Magnetotransport Properties of Graphene Nanoribbons with Zigzag Edges.

The determination of the electronic structure by edge geometry is unique to graphene. In theory, an evanescent nonchiral edge state is predicted at the zigzag edges of graphene. Up to now, the approach used to study zigzag-edged graphene has mostly been limited to scanning tunneling microscopy. The transport properties have not been revealed. Recent advances in hydrogen plasma-assisted "top-down" fabrication of zigzag-edged graphene nanoribbons (Z-GNRs) have allowed us to investigate edge-related transport properties. In this Letter, we report the magnetotransport properties of Z-GNRs down to ∼70 nm wide on an h-BN substrate. In the quantum Hall effect regime, a prominent conductance peak is observed at Landau ν=0, which is absent in GNRs with nonzigzag edges. The conductance peak persists under perpendicular magnetic fields and low temperatures. At a zero magnetic field, a nonlocal voltage signal, evidenced by edge conduction, is detected. These prominent transport features are closely related to the observable density of states at the hydrogen-etched zigzag edge of graphene probed by scanning tunneling spectroscopy, which qualitatively matches the theoretically predicted electronic structure for zigzag-edged graphene. Our study gives important insights for the design of new edge-related electronic devices.

Graphene-Contacted Ultrashort Channel Monolayer MoS2 Transistors.

2D semiconductors are promising channel materials for field-effect transistors (FETs) with potentially strong immunity to short-channel effects (SCEs). In this paper, a grain boundary widening technique is developed to fabricate graphene electrodes for contacting monolayer MoS2 . FETs with channel lengths scaling down to ≈4 nm can be realized reliably. These graphene-contacted ultrashort channel MoS2 FETs exhibit superior performances including the nearly Ohmic contacts and excellent immunity to SCEs. This work provides a facile route toward the fabrication of various 2D material-based devices for ultrascaled electronics.

Precisely Aligned Monolayer MoS2 Epitaxially Grown on h-BN basal Plane.

Control of the precise lattice alignment of monolayer molybdenum disulfide (MoS2 ) on hexagonal boron nitride (h-BN) is important for both fundamental and applied studies of this heterostructure but remains elusive. The growth of precisely aligned MoS2 domains on the basal plane of h-BN by a low-pressure chemical vapor deposition technique is reported. Only relative rotation angles of 0° or 60° between MoS2 and h-BN basal plane are present. Domains with same orientation stitch and form single-crystal, domains with different orientations stitch and from mirror grain boundaries. In this way, the grain boundary is minimized and a continuous film stitched by these two types of domains with only mirror grain boundaries is obtained. This growth strategy is also applicable to other 2D materials growth.

Rolling Up a Monolayer MoS2 Sheet.

MoS2 nanoscrolls are formed by argon plasma treatment on monolayer MoS2 sheet. The nanoscale scroll formation is attributed to the partial removal of top sulfur layer in MoS2 during the argon plasma treatment process. This convenient, solvent-free, and high-yielding nanoscroll formation technique is also feasible for other 2D transition metal dichalcogenides.

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.

Gate tunable WSe2-BP van der Waals heterojunction devices.

Due to the weak screening effect, the concentration and type of charge carriers in 2D semiconductor heterostructures can be effectively tuned by electrostatic gating, enabling us to realize different types of heterojunctions in a single device. Such 'type tunable' properties are useful for designing novel electrical or optoelectrical devices. Here, we demonstrate a 'type tunable' heterojunction device construct with two pieces of ambipolar 2D semiconductors: WSe2 and black phosphorus (BP). This heterojunction could be tuned to either the p-p junction or n-n junction by gate modulation. The p-p junction shows a large current rectification ratio while the n-n junction shows a negligible current rectification ratio, indicating a large valence band offset and a small conduction band offset at the WSe2/BP interface. In the optoelectrical measurements, we found the amplitude and even the polarity of photocurrent could be modulated by electrostatic gating. Our study could further enhance the understanding of designing devices based on these 'type tunable' van der Waals heterojunctions. Moreover, the properties of the WSe2/BP interface were also experimentally identified through the electrical and optoelectrical measurements in our study.

Nanographene charge trapping memory with a large memory window.

Nanographene is a promising alternative to metal nanoparticles or semiconductor nanocrystals for charge trapping memory. In general, a high density of nanographene is required in order to achieve high charge trapping capacity. Here, we demonstrate a strategy of fabrication for a high density of nanographene for charge trapping memory with a large memory window. The fabrication includes two steps: (1) direct growth of continuous nanographene film; and (2) isolation of the as-grown film into high-density nanographene by plasma etching. Compared with directly grown isolated nanographene islands, abundant defects and edges are formed in nanographene under argon or oxygen plasma etching, i.e. more isolated nanographene islands are obtained, which provides more charge trapping sites. As-fabricated nanographene charge trapping memory shows outstanding memory properties with a memory window as wide as ∼9 V at a relative low sweep voltage of ±8 V, program/erase speed of ∼1 ms and robust endurance of >1000 cycles. The high-density nanographene charge trapping memory provides an outstanding alternative for downscaling technology beyond the current flash memory.

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.

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.