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Hybrid devices consisting of graphene or transition metal dichalcogenides (TMDs) and semiconductor quantum dots (QDs) were widely studied for potential photodetector and photovoltaic applications, while for photodetector applications, high internal quantum efficiency (IQE) is required for photovoltaic applications and enhanced carrier diffusion length is also desirable. Here, we reported the electrical measurements on hybrid field-effect optoelectronic devices consisting of compact QD monolayer at controlled separations from single-layer graphene, and the structure is characterized by high IQE and large enhancement of minority carrier diffusion length. While the IQE ranges from 10.2% to 18.2% depending on QD-graphene separation, ds, the carrier diffusion length, LD, estimated from scanning photocurrent microscopy (SPCM) measurements, could be enhanced by a factor of 5-8 as compared to that of pristine graphene. IQE and LD could be tuned by varying back gate voltage and controlling the extent of charge separation from the proximal QD layer due to photoexcitation. The obtained IQE values were remarkably high, considering that only a single QD layer was used, and the parameters could be further enhanced in such devices significantly by stacking multiple layers of QDs. Our results could have significant implications for utilizing these hybrid devices as photodetectors and active photovoltaic materials with high efficiency.

Graphene (Gr)–transition metal dichalcogenide (TMDC) hybrids are promising platforms for achieving sensitive and ultra-fast photodetection. The process of photo-detection in such van der Waals hybrids is usually dictated by the formation of excitons followed by the transfer of charge and energy from the TMDC layer to graphene, but they have not been explored simultaneously in the same device before. In this work, we have investigated optically excited Gr–WS2(tungsten disulfide) heterostructures using both standard electrical transport and Johnson noise thermometry. At large negative gate voltages, the experimentally observed photoresponse cannot be explained from conventional photogating but was found to host an increase in electron temperature as large as [Formula: see text] K. Time dependence of the transport and the noise reveals that the change in temperature and photoresistance can originate from distinct microscopic processes. The findings can be exploited for the development of Gr–TMDC based ultra-fast bolometers.

Au–Ag nanostructures comprising of ∼1 nm Ag nanoparticles embedded into an Au matrix show several unconventional optical, electric and magnetic properties. Here, we review progress made towards the preparation of these materials as well as analysis of their structure. Further, electrical and magnetic transitions in these materials are discussed. Finally, we review the properties of these materials as revealed from optical and electron microscopic probes.

Polyethylene terephthalate-based nanocomposites with hexagonal boron nitride nanosheets (BNNs) were prepared by a solution casting method with varying concentrations of BNNs from 0.5 wt% to 4 wt%. Melting and crystallization behaviour of the composites were investigated by differential scanning calorimetry, which suggests that with increasing presence of nanosheets, the crystallinity increases and hence the polyethylene terephthalate chain mobility gets restricted, which leads to suppression of crystal growth. The nanoindentation measurements on the composite films exhibit improved mechanical properties. Enhancement of 33.3% of elastic modulus and 32.4% of hardness was observed with 2 wt% infusion of boron nitride nanosheets in polyethylene terephthalate.

Despite the many fascinating discoveries of fundamental significance and device applications involving graphene, one area that has been lacking is graphene-based displays and emissive devices. Since graphene by itself has weak and wavelength-independent absorption and no emission in the visible range, such devices must rely on synergistic combination with other highly sensitive optical materials such as quantum dots. However, the well-known strong nonradiative energy transfer between emitters and quantum dots and graphene makes it impossible to create such devices due to strong emission quenching. Here we report the first demonstration of enhanced photoluminescence of quantum dots in close proximity to graphene field effect transistor devices, which are electrically and spectrally tunable. The enhanced emission originates from super-radiance between closely packed quantum dots placed close to single-layer graphene, which overcomes the strong nonradiative quenching observed earlier. Finite difference time ...

