Pressure-induced structural phase transition of vanadium: a revisit from the perspective of ensemble theory.

For realistic crystals, the free energy strictly formulated in ensemble theory can hardly be obtained because of the difficulty in solving the high-dimension integral of the partition function, the dilemma of which makes it even a doubt if the rigorous ensemble theory is applicable to phase transitions of condensed matters. In the present work, the partition function of crystal vanadium under compression up to 320 GPa at room temperature is solved by an approach developed very recently, and the derived equation of state is in a good agreement with all the experimental measurements, especially the latest one covering the widest pressure range up to 300 GPa. Furthermore, the derived Gibbs free energy proves the very argument to understand most of the experiments reported in the past decade on the pressure-induced phase transition, and, especially, a novel phase transition sequence concerning three different phases observed very recently and the measured angles of two phases agree with our theoretical results excellently.

Efficient approaches to solutions of partition function for condensed matters.

The key problem of statistical physics standing over one hundred years is how to exactly calculate the partition function (or free energy), which severely hinders the theory to be applied to predict the thermodynamic properties of condensed matters. Very recently, we developed a direct integral approach (DIA) to the solutions and achieved ultrahigh computational efficiency and precision. In the present work, the background and the limitations of DIA were examined in details, and another method with the same efficiency was established to overcome the shortage of DIA for condensed system with lower density. The two methods were demonstrated with empirical potentials for solid and liquid cooper, solid argon and C60 molecules by comparing the derived internal energy or pressure with the results of vast molecular dynamics simulations, showing that the precision is about ten times higher than previous methods in a temperature range up to melting point. The ultrahigh efficiency enables the two methods to be performed with ab initio calculations and the experimental equation of state of solid copper up to ∼600 GPa was well reproduced, for the first time, from the partition function via density functional theory implemented.

How accurate for phonon models to predict the thermodynamics properties of crystals.

Previous work has shown that thermodynamics properties calculated by phonon model with quasi-harmonic approximation (QHA) may differ badly from experiment in some cases. The inaccuracy was examined in the present work by comparing the results of QHA for argon and copper crystal with the ones of molecular dynamics simulations, partition functions obtained by a new method or experiment. It is shown that QHA works well for the systems of atomic volume smaller than 22 Å3/atom and the accuracy gets lower and lower gradually with increasing of the atomic volume. Based on this fact, the disagreement (or agreement) between the thermodynamics properties of MgO, Si, CaO, ZrO2 calculated in previous work by QHA and the experiments can be well understood.

Statistical model for small clusters transforming from one isomer to another.

Based on the fact that the kinetic energy of one atom in small cluster still obeys the Boltzmann distribution, a statistical model is developed to predict the time consumed by a small cluster transforming from one isomer to another and is tested by vast molecular dynamics simulations of C(12) isomers transformation in helium gas at high temperatures (2000-3500 K). Extrapolating the model to lower temperatures, we found that the time for the most probable isomer of C(12) formed at 2500 K turning into the most stable one is more than 10(12) years at room temperature.

Evaluating the ability to form single crystal.

Design of crystal materials requires predicting the ability of bulk materials to form single crystals, challenging current theories of material design. By introducing a concept of condensing potential (CP), it is shown via vast simulations of crystal growth for fcc (Ni, Cu, Al, Ar) and hcp (Mg), that materials with larger CP can grow into perfect single crystal more easily. Due to the simplicity of the calculation of CP, this method might prove a convenient way to evaluate the ability of materials to form single crystal.

Rapid and Wide-Range Detection of NOx Gas by N-Hyperdoped Silicon with the Assistance of a Photovoltaic Self-Powered Sensing Mode.

Wide-dynamic-range NOx sensors are vital for the environment and health purposes, but few sensors could achieve wide-range detection with ultralow and ultrahigh concentrations at the same time. In this article, the microstructured and nitrogen-hyperdoped silicon (N-Si) for NOx gas sensing is investigated systematically. Working by the change of surface conductivity, the sensor is ultrasensitive to low concentrations of NOx down to 11 ppb and shows a rapid response/recovery time of 22/33 s for 80 ppb. When the NOx concentration increases and exceeds a threshold value (10-50 ppm), an n-p conduction-type transition is observed due to the inversion of the conduction type of major carriers, which limits the dynamic range of the sensor at high concentration. However, when the sensor works in a photovoltaic self-powered mode under the asymmetric light illumination, the limitation can be successfully overcome. Therefore, with the combination of the two working principles, a wide dynamic range stretching over 6 orders of magnitude (∼0.011-4000 ppm) can be achieved.

A New Model to Predict Optimum Conditions for Growth of 2D Materials on a Substrate.

Deposition of atoms or molecules on a solid surface is a flexible way to prepare various novel two-dimensional materials if the growth conditions, such as suitable surface and optimum temperature, could be predicted theoretically. However, prediction challenges modern theory of material design because the free energy criteria can hardly be applied to this issue due to the long-standing problem in statistical physics of the calculations of the free energy. Herein, we present an approach to the problem by the demonstrations of graphene and γ-graphyne on the surface of copper crystal, as well as silicene on a silver substrate. Compared with previous state-of-the-art algorithms for calculations of the free energy, our approach is capable of achieving computational precisions at least 10-times higher, which was confirmed by molecular dynamics simulations, and working at least four orders of magnitude faster, which enables us to obtain free energy based on ab initio calculations of the interaction potential instead of the empirical one. The approach was applied to predict the optimum conditions for silicene growth on different surfaces of solid silver based on density functional theory, and the results are in good agreement with previous experimental observations.

