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Projects for Physics Students 2017/18. 1. Light Trapping Photovoltaics. Supervisor: Professor Werner Blau. Location: TCD

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Projects for Physics Students 2017/18 1. Light Trapping Photovoltaics Supervisor: Professor Werner Blau Location: TCD Incorporating Carbon Nanotubes (CNT) into photovoltaic devices can play two important roles. First, doing so provides a means to measure relevant physical (e.g., electronic, optical) properties of the CNTs themselves. Second, such devices have great potential for applications. Many envisaged practical applications can be based on dense Carbon Nanotube (CNT) carpets. Thus, a device structure of note is the light-trapping design envisaged here. The CNTs are grown vertically in a pattern on Si. The remainder of the device is fabricated with subsequent perovskite films followed by a transparent conductive-oxide (TCO). The benefit is the multiple scattering of the incident photons, thus ensuring nearly complete absorption in the thin active layer. A simple planar device has one opportunity for a photon to be absorbed and create an electron-hole pair. This design enhances the absorption by light trapping. This does not take advantage of the PV effect of the CNTs themselves, but relies on the high conductivity of the CNTs so that it acts simply as a charge carrier. Its advantage here over conductors is its high aspect ratio, which cannot be achieved currently with metals. 2. All-Optical Switching Supervisor: Professor Werner Blau Location: TCD The all-optical switch is a key component in high-speed optical communication networks. In a switch device, due to the nonlinear optical interaction of light and matter, the input optical signal can be controlled between “ON” and “OFF” states by another pump beam, realising optical signal switching. The unique nonlinear optical responses of 2D nanostructures make them very interesting candidates for all-optical switching devices. The high intensity pump beam will change the state of excitons in these nanostructures, resulting in a modulation of the signal pulses. In this project, planar films will be adopted for the initial optical switching test. For further improving switch performance, we will try two additional designs: 1) Employing multi-spin-coating or extrusion process technique to fabricate a 1-D photonic crystal optical switch 2) Using ion beam etching technique to fabricate a 2-D photonic crystal structure in biomaterial-polymer films. The optical switches will be tested in the CRANN ultrafast laser pump-probe experiments.

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3. Orientation of Nanocarbons in Composites Supervisor: Professor Werner Blau Location: TCD In photonic device applications, it is desirable to have the active nanoparticles oriented preferentially in a certain orientation as opposed to a random orientation. This will allow us to capitalize on their unique attributes: polarized emission and absorption, angular dependence of optical nonlinearity, external electric field modulation of fluorescence, etc. An oriented ensemble dispersed in an optical waveguide will show anisotropy in the linear and nonlinear response, as well as property changes in response to an externally applied electric field. After the polymer is spin cast onto the substrate, an electric field will be applied that serves to orient the dopants. The electric field is present throughout the polymer bake process, thus trapping the Nanocarbon in the oriented state. The buried electrodes can also be used to apply an external electric field to induce switching when the photonic device is in operation mode. A compromise is necessary in this technique, as the conductive electrodes have to be set sufficiently distant from the optical mode guided by the waveguide to eliminate free carrier absorption and shorting by the conducting Nanocarbons. However, for the voltages to remain reasonable, it is desirable to minimize their distance. Using an electric field of 10V/m as a rough guide and a 30 µm gap size for sufficient isolation from the optical mode, then the required alignment voltage is 300 V. 4. Plasmonics for enhanced light emitting devices Supervisor: Professor Louise Bradley Location: TCD Current down conversion based light emitting diodes depend on radiative energy transfer from the electrically pumped quantum well to the light emitting quantum dots. Nonraditave energy transfer has the potential to be more efficient but suffers from a very limited energy transfer distance. Work in the Bradley group has shown that arrays of nanoscale metallic features can be used to increase the energy transfer distance and efficiency. The principle has been validated in optically pumped QW_QD devices. These plasmon-coupled systems can offer new functionalities and improved performances in terms of light emission, colour conversion and light harvesting. This project will extend this study to novel electrically contacted QW devices. The plasmonic arrays will be fabricated using e-beam lithography or He-ion lithography. The QDs are deposited using the spin coating technique. Complementary photoluminescence (spectral and time-resolved) and electroluminescence will be used to investigate the energy transfer for light emission and light harvesting.

