Post-deposition Thermal Treatment Research Articles

Enhanced Oxygen Evolution Reaction Activity of Nanoporous SnO2/Fe2O3/IrO2 Thin Film Composite Electrodes with Ultralow Noble Metal Loading

AbstractA conductive SnO2 layer and small quantities of IrO2 surface cocatalyst enhance the catalytic efficiency of nanoporous Fe2O3 electrodes in the oxygen evolution reaction at neutral pH. Anodic alumina templates are therefore coated with thin layers of SnO2, Fe2O3, and IrO2 by atomic layer deposition. In the first step, the Fe2O3 electrode is modified with a conductive SnO2 layer and submitted to different postdeposition thermal treatments in order to maximize its catalytic performance. The combination of steady‐state electrolysis, electrochemical impedance spectroscopy, X‐ray crystallography, and X‐ray photoelectron spectroscopy demonstrates that catalytic turnover and e− extraction are most efficient if both layers are amorphous in nature. In the second step, small quantities of IrO2 with extremely low iridium loading of 7.5 µg cm−2 are coated on the electrode surface. These electrodes reveal favorable long‐term stability over at least 15 h and achieve maximized steady‐state current densities of 0.57 ± 0.05 mA cm−2 at η = 0.38 V and pH 7 (1.36 ± 0.10 mA cm−2 at η = 0.48 V) in dark conditions. This architecture enables charge carrier separation and reduces the photoelectrochemical water oxidation onset by 300 mV with respect to pure Fe2O3 electrodes of identical geometry.

Toward reproducible metal-insulator transition characteristics in V2O3 thin films sputter-deposited on glass

Vanadium sesquioxide (V2O3) exhibits remarkable property changes through its metal-insulator transition near 150 K and is a very promising candidate for device applications. Thin V2O3 films were deposited on SiO2 glass by reactive DC magnetron sputtering. While resistivity changes over four orders of magnitude were demonstrated, films deposited under nominally identical conditions exhibited significant differences in electrical characteristics, which would hinder reproducibility under deposition techniques appropriate for industrial scale production with standard control features. These differences were attributed to small deviations from exact stoichiometry. A post-deposition thermal treatment consistent with equilibrium temperature and oxygen pressure conditions for V2O3 applied to the samples succeeded in nearly equalizing their characteristics within a relatively short time and without negatively impacting the glass substrate or film continuity. Analysis of film structure, morphology, and resistivity measured from room temperature through the metal-insulator transitions, both before and after the thermal process, revealed information about the interplay between non-stoichiometry, residual stress, and electrical characteristics of the films. The approach employed can lead to reproducible results for V2O3 films and is applicable to similar materials which exhibit metal-insulator transitions.

Enhanced uniformity of sputtered oxygenated cadmium sulfide (CdS:O) films for large area photovoltaic applications

The possible beneficial effects of replacing CdS with CdS:O in photovoltaic applications have been reported and commented by many authors. However, the effective scalability of the CdS oxygenation process by RF reactive sputtering has never been addressed. In this paper, we studied the uniformity of the CdS oxygenation on 100 cm2 samples as well as the effect of a Post-Deposition Thermal Treatment (PDTT) on the film properties. The samples were analyzed by profilometry, X-ray diffraction spectroscopy, X-ray photoemission spectroscopy and atomic force microscopy (AFM). Moreover, an ad-hoc system was developed to map the optical transmission on the sample area, hence the CdS:O band gap (Eg) profile was calculated. The as-grown films presented poor oxygenation uniformity. However, 375 °C was identified as the threshold temperature for oxygen expulsion. Hence, we propose a possible method to improve the film uniformity based on an initial over-oxygenation followed by a thermal treatment at the threshold temperature. A 10 × 10 cm2 CdS:O film with remarkable uniformity and Eg = 2.60 ± 0.05 eV was obtained.

