Synthesis, Characterization and Properties of Functional Nanomaterials: Metal Oxides and Layered Materials.

摘要

Nanomaterials have played an important role in the advances of modern technologies. By shrinking the materials to nanoscale, it not only provides higher packing density, but also changes their fundamental properties which can be utilized for different purposes. In my Ph.D. research, I and my colleagues focused on two kinds of nanomaterials: nanopatterned metal oxide dots, and two-dimensional layered materials.;Functional metal oxides have received much attention due to their rich varieties. However, literatures about nanopatterned metal oxides are scarce due to the difficulty in fabrication and property measurement. We combined the new patterning technique: Beam Pen Lithography, along with sol-gel deposition, to build up a process to make metal oxide nanopatterns with flexible control. Beam Pen Lithography renders flexible shape control, large-area patterning and nanoscale resolution. Sol-gel deposition provides large-area coverage and easy tuning of compositions. Although solution process is believed to easily result in undesired morphology upon drying process as seen in ink-jet printing, we showed that by controlling surface chemistry and solvents of sol-gel solution, we could turn this disadvantage into an advantage to make oxide nanodots smaller than the size defined by initial patterning. Several material systems were demonstrated, and more importantly, single crystalline epitaxial growth could be achieved. Photolithography was also used to make nanopatterns, and with the help from control of surface chemistry, nanodots with size much smaller than resolution limit of photolithography were achieved. Scanning probe microscope techniques as well as collective measurements were then used to study the properties of oxide nanodots.;Another class of nanomaterials we have studied, two-dimensional layered materials, is an emerging field since the discovery of graphene, and they are promising to make advances in many technological applications. In particular, we exploited their high surface-to-volume ratio to make gas sensors. We first studied gas sensing properties of graphene by using Hummer's method to make large size graphene sheets. We can further improve graphene's sensitivity by introducing more edge sites, or by adding zinc oxide nanoparticles. Other than graphene, molybdenum disulfide (MoS2) is another layered material with the advantage of having finite bandgap. We found out for MoS 2, five-layer device actually gave higher sensitivity than single-layer device, showing that surface-to-volume ratio is not the only important parameter in gas-sensing. We also utilized its semiconducting nature to investigate gas sensing properties under gate-bias or light illumination, which substantially change sensitivity, selectivity, and recovery. These results show promising vision to replace traditional metal oxide sensors with such layered materials. At last, to study new layered materials more efficiently, we have developed vapor-transport based techniques and applied on iodide and chalcogenide layered material systems to make thin flakes with large lateral sizes. In-situ TEM was employed to directly observe changes in morphology and microstructure. Transport and optical properties were also measured.