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

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.