Growth, structure and electrical properties of tungsten oxide nanorods

Tungsten trioxide has shown good sensing properties towards various gases. Recently thin nanostructured WO₃ films have been tested. Due to their large surface area to volume ratio they exhibit good sensitivity depending on the grain size. However in conventional WO₃ thin films the average grain size exceeds the thickness of the surface space charge layer, so the electrical conduction is mainly controlled by the carriers transport across the grain boundaries. An alternative way seems to be in a monocrystalline material with nanometric dimensions. Our objective is to fabricate nanosized tungsten oxide rods and to test their sensing properties under gas adsorption. In this work, we focus on the growth, the structure and the electrical properties of tungsten nanorods. The tungsten oxide nanorods were grown by vapour transport from a WO₃ layer onto a substrate (Mica). The nanorods growth was controlled by the temperature gradient between the WO₃ layer and the substrate. Their morphology was investigated by AFM and their structure by TED and TEM. We have investigated the conductivity of the WO₃ nanorods with a technique derived from Atomic Force Microscopy operating in contact mode with a conductive tip (Resiscope).     

Growth, structure and electrical conduction of WO3 nanorods

We present a very simple method to obtain tungsten trioxide nanorods. The nanorods are epitaxially grown on a mica substrate in low supersaturation conditions. Investigations of morphology, crystallographic structure and chemical composition of the nanorods allow us to propose a growth model in which the potassium ions of the substrate play a major role inducing the one-dimensional structure. The nanorod growth is initiated by the formation of a hexagonal tungsten bronze (HTB) epitaxially oriented on the mica. By using a conductive atomic force microscopy technique, we characterise the electrical conduction of WO₃ networks.     

Nanostructure evolution in cluster-assembled WO x films synthesized by radio-frequency assisted laser ablation

We deposited WO _x films by radio frequency assisted pulsed laser deposition (RF-PLD). We explored the roles of substrate temperature and ambient gas pressure-composition on film nanostructure to maximise the film surface area in view of gas sensing applications. A W target was ablated in highly reactive O₂ at 5 Pa and 700 Pa, as well as in a mixed oxygen-helium atmosphere at 700 Pa. Corning glass was used as the substrate, at increasing temperatures. Process parameters such as laser fluence and wavelength, radio-frequency power, target to substrate distance were kept fixed. The influence of the deposition parameters on roughness, morphology, nanostructure, and bond coordination of the deposited films were studied by Atomic Force Microscopy, Scanning Electron Microscopy, Transmission Electron Microscopy, and micro-Raman spectroscopy. Nanoparticle formation, the development of an extended, open nanostructure and its evolution toward compact films are discussed.     

Structure and electrical properties of tungsten oxide nanorods epitaxially organized on a mica substrate

Our objective was to fabricate nanosized tungsten oxide rods and to test their sensing properties. In the present report, we focus on the crystallographical structure and the electrical properties of tungsten nanorods. The tungsten oxide nanorods were grown by vapor transport from a WO₃ layer onto a substrate (Mica). The nanorods growth was controlled by the temperature gradient between the WO₃ layer and the substrate. Their morphology was investigated by AFM and their structure by TED and TEM. We have investigated the conductivity of the WO₃ nanorods with a technique derived from atomic force microscopy operating in contact mode with a conductive tip (C-AFM). Its provides at the same time a classical topographic image of the sample surface and an image representative of the local electrical resistance between the tip and a metallic contact on the substrate. We also investigated the electrical properties of the WO₃ nanorods by the current-voltage responses in a bias range of 0 ± 1 V. We have performed experiments in an environmental chamber and characterized the role of water vapor on the electrical conductivity of WO₃ nanorods.     

Comparative study of Pt, Au and Ag growth on Fe3O4(0 0 1) surface

The epitaxial growth of Pt, Au and Ag layers on Fe₃O₄(0 0 1) as a function of temperature and thickness have been studied. The layers were deposited by sputtering in an ultra high vacuum chamber and the structural properties were investigated by Reflection High Energy Electron Diffraction, X-ray reflectivity and diffraction, High Resolution Transmission Electron Microscopy and Atomic Force Microscopy. Our studies give evidence for three different growth behaviours depending both on the nature of the metals and the temperature. Comparison between the growth modes of the three metals will be discussed in relation with surface and interfaces energies.     

