UC Riverside

Two-dimensional (2D) nanomaterials, such as graphene, hold great potential for next-generation electronics owing to their unique properties such as high carrier mobility, fast response time and high transparency. However, the absence of an electrical band gap in graphene limits its applications as a field-effect transistor (FET). To overcome this limitation, other 2D nanomaterials such as transitional metal dichalcogenides (TMDs) have been studied and attracted significant research interests. TMDs consist of 2D stacked layers of covalently bonded transition metal and dichalcogenide atoms arranged in a hexagonal lattice where adjacent layers are held together by relatively weak van der Waals forces. MoS2 is one such example of TMDs that exhibits a direct band gap of 1.8 eV for single-layer MoS2 and an indirect band gap of 1.2 eV for multiple-layer structure. FETs based on single-layer MoS2 exhibit an on/off ratio of > 108 at room temperature. However, unlike graphene-based FETs which typically exhibit a high mobility of 5,000 - 10,000 cm2V-1s-1, MoS2-based FETs show a much lower mobility of ~100 -200 cm2V-1s-1.

One possible way of tailoring the electrical properties of a material is by stacking individual layers of different 2D materials to form a vertical heterostructure. In this dissertation, we explored the electrical properties of graphene - MoS2 hybrid films synthesized by direct growth of single-layer and multi-layer MoS2 on graphene using chemical vapor deposition (CVD). By taking advantage of the high charge carrier mobility of graphene and the finite band gap of MoS2, we fabricate FETs which consist of graphene - MoS2 hybrid as the conducting channel. We explore the potential of this new hybrid structure for electro-chemical sensing by testing its selectivity and sensitivity for different volatile organic compounds (VOCs). In addition, we study an optoelectronic sensor using graphene as electrodes and MoS2 as conducting/sensing channel. The red-light with matching energy to band gap in single-layer MoS2 increases electron concentration in MoS2 and hence promotes the current in the sensor. The optoelectronic sensor was used for NO2 detection. In addition, another heterostructure of MoS2 and metal oxide was also investigated for DNA sensor. Finally, we studied the potentials of MoS2 and graphene heterostructure for a potential flexible and on-skin applications for NO2 and NO detection.