Screw dislocation-driven growth of nanostructures of two-dimensional (2D) transition metal dichalcogenides (TMDs) can feature special physical behaviour like unconventional superconductivity and Moiré excitons. Three of the outstanding representatives of TMDs are WSe₂, MoSe₂, and WS₂ as they can exhibit intriguing new size- and shape-depended properties compared to its bulk counterpart. Crystal growth control in nanostructures is central for exploiting their structure-related properties. However, bottom-up syntheses of 2D TMDs usually contain ‘trial and error’ approaches. This work addresses the rational synthesis planning and realizing for the binary systems W:Se, Mo:Se and W:S to achieve nanosized flakes by chemical vapor transport (CVT). For that purpose, key parameters for the CVT were modelled based on thermodynamic datasets and transferred into actual experiments. Regarding WSe₂, crystal growth by CVT under addition of SeCl₄ succeeds for bulk-WSe₂ from 900 °C to 820 °C with a dwell time of 72 h. High-crystalline, right-handed spirals with step heights of around 10 nm are obtained from 850 °C to 800 °C with a dwell time of 60 min, while left-handed spirals occur from 915 °C to 860 °C. These characteristics are linked to theoretical considerations. The terrace widths correlate linearly to the supersaturation condition during the crystal growth, which is represented by the temperature-depended equilibrium constant. Chirality of WSe₂ with screw dislocation-driven growth was investigated by circular-polarised Raman Spectroscopy and showed an intensity increase up to 29 % for the E¹₂ᶢ mode. For MoSe₂, the primary choice of substrate dictates the resulting crystal shapes. Under essentially identical growth conditions, i.e., 710 °C → 685 °C and a dwell time of 30 min, SiO₂@Al₂O₃(0001) substrates can be used for the controlled precipitation of both hexagonally and triangular shaped nano-MoSe₂. Compared to that, SiO₂@Si(100) substrates enable the growth of predominantly triangular crystals. Individual steps vary from 0.9 nm to 2.9 nm. Nano-MoSe₂ exhibits a twist angles sequence of ca. 5°. Additionally, a twist angle of 19° can be obtained with respect to the 010 zone axis. Supertwisted WS₂ flakes occur from 810 °C to 770 °C under addition of WCl₄(s) and a dwell time of 40 min, while the bulk counterpart in the 2H modification can be obtained from 1030 °C to 950 °C with a dwell time of 96 h. Tailored non-Euclidean Si(100) substrate surfaces with seeded SiO₂ nanoparticles induce WS₂ spirals with twist angles of – 19° for right-handed spirals with individual step heights in the monolayer range.
Philip Putze (Thu,) studied this question.