Synthesis,Electrical And Optical Properties Of Graphene

Graphene is a novel two dimensional material that is consisted of one-atom-thick planner sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. Because of its unique mechanical electronic and optical properties, especially the giant potential electronic device applications, graphene has attracted considerable interest for physical fundamental studies, material science research and also the commercial industry since graphene was first discovered in2004, therefore, high quality graphene samples are widely desired for both fundamental experiment and industry applications. However, the traditional micromechanical exfoliation method can only prepare micron size graphene, with this method the productivity is very low, and has an expensive cost, this problem highly limited the progress in graphene research and application, therefore, how to prepare high quality and large scale graphene, has become a hot topic in the research society and also a very important subject for both the fundamental studies and industrial application.In this paper we described the synthesis of monolayer and multilayer graphene by chemical vapor deposition, compared the surface morphology of graphene prepared by different transfer methods, finally find the most efficient parameter to achieve high quality graphene. Then we studied the transport and optical properties of both the monolayer and bilayer graphene, we fabricated a graphene based broadband optical modulator and observed several unique phenomenon from the electronically dope graphene device. The content, results is listed as follows:1. Graphene synthesis by chemical vapor deposition. We grew the multi-layer graphene with Ni film and grew the monolayer graphene with copper foil; respectively, in the Ni CVD process, we could control the graphene layer thickness by adjusting the cooling rate, but the graphene film we got is patched by small graphene flakes. With the copper CVD process, we could got high quality and large scale monolayer graphene, and because of the special growth mechanism, the graphene grown on copper is self-limited at monolayer.2. Comparison and analysition between different graphene transfer methods. We systematically studied the dry transfer method with PDMS as protect layer and the wet transfer method with PMMA or PS as protect layer in the graphene transfer technique, the experimental results prove that using the liquid phase PDMS to fabricate the protect layer could improve the sample quality when transfer with dry method, and with PS as protect layer and chloroform as solvent could get the cleanest sample transferred by wet method.3. Studied the electrical and optical properties of monolayer graphene. By applying a bias voltage to electrically dope the graphene, the graphene’s Fermi level could be changed, we could tune the Fermi level to0.9eV with ion gel as bias electrode, By transfer the graphene onto a waveguide to increase the optical interaction distance, we fabricated an graphene based broadband optical modulator with a working frequency as high as1GHz; we controlled the inelastic light scattering pathways by tuning the bias voltage and found the transition pathways interfered with each other, we also studied the polarization dependence of the hot luminescence in graphene in the highly doped graphene.4. Studied the electrical and optical properties of bilayer graphene. By applying dual gate voltage through the bilayer graphene, we could tune the carrier density and band gap independently. We mainly studied the tunable band gap in bilayer graphene by controlling the average electric field in between the graphene layers, with this structure we could open a band gap as high as250meV, and we also studied the fano resonance between the phonon and electron-hole excitations when the bilayer’s band gap is close to the phonon vibration energy.