Plasmonic Near-Infrared-Response S-Scheme ZnO/CuInS2 Photocatalyst for H2O2 Production Coupled with Glycerin Oxidation.

Solar fuel synthesis is intriguing because solar energy is abundant and this method compensates for its intermittency. However, most photocatalysts can only absorb UV-to-visible light, while near-infrared (NIR) light remains unexploited. Surprisingly, the charge transfer between ZnO and CuInS2 quantum dots (QDs) can transform a NIR-inactive ZnO into a NIR-active composite. This strong response is attributed to the increased concentration of free charge carriers in the p-type semiconductor at the interface after the charge migration between ZnO and CuInS2, enhancing the localized surface plasmon resonance (LSPR) effect and the NIR response of CuInS2. As a paradigm, this ZnO/CuInS2 heterojunction is used for H2O2 production coupled with glycerin oxidation and demonstrates supreme performance, corroborating the importance of NIR response and efficient charge transfer. Mechanistic studies through contact potential difference (CPD), Hall effect test, and finite element method (FEM) calculation allow for the direct correlation between the NIR response and charge transfer. This approach bypasses the general light response issues, thereby stepping forward to the ambitious goal of harnessing the entire solar spectrum.

Template-Assisted Synthesis of 2D Perovskite Grating Single Crystal Films at Low Temperatures for UV Polarization-Sensitive Photodetectors.

2D perovskites have attracted tremendous attention due to their superior optoelectronic properties and potential applications in optoelectronic devices. Especially, the larger bandgap of 2D perovskite means that they are suitable for UV photodetection. However, the layered structure of 2D perovskites hinders the interlayer carrier transport, which limits the improvement of device performance. Therefore, nanoscale structures are normally used to enhance the light absorption ability, which is an effective strategy to improve the photocurrent in 2D perovskite-based photodetectors. Herein, a template-assisted low-temperature method is proposed to fabricate 2D perovskite ((C6H5C2H4NH3)2PbBr4, (PEA)2PbBr4) grating single crystal films (GSCFs). The crystallinity of the (PEA)2PbBr4 GSCFs is significantly improved due to the slow evaporation of the precursor solution under low temperatures. Based on this high crystalline quality and extremely ordered microstructures, the metal-semiconductor-metal photodetectors are assembled. Finite-different time-domain (FDTD) simulation and experiment indicate that the GSCF-based photodetectors exhibit significantly improved performance in comparison with the plane devices. The optimized 2D perovskite photodetectors are sensitive to UV light and demonstrate a responsivity and detectivity of 28.6 mA W-1 and 2.4 × 1011 Jones, respectively. Interestingly, the photocurrent of this photodetector varies as the angle of the incident polarized light, resulting in a high polarization ratio of 1.12.

Surface Microstructure Engineering in MAPbBr3 Microsheets for Performance-Enhanced Photodetectors.

Metal halide-perovskite-based photodetectors have recently emerged as a class of promising optoelectronic devices in various fields. Meanwhile, nano/microstructuring perovskite-based photodetectors are a facile integration with complementary metal-oxide semiconductors for miniaturized imaging systems. However, there are still challenges to be overcome in reducing the losses caused by light reflection on the surface of microstructural perovskites. In this work, surface microstructure engineering is employed in MAPbBr3 microsheets for reducing light reflection and improving light absorption, resulting in high-performance perovskite photodetectors. MAPbBr3 microsheets, which possess different surface morphologies of flat, upright hemisphere arrays and inverted hemisphere arrays (IHAs), are fabricated by a simple microstructure template-assisted space confinement process. The light absorption capacity of IHA MAPbBr3 is significantly higher than that of the other two structures. Hence, IHA photodetectors with excellent figures of merit, including low dark current, decent responsivity, and fast speed, are achieved. Furthermore, the noise of the IHA photodetectors is only ∼10-13 A/Hz, which results in the superior sensitivity for weak light detection with a specific detectivity up to 1011 Jones. Our results demonstrate that surface engineering is a simple, low-cost, yet effective approach to improve the performance of nano-/micro-optoelectronic devices.

