Observation of negative differential resistance in mesoscopic graphene oxide devices.

The fractions of various functional groups in graphene oxide (GO) are directly related to its electrical and chemical properties and can be controlled by various reduction methods like thermal, chemical and optical. However, a method with sufficient controllability to regulate the reduction process has been missing. In this work, a hybrid method of thermal and joule heating processes is demonstrated where a progressive control of the ratio of various functional groups can be achieved in a localized area. With this precise control of carbon-oxygen ratio, negative differential resistance (NDR) is observed in the current-voltage characteristics of a two-terminal device in the ambient environment due to charge-activated electrochemical reactions at the GO surface. This experimental observation correlates with the optical and chemical characterizations. This NDR behavior offers new opportunities for the fabrication and application of such novel electronic devices in a wide range of devices applications including switches and oscillators.

Photodetector Based on Multilayer SnSe2 Field Effect Transistor.

We demonstrate a high-performance photodetector with multilayer tin diselenide (SnSe2) exfoliated from a high-quality crystal which was synthesized by the temperature gradient growth method. This SnSe2 photodetector exhibits high photoresponsivity of 5.11 × 105 A W-1 and high specific detectivity of 2.79 × 1013 Jones under laser irradiation (λ = 450 nm). We also observed a reproducible and stable time-resolved photoresponse to the incident laser beam from this SnSe2 photodetector, which can be used as a promising material for future optoelectronic applications.

Hole dephasing caused by hole-hole interaction in a multilayered black phosphorus.

We study the magnetotransport of holes in a multilayered black phosphorus in a temperature range of 1.9 to 21.5 K. We observed a negative magnetoresistance at magnetic fields up to 1.5 T. This negative magetoresistance was analyzed by weak localization theory in diffusive regime. At the lowest temperature and the highest carrier density we found a phase coherence length of 48 nm. The linear temperature dependence of the dephasing rate shows that the hole-hole scattering processes with small energy transfer are the dominant contribution in breaking the carrier phase coherence.

Molybdenum disulfide nanoparticles decorated reduced graphene oxide: highly sensitive and selective hydrogen sensor.

In this work, we report on the hydrogen (H2) sensing behavior of reduced graphene oxide (RGO)/molybdenum disulfide (MoS2) nano particles (NPs) based composite film. The RGO/MoS2 composite exhibited a highly enhanced H2 response (∼15.6%) for 200 ppm at an operating temperature of 60 °C. Furthermore, the RGO/MoS2 composite showed excellent selectivity to H2 with respect to ammonia (NH3) and nitric oxide (NO) which are highly reactive gas species. The composite's response to H2 is 2.9 times higher than that of NH3 whereas for NO it is 3.5. This highly improved H2 sensing response and selectivity of RGO/MoS2 at low operating temperatures were attributed to the structural integration of MoS2 nanoparticles in the nanochannels and pores in the RGO layer.

Unimer-Assisted Exfoliation for Highly Concentrated Aqueous Dispersion Solutions of Single- and Few-Layered van der Waals Materials.

We suggest a unimer-assisted exfoliation method for the exfoliation of van der Waals two-dimensional (2D) materials such as graphene, MoS2, and h-BN and show that the micellar size is a critical parameter for enhancing the exfoliation efficiency. To explain the effectiveness of the unimers in the exfoliation, the influence of the micellar size of a biocompatible block copolymer, Pluronic F-68, is evaluated in view of the yield and thickness of exfoliated 2D flakes. By the addition of water-soluble alcohols, the surfactants exist in the form of a unimer, which facilitates the intercalation into the layered materials and their exfoliation. The results showed that the high exfoliation efficiency could be achieved by controlling the micellar size mostly to be unimers; the average yield rate of MoS2 exfoliation was 4.51% per hour, and the very high concentration of 1.45 mg/mL was obtained by sonication for 3 h. We also suggested the dielectrophoresis technique as a method for forming a film composed of 2D flakes for diverse applications requiring electrical signals. The unimer-assisted exfoliation method will be substantially utilized to achieve highly concentrated aqueous dispersion solutions of 2D materials.

Tunable electrical properties of multilayer HfSe2 field effect transistors by oxygen plasma treatment.

