Cover Picture: Sequential Nucleation and Growth of Complex Nanostructured Films (Adv. Funct. Mater. 3/2006)

A sequential nucleation and growth process has been developed to construct complex nanostructured films step‐by‐step from aqueous solutions, as reported by Liu, Voigt, and co‐workers on p. 335. This method can be applied to a wide range of materials, and can be combined with top–down techniques to create spatially resolved micropatterns. The cover figure shows images of oriented nanowires, nanoneedles, nanotubes, nanoplates and stacked columns, wagon‐wheels, hierarchical films based on wagon‐wheels, hierarchically ordered mesophase silicate, and micropatterned flower‐like structures. Nanostructured films with controlled architectures are desirable for many applications in optics, electronics, biology, medicine, and energy/chemical conversions. Low‐temperature, aqueous chemical routes have been widely investigated for the synthesis of continuous films, and arrays of oriented nanorods and nanotubes. More recently, aqueous‐phase routes have been used to produce films composed of more complex crystal structures. In this paper, we discuss recent progress in the synthesis of complex nanostructures through sequential nucleation and growth processes. We first review the use of multistage, seeded‐growth methods to synthesize a wide range of nanostructures, including oriented nanowires, nanotubes, and nanoneedles, as well as laminated films, columns, and multilayer heterostructures. We then describe more recent work on the application of sequential nucleation and growth to the systematic assembly of large arrays of hierarchical, complex, oriented, and ordered crystal architectures. The multistage aqueous chemical route is shown to be applicable to several technologically important materials, and therefore may play a key role in advancing complex nanomaterials into applications.     

Surface Wettability of Nanostructured Zinc Oxide Films

Zinc oxide (ZnO) nanograin and nanorod films were prepared by magnetron sputter deposition and an aqueous solution growth method. Their surface wettability was studied in relation to their surface morphologies. While the surfaces of both films were hydrophobic, the nanorod films exhibited higher surface hydrophobicity. A superhydrophobic surface was obtained on a ZnO nanorod film with a water contact angle of 151 deg. Results have shown that their surface wettability was influenced by the morphology of ZnO nanostructures, including the grain size, the length, and density of nanorods. Both types of ZnO films showed switchable wettability under ultraviolet irradiation and dark storage.     

Solvent Dependent Shape and Magnetic Properties of Doped ZnO Nanostructures

This study reports on a new solution phase synthesis leading to cobalt and manganese doped ZnO which have been theoretically predicted ferromagnetic at room temperature. The solvothermal synthesis involving the reaction of zinc and cobalt acetate or manganese oleate with benzyl alcohol leads to pure inorganic nanoparticles that are diluted magnetic semiconductors. The addition of an inert solvent, that is used in order to control the amount of benzyl alcohol, drastically influences the particles morphology and strongly affects the magnetic behaviors. Cobalt doped particles are paramagnetic or ferromagnetic depending on the synthesis conditions. In order to exclude the formation of secondary phases and/or metal clusters and to understand the role of the solvent on the magnetic properties, the local structure of Co²⁺ and Mn²⁺ in the wurtzite ZnO matrix were characterized by XRD, UV‐visible diffuse reflectance and electron paramagnetic resonance.     

Solid-State Self-Assembly of Nanostructured Oxide as a Candidate High-Performance Thermoelectric Material

We have focused on the recently reported nanostructured bulk ZnMn_2−x Ga _x O₄ to evaluate whether this type of nanostructured oxide can effectively reduce thermal conductivity. Firstly, powdered samples of ZnMn_2−x Ga _x O₄ (x = 0 to 2) were prepared and the effect of heat treatment on the obtained phases was examined. Secondly, we have picked out the composition of ZnMnGaO₄, in which two distinct types of rectangular nanorods with different compositions spontaneously interlace to form a cross-sectional checkerboard pattern. To confirm the effect of nanostructure on thermal transport properties, the room-temperature thermal conductivity of this nanostructured oxide was evaluated.     

