Post-deposition Thermal Treatment Research Articles

Stability and Surface Reactivity of Anatase TiO2 Films

Atomic Layer Deposition has been used to deposit a series of TiO2 films on native oxide Si(100) at temperatures ranging from 100 to 300 °C using tetrakis(dimethylamino) titanium and H2O. The films were subjected to post-deposition thermal treatment and their structure, morphology and composition was examined. As-deposited films were amorphous and began to crystallize after thermal treatment at 500 °C. The films deposited at 100 °C and treated thermally at temperatures up to 900 °C for 10 min crystallized exclusively in the anatase phase forming crystallites with lateral size in excess of 200 nm. The films deposited at higher temperatures formed rutile crystallites with no mixing of the two phases. The films containing anatase grains exhibited significantly higher surface activity as evidenced by the detection of surface bound species such as carbonates (CO), carbon-nitrogen (CN) and nitrosyls. Although 100 °C is not within the ALD window for optimal film growth, it appears that the poor film adhesion to the substrate as well as the low film density and possibly the presence of carbon containing impurities result in the stabilization of anatase crystallites with lateral size at least one order of magnitude higher than it is widely reported in the literature.

Plasma Based Synthesis, Electron Microscopy, and Optical Characterization of Au-, Ag-, and Ag/Au-Core–Shell Nanoparticles

Metal nanoparticles are commonly used for a wide range of applications. This paper presents a novel synthesis method using plasma vapor deposition and a subsequent thermal treatment to produce homogeneous particles within the nanometer range. A combination of high resolution transmission electron microscopy and scanning electron microscopy studies as well as ultraviolet/visible light transmission measurements are employed in order to characterize crystallographic, microstructural, and optical properties of gold, silver, and core–shell bimetallic nanostructures on a titania surface. The nanostructural size and composition of the core–shell nanostructures and therefore their optical properties could be directly adjusted by the plasma process parameters and/or the postdeposition thermal treatment of the samples.

Optical, morphological and electrical studies of thermally vacuum evaporated CdTe thin films for photovoltaic applications

Cadmium telluride (CdTe) semiconductor thin films were obtained by thermal vacuum evaporation (TVE) on optical glass substrates. The optical characterizations of CdTe thin films were made by spectroscopic ellipsometry (SE) and optical spectrophotometry. The optical constants (refractive indices and extinction coefficients) were obtained for CdTe thin films of different thicknesses. High absorption coefficients of order 105cm−1 and suitable optical bandgaps values of 1.5eV were obtained. The surface morphology of the obtained films was investigated by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) techniques. The influences of post-deposition thermal treatment on CdTe morphological properties were discussed. The current–voltage characteristics of CdTe films were recorded at temperatures in the range 150–280K. An electrical analysis of the conduction mechanisms specific for different voltage ranges was also employed.

Elaboration of tin oxide nano-islands through post-deposition thermal treatment

Tin dioxide (SnO2) films have been grown onto (006) sapphire substrates by sol–gel dip-coating using tin alkoxide solutions. It is shown, using transmission electron microscopy and atomic force microscopy, that the thickness of the layers is easily controlled with tin concentration. It can vary after the deposition process between few nanometers and few tens of nanometers according to the concentration of the precursor in the sol. A post-deposition thermal treatment forces the continuous precursor film to split into isolated islands. A decrease of the tin concentration in the precursor sol to 100μM allows the formation of islands of a few nanometers (3nm) in height and several tens of nanometers in diameter (30nm). Such height value is very close to the Bohr radius of SnO2.

Structural and Optical Properties of Luminescent Silicon Carbonitride Thin Films

The composition, structure, and optical characteristics of amorphous hydrogenated silicon carbonitride (a-SiCN:H) thin films were investigated as a function of nitrogen content. The electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR PECVD) technique was utilized to fabricate two different types of a-SiCN:H thin films including films with varying nitrogen contents and films co-doped with cerium and terbium. The intensity and position of the photoluminescence (PL) peak could be tuned in the visible range by controlling the film composition, deposition conditions, and post-deposition thermal treatment. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy at the Si K- and N K-edge were used to perform a comparative structural study on a series of CVD grown a-SiCN:H thin films. A correlation was obtained between the amount of nitrogen in the matrix and the PL behaviour, which also supported the trend in the NEXAFS results.

