Investigation of the Electronic Structure of CdS Nanoparticles with Sum Frequency Generation and Photoluminescence Spectroscopy

The photocatalytic performance of semiconductor nanoparticles (NPs) is sensitive to their electronic configuration, especially the surface states, which call for effective analysis methods. The electronic structures of CdS NPs have been investigated with traditional spectroscopic methods: ultraviolet-visible diffuse relectance (UV-Vis DRS) and fluorescence excitation emission matrix spectroscopy (EMMs), and nonlinear methods: two-photon luminescence (TPL) and broadband sum frequency generation (BB-SFG) spectroscopy. The satisfactory band gap of hexagonal CdS NPs is identified through nonlinear spectroscopy. The bandgap value is specified as 2.38 eV with SFG induced photoluminescence (SFG-PL). Two surface states (2.18 eV - 2.43 eV and 1.97 eV - 2.12 eV) have been observed with wavelength-dependent BB-SFG (not shown in UV-Vis DRS), and may be attributed to shallow trapped states from S2- vacancy defects. BB-SFG spectroscopy can be a very useful complementary technique to traditional methods to access the “d...