Single-crystal Nanowires Research Articles

Structure, morphology and crystallization behavior of PVDF/Co nanowire nanocomposites under horizontal and vertical magnetic fields

So far, very limited research has been carried out on the fabrication of polyvinylidene fluoride (PVDF) composited with anisotropic magnetic nanostructures such as nanowires or nanorods, which can potentially provide specific reactions to magnetic fields. Here, single crystal Co nanowires (CNWs) with an average length and diameter of about 220 and 12nm, respectively, are synthesized using a solvothermal method, followed by adding them to PVDF matrix in order to prepare flexible PVDF/CNW nanocomposites by solvent casting and solution crystallization processes at room temperature. The nucleation and growth of PCNW nanocomposites are investigated in the absence and presence of horizontal and vertical magnetic fields. According to FESEM images, in addition to the direction of the applied magnetic field, the presence or absence of CNWs induces different effects on the size of spherulites and the configuration of polymer chains. By adding CNW to the PVDF matrix in the absence of a magnetic field and under a horizontal magnetic field, the size of the spherulites in the pure PVDF film increases about 67% (from 3 to 5 μm) and 300% (from 3 to 12 μm), respectively. Under a vertical magnetic field, the addition of CNW reduces the size of the spherulites to 1 μm, thereby forming a film with a combination of spherulites and filamentary structure likely due to the nanowire-nanowire, nanowire-chain and chain-chain interactions. Also non-contact flexible triboelectric nanogenerators of magnetic force are fabricated.

Uncovering the Size-Dependent Thermal Solid Transformation of Akaganéite.

Investigating the structural evolution and phase transformation of iron oxides is crucial for gaining a deeper understanding of geological changes on diverse planets and preparing oxide materials suitable for industrial applications. In this study, in-situ heating techniques are employed in conjunction with transmission electron microscopy (TEM) observations and ex-situ characterization to thoroughly analyze the thermal solid-phase transformation of akaganéite 1D nanostructures with varying diameters. These findings offer compelling evidence for a size-dependent morphology evolution in akaganéite 1D nanostructures, which can be attributed to the transformation from akaganéite to maghemite (γ-Fe2O3) and subsequent crystal growth. Specifically, it is observed that akaganéite nanorods with a diameter of ∼50nm transformed into hollow polycrystalline maghemite nanorods, which demonstrated remarkable stability without arresting crystal growth under continuous heating. In contrast, smaller akaganéite nanoneedles or nanowires with a diameter ranging from 20 to 8nm displayed a propensity for forming single-crystal nanoneedles or nanowires through phase transformation and densification. By manipulating the size of the precursors, a straightforward method is developed for the synthesis of single-crystal and polycrystalline maghemite nanowires through solid-phase transformation. These significant findings provide new insights into the size-dependent structural evolution and phase transformation of iron oxides at the nanoscale.

Manipulation and applications of ultrafast all-optical switching based on transient absorption and dispersion in KTN crystals.

Potassium tantalate niobate (KTN) represents a noteworthy category of optical crystals known for their superior nonlinear optical properties. In this study, we conducted measurements of femtosecond time-resolved transient absorption (TA) spectra in KTa0.57Nb0.43O3 crystals. Notably, a rapid and pronounced "plateau" phase, ∼1.5 ps in duration, was detected at the onset of the TA kinetics and succeeded by two distinct decay components, exhibiting lifetimes of ∼140 ps and over 10 ns, respectively. We attribute these observations to a decay process involving two-photon absorption, dispersion characteristics, and excited state absorption. Based on this unique TA characteristic of KTN crystals, an all-optical switching strategy was proposed and utilized to measure the ultrafast lasing dynamics of single-crystal CH3NH3PbBr3 nanowires. This polarization-independent TA gate approach offers an adjustable gate width combining ps and ns time scales and introduces a versatile tool for advanced optical applications.

Micronano Lasers Based on Aggregation-Induced Emission Molecules: Diverse Resonant Cavities Investigation.