Integration of plasmonic Au nanoparticles in TiO2 hierarchical structures in a single-step pulsed laser co-deposition

The plasmonic resonance of noble metal nanoparticles (NPs) can be exploited to enhance the photoresponse of wide band gap oxides in view of several solar energy applications. Here, we demonstrate single-step synthesis of plasmonic Au nanoparticles integrated in TiO2 hierarchical nanoporous layers through a vapor phase pulsed laser co-deposition approach. Specifically, we report the fabrication and characterization of Au NPs-decorated TiO2 forest-like systems with tunable porosity and density as well as the morphological/structural evolution as a function of Au content and we discuss the corresponding optical properties. The effect of post-deposition thermal treatment has been investigated as well in order to control TiO2 crystallization and Au NPs nucleation and growth. Optical analyses show the onset of characteristic plasmonic resonance of Au NPs with the increase of film absorption in the visible range. Preliminary tests of photodegradation of methyl orange dye indicate that the integration of Au NPs leads to a significant increase of the catalytic activity of nanoporous TiO2. Our results suggest the potentiality of this approach for the synthesis and the integration of metallic NPs within wide band gap semiconductors, while paving the way toward novel plasmonic-based devices.

Tuning the electrical properties of the p-type transparent conducting oxide Cu1\u2212xCr1+xO2 by controlled annealing

Off-stoichiometric copper chromium oxide delafossite received lately a great interest due to its high p-type electrical conductivity and adequate optical transmittance in the visible range. However, for a suitable integration in active devices such as p-n junctions, transistors or optoelectronic devices, the electronic properties must be efficiently tailored. Here, post-deposition thermal treatment is proven as an adequate approach for finely controlling the electrical properties of this former degenerate semiconducting material. The energetics of the annealing process are investigated using two different approaches, as a function of the annealing temperature and as a function of the annealing time, allowing the accurate determination of the activation energy of the annealing of defects. By using this method, the electrical carrier concentration was varied in the 1021 – 1017 cm−3 range while the recorded changes in the drift mobility covered three orders of magnitude. Moreover, we demonstrate the ability to accurately manipulate the Fermi level of such materials, which is of great importance in controlling the carrier injection and extraction in optoelectronic active layers.

Effect on the electrical and morphological properties of Bi incorporation into ZnO:Ga and ZnO:Al thin films deposited by confocal magnetron sputtering

This paper reports the effect on the electrical and morphological properties of co-doping ZnO thin films with Bi and Al or Ga. To do so, a confocal sputtering geometry was used with a Bi target and two intrinsically doped ZnO:Ga and ZnO:Al targets. By depositing at an intentional heating of 200 °C and applying a post-deposition thermal treatment at 350 °C and 300 °C, for ZnO:Ga,Bi and ZnO:Al,Bi, respectively, electrical resistivity values of 1.3 × 10−3 Ω cm and 4.8 × 10−4 Ω cm were achieved, with an optical transmittance above 80%. The X-ray diffraction data shows that all doped ZnO films have a wurtzite hexagonal structure with preferential crystal growth perpendicular to the (002) plane. The Seebeck coefficient was measured for the ZnO:Al,Bi films, where a maximum value of −48 μV K−1 was registered. The optimized electrical properties were correlated with the preferential crystalline texture along [001] and the corresponding current density applied to the Bi dopant target, J(Bi). ZnO:Al,Bi films present out-of-plane compression stress, which concomitantly increases with J(Bi), due to higher compact volume of unit cell with lower lattice parameter c when compared with the undoped ZnO. By controlling the incorporation of Bi, the deposition temperature and the post-deposition thermal treatment temperature, improvements on the thermoelectric power factor of ZnO:Ga and ZnO:Al thin films can be achieved.

Solution processed As30Se70chalcogenide glass thin films with specular optical quality: multi-component solvent approach

We present the study of volatile amine-based single or multi-component solvent formulations and their influence on As30Se70 chalcogenide glass solubility and dissolution mechanism. The source As30Se70 bulk glass was dissolved in various amine based solvents, and solutions of quantitatively dissolved bulk glass were used for thin films deposition by the spin-coating technique. The methanol – n-propylamine solvent mixture proved to be suitable for preparation of thin films in specular optical quality, low surface roughness and targeted composition. The post-deposition thermal treatment has a high impact on the thickness, optical properties and structure of the studied thin films. The content of the organic residuals was also significantly reduced.

