Textured Tubular Nanoparticle Structures: Precursor‐Templated Synthesis of GaN Sub‐micrometer Sized Tubes

Porous and sub‐micrometer tubes made of textured GaN nanoparticles have been synthesized by an in situ chemical reaction and characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL) and Raman spectroscopies. The in situ reaction involves thermal decomposition and nitridation of 1D gallium oxyhydroxide (GaOOH) at temperatures in the range of 700–900 °C. The 1D shape of the precursor GaOOH is maintained in the resultant GaN tubes. The GaN nanocrystals (estimated to be about 15 nm in size) are found to be highly oriented with respect to each other in the tube structure, with the [110] GaN direction parallel to the tube axis. The growth mechanism of the tube structure has also been studied. β‐Ga₂O₃ is found to be an intermediate phase between the starting GaOOH precursor and the final GaN product. The growth mechanism involves decomposition of GaOOH, which produces β‐Ga₂O₃ tubes with hollow interiors, and nitridation of β‐Ga₂O₃, which leads to growth of textured GaN nanocrystals. Based on the growth mechanism, tubular structures with either quasi‐circular or rectangular cross section are selectively synthesized by controlling the heating rate and calcination temperature. This in situ chemical reaction method provides a new route for synthesizing 1D hollow nanostructures.     

Systematic Study of the Growth of Aligned Arrays of α‐Fe2O3 and Fe3O4 Nanowires by a Vapor–Solid Process

Growth of aligned and uniform α‐Fe₂O₃ nanowire (NW) arrays has been achieved by a vapor–solid process. The experimental conditions, such as type of substrate, local growth and geometrical environment, gas‐flow rate, and growth temperature, under which the high density α‐Fe₂O₃ NW arrays can be grown by a vapor–solid route via the tip‐growth mechanism have been systematically investigated. The density of the α‐Fe₂O₃ NWs can be enhanced by increasing the concentration of Ni atoms inside the alloy substrate. The synthesized temperature can be as low as 400 °C. Fe₃O₄ NWs can be produced by converting α‐Fe₂O₃ NWs in a reducing atmosphere at 450 °C. The transformation of phase and structure have been observed by in situ transmission electron microscopy. The magnetic and field‐emission properties of the NWs indicate their potential applications in nanodevices.     

Low‐Temperature Growth of SnO2 Nanorod Arrays and Tunable n–p–n Sensing Response of a ZnO/SnO2 Heterojunction for Exclusive Hydrogen Sensors

Uniform SnO₂ nanorod arrays have been deposited at low temperature by plasma‐enhanced chemical vapor deposition (PECVD). ZnO surface modification is used to improve the selectivity of the SnO₂ nanorod sensor to H₂ gas. The ZnO‐modified SnO₂ nanorod sensor shows a normal n‐type response to 100 ppm CO, NH₃, and CH₄ reducing gas whereas it exhibits concentration‐dependent n–p–n transitions for its sensing response to H₂ gas. This abnormal sensing behavior can be explained by the formation of n‐ZnO/p‐Zn‐O‐Sn/n‐SnO₂ heterojunction structures. The gas sensors can be used in highly selective H₂ sensing and this study also opens up a general approach for tailoring the selectivity of gas sensors by surface modification.     

Electrochemical Method for Synthesis of a ZnFe2O4/TiO2 Composite Nanotube Array Modified Electrode with Enhanced Photoelectrochemical Activity

An electrode with intimate and well‐aligned ZnFe₂O₄/TiO₂ composite nanotube arrays is prepared via electrochemical anodization of pure titanium foil in fluorine‐containing ethylene glycol, followed by a novel cathodic electrodeposition method. The deposition of ZnFe₂O₄ is promoted in the self‐aligned, vertically oriented TiO₂ nanotube arrays but minimized at the tube entrances. Thus, pore clogging is prevented. Environmental scanning electron microscopy, energy‐dispersive X‐ray spectra, high‐resolution transmission electron microscopy, X‐ray diffraction patterns, and X‐ray photoelectron spectroscopy indicate that the as‐prepared samples are highly ordered and vertically aligned TiO₂ nanotube arrays with ZnFe₂O₄ nanoparticles loading. The TiO₂ nanotubes are anatase with the preferential orientation of <101> plane. Enhanced absorption in both UV and visible light regions is observed for the composite nanotube arrays. The current–voltage curve of ZnFe₂O₄‐loaded TiO₂ nanotube arrays reveals a rectifying behavior. The enhanced separation of photoinduced electrons and holes is demonstrated by surface photovoltage and photocurrent measurements. Meanwhile, the photoelectrochemical investigations verify that the ZnFe₂O₄/TiO₂ composite nanotube array modified electrode has a more effective photoconversion capability than the aligned TiO₂ nanotube arrays alone. In addition, the photoelectrocatalytic ability of the novel electrode is found enhanced in the degradation of 4‐chlorophenol.     

