Emergence of two distinct phase transitions in monolayer CoSe2 on graphene.

Dimensional modifications play a crucial role in various applications, especially in the context of device miniaturization, giving rise to novel quantum phenomena. The many-body dynamics induced by dimensional modifications, including electron-electron, electron-phonon, electron-magnon and electron-plasmon coupling, are known to significantly affect the atomic and electronic properties of the materials. By reducing the dimensionality of orthorhombic CoSe2 and forming heterostructure with bilayer graphene using molecular beam epitaxy, we unveil the emergence of two types of phase transitions through angle-resolved photoemission spectroscopy and scanning tunneling microscopy measurements. We disclose that the 2 × 1 superstructure is associated with charge density wave induced by Fermi surface nesting, characterized by a transition temperature of 340 K. Additionally, another phase transition at temperature of 160 K based on temperature dependent gap evolution are observed with renormalized electronic structure induced by electron-boson coupling. These discoveries of the electronic and atomic modifications, influenced by electron-electron and electron-boson interactions, underscore that many-body physics play significant roles in understanding low-dimensional properties of non-van der Waals Co-chalcogenides and related heterostructures.

Highly Dispersed Pt Clusters on F-Doped Tin(IV) Oxide Aerogel Matrix: An Ultra-Robust Hybrid Catalyst for Enhanced Hydrogen Evolution.

Dispersing the minuscule mass loading without hampering the high catalytic activity and long-term stability of a noble metal catalyst results in its ultimate efficacy for the electrochemical hydrogen evolution reaction (HER). Despite being the most efficient HER catalyst, the use of Pt is curtailed due to its scarcity and tendency to leach out in the harsh electrochemical reaction environment. In this study, we combined F-doped tin(IV) oxide (F-SnO2) aerogel with Pt catalyst to prevent metallic corrosion and to achieve abundant Pt active sites (approximately 5 nm clusters) with large specific surface area (321 cm2·g-1). With nanoscopic Pt loading inside the SnO2 aerogel matrix, the as-synthesized hybrid F-SnO2@Pt possesses a large specific surface area and high porosity and, thus, exhibits efficient experimental and intrinsic HER activity (a low overpotential of 42 mV at 10 mA·cm-2 in 0.5 M sulfuric acid), a 22-times larger turnover frequency (11.2 H2·s-1) than that of Pt/C at 50 mV, and excellent robustness over 10,000 cyclic voltammetry cycles. The existing metal support interaction and strong intermolecular forces between Pt and F-SnO2 account for the catalytic superiority and persistence against corrosion of F-SnO2@Pt compared to commercially used Pt/C. Density functional theory analysis suggests that hybridization between the Pt and F-SnO2 orbitals enhances intermediate hydrogen atom (H*) adsorption at their interface, which improves the reaction kinetics.

Emergent Topological Hall Effect from Exchange Coupling in Ferromagnetic Cr2Te3/Noncoplanar Antiferromagnetic Cr2Se3 Bilayers.

The topological Hall effect has been observed in magnetic materials of complex spin structures or bilayers of trivial magnets and strong spin-orbit-coupled systems. In view of current attention on dissipationless topological electronics, the occurrence of the topological Hall effect in new systems or by an unexpected mechanism is fascinating. Here, we report a robust topological Hall effect generated in bilayers of a ferromagnet and a noncoplanar antiferromagnet, from the interfacial Dzyaloshinskii-Moriya interaction due to the exchange coupling of magnetic layers. Molecular beam epitaxy has been utilized to fabricate heterostructures of a ferromagnetic metal Cr2Te3 and a noncoplanar antiferromagnet Cr2Se3. A significant topological Hall effect at low temperature implies the development of nontrivial spin chirality, and density functional theory calculations explain the correlation of the Dzyaloshinskii-Moriya interaction increase and inversion symmetry breaking at the interface. The presence of noncoplanar ordering in the antiferromagnet plays a pivotal role in producing the topological Hall effect. Our results suggest that the exchange coupling in ferromagnet/noncoplanar antiferromagnet bilayers could be an alternative mechanism toward topologically protected magnetic structures.

Coherent control of interlayer vibrations in Bi2Se3 van der Waals thin-films.