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5. Energy transfer in novel optically activated nanostructures Supervisor: Professor Louise Bradley Location: TCD New quantum dot structures such as the dot in a rod or balalaika shaped-quantum dot have been recently synthesized by the Gun’ko group in the School of Chemistry. This project will explore the concentration dependence of nonradiative energy transfer between these novel structures in monolayer and bilayer structures. The Layer-by-Layer technique will be used to fabricate the layered structures. The energy transfer process will be characterized using absorption, photoluminescence excitation, photoluminescence and time-resolved photoluminescence measurements. These techniques can be used to quantify the energy transfer rate and efficiency. These novel quantum dots also exhibit signatures of chirality. Chirality can be detected using circular dichroism (CD) spectroscopy, with a difference in the absorbance for left and right circularly polarised light evident for samples with chiral molecules. The origin of the chirality will also be explored using a variety of techniques techniques and the possibility to enhance the chirality in the presence of enhanced of local electromagnetic field in proximity to plasmonic components will also be investigated. 6. Printed transistors from networks of nano-materials Supervisor: Professor Jonathan Coleman Location: TCD With the advent of the internet of things, printed electronics is becoming an increasing important research area. Most printed transistors are fabricated from organic molecules which suffer from relatively low mobility. Recently, it has become clear that nanomaterials can easily be printed using standard inkjet printers and that the resultant nanosheet networks can be used as active channels in transistors. This project will involve printing networks of semiconducting nanotubes of the inorganic compound, WS2. These networks will be characterised as active channel materials by measuring the source-drain voltage as a function of gate voltage in a field-effecttransistor arrangement. Once switching of current has been measured, we will attempt to print all-printed, all-nanotube transistors using carbon nanotubes as the electrodes, WS2 nanotubes as the channel and Boron Nitride nanotubes as the dielectric.

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7. Medical sensors from silver nano-platelet/polymer composites Supervisor: Professor Jonathan Coleman Location: TCD Sensors which can measure strain, pressure and impact are becoming increasingly important for applications such as wearable health monitors which can track blood pressure and breathing. The simplest way to produce such sensors is to mix a conductive nanomaterial with an elastomer (a very stretchy polymer). While the nanomaterial renders the polymer conductive, deforming the polymer disrupts the connections between nanoparticles thus increasing the composite resistance in proportion to the deformation. However, such composites are generally too resistive for most applications. This project will explore new, highly conductive, composites by mixing silver nano-platelets (2D sheets of silver) with elastomers. The first step will be to measure the composite conductivity as a function of silver content. Here we would expect a sharp increase above some threshold silver content: the percolation threshold. You will then deform the composites and measure how the resistance changes with strain, leading to the sensor sensitivity. We expect the sensitivity to be maximised at the percolation threshold, falling off at higher contents. The aim is to find the optimum silver content where the both conductivity and sensitivity are sufficiently high. Once this sweet spot is identified, you will print composite sensors using inkjet printers with the aim of creating a wearable device. 8. Mechanics of nanotube-nanosheet composites. Supervisor: Professor Jonathan Coleman Location: TCD 2D nanosheets are important for a number of applications such as battery or catalytic electrodes, where they are found in the form of networks of billions of weakly bonded nanosheets. However, such applications can involve the generation of large stresses which typically result in the mechanical failure of the network (it breaks). Typically, there is a critical crack thickness (CCT), above which the network will fail. In practise, one wants the CCT to be as large as possible. We have shown that the CCT can be increased dramatically by adding nanotubes to the nanosheet network. However, it is unknown how big the CCT can be or how it is related to nanotube content. This project will incorporate a number of related strands. First, it will measure the CCT for composites of graphene nanosheets mixed with nanotubes as a function of nanosheet content. In addition, for composites with large CCT, we will test whether the composite strength is thickness independent (as continuum mechanics would predict), gaining knowledge which will be important for practical electrode design. Finally, some nanosheet networks needed to be reinforced without increasing their electrical conductivity (as adding nanotubes does). We will attempt to achieve this by preparing composites of boron nitride (BN) nanosheets mixed with BN nanotubes and studying their mechanics. Page 4 of 22

9. Fresnel Lens in silicon nitride waveguides Supervisor: Professor John Donegan Location: TCD Light travelling in waveguides excites various modes that are determined by the waveguides size, material composition and refractive index. For many applications, it is important to be able to focus light within the waveguide rather than using an external optical lens system. A Fresnel lens is an optical device in which a pattern of holes in the waveguide are designed to focus the light to a point. The pattern is determined by the light wavelength and the focal length. In this project, we will look first at the diffraction of light within the waveguide with a single slot and then we will look at the use of various patterns in the focussing of the light. We will examine the side modes that are produced in such a design and how interference effects can be used to minimise such side modes. Our work will be based on the use of silicon nitride waveguides. The project will involve the fabrication of the Fresnel lens structures, the analysis of waveguide modes and the study of focussing properties of the lens.

