg-Brentano () and glancing angle X-ray diffraction (GAXRD), respectively. The film thickness, electron density and surface roughness were investigated by simulating the specular X-ray reflectivity (XRR) spectra using the PANalytical X’Pert Reflectivity software (ver. 1.2 with segmented fit).

having the lowest free energy therefore it is more favourable to grow. Its growth depends on the size of island sizes (large number of grain boundaries are preferred) and the energy of the deposited/growing nuclei. The high power (100W) ion-beam growth can fulfil all these requirements. The high growth rate of the growing thin films on amorphous substrate results in small sized grains and hence in higher number of grain boundaries. Subsequently, it favours the growth with (110) textures. Further, ion beam sputtering is an energyenhanced process, compared to other deposition methods, in which the sputtered atoms/ions carry relatively higher energies resulting in higher grain boundary migration during the film growth leading to energetically favourable grain-orientation. Further enhancement of the 110 texture can be done by post deposition annealing process as executed in the present sample growth. Since, the nucleation depends on the kinetic energy of addatom and their mobility, therefore, deposition at 100W RF-power having higher growth rate compared to 85 W deposition leads to the growth of (110) texture of Cr thin films. However at low RF-power (85W) the nucleation rate is small, thus the add-atom gets more time for surface diffusion and are able to to contribute in the growth of bigger grains of (002) orientation in the beginning of deposition. Thus the equilibrium island growth occurs which possess the (002) texture. Therefore, the Cr thin film grown at 85 W power results in (002) texture in comparison to the high power (100W) sputtered films which exhibited (011) orientation.

3. Results and Discussion 3.1 Chromium (Cr) Figure 1 shows the  X-ray diffraction patterns of Si/SiOx/Cr(41nm) thin film deposited at 100W and 85W RF-powers at RT and subsequently annealed at 500°C. It is clearly evident that 110W power deposited sample results in (110) crystallographic orientation although the 85W power deposited sample exhibits (002) orientation. The observed changes of Cr texture are attributed to the film growth rate, changes of RF sputtering power, as the annealing temperature and time were kept constant. This growth rate assisted changes in the post anneal Cr films crystallographic orientation can be understand by the growth models explained in Feng et al [5]. In film growth mechanism the high sputtering/growth rate favours the faster grain growth and therefore faster nucleation of atoms which results the smaller grains on amorphous systems. The planes of the grains having low free energy at their surface will grow faster compared to others ignoring the fact which texture start nucleating at the substrate surface. The BCC Cr thin film (110) texture is

Fig.1: XRD spectra of Si/SiOx/Cr(40nm) thin films grown at different RF-power. 42

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In order to precisely determine the thickness, density, and the amount of surface roughness of Cr thin films specular XRR spectra were recorded as shown in Fig.2. The estimated values of film thickness, density and associated surface roughness are presented in Table I. The density of Cr was found to be 6.64gm/cc which is comparable to the bulk value of density of Cr i.e.,7.19gm/cc. The very small

of a single peak corresponding to (200) orientation on both the substrates indicates the preferred oriented growth of MgO thin film. The stoichiometric phase of MgO films deposited at RT are optimized by varying various parameters such as sputtering power, and O2 ions energy at different oxygen partial pressures using ion assisted gun. Here, in present case the MgO thin films was prepared at O2 partial pressure of 1.210-4 Torr at 75 W of RF-power with Ar partial pressure of 1.910-4 Torr for Mg metal sputtering at 100W. The systematic study of MgO thin film at various O2 ion energy and partial pressures were reported by Braj et al [6].

Fig.2: XRR spectra of Si/SiOx/Cr(41nm) thin films grown at 100W RF-power.

roughness (4Å) and nearly equal bulk density of these films indicate the excellent film quality. These excellent sample quality in terms of surface roughness, density and crystallographic orientations are associated to energy enhanced growth technique where sputtered atoms have high 10-20eV energy compared to other deposition technique as discusses above.

Fig.3: XRD spectra of Si/SiOx/MgO(30nm) thin film.

It is observed that the (200) diffraction peak of MgO thin film deposited on glass substrate is not very sharp compared to the thermally oxidized Si. It is attributed to fact that the roughness of the glass surface is significantly higher than the thermally oxidized Si which requires larger formation energy for crystallization on glass substrate.