New bio-active, antimicrobial and adherent coatings of nanostructured carbon double-reinforced with silver and silicon by Matrix-Assisted Pulsed Laser Evaporation for medical applications

We report on Matrix-Assisted Pulsed Laser Evaporation (MAPLE) deposition of Carbon thin films, simple or reinforced with intended concentrations of Ag and Si. A KrF∗ (λ = 248 nm, τFWHM ≤ 25 ns, ν = 10 Hz) excimer laser was used for irradiation. The effect of a post-deposition thermal treatment in vacuum was studied. Besides detailed morphological, compositional, structural and pull-out adherence characterizations, the potential of the carbonaceous films for medical applications was investigated in vitro by anti-biofilm and cytocompatibility assays. The microscopic images evidenced no delaminations. Micro-Raman spectroscopy revealed a graphitization tendency depending on preparation conditions, thermal treatment and reinforcing agents’ presence. Adherence values improved considerably after thermal treatment. In vitro biological evaluation showed that the films containing ∼1.85 at.% Ag were non-cytotoxic for MG63 cells, while eliciting a limited antimicrobial activity. The increase of Ag content to 3.6 at.% results in a significant enhancement of antimicrobial activity, whilst maintaining the cytotoxic action and adherence characteristics at acceptable levels.We propose a new class of metamaterials based on C reinforced with Ag and Si obtained by MAPLE for medical applications, i.e. the prevention and treatment of various infections associated with biofilms developed on implants and other medical equipments.

Post-deposition thermal treatment of sprayed ZnO:Al thin films for enhancing the conductivity

Here, we report the enhanced conductivity of Aluminium doped (2at.%) zinc oxide thin films prepared by simple spray pyrolysis technique. The structural, optical, electrical, morphological and compositional investigations confirm the better quality of films that can be a potential candidate for application in transparent electronics. Most importantly, the film demonstrates an average transmittance of 90 percent with a low resistivity value which was dropped from 1.39×10−2 to 5.10×10−3Ω.cm, after annealing, and a very high carrier concentration in the order of 10×20cm−3. Further, we have used the Swanepoel envelop method to calculate thickness, refractive index and extinction coefficient from the interference patterns observed in the transmission spectra. The calculated figure of merit of the as-deposited sample was 1.4×10−3Ω−1 which was improved to 2.5×10−3Ω−1 after annealing.

Thermal dependence of photo-induced effects in spin-coated As20Ge12.5S67.5 thin films

Chalcogenide glass thin films of ternary As20Ge12.5S67.5 composition were prepared in specular quality using spin-coating technique from n-butylamine based glass solution. The content of organic residuals was notably reduced by post-deposition thermal treatment. The annealing process induced significant structural changes resulting in thin films with the structure close to the source bulk glass. The thickness of deposited thin films was decreasing with increasing annealing temperature. Contrary, the refractive index, optical bandgap, roughness and chemical stability were increasing. The experimental data also proved that deposited thin films were photo-sensitive as opto-physical properties and chemical resistance were changed after UV light exposure. The exposed thin films annealed bellow 90°C were etched slower than unexposed ones (negative etching) and the thin films annealed above 90°C were etched faster (positive etching).

Straightforward prediction of the Ni1−xO layers stoichiometry by using optical and electrochemical measurements

In this study, we propose a straightforward method for x determination in sub-stoichiometric nickel oxide (Ni1−xO) films prepared by ultrasonic spray pyrolysis on fluor-tin oxide (FTO) substrates by varying the post-deposition thermal treatment. The Ni3+ concentration, the flat band potential (Φfb) and the open circuit potential (Voc) were determined by electrochemical impedance analysis in aqueous media and correlated to the transmission of Ni1−xO films. An x-ray photoelectron spectroscopy study was also performed to quantify the amount of Ni3+ in the films and compare it with the one determined by electrochemical analysis. The electrochromic behavior of the Ni1−xO films in non-aqueous electrolyte was investigated as well.With increasing Ni3+ concentration the films became more brownish and more conductive, both Voc and Φfb values increased. Calibration curves of transmission at 550 nm or open circuit potential versus carrier concentration were plotted and allowed the prediction of x in an unknown Ni1−xO sample. The Ni1−xO films characterized by the highest Ni3+ concentration have a darker colored state but lower transmission modulation, due to their reduced specific surface and increased crystallinity.

