1D MXenes: Synthesis, Properties, and Applications.

The fascinating properties and versatile nature of 2D MXenes have generated significant interest in the scientific community. This has led to extensive research on expanding these materials into 1D and 0D forms. This review investigates the synthesis, properties, and applications of 1D MXenes, elucidating their potential across various fields. 1D MXenes, including nanowires, nanoribbons, nanorods, and nanotubes, inherit the remarkable properties of their 2D counterparts while also exhibiting unique anisotropic characteristics that enhance their performance in various applications. The review explores various methods for synthesizing 1D MXenes and examines their structural, electronic, and optical properties. The transition from 2D to 1D results in MXenes that offer superior properties, which are advantageous for various next-generation systems. The increased aspect ratio and surface area of 1D MXenes broaden their usage in energy storage, photothermal therapy, oxygen evolution reactions (OER), hydrogen evolution reactions (HER), oxygen reduction reactions (ORR), microwave absorption, filtration membranes, gas sensors, metal detection, etc. The review also addresses the challenges associated with 1D MXenes, such as limited synthesis methods, scalable production, size customization, preservation of structural integrity, and stability. Furthermore, potential opportunities and future directions in the field of 1D MXenes have also been proposed.

Bone Reconstruction Using Two-Layer Porcine-Derived Bone Scaffold Composed of Cortical and Cancellous Bones in a Rabbit Calvarial Defect Model.

In this study, we aimed to investigate the bone regeneration efficiency of two-layer porcine-derived bone scaffolds composed of cancellous and cortical bones in a rabbit calvarial defect model. Four circular calvaria defects were formed on cranium of rabbit and were filled with block bone scaffolds of each group: cortical bone block (Cortical group), cancellous bone block (Cancellous group), and two-layer bone block (2layer group). After 8 weeks, new bones were primarily observed in cancellous parts of the Cancellous and 2layer groups, while the Cortical group exhibited few new bones. In the results of new bone volume and area analyses, the Cancellous group showed the highest value, followed by the 2layer group, and were significantly higher than the Cortical group. Within the limitations of this study, the cancellous and two-layer porcine-derived bone scaffolds showed satisfactory bone regeneration efficiency; further studies on regulating the ratio of cortical and cancellous bones in two-layer bones are needed.

Optoelectronics of Multijunction Heterostructures of Transition Metal Dichalcogenides.

Among p-n junction devices with multilayered heterostructures with WSe2 and MoSe2, a device with the MoSe2-WSe2-MoSe2 (NPN) structure showed a remarkably high photoresponse, which was 1000 times higher than the MoSe2-WSe2 (NP) structure. The ideality factor of the NPN structure was estimated to be ∼1, lower than that of the NP structure. It is claimed that the NPN structure formed a thinner depletion region than that of the NP structure because of the difference of carrier concentrations of MoSe2 and WSe2. Hence, the built-in electric field was weaker, and the motion of the photocarriers was facilitated. These behaviors were confirmed experimentally from a photocurrent mapping analysis and Kelvin probe force microscopy. The work function depended on the wavelength of the illuminator, and quasi-Fermi level was estimated. The surface photovoltage on the MoSe2 region was higher than that on WSe2 because the lower bandgap of MoSe2 induces more electron-hole pair generation.

Visibility of hexagonal boron nitride on transparent substrates.

The high transmittance and low reflectance of monolayer hexagonal boron nitride (hBN) lead to its invisibility under white-light, causing serious troubles in the search, transfer, and fabrication of 2D material devices. In this work, we demonstrate enhancing the contrast of hBN on a transparent substrate by simulation and experimental observation, where the highest contrast is obtained by using a polymer-based interfacial layer on a polydimethylsiloxane (PDMS) substrate. The simulation result reveals that the contrast under short wavelength light is higher than that under long wavelength. To confirm this, the red-green-blue components are extracted from the optical color image. The blue component image shows an hBN flake clearly on the substrate, while the hBN flake fades on the green and red components. Moreover, the contrast on transparent substrates have only positive value, while opaque substrates cause both negative and positive contrast depending on the thickness of the interfacial layer. Thus, the high contrast (∼4.5%) of hBN on the PDMS substrate enables us to observe mono- and few-layer hBN flakes under white-light illumination by an optical microscope.

CVD-graphene for low equivalent series resistance in rGO/CVD-graphene/Ni-based supercapacitors.