Table I: The XRR simulated parameters for Cr and MgO thin films; density , thickness t, and surface roughness . Si/SiOx/Cr

Si/SiOx/MgO

Layer

SiOx

Cr

(g/cc)±0.06

3.26

6.64

t(nm)±0.01 (nm) ±0.03

Cr2O3

SiOx

MgO

3.26

2.96

60000 41.80 1.62

60000

31.67

0.43

0.37

5.57

0.41 0.28

1.49

3.2 Magnesium oxide (MgO) Fig.4: XRD spectra of Si/SiOx/MgO(30nm) thin film.

Figure 3 and 4 show the XRD spectra of MgO thin film deposited at room temperature on oxidize silicon and glass substrates, respectively. Presence

The XRR spectra recorded on Si/SiOx/MgO thin film grown at room temperature is shown in 43

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Fig.(5). The observed surface roughness for MgO thin films is relatively higher as compared to the Cr thin film. It could be inferred by the fact that the Mg is hygroscopic in nature and it forms the hydroxide when it comes in contact with atmosphere, thereby resulting in higher surface roughness. However, we would like to mention that the interfacial roughness of surface, which is of the order of half a monolayer, protected the ultrathin MgO layer[7].

Acknowledgements SH thankfully acknowledges the DST, India for providing the INSPIRE fellowship for research. References [1] Tezuka, N., Ikeda, N., Mitsuhashi, F. & Sugimoto, S. Improved TMR JCs with Heusler Co2FeAl0.5Si0.5 electrodes fabricated by molecular beam epitaxy. Appl. Phys. Lett. 2009, 94, 162504. [2] Husain, S., Akansel, S., Kumar, A., Svedlindh, P. and Chaudhary, S., Growth of Co2FeAl Heusler alloy thin films on Si(100) having very small Gilbert damping by Ion beam sputtering Sci. Rep. 6, 28692 (2016). [3] Ortiz, G. et al. Growth, structural, and magnetic characterization of epitaxial Co2MnSi films deposited on MgO and Cr seed layers. J. Appl. Phys. 2013, 113, 043921. [4] Schmid, M., Pinczolits, M., Hebenstreit, W. & Varga, P. Segregation of impurities on Cr(100) studied by AES and STM. Surf. Sci. 1997, 377379, 1023–1027.

Fig.5: XRR spectra of Si/SiOx/MgO(30nm) thin film grown at room temperature.

4. Conclusions

[5] Feng, Y.C., Laughlin and lambeth D.N. Formation of crystellographic texture in RF sputtered Cr thin films, J. Appl. Phys 1994 76, 7311.

The oriented thin films of Cr and MgO have been deposited on oxidised Si and glass substrate using ion assisted ion beam sputtering technique. It has been observed that the crystalline orientation of Cr critically depends on the growth/sputtering rate. The (200) orientation of MgO thin films is obtained at room temperature without requiring any post annealing treatment. These Cr and MgO oriented thin films are indispensable for spintronic devices as an underlayer and MTJ barrier, respectively.

[6] Singh, B. B., Agrawal, V., Joshi, A. G. & Chaudhary, S. XPS and CAFM investigations on dual ion beam sputtered MgO ultrathin films. Thin Solid Films 2012, 520, 6734–6739. [7] The multilayer structure Si/Ta(10nm)/ Co2FeAl (1.8nm)/MgO(2.2)/Ta(2nm) was prepared for PMA and XRR simulation provide the interfacial roughness less then 3Å.

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1. Introduction: Bio ceramics are materials which include Bio active glasses as well. They are a group of glass ceramic materials having surface reactivity. The biophysical properties of these glasses has led them to be studied in detail to be used as implant material. Ceramics show many applications due to their physico-chemical properties. They have the advantage of being inactive in the human body. The resistance to abrasion makes them useful for bones and tooth replacement. A material is said to be bioactive, if it gives an appropriate response to stmulii and results in the formation of a bond between material and the body tissue. Bioactive glasses are silicate based, containing calcium and phosphate1.Hench was the first to develop bioactive glasses, which were found to able to bond to tissues2.The morphology of the gel surface layer was a key point in determining the response of bioactive glass . The ability of bonding to bone also known as Biocompatibility was increased for a certain compositions of bioactive glasses.These bioactive glasses mainly contained SiO2, Na2O, CaO and P2O5. Synthetic bone graft material for general orthopaedics and dentistry are some of the application of bioactive materials.

