Energy Surfaces Of States Research Articles

Surface states of wet chemically etched n-Si(1 1 1):H surfaces

Impedance measurements at wet chemically etched n-Si(111):H surfaces in contact with the aqueous electrolyte 0.1M H2SO4 reveal a surface state related capacitance extending over a potential range of 350mV in the n-Si(111):H band gap. A detailed analysis of the impedance data allows to identify three different surface state energy levels contributing to the observed surface state capacitance. These energy levels coincide with electronic states of hydrogen interdiffused into the n-Si(111):H subsurface region, which occurs, for example, during the wet chemical preparation of n-Si(111):H surfaces. The density of the observed states in the n-Si(111):H subsurface region is of the order of (1014–1015)cm−3.

An ab initio based full-dimensional global potential energy surface for FH2O(X2A′) and dynamics for the F + H2O → HF + HO reaction

A global potential energy surface (PES) for the ground electronic state of FH(2)O is constructed based on more than 30,000 ab initio points at the multi-reference configuration interaction level. The PES features a pre-reaction van der Waals well and two post-reaction hydrogen-bonded complexes, as well as a "reactant-like" transition state with a classical barrier of 3.8 kcal/mol. The adiabatic F + H(2)O → HF + OH reaction dynamics on this PES was investigated using a standard quasi-classical trajectory method. In agreement with experiment, the HF product contains significant vibrational excitation with limited rotational excitation, while the OH product is internally cold, reflecting its spectator role in the reaction. The products are primarily scattered in the backward direction, consistent with a direct abstraction mechanism.

Tribological properties of N+ ion implanted ultrananocrystalline diamond films

Tribological properties of ultra nanocrystalline diamond (UNCD) films have chemically been modified by N+ ion implantation and subsequent annealing processes. Friction coefficient is found to be 0.15 in as-prepared film comparing to 0.09 and 0.05 in N+ ion implanted and post-annealed films, respectively. Such a modification of friction coefficient is a characteristic of the transformation of sp3 to graphitized/amorphized sp2 bonded carbon network. Transformation of sp3 to sp2 carbon network causes conversion of higher surface energy state (hydrophilic) to lower (hydrophobic) one which results in ultra low friction coefficient. Graphitization/amorphization in wear track observed by micro Raman spectroscopy is found to be the prominent mechanism for the reduction in friction coefficient.

Exploring Potential Energy Surfaces of Electronic Excited States in Solution with the EOM-CCSD-PCM Method.

The effect of the solvent on the structure of a molecule in an electronic excited state cannot be neglected. However, the computational cost of including explicit solvent molecules around the solute becomes rather onerous when an accurate method such as the equation of motion coupled cluster singles and doubles (EOM-CCSD) is employed. Solvation continuum models like the polarizable continuum model (PCM) provide an efficient alternative to explicit models, since the solvent conformational average is implicit and the solute-solvent mutual polarization is naturally accounted for. In this work, the coupling of EOM-CCSD and PCM in a state specific approach is presented for the evaluation of energy and analytic energy gradients. Also, various approximations are explored to maintain the computational cost comparable to gas phase EOM-CCSD. Numerical examples are used to test the different schemes.

Connection between the Upper and Lower Energy Regions of the Potential Energy Surface of the Ground Electronic State of the HSO2 System

The importance of the HSO(2) system in atmospheric and combustion chemistry has motivated several works dedicated to the study of associated structures and chemical reactions. Nevertheless, controversy still exists about a possible connection between the upper and lower energy regions of the potential energy surface (PES) for the ground electronic state of the system. Very recently, a path to connect these regions was proposed based on studies at the CASPT2/aug-cc-pV(T+d)Z level of calculation but the small energy difference between some of the transitions states along that path suggested the necessity of calculations at a higher level of theory. In the present work, we report a CCSD(T)/aug-cc-pV(T+d)Z study of the stationary states associated to the proposed connection path, including assessment of the most reliable complete basis set (CBS) extrapolation scheme for the system. Among the new features, the present calculations show that there are no structures corresponding to the HSO(2)(b) minimum and the TS3 saddle point obtained at the CASPT2 level and that the connection path between the upper and lower energy regions of the PES for the ground electronic state involves only one transition state and most probably more than one electronic state.

