In Situ Nanomechanics: Opportunities Based on Superplastic Nanomolding

In situ nanomechanics, referring to the real-time monitoring of nanomechanical deformation during quantitative mechanical testing, is a key technology for understanding the physical and mechanical properties of nanoscale materials. This perspective reviews the progress of in situ nanomechanics from the aspects of preparation and testing of nanosamples, with a major focus on one-dimensional (1D) nanostructures and discussions of their challenges. We highlight the opportunities provided by in situ nanomechanics combined with the superplastic nanomolding technique, especially in the aspects of regulating physical and chemical properties which are highly exploitable for mechanoelectronics, mechanoluminescence, piezoelectronics, piezomagnetism, piezothermography, and mechanochemistry.

Growth of 1D TiO2nanostructures on Ti substrates incorporated with residual stress through humid oxidation and their characterizations

Different Ti substrates, such as particles (as-received and ball milled), plate and TEM grid were oxidized for the growth of one dimensional (1D) TiO₂nanostructures. The Ti substrates were oxidized for 4 h at temperatures of 700 °C-750 °C in humid and dry Ar containing 5 ppm of O₂. The effects of residual stress on the growth of 1D TiO₂nanostructures were investigated. The residual stress inside the Ti particles was measured by XRD-sin²ψtechnique. The oxidized Ti substrates were characterized using field emission scanning electron microscope equipped with energy dispersive x-ray spectroscope, transmission electron microscope, x-ray diffractometer and x-ray photoelectron spectroscope. Results revealed that humid environment enhances the growth of 1D TiO₂nanostructures. Four different types of 1D morphologies obtained during humid oxidation, e.g. stacked, ribbon, plateau and lamp-post shaped nanostructures. The presence of residual stress significantly enhances the density and coverage of 1D nanostructures. The as-grown TiO₂nanostructures possess tetragonal rutile structure having length up to 10μm along the 〈1 0 1〉 directions. During initial stage of oxidation, a TiO₂layer is formed on Ti substrate. Lower valence oxides (Ti₃O₅, Ti₂O₃and TiO) then form underneath the TiO₂layer and induce stress at the interface of oxide layers. The induced stress plays significant role on the growth of 1D TiO₂nanostructures. The induced stress is relaxed by creating new surfaces in the form of 1D TiO₂nanostructures. A diffusion based model is proposed to explain the mechanism of 1D TiO₂growth during humid oxidation of Ti. The 1D TiO₂nanostructures and TiO₂layer is formed by the interstitial diffusion of Ti⁴⁺ions to the surface and reacts with the surface adsorbed hydroxide ions (OH^-). Lower valence oxides are formed at the metal-oxide interface by the reaction between diffused oxygen ions and Ti ions.

Synthesis of Co-Fe 1D Nanocone Array Electrodes Using Aluminum Oxide Template

Porous anodic alumina oxide (AAO) obtained via two-step anodization is a material commonly used as a template for fabricating 1D nanostructures. In this work, copper and cobalt-iron 1D nanocones were obtained by an electrodeposition method using AAO templates. The templates were produced using two-step anodization in H₂C₂O₄. The Co–Fe nanostructures are characterized by homogeneous pore distribution. The electrocatalytic activity of the produced nanomaterials was determined in 1 M NaOH using the linear sweep voltammetry (LSV) and chronopotentiometry (CP) methods. These materials can be used as catalysts in the water-splitting reaction. The sample’s active surface area was calculated and compared with bulk materials.

Mechanical design of brush coating technology for the alignment of one-dimension nanomaterials

Widespread approaches to fabricate surfaces with aligned nanostructured topographies have been stimulated by opportunities to enhance interface performance by combing physical and chemical effects, in which brush-coating technology (BCT) is a cost-effective and feasible method for aligned film and large-scale production. Here, we reported a BCT process to realize the alignment of various 1D nanostructures through mechanical design that provides a more precise and higher shear force. By regulating the viscosity of dispersion, shear force is proved to be 24 and 20.3 times larger (when the volume ratio of water and glycerol is 1:3) according to the theoretical calculation and ANSYS simulating calculation results respectively, which plays a vital role in brush coating process. The universality was demonstrated by the alignment of one-dimension nanomaterials with different diameters, including silver nanowires (~80 nm), molybdenum trioxide nanobelts (~150 nm), vanadium pentoxide nanobelts (~150 nm) and bismuth sulfide nanobelts (~200 nm), et al., which in consequence have different alignment ratios. Meanwhile, anisotropic and flexible electrical conductors (the resistance anisotropic ratio was 2) and thermoelectric films (Seebeck coefficient was calculated to be 56.7 µV/K) were demonstrated.

