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In this paper, we report a facile process to fabricate CuO/Au core/shell nanowires, where CuO core and Au shell were prepared by thermal oxidation and sputtering, respectively. The as-prepared CuO/Au nanowires are highly sensitive surface-enhanced-Raman-scattering (SERS) substrates, which can detect methylene blue down to a very low concentration of 10−13 M. The major advantages of SERS substrates based on CuO/Au core/shell nanowires compared with others SERS substrates are the high sensitivity, uniformity, and purity due to the absence of any organic surfactants in the synthesis process.

Noncentrosymmetric transition-metal dichalcogenides, particularly their 3R polymorphs, provide a robust setting for valleytronics. Here, we report on the selective growth of monolayers and bilayers of MoS2, which were acquired from two closely but differently oriented substrates in a chemical vapor deposition reactor. It turns out that as-grown bilayers are predominantly 3R-type, not more common 2H-type, as verified by microscopic and spectroscopic characterization. As expected, the 3R bilayer showed a significantly higher valley polarization compared with the centrosymmetric 2H bilayer, which undergoes efficient interlayer scattering across contrasting valleys because of their vertical alignment of the K and K' points in momentum space. Interestingly, the 3R bilayer showed even higher valley polarization compared with the monolayer counterpart. Moreover, the 3R bilayer reasonably maintained its valley efficiency over a very wide range of excitation power density from ∼0.16 kW/cm2 to ∼0.16 MW/cm2 at both low and room temperatures. These observations are rather surprising because valley dephasing could be more efficient in the bilayer via both interlayer and intralayer scatterings, whereas only intralayer scattering is allowed in the monolayer. The improved valley polarization of the 3R bilayer can be attributed to its indirect-gap nature, where valley-polarized excitons can relax into the valley-insensitive band edge, which otherwise scatter into the contrasting valley to effectively cancel out the initial valley polarization. Our results provide a facile route for the growth of 3R-MoS2 bilayers that could be utilized as a platform for advancing valleytronics.

The power conversion efficiency (PCE) of solution-processed organic–inorganic hybrid perovskite solar cells has been drastically improved. Despite this considerable progress, systematic research on...

We performed polarized Raman spectroscopy on mechanically exfoliated few-layer MoTe2 samples and observed both 1T' and Td phases at room temperature. Few-layer 1T' and Td MoTe2 exhibited a significant difference especially in interlayer vibration modes, from which the interlayer coupling strengths were extracted using the linear chain model: strong in-plane anisotropy was observed in both phases. Furthermore, temperature-dependent Raman measurements revealed a peculiar phase transition behavior in few-layer 1T' MoTe2. In contrast to bulk 1T' MoTe2 crystals where the phase transition to the Td phase occurs at ~250 K, the temperature-driven phase transition to the Td phase is increasingly suppressed as the thickness is reduced, and the transition and the critical temperature varied dramatically from sample to sample even for the same thickness. Raman spectra of intermediate phases that correspond to neither 1T' nor Td phase with different interlayer vibration modes were observed, which suggests that several metastable phases exist with similar total energies., Comment: 58 pages (32 pages of main text, 26 pages of Supporting Information), to be published in ACS Nano

The evolution of the optical phonons in layered semiconductor alloys SnSe1-xSx is studied as a function of the composition by using polarized Raman spectroscopy with six different excitation wavelengths (784.8, 632.8, 532, 514.5, 488, and 441.6 nm). The polarization dependences of the phonon modes are compared with transmission electron diffraction measurements to determine the crystallographic orientation of the samples. Some of the Raman modes show significant variation in their polarization behavior depending on the excitation wavelengths. It is established that the maximum intensity direction of the Ag2 mode of SnSe1-xSx (0, X

