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In this research, dispersion of a new type of one-dimensional inorganic material Nb2Se9, composed of van der Waals bonds, in aqueous solution for bio-application study were studied. To disperse Nb2Se9, which exhibits hydrophobic properties in water, experiments were carried out using a block copolymer (poloxamer) as a dispersant. It was confirmed that PPO, the hydrophobic portion of Poloxamer, was adsorbed onto the surface of Nb2Se9, and PEO, the hydrophilic portion, induced steric hinderance to disperse Nb2Se9 to a size of 10 nm or less. To confirm the adaptability of muscle cells C2C12 to the dispersed Nb2Se9 using poloxamer 188 as dispersant, a MTT assay and a live/dead assay were performed, demonstrating improvement in the viability and proliferation of C2C12 cells.

A hydrogen sensor was fabricated by utilizing a bundle of metal oxide nanostructures whose growth positions were selectively controlled by utilizing graphene, which is a carbon of atomic-unit thickness. To verify the reducing ability of graphene, it was confirmed that the multi-composition metal oxide V2O5 was converted into VO2 on the graphene surface. Because of the role of graphene as a reducing catalyst, it was confirmed that ZnO and MoO3 nanostructures were grown at high density only on the graphene surface. The fabricated gas sensor showed excellent sensitivity.

Three-dimensional (3D) plasmonic structures have attracted great attention because abnormal wetting behavior of plasmonic nanoparticles (NPs) on 3D nanostructure can enhance the localized surface plasmons (LSPs). However, previous 3D plasmonic nanostructures inherently had weak plasmonic light absorption, low electrical conductivity, and optical transmittance. Here, we fabricated a novel 3D plasmonic nanostructure composed of Ag NPs as the metal for strong LSPs and 3D nano-branched indium tin oxide (ITO BRs) as a transparent and conductive framework. The Ag NPs formed on the ITO BRs have a more dewetted behavior than those formed on the ITO films. We experimentally investigated the reasons for the dewetting behavior of Ag NPs concerning the geometry of ITO BRs. The spherical Ag NPs are spatially separated and have high density, thereby resulting in strong LSPs. Finite-domain time-difference simulation evidenced that spatially-separated, high-density and spherical Ag NPs formed on ITO BRs dramatically boost the localized electric field in the active layer of organic solar cells (OSCs). Photocurrent of PTB7:PCBM OSCs with the ITO BRs/Ag NPs increased by 14%.

Though it is well recognized that the space between graphene cover and the metal substrate can act as a two-dimensional (2D) nanoreactor, several issues are still unresolved, including the role of the metal substrate, the mechanisms ruling water intercalation and the identification of sites at which water is decomposed. Here, we solve these issues by means of density functional theory and high-resolution electron energy loss spectroscopy experiments carried out on graphene grown on (111)-oriented Cu foils. Specifically, we observe decomposition of H2O at room temperature with only H atoms forming bonds with graphene and with buried OH groups underneath the graphene cover. Our theoretical model discloses physicochemical mechanisms ruling the migration and decomposition of water on graphene/Cu. We discover that the edge of graphene can be easily saturated by H through decomposition of H2O, which allows H2O to migrate in the subsurface region from the decoupled edge, where H2O decomposes at room temperature. Hydrogen atoms produced by the decomposition of H2O initially form a chemical bond with graphene for the lower energy barrier compared with other routes. These findings are essential to exploit graphene/Cu interfaces in catalysis and in energy-related applications.

MgO/Mg(OH)2 based thermal energy storage materials are studied. MgO micro-beams were extensively formed on the surface of the millimeter size MgO pellets by electron beam irradiation, and Mg(OH)2 flakes were synthesized in large amounts by inducing reaction with water molecules on the increased MgO surface area. It was confirmed that the mass reduction tendency of Mg(OH)2 flakes synthesized in large quantities was remarkably increased in the course of decomposition into MgO and H2O through an endothermic process. It is expected that this process can be applied to various thermal energy storage systems in the future because there is no dispersion and fine powder, and large-area processing is possible.

By using a MoI2 precursor, a one-dimensional linear chain material, Mo6S3I6, was synthesized on a centimeter scale. As MoI2 can be used to form Mo6S3I6 through a gas-phase reaction, a larger amount of single crystals can be synthesized; moreover, these crystals are significantly longer than those synthesized by using Mo, S, and I powder. The synthesized Mo6S3I6 exhibited the characteristics of paramagnetic semiconductors with excellent thermal stability and a low bandgap. Furthermore, Mo6S3I6 can be separated into long nanometer-scale chains by a solution dispersion process.

