Abstract: Conventional superconductivity is inevitably suppressed in ultra-small metallic grains for characteristic sizes smaller than the Anderson limit. Experiments have shown that above the Anderson limit the critical temperature may be either enhanced or reduced when decreasing the particle size, depending on the superconducting material. In addition, there is experimental evidence that whether an enhancement or a reduction is found depends on the strength of the electronphonon interaction in the bulk. We reveal how the strength of the e-ph interaction interplays with the quantum-size effect and theoretically obtain the critical temperature of the superconducting nanograins in excellent agreement with experimental data. We demonstrate that strong e-ph scattering smears the peak structure in the electronic density-of-states of a metallic grain and enhances the electron mass, and thereby limits the highest T-c achievable by quantum confinement.

Abstract: In tunnel field-effect transistors, trap-assisted tunneling (TAT) is one of the probable causes for degraded subthreshold swing. The accurate quantum-mechanical (QM) assessment of TAT currents also requires a QM treatment of phonon-assisted tunneling (PAT) currents. Therefore, we present a multi-band PAT current formalism within the framework of the quantum transmitting boundary method. An envelope function approximation is used to construct the electron-phonon coupling terms corresponding to local Frohlich-based phonon-assisted inter-band tunneling in direct-bandgap III-V semiconductors. The PAT current density is studied in up to 100 nm long and 20 nm wide p-n diodes with the 2- and 15-band material description of our formalism. We observe an inefficient electron-phonon coupling across the tunneling junction. We further demonstrate the dependence of PAT currents on the device length, for our non-self-consistent formalism which neglects changes in the electron distribution function caused by the electron-phonon coupling. Finally, we discuss the differences in doping dependence between direct band-to-band tunneling and PAT current. Published under license by AIP Publishing.

Abstract: The Zener tunneling current flowing through a biased, abrupt p-n junction embedded in a cylindrical silicon nanowire is calculated. As the band gap becomes indirect for sufficiently thick wires, Zener tunneling and its related transitions between the valence and conduction bands are mediated by short-wavelength phonons interacting with mobile electrons. Therefore, not only the high electric field governing the electrons in the space-charge region but also the transverse acoustic (TA) and transverse optical (TO) phonons have to be incorporated in the expression for the tunneling current. The latter is also affected by carrier confinement in the radial direction and therefore we have solved the Schrodinger and Poisson equations self-consistently within the effective mass approximation for both conduction and valence band electrons. We predict that the tunneling current exhibits a pronounced dependence on the wire radius, particularly in the high-bias regime. (C) 2012 Elsevier Ltd. All rights reserved.

Abstract: The superconducting properties of two recently synthesized ternary silicides with unit formula X₄CoSi (X = Nb, Ta) are investigated through ab initio calculations combined with Eliashberg theory. Interestingly, their crystal structure comprises interlocking honeycomb networks of Nb/Ta atoms. Nb₄CoSi is found to harbor better conditions for phonon-mediated superconductivity, as it possesses a higher density of states at the Fermi level, fostering stronger electron-phonon coupling. The superconducting critical temperatures (T_c) follow the same trend, with Nb₄CoSi having a twice higher value than Ta₄CoSi. Furthermore, the calculated T_c values (5.9 K vs 3.1 K) agree excellently with the experimentally obtained ones, establishing superconductivity in this new materials class as mediated by the electron-phonon coupling. Furthermore, my calculations show that the superconducting properties of these compounds do not simply correlate with the parameters of their honeycomb networks, contrary to proposals raised in the literature. Rather, their complete fermiology and phonon spectrum should be taken into account in order to explain their respective superconducting properties.

Abstract: We present a theoretical study of the optoelectronic properties of monolayer graphene. Including the effect of the electron-photon-phonon scattering, we employ the mass- and energy-balance equations derived from the Boltzmann equation to evaluate self-consistently the carrier densities, optical conductance and transmission coefficient in graphene in the presence of linearly polarized radiation field. We find that the photo-excited carrier density can be increased under infrared radiation and depend strongly on radiation intensity and frequency. For short wavelengths (lambda <3 mu m), the universal optical conductance sigma(0) = e(2)/4h is obtained and the light transmittance is about 0.97-0.98. Interestingly, there is an optical absorption window in the range 4-100 mu m which is induced by different transition energies required for inter- and intra-band optical absorption. The position and width of this absorption window depend sensitively on temperature and carrier density of the system. These results are relevant for applications of recently developed graphene devices in advanced optoelectronics such as the infrared photodetectors. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.

