Effect of hydrogen on electronic properties of two dimensional Graphene, Graphene oxide and MoS2

Abstract

Two-dimensional(2D) materials have attracted great scientific attention due to their extraordinary and fascinating properties. Graphene and transition metal dichalcogenides such as MoS2, have shown promising candidate for gas sensing, flexible electronics, energy harvesting due to their high mechanical properties, high surface-to-volume ratio, low noise and sensitivity of electronic properties to the changes in the surroundings. It has been intensively investigated on the interaction between hydrogen and 2D materials both experimentally and theoretically. Therefore, it is required that studies of the interaction between hydrogen and 2D materials have been increasingly required due to the indispensable modification of the electronic structure of 2D materials for device applications and the possibility of using 2D materials as hydrogen storage materials. In this dissertation, we will describe the effect hydrogen on the electronic properties of 2D materials such as Graphene, Graphene Oxide, MoS2 using electrical conductivity, thermoelectric power (TEP), and Raman spectroscopy. The first part of this thesis describes presenting a simple method to fabricate p-n junction in a single layer graphene by means of the selective H2 exposure. This is achieved by the fabrication of poly methyl methacrylate (PMMA) window on the half region of the graphene. The gate voltage (VG)- dependent resistance of single layer graphene is measured as a function of H2 (12 bar) exposure time at 350 K. As hydrogen exposed sample, n-doping of the window region shifts to the negative VG region prominently compared with that of the PMMA-covered region. The temporal evolution of Dirac point both PMMA region and window region follows first order Langmuir adsorption model. Consequently, a single graphene p-n junction is obtained by measuring the VG-dependent resistance of the whole graphene region. The second part of this thesis describe the electrical transport properties of single layer reduced graphene oxide(RGO) and its hydrogenation. The single layer RGO is obtained by bubble deposition method and thermal reduction. The structure and reduction efficiency is confirmed by X-ray photoelectron spectroscopy and Raman spectroscopy, respectively. The RGO contains 82 % of C=C and C-C species and have 2 nm of defect distance. The transfer characteristic of RGO shows ambipolar transport for temperature range (10 K < T < 300 K). The conductance of Ohmic regime is rapidly decrease as decrease temperature which means a sign of variable range hopping. The Efros-Skhlovskii variable range hopping (ES-VRH) is a dominant charge transport (T < 70 K). With increase temperature (T > 70 K), the charge transport become 2D-VRH due to absence of coulomb gap. The ES-VRH is additionally confirmed by electric field dependent conductance of non-Ohmic regime at low temperature. The TEP exhibits that the dominant charge carrier in RGO is switched at charge neutrality point. The Seebeck coefficient is proportional to T1/3 (50 K < T < 300 K) suggesting a 2D VRH conduction. Hydrogenation of RGO shows n-type doing resulting from hydrogen adsorption. Maximum resistance of transfer curve is reduced by improvement of reduction efficiency. The third part of this thesis will introduce the effect of hydrogen on geometric and electronic structure of single layer MoS2. MoS2 is well known catalyst for hydrodesulfurization process and the hydrogen adsorbed plays an important role in its activation. So that, investigations of the interaction between molecular hydrogen and molybdenum disulfide have been increasingly demanded for the understanding of the HDS process, especially the hydrogen adsorption on MoS2 and creation of sulfur vacancies in MoS2. Electrical transport properties were measured as a function of time at 350 K and 12 bar of hydrogen atmosphere. Upon to hydrogen exposure, the threshold voltage of MoS2 is shifted toward negative bias, which indicates n-type doing by thiol bonding and sulfur vacancy. The temporal evolution of threshold voltage follows double exponential first adsorption model, which consist of thiol bonding term and sulfur vacancy term. The mobility has increase as exposure to hydrogen, which result from screening of long range scatterer by intercalated hydrogen between MoS2 and substrate. The n-type doping of MoS2 by hydrogen is confirmed by TEP measurement. The TEP curve is shifted toward negative gate bias region likewise transfer characteristics. We observe the red shift of the two prominent peaks in Raman spectroscopy by electron doping and strain.