Nanostructured CdSe Films in Low Size-Quantization Regime: Temperature Dependence of the Band Gap Energy and Sub-Band Gap Absorption Tails

摘要

Temperature dependence of the band gap energy and sub-band gap absorption tails in 3D assemblies of close packed weakly quantized CdSe quantum dots deposited as thin films was studied. The range from cryogenic temperatures (～10 K) up to 340 K was covered. Excitonic absorption peaks were not observed even at temperatures as low as 11 K, which was attributed to the finite particle size distribution and interdot electronic couphng effects. The temperature coefficient of the band gap energy of the nanostructured fihnsof9.4 x 10~(-4) eV K~(-1) ishigherbyafactorof 1.35 than the correspondingvalueforthe bulk CdSespecimen.Ascompared to the case offilms constituted of strongly quantized ZnSe QDs, where the factor α_(nanocrystal)/α_(bulk) was found to be 1.82 (Pejova, B.; Abay, B.; Bineva, I. /. Phys. Chem. C 2011, 115, 37), the present findings imply that this ratio increases upon enhancement of size quantization effects in semiconductor nanocrystals. Analysis of the temperature-dependent optical absorption data within the Bose-Einstein model implies that no phonon confinement effects influence the phonon spectrum in the presently studied material due to the very small size-quantization effects. This situation is opposite to what we have recently found in the case of 3D arrays of strongly quantized ZnSe films. The characteristic Einstein temperature of the presently studied material corresponds to phonon frequency of about 220 cm" , in excellent agreement with the LO mode frequency of bulk CdSe (210-214cm~(-1)). It is demonstrated that the Urbach rule is vahd in the presently studied nanostructured material in low size-quantization regime. Urbach energies are several times higher than the values characteristic for macrocrystalline materials, due to the relatively high degree of inherent structural disorder in the studied QD sohds. At the same time, however, these values are approximately three times smaller than those reported for strongly quantized ZnSe films in our previous study. The dynamical (temperature-dependent) term accounts for only about 22% of the overall Urbach energy values, though this value is higher than the corresponding ratio in the case of strongly quantized ZnSe films.