Spatially Controlled Fabrication and Mechanisms of Atomically Thin Nanowell Patterns in Bilayer WS₂ Using in Situ High Temperature Electron Microscopy

Abstract

We show controlled production of atomically thin nanowells in bilayer WS2 using an in situ heating holder combined with a focused electron beam in a scanning transmission electron microscope (STEM). We systematically study the formation and evolvement mechanism involved in removing a single layer of WS2 within a bilayer region with 2 nm accuracy in location and without punching through to the other layer to create a hole. Best results are found when using a high temperature of 800 °C, because it enables thermally activated atomic migration and eliminates the interference from surface carbon contamination. We demonstrate precise control over spatial distributions with 5 nm accuracy of patterning and the width of nanowells adjustable by dose-dependent parameters. The mechanism of removing a monolayer of WS2 within a bilayer region is different than removing equivalent sections in a monolayer film due to the van der Waals interaction of the underlying remaining layer in the bilayer system that stabilizes the excess W atom stoichiometry within the edges of the nanowell structure and facilitates expansion. This study offers insights for the nanoengineering of nanowells in two-dimensional (2D) transitional metal dichalcogenides (TMDs), which could hold potential as selective traps to localize 2D reactions in molecules and ions, underpinning the broader utilization of 2D material membranes.

In Situ TEM Investigations on the Controlled Phase Transformation of Vertically Aligned WS2 at Designated Locations on an Atomic Scale

“…In order to find out if there is an intermediate structure between layered WS 2 and nanocrystalline W, we have carefully analyzed the initial stage of structural transformation of WS 2 . Detailed results suggest that nonlayered crystal, such as nonlayer WS crystals reported in the previous literature, was not observed here. This could be because of the high-temperature environment used in the experiment, which is helpful to the rapid migration of atoms as suggested by Warner .…”

“…Detailed analyses indicate that the vertically aligned WS 2 is transformed into horizontal WS 2 with small domain sizes and high degree of disorder. 32 Figure S4c shows the inverse FFT image of another area marked by the pale pink virtual box in Figure S4a, which suggests the formation of amorphous W. 36 In addition, it can be noted that there is no new diffraction ring or pattern shown in the FFT image (bottom of Figure 5b), which implies the amorphous region transformed from vertical WS 2 may be amorphous W. At the time of 572 s (Figure 5c), the horizontal WS 2 NCs with a high degree of disorder begins to crystallize, and the lattice spacing of newly formed crystal is ∼0.22 nm, corresponding to the (110) plane of bcc W. Moreover, a new and weak diffraction ring emerges in the FFT pattern (bottom of Figure 5c), which can be indexed as the (110) plane of W as well. As time goes on, more and more vertically oriented WS 2 layers disappear, such as the areas circled by the orange and light blue virtual frames.…”

Selective Vertical and Horizontal Growth of 2D WS2 Revealed by In Situ Thermolysis using Transmission Electron Microscopy

“…[63] Electron beam-induced crystallization and reconstruction of amorphous MoS 2 into crystalline MoS 2 were also studied through atomic scale in situ TEM. [64] In situ TEM was employed on WS 2 layers as well, through the fabrication of atomically thin nanowell patterns [28] and atomic-scale localized thinning of 2D WS 2 layers. [55] Vertical layers of WS 2 were grown in an in situ TEM experiment through thermolysis of K 2 WS 4 precursor, [65] but a detailed understanding of the growth of horizontal WS 2 layers was not reported yet and is still lacking.…”

“…The conventional CVD method is the most commonly used technique to grow large area, pure, and monolayer thin 2D TMDs, including WS 2 layers grown on a SiO 2 /Si substrate. [11,[16][17][18]28,29] Growth of WS 2 on the amorphous and inert SiO 2 /Si substrate usually forms randomly oriented domains with high angle grain boundaries, [14] having low quality and uncontrolled structure. [11][12][13]16,30] Also, in order to transfer WS 2 materials grown via CVD from an inert substrate, it has to go through a substrate etching process by using acidic or basic solutions.…”

Step‐By‐Step Atomic Insights into Structural Reordering from 2D to 3D MoS2

“…In recent times, in situ techniques combining elevated temperatures and electron irradiation have also been explored for nanopatterning of 2D structures (e.g., refs. [22][23][24][25][26][27][28]). While mono-and bilayer TMDs have been among the studied structures up to temperatures of 1200 °C, [29,30] their heterostructures and observations at higher temperatures have received less attention.…”