A key challenge in the development of two-dimensional (2D) devices is the fabrication of metal–semiconductor junctions with minimal contact resistance and depinned energy levels. An ideal solution for practical applications is to make contacts between 2D van der Waals semiconductors and 2D van der Waals metals. Here we report the wafer-scale production of patterned layers of metallic transition metal ditellurides on different substrates. Our tungsten ditelluride and molybdenum ditelluride layers, which are grown using a tellurization process applied to a precursor transition metal layer, have an electronic performance comparable to that of mechanically exfoliated flakes and can be combined with the 2D semiconductor molybdenum disulfide. The resulting metal–semiconductor junctions are free from significant disorder effects and Fermi-level pinning, and are used to create monolayer molybdenum disulfide field-effect transistors. The Schottky barrier heights of the devices also largely follow the trend of the Schottky–Mott limit. Two-dimensional metallic WTe2 and MoTe2 layers can be combined with a semiconducting MoS2 monolayer to create metal–semiconductor junctions that are free from substantial disorder effects and Fermi-level pinning.

Noncentrosymmetric monolayers (MLs) of transitional metal dichalcogenides (TMDCs) and their 3R-type vertical stacks provide an ideal platform for studying atomic-scale nonlinear light-matter...

Spin–phonon coupling is one of the fundamental interactions in functional materials, indispensable for understanding their unexpected magnetic ground states. Ferromagnetic SrRuO3 is a correlated metal with the potential for utilization in novel spintronic devices and serves as a promising platform for studying spin–phonon interactions. In this study, we used Raman spectroscopy to identify spin–phonon coupling in SrRuO3 heterostructures. We deliberately decreased the exchange interactions within SrRuO3 by reducing system dimensions, which was coherently observed in both temperature-dependent magnetization measurements and phonon spectra. To collect the Raman signals from the very thin (quasi-2D) SrRuO3 layers while maintaining the layer thickness, we fabricated epitaxial oxide superlattices with 50 repetitions of the layers. We also present polarization-dependent Raman spectra of SrRuO3, with accurate identification of the Raman modes. These results show that the phonon dynamics of SrRuO3 is strongly influenced by the spin ordering, which can be efficiently tailored via atomically controlled epitaxial heterostructuring.

The conventional precursor mixture based on highly pure and expensive PbI2 (purity >99.99%) is problematic in commercializing perovskite solar cells (PSCs) because of a large deviation in the batch-to-batch photovoltaic performance due to underlying nonstoichiometry and/or the nonperovskite phase. Here, we report on reproducibly efficient (>21%) PSCs based on the ambient-temperature-stable δ-phase FAPbI3 powder, synthesized by reacting PbI2 (either homemade or low-grade commercial product with purity

The family of group IV-VI monochalcogenides has an atomically puckered layered structure, and their atomic bond configuration suggests the possibility for the realization of various polymorphs. Here, we report the synthesis of the first hexagonal polymorph from the family of group IV-VI monochalcogenides, which is conventionally orthorhombic. Recently predicted four-atomic-thick hexagonal GeSe, so-called {\gamma}-GeSe, is synthesized and clearly identified by complementary structural characterizations, including elemental analysis, electron diffraction, high-resolution transmission electron microscopy imaging, and polarized Raman spectroscopy. The electrical and optical measurements indicate that synthesized {\gamma}-GeSe exhibits high electrical conductivity of 3x10^5 S/m, which is comparable to those of other two-dimensional layered semimetallic crystals. Moreover, {\gamma}-GeSe can be directly grown on h-BN substrates, demonstrating a bottom-up approach for constructing vertical van der Waals heterostructures incorporating {\gamma}-GeSe. The newly identified crystal symmetry of {\gamma}-GeSe warrants further studies on various physical properties of {\gamma}-GeSe., Comment: 24 pages, 5 figures

