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Projects for Physics Students 2016/17 1. Ultrafast Spectroscopy OF 2D Nanomaterials Supervisor Professor Werner Blau Location: TCD Femtosecond laser characterisation will be used to study the relaxation dynamics of nonequilibrium excitons in 2D-nanocrystals, which dominates the photonic and optoelectronic applicability of these materials. A fast relaxation time implies that the materials can realise electron-hole recombination or electron transition in a very short time, typically sub-ps magnitude and return back to an equilibrium state. Materials with an ultrafast response time could modulate input optical signals quickly and speed up information processing. We will use the CRANN fs laser systems to perform pump-probe experiments and achieve dynamic information after fitting with appropriate models. 2. Saturable Absorption in Molecular Nanoaggregates Supervisor Professor Werner Blau Location: TCD Laser sources producing ns to sub-ps optical pulses are a major component in the portfolio of leading laser manufacturers. They are deployed in a variety of applications ranging from basic scientific research to plastic material processing, from eye surgery to printed circuit board manufacturing, from metrology to the trimming of electronic components. Regardless of the wavelength, the majority of ultrashort (ps-fs) laser systems employ a mode-locker, i.e. a NLO element - called a saturable absorber - that turns the laser continuous wave output into a train of ultrashort optical pulses. The key point for the nonlinear optical study is to measure the third order susceptibility χ(3) of the nanomaterials in question. χ(3) is the central parameter for measuring the strength of optical nonlinearity. Z-scan is a quick and powerful technique to characterise the nonlinear optical properties of materials, including nonlinear absorption, scattering, or refraction. It measures the transmittance through the sample as a function of incident laser intensity, while the sample is gradually moved through the focus of a lens (along the z-axis). We will use both closed and open aperture Z-scan techniques to measure the real and imaginary parts of χ(3), the nonlinear refractive index, the nonlinear absorption coefficient and the excited state absorption cross section, etc.

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3. Ethanol Concentration by Forward Osmosis Supervisor: Professor Werner Blau Location: TCD With increasing attention of alternative energy sources, bioethanol is considered that one of the most promising fuel for future studies. As a production of fuel-grade ethanol, mainly fermentation method, highly energy intensive, is used to remove surplus water from the ethanol. Nevertheless, the concentrate of ethanol solution produced by using this technology is very low and requires running cost in the production of bioethanol. This project will investigate an alternative approach to concentrate ethanol solution by using osmosis which cause the transfer of water from the ethanol solution and into the draw solution. 4. Hybrid material systems Supervisor: Professor Louise Bradley Location: TCD Recent development of a host 2D materials brings exciting possibilities for novel photonic structures. These materials, for example graphene, BN and MoS2, have diverse range of electronic and optical properties. This project will investigate hybrid systems incorporating 2D materials with other photonic materials such as quantum wells and quantum dots, as well as colloidal plasmonic structures. These coupled systems can offer new functionalities and improved performances in terms of light emission, colour conversion and light harvesting. The coupling between the materials can be investigated using time-resolved confocal microscopy and, subsequently, structures which take advantage of mechanisms such as fluorescence energy transfer or charge transfer can be designed with specific applications in mind. The project will investigate both the optical and electrical properties of the hybrid materials for light emission and light detection. 5. Plasmonics for structural colour Supervisor: Professor Louise Bradley Location: TCD There are many examples of structural colour occurring in nature, such as the colour of butterfly wings, or peacock feathers. These beautiful colours arise from the interaction of the ambient light with complex structure on the surface. It has been known since antiquity that nanoscale metallic particles can also produce vibrant colours, very different to the bulk. With advances in nanofabrication the properties of the metal nanostructures can be tailored for specific applications. This project will investigate the potential to modify the colour of surfaces in ambient light conditions. A range of metal nanoparticle materials, sizes and geometries on different substrates will be studied. A finite element modelling tool will be used to computationally

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investigate the parameter space and design structures taking consideration of the interaction between the local surface plasmon resonances, the sensitivity to the dielectric environment, polarization and angle dependences.

