Architecture for Surface-Emitting Lasers with On-Demand Lasing Wavelength by Nanowire Optical Cavities.

Despite the importance and exciting progress of surface-emitting (SE) semiconductor lasers, we have limited choices of lasing wavelength even today. From an application viewpoint, it is desirable to have an architecture that can allow SE lasing in a wide spectral range, based on the need of applications. Herein, we demonstrate a path for SE lasers with lasing wavelength on demand by exploiting III-nitride nanowire optical cavities formed by low-temperature selective area epitaxy (SAE), combined with fine-tuning of substrate patterns and photonic bands. Moreover, in this study, we focus on the device demonstration in the ultraviolet (UV) spectral range, considering the severe lag in developing SE lasers in the UV wavelength range compared to longer wavelengths, e.g., near-infrared (NIR), as well as the potential applications enabled by UV lasers such as solar blind optical wireless communications. Ultralow threshold wavelength-tunable SE UV lasing is achieved by optical pumping. Moreover, SE UV lasing under direct electric current injection is also achieved. This study not only represents an important step in the journey of SE UV laser development but, more importantly, it lays the ground for SE lasers with lasing wavelength on demand, broadly from NIR to UV.

Facile Semiconductor p-n Homojunction Nanowires with Strategic p-Type Doping Engineering Combined with Surface Reconstruction for Biosensing Applications.

Photosensors with versatile functionalities have emerged as a cornerstone for breakthroughs in the future optoelectronic systems across a wide range of applications. In particular, emerging photoelectrochemical (PEC)-type devices have recently attracted extensive interest in liquid-based biosensing applications due to their natural electrolyte-assisted operating characteristics. Herein, a PEC-type photosensor was carefully designed and constructed by employing gallium nitride (GaN) p-n homojunction semiconductor nanowires on silicon, with the p-GaN segment strategically doped and then decorated with cobalt-nickel oxide (CoNiOx). Essentially, the p-n homojunction configuration with facile p-doping engineering improves carrier separation efficiency and facilitates carrier transfer to the nanowire surface, while CoNiOx decoration further boosts PEC reaction activity and carrier dynamics at the nanowire/electrolyte interface. Consequently, the constructed photosensor achieves a high responsivity of 247.8 mA W-1 while simultaneously exhibiting excellent operating stability. Strikingly, based on the remarkable stability and high responsivity of the device, a glucose sensing system was established with a demonstration of glucose level determination in real human serum. This work offers a feasible and universal approach in the pursuit of high-performance bio-related sensing applications via a rational design of PEC devices in the form of nanostructured architecture with strategic doping engineering.

Ultralow threshold surface emitting ultraviolet lasers with semiconductor nanowires.

Surface-emitting (SE) semiconductor lasers have changed our everyday life in various ways such as communication and sensing. Expanding the operation wavelength of SE semiconductor lasers to shorter ultraviolet (UV) wavelength range further broadens the applications to disinfection, medical diagnostics, phototherapy, and so on. Nonetheless, realizing SE lasers in the UV range has remained to be a challenge. Despite of the recent breakthrough in UV SE lasers with aluminum gallium nitride (AlGaN), the electrically injected AlGaN nanowire UV lasers are based on random optical cavities, whereas AlGaN UV vertical-cavity SE lasers (VCSELs) are all through optical pumping and are all with large lasing threshold power densities in the range of several hundred kW/cm2 to MW/cm2. Herein, we report ultralow threshold, SE lasing in the UV spectral range with GaN-based epitaxial nanowire photonic crystals. Lasing at 367 nm is measured, with a threshold of only around 7 kW/cm2 (~ 49 µJ/cm2), a factor of 100× reduction compared to the previously reported conventional AlGaN UV VCSELs at similar lasing wavelengths. This is also the first achievement of nanowire photonic crystal SE lasers in the UV range. Further given the excellent electrical doping that has already been established in III-nitride nanowires, this work offers a viable path for the development of the long-sought-after semiconductor UV SE lasers.

Vertical semiconductor deep ultraviolet light emitting diodes on a nanowire-assisted aluminum nitride buffer layer.