Optimisation of HiPIMS photocatalytic titania coatings for low temperature deposition

Titanium dioxide in its anatase form is widely used in photocatalytic applications due to its high photocatalytic activity, stability and low cost. Titania coatings directly deposited by conventional magnetron sputtering tend to have an amorphous microstructure. For the anatase structure to develop, substrate heating or post-deposition thermal treatment is usually required, with the anatase crystal phase generally forming at temperatures in excess of 400°C. This precludes the choice of thermally sensitive substrate materials for the photoactive coating.Depending on the nature of the driving voltage waveform, high power impulse magnetron sputtering (HiPIMS) has been shown to deliver a relatively low thermal flux to the substrate, whilst still allowing the direct deposition of crystalline titania coatings. Consequently, this technique offers the potential to deposit photocatalytically active titania coatings directly onto polymeric substrates and, therefore, opens up a range of new applications. In the present work a range of titanium dioxide thin films were deposited by HiPIMS onto glass substrates in order to study the influence of various process parameters, such as pressure, pulse frequency and pulse duration on coating structure and photocatalytic properties. The photocatalytic properties of the coatings were assessed by their ability to degrade the organic dye methylene blue under UV and fluorescent light irradiation. The degradation rate of methylene blue was calculated by measuring its absorption peak height at 665nm in continuous mode under UV/fluorescent light source. The hydrophilic properties of the coatings were also investigated by measuring the contact angle of water droplets on the coating surfaces. Experimentally, the optimum conditions to maximise the photocatalytic performance of the coatings were found. The influence of various deposition parameters on the photocatalytic properties and crystal structure of the coatings is discussed.Optimised coatings then were deposited onto polymeric substrates, such as polyethylene terephthalate (PET) and polycarbonate, to assess the suitability of using this method for high-energy, low-temperature deposition of photoactive titania coatings and the relevance of the optimised condition was tested for other types of substrates other than glass. It was found that titania coatings deposited by HiPIMS directly onto polymeric substrates showed relatively high levels of activity in their as-deposited state.The ability to deposit crystalline titania with photocatalytic functionality at temperatures low enough to enable the use of polymer substrates is a significant advancement in the field. It could potentially allow the production of high volumes of photocatalytic material on substrates, such as polymer web, which is not possible with current deposition techniques.

Optical phonons in nanostructured thin films composed by zincblende zinc selenide quantum dots in strong size-quantization regime: Competition between phonon confinement and strain-related effects

Raman scattering in combination with optical spectroscopy and structural studies by X-ray diffraction was employed to investigate the phonon confinement and strain-induced effects in 3D assemblies of variable-size zincblende ZnSe quantum dots close packed in thin film form. Nanostructured thin films were synthesized by colloidal chemical approach, while tuning of the nanocrystal size was enabled by post-deposition thermal annealing treatment. In-depth insights into the factors governing the observed trends of the position and half-width of the 1LO band as a function of the average QD size were gained. The overall shifts in the position of 1LO band were found to result from an intricate compromise between the influence of phonon confinement and lattice strain-induced effects. Both contributions were quantitatively and exactly modeled. Accurate assignments of the bands due to surface optical (SO) modes as well as of the theoretically forbidden transverse optical (TO) modes were provided, on the basis of reliable physical models (such as the dielectric continuum model of Ruppin and Englman). The size-dependence of the ratio of intensities of the TO and LO modes was studied and discussed as well. Relaxation time characterizing the phonon decay processes in as-deposited samples was found to be approximately 0.38ps, while upon post-deposition annealing already at 200°C it increases to about 0.50ps. Both of these values are, however, significantly smaller than those characteristic for a macrocrystalline ZnSe sample.

Thermally induced structural modifications of nano-sized anatase films and the effects on the dye-TiO2 surface interactions

Nano-sized TiO2 films have been grown by DC-Reactive Sputtering at ∼150°C on [0001] ZnO:Al substrates to be applied in Dye Sensitized Solar Cells. The effects of post-deposition thermal treatment at 500°C on the optical and structural (density, grain size) properties of the TiO2/ZnO:Al bi-layers have been investigated in details. The effects of such modifications on the surface properties, have been evaluated by dye-sensitization with 5,10,15,20 Tetrakis(4-carboxyphenyl) porphyrin (TCPP). Different interactions of the probe-molecules with the as grown and annealed TiO2 surfaces have been observed. In particular, Quantum yield (QY) analysis of TCPP-sensitized films showed that the emitted/absorbed photons ratio in the as grown layers is a factor of ∼3 lower than in the annealed layers. Based on simplified test solar cells, the electron injection is thought to be the most appropriate mechanism to explain the QY results. Our approach gives particular emphasis to the role of the surfaces and the interfaces involved in the TCPP/TiO2/AZO multilayer structures with the intent of proposing the as sputtered TiO2 layer as blocking layer in complete architecture for low thermal budget solutions (<200°C).

