Meniscus-assisted solution printing of large-grained perovskite films for high-efficiency solar cells.

Control over morphology and crystallinity of metal halide perovskite films is of key importance to enable high-performance optoelectronics. However, this remains particularly challenging for solution-printed devices due to the complex crystallization kinetics of semiconductor materials within dynamic flow of inks. Here we report a simple yet effective meniscus-assisted solution printing (MASP) strategy to yield large-grained dense perovskite film with good crystallization and preferred orientation. Intriguingly, the outward convective flow triggered by fast solvent evaporation at the edge of the meniscus ink imparts the transport of perovskite solutes, thus facilitating the growth of micrometre-scale perovskite grains. The growth kinetics of perovskite crystals is scrutinized by in situ optical microscopy tracking to understand the crystallization mechanism. The perovskite films produced by MASP exhibit excellent optoelectronic properties with efficiencies approaching 20% in planar perovskite solar cells. This robust MASP strategy may in principle be easily extended to craft other solution-printed perovskite-based optoelectronics.

Carrier dynamics and the role of surface defects: Designing a photocatalyst for gas-phase CO2 reduction.

In2O3-x(OH)y nanoparticles have been shown to function as an effective gas-phase photocatalyst for the reduction of CO2 to CO via the reverse water-gas shift reaction. Their photocatalytic activity is strongly correlated to the number of oxygen vacancy and hydroxide defects present in the system. To better understand how such defects interact with photogenerated electrons and holes in these materials, we have studied the relaxation dynamics of In2O3-x(OH)y nanoparticles with varying concentration of defects using two different excitation energies corresponding to above-band-gap (318-nm) and near-band-gap (405-nm) excitations. Our results demonstrate that defects play a significant role in the excited-state, charge relaxation pathways. Higher defect concentrations result in longer excited-state lifetimes, which are attributed to improved charge separation. This correlates well with the observed trends in the photocatalytic activity. These results are further supported by density-functional theory calculations, which confirm the positions of oxygen vacancy and hydroxide defect states within the optical band gap of indium oxide. This enhanced understanding of the role these defects play in determining the optoelectronic properties and charge carrier dynamics can provide valuable insight toward the rational development of more efficient photocatalytic materials for CO2 reduction.

Spatial Separation of Charge Carriers in In2O3-x(OH)y Nanocrystal Superstructures for Enhanced Gas-Phase Photocatalytic Activity.

The development of strategies for increasing the lifetime of photoexcited charge carriers in nanostructured metal oxide semiconductors is important for enhancing their photocatalytic activity. Intensive efforts have been made in tailoring the properties of the nanostructured photocatalysts through different ways, mainly including band-structure engineering, doping, catalyst-support interaction, and loading cocatalysts. In liquid-phase photocatalytic dye degradation and water splitting, it was recently found that nanocrystal superstructure based semiconductors exhibited improved spatial separation of photoexcited charge carriers and enhanced photocatalytic performance. Nevertheless, it remains unknown whether this strategy is applicable in gas-phase photocatalysis. Using porous indium oxide nanorods in catalyzing the reverse water-gas shift reaction as a model system, we demonstrate here that assembling semiconductor nanocrystals into superstructures can also promote gas-phase photocatalytic processes. Transient absorption studies prove that the improved activity is a result of prolonged photoexcited charge carrier lifetimes due to the charge transfer within the nanocrystal network comprising the nanorods. Our study reveals that the spatial charge separation within the nanocrystal networks could also benefit gas-phase photocatalysis and sheds light on the design principles of efficient nanocrystal superstructure based photocatalysts.

Photoexcited Surface Frustrated Lewis Pairs for Heterogeneous Photocatalytic CO2 Reduction.

