Surfactant mediated optical properties of cytosine capped CdSe quantum dots

a r t i c l e i n f o This letter demonstrates the use of one of the nucleobases, 'cytosine' as a new capping agent in controlling the size of the nanoparticles. A size dependent blue shift in optical absorption with enhanced luminescence is observed. Since the calculated density of states do not show any change in the band gap of as-prepared quantum dots after capping, the observed blue shift of the absorption peak can solely be attributed to the so-called size-effect whereas the enhancement in luminescence to surfactant mediated defect passivation. It is expected that the observed properties of the cytosine capped CdSe quantum dots would facilitate a better bio-compatibility of tailor-made nanoparticles for bio-imaging applications. In the recent past, synthesis and characterization of II–VI semiconductor materials at nanometer scale have been the focus of several workers. Due to unique optical and electronic properties, CdSe quantum dots (QDs) have been studied and used as biological labels for multiplexed biological detection, imaging and in molecular cell biology [1–5]. Scientific literatures have shown a variety of methods for synthesis of CdSe nanocrystals [6–10]. But these nanocrystals have shown low quantum yields due to the non-radiative recombination of light-generated charge carriers at surface traps caused by surface defects [11,12]. It has been suggested that the passivation of surface defects with ligands or shells of a high band gap semiconductor can reduce the surface trap densities; thereby enhancing the quantum yield of CdSe QDs. Murray and co-workers [6] have achieved the enhancement in the quantum efficiency of CdSe quantum dots by coating them with a higher band gap semiconducting compounds, such as CdS and ZnS. Similarly, CdSe quantum dots embedded in the SiO 2 matrix was found to give a highly intense emission peak in the visible region [5]. In the present study, we follow an alternative route by which passivation of CdSe QDs is achieved employing one of the nucleobases, 'cytosine' that makes the CdSe QDs more biocompatible for its versatility. Cytosine per se does not contribute to fluorescence yield [13], as the change in electronic and optical properties of cytosine capped CdSe QDs are insignificant, rather cytosine will only act as a new capping agent for reducing the surface defect density of QDs. 2. Experimental details 2.1. Material and methods For synthesizing CdSe QDs, 45 mmol of cadmium acetate di-hydrate was dissolved in 100 ml of methanol and refluxed …