Quantum chemical studies of semiconductor surface chemistry using cluster models

Modern quantum chemical methods can be used to investigate many properties of novel molecules and materials with predictive power. We have carried out accurate quantum chemical calculations with cluster models to investigate chemical reactions on semiconductor surfaces. The structure–property relationships that emerge from these studies are illustrated with particular emphasis on silicon as well as indium phosphide surface chemistry. Some new strategies that we have developed to provide a proper balance between covalent and dative bonding in compound semiconductors are discussed. Embedded cluster models have been used in some cases to include the effects of the surroundings on the active region. The structural and mechanistic understanding that emerges from our studies is illustrated by selected results on atomic layer deposition of Al₂O₃ on silicon and hydrogenation of P-rich and In-rich indium phosphide surfaces.     

Quantum chemical studies of the OH-initiated oxidation reactions of propenols in the presence of O2

ABSTRACT

The atmospheric oxidation mechanisms of 1- and 2-propenol initiated by OH radical have been theoretically investigated at the CCSD(T)//BH&;HLYP/6-311?+?+G(d,p) level of theory. Conventional transition state theory was employed to predict the rate constants for the initial reaction channels. The calculations clearly indicate that OH-addition channels contribute maximum to the total reaction, both for 1- and 2-propenol, while H-abstraction channels can be neglected at the temperature range of 220–520?K. The calculated total rate constants at 298?K are 1.66?×?10^?11 and 7.69?×?10^?12 cm³?molecule^?1?s^?1 respectively for 1- and 2-propenol, which are in reasonable agreement with the experimental values of similar systems (vinyl ethers?+?OH reactions). The deduced Arrhenius expressions are k(OH?+?1-propenol)?=?1.43?×?10^?12 exp[(743.7?K)/T] and k(OH?+?2-propenol)?=?2.86?×?10^?12 exp[(310.5?K)/T] cm³?molecule^?1?s^?1. Under atmospheric condition, the OH-addition intermediates (CH₃C?HCH(OH)₂, CH₃CH(OH)C?H(OH), CH₃CH(OH)₂?CH₂, CH₃?C(OH)CH₂(OH)) are likely to react rapidly with O₂, the theoretically identified major products for 1-propenol are HCOOH, CH₃CHO and CH₃CH(OH)CHO, and the dominant products for 2-propenol are CH₃COOH, HCHO and CH₃COCH₂OH, both companied with the regeneration of OH and HO₂ radicals (crucial reactive radicals in the atmosphere).     

Quantum chemical modelling case studies relevant to metal oxide dissolution and catalysis

Over the past 20 years quantum-chemical methods have been developed sufficiently so that they can now be applied to the elucidation of the complex mechanistic processes that occur during metal oxide dissolution and catalysis reactions. Many of the reactions occurring during these processes are not directly accessible to experimental techniques and therefore quantum-chemical modelling can be applied to probe the individual reaction steps involved in the overall mechanism. Quantum chemistry provides the means of calculating the electronic properties of solids (e.g. band structures) structural properties of solids and surfaces (for instance surface relaxation and rumpling) heats of formation and reaction, activation energies, spectroscopic excitation energies and vibrational frequencies. Three case studies are described, which have been chosen to cover a range of quantum chemical applications and methodologies. These case studies are a) the dissolution mechanism of MgO, b) the parameterisation of titanium dioxide for the determination of electronic properties and c) the mechanism and energetics of adsorption of Pd onto rutile. These case studies utilise Hartree-Fock semiempirical andab initio quantum-chemical methods as well as density functional methodologies. A range of model types are used, namely cluster models embedded in pseudo-atoms, 3-dimensional periodic models and 2-dimensional periodic surface models.     

Quantum Games in ion traps

We propose a general, scalable framework for implementing two-choices-multiplayer Quantum Games in ion traps. In particular, we discuss two famous examples: the Quantum Prisoners' Dilemma and the Quantum Minority Game. An analysis of decoherence due to intensity fluctuations in the applied laser fields is also provided.     

Molecular dynamics computation of solvent contribution to work of ion solvation

Clusters of water with various (charged and neutral) solute particles were simulated by molecular dynamics, and the radial profiles of local density, energy, electric potential, normal pressure, and polarization were obtained. The work of cluster formation was calculated. On the basis of an estimate of the surface potential for the vacuum-liquid and liquid-solid interfaces, the linear contribution of the ion charge to the chemical work of solvation was determined. In the case of the K⁺ ion, the linear contribution to the total work of solvation proved to be practically negligible.     

He-CO散射截面的量子力学研究

采用精确度较高的密耦方法(Close-Coupling)计算了He-CO碰撞的散射截面,能量从5meV～70meV,计算结果与M.Keil等的实验结果(64meV)基本相符,研究表明:势能的零点能位置、势阱深度、势阱位置、排斥势强度以及势能在势阱附近的方向性都对散射截面有较大的影响.     

Quantum chemical calculations of the molecular structure of diphenylguanidine and experimental studies of its tautomerism in solution

MNDO-PM3 and AMI quantum chemical calculations of the molecular structure of N,N’-diphenylguanidine (DPG) prove that the molecule preferably exists as an asymmetric tautomer with a basic center C=N-. IR and UV spectral data, dipole moments, and average molecular weights of DPG solutions in nonpolar and low-polar solvents confirm the results of calculations, indicating formation of hydrogenbonded cyclic self-associates of DPG in solution. Translated fromZhurnal Struktumoi Khimii, Vol. 39, No. 4, pp. 618–625, July–August, 1998.     