Thickness dependence oscillations of transport properties in thin films of a topological insulator Bi91Sb9

The dependences of the electrical conductivity, Hall coefficient, magnetoresistance, and Seebeck coefficient on the thickness d (d = 15–400 nm) of the topological insulator Bi91Sb9 thin films grown on mica substrates were obtained at room temperature. In addition to the oscillations with a period Δd = (105 ± 5) nm in the thickness range d = 100–400 nm which are attributed to the quantization of the semiconductor electron energy spectrum, oscillations with a period Δd = (8 ± 2) nm in the range d = 15–60 nm were also revealed. It is suggested that the existence of the high-frequency oscillations in the thin films may be connected with the quantization of the metallic surface states energy spectrum.

Direct measurement of surface states density and energy distribution in individual InAs nanowires

InAs nanowires are candidates for future high-speed electronic and optoelectronic applications due to their high electron mobility and large coherence length. However, InAs surfaces are known to possess a high concentration of donor-type surface states, which results in an electron accumulation layer and, consequently, Fermi level pinning. Since the surface to volume ratio in nanowires is very large, the effect of surface states is greatly enhanced. We present a method for directly determining the density and energy distribution of single nanowire surface states using Kelvin probe force microscopy measured on a nanowire field-effect transistor and interpreted by electrostatic modeling. Here, the method is applied to individual InAs nanowires, which similarly to bulk InAs exhibit a prominent accumulation layer consisting of a large concentration of donor-type surface states. Nevertheless, due to the small diameter of the nanowires, the electron accumulation and Fermi level pinning take place within the entire nanowire.

Effect of electron–phonon interaction on surface states in wurtzite nitride semiconductors under pressure

A variational theory is proposed to study the electronic surface states in semi-infinite wurtzite nitride semiconductors under the hydrostatic pressure. The electronic surface state energy level is calculated, by taking the effects of the electron–Surface–Optical–phonon interaction, structural anisotropy and the hydrostatic pressure into account. The numerical computation has been performed for the electronic surface state energy levels, coupling constants and the average penetrating depths of the electronic surface state wave functions under the hydrostatic pressure for wurtzite GaN, AlN and InN, respectively. The results show that electron–Surface–Optical–phonon interaction lowers the electronic surface state energy levels. It is also found that the electronic surface state energy levels decrease with the hydrostatic pressure in wurtzite GaN and AlN. But for wurtzite InN, the case is contrary. It is shown that the hydrostatic pressure raised the influence of electron–phonon interaction on the electronic surface states obviously. The effect of electron–Surface–Optical–phonon interaction under the hydrostatic pressure on the electronic surface states cannot be neglected, in specially, for materials with strong electron–phonon coupling and wide band gap.

Ab initio-based double many-body expansion potential energy surface for the first excited triplet state of the ammonia molecule

A global single-sheeted double many-body expansion potential energy surface is reported for the first excited triplet state of NH(3). It employs an approximate cluster expansion of the molecular potential that utilizes previously reported functions of the same family for the triatomic fragments. Four-body energy terms have been calibrated from extensive accurate ab initio data so as to reproduce the main features of the title system. A new switching function formalism has been reported to approximate the true multisheeted nature of NH(3)((3)A(2) ('')) potential energy surface, thus allowing the correct behavior at the NH(2)((2)A(")) + H((2)S) and NH(2)((4)A(")) + H((2)S) dissociation limits. The resulting fully six-dimensional potential energy function reproduces the correct symmetry under the permutation of identical atoms, and predicts the correct behavior at all dissociation channels while providing a realistic representation at all interatomic separations. The major attributes of the NH(3) double many-body expansion potential energy surface have also been characterized, and found to be in good agreement, both with the calculated ones from the raw ab initio energies and the theoretical results available in the literature.

