Origin of Visible Photoluminescence of ZnO Quantum Dots: Defect-Dependent and Size-Dependent

Abstract

To get a real understanding on the complexity of origin and mechanism of visible emission for ZnO quantum dots (QDs), we systematically property of visible emission of ZnO QDs with tunable diameters in a range of 2.2−7.8 nm synthesized via a sol−gel route using self-made zinc−oleate complex as a precursor. It is indicated that the visible emission of ZnO QDs can be ascribed to singly ionized oxygen vacancies, which is associated with the paramagnetic centers with electron paramagnetic resonance (EPR) value of g = 2.0056. The visible emission property of the ZnO QDs displays highly size-dependent behavior. With ZnO QDs size decreasing, the visible emission peaks blue-shift to the positions with shorter wavelength due to quantum size effect, however, is different from that of band gap. Quantitative investigation shows that the visible emission can correspond to a transition of holes from the valence band to the preexisting deep donor energy level, which is different from the well-known conclusion that the visible emission is due to the transition of an electron from the conduction band to a deep trap. Two important points can be obtained: the defects of singly ionized oxygen vacancies determine the origin and intensity of visible emission of ZnO QDs; and the visible emission peak position of ZnO QDs is decided by their size, and a transition of holes from the valence band to the preexisting deep donor energy level is responsible for the visible emission of the ZnO QDs.

Structural, Optical and Electrical Properties of Cobalt Doped ZnO Quantum Dots

“…In addition, formula based on effective mass given below (Eqn. 3) can describe why band gap changes with doping [4,17]. …”

“…The important technological applications of quantum dots are due to the fact that their optical absorption depends on the size. The size of quantum dots in turn can be easily tuned by changing the reaction time and annealing temperature during synthesis process [4]. For ZnO quantum dots, there are two prominent emission bands observed in the photoluminescence (PL) spectrum.…”

“…For ZnO quantum dots, there are two prominent emission bands observed in the photoluminescence (PL) spectrum. One band is centered in the ultra violet (UV) region, around 395 nm, and the other is centered in visible region centered at 468 nm [4]. The UV emission depends on near band radiative recombination of electron from conduction band and holes in the valence band.…”

Controllable fabrication and optical properties of Sn-doped ZnO hexagonal microdisk for whispering gallery mode microlaser

“…It has been reported that the electron-depletion layer where the defects are trapped becomes thinner when the excitation intensity increases, therefore, the relative intensity of defect emission is decreased. 36,37 In addition, two emission subbands at about 379 nm (3.271 eV) and 390 nm (3.18 eV) on the UV emission band can be distinguished, which are related to different photoluminescence mechanisms. The emission subband at 379 nm increased when the excitation power density increased to 60 KW/cm 2 , then decreased and disappeared when the excitation power density further increased to 100 KW/cm 2 , meanwhile the emission subband at 390 nm increased continuously.…”

Room Temperature Synthesis of ZnO Quantum Dots by Polyol Methods

“…ZnO nanoparticles exhibit quantum size effects when the particle size is smaller than the exciton-Bohr diameter of ~ 8 nm. In the past, many methods to synthesize ZnO quantum dots were investigated, (Zhang et al 2010;Xiong et al 2005;Tang et al 2009;Tsuzuki & McCormick, 2001). Among them, wet chemical synthesis methods, such as sol-gel, hydrothermal and solvothermal methods, are widely used to obtain free ZnO quantum dots that are detached from substrates.…”