Orientation‐Controlled Selective‐Area Epitaxy of III–V Nanowires on (001) Silicon for Silicon Photonics

Monolithic integration of III–V nanowires on silicon platforms has been regarded as a promising building block for many on‐chip optoelectronic, nanophotonic, and electronic applications. Although great advances have been made from fundamental material engineering to realizing functional devices, one of the remaining challenges for on‐chip applications is that the growth direction of nanowires on Si(001) substrates is difficult to control. Here, catalyst‐free selective‐area epitaxy of nanowires on (001)‐oriented silicon‐on‐insulator (SOI) substrates with the nanowires aligned to desired directions is proposed and demonstrated. This is enabled by exposing {111} planes on (001) substrates using wet chemical etching, followed by growing nanowires on the exposed planes. The formation of nanowire array‐based bottom‐up photonic crystal cavities on SOI(001) and their coupling to silicon waveguides and grating couplers, which support the feasibility for on‐chip photonic applications are demonstrated. The proposed method of integrating position‐ and orientation‐controllable nanowires on Si(001) provides a new degree of freedom in combining functional and ultracompact III–V devices with mature silicon platforms.     

Virus‐Templated Self‐Mineralization of Ligand‐Free Colloidal Palladium Nanostructures for High Surface Activity and Stability

In solution‐based synthesis of colloidal nanostructures, additions of ligands, stabilizers, and redox reagents are generally required to obtain desirable structures, though ligands and stabilizers on the surface of nanostructures can substantially affect the surface‐related activity. Accordingly, an extensive rinsing process is usually required to remove residual reagents and stabilizers. This study reports a spontaneous self‐biomineralization of palladium (Pd) ions on a filamentous virus to form ligand‐free Pd nanowires under ambient conditions. No reducing reagents or additional surface stabilizers are used; the genetically modified virus alone supports the polycrystalline Pd nanowires within the nanostructure, maintaining the clean surface even without a rinsing process. The advantage of the ligand‐free Pd nanowires is found in the Suzuki‐coupling reaction, in which the nanowire catalytic activity is maintained after repeated reactions, while conventional Pd colloids undergo surface contamination by the stabilizer and lose their catalytic activity during repeated uses. The ligand‐free surface, high electronic connectivity, and structural stability of the Pd nanowires also allow high sensitivity and selectivity in hydrogen gas sensing analysis. This work emphasizes the importance of the ligand‐free surface of biotemplated nanostructures in maintaining functionalities without surface contamination.     

Multiarray Nanopattern Electronic Nose (E‐Nose) by High‐Resolution Top‐Down Nanolithography

An electronic nose (E‐nose) is an artificial sensing device that mimics the human olfactory system using a multiarray sensor system. However, since the design and fabrication of multiarray sensing channels are significantly limited because of the requirement of time‐consuming and nonuniversal processes, the development of commercializable and high‐throughput fabrication approaches are critically required. Herein, high‐resolution top‐down lithography is developed for E‐nose fabrication for the first time. Five different metal oxide semiconductor (MOS) nanopattern channels (NiO, CuO, Cr₂O₃, SnO₂, and WO₃) are fabricated into multiarray sensors with high‐throughput using a unique lithographic approach that utilizes the sputtering of grains of the metals via low‐energy ion plasma bombardment. The nanopattern channels show i) high‐resolutions (15 nm scale), ii) high‐aspect‐ratios (11; 14 nm width and 150 nm height), and iii) ultrasmall grains (5.1 nm) with uniformity on a cm² scale, resulting in high sensitivity toward the target analytes. The E‐nose system, which is composed of five MOS nanopattern channels, can successfully distinguish seven different hazardous analytes, including volatile organic compounds and nitrogen‐containing compounds. It is expected that this unique lithography approach can provide a simple and reliable method for commercializable channel fabrication, and the E‐noses can have further applications in real‐life situations.     

Electric‐Driven Rotation of Silicon Nanowires and Silicon Nanowire Motors

In this work, the first highly controllable assembly and rotation of silicon nanowires and nanomotors in suspension are reported. Si and Si composite nanowires are fabricated with precisely controlled dimensions via colloidal assisted catalytic etching. The nanowires can be rotated with deterministic speed and chirality. The rotation speed and orientation not only depend on the applied AC electric frequency, but also on the electronic type, geometry, surface coating, as well as the electric conductance of suspension mediums. Theoretical analysis is used to understand the rotation of Si nanowires, and also the electric resistivity of Si nanowires is determined from their mechanical rotation. The Si nanowires are precisely assembled into nanomotors that can be rotated with controlled speeds and orientations at prescribed locations. This work provides a new paradigm for designing and actuating various Si‐based nanoelectromechanical system (NEMS) devices, which are relevant to man‐made nanomotors, nanorobots, and nanoengines.     

