Enhancing the Hydrogen Evolution Performance of Tungsten Diphosphide on Carbon Fiber through Ruthenium Modification.

Hydrogen-based energy systems hold promise for sustainable development and carbon neutrality, minimizing environmental impact with electrolysis as the preferred fossil-fuel-free hydrogen generation method. Effective electrocatalysts are required to reduce energy consumption and improve kinetics, given the need for additional voltage (overpotential, η) despite the theoretical water splitting potential of 1.23 V. To date, platinum has been acknowledged as the most effective but expensive hydrogen evolution reaction (HER) catalyst. Hence, we introduce a cost-effective (∼2-fold cheaper) ruthenium-modified tungsten diphosphide (Ru/WP2) catalyst on carbon fiber for HER in ∼0.5 M H2SO4, with η ≈ 34 mV at -10 mA cm-2 which can be comparable (only ∼2-fold higher) to benchmark Pt/C (η ≈ 17 mV). The HER performance of WP2 can be enhanced through the modification of ruthenium, as indicated by the electrochemical characterizations. Considering the Tafel value of ∼40 ± 0.2 mV dec-1, it can be inferred that Ru/WP2 follows the Volmer-Heyrovsky reaction pathway for hydrogen generation. Furthermore, the Faradaic efficiency estimation indicates that Ru/WP2 demonstrates a minimal loss of electrons during the electrochemical reaction with an estimated value of ∼98.7 ± 1.4%. Therefore, this study could emphasize the potential of the Ru/WP2 electrode in advancing sustainable hydrogen production through water splitting.

On-Chip Monolithically Integrated Ultraviolet Low-Threshold Plasmonic Metal-Semiconductor Heterojunction Nanolasers.

The metal-semiconductor heterojunction is imperative for the realization of electrically driven nanolasers for chip-level platforms. Progress in developing such nanolasers has hitherto rarely been realized, however, because of their complexity in heterojunction fabrication and the need to use noble metals that are incompatible with microelectronic manufacturing. Most plasmonic nanolasers lase either above a high threshold (101 -103 MW cm-2 ) or at a cryogenic temperature, and lasing is possible only after they are removed from the substrate to avoid the large ohmic loss and the low modal reflectivity, making monolithic fabrication impossible. Here, for the first time, record-low-threshold, room-temperature ultraviolet (UV) lasing of plasmon-coupled core-shell nanowires that are directly grown on silicon is demonstrated. The naturally formed core-shell metal-semiconductor heterostructure of the nanowires leads to a 100-fold improvement in growth density over previous results. This unprecedentedly high nanowire density creates intense plasmonic resonance, which is outcoupled to the resonant Fabry-Pérot microcavity. By boosting the emission strength by a factor of 100, the hybrid photonic-plasmonic system successfully facilitates a record-low laser threshold of 12 kW cm-2 with a spontaneous emission coupling factor as high as ≈0.32 in the 340-360 nm range. Such architecture is simple and cost-competitive for future UV sources in silicon integration.

Binder free cobalt iron phosphate thin films as efficient electrocatalysts for overall water splitting.

Designing nanostructure based robust catalyst for the electrochemical water splitting is the great task in the energy conversion field to accomplish high electrical conductivity, low overpotential and long lasting activity. Herein, the electrochemical overall water splitting is reported by using the hydrothermally synthesized binder free cobalt iron phosphate thin films on low cost stainless steel substrates as a conducting backbone for the first time. The effect of composition ratio variation of cobalt and iron was studied on the structural, compositional, morphological, and surface electronic properties by conducting various characterizations which results in amorphous hydrous cobalt iron phosphate having mesoporosity. The as synthesized cobalt iron phosphate having composition ratio (50:50 of Co:Fe) exhibits excellent electrochemical OER and HER catalytic water splitting performance. Best performing electrode exhibits smallest overpotentials of 251.9 mV and 55.5 mV for OER and HER respectively at 10 mA/cm2 current density. To split water molecule into the H2 and O2 by overall water splitting in same alkaline medium, the potential of 1.75 V was required after long duration (100 h) catalysis. Overall analysis confirms the cobalt iron phosphate thin films are outstanding and robust for the hydrogen production as clean renewable energy source.

Multifunctional plasmonic gold nanostars for cancer diagnostic and therapeutic applications.

