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In this work, we, for the first time, observed the remarkable thermoelectric properties of a few high-quality PtSe2 layered films fabricated by a post selenization of Pt thin films. An excellent po... more...

Recent advances in nanotechnology, especially the development of integrated nanostructured materials, have offered unprecedented opportunities for photocatalytic CO2 reduction. Compared to bulk semiconductor photocatalysts, most of these nanostructured photocatalysts offer at least one advantage in areas such as photogenerated carrier kinetics, light absorption, and active surface area, supporting improved photochemical reaction efficiencies. In this review, we briefly cover the cutting-edge research activities in the area of integrated nanostructured catalysts for photochemical CO2 reduction, including aqueous and gas-phase reactions. Primarily explored are the basic principles of tailor-made nanostructured composite photocatalysts and how nanostructuring influences photochemical performance. Specifically, we summarize the recent developments related to integrated nanostructured materials for photocatalytic CO2 reduction, mainly in the following five categories: carbon-based nano-architectures, metal-organic frameworks, covalent-organic frameworks, conjugated porous polymers, and layered double hydroxide-based inorganic hybrids. Besides the technical aspects of nanostructure-enhanced catalytic performance in photochemical CO2 reduction, some future research trends and promising strategies are addressed. more...

Interface engineering of CdS/CZTS(Se) is an important aspect of improving the performance of buffer/absorber heterojunction combination. It has been demonstrated that the crossover phenomenon due to the interface recombination can be drastically eliminated by interface modification. Therefore, in-depth studies across the CdS/CZTS(Se) junction properties, as well as effective optimization processes, are very crucial for achieving high-efficiency CZTSSe solar cells. Here, we present a comprehensive study on the effects of soft-baking (SB) temperature on the junction properties and the corresponding optoelectronic and interface-structural properties. Based on in-depth photoemission studies corroborated with structural and composition analysis, we concluded that interdiffusion and intermixing of CZTSSe and CdS phases occurred on the Cu-poor surface of CZTSSe at elevated SB temperatures, and the interface dipole moments induced by electrostatic potential fluctuation were thus significantly eliminated. In contrast, with low SB temperature, the CdS/CZTSSe heterojunction revealed very sharp interface with very short interdiffusion, forming interface dipole moments and drastically deteriorating device performance. These post thermal treatments also significantly suppress defect energy level of interface measured by admittance spectroscopy from 294 to 109 meV due to CdS/CZTSSe interdiffusion. Meanwhile, the interdiffusion effects on the shift of valence band maximum, conduction band minimum and band offset across the heterojunction of thermally treated CdS/CZTSSe interface are spatially resolved at the atomic scale by measuring the local density of states with cross-sectional scanning tunneling microscopy and spectroscopy. A significant enhancement in the power conversion efficiency from 4.88% to 8.48% is achieved by a facile interface engineering process allowing a sufficient intermixing of CdS/Cd and CZTSSe/Se phases without detrimental recombination centers. more...

Chemical alloying is a powerful approach to tune the electronic structure of semiconductors and has led to the synthesis of ternary and quaternary two-dimensional (2D) dichalcogenide semiconductor alloys (e.g., MoSSe₂, WSSe₂, etc.). To date, most of the studies have been focused on determining the chemical composition by evaluating the optical properties, primarily via photoluminescence and reflection spectroscopy of these materials in the 2D monolayer limit. However, a comprehensive study of alloying in multilayer films with direct measurement of electronic structure, combined with first-principles theory, is required for a complete understanding of this promising class of semiconductors. We have combined first-principles density functional theory calculations with experimental characterization of MoS_(2(1-x))Se_(2x) (where x ranges from 0 to 1) alloys using X-ray photoelectron spectroscopy to evaluate the valence and conduction band edge positions in each alloy. Moreover, our observations reveal that the valence band edge energies for molybdenum sulfide/selenide alloys increase as a function of increasing selenium concentration. These experimental results agree well with the results of density functional theory calculations showing a similar trend in calculated valence band edges. Our studies suggest that alloying is an effective technique for tuning the band edges of transition-metal dichalcogenides, with implications for applications such as solar cells and photoelectrochemical devices. more...

The present investigation mainly addresses the open circuit voltage (Voc) issue in kesterite based Cu2ZnSn(S,Se)4 solar cells by simply introducing alkali metal fluoride nanolayers (~ several nm NaF, or LiF) to lower the work functions of the front ITO contacts without conventional hole-blocking ZnO layers. Kelvin probe measurements confirmed that the work function of the front ITO decreases from 4.82 to 3.39 and 3.65 eV for NaF and LiF, respectively, resulting in beneficial band alignment for electron collection and/or hole blocking on top electrodes. Moreover, a 10.4% power conversion efficiency (~ 11.5% in the cell effective area) CZTSSe cell with improved Voc of up to 90 mV has been attained. This demonstration may provide a new direction of further boosting the performance of copper chalcogenide based solar cells as well. more...

