Low-Power Complementary Inverter Based on Graphene/Carbon-Nanotube and Graphene/MoS2 Barristors.

The recent report of a p-type graphene(Gr)/carbon-nanotube(CNT) barristor facilitates the application of graphene barristors in the fabrication of complementary logic devices. Here, a complementary inverter is presented that combines a p-type Gr/CNT barristor with a n-type Gr/MoS2 barristor, and its characteristics are reported. A sub-nW (~0.2 nW) low-power inverter is demonstrated with a moderate gain of 2.5 at an equivalent oxide thickness (EOT) of ~15 nm. Compared to inverters based on field-effect transistors, the sub-nW power consumption was achieved at a much larger EOT, which was attributed to the excellent switching characteristics of Gr barristors.

Indirect Band Gap in Scrolled MoS2 Monolayers.

MoS2 nanoscrolls that have inner core radii of ∼250 nm are generated from MoS2 monolayers, and the optical and transport band gaps of the nanoscrolls are investigated. Photoluminescence spectroscopy reveals that a MoS2 monolayer, originally a direct gap semiconductor (∼1.85 eV (optical)), changes into an indirect gap semiconductor (∼1.6 eV) upon scrolling. The size of the indirect gap for the MoS2 nanoscroll is larger than that of a MoS2 bilayer (∼1.54 eV), implying a weaker interlayer interaction between concentric layers of the MoS2 nanoscroll compared to Bernal-stacked MoS2 few-layers. Transport measurements on MoS2 nanoscrolls incorporated into ambipolar ionic-liquid-gated transistors yielded a band gap of ∼1.9 eV. The difference between the transport and optical gaps indicates an exciton binding energy of 0.3 eV for the MoS2 nanoscrolls. The rolling up of 2D atomic layers into nanoscrolls introduces a new type of quasi-1D nanostructure and provides another way to modify the band gap of 2D materials.

Simulation of Figures of Merit for Barristor Based on Graphene/Insulator Junction.

We investigated the tunneling of graphene/insulator/metal heterojunctions by revising the Tsu-Esaki model of Fowler-Nordheim tunneling and direct tunneling current. Notably, the revised equations for both tunneling currents are proportional to V3, which originates from the linear dispersion of graphene. We developed a simulation tool by adopting revised tunneling equations using MATLAB. Thereafter, we optimized the device performance of the field-emission barristor by engineering the barrier height and thickness to improve the delay time, cut-off frequency, and power-delay product.

Charge Transport in UV-Oxidized Graphene and Its Dependence on the Extent of Oxidation.

Graphene oxides with different degrees of oxidation are prepared by controlling UV irradiation on graphene, and the charge transport and the evolution of the transport gap are investigated according to the extent of oxidation. With increasing oxygenous defect density nD, a transition from ballistic to diffusive conduction occurs at nD≃1012 cm-2 and the transport gap grows in proportion to nD. Considering the potential fluctuation related to the e-h puddle, the bandgap of graphene oxide is deduced to be Eg≃30nD(1012cm-2) meV. The temperature dependence of conductivity showed metal-insulator transitions at nD≃0.3×1012 cm-2, consistent with Ioffe-Regel criterion. For graphene oxides at nD≥4.9×1012 cm-2, analysis indicated charge transport occurred via 2D variable range hopping conduction between localized sp2 domain. Our work elucidates the transport mechanism at different extents of oxidation and supports the possibility of adjusting the bandgap with oxygen content.

Atomic layer deposited Al2O3passivation layer for few-layer WS2field effect transistors.

We have investigated the effect of an Al2O3passivation layer on the performance of few-layer WS2FETs. While the performance of WS2FETs is often limited by a substantial decrease in carrier mobility owing to charged impurities and a Schottky barrier between the WS2and metal electrodes, the introduction of an Al2O3overlayer by atomic layer deposition (ALD) suppressed the influence of charged impurities by high-κdielectric screening effect and reduced the effective Schottky barrier height. We argue that n-doping of WS2, induced by positive fixed charges formed at Al2O3/WS2interface during the ALD process, is responsible for the reduction of the effective Schottky barrier height in the devices. In addition, the Al2O3passivation layer protected the device from oxidation, and maintained stable electrical performance of the WS2FETs over 57 d. Thus, the ALD of Al2O3overlayer provides a facile method to enhance the performance of WS2FETs and to ensure ambient stability.

