Distinguishing the Photothermal and Photoinjection Effects in Vanadium Dioxide Nanowires

Vanadium dioxide (VO₂) has drawn significant attention for its unique metal-to-insulator transition near the room temperature. The high electrical resistivity below the transition temperature (∼68 °C) is a result of the strong electron correlation with the assistance of lattice (Peierls) distortion. Theoretical calculations indicated that the strong interelectron interactions might induce intriguing optoelectronic phenomena, such as the multiple exciton generation (MEG), a process desirable for efficient optoelectronics and photovoltaics. However, the resistivity of VO₂ is quite temperature sensitive, and therefore, the light-induced conductivity in VO₂ has often been attributed to the photothermal effects. In this work, we distinguished the photothermal and photoinjection effects in VO₂ nanowires by varying the chopping frequency of the optical illumination. We found that, in our VO₂ nanowires, the relatively slow photothermal processes can be well suppressed when the chopping frequency is >2 kHz, whereas the fast photoinjection component (direct photoexcitation of charge carriers) remains constant at all chopping frequencies. By separating the photothermal and photoinjection processes, our work set the basis for further studies of carrier dynamics under optical excitations in strongly correlated materials

Widely Tunable Distributed Bragg Reflectors Integrated into Nanowire Waveguides

Periodic structures with dimensions on the order of the wavelength of light can tailor and improve the performance of optical components, and they can enable the creation of devices with new functionalities. For example, distributed Bragg reflectors (DBRs), which are created by periodic modulations in a structure’s dielectric medium, are essential in dielectric mirrors, vertical cavity surface emitting lasers, fiber Bragg gratings, and single-frequency laser diodes. This work introduces nanoscale DBRs integrated directly into gallium nitride (GaN) nanowire waveguides. Photonic band gaps that are tunable across the visible spectrum are demonstrated by precisely controlling the grating’s parameters. Numerical simulations indicate that in-wire DBRs have significantly larger reflection coefficients in comparison with the nanowire’s end facet. By comparing the measured spectra with the simulated spectra, the index of refraction of the GaN nanowire waveguides was extracted to facilitate the design of photonic coupling structures that are sensitive to phase-matching conditions. This work indicates the potential to design nanowire-based devices with improved performance for optical resonators and optical routing

Plasmon-Enhanced Photocatalytic Activity of Iron Oxide on Gold Nanopillars

Photocatalytic water splitting represents a promising way to produce renewable hydrogen fuel from solar energy. Ultrathin semiconductor electrodes for water splitting are of particular interest because the optical absorption occurs in the region where photogenerated charge carriers can effectively contribute to the chemical reactions on the surface. It is therefore important to manipulate and concentrate the incident light so that more photons can be absorbed within the thin film. Here we show an enhanced photocurrent in a thin-film iron oxide photoanode coated on arrays of Au nanopillars. The enhancement can be attributed primarily to the increased optical absorption originating from both surface plasmon resonances and photonic-mode light trapping in the nanostructured topography. The resonances can be tuned to a desirable wavelength by varying the thickness of the iron oxide layer. A net enhancement as high as 50% was observed over the solar spectrum

Strong Chiroptical Activities in Gold Nanorod Dimers Assembled Using DNA Origami Templates

Asymmetric three-dimensional (3D) nanoarchitectures that cannot coincide with their mirrored-symmetric counterparts are known as chiral objects. Numerous studies have focused on chiral plasmonic nanoarchitectures created intentionally with 3D asymmetric configurations, whose plasmonic chirality is promising for various nanoplasmonic and nanophotonic applications. Here, we show that gold nanorod (AuNR) plasmonic nanoarchitectures assembled on a soft 2D DNA origami template, which was often simplified to be a rigid rectangle, can exhibit strong chiroptical activities. The slight flexibility of the origami templates was found to play a critical role in inducing the plasmonic chirality of the assembled nanoarchitectures. Our study set a new example of reflecting the native conformation of nanostructures using chiral spectroscopy and can inspire the exploration of the softness of DNA templates for the future design of assembled chiral nanoarchitectures

Effect of Thermal Annealing in Ammonia on the Properties of InGaN Nanowires with Different Indium Concentrations

