Hydrogen sensing properties of Pd-doped SnO2 sputtered films with columnar nanostructures

Pd-doped SnO₂ sputtered films with columnar nanostructures were deposited using reactive magnetron sputtering at the substrate temperature of 300 °C and the discharge gas pressures of 1.5, 12, and 24 Pa. Structural characterization by means of X-ray diffraction and scanning electron microscopy shows that the films composed of columnar nanograins have a tetragonal SnO₂ structure. The films become porous as the discharge gas pressure increases. Gas sensing measurements demonstrate that the films show reversible response to H₂ gas. The sensitivity increases as the discharge gas pressure increases, and the operating temperature at which the sensitivity shows a maximum is lowered. The highest sensitivity defined by (R_a − R_g) / R_g, where R_a and R_g are the resistances before and after exposure to H₂, 84.3 is obtained for the Pd-doped film deposited at 24 Pa and 300 °C upon exposure to 1000 ppm H₂ gas at the operating temperature of 200 °C. The improved gas sensing properties were attributed to the porosity of columnar nanostructures and catalytic activities of Pd doping.     

Synthesis and properties of epitaxial SnO2 films deposited on MgO (100) by MOCVD

Epitaxial tin oxide (SnO₂) thin films have been prepared on MgO (100) substrates at 500-600 °C by metalorganic chemical vapor deposition method. Structural and optical properties of the films have been investigated in detail. The obtained films were pure SnO₂ with the tetragonal rutile structure. An in-plane orientation relationship of SnO₂ (110) [010]//MgO (200) [110] between the film and substrate was determined. Two variant structure of SnO₂ were analyzed. The structure of the film deposited at 600 °C was investigated by high-resolution transmission electron microscopy, and an epitaxial structure was observed. The absolute average transmittance of the SnO₂ film at 600 °C in the visible range exceeded 90%. The optical band gap of the film was about 3.93 eV.     

Synthesis and field emission of patterned SnO2 nanoflowers

This article reports the synthesis and field emission of patterned SnO₂ nanoflowers obtained by a simple method. A patterned Au catalyst film was prepared on the silicon wafer by radio frequency (RF) magnetron sputtering and photolithographic patterning processes. The patterned SnO₂ nanoflowers arrays, with a unit diameter of ∼ 50 μm, were synthesized via vapor phase transport method. Field emission scanning electron microscopy (SEM) and X-ray diffraction (XRD) are used to identify the surface morphology and composition of the as-synthesized SnO₂ nanostructures. The mechanism of formation of SnO₂ nanostructures was also discussed. The measurement of field emission (FE) showed that the as-synthesized SnO₂ nanostructure arrays have a lower turn-on field of 2.6 V/μm at the current density of 0.1 μA/cm². This approach must have a wide variety of applications such as fabrications of micro-optical components and micropatterned oxide thin films used in FE-based flat panel displays and sensor arrays.     

Growth of Al2O3 thin film by oxidation of resistively evaporated Al on top of SnO2, and electrical properties of the heterojunction SnO2/Al2O3

Aiming for the investigation of insulating properties of aluminum oxide (Al₂O₃) layers, as well as the combination of this oxide with tin dioxide (SnO₂) for application in transparent field effect transistors, Al thin films are deposited by resistive evaporation on top of SnO₂ thin films deposited by sol–gel dip-coating process. The oxidation of Al films to Al₂O₃ are carried out by thermal annealing at 500 °C in room conditions or oxygen atmosphere. X-ray diffraction data indicate that tetragonal Al₂O₃ is indeed obtained. A simple device and electric circuit is proposed to measure the insulating properties of aluminum oxide and the transport properties of SnO₂ as well. Results indicate a fair insulation when four layers or Al₂O₃ are grown on the tin dioxide film, concomitant with thermal annealing between each layer. The current magnitude through the insulating layer is only 0.2% of the current through the semiconductor film, even though the conductivity of the SnO₂ alone is not very high (the average resistivity is 2 Ω cm), because no doping is used. The presented results are a good indication that this combination may be useful for transparent devices.     

Electrical and optical properties of indium tin oxide films prepared on plastic substrates by radio frequency magnetron sputtering

The influence of deposition power, thickness and oxygen gas flow rate on electrical and optical properties of indium tin oxide (ITO) films deposited on flexible, transparent substrates, such as polycarbonate (PC) and metallocene cyclo-olefin copolymers (mCOC), at room temperature was studied. The ITO films were prepared by radio frequency magnetron sputtering with the target made by sintering a mixture of 90 wt.% of indium oxide (In₂O₃) and 10 wt.% of tin oxide (SnO₂). The results show that (1) average transmission in the visible range (400-700 nm) was about 85%-90%, and (2) ITO films deposited on glass, PC and mCOC at 100 W without supplying additional oxygen gas had optimum resistivity of 6.35 × 10⁻⁴ Ω-cm, 5.86 × 10⁻⁴ Ω-cm and 6.72 × 10⁻⁴ Ω-cm, respectively. In terms of both electrical and optical properties of indium tin oxide films, the optimum thickness was observed to be 150-300 nm.     