Photoelectrochemical Study of Nanostructured ZnO Thin Films for Hydrogen Generation from Water Splitting

Photoelectrochemical cells based on traditional and nanostructured ZnO thin films are investigated for hydrogen generation from water splitting. The ZnO thin films are fabricated using three different deposition geometries: normal pulsed laser deposition, pulsed laser oblique‐angle deposition, and electron‐beam glancing‐angle deposition. The nanostructured films are characterized by scanning electron microscopy, X‐ray diffraction, UV‐vis spectroscopy and photoelectrochemical techniques. Normal pulsed laser deposition produces dense thin films with ca. 200 nm grain sizes, while oblique‐angle deposition produces nanoplatelets with a fishscale morphology and individual features measuring ca. 900 by 450 nm on average. In contrast, glancing‐angle deposition generates a highly porous, interconnected network of spherical nanoparticles of 15–40 nm diameter. Mott‐Schottky plots show the flat band potential of pulsed laser deposition, oblique‐angle deposition, and glancing‐angle deposition samples to be ?0.29, ?0.28 and +0.20 V, respectively. Generation of photocurrent is observed at anodic potentials and no limiting photocurrents were observed with applied potentials up to 1.3 V for all photoelectrochemical cells. The effective photon‐to‐hydrogen efficiency is found to be 0.1%, 0.2% and 0.6% for pulsed laser deposition, oblique‐angle deposition and glancing‐angle deposition samples, respectively. The photoelectrochemical properties of the three types of films are understood to be a function of porosity, crystal defect concentration, charge transport properties and space charge layer characteristics.     

Site‐Selective Deposition of Nanostructured ZnO Thin Films from Solutions Containing Polyvinylpyrrolidone

When soluble zinc salts are hydrolyzed in water, usually elongated micrometer‐sized zincite crystals are formed. In this study, polyvinylpyrrolidone (PVP) in a methanolic solution is used as an agent to control the morphology of the deposition product. It prevents crystal growth and yields zinc oxide nanocrystals. Thin films consisting of zinc oxide nanocrystals are formed on self‐assembled monolayers (SAMs) of sulfonate‐terminated alkylsiloxanes. Patterned films are deposited after local decomposition of the SAM by UV irradiation. The films fabricated from methanolic solutions containing PVP are particularly smooth, uniform and stable. Their thickness is determined by the deposition time and the molar ratio [PVP]:[Zn²⁺], so that films of arbitrary thickness and nearly constant roughness can be obtained. The crystal grains are oriented preferentially with 〈001〉 direction perpendicular to the substrate surface. The films show ultraviolet, orange‐red and green‐yellow photoluminescence; the latter is quenched by heat treatment. Based on the obtained experimental results, a deposition mechanism is suggested.     

Enhanced Ionic Conductivity in Nanostructured,Heavily Doped Ceria Ceramics

The electrical properties of nanostructured, heavily yttria‐ or samaria‐doped ceria ceramics are studied as a function of grain size using electrochemical impedance spectroscopy (EIS). A remarkable enhancement in the total ionic conductivity of about one order of magnitude is found in nanostructured samples, compared with the intrinsic bulk conductivity of conventional microcrystalline ceramics. This effect is attributed to the predominance of grain‐boundary conduction in the nanostructured materials, coupled with an increase in the grain‐boundary ionic diffusivity with decreasing grain size.     

Facile Fabrication and Integration of Patterned Nanostructured TiO2 for Microsystems Applications

A simple technique to fabricate integrated crack‐free and crystalline nanostructured titania (ns‐titania) in microsystems devices is presented. In this technique, crack elimination is achieved by oxidizing Ti films, pre‐patterned below a threshold dimension, in aqueous hydrogen peroxide solution. Amorphous ns‐titania with walls of pores having thicknesses and pore diameters ranging from 25 nm–50 nm and 50 nm–200 nm, respectively, is formed after oxidation and transformed to anatase after thermal annealing. We demonstrate the functionality of ns‐titania formed and compatibility of this technique with microsystems device manufacturing practices by fabricating a prototype device for gas sensing using integrated ns‐titania features as sensing elements.     