Reduced equivalent series resistance (ESR) is necessary, particularly at a high current density, for high performance supercapacitors, and the interface resistance between the current collector and electrode material is one of the main components of ESR. In this report, we have optimized chemical vapor deposition-grown graphene (CVD-G) on a current collector (Ni-foil) using reduced graphene oxide as an active electrode material to fabricate an electric double layer capacitor with reduced ESR. The CVD-G was grown at different cooling rates-20 °C min-1, 40 °C min-1 and 100 °C min-1-to determine the optimum conditions. The lowest ESR, 0.38 Ω, was obtained for a cell with a 100 °C min-1 cooling rate, while the sample without a CVD-G interlayer exhibited 0.80 Ω. The CVD-G interlayer-based supercapacitors exhibited fast CD characteristics with high scan rates up to 10 Vs-1 due to low ESR. The specific capacitances deposited with CVD-G were in the range of 145.6 F g-1-213.8 F g-1 at a voltage scan rate of 0.05 V s-1. A quasi-rectangular behavior was observed in the cyclic voltammetry curves, even at very high scan rates of 50 and 100 V s-1, for the cell with optimized CVD-G at higher cooling rates, i.e. 100 °C min-1.

Van der Waals heterojunction diode composed of WS2 flake placed on p-type Si substrate.

P-N junctions represent the fundamental building blocks of most semiconductors for optoelectronic functions. This work demonstrates a technique for forming a WS2/Si van der Waals junction based on mechanical exfoliation. Multilayered WS2 nanoflakes were exfoliated on the surface of bulk p-type Si substrates using a polydimethylsiloxane stamp. We found that the fabricated WS2/Si p-n junctions exhibited rectifying characteristics. We studied the effect of annealing processes on the performance of the WS2/Si van der Waals p-n junction and demonstrated that annealing improved its electrical characteristics. However, devices with vacuum annealing have an enhanced forward-bias current compared to those annealed in a gaseous environment. We also studied the top-gate-tunable rectification characteristics across the p-n junction interface in experiments as well as density functional theory calculations. Under various temperatures, Zener breakdown occurred at low reverse-bias voltages, and its breakdown voltage exhibited a negative coefficient of temperature. Another breakdown voltage was observed, which increased with temperature, suggesting a positive coefficient of temperature. Therefore, such a breakdown can be assigned to avalanche breakdown. This work demonstrates a promising application of two-dimensional materials placed directly on conventional bulk Si substrates.

Study of Grains and Boundaries of Molybdenum Diselenide and Tungsten Diselenide Using Liquid Crystal.

Direct observation of grains and boundaries is a vital factor in altering the electrical and optoelectronic properties of transition metal dichalcogenides (TMDs), that is, MoSe2 and WSe2. Here, we report visualization of grains and boundaries of chemical vapor deposition grown MoSe2 and WSe2 on silicon, using optical birefringence of two-dimensional layer covered with nematic liquid crystal (LC). An in-depth study was performed to determine the alignment orientation of LC molecules and their correlation with other grains. Interestingly, we found that alignment of liquid crystal has discrete preferential orientations. From computational simulations, higher adsorption energy for the armchair direction was found to force LC molecules to align on it, compared to that of the zigzag direction. We believe that these TMDs with three-fold symmetric alignment could be utilized for display applications.

Flexible Transparent Conductive Electrodes: Unveiling Growth Mechanisms, Material Dimensions, Fabrication Methods, and Design Strategies.

Flexible transparent conductive electrodes (FTCEs) constitute an indispensable component in state-of-the-art electronic devices, such as wearable flexible sensors, flexible displays, artificial skin, and biomedical devices, etc. This review paper offers a comprehensive overview of the fabrication techniques, growth modes, material dimensions, design, and their impacts on FTCEs fabrication. The growth modes, such as the "Stranski-Krastanov growth," "Frank-van der Merwe growth," and "Volmer-Weber growth" modes provide flexibility in fabricating FTCEs. Application of different materials including 0D, 1D, 2D, polymer composites, conductive oxides, and hybrid materials in FTCE fabrication, emphasizing their suitability in flexible devices are discussed. This review also delves into the design strategies of FTCEs, including microgrids, nanotroughs, nanomesh, nanowires network, and "kirigami"-inspired patterns, etc. The pros and cons associated with these materials and designs are also addressed appropriately. Considerations such as trade-offs between electrical conductivity and optical transparency or "figure of merit (FoM)," "strain engineering," "work function," and "haze" are also discussed briefly. Finally, this review outlines the challenges and opportunities in the current and future development of FTCEs for flexible electronics, including the improved trade-offs between optoelectronic parameters, novel materials development, mechanical stability, reproducibility, scalability, and durability enhancement, safety, biocompatibility, etc.

