First-principles Local Density Functional Calculations Research Articles

Strain effects on the band gap and work function of zigzag single-walled carbon nanotubes and graphene: A comparative study

First-principles local density functional calculation of the electronic structure and work function of zigzag single-wall carbon nanotubes (SWCNTs) and graphene under strain are presented. We found that there is a small circumference strain even for unstrained zigzag SWCNTs that release the curvature energy of the tube. Therefore, we propose that there are two effects contributing to the opening of the band gap of unstrained (3q, 0) SWCNTs: the electron transfer enhancement effect and the circumference strain effect. We show that the band gap and work function of strained zigzag SWCNTs can be successfully estimated from the band structure and work function of strained graphene based on the zone folding method with circumference strain effect. The circumference strain effect is crucial to obtain a correct result. The rate of change of the band gap with respect to the strain for zigzag SWCNTs is almost independent of the value of the strain and the radius of the tube.

Bulk Modulus and Electronic Band Structure of ZnGa2X4 (X = S,Se): a First-Principles Study

First-principles local density functional calculations are presented for the compounds ZnGa2X4 (X = S,Se). We investigate the bulk moduli and electronic band structures in a defect chalcopyrite structure. The lattice constants and internal parameters are optimized. The electronic structures are analysed with the help of total and partial density of states. The relation between the cohesive energy and the unit cell volume is obtained by fully relaxed structures. We derive the bulk modulus of ZnGa2X4 by fitting the Birch–Murnaghan's equation of state. The extended Cohen's empirical formula agrees well with our ab initio results.

Spin-dependent transport in Fe andFe∕Aumultilayers

In situ resistance measurements of epitaxial Fe layers and $\mathrm{Au}∕\mathrm{Fe}$ bilayers were used to quantify the scattering in giant magnetoresistance (GMR) spin valve structures. The semiclassical Boltzmann transport equation, incorporating first-principles local density functional calculations, fitted the thickness dependence of the conductivity. Fits to the data indicate that Fe has a large spin asymmetry with bulk relaxation times ${\ensuremath{\tau}}_{\ensuremath{\downarrow}}=3.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ and ${\ensuremath{\tau}}_{\ensuremath{\uparrow}}=2.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$. These give a conductivity equal to that of bulk Fe. The interface scattering from the $\mathrm{Fe}∕\mathrm{GaAs}$, the $\mathrm{Fe}∕\text{vacuum}$, and the $\mathrm{Au}∕\text{vacuum}$ interfaces is purely diffuse. This is in contrast to the high electron reflection coefficients determined from kinematical calculations using scanning tunneling microscope images. Fits to conductivity measurements of $\mathrm{Au}∕\mathrm{Fe}∕\mathrm{GaAs}(001)$ indicate that the Au films have the conductivity of bulk material modified only by interface scattering. The GMR of $\mathrm{Au}∕\mathrm{Fe}∕\mathrm{Au}∕\mathrm{Fe}∕\mathrm{GaAs}(001)$ structures is 1.8% at room temperature and 2.9% at 10 K. The magnetoresistance is reduced by the presence of partial diffuse scattering at the inner interfaces, as indicated by the fits to both the GMR and the Au conductivity. The GMR in $\mathrm{Fe}∕\mathrm{Au}$ structures is intrinsically low due to a large electron band mismatch between Au and Fe band structures.

Generalized Kubas Complexes as a Novel Means for Room Temperature Molecular Hydrogen Storage

ABSTRACTWe propose that generalized Kubas complexes of molecular hydrogen with light metal elements, such as B and Be embedded in carbon nanostructures, or related Be and B materials, could offer breakthrough performance in room temperature hydrogen storage. First-principles local-density functional calculations show that hydrogen bound to these materials are intact, in similarity to physisorbed H2, but with a greatly enhanced adsorption energy in the range of 0.2–0.7 eV. The metal-H2 binding is attributed to the Coulombic interaction between holes created at the metal sites and Σ electrons of the H2. Management of the hole density and electron-hole orbital overlap thus enables us to control the binding strength of H2 for optimal storage properties.