Aggregation-induced emission (AIE) molecules have great potential to enhance the performance of micronano lasers due to their excellent aggregated luminescence properties, so it is valuable to expand their applications in micronano lasers. In this work, a typical AIE active fluorescent dye motif 9,10-bis(2,2-diphenylvinyl) anthracene (BDPVA) was selected as the gain medium. First, drop-casting was used to fabricate BDPVA single-crystal nanowires, which can be used as Fabry-Perot (FP)-type resonators with a lasing threshold of 49.4 μJ/cm2. Furthermore, we innovatively doped BDPVA molecules as gain mediums into external polymer Whispering-Gallery-Mode (WGM)-type resonators via the emulsion self-assembly method. Fabricated BDPVA-doped polystyrene (PS) microspheres exhibit a much lower lasing threshold of 9.04 μJ/cm2. These results prove that the BDPVA molecules, in addition to realizing the reported AIE single-crystal lasers, can also be used as a guest-doped gain medium in the resonant cavity for obtaining better fluorescence gain. In addition, multimode tunability of two types of lasers has been successfully achieved by tuning the structure of the resonant cavity. This work further expands the application potential of AIE materials and will provide a useful reference for the rational design and fabrication of photonic micronano laser components using AIE materials.

Highly uniform organic nanowire synaptic arrays with excellent performance for associative memory

Synaptic devices of optoelectronic nature, which combine light-detection and data-storage capabilities, hold significant promise in the realm of neuromorphic computing. They are especially beneficial for the processing of visual data and the execution of intricate cognitive functions akin to learning, memory retention, and logical reasoning. However, current research is mostly confined to the level of individual devices, with corresponding studies on synaptic arrays being relatively scarce. Type II heterojunctions are widely used in optoelectronics. Bandgap engineering enables efficient separation and trapping of photogenerated excitons in selected materials, reducing carrier recombination. This prolongs carrier retention in the channel, delaying photocurrent decay, crucial for artificial optoelectronic synapses. We designed an organic nanowire/perovskite Type II heterojunction where CsPbBr3 perovskite films and TIPS nanowires serve as the photosensitive and channel layers, respectively. The composite synaptic array was successfully fabricated by in-situ growth of TIPS nanowires on the surface of perovskite thin films. It was further used to fabricate a photonic synapse array that exhibits characteristic photocurrent and stable optical response. Achieving high-performance photonic synapses was facilitated by characteristics such as high mobility and high on/off ratios, stemming from the formation of complete single-crystal nanowires on the perovskite films. Exhibiting synaptic behaviors such as photo-induced enhancements and paired-pulse facilitation, the device enabled a visual sensing system with a 5 × 5 pixel array for simulating memory processes and restoring associative memories. A novel photocurrent model was established for understanding device characteristics. This work provides valuable insights for future utilization of composite organic nanowire arrays in simulating brain-like computations and realizing large-scale intelligent visual sensing systems.

Na2Ti3O7@RF@Ag Heterostructures as Efficient Substrates for SERS and Photocatalytic Applications.

A multi-step procedure was effectively employed to synthesize innovative three-dimensional (3D) heterostructures encompassing sodium titanate (Na2Ti3O7) nanowire cores, an intermediate resorcinol-formaldehyde (RF) layer, and outer silver (Ag) nanoparticle sheaths, referred to as Na2Ti3O7@RF@Ag heterostructures. Initially, a one-step hydrothermal technique facilitated the direct growth of single-crystal Na2Ti3O7 nanowires onto a flexible Ti foil. Subsequently, a two-step wet chemical process facilitated the sequential deposition of an RF layer and Ag nanoparticles onto the Na2Ti3O7 nanowires at a low reaction temperature. Optimal concentrations of silver nitrate and L-ascorbic acid can lead to the cultivation of Na2Ti3O7@RF@Ag heterostructures exhibiting heightened surface-enhanced Raman scattering (SERS), which is particularly beneficial for the detection of rhodamine B (RhB) molecules. This phenomenon can be ascribed to the distinctive geometry of the Na2Ti3O7@RF@Ag heterostructures, which offer an increased number of hot spots and surface-active sites, thereby showcasing notable SERS enhancement, commendable reproducibility, and enduring stability over the long term. Furthermore, the Na2Ti3O7@RF@Ag heterostructures demonstrate remarkable follow-up as first-order chemical kinetic and recyclable photocatalysts for the photodecomposition of an RhB solution under UV light irradiation. This result can be attributed to the enhanced inhibition of electron-hole pair recombination and increased surface-active sites.