Nanolithography on graphene by using scanning tunneling microscopy in a methanol environment.

Since it was discovered in 2004, graphene has attracted enormous attention as an emerging material for future devices, but it has been found that conventional lithographic processes based on polymer resist degrade its intrinsic performance. Recently, our group studied a resist-free scanning tunneling microscopy-based lithography in various atmospheres by injecting volatile liquids into a chamber. In this study, multilayer graphene was scanned and etched by controlling bias voltage under methanol pressure. We focused on improving patterning results in terms of depth and line width, while the previous study was performed to find an optimum gas environment for patterning on a graphite surface. Specifically, we report patterning outputs depending on conditions of voltage, current, and pressure. The optimum conditions for methanol environment etching were a gas pressure in the range of 41-50 torr, a -4 V tip bias, and a 2 nA tunneling current.

Effect of Graphite Nanoplatelet Size and Dispersion on the Thermal and Mechanical Properties of Epoxy-Based Nanocomposites.

This study investigated the effect of graphite nanoplatelet (GNP) size and dispersion on the thermal conductivities and tensile strengths of epoxy-based composites. GNPs of four different platelet sizes, ranging from 1.6 to 3 µm, were derived by mechanically exfoliating and breaking expanded graphite (EG) particles using high-energy bead milling and sonication. The GNPs were used as fillers at loadings of 0-10 wt%. As the GNP size and loading amount increased, the thermal conductivities of the GNP/epoxy composites increased, but their tensile strengths decreased. However, interestingly, the tensile strength reached a maximum value at the low GNP content of 0.3% and thereafter decreased, irrespective of the GNP size. Our observations of the morphologies and dispersions of the GNPs in the composites indicated that the thermal conductivity was more likely related to the size and loading number of fillers, whereas the tensile strength was more influenced by the dispersion of fillers in the matrix.

Nanoscale investigation of charge transport at the grain boundaries and wrinkles in graphene film.

The influence of grain boundaries and mechanical deformations in graphene film on the electric charge transport is investigated at nanoscale with conductive atomic force microscopy. Large area monolayer graphene samples were prepared by the chemical vapor deposition technique. Field emission scanning electron microscopy confirmed the formation of grain boundaries and the presence of wrinkles. The presence of the D-band in the Raman spectrum also indicated the existence of sharp defects such as grain boundaries. Extremely low conductivity was found at the grain boundaries and the wrinkled surface was also more resistive in comparison to the plain graphene surface. Many samples were experimented with to justify our findings by selecting different areas on the graphene surface. Uniform conductivity was found on grain boundary and wrinkle free graphene surfaces. We made channels of varied lengths by local anodic oxidation to confine the charge carrier to the smallest dimensions to better confirm the alteration in current due to grain boundaries and wrinkles. The experimental findings are discussed with reference to the implementation of graphene as transparent conductive electrode.

NaCl-Assisted Temperature-Dependent Controllable Growth of Large-Area MoS2 Crystals Using Confined-Space CVD.

Due to its semiconducting nature, controlled growth of large-area chemical vapor deposition (CVD)-grown two-dimensional (2D) molybdenum disulfide (MoS2) has a lot of potential applications in photodetectors, sensors, and optoelectronics. Yet the controllable, large-area, and cost-effective growth of highly crystalline MoS2 remains a challenge. Confined-space CVD is a very promising method for the growth of highly crystalline MoS2 in a controlled manner. Herein, we report the large-scale growth of MoS2 with different morphologies using NaCl as a seeding promoter for confined-space CVD. Changes in the morphologies of MoS2 are reported by variation in the amount of seeding promoter, precursor ratio, and the growth temperature. Furthermore, the properties of the grown MoS2 are analyzed using optical microscopy, scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX), and atomic force microscopy (AFM). The electrical properties of the CVD-grown MoS2 show promising performance from fabricated field-effect transistors. This work provides new insight into the growth of large-area MoS2 and opens the way for its various optoelectronic and electronic applications.

Graphite patterning in a controlled gas environment.

