Aligned Nanowires Research Articles

Dispersion and Tunable Alignment of Colloidal Silver Nanowires in a Nematic Liquid Crystal for Applications in Electric–Optic Devices

The dispersion and tunable alignment of colloidal nanomaterials is desirable for practical applications in electric-optic (E-O) devices; however, it remains challenging for large one-dimensional nanomaterials with a large aspect ratio. Here, we demonstrate a large-scale, simple, multi-microdomain, and noncontact photoalignment technology to align colloidal silver nanowires (AgNWs, length ∼4.5 μm, diameter ∼70.6 nm) in a liquid crystal (LC) with a high two-dimensional order parameter (about 0.9). The AgNWs are precisely self-assembled via photomasks with twisted nematic and planar alignment models in microdomain regions. The AgNW orientation is tuned with an electric field, through the rotation of an LC director n, which allows three-dimensional (3D) tunable orientation combined with photoalignment. The colloidal dispersions of AgNWs in the LC cell influenced the ion transfer, elastic constant, dielectric anisotropy, and near LC alignment, changing the E-O properties of the LC devices. The 3D tunable orientation of an AgNW by photoalignment and an electric field could provide a new way to assemble large colloidal nanomaterials and fabricate functional E-O devices.

Surface steered aligned gradient inorganic lead halide perovskite CsPbBrxI3−x nanowires for use in photodetectors

Explicit control over the orientation and yield of aligned planar nanowires (NWs) offers a unique blueprint for the facile integration of sophisticated functional devices with ultra-high compactness. In this report, vapor-phase synthesis produces a massive parallel assembly of nanowires owing to the graphoepitaxial effect exhibited by the annealed M-plane sapphire surface. The nanochannels on the substrate surface serve as natural templates and define the position, growth direction, and length of NWs. Microstructure characterizations and strong band-edge photoluminescence (PL) reveal the superior quality crystal structure of CsPbBrxI3-x NWs. The NW's axially asynchronous bromide-iodide composition provides a graded bandgap with two advantages: broad gamut absorption and a dominant unidirectional built-in electric field. The electric field promotes charge carrier transportation via an energy-funneling process, as demonstrated by the outstanding performance of the CsPbBrxI3-x NW photodetector, which has a responsivity of 370 A/W and a brisk photoresponse time of about 4 ms. Bright PL observation even after 100 days of storage at room temperature (RT) and 25 % relative humidity (RH) conditions indicates a boost in monoclinic phase stability and improved environmental degradation resistance. This work presents the feasibility of massive-scale synthesis of parallel NWs and their direct integration into circuits and functional systems for smart and efficient spectrometers, image sensors, optical communication, chemical or biological detection, and other optoelectronic applications.

Horizontally‐Oriented Growth of Organic Crystalline Nanowires on Polymer Films for In‐Situ Flexible Photodetectors with Vis‐NIR Response and High Bending Stability

AbstractVarious epitaxial mechanisms have been proposed to control the growth orientation of vapor‐deposited nanowires, yet the required lattice matching between target nanowires and supporting substrates limits their applicability. In this work, a versatile hot stamping protocol for fabricating parallel hydrophobic nanogrooves on flexible polymer films (e.g., polyimide (PI), polyethylene naphthalate (PEN), polydimethylsiloxane (PDMS)) is proposed. More interestingly, various organic small molecules, including several metal phthalocyanines (MPc, M = Cu, Zn, Fe, Ni, Co), 9,10‐bis(phenylethynyl)anthracene (BPEA), 9,10‐diphenylanthracene (DPA), and tris‐(8‐hydroxyquinoline)aluminium (Alq3), are directly assembled into horizontally‐oriented nanowires along the hot‐stamped nanogrooves on a flexible PI film, thereby breaking the lattice‐matching limitation for oriented nanowire growth. These submillimeter‐long horizontally oriented nanowires can be integrated into flexible photodetectors directly on their growth film, eliminating the need for laborious post‐growth transfer and alignment steps and the associated structural damage and contamination. Consequently, the in situ integrated flexible photodetector made of aligned CuPc nanowires maintains a stable and fast photoresponse to a spectrum in the region of 405‐980 nm even when the detector is bent to a radius of curvature of 2.5 mm and 1000 times. This work will open new opportunities to develop in situ integrated flexible devices based on organic crystalline nanowires for practical applications.

Structural investigation of titanium oxide nanowires with unconventional optoelectronic behaviour.

