Photo-induced Contraction of Layered Materials
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.127
Photo-induced Contraction of Layered Materials Hiroyuki Kumazoe1,2, Aravind Krishnamoorthy2, Lindsay Bassman2, Fuyuki Shimojo1, Rajiv K. Kalia2, Aiichiro Nakano2, and Priya Vashishta2 1 Department of Physics, Kumamoto University, Kumamoto 860-8555, Japan, 2Collaboratory for Advanced Computing and Simulations, University of Southern California, Los Angeles, CA 90089
ABSTRACT
Ultrafast atomic dynamics induced by electronic and optical excitation opens new possibilities for functionalization of two-dimensional and layered materials. Understanding the impact of perturbed valence band populations on both the strong covalent bonds and relatively weaker van der Waals interactions is important for these anisotropic systems. While the dynamics of strong covalent bonds has been explored both experimentally and theoretically, relatively fewer studies have focused on the impact of excitation on weak bonds like van der Waals and hydrogen-bond interactions. We perform nonadiabatic quantum molecular dynamics (NAQMD) simulations to study photo-induced dynamics in MoS 2 bilayer. We observe photo-induced non-thermal contraction of the interlayer distance in the MoS 2 bilayer within 100 femtoseconds after photoexcitation. We identify a large photo-induced redistribution of electronic charge density, whose Coulombic interactions could explain the observed inter-layer contraction.
INTRODUCTION Two-dimensional transition metal dichalcogenides (TMDC) are emerging functional materials with potential applications in ultraresponsive photodectection [1], valleytronics [2], and as field effect transistors for nanoelectronic applications [3]. In addition to traditional methods like chemical doping [4] and thermal [5] and strain [6] modulation of electronic properties, optical and electronic excitation has been proposed as a potential technique to induce structural distortion and phase transformation in an effort to form chemically heterophase homojunctions in these two dimensional materials. There have been several recent experimental and theoretical studies of atomic dynamics of monoand few-layer graphene and semiconductors like TMDCs. Recent ultrafast structural analysis using ultrafast electron diffraction shows softening of specific vibrational modes and induction of structural phase transformations, reflecting collective distortion of strong covalent bonds [7,8]. Higher values of optical fluence and excited charge carrier densities can cause bond-breaking and can induce chemical reactions [9]. While the impact of moderate and strong electronic excitation on strong covalent bonds is well characterized, the effect on weaker long-range interactions like van der Waals and hydrogen-bonded interactions is not well understood, even for systems containing many weak interactions like polymers and two-dimensional materials. In order to resolve this issue, we have performed non-adiabatic quantum molecular dynamics simulations (NAQMD) on bilayer MoS 2 simulation supercells s
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