New Approach to Unveiling Individual Atomic Layers of 2D Materials and Their Heterostructures

Abstract

Visualization of the chemical structures of two-dimensional (2D) materials and their interfaces at the virtually atomic scale is an imperative step toward devising highly efficient ultrathin optoelectronic devices. Herein, we demonstrate a universal method featuring time-of-flight secondary ion mass spectrometry (ToF-SIMS), coupled with the structure simplicity of 2D materials, as a versatile tool to reveal the vertical atomic layers of various two-dimensional (2D) materials including graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDs). We demonstrated that the vertical atomic layers of those 2D materials can be unveiled layer-by-layer using a strategy of ToF-SIMS three-dimensional (3D) analysis developed in this work. Moreover, we found that the extreme surface sensitivity and chemical specificity of ToF-SIMS also enables the examination of the lateral uniformity of 2D materials. During this process, we first removed interference of adsorbed organic contamination by annealing, which allows the high quality signals specific to the 2D materials which were accumulated along the sputtering depth. The accumulated signal was found to be linearly proportional to the number of atomic layers in the z-direction, thus providing a chemical intensity contrast that can be directly used to reveal the number of atomic layers in the vertical direction. For the case of CVD-grown graphene, up to six individual adlayers have been resolved. The technique developed in this work is substrate-independent and can be directly applied to the grown substrate, which circumvents the time-consuming transfer process and avoids any potential hazard to the delicate 2D materials. Our approach provides an efficient characterization tool for analyzing the 2D materials and their heterostructures, with extensive implications toward preparation of high-quality 2D crystals for atomically thin optoelectronic devices.

One-Pot Selective Epitaxial Growth of Large WS2/MoS2 Lateral and Vertical Heterostructures

“…To investigate the selective growth mechanism of lateral and vertical heterostructures, ToF-SIMS depth profiling and chemical maps were employed to reveal the surface and vertical chemical composition of the MoS 2 monolayer of different WS 2 /MoS 2 heterostructures. , An as-obtained single MoS 2 monolayer was also analyzed by ToF-SIMS, and the depth profile in Figure a exhibits the knock-on sequence of OH – , S – , Mo – , S – , and Al – (sapphire substrate) from the top to the bottom, confirming the MoS 2 –OH bilayer structure. As shown in the lateral heterostructures (Figure b), the corresponding depth profile of the inner MoS 2 part shows no change with the MoS 2 –OH bilayer, while the outer part of the WS 2 monolayer shows a distinct difference.…”

Segregation-Enhanced Epitaxy of Borophene on Ir(111) by Thermal Decomposition of Borazine

“…ToF-SIMS has previously been demonstrated to be suitable for analyzing 2D materials at surfaces with monolayer sensitivity . Dual beam depth profiling via ToF-SIMS was carried out using a TOF.SIMS 5-100 (IONTOF).…”

“…ToF-SIMS has previously been demonstrated to be suitable for analyzing 2D materials at surfaces with monolayer sensitivity. 44 Dual beam depth profiling via ToF-SIMS was carried out using a TOF.SIMS 5-100 (IONTOF). Parameters were chosen in order to ensure high sputter rate ratios, low transient widths, and high vertical resolution.…”

Thermal Transport in Two-Dimensional Heterostructures

“…Benefitting from the progress of ingenious experimental methods, the synthesis of vdW heterostructures via self-assembly technology has achieved an unprecedented degree of control and accuracy, and provides new insights to explore structurefunction relationship (Frisenda et al, 2018) and to assemble complex devices (Abidi et al, 2018;Ding et al, 2018b) which are able to be tuned by stacking sequence, orientation, interlayer coupling, and external field. For example, graphene/hBN vdW heterostructures were prepared via an all-dry poly (methyl methacrylate) transfer process using polyvinylalcohol as the water-soluble sacrificial layer, which can avoid the polymer residual problem and prevent contamination from chemical solutions (Tien et al, 2016).…”