Interlayer vibrations with discrete quantized modes in two-dimensional (2D) materials can be excited by ultrafast light due to the inherent low dimensionality and van der Waals force as a restoring force. Controlling such interlayer vibrations in layered materials, which are closely related to fundamental nanomechanical interactions and thermal transport, in spatial- and time-domain provides an in-depth understanding of condensed matters and potential applications for advanced phononic and photonics devices. The manipulation of interlayer vibrational modes has been implemented in a spatial domain through material design to develop novel optoelectronic and phononic devices with various 2D materials, but such control in a time domain is still lacking. We present an all-optical method for controlling the interlayer vibrations in a highly precise manner with Bi2Se3 as a promising optoelectronic and thermoelasticity material in layered structures using a coherently controlled pump and probe scheme. The observed thickness-dependent fast interlayer breathing modes and substrate-induced slow interfacial modes can be exactly explained by a modified linear chain model including coupling effect with substrate. In addition, the results of coherent control experiments also agree with the simulation results based on the interference of interlayer vibrations. This investigation is universally applicable for diverse 2D materials and provides insight into the interlayer vibration-related dynamics and novel device implementation based on an ultrafast timescale interlayer-spacing modulation scheme.

Engineering MoSe2/WS2 Hybrids to Replace the Scarce Platinum Electrode for Hydrogen Evolution Reactions and Dye-Sensitized Solar Cells.

In recent times, two-dimensional transition-metal dichalcogenides (TMDs) have become extremely attractive and proficient electrodes for dye-sensitized solar cells (DSSCs) and water electrolysis hydrogen evolution as alternatives to the scarce metal platinum (Pt). The active TMD molybdenum selenide (MoSe2) and tungsten disulfide (WS2) are inspiring systems owing to their abundance of active sulfur and selenium sites, but their outputs are lacking due to their inactive basal planes and ineffective transport behavior. In this work, van der Waals interrelated MoSe2/WS2 hybrid structures were constructed on conducting glass substrates by chemicophysical methodologies. For the first time, the constructed MoSe2/WS2 structures were effectively used as a counter electrode for DSSCs and an active electrode for hydrogen evolution to replace the nonabundant Pt. The assembled DSSCs using the designed MoSe2/WS2 heterostructure counter electrode provided a superior power-conversion efficiency of 9.92% and a photocurrent density of 23.10 mA·cm-2, unmatchable by most of the TMD-based structures. The MoSe2/WS2 heterostructure displayed excellent electrocatalytic hydrogen evolution behavior with a 75 mV overpotential to drive a 10 mA·cm-2 current density, a 60 mV·dec-1 Tafel slope, and an over 20 h durable process in an acidic medium. The results demonstrated the advantages of the MoSe2/WS2 hybrid development for generating interfacial transport and active facet distribution and enriching the electrocatalytic activity for DSSCs and the water-splitting hydrogen evolution process.

Skyrmion Phase in MnSi Thin Films Grown on Sapphire by a Conventional Sputtering.

Topologically protected chiral skyrmions are an intriguing spin texture that has attracted much attention because of fundamental research and future spintronic applications. MnSi with a non-centrosymmetric structure is a well-known material hosting a skyrmion phase. To date, the preparation of MnSi crystals has been investigated by using special instruments with an ultrahigh vacuum chamber. Here, we introduce a facile way to grow MnSi films on a sapphire substrate using a relatively low vacuum environment of conventional magnetron sputtering. Although the as-grown MnSi films have a polycrystalline nature, a stable skyrmion phase in a broad range of temperatures and magnetic fields is observed via magnetotransport properties including phenomenological scaling analysis of the Hall resistivity contribution. Our findings provide not only a general way to prepare the materials possessing skyrmion phases but also insight into further research to stimulate more degrees of freedom in our inquisitiveness.

One-Pot Synthesis of W2C/WS2 Hybrid Nanostructures for Improved Hydrogen Evolution Reactions and Supercapacitors.

Tungsten sulfide (WS2) and tungsten carbide (W2C) are materialized as the auspicious candidates for various electrochemical applications, owing to their plentiful active edge sites and better conductivity. In this work, the integration of W2C and WS2 was performed by using a simple chemical reaction to form W2C/WS2 hybrid as a proficient electrode for hydrogen evolution and supercapacitors. For the first time, a W2C/WS2 hybrid was engaged as a supercapacitor electrode and explored an incredible specific capacitance of ~1018 F g-1 at 1 A g-1 with the outstanding robustness. Furthermore, the constructed symmetric supercapacitor using W2C/WS2 possessed an energy density of 45.5 Wh kg-1 at 0.5 kW kg-1 power density. For hydrogen evolution, the W2C/WS2 hybrid produced the low overpotentials of 133 and 105 mV at 10 mA cm-2 with the small Tafel slopes of 70 and 84 mV dec-1 in acidic and alkaline media, respectively, proving their outstanding interfaced electrocatalytic characteristics. The engineered W2C/WS2-based electrode offered the high-performance for electrochemical energy applications.