Vertical light-emitting diodes (LEDs) have many advantages such as uniform current injection, excellent scalability of the chip size, and simple packaging process. Hitherto, however, technologically important semiconductor aluminum gallium nitride (AlGaN) deep ultraviolet (UV) LEDs are mainly through lateral injection. Herein, we demonstrate a new and practical path for vertical AlGaN deep UV LEDs, which exploits a thin AlN buffer layer formed on a nanowire-based template on silicon (Si). Such a buffer layer enables in situ formation of vertical AlGaN deep UV LEDs on Si. Near Lambertian emission pattern is measured from the top surface. The decent reflectivity of Si in the deep UV range makes such a configuration a viable low-cost solution for vertical AlGaN deep UV LEDs. More importantly, the use of such a thin AlN buffer layer can allow an easy transfer of device structures to other carrier wafers for vertical AlGaN deep UV LEDs with ultimately high electrical and optical performance.

Field emission from AlGaN nanowires with low turn-on field.

We fabricate AlGaN nanowires by molecular beam epitaxy and we investigate their field emission properties by means of an experimental setup using nano-manipulated tungsten tips as electrodes, inside a scanning electron microscope. The tip-shaped anode gives access to local properties, and allows collecting electrons emitted from areas as small as 1 µm2. The field emission characteristics are analysed in the framework of Fowler-Nordheim theory and we find a field enhancement factor as high as ß = 556 and a minimum turn-on field [Formula: see text] = 17 V µm-1 for a cathode-anode separation distance [Formula: see text] = 500 nm. We show that for increasing separation distance, [Formula: see text] increases up to about 35 V µm-1 and ß decreases to ∼100 at [Formula: see text] = 1600 nm. We also demonstrate the time stability of the field emission current from AlGaN nanowires for several minutes. Finally, we explain the observation of modified slope of the Fowler-Nordheim plots at low fields in terms of non-homogeneous field enhancement factors due to the presence of protruding emitters.

Decoupling Strategy for Enhanced Syngas Generation from Photoelectrochemical CO2 Reduction.

Photoelectrochemical CO2 reduction into syngas (a mixture of CO and H2) provides a promising route to mitigate greenhouse gas emissions and store intermittent solar energy into value-added chemicals. Design of photoelectrode with high energy conversion efficiency and controllable syngas composition is of central importance but remains challenging. Herein, we report a decoupling strategy using dual cocatalysts to tackle the challenge based on joint computational and experimental investigations. Density functional theory calculations indicate the optimization of syngas generation using a combination of fundamentally distinctive catalytic sites. Experimentally, by integrating spatially separated dual cocatalysts of a CO-generating catalyst and a H2-generating catalyst with GaN nanowires on planar Si photocathode, we report a record high applied bias photon-to-current efficiency of 1.88% and controllable syngas products with tunable CO/H2 ratios (0-10) under one-sun illumination. Moreover, unassisted solar CO2 reduction with a solar-to-syngas efficiency of 0.63% is demonstrated in a tandem photoelectrochemical cell.

AlGaN Nanowires for Ultraviolet Light-Emitting: Recent Progress, Challenges, and Prospects.

In this paper, we discuss the recent progress made in aluminum gallium nitride (AlGaN) nanowire ultraviolet (UV) light-emitting diodes (LEDs). The AlGaN nanowires used for such LED devices are mainly grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD); and various foreign substrates/templates have been investigated. Devices on Si so far exhibit the best performance, whereas devices on metal and graphene have also been investigated to mitigate various limitations of Si substrate, e.g., the UV light absorption. Moreover, patterned growth techniques have also been developed to grow AlGaN nanowire UV LED structures, in order to address issues with the spontaneously formed nanowires. Furthermore, to reduce the quantum confined Stark effect (QCSE), nonpolar AlGaN nanowire UV LEDs exploiting the nonpolar nanowire sidewalls have been demonstrated. With these recent developments, the prospects, together with the general challenges of AlGaN nanowire UV LEDs, are discussed in the end.

Effect of growth temperature on the structural and optical properties of few-layer hexagonal boron nitride by molecular beam epitaxy.