Organic solar cells: Study of combined effects of active layer nanostructure and electron and hole transport layers

An organic solar cell based on poly (3-hexathiophine-2,5-diyl) and [6,6]-phenyl C61 butyric acid methyl ester has been subjected to all layers treatment and was investigated for combined effects of the these layers on device performance. These treatment included optimization of active layer morphology and thickness and improving the structure of the hole and electron transport layers, as well as subjecting the full device to optimum post deposition thermal treatment. Such a device has shown an increase in the optical absorption intensity in the near infrared region compared to the reference device, which is thought to be advantageous for producing high current density. The increase in the current density has also been correlated with light trapping within the active layer and the possibility of the occurrence of total internal reflection, which was explained using total internal reflection spectroscopic ellipsometry measurements. The current density-voltage characteristics have been measured in dark and under illumination. Power conversion efficiency as high as 7% has been achieved correlated with a fill factor of 71%.

Structural and optical properties of thermally treated CdO thin films deposited on ITO coated glass substrates for photovoltaic applications

In this study, we have reported effects of post-deposition thermal treatment of cadmium oxide thin films on indium tin oxide coated glass substrates by sol–gel technique using spin coating method. To prepare CdO thin films, a precursor solution based on a derivative from cadmium acetate was gelated and subsequently coated on ITO/glass substrates. Spin coated CdO films were annealed in air at varied temperatures of 300, 400, and 500 °C. The crystallite size, microstrain and dislocation density were estimated from high resolution X-ray diffraction analysis. X-ray diffraction patterns confirmed cubic phase formation in polycrystalline CdO films. Cross-sectional and top surface morphology of the films was studied by field-enhanced scanning electron microscopy. The as-deposited and thermally treated CdO thin films for various annealing temperatures showed an increase in the optical transmittance to 79% from that of 63% of CdO film in the visible and near infrared region. Shrinkage in band gap from 2.6 to 2.4 eV was reported for temperature variation of 300–500 °C. The emission properties of as-deposited and thermally treated CdO thin films were studied by photoluminescence spectrum recorded at 298 K.

Post-deposition thermal treatment of sprayed SnS films

SnS films were grown in air by chemical spray pyrolysis method using aqueous solutions containing SnCl2 and SC(NH2)2 at molar ratios of 1:1 and 1:8 and a substrate temperature of 200°C. As-deposited films were thermally treated at 450°C in nitrogen or vacuum atmospheres. All samples were studied using X-ray diffractometry, Raman spectroscopy, energy-dispersive X-ray analysis, and ultraviolet–visible spectroscopy. The as-grown films consisted of cubic SnS as the only crystalline phase regardless of the molar ratio of the precursors in the spray solution. Annealing of the 1:1 films (derived from the 1:1 solution) in vacuum yielded metallic Sn, whereas annealing in N2 produced films composed of a mixture of cubic SnS and SnO2 phases, indicating the presence of oxygen-containing non-crystalline phases in the as-grown films. Thermal treatment of the 1:8 films in nitrogen yielded films composed of Sn2S3, whereas vacuum annealing produced films consisting of orthorhombic SnS with a bandgap energy of 1.4eV.

