Nonlinear optical properties of colloidal "quantum-dot" composite materials with tailored organic coatings

Abstract

Colloidal, "quantum-dot" semiconductormaterials, as exemplified by the sharp-cut yellow-red filter glasses which contain small (less than 100 A diameter) crystallites of cadmium sulfide and/or cadmium selenide, are of current interest because of the large near-resonant third-order nonlinear optical response they exhibit. Because these semiconductors are generally soluble only in inorganic media, such as silicate glasses, they have not been considered widely as candidates for inclusion in organic polymer waveguides. We report here the initial results of a continuing study in which very small colloidal particles of both cadmium sulfide and cadmium selenide (with crystallite diameters of typically 30-50 A) have been prepared by the reverse-micellar technique and then "capped" with a variety of organic reagents, using the methodology described several years ago by Brus and Steigerwa1d14. By varying the organic capping reagent, we find that we are able to produce significant variations in both the solublilty and the nonlinear optical response of the resulting composite materials. In particular, we find that the solubility of the particles in organic polymers such as PMMA and polycarbonate varies greatly with the nature of the capping reagent, with corresponding changes in the nonlinear refractive index of the composites. The feasibility of developing organic nonlinear optical waveguides based on this approach will be discussed.

Large Nonlinear Absorption in Coated Ag2S/CdS Nanoparticles by Inverse Microemulsion

“…For comparison reasons, we have measured the nonlinear absorption in the CdS nanoparticles. , Parts a and b of Figure are the normalized transmittance of the CdS nanoparticles in the microemulsion with the 25-ps and 7-ns pulses, respectively. The two Z scans are obtained with the same fluence (1.8 J/cm 2 ) but different irradiances (67 and 0.25 GW/cm 2 ) due to the different pulse duration .…”

Nanocomposite Photonic Polymers. 1. Third-Order Nonlinear Optical Properties of Capped Cadmium Sulfide Nanocrystals in an Ordered Polydiacetylene Host

“…CdS Quantum Dots. CdS QD particles having a mean diameter of 33 ± 9 Å were prepared by the reverse micelle method of Steigerwald and Brus 4 and capped with thiophenol, which has been shown to give the particles good solubility in organic solvents such as pyridine and chloroform. , A representative 400 keV transmission electron micrograph (TEM) of these particles (deposited on a carbon support from pyridine solution) is shown in Figure . Cadmium lattice planes are visible in some of the particles; the size distribution was estimated by measuring a number of the particles in the TEM image.…”

“…The sign and magnitude of the nonlinear refractive index ( n 2 ) of the particles were measured by pulsed Z-scan 8 and by pulsed degenerate four-wave mixing (DFWM) techniques, respectively, in pyridine solution at several concentrations ranging from 5% (w/v) to 20% (w/v) and at several wavelengths from 528 to 490 nm, as reported previously; a nitrogen-pumped dye laser having a pulse energy of 20 μJ and a pulse duration of 5 ns was used. In those studies, we found that the observed (negative) values of n 2 increased smoothly as the wavelength was decreased (that is, as the pulse wavelength moved more and more into the absorption band), indicating resonant enhancement of the NLO response, and ranged from around −4 × 10 -8 cm 2 /MW for a 5% (w/v) solution at 528 nm to around −14 × 10 -8 cm 2 /MW for a 20% (w/v) solution at 490 nm . A typical Z-scan curve for these solutions is shown in Figure , demonstrating the characteristic shape for a negative nonlinearity …”

“…Materials. The capped CdS QD particles and the diacetylene monomers were prepared by standard procedures, as reported before. , Freshly obtained spectroscopic grade pyridine, CHCl 3 , and CS 2 (all from Aldrich) were used as received for all experiments. Deionized water was prepared in a Class 100 clean room using a Milli-Q apparatus (Millipore Corp.) and was further microfiltered through 0.2 μm filters immediately prior to each use.…”

“…Our interest in these materials stems from the ease with which the size and composition of the core semiconductor particle can be varied, while independently controlling the chemical properties of the particle by “capping” the surface with functionalized thiol reagents which form a monolayer or partial monolayer of organic groups. , This independent control of the optical properties and the chemical behavior of these particles provides an unprecedented opportunity to design new materials in which quantum-dot semiconductors having specific optical and electronic properties are combined with other components to create tailored nanocomposite materials for specific device applications.…”