A nonstoichiometric Si1− x Ge x with composition ratio dependent saturable absorption prepared by vaporized synthesis and chemical exfoliation is performed to passively mode-lock the Erbium-doped fiber laser (EDFL). The Si1− x Ge x with varied Ge/Si composition ratio from 3 to 16 exhibits tunable nonlinear modulation depth from 17% to 22%, where the Si1− x Ge x with the highest Ge content performs the largest nonlinear modulation depth. When operating the EDFL in the self-amplitude modulation region, the Si1− x Ge x with Ge/Si composition ratios of 3, 9, and 16 self-starts the EDFL pulsation with pulsewidths of 820, 760, and 730 fs. When operating the EDFL in high gain region, the self-phase modulation induced soliton compression dominates the repulsation of passively mode-locked EDFL, which slightly shrinks the EDFL pulsewidth from 346 to 338 fs. All these demonstrations are premier and important to explore the superior nonstoichiometric Si 1− x Ge x saturable absorbers for ultrafast fiber lasers. more...

Photocatalytic formation of hydrocarbons using solar energy via artificial photosynthesis is a highly desirable renewable-energy source for replacing conventional fossil fuels. Using an l-cysteine-based hydrothermal process, here we synthesize a carbon-doped SnS2 (SnS2-C) metal dichalcogenide nanostructure, which exhibits a highly active and selective photocatalytic conversion of CO2 to hydrocarbons under visible-light. The interstitial carbon doping induced microstrain in the SnS2 lattice, resulting in different photophysical properties as compared with undoped SnS2. This SnS2-C photocatalyst significantly enhances the CO2 reduction activity under visible light, attaining a photochemical quantum efficiency of above 0.7%. The SnS2-C photocatalyst represents an important contribution towards high quantum efficiency artificial photosynthesis based on gas phase photocatalytic CO2 reduction under visible light, where the in situ carbon-doped SnS2 nanostructure improves the stability and the light harvesting and charge separation efficiency, and significantly enhances the photocatalytic activity. Photocatalytic reduction of CO2 to hydrocarbons is a promising route to both CO2 utilization and renewable fuel production. Here the authors identify that carbon-doped SnS2 possesses a high catalytic efficiency towards CO2 reduction owing to low photogenerated charge recombination rates. more...

The thickness of transition metal dichalcogenides (TMDs) plays a key role in enhancing their photocatalytic CO2 reduction activity. However, the optimum thickness of the layered TMDs that is required to achieve sufficient light absorption and excellent crystallinity has still not been definitively determined. In this work, ultra-thin molybdenum disulfide films (MoS2TF) with 25 nm thickness presented remarkable photocatalytic activity, and the product yield increased by about 2.3 times. The photocatalytic mechanism corresponding to the TMDs’ thickness was also proposed. This work demonstrates that the thickness optimization of TMDs provides a cogent direction for the design of high-performance photocatalysts. more...

Kesterite-type Cu 2 ZnSnS 4 (CZTS) has been attracting a lot of attention in thin-film solar cells due to its relatively low cost, earth abundant and environmentally benign nature compared to its analog Cu(In, Ga)Se 2 materials. Until now, solution-based processes are considered as promising methodologies for mass production of CZTS materials for industrial demands. However, most material sources are highly toxic as well as dangerous. In this study, we proposed a facile and green synthesis strategy for the CZTS absorber by taking advantage of an ultrasonic spray technique where we adopted metallic nanopowders and ethanol as our nontoxic precursors as well as solvents, respectively. The phase formation and transition mechanism of the synthesized CZTS film are investigated through varying sulfurization temperatures and analyzed using X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. The solar cell device can be made at a scale as large as 20 × 20 mm 2 , and the cell shows a power conversion efficiency of 1.5%. more...

We have demonstrated a nonlinear behavior for the bandgap opening of doped graphene by controlling the concentration of B and N co-dopants. X-ray absorption and emission spectra reveal that the ban ... more...

A cadmium sulfide (CdS) layer with a thickness of 37±5 nm is deposited onto a Cu2ZnSn(SSe)4 absorber layer using Cd(NO3)2 precursor at pH 11.8 via chemical bath deposition. Full devices fabricated with the thin CdS layer show improved champion efficiency of 6.97%, compared with the 5.91% efficiency of the control device, which can be attributed to the increased current density from reduced light reflection in the visible region and enhanced charge collection in the shorter wavelength region. more...

