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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. In field-effect transistors, a semiconducting channel is indispensable for device switching. Here, the authors demonstrate semiconductor-less switching via modulation of the field emission barrier height across a graphene-hBN interface with ON/OFF ratio of 106.

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.

We have investigated the effect of an Al

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., In field-effect transistors, a semiconducting channel is indispensable for device switching. Here, the authors demonstrate semiconductor-less switching via modulation of the field emission barrier height across a graphene-hBN interface with ON/OFF ratio of 106.

The electrical performance of suspended few-layer MoTe2 field-effect-transistors with ionic liquid gating has been investigated. The suspended structure not only enhances the mobility of MoTe2 by removing the influence of the substrate but also allows ions to accumulate on both the top and the bottom surface of MoTe2. The consequent increase in the gate capacitance resulted in an improved subthreshold swing (∼73 mV/dec) and on-off ratio (106) at room temperature for suspended MoTe2 compared to substrate-supported devices. Suspended transistors with ionic liquid gating enable a larger charge density compared to ionic liquid gated supported devices and may provide a useful platform to study screening physics in 2D materials.

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.

We have investigated the performances of barristors with a graphene-tungsten disulfide (WS 2 ) junction by varying the thickness of WS 2 and gate oxide. On-current density ( J ON ) and on- and off-current ratio ( J ON / J OFF ) increases, and sub-threshold swing ( V SS ) decreases with the WS 2 thickness. Also, barristors with thicker WS 2 required less workfunction shift, to switch the barristors. Therefore, unlike the traditional devices, V SS of barristor with gate dielectric 300 nm was smaller than that of 90 nm, when the former is fabricated with thicker WS 2 than the latter. Since materials properties of 2-dimensional semiconductors generally vary with their thickness, the thickness of 2D semiconductors could become a key parameter to engineer the performance of barristors with graphene and the 2D semiconductors.

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.

The temperature ($\it T$) dependence of electrical resistivity in graphene has been experimentally investigated between 10 and 400 K for samples prepared on various substrates; HfO$_2$, SiO$_2$ and h-BN. The resistivity of graphene shows a linear $\it T$-dependence at low $\it T$ and becomes superlinear above a substrate-dependent transition temperature. The results are explained by remote interfacial phonon scattering by surface optical phonons at the substrates. The use of an appropriate substrate can lead to a significant improvement in the charge transport of graphene.

We have investigated the current distribution between individual layers of multilayer MoS2 in a field-effect transistor structure with source and drain electrodes on the top layer. By employing a laser thinning technique, we partially etched down the multilayer in the middle of the device and monitored the electrical current after each etching step. The current was decreased by several orders of magnitude when the upper layers were partially etched away. Our observation suggests that the electrical current of multilayer MoS2 flows mainly through the upper layers close to the source/drain contacts due to relatively large interlayer resistance of MoS2.

We have investigated the effect of laser irradiation on monolayer MoS2 and observed the swellingup of the monolayer from the SiO2 substrate upon laser illumination. The mismatch in the thermal expansion between the substrate and MoS2 can result in the structural deformation. Employing this method, one can induce structural deformation in a desired pattern, and one can demonstrate the patterning of periodic ripples in monolayer MoS2 by using laser irradiation. The controlled fabrication of the ripple structure may be instrumental in understanding the effect of ripples on the interesting physical properties of monolayer MoS2.

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.

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 th...

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.

We have investigated the longitudinal magnetoresistance (MR) of thin films of single-walled carbon nanotubes, in pulsed magnetic field up to 32 T. We report the longitudinal MR markedly different from the transverse MR. At T = 38.5 K, the longitudinal MR exhibits a pronounced positive MR and saturates around 20 T, while the transverse MR remained negative for similar carbon nanotube films. With decreasing temperature down to 11 K, the negative contribution to the MR increases at low field and the positive contribution diminishes. The minimum of the MR, thus, appears at a higher field with lowering T , for the temperature range between 11 and 38.5 K. On the other hand, below 10 K, the longitudinal MR behaves similar to the transverse MR. The behaviors of the longitudinal MR are distinguished across T ∼ 11 K, and they might have different origin. We suggest Aharonov–Bohm effect, which tunes the band structure of the CNTs by the field parallel to the tube axis, may play a role.

Ethylenediamine-modified -cyclodextrin (Et-beta-CD) was immobilized on aggregated silver nanoparticle (NP)-embedded silica NPs (SiO2@Ag@Et--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-beta-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-beta-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, beta-estradiol, isopropyl alcohol, naphthalene, and toluene. In addition, the SERS signal for luteolin captured by the SiO2@Ag@Et-beta-CD NPs remained even after repeated washing. These results indicated that the SiO2@Ag@Et-beta-CD NPs can be used as a rapid, sensitive, and selective sensor for flavonoids.

We have investigated random telegraph noise (RTN) observed in individual semiconducting carbon nanotubes (CNTs) in Coulomb-blockade regime. RTN characteristics are studied as a function of gate-voltage and drain–source voltage. Our results are explained by the capture and emission of carriers by charge traps in the vicinity of CNTs. Because of the large RTN amplitude, often greater than ∼50% of the total current, RTN measurements can be developed into an effective tool to estimate the trap energy and the location of the trap in CNT quantum dot devices.

We have investigated Hall coefficient and magnetoresistance in thin films of single-walled carbon nanotubes, prepared in four different ways. Hall voltages are linear for all samples in magnetic fields up to 6 T, and the measured carrier density lies in ∼10 21 –10 22 cm −3 . Whereas earlier Hall-effect experiments reported ∼10 18 –10 19 cm −3 for the carrier density, our results are consistent with the theoretically predicted value of ∼10 22 cm −3 , calculated for the aligned metallic CNTs. The signs of the Hall coefficients are positive in general, indicating that majority carriers are holes in these films. In a nanotube film with the lowest conductivity, however, we find the Hall coefficient reverses the sign at low temperature around T = 15 K. The origin of the sign change is not clear. In strongly localized regime, the Hall effect can be anomalous.