High-Field Electron Transport and High Saturation Velocity in Multilayer Indium Selenide Transistors.

Creating a high-frequency electron system demands a high saturation velocity (υsat). Herein, we report the high-field transport properties of multilayer van der Waals (vdW) indium selenide (InSe). The InSe is on a hexagonal boron nitride substrate and encapsulated by a thin, noncontinuous In layer, resulting in an impressive electron mobility reaching 2600 cm2/(V s) at room temperature. The high-mobility InSe achieves υsat exceeding 2 × 107 cm/s, which is superior to those of other gapped vdW semiconductors, and exhibits a 50-60% improvement in υsat when cooled to 80 K. The temperature dependence of υsat suggests an optical phonon energy (ℏωop) for InSe in the range of 23-27 meV, previously reported values for InSe. It is also notable that the measured υsat values exceed what is expected according to the optical phonon emission model due to weak electron-phonon scattering. The superior υsat of our InSe, despite its relatively small ℏωop, reveals its potential for high-frequency electronics, including applications to control cryogenic quantum computers in close proximity.

Drain-Induced Multifunctional Ambipolar Electronics Based on Junctionless MoS2.

Applying a drain bias to a strongly gate-coupled semiconductor influences the carrier density of the channel. However, practical applications of this drain-bias-induced effect in the advancement of switching electronics have remained elusive due to the limited capabilities of its current modulation known to date. Here, we show strategies to largely control the current by utilizing drain-bias-induced carrier type switching in an ambipolar molybdenum disulfide (MoS2) field-effect transistor with Pt bottom contacts. Our CMOS-compatible device architecture, incorporating a partially gate-coupled p-n junction, achieves multifunctionality. The ambipolar MoS2 device operates as an ambipolar transistor (on/off ratios exceeding 107 for both NMOS and PMOS), a rectifier (rectification ratio of ∼3 × 106), a reversible negative breakdown diode with an adjustable breakdown voltage (on/off ratio exceeding 109 with a maximum current as high as 10-4 A), and a photodetector. Finally, we demonstrate a complementary inverter (gain of ∼24 at Vdd = 1.5 V), which is highly facile to fabricate without the need for complex heterostructures and doping processes. Our study provides strategies to achieve high-performance ambipolar MoS2 devices and to effectively utilize drain bias for electrical switching.

Identification of geographical origins of soybean pastes using headspace gas chromatography-mass spectrometry by selecting sample-descriptive components with an Incremental Association Markov Blanket.

The identification of geographical origins of soybean pastes using headspace gas chromatography-mass spectrometry was attempted in this study. Since soybean paste was odor-rich, 36 components were identified in the imported and domestic soybean samples. t-Test, variable importance in projection (VIP), and Incremental Association Markov Blanket (IAMB) were employed to select proper components that could effectively discriminate the two sample groups. The discrimination accuracies were below 87.3 % when all 36 components were fed for either LDA, k-NN, or SVM. When the five t-test-selected components or six VIP score-selected components were employed, the accuracies improved to 95.2-96.2 %. The IAMB selected three different components were 3-methylbutanal, 4-methylnonane, and 2,3-pentanedione, and the correlations among their peak areas were not significant. This suggests that these three components were independently relevant for the discrimination. The accuracy obtained using these three components was superior, 97.7 %, as undescriptive and/or redundant components for the discrimination were excluded.

Effective inactivation of Bacillus atrophaeus spores and Escherichia coli on disposable face masks using ultraviolet laser irradiation.

Due to the widespread emergence of COVID-19, face masks have become a common tool for reducing transmission risk between people, increasing the need for sterilization methods against mask-contaminated microorganisms. In this study, we measured the efficacy of ultraviolet (UV) laser irradiation (266 nm) as a sterilization technique against Bacillus atrophaeus spores and Escherichia coli on three different types of face mask. The UV laser source demonstrated high penetration of inner mask layers, inactivating microorganisms in a short time while maintaining the particle filtration efficiency of the masks. This study demonstrates that UV laser irradiation is an efficient sterilization method for removing pathogens from face masks.

