Sensor properties of Pt doped SnO2 thin films for detecting CO

Polycristalline Pt-doped SnO₂ thin films have been integrated to silicon substrate by ultrasonic spray deposition. This deposition technique differs from the usual SnO₂ deposition methods by using a liquid source. It allows one to obtain a very fine and homogeneous dispersion of Pt aggregates which act as a catalyst for the low temperature CO detection (25–100°C) by conductance change. The influence of synthesis temperature (460–560°C), concentration of Pt additive (0.1–5 at.%) on gas sensitivity has been studied. The realisation of gas sensor includes a gas sensitive highly porous layer (SnO₂/Pt, thickness: ∼1 μm). The results of electrical measurements under 300 ppm of CO for thin films in a dynamic and quasistatic regime are discussed. The narrow peak of gas sensitivity in the range of low temperatures (25–100°C) is obtained for about 2 at.% Pt in the SnO₂ film.     

Spectroscopic,microscopic, and electrical characterization of nanostructured SnO2:Co thin films prepared by sol–gel spin coating technique

Cobalt doped SnO₂ thin films were prepared by sol–gel spin coating technique and influence of dopant concentration on structural, morphological and optical properties of thin films were investigated by XRD, XPS, FTIR, SEM, AFM, PL, UV–vis, and Hall effect measurement. All samples have a tetragonal rutile structure and the grain size decreases with increasing the doping concentration. XPS results clearly showed the presence of Co²⁺ ions into the SnO₂. The SEM and AFM images reveal that the morphology of samples was affected by dopant. Conductivity type of the films changes from n-type to p-type with increasing Co-dopant above 3 mol% and electrical resistivity increases with increasing Co content. The optical band gap gradually decreases with improved cobalt concentration from 3.91 eV to 3.70 eV. The PL measurements revealed the decrease in intensity of blue emission lines and increase in green emission when content of Co is enhanced in the thin films.     

Transport,structural and optical properties of SnO<Subscript>2</Subscript> transparent semiconductor thin films alloyed with chromium: carrier type conversion

In this work, tin oxide thin films alloyed with chromium up to 50 at.% were prepared via spray pyrolysis method on preheated glass substrates of 480?°C, and the effect of Cr on the structural, optical, electrical and thermo-electrical properties of SnO₂:Cr thin films was studied. The results show that all the films grow in polycrystalline form with tetragonal rutile structure. The lattice volume of SnO₂:Cr films was found to be minimum at the critical Cr concentration of 15% indicating to two different mechanisms for Cr addition: For low Cr concentration (<15%) substitutional doping is the involving mechanism, while for more Cr addition, interstitial doping is the dominant one in the studied films. Cr addition in SnO₂ films results in reduction of crystallite size, transparency and band-gap, and also varies resistivity and carrier concentration. The remarkable effect of Cr addition was revealed as achieving p-type conductivity in SnO₂:Cr films with Cr content ranging 5–15%.     

The effect of Au and Pt nanoclusters on the structural and hydrogen sensing properties of SnO2 thin films

Au and Pt nanoparticle modified SnO₂ thin films were prepared by the sol-gel method on glass substrates targeting sensing applications. Structural and morphological properties of these films were studied using X-ray Diffraction and Scanning Electron Microscopy. It was proved that the films crystallized in tetragonal rutile SnO₂ crystalline structure. Scanning Electron Microscopy observations showed that the metallic clusters' dimensions and geometry depend on the kind of the metal (Au or Pt) while SnO₂ films surface remains almost the same: nanostructured granular very smooth. Optical properties of the films were studied using UV-visible spectroscopy. The modified SnO₂ films were tested as hydrogen sensors. The response of SnO₂, SnO₂-Au and SnO₂-Pt thin films against hydrogen was investigated at different operating temperatures and for different gas concentrations. The addition of metal nanoparticles was found to decrease the detection limit and the operating temperature (from 180 °C to 85 °C), while increasing the sensing response signal.     