For scalable perovskite solar cells (PSCs), deposition of a homogeneous and high-quality perovskite film on a large area (>100 cm2) is a prerequisite. Conventional solutions for spin-coating on small areas usually contain polar aprotic solvents with high boiling point, which is difficult to adopt for large-area bar coating because of the uncontrollable and slow drying process due to the strong interaction between polar aprotic solvent and Lewis acidic PbI2 or perovskite. Thus, the precursor solution plays a vital role in the success of large-area coating. Here we report a coating solution suitable for large-area perovskite films. The coating solutions prepared via gas-mediated solid–liquid conversion contain preformed perovskite clusters as confirmed by rotational mode of methylammonium cation in the PbI3– framework from Raman spectroscopy. CH3NH3PbI3 (MAPbI3) films formed by D-bar coating within 20 s on the area over 100 cm2 exhibit tetragonal/cubic superlattice structure with highly preferred orientatio...

CZTSSe absorbers are synthesized by a one-step annealing process from Cu, SnS and ZnS stacks prepared by sputtering deposition. CZTSSe thin films are fabricated by a selenization process using an optimized precursor stack structure. The selenization temperature has a strong influence on the phase and reactions of the elements in the films. CZTSSe formation is initially triggered by the interaction of Cu with Sn(S, Se) to form CuSn(S, Se) compounds, which then react with ZnS. CZTSSe films obtained at 590 °C exhibit a kesterite structure. Sulfo-selenization is performed using an optimized selenization process. Based on the mass ratio of the chalcogen sources, selenium disulfide and selenium, the effect of the S/(S + Se) compositional ratio on the structural and morphological properties of the as-grown films and the optoelectronic parameters of solar cells fabricated using these absorber films are studied. Using this single-step sulfo-selenization method, CZTSSe devices with conversion efficiencies up to 9.47% are obtained.

Lattice matching has been supposed to play an important role in the coupling between two materials in a vertical heterostructure (HS). To investigate this role, we fabricated a heterojunction device with a few layers of p-type WSe2 and n-type MoSe2 with different crystal orientation angles. The crystal orientations of WSe2 and MoSe2 were estimated using high-resolution X-ray diffraction. Heterojunction devices were fabricated with twist angles of 0, 15, and 30°. The I–V curve of the sample with the twist angle of 0° under the dark condition showed a diodelike behavior. The strong coupling due to lattice matching caused a well-established p–n junction. In cases of 15 and 30° samples, the van der Waals gap was built because of lattice mismatching, which resulted in the formation of a potential barrier. However, when the light-emitting diode light of 365 nm (3.4 eV) was illuminated, it was possible for excited electrons and holes to jump beyond the potential barrier and the current flowed well in both forwar...

As a 3D topological insulator, bismuth selenide (Bi2Se3) has potential applications for electrically and optically controllable magnetic and optoelectronic devices. Understanding the coupling with its topological phase requires studying the interactions of carriers with the lattice on time scales down to the subpicosecond regime. Here, we investigate the ultrafast carrier-induced lattice contractions and interlayer modulations in Bi2Se3 thin films by time-resolved diffraction using an X-ray free-electron laser. The lattice contraction depends on the carrier concentration and is followed by an interlayer expansion accompanied by oscillations. Using density functional theory and the Lifshitz model, the initial contraction can be explained by van der Waals force modulation of the confined free carrier layers. Our theoretical calculations suggest that the band inversion, related to a topological phase transition, is modulated by the expansion of the interlayer distance. These results provide insights into the topological phase control by light-induced structural change on ultrafast time scales.

Multiple quantum wells based on 2D semiconductors are realized by monolithic bandgap engineering and van der Waals stacking., Quantum wells (QWs), enabling effective exciton confinement and strong light-matter interaction, form an essential building block for quantum optoelectronics. For two-dimensional (2D) semiconductors, however, constructing the QWs is still challenging because suitable materials and fabrication techniques are lacking for bandgap engineering and indirect bandgap transitions occur at the multilayer. Here, we demonstrate an unexplored approach to fabricate atomic–layer–confined multiple QWs (MQWs) via monolithic bandgap engineering of transition metal dichalcogenides and van der Waals stacking. The WOX/WSe2 hetero-bilayer formed by monolithic oxidation of the WSe2 bilayer exhibited the type I band alignment, facilitating as a building block for MQWs. A superlinear enhancement of photoluminescence with increasing the number of QWs was achieved. Furthermore, quantum-confined radiative recombination in MQWs was verified by a large exciton binding energy of 193 meV and a short exciton lifetime of 170 ps. This work paves the way toward monolithic integration of band-engineered heterostructures for 2D quantum optoelectronics.

