Guibai Xie

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.

Ultra-sensitive strain sensors based on piezoresistive nanographene films

Graphene shows promise on strain sensor applications, but the piezoresistive sensitivity of perfect graphene is low due to its weak electrical conductivity response upon structural deformation. In this paper, we used nanographene films for ultra-sensitive strain sensors. The piezoresistive sensitivity of nanographene films with different thicknesses and conductivities was systematically investigated and a nearly inverse proportional correlation was found. A gauge factor over 300, the highest so far for graphene-based strain sensors, was achieved. A charge tunneling model was used to explain the piezoresistive characteristics of nanographene films, which indicates our results provide a different rout toward ultra-sensitive strain sensors.

Scalable Growth of High-Quality Polycrystalline MoS2 Monolayers on SiO2 with Tunable Grain Sizes

We report a scalable growth of monolayer MoS2 films on SiO2 substrates by chemical vapor deposition. As-grown polycrystalline MoS2 films are continuous over the entire substrate surface with a tunable grain size from ∼20 nm up to ∼1 μm. An obvious blue-shift (up to 80 meV) of photoluminescence peaks was observed from a series samples with different grain sizes. Back-gated field effect transistors based on a polycrystalline MoS2 film with a typical grain size of ∼600 nm shows a field mobility of ∼7 cm(2)/(V s) and on/off ratio of ∼10(6), comparable to those achieved from exfoliated MoS2. Our work provides a route toward scaled-up synthesis of high-quality monolayer MoS2 for electronic and optoelectronic devices.

Tunable Piezoresistivity of Nanographene Films for Strain Sensing

Graphene-based strain sensors have attracted much attention recently. Usually, there is a trade-off between the sensitivity and resistance of such devices, while larger resistance devices have higher energy consumption. In this paper, we report a tuning of both sensitivity and resistance of graphene strain sensing devices by tailoring graphene nanostructures. For a typical piezoresistive nanographene film with a sheet resistance of ∼100 KΩ/□, a gauge factor of more than 600 can be achieved, which is 50× larger than those in previous studies. These films with high sensitivity and low resistivity were also transferred on flexible substrates for device integration for force mapping. Each device shows a high gauge factor of more than 500, a long lifetime of more than 10(4) cycles, and a fast response time of less than 4 ms, suggesting a great potential in electronic skin applications.

Observation of Strong Interlayer Coupling in MoS2/WS2 Heterostructures

Epitaxial growth of A-A and A-B stacking MoS2 on WS2 via a two-step chemical vapor deposition method is reported. These epitaxial heterostructures show an atomic clean interface and a strong interlayer coupling, as evidenced by systematic characterization. Low-frequency Raman breathing and shear modes are observed in commensurate stacking bilayers for the first time; these can serve as persuasive fingerprints for interfacial quality and stacking configurations.

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.

Tunable electroluminescence in planar graphene/SiO(2) memristors.

Electroluminescence and resistive switching are first realized simultaneously in graphene/SiO2 memristor devices. The electroluminescence peaks can be tuned between 550 nm and 770 nm reliably via setting the device to different resistance states by applying different voltages. The combination of resistive switching and electroluminescence may bring new functionalities for these memristor devices which are fully compatible with silicon-based electronics.

Thermally Induced Graphene Rotation on Hexagonal Boron Nitride.

In this Letter, we report the observation of thermally induced rotation of graphene on hexagonal boron nitride (h-BN). After the rotation, two thermally stable configurations of graphene on h-BN with a relative lattice twisting angle of 0° (most stable) and 30° (metastable), respectively, were found. Graphene on h-BN with a twisting angle below (above) a critical angle of ∼12±2° tends to rotate towards 0° (30°) at a temperature of >100 °C, which is in line with our theoretical simulations. In addition, by manipulating the annealing temperature and the flake sizes of graphene, moiré superlattices with large spatial periods of graphene on h-BN are achieved. Our studies provide a detailed understanding of the thermodynamic properties of graphene on h-BN and a feasible approach to obtaining van der Waals heterostructures with aligned lattices.

Graphene Edge Lithography

Fabrication of graphene nanostructures is of importance for both investigating their intrinsic physical properties and applying them into various functional devices. In this paper, we report a scalable fabrication approach for graphene nanostructures. Compared with conventional lithographic fabrication techniques, this new approach uses graphene edges as the templates or masks and offers advantage in technological simplicity and capability of creating small features below 10 nm scale. Moreover, mask layers used in the fabrication process could be simultaneously used as the dielectric layers for top-gated devices. The as-fabricated graphene nanoribbons (GNRs) are of high quality with the carrier mobility ∼400 cm(2)/(V s) for typical 15 nm wide ribbons. Our technique allows easy and reproducible fabrication of various graphene nanostructures, such as ribbons and rings, and can be potentially extended to other materials and systems by use of their edges or facets as templates.

A route toward digital manipulation of water nanodroplets on surfaces.

Manipulation of an isolated water nanodroplet (WN) on certain surfaces is important to various nanofluidic applications but challenging. Here we present a digital nanofluidic system based on a graphene/water/mica sandwich structure. In this architecture, graphene provides a flexible protection layer to isolate WNs from the outside environment, and a monolayer ice-like layer formed on the mica surface acts as a lubricant layer to allow these trapped WNs to move on it freely. In combination with scanning probe microscope techniques, we are able to move, merge, and separate individual water nanodroplets in a controlled manner. The smallest manipulatable water nanodroplet has a volume down to yoctoliter (10(-24) L) scale.