Thin‐Wall Assembled SnO2 Fibers Functionalized by Catalytic Pt Nanoparticles and their Superior Exhaled‐Breath‐Sensing Properties for the Diagnosis of Diabetes

Hierarchical SnO₂ fibers assembled from wrinkled thin tubes are synthesized by controlling the microphase separation between tin precursors and polymers, by varying flow rates during electrospinning and a subsequent heat treatment. The inner and outer SnO₂ tubes have a number of elongated open pores ranging from 10 nm to 500 nm in length along the fiber direction, enabling fast transport of gas molecules to the entire thin‐walled sensing layers. These features admit exhaled gases such as acetone and toluene, which are markers used for the diagnosis of diabetes and lung cancer. The open tubular structures facilitated the uniform coating of catalytic Pt nanoparticles onto the inner SnO₂ layers. Highly porous SnO₂ fibers synthesized at a high flow rate show five‐fold higher acetone responses than densely packed SnO₂ fibers synthesized at a low flow rate. Interestingly, thin‐wall assembled SnO₂ fibers functionalized by Pt particles exhibit a dramatically shortened gas response time compared to that of un‐doped SnO₂ fibers, even at low acetone concentrations. Moreover, Pt‐decorated SnO₂ fibers significantly enhance toluene response. These results demonstrate the novel and practical feasibility of thin‐wall assembled metal oxide based breath sensors for the accurate diagnosis of diabetes and potential detection of lung cancer.     

A rapid growth of aligned carbon nanotube films and high-aspect-ratio arrays

The remarkable properties of carbon nanotubes (CNTs) make them attractive for microelectronic applications, especially for interconnects and nanoscale devices. In this paper, we report an efficient process to grow well-aligned CNT films and high-aspect-ratio CNT arrays with very high area distribution density (>1600 μm⁻²). Chemical vapor deposition (CVD) was invoked to deposit highly aligned CNTs on Al₂O₃/Fe coated silicon substrates of several square centimeter area using ethylene as the carbon source, and argon and hydrogen as carrier gases. The nanotubes grew at a high rate of ∼100 μm/min. for nanotube films at 800°C, while the nanotube arrays grew at ∼140 μm/min. even at 750°C, due to the base growth mode. The CNTs were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and x-ray photoelectron spectroscopy (XPS). The results demonstrated that the CNTs are of high purity and form densely aligned arrays with controllable size and height. The as-grown CNT structures have considerable potential for thermal management and electrical interconnects for microelectronic devices.     

White‐Light Emitting Diode Array of p+‐Si/Aligned n‐SnO2 Nanowires Heterojunctions

We report on the fabrication and optoelectronic properties of p‐n heterojunction arrays of p⁺‐type Si and aligned n‐type SnO₂ nanowires with high rectification ratios of >10⁴ at ±15 V. The electrical stability of the p‐n heterojunction devices was improved by coating the junction with poly(methylmethacrylate) to minimize the degradation of the interface layer at the junction. As a photodiode an enhanced UV photosensitivity higher than 10² was recorded under reverse bias. Using a large forward bias in the light‐emitting diode mode white light was emitted from the large‐scale heterojunction devices with at least three broad peaks in the visible range, which can be attributed to the interband transitions of the injected electrons or holes mediated by an interfacial SiO₂ layer with a contribution of trap‐level energies. These results indicate the high potential of Si/SnO₂ nanowires heterojunctions as optoelectronic devices with proper tuning of the recombination center at the junctions.     

Vertically Well‐Aligned ZnO Nanowires on c‐Al2O3 and GaN Substrates by Au Catalyst

In this letter, we report that vertically well‐aligned ZnO nanowires were grown on GaN epilayers and c‐plane sapphire via a vapor‐liquid‐solid process by introducing a 3 nm Au thin film as a catalyst. In our experiments, epitaxially grown ZnO nanowires on Au‐coated GaN were vertically well‐aligned, while nanowires normally tilted from the surface when grown on Au‐coated c‐Al₂O₃ substrates. However, pre‐growth annealing of the Au thin layer on c‐Al₂O₃ resulted in the growth of well‐aligned nanowires in a normal surface direction. High‐resolution transmission electron microscopy measurements showed that the grown nanowires have a hexagonal c‐axis orientation with a single‐crystalline structure.     

