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

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

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

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

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 (β-Sn) was produced upon lithiation of SnS and SnO₂ NCs. Remarkably, a cubic phase of diamond-type Sn (α-Sn) coexisting with β-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 β-Sn (Sn₄) indicate that Sn₄Li_<i>x</i> (<i>x</i> ≤ 3) is thermodynamically more stable than Sn₈Li_<i>x</i> (<i>x</i> ≤ 6) when both have the same composition. α-Sn maintains its crystalline form, while β-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

<i>In Situ</i> Temperature-Dependent Transmission Electron Microscopy Studies of Pseudobinary <i>m</i>GeTe·Bi₂Te₃ (<i>m</i> = 3–8) Nanowires and First-Principles Calculations

Phase-change nanowires (NWs) have emerged as critical materials for fast-switching nonvolatile memory devices. In this study, we synthesized a series of <i>m</i>GeTe·Bi₂Te₃ (GBT) pseudobinary alloy NWsGe₃Bi₂Te₆ (<i>m</i> = 3), Ge₄Bi₂Te₇ (<i>m</i> = 4), Ge₅Bi₂Te₈ (<i>m</i> = 5), Ge₆Bi₂Te₉ (<i>m</i> = 6), and Ge₈Bi₂Te₁₁ (<i>m</i> = 8)and investigated their composition-dependent thermal stabilities and electrical properties. As <i>m</i> decreases, the phase of the NWs evolves from the cubic (C) to the hexagonal (H) phase, which produces unique superlattice structures that consist of periodic 2.2–3.8 nm slabs for <i>m</i> = 3–8. <i>In situ</i> temperature-dependent transmission electron microscopy reveals the higher thermal stability of the compositions with lower <i>m</i> values, and a phase transition from the H phase into the single-crystalline C phase at high temperatures (400 °C). First-principles calculations, performed for the superlattice structures (<i>m</i> = 1–8) of GBT and <i>m</i>GeTe·Sb₂Te₃ (GST), show an increasing stability of the H phase (versus the C phase) with decreasing <i>m</i>; the difference in stability being more marked for GBT than for GST. The calculations explain remarkably the phase evolution of the GBT and GST NWs as well as the composition-dependent thermal stabilities. Measurement of the current–voltage curves for individual GBT NWs shows that the resistivity is in the range 3–25 mΩ·cm, and the resistivity of the H phase is lower than that of the C phase, which has been supported by the calculations

Polymorphism of GeSbTe Superlattice Nanowires

Scaling-down of phase change materials to a nanowire (NW) geometry is critical to a fast switching speed of nonvolatile memory devices. Herein, we report novel composition-phase-tuned GeSbTe NWs, synthesized by a chemical vapor transport method, which guarantees promising applications in the field of nanoscale electric devices. As the Sb content increased, they showed a distinctive rhombohedral–cubic–rhombohedral phase evolution. Remarkable superlattice structures were identified for the Ge₈Sb₂Te₁₁, Ge₃Sb₂Te₆, Ge₃Sb₈Te₆, and Ge₂Sb₇Te₄ NWs. The coexisting cubic–rhombohedral phase Ge₃Sb₂Te₆ NWs exhibited an exclusively uniform superlattice structure consisting of 2.2 nm period slabs. The rhombohedral phase Ge₃Sb₈Te₆ and Ge₂Sb₇Te₄ NWs adopted an innovative structure; 3Sb₂ layers intercalated the Ge₃Sb₂Te₆ and Ge₂Sb₁Te₄ domains, respectively, producing 3.4 and 2.7 nm period slabs. The current–voltage measurement of the individual NW revealed that the vacancy layers of Ge₈Sb₂Te₁₁ and Ge₃Sb₂Te₆ decreased the electrical conductivity

Photoluminescence and Photocurrents of GaS_1–<i>x</i>Se_<i>x</i> Nanobelts

Two-dimensional layered structures have recently drawn worldwide attention because of their intriguing optical and electrical properties. GaS and GaSe are attractive layered materials owing to their wide band gap. Herein, we synthesized GaS_1–<i>x</i>Se_<i>x</i> belt-type multilayers (nanobelts) with uniform morphology ([21̅1̅0] hexagonal-phase long axis) by a chemical vapor transport method, and investigate their composition-dependent optical and optoelectronic properties. The GaS_1–<i>x</i>Se_<i>x</i> exhibited strong visible-range photoluminescence at 490–620 nm (2.0–2.5 eV), with a unique composition dependence: longer decay time for the S-rich compositions (<i>x</i> ≤ 0.5). Photocurrent measurements were performed on individual nanobelts by fabricating photodetector devices; higher photocurrents were found for <i>x</i> ≤ 0.5. First-principles calculations predicted that oxygen chemisorption can cause the direct and indirect band gaps of GaS to converge, similar to the band structures of GaSe, and thus enhance the optical properties. On the basis of the band alignment (predicted by calculation) for the Schottky barriers in the metal–semiconductor–metal photodetector, we proposed the origin of the higher photocurrent for GaS than for GaSe