A systematic exfoliation technique for isolating large and pristine samples of 2D materials

The device conceptualization and proof-of-concept testing of two-dimensional (2D) materials are performed with their pristine forms that are obtained through the micromechanical cleaving of bulk natural crystals, i.e., the so-called Scotch tape method. However, obtaining large 2D sheets is very difficult and time consuming. We developed a systematic exfoliation technique for producing sub-millimeter-sized (the largest lateral dimension ever reported) pristine 2D sheets with high throughput. It requires the treatment of both the bulk crystal and receiving substrate. Contrary to the conventional Scotch tape technique that involves the repeated folding and unfolding of an adhesive tape, the flake is stamped onto an adhesive tape to preserve the lateral size of the bulk crystal, to improve the surface flatness, and to reduce the amount of residue on the surface of the samples. When applied to graphene, the method produced monolayer and few layer graphene samples that were several hundreds of microns in length. Surprisingly, the biggest monolayer graphene sample of 367 mu m in length was easily produced. The technique was also applied to produce pristine MoS2 and phosphorene sheets of about 45 mu m and 95 mu m in length, respectively.