Theoretical modelling of Jahn-Teller distorted C60 anions on a surface

Topological Defects Induced High-Spin Quartet State in Truxene-Based Molecular Graphenoids

Atomic Scale STM Imaging of Alloy Surfaces With Chemical Resolution

The scanning tunneling microscope (STM) images of benzene on Pt(111) have been calculated with different adsorption sites (hollow, top, and bridge sites). Our aim was to get a qualitative understanding with a molecular-orbital (MO) approach of the factors that govern the STM image pattern and shape in the case of a molecular adsorbate. The calculated images strongly depend on the chemisorption site and they allow the assignment of each experimental image of benzene to a given site and orientation of the molecule. The contributions to the tunnel current of each molecular orbital were calculated and analyzed with the help of a simple analytic model of tunneling through a molecule. It is not only the orbitals close to the Fermi level that have a significant contribution to the current. Indeed, the shape of the orbital (especially the number of nodal planes perpendicular to the surface) also has a great importance. The final image results from the electronic interferences between these individual MO contributions and with the direct tip surface electronic current, as explained by the model. The main interference effect is between the \ensuremath{\sigma} and the \ensuremath{\pi} orbitals of benzene with a given symmetry, since these orbitals have a different phase behavior across the tunnel junction (respectively, symmetric and antisymmetric). The site differentiation in the STM pattern results from the effective symmetry of the adsorption site and, for the hollow case, only appears after interference of the MO contributions. \textcopyright{} 1996 The American Physical Society.

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) images of the charge density wave (CDW) materials 1T-TaX[sub 2] (X = S, Se) were examined by calculating the partial electron density p(r[sub o], e[sub f]) and total electron density p(r[sub o]) of a single 1T-TaSe[sub 2] layer. Our calculations show that the bright spots of the STM and AFM images are associated with the surface chalcogen atoms, and that both images should possess a pattern of six-chalcogen-atom triangles at CDW maxima. The present study explains why STM images of the CDW's have apparent height amplitudes an order of magnitude greater than the true atomic displacements of the surface chalcogen atoms, why the bright spots of the STM images have a circular shape with a radius smaller than about 2 A, and why these bright spots appear as three-atom clusters in some images. The patterns of the STM and AFM images predicted from our calculations agree well with those of the observed images. 26 refs., 11 figs.

The highest occupied molecular orbitle(HOMO) and lowest unoccupied molecular orbitle(LUMO) charge density distributions of adsorbed C60 with different orientations are calculated by using discreted variational-local density functional (DV-LDF) method. The theoretical simulations of scanning tunneling microscope (STM) images are based on Tersoff-Hamann theory. The results show that the charge density distribution parttens have their own‘fingerprint’. The LUMO charge distributions are more reliable than HOMO distributions when using them to simulate STM images. The molecular orientations can be easily determined by comparing with observed STM images in experiments and theoretical calculations in the literatures. Morevoer, according to the STM images obtained from our experiments, a new adsorbed orientation of C60 on Si(111)-7×7 surface has been found. Namely, C60 can adsorb with its 5—6 bond facing towards the substrate surface. At the same time, experimentalist can get some useful imformation from the results of this paper, and the orientations of the other molecules can also be determined by using the method.

In this article we report on investigations of the nucleation and growth of TiN on tungsten by use of scanning tunneling microscopy (STM) in distilled water. In water tip wear is significantly reduced and the STM images are reproducible by successive scannings. Simultaneous measurements of topography and tunneling barrier signals give new information on material compositions, which is supplementary to the results obtained by transmission electron microscopy and scanning electron microscopy combined with energy dispersive x-ray analysis and Auger electron spectroscopy. The combination of methods give an improved understanding of the nucleation and growth of TiN on W not available by the different methods separately.