Gas sensors, which play an important role in the safety of human life, cover a wide range of applications including intelligent systems and detection of harmful and toxic gases. It is known that graphene is an ideal and attractive candidate for gas sensing due to its high surface area and excellent mechanical, electrical, optical, and thermal properties. However, in order to fully realize its potential as a commercial gas sensor, demand for a graphene-based device of low-limit detection, high sensitivity, and fast response time needs to be met. Here, we demonstrate a metal/insulator/semiconductor (MIS) based gas sensor consisting of as-grown epitaxial graphene nanowalls (EGNWs)/silicon carbide (SiC)/silicon (Si) structure. The unique edge dominant three-dimensional (3D) EGNWs based MIS device achieved an extraordinarily low limit of detection (0.5 ppm) and unprecedented sensitivity (82 μA/ppm/cm more...

The photoconduction properties of (non-polar) m-plane and (polar) c-plane ZnO thin films epitaxially grown on relatively lattice-matched LiGaO2 substrates by chemical vapor deposition have been investigated and compared. Structural and photoluminescence characterizations indicate that the c-plane ZnO films possess higher crystallinity than the m-plane ZnO films. Owing to their superior crystallinity and lower dark current, the c-plane ZnO films exhibit significantly higher optimal sensitivity (S = 4650) to ultraviolet light than the m-plane ZnO films (S = 10). However, in terms of photocurrent generation efficiency, the m-plane ZnO films exhibit over one order of magnitude higher responsivity (R = 0.05–3.2 A W−1) than the c-plane ZnO films (R = 0.009–0.035 A W−1) in the light intensity range 1.6–320 W m−2. The probable mechanisms causing the aforementioned differences based on surface-dominant photoconduction and bulk-dominant structural and photoluminescence properties are also discussed. more...

Manipulating the nature of defects and carrier dynamics in kesterite Cu2ZnSnS4 (CZTS) solar cells is challenging because of the complex behavior of defects. Among the various strategies used to reduce the defect levels, sodium-incorporated CZTS absorbers are effective and beneficial for defect passivation. In this study, we proposed a bifacial sodium-incorporated treatment (BSIT) in CZTS for the control of defect levels. The defect energy levels of the absorber measured by admittance spectroscopy decrease from 263 to 112 meV with increasing Na contents. In addition, impedance measurements of the sample after the BSIT showed an improved carrier dynamics with a prolonged minority carrier lifetime from 0.9 to 1.8 μs. This suggests that the post treatment of NaF diffused from the top and bottom surfaces of the CZTS absorbers is beneficial for carrier transport behaviors. Our results demonstrated that by manipulating the sodium content in the BSIT, defect passivation and effective reduction of carrier recombination can be attained, resulting in enhanced CZTS device performance from 4.1% to 5.6%. more...

In this study, we combine graphene with gold oxide (AuOx), a transparent and high-work-function electrode material, to achieve a high-efficient, low-bias, large-area, flexible, transparent, broadband, and bifacial-operable photodetector. The photodetector operates through hot electrons being generated in the graphene and charge separation occurring at the AuOx-graphene heterojunction. The large-area graphene covering the AuOx electrode efficiently prevented reduction of its surface; it also acted as a square-centimeter-scale active area for light harvesting and photodetection. Our graphene/AuOx photodetector displays high responsivity under low-intensity light illumination, demonstrating picowatt sensitivity in the ultraviolet regime and nanowatt sensitivity in the infrared regime for optical telecommunication. In addition, this photodetector not only exhibited broadband (from UV to IR) high responsivity-3300 A W(-1) at 310 nm (UV), 58 A W(-1) at 500 nm (visible), and 9 A W(-1) at 1550 nm (IR)-but also required only a low applied bias (0.1 V). The hot-carrier-assisted photoresponse was excellent, especially in the short-wavelength regime. In addition, the graphene/AuOx photodetector exhibited great flexibility and stability. Moreover, such vertical heterojunction-based graphene/AuOx photodetectors should be compatible with other transparent optoelectronic devices, suggesting applications in flexible and wearable optoelectronic technologies. more...

For graphene-based electrode materials, N doping is one of the leading approaches for enhancing the performance of supercapacitors. However, such an outstanding performance is suppressed by the agglomeration of graphene and unspecified N incorporation. Here, we demonstrate a direct growth of vertically epitaxial graphene nanowalls (GNWs) on flexible carbon cloths (CCs) via microwave plasma-enhanced chemical vapor deposition, whereby predominantly N doping was sequentially achieved by introducing in situ NH 3 plasma, to form N-doped GNWs (NGNWs). The vertically aligned three-dimensional (3D) architecture of epitaxial NGNWs and their unique selectivity to the specific N dopants make such electrodes an ideal platform, not only for enhancing the capacitive performance but also for studying the role of the C N bonding configuration in its performance. Remarkably, NGNW supercapacitors exhibit an excellent specific capacitance of 991.6 F/g (estimation based on the actively contributing component) and an apparent area-normalized capacitance of 1488.9 mF/cm 2 , at a specific current of 14.8 A/g. This approach allows us to achieve an energy density of 275.4 Wh/kg at a power density of 14.8 kW/kg (specific current of 14.8 A/g), and a power density of 74.1 kW/kg at an energy density of 232.6 Wh/kg (specific current of 74.1 A/g) in 1 M H 2 SO 4 . more...

