Semiconductors behavior

Besides these chemical effects, which are understood in terms of the established theories in semiconductor physics and chemical kinetics, new physico-chemical phenomena are observed in the case of extremely small particles. The metal or semiconductor behavior is gradually lost with decreasing size, the consequences being drastic changes in the optical properties of the materials and also in their photocatalytic effects. 

The most thoroughly studied mixed S jA-ligand is pyridine-2-thiolate because of its versatile coordination modes. The compound [Ag(SPy)] has a graphite-like array of silver(I) ions and has a semiconductor behavior. The PyS ligand acts as a fi3-N,S,S bridge. [Ag5(SPy)(HSPy)BF4] has a layered structure with the coordination modes of PyS showed in Figure 14.1 04... 

This review will include both types of studies, but will not discuss in any detail optically pumped NMR of semiconductors, which has been well-reviewed [5, 11, 12,14], or other unconventional techniques for detection of NMR signals. Physics-related NMR studies of more complicated semiconductor behavior such as Kondo insulators or semiconductors and other unusual semiconducting phases, and semiconducting phases of high-Tc superconductors, while very important in physics, will be neglected here. I have deemed it of some value to provide rather extensive citation of the older as well as of the more recent literature, since many of the key concepts and approaches relevant to current studies (e.g., of nanoparticle semiconductors) can be found in the older, often lesser-known, literature. My overall aim is to provide a necessarily individual perspective on experimental and theoretical approaches to the study of semiconductors by NMR techniques that will prove useful to chemists and other scientists. 

An STM probe has been used to isolate individual MS (M = Cd, Pb) particles and to measure electronic phenomena (55,56,81). The MS films were prepared either by exposure of metal ion/fatty acid films to H2S (55,56) or by transfer of a compressed DDAB-complexed CdS monolayer (81). All the films were transferred onto highly oriented pyrolytic graphite (HOPG) for the STM measurements. A junction was created at an individual CdS particle with the STM tip as one electrode and the graphite as the other, and the current/voltage characteristics of the panicles were measured. For the particle prepared in the fatty acid films the I/V curves exhibit step-like features characteristic of monoelectron phenomena. In the case of the DDAB-coated CdS particles the I/V measurements demonstrated n-type semiconductor behavior. The absence of steps in this system is probably a reflection of the larger size of the particles in the DDAB films (8 nm by AFM) compared to the 2-nm particle size typically found for MS particles formed in fatty acid films. 

There is a growing tendency to invoke surface states to explain electron transfer at semiconductor-electrolyte interfaces. Too frequently the discussion of surface states is qualitative with no attempt to make quantitative estimates of the rate of surface state reactions or to measure any of the properties of these surface states. This article summarizes earlier work in which charge transfer at the semiconductor-electrolyte interface is analyzed as inelastic capture by surface states of charge carriers in the semiconductor bands at the surface. This approach is shown to be capable of explaining the experimental results within the context of established semiconductor behavior without tunneling or impurity conduction in the bandgap. Methods for measuring the density and cross section of surface states in different circumstances are discussed. 

Recently, a detailed study4,5 of AB02 compounds confirms that AgFe02 has the CuFe02 (delafossite) structure. The cell dimensions, determined with a Guinier camera, are found to be a = 3.0391 2 A. and c = 18.590 2 A. Single crystals are used to measure resistivity as a function of temperature.6 Silver fer-rate(III) exhibits semiconductor behavior with an activation energy of 0.7 e.V. An unusual anisotropy in resistivity is found withp(j c) = 3 X 107 S2-cm. and P( c) = 2 X 1010 J2-cm. 

The meso-functionalized bis(thien-2-yl)methanes 267 (X=S) and bis(furan-2-yl)methanes 267 (X=0) were used for synthesizing the first neutral meso-functionalized tetrathia[22]annulene derivatives 271 (X=S) and tetraoxa[22]annulene derivatives 271 (X=0) (Scheme 105). The compounds were tested for organic field effect transistor (OFET) studies and have shown good mobilities with p-type semiconductor behavior (11JCS(CC)905,12JCS(CC)121). 

Photocurrent measurements performed for unmodified P25 point to an ra-type semiconductor behavior. Calculated IPCE values for P25... 

Functionalization of pentacene with the specific aim of improving performance in devices is a recent endeavor - the first use of a functionalized pentacene in a field-effect transistor was reported only recently (2003) [26], Functionalization of pentacene has led to the ability to engineer the solid-state arrangement, electronic, and solubility properties of this important semiconductor and to improve its stability and film-forming ability. Recent functionalized pentacene materials have yielded devices with properties comparable with those of the parent acene, have enabled the formation of devices from solution-deposited films, and have even changed the semiconductor behavior of this organic molecule from p-type to n-type. As functionalization strategies are refined, materials with all of the properties necessary for commercial device applications should soon be developed. 