6. Chiral plasmonics Supervisor: Professor Louise Bradley Location: TCD Chirality is a property of a system that cannot be superimposed onto its mirror image, for example the left hand cannot be superimposed on a mirror image of the right hand. Chiral molecules can exist in two different states, with similar chemical and physical properties but with different optical properties. Chiral molecules will rotate linearly polarised light in opposite directions, a property known as optical activity. 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. Many biomolecules and drugs are chiral. Chirality at the single molecule level is understood but the creation and understanding of artificial inorganic chiral structures is a hot topic of research, expected to bring new possibilities for photonic and biological applications. Recent years have seen enormous focus on the search for and demonstration of nanoscale chirality based on metallic structures. This project will involve the design of different types of novel artificial chiral metallic structures. 7. Inkjet printed graphene-based composites as high-strain, strain sensors Supervisor: Professor Jonathan Coleman Location: TCD Electromechanical sensors or strain sensors are extremely useful for measuring the displacement, velocity or acceleration of one point relative to another. These are generally made of metal foil and have the disadvantage that they tend to break at strains of ~5%. High strain, low stiffness sensors are hard to make cheaply but would be extremely valuable for monitoring human muscular and skeletal motion. If sensitive enough, such sensors could even measure breathing and pulse. This project will investigate composites of graphene mixed with the soft polymer polydimethylsiloxane for use as strain sensors. You will prepare composite films with different graphene contents and measure their electrical properties. You will then characterise how their resistance varies with strain and assess their performance as strain sensors. Finally the project will investigate whether such composites can be deposited on plastic surfaces by inkjet printing to produce printed sensors.

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8. Graphene-MoS2 nanosheet composites as printed photodetectors Supervisor: Professor Jonathan Coleman Location: TCD Future developments in communications such as the Internet of Things will require the development of printed electronics (PE). Many believe the most promising printable active materials for PE are liquid dispersed 2D nanosheets of materials such as MoS2. To date such materials have been demonstrated as all printed photo-detectors and capacitors. However, printed devices tend to have long channel lengths which tend to make them very resistive. One solution might be to add nanoconductors such as graphene to the MoS2 network, increasing the effective carrier mobility. This project will investigate the dependence of photoconductivity of MoS2/graphene composite films on graphene content. You will find the optimum graphene content where the photoresponse is maximised. You will then take this optimised composition and fabricate all-printed photodetectors by printing combinations of graphene electrodes and MoS2/graphene active materials. 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

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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. 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, ideally using Mathematica. OR A quantum-mechanical system, like a pair of spins, is entangled when its overall 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. In this project you will calculate the amount of entanglement present in models of solid-state systems such as quantum dots. You will explore how entanglement can be generated, how it is destroyed, and the validity of different theoretical methods. This project is theoretical and will involve both analytical and numerical work. 12. The Music of Quantum Mechanics (theory/computational) Supervisor: Professor Mauro Ferreira Location: TCD Particle confinement in quantum mechanics is well described by bound-state wave functions, which are solutions of the time-independent Schroedinger equation. While this is mathematically simple in the case of simple confining wells, it gets more challenging when the confining geometry is more involved. In this project, numerical solutions of the Schroedinger equation will be carried out to map the spatial distribution of wave function nodes in several different geometries. In acoustics, node lines of vibrating musical instruments are called Chladni figures and are used to assess the quality of the instrument. Here by implementing a numerical method of

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obtaining Chladni figures for quantum objects, we will be able to map how the wave function vibrations are spatially distributed. This is an analogous and indirect way of “listening" to the music played by quantum objects. The project requires good knowledge of quantum mechanics, a good mathematical background and a reasonably good ability to code in any language (student’s choice). 13. Experimental studies of foam-fibre interactions Supervisor: Professor Stefan Hutzler Location: TCD This experimental project concerns structure and flow-properties of foam-fibre dispersions. This is relevant in the context of the so-called foam forming process for paper making. It can also be used to produce novel fibreous materials made from natural fibres, such as found in peat. 14. Computer simulations of foam structures Supervisor: Professor Stefan Hutzler Location: TCD Foam structure is determined by the minimisation of surface area for given bubble volumes, together with external constraints (e.g. cross-section of tube containing the foam). This computational project concerns computer simulations of ordered foam structures, using software such as the Surface Evolver (http://www.susqu.edu/brakke/evolver/evolver.html), but also methods where bubbles are approximated as (overlapping) spheres, interacting via Hooke's law under compression only. 15. Plasma assisted atmospheric pulsed laser deposition of thin films Supervisor: Professor James G. Lunney 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. The Laser and Plasma Applications group are exploring the potential of doing PLD in a gas at atmospheric pressure. The aim of this project is to investigate how process of atmospheric PLD can be assisted by transporting the laser ablated material through a secondary atmospheric plasma.