Gold nanostar (AuNSt) has gained great attention in bioimaging and cancer therapy due to their tunable surface plasmon resonance across the visible-near infrared range. Photothermal treatment and imaging capabilities including fluorescence lifetime imaging at two-photon excitation (TP-FLIM) and dark-field microscopic imaging are considered in this work. Two types of AuNSts having plasmon absorption peaks centred at 600 and 750 nm wavelength were synthesized and studied. Both NSts exhibited low cytotoxicity on A549 human lung carcinoma cells. A strong emission at two-photon excitation was observed for both NSts, well-distinguishable from lifetimes of bio-object autofluorescence. High efficiency in raising the temperature in the NSts environment with the irradiation of near infrared, AuNSts triggered photothermal effect. The decreased cell viability of A549 observed via MTT test and the cell membrane damaging was demonstrated with trypan blue staining. These results suggest AuNSts can be agents with tunable plasmonic properties for imaging and photothermal therapy.

Ultralow-Threshold Continuous-Wave Room-Temperature Crystal-Fiber/Nanoperovskite Hybrid Lasers for All-Optical Photonic Integration.

Continuous-wave (CW) room-temperature (RT) laser operation with low energy consumption is an ultimate goal for electrically driven lasers. A monolithically integrated perovskite laser in a chip-level fiber scheme is ideal. However, because of the well-recognized air and thermal instabilities of perovskites, laser action in a perovskite has mostly been limited to either pulsed or cryogenic-temperature operations. Most CW laser operations at RT have had poor durability. Here, crystal fibers that have robust and high-heat-load nature are shown to be the key to enabling the first demonstration of ultralow-threshold CW RT laser action in a compact, monolithic, and inexpensive crystal fiber/nanoperovskite hybrid architecture that is directly pumped with a 405 nm diode laser. Purcell-enhanced light-matter coupling between the atomically smooth fiber microcavity and the perovskite nanocrystallites gain medium enables a high Q (≈1500) and a high ß (0.31). This 762 nm laser outperforms previously reported structures with a record-low threshold of 132 nW and an optical-to-optical slope conversion efficiency of 2.93%, and it delivers a stable output for CW and RT operation. These results represent a significant advancement toward monolithic all-optical integration.

Doping-free bandgap tunability in Fe2O3 nanostructured films.

A tunable bandgap without doping is highly desirable for applications in optoelectronic devices. Herein, we develop a new method which can tune the bandgap without any doping. In the present research, the bandgap of Fe2O3 nanostructured films is simply tuned by changing the synthesis temperature. The Fe2O3 nanostructured films are synthesized on ITO/glass substrates at temperatures of 1100, 1150, 1200, and 1250 °C using the hot filament metal oxide vapor deposition (HFMOVD) and thermal oxidation techniques. The Fe2O3 nanostructured films contain two mixtures of Fe2+ and Fe3+ cations and two trigonal (α) and cubic (γ) phases. The increase of the Fe2+ cations and cubic (γ) phase with the elevated synthesis temperatures lifted the valence band edge, indicating a reduction in the bandgap. The linear bandgap reduction of 0.55 eV without any doping makes the Fe2O3 nanostructured films promising materials for applications in bandgap engineering, optoelectronic devices, and energy storage devices.

Correction: Doping-free bandgap tuning in one-dimensional Magnéli-phase nanorods of Mo4O11.

Correction for 'Doping-free bandgap tuning in one-dimensional Magnéli-phase nanorods of Mo4O11' by Duy Van Pham et al., Nanoscale, 2016, 8, 5559-5566.

Temperature-dependent ultraviolet photoluminescence in hierarchical Zn, ZnO and ZnO/Zn nanostructures.

The temperature-dependent effects on the ultraviolet (UV) photoluminescence (PL) enhancement, blueshift, thermal quenching, and chromaticity of the two-dimensional (2D) Zn nanosheets, 2D-hierarchical ZnO nanostructures, and 2D-hierarchical ZnO/Zn nanostructures are presented. In this study, 2D Zn nanosheets were synthesized using a hot-plate metal vapor deposition technique, after which 2D-hierarchical ZnO nanostructures and ZnO/Zn were prepared from the 2D Zn nanosheets by a simple thermal annealing method. The enhancement and blueshift of the UV PL emissions from the three nanostructures at low temperatures arose from three distinct PL mechanisms. For the ZnO nanostructures, the UV PL emission enhancements and blueshifts at low temperature were due to the conversion of the free excitons (FXs) to neutral-donor-bound-excitons (D0Xs). The ZnO/Zn nanostructures possessed the highest UV PL intensities among the three nanostructures, because the free electrons from the Zn portions across the metal-semiconductor heterojunctions greatly assisted in enhancing the PL emissions. The enhancement and thermal quenching were quantitatively analyzed with simple normalization methods. The results show that all three kinds of nanostructures are excellent candidates for use in UV light emitters.