One-Dimensional Nanostructures of Polypyrrole for Shielding of Electromagnetic Interference in the Microwave Region

Polypyrrole one-dimensional nanostructures (nanotubes, nanobelts and nanofibers) were prepared using three various dyes (Methyl Orange, Methylene Blue and Eriochrome Black T). Their high electrical conductivity (from 17.1 to 60.9 S cm-1), good thermal stability (in the range from 25 to 150 °C) and resistivity against ageing (half-time of electrical conductivity around 80 days and better) were used in preparation of lightweight and flexible composites with silicone for electromagnetic interference shielding in the C-band region (5.85-8.2 GHz). The nanostructures' morphology and chemical structure were characterized by scanning electron microscopy, Brunauer-Emmett-Teller specific surface measurement and attenuated total reflection Fourier-transform infrared spectroscopy. DC electrical conductivity was measured using the Van der Pauw method. Complex permittivity and AC electrical conductivity of respective silicone composites were calculated from the measured scattering parameters. The relationships between structure, electrical properties and shielding efficiency were studied. It was found that 2 mm-thick silicone composites of polypyrrole nanotubes and nanobelts shield almost 80% of incident radiation in the C-band at very low loading of conductive filler in the silicone (5% w/w). Resulting lightweight and flexible polypyrrole composites exhibit promising properties for shielding of electromagnetic interference in sensitive biological and electronic systems.

One Dimensional ZnO Nanostructures: Growth and Chemical Sensing Performances

Recently, one-dimensional (1D) nanostructures have attracted the scientific community attention as sensitive materials for conductometric chemical sensors. However, finding facile and low-cost techniques for their production, controlling the morphology and the aspect ratio of these nanostructures is still challenging. In this study, we report the vapor-liquid-solid (VLS) synthesis of one dimensional (1D) zinc oxide (ZnO) nanorods (NRs) and nanowires (NWs) by using different metal catalysts and their impact on the performances of conductometric chemical sensors. In VLS mechanism, catalysts are of great interest due to their role in the nucleation and the crystallization of 1D nanostructures. Here, Au, Pt, Ag and Cu nanoparticles (NPs) were used to grow 1D ZnO. Depending on catalyst nature, different morphology, geometry, size and nanowires/nanorods abundance were established. The mechanism leading to the VLS growth of 1D ZnO nanostructures and the transition from nanorods to nanowires have been interpreted. The formation of ZnO crystals exhibiting a hexagonal crystal structure was confirmed by X-ray diffraction (XRD) and ZnO composition was identified using transmission electron microscopy (TEM) mapping. The chemical sensing characteristics showed that 1D ZnO has good and fast response, good stability and selectivity. ZnO (Au) showed the best performances towards hydrogen (H2). At the optimal working temperature of 350 °C, the measured response towards 500 ppm of H2 was 300 for ZnO NWs and 50 for ZnO NRs. Moreover, a good selectivity to hydrogen was demonstrated over CO, acetone and ethanol.

Aerobic Oil-Phase Cyclic Magnetic Adsorption to Synthesize 1D Fe2O3@TiO2 Nanotube Composites for Enhanced Visible-Light Photocatalytic Degradation

In this work, Fe₂O₃@TiO₂ nanostructures with staggered band alignment were newly designed by an aerobic oil-phase cyclic magnetic adsorption method. XRD and TEM analyses were performed to verify the uniform deposition of Fe₂O₃ nanoparticles on the nanotube inner walls of TiO₂. The steady-state degradation experiments exhibited that 1FeTi possessed the most superior performance, which might be ascribable to the satisfying dark adsorption capacity, efficient photocatalytic activity, ease of magnetic separation, and economic efficiency. These results indicated that the deposition of Fe₂O₃ into TiO₂ nanotubes significantly enhanced the activity of Fe₂O₃, which was mainly ascribed to the Fe₂O₃-induced formation of staggered iron oxides@TiO₂ band alignment and thus efficient separation of h⁺ and e^-. Furthermore, the PL intensity and lifetime of the decay curve were considered as key criterions for the activity's evaluation. Finally, the leaching tests and regeneration experiments were also performed, which illustrated the inhibited photodissolution compared with TiO₂/Fe₃O₄ and stable cycling ability, enabling 1FeTi to be a promising magnetic material for photocatalytic water remediation.

