Experimental studies on the defect states at the interface between nanocrystalline CdSe and amorphous SiO_x

Abstract

Superlattices of a-SiO x /nc-CdSe and thin composite films of SiO x doped with CdSe nanocrystals have been investigated. The CdSe nanocrystals size in both kinds of samples was determined by x-ray diffraction and HREM measurements. A significant difference has been found in the size values determined by both methods, which has been ascribed to appreciable nanocrystal lattice deformations. Subband absorption, room-temperature photoluminescence and thermally stimulated currents have been measured. It has been observed that in the superlattices the absorption in the tail region increases as sublayer thickness decreases. A new photoluminescence band has also appeared in the superlattices having thinnest (2.5 nm) CdSe sublayers. Two new maximums at about 220 K and 240 K, not existing in the CdSe single layers studied, have been found in thermally stimulated current spectra of the composite films. Both maximums are less expressed in the superlattices. The described results have been connected with a size-induced increase in the concentration of interface defect states in CdSe nanocrystals. It has been estimated that these defects are disposed at about 0.35 eV above the highest occupied molecular orbit in CdSe.

Nanoparticle layers of CdSe buried in oxide and chalcogenide thin film matrices

“…Electron micrographs were recorded at 40,000, 110,000 and 500,000 times magnification using optimum contrast conditions (near Scherzer defocus). The constant photocurrent method was employed [9,11] in order to study the absorption of CdSe nanoparticles in the region 1.45-3.1 eV. Planar contacts from aluminum (on the SiO x -CdSe films), carbon (on the GeS 2 -CdSe films) and sputtered gold or In-Ga (on the ZnSe-CdSe films) have been used in those measurements.…”

“…Figs.1a and b show that at low substrate temperatures and a thickness ratio of d matrix =d CdSe ¼ 20 the surface roughness of the 'matrix' layers is sufficient to ensure the formation of quasi-isolated CdSe nanoparticles. However, multilayer structures fabricated as described above, but having a thickness ratio d matrix =d CdSe E1 (d ¼ 2210 nm) and a smooth surface of the 'matrix' layers behave as high quality amorphous/nanocrystalline multiple quantum wells [11,17,18]. These results prove the correctness of the suggested mechanism of selforganized nanocluster formation on a rough surface and indicate that the multilayer vapor deposition technique may be applied for fabrication of both II-VI nanoclusters in various matrices and multiple quantum well structures from various materials.…”

Study of structure and luminescence of CdSe Nanocrystals obtained by ball milling

“…1 Recently, semiconductor nanocrystals have attracted attention due to their potential application in new optoelectronic devices such as light-emitting diodes, 2 nonlinear optical devices, 3 and fluorescent biological labels. 4 In particular, nanocrystalline CdSe exhibits quantum confinement effects which enables bandgap tuning from the near infrared ͑1.8 eV͒ to the blue-green region, thus being a good candidate for the fabrication of a variety of nanodevices.…”

Study of modified solid surfaces by nanostructured CdSe in SiO x thin films

“…The common thickness of the grown structure on Ti is approximately 1.5 µm. In order to crystallize CdSe sublayers as well as to reduce the inherent mechanical strains in the SiO x films, the structures were annealed in atmosphere of technical Ar for 60 min at 673 K. Highresolution electron microscopy (HREM) studies of typical SiO x :CdSe MLs show a very good periodicity after annealing [11,12]. The second group of samples-SiO x :CdSe CFs was based on the assumption [8] that during the deposition of superthin films of thermally evaporated CdSe on a rough surface, selforganized formation of CdSe nanoparticles rather than growth of continuous thin films will be favoured.…”