(Invited) Multifunctional Plasma-Enabled Low Dimensional Nanoarchitectures: From Synthesis to Devices

In this presentation, we will give an overview on our last advances in the exploitation of plasma and vacuum deposition methods for the nanoengineering of thin films and supported low-dimensional materials. A key step in the development of such nanoarchitectures is the implementation of a soft-template procedure based on the use of single-crystal small-molecule nanowires grown on processable substrates under mild conditions. We will demonstrate the application of vacuum and plasma-assisted deposition techniques to develop complex nanowires (NWs) and nanotubes (NTs) with a core@multishell morphology where each shell adds functionality or multifunctionality to the system. The steps required for the integration of these nanomaterials as supported or in-device applications will be presented. Two final applications will drive the presentation, namely, the development of multisource energy harvesters (and self-powered sensors) and novel approaches for smart and efficient de-icing by acoustic waves covering the topics of the 3DScavengers and SOUNDofICE EU projects. The universal character of vacuum and plasma methods allows for the deposition of an unprecedented variety of possibilities, including, organic (small-molecules, organic nanocomposites, polymeric layers), inorganic (metal and metal oxides), hybrid (hybrid perovskite) materials acting as conducting, semiconducting, dielectric, photo-absorbent, piezoelectric or plasmonic components in a multilayer or radial configuration. We will share the results accomplished during the last years in the field of energy harvesting (solar cells,[1-4] piezoelectric and triboelectric nanogenerators[5-7]), semi-transparent nanoelectrodes,[8] wetting [9], de-icing and anti-icing.[10-11] References Filippin, A., Sanchez-Valencia, J., Borras, A. et al. (2017) Nanoscale, 9:8133-8141.Barranco, A., Borras, A., Sanchez-Valencia J. R. et al. (2020) Adv. Ener. Mater 10:1901524.Castillo, J., Borras, A., Sanchez-Valencia J.R. et al. (2022) Adv. Mater. 34:2107739.Obrero, J., Borras A., Barranco, A. (2022) Adv. Ener. Mater. 12:2200812.Filippin, A. N., Borras, A. et al. (2019) Nano Energy 58:476-483.García-Casas, X., Borras, A. et al. (2022) Nano Energy 91:106673.García-Casas, X. Borras, A. et al. Patent application Self-powered triboelectric transductor device for drops and liquids. EP23382176.8Castillo-Seoane, J., Borras, A. et al. (2021) Nanoscale 13:13882.Montes, L., Borras, A. et al. (2021) Adv. Mater. Interf. 21:2100767.Lopez-Santos, C., Borras, A. et al. (2019) Langmuir 35:16876.Del Moral, J., Montes, L., Borras, A. et al. (2023) Adv. Funct. Mater. 2023, 2209421

Low Redox Potential InN Nanowires as High-Energy-Density Anode for Lithium-Ion Batteries

