Fast-Growth Polymer: Fullerene Bulk-Heterojunction Thin Films for Efficient Organic Photovoltaics.

The bulk-heterojunction (BHJ) system that uses a π-conjugated polymer as an electron donor, and a fullerene derivative as an electron acceptor, is widely used in organic solar cells (OSCs) to facilitate efficient charge separation and extraction. However, the conventional BHJ system still suffers from unwanted phase segregation caused by the existence of significant differences in surface energy between the two BHJ components and the charge extraction layer during film formation. In the present work, we demonstrate a sophisticated control of fast film-growth kinetics that can be used to achieve a uniform distribution of donor and acceptor materials in the BHJ layer of OSCs without undesirable phase separation. Our approach involves depositing the BHJ solution onto a spinning substrate, thus inducing rapid evaporation of the solvent during BHJ film formation. The fast-growth process prevents the fullerene derivative from migrating toward the charge extraction layer, thereby enabling a homogeneous distribution of the fullerene derivative within the BHJ film. The OSCs based on the fast-growth BHJ thin film are found to exhibit substantial increases in JSC, fill factor, and a PCE up to 11.27 mA/cm2, 66%, and 4.68%, respectively; this last value represents a remarkable 17% increase in PCE compared to that of conventional OSCs.

Correction to "Reversible Decomposition of Single-Crystal Methylammonium Lead Iodide Perovskite Nanorods".

[This corrects the article DOI: 10.1021/acscentsci.0c00385.].

Polar Metal Phase Induced by Oxygen Octahedral Network Relaxation in Oxide Thin Films.

ABO3 perovskite materials and their derivatives have inherent structural flexibility due to the corner sharing network of the BO6 octahedron, and the large variety of possible structural distortions and strong coupling between lattice and charge/spin degrees of freedom have led to the emergence of intriguing properties, such as high-temperature superconductivity, colossal magnetoresistance, and improper ferroelectricity. Here, an unprecedented polar ferromagnetic metal phase in SrRuO3 (SRO) thin films is presented, arising from the strain-controlled oxygen octahedral rotation (OOR) pattern. For compressively strained SRO films grown on SrTiO3 substrate, oxygen octahedral network relaxation is accompanied by structural phase separation into strained tetragonal and bulk-like orthorhombic phases, and the asymmetric OOR evolution across the phase boundary allows formation of the polar phase, while bulk metallic and ferromagnetic properties are maintained. From the results, it is expected that other oxide perovskite thin films will also yield similar structural environments with variation of OOR patterns, and thereby provide promising opportunities for atomic scale control of material properties through strain engineering.

Reversible Decomposition of Single-Crystal Methylammonium Lead Iodide Perovskite Nanorods.

Perovskite solar cells offer remarkable performance, but further advances will require deeper understanding and control of the materials and processing. Here, we fabricate the first single crystal nanorods of intermediate phase (MAI-PbI2-DMSO), allowing us to directly observe the phase evolution while annealing in situ in a high-vacuum transmission electron microscope, which lets up separate thermal effects from other environmental conditions such as oxygen and moisture. We attain the first full determination of the crystal structures and orientations of the intermediate phase, evolving perovskite, precipitating PbI2, and e-beam induced PbI2 during phase conversion and decomposition. Surprisingly, the perovskite decomposition to PbI2 is reversible upon cooling, critical for long-term device endurance due to the formation of MAI-rich MAPbI3 and PbI2 upon heating. Quantitative measurements with a thermodynamic model suggest the decomposition is entropically driven. The single crystal MAPbI3 nanorods obtained via thermal cycling exhibit excellent mobility and trap density, with full reversibility up to 100 °C (above the maximum temperature for solar cell operation) under high vacuum, offering unique potential for high-performance flexible solar cells.

Exceptional Tunability over Size and Density of Spontaneously Formed Nanoparticles via Nucleation Dynamics.

