Tin(IV) oxide (SnO2) is a metal oxide renowned for its excellent optoelectronic properties. With the use of simple post-processing methods, the characteristics of SnO2 may be easily modified. In the current work, SnO2 thin films were prepared using the spray pyrolysis technique and were subjected to post-UV-ozone (UVO) treatment for different durations. Characterization techniques including x-ray diffraction, Raman spectroscopy, scanning electron microscopy, energy-dispersive spectroscopy, UV–visible spectroscopy, and photoluminescence spectroscopy were employed to assess the effects of UVO treatment. It was found that UVO treatment had no significant impact on the film's structural characteristics. However, after exposure to UVO, the bandgap was seen to decrease from 3.04 eV to 2.84 eV. Also, photoluminescence investigations revealed that UVO treatment increased the defects in the films with a decrease in the ratio between band-to-band emission and defect emissions. The results indicate that UVO treatment is an effective strategy for tuning the optical properties of SnO2 thin films by precisely managing the bandgap.
Notes
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Introduction
Tin oxide, which possesses exceptional optoelectronic properties, is one of the most well-known metal oxides. These materials are suitable for a range of applications, including solar cells, flat-panel displays, transistors, sensor devices, and other power electronics, owing to their broad bandgap, low electrical resistance, and high chemical stability.1 The properties of tin oxides such as bandgap, electrical resistivity, and defect level can be readily tuned by employing several approaches, including doping,2 surface modification,3 and nano-structuring.4 Postprocessing techniques such as annealing,5 plasma treatments,6 and irradiation7 can also modulate the material properties. Among the several postprocessing methods available for thin films, UV-ozone/O3 (UVO) treatment is a well-recognized technique that has the potential to tune the characteristics of the material. These changes mainly involve the surface properties and electrical properties as a result of changes in the defect levels.8,9 Dong et al. used post-UVO treatment on SnO2 nanocrystals and found an improvement in the optical and electrical properties after UVO treatment.10 Oluwabi et al. observed that at an optimized UVO treatment time, it is possible to reduce oxygen vacancies and hence achieve better dielectric properties for sprayed zirconium oxide films.11 Mendez et al. showed that UVO-treated SnO2 film could perform well as electron transport layer in a perovskite solar cell, with a decreased recombination rate after treatment.12 However, UVO treatment can modulate the material properties, and the time of exposure or treatment should be optimized.
There are several synthesis techniques for preparing SnO2 thin films, including both physical13 and chemical methods.14 Spray pyrolysis is the most effective chemical method for the synthesis of SnO2 thin films, as it is a cost-effective vacuum-free technique that can provide high-quality films even at a large scale.15 The material characteristics of the synthesized thin film are largely determined by various deposition parameters. To the best of our knowledge, there have been no reports on the impact of UVO treatment on spray-pyrolyzed SnO2 films.
Advertisement
In this work, we report an effective strategy for tuning the bandgap of SnO2 thin films through the use of UVO treatment as an accessible postprocessing technique. The modifications achieved by UVO treatment were investigated by utilizing different structural, morphological, and optical studies.
Experimental Details
Utilizing the spray pyrolysis technique (Holmarc, model HO-TH-04), tin oxide thin films were deposited onto soda–lime glass (SLG) substrates, which are stable and can withstand up to around 600°C.16‐19 The initial precursor solution was made by dissolving SnCl4·5H2O in double-distilled water to obtain 0.1 M solution, and 100 µL of concentrated HCl was added to ensure the salt was completely dissolved in the solvent. The pH of the solution was then found to be 1. Then, it was magnetically stirred for 1 h to obtain a homogeneous solution. Prior to the deposition, SLG substrates were subjected to acid treatment for 48 h, followed by cleaning through ultrasonication in soap solution, acetone, and isopropyl alcohol separately for 15 min each. Cleaned glass slides were used as substrate and the precursor was sprayed on the substrate, which was maintained at a substrate temperature of 400°C. Air was used as carrier gas, which was set to a pressure of 1.2 bar. Throughout the deposition, the flow rate was held constant at 1 mL/min. The synthesized films were then subjected to UVO treatment for different durations (30 min and 60 min) using a Jelight model 30 with a low-pressure mercury grid lamp of wavelength 253.7 nm (average intensity 28–32 mW/cm2). The distance between the sample and the UV source was around 30 mm. All precautions were taken prior to the experiment since both UV radiation and ozone gases are harmful. Figure 1 shows the setup for the UVO treatment.
Fig. 1
Photographs of experimental setup for UVO treatment: (a) open setup with the sample (UV radiation is OFF), (b) closed setup (UV radiation is ON).
