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

Abstract

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 coupling effects. The temperature coefficient of the band gap energy of the nanostructured films of 9.4 Â 10 À4 eV K À1 is higher by a factor of 1.35 than the corresponding value for the bulk CdSe specimen. As compared to the case of films 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. J. 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 À1 , in excellent agreement with the LO mode frequency of bulk CdSe (210À214 cm À1 ). It is demonstrated that the Urbach rule is valid 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 solids. 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.

Sonochemically assisted colloidal route to CdSe quantum dot assemblies: an alternative way to further fine-tune the size-dependent properties

“…In the case of bulk cadmium selenide, the thermodynamically most stable modification at ambient temperature and pressure is the hexagonal (wurtzite) polymorph of this compound. The situation, however, that we encounter in the present case, is similar to those that we have reported before in the case of other compound semiconductor systems [59][60][61]. Namely, as a consequence of the ultrasmall dimensions of nanocrystals constituting the 3D array in the form of compact thin film or bulk precipitate, it is the cubic form that is actually being observed under ambient conditions.…”

“…We have further proceeded with careful analysis of the semiconductor absorption function of the form (28) to determine the band gap energy of the films deposited under various conditions, as well as of the thermally annealed films. We did this by a combined interpolation-extrapolation procedure, described in details in our previous publications [59][60][61]73]. It is necessary to implement such algorithm as the parabolic approximation for the dispersion relation is, strictly speaking, valid only in the neighborhood of the borders of the first Brillouin zone.…”

Conversion of above- and below-bandgap photons via InAs quantum dot media embedded into GaAs solar cell

“…18 Various mechanisms of Urbach-Martienssen tails in nanostructured GaAs may also provide substantial contribution in this range. 19 In the range above 900 nm, only QD devices with WLs show substantial contribution to the electric current. The 2D quasi-localized WL electron states provide effective coupling between the 0D localized QD states and 3D conducting states in the matrix.…”

Sharp exponential band tails in highly disordered lead sulfide quantum dot arrays

“…Reported E U for various single crystal, amorphous, and nanocrystalline material systems are compared in Table 1 illustrating that the more disorder the system exhibits, the higher E U . Early work on E U in QD arrays found evidence of broad Urbach tails in nanostructured CdSe [38] and ZnSe [39]. Optical absorption of colloidal CdTe QD films show a broad band tail (97 meV), which reduces (to 30 meV) when the film is sintered to form larger grains [36].…”