Lithium-ion batteries (LIBs) have become the most predominant energy sources for various digital products and electric vehicles due to the rechargeable, high capacity, and no memory effect characteristics. Charging to 80% of the full capacity time is commonly required to be less than 30 minutes in the future as technological development. Thus, it is essential to improve the dynamics of the active material [1]. The current approach to increasing the energy density of LIBs is to increase the electrode loading mass. This has the effect of greatly increasing the thickness of the electrodes, making it difficult for Li-ions to quickly pass through the electrode. Graphite anode is the most widely used anode material due to its low cost, easy preparation, and applicable working voltage below 0.3 V (vs Li/Li+). However, its low capacity and disordered staging behavior at large currents limit the fast charging performance. Moreover, due to the low density of graphite, when designed as a high-energy density cell, its thickness increases considerably, significantly reducing the Li-ions diffusion. Therefore, new active materials with high capacity and fast Li-ions diffusion should be developed to achieve high-energy-density cells.As a well-known semiconductor, Indium Nitride (InN) has been widely used for various applications such as high-frequency terahertz and solar energy devices. When it is denoted as an anode for LIBs, the InN can deliver a theory specific capacity over 1200 mAh g-1, which is nearly four times of the conventional graphite anodes (~372 mAh g-1). Meanwhile, it shows a relatively low redox potential (below 0.7 V vs. Li+/Li), affording a wide voltage window for a full cell. Remarkably, the InN is a narrow band (0.69 eV) semiconductor showing good electron transfer properties. Thus, the InN could be considered as one of the most promising anode for the next-generation HED-LIBs. However, the inherent volume change of InN particles during the conversion and alloying/dealloying process with Li+ maybe lead to many adverse effects, including pulverization of InN particles electrode, tearing solid electrolyte interphase (SEI). The fact finally worsens the Li-storage performance, greatly hindering the commercial application of InN anode. In addition, the sluggish ion diffusion of InN particles also limits the rate capability. To address the issues, approaches like constructing micro/nano composite structures, which have been broadly explored to alleviate the volume expansion and improve ion diffusion [2]. Among these micro/nanostructures, the 1D nanostructure shows superior physicochemical properties in favor of improving Li-storage performance. Firstly, the One-dimensional (1D) nanostructures can accommodate the strain of Li-ions delidiation/lidiation along specified directions. Secondly, the gap of the 1D nanostructure is just enough to provide a buffer during the charge/discharge process. Thirdly, the 1D nanostructure is beneficial to the Li-ions diffusion leading to good rate performance [3]. Moreover, the 1D structure has the advantage of self-forming film, which greatly reduces the weight of the electrode. It is worth noting that the large ratio aspect of 1D nanostructure can lower the ion diffusion paths accelerating the rate performance.Herein, we elaborately constructed the single-crystal InN nanowires on carbon fibers (InN/Au-CFs). Firstly, the CFs are decorated with Au nanoparticles by a magnetron sputtering method, which can afford many stable growth sites for InN nanowires. And then, the InN nanowires in situ grown on the Au nanoparticle surface by chemical vapor deposition successfully improved the adhesive force active material and substrate. The structural properties and Li-storage performance of InN/Au-CFs are systematically studied. In addition, the density of state and adatom adsorption of InN nanowires are calculated by density functional theory. The ion diffusion, reactivity, and electrode stability are successfully enhanced based on the results. We also built two-terminal electrodes to decelerate the metal electrode fatigue at large rate for a full cell. Particularly, the as-prepared InN/Au-CFs anode can deliver the capacities of 624.2 mAh g-1 at 0.1 A g-1 after 500 cycles and 416 mAh g-1 at 30 A g-1 for the rate assessment, respectively. Our work suggests that the embellishment one dimentional InN nanowire and two-terminal electrodes are a successful strategy for HED-LIBs contributing excellent long cycling performance and large rate capability.[1] J. Kim et al, Nat. Rev. Mater. 8(1) (2023) 54-70.[2] A. M. Bates, et al, Joule 6(4) (2022) 742-755.[3] L. Xing et al, Nano Energy 79 (2021) 105384. Figure 1

Simultaneous Growth Strategy of High-Optical-Efficiency GaN NWs on a Wide Range of Substrates by Pulsed Laser Deposition.

