Projects for Physics Students 2017/18 · Page 1 of 2 Projects for Physics Students 2017/18 . 1....

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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-effect-transistor 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.

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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

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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

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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.

http://www.sciencedirect.com/science/article/pii/S0927775717302315

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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] 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

[2] 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 magneto-optic 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

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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.

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[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 ultra-high 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 (<6ML) and thicker islanded growths. Results from the single crystal Ru(0001) surfaces will be compared to those previously obtained from Mn on Ru thin films generated by atomic-layer-deposition. In addition a manganese/ruthenium bimetallic alloy may be studied. The student will become skilled in many aspects of surface science, vacuum technology and analytical techniques. Or

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Calculation of the electronic structure, valence band and core level spectra and transport in differing metal porphyrin incorporated graphene nanoribbons. (Computational) Supervisor: Professor Cormac McGuinness Location: TCD This project will use appropriate Density Functional Theory (DFT) codes to simulate the electronic structure of transition metal porphyrins incorporated into graphene nanoribbons. Transition metal porphyrins accommodate a range of divalent metal ions at the center of the porphyrin macrocycle giving differing band-gaps and differing optical responses with metal ions ranging from Zn, Ni, Fe, Mn, Cr and Mg, and give rise to e.g. heme and chlorophyll. Self-assembled graphene nano-ribbons can be obtained through on-surface synthesis with the thermal dehalogenation, polymeric assembly and cyclodehrogenation of precursor molecules such as di-bromo-bi-anthracene into a 7-carbon atom wide armchair graphene nanoribbon [1]. Brominated porphyrin molecules can participate in this self-assembly giving rise to molecular nanostructures like that shown in figure 1. The electronic bandstructure of such a molecular nanostructure will be calculated for a variety of differing transition metal (M) species in the functionalised graphene nanoribbon. The valence band occupied density of states (DOS) and the core-level binding energies will be computed for future comparison to measurements of these systems obtained through valence band photoemission (UPS) and core–level x-ray photoemission (XPS) respectively in laboratory or of the electronic bandstructure at synchrotron radiation based ARPES experiments. A desirable end goal would be the calculation of the electronic transport of the molecular nanostructure for voltages applied across its ends and the dependence of this transport on either the type or number of metal porphyrins incorporated within the nanostructure. Physical insight, experience with unix/linux, some programming or scripting ability and careful thought will be required for this project. [1] J. Cai, P. Ruffieux, R. Jaafar, M. Bieri, T. Braun, S. Blankenburg, M. Muoth, A.P. Seitsonen, M. Saleh, X. Feng, K. Müllen, and R. Fasel, Nature 466, 470 (2010).

Figure 1: A scheme for integrating a transition metal porphyrin molecule into a self-assembled graphene nanoribbon composed from pre-cursor molecules of di-bromo-bi-anthracene (on left in blue) and di-brominated transition metal porphyrins (on left in red) into a porphyrin integrated

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19. The comparability of total-energies calculated using self-consistent quantum-mechanical simulation in the DFT+U formalism Supervisor: Professor David O’Regan Location: TCD Density-functional theory (DFT) is today a very widely-used theoretical framework for calculating the electronic properties of materials and molecules. Both in its development and application, DFT is experiencing a period of substantial growth. At present, however, the predictions yielded by standard approximations within DFT are often unreliable, even qualitatively, due to systematic errors which are well understood but difficult to correct. One established approach for doing just that is known as DFT+U, where parameters known as the Hubbard U are introduced, parameters which determine the magnitude of an additional energy term which corrects the most dominant errors. A number of methods have been suggested for calculating these Hubbard U, in which case the theory is restored to its parameter-free status. This approach has proven to be successful for correcting spectroscopic properties such as the insulating gap, but more fundamental ground-state properties such as energy differences and the structural stability of crystals have received less attention, and enjoyed less success. If the Hubbard U can be calculated in an appropriate way then the energies should, in principle, be comparable from one crystal structure to another. The aim of this project is to directly test this type of approach on transition-metal oxides, first calculating their spectroscopic properties, followed by magnetic exchange coupling parameters, up to complete phase stability diagrams which explicitly require the comparability of energies. This project will not pre-suppose familiarity with high-performance computing, but familiarity with Linux and gnuplot (or similar) would be helpful, and the careful management of a large number of simultaneous supercomputer calculations is essential. Phys. Rev. B 94, 220104(R) (2016). See https://arxiv.org/abs/1608.07320 20. Application of geometric correction methods to accelerate quantum electronic structure calculations Supervisor: Professor David O’Regan Location: TCD In certain advanced density-functional theory based approaches for calculating the electronic properties of materials and molecules, the orbitals in which the electrons are represented are allowed to evolve numerically. To give a picture, imagine two atoms moving together. Their electronic states hybridise, and the orbitals used to represent those states must be relaxed from their atomic starting point in order to find the ground state of the system. A long-standing open question in this area is whether we can simultaneously evolve the matrix representation of operators of interest, such the Hamiltonian, in order to compensate for this. Otherwise, leaving these matrices fixed while the orbitals evolve, as is conventional practice, can lead the calculation to become slowly converging or even unstable. This is a particularly