High-Efficiency Perovskite Solar Cells Enabled by Anatase TiO2 Nanopyramid Arrays with an Oriented Electric Field

One-dimensional (1D) nanostructured oxides are proposed as excellent electron transport materials (ETMs) for perovskite solar cells (PSCs); however, experimental evidence is lacking. A facile hydrothermal approach was employed to grow highly oriented anatase TiO₂ nanopyramid arrays and demonstrate their application in PSCs. The oriented TiO₂ nanopyramid arrays afford sufficient contact area for electron extraction and increase light transmission. Moreover, the nanopyramid array/perovskite system exhibits an oriented electric field that can increase charge separation and accelerate charge transport, thereby suppressing charge recombination. The anatase TiO₂ nanopyramid array-based PSCs deliver a champion power conversion efficiency of approximately 22.5 %, which is the highest power conversion efficiency reported to date for PSCs consisting of 1D ETMs. This work demonstrates that the rational design of 1D ETMs can achieve PSCs that perform as well as typical mesoscopic and planar PSCs.

1D Carbon-Based Nanocomposites for Electrochemical Energy Storage

Electrochemical energy storage (EES) devices have attracted immense research interests as an effective technology for utilizing renewable energy. 1D carbon-based nanostructures are recognized as highly promising materials for EES application, combining the advantages of functional 1D nanostructures and carbon nanomaterials. Here, the recent advances of 1D carbon-based nanomaterials for electrochemical storage devices are considered. First, the different categories of 1D carbon-based nanocomposites, namely, 1D carbon-embedded, carbon-coated, carbon-encapsulated, and carbon-supported nanostructures, and the different synthesis methods are described. Next, the practical applications and optimization effects in electrochemical energy storage devices including Li-ion batteries, Na-ion batteries, Li-S batteries, and supercapacitors are presented. After that, the advanced in situ detection techniques that can be used to investigate the fundamental mechanisms and predict optimization of 1D carbon-based nanocomposites are discussed. Finally, an outlook for the development trend of 1D carbon-based nanocomposites for EES is provided.

Stress-Tolerant Nanoporous Germanium Nanofibers for Long Cycle Life Lithium Storage with High Structural Stability

Nanowires (NWs) synthesized via chemical vapor deposition (CVD) have demonstrated significant improvement in lithium storage performance along with their outstanding accommodation of large volume changes during the charge/discharge process. Nevertheless, NW electrodes have been confined to the research level due to the lack of scalability and severe side reactions by their high surface area. Here, we present nanoporous Ge nanofibers (NPGeNFs) having moderate nanoporosity via a combination of simple electrospinning and a low-energetic zincothermic reduction reaction. In contrast with the CVD-assisted NW growth, our method provides high tunability of macro/microscopic morphologies such as a porosity, length, and diameter of the nanoscale 1D structures. Significantly, the customized NPGeNFs showed a highly suppressed volume expansion of less than 15% (for electrodes) after full lithation and excellent durability with high lithium storage performance over 500 cycles. Our approach offers effective 1D nanostructuring with highly customized geometries and can be extended to other applications including optoelectronics, catalysis, and energy conversion.

Electrospun one-dimensional nanostructures: a new horizon for gas sensing materials

Electrospun one-dimensional (1D) nanostructures are rapidly emerging as key enabling components in gas sensing due to their unique electrical, optical, magnetic, thermal, mechanical and chemical properties. 1D nanostructures have found applications in numerous areas, including healthcare, energy storage, biotechnology, environmental monitoring, and defence/security. Their enhanced specific surface area, superior mechanical properties, nanoporosity and improved surface characteristics (in particular, uniformity and stability) have made them important active materials for gas sensing applications. Such highly sensitive and selective elements can be embedded in sensor nodes for internet-of-things applications or in mobile systems for continuous monitoring of air pollutants and greenhouse gases as well as for monitoring the well-being and health in everyday life. Herein, we review recent developments of gas sensors based on electrospun 1D nanostructures in different sensing platforms, including optical, conductometric and acoustic resonators. After explaining the principle of electrospinning, we classify sensors based on the type of materials used as an active sensing layer, including polymers, metal oxide semiconductors, graphene, and their composites or their functionalized forms. The material properties of these electrospun fibers and their sensing performance toward different analytes are explained in detail and correlated to the benefits and limitations for every approach.