(Cu,Fe, Co,or Ni)-doped tin dioxide films deposited by spray pyrolysis: Doping influence on thermal stability of the film structure

The results of doping influence on thermal stability of the SnO₂ film morphology are presented in this article. The SnO₂ films doped by Fe, Cu, Ni, Co (16 at.%) were deposited by spray pyrolysis from 0.2 M SnCl₄–water solution at T_pyr 350–450 °C. The annealing at 850–1030 °C was carried out in the atmosphere of the air. The change of such parameters as film morphology, the grain size, texture and the intensity of X-ray diffraction (XRD) peaks have been controlled. For structural analysis of tested films we have been using X-ray diffraction, Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM) techniques. It was established that the doping does not improve thermal stability of both film morphology and the grain size. It was made a conclusion that the increased contents of the fine dispersion phase of tin oxide in the doped metal oxide films, and the coalescence of this phase during thermal treatment are the main factors, responsible for observed changes in the morphology of the doped SnO₂ films.     

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.     

Selective detection of hydrogen sulfide using copper oxide-doped tin oxide based thick film sensor array

In this work, copper oxide-doped (1, 3 and 5 wt%) tin oxide powders have been synthesised by sol–gel method and thick film sensor array has been developed by screen printing technique for the detection of H₂S gas. Powder X-ray diffraction pattern shows that the tin oxide (SnO₂) doped with 3 wt% copper oxide (CuO) has smaller crystallite size in comparison to 0, 1 and 5 wt% CuO-doped SnO₂. Furthermore, field emission scanning electron microscopy manifests the formation of porous film consisting of loosely interconnected small crystallites. The effect of various amounts of CuO dopant has been studied on the sensing properties of sensor array with respect to hydrogen sulfide (H₂S) gas. It is found that the SnO₂ doped with 3 wt% CuO is extremely sensitive (82%) to H₂S gas at 150 °C, while it is almost insensitive to many other gases, i.e., hydrogen (H₂), carbon monoxide (CO), sulphur dioxide (SO₂) and liquefied petroleum gas (LPG). Moreover, at low concentration of gas, it shows fast recovery as compared to response time. Such high performance of 3 wt% CuO-doped SnO₂ thick film sensor is probably due to the diminishing of the p–n junction and the smallest crystallite size (11 nm) along with porous structure.     

Fabrication of resistive CO gas sensor based on SnO2 nanopowders via low frequency AC electrophoretic deposition

A resistive CO gas sensor has been fabricated using AC electrophoretic deposition (ACEPD) technique. SnO₂ thick films are deposited by applying low frequency (0.01–1,000 Hz) AC electric field to a stable suspension of SnO₂ nanoparticles in acetyl acetone. A carbon film base electrode is used as deposit substrate. Effect of CO gas exposure on conductivity of the SnO₂ film at 300 °C is investigated. Results show that the sensor is sensitive and its response is repeatable. This work shows that ACEPD can be used as an easy and cheap technique for fabrication of electronic devices such as ceramic gas sensors.     

TiO2-SnO2 system for gas sensing—Photodegradation of organic contaminants

Catalytic photodegradation of organic contaminants by means of UV light has been demonstrated for gas sensors based on composites of TiO₂-SnO₂. Thin film resistive-type gas sensors of TiO₂-SnO₂ have been deposited at 350 °C by RF sputtering from a Ti-SnO₂ target with varying surface ratio of SnO₂/Ti. Photodegradation experiments of bromothymol blue by UV light have been performed by means of the optical spectrophotometry over the wavelength range extending from 300 nm to 600 nm. The influence of the UV illumination on the sensor response to 100-20,000 ppm of H₂ has been investigated in situ on motor oil contaminated sensors. It has been found that sequential exposures to UV light lead to a partial recovery of the sensor signal to hydrogen.     

