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A Cu2ZnSnSe4 (CZTSe) photovoltaic absorber thin films were prepared using a 2-step selenization process on a Ti substrate including a Na precursor layer and a Na-free Ti substrate, and the effect of Na on the solar cell performance was compared. A CZTSe flexible solar cell fabricated on a Ti foil substrate exhibited an efficiency of 3.06%, which was less than half that of a solar cell fabricated on a soda lime glass substrate. This was attributed to the absence of Na and severe Zn crowding near the back contact. By depositing a 100‐nm-thick sodalime glass thin film on a Ti substrate to supply Na, the efficiency increased up to 5.59%. In the Na-doped CZTSe absorber layer grown on the Ti substrate, the back crowding of Zn was eliminated and the upper part of the absorption layer was converted to a Zn-rich environment, which prevented the formation of CuZn antisite defects.

Sb2Se3 is a promising candidate for thin-film photovoltaics, with a suitable band gap, benign grain boundaries, Earth-abundant and nontoxic constituents, and excellent stability. However, the low doping density (1013 cm–3) of Sb2Se3 absorber and back contact barrier limit its efficiency. Here we introduced a PbS colloidal quantum dot (CQD) film as the hole-transporting layer (HTL) to construct a n-i-p configured device and overcame these problems. Through simulation-guided optimization, we have significantly improved the efficiency of a Sb2Se3 thin-film solar cell to a new certified record of 6.5%. The PbS CQD HTL not only minimized carrier recombination loss at the back contact and boosted carrier collection efficiency but also contributed photocurrent by its own near-infrared absorption. Furthermore, these n-i-p devices also demonstrated improved device uniformity, achieving 6.39% in a 1.02 cm2 device.

To commercialize Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells, it is necessary to improve their efficiency and to develop the technological ability to produce large-area modules. Defect formation due to the secondary phase is considered to be one of the main reasons for decreased CZTSSe thin-film solar-cell efficiency. This study explores the potential capabilities of large-area thin-film solar cells by controlling the defect formation using various CZTSSe precursor designs, and by improving the characteristic uniformity within the thin-film solar cells. Alloying the precursor as a stack of discrete layers can result in lateral segregation of elements into stable-phase islands, yielding a non-uniform composition on small length scales. It is found that the application of an indiscrete layer by minimizing the precursor-layer thickness allows avoiding Zn rich inhomogeneities in the absorber that would favor formation of detrimental ZnS-ZnSe secondary phases and deep defects. Among the various precursor layers designed by considering the reaction mechanism under annealing, a sample with 15 precursor layers is found to exhibit a shallow electron-acceptor energy level, high photovoltaic conversion efficiency, and uniform characteristics over the corresponding thin-film solar cell. Based on such improvements in both the efficiency and characteristic distribution, it is expected that the commercialization of CZTSSe thin-film solar cells can be advanced.

In this study, we investigate the electrical, structural, and optical properties of band gap front-graded Cu2ZnSn(S,Se)4 (CZTSSe) thin films grown by a modified single-step sulfo-selenization process from copper-poor and zinc-rich precursor metallic stacks prepared by co-evaporation. To investigate how the bandgap was graded in connection with the compositional distribution, we calculated the bandgap energy distribution along the film thickness, based on the transmission electron microscopy and energy-dispersive X-ray spectroscopy composition profile. The band gap of the CZTSSe phase with high S content near the surface layer is determined to be 1.161 eV. From the surface to the bottom, there is a decrease in the S content of the CZTSSe phase, and the band gap subsequently decreases to, 1.029 eV, close to the value of CZTSe. From the results of dimpling-Raman and scanning transmission electron microscopy line scanning, we confirm that the S content drastically increases from the bottom to the top surface of the CZTSSe thin film. The CZTSSe thin-film solar cell exhibits a power conversion efficiency (PCE) of 10.33%, with an open-circuit voltage (Voc) of 0.505 V, short-circuit current density (Jsc) of 31.61 mA/cm2, fill factor (FF) of 64.6%, and Voc deficit of 525 mV. Compared with the performance of the CZTSe solar cell, which had PCE of 7.23%, Voc of 0.424 V, Jsc of 32.83 mA cm−2, FF of 51.9%, and Voc deficit of 576 mV, the Voc and Voc deficit of the CZTSSe cell improved considerably. The high Voc, low Voc deficit, and less loss of Jsc are attributed to the effect of band gap front-grading induced by S grading into the CZTSSe thin film.

In this study, we investigate methods of improving the efficiency of Cu 2 ZnSn(S,Se) 4 (CZTS)-based solar cells by comparing Cu(In,Ga)Se 2 (CIGSe)- and CZTS-based absorber layers. In particular, the CZTS-based absorber exhibits lower current characteristics than the CIGSe absorber layer in terms of the band gap alignment and electron-hole recombination at the CdS-absorber interface. Moreover, we demonstrate that defects are one of the causes of the voltage loss. In order to improve the efficiency of CZTS-based solar cells, it is important to control the band gap alignment at the CdS-absorber layer interface and to suppress the formation of secondary phases inside the absorber.

