Semiconductors, surface attachment

Bent, S. F. Attaching organic layers to semiconductor surfaces. Journal of Physical Chemistry 106, 2830-2842 (2002). 

Photosensitive substances adsorbed on the semiconductor surface are especially efficient in sensitization reactions. Thus, sensitizing effect can be enhanced if a sensitizer is attached to the semiconductor surface by a chemical bond. For this purpose one has to create either the ether bond -O-between the semiconductor and reactant, using natural OH groups, which exist on the surface of, for example, oxide semiconductors (Ti02, ZnO) or oxidized materials (Ge, GaAs, etc.) in aqueous solutions, or the amide bond -NH- in the latter case a monolayer of silane compounds with amido-groups is preliminarily deposited on the semiconductor surface (see, for instance, Osa and Fujihira, 1976). With such chemically modified electrodes the photocurrent is much higher than with ordinary (naked) semiconductor electrodes. 

For covalently attached dyes, the mechanism is more complex, involving trap states as intermediates and entailing coupled proton transfer. Why do the mechanisms differ It appears likely that phosphonate- and carboxylate-binding perturb semiconductor surfaces sufficiently to create new trap states that can be rapidly populated following injection. The states are necessarily spatially proximal to the attached dyes, but apparently sufficiently separated from each other to preclude fast trap-to-trap hopping. 

Attached molecules. A molecule attached to a semiconductor surface could behave as a surface state if it has an energy level within the... 

There are also some very important scientific conclusions to be drawn from this work. The attachment of photoactive molecular components to TiC>2 surfaces has shown that the photophysical properties of the resulting interfacial ensemble are significantly different from those observed for the individual components. The semiconductor surface clearly acts as a rectifying one. In order to fully understand the balance between the intramolecular and the interfacial processes, more experiments are needed where the nature and connectivity of the molecular components, as well as the nature of the contacting solution, are systematically varied. 

Mui et al.36 report a comparative experimental - theoretical study of amines on both the Si(001)-(2x 1) and the Ge(001)-(2x 1) surface. Both substrates were modeled by X9H12 (X = Si, Ge) clusters, utilizing DFT at the BLYP/6-31G(d) level of theory. For both, the Si and the Ge substrate, formation of a X-N dative bond (X = Si, Ge) is the initial step of the reaction between the considered amine species and the semiconductor surface. Flowever, while primary and secondary amines display N-H dissociation when attached to Si(001)-(2 x 1), no such trend is observed for the Ge counterpart of this system. This deviating behavior may be understood in terms of the energy barrier that separates the physisorption from the chemisorption minimum, involving the cleavage of an H atom. For dimethylamine adsorption, this quantity turned out to be about 50% higher for the Ge than for the Si surface. The authors relate this characteristic difference between the two substrates to the different proton affinities of Si and Ge. 

Most studies on deposition of fullerenes on semiconductor surfaces have focused on silicon9 28 and, to a much less extent, on GaAs. Earlier experimental work using HREELS (high-resolution electron energy loss spectroscopy) on C60 adsorption on Si (100)2 x 1 surface suggested that the molecular attachment to the... 

Bent SF (2002) Attaching Organic Layers to Semiconductor Surfaces, J Phys Chem B 106 2830-2842... 

Much attention has been devoted to the development of optimal photo sensitizers of semiconductor electrodes [36, 43]. Ruthenium(II) polypyridine complexes are especially well suited for this purpose. They are strong light absorbers in the visible spectral region and bpy or tpy ligands can be easily derivatized with anchoring groups. Moreover, localization of the excited electron on the ligand which is attached to the semiconductor surface facilitates the electron injection. 

Titration experiments show that PL of semiconductor CdSe/ZnS and CdSe quantum dots is quenched by surfacely attached porphyrins (Fig. 2) [7] PL integrated intensity becomes smaller accompanied by PL non-exponential decay shortening upon the increase of the molar ratio x = [Cp]/[Cqd]. 

The theme of photosensitizing semiconductor electrodes introduced in Section 57.3.2.5(iii) may be developed with an example from ruthenium—bipyridyl chemistry. The sequence (40) is well known. The effectiveness of the photosensitization should be increased by the covalent attachment of the tris(bipyridyl)ruthenium(II) entity to the semiconductor surface, for example to Sn02. This has been achieved using the versatile halosilane chemistry shown in equation (41). The coimter anion was PFg . Cyclic voltammetry showed that the behaviour of the sjretems Sn02/aqueous [Ru(bipy)3] " and Sn02(Alm)/electroly te were very similar but with a -1-0.05 V shift in E°. The coated electrode gives a photocurrent with a red shift of 10 nm which is twice as large as for the non-coated electrode. Unfortunately the current falls off due to promotion of the hydrolysis of the Aim. 

Surface chemistry, in general, is an area in which the ability to selectively modify the chemical and physical properties of an interface is highly desirable. The synthetic chemistry of surfaces is now in a developing stage, particularly with respect to the attachment of electroactive redox sites to metal or semiconductor surfaces (L-3). Single component and bilayer (4) electroactive films have been a field of intense research activity since their applications are apparent in catalysis, solar energy conversion, directed charge transfer, electrochromic devices, and trace analysis. 

Unfortunately, many of the semiconductor materials which would satisfy requirement (a) and (b) are not stable and undergo light-assisted corrosion instead of driving the desired redox reaction. Several important methods have been devised to chemically protect the surfaces of such materials as Si, CdS or CdSe, and the GaAs with the result however that the redox reactions that can be light-assisted are dictated by the redox e.m.f. of the surface-attached species (6). 

Research in our laboratory and by Osa and Fujihira showed that it is possible to covalently attach monolayers of chromo-phores to metal-oxide semiconductor surfaces — with no compromise in quantum efficiency to energy conversion compared with dyes adsorbed from solution (9-11). The quantum efficiency for these systems (ratio of photo-generated current to photons adsorbed in the dye layer, ne/np) is quite low, in the range of 10 5 to 10 4 and argues against device applications of these simple modified electrodes without further improvements, such as linear, multielectrode stacks of dye-modified, semi-transparent electrodes (10). 

The absorption of organic molecules on semiconductor surface has been drawing attention in the past years for the possibility of combining the wide range of functionahty of organic molecules with existing semiconductor-based infrastractures [7, 8]. It is also cmcial to future molecular devices since the attachment of the molecules onto the surface of electrodes also play important roles in determining the functionality of the devices [9]. 

In this section some optical properties of composite semiconductor particles are reviewed. We shall restrict ourselves to aspects of surface modification of given particles and the formation of core-shell structures. We are leaving out of consideration the field of sandwich colloids (i.e., two different semiconductor particles attached to each other) since their properties have been reviewed in detail before [5,11,12]. 

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