Electronic structure and Photophysical study of semiconducting nanocrystallites

Abstract

Sizable photocurrent was observed from the CdS-4.5 and CdS-2.2 nanocrystalline films, although the EL efficiency was weak. The I-V characteristic of CdS-4.5 and CdS-2.2 has a typical diode behavior with the onset voltage depending on the nanocrystallite size. We have also discussed the probable origins of the photocurrent generation in such devices. Further studies like field-dependent photoconductivity and temperature-dependent I-V characteristics are being planned to understand the detailed mechanism of the photocurrent generation and emission from such devices. The photoluminescence emission from such size-selected nanocrystallites is broad and considerably red-shifted from the corresponding excitation spectra due to surface state-mediated emission. In this section, we have reported the synthesis and characterization of free-standing powders of ZnS nanocrystallites of three different sizes with relatively narrow size distributions. The synthesized particles have sizes lower than the bulk Bohr exciton diameter and consequently exhibit strong quantum confinement effects. Average particle sizes of the nanocrystallites, as determined from X-ray diffraction and transmission electron microscopy (TEM), are about 3.5, 2.5, and 1.8 nm, respectively. X-ray diffraction and TEM studies showed that the crystallites had a zinc blende structure. The larger crystallites show intrinsic defects like twinning and dislocations, which may explain the moderate discrepancy between the sizes estimated from X-ray diffraction and TEM. The shift of the band gap with respect to the size of the nanocrystallites, as observed from optical absorption spectra, was compared with the results from model calculations that relate the band gap to the size of the crystallites. The synthesis of such size-selected and well-characterized nanocrystallites of various sizes, obtained as stable free-standing powders, provides an opportunity not only for potential applications in areas such as photovoltaic devices, but also for studying size-dependent optical and electronic properties of these nanocrystallites as demonstrated in the subsequent sections of this chapter. In this section, we have reported preliminary photoconductivity and luminescence results of ZnS nanocrystallites of various sizes. The importance of surface states on these properties is discussed. Further studies are in progress to correlate the observed photocurrent and luminescence behavior in order to get a better understanding of the role played by the surfaces on the physical properties of the nanocrystallites. We have synthesized various sizes of PbS nanocrystallites in different polymeric matrices as well as with thio-glycerol as a capping agent. The synthesized particles have a rock salt structure, same as that of bulk PbS. The particle sizes were determined from the X-ray diffraction and UV-Vis optical absorption spectra. The PVA and PEO methods yield particles with narrow size distributions, while the thio-glycerol method gives rise to a very wide distribution of particle sizes. The wide scan survey of the nanocrystallites shows sulfur-to-lead ratio of 1:1 within the experimental error, except in the case of PbS-15, where there was additional sulfur contribution from SDS. The photoemission core-level spectra of PbS-25 and bulk were similar, giving evidence for the formation of stable PbS particles in polymers. The valence band density of states, calculated using the method of LMTO-ASA, showed that the valence band is derived primarily from S 3p with some small contributions from Pb s and p states. The calculated total density of states matches with the XPS valence band of the PbS-25. The valence band of PbS-25 shows a shift of about 0.07 eV towards higher energy, compared to the bulk PbS, in accordance with the quantum size effect. Further attempts are being made to synthesize PbS nanoparticles of smaller sizes, but with a lower polymeric content in order to perform detailed valence and conduction band studies using photoemission and inverse photoemission techniques. In this chapter, we have discussed the electron spectroscopic results of two different sizes of CdSe nanocrystallites, synthesized by both chemical and electro-deposition methods. Figure 6.16: (a) The calculated DOS of the unoccupied states obtained from LMTO-ASA. The solid line shows the total DOS and the dotted lines are the partial DOS. (b) The experimental BIS spectra of different sizes of CdSe nanocrystalline films. While the Cd 3d core levels of all the sizes suggest a single Cd species, the Se 3d core level exhibits multiple features, arising from various distinctly different selenium species. These are interpreted as Se ions in the core region and Se ions at the surface of the nanocrystallites. Additionally, in the case of the 5 nm sample, there is another component of selenium arising from elemental selenium adsorbed on the substrate. Spectroscopic evidence suggests that the SeO? layer formed on the surface of the nanocrystallites is removed by treating the surface with KCN solution. The valence band spectral features of the nanocrystallites match with those in the density of states calculated using LMTO-ASA for bulk CdSe. The valence band of the smaller 5 nm sample exhibits a shift of about 0.1–0.2 eV towards the higher energy side compared to the 15 nm samples, showing the signature of quantum confinement effects. To our knowledge, this is the first experimental observation of the shift of the valence band in the case of CdSe nanocrystallites. Treating the surface of the nanocrystallites with KCN leads to the passivation of the surface, removing the SeO? layer. The valence band of such surface-treated samples showed a narrowing of the spectral features and a shift of about 0.05 eV towards the higher binding energy compared to the untreated sample. Preliminary UPS and BIS spectroscopic results of the CdSe nanocrystallites are briefly discussed.