D-bands

Fig. XVIII-18. Interaction of the a and n molecular orbitals with the Pt d band Ef is the Fermi level. (From Ref. 172.)...

There are two main kinds of dye aggregates, characterized by their typical spectral properties J-aggregates and H-aggregates. The absorption band maximum (f-band) of the J-aggregates is shifted bathochromicaHy with respect to that of an isolated molecule (M-band) the absorption maximum of the H-aggregates is shifted hypsochromicaHy (H-band). The dyes can also form dimers with a shorter absorption wavelength (D-band). 

Similar results were found by Bacsa el al. [26] for cathode core material. Raman scattering spectra were reported by these authors for material shown in these figures, and these results are discussed below. Their HRTEM images showed that heating core material in air induces a clear reduction in the relative abundance of the carbon nanoparticles. The Raman spectrum of these nanoparticles would be expected to resemble an intermediate between a strongly disordered carbon black synthesized at 850°C (Fig. 2d) and that of carbon black graphitized in an inert atmosphere at 2820°C (Fig. 2c). As discussed above in section 2, the small particle size, as well as structural disorder in the small particles (dia. —200 A), activates the D-band Raman scattering near 1350 cm . ... 

Several Raman studies have been carried out on nested nanotubes [23-26]. The first report was by Hiura et al. [23], who observed a strong first-order band at 1574 cm and a weaker, broader D-band at 1346... 

Kastner et al. [25] also reported Raman spectra of cathode core material containing nested tubules. The spectral features were all identified with tubules, including weak D-band scattering for which the laser excitation frequency dependence was studied. The authors attribute some of the D-band scattering to curvature in the tube walls. As discussed above, Bacsa et al. [26] reported recently the results of Raman studies on oxidatively purified tubes. Their spectrum is similar to that of Hiura et al. [23], in that it shows very weak D-band scattering. Values for the frequencies of all the first- and second-order Raman features reported for these nested tubule studies are also collected in Table 1. 

Prominent in both first-order Raman spectra Fig. 10a is the broad D-band centered at 1341 cm. Two second-order features, one at 2681 cm = 2(1341... 

In Fig. 11 we show the Raman speetrum of earbo-naeeous soot eontaining l-2 nm diameter, singlewall nanotubes produeed from Co/Ni-eatalyzed carbon plasma[28). These samples were prepared at MER, Inc. The sharp line components in the spectrum are quite similar to that from the Co-catalyzed carbons. Sharp, first-order peaks at 1568 cm and 1594 cm , and second-order peaks at -2680 cm" and -3180 cm are observed, and identified with single-wall nanotubes. Superimposed on this spectrum is the contribution from disordered sp carbon. A narrowed, disorder-induced D-band and an increased intensity in the second-order features of this sample indicate that these impurity carbons have been partially graphitized (i.e., compare the spectrum of carbon black prepared at 850°C, Fig. Id, to that which has been heat treated at 2820°C, Fig. Ic). 

Considering all the spectra from nested tubule samples first, it is clear from Table 1 that the data from four different research groups are in reasonable agreement. The spectral features identified with tubules appear very similar to that of graphite with sample-dependent variation in the intensity in the D (disorder-induced) band near 1350 cm" and also in the second-order features associated with the D-band (i.e., 2 X D <= 2722 cm ) and -f- D 2950 cm . Sample-dependent D-band scattering may stem from the relative admixture of nanoparticles and nanotubes, or defects in the nanotube wall. 

It is possible to observe spin-allowed, d d bands in the visible region of the. spectra of low-spin cobalt(lll) complexes because of the small value of 0Dq, (A), which is required to induce spin-pairing in the cobalt(lll) ion. This means that the low-spin configuration occurs in complexes with ligands which do not cause the low -energy charge transfer bands whieh so often dominate the spectra of low-spin complexes. 

It is relevant to note at this point that, because the metal ions are isoelcctronic, the spectra of low-spin Fe complexes might be expected to be similar to those of low-spin Co ". However, Fe" requires a much stronger crystal field to effect spin-pairing and the ligands which provide such a field also give rise to low-energy charge-transfer bands which almost always obscure the d-d bands. Nevertheless, the spectrum of the pale-yellow [Fe(CN)f,] shows a shoulder at... 

Figure 2. The structural energy difference (a) and the magnetic moment (b) as a function of the occupation of the canonical d-band n corresponding to the Fe-Co alloy. The same lines as in Fig. 1 are used for the different structures. In (b) the concentration dependence of the Stoner exchange integral Id used for the spin-polarized canonical d-band model calculations is shown as a thin dashed line with the solid circles. The value of Id for pure Fe and Co, calculated from LSDA and scaled to canonical units, are also shown in (b) as solid squares.

In a previous work we showed that we could reproduce qualitativlely the LMTO-CPA results for the Fe-Co system within a simple spin polarized canonical band model. The structural properties of the Fe-Co alloy can thus be explained from the filling of the d-band. In that work we presented the results in canonical units and we could of course not do any quantitative comparisons. To proceed that work we have here done calculations based on the virtual crystal approximation (VGA). In this approximation each atom in the alloy has the same surrounding neighbours, it is thus not possible to distinguish between random and ordered alloys, but one may analyze the energy difference between different crystal structures. 

From the analysis of the DOS combined with the interpretation of the spectral details in FeAl, CoAl and NiAl (Botton et al., 1996a) the features at the edge threshold (71-73 eV) have been shown to arise from d character being introduced at the Al sites by the strong interaction with the TM d bands and thus by the presence of "covalent" character. This interaction causes the... 

The general understanding of the electronic structure and the bonding properties of transition-metal silicides is in terms of low-lying Si(3.s) and metal-d silicon-p hybridization. There are two dominant contributions to the bonding in transition-metal compounds, the decrease of the d band width and the covalent hybridization of atomic states. The former is caused by the increase in the distance between the transition-metal atoms due to the insertion of the silicon atoms, which decreases the d band broadening contribution to the stability of the lattice. 

The Cu majority Fermi surface is the same as the minority, but for Co the majority and minority Fermi surfaces are quite different. The Co minority Fermi surface (not shown) is very complicated because it lies in the d-bands. Our calculations show that for every value of ky in Cu there is an allowed value of kj on the minority Co Fermi surface. This means that total internal reflection does not occur at the Co-Cu interface for the minority electrons even though the difference in electronic structures is larger in this channel than in the majority channel. 

Ed is the center of gravity of the