Atom-Dependent Edge-Enhanced Second-Harmonic Generation on MoS2 Monolayers
Kuang-I Lin1*, Yen-Hung Ho2, Shu-Bai Liu1, Jian-Jhih Ciou3, Bo-Ting Huang1, Christopher Chen4, Han-Ching Chung3, Chien-Liang Tu3, Chang-Hsiao Chen3
1Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, Taiwan
2National Center for Theoretical Sciences and Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
3Department of Automatic Control Engineering, Feng Chia University, Taichung, Taiwan
4Department of Electrical Engineering, University of California, Los Angeles, CA 90095, USA
* Presenter:Kuang-I Lin, email:kilin@mail.ncku.edu.tw
Nonlinear optical properties of two-dimensional (2D) insulating h-BN (1), semi-metallic graphene (2), and semiconducting layered materials such as GaTe and MoS2, a transition metal dichalcogenide (TMD) (3), have been extensively investigated using multiphoton microscopy. Of these properties, second-harmonic generation (SHG), a second-order optical nonlinearity, is only allowed in materials without inversion symmetry. Using mechanically exfoliated few-layer MoS2, layer number dependent optical SHG has been discussed (4). At the exciton resonance energies, the SHG intensity can be enhanced by up to three orders of magnitude, due to the unusual combination of electric and magnetic dipole transitions (5). In addition, SHG from MoS2 bilayers formed by artificial stacking with varying stacking angles allows characterization for the crystal orientation, stacking orientation, uniformity, and domain boundary of TMDs (6), even over an entire triangular WSe2-MoS2 heterostructure (7) or in industrial-scale production (8).
Recently, one-dimensional (1D) nonlinear optical edge states of a semiconducting MoS2 monolayer have been discovered by Yin et al. (9). The electronic structural changes at the edges of the MoS2 monolayer result in strong resonant SHG. This phenomenon has been attributed to a two-photon resonance due to the subband transitions from the valence bands to the localized edge states originating from the Mo-zigzag edges. These localized mid-gap states are important because they can affect the optical and transport properties of 2D TMDs (10). Many kinds of domain shapes of MoS2 monolayers have been synthesized in the chemical vapor deposition (CVD) method (11). The shape and edge evolution of domains is attributed to the Mo:S ratio, growth temperature, and their influence on the kinetic growth dynamics of edges (11). For monolayer MoS2, the edge structures are commonly believed to be zigzag terminations (10,11). However, very recently, bare Mo atoms protruding from a S-zigzag edge, similar to the so-called Klein edge in graphene, have been theoretically predicted and experimentally observed (12,13,14). In this work, edge morphology and lattice orientation of single-crystal MoS2 monolayers possessing a triangular shape with different edges grown by CVD are characterized by atomic force microscopy and transmission electron microscopy. Multiphoton laser scanning microscopy is utilized to study 1D atomic edges of MoS2 monolayers with localized mid-gap electronic states, which result in greatly enhanced optical SHG. Microscopic S-zigzag edge and S-Mo Klein edge terminations and the edge-atom dependent resonance energies can therefore be deduced based on SHG images. Theoretical calculations based on density functional theory clearly explain the interesting nonlinear optical phenomenon. Characterization of the atomic-scale variation of edge-enhanced SHG is a big step forward in this full-optical and high-yield technique of atomic-layer TMDs.
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Keywords: MoS2 monolayers, second-harmonic generation, edge terminations, mid-gap states, density functional theory