Visualizing Orbital Content of Electronic Bands in Anisotropic 2D Semiconducting ReSe2.

Many properties of layered materials change as they are thinned from their bulk forms down to single layers, with examples including indirect-to-direct band gap transition in 2H semiconducting transition metal dichalcogenides as well as thickness-dependent changes in the valence band structure in post-transition-metal monochalcogenides and black phosphorus. Here, we use angle-resolved photoemission spectroscopy to study the electronic band structure of monolayer ReSe2, a semiconductor with a distorted 1T structure and in-plane anisotropy. By changing the polarization of incoming photons, we demonstrate that for ReSe2, in contrast to the 2H materials, the out-of-plane transition metal dz2 and chalcogen pz orbitals do not contribute significantly to the top of the valence band, which explains the reported weak changes in the electronic structure of this compound as a function of layer number. We estimate a band gap of 1.7 eV in pristine ReSe2 using scanning tunneling spectroscopy and explore the implications on the gap following surface doping with potassium. A lower bound of 1.4 eV is estimated for the gap in the fully doped case, suggesting that doping-dependent many-body effects significantly affect the electronic properties of ReSe2. Our results, supported by density functional theory calculations, provide insight into the mechanisms behind polarization-dependent optical properties of rhenium dichalcogenides and highlight their place among two-dimensional crystals.

Facile electrodeposition of V-doped CoP on vertical graphene for efficient alkaline water electrolysis.

In this work, we demonstrate a highly enhanced electrocatalytic activity of vanadium-doped CoP (V-CoP), directly grafted on a vertical graphene/carbon cloth electrode (VG/CC) by a facile electrochemical deposition method. Impressively, V-CoP/VG/CC exhibited a superior catalytic activity toward the hydrogen evolution reaction (HER) in alkaline solution. Compared to CoP/VG/CC, V-doping decreased the overpotential for HER at 10 mA cm-2 by more than half to 40 mV. The new catalyst even outperformed Pt/C beyond 150 mA cm-2. The overpotential for OER at 50 mA cm-2 was merely 314 mV, more than 100 mV lower than that of IrO2. Moreover, our novel catalyst worked as an excellent bifunctional catalyst with a low cell voltage of 1.69 V to achieve a current density of 50 mA cm-2. Detailed characterizations revealed that the V-doping in CoP resulted in improved electrical conductivity and increased active sites. Our findings highlight the significant advantage of V doping on the catalytic activities of CoP, already boosted by VG. Furthermore, concurrent doping with the electrodeposition of catalyst offers a new approach for practical water electrolysis.

Synthesis of Mo2C and W2C Nanoparticle Electrocatalysts for the Efficient Hydrogen Evolution Reaction in Alkali and Acid Electrolytes.

The synthesis of low cost, high efficacy, and durable hydrogen evolution electrocatalysts from the non-noble metal group is a major challenge. Herein, we establish a simple and inexpensive chemical reduction method for producing molybdenum carbide (Mo2C) and tungsten carbide (W2C) nanoparticles that are efficient electrocatalysts in alkali and acid electrolytes for hydrogen evolution reactions (HER). Mo2C exhibits outstanding electrocatalytic behavior with an overpotential of -134 mV in acid medium and of -116 mV in alkaline medium, while W2C nanoparticles require an overpotential of -173 mV in acidic medium and -130 mV in alkaline medium to attain a current density of 10 mA cm-2. The observed results prove the capability of high- and low-pH active electrocatalysts of Mo2C and W2C nanoparticles to be efficient systems for hydrogen production through HER water electrolysis.

Fabrication of Robust Hydrogen Evolution Reaction Electrocatalyst Using Ag2Se by Vacuum Evaporation.

