Stabilized High-Membered and Phase-Pure 2D All Inorganic Ruddlesden-Popper Halide Perovskites Nanocrystals as Photocatalysts for the CO2 Reduction Reaction.

In contrast to the 2D organic-inorganic hybrid Ruddlesden-Popper halide perovskites (RPP), a new class of 2D all inorganic RPP (IRPP) has been recently proposed by substituting the organic spacers with an optimal inorganic alternative of cesium cations (Cs+ ). Nevertheless, the synthesis of high-membered 2D IRPPs (n > 1) has been a very challenging task because the Cs+ need to act as both spacers and A-site cations simultaneously. This work presents the successful synthesis of stable phase-pure high-membered 2D IRPPs of Csn+1 Pbn Br3n+1 nanosheets (NSs) with n = 3 and 4 by employing the strategy of using additional strong binding bidentate ligands. The structures of the 2D IRPPs (n = 3 and 4) NSs are confirmed by powder X-ray diffraction and high-resolution aberration-corrected scanning transmission electron microscope measurements. These 2D IRPPs NSs exhibit a strong quantum confinement effect with tunable absorption and emission in the visible light range by varying their n values, attributed to their inherent 2D quantum-well structure. The superior structural and optical stability of the phase-pure high-membered 2D IRPPs make them a promising candidate as photocatalysts in CO2 reduction reactions with outstanding photocatalytic performance and long-term stability.

Self-assembly nuclei with a preferred orientation at the extended hydrophobic surface toward textured growth of ZnO nanorods in aqueous chemical bath deposition.

Textured growth of ZnO nanorods with no restriction of the substrate material is beneficial to their applications. The approaches to grow ZnO nanorods with texture are based on preparing suitable surface structure on the growth substrate, e.g. using a crystalline substrate with a specific surface structures or pre-depositing seed layers by high-temperature annealing of precursors. In the aqueous nutrient solution of the chemical bath deposition (CBD) process for ZnO growth, the concentration of Zn2+ ions at the extended hydrophobic surface is sufficiently high for forming self-assembly nuclei with a preferred orientation, resulting in the subsequent textured growth of ZnO nanorods. In this research, the hydrophobic surface is prepared by modifying Si surface with a self-assembly octadecyltrimethoxysilane (OTMS) monolayer. The formation mechanism of the nuclei on this hydrophobic surface for the textured growth of ZnO nanorods is investigated. It is shown that the nuclei form at the beginning of the CBD process and later transform into the Wurtzite structure to seed ZnO growth. An alternative approach to prepare seed layers is therefore involved in the aqueous CBD process, which is applicable to a range of hydrophobic substrates for textured growth of ZnO nanorods.

Synthesis of Au@Cu2O Core-Shell Nanoparticles with Tunable Shell Thickness and Their Degradation Mechanism in Aqueous Solutions.

Metal@semiconductor core-shell nanoparticles (NPs) are widely used in photocatalysts, sensors, and optical applications owing to their unique metal-semiconductor interface and the integration of the properties from both core and shell materials. Although many efforts have been made toward the precise synthesis of Au@Cu2O core-shell structures, the chemical stability of Au@Cu2O aqueous suspensions, which is of great significance in many related applications, is not mentioned in any published research. Herein we report the synthesis of Au@Cu2O core-shell NPs with small shell thickness from 2 to 40 nm through a wet-chemistry method. The UV-vis absorption properties are found to be tunable with Cu2O thickness in the range of 2-40 nm. Furthermore, the chemical stability of Au@Cu2O core-shell nanoparticle suspensions in water/ethanol mixed solvents is investigated. It is found that water/ethanol mixed solvents with a larger amount of water are more likely to deteriorate the stability of Au@Cu2O NPs by oxidizing Cu2O to CuO. The results from this work may provide useful information for the preparation of metal@Cu2O water-based suspensions that are expected to be used for SERS, photocatalyst, or photothermal applications.

Lithiation-induced fracture of silicon nanowires observed by in-situ scanning electron microscopy.

Silicon is expected to be a useful anode material in lithium ion batteries for future energy storage applications, because of its high theoretical charge storage density of Li+ ions. However, volume expansion due to lithiation fractures the Si anode material, leading to poor cycle stability of battery operation. The approaches to overcome the problem include using Si nanowires to relieve the stress induced by volume expansion and coating a protective layer on the Si anode to prevent delamination. In this study, we use in-situ scanning electron microscopy to monitor the morphological changes of 90 nm thick pristine Si nanowires and the Si nanowires coated with amorphous TiO2, respectively, during electrochemical lithiation. The results of in-situ observation show that both kinds of Si nanowires exhibit a larger thickness after 10 h lithiation and suffer fracture after 25 h. It is also found that the TiO2 layer is not strong enough to prevent Si nanowires from fracture. Since the TiO2 layer can not be elastically deformed, this surface shell fractures earlier in the lithiation process than pristine Si nanowires. Transformation of the crystalline Si nanowires to an amorphous phase and lithium composition detected in the nanowires support that the observed fracture indeed results from lithiation.

