Fabrication and characterization of metal-cluster decorated single layer graphene

This work is devoted to investigating the changes of electronic properties of graphene devices by introducing defect states, such as metal clusters deposited on graphene device surfaces. By measuring the charge carriers transport properties of the graphene devices, we have obtained some interesting results, such as Fermi level shifts, weak localization and photocurrent which provide us better understanding on the interaction between metal clusters and graphene, charge carriers transport mechanism in graphene. After metal clusters deposition on graphene surface, a Fermi level shift is detected. I firstly explain with a work function comparison (this is a rough interpretation), then the details are given by density functional theory(DFT), which is a very popular and successful computation method to analyze the ground state of a interacting system of fermions. They both indicate graphene is p doped when Au clusters are deposited on it, that is, Fermi level shifts to lower energy than the Dirac point. DFT does not only tells us whether Au favors to exist as clusters not particles on graphene, but also shows the details of the bond between Au and graphene. When metal clusters deposit on graphene, two main effects is formed: charge (electron) transfer between graphene and metal clusters, and metal clusters as scattering centers. The charge (electron) transfer direction determines whether it is a p doped or n doped. These two effects actually can be understood from the bond formed between graphene and metal. The measurement of magnetoresistance at low temperature (about 2K) with a PPMS (Physics Property Measurement System) always shows weak localization after Au clusters were deposited on graphene. Actually, in theoretical prediction weak localization is strongly suppressed in intrinsic graphene where no impurities, defects, adsorptions and so on exist. Actually, intrinsic graphene even has anti-weak localization which means conductivity has a correction of increase so if a magnetic field is applied to the device, the conductivity will decrease. The reasons of this are the chirality of charge carries and an additional quantum number, the pseudospin, in graphene, which originate from its honeycomb lattice structure. Because of chirality nature of charge carrier a Berry phase π adds to the phase of wave, this causes destructive quantum interference. Metal clusters deposited on graphene can restore weak localization because they can increase elastic intervalley and intravalley scattering. The both scatterings can break chiral symmetry to suppress the Berry phase, so that the weak localization can be restored. Finally, a phenomenon that continuous charge carrier transfer from graphene to Au clusters when light is applied onto devices is observed. The continuous charge carrier transfer will actually form a current. Generally, the generation of photocurrent is attributed to the presence of a local, in-plane electric field within the device. Charged impurities on the graphene can form a local potential step which is modeled by an interface dipole to shift locally the Fermi level. That is the reason why photocurrent occurs between metal clusters and graphene