Structural Engineering for Controlling Thermal and Electrical Properties of Carbon Nanosheet and Its Application to Electronic Devices

Abstract

Carbon nanosheet (CNS), which has a structure similar to that of graphene, has recently received attention because of advantages in its synthetic method, such as a catalyst- and transfer-free process, despite having more structural defects compared to graphene grown by chemical vapor deposition (CVD). In previous studies, CNS has controlled its structural, electrical, and optical properties by modulating solid precursors, such as polyacrylonitrile (PAN), polymers of intrinsic microporosity (PIM), pitch, and polyethylene, and has been applied as a transparent conductive electrode in organic photovoltaics (OPV). However, it remains necessary to improve various characteristics of CNS for realization of its practical application while maintaining the advantages of its synthetic method. I introduce structural engineering approaches that can control thermal and electrical properties of CNSs to realize their practical application in electronic devices. Chapter 1 describes the background and objective of this dissertation. The research presented in chapter 2 is focused on structural engineering to improve thermal conduc-tivity of CNS as a heat sink material. The thermal conductivity of CNS was calculated by using optothermal Raman measurement for the first time. As the thickness of CNS decreased from 23 to 1.4 nm, the thermal conductivity of CNS increased from 40 to 100 W/m•K. The systematic structural analysis of CNS, including XPS, Raman, and TEM, revealed that the ratio of sp2- and sp3-bonded C in the CNS decreased with in-creasing thickness of CNS. Overall, it can be confirmed that the thickness and C bond-ing configuration of CNS closely influence heat transfer in CNS. Based on these results, in order to improve thermal conductivity of CNS, structural engineering is required in the direction of fabricating thin and high sp2/sp3 C CNS. To do this, structural engineer-ing of CNS was attempted by controlling carbonization holding time. CNS with self-emerged graphitic carbon dots was fabricated by inducing rearrangement of C atoms. From the structural engineering, the CNS exhibited the enhanced thermal conductivity of ~540 W/m•K near room temperature. In addition, when the CNS was applied to heat evaluation devices as the heat sink material, the devices showed temperature cooling effect of ~14 °C compared to the case without the CNS. The research in chapter 3 presents morphological engineering for modulat-ing electrical properties of CNS. The main strategy is the phase-separation phenomenon of immiscible polymer blends. Direct patterning into mesh-type CNS can be realized by adjusting control factors of polymer blends. In particular, this chapter is divided into two main parts depending on mesh size as follows: (3.1) carbon nanomesh (CNM) with nanometer-sized holes and (3.2) micromesh-CNS (M-CNS) with micrometer-sized holes. Chapter 3.1 introduces the direct patterning approach of graphene-related mate-rials by using the phase-separated PAN/poly(methyl methacrylate) (PMMA) blends. From the direct patterning, CNMs with various neck widths less than 100 nm were ob-tained by controlling the mixing ratio of PAN to PMMA. Interestingly, gate-dependent ambipolar transport behavior was observed even though the CNM exhibited imperfect crystallinity due to the catalyst-free preparation. Based on the systematic investigation of the temperature-dependent electrical transport of the CNM-based FET, it was demonstrated that the charge conduction in CNMs is dominated by variable range hop-ping between localization states. Chapter 3.2 suggests morphological engineering for improving efficiency of the CNS electrode based-OPV. To improve the high sheet resistance and low transmit-tance of conventional CNS electrodes, thick CNS with a micrometer-sized hole array (i.e. M-CNS) was fabricated using the phase-separated PAN/PMMA blends. The mi-crometer-sized holes prevented the decrease of transmittance of CNS due to the in-creased thickness compared to the conventional CNS. The resulting M-CNS electrode showed ~50% improved sheet resistance without loss of transmittance and device effi-ciency of 2.07%, which is about 18% higher than that of OPV based on the conven-tional CNS electrode. This dissertation offers effective and facile structural engineering approach-es that can control the thermal and electrical properties of the CNS. This is expected to enable more practical applications of CNS to electronic devices.