Lianchang Zhang

Epitaxial growth of single-domain graphene on hexagonal boron nitride

Hexagonal boron nitride (h-BN) has recently emerged as an excellent substrate for graphene nanodevices, owing to its atomically flat surface and its potential to engineer graphenes electronic structure. Thus far, graphene/h-BN heterostructures have been obtained only through a transfer process, which introduces structural uncertainties due to the random stacking between graphene and h-BN substrate. Here we report the epitaxial growth of single-domain graphene on h-BN by a plasma-assisted deposition method. Large-area graphene single crystals were successfully grown for the first time on h-BN with a fixed stacking orientation. A two-dimensional (2D) superlattice of trigonal moiré pattern was observed on graphene by atomic force microscopy. Extra sets of Dirac points are produced as a result of the trigonal superlattice potential and the quantum Hall effect is observed with the 2D-superlattice-related feature developed in the fan diagram of longitudinal and Hall resistance, and the Dirac fermion physics near the original Dirac point is unperturbed. The macroscopic epitaxial graphene is in principle limited only by the size of the h-BN substrate and our synthesis method is potentially applicable on other flat surfaces. Our growth approach could thus open new ways of graphene band engineering through epitaxy on different substrates.

An Anisotropic Etching Effect in the Graphene Basal Plane

A highly controllable, dry, anisotropic etching technique for graphene sheets has been achieved using hydrogen plasma etching. Zigzag edge formation was achieved by starting the etching at edges and defects and depends strongly on crystallographic orientation of the graphene. This dry, anisotropic etching approach combined with the standard lithographic technique is ideal for scalable graphene tailoring because the etching rates can be precisely controlled and the quality of the graphene can be preserved.

Patterning Graphene with Zigzag Edges by Self-Aligned Anisotropic Etching

A top-down approach for controlled tailoring of graphene nanostructures with zigzag edges is presented. It consists of two key steps: artificial defect patterning and hydrogen-plasma etching. With this approach, various graphene nanostructures with sub-10 nm features and identical zigzag edges are reliably achieved. This approach shows great promise for making future graphene devices or circuits.

Multilevel Resistive Switching in Planar Graphene/SiO2 Nanogap Structures

We report a planar graphene/SiO(2) nanogap structure for multilevel resistive switching. Nanosized gaps created on a SiO(2) substrate by electrical breakdown of nanographene electrodes were used as channels for resistive switching. Two-terminal devices exhibited excellent memory characteristics with good endurance up to 10(4) cycles, long retention time more than 10(5) s, and fast switching speed down to 500 ns. At least five conduction states with reliability and reproducibility were demonstrated in these memory devices. The mechanism of the resistance switching effect was attributed to a reversible thermal-assisted reduction and oxidation process that occurred at the breakdown region of the SiO(2) substrate. In addition, the uniform and wafer-size nanographene films with controlled layer thickness and electrical resistivity were grown directly on SiO(2) substrates for scalable device fabrications, making it attractive for developing high-density and low-cost nonvolatile memories.

Growth, Characterization, and Properties of Nanographene

A systematic study on nanographene grown directly on silicon dioxide substrates is reported. The growth is carried out in a remote plasma-enhanced chemical vapor deposition system at a low temperature of around 550 °C with methane gas as the carbon source. Atomic force microscopy is used to characterize the nanographene morphology, and Raman spectroscopy, X-ray photoelectron spectroscopy, and scanning tunneling microscopy are exploited to identify the in-plane sp(2) bonding structures of nanographene samples. Electrical transport properties are measured at various temperatures down to 4 K. Tunneling effects, minimal conductance at the charge-neutral point, sheet resistances, and Dirac point position at different channel lengths are investigated. In addition, nanographene film possesses high transmittance properties, as indicated by transmittance spectra. Nanographene devices are fabricated from rippled structures, and show great promise for strain-gauge sensor applications.