Large-scale industrial application of flexible device has called for development of transfer methods that deliver high yield and stability. Here, we show an ultrafast and chemically stable transfer method by using a water-soluble NaCl sacrificial layer. Extremely thin (10 nm) and large-area (4 in. wafer) free-standing Au nanomembranes (NMs) prepared on silicon substrate were successfully transferred to flexible PDMS substrate by dissolving the NaCl sacrificial layer. This transfer method enables highly transparent and electrically conductive Au NMs on PDMS substrate. To transfer a multilayered optoelectronic device, we fabricated flexible hydrogenated amorphous silicon (a-Si:H) solar cell on a glass substrate and transferred it to a PDMS substrate. There was no degradation of the electrical characteristic of the solar cell after the transfer. This approach enables the integration of high-temperature-processed a-Si:H solar cell onto low-temperature tolerant flexible polymer substrate without chemical contamination or damage.

We report the synthesis of single crystalline alkali metal vanadate nanowires, Li-vanadate (Li4V10O27), Na-vanadate (NaV6O15), and K-vanadate (KV4O10) and their electrical properties in a single nanowire configuration. Alkali metal vanadate nanowires were obtained by a simple thermal annealing process with vanadium hydroxides(V(OH)3) nanoparticles containing Li+, Na+, and K+ ions and further the analysis of the migration of charged particles (Li+, Na+, and K+) in vanadate by measuring the conductivity of them. We found that their ionic conductivities can be empirically explained by the Rasch-Hinrichsen resistivity and interpreted on the basis of transition state theory. Our results thus indicate that the Li ion shows the lowest potential barrier of ionic conduction due to its small ionic size. Additionally, Na-vanadate has the lowest ion number per unit V2O5, resulting in increased distance to move without collision, and ultimately in low resistivity at room temperature.

Detection of hydrogen (H2) over a wide concentration range (from parts-per-million levels to 4%) is necessary for safety in using hydrogen-fuel cell systems. For this purpose, our development of H2 sensitive nanohybrids of reduced graphene oxide (rGO) and chemochromic palladium-decorated molybdenum trioxide (Pd:MoO3) nanoflakes is presented. Additionally, the nanohybrids were employed as the active channel in a thin film transistor platform to invest informative characteristics for understanding their electrical properties and sensing mechanism. Through sensing measurements, (i) phenomena relating to response saturation and reversed behavior of gas sensors towards high exposure doses (i.e., p-to-n transition) and (ii) doping effects on the p-to-n transition and detection range of the hybrid materials are clarified. With combining electrical and visual output signals, the bimodal devices using the hole-doped nanohybrids can detect H2 over a wide concentration range (from 10 ppm to over 10%) at room temperature. Our results reveal opportunities for further development and improvement of hybrid nanomaterials and chemical sensors having a large sensing capability.

In this paper, a nickel (Ni) catalyst with large surface area supported by a graphitic layer is proposed. The graphitic layer is formed on a Ni foil by chemical vapor deposition (CVD) using CH4 as a source. The Ni foil is partially etched by a chemical etchant to increase the surface to volume ratio. The H2 generation rates of the catalysts for different Ni etching times are measured and their surfaces are observed using an optical microscope. The optimized Ni etching time is 6 h, and the catalyst prepared by 6 h of etching shows an H2 generation rate of about 600 ml/min⋅(g metal). The 6-h etched Ni/graphitic layer (Ni/G) is reused five times to confirm durability; it shows a high performance without any degradation. The H2 generation rate for different concentrations of NaBH4 is measured. The reaction follows first order kinetics, which means that the higher the concentration of NaBH4 is, the faster the H2 generation is.

Chemical vapor deposition of graphene on transition metals is the most favored method to get large scale homogenous graphene films to date. However, this method involves a very critical step of transferring as grown graphene to desired substrates. A sacrificial polymer film is used to provide mechanical and structural support to graphene, as it is detached from underlying metal substrate, but, the residue and cracks of the polymer film after the transfer process affects the properties of the graphene. Herein, a simple mixture of polystyrene and low weight plasticizing molecules is reported as a suitable candidate to be used as polymer support layer for transfer of graphene synthesized by chemical vapor deposition (CVD). This combination primarily improves the flexibility of the polystyrene to prevent cracking during the transfer process. In addition, the polymer removal solvent can easily penetrate between the softener molecules, so that the polymer film can be easily dissolved after transfer of graphene, thereby leaving no residue. This facile method can be used freely for the large-scale transfer of 2D materials.

Chemical vapor deposition (CVD) is one of the best ways to scalably grow low cost, high quality graphene on metal substrates; unfortunately, it not ideal for producing graphene on dielectric substrates. Here, we demontrate production of a high quality graphene layer on Sapphire using CVD with a copper catalyst. The catalyst consists of a thin copper film grown epitaxially on α- Al 2 O 3 (0001). After CVD growth of Graphene, the copper can be removed by simple evaporation in the presence of a carbon source ( C 2 H 4 ). We characterized the resulting graphene layer using Raman spectroscopy, atomic force microscopy (AFM), optical transmission and helium atom scattering (HAS). The sample exhibited a reduced Raman D peak and an excellent 2D to G ratio. AFM and HAS show large graphene domains over a macroscopic region. We measured > 86 % transparency over the visible spectrum.