Much research has been done on reliable and low-cost electrocatalysts for hydrogen generation by water splitting. In this study, we synthesized thin films of silver selenide (Ag2Se) using a simple thermal evaporation route and demonstrated their electrocatalytic hydrogen evolution reaction (HER) activity. The Ag2Se catalysts show improved electrochemical surface area and good HER electrocatalytic behavior (367 mV overpotential @ 10 mA·cm-2, exchange current density: ~1.02 × 10-3 mA·cm-2, and Tafel slope: 53 mV·dec-1) in an acidic medium). The reliability was checked in 0.5 M sulfuric acid over 20 h. Our first-principles calculations show the optimal energy of hydrogen adsorption, which is consistent with experimental results. The works could be further extended for finding a new catalyst by associating the selenide, sulfide or telluride-based materials without complex catalyst synthesis procedures.

Novel polymorphic phase of two-dimensional VSe2: the 1T' structure and its lattice dynamics.

Polymorphisms allowing multiple structural phases are among the most fascinating properties of transition metal dichalcogenides (TMDs). Herein, the polymorphic 1T' phase and its lattice dynamics for bilayer VSe2 grown on epitaxial bilayer graphene are investigated via low temperature scanning tunneling microscopy (STM). The 1T' structure, mostly observed in group-6 TMDs, is unexpected in VSe2, which is a group-5 TMD. Emergence of the 1T' structure in bilayer VSe2 suggests the important roles of interface and layer configurations, providing new possibilities regarding the polymorphism of TMDs. Detailed topographical analysis elucidates the microscopic nature of the 1T' structure, confirming that Se-like and V-like surfaces can be resolved depending on the polarity of the sample bias. In addition, bilayer VSe2 can transit from a static state of the 1T' phase to a dynamic state consisting of lattice vibrations, triggered by tunneling current from the STM tip. Topography also shows hysteretic behavior during the static-dynamic transition, which is attributed to latent energy existing between the two states. The observed lattice dynamics involve vibrational motion of the Se atoms and the middle V atoms. Our observations will provide important information to establish in-depth understanding of the microscopic nature of 1T' structures and the polymorphism of two-dimensional TMDs.

Prospective Evaluation of Atrophic Acne Scars on the Face With Needle-Free High-Pressure Pneumatic Injection: Quantitative Volumetric Scar Improvement.

BACKGROUND: Atrophic acne facial scars still pose a treatment challenge. Needle-free high-pressure pneumatic injection has recently been introduced; however, few studies exist regarding its effectiveness. OBJECTIVE: To evaluate the efficacy and safety of pneumatic injection for treating atrophic acne scars using a 3-dimensional optical profiling system. METHODS AND MATERIALS: A pneumatic injection device with a 0.2-mm nozzle diameter, 50% pressure power, and 85-µL injection volume was used. The degree of depression was examined and analyzed using a 3-dimensional optical profiling system and clinical photographs. The patients also evaluated any side effects. Each subject underwent a single treatment session and was followed up after 1 and 2 months. RESULTS: A total of 13 atrophic acne scars from 10 Korean men and women aged 20 to 29 (mean age 25.8 ± 2.4) years were studied. The mean scar volume values were 0.964, 0.741, and 0.566 mm, respectively, at baseline, 1 month, and 2 months after the injection. Scar volumes after 2 months were significantly different compared with baseline volumes. However, there was no significant difference between the baseline and 1-month volumes. CONCLUSION: Treatment with pneumatic injection is safe and effective in reducing atrophic acne facial scars; it results in quantitative improvement in scar volumes.

Needle-free pneumatic injection device; histologic assessment using a rat model and parameter comparison in predicting collagen synthesis degree.

BACKGROUND: Needle-free pneumatic injections have been recently introduced to the field of dermatology to inject such substances as hyaluronic acid. However, data on the influence of various pneumatic injection parameters on collagen synthesis are lacking. OBJECTIVE: Compare the effect of diameter, pressure, and volume of a pneumatic injection jet on collagen synthesis and fluid dispersion pattern using a rat model. Investigate if the total work force of the injection jet is useful in predicting the degree of collagen synthesis. MATERIALS AND METHODS: We injected fluid with 1 mg/ml of hyaluronic concentration to adult rats. Different injection pressures and volumes were tested using devices with nozzles of different diameters. Collagen synthesis areas were then measured, and statistical analyses were performed. RESULTS: The area of collagen fibers increased for up to two months. The injection pressure and volume did not correlate with the degree of collagen synthesis. The nozzle diameter showed a significant after two and four weeks of injection. The total work force correlated with collagen synthesis 2, 4, and 8 weeks post-injection. (P = 0.043, 0.027, and 0.000, respectively). CONCLUSION: Collagen formation is more prominent 2 months post-hyaluronic acid injection than after 1 month when using a needle-free pneumatic injection device. The total work force, which is affected by both the nozzle diameter and injection pressure, can be helpful in predicting the degree of collagen synthesis. Lasers Surg. Med. 51:278-285, 2019. © 2019 Wiley Periodicals, Inc.

