Low-Threshold Lasing from 2D Homologous Organic-Inorganic Hybrid Ruddlesden-Popper Perovskite Single Crystals.

Organic-inorganic hybrid two-dimensional (2D) perovskites have recently attracted great attention in optical and optoelectronic applications due to their inherent natural quantum-well structure. We report the growth of high-quality millimeter-sized single crystals belonging to homologous two-dimensional (2D) hybrid organic-inorganic Ruddelsden-Popper perovskites (RPPs) of (BA)2(MA) n-1Pb nI3 n+1 ( n = 1, 2, and 3) by a slow evaporation at a constant-temperature (SECT) solution-growth strategy. The as-grown 2D hybrid perovskite single crystals exhibit excellent crystallinity, phase purity, and spectral uniformity. Low-threshold lasing behaviors with different emission wavelengths at room temperature have been observed from the homologous 2D hybrid RPP single crystals. Our result demonstrates that solution-growth homologous organic-inorganic hybrid 2D perovskite single crystals open up a new window as a promising candidate for optical gain media.

Two-Dimensional Layered Hydroxide Nanoporous Nanohybrids Pillared with Zero-Dimensional Polyoxovanadate Nanoclusters for Enhanced Water Oxidation Catalysis.

The oxygen-evolution reaction (OER) is critical in electrochemical water splitting and requires an efficient, sustainable, and cheap catalyst for successful practical applications. A common development strategy for OER catalysts is to search for facile routes for the synthesis of new catalytic materials with optimized chemical compositions and structures. Here, nickel hydroxide Ni(OH)2 2D nanosheets pillared with 0D polyoxovanadate (POV) nanoclusters as an OER catalyst that can operate in alkaline media are reported. The intercalation of POV nanoclusters into Ni(OH)2 induces the formation of a nanoporous layer-by-layer stacking architecture of 2D Ni(OH)2 nanosheets and 0D POV with a tunable chemical composition. The nanohybrid catalysts remarkably enhance the OER activity of pristine Ni(OH)2 . The present findings demonstrate that the intercalation of 0D POV nanoclusters into Ni(OH)2 is effective for improving water oxidation catalysis and represents a potential method to synthesize novel, porous hydroxide-based nanohybrid materials with superior electrochemical activities.

Intrinsic Carrier Transport of Phase-Pure Homologous 2D Organolead Halide Hybrid Perovskite Single Crystals.

This work reveals the intrinsic carrier transport behavior of 2D organolead halide perovskites based on phase-pure homologous (n = 1, 2, and 3) Ruddelsden-Popper perovskite (RPP) (BA)2 (MA)n -1 Pbn I3n+1 single crystals. The 2D perovskite field effect transistors with high-quality exfoliated 2D perovskite bulk crystals are fabricated, and characteristic output and transfer curves are measured from individual single-crystal flakes with various n values under different temperatures. Unipolar n-type transport dominated the electrical properties of all these 2D RPP single crystals. The transport behavior of the 2D organolead halide hybrid perovskites exhibits a strong dependence on the n value and the mobility substantially increases as the ratio of the number of inorganic perovskite slabs per organic spacer increases. By extracting the effect of contact resistances, the corrected mobility values for n = 1, 2, and 3 are 2 × 10-3 , 8.3 × 10-2 , and 1.25 cm2 V-1 s-1 at 77 K, respectively. Furthermore, by combining temperature-dependent electrical transport and optical measurements, it is found that the origin of the carrier mobility dependence on the phase transition for 2D organolead halide perovskites is very different from that of their 3D counterparts. Our findings offer insight into fundamental carrier transport behavior of 2D organic-inorganic hybrid perovskites based on phase-pure homologous single crystals.

Solution-derived ZnO homojunction nanowire films on wearable substrates for energy conversion and self-powered gesture recognition.