Enhancement of fatigue resistance of Bi-Sb-Te films on flexible substrates by current-assisted thermal annealing

Brittle nature of chalcogenide compounds is a major concern for implementation of bismuth telluride based compound films in flexible thermoelectric devices. In this study we report that the fatigue resistance of Bi-Sb-Te films sputtered on a polyimide substrate is improved by applying a high-density electric current (1–2×103A/cm2) through the film during post-deposition thermal treatment. The electrically stressed film was found to suffer less conductivity degradation caused by cyclic stretching than the film thermally annealed at the same temperature. The increase of electrical resistance is well correlated to the enlarging crack width observed in the stretched film. It is proposed that the electromigration-induced Sb-rich precipitates suppress the propagation of microcracks along grain boundaries during cyclic stretching and thus improves the fatigue resistance of the sputtered Bi-Sb-Te films.

Effect of post-deposition annealing on the optical and mechanical properties of amorphous tungsten oxynitride thin films

The effect of post-deposition thermal treatment on the optical and mechanical properties of WON thin films is reported. WON thin films were deposited onto silicon (100) and quartz substrates utilizing direct current (DC) sputtering under the variable flow of reactive gas mixture. Post-deposition annealing was performed at a temperature (Ta) of 400°C under inert (argon) and oxygen (air) atmosphere. For annealing in an inert atmosphere, spectroscopic ellipsometry (SE) revealed changes to the dispersion profiles and magnitudes of the index of refraction (n) and extinction coefficient (k) as a result of thermal treatment. X-ray photoelectron spectroscopy (XPS) measurements revealed the loss of both nitrogen and oxygen during the annealing process in an inert atmosphere while the samples remain amorphous as evidenced in X-ray diffraction measurements. The chemical changes correlate with the changes seen in the optical constants for annealed WON films. Furthermore, the physical changes during post thermal treatment probed by calculating the film-density using the Lorentz-Lorenz relation indicate a decrease in density that supports the changes in chemical composition and optical properties. Density values for annealed WON films vary in the range ~9–12g/cm3, the low-to-high end of density values are noted for low and high values of nitrogen flow rate, respectively, employed during deposition. Mechanical properties of WON films were demonstrated a drastic improvement as a result of post thermal treatment. The hardness values generally were improved but the WON films annealed in air showed the significant improvement, up to ~10GPa. XRD measurements indicated the amorphous-to-crystalline transition with m-WO3 phase as dominant component for samples annealed in an air due to the abundance of oxygen absorbed during annealing.

Improved Silicon Surface Passivation of APCVD Al2O3 by Rapid Thermal Annealing

Short-duration post-deposition thermal treatments at temperatures above those normally used for annealing activation have the potential to further improve the already excellent passivation of crystalline silicon (c-Si) achieved by Al2O3, but have so far received little attention. In this work we investigate the influence of rapid thermal annealing (RTA) on the surface passivation of c-Si by Al2O3 deposited by atmospheric pressure chemical vapour deposition (APCVD) as a function of RTA peak temperature between 500 and 900°C, and for Al2O3 deposition temperatures between 325 and 440°C. The saturation current density J0 of undiffused p-type surfaces is observed either to increase or decrease following RTA depending on the Al2O3 deposition temperature and the RTA peak temperature. The optimum deposition temperature depends on the post-deposition thermal processing to be applied. Films deposited at lower temperatures provide worse passivation after low temperature heat treatment, but maintain this passivation better at higher RTA temperatures. An exceptionally low J0 of 7 fA cm−2, due to the combination of a very low interface state density Dit and unusually high negative fixed charge density Qf, is achieved by the use of a short 500–550°C RTA combined with optimised deposition conditions.

Thermal evolution of physical properties of vacuum evaporated polycrystalline CdTe thin films for solar cells