Continental‐scale water and energy flux analysis and validation for the North American Land Data Assimilation System project phase 2 (NLDAS‐2): 1. Intercomparison and application of model products

Results are presented from the second phase of the multiinstitution North American Land Data Assimilation System (NLDAS‐2) research partnership. In NLDAS, the Noah, Variable Infiltration Capacity, Sacramento Soil Moisture Accounting, and Mosaic land surface models (LSMs) are executed over the conterminous U.S. (CONUS) in realtime and retrospective modes. These runs support the drought analysis, monitoring and forecasting activities of the National Integrated Drought Information System, as well as efforts to monitor large‐scale floods. NLDAS‐2 builds upon the framework of the first phase of NLDAS (NLDAS‐1) by increasing the accuracy and consistency of the surface forcing data, upgrading the land surface model code and parameters, and extending the study from a 3‐year (1997–1999) to a 30‐year (1979–2008) time window. As the first of two parts, this paper details the configuration of NLDAS‐2, describes the upgrades to the forcing, parameters, and code of the four LSMs, and explores overall model‐to‐model comparisons of land surface water and energy flux and state variables over the CONUS. Focusing on model output rather than on observations, this study seeks to highlight the similarities and differences between models, and to assess changes in output from that seen in NLDAS‐1. The second part of the two‐part article focuses on the validation of model‐simulated streamflow and evaporation against observations. The results depict a higher level of agreement among the four models over much of the CONUS than was found in the first phase of NLDAS. This is due, in part, to recent improvements in the parameters, code, and forcing of the NLDAS‐2 LSMs that were initiated following NLDAS‐1. However, large inter‐model differences still exist in the northeast, Lake Superior, and western mountainous regions of the CONUS, which are associated with cold season processes. In addition, variations in the representation of sub‐surface hydrology in the four LSMs lead to large differences in modeled evaporation and subsurface runoff. These issues are important targets for future research by the land surface modeling community. Finally, improvement from NLDAS‐1 to NLDAS‐2 is summarized by comparing the streamflow measured from U.S. Geological Survey stream gauges with that simulated by four NLDAS models over 961 small basins.

Theoretical study of two states reactivity of methane activation on iron atom and iron dimer

Density functional theory (DFT) calculations have been carried out to explore the catalytic activation of C–H bonds in methane by the iron atom, Fe, and the iron dimer, Fe2. For methane activation on an Fe atom, the calculations suggest that the activation of the first C–H bond is mediated via the triplet excited-state potential energy surface (PES), with initial excitation of Fe to the triplet state being necessary for the reaction to be energetically feasible. Compared with the breaking of the first C–H bond, the cleavage of the second C–H bond is predicted to involve a significantly higher barrier, which could explain experimental observations of the HFeCH3 complex rather than CH2FeH2 in the activation of methane by an Fe atom. For methane activation on an iron dimer, the cleavage of the first C–H bond is quite facile with a barrier only 11.2, 15.8 and 8.4kcal/mol on the septet state energy surface at the B3LYP/6-311+G(2df,2dp), BPW91/6-311+G(2df,2dp) and M06/B3LYP level, respectively. Cleavage of the second C–H bond from HFe2CH3 involves a barrier calculated respectively as 18.0, 10.7 and 12.4kcal/mol at the three levels. The results suggest that the elimination of hydrogen from the dihydrogen complex is a rate-determining step. Overall, our results indicate that the iron dimer Fe2 has a stronger catalytic effect on the activation of methane than the iron atom.

Energetics and Electronic Properties of a Neutral Diuranium Molecule Encapsulated in C90 Fullerene

Scientists have found that the size of the cavity of endohedral metallofullerenes (EMFs) can influence the properties of the inner molecule. In this work, the neutral diuranium molecule was encapsulated into the C90 fullerene. The neutral U-U dissociation potential energy surfaces of different electronic states confined in the C90 fullerene were scanned and the quintuple and the septuplet of these states were found to be low-lying. In the fullerene cage, the U2 molecule easily disintegrates along the axis of the fullerene, and then is chemically adsorbed on the both ends of the fullerene. The charge distribution and the molecular orbit properties of the complex were also uncovered.