Here, we explore a catalyst-free single-step growth strategy that results in high-quality self-assembled single-crystal vertical GaN nanowires (NWs) grown on a wide range of common and novel substrates (including GaN, Ga2O3, and monolayer two-dimensional (2D) transition-metal dichalcogenide (TMD)) within the same chamber and thus under identical conditions by pulsed laser deposition. High-resolution transmission electron microscopy and scanning transmission electron microscopy (HR-STEM) and grazing incidence X-ray diffraction measurements confirm the single-crystalline nature of the obtained NWs, whereas advanced optical and cathodoluminescence measurements provide evidence of their high optical quality. Further analyses reveal that the growth is initiated by an in situ polycrystalline layer formed between the NWs and substrates during growth, while as its thickness increases, the growth mode transforms into single-crystalline NW nucleation. HR-STEM and corresponding energy-dispersive X-ray compositional analyses indicate possible growth mechanisms. All samples exhibit strong band edge UV emission (with a negligible defect band) dominated by radiative recombination with a high optical efficiency (∼65%). As all NWs have similar structural and optical qualities irrespective of the substrate used, this strategy will open new horizons for developing III-nitride-based devices.

One-step synthesis of single-crystal Si3N4 nanowires-amorphous SiO2 beads nanochains by chemical vapor deposition

One-dimensional (1D) nanochain heterojunctions have attracted wide attention due to their unique optical and electrical properties, while efficient and controllable synthesis of nanochains is still difficult. Herein, Si3N4/SiO2 nanochain heterojunctions (Si3N4 single-crystal nanowires and amorphous SiO2 beads) were successfully synthesized by a one-step chemical vapor deposition (CVD) process without using any templates and catalysts. A two-stage growth mode was proposed to explain the formation mechanism of the special nanochain. We found that carbon vapor induced the formation of Si3N4 nanowires and suppressed the formation of Si2N2O. The Si3N4/SiO2 nanochains can only be synthesized within a suitable concentration range of carbon vapor. The as-synthesized fascinating nanochain exhibits an extra strong emission at about 3.02 eV, showing great promise for optoelectronic nanodevices.

Nanowire breakup via a morphological instability enhanced by surface electromigration

Using a recent continuum model of a single-crystal nanowire morphological evolution in the applied axial electric field, an axisymmetric evolution of a microscopically rough nanowire surface is computed. Morphological evolution results in a wire breakup into a cylindrical segments (particles). Breakup time and the number of particles are characterized for various levels of the radial and axial surface roughness. It is shown that electromigration and larger surface roughness lead to a shorter breakup time and the increased number of particles.

Enhancement of efficiency of solar water splitting based on highly dense hematite single-crystal nanowires

Enhancement of efficiency of solar water splitting based on highly dense hematite single-crystal nanowires

Defect formation mechanisms in metal nanowire under cyclic loading: a molecular dynamics study

A series of molecular dynamics simulations were conducted to reveal the fatigue mechanisms in metal nanowires. We applied axial cyclic loading deformation on a copper single-crystal nanowire model and observed the deformation process during cycle evolution. The detailed observation revealed that the deformation mechanisms in the nanowire is essentially different from the case of the macro- and micro-scaled materials because of the lack of dislocation sources. We also found that atomic vacancies were formed continually by dislocation motion even under a simple single-slip condition. The accumulation of vacancies is expected to be a probable mechanism of fatigue in nanomaterials.

Atomistic simulation for initiation of crystal slip deformation from surface of nanoscale copper single-crystal nanowires

In nano/micrometer-scale materials, dislocations accounting for crystal slip deformation are introduced from the surface because of lack of pre-existing dislocations inside the materials. The dislocation emission from the surface must play an important role in the formation of dislocation network structures in the nano/micrometer-scale materials under cyclic loading, which should differ from that in macroscale materials. The purpose of this study is to investigate the effect of the atomic-level structure of surfaces on the initiation of crystal slip deformation. We create copper single-crystal nanowire models with different surface structures by varying the cross-section shape and crystal orientation. To observe the initiation of the slip deformation from the surface, we conduct atomistic simulations of tensile deformation for these models. We also discuss the effect of the surface structure on the critical shear stress for slip occurrence from the surface. Our analyses reveal that the orientation of surface steps and the normal stress on the active slip plane are important factors affecting the slip occurrence. Thus, the slip occurs in a slip system so as to reduce the height of a surface step. The active slip system is affected by the cross-section shape and crystal orientation because the slip system is dependent on the relationship between slip planes and the orientation of the surface step. Our results also suggest that the cross-section shape and crystal orientation affect the critical shear stress because of the difference of the normal stress on the active slip plane according to these factors.