Inkjet catalyst printing and electroless copper deposition for low-cost patterned microwave passive devices on paper

A scalable, low-cost process for fabricating copper-based microwave components on flexible, paper-based substrates is demonstrated. An inkjet printer is used to deposit a catalyst-bearing solution (tailored for such printing) in a desired pattern on commercially-available, recyclable, non-toxic (Teslin®) paper. The catalystbearing paper is then immersed in an aqueous copper-bearing solution to allow for electroless deposition of a compact and conformal layer of copper in the inkjet-derived pattern. Meander monopole antennas comprised of such electroless-deposited copper patterns on paper exhibited comparable performance as for antennas synthesized via inkjet printing of a commercially-available silver nanoparticle ink. However, the solution-based patterning and electroless copper deposition process avoids nozzle-clogging problems and costs associated with noble metal particle-based inks. This process yields compact conductive copper layers without appreciable oxidation and without the need for an elevated temperature, post-deposition thermal treatment commonly required for noble metal particle-based ink processes. This low-cost copper patterning process is readily scalable on virtually any substrate and may be used to generate a variety of copper-based microwave devices on flexible, paper-based substrates.

Secondary phases dependence on composition ratio in sprayed Cu2ZnSnS4 thin films and its impact on the high power conversion efficiency

In this work we report the relation between different secondary phases formation and the values of Cu/(Zn+Sn) and Zn/Sn ratios of Cu2ZnSnS4 (CZTS) thin films deposited by pneumatic spray pyrolysis using air as carrier gas. The films were grown on soda lime glass substrates via (CH3COO)2·Zn·2H2O, CuCl2·2H2O, SnCl2·2H2O and thiourea as precursors salts in the solution. The secondary phases present in the films were determined from Raman spectroscopy and the chemical composition of the elements from X-ray fluorescence (XRF) measurements. Varying the growth parameters and post-deposition thermal and chemical KCN treatments we have studied the different secondary phases formation dependence on the values of the Cu/(Zn+Sn) and Zn/Sn ratios and its impact on the properties of polycrystalline thin films solar cells where CZTS is used as the absorber layer.

Thermoelectric properties of V2O5 thin films deposited by thermal evaporation

This work reports the structural, optical, electrical and thermoelectric properties of vanadium pentoxide (V2O5) thin films deposited at room temperature by thermal evaporation on Corning glass substrates. A post-deposition thermal treatment up to 973K under atmospheric conditions induces the crystallization of the as-deposited amorphous films with an orthorhombic V2O5 phase with grain sizes around 26nm. As the annealing temperature rises up to 773K the electrical conductivity increases. The films exhibit thermoelectric properties with a maximum Seebeck coefficient of −218μV/K and electrical conductivity of 5.5 (Ωm)−1. All the films show NIR-Vis optical transmittance above 60% and optical band gap of 2.8eV.

Sonochemically Synthesized 3D Assemblies of Close-Packed In2S3 Quantum Dots: Structure, Size Dependent Optical and Electrical Properties

Template-free conventional chemical and sonochemical approaches to 3D assemblies of indium(III) sulfide quantum dots were developed that allow deposition of strongly quantized cubic α-In2S3 nanocrystals close packed in thin film form. Our observation of metastable cubic structure at room temperature (instead of the thermodynamically most stable tetragonal β modification in the case of bulk material) was related to the very small crystal size. Because of heterogeneous sonochemical effects, the average crystal radius of the QD solids reduces from 2.5 to 2.0 nm upon sonification of the reaction system by continuous high-intensity ultrasound. Upon postdeposition annealing treatment, these values increase to 4.1 nm. Structural, optical and electrical properties of the synthesized QD solids were studied in details. The band gap energy value of 2.85 eV for the as-deposited QD solids in thin film form is strongly blue-shifted (by 0.85 eV) with respect to the value characteristic for a macrocrystalline specimen. In the case of as-deposited films by sonochemical approach, band gap value is 3.00 eV, indicating the possibility for further control of the optoelectronic properties of this material by sonochemical approach. Upon postdeposition thermal treatment at 150 and 200 °C, band gap energy red shifts to 2.20 and 2.00 eV were observed. Analysis of the size-quantization effects in the synthesized QD solids deposited in thin film form enabled us to estimate that the Bohr’s excitonic radius in the studied semiconductor lies in the range from 2.5 to 4.1 nm. The absence of clearly defined excitonic peaks in the absorption spectra of the studied QD assemblies was attributed to the size-distribution of the nanoparticles and to the interdot electronic coupling effects. Analysis of the charge carrier transport properties in the QD assemblies within the Kazmerski’s model indicated that the intercrystalline barrier height decreases by 0.04 eV upon thermal treatment of the films. Conductivity activation energy was found to be 0.82 eV, while the thermal band gap energy, calculated from the thermoelectrical measurements in the region where intrinsic conductivity mechanism is activated, was 2.22 eV. AFM measurements have shown that QD assemblies constituting the sonochemically deposited films show stronger tendency toward coagulation than those synthesized by conventional chemical approach.