The ex-solution process, in which metal nanoparticles are grown on a host oxide, can be used to synthesize nanocatalysts with excellent thermal and chemical durability via spontaneous heterogeneous nucleation. However, this technique lacks a means to control the particle size and density because the amounts of ex-solved metal elements vary with the reduction conditions. Here, we devise a strategy to achieve small particle sizes and high particle densities concurrently by controlling the temperature (T), oxygen partial pressure (pO2) and ramping rate of the temperature. Quantitative analyses of Co particles ex-solved on Sr0.98Ti0.95Co0.05O3-δ thin films using ex situ SEM and in situ TEM reveal that the increasing T and decreasing the pO2 lead to smaller particle sizes with higher density levels and vice versa, contrary to common ex-solution examples. We find that nucleation thermodynamics dictates such counterintuitive behaviors of particle characteristics, which are attributed to our specific ex-solution conditions in which particle interactions are minimized and all the Co atoms are ex-solved under highly reducible conditions. We also demonstrated the feasibility of our strategy via CO oxidation with typical powder-based catalysts, suggesting that this method can be extended to various chemical/electrochemical applications.

Unsymmetrical Small Molecules for Broad-Band Photoresponse and Efficient Charge Transport in Organic Phototransistors.

Organic photosensitizers have been investigated as effective light-sensing elements that can promote strong absorption with high field-effect mobility in organic phototransistors (OPTs). In this study, a novel organic photosensitizer is synthesized to demonstrate broad-band photoresponse with enhanced electrical performance. An unsymmetrical small molecule of a solubilizing donor (Dsol)-acceptor (A)-dye donor (Ddye) type connected with a twisted conjugation system is designed for broad-band detection (ranging from 250 to 700 nm). This molecule has high solubility, thereby facilitating the formation of uniformly dispersed nanoparticles in an insulating polymer matrix, which is deposited on top of OPT semiconductors by a simple solution process. The broad-band photodetection shown by the organic photosensitizer is realized with improved mobility close to an order of magnitude and high on/off current ratio (∼105) of the organic semiconductor. Furthermore, p-type charge transport behavior in the channel of the OPT is enhanced through the intrinsic electron-accepting ability of the organic photosensitizer caused by the unique molecular configuration. These structural properties of organic photosensitizers contribute to an improvement in broad-band photosensing systems with new optoelectronic properties and functionalities.

Ultrafast Infrared Photoresponse from Heavily Hydrogen-Doped VO2 Single Crystalline Nanoparticles.

Infrared photodetectors are sought for diverse applications and their performance relies on photoactive materials and photocurrent generation mechanisms. Here, we fabricate IR photodetectors with heavily hydrogen-doped VO2 (i.e., HVO2) single-crystalline nanoparticles which show two orders greater resistivities than pure VO2. The I-V plots obtained under IR light irradiation are expressed by space charge limited current mechanism and the increase in photocurrent occurs due to the increase in the number of photoinduced trap sites. This phenomenon remarkably improves the key parameters at λ = 780 nm of high responsivity of 35280 A/W, high detectivity of 1.12 × 1013 Jones, and strikingly fast response times of 0.6-2.5 ns, that is, 3 orders of magnitude faster than the best records of two-dimensional structures and heterostructures. Density functional theory calculations illustrate that the generation of photoinduced trap sites is attributed to the movement of hydrogen atoms to less stable interstitial sites in VO2 under light exposure.

Reversible Polymorphic Transition and Hysteresis-Driven Phase Selectivity in Single-Crystalline C8-BTBT Rods.

Organic semiconductors (OSCs) are highly susceptible to the formation of metastable polymorphs that are often transformed by external stimuli. However, thermally reversible transformations in OSCs with stability have not been achieved due to weak van der Waals forces, and poor phase homogeneity and crystallinity. Here, a polymorph of a single crystalline 2,7-dioctyl[1] benzothieno[3,2-b][1]benzothio-phene rod on a low molecular weight poly(methyl methacrylate) (≈120k) that limits crystal coarsening during solvent vapor annealing is fabricated. Molecules in the polymorph lie down slightly toward the substrate compared to the equilibrium state, inducing an order of greater resistivity. During thermal cycling, the polymorph exhibits a reversible change in resistivity by 5.5 orders with hysteresis; this transition is stable toward bias and thermal cycling. Remarkably, varying cycling temperatures leads to diverse resistivities near room temperature, important for nonvolatile multivalue memories. These trends persist in the carrier mobility and on/off ratio of the polymorph field-effect transistor. A combination of in situ grazing incident wide angle X-ray scattering analyses, visualization for electronic and structural analysis simulations, and density functional theory calculations reveals that molecular tilt governs the charge transport characteristics; the polymorph transforms as molecules tilt, and thereby, only a homogeneous single-crystalline phase appears at each temperature.