Germanium antimony telluride (GeSbTe or GST), a popular material in optical and non-volatile memory devices, attracted renewed attention due to its potential for thermoelectric applications. In this study, we have employed a two-stage engineering process to enhance the thermoelectric properties of GeTe-rich GeSbTe. First, we introduced vacancy into the material by modifying the germanium content without disrupting the crystal structure. This influenced the electronic properties of the GeTe-rich GeSbTe and improved the overall dimensionless figure of merit, zT, from 0.7 to 1.1. Second, we rapidly cooled the material after annealing, further enhancing the zT value from 1.1 to 1.48 – one of the highest values reported for this material. In-depth studies suggest that disorder in the crystal structure was created via rapid cooling enhanced phonon scattering and effectively reduced the thermal conductivity, which, in turn, enhanced the thermoelectric performance. more...

We demonstrated the catalytic growth of m-axial InxGa1−xN (0.10 ≤ x ≤ 0.17) nanocolumn arrays with high crystallinity on silicon substrates using metal–organic chemical vapor deposition with trimethylindium (TMIn), triethylgallium (TEGa), and ammonia as precursors. The high quality of InGaN nanocolumns (NCs) were believed to be due to the utilization of TEGa that achieved less carbon impurities and offered more comparable vapor pressure with that of TMIn at low temperature. In addition, these NCs were grown in non-polar m-axis, which the internal electric field of the InGaN that often deteriorates the device performances might be able to be eliminated. Furthermore, the bandgap of this InGaN can be modulated from UV to visible region simply by tuning the ratio of the precursor during the fabrication. Our results suggest an approach to the fabrication of large-area NCs with a tunable bandgap on a silicon substrate by the standard MOCVD method that offers an immense opportunity for electronic and photonic applications and allows the scale-up from a research laboratory to industrial scale. more...

Practically, graphene is often deposited on substrates. Given the major substrate-induced modification of properties and considerable energy transfer at the interface, the graphene-substrate interaction has been widely discussed. However, the proposed mechanisms were restricted to the two-dimensional (2D) plane and interface, while the energy conduction in the third dimension is hardly considered. Herein, we disclose the transfer of energy perpendicular to the interface of the combined system of the 2D graphene and the 3D base. More precisely, our observation of the energy dissipation of optically excited graphene via emitting out-of-plane longitudinal acoustic phonon into the substrate is presented. By applying nanoultrasonic spectroscopy with a piezoelectric nanolayer embedded in the substrate, we found that under photoexcitation by a femtosecond laser pulse graphene can emit longitudinal coherent acoustic phonons (CAPs) with frequencies over 1 THz into the substrate. In addition, the waveform of the CAP pulse infers that the photocarriers and sudden lattice heating in graphene caused modification of graphene-substrate bond and consequently generated longitudinal acoustic phonons in the substrate. The direct observation of this unexplored graphene-to-substrate vertical energy transfer channel can bring new insights into the understanding of the energy dissipation and limited transport properties of supported graphene. more...

A planar heterojunction organic solar cell (OSC) with high internal quantum efficiency (IQE=95.6% at 590 nm) was demonstrated based on boron subphthalocyanine chloride (SubPc) and C 60 as the electron donor and acceptor materials. A thin layer of N,N-dicarbazolyl-3,5-benzene (mCP), acting as an electron and exciton blocking layer, was inserted between the ITO anode and the SubPc layer to prevent leakage current and exciton quenching. In the optimized device, a mCP thickness of 3 nm was thick enough to block the excitons (as suggested by a high IQE and longer-lived photoluminescence), resulting in an open circuit voltage of 1.09 V, a short circuit current of 7.87 mA/cm 2 , a fill factor of 59, and a power conversion efficiency of 5.08%. Increasing the mCP thickness resulted in increased serial resistance and SubPc crystallization that reduced the fill factor, V OC , and J SC . Due to the narrow absorption band of SubPC (71 nm at the full-width-at-half-maximum), this high performance (both in peak IQE and V OC ) device has the potential to be used as a subcell in a spectral splitting solar cell module. more...

This article outlines state-of-the-art energy technologies, including production and storage, available to us through semiconductor nanomaterials. The nanostructure growth processes have been illustrated in detail, with emphasis on the latest developments in hierarchical and radial-composition modulated nanostructures. On the energy efficiency and generation part, light-emitting diodes, photovoltaics, photoelectrochemistry, thermoelectric, and fuel cells have been discussed. In the energy storage part, supercapacitors and lithium batteries have been discussed. more...