In this study we investigated, theoretically and experimentally, the unique photoactive behavior of pristine and defected indium oxide surfaces providing fundamental insights into their excited state properties as well as an explanation for the experimentally observed enhanced activity of defected indium oxide surfaces for the gas-phase reverse water gas shift reaction, CO2 + H2 + hν→ CO + H2O in the light compared to the dark. To this end, a detailed excited-state study of pristine and defected forms of indium oxide (In2O3, In2O3-x, In2O3(OH)y and In2O3-x(OH)y) surfaces was performed using time dependent density functional theory (TDDFT) calculations, the results of which were supported experimentally by transient absorption spectroscopy and photoconductivity measurements. It was found that the surface frustrated Lewis pairs (FLPs) created by a Lewis acidic coordinately unsaturated surface indium site proximal to an oxygen vacancy and a Lewis basic surface hydroxide site in In2O3-x(OH)y become more acidic and basic and hence more active in the ES compared to the GS. This provides a theoretical mechanism responsible for the enhanced activity and reduced activation energy of the photochemical reverse water gas shift reaction observed experimentally for In2O3-x(OH)y compared to the thermochemical reaction. This fundamental insight into the role of photoexcited surface FLPs for catalytic CO2 reduction could lead to improved photocatalysts for solar fuel production.

Electronic and vibrational dynamics of hollow au nanocages embedded in cu2 o shells.

We have synthesized hollow Au nanocages embedded within thick porous shells of cuprous oxide (Cu2 O). The shell causes a significant redshift of the localized surface plasmon resonance of Au into the near-IR. Electron-phonon coupling in the Au nanocage is 3-6 times faster in the core-shell structure due to the higher thermal conductivity of Cu2 O compared to water. Coherent phonon oscillations within the Au lattice are characterized by a breathing mode of the entire structure for both bare and core-shell nanocages, an assignment made through the use of structural mechanics simulations. The experimental frequencies are obtained through simulations by selectively applying a force to the shell of the core-shell structure. We interpret this as rapid thermal expansion of the gold leading to a mechanical force that acts on the shell.

Self-assembled nanostructured photoanodes with staggered bandgap for efficient solar energy conversion.

Vertically oriented Ta-W-O nanotube array films were fabricated via the anodization of Ta-W alloy foils in HF-containing electrolytes. HF concentration is a key parameter in achieving well-adhered nanotube array structure. X-ray photoelectron spectroscopy (XPS) and diffuse reflectance measurements confirm the staggered band-alignment between Ta2O5 and WO3, which facilitates the separation of charge carriers. The nanotubes made of Ta-W films containing 10% W showed 100-fold improvement in the measured photocurrent compared to pristine Ta2O5 upon their use to split water photoelectrochemically. This enhancement was related to the efficient charge transport and the red shift in absorption spectrum with increase of the W content, which was asserted by ultrafast transient absorption (TA) spectroscopy measurements. The TA measurements showed the elimination of trap states upon annealing Ta-W-O nanotubes and, hence, minimizing the charge carrier trapping, whereas the trap states remain in pristine Ta2O5 nanotubes even after annealing.

Energy-transfer efficiency in Eu-doped ZnO thin films: the effects of oxidative annealing on the dynamics and the intermediate defect states.

We have studied ultrafast dynamics in thin films of Eu-doped zinc oxide (ZnO), prepared by radio-frequency sputtering onto sapphire substrates. Following UV excitation of ZnO, a red emission is observed. Postdeposition annealing in an oxygen atmosphere improves the crystallinity and emission intensity of the films, which are highly sensitive to the dopant concentration. Transient-absorption spectroscopy shows that the excited semiconductor host transfers energy to rare-earth ions on a time scale of only a few picoseconds. The dynamics as a function of the probe wavelength change dramatically after annealing, with annealed films showing the fastest dynamics at much lower wavelengths. Our results show that annealing greatly affects the defect energy levels of the films and the dynamics of the trapped carriers. Unannealed films show dynamics consistent with energy transfer from O vacancies to the dopant, while energy transfer in annealed samples involves acceptor-type defects such as Zn vacancies as intermediates.

Role of solvent-oxygen ion pairs in photooxidation of CdSe nanocrystal quantum dots.