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important problem when relaxing crystalline geometries, or performing molecular dynamics calculations in which the ions move at finite temperature, in which case it can badly slow calculations. This project involves testing and comparing the available approaches, including one recently introduced in our group, across a range of systems from small molecules, to metals, to magnetic oxides. The theory involved in this project is new and very enjoyable to work with, the project brings hands-on experience with high-performance computing, and one bringing an interest in carrying out advanced numerical work together with a careful, methodical approach to this project would be rewarded. See Phys. Rev. B 95, 115155 (2017). https://arxiv.org/abs/1608.05300 21. Quantum-mechanical simulation of magneto-optical spectroscopy Supervisor: Professor David O’Regan Location: TCD Certain materials exhibit the phenomenon of rotating linearly-polarised light passing through them, an effect known as optical activity or magneto-optical response. The related spectroscopic techniques, in particular, electronic circular dichroism (ECD) and optical rotatory dispersion (ORD) directly probe the chirality of the material in question. These techniques are particularly important for characterising similar molecules or nanostructures with different chirality, whose absorption spectra may be identical but whose magneto-optical may differ enormously. ECD also plays a role in the study of magnetic materials with a strong orbital component to magnetism. Experimental ECD signals are often very clear, but can be difficult to understand without some prior knowledge of the crystal structure. This is why a theoretical description, necessarily quantum-mechanical, is an essential, but hitherto almost entirely absent, companion to experimental magneto-optical spectroscopy. Up to now, relatively very few first-principles quantum calculations of magneto-optical spectra have been carried out, and code for carrying them out is not very widely available. The proposed project first entails a study of the rather interesting and under-developed quantum-mechanical theory of angular momentum and related magneto-optical response, and how they can be computed using the output of computer simulations using the widely used atomistic simulation method known as density-functional theory (DFT), and its linear-scaling generalisation. This project aims to bring to completion research carried out by summer interns, involving abstract theory and its software implementation in parallelised code for high-performance computing architectures. See https://arxiv.org/abs/1703.05056