10. Novel plasmonic materials based on Au alloy materials Supervisor: Professor John Donegan Location: TCD Gold (Au) and silver (Ag) are the key plasmonic elements exhibiting resonances in the visible region of the spectrum. For several applications, these metallic structures will be put under extreme conditions where they will be used at high temperature and under high-intensity light. Recent studies show that the plasmonic materials degrade rapidly in applications such as heat-assisted magnetic recording. Alloying the Au and Ag with other elements including copper will be examined in this project. Alloying generally improves the mechanical properties of metallic films and will be examined in this project to see how high temperature and high optical intensity affect its operation. In the project, films of different alloy composition will be deposited and studies of degradation of the films under intense optical excitation will be carried out. 11. Novel perovskite materials for photonic applications Supervisor: Professor John Donegan Location: TCD There has been a very large amount of research work on the optical properties of both organic and inorganic perovskite materials. The major application area is in solar cells, but there are also many other applications where arrays of both lasers and photodetectors are required. In this project, we will first synthesise a range of perovskites materials in single crystal form. Next, these materials will be processed into layer structures and an array of laser and photodetector devices will be developed. We will study how the preparation conditions including thermal annealing Page 5 of 22

can be used to improve the quantum efficiency of the devices. A study of the long term stability of the materials and the devices will be carried out. 12. Theory of optical topological insulators Supervisor: Professor Paul Eastham Location: TCD Topological insulators are materials where the electrons orbit in knots. They behave much like ordinary insulators, except at an edge, where there has to be a conducting region. While this classification is now quite well understood for electrons, it should apply to other waves too, and in particular to light. In this project you will develop and analyze models of light propagating in structured materials, identify the structures where the photonic states have non-trivial topology, and demonstrate the physical consequences of this at an edge. This is a theoretical project which will require both analytical work as well as the development of simulations using Mathematica and other tools. OR Entanglement in open quantum systems A quantum-mechanical system, like a pair of spins, is entangled when its wavefunction does not factorize into components representing its constituent parts. Entanglement is the resource used by quantum computers to outperform classical computers, and the most radical difference between quantum and classical mechanics. Unfortunately entanglement is fragile, and destroyed by the interactions between a quantum system and its environment. In this project you will write a Python code to calculate how this occurs in an exactly solvable model. You will use this code to explore how entanglement is destroyed, how it can be protected against the effects of an environment, and the validity of different theoretical methods. This project is theoretical and will involve both analytical and numerical work. 13. Physical properties of disordered networks Supervisor: Mauro Ferreira Location: TCD Thin films composed of networks made of an array of low-dimensional objects (nanowires, 2D nano-sheets, etc) have been attracting a lot of attention due to their promising physical properties. The goal of the present project is to develop simple theoretical models capable of describing the physical properties of such networks. Transport, optical, thermal and magnetic are some of the possible physical properties to be investigated. In order to achieve this, we must separate the project in two complementary parts: one involving the development of a macroscopic model and another which consists of the microscopic details of the network. The student Page 6 of 22

will be in charge of developing such models and will involve good analytical and numerical skills. 14. Computer simulations of foam-fibre dispersions Supervisor: Stefan Hutzler Location: TCD Liquid foams are used in the production process of novel fibrous materials. A model has recently been developed by the TCD Foams group for the flow of foam fibre dispersions in two dimensions. The aims of this project are firstly the introduction of fibre roughness into the model and secondly its extension to three dimensions. The work will be carried out in close collaboration with a PhD student. VJ Langlois and S Hutzler, Dynamics of a flexible fibre in a sheared two-dimensional foam: numerical simulations, Colloids and Surfaces A: Physicochemical and Engineering Aspects (in press, 2017). http://www.sciencedirect.com/science/article/pii/S0927775717302315

15. Experiments on foam drainage Supervisor: Stefan Hutzler Location: TCD Once a foam is formed, liquid drains from it, driven by gravity. This mainly experimental project will examine several aspects of foam drainage, including its role in overall stability of the foam, foam fractionation (separation of surface active material out of a bulk solution), and bubble rearrangements. The work, which will also contribute to the setting-up of an apparatus for measurement of electrical conductivity of a foam, will be carried out in close collaboration with a postdoctoral researcher. Hutzler S, Lösch D, Carey E, Weaire D, Hloucha M and Stubenrauch C (2011), Evaluation of a steady-state test of foam stability', Philosophical Magazine, 91, 537-552.