Crystal Growth and Magnetic Properties of Topological Nodal-Line Semimetal GdSbTe with Antiferromagnetic Spin Ordering.

We report crystal growth, AC and DC magnetic susceptibilities [χ(T, H)], magnetization [M(T, H)], and heat capacity [CP(T, H)] measurement results of GdSbTe single crystal. GdSbTe is isostructural to the confirmed nonmagnetic nodal-line semimetal ZrSiS of noncentrosymmetric tetragonal crystal structure in space group P4/nmm (No. 129), but it shows additional long-range antiferromagnetic spin ordering for the Gd spins of S = 7/2 below TN. Both χ(T, H) and CP(T, H) measurements confirm the existence of a long-range antiferromagnetic (AFM) spin ordering of Gd spins below TN ∼ 12 K, and an additional spin reorientation/recovery (sr) behavior is identified from the change of on-site spin anisotropy between Tsr1 ∼ 7 and Tsr2 ∼ 4 K. The anisotropic magnetic susceptibilities of χ(T, H) below TN clearly demonstrate that the AFM long-range spin ordering of GdSbTe has an easy axis parallel to the ab-plane direction. The field- and orientation-dependent magnetization of M(T, H) at 2 K shows two plateaus to indicate the spin-flop transition for H||ab near ∼2.1 T and a metamagnetic state near ∼5.9 T having ∼1/3 of the fully polarized magnetization by the applied field. The heat capacity measurement results yield Sommerfeld coefficient of γ ∼ 7.6(4) mJ/mol K2 and θD ∼ 195(2) K being less than half of that for the nonmagnetic ZrSiS. A three-dimensional (3D) AFM spin structure is supported by the ab initio calculations for Gd having magnetic moment of 7.1 µB and the calculated AFM band structure indicates that GdSbTe is a semimetal with bare density of states (0.36 states/eV fu) at the Fermi level, which is 10 times smaller than the measured one to suggest strong spin-fluctuation.

Growth of heterojunctions in Si-Ge alloy nanowires by altering AuGeSi eutectic composition using an approach based on thermal oxidation.

Understanding the growth mechanism of heterojunctions in silicon-germanium alloy (Si-Ge) nanowires is helpful for designing adequate physical properties in the material for device applications. We examine the formation of the heterojunction in low Ge-content Si-Ge nanowires by an approach of thermal oxidation, which produces an atomically abrupt interface with an obvious concentration change. Forming heterojunctions in Si-Ge nanowires by this approach involves more complicated reaction routes than direct growth of heterojunction nanowires using the vapor-liquid-solid method. At the beginning of the oxidation process, the AuGeSi eutectic liquid at the nanowire tip significantly etches the Si-Ge alloy nanowires. Selective oxidation of Si results in a change of the relative amount of Ge to Si in the eutectic liquid, which further modulates the solubility of Ge and Si atoms. The compositional variation in the Au-Ge-Si ternary alloy system during the oxidation process accounts for the observed concentration profile in the heterojunction nanowire. The thermal oxidation approach is applied on a low Ge-content Si-Ge thin film that is coated with Au nanoparticles. Si-Ge nanodots, which exhibit a higher Ge concentration, are precipitated epitaxially in the film, as a result of compositional modulation in the AuGeSi eutectic liquid.

Producing Atomically Abrupt Axial Heterojunctions in Silicon-Germanium Nanowires by Thermal Oxidation.

Compositional abruptness of the interfaces is one of the important factors to determine the performance of Group IV semiconductor heterojunction (Si/Ge or Si/SiGe) nanowire devices. However, forming abrupt interfaces in the nanowires using the common vapor-liquid-solid (VLS) method is restricted because large solubility of Si and Ge in the Au eutectic liquid catalyst makes gradual composition change at the heterojunction after switching the gas phase components. According to the VLS growth mechanism, another possible approach to form an abrupt interface is making a change of the semiconductor concentration in the eutectic liquid before precipitation of the second phase. Here we show that the composition in AuSiGe eutectic liquid on SiGe nanowires of low Ge concentration (≤6%) can be altered by thermal oxidation at 700 °C. During the oxidation process, only Si is oxidized on the surface of the eutectic liquid, and the Ge/Si ratio in the eutectic liquid is increased. The subsequently precipitated SiGe step at the liquid/solid interface has a higher Ge concentration (∼20%), and a compositionally abrupt interface is produced in the nanowires. The growth mechanism of the heterojunction includes diffusion of Si and Ge atoms on nanowire surface into the AuSiGe eutectic liquid and step nucleation at the liquid/nanowire interface.