Fingerprint‐Inspired Conducting Hierarchical Wrinkles for Energy‐Harvesting E‐Skin

In the field of bionics, sophisticated and multifunctional electronic skins with a mechanosensing function that are inspired by nature are developed. Here, an energy‐harvesting electronic skin (energy‐E‐skin), i.e., a pressure sensor with energy‐harvesting functions is demonstrated, based on fingerprint‐inspired conducting hierarchical wrinkles. The conducting hierarchical wrinkles, fabricated via 2D stretching and subsequent Ar plasma treatment, are composed of polydimethylsiloxane (PDMS) wrinkles as the primary microstructure and embedded Ag nanowires (AgNWs) as the secondary nanostructure. The structure and resistance of the conducting hierarchical wrinkles are deterministically controlled by varying the stretching direction, Ar plasma power, and treatment time. This hierarchical‐wrinkle‐based conductor successfully harvests mechanical energy via contact electrification and electrostatic induction and also realizes self‐powered pressure sensing. The energy‐E‐skin delivers an average output power of 3.5 mW with an open‐circuit voltage of 300 V and a short‐circuit current of 35 µA; this power is sufficient to drive commercial light‐emitting diodes and portable electronic devices. The hierarchical‐wrinkle‐based conductor is also utilized as a self‐powered tactile pressure sensor with a sensitivity of 1.187 mV Pa^‐1 in both contact‐separation mode and the single‐electrode mode. The proposed energy‐E‐skin has great potential for use as a next‐generation multifunctional artificial skin, self‐powered human–machine interface, wearable thin‐film power source, and so on.     

Palladium Nanowire from Precursor Nanowire: Crystal‐to‐Crystal Transformation via In Situ Reduction by Polymer Matrix

Precursor nanowires of potassium palladium(II ) chloride crystallized inside a poly(vinyl alcohol) film are reduced to palladium nanowires by the polymer itself under mild thermal annealing. The chemical reaction occurring in situ inside the polymer film, including byproduct formation, is investigated through electronic absorption and X‐ray photoelectron spectroscopy together with atomic force and electron microscopy. The overall process can be described as a novel case of crystal‐to‐crystal transformation at the nanoscopic level. Optical limiting characteristics of the nanowire‐embedded polymer film are explored. The fabrication procedure developed, involving chemistry inside a polymer matrix mediated by the polymer, opens up a convenient route to the fabrication of free‐standing metal nanowire‐embedded thin films.     

Photoinduced Electron Transfer in Dye‐Sensitized SnO2 Nanowire Field‐Effect Transistors

Electron transfer from excited dye molecules (chlorophyll or fluorescein) to a semiconductor is demonstrated by photoaction and photoluminescence spectra on field‐effect transistors consisting of dye‐sensitized individual SnO₂ nanowires. The photoaction spectrum shows a much better resolution for nanowires non‐covalently functionalized with dye molecules than for dyes deposited on SnO₂ nanoparticle‐films. Possible reasons for the deviation between the photoaction spectra and ordinary optical absorption spectra as well as for the current‐tail appearing along the falling edge are addressed. In dye‐sensitized nanowires, electron transfer from photo‐excited dyes to nanowires is analyzed by comparing gate‐voltage dependences in photoaction and photoluminescence spectra. The importance of this study is in the understanding of electron injection and recombination provided, as well as the performance optimization of nanowire‐based dye‐sensitized solar cells.     

Low‐Resistant Electrical and Robust Mechanical Contacts of Self‐Attachable Flexible Transparent Electrodes with Patternable Circuits

Flexible, transparent, conductive electrodes are key elements of emerging flexible electronic and energy devices. Such electrodes should form an intimate physical contact with various active components of flexible devices to ensure stable, low‐resistant electrical contacts. However, contact formation techniques are based largely on conventional soldering, conductive pastes, mechanical clamping, and thin film deposition. These generally result in damaged, contaminated, bulky, and uncontrollable contact interfaces. A self‐attachable, flexible, transparent, and conductive electrode that is based on a distinctive design of regular grid patterns into which bioinspired adhesive architectures and percolating Ag nanowires are integrated is proposed. Based on this integrated design, the proposed electrode forms reliable, low‐resistant electrical contacts; strong mechanical adhesive contacts; and ultra‐clean, damage‐free contact interfaces with active device components by attaching onto the components without using additional conductive pastes, mechanical pressing, or vacuum deposition processes. The contact interfaces of the electrode and device components remain stable even when the electrode is extremely bent. Moreover, specific electronic circuits can be generated on the electrode surface by a selective deposition of Ag nanowires. This enables simple interconnections of diverse electronic components on its surface.     

