Band Gap Tuning of Twinned GaAsP Ternary Nanowires

GaAs_1–<i>x</i>P_<i>x</i> ternary alloy nanowires have drawn much interest because their tunable band gaps, which range from the near-infrared to visible region, are promising for advanced and integrated nanoscale optoelectronic devices. In this study, we synthesized compositionally tuned GaAs_1<i>–x</i>P_<i>x</i> (0 ≤ <i>x</i> ≤ 1) alloy nanowires with two average diameters of 60 and 120 nm by vapor transport method. The nanowires exhibit exclusively twinned superlattice structures, consisting of zinc blende phase twinned octahedral slice segments between wurtzite phase planes. Smaller diameter and higher P content (<i>x</i>) result in shorter periodic superlattice structures. The band gap of the smaller diameter nanowires is larger than that of the larger diameter nanowires by about 90 meV, suggesting that the twinned superlattice structure increases the band gap. The increase in band gap is ascribed to the higher band gap of the wurtzite phase than that of the zinc blende phase

Transition-Metal Doping of Oxide Nanocrystals for Enhanced Catalytic Oxygen Evolution

Catalysts for the oxygen reduction and evolution reactions are central to key renewable-energy technologies including fuel cells and water splitting. Despite tremendous effort, the development of oxygen electrode catalysts with high activity at low cost remains a great challenge. In this study, we report a generalized sol–gel method for the synthesis of various oxide nanocrystals (TiO₂, ZnO, Nb₂O₅, In₂O₃, SnO₂, and Ta₂O₅) with appropriate transition metal dopants for an efficient electrocatalytic oxygen evolution reaction (OER). Although TiO₂ and ZnO nanocrystals alone have little activity, all the Mn-, Fe-, Co-, and Ni-doped nanocrystals exhibit greatly enhanced OER activity. A remarkable finding is that Co dopant produces higher OER activity than the other doped metals. X-ray photoelectron and X-ray absorption spectroscopies revealed the highly oxidized metal ions that are responsible for the enhanced catalytic reactivity. The excellent OER activity of the Co-doped nanocrystals was explained by a synergistic effect in which the oxide matrix effectively guards the most active Co dopants at higher oxidation states by withdrawing the electrons from the metal dopants. The metal-doped NCs exhibit enhanced catalytic activity under visible light irradiation, suggesting their potential as efficient solar-driven OER photoelectrocatalysts

Germanium and Tin Selenide Nanocrystals for High-Capacity Lithium Ion Batteries: Comparative Phase Conversion of Germanium and Tin

Germanium and tin sulfide nanostructures are considered the most promising candidates for useful alternative materials in commercial Li–graphite anodes of lithium ion batteries. Selenides have received less attention, but the electrochemical reaction mechanism is still being debated. We report the novel synthesis of GeSe_<i>x</i> and SnSe_<i>x</i> (<i>x</i> = 1 and 2) nanocrystals by a gas-phase laser photolysis reaction and their excellent reversible capacity for lithium ion batteries. The capacity was 400–800 (mA h)/g after 70 cycles, which is close to the theoretical capacity (Li_4.4Ge or Li_4.4Sn). Remarkably, SnSe_<i>x</i> exhibited higher rate capabilities than GeSe_<i>x</i>. Ex situ X-ray diffraction and Raman spectroscopy revealed the <i>cubic</i>–<i>tetragonal</i> phase conversion of Ge and Sn upon lithiation/delithiation to support their distinctive lithium ion battery capacities. First-principles calculations of the Li intercalation volume change indicate that the smallest volume expansion in the cubic Sn phase can guarantee the enhanced cycling capability of the Sn compounds

GaP–ZnS Pseudobinary Alloy Nanowires

Multicomponent nanowires (NWs) are of great interest for integrated nanoscale optoelectronic devices owing to their widely tunable band gaps. In this study, we synthesize a series of (GaP)_1–<i>x</i>(ZnS)_<i>x</i> (0 ≤ <i>x</i> ≤ 1) pseudobinary alloy NWs using the vapor transport method. Compositional tuning results in the phase evolution from the zinc blende (ZB) (<i>x</i> < 0.4) to the wurtzite (WZ) phase (<i>x</i> > 0.7). A coexistence of ZB and WZ phases (<i>x</i> = 0.4–0.7) is also observed. In the intermediate phase coexistence range, a core–shell structure is produced with a composition of <i>x</i> = 0.4 and 0.7 for the core and shell, respectively. The band gap (2.4–3.7 eV) increases nonlinearly with increasing <i>x</i>, showing a significant bowing phenomenon. The phase evolution leads to enhanced photoluminescence emission. Strikingly, the photoluminescence spectrum shows a blue-shift (70 meV for <i>x</i> = 0.9) with increasing excitation power, and a wavelength-dependent decay time. Based on the photoluminescence data, we propose a type-II pseudobinary heterojunction band structure for the single-crystalline WZ phase ZnS-rich NWs. The slight incorporation of GaP into the ZnS induces a higher photocurrent and excellent photocurrent stability, which opens up a new strategy for enhancing the performance of photodetectors