Global optimization method for cluster structures.

A time-going-backward quasidynamics method is developed for global optimization of cluster structures, and its merits are examined by a simple classical mechanics model, indicating that the probability for the system to jump over high potential barriers by this method is much higher than that by common annealing methods. The method is then used to investigate the isomers of a Lennard-Jones cluster containing 38 atoms and the C60 cluster with the Brenner potential, and can easily give the most stable structures, which are difficult to obtain by common annealing methods. In addition, for small carbon clusters C_{n} (n=21-30) , most of the potential energies optimized by this method are much lower than those obtained by a genetic algorithm.

"Infinite Sensitivity" of Black Silicon Ammonia Sensor Achieved by Optical and Electric Dual Drives.

The microstructured and hyperdoped silicon as a superior photoelectric and photovoltaic material is first studied as a gas-sensing material. The material is prepared by femtosecond-laser irradiation on selenium-coated silicon and then fabricated as a conductive gas sensor, targeting ammonia. At room temperature, the sensitivity, response time, repeatability, distinguishability, selectivity, and natural aging effect of the sensor have been systematically studied. Results show that such black silicon has good potential for application as an ammonia-sensing material. On the basis of its unique optoelectronic properties, an additional optical drive is proposed for the formation of an optical and electric dual-driven sensor, which is achieved by asymmetric light illumination between the two electrode regions. In a certain range of applied voltage, the sensitivity is enhanced dramatically and even tends to be infinite. For the aged device with degraded sensitivity, a two-order increment is obtained for 500 ppm of NH3 under the extra optical drive. A mechanism based on Dember effect is proposed for explaining such a phenomenon.

Electronic Band Structure and Sub-band-gap Absorption of Nitrogen Hyperdoped Silicon.

We investigated the atomic geometry, electronic band structure, and optical absorption of nitrogen hyperdoped silicon based on first-principles calculations. The results show that all the paired nitrogen defects we studied do not introduce intermediate band, while most of single nitrogen defects can introduce intermediate band in the gap. Considering the stability of the single defects and the rapid resolidification following the laser melting process in our sample preparation method, we conclude that the substitutional nitrogen defect, whose fraction was tiny and could be neglected before, should have considerable fraction in the hyperdoped silicon and results in the visible sub-band-gap absorption as observed in the experiment. Furthermore, our calculations show that the substitutional nitrogen defect has good stability, which could be one of the reasons why the sub-band-gap absorptance remains almost unchanged after annealing.

Site-selective substitutional doping with atomic precision on stepped Al (111) surface by single-atom manipulation.

In fabrication of nano- and quantum devices, it is sometimes critical to position individual dopants at certain sites precisely to obtain the specific or enhanced functionalities. With first-principles simulations, we propose a method for substitutional doping of individual atom at a certain position on a stepped metal surface by single-atom manipulation. A selected atom at the step of Al (111) surface could be extracted vertically with an Al trimer-apex tip, and then the dopant atom will be positioned to this site. The details of the entire process including potential energy curves are given, which suggests the reliability of the proposed single-atom doping method.

Mass dependence of the Soret coefficient for atomic diffusion in condensed matter.

Particle diffusion in condensed matters driven by thermal gradient, the so-called Ludwig-Soret effect, has been investigated for about 160 years, but up to the present, seldom do theories on atomic level understand a series of puzzles in relevant experiments. In this work, we derived an expression of Soret coefficient for atomic diffusion in condensed matter from a single atom statistic model with relevant parameters expressed in terms of atomic mass and the potential profile felt by the guest atom without empirical parameters. The reality of the model was strictly tested by molecular dynamics simulations, especially the result for He atom diffusing on graphene sheet, which suggests the Soret effect may be used to separate (3)He from (4)He.

Isomers of C36 and free energy criteria for cluster growth.

A molecular dynamics procedure is developed to search for cluster isomers and is used to study the isomer spectrum of C36 with the Brenner potential. Beginning with isolated carbon atom, the procedure quickly arrives at the D6h cage with the lowest potential and produces other 410 isomers. Among these isomers, we selected ones of typical cage, bowl, and sheet structures to calculate their free energies at 2300 K and performed molecular dynamics simulations starting either from 36 free carbon atoms diluted in He buffer gas kept at 2300 K or from the D6h cage under the same conditions, which show that the microsystem reaches a kinetic equilibrium within about 100 ns and that the isomer of the lowest free energy rather than the D6h cage of the lowest potential energy dominates in the resultant cluster.

Evolution spectrum of C60 isomers in buffer gas.

The energy spectrum of C60 nonclassic fullerenes with single heptagon defects calculated by Brenner empirical potential is found to submerge into the spectrum of classic fullerenes. Geometry analysis indicates that these nonclassic fullerene isomers can be more attainable than classic fullerenes at higher Stone-Wales (SW) stacks. Molecular dynamic simulations of the C60 isomer evolution in He buffer gas at 2500 K demonstrate that nonclassic fullerenes, especially those with heptagon defects, play an important role in the dynamics of C60 annealing, and that the Stone-Wales stack-by-stack transition mainly occurs at lower SW stacks. A non-SW multistep rearrangement is first observed in the simulation with its transition sequence and intermediate state presented in detail.

Isomer abundance of small carbon clusters formed in buffer He gas.

We calculated the isomer spectrum of carbon clusters of 3-36 atoms, and performed molecular dynamics simulations of the cluster growth in buffer helium gas, showing that the isomers with potentials higher than those of the most stable clusters form with considerable probabilities under common experimental conditions.