The combination of field tunable bandgap, topological edge states, and valleys in the band structure, makes insulating bilayer graphene a unique localized system, where the scaling laws of dimensionless conductance g remain largely unexplored. Here we show that the relative fluctuations in ln g with the varying chemical potential, in strongly insulating bilayer graphene (BLG) decay nearly logarithmically for channel length up to L/${\xi}$ ${\approx}$ 20, where ${\xi}$ is the localization length. This 'marginal' self averaging, and the corresponding dependence of on L, suggest that transport in strongly gapped BLG occurs along strictly one-dimensional channels, where ${\xi}$ ${\approx}$ 0.5${\pm}$0.1 ${\mu}$m was found to be much longer than that expected from the bulk bandgap. Our experiment reveals a nontrivial localization mechanism in gapped BLG, governed by transport along robust edge modes., Comment: This document is the Author's version of a submitted work that was subsequently accepted for publication in Physical Review Letters

Ultrathin Au nanowires (similar to 2 nm diameter) are interesting from a fundamental point of view to study structure and electronic transport and also hold promise in the field of nanoelectronics, particularly for sensing applications. Device fabrication by direct growth on various substrates has been useful in demonstrating some of the potential applications. However, the realization of practical devices requires device fabrication strategies that are fast, inexpensive, and efficient. Herein, we demonstrate directed assembly of ultrathin Au nanowires over large areas across electrodes using ac dielectrophoresis with a mechanistic understanding of the process. On the basis of the voltage and frequency, the wires either align in between or across the contact pads. We exploit this assembly to produce an array of contacting wires for statistical estimation of electrical transport with important implications for future nanoelectronic/sensor applications.

Van der Waals hybrids of graphene and transition metal dichalcogenides exhibit an extremely large response to optical excitation, yet counting of photons with single-photon resolution is not achieved. Here, a dual-gated bilayer graphene (BLG) and molybdenum disulphide (MoS2 ) hybrid are demonstrated, where opening a band gap in the BLG allows extremely low channel (receiver) noise and large optical gain (≈1010 ) simultaneously. The resulting device is capable of unambiguous determination of the Poissonian emission statistics of an optical source with single-photon resolution at an operating temperature of 80 K, dark count rate 0.07 Hz, and linear dynamic range of ≈40 dB. Single-shot number-resolved single-photon detection with van der Waals heterostructures may impact multiple technologies, including the linear optical quantum computation.

Here we present the fabrication and characterization of a new class of hybrid devices where the constituents are graphene and ultrathin molybdenum di-sulphide (MoS2). This device is one of the simplest member of a family of hybrids where the desirable electrical characteristics of graphene such as high mobility are combined with optical activity of semiconductors. We find that in the presence of an optically active substrate, considerable photoconductivity is induced in graphene which is persistent up to a time scale of at least several hours. This photo induced memory can be erased by the application of a suitable gate voltage pulse. This memory operation is stable for many cycles. We present a theoretical model based on localized states in MoS2 which explains the data. (C) 2013 Elsevier Ltd. All rights reserved.

Combining the electronic properties of graphene(1,2) and molybdenum disulphide (MoS2)(3-6) in hybrid heterostructures offers the possibility to create devices with various functionalities. Electronic logic and memory devices have already been constructed from graphene-MoS2 hybrids(7,8), but they do not make use of the photosensitivity of MoS2, which arises from its optical-range bandgap(9). Here, we demonstrate that graphene-on-MoS2 binary heterostructures display remarkable dual optoelectronic functionality, including highly sensitive photodetection and gate-tunable persistent photoconductivity. The responsivity of the hybrids was found to be nearly 1 x 10(10) A W-1 at 130 K and 5 x 10(8) A W-1 at room temperature, making them the most sensitive graphene-based photodetectors. When subjected to time-dependent photoillumination, the hybrids could also function as a rewritable optoelectronic switch or memory, where the persistent state shows almost no relaxation or decay within experimental timescales, indicating near-perfect charge retention. These effects can be quantitatively explained by gate-tunable charge exchange between the graphene and MoS2 layers, and may lead to new graphene-based optoelectronic devices that are naturally scalable for large-area applications at room temperature.

The zigzag edges of single- or few-layer graphene are perfect one-dimensional conductors owing to a set of gapless states that are topologically protected against backscattering. Direct experimental evidence of these states has been limited so far to their local thermodynamic and magnetic properties, determined by the competing effects of edge topology and electron–electron interaction. However, experimental signatures of edge-bound electrical conduction have remained elusive, primarily due to the lack of graphitic nanostructures with low structural and/or chemical edge disorder. Here, we report the experimental detection of edge-mode electrical transport in suspended atomic-scale constrictions of single and multilayer graphene created during nanomechanical exfoliation of highly oriented pyrolytic graphite. The edge-mode transport leads to the observed quantization of conductance close to multiples of G0 = 2e2/h. At the same time, conductance plateaux at G0/2 and a split zero-bias anomaly in non-equilibrium transport suggest conduction via spin-polarized states in the presence of an electron–electron interaction. Graphene nanoconstrictions achieved via mechanical exfoliation of highly oriented pyrolytic graphite evidence clear signatures of one-dimensional transport via zigzag edges.