Polarized Raman spectroscopy on few-layer ReS2 and ReSe2 was carried out to determine the crystallographic orientations. Since monolayer ReX2 (X=S or Se) has a distorted trigonal structure with only an inversion center, there is in-plane anisotropy and the two faces of a monolayer crystal are not equivalent. Since many physical properties vary sensitively depending on the crystallographic orientation, it is important to develop a reliable method to determine the crystal axes of ReX2. By comparing the relative polarization dependences of some representative Raman modes measured with three different excitation laser energies with high-resolution scanning transmission electron microscopy, we established a reliable procedure to determine the all three principal directions of few-layer ReX2 including a way to distinguish the two types of faces: a 2.41-eV laser for ReS2 or a 1.96-eV laser for ReSe2 should be chosen as the excitation source of polarized Raman measurements; then the relative directions of the maximum intensity polarization of the Raman modes at 151 and 212 cm-1 (124 and 161 cm-1) of ReS2 (ReSe2) can be used to determine the face types and the Re-chain direction unambiguously., Comment: 22 pages, 4 figures, supplementary information 9 pages

The resonance Raman effects in two dimensional materials including transition metal dichalcogenides and black phosphorus are reviewed. The Raman intensities of high-frequency intra-layer vibration modes are enhanced near resonance with exciton states. Some Raman peaks that are either forbidden or weak in non-resonant cases show strong enhancement near resonances. In the low-frequency Raman spectra, some unusual features, in addition to shear and breathing modes, appear near resonance with exciton states. Some intra-layer vibration modes exhibit Davydov splitting due to inter-layer interactions when the excitation energy is close to resonances. The polarization behaviors of some Raman modes in anisotropic two-dimensional materials have peculiar dependences on the excitation energy, which is related to the resonance effect.

To observe the superconducting current and structural properties of high critical temperature ( T c ) superconductors (HTS), we suggest the following imaging methods: Room temperature imaging (RTI) through thermal heating, low-temperature bolometric microscopy (LTBM) and Raman scattering imaging. RTI and LTBM images visualize thermal-electric voltages as different thermal gradients at room temperature (RT) and superconducting current dissipation at near- T c , respectively. Using RTI, we can obtain structural information about the surface uniformity and positions of impurities. LTBM images show the flux flow in two dimensions as a function of the local critical currents. Raman imaging is transformed from Raman survey spectra in particular areas, and the Raman vibration modes can be combined. Raman imaging can quantify the vibration modes of the areas. Therefore, we demonstrate the spatial transport properties of superconducting materials by combining the results. In addition, this enables visualization of the effect of current flow on the distribution of impurities in a uniform superconducting crystalline material. These imaging methods facilitate direct examination of the local properties of superconducting materials and wires.

We found a clear correlation between the solar conversion efficiency and the distribution of the ZnS secondary phase in Cu 2 ZnSnS 4 (CZTS) solar cells. Five CZTS solar cells were prepared by the two-step process in which sputtered precursor multilayers were sulfurized at different temperatures. In higher efficiency solar cells, the ZnS secondary phase was found to accumulate near the interface between CZTS and MoS 2 formed on top of the Mo layer. It was also found that a solar cell with a higher efficiency showed weaker ZnS Raman signal in the MoS 2 layer. The highest conversion efficiency of 7.5% was obtained from a solar cell prepared at a sulfurization temperature of 570 °C, in which no ZnS was detected in the MoS 2 layer. We suggest that in order to obtain a high efficiency CZTS solar cell, it is critical to find a condition that keeps ZnS from the MoS 2 layer.

Cu 2 ZnSnSe 4 photovoltaic absorber layers were prepared by the vapor selenization of stacked Cu/Sn/CuZn/Mo/glass precursor films with different Cu/(Zn + Sn) compositional ratios (CZT = 0.8, 0.85 and 0.9). The absorber was ZnSe-free at CZT = 0.9 but the films obtained from CZT = 0.85 or 0.8 contained a ZnSe phase mainly at the bottom or top of the absorber film, respectively. Although the solar cell fabricated from the CZT = 0.9 absorber showed poor-shunt characteristics (efficiency = 1.94%), that produced from CZT = 0.85 exhibited significantly distorted current–voltage characteristics (efficiency = 4.23%). The CZT = 0.8 ratio showed the highest efficiency of 5.8% and normal pn diode behavior. This suggests that the existence or location of the ZnSe phase within the absorber is not related directly to the solar cell performance. Instead, the ZnSe phase may alter the defect distribution near the space charge region. Several aspects of the device behavior depending on the CZT ratio are discussed with respect to the relative position of the ZnSe phase.