HfSe2 field effect transistors are systematically studied in order to selectively tune their electrical properties by optimizing layer thickness and oxygen plasma treatment. The optimized plasma-treated HfSe2 field effect transistors showed a high on/off ratio improvement of four orders of magnitude, from 27 to 105, a field effect mobility increase from 2.16 to 3.04 cm2 V-1 s-1, a subthreshold swing improvement from 30.6 to 4.8 V dec-1, and a positive threshold voltage shift between depletion mode and enhancement mode, from -7.02 to 11.5 V. The plasma-treated HfSe2 photodetector also demonstrates a reasonable photoresponsivity from the visible to the near-infrared region of light.

Hopping conduction and random telegraph signal in an exfoliated multilayer MoS2 field-effect transistor.

We investigate the hopping conduction and random telegraph signal caused by various species of interface charge scatterers in a MoS2 multilayer field-effect transistor. The temperature dependence of the channel resistivity shows that at low temperatures and low carrier densities the carrier transport is via Mott variable range hopping with a hopping length changing from 41 to 80 nm. The hopping conduction was due to electron tunneling through localized band tail states formed by the scatterers located in the vicinity of the MoS2 layer. In the temperature range of 40-70 K, we observed random telegraph signal (RTS) that is caused by the capture and emission of a carrier by the interface traps that are located away from the layer. These traps form strong potential that interact with the layer and change the potential profile of the electron system. The characteristics of RTS depend strongly on gate bias and temperature, as well as the application of a magnetic field.

Gate-Tunable Hole and Electron Carrier Transport in Atomically Thin Dual-Channel WSe2 /MoS2 Heterostructure for Ambipolar Field-Effect Transistors.

An ambipolar dual-channel field-effect transistor (FET) with a WSe2 /MoS2 heterostructure formed by separately controlled individual channel layers is demonstrated. The FET shows a switchable ambipolar behavior with independent carrier transport of electrons and holes in the individual layers of MoS2 and WSe2 , respectively. Moreover, the photoresponse is studied at the heterointerface of the WSe2 /MoS2 dual-channel FET.

Spin diffusion and non-local spin-valve effect in an exfoliated multilayer graphene with a Co electrode.

We fabricated a non-local spin valve with a thin layer of graphite with Co transparent electrodes. The spin-valve effect and spin precession were observed at room temperature. The magnitude of the mangetoresistance increases when temperature decreases. The spin-relaxation time, [Formula: see text], obtained from the fitting of the Hanle curves increases with decreasing temperature with a weak dependence [Formula: see text] while the spin-diffusion constant D decreases. At room temperature, [Formula: see text] exceeds 100 ps and the spin-diffusion length, [Formula: see text], is ∼2 µm. The temperature dependence of [Formula: see text] is not monotonic, and it has the largest value at room temperature. Our results show that multilayer graphene is a suitable material for spintronic devices.

High performance MoS2-based field-effect transistor enabled by hydrazine doping.

We investigated the n-type doping effect of hydrazine on the electrical characteristics of a molybdenum disulphide (MoS2)-based field-effect transistor (FET). The threshold voltage of the MoS2 FET shifted towards more negative values (from -20 to -70 V) on treating with 100% hydrazine solution with the channel current increasing from 0.5 to 25 µA at zero gate bias. The inverse subthreshold slope decreased sharply on doping, while the ON/OFF ratio increased by a factor of 100. Gate-channel coupling improved with doping, which facilitates the reduction of channel length between the source and drain electrodes without compromising on the transistor performance, making the MoS2-based FET easily scalable.

Non-degenerate n-type doping by hydrazine treatment in metal work function engineered WSe2 field-effect transistor.

We report a facile and highly effective n-doping method using hydrazine solution to realize enhanced electron conduction in a WSe2 field-effect transistor (FET) with three different metal contacts of varying work functions-namely, Ti, Co, and Pt. Before hydrazine treatment, the Ti- and Co-contacted WSe2 FETs show weak ambipolar behaviour with electron dominant transport, whereas in the Pt-contacted WSe2 FETs, the p-type unipolar behaviour was observed with the transport dominated by holes. In the hydrazine treatment, a p-type WSe2 FET (Pt contacted) was converted to n-type with enhanced electron conduction, whereas highly n-doped properties were achieved for both Ti- and Co-contacted WSe2 FETs with on-current increasing by three orders of magnitude for Ti. All n-doped WSe2 FETs exhibited enhanced hysteresis in their transfer characteristics, which opens up the possibility of developing memories using transition metal dichalcogenides.

Raman shift and electrical properties of MoS2 bilayer on boron nitride substrate.