Grazing incidence wide angle X-ray scattering measurements on aligned titanium oxide nanowires displaying anisotropic optical-electronic properties are carried out. Elemental and thermal analyses provide a chemical composition corresponding to H2Ti3O7·nH2O with n ≈ 1 while the crystallographic data indicate a monoclinic cell with a lamellar substructure. Cell parameters are close to those of H2Ti3O7 notwithstanding a doubling of the lattice in the layer plane. A comparison of the band gap energy values and the electronic transition modes between the two polymorphs displays differences that could be ascribed to the structural variation.

Engineering 2D Aligned Nanowires Assembled Porous Hetero-Membrane for Smart Ion Transport.

Engineering 2D nanosheets with well-defined porous structures and their assembled heterostructure membrane is a promising method to improve osmotic energy conversion. However, it is still a great challenge to directly fabricate 2D nanosheets with regular parallel nanochannels in aqueous media. Here, the desired functional nanosheets and heterostructure membrane device are successfully prepared through a simple interfacial assembly strategy. In this method, monolayer cylindrical monomicelles closely arrange and assemble on the surfaces of graphene oxide, and the resulting nanosheets with monolayered aligned nanowire polymer arrays parallel to the substrate surfaces are then obtained. Subsequently, a heterostructured membrane is constructed by assembling these 2D nanosheets on macroporous alumina. The nanofluidic membrane device with asymmetric geometry and charge polarity exhibits smart ion transport properties, and the output osmotic power density is ≈1.22 and 1.63 times over the reported pure 2D graphene oxide and biomass-derived membranes, respectively. In addition, theoretical calculations are carried out to reveal the mechanisms for ion selectivity and salinity gradient energy conversion. This monolayered interfacial assembly approach can open up new avenues for the synthesis of functional porous low-dimensional nanomaterials and membrane devices, and expand the palette of materials selection for many applications.

Line‐Shaped Laser Lithography for Efficiently Fabricating Flexible Transparent Electrodes with Hierarchical Metal Grids across λ/10 to Microscale

AbstractHierarchical metal grids are proven to be a promising solution for achieving high‐performance flexible transparent electrodes (FTEs) due to their significantly enhanced conductivity without noticeably sacrificing transparency. This work develops a one‐step mask‐free line‐shaped laser lithography by separated pulse laser ablation to efficiently fabricate large‐area FTEs composed of hierarchical metal grids, namely microscale grids interconnected with aligned nanowire arrays. The linewidth of aligned wires is highly controllable from the nanometer scale far beyond the diffraction limit (<λ/10) to the micrometer scale. This work experimentally studies the overall performance of FTEs to provide a guideline for selecting the layout and feature sizes of hierarchical metal grids. Hierarchical metal grids with 50 nm nanowires aligned inside the microscale grids show outstanding optoelectronic properties and mechanical stability, with sheet resistance of 4.6 Ω sq−1, transmittance of 82.9%, and a tiny increase in sheet resistance less than 4% after 1000 cycles of bending tests. These results prove that the line‐shaped laser lithography technique is a facile, low‐cost, and high‐throughput fabrication method for high‐performance FTEs.

Langmuir‐Blodgett Assembly to order Nanoparticles and Colloidal Objects

Bottom-up assembly of nanoparticles and colloidal objects pose a formidable challenge when processing devices. Speed, compatibility with various materials, defect tolerance and cost effectiveness are among the desired properties of a suitable nanoscale assembly process. In this regard, the Langmuir‐Blodgett (LB) technique is a highly sought-after candidate which aids in arranging a large number of nanostructures on solid surfaces. This mini-review aims to provide a concise account on the LB technique and four distinct ways of how it allows to assemble systems made of nanoparticles and colloidal objects: namely, close-packed nanoparticle superlattices by compression, micrometer scale nanoparticle fingering patterns by dip coating, single nanoparticle lines by stick-slip deposition and one-step patterning of aligned nanowire arrays.

Controlled Alignment of Nanowires for Transparent Conductive Films: Methods and Applications

