Zn3P2-Zn3As2 solid solution nanowires.

Semiconductor alloy nanowires (NWs) have recently attracted considerable attention for applications in optoelectronic nanodevices because of many notable properties, including band gap tunability. Zinc phosphide (Zn3P2) and zinc arsenide (Zn3As2) belong to a unique pseudocubic tetragonal system, but their solid solution has rarely been studied. Here In this study, we synthesized composition-tuned Zn3(P1-xAsx)2 NWs with different crystal structures by controlling the growth conditions during chemical vapor deposition. A first type of synthesized NWs were single-crystalline and grew uniformly along the [110] direction (in a cubic unit cell) over the entire compositional range (0 ≤ x ≤ 1) explored. The use of an indium source enabled the growth of a second type of NWs, with remarkable cubic-hexagonal polytypic twinned superlattice and bicrystalline structures. The growth direction of the Zn3P2 and Zn3As2 NWs was also switched to [111] and [112], respectively. These structural changes are attributable to the Zn-depleted indium catalytic nanoparticles which favor the growth of hexagonal phases. The formation of a solid solution at all compositions allowed the continuous tuning of the band gap (1.0-1.5 eV). Photocurrent measurements were performed on individual NWs by fabricating photodetector devices; the single-crystalline NWs with [110] growth direction exhibit a higher photoconversion efficiency compared to the twinned crystalline NWs with [111] or [112] growth direction.

Reversible Halide Exchange Reaction of Organometal Trihalide Perovskite Colloidal Nanocrystals for Full-Range Band Gap Tuning.

In recent years, methylammonium lead halide (MAPbX3, where X = Cl, Br, and I) perovskites have attracted tremendous interest caused by their outstanding photovoltaic performance. Mixed halides have been frequently used as the active layer of solar cells, as a result of their superior physical properties as compared to those of traditionally used pure iodide. Herein, we report a remarkable finding of reversible halide-exchange reactions of MAPbX3, which facilitates the synthesis of a series of mixed halide perovskites. We synthesized MAPbBr3 plate-type nanocrystals (NCs) as a starting material by a novel solution reaction using octylamine as the capping ligand. The synthesis of MAPbBr(3-x)Clx and MAPbBr(3-x)Ix NCs was achieved by the halide exchange reaction of MAPbBr3 with MACl and MAI, respectively, in an isopropyl alcohol solution, demonstrating full-range band gap tuning over a wide range (1.6-3 eV). Moreover, photodetectors were fabricated using these composition-tuned NCs; a strong correlation was observed between the photocurrent and photoluminescence decay time. Among the two mixed halide perovskite series, those with I-rich composition (x = 2), where a sole tetragonal phase exists without the incorporation of a cubic phase, exhibited the highest photoconversion efficiency. To understand the composition-dependent photoconversion efficiency, first-principles density-functional theory calculations were carried out, which predicted many plausible configurations for cubic and tetragonal phase mixed halides.

Laser-induced graphitization of colloidal nanodiamonds for excellent oxygen reduction reaction.

Graphitized nanodiamonds were conveniently prepared by the laser irradiation of colloidal solution using various solvents. The nanodiamonds were converted into a fully graphitized onion-like structure, which became a cage-like mesoporous structure by the degradation of graphitic layers. Alcohols, acetone, and acetonitrile are more efficient solvents for the graphitization compared to water and hydrocarbons. Therefore the number and morphology of the graphitic layers can be simply controlled by the solvent and laser-irradiation duration. We suggest a graphitization model, in which the photocatalytic oxidation of the solvent accelerates the graphitization of nanodiamonds. The graphitized nanodiamonds were easily doped with the nitrogen and sulfur atoms in a controlled manner. In particular, the spherical graphitic layers were preferentially doped with the pyrrolic nitrogen that enhances remarkably electrocatalytic activity for the oxygen reduction reaction.

Hydrogen and carbon monoxide generation from laser-induced graphitized nanodiamonds in water.

Nanodiamonds (ND) were found to generate hydrogen (H2) and carbon monoxide (CO) from water at a remarkable rate under pulsed laser (532 nm) irradiation. The transformation of diamond structure into graphitic layers takes place to form an onion-like carbon structure. The CO generation suggests the oxidative degradation reaction of graphitic layers, C + H2O → CO + 2H(+) + 2e(-), which produced a unique laser-induced reaction: C + H2O → CO + H2. Au, Pt, Pd, Ag, and Cu nanoparticles on the ND enhance both gas evolution rates (~2 times for Au) and graphitization and, specifically, Au was found to be the most efficient amongst other nanoparticles. The enhancement effect was ascribed to effective charge separation between the metal nanoparticles and ND. The Au-ND hybrid on the reduced graphene oxide produced consistently a greater photocurrent than the ND upon visible light irradiation.