A 3D trenched-structure metal-insulator-metal (MIM) nanocapacitor array with an ultrahigh equivalent planar capacitance (EPC) of ~300 μF cm(-2) is demonstrated. Zinc oxide (ZnO) and aluminum oxide (Al2O3) bilayer dielectric is deposited on 1 μm high biomimetic silicon nanotip (SiNT) substrate using the atomic layer deposition method. The large EPC is achieved by utilizing the large surface area of the densely packed SiNT (!5 × 10(10) cm(-2)) coated conformally with an ultrahigh dielectric constant of ZnO. The EPC value is 30 times higher than those previously reported in metal-insulator-metal or metal-insulator-semiconductor nanocapacitors using similar porosity dimensions of the support materials. more...

In this study, we find that the optical anisotropy of graphene films could be used as an alternative quality factor for the rapid characterization of large-area graphene films prepared through chemical vapor deposition. We develop an angle-variable spectroscopic method to rapidly determine the optical anisotropy of graphene films. Unlike approaches using Raman scattering spectroscopy, this optical anisotropy method allows ready characterization of the structural quality of large-area graphene samples without the application of high-intensity laser irradiation or complicated optical setups. Measurements of optical anisotropy also allow us to distinguish graphene samples with different extents of structural imperfections; the results are consistent with those obtained from using Raman scattering spectroscopy. In addition, we also study the properties of graphene-based transparent conductive films at wide incident angles because of the advantage of the optical anisotropic properties of graphene. The transmittance of graphene is much higher than that of indium tin oxide films, especially at large incident angles. more...

In this Letter, we investigated the effect of the molecular stacking orientation on the open circuit voltage (VOC) of pentacene-based organic solar cells. Two functionalized pentacenes, namely, 6,13-diphenyl-pentacene (DP-penta) and 6,13-dibiphenyl-4-yl-pentacene (DB-penta), were utilized. Different molecular stacking orientations of the pentacene derivatives from the pristine pentacene were identified by angle-dependent near-edge X-ray absorption fine structure measurements. It is concluded that pentacene molecules stand up on the substrate surface, while both functionalized pentacenes lie down. A significant increase of the VOC from 0.28 to 0.83 V can be achieved upon the utilization of functionalized pentacene, owing to the modulation of molecular stacking orientation, which induced a vacuum-level shift. more...

The effect of direct current (dc) substrate bias on the promotion of nanocrystallization in Si network has been studied, specifically within He-diluted SiH4 plasma in radio frequency (RF)-plasma-enhanced chemical vapor deposition. In view of organizing nanocrystallinity, controlled transmission of energy to the growing surface is needed and that is obtainable from metastable helium (He*) bombardment and, in particular, ionic helium (He+) bombardment under negative substrate bias. The structural morphology has been adequately regulated to a homogeneous network restraining from an exclusive columnar structure that is coherent to low-temperature growth. Notable improvements in the film quality in terms of enhanced crystallinity with low hydrogen content as well as reduced incubation volume, bulk void, and surface roughness have been demonstrated, even at low substrate temperature and low RF power. Use of appropriate dc substrate-bias has been identified as a supplementary parameter efficiently organizing the growth, making it more device-friendly. more...

This study investigates the effect of substrate temperature (T s ) on the boron subphthalocyanine chloride (SubPc) thin film and its power conversion efficiency in SubPc/C 60 heterojunction photovoltaic cells. The orientations of SubPc molecules in thin films determined by X-ray diffraction is strongly correlated with the electronic properties of the organic thin films, and can be controlled by the substrate temperature during the vapor deposition. An optimal substrate temperature of 120 °C has been concluded to induced (221) molecular orientation over the (122) orientation and significantly improve the carrier transport of the SubPc thin film. A SubPc/C 60 heterojunction photovoltaic cells thus fabricated shows higher open-circuit voltage and up to 1.55% conversion efficiency has been achieved, which is attributed to preferential (221) orientation of the SubPc deposited at the elevated temperature. more...

An on-chip growth technique aiming at large-scale production of few-layer epitaxial graphene nanowall (EGNW) arrays by microwave plasma enhanced chemical vapor deposition has been demonstrated. This hetero-architecture is formed by growing edge-oriented SiC nanowalls on Si substrates, followed by surface graphitization, consequently, thus resulting in a heterojunction composed of a 2H-SiC nanowall sheathed by few-layer strained graphene. Similar to epitaxial graphene grown on SiC in an ultrahigh vacuum ambient, structural compressive strain was found in the EGNW and can be relaxed as the layer number of graphene layers increases. More significantly, the SiC-supported strained graphene nanowalls show a remarkably improved catalytic activity ∼425 A/g and low onset potential ∼0.23 V (vs. Ag/AgCl) for the electro-oxidation of methanol as well as excellent pH sensitivity, thus demonstrating their potential applications in sensors, catalyst supports, and other electrochemical devices. more...

