Defect Passivation of 2D Semiconductors by Fixating Chemisorbed Oxygen Molecules via h-BN Encapsulations

Hexagonal boron nitride (h-BN) is a key ingredient for various 2D van der Waals heterostructure devices, but the exact role of h-BN encapsulation in relation to the internal defects of 2D semiconductors remains unclear. Here, it is reported that h-BN encapsulation greatly removes the defect-related gap states by stabilizing the chemisorbed oxygen molecules onto the defects of monolayer WS2 crystals. Electron energy loss spectroscopy (EELS) combined with theoretical analysis clearly confirms that the oxygen molecules are chemisorbed onto the defects of WS2 crystals and are fixated by h-BN encapsulation, with excluding a possibility of oxygen molecules trapped in bubbles or wrinkles formed at the interface between WS2 and h-BN. Optical spectroscopic studies show that h-BN encapsulation prevents the desorption of oxygen molecules over various excitation and ambient conditions, resulting in a greatly lowered and stabilized free electron density in monolayer WS2 crystals. This suppresses the exciton annihilation processes by two orders of magnitude compared to that of bare WS2 . Furthermore, the valley polarization becomes robust against the various excitation and ambient conditions in the h-BN encapsulated WS2 crystals.

Reversible Transition of Semiconducting PtSe2 and Metallic PtTe2 for Scalable All-2D Edge-Contacted FETs

Two-dimensional (2D) transition metal dichalcogenide (TMD) layers are highly promising as field-effect transistor (FET) channels in the atomic-scale limit. However, accomplishing this superiority in scaled-up FETs remains challenging due to their van der Waals (vdW) bonding nature with respect to conventional metal electrodes. Herein, we report a scalable approach to fabricate centimeter-scale all-2D FET arrays of platinum diselenide (PtSe2) with in-plane platinum ditelluride (PtTe2) edge contacts, mitigating the aforementioned challenges. We realized a reversible transition between semiconducting PtSe2 and metallic PtTe2 via a low-temperature anion exchange reaction compatible with the back-end-of-line (BEOL) processes. All-2D PtSe2 FETs seamlessly edge-contacted with transited metallic PtTe2 exhibited significant performance improvements compared to those with surface-contacted gold electrodes, e.g., an increase of carrier mobility and on/off ratio by over an order of magnitude, achieving a maximum hole mobility of ∼50.30 cm2 V-1 s-1 at room temperature. This study opens up new opportunities toward atomically thin 2D-TMD-based circuitries with extraordinary functionalities.

Optimal ropivacaine concentration for ultrasound-guided erector spinae plane block in patients who underwent video-assisted thoracoscopic lobectomy surgery

Background

An ultrasound-guided erector spinae plane block (ESPB) has emerged as an effective way to control postoperative pain and may be a good alternative way to an epidural block. However, relevant research on the appropriate concentration of local anesthetics for an ESPB remains scarce.

Aims

This study aimed to investigate the optimal concentration of ropivacaine for an ESPB in patients undergoing video-assisted thoracoscopic surgery (VATS).

Methods

A total of 68 patients who underwent a VATS lobectomy were enrolled. An ipsilateral ultrasound-guided ESPB was performed with three different ropivacaine concentrations as a local anesthetic: 0.189% (G1), 0.375% (G2), and 0.556% (G3). The total amount of perioperative remifentanil administered, patient-controlled analgesia (PCA) applied, and rescue drugs for postoperative analgesia during the 24 h after surgery were acquired, and numeric rating scale (NRS) scores were obtained.

Results

The total amount of intraoperative remifentanil administered was 7.20 ± 3.04 mcg/kg, 5.32 ± 2.70 mcg/kg, and 4.60 ± 1.75 in the G1, G2, and G3 groups, respectively. G2 and G3 had significantly lower amounts of remifentanil administered than the G1 group (P = 0.02 vs. G2; P = 0.003 vs. G3). The G3 group needed more inotropes than the G1 and G2 groups in the perioperative period (P = 0.045). The NRS scores, PCA, and rescue drug were not significantly different in the three groups.

Conclusion

The optimal concentration of ropivacaine recommended for an ESPB was 0.375%, which was effective in controlling pain and reducing the intraoperative opioid requirements with minimal adverse reactions such as hypotension.

Hemorrhagic morbidity in nulliparous patients with placenta previa without placenta accrete spectrum disorders

Background

Placental adhesion spectrum (PAS) is a disease in which the trophoblast invades the myometrium, and is a well-known high-risk condition associated with placental previa.

Aim

The morbidity of nulliparous women with placenta previa without PAS disorders is unknown.

Patients and Methods

The data from nulliparous women who underwent cesarean delivery were collected retrospectively. The women were dichotomized into malpresentation (MP) and placenta previa groups. The placenta previa group was categorized into previa (PS) and low-lying (LL) groups. When the placenta covers the internal cervical os, it is called placenta previa, when the placenta is near the cervical os, it is called the low-lying placenta. Their maternal hemorrhagic morbidity and neonatal outcomes were analyzed and adjusted using multivariate analysis based on univariate analysis.

Results

A total of 1269 women were enrolled: 781 women in the MP group and 488 women in the PP-LL group. Regarding packed red blood cell transfusion, PP and LL had adjusted odds ratio (aOR) of 14.7 (95% confidence interval (CI): 6.6 - 32.5), and 11.3 (95% CI: 4.9 - 26) during admission, and 51.2 (95% CI: 22.1 - 122.7) and 10.3 (95% CI: 3.9 - 26.6) during operation, respectively. For intensive care unit admission, PS and LL had aOR of 15.9 (95% CI: 6.5 - 39.1) and 3.5 (95% CI: 1.1 - 10.9), respectively. No women had cesarean hysterectomy, major surgical complications, or maternal death.

Conclusion

Despite placenta previa without PAS disorders, maternal hemorrhagic morbidity was significantly increased. Thus, our results highlight the need for resources for those women with evidence of placenta previa including a low-lying placenta, even if those women do not meet PAS disorder criteria. In addition, placenta previa without PAS disorder was not associated with critical maternal complications.

Large-area vertically aligned 2D MoS2layers on TEMPO-cellulose nanofibers for biodegradable transient gas sensors

Crystallographically anisotropic two-dimensional (2D) molybdenum disulfide (MoS₂) with vertically aligned (VA) layers is attractive for electrochemical sensing owing to its surface-enriched dangling bonds coupled with extremely large mechanical deformability. In this study, we explored VA-2D MoS₂layers integrated on cellulose nanofibers (CNFs) for detecting various volatile organic compound gases. Sensor devices employing VA-2D MoS₂/CNFs exhibited excellent sensitivities for the tested gases of ethanol, methanol, ammonia, and acetone; e.g. a high response rate up to 83.39% for 100 ppm ethanol, significantly outperforming previously reported sensors employing horizontally aligned 2D MoS₂layers. Furthermore, VA-2D MoS₂/CNFs were identified to be completely dissolvable in buffer solutions such as phosphate-buffered saline solution and baking soda buffer solution without releasing toxic chemicals. This unusual combination of high sensitivity and excellent biodegradability inherent to VA-2D MoS₂/CNFs offers unprecedented opportunities for exploring mechanically reconfigurable sensor technologies with bio-compatible transient characteristics.

Multiwavelength Optoelectronic Synapse with 2D Materials for Mixed-Color Pattern Recognition

Neuromorphic visual systems emulating biological retina functionalities have enormous potential for in-sensor computing, with prospects of making artificial intelligence ubiquitous. Conventionally, visual information is captured by an image sensor, stored by memory units, and eventually processed by the machine learning algorithm. Here, we present an optoelectronic synapse device with multifunctional integration of all the processes required for real time object identification. Ultraviolet-visible wavelength-sensitive MoS₂ FET channel with infrared sensitive PtTe₂/Si gate electrode enables the device to sense, store, and process optical data for a wide range of the electromagnetic spectrum, while maintaining a low dark current. The device exhibits optical stimulation-controlled short-term and long-term potentiation, electrically driven long-term depression, synaptic weight update for multiple wavelengths of light ranging from 300 nm in ultraviolet to 2 μm in infrared. An artificial neural network developed using the extracted weight update parameters of the device can be trained to identify both single wavelength and mixed wavelength patterns. This work demonstrates a device that could potentially be used for realizing a multiwavelength neuromorphic visual system for pattern recognition and object identification.