Nanostructured ZnO gas sensors obtained by green method and combustion technique

The present study reports on the synthesis of nano ZnO using two different processes i.e., biological (green synthesis) and chemical synthesis (solution combustion technique). The prepared materials were characterized by microscopy and spectroscopy techniques, such as Field emission scanning electron microscope (FE-SEM), UV–visible spectroscopy, X-Ray Diffraction (XRD) and Fourier Transform-Infrared (FT-IR) spectroscopy. The two synthesized materials were analyzed and their liquefied petroleum gas (LPG) sensing properties to their sensing characteristics were compared, to highlight the suitable one for chemiresistor. The dynamic gas sensing analysis, sensitivity and resistance were studied. For optimization, sensing characterization was monitored at various operating temperatures in different LPG concentration. Eventually, we found out that the green synthesis route, to fabricate sensor devices is more advantageous as it is cost-effective, eco-friendly and simple.     

Electrodeposited Zinc Oxide/Phthalocyanine Films – An Inorganic/Organic Hybrid System with Highly Variable Composition

Hybrid films of zinc oxide (ZnO) and tetrasulfonated nickel phthalocyanine (TSPcNi) have been electrodeposited by cathodic electrodeposition from aqueous O₂‐saturated solutions of ZnCl₂ and TSPcNi. The TSPcNi content of the films can be varied in a wide range by variation of dye concentration in the electrodeposition bath – from single TSPcNi molecules embedded in compact crystalline ZnO to films based on an amorphous TSPcNi framework. With increasing dye content the colour of the films changes from green, indicating the presence of the TSPcNi monomers, to blue, caused by TSPcNi aggregates. At the same time, changes in the electrical and photoelectrical properties of the films are observed, enabling the tuning of these properties in view of optoelectronic applications.     

退火处理对氧化锌透明导电薄膜的结构及电学性能的影响

采用真空蒸发技术，以醋酸锌和氯化铝作为蒸发物质，在加热的玻璃衬底上制备出氧化锌和铝掺杂的氧化锌透明导电薄膜。研究了氢气、空气和真空退火处理对制备薄膜的结构及电学性能的影响。     

A Three‐Dimensional and Sensitive Bioassay Based on Nanostructured Quartz Combined with Viral Nanoparticles

An effective mask‐free method for fabricating high‐aspect‐ratio pillarlike nanostructures over a large area of a quartz surface via a simple O₂ and CF₄ two‐step reactive ion etching (RIE) procedure is developed. The nanostructured quartz surfaces are successfully combined with the engineered viral particles derived from hepatitis B virus capsid, yielding a novel 3D assay system with attomolar sensitivity, which has great potential for use in sensitive and early detection of various disease markers.     

Spatially Graded Nanostructured Chiral Films as Tunable Circular Polarizers

In this paper, we introduce a simple single‐step method for creating spatially graded helical nanostructured thin films. The films mimic some of the interesting polarization and coloration properties found in nature and enhance the application prospects for helically structured thin films. Our helical nanostructures, fabricated using a variant of the glancing angle deposition technique (GLAD), are spatially graded with thicknesses that vary by several microns across substrate lengths of several tens of centimeters. These thickness gradations are predicted by simulation and verified by scanning electron microscopy (SEM). The resultant films act as Bragg multilayers and can be employed as optical filters which not only preferentially transmit one handedness of circularly polarized light, but also allow for spatially determined frequency tunability. Through spectroscopic measurements, we demonstrate that when appropriate deposition conditions are chosen these nanostructures exhibit strong polarization selectivity, concurrent with excellent frequency tunability. The preferentially transmitted peak wavelength can be changed from approximately 620 to 690 nm by translating the film over a spatial distance of 30 mm. These graded nanostructures can be incorporated into photonic and sensing devices. The graded helical nanostructures may also be useful for providing a graded scaffolding to support liquid crystals.     