Rapid formation of radial p–n junctions on electroless-etched silicon nanowires (SiNWs) was successfully demonstrated. With a low-cost objective, a homemade nonhazardous diffusion source of high phosphor concentration annealed at a small thermal budget was used. The SiNW solar cell, with Au electrodes, has shown a power conversion efficiency of 8.41%, which is higher by 30% compared with its planar counterpart. The SiNW solar cell incorporates an inherent antireflection property, reduced diffusion length requirement, and broad-band spectral quantum efficiency. The evidence of a successful radial p–n junction formation in the NWs has been revealed through the help of a conducting atomic force microscope (AFM) scanning for the photogenerated currents on the fractured surfaces of the NWs. The demonstrated radial junction fabrication technique is believed to reduce the cost of production and promote widespread use of them. more...

Electronic excitations of single ZnO rectangular nanorod have been investigated by electron energy-loss spectroscopy in conjunction with scanning transmission electron microscopy (STEM-EELS). We focus primarily on the surface excitations greatly enhanced at the grazing incidence parallel to the surfaces of ZnO nanorods. An uncommon kind of surface excitation known as surface exciton polaritons occurring near interband transitions is found to dominate in the spectral range between the band gap at 3.4 eV and the surface plasmon peak at 15.8 eV. In addition, the dielectric function of ZnO up to 25 eV has also been derived from the bulk excitation spectra using the Kramers–Kronig analysis on a single nanorod. Theoretical EELS simulations are also compared with the experimental results and good agreements are obtained. more...

One-dimensional semiconductor nanomaterials form the basis for new technologies as well as driving the evolution of existing ones. Although these various technologies are in different stages of development, from nucleation to pilot production, it is difficult to ignore the tremendous potential they carry, overall, for next-generation concepts. One of the several areas that have been revolutionized by one-dimensional nanostructures, and which could not have happened at a more critical time, is energy. This review outlines the impact that one-dimensional semiconductor materials are having on energy production and conversion technologies. Instead of being extensive, we provide key developments in the areas of light-emitting diodes, solar cells, photoelectrochemistry and fuel cells. more...

Systematic investigations of photoelectrochemical behavior between Au nano-particle modified n- and p-GaN were reported in this study. With Au nanoparticles sputtered on the surface, strong Fermi level pinning caused by the creation of metal induced gap states alters the behavior of electrolyte/GaN interface. Under illumination, the photocurrent of p-GaN at zero bias exhibited 25 times enhancement, whereas that of n-GaN showed slight decrease. The overall hydrogen generation efficiency of p-GaN in HCl solution was increased from 0.02% to around 0.59%. The enhancement can be attributed to the different energy shift of the surface band edge at the interface according to the doping of GaN. Keywords: Photoelectrochemistry, Water splitting, GaN, Gold nanoparticle, Hydrogen generation more...

Spectroscopic investigations of individual single-crystalline GaN nanowires with a lateral dimensions of approximately 30-90nm were performed using the spatially resolved technique of electron energy-loss spectroscopy in conjunction with scanning transmission electron microscope showing a 2-A electron probe. Positioning the electron probe upon transmission impact and at aloof setup with respect to the nanomaterials, we explored two types of surface modes intrinsic to GaN, surface exciton polaritons at approximately 8.3eV (approximately 150nm) and surface guided modes at 3.88eV (approximately 320nm), which are in visible/ultra-violet spectral regime above GaN bandgap of approximately 3.3eV (approximately 375nm) and difficult to access by conventional optical spectroscopies. The explorations of these electromagnetic resonances might expand the current technical interests in GaN nanomaterials from the visible/UV range below approximately 3.5eV to the spectral regime further beyond. more...

We propose a nanostructured near infrared photodetector based on indium nitride (InN) nanorod/poly(3-hexylthiophene) hybrids. The current–voltage characteristic of the hybrid device demonstrates the typical p–n heterojunction diode behavior, consisting of p-type polymer and n-type InN nanorods. The device shows a photoresponse range of 900–1260 nm under various reverse biases. An external quantum efficiency of 3.4% at 900 nm operated at −10 V reverse bias was obtained, which is comparable with devices based on lead sulfide and lead selenide hybrid systems. more...

Flexible supercapacitor is successfully fabricated using polyaniline nanowires/carbon cloth (PANI-NWs/CC) nanocomposite. High gravimetric capacitance of 1079 F g−1 at a specific energy of 100.9 Wh kg−1 and a specific power of 12.1 kW kg−1 is obtained. Moreover, this approach also offers an exceptionally high area-normalized capacitance of 1.8 F cm−2. The diffusion length of protons within the PANI-NWs is estimated to be about 60 nm by electrochemical impedance analysis, which indicates that the electrochemical performance of the electrode is not limited by the thickness of PANI-NWs. The electrochemical performance of PANI-NWS/CC remains without any deterioration, even when the cell is bent under high curvature. These results clearly present a cost-effective and simple method of fabrication of the nanostructured polymers with enormous potential in flexible energy storage device applications. more...