Solid phase growth of tin oxide nanostructures

For the first time, single-crystalline SnO₂ nanostructures comprising of nanobelts, nanowires and nanosheets have been synthesised by solid phase crystal growth from tin oxide single crystals. The product was characterised by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy, selected area electron diffraction, and Raman spectroscopy. The procedure consisted of two stages. In the first stage, a mixture of SnO₂ polyhedral single crystals attached with graphite particles were produced by heating a mixture of SnCl₂ and graphite. Then, the SnO₂ single crystals were grown into nanobelts, nanowires and nanosheets by further heating. The role of graphite in the process is also discussed to be the surface reduction of SnO₂ into oxides with lower oxygen content which provide a driving force for surface diffusion and subsequent crystal growth of tin oxide into the one and two dimensional nanostructures. The results provide insights for both fundamental research as well as technological production of SnO₂ nanostructures.     

A heterostructured SnO2–TiO2 thin film prepared by Langmuir–Blodgett technique

SnO₂–TiO₂ heterostructure films were prepared through Langmuir–Blodgett (LB) route. LB films of octadecyl amine (ODA)–titanyl oxalate multilayer deposited on Si (100) and decomposed at 600 °C showed rutile and anatase phases of ultrathin TiO₂ film. Subsequently, multilayer LB film of ODA–stannate deposited on the pre deposited TiO₂ film after decomposition at 600 °C resulted in thin SnO₂ films on the TiO₂ thin film. The phase analysis of the SnO₂–TiO₂ film showed cassiterite phase of SnO₂ as well as the rutile/anatase mixture of TiO₂ indicating a SnO₂–TiO₂ heterostructured film. Surface morphology of the pure TiO₂ film and SnO₂–TiO₂ film were analyzed by using AFM. Electrical characterization by AC impedance analysis suggested SnO₂–TiO₂ heterostructure formation. DC current voltage measurement showed increase in photocurrent indicating visible light absorption and efficient charge separation under the sunlight type radiation.     

Structural and electrical properties of sputtered indium-zinc oxide thin films

In this study, indium-zinc oxide (IZO) thin films have been prepared at a room temperature, 200 and 300 °C by radio frequency magnetron sputtering from a In₂O₃-12 wt.% ZnO sintered ceramic target, and their dependence of electrical and structural properties on the oxygen content in sputter gas, the substrate temperature and the post-heat treatment was investigated. X-ray diffraction measurements showed that amorphous IZO films were formed at room temperature (RT) regardless of oxygen content in sputter gas, and micro-crystalline and In₂O₃-oriented crystalline films were obtained at 200 and 300 °C, respectively. From the analysis on the electrical and the structural properties of annealed IZO films under Ar atmosphere at 200, 300, 400 and 500 °C, it was shown that oxygen content in sputter gas is a critical parameter that determines the local structure of amorphous IZO film, stability of amorphous phase as well as its eventual crystalline structure, which again decide the electrical properties of the IZO films. As-prepared amorphous IZO film deposited at RT gave specific resistivity as low as 4.48 × 10^− 4 Ω cm, and the highest mobility value amounting to 47 cm²/V s was obtained from amorphous IZO film which was deposited in 0.5% oxygen content in sputter gas and subsequently annealed at 400 °C in Ar atmosphere.     

XRD and SEM studies of reactively deposited tin oxide thin films

Stoichiometric polycrystalline tin oxide thin films were deposited by the reactive evaporation of tin and the SnO₂ formation was found to be strongly dependent on the deposition parameters. The preferred orientation of the SnO₂ films deposited on different substrates was varying due to the dislocation defects arising during the thin film formation. The X-ray diffraction (XRD) studies identified a tetragonal structure while the scanning electron microscopic (SEM) studies revealed a polycrystalline surface for the SnO₂ films reactively deposited.     

Gas sensing properties of lead doped tin oxide thick films

The present investigation deals with the fabrication of liquid petroleum gas (LPG) sensor materials based on semiconducting oxide SnO₂. The gas sensor materials have been prepared by conventional solid-state route. The effect of Pb incorporation, operating temperature, morphology, and sensitivity is discussed using the results of X-ray diffraction (XRD), along with sensing performance. Out of various sensor compositions, Pb doped SnO₂ sintered at 1000 °C for 2 h has shown high sensitivity towards LPG at an operating temperature of 150 °C. Different characterization techniques have been employed, such as surface area analyzer, X-ray diffraction (XRD), to study the formation of SnO₂, surface area and crystallite size, respectively. The results suggested the possibility of utilizing the sensor element for the detection of LPG.     