Quantum chemical studies on (ZnO)n/(NiO)n heterostructured nanoclusters

The structural stability and electronic properties of (ZnO)_n, (NiO)_n, (ZnO)_n/(NiO)_n for n = (1 to 4) and 3D structures were studied using density functional theory. The geometrical optimisation of clusters implies that when the atoms in the cluster increase it leads to an increase in its stability. The stability drastically increases for the heterostructure of (ZnO)_n/(NiO)_n. The dipole moment of the clusters depends on the geometry of the cluster and it is found to be minimum for heterostructures representing more neutralised clusters. HOMO-LUMO energies, ionisation potential, electron affinity, chemical hardness, binding energies and vibrational analysis of different clusters are calculated and reported. The adsorption of CO on the different sites of nanoclusters are studied and discussed.     

Quantum logics and chemical kinetics

A statistical theory of chemical kinetics is presented based on the quantum logical concept of chemical observables. The apparatus of Boolean algebra $\text{B}$ is applied for the construction of appropriate composition polynomials referring to any stipulated arrangement of the atomic constituents. A physically motivated probability measure μ(F) is introduced on the field $\text{B}$ of chemical observables, which considers the occurrence of the yes response of a given F ? $\text{B}$. The equations for the time evolution of the species density operators and the master equations for the corresponding number densities are derived. The general treatment is applied to a superposition of elementary substitution reactions (AB)_α + C ? (AC)_β + B. The expressions for the reaction rate coefficients are established.     

Quantum zigzag transition in ion chains

A string of trapped ions at zero temperature exhibits a structural phase transition to a zigzag structure, tuned by reducing the transverse trap potential or the interparticle distance. The transition is driven by transverse, short wavelength vibrational modes. We argue that this is a quantum phase transition, which can be experimentally realized and probed. Indeed, by means of a mapping to the Ising model in a transverse field, we estimate the quantum critical point in terms of the system parameters, and find a finite, measurable deviation from the critical point predicted by the classical theory. A measurement procedure is suggested which can probe the effects of quantum fluctuations at criticality. These results can be extended to describe the transverse instability of ultracold polar molecules in a one-dimensional optical lattice.     

Quantum statistical theory of chemical reactions

Exact rate equations for chemical reactions taking place in a closed homogeneous system arbitrarily far from equilibrium are derived by use of a recently developed method. The possibly different kinetic temperatures of the species are taken into account. The diagram expansion of the integral kernels of the rate equations is given and the renormalization of the interaction carried out. In lowest order non-linear non-markovian equations are obtained which reduce after suitable approximations to the usual collision theory results.     

Quantum chaos of an ion trapped in a linear ion trap

We describe the transition to quantum chaos of an ion trapped in a linear ion trap and interacting with two laser fields. Under the conditions of adiabatic illumination of the upper level of the ion, and when the frequencies of the two laser beams are slightly different, the system is reduced to a quantum linear oscillator interacting with a monochromatic wave. The property of localization over the quantum resonance cells is proposed to exploit in order to facilitate the process of measurement of the probability distribution of an ion on the vibrational levels. In the regime of strong chaos the time-averaged values of the energy and dispersion of energy are computed and compared with the corresponding classical quantities for different values of the perturbation amplitude. In the exact resonance case, the classical analog of the system possesses an infinite inhomogeneous stochastic web. We analyze the quantum dynamics inside the inhomogeneous web. It is shown that the quantum system mimics on average the dynamics of the corresponding classical system. Formation of the quantum resonance cells is illustrated in the case of a finite detuning from the exact resonance, and under increasing of the wave amplitude. The parameters of the model and the initial conditions are close to the real physical situation which can be realized in the system of cold trapped ion perturbed by two lasers fields with close frequencies. (c) 2000 American Institute of Physics.     

含高阶项Trapped离子振动态的崩塌-回复特性

解析分析了“捕陷”离子在驻波光场作用下的动力学。离子的量子平均能量随时间的变化呈现崩塌 -回复的特征 ,表明这个系统的振动态具有压缩效应。哈密顿量中的高阶项使得离子平均能量呈现不规则的崩塌 -回复 ,严重抑制离子的振动态的相干性。     

取代硝基苯炸药的理论研究

在133LYP/6.31 G*理论水平上优化了一系列取代硝基苯类化舍物的几何构型,计算了它们的电子结构和Wiberg键级,结果表明,随着氨基的引入,起爆引发键C-NO2键的强度有可能得到加强,炸药的撞击感度减小.讨论了分子内氢键与其撞击感度的关系,结果表明氢键的形成可能对分子的撞击感度起钝化作用.讨论了C-NO2键的强弱与其位置的关系.     

Synthesis and characterization of cadmium hydroxide nano-nest by chemical route

A facile chemical route based on room temperature chemical bath deposition (CBD) was developed to deposit the Cd(OH)₂ nano-nest. The growth mechanism follows two-stage crystallization with initial growth of nucleation centers, followed by subsequent anisotropic growth. The nano-nest morphological evolution of Cd(OH)₂ on different substrates has been carried out. These films have been characterized by the techniques; such as X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), optical absorption, contact angle measurement and thermoelectric power (TEP) measurements. The X-ray diffraction study revealed that the as deposited film consists of cadmium hydroxide (Cd(OH)₂) phase. The nano-nest consisted of wires with nearly uniform in dimensions, with diameter around 30 nm and length of few microns. As-deposited Cd(OH)₂ film used in this study showed water contact angle of 66°. The optical bandgap was found to be 3.2 eV, with n-type electrical conductivity as confirmed from thermo-emf measurements.     

Quantum entanglement in a two—dimensional ion trap

In this paper,we investigate the quantum entanglement in a two-dimensional ion trap system.we discuss the quantum entanglement between the ion and phonons by using reduced entropy,and that between two degrees of freedom of the vibrational motion along x and y directions by using quantum relative entropy.We discuss also the influence of initial state of the system on the quantum entanglement and the relation between two entanglements in the trapped ion system.