Abstract

Nickel oxide (NiO) and manganese dioxide (MnO₂) are naturally abundant, environment-friendly low-cost, and technologically important compounds having p-type and n-type semiconducting nature, respectively [1, 2]. In this work, MnO₂/NiO bilayer composite thin films were deposited onto glass substrates by spin coating technique. The film thickness variation was achieved via the change of rotation per minute (rpm) during spin coating. The NiO films deposited at 3000, 3500, and 4000 rpm, were used as the substrate for the deposition of MnO₂/NiO thin films. The as-deposited films were annealed in the air at 600 ℃ for 1 hour. The structure of the thin film was explored using X-ray diffraction technique. The MnO₂/NiO film showed the existence of (211) peak related to α-MnO₂ crystalline phase along with the (200) and (220) peaks related to the face centered cubic structure of NiO. Field emission scanning electron microscopy analysis of MnO₂/NiO thin films showed porous surface morphology and the grain size was obtained in the rage of 20 - 62 nm. UV-vis-NIR spectroscopy analysis was carried out to record the transmittance data in the spectral range of 300 - 1100 nm. In the NIR region of light, the maximum transmittance of the MnO₂/NiO film was 54%. The optical band gap was estimated in the range of 2.54 - 2.71 eV for MnO₂/NiO films. The photoluminescence (PL) spectra of MnO₂/NiO films were studied using an excitation wavelength 270 nm. The deep and low-level emission bands were found in the wavelength ranges 368 - 367 nm and 620 - 623 nm, respectively. The electrical resistivity of MnO₂/NiO thin films was recorded using the two-point probe method. The electrical resistivity was found in the order 10⁴ Ω-cm.

Abstract

Composite thin films with a bilayer structure composed of nickel oxide (NiO) and tri-manganese tetra-oxide (Mn₃O₄) were deposited using the spray pyrolysis technique. NiO thin film was deposited on to a glass substrate at the temperature of 450 ℃ as the first layer and Mn₃O₄ thin film was deposited on it as the second layer under the same deposition conditions. The X-ray diffraction analysis of Mn₃O₄/NiO film revealed the existence of the diffraction peaks (011) corresponding to Mn₃O₄ phase and (111), (200), and (220) corresponding to NiO phase. In field emission scanning electron microscopic analysis, the surface of the composite film was observed to be comprised of cube-like agglomerated nanoparticles. The optical properties of the deposited films were investigated in the spectral range 300 - 1100 nm using UV–vis spectroscopy. The transmittance of composite film decreased significantly compared to that of NiO film. The optical band gaps of NiO, Mn₃O₄, and Mn₃O₄/NiO thin films were found 3.33, 3.12 and 3.03 eV, respectively. The Urbach energy of the Mn₃O₄/NiO film was found 1.99 eV. Extinction coefficient, optical conductivity and dielectric constants of composite film was higher than those of NiO and Mn₃O₄ thin films. The room temperature electrical resistivity of all the films was found in the order of 10⁵ W-cm and it was found higher for Mn₃O₄/NiO film compared to the NiO and Mn₃O₄ films. Electrical conductivity showed nonlinear variation with temperature with three conduction regions.  It can be said from the results that the Mn₃O₄/NiO bilayer has feasibility in optoelectronic applications.

