Electrochemical CO2 reduction using renewable sources of electrical energy holds promise for converting CO2 into fuels and chemicals. The complex interactions among chemical/electrochemical reactions and mass transport make it difficult to analyze the effect of an individual process on electrode performance based only on experimental methods. Here, we developed a generalized steady-state simulation to describe an electrode surface in which sequential cascade catalysts are patterned in a periodic trench design. If appropriately constructed, this trench geometry is hypothesized to be able to yield a higher net current density for a CO2 reduction (CO2R) cascade reaction. We have used realistic experimental reaction kinetics to investigate the role of trench geometry in mass transport, local microenvironments, and selectivity for a model CO2R cascade reaction. The model considers local concentration gradients of bicarbonate species at quasi-equilibrium and catalytic surface reactions based on concentration-dependent Butler-Volmer kinetics. Our results suggest that varying the spatial distribution of active sites plays a significant role in facilitating effective mass transport between active sites, modulating selectivity for the cascade reaction, and enhancing the yield of desirable cascade products. Moreover, we observe that this trench geometry significantly alters the cascade reaction rate by affecting the local pH, which can cause inadvertent depletion of available aqueous CO2 to limit the CO2R cascade kinetics and modest suppression of the hydrogen evolution reaction (HER). The results highlight the trade-offs between mass transport, pH, and reaction kinetics that become apparent only when considering the coupled physics of all processes at the electrode surface. This model can thus serve as a primary tool to build more selective and efficient patterned architectures for the CO2R cascade catalysis.

Improving the photon-magnon coupling strength can be done by tuning the structure of microwave resonators to better interact with the magnon counterpart. Planar resonators accommodating unconventional photon modes beyond the half- and quarter-wavelength designs have been explored due to their optimized mode profiles and potentials for on-chip integration. Here, we designed and fabricated an actively controlled ring resonator supporting the spoof localized surface plasmons (LSPs), and implemented it in the investigation of photon-magnon coupling for hybrid magnonic applications. We demonstrated gain-assisted photon-magnon coupling with the YIG magnon mode under several different sample geometries. The achieved coupling amplification largely benefits from the high quality factor (Q-factor) due to the additional gain provided by a semiconductor amplifier, which effectively increases the Q-factor from a nearly null state (passive resonance) to more than 1000 for a quadrupole LSP mode. Our results suggest an additional control knob for manipulating photon-magnon coupled systems exploiting external controls of gain and loss.

On-demand drug delivery holds great promise to optimize pharmaceutical efficacy while minimizing the side effects. However, existing on-demand drug delivery systems often require complicated manufacturing processes that preclude their wide implementation of a broad range of drugs. In this work, we demonstrate the introduction of MXene-coated microneedles (MNs) into bioelectronics for digitally controllable gate-valve drug delivery. MXenes, featuring high electronic conductivity, excellent biocompatibility, and solution processibility, enable low-cost scalability for printable bioelectronics. In an electrolytic state (e.g., body fluid), the coated MXene is oxidized and desorbed due to redox reactions caused by electrical bias, allowing the underlying drug to be controllably released. The MXene-incorporated drug delivery system not only demonstrates excellent biocompatibility and operational stability, but also features low-cost construction and sustainable usage. Besides, these MXene-coated MNs allow both on-demand transformation and local-region customization, further increasing the structural versatility and capability of multidrug delivery systems. Water-processable MXene layer applied to microneedle toward on-demand drug delivery bioelectronics.

VO2 has been widely used in optical modulation devices due to its large change in permittivity across its first-order metal-semiconductor phase transition. People have proved that VO2's phase transition can be driven by plasmonic hot-electron injection, providing a new approach to realizing ultrafast all-optical devices with a low pump fluence. Here, we report on ultrafast VO2 all-optical modulation with a dramatically low pump fluence utilizing the composite structure of a self-supported VO2 film on a gold nanoshell grating. Plasmonic hot electrons in the gold excited by femtosecond pulses are injected into the VO2 film and trigger its phase transition, reducing the pump fluence threshold for the structural transformation to only 147.8 mu J/cm2. With a pump fluence above this threshold, ultrafast phase transition can be triggered within 1 ps, and a high modulation depth of 50% can be achieved. Moreover, an ultrafast modulation with an on-off time of 650 fs can also be achieved using a pump fluence below the threshold. This work may explore new applications of plasmonic hot-electron-driven phase transition in optical modulation devices and configurable devices.

