Xiaojie Chen

Rigid unit modes in the molecular dynamics simulation of quartz and the incommensurate phase transition

A molecular dynamics simulation of quartz at different temperatures both in the a and in the β phase has been conducted. The α-β phase transition could be observed. A phonon analysis of the β-phase confirms and rounds out in a quantitative way the origin of the incommensurate (ic) modulated phase. In particular it traces the optic soft mode at Γ becoming (to a good approximation) a so-called rigid unit mode (RUM) at q≠0, and elucidates its coupling to the transverse acoustic mode which precipitates the incommensurate transition. This success underpins and illuminates the concept of RUMs and their role in structural phase transitions.

Fluorination of diamond — C4F9I and CF3I photochemistry on diamond (100)

Abstract The radiation-induced decomposition of C4F9I and CF3I overlayers at 119 K on diamond (100) surfaces has been shown to be an efficient route to fluorination of the diamond surface. X-ray photoelectron spectroscopy has been used for photoactivation as well as for studying the photodecomposition of the fluoroalkyl iodide molecules, the attachment of the photofragments to the diamond surface, and the thermal decomposition of the fluoroalkyl ligands. Measured chemical shifts agree well with ab initio calculations of both C 1s and F 1s binding energies. It is found that chemisorbed CF3 groups on diamond (100) decompose by 300 K whereas C4F9 groups decompose over the range 300 to ∼ 700 K and this reactivity difference is rationalized on steric grounds. Both of these thermal decomposition processes produce surface Cue5f8F bonds on the diamond. The surface Cue5f8F species thermally decompose over a wide temperature range extending up to 1500 K. Hydrogen passivation of the diamond surface is ineffective in preventing free radical attack from the photodissociated products of the fluoroalkyl iodides; I atoms produced photolytically abstract H from surface Cue5f8H bonds to yield hydrogen iodide at 119 K allowing diamond fluorination. The attachment of chemisorbed F species to the diamond (100) surface causes band bending as the surface states are occupied as a result of chemisorption. This results in a shift to higher binding energy of the diamond-related C 1s levels present in the surface and subsurface regions which are sampled by XPS on the diamond. The use of photoactivation of fluoroalkyl iodides for the fluorination of diamond surfaces provides a convenient route compared to other methods involving the action of atomic F, molecular F2, XeF2 and F-containing plasmas.

Molecular dynamics study of diffusion in a molten surface

Abstract Using molecular dynamics simulations, we have studied atomic diffusion in a molten Lennard-Jones fcc (110) surface, at temperatures where the thickness of the quasi-liquid layer (QLL) is small. Both short-time (displacement) and long-time (diffusion) dynamics have been found to have anisotropy effects in the plane (between the [110] and [001] directions) due to the underlying anisotropy of the crystalline periodic potential. The dynamics in the [001] direction is found to be faster in the short-time regime, but slower in the long-time regime. The amount of anisotropy, and the cross-over time are found to decrease as the temperature is increased. Atoms in the QLL are found to diffuse via both single and double jumps along [110], and mainly via single jumps along [001]. Another, more obvious, anisotropy between the in-plane and vertical motions is found to be very strong when the QLL is thin, and to vanish gradually in the centre of the QLL when the QLL becomes thick. This kind of anisotropy persists up to the triple point close to both the solid-QLL and the QLL-vapour interface.

Surface supermelting with a long-range potential: a molecular dynamics study

Abstract A molecular dynamics simulation scheme suitable for treating long-range interactions in the presence of a surface is described. The method is applied to study the surface melting of the (100) surface in an fcc crystal formed by particles interacting through a potential with a long-range tail ∼ r −4 . A quasi-liquid film of very pronounced thickness (“supermelting”) is obtained well below the bulk melting point, as was predicted by Trayanov and Tosatti by mean-field methods. Static and dynamic properties as well as fluctuations of the quasi-liquid film are studied. The expected crossover from a “slow”, logarithmic-like increase at low temperatures to a “fast”, power-law increase closer to the triple point is found to occur. In the power-law increase regime the exponent approaches 1, as expected. The absolute film thickness, however, is underestimated by mean-field theory and fluctuations are found to give important corrections. Some effects of fluctuations are studied via molecular dynamics simulations, and discussed in comparison with recent theoretical models.

Hamaker constant calculations and surface melting of metals, semimetals and semiconductors

SummarySurface melting is strongly influenced by the interaction between the solid-liquid interface and the liquid-vapour interface. Unretarded electromagnetic dispersion forces yield a potential of the formH/l2, wherel is the distance between the interfaces (thickness of the quasi-liquid film) andH is the appropriate Hamaker constant. We have adopted the well-known treatment of dispersion forces of Lifshitzet al., where the frequency-dependent conductivities of solid and liquid are the primary quantities for the calculation of this Hamaker constant. Using the dielectric functions obtained from experimental data, numerical values forH are obtained for Al, Pb, Au, Bi and Ge. The Hamaker constant of the metals is found to be positive and is discussed in connection with recent experimental data on surface melting, particularly of Pb(110) and Al(110). The Hamaker constants of Bi (semimetal) and Ge (semiconductor) are negative, allowing at most blocked surface melting.

Landau theory revisited

Abstract The Landau free energy GL(T,Q) is an important concept in structural phase transitions because it is often the meeting point of experiment and the development of microscopic models. We review recent work on the real Q and T dependence of GL, the origin of non-classical non-critical exponents for T below Tc, the cross-over from soft mode to O/D behaviour, local bifurcation at T* and relation to ordering kinetics. Framework structures such as many silicates and the perovskite structure allow special geometrical Rigid Unit phonon Modes. The material is then likely to be near to the soft mode limit, resulting in classical behaviour of Q(T) over a very wide T range. Long range correlations result in a very small Ginzburg interval.

First principles studies of band offsets at heterojunctions and of surface reconstruction using Gaussian dual‐space density functional theory

The use of localized Gaussian basis functions for large scale first principles density functional calculations with periodic boundary conditions (PBC) in 2 dimensions and 3 dimensions has been made possible by using a dual space approach. This new method is applied to the study of electronic properties of II–VI (II=Zn, Cd, Hg; VI=S, Se, Te, Po) and III–V (III=Al, Ga; V=As, N) semiconductors. Valence band offsets of heterojunctions are calculated including both bulk contributions and interfacial contributions. The results agree very well with available experimental data. The p(2 × 1) cation terminated surface reconstructions of CdTe and HgTe (100) are calculated using the local density approximation (LDA) with two-dimensional PBC and also using the ab initio Hartree–Fock (HF) method with a finite cluster. The LDA and HF results do not agree very well.

Surface supermelting with long-range potential

A slab of particles interacting through a potential with a long-range tail — ar −4 is studied by molecular dynamics. A very pronounced surface melting (supermelting), is observed well below bulk point. The expected cross-over from a ‘slow’, logarithmic-like growth of the quasi-liquid film at low temperatures to a ‘fast’, power-law growth closer to the triple point is found to occur. In the power-law growth regime the exponent approaches 1, as expected from the phenomenological theory. The absolute film thickness, however, is severely underestimated by mean-field theory.