Electroluminescence Transition From Visible- to Ultraviolet-Dominant Mode in $\hbox{n-Mn}_{0.04}\hbox{Zn}_{0.96}\hbox{O/i-ZnGa}_{2}\hbox{O}_{4}\hbox{/} \hbox{n-GaN}$ Structure With Highly Ultraviolet Detection Performance

We report an n-Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.04</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.96</sub> O(MZO) /i-ZnGa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> / n-GaN structure both as a light-emitting diode and as an ultraviolet (UV) photodetector at forward and reverse biases, respectively. The n-MZO films were prepared on n-GaN coated sapphire substrates, followed by postdeposition thermal annealing treatment at 700 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C, and an i-ZnGa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> interface layer was formed at the MZO/GaN interface after the annealing treatment. We found that the electroluminescence characteristics showed a transition from visible- to UV-dominant mode when the deposition temperature of the MZO film is from 100 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C to 300 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. For the UV detection performance, the devices based on the low-temperature deposition (100 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C) of the MZO film showed the lowest dark current and the biggest ratio of photocurrent to dark current with a selectivity detectivity for 365 nm around light.

Enhanced charge generation of the ZnO nanowires/PZT hetero-junction based nanogenerator

We fabricated an alternative nanogenerator device with distinguished structure. Representative piezoelectric materials of ZnO nanowires and PZT thin films were tried to be combined to form a hetero-junction structure for fabrication of an alternative nanogenerator device to possibly obtain a synergy effect and then improved performance. The ZnO nanowires were grown by a hydrothermal synthesis technique and then PZT thin films were deposited on the surface of the ZnO nanowires by rf magnetron sputtering process. The PZT thin films were annealed to be crystallized with different conditions for post-deposition thermal treatment process. The hetero-junction structure was polarized by a Corona poling process to obtain a unidirectional orientation of dipole moments to enhance their piezoelectric property. Structure and morphology of the grown ZnO nanowires were investigated to achieve appropriate characteristics to achieve performance improvement for the resulting nanogenerator device. To confirm effect of the hetero-junction structure on improvement of power generation performance of the resulting nanogenerator device, current generating properties were comparatively investigated with those of nanogenerator device with only ZnO nanowires or PZT thin films as active piezoelectric component, respectively. The nanogenerator device with a hetero-junction structure of ZnO nanowires/PZT revealed distinctively improved average currents of 270nm, which is quite higher than those of the nanogenerator devices with pristine ZnO nanowires, or with pristine PZT thin films, respectively. Possible factors contributed to improvement of the current generation properties were discussed for the presented nanogenerator device.

Electrochemically synthesised CdSe nanofibers and pearl-chain nanostructures for photovoltaic applications

Self-assembled arrays of CdSe nanofibers with diameter of 40nm and length about 2μm were electrodeposited onto glass/ITO substrates in aqueous media. Thermal treatments of CdSe nanofibers were applied and the influence of treatment on morphology, optical and electrical parameters were investigated. Scanning electron microscopy (SEM) study of CdSe structures revealed the change in the morphology of CdSe structures from nanofibers to pearl chain-like nanostructures after thermal treatment at 350°C in air. Also CdSe nanostructures were characterized by UV–vis absorption spectroscopy and X-ray diffraction spectroscopy (XRD). The poly(3-octylthiophene) (P3OT) functional layers were deposited on CdSe nanowires by spin-casting technique to prepare hybrid photovoltaic (PV) structures. Obtained CdSe nanowires/P3OT PV structures were characterized and partially optimized by additional post-deposition thermal treatment.