Van der Waals epitaxy on the surface of two-dimensional (2D) layered crystals has gained significant research interest for the assembly of well-ordered nanostructures and fabrication of vertical heterostructures based on 2D crystals. Although van der Waals epitaxial assembly on the hexagonal phase of transition metal dichalcogenides (TMDCs) has been relatively well characterized, a comparable study on the distorted octahedral phase (1T' or Td) of TMDCs is largely lacking. Here we investigate the assembly behavior of one-dimensional (1D) AgCN microwires on various distorted TMDC crystals, namely 1T'-MoTe2, Td-WTe2, and 1T'-ReS2. The unidirectional alignment of AgCN chains is observed on these crystals, reflecting the symmetry of underlying distorted TMDCs. Polarized Raman spectroscopy and transmission electron microscopy directly confirm that AgCN chains display the remarkable alignment behavior along the distorted chain directions of underlying TMDCs. The observed unidirectional assembly behavior can be attributed to the favorable adsorption configurations of 1D chains along the substrate distortion, which is supported by our theoretical calculations and observation of similar assembly behavior from different cyanide chains. The aligned AgCN microwires can be harnessed as facile markers to identify polymorphs and crystal orientations of TMDCs., 23 pages, 6 main figures

The behavior of 2-dimensional (2D) van der Waals (vdW) layered magnetic materials in the 2D limit of the few-layer thickness is an important fundamental issue for the understanding of the magnetic ordering in lower dimensions. The antiferromagnetic transition temperature TN of the Heisenberg-type 2D magnetic vdW material MnPS3 was estimated as a function of the number of layers. The antiferromagnetic transition was identified by temperature-dependent Raman spectroscopy, from the broadening of a phonon peak at 155 cm−1, accompanied by an abrupt redshift and an increase of its spectral weight. TN is found to decrease only slightly from ~78 K for bulk to ~66 K for 3L. The small reduction of TN in thin MnPS3 approaching the 2D limit implies that the interlayer vdW interaction is playing an important role in stabilizing magnetic ordering in layered magnetic materials.

Gallium selenide (GaSe) is one of layered group-III metal monochalcogenides, which has an indirect bandgap in monolayer and direct bandgap in bulk unlike other conventional transition metal dichalcogenides (TMDs) such as MoX2 and WX2 (X=S and Se). Four polytypes of bulk GaSe, designated as beta-, epsilon-, gamma-, and delta-GaSe, have been reported. Since different polytypes result in different optical and electrical properties even for the same thickness, identifying the polytype is essential in utilizing this material for various optoelectronic applications. We performed polarized Raman measurement on GaSe and found different ultra-low-frequency Raman spectra of inter-layer vibrational modes even for the same thickness due to different stacking sequences of the polytypes. By comparing the ultra-low-frequency Raman spectra with theoretical calculations and high-resolution electron microscopy measurements, we established the correlation between the ultra-low-frequency Raman spectra and the stacking sequences for trilayer GaSe. We further found that the AB-type stacking is more stable than the AA'-type stacking in GaSe.