Germanium-tin alloy nanocrystals for high-performance lithium ion batteries.

Germanium-tin (Ge(1-x)Sn(x)) alloy nanocrystals were synthesized using a gas-phase laser photolysis reaction of tetramethyl germanium and tetramethyl tin. A composition tuning was achieved using the partial pressure of precursors in a closed reactor. For x < 0.1, cubic phase alloy nanocrystals were exclusively produced without separation of the tetragonal phase Sn metal. In the range of x = 0.1-0.4, unique Ge(1-x)Sn(x)-Sn alloy-metal hetero-junction nanocrystals were synthesized, where the Sn metal domain becomes dominant with x. Thin graphitic carbon layers usually sheathed the nanocrystals. We investigated the composition-dependent electrochemical properties of these nanocrystals as anode materials of lithium ion batteries. Incorporation of Sn (x = 0.05) significantly increased the capacities (1010 mA h g(-1) after 50 cycles) and rate capabilities, which promises excellent electrode materials for the development of high-performance lithium batteries.

Three-dimensional structure of twinned and zigzagged one-dimensional nanostructures using electron tomography.

Electron tomography and high-resolution transmission electron microscopy were used to characterize the unique three-dimensional (3D) structures of twinned Zn(3)P(2) (tetragonal) and InAs (zinc blende) nanowires synthesized by the vapor transport method. The Zn(3)P(2) nanowires adopt a unique superlattice structure that consists of twinned octahedral slice segments having alternating orientations along the axial [111] direction of a pseudo cubic unit cell. The apexes of the octahedral slice segment are indexed as six equivalent <112> directions at the [111] zone axis. At each 30 degrees turn, the straight and zigzagged morphologies appear repeatedly at the <112> and <011> zone axes, respectively. The 3D structure of the twinned Zn(3)P(2) nanowires is virtually the same as that of the twinned InAs nanowires. In addition, we analyzed the 3D structure of zigzagged CdO (rock salt) nanowires and found that they include hexahedral segments, whose six apexes are matched to the <011> directions, linked along the [111] axial direction. We also analyzed the unique 3D structure of rutile TiO(2) (tetragonal) nanobelts; at each 90 degrees turn, the straight morphology appears repeatedly, while the in-between twisted form appears at the [011] zone axis. We suggest that the TiO(2) nanobelts consist of twinned octahedral slices whose six apexes are indexed by the <011>/<001> directions with the axial [010] direction.

CoSe2 and NiSe2 Nanocrystals as Superior Bifunctional Catalysts for Electrochemical and Photoelectrochemical Water Splitting.

Catalysts for oxygen evolution reactions (OER) and hydrogen evolution reactions (HER) are central to key renewable energy technologies, including fuel cells and water splitting. Despite tremendous effort, the development of low-cost electrode catalysts with high activity remains a great challenge. In this study, we report the synthesis of CoSe2 and NiSe2 nanocrystals (NCs) as excellent bifunctional catalysts for simultaneous generation of H2 and O2 in water-splitting reactions. NiSe2 NCs exhibit superior electrocatalytic efficiency in OER, with a Tafel slope (b) of 38 mV dec(-1) (in 1 M KOH), and HER, with b = 44 mV dec(-1) (in 0.5 M H2SO4). In comparison, CoSe2 NCs are less efficient for OER (b = 50 mV dec(-1)), but more efficient for HER (b = 40 mV dec(-1)). It was found that CoSe2 NCs contained more metallic metal ions than NiSe2, which could be responsible for their improved performance in HER. Robust evidence for surface oxidation suggests that the surface oxide layers are the actual active sites for OER, and that CoSe2 (or NiSe2) under the surface act as good conductive layers. The higher catalytic activity of NiSe2 is attributed to their oxide layers being more active than those of CoSe2. Furthermore, we fabricated a Si-based photoanode by depositing NiSe2 NCs onto an n-type Si nanowire array, which showed efficient photoelectrochemical water oxidation with a low onset potential (0.7 V versus reversible hydrogen electrode) and high durability. The remarkable catalytic activity, low cost, and scalability of NiSe2 make it a promising candidate for practical water-splitting solar cells.

Light-Matter Interactions in Cesium Lead Halide Perovskite Nanowire Lasers.

