Some aspects of substrate pretreatment for epitaxial Si nanowire growth

We report on the influence of the surface pretreatment for vapor-liquid-solid growth of epitaxial silicon nanowires with gold catalyst and silane precursor on Si(111) substrates. In this paper we make it obvious that a thin native oxide layer on the Si substrate-as is present under most technological conditions-or a thin layer of oxide formed on top of the catalytic gold particle restrain nucleation and nanowire growth. High resolution transmission electron microscopy, and electron energy loss spectroscopy were utilized to demonstrate Si diffusion from the substrate through the catalytic Au layer and further the formation of a thin oxide layer atop. Based on this observation we present a sample pretreatment practice, making the catalyst insensitive for further oxide formation, thereby preserving epitaxy for nanowire synthesis.     

Coexistence of Vapor–Liquid–Solid and Vapor–Solid–Solid Growth Modes in Pd‐Assisted InAs Nanowires

During the growth of InAs nanowires from Pd catalyst particles on InAs(111)A substrates, two distinct classes of nanowires are observed with smooth or zigzagged sidewalls. It is shown that this is related to a bimodal distribution of the wire‐tip diameter: above a critical diameter wires grow with smooth sidewalls, and below with zigzagged morphology. Transmission electron microscopy analysis shows that the catalyst particles at the tip of zigzagged wires are smooth and have a higher aspect ratio than those at the tip of smooth wires. Zigzagged wires grow from liquid particles in the vapor–liquid–solid (VLS) mode whereas the smooth ones grow from solid particles in the vapor–solid–solid (VSS) mode.     

Formation of Single Tiers of Bridging Silicon Nanowires for Transistor Applications Using Vapor–Liquid–Solid Growth from Short Silicon‐on‐Insulator Sidewalls

Single tiers of silicon nanowires that bridge the gap between the short sidewalls of silicon‐on‐insulator (SOI) source/drain pads are formed. The formation of a single tier of bridging nanowires is enabled by the attachment of a single tier of Au catalyst nanoparticles to short SOI sidewalls and the subsequent growth of epitaxial nanowires via the vapor–liquid–solid (VLS) process. The growth of unobstructed nanowire material occurs due to the attachment of catalyst nanoparticles on silicon surfaces and the removal of catalyst nanoparticles from the SOI‐buried oxide (BOX). Three‐terminal current–voltage measurements of the structure using the substrate as a planar backgate after VLS nanowire growth reveal transistor behaviour characteristics.     

Key Factors Achieving Large Recovery Strains in Polycrystalline Fe–Mn–Si‐Based Shape Memory Alloys: A Review

Fe–Mn–Si‐based shape memory alloys are the most favorable for large‐scale applications owing to low cost, good workability, good machinability, and good weldability. However, polycrystalline Fe–Mn–Si‐based shape memory alloys have low recovery strains of only 2–3% after solution treatment, although monocrystalline ones reach a large recovery strain of ≈9%. This review gives an overview of the improvement of recovery strains for polycrystalline Fe–Mn–Si‐based shape memory alloys. It is proposed that two fundamental aspects, that is, composition design and microstructure design, shall be satisfied for obtaining large recovery strains of above 6%. Alloying compositions determining the ceiling of recovery strains shall follow three guidelines: (i) Si content is 5–6 wt%; (ii) 20 wt% ≤ Mn ≤ 32 wt%; (iii) addition of elements strongly strengthening austenite matrix. Microstructure design includes coarsening austenitic grains and reducing twin boundaries as far as possible together with introducing a high density of stacking faults and second phases of strengthening austenite.

     

Growth of germanium nanowires on silicon(111) substrates by molecular beam epitaxy

Heteroepitaxial growth of Ge nanowires was carried out on Si(111) substrates by MBE. Au seeds were used as precursor for the VLS growth of the nanowires. Even if the Au droplets do not act as catalyst for the dissociation of gas, they are local preferential areas where the energetic barrier of Ge nucleation is lowered compare to the remaining non activated surface. Two sets of Au seeds were used as precursors for the VLS process. The first set have an average diameter of 125 nm and the second of 25 nm. In-situ RHEED monitoring showed a Au wetting layer between these seeds before the nanowires growth as well as at the end of the Ge nanowires growth. It means that the wetting layer acted as a surfactant from the Si(111) surface to the Ge grown layer between the nanowires. Analysis of SEM images brought the fact that the diffusion of gold from the droplets on the surface and the sidewalls of the nanowires via the Ostwald ripening is a key parameter of the growth of the nanowires.     

