Inder P. Batra

Band structure of polyacetylene, (CH)x

The one-electron energy bands and densities of states of polyacetylene in both cis- and trans-conformations have been investigated. The principal issue addressed is whether the itinerant picture alone is sufficient to explain the experimental properties of this material. We conclude that the one-electron model provides an excellent zeroth-order explanation of current observations of optical and transport effects in both pure and doped forms of this unusual polymer.

Chemisorption of oxygen on aluminum surfaces

Abstract A critical review of the oxygen—aluminum system is presented. The primary emphasis concerns the electronic properties of aluminum surfaces exposed to oxygen. The chemisorption and oxidation aspects are considered. Cluster and slab model calculations are discussed fully and the results are related to relevant experimental data. Some of the unresolved issues are listed. A comprehensive guide to the oxygen—aluminum literature is provided.

A STUDY OF GRAPHITE SURFACE WITH STM AND ELECTRONIC STRUCTURE CALCULATIONS

Abstract The (0001) surface of graphite has been investigated using the scanning tunneling microscope (STM) over a relatively wide area containing many unit cells. We do not observe trigonal symmetry but rather find one preferred direction which remains unaffected even by extended defect areas. Corrugations over several angstroms are easily discernible. We have performed an extensive set of self-consistent electronic structure calculations for a monolayer and three-layer graphite slabs with the ideal and slipped configurations. Based on the detailed charge-density analysis we find that the large corrugation arises neither from the total charge density, nor from the local density of states around the Fermi surface. Somewhat larger corrugation (but still smaller than the one observed) and the absence of trigonal symmetry can be explained by an atomic configuration, in which the top layer of graphite is slipped relative to sub-surface layers. Our results point to the fact that the huge corrugation observed may have several origins, one possibility being the elastic deformations induced by the tip.

Interpretation of the photoemission spectrum of chemisorbed carbon monoxide on Ni (100)

Abstract Cluster model calculations suggest that the molecular orbitals of CO are significantly perturbed upon chemisorption on Ni. The CO-Ni interaction is found to involve the Ni- d electrons in addition to the s -and p -derived states, contrary to some earlier suggestions. Based on our results, a new interpretation is proposed for the observed photoemission spectra of CO on Ni, which is consistent with the large bonding and relaxation shifts in other systems.

Thermodynamic stability of thin ferroelectric films

Abstract It is shown that the residual depolarization field in a ferroelectric thin film which arises from the incomplete cancellation of polarization and compensation charge in a semiconducting electrode, introduces modifications in the spontaneous polarization and transition temperature. It is also found that below a certain critical thickness the ferroelectric polar state is thermodynamically unstable. For triglycine sulphate the critical thickness is calculated to be 0.4 μm.

Theoretical scanning tunneling microscopy and atomic force microscopy study of graphite including tip–surface interaction

The graphite surface, due to its unusual electronic structure, offers challenges and opportunities for scanning tunneling microscopy (STM) and atomic force microscopy (AFM). To draw a measurable current at small voltages (1 nA at 50 mV) the tip has to approach close to the surface; estimated tip‐to‐surface distance is ∼2 A. At such distances repulsive interatomic forces between the tip and surface set in (the basis of AFM) and it is important to consider the tip–surface interaction. Therefore, we have calculated interatomic forces and charge densities including the tip (atom) as an integral part of the system rather than treating it perturbatively. Our calculations reveal that the tip at close proximity to the surface disturbs the states near the Fermi level, and induces localized states which in turn influence the STM images. The tunneling barrier appears to collapse at small tip‐to‐surface distances. Some experimental evidence for this effect is cited. The repulsive force on the tip has a value in the r...

Cluster model calculation and photoemission studies of molecularly adsorbed NO on Ni

Self-consistent scattered wave cluster model calculations for the NONi system are presented with NO bonded N-down in a fourfold coordination site. The results suggest that the charge is transferred from metal into the single occupied 2π∗ molecular orbital of NO which is thus significantly perturbed. The interaction involves the metal s, p, and d-derived states. We calculate total and local “density of states” defined within the framework of the cluster model and compare the results with Ultraviolet Photoemission experiments recorded at 80 K. At 300 K partial dissociation occurs as judged from UPS and XPS. Increased photoemission observed at about 2 eV below the Fermi level in the molecularly adsorbed state is attributed to the alteration of density of states due to the interaction of the 2π∗ molecular orbital of NO with the substrate. A detailed analysis of the nature of the relevant molecular orbitals is also given.

Charge‐Carrier Dynamics Following Pulsed Photoinjection

The transient behavior of charge carriers generated by pulse illumination of arbitrary intensity is investigated. The configuration treated is that in which one surface of a photoinsulating sample is grounded and the other charged to an initial potential V0 and left floating. Within the framework of the assumed model, expressions have been derived which completely describe the spatial and temporal dependence of the carrier density, electric field, and conduction current density. These expressions provide a physical picture of the charge‐carrier dynamics within the photoinsulator and allow a qualitative discussion of possible trapping effects. Solutions are also obtained for the physically observable surface potential and its time rate of change. The geometry considered here is that usually employed in electrophotographic applications of photoinsulators for information copying and image conversion devices.

Pentagonal nanowires: A first-principles study of the atomic and electronic structure

We performed an extensive first-principles study of nanowires in various pentagonal structures by using pseudopotential plane wave method within the density functional theory. Our results show that nanowires of different types of elements, such as alkali, simple, transition, and noble metals and inert gas atoms, have a stable structure made from staggered pentagons with a linear chain perpendicular to the planes of the pentagons and passing through their centers. This structure exhibits bond angles close to those in the icosahedral structure. However, silicon is found to be energetically more favorable in the eclipsed pentagonal structure. These quasi-one-dimensional pentagonal nanowires have higher cohesive energies than many other one-dimensional structures and hence may be realized experimentally. The effects of magnetic state are examined by spin-polarized calculations. The origin of the stability is discussed by examining optimized structural parameters, charge density and electronic band structure, and by using analysis based on the empirical Lennard-Jones-type interaction. Electronic band structure of pentagonal wires of different elements are discussed and their effects on quantum ballistic conductance are mentioned. It is found that the pentagonal wire of silicon exhibits metallic band structure.

Electronic structure of conducting π-electron systems

Abstract We will discuss the application of one-electron band theory to several systems where π-electron delocalization has been experimentally established. We will use polyacetylene as our prototype and extend results on it to related polymers such as (SN)x, polypyrrole and poly(p-phenylene). We will particularly concentrate on those aspects of chemical bonding and symmetry which govern whether a given system will be semiconducting or metallic, and which scale its transport properties.