Third-order nonlinear optical response in PbS-coated CdS nanocomposites
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Heping Li, Yee Loy Lam, Chan Hin Kam, and Wen Xiu Que School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Republic of Singapore 639798 (Received 12 September 2000; accepted 19 March 2001)
PbS-coated CdS nanocomposite particles were synthesized by an ion displacement method in an inverse microemulsion. Their growth kinetics were studied with UV–vis spectroscopy. Transmission electron microscopic characterization shows that PbS-coated CdS particles are uniform in size with a mean diameter of 6 nm. The electron diffraction patterns demonstrate their crystalline nature. Third-order nonlinear optical properties in the samples were investigated using the Z-scan technique with femtosecond laser pulses at 780-nm wavelength. The nonlinear refractive index of PbS-coated CdS nanocomposite particles in microemulsion varied with the molar ratio of Cd/Pb ions and reached a maximum of 5.3 × 10−12 cm2/GW for the sample with a Cd/Pb ion ratio of 1 to 2. The observed large refractive nonlinearity in these nanocomposite particles may be attributed to the optical Stark effect and strong interfacial and inter-nanoparticle interactions.
I. INTRODUCTION
Nanometer semiconductor materials have been a new generation of advanced materials due to their novel electronic and optical properties, originating from size quantization.1,2 The properties of such nanomaterials are determined not only by their atomic scale structures but also by their interface and surface structures. The reduction in dimensionality of systems from bulk to nanoscale for semiconductors under certain conditions can lead to an enhanced nonlinearity.3 Materials with large thirdorder optical nonlinearity and fast response time are essential for future optical device applications in optical computing, real time holography, optical correlators, and phase conjugators.4 CdS and PbS semiconductor nanoparticles as “quantum dot” materials are of interest for their large thirdorder nonlinear optical (NLO) response and have been extensively investigated.5–8 However, the synthesis and characterization of nanocomposite materials of mixed semiconductor nanoparticles have only been reported recently.9–11 To further improve NLO response in CdS or PbS nanoparticles, one can change their electronic properties in beneficial ways by surface modification, since surface electronic states dominate the linear and nonlinear optical properties of these nanomaterials.12 a)
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J. Mater. Res., Vol. 16, No. 6, Jun 2001
The synthesis of coated nanoparticles is a new direction in the study of NLO properties of nanometer-sized materials. An approach to meet the purposes mentioned above is the use of microemulsion. The reagents of surfactants in microemulsion act as the physical boundaries. Normal vehicles using bis(2-ethylhexyl)sulfosuccinate salts (AOT) allow the formation of small water pools (99%] lead nitrate [Pb(NO3)2, >99%], ammonia sulfide (20 wt% in aqueous solution), and 1-dodecanthiol (thiol, >97%)
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