Mitigating substrate effects of van der Waals semiconductors using perfluoropolyether self-assembled monolayers.

The properties of transition metal dichalcogenides (TMDCs) are critically dependent on the dielectric constant of substrates, which significantly limits their application. To address this issue, we used a perfluorinated polyether (PFPE) self-assembled monolayer (SAM) with low surface energy to increase the van der Waals (vdW) gap between TMDCs and the substrate, thereby reducing the interaction between them. This resulted in a reduction in the subthreshold swing value, an increase in the photoluminescence intensity of excitons, and a decrease in the doping effect by the substrate. This work will provide a new way to control the TMDC/dielectric interface and contribute to expanding the applicability of TMDCs.

Interface Trap Suppression and Electron Doping in Van der Waals Materials Using Cross-Linked Poly(vinylpyrrolidone).

The instability of van der Waals (vdW) materials leads to spontaneous morphological and chemical transformations in the air. Although the passivation of vdW materials with other resistive materials is often used to solve stability issues, this passivation layer can block carrier injection and thus interfere with charge transfer doping. In this study, a facile method is proposed for n-doping and mediation of Se vacancies in tungsten diselenide (WSe2) by poly(vinylpyrrolidone) (PVP) coating. The major carrier type of the PVP-coated WSe2-based field-effect transistor (FET) was converted from hole (p-type) to electron (n-type). Furthermore, the vacancy-induced interface trap density was reduced by approximately 500 times. This study provides a practical doping and passivation method for the van der Waals materials, as well as a comprehensive understanding of the chemical reaction and electronic transport in these materials.

Modulation of Junction Modes in SnSe2/MoTe2 Broken-Gap van der Waals Heterostructure for Multifunctional Devices.

We study the electronic and optoelectronic properties of a broken-gap heterojunction composed of SnSe2 and MoTe2 with gate-controlled junction modes. Owing to the interband tunneling current, our device can act as an Esaki diode and a backward diode with a peak-to-valley current ratio approaching 5.7 at room temperature. Furthermore, under an 811 nm laser irradiation the heterostructure exhibits a photodetectivity of up to 7.5 × 1012 Jones. In addition, to harness the electrostatic gate bias, Voc can be tuned from negative to positive by switching from the accumulation mode to the depletion mode of the heterojunction. Additionally, a photovoltaic effect with a fill factor exceeding 41% was observed, which highlights the significant potential for optoelectronic applications. This study not only demonstrates high-performance multifunctional optoelectronics based on the SnSe2/MoTe2 heterostructure but also provides a comprehensive understanding of broken-band alignment and its applications.

Fabrication of pyramidal (111) MAPbBr3 film with low surface defect density using homogeneous quantum-dot seeds.

Nucleation and seeding of organometal halide perovskite (OHP) films have been extensively investigated for forming high-density, large-crystalline, and low-defect films. In this study, CH3NH3PbBr3 (MAPbBr3) films with a low defect density are synthesized via a molecular exchange mechanism using MAPbBr3 quantum dots as seeds. The synthesized films exhibit a pyramidal morphology with a (111) crystal plane. The distribution of the (111) plane is controlled by adjusting the seed concentration. The pyramidal MAPbBr3 films exhibit improved photoluminescence intensity and uniformity compared with films produced using seedless toluene. When the seeds are employed, the surface trap density is reduced by a factor of 3.5, suppressing the photocurrent hysteresis and nonsaturated response of photodetectors. Additionally, the films formed using the seeds have improved stability owing to the chain decomposition reaction induced by electron beam heating.

Encapsulation of a Monolayer WSe2 Phototransistor with Hydrothermally Grown ZnO Nanorods.

Transition metal dichalcogenides (TMDCs) are promising two-dimensional (2D) materials for realizing next-generation electronics and optoelectronics with attractive physical properties. However, monolayer TMDCs (1LTMDCs) have various serious issues, such as instability under ambient conditions and low optical quantum yield from their extremely thin thickness of ∼0.7 nm. To overcome these issues, we constructed a hybrid structure (HS) by growing zinc oxide nanorods (ZnO NRs) on a monolayer tungsten diselenide (1LWSe2) using the hydrothermal method. Consequently, we confirmed not only enhanced photoluminescence of 1LWSe2 but also improved optoelectronic properties by fabricating the HS phototransistor. Through various investigations, we found that these phenomena were due to the antenna and p-type doping effects attributed to the ZnO NRs. In addition, we verified that the optoelectronic properties of 1LTMDCs are maintained for 2 weeks in ambient condition through the sustainable encapsulation effect induced by our HS. This encapsulation method with inorganic materials is expected to be applied to improve the stability and performance of various emerging 2D material-based devices.

Modulating the Functions of MoS2/MoTe2 van der Waals Heterostructure via Thickness Variation.