Although Cu2ZnSn(S,Se)4 (CZTSSe) has attracted attention as an alternative to CuInGaSe2 (CIGS) as an absorber material in solar cells, its low efficiency is a serious shortcoming preventing its commercialization. To realize a high-efficiency CZTSSe solar cell, improved grain crystallinity, inhibited secondary-phase formation, controlled defect generation, adequate Na content, and band gap grading are required in the absorber layer. Few studies have focused specifically on band gap grading. In this study, a method of using SeS2, a new potential chalcogenization source material, to control the S and Se contents in a CZTSSe absorber and its effects were investigated. Using an appropriate SeS2/Se weight ratio, band gap grading was realized within the depletion region. By increasing the value of VOC through band gap grading in the depletion region, a record VOC deficit of 0.576 V was achieved. Furthermore, the possibility of enhancing JSC through the formation of a type-inverted n-type phase at the absorber surface in response to an appropriate alignment of the conduction-band minimum energy level and the Fermi energy pinning level is discussed. By introducing the chalcogenization source material SeS2 during the annealing process, CZTSSe solar cells with a maximum efficiency of 12.3% were obtained.

We report a new way to enhance the electrical performances of large area CVD-grown graphene through controlling the ripple density and heights after transfer onto SiO2/Si substrates by employing different cooling rates during fabrication. We find that graphene films prepared with a high cooling rate have reduced ripple density and heights and improved electrical characteristics such as higher electron/hole mobilities as well as reduced sheet resistance. The corresponding Raman analysis also shows a significant decrease of the defects when a higher cooling rate is employed. We suggest a model that explains the improved morphology of the graphene film obtained with higher cooling rates. From these points of view, we can suggest a new pathway toward a relatively lower density and heights of ripples in order to reduce the flexural phonon-electron scattering effect, leading to higher lateral carrier mobilities.

Raman spectroscopy has been established as one of the core experimental tools to study two-dimensional materials (2DMs) including graphene, black phosphorus, transitional metal chalcogenides, and other layered materials. If the polarization of the incident photons and the scattered photons are carefully controlled, the selection rules for the Raman scattering from phonon modes allow accurate mode assignments, which is not always possible in Raman scattering measurements using unpolarized light. Furthermore, polarized Raman spectroscopy can be used to determine the crystallographic orientation of isotropic 2DMs with in-plane strain or anisotropic 2DMs. This review explains the basics of polarized Raman spectroscopy, especially in the context of 2DMs research, and survey some of the most important applications of polarized Raman spectroscopy in isotropic and anisotropic 2DMs studies.

Three types of carbon nanoscroll (CNS) structures that are formed when scrolling up graphene sheets are investigated using Raman spectroscopy and atomic force microscopy (AFM). The CNSs were produced from exfoliated monolayer graphene deposited on a Si chip by applying a droplet of isopropyl alcohol (IPA) solution. The three types of CNS are classified as single-elliptical-core, double-elliptical-core (both with large internal volumes) and collapsed ribbon-like, based on AFM surface profile measurements. We discuss the structure and formation of CNS with much larger hollow cores than is commonly assumed and relate this to the large effective 2D bending stiffness of graphene in the IPA solution. The large elliptical core structures show Raman spectra similar to those previously reported for CNS and indicate little interaction between the scrolled layers. The Raman spectra from ribbon-like structures show additional features that are similar to that of folded graphene. These new features can be related to layer breathing modes combined with some resonance enhancement at specific regions of the ribbon-like CNSs that are due to specific twist angles produced when the structure folds/collapses.

The atomic or molecular assembly on 2D materials through the relatively weak van der Waals interaction is quite different from the conventional heteroepitaxy and may result in unique growth behaviors. Here, it is shown that straight 1D cyanide chains display universal epitaxy on hexagonal 2D materials. A universal oriented assembly of cyanide crystals (AgCN, AuCN, and Cu0.5Au0.5CN) is observed, where the chains are aligned along the three zigzag lattice directions of various 2D hexagonal crystals (graphene, h‐BN, WS2, MoS2, WSe2, MoSe2, and MoTe2). The potential energy landscape of the hexagonal lattice induces this preferred alignment of 1D chains along the zigzag lattice directions, regardless of the lattice parameter and surface elements as demonstrated by first‐principles calculations and parameterized surface potential calculations. Furthermore, the oriented microwires can serve as crystal orientation markers, and stacking‐angle‐controlled vertical 2D heterostructures are successfully fabricated by using them as markers. The oriented van der Waals epitaxy can be generalized to any hexagonal 2D crystals and will serve as a unique growth process to form crystals with orientations along the zigzag directions by epitaxy., 1D cyanide chains show universal oriented van der Waals epitaxy on various hexagonal 2D crystals. The chain directions are always aligned along the zigzag lattice directions of the 2D substrates, regardless of differing lattice parameters and surface elements. Stacking‐angle‐controlled vertical 2D heterostructures can also be fabricated using cyanide microwire markers for identification of crystal orientations.

We investigated interlayer phonon modes of mechanically exfoliated few-layer 2H-SnS2 samples by using room temperature low-frequency micro-Raman spectroscopy. Raman measurements were performed using laser wavelengths of 441.6, 514.4, 532 and 632.8 nm with power below 100 μW and inside a vacuum chamber to avoid photo-oxidation. The intralayer Eg and A1g modes are observed at ~206 cm−1 and 314 cm−1, respectively, but the Eg mode is much weaker for all excitation energies. The A1g mode exhibits strong resonant enhancement for the 532 nm (2.33 eV) laser. In the low-frequency region, interlayer vibrational modes of shear and breathing modes are observed. These modes show characteristic dependence on the number of layers. The strengths of the interlayer interactions are estimated by fitting the interlayer mode frequencies using the linear chain model and are found to be 1.64 × 1019 N · m−3 and 5.03 × 1019 N · m−3 for the shear and breathing modes, respectively.