Many properties of layered materials change as they are thinned from their bulk forms down to single layers, with examples including indirect-to-direct band gap transition in 2H semiconducting transition metal dichalcogenides as well as thickness-dependent changes in the valence band structure in post-transition metal monochalcogenides and black phosphorus. Here, we use angle-resolved photoemission spectroscopy to study the electronic band structure of monolayer ReSe$_{2}$, a semiconductor with a distorted 1T structure and in-plane anisotropy. By changing the polarization of incoming photons, we demonstrate that for ReSe$_{2}$, in contrast to the 2H materials, the out-of-plane transition metal $d_{z^{2}}$ and chalcogen $p_{z}$ orbitals do not contribute significantly to the top of the valence band which explains the reported weak changes in the electronic structure of this compound as a function of layer number. We estimate a band gap of 1.7 eV in pristine ReSe$_{2}$ using scanning tunneling spectroscopy and explore the implications on the gap following surface-doping with potassium. A lower bound of 1.4 eV is estimated for the gap in the fully doped case, suggesting that doping-dependent many-body effects significantly affect the electronic properties of ReSe$_{2}$. Our results, supported by density functional theory calculations, provide insight into the mechanisms behind polarization-dependent optical properties of rhenium dichalcogenides and highlight their place amongst two-dimensional crystals., Comment: 37 pages (including Supporting Information), 7 figures in the main text

The origin of the variation of photoluminescence (PL) spectra of monolayer tungsten disulfide (WS2) is investigated systematically. Dependence of the PL spectrum on the excitation power show that the relatively sharp component corresponds to excitons whereas the broader component at slightly lower energy corresponds to negatively charged trions. PL imaging and second harmonic generation measurements show that the trion signals are suppressed more than the exciton signals near the edges, thereby relatively enhancing the excitonic feature in the PL spectrum and that such relative enhancement of the exciton signals is more pronounced near approximately armchair edges. This effect is interpreted in terms of depletion of free electrons near the edges caused by structural defects and adsorption of electron acceptors such as oxygen atoms., 13 pages, 4 figures

Herein, we report a facile process, i.e., controlling the initial chamber pressure during the postdeposition annealing, to effectively lower the band tail states in the synthesized CZTSSe thin films. Through detailed analysis of the external quantum efficiency derivative (dEQE/dλ) and low-temperature photoluminescence (LTPL) data, we find that the band tail states are significantly influenced by the initial annealing pressure. After carefully optimizing the deposition processes and device design, we are able to synthesize kesterite CZTSSe thin films with energy differences between inflection of d(EQE)/dλ and LTPL as small as 10 meV. These kesterite CZTSSe thin films enable the fabrication of solar cells with a champion efficiency of 11.8% with a low Voc deficit of 582 mV. The results suggest that controlling the annealing process is an effective approach to reduce the band tail in kesterite CZTSSe thin films.

The structural and electro-optical influence of marcasite on the properties of solution-processed iron pyrite thin films was investigated. Marcasite has a strong tendency to form simultaneously with pyrite upon sulfurization of amorphous iron-oxide precursor films, leading to a mixed-phase structure in which pyrite grains are surrounded by nanocrystalline marcasite boundaries. The optical analysis in combination with spectroscopic ellipsometry revealed that marcasite should have a bandgap of approximately 0.85–0.88 eV with a higher absorption coefficient than pyrite, differing strongly from the prior belief that marcasite has a bandgap of less than 0.4 eV. In addition, the pyrite/marcasite film has been found to have a larger diffusion coefficient for photogenerated minority carriers than the phase-pure pyrite film from electrochemical impedance analyses, resulting in a higher photocurrent density, as determined through photoelectrochemical measurement. The facile transport of a minority carrier along the marcasite boundaries is the putative origin of the observed improvement in the photoactivity of the pyrite/marcasite mixture films.

We report on the thickness-dependent Raman spectroscopy of ultrathin silicon (Si) nanomembranes (NMs), whose thicknesses range from 2 to 18 nm, using several excitation energies. We observe that the Raman intensity depends on the thickness and the excitation energy due to the combined effects of interference and resonance from the band-structure modulation. Furthermore, confined acoustic phonon modes in the ultrathin Si NMs were observed in ultralow-frequency Raman spectra, and strong thickness dependence was observed near the quantum limit, which was explained by calculations based on a photoelastic model. Our results provide a reliable method with which to accurately determine the thickness of Si NMs with thicknesses of less than a few nanometers.