Role of cobalt cations in short range antiferromagnetic Co3O4 nanoparticles: a thermal treatment approach to affecting phonon and magnetic properties.

We report the phonon and magnetic properties of various well-stabilized Co3O4 nanoparticles. The net valence in cobalt (II)/(III) cation can be obtained by subtracting the Co2+ ions in tetrahedral interstices and Co3+ ions in the octahedral interstices, respectively, which will possess spatial inhomogeneity of its magnetic moment via Co2+ in tetrahedra and Co3+ in octahedral configurations in the normal spinel structure. Furthermore, the distribution of Co2+/Co3+ governed by various external (magnetic field and temperature) and internal (particle size and slightly distorted CoO6 octahedra) sources, have led to phenomena such as a large redshift of phonon-phonon interaction and short-range magnetic correlation in the inverse spinel structure. The outcome of our study is important in terms of the future development of magnetic semiconductor spintronic devices of Co3O4.

Probing the Mechanism for Bipolar Resistive Switching in Annealed Graphene Oxide Thin Films.

The bipolar resistive switching (BRS) between a metallic low resistance state (LRS) and an insulating high resistance state (HRS) is demonstrated for annealed graphene oxide (GO) thin film-based device structures with aluminum (Al) as one of the contact electrodes. An optimal switching of ∼104 order is recorded for Al/GO (200 °C)/indium tin oxide (ITO) among the device structures in metal (M2)/GO (T)/metal (M1) configurations (M1 = Al, Au, or ITO and M2 = Au or Al), fabricated using GO (T)/metal (M1), annealed at different temperatures, T = 100, 200, 300, and 400 °C. The initial Ohmic conduction for electronic transport and the presence of metal contents through GO thin films in the X-ray photoelectron spectroscopy support the physical evidence of Al filament formation between the two electrodes as imaged by the high-resolution transmission electron microscopy. The speculated mechanism for BRS in repeated voltage sweep cycles is attributed to the current triggered breaking of metal filaments because of the combined effect of Joule's heating and Peltier heat generation at LRS → HRS transition, and electric field induced migration of metal atoms, leading to the formation of metal filaments through the GO film at the HRS → LRS transition. The higher switching ratio exhibited in the current study could be translated to engineer simple and low-cost resistive memory devices.

Copper Oxide/Reduced Graphene Oxide Nanocomposite-Catalyzed Synthesis of Flavanones and Flavanones with Triazole Hybrid Molecules in One Pot: A Green and Sustainable Approach.

An efficient, green, and sustainable synthesis of new hybrid molecules containing flavanone with triazole by merging the Michael addition and Click reaction using a copper oxide/reduced graphene oxide nanocomposite in one pot is reported. The catalyst can easily be recycled and reused in seven consecutive runs without compromising the product yields. Other notable advantages include using water as a reaction medium and obtaining good to excellent yields, low catalyst loading, high atom efficiency, high substrate variation, and good results in the gram scale reaction.

Spitzer shaped ZnO nanostructures for enhancement of field electron emission behaviors.

We observed enhanced field emission (FE) behavior for spitzer shaped ZnO nanowires synthesized via a hydrothermal approach. The spitzer shaped and pointed tipped 1D ZnO nanowires of average diameter 120 nm and length ∼5-6 µm were randomly grown over an ITO coated glass substrate. The turn-on field (E on) of 1.56 V µm-1 required to draw a current density of 10 µA cm-2 from these spitzer shaped ZnO nanowires is significantly lower than that of pristine and doped ZnO nanostructures, and MoS2@TiO2 heterostructure based FE devices. The orthodoxy test that was performed confirms the feasibility of a field enhancement factor (ß FE) of 3924 for ZnO/ITO emitters. The enhancement in FE behavior can be attributed to the spitzer shaped nanotips, sharply pointed nanotips and individual dispersion of the ZnO nanowires. The ZnO/ITO emitters exhibited very stable electron emission with average current fluctuations of ±5%. Our investigations suggest that the spitzer shaped ZnO nanowires have potential for further improving in electron emission and other functionalities after forming tunable nano-hetero-architectures with metal or conducting materials.

Nano-Heteroarchitectures of Two-Dimensional MoS2@ One-Dimensional Brookite TiO2 Nanorods: Prominent Electron Emitters for Displays.

