Nanowire and graphene-based vapor sensors for electronic nose applications

The development of a sophisticated electronic nose (E-nose) system would open vast markets spanning a wide variety of applications. Despite several decades of intensive research, the goal of creating such an E-nose system has yet to be achieved. The major difficulty is to create a sensor array with the extremely large chemical diversity and massive parallelism that is characteristic of mammalian olfactory systems. In this thesis, we have moved one step closer to this goal by synthesizing, characterizing and integrating heterogeneous nanowires on chip. The work presented in this thesis is divided into three parts with the first part focused on single nanowire chemical gas sensors, the second part on heterogeneous nanowire integration and the third part on graphene vapor sensors. In the first part of work, we synthesized single nanowire sensors of conducting polymers (PEDOT/PSS) and metal oxides (TiO2) using electrochemical-templating method. The electronic and gas-sensing properties including thermal effects of these nanowires were also investigated. This part of work laid a solid foundation for the nanowire integration in the second part of work. The second part started with analysis of the challenges to develop a sophisticated E-nose system. To address these challenges, we proposed and demonstrated a novel electrochemical approach by integrating silver, gold, tin and TiO2 two- and multiple terminal nanowires on chip. This approach has a potential to realize such a sophisticated E-nose system. The third part of work is to explore properties of graphene sensors. We found that the contamination layer left by conventional nanolithographic processing both degraded the electronic properties of the graphene and masked graphene's intrinsic sensor responses. The contamination layer chemically doped the graphene, enhanced carrier scattering, and acted as an absorbent layer that concentrated analyte molecules at the graphene surface, thereby enhancing the sensor response. We demonstrated a cleaning process that verifiably removed the contamination on the device structure and allowed the intrinsic chemical responses of graphene to be measured