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16 Z-scan measurements of gold nanoparticle films made by pulsed laser deposition Supervisor: Professor James Lunney and Professor Werner Blau Location: TCD Previously we have used pulsed laser deposition (PLD) to make nanoparticles of gold and other noble metals. We used the Z-scan technique to investigate the nonlinear optical response of these films. The aim of this project is to use PLD to make nanoparticle gold films, and then apply the Z-scan technique, with different laser repetition rates, to clarify the physical origin of the nonlinear response. 17. May be especially suitable for NPCAM students A study of growth and properties of Au-capped Fe atomic-width nanowires 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. 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).

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18 Memristive current-voltage characteristics and spatial distribution of electromigrated oxygen vacancies in undoped TiO2 and transition metal doped TiO2. Supervisor: Professor Cormac McGuinness 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. [1] D.B. Strukov, G.S. Snider, D.R. Stewart, and R.S. Williams, Nature 453, 80 (2008).

19. A study of Mn and MnO thin films on Ru surfaces – investigation into lowered MnO reduction due to bimetallic catalysis Supervisor: Professor Cormac 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

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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. 20. 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

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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).

21. The reliability of total-energies calculated using self-consistent DFT+U Supervisor: 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

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

h ibb l l t t ( i ht)

BrBr

n nn

MN

N N

N

R

R

BrBr MN

N N

N

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R

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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. 22. Application of geometric correction methods to accelerate 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 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. 23. Quantum-mechanical simulation of magneto-optical spectroscopy Supervisor: 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

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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. 24. Simulation of photoluminescence in ZnO Supervisor: Professor Charles Patterson Location: TCD Luminescence is the process whereby light is emitted by matter after it has been excited, either by passing a current through it (electroluminescence, e.g. LED's) or by illuminating it with light (photoluminescence, PL). The wavelength of luminescence light from crystals is commonly in the sub-optical band gap range. This may be because the luminescence process occurs via defects in crystals such as vacancies or interstitial ions which have energy levels in the band gap. PL in ZnO is a well known and studied phenomenon but there is as yet no definite mechanism for it. Single ion vacancies do not have optical transition energies in the band gap of ZnO. However, ion vacancy-interstitial ion pairs such as the zinc-vacancy + zinc-interstitial pair have energy levels which differ by an energy equivalent to half the band gap of ZnO. They also can have strong optical transitions as they are donor-acceptor pairs. In this project, you will investigate PL in ZnO using density functional theory techniques and model Hamiltonians. The project is computational rather than experimental and you will learn to use high performance computing techniques. 25. Thermal effects in optical spectra Supervisor: Professor Charles Patterson Location: TCD Reflectance anisotropy (RA) is a technique in which the optical reflectance of a surface is measured in normal incidence with the electric vector in two perpendicular directions. An RA spectrum consists of normalised differences of reflectance over a range of several eV from the IR to the near UV. We have experience in calculating RA spectra of a wide range of ordered surfaces for interpretation of experiments.

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Generally it is found that features in experimental spectra are much broader than in theory. However, in cases where RA spectra have been measured at low temperature, they can sharpen considerably. The broad features may therefore be caused by thermal effects. In this project you will calculate phonons of a surface (for which the RA spectrum is available) using density functional theory techniques. The RA spectrum for an ensemble of structures with finite displacements of ions from their equilibrium positions will be used to simulate the effects of thermal broadening on the RA spectrum. The project is computational rather than experimental and you will learn to use high performance computing techniques. 26. 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). 27. Characterisation of Toggle Magnetic Random Access Memory (TMRAM) Arrays for use as Magnetic Sensors with Sub-micron Resolution and Parallel Readout 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. The project will involve the analytical and numerical simulation of the magnetic field response of commercial MRAM – 1 Mb EverspinTM , and the experimental characterisation of the same. A small set of Helmholtz orthogonal coil pairs will be used to provide rotating magnetic field of controllable amplitude (ring down) while reading the state of the memory cells and asserting their switching state in the device plane. Once the rotational hysteresis characterisation of the arrays is complete, tests will be undertaken to quantify the response to micron-sized magnetic phantoms placed on the sensitive area of the test chips. Conclusions will be reached