In this article, the authors present the synthesis of Sn–1.0Ag–0.5Cu (SAC105) alloy embedded with zirconium oxide nanoparticles using simple mechanical blending and casting route. The cast samples were characterized in terms of microstructural evolution, wetting, microhardness, and drop test reliability. The characterizations were performed by using a tabletop X-ray diffraction, field emission scanning electron microscope, and the compositions were identified by energy dispersive spectroscopy. The results showed that addition of ZrO2 nanoparticles significantly refines the grain size, Ag3Sn, and Cu6Sn5 intermetallic compounds thickness by 46, 14, and 26% respectively as compared to the single component SAC105 alloy. The results were also compared with those of standard Sn–3.0Ag–0.5Cu (SAC305) alloy. Although the spreading and microhardness of SAC105 is found to be slightly poor or comparable to SAC305 yet drop test reliability can be improved significantly after addition of ZrO2 nanoparticles appreciably. This kind of refinement results in a cheap alternative to SAC305 with comparable mechanical strength and high solder joint reliability.

We developed a breakthrough method for preparing metallic nanostructures using a self-assembled NaCl nanosized sacrificial crystal and applied it to (i) transparent flexible electrodes and (ii) as catalysts for Si nanorod fabrication. Au nanostructures fabricated using the NaCl sacrificial crystal on a polyethylene terephthalate substrate showed low resistance (40 Ω/sq), relatively high transmittance (73%), and excellent mechanical stability. Wet chemical etching of Si with Ag nanostructures formed on a Si(100) substrate using the NaCl sacrificial crystal produced uniform vertical Si microrods with a thickness of 1–2 μm and length of about 3–4 μm. These Si microrods showed high light absorption (over 90%) in the wavelength range of 350–800 nm, and various applications in optoelectronic devices are expected in the future.

The secondary electron emission (SEE) properties of Zn-doped MgO thin films (~700 nm thickness) grown via electron-beam evaporation have been studied based on Paschen curve measurements. The Zn-doped (1 at.%) MgO film shows an improved SEE coefficient (γ) compared to that of non-doped MgO from 0.02 to 0.05 in the 100 V Pa−1 m−1 region. The origin of a superior γ is due to the reduction in the band-gap from 6.79 eV to 6.26 eV (the values are measured via ultra-violet photoemission spectroscopy; UPS) because γ is normally determined by the Auger neutralization process, which uses the neutralization energy of ion valence band electron emission. This thin film can be used in new kinds of plasma devices, such as flat-fluorescent lamps, plasma displays, electron emitters, and ionized mass spectrometers.

We studied the optimum dispersion solvent for bulk V2Se9 material, which can be used as a new one-dimensional (1D) material, to separate into 1D chain units. Selected twelve solvents, which have different dielectric constants and surface tensions, were tested to exfoliate bulk V2Se9 into nano-scale chains. The atomic level (∼1 nm, mono-chain) exfoliation of V2Se9 was performed using acetone as the solvent. The dispersion concentration was high in solvents having medium dielectric constants ranging from 20 to 40 with surface tensions ranging from 25 to 35 mJ m−2. This result is similar to the dispersion results of previous transition metal dichalcogenides (TMDCs) such as MoS2, WS2, MoSe2, MoTe2, TaSe2, NbSe2, and NiTe2, indicating that the V2Se9 material and its dispersion to 1D units can be expected to play an important role in opening opportunities for new low-dimensional material studies.

A novel semiconductor 1D nanomaterial, Nb2Se9, was synthesized on a bulk scale via simple vapor transport reaction between niobium and selenium. Needle-like single crystal Nb2Se9 contains numerous single Nb2Se9 chains linked by van der Waals interactions, and we confirmed that a bundle of chains can be easily separated by mechanical cleavage. The exfoliated Nb2Se9 flakes exhibit a quasi-two-dimensional layered structure, and the number of layers can be controlled using the repeated-peeling method. The work function varied depending on the thickness of the Nb2Se9 flakes as determined by scanning Kelvin probe microscopy. Moreover, we first implemented a field effect transistor (FET) based on nanoscale Nb2Se9 flakes and verified that it has p-type semiconductor characteristics. This novel 1D material can form a new family of 2D materials and is expected to play important roles in future nano-electronic devices.

