Dioxygen (O2) and other gas molecules have a fundamental role in a variety of enzymatic reactions. However, it is only poorly understood which O2 uptake mechanism enzymes employ to promote efficient catalysis and how general this is. We investigated O2 diffusion pathways into monooxygenase and oxidase flavoenzymes, using an integrated computational and experimental approach. Enhanced-statistics molecular dynamics simulations reveal spontaneous protein-guided O2 diffusion from the bulk solvent to preorganized protein cavities. The predicted protein-guided diffusion paths and the importance of key cavity residues for oxygen diffusion were verified by combining site-directed mutagenesis, rapid kinetics experiments, and high-resolution X-ray structures. This study indicates that monooxygenase and oxidase flavoenzymes employ multiple funnel-shaped diffusion pathways to absorb O 2 from the solvent and direct it to the reacting C4a atom of the flavin cofactor. The difference in O 2 reactivity among dehydrogenases, monooxygenases, and oxidases ultimately resides in the fine modulation of the local environment embedding the reactive locus of the flavin.computational biochemistry ͉ enzymology ͉ flavin ͉ oxygen reactivity

Light emission from biased tunnel junctions has recently gained much attention owing to its unique potential to create ultracompact optical sources with terahertz modulation bandwidth 1-5. The emission originates from an inelastic electron tunnelling process in which electronic energy is transferred to surface plasmon polaritons and subsequently converted to radiation photons by an optical antenna. Because most of the electrons tunnel elastically, the emission efficiency is typically about 10 −5-10 −4. Here, we demonstrate efficient light generation from enhanced inelastic tunnelling using nanocrystals assembled into metal-insulator-metal junctions. The colour of the emitted light is determined by the optical antenna and thus can be tuned by the geometry of the junction structures. The efficiency of far-field free-space light generation reaches ~2%, showing an improvement of two orders of magnitude over previous work 3,4. This brings on-chip ultrafast and ultracompact light sources one step closer to reality. Electrons can tunnel through a metal-insulator-metal (MIM) junction either elastically or inelastically. For elastic tunnelling, electrons tunnel across the barrier layer without energy loss 6. However, the inelastic tunnelling process may create either phonons or photons as the electrons lose part of their energy in the gap and transition to a lower energy state in the metal counter-electrode. This process can be enhanced in the presence of surface plasmon polaritons around the MIM junction, as first discovered in 1976 1. Later theoretical 2 and experimental 7-10 studies increased the appeal of the MIM junction because of its ultra-small footprint and ultra-large modulation bandwidth. However, the main challenge for light generation from inelastic electron tunnelling is its low external quantum efficiency (EQE), a production of internal quantum efficiency (IQE) and radiation efficiency. Generally, the IQE describes the efficiency of the inelastic tunnelling event and can be increased by designing a plasmonic structure with a large local density of optical states (LDOS) 7,11,12 , and the radiation efficiency can be improved by introducing a high-quality optical antenna 13,14. Recently, light emission from electrically driven optical antennas made by amorphous (polycrystalline) plasmonic material has been demonstrated 3,4 with quantum efficiencies up to 10 −4. Compared with amorphous or polycrystalline plasmonic material, single-crystalline material has lower plasmonic loss 15 , which can further enhance the performance of the inelastic tunnel junction. Here, we use single-crystalline silver (Ag) nanocrystals to form tunnel junctions with gap distances of ~1.5 nm. Through geometrical engineering of the junctions to optimize the LDOS and radiation efficiency, we obtain a far-field light that the device could be integrated into photonics and/or plasmonic systems for on-chip applications 23-26. In principle, the emission frequency of the MIM junction device could cover a range from ultraviolet to mid-infrared, a...

We report a facile and large-scale fabrication of three-dimensional (3D) ZnO/CuO heterojunction branched nanowires (b-NWs) and their application as photocathodes for photoelectrochemical (PEC) solar hydrogen production in a neutral medium. Using simple, cost-effective thermal oxidation and hydrothermal growth methods, ZnO/CuO b-NWs are grown on copper film or mesh substrates with various ZnO and CuO NWs sizes and densities. The ZnO/CuO b-NWs are characterized in detail using high-resolution scanning and transmission electron microscopies exhibiting single-crystalline defect-free b-NWs with smooth and clean surfaces. The correlation between electrode currents and different NWs sizes and densities are studied in which b-NWs with longer and denser CuO NW cores show higher photocathodic current due to enhanced reaction surface area. The ZnO/CuO b-NW photoelectrodes exhibit broadband photoresponse from UV to near IR region, and higher photocathodic current than the ZnO-coated CuO (core/shell) NWs due to improved surface area and enhanced gas evolution. Significant improvement in the photocathodic current is observed when ZnO/CuO b-NWs are grown on copper mesh compared to copper film. The achieved results offer very useful guidelines in designing b-NWs mesh photoelectrodes for high-efficiency, low-cost, and flexible PEC cells using cheap, earth-abundant materials for clean solar hydrogen generation at large scales.

When engineered on scales much smaller than the operating wavelength, metal-semiconductor nanostructures exhibit properties unobtainable in nature. Namely, a uniaxial optical metamaterial described by a hyperbolic dispersion relation can simultaneously behave as a reflective metal and an absorptive or emissive semiconductor for electromagnetic waves with orthogonal linear polarization states. Using an unconventional multilayer architecture, we demonstrate luminescent hyperbolic metasurfaces, wherein distributed semiconducting quantum wells display extreme absorption and emission polarization anisotropy. Through normally incident micro-photoluminescence measurements, we observe absorption anisotropies greater than a factor of 10 and degree-of-linear polarization of emission >0.9. We observe the modification of emission spectra and, by incorporating wavelength-scale gratings, show a controlled reduction of polarization anisotropy. We verify hyperbolic dispersion with numerical simulations that model the metasurface as a composite nanoscale structure and according to the effective medium approximation. Finally, we experimentally demonstrate >350% emission intensity enhancement relative to the bare semiconducting quantum wells.