Bulk Micromachining of Si by Metal‐assisted Chemical Etching

Bulk micromachining of Si is demonstrated by the well‐known metal‐assisted chemical etching (MaCE). Si microstructures, having lateral dimension from 5 μm up to millimeters, are successfully sculpted deeply into Si substrate, as deep as >100 μm. The key ingredient of this success is found to be the optimizations of catalyst metal type and its morphology. Combining the respective advantages of Ag and Au in the MaCE as a Ag/Au bilayer configuration leads to quite stable etch reaction upon a prolonged etch duration up to >5 h. Further, the permeable nature of the optimized Ag/Au bilayer metal catalyst enables the etching of pattern features having very large lateral dimension. Problems such as the generation of micro/nanostructures and chemical attacks on the top of pattern surface are successfully overcome by process optimizations such as post‐partum sonication treatment and etchant formulation control. The method can also be successful to vertical micromachining of Si substrate having other crystal orientations than Si(100), such as Si(110) and Si(111). The simple, easy, and low‐cost nature of present approach may be a great help in bulk micromachining of Si for various applications such as microelectromechanical system (MEMS), micro total analysis system (μTAS), and so forth.     

Gold nanocluster distribution on faceted and kinked Si nanowires

Si nanowires were grown on (111) substrates by ultra high vacuum chemical vapor deposition using the Au-catalyzed vapor-liquid-solid (VLS) technique. Depending on the growth temperature, the nanowires can be straight in the <111> direction or kinked towards <112>. We present a transmission electron microscopy investigation of the <112> Si nanowires. Results exhibit the relationship between the morphology of nanowires and the distribution of gold on sidewalls bounding the nanowires. The distribution of Au nanoclusters is used as a probe to investigate the growth mechanisms of the VLS process. Our observations are consistent with the model of nucleation and step flow related to the oscillatory behavior of the catalyst droplet.     

Reactive Atmosphere Synthesis of Polymer‐Derived Si–O–C–N Aerogels and Their Cr Adsorption from Aqueous Solutions

Polymer derived ceramic (PDCs) aerogels belonging to the Si–O–C–N system are synthesized by crosslinking a preceramic polymer in a diluted solution followed by supercritical or atmospheric drying and pyrolysis in inert (N₂) or reactive (NH₃/CO₂) atmosphere. Accordingly, aerogels with hierarchical porosity ranging from some microns to few nanometers together with high specific surface area in the range 30–400 m² g^?1 have been obtained. Moreover, their surface contains a broad range of moieties (Si–OH, Si–NH, C=O, etc.) that can interact and bind metal ions thanks to electrostatic interactions. This combination of hierarchical porosity, high SSA, and broad range of chemical functionalities makes these PDCs aerogels interesting candidates for water purification. In this work, SiOC and SiCN aerogels have been tested as adsorbents for Cr(III)/(VI) ions from aqueous solutions with promising results for the SiOC aerogel pyrolyzed in N₂ and the SiCN treated in NH₃. Correlations and similarities among the Cr(VI)/(III) adsorption capacity with the main features of the porous substrates (SSA, N₂ TPV, amount of free C, bulk density, isoelectric point, main IR peaks (Si–OH, OH, NH, C=O, C=C, Si–O, C–N, Si–N) have been investigated by applying the Principal Component Analysis (PCA).

     

Nanoparticle formation in Au thin films by electron-beam-induced dewetting

We carried out investigations on electron-beam-induced nanoparticle formation in thin (5-30?nm) Au films on smooth SiO(2)/Si substrates. When the Au films were irradiated with an electron beam, the Au films broke up into nanoparticles through the dewetting process. The dominant wavelengths of the surface (corresponding to the pitch between nanoparticles) were closely related with the thickness of the Au. We then developed a new technique for the formation of periodically arranged Au nanoparticles using a holed substrate. The nanoholes induced heterogeneous nucleation and helped to form ordered nanoparticles between the holes. Two-dimensionally, periodically arranged Au nanoparticles with a pitch of 100?nm were?obtained.     

From UV to Near‐Infrared Light‐Responsive Metal–Organic Framework Composites: Plasmon and Upconversion Enhanced Photocatalysis

The exploitation of photocatalysts that harvest solar spectrum as broad as possible remains a high‐priority target yet grand challenge. In this work, for the first time, metal–organic framework (MOF) composites are rationally fabricated to achieve broadband spectral response from UV to near‐infrared (NIR) region. In the core–shell structured upconversion nanoparticles (UCNPs)‐Pt@MOF/Au composites, the MOF is responsive to UV and a bit visible light, the plasmonic Au nanoparticles (NPs) accept visible light, whereas the UCNPs absorb NIR light to emit UV and visible light that are harvested by the MOF and Au once again. Moreover, the MOF not only facilitates the generation of “bare and clean” Au NPs on its surface and realizes the spatial separation for the Au and Pt NPs, but also provides necessary access for catalytic substrates/products to Pt active sites. As a result, the optimized composite exhibits excellent photocatalytic hydrogen production activity (280 µmol g^?1 h^?1) under simulated solar light, and the involved mechanism of photocatalytic H₂ production under UV, visible, and NIR irradiation is elucidated. Reportedly, this is an extremely rare study on photocatalytic H₂ production by light harvesting in all UV, visible, and NIR regions.     