The synthesis and nanoscale dispersion of a new one-dimensional (1D) material, V2Se9, in which chain-shaped molecular units with strong covalent bonds are assembled in a crystal structure via weak van der Waals attraction between chains, were successfully carried out. V2Se9 is synthesized by solid-state reaction and additional heat treatment. The synthesized V2Se9 has excellent thermal stability up to 400 °C and has been experimentally confirmed to be dispersed up to ∼20 nm or less through a specific solvent dispersion. The covalently bonded V2Se9 chain, when isolated from its bulk material, is expected to exhibit unique quantum physical properties owing to the confinement of electrons in the 1D chain structure and the absence of inter-chain interactions, as demonstrated in the case of graphene and other 2D materials. Therefore, the novel 1D material of V2Se9, as an extension of the current 2D material, is expected to create a new class of materials that will be of significant interest to the materials science and nanotechnology communities for new device applications.

For flexible organic light emitting diodes (OLEDs), roll-to-roll production enables low-cost fabrication and wide-ranging applications. Choosing an appropriate substrate material is one of the critical issues in the fabrication of flexible OLEDs. We demonstrated top-emitting OLEDs with a highly reflective distributed Bragg reflector (DBR) using a metal foil substrate. The DBR, made of seven pairs of SiO2/ZrO2, was formed by electron-beam evaporation on metal foil and showed high reflectivity of 90.5% at λ = 500 nm. The DBR served not only as the optical reflector, but also the substrate insulating layer which enabled the electrical isolation and prevented crosstalk. The OLEDs showed an operation voltage of 6.5 V at a current density of J = 10 mA cm−2 and maximum luminance of 17 400 cd m−2 at J = 225 mA cm−2. The electroluminescence property of the device could be maintained under the tensile bending condition.

Nb2O5 nanostructures with excellent crystallinities were grown on c-plane sapphire and employed for ultraviolet-(UV)-radiation detection. The triangular radial Nb2O5 grown on the c-sapphire substrate had a 6-fold symmetry with domain matching epitaxy on the substrate. Owing to the radial growth, the nanorods naturally connected when the deposition time increased. This structure can be used as a UV-detector directly by depositing macroscale electrodes without separation of a single nanorod and e-beam lithography process. It was confirmed that electric reactions occur at different UV irradiation wavelengths (254 nm and 365 nm).

The fundamental issues related to the formation of mechanical cracks and the chemical residue during the transfer process of large-area CVD graphene by polymeric carrier-films are addressed in this work. This paper presents a method to design a new polymer carrier-film (using polystyrene (PS) and 4,4'-diisopropylbiphenyl (DIPB)) that is free from mechanical cracks and polymer residue during the transfer of large-area graphene from a metal catalyst. This new polymer carrier film shows excellent mechanical flexibility and good solubility in tetrahydrofuran solvent without any residue and it is confirmed that the graphene transfer process is excellent without mechanical destruction even over a large area. Our result gave a technical milestone for the real industrial application of graphene in many application areas (not only graphene but also several two-dimensional materials such as boron nitride, transition metal di-chalcogenide, and black phosphorus).

We studied dispersants for exfoliating bulk V2Se9 into one dimensional chain units. V2Se9, with a negative zeta potential of −30.1 mV, was acidic and strongly anchored with the amine head of octadecylamine by charge transfer reaction. The steric hindrance by the octadecyl tail promoted the dispersion of V2Se9 to the nano level.

The thermal transport growth of various metal oxide (MOx) nanostructures using graphene as a catalytic layer was studied. Graphene was synthesized by Cu-catalyzed chemical vapor deposition and transferred onto a SiO2-covered Si substrate using bubble transfer methods. Due to the catalytic activity of the atomic-thick carbon layer, control of the position of the MOx nanostructures as well as the growth parameters, such as nucleation density and growth rate, could be achieved. The position-selective and density-controlled MOx nanostructures were evaluated for hydrogen gas sensor applications, where different responses were obtained with hydrogen molecules.

In the ternary Ta-Ni-Se materials group, the conditions for selectively synthesizing Ta2NiSe5 and Ta2NiSe7 that exhibit a low-dimensional layered structure (due to van der Waals bonding between layers) were studied. Control of both the stoichiometric ratio of selenium and synthesis temperature facilitated the selective growth of high-quality Ta2NiSe5 and Ta2NiSe7 crystals. The synthesized low-dimensional materials could be exfoliated in a layered form, with the Ta2NiSe5 and Ta2NiSe7 layers having semiconductor and metallic electrical characteristics, respectively, as verified through the change in electrical resistance with temperature. The synthesized materials are expected to expand the repertoire of semiconducting and conducting building blocks for use in various low-dimensional devices.

Three bismuth sillenite compounds with the general composition, Bi 12 XO 20 (X = Si, Ge, Ti), were prepared by a conventional solid state synthesis route and their local crystal structure, light absorption and visible-light induced photocatalytic activities were systematically investigated. The prepared powder samples show comparable particle size distribution (~ 0.5–5 µm) and BET surface area (~ 0.8 m 2 /g). The optical band gap energies were estimated from diffuse reflectance spectra to be in the range 2.6–2.9 eV. The local crystal structure variation and its influence on the electronic band structure was elucidated in the three sillenite compounds by Raman analysis and density functional theory (DFT). The visible-light induced photocatalytic activity was evaluated by monitoring the degradation of rhodamine B dye solution under visible-light irradiation (> 420 nm). We found that the Bi 12 TiO 20 powder shows the highest photocatalytic activity, which is attributed to the higher visible light absorption and the smaller band gap resulting from its distinct local crystal structure (longer Bi-O distance).