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16. Pulsed laser deposition of nanoparticle films of titanium nitride Supervisors: Professor James G Lunney and Professor Louise Bradley Location: TCD Pulsed laser deposition (PLD) provides a relatively simple and convenient method for the preparation of thin films of functional materials for research. Both nanosecond and femtosecond lasers can be used [1]. Previously we have used PLD to nanoparticle films of silver and gold. These NP films display a plasmonic resonance in the visible and can be used for optical application such as surface enhanced Raman spectroscopy (SERS). We have also demonstrated that plasmonic silver can be made using PLD at atmospheric pressure. This project will explore the feasibility of using PLD to make NP films of titanium nitride, which also has a plasmonic resonance in the visible, but is more is a more robust material for some applications [2]. [1]

[2]

I. Mirza, G. O’Connell, J. J. Wang and J. G. Lunney J G 2014 Comparison of nanosecond and femtosecond pulsed laser deposition of silver nanoparticle films Nanotechnology 25 265301 V. N. Gururaj, J. L. Schroeder, X. Ni, A. V. Kildishev, T. D. sands and A. Boltasseva, 2012. Titanium nitride as a plasmonic materials for visible and near-infrared wavelengths Optical Materials Express, 2 478.

17. Magnetohydrodynamic heating and control of laser produced plasma Supervisor: Professor James G Lunney Location: TCD We have recently demonstrated that a pulsed magnetic field can be used to inductively heat and focus a laser produced plasma in vacuum [1]. Arising from that work, we wish to explore some new approaches to using a combination of high discharge currents and strong magnetic fields to heat, and control the expansion dynamics of, a laser produced plasma. [1}

J. R. Creel, T. Donnelly and J. G. Lunney 2016 Heating and compression of a laser produced plasma in a pulsed a magnetic field, Applied Physics Letters 109 071104.

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18. A study of growth and magnetic properties of Au-capped Fe atomic-width nanowire arrays self-assembled on a vicinal platinum single crystal surface. Supervisor: Professor Cormac McGuinness Location: TCD The magnetic properties of bare Fe atomic-width and height nanowires grown by self-assembly at the step edges of platinum vicinal single crystal stepped surfaces such as Pt(997) have been investigated in the past [1]. Capping such self-assembled nanowire arrays by a few monolayers of gold is expected to change greatly the magnetic behaviour of these systems as has been observed to occur for cobalt nanowires [2]. The self-assembled growth of Fe nanowires on Pt(997) will be attempted and these nanowires will be capped with an ultra-thin Au layer. Preparation of the Pt(997) surface and the growth of these nanowires will occur in ultra-high vacuum (UHV) chambers. In UHV the growth will be characterised by low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) and also by in-situ reflection anisotropy spectroscopy (RAS) in the visible and near-visible regions. Upon successful growth then the capping layer prevents oxidation upon removal from the chamber and ex-situ magnetic measurements such as magnetooptic Kerr effect (or RAS-MOKE) measurements will measure the magnetic hysteresis of the Au-capped Fe nanowire arrays at room temperature and at a range of temperatures below room temperature. In addition, further ex-situ measurements by x-ray photoemission spectroscopy (XPS) can serve to confirm the electronic structure and metallicity of the capped Fe nanowires. It is the intention that these samples will then be studied at synchrotron radiation sources by x-ray magnetic circular dichroism (XMCD) techniques. The student will become skilled in many aspects of surface science, vacuum technology and analytical techniques. [1] R. Cheng, K.Y. Guslienko, F.Y. Fradin, J.E. Pearson, H.F. Ding, D. Li, and S.D. Bader, Phys. Rev. B 72, 014409 (2005). [2] M.J. Duignan, J.P. Cunniffe, P.-A. Glans, E. Arenholz, C. McGuinness, and J.F. McGilp, Phys. Status Solidi 253, 241 (2016).

Or A study of in-situ and in-operando OFET device relevant thin films and their application in diagnostics. Supervisor: Professor Cormac McGuinness with Maria Daniela Angione (AMBER/Chemistry) Location: TCD Advances, over the past two decades, in electronic and functional materials development has seen Organic-Field Effect Transistors (OFET) devices emerging as a powerful platform for applications in sensing and diagnostics [1]. OFETs have been fabricated on SiO2/Si substrates having as electronic active layer polythiophene-based organic semiconductors. Selectivity, sensitivity and single molecule detection has been achieved through an ad hoc carbohydrates modification Page 9 of 22