Abstract

Nickel oxide (NiO) and 2at% manganese (Mn)-doped NiO thin films were synthesized by the spray pyrolysis technique at 450 ℃. The surface morphology, structure, optical properties, and methanol gas sensing properties were investigated using FESEM, XRD, UV-vis spectroscopy, and the two-point probe method, respectively. The deposited thin films showed a face-centered cubic structure with the preferred orientation along the (111) plane. Porous morphology with the agglomerations of nanoparticles was observed in scanning electron microscopy analysis. The band gap of NiO thin film increased from 3.14 to 4.03 eV for 2at% Mn-doping. To study methanol sensing properties of undoped and 2 at% Mn-doped NiO thin films, 1 to 5 mL methanol gas was used with bias voltage 20 V to undoped at the ambient temperature of 303 K. The highest sensitivity for NiO film was around 800% for 4 mL methanol, whereas the highest sensitivity increased to 3200% for 5 mL for 2at% Mn-doped NiO film. The response and recovery time were 8.5 and 5.87 minutes for NiO film for 4 mL methanol, respectively. For 2 at% Mn-doped NiO film, the response and recovery time were 14.07 minutes and 9.38 minutes, respectively, for 5 mL. These findings indicate 2 at% Mn-doped NiO can be a crucial compound for methanol sensing at ambient temperature.

Abstract

Nickel oxide (NiO) and manganese dioxide (MnO2) are two technologically important metal oxides showing p-type and n-type semiconducting behaviors, respectively. In this work, NiO thin films are deposited onto glass substrates by sol-gel spin coating technique. NiO films were deposited at 3000, 3500, and 4000 rpm, respectively. NiO films were used as the substrate for the deposition of MnO2/NiO bilayer thin films. The as-deposited films were pre-heated at 350 ℃ for 10 min before annealing in the air at 600 ℃ for 1 hour. The film thickness variation was achieved via the change of rotation per minute (rpm) during spin coating. The structure of the thin film was explored using X-ray diffraction (XRD) technique. The NiO film prepared at 3000 rpm showed XRD peaks for (002) plane related to the hexagonal Ni2O3 structure. NiO films prepared at 3500 and 4000 rpm show peaks for (111), (200) and (220) planes related to face-centered cubic NiO structure. The MnO2/NiO film showed the existence of (211) peak related to α-MnO2 crystalline phase along with the (200) and (220) peaks for the NiO substrate. Field emission scanning electron microscopy analysis of NiO films revealed that the film surface contained agglomerated nanoparticles. The shape of the particles changed from spherical to uniformly distributed cubic particles in the MnO2/NiO film. UV-vis spectroscopy analysis was carried out to record the transmittance data in the spectral range of 300 - 1100 nm. In the visible region of light, the maximum transmittance of the NiO films was found 90%, whereas the highest transmittance dropped to 54% for MnO2/NiO film.  The optical band gap was estimated in the range of 3.38 - 3.48 eV for the NiO films and 1.48 – 2.10 eV for MnO2/NiO films. The I-V characteristic curves were recorded using the two-point probe method. Resistance of the films was found in the ranges 205- 566 MΩ for NiO films and 123 -491 MΩ for the MnO2/NiO films, respectively.

Abstract

Bilayer composite thin films composed of manganese oxide (MnO_x) and nickel oxide (NiO) were synthesized using the spray pyrolysis technique. First, the NiO thin film was deposited on to glass substrate at the temperature 450 ℃. The MnO_x thin film was deposited on the NiO film as the second layer under the same deposition conditions. The surface morphology of the film was investigated by field emission scanning electron microscope. The film surface comprised of agglomerated nanoparticles. Energy dispersive X-ray analysis confirmed the elemental composition of the film. The X-ray diffraction analysis of NiO and MnO_x/NiO films were performed which revealed the existence of the required MnO_x/NiO composite. The diffraction peaks were observed at 2q values: 18.3883, 37.2359, 43.2653, and 62.8769º, corresponding to (011), (111), (200), and (220) planes, respectively. The (011) peak appeared for the Mn₃O₄ phase and the rest of the peaks were for NiO among which (111) was the predominant one. UV-vis spectroscopy was used to investigate the optical properties in the spectral range of 300 - 1100 nm. The reflectance of the film increased by 32% in the IR region. The band gap of the composite thin film was direct, and the determined value was 2.86 eV. It can be said from the results that the MnO_x/NiO bilayer has probable use in optoelectronic applications.

