Engineering of the structures of carbon nanotubes and graphene-based materials for electronic applications
Date of Issue2015
School of Chemical and Biomedical Engineering
Carbon nanotubes and graphene have been studied extensively for transparent conductors as replacement of ITO, high performance transistors as replacement of silicon and other electronic applications. The current graphene-based transparent conductors face three main problems: 1) high cost and limited size for the epitaxial grown graphene films; 2) low conductivity for chemically exfoliated graphene films due to the defects on graphene sheets and sheet-sheet junction resistance; 3) instability of doping. The junction resistance and instability of doping causes troubles for the SWNT based transparent conductors as well. To address the above problems, several types of solution-based flexible transparent conductors have been developed in this thesis, including reduced graphene oxide films, hybrid films of self-assembled silver network with unsorted SWNTs, sorted metallic SWNTs, and CVD-grown graphene. While all the hybrid films show similarly excellent performance in terms of sheet resistance, transmittance, stability in ambient conditions and bendability, unsorted SWNT-Ag hybrid films (R; --5.8 n sq" at T --83.7%) are more suitable for industrial applications due to the all solution-based scalable process and low cost. The application of the hybrid films as photovoltaic electrodes is also demonstrated. In addition, reduced graphene oxide (RGO) thin films were applied as platforms for gold nanostructure synthesis. By different reduction methods, viz. hydrazine vapor reduction, H2 annealing, and ethanol/He reduction, the RGO reduction degree was controlled from low, to moderate, and to high, respectively. By simply immersing the RGO thin films into Au3 + solution, Au nanoparticles, long Au nanowires and semicircularshaped Au nanoplates were produced on slightly-, moderately-, and highly-reduced graphene oxide surfaces, respectively, without the need of any templating agent. The growth of various gold nanostructures is related to size and distribution of the _Sp2 / _Sp3 domains of reduced graphene oxide with different reduction degree. Semiconducting SWNTs are considered to be the most promising material to replace single crystalline silicon in high performance transistors. However, all current methods for semiconducting/metallic SWNT sorting suffer from either high cost or low efficiency. In this thesis, we studied the sorting of large diameter arc-discharge SWNTs into semiconducting and metallic fractions with high purity via a simple repulsion chromatography method using non-ionic surfactants. Semiconducting SWNTs with >99.9% purity or metallic SWNTs with >90% purity can be obtained after a single-pass separation though functionalized agarose beads. Network field-effect transistor (FET) devices were fabricated with the sorted semi-SWNTs, and the performance of a typical network FET device reaches on/off ratio 104 - 105 and mobility 15-30 cm2 V-IS-I. The method which is simple, fast, cost-effective and easy to scale up for industrial applications is envisaged to reduce the price of sorted carbon nanotubes to a fraction of today's price.