From anisotropic dots to smooth RFe 2 (110) single crystal layers (R = rare earth)

Single crystal RFe₂(110) films were grown by molecular beam epitaxy to a total thickness of 1000 ? at different substrate temperatures ranging from 450 ^° C to 660 ^° C. The first stages of growth and the surface morphology of the deposited layers have been studied using Reflection High Energy Electron Diffraction (RHEED) and Atomic Force Microscopy (AFM). The growth is first strained but further deposit induces the formation of three-dimensional fully relaxed islands. Subsequently, the morphology of the RFe₂(110) nanosystems evolves from anisotropic dots to a smooth surface, as a function of the preparation parameters, i.e. nominal thickness and substrate temperature. It also depends on the rare earth involved in the compound. Received 29 June 2000     

Advantages and limitations of OM, SEM, TEM and AFM in the study of ancient decorated pottery

This paper presents results from the study of two fragments of pre-Hispanic pottery, decorated with red pigment, using Optical Microscopy (OM), Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HRTEM), Atomic Force Microscopy (AFM) and Magnetic Force Microscopy (MFM). Capabilities and limitations of these techniques in the analysis of archaeological material are highlighted with special emphasis on TEM, AFM and MFM due to their contribution in the study of the pigment layer at micro and nano scale. The analyzed samples come from the archaeological sites of El Tajin and Xochicalco, both in Mexico. Results of conventional TEM and HRTEM analysis of the red pigment showed nanometric Fe₂O₃ particles in both samples but different particle shape and size distributions: specimen from El Tajin presented irregular particles between 50–100 nm while that from Xochicalco exhibited semispherical shapes in the 3–25 nm range. AFM images showed the topography of the pigments, which are related to the texture of their surface and thus to the production process. Finally, MFM showed different contrast regions suggesting the presence of ferromagnetic elements forming clusters and domain orientations on the color layer.     

Aqueous chemical growth of free standing vertical ZnO nanoprisms,nanorods and nanodiskettes with improved texture co-efficient and tunable size uniformity

Tuning the morphology, size and aspect ratio of free standing ZnO nanostructured arrays by a simple hydrothermal method is reported. Pre-coated ZnO seed layers of two different thicknesses (≈350 nm or 550 nm) were used as substrates to grow ZnO nanostructures for the study. Various parameters such as chemical ambience, pH of the solution, strength of the Zn²⁺ atoms and thickness of seed bed are varied to analyze their effects on the resultant ZnO nanostructures. Vertically oriented hexagonal nanorods, multi-angular nanorods, hexagonal diskette and popcorn-like nanostructures are obtained by altering the experimental parameters. All the produced nanostructures were analysed by X-ray powder diffraction analysis and found to be grown in the (002) orientation of wurtzite ZnO. The texture co-efficient of ZnO layer was improved by combining a thick seed layer with higher cationic strength. Surface morphological studies reveal various nanostructures such as nanorods, diskettes and popcorn-like structures based on various preparation conditions. The optical property of the closest packed nanorods array was recorded by UV-VIS spectrometry, and the band gap value simulated from the results reflect the near characteristic band gap of ZnO. The surface roughness profile taken from the Atomic Force Microscopy reveals a roughness of less than 320 nm.     

Morphology and orientational disorder of C70 single crystals

C₇₀ single crystals, free from solvent contamination, were grown via vapor phase transport technique. The (0001) face of the C₇₀ crystal was imaged with Atomic Force Microscopy (AFM) under ambient atmosphere. Low-resolution images of a freshly prepared crystal revealed well-ordered faceted regions with multiple terraces extending over a region of several hundred square nanometers. The terraced structure in the images indicates that the crystals formed as a layer growth. In the high-resolution images, C₇₀ molecules were resolved in a hexagonal close packed (hcp) structure with an average center-to-center distance of 10.5±0.5 Å. After one month's exposure to the ambient atmosphere, the molecules rearranged into a mixed structure consisting of hcp and face centered cubic (fcc) regions.     

Controlled synthesis and luminescence properties of LaPO4:Eu phosphors

Uniform single crystal LaPO₄ phosphors were selectively synthesized using a facile hydrothermal method without the aid of templates or catalysts. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectra were used to characterize the nanocrystal materials. The morphological evolution from nanorods to nanoparticles was brought about by altering the pH. In addition, the crystal structure was changed from hexagonal phase to monoclinic one by increasing the hydrothermal temperature. We propose a possible growth mechanism for LaPO₄ nanorods based on the results of our analyses. Furthermore, we compared the photoluminescence properties of LaPO₄:Eu with different morphologies. The results showed that the fluorescence intensity of monoclinic LaPO₄:Eu nanorods is stronger than that of nanoparticles.     