We report comparative field electron emission (FE) studies on a large-area array of two-dimensional MoS2-coated @ one-dimensional (1D) brookite (ß) TiO2 nanorods synthesized on Si substrate utilizing hot-filament metal vapor deposition technique and pulsed laser deposition method, independently. The 10 nm wide and 760 nm long 1D ß-TiO2 nanorods were coated with MoS2 layers of thickness ∼4 (±2), 20 (±3), and 40 (±3) nm. The turn-on field (E on) of 2.5 V/µm required to a draw current density of 10 µA/cm2 observed for MoS2-coated 1D ß-TiO2 nanorods emitters is significantly lower than that of doped/undoped 1D TiO2 nanostructures, pristine MoS2 sheets, MoS2@SnO2, and TiO2@MoS2 heterostructure-based field emitters. The orthodoxy test confirms the viability of the field emission measurements, specifically field enhancement factor (ßFE) of the MoS2@TiO2/Si emitters. The enhanced FE behavior of the MoS2@TiO2/Si emitter can be attributed to the modulation of the electronic properties due to heterostructure and interface effects, in addition to the high aspect ratio of the vertically aligned TiO2 nanorods. Furthermore, these MoS2@TiO2/Si emitters exhibit better emission stability. The results obtained herein suggest that the heteroarchitecture of MoS2@ß-TiO2 nanorods holds the potential for their applications in FE-based nanoelectronic devices such as displays and electron sources. Moreover, the strategy employed here to enhance the FE behavior via rational design of heteroarchitecture structure can be further extended to improve other functionalities of various nanomaterials.

Impact of cuticle photoluminescence on the color morphism of a male damselfly Ischnura senegalensis (Rambur, 1842).

In this study the damselfly Ischnura senegalensis (Rambur, 1842) was first found to produce strong photoluminescence (PL) emissions from various colored-body portions, such as the eighth abdominal segment of the tail. The colors of the colored-body portions can be enhanced or modified by the PL emissions for assistance in reducing intrasexual and male harassment, and improving mature mating and conspecific identity. Therefore, the PL emissions that contribute to the color modification and coloration are involved in the cuticle evolution of the damselflies. The micro-PL confocal images verify that the PL emissions can strongly influence the surface colors of the cuticle, and demonstrate why the damselfly Ischnura senegalensis is called a bluetail.

Atomically smooth hybrid crystalline-core glass-clad fibers for low-loss broadband wave guiding.

We demonstrate direct evidence for the first realization of atomically smooth sapphire crystalline fiber cores with a surface variation of only ~1.9 Å. The hybrid glass-clad crystalline cores were grown by a laser-based fiber drawing technique. Because of the improvement in crystal fiber quality, we were able, for the first time, to comprehensively and quantitatively elucidate the correlation between fiber nanostructure and optical loss. We also experimentally demonstrated that high-temperature treatment has a significant impact on defect relaxation and promotes excellent crystallinity, and hence enables low-loss optical wave guiding. The experimentally measured propagation losses in the order of 0.01-0.1 dB/cm are the lowest ever reported among conventional Ti:sapphire channel waveguides and ultrafast-laser-inscribed waveguides, and agree well with the theory. Through experiments and numerical calculation, we have demonstrated that low threshold and high efficiency of Ti:sapphire crystal fiber lasers are possible with the atomic-level roughness, low-loss propagation, and high crystallinity of the Ti:sapphire crystalline core.

Terahertz spin-wave waveguides and optical magnonics in one-dimensional NiO nanorods.

The two-magnon (2M) spin waves with a magnon frequency of 43 THz, generated by a polarized laser, were first observed in one-dimensional (1D) NiO nanorods. The 1D NiO nanorods of ∼700 nm length, which have perfectly in-plane antiferromagnetic spins lying on the (200) and (100) faces, are the smallest spin-wave waveguides. Due to the magneto-optical Faraday effect (MOFE), the significant change in the Faraday intensity can show the 2M information in the NiO nanorods. There are only two 2M-on and 2M-off states at various applied alternating-current magnetic fields and laser-incident angles, which make the 1D NiO nanorods excellent optical nanomagnonics.

Impact of Nanosize on Supercapacitance: Study of 1D Nanorods and 2D Thin-Films of Nickel Oxide.