Microwave-assisted thermochemical conversion of Zr-FeOOH nanorods to Zr-ZnFe2O4 nanorods for bacterial disinfection and photo-Fenton catalytic degradation of organic pollutants

Herein, we report a CoO_x-loaded Zr-doped ZnFe₂O₄ (CoO_x/Zr-ZFO) NR photocatalyst synthesized by successive microwave and wet impregnation methods for bacterial inactivation and degradation of organic pollutants. For the first time, microwave treatment was used for Zn attachment on hydrothermally synthesized self-assembled Zr-FeOOH NRs to produce Zr-doped ZnFe₂O₄ (Zr-ZFO) NRs. The lowest bandgap energy (1.96 eV) enables for significant absorption in the visible light region, which helps to improve bacteria degradation inactivation efficiency. Further, various metal oxides (Cu, Ag and Co) were loaded onto the surface of photocatalysts (Zr-ZFO NRs) by a wet impregnation method. As-synthesized CoO_x/Zr-ZFO-3 NRs were systematically characterized and used as photocatalysts for inactivation of E. coli and S. aureus and degradation of organic pollutants. The CoO_x/Zr-ZFO-3 NR photocatalyst exhibited better inactivation efficiency (99.4 %) than other metal oxide-loaded Zr-ZFO NRs (Ag₂O_x-loaded Zr-ZFO NRs (33.6 %), CuO_x-loaded Zr-ZFO NRs (77.6 %)). Additionally, the optimum CoO_x/Zr-ZFO-3 NR photocatalyst showed 98.3 % and 98.1 % degradation efficiencies for BPA and orange II dye, respectively, under visible light irradiation (λ ≥ 420 nm). Therefore, this work affords a novel, simple and rapid approach for the development of photocatalysts which active in visible light for bacterial disinfection and organic degradation.

Peel-and-Stick Integration of Atomically Thin Nonlayered PtS Semiconductors for Multidimensionally Stretchable Electronic Devices

Various near-atom-thickness two-dimensional (2D) van der Waals (vdW) crystals with unparalleled electromechanical properties have been explored for transformative devices. Currently, the availability of 2D vdW crystals is rather limited in nature as they are only obtained from certain mother crystals with intrinsically possessed layered crystallinity and anisotropic molecular bonding. Recent efforts to transform conventionally non-vdW three-dimensional (3D) crystals into ultrathin 2D-like structures have seen rapid developments to explore device building blocks of unique form factors. Herein, we explore a "peel-and-stick" approach, where a nonlayered 3D platinum sulfide (PtS) crystal, traditionally known as a cooperate mineral material, is transformed into a freestanding 2D-like membrane for electromechanical applications. The ultrathin (∼10 nm) 3D PtS films grown on large-area (>cm²) silicon dioxide/silicon (SiO₂/Si) wafers are precisely "peeled" inside water retaining desired geometries via a capillary-force-driven surface wettability control. Subsequently, they are "sticked" on strain-engineered patterned substrates presenting prominent semiconducting properties, i.e., p-type transport with an optical band gap of ∼1.24 eV. A variety of mechanically deformable strain-invariant electronic devices have been demonstrated by this peel-and-stick method, including biaxially stretchable photodetectors and respiratory sensing face masks. This study offers new opportunities of 2D-like nonlayered semiconducting crystals for emerging mechanically reconfigurable and stretchable device technologies.

MoS2 Synapses with Ultra-low Variability and Their Implementation in Boolean Logic

Brain-inspired computing enabled by memristors has gained prominence over the years due to the nanoscale footprint and reduced complexity for implementing synapses and neurons. The demonstration of complex neuromorphic circuits using conventional materials systems has been limited by high cycle-to-cycle and device-to-device variability. Two-dimensional (2D) materials have been used to realize transparent, flexible, ultra-thin memristive synapses for neuromorphic computing, but with limited knowledge on the statistical variation of devices. In this work, we demonstrate ultra-low-variability synapses using chemical vapor deposited 2D MoS₂ as the switching medium with Ti/Au electrodes. These devices, fabricated using a transfer-free process, exhibit ultra-low variability in SET voltage, RESET power distribution, and synaptic weight update characteristics. This ultra-low variability is enabled by the interface rendered by a Ti/Au top contact on Si-rich MoS₂ layers of mixed orientation, corroborated by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS). TEM images further confirm the stability of the device stack even after subjecting the device to 100 SET-RESET cycles. Additionally, we implement logic gates by monolithic integration of MoS₂ synapses with MoS₂ leaky integrate-and-fire neurons to show the viability of these devices for non-von Neumann computing.

Efficient Suppression of Charge Recombination in Self-Powered Photodetectors with Band-Aligned Transferred van der Waals Metal Electrodes

Recombination of photogenerated electron-hole pairs dominates the photocarrier lifetime and then influences the performance of photodetectors and solar cells. In this work, we report the design and fabrication of band-aligned van der Waals-contacted photodetectors with atomically sharp and flat metal-semiconductor interfaces through transferred metal integration. A unity factor α is achieved, which is essentially independent of the wavelength of the light, from ultraviolet to near-infrared, indicating effective suppression of charge recombination by the device. The short-circuit current (0.16 μA) and open-circuit voltage (0.72 V) of the band-aligned van der Waals-contacted devices are at least 1 order of magnitude greater than those of band-aligned deposited devices and 2 orders of magnitude greater than those of non-band-aligned deposited devices. High responsivity, detectivity, and polarization sensitivity ratio of 283 mA/W, 6.89 × 10¹² cm Hz^1/2 W^-1, and 3.05, respectively, are also obtained for the device at zero bias. Moreover, the efficient suppression of charge recombination in our air-stable self-powered photodetectors also results in a fast response speed and leads to polarization-sensitive performance.

Crystal Phase Transition Creates a Highly Active and Stable RuCX Nanosurface for Hydrogen Evolution Reaction in Alkaline Media

Although metastable crystal structures have received much attention owing to their utilization in various fields, their phase-transition to a thermodynamic structure has attracted comparably little interest. In the case of nanoscale crystals, such an exothermic phase-transition releases high energy within a confined surface area and reconstructs surface atomic arrangement in a short time. Thus, this high-energy nanosurface may create novel crystal structures when some elements are supplied. In this work, the creation of a ruthenium carbide (RuC_X , X < 1) phase on the surface of the Ru nanocrystal is discovered during phase-transition from cubic-close-packed to hexagonal-close-packed structure. When the electrocatalytic hydrogen evolution reaction (HER) is tested in alkaline media, the RuC_X exhibits a much lower overpotential and good stability relative to the counterpart Ru-based catalysts and the state-of-the-art Pt/C catalyst. Density functional theory calculations predict that the local heterogeneity of the outermost RuC_X surface promotes the bifunctional HER mechanism by providing catalytic sites for both H adsorption and facile water dissociation.

Efficient and additive-free synthesis of morphology variant iron oxyhydroxide nanostructures for phosphate adsorption application

Iron oxyhydroxide (FeOOH) nanostructures of different shapes were successfully synthesized on flexible textile cloth of polyester using a novel and simple technique based on hydrolysis method. The technique used herein is newly designed specifically to improve the efficiency in terms of energy, simplicity and cost involved in large scale synthesis of nanostructured thin films. Additionally, the morphology of nano-sized iron oxyhydroxide could be tuned into different shapes through variation in the type of precursors used for synthesis. The uniformity and adhesion of the depositions were also found to be excellent as examined by qualitative techniques. The as-deposited samples exhibited monoclinic and orthorhombic structures of FeOOH. A significant variation in the shape of as-deposited FeOOH nanostructures with change in precursor was observed through morphological studies, which displayed lance-shaped, rounded clusters and rod-like growth features in different cases. The nanocrystalline FeOOH can be directly applied to attract and trap phosphate from water reservoirs, thus contributing to environmental solutions. The proposed technique can also be utilized to deposit larger areas, which could be suitable for practical applications.