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22. Quantum many-body theory of Van der Waals forces Supervisor: Professor Charles Patterson Location: TCD Quantum physics tells us that any system of particles has a zero point energy associated with quantum fluctuations of the system. For electrons in molecules or solids the zero point energy associated with fluctuations in the electron density causes Van der Waals forces between molecules. These fluctuations are not included in mean field theories of electrons such as Hartrree-Fock or density functional theories. They are included in many-body theories such as the random phase approximation (RPA). In this project you will learn some many-body theory and some computational physics. There will be opportunities to learn about (and do) some high performance computing code development too. The project will be to investigate Van der Waals forces between pairs of small molecules such as benzene and to relate the computed forces and potential energies to available experimental data. 23. Quantum theory of excitonic interactions in molecules Supervisor: Professor Charles Patterson Location: TCD Optical excitations in matter are commonly represented as single particle transitions in which an electron jumps from one level to another. This is an oversimplification of optical excitations. An alternative picture, which comes from many-body theory, is that an optical excitation is creation of an electron-hole pair by a photon. In a single particle theory the electron and hole move independently before deexciting. In a many-particle theory the electron and hole created by photon absorption interact via Coulombic forces and can bind to form excitons. In this project you will learn some many-body theory and some computational physics. There will be opportunities to learn about (and do) some high performance computing code development too. The project will be to investigate the importance of this interaction in determining the optical spectra of a range of molecules. 24. High Entropy Magnetic Alloys Supervisor: Professor Stefano Sanvito Location: TCD Steel is usually considered as a highly mechanically performing material, with a large yield strength and fracture toughness. Recently a new class of metallic alloys have seriously challenged such performances and have established themselves as new star materials for a wide range of applications. These are the so-called high-entropy alloys [1, 2], made by mixing in equal proportion five or more transition metals (either from the 3d or the 4d family). Remarkably such alloys, whose thermodynamical

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stability at high temperature is driven by their large entropy, have remarkable mechanical properties. In this project we will use advanced electronic structure theory (density functional theory) for understanding the origin of such remarkable properties and for designing at least one new high entropy alloy presenting a magnetic order. In particular the students will:

1. Learn how to use a density functional theory code to calculate the electronic structure of metals

2. Run calculations for random alloys 3. Extract the magnetic properties of such alloys from the calculations.

References

[1] Bernd Gludovatz et al., A fracture-resistant high-entropy alloy for cryogenic applications, Science 345, 1153 (2016).

[2] X. Lim, Metal Mixology, Nature 533, 306 (2016). 25. High-Field Point Contact Andreev Reflection from Zero-Moment Half Metals Supervisor: Professor Plamen Stamenov Location: TCD Point Contact Andreev Reflection (PCAR) is a method for determination of the magnitude of the electron spin polarisation close to the Fermi level in magnetic metals and degenerate semiconductors – a parameter of critical importance for their applications in spin electronic devices. The experiments involve the accurate measurements of the low-temperature differential conductance of superconductor – metal junctions and the determination of the characteristic current conversion at the interface (from Cooper pairs to normal quasi-electrons). The project will involve the experimental investigation of a relatively new class of magnetic materials – the Zero-Moment Half-Metals (ZMHM), using their prototype Mn2-xRuxGa, in a thin film form. ZMHMs have the potential to offer the rather unique for spin electronics combination of high bulk spin polarisation, stray field immunity and intrinsically high resonance and switching frequencies. Nano-scale memory elements and terahertz oscillators are only two examples of possible applications. Films prepared by sputtering at different conditions (temperature, Ru-target current, etc.) will be used to demonstrate the sensitivity of the high-field Andreev reflection technique towards the sign (and not only the magnitude) of the Fermi level spin polarisation of the ZMHM. Two distinct sets of parameters will be used to achieve both signs of the polarisation in the same material system. The newly developed spectrometer for use in high magnetic fields (up to 14 Tesla) will be utilised and theoretical understanding and modelling (fitting) of the data will be sought in the framework of a modified BTK theory.