Mutation detection of urinary cell-free DNA via catch-and-release isolation on nanowires for liquid biopsy

Cell-free DNA (cfDNA) and extracellular vesicles (EVs) are molecular biomarkers in liquid biopsies that can be applied for cancer detection, which are known to carry information on the necessary conditions for oncogenesis and cancer cell-specific activities after oncogenesis, respectively. Analyses for both cfDNA and EVs from the same body fluid can provide insights into screening and identifying the molecular subtypes of cancer; however, a major bottleneck is the lack of efficient and standardized techniques for the isolation of cfDNA and EVs from clinical specimens. Here, we achieved catch-and-release isolation by hydrogen bond-mediated binding of cfDNA in urine to zinc oxide (ZnO) nanowires, which also capture EVs by surface charge, and subsequently we identified genetic mutations in urinary cfDNA. The binding strength of hydrogen bonds between single-crystal ZnO nanowires and DNA was found to be equal to or larger than that of conventional hydrophobic interactions, suggesting the possibility of isolating trace amounts of cfDNA. Our results demonstrated that nanowire-based cancer screening assay can screen cancer and can identify the molecular subtypes of cancer in urine from brain tumor patients through EV analysis and cfDNA mutation analysis. We anticipate our method to be a starting point for more sophisticated diagnostic models of cancer screening and identification.

Dialysate regeneration via urea photodecomposition with TiO2 nanowires at therapeutic rates.

The standard weekly treatment for end-stage renal disease patients is three 4-h-long hemodialysis sessions with each session c'onsuming over 120 L of clean dialysate, which prevents the development of portable or continuous ambulatory dialysis treatments. The regeneration of a small (~1L) amount of dialysate would enable treatments that give conditions close to continuous hemostasis and improve patient quality of life through mobility. Small-scale studies have shown that nanowires of TiO2 are highly efficient at photodecomposing urea into CO2 and N2 when using an applied bias and an air permeable cathode. To enable the demonstration of a dialysate regeneration system at therapeutically useful rates, a scalable microwave hydrothermal synthesis of single crystal TiO2 nanowires grown directly from conductive substrates was developed. These were incorporated into 1810 cm2 flow channel arrays. The regenerated dialysate samples were treated with activated carbon (2min at 0.2g/mL). The photodecomposition system achieved the therapeutic target of 14.2g urea removal in 24 h. TiO2 electrode had a high urea removal photocurrent efficiency of 91%, with less than 1% of the decomposed urea generating NH4 + (1.04 μg/h/cm2 ), 3% generating NO3 - and 0.5% generating chlorine species. Activated carbon treatment could reduce total chlorine concentration from 0.15 to <0.02 mg/L. The regenerated dialysate showed significant cytotoxicity which could be removed by treatment with activated carbon. Additionally, a forward osmosis membrane with sufficient urea flux can cut off the mass transfer of the by-products back into the dialysate. Urea could be removed from spent dialysate at a therapeutic rate using a TiO2 based photooxidation unit, which can enable portable dialysis systems.