Design of compact dual-mode photoelectric modulator with high process tolerance based on vanadium dioxide.

Opto-electro modulators with nanometer-scale footprint are indispensable in integrated photonic electronic circuits. Due to weak light-matter interactions and limits of micro-nano fabrication technology, it is challenging to shrink a modulator to subwavelength size. In recent years, hybrid modulators based on surface plasmons have been proposed to solve this problem. Although the introduced high lossy surface plasmons provide large modulation depth, the polarization selectivity limits its application. Toward this end, in this paper, we present a design of an ultra-compact vanadium oxide (VO2)-based plasmonic waveguide modulator for both transverse electric (TE) and transverse magnetic (TM) modes. The device consists of two silicon tapers and a silicon waveguide embedded with a VO2 wedge. When electrical signals put on the device change the phase of VO2 from a metal to an insulator, the output optical signals along the waveguide are significantly modulated. For a 1.5 µm length modulator operating at 1.55 µm wavelength, the extinction ratio is 11.62 dB for the TE mode and 8.86 dB for the TM mode, while the insertion loss is 4.31 dB for the TE mode and 4.12 dB for the TM mode. Furthermore, the proposed design has excellent tolerance for fabrication process error, which greatly increases the yield rate of products and indicates a promotable application prospect.

Porous Pure MXene Fibrous Network for Highly Sensitive Pressure Sensors.

Wearable and elastic pressure sensors have caused widespread concern due to the popularity of smart terminals and human health monitoring. To obtain a flexible pressure sensor with a wide detection region and outstanding sensitivity, exploring new materials and novel structures has become the first choice for the research. Here, a wearable and flexible MXene fibrous network pressure sensor (MFNS) with a high sensitivity and wide detection region is reported. The holistic fiber network is composed of pure MXene fibers; among them, MXene fibers were prepared by wet-spinning of MXene nanosheets. The MFNS exhibits a high sensitivity in a wide detection region (51 kPa-1 for 14.7 kPa and 427 kPa-1 within the 14.7-19.9 kPa range), a low detection limit (8 Pa), a robust durability (10,000 cycles), and a prompt response (95 ms). Due to the superior performance of MFNS, it also proves prospective applications for human motion signal detection (such as swallowing, pulse beat, and joint motion) and measuring pressure distribution. This work provides an effective way to fabricate a high-performance pressure sensor for human-machine interactions, personal healthcare monitoring, and multitouch devices.

Upside-Down Molding Approach for Geometrical Parameter-Tunable Photonic Perovskite Nanostructures.

Considering that the periodic photonic nanostructures are commonly realized by expensive nanofabrication processes and the tunability of structure parameters is limited and complicated, we demonstrate a solution-processed upside-down molding method to fabricate photonic resonators on perovskites with a pattern geometry controllable to a certain extent. This upside-down approach not only reveals the effect of capillary force during the imprinting but also can control the waveguide layer thickness due to the inversion of the perovskite membranes.

Optical-electrical-thermal optimization of plasmon-enhanced perovskite solar cells.

Metal nanoparticles associated with local surface plasmon (LSP) resonance, i.e. highly confined electric field and large scattering cross-sections (σ), have been widely used to enhance the light-harvesting of solar cells toward high optoelectronic performance. However, the metal nanoparticles embedded into the solar cells suffer from parasitic ohmic loss that subsequently causes the local temperature to rise, which, in turn, reduces the photoelectric conversion efficiency and stability of solar cells. Previous studies on plasmon-enhanced solar cells have rarely considered the negative effects of metal nanoparticles' ohmic losses and temperature rise on solar cell performance optimization. Therefore, it is of great interest to alleviate the ohmic loss and temperature rise that are critical for high-performance solar cells. Herein, we propose a model to comprehensively study and optimize the performance of plasmon-enhanced perovskite solar cells (PSCs) from simultaneous optical-electrical-thermal aspects. First, the optical simulation results indicated that the geometric tuning of metal nanoparticles can make full use of the plasmonic effect and significantly improve PSCs' light absorption. The analysis showed that the embedded nanoparticles with optimal geometry in PSC devices can significantly increase the optical absorption by 17% (41%) compared to non-optimal nanostructures (devices without nanoparticles). Then, we explored the influence of the temperature-dependent carrier mobility on PSC performance from the coupled electrical and thermal studies. Our results indicated that the optimization of the geometrical parameters of metal nanoparticles can minimize energy dissipation, thereby redusing the heat loss and then lowering the local cell temperature. Interestingly, PSCs' electrical properties such as carrier transportation significantly improved. Consequently, the PSC performance improved with increment in the short-circuit current by 23% and the power conversion efficiency by 38%.