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

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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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about the sensitivity, resolution and speed performance of this parallel magnetic sensing scheme. 28. 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.

29. 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. 30. Helium-ion beam for nanofabrication Supervisor: Professor Hongzhou Zhang

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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 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. 31. THz-Zentrum Supervisor: Professor Sergey Ganichev Location: University of Regensburg Local TCD Contact: Professor Werner Blau Spectroscopy of materials in the far-infrared (FIR) (wavelengths extending from 30 to 1000 µm, which corresponds to photon energies from 35 to 1 meV) is of importance because the characteristic energies of many elementary excitations of solids are lying in this spectral range. Among them are the plasma oscillation energy, ionization energies of typical shallow donors and acceptors, cyclotron resonance and spin flip energies, the characteristic size-quantization energies of low dimensional electron systems, optical phonon energies etc. The project will be in one of the following areas: terahertz photoresponse of topological insulators, artificial lateral superlattices and microwave induced resistance oscillations in 2D systems. 32. SIPs Seebeck ion pumps Supervisor: Professor David Carroll Location: Wake Forest University, North Carolina Local TCD Contact: Professor Werner Blau SIPs ~ Seebeck ion pumps use the seebeck effect to drive ion separation in electrolytic solutions. By introducing nanophase materials into such systems, and tailoring surface interactions with that nanophase, ion separation can be dramatically enhanced. In this program heterogeneous films of Seebeck active nanodispersions

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are employed to separate salts (such as NaCl) from water (desalination) using thermal gradients. Preliminary data has suggested that the desalination efficiency of such an approach may significantly exceed that of methods currently used. If correct, this may pose a possible new approach to fresh water access in developing regions of the world. 33. TPEGs ~ thermo/piezo – electric generators Supervisor: Professor David Carroll Location: Wake Forest University, North Carolina Local TCD Contact: Professor Werner Blau TPEGs ~ thermo/piezo – electric generators are flexible, conformal systems, that combine an organic piezoelectric (PVDF derivatives) with thermoelectric electrodes (organic or inorganic heterogeneous) into a large scale meta--‐material. The combination of both thermoelectric and piezoelectric properties into one structure yields specific synergisms between components that allow for a significant enhancement of power generation above that expected from the simple linear addition of piezoelectric and thermoelectric effects. We know that this synergism is strongly dependent on the dimensionality of the materials used to create the thermoelectric components, so in this program we explore the use of ternary metal chalcogenide nanostructures in the TPEG which gives control over dimension, and carrier density. 34. OLEVs ~ organic light emitting varacters Supervisor: Professor David Carroll Location: Wake Forest University, North Carolina Local TCD Contact: Professor Werner Blau OLEVs ~ organic light emitting varacters are field activated (AC--‐driven) organic lighting devices capable of extreme efficiency and brightness. Recently we have discovered that the internal magnetic field of the devices plays a central role in the energy transfer mechanisms of the OLEV allowing for control over triplet lifetimes, polaronic diffusion rates, intersystem crossing rates, and more. In this study we examine ways in which the magnetic interactions might be used to increase gain in such systems, leading ultimately to novel concepts in organic, electrically driven, lasers. 35. Optical nonlinearity in layered transition metal dichalcogenides Supervisor: Professor Jun Wang Location: Key Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Local TCD Contact: Dr. Niamh McGoldrick