Various functional devices including p-n forward, backward, and Zener diodes are realized with a van der Waals heterostructure that are composed of molybdenum disulfide (MoS2) and molybdenum ditelluride (MoTe2) by changing the thickness of the MoTe2 layer and common gate bias. In addition, the available negative differential transconductance of the heterostructure is utilized to fabricate a many-valued logic device that exhibits three different logic states ( i.e., a ternary inverter). Furthermore, the multivalued logic device can be transformed into a binary inverter using laser irradiation. This work provides a comprehensive understanding of the device fabrication and electronic-device design utilizing thickness control.

Photovoltaic effect in a few-layer ReS2/WSe2 heterostructure.

Two-dimensional transition-metal dichalcogenides (TMDCs) are notable materials owing to their flexibility, transparency, and appropriate bandgaps. Because of their unique advantages, TMDC p-n diodes have been studied for next-generation electronics and optoelectronics. However, their efficiency must be increased for commercialization. In this study, we demonstrated a heterostructure composed of few-layer ReS2 and WSe2. This few-layer ReS2/WSe2 heterostructure exhibits a p-n junction and an n-n junction in different gate-bias regimes. In the p-n junction regime, the heterostructure shows outstanding rectification behavior. Additionally, we identify three carrier-transfer mechanisms - direct tunneling, Fowler-Nordheim tunneling, and the space charge region - depending on the drain bias. Furthermore, the photovoltaic effect is observed in this few-layer ReS2/WSe2 heterostructure. As a result, a high fill factor (≈ 0.56), power conversion (≈ 1.5%), and external quantum efficiency (≈ 15.3%) were obtained. This study provides new guidelines for flexible optoelectronic devices.

Parameter control for enhanced peak-to-valley current ratio in a MoS2/MoTe2 van der Waals heterostructure.

The Ids-Vds properties of a van der Waals cross-junction of few layered MoS2/MoTe2 were investigated, and the physical device parameters were altered in order to transform the conduction mechanism from thermionic emission to interband tunneling. The pristine heterostructure demonstrated rectification behavior of typical p-n junction diodes, because of the p-type and n-type nature of MoTe2 and MoS2, respectively. Lowering the contact resistance between the metal and channel materials, by changing the electrode metals from Au to Pd and Ti, alone did not give rise to carrier conduction through the hetero-interband tunneling between MoTe2 and MoS2. In addition to the reduction in contact resistance, the chemical doping of MoS2 using Benzyl Viologen (BV) achieves hetero-interband tunneling between MoTe2 and MoS2, which probably narrows the depletion layer by degenerating MoS2. The peak-to-valley ratio of the tunneling current of the BV-doped heterostructure of MoS2/MoTe2 is about 4.8, which is comparable to that of the commercially available Si tunneling diode.

Augmented Quantum Yield of a 2D Monolayer Photodetector by Surface Plasmon Coupling.

Monolayer (1L) transition metal dichalcogenides (TMDCs) are promising materials for nanoscale optoelectronic devices because of their direct band gap and wide absorption range (ultraviolet to infrared). However, 1L-TMDCs cannot be easily utilized for practical optoelectronic device applications (e.g., photodetectors, solar cells, and light-emitting diodes) because of their extremely low optical quantum yields (QYs). In this investigation, a high-gain 1L-MoS2 photodetector was successfully realized, based on the surface plasmon (SP) of the Ag nanowire (NW) network. Through systematic optical characterization of the hybrid structure consisting of a 1L-MoS2 and the Ag NW network, it was determined that a strong SP and strain relaxation effect influenced a greatly enhanced optical QY. The photoluminescence (PL) emission was drastically increased by a factor of 560, and the main peak was shifted to the neutral exciton of 1L-MoS2. Consequently, the overall photocurrent of the hybrid 1L-MoS2 photodetector was observed to be 250 times better than that of the pristine 1L-MoS2 photodetector. In addition, the photoresponsivity and photodetectivity of the hybrid photodetector were effectively improved by a factor of ∼1000. This study provides a new approach for realizing highly efficient optoelectronic devices based on TMDCs.

Highly Enhanced Photoresponsivity of a Monolayer WSe2 Photodetector with Nitrogen-Doped Graphene Quantum Dots.

Hybrid structures of two-dimensional (2D) materials and quantum dots (QDs) are particularly interesting in the field of nanoscale optoelectronic devices because QDs are efficient light absorbers and can inject photocarriers into thin layers of 2D transition-metal dichalcogenides, which have high carrier mobility. In this study, we present a heterostructure that consists of a monolayer of tungsten diselenide (ML WSe2) covered by nitrogen-doped graphene QDs (N-GQDs). The improved photoluminescence of ML WSe2 is attributed to the dominant neutral exciton emission caused by the n-doping effect. Owing to strong light absorption and charge transfer from N-GQDs to ML WSe2, N-GQD-covered ML WSe2 showed up to 480% higher photoresponsivity than that of a pristine ML WSe2 photodetector. The hybrid photodetector exhibits good environmental stability, with 46% performance retention after 30 days under ambient conditions. The photogating effect also plays a key role in the improvement of hybrid photodetector performance. On applying the back-gate voltage modulation, the hybrid photodetector shows a responsivity of 2578 A W-1, which is much higher than that of the ML WSe2-based device.