Improving Hydrogen Evolution Activity of Earth-Abundant Cobalt-Doped Iron Pyrite Catalysts by Surface Modification with Phosphide.

Hydrogen is considered as sustainable and environmentally friendly energy for global energy demands in the future. Here a Co-FeS2 catalyst with surface phosphide doping (P/Co-FeS2 ) for hydrogen evolution reaction (HER) in acidic solutions is developed. The P/Co-FeS2 exhibits superior HER electrochemical performance with overpotential of -90 mV at 100 mA cm-2 and Tafel slope of 41 mV/decade and excellent durability.

Strain and stability of ultrathin Ge layers in Si/Ge/Si axial heterojunction nanowires.

The formation of abrupt Si/Ge heterointerfaces in nanowires presents useful possibilities for bandgap engineering. We grow Si nanowires containing thick Ge layers and sub-1 nm thick Ge "quantum wells" and measure the interfacial strain fields using geometric phase analysis. Narrow Ge layers show radial compressive strains of several percent, while stress at the Si/Ge interface causes lattice rotation. High strains can be achieved in these heterostructures, but we show that they are unstable to interdiffusion.

Strategies to control morphology in hybrid group III-V/group IV heterostructure nanowires.

By combining in situ and ex situ transmission electron microscopy measurements, we examine the factors that control the morphology of "hybrid" nanowires that include group III-V and group IV materials. We focus on one materials pair, GaP/Si, for which we use a wide range of growth parameters. We show through video imaging that nanowire morphology depends on growth conditions, but that a general pattern emerges where either single kinks or inclined defects form some distance after the heterointerface. We show that pure Si nanowires can be made to exhibit the same kinks and defects by changing their droplet volume. From this we derive a model where droplet geometry drives growth morphology and discuss optimization strategies. We finally discuss morphology control for material pairs where the second material kinks immediately at the heterointerface and show that an interlayer between segments can enable the growth of unkinked hybrid nanowires.

In situ TEM creation and electrical characterization of nanowire devices.

We demonstrate the observation and measurement of simple nanoscale devices over their complete lifecycle from creation to failure within a transmission electron microscope. Devices were formed by growing Si nanowires, using the vapor-liquid-solid method, to form bridges between Si cantilevers. We characterize the formation of the contact between the nanowire and the cantilever, showing that the nature of the connection depends on the flow of heat and electrical current during and after the moment of contact. We measure the electrical properties and high current failure characteristics of the resulting bridge devices in situ and relate these to the structure. We also describe processes to modify the contact and the nanowire surface after device formation. The technique we describe allows the direct analysis of the processes taking place during device formation and use, correlating specific nanoscale structural and electrical parameters on an individual device basis.

Studies of high-membered two-dimensional Ruddlesden-Popper Cs7Pb6I19 perovskite nanosheets via kinetically controlled reactions.

Two-dimensional (2D) all-inorganic Ruddlesden-Popper (RP) perovskite Cs7Pb6I19 nanosheets (NSs) were successfully developed for the first time by employing a structural recrystallization process with additional passivation of small organic sulfide molecules. The structure of Cs7Pb6I19 NSs is confirmed by powder X-ray diffraction measurements, atomically-resolved STEM measurements and atomic force microscopy (AFM) studies. Cs7Pb6I19 NSs with a specific n value of 6 exhibits unique absorption and emission spectra with intense excitons at 560 nm due to quantum confinement effects in 2D perovskite slabs. The formation mechanisms of 2D Cs7Pb6I19 NSs and 3D γ-CsPbI3 phases were investigated by in situ photoluminescence (PL) spectroscopy and the activation energies of their formation reactions were calculated to be 151 kJ mol-1 and 95.3 kJ mol-1, respectively. The phase stability of 2D Cs7Pb6I19 NSs can be maintained at temperatures below 14 °C for more than 4 weeks. The overall results indicate that 2D Cs7Pb6I19 NSs demonstrate unique optical properties and structural stability compared with other 3D perovskite materials. We have opened a new path to the future discovery of 2D perovskite structures with metastable phases by using this recrystallization method and the assistance of sulfur-derived organic molecules.