Nanostructured CuO: Facile synthesis,optical absorption and defect dependent electrical conductivity

Nanostructured CuO with different average crystallite sizes in the range ~11–48 nm is synthesized by thermolysis of carbonate precursor at different temperatures. Structural characterization of the samples is done using X-ray powder Diffraction and Transmission Electron Microscopy techniques. Analysis of UV–Visible absorption spectra of the samples reveal direct band gaps in the range 1.49–1.79 eV which is blue shifted in comparison with that corresponding to single crystalline CuO. DC electrical conductivity of the samples is found to increase with increase in crystallite size/decomposition temperature. The conduction mechanism is found to be defect dependent with holes associated with uncompensated Cu²⁺ vacancies being the charge carriers. A comparison of the DC electrical conductivities in vacuum and air ambience reveal that there is a decrease in the concentration of O^2- vacancies with increase in the decomposition temperature. X-ray photoelectron spectral studies confirm an increase in the concentration of Cu³⁺ ions in samples with higher electrical conductivity. Analysis of Raman spectra indicates a decrease in the concentration of O^2- vacancies with increase in the decomposition temperature confirming the proposed role of Cu²⁺ and O^2- vacancies in determining the electrical conductivity.     

Second Generation Nanostructured Metal Oxide Matrices to Increase the Thermal Stability of CO and NO2 Sensing Layers Based on Iron(II) Phthalocyanine

An iron(II) phthalocyanine (FePc) complex solubilized by decylamine (DA) and benzylamine (BA) is incorporated into a nanoparticulate metal oxide matrix to develop optical sensor films sensitive to NO₂ and CO. Eleven amine solvents have been tested as N‐donor ligands that permit ligand exchange with the gas molecules. We have systematically investigated the suitability of different N‐donor ligands, studied the thermal stability of the NO₂‐ and CO‐sensing films at 4, 25, 60, and 80 °C by photometry, and corroborated our findings by using NMR experiments. A satisfactory thermal stability of the films has not been obtained for chemically unmodified nanoparticulate metal oxide matrices. We have therefore developed a second generation of nanostructured metal oxide supports that show increased thermal stability and adequate sensitivity to NO₂ and CO. These novel nanostructured matrices have been chemically modified using amines, alumina oligomers, and/or anti‐gas‐fading agents. These components have been integrated into the metal oxide matrices to avoid degradation of the optical films and to preserve their sensitivity.     

Well‐Aligned ZnO Nanorods via Hydrogen Treatment of ZnO Films

The formation of well‐aligned ZnO nanorods has been achieved via H₂ treatment of as‐grown ZnO films. Structural analyses reveal that the ZnO nanorods on the ZnO films are preferentially oriented along the c‐axis direction and exhibit a single‐crystalline wurtzite structure. To investigate the mechanism of formation of ZnO nanorods on the film, further H₂ treatment of the as‐grown ZnO nanorods was performed. Thinner and longer ZnO nanorods were obtained after certain periods of H₂ treatment. It is proposed that both etching and re‐deposition processes are taking place during the process, resulting in the aspect‐ratio enhancement of the ZnO nanorods and the formation of ZnO nanorods on the ZnO films. It is suggested that an appropriate concentration of the etching products remaining from the initial rod‐forming H₂ treatment allows subsequent re‐deposition of the ZnO nanorods with enhanced differentiation of the growth rates on the 〈001〉 and 〈100〉 crystal facets.     