A scanning tunneling microscopy (STM) image of a hydrogen-adsorbed Si(001) surface is studied using first-principles electron-conduction calculation. The resultant STM image and scanning tunneling spectroscopy spectra are in agreement with experimental results. The contributions of the $\pi$ states of bare dimers to the tunnel current are markedly large, and the $\sigma$ states of the dimers rarely affect the STM images. The tunnel currents do not pass through the centers of the dimers but go through the edges of the dimers with local loop currents. In addition, when the tip exists above the hydrogen-adsorbed dimer, there are certain contributions from the $\pi$ state of the adjacing bare dimers to the tunnel current. This leads to the STM image in which the hydrogen-adsorbed dimers neighboring bare dimers look higher than those surrounded by hydrogen-adsorbed dimers. These results are consistent with the experimental images observed by STM.

Scanning tunneling microscopy investigation of the surface structures of natural MoS 2

We study the electronic structures and scanning tunneling microscopy (STM) images of heterostructures consisting of pristine h-BN, graphene, and C-doped h-BN layers using first-principles total-energy calculations within the framework of the density functional theory. We find that substitutional doping of a C atom at B and N sites in the h-BN layer induces the different charge carrier types and asymmetric carrier densities on the graphene layer. We also find that C-atom impurity state can be clearly observed in the STM image when a C atom is doped at the N site in the h-BN layer. We discuss the relationship between the STM images and the spatial distribution of the C-induced impurity state.

In this paper we review the concepts of dynamic and static Jahn-Teller (JT) effects and study their influence in the electron paramagnetic resonance (EPR) spectra of transition metal impurities in wide gap insulators. We show that the key quantities involved in this problem are the tunneling splitting, usually denoted 3Γ, and the splitting, δ, between∼​3z 2 −r 2 and∼​ x 2 −y 2 energy levels due to the random strain field which is present in every real crystal. It is pointed out that in the E⊗e JT problem the kinetic energy of nuclei involved in the ground state plays a key role for understanding the actual value of 3Γ and thus the existence of dynamic JT effect. The results of ab initio calculations on a variety of JT systems show that 3Γ values span a much larger range than previously suggested. In particular, we find that 3Γ=235cm−1 for MgO:Cu2+ while 3Γ=10−4cm−1 for KCl:Ag2+. We also show that the dynamic JT effect can only appear for such large values of 3Γ as those found in MgO:Cu2+ since usual strain fields lead to δ values of the order of 10cm−1 that would otherwise localize the system in only one of the JT wells. The present results explain satisfactorily why the JT effect for Cu2+ and Ag2+ impurities in MgO is dynamic while static for Ag2+-doped CaO and alkali chlorides. The origin of such a difference is discussed in detail.

Scanning tunneling microscope (STM) images of random-sequence nucleic acid polymers under water show internal structure which depends strongly on the packing density of the polymer. Images of dense aggregates have a semicrystalline order with the individual polymers adopting simple periodic structures. Loose aggregates (or isolated molecules) show structural variability with considerable local bending and curving on a nanometer scale. It is not clear to what extent this structure is induced by the operation of the microscope. In order to investigate the possibility that the structure is sequence directed, we have imaged various DNA and RNA polymers at low packing densities. We present results here for random sequence DNA, poly(dAT).poly(dAT), poly(dA).poly(dT), poly(dCG).poly(dCG) and for random sequence RNA and poly(U). In contrast to loose aggregates of the random sequence material, the homopolymers show few sharp bends. Furthermore, the homopolymers appear to yield characteristic backbone patterns, usually at resolutions in excess of that obtained with random sequence polymers. The random sequence polymers show much more evidence of image distortion due to tip-molecule interactions, suggesting that they are, on average, mechanically less stable in the STM tunnel-gap than the homopolymers. Thus, while some of the structure observed in STM images is a consequence of tip-molecule interactions, it is related to sequence-directed properties of the polymer.

