Showing papers on "Evaporation (deposition) published in 2006"

Kinetically driven self assembly of highly ordered nanoparticle monolayers

Abstract: When a drop of a colloidal solution of nanoparticles dries on a surface, it leaves behind coffee-stain-like rings of material with lace-like patterns or clumps of particles in the interior. These non-uniform mass distributions are manifestations of far-from-equilibrium effects, such as fluid flows and solvent fluctuations during late-stage drying. However, recently a strikingly different drying regime promising highly uniform, long-range-ordered nanocrystal monolayers has been found. Here we make direct, real-time and real-space observations of nanocrystal self-assembly to reveal the mechanism. We show how the morphology of drop-deposited nanoparticle films is controlled by evaporation kinetics and particle interactions with the liquid-air interface. In the presence of an attractive particle-interface interaction, rapid early-stage evaporation dynamically produces a two-dimensional solution of nanoparticles at the liquid-air interface, from which nanoparticle islands nucleate and grow. This self-assembly mechanism produces monolayers with exceptional long-range ordering that are compact over macroscopic areas, despite the far-from-equilibrium evaporation process. This new drop-drying regime is simple, robust and scalable, is insensitive to the substrate material and topography, and has a strong preference for forming monolayer films. As such, it stands out as an excellent candidate for the fabrication of technologically important ultra thin film materials for sensors, optical devices and magnetic storage media.

Aligned Ultralong ZnO Nanobelts and Their Enhanced Field Emission

Abstract: One-dimensional (1D) semiconducting nanoscale materials have attracted considerable attention because of their importance in understanding the fundamental properties of low dimensionality in materials as well as in nanodevice applications. Many methods, including vapor–liquid–solid (VLS), vapor–solid (VS), and solution-based, have been developed to synthesize 1D semiconducting nanoscale materials such as nanoscale wires, belts, rods, tubes, and needles. Usually, these methods require templates/catalysts and tedious operational procedures. Here, we demonstrate a new strategy for the growth of aligned ultralong ZnO nanobelts, yielding an average length of 3.3 mm and widths up to 6 lm, on metal substrates in a one-step process via molten-salt-assisted template-free thermal evaporation. These ultralong nanobelts show enhanced field emission. The electric field for an emission current density of 1 mA cm is 2.9 V lm, the lowest value ever reported for pure 1D ZnO nanostructures grown on flat surfaces, corresponding to a field-enhancement factor of about 1.4 × 10. This approach is simple, efficient, and inexpensive, which significantly facilitates device fabrication. By combining a general molten-salt process, which is usually used to prepare micrometer-scale ceramic powders (although it was also used for the synthesis of ZnO nanorods in a thermal evaporation process), we have designed a new approach, molten-salt-assisted thermal evaporation, and we demonstrate that this approach can produce aligned ultralong ZnO nanobelts over a large area. The key point of this new approach is the evaporation of Zn metal powder in a liquid environment of molten sodium chloride (NaCl) salt. A side-view camera photograph of the as-grown ZnO nanobelts on the Au substrate is shown in Figure 1a, indicating that the nanobelts can grow to several millimeters in length. Figure 1b shows a top-view optical microscopy photograph, demonstrating that the ZnO nanobelts are also transparent under an optical microscope. A higher-magnification optical microscopy image of the side-view is shown in Figure 1c, indicating nominal, though imperfect, alignment. Figure 2 shows field-emission scanning electron microscopy (SEM) images of the as-grown ZnO nanobelts under different magnifications. The low-magnification image shown in FigC O M M U N IC A IO N

Fabrication of Cu2ZnSnS4 thin films by co-evaporation

Abstract: Cu2ZnSnS4 (CZTS) thin films were fabricated by co-evaporation of elemental sources on quartz glass substrates. The deposition was performed at the substrate temperature between 400 oC and 600 oC. From the results of electron probe microanalysis, it was confirmed that the nearly stoichiometric CZTS thin films were obtained in all substrate temperature. The X-ray diffraction patterns revealed that CZTS thin films have a kesterite structure with a strong preferred orientation. From the scanning electron microscope observation, the grain size becomes larger with increasing the substrate temperature. The CZTS thin films showed p -type conductivity. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Zinc oxide thin films prepared by thermal evaporation deposition and its photocatalytic activity