Cu-based compounds can be a good candidate for a low cost solar cell material. In particular, Cu x O (x : 1–2) has a good visible light absorbing bandgap at 1–2 eV. As for using nanostructures in solar cell applications, metal nanoparticle-induced localized plasmon resonance is a promising way to increase light absorbance, which can help improve the efficiency of solar cells. We fabricated ZnO nanorod/Au nanoparticles/Cu2O nanostructures to study their solar cell performance. ZnO nanorods and Cu2O layer were synthesized by the electrodeposition method. Size-controlled Au nanoparticles were deposited using E-beam evaporator for localized surface plasmon resonance (LSPR) effect. By inserting Au plasmon nanoparticles and annealing Au NPs in solar cells, we could tune the maximum incident photon-to-current efficiency wavelength. However, the potential well formed by Au NP at the ZnO/Cu2O junction leads to charge-trapping, based on the constructed electronic band analysis. LSPR-induced hot carrier generation is proposed to promote carrier transport further in the presence of Au NPs.

We report a modified TiO2/p-Si electronic structure that uses ultraviolet exposure for the incorporation of H. This structure was characterized using various photoelectron spectroscopic techniques. The ultraviolet (UV) exposure of the TiO2 surface allowed the Fermi energy level to be tuned by the insertion of H radicals, which induced changes in the heterojunction TiO2/p-Si diode properties. The UV exposure of the TiO2 surface was performed in air. On UVexposure, a photochemical reaction involving the incorporation of UV-induced H radicals led to the creation of a surface Ti-O-OH group and caused interstitial H doping (Ti-H-O) in the bulk, which modified the electronic structures in different ways, depending on the location of the H. On the basis of the band alignment determined using a combined spectroscopic analysis, it is suggested that the UV-induced H incorporation into the TiO2 could be utilized for the systematic tuning of the heterojunction property for solar cells, photocatalytic applications, and capacitors.

The catalytic activity of transition metals with regard to carbo-hydroxyl molecules (CxHy) has triggered new technological developments in graphene growth. Both the opening of the Dirac-point by controlling the number of graphene layers as well as the patterning of the graphene are critical for applications such as transistor-based electronics. In this work, we have developed a method to control the position and number of layers (n = 0, 1, and more than 2) during graphene growth based on our previous key ideas. This was achieved by using pre-patterned (Ni pre-patterned for more than 2 layers due to its high carbon solubility compared to Cu and MgO pre-patterned for 0 layer graphene due to the low catalytic activity and carbon solubility) ultra-flat Cu foils made using the peeled off method from a c-plane sapphire substrate.

We introduce a facile growth process for mixed metal oxide nanorods of Ir1−xVxO2 in the broad range of metal contents between iridium and vanadium via a vapor phase transport process and their electrochemical performance. The growth process of Ir1−xVxO2 nanorods was performed by modulating the ratios of two precursors of VO2 and IrO2, the growth temperature, the position of metal oxide precursors in a furnace, and the flow rate of carrier gases. The results indicated that quasi one-dimensional nanostructures of Ir1−xVxO2 can be obtained of a single crystalline nature with the tetragonal rutile crystal structure by randomly incorporating Ir4+ and V4+ metal ions into the same lattice unit of the tetragonal structure owing to similar ionic sizes. In addition, the fundamental electrochemical performances for Ir1−xVxO2 nanorods grown directly on an Au microelectrode of 25 μm in diameter were carefully tested in order to explore the nonenzymatic amperometric sensing capacity of ascorbic acid as a biologically important molecule.

We present an efficient growth process of rhenium disulfide (ReS2) nanosheets with a few layers at relatively low temperature under ambient pressure. Highly single-crystalline two-dimensional ReS2 nanostructures are obtained from a direct chemical reaction between rhenium atoms (Re) originating from the decomposition of ReCl3 and sulfur atoms (S) on a SiO2 substrate under helium (He) gas flow at 450 °C. The synthetic approach for the ReS2 nanosheets exhibits several favorable features: it is simple, cost-effective, and versatile for the development of new functional platforms toward highly single-crystalline 2D layered materials.

We determined the optimum dispersant to separate bulk Nb2Se9 material into 1D chain units. The Nb2Se9, which had a negative zeta potential (−43.3 mV), showed acidic characteristics and strongly bonded with the amine head of octadecyl amine through a charge transfer (from the amine to Se atoms) reaction. The steric hindrance of the octadecyl tail resulted in excellent dispersion of Nb2Se9 (down to nanometre-sized mono-chains).