Optical reflection, or in other words the loss of reflection, from a surface becomes increasingly crucial in determining the extent of the light-matter interaction. The simplest example of using an anti-reflecting (AR) surface is possibly the solar cell that incorporates an AR coating to harvest sunlight more effectively. Researchers have now found ways to mimic biological structures, such as moth eyes or cicada wings, which have been used for the AR purpose by nature herself. These nanoscopic biomimetic structures lend valuable clues in fabricating and designing gradient refractive index materials that are efficient AR structures. The reflectance from a selected sub-wavelength or gradient index structures have come down to below 1% in the visible region of the spectrum and efforts are on to achieve broader bands of such enhanced AR regime. In addition to the challenge of broader bands, the performance of AR structures is also limited by factors such as omnidirectional properties and polarization of incident light. This review presents selected state-of-the-art AR techniques, reported over the last half a century, and their guiding principles to predict a logical trend for future research in this field. more...

Nanocrystalline diamond (NCD) films were deposited on Si substrates by microwave plasma-enhanced chemical vapor deposition (MPECVD) using methane/hydrogen/oxygen (30/169/0.2 sccm) as process gases. Subsequently a thin (0.33 μm) and a thick (1.01 μm) NCD films were irradiated with XeF excimer laser ( λ = 351 nm) with 300 and 600 mJ cm − 2 of energy densities in air. The NCD films became rougher after laser irradiations. Fraction of graphitic clusters decreased but oxygen content increased in the thin NCD film after laser irradiation. Opposite phenomena were observed for the thick NCD films. Effect of laser irradiation to oxygenation and graphitization of NCD films was correlated with structural properties of free surface and grain boundaries of the thin and thick NCD films. more...

[*] Dr. K. H. Chen, M. H. Wu, W. C. Chen Institute of Atomic and Molecular Sciences, Academia Sinica No. 1, Sec. 4, Roosevelt Rd., Taipei 106 (Taiwan) E-mail: chenkh@pub.iams.sinica.edu.tw Dr. L. C. Chen, Dr. C. H. Chen, Dr. K. H. Chen Center for Condensed Matter Sciences, National Taiwan University No. 1, Sec. 4, Roosevelt Rd., Taipei 106 (Taiwan) E-mail: chenlc@ntu.edu.tw C. H. Lin, Prof. J. H. Lee Graduate Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University No. 1, Sec. 4, Roosevelt Rd., Taipei 106 (Taiwan) more...

Mg þ ions (60 keV) were implanted into GaN nanowires (NWs) with total fluxes of 5 � 10 12 –5 � 10 14 cm �2 followed by thermal annealing at 700 � Ci n N 2 ambient. Transmission electron microscopic images showed amorphous layer formation and defect accumulation in the higher dose Mg-implanted GaN NWs after annealing. Photoluminescence spectra (300 K) of the annealed Mg-implanted GaN NWs exhibited near-band-edge (NBE) emission, donor–acceptor pair (DAP) emission, and defect-related yellow luminescence. With increasing dose, the NBE and DAP emissions are red shifted. Similar phenomena were observed in samples implanted with Ar to produce similar amounts of lattice disorder. The NWs show a much higher sensitivity to defect accumulation than GaN thin films. more...

We report the structure and emission properties of ternary (In,Ga)N nanowires (NWs) embedded with self-assembled quantum dots (SAQDs). InGaN NWs are fabricated by the reaction of In, Ga and NH 3 via a vapor-liquid-solid (VLS) mechanism, using Au as the catalyst. By simply varying the growth temperature, In-rich or Ga-rich ternary NWs have been produced. X-ray diffraction, Raman studies and transmission electron microscopy reveal a phase-separated microstructure wherein the isovalent heteroatoms are self-aggregated, forming SAQDs embedded in NWs. The SAQDs are observed to dominate the emission behavior of both In-rich and Ga-rich NWs. Temperature-dependent photoluminescence (PL) measurements indicate relaxation of excited electrons from the matrix of the Ga-rich NWs to their embedded SAQDs. A multi-level band schema is proposed for the case of In-rich NWs, which showed an anomalous enhancement in the PL peak intensity with increasing temperature accompanies with red shift in its peak position. more...