X-ray photoelectron spectroscopy study and humidity sensing properties of Zn doped SnO2 thin films

Undoped and Zn-doped SnO₂ thin films are deposited onto glass substrates by sol–gel spin coating method. All the films are characterized by X-ray photon spectroscopy (XPS) and Fourier transform infra-red spectroscopy (FTIR). XPS shows that Sn presence as valence of Sn⁴⁺ in the prepared SnO₂ thin films instead of Sn²⁺. In addition, it also exhibits the amount of Zn in SnO₂ thin films, which increases with increasing Zn doping percentage. The Zn (2P_3/2) peak is symmetric and centred at around 1,021.73 eV which shifts to the lower binding energy of 1,020.83 eV for 15 at.% Zn doped SnO₂ thin film. FTIR study is used to describe the local environment of undoped and Zn-doped SnO₂ thin films which also confirms the synthesis of undoped and Zn-doped SnO₂ thin films. It is found that the resistance of SnO₂ thin films increases as Zn doping concentration increases at room humidity. The resistance of all the samples increases as relative humidity (RH) increases. The sensitivity of SnO₂ thin films increases as RH increases while it decreases as Zn doping percentage increases. Response time of SnO₂ thin film decreases as Zn doping percentage increases and recovery time slightly increases with doping percentage.     

The variation of the features of SnO2 and SnO2:F thin films as a function of V dopant

V doped SnO₂ and SnO₂:F thin films were successfully deposited on glass substrates at 500 °C with spray pyrolysis. It was observed that all films had SnO₂ tetragonal rutile structure and the preferential orientation depended on spray solution chemistry (doping element and solvent type) by X-ray diffraction measurements. The lowest sheet resistance and the highest optical band gap, figure of merit, infrared (IR) reflectivity values of V doped SnO₂ for ethanol and propane-2-ol solvents and V doped SnO₂:F films were found to be 88.62 Ω–3.947 eV–1.02 × 10^?4 Ω^?1–65.49 %, 65.35 Ω–3.955 eV–8.54 × 10^?4 Ω^?1–72.58 %, 5.15 Ω–4.076 eV–6.15 × 10^?2 Ω^?1–97.32 %, respectively, with the electrical and optical measurements. Morphological properties of the films were investigated by atomic force microscope and scanning electron microscope measurements. From these analysis, the films consisted of nanoparticles and the film morphology depended on doping ratio/type and solvent type. It was observed pyramidal, polyhedron, needle-shaped and spherical grains on the films’ surfaces. The films obtained in present study with these properties can be used as front contact for solar cells and it can be also one of appealing materials for other optoelectronic and IR coating applications.     

Structural and optoelectronic properties of indium doped SnO2 thin films deposited by sol gel technique

Indium doped tin oxide (SnO₂:In) thin films were deposited on glass substrates by sol–gel dip coating technique. X-ray diffraction pattern of SnO₂:In thin films annealed at 500 °C showed tetragonal phase with preferred orientation in T (110) plane. The grain size of tin oxide (SnO₂) in SnO₂:In thin films are found to be 6 nm which makes them suitable for gas sensing applications. AFM studies showed an inhibition of grain growth with increase in indium concentration. The rms roughness value of SnO₂:In thin films are found to 1 % of film thickness which makes them suitable for optoelectronic applications. The film surface revealed a kurtosis values below 3 indicating relatively flat surface which make them favorable for the production of high-quality transparent conducting electrodes for organic light-emitting diodes and flexible displays. X-ray photoelectron spectroscopy gives Sn 3d, In 3d and O 1s spectra on SnO₂:In thin film which revealed the presence of oxygen vacancies in the SnO₂:In thin film. These SnO₂:In films acquire n-type conductivity for 0–3 mol% indium doping concentration and p type for 5 and 7 mol% indium doping concentration in SnO₂ films. An average transmittance of >80 % (in ultra-violet–Vis region) was observed for all the SnO₂:In films he In doped SnO₂ thin films demonstrated the tailoring of band gap values. Photoluminescence spectra of the films exhibited an increase in the emission intensity with increase in indium doping concentration which may be due structural defects or luminescent centers, such as nanocrystals and defects in the SnO₂.     

Synthesis of SnO2/Pd composite films by PVD route for a liquid petroleum gas sensor

Synthesis of SnO₂/Pd composite films was carried out by depositing antimony doped SnO₂ films by magnetron sputtering technique and evaporating a thin layer of palladium on the top of it. This bi-layer structure was subjected to rapid thermal annealing for the incorporation of Pd in SnO₂. The films thus obtained were characterized by measuring electrical, optical and microstructural properties. Liquid petroleum gas-sensing properties were also investigated.     

