Chapter - 7
SEMICONDUCTOR QUANTUM DOTS
Figure 1. Quantization of the electronic density of states as a result of variation in the dimensionality of materials.
Figure 2. Typical chemical synthesis approach for making nanoparticles, especially QDs. These are made at higher temperatures, produced by a heating mantle. The process is carried out in an inert atmosphere. Solutions may be added simultaneously.
Figure 3. The change in the electronic energy levels of the system when number of structural units increase as in the case of a change from one double bond to many double bonds. The energy gap between the levels, corresponding to the first excitation energy, ∆E decreases. When the number of double bonds increase to very large number, the energy levels merge and the gap becomes comparable to thermal energy.
Figure 4. The electronic states of a nanocrystal. The allowed optical transitions are marked.
Figure 5. Absorption and emission spectra of InP nanoparticles, from O. I. Mićić, J. R. Sprague, Z. Lu and A. J. Nozik, Appl. Phys. Lett., 68 (1996) 3150.
Figure 6. X-ray diffraction patterns of colloidal InP quantum dots as a function of particle size, (a) 2.5 nm, (b) 3.5 nm and (c) 4.5 nm. The data are compared with the data of bulk InP of zincblende structure (d). While the peak positions and intensities are the same as that of the bulk, the peaks broaden with decrease in particle size. Data from O. I. Mićić, J. R. Sprague, C. J.
Curtis, K. M. Jones, J. L. Machol and A. J. Nozik, J. Phys. Chem., 99 (1995) 7754.
Figure 7. A collection of CdSe nanoparticles synthesized by chemical route. A larger area image of the particles is shown as the inset. This gives a lattice resolved image of the particle. Individual lattice points are observable. Data from the author’s laboratory.
Figure 8. Shift in the absorption spectra of CdS measured in solution, as a function of the particle dimension. The curves, a,b,c and d correspond to particle dimensions, <25, 30, 37 and >42 A, respectively. From, P. V. Kamat, N. M. Dimitrijevic and R. W. Fessenden, J. Phys. Chem. 91 (1987) 396.
Figure 9. Diverse applications of quantum dots.