Mechanically rollable photodetectors enabled by centimetre-scale 2D MoS2 layer/TOCN composites

Two-dimensional (2D) molybdenum disulfide (MoS2) layers are suitable for visible-to-near infrared photodetection owing to their tunable optical bandgaps. Also, their superior mechanical deformability enabled by an extremely small thickness and van der Waals (vdW) assembly allows them to be structured into unconventional physical forms, unattainable with any other materials. Herein, we demonstrate a new type of 2D MoS2 layer-based rollable photodetector that can be mechanically reconfigured while maintaining excellent geometry-invariant photo-responsiveness. Large-area (>a few cm2) 2D MoS2 layers grown by chemical vapor deposition (CVD) were integrated on transparent and flexible substrates composed of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibers (TOCNs) by a direct solution casting method. These composite materials in three-dimensionally rollable forms exhibited a large set of intriguing photo-responsiveness, well preserving intrinsic opto-electrical characteristics of the integrated 2D MoS2 layers; i.e., light intensity-dependent photocurrents insensitive to illumination angles as well as highly tunable photocurrents varying with the rolling number of 2D MoS2 layers, which were impossible to achieve with conventional photodetectors. This study provides a new design principle for converting 2D materials to three-dimensional (3D) objects of tailored functionalities and structures, significantly broadening their potential and versatility in futuristic devices.

Scalable Van der Waals Two-Dimensional PtTe2 Layers Integrated onto Silicon for Efficient Near-to-Mid Infrared Photodetection

In recent years, there has been increasing interest in leveraging two-dimensional (2D) van der Waals (vdW) crystals for infrared (IR) photodetection, exploiting their unusual optoelectrical properties. Some 2D vdW materials with small band gap energies such as graphene and black phosphorus have been explored as stand-alone IR responsive layers in photodetectors. However, the devices incorporating these IR-sensitive 2D layers often exhibited poor performances owing to their preparation issues such as limited scalability and air instability. Herein, we explored wafer-scale 2D platinum ditelluride (PtTe₂) layers for near-to-mid IR photodetection by directly growing them onto silicon (Si) wafers. 2D PtTe₂/Si heterojunctions exhibited wavelength- and intensity-dependent high photocurrents in a spectral range of ∼1-7 μm, significantly outperforming stand-alone 2D PtTe₂ layers. The observed superiority is attributed to their excellent Schottky junction characteristics accompanying suppressed carrier recombination as well as optical absorbance competition between 2D PtTe₂ layers and Si. The direct and scalable growth of 2D PtTe₂ layers was further extended to demonstrate mechanically flexible IR photodetectors.

Large-area 2D PtTe2/silicon vertical-junction devices with ultrafast and high-sensitivity photodetection and photovoltaic enhancement by integrating water droplets

2D PtTe2 layers, a relatively new class of 2D crystals, have unique band structure and remarkably high electrical conductivity promising for emergent opto-electronics. This intrinsic superiority can be further leveraged toward practical device applications by merging them with mature 3D semiconductors, which has remained largely unexplored. Herein, we explored 2D/3D heterojunction devices by directly growing large-area (>cm2) 2D PtTe2 layers on Si wafers using a low-temperature CVD method and unveiled their superior opto-electrical characteristics. The devices exhibited excellent Schottky transport characteristics essential for high-performance photovoltaics and photodetection, i.e., well-balanced combination of high photodetectivity (>1013 Jones), small photo-responsiveness time (∼1 μs), high current rectification ratio (>105), and water super-hydrophobicity driven photovoltaic improvement (>300%). These performances were identified to be superior to those of previously explored 2D/3D or 2D layer-based devices with much smaller junction areas, and their underlying principles were confirmed by DFT calculations.

Vertically Aligned 2D MoS2 Layers with Strain-Engineered Serpentine Patterns for High-Performance Stretchable Gas Sensors: Experimental and Theoretical Demonstration

Two-dimensional (2D) molybdenum disulfide (MoS₂) with vertically aligned (VA) layers exhibits significantly enriched surface-exposed edge sites with an abundance of dangling bonds owing to its intrinsic crystallographic anisotropy. Such structural variation renders the material with exceptionally high chemical reactivity and chemisorption ability, making it particularly attractive for high-performance electrochemical sensing. This superior property can be further promoted as far as it is integrated on mechanically stretchable substrates well retaining its surface-exposed defective edges, projecting opportunities for a wide range of applications utilizing its structural uniqueness and mechanical flexibility. In this work, we explored VA-2D MoS₂ layers configured in laterally stretchable forms for multifunctional nitrogen dioxide (NO₂) gas sensors. Large-area (>cm²) VA-2D MoS₂ layers grown by a chemical vapor deposition (CVD) method were directly integrated onto a variety of flexible substrates with serpentine patterns judiciously designed to accommodate a large degree of tensile strain. These uniquely structured VA-2D MoS₂ layers were demonstrated to be highly sensitive to NO₂ gas of controlled concentration preserving their intrinsic structural and chemical integrity, e.g., significant current response ratios of ∼160-380% upon the introduction of NO₂ at a level of 5-30 ppm. Remarkably, they exhibited such a high sensitivity even under lateral stretching up to 40% strain, significantly outperforming previously reported 2D MoS₂ layer-based NO₂ gas sensors of any structural forms. Underlying principles for the experimentally observed superiority were theoretically unveiled by density functional theory (DFT) calculation and finite element method (FEM) analysis. The intrinsic high sensitivity and large stretchability of VA-2D MoS₂ layers confirmed in this study are believed to be applicable in sensing diverse gas species, greatly broadening their versatility in stretchable and wearable technologies.

Subcallosal haemorrhage as a sign of diffuse axonal injury in patients with traumatic brain injury

AIM

To identify the relationship between subcallosal haemorrhage and diffuse axonal injury (DAI) grading.

MATERIALS AND METHODS

Computed tomography (CT) and magnetic resonance imaging (MRI) images of all patients with traumatic brain injury over the past 5 years were reviewed. Subcallosal haemorrhage was defined as the presence of haemorrhage on admission CT underneath the corpus callosum. Grading of DAI was performed using MRI or CT exclusive of subcallosal haemorrhage status. The association of demographic factors, mechanism of injury, Glasgow Coma Scale (GCS) on admission, and positive subcallosal haemorrhage status with the presence of moderate-severe DAI was assessed. Receiver operating characteristic (ROC) curve analysis was used to evaluate the performance of subcallosal haemorrhage status in predicting DAI severity. Median modified Rankin Scale (mRS) scores were compared between subcallosal haemorrhage positive and negative cases.

RESULTS

The images of 1,150 patients were reviewed with 301 patients showing DAI. Of those, 64 patients (21.2%) and 237 patients (78.7%) were positive and negative for subcallosal haemorrhage, respectively. Isolated subcallosal haemorrhage was noted in 15 patients (23.4%). A subcallosal haemorrhage positive status (OR=5.16, p < 0.001) was statistically significantly associated with moderate-severe DAI. The ROC curve for predicting moderate-severe DAI with subcallosal haemorrhage status showed an area under the curve of 0.625 (95% confidence interval [CI]: 0.561-0.688, p < 0.001). The median mRS score was significantly higher (p < 0.001) in the subcallosal haemorrhage positive group (median 4.5, interquartile range [IQR] 2-6) versus the negative group (median 2, IQR 2-3). Isolated subcallosal haemorrhage group showed moderate-severe DAI in 80% (12/15) of cases.

CONCLUSION

Subcallosal haemorrhage is a highly specific radiographic predictor of moderate-severe DAI (grade 2-3).

High-performance flexible asymmetric supercapacitor based on rGO anode and WO3/WS2 core/shell nanowire cathode

Flexible smart electronics require their energy storage device to be flexible in nature. Developing high-performance flexible energy storage devices require direct integration of electrode active materials on current collectors to satisfy the high electronic/ionic conductivity and long-term durability requirements. Herein, we develop a flexible all-solid-state asymmetric supercapacitor comprised of reduced graphene oxide (rGO) and core/shell tungsten trioxide/tungsten disulfide (WO₃/WS₂) nanowire based electrodes. The electrodes synthesized via electrochemical deposition and chemical vapor deposition avoided the necessity to use non-conductive binders and offered excellent cyclic stability. The structural integrity provided by the rGO and WO₃/WS₂ electrodes facilitated excellent electrochemical stability with capacitance retention of 90% and 100% after 10 000 charge-discharge cycles, respectively. An all-solid-state device provides a voltage window of 1.5 V and more than 70% capacitance retention after 10 000 charge-discharge cycles. Providing 97% capacitance retention upon mechanical bending reveals its potential to be used as an energy storage devices in flexible electronics.