Following the celebrated discovery of graphene, considerable attention has been directed toward the rich spectrum of properties offered by van der Waals crystals. However, studies have been largely limited to their 2D properties due to lack of 1D structures. Here, the growth of high-yield, single-crystalline 1D nanobelts composed of transition metal ditellurides at low temperatures (T ≤ 500 °C) and in short reaction times (t ≤ 10 min) via the use of tellurium-rich eutectic metal alloys is reported. The synthesized semimetallic 1D products are highly pure, stoichiometric, structurally uniform, and free of defects, resulting in high electrical performances. Furthermore, complete compositional tuning of the ternary ditelluride nanobelts is achieved with suppressed phase separation, applicable to the creation of unprecedented low-dimensional materials/devices. This approach may inspire new growth/fabrication strategies of 1D layered nanostructures, which may offer unique properties that are not available in other materials.

Variously shaped graphene domains are of significant interest since the electronic properties of pristine graphene are strongly dependent on its size, shape, and edge structures. With the consideration that the reactivity of graphene is governable by the p-electron structure at its edge, a number of attempts have been made to grow variously shaped graphene domains and to define their edge structures. In this work, we explored the anisotropic behavior of hydrogen in the formation of graphene domains during atmospheric pressure chemical vapor deposition. As increasing the hydrogen or reducing the methane partial pressure, the formation of pentagonal graphene domains was accelerated through anisotropic growth and etching. Their edge structures were characterized using polarization-dependent D and G peak Raman spectroscopy. This work contributes significantly to improving graphene-based engineering by allowing graphene shapes and domain edges to be tuned, and also provides greater insight into the electronic properties of graphene devices.

Molybdenum disulfide (MoS 2 ) nano-sheets have been successfully deposited directly on the cleaved facet of a silica optical fiber using optical trapping forces. By controlling the incident laser power, various types of MoS 2 films were deposited near the fiber core. An area selective deposition was successfully realized during the optical deposition process by using an optical reflectometry method. The presence of MoS 2 deposited layer at the fiber end was confirmed by microscopic Raman spectroscopy as well as scanning electron microscope images. Nonlinear response and power dependent transmission of the prepared sample was observed at the wavelength of 1552 nm, which can be used as an all-fiber saturable absorber. Moreover, the fabricated sample was inserted in a Er-doped fiber ring laser cavity and stable Q-switched pulses with the repletion rate from 26.6 kHz to 40.9 kHz were generated, which confirmed the potential of MoS 2 as a saturable absorber.

Graphene and related two-dimensional materials are promising candidates for atomically thin, flexible, and transparent optoelectronics. In particular, the strong light-matter interaction in graphene has allowed for the development of state-of-the-art photodetectors, optical modulators, and plasmonic devices. In addition, electrically biased graphene on SiO2 substrates can be used as a low-efficiency emitter in the mid-infrared range. However, emission in the visible range has remained elusive. Here we report the observation of bright visible-light emission from electrically biased suspended graphenes. In these devices, heat transport is greatly minimised; thus hot electrons (~ 2800 K) become spatially localised at the centre of graphene layer, resulting in a 1000-fold enhancement in the thermal radiation efficiency. Moreover, strong optical interference between the suspended graphene and substrate can be utilized to tune the emission spectrum. We also demonstrate the scalability of this technique by realizing arrays of chemical-vapour-deposited graphene bright visible-light emitters. These results pave the way towards the realisation of commercially viable large-scale, atomically-thin, flexible and transparent light emitters and displays with low-operation voltage, and graphene-based, on-chip ultrafast optical communications., Comment: 63 page

The optical and structural properties of aluminium-doped zinc oxide (AZO) films were investigated by photoluminescence (PL) spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy. Pure zinc oxide (ZnO) and AZO composite films were deposited using vacuum evaporation method. The films exhibited different morphologies and crystallinity depending on the Al-doping. The SEM micrographs showed that a granular and compact structure could be seen for the ZnO film, while a nanoleaf structure with relatively porous nature was observed for the AZO composite film. The XRD patterns indicated that the crystalline growth orientation would be significantly affected by addition of Al. Compared with pure ZnO, the XRD peak intensity of the AZO composite was stronger and the line-width was narrower. Two-probe resistivity measurements showed that the AZO composites could be used as transparent conducting materials. The PL spectra revealed that the PL intensities of the AZO composites were stronger than that of the pure ZnO. The PL enhancement might be ascribed to the surface plasmon resonance of metal nanoclusters within the composite. Another possible reason of the PL enhancement would be the metal-induced crystallization caused by doping Al to ZnO matrix.

