Three-dimensional confinement effects in semiconducting zinc selenide quantum dots deposited in thin-film form

Abstract

The three-dimensional confinement effects in chemically deposited ZnSe quantum dots in thin film form are experimentally detected and analyzed. Experimentally measured band gap shifts with respect to the bulk value for quantum dot thin films with various average nanocrystal sizes are compared with the predictions of the effective mass approximation model (i.e., Brus model), the hyperbolic band model, as well as of the Nosaka's approach. It is found that the original Brus model fails to predict correctly the Δ E g( R) dependence, the deviations from experimental data being largest for the smallest nanocrystals. However, using the Brus equations with a single modified parameter—the relative dielectric constant of the material (setting it to approximately four times smaller value than in the case of the bulk material), leads to an excellent agreement with the experimental observations. This seems to be in line with some existing indications that the nanocrystal ɛ r values should be smaller than the corresponding values for bulk specimen, due to the inability of the lattice polarization to follow the more rapid electron and hole motions associated with a smaller crystal radius. On the other hand, application of the hyperbolic band model leads to only a moderate improvement of the agreement with the experimental data in comparison to the Brus model, implying that the electron and hole band non-parabolicity is of minor importance in the case of the studied nanocrystalline material. The possibility of an electron leakage outside the nanoparticle seems to be another effect of key importance for the presently studied system, besides the reduction of ɛ r value upon crystal size decrease. This conclusion is drawn on the basis of excellent agreement of the predicted Δ E g( R) dependence according to Nosaka's approach with our experimental data for chemically deposited ZnSe quantum dots in thin film form.