We report an ultrathin NiOx catalyzed Si np(+) junction photoanode for a stable and efficient solar driven oxygen evolution reaction (OER) in water. A stable semi-transparent ITO/Au/ITO hole conducting oxide layer, sandwiched between the OER catalyst and the Si photoanode, is used to protect the Si from corrosion in an alkaline working environment, enhance the hole transportation, and provide a pre-activation contact to the NiOx catalyst. The NiOx catalyzed Si photoanode generates a photocurrent of 1.98 mA cm(-2) at the equilibrium water oxidation potential (EOER = 0.415 V vs. NHE in 1 M NaOH solution). A thermodynamic solar-to-oxygen conversion efficiency (SOCE) of 0.07% under 0.51-sun illumination is observed. The successful development of a low cost, highly efficient, and stable photoelectrochemical electrode based on earth abundant elements is essential for the realization of a large-scale practical solar fuel conversion.

Broadband absorbers are essential components of many light detection, energy harvesting, and camouflage schemes. Current designs are either bulky or use planar films that cause problems in cracking and delamination during flexing or heating. In addition, transferring planar materials to flexible, thin, or low-cost substrates poses a significant challenge. On the other hand, particle-based materials are highly flexible and can be transferred and assembled onto a more desirable substrate but have not shown high performance as an absorber in a standalone system. Here, we introduce a class of particle absorbers called transferable hyperbolic metamaterial particles (THMMP) that display selective, omnidirectional, tunable, broadband absorption when closely packed. This is demonstrated with vertically aligned hyperbolic nanotube (HNT) arrays composed of alternating layers of aluminum-doped zinc oxide and zinc oxide. The broadband absorption measures >87% from 1,200 nm to over 2,200 nm with a maximum absorption of 98.1% at 1,550 nm and remains large for high angles. Furthermore, we show the advantages of particle-based absorbers by transferring the HNTs to a polymer substrate that shows excellent mechanical flexibility and visible transparency while maintaining near-perfect absorption in the telecommunications region. In addition, other material systems and geometries are proposed for a wider range of applications.hyperbolic metamaterials | perfect absorber | nanoparticle | nanowire | photonic hypercrystal S elective and broadband perfect absorbers generally consist of plasmonic cavities coupled to metallic reflectors separated by dielectric spacers. These geometries have led to many exciting applications such as thermophotovoltaics (TPV) (1, 2), thermal emitters (3, 4), camouflage (5), and thermal detectors (6). However, the ability to be scaled up to larger surface area devices and transferred to more desirable substrates is a major limitation of absorbers that rely on planar reflectors. Here, we introduce a class of standalone particles, transferrable hyperbolic metamaterial particles (THMMP), that display broadband, selective, omnidirectional absorption and can be transferred to secondary substrates, allowing enhanced flexibility and selective transmission. This is demonstrated using vertically aligned hyperbolic metamaterial nanotube (HNT) arrays. We first realize the concept by fabricating the HNTs on silicon substrates and then transfer the arrays to a thin elastomer to create a mechanically flexible, visibly transparent material that maintains near-perfect absorption at telecommunication wavelengths (∼1,550 nm). In addition, different materials systems and geometries are discussed, leading to a broader range of applications.Currently, carbon nanotubes (CNTs) provide perfect ultrabroadband absorption (7). However, CNT films are relatively thick and do not allow any control over the operating wavelengths of the absorber, which is critical for creating visibly transparent IR absorbers and materials for many other a...

Aluminum-doped zinc oxide (AZO) is a tunable low-loss plasmonic material capable of supporting dopant concentrations high enough to operate at telecommunication wavelengths. Due to its ultrahigh conformality and compatibility with semiconductor processing, atomic layer deposition (ALD) is a powerful tool for many plasmonic applications. However, despite many attempts, high-quality AZO with a plasma frequency below 1550 nm has not yet been realized by ALD. Here a simple procedure is devised to tune the optical constants of AZO and enable plasmonic activity at 1550 nm with low loss. The highly conformal nature of ALD is also exploited to coat silicon nanopillars to create localized surface plasmon resonances that are tunable by adjusting the aluminum concentration, thermal conditions, and the use of a ZnO buffer layer. The high-quality AZO is then used to make a layered AZO/ZnO structure that displays negative refraction in the telecommunication wavelength region due to hyperbolic dispersion. Finally, a novel synthetic scheme is demonstrated to create AZO embedded nanowires in ZnO, which also exhibits hyperbolic dispersion.

, and novel light-matter interaction (e.g., metamaterials) [7]. Most plasmonic materials consist of noble metals such as gold or silver due to their strong resonances in the visible part of the electromagnetic spectrum. However, noble metals pose an immediate problem for many plasmonic devices since they are expensive, possess high loss at near infrared (NIR) wavelengths and are not well-suited for complementary metal-oxide-semiconductor (CMOS) processing. An alternative approach is to use highly doped semiconductors which have tunable plasmon resonances in the NIR to mid-IR region (0.8-3 µm) [8].In order to be a viable candidate for many plasmonic applications a semiconductor should fulfill two criteria: (1) a carrier density large enough to display reso-