Background: Nanowires (NWs) have received extensive attention as the candidate materials for transparent conductive films (TCFs) in recent years. To date, the aligned nanowire (NW)- based TCFs with the same arrangement direction have shown superior characteristics to their random counterparts in applications. Objective: To fully develop the potential of NW TCFs in devices and provide inspiration for the development of subsequent NW alignment processes, this review summarizes state-of-the-art alignment techniques and emphasizes their mechanisms in detail from multiple perspectives. Methods: According to the mechanism of NW alignment, this review divides these techniques into seven categories, i.e., the assisted assembly of fluid flow, meniscus, pressure, template, electromagnetic field, contact and strain, and analyzes the characteristics of these techniques. Moreover, by briefly enumerating the applications of aligned NW films in solar cells, organic light-emitting diodes, and touch screens, the superiority of aligned NW films over random NW films is also addressed. Results: Contact-assisted assembly exhibits the best arrangement effect, reaching a 98.6% alignment degree within ±1°. Under the same conditions, shorter NWs show better alignment in several cases. The combination of various assembly techniques is also an effective means to improve the alignment effect. Conclusion: There is still room for improvement in the precise control of NW position, density, and orientation in a simple, efficient and compatible process. Therefore, follow-up research work is needed to conquer these problems. Moreover, a process that can realize NWs’ alignment and film patterning simultaneously is also a desirable scheme for fabricating personalized devices.

Texturing of nanocoatings for surface acoustic wave-based sensors for volatile organic compounds

Abstract An approach for texturing of gas-sensitive nanocoatings by using surface acoustic waves (SAW) is presented in this article. The objective of the work is to enhance the performance of precise SAW-based gas sensors due to the increased specific area of the sensitive nanocoating, induced during its growth and to replace the expensive lithographic techniques for nanopatterning, typically used for this purpose. The technique can be used for tuneable alignment of nanoparticles or nanowires and it is scale-independent. To control the texture of the sensitive nanocoating, a specific electrode topology was used to generate waves with a specific space distribution, which in turn caused assembling of the nanoparticles increasing the adsorption capacity. In this way, a broader dynamic range of 7,000 ppm was achieved (three times extended as compared to the non-textured sensing film), measurement error of 0.6% against 4% for the non-patterned, faster response time in the sub-seconds range (970 ms vs 1.1 s), negligible hysteresis of 10 mV (against >100 mV), and very good sensitivity of 5 µV per ppm, which are in line with the current standards for ethanol sensors. The enhanced sensor parameters were achieved by implementation of conventional patterning technologies without the need for nanolithographic techniques for the texturing the nanocoating. The method is low-cost, and applicable in a variety of sensing structures despite the sensing coating (optical, biological, etc.).

Direct Ink Writing for High-Efficiency Microwave Attenuation with Nanofibers Alignment.

One-dimensional (1D) fibers have been widely used in composites reinforcement for microwave attenuation due to their outstanding mechanical and electromagnetic properties, especially in the axial direction. However, the precise control of fiber alignment in a polymer matrix remains a challenge. In this work, we successfully demonstrated the well-controlled alignment of silicon carbide nanowires (SiCNW) in a silicone matrix by using direct ink writing (DIW)-based 3D printing. It is proven that the printed multilayer material with fiber alignment could show a dramatic improvement in both reflection loss (RL) and effective attenuation bandwidth (EAB, RL < -10 dB). In particular, a uniaxial in-plane orientation is found to be the optimal alignment among other planar and also out-of-plane orientations. Benefiting from the optimized alignment, the 3D-printed SiC composite could show an EAB (∼6.4 GHz)1.6 times broader than that of the randomly mixed composite at the same thickness without alignment, associated with a minimum RL of -48 dB at 14.3 GHz. In addition, it is demonstrated that DIW could print different materials, such as SiCNW and multiwall carbon nanotube (MWCNT), in alternating layers for multiple-frequency-band attenuation benefiting from the distinct property of each material. Considering the one-step control of fiber alignment and material selectivity, DIW could play an important role in materials design for high-efficiency microwave attenuation.

Alignment of Metal Halide Perovskite Nanowires and Their Application in Photodetectors

Alignment of Metal Halide Perovskite Nanowires and Their Application in Photodetectors

Light Trapping of Inclined Si Nanowires for Efficient Inorganic/Organic Hybrid Solar Cells.

Light/matter interaction of low-dimensional silicon (Si) strongly correlated with its geometrical features, which resulted in being highly critical for the practical development of Si-based photovoltaic applications. Yet, orientation modulation together with apt control over the size and spacing of aligned Si nanowire (SiNW) arrays remained rather challenging. Here, we demonstrated that the transition of formed SiNWs with controlled diameters and spacing from the crystallographically preferred <100> to <110> orientation was realized through the facile adjustment of etchant compositions. The underlying mechanism was found to correlate with the competing reactions between the formation and removal of oxide at Ag/Si interfaces that could be readily tailored through the concentration ratio of HF to H2O2. By employing inclined SiNWs for the construction of hybrid solar cells, the improved cell performances compared with conventional vertical-SiNW-based hybrid cells were demonstrated, showing the conversion efficiency of 12.23%, approximately 1.12 times higher than that of vertical-SiNW-based hybrid solar cells. These were numerically and experimentally interpreted by the involvement of excellent light-trapping effects covering the wide-angle light illuminations of inclined SiNWs, which paved the potential design for next-generation optoelectronic devices.