A Novel and Facile Route to Synthesize Atomic-Layered MoS2 Film for Large-Area Electronics.

High-quality and large-area molybdenum disulfide (MoS2 ) thin film is highly desirable for applications in large-area electronics. However, there remains a challenge in attaining MoS2 film of reasonable crystallinity due to the absence of appropriate choice and control of precursors, as well as choice of suitable growth substrates. Herein, a novel and facile route is reported for synthesizing few-layered MoS2 film with new precursors via chemical vapor deposition. Prior to growth, an aqueous solution of sodium molybdate as the molybdenum precursor is spun onto the growth substrate and dimethyl disulfide as the liquid sulfur precursor is supplied with a bubbling system during growth. To supplement the limiting effect of Mo (sodium molybdate), a supplementary Mo is supplied by dissolving molybdenum hexacarbonyl (Mo(CO)6 ) in the liquid sulfur precursor delivered by the bubbler. By precisely controlling the amounts of precursors and hydrogen flow, full coverage of MoS2 film is readily achievable in 20 min. Large-area MoS2 field effect transistors (FETs) fabricated with a conventional photolithography have a carrier mobility as high as 18.9 cm2 V-1 s-1 , which is the highest reported for bottom-gated MoS2 -FETs fabricated via photolithography with an on/off ratio of ≈105 at room temperature.

Metal-Insulator-Semiconductor Diode Consisting of Two-Dimensional Nanomaterials.

We present a novel metal-insulator-semiconductor (MIS) diode consisting of graphene, hexagonal BN, and monolayer MoS2 for application in ultrathin nanoelectronics. The MIS heterojunction structure was fabricated by vertically stacking layered materials using a simple wet chemical transfer method. The stacking of each layer was confirmed by confocal scanning Raman spectroscopy and device performance was evaluated using current versus voltage (I-V) and photocurrent measurements. We clearly observed better current rectification and much higher current flow in the MIS diode than in the p-n junction and the metal-semiconductor diodes made of layered materials. The I-V characteristic curve of the MIS diode indicates that current flows mainly across interfaces as a result of carrier tunneling. Moreover, we observed considerably high photocurrent from the MIS diode under visible light illumination.

Semiconductor-Insulator-Semiconductor Diode Consisting of Monolayer MoS2, h-BN, and GaN Heterostructure.

We propose a semiconductor-insulator-semiconductor (SIS) heterojunction diode consisting of monolayer (1-L) MoS2, hexagonal boron nitride (h-BN), and epitaxial p-GaN that can be applied to high-performance nanoscale optoelectronics. The layered materials of 1-L MoS2 and h-BN, grown by chemical vapor deposition, were vertically stacked by a wet-transfer method on a p-GaN layer. The final structure was verified by confocal photoluminescence and Raman spectroscopy. Current-voltage (I-V) measurements were conducted to compare the device performance with that of a more classical p-n structure. In both structures (the p-n and SIS heterojunction diode), clear current-rectifying characteristics were observed. In particular, a current and threshold voltage were obtained for the SIS structure that was higher compared to that of the p-n structure. This indicated that tunneling is the predominant carrier transport mechanism. In addition, the photoresponse of the SIS structure induced by the illumination of visible light was observed by photocurrent measurements.

Comparison of the impact of the anesthesia induction using thiopental and propofol on cardiac function for non-cardiac surgery.

BACKGROUND: Thiopental and propofol have been widely used for general anesthesia induction, but their impacts on cardiac function have not been well described. A recent study speculated that anesthesia induction using propofol 2 mg/kg transiently reduced left ventricular (LV) contraction by analyzing tissue Doppler-derived imaging (TDI) during induction phase. The purpose of this study was to analyze and to compare the impacts of propofol- and thiopental-induction on LV function. METHODS: Twenty-four female patients with normal LV function undergoing non-cardiac surgery were randomly administered intravenous bolus thiopental (5 mg/kg, Thiopental-group, n = 12) or propofol (2 mg/kg, Propofol-group, n = 12) for anesthesia-induction. TDI of septal mitral annular velocity during systole (S'), early diastole (e') and atrial contraction (a') were determined by transthoracic echocardiography before and 1, 3, and 5 minutes after thiopental/propofol administration (T0, T1, T2, and T3, respectively). RESULTS: The bispectral index and systolic blood pressure declined significantly during anesthesia induction in both groups, however, more depressed in Thiopental-group compared with those in Propofol-group at T2 and T3 (all, p < 0.05). Among TDI two parameters demonstrated a significant inter-group difference: the S' in propofol was lower than that in Thiopental-group at T3 (p = 0.002), and a' velocities were persistently lower in Propofol-group, compared with same time values in Thiopental-group (T1, T2, and T3: p = 0.025, 0.007, and 0.009, respectively). CONCLUSION: Anesthesia induction using propofol revealed a more persistent and profound decline of LV and atrial contraction than that using thiopental. Further studies are needed to understand the clinical implication.