Graphene domains with different sizes and densities were successfully grown on Cu foils with use of a chemical vapor deposition method. We investigated the effects of volume ratios of argon to hydrogen during the annealing process on graphene growth, especially as a function of hydrogen partial pressure. The mean size and density of graphene domains increased with an increase in hydrogen partial pressure during the annealing time. In addition, we found that annealing with use of only hydrogen gas resulted in snowflake-shaped carbon aggregates. Energy-dispersive X-ray spectroscopy (EDX) and high-resolution photoemission spectroscopy (HRPES) revealed that the snowflake-shaped carbon aggregates have stacked sp2 carbon configuration. With these observations, we demonstrate the key reaction details for each growth process and a proposed growth mechanism as a function of the partial pressure of H2 during the annealing process.

We report a method to characterize z-directional surface roughness of chemical vapor deposition-grown graphene monolayers with the aid of surface-enhanced Raman scattering spectral signatures of “out-of-plane” phonon modes observed at Au nanoparticle (NP)–graphene–Au thin film junctions. This method reveals that intensities of the out-of-plane mode (Radial Breathing Like Mode) are strongly correlated with the Full Width at Half Maximum (FWHM) of the de-convoluted 2D peak. On the basis of our findings, in-plane 2D peak shape can be used as a straightforward, quantitative indicator in estimating surface roughness of graphene without loading Au NPs by calculating the FWHM [2D−]/FWHM [2D+] value. Furthermore, we examine the different ripple (RP) shapes on graphene by employing atomic force microscopy-correlated Raman microscopy to identify “threading” and “surrounded” RP types for further investigation on the relationship between spectral features and structural aspects. Electrostatic force microscopy (EFM)-based investigation further substantiates that threading-type RPs in graphene show higher EFM amplitude, indicating that the threading domains tend to be more neutral with a few more sp3 type defects than the surroundings.

As the elements of integrated circuits are downsized to the nanoscale, the current Cu‐based interconnects are facing limitations due to increased resistivity and decreased current‐carrying capacity because of scaling. Here, the bottom‐up synthesis of single‐crystalline WTe2 nanobelts and low‐ and high‐field electrical characterization of nanoscale interconnect test structures in various ambient conditions are reported. Unlike exfoliated flakes obtained by the top‐down approach, the bottom‐up growth mode of WTe2 nanobelts allows systemic characterization of the electrical properties of WTe2 single crystals as a function of channel dimensions. Using a 1D heat transport model and a power law, it is determined that the breakdown of WTe2 devices under vacuum and with AlOx capping layer follows an ideal pattern for Joule heating, far from edge scattering. High‐field electrical measurements and self‐heating modeling demonstrate that the WTe2 nanobelts have a breakdown current density approaching ≈100 MA cm−2, remarkably higher than those of conventional metals and other transition‐metal chalcogenides, and sustain the highest electrical power per channel length (≈16.4 W cm−1) among the interconnect candidates. The results suggest superior robustness of WTe2 against high‐bias sweep and its possible applicability in future nanoelectronics.

We measured the polarization dependence of the photocurrent in symmetric Pd-graphene-Pd photodevices. The photocurrent is maximum when the polarization angle of the incident light is parallel to the edge of an electrode. On the other hand, when the polarization direction is parallel to the graphene channel, the photocurrent is minimum. This polarization dependence of the photocurrent is similar to what has been observed in an asymmetric Pd-graphene-Ti device and results from an anisotropic electron-photon interaction in graphene, which generates photocarriers with momenta predominantly in the direction perpendicular to the polarization direction.

The influence of the conventional depositing processes on the optical properties of Cu(In,Ga)Se 2 (CIGS) thin films in solar cell structures was investigated by measuring the photoluminescence (PL) and Raman spectra of the CIGS layer at each stage of the solar cell deposition process. The intensities of the PL and the Raman A 1 mode increase after the CdS buffer layer is deposited, suggesting that the CdS layer either improves the optical quality of the CIGS film or protects it from degradation due to environmental factors. The temperature and excitation power dependences of the PL for the bare CIGS sample are very different from those for the samples with the CdS layer, reflecting different characters of the luminescence centers near the surface of the CIGS layer. On the other hand, the lateral homogeneity, as seen in the micro-PL and micro-Raman images, does not seem to improve. After the ZnO window layer is deposited, the overall PL and Raman intensities do not change much, although the intensity distribution becomes more inhomogeneous.

Crystallographic orientations of early multi-lobe graphene domains in CVD-grown graphene are investigated. Partially grown graphene domains were transferred onto flexible substrates and uniaxial tensile strain was applied. From the polarization dependence of the Raman G band, split due to strain, the crystallographic orientation was determined. It was found that there are some single crystal domains. However, lobes have different orientations in some other multi-lobe domains. Within a given lobe, the orientation does not change as the domain grows, indicating that the orientations present in a graphene domain are determined at an early stage of growth.