A core-shell approach that utilizes a high-surface-area conducting core and an outer semiconductor shell is exploited here to prepare p-type dye-sensitized solar energy cells that operate with a minimal applied bias. Photocathodes were prepared by coating thin films of nanocrystalline indium tin oxide with a 0.8 nm Al2O3 seeding layer, followed by the chemical growth of nonstoichiometric strontium titanate. Films were annealed and sensitized with either a porphyrin chromophore or a chromophore-catalyst molecular assembly consisting of the porphyrin covalently tethered to the ruthenium complex. The sensitized photoelectrodes produced cathodic photocurrents of up to -315 μA/cm2 under simulated sunlight (AM1.5G, 100 mW/cm2) in aqueous media, pH 5. The photocurrent was increased by the addition of regenerative hole donors to the system, consistent with slow interfacial recombination kinetics, an important property of p-type dye-sensitized electrodes.

Alternating block copolymers consisting of diketopyrrolopyrrole and benzothiadiazole electron acceptor units linked together via aromatic five-membered donor heterocycles are studied using a combination of computer simulation techniques and experiments. Four copolymers are modeled starting from their monomers to stacked macromolecules: with two different linkers-thiophene or furan, connecting electron-withdrawing core units-and two different alkyl substituents at lactam nitrogens of diketopyrrolopyrrole-linear dodecyl and branched 2-octyldodecyl chains. In our experiments, we aim at characterization of the optical and electrochemical properties of two copolymers with branched side chains differing in the linker, since as the literature survey shows the data published on these copolymers are very sparse. These properties can be easily interpreted and later compared with theoretical predictions. The results of simulations supported by experiments show that monomers of these polymers have very similar electronic and optical properties, and the main difference between them consists in various chain curvature defined by the linker. More curved furan-containing monomers and more stretched thiophene-linked molecules are characterized by different energetics of the stack formation and diverse in charge carrier mobilities. The branching of the side chains affects the planarity of the macromolecules, leads to longer pi-pi stacking distance and lamellar interval in the ordered arrays of polymers, and defines the stacking patterns of the conjugated backbones. The ambipolar transport is predicted for the majority of considered copolymer morphologies, and a quantitatively satisfactory agreement between experiment and computation is achieved.

ZnO films are commonly employed as n-type semiconductors in heterojunctions with PbS colloidal quantum-dot (CQDs) films because of their outstanding optical transparency and electron acceptor properties, yet studies of the impact of ZnO film microstructure, composition, and defect qualities on the solar-cell performance are quite limited. Here we report on the fabrication of ZnO films via pulsed-laser deposition and use these films to investigate how different morphologies affect the PbS CQD solar-cell performance. By modification of the background gas O2 pressures during the ZnO deposition process, the device performance approaching a 7.8% energy conversion efficiency is achieved with an O2 pressure of 100 mTorr. Higher or lower O2 pressures led to significantly lower device efficiency. We employ various materials and device characterizations to highlight the differences in the physical properties introduced by the fabrication oxygen pressure. In particular, we have found that the differences in the type and density of ZnO oxygen defects are the key factors behind the dispersion in solar-cell performances.

The oxidation speed of PbS quantum dots has been a subject of controversy for some time. In this study, we reveal the precise functional form of the oxidation rate constant for bare quantum dots through analysis of their photoluminescence as a function of temperature, oxygen pressure, and excitation-laser intensity. The combined effect of these factors results in a reduced energy barrier that allows the oxidation to proceed at a high rate. Each absorbed photon is found to have a 10−8 probability of oxidizing a PbS atomic pair. This highlights the importance of photo-excitation on the speed of the oxidation process, even at low illumination conditions. The procedure used here may set up a quantitative standard useful for characterizing the stability of quantum dots coated with ligands/linkers, and to compare different protection schemes in a fair quantitative way.

