Films colloidal semiconductor

In general, the absorption spectra of sensitizers bound to colloidal semiconductor films closely resemble those measured in fluid solution. In some cases small spectral shifts have been observed and attributed to Stark effects, acid-base chemistry or stabilization of the sensitizer excited states by the semiconductor surface. However, the effects are small, typically a few nanometers in the visible region. 

A fascinating aspect of the sensitized colloidal semiconductor films is that injected electrons created throughout the semiconductor network are collected in the external circuit with high efficiency. This implies that carrier transport through the 10 pm thick film occurs with no measurable recombination loss. The mechanisms of carrier transport have been studied in some detail. Carrier transport in a semiconductor film can be described by the continuity equation [155] ... 

The long effective pathlength and high surface area afforded by these colloidal semiconductor materials allow spectroscopic characterization of interfacial electron transfer in molecular detail that was not previously possible. It is likely that within the next decade photoinduced interfacial electron transfer will be understood in the same detail now found only in homogeneous fluid solution. In many cases the sensitization mechanisms and theory developed for planar electrodes" are not applicable to the sensitized nanocrystalline films. Therefore, new models are necessary to describe the fascinating optical and electronic behavior of these materials. One such behavior is the recent identification of ultra-fast hot injection from molecular excited states. Furthermore, with these sensitized electrodes it is possible to probe ultra-fast processes using simple steady-state photocurrent action spectrum. 

More recently, Fendler and coworkers have used surfactant vesicles as hosts for colloidal semiconductor particles [83]. For instance, dihexadecyl phosphate (DHP) and 2Ci8Br vesicles incorporating CdS, ZnS and mixed CdS-ZnS particles have been obtained and used for the realization of nanoscale photoelectric devices (see later). It is important to point out, however, that the presence of the closed vesicle does not appear, at least with these systems, to be of critical importance for the control of the size of the particles, as similar phenomena also occurs in Langmuir-Blodgett films [84], suggesting that a crucial role in nucleation is played mainly by the surface. 

Passive films (corrosion) Photoredox processes with colloidal semiconductor particles as photocatalyst (e.g., degradation of refractory organic substances) Photoelectrochemistry (e.g., photoredox processes at semiconductor electrodes)... 

In the most commonly studied configuration of the DSSC, the electron conductor is a wide band gap, nanocrystalhne, metal oxide film. Colloidal titanium dioxide is most often used although other wide band gap metal oxides are candidates. The sensitizer is a transition metal-based (usually ruthenium), organic dye, with excited state free energy sufficient to reduce the semiconductor, and containing ligands, such as carboxylates or phosphonates, which facilitate bonding to the semiconductor... 

The charge injection and recombination dynamics have been studied for various dyes on colloidal semiconductors using luminescence and laser photolysis in conjunction with time-resolved optical spectroscopy. The transparent/translucent nature ofthe finely dispersed colloids allow facile measurements of the luminescence of the sensitizer and the excited state quenching by the semiconductor [60-65]. Sensitization of Ti02 membrane films by the anionic porphyrin [tetrakis(p-carboxyphenyl)porphyrinato Zn(II)], ZnTPPC. Examination of the photophysical behaviour of the porphyrin in the presence of colloidal Ti02 allows identification of the excited state involved in the charge injection process. 

Another approach is to use the LB film as a template to limit the size of growing colloids such as the Q-state semiconductors that have applications in nonlinear optical devices. Furlong and co-workers have successfully synthesized CdSe [186] and CdS [187] nanoparticles (<5 nm in radius) in Cd arachidate LB films. Finally, as a low-temperature ceramic process, LB films can be converted to oxide layers by UV and ozone treatment examples are polydimethylsiloxane films to make SiO [188] and Cd arachidate to make CdOjt [189]. 

Generally, the experimental results on electrodeposition of CdS in acidic solutions of thiosulfate have implied that CdS growth does not involve underpotential deposition of the less noble element (Cd), as would be required by the theoretical treatments of compound semiconductor electrodeposition. Hence, a fundamental difference exists between CdS and the other two cadmium chalcogenides, CdSe and CdTe, for which the UPD model has been fairly successful. Besides, in the present case, colloidal sulfur is generated in the bulk of solution, giving rise to homogeneous precipitation of CdS in the vessel, so that it is quite difficult to obtain a film with an ordered structure. The same is true for the common chemical bath CdS deposition methods. 

Thin film coatings of nanocrystalline semiconductors, as collections of quantum dots (QD or Q-dot) attached to a solid surface, resemble in many ways semiconductor colloids dispersed in a liquid or solid phase and can be considered as a subsection of the latter category. The first 3D quantum size effect, on small Agl and CdS colloids, was observed and correctly explained, back in 1967 [109]. However, systematic studies in this field only began in the 1980s. 