Layered materials such as graphite and transition metal dichalcogenides have extremely anisotropic mechanical properties owing to orders of magnitude difference between in-plane and out-of-plane interatomic interaction strengths. Although effects of mechanical perturbations on either intra- or inter-layer interactions have been extensively investigated, mutual correlations between them have rarely been addressed. Here we show that layered materials have an inevitable coupling between in-plane uniaxial strain and interlayer shear. Because of this, the uniaxial in-plane strain induces an anomalous splitting of the degenerate interlayer shear phonon modes such that the split shear mode along the tensile strain is not softened but hardened contrary to the case of intralayer phonon modes. We confirm the effect by measuring Raman shifts of shear modes of bilayer MoS2 under strain. Moreover, by analyzing the splitting, we obtain an unexplored off-diagonal elastic constant, demonstrating that Raman spectroscopy can determine almost all mechanical constants of layered materials., Comment: 36 pages, 10 figures

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.

Interlayer excitons were observed at the heterojunctions in van der Waals heterostructures (vdW HSs). However, it is not known how the excitonic phenomena are affected by the stacking order. Here, we report twist-angle-dependent interlayer excitons in MoSe2/WSe2 vdW HSs based on photoluminescence (PL) and vdW-corrected density functional theory calculations. The PL intensity of the interlayer excitons depends primarily on the twist angle: It is enhanced at coherently stacked angles of 0° and 60° (owing to strong interlayer coupling) but disappears at incoherent intermediate angles. The calculations confirm twist-angle-dependent interlayer coupling: The states at the edges of the valence band exhibit a long tail that stretches over the other layer for coherently stacked angles; however, the states are largely confined in the respective layers for intermediate angles. This interlayer hybridization of the band edge states also correlates with the interlayer separation between MoSe2 and WSe2 layers. Furthermo...

The defects states and carrier density of CZTSe absorber layers are two of the crucial factors that decide the photovoltaic performance of CZTSe thin film solar cells. Fine tailoring the defects and carrier density is a key to push the power conversion efficiency of CZTSe solar cells to a more competitive level. In this work, the phase properties, defect states, and carrier density of CZTSe thin film are well controlled by fine tuning the ratio of Zn/Sn in the range from 0.75 to 1.27. Capacity-Voltage measurements and Admittance Spectroscopy are used to characterize the carrier density, depletion region width, and defect states of the CZTSe solar cells. The results indicate that the defects states and carrier density of CZTSe layer are very sensitive to the ratio of Zn/Sn. Combining experimental results and numerical simulation, the statistic regularities of the photovoltaic parameters of the CZTSe solar cells with different ratios of Zn/Sn is well explained. The increase of VOC of CZTSe solar cells with the ratio of Zn/Sn is related to both the increased carrier density and the decreased deep level defects states. The decline of JSC of the Zn-rich solar cells is caused by both the shrunken depletion region width and a large barrier caused by ZnSe phase. This barrier is the cause for a low fill factor in the Zn-rich solar cells. Overall, the CZTSe solar cells with a stoichiometric ratio of Zn/Sn=1.02 have favorable defects property and carrier density, thus resulting in the highest photovoltaic efficiency of 10.21%.

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.

State Fund for Fun- damental Research; Ministry of Education and Science of Ukraine; New&Renew- able Energy of the Korea Institute of Energy Technology Evaluation and Planning

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.