strategy of the organic semiconductor thin film. These glycosylated OFET devices have demonstrated exceptional sensitivity to measure very small concentration of swine flu virus (H1N1) with a strong effect on the OFET device I-V behaviour following virus detection. A surface science investigation of the functional attachment chemistry, influencing the sensitivity of these OFET devices, will proceed via measurements from spin-coated thin films studied through both x-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). Optical surface science studies by reflectance anisotropy spectroscopy (RAS) of these thin films, obtained in situ and simultaneous with the photoemission data will be acquired. These will be compared with RAS studies of these thin films and OFET devices obtained in operando, i.e. while the OFET device is in operation, measuring the RAS before, after and during the exposure to the H1N1 analyte. The design and implementation of the optical setup for such an in-operando RAS measurement is one of the key necessities and anticipated outcomes of the project. The student will become skilled in many aspects of surface science, vacuum technology and analytical techniques. [1] Lin P, Yan F., Adv Mater. 2012, 24(1):34-51

Or Memristive current-voltage characteristics and spatial distribution of electromigrated oxygen vacancies in undoped TiO2 and transition metal doped TiO2. Supervisor: Professor Cormac McGuinness and Professor HongZhou Zhang Location: TCD Oxygen vacancies in titanium dioxide are of interest as their spatial distribution can be manipulated by electric fields giving rise to hysteretic current-voltage behaviours, dubbed memristance, an effect which can serve as the basis for non-volatile memories [1]. Oxygen vacancies in titanium dioxide bulk or thin-film samples can be produced by high temperature annealing in vacuum. Voltages across small length scales give very high electric fields and can cause oxygen anions to electromigrate towards an anode with the vacancy in the lattice migrating in the opposite direction. At high-temperature the energy barrier against vacancy diffusion is overcome through thermal energy and electromigration across large length scales with small electric fields is possible. In this experiment an ultra-high vacuum purpose built electromigration chamber will be used to produce vacancies, manipulate vacancies and produce inhomogeneous oxygen vacancy distributions that will freeze out as temperature is reduced. The student will investigate the resultant I-V behaviour in both bulk titanium dioxide crystals and in thin films of titanium dioxide, some of the latter of which are to be doped with other transition metals. The spatial distribution of vacancies in these electromigrated TiO2 materials will be probed by optical methods, x-ray photoemission spectroscopy (XPS) methods and by electron-beam based cathodoluminescence (CL) methods available at electron microscopes in the Advanced Microscopy Laboratory (AML). The student will become skilled in many aspects of surface science, vacuum technology and analytical techniques. Page 10 of 22

[1] D.B. Strukov, G.S. Snider, D.R. Stewart, and R.S. Williams, Nature 453, 80 (2008).

Or A study of Mn and MnO thin films on Ru surfaces – investigation into lowered MnO reduction due to bimetallic catalysis Supervisor: C. McGuinness Location: TCD Solid oxide fuel cells (SOFC) are a strong candidate for use as a future source of environmentally stable renewable energy. The basic operation of SOFCs requires only the input of air as a source of oxygen, which undergoes an oxygen reduction reaction (ORR) catalysed by the cathode electrode. This ORR can be expressed in simple terms as a dissociation/reduction interaction between gaseous oxygen and the cathode surface, which converts O2 into negatively charged oxygen ions. However, the high operational temperature (>800 °C) required for current, state of the art, SOFC cathodes has been identified as the major barrier to widespread SOFC use. As such, a research goal is to improve SOFC efficiency by identifying alternative cathode materials capable of catalysing the ORR at lower temperatures. Promising results have recently been achieved, with manganese/ruthenium cathode surfaces showing evidence for ORR at temperatures as low as 500 °C. To further the understanding of this behaviour monolayers (ML) or several monolayers of manganese on a single crystal Ru(0001) surface, their oxidation to MnO and the subsequent reduction to Mn and the temperature dependence of this will be studied. Mn layers are known to grow pseuodmorphically with the underlying Ru surface until islanding occurs after 6 ML. This investigation will occur via ultrahigh vacuum (UHV) surface science analysis techniques, inclusive of x-ray and ultraviolet photoemission spectroscopies (XPS and UPS), and low energy electron diffraction (LEED) as part of a fully in-situ growth and analysis experimental procedure. The three stage procedure will involve the cleaning and preparation of a clean Ru(0001) surface, the in-situ growth of Mn layers on that surface, controlled O2 exposure to oxidise, followed by high temperature UHV annealing cycles to ascertain the lowest temperature at which the oxygen reduction reaction can be achieved. Crucially, all stages of sample cleaning, sample growth, O2 catalysis and sample analysis will be performed in-situ within a UHV environment with XPS and UPS at each step to evaluate the validity of the d-band model of catalysis to this Mn/Ru system and to evaluate the result for differing thin films (

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