Improved visible-light photocatalytic activity of titania activated by nitrogen and indium modification

Nitrogen and indium co-modified TiO2 was prepared by a simple sol–gel method. Under visible-light irradiation, the modified catalysts exhibited much higher photocatalytic activity for 4-chlorophenol photodegradation than both the as-prepared TiO2 and nitrogen-doped TiO2. The structure and properties of the resultant catalysts were characterized by XRD, BET, XPS, UV-vis DRS and PL techniques. It was found that unique chemical species, such as N–O and O–In–Clx (x = 1 or 2), existed on the surface of the nitrogen and indium co-modified TiO2. The surface-state energy levels introduced by these surface species were located close to the valence and conduction bands, respectively, which could lead to significant absorption in the visible-light region and facilitate the separation of photogenerated electrons and holes. Thus, the visible-light photocatalytic activity of TiO2 can be greatly improved by nitrogen and indium modification. We envision the opportunities for using such a method to develop TiO2-based visible-light photocatalysts suitable for practical applications.

Improved Potential Energy Surface of Ozone Constructed Using the Fitting by Permutationally Invariant Polynomial Function

New global potential energy surface for the ground electronic state of ozone is constructed at the complete basis set level of the multireference configuration interaction theory. A method of fitting the data points by analytical permutationally invariant polynomial function is adopted. A small set of 500 points is preoptimized using the old surface of ozone. In this procedure the positions of points in the configuration space are chosen such that the RMS deviation of the fit is minimized. New ab initio calculations are carried out at these points and are used to build new surface. Additional points are added to the vicinity of the minimum energy path in order to improve accuracy of the fit, particularly in the region where the surface of ozone exhibits a shallow van der Waals well. New surface can be used to study formation of ozone at thermal energies and its spectroscopy near the dissociation threshold.

Combinatorial growth of Si nanoribbons

Silicon nanoribbons (Si NRs) with a thickness of about 30 nm and a width up to a few micrometers were synthesized. Systematic observations indicate that Si NRs evolve via the following sequences: the growth of basal nanowires assisted with a Pt catalyst by a vapor-liquid-solid (VLS) mechanism, followed by the formation of saw-like edges on the basal nanowires and the planar filling of those edges by a vapor-solid (VS) mechanism. Si NRs have twins along the longitudinal < 110 > growth of the basal nanowires that also extend in < 112 > direction to edge of NRs. These twins appear to drive the lateral growth by a reentrant twin mechanism. These twins also create a mirror-like crystallographic configuration in the anisotropic surface energy state and appear to further drive lateral saw-like edge growth in the < 112 > direction. These outcomes indicate that the Si NRs are grown by a combination of the two mechanisms of a Pt-catalyst-assisted VLS mechanism for longitudinal growth and a twin-assisted VS mechanism for lateral growth.

Effects of ultraviolet treatment on the optical and structural properties of ZnO nanoparticles

This study investigates the effect of ultraviolet treatment on the structural and optical properties of the ZnO nanoparticles by X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy and photoluminescence measurements. A dependence of the luminescence behavior and crystal structure upon ultraviolet treatment has been found. It is shown that ultraviolet treatment may lead to the formation of surface oxygen vacancies, producing hydroxyl adsorption. The surface modification may lead to transition from hydrophobic (low surface energy) to hydrophilic (high surface energy) state, resulting in uniform distribution of the ZnO nanoparticles on the substrate.

Structural characterization and surface heterogeneity of bimodal mesoporous silicas functionalized with aminopropyl groups and loaded aspirin

Bimodal mesoporous materials were modified with different amount of 3-aminopropyltriethoxysilane and employed as aspirin carriers. The modified and drug loaded bimodal mesoporous materials were characterized with XRD, FT-IR, TEM and elemental analysis methods to explore the influence of amino groups and drug molecules on the mesoporous surface. Meanwhile, the mesoporous surface energy states was calculated by the density functional theory based on the N 2 sorption analysis. According to the surface energy distributions of samples including before and after modification and drug loading, it can be deduced that through superficial modification with amino groups, the surface energy moves to high state, implying the introduction of amino moieties could provide the mesoporous surface with much active sites. Besides, the interaction between the amino groups and drug molecules is weak, and hence the controlled drug delivery would be possible.