Emerging applications in wearable technology, pervasive computing, human-machine interfacing, and implantable biomedical devices demand an appropriate power source that can sustainably operate for extended periods of time with minimal intervention (Wang, Z. L.; et al. Angew. Chem., Int. Ed. 2012, 51, 11700). Self-powered nanosystems, which harvest operating energy from its host (i.e., the human body), may be feasible due to their extremely low power consumption (Tian, B. Z.; et al. Nature 2007, 449, 885. Javey, A.; et al. Nature 2003, 424, 654. Cui, Y.; et al. Science 2001, 291, 851). Here we report materials and designs for wearable-on-skin piezoelectric devices based on ultrathin (2 µm) solution-derived ZnO p-n homojunction films for the first time. The depletion region formed at the p-n homojunction effectively reduces internal screening of strain-induced polarization charges by free carriers in both n-ZnO and Sb-doped p-ZnO, resulting in significantly enhanced piezoelectric output compared to a single layer device. The p-n structure can be further grown on polymeric substrates conformable to a human wrist and used to convert movement of the flexor tendons into distinguishable electrical signals for gesture recognition. The ZnO homojunction piezoelectric devices may have applications in powering nanodevices, bioprobes, and self-powered human-machine interfacing.

Contribution of Dislocations in SiC Seed Crystals on the Melt-Back Process in SiC Solution Growth.

The melt-back process has a significant effect on the quality of solution-grown SiC crystals. However, the phenomena surrounding the SiC dissolution into the molten alloy during the melt-back process have not been clarified. In this study, the behavior of 4H-SiC dissolution into molten alloy was investigated by using high-temperature in situ observation and subsequent KOH etching, and the effects of different doping conditions and crystal polarity were studied. Local dissolutions with hexagonal pyramid-shape originating from threading screw dislocation (TSD) were observed on the C face of n-type SiC with light nitrogen doping. Our analysis of their behavior revealed that the process was governed by the spiral dissolution. In addition to the dissolution at TSD, local dissolutions at threading-edge dislocations were observed on the Si face of the same crystal. The shape of the local dissolution at the dislocation was significantly affected by the doping conditions and the polarity of the SiC crystal. This local dissolution may occur during the melt-back process, suggesting that it is important to promote the dissolution while maintaining a smooth interface through the selection of the seed crystal and by keeping the degree of interface undersaturation small.

Screening of metal flux for SiC solution growth by a thin-film combinatorial method.

4H-SiC is a wide-bandgap semiconductor with potential applications in power devices. The lack of a liquid phase in SiC hinders conventional crystal growth from the melt; consequently, SiC wafers still have low quality and are nearly 100 times more expensive than Si wafers. To take advantage of the solution growth for improving the quality and reducing the cost of SiC, Ni addition to Si-Ti flux has been investigated. A combinatorial approach was employed to accelerate the screening of metal flux for the SiC solution growth.

Electronic-Grade High-Quality Perovskite Single Crystals by a Steady Self-Supply Solution Growth for High-Performance X-ray Detectors.

High-quality perovskite single crystals with large size are highly desirable for the fundamental research and high energy detection application. Here, a simple and convenient solution method, featuring continuous-mass transport process (CMTP) by a steady self-supply way, is shown to keep the growth of semiconductor single crystals continuously stable at a constant growth rate until an expected crystal size is achieved. A significantly reduced full width at half-maximum (36 arcsec) of the (400) plane from the X-ray rocking curve indicates a low angular dislocation of 6.8 × 106 cm-2 and hence a higher crystalline quality for the CH3 NH3 PbI3 (MAPbI3 ) single crystals grown by CMTP as compared to the conventional inverse temperature crystallization (ITC) method. Furthermore, the CMTP-based single crystals have lower trap density, reduced by nearly 200% to 4.5 × 109 cm-3 , higher mobility increased by 187% to 150.2 cm2 V-1 s-1 , and higher mobility-lifetime product increased by around 450% to 1.6 × 10-3 cm2 V-1 , as compared with the ITC-grown reference sample. The high performance of the CMTP-based MAPbI3 X-ray detector is comparable to that of a traditional high-quality CdZnTe device, indicating the CMTP method as being a cost-efficient strategy for high-quality electronic-grade semiconductor single crystals.

Improvement of SiC Crystal Growth Rate and Uniformity via Top-Seeded Solution Growth under External Static Magnetic Field: A Numerical Investigation.