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References: G. E. Blonder, M. Tinkham, and T. M. Klapwijk, Phys. Rev. B 25, 4515 (1982). I. I. Mazin, A. A. Golubov, and B. Nadgorny, Journ. Appl. Phys. 89, 7576 (2001). G. T.Woods, R. J. Soulen, I. Mazin, B. Nadgorny, M. S. Osofsky, J. Sanders, H. Srikanth, W. F. Egelho, and R. Datla, Phys. Rev. B 70, 154416 (2004). 26. Development of High-speed Parallel Readout Interface to Toggle Magnetic Random Access Memory (TMRAM) Arrays for use as Magnetic Sensors with Sub-Micron Resolution for Detection of Nanowires Supervisor: Professor Plamen Stamenov Location: TCD Spin-Valves (SVs) and Magnetic Tunnel Junctions (MTJs) are, in their simplest forms, sandwiches of two conducting and magnetic layers, separated by a nonmagnetic conductor or a nonmagnetic insulator, respectively. Their primary uses in spin electronics have been concentrated in the area of external magnetic field sensing. Another strand of spin electronics, however, relies on large arrays of SVs or MTJs, designed particularly to be insensitive towards the external magnetic field, as the storage elements in the so-called Magnetic Random Access Memory (MRAM). The two branches of the same field have, so far, had little interaction, but to the optimisation of the very SVs and MTJs used. The development of arrays of magnetic sensors should take the best of both worlds and provide useful measurement platforms for fields like magnetic bio-marking and imaging magnetometry. Following a successful project (2016/17) the work will involve the interfacing to an already characterised commercial MRAM – 1 Mb EverspinTM . The magnetic field source – a small vector electromagnet is also already in place and characterised. Once the rotational field evolution aquisitoin times are brought to about 0.1 – 0.3 s per complete array acquisition, tests will be undertaken to quantify the response to micron-sized magnetic nanowires, placed on the sensitive area of the test chips. The working signal-to-noise ratio and the imaging throughput of the new acquisition scheme will be then objectively verified. Images of the stray field of actual nanowires will be compared to quasi-analytical models.

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 60.0

0.2

0.4

0.6

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1.0

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1.6

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CuCrSeBr / NbT = 3.00(5) K∆T * = 0.95(5) K∆1* = 1.378(4) meV∆2 = 1.5(2) meVZ *= 0.43(2) P * = 0.40(1)

Co / NbT = 3.00(5) K∆T * = 3.0(8) K∆1* = 1.45(5) meV∆2 = 1.5(2) meVZ *= 0.39(9) P * = 0.42(9)

Ni / NbT = 2.50(5) K∆T * = 3.5(8) K∆1* = 1.40(5) meV∆2 = 1.5(2) meVZ *= 0.26(9) P * = 0.45(9)

CrO2 Data CrO2 Fit Cu Data Cu Fit Fe Data Fe Fit

Ni DataNi FitCo DataCo FitCCSB DataCCSB Fit

Fe / NbT = 6.80(5) K∆T * = 1.7(8) K∆1* = 1.19(2) meV∆2 = 1.5(2) meVZ *= 0.21(9) P * = 0.45(3)

Cu / NbT = 4.20(5) K∆T * = 0.0(8) K∆1* = 1.26(1) meV∆2 = 1.5(2) meVZ *= 0.00(1) P * = 0.00(1)

G /G

N

Applied Bias (∆)

CrO2 / NbT = 2.60(5) K∆T * = 2.9(8) K∆1* = 1.32(5) meV∆2 = 1.5(2) meVZ *= 0.42(8) P * = 0.86(9)

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27. An ab-initio calculation of electron energy loss spectroscopy for localized irradiation effect of Co3O4 nanostructures Supervisor: Professor Hongzhou Zhang Location: TCD The ultimate goal of this project is to evaluate the capability of nanoscale modification using a focused 30-KeV He+ beam. We plan to evaluate the preferential milling of oxygen atoms in Co3O4. The irradiation effect is then studied by calculating excitation spectra of the irradiated CO3O4 nanostructures by using FEFF[1], an ab initio multiple-scattering code. We hope to acquire electron energy loss spectra (EELS) from Co3O4 nanostructures that undergoes a range of irradiation conditions [2]. Specifically, the dependence of near-edge fine structures on the defect concentration will be investigated. We will assess the viability of using HIM for the nanoscale modification and the theoretical work will provide guidance for the design of our further experiments on both the beam modification and EELS characterisation. [1] http://feffproject.org/ [2] Pearson, D.H., C.C. Ahn, and B. Fultz, White lines and \textit{d} -electron occupancies for the 3 \textit{d} and 4 \textit{d} transition metals. Physical Review B, 1993. 47(14): p. 8471-8478. 28. Simulation of Helium-ion Images Supervisor: Professor Hongzhou Zhang Location: TCD Helium-ion microscope (HIM) is a newly-developed charged-beam microscope that offers sub-nanometer lateral resolution with high surface sensitivity. Our recent work on HIM imaging of graphene samples indicate this tool can be used to distinguish the thickness of graphene samples. In this project, we intend to develop a numerical approach that can clarify the image formation mechanism in the HIM, especially for the thickness contrast. The contribution of the work function and the attenuation of the electrons will be studied in detail. Our aim is to evaluate the possibility of using the HIM as a quantitative tool for the extraction of physical quantities of materials. 29. Helium-ion beam for nanofabrication Supervisor: Professor Hongzhou Zhang Location: TCD The precise creation of surface structure is crucial to the future of the nanotechnology. High resolution Ion beam machining is one of the key enabling methodologies allowing for the creation of 10nm fine structures. However as the demands for finer structures begin to approach the maximum capabilities of current machinery alternatives must be investigated. The Helium-ion Microscope with its sub nm spot size and milling capabilities shows excellent promise in this area. The