Atomic Structures of Precipitates in Al–Mg–Si Alloys with Small Additions of Other Elements

In Al–Mg–Si alloys, additions of only a few weight percent of Mg and Si enable formation of hardening precipitates during heat treatment. The precipitation is complex and is influenced by chemical compositions and thermo‐mechanical treatment. Structural analysis at the atomic scale has played an important role for understanding the Al–Mg–Si system. This review paper gives a summary of the influence of elements on the precipitate structures of Al–Mg–Si alloys at the atomic scale. The structures are modified by small additions of different elements, but all the encountered precipitates are structurally connected with the Si network, except for the main hardening phase which exhibit a partially discontinuous Si network. The influence of the selected elements (Li, Cu, Zn, Ge, Ag, Ni, Co, and Au) is discussed in detail.

     

Refinement of Mg2Si reinforcement in a commercial Al–20%Mg2Si in-situ composite with bismuth,antimony and strontium

Refinement by addition elements of Al–Mg₂Si alloys is known to result in a change of primary Mg₂Si morphology. In this paper, the effects of Bi, Sb and Sr on the characteristic parameters of Al–20%Mg₂Si in-situ composite have been investigated by computer aided cooling curve thermal analysis and microstructural inspection. Size, density and aspect ratio measurements showed that additions of 0.4 wt.% Bi, 0.8 wt.% Sb and 0.01 wt.% Sr refined the Mg₂Si reinforcement. Exceeding these concentrations, however, resulted in coarsening of Mg₂Si particles with no change in the morphology. The results also showed that addition elements caused a decrease in the nucleation and growth temperatures of Mg₂Si particles. The refining effect of Bi, Sb and Sr is likely to be related to the effect of oxide bifilms suspended in the composite melt as favored nucleation substrates for Mg₂Si particles.     

Seed‐Induced Growth of Flower‐Like Au–Ni–ZnO Metal–Semiconductor Hybrid Nanocrystals for Photocatalytic Applications

The combination of metal and semiconductor components in nanoscale to form a hybrid nanocrystal provides an important approach for achieving advanced functional materials with special optical, magnetic and photocatalytic functionalities. Here, a facile solution method is reported for the synthesis of Au–Ni–ZnO metal–semiconductor hybrid nanocrystals with a flower‐like morphology and multifunctional properties. This synthetic strategy uses noble and magnetic metal Au@Ni nanocrystal seeds formed in situ to induce the heteroepitaxial growth of semiconducting ZnO nanopyramids onto the surface of metal cores. Evidence of epitaxial growth of ZnO{0001} facets on Ni {111} facets is observed on the heterojunction, even though there is a large lattice mismatch between the semiconducting and magnetic components. Adjustment of the amount of Au and Ni precursors can control the size and composition of the metal core, and consequently modify the surface plasmon resonance (SPR) and magnetic properties. Room‐temperature superparamagnetic properties can be achieved by tuning the size of Ni core. The as‐prepared Au–Ni–ZnO nanocrystals are strongly photocatalytic and can be separated and re‐cycled by virtue of their magnetic properties. The simultaneous combination of plasmonic, semiconducting and magnetic components within a single hybrid nanocrystal furnishes it multifunctionalities that may find wide potential applications.     

Surface Supported Gold–Organic Hybrids: On‐Surface Synthesis and Surface Directed Orientation

The surface‐assisted synthesis of gold–organic hybrids on Au (111) and Au (100) surfaces is repotred by thermally initiated dehalogenation of chloro‐substituted perylene‐3,4,9,10‐tetracarboxylic acid bisimides (PBIs). Structures and surface‐directed alignment of the Au–PBI chains are investigated by scanning tunnelling microscopy in ultra high vacuum conditions. Using dichloro‐PBI as a model system, the mechanism for the formation of Au–PBI dimer is revealed with scanning tunnelling microscopy studies and density functional theory calculations. A PBI radical generated from the homolytic C‐Cl bond dissociation can covalently bind a surface gold atom and partially pull it out of the surface to form stable PBI‐Au hybrid species, which also gives rise to the surface‐directed alignment of the Au–PBI chains on reconstructed Au (100) surfaces.     

Nucleation of small metal particles on ultrathin SiO2 films on Si

Metal particles of Pt, Ni, Au and Ag, typically less than 50 Å in diameter, were deposited by vacuum evaporation onto substrates of ultrathin (～25 Å) SiO₂ on Si. The particle size and charge were determined by transmission electron microscopy and MOS type capacitance-voltage measurements, respectively. It was observed that the Pt and Ni particles were smaller and more closely spaced than Au and Ag particles under comparable deposition conditions. The particle charge for Pt and Ni appeared to exceed that for Au and Ag for the same substrate coverage, suggesting possible charge contributions to heterogeneous nucleation. Since Pt and Ni particles were observed to carry a positive charge, they are presumably stabilized at negative substrate sites; the opposite appears to be true for Au and Ag.     

