EMSL: Fei Gao's Publications

Chen ZJ, HY Xiao, X Zu, L Wang, F Gao, J Lian, and RC Ewing. 2008. "Structural and bonding properties of stannate pyrochlores: a density functional theory investigation ." Computational Materials Science 42(4):653-658. doi:10.1016/j.commatsci.2007.09.019 Abstract First-principle calculations have been completed on a series of Ln2Sn2O7 (Ln= Sm, Gd, Tb, Ho, Er, Lu, Y, La, Pr and Nd) pyrochlores to study the effect of structural geometry and bond-type on the stability of the pyrochlore structure-type. Overlap population analysis showed that the <Sn–O48f> bonds in stannate pyrochlores are much more covalent than the <Ln–O48f> bonds, and a nonlinear relationship is observed between the <Sn–O48f> or <Ln–O48f> bond lengths and the Ln cation radii. The <Ln-O8b> bonds are the most ionic among the metal-oxygen bonds. These results are consistent with experimental results. We note other factors, in addition to bond-type, that affect the stability of the pyrochlore structure.

Gao F, Y Zhang, M Posselt, and WJ Weber. 2008. "Computational Study of Anisotropic Epitaxial Recrystallization in 4H-SiC." Journal of Physics. Condensed matter 20(12):Art. No. 125203. doi:10.1088/0953-8984/20/12/125203 Abstract Two nano-sized amorphous layers were employed within a crystalline cell to study anisotropic expitaxial recrystallization using molecular dynamics (MD) methods in 4H-SiC. Both amorphous layers were created with the normal of the amorphous-crystalline (a-c) interfaces along the [0001] direction, but one with a microscopic extension long the [0001] direction, i.e. the dimension along the [-12-10] direction is much larger than that along the [-12-10] direction (Ix model), and another with a microscopic extension long the [-1010] direction (Iy model). The amorphous layer within the Ix model can be completely recrystallized at 2000 K within achievable simulation time, and the recrystallization is driven by a step-regrowth mechanism. On the other hand, the nucleation and growth of secondary ordered phases are observed at high temperatures in the Iy model. The temperature for recrystallization of the amorphous layer into high quality 4H-SiC is estimated to be below 1500 K. As compared with other models, it is found that the regrowth rates and recrystallization mechanisms strongly depend on the orientation of 4H-SiC, whereas the activation energy spectra for recrystallization processes are independent on a specific polytypic structure, with activation energies ranging from 0.8 to 1.7 eV.

Gao F, LW Campbell, YL Xie, R Devanathan, AJ Peurrung, and WJ Weber. 2008. "Electron-Hole Pairs Created by Photons and Intrinsic Properties in Detector Materials." IEEE Transactions on Nuclear Science 55(3):1079-1085. doi:10.1109/TNS.2007.908917 Abstract A Monte Carlo (MC) code has been developed to simulate the interaction of gamma-rays with semiconductors and scintillators, and the subsequent energy partitioning of fast electrons. The results provide insights on the processes involved in the electron-hole pair yield and intrinsic variance through simulations of full electron energy cascades. The MC code has been applied to simulate the production of electron-hole pairs and to evaluate intrinsic resolution in a number of semiconductors. In addition, the MC code is also able to consider the spatial distribution of electron-hole pairs induced by photons and electrons in detector materials, and has been employed to obtain details of the spatial distribution of electron-hole pairs in Ge, as a benchmark case. The preliminary results show that the distribution of electron-hole pairs exhibit some important features; (a) the density of electron-hole pairs along the main electron track is very high and (b) most electron-hole pairs produced by interband transitions are distributed at the periphery of the cascade volume. The spatial distribution and density of thermalized electron-hole pairs along the primary and secondary tracks are important for large scale simulations of electron-hole pair transport.

