Husong Zheng1#, Salvador Valtierra2#, Nana Ofori-Opoku4,5#, Chuanhui Chen1, Lifei Sun3, Liying Jiao3, Kirk H. Bevan2*, and Chenggang Tao1* 1Department of Physics, Virginia Tech, Blacksburg, Virginia 24061, USA
2Materials Engineering, McGill University, Montreal, Quebec H3A 0C5, Canada 3Department of Chemistry, Tsinghua University, Beijing 100084, China 4Materials Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA
5Center for Hierarchical Materials Design, Northwestern University, Evanston, IL 60208, USA

These authors contributed equally to this work.

• Corresponding to: kirk.bevan@mcgill.ca and cgtao@vt.edu.

2 ABSTRACT To ensure the practical application of atomically thin transition metal dichalcogenides, it is essential to characterize their structural stability under external stimuli such as electric fields and currents. Using vacancy monolayer islands on TiSe2 surfaces as a model system, for the first time we have observed a shape evolution and growth from triangular to hexagonal driven by scanning tunneling microscopy (STM) electrical stressing. The size of islands shows linear growth with a rate of (3.00 ± 0.05) ´ 10-3 nm/s, when the STM scanning parameters are held fixed at Vs = 1.0 V and I = 1.8 nA. We further quantified how the growth rate is related to the magnitude of the tunneling current. Our simulations of monolayer island evolution using phase-field modeling are in good agreement with our experimental observations, and point towards preferential edge atom dissociation under STM scanning driving the observed growth. The results could be potentially important for device applications of ultrathin transition metal dichalcogenides and related 2D materials subject to electrical stressing under device operating conditions.

PACS numbers: 68.43.Jk, 68.37.Ef, 68.65.-k, 64.35.Md

I. INTRODUCTION Emerging 2D materials, such as atomically thin transition metal dichalcogenides (TMDs), have been the subject of intense research due to their fascinating properties and potential practical applications [1-3]. TMDs have a layered structure in which a transition metal atom layer is sandwiched between two chalcogen atom layers and adjacent layers are stacked via Van der Waals forces. Atomically thin TMDs vary from metallic, semimetallic, to semiconducting, and can be used in electronic devices, phototransistors, solar cells and gas sensors [4-9]. As a member of the TMDs, 1T-TiSe2 is a widely studied charge density wave (CDW) material. Below 200 K, 1T-TiSe2 undergoes a phase

3 transition to a CDW state, showing a 2 × 2 × 2 commensurate superlattice [10-12]. Such a transition in 1T-TiSe2 implies great potential applications in optoelectronics [13], voltage-controlled oscillators [14] and ultrafast electronics [15]. However, the properties of atomically thin TMDs and other 2D materials are both sensitive to and governed by defects and interfaces, such as domain boundaries and edges [16-19]. Usually these interfaces are more susceptible to thermal fluctuations and external stimuli, than the bulk of the material [20,21]. Thus, for practical applications of mono- and few-layer TMDs, it is essential to characterize their structural stability when subjected to the external stimuli present in devices, such as: electrical fields, irradiation and other forcing conditions. In situ investigations of said systems are usually difficult given the dynamical conditions, and so far quantitative characterizations of structural stability of these systems are still lacking. Furthermore, it is important to utilize theoretical modeling to interpret the results and understand the impact of measurement parameters on the observed trends. Numerical techniques such as molecular dynamics (MD) and most recently phase-field modeling enable one to calculate the dynamical evolution of materials. However, the phase-field method, unlike MD, can explore the diffusive time and length scales appropriate for studying microstructure evolution in electronic materials. Using monolayer vacancy islands on titanium diselenide (TiSe2) surfaces as a model system as shown in Fig. 1, we experimentally and theoretically investigated their shape evolution and growth rate driven by scanning tunneling microscopy (STM) electrical stressing. The equilibrium triangular monolayer vacancy islands evolve to a hexagonal shape and the island area shows a non-linear area growth dependence with respect to time (when electrically stressed by a STM tip). The growth rate dependence on the tunneling current is experimentally determined. Our simulations of shape and size evolution using a phase-field model are consistent with our experimental observations, and suggest that the STM driven vacancy island growth is driven by island edge atom dissociation under

4 electrical stressing. T

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