Bio ceramics: Future implant material Aruna Dani Asso. Prof. (App Physics) Priyadarshini College of Engineering, Nagpur-440019, INDIA [email protected] Abstract: Ceramics exhibit many applications as biomaterials due to their varied properties. Glass ceramics possess many properties, similar to both glass and ceramics as well. They have the property of being inert in the human body. Because of their quality of being hard and resistant to abrasion they become the best option for tooth and bone replacement. Some ceramics which are resistant to friction, makes them useful as replacement materials for malfunctioning joints. Aluminum oxide has been used in orthopedic surgery for more than 20 years as the joint replacement material due to its exceptionally low coefficient of friction and minimum wear and tare. Bioactive glasses are composed of calcium and phosphate which are present in a proportion that is similar to that of bone in human body. These glasses bond to the tissue and are biocompatible. They have large medical and dental applications. Since bioactive glasses and glass ceramics are brittle materials they are specially used in the field of small bone defects. Following inorganic processess occur when a bioactive glass is immersed in a physiological environment: 1. ion exchange 2. Hydrolysis 3. Condensation 4. Precipitation and 5. Mineralization. This article reviews various properties of bioactive glasses and their applications

2. Experimental 2.1 Materials Bioactive glasses are classified into different groups and each group has a different composition. Some bioactive glasses, for ex. 45S5, are now being used as bone grafting material3. 45S5 bioactive glass is composed of SiO2 (46.1 mol%), CaO (26.9 mol%), Na2O (24.4 mol%) and P2O5 (2.6 mol%)4 . 45S5 is able to form HCAP (hydroxyl carbonated apatite) in less than 2 hours and binds to tissues1. It is essential that a bioactive glass forms without getting crystallized. If a bioactive glass crystallizes, it becomes less bioactive because the ion exchange between the glass and aqueous solution is resisted by the crystalline phases 2.2 Preparation of Samples: Bio active glasses were initially obtained by the process of melting at higher temperatures. The process for the formation of bioactive glasses are melting at

Keywords: Bioactive glasses, Bio Ceramics, Implant material, biocompatible, Glass transition

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Table Composition of bioactive glasses and glass ceramics used for medical and dental applications Composit ion Wt% Na2O CaO CaF2 MgO P2O5 SiO2 Phases

45S5 Bio glass 24.5 24.5 0 0 6 45 glass

S53P4

A-W glass ceramic

23 20 0 0 4 53 glass

0 44.7 0.5 4.6 16.2 34 Betawollstonite glass

For example, when a particulate of bioactive glass is used to fill a bone defect there is rapid regeneration of bone that matches the architecture and mechanical properties of bone at the site of repair. 4. Conclusions Bioactive glasses with various compositions are now used for wide range of applications. Bioactive glasses have become an area of interest for researchers from the field of medicine and dentistry. The growing requirement of tough, strong and stable bioinert glasses/ ceramics could be met either by nano-structured ceramics or composites. References

higher temperature and sol-gel. It was later demonstrated that the formation of bioactive glasses with a composition of SiO2- CaO-P2O5 by sol-gel process was possible and it was also observed that glasses were formed at lower temperatures in sol-gel process as compared to conventional melting method5,6.Glass transition temperature (Tg), is a characteristic of any glass, indicating a range of transformation when an amorphous solid is changed into a super cooled liquid on heating. In case of a bioactive glass a linear relationship exists between Tg and hardness of the glass. Reduction in Tg of a bioactive glass indicates that the glass has reduced hardness.

[1] Hench LL, Wilson J. An introduction to bio ceramics. Singapore: World Scientific Publishing, 1993 [2] Hench LL. The story of Bioglass TM. J Mater Sci: Mater Med 200617, 967-78 [3] Paolinelis G, Banarjee A, Watson TF. An in vitro investigation of the effect and retention of bioactive glass air-abrasive on sound and carious dentine. Journal of Dentistry 2008,36,214-18 [4] Masahiro Kobayashi, Hiroaki Saito, Takatsune Mase, Taketo Sasaki, Wei Wang, Yumi Tanaka, et al. Polarization of hybridized calcium phospho aluminosilicates with 45S5-type bioglasses. Biomed Mater 2010 ,5,025001

3. Results and Discussion Addition of fluoride increases re mineralization and reduces demineralization. CaF2 concentration was increased in SiO2-CaO-P2O5-Na2O system while network connectivity was kept constant. It was observed that due to addition of fluorine in bioactive glass, there is decrease in Tg which means that the glass has reduced hardness and is more bioactive 7. For the prevention of caries, the role of fluoride is very important. This substitution has a profound effect on solubility of enamel8. As the addition of fluoride is essential, its incorporation in bioactive glasses is of immense importance. It was observed that incorporation of fluorine in bioactive glass, decreased its Tg which indicates that the glass has reduced its hardness and is more bioactive. Alternately, the onset of crystallization and peak temperatures were decreased when CaF2 was increased9 .