Atomistic real-space observation of the van der Waals layered structure and tailored morphology in VSe2.

Two-dimensional (2D) transition metal dichalcogenides with van der Waals gaps have attracted much attention due to their peculiarly distinctive physical properties from their bulk counterparts. Among them, vanadium diselenide (VSe2) has been considered to be a promising candidate for future spintronic devices, as room temperature ferromagnetism was reported recently. However, detailed crystallography and properties of VSe2 nanosheets have been less explored. Here, we report the atomistic real-space observation of the van der Waals layered structure of VSe2 for the first time. Furthermore, simply by controlling the carrier gas flow rate, a morphological variation of the surface area and thickness of VSe2 nanosheets was observed. The room temperature ferromagnetic feature of single VSe2 nanosheets was also revealed by magnetic force microscopy measurements. Our findings will play a significant role in the research of intrinsic 2D ferromagnetic materials.

Successfully Treated Severe Acne Using Selective Electrothermolysis in a Patient with Nephrotic Syndrome.

Acne is very common in adolescents and young adults. Although there are various conventional treatment modalities, some patients are prone to side effects and need alternative options. A 22-year-old male patient who were treated with high dose systemic steroid for his nephrotic syndrome, encountered severe acne on his face and neck. Since the patient's medical condition was unable to administer systemic agent, he was treated with the selective electrothermolysis device. The sebaceous gland targeting treatment by selective electrothermolysis with microneedle radiofrequency device, had minimal adverse effect and the skin lesions improved dramatically. The patient was satisfied and did not want further treatment.

WS(1-x)Sex Nanoparticles Decorated Three-Dimensional Graphene on Nickel Foam: A Robust and Highly Efficient Electrocatalyst for the Hydrogen Evolution Reaction.

To find an effective alternative to scarce, high-cost noble platinum (Pt) electrocatalyst for hydrogen evolution reaction (HER), researchers are pursuing inexpensive and highly efficient materials as an electrocatalyst for large scale practical application. Layered transition metal dichalcogenides (TMDCs) are promising candidates for durable HER catalysts due to their cost-effective, highly active edges and Earth-abundant elements to replace Pt electrocatalysts. Herein, we design an active, stable earth-abundant TMDCs based catalyst, WS(1-x)Sex nanoparticles-decorated onto a 3D porous graphene/Ni foam. The WS(1-x)Sex/graphene/NF catalyst exhibits fast hydrogen evolution kinetics with a moderate overpotential of ~-93 mV to drive a current density of 10 mA cm-2, a small Tafel slope of ~51 mV dec-1, and a long cycling lifespan more than 20 h in 0.5 M sulfuric acid, which is much better than WS2/NF and WS2/graphene/NF catalysts. Our outcomes enabled a way to utilize the TMDCs decorated graphene and precious-metal-free electrocatalyst as mechanically robust and electrically conductive catalyst materials.

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

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

Recurrent Dermatofibrosarcoma Protuberans of Scalp in a Distant Location 10 Years after Primary Excision.

Dermatofibrosarcoma protuberans (DFSP) is a slow growing low-grade cutaneous sarcoma. Local recurrence after excision is common due to the poorly defined periphery that renders histological control of surgical margin difficult, Mohs micrographic surgery is the optimal method for treatment. A 41 years old male patient, who had a previous history of DFSP, came to our dermatology clinic for evaluation of an asymptomatic firm flesh-colored nodule on the forehead. Total excision biopsy was done and the mass was histologically proved as DFSP. Wide excision with reconstruction was performed and showed no sign of recurrence till 18-month follow up. Local recurrence is known to be common for DFSP but a new visible lesion distant from the initial site may be confused as a de novo lesion or a benign neoplasm especially in scalp area, and thus interrupt early detection of DFSP. Herein, we report a case of recurrent DFSP of scalp which recurred distant from the original lesion.