Li DF, HY Xiao, XT Zu, and F Gao. 2008. "First-principles study of sulfur passivation of GaP(001) surfaces at one-monolayer coverage." Solid State Communications 147(3-4):141-145. doi:10.1016/j.ssc.2008.04.039 Abstract Using first-principles total energy method, we have studied the structural and electronic properties of Ga- and P-terminated GaP(001)(1×2) surfaces adsorbed with one monolayer of sulfur. It was found that the sulfur atoms prefer to occupy bridge sites and the periodicity becomes (1 × 1) on both Ga- and Pterminated surfaces. The S–Ga bond was confirmed to be stronger than the S–P bond. The electronic analysis showed that the surface state within the energy gap on the Ga-terminated GaP surface was noticeably reduced by the sulfur adsorption, while such reduction does not occur on the P-terminated surface due to the S–P antibonding state. The nearly filled S dangling bonds on the Ga-terminated surface make this surface resistant to contamination.

Nie JL, H Xiao, XT Zu, and F Gao. 2008. "Hydrogen Adsorption, Dissociation and Diffusion on the α-U(001) Surface." Journal of Physics. Condensed matter 20(44):445001. doi:10.1088/0953-8984/20/44/445001 Abstract First-principles pseudopotential plane wave calculations based on density functional theory and the generalized gradient approximation have been used to study the adsorption, dissociation, and diffusion of hydrogen on the α-U(001) surface. Weak molecular chemisorption was observed for H2 approaching with its molecular axis parallel to the surface. The optimization of the adsorption geometries on the threefold hollow sites yields final configurations with H2 molecules move towards the top site at both coverages of 0.25 and 0.5 monolayer. A low dissociation barrier of 0.081 eV was determined for H2 dissociated from onefold top site with the H atoms falling into the two adjacent threefold hollow sites. The density of states analysis along the dissociation paths show that the hybridization of U 5f and H 1s states only occurs when H2 molecule is dissociated.

Posselt M, F Gao, and H Bracht. 2008. "Correlation between self-diffusion in Si and the migration mechanisms of vacancies and self-interstitials: An atomistic study." Physical Review. B, Condensed Matter 78:035208. doi:10.1103/PhysRevB.78.035208 Abstract The migration of point defects in silicon and the corresponding atomic mobility are investigated by classical molecular dynamics simulations using the Stillinger-Weber potential and the Tersoff potential. In contrast to most of the previous studies both the point defect diffusivity and the self-diffusion coefficient per defect are calculated separately so that the diffusion-correlation factor can be determined. Simulations with both the Stillinger-Weber and the Tersoff potential show that vacancy migration is characterized by the transformation of the tetrahedral vacancy to the split vacancy and vice versa and the diffusion-correlation factor is about 0.5. This value was also derived by the statistical diffusion theory under the assumption of the same migration mechanism. The mechanisms of self-interstitial migration are more complex. The detailed study, including a visual analysis and investigations with the nudged elastic band method, reveals a variety of transformations between different self-interstitial configurations. Molecular dynamics simulations using the Stillinger-Weber potential show, that the self-interstitial migration is dominated by a dumbbell mechanism, whereas the interstitialcy mechanism prevails with the Tersoff potental. The corresponding values of the correlation factor are different, namely 0.59 and 0.69 for the dumbbell and the interstitialcy mechanism, respectively. The latter value is nearly equal to that obtained by the statistical theory which assumes the interstitialcy mechanism. Recent analysis of experimental results demonstrated, that in the framework of state-of-the-art diffusion and reaction models the best interpretation of point defect data can be given by assuming . The comparison with the present atomistic study leads to the conclusion that a dumbbell mechanism governs the self-interstitial migration in Si. Simulations using the Stillinger-Weber potential reveal two dominating migration paths which are characterized by transformation between the extended <110> dumbbell and the <110> dumbbell and vice versa. This process occurs either in a single {110} plane or includes a change into an equivalent {110} plane.