Zero power infrared sensing in 2D/3D-assembled heterogeneous graphene/In/InSe/Au.

Low- or self-powered infrared sensors can be used in a broad range of applications, including networking mobile edge devices and image recognition for autonomous driving technology. Here, we show state-of-the-art self-powered near-infrared (NIR) sensors using graphene/In/InSe/Au as a photoactive region. The self-powered NIR sensors show outstanding performance, achieving a photoresponsivity of ∼8.5 A W-1 and a detectivity of ∼1012 Jones at 850 nm light. Multiple self-powered InSe photodetectors with different device structures and contacts were systematically investigated. In particular, the asymmetrically assembled graphene/In/InSe/Au vertical heterostructure offers a high built-in field, which gives rise to efficient electron-hole pair separation and transit time that is shorter than the photocarrier lifetime. The built-in potential across the InSe was estimated using the Schottky barrier height at each metal contact with InSe, obtained using density functional theory calculations. We also demonstrate InSe vertical field-effect transistors and provide an out-of-plane carrier mobility of InSe. Using the out-of-plane mobility and structural parameters of each device, the built-in field, drift velocity, and corresponding transit time are estimated.

Sterilization effects of UV laser irradiation on Bacillus atrophaeus spore viability, structure, and proteins.

Bacillus spores are highly resistant to toxic chemicals and extreme environments. Because some Bacillus species threaten public health, spore inactivation techniques have been intensively investigated. We exposed Bacillus atrophaeus spores to a 266 nm Nd:YVO4 laser at a laser power of 1 W and various numbers of scans. As a result, the UV laser reduced the viability of Bacillus atrophaeus spores. Although the outer coat of spores remained intact after UV laser irradiation of 720 scans, damage inside the spores was observed. Spore proteins were identified by matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry during the course of UV laser irradiation. Photochemical and photothermal processes are believed to be involved in the UV laser sterilization of Bacillus spores. Our findings suggest that a UV laser is capable of sterilizing Bacillus atrophaeus spores.

Bias-controlled multi-functional transport properties of InSe/BP van der Waals heterostructures.

Van der Waals (vdW) heterostructures, consisting of a variety of low-dimensional materials, have great potential use in the design of a wide range of functional devices thanks to their atomically thin body and strong electrostatic tunability. Here, we demonstrate multi-functional indium selenide (InSe)/black phosphorous (BP) heterostructures encapsulated by hexagonal boron nitride. At a positive drain bias (VD), applied on the BP while the InSe is grounded, our heterostructures show an intermediate gate voltage (VBG) regime where the current hardly changes, working as a ternary transistor. By contrast, at a negative VD, the device shows strong negative differential transconductance characteristics; the peak current increases up to ~5 µA and the peak-to-valley current ratio reaches 1600 at VD = -2 V. Four-terminal measurements were performed on each layer, allowing us to separate the contributions of contact resistances and channel resistance. Moreover, multiple devices with different device structures and contacts were investigated, providing insight into the operation principle and performance optimization. We systematically investigated the influence of contact resistances, heterojunction resistance, channel resistance, and the thickness of BP on the detailed operational characteristics at different VD and VBG regimes.

Substitutional Fluorine Doping of Large-Area Molybdenum Disulfide Monolayer Films for Flexible Inverter Device Arrays.

Reliable and controllable doping of transition metal dichalcogenides (TMDCs) is a mandatory requirement for practical large-scale electronic applications. However, most of the literature on the doping methodologies of TMDCs has focused on n-type doping and multilayer TMDC rather than a monolayer one enabling large-scale growth. Herein, we report substitutional fluorine doping of a chemical vapor deposition (CVD)-grown molybdenum disulfide (MoS2) monolayer film using a delicate SF6 plasma treatment. Our SF6-treated MoS2 monolayer shows a p-type doping effect with fluorine substitution. The doping concentration is controlled by the plasma treatment time (2-4.9 atom %) while maintaining the structural integrity of the MoS2 monolayer. Such reliable and tunable substitutional doping is attributed to preventing direct ion bombardment to the MoS2 monolayer by our gentle plasma treatment system. Finally, we fabricated MoS2 homojunction flexible inverter device arrays based on the pristine and SF6-treated MoS2 monolayer. A clear switching behavior is observed, and the voltage gain is approximately 8 at an applied VDD of 2 V, which is comparable to that of CVD-grown MoS2-based inverter devices reported previously. Obtained voltage gain is also stable even after 500 bending cycles at an applied strain of 0.5%.