氘化及氦离子注入对钪膜的表面形貌和相结构的影响

采用XRD, SEM, AFM等详细研究了氘化及氦离子注入对钪膜的表面形貌和相结构的影响. 结果表明,在单晶硅及抛光Mo基片上制备的钪膜均具有(002)晶面择优取向;钪膜氘化后表面会出现大量孔洞, 氘化后氘化钪(ScD₂)晶粒长大,但内部会残留少量未完全氘化反应的晶粒尺寸较小的 ScD_0.33/Sc晶粒;氦离子注入对钪及氘化钪的表面形貌没有明显影响, 离子注入的氦将在钪及氘化钪晶格中聚集成泡,导致氦离子注入层中的钪及氘化钪衍射峰向低角度偏移, 并且氦泡的聚集具有择优取向性.     

X-PEEM/NEXAFS and AFM of polypyrrole and copper micro-patterns on insulating fluoropolymer substrates

Micro-patterns (80 μm and 10 μm) of copper and semi-conducting polypyrrole on insulating fluorinated ethylene propylene substrates were characterized using synchrotron-based X-ray Photoemission Electron Microscopy (X-PEEM), Near Edge X-ray Absorption Fine Structure (NEXAFS), and Atomic Force Microscopy (AFM). Electronic states in the polypyrrole are verified using the NEXAFS data, and sample degradation upon irradiation is addressed. X-PEEM images show homogeneous distributions of the corresponding elements in the patterns. They do not exhibit dichroic effects and give information about the growth of copper and polypyrrole (i.e. nucleation of Cu, overgrowth of PPy, formation of PPy granules). AFM results are used to verify the topography of the patterns and support the findings on pattern growth.     

Electrodeposition of diamond-like carbon (DLC) films on Ti

Diamond-like carbon films (DLC) were deposited on titanium substrates in acetonitrile and N,N-dimethyl formamide (DMF) liquids by the liquid-phase electrodeposition technique at ambient pressure and temperature. The applied voltage between the electrodes was high (1200 V) due to the use of resistive organic liquids. The surface morphology was examined by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Corrosion performance of the coatings was investigated by potentiodynamic polararization tests in phosphate buffer saline solution. Raman spectroscopy analysis of the films revealed two broad bands at approximately 1360 cm⁻¹ and 1580 cm⁻¹, related to D and G-band of DLC, respectively. The coated Ti was tested in a ball-on-plate type wear test machine with Al₂O₃ balls. The films presented a low friction coefficient (about 0.1), and the films deposited from DMF presented the best wear resistance.     

One-step,template-free hydrothermal synthesis of CuO tetrapods

The present investigation reports one-step template-free hydrothermal synthesis of CuO tetrapods (CuO-T) and its characterization. The CuO tetrapods have been prepared in moderate condition without using any surfactant. The prepared sample was characterized by powder X-ray diffraction (PXRD), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR) spectroscopy, UV–vis–NIR (DRS) spectroscopy and Photoluminescence (PL) spectroscopy. The X-ray diffraction and Fourier transform infrared spectrum analysis confirm clearly the formation of a pure phase high-quality CuO with monoclinic crystal structure. X-ray peak broadening analysis was used to evaluate the average crystallite size (∼30 nm) and lattice strain by the Williamson–Hall (W–H) method. Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) result reveals that the particles are tetrapods in shape with an average length ∼50 nm. Additionally the optical properties were investigated by using UV–vis reflectance spectra with considerable blue-shift in the optical band gap (Eg = 1.45 eV) due to quantum confinement effect. Photoluminescence (PL) spectrum showed both UV as well as visible emission peaks indicating their good optical properties.     

Observation of the mica surface by atomic force microscopy

Freshly cleaved mica and a mica surface treated with pure water and dilute-salt solution have been investigated by Atomic Force Microscopy (AFM). On the bare mica surface (after repeated scanning), small dots and islands were observed. The disappearance of these dots and islands has also been captured by AFM. We believe these structures to be condensed water. The water meniscus between AFM tip and mica surface is considered as the source of this water structure. On the mica surface treated with pure water and dilute-salt solution, network structures are frequently observed by AFM.     

