Increase in graphitization and electrical conductivity of glassy carbon nanowires by rapid thermal annealing

Abstract

Glassy carbon (GC) exhibits numerous desirable properties such as high thermal and chemical stabilities, good hardness, and good thermal and electrical conductivities. Moreover, GC can be manufactured into micro-/nanostructures through the versatile microfabrication technology, or carbon-micro electromechanical systems, which includes polymer patterning and pyrolysis. However, despite these advantages, there are growing demands for enhancing the electrical conductivity of GC, so that it can compensate or be substituted for other carbon allotropes such as graphite, carbon nanotubes, and graphene. In this study, we demonstrated that simple rapid thermal annealing (RTA) can dramatically enhance the electrical conductivity of pyrolyzed GC nanostructures by ∼ 300%. In this research, two different architectures of 1D carbon nanostructures such as a suspended nanowire that was separated from the substrate with a fixed distance and a substrate-bound nanowire were fabricated using conventional UV-lithography and pyrolysis processes, and their conductivity enhancement behaviors via RTA were studied. After the RTA process, the carbon/oxygen content and G-/D-band intensity ratios, which are correlated to the electrical conductivity, were enhanced, depending on the pyrolysis temperature. GC structures pyrolyzed at relatively low temperatures became more electrically conductive after the RTA process owing to their relatively higher oxygen content. This is because carbon atoms interconnected to oxygen atoms tend to align more readily than those corresponding to other carbon compositions because of the graphene healing mechanism. In addition, the architecture of the carbon nanostructures (i.e., whether they were suspended or substrate-bound nanowires) influenced the RTA-induced increase in electrical conductivity; the former showed a greater increase in electrical conductivity owing to its larger portion of well-aligned carbon atoms at the surface compared to the latter carbon structure. This is because graphitization is initiated on the surface and then proceeds to the carbon core in the heat treatment. In addition, tensile stress generated only at the suspended carbon nanowires during the pyrolysis process is assumed to enhance further the electrical conductivity via RTA. For instance, the electrical conductivity of the suspended carbon nanowires formed by pyrolysis at 600 °C was enhanced to ∼59,000 S/m after the RTA process.