Gas Sensors: Ultrasensitive and Highly Selective Gas Sensors Based on Electrospun SnO2 Nanofibers Modified by Pd Loading (Adv. Funct. Mater. 24/2010)

This work presents a new route to suppress grain growth and tune the sensitivity and selectivity of nanocrystalline SnO₂ fibers. Unloaded and Pd‐loaded SnO₂ nanofiber mats are synthesized by electrospinning followed by hot‐pressing at 80 °C and calcination at 450 or 600 °C. The chemical composition and microstructure evolution as a function of Pd‐loading and calcination temperature are examined using EDS, XPS, XRD, SEM, and HRTEM. Highly porous fibrillar morphology with nanocrystalline fibers comprising SnO₂ crystallites decorated with tiny PdO crystallites is observed. The grain size of the SnO₂ crystallites in the layers that are calcined at 600 °C decreases with increasing Pd concentration from about 15 nm in the unloaded specimen to about 7 nm in the 40 mol% Pd‐loaded specimen, indicating that Pd‐loading could effectively suppress the SnO₂ grain growth during the calcination step. The Pd‐loaded SnO₂ sensors have 4 orders of magnitude higher resistivity and exhibit significantly enhanced sensitivity to H₂ and lower sensitivity to NO₂ compared to their unloaded counterparts. These observations are attributed to enhanced electron depletion at the surface of the PdO‐decorated SnO₂ crystallites and catalytic effect of PdO in promoting the oxidation of H₂ into H₂O. These phenomena appear to have a much larger effect on the sensitivity of the Pd‐loaded sensors than the reduction in grain size.     

Ultrasensitive and Highly Selective Gas Sensors Based on Electrospun SnO2 Nanofibers Modified by Pd Loading

This work presents a new route to suppress grain growth and tune the sensitivity and selectivity of nanocrystalline SnO₂ fibers. Unloaded and Pd‐loaded SnO₂ nanofiber mats are synthesized by electrospinning followed by hot‐pressing at 80 °C and calcination at 450 or 600 °C. The chemical composition and microstructure evolution as a function of Pd‐loading and calcination temperature are examined using EDS, XPS, XRD, SEM, and HRTEM. Highly porous fibrillar morphology with nanocrystalline fibers comprising SnO₂ crystallites decorated with tiny PdO crystallites is observed. The grain size of the SnO₂ crystallites in the layers that are calcined at 600 °C decreases with increasing Pd concentration from about 15 nm in the unloaded specimen to about 7 nm in the 40 mol% Pd‐loaded specimen, indicating that Pd‐loading could effectively suppress the SnO₂ grain growth during the calcination step. The Pd‐loaded SnO₂ sensors have 4 orders of magnitude higher resistivity and exhibit significantly enhanced sensitivity to H₂ and lower sensitivity to NO₂ compared to their unloaded counterparts. These observations are attributed to enhanced electron depletion at the surface of the PdO‐decorated SnO₂ crystallites and catalytic effect of PdO in promoting the oxidation of H₂ into H₂O. These phenomena appear to have a much larger effect on the sensitivity of the Pd‐loaded sensors than the reduction in grain size.     

Self‐Aligned and Scalable Growth of Monolayer WSe2–MoS2 Lateral Heterojunctions

2D layered heterostructures have attracted intensive interests due to their unique optical, transport, and interfacial properties. The laterally stitched heterojunction based on dissimilar 2D transition metal dichalcogenides forms an intrinsic p–n junction without the necessity of applying an external voltage. However, no scalable processes are reported to construct the devices with such lateral heterostructures. Here, a scalable strategy, two‐step and location‐selective chemical vapor deposition, is reported to synthesize self‐aligned WSe₂–MoS₂ monolayer lateral heterojunction arrays and demonstrates their light‐emitting devices. The proposed fabrication process enables the growth of high‐quality interfaces and the first successful observation of electroluminescence at the WSe₂–MoS₂ lateral heterojunction. The electroluminescence study has confirmed the type‐I alignment at the interface rather than commonly believed type‐II alignment. This self‐aligned growth process paves the way for constructing various 2D lateral heterostructures in a scalable manner, practically important for integrated 2D circuit applications.     