Semiconductor-less vertical transistor with ION/IOFF of 106.

Semiconductors have long been perceived as a prerequisite for solid-state transistors. Although switching principles for nanometer-scale devices have emerged based on the deployment of two-dimensional (2D) van der Waals heterostructures, tunneling and ballistic currents through short channels are difficult to control, and semiconducting channel materials remain indispensable for practical switching. In this study, we report a semiconductor-less solid-state electronic device that exhibits an industry-applicable switching of the ballistic current. This device modulates the field emission barrier height across the graphene-hexagonal boron nitride interface with ION/IOFF of 106 obtained from the transfer curves and adjustable intrinsic gain up to 4, and exhibits unprecedented current stability in temperature range of 15-400 K. The vertical device operation can be optimized with the capacitive coupling in the device geometry. The semiconductor-less switching resolves the long-standing issue of temperature-dependent device performance, thereby extending the potential of 2D van der Waals devices to applications in extreme environments.

Large Temperature-Independent Magnetoresistance without Gating Operation in Monolayer Graphene.

Temperature-independent magnetoresistance (TIMR) has been studied for applications in magnetic field sensors operating in wide temperature ranges. Graphene is considered as one of the best candidates for achieving nonsaturating and large TIMR through engineering disorders. Nevertheless, large TIMR has not been achieved in disordered graphene with intrinsic defects, such as chemical doping and atomic dislocations. In this work, by introducing extrinsic defects, we realize nonsaturating and large TIMR in monolayer graphene transferred on a BiFeO3 nanoisland array (G/BFO-NIA). Furthermore, the G/BFO-NIA device exhibits a significantly larger MR (∼250% under 9 T) than other materials without gating operation, demonstrating its application feasibility. It is shown that the large MR is a result of the coexistence of electrons and holes with almost the same density, and the observed TIMR originates from the temperature dependence of carrier transport in graphene and of the dielectric property of BFO-NIA.

Structural configurations and Raman spectra of carbon nanoscrolls.

Three types of carbon nanoscroll (CNS) structures that are formed when scrolling up graphene sheets are investigated using Raman spectroscopy and atomic force microscopy (AFM). The CNSs were produced from exfoliated monolayer graphene deposited on a Si chip by applying a droplet of isopropyl alcohol (IPA) solution. The three types of CNS are classified as single-elliptical-core, double-elliptical-core (both with large internal volumes) and collapsed ribbon-like, based on AFM surface profile measurements. We discuss the structure and formation of CNS with much larger hollow cores than is commonly assumed and relate this to the large effective 2D bending stiffness of graphene in the IPA solution. The large elliptical core structures show Raman spectra similar to those previously reported for CNS and indicate little interaction between the scrolled layers. The Raman spectra from ribbon-like structures show additional features that are similar to that of folded graphene. These new features can be related to layer breathing modes combined with some resonance enhancement at specific regions of the ribbon-like CNSs that are due to specific twist angles produced when the structure folds/collapses.

The evolution of surface cleanness and electronic properties of graphene field-effect transistors during mechanical cleaning with atomic force microscopy.

The evolution of surface cleanliness and the electronic properties-Dirac voltage(V Dirac), hysteresis and mobility (µ) of a graphene field-effect transistor (GFET)-were monitored by measuring lateral force microscopy and drain current (I D) as a function of gate voltage (V G), after mechanically cleaning the surface, scan-by-scan, with contact-mode atomic force microscopy. Both the surface cleanliness and the electronic properties evolved, showing a sudden improvement and then saturation for a mobility of around 2200 cm2 V-1 s-1. We found that the mobility suppression of the as-fabricated GFET deviated from a randomly distributed impurities model, which predicted a greater mobility than obtained from the measured V Dirac. Therefore, the substrate impurities are excluded from the origins of the extraordinary suppression of the mobility, and the possible origin will be discussed.

Chemical Vapor-Deposited Vanadium Pentoxide Nanosheets with Highly Stable and Low Switching Voltages for Effective Selector Devices.