Wafer-Scale Two-Dimensional MoS2 Layers Integrated on Cellulose Substrates Toward Environmentally Friendly Transient Electronic Devices

We explored the feasibility of wafer-scale two-dimensional (2D) molybdenum disulfide (MoS₂) layers toward futuristic environmentally friendly electronics that adopt biodegradable substrates. Large-area (> a few cm²) 2D MoS₂ layers grown on silicon dioxide/silicon (SiO₂/Si) wafers were delaminated and integrated onto a variety of cellulose-based substrates of various components and shapes in a controlled manner; examples of the substrates include planar papers, cylindrical natural rubbers, and 2,2,6,6-tetramethylpiperidine-1-oxyl-oxidized cellulose nanofibers. The integrated 2D layers were confirmed to well preserve their intrinsic structural and chemical integrity even on such exotic substrates. Proof-of-concept devices employing large-area 2D MoS₂ layers/cellulose substrates were demonstrated for a variety of applications, including photodetectors, pressure sensors, and field-effect transistors. Furthermore, we demonstrated the complete "dissolution" of the integrated 2D MoS₂ layers in a buffer solution composed of baking soda and deionized water, confirming their environmentally friendly transient characteristics. Moreover, the approaches to delaminate and integrate them do not demand any chemicals except for water, and their original substrates can be recycled for subsequent growths, ensuring excellent chemical benignity and process sustainability.

Wafer-scale 2D PtTe2 layers for high-efficiency mechanically flexible electro-thermal smart window applications

Two-dimensional (2D) transition metal dichalcogenide (TMD) layers have gained increasing attention for a variety of emerging electrical, thermal, and optical applications. Recently developed metallic 2D TMD layers have been projected to exhibit unique attributes unattainable in their semiconducting counterparts; e.g., much higher electrical and thermal conductivities coupled with mechanical flexibility. In this work, we explored 2D platinum ditelluride (2D PtTe2) layers - a relatively new class of metallic 2D TMDs - by studying their previously unexplored electro-thermal properties for unconventional window applications. We prepared wafer-scale 2D PtTe2 layer-coated optically transparent and mechanically flexible willow glasses via a thermally-assisted tellurization of Pt films at a low temperature of 400 °C. The 2D PtTe2 layer-coated windows exhibited a thickness-dependent optical transparency and electrical conductivity of >106 S m-1 - higher than most of the previously explored 2D TMDs. Upon the application of electrical bias, these windows displayed a significant increase in temperature driven by Joule heating as confirmed by the infrared (IR) imaging characterization. Such superior electro-thermal conversion efficiencies inherent to 2D PtTe2 layers were utilized to demonstrate various applications, including thermochromic displays and electrically-driven defogging windows accompanying mechanical flexibility. Comparisons of these performances confirm the superiority of the wafer-scale 2D PtTe2 layers over other nanomaterials explored for such applications.

Automated Assembly of Wafer-Scale 2D TMD Heterostructures of Arbitrary Layer Orientation and Stacking Sequence Using Water Dissoluble Salt Substrates

We report a novel strategy to assemble wafer-scale two-dimensional (2D) transition metal dichalcogenide (TMD) layers of well-defined components and orientations. We directly grew a variety of 2D TMD layers on "water-dissoluble" single-crystalline salt wafers and precisely delaminated them inside water in a chemically benign manner. This manufacturing strategy enables the automated integration of vertically aligned 2D TMD layers as well as 2D/2D heterolayers of arbitrary stacking orders on exotic substrates insensitive to their kind and shape. Furthermore, the original salt wafers can be recycled for additional growths, confirming high process sustainability and scalability. The generality and versatility of this approach have been demonstrated by developing proof-of-concept "all 2D" devices for diverse yet unconventional applications. This study is believed to shed a light on leveraging opportunities of 2D TMD layers toward achieving large-area mechanically reconfigurable devices of various form factors at the industrially demanded scale.

Thickness-Independent Semiconducting-to-Metallic Conversion in Wafer-Scale Two-Dimensional PtSe2 Layers by Plasma-Driven Chalcogen Defect Engineering

Platinum diselenide (PtSe₂) is an emerging class of two-dimensional (2D) transition-metal dichalcogenide (TMD) crystals recently gaining substantial interest, owing to its extraordinary properties absent in conventional 2D TMD layers. Most interestingly, it exhibits a thickness-dependent semiconducting-to-metallic transition, i.e., thick 2D PtSe₂ layers, which are intrinsically metallic, become semiconducting with their thickness reduced below a certain point. Realizing both semiconducting and metallic phases within identical 2D PtSe₂ layers in a spatially well-controlled manner offers unprecedented opportunities toward atomically thin tailored electronic junctions, unattainable with conventional materials. In this study, beyond this thickness-dependent intrinsic semiconducting-to-metallic transition of 2D PtSe₂ layers, we demonstrate that controlled plasma irradiation can "externally" achieve such tunable carrier transports. We grew wafer-scale very thin (a few nm) 2D PtSe₂ layers by a chemical vapor deposition (CVD) method and confirmed their intrinsic semiconducting properties. We then irradiated the material with argon (Ar) plasma, which was intended to make it more semiconducting by thickness reduction. Surprisingly, we discovered a reversed transition of semiconducting to metallic, which is opposite to the prediction concerning their intrinsic thickness-dependent carrier transports. Through extensive structural and chemical characterization, we identified that the plasma irradiation introduces a large concentration of near-atomic defects and selenium (Se) vacancies in initially stoichiometric 2D PtSe₂ layers. Furthermore, we performed density functional theory (DFT) calculations and clarified that the band-gap energy of such defective 2D PtSe₂ layers gradually decreases with increasing defect concentration and dimensions, accompanying a large number of midgap energy states. This corroborative experimental and theoretical study decisively verifies the fundamental mechanism for this externally controlled semiconducting-to-metallic transition in large-area CVD-grown 2D PtSe₂ layers, greatly broadening their versatility for futuristic electronics.

Wafer-Scale Growth of 2D PtTe2 with Layer Orientation Tunable High Electrical Conductivity and Superior Hydrophobicity

Platinum ditelluride (PtTe₂) is an emerging semimetallic two-dimensional (2D) transition-metal dichalcogenide (TMDC) crystal with intriguing band structures and unusual topological properties. Despite much devoted efforts, scalable and controllable synthesis of large-area 2D PtTe₂ with well-defined layer orientation has not been established, leaving its projected structure-property relationship largely unclarified. Herein, we report a scalable low-temperature growth of 2D PtTe₂ layers on an area greater than a few square centimeters by reacting Pt thin films of controlled thickness with vaporized tellurium at 400 °C. We systematically investigated their thickness-dependent 2D layer orientation as well as its correlated electrical conductivity and surface property. We unveil that 2D PtTe₂ layers undergo three distinct growth mode transitions, i.e., horizontally aligned holey layers, continuous layer-by-layer lateral growth, and horizontal-to-vertical layer transition. This growth transition is a consequence of competing thermodynamic and kinetic factors dictated by accumulating internal strain, analogous to the transition of Frank-van der Merwe (FM) to Stranski-Krastanov (SK) growth in epitaxial thin-film models. The exclusive role of the strain on dictating 2D layer orientation has been quantitatively verified by the transmission electron microscopy (TEM) strain mapping analysis. These centimeter-scale 2D PtTe₂ layers exhibit layer orientation tunable metallic transports yielding the highest value of ∼1.7 × 10⁶ S/m at a certain critical thickness, supported by a combined verification of density functional theory (DFT) and electrical measurements. Moreover, they show intrinsically high hydrophobicity manifested by the water contact angle (WCA) value up to ∼117°, which is the highest among all reported 2D TMDCs of comparable dimensions and geometries. Accordingly, this study confirms the high material quality of these emerging large-area 2D PtTe₂ layers, projecting vast opportunities employing their tunable layer morphology and semimetallic properties from investigations of novel quantum phenomena to applications in electrocatalysis.