Abstract: Thin zinc oxide (ZnO) films have been grown on silicon substrates by thermal physical vapor deposition approach. X-ray diffraction (XRD) analyses reveal that the deposited films are polycrystalline ZnO phase. Atomic force microscopy images (AFM) show needle-like shape highly oriented ZnO crystals. Thin film thickness ranges from 10 to 80 nm. X-ray photoelectron spectroscopy (XPS) results declare that the films compose mainly of Zn and O. Nevertheless, Si is not detected in the films and consequently no possibility of any silicide formation as is confirmed by XRD analysis. Photocatalytic decomposition of azo-reactive dye on ZnO films is tested. The results show that the dye decomposition efficiency increases with decreasing pH. Maximum photodecomposition, 99.6% is obtained at pH 2 with 10 mg/l dye concentration.

Method and apparatus for reducing particle contamination in a deposition system

Abstract: A method and system is described for reducing particle contamination of a substrate in a deposition system. The deposition system comprises one or more particle diffusers disposed therein and configured to prevent or partially prevent the passage of film precursor particles, or break-up or partially break-up film precursor particles. The particle diffuser may be installed in the film precursor evaporation system, or the vapor delivery system, or the vapor distribution system, or two or more thereof.

Influence of the water content in dental enamel and dentin on ablation with erbium YAG and erbium YSGG lasers.

Abstract: The theory of the ablation of dental hard tissue with erbium lasers is based on a process of thermomechanical interaction, which is explained by the absorption of the radiation in the water component of the tissue. The abrupt evaporation of the water is the cause of tissue fragments being blasted out of the tooth structure. The aim of this study is to examine the effect of the water contained in dental hard tissues on the efficiency of ablation. 192 specimens of both bovine dental enamel and bovine dentin are irradiated with an Er:YAG and an Er,Cr:YSGG laser. Half of the specimens are dehydrated beforehand. Irradiation is carried out in subgroups: without water spray and with water spray at flow rates of 0.8 and 3 mls. The ablated volume is determined following histological preparation. Only in dentin, and then only with irradiation with the Er:YAG laser, is the water contained in the tissue found to have a significant influence (p < 0.0001) on the ablated volume. The water content has no effect on the efficiency of laser ablation in any of the other test groups. In contrast, the externally supplied water always has a significant influence on the effectiveness of the ablation process.

Simple approach to β-SiC nanowires: Synthesis, optical, and electrical properties

Abstract: High-quality β-SiC nanowires were synthesized on activated carbon fiber via a thermal evaporation method without the use of metal catalyst. Their structure and chemical composition were studied by Raman spectroscopy and high-resolution electron microscopy. Field effect transistors were fabricated to investigate the β-SiC nanowire electrical behavior possessing n-channel characterization. The carrier mobility of the devices was 15.9cm2∕Vs when the Vds is 0.01V. This result contributes to the development of efficient nanodevices based on β-SiC nanowires, as well as nanocomposites.

Study on synthesis and blue emission mechanism of ZnO tetrapodlike nanostructures

Abstract: Zinc oxide (ZnO) tetrapodlike nanostructures were synthesized by thermal evaporation of Zn powder at different flow rates of argon. Scanning electron microscopy, x-ray diffraction, transmission electron microscopy, and photoluminescence were employed to study the structural features and optical properties of the product. Results show that the flow rate of argon gas has a comparative great influence on the morphology of ZnO nanostructures. As the flow rate increases, nano-ZnO morphology changes from an initial mixture of tetrapod, nanowire, nanosheet, and nanodendritic forms to a uniform tetrapodlike morphology. The fact that the ultraviolet emission weakens and the blue emission strengthens with increase of the argon flow rate suggests that the oxygen vacancy is most likely responsible for the blue emission. This was confirmed by the experimental results: the blue light emission disappears after the products are oxidized in air at 700°C, and then reappears after the oxidized products are deoxidized in H2 ...

