Nanomechanical and Nanoelectrical Mappings of Buried Defects in Lateral and Vertical Van der Waals Heterostructures

Mucientes, Marta and Kolosov, Oleg (2019) Nanomechanical and Nanoelectrical Mappings of Buried Defects in Lateral and Vertical Van der Waals Heterostructures. In: 2019 MRS Fall Meeting & Exhibit, 2019-12-01 - 2019-12-06, Hynes Convention Center.

Abstract

Two dimensional (2D) transition metal dichalcogenides (TMDs) have generated strong interest between a wide scientific audience due to the variety of promising applications for nanoelectronics, photonics, sensing, energy storage, and opto-electronics, to name a few. In particular, tungsten disulfide (WS2), tungsten diselenide (WSe2) and molybdenum disulfide (MoS2), individually or their combination to form vertical and lateral heterostructures, present unique mechanical, electrical and optical properties, being exceptional candidates for the fabrication of opto-electronic devices. The physical properties of these structures are strongly dependent of the quality of the materials and presence of defects, limiting somehow the performance in devices. 1 We have studied the subsurface defects with diverse Scanning Probe Microscopies (SPM) based in the Atomic Force Microscopy (AFM). The combination of the AFM with ultrasonic excitation of the sample or/and the cantilever allow the mapping of the mechanical properties with nanoscale resolution – namely, Ultrasonic Force Microscopy (UFM), Waveguide UFM (W-UFM) and Heterodyne Force Microscopy (HFM), respectively.2 Similarly, by the electrical excitation of the sample, we can probe the dielectric properties and materials work function, using Dielectric Electrostatic Force Microscopy (D-EFM) and Kelvin Probe Force Microscopy (KPFM). The preliminary results of the top surface nanomechanical maps of vertical heterostructures of WS2 and WSe2 with pyramidal shape, show clear contrast corresponding to missing planes in the buried layers, subsurface mis-orientation of the crystallographic axis and edge dislocations. To complete the study, the pyramids have been sectioned with the Beam Exit Cross-sectional Polishing (BEXP) permitting the direct scanning of the inner part of the 3D structures, identifying the layers and revealing inhomogeneities in the layer stack via local mechanical and electrical properties mapping. For lateral heterostructures of WS2 and MoS2 we use nanomechanical maps to reveal the unique mechanical behaviour creating ripples at the interface of the two materials, likely corresponding with the strain produced by the lattice mismatch.3 1. W. Choi, N. Choudhary, G. H. Han, J. Park, D. Akinwande and Y. H. Lee, Materials Today 20 (3), 116-130 (2017). 2. M. T. Cuberes, H. E. Assender, G. A. D. Briggs and O. V. Kolosov, Journal of Physics D-Applied Physics 33 (19), 2347-2355 (2000). 3. Y. Han, K. Nguyen, M. Cao, P. Cueva, S. Xie, M. W. Tate, P. Purohit, S. M. Gruner, J. Park and D. A. Muller, Nano Letters 18 (6), 3746-3751 (2018).