We report on the anisotropic properties of solution-processed thin-film transistors (TFTs) fabricated by the vertical flowing of the viscous 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) and isotactic (i-) poly-(methyl methacrylate) (PMMA) blend. During film formation processes, lateral crystalline growth of TIPS-pentacene and TIPS-pentacene/i-PMMA phase-separation were observed. Such anisotropic crystallization of TIPS-pentacene channel caused anisotropic mobility with respect to the channel orientation, so that it could be used for composing of a logic inverter where two TFTs with different channel orientations were serially connected.

We found Raman scattering enhancement of the Raman G band in folded bilayer graphene. Its Raman intensity is more than 40 times greater than that of AB stacked Bernal bilayer graphene. The enhancement is interpreted as being caused by resonant Raman scattering via the π 2 * and the π 1 * states in the modified electronic band structure of the folded bilayer graphene. This enhancement of the Raman G band is a critical indicator of the strength of the coupling between the upper and the lower graphene sheets.

We investigated polarization dependence of the Raman modes in black phosphorus (BP) using five different excitation wavelengths. The crystallographic orientation was determined by comparing polarized optical microscopy with high-resolution transmission electron microscope analysis. In polarized Raman spectroscopy, the B2g mode shows the same polarization dependence regardless of the excitation wavelength or the sample thickness. On the other hand, the Ag1 and Ag2 modes show a peculiar polarization behavior that depends on the excitation wavelength and the sample thickness. The thickness dependence can be explained by considering the anisotropic interference effect due to birefringence and dichroism of the BP crystal, but the wavelength dependence cannot be explained. We propose a simple and fail-proof procedure to determine the orientation of a BP crystal by combining polarized Raman scattering with polarized optical microscopy., Comment: 22+7 pages, 8+5 figures

Monolayer graphene is one of the most interesting materials applicable to next-generation electronic devices due to its transport properties. However, realization of graphene devices requires suitable nanoscale lithography as well as a method to open a band gap in monolayer graphene. Nanoscale hydrogenation and oxidation are promising methods to open an energy band gap by modification of surface structures and to fabricate nanostructures such as graphene nanoribbons (GNRs). Until now it has been difficult to fabricate nanoscale devices consisting of both hydrogenated and oxidized graphene because the hydrogenation of graphene requires a complicated process composed of large-scale chemical modification, nanoscale patterning, and etching. We report on nanoscale hydrogenation and oxidation of graphene under normal atmospheric conditions and at room temperature without etching, wet process, or even any gas treatment by controlling just an external bias through atomic force microscope lithography. Both the lithographically defined nanoscale hydrogenation and oxidation have been confirmed by micro-Raman spectroscopy measurements. Patterned hydrogenated and oxidized graphene show insulating behaviors, and their friction values are several times larger than those of graphene. These differences can be used for fabricating electronic or electromechanical devices based on graphene.

We studied the effects of seed layers on the structural and optical properties of ZnO nanorods. ZnO and Ag-doped ZnO (ZnO:Ag) seed layers were deposited on glass substrates by magnetron co-sputtering. ZnO nanorods were grown on these seed layers by the chemical bath deposition in an aqueous solution of Zn(NO3)2 and hexamethyltetramine. SEM micrographs clearly reveal that ZnO nanorods were successfully grown on both kinds of seed layers. The XRD patterns indicate that crystallization of ZnO nanorods is along the c-axis. Meanwhile, the packing density and the vertical alignment of the ZnO nanorods on the ZnO seed layer are better than those of the ZnO nanorods on ZnO:Ag. The enhanced growth of nanorods is thought to be due to the fact that the ZnO layer exhibits a higher crystalline quality than the ZnO:Ag layer. According to the low-temperature photoluminescence spectra, the ZnO nanorods on the ZnO seed layer show a narrow strong ultraviolet emission band centered at 369 nm, while those on ZnO:Ag exhibit multiple bands. These results are thought to be related with the crystallinity of ZnO nanorods, the morphologies of ZnO nanorods, and the reflectivities of seed layers. More detailed studies for clarification of the seed layer effect on the growth of ZnO nanorods are desirable.

Femtosecond laser interference crystallization was studied in 500-nm-thick amorphous silicon (a-Si) films prepared on glass by using plasma-enhanced chemical-vapor deposition. The efficient crystallization of 500-nm-thick a-Si films was found to require post thermal annealing as well as laser annealing. Femtosecond laser interference technique was used to produce the seed pattern for the spatially-selected crystallization of a-Si. Post thermal annealing of the seed pattern was performed at 550 ◦C for 20 hours under a nitrogen atmosphere. By applying post thermal annealing to laser-crystallized silicon, the degree of crystallization was enhanced. The femtosecond laser-induced grating can be regarded as a pattern of alternating a-Si and μc-Si (microcrystalline silicon) bands with a period of about 2 μm. Probe-beam diffraction, micro-Raman spectroscopy, and transmission electron microscopy were used to investigate the diffraction behavior and to confirm the spatiallyselected crystallization of a-Si.

