Quantum dots provide an opportunity to investigate semiconductor behavior in the finite size regime. Due to quantum confinement these materials have many fundamentally interesting and potentially useful optical properties. We are currently studying the synthesis and optical properties of CdSe quantum dots for potential applications in opto-electronics. Experimentally, the stoichiometric amount of reagents of Cd and Se elemental powders were sealed in hardened alloy vials together with solid organic medium on a Spex 8000D miller machine. The mass ratio of reagents to balls was 1:20. Small amounts of as-milled powders were taken out of the vial within different intervals for structural and optical measurement. XRD measurement results revealed that small amount of CdSe with wurtzite structure was fabricated by mechanical alloying process for only 15 minutes, mixing with the majority phases of Cd and Se elemental powders being indexed in the XRD pattern. The intensity of the diffraction peaks from elemental Cd and Se powders decreases with ball milling time, which disappeared after 6 hours reaction between the elemental powders during the ball milling process. On the other hand, the intensity indexing from CdSe diffraction peaks increases with milling time. Meanwhile, the phase transition for CdSe nanocrystals from wurtzite structure to zinc blende structure during ball milling process was observed. Wurtzite structural CdSe particles are the main products during the initial stage of ball milling process (from begin to 2 hours). Longer ball milling period than 2 hours leads to the phase transition for CdSe from wurtzite structure to zinc blende structure. Single phase CdSe nanocrystals with zinc blende structure were fabricated after ball milling for more than 8 hours. The as-milled powders were first cleaned by acetone and then dispersed in hexane solution, which exhibits red colorization. Big particles deposit onto the bottom, only small particles dispersed in the solution. The visible absorption spectrum of the dispersion solution was shown in Figure 1. As we can see that the colloid dispersion solution containing CdSe nanocrystals as milled for 2 hours exhibit a small absorption peak at 626 nm (Fig.1(a)). These particles have wurtzite structure, whose bulk bandgap is 1.751 eV (708nm), indicating that the particle size is near its Bohr exciton radius. The disperse solution containing CdSe nanocrystals as milled for 32 hours exhibits a wide absorption platform (from 400nm to 700nm) with two absorption bands at 601nm and 704 nm respectively. This result reveals a wide size distribution of zinc blende CdSe nanoparticles in the solution. The optical properties of CdSe nanocrystals prepared by chemical method will also be presented.
Figure 1. UV visible spectrum of CdSe nanocrystals dispersed in Hexane
Physics Department, Hampton University
Settler Landing, Hampton, Virginia 23668 USA
Phone: 757-7275997 Fax: 757-7286910