Creating chirality in WSe2 through screw dislocations by chemical vapor transport†

Abstract

Screw dislocation-driven nanostructures of two-dimensional transition metal dichalcogenides (2D TMDs) can feature chirality that enables prominent asymmetric optical properties. One of the outstanding representatives is WSe₂ as it can exhibit intriguing new size and shape-dependent chemical and physical properties compared to its bulk counterpart. Crystal growth control in nanostructures with screw dislocation-driven growth is central for exploiting their structure-related properties. However, bottom-up syntheses of 2D TMDs usually contain ‘trial and error’ approaches. Here we report on the rational synthesis planning and realizing for the binary system W:Se to achieve chirality in nano-scale crystals by chemical vapor transport (CVT). For that purpose, key parameters were modelled based on thermodynamic datasets. Thus, crystal growth by CVT under addition of SeCl₄ succeeds for right-handed spiral nanocrystals from 850 °C to 800 °C with a dwell time of 60 min, while left-handed spirals are obtained from 915 °C to 860 °C. Surface-fused SiO₂ nanoparticles on an Si(100) substrate served as potential nucleation points. Chirality of screwed WSe₂ was unprecedentedly investigated by circular-polarized Raman Spectroscopy and showed an intensity increase of the E¹_2g mode of 29% and 15% for right and left-handed spirals, respectively. Pyramid-like WSe₂ analyzed by atomic force microscopy exhibits step heights of around 10 nm. Electron backscatter diffraction patterns reveal a convex curvature for WSe₂ with the curvature radii determined as R_x = (270 ± 32) μm and R_y = (141 ± 9) μm, respectively.