Trapping states in CdS:Eu nanobelts studied by excitation-dependent photoluminescence

Abstract

Pure and Eu-doped CdS nanobelts are synthesized by a thermal evaporation method. For the undoped CdS reference nanobelt, it only exhibits the emission related to free-excitons, very close to the energy of exciton absorption band, and moreover, the excitation power dependent photoluminescence (PL) data show a superlinear increase in integrated intensity with power. For the doped nanobelts, energy cannot transfer effectively between CdS host and incorporated Eu3+ ions. However, incorporated Eu3+ ions can form shallow level trap below the conduction band minimum (CBM), leading to a strong dependence of PL spectra on excitation power and energy. Under the 325 nm He–Cd laser excitation above the band gap energy, free-excitons ionize, and moreover, photogenerated electrons can relax rapidly from CBM to shallow level trap. Thus, the near-band edge (NBE) emission not only redshifts due to the renormalization of band gap but broadens more asymmetrically on its lower-energy side with power. Moreover, the power dependent NBE emission shows a sublinear property. In addition, a trap-related emission appears at higher excitation power, while the two emissions related to free-excitons and deep-levels decrease. Under the 514.5 nm Ar+ laser excitation below the band gap energy, it is just the doped nanobelts that can show the emission related to shallow level trap under the higher excitation condition.