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This project focuses on the optical nonlinearity in transition metal dichalcogenides (TMDs). We need to characterize the morphology, structure and optical properties of TMDs. Investigate the optical nonlinearity of mono- and few-layer TMDs (like WS2 and MoS2, etc.) using the Z-scan technique with femtosecond pulses ranging from visible to infrared. Observe the nonlinear response such as single photon saturable absorption, two-photon absorption and its saturation. The optical nonlinear parameters such as saturable intensity, saturable absorption or two-photon absorption coefficients, the third order susceptibility, figure of merit, damage threshold need to be obtained. The layer number and excitation wavelength dependence of the optical nonlinearity should be studied systematically. The intrinsic mechanisms of optical nonlinearity will be clarified. 1. Yuanxin Li, Ningning Dong, Saifeng Zhang, Xiaoyan Zhang, Yanyan Feng, Kangpeng Wang, Long Zhang, Jun Wang*, Giant Two-Photon Absorption in Monolayer MoS2, Laser & Photonics Reviews 9(4), 427-434 (2015). 2. Saifeng Zhang, Ningning Dong, Niall McEvoy*, Maria O'Brien, Sinéad Winters, Nina C. Berner, Chanyoung Yim, Xiaoyan Zhang, Zhanghai Chen, Long Zhang, Georg S. Duesberg, Jun Wang*, Direct observation of degenerate two photon absorption and its saturation of WS2 and MoS2 monolayer and few-layer films, ACS Nano 9 (7), 7142-7150 (2015).

36. Transition metal dichalcogenide thin films for ultrafast photonic applications Supervisor: Professor Jun Wang Location: Key Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Local TCD Contact: Dr. Niamh McGoldrick

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This project focuses on transition metal dichalcogenides (TMDs) based composite thin films for ultrafast saturable absorption (SA) application. We need to characterize the morphology, structure, surface roughness, thickness and optical properties of the composite thin films. Investigate the SA performance of the composite thin films (like MoS2/Graphene, WS2/Graphene and MoS2/WS2) using the Z-scan technique with femtosecond pulses ranging from visible to infrared, in compared to the SA performance of single component thin films. The optical nonlinear parameters such as saturable intensity, saturable absorption coefficients, the third order susceptibility, figure of merit, damage threshold need to be obtained. The film thickness and components, and excitation wavelength dependence of the SA should be studied systematically. 1. Xiaoyan Zhang, Saifeng Zhang, Chunxia Chang, Yanyan Feng, Yuanxin Li, Ningning Dong, Kangpeng Wang, Long Zhang, Werner J. Blau, Jun Wang*, Facile fabrication of wafer-scale MoS2 neat films with enhanced third-order nonlinear optical performance, Nanoscale, 7, 2978-2986 (2015). 2. Xiaoyan Zhang, Yang Chen, Bohua Chen, Hao Wang, Kan Wu, Saifeng Zhang, Jintai Fan, Shen Qi, Xiaoli Cui, Long Zhang, Jun Wang*, Direct synthesis of large-scale hierarchical MoS2 films nanostructured with orthogonally oriented vertically and horizontally aligned layers, Nanoscale 8, 431 (2016).

37. Magnetic and Electrical Dead Layers Supervisors: Professor Mike Coey and Dr. M. Venkatesan Location: TCD Ultra-thin films are dominated by their surface properties. In particular, very thin ferromagnetic films may be non-magnetic and very thin metallic films may be insulating. Many investigations have focused on La0.7Sr0.3MnO3 (LSMO), which shows the highest Curie temperature among the family of manganites (TC ~ 370 K) [1], a much-studied family of ferromagnetic conducting oxides. An extensive number of studies have demonstrated the fabrication of these manganites in the form of thin films, which may display very different properties as compared to bulk. Lattice mismatch can cause structural modifications at the interface between the film and the substrate, strongly affecting the magnetic properties. A surface 'dead-layer' is thought to exist where electrical conduction and magnetic order are surpressed. The

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thicknesses of the two dead layers may be different, and they can differ at the surface and interface of the manganite film with the substrate. This project involves growing films on different substrates (SrTiO3, LaAlO3) by pulsed-lased deposition using a KrF excimer laser ( = 248 nm)

will be determined by small-angle X-ray reflectivity measurements. All the films will be checked by standard characterization techniques: X-ray diffraction, AFM and SQUID magnetometry. Detailed electrical measurements and magnetotransport studies will be carried out, including the effect of an electric gate which adds or subtracts electrons from the manganite d-band. In this way we hope to understand the relation between ferromagnetism and conductivity in these fascinating materials. References [1] J.M.D. Coey, M Viret and S Von Molnar, Mixed-valence Manganites, Advances in Physics 58, 571-697 (2009).