The role of interfaces in the magnetoresistance of Au/Fe/Au/Fe/GaAs(001)

The electron transport and magnetoresistance (MR) were investigated in high quality crystalline epitaxial Fe(001) and Au(001) films and exchange coupled Au/Fe/Au/Fe/GaAs(001) trilayer structures. Fits to the experimental data were based on the semiclassical Boltzmann equation, which incorporates the electronic properties obtained from first-principles local density functional calculations. The fits require a surprisingly high asymmetry for the spin dependent electron lifetimes in Fe, τ↓/τ↑=10 at room temperature. Despite the large atomic terraces at the Au/vacuum and Fe/GaAs interfaces the scattering at the outer interfaces was found to be diffuse. The origin of MR in Au/Fe/Au/Fe/GaAs(001) structures is due to electron channeling in the Au spacer layer. The measured MR is consistent with the diffusivity parameters s↑=0.55, s↓=0.77 at the metal–metal interfaces.

Interfacial atomic composition and Schottky barrier heights at the Al/GaAs(001) interface

The influence of different interfacial chemical compositions on the Schottky barrier heights across the Al/GaAs(001) interface is studied using first-principles local density functional calculations. The barrier heights are calculated for seven different interfacial chemical compositions, including the chemically abrupt As-terminated and Ga-terminated interfaces, and also several other interfaces related by Al↔Ga place exchange or containing As antisites. We find p-type barrier heights φp that vary by 0.4 eV, demonstrating a significant influence of the interface composition on the resulting barrier heights. The barrier height variation is explained by the different chemical bonding at the interface in the various cases. The metal induced gap states (MIGS) of two structures with different barrier heights are compared in order to demonstrate why such states do not result in barrier heights independent of interfacial chemical composition. It is thus suggested that the reason an experimental value of φp=0.65 eV is generally found for Al/GaAs is not due to several possible interfacial structures all having their Fermi level pinned by MIGS at the same value, but rather due to the thermodynamic preference for certain structures over others with significantly different barriers. This proposal offers a potential explanation for recent photoemission experiments that find the Fermi level position in the gap varying by several tenths of a volt as a function of the initial surface structure of the GaAs substrate: some of the samples are likely to have interfacial compositions which are metastable with respect to structures that give the standard barrier heights.

Interfacial atomic structure and band offsets at semiconductor heterojunctions

The relationship between interfacial atomic structure and band offsets at semiconductor heterojunctions is explored through first-principles local density functional calculations. In particular, the effects of variations in interfacial geometry are analyzed for (001) interfaces between III–V/III–V materials. For the AC/BC case of a common atom, isovalent A–B intermixing in the noncommon atomic planes near the interface does not affect the band offset, even in the case of large lattice-mismatched systems. For quaternary AB/CD systems, there are two possible chemically abrupt interfaces (A–D or B–C); these can have offsets that differ by up to 80 meV. In those cases where the chemically abrupt AB/CD offset depends on the interfacial identity, intermixing leads to offset variations which are directly related to the offset difference between the chemically abrupt A–D and B–C interfaces. The differing behavior of common-atom versus noncommon-atom systems is analyzed in terms of the symmetry of the nearest-neighbor environment surrounding an atomic site of a composition change.

Calculation of the Stability and the Polarizability of Isolated Fullerene Molecules as a Function of Charge State

ABSTRACTWe present first-principles local density functional calculations of the electronic structure and energetics of neutral and negatively charged fullerene molecules. We find thatthe negatively charged -1 state is stable relative to the neutral molecule and that the -2 state is stable relative to the neutral molecule but not to the -1 state of the molecule. We have also performed calculations of the electronic polarizabilities for different charged states and developed a simple model to estimate the dielectric constant of fullerene based crystals.

Effect of tip atomic and electronic structure on scanning tunneling microscopy/spectroscopy

The current image and the tunnel conductance in the scanning tunneling microscopy/spectroscopy (STM/STS) of graphite are simulated by the first-principles local density functional calculation of the electronic states. As models of the tip, W 10[111], W 14[110], W 13[110] and Pt 10[111] clusters are used. For the tips with a single apex atom, normal STM images are obtained. However, abnormal images are generated by the removal of the apex atom. STS spectra are changed in a systematic way by the apex angle of the tip. The negative differential resistance observed in the STS of the Si(111) 3 × 3 -B/defect surface is reproduced by the theoretical simulation with the cluster models of the tip.