×
The untreated and UVO-treated thin films were characterized through various techniques including x-ray diffraction (XRD) [Rigaku MiniFlex 600 diffractometer with Cu-Kα radiation, with λ = 1.5406 Å], Raman spectroscopy [Renishaw inVia Raman microscope, 532 nm excitation wavelength with laser power of 5mW], scanning electron microscopy (SEM) [Zeiss Sigma 5 keV accelerating voltage], energy-dispersive spectroscopy (EDS) [Zeiss Sigma, 15 keV accelerating voltage], UV–visible spectroscopy [UV-Shimadzu 1800], and photoluminescence spectroscopic (PL) [Jasco spectrofluorometer FP-8500: excitation wavelength 265 nm] measurements. The thickness of the prepared thin films was obtained using a Bruker DektakXT stylus profilometer and was found to be 600 nm ± 20 nm for all the films irrespective of the UVO treatment time.
Results and Discussion
X-Ray Diffraction Studies
Figure 2 illustrates the XRD pattern of SnO2 thin films which were subjected to UVO treatment for different time intervals. We may deduce from the pattern that all the samples were polycrystalline, exhibiting a tetragonal crystal structure. The observed planes (110), (101), (200), (211), (220), (310), and (310) at 26.25°, 33.46°, 37.49°, 51.43°, 54.29°, 61.43°, and 65.04° match well with the standard JCPDS no. 41-1445. For all the samples, the preferred orientation is along the (110) plane, and the UVO treatment did not affect the crystal plane orientations. A similar effect was observed by Mendez et al. in their work on the SnO2 electron transport layer.12
Fig. 2
XRD pattern of untreated and UVO-treated SnO2 thin films.
×
Advertisement
The structural parameters were computed and are tabulated in Table I. The crystallite size was calculated using Scherrer’s formula, given by D = Kλ / βcos θ , where D is the crystallite size, λ refers to the wavelength x-ray used, β represents the full width at half maximum (FWHM), θ is the peak position, and K is the constant.20 The micro-strain and dislocation density were determined using the relations \(\varepsilon\)= β / 4tan θ and \(\delta\)= (1/D)2, respectively. The crystallite size was also estimated using a size-strain plot (SSP) (Fig. 3) employing the equation (dβcosθ / λ)2 = (Kλ / D) ( d2βcosθ / λ2) + (\(\varepsilon\) / 2)2 , where d is the lattice spacing. It was observed that the crystallite size varied very slightly as a result of UVO treatment, and the micro-strain and dislocation density varied accordingly. There were no significant structural changes following UVO treatment, and the computed lattice constant and lattice parameters (a = b and c) were in agreement with the standard values.21
Figure 4 presents the Raman spectra of the SnO2 thin films taken at room temperature in the 200–1000 cm−1 region utilizing an excitation wavelength of 532 nm. Different vibrational modes were visible in the spectrum at approximately 308.9 cm−1, 453.8 cm−1, 570.4 cm−1, 632.7 cm−1, 725.0 cm−1, and 778.8 cm−1, which correspond to the Eu (TO), Eg, B1u, A1g, Eu (LO), and B2g, respectively. All these detected vibrational modes coincide with those of pure SnO2. The Raman-active A1g mode and B2g mode correspond to non-degenerative modes which could be connected to the vibration of Sn-O bonds in SnO2.22,23 The vibration of the oxygen atoms in SnO2 corresponds to another Raman-active mode, Eg, which is doubly degenerate.24 It is apparent from the spectra that following exposure to UVO treatment, the intensity and FWHM of the Eg mode changed. The Eg mode is quite sensitive to oxygen, and this variation might be triggered by oxygen vacancies or oxygen-related defects.25 The Eu (TO) and Eu (LO) modes (TO—transverse optical phonon, LO—longitudinal optical phonon) are infrared (IR)-active, whereas the B1u is a forbidden mode. These modes often appear when the crystallite size is at the nano level due to the size effect, which is consistent with our observed crystallite size values from XRD.26 From the spectra it is evident that the B1u mode weakened as the UVO treatment time increased from 30 min to 60 min. This might be because of the increase in the crystallite size.22 Further, the appearance of IR-active mode is usually related to the disorder activation.27 This implies that the sample that underwent 60 min of UVO treatment had more disorders than the untreated SnO2 thin film.
Fig. 4
Raman spectra of untreated and UVO-treated SnO2 thin films.