Two-Dimensional Bis(dithiolene)iron(II) Self-Powered UV Photodetectors with Ultrahigh Air Stability.

Organometallic two-dimensional (2D) nanosheets with tailorable components have recently fascinated the optoelectronic communities due to their solution-processable nature. However, the poor stability of organic molecules may hinder their practical application in photovoltaic devices. Instead of conventional organometallic 2D nanosheets with low weatherability, an air-stable π-conjugated 2D bis(dithiolene)iron(II) (FeBHT) coordination nanosheet (CONASH) is synthesized via bottom-up liquid/liquid interfacial polymerization using benzenehexathiol (BHT) and iron(II) ammonium sulfate [Fe(NH4)2(SO4)2] as precursors. The uncoordinated thiol groups in FeBHT are easily oxidized, but the Fe(NH4)2(SO4)2 dissociation rate is slow, which facilitates the protection of sulfur groups by iron(II) ions. The density functional theory calculates that the resultant FeBHT network gains the oxygen-repelling function for oxidation suppression. In air, the FeBHT CONASH exhibits self-powered photoresponses with short response times (<40 ms) and a spectral responsivity of 6.57 mA W-1, a specific detectivity of 3.13 × 1011 Jones and an external quantum efficiency of 2.23% under 365 nm illumination. Interestingly, the FeBHT self-powered photodetector reveals extremely high long-term air stability, maintaining over 94% of its initial photocurrent after aging for 60 days without encapsulation. These results open the prospect of using organometallic 2D materials in commercialized optoelectronic fields.

Electrocatalytic Reduction of NO3- to Ultrapure Ammonia on {200} Facet Dominant Cu Nanodendrites with High Conversion Faradaic Efficiency.

Nitrate (NO3-) reduction reaction (NtRR) is considered as a green alternative method for the conventional method of NH3 synthesis (Haber-Bosch process), which is known as a high energy consuming and large CO2 emitting process. Herein, the copper nanodendrites (Cu NDs) grown along with the {200} facet as an efficient NtRR catalyst have been successfully fabricated and investigated. It exhibited high Faradaic efficiency of 97% at low potential (-0.3 V vs RHE). Furthermore, the 15NO3- isotope labeling method was utilized to confirm the formation of NH3. Both experimental and theoretical studies showed that NtRR on the Cu metal nanostructure is a facet dependent process. Dissociation of NO bonding is supposed to be the rate-determining step as NtRR is a spontaneously reductive and protonation process for all the different facets of Cu. Density functional theory (DFT) calculations revealed that Cu{200} and Cu{220} offer lower activation energy for dissociation of NO compared to that of Cu{111}.

Atomic-Layer Controlled Interfacial Band Engineering at Two-Dimensional Layered PtSe2/Si Heterojunctions for Efficient Photoelectrochemical Hydrogen Production.

Platinum diselenide (PtSe2) is a group-10 two-dimensional (2D) transition metal dichalcogenide that exhibits the most prominent atomic-layer-dependent electronic behavior of "semiconductor-to-semimetal" transition when going from monolayer to bulk form. This work demonstrates an efficient photoelectrochemical (PEC) conversion for direct solar-to-hydrogen (H2) production based on 2D layered PtSe2/Si heterojunction photocathodes. By systematically controlling the number of atomic layers of wafer-scale 2D PtSe2 films through chemical vapor deposition (CVD), the interfacial band alignments at the 2D layered PtSe2/Si heterojunctions can be appropriately engineered. The 2D PtSe2/p-Si heterojunction photocathode consisting of a PtSe2 thin film with a thickness of 2.2 nm (or 3 atomic layers) exhibits the optimized band alignment and delivers the best PEC performance for hydrogen production with a photocurrent density of -32.4 mA cm-2 at 0 V and an onset potential of 1 mA cm-2 at 0.29 V versus a reversible hydrogen electrode (RHE) after post-treatment. The wafer-scale atomic-layer controlled band engineering of 2D PtSe2 thin-film catalysts integrated with the Si light absorber provides an effective way in the renewable energy application for direct solar-to-hydrogen production.

Measurement of local Si-nanowire growth kinetics using in situ transmission electron microscopy of heated cantilevers.

A technique to study nanowire growth processes on locally heated microcantilevers in situ in a transmission electron microscope has been developed. The in situ observations allow the characterization of the nucleation process of silicon wires, as well as the measurement of growth rates of individual nanowires and the ability to observe the formation of nanowire bridges between separate cantilevers to form a complete nanowire device. How well the nanowires can be nucleated controllably on typical cantilever sidewalls is examined, and the measurements of nanowire growth rates are used to calibrate the cantilever-heater parameters used in finite-element models of cantilever heating profiles, useful for optimization of the design of devices requiring local growth.