Template‐Assisted Synthesis of π‐Conjugated Molecular Organic Nanowires in the Sub‐100 nm Regime and Device Implications

π‐conjugated molecular organics such as rubrene, Alq₃, fullerene, and PCBM have been used extensively over the last few decades in numerous organic electronic devices, including solar cells, thin‐film transistors, and large‐area, low‐cost flexible displays. Rubrene and Alq₃, have emerged as promising platforms for spin‐based classical and quantum information processing, which has triggered significant research activity in the relatively new area of organic spintronics. Synthesis of these materials in a nanowire geometry, with feature sizes in the sub‐100 nm regime, is desirable as it often enhances device performance and is essential for development of high‐density molecular electronic devices. However, fabrication techniques that meet this stringent size constraint are still largely underdeveloped. Here, a novel, versatile, and reagentless method that enables growth of nanowire arrays of the above‐mentioned organics in the cylindrical nanopores of anodic aluminum oxide (AAO) templates is demonstrated. This method 1) allows synthesis of high‐density organic nanowire arrays on arbitrary substrates, 2) provides electrical access to the nanowire arrays, 3) offers tunability of the array geometry in a range overlapping with the relevant physical length scales of many organic devices, and 4) can potentially be extended to synthesize axially and radially heterostructured organic nanowires. Thus prepared nanowires are characterized extensively with an aim to identify their potential applications in diverse areas such as organic optoelectronics, photovoltaics, molecular nanoelectronics, and spintronics.     

Lattice‐Symmetry‐Driven Epitaxy of Hierarchical GaN Nanotripods

Lattice‐symmetry‐driven epitaxy of hierarchical GaN nanotripods is demonstrated. The nanotripods emerge on the top of hexagonal GaN nanowires, which are selectively grown on pillar‐patterned GaN templates using molecular beam epitaxy. High‐resolution transmission electron microscopy confirms that two kinds of lattice‐symmetry, wurtzite (wz) and zinc‐blende (zb), coexist in the GaN nanotripods. Periodical transformation between wz and zb drives the epitaxy of the hierarchical nanotripods with N‐polarity. The zb‐GaN is formed by the poor diffusion of adatoms, and it can be suppressed by improving the ability of the Ga adatoms to migrate as the growth temperature increased. This controllable epitaxy of hierarchical GaN nanotripods allows quantum dots to be located at the phase junctions of the nanotripods and nanowires, suggesting a new recipe for multichannel quantum devices.     

Multifunctional ZnO‐Nanowire‐Based Sensor

A simple fabrication of ZnO‐nanowire‐based device and their implementation as a pH sensor, temperature sensor, and photo detector is reported. The presented multifunctional ZnO multiple‐nanowire sensor platform contains a Au finger structure, which is realized by conventional photolithography on a SiO₂ substrate. The nanowires are grown using thermal chemical vapor deposition. In order to detect the physical signals, changes in electrical signals were measured (conductance and current). For temperature sensing, the current behavior from 90 to 380 K under vacuum conditions exhibit a tunneling behavior between spaced nanowires. For photo sensing, the current response between the “on” and “off” states of light was measured when exposed to different wavelengths ranging from UV to visible light. Finally, for pH sensing the conductance was measured between a pH of 5 and 8.5. The ZnO nanowires were protected from chemical attacks by a thin layer of C₄F₈‐plasma‐based coating.     

Wound‐State Monitoring for Burn Patients Using E‐Nose/SPME System

Array‐based gas sensors now offer the potential of a robust analytical approach to odor measurement for medical use. We are developing a fast reliable method for detection of microbial infection by monitoring the headspace from the infected wound. In this paper, we present initial results obtained from wound‐state monitoring for burn patients using an electronic nose incorporating an automated solid‐phase microextraction (SPME) desorption system to enable the system to be used for clinical validation. SPME preconcentration is used for sampling of the headspace air and the response of the sensor module to variable concentrations of volatiles emitted from SPME fiber is evaluated. Gas chromatography‐mass spectrometry studies prove that living bacteria, the typical infectious agents in clinical practice, can be distinguished from each other by means of a limited set of key volatile products. Principal component analysis results give the first indication that infected patients may be distinguished from uninfected patients. Microbial laboratory analysis using clinical samples verifies the performance of the system.     