×
Morphological Studies
Figure 5 displays the SEM micrographs of the untreated and UVO-treated SnO2 thin film at magnification of ×10,000. It was observed that the as-deposited film was uniform and free from pinholes and cracks. As the film was subjected to UVO treatment, the surface became nonuniform due to agglomeration of SnO2 grains. These changes were observed to increase with the duration of UVO treatment. Similar results were observed by Dong et al. in their work on sol–gel-deposited SnO2 nanocrystals.10
Fig. 5
SEM images of (a) untreated, (b) 30-min UVO-treated (c) 60-min UVO-treated SnO2 thin films.
×
Compositional Studies
Energy-dispersive spectroscopy (EDS) was adopted to analyze the samples to determine their elemental composition. Figure 6 shows the EDS spectra of untreated and UVO-treated SnO2 thin films. All the spectra displayed peaks only for Sn and O, with no further peaks for the impurity element. This suggests that the prepared samples are phase pure. The inset of the figure shows the atomic percentage of Sn and O in the prepared samples. The observed atomic percentage of the untreated sample implies that the sample is almost stoichiometric and agrees with the earlier reports.28 A minor increase in the oxygen composition following UVO treatment might be attributed to chemisorbed oxygen on the surface of the SnO2 thin film, which was caused by the interaction of UV-generated reactive species with the film surface.29
The optical properties of the obtained thin films were investigated using UV–visible spectroscopy. Figure 7 shows the Tauc plot for both untreated and UVO-treated SnO2 films for different durations, and the absorbance of the films is displayed in the inset as a function of wavelength. The bandgap was estimated using Tauc’s relation followed by extrapolating the tangent line drawn at the linear region of the graph to the energy axis, which gives the bandgap value. The observed bandgap for the untreated SnO2 film was 3.04 eV, which is consistent with prior reported values.30 After the UVO treatment, we found a slight reduction in the bandgap, where we observed a bandgap of 2.99 eV for 30-min-treated film and 2.84 eV for 60-min-treated film. This bandgap lowering might be due to the appearance of defects within the bandgap as a consequence of UVO treatment.31,32
Fig. 7
Tauc plots for the untreated and UVO-treated SnO2 thin films [inset: absorbance plot].
×
Photoluminescence Studies
Figure 8 depicts the deconvoluted PL spectra of untreated and UVO-treated SnO2 thin films. Deconvolution of the spectra using the Gaussian function revealed five peaks in each spectrum. The peak P1 which is located around 409.41 nm may be related to the band-to-band transitions.33 The peak P2 at 434 nm can be ascribed to tin interstitials, and the other two peaks, P3 and P4, at 463.40 and 488.31 nm, respectively, may be due to other Sn-related defects or point defects such as oxygen vacancies.34 The peak P5 at 573.79 nm is because of oxygen vacancies present in the sample.35 The intensity of peak P1 was reduced and the peak broadened after UVO treatment. This might be due to the reduced near-band-edge emission as defects in the films increased. This can be again seen from variations in the peak intensities (P3 and P4) of defect-related emissions. Also, the increase in the intensity of P5 peaks after UVO treatment implies enhanced oxygen vacancies in the films, which is also reflected in Raman measurements, where the Eg peak was intensified after treatment. The integral intensity ratio between the band-to-band emission and defect emissions was found to be 0.31 for untreated film, whereas it decreased to 0.24 and then to 0.19 with UVO treatment for 30 min and 60 min, respectively. This decrease in the ratio indicates that after UVO treatment, the films become more defective, thereby suppressing the band-to-band emission.
In conclusion, tin oxide thin films were deposited utilizing the spray pyrolysis technique and were post-treated in a UVO environment for different time durations. The XRD studies showed that UVO treatment did not affect the structural characteristics of the synthesized films. Raman spectroscopy displayed peaks corresponding to the vibrations in SnO2, and with UVO treatment, the intensities of the peaks varied. SEM images showed some nonuniformity after UVO treatment, while EDS studies showed a slight variation in the oxygen composition, which might be due to the chemisorbed oxygen. The bandgap was observed to decrease slightly because of UVO treatment due to the appearance of defects. The PL studies revealed that the near-band-edge emission intensity decreased with an increase in the emission intensity from defect levels, and the ratio between the band-to-band emission and defect emissions also decreased from 0.31 to 0.19 because of UVO treatment. However, the results indicated that optimization of UVO-treatment time is crucial for modulating the properties of SnO2 thin films to obtain the desired properties. These UVO-treated SnO2 thin films can find a variety of applications in optical devices.
Acknowledgments
The authors, Ms Pramitha A and Ms Srijana G Rao, would like to thank the Manipal Academy of Higher Education (MAHE) for the support to carry out this work by providing necessary research and library facilities.
Conflict of interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Ethical Approval
Not applicable.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
