Investigation on structural and photoluminescence properties of ($\mathrm{Co}$, $\mathrm{Al}$) $\mathrm{Co}$-doped $\mathrm{SnO}_2$ nanoparticles

Pure and ($\mathrm{Co}$, $\mathrm{Al}$) co-doped ($\mathrm{Co}=1,3,5$ mol %, and $\mathrm{Al}=5$ mol % as constant) $\mathrm{SnO}_2$ nanoparticles were synthesized in aqueous solution by the chemical coprecipitation method using polyethylene glycol (PEG) as a stabilizer. The effects of structural and photoluminescence of ($\mathrm{Co}$, $\mathrm{Al}$) co-doped $\mathrm{SnO}_2$ nanoparticles are investigated. The XRD pattern reveals that the samples are in a single phase rutile type tetragonal crystalline structure of $\mathrm{SnO}_2$. The peak positions with $\mathrm{Co}$ concentration are slightly shifted to lower $2\theta$ values and size of particles from XRD calculations are in between $20$ – $30$ nm. The Raman studies of the samples reveal that the Raman peaks are shifted towards lower wave numbers, when compared to those of pure $\mathrm{SnO}_2$ at $150$ cm$^{-1}$, $303$ cm$^{-1}$, $476$ cm$^{-1}$, $630$ cm$^{-1}$, and $765$ cm$^{-1}$ respectively. Photoluminescence studies show that pure $\mathrm{SnO}_2$ has an emission peak at $444$ nm and ($\mathrm{Co}$, $\mathrm{Al}$) co-doped samples show emission peaks at $417$ nm, $433$ nm and $485$ nm with exciting wave length $320$ nm. The PL intensity increases and broadening of peaks for co-doped samples with increase of $\mathrm{Co}$ concentration indicates the decrease of size of the crystallinity. The UV absorption spectrum exhibits absorption at $310$ nm, and is in agreement with the emission spectra.