Strontium titanate (STO) is an oxide that has found application in several technological areas and is a candidate electrode for photoelectrochemical cells (PECs). In this study, STO thin films were prepared via pulsed laser deposition to function as PEC electrodes. Effects of post-deposition annealing in a reducing environment on the photocatalytic activity of these electrodes are characterized by measurements of optical and electrochemical properties of the films. These observations are used to obtain insight into how the crystal and electronic properties of these electrodes are affected by the annealing process. Low annealing temperatures produce non-crystalline STO films that exhibit low photocatalytic activity. Annealing at 500(degrees)C and higher allowed the formation of crystalline STO, which showed substantially higher ultraviolet-driven photocurrent densities. Electrochemical impedance spectroscopy reveals large decreases in charge transfer resistance that is associated with improved performance of these films. Oxygen evolution at these electrodes was confirmed with a rotating ring-disk electrode setup. Porous versions of the STO films were investigated to evaluate the effect larger surface area can have in enhancing the water oxidation performance.

Indium tin oxide (ITO) is the premier choice for transparent conductive electrodes in optoelectronic devices despite its inherent brittleness. Here we report the fabrication of a grating-like structure that obviates ITO's mechanical limitations while retaining its resistivity and optical qualities. ITO nanopatterned films exhibited a resistivity <1.3 × 10-3 Ω cm, which surpassed all previously reported values for flexible ITO, with a normal transmission >90% across the whole visible spectrum range. We demonstrate the nanopatterned ITO retains extraordinary flexibility and durability on heat-sensitive substrates, accommodating cyclic bending to a curvature diameter of at least 3.2 mm for over 50 cycles of compressive and decompressive flexing without significant deterioration of its resistivity or optical properties. Moreover, 2-dimensional extrapolation shows that multiaxial bending is also feasible while maintaining mechanical flexibility, durability, and optical transparency.

Laser deposition of ZnO under different O2 pressures controls the performance of the n-type semiconductor in PbS quantum dots solar cells. Detailed characterization shows the mechanism behind it and allows to obtain important device improvement.

We study the line roughness in poly(styrene-b-methyl methacrylate) symmetric block copolymer thin films and propose a phenomenological model to fit and describe the observed line edge, width, and placement roughness. Owing to the layering structure of symmetric block copolymers, we build from the model used to describe the thermal fluctuations in bilayer membranes and add a term for the bulk composition fluctuations in a phase segregated system. We use the peristaltic and undulatory modes of bilayer membranes to describe the width and placement roughness, respectively. We also include the correlations between adjacent domains to capture the cross-talk between alternating domains. We find that the model reproduces most of the main features observed in the power spectral density of our block copolymer films, providing a baseline to understand the physical properties that influence line roughness in a system relevant to nanolithography.

Vanadium dioxide (VO2) is a common material for use in thermochromic windows due to a semiconductor‐to‐metal transition (SMT) that is coupled with a change in infrared opacity. Commercialization of VO2‐based thermochromic technology is hampered by relatively expensive synthesis and film fabrication techniques as well as overall low performance as a window material. Here, simulations that indicate the plasmon resonance of VO2 nanoparticles in a composite film, which can be tuned to achieve record performance values, are reported. These simulations are experimentally verified by fabricating a VO2 nanoparticle composite in an elastomeric matrix using low‐temperature and atmospheric processing conditions. The optical properties of the films are analyzed, yielding visible transmittance and infrared modulation values within the range of top‐performing thermochromic windows. In addition, an improvement in performance is observed upon stretching the films, an effect that can be attributed to a local refractive index modulation. The results highlight the potential use of elastomeric composites as a low‐cost route to higher‐performance smart windows.

