General Approach for Fabricating Inorganically Coupled Colloidal Quantum Dots and Rods Developed
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yield nanoparticles soluble in organic agents or water. The amine in the oleylamine provides the reducing equivalents both for tetrachloroauric acid and silver acetate. The carbon double bond does not play a significant role in the process, since other aliphatic amines work equally in the reaction. Oleylamine was chosen because it is commercially available and is inexpensive, said the researchers. The reaction typically proceeds over the course of ~2–24 h. The size of the metal nanoparticles can be controlled by varying the concentrations of the metal precursor and the amine and by adequately choosing the growth temperature. The researchers also show that the resulting particles have oleylamine weakly adsorbed on the surface and that this surfactant can be easily exchanged with aliphatic and functional thiols, thus allowing the solution of gold and silver nanoparticles in both aqueous and nonaqueous media. ROSALÍA SERNA
General Approach for Fabricating Inorganically Coupled Colloidal Quantum Dots and Rods Developed A.P. Alivisatos of the University of California, Berkeley, and colleagues there and at Lawrence Berkeley National Laboratory (LBNL) have fabricated coupled quantum dots and segmented nanorods into tunable, linear, and multiple-branching forms. The linear and branched topology can be created at any
a
point during the growth of heterostructures, not just at nucleation. To calculate the electronic interactions of the branches in three dimensions, the researchers developed a method that draws on the technique of local density approximation (LDA), extended to the entire system. As reported in the July 8 issue of Nature (p. 190), the Alivisatos group used suspensions of cadmium, selenium, tellurium, sulfur, and other constituents. When these are assembled from liquid solution, crystals of different cadmium compounds are formed. A given structure begins with a quantum dot. The dot is then made to sprout four arms made of the same or a different compound. The arms of such a tetrapod can be lengthened into nanorods containing segments of different compounds. Nanorods, whether or not they are the arms of tetrapods, can be induced to branch into separate rods of the same or a different compound (see Figure 1a). “Linear heterojunctions were formed when precursors for a second material were added to a growth solution containing preformed nanorods or tetrapods,” said the researchers. The research group synthesized linearly extended rods with CdS rods and CdSe extensions. They synthesized branched rods with CdS rods and CdTe branches. “Nanoprobe x-ray energy dispersive spectrometry…confirm[ed] the presence of Te at either end of branched-rod heterostructures, Se in the central rod, and Cd throughout,” the researchers said. These
results were further corroborated by powder x-ray diffraction, transmission electron microscopy (TEM), and high-resolution TEM (HRTEM). HRTEM revealed a continuation of anisotropic wurtzite growth in the second semiconductor, the researchers said. They concluded that “a branched junction fo
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