Effect of SHI irradiation on the morphology of SnO2 thin film prepared by reactive thermal evaporation

SnO₂ thin films prepared by reactive thermal evaporation on glass substrates were subjected to 120 MeV Ag⁹⁺ ion irradiation. The surface topography progression using the swift heavy ion irradiation was studied. It shows creation of unique surface morphologies and regular structures on the surface of the SnO₂ thin film at particular fluences. Field Emission Scanning electron microscopy (FE-SEM) and Atomic force microscopy (AFM) are used for investigating the effect of Ag ions at different fluences on the surface of SnO₂. The morphological changes suggest that ion assisted/induced diffusion process play a significant role in the evolution of nanostructures on SnO₂ surface. The roughness increases from 9.4 to 14.9 with fluence upto 1 × 10¹² ions/cm² and beyond this fluence, the roughness decreases. Ion-beam induced recrystallization at lower fluences and amorphization or disordering of crystals at higher fluences are understood based on the thermal spike model.     

Nanocrystalline SnO2 and Au:SnO2 thin films prepared by direct current magnetron reactive sputtering

Nanocrystalline pure and gold doped SnO₂(Au:SnO₂) films were prepared on unheated glass substrates by dc magnetron reactive sputtering and, subsequently, the as deposited films were annealed in air. The films structure, surface morphology, photoluminescence, electrical and optical properties were investigated. After annealing the as deposited SnO₂ films, crystallinity increased and the surface roughness decreased. The intensity of PL peaks increases sharply with the annealing temperature. The optical transmittance of the films was around 89% after annealing the as deposited SnO₂ films at 450 °C. The as deposited Au:SnO₂ films show better crystallinity than the as deposited SnO₂ films, the average grain size was around 4.4 nm. The emission peaks of Au:SnO₂ films are slightly blue shifted as compare to undoped SnO₂ films. The Au:SnO₂ films show the lowest electrical resistivity of 0.001 Ωcm with optical transmittance of 76%, after annealing at 450 °C.     

Microstructure-Property relationships in thin film ITO

Polycrystalline tin-doped indium oxide (ITO) thin films were prepared by pulsed laser deposition (PLD) with an ITO (In₂O₃-10 wt.% SnO₂) target and deposited on borosilicate glass substrates. By changing independently the deposition temperature and the oxygen pressure, a variety of microstructures were deposited. These different microstructures were mainly investigated not only by transmission electron microscopy (TEM) with cross-section and plan-view electron micrographs, but also by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction. Composition changes in ITO thin films grown under different deposition conditions were characterized by energy dispersive X-ray spectroscopy (EDX). The optical and electrical properties were studied respectively by UV-visible spectrophotometry and a four-point probe. The best compromise in terms of high transmittance (T) in the visible range and low resistivity (ρ) was obtained for films deposited between 0.66 and 2 Pa oxygen pressure (P_O2) at 200 °C substrate temperature (T_s). The influence of P_O2 and T_s on the microstructure and ITO film properties is discussed.     

Gas sensing selectivity of oxygen-regulated SnO2 films with different microstructure and texture

The selectivity of gas sensing materials is increasingly important for their applications. The oxygen-regulated SnO₂ films with (110) and (101) preferred orientation were obtained through magnetron sputtering, followed by annealing treatment. Their micro-structure, surface morphology and gas response were investigated by advanced structural characterization and property measurement. The results showed that the as-prepared (110)-oriented SnO₂ film was oxygen-rich and had more adsorption sites while the as-prepared (101)-oriented SnO₂ film was oxygen-poor and more sensitive to de-oxidation. H₂ gas sensitivity, response speed, selectivity between H₂ and CO of the (110)-orientated SnO₂ film was superior to that of the (101)-orientated SnO₂ film. After treated at high temperature and high vacuum, the reduction of gas-sensing properties of the annealed (110) SnO₂ film was much more than that of the annealed (101) SnO₂ film. The lattice oxygen was responsible for the difference in gas-sensing response between (110) and (101)-oriented SnO₂ films under oxygen regulation. This work indicated the gas-sensing selectivity of the different crystal planes in SnO₂ film, providing a significant reference for design and extension of the related materials.     

H2S sensing property of porous SnO2 sputtered films coated with various doping films

Pure SnO₂ films and Ag-, Cu-, Pt-, and Pd-doped SnO₂ films were investigated for H₂S sensing properties. SnO₂ films were deposited by DC magnetron sputtering at various substrate temperatures and discharge gas pressures. As the discharge gas pressure increased and the substrate temperature decreased, the film became porous. Doping with Cu or Ag film improved the sensitivity, and the highest sensitivity was obtained in the porous SnO₂ film coated with an Ag film 16 nm thick. According to the X-ray diffraction (XRD) pattern, Ag deposited on SnO₂ film transformed to Ag₂S upon exposure to H₂S. When the Ag-doped film sensor was operated at a low temperature, the sensitivity was extremely high, but the recovery was insufficient. By increasing the operation temperature, the recovery was improved but the sensitivity decreased.