[5] Rounan Li, Clarke, Hench. An investigation of bioactive glass powders by sol-gel processing. J Appl Biomater 1991,2(4), 231-39. [6] Peltola T, Jokinen M, Rahiala H, Levänen E, Rosenholm, Kangasniemi, Yli-Urpo. Journal of Biomedical Material Research 1999, 44(1), 12-21. [7] Featherstone JDB. The science and practice of caries prevention. J Am Dental Assoc JADA 2000, 131, 88799. [8] Wei M, Evans JH, Bostrom T, Grondahl L. Synthesis and characterization of hydroxyl apatite, fluoridesubstituted hydroxyl apatite and fluorapatite. J Mater Sci Mater Med 2003, 14, 311-20. [9] Brauer D, Karpukhina N, Law R, Hill R. Structure of fluoride containing bioactive glasses. J Mater Chem 2009, 19,5629-36. 46

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illumination and communication. ITS using visible light would enable to use the light from the street lights as a source to communicate. In VLC system, modulation of intensity of light is done in such a way that it is undetectable to human eye and have no effect on the illumination functionality. LEDs are used for transmission purpose because of its certain advantages such as high lightening efficiency, long durability, being environment friendly and low power consumption. The transmitter and receivers used have same configuration as most of the general analog communication systems as shown. LEDs are used in head/tail lights of vehicles, street lights and traffic lights which will make the deployment of these Smart and Intelligent Transportation System easy and using VLC technology. Using this technology the vehicles will be able to communicate about the speed, routes, destination, and traffic conditions. Vehicular Communication can be Vehicle to Vehicle, Vehicle to Infrastructure, Infrastructure to Vehicle. Most Challenging Project which is under consideration of many scientist is development of Visible Light Communication for Advanced Driver Assistance. There are also many applications listed in this paper like the Smart Obstacle Intimation System [4], Blind Turn Assistance etc. Use of VLC in Transportation System will be very advantageous since it will make the transportation system faster, easier and safer. ITS holds a promising sustainable future , it can play a vital role in reducing pollution, better traffic management and better on road security. A great initiative has been taken in the field of ITS.

Intelligent Transportation System Shubham Sehgal, Akshat Mathur*, Mona Aggrawal, Ram Sharma Department of Electrical Electronics and Communication Engineering The NorthCap University, Gurgaon Corresponding Author: *[email protected], Abstract - With the emerging advancements in transportation system, need for effective assistance during this process has emerged. There is no technology being used presently that assists transportation through visible light. So we discussed about Intelligent Transportation System (ITS) that can be used as a potential element in traffic control management. According to World Health Organization’s figures, major cause of death after the year 2000 are road accidents. VLC using LED is technology which will help for high speed and low cost wireless communication which will be helpful in this study of Intelligent Transportation System. The unique features and benefits of VLC make it the most important technological innovations in communication system. In this paper we are discussing this concept of Visible Light Communication to develop some techniques in the field of transportation system. Keywords – Visible Light Communication, Vehicular Communication, Intelligent Transportation System

2. Working LEDs act as a transmitter as shown in diagram, the transmitter and receiver configuration is similar to the analog communication systems. Digital Modulation Technique are used modulation of light beam. Data transmission will be in binary form since the two states of LED on and off can only denote max two states. LED in on state will denote binary ‘1’ similarly off state of LED will denote binary ‘0’. At the receiver’s end we use a light sensitive device like photodiodes for receiving this encoded signal.

1. Introduction With the advent of smart technology in every field, it is imperative to establish the transportation system as smart, now considering the extent of spread of transport network, a method needs to be devised which can easily be used with the present technology. The following texts focuses on Intelligent Transportation System using Visible Light. In Visible Light Communication (VLC), communication takes place using visible light in which LEDs perform two functions simultaneously 47

ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

These traffic lights will help in traffic management. There will continuous communication between these lights and vehicles coming towards these lights. The light will signal green to that lane of road where the traffic density is more in short amount of time keeping on lane as initial starting lane. The decision making will be according to the traffic density. This will reduce the traffic jams and will increase mobility on road.