We synthesized unique one-dimensional (1D) nanorods and two-dimensional (2D) thin-films of NiO on indium-tin-oxide thin-films using a hot-filament metal-oxide vapor deposition technique. The 1D nanorods have an average width and length of ∼100 and ∼500 nm, respectively, and the densely packed 2D thin-films have an average thickness of ∼500 nm. The 1D nanorods perform as parallel units for charge storing. However, the 2D thin-films act as one single unit for charge storing. The 2D thin-films possess a high specific capacitance of ∼746 F/g compared to 1D nanorods (∼230 F/g) using galvanostatic charge-discharge measurements at a current density of 3 A/g. Because the 1D NiO nanorods provide more plentiful surface areas than those of the 2D thin-films, they are fully active at the first few cycles. However, the capacitance retention of the 1D nanorods decays faster than that of the 2D thin-films. Also, the 1D NiO nanorods suffer from instability due to the fast electrochemical dissolution and high nanocontact resistance. Electrochemical impedance spectroscopy verifies that the low dimensionality of the 1D NiO nanorods induces the unavoidable effects that lead them to have poor supercapacitive performances. On the other hand, the slow electrochemical dissolution and small contact resistance in the 2D NiO thin-films favor to achieve high specific capacitance and great stability.

Doping-free bandgap tuning in one-dimensional Magnéli-phase nanorods of Mo4O11.

We synthesized one-dimensional (1D) Magnéli-phase nanorods of Mo4O11 using the hot filament metal-oxide vapor deposition technique. The 1D Magnéli-phase Mo4O11 nanorods synthesized at 1000, 1050, 1100, 1150, and 1200 °C contain varying combinations of two orthorhombic (α) and monoclinic (η) phases, and various mixtures of Mo(4+), Mo(5+) and Mo(6+) cations, while those synthesized at a higher temperature look bluer. The shifts of the transmittance maximum and absorbance minimum of the 1D Magnéli-phase Mo4O11 nanorods are inversely and linearly proportional to the elevated temperature, verifying that the bandgaps (Eg) are inversely proportional to the elevated temperature. The bandgap (Eg) of the 1D Magnéli-phase Mo4O11 nanorods can be tuned by simply controlling the synthesis temperature without doping with other materials, giving the 1D Magnéli-phase Mo4O11 nanorods good potential for use in optoelectronic nanodevices and bandgap engineering.

Size-controllable synthesis of Bi/Bi2O3 heterojunction nanoparticles using pulsed Nd:YAG laser deposition and metal-semiconductor-heterojunction-assisted photoluminescence.

We synthesized Bi/Bi2O3 heterojunction nanoparticles at various substrate temperatures using the pulsed laser deposition (PLD) technique with a pulsed Nd:YAG laser. The Bi/Bi2O3 heterojunction nanoparticles consisted of Bi nanoparticles and Bi2O3 surface layers. The average diameter of the Bi nanoparticles and the thickness of the Bi2O3 surface layer are linearly proportional to the substrate temperature. The heterojunctions between the Bi nanoparticles and Bi2O3 surface layers, which are the metal-semiconductor heterojunctions, can strongly enhance the photoluminescence (PL) of the Bi/Bi2O3 nanoparticles, because the metallic Bi nanoparticles can provide massive free Fermi-level electrons for the electron transitions in the Bi2O3 surface layers. The enhancement of PL emission at room temperature by metal-semiconductor-heterojunctions make the Bi/Bi2O3 heterojunction nanoparticles potential candidates for use in optoelectronic nanodevices, such as light-emitting diodes (LEDs) and laser diodes (LDs).

Ligand-Driven and Full-Color-Tunable Fiber Source: Toward Next-Generation Clinic Fiber-Endoscope Tomography with Cellular Resolution.

In many biomedical applications, broad full-color emission is important, especially for wavelengths below 450 nm, which are difficult to cover via supercontinuum generation. Single-crystalline-core sapphires with defect-driven emissions have potential roles in the development of next-generation broadband light sources because their defect centers demonstrate multiple emission bands with tailored ligand fields. However, the inability to realize high quantum yields with high crystallinity by conventional methods hinders the applicability of ultra-broadband emissions. Here, we present how an effective one-step fiber-drawing process, followed by a simple and controllable thermal treatment, enables a low-loss, full-color, and crystal fiber-based generation with substantial color variability. The broad spectrum extends from 330 nm, which is over 50 nm further into the UV region than that in previously reported results. The predicted submicrometer spatial resolutions demonstrate that the defect-ligand fields are potentially beneficial for achieving in vivo cellular tomography. It is also noteworthy that the efficiency of the milliwatt-level full-color generation, with an optical-to-optical efficiency of nearly 5%, is the highest among that of the existing active waveguide schemes. In addition, direct evidence from high-resolution transmission electron microscopy together with electron energy loss spectroscopy and crystal-field ligands reveals an excellent crystalline core, atomically defined core/cladding interfacial roughness, and significant enhancements in new laser-induced electronic defect levels. Our work suggests an inexpensive, facile, and highly scalable route toward achieving cellular-resolution tomographic imaging and represents an important step in the development of endoscope-compatible diagnostic devices.