First-principles simulation of elastic constants and electronic properties of GaN

The electronic and structural properties and elastic constants of the wurtzite phase of GaN, was investigated by computer simulation at Density Functional Theory level, with B3LYP and B3PW hybrid functional. The electronic properties were investigated through the analysis of the band structures and density of states, and the mechanical properties were studied through the calculus of the elastic constants: C11, C33, C44, C12, and C13. The results show that the maximum of the valence band and the minimum of the conduction band are both located at the Γ point, indicating that GaN is a direct band gap semiconductor. The following constants were obtained for B3LYP and B3PW (in brackets): C11 = 366.9 [372.4], C33 = 390.9 [393.4], C44 = 99.1 [96.9], C12 = 143.6 [155.2], and C13 = 107.6 [121.4].

The Structural and Electronic Properties of Tin Oxide Nanowires: An Ab Initio Investigation

We performed an ab initio investigation on the properties of rutile tin oxide (SnOx) nanowires. We computed the wire properties determining the equilibrium geometries, binding energies, and electronic band structures for several wire dimensions and surface facet configurations. The results allowed us to establish scaling laws for the structural properties, in terms of the nanowire perimeters. The results also showed that the surface states control most of the electronic properties of the nanowires. Oxygen incorporation in the nanowire surfaces passivated the surface-related electronic states, and the resulting quantum properties and scaling laws were fully consistent with electrons confined inside the nanowire. Additionally, oxygen incorporation in the wire surfaces generated an unbalanced concentration of spin up and down electrons, leading to magnetic states for the nanowires.

Structure and electronic properties of hydrated mesityl oxide: a sequential quantum mechanics/molecular mechanics approach

The hydration of mesityl oxide (MOx) was investigated through a sequential quantum mechanics/molecular mechanics approach. Emphasis was placed on the analysis of the role played by water in the MOx syn-anti equilibrium and the electronic absorption spectrum. Results for the structure of the MOx-water solution, free energy of solvation and polarization effects are also reported. Our main conclusion was that in gas-phase and in low-polarity solvents, the MOx exists dominantly in syn-form and in aqueous solution in anti-form. This conclusion was supported by Gibbs free energy calculations in gas phase and in-water by quantum mechanical calculations with polarizable continuum model and thermodynamic perturbation theory in Monte Carlo simulations using a polarized MOx model. The consideration of the in-water polarization of the MOx is very important to correctly describe the solute-solvent electrostatic interaction. Our best estimate for the shift of the pi-pi* transition energy of MOx, when it changes from gas-phase to water solvent, shows a red-shift of -2,520 +/- 90 cm(-1), which is only 110 cm(-1) (0.014 eV) below the experimental extrapolation of -2,410 +/- 90 cm(-1). This red-shift of around -2,500 cm(-1) can be divided in two distinct and opposite contributions. One contribution is related to the syn -> anti conformational change leading to a blue-shift of similar to 1,700 cm(-1). Other contribution is the solvent effect on the electronic structure of the MOx leading to a red-shift of around -4,200 cm(-1). Additionally, this red-shift caused by the solvent effect on the electronic structure can by composed by approximately 60 % due to the electrostatic bulk effect, 10 % due to the explicit inclusion of the hydrogen-bonded water molecules and 30 % due to the explicit inclusion of the nearest water molecules.

Electronic Properties of Water in Liquid Environment. A Sequential QM/MM Study Using the Free Energy Gradient Method

There is a continuous search for theoretical methods that are able to describe the effects of the liquid environment on molecular systems. Different methods emphasize different aspects, and the treatment of both the local and bulk properties is still a great challenge. In this work, the electronic properties of a water molecule in liquid environment is studied by performing a relaxation of the geometry and electronic distribution using the free energy gradient method. This is made using a series of steps in each of which we run a purely molecular mechanical (MM) Monte Carlo Metropolis simulation of liquid water and subsequently perform a quantum mechanical/molecular mechanical (QM/MM) calculation of the ensemble averages of the charge distribution, atomic forces, and second derivatives. The MP2/aug-cc-pV5Z level is used to describe the electronic properties of the QM water. B3LYP with specially designed basis functions are used for the magnetic properties. Very good agreement is found for the local properties of water, such as geometry, vibrational frequencies, dipole moment, dipole polarizability, chemical shift, and spin-spin coupling constants. The very good performance of the free energy method combined with a QM/MM approach along with the possible limitations are briefly discussed.