Hollow Porous Gold Nanoshells with Controlled Nanojunctions for Highly Tunable Plasmon Resonances and Intense Field Enhancements for Surface-Enhanced Raman Scattering.

Plasmonic metal nanostructures with nanogaps have attracted great interest owing to their controllable optical properties and intense electromagnetic fields that can be useful for a variety of applications, but precise and reliable control of nanogaps in three-dimensional nanostructures remains a great challenge. Here, we report the control of nanojunctions of hollow porous gold nanoshell (HPAuNS) structures by a facile oxygen plasma-etching process and the influence of changes in nanocrevices of the interparticle junction on the optical and sensing characteristics of HPAuNSs. We demonstrate a high tunability of the localized surface plasmon resonance (LSPR) peaks and surface-enhanced Raman scattering (SERS) detection of rhodamine 6G (R6G) using HPAuNS structures with different nanojunctions by varying the degree of gold sintering. As the neck region of the nanojunction is further sintered, the main LSPR peak shifts from 785 to 1350 nm with broadening because the charge transfer plasmon mode becomes more dominant than the dipolar plasmon mode, resulting from the increase of conductance at the interparticle junctions. In addition, it is demonstrated that an increase in the sharpness of the nanojunction neck can enhance the SERS enhancement factor of the HPAuNS by up to 4.8-fold. This enhancement can be ascribed to the more intense local electromagnetic fields at the sharper nanocrevices of interparticle junctions. The delicate change of nanojunction structures in HPAuNSs can significantly affect their optical spectrum and electromagnetic field intensity, which are critical for their practical use in a SERS-based analytical sensor as well as multiple-wavelength compatible applications.

In Situ Growth of Nanostructured BiVO4-Bi2O3 Mixed-Phase via Nonequilibrium Deposition Involving Metal Exsolution for Enhanced Photoelectrochemical Water Splitting.

Nonequilibrium deposition is a remarkable method for the in situ growth of unique nanostructures and phases for the functionalization of thin films. We introduce a distinctive structure of a mixed-phase, composed of BiVO4 and ß-Bi2O3, for photoelectrochemical water splitting. The mixed-phase is fabricated via nonequilibrium deposition by adjusted oxygen partial pressure. According to density functional theory calculations, we find that vanadium exsolution can be facilitated by introducing oxygen vacancies, enabling the fabrication of a nanostructured mixed-phase. These unique structures enhance charge migration by increasing the interfacial area and properly aligning the band offset between two crystalline phases. Consequently, the photocurrent density of the nanostructured mixed-phase thin films is about twice that of pristine BiVO4 thin films at 1.23 VRHE. Our work suggests that nonequilibrium deposition provides an innovative route for the structural engineering of photoelectrodes for the understanding of fundamental properties and improving the photocatalytic performance for solar water splitting.

Growth Kinetics of Individual Au Spiky Nanoparticles Using Liquid-Cell Transmission Electron Microscopy.

Precise control over the size and morphology of the Au spiky nanoparticle (SNP) is essential to obtain narrow and tunable surface plasmon resonance (SPR). However, these challenges require a fundamental understanding of the particle growth mechanism and kinetics as well as its morphological transition, which can only be achieved by real-time observation at nanometer resolution. Here, we report in situ liquid cell transmission electron microscopy studies of single and multiple Au SNP growth at various conditions of such parameters as size and dose rate of electron beam and HAuCl4 solution concentration. The particle evolves via a mixture of reaction and Au formation-limited growth, followed by Au formation-limited growth-a transition from faceted to roughened surfaces, confirmed by the analysis with different beam sizes and the UV-vis spectra that feature a unique trend of short- and long-wavelength plasmon band shift. Quantitative analyses combined with a theoretical model determine the transition time (tc) of the two regimes and estimate the surface concentration (ci) of the particle with time. Interestingly, tc can be manipulated by the particle density, which alters the surface roughening rate, and the density is modulated by tuning the aforementioned parameters based on DLVO theory. These results suggest a new method for synthesizing a Au SNP whose size, morphology, SPR, and density can be sensibly manipulated without adding reducing or capping agents.