Light-matter interactions in inorganic perovskite nanolasers are investigated using single-crystalline cesium lead halide (CsPbX3, X = Cl, Br, and I) nanowires synthesized by the chemical vapor transport method. The perovskite nanowires exhibit a uniform growth direction, smooth surfaces, straight end facets, and homogeneous composition distributions. Lasing occurs in the perovskite nanowires at low thresholds (3 µJ/cm(2)) with high quality factors (Q = 1200-1400) under ambient atmospheric environments. The wavelengths of the nanowire lasers are tunable by controlling the stoichiometry of the halide, allowing the lasing of the inorganic perovskite nanowires from blue to red. The unusual spacing of the Fabry-Pérot modes suggests strong light-matter interactions in the reduced mode volume of the nanowires, while the polarization of the lasing indicates that the Fabry-Pérot modes belong to the same fundamental transverse mode. The dispersion curve of the exciton-polariton model suggests that the group refractive index of the polariton is significantly enhanced.

Red-to-Ultraviolet Emission Tuning of Two-Dimensional Gallium Sulfide/Selenide.

Graphene-like two-dimensional (2D) nanostructures have attracted significant attention because of their unique quantum confinement effect at the 2D limit. Multilayer nanosheets of GaS-GaSe alloy are found to have a band gap (Eg) of 2.0-2.5 eV that linearly tunes the emission in red-to-green. However, the epitaxial growth of monolayers produces a drastic increase in this Eg to 3.3-3.4 eV, which blue-shifts the emission to the UV region. First-principles calculations predict that the Eg of these GaS and GaSe monolayers should be 3.325 and 3.001 eV, respectively. As the number of layers is increased to three, both the direct/indirect Eg decrease significantly; the indirect Eg approaches that of the multilayers. Oxygen adsorption can cause the direct/indirect Eg of GaS to converge, resulting in monolayers with a strong emission. This wide Eg tuning over the visible-to-UV range could provide an insight for the realization of full-colored flexible and transparent light emitters and displays.

High-turnover visible-light photoreduction of CO2 by a Re(I) complex stabilized on dye-sensitized TiO2.

Hybrid systems prepared by fixing a Re(i) complex and a dye on three types of TiO2 nanoparticles in two different ways commonly revealed persistent photocatalysis of the CO2 reduction to CO with no levelling-off tendency under visible-light irradiation in DMF, giving a turnover number of ≥435.

Phase evolution of tin nanocrystals in lithium ion batteries.

Sn-based nanostructures have emerged as promising alternative materials for commercial lithium-graphite anodes in lithium ion batteries (LIBs). However, there is limited information on their phase evolution during the discharge/charge cycles. In the present work, we comparatively investigated how the phases of Sn, tin sulfide (SnS), and tin oxide (SnO2) nanocrystals (NCs) changed during repeated lithiation/delithiation processes. All NCs were synthesized by a convenient gas-phase photolysis of tetramethyl tin. They showed excellent cycling performance with reversible capacities of 700 mAh/g for Sn, 880 mAh/g for SnS, and 540 mAh/g for SnO2 after 70 cycles. Tetragonal-phase Sn (ß-Sn) was produced upon lithiation of SnS and SnO2 NCs. Remarkably, a cubic phase of diamond-type Sn (α-Sn) coexisting with ß-Sn was produced by lithiation for all NCs. As the cycle number increased, α-Sn became the dominant phase. First-principles calculations of the Li intercalation energy of α-Sn (Sn8) and ß-Sn (Sn4) indicate that Sn4Li(x) (x ≤ 3) is thermodynamically more stable than Sn8Li(x) (x ≤ 6) when both have the same composition. α-Sn maintains its crystalline form, while ß-Sn becomes amorphous upon lithiation. Based on these results, we suggest that once α-Sn is produced, it can retain its crystallinity over the repeated cycles, contributing to the excellent cycling performance.

Photo-induced cation exchange reaction of germanium chalcogenide nanocrystals synthesized using gas-phase laser photolysis reaction.

Germanium chalcogenide GeS(x)Se(1-x) nanocrystals (NC) were synthesized using a novel gas-phase laser photolysis reaction. The composition was simply controlled by the partial pressure of precursors in a closed reactor. Remarkably, these ligand-free NC undergo the photo-induced cation exchange reaction to produce a series of Cd, Zn, Pb, and Ag chalcogenide NC in aqueous solution, which is governed by the thermodynamic driving force based on solubility.

Tetragonal phase germanium nanocrystals in lithium ion batteries.