Abstract

Copper (II) oxide (CuO) is a semiconductor with p-type conductivity and monoclinic structure. Availability in nature, environment friendly behavior, band gap in the visible region of light and ability to form porous microstructure has made CuO an interesting material to the researchers. In recent years, transition metal-doped CuO thin films have been prepared using several deposition techniques and the properties of the films have been studied by the scientific community for device applications. In this work, cobalt (Co)-doped CuO thin films have been synthesized using a simple and low-cost spray pyrolysis technique. CuO films have been prepared with the Co concentrations 0, 2, 4, 6 and 8 at%. Field emission scanning electron microscopic images of Co-doped CuO thin films, show the aggregations of nanoparticles on the film surfaces. For 6 at% Co doping, the surface morphology of CuO turns into porous and sponge-like nanostructures. In X-ray diffraction analysis, CuO film shows monoclinic structure with the preferential orientation along (11) direction. No other peak corresponding to Co impurity or any other crystalline phase of copper oxide are found up to 4 at% of Co doping. The (11) peak loses its predominance with the appearance of the (311) peak corresponding to Co₃O₄ and (200) peak related to Cu₂O upon 6 and 8 at% Co doping. The crystallite size decreases from 11 to 5 nm with Co doping. In UV-visible spectroscopy, the optical transmittance of Co-doped CuO films increases with wavelength and saturates in the near-infrared light region. The highest transmittance of about 96% is found for the 6 at% Co- doped CuO film. The optical band gap of the films increases from 2.14 to 2.91 eV with Co doping. Refractive index, dielectric constants, and electron effective mass reduces with Co doping. Electrical resistivity of the films decreases with Co doping. The films shows p-type conductivity up to 2 at% Co doping, then for further doping the conductivity changes into n-type with the reduction of Hall coefficient and carrier mobility. Remarkable changes occur in the structural, morphological, optical and electrical properties of CuO thin films because of Co doping. It may be said that Co-doped CuO thin films can be used for optoelectronic applications.

Abstract

Nickel oxide (NiO) is a semiconductor with face-centered cubic (fcc) crystal structure. Because of the wide band gap, high transparency, and porous surface morphology of NiO, it shows high-performance in various electronic and optoelectronic devices. For this reason, recently metal doped NiO thin films has drawn the attention of the researchers. In this work, NiO and manganese (Mn) doped NiO thin films have been deposited onto glass substrates at 723 K temperature using a simple and low-cost spray pyrolysis technique. The amount of Mn doping is varied from 0 to 4 at% at the step of 1 at% in NiO films. Structure, morphology, chemical composition as well as the optical and electrical properties of the deposited films are studied. X-ray diffraction analysis shows that NiO and Mn-doped NiO films has fcc structure with the predominance of (111) peak. Crystallite size decreases from 31 to 22 nm with Mn doping. Surface morphology was investigated by a scanning electron microscope (SEM). In SEM images all the samples show uniform distribution of particles with porous structure. Chemical composition and stoichiometry the deposited thin films have been confirmed by Energy dispersive X-ray spectroscopy. The optical transmittance is measured in the wavelength range 200 - 1100 nm using an UV-Visible spectrophotometer. Highest transparency around 90% and band gap of 4.03 eV is obtained for 2 at% Mn-doped NiO thin film. Extinction coefficient and dielectric loss tangent in the visible-NIR region of light reduce with Mn doping. 1 at% Mn doped NiO film shows the highest optical conductivity and refractive index in the visible light. The room temperature resistivity increases with Mn-doping up to 2 at%. Investigation of the temperature dependent electrical conductivity reveals that almost all the films show three regions of conduction except the 2 at% Mn-doped film. Two regions of conductivity are found for 2 at% Mn-doped NiO thin film. The activation energies are calculated in the temperature range of 300 - 373 K. All the samples show low activation energy at the higher temperature regions. Formation of transparent and crystalline NiO thin films with porous surface morphology, wide band gap, high resistivity and low activation energy via Mn-doping indicated the suitability of this material in fabrication of sensors and high-power devices.    