In this study, CZTSSe precursors (Se/Cu/SnS/ZnS) were deposited by sputtering and evaporation and then annealed without added chalcogen substances in an argon-filled atmospheric chamber using rapid thermal processing (RTP). A low-temperature pre-annealing treatment (PAT) was applied to induce a pre-reaction between Se and Cu/SnS/ZnS. It was found that the PAT leads to the formation of CuSe and Cu2−xSe and enhances Se incorporation during RTP chalcogenization. Scanning electron microscopy (SEM), X-ray diffractometry, Raman analysis and scanning transmission electron microscopy with energy dispersive spectroscopy (STEM-EDS) were applied to investigate the differences between the absorber with the PAT and the one without the PAT. Better morphologies on the surface and in the cross section were obtained for the CZTSSe absorber annealed using the PAT compared with the absorber annealed without the PAT. Both absorbers showed nearly pure kesterite phases in their upper regions, as confirmed by Raman analysis. However, STEM-EDS maps revealed that when the absorber was prepared without the PAT, Zn- and S-rich secondary phases and voids were easily formed near the back contact. The electrical characteristics and efficiencies of the CZTSSe thin films showed drastic changes; the CZTSSe solar cell without the PAT showed no diode response, but the cell with the PAT showed an efficiency of 6.77% with an open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF) of 376 mV, 31.39 mA/cm2 and 57%, respectively.

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.

Surface enhanced-Raman spectroscopy (SERS) is greatly affected by the substrate where excitation of the localized metal surface plasmon resonance enhances the scattering signals. This report presents the results of preparation of CuO/Ag nanowires by thermal oxidation and sputtering method. The nanowires were coated with a layer of Ag by sputtering. The samples were investigated with different characterization techniques including X-ray diffraction, scanning electron microscopy, X-ray energy dispersive spectroscopy, X-ray photoemission spectroscopy, and Raman spectroscopy. The effect of the coating layer thickness, ranging from 20 to 240 nm, on SERS efficiency was studied. The results show that the CuO/Ag nanowires are very sensitive SERS substrates, which can detect the signal of Methylene Blue at low concentration of 10-10 M. The reason for high sensitivity of the as-prepared CuO/Ag nanowires is believed to be the surface plasmon resonance in silver layer, which is further enhanced due to the charge transfer transition from Ag shell to CuO core.

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.

Nanocrystalline ZnO films were deposited onto glass substrates in the temperature range of 473–673 K using pulsed spray pyrolysis. The structural, substructural, and optical properties of the films were investigated by means of X-ray diffraction analysis, Raman scattering, and Fourier transform infrared (FTIR) spectroscopy. The effect of the substrate temperature (Ts) on the coherent scattering domain (CSD) sizes L, microstrains ϵ, and microstress σ grades, and the average density of dislocations ρ in the films were estimated through the broadening of X-ray lines using the Cauchy and Gauss approximations and the threefold function convolution method. The ZnO films grown at Ts = 623–673 K possessed the highest values of L, and the lowest of ϵ, σ, and ρ, indicating high-crystalline quality. The Raman spectra showed peaks located at 95–98, 333–336, 415, 439–442, 572, and 578–584 cm−1, which were interpreted as E2low(Zn), (E2high−E2low), E1(TO), E2high(O), A1(LO) and E1(LO) phonon modes of the ZnO wurtzite phase. The FTIR spectra showed relatively weak signals at 856, 1405, and 1560 cm−1, corresponding to the C–H and C–O stretching modes, in addition to the main Zn–O mode at 475 cm−1, indicating a low content of precursor residues.

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.

ZnSe thin films were deposited on well-cleaned glass substrates by the close-spaced vacuum sublimation technique. Various structural, sub-structural and optical properties have been investigated through scanning electron microscopy (SEM), X-ray diffraction (XRD), spectral photometry, Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. Electro-physical studies were performed using an ITO/ZnSe/In “sandwich” structure. The correlation between the films structure and their optical and electro-physical properties was estimated. The evaluated films were fine-crystalline, with their grain size depending on the substrate temperature. The optical band gap was estimated through optical measurements and the high optical qualities of the ZnSe films were confirmed by Raman and FTIR analyses.