Gold clusters on oxygen plasma functionalized carbon nanotubes: XPS and TEM studies

Abstract: Oxygen plasma treated multi-walled carbon nanotubes (MWCNTs) have been decorated with gold nanoclusters by thermal evaporation. Transmission electron microscopy (TEM) shows that the nature and extent of gold coverage can be varied by simultaneously changing the parameters used for the plasma treatment and the gold evaporation time. The evaporated gold clusters on oxygen plasma treated MWCNTs have a more dense distribution than the clusters evaporated on as-synthesized MWCNTs. Analyses of the valence band and the core levels by x-ray photoelectron spectroscopy (XPS) suggest poor charge transfer between the gold clusters and the MWCNTs.

The Anomalous Infrared Transmission of Gold Films on Two‐Dimensional Colloidal Crystals

Abstract: Tailoring optical response using periodic nanostructures is one of the key issues in the current research on functional composite materials. The anomalous light transmission through metallic films that have a regular array of submicrometer holes has stimulated much interest. This interest stems from both the underlying physics and also the perceived potential for applications in nanophotonics, quantum-information processing, nanolithography, and surface-enhanced Raman scattering. Extraordinary transmission of light through an optically opaque metal film perforated with a 2D array of subwavelength holes was first reported by Ebbesen et al. This unusual phenomenon can be understood as a result of diffractive coupling to evanescent surface plasmon polaritons (SPPs) that leads to a strong concentration of light at the metal surface, which then weakly tunnels through the holes in the film, reradiating by the inverse process on the exit side. In order to explore the SPP properties of microstructured metal films, extensive efforts have been made to study their spectral response and dependence on geometrical parameters, such as the type of lattice symmetry, metal film thickness, and adjacent dielectric media. Recent studies show that the hole shape has a significant effect on the optical transmission. Nearly all the metallic films studied have been on a flat substrate and the hole arrays were made using focused ion-beam milling, and electron-beam lithography or interferometric lithography combined with reactive ion etching. Here we use nanosphere lithography as the sample production technique. This approach has several advantages over the conventional lithographic and machining techniques, including the relative ease of casting large, high-quality, ordered nanomaterials and the low cost of implementation. Ordered arrays of gold half shells and nanocaps have been constructed by controlled gold vapor deposition with thicknesses less than 20 nm by using a 2D colloidal crystal (CC) as a substrate. Baumberg’s group has fabricated metallic nanocavity arrays by electrodeposition within the pores of CC templates and observed the excitation of the SPPs in metallic cavities that led to rich features in reflectivity spectra. Very recently, Landstrom et al. have shown that the transmission spectra through a metal film formed on a 2D CC substrate are quite similar to those observed through subwavelength hole arrays in metal films. In this communication, we report a study on the infrared transmission properties of gold films patterned on 2D CCs. The fabricated metallodielectric structures have a strong surface corrugation as well as a 2D periodic pore array. We show that the SPPs on these curved surfaces display unusual dispersion properties, compared to those of metal films on flat substrates studied before. The dielectric property of the template spheres is also found to have a substantial effect on the transmission. More importantly, the transmission features vary dramatically as the gold film thickness is increased, with an apparent transition from the excitation of localized SPP resonance to extended SPP propagation at a critical metal film thickness. Our results will be useful for designing and fabricating new optical devices based on SPP excitation and this will stimulate further studies on the optical properties of metallic microstructures deposited on 2D CCs. The ordered metallic microstructures were prepared by sputtering a thin gold layer onto a monolayer of dielectric microspheres self-assembled onto a quartz chip. The 2D sphere arrays were crystallized by controlled evaporation from a colloidal solution within a channel formed using two quartz chips. The microbeads were hemispherically covered with metal and the resulting gold film consists of a hexagonally close-packed (HCP) array of gold half-shells with a size dictated by the template spheres. The diameters of these hemispherical shells can be conveniently controlled from 200 nm to several micrometers by choosing colloidal beads with different sizes. Figure 1 shows the scanning electron microscopy (SEM) image of a typical sample with a thin gold layer on a 2D silica CC substrate. In the center of the image, there is a vacancy C O M M U N IC A TI O N S

Fabrication of zinc ferrite nanocrystals by sonochemical emulsification and evaporation: observation of magnetization and its relaxation at low temperature.