Abstract

Thin films of Ba_1-xCa_xTiO₃ with various calcium (Ca) contents (x = 0, 2, 4, 6, and 8 at.%) were successfully synthesized onto the glass substrate by a thermal spray pyrolysis technique. Field emission scanning electron microscopy images revealed that the average grain size varies from 1.38 to 4.95 μm. The presence of all the elements was confirmed by energy dispersive X-ray analysis. The X-ray diffraction analysis confirmed the formation of the hexagonal phase of as-deposited samples. The crystallite sizes were found to reduce from 189 to 116 nm as a result of Ca doping. Atomic force microscopy indicated surface roughness varies from 86 to 187 nm. The 2 at.% Ca doped thin films exhibited transparency of about 47%. From the UV-visible spectra, an increment of optical conductivity is calculated from 2.9 to 7.3 s^-1. The optical band gap varies from 3.90 to 3.80 eV with the increase of Ca substitution. The electrical resistivity of the films decreased from 11.95×10³ to 7.54×10³ Ωm with increasing Ca content. Nanostructured thin films with uniform surfaces and well-distributed grains are grown in the present research. The results obtained from Ba_1-xCa_xTiO₃ thin films could be useful for various optoelectronic applications.  

Abstract

Nickel oxide (NiO) is a p-type metal oxide semiconductor with a band gap in the range of 3.60 – 4.00 eV. In this work, a simple and low-cost spray pyrolysis technique was used for the deposition of NiO thin films with 0, 1, 2, 3, and 4 at% manganese (Mn) concentrations. In the X-ray diffraction (XRD) analysis of the films, peaks correspond to (002), (111), (200), (220), (311) and (222) crystalline planes appeared. The films exhibited face-centered cubic (fcc) structure with the predominance of (111) peak. The intensity of (111) peak maximized for 1 at% Mn doping in NiO indicating improvement in crystallinity in comparison with NiO film. No other peaks corresponding to Mn impurities were found in the XRD analysis. No further improvement was found in crystalline structure upon 4 at% Mn doping. The optical properties of the deposited films were studied in the spectral range 300 – 1100 nm using a UV-vis spectrophotometer. NiO and Mn-doped NiO films were transparent in the visible and near infrared region. The transmittance of the film increased significantly with Mn doping. The transmittance of 2 at% Mn doped NiO film reached a maximum of about 87%, which is much higher than the maximum transmittance of NiO film. The optical band gap of the films was in the range of 3.17- 4.03 eV. The maximum band gap was found for 2 at% Mn doped NiO thin film. The increase of transparency and optical band gap indicates the applicability of this material in the field of optoelectronics devices.

Abstract

Zinc oxide (ZnO) and boron (B) doped ZnO thin films were deposited onto microscope glass slides by spray pyrolysis technique. The amount of B doping was varied between 1 and 5 at% in ZnO thin films. An evolution in surface morphology from nanofibrous to nanoflakes pattern has been observed in B doped ZnO thin films by field emission scanning electron and atomic force microscopies images. The chemical structure and composition of the films are confirmed by Energy dispersive X-ray and X-ray photoelectron spectroscopy analyses. The films show a polycrystalline wurtzite structure with the preferential (002) orientation in X-ray diffraction analysis. Crystallite size is found in the range of 18 to 32 nm. The films are transparent in the Vis-NIR region and the optical band gap is found between 3.20 and 3.29 eV. Direct current electrical resistivity and carrier concentration of B doped ZnO thin films are found in the order 10^-2 Ω-cm and 10²⁰ cm-3, respectively. The films show a change of majority charge carriers from n-type to p-type for 5 at% B doping. The highest carrier mobility is found to be 1.44 cm²V^-1s^-1 for 1 at% B doped ZnO thin film. The results of various investigations presented in the paper suggest the suitability of B doped ZnO thin films in optoelectronic applications. 