An unexpected surfactant role of immiscible nitrogen in the structural development of silver nanoparticles: an experimental and numerical investigation

Artificially designing the crystal orientation and facets of noble metal nanoparticles is important to realize unique chemical and physical features that are very different from those of noble metals in bulk geometries. However, relative to their counterparts synthesized in wet-chemical processes, vapor-depositing noble metal nanoparticles with the desired crystallographic features while avoiding any notable impurities is quite challenging because this task requires breaking away from the thermodynamically favorable geometry of nanoparticles. We used plasma-generated N atoms as a surface-active agent, a so-called surfactant, to control the structural development of Ag nanoparticles supported on a chemically heterogeneous ZnO substrate. The N-surfactant-facilitated sputter deposition provided strong selectivity for crystalline orientation and facets, leading to a highly flattened nanoparticle shape that clearly deviated from the energetically favorable spherical polyhedra, due to the drastic decreases in the surface free energies of Ag nanoparticles in the presence of the N surfactant. The Ag nanoparticles successively developed a nearly unidirectional (111) orientation aligned by stimulating the crystalline coupling of Ag along the orientation of the ZnO substrate. The experimental and simulation results not only offer new insights into the advantages of N as a surfactant for the orientation and shape-controlled synthesis of Ag nanoparticles via sputter deposition but also provide the first solid evidence validating that immiscible, nonresidual gaseous surfactants can be used in the vapor deposition processes of noble metal nanoparticles to manipulate their surface free energies.

Simultaneous and synergistic effect of heavy metal adsorption on the enhanced photocatalytic performance of a visible-light-driven RS-TONR/TNT composite

A hydrothermally synthesized rhodium/antimony co-doped TiO₂ nanorod and titanate nanotube (RS-TONR/TNT) composite was prepared for removal of heavy metals and organic pollutants from water under visible light irradiation. The composite provides the dual function of simultaneous adsorption of heavy metal ions and enhanced degradation of dissolved organic compounds. Acid treatment transformed titanate nanotubes to irregular tubular structures distributed homogeneously over untransformed RS/TONRs. Synergistic removal and degradation was studied with various heavy metals, Orange (II) dye, and Bisphenol A. The adsorption capacity of the composite for heavy metal ions was Pb(II) > Cd(II) > Cu(II) > Zn(II). The adsorbed metals enhanced photocatalytic degradation of the organic pollutants, but Cu was most effective, with degradation exceeding 70% for the dye and 80% for Bisphenol A after 5 h of treatment. Photocatalytic activity was enhanced more by adsorption than photodeposition of Cu ions. A decrease in XRD rutile peak intensity with adsorbed metal indicates a change in crystallinity which may enhance photocatalytic activity. Thick and bulging nanostructures in FE-SEM images signify ion adsorption within titanate pores. BET analysis indicated titanate nanotubes with adsorbed metal are mesoporous but their tubular structure persists. XPS showed more active Cu 2p_3/2 states under light, supporting an active role of Cu⁺ in photocatalytic ROS generation. Detection of ROS and Cu species using methanol, EDTA, pCBA, and benzoic acid probes provided strong evidence for degradation via a charge transfer mechanism. Findings demonstrate the potential of the RS-TONR/TNT composite for simultaneous removal of heavy metals and degradation of organic pollutants.

Multifunctional Two-Dimensional PtSe2-Layer Kirigami Conductors with 2000% Stretchability and Metallic-to-Semiconducting Tunability

Two-dimensional transition-metal dichalcogenide (2D TMD) layers are highly attractive for emerging stretchable and foldable electronics owing to their extremely small thickness coupled with extraordinary electrical and optical properties. Although intrinsically large strain limits are projected in them (i.e., several times greater than silicon), integrating 2D TMDs in their pristine forms does not realize superior mechanical tolerance greatly demanded in high-end stretchable and foldable devices of unconventional form factors. In this article, we report a versatile and rational strategy to convert 2D TMDs of limited mechanical tolerance to tailored 3D structures with extremely large mechanical stretchability accompanying well-preserved electrical integrity and modulated transport properties. We employed a concept of strain engineering inspired by an ancient paper-cutting art, known as kirigami patterning, and developed 2D TMD-based kirigami electrical conductors. Specifically, we directly integrated 2D platinum diselenide (2D PtSe₂) layers of controlled carrier transport characteristics on mechanically flexible polyimide (PI) substrates by taking advantage of their low synthesis temperature. The metallic 2D PtSe₂/PI kirigami patterns of optimized dimensions exhibit an extremely large stretchability of ∼2000% without compromising their intrinsic electrical conductance. They also present strain-tunable and reversible photoresponsiveness when interfaced with semiconducting carbon nanotubes (CNTs), benefiting from the formation of 2D PtSe₂/CNT Schottky junctions. Moreover, kirigami field-effect transistors (FETs) employing semiconducting 2D PtSe₂ layers exhibit tunable gate responses coupled with mechanical stretching upon electrolyte gating. The exclusive role of the kirigami pattern parameters in the resulting mechanoelectrical responses was also verified by a finite-element modeling (FEM) simulation. These multifunctional 2D materials in unconventional yet tailored 3D forms are believed to offer vast opportunities for emerging electronics and optoelectronics.

Experimental Realization of Few Layer Two-Dimensional MoS2 Membranes of Near Atomic Thickness for High Efficiency Water Desalination

A globally imminent shortage of freshwater has been demanding viable strategies for improving desalination efficiencies with the adoption of cost- and energy-efficient membrane materials. The recently explored 2D transition metal dichalcogenides (2D TMDs) of near atomic thickness have been envisioned to offer notable advantages as high-efficiency membranes owing to their structural uniqueness; that is, extremely small thickness and intrinsic atomic porosity. Despite theoretically projected advantages, experimental realization of near atom-thickness 2D TMD-based membranes and their desalination efficiency assessments have remained largely unexplored mainly due to the technical difficulty associated with their seamless large-scale integration. Herein, we report the experimental demonstration of high-efficiency water desalination membranes based on few-layer 2D molybdenum disulfide (MoS₂) of only ∼7 nm thickness. Chemical vapor deposition (CVD)-grown centimeter-scale 2D MoS₂ layers were integrated onto porous polymeric supports with well-preserved structural integrity enabled by a water-assisted 2D layer transfer method. These 2D MoS₂ membranes of near atomic thickness exhibit an excellent combination of high water permeability (>322 L m^-2 h^-1 bar^-1) and high ionic sieving capability (>99%) for various seawater salts including Na⁺, K⁺, Ca²⁺, and Mg²⁺ with a range of concentrations. Moreover, they present near 100% salt ion rejection rates for actual seawater obtained from the Atlantic coast, significantly outperforming the previously developed 2D MoS₂ layer membranes of micrometer thickness as well as conventional reverse osmosis (RO) membranes. Underlying principles behind such remarkably excellent desalination performances are attributed to the intrinsic atomic vacancies inherent to the CVD-grown 2D MoS₂ layers as verified by aberration-corrected electron microscopy characterization.

Two-Dimensional/Three-Dimensional Schottky Junction Photovoltaic Devices Realized by the Direct CVD Growth of vdW 2D PtSe2 Layers on Silicon

Two-dimensional (2D) platinum diselenide (PtSe₂) layers are a new class of near-atom-thick 2D crystals in a van der Waals-assembled structure similar to previously explored many other 2D transition-metal dichalcogenides (2D TMDs). They exhibit distinct advantages over conventional 2D TMDs for electronics and optoelectronics applications such as metallic-to-semiconducting transition, decently high carrier mobility, and low growth temperature. Despite such superiority, much of their electrical properties have remained mostly unexplored, leaving their full technological potential far from being realized. Herein, we report 2D/three-dimensional Schottky junction devices based on vertically aligned metallic 2D PtSe₂ layers integrated on Si wafers. We directly grew 2D PtSe₂ layers of controlled orientation and carrier transport characteristics via a low-temperature chemical vapor deposition process and investigated 2D PtSe₂/Si Schottky junction properties. We unveiled a comprehensive set of material parameters, which decisively confirm the presence of excellent Schottky junctions, i.e., high-current rectification, small ideality factor, and temperature-dependent variation of Schottky barrier heights. Moreover, we observed strong photovoltaic effects in the 2D PtSe₂/Si Schottky junction devices and extended them to realize flexible photovoltaic devices. This study is believed to significantly broaden the versatility of 2D PtSe₂ layers in practical and futuristic electronic devices.

Surface-diffusion-limited growth of atomically thin WS2 crystals from core-shell nuclei

Atomically thin transition metal dichalcogenides (TMDs) have recently attracted great attention since the unique and fascinating physical properties have been found in various TMDs, implying potential applications in next-generation devices. The progress towards developing new functional and high-performance devices based on TMDs, however, is limited by the difficulty in producing large-area monolayer TMDs due to a lack of knowledge of the growth processes of monolayer TMDs. In this work, we have investigated the growth processes of monolayer WS2 crystals using a thermal chemical vapor deposition method, in which the growth conditions were adjusted in a systematic manner. It was found that, after forming WO3-WS2 core-shell nanoparticles as nucleation sites on a substrate, the growth of three-dimensional WS2 islands proceeds by ripening and crystallization processes. Lateral growth of monolayer WS2 crystals subsequently occurs by the surface diffusion process of adatoms toward the step edge of the three-dimensional WS2 islands. Our results provide understanding of the growth processes of monolayer WS2 by using chemical vapor deposition methods.