Three‐Dimensional Branched Nanowire Heterostructures as Efficient Light‐Extraction Layer in Light‐Emitting Diodes

A facile method to fabricate three‐dimensional branched ZnO/MgO nanowire heterostructures and their application as the efficient light‐extraction layer in light‐emitting diodes are reported. The branched MgO nanowires are produced on the hydrothermally‐grown ZnO nanowires with a small tapering angle towards the tip (≈6°), by the oblique angle flux incidence of MgO. The structural evolution during the growth verifies the formation of the MgO nanoscale islands with strong (111) preferred orientation on very thin (5–7 nm) MgO (110) layer. The MgO nanobranches, then grown on the islands, are polycrystalline consisting of many grains oriented in specific directions of <200> and <220>, supported by the nucleation theory. The LEDs with the branched ZnO/MgO nanowire arrays show a remarkable enhancement in the light output power by 21% compared with that of LEDs with pristine ZnO nanowires. Theoretical calculations using a finite‐difference time‐domain method reveal that the nanostructure is very effective in breaking the wave‐guiding mode inside the ZnO nanowires, extracting more light especially in radial direction through the MgO nanobranches.     

Ionic‐Liquid Gating of InAs Nanowire‐Based Field‐Effect Transistors

Here, the operation of a field‐effect transistor based on a single InAs nanowire gated by an ionic liquid is reported. Liquid gating yields very efficient carrier modulation with a transconductance value 30 times larger than standard back gating with the SiO₂/Si₊₊ substrate. Thanks to this wide modulation, the controlled evolution from semiconductor to metallic‐like behavior in the nanowire is shown. This work provides the first systematic study of ionic‐liquid gating in electronic devices based on individual III–V semiconductor nanowires: this architecture opens the way to a wide range of fundamental and applied studies from the phase transitions to bioelectronics.     

Studies of Surface‐Modified Gold Nanowires Inside Living Cells

The cytotoxicity of various surface‐functionalized gold nanowires with different aspect ratios is investigated by (3‐(4,5‐dimethylthiazol‐2‐yl)2,5‐diphenyltetrazolium bromide) (MTT) assays for two cell lines, fibroblast and HeLa. It is found that functionalized gold nanowires with a diameter of 200 nm and lengths up to a few micrometers can be readily internalized by both types of cells regardless of the type of surface functionalization. However, the cytotoxicity of the gold nanowires is observed to depend on their surface modification. Serum‐coated gold nanowires are the least toxic, whereas more than 50 % of the cells are damaged in the presence of mercapto‐acid‐modified gold nanowires even at very low concentrations (10³ nanowires mL^–1). Nanowires with different aspect ratios exhibit the same cytotoxicity within limits of experimental error. However, the uptake efficiency is found to be higher for shorter nanowires as compared to their longer counterparts. Therefore, we conclude that internalized nanowires with high aspect ratios are more toxic to cells than nanowires with low aspect ratios. Positively charged aminothiol‐modified gold nanowires are employed to deliver both plasmid DNA and probe molecules into cells without compromising the viability of the cells. The local environment of individual nanowires within the cells is studied by monitoring the fluorescence signal from probe molecules attached to the nanowires.     

Self‐Assembled In‐Plane Growth of Mg2SiO4 Nanowires on Si Substrates Catalyzed by Au Nanoparticles

In‐plane growth of Mg₂SiO₄ nanowires on Si substrates is achieved by using a vapor transport method with Au nanoparticles as catalyst. The self‐assembly of the as‐grown nanowires shows dependence on the substrate orientation, i.e., they are along one, two, and three particular directions on Si (110), (100), and (111) substrates, respectively. Detailed electron microscopy studies suggest that the Si substrates participate in the formation of Mg₂SiO₄, and the epitaxial growth of the nanowires is confined along the Si <110> directions. This synthesis route is quite reliable, and the dimensions of the Mg₂SiO₄ nanowires can be well controlled by the experiment parameters. Furthermore, using these nanowires, a lithography‐free method is demonstrated to fabricate nanowalls on Si substrates by controlled chemical etching. The Au nanoparticle catalyzed in‐plane epitaxial growth of the Mg₂SiO₄ nanowires hinges on the intimate interactions between substrates, nanoparticles, and nanowires, and our study may help to advance the developments of novel nanomaterials and functional nanodevices.     