Pu J, L Yang, F Gao, HL Heinisch, RJ Kurtz, and XT Zu. 2008. "Interaction of displacement cascade with helium bubbles in alpha-iron: Computer simulation." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 266(18):3993-3999. doi:10.1016/j.nimb.2008.07.017 Abstract Molecular dynamics (MD) method has been performed to study the interaction of displacement cascade with He bubbles with two sets of potentials. The results show that the stability of He bubbles depends much on the initial He-vacancy (He/V) ratio and the recoil energy. For an initial He/V ratio of 3, the cascade leads to the increase in the number of vacancies in the He bubble and the decrease in the He/V ratio. For an initial He/V ratio of 0.5, the interaction of a cascade with the He/V bubble results in the decrease in the number of vacancies and the increase in the He/V ratio. For an initial He/V ratio of 1, the stability of the bubbles slightly depends on the primary knock-on atom (PKA) energy. Furthermore, a large number of self-interstitial atom clusters are formed after cascade collision for the He/V ratio of 3, while large vacancy clusters are observed for the He/V ratio of 0.5. However, some differences of defect production and clustering between the two sets of potentials are observed, which may be associated the formation energies of He-V clusters, the binding energies of vacancies and He atoms to the clusters and the probability of subcascade formation.

Wang Z, XT Zu, Z Li, and F Gao. 2008. "Amorphous layer coating induced brittle to ductile transition in single crystalline SiC nanowires: an atomistic simulation." Journal of Physics D. Applied Physics 41(15):155419. doi:10.1088/0022-3727/41/15/155419 Abstract Molecular dynamics simulations with Tersoff potentials were used to study the response of SiC nanowires with and without amorphous coating to a tensile strain along the axial direction. The uncoated nanowires show brittle properties and fail through bond breaking. Although the amorphous coating leads to the decrease of Young’s modulus of nanowires, yet it also leads the appearance of plastic deformation under axial strain. These results provide an effective way to modify the brittle properties of some other semiconductor nanowires.

Wang Z, XT Zu, F Gao, and WJ Weber. 2008. "Atomistic Level Studies on the Tensile Behavior of GaN Nanotubes under Uniaxial Tension." The European Physical Journal. B. 61(4):413-418. doi:10.1140/epjb/e2008-00091-3 Abstract Molecular dynamics method with the Stillinger-Weber (SW) potential has been employed to study the responses of GaN nanotubes (Ga NNTs) to a uniaxial tensile load along the axial direction. It has been revealed that GaNNTs exhibits a completely different tensile behavior at different temperatures, i.e., ductility at higher deformation temperatures and brittleness at lower temperatures, leading to a brittle to ductile transition (BDT). Both the BDT temperature and the critical stress increases with increasing thickness of GaNNTs and the critical stress at higher tmperature are lower than those at lower temperature. These results on the tensile behaviors of GaNNTs in an atomic level will provide a good reference to its promising applications.

Wang Z, XT Zu, F Gao, and WJ Weber. 2008. "Atomistic simulations of the mechanical properties of silicon carbide nanowires." Physical Review. B, Condensed Matter and Materials Physics 77(22):224113, 1-10. doi:10.1103/PhysRevB.77.224113 Abstract Molecular dynamics methods using the Tersoff bond-order potential are performed to study the nanomechanical behavior of [111]-oriented β-SiC nanowires under tension, compression, torsion, combined tension-torsion and combined compression-torsion. Under axial tensile strain, the bonds of the nanowires are just stretched before the failure of nanowires by bond breakage. The failure behavior is found to depend on size and temperatures. Under axial compressive strain, the collapse of the SiC nanowires by yielding or column buckling mode depends on the length and diameters of the nanowires, and the latter is consistent with the analysis of equivalent continuum structures using Euler buckling theory. The nanowires collapse through a phase transformation from crystal to amorphous structure in several atomic layers under torsion strain. Under combined loading the failure and buckling modes are not affected by the torsion with a small torsion rate, but the critical stress decreases with increasing the torsion rate. Torsion buckling occurs before the failure and buckling with a big torsion rate. Plastic deformation appears in the buckling zone with further increasing the combined loading.