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Dopamine covalently chelated ZnO nanoparticles were synthesized by a nonaqueous one-step chemical process at a temperature as low as 60 °C. The formation of ZnO/dopamine hybrid structure was proved by x-ray powder diffraction (XRD), transmission electron microscopy (TEM), and Fourier transform infrared (FTIR) techniques. Detailed absorption, luminescence, and time-resolved decay studies were performed for these ZnO/dopamine hybrid nanoparticles. We observed an enhanced green emission, which could be assigned to a new band-gap emission based on the fast of nanosecond lifetime of the green emission. Our results demonstrated that the change of optical properties of ZnO nanoparticles after covalently chelated by dopamine ligands is closely associated with the formation of new band structure.

I. INTRODUCTION

As a highly efficient luminescent material, ZnO has been studied for decades, but recent applications, such as in field-emission displays (FEDs),1 have inspired new interest in the luminescent performance of ZnO. Previous work on luminescent properties of ZnO revealed two emission bands: one a narrow ultraviolet (UV)-emission near the band edge, the other a broad, visible emission, normally in the green band. The lifetime of UV emission is short, on the order of several tens to hundreds of picoseconds.2 The lifetime of visible emission is much longer, in the microsecond range.3 However, the mechanism behind the luminescence of ZnO varies greatly from sample to sample and when grown by different techniques. The main reason for such variety is that surface defects that originate from different fabrication processes may complicate the situation. These defects on the surface of ZnO particles normally include interstitial zinc ions, zinc vacancies, oxygen vacancies, chemisorbed oxygen, impurities, etc.4–7 Among different ways that could be used to remove or suppress the influence from surface defect effects, surface organic modification has been shown to be one of the most effective. The often-used surface organic modification involves capping the surface of a semiconductor particle with a layer of organic complexant, such as oleic a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0243 1946

http://journals.cambridge.org

J. Mater. Res., Vol. 23, No. 7, Jul 2008 Downloaded: 04 Sep 2014

acid,8 trioctylphosphine oxide (TOPO),9 poly(vinylpyrrolidone) (PVP),10 or dodecylamine (DDA),11 for examples. Capping an organic complexant onto the surface of the ZnO particles is effective in removing surface defects and in reducing the density of surface defects. Therefore, intensity of UV emission of ZnO nanoparticles was enhanced at the cost of suppressing or weakening defect-related visible emissions. The shortcoming of these surface organic modifications, however, is that they usually involve a complicated synthesis process, and sometimes a higher temperature (e.g., 200 °C) or an extremely low temperature (e.g., 0 °C) is needed to cap the organic complexant onto the surface of the