Recently, attempts to overcome the physical limits of memory devices have led to the development of promising materials and architectures for next-generation memory technology. The selector device is one of the essential ingredients of high-density stacked memory systems. However, complicated constituent deposition conditions and thermal degradation are problematic, even with effective selector device materials. Herein, we demonstrate the highly stable and low-threshold voltages of vanadium pentoxide (V2O5) nanosheets synthesized by facile chemical vapor deposition, which have not been previously reported on the threshold switching (TS) properties. The electrons occupying trap sites in poly-crystalline V2O5 nanosheet contribute to the perfectly symmetric TS feature at the bias polarity and low-threshold voltages in V2O5, confirmed by high-resolution transmission electron microscopy measurements. Furthermore, we find an additional PdO interlayer in V2O5 nanodevices connected with a Pd/Au electrode after thermal annealing treatment. The PdO interlayer decreases the threshold voltages, and the Ion/ Ioff ratio increases because of the increased trap density of V2O5. These studies provide insights into V2O5 switching characteristics, which can support low power consumption in nonvolatile memory devices.

SERS-Based Flavonoid Detection Using Ethylenediamine-ß-Cyclodextrin as a Capturing Ligand.

Ethylenediamine-modified ß-cyclodextrin (Et-ß-CD) was immobilized on aggregated silver nanoparticle (NP)-embedded silica NPs (SiO2@Ag@Et-ß-CD NPs) for the effective detection of flavonoids. Silica NPs were used as the template for embedding silver NPs to create hot spots and enhance surface-enhanced Raman scattering (SERS) signals. Et-ß-CD was immobilized on Ag NPs to capture flavonoids via host-guest inclusion complex formation, as indicated by enhanced ultraviolet absorption spectra. The resulting SiO2@Ag@Et-ß-CD NPs were used as the SERS substrate for detecting flavonoids, such as hesperetin, naringenin, quercetin, and luteolin. In particular, luteolin was detected more strongly in the linear range 10-7 to 10-3 M than various organic molecules, namely ethylene glycol, ß-estradiol, isopropyl alcohol, naphthalene, and toluene. In addition, the SERS signal for luteolin captured by the SiO2@Ag@Et-ß-CD NPs remained even after repeated washing. These results indicated that the SiO2@Ag@Et-ß-CD NPs can be used as a rapid, sensitive, and selective sensor for flavonoids.

Engineering Optical and Electronic Properties of WS2 by Varying the Number of Layers.

The optical constants, bandgaps, and band alignments of mono-, bi-, and trilayer WS2 were experimentally measured, and an extraordinarily high dependency on the number of layers was revealed. The refractive indices and extinction coefficients were extracted from the optical-contrast oscillation for various thicknesses of SiO2 on a Si substrate. The bandgaps of the few-layer WS2 were both optically and electrically measured, indicating high exciton-binding energies. The Schottky-barrier heights (SBHs) with Au/Cr contact were also extracted, depending on the number of layers (1-28). From an engineering viewpoint, the bandgap can be modulated from 3.49 to 2.71 eV with additional layers. The SBH can also be reduced from 0.37 eV for a monolayer to 0.17 eV for 28 layers. The technique of engineering materials' properties by modulating the number of layers opens pathways uniquely adaptable to transition-metal dichalcogenides.

Effect of SOCl2 treatment on electrical and mechanical properties of single-wall carbon nanotube networks.

Chemical modification by SOCl2 of an entangled network of purified single-wall carbon nanotubes, also known as 'bucky paper', is reported to profoundly change the electrical and mechanical properties of this system. Four-probe measurements indicate a conductivity increase by up to a factor of 5 at room temperature and an even more pronounced increase at lower temperatures. This chemical modification also improves the mechanical properties of SWNT networks. Whereas the pristine sample shows an overall semiconducting character, the modified material behaves as a metal. The effect of SOCl2 is studied in terms of chemical doping of the nanotube network. We identified the microscopic origin of these changes using SEM, XPS, NEXAFS, EDX, and Raman spectroscopy measurements and ab initio calculations. We interpret the SOCl2-induced conductivity increase by p-type doping of the pristine material. This conclusion is reached by electronic structure calculations, which indicate a Fermi level shift into the valence band, and is consistent with the temperature dependence of the thermopower.