The oblique angle deposition of Ag with different deposition rates and substrates was studied for surface-enhanced Raman spectroscopy (SERS) efficiency. The deposition rate for the Ag substrate with maximum SERS efficiency was optimized to 2.4 &Aring, /s. We also analyzed the morphology of Ag nanorods deposited at the same rate on various substrates and compared their SERS intensities. Ag deposited on SiO2, sapphire, and tungsten showed straight nanorods shape and showed relatively high SERS efficiency. However, Ag deposited on graphene or plasma-treated SiO2 substrate was slightly or more aggregated (due to high surface energy) and showed low SERS efficiency.

A new of 2D transition metal thiophosphates, Nb4P2S21, was synthesized through a vapor transport reaction. High purity and centimeter-scale large Nb4P2S21 crystals were obtained by controlling the growth temperature and stoichiometric ratio between niobium, phosphorus, and sulfur (using excess sulfur condition, Nb: P: S = 4: 2: 23). It was confirmed that large Nb4P2S21 crystals could be separated to a nanometer scale through mechanical exfoliation, and the exfoliated Nb4P2S21 layer exhibits semiconducting characteristics by measuring the change in electrical conductivity with temperature (increased conductivity with temperature in the range of 93–333 K). Furthermore, the device characteristics for detecting visible ray laser (635 nm) and NO2 gas molecules were verified. Based on these characteristics, the synthesized 2D material is expected to be a potential candidate for additional 2D materials and in multi-functional sensor devices, including optoelectronics and gas sensing applications.

A room-temperature (∼26 °C) NO2 gas sensor was fabricated using tellurium (Te) nanostructures. Te forms a helical chain with strong covalent bonds in the c-axis and weak van der Waals bonds between the chains. Therefore, the energy required for gas adsorption and desorption is low, thus, giving it the advantage of operating at room temperature. Since the shape and size of the Te helical chains can be freely controlled by adjusting the growth parameters, the electrical reactivities of Te to a specific gas can be controlled. In this study, the reactivity of Te to NO2 gas was controlled by adjusting the number of Te nanotube walls and lengths. It was also verified that the gas detection performance was maintained even with increasing humidity.

A new type of 1D inorganic material, MoI3, which can be separated into individual 1D chains, was synthesized through a vapor transport reaction between molybdenum and iodine. High-purity and centimeter-scale large MoI3 crystals were obtained by controlling the growth temperature and the stoichiometric ratio between molybdenum and iodine. Their semiconductor characteristics were confirmed by measuring the electrical conductivity as a function of temperature (conductivity increased with increased temperature across the range −70 to 30 °C). Furthermore, it was confirmed that MoI3 with a 1.27 eV band gap can be separated to a few nanometer scale through mechanical and chemical exfoliation methods and can be used as a photodetector in the near IR (less than 1000 nm)-to-visible light region. This novel 1D material is expected to play an important role in the development of additional low-dimensional materials and in advanced nano-optoelectronic devices.

Growth of ZnO thin film on conventional Si (100) substrate using atomically thin graphene as a buffer layer has been studied. The graphene buffer layer was synthesized by Cu-catalyzed chemical vapor deposition method and transferred on Si (100) using well-established polymer coating and Cu etching techniques. The ZnO film with (0002) preferred orientation was developed on graphene buffered Si (100) compared to ZnO film grown on Si (100) directly, which showed random orientation distribution. The graphene layer acts as not only lattice matching with ZnO but also diffusion barrier between ZnO and Si substrate, resulting in high electron mobility and photoluminescence intensity of ZnO film.

By using Kelvin Probe Force Microscopy with an additional applied electric field we investigate the local voltage drop in graphene on SiO 2 under ambient conditions. We are able to quantify the variation of the local sheet resistance and to resolve localized voltage drops at line defects. Our data demonstrates that the resistance of line defects has been overestimated so far. Moreover, we show that wrinkles have the largest resistance, ρ Wrinkle Ω μ m . Temperature-dependent measurements show that the local monolayer sheet resistance reflects the macroscopic increase in resistance with temperature while the defect resistance for folded wrinkles is best described by a temperature-independent model which we attribute to interlayer tunneling.

We developed a kinetic model to predict the preferred orientation for the case of rock-salt MgO crystal by assuming random walk diffusion of the adatom between (111) and (100) planes during growth. This model suggests that the tendency of the (111) preferred orientation increases under high deposition rate and low activation energy for diffusion of the adatom between (111) and (100) grains because the adatoms on the (111) surface stick to the surface more strongly compared to those on the (100) plane. The theoretical model was confirmed experimentally by growing several polycrystalline MgO films using the electron beam evaporation method. We could easily control the flux and energy of MgO adatoms by modulating the emission current of the electron gun, substrate temperature, and extra energy source such as ion beam assisted deposition. Our experimental results show good qualitative agreement with the model for the various polycrystalline MgO films deposited at several growth conditions.

