Enhancing thermoelectric performance of graphene through nanostructuring

© 2017 Dr. Md Sharafat HossainThe field of thermoelectrics has the potential to solve many problems that are predominant in the electronics industry. However, due to low-efficiency, high material cost, and toxicity, this field is yet to meet its expectations. On another, graphene, a two-dimensional allotrope of carbon has attracted a great deal of attention due to its unique electronic, thermal and mechanical properties. In this work, we explore the suitability of planar materials like graphene for thermoelectric application and propose techniques that have the potential to address the issues that are holding back thermoelectrics from large scale applications. Due to the high thermal conductivity and lack of electrical band gap graphene has not been thoroughly investigated for thermoelectric application. In this work, the inherent properties of graphene are modified through nanostructuring in order to make it suitable for the thermoelectric application. First, the graphene sheet is nanostructured into graphene nanoribbon (GNR). The focus is given to the electronic properties that affect the thermoelectric performance. Graphene nano-ribbon with different width and array combinations are analyzed. To explain the experimental results, a model that considers the effect of scattering mechanism and random charge carrier fluctuations is proposed. Based on the model, a route to further enhance the thermoelectric properties of graphene is presented. In the next part of this thesis, further nanostructuring of graphene nano-ribbon is investigated. Simulation is carried out for GNR with pores. Pores impede the phonon transmission while electron transmission continues to take place at the edges. There have been several reports on utilizing GNRs with pores for thermoelectric application, but the design methodology of such structures has not been thoroughly investigated. In this work, the effect of pore dimensions on thermoelectric parameters is studied through quantum mechanical simulations. The results report a surprising relation between pore width and TE parameters which is later explained using physical insights. Similarly, another approach of nano-structuring GNRs through the introduction of break junctions is investigated. The motivation behind this work is to utilize the tunneling mechanism that is observed in GNR with break junction, in order to achieve delta like transmission spectra. Moreover, the nano-break impedes phonon transmission. As a result, the overall thermoelectric performance of the device is enhanced significantly. Finally, a novel approach of bio-sensing based on the Seebeck coefficient measurement of graphene is proposed and validated using quantum mechanical simulation. This proof of concept study indicates the wide range of applications that are enabled through exploiting thermoelectric properties of planar devices. Overall, the techniques and insights presented throughout the thesis are based on graphene but can be applied and investigated on other two-dimensional materials as well