Wavelength‐Emission Tuning of ZnO Nanowire‐Based Light‐Emitting Diodes by Cu Doping: Experimental and Computational Insights

The band‐gap engineering of doped ZnO nanowires is of the utmost importance for tunable light‐emitting‐diode (LED) applications. A combined experimental and density‐functional theory (DFT) study of ZnO doping by copper (Zn²⁺ substitution by Cu²⁺) is presented. ZnO:Cu nanowires are epitaxially grown on magnesium‐doped p‐GaN by electrochemical deposition. The heterojunction is integrated into a LED structure. Efficient charge injection and radiative recombination in the Cu‐doped ZnO nanowires are demonstrated. In the devices, the nanowires act as the light emitters. At room temperature, Cu‐doped ZnO LEDs exhibit low‐threshold emission voltage and electroluminescence emission shifted from the ultraviolet to violet–blue spectral region compared to pure ZnO LEDs. The emission wavelength can be tuned by changing the copper content in the ZnO nanoemitters. The shift is explained by DFT calculations with the appearance of copper d states in the ZnO band‐gap and subsequent gap reduction upon doping. The presented data demonstrate the possibility to tune the band‐gap of ZnO nanowire emitters by copper doping for nano‐LEDs.     

Well‐Defined Fullerene Nanowire Arrays

Fullerene nanowire arrays with well‐defined size and length have been prepared by a controllable technique. Fullerene molecules such as C₆₀ are introduced into the pores of anodic aluminum oxide (AAO) templates under a direct current (DC) electric field and polymerized in the pores. Structure analysis shows that the C₆₀ nanowires are mainly polycrystalline, and a rhombohedral polymeric phase is observed in their vibration spectra. The electrical conductivity of so‐prepared nanowire arrays show a semiconducting behavior. The ability to fabricate the fullerene nanowire arrays with controlled structures represents an important step toward the development of chemical sensors and nanoscale electronic devices based on fullerenes.     

Growth of Wurtzite ZnS Micrometer‐Sized Diskettes and Nanoribbon Arrays with Improved Luminescence

We report on the synthesis of wurtzite ZnS micrometer‐sized diskettes (including those lined up with ZnS nanowires) and ZnS nanoribbon arrays. Using ZnS powder as a source material, a vapor–solid growth based on a two‐stage temperature‐controllable thermal evaporation and condensation process is realized. Significant enhancement of luminescence compared to the ZnS source material is observed from these ZnS micro‐ and nanometer‐sized structures. The structures may serve as ideal model systems in the nano‐ to micrometer range for studying the optical and electronic properties of ZnS material. They can also be treated as prospective building blocks of two‐ and/or three‐dimensional arrays and are promising candidates for fabricating novel electronic and optoelectronic devices.     

Tunable Near‐Infrared Organic Nanowire Nanolasers

Organic semiconductor nanowires have inherent advantages, such as amenability to low‐cost, low‐temperature processing, and inherent four‐level energy systems, which will significantly contribute to the organic solid‐state lasers (OSSLs) and miniaturized laser devices. However, the realization of near‐infrared (NIR) organic nanowire lasers is always a big challenge due to the difficultly in fabrication of organic nanowires with diameters of ≈100 nm and material issues such as low photoluminescence quantum efficiency in the red‐NIR region. What is more, the achievement of wavelength‐tunable OSSLs has also encountered enormous challenge. This study first demonstrates the 720 nm NIR lasing with a low lasing threshold of ≈1.4 µJ cm^?2 from the organic single‐crystalline nanowires, which are self‐assembled from small organic molecules of (E )‐3‐(4‐(dimethylamino)‐2‐methoxyphenyl)‐1‐(1‐hydroxynaphthalen‐2‐yl)prop‐2‐en‐1‐one through a facile solution‐phase growth method. Notably, these individual nanowires' Fabry–Pérot cavity can alternatively provide the red‐NIR lasing action at 660 or 720 nm from the 0–1 or 0–2 radiative transition channels, and the single (660 or 720 nm)/dual‐wavelength (660 and 720 nm) laser action can be achieved by modulating the length of these organic nanowires due to the intrinsic self‐absorption. These easily‐fabricated organic nanowires are natural laser sources, which offer considerable promise for coherent light devices integrated on the optics microchip.