Understanding the mechanisms for photodegradation of nanocrystal quantum dots is an important step toward their application in real-world technologies. A usual assumption is that photochemical modifications in nanocrystals, such as their photooxidation, are triggered by absorption of a photon in the dot itself. Here, we demonstrate that, contrary to this commonly accepted picture, nanocrystal oxidation can be initiated by photoexcitation of solvent-oxygen ion pairs that relax to produce singlet oxygen, which then reacts with the nanocrystals. We make this conclusion on the basis of photolysis studies of solutions of CdSe nanocrystals. Our measurements indicate a sharp spectral onset for photooxidation, which depends on solvent identity and is 4.8 eV for hexane and 3.4 eV for toluene. Importantly, the photooxidation onset correlates with the position of a new optical absorption feature, which develops in a neat solvent upon its exposure to oxygen. This provides direct evidence that nanocrystal photooxidation is mediated by excitation of solvent-oxygen pairs and suggests that the stability of the nanocrystals is defined by not only the properties of their surfaces (as has been commonly believed) but also the properties of their environment, that is, of the surrounding solvent or matrix.

Different Methods of Increasing the Mechanical Strength of Gold Nanocages.

Using the ultrafast coherent modulation of the surface plasmon band intensity with the totally symmetric lattice vibration of gold nanocages, we were able to determine and use their frequencies as a measure of the cage's mechanical stability. The presence of an inner "stiff" transition-metal nanoshell with a higher value of the elastic modulus is found to increase the frequency of the lattice vibration of the outer soft gold nanoshell. This could also explain the observed increase in both the gold lattice vibrational frequency as well as the lattice vibration relaxation time in the Au-Pt and Au-Pd double-shell nanocages. It is also found that when these nanoparticles are assembled into monolayers on quartz substrates by the Langmuir-Blodgett technique, the oscillation frequency of the gold shell with the transition metal having the largest elastic constant suffers the least change in its oscillation frequency as a result of its resistance to distortion as a result of binding to the substrate.

Tailoring plasmonic and electrostatic field effects to maximize solar energy conversion by bacteriorhodopsin, the other natural photosynthetic system.

We have explored the plasmonic field enhancement of current production from bacteriorhodopsin (bR) by maximizing the blue light effect, where the influx of blue photons absorbed by the long-lived M intermediate drastically shortens the time scale of the bR photocycle, leading to current enhancement. To this end, we used three approaches in our solution-based cell: (1) We improved the charge carrier separation in solution through the use of a proton-selective Nafion membrane. (2) We maximized the plasmonic surface field effects by selecting the capping polymer with minimum surface field screening and best nanoparticle stability. (3) We selected the plasmonic nanoparticle with the strongest plasmonic field whose surface plasmon resonance has the largest spectral overlap with the blue light absorbing M-intermediate. Theoretical models are used to explain experimental results, which show a 40 nm cuboidal nanoparticle capped with 55k PVP polymer to give the best photocurrent enhancement. Enhanced by this particle, bR in our Nafion membrane solution cell gave a photocurrent of 0.21 µA/cm(3), which is 5000 times larger than the published results for thin film bR electrochemical cells even with an applied bias. Additional possible enhancements are proposed.

Effect of organic passivation on photoinduced electron transfer across the quantum dot/TiO2 interface.

We report a study of the internal quantum efficiency (IQE) of CdSe quantum-dot (QD)-sensitized solar cells prepared by direct adsorption of pre-synthesized QDs, passivated with either tri-n-octylphosphine oxide (TOPO) or n-butylamine (BA), onto a nanocrystalline TiO(2) film.

Development and validation of an improved inducer-regulator protein complex in the pBRES-regulated expression system.

