Raman analysis of bilayer graphene film prepared on commercial Cu(0.5 at% Ni) foil

This study reports the Raman analysis of bilayer graphene films prepared on commercial dilute Cu(0.5 at% Ni) foils using atmospheric pressure chemical vapor deposition. A bilayer graphene film obtained on Cu foil is known to have small areas of bilayer (islands) with a significant fraction of non‐Bernal stacking, while that obtained on Cu/Ni is known to grow over a large area with Bernal stacking. In the Raman optical microscope images, a wafer‐scale monolayer and large‐area bilayer graphene films were distinguished and confirmed with Raman spectra intensities ratios of 2D to G peaks. The large‐area part of bilayer graphene film obtained was assisted by Ni surface segregation because Ni has higher methane decomposition rate and carbon solubility compared with Cu. The Raman data suggest a Bernal stacking order in the prepared bilayer graphene film. A four‐point probe sheet resistance of graphene films confirmed a bilayer graphene film sheet resistance distinguished from that of monolayer graphene. A relatively higher Ni surface concentration in Cu(0.5 at% Ni) foil was confirmed with time‐of‐flight secondary ion mass spectrometry. The inhomogeneous distribution of Ni in a foil and the diverse crystallographic surface of a foil (confirmed with proton‐induced X‐ray emission and electron backscatter diffraction, respectively) could be a reason for incomplete wafer‐scale bilayer graphene film. The Ni surface segregation in dilute Cu(0.5 at% Ni) foil has a potential to impact on atmospheric pressure chemical vapor deposition growth of large‐area bilayer graphene film. Copyright © 2015 John Wiley & Sons, Ltd.     

Band structure of Au layers on the Ru(0001) and graphene/Ru(0001) surfaces

The electronic structures of Au monolayers on the Ru(0001) and graphene-coated Ru(0001) surfaces have been calculated by DFT method using the supercell (repeated-slab) approach. The local densities of states (LDOS) and band structures of the monolayer and bilayer Au films adsorbed on the graphene/Ru(0001) and those of free hexagonal Au layers are found to be very similar. This result indicates that the monolayer graphene almost completely screens the Au layers from the Ru(0001) substrate surface, so that electronic properties of Au films adsorbed on graphene are determined predominantly by the electronic structure of the Au adlayers, essentially independent on the electronic structure of the substrate surface.     

Electronic transport of bilayer graphene with asymmetry line defects

In this paper,we study the quantum properties of a bilayer graphene with(asymmetry) line defects.The localized states are found around the line defects.Thus,the line defects on one certain layer of the bilayer graphene can lead to an electric transport channel.By adding a bias potential along the direction of the line defects,we calculate the electric conductivity of bilayer graphene with line defects using the Landauer-Biittiker theory,and show that the channel affects the electric conductivity remarkably by comparing the results with those in a perfect bilayer graphene.This one-dimensional line electric channel has the potential to be applied in nanotechnology engineering.     

1064nm纳秒激光对石墨烯的损伤效应研究

通过化学气相沉积法制备,并转移到基片得到1~3层石墨烯样品。利用霍尔效应及微区拉曼光谱测量,结合光学显微镜观察,分析了不同层数石墨烯在1064nm纳秒激光辐照下的损伤特性。实验发现1~3层石墨烯的激光损伤阈值依次降低,分别为:单层0.45J/cm2,2层0.34J/cm2,3层0.23J/cm2。激光强度超过阈值时,石墨烯薄膜电阻增大,载流子迁移率降低。通过光学显微镜观察发现局部区域破损,破损区域的拉曼光谱中1580cm-1左右的G峰和2700cm-1左右的2D峰高度比发生变化。实验结果表明1064nm纳秒激光辐照石墨烯主要为剥离作用。     