Graphene-protected Ag nanowires have been studied for use in transparent flexible electrodes due to their superior thermal stability and uniform transparency. The protective graphene layers were synthesized via chemical vapor deposition and were transferred to both sides of spin-coated Ag nanowires. The graphene-protected Ag nanowires exhibit a stable sheet resistivity, even at 170 °C for several hundred hours, while pure Ag nanowires suffered from degradation at only 150 °C for 1 hour. The degradation model of pure Ag nanowires is suggested according to the surface diffusion kinetics, and the graphene layers were observed to block the surface plasmonic absorption of Ag nano-structures near the 350 to 400 nm range, resulting in uniform transparency at a wide range of wavelengths (300 to 1000 nm).

Bismuth vanadate (BiVO4, BVO) is a promising photoanode material for photoelectrochemical water-splitting, and it is mostly prepared using a sol-gel spin-coating method. BVO often exhibits poor PEC performance without modifications such as doping, co-catalyst deposition, and heterojunction formation. Herein, we report an alternative method to deposit a phase-pure BVO film using the electron-beam evaporation (EB) method. Specifically, electron-beam irradiation on the BVO source generates BVO precursor vapors, depositing an amorphous BVO film. The substrate temperature and emission current (of the electron-beam) were varied to control the phase-purity and grain size of the BVO film. The surface chemical state, optical, and electrochemical properties of the BVO films were characterized using X-ray photoelectron spectroscopy, ultraviolet–visible (UV–Vis) spectroscopy, and electrochemical impedance spectroscopy measurements, respectively. Interestingly, we found that BVO prepared under optimal condition has large grains (~400 nm in lateral size) and oxygen vacancies, thus exhibiting enhanced PEC performance. The photocurrent density of ~1.0 mA/cm2 at 1.23 V versus a reversible hydrogen electrode was obtained, which is 50% higher than the sol-gel derived BVO. The photocurrent density increased further to 2.4 mA/cm2 via CoOx co-catalyst deposition. More importantly, the photocurrent stability of EB-BVO was much higher than the sol-gel BVO.

The two-dimensional (2D) inorganic material Nb3I8, which can be separated into atomic planes, was synthesized through a vapor transport reaction between niobium and iodine. High-purity and cm scale large Nb3I8 crystals were obtained by controlling the growth temperature (600–700 °C) and the stoichiometric ratio between niobium and iodine (using excess I2 condition, niobium: iodine = 3:9). The semiconductor characteristics were measured by the change of electrical conductivity with temperature (increased conductivity with temperature in the range of 100 K–300 K), and have a band gap of ∼1 eV (∼1200 nm) through Infrared (IR)-visible light absorption analysis. Based on these characteristics, the device characteristics of detecting IR near 1200 nm region were verified. Besides few-layers of Nb3I8 was acquired by the mechanical exfoliation method, and experimentally observed by atomic force microscopy (AFM). This 2D material is expected to have an important role in developing another 2D materials and in new nano-optoelectronic devices using it.

We demonstrate the differential detection of UV-A (ultra-violet 320-400 nm region) and UV-C (100-280 nm) using porous two-dimensional (2D) Nb2O5 and additional Ag nano-particle decoration. The 2D Nb2O5, which has band-absorption edge near the UV-A zone, was synthesized by thermodynamic conversion of 2D material NbSe2 (Nb2O5 has lower Gibbs formation energy than NbSe2). For the differential detection (to distinguish with UV-C absorption), we decorated the Ag nano-particles on the Nb2O5 surface. By coating Ag nano-particles, we can expect (i) a decrease in the area of light absorption by the Ag-coated area, and (ii) an increase of surface plasmon absorption by Ag nano-particles, especially the UV-A region, resulting in strong intensity ratio change UV-A/UV-C.

The optimum solvent for Nb2Se9 dispersion, which is a new type of one dimensional (1D) material, is investigated. Among several solvents (16 solvents in total), strong dispersion was observed in benzyl alcohol, isopropyl alcohol, isobutyl alcohol, and diacetone alcohol, which have medium dielectric constants in the range of 10 to 30 and surface tension in the range of 25 to 35 mJ m&minus, 2. 1D Nb2Se9 chains, whose size is less than 10 nm, are well dispersed and it is possible to disperse mono-chains of 1 nm or less in a specific dispersion region. The 1D unit chain with dangling bond free surface and high volume to area ratio is expected to be used in applications that utilize the surface of the material. Such dispersion is an important first step towards various potential applications and is an indispensable scientific goal for the practical applications of Nb2Se9.

