Cadmium sulfide and lead sulfide quantum dots in glass: Processing, growth, and optical absorption

Glasses containing cadmium sulfide and lead sulfide particles were prepared, and their properties were studied. These particles exhibit quantum confinement behavior when they are smaller than their Bohr exciton radii. Quantum confinement leads to size dependence in the optical absorption of particles. This size dependence can tune the optical absorption of the material to a particular wavelength or energy and possibly enhances the nonlinear optical absorption of the particles. These properties have potential applications in photonic devices.; To control the growth of these semiconductor particles in glass, the glass processing conditions were studied. CdS-doped glasses were initially prepared with CdO and ZnS. The sublimation temperature for ZnS is at 1185°C; whereas, CdO sublimes at 1559°C, and CdS at 980°C. Loss of both cadmium and sulfur was observed in open crucible melts, even when CdO and ZnS were used. Improvements in glass processing were made by use of preheat and a cover during the glass melting, resulting in better retention of both dopants. Direct CdS addition to the glasses was possible with these improvements, thus eliminating complications of zinc incorporation during the growth of the semiconductor particles. These methods were successfully applied to the synthesis of PbS-doped glasses.; CdS and PbS particles were grown in alkali borosilicate glasses, and their optical absorption spectra were measured as a function of heat treatment temperature and time. The position of the absorption peak and edge shifted to longer wave-lengths, or lower energies, with longer heat treatments at a constant temperature. Both CdS and PbS particles exhibited quantum confinement. These measurements were used to calculate particle sizes from quantum confinement models. Comparisons with transmission electron microscopy (TEM) demonstrated that the 1-term effective-mass approximation was appropriate for estimating CdS particle sizes. A sophisticated four-band envelope-function was used for estimation of PbS particle sizes. These sizes were compared and found to be consistent with TEM and x-ray diffraction measurements.; The growth kinetics of these particles were studied. Both direct diffusion and coarsening growth mechanisms were observed with the growth of the CdS particles. Growth of particles in a narrow size range resulted in the development of narrow optical absorption peaks. These particle size distributions were calculated from the optical absorption peak and suggested that diffusion-controlled growth dominated during early stages of heat treatment. After long times, the growth behavior of the particles indicated that there was depletion of solute in the matrix. The magnitude of the optical absorption of the PbS-doped glasses increased with increasing heat treatment time, indicating that there was diffusion of dopant species from the matrix to the particles. Narrow size distributions were also calculated from the sharp optical absorption peaks of PbS particles in glass. The mean and largest particle sizes were close, and the largest particle sizes evaluated at two temperatures were linear with the square root of time. Diffusion-controlled growth was concluded for PbS particles. A diffusion coefficient of 1.8 × 10−16 cm2/s at 630°C, and an activation energy of 33 kJ/mole were calculated.