Asynchronous propagation of atomic force and excited electronic charge in GaAs under proton irradiation.

The studies for the interaction of energetic particles with matter have greatly contributed to the exploration of material properties under irradiation conditions, such as nuclear safety, medical physics and aerospace applications. In this work, we theoretically simulate the non-adiabatic process for GaAs upon proton irradiation using time-dependent density functional theory, and find that the radial propagation of force on atoms and the excitation of electron in GaAs are non-synchronous process. We calculated the electronic stopping power on proton with the velocity of 0.1-0.6 a.u., agreement with the previous empirical results. After further analyzing the force on atoms and the population of excited electrons, we find that under proton irradiation, the electrons around the host atoms at different distances from the proton trajectories are excited almost simultaneously, especially those regions with relatively high charge density. However, the distant atoms have a significant hysteresis in force, which occurs after the surrounding electrons are excited. In addition, hysteresis in force and electron excitation behavior at different positions are closely related to the velocity of proton. This non-synchronous propagation reveals the microscopic dynamic mechanism of energy deposition into the target material under ion irradiation.

Antiferroelectricity-Induced Negative Thermal Expansion in Double Perovskite Pb2 CoMoO6.

Materials with negative thermal expansion (NTE) attract significant research attention owing to their unique physical properties and promising applications. Although ferroelectric phase transitions leading to NTE are widely investigated, information on antiferroelectricity-induced NTE remains limited. In this study, single-crystal and polycrystalline Pb2 CoMoO6 samples are prepared at high pressure and temperature conditions. The compound crystallizes into an antiferroelectric Pnma orthorhombic double perovskite structure at room temperature owing to the opposite displacements dominated by Pb2+ ions. With increasing temperature to 400 K, a structural phase transition to cubic Fm-3m paraelectric phase occurs, accompanied by a sharp volume contraction of 0.41%. This is the first report of an antiferroelectric-to-paraelectric transition-induced NTE in Pb2 CoMoO6 . Moreover, the compound also exhibits remarkable NTE with an average volumetric coefficient of thermal expansion αV = -1.33 × 10-5 K-1 in a wide temperature range of 30-420 K. The as-prepared Pb2 CoMoO6 thus serves as a prototype material system for studying antiferroelectricity-induced NTE.

Crossover of energy deposition and semi-stable chemical bonding in the nonadiabatic dynamics of GaN under proton irradiation.

Understanding the ion-solid interactions of charged particles in materials facilitates the development of ion beam irradiation techniques. Combining Ehrenfest dynamics and time-dependent density-functional theory, we investigated the electronic stopping power (ESP) of an energetic proton in GaN crystal and studied the ultrafast dynamic interaction between the proton and target atoms during the nonadiabatic process. We found a crossover phenomenon of ESP at 0.36 a.u. along the <100> and <110> channels, which is interpreted by the charge transfer between the host material and the projectile and the stopping force exerted on the proton. At velocities of 0.2 and 1.7 a.u., we demonstrated that the reversal of the average number of charge transfer and the average axial force resulted in the reversed energy deposition rate and ESP in the corresponding channel. Further analysis of the evolution of non-adiabatic electronic states revealed the existence of the transient and semi-stable N-H chemical bonding during irradiation process, which is introduced by the electron clouds overlap of Nsp3hybridization and thesorbitals of the proton. These results provide meaningful information for the interactions between energetic ions and matter.

Stretching effects on non-adiabatic electron dynamic behavior in poly(dG)-poly(dC) DNA upon the proton irradiation.

The electronic excitations caused by DNA when exposed to ion radiation is essential to DNA damage. In this paper, we investigated the energy deposition and electron excitation process of DNA with reasonable stretching range upon proton irradiation based on time-dependent density functional theory. Stretching changes the strength of hydrogen bonding between the DNA base pairs, which in turn affects the Coulomb interaction between the projectile and DNA. As a semi-flexible molecule, the way of energy deposition is weakly sensitive to the stretching rate of DNA. However, the increase of stretching rate causes the increase of charge density along the trajectory channel, sequentially resulting in an increase in proton resistance along the intruding channel. The Mulliken charge analysis indicates that the guanine base and guanine ribose are ionized, meanwhile the cytosine base and cytosine ribose are reduced at all stretching rates. In a few femtoseconds, there exists an electron flow passing through the guanine ribose, guanine, cytosine base and the cytosine ribose in turn. This electron flow increases electron transfer and DNA ionization, promoting the side chain damage of the DNA upon ion irradiation. Our results provide a theoretical insight for deciphering the physical mechanism of the early stage of the irradiation process, and are also of great significance for the study of particle beam cancer therapy in different biological tissues.

Dynamic interplay between thionine and DNA under carbon ion irradiation: a real-time first-principles study.

Understanding the interactions between deoxyribonucleic acid (DNA) and photosensitizer under ion irradiation benefits the development of aptasensors, DNA biosensors and cancer diagnosis. Using real-time time-depended density functional theory, by simulating high-energy C ion passing through DNA with poly(dG)·poly(dC) sequence and that with embedded thionine (3,7-diamino-5-phenothiazinium, TH), we compared the electronic stopping power (ESP), evolution of the structure and charge, and absorption spectrum. TH inserting leads the increase in space charge density, a larger electron de-excitation and a larger ESP, but the speed corresponding to the maximum ESP is almost same. When C ion passes through TH-DNA, the structure of TH slightly changes and there still exists noncovalent interaction between TH and DNA, but the absorption coefficient depends on the electron occupied state of TH when the ion passes through. These results indicate that at low radiation doses, TH still can be a DNA detector, although its response wavelength and intensity have been slightly changed, and provide a theoretical reference to improve the possible application of phenothiazine dye in DNA biosensor under ion irradiation.

Real-time first-principles calculations of ultrafast carrier dynamics of SnSe/TiO2heterojunction under Li+implantation.

Ion implantation has been widely used in biomaterials, alloys, and semiconductors modification. Basing on the studying of trapping states in the equilibrium state, we investigate the ultrafast carrier dynamics of SnSe/TiO2and SnSe/Li/TiO2heterojunctions under Li+implantation by the real-time time-dependent density functional theory. The special type II band alignment and Li+interfacial states in SnSe/TiO2heterojunction effectively facilitate the exciton dissociation in a benign process and suppresses the interfacial nonradiative recombination. By monitoring the instantaneous ion-solid interaction energy, electronic stropping power and the excitation electron evolution, we find that atomic reconstruction introduced by the Li inserting layer changes the charge density and crystal potential field in the injection channel, and thus weakens the violent oscillation force and electron excitation on the Ti and O atoms. There exists a weaker and shorter charge excitation at the interface for SnSe/Li/TiO2implantation system, which suggests that the Li ion layer weakens the e-ph coupling between the interface electrons and the moving ion. Meanwhile, only the hot electrons are produced in the interface region, reducing the probability of carrier recombination. These results provide an understanding for the behavior of carriers in SnSe based heterojunctions and the electron-phonon coupling mechanism at the phase/grain boundary under ion implantation.