Widespread adaptation of small molecule-regulated expression systems requires the development of selective inducer molecules that do not have any significant side effects on the endogenous receptors from which the regulated expression system is derived. Here we report the identification and in vitro validation of a novel inducer-receptor pair for the single-plasmid regulated expression system termed pBRES, which contains the ligand-binding domain from the human progesterone receptor (hPR). A small molecule inducer, BLX-913, has been identified as having a 30-fold lower IC(50) for the human progesterone receptor than mifepristone (MFP), the previously best characterized inducer for pBRES. Using modeling-guided protein engineering, compensatory mutations were installed at positions W755 and V729 (hPR numbering) in the ligand-binding pocket of the pBRES regulator protein (pBRES RP) to accommodate the new inducer and allow induction of transgene expression to levels previously seen with MFP. The improved inducer-pBRES RP complex was validated in vitro by monitoring the induction of luciferase, murine secreted alkaline phosphatase, and human interferon beta transgenes in mouse skeletal muscle cells. The engineered pBRES demonstrated low levels of transgene expression in the absence, and high expression levels in the presence, of the new BLX-913 inducer. Findings presented here allow induction of the pBRES-regulated gene expression system by a compound with markedly lower anti-hPR activity than MFP, the previously best characterized inducer.

Temperature-dependent femtosecond photoinduced desorption in CO/Pd(111).

The desorption of CO from a Pd(111) surface following absorption of 120 fs pulses of 780 nm light occurs on two distinct and well-separated time scales. Two-pulse correlation measurements show a fast subpicosecond decay followed by a slower, approximately 40 ps decay. Simulations based on the two-temperature model of electron and phonon heat baths within the substrate, and an empirical friction model to treat coupling to the adsorbate, support the assignment of the desorption mechanism as an electron-mediated process. The photodesorption yield and overall width of the temporal response exhibit a marked dependence on the initial surface temperature in the 100-375 K range despite the much higher transient electronic temperatures (approximately 7000 K) achieved. The observed temperature dependences can be attributed directly to variations in the initial temperature within the frictional coupling picture. Simulations of this extended data set imply that the activation barrier to photoinduced desorption is equal in magnitude to that derived from thermal desorption experiments for this system within the limits of a one-dimensional Arrhenius desorption model. The simulations also imply that the slower decay is not the result of phonon-driven desorption. Though we cannot unambiguously determine the strength of the adsorbate-phonon coupling, our results suggest that its role is to moderate the degree of the adsorbate excitation.

Adsorption-state-dependent subpicosecond photoinduced desorption dynamics.

Femtosecond laser excitation has been used to initiate desorption of molecular oxygen from the (111) surface of Pd and to study the adsorption-state dependence of the substrate-adsorbate coupling. The relative populations of the two chemical states, peroxo (O2(2-)) and superoxo (O2-), were varied by changing the total coverage. Two-pulse correlation measurements exhibit a dominant 400 fs response and a slower 10 ps decay that are relatively independent of the initial O2 coverage. In contrast, the photodesorption yield and the nonlinearity of the fluence dependence show a systematic coverage dependence. The coverage-independent subpicosecond response indicates that the photoinduced desorption from the two states is driven primarily by the same electron-mediated mechanism, while the coverage dependence of the yield indicates that the desorption efficiency from the superoxo state is greater than that from the peroxo state. These results are discussed in the context of the electron-phonon two-temperature model with an empirical adsorbate-electron frictional coupling that depends on both the electronic temperature and the activation energy for desorption. With a coupling strength that decreases as the activation energy decreases, the trends with varying coverage, absorbed fluence, and time delay can all be reproduced. The model is consistent with a transition from a resonantly enhanced (diabatic) regime to an adiabatic regime as the system relaxes, accounting for the biexponential correlation behavior.

Development and validation of a robust and versatile one-plasmid regulated gene expression system.

We have developed a one-plasmid regulated gene expression system, pBRES, based on a mifepristone (MFP)-inducible two-plasmid system. The various expression elements of the pBRES system (promoters, 5' and 3' untranslated regions (UTRs), introns, target gene, and polyA sequences) are bounded by restriction enzyme sites so that each module can be conveniently replaced by alternate DNA elements in order to tailor the system for particular tissues, organs, or conditions. There are four possible orientations of the two expression units relative to each other, and insertion of a variety of expression elements and target genes into the four different orientations revealed orientation- and gene-dependent effects on induced and uninduced levels of gene expression. Induced target gene expression from the pBRES system was shown to be comparable to the two-plasmid system and higher than the expression from the cytomegalovirus (CMV) promoter in vivo, while maintaining low uninduced levels of expression. Finally, a pBRES expression cassette was transferred to an adeno-associated virus (AAV) vector and shown to be capable of regulated gene expression in vivo for nearly 1 year.