Nanoscale surface properties and interaction with fundamental biomolecules of chlorite and phlogopite

The surface properties of minerals have important implications in geology, environment, industry and biotechnology and for certain aspects in the research on the origin of life. This research project aims to widen the knowledge on the nanoscale surface properties of chlorite and phlogopite by means of advanced methodologies, and also to investigate the interaction of fundamental biomolecules, such as nucleotides, RNA, DNA and amino acid glycine with the surface of the selected phyllosilicates. Multiple advanced and complex experimental approaches based on scanning probe microscopy and spatially resolved spectroscopy were used and in some cases specifically developed. The results demonstrate that chlorite exposes at the surface atomically flat terraces with 0.5 nm steps typically generated by the fragmentation of the octahedral sheet of the interlayer (brucitic-type). This fragmentation at the nanoscale generates a high anisotropy and inhomogeneity with surface type and isomorphous cationic substitutions determining variations of the effective surface potential difference, ranging between 50-100 mV and 400-500 mV, when measured in air, between the TOT surface and the interlayer brucitic sheet. The surface potential was ascribed to be the driving force of the observed high affinity of the surface with the fundamental biomolecules, like single molecules of nucleotides, DNA, RNA and amino acids. Phlogopite was also observed to present an extended atomically flat surface, featuring negative surface potential values of some hundreds of millivolts and no significant local variations. Phlogopite surface was sometimes observed to present curvature features that may be ascribed to local substitutions of the interlayer cations or the presence of a crystal lattice mismatch or structural defects, such as stacking faults or dislocation loops. Surface chemistry was found similar to the bulk. The study of the interaction with nucleotides and glycine revealed a lower affinity with respect to the brucite-like surface of chlorite.

Electronic properties of Van der Waals heterostructures

L’interazione spin-orbita (SOI) nel grafene è attualmente oggetto di intensa ricerca grazie alla recente scoperta di una nuova classe di materiali chiamati isolanti topologici. Questi materiali, la cui esistenza è strettamente legata alla presenza di una forte SOI, sono caratterizzati dall’interessante proprietà di avere un bulk isolante ed allo stesso tempo superfici conduttrici. La scoperta teorica degli isolanti topologici la si deve ad un lavoro nato con l’intento di studiare l’influenza dell’interazione spin-orbita sulle proprietà del grafene. Poichè questa interazione nel grafene è però intrinsecamente troppo piccola, non è mai stato possibile effettuare verifiche sperimentali. Per questa ragione, vari lavori di ricerca hanno recentemente proposto tecniche volte ad aumentare questa interazione. Sebbene alcuni di questi studi abbiano mostrato un effettivo aumento dell’interazione spin-orbita rispetto al piccolo valore intrinseco, sfortunatamente hanno anche evidenziato una consistente riduzione della qualità del grafene. L’obbiettivo che ci si pone in questa tesi è di determinare se sia possibile aumentare l’interazione spin-orbita nel grafene preservandone allo stesso tempo le qualità. La soluzione proposta in questo lavoro si basa sull’utilizzo di due materiali semiconduttori, diselenio di tungsteno WSe2 e solfuro di molibdeno MoS2, utilizzati da substrato su cui sopra verrà posizionato il grafene formando così un’eterostruttura -nota anche di “van der Waal” (vdW)-. Il motivo di questa scelta è dovuto al fatto che questi materiali, appartenenti alla famiglia dei metalli di transizione dicalcogenuri (TMDS), mostrano una struttura reticolare simile a quella del grafene, rendendoli ideali per formare eterostrutture e ancora più importante, presentano una SOI estremamente grande. Sostanzialmente l’idea è quindi di sfruttare questa grande interazione spin-orbita del substrato per indurla nel grafene aumentandone così il suo piccolo valore intrinseco.

Electronic properties of zig-zag carbon nanotubes : a first-principles study

Carbon nanotube (CNT) is a one dimensional (1-D) nanostructured material, which has been the focal point of research over the past decade for intriguing applications ranging from nanoelectronics to chemical and biological sensors. Using a first-principles gradient corrected density functional approach, we present a comprehensive study of the geometry and energy band gap in zig-zag semi-conducting (n,0) carbon nanotubes (CNT) to resolve some of the conflicting findings. Our calculations confirm that the single wall (n,0) CNTs fall into two distinct classes depending upon n mod 3 equal to 1 (smaller band gaps) or 2 (larger gaps). The effect of longitudinal strain on the band gap further confirms the existence of two distinct classes: for n mod 3 = 1 or 2, changing Eg by ~ ±110 meV for 1% strain in each case. We also present our findings for the origin of metallicity in multiwall CNTs.