Effects of grain refinement on age hardening behavior and precipitation kinetics of Al–Si–Cu–Mg alloys

The influences of grain refinement on precipitation kinetics were investigated for an Al–11 wt% Si–1.5 wt% Cu–0.3 wt% Mg casting alloy doped with B and and with La–B respectively by microstructure observation, hardness test and Johnson–Mehl–Avrami (JMA) equation. Co‐alloying of La–B facilitates the faster hardening response with higher hardness value for the alloy. The calculated Avarmi exponent indicates that the nucleation of θ′‐Al₂Cu precipitates occurs on grain boundaries for the refined alloys. The activation energies for the precipitation are of 42 kJ/mol and 30 kJ/mol for B‐doped and La–B co‐doped alloys, respectively.     

Amorphizing of Au Nanoparticles by CeOx–RGO Hybrid Support towards Highly Efficient Electrocatalyst for N2 Reduction under Ambient Conditions

Ammonia synthesis is one of the most kinetically complex and energetically challenging chemical processes in industry and has used the Harber–Bosch catalyst for over a century, which is processed under both harsh pressure (150–350 atm) and hightemperature (623–823 K), wherein the energy and capital intensive Harber–Bosch process has a huge energy cost accounting for about 1%–3% of human's energy consumption. Therefore, there has been a rough and vigorous exploration to find an environmentally benign alternative process. As the amorphous material is in a metastable state and has many “dangling bonds”, it is more active than the crystallized one. In this paper, CeO_x ‐induced amorphization of Au nanoparticles anchored on reduced graphite oxide (a‐Au/CeO_x –RGO) has been achieved by a facile coreduction method under ambient atmosphere. As a proof‐of‐concept experiment, a‐Au/CeO_x –RGO hybrid catalyst containing the low noble metal (Au loading is 1.31 wt%) achieves a high Faradaic efficiency (10.10%) and ammonia yield (8.3 μg h^?1 mg^?1_cat.) at ?0.2 V versus RHE, which is significantly higher than that of the crystalline counterpart (c‐Au/RGO), and even comparable to the yields and efficiencies under harsh temperatures and/or pressures.     

Minimizing the Gibbs-Thomson effect in the low-temperature plasma synthesis of thin Si nanowires

An advanced combination of numerical models, including plasma sheath, ion- and radical-induced species creation and plasma heating effects on the surface and within a Au catalyst nanoparticle, is used to describe the catalyzed growth of Si nanowires in the sheath of a low-temperature and low-pressure plasma. These models have been used to explain the higher nanowire growth rates, low-energy barriers, much thinner Si nanowire nucleation and the less effective Gibbs-Thomson effect in reactive plasma processes, compared with those of neutral gas thermal processes. The effects of variation in the plasma sheath parameters and substrate potential on Si nanowire nucleation and growth have also been investigated. It is shown that increasing the plasma-related effects leads to decreases in the nucleation energy barrier and the critical nanoparticle radius, with the Gibbs-Thomson effect diminished, even at low temperatures. The results obtained are consistent with available experimental results and open a path toward the energy- and matter-efficient nucleation and growth of a broad range of one-dimensional quantum structures.     

Edge‐Epitaxial Growth of InSe Nanowires toward High‐Performance Photodetectors

Semiconducting nanowires offer many opportunities for electronic and optoelectronic device applications due to their unique geometries and physical properties. However, it is challenging to synthesize semiconducting nanowires directly on a SiO₂/Si substrate due to lattice mismatch. Here, a catalysis‐free approach is developed to achieve direct synthesis of long and straight InSe nanowires on SiO₂/Si substrates through edge‐homoepitaxial growth. Parallel InSe nanowires are achieved further on SiO₂/Si substrates through controlling growth conditions. The underlying growth mechanism is attributed to a selenium self‐driven vapor–liquid–solid process, which is distinct from the conventional metal‐catalytic vapor–liquid–solid method widely used for growing Si and III–V nanowires. Furthermore, it is demonstrated that the as‐grown InSe nanowire‐based visible light photodetector simultaneously possesses an extraordinary photoresponsivity of 271 A W^?1, ultrahigh detectivity of 1.57 × 10¹⁴ Jones, and a fast response speed of microsecond scale. The excellent performance of the photodetector indicates that as‐grown InSe nanowires are promising in future optoelectronic applications. More importantly, the proposed edge‐homoepitaxial approach may open up a novel avenue for direct synthesis of semiconducting nanowire arrays on SiO₂/Si substrates.