Abstract: Due to the potential application of DNA for biophysics and optoelectronics, the electronic energy states and transitions of this genetic material have attracted a great deal of attention recently. However, the fluorescence and corresponding physical process of DNA under optical excitation with photon energies below ultraviolet are still not fully clear. In this work, we experimentally investigate the photoluminescence (PL) properties of single-stranded DNA (ssDNA) samples under near-ultraviolet (NUV) and visible excitations (270 similar to 440 nm). Based on the dependence of the PL peak wavelength (lem) upon the excitation wavelength (lex), the PL behaviors of ssDNA can be approximately classified into two categories. In the relatively short excitation wavelength regime, lem is nearly constant due to exciton-like transitions associated with delocalized excitonic states and excimer states. In the relatively long excitation wavelength range, a linear relation of lem = Alex + B with A 0 or A < 0 can be observed, which comes from electronic transitions related to coupled vibrational-electronic levels. Moreover, the transition channels in different excitation wavelength regimes and the effects of strand length and base type can be analyzed on the basis of these results. These important findings not only can give a general description of the electronic energy states and transitional behaviors of ssDNA samples under NUV and visible excitations, but also can be the basis for the application of DNA in nanoelectronics and optoelectronics.

Abstract: We report on the electrical, optical and physical properties of Cu2ZnSnSe4 solar cells using an absorber layer fabricated by selenization of sputtered Cu, Zn and Cu10Sn90 multilayers. A maximum active-area conversion efficiency of 10.4% under AM1.5G was measured with a maximum short circuit current density of 39.7 mA/cm(2), an open circuit voltage of 394 mV and a fill factor of 66.4%. We perform electrical and optical characterization using photoluminescence spectroscopy, external quantum efficiency, current-voltage and admittance versus temperature measurements in order to derive information about possible causes for the low open circuit voltage values observed. The main defects derived from these measurements are strong potential fluctuations in the absorber layer as well as a potential barrier of the order of 133 meV at the back side contact. (C) 2014 Elsevier B.V. All rights reserved.

Abstract: We study the mobility of Dirac fermions in monolayer graphene on a GaAs substrate, limited by the combined action of the extrinsic potential of piezoelectric surface acoustical phonons of GaAs (PA) and of the intrinsic deformation potential of acoustical phonons in graphene (DA). In the high-temperature (T) regime, the momentum relaxation rate exhibits the same linear dependence on T but different dependencies on the carrier density n, corresponding to the mobility mu proportional to 1 root n and 1/n, respectively for the PA and DA scattering mechanisms. In the low-T Bloch-Gruneisen regime, the mobility shows the same square-root density dependence mu proportional to root n, but different temperature dependencies mu proportional to T-3 and T-4, respectively for PA and DA phonon scattering. DOI: 10.1103/PhysRevB.87.075443

Abstract: The elastic constant C-11 and piezoelectric stress constant e(1),(11) of two-dimensional (2D) dielectric materials comprising h-BN, 2H-MoS2, and other transition-metal dichalcogenides and dioxides are calculated using lattice dynamical theory. The results are compared with corresponding quantities obtained with ab initio calculations. We identify the difference between clamped-ion and relaxed-ion contributions with the dependence on inner strains which are due to the relative displacements of the ions in the unit cell. Lattice dynamics allows us to express the inner-strain contributions in terms of microscopic quantities such as effective ionic charges and optoacoustical couplings, which allows us to clarify differences in the piezoelectric behavior between h-BN and MoS2. Trends in the different microscopic quantities as functions of atomic composition are discussed.

Abstract: We present a theoretical study of the plasmon and coupled plasmon-phonon modes induced by intraband electron-electron interaction in graphene in the presence of driving dc electric field. We find that the electric field dependence of these collective excitation modes in graphene differs significantly from that in a conventional two-dimensional electron gas with a parabolic energy spectrum. This is due mainly to the fact that graphene has a linear energy spectrum and the Fermi velocity of electrons in graphene is much larger than the drift velocity of electrons. The obtained results demonstrate that the plasmon and coupled plasmon-phonon modes in graphene can be tuned by applying not only the gate voltage but also the source-to-drain field. The manipulation of plasmon and coupled plasmon-phonon modes by source-to-drain voltage can let graphene be more conveniently applied as an advanced plasmonic material.

