Optical properties of nanometer-sized CdO organosol
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Optical properties of nanometer-sized CdO organosol Wu Xiaochun, Wang Rongyao, Zou Bingsuo, Wang Li, Liu Shaomei, and Xu Jiren Institute of Physics, Chinese Academy of Sciences, Beijing, 100080 People’s Republic of China
Huang Wei Institute of Materials Research and Engineering, National University of Singapore, Singapore, 119260 Singapore (Received 8 July 1996; accepted 23 April 1997)
In this paper, nanometer-sized CdO organosol was prepared by using microemulsion methods. Its electronic structure and optical properties are characterized through UV-visible light absorption, photoluminescence, and Z-scan techniques. Some new features were observed. Fluorescence and nonlinear optical responses are mainly related to surface trapped states.
dropping OH2
I. INTRODUCTION
°°°°°°°°°°°! CdsOHd2 microemulsion
Materials with large optical nonlinearities are in great need for the development of nonlinear optical technology and are being studied intensively.1 In general, the materials with a large linear refractive index (n0 ) and a narrow band gap (Eg ) in the bulk could have a large third-order optical nonlinearity at the nonresonant region, therefore CdO with n0 2.49 and Eg 2.3 eV has a large nonlinear refractive index (n2 ).2 As the particle size decreases down to the nanometer scale, its nonlinear optical response is further enhanced due to the quantum size effect and other mesoscopic effects.3 Recently, optical studies on nanometer-sized particles of indirect band gap semiconductors, such as Si nanocrystal, have attracted much attention because of possible changes of electronic transitions arisen from the size effect and surface states. CdO in the bulk (rocksalt type) is mainly a direct band gap (2.3 eV) semiconductor with a small indirect band at 1.36 eV. The study of nanometer-sized CdO is beneficial for us to understand the change of its structure and properties in the mesoscopic dimension. In addition, some new applications may be found from the study. In this paper nanometer-sized CdO organosol was prepared by using the microemulsion method. Its electronic energy level structure is characterized through absorption and fluorescence spectra. Its nonresonant optical nonlinearity was measured by using the Z-scan technique with fs pulse out of a Ti : sapphire laser. II. EXPERIMENTAL
refluxing
°°°°°°°°°°°°°°°°°!2 red nanometer-sized distilling to remove OH
CdO organosol The size distribution of the nanometer-sized CdO organosol was further narrowed by using size-selective separation. A sample with particle diameters centered at 6 nm was obtained, as shown in Fig. 1. B. Apparatus and measurement methods
The absorption spectrum was measured by using a CARY 2390 spectrophotometer. Conventional photoluminescence spectra were performed on a Plekim Elmek luminescence spectrometer (type LS 50B). The laser fluorescence spectrum was measured using double frequency of Nd : YAG laser of 10 ns pulse duration and 10 Hz repetition rate as the excitation light source. The optica
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