Materials informatics is a rapidly emerging data- and knowledge-driven approach for the identification of novel materials for a range of applications, including solar energy conversion. Despite significant experimental effort, the development of highly efficient, stable, and cost-effective photovoltaic materials remains a challenging scientific problem. The quest for precisely defined semiconductor properties revolves around an immensely broad landscape of structural parameters. Here, we have resolved this challenge by applying material informatics to design a novel photocathode material for dye-sensitized solar cells (DSSCs). By conducting a virtual screening of 50,000 known inorganic compounds, we have identified lead titanate (PbTiO3), a perovskite, as the most promising photocathode material. Notably, lead titanate is significantly different from the traditional base elements or crystal structures used for photocathodes. The fabricated PbTiO3 DSSC devices exhibited the best performance in aqueous solution, showing remarkably high fill factors compared to typical photocathode systems. The results highlight the pivotal role materials informatics can play in streamlining the experimental development of materials with the desired properties.

Colloidal quantum dot (CQD) solar cells prove to be promising devices for optoelectronic applications due to their tunable absorption range, deep infrared absorption capabilities, and straightforward processability. However, there remains a need to further enhance their device performance-particularly when one has to adhere to strict physical limitations on their physical structure. Here we present a three-dimensional numerical model of CQD solar cells in COMSOL Multiphysics based on the finite element method. With this model we have simulated the optical characteristics of several CQD solar cells across varying photonic structures and physical parameters to investigate how distinct photonic structures may enhance the light absorption and current output of CQD solar cells using identical physical parameters. Of the many cells simulated, one notable model increased the predicted current in the active layer PbS by 69.33% as compared to a flat solar cell with identical physical parameters, and produced a current of 24.18 mA cm(-2) by implementing a cross-shaped photonic structure built on top of a flat substrate of glass and ITO. This cross-shaped model serves as a key example of how unique photonic structures can be implemented to further enhance light absorption.

Corrugated structures are integral to many types of photoelectronic devices, used essentially for the manipulation of optical energy inputs. Here, we have investigated the gains and losses incurred by this microscale geometrical change. We have employed nanostructured electrode gratings of 600 nm pitch in PbS colloidal quantum dot (PbS-CQD) solar cells and investigated their effect on photovoltaic properties. Solar cells employing grating structure achieved a 32% and 20% increase in short-circuit current density (J(sc)) and power conversion efficiency, respectively, compared to nonstructured reference cells. The observed photocurrent increase of the structured devices mainly stems from the enhancement of photon absorption due to the trapping of optical energy by the grating structures. This optical absorption enhancement was particularly high in the near-infrared portion of the sun spectrum where PbS-based solar cells commonly present poor absorption. We have interestingly observed that the open-circuit voltage of all the devices increase with the increase in the absorbed photon energy (at a fixed light intensity), indicating a significant shift in Fermi energy level due to localization of low photon energy generated carriers in the tail of the density of states. We elucidate the role of the grating structure on charge dynamics and discuss the feasibility of these structures for construction of cheap and efficient photovoltaic devices.

The brilliant blue coloration of the Morpho rhetenor butterfly originates from complex nanostructures found on the surface of its wings. The Morpho butterfly exhibits strong short-wavelength reflection and a unique two-lobe optical signature in the incident (theta) and reflected (phi) angular space. Here, we report the large-area fabrication of a Morpho-like structure and its reproduction in perfluoropolyether. Reflection comparisons of periodic and quasi-random 'polymer butterfly' nanostructures show similar normal-incidence spectra but differ in the angular theta-phi dependence. The periodic sample shows strong specular reflection and simple diffraction. However, the quasi-random sample produces a two-lobe angular reflection pattern with minimal specular refection, approximating the real butterfly's optical behavior. Finite-difference time-domain simulations confirm that this pattern results from the quasi-random periodicity and highlights the significance of the inherent randomness in the Morpho's photonic structure.

Structural coloration has attracted great interest from scientists and engineers in recent years, owing to fascination with various brilliant examples displayed in nature as well as to promising applications of bio-inspired functional photonic structures and materials. Much research has been done to reveal and emulate the physical mechanisms that underlie the structural colors found in nature. In this article, we review the fundamental physics of many natural structural colors displayed by living organisms as well as their bio-inspired artificial counterparts, with emphasis on their connections, tunability strategies, and proposed applications, which aim to maximize the technological benefits one could derive from these photonic nanostructures. WIREs Nanomed Nanobiotechnol 2016, 8:758-775. doi: 10.1002/wnan.1396 For further resources related to this article, please visit the WIREs website.