Silicon carbide (SiC) is an ideal material for highpower and highperformance electronic applications. Topseeded solution growth (TSSG) is considered as a potential method for bulk growth of highquality SiC single crystals from the liquid phase source material. The crystal growth performance, such as growth rate and uniformity, is driven by the fluid flow and constitutional flux in the solution. In this study, we numerically investigate the contribution of the external static magnetic field generated by Helmholtz coils to the fluid flow in the silicon melt. Depending on the setup of the Helmholtz coils, four static magnetic field distributions are available, namely, uniform vertical upward/downward and vertical/horizontal cusp. Based on the calculated carbon flux coming to the crystal surface, the vertical downward magnetic field proved its ability to enhance the growth rate as well as the uniformity of the grown crystal.

Planar Perovskite Solar Cells with High Efficiency and Fill Factor Obtained Using Two-Step Growth Process.

Hybrid organic-inorganic perovskite solar cells (PSCs) have been regarded as the most promising next-generation photovoltaics (PVs) because of their potential for low-cost fabrication and advances in their development. Superior quality of the photoactive perovskite layer is a main factor for further increasing the PV performance of the organic-inorganic perovskite solar cells (PSCs). Herein, we successfully obtained perovskite with a high crystallinity and large grain size by utilizing excess PbI2 and SSGP technique and demonstrated a superior PV performance of normal-architecture planar PSCs. The SSGP PSCs with the highest fill factor (FF) reported thus far (83.4%) to our knowledge were obtained without sacrificing other PV parameters. Moreover, a high efficiency of 21.3% (21.6%) with a high FF of 80.0% (81.2%) in forward (reverse) scan was achieved. The unencapsulated SSGP PSCs showed robust continuous light-soaking and thermal stability under harsh characterization conditions. Additionally, we achieved a high efficiency of 20.1% with a negligible hysteresis on the large active area SSGP PSCs (∼1 cm2). The optical properties, efficient carrier extraction, and reduction of recombination loss of the SSGP perovskite significantly contribute to the high PV performance and robust stability of SSGP PSCs.

Molecular Sieve Induced Solution Growth of Li2O2 in the Li-O2 Battery with Largely Enhanced Discharge Capacity.

The formation of the insulated film-like discharge products (Li2O2) on the surface of the carbon cathode gradually hinders the oxygen reduction reaction (ORR) process, which usually leads to the premature death of the Li-O2 battery. In this work, by introducing the molecular sieve powder into the ether electrolyte, the Li-O2 battery exhibits a largely improved discharge capacity (63 times) compared with the one in the absence of this inorganic oxide additive. Meanwhile, XRD and SEM results qualitatively demonstrate the generation of the toroid Li2O2 as the dominated discharge products, and the chemical titration quantifies a higher yield of the Li2O2 with the presence of the molecular sieve additive. The addition of the molecular sieve controls the amount of the free water in the electrolyte, which distinguishes the effect of the molecular sieve and the free water on the discharge process. Hence, a possible mechanism has been proposed that the adsorption of the molecular sieves toward the soluble lithium superoxides improves the disproportionation of the lithium superoxides and consequently enhances the solution-growth of the lithium peroxides in the low donor number ether electrolyte. In general, the application of the molecular sieve triggers further studies concerning the improvement of the discharge performance in the Li-O2 battery by adding the inorganic additives.

Controllable Electrochemical Fabrication of KO2-Decorated Binder-Free Cathodes for Rechargeable Lithium-Oxygen Batteries.

Understanding the electrochemical property of superoxides in alkali metal oxygen batteries is critical for the design of a stable oxygen battery with high capacity and long cycle performance. In this work, a KO2-decorated binder-free cathode is fabricated by a simple and efficient electrochemical strategy. KO2 nanoparticles are uniformly coated on the carbon nanotube film (CNT-f) through a controllable discharge process in the K-O2 battery, and the KO2-decorated CNT-f is innovatively introduced into the Li-O2 battery as the O2 diffusion electrode. The Li-O2 battery based on the KO2-decorated CNT-f cathode can deliver enhanced discharge capacity, reduced charge overpotential, and more stable cycle performance compared with the battery in the absence of KO2. In situ formed KO2 particles on the surface of CNT-f cathode assist to form Li2O2 nanosheets in the Li-O2 battery, which contributes to the improvement of discharge capacity and cycle life. Interestingly, the analysis of KO2-decorated CNT-f cathodes, after discharge and cycle tests, reveals that the electrochemically synthesized KO2 seems not a conventional electrocatalyst but a partially dissolvable and decomposable promoter in Li-O2 batteries.