Various germanium-based nanostructures have recently demonstrated outstanding lithium ion storage ability and are being considered as the most promising candidates to substitute current carbonaceous anodes of lithium ion batteries. However, there is limited understanding of their structure and phase evolution during discharge/charge cycles. Furthermore, the theoretical model of lithium insertion still remains a challenging issue. Herein, we performed comparative studies on the cycle-dependent lithiation/delithiation processes of germanium (Ge), germanium sulfide (GeS), and germanium oxide (GeO2) nanocrystals (NCs). We synthesized the NCs using a convenient gas phase laser photolysis reaction and attained an excellent reversible capacity: 1100-1220 mAh/g after 100 cycles. Remarkably, metastable tetragonal (ST12) phase Ge NCs were constantly produced upon lithiation and became the dominant phase after a few cycles, completely replacing the original phase. The crystalline ST12 phase persisted through 100 cycles. First-principles calculations on polymorphic lithium-intercalated structures proposed that the ST12 phase Ge12Lix structures at x ≥ 4 become more thermodynamically stable than the cubic phase Ge8Lix structures with the same stoichiometry. The production and persistence of the ST12 phase can be attributed to a stronger binding interaction of the lithium atoms compared to the cubic phase, which enhanced the cycling performance.

Germanium sulfide(II and IV) nanoparticles for enhanced performance of lithium ion batteries.

Germanium sulfide (GeS and GeS2) nanoparticles were synthesized by novel gas-phase laser photolysis and subsequent thermal annealing. They showed excellent cycling performance for lithium ion batteries, with a maximum capacity of 1010 mA h g(-1) after 100 cycles. Metastable tetragonal phase Ge nanoparticles were suggested as active materials for a reversible lithium insertion-extraction process.

Charge-selective surface-enhanced Raman scattering using silver and gold nanoparticles deposited on silicon-carbon core-shell nanowires.

The deposition of silver (Ag) or gold (Au) nanoparticles (NPs) on vertically aligned silicon-carbon (Si-C) core-shell nanowires (NWs) produces sensitive substrates for surface-enhanced Raman spectroscopy (SERS). The undoped and 30% nitrogen (N)-doped graphitic layers of the C shell (avg thickness of 20 nm) induce a higher sensitivity toward negatively (-) and positively (+) charged dye molecules, respectively, showing remarkable charge selectivity. The Ag NPs exhibit higher charge selectivity than the Au NPs. The Ag NPs deposited on p- and n-type Si NWs also exhibit (-) and (+) charge selectivity, respectively, which is higher than that of the Au NPs. The X-ray photoelectron spectroscopy analysis indicates that the N-doped graphitic layers donate more electrons to the metal NPs than the undoped ones. More distinct electron transfer occurs to the Ag NPs than to the Au NPs. First principles calculations of the graphene-metal adducts suggest that the large electron transfer capacity of the N-doped graphitic layers is due to the formation of a N→Ag coordinate bond involving the lone pair electrons of the N atoms. We propose that the more (-) charged NPs on the N-doped graphitic layers prefer the adsorption of (+) charged dyes, enhancing the SERS intensity. The charge selectivity of the Si NW substrates can also be rationalized by the greater electron transfer from the n-type Si to the metal NPs.

Nanodiamonds as photocatalysts for reduction of water and graphene oxide.

Detonated nanodiamonds (NDs) exhibit remarkable photocatalytic activity towards the hydrogen gas generation upon 532 nm laser pulse irradiation. Hydrogenation dramatically increases the quantum yield, suggesting that hydrogen-terminated sites work as electron reservoirs. NDs can also be used as effective photocatalysts to reduce graphene oxide. The resulting composites exhibit high and stable photocurrent generation upon visible light irradiation.

Composition-tuned ZnO--CdSSe core--shell nanowire arrays.

Vertically aligned ZnO--CdSSe core-shell nanocable arrays were synthesized with a controlled composition and shell thickness (10-50 nm) by the chemical vapor deposition on the pregrown ZnO nanowire arrays. They consisted of a composition-tuned single-crystalline wurtzite structure CdS1-xSex (x=0, 0.5, and 1) shell whose [0001] direction was aligned along the [0001] wire axis of the wurtzite ZnO core. The analysis of structural and optical properties shows the formation of Zn containing alloy in the interface region between the ZnO core and shell, which can facilitate the growth of single-crystalline shell layers by reducing both the lattice mismatch and the number of defect sites. In contrast, the TiO2 (rutile) nanowire array can form the polycrystalline shell under the same condition. The photoelectrochemical cell using the ZnO--CdS photoelectrode exhibits a higher photocurrent and hydrogen generation rate than that using the TiO2-CdS one. We suggest that the formation of the CdZnSSe intermediate layers contributes to the higher photoelectrochemical cell performance of the ZnO--CdSSe nanocables.