Deposition of Na2WO4 films by ultrasonic spray pyrolysis: effect of thickness on the crystallographic and sensing properties

Na₂WO₄ films have been grown on Si (1 0 0) and glass substrate using ultrasonic spray pyrolysis. The films are prepared from aqueous solution containing Na₂WO₄·2H₂O at 475 °C temperature and characterized by XRD and SEM techniques and the chemical composition of the films have been verified by EDX and PIXE and its formula Na₂WO₄ is confirmed by XRD. The evolution of the crystallinity was studied as a function of film thickness ranging from 2500 to 4200 nm, which corresponds to a deposition time from 10 to 30 min, respectively. The crystalline quality was found to improve, where the grain size values increased with increasing thickness. Atomic Force Microscopy (AFM) was used to study the morphology evolution with the deposition time, where porous films were found due to the synthesis parameters, and a better sensing response to gases was developed with increasing thickness. Thus, this study demonstrates the possibility of utilizing Na₂WO₄ thick films as a sensor element for the detection of ethanol vapor at room temperature, where thicker films exhibit excellent ethanol vapor sensing properties with a maximum sensitivity at 25 °C in air atmosphere with fast response time.     

Polar properties of hydrothermally synthesized BiFeO3 thin films

Multiferroic bismuth ferrite (BiFeO₃) has attracted considerable attention due to applications related to the bulk photovoltaic effect in which the direction of polarization determines the direction of the photocurrent. Epitaxial thin films are produced by means of techniques that usually require high temperature processes. The hydrothermal method can be seen as an alternative route to obtain highly textured thin films in quantities compatible with batch processing; nevertheless, the structural, dielectric and electric properties are generally affected by the presence of hydrogen and other reaction by-products. In this work, functional and highly textured BiFeO₃ films were successfully produced on metallic SrTiO₃:Nb (0.5 wt.%) (100) substrates via hydrothermal synthesis. X-ray diffraction (XRD) and Atomic Force Microscopy (AFM) were used to analyze the structural properties of the films. Piezoresponse Force Microscopy (PFM) and Photoconductive Atomic Force Microscopy (Pc-AFM) were used to determine their functional properties. We show the polarization switching and confirm the presence of the bulk photovoltaic effect for the first time in hydrothermally synthesized BiFeO₃.     

The effect of low temperature AlN interlayers on the growth of GaN epilayer on Si (111) by MOCVD

Low temperature (LT) AlN interlayers were used to effectively reduce the tension stress and micro-cracks on the surface of the GaN epilayer grown on Si (111) substrate. Optical Microscopy (OM), Atomic Force Microscopy (AFM), Surface Electron Microscopy (SEM) and X-Ray Diffraction (XRD) were employed to characterize these samples grown by metal-organic chemical vapor deposition (MOCVD). In addition, wet etching method was used to evaluate the defect of the GaN epilayer. The results demonstrate that the morphology and crystalline properties of the GaN epilayer strongly depend on the thickness, interlayer number and growth temperature of the LT AlN interlayer. With the optimized LT AlN interlayer structures, high quality GaN epilayers with a low crack density can be obtained.     

Electrical conductivity dependence of thin metallic films of Au and Pd as a top electrode in capacitor applications

Electrical conductivity dependence of thin metallic films of Au and Pd over the different perovskites was investigated. It is found from electrical properties that crystallographic growth orientation of Au and Pd thin layers attained from X-ray diffraction results indicate the slop of current (I)-voltage (V) plots. Besides, surface morphology and topography was considered using Field Emission Scanning Electron Microscopy and Atomic Force Microscopy, respectively. Obtained results showed the Stranski-Krastanov growth of the Pd and Au. Indeed, diminishing of the root-mean-square roughness of Pd/BiMnO₃/SrTiO₃ following by Au deposition should be concerned due to growth of Au onto the crack-like parts of the substrate. These crack-like parts appeared due to parasitic phases of the Bi-Mn-O system mainly Mn₃O₄ (l 0 l) and Mn₃O₄ (0 0 4 l).The different response in the electrical properties of heterostructures suggests that electrical conductance of the Au and Pd thin metallic films have the crystallographic orientation dependence. Furthermore, polycrystallinity of the thin metallic films are desired in electrode applications due to increase the conductivity of the metallic layers.     

Conversion Electron Mössbauer Study of Mixing Induced by Swift Heavy Ions at the Fe/Si Interface

Conversion Electron Mössbauer Spectroscopy (CEMS) was employed in order to study the mixing induced by swift Au and Ag ions at the Fe/Si interface in the Fe/Fe⁵⁷/Si system. An increase in the amount of mixing with ion fluence and electronic energy loss (S _e) has been observed. Atomic Force Microscopy (AFM) results provided indirect evidence to support the above observations.