Growth Kinetics of Individual Co Particles Ex-solved on SrTi0.75Co0.25O3-δ Polycrystalline Perovskite Thin Films.

A precise control of the size, density, and distribution of metal nanoparticles dispersed on functional oxide supports is critical for promoting catalytic activity and stability in renewable energy and catalysis devices. Here, we measure the growth kinetics of individual Co particles ex-solved on SrTi0.75Co0.25O3-δ polycrystalline thin films under a high vacuum, and at various temperatures and grain sizes using in situ transmission electron microscopy. The ex-solution preferentially occurs at grain boundaries and corners which appear essential for controlling particle density and distribution, and enabling low temperature ex-solution. The particle reaches a saturated size after a few minutes, and the size depends on temperature. Quantitative measurements with a kinetic model determine the rate limiting step, vacancy formation enthalpy, ex-solution enthalpy, and activation energy for particle growth. The ex-solved particles are tightly socketed, preventing interactions among them over 800 °C. Furthermore, we obtain the first direct clarification of the active reaction site for CO oxidation-the Co-oxide interface, agreeing well with density functional theory calculations.

Crystal-Structure-Dependent Piezotronic and Piezo-Phototronic Effects of ZnO/ZnS Core/Shell Nanowires for Enhanced Electrical Transport and Photosensing Performance.

We report the crystal-structure-dependent piezotronic and piezo-phototronic effects of ZnO/ZnS core/shell nanowires (CS NWs) having different shell layer crystalline structures. The wurtzite (WZ) ZnO/WZ ZnS CS NWs showed higher electrical transport and photosensing properties under external strain than the WZ ZnO/zinc blende (ZB) ZnS CS NWs. The WZ ZnO/WZ ZnS CS NWs under a compressive strain of -0.24% showed 4.4 and 8.67 times larger increase in the output current (1.93 × 10-4 A) and photoresponsivity (8.76 × 10-1 A/W) than those under no strain. However, the WZ ZnO/ZB ZnS CS NWs under the same strain condition showed 3.2 and 2.16 times larger increase in the output current (1.13 × 10-4 A) and photoresponsivity (2.16 × 10-1 A/W) than those under no strain. This improvement is ascribed to strain-induced piezopolarization charges at both the WZ ZnO NWs and the grains of the WZ ZnS shell layer in WZ ZnO/WZ ZnS CS NWs, whereas piezopolarization charges are induced only in the ZnO core region of the WZ ZnO/ZB ZnS CS NWs. These charges can change the type-II band alignment in the ZnO and ZnS interfacial region as well as the Schottky barrier height at the junction between the semiconductor and the metal, thus facilitating electrical transport and reducing the recombination probability of charge carriers under UV irradiation.

Radial multi-quantum well ZnO nanorod arrays for nanoscale ultraviolet light-emitting diodes.

Since semiconducting ZnO has attractive properties such as wide bandgap and large exciton binding energy, it has motivated us to realize efficient ultraviolet (UV) light-emitting diodes (LEDs). Furthermore, facile growth of ZnO nanostructures has triggered numerous research studies to examine them as nanoscale building blocks for optoelectronic devices. Here, we demonstrate the growth of ZnO-based core-shell p-n homojunction nanorod arrays with radial MgZnO/ZnO multiple quantum wells (MQWs) and report the characteristics of a core-shell ZnO nanorod LED. The shell layers of MgZnO/ZnO MQWs and p-type antimony-doped MgZnO were epitaxially grown on the surface of ZnO core nanorod arrays. By introducing the radial MQWs, the photoluminescence intensity was greatly increased by 4 times, compared to that of the bare ZnO nanorod array, suggesting that the core-shell MQWs can be used to realize the nanoscale ZnO LEDs with high internal quantum efficiency. As the injection current increased, the EL intensity of UV emission at 375 nm from the MgZnO/ZnO MQWs strongly increased without shifting of the emission peak because of the non-polar nature of MQWs grown on the side walls of the ZnO nanorods. These results highlight the potential of an integrated nanoscale UV light emitter in various photonic devices.

Close-packed polymer crystals from two-monomer-connected precursors.

