Controlled Preferential Oxidation of Grain Boundaries in Monolayer Tungsten Disulfide for Direct Optical Imaging

Abstract

Synthetic 2D crystal films grown by chemical vapor deposition are typically polycrystalline, and determining grain size within domains and continuous films is crucial for determining their structure. Here we show that grain boundaries in the 2D transition metal dichalcogenide WS2, grown by CVD, can be preferentially oxidized by controlled heating in air. Under our developed conditions, preferential degradation at the grain boundaries causes an increase in their physical size due to oxidation. This increase in size enables their clear and rapid identification using a standard optical microscope. We demonstrate that similar treatments in an Ar environment do no show this effect, confirming that oxidation is the main role in the structural change. Statistical analysis of grain boundary (GB) angles shows dominant mirror formation. Electrical biasing across the GB is shown to lead to changes at the GB and their observation under an optical microscope. Our approach enables high-throughput screening of as-synthesized WS2 domains and continuous films to determine their crystallinity and should enable improvements in future CVD growth of these materials.

E’’ Raman Mode in Thermal Strain-Fractured CVD-MoS2

“…Figure 2b displays a representative OM image of OF-MoS2, showing that fracture only occurs along the grain boundaries between single crystalline MoS2 domains due to their higher chemical reactivity. This is similar to the case of WS2 annealing under ambient condition [23]. The local AFM image in Figure 2c shows the edge of single layered OF-MoS2 in the flat and smooth zigzag facets instead of nanoscolls.…”

“…Thus, thermal strain cannot be released sufficiently during the fast quenching process at the end of CVD synthesis process. Apparently, these behaviors differ from oxidation-induced intergranular fracture behaviors in MoS2 [23,24]. The inset height profile along It is well known that the amount of catalytic active sites along the edges of MoS 2 is one of the key factors for the hydrogen evolution reaction (HER).…”

Engineering the Electronic Properties of Two‐Dimensional Transition Metal Dichalcogenides by Introducing Mirror Twin Boundaries

“…[14] Alternatively, oxidation can be used to make the grain boundaries easier to distinguish under atomic force microscopy, scanning electron microscopy, or optical microscopy. [22,23] Furthermore, they can be found by forming a bilayer structure with another layer of the same material, since the PL wavelength is sensitive to the relative orientations of the two layers. [5] These methods provide quick and efficient ways to locate GBs and assess their concentration, but offer very little information about the type of GB.…”

Vertically Oriented Arrays of ReS2 Nanosheets for Electrochemical Energy Storage and Electrocatalysis

“…[8][9][10][11][12][13][14][15][16][17][18][19] Here we report that while MoS 2 and WS 2 films tend to grow parallel to the substrate, under similar CVD growth conditions, ReS 2 nanosheets show a remarkable tendency to bend and stand vertically on the growth substrate. In fact, the propensity for ReS 2 to orient itself perpendicular to the growth substrate is independent of the substrate material used or the packing density (i.e.…”

“…This approach has been used in the literature [8][9][10][11][12][13][14][15][16][17][18][19] to produce a variety of monolayer TMD materials including MoS 2 , WS 2 , ReS 2 , MoSe 2 , and WSe 2 . Supplementary Figure S1 shows typical monolayer WS 2 films grown by this approach in our Lab (the TMD growth is always parallel to the growth substrate).…”