The surfaces of commensurate layered tellurides NbGe{sub 2/5}Te{sub 2}, NbGe{sub 1/3}Te{sub 2}, and NbSi{sub 1/2}Te{sub 2} were examined by scanning tunneling microscopy (STM) and atomic force microscopy (AFM). The observed atomic-resolution STM and AFM images were analyzed in terms of the partial and total electron density plots of their surfaces calculated by the extended Hueckel tight-binding electronic band structure method. The STM and AFM images of these phases are characterized by a pseudohexagonal arrangement of spots representing the surface Te atoms, and the surface unit-cell parameters of these commensurate MA{sub x}Te{sub 2} phases are well reproduced by STM and AFM. The atomic size patterns of the AFM and STM images differ in their relative contrasts from those expected on the basis of the surface Te-atom corrugations. This discrepancy is explained if the surface Te-atom sheets undergo a slight height reconstruction, most likely because the short interlayer Te{center_dot}{center_dot}{center_dot}Te contacts present in the bulk crystals are truncated at the surface. 12 refs., 13 figs.

The Jahn-Teller (J-T) effect in systems of four-fold symmetry is well known to differ from that in all other point groups with respect to the nature of the J-T active normal modes of vibration. The present report addresses some previously unnoticed features which are of intrinsic importance in recognizing and understanding the unique manifestations of quadrate symmetry in both the static and dynamic Jahn-Teller effects. We first consider the nature of the static J-T potential surfaces when coupling to and strains in two modes, b 1 and b 2, are included in the hamiltonian. The second part of this paper is devoted to an examination of the dynamic J-T effect in four-fold systems. Utilizing both perturbation theory and numerical solution to the Schrödinger equation, we examine the spin-hamiltonian parameters for a metalloporphyrin 3 Eu triplet state and discuss some dynamical processes, including reorientation of the system between minima, spin-lattice relaxation, and the dependences of these phenomena on the nature and magnitude of the off-diagonal terms in the hamiltonian. There emerge from this analysis several signal differences between the Jahn-Teller effect for a doubly degenerate state in four-fold systems and in the more usual cubic or tetrahedral situation.

Nominally pure black phosphorus (BP) is commonly found to be a p-type semiconductor, suggesting the ubiquitious presence of impurity species or intrinsic, charged defects. Moreover, scanning tunneling microscopy (STM) images of black phosphorus reveal the presence of long-range double-lobed defect features superimposed onto the surface atomic lattice. We show that both the p-type doping of BP and the defect features observed in STM images can be attributed to substitutional tin impurities. We show that black phosphorus samples produced through two common synthesis pathways contain tin impurities, and we demonstrate that the ground state of substitutional tin impurities is negatively charged for a wide range of Fermi level positions within the BP band gap. The localized negative charge of the tin impurities induces hydrogenic states in the band gap, and it is the 2p level that sits at the valence band edge that gives rise to the double-lobed features observed in STM images.

In the present work we write a general expression for the local density of states (LDOS) due to a commensurate charge density wave (CDW). The main goal is to investigate the voltage dependence of the contrast in the scanning tunneling microscope (STM) images of materials showing CDW's. For layered materials, having nearly two-dimensional electronic structures, the problem is complicated by the many-band situation near the Fermi level, and by the incomplete band gapping. Nevertheless, a simple perturbation method allows one to relate the amplitude and phase of the CDW to features of the band structure. We emphasize the role of particular characteristic energies, at which the CDW has a large contribution from special k points of the surface Brillouin zone, leading to different modulations in the STM image. In a second part of the paper, we consider the voltage-dependent contrast of ${\mathrm{NbSe}}_{2}.$ For this material, we find that the amplitude and the phase of the CDW change significantly as a function of energy (or voltage), resulting in a number of different possible motifs. For example, the direct comparison between occupied and empty states reveals that new states on the order of ${E}_{F}\ifmmode\pm\else\textpm\fi{}\ensuremath{\Delta},$ giving a dominant contribution to the LDOS, have different phases. As a result, in the corresponding STM images, the maxima of the corrugation have shifted positions along the diagonal of the conventional unit cell.