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.

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.

Nanoscale dynamic behavior of surface magnetic domains on a La0.7Sr0.3MnO3 thin film.

An oscillating magnetic tip can be used to induce the striped magnetic ripple pattern with alternating up-and-down striped magnetic domains on a ferromagnetic La0.7Sr0.3MnO3 (LSMO) thin film surface. Magnetic force microscopy (MFM) images show that the surface magnetic domains (SMDs) can be aligned in a well-ordered alternating up-and-down c(2 x 2) structure on the stripe magnetic domains, indicating that the oscillating magnetic tip turns the ferromagnetic LSMO surface into a canted antiferromagnetic state. The orientation of the SMDs is determined by their discrete phase distribution. A three-dimensional (3D) SMD orientation model is built to understand dynamic behavior of the SMDs.

Effective photoluminescence in a large-area array of Ta2O5 nanodots.

We describe here the synthesis of a large-area Ta2O5 nanodot array by utilizing the hot filament metal vapor deposition technique. The Ta2O5 nanodots arranged in a large-area array on a Si wafer had an average diameter of -8 nm. X-ray photoemission spectroscopy (XPS) revealed the stoichiometric Ta and O compositions of the Ta2O5 nanodots. Raman spectroscopy showed the Ta2O5 nanodots to be of orthorhombic (beta) crystal. Photoluminescence (PL) spectroscopy showed the green and red light emissions of the beta-Ta2O5 nanodots at room temperature.

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.

Enhancement of green-light photoluminescence of Ta2O5 nanoblock stacks.

In this study we have explored the structural, electronic, and photoluminescence (PL) properties of Ta(2)O(5) nanoblock stacks. The Ta(2)O(5) nanoblocks were synthesized by the hot filament metal-oxide vapor deposition (HFMOVD) technique and randomly arranged in large-area stacks. Field-emission scanning electron microscopy (FESEM) showed most of the stacking Ta(2)O(5) nanoblocks to be 21 nm wide. Energy dispersive spectroscopy (EDS) analysis verified the presence of only the elements Ta and O. X-Ray photoemission spectroscopy (XPS) not only revealed the electronic structures and chemical properties of the stacking Ta(2)O(5) nanoblocks but also their stoichiometric Ta/O ratio of ∼0.416 (i.e. Ta:O = 2.08 : 5). Photoluminescence (PL) spectroscopy showed very strong green-light emissions, which emerged from the trap-levels of the oxygen vacancies within the Ta(2)O(5) bandgap. The PL intensities were linearly enhanced by increasing the laser power and the excitation time. The PL results suggest that the nanoblocks are excellent visible-light emitters.

Scanning photoemission spectromicroscopic study of 4-nm ultrathin SiO(3.4) protrusions probe-induced on the native SiO(2) layer.

Atomic force microscopy probe-induced large-area ultrathin SiO(x) (x ≡ O/Si content ratio and x > 2) protrusions only a few nanometers high on a SiO(2) layer were characterized by scanning photoemission microscopy (SPEM) and X-ray photoemission spectroscopy (XPS). SPEM images of the large-area ultrathin SiO(x) protrusions directly showed the surface chemical distribution and chemical state specifications. The peak intensity ratios of the XPS spectra of the large-area ultrathin SiO(x) protrusions provided the elemental quantification of the Si 2p core levels and Si oxidation states (such as the Si(4+), Si(3+), Si(2+), and Si(1+) species). The O/Si content ratio (x) was evidently determined by the height of the large-area ultrathin SiO(x) protrusions.

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.

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.

Spontaneous self-organization of Cu2O/CuO core-shell nanowires from copper nanoparticles.

We report on the formation and spontaneous self-organization of Cu(2)O/CuO core-shell nanowires from individual copper nanoparticles. The growth process is interpreted using the results of time-dependent in situ x-ray diffraction. High-resolution transmission electron microscopy is used to observe the intermediate state of pearl-necklace-like aggregates that form a chain-like configuration of Cu(2)O nanoparticles intertwined into nanowires. The existence of an amorphous CuO shell is confirmed by the XANES technique and explained through an intensity simulation using a proposed core-shell nanowire model.

Creation and manipulation of surface magnetic domains in an alternating up-and-down pattern.

According to Lenz's law, the magnetic field from the oscillating magnetic probe will induce out-of-plane surface magnetic domains (SMDs) from the in-plane magnetization at the locally tapped points on a ferromagnetic La(0.7)Sr(0.3)MnO3 (LSMO) thin film. It was possible to control and manipulate the out-of-plane SMDs by varying the tapping intervals and changing the scanning direction. We also found that the anisotropic stresses from the out-of-plane SMDs caused the appearance of large-area straight striped domain structures on the order of several micrometers. Smaller oscillating magnetic probe tapping intervals produced larger periods (or widths) of the straight striped domain structure.

Comparative atomic force and scanning electron microscopy: an investigation of structural differentiation of hepatic stellate cells.

The molecular mechanism leading to the transdifferentiation of hepatic stellate cells (HSC) into myofibroblast-like cells following liver injury is not well understood. The state of cultured rat HSCs was determined using primarily fluorescence microscopy (UV), immunofluorescence (IF) (Glial fibrillary acidic protein (GFAP), Desmin, alpha-smooth muscle actin (alpha-SMA), F-actin) and immunocytochemistry (ICC) (GFAP, Desmin, alpha-SMA, Fibulin-2). Additionally, tapping-mode atomic force microscopy (TM-AFM) and field-emission scanning electron microscopy (FE-SEM) with low-resistivity indium-tin-oxide (ITO) thin-film were performed to observe the micro-morphological character of cells during HSC differentiation. Quiescent HSCs changed to the activated state were identified via UV, IF, and ICC observations. Normal rat HSCs (NHSCs) and thioacetamide-induced rat HSCs (THSCs) were demonstrated to be UV(-), GFAP(+), Desmin(+), alpha-SMA(+) and Fibulin-2(-). After F-actin staining, lamellipodia and filopodia were found in both NHSCs and THSCs, but membrane ruffles were only seen in THSCs. The micro-structures of lamellipodia and filopodia in both NHSCs and THSCs were confirmed using FE-SEM and TM-AFM with ITO; in contrast, the micro-projection was not found. Moreover, "aerial root" structures were observed for the first time in the filopodia of THSCs using TM-AFM. These results reveal that HSC transdifferentiation to a myofibroblastic-like cell (activated HSC) from thioacetamide-induced rat HSC induces extensive changes in the cytoskeleton.

High room-temperature photoluminescence of one-dimensional Ta2O5 nanorod arrays.

In this study we analyzed the structural and electronic properties of a new morphological form, one-dimensional (1D) Ta2O5 nanorod arrays, which were synthesized by hot filament metal vapor deposition. Field-emission scanning electron microscopy (FESEM) showed the 1D Ta2O5 nanorods to be arranged in a large-area high-density array about 50 nm wide and approximately 550 nm long. X-ray photoemission spectroscopy (XPS) revealed not only the electronic structures and chemical properties of the 1D Ta2O5 nanorods but also their stoichiometric Ta and O compositions. Photoluminescence (PL) spectra showed intensive green-light, yellow-light and red-light emissions at room temperature. These emissions simultaneously emerged from the trap levels of oxygen vacancies within the Ta2O5 bandgap. The emission results strongly indicate that the 1D Ta2O5 nanorods are good room-temperature visible-light emitters.

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.

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.

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.