The transition of PbS from molecular to bulk form has been observed in polymer films. As the particle size decreases the band gap shifts to the blue and eventually approaches the transition energy of the first allowed excited state, X→A, of a PbS molecule. Discrete absorption bands also appear. The electron-hole-in-a-box model with effective mass approximation cannot explain the observed size dependence. We have developed two theoretical models, both including the effect of band nonparabolicity, that successfully explain the observed size dependence down to about 25 Å.
We have studied the room temperature optical reflectivity of MgO, MgA1,0,, and a-Al,O, from 5 to 40eV using a novel spectrophotometer with a laser plasma light source. Structure in the imaginary component of the dielectric response is analysed using critical point line shapes, and the origins of the major transitions in MgO and MgA1,0, are determined using an ab initio pseudofunction band structure calculation of MgO. The exciton reflectivity has been studied in the three materials at temperatures between 300 and I500 K, and exciton-phonon coupling appears to increase from MgO to a-A1203. The temperature dependence of the higher lying interband transitions in MgO has been determined to 1100 K , and we find that while the temperature dependence of the onset transitions at r and X are nearly identical (-1.22meV/K at r), higher lying transitions have very different temperature dependences. Furthermore with increasing temperature the X point valence band separation increases at a rate of 0.38 meV/K, while the conduction band separation at X decreases at -0.41 meV/K.
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