Lowering the Schottky Barrier Height by Graphene/Ag Electrodes for High-Mobility MoS2 Field-Effect Transistors.

2D transition metal dichalcogenides (TMDCs) have emerged as promising candidates for post-silicon nanoelectronics owing to their unique and outstanding semiconducting properties. However, contact engineering for these materials to create high-performance devices while adapting for large-area fabrication is still in its nascent stages. In this study, graphene/Ag contacts are introduced into MoS2 devices, for which a graphene film synthesized by chemical vapor deposition (CVD) is inserted between a CVD-grown MoS2 film and a Ag electrode as an interfacial layer. The MoS2 field-effect transistors with graphene/Ag contacts show improved electrical and photoelectrical properties, achieving a field-effect mobility of 35 cm2 V-1 s-1 , an on/off current ratio of 4 × 108 , and a photoresponsivity of 2160 A W-1 , compared to those of devices with conventional Ti/Au contacts. These improvements are attributed to the low work function of Ag and the tunability of graphene Fermi level; the n-doping of Ag in graphene decreases its Fermi level, thereby reducing the Schottky barrier height and contact resistance between the MoS2 and electrodes. This demonstration of contact interface engineering with CVD-grown MoS2 and graphene is a key step toward the practical application of atomically thin TMDC-based devices with low-resistance contacts for high-performance large-area electronics and optoelectronics.

Effect of ribbon width on electrical transport properties of graphene nanoribbons.

There has been growing interest in developing nanoelectronic devices based on graphene because of its superior electrical properties. In particular, patterning graphene into a nanoribbon can open a bandgap that can be tuned by changing the ribbon width, imparting semiconducting properties. In this study, we report the effect of ribbon width on electrical transport properties of graphene nanoribbons (GNRs). Monolayer graphene sheets and Si nanowires (NWs) were prepared by chemical vapor deposition and a combination of nanosphere lithography and metal-assisted electroless etching from a Si wafer, respectively. Back-gated GNR field-effect transistors were fabricated on a heavily p-doped Si substrate coated with a 300 nm-thick SiO2 layer, by O2 reactive ion etching of graphene sheets using etch masks based on Si NWs aligned on the graphene between the two electrodes by a dielectrophoresis method. This resulted in GNRs with various widths in a highly controllable manner, where the on/off current ratio was inversely proportional to ribbon width. The field-effect mobility decreased with decreasing GNR widths due to carrier scattering at the GNR edges. These results demonstrate the formation of a bandgap in GNRs due to enhanced carrier confinement in the transverse direction and edge effects when the GNR width is reduced.

Sulfur vacancy-induced reversible doping of transition metal disulfides via hydrazine treatment.

Chemical doping of transition metal dichalcogenides (TMDCs) has drawn significant interest because of its applicability to the modification of electrical and optical properties of TMDCs. This is of fundamental and technological importance for high-efficiency electronic and optoelectronic devices. Here, we present a simple and facile route to reversible and controllable modulation of the electrical and optical properties of WS2 and MoS2via hydrazine doping and sulfur annealing. Hydrazine treatment of WS2 improves the field-effect mobilities, on/off current ratios, and photoresponsivities of the devices. This is due to the surface charge transfer doping of WS2 and the sulfur vacancies formed by its reduction, which result in an n-type doping effect. The changes in the electrical and optical properties are fully recovered when the WS2 is annealed in an atmosphere of sulfur. This method for reversible modulation can be applied to other transition metal disulfides including MoS2, which may enable the fabrication of two-dimensional electronic and optoelectronic devices with tunable properties and improved performance.