Nature routinely produces nanostructured surfaces with useful properties, such as the self-cleaning lotus leaf, the colour of the butterfly wing, the photoreceptor in brittlestar and the anti-reflection observed in the moth eye. Scientists and engineers have been able to mimic some of these natural structures in the laboratory and in real-world applications. Here, we report a simple aperiodic array of silicon nanotips on a 6-inch wafer with a sub-wavelength structure that can suppress the reflection of light at a range of wavelengths from the ultraviolet, through the visible part of the spectrum, to the terahertz region. Reflection is suppressed for a wide range of angles of incidence and for both s- and p-polarized light. The antireflection properties of the silicon result from changes in the refractive index caused by variations in the height of the silicon nanotips, and can be simulated with models that have been used to explain the low reflection from moth eyes. The improved anti-reflection properties of the surfaces could have applications in renewable energy and electro-optical devices for the military. more...

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The control of nucleation site size and density for Au catalyst-driven growth of GaN nanowires is reported. By using initial Au film thicknesses of 15–50 A we have shown that annealing between 300 and 900 °C creates Au cluster size in the range 30–100 nm diameter with a cluster density from 300 to 3500 μm−2.Conventional optical lithography to create parallel Au stripes shoes that a minimum separation of ∼15 μm is needed to avoid overlap of wires onto neighboring lines with our growth conditions that yield wires of this same length. The GaN nanowires exhibit strong band-edge photoluminescence and total resistances of 1.2 × 108–5.5 × 106 Ω in the temperature range from 240 to 400 K, as determined for the temperature-dependent current–voltage characteristics. more...

This paper introduces a photo-assisted atomic force microscope (AFM) local oxidation technique which is capable of producing highly smooth oxide patterns with heights reaching several tens of nanometres on both n- and p-types of GaN (and in principle on most semiconductors) without the use of chemicals. The novel methodology relies on UV illumination of the surface of the substrate during conventional AFM local oxidation. A low 1.2 V threshold voltage for n-type GaN was obtained, which can be explained by UV photo-generation of excess electron–hole pairs in the substrate near the junction, thereby reducing the electric field required to drive carrier flow through the tip–sample Schottky barrier. It was demonstrated that the presence or absence of light alone was sufficient to switch the growth of the oxide on or off. The photo-assisted AFM oxidation technique is of immediate interest to the semiconductor industry for the fabrication of GaN-based complementary metal–oxide–semiconductor devices and nanodevices, improves chances for AFM-type data storage, and presents new degrees of freedom for process control technique. more...

Top emission polymer light-emitting devices (PLEDs) with different heat treatments are investigated. Due to the high work function of an indium–tin-oxide (ITO) cathode as a top electrode, LiF is employed as an interlayer to increase electron injection. The PLEDs fabricated at room temperature show better electrical and optical characteristics such as current density–voltage, luminance, and current efficiency, etc., than that of PLED devices with heat treatment. To investigate the Li diffusion in the polyfluorene layer, X-ray photoelectron spectroscopy measurement before and after heat treatment is performed. The poor performance of PLEDs with heat treatments is observed due to the high trap density generated in the film because of the Li diffusion in the polyfluorene layer. more...

Single-crystalline InN nanobelts have been synthesized using Au as the catalyst by a guided-stream thermal chemical vapor deposition technique. The resultant InN nanobelts typically have widths ranging from 20 to 200 nm, a width to thickness ratio of 2-10, and lengths of up to several tens of micrometers. Structural analysis shows that these InN nanobelts have a wurtzite structure and exhibit a rectangular cross section with self-selective facets, i.e., the nanobelts are enclosed only by ±(001) and ±(110) planes with [110] being the exclusive growth direction along their long axis. This facet selectivity can be understood by the differences in the surface energies of the different facets. Photoluminescence (PL) spectra of InN nanobelts show a sharp infrared emission peak at 0.76 eV with a full width at half maximum of 14 meV, narrower than the values reported for InN epilayers. The integrated PL intensity is found to increase linearly with the excitation power, which suggests that the observed PL can be attributed to direct band-to-band emission. more...

InGaN/GaN multiple quantum well structures have been grown on sapphire substrates by metalorganic chemical vapor deposition, for wide range visible light emitting diode application. The compositions and sizes within quantum wells were designed according to the requirements on the LED performance. Samples were investigated by a variety of characterization techniques. Optimization of the growth parameters and process was realized and evidenced by high resolution X-ray diffraction measurements. Optical spectroscopic properties were further studied and quantum confined stokes shift was observed from room temperature photoluminescence and photoluminescence excitation measurements. more...

The formation of voids and bubbles in the energetic ion implantation process is an important issue in material science research, involving swelling induced embrittlement of materials in nuclear reactors, catalytic activities in the nanopores of the bubble, etc. We report here the formation and in situ dynamics of metallic nanoblisters in GaN nanowires under self-ion implantation using a Ga + focused ion beam. High-resolution transmission electron microscopes equipped with electron energy loss spectroscopy and energy filtering are used to identify the constituents of the blister. In situ monitoring, with focused ion beam imaging, revealed the translation and rotation dynamics of the blisters. more...