Let us add here that the fabrication of polycrystalline semiconductive films with enhanced photoresponse and increased resistance to electrochemical corrosion has been attempted by introducing semiconductor particles of colloidal dimensions to bulk deposited films, following the well-developed practice of producing composite metal and alloy deposits with improved thermal, mechanical, or anti-corrosion properties. Eor instance, it has been reported that colloidal cadmium sulfide [105] or mercuric sulfide [106] inclusions significanfly improve photoactivity and corrosion resistance of electrodeposited cadmium selenide. 

Semiconductor photocatalysts in a form of colloids, powders, porous granules, thin films or bulk solids including single crystals (used in model studies) provide both liquid phase and gas phase transformations. Comprehensive reviews in this field can be found in monographs [4] (Chapters by N.S.Lewis and M.L.Rosenbluth M.Gratzel M.Schiavello and A.Sclafani P.Pichat and J.-M.Herrmann G.A.Somorjai T.Sakata H.Tributsch M.A.Fox H.Al-Ekabi and N.Serpone D.F.Ollis, E.Pelizzetti and N.Serpone) [8] (Chapter by Yu.A.Gruzdkov, E.N.Savinov and V.N.Parmon) and [3]. 

Fig. 1 Schematic drawing to show the concept of system dimensionality (a) bulk semiconductors, 3D (b) thin film, layer structure, quantum well, 2D (c) linear chain structure, quantum wire, ID (d) cluster, colloid, nanocrystal, quantum dot, OD. In the bottom, it is shown the corresponding density of states [A( )] versus energy (E) diagram (for ideal cases).

Kida et al., 2004). Semiconductors have been utilized for this purpose in the form of electrodes (Desilvestro and Neumaimspallart, 1985 Mackor and Blasse, 1981 Gringue et al., 1987 Ashokkumar et al., 1994), colloids (Keimedy and Duimwald, 1983 Lee et al., 1984 Kamat and Fox, 1983 Kamat, 1989), powders (Ashokkumar and Maruthamuthu, 1989 Herrmarm et al., 1986 Okamoto et al., 1985 Oosawa, 1984) and thin films (Fonash, 1981 Green, 1982 Faherburch and Bube, 1983). 

Application of amphiphilic block copolymers for nanoparticle formation has been developed by several research groups. R. Schrock et al. prepared nanoparticles in segregated block copolymers in the sohd state [39] A. Eisenberg et al. used ionomer block copolymers and prepared semiconductor particles (PdS, CdS) [40] M. Moller et al. studied gold colloidals in thin films of block copolymers [41]. M. Antonietti et al. studied noble metal nanoparticle stabilized in block copolymer micelles for the purpose of catalysis [36]. Initial studies were focused on the use of poly(styrene)-folock-poly(4-vinylpyridine) (PS-b-P4VP) copolymers prepared by anionic polymerization and its application for noble metal colloid formation and stabilization in solvents such as toluene, THF or cyclohexane (Fig. 6.4) [42]. 

The technique of alternating polyelectrolyte film construction has also been adapted to incorporate semiconductors into layered films. For example, multilayer films have been constructed by alternately dipping a quartz substrate into a solution of poly(diallylmethylammonium chloride) and then a solution of a stabilized CdS or PbS colloid (41). The layer-by-layer self-assembly of alternating polymer and metal sulfide is at least partially driven by the electrostatic attraction of the cationic polymer and the negative charge of the stabilized MC colloid particles. 

Optical absorption spectroscopy is often carried out on CD films to verify that the films have a bandgap expected from the deposited semiconductor. Additionally, since CD films are often nanocrystaUine and the most apparent effect of very small crystal size is the increasing bandgap due to size quantization (the effect is visible to the eye if the bandgap is in the visible region of the spectrum), absorption (or transmission) optical spectroscopy is clearly a fast and simple pointer to crystal size, since bandgap-size correlations have been made for a number of semiconductor colloids and films. 

Virtually all the semiconductors deposited by CD are compound semiconductors, the one exception being elemental Se. This has been deposited from solutions of selenosulphate, which rapidly form Se if acidified. By control of the pH, this reaction can be controlled to allow Se deposition to occur. Se films have also been deposited from colloids of Se (prepared by reducing SeOi solutions) by photodeposition, whereby the light activates the formation of films. 

A colloid chemical approach to CdS/HgS/CdS spherical quantum wells was described [79]. Size-dependent third-order non-linear susceptibilities of CdS clusters were investigated [80]. Reviews appeared on size-quantized nanocrystalline semiconductor films [81] and on the quantum size effects and electronic properties of semiconductor microcrystallites [82]. 

In order to take advantage of nanometer-sized semiconductor clusters, one must provide an electron pathway for conduction between the particles. This has been achieved by sintering colloidal solutions deposited on conductive glasses. The resulting material is a porous nanostructured film, like that shown in Fig. 1, which retains many of the characteristics of colloidal solutions, but is in a more manageable form and may be produced in a transparent state. Furthermore, the Fermi level within each semiconductor particle can be controlled potentiostati-cally, a feature which is fundamental for the functioning of the electrochromic devices described in Section III. 

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