Efficient Raman Enhancement in Molybdenum Disulfide by Tuning the Interlayer Spacing.

Two-dimensional nanomaterials, such as graphene and molybdenum disulfide (MoS2), have recently attracted widespread attention as surface-enhanced Raman scattering (SERS) substrates. However, their SERS enhancement is of a smaller magnitude than that of noble metal nanomaterials, and therefore, the detection sensitivity still needs to be substantially improved for practical applications. Here, we present the first detailed studies on the effect of the (MoS2) interlayer distances on the SERS intensity enhancement. We find that MoS2 with smaller interlayer distances achieves an SERS enhancement factor as high as 5.31 × 105, which is one of the highest enhancement factors to date among the two-dimensional nanomaterial SERS sensors. This remarkable SERS sensitivity is attributed to the highly efficient charge transfer from MoS2 to probe molecules. The charge-transfer ability directly determines the variable quantity dz2 orbitals of Mo elements in the MoS2-molecule system and then tunes the Raman intensity of probe molecules. Our work contributes to reveal the influence of MoS2 interlayer spacing on SERS detection and to open a new way for designing a highly sensitive nonmetal SERS technology.

Linear and nonlinear spin-orbital coupling in golden-angle spiral quasicrystals.

The appealing characteristics of quasi-crystalline nanostructure offer tremendous possibilities to tailor the transmission of the angular momenta. Moreover, the second harmonic generation existing in nonlinear nanostructures also exhibits remarkable potential in the fundamental and applied research areas of the angular momenta conversion. By systematically studying the general angular momenta conservation law, we show that the high-dimensional angular momenta transformation and spin-orbital coupling are realized by the nonlinear sunflower-type quasicrystals, which feature the high-fold rotational symmetry and possess an increasing degree of rotational symmetry in Fourier space. Interestingly, since the sequential Fibonacci numbers are essentially encoded in the distinctive nonlinear sunflower-type patterns, the high-fold angular momenta transformation regularly occurs at both linear and nonlinear wavelengths. The investigations of fundamental physics for the unique quasi-crystals reveal scientific importance for manipulating the angular momenta of nonlinear optical signals, which plays a key role in the promotion and development of modern physics.

Synergetic light trapping effects in organic solar cells with a patterned semi-transparent electrode.

The dielectric/ultra-thin metal/dielectric structure has been widely used for semi-transparent electrodes in organic solar cells (OSCs) due to its potential replacement of the transparent conductive oxide indium tin oxide. Here, we introduce the dual light trapping structures, i.e. the nanopatterned MoO3/Ag/MoO3 (MAM) as the anode and a short-pitched metallic grating as the cathode, to cooperatively improve the OSC performance. The optical and electrical properties of the OSCs have been investigated via solving the coupled Maxwell and semiconductor equations by the finite-difference method. The results indicate that the optical light absorption of the active layer and the electrical carrier collection have been significantly enhanced after the adoption of the proposed dual light trapping structures. We have shown that the optical and electrical improvements are attributed to the synergetic effects of surface plasmon resonance of the grating patterned cathode and scattering effect of the nanopatterned MAM anode. Our results have further revealed that the short-pitched metal grating can induce considerable field confinement due to the mode coupling and hybridization of the surface plasmons in-between the adjacent short-distance metal strips. With the optimized structural parameters of the dual light trapping structure, the power conversion efficiency (PCE) of the OSCs has been substantially enhanced by 39% in comparison with a flat cell. Besides the efficiency improvement, the OSCs with the proposed dual light harvesting structures reveal an alleviated angular dependence of electrical properties as the light is beyond the normal incident angle. Our results contribute to the further development of ITO-free OSCs and are promising for semi-transparent optoelectronics.