Abstract

Cupric oxide (CuO), an environment friendly p-type transition metal oxide was studied in undoped and doped nanostructured forms by researchers for several potential applications such as solar cells, transistors, gas sensors, etc. In this work, manganese (Mn) doped CuO thin films were prepared onto glass substrates at the substrate temperature of 523 K using spray pyrolysis technique. The amount of Mn concentration was varied from 0 to 6 at% in CuO thin films. The prepared samples were annealed at 723 K in the air for 1 hour. The film surface was observed to be comprised of aggregations of nanoparticles in the Scanning Electron Microscopic images. Elemental composition of the films was confirmed by Energy Dispersive X-ray analysis. The as-deposited films showed amorphous structure, whereas the annealed films showed monoclinic structure with the preferential orientation along ( 1 11) and (111) planes. Crystallite size was found between 10 and 19 nm. Optical properties were measured using a UV-vis spectrophotometer. Transmittance of CuO thin film increased with Mn concentration in the visible-infrared region of light. Optical band gap of the films broadened from 2.09 to 2.45 eV with increase in Mn doping. Electrical resistivity of the samples raised from 0.64×10³ to 1.62×10³ Ω-m with the increase of Mn doping. The films showed two regions of electrical conduction in the temperature ranges 383 - 423 K and 323 - 383 K, respectively. Activation energy varied from 0.08 to 0.29 eV in the temperature ranges 323 - 383 K, whereas it varied from 0.29 to 0.40 eV in the temperature ranges 383 - 423 K. The results suggest that Mn doped CuO thin films can be suitable for optoelectronic devices.

Abstract

Magnesium (Mg) doped Ferric oxide (Fe₂O₃) thin films were synthesized onto soda lime glass substrates at the temperature of 350 ℃ by the thermal spray pyrolysis technique. Mg doped Fe₂O₃ thin films were prepared with 0, 2, 4, 6, 8 and 10 at% Mg concentrations. Mg doped Fe₂O₃ thin films are found to have porous surface with uniform distribution of peaks and valleys in atomic force microscopy images. The average roughness of Mg doped Fe₂O₃ thin films increases in the range 11 to 66 nm with Mg concentration. X-ray diffraction spectra of Mg doped Fe₂O₃ thin films show that the films have corundum structure with preferential (104) peak. Mg doping causes shifting and splitting of (104) peak due to the generation of lattice asymmetry in Fe₂O₃ structure. Raman spectroscopy of Mg doped Fe₂O₃ thin films shows the presence of hematite phases in the range of wavenumber 200 - 1320 cm^-1. The optical band gap widens from 2.11 - 2.75 eV with the increase of Mg concentration. Refractive index and extinction coefficient at the wavelength 750 nm decreases with Mg concentration. Photoluminescence (PL) property of Fe₂O₃ thin film is enhanced via Mg doping and that promotes the blue emission in Fe₂O₃ thin films. Band gap widening is also evident from the PL spectra. The exclusive topographical features, improved crystalline structure, broadened band gap and enriched PL behavior of Mg doped Fe₂O₃ thin films suggest its suitability in the fabrication of optoelectronic devices.