Solution Growth of Ultralong Gold Nanohelices.

Metallic nanohelices are extremely rare and, to date, have never been synthesized by a direct solution method. In this work, we report ultralong Au nanohelices grown in solution under ambient conditions. They are ultralong with several tens of micrometers in length, with extraordinary aspect ratio (length/diameter greater than 22 300) and the number of pitches (more than 22 000 pitches). The pitch and width are uniform within each helix but vary widely among the helices. Crystal analyses showed that the facets, twin boundaries, grain sizes, and orientations are aperiodic along the helices. The apparent smooth curving is only possible with a large number of surface steps, suggesting that these structural features are the mere consequence of the helix formation rather than the cause. We propose that the nanowires are formed by the active surface growth mechanism and that the helicity originates from the random and asymmetrical blocking of nuclei embedded within the floccules of ligand complexes, in the form of either asymmetric binding of ligands or asymmetric diffusion of growth materials through the floccules. The separate growth environment of these nuclei causes constant helicity within each helix but differing helicity among the individuals. The embedding also provides a robust environment for the sustained growth of the nanohelices, leading to their record length and consistency.

Patterning Multicolored Microdisk Laser Arrays of Cesium Lead Halide Perovskite.

A polydimethylsiloxane cylindrical-hole-template-confined solution-growth method is developed to fabricate densely packed CsPbCl3-x Br x microdisk laser arrays. Furthermore, a strategy to integrate multicolored microdisk laser (MDL) arrays is demonstrated that simultaneously lase in the deep blue, blue, cyan, and green by means of gas-phase replacement of Cl by Br from initial CsPbCl3 MDLs in HBr vapor.

Organic-Inorganic Hybrid Perovskite Nanowire Laser Arrays.

Fabrication of semiconductor nanowire laser arrays is very challenging, owing to difficulties in direct monolithic growth and patterning of III-V semiconductors on silicon substrates. Recently, methylammonium lead halide perovskites (MAPbX3, X = Cl, Br, I) have emerged as an important class of high-performance solution-processed optoelectronic materials. Here, we combined the "top-down" fabricated polydimethylsiloxane rectangular groove template (RGT) with the "bottom-up" solution self-assembly together to prepare large-scale perovskite nanowire (PNW) arrays. The template confinement effect led to the directional growth of MAPbX3 along RGTs into PNWs. We achieved precise control over not only the dimensions of individual PNWs (width 460-2500 nm; height 80-1000 nm, and length 10-50 µm) but also the interwire distances. Well-defined dimensions and uniform geometries enabled individual PNWs to function as high-quality Fabry-Perot nanolasers with almost identical optical modes and similarly low-lasing thresholds, allowing them to ignite simultaneously as a laser array. Optical tests demonstrated that PNW laser arrays exhibit good photostabillity with an operation duration exceeding 4 × 107 laser pulses. Precise placement of PNW arrays at specific locations makes our method highly compatible with lithographic techniques, which are important for integrating PNW electronic and photonic circuits.

Solution-Grown ZnO Films toward Transparent and Smart Dual-Color Light-Emitting Diode.

An individual light-emitting diode (LED) capable of emitting different colors of light under different bias conditions not only allows for compact device integration but also extends the functionality of the LED beyond traditional illumination and display. Herein, we report a color-switchable LED based on solution-grown n-type ZnO on p-GaN/n-GaN heterojunction. The LED emits red light with a peak centered at ∼692 nm and a full width at half-maximum of ∼90 nm under forward bias, while it emits green light under reverse bias. These two lighting colors can be switched repeatedly by reversing the bias polarity. The bias-polarity-switched dual-color LED enables independent control over the lighting color and brightness of each emission with two-terminal operation. The results offer a promising strategy toward transparent, miniaturized, and smart LEDs, which hold great potential in optoelectronics and optical communication.

Growth and characterization of Cadmium Thiosemicarbazide Bromide crystals for antibacterial and nonlinear optical applications.