Unprecedented Centimeter-Long Carbon Nitride Needles: Synthesis, Characterization and Applications

Free standing centimeter-long 1D nanostructures are highly attractive for electronic and optoelectronic devices due to their unique photophysical and electrical properties. Here a simple, large-scale synthesis of centimeter-long 1D carbon nitride (CN) needles with tunable photophysical, electric, and catalytic properties is reported. Successful growth of ultralong needles is acquired by the utilization of 1D organic crystal precursors comprised of CN monomers as reactants. Upon calcination at high temperatures, the shape of the starting crystal is fully preserved while the CN composition and porosity, and optical and electrical properties can be easily tuned by tailoring the starting elements ratio and final calcination temperature. The facile manipulation and visualization of the CN needles endow their direct electrical measurements by placing them between two conductive probes. Moreover, the CN needles exhibit good photocatalytic activity for hydrogen production owing to their improved light harvesting properties, high surface area, and advantageous energy bands position. The new growth strategy developed here may open opportunities for a rational design of CN and other metal-free materials with controllable directionality and tunable photophysical and electronic properties, toward their utilization in (photo)electronic devices.

SnO2 Nanostructures: Effect of Processing Parameters on Their Structural and Functional Properties

Zero- and 1D (one-dimensional) tin (IV) oxide nanostructures have been synthesized by thermal evaporation method, and a comparison of their morphology, crystal structure, sorption properties, specific surface area, as well as electrical characteristics has been performed. Synthesized SnO₂ nanomaterials were studied by X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM), N₂ sorption/desorption technique, IR spectroscopy and, in addition, their current-voltage characteristics have also been measured. The single crystalline structures were obtained both in case of 0D (zero-dimensional) SnO₂ powders and in case of 0D nanofibers, as confirmed by electron diffraction of TEM. It was found that SnO₂ synthesis parameters significantly affect materials' properties by contributing to the difference in morphology, texture formation, changes in IR spectra of 1D structure as compared to 0D powders, increases in the specific surface area of nanofibers, and the alteration of current-voltage characteristics 0D and 1D SnO₂ nanostructures. It was established that gas sensors utilizing of 1D nanofibers significantly outperform those based on 0D powders by providing higher specific surface area and ohmic I-V characteristics.

Metallic Nanobrushes Made using Ambient Droplet Sprays

An ambient solution-state method for making uniform nanobrushes composed of oriented 1D silver nanowires (NWs) with aspect ratios of 10(2) -10(4) is reported. These structures are grown over cm(2) areas on conducting surfaces. Assemblies of NWs form uniform nanobrush structures, which can capture micrometer-sized objects, such as bacteria and particulate matter. Variation in composition produces unique structures with catalytic properties.

Quantitative 3D electromagnetic field determination of 1D nanostructures from single projection

One-dimensional (1D) nanostructures have been regarded as the most promising building blocks for nanoelectronics and nanocomposite material systems as well as for alternative energy applications. Although they result in confinement of a material, their properties and interactions with other nanostructures are still very much three-dimensional (3D) in nature. In this work, we present a novel method for quantitative determination of the 3D electromagnetic fields in and around 1D nanostructures using a single electron wave phase image, thereby eliminating the cumbersome acquisition of tomographic data. Using symmetry arguments, we have reconstructed the 3D magnetic field of a nickel nanowire as well as the 3D electric field around a carbon nanotube field emitter, from one single projection. The accuracy of quantitative values determined here is shown to be a better fit to the physics at play than the value obtained by conventional analysis. Moreover the 3D reconstructions can then directly be visualized and used in the design of functional 3D architectures built using 1D nanostructures.

