Surface-Enhanced Raman Scattering (SERS) on Indium-Doped CdO (ICO) Substrates: A New Charge-Transfer Enhancement Contribution from Electrons in Conduction Bands

Abstract

Surface-enhanced Raman scattering (SERS) substrates have shown rapid development in recent years, but the use of transparent conductive oxides (TCOs) in SERS is still limited. Here, a series of In-doped CdO (ICO) NPs with different In3+ doping contents x (x = 0, 0.025, 0.050, 0.075, 0.100, 0.125, 0.150, and 0.200) were synthesized by a hydrothermal method and first employed as a SERS platform. The structures and properties of the ICO NPs influenced by dopant concentration of In3+ ions were characterized by powder X-ray diffraction (XRD), inductively coupled plasma (ICP) analysis, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. Furthermore, the SERS spectra of ICO-MPY were investigated. Interestingly, the enhancement was dominated by the charge-transfer (CT) contribution through the free electrons in conduction bands (CBs). The changes in electronic distributions in energy levels of ICO NPs at different doping contents were explored, and the corresponding effect on SERS was discussed by employing 633 and 785 nm excitation lines, respectively. The CT route was found to be the excitation of electrons from CBs of semiconductor to the LUMO of molecule, which provides a new perspective in understanding SERS mechanisms. The present work provides a new approach to develop potential semiconductive SERS substrates.