Warping caused by surface elasticity in a nanowire under torsion

By combining molecular statics simulations and continuum mechanics-based modeling, we show in this paper that the torsion of a $\ensuremath{\langle}001\ensuremath{\rangle}$ single-crystal copper nanowire with a circular cross section gives rise to a warp, i.e., to a displacement field along the wire axis that renders the cross section nonplanar. This behavior, which is in apparent contradiction with what is predicted by continuum mechanics for an isotropic cylinder, can be well explained if we take into account the elastic response of the wire lateral surface. The latter is characterized by the anisotropy of the surface elastic constants and, more specifically in the case of torsion, by the surface shear constant ${C}_{44}^{S}$ whose strength as a function of the local orientation of the lateral surface is estimated independently from atomistic calculations on slabs presenting different vicinal surfaces. The solution of the torsion problem is then obtained by adopting a semi-inverse method in the framework of the finite strain theory in linear elasticity with Gurtin-Murdoch boundary conditions linking surface stress and bulk stress. It is shown that such an approach is well suited to explain quantitatively the warp obtained in our atomistic simulations and to prove the preponderant role played by the surface elastic constant ${C}_{44}^{S}$.

Atomically Sharp Internal Interface in a Chiral Weyl Semimetal Nanowire

Internal interfaces in Weyl semimetals (WSMs) are predicted to host distinct topological features that are different from the commonly studied external interfaces (crystal-to-vacuum boundaries). However, the lack of atomically sharp and crystallographically oriented internal interfaces in WSMs makes it difficult to experimentally investigate topological states buried inside the material. Here, we study a unique internal interface known as merohedral twin boundary in chemically synthesized single-crystal nanowires (NWs) of CoSi, a chiral WSM of space group P213 (No. 198). Scanning transmission electron microscopy reveals that this internal interface is a (001) twin plane which connects two enantiomeric counterparts at an atomically sharp interface with inversion twinning. Ab initio calculations show localized internal Fermi arcs at the (001) twin plane that can be clearly distinguished from both external Fermi arcs and bulk states. These merohedrally twinned CoSi NWs provide an ideal platform to explore topological properties associated with internal interfaces in WSMs.

Ultrafine Filteration of Metal Catalysts Enables Fabrication of Silicon Nanowires with Diameter Approaching a Unit Cell Size

Advanced functionalities of silicon nanowires are size-dependent and downscaling of the nanostructure often leads to higher device performances. Single-crystal silicon nanowires with diameters approaching a single unit cell are fabricated using membrane-filtrated catalyst assisted chemical etching. Atomically filtrated gold is used as uniform pattern to direct anisotropic etching of dense silicon nanowire arrays. The size of the nanowires can be controlled by engineering the molecular weight of Poly(methyl methacrylate) used to fabricate the polymer globule membranes. The smallest silicon nanowires with 0.9 nm diameters exhibit direct, and wide band gap of 3.55 eV and establishes a new record. The experimentally obtained silicon nanowires in this size fill the valuable gap below the few-nanometer regime where to date only theoretical predictions have been available. This fabrication approach could provide facile access to atomic-scale silicon, which can bring further advancement to next generation nanodevices.

Carbon quantum dots induced one-dimensional ordered growth of single crystal TiO2 nanowires while boosting photoelectrochemistry properties

TiO2 nanowire arrays (TNWAs) thin film of flexible substrates has great application potential in photoelectrochemical (PEC) hydrogen evolution. Moreover, the photoquantum efficiency of TNWAs can be effectively enhanced by reducing the nanoscale as much as possible or combining with other co-catalysts. This work presents a simple and effective strategy for the synthesis of ultrafine TiO2 nanowire arrays by a one-step hydrothermal method using CQDs as an inducer for single crystal growth. Meanwhile, CQDs recombine on the surface of TNWAs to form CQDs/TNWAs heterojunction during the synthesis process. The synthesized TNWAs have well ordered separability, whose average diameter is regulated at 11.21 nm with an average aspect ratio of 1097. PEC analysis showed that the performance of TNWAs was significantly improved compared with initial TiO2 with unregulated morphology. The charge separation efficiency increased by 120%, applied bias photon-to-current efficiency (ABPE) reached 0.37%(0.68 v vs RHE), and incident photon-to-current conversion efficiency (IPCE) reached 50.31% (380 nm). The influence of TiO2 nano-morphology optimization and formation of CQDs/TiO2 heterojunction on the PEC performance were discussed in detail.