We have fabricated a bilayer molybdenum disulphide (MoS2) transistor on boron nitride (BN) substrate and performed Raman spectroscopy and electrical measurements with this device. The characteristic Raman peaks show an upshift about 2.5 cm(-1) with the layer lying on BN, and a narrower line width in comparison with those on a SiO2 substrate. The device has a maximum drain current larger than 1 µA and a high current on/off ratio of greater than 10(8). In the temperature range of 100 K-293 K, the two terminal gate effect mobility and the carrier density do not change significantly with temperature. Results of the Raman and electrical measurements reveal that BN is a suitable substrate for atomic layer electrical devices.

Alternating Current Dielectrophoresis Optimization of Pt-Decorated Graphene Oxide Nanostructures for Proficient Hydrogen Gas Sensor.

Alternating current dielectrophoresis (DEP) is an excellent technique to assemble nanoscale materials. For efficient DEP, the optimization of the key parameters like peak-to-peak voltage, applied frequency, and processing time is required for good device. In this work, we have assembled graphene oxide (GO) nanostructures mixed with platinum (Pt) nanoparticles between the micro gap electrodes for a proficient hydrogen gas sensors. The Pt-decorated GO nanostructures were well located between a pair of prepatterned Ti/Au electrodes by controlling the DEP technique with the optimized parameters and subsequently thermally reduced before sensing. The device fabricated using the DEP technique with the optimized parameters showed relatively high sensitivity (∼10%) to 200 ppm hydrogen gas at room temperature. The results indicates that the device could be used in several industry applications, such as gas storage and leak detection.

Tunable Electrical and Optical Characteristics in Monolayer Graphene and Few-Layer MoS2 Heterostructure Devices.

Lateral and vertical two-dimensional heterostructure devices, in particular graphene-MoS2, have attracted profound interest as they offer additional functionalities over normal two-dimensional devices. Here, we have carried out electrical and optical characterization of graphene-MoS2 heterostructure. The few-layer MoS2 devices with metal electrode at one end and monolayer graphene electrode at the other end show nonlinearity in drain current with drain voltage sweep due to asymmetrical Schottky barrier height at the contacts and can be modulated with an external gate field. The doping effect of MoS2 on graphene was observed as double Dirac points in the transfer characteristics of the graphene field-effect transistor (FET) with a few-layer MoS2 overlapping the middle part of the channel, whereas the underlapping of graphene have negligible effect on MoS2 FET characteristics, which showed typical n-type behavior. The heterostructure also exhibits a strongest optical response for 520 nm wavelength, which decreases with higher wavelengths. Another distinct feature observed in the heterostructure is the peak in the photocurrent around zero gate voltage. This peak is distinguished from conventional MoS2 FETs, which show a continuous increase in photocurrent with back-gate voltage. These results offer significant insight and further enhance the understanding of the graphene-MoS2 heterostructure.

Construction and characterization of Cu²âº, Ni²âº, Zn²âº, and Co²âº modified-DNA crystals.

We studied the physical characteristics of modified-DNA (M-DNA) double crossover crystals fabricated via substrate-assisted growth with various concentrations of four different divalent metallic ions, Cu(2+), Ni(2+), Zn(2+), and Co(2+). Atomic force microscopy (AFM) was used to test the stability of the M-DNA crystals with different metal ion concentrations. The AFM images show that M-DNA crystals formed without deformation at up to the critical concentrations of 6 mM of [Cu(2+)], 1.5 mM of [Ni(2+)], 1 mM of [Zn(2+)], and 1 mM of [Co(2+)]. Above these critical concentrations, the M-DNA crystals exhibited deformed, amorphous structures. Raman spectroscopy was then used to identify the preference of the metal ion coordinate sites. The intensities of the Raman bands gradually decreased as the concentration of the metal ions increased, and when the metal ion concentrations increased beyond the critical values, the Raman band of the amorphous M-DNA was significantly suppressed. The metal ions had a preferential binding order in the DNA molecules with G-C and A-T base pairs followed by the phosphate backbone. A two-probe station was used to measure the electrical current-voltage properties of the crystals which indicated that the maximum currents of the M-DNA complexes could be achieved at around the critical concentration of each ion. We expect that the functionalized ion-doped M-DNA crystals will allow for efficient devices and sensors to be fabricated in the near future.

Postfabrication annealing effects on insulator-metal transitions in VO2 thin-film devices.