Generation and Self-Organization of Bimetallic Pd/Au Nanoparticles on SiO2 by Sequential Sputtering Depositions and Annealing Processes

PdAu nanoparticles have been grown on SiO2 by room-temperature sequential sputtering depositions. The nucleation and growth kinetics have been determined crossing atomic force and scanning electron microscopy measurements. From these measurements the mechanisms of the nucleation and growth have been determined. In particular: (1) during the deposition of the first metal (Pd), atoms adsorbed on the substrate are readily trapped on the substrate defects, forming stable nuclei which grow further. During the deposition of the second metal (Au), adsorbed atoms are captured by the clusters formed during the first deposition, before they have time to form a stable nucleus of pure second metal on the surface sites. So, the nucleation is mainly controlled by the Pd and the Au atoms are incorporated essentially by direct impingement of the vapour atoms on the already formed particles. (2) fixing the amount of Pd and Au, during post-deposition thermal treatments, a surface diffusion limited ripening of the NPs occurs. Applying the standard ripening growth theory several parameters characterizing the process were determined, in particular, the growth exponent n and the activation energy E(a). n was found to be dependent on temperature and amount of Au deposited. E(a) was found to linearly increase with the amount of Au deposited. Such a dependence is discussed separating E(a) in two terms: one describing the activation energy for atomic surface diffusion (independent on the amount of Au deposited), the other one describing the activation energy for the film clustering process (dependent on the amount of Au deposited).

Influence of doping on the photoactive properties of magnetron-sputtered titania coatings: Experimental and theoretical study

Titanium dioxide (TiO2) thin films, doped with chromium (Cr) and codoped with chromium-carbon (Cr, C) and chromium-nitrogen (Cr, N) of various concentrations, were deposited using magnetron sputtering. Postdeposition thermal treatments were carried out at 450 ◦ C for 5 h to change the as-deposited amorphous coatings to their crystalline form. The crystalline phase was found to be dependent on the amount and type of dopant present. Ultraviolet-visible (UV-Vis) absorption data and band gap energies calculated by spectroscopic ellipsometry showed that, on increasing Cr concentration, a shift of the absorption edge towards visible light and a reduction of the band gap occurred. This was further improved by codoping of Cr with either N or C, with the latter case exhibiting more photo-activity towards visible light. In addition, hybrid density functional theory (DFT) calculations were performed for Cr-, N-. and C-monodoping, together with Cr-C and Cr-N codoping, in both rutile and anatase phases of crystalline titania. Using this method, absorption coefficients and band gaps were determined to explore photo-activity. Very good, semiquantitative agreement was found between the DFT and experimental approaches for these quantities, underlining the key role of state-of-the-art quantum calculations in interpreting and guiding experimental studies of doping in metal oxides.

Nanointerlayer Induced Electroluminescence Transition from Ultraviolet- to Red-Dominant Mode for n-Mn:ZnO/N-GaN Heterojunction

High-quality Mn:ZnO (MZO) film had been prepared on N-GaN coated sapphire substrates followed by postdeposition thermal annealing treatment at 700 °C. For the annealed MZO/GaN heterojunction, a 15 nm cubic structural ZnGa(2)O(4) layer was observed at the MZO/GaN interface through transmission electron microscope analysis. Through electroluminescence (EL) measurement, the formation of the nanointerface results in an EL transition from ultraviolet- to red-dominant mode for n-Mn:ZnO/N-GaN heterojunction light-emitting diodes (LEDs). The heterojunction LED showed a rectification ratio of ∼2.0 × 10(5) at ±2 V, a dark current of 3.5 nA at -2 V and a quite strong red EL with a low turn-on voltage of 3 V. On the basis of the energy band diagram, we think the EL transition from ultraviolet- to red-dominant mode is mainly due to the formation of a thin oxide blocking nanolayer at the MZO/GaN interface during the annealing process.

Interaction of reducing gases with tin oxide films prepared by reactive evaporation techniques

SnO2−x films were prepared by reactive thermal and e-beam evaporation of Sn on alumina substrates and by post deposition thermal treatment. X-ray diffraction measurements found that films are tin dioxide (SnO2) phase with small amounts of SnO phase. The surface conductivity of films was measured in air and in presence of H2S, H2 and C2H5OH vapors at four sensor operating temperatures of 433–493 K. The resistance of SnO2−x films decreases on exposure to H2S but shows no change with hydrogen and ethanol. H2S response decreases with rise in sensor temperature while both response and recovery times improve. H2S signal enhances with increase in resistivity of SnO2−x coatings. Our experiments conclude that increase in film conductance is due to chemical reaction between H2S and SnO2−x surface and there is little or no role of interaction of gas molecules with surface adsorbed charged oxygen species.

Effect of Thermal Treatment on the Crystallographic, Surface Energetics, and Photoelectrochemical Properties of Reactively Cosputtered Copper Tungstate for Water Splitting

In this Article, we report the effect of postdeposition thermal treatment on reactively cosputtered copper tungstate (CuWO4) thin films and its impact on photoelectrochemical performances. This stu...