This paper reports the change in optical, structural, morphological and electrical properties of CdTe thin films with post-deposition thermal treatment in air atmosphere. The polycrystalline cadmium telluride (CdTe) thin films of thickness 850 nm were deposited on glass and ITO coated glass substrates by thermal vacuum evaporation technique followed by annealing at different temperature 150, 250 and 350 °C. The physical properties have been investigated using characterization tools like UV–Vis spectrophotometer, X-ray diffraction, scanning electron microscopy coupled with EDS and source meter. The optical transmittance was observed to increase with post-deposition heat treatment which revealed the systematic reduction in optical energy band gap from 1.78 to 1.54 eV. The Swanepoel and Herve–Vandamme models as well as the dielectric theory were used to calculate various optical and dielectric constants. The thermal annealing enhances the crystallinity (zinc blende cubic structure) of films with preferred orientation (111). A number of crystallographic parameters were also evaluated and studied in depth with heat treatment. The surface morphology studies indicate that the films are homogeneous, densely packed, uniform and free from crystal defects. The current–voltage measurements exhibited ohmic behavior and the conductivity was found to increase with annealing temperature.

Control of High Quality SrVO3 Electrode in Oxidizing Atmosphere

SrVO3 (SVO) is a model system for strongly correlated oxides and is highly promising as conductive layer in heterostructures. Therefore, the control of electronic properties and morphology are essential for the advanced applications. Here, the oxygen stability during and after the deposition is explored, as SVO film is expected to undergo different postdeposition thermal and atmosphere treatments during its integration in a heterostructure. Hence, the influence of oxygen stability on morphology and electrical properties of the metallic SrVO3 grown by pulsed laser deposition has been investigated. Films grown under vacuum (SrVO3− δ ) exhibit a very smooth surface while films grown under higher oxygen pressure roughen and present nanostructures at the surface. These nanostructures are found to be of Sr3V2O8 phase and their apparition can be controlled by the oxygen supply. Subsequent thermal treatments at different temperatures under same oxygen pressure prevent formation of the Sr3V2O8 phase, lead to the stoichiometric SrVO3, and thus improve the transport properties. In this study is shown the extreme sensitivity of SVO to oxygen and the conditions to obtain high quality smooth SVO films with improved electrical properties for electrode application.

Sonochemically assisted colloidal route to CdSe quantum dot assemblies: an alternative way to further fine-tune the size-dependent properties

Continuous-wave heterogeneous sonochemical method was developed to synthesize 3D assemblies composed by QDs of metastable (cubic) modification of CdSe with sphalerite structural type. 3D QD assemblies were deposited in thin film form and as bulk precipitates. Structure, surface morphology, optical absorption as well as charge carriers’ and phonon confinement effects were studied in details. The average nanocrystal radius, calculated by the Scherrer and Williamson–Hall methods, diminishes from 2.7 nm for samples synthesized by conventional colloidal chemical route to 2.0 nm for samples synthesized by sonochemical route. Upon post-deposition thermal annealing this value increases to 12.0 nm. QD size decrease is followed by increase of the uni-directional lattice strain and dislocation density (as defined by Hirsch), as well as by decrease of the lattice constant value. Previous findings were justified by analysis of X-ray diffraction peaks shape, which were found to be dominated by the particle size-distribution, instead of non-uniform strain. The energy of the fundamental direct \(\Gamma_{8}^{v} \to \Gamma_{6}^{c}\) interband electronic transition, computed from optical absorption data within parabolic approximation for the dispersion relation, shifts from 2.67 eV for sonochemically deposited samples and 2.08 eV for chemically deposited ones, to 1.77 eV upon post-deposition thermal treatment (close to the bulk value of 1.74 eV). The type of “interband” transitions does not change upon dimensionality reduce. Enhanced band gap energy blue shift upon average QD size decrease is followed by oscillator strength increase. Such trends were rationalized in terms of 3D confinement effects on charge carriers’ motion. Experimentally observed band gap energies agree very well with the predictions of the effective mass model of Brus. The next direct interband electronic transition, from the spin–orbit split component of the valence band to the conduction band, has been detected and analyzed. The 1LO bands in the Raman spectra of chemically and sonochemically deposited samples appear at 206 and 204 cm−1, as compared to the bulk value of 210 cm−1. The extent of homogeneous broadening of the corresponding band is larger in sonochemically deposited samples, indicating enhanced frequency of phonon–phonon collisions in smaller QDs. The observed frequency shifts are dominated by phonon-dispersion terms, instead of lattice contraction.