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milling rate of the HIM is a factor of a hundred times slower than the commonly used Focused-ion beam system. This low removal rate allows for more controlled amounts of material to be removed resulting in finer etching, while beam damage and contamination must be evaluated. In this project, we will study the milling process of the HIM with an objective of understanding its capability and limitation in nanoscale fabrication. 30. Optical properties of Transition Metal Dichalcogenide Quantum Dots Supervisor: Professor P.N. Ajayan Location: Rice University, Houston, TX, USA Local TCD Contact: Professor Werner Blau Recent advances in the development of atomically thin layers of van der Waals bonded solids have opened up new possibilities for the exploration of 2D physics as well as for materials for applications. Among them, semiconductor transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se), have bandgaps in the near-infrared to the visible region, in contrast to the zero bandgap of graphene. In the monolayer limit, these materials have been shown to possess direct bandgaps, a property well suited for photonics and optoelectronics applications. This project will investigate their optical properties with particular emphasis on additional lateral nanoscale confinement which will further alter the electronic and hence optical properties. 31. 2D topological insulators for thermoelectric applications Supervisor: Professor David Carroll Location: Wake Forest University, Winston-Salem NC, USA Local TCD Contact: Professor Werner Blau Electrical charges on the boundaries of topological insulators favour forward motion over back-scattering at impurities, produc¬ing low-dissipation, metallic states that exist up to room temperature in ambient conditions. These states have the promise to impact a broad range of applications from electronics to the production of energy. Almost all the proposed topological insulators are also thermoelectric materials. This is not a coincidence. Advanced thermoelectric materials have an optimized efficiency through low thermal conductivity and excellent electrical conductiv¬ity. For topological insulators, spin–orbit coupling must be strong enough to modify the electronic structure — as spin–orbit coupling strength increases with atomic mass, this indicates that narrow-band¬gap compounds consisting of heavy elements are the most promising candidates. The project will explore the potential impact of the surface states of topological insulators on thermoelectricity in nanostructurally engineered high-performance thermoelectric materials.

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32. High performance organic light sources based on magnetic control of exciton lifetimes Supervisor: Professor David Carroll Location: Wake Forest University, Winston-Salem NC, USA Local TCD Contact: Professor Werner Blau Our “lighting” research focusses on the use of matrix nanocomposite organic emitters in OLED and newly developed FIPEL topologies. We have shown that internal symmetries and dimensionality of the nanophase can be used to “engineer” properties of the emitter’s excited states opening the possibility of super - efficiency illumination, optical amplification, dynamic color engineering, and novel lighting configurations. Thios project will investigate the new possibility of magnetically controlling the exciton lifetimes and hence device efficiencies. 33. Solar-thermal technologies utilizing spectral splitting Supervisor: Professor David Carroll Location: Wake Forest University, Winston-Salem NC, USA Local TCD Contact: Professor Werner Blau As everyone knows, the big problem with photovoltaics is that they do not work at night (or on very cloudy days). The Carroll group has developed a number of hybrid systems that create electrical energy from BOTH the sun and ambient heat. But what are the limitations of these approaches? This project hopes to show that thermodynamic considerations lead naturally to avenues of efficient and cost effective PV-T. 34. Photocurrent spectra of organic semiconductors Supervisor: Professor Andreas Opitz Location: Humboldt University, Berlin, Germany Local TCD Contact: Professor Werner Blau Measurement of photocurrent spectra for organic semiconductors. This data allowsd the determination of the transport gap and exciton binding energy. Suitable for Bachelor and Master thesis work.