Advantages of VLC over RF 1. RF Band – 3 KHz to 300 GHz for wireless communication whereas for VLC it is 400 THz to 800 THz, very large as compared to RF Band.[1] 2. VLC provide a highly secure, low speed and high speed communication, data rates greater than 100 Mbps can be achieved using high speed LEDs. 3. VLC could be implemented using cheap components for transmitter and receiver purposes unlike the costly hardware using in wireless communication using RF technology. 4. Visible light does not create the phenomenon Electromagnetic Interference (EMI) [1] 5. Visible light is environment friendly as compared to Radio Frequencies.

ii) Speed Control mechanism – Similar to above application we create a continuous link between vehicle and street lights on road, this data will be given to the police control room which will help them in reducing road accidents. An internal mechanism can be designed which could track the state of the driver. If the driver is drunk an immediate signal could be transferred to the police control room and they can track him down or if a driver jumps a traffic signal then also he can be tracked down by communication the car using visible light communication. iii) Smart Obstacle Intimation System – This system will help the driver about obstacles coming near to him. During night or fog time many of the obstacles are not visible to drivers which lead to accidents but using this intimation system obstacles would be detected coming in the range of head lights and an immediate braking system (if planted) will get activated and brakes will be applied to reduce the amount of damage or even eliminate it. Vehicle to Vehicle communication will also be playing a major role in this system. The brakes will be applied after giving the signal to driver.

3. Applications i) Smart Traffic Management System – Traffic can be managed using VLC by making use of the Smart Traffic Lights [4]. 48

ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

communicate with the device planted at those booths and a certain amount of money is deducted from the bank account of that driver.

iv) Pollution Level Marker – This could be implemented using LEDs planted in the street lights. The pollution

4. Improvements/ Suggestions 1. Improvement in the outage area of the beam can significantly improve the extent of area covered and also provide better connectivity. 2. A convex approach towards transmission and reception must be adopted, inclusion of photodetectors at the frontal extremities of the vehicle can also improve connectivity. 3. Development of standards and protocols that would contribute in improving the Interference, Sound to Noise Ratio(SNR), which can be achieved by developing Medium Access Control (MAC) [1].

level in the environment will be marked according to the intensity of light. A scale can be made which will keep track of light intensity of street light, divided into section marked with the intensity columns and corresponding pollution level. The more the illumination of street lamps, the lesser will be the pollution level in environment.

5. Shortcomings 1. Low bandwidth in modulation –

LEDs are responsible for producing low modulation bandwidth which in turn is responsible for lower data rate. Pre and post equalisation however can increase BW upto 50MHz[3] adaptive equalization can help to compensate for Inter-symbol Interference (ISI), improving the data rates and the biterror-rate 2. Interference and noise – Visible light is susceptible to interference from external factors and data can be affected due to destructive interference from sunlight 3. Non linearity – LEDs transmission and detection is most efficient in LOS line of sight. Also LEDs can produce non-linear characteristic graphs. So it is important to search for an optimum DC operating point 4. Cross path interference – Considering similar vehicles work on similar signals, there is a possibility of reception of signal from another source, this crates distortion that degrades performance.

v) Blind Turn Assistance – This technique will use the basic concept of interference. Major amount of accidents take place on blind cuts where the cars coming from the other side are not visible. So this technique will help in reducing such type of accidents. The lights coming from the head lights of cars coming from the opposite ends will interfere and will produce max and min. level which can be detected and will intimate the driver regarding an obstacle as described above. The driver can then apply the brakes or it will be automatically applied. vi) Toll Collection – Toll Booths are area of road where max amount of jams take place. The service of people on those booths is too slow which make this happen. So to reduce this we could apply VLC technique here. In this technique Vehicle to Infrastructure technique [5] will be used. As soon as vehicle reaches the booth, a vehicle will 49

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6. Conclusion [2] Kashif Naseer Qureshi and Abdul Hanan Abdullah ” A Survey on Intelligent Transportation Systems” Middle-East Journal of Scientific Research 15 (5): 629-642, 2013

In this paper we mainly reviewed the fundamentals and application of visible light communication is the area of transportation. The results of research done in the area of ITS have been promising and ITS using VLC till now has also proven to be an upcoming technology that can benefit many areas of transportation like data collection, toll collection, accidents safety etc. and benefits include mobility, efficiency, safety etc. With further research in this area VLC can definitely replace DSRC in intelligent transportation system. Many features provide VLC in ITS an edge over other technologies most importantly, unlike most emerging technologies, cost of establishment with this would be much less, since it can use the present infrastructure

[3] Carlos Medina, Mayteé Zambrano and Kiara Navarro “Led based visible light communication: technology, applications and challenges – a survey” International Journal of Advances in Engineering & Technology, Aug., 2015. [4] Navin Kumar “Visible Light Communication in Intelligent Transportation Systems” IEEE International Conference on Communications [5]Anitha Chepuru , Dr.K.Venugopal Rao “A Survey on IOT Applications for Intelligent Transport Systems” Technical Research Organization India

References [1]Navin Kumar “An Emerging Visible Light Communication System for Driver Assistance”

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