Horizontal-to-Vertical Transition of 2D Layer Orientation in Low-Temperature Chemical Vapor Deposition-Grown PtSe2 and Its Influences on Electrical Properties and Device Applications

Two-dimensional (2D) transition-metal dichalcogenides (2D TMDs) in the form of MX₂ (M: transition metal, X: chalcogen) exhibit intrinsically anisotropic layered crystallinity wherein their material properties are determined by constituting M and X elements. 2D platinum diselenide (2D PtSe₂) is a relatively unexplored class of 2D TMDs with noble-metal Pt as M, offering distinct advantages over conventional 2D TMDs such as higher carrier mobility and lower growth temperatures. Despite the projected promise, much of its fundamental structural and electrical properties and their interrelation have not been clarified, and so its full technological potential remains mostly unexplored. In this work, we investigate the structural evolution of large-area chemical vapor deposition (CVD)-grown 2D PtSe₂ layers of tailored morphology and clarify its influence on resulting electrical properties. Specifically, we unveil the coupled transition of structural-electrical properties in 2D PtSe₂ layers grown at a low temperature (i.e., 400 °C). The layer orientation of 2D PtSe₂ grown by the CVD selenization of seed Pt films exhibits horizontal-to-vertical transition with increasing Pt thickness. While vertically aligned 2D PtSe₂ layers present metallic transports, field-effect-transistor gate responses were observed with thin horizontally aligned 2D PtSe₂ layers prepared with Pt of small thickness. Density functional theory calculation identifies the electronic structures of 2D PtSe₂ layers undergoing the transition of horizontal-to-vertical layer orientation, further confirming the presence of this uniquely coupled structural-electrical transition. The advantage of low-temperature growth was further demonstrated by directly growing 2D PtSe₂ layers of controlled orientation on polyimide polymeric substrates and fabricating their Kirigami structures, further strengthening the application potential of this material. Discussions on the growth mechanism behind the horizontal-to-vertical 2D layer transition are also presented.

Intraoperative Management to Prevent Cardiac Collapse in a Patient With a Recurrent, Large-volume Pericardial Effusion and Paroxysmal Atrial Fibrillation During Liver Transplantation: A Case Report

BACKGROUND

Pericardial effusion is a common feature of end-stage liver disease. In this case report we describe the intraoperative management of recurrent pericardial effusion, without re-pericardiocentesis, to prevent circulatory collapse during a critical surgical time-point; that is, during manipulation of the major vessels and graft reperfusion.

METHODS

A 47-year-old woman with hepatitis B was scheduled to undergo deceased donor liver transplantation (LT). A large pericardial effusion was preoperatively identified using transthoracic echocardiography (TTE). The patient also had paroxysmal atrial fibrillation. Two days before surgery, preemptive pericardiocentesis was performed and the 1150-mL effusion was drained. Intraoperatively, recurrence of the large pericardial effusion was identified using transesophageal echocardiography (TEE). During inferior vena cava manipulation, the surgeon consulted the anesthesiologist to evaluate the hemodynamic changes in the patient. After 3 attempts, the transplant team was able to determine the most appropriate anastomosis site, defined as that with the least impact on cardiac function. To prevent the development of severe postreperfusion syndrome, 10% MgSO₄ (2 g) was gradually infused 20 minutes before portal vein declamping, and immediately before graft reperfusion a 100-μg bolus of epinephrine was administered.

RESULTS

During graft reperfusion, there was no evidence of heart chamber collapse or flow disturbance, as seen on the TEE findings. Postoperatively, the patient recovered completely and was discharged from the hospital. Six months after surgery, there was no sign of pericardial effusion on follow-up TTE.

CONCLUSION

Our intraoperative strategy may prevent cardiac collapse in patients with pericardial effusion detected during LT. Intraoperative TEE plays an important role in guiding hemodynamic management.

Centimeter-scale Green Integration of Layer-by-Layer 2D TMD vdW Heterostructures on Arbitrary Substrates by Water-Assisted Layer Transfer

Two-dimensional (2D) transition metal dichalcogenide (2D TMD) layers present an unusually ideal combination of excellent opto-electrical properties and mechanical tolerance projecting high promise for a wide range of emerging applications, particularly in flexible and stretchable devices. The prerequisite for realizing such opportunities is to reliably integrate large-area 2D TMDs of well-defined dimensions on mechanically pliable materials with targeted functionalities by transferring them from rigid growth substrates. Conventional approaches to overcome this challenge have been limited as they often suffer from the non-scalable integration of 2D TMDs whose structural and chemical integrity are altered through toxic chemicals-involved processes. Herein, we report a generic and reliable strategy to achieve the layer-by-layer integration of large-area 2D TMDs and their heterostructure variations onto a variety of unconventional substrates. This new 2D layer integration method employs water only without involving any other chemicals, thus renders distinguishable advantages over conventional approaches in terms of material property preservation and integration size scalability. We have demonstrated the generality of this method by integrating a variety of 2D TMDs and their heterogeneously-assembled vertical layers on exotic substrates such as plastics and papers. Moreover, we have verified its technological versatility by demonstrating centimeter-scale 2D TMDs-based flexible photodetectors and pressure sensors which are difficult to fabricate with conventional approaches. Fundamental principles for the water-assisted spontaneous separation of 2D TMD layers are also discussed.

Artificial Neuron using Vertical MoS2/Graphene Threshold Switching Memristors

With the ever-increasing demand for low power electronics, neuromorphic computing has garnered huge interest in recent times. Implementing neuromorphic computing in hardware will be a severe boost for applications involving complex processes such as image processing and pattern recognition. Artificial neurons form a critical part in neuromorphic circuits, and have been realized with complex complementary metal-oxide-semiconductor (CMOS) circuitry in the past. Recently, metal-insulator-transition materials have been used to realize artificial neurons. Although memristors have been implemented to realize synaptic behavior, not much work has been reported regarding the neuronal response achieved with these devices. In this work, we use the volatile threshold switching behavior of a vertical-MoS2/graphene van der Waals heterojunction system to produce the integrate-and-fire response of a neuron. We use large area chemical vapor deposited (CVD) graphene and MoS2, enabling large scale realization of these devices. These devices can emulate the most vital properties of a neuron, including the all or nothing spiking, the threshold driven spiking of the action potential, the post-firing refractory period of a neuron and strength modulated frequency response. These results show that the developed artificial neuron can play a crucial role in neuromorphic computing.

Uniform Vapor-Pressure-Based Chemical Vapor Deposition Growth of MoS2 Using MoO3 Thin Film as a Precursor for Coevaporation

Chemical vapor deposition (CVD) is a powerful method employed for high-quality monolayer crystal growth of 2D transition metal dichalcogenides with much effort invested toward improving the growth process. Here, we report a novel method for CVD-based growth of monolayer molybdenum disulfide (MoS₂) by using thermally evaporated thin films of molybdenum trioxide (MoO₃) as the molybdenum (Mo) source for coevaporation. Uniform evaporation rate of MoO₃ thin films provides uniform Mo vapors which promote highly reproducible single-crystal growth of MoS₂ throughout the substrate. These high-quality crystals are as large as 95 μm and are characterized by scanning electron microscopy, Raman spectroscopy, photoluminescence spectroscopy, atomic force microscopy, and transmission electron microscopy. The bottom-gated field-effect transistors fabricated using the as-grown single crystals show n-type transistor behavior with a good on/off ratio of 10⁶ under ambient conditions. Our results presented here address the precursor vapor control during the CVD process and is a major step forward toward reproducible growth of MoS₂ for future semiconductor device applications.