The utility of an annealing procedure in ammonia ambient is investigated for improving the optical characteristics of In_<i>x</i>Ga_1–<i>x</i>N nanowires (0.07 ≤ <i>x</i> ≤ 0.42) grown on c-Al₂O₃ using a halide chemical vapor deposition method. Morphological studies using scanning electron microscopy confirm that the nanowire morphology is retained after annealing in ammonia at temperatures up to 800 °C. However, significant indium etching and composition inhomogeneities are observed for higher indium composition nanowires (<i>x</i> = 0.28, 0.42), as measured by energy-dispersive X-ray spectroscopy and <i>Z</i>-contrast scanning transmission electron microscopy. Structural analyses, using X-ray diffraction and high-resolution transmission electron microscopy, indicate that this is a result of the greater thermal instability of higher indium composition nanowires. The effect of these structural changes on the optical quality of InGaN nanowires is examined using steady-state and time-resolved photoluminescence measurements. Annealing in ammonia enhances the integrated photoluminescence intensity of In_<i>x</i>Ga_1–<i>x</i>N nanowires by up to a factor of 4.11 ± 0.03 (for <i>x</i> = 0.42) by increasing the rate of radiative recombination. Fitting of photoluminescence decay curves to a Kohlrausch stretched exponential indicates that this increase is directly related to a larger distribution of recombination rates from composition inhomogeneities caused by annealing. The results demonstrate the role of thermal instability on the improved optical properties of InGaN nanowires annealed in ammonia

Composite Perovskites of Cesium Lead Bromide for Optimized Photoluminescence

The halide perovskite CsPbBr₃ has shown its promise for green light-emitting diodes. The optimal conditions of photoluminescence and the underlying photophysics, however, remain controversial. To address the inconsistency seen in the previous reports and to offer high-quality luminescent materials that can be readily integrated into functional devices with layered architecture, we created thin films of CsPbBr₃/Cs₄PbBr₆ composites based on a dual-source vapor-deposition method. With the capability of tuning the material composition in a broad range, CsPbBr₃ is identified as the only light emitter in the composites. Interestingly, the presence of the photoluminescence-inactive Cs₄PbBr₆ can significantly enhance the light emitting efficiency of the composites. The unique negative thermal quenching observed near the liquid nitrogen temperature indicates that a type of shallow state generated at the CsPbBr₃/Cs₄PbBr₆ interfaces is responsible for the enhancement of photoluminescence

Femtosecond M_2,3-Edge Spectroscopy of Transition-Metal Oxides: Photoinduced Oxidation State Change in α‑Fe₂O₃

Oxidation-state-specific dynamics at the Fe M_2,3-edge are measured on the sub-100 fs time scale using tabletop high-harmonic extreme ultraviolet spectroscopy. Transient absorption spectroscopy of α-Fe₂O₃ thin films after 400 nm excitation reveals distinct changes in the shape and position of the 3p → valence absorption peak at ∼57 eV due to a ligand-to-metal charge transfer from O to Fe. Semiempirical ligand field multiplet calculations of the spectra of the initial Fe³⁺ and photoinduced Fe²⁺ state confirm this assignment and exclude the alternative d–d excitation. The Fe²⁺ state decays to a long-lived trap state in 240 fs. This work establishes the ability of time-resolved extreme ultraviolet spectroscopy to measure ultrafast charge-transfer processes in condensed-phase systems

Fully Printed Halide Perovskite Light-Emitting Diodes with Silver Nanowire Electrodes

Printed organometal halide perovskite light-emitting diodes (LEDs) are reported that have indium tin oxide (ITO) or carbon nanotubes (CNTs) as the transparent anode, a printed composite film consisting of methylammonium lead tribromide (Br-Pero) and poly­(ethylene oxide) (PEO) as the emissive layer, and printed silver nanowires as the cathode. The fabrication can be carried out in ambient air without humidity control. The devices on ITO/glass have a low turn-on voltage of 2.6 V, a maximum luminance intensity of 21014 cd m^–2, and a maximum external quantum efficiency (EQE) of 1.1%, surpassing previous reported perovskite LEDs. The devices on CNTs/polymer were able to be strained to 5 mm radius of curvature without affecting device properties

Fully Printed Halide Perovskite Light-Emitting Diodes with Silver Nanowire Electrodes

Printed organometal halide perovskite light-emitting diodes (LEDs) are reported that have indium tin oxide (ITO) or carbon nanotubes (CNTs) as the transparent anode, a printed composite film consisting of methylammonium lead tribromide (Br-Pero) and poly­(ethylene oxide) (PEO) as the emissive layer, and printed silver nanowires as the cathode. The fabrication can be carried out in ambient air without humidity control. The devices on ITO/glass have a low turn-on voltage of 2.6 V, a maximum luminance intensity of 21014 cd m^–2, and a maximum external quantum efficiency (EQE) of 1.1%, surpassing previous reported perovskite LEDs. The devices on CNTs/polymer were able to be strained to 5 mm radius of curvature without affecting device properties