We investigated the photo-structuring of silver-oxide lms by using femtosecond laser pulses. The silver-oxide lms were prepared by magnetron sputtering. The physical properties of the prepared silver-oxide lms were investigated by using absorption spectroscopy. For the sample that underwent the photo-structuring of the silver-oxide lms by two interfering femtosecond laser pulses, scanning and transmission electron microscopy measurements showed that a metal grating was formed through the photoreduction of silver oxide. Micro-photoluminescence spectroscopy showed spationally selective photoluminescence from the laser-modi ed region. In order to nd the nature of laser-modi ed region, we compared the metal grating formed by laser interference structuring with the thermally annealed lm.

Metal–graphene–metal photodetectors utilize photocurrent generated near the graphene/metal junctions and have many advantages including high speed and broad-band operation. Here, we report on photocurrent generation at ABA/ABC stacking domain junctions in tri-layer graphene with a responsivity of 0.18 A/W. Unlike usual metal–graphene–metal devices, the photocurrent is generated in the middle of the graphene channel, not confined to the vicinity of the metal electrodes. The magnitude and the direction of the photocurrent depend on the back-gate bias. Theoretical calculations show that there is a built-in band offset between the two stacking domains, and the dominant mechanism of the photocurrent is the photo-thermoelectric effect due to the Seebeck coefficient difference.

The dependence of the Raman spectrum on the excitation energy has been investigated for ABA-and ABC- stacked few-layer graphene in order to establish the fingerprint of the stacking order and the number of layers, which affect the transport and optical properties of few-layer graphene. Five different excitation sources with energies of 1.96, 2.33, 2.41, 2.54 and 2.81 eV were used. The position and the line shape of the Raman 2D, G*, N, M, and other combination modes show dependence on the excitation energy as well as the stacking order and the thickness. One can unambiguously determine the stacking order and the thickness by comparing the 2D band spectra measured with 2 different excitation energies or by carefully comparing weaker combination Raman modes such as N, M, or LOLA modes. The criteria for unambiguous determination of the stacking order and the number of layers up to 5 layers are established., Comment: 25 pages, 14 figures

A novel one-pot process that can produce freestanding reduced graphene oxide (RGO) sheets in large scale through a mechanochemical method is presented, which is based on a 1:1 adduct of hydrazine and carbon dioxide (H_3N^+NHCO_2^−, solid hydrazine). We were able to synthesize RGO sheets by grinding solid hydrazine with graphene oxide (GO), followed by storing the mixed powder at 50 °C for 10 min. No solvents, nor large vessels, nor post-annealing at high temperatures are required. The resulting RGO sample was characterized by elemental analysis, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, Brunauer–Emmett–Teller measurement, thermo gravimetric analysis, Fourier transform infrared spectroscopy, solid state nuclear magnetic resonance spectroscopy, and conductivity measurement. It exhibits excellent conductivity and possesses a high specific surface area. This reduction method was successfully applied for the fabrication of inkjet-printed RGO devices on a flexible substrate.

The polarization dependence of the double resonant Raman scattering (2D) band in bilayer graphene (BLG) is studied as a function of the excitation laser energy. It has been known that the complex shape of the 2D band of BLG can be decomposed into four Lorentzian peaks with different Raman frequency shifts attributable to four individual scattering paths in the energy-momentum space. From our polarization dependence study, however, we reveal that each of the four different peaks is actually doubly degenerate in its scattering channels, i.e., two different scattering paths with similar Raman frequency shifts for each peak. We find theoretically that one of these two paths, ignored for a long time, has a small contribution to their scattering intensities but are critical in understanding their polarization dependences. Because of this, the maximum-to-minimum intensity ratios of the four peaks show a strong dependence on the excitation energy, unlike the case of single-layer graphene (SLG). Our findings thus reveal another interesting aspect of electron-phonon interactions in graphitic systems., 19 pages, 5 figures

We report the novel role of graphene substrates in obstructing the aging propagation in both the basal planes and edges of two-dimensitional sheets of transition metal dichalcogenides (TMDs). Even after 300 d in ambient air conditions, the epitaxially grown WS2/graphene samples have a clean, uniform surface without any encapsulation. We show that high crystallinity is an effective factor that determines the excellent air stability of WS2/graphene, and we present impressive experimental evidence of the relation between defects and the aging phenomena. Moreover, we reveal the strong interlayer charge interaction as an additional factor for the enhanced air stability as a result of charge transfer-induced doping. This work not only proposes a simple method to create highly stable TMDs by the selection of a suitable substrate but also paves the way for the realization of practical TMDs-based applications.

The structure of water confined in nanometer-sized cavities is important because, at this scale, a large fraction of hydrogen bonds can be perturbed by interaction with the confining walls. Unusual fluidity properties can thus be expected in the narrow pores, leading to new phenomena like the enhanced fluidity reported in carbon nanotubes. Crystalline mica and amorphous silicon dioxide are hydrophilic substrates that strongly adsorb water. Graphene, on the other hand, interacts weakly with water. This presents the question as to what determines the structure and diffusivity of water when intercalated between hydrophilic substrates and hydrophobic graphene. Using atomic force microscopy, we have found that while the hydrophilic substrates determine the structure of water near its surface, graphene guides its diffusion, favouring growth of intercalated water domains along the C-C bond zigzag direction. Molecular dynamics and density functional calculations are provided to help understand the highly anisotropic water stripe patterns observed.