Growth system, structure, and doping of aluminum-seeded epitaxial silicon nanowires.

We have examined the formation of silicon nanowires grown by self-assembly from Si substrates with thin aluminum films. Postgrowth and in situ investigations using various Al deposition and annealing conditions suggest that nanowire growth takes place with a supercooled liquid droplet (i.e., the vapor-liquid-solid system), even though the growth temperatures are below the bulk Al/Si eutectic temperature. Wire morphology as a function of processing conditions is also described. It is shown that when Al environmental exposure is prevented before wire growth a wide process window for wire formation can be achieved. Under optimum growth conditions, it is possible to produce excellent crystal quality nanowires with rapid growth rates, high surface densities, low diameter dispersion, and controlled tapering. Photoelectron spectroscopy measurements indicate that the use of Al leads to active doping levels that depend on the growth temperature in as-grown nanowires and increase when annealed. We suggest that these structural and electronic properties will be relevant to photovoltaic and other applications, where the more common use of Au is believed to be detrimental to performance.

In situ TEM observation of liquid-state Sn nanoparticles vanishing in a SiO2 structure: a potential synthetic tool for controllable morphology evolution from core-shell to yolk-shell and hollow structures.

Precise design of hollow nanostructures can be realized via various approaches developed in the last two decades, endowing nanomaterials with unique structures and outstanding performances, showing their usefulness in a broad range of fields. Herein, we demonstrate the formation of SnO2@SiO2 hollow nanostructures, for the first time, by interaction between liquid state Sn cores and SiO2 shell structures inside Sn@SiO2 core-shell nanoparticles with real-time observation via in situ transmission electron microscopy (TEM). Based on the in situ results, designed transformation of the nanoparticle structure from core-shell Sn@SiO2 to yolk-shell Sn@SiO2 and hollow SnO2@SiO2 is demonstrated, showing the controllable structure of core-shell Sn@SiO2 nanoparticles via fixing liquid-state Sn inside a SiO2 shell which has a certain Sn containing capacity. The present approach expands the toolbox for the design and preparation of yolk-shell and hollow nanostructures, thus providing us with a new strategy for fabrication of more complicated nanostructures.

Exploring the Origin of Phase-Transformation Kinetics of CsPbI3 Perovskite Nanocrystals Based on Activation Energy Measurements.

Perovskite α-CsPbI3 nanocrystals (NCs) with a high fluorescence quantum yield (QY) typically undergo a rapid phase transformation to a low-QY δ-CsPbI3 phase, thus limiting their optoelectronic applications. In this study, organic molecule hexamethyldisilathiane (HMS) is used as a unique surfactant to greatly enhance the stability of the cubic phase of CsPbI3 NCs (HMS-CsPbI3) under ambient conditions. The reaction kinetics of the phase transformation of CsPbI3 NCs are systemically investigated through in situ photoluminescence (PL), X-ray diffraction, and transmission electron microscope (TEM) measurements under moisture. The activation energy of HMS-CsPbI3 NCs is found to be 14 times larger than that of CsPbI3 NCs capped by olyelamine (OLA-CsPbI3 NCs). According to density functional theory calculations, the bonding between HMS and CsPbI3 NCs is stronger than that between OLA and CsPbI3 NCs, preventing the subsequent phase transformation. Our study presents a clear pathway for achieving highly stable CsPbI3 NCs for future applications.

Fast growth of large-grain and continuous MoS2 films through a self-capping vapor-liquid-solid method.

Most chemical vapor deposition methods for transition metal dichalcogenides use an extremely small amount of precursor to render large single-crystal flakes, which usually causes low coverage of the materials on the substrate. In this study, a self-capping vapor-liquid-solid reaction is proposed to fabricate large-grain, continuous MoS2 films. An intermediate liquid phase-Na2Mo2O7 is formed through a eutectic reaction of MoO3 and NaF, followed by being sulfurized into MoS2. The as-formed MoS2 seeds function as a capping layer that reduces the nucleation density and promotes lateral growth. By tuning the driving force of the reaction, large mono/bilayer (1.1 mm/200 µm) flakes or full-coverage films (with a record-high average grain size of 450 µm) can be grown on centimeter-scale substrates. The field-effect transistors fabricated from the full-coverage films show high mobility (33 and 49 cm2 V-1 s-1 for the mono and bilayer regions) and on/off ratio (1 ~ 5 × 108) across a 1.5 cm × 1.5 cm region.