Session: Rising Stars of Mechanical Engineering Celebration & Showcase

Paper Number: 148558

148558 - Stress Modulated Phase Transition in 2d Materials and Silicon 

In this poster, we will present phase transition of 2D materials and Silicon, covering two projects funded by NSF, including CAREER Award.

The first part is on phase transition of 2D TMD mateirlas. Monolayer group-VI transition metal dichalcogenide (TMD) materials have multiple crystal phases with different physical properties. The generalized formula of these TMD materials is MX2, where M represents a transition metal (Mo or W) in group-VI and X represents a chalcogen (S, Se or Te). These 2D materials have recently shown great promise for phase engineering applications at the atomically thin limits, which can lead to revolutionary devices such as memory devices, reconfigurable circuits and topological transistors. Monolayer MoTe2 is one of the members inside this 2D material family. One of the stable phases of monolayer MoTe is 2H, the monolayer is composed of a layer of hexagonally arranged transition-metal atoms (blue color), sandwiched between two layers of chalcogen atoms (orange and yellow colors). Monolayers of MoTe2 in 2H phase are semiconductors with a direct band gap. A hexagonal 1T phase is generated by shifting one of the Te layers around Mo atoms. This 1T structure is energetically unstable, so it turns into conducting 1T phase with monoclinic symmetry. Inside the 2D TMD family, MoTe has the least energy difference between 2H and 1T at the ground state, thus it exhibits great potential for phase engineering applications. Stress plays important role in the phase transition of monolayer MoTe2 and can be utilized as a useful tool in the phase engineering applications. In this study, the transition barriers and pathways of a pristine MoTe2 are calculated by using finite deformation nudged elastic band method and density functional theory. The kinetic and thermodynamic analysis are conducted for both phase nucleation and propagation processes.

The second part is on phase transition of silicon. In recent years, there have been widespread applications of nanostructured Si, such asnanoparticles, nanopillars, nanocubes and nanowires, which possess unique mechanical, optical and electricalproperties. The ground state of Si has a diamond cubic (Si-I) phase. Under hydrostatic pressure, bulk Siexperiences a series of sequential phase transitions. When deviatoric stress exists, Si could deform plasticity.Recent experiments have shown many unique features of phase transition in nanostructured Si that aredifferent from the bulk Si in many aspects. For example, the direct transitions from Si-I to Si-II, Si-V andSi-IV were respectively observed in Si nanoparticles, nanocubes and nanopillar. On the computational side, molecular dynamics (MD) simulations have been applied to study atomisticdeformation mechanism in nanostructured Si, which however have yield contradictory results. For example,MD simulations using the Tersoff potential suggested that Si nanoparticles (5-40 nm) under uniaxialcompression experiences a Si-I to Si-II phase transition. By contrast, in the MD simulation using theStillinger-Weber potential, the Si nanoparticles (5-10 nm) under the same loading condition only showeddislocation-driven plasticity in Si-I without any phase transitions. The central goal of this project is to quantitatively determine the effects of stress,structural geometry and dislocations on the phase transition of Si nanostructures, using a novel mechanicsinformedmachine learning potential, to close the gap between experiments and atomistic modeling, and tosuggest approaches for phase engineering of Si nanostructures. Specifically, three Aims will be pursued: (1)Compute the minimum energy paths of concerted phase transitions of Si under various stresses, using finitedeformation nudged elastic band (FD-NEB) method, in order to generate essential dataset used for potentialdevelopment. (2) Develop a mechanics-informed machine learning potential for Si phase transition. (3)Determine the atomistic mechanism of phase nucleation and propagation, using combined MD and FDNEBsimulations as well as a proposed new method called finite deformation Dimer method.

Presenting Author: Wei Gao Texas A&M University

Presenting Author Biography: Dr. Gao is an Associate Professor of Mechanical Engineering at Texas A&M University. He earned a Bachelor degree in Engineering Mechanics from Sichuan University in 2003, and a Master degree in Solid Mechanics from Tsinghua University in 2006. Upon receiving his second Master in Mechanical Engineering from University of California, Irvine in 2007, Gao worked in industry for two years on the drive train design for electrical vehicles. Furthering his academic pursuit, he received Ph.D. in Engineering Mechanics from University of Texas at Austin in 2014 and conducted his postdoctoral training at Northwestern University. Before joining TAMU, Dr. Gao worked as an Assistant Professor at University of Texas at San Antonio from 2016 to 2022. Dr. Gao received the NSF CAREER award in 2021.

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