Although a number of methods using scanning probe lithography to pattern graphene have already been introduced, the fabrication of real devices still faces limitations. We report graphite patterning using scanning probe lithography with control of the gas environment. Patterning processes using scanning probe lithography of graphite or graphene are normally performed in air because water molecules forming the meniscus between the tip and the sample mediate the etching reaction. This water meniscus, however, may prevent uniform patterning due to its strong surface tension or large contact angle on surfaces. To investigate this side effect of water, our experiment was performed in a chamber where the gas environment was controlled with methyl alcohol, oxygen or isopropanol gases. We found that methyl alcohol facilitates graphite etching, and a line width as narrow as 3 nm was achieved as methyl alcohol also contains an oxygen atom which gives rise to the required oxidation. Due to its low surface tension and highly adsorptive behavior, methyl alcohol has advantages for a narrow line width and high speed etching conditions.

Application of scanning probe lithography to graphite patterning.

We applied the scanning probe lithographic technique to a graphite patterning in air and analyzed the patterned sample with the lateral force microscopy and Raman spectroscopy. The local electric field generated from a tip caused either etching or oxidization depending on the electric field intensity in air. We have found that the frictional force between the tip and local oxidized graphite surface was increased remarkably from lateral force analysis. Also, it was found that the graphene layer was peeled from the graphite surface in the etching process, which could be a potential tool as a top-down nano-fabrication process for the graphene nano device without contamination.

Ridge formation and removal via annealing in exfoliated graphene.

It is well known that graphene is a very promising material due to its excellent physical, chemical, and thermal properties. Previously, ridges in graphene on a substrate were found in epitaxial graphene on a SiC substrate. It was found in this study that ridges can be made on a graphene layer via mechanical exfoliation on a sapphire substrate, and that ridges can be created or removed through heating and cooling. Due to the difference of the thermal-expansion coefficients of the substrate and graphene, it can be said that thermal cycling causes compressive strain, which is released by forming ridges. Annealing was carried out in a vacuum chamber within the pressure range of 10(-3)-10(-6) Torr and at 900-1100 degrees C. To analyze the shapes and mechanical properties of the ridges, Raman spectroscopy and AFM measurement were performed. It was found that the ridges can be extended by defect as a nucleation center, and the graphene layer can be folded along the preexisting ridge during heating and cooling.

Supercapacitors based on Ti3C2Tx MXene extracted from supernatant and current collectors passivated by CVD-graphene.

An ultrahigh capacity supercapacitor is fabricated using a nano-layered MXene as an active electrode material, and Ni-foil is used as a current collector. The high-quality Ti3C2Tx obtained from supernatant during etching and washing processes improves the specific capacitance significantly. As another strategy, the surface of Ni-foil is engineered by coating chemical vapor deposition-grown graphene. The graphene grown directly on the Ni-foil is used as a current collector, forming the electrode structure of Ti3C2Tx/graphene/Ni. The surface passivation of the current collectors has a high impact on charge-transfer, which in turn increases the capacitance of the supercapacitors. It is found that the capacitance of the graphene-based supercapacitors is more than 1.5 times of the capacitance without graphene. A high specific capacitance, ~ 542 F/g, is achieved at 5 mV/s scan rate based on cyclic voltammetry analysis. Also, the graphene-based supercapacitor exhibits a quasi-rectangular form in cyclic voltammetry curves and a symmetric behavior in charge/discharge curves. Furthermore, cyclic stability up to 5000 cycles is confirmed with high capacitance retention at high scan rate 1000 mV/s. A reduced series resistance with a high limit capacitance is revealed by equivalent circuit analysis with the Nyquist plot.

Self-standing SnS nanosheet array: a bifunctional binder-free thin film catalyst for electrochemical hydrogen generation and wastewater treatment.

Hydrogen generation during wastewater treatment has remained a long-standing challenge for the environment preservation welfare. In the present work, we have fabricated a promising bifunctional thin film-based catalyst for hydrogen generation with concurrent wastewater treatment. The prepared catalyst film is a vertically oriented thin SnS (tin monosulfide) nanosheet array on a Ni-foam (SnS/NF) obtained via a solution process, demonstrating a promising electrocatalytic activity towards the generation of green H2 fuel at the cathodic side and the decomposition of urea waste at the anodic side. Notably, while assembling two identical electrodes as cathode and anode together with a reference electrode (i.e., SnS/NF∥SnS/NF vs. RHE assembly) in 1 M KOH aqueous electrolyte containing 0.33 M urea, the electrolyzer electrolyzed urea at a lower cell potential of 1.37 and 1.43 V (vs. RHE) to deliver a current density of 10 mA cm-2 and 50 mA cm-2, respectively, for the decomposition of urea at the anodic SnS/NF electrode and green hydrogen fuel generation at the cathodic SnS/NF electrode. This activity on electrocatalytic urea decomposition lies within the best performance to those of the previously reported sulfide-based and other catalytic materials. The promising catalytic activities of the SnS catalyst film are attributed to its combined effect of self-standing nanosheet array morphology and high crystallinity, which provides abundant active sites and a facile charge transfer path between the nanosheet arrays and the electrolyte. Thus, the present work offers a green avenue to the waste-urea treatment in water and sustainable hydrogen energy production.