Assessment of bilayer silicene to probe as quantum spin and valley Hall effect

Silicene takes precedence over graphene due to its buckling type structure and strong spin orbit coupling. Motivated by these properties, we study the silicene bilayer in the presence of applied perpendicular electric field and intrinsic spin orbit coupling to probe as quantum spin/valley Hall effect. Using analytical approach, we calculate the spin Chern-number of bilayer silicene and then compare it with monolayer silicene. We reveal that bilayer silicene hosts double spin Chern-number as compared to single layer silicene and therefore accordingly has twice as many edge states in contrast to single layer silicene. In addition, we investigate the combined effect of intrinsic spin orbit coupling and the external electric field, we find that bilayer silicene, likewise single layer silicene, goes through a phase transitions from a quantum spin Hall state to a quantum valley Hall state when the strength of the applied electric field exceeds the intrinsic spin orbit coupling strength. We believe that the results and outcomes obtained for bilayer silicene are experimentally more accessible as compared to bilayer graphene, because of strong SO coupling in bilayer silicene.     

Comparative Study of Monolayer and Bilayer Epitaxial Graphene Field-Effect Transistors on SiC Substrates

Monolayer and bilayer graphenes have generated tremendous excitement as the potentially useful electronic materials due to their unique features.We report on monolayer and bilayer epitaxial graphene field-effect transistors(GFETs)fabricated on SiC substrates.Compared with monolayer GFETs,the bilayer GFETs exhibit a significant improvement in dc characteristics,including increasing current density Ids,improved transconductance g_m,reduced sheet resistance R_(on),and current saturation.The improved electrical properties and tunable bandgap in the bilayer graphene lead to the excellent dc performance of the bilayer GFETs.Furthermore,the improved dc characteristics enhance a better rf performance for bilayer graphene devices,demonstrating that the quasifree-standing bilayer graphene on SiC substrates has a great application potential for the future graphene-based electronics.     

Preparation of graphene on Cu foils by ion implantation with negative carbon clusters

We report on few-layer graphene synthesized on Cu foils by ion implantation using negative carbon cluster ions,followed by annealing at 950?C in vacuum.Raman spectroscopy reveals IG/I2Dvalues varying from 1.55 to 2.38 depending on energy and dose of the cluster ions,indicating formation of multilayer graphene.The measurements show that the samples with more graphene layers have fewer defects.This is interpreted by graphene growth seeded by the first layers formed via outward diffusion of C from the Cu foil,though nonlinear damage and smoothing effects also play a role.Cluster ion implantation overcomes the solubility limit of carbon in Cu,providing a technique for multilayer graphene synthesis.     

Faraday rotations,ellipticity, and circular dichroism in magneto-optical spectrum of moiré superlattices

We study the magneto-optical conductivity of a number of van der Waals heterostructures, namely, twisted bilayer graphene, AB-AB and AB-BA stacked twisted double bilayer graphene and monolayer graphene and AB-stacked bilayer graphene on hexagonal boron nitride. As the magnetic field increases, the absorption spectrum exhibits a self-similar recursive pattern reflecting the fractal nature of the energy spectrum. Whilst twisted bilayer graphene displays only weak circular dichroism, the other four structures display strong circular dichroism with monolayer graphene and AB-stacked bilayer graphene on hexagonal boron nitride being particularly pronounced owing to strong inversion symmetry breaking properties of the hexagonal boron nitride layer. As the left and right circularly polarized light interact with these structures differently, plane-polarized incident light undergoes a Faraday rotation and gains an ellipticity when transmitted. The size of the respective angles is on the order of a degree.     

Localized states at zigzag edges of bilayer graphene

We report the existence of zero-energy surface states localized at zigzag edges of bilayer graphene. Working within the tight-binding approximation we derive the analytic solution for the wave functions of these peculiar surface states. It is shown that zero-energy edge states in bilayer graphene can be divided into two families: (i) states living only on a single plane, equivalent to surface states in monolayer graphene and (ii) states with a finite amplitude over the two layers, with an enhanced penetration into the bulk. The bulk and surface (edge) electronic structure of bilayer graphene nanoribbons is also studied, both in the absence and in the presence of a bias voltage between planes.