Novel strategies for the enhancement of the NIR emission in the Ca doped ZnO system: Doping with Cr or adding Sb2S3

Abstract

We report two novel strategies to enhance the NIR emission in the ZnO:Ca system. In the first strategy, we synthesized ZnO:Ca nanoparticles using a wet chemical method and doped them with various concentration of Cr. SEM images showed that the ZnO:Ca,Cr nanoparticles had a grain-like morphology and their sizes increased from 33 to 110 nm after incrementing the Cr concentration from 4 to 17 mol%. The analysis by X-ray diffraction (XRD) revealed that he ZnO:Ca,Cr sample made with 4 mol% of Cr had a pure hexagonal phase but the other samples synthesized with Cr concentration above 11 mol% presented the hexagonal phase and a small content of Cr2O3 (<3%). Moreover, the photoluminescence (PL) spectra of these samples showed two main emission peaks in the blue and NIR regions (λexc = 265 and 365 nm). In particular, the ZnO:Ca,Cr sample made with 17 mol% of Cr had the strongest NIR emission in the 700–850 nm region and only this sample showed a prominent NIR emission peak at 805 nm. Moreover, the CIE maps showed that the emissions of the ZnO:Ca,Cr samples are located in the blue region under excitation at 265 nm, but it was tuned to the orange region under excitation at 365 nm. In the second strategy, different amounts of Sb2S3 was added during the synthesis of ZnO:Ca to form ZnO:Ca/Sb2S3 composites. In this case, we obtained different morphologies of grains, rod-like and sheet-like for Sb2S3 contents of 10, 28 and 34 wt%, respectively. In addition, the composites presented a mixture of hexagonal and orthorhombic phases, which corresponded to the ZnO and Sb2S3, respectively. The ZnO:Ca/Sb2S3 composites had NIR emissions peaks in the 650–800 nm region and their intensity increased with the content of Sb2S3. Furthermore, their visible emission was tuned from blue to orange-red or to cold white light according to the CIE maps. Overall, the best ZnO:Ca,Cr sample had 312% and 37% higher emission than the ZnO:Ca reference sample and the best ZnO:Ca/Sb2S3 composite (under 265 nm excitation), respectively. In general, XPS studies showed the existence of oxygen vacancy defects, which produced the visible-NIR emissions in all the samples. The strategies presented here for NIR emission enhancement are feasible at low cost and the NIR emissions produced here occur within the first biological window (700–1000 nm), which is of interest for biomedical applications.