Hydrothermally Treated SnO2 as the Electron Transport Layer in High‐Efficiency Flexible Perovskite Solar Cells with a Certificated Efficiency of 17.3%

Perovskite solar cells (PSCs) are one of the most promising solar energy conversion technologies owing to their rapidly developing power conversion efficiency (PCE). Low‐temperature solution processing of the perovskite layer enables the fabrication of flexible devices. However, their application has been greatly hindered due to the lack of strategies to fabricate high‐quality electron transport layers (ETLs) at the low temperatures (≈100 °C) that most flexible plastic substrates can withstand, leading to poor performances for flexible PSCs. In this work, through combining the spin‐coating process with a hydrothermal treatment method, ligand‐free and highly crystalline SnO₂ ETLs are successfully fabricated at low temperature. The flexible PSCs based on this SnO₂ ETL exhibit an excellent PCE of 18.1% (certified 17.3%). The flexible PSCs maintained 85% of the initial PCE after 1000 bending cycles and over 90% of the initial PCE after being stored in ambient air for 30 days without encapsulation. The investigation reveals that hydrothermal treatment not only promotes the complete removal of organic surfactants coated onto the surface of the SnO₂ nanoparticles by hot water vapor but also enhances crystallization through the high vapor pressure of water, leading to the formation of high‐quality SnO₂ ETLs.     

Optimal Doping for Enhanced SnO2 Sensitivity and Thermal Stability

Tin oxide nanocrystals (5–10 nm) doped with silica (0–15 wt %) were made by flame‐spray‐pyrolysis direct deposition onto the sensing electrodes and in situ stabilization by rapid flame annealing. Although increased SiO₂‐doping reduced the SnO₂ crystal and grain size, its sensing performance to ethanol vapor (0.1–50 ppm) exhibited an optimum with respect to SiO₂ content. The thermal stability and morphology of SiO₂‐doped SnO₂ nanoparticles were evaluated by sintering at 200–900 °C for 4–24 h in air. At low SiO₂ content, sintering of SnO₂ was prevented only partially resulting in small sinter necks (bottlenecks) between SnO₂ primary particles (smaller than twice the Debye length). This morphology drastically enhanced the sensitivity toward the analyte by maintaining a thermally stable high surface area and fully depleted connections at the primary particle necks. This enhancement is attributed mostly to the decreasing neck size of the SnO₂ SiO₂ heterojunctions rather than the decreasing SnO₂ crystallite and grain sizes with increasing SiO₂ doping. At high SiO₂ contents, SnO₂ sintering was inhibited as its grains were separated effectively by dielectric SiO₂; this resulted in isolated SnO₂ nanocrystals with drastically reduced sensitivity, thereby effectively being insulators.     

Tin Dioxide Sensing Layer Grown on Tubular Nanostructures by a Non‐Aqueous Atomic Layer Deposition Process

A new atomic layer deposition (ALD) process for nanocrystalline tin dioxide films is developed and applied for the coating of nanostructured materials. This approach, which is adapted from non‐hydrolytic sol‐gel chemistry, permits the deposition of SnO₂ at temperatures as low as 75 °C. It allows the coating of the inner and outer surface of multiwalled carbon nanotubes with a highly conformal film of controllable thickness. The ALD‐coated tubes are investigated as active components in gas‐sensor devices. Due to the formation of a p‐n heterojunction between the highly conductive support and the SnO₂ thin film an enhancement of the gas sensing response is observed.     

Fabrication and Optical Characteristics of Position‐Controlled ZnO Nanotubes and ZnO/Zn0.8Mg0.2O Coaxial Nanotube Quantum Structure Arrays

The position‐controlled growth and structural and optical characteristics of ZnO nanotubes and their coaxial heterostructures are reported. To control both the shape and position of ZnO nanotubes, hole‐patterned SiO₂ growth‐mask layers on Si(111) substrates with GaN/AlN intermediate layers using conventional lithography are prepared. ZnO nanotubes are grown only on the hole patterns at 600 °C by catalyst‐free metal–organic vapor‐phase epitaxy. Furthermore, the position‐controlled nanotube growth method allows the fabrication of artificial arrays of ZnO‐based coaxial nanotube single‐quantum‐well structures (SQWs) on Si substrates. In situ heteroepitaxial growth of ZnO and Zn_0.8Mg_0.2O layers along the circumference of the ZnO nanotube enable an artificial formation of quantum‐well arrays in a designed fashion. The structural and optical characteristics of the ZnO nanotubes and SQW arrays are also investigated using synchrotron radiation X‐ray diffractometry and photoluminescence and cathodoluminescence spectroscopy.     

