8727
J. Phys. Chem. 1993,97, 8727-8731
Surface Effects on the Optical Properties of Cadmium Selenide Quantum Dots S. A. Majetich' and A. C. Carter Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received: January 19, 1993; In Final Form: June 1 , 1993
Cadmium selenide quantum dots were synthesized with different functional groups attached to their surfaces. The effect of surface chemistry on the optical properties was determined through optical hole-burning and luminescence measurements. Nanosecond hole burning probed ground-state recovery due to recombination at trap states, and the results indicate that nanocrystallites with better surface passivation and stronger bonds to terminating ligands have longer ground-state recovery times. Better surface passivation and stronger bond strength were also correlated with increased luminescence intensity, which shows the relative amount of radiative recombination. The luminescence spectrum changed very little as a function of the terminating ligand, indicating little or no excitation transfer from the crystallite to the ligand for these samples.
I. Introduction The size-dependent absorption in quantum dots is wellunderstood, but little is known concerning the equally important surface effects which control the electron-hole recombination. These semiconductor nanocrystallites are among the smallest species for which a surface can be meaningfullydefined,so surface effects are more pronounced. Small quantum dots may have one-third to one-half of their atoms at the surface. Unlike materials grown by molecular beam epitaxy or chemical vapor deposition, the quantum dots studied here are grown in solution and have numerous surface defects. However, since many interesting effects occur at nonideal surfaces, it is important to understand the processes occurring at these interfaces. The quantum dots used here are a sensitive system for investigating surface effects, because their surface can be modified in many ways. Here we report how changing the surfacechemistryof cadmium selenide quantum dots affects the mechanisms and the rates of electron-hole recombination. The nanocrystallite surface was varied using modificationsof the microemulsion growth technique of Steigerwald and Brus.' We synthesized CdSe quantum dots in micelle solutions and capped the crystallites with different terminating ligands. Some of these samples were later heattreated to anneal both the surface and the interior. The resulting materialswerecharacterized todeterminetheir sizeand thenature of the bonding at the surface. The effect of the surface chemistry on the optical properties of the small quantum dots was then determined through hole-burning and luminescence measurements.
II. Experimental Section A. Synthesis. A series of CdSe quantum dot samplesdiffering only in their surfaces were prepared. To a microemulsion of heptane, water, and AOT surfactant were added cadmium perchlorate and trimethylsilylseleniumsolutions to grow clusters within the micelles. Details of this nanocrystallite preparation have been published elsewhere.' To prevent coalescence when extracting the quantum dots from the micelles, they were capped with ligands which bond to their surfaces. In order to minimize the differences other than at the surface, a single batch of colloidal CdSe was divided into six parts, and a different terminating ligand was added to each. This was done by adding a particular capping To whom correspondence should be addressed.
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ligand dissolved in heptane to the colloidal solution. Equimolar amountsof thedifferent capping agents were added. The amounts corresponded to an excess of 50% over the number of available Cd*+surface sites. The crystallites isolated had a mean diameter of 35 A, with a 10% size distribution. Once capped, the quantum dots were isolated from the microemulsion for characterization. The solution was first rotary evaporated to dryness, and the resulting mixture was redissolved in pyridine. Petroleum ether was added until a precipitate formed. Following filtration the powder was washed extensively with petroleum ether to remove byproducts and residual AOT. Some characterization methods, such as X-ray diffraction and IR absorption, utilized this powder directly, but for optical measurements the quantum dots were dispersed in transparent media. The quantum dot surface was varied in three ways. First, the steric hindrance of the capping agent was changed to determine the importance of surface passivation. More steric hindrance in the terminating ligand will reduce the surface passivation of the quantum dot (Figure 1). Quantum dots were grown terminated through the sulfur atoms with butanethiol, thiophenol, and thio2-naphthol (Chart I). Here the weak sulfur-hydrogen bond breaks and is replaced with a bond between the sulfur and a surface cadmium ion. In a second series, steric effectswerevaried by capping with acetonitrile, pyridine, and quinoline. Here the nitrogen lone pair forms a dative bond to the cadmium ions at the surface. The effect of the bond strength to the quantum dot surface was ascertained by comparing the properties of the sulfurand nitrogen-bonded samples. The third way the quantum dot surface was varied was to heat treat some of the samples prepared at room temperature. This was done by redissolvingthe purified quantum dot powder in pyridine and refluxing the solution at 115 OC for 1 h. This annealed both surface and interior defects and allowed pyridine molecules from the solvent to further passivate the quantum dot surface. Low-temperature hole-burning and luminescence measurements were performed on quantum dots dispersed in either epoxy or poly(methy1 methacrylate) (PMMA). For the epoxy samples, a small amount of the quantum dot powder was added to the hardener of Bicron BC-600 optical quality epoxy (