The effect of graphene coating on the growth of grains on bulk copper film was studied. When methane gas is catalytically decomposed on the surface of copper, and a carbon&ndash, copper solid solution is formed at high temperature, precipitated carbon on the copper surface forms graphene during rapid cooling through strong sp2 covalent bonding. The graphene layer can prevent the growth of grains by suppressing the diffusion of copper atoms on the surface, even after continuous heat treatment at high temperatures. The actual size of the copper grains was analyzed in terms of repetitive high-temperature heat treatment processes, and the grain growth process was simulated by using thermodynamic data, such as surface migration energy and the binding energy between copper and carbon. In general, transition metals can induce graphene growth on surfaces because they easily form carbon solid solutions at high temperatures. It is expected that the process of graphene growth will be able to suppress grain growth in transition metals used at high temperatures and could be applied to materials that are prone to thermal fatigue issues such as creep.

We demonstrated a single-material multifunctional sensor using ZnO nanorods, which is capable of mimicking the five human senses. A simple method for the growth of ZnO nanorods and their application as a multifunctional sensor was studied. High-density ZnO nanorods were synthesized by the carbothermal transport method on a graphene catalyst. To optimize the sensing mechanisms, we investigated ZnO nanostructure growth under two oxygen flow rates. ZnO nanorods as sensors for smell, light, sound, touch, and taste, the five human senses, showed stable electrical signals under standard ambient conditions.

Mass production of one-dimensional, V2Se9 crystals, was successfully synthesized using the solid-state reaction of vanadium and selenium. Through the mechanical exfoliation method, the bulk V2Se9 crystal was easily separated to nanoribbon structure and we have confirmed that as-grown V2Se9 crystals consist of innumerable single V2Se9 chains linked by van der Waals interaction. The exfoliated V2Se9 flakes can be controlled thickness by the repeated-peeling method. In addition, atomic thick nanoribbon structure of V2Se9 was also obtained on a 300 nm SiO2/Si substrate. Scanning Kelvin probe microscopy analysis was used to explore the variation of work function depending on the thickness of V2Se9 flakes. We believe that these observations will be of great help in selecting suitable metal contacts for V2Se9 and that a V2Se9 crystal is expected to have an important role in future nano-electronic devices.

Herein, we fabricated a super-hydrophobic SERS substrate using Sn-doped indium oxide (Indium-tin-oxide: ITO) nano-branches as a template. ITO nano-branches with tens of nanometer diameter are first fabricated through the vapor–liquid–solid (VLS) growth to provide roughness of the substrate. 10 nm thickness of Ag thin film was deposited and then treated with the post-annealing process to create numerous air-pockets in the Ag film, forming a hierarchical Ag nanostructures. The resulting substrate obtained Cassie's wetting property with a water contact angle of 151°. Compared to the normal hydrophobic Ag nanoparticle substrate, increase of about 4.25-fold higher SERS signal was obtained for 7 μL of rhodamine 6G aqueous solutions.

We report the method of fabrication of nano-gaps (known as hot spots) in Ag thin film using a sodium chloride (NaCl) sacrificial layer for Raman enhancement. The Ag thin film (20-50 nm) on the NaCl sacrificial layer undergoes an interfacial reaction due to the AgCl formed at the interface during water molecule intercalation. The intercalated water molecules can dissolve the NaCl molecules at interfaces and form the ionic state of Na+ and Cl-, promoting the AgCl formation. The Ag atoms can migrate by the driving force of this interfacial reaction, resulting in the formation of nano-size gaps in the film. The surface-enhanced Raman scattering activity of Ag films with nano-size gaps has been investigated using Raman reporter molecules, Rhodamine 6G (R6G).

The excitation spectrum of graphene on Cu(111), probed by momentum-resolved electron-energy-loss spectroscopy, exhibits multiple acoustic surface plasmons (ASP), arising from both the graphene overlayer and the Cu(111) substrate. We have investigated the mutual interaction between the ASP of the graphene overlayer and that of the underlying Cu(111) substrate. The weak interaction between graphene and Cu(111) implies that the ASP of Cu(111) survives, with only slight changes in its group velocity. Remarkably, we find that the damping processes of ASP are mitigated as compared to the cases of other graphene/metal interfaces. Therefore, this interface is an ideal graphene/metal contact for graphene-based plasmonics. Moreover, the presence of graphene coating protects the ASP of Cu(111) from quenching in an ambient atmosphere.

We studied growth mechanisms of water-soluble NaCl thin films on single crystal substrates. Epitaxial growth of NaCl(100) on Si(100) and domain-matched growth of NaCl(111) on c-sapphire were obtained at thicknesses below 100 nm even at room temperature from low lattice mismatches in both cases. NaCl thin film, which demonstrates high solubility selectivity for water, was successfully applied as a water-soluble sacrificial layer for fabrication of several functional materials, such as WO3 nano-helix and Sn doped In2O3 nano-branches.