Chemical Sensing in Two-Dimensional (2D) Nanomaterials

The development of highly sensitive portable chemical sensors is of a fundamental importance in environmental monitoring, medical diagnostics, and other areas. In this respect, two-dimensional (2D) nanomaterials have attracted great attention due to their excellent electrical and structural properties. As the main representative of the 2D materials, graphene has exhibited great performance in a wide range of sensing applications over the past decade. Particularly, it is unraveled that structural defects play a crucial role on the sensing performance of the graphene-based sensing devices. In this thesis, the roles of internal and external structural defects on the sensing performance of graphene-based chemical sensors have been elucidated. In the beginning, it is shown that the sensitivity (in terms of modulation in electrical conductivity) of pristine graphene chemical field-effect transistors (chemFETs) is not necessarily intrinsic to graphene, but rather it is facilitated by external defects in the insulating substrate, which can modulate the electronic properties of graphene. We disclose a mixing effect caused by partial overlap of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of adsorbed gas molecules to explain graphene’s ability to detect adsorbed molecules. In the next phase, graphene grain boundaries are extensively studied as they are known to desirably alter the electronic properties and chemical reactivity of graphene structures. It is observed that an isolated graphene grain boundary has ~300 times higher sensitivity to the adsorbed gas molecules than a single crystalline graphene grain. Electronic structure and transport modeling reveal that the ultra-sensitivity in grain boundaries is caused by a synergetic combination of gas molecules accumulation at grain boundary, together with the existence of a sharp onset energy in the transmission spectrum of its conduction channels. Moving beyond the graphene-based devices, several other members of 2D nanomaterials are also tested for their potential in chemical sensing applications. In particular, the sensing characteristics of black phosphorus (BP) films of stacked atomic layers made by liquid exfoliation and vacuum filtration are studied. Interestingly, BP films are observed to exhibit an ultra-sensitive and selective response towards humid air with a trace-level detection capability and a negligible drift over time. Lifetime analysis predicts that BP film sensors can function stably for several years in highly humid environments. This study opens up the route for utilizing BP stacked films in many applications such as energy generation/storage systems, electrocatalysis, and chemical/bio-sensing. It is believed that the family of 2D materials consists of numerous unexplored members that can revolutionize the chemical and biological sensing industry.