We studied the development of interfacial energy γ of alkanethiol self-assembled monolayers on Ag/mica substrates by force-displacement spectroscopy using an atomic force microscope. Self-assembled monolayers of decanethiol [CH3 (CH2)9SH] and octadecanethiol [CH3 (CH2)17SH] were studied. The growth of the monolayer as a function of time was studied by an ellipsometer for three different concentrations of decanethiol and octadecanethiol. The thiols led to well-ordered self-assembled monolayer that showed atomic resolution images by lateral force microscopy mode. The behaviour of γ showed non-monotonic variation in stages of adsorption, which was more prominent for short chain length decanethiol. The γ reached a limiting value of 12 mJ m−2 for decanethiol and 7 mJ m−2 for octadecanethiol.

Ultra-thin flakes of layered materials have recently been attracting widespread research interest due to their exotic properties. In this work, we study the optoelectronic response of a hybrid of two such materials – graphene and MoS2. Our devices consist of mechanically exfoliated graphene flakes transferred on top of similarly exfoliated MoS2. The electrical response of the hybrid is studied in the presence of white light. We show that the four-point resistance of graphene is modulated in the presence of light. This effect is observed to be a strong function of gate voltage. We have also extended our studies to CVD (chemical vapor deposition) - grown graphene transferred onto MoS2 which show qualitatively similar features, thereby attesting to the scalability of the device architecture.

We experimentally demonstrate that low-frequency electrical noise in silver nanowires is heavily suppressed when the crystal structure of the nanowires is hexagonal closed pack (hcp) rather than face centered cubic (fcc). Using a low-potential electrochemical method we have grown single crystalline silver nanowires with hcp crystal structure, in which the noise at room temperature is two to six orders of magnitude lower than that in the conventional fcc nanowires of the same diameter. We suggest that motion of dislocations is probably the primary source of electrical noise in metallic nanowires, which is strongly diminished in hcp crystals., Comment: 7 pages, 4 figures

Molecular wires of charge transfer molecules were formed by co-evaporating the 7 7 8 8-Tetracyanoquinodimethane [TCNQ] (acceptor) and Tetrathiafulvalene [TTF] (donor) molecules across prefabricated metal electrodes. Molecular wires of TTF TCNQ were also formed by evaporating single complex of TTF:TCNQ across prefabricated metal electrodes The prefabricated metal electrodes were made using electron beam lithography on SiO2 and glass cover slip substrates. Even though TTF: TCNQ wires grown from both co-evaporation and evaporation techniques show semiconductor like behavior in temperature dependence of resistance they show different activation energies due the difference in stoichiometry of TTF and TCNQ.

We present an experimental investigation of the variation of the amplitude of vibrating microcantilever, as a function of distance (h) between the microcantilever and the sample in a Dynamic Force Microscopy (DFM) and explain the observations with a theoretical model. In DFM, as the cantilever tip approaches the sample, neither the force nor the response of the cantilever is in the linear regime. We present an exact numerical solution to the equation of motion of the oscillations of the microcantilever and present a quantitative explanation to the observed force versus distance curves, in terms of the resonance curves. We show that the change in the resonance frequency of the cantilever due to the atomic forces is highly nonlinear.

We report the growth of nanowires of the charge transfer complex tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) with diameters as low as 130 nm and show that such nanowires can show Peierls transitions at low temperatures. The wires of sub-micron length were grown between two prefabricated electrodes (with sub-micron gap) by vapor phase growth from a single source by applying an electric field between the electrodes during the growth process. The nanowires so grown show a charge transfer ratio approximately 0.57, which is close to that seen in bulk crystals. Below the transition the transport is strongly nonlinear and can be interpreted as originating from de-pinning of CDW that forms at the Peierls transition.