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

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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.
Funding Information
  • China Postdoctoral Science Foundation (2018M640576)
  • China Postdoctoral Science Foundation (2019T120534)
  • Natural Science Foundation of Ningbo (2018A610218)
  • Natural Science Foundation of Ningbo (2019A610024)
  • National Natural Science Foundation of China (21875008)
  • National Natural Science Foundation of China (31971292)
  • National Natural Science Foundation of China (51532001)
  • National Natural Science Foundation of China (51801007)
  • National Natural Science Foundation of China (U1432114)
  • Chinese Academy of Sciences (KFJ-STS-ZDTP-061)
  • Chinese Academy of Sciences (XDB36000000)

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