Electronic properties of doped magnesium thioindate ternary spinel in the normal and in the inverse structure

We present a theoretical study of the structural and electronic properties of the M-doped MgIn2S4 ternary spinel semiconductor with M = V, Cr, and Mn. All substitutions, in the normal and in the inverse structure, are analyzed. Some of these possible substitutions present intermediate-band states in the band gap with a different occupation for a spin component. It increases the possibilities of inter-band transitions and could be interesting for applications in optoelectronic devices. The contribution to, and the electronic configuration of, these intermediate bands for the octahedral and tetrahedral sites is analyzed and discussed. The study of the substitutional energies indicates that these substitutions are favorable. Comparison between the pure and doped hosts absorption coefficients shows that this deeper band opens up more photon absorption channels and could therefore increase the solar-light absorption with respect to the host.

Electronic properties of the MoS2-WS2 heterojunction

We study the electronic structure of a heterojunction made of two monolayers of MoS2 and WS2. Our first-principles density functional calculations show that, unlike in the homogeneous bilayers, the heterojunction has an optically active band gap, smaller than the ones of MoS2 and WS2 single layers. We find that the optically active states of the maximum valence and minimum conduction bands are localized on opposite monolayers, and thus the lowest energy electron-holes pairs are spatially separated. Our findings portray the MoS2-WS2 bilayer as a prototypical example for band-gap engineering of atomically thin two-dimensional semiconducting heterostructures.

Modification of Hematite Electronic Properties with Trimethyl Aluminum to Enhance the Efficiency of Photoelectrodes

The electronic properties of hematite were investigated by means of synchrotron radiation photoemission (SR-PES) and X-ray absorption spectroscopy (XAS). Hematite samples were exposed to trimethyl aluminum (TMA) pulses, a widely used Al-precursor for the atomic layer deposition (ALD) of Al2O3. SR-PES and XAS showed that the electronic properties of hematite were modified by the interaction with TMA. In particular, the hybridization of O 2p states with Fe 3d and Fe 4s4p changed upon TMA pulses due to electron inclusion as polarons. The change of hybridization correlates with an enhancement of the photocurrent density due to water oxidation for the hematite electrodes. Such an enhancement has been associated with an improvement in charge carrier transport. Our findings open new perspectives for the understanding and utilization of electrode modifications by very thin ALD films and show that the interactions between metal precursors and substrates seem to be important factors in defining their electronic and photoelectrocatalytic properties.

Electronic properties of transition metal atoms on Cu2N/Cu(100)

We study the nature of spin excitations of individual transition metal atoms (Ti, V, Cr, Mn, Fe, Co, and Ni) deposited on a Cu2N/Cu(100) surface using both spin-polarized density functional theory (DFT) and exact diagonalization of an Anderson model derived from DFT. We use DFT to compare the structural, electronic, and magnetic properties of different transition metal adatoms on the surface. We find that the average occupation of the transition metal d shell, main contributor to the magnetic moment, is not quantized, in contrast with the quantized spin in the model Hamiltonians that successfully describe spin excitations in this system. In order to reconcile these two pictures, we build a zero bandwidth multi-orbital Anderson Hamiltonian for the d shell of the transition metal hybridized with the p orbitals of the adjacent nitrogen atoms, by means of maximally localized Wannier function representation of the DFT Hamiltonian. The exact solutions of this model have quantized total spin, without quantized charge at the d shell. We propose that the quantized spin of the models actually belongs to many-body states with two different charge configurations in the d shell, hybridized with the p orbital of the adjacent nitrogen atoms. This scenario implies that the measured spin excitations are not fully localized at the transition metal.

Spin-charge conductance in nanoscale electronic devices

We introduce a spin-charge conductance matrix as a unifying concept underlying charge and spin transport within the framework of the Landauer-Buttiker conductance formula. It turns out that the spin-charge conductance matrix provides a natural and gauge covariant description for electron transport through nanoscale electronic devices. We demonstrate that the charge and spin conductances are gauge invariant observables which characterize transport phenomena arising from spin-dependent scattering. Tunnelling through a single magnetic atom is discussed to illustrate our theory.

Electronic properties of homoepitaxial (111) highly boron-doped diamond films

The use of diamond as a semiconductor for the realization of transistor structures, which can operate at high temperatures (>700 K), is of increasing interest. In terms of bipolar devices, the growth of n-type phosphorus doped diamond is more efficient on the (111) growth plane; p-type boron-doped diamond growth has been most usually grown in the (100) direction and, hence, this study into the electronic properties, at high temperatures, of boron-doped diamond (111) homoepitaxial layers. It is shown that highly doped layers (hole carrier concentrations as high as 2×1020 cm-3) can be produced without promoting the onset of (unwanted) hopping conduction. The persistence of valance-band conduction in these films enables relatively high mobility values to be measured ( ~ 20 cm2/V?s) and, intriguingly, these values are not significantly reduced at high temperatures. The layers also display very low compensation levels, a fact that may explain the high mobility values since compensation is required for hopping conduction. The results are discussed in terms of the potential of these types of layers for use with high temperature compatible diamond transistors.