Abstract: We study Dirac plasmons and their damping in spatially separated N-layer graphene structures at finite doping and temperatures. The plasmon spectrum consists of one optical excitation with square-root dispersion and N – 1 acoustical excitations with linear dispersion, which are undamped at zero temperature and finite doping within a triangular energy region outside the electron-hole continuum. In the long-wavelength limit the energy and weight of the optical plasmon modes increase, respectively, as the square root and linearly with N in agreement with recent experimental findings. The energy and weight of the upper-lying acoustical branches also increase with N. This increase is strongest for the uppermost acoustical mode, and we find that its energy can exceed at some value of momentum the plasmon energy in an individual graphene sheet. Meanwhile, the energy of the low-lying acoustical branches decreases weakly with N as compared with the single acoustical mode in double-layer graphene structures. Our numerical calculations provide a detailed understanding of the overall behavior of the wave-vector dependence of the optical and acoustical multilayer plasmon modes and show how their dispersion and damping are modified as a function of temperature, interlayer spacing, and inlayer carrier density in (un)balanced graphene multilayer structures. DOI: 10.1103/PhysRevB.87.085401

Abstract: This thesis presents, in its first part, an investigation on the trembling motion of wave packets known as zitterbewegung (ZBW), in multilayer graphene, as well as in moiré excitons in twisted MoS2/WSe2 hetero-bilayers. In the last few decades, the dynamics of wave packets has been subject of many theoretical and experimental studies in various types of systems such as semiconductors, superconductors, crystalline solids and cold atoms. The discovery of graphene and moiré excitons in twisted hetero-bilayers, brought two new platforms for the investigation on time evolution of wave packets and possible observation of ZBW. This trembling motion was first theoretically predicted by Schrödinger for wave packets describing particles that obey the Dirac equation. This is exactly the case of low energy electrons in graphene, as well as of moiré exciton in twisted MoS2/WSe2 under an external applied electromagnetic field. ZBW in multilayer graphene was studied both analytically and computationally, respectively, through the Green's function and split-operator methods. In this system, it is found that ZBW depends not only on the wave packet width and initial pseudospin polarization, but also on the number of layers. Furthermore, the analytical and numerical methods proposed here allow to investigate wave packet dynamics in graphene systems with an arbitrary number of layers and arbitrary potential landscapes. For moiré excitons in twisted MoS2/WSe2 hetero-bilayers, it is shown that, analogously to other Dirac-like particles, this system also exhibits ZBW when under a perpendicular applied field. In this case, the ZBW presents long timescales that are compatible with current experimental techniques for exciton dynamics. This promotes the study of the dynamics of moiré excitons in van der Waals heterostructures as an advantageous solidstate platform to probe zitterbewegung, broadly tunable by gating and inter-layer twist angle. In the second part of this thesis, a study into graphene plasmonic in van der Waals heterostructure (vdWhs) are treated in a linear response framework within the Random Phase Approximation and with support of the quantum electrostatic heterostructure (QEH), a DFT-based method. Since Dirac plasmons in graphene are very sensitive to the dielectric properties of the environment, it is possible to explore this property to probe the structure and composition of van der Waals heterostructures (vdWh) placed underneath a single graphene layer. In this way, one can achieve a layer sensitivity of a single layer and differentiate between different TMDs for heterostructures thicker than 2 layers. As a consequence of this, study, the hybridization of Dirac plasmons in graphene with phonons of transition metal dichalcogenides (TMDs), when the materials are combined in so-called van der Waals heterostructures (vdWh) forming surface plasmon-phonon polaritons (SPPPs) are also investigated. It was found that it is possible to realize both strong and ultrastrong coupling regimes by tuning graphene’s Fermi energy and changing TMD layer number.

Abstract: The reduction of dimensionality is a very effective way to achieve appealing properties in two-dimensional materials (2DMs). First-principles calculations can greatly facilitate the prediction of 2DM properties and find possible approaches to enhance their performance. We employed first-principles calculations to gain insight into the impact of different types of point defects (vacancies and substitutional dopants) on the electronic and magnetic properties of a ZnSnN2 (ZSN) monolayer. We show that Zn, Sn, and N + Zn vacancy-defected structures are p-type conducting, while the defected ZSN with a N vacancy is n-type conducting. For substitutional dopants, we found that all doped structures are thermally and energetically stable. The most stable structure is found to be B-doping at the Zn site. The highest work function value (5.0 eV) has been obtained for Be substitution at the Sn site. Li-doping (at the Zn site) and Be-doping (at the Sn site) are p-type conducting, while B-doping (at the Zn site) is n-type conducting. We found that the considered ZSN monolayer-based structures with point defects are magnetic, except those with the N vacancy defects and Be-doped structures. The ab initio molecular dynamics simulations confirm that all substitutionally doped and defected structures are thermally stable. Thus, our results highlight the possibility of tuning the magnetism in ZnSnN2 monolayers through defect engineering.