We have studied the epitaxy of few-layer hexagonal boron nitride (h-BN) by plasma-assisted molecular beam epitaxy (MBE) using a low growth rate and nitrogen-rich condition. It has been determined that under such conditions, the growth temperature is the factor having the most significant impact on the structural and optical quality of the material. When grown at temperatures <1000 °C, the h-BN film is polycrystalline, and defect-related photoluminescence (PL) emission dominates. Epitaxial domains of exceptional crystalline quality are obtained at elevated substrate temperatures of ~1300 °C, which exhibit strong band-edge PL emission at ~220 nm and negligible defect-related emission at room temperature. Our atomistic calculations reveal that, even though the gap of h-BN is indirect, it luminesces as strongly as direct-gap materials. Experimentally, the luminescence intensity of such a few-layer h-BN sample is measured to be two orders of magnitude stronger than that of a 4-µm thick commercially grown AlN template on sapphire, demonstrating the extraordinary potential of epitaxial h-BN for deep ultraviolet (UV) optoelectronics and quantum photonics.

Magnetic Field Enhanced Superconductivity in Epitaxial Thin Film WTe2.

In conventional superconductors an external magnetic field generally suppresses superconductivity. This results from a simple thermodynamic competition of the superconducting and magnetic free energies. In this study, we report the unconventional features in the superconducting epitaxial thin film tungsten telluride (WTe2). Measuring the electrical transport properties of Molecular Beam Epitaxy (MBE) grown WTe2 thin films with a high precision rotation stage, we map the upper critical field Hc2 at different temperatures T. We observe the superconducting transition temperature T c is enhanced by in-plane magnetic fields. The upper critical field Hc2 is observed to establish an unconventional non-monotonic dependence on temperature. We suggest that this unconventional feature is due to the lifting of inversion symmetry, which leads to the enhancement of Hc2 in Ising superconductors.

Selective area epitaxy of AlGaN nanowire arrays across nearly the entire compositional range for deep ultraviolet photonics.

Semiconductor light sources operating in the ultraviolet (UV)-C band (100-280 nm) are in demand for a broad range of applications but suffer from extremely low efficiency. AlGaN nanowire photonic crystals promise to break the efficiency bottleneck of deep UV photonics. We report, for the first time, site-controlled epitaxy of AlGaN nanowire arrays with Al incorporation controllably varied across nearly the entire compositional range. It is also observed that an Al-rich AlGaN shell structure is spontaneously formed, significantly suppressing nonradiative surface recombination. An internal quantum efficiency up to 45% was measured at room-temperature. We have further demonstrated large area AlGaN nanowire LEDs operating in the UV-C band on sapphire substrate, which exhibit excellent optical and electrical performance, including a small turn-on voltage of ~4.4 V and an output power of ~0.93 W/cm2 at a current density of 252 A/cm2. The controlled synthesis of AlGaN subwavelength nanostructures with well-defined size, spacing, and spatial arrangement and tunable emission opens up new opportunities for developing high efficiency LEDs and lasers and promises to break the efficiency bottleneck of deep UV photonics.

AlN/h-BN Heterostructures for Mg Dopant-Free Deep Ultraviolet Photonics.

Aluminum-rich AlGaN is the ideal material system for emerging solid-state deep-ultraviolet (DUV) light sources. Devices operating in the near-UV spectral range have been realized; to date, however, the achievement of high-efficiency light-emitting diodes (LEDs) operating in the UV-C band (200-280 nm specifically) has been hindered by the extremely inefficient p-type conduction in AlGaN and the lack of DUV-transparent conductive electrodes. Here, we show that these critical challenges can be addressed by Mg dopant-free Al(Ga)N/h-BN nanowire heterostructures. By exploiting the acceptor-like boron vacancy formation, we have demonstrated that h-BN can function as a highly conductive, DUV-transparent electrode; the hole concentration is ∼1020 cm-3 at room temperature, which is 10 orders of magnitude higher than that previously measured for Mg-doped AlN epilayers. We have further demonstrated the first Al(Ga)N/h-BN LED, which exhibits strong emission at ∼210 nm. This work also reports the first achievement of Mg-free III-nitride LEDs that can exhibit high electrical efficiency (80% at 20 A/cm2).

Controlled Coalescence of AlGaN Nanowire Arrays: An Architecture for Nearly Dislocation-Free Planar Ultraviolet Photonic Device Applications.