Typical values of transfer coefficients a and ji thus obtained are listed in Table 4 for single crystal and polycrystalline thin-film electrodes [69] and for a HTHP diamond single crystal [77], We see for Ce3+/ 41 system (as well as for Fe(CN)63 /4 and quinone/hydroquinone systems [104]), that, on the whole, the transfer coefficients are small and their sum is less than 1. We recall that an ideal semiconductor electrode must demonstrate a rectification effect in particular, a reaction proceeding via the valence band has transfer coefficients a = 0, / =l a + / = 1 [6], Actually, the ideal behavior is rarely the case even with single crystal semiconductor materials fabricated by advanced technologies. Departure from the ideal semiconductor behavior is likely because the interfacial potential drop is located in part in the Helmholtz layer (due e.g. to a high density of surface states), or because the surface states participate in the reaction. As a result, the transfer coefficients a and ji take values intermediate between those characteristic of a semiconductor (0 or 1) and a metal ( 0.5). 

Moderately doped diamond demonstrates almost ideal semiconductor behavior in inert background electrolytes (linear Mott -Schottky plots, photoelectrochemical properties (see below), etc.), which provides evidence for band edge pinning at the semiconductor surface. By comparison in redox electrolytes, a metal-like behavior is observed with the band edges unpinned at the surface. This phenomenon, although not yet fully understood, has been observed with numerous semiconductor electrodes (e.g. silicon, gallium arsenide, and others) [113], It must be associated with chemical interaction between semiconductor material and redox system, which results in a large and variable Helmholtz potential drop. 

LuRh03 2.2 Distorted perovskite structure with p-type semiconductor behavior. 429... 

Armelao et al. (2005) fabricated LaCoOs thin films by the combination of chemical vapor deposition (CVD) and sol-gel methods. Two sequences were adopted to prepare the target film (i) sol-gel of Co-O on CVD La-O (ii) CVD of Co-O on sol-gel La-O. Losurdo et al. (2005) further investigated the spectroscopic properties of these films by ellipsometry in the near-IR and UV range. The former film has a larger crystallite size, a lower refractive index, and a higher extinction coefficient. It also presents a semiconductor-to-metal transition at a temperature of 530 K. Contrarily, the latter film has a smaller crystallite size, a higher refractive index, a lower extinction coefficient and a semiconductor behavior. 

When considering the synthesis of group 13-15 compounds for electronic applications, the very nature of semiconductor behavior demands the use of high-purity single-crystal... 

Gallium arsenide s native oxide is found to be a mixture of nonstoichiometric galhum and arsenic oxides and elemental arsenic. Thus, the electronic band structure is found to be severely disrupted, causing a breakdown in normal semiconductor behavior on the GaAs surface. As a consequence, the GaAs MISFET (metal insulator semiconductor field-effect transistor) equivalent to the technologically important Si-based MOSFET (metal-oxide semiconductor field-effect transistor) is, therefore, presently unavailable. 

It should be noted that these effects are generally not observed at potentials for redox couples located at potentials negative (n-type semiconductors) or positive (p-type semicoductors) of the flat band potential. In these cases, the majority carriers (electrons for n-type semicoductors, holes for p-type ones) tend to accumulate near the semiconductor/electrolyte interface and the semiconductor behavior approaches that of a metal electrode. 

Janzen, D.E. et al.. Preparation and characterization of Jl-stacking quinodimethane oligothiophenes. Predicting semiconductor behavior and bandwidths from crystal structures and molecular orbital calculations, J. Am. Chem. Soc. 126, 15295-15308, 2004. 

Photoeffects are generally not observed at n-type materials for redox couples located at potentials negative of In this case, the bands are bent downwards, the majority carrier tends to accumulate near the surface (i.e., an accumulation layer forms), and the semiconductor behavior approaches that of an inert metal electrode. Similarly, a (hole) accumulation layer forms in a p-type material for couples located positive of Ef y. 

Semiconductor behavior is to be found in many other types of compound, as for example in Fe O and Fej-yS where electron transfer from Fe2+ to Fe3+ causes a virtual migration of Fe3 + ions and hence p-type conduction. 

The same type of methodology was also used to prepare ferrocene-containing arylidene polyesters 122 in good yields from dicarboxyl ferrocenes and organic diols. These materials were characterized by elemental analysis, IR spectroscopy, viscometry, and WAXS. The polymers were found to be semicrystalline but were soluble in polar organic solvents. Conductivity studies showed an n-type semiconductor behavior (cr = 3 x 10 Scm at room temperature) that followed a one-term Arrhenius-type equation with increasing conductivity over the range 25-220 °G. 

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