Effect of Li doping on the structural, optical and electrical properties of spray deposited SnO2 thin films

Studies on spray deposited transparent conducting Li doped SnO₂ thin films are scarce. Li (0 to 5 wt.%) doped SnO₂ thin films spray deposited onto glass substrates at 773 K in air from chloride precursors were studied for their structural, optical and temperature dependent electrical behaviors. X-ray diffraction patterns indicated single phase with polycrystalline nature. Systematic variation in surface morphology on Li doping was examined by scanning electron microscopy and atomic force microscopy. Film thickness, optical band gap (direct and indirect), sheet resistance and figure of merit were computed from spectral transmittance and temperature dependent resistivity data. Lithium doping was found to decrease the value of sheet resistance by an order in magnitude. Activation energy was computed from temperature dependent electrical resistivity data measured in the range 300 to 448 K. The 4 wt.% Li doped SnO₂ film was found to have a high value of figure of merit among other films. The results are discussed.     

Structural, optical, electrical properties and FTIR studies of fluorine doped SnO2 films deposited by spray pyrolysis

Thin films of SnO₂:F were prepared by ultrasonic spray pyrolysis method. The effect of fluorine concentration on the structural, optical, and electrical properties of SnO₂:F films was investigated. The X-ray diffraction results showed the preferred growth along (110). FTIR was employed to study the defects in SnO₂ lattice. The evidence of oxygen vacancy and substitution of fluorine for oxygen in FTIR were investigated. It was found that at low doping levels, fluorine ions preferred to replace the oxygen in the lattice. While beyond a certain doping level, fluorine ions started to occupy interstitial site, which had a negative effect on carrier concentration that, in turn, affected the infrared reflectivity of SnO₂:F films. The increased disorder of SnO₂ at high doping levels was also shown by FTIR. The discussion of carrier scattering suggested that ionized impurity and/or neutral impurity scattering were the dominant scattering mechanisms in SnO₂:F films.     

Fast response time alcohol gas sensor using nanocrystalline F-doped SnO2 films derived via sol–gel method

Pure and fluorine-modified tin oxide (SnO₂) thin films (250–300 nm) were uniformly deposited on corning glass substrate using sol–gel technique to fabricate SnO₂-based resistive sensors for ethanol detection. The characteristic properties of the multicoatings have been investigated, including their electrical conductivity and optical transparency in visible IR range. Pure SnO₂ films exhibited a visible transmission of 90% compared with F-doped films (80% for low doping and 60% for high doping). F-doped SnO₂ films exhibited lower resistivity (0· 12 × 10^???4 Ω  cm) compared with the pure (14·16 × 10^???4 Ω  cm) one. X-ray diffraction and scanning electron microscopy techniques were used to analyse the structure and surface morphology of the prepared films. Resistance change was studied at different temperatures (523–623 K) with metallic contacts of silver in air and in presence of different ethanol vapour concentrations. Comparative gas-sensing results revealed that the prepared F-doped SnO₂ sensor exhibited the lowest response and recovery times of 10 and 13 s, respectively whereas that of pure SnO₂ gas sensor, 32 and 65 s, respectively. The maximum sensitivities of both gas sensors were obtained at 623 K.     

Effect of heavy doping in SnO2:F films

Thin films of fluorine-doped tin dioxide (SnO₂:F) were deposited by a spray pyrolysis technique on soda lime glass substrates. Structural and electronic transport properties of the films deposited with different doping levels of fluorine (zero to 350 at %) were investigated. X-ray diffraction technique and Hall effect measurements were used for this work. Growth rate of the films was considerably affected by doping, specially at higher doping levels. The films were polycrystalline and preferentially oriented along [200]. This preferred growth played a dominant role in determining the transport properties. Notably the charge carrier mobility was directly governed by this preferred growth. The electrical conductivity was totally governed by the carrier concentration. The respective changes in carrier concentration were used to suggest the site selection of the fluorine dopant in the SnO₂ lattice.     

