Conduction band, nanostructures

The last section revealed that semiconductor band gaps for nanostructures vary with the size of the structure. The wavelength of light emitted when an electron in the conductance band returns to the valence band will therefore also vary. Thus, different colour fluorescence emission can be obtained from different-sized particles of the same substance (e.g., different sized quantum dots of CdSe irradiated with UV emit different colours of light). To produce fluorescence, light of greater photon energy than the band gap is shone onto the nanocrystal. An electron is excited to a... 

Investigating electron migration in nanostructured anatase Ti02 films with intensity-modulated photocurrent spectroscopy [288], it was found that, upon illumination, a fraction of the electrons accumulated in the nanostructured film is stored in deep surface states, whereas another fraction resides in the conduction band and is free to move. These data indicate that the average concentration of the excess conduction band electrons equals about one electron per nanoparticle, irrespective of the type of electrode, the film thickness, or the irradiation intensity. 

Figure 1. P L2 3XANES of nanostructures with InP quantum dots with different number of monolayers, Ec is the bottom of conduction band O ft) and P Ly USXES of Ino.5Gao.5P alloy, is the valence band... 

Comparison of the energy gap —1.9eV for Ino.5Gao.5P determined as the difference between the valence band top and the bottom of the conduction band with the energy of the photoluminescence peak demonstrates rather good accordance [2]. For InP quantum dots one can observe a decrease in conduction band bottom energy by the value 0.2 eV that results in reducing of the band gap in these nanostructures. 

Fig. 6 Schematic of DSSC operation for conversion of light into electrical energy, a Represents the different steps involved in energy conversion. The horizontal line represents the Fermi level/redox potential/electrochemical potential of the electron. Step 1 represents the regeneration of iodide by the reduction of tri-iodide at the counter electrode, step 2 diffusion of 1 to dye-sensitized electrode, step 3 restoration of dye to the original state through electron donation by the electrolyte, step 4 photoexcitation of the dye and the resulting injection into the conduction band on the semi-conductiong oxide, and finally, step 5 recombination, b Represents the dye molecule adsorbed on nanoparticles (based on appropriate semiconducting oxide) of the nanostructured electrode...

A quantum dot is made from a semiconductor nanostructure that confines the motion of conduction band electrons, valence band holes, or excitons (bound pairs of conduction band electrons and valence band holes) in all three spatial directions. A quantum dot contains a small finite number (of the order of 1 to 100) of conduction band electrons, valence band holes, or excitons, that is, a finite number of elementary electric charges (Scheme 16.2). The reason for the confinement is either the presence of an interface between different semiconductor materials (e.g. in coie-sheU nanocrystal systems) or the existence of the semiconductor surface (e.g. semiconductor nanocrystal). Therefore, one quantum dot or numerous quantum dots of exactly the same size and shape have a discrete quantized energy spectrum. The corresponding wave functions are spatially localized within the quantum dot, but they always extend over many periods of the crystal lattice (5). 

Therefore, only certain wavelengths are allowed, i.e., those that have a node plane at the boundaries of the three-dimensional conduction band. Hence, the conductivity is quantum size Kmited. We observe the quantum effects of conductivity. These results motivated us to work together with the Cologne group and to find out whether there are similarities between mesoscopic metals and organic metals [5], as we suspected that the conductivity phenomena in conductive polymers may be better understood taking nanostructures into account. 

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