Semiconductors surface modification

In addition, the rate of Oz reduction, forming 02 by electron, is of importance in preventing carrier recombination during photocatalytic processes utilizing semiconductor particles. 02 formation may be the slowest step in the reaction sequence for the oxidation of organic molecules by OH radicals or directly by positive holes. Cluster deposition of noble metals such as Pt, Pd, and Ag on semiconductor surfaces has been demonstrated to accelerate their formation because the noble metal clusters of appropriate loading or size can effectively trap the photoinduced electrons [200]. Therefore, the addition of a noble metal to a semiconductor is considered as an effective method of semiconductor surface modification to improve the separation efficiency of photoinduced electron and hole pairs. 

L. Spanhel, M. Haase, H. Weller, A. Henglein, Photochemistry of colloidal semiconductors surface modification and stability of strong luminescing CdS particles, J. Am. Chem. Soc.109 (1987)5649-5655. 

Dominey RN, Lewis NS, Bruce JA, Bookbinder DC, Wrighton MS (1982) Improvement of photoelectrochemical hydrogen generation by surface modification of p-type silicon semiconductor photo-cathodes. J Am Chem Soc 104 467 82... 

On the other hand, modification of substrate surfaces, especially semiconductor surfaces, has been an intensively... 

GaAs, CuInS2, CuInSe2- Semiconductor electrodes have received increasing attention as a consequence of their potential application in photoelectrochemical energy conversion devices. In order to achieve optimum efficiency, the knowledge of the surface composition plays a crucial role. Surface modifications may occur during operation of the photo electrode, or may be the result of a chemical or electrochemical treatment process prior to operation. 

One additional problem at semiconductor/liquid electrolyte interfaces is the redox decomposition of the semiconductor itself.(24) Upon Illumination to create e- - h+ pairs, for example, all n-type semiconductor photoanodes are thermodynamically unstable with respect to anodic decomposition when immersed in the liquid electrolyte. This means that the oxidizing power of the photogenerated oxidizing equivalents (h+,s) is sufficiently great that the semiconductor can be destroyed. This thermodynamic instability 1s obviously a practical concern for photoanodes, since the kinetics for the anodic decomposition are often quite good. Indeed, no non-oxide n-type semiconductor has been demonstrated to be capable of evolving O2 from H2O (without surface modification), the anodic decomposition always dominates as in equations (6) and (7) for... 

The addition of a second species can cause a decrease in charge recombination and an increase in the TiOz photocatalytic efficiency. Such behavior was examined by loading a series of species on the surface or into the crystal lattice of photocatalysts inorganic ions [148-152], noble metals [153,154], and other semiconductor metal oxides [155], It was thus proven that modifications produced by these species can change semiconductor surface properties by altering interfacial electron-transfer events and thus the photocatalytic efficiency. 

The optical properties of semiconductor QDs (Fig. la-c, Tables 1 and 2) are controlled by the particle size, size distribution (dispersity), constituent material, shape, and surface chemistry. Accordingly, their physico-chemical properties depend to a considerable degree on particle synthesis and surface modification. Typical diameters of QDs range between 1 and 6 nm. The most prominent optical features of QDs are an absorption that gradually increases toward shorter... 

The limiting factors that control photocatalysis efficiency are rapid recombination between photo-generated charge carriers, and the backward reaction leading to recombination of the formed molecular hydrogen and oxygen. To retard these processes efforts have typically focused on surface modification of the semiconductor particles using metals or metal oxides. 

The transducing mechanism of semiconductor luminescence involves the modification of the semiconductors surface electrical properties through molecular adsorption. Changes in solid-state electro-optical properties result from adsorption of the molecule of interest onto the semiconductor surface. 

Thus hole or electron transfer can follow a number of pathways across the semiconductor/electrolyte interface. First, one can have direct oxidative or reductive charge transfer to solution species resulting in desired product formation. Second, one can have direct charge transfer resulting in surface modification, such as oxide film growth on GaP or CdS in aqueous PECs. Finally, one can have photoemission of electrons or holes directly into the electrolyte. All of these processes provide some information about the electronic structure of the interface. 

In addition to the illumination of the catalyst surface, another simple method is used for the alteration of the electron concentration and the occupation of the bond orbitals in the semiconductor surface. This method is a modification of the inverse mixed catalysts introduced by Schwab 89 9 . The electron concentration and distribution upon the bond states is achieved 1. by putting the surface bonds into the potential of a boundary layer of a metal-semiconductor junction and 2. by illumination of the semiconductor-metal junction with ultraviolet light (photovoltaic effect). 

The consistent use of top-contact architecture with development of a variety of surface modifications to alter the interface where the semiconductor is deposited, detailed below, have contributed to reports of steadily increasing performance in pentacene and other organic materials. 

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