Preparation of Sb:SnO2 thin films and its effect on opto-electrical properties

The present study focuses on pure and antimony (Sb)-doped tin oxide thin film and its influence on their structural, optical, and electrical properties. Both undoped and Sb-doped SnO₂ thin films have been grown by using simple, inexpensive pyrolysis spray technique. The deposition temperature was optimized to 450 °C. X-ray diffractions pattern have revealed that the films are polycrystalline and have tetragonal rutile-type crystal structure. Undoped SnO₂ films grow along (110) preferred orientation, while the Sb-doped SnO₂ films grow along (200) direction. The size of Sb-doped tin oxide crystals changes from 26.3 to 58.0 nm when dopant concentration is changed from 5 to 25 wt%. The transmission spectra revealed that all the samples are transparent in the visible region, and the optical bandgap varies between 3.92 and 3.98 eV. SEM analysis shows that the surface morphology and grain size are affected by the doping rate. All the films exhibit a high transmittance in the visible region and show a sharp fundamental absorption edge at about 0.38–0.40 nm. The maximum electrical conductivity of 362.5 S/cm was obtained for the film doped with 5 wt% Sb. However, the carrier concentration is increased from 0.708?×?10¹⁸ to 4.058?×?10²⁰ cm³. The electrical study reveals that the films have n-type electrical conductivity and depend on Sb concentration. We observed a decrease in sheet resistance and resistivity with the increase in Sb dopant concentration. For the dopant concentration of 5 wt% of Sb in SnO₂, the Rs and ρ were found minimum with the values of 88.55 (Ω cm^?2) and 2.75 (Ω cm), respectively. We observed an increase in carrier concentration and a decrease in mobility with the addition of Sb up to 25 wt%. The highest figure of merit values 2.5?×?10^–3 Ω^?1 is obtained for the 5wt% Sb, which may be considered potential materials for solar cells' transparent windows.

     

Preparation and application in p-n homojunction diode of p-type transparent conducting Ga-doped SnO2 thin films

The direct preparation of p-type transparent conducting Ga-doped SnO₂ thin films and their fundamental application in transparent p-n homojunction diode were realized. The films were grown in an active oxygen ambient using reactive rf magnetron sputtering without post-deposition annealing involved. This method improved the electrical properties of the films while maintaining their optical transparency. By growing a p-type thin film on commercial n-type SnO₂:F-coated glass, transparent p-n homojunction diode was obtained. It exhibits a distinct current-voltage rectifying characteristic, manifesting this p-type thin film and the fabrication technology are suitable for industrial applications.     

A novel templating process for fabricating SnO2 hollow hemispherical arrays by pulsed laser deposition

Well ordered monolayer arrays of hollow SnO₂ shell were fabricated by combining colloidal templating and pulsed laser deposition methods. With increasing the thickness of SnO₂ hollow films, their half-spherical morphologies gradually develop to a bell-shaped one. While the surface of SnO₂ thin films is fairly flat, the surface of hollow SnO₂ shells consists of protruding crystals, indicating morphological dependency on the presence of substrate. Because of the low population of contiguous hollow SnO₂ shells, arrays of hollow SnO₂ showed higher dc electrical resistance than those of thin films.     

Influence of Zn doping on electrical and optical properties of multilayered tin oxide thin films

In this study, the electrical and optical properties of Zn doped tin oxide films prepared using sol-gel spin coating process have been investigated. The SnO² : Zn multi-coating films were deposited at optimum deposition conditions using a hydroalcoholic solution consisting of stannous chloride and zinc chloride. Films with Zn doping levels from 0–10 wt% in solution are developed. The results of electrical measurements indicate that the sheet resistance of the deposited films increases with increasing Zn doping concentration and several superimposed coatings are necessary to reach expected low sheet resistance. Films with three coatings show minimum sheet resistance of 1–479 kΩ/ in the case of undoped SnO₂ and 77 kΩ/ for 5 wt% Zn doped SnO₂ when coated on glass substrate. In the case of single layer SnO₂ film, absorption edge is 3.57 eV and when doped with Zn absorption edge shifts towards lower energies (longer wavelengths). The absorption edge lies in the range of 3.489-3.557 eV depending upon the Zn doping concentration. The direct and indirect transitions and their dependence on dopant concentration and number of coatings are presented.     