The design of crystalline polymers is intellectually stimulating and synthetically challenging, especially when the polymerization of any monomer occurs in a linear dimension. Such linear growth often leads to entropically driven chain entanglements and thus is detrimental to attempts to realize the full potential of conjugated molecular structures. Here we report the polymerization of two-monomer-connected precursors (TMCPs) in which two pyrrole units are linked through a connector, yielding highly crystalline polymers. The simultaneous growth of the TMCP results in a close-packed crystal in polypyrrole (PPy) at the molecular scale with either a hexagonal close-packed or face-centred cubic structure, as confirmed by high-voltage electron microscopy, and the structure that formed could be controlled by simply changing the connector. The electrical conductivity of the TMCP-based PPy is almost 35 times that of single-monomer-based PPy, demonstrating its promise for application in diverse fields.

Titanium oxide nanotube arrays for high light extraction efficiency of GaN-based vertical light-emitting diodes.

TiO2 nanotube (NT) arrays were fabricated on the surface of n-GaN through a liquid-phase conversion process using ZnO nanorods (NRs) as a template for high-efficiency InGaN/GaN multiple quantum well (MQW) vertical light-emitting diodes (VLEDs). The optical output power of the VLEDs with TiO2 NTs was remarkably enhanced by 23% and 189% at an injection current of 350 mA compared to those of VLEDs with ZnO NRs and planar VLEDs, respectively. The large enhancement in optical output is attributed to a synergistic effect of efficient light injection from the n-GaN layer of the VLED to TiO2 NTs because of the well-matched refractive indices and superior light extraction into air at the end of the TiO2 NTs. Light propagation along various configurations of TiO2 NTs on the VLEDs was investigated using finite-difference time domain simulations and the results indicated that the wall thickness of the TiO2 NTs should be maintained close to 20 nm for superior light extraction from the VLEDs.

Organic Single-Crystal Semiconductor Films on a Millimeter Domain Scale.

Nucleation and growth processes can be effectively controlled in organic semiconductor films through a new concept of template-mediated molecular crystal seeds during the phase transition; the effective control of these processes ensures millimeter-scale crystal domains, as well as the performance of the resulting organic films with intrinsic hole mobility of 18 cm(2) V(-1) s(-1).

Observation and tunability of room temperature photoluminescence of GaAs/GaInAs core-multiple-quantum-well shell nanowire structure grown on Si (100) by molecular beam epitaxy.

We report the observation of room temperature photoluminescence (PL) emission from GaAs/GaInAs core-multiple-quantum-well (MQW) shell nanowires (NWs) surrounded by AlGaAs grown by molecular beam epitaxy (MBE) using a self-catalyzed technique. PL spectra of the sample show two PL peaks, originating from the GaAs core NWs and the GaInAs MQW shells. The PL peak from the shell structure red-shifts with increasing well width, and the peak position can be tuned by adjusting the width of the MQW shell. The GaAs/GaInAs core-MQW shell NW surrounded by AlGaAs also shows an enhanced PL intensity due to the improved carrier confinement owing to the presence of an AlGaAs clad layer. The inclined growth of the GaAs NWs produces a core-MQW shell structure having a different PL peak position than that of planar QWs. The PL emission by MQW shell and the ability to tune the PL peak position by varying the shell width make such core-shell NWs highly attractive for realizing next generation ultrasmall light sources and other optoelectronics devices. PACS: 81.07.Gf; 81.15.Hi; 78.55.Cr.

Directly-grown and square-patterned arrays of metal oxide nanowires for high-performance catalyst support platforms.

This research reports novel and efficient electrocatalyst support systems. Tin dioxide nanowires grown directly on current collecting substances are introduced as high-performance support platforms. For this propose, palladium or platinum catalysts are impregnated on these nanowire scaffolds and exhibit improved electrocatalytic performance for methanol oxidation in alkaline and acidic environments. These nanowire support platforms could be demonstrated to maximize the electrocatalytic activity because of the effective charge transport provided by the direct connection between the nanowire supports and current collectors. More significantly, grid-patterned nanowire arrays grown directly on current collectors are, for the first time, demonstrated as a milestone to enhance the electrocatalytic performance. The empty space between the patterned nanowire arrays acts as a channel to facilitate the electrolyte diffusion. The metal catalysts incorporated into the patterned nanowire supports show an 8-fold improvement in the catalytic performance for methanol electrooxidation, most likely because of the synergetic effects of the enhanced charge transport and mass transfer attributed to the structural advantages of the patterned nanowire array supports.