Abstract: A new ultrasound assisted emulsion (consisting of rapeseed oil and aqueous solution of Zn2+ and Fe2+ acetates) and evaporation protocol has been developed for the synthesis of zinc ferrite (ZnFe2O4...

Growth of high-quality large-area MgB2 thin films by reactive evaporation

Abstract: We report a new in situ reactive deposition thin film growth technique for the production of MgB2 thin films which offers several advantages over all existing methods and is the first deposition method to enable the production of high-quality MgB2 films for real-world applications. We have used this growth method, which incorporates a rotating pocket heater, to deposit MgB2 films on a variety of substrates, including single-crystalline, polycrystalline, metallic, and semiconductor materials up to 4 inch in diameter. This technique allows growth of double-sided, large-area films in the intermediate temperature range of 400–600 °C. These films are clean, well-connected, and consistently display Tc values of 38–39 K with low resistivity and residual resistivity values. They are also robust and uncommonly stable upon exposure to atmosphere and water.

Infrared spectroscopic characterization of [2]rotaxane molecular switch tunnel junction devices.

Abstract: Langmuir−Blodgett monolayers of a bistable [2]rotaxane were prepared at packing densities of 118, 73, and 54 A^2/molecule. The monolayers were both characterized via infrared spectroscopy before and after evaporation of a 2 nm film of titanium and incorporated into molecular switch tunnel junction devices. The study suggests that the evaporation process primarily affects portions of the molecule exposed to the metal atom source. Thus, in tightly packed monolayers (73 and 54 A^2/molecule), only the portions of the [2]rotaxane that are present at the molecule/air interface are clearly affected, leaving key functionality necessary for switching intact. Monolayers transferred at a lower pressure (118 A^2/molecule) exhibit nonspecific damage and poor switching behavior following Ti deposition. These results indicate that tightly packed monolayers and sacrificial functionality displayed at the molecule/air interface are important design principles for molecular electronic devices.

Evaporation‐Induced Self‐Assembly (EISA) at Its Limit: Ultrathin, Crystalline Patterns by Templating of Micellar Monolayers

