Low temperature-boosted high efficiency photo-induced charge transfer for remarkable SERS activity of ZnO nanosheets

Jie Lin; Jian Yu; Ozioma Udochukwu Akakuru; Xiaotian Wang; Bo Yuan; Tianxiang Chen; Lin Guo; Aiguo Wu

doi:10.1039/d0sc02712j

Low temperature-boosted high efficiency photo-induced charge transfer for remarkable SERS activity of ZnO nanosheets

Chem Sci. 2020 Aug 13;11(35):9414-9420. doi: 10.1039/d0sc02712j.

Authors

Jie Lin¹, Jian Yu², Ozioma Udochukwu Akakuru¹, Xiaotian Wang², Bo Yuan¹, Tianxiang Chen¹, Lin Guo², Aiguo Wu¹

Affiliations

¹ Cixi Institute of Biomedical Engineering, Chinese Academy of Science (CAS) Key Laboratory of Magnetic Materials and Devices, Zhejiang Engineering Research Center for Biomedical Materials, Ningbo Institute of Materials Technology and Engineering, CAS 1219 Zhongguan West Road Ningbo 315201 P. R. China aiguo@nimte.ac.cn chentx@nimte.ac.cn.
² School of Chemistry, Beihang University Beijing 100191 P. R. China wangxt@buaa.edu.cn guolin@buaa.edu.cn.

Abstract

Improving the photo-induced charge transfer (PICT) efficiency is the key factor for boosting the surface-enhanced Raman scattering (SERS) performance of semiconductor nanomaterials. Introducing plentiful surface defect states in porous ZnO nanosheets (d-ZnO NSs) effectively provides additional charge transfer routes for highly efficient PICT within the substrate-molecule system. Significantly, an interesting phenomenon of low temperature-boosted SERS activity of these d-ZnO NSs is consequently observed. The enhanced SERS activity can be attributed to the efficient PICT processes due to the significant reduction of non-radiative recombination of surface defects at a low temperature. This is carefully investigated through combining in situ low-temperature SERS measurements with temperature-dependent photoluminescence (PL) emission spectroscopy. Our results clearly demonstrate that the weakened lattice thermal vibration at a low temperature effectively suppresses the phonon-assisted relaxation and reduces carrier traps, resulting in the increase of PL intensity. The decreased traps of photo-induced electrons at surface defect states effectively facilitate the PICT efficiency within the substrate-molecule system. An ultrahigh enhancement factor of 7.7 × 10⁵ and low limit of detection (1 × 10^-7 M) for a 4-mercaptopyridine molecule at a temperature of 77 K are successfully obtained. More importantly, the low temperature-enhanced SERS effect is also obtainable in other metal oxide semiconductors, such as d-TiO₂ and d-Cu₂O nanoparticles. To the best of our knowledge, this is the first time the low temperature-boosted SERS activity of semiconductors has been observed. This study not only provides a deep insight into the chemical SERS mechanism, but also develops a novel strategy for improving semiconductor SERS sensitivity. The strong SERS activity at a low temperature reported here may open new avenues for developing non-metal SERS substrates with new functionalities, especially for the research on cryogenic sensing and hypothermal medicine.