Nearly dislocation-free semipolar AlGaN templates are achieved on c-plane sapphire substrate through controlled nanowire coalescence by selective-area epitaxy. The coalesced Mg-doped AlGaN layers exhibit superior charge-carrier-transport properties. Semipolar-AlGaN ultraviolet light-emitting diodes demonstrate excellent performance. This work establishes the use of engineered nanowire structures as a viable architecture to achieve large-area, dislocation-free planar photonic and electronic devices.

Nanogenerators based on vertically aligned InN nanowires.

Piezoelectric nanogenerators (NGs) based on vertically aligned InN nanowires (NWs) are fabricated, characterized, and evaluated. In these NGs, arrays of p-type and intrinsic InN NWs prepared by plasma-assisted molecular beam epitaxy (MBE) demonstrate similar piezoelectric properties. The p-type NGs show 160% more output current and 70% more output power product than the intrinsic NGs. The features driving performance enhancement are reduced electrostatic losses due to better NW array morphology, improved electromechanical energy conversion efficiency due to smaller NW diameters, and the higher impedance of intrinsic NGs due to elevated NW surface charge levels. These findings highlight the potential of InN based NGs as a power source for self-powered systems and the importance of NW morphology and surface state in overall NG performance.

Optical constants of In(x)Ga(1-x)N (0 ≤ x ≤ 0.73) in the visible and near-infrared wavelength regimes.

Complex refractive indices of In(x)Ga(1-x)N epitaxial layers have been determined from analysis of data obtained by spectroscopic ellipsometry. The measurements were made in the wavelength range of 400-1687 nm. The samples were grown by plasma-assisted molecular beam epitaxy on (001) silicon substrate and are of the wurtzite crystalline form. A comparison of the fundamental absorption edge derived from analysis of measured data and the measured photoluminescence peak emission energy indicates a Stokes shift present in the alloys.

One-step overall water splitting under visible light using multiband InGaN/GaN nanowire heterostructures.

The conversion of solar energy into hydrogen via water splitting process is one of the key sustainable technologies for future clean, storable, and renewable source of energy. Therefore, development of visible light-responsive and efficient photocatalyst material has been of immense interest, but with limited success. Here, we show that overall water splitting under visible-light irradiation can be achieved using a single photocatalyst material. Multiband InGaN/GaN nanowire heterostructures, decorated with rhodium (Rh)/chromium-oxide (Cr2O3) core-shell nanoparticles can lead to stable hydrogen production from pure (pH ∼ 7.0) water splitting under ultraviolet, blue and green-light irradiation (up to ∼560 nm), the longest wavelength ever reported. At ∼440-450 nm wavelengths, the internal quantum efficiency is estimated to be ∼13%, the highest value reported in the visible spectrum. The turnover number under visible light well exceeds 73 in 12 h. Detailed analysis further confirms the stable photocatalytic activity of the nanowire heterostructures. This work establishes the use of metal-nitrides as viable photocatalyst for solar-powered artificial photosynthesis for the production of hydrogen and other solar fuels.

Large-scale cubic InN nanocrystals by a combined solution- and vapor-phase method under silica confinement.

Large-scale cubic InN nanocrystals were synthesized by a combined solution- and vapor-phase method under silica confinement. Nearly monodisperse cubic InN nanocrystals with uniform spherical shape were dispersed stably in various organic solvents after removal of the silica shells. The average size of InN nanocrystals is 5.7 ± 0.6 nm. Powder X-ray diffraction results indicate that the InN nanocrystals are of high crystallinity with a cubic phase. X-ray photoelectron spectroscopy and energy-dispersive spectroscopy confirm that the nanocrystals are composed of In and N elements. The InN nanocrystals exhibit infrared photoluminescence at room temperature, with a peak energy of ~0.62 eV, which is smaller than that of high-quality wurtzite InN (~0.65-0.7 eV) and is in agreement with theoretical calculations. The small emission peak energy of InN nanocrystals, as compared to other low-cost solution or vapor methods, reveals the superior crystalline quality of our samples, with low or negligible defect density. This work will significantly promote InN-based applications in IR optoelectronic device and biology.