We prepared CZTSe thin films by the co-evaporation method. In order to investigate the presence of various kinds of secondary phases, we also prepared reference ZnSe, SnSe, CuSnSe, and CuSe thin films in the same way. The depth dependence of the secondary phases was investigated by preparing samples that are sputtered to various depths. We used Raman spectroscopy measurement with several lasers to detect secondary phases. CZTSe films were compared with the reference films, and only ZnSe and MoSe2 phases were detected. The relative amount of ZnSe varied with the depth from the sample surface. The ZnSe Raman peak seems to originate from the top and the bottom of the sample since the signal decreased initially and then increased as deeper parts of the film were probed. Intensity of CZTSe peak did not change much until the bottom part of the sample was reached and the MoSe2 peak appeared.

Fano resonances and their strong doping dependence are observed in Raman scattering of single-layer graphene (SLG). As the Fermi level is varied by a back-gate bias, the Raman G band of SLG exhibits an asymmetric line shape near the charge neutrality point as a manifestation of a Fano resonance, whereas the line shape is symmetric when the graphene sample is electron or hole doped. However, the G band of bilayer graphene (BLG) does not exhibit any Fano resonance regardless of doping. The observed Fano resonance can be interpreted as interferences between the phonon and excitonic many-body spectra in SLG. The absence of a Fano resonance in the Raman G band of BLG can be explained in the same framework since excitonic interactions are not expected in BLG., Comment: 18 pages, 3 figures, to appear in Carbon

A study has been conducted of (crystallized a-Ge :H)/a-SiN x :H multilayers which display room temperature photoluminescence in the visible range. The a-Ge :H/a-SiN x :H multilayers were prepared by glow discharge chemical vapor deposition by changing gas flows in a continuous deposition using a computer. Crystallization of the a-Ge :H layers was achieved by either thermal annealing in vacuum or exposure to a scanning laser light. The multilayer structure was confirmed by glancing angle X-ray diffraction. Crystallization was confirmed by Raman scattering. When crystallization of the a-Ge :H is achieved using the laser-scanning technique, strong visible photoluminescence with a peak centered around 625 nm, independent of the a-Ge :H layer thickness, is observed. The crystallization is accompanied by severe cracking and pitting of the film. A study of several a-Ge :H/a-SiN x :H layered structures reveals that this laser-induced crystallization only occurs when the a-Ge :H is in a state of high stress. Further study reveals that a-SiN x :H also photoluminesces with a similarly broad peak in the same wavelength range as observed from the crystallized multilayers. Using slow thermal annealing, it was possible to crystallize the a-Ge :H with minimal physical damage to the film. Photoluminescence measurements of these quantum-well structures yield a low intensity broad peak in the visible range before crystallization which does not change after crystallization. One is led to conclude that the photoluminescence observed in the laser-crystallized a-Ge :H/a-SiN x :H is from the a-SiN x :H and cannot be attributed to effects of quantum confinement.

We examine the origin of the surface-enhanced Raman scattering (SERS) background mediating with a graphene monolayer at Au nanoparticle-Au thin film junctions. We find the degree of the asymmetric shape of the radial breathing like mode (RBLM) peak at 150 cm−1 is strongly correlated with the accompanied SERS background intensities. The SERS investigation with the out-of-plane modes (300~700 cm−1) of graphene gives significant clues that the SERS background, the degree of the tilted formation from the analysis of the transverse acoustic peaks, and the spectral shape of the RBLM peak are strongly correlated with each other, indicating that electron–phonon coupling plays a crucial role in increasing the SERS background.

The Young's modulus of graphene is estimated by measuring the strain applied by a pressure difference across graphene membranes using Raman spectroscopy. The strain induced on pressurized graphene balloons can be estimated directly from the peak shift of the Raman G band. By comparing the measured strain with numerical simulation, we obtained the Young's modulus of graphene. The estimated Young's modulus values of single- and bi-layer graphene are 2.4\pm0.4 TPa and 2.0\pm0.5 TPa, respectively.

The dependence of the photocurrent generated in a Pd/graphene/Ti junction device on the incident photon polarization is studied. Spatially resolved photocurrent images were obtained as the incident photon polarization is varied. The photocurrent is maximum when the polarization direction is perpendicular to the graphene channel direction and minimum when the two directions are parallel. This polarization dependence can be explained as being due to the anisotropic electron-photon interaction of Dirac electrons in graphene., 11 pages, 4 figures

Raman spectroscopy is one of the most widely used characterization tool in the study of graphene, a two-dimensional hexagonal crystal of carbon atoms. Owing to the resonant nature of the light scattering process, fairly strong signals are detected in spite of the fact that only one monolayer of atoms is probed. In graphene research, Raman spectroscopy is used not only as an indispensible characterization tool to identify the number of layers, but also as a probe to investigate the mechanical, electrical, and optical properties. Here, important recent progress in Raman spectroscopic characterization of graphene is reviewed.