Effects of Post-Synthesis Treatments on SnO₂ Aerogel for Electronic Applications

Aerogel constitutes a unique class of materials characterized by self-assembled arrays of interconnected crystallites, but while the mechanical properties of aerogels have been extensively explored, their electrical properties remain relatively unexplored. This study focuses on the synthesis and characterization of a semiconducting tin oxide (SnO₂) aerogel for electronic applications. SnO₂ is a wide bandgap, n-type semiconductor known for its high optical transparency and low electrical resistance. These SnO₂ properties in combination with the unique characteristics of aerogel enable the possibility for innovative applications in solar cell technologies, memory devices (e.g., resistive random-access memory), and neuromorphic computing. However, a challenge remains; while bulk SnO₂ exhibits an optical bandgap energy (E_g) of around 3.6 eV, previous literature indicates that SnO₂ in aerogel form displays a larger apparent E_g of around 4.6 eV. This increase in E_g is likely attributable to defects within the material such as dangling bonds likely caused by a Burstein-Moss shift due to the Sn-rich surface. To minimize defects within the material this research characterizes the aerogels after a post-annealing process in an environment with oxygen present and the effects of using H₂O₂ as a surface treatment without a post-annealing process.<br/>An epoxide method was used to synthesize the SnO₂ aerogels. The material underwent annealing at atmospheric pressure in a tube furnace at temperatures of 100°C, 200°C, 300°C, and 400°C for 30 minutes each. The samples were exposed to oxygen during the annealing process to attempt to passivate any dangling bonds without surface treatment. Additionally, a sample without annealing was aged in a 3% H₂O₂ and H₂O solution to compare with the unannealed sample aged in only ethanol. Fourier transform infrared (FTIR) spectroscopy was utilized to identify any potential contaminants and unreacted precursors in each sample. X-ray diffraction (XRD) was used to calculate the crystallite size using the Debye-Scherrer equation and the microstrain on the crystal structure using the Williamson-Hall method. Finally, UV-Vis was performed to calculate the E_gthrough Tauc’s equation and the Urbach energy (E_U) through the Urbach tail equation, which quantifies the disorder in the band edges of a semiconductor.<br/>The results of this study determined that annealing removes the impurities of the sample at each temperature and the apparent bandgap reduces with a linear trend from 5.32 eV to 4.06 eV with temperature. Shifts in the XRD peaks suggest that there is a phase change at 300°C from SnO₂ to SnO, and the E_U was seen to increase with annealing, the largest value being 1.13 eV at 300°C. This indicates a large amount of disorder and the presence of more localized states within the bandgap, and when the structure stabilizes after the phase change at 400°C, the structure displays the value of disorder of 0.42 eV which exhibits a sharper absorption edge with fewer localized states within the bandgap. In comparison, the use of H₂O₂ as a surface treatment resulted in a reduction of E_U from 0.2207 eV to 0.147 eV and decreased E_g from 5.32 eV to 3.64 eV.<br/>This study concludes that while annealing improves some qualities of the material, this is likely due to the phase change within the material and not the reduction of defects. Whereas significant improvements in the optical characteristics using an H₂O₂ surface treatment were observed. The passivation of dangling bonds on the SnO₂ aerogel surface is the solution to minimizing the apparent E_g for future solar and neuromorphic applications. Future work of this project consists of incorporating impurities such as indium chloride into the synthesized SnO₂ aerogels to further reduce the E_g for solar cells and neuromorphic technology.