Optical Properties of Colloidal CdSe/ZnS core/shell Nanocrystals Embedded in a UV Curable Resin
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Optical Properties of Colloidal CdSe/ZnS core/shell Nanocrystals Embedded in a UV Curable Resin Abhishek Joshi1, Edwin Davis2, Kaushik Narsingi1, Omar Manasreh1, and B. D. Weaver3 1 Department of Electrical Engineering, University of Arkansas, 3217 Bell Engineering Center, Fayetteville, AR, 72701 2 Department of Optical Engineering, Norfolk State University, 700 Par Ave., Norfolk, VA, 23504 3 Naval Research Laboratory, Code 6818, 4555 Overlook Avenue, SW Washington, DC, 20375 ABSTRACT Optical absorption and photoluminescence techniques were used to investigate the band gap of colloidal CdSe/ZnS core/shell nanocrystals matrixed in a UV curable resin. The band gap was measured for several nanocrystals with size ranging between 1.9 and 4.0 nm. The band gap (Eg) was determined from the first exciton peaks observed in the optical absorption spectra. Both Debye and Einstein temperatures were estimated from fitting the energy band gap vs. temperature using two different empirical expressions.
INTRODUCTION In recent years, colloidal semiconductor nanocrystals are investigated for applications in optoelectronic devices such as solar cells [1-3], light-emitting diodes [4], fluorescence labeling of cells [5-6], and for magnetic resonance imaging [7-8]. To employ these nanocrystals, device fabrication requires knowledge of their optical and electronic properties. To determine these properties, nanocrystals were matrixed into polymers to form a composite that can be easily handled and measured [9-14]. Nanocrystals are a class of quantum dots whose radii is comparable to the bulk exciton Bohr radius [15-18]. Unlike bulk material, the property of quantum confinement leads to an increase in the effective band gap as the nanocrystal diameter decreases. By coating the nanocrystal core with a shell of higher-band gap material, higher photoluminescence yields are obtained [19]. In this study, optical absorption and photoluminescence/emission measurements were made for colloidal CdSe/ZnS core/shell nanocrystals matrixed into a UV curable resin. From the optical absorption spectra, the energy of the first exciton peak wavelength was determined. The nanocrystal diameters were in the range of 1.9-4.0 nm and the temperature range was 10-300 K.
EXPERIMENT Samples of CdSe/ZnS core/shell nanocrystals embedded in a UV curable resin were obtained commercially from Evident Technologies [20]. The diameter of these nanocrystals is quoted in their respective datasheets and was estimated from the high-resolution tunneling electron microscopy images by the manufacturer. The composite was deposited on several cplane sapphire substrates and then were exposed to a 200W UV light for 90 minutes. A Varian Cary 500 spectrophotometer was used with a closed-cycle refrigerator to measure the absorption spectra. Temperature dependent measurements were taken in the range of 10 – 300K. The Photoluminescence Spectra (PL) were measured at 300K by using a Bomem DA8 Fouriertransform spectrometer. RESULTS AND DISCUSSION Four CdSe/ZnS core/shell samples
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