Application of Titanium-Carbide MXene-Based Transparent Conducting Electrodes in Flexible Smart Windows.

Among various available materials used in transparent and flexible devices, MXenes are attracting attention as a brand-new candidate in this category. Ti3C2Tx MXene as a 2D material has exceptional properties, making it a potential material having numerous applications in different areas. Because of its high conductivity, it can be used in transparent conducting electrodes (TCEs). In this study, the MXenes etched by highly concentrated acid at 50 °C,were spin-coated on polyethylene terephthalate (PET) film and annealed at moderate temperatures up to 170 °C. The adhesion of MXene to PET was found to be remarkably improved by annealing. These TCEs exhibited a sheet resistance of ∼424 Ω/sq. and transmittance of ∼87%. The aging stability of MXene-coated PET films against oxidation under ambient conditions was studied up to 28 days and resistance change was found ∼30% during this period. The flexibility test showed low bending resistance change (∼1.5%) at 1000th cycle and cumulative resistance change of ∼20% at a bending radius of ∼3.9 mm after 1000 cycles. These transparent, flexible, and conducting electrodes were used to fabricate polymer dispersed liquid crystal (PDLC)-based flexible smart windows. The smart windows fabricated by curing PDLC mixture sandwiched between the MXene electrodes were also found flexible in ON/OFF states. The MXene-based flexible smart windows resulted in good opacity in the OFF state and high transparency in the ON state, exhibiting low threshold voltage <10 V and high transmittance ∼80% at 60 V. The flexible smart windows operated normally even at ∼4 mm bending radius.

NIR self-powered photodetection and gate tunable rectification behavior in 2D GeSe/MoSe2 heterojunction diode.

Two-dimensional (2D) heterostructure with atomically sharp interface holds promise for future electronics and optoelectronics because of their multi-functionalities. Here we demonstrate gate-tunable rectifying behavior and self-powered photovoltaic characteristics of novel p-GeSe/n-MoSe2 van der waal heterojunction (vdW HJ). A substantial increase in rectification behavior was observed when the devices were subjected to gate bias. The highest rectification of ~ 1 × 104 was obtained at Vg = - 40 V. Remarkable rectification behavior of the p-n diode is solely attributed to the sharp interface between metal and GeSe/MoSe2. The device exhibits a high photoresponse towards NIR (850 nm). A high photoresponsivity of 465 mAW-1, an excellent EQE of 670%, a fast rise time of 180 ms, and a decay time of 360 ms were obtained. Furthermore, the diode exhibits detectivity (D) of 7.3 × 109 Jones, the normalized photocurrent to the dark current ratio (NPDR) of 1.9 × 1010 W-1, and the noise equivalent power (NEP) of 1.22 × 10-13 WHz-1/2. The strong light-matter interaction stipulates that the GeSe/MoSe2 diode may open new realms in multi-functional electronics and optoelectronics applications.

Gel-type autologous chondrocyte (Chondron) implantation for treatment of articular cartilage defects of the knee.

BACKGROUND: Gel-type autologous chondrocyte (Chondron) implantations have been used for several years without using periosteum or membrane. This study involves evaluations of the clinical results of Chondron at many clinical centers at various time points during the postoperative patient follow-up. METHODS: Data from 98 patients with articular cartilage injury of the knee joint and who underwent Chondron implantation at ten Korean hospitals between January 2005 and November 2008, were included and were divided into two groups based on the patient follow-up period, i.e. 13~24-month follow-up and greater than 25-month follow-up. The telephone Knee Society Score obtained during telephone interviews with patients, was used as the evaluation tool. RESULTS: On the tKSS-A (telephone Knee Society Score-A), the score improved from 43.52 +/- 20.20 to 89.71 +/- 13.69 (P < 0.05), and on the tKSS-B (telephone Knee Society Score-B), the score improved from 50.66 +/- 20.05 to 89.38 +/- 15.76 (P < 0.05). The total improvement was from 94.18 +/- 31.43 to 179.10 +/- 24.69 (P < 0.05). CONCLUSION: Gel-type autologous chondrocyte implantation for chondral knee defects appears to be a safe and effective method for both decreasing pain and improving knee function.