Bandgap excitations

It is generally accepted that three major processes limit the photoelectrochemical current in semiconductors after a bandgap excitation [76]. These processes are schematically illustrated in the band diagram shown in Fig. 3.2. The bold arrows show the desired processes for efficient water splitting PEC cell after a bandgap excitation the transport of electrons to the back contact, the transfer of the hole to the semiconductor surface and the oxidation of water at the semiconductor/electrolyte interface. The three major limiting processes are a) bulk recombination via bandgap states, or b) directly electron loss to holes in the... 

The direct charge transfer to dichloroacetate proposed in reaction (7.21) requires that the scavenging molecules are adsorbed on the Ti02 surface prior to the adsorption of the photon. Otherwise, this reaction could not compete with the normal hole-trapping reactions (7.9) and (7.10). So the adsorption of the model compound DCA on the titanium dioxide surface prior to the bandgap excitation appears to be a prerequisite for an efficient hole scavenging. 

Figure 1 Photoinduced charge transfer processes in semiconductor nanoclusters, (a) Under bandgap excitation and (b) sensitized charge injection by exciting adsorbed sensitizer (S). CB and VB refer to conduction and valence bands of the semiconductor and et and ht refer to trapped electrons and holes, respectively.

An interesting aspect of composite semiconductor nanoclusters is their ability to rectify the charge carrier flow following the bandgap excitation of the... 

Mason et al supported the two step process based on the decrease in relative intensity with below bandgap excitation [19]. Partially based cm their PL lifetime studies, Hofmann and co-workers have argued that the spin-dependent process is a radiative transition from a shallow donor to a much deeper single donor [20,21], The basic argument that there is a level in the lower half of the bandgap with a broad line having a g value a little less than two has been supported by Reinacher et al based cm their LESR results [13],... 

Site selective PL was performed by identifying four distinct Er sites in implanted and annealed GaN [18]. This work clearly demonstrates that different Er sites are sensitive to different optical pump wavelengths. Apparently, all but 1/100 - 1/1000 of the Er ions are situated on Ga substitutional sites and only pumped with below bandgap excitation (direct 4f-shell absorption) [19], This leaves preciously few Er ions susceptible to the desired above bandgap excitation (electron-hole pair mediated excitation). This has serious implications for a potential forward biased GaN Er LED as explained in the next paragraph. 

The valence band holes (h+) created by bandgap excitation of the WO3 or Fe203 serve to oxidize water to oxygen ... 

Under bandgap excitation and (b) sensitized and ht refer to trapped electrons and holes,... 

As confirmed from the spectroelectrochemical study of Ti02 particulate films [148, 149], the inherent semiconductor properties such as trapping of electrons at the defect sites is responsible for the coloration effects (Figure 19.8). Electrochemical and photoelectrochemical approaches have supported this mechanism for coloration in WO3 colloids [137] and nanostmctured films [143]. The net color change was found to be spectrally similar whether one employed direct bandgap excitation of WO3 nanopartides or subjected them to a negative electrochemical bias. 

Usually the defects reduce the lifetime of non-equilibrium carriers and, consequently, their diffusion length in semiconductors. However, because of the presence of the closely spaced AlGaAs barriers, the carrier capture by the QDs in our samples is not diffusion-limited. That is why a difference in the quenching factor of the PL intensity at a given irradiation dose for the above- and below-bandgap excitation for all energies above the m = 2 QD excited state (Fig. lb) is not observed. Thus, the loss of carriers occurs mainly in the dots due to tunneling of caniers from the dots to adjacent non-radiative recombination centers. 

The results of the TRPL measurements performed on sample A as a function of the irradiation dose, for resonant and non-resonant excitation, corroborate the PLE data. No influence of the irradiation on the PL decay kinetics from the ground state is observed (Fig. 3). However, the rise time shortens by about a factor of 4 for the maximum dose used. This means that the rise time shortening upon above-bandgap excitation is caused by a carrier loss in the QDs and not by any reduction of the diffusion length in the barrier or the WL. The effect can be explained by tuimel escape of the carriers to adjacent defects. The groimd state, having a more localized wavefimction than the excited ones, remains essentially undamaged . Contrary to earlier measurements on electron irradiated QDs... 

Domen et al. studied the photocatalytic activities of several laya-ed metal oxide semiconductors, mainly K4Nb60n, for water cleavage by bandgap excitation (272). Motivated by the success, a wide variety of photocatalysts based on the layered transition metal oxides have been investigated (273-280). The unique structure has successfiilly been utilized for efficient photocatalytic reactions. In some cases, metal or oxide clusters have been deposited on K4Nb60i7. 

Inorganic semiconductors such as TiOa have been widely studied as photocatalysts to carry out the chemical transformation of organic and inorganic compounds in aqueous and non-aqueous media. Under bandgap excitation, the semiconductor, particles act as short-circuited microelectrodes and initiate the oxidation and reduction processes of the adsorbed substrates. 

Bandgap excitation of semiconductor particles suspended in water causes electronic transitions from the valence band to the conduction band, leaving holes in the former. These electrons and holes then either migrate to the particle surface and become involved in redox reactions or they recombine and simply liberate heat. 

The above features were all demonstrated in thermal catalysis using oxides. However, they can be transposed to photocatalysis, at room temperature, where the simultaneous formation of photoelectrons and holes occurs. Therefore, metal oxide semiconductors have also been widely used as photocatalysts. Under these conditions, the redox reactions are based on photoinduced electronic processes + hi/ -> e -I- h (hv > Eq bandgap). By these bandgap excitations,... 

Frei et al. [34[ examined the surface chelation of phenylfluorene on Ti02 using FT-IR and laser photolysis techniques. The surface chelate has its visible absorption band maximum located at 476 nm (e = 3.6x10 M cm ). Electronic excitation in the visible absorption band results in extremely rapid and efficient injection in the conduction band of the semiconductor. A lower limit for the rate constant of interfacial electron transfer was determined as 10 s l and the back electron transfer was found to occur with a specific rate of 2.8x10 s l. The injected electrons in the conduction band readily reduce electron acceptors such as methyl viologen efficiently in the same manner as when they are produced by bandgap excitation of the semiconductor. 

Dye sensitization, i.e., charge injection from an electronically excited adsorbed dye, is a well established technique [17,18] that permits to drive photoelectrochemical and photocatalytic processes on wide-bandgap semiconductors using sub-bandgap excitation. This feature is of obvious relevance to the use of semiconductors in solar energy conversion [19]. The main... 

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