Kinetics and Atomic Mechanisms of Structural Phase Transformations in Photoexcited Monolayer TMDCs
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.122
Kinetics and Atomic Mechanisms of Structural Phase Transformations in Photoexcited Monolayer TMDCs Aravind Krishnamoorthy1, Lindsay Bassman1, Rajiv K. Kalia1, Aiichiro Nakano1, Fuyuki Shimojo2, Priya Vashishta1 1 Collaboratory for Advanced Computing and Simulations, University of Southern California, Los Angeles, CA 90089
2
Department of Physics, Kumamoto University, Kumamoto 860-8555, Japan
ABSTRACT
Rapid transitions between semiconducting and metallic phases of transition-metal dichalcogenides are of interest for 2D electronics applications. Theoretical investigations have been limited to using thermal energy, lattice strain and charge doping to induce the phase transition, but have not identified mechanisms for rapid phase transition. Here, we use density functional theory to show how optical excitation leads to the formation of a lowenergy intermediate crystal structure along the semiconductor-metal phase transition pathway. This metastable crystal structure results in significantly reduced barriers for the semiconducting-metal phase transition pathway leading to rapid transition in optically excited crystals.
INTRODUCTION Structural transition between semiconducting (2H) and semi-metallic (1T') polymorphs of transition metal dichalcogenides (TMDC; MX 2 where M = Mo, W and X = S, Se, Te) are of great interest for two-dimensional electronics [1]. Rapid phase transformation between the 2H and 1T' phases would enable the formation of defect-free semiconducting-metallic lateral junctions in TMDC monolayers, which is of fundamental importance for nano-electronics [2,3], optoelectronics [4], sensors [5] and catalysis applications [6,7]. Recently, it has been demonstrated that 2H – 1T' phase transition in this family of materials can be triggered by thermal annealing [8], alloying [9-14], adsorption [15], strain engineering [3,16] and electron injection [17,18]. However, these phase transitions are associated with slow kinetics and concomitantly large timescales (on the order of 102 - 103 seconds) [18-20], which limits the experimental applicability of such techniques. Concurrently, theoretical investigations into improving the slow kinetics of this reaction have focused primarily on improving the driving force for the phase transition (i.e. the relative stability of the semiconducting and metallic phases) through modulation of external variables like strain [3], atomic and charge doping [21] while
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explicit quantification of kinetic parameters like activation barriers remains scarce. In contrast to these approaches, recent pump-probe experiments [22] as well as ab initio calculations [23] on TMDC systems suggest that strong electronic excitation could result in ultrafast changes in the crystal struc
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