Mx1 and IP-10: biomarkers to measure IFN-beta activity in mice following gene-based delivery.

Recombinant interferon-beta (IFN-beta) protein is used successfully for the treatment of multiple sclerosis (MS). Gene therapy might be an alternative approach to overcome drawbacks occurring with IFN-beta protein therapy. A critical issue in developing a new approach is detection of biologically active IFN-beta in preclinical models. The goal of the present study was to determine if Mx1 and IP-10, which are known to be activated after IFN-beta treatment in humans, can be used as biomarkers in mice. In three in vivo experiments, the correlation between different methods of murine IFN-beta (MuIFN-beta) delivery and biomarker induction was studied: (1) bolus protein delivery by intravenous (i.v.) or intramuscular (i.m.) injection, (2) gene-based delivery of IFN- beta by i.m. injection of plasmid DNA, followed by electroporation, and (3) gene-based delivery of IFN-beta by i.m. injection of adenovirus-associated type 1 (AAV1). Short-term induction of Mx1 mRNA and IP-10 was observed after treatment with bolus MuIFN-beta protein. Long-term induction of both biomarkers was observed after IFN-beta plasmid DNA delivery or when AAV1 was used as the vector. The experiments demonstrate that gene-based delivery provides sustained levels of IFN-beta compared with bolus protein injection and that Mx1 RNA and IP-10 can be used to monitor biologically active circulating plasma MuIFN-beta protein in mice.

Gene-based delivery of IFN-beta is efficacious in a murine model of experimental allergic encephalomyelitis.

Experimental allergic encephalomyelitis (EAE) is a model of central nervous system (CNS) inflammation that follows immunization with certain CNS antigens. The course and clinical manifestations of EAE are similar to those of multiple sclerosis (MS) in humans; therefore, EAE has become an accepted animal model to study MS. The purpose of this study was to demonstrate that systemic expression of murine interferon-beta (IFN-beta) (MuIFN-beta), following intramuscular (i.m.) delivery of plasmid DNA encoding MuIFN-beta to the hind limb of mice, is effective in reducing the clinical manifestations of disease in a model of EAE. The results of the study demonstrate that gene-based delivery of MuIFN-beta caused significantly decreased clinical scores compared with delivery of the null vector. A single injection of the MuIFN-beta plasmid was as effective in reducing the severity of the disease as an every other day injection of MuIFN-beta protein.

Determination of band curvatures by angle-resolved two-photon photoemission in thin films of C60 on Ag(111).

The thickness-dependent interfacial band structure was determined for thin films of C(60) on Ag(111) by angle-resolved two-photon photoemission spectroscopy. Dispersions of molecular-orbital derived bands (HOMO, LUMO+1, and LUMO+2) were acquired, and limits were placed on their possible effective masses. A group theoretic approach is also incorporated to further understand the properties of these states. The HOMO, LUMO+1, and LUMO+2 bands possess (best-fit) effective masses of -7 m(e), -7 m(e), and -12 m(e), respectively. These values are consistent with theoretical calculations, averaged over the closely spaced subbands for each state, and provide practical limits on the effective fundamental charge-transport properties of C(60) films.

Ultrafast electron dynamics at metal interfaces: intraband relaxation of image state electrons as friction.

Two-photon photoemission of image potential states above monolayers of p-xylene/Ag(111) shows that electrons with different momenta have very different rise and decay rates as a function of parallel momentum. The dynamics are due to energy and momentum loss (intraband relaxation), which we model as a stochastic process isomorphic to the overdamped motion of a harmonic oscillator. The method extracts a friction coefficient from the data which can be explained by electron-electron scattering in a formalism based on the Lindhard dielectric function. One-electron excitations (interband transistions) dominate the dissipation mechanism, with a smaller contribution from collective electronic excitations (plasmons).