Modeling the optical properties of tin oxide thin films

Searching the many papers reporting on the optical characteristics of tin oxide thin films, an obvious question arises: what is the origin of the very large differences in the reported optical and electrical properties of these films? The objective of the present work is to resolve this question by applying a modeling approach, simulating the refractive index of SnO, SnO₂, SnO + SnO₂, and porous tin oxide films in the visible range of the spectrum under various structure and composition conditions. Using the semi-empirical model of Wemple and DiDomenico for the dielectric function below the interband absorption edge of ionic and covalent solids, and the effective-medium theory of Bruggeman, the refractive indices of SnO, SnO₂, several mixtures of SnO and SnO₂ and various porous tin oxide films were calculated. The resulting data are compared with some published data to suggest the compositional and structural characteristics of the reported oxides. The correlation between the optical properties of the studied thin films and film composition is also indicated. It is proposed that the large spread in reported optical data is possibly a spread in the composition of the samples.     

(Cu, Fe, Co, or Ni)-doped tin dioxide films deposited by spray pyrolysis: doping influence on film morphology

The results of structural characterization of SnO₂ films doped by impurities such as Fe, Cu, Ni, and Co during spray pyrolysis deposition from 0.2 M SnCl₄–water solutions are presented. The change of parameters such as film morphology, the grain size, texture, and the intensity of X-ray diffraction peaks were controlled. For structural analysis of tested films, we used X-ray Diffraction, Scanning Electron Microscopy, and Atomic Force Microscopy techniques. It was shown that the doping promoted the change of the film morphology and the decrease of the SnO₂ grain size; however, these changes were not great. The doping influence becomes apparent more obviously for thin films and the films deposited at low temperatures (T _pyr ∼ 350 °C). At higher pyrolysis temperatures (T _pyr ∼ 450 °C), the influence of the doping on both the grain size and the film morphology was weakened. We concluded that used additives had dominant influence on the structural properties of SnO₂ at the initial stages of the film growth, as well as at the stages of twinning and agglomeration of the SnO₂ crystallites. It was shown that the increase in the contents of the fine dispersion phase in as-deposited film is an important consequence of the SnO₂ doping.     

An investigation of super-hydrophilic properties of TiO2/SnO2 nano composite thin films

A sol-gel dip coating technique was used to fabricate TiO₂/SnO₂ nano composite thin films on soda-lime glass. The solutions of SnO₂ and TiO₂ were mixed with different molar ratios of SnO₂:TiO₂ as 0, 3, 4, 6, 8, 9, 10.5, 13, 15, 19.5, 25 and 28 mol.% then the films were prepared by dip coating of the glasses. The effects of SnO₂ concentration, number of coating cycles and annealing temperature on the hydrophilicity of films were studied using contact angle measurement. The films were characterized by means of scanning electron microscopy, X-ray diffraction and atomic force microscopy measurements. The nano composite thin films fabricated with 8 mol.% of SnO₂, four dip coating cycles and annealing temperature of 500 °C showed super-hydrophilicity.     

Synthesis of Fe alloyed PbS thin films and investigation of their photovoltaic properties

Lead sulfide (PbS) and iron (Fe)-alloyed PbS thin films with different Fe concentrations were synthesized by chemical bath deposition (CBD) technique, which is suitable for cost on glass substrates at room temperature. The structural, elemental and optical properties of the synthesized thin films were characterized by X-ray diffraction (XRD), energy dispersive X-ray (EDX) and optical absorption measurements, respectively. It was observed that Fe dopant alters crystal size and energy band gap of PbS although it does not change the structure of PbS. PbS and Fe-alloyed PbS thin films with different Fe concentrations were grown on zinc tin oxide (Zn₂SnO₄) substrates coated on fluorine-alloyed tin oxide conductive glasses to investigate their photovoltaic properties. Incident photon-to-current efficiency (IPCE) and current density (J)–voltage (V) measurements were carried out to determine IPCE (%) and power conversion efficiency (η%) values of thin films. As a result, it was observed that as the concentration of Fe dopant is increased in Fe-alloyed PbS thin films, there is an increase in η%.