Fluorescent/SERS dual-sensing and imaging of intracellular Zn2.

A fluorescent and surface-enhanced Raman spectroscopy (SERS) dual-mode probe is developed for imaging of intracellular Zn2+ based on N-(2-(bis(pyridine-2-ylmethyl)amino)ethyl)-2-mercaptoacetamide (MDPA) modified gold nanoparticles (MDPA-GNPs). Benefiting from the chelation-enhanced fluorescence (CHEF) between MDPA-GNPs and Zn2+, the fluorescent intensities of MDPA-GNPs are substantially enhanced with the increment of Zn2+ concentrations, which can be clearly observed by the naked eye. Under physiological conditions, the probe exhibits a stable response for Zn2+ from 1 µM to 120 µM, with a detection limit of 0.32 µM in aqueous solutions. The resultant MDPA-GNPs can be used for ultrasensitive SERS detection of Zn2+ because of the strong inter-particle plasmonic coupling generated in the process of Zn2+-triggered MDPA-GNPs self-aggregation, with a low detection limit of 0.28 pM, which is eight order of magnitude lower than the United States Environmental Protection Agency (US EPA)-defined limit (76 µM) in drinkable water. More importantly, the proposed probe can be applied for efficient detection of intracellular Zn2+ with excellent biocompatibility and cellular imaging capability. Therefore, a highly sensitive and selective nanosensor has been demonstrated for both reliable quantitative detection of Zn2+ in aqueous solution and real-time imaging of intracellular Zn2+, suggesting its significant potential utility in bioanalysis and biomedical detection in the future.

Emerging Novel Metal Electrodes for Photovoltaic Applications.

Emerging novel metal electrodes not only serve as the collector of free charge carriers, but also function as light trapping designs in photovoltaics. As a potential alternative to commercial indium tin oxide, transparent electrodes composed of metal nanowire, metal mesh, and ultrathin metal film are intensively investigated and developed for achieving high optical transmittance and electrical conductivity. Moreover, light trapping designs via patterning of the back thick metal electrode into different nanostructures, which can deliver a considerable efficiency improvement of photovoltaic devices, contribute by the plasmon-enhanced light-mattering interactions. Therefore, here the recent works of metal-based transparent electrodes and patterned back electrodes in photovoltaics are reviewed, which may push the future development of this exciting field.

High Efficiency Organic Solar Cells Achieved by the Simultaneous Plasmon-Optical and Plasmon-Electrical Effects from Plasmonic Asymmetric Modes of Gold Nanostars.

The plasmon-optical effects have been utilized to optically enhance active layer absorption in organic solar cells (OSCs). The exploited plasmonic resonances of metal nanomaterials are typically from the fundamental dipole/high-order modes with narrow spectral widths for regional OSC absorption improvement. The conventional broadband absorption enhancement (using plasmonic effects) needs linear-superposition of plasmonic resonances. In this work, through strategic incorporation of gold nanostars (Au NSs) in between hole transport layer (HTL) and active layer, the excited plasmonic asymmetric modes offer a new approach toward broadband enhancement. Remarkably, the improvement is explained by energy transfer of plasmonic asymmetric modes of Au NS. In more detail, after incorporation of Au NSs, the optical power in electron transport layer transfers to active layer for improving OSC absorption, which otherwise will become dissipation or leakage as the role of carrier transport layer is not for photon-absorption induced carrier generation. Moreover, Au NSs simultaneously deliver plasmon-electrical effects which shorten transport path length of the typically low-mobility holes and lengthen that of high-mobility electrons for better balanced carrier collection. Meanwhile, the resistance of HTL is reduced by Au NSs. Consequently, power conversion efficiency of 10.5% has been achieved through cooperatively plasmon-optical and plasmon-electrical effects of Au NSs.