Nitrogen-Mediated Growth of Silver Nanocrystals to Form UltraThin, High-Purity Silver-Film Electrodes with Broad band Transparency for Solar Cells

Controlling the shape and crystallography of nanocrystals during the early growth stages of a noble metal layer is important because of its correlation with the final layer morphology and optoelectrical features, but this task is unattainable in vapor deposition processes dominated by artificially uncontrollable thermodynamic free energies. We report on experimental evidence for the controllable evolution of Ag nanocrystals as induced by the addition of nitrogen, presumed to be nonresidual in the Ag lattice given its strong float-out behavior. This atypical formation of energetically stable Ag nanocrystals with significantly improved wetting abilities on a chemically heterogeneous substrate promotes the development of an atomically flat, ultrathin, high-purity Ag layer with a thickness of only 5 nm. This facilitates the fabrication of Ag thin-film electrodes exhibiting highly enhanced optical transparency over a broad spectral range in the visible and near-infrared spectral range. An Ag thin-film electrode with a ZnO/Ag/ZnO configuration exhibits an average transmittance of about 95% in the spectral range of 400-800 nm with a maximum transmittance of over 98% at 580 nm, which is comparable with the best transparency values so far reported for transparent electrodes. This degree of optical transparency provides an excellent chance to improve the photon absorption of photovoltaic devices employing an Ag thin film as their window electrode. This is clearly confirmed by the superior performance of a flexible organic solar cell with a power conversion efficiency of 8.0%, which is far superior to that of the same solar cell using a conventional amorphous indium tin oxide electrode (6.4%).

Three dimensionally-ordered 2D MoS2 vertical layers integrated on flexible substrates with stretch-tunable functionality and improved sensing capability

The intrinsically anisotropic crystallinity of two-dimensional (2D) transition metal dichalcogenide (2D TMD) layers enables a variety of intriguing material properties which strongly depend on the physical orientation of constituent 2D layers. For instance, 2D TMDs with vertically-aligned layers exhibit numerous dangling bonds on their 2D layer edge sites predominantly exposed on the surface, projecting significantly improved physical and/or chemical adsorption capability compared to their horizontally-oriented 2D layer counterparts. Such property advantages can be further promoted as far as the material can be integrated onto unconventional substrates of tailored geometry/functionality, offering vast opportunities for a wide range of applications which demand enhanced surface area/reactivity and mechanical flexibility. Herein, we report a new form of 2D TMDs, i.e., three-dimensionally ordered 2D molybdenum disulfide (2D MoS2) with vertically-aligned layers integrated on elastomeric substrates and explore their tunable multi-functionalities and technological promise. We grew large-scale (>2 cm2) vertically-aligned 2D MoS2 layers using a three-dimensionally patterned silicon dioxide (SiO2) template and directly transferred/integrated them onto flexible polydimethylsiloxane (PDMS) substrates by taking advantage of the distinguishable water-wettability of 2D MoS2vs. SiO2. The excellent structural integrity of the integrated vertical 2D MoS2 layers was confirmed by extensive spectroscopy/microscopy characterization. In addition, the stretch-driven unique tunability of their optical and surface properties was also examined. Moreover, we applied this material for flexible humidity sensing and identified significantly improved (>10 times) sensitivity over conventionally-designed horizontal 2D MoS2 layers, further confirming their high potential for unconventional flexible technologies.

Centimeter-Scale Periodically Corrugated Few-Layer 2D MoS2 with Tensile Stretch-Driven Tunable Multifunctionalities

Two-dimensional (2D) transition metal dichalcogenide (TMD) layers exhibit superior optical, electrical, and structural properties unattainable in any traditional materials. Many of these properties are known to be controllable via external mechanical inputs, benefiting from their extremely small thickness coupled with large in-plane strain limits. However, realization of such mechanically driven tunability often demands highly complicated engineering of 2D TMD layer structures, which is difficult to achieve on a large wafer scale in a controlled manner. Herein, we explore centimeter-scale periodically corrugated 2D TMDs, particularly 2D molybdenum disulfide (MoS₂), and report their mechanically tunable multifunctionalities. We developed a water-assisted process to homogeneously integrate few layers of 2D MoS₂ on three-dimensionally corrugated elastomeric substrates on a large area (>2 cm²). The evolution of electrical, optical, and structural properties in these three-dimensionally corrugated 2D MoS₂ layers was systematically studied under controlled tensile stretch. We identified that they present excellent electrical conductivity and photoresponsiveness as well as systematically tunable surface wettability and optical absorbance even under significant mechanical deformation. These novel three-dimensionally structured 2D materials are believed to offer exciting opportunities for large-scale, mechanically deformable devices of various form factors and unprecedented multifunctionalities.

Optical Transmittance Enhancement of Flexible Copper Film Electrodes with a Wetting Layer for Organic Solar Cells

The development of highly efficient flexible transparent electrodes (FTEs) supported on polymer substrates is of great importance to the realization of portable and bendable photovoltaic devices. Highly conductive, low-cost Cu has attracted attention as a promising alternative for replacing expensive indium tin oxide (ITO) and Ag. However, highly efficient, Cu-based FTEs are currently unavailable because of the absence of an efficient means of attaining an atomically thin, completely continuous Cu film that simultaneously exhibits enhanced optical transmittance and electrical conductivity. Here, strong two-dimensional (2D) epitaxy of Cu on ZnO is reported by applying an atomically thin (around 1 nm) oxygen-doped Cu wetting layer. Analyses of transmission electron microscopy images and X-ray diffraction patterns, combined with first-principles density functional theory calculations, reveal that the reduction in the surface and interface free energies of the wetting layers with a trace amount (1-2 atom %) of oxygen are largely responsible for the two-dimensional epitaxial growth of the Cu on ZnO. The ultrathin 2D Cu layer, embedded between ZnO films, exhibits a highly desirable optical transmittance of over 85% in a wavelength range of 400-800 nm and a sheet resistance of 11 Ω sq^-1. The validity of this innovative approach is verified with a Cu-based FTE that contributes to the light-to-electron conversion efficiency of a flexible organic solar cell that incorporates the transparent electrode (7.7%), which far surpasses that of a solar cell with conventional ITO (6.4%).

Noble metal-coated MoS2 nanofilms with vertically-aligned 2D layers for visible light-driven photocatalytic degradation of emerging water contaminants

Two-dimensional molybdenum disulfide (2D MoS2) presents extraordinary optical, electrical, and chemical properties which are highly tunable by engineering the orientation of constituent 2D layers. 2D MoS2 films with vertically-aligned layers exhibit numerous 2D edge sites which are predicted to offer superior chemical reactivity owing to their enriched dangling bonds. This enhanced chemical reactivity coupled with their tunable band gap energy can render the vertical 2D MoS2 unique opportunities for environmental applications that go beyond the conventional applications of horizontal 2D MoS2 in electronics/opto-electronics. Herein, we report that MoS2 films with vertically-aligned 2D layers exhibit excellent visible light responsive photocatalytic activities for efficiently degrading organic compounds in contaminated water such as harmful algal blooms. We demonstrate the visible light-driven rapid degradation of microcystin-LR, one of the most toxic compounds produced by the algal blooms, and reveal that the degradation efficiency can be significantly improved by incorporating noble metals. This study suggests a high promise of these emerging 2D materials for water treatment, significantly broadening their versatility for a wide range of energy and environmental applications.

Centimeter-Scale 2D van der Waals Vertical Heterostructures Integrated on Deformable Substrates Enabled by Gold Sacrificial Layer-Assisted Growth

Two-dimensional (2D) transition metal dichalcogenides (TMDs) such as molybdenum or tungsten disulfides (MoS₂ or WS₂) exhibit extremely large in-plane strain limits and unusual optical/electrical properties, offering unprecedented opportunities for flexible electronics/optoelectronics in new form factors. In order for them to be technologically viable building-blocks for such emerging technologies, it is critically demanded to grow/integrate them onto flexible or arbitrary-shaped substrates on a large wafer-scale compatible with the prevailing microelectronics processes. However, conventional approaches to assemble them on such unconventional substrates via mechanical exfoliations or coevaporation chemical growths have been limited to small-area transfers of 2D TMD layers with uncontrolled spatial homogeneity. Moreover, additional processes involving a prolonged exposure to strong chemical etchants have been required for the separation of as-grown 2D layers, which is detrimental to their material properties. Herein, we report a viable strategy to universally combine the centimeter-scale growth of various 2D TMD layers and their direct assemblies on mechanically deformable substrates. By exploring the water-assisted debonding of gold (Au) interfaced with silicon dioxide (SiO₂), we demonstrate the direct growth, transfer, and integration of 2D TMD layers and heterostructures such as 2D MoS₂ and 2D MoS₂/WS₂ vertical stacks on centimeter-scale plastic and metal foil substrates. We identify the dual function of the Au layer as a growth substrate as well as a sacrificial layer which facilitates 2D layer transfer. Furthermore, we demonstrate the versatility of this integration approach by fabricating centimeter-scale 2D MoS₂/single walled carbon nanotube (SWNT) vertical heterojunctions which exhibit current rectification and photoresponse. This study opens a pathway to explore large-scale 2D TMD van der Waals layers as device building blocks for emerging mechanically deformable electronics/optoelectronics.