In order to investigate the metal-insulator transition characteristics of VO2 devices annealed in reducing atmosphere after device fabrication at various temperature, electrical, chemical, and thermal characteristics are measured and analyzed. It is found that the sheet resistance and the insulator-metal transition point, induced by both voltage and thermal, decrease when the devices are annealed from 200 to 500 °C. The V 2p3/2 peak variation in X-ray photoelectron spectroscopy (XPS) characterization verifies the reduction of thin-films. A decrease of the transition temperature from voltage hysteresis measurements further endorse the reducing effects of the annealing on VO2 thin-film.

Nanocrystalline-graphene-tailored hexagonal boron nitride thin films.

Unintentionally formed nanocrystalline graphene (nc-G) can act as a useful seed for the large-area synthesis of a hexagonal boron nitride (h-BN) thin film with an atomically flat surface that is comparable to that of exfoliated single-crystal h-BN. A wafer-scale dielectric h-BN thin film was successfully synthesized on a bare sapphire substrate by assistance of nc-G, which prevented structural deformations in a chemical vapor deposition process. The growth mechanism of this nc-G-tailored h-BN thin film was systematically analyzed. This approach provides a novel method for preparing high-quality two-dimensional materials on a large surface.

Conductance control in VO2 nanowires by surface doping with gold nanoparticles.

The material properties of semiconductor nanowires are greatly affected by electrical, optical, and chemical processes occurring at their surfaces because of the very large surface-to-volume ratio. Precise control over doping as well as the surface charge properties has been demonstrated in thin films and nanowires for fundamental physics and application-oriented research. However, surface doping behavior is expected to differ markedly from bulk doping in conventional semiconductor materials. Here, we show that placing gold nanoparticles, in controlled manner, on the surface of an insulating vanadium dioxide nanowire introduces local charge carriers in the nanowire, and one could, in principle, completely and continuously alter the material properties of the nanowire and obtain any intermediate level of conductivity. The current in the nanowire increased by nearly 3 times when gold nanoparticles of 10(11) cm(-2) order of density were controllably placed on the nanowire surface. A strong quadratic space-charge limited (SCL) transport behavior was also observed from the conductance curve suggesting the formation of two-dimensional (2D) electron-gas-like confined layer in the nanowire with adsorbed Au NPs. In addition to stimulating scientific interest, such unusual surface doping phenomena may lead to new applications of vanadium dioxide-based electronic, optical, and chemical sensing nanodevices.

Poly-4-vinylphenol and poly(melamine-co-formaldehyde)-based graphene passivation method for flexible, wearable and transparent electronics.

Next generation graphene-based electronics essentially need a dielectric layer with several requirements such as high flexibility, high transparency, and low process temperature. Here, we propose and investigate a flexible and transparent poly-4-vinylphenol and poly(melamine-co-formaldehyde) (PVP/PMF) insulating layer to achieve intrinsic graphene and an excellent gate dielectric layer at sub 200 °C. Chemical and electrical effects of PVP/PMF layer on graphene as well as its dielectric property are systematically investigated through various measurements by adjusting the ratio of PVP to PMF and annealing temperature. The optimized PVP/PMF insulating layer not only removes the native -OH functional groups which work as electron-withdrawing agents on graphene (Dirac point close to zero) but also shows an excellent dielectric property (low hysteresis voltage). Finally, a flexible, wearable, and transparent (95.8%) graphene transistor with Dirac point close to zero is demonstrated on polyethylene terephthalate (PET) substrate by exploiting PVP/PMF layer which can be scaled down to 20 nm.

Large-scale synthesis of high-quality hexagonal boron nitride nanosheets for large-area graphene electronics.

Hexagonal boron nitride (h-BN) has received a great deal of attention as a substrate material for high-performance graphene electronics because it has an atomically smooth surface, lattice constant similar to that of graphene, large optical phonon modes, and a large electrical band gap. Herein, we report the large-scale synthesis of high-quality h-BN nanosheets in a chemical vapor deposition (CVD) process by controlling the surface morphologies of the copper (Cu) catalysts. It was found that morphology control of the Cu foil is much critical for the formation of the pure h-BN nanosheets as well as the improvement of their crystallinity. For the first time, we demonstrate the performance enhancement of CVD-based graphene devices with large-scale h-BN nanosheets. The mobility of the graphene device on the h-BN nanosheets was increased 3 times compared to that without the h-BN nanosheets. The on-off ratio of the drain current is 2 times higher than that of the graphene device without h-BN. This work suggests that high-quality h-BN nanosheets based on CVD are very promising for high-performance large-area graphene electronics.