Recent Advances in Organic Photovoltaics: Device Structure and Optical Engineering Optimization on the Nanoscale.

Organic photovoltaic (OPV) devices, which can directly convert absorbed sunlight to electricity, are stacked thin films of tens to hundreds of nanometers. They have emerged as a promising candidate for affordable, clean, and renewable energy. In the past few years, a rapid increase has been seen in the power conversion efficiency of OPV devices toward 10% and above, through comprehensive optimizations via novel photoactive donor and acceptor materials, control of thin-film morphology on the nanoscale, device structure developments, and interfacial and optical engineering. The intrinsic problems of short exciton diffusion length and low carrier mobility in organic semiconductors creates a challenge for OPV designs for achieving optically thick and electrically thin device structures to achieve sufficient light absorption and efficient electron/hole extraction. Recent advances in the field of OPV devices are reviewed, with a focus on the progress in device architecture and optical engineering approaches that lead to improved electrical and optical characteristics in OPV devices. Successful strategies are highlighted for light wave distribution, modulation, and absorption promotion inside the active layer of OPV devices by incorporating periodic nanopatterns/nanostructures or incorporating metallic nanomaterials and nanostructures.

Broadband near-field enhancement in the macro-periodic and micro-random structure with a hybridized excitation of propagating Bloch-plasmonic and localized surface-plasmonic modes.

We demonstrate that the silver nanoplate-based macroscopically periodic (macro-periodic) and microscopically random (micro-random) structure has a broadband near-field enhancement as compared to conventional silver gratings. The specific field enhancement in a wide spectral range (from UV to near-infrared) originates from the abundance of localized surface-plasmonic (LSP) modes in the microscopically random distributed silver nanoplates and propagating Bloch-plasmonic (PBP) modes from the macroscopically periodic pattern. The characterization of polarization dependent spectral absorption, surface-enhanced Raman spectroscopy (SERS), as well as theoretical simulation was conducted to comprehensively understand the features of the broadband spectrum and highly concentrated near-field. The reported macro-periodic and micro-random structure may offer a new route for the design of plasmonic systems for photonic and optoelectronic applications.

An all-copper plasmonic sandwich system obtained through directly depositing copper NPs on a CVD grown graphene/copper film and its application in SERS.

A simple, low-cost, all-copper sandwich system has been obtained through directly depositing Cu nanoparticles (NPs) onto a graphene sheet, which has already been grown on a Cu foil (Cu-NGF). The new design inherits two key advantages: (1) the materials of the NGF coupling system are composed of only cheaper Cu instead of Au and Ag, (2) direct fabrication of the system without transferring graphene will greatly lower the fabrication cost. More importantly, the Cu-NFG system shows a high sensitivity in surface-enhanced Raman scattering (SERS) with the highest enhancement factor (EF, over 1.89 × 10(7)) reported to date in Cu plasmonic systems. Experimental and theoretical results reveal that the strong EF is mainly because of the strong near-field coupling between Cu NPs and Cu films at the optimal angle of incidence, opening up a new route for Cu materials in SERS applications.

Experimental and theoretical investigation of macro-periodic and micro-random nanostructures with simultaneously spatial translational symmetry and long-range order breaking.

Photonic and plasmonic quasicrystals, comprising well-designed and regularly-arranged patterns but lacking spatial translational symmetry, show sharp diffraction patterns resulting from their long-range order in spatial domain. Here we demonstrate that plasmonic structure, which is macroscopically arranged with spatial periodicity and microscopically constructed by random metal nanostructures, can also exhibit the diffraction effect experimentally, despite both of the translational symmetry and long-range order are broken in spatial domain simultaneously. With strategically pre-formed metal nano-seeds, the tunable macroscopically periodic (macro-periodic) pattern composed from microscopically random (micro-random) nanoplate-based silver structures are fabricated chemically through photon driven growth using simple light source with low photon energy and low optical power density. The geometry of the micro-structure can be further modified through simple thermal annealing. While the random metal nanostructures suppress high-order Floquet spectra of the spatial distribution of refractive indices, the maintained low-order Floquet spectra after the ensemble averaging are responsible for the observed diffraction effect. A theoretical approach has also been established to describe and understand the macro-periodic and micro-random structures with different micro-geometries. The easy fabrication and comprehensive understanding of this metal structure will be beneficial for its application in plasmonics, photonics and optoelectronics.