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38. Thermal annealing effects on the compensated ferrimagnetic half-metal Mn2RuxGa Supervisors: Professor Mike Coey and Dr. Karsten Rode Location: TCD A final year project is available in the “Magnetism and Spin electronics” group of Trinity College Dublin. The subject of the project will be to investigate the effect of thermal treatment on the compensated ferrimagnetic half metal (CFHM), Mn2RuxGa, as well as its transport spin polarisation, crystal structure and magnetic properties. Magnetism and magnetic materials are foundations of a large part of modern information and telecommunication technology. All the data stored on server farms across the globe, is recorded in the orientation of nano-sized magnetic elements, and is both written and read using magnetic materials arranged in thin-film multilayer stacks. Currently, an intense research effort is dedicated to improving the scalability and energy consumption of these technologies. Several routes are investigated, and one of the more promising ones is based on magnetisation switching by spin-transfer torque (STT). STT is also the driving force of nano-sized oscillators capable of current modulation at the magnetic resonance frequency (FMR), which is of order ~ 10 GHz for conventional ferromagnetic materials. The material properties required to effectively excite oscillations and increase the FMR frequency which could lead to far faster data rates are: high spin polarisation P; low magnetic damping α high anisotropy field Ha and low magnetisation Ms. The ideal choice appears to be the compensated ferrimagnetic half-metal, first proposed by van Leuken and de Groot [1] in 1995. Despite numerous attempts, it was only 20 years later, that Mn2RuxGa (MRG), the first member of this new class of magnetic materials, was experimentally demonstrated in our lab.[2] We have shown that MRG exhibits clear signs of being a CFHM: high spin polarisation;[2] an unusually high anomalous Hall angle;[3] full ferromagnetic compensation;[4]; and the possibility to tune the Fermi level to fall in the spin gap.[5] The upwards shift of the Fermi level is achieved through electronic doping originating from the first 10 monolayers of growth, where the crystal structure is disordered and excess free carriers are generated via Mn-Ga anti-site defects.[5] We furthermore fabricated magnetic tunnel junction stacks with MRG as one of two magnetic electrodes, and demonstrated up to 40% tunnel magnetoresistance (TMR) at low temperature (T = 10K).[6] In figure 1 we show TMR loops recorded at room temperature for a sample annealed at successively higher temperatures Ta. The TMR is increasing as Ta is increased, with values from −4 to −12%. The origin of this increase is partly due to the induction of perpendicular magnetic anisotropy in the magnetic counter electrode, CoFe, but

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more importantly to changes in the MRG itself. The proposed project will be dedicated to the understanding of the changes in MRG during annealing, responsible for this improvement, and to enhance the TMR ratio. For further information, do not hesitate to contact: Dr. K. Rode (rodek@tcd.ie) or Prof. J. M.D Coey (jcoey@tcd.ie), Magnetism and spin electronics group, School of Physics, Trinity College Dublin. [1] H. van Leuken and R. A. de Groot, Phys. Rev. Lett. 74:1171 (1995) [2] H. Kurt, K. Rode, P. Stamenov, et al., Phys. Rev. Lett. 112:027201 (2014) [3] N. Thiyagarajah, Y.-C. Lau, D. Betto, et al., Applied Physics Letters 106(12):122402 (2015) [4] D. Betto, N. Thiyagarajah, Y.-C. Lau, et al., Phys. Rev. B 91:094410 (2015) [5] M. Zic, K. Rode, N. Thiyagarajah, et al., Phys. Rev. B 93:140202 (2016) [6] K. Borisov, D. Betto, Y.-C. Lau, et al. (2016), unpublished

Figure 1: Room-temperature TMR effect measured for a MRG/MgO/CoFe magnetic tunnel junction. Positive TMR is observed for an applied bias voltage of U = 10mV, whereas it is negative when U = −1V. The highest TMR is observed for U = −1V after thermal annealing at Ta = 325 °C.