Abstract: Patterning of surfaces at the nanoscale is important in many fields of application, e.g., for electronic devices, storage media, and as a starting base for the generation of more complex structures. To exemplify the latter subject, it has been shown that a periodic surface structure with periodicity of surface energy and topology allows for the alignment of liquid crystals or the consecutive construction of channels or pillars being vertically aligned with respect to the substrate, thus making the application of strong external fields redundant. Structures in the range of 45–300 nm are predominantly fabricated by either opticalor electron-beam lithography. To reach the sub-45 nm range, self-assembled block-copolymer mesophases are employed as templates within lithographic processes. For instance, Adamson and co-workers reported on the use of block copolymers in a lithography etching process to generate dense periodic arrays of holes and dots in a silicon nitride coated silicon wafer. However, this process deals only with the structuring of a given surface, i.e., the physical properties of the final material are limited by the choice of the starting material and the restrictions of lithography. Herein, we present a straightforward approach towards a whole range of ultrathin, mesostructured metal oxide layers by sol–gel processing via a modified evaporation-induced selfassembly (EISA) approach introduced by Ogawa et al. and Brinker et al. several years ago. EISA usually provides mesostructured films with a thickness of 50–1000 nm, which therefore consist of several stacked layers (multilayers of micellar aggregates). However, it is challenging to use this well-established procedure to generate corresponding ultrathin films with ordered mesoporosity, although it is usually assumed that such fine control can only be achieved by Langmuir–Blodgett (LB) techniques. Kunitake and co-workers reported the successful preparation of ultrathin, self-supporting films of different oxides by sequential sol–gel processing using polymer underlayers and spin-coating, but the porosity was disordered and the metal oxides amorphous. In the present study, it is shown that a modified EISA procedure allows for the preparation of crack-free, ultrathin, crystalline metal oxide films with highly ordered in-plane mesostructure and a tunable film thickness on the scale of only several nanometers, even achieving the sequential formation of mesostructured metal oxides with exactly one micellar monolayer in height. Different structures (e.g., circular pores or line patterns) of the final surface layer material can be simply adjusted by varying the assembly structure of the template, and by using sufficiently small templates, the periodicity of the patterns can be downsized to about 4–5 nm. In addition, the structure is instantaneously generated on macroscopic length scales (i.e., not serial/slow). This has been beyond the reach of common lithography techniques to date. Furthermore, in contrast to physical methods such as molecular-beam epitaxy, pulsed laser deposition, and chemical vapor deposition, which are often used in combination with lithography and are known to suffer from elaborate preparation techniques, the present sol–gel approach also allows one to address more complex metal oxides (e.g., BaTiO3) by the ease of “beaker” chemistry and liquid-coating processing. For the generation of ultrathin, mesoporous metal oxide layers with periodicities in two dimensions and one dimension, respectively, we used two different types of structure-directing agents, namely a novel block copolymer of the “KLE” family (poly(ethylene-co-butylene)-block-poly(ethylene oxide)), which has already enabled the synthesis of various mesoporous, crystalline metal oxide thin films (e.g., CeO2, [10] TiO2, [11] c-Al2O3, [12] FeOx, [13] SrTiO3 ) and the well-known classical nonionic surfactant “Brij 58”. The thickness of C O M M U N IC A TI O N S

Gold surface with sub-nm roughness realized by evaporation on a molecular adhesion monolayer

Abstract: This letter describes a technique for realizing a gold (Au) surface with roughness at the atomic scale using techniques compatible with integrated device fabrication The Au layer is electron-beam evaporated on a self-assembled monolayer of (3-Mercaptopropyl) trimethoxysilane on an oxidized silicon substrate and shows a root-mean-square surface roughness of ∼2A over a 1μm2 area The physical stability of the Au film toward commonly used chemicals and processes for photolithography and self-assembly, and its suitability for formation of well-ordered organic monolayers indicate that the films are well suited as substrates for future device fabrication in molecular electronics or other devices involving self-assembled monolayers

Direct formation of catalyst-free ZnO nanobridge devices on an etched Si substrate using a thermal evaporation method.

Abstract: Well-aligned single crystalline ZnO nanobridges have been synthesized selectively across the prefabricated electrodes on silicon substrates by a single-step thermal evaporation method without using any metal catalysts or a predeposited ZnO seed layer that was a prerequisite for such synthesis. The growth region was self-defined by the anisotropic surface of the silicon substrate as initially postulated. Careful control of the reaction time and the substrate temperature allows the nanobridges to form almost exclusively across the electrodes. The photoresponses as well as the current-voltage characteristics of the device confirm that this single-step process indeed offers a simple and a cost-effective way to integrate self-assembled nanodevices based on individual and/or a large number of ZnO nanowires with conventional circuits without using e-beam lithography techniques and/or additional costly deposition processes.

Morphology, structures and properties of ZnO nanobelts fabricated by Zn-powder evaporation without catalyst at lower temperature

Abstract: Uniform ZnO normal nanobelts and toothed-nanobelts have been successfully synthesized respectively through pure zinc powder evaporation without catalyst at 600°C. Experimental results indicate that the key to the fabricating method is to control the gas flow rates and the partial pressures of argon, oxygen and zinc vapor. Scanning electron microscopy and high-resolution transmission electron microscopy observations show that the ZnO nanobelts have several types of single crystalline morphology. HRTEM images reveal that there are numerous screw dislocations and the growth is around the dislocations in the toothed-nanobelts. The growth of ZnO nanobelts is controlled by vapor-solid and screw dislocation mechanisms. Room temperature photoluminescence spectra of the toothed-nanobelts showed a UV emission at ∼390 nm and a broad green emission with 4 subordinate peaks at 455–495 nm.