Scattering of current carriers on a Mo(110) surface covered with hydrogen and deuterium submonolayers

The paper reports the experimental study of current carrier transitions between surface and bulk energy states and their influence on the magneto-resistances of an atomically pure Mo(110) plate and a similar plate covered with adsorbed hydrogen and deuterium monolayer films. Effect of adsorbate submonolayer ordering on the surface scattering of current carriers is investigated. The experiment employs the static skin effect technique under ultrahigh vacuum conditions.

The electrochemical corrosion of bulk nanocrystalline ingot iron in HCl solutions with different concentrations

We studied the corrosion properties of bulk nanocrystalline ingot iron (BNII) and conventional polycrystalline ingot iron (CPII) in HCl solutions from 0.1 mol L −1 to 0.4 mol L −1 at room temperature. The corrosion resistance of BNII was enhanced in comparison with CPII. We investigated the surface energy state densities of BNII and CPII with ultra-violet photoelectron spectroscopy. The energy state density of BNII 4 s electrons was 13.73% less than that of CPII. The function work of BNII was 0.47 eV larger that of CPII. The corrosion resistance of BNII was enhanced in comparison with CPII due to its less energy state density of 4 s electrons, larger work function and weaker Cl − specific adsorption.

Ab initio determination of atomic structure and energy of surface states of bare and hydrogen covered GaN (0001) surface — Existence of the Surface States Stark Effect (SSSE)

Density Functional Theory (DFT) calculations indicate that energetically stable structure of clean GaN(0001) surface posses (2 × 1) reconstruction, having every second row of Ga located near plane of N atoms, that gives rise to Ga-related dispersionless surface electronic state, already identified by angle resolved photoelectron spectroscopy (ARPES) measurements [S.S. Dhesi et al. Phys. Rev. B 56 (1997) 10271, L. Plucinski et al. Surf. Sci 507-10 (2002) 223, S. M. Widstrand et al. Surf. Sci. 584 (2005) 169]. The energy reduction in reconstruction proceeds via change of the hybridization of the occupied Ga surface states from sp 3 to sp 2, transforming the empty states to p z type. It is also shown that the electric subsurface field, modeled in new slab model which allows to simulate electric fields at the semiconductor surfaces [P. Kempisty et al., J. Appl. Phys. 106 (2009) 054901], strongly affects the energy of electronic states of GaN(0001) surfaces. The change of the field may shift the energy of surface states of bare and hydrogen covered GaN(0001) surface, by several eV with respect to the band states. The phenomenon, denoted as Surface States Stark Effect (SSSE), explains various band bending values, measured at differently doped n-type GaN(0001) surfaces. It is shown also that, for the adsorbate density up to one H atom for each Ga surface atom i.e. 1 monolayer coverage (1 ML), the hydrogen adatoms are located at the on-top positions, i.e. directly above Ga atoms. For these adsorbate densities, the H-related quantum surface state is located slightly below the valence band maximum (VBM) in the case of p-type GaN surface. For n-type GaN, the H-related surface state is located deeply in the valence band, about 2 eV below VBM. For higher, 1.25 ML hydrogen coverage, the two H adatoms create either surface attached H 2 ad-molecule (energetically stable) or triple bridge configuration is created (metastable). The H 2 ad-molecule is weekly attached to the surface, having the desorption energy barrier equal to 0.16 eV. For 1.25 ML coverage the DFT results were obtained for p-type GaN only. They show that in the ad-molecule case, a new surface electronic state arises which is located about 6.7 eV below VBM. In the case of the bridge configuration, the bridge related surface state is located closely to the conduction band minimum (CBM).