Tailored Uniaxial Alignment of Nanowires Based on Off-Center Spin-Coating for Flexible and Transparent Field-Effect Transistors.

The alignment of nanowires (NWs) has been actively pursued for the production of electrical devices with high-operating performances. Among the generally available alignment processes, spin-coating is the simplest and fastest method for uniformly patterning the NWs. During spinning, the morphology of the aligned NWs is sensitively influenced by the resultant external drag and inertial forces. Herein, the assembly of highly and uniaxially aligned silicon nanowires (Si NWs) is achieved by introducing an off-center spin-coating method in which the applied external forces are modulated by positioning the target substrate away from the center of rotation. In addition, various influencing factors, such as the type of solvent, the spin acceleration time, the distance between the substrate and the center of rotation, and the surface energy of the substrate, are adjusted in order to optimize the alignment of the NWs. Next, a field-effect transistor (FET) incorporating the highly aligned Si NWs exhibits a high effective mobility of up to 85.7 cm2 V−1 s−1, and an on-current of 0.58 µA. Finally, the single device is enlarged and developed in order to obtain an ultrathin and flexible Si NW FET array. The resulting device has the potential to be widely expanded into applications such as wearable electronics and robotic systems.

Soft-Shear-Aligned Vertically Oriented Lamellar Block Copolymers for Template-Free Sub-10 nm Patterning and Hybrid Nanostructures.

The template-free unidirectional alignment of lamellar block copolymers (l-BCPs) for sub-10 nm high-resolution patterning and hybrid multicomponent nanostructures is important for technological applications. We demonstrate a modified soft-shear-directed self-assembly (SDSA) approach for aligning pristine l-BCPs and l-BCPs with incorporated polymer-grafted nanoparticles (PGNPs), as well as the l-BCP conversion to aligned gold nanowires, and hybrid of metallic gold nanowire and dielectric silica nanoparticle in the form of line-dot nanostructures. The smallest patterns have a half-pitch as small as 9.8 nm. In all cases, soft-shear is achieved using a high-molecular-mass polymer topcoat layer, with support on a neutral bottom layer. We also show that the hybrid line-dot nanostructures have a red-shifted plasmonic response in comparison to neat gold nanowires. These template-free aligned BCPs and nanowires have potential use in nanopatterning applications, and the line-dot nanostructures should be useful in the sensing of biomolecules and other molecular species based on the plasmonic response of the nanowires.

Directed high‐χ block copolymer self‐assembly by laser writing on silicon substrate

AbstractPolymer self‐assembly commonly suffers from retarded equilibrium structure formation caused by the large diffusion barrier of long‐chain molecules. In particular, the defect annihilation kinetics of high‐χ block copolymers (BCPs) are generally sluggish because of a slow inter‐block chain diffusion process from strong inter‐block segregation. Therefore, long‐range order of high‐χ BCPs still hard to be obtained by conventional approaches. Here, we introduce near‐infrared laser photothermal treatment to effectively promote high‐χ BCP self‐assembly and demonstrating highly aligned nanoscale patterned structures on silicon substrates. Adequate molecular weight selection of high‐χ PS‐b‐P2VP system enables one‐time laser hot‐zone annealing, resulting in highly ordered nanodomains along laser writing direction. Facile sub‐sequential metal ion loading to P2VP cylinders enables the formation of highly aligned metal nanowires. Moreover, a commonly used silicon substrate without a photoabsorbing layer is employed as a photo‐thermal substrate, demonstrating that the laser writing process is compatible with conventional semiconductor processes.

Semiconducting Polymer Nanowires with Highly Aligned Molecules for Polymer Field Effect Transistors

Conjugated polymers have emerged as promising materials for next-generation electronics. However, in spite of having several advantages, such as a low cost, large area processability and flexibility, polymer-based electronics have their own limitations concerning low electrical performance. To achieve high-performance polymer electronic devices, various strategies have been suggested, including aligning polymer backbones in the desired orientation. In the present paper, we report a simple patterning technique using a polydimethylsiloxane (PDMS) mold that can fabricate highly aligned nanowires of a diketopyrrolopyrrole (DPP)-based donor–acceptor-type copolymer (poly (diketopyrrolopyrrole-alt-thieno [3,2-b] thiophene), DPP-DTT) for high-performance field effect transistors. The morphology of the patterns was controlled by changing the concentration of the DPP-based copolymer solution (1, 3, 5 mg mL−1). The molecular alignment properties of three different patterns were observed with a polarized optical microscope, polarized UV-vis spectroscopy and an X-ray diffractometer. DPP-DTT nanowires made with 1 mg mL−1 solution are highly aligned and the polymer field-effect transistors based on nanowires exhibit more than a five times higher charge carrier mobility as compared to spin-coated film-based devices.