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35. Development of novel photodetectors (hardware, simulation) Supervisor: Professor Heiko Lacker Location: Humboldt University, Berlin, Germany Local TCD Contact: Professor Werner Blau A Wavelength-shifting Optical Module (WOM) is a novel photodetector and has been proposed for the large-scale expansion of the IceCube experiment (*). We are working closely with Kowalski in this area. The principle: A primary photon of small wavelength applies to a large-area (quartz) tube which is thinly coated with a wavelength-shifting material. Most of the photons emitted thereafter with longer wavelengths are transported through the total reflection to the end of the tube, where they are detected by the photomultiplier. While Kowalski is investigating WOMs for the detection of Cherenkov photons, eg at IceCube, we are investigating the use of liquid scintillator detectors. Possible working areas include: * Detection efficiency and time resolution of the detector * Influence of the layer thickness of the wavelength-shifting material * Mirroring of one side * Influence of the tube surface * Understanding of the detection by means of simulation Who has fun with hardware and / or simulation is just right in our research and development work in this area. 36. X-ray spectroscopy of oligothiophene molecules and crystals Supervisor: Professor Claudia Draxl and Professor Caterina Cocchi Location: Humboldt University, Berlin, Germany Local TCD Contact: Professor Werner Blau This project is aimed at identifying spectroscopic fingerprints in organic molecular crisps by means of core-level spectroscopy. It concerns the ab initio of excitation, like the sufur K (1s electron) and L2.3 (2p electrons) in oligo-thiophenes, based on manybody perturbation theory. We are interested in intermolecular interactions. 37. Printing processes in organic electronics Supervisor: Patrick Barkowski Location: Humboldt University, Berlin, Germany Local TCD Contact: Professor Werner Blau Organic electronics is concerned with the implementation of molecular electronic components such as light-emitting diodes, solar cells, transistors and the like. The molecular hydrocarbon-based structures allow nasemic processing and direct printing. Within the scope of the Bachelor's or Master's thesis, various novel materials are characterized and implemented in printed organic light-emitting diodes. The work is a co-operation between the group of hybrid components and the company Inuru.

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38. Experiment and theory on the atomic structure of matter and on 2D and 3D imaging with electrons and photons Supervisor: Professor Christoph Koch Location: Humboldt University, Berlin, Germany Local TCD Contact: Professor Werner Blau The AG Structure Reserach and Electron Microscopy offers a number of topics that can be tailored to fit either a Master thesis or a Bachelor thesis. The spectrum of available projects is very broad and involves experiment and theory for explore the atomic structure of matter by electron microscopy (imaging, diffraction, and spectroscopy with electrons in the transmission electron microscope [TEM], or the scanning electron microscope [SEM]), but also the development of new techniques for measuring three-dimensional objects with electrons or light. A selection of available projects can be found here: https://www.physik.hu-berlin.de/en/sem/StudentProjects But, depending on personal interests, more projects are also available. The language can freely be chosen to be either German, or English. Some of these projects are more theoretical and involve quite a bit of programming (Matlab, Python, or C/C++) while others are more focused on hands-on experiments, either at the electron microscope, or various optical microscopes. 39. Updated 2nd May Electrical conductivity of compressed granular media Supervisor: Professor Matthias Mobius Location: TCD In this project you will investigate how the electrical conductivity of a granular packing of conducting spheres changes with compression. As the pressure increases, the coordination number of the particles increases as well as the contact areas which leads to a decrease in electrical resistance of the packing. This effect has been exploited in carbon microphones for example. Here, you will study a macroscopic model system made from ball bearings. Analogous experiments will be performed with rods and disks. How does the shape of the particles affect the changes in the electrical resistance due to compression?

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