Anisotropy of crystal-melt interfacial free energy of silicon by simulation

We extend the cleaving wall method to a nonpairwise additive potential. Using this method, we compute the anisotropy of crystal-melt interfacial free energy γ for Stillinger–Weber potential of silicon [F. H. Stillinger and T. A. Weber, Phys. Rev. B 31, 5262 (1985)]. The calculated γ for (100), (111), and (110) orientations are 0.42±0.02, 0.34±0.02, and 0.35±0.03 J &#;m2, respectively. The anisotropy in γ we found is consistent with the experimental observation that Si(100)-melt interface develops (111) facets and also helps in explaining a higher undercooling observed for Si(111)-melt interface in Czochralski method

Nonmonotonic dependence of the absolute entropy on temperature in supercooled Stillinger-Weber silicon

Using a recently developed thermodynamic integration method, we compute the precise values of the excess Gibbs free energy (G^e) of the high density liquid (HDL) phase with respect to the crystalline phase at different temperatures (T) in the supercooled region of the Stillinger-Weber (SW) silicon [F. H. Stillinger and T. A. Weber, Phys. Rev. B. 32, 5262 (1985)]. Based on the slope of G^e with respect to T, we find that the absolute entropy of the HDL phase increases as its enthalpy changes from the equilibrium value at T \ge 1065 K to the value corresponding to a non-equilibrium state at 1060 K. We find that the volume distribution in the equilibrium HDL phases become progressively broader as the temperature is reduced to 1060 K, exhibiting van-der-Waals (VDW) loop in the pressure-volume curves. Our results provides insight into the thermodynamic cause of the transition from the HDL phase to the low density phases in SW silicon, observed in earlier studies near 1060 K at zero pressure.Comment: This version is accepted for publication in Journal of Statistical Physics (11 figures, 1 table

The freezing tendency towards 4-coordinated amorphous networks causes an increase in the heat capacity of supercooled Stillinger–Weber silicon

Supercooled liquid silicon (Si), modeled by the Stillinger–Weber (SW) potential, has been shown to undergo transition to low density amorphous phases at 1060 K in previous studies. Furthermore, the constant pressure heat capacity Cp has been found to exhibit a large increase as the liquid is cooled to 1060 K. In this work, we examine the nature of the equilibrium and the relaxation process of supercooled SW Si in the temperature range of 1060 K to 1070 K at zero pressure. We find that the relaxation of the supercooled liquid leads to a sharp irreversible decrease in the fluctuation of the two body energy of the largest connected network of 4-coordinated particles. Such a process implies a tightening of the bonds (i.e. freezing or jamming) of the network, and is accompanied by a sharp increase in the fraction of the 4-coordinated particles in the system. We find that the jamming (or freezing) process shows a sudden acceleration across a dynamical instability point that occurs at a unique potential energy state of the network. Further, we find that the occurrence of the dynamical instability is associated with the appearance of a straight line region in the cumulative potential energy distributions with a configurational temperature close to 1060 K. We conclude that the supercooled liquid state must be regarded as a constrained equilibrium state, since the accessible microstates are constrained by the inherent tendency of the system to approach the dynamical instability point. Thus all properties of supercooled liquid SW-Si, including the rise in Cp at 1060 K, can be attributed to the freezing tendency of the 4-coordinated particle network

Structural Phase Transformation in Strained Monolayer MoWSe2, Alloy

Two-dimensional (2D) materials exhibit different mechanical properties from their bulk counterparts owing to their monolayer atomic thickness. Here, we have examined the mechanical behavior of 2D molybdenum tungsten diselenide (MoWSe2) precipitation alloy grown using chemical vapor deposition and composed of numerous nanoscopic MoSe2 and WSe2 regions. Applying a bending strain blue-shifted the MoSe2 and WSe2 A1g Raman modes with the stress concentrated near the precipitate interfaces predominantly affecting the WSe2 modes. In situ local Raman measurements suggested that the crack propagated primarily thorough MoSe2-rich regions in the monolayer alloy. Molecular dynamics (MD) simulations were performed to study crack propagation in an MoSe2 monolayer containing nanoscopic WSe2 regions akin to the experiment. Raman spectra calculated from MD trajectories of crack propagation confirmed the emergence of intermediate peaks in the strained monolayer alloy, mirroring experimental results. The simulations revealed that the stress buildup around the crack tip caused an irreversible structural transformation from the 2H to 1T phase both in the MoSe2 matrix and WSe2 patches. This was corroborated by high-angle annular dark-field images. Crack branching and subsequent healing of a crack branch were also observed in WSe2, indicating the increased toughness and crack propagation resistance of the alloyed 2D MoWSe2 over the unalloyed counterparts.by Amey Apte, Vidya Kochat, Pankaj Rajak, Aravind Krishnamoorthy , Praveena Manimunda , Jordan A. Hachtel, Juan Carlos Idrobo, Syed Asif Syed Amanulla, Priya Vashishta , Aiichiro Nakano , Rajiv K. Kalia, Chandra Sekhar Tiwary and Pulickel M. Ajaya

Structural Phase Transformation in Strained Monolayer MoWSe₂ Alloy

Two-dimensional (2D) materials exhibit different mechanical properties from their bulk counterparts owing to their monolayer atomic thickness. Here, we have examined the mechanical behavior of 2D molybdenum tungsten diselenide (MoWSe₂) precipitation alloy grown using chemical vapor deposition and composed of numerous nanoscopic MoSe₂ and WSe₂ regions. Applying a bending strain blue-shifted the MoSe₂ and WSe₂ A_1g Raman modes with the stress concentrated near the precipitate interfaces predominantly affecting the WSe₂ modes. <i>In situ</i> local Raman measurements suggested that the crack propagated primarily thorough MoSe₂-rich regions in the monolayer alloy. Molecular dynamics (MD) simulations were performed to study crack propagation in an MoSe₂ monolayer containing nanoscopic WSe₂ regions akin to the experiment. Raman spectra calculated from MD trajectories of crack propagation confirmed the emergence of intermediate peaks in the strained monolayer alloy, mirroring experimental results. The simulations revealed that the stress buildup around the crack tip caused an irreversible structural transformation from the 2H to 1T phase both in the MoSe₂ matrix and WSe₂ patches. This was corroborated by high-angle annular dark-field images. Crack branching and subsequent healing of a crack branch were also observed in WSe₂, indicating the increased toughness and crack propagation resistance of the alloyed 2D MoWSe₂ over the unalloyed counterparts