Observation of Raman G-Peak Split for Graphene Nanoribbons with Hydrogen-Terminated Zigzag Edges

Raman scattering of individual hydrogen-terminated zigzag-edged graphene nanoribbons (Z-GNRs) was studied with focus on the G-peak. In addition to the bulk graphene G-peak appearing at ∼1594 cm(-1) (G(+)), an edge-related G-peak at ∼1583 cm(-1) (G(-)) was observed for Z-GNRs. This additional Raman vibrational mode originates from the zigzag edges where localized metallic edge states are present. The relative intensity ratio G(-)/G(+) displays a strong dependence on the ribbon width (W). It increases gradually with decreasing W, and the G(+) finally vanishes at W = 5(±3) nm. Polarized Raman scattering was also employed to confirm the four-fold symmetry of the split TO modes, and the results are in good agreement with previous theoretical predictions. Our work offers the first direct experimental evidence to confirm the validity of predicted Raman scattering of GNRs.

Studies of graphene-based nanoelectromechanical switches

AbstractElectromechanical switch devices employing suspended graphene as movable elements have been developed. Their on and off states can be controlled by modulating the electrostatic force applied to the graphene. The devices exhibit on-off ratios of up to 104 and lifetimes of over 500 cycles. The prototype device demonstrates the feasibility of using multilayer graphene in electromechanical systems. Measurements of the mechanical properties of the free-standing monolayer graphene gave a value of 0.96 TPa for the Young’s modulus and a van der Waals force with silicon oxide of 0.17 nN/nm2.

Vapour-phase graphene epitaxy at low temperatures

AbstractWe report an epitaxial growth of graphene, including homo- and hetero-epitaxy on graphite and SiC substrates, at a temperature as low as ∼540 °C. This vapour-phase epitaxial growth, carried out in a remote plasma-enhanced chemical vapor deposition (RPECVD) system using methane as the carbon source, can yield large-area high-quality graphene with the desired number of layers over the entire substrate surfaces following an AB-stacking layer-by-layer growth model. We also developed a facile transfer method to transfer a typical continuous one layer epitaxial graphene with second layer graphene islands on top of the first layer with the coverage of the second layer graphene islands being 20% (1.2 layer epitaxial graphene) from a SiC substrate onto SiO2 and measured the resistivity, carrier density and mobility. Our work provides a new strategy toward the growth of graphene and broadens its prospects of application in future electronics.

Effects of Pretreatment on the Electronic Properties of Plasma Enhanced Chemical Vapor Deposition Hetero-Epitaxial Graphene Devices

Quasi-monolayer graphene is successfully grown by the plasma enhanced chemical vapor deposition heteroepitaxial method we reported previously. To measure its electrical properties, the prepared graphene is fabricated into Hall ball shaped devices by the routine micro-fabrication method. However, impurity molecules adsorbed onto the graphene surface will impose considerable doping effects on the one-atom-thick film material. Our experiment demonstrates that pretreatment of the device by heat radiation baking and electrical annealing can dramatically influence the doping state of the graphene and consequently modify the electrical properties. While graphene in the as-fabricated device is highly p-doped, as confirmed by the position of the Dirac point at far more than +60 V, baking treatment at temperatures around 180°C can significantly lower the doping level and reduce the conductivity. The following electrical annealing is much more efficient to desorb the extrinsic molecules, as confirmed by the in situ measurement, and as a result, further modify the doping state and electrical properties of the graphene, causing a considerable drop of the conductivity and a shifting of Dirac point from beyond +60 V to 0 V.

Reducing the contact resistance of SiNW devices by employing a heavily doped carrier injection layer

Silicon nanowires (SiNWs) are promising building blocks for future electronic devices. In SiNW-based devices, reducing the contact resistance of SiNW-metal as much as possible is critically important. Here we report a simple fabrication approach for SiNW field effect transistors (FETs) with low contact resistances by employing a heavily doped carrier injection layer wrapped around SiNWs at the contact region. Both n- and p-type SiNW-FET devices with carrier injection layers were investigated, the contact resistances were one order smaller than those without carrier injection layers and only contribute less than 14.8% for n-type devices and 11.4% for p-type devices, respectively, to the total resistance. Such low contact resistance guarantees the device characteristics mainly from the channel region of SiNW-based devices.