Selective Angle Electroluminescence of Light‐Emitting Diodes based on Nanostructured ZnO/GaN Heterojunctions

Selective angle electroluminescence of violet light with a peak wavelength of 405 nm from light‐emitting diodes based on nanostructured p‐GaN/ZnO heterojunctions is reported. The fabrication of well‐aligned nanobottles with excellent crystalline quality is achieved by chemical vapor deposition at temperatures as low as 450 °C with a specially designed upside‐down arrangement of substrate configuration. Selective angle light sources are essential in our daily life. With the geometry of the nanobottle waveguides, it is very easy to realize such a practical application. Therefore, the discovery reported here should be very useful for the future development of many unique optoelectronic devices.     

Nanostructured Titanium Oxynitride Porous Thin Films as Efficient Visible‐Active Photocatalysts

Nanocrystalline mesoporous N‐doped titania films have been prepared for the first time. The introduction of nitrogen into the anatase structure starts at 500 °C, with N bonding to titanium via oxygen substitution. Increasing the treatment temperature leads to the formation of TiN (TiN_1–xO_x) and N‐doped rutile showing mixed‐valence Ti states. Microstructural characterization shows that the ordered mesoporosity is maintained until 700 °C, where TiN (TiN_1–xO_x) begins to form. Optical characterization shows that the discrete introduction of N is able to shift the titania absorption edge. The photocatalytic tests give the best results under visible light excitation for the film nitrided at 500 °C. At this temperature the concentration of nitrogen in the structure is optimal since oxygen vacancies are still not important enough to promote the recombination of the photogenerated electrons and holes.     

Functional Free‐Standing Graphene Honeycomb Films

Fabricating free‐standing, three‐dimensional (3D) ordered porous graphene structure can service a wide range of functional materials such as environmentally friendly materials for antibacterial medical applications and efficient solar harvesting devices. A scalable solution processable strategy is developed to create such free‐standing hierarchical porous structures composed of functionalized graphene sheets via an “on water spreading” method. The free‐standing film shows a large area uniform honeycomb structure and can be transferred onto any substrate of interest. The graphene‐based free‐standing honeycomb films exhibit superior broad spectrum antibacterial activity as confirmed using green fluorescent protein labeled Pseudomonas aeruginosa PAO1 and Escherichia coli as model pathogens. Functional nanoparticles such as titanium dioxide (TiO₂) nanoparticles can be easily introduced into conductive graphene‐based scaffolds by premixing. The formed composite honeycomb film electrode shows a fast, stable, and completely reversible photocurrent response accompanying each switch‐on and switch‐off event. The graphene‐based honeycomb scaffold enhances the light‐harvesting efficiency and improves the photoelectric conversion behavior; the photocurrent of the composite film is about two times as high as that of the pure TiO₂ film electrode. Such composite porous films combining remarkably good electrochemical performance of graphene, a large electrode/electrolyte contact area, and excellent stability during the photo‐conversion process hold promise for further applications in water treatment and solar energy conversion.     

Growth and Characterization of Zn-Incorporated Copper Oxide Films

In this work, undoped and Zn-doped copper oxide films were deposited on glass substrates at a substrate temperature of 250 ± 5°C by using an ultrasonic spray pyrolysis technique. Electrical, optical, and structural properties of the films were investigated, and the effect of Zn incorporation on these properties are presented. The variations of electrical conductivities and electrical conduction mechanisms of all films were investigated in the dark and in the light. Optical properties of the produced films were analyzed by transmission, linear absorption coefficient, and reflection spectra. The band gaps of the films were determined by an optical method. The film structures were studied by x-ray diffraction. To obtain information about structural properties in detail, the grain size (D), dislocation density (δ), and lattice parameters for preferential orientations were calculated. The elemental analyses were performed using energy-dispersive x-ray spectroscopy. It was concluded that Zn has a strong effect, especially on the electrical and structural properties, and the undoped and Zn-doped copper oxide (at 3%) films may be used as absorbing layers in solar cells due to their low resistivities and suitable linear absorption coefficient values.