Vacuum-assisted thermal annealing of CH3NH3PbI3 for highly stable and efficient perovskite solar cells.

Solar cells incorporating lead halide-based perovskite absorbers can exhibit impressive power conversion efficiencies (PCEs), recently surpassing 15%. Despite rapid developments, achieving precise control over the morphologies of the perovskite films (minimizing pore formation) and enhanced stability and reproducibility of the devices remain challenging, both of which are necessary for further advancements. Here we demonstrate vacuum-assisted thermal annealing as an effective means for controlling the composition and morphology of the CH(3)NH(3)PbI(3) films formed from the precursors of PbCl(2) and CH(3)NH(3)I. We identify the critical role played by the byproduct of CH(3)NH(3)Cl on the formation and the photovoltaic performance of the perovskite film. By completely removing the byproduct through our vacuum-assisted thermal annealing approach, we are able to produce pure, pore-free planar CH(3)NH(3)PbI(3) films with high PCE reaching 14.5% in solar cell device. Importantly, the removal of CH(3)NH(3)Cl significantly improves the device stability and reproducibility with a standard deviation of only 0.92% in PCE as well as strongly reducing the photocurrent hysteresis.

Selective growth and integration of silver nanoparticles on silver nanowires at room conditions for transparent nano-network electrode.

Recently, metal nanowires have received great research interests due to their potential as next-generation flexible transparent electrodes. While great efforts have been devoted to develop enabling nanowire electrodes, reduced contact resistance of the metal nanowires and improved electrical stability under continuous bias operation are key issues for practical applications. Here, we propose and demonstrate an approach through a low-cost, robust, room temperature and room atmosphere process to fabricate a conductive silver nano-network comprising silver nanowires and silver nanoparticles. To be more specific, silver nanoparticles are selectively grown and chemically integrated in situ at the junction where silver nanowires meet. The site-selective growth of silver nanoparticles is achieved by a plasmon-induced chemical reaction using a simple light source at very low optical power density. Compared to silver nanowire electrodes without chemical treatment, we observe tremendous conductivity improvement in our silver nano-networks, while the loss in optical transmission is negligible. Furthermore, the silver nano-networks exhibit superior electrical stability under continuous bias operation compared to silver nanowire electrodes formed by thermal annealing. Interestingly, our silver nano-network is readily peeled off in water, which can be easily transferred to other substrates and devices for versatile applications. We demonstrate the feasibly transferrable silver conductive nano-network as the top electrode in organic solar cells. Consequently, the transparent and conductive silver nano-networks formed by our approach would be an excellent candidate for various applications in optoelectronics and electronics.

Designing and fabricating double resonance substrate with metallic nanoparticles-metallic grating coupling system for highly intensified surface-enhanced Raman spectroscopy.

Recently, nanoparticle-film coupling systems in which metal nanoparticles (supported localized surface plasmons, LSPs) are separated from a flat metal film (supported surface plasmon polaritons, SPPs) by a spacer have been widely reported due to its strong local enhancement field. However, there is are limited studies, which employ the design of combing metal grating into the nanoparticle-film gap system. Here, we propose and fabricate a novel double-resonance SERS system by strategically assembling Au NPs separated by a MoO3 nanospacer from an Ag grating film. The Ag grating with clear SPP effect is used for the first time in a double-resonance system, and the monolayer Au NPs array is well assembled onto the top of the Ag grating with a compact and uniform distribution (inter-particles gap of about 5 nm). As a result, we experimentally and theoretically demonstrate a significant near-field enhancement. The very strong near-field produced in the proposed SERS substrates is due to multiple couplings, including the Au NPs-Ag grating film coupling and Au NPs-Au NPs coupling. In addition, the as-proposed SERS substrates show good reproducibility of SERS, which have potential applications in plasmonic sensing and analytical science.