SnO2 gas sensor fabricated by low frequency alternating field electrophoretic deposition

SnO₂ thick film gas sensor has been prepared by applying low frequency (0.1 Hz) AC electric fields to a stable suspension of SnO₂ nanoparticles in acetylacetone. Parallel gold electrodes were used as the deposition substrate. Effect of CO, O₂ and H₂ gas exposure as well as ethanol vapor on conductivity of the SnO₂ film at 300 °C is investigated. Results show that the sensor is sensitive and its response is repeatable. This work shows that ACEPD can be used as an easy and cheap technique for fabrication of electronic devices such as ceramic-based gas sensors.     

Laser‐Ablation Growth and Optical Properties of Wide and Long Single‐Crystal SnO2 Ribbons

Wide and long ribbons of single‐crystalline SnO₂ have been achieved via laser ablation of a SnO₂ target. Transmission electron microscopy (TEM) shows the as‐grown SnO₂ ribbons are structurally perfect and uniform, with widths of 300–500 nm, thicknesses of 30–50 nm (width‐to‐thickness ratio of ～ 10), and lengths ranging from several hundreds of micrometers to the order of millimeters. X‐ray diffraction (XRD) pattern and energy‐dispersive X‐ray spectroscopy (EDS) spectral analysis indicate that the ribbons have the phase structure and chemical composition of the rutile form of SnO₂. Selected‐area electron diffraction (SAED) patterns and high‐resolution TEM images reveal that the ribbons are single crystals and grow along the [100] crystal direction. Photoluminescence measurements show that the synthesized SnO₂ ribbons have one strong emission band at ～ 605 nm and a red‐shift of ～ 30 nm, as compared to standard SnO₂ powder, which may be attributed to crystal defects and residual strains accommodated during the growth of the ribbons.     

Epitaxial Growth of Branched α‐Fe2O3/SnO2 Nano‐Heterostructures with Improved Lithium‐Ion Battery Performance

We report the synthesis of a novel branched nano‐heterostructure composed of SnO₂ nanowire stem and α‐Fe₂O₃ nanorod branches by combining a vapour transport deposition and a facile hydrothermal method. The epitaxial relationship between the branch and stem is investigated by high resolution transmission electron microscopy (HRTEM). The SnO₂ nanowire is determined to grow along the [101] direction, enclosed by four side surfaces. The results indicate that distinct crystallographic planes of SnO₂ stem can induce different preferential growth directions of secondary nanorod branches, leading to six‐fold symmetry rather than four‐fold symmetry. Moreover, as a proof‐of‐concept demonstration of the function, such α‐Fe₂O₃/SnO₂ composite material is used as a lithium‐ion batteries (LIBs) anode material. Low initial irreversible loss and high reversible capacity are demonstrated, in comparison to both single components. The synergetic effect exerted by SnO₂ and α‐Fe₂O₃ as well as the unique branched structure are probably responsible for the enhanced performance.     

An Electrical Switch‐Driven Flexible Electromagnetic Absorber

The development of a thin, tunable, and high‐performance flexible electromagnetic (EM) absorbing device that aims to solve signal interference or EM pollution is highly desirable but remains a great challenge. Herein, demonstrated is a flexible electrical‐driven device constructed by an insulated organic‐polymer substrate, carrier transmission layer, and core–shell structured absorber, enabling a narrow and tunable effective absorption region (f_E < 2.0 GHz) by controlling the external voltage toward this challenge. As a key design element, the selected absorber consists of an Sn/SnS/SnO₂ core and C shell, which exhibits an exceptional dielectric‐response ability at a small voltage, which is attributed to desirable carrier mobility and excitable carriers. Multiple f_E‐tuning regions (maximum up to 7.0), covering 90% of C‐band can be achieved for Sn/SnS/SnO₂@C‐based flexible device by selecting a low voltage (2–12 V). The strategy developed here may open a new avenue toward the design of flexible intelligent EM device for practical applications.     

Low‐Temperature Formation of Well‐Aligned Nanocrystalline Si/SiOx Composite Nanowires

Well‐aligned nanocrystalline (nc)‐Si/SiO_x composite nanowires have been deposited on various substrates at 120 °C using SiCl₄/H₂ in a hot‐filament chemical vapor deposition reactor. Structural and compositional analyses reveal that silicon nanocrystals are embedded in the amorphous SiO_x nanowires. The nc‐Si/SiO_x composite nanowires are transparent in the range 500–900 nm. Photoluminescence spectra of the nc‐Si/SiO_x composite nanowires have a broad emission band, ranging from 420 to 525 nm. Water vapor from the chamber wall plays a crucial role in the formation of the well‐aligned nanowires. A possible mechanism for the formation of the composite nanowires is suggested.