Layered of transition metals

A thin layer deposited between the electrode and the charge transport material can be used to modify the injection process. Some of these arc (relatively poor) conductors and should be viewed as electrode materials in their own right, for example the polymers polyaniline (PAni) [81-83] and polyethylenedioxythiophene (PEDT or PEDOT) [83, 841 heavily doped with anions to be intrinsically conducting. They have work functions of approximately 5.0 cV [75] and therefore are used as anode materials, typically on top of 1TO, which is present to provide lateral conductivity. Thin layers of transition metal oxide on ITO have also been shown [74J to have better injection properties than ITO itself. Again these materials (oxides of ruthenium, molybdenum or vanadium) have high work functions, but because of their low conductivity cannot be used alone as the electrode. 

The structure of layered compounds often can be described by the ABC notation, shown in conjunction with the sites between close-packed layers in Fig. 7.1. Given a close-packed layer of atoms, with atoms at positions we call A, there are three possible places to put the atoms of the next layer directly above the first layer (also in the A positions), or fitted into the interstices of the first layer, either in B or C positions. In this notation, a trigonal prism sandwich is denoted AbA, and an octahedral sandwich is AbC, where the uppercase letters denote the layers of chalcogen or oxygen, and the lowercase letters the layers of transition metal atoms. 

Figure 4.17 Electrical resistivity parallel to the layers of transition-metal dichalcogenides. (After Wilson et al, 1975.)...

Wear-resistant layers of transition metal carbides that have been deposited by CVD (see Chemical Vapor Deposition) and PVD processes on the surface of WC-Co hardmetals further reduce the wear in cutting applications. About 80% of... 

The thin layer of transition metal macrocycles attached to carbon generally lack long-term stability in concentrated acid and alkaline solutions. This drawback can be overcome by thermal treatment at 450-900°C for cobalt tetramethoxy phenyl porphyrin (Co-TMPP) [65]. Under these conditions, the Co-TMPP is substantially degraded to cobaltous oxide. Pyrolyzed layers involve high-area carbonaceous materials with a significant surface nitrogen and the transition metals as small oxide and metallic particles dispersed on the high-area substrate. These layers catalyze peroxide elimination in alkahne solutions. 

Diffusion layers of transition metal nitrides are (similar to case hardened steels, which should not be discussed here) prepared by heating metals and alloys in nitrogen or ammonia, occasionally diluted with an inert gas. Mainly titanium and... 

Sections 5 and 6 are devoted to the electronic and magnetic properties, respectively. The physical properties of this series are believed to be determined mainly by the layers of transition metal ions. They are characterized by a triangular net perpendicular to the c-axis. The frame of the crystal lattice is, on the other hand, determined by the rare earth elements. The character of the R ions is reflected in the properties indirectly but definitely. 

Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

The nucleation behavior of transition metal particles is determined by the ratio between the thermal energy of the diffusing atoms and the interaction of the metal atoms at the various nucleation sites. To create very small particles or even single atoms, low temperatures and metal exposures have to be used. The metal was deposited as metal atoms impinging on the surface. The metal exposure is given as the thickness (in monolayer ML) of a hypothetical, uniform, close-packed metal layer. The interaction strength of the metals discussed here was found to rise in the series from Pd < Rh < Co ( Ir) < V [17,32]. Whereas Pd and Rh nucleate preferentially at line defects at 300 K and decorate the point defects at 90 K, point defects are the predominant nucleation center for Co and V at 300 K. At 60 K, Rh nucleates at surface sites between point defects [16,33]. 

Vassilev P, van Santen RA, Koper MTM. 2005. Ah initio studies of a water layer at transition metal surfaces. J Chem Phys 122. 

Nilekar AU, Mavrikakis M. 2008. Improved oxygen reduction reactivity of platinum mono-layers on transition metal surfaces. Surf Sci 602 L89-L94. 

To dissociate molecules in an adsorbed layer of oxide, a spillover (photospillover) phenomenon can be used with prior activation of the surface of zinc oxide by particles (clusters) of Pt, Pd, Ni, etc. In the course of adsorption of molecular gases (especially H2, O2) or more complex molecules these particles emit (generate) active particles on the surface of substrate [12], which are capable, as we have already noted, to affect considerably the impurity conductivity even at minor concentrations. Thus, the semiconductor oxide activated by cluster particles of transition metals plays a double role of both activator and analyzer (sensor). The latter conclusion is proved by a large number of papers discussed in detail in review [13]. The papers cited maintain that the particles formed during the process of activation are fairly active as to their influence on the electrical properties of sensors made of semiconductor oxides in the form of thin sintered films. 

A number of transition metal oxides can also be intercalated by lithium. One of the best known examples is VsOi3. The VgOu structure, shown in Fig. 11.19, consists of alternate double and single layers of V2OS ribbons. The layers are connected by vertex sharing of octahedral sites, and this leads to a relatively open framework structure (Wilhelmi, Waltersson and Kihlburg, 1971). Insertion of lithium into the oxide matrix... 

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