Hyperbranched quasi-1D TiO2 nanostructure for hybrid organic-inorganic solar cells

The performance of hybrid solar cells is strongly affected by the device morphology. In this work, we demonstrate a poly(3-hexylthiophene-2,5-diyl)/TiO2 hybrid solar cell where the TiO2 photoanode comprises an array of tree-like hyperbranched quasi-1D nanostructures self-assembled from the gas phase. This advanced architecture enables us to increase the power conversion efficiency to over 1%, doubling the efficiency with respect to state of the art devices employing standard mesoporous titania photoanodes. This improvement is attributed to several peculiar features of this array of nanostructures: high interfacial area; increased optical density thanks to the enhanced light scattering; and enhanced crystallization of poly(3-hexylthiophene-2,5-diyl) inside the quasi-1D nanostructure.

3D and 2D structural characterization of 1D Al/Al2 O3 biphasic nanostructures

1D Al/Al2 O3 nanostructures have been synthesized by chemical vapour deposition (CVD) of the molecular precursor [(t) BuOAlH2 ]2 . The deposited nanostructures grow chaotically on the substrate forming a layer with a high porosity (80%). Depending on the deposition time, diverse nanostructured surfaces with different distribution densities were achieved. A three-dimensional (3D) reconstruction has been evaluated for every nanostructure density using the Focus Ion Beam (FIB) tomography technique and reconstruction software tools. Several structural parameters such as porosity, Euler number, geometrical tortuosity and aspect ratio have been quantified through the analysis with specified software of the reconstructions. Additionally roughness of the prepared surfaces has been characterized at micro- and nanoscale using profilometry and AFM techniques, respectively. While high aspects ratio around 20-30 indicates a strong anisotropy in the structure, high porosity values (around 80%) is observed as a consequence of highly tangled geometry of such 1D nanostructures.

Controlling DNA Bundle Size and Spatial Arrangement in Self-assembled Arrays on Superhydrophobic Surface

The use of superhydrophobic surfaces (SHSs) is now emerging as an attractive platform for the realization of one-dimensional (1D) nanostructures with potential applications in many nanotechnological and biotechnological fields. To this purpose, a strict control of the nanostructures size and their spatial arrangement is highly required. However, these parameters may be strongly dependent on the complex evaporation dynamics of the sessile droplet on the SHS. In this work, we investigated the effect of the evaporation dynamics on the size and the spatial arrangement of self-assembled 1D DNA bundles. Our results reveal that different arrangements and bundle size distributions may occur depending on droplet evaporation stage. These results contribute to elucidate the formation mechanism of 1D nanostructures on SHSs.

Self-assembled hierarchical nanostructures for high-efficiency porous photonic crystals

The nanoscale modulation of material properties such as porosity and morphology is used in the natural world to mold the flow of light and to obtain structural colors. The ability to mimic these strategies while adding technological functionality has the potential to open up a broad array of applications. Porous photonic crystals are one such technological candidate, but have typically underachieved in terms of available materials, structural and optical quality, compatibility with different substrates (e.g., silicon, flexible organics), and scalability. We report here an alternative fabrication method based on the bottom-up self-assembly of elementary building blocks from the gas phase into high surface area photonic hierarchical nanostructures at room temperature. Periodic refractive index modulation is achieved by stacking layers with different nanoarchitectures. High-efficiency porous Bragg reflectors are successfully fabricated with sub-micrometer thick films on glass, silicon, and flexible substrates. High diffraction efficiency broadband mirrors (R≈1), opto-fluidic switches, and arrays of photonic crystal pixels with size<10 μm are demonstrated. Possible applications in filtering, sensing, electro-optical modulation, solar cells, and photocatalysis are envisioned.

Self-integration of nanowires into circuits via guided growth

The ability to assemble discrete nanowires (NWs) with nanoscale precision on a substrate is the key to their integration into circuits and other functional systems. We demonstrate a bottom-up approach for massively parallel deterministic assembly of discrete NWs based on surface-guided horizontal growth from nanopatterned catalyst. The guided growth and the catalyst nanopattern define the direction and length, and the position of each NW, respectively, both with unprecedented precision and yield, without the need for postgrowth assembly. We used these highly ordered NW arrays for the parallel production of hundreds of independently addressable single-NW field-effect transistors, showing up to 85% yield of working devices. Furthermore, we applied this approach for the integration of 14 discrete NWs into an electronic circuit operating as a three-bit address decoder. These results demonstrate the feasibility of massively parallel "self-integration" of NWs into electronic circuits and functional systems based on guided growth.