Semiorganic nonlinear optical crystals of Cadmium Thiosemicarbazide Bromide was grown by slow evaporation solution growth technique. The unit cell parameters were estimated by subjecting the crystals to single crystal X-ray diffraction. The grown crystals were subjected to Powder X-ray diffraction for analyzing the crystalline nature of the sample. FTIR studies reveal the functional groups and the optical characters were analyzed by UV-Vis spectral studies. Mechanical stability of the sample was assessed by Vicker's micro hardness test. The presence of surface dislocations was identified by chemical etching technique. Antibacterial study was carried out against ACDP declared harmful pathogens. SHG efficiency of CTSB crystal was tested using Nd: YAG laser and it was found to be ∼1.8 times that of potassium dihydrogen phosphate.

Synthesis, growth and spectral, optical and thermal characterization studies on L-Tryptophan p-nitrophenol (LTPN) single crystals for NLO applications.

The novel nonlinear optical single crystal of L-Tryptophan p-nitrophenol (LTPN) has been successfully synthesized by taking the appropriate amount of L-Tryptophan and p-nitrophenol. The single crystals have been grown by slow evaporation solution growth technique. The single crystal XRD studies confirmed that the grown crystal belongs to the monoclinic system. The various functional groups presented in the crystal were confirmed by FT-IR and (1)H NMR spectroscopic studies. The absorptions of the grown crystals were analyzed using UV-Vis-NIR spectral studies. The thermal analysis was performed to study the thermal stability of the grown crystals. The second harmonic generation behavior of L-Tryptophan p-nitrophenol crystal was tested by Kurtz-Perry powder technique.

Structural, thermal and optical properties of KTi(0.92)La(0.08)OPO4 and KTi(0.94)Nd(0.06)OPO4.

KTi0.92La0.08OPO4 (KTP:La) and KTi0.94Nd0.06OPO4 (KTP:Nd) single crystals are grown using high temperature top seeded flux growth technique. The strain derived from doping is calculated from Williamson-Hall relation. The packing structure and lattice parameter of the grown crystals are analyzed using single crystal X-ray diffraction. The bonding, distortion and change in inter-atomic distances by strain effects of doping are assessed by Raman spectroscopy. Thermal stabilities of grown crystals are evaluated by specific heat capacity measurement. Pronounced high specific heat capacity is recorded as 1.16 J/gK at 498 K for KTP:Nd. Second harmonic generation intensities are measured for KTP:Nd and KTP:La single crystal.

Direct Low-Temperature Growth of Single-Crystalline Anatase TiO2 Nanorod Arrays on Transparent Conducting Oxide Substrates for Use in PbS Quantum-Dot Solar Cells.

We report on the direct growth of anatase TiO2 nanorod arrays (A-NRs) on transparent conducting oxide (TCO) substrates that can be directly applied to various photovoltaic devices via a seed layer mediated epitaxial growth using a facile low-temperature hydrothermal method. We found that the crystallinity of the seed layer and the addition of an amine functional group play crucial roles in the A-NR growth process. The A-NRs exhibit a pure anatase phase with a high crystallinity and preferred growth orientation in the [001] direction. Importantly, for depleted heterojunction solar cells (TiO2/PbS), the A-NRs improve both electron transport and injection properties, thereby largely increasing the short-circuit current density and doubling their efficiency compared to TiO2 nanoparticle-based solar cells.

L-Argininium phosphite - a new candidate for NLO materials.

In order to investigate the possibility of salt formation in the L-Arg-H3PO3-H2O system, single crystals of L-argininium phosphite, C6H15N4O2(+)·H2PO3(-), were prepared by evaporation of an aqueous solution containing equimolar quantities of L-arginine and phosphorous acid. The asymmetric unit contains one L-argininium(+) cation and one phosphite [HPO2(OH)](-) anion. The phosphite anions form chains parallel to [010] by O-H...O hydrogen bonding, with an O...O distance of 2.630 (3) Å. The protonated amine and guanidyl groups of the L-argininium(+) cations form N-H...O hydrogen bonds with the carboxylate groups and anions. The IR and Raman spectra are discussed in relation to the crystal structure. The salt displays nonlinear optical (NLO) properties. Another salt was obtained from a solution with a 1:2 molar ratio of components, but was characterized by vibrational spectra only.