Influence of Coloring Voltage and Thickness on Electrochromical Properties of e-beam Evaporated WO3 Thin Films

Abstract: In this investigation, the effect of coloring voltage and thickness on optical and also electrochromical properties of WO 3 thin films has been studied. The WO 3 thin films were grown on glass and indium tin oxide coated conducting glass substrates by e-beam evaporation at different thicknesses of 200, 400, and 700 nm. Optical properties of the deposited samples were characterized in the ultraviolet-visible range (300-1100 nm). The optical bandgap energy of the WO 3 was obtained in a range of 3.3-3.5 eV showing its increase by decreasing the film thickness. The refractive index of the WO 3 films was measured around 2 in the visible range. Surface chemical states of the films were studied by X-ray photoelectron spectroscopy, which showed the stoichiometry of our deposited tungsten oxide thin films is acceptable. Atomic force microscopy was used for studying surface morphology of the deposited films. The electrochromic properties of the WO 3 films were characterized using a lithium-based electrolyte. It was shown that there is an optimum coloring voltage for each film thickness, which maximizes the change in optical density during electrochromic process. The coloration efficiency of the samples at the optimum voltage was linearly improved by increasing the film thickness at a constant wavelength (500 nm).

Low-voltage-driven top-gate ZnO thin-film transistors with polymer/high-k oxide double-layer dielectric

Abstract: The authors report on the fabrication of a low-voltage-driven top-gate ZnO thin-film transistor with a polymer/high-k oxide double-layer dielectric. Hybrid double-layer dielectric (k=∼9.8) was formed on patterned ZnO through sequential deposition processes: spin casting of 45-nm-thin poly-4-vinylphenol and e-beam evaporation of 50-nm-thick amorphous high-k oxide (CeO2–SiO2 mixture). Room-temperature-deposited ZnO channel exhibits much rougher surfaces compared to that of 100°C deposited ZnO, so that enhanced device performances were achieved from a ZnO thin-film transistor (TFT) prepared with 100°C deposited ZnO: ∼0.48cm2∕Vs for field-effect mobility and ∼5×103 for on/off current ratio. Adopting our top-gate ZnO-TFT, a load-resistance inverter was set up and demonstrated decent static and dynamic operations at 3V.

Controlled synthesis and field emission properties of ZnO nanostructures with different morphologies.

Abstract: By simply controlling atmosphere, rods, tetraleg-rods, and arrays of ZnO nanostructures have been fabricated respectively through pure zinc powder evaporation without catalyst at temperature of 650 - 700 degrees C. Investigations through HRTEM and XRD showed that the growth of the synthesized ZnO nanostructures was controlled by vapor-solid mechanism. Field emission measurements revealed that all of the structures, owing to their very low turn-on voltage, sufficient emission current and proper linearity of 1/V - Ln(l/V2), are likely to be potential candidates as a field emitter. The results also indicated that field emission properties are relative to morphology and size of the tips of ZnO nanostructures, and the nanorods with sharp tips possess the first-class FE property.

Copper phthalocyanine thin-film transistors with polymeric gate dielectric

Abstract: Copper phthalocyanine (CuPc) thin-film transistors have been fabricated using polymethyl methacrylate (PMMA) as gate dielectric. A bottom gate, staggered structure was selected to study the device performance. CuPc thin-films were deposited by thermal evaporation in a high vacuum system. The maximum process temperature achieved was 100 � C, corresponding to the baking of the PMMA. The devices showed satisfactory p-type electrical characteristics with field-effect mobility and threshold voltage values around 0.2 · 10 � 4 cm 2 V � 1 s � 1 and 6 V, respectively. The device electrical characteristics were correlated with the structural and morphological