Strain Mapping and Raman Spectroscopy of Bent GaP and GaAs Nanowires

Strain engineering of nanowires (NWs) has been recognized as a powerful strategy for tuning the optical and electronic properties of nanoscale semiconductors. Therefore, the characterization of the strains with nanometer-scale spatial resolution is of great importance for various promising applications. In the present work, we synthesized single-crystalline zinc blende phase GaP and GaAs NWs using the chemical vapor transport method and visualized their bending strains (up to 3%) with high precision using the nanobeam electron diffraction technique. The strain mapping at all crystallographic axes revealed that (i) maximum strain exists along the growth direction ([111]) with the tensile and compressive strains at the outer and inner parts, respectively; (ii) the opposite strains appeared along the perpendicular direction ([2̅11]); and (iii) the tensile strain was larger than the coexisting compressive strain at all axes. The Raman spectrum collected for individual bent NWs showed the peak broadening and red shift of the transverse optical modes that were well-correlated with the strain maps. These results are consistent with the larger mechanical modulus of GaP than that of GaAs. Our work provides new insight into the bending strain of III–V semiconductors, which is of paramount importance in the performance of flexible or bendable electronics

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(P_1–<i>x</i>As_<i>x</i>)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 ≤ <i>x</i> ≤ 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

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/cm²) with high quality factors (<i>Q</i> = 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

IrO₂–ZnO Hybrid Nanoparticles as Highly Efficient Trifunctional Electrocatalysts

Development of high-performance catalysts is very crucial for the commercialization of sustainable energy conversion technologies. Searching for stable, highly active, and low-cost multifunctional catalysts has become a critical issue. In this study, we report the synthesis of IrO₂–ZnO hybrid nanoparticles and their highly efficient electrocatalytic activities toward oxygen/hydrogen evolution reaction (OER/HER) as well as oxygen reduction reaction (ORR). For comparison, we synthesized RuO₂–ZnO, showing a smaller catalytic activity than IrO₂–ZnO, which provides robust evidence for the unique synergic effect of these hybrid structures. IrO₂–ZnO and RuO₂–ZnO exhibit excellent OER catalytic performance with Tafel slopes of 57 and 59 mV decade^–1, respectively. For HER, IrO₂–ZnO shows a higher catalytic activity than RuO₂–ZnO. The numbers of electrons involved in the ORR were 3.7 and 2.8, respectively, for IrO₂–ZnO and RuO₂–ZnO. The remarkable catalytic performance of IrO₂–ZnO would be ascribed to the abundant oxygen vacancies and the metallic states of Ir, which ensure excellent catalytic activity and stability

Surface-Modified Ta₃N₅ Nanocrystals with Boron for Enhanced Visible-Light-Driven Photoelectrochemical Water Splitting

Photocatalysts for water splitting are the core of renewable energy technologies, such as hydrogen fuel cells. The development of photoelectrode materials with high efficiency and low corrosivity has great challenges. In this study, we report new strategy to improve performance of tantalum nitride (Ta₃N₅) nanocrystals as promising photoanode materials for visible-light-driven photoelectrochemical (PEC) water splitting cells. The surface of Ta₃N₅ nanocrystals was modified with boron whose content was controlled, with up to 30% substitution of Ta. X-ray photoelectron spectroscopy revealed that boron was mainly incorporated into the surface oxide layers of the Ta₃N₅ nanocrystals. The surface modification with boron increases significantly the solar energy conversion efficiency of the water-splitting PEC cells by shifting the onset potential cathodically and increasing the photocurrents. It reduces the interfacial charge-transfer resistance and increases the electrical conductivity, which could cause the higher photocurrents at lower potential. The onset potential shift of the PEC cell with the boron incorporation can be attributed to the negative shift of the flat band potential. We suggest that the boron-modified surface acts as a protection layer for the Ta₃N₅ nanocrystals, by catalyzing effectively the water splitting reaction

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 (<i>E</i>_g) 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 <i>E</i>_g to 3.3–3.4 eV, which blue-shifts the emission to the UV region. First-principles calculations predict that the <i>E</i>_g 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 <i>E</i>_g decrease significantly; the indirect <i>E</i>_g approaches that of the multilayers. Oxygen adsorption can cause the direct/indirect <i>E</i>_g of GaS to converge, resulting in monolayers with a strong emission. This wide <i>E</i>_g tuning over the visible-to-UV range could provide an insight for the realization of full-colored flexible and transparent light emitters and displays