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Ferric oxide (Fe₂O₃) is an intrinsic n-type semiconductor having a wide band gap and environment friendly nature. Magnesium (Mg) is one of the p-type dopants used to enhance electrical conductivity of Fe₂O₃. In this work, Mg doped Fe₂O₃ thin films were synthesized with 0, 2, 4, 6, 8 and 10 at% Mg concentrations by a simple spray pyrolysis technique. The surface of the films comprised of uniformly distributed nanoparticles along with some cubic shaped nanocrystals, which were found up to 6 at% Mg doping. Corundum structure with the favored orientation along (104) plane is found in the X-ray diffraction patterns of Mg doped Fe₂O₃ thin films. Raman spectra of Fe₂O₃ thin films showed the existence of pure α-Fe₂O₃ or hematite phases in the film, which mostly disappeared after 10 at% Mg doping. Transmittance of Mg doped Fe₂O₃ thin films increased with the rise of Mg concentration. The maximum transmittance was about 83 %, found in 10 at% Mg doped Fe₂O₃ thin films. The optical band gap widened from 2.14 to 2.75 eV with the increment of Mg concentration up to 8 at%. The room temperature electrical resistivity of Mg doped Fe₂O₃ thin films increased from 4.9 × 10⁵ - 12.2 × 10⁵ Ω-cm with the increase of Mg concentration. The n-type conductivity of Fe₂O₃ thin film is changed to p-type above 6 at% Mg doping and carrier mobility decreased with the raise of Mg concentration. The hysteresis loop of Mg doped Fe₂O₃ thin films recommended that Mg doping softened the ferromagnetic properties of Fe₂O₃. The unique surface morphology, better crystallinity, high transparency, wide band gap, p-type conductivity and ferromagnetic behavior of Mg doped Fe₂O₃ thin films recommended its versatile use in manufacturing optoelectronic devices.

Abstract

Ferric oxide (Fe₂O₃) is a large band gap material having the versatility of applications in various devices. Aluminum (Al) is a popular dopant used for Fe₂O₃ to modify the structure and properties of the material. In this work, Al doped Fe₂O₃ thin films were prepared onto clean glass substrates at the substrate temperature of 350 °C with 0, 2, 4, 6, 8 and 10 at% Al concentration by spray pyrolysis technique. Field emission scanning electron microscopy and atomic force microscopy images showed the presence of nanoparticle agglomerates and regular distribution of pores covering the scanned areas of Al doped Fe₂O₃ thin films. X-ray diffraction patterns of Al doped Fe₂O₃ thin films showed polycrystalline Corundum structure with a predominant orientation along (104) plane, which disappeared at 10 at% Al doping. Raman spectroscopy of Fe₂O₃ thin films suggested the presence of pure α-Fe₂O₃ or hematite phases in the film. Optical transmittance of Al doped Fe₂O₃ thin films increased with the rise of Al concentration up to 6 at% and the maximum transmittance was found to be 87 %. The optical band gap increased from 2.19 to 2.98 eV with the increment of Al concentration up to 6 at%. The room temperature electrical resistivity of Al doped Fe₂O₃ thin films was found between 5.90 x 10⁵ and 8.92 x 10⁵ Ω-cm. Al doped Fe₂O₃ thin films showed n-type electrical conductivity and carrier concentration in the order of ~ 10¹⁷ cm^-3 in Hall effect measurements. Ferromagnetic properties of Fe₂O₃ thin films were softened upon Al doping. Homogeneous surface, regular distribution of pores, better crystalline structure, high transparency, wide band gap, n-type conductivity and ferromagnetic behavior of Al doped Fe₂O₃ thin films recommends its suitability in optoelectronic devices.

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Electrical conductivity of p-type single crystal Gallium Arsenide was measured in the temperature ranges 170K-300K. It was found that electrical conductivity decreases with the increase of temperature. The logarithmic graph of conductivity (lnσ) versus the inverse of temperature (in K^-1) was found to be almost linear in ranges 185K-195K (below room temperature). The acceptor ionization energy (ΔE‪_A) obtained from the graphs was 0.0308 eV. Photoconductivity of the same samples was also measured under different conditions. The effect of varying sample current (I), light intensity (I_L) and temperature (T in K) on photoconductivity at a constant chopping frequency 45.60 Hz were investigated. It has been seen that photoconductivity increases nonlinearly with light intensity and it increases linearly with temperature. Absorption coefficient (α) of the same samples was also measured at the room temperature within the wavelength range 300 nm-2500 nm and the optical band gap energy was calculated to be 1.425 eV from the plot of (αhν) ² versus photon energy (E).