Characterization of Rotational Stacking Layers in Large-Area MoSe2 Film Grown by Molecular Beam Epitaxy and Interaction with Photon

Transition metal dichalcogenides (TMDCs) are promising next-generation materials for optoelectronic devices because, at subnanometer thicknesses, they have a transparency, flexibility, and band gap in the near-infrared to visible light range. In this study, we examined continuous, large-area MoSe₂ film, grown by molecular beam epitaxy on an amorphous SiO₂/Si substrate, which facilitated direct device fabrication without exfoliation. Spectroscopic measurements were implemented to verify the formation of a homogeneous MoSe₂ film by performing mapping on the micrometer scale and measurements at multiple positions. The crystalline structure of the film showed hexagonal (2H) rotationally stacked layers. The local strain at the grain boundaries was mapped using a geometric phase analysis, which showed a higher strain for a 30° twist angle compared to a 13° angle. Furthermore, the photon-matter interaction for the rotational stacking structures was investigated as a function of the number of layers using spectroscopic ellipsometry. The optical band gap for the grown MoSe₂ was in the near-infrared range, 1.24-1.39 eV. As the film thickness increased, the band gap energy decreased. The atomically controlled thin MoSe₂ showed promise for application to nanoelectronics, photodetectors, light emitting diodes, and valleytronics.

Thermodynamically Stable Synthesis of Large-Scale and Highly Crystalline Transition Metal Dichalcogenide Monolayers and their Unipolar n-n Heterojunction Devices

Transition metal dichalcogenide (TMDC) monolayers are considered to be potential materials for atomically thin electronics due to their unique electronic and optical properties. However, large-area and uniform growth of TMDC monolayers with large grain sizes is still a considerable challenge. This report presents a simple but effective approach for large-scale and highly crystalline molybdenum disulfide monolayers using a solution-processed precursor deposition. The low supersaturation level, triggered by the evaporation of an extremely thin precursor layer, reduces the nucleation density dramatically under a thermodynamically stable environment, yielding uniform and clean monolayer films and large crystal sizes up to 500 µm. As a result, the photoluminescence exhibits only a small full-width-half-maximum of 48 meV, comparable to that of exfoliated and suspended monolayer crystals. It is confirmed that this growth procedure can be extended to the synthesis of other TMDC monolayers, and robust MoS₂ /WS₂ heterojunction devices are easily prepared using this synthetic procedure due to the large-sized crystals. The heterojunction device shows a fast response time (≈45 ms) and a significantly high photoresponsivity (≈40 AW^-1 ) because of the built-in potential and the majority-carrier transport at the n-n junction. These findings indicate an efficient pathway for the fabrication of high-performance 2D optoelectronic devices.

Boosting Photocatalytic Performance of Inactive Rutile TiO2 Nanorods under Solar Light Irradiation: Synergistic Effect of Acid Treatment and Metal Oxide Co-catalysts

In the present work, we accomplish the boosting of photocatalytic performance by the synergistic effect of acid treatment and transition metal oxide co-catalysts on molten salt rutile TiO₂ nanorods. FT-IR and XPS (oxygen deconvolution) results confirmed that the amount of hydroxyl groups increased on the surface of rutile TiO₂ nanorods (TO-NRs) after acid treatment. HR-TEM analysis revealed fine dispersion of metal oxide on the surface of acid treated TiO₂ nanorods (ATO-NRs). The photocatalytic activities of as-prepared (TO-NRs), acid treated (ATO-NRs), metal oxide loaded (MTO-NRs), and both acid treated and metal oxide loaded (MATO-NRs) nanorods were compared based on the rate kinetics and dye degradation efficiencies. Cobalt oxide (1 wt %) loaded and 1.0 M acid treated TiO₂ nanorods (Co/ATO-NR) exhibited the higher photocatalytic degradation efficiency for Orange-II dye degradation and inactivation of S. typhimurium pathogen compared to other photocatalysts under solar irradiation. Photoelectrochemical analysis demonstrated that the charge transfer process in Co/ATO-NR is significantly higher than that in the untreated samples. The improved photocatalytic activity of inactive TO-NRs might be due to enhanced charge transfer of finely dispersed metal oxides on the OH-rich surface of acid treated TiO₂ nanorods.

CdS/Zr:Fe2 O3 Nanorod Arrays with Al2 O3 Passivation Layer for Photoelectrochemical Solar Hydrogen Generation

CdS-sensitized 1 D Zr:Fe₂ O₃ nanorod arrays were synthesized on fluorine-doped tin oxide substrates by a two-step hydrothermal method. The photoelectrochemical results demonstrate that the current density (4.2 mA cm^-2 at 0 V vs. Ag/AgCl) recorded under illumination for the CdS/1 D Zr:Fe₂ O₃ photoanodes is 2.8 time higher than the bare 1 D Zr:Fe₂ O₃ . The extended absorbance spectrum, the reduced recombination, and the effective transport of photogenerated holes in CdS to the electrolyte facilitate enhancement in the photoelectrochemical performance. From X-ray photoelectron spectroscopy and TEM observations of the bare and aluminum oxide-treated CdS/1 D Zr:Fe₂ O₃ photoanodes, we could confirm that the 1 D Zr:Fe₂ O₃ nanorods were covered by the CdS layer and Al₂ O₃ layer present on surface of CdS. Furthermore, the photocurrent and stability of the CdS/1 D Zr:Fe₂ O₃ nanorods was significantly enhanced by Al₂ O₃ compared to bare CdS/1 D Zr:Fe₂ O₃ heterojunction owing to its ability to act as an effective holetransport- as well as photocorrosion-protecting layer. These remarkable enhancements in light-energy harvesting, improvement in charge transport, and stability directly suggest the usefulness of photoanodes for solar hydrogen generation.

Effect of Nb Doping on Chemical Sensing Performance of Two-Dimensional Layered MoSe2

Here, we report that Nb doping of two-dimensional (2D) MoSe₂ layered nanomaterials is a promising approach to improve their gas sensing performance. In this study, Nb atoms were incorporated into a 2D MoSe₂ host matrix, and the Nb doping concentration could be precisely controlled by varying the number of Nb₂O₅ deposition cycles in the plasma enhanced atomic layer deposition process. At relatively low Nb dopant concentrations, MoSe₂ showed enhanced device durability as well as NO₂ gas response, attributed to its small grains and stabilized grain boundaries. Meanwhile, an increase in the Nb doping concentration deteriorated the NO₂ gas response. This might be attributed to a considerable increase in the number of metallic NbSe₂ regions, which do not respond to gas molecules. This novel method of doping 2D transition metal dichalcogenide-based nanomaterials with metal atoms is a promising approach to improve the performance such as stability and gas response of 2D gas sensors.

High-Performance One-Body Core/Shell Nanowire Supercapacitor Enabled by Conformal Growth of Capacitive 2D WS2 Layers

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have emerged as promising capacitive materials for supercapacitor devices owing to their intrinsically layered structure and large surface areas. Hierarchically integrating 2D TMDs with other functional nanomaterials has recently been pursued to improve electrochemical performances; however, it often suffers from limited cyclic stabilities and capacitance losses due to the poor structural integrity at the interfaces of randomly assembled materials. Here, we report high-performance core/shell nanowire supercapacitors based on an array of one-dimensional (1D) nanowires seamlessly integrated with conformal 2D TMD layers. The 1D and 2D supercapacitor components possess "one-body" geometry with atomically sharp and structurally robust core/shell interfaces, as they were spontaneously converted from identical metal current collectors via sequential oxidation/sulfurization. These hybrid supercapacitors outperform previously developed any stand-alone 2D TMD-based supercapacitors; particularly, exhibiting an exceptional charge-discharge retention over 30,000 cycles owing to their structural robustness, suggesting great potential for unconventional energy storage technologies.