Epitaxial growth of aligned MgO nanowire arrays on a single crystalline substrate.

MgO nanostructures with different morphologies were fabricated by regulating the composition of raw materials, using the chemical vapor deposition method with gold as a catalyst. Polycrystalline MgO nanopillars were prepared using only Mg3N2 as the raw material. Single crystalline nanowire arrays can be obtained by adding carbon powder into the raw material. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy were used for the detailed characterization of (100) nanowire arrays. Besides the vertical growth direction [100], the nanowires change into two kinds of orientations after growing to a certain length: 〈111〉 and 〈001〉. The epitaxial growth was attributed to the lattice match and regulation of the concentration of the Mg source. The variation of growth orientations was probably caused by the difference in surface energy and reaction temperature.

Single-Crystal Nanostructure Arrays Forming Epitaxially through Thermomechanical Nanomolding.

For nanostructures in advanced electronic and plasmonic systems, a single-crystal structure with controlled orientation is essential. However, the fabrication of such devices has remained challenging, as current nanofabrication methods often suffer from either polycrystalline growth or the difficulty of integrating single crystals with substrates in desired orientations and locations to create functional devices. Here we report a thermomechanical method for the controlled growth of single-crystal nanowire arrays, which enables the simultaneous synthesis, alignment, and patterning of nanowires. Within such diffusion-based thermomechanical nanomolding (TMNM), the substrate material diffuses into nanosized cavities under an applied pressure gradient at a molding temperature of ∼0.4 times the material's melting temperature. Vertically grown face-centered cubic (fcc) nanowires with the [110] direction in an epitaxial relationship with the (110) substrate are demonstrated. The ability to control the crystal structure through the substrate takes TMNM a major step further, potentially allowing all fcc and body-centered cubic (bcc) materials to be integrated as single crystals into devices.

Magnetic field-induced alignment of nickel-coated copper nanowires in epoxy composites for highly thermal conductivity with low filler loading

In this work, the large-scale produced copper nanowires (Cu NWs) were coated by nickel nanoparticles (Ni NPs) lay, i.e., Cu NWs@Ni. And under external magnetic field condition, Cu NWs@Ni as thermally conductive fillers were filled into epoxy (EP) resin to prepare the composites with high thermal conductivity (TC). The results showed that, when the magnetic field was 80 mT, Cu NWs@Ni (molar ratio of Cu:Ni = 10:5, and saturation magnetization (Ms) = 20.3 emu/g) can distribute in EP resin in an orderly manner along the direction of the magnetic field, endowing the composite with excellent TC at lower loading. Especially, when only 9 vol% Cu NWs@Ni was filled, the TC along the direction of the magnetic field can reach 2.90 W/mK, which was 12.81 times than that of neat EP resin. Even so, the composite still maintained outstanding insulation properties, namely, the volume resistivity and the dielectric strength were 1.79 × 1015 Ω cm and 12.88 kV/mm, respectively. In addition, compared to that of neat EP resin, the composite still possessed splendid heat stability and mechanical properties. For the above positive improvement, the corresponding mechanisms were also discussed in detail.

Layer‐by‐Layer Alignment of Silver Nanowires for Transparent and Flexible Energy‐Saving Windows

AbstractThe energy consumption in buildings through the thermal radiation of windows has become a big challenge in modern life. The low mid‐infrared emissivity (εMIR) and near‐infrared blocking windows with high visible transmittance (Tvis) are highly required in modern energy‐saving buildings, while most existing low εMIR windows are severely limited by their complex and high‐cost processing conditions and could not be retrofitted to existing windows. Herein low εMIR foils based on aligned silver nanowires (AgNWs) fabricated via an economical solution process are reported with good energy‐saving performance. The foils with aligned AgNWs give lower εMIR and higher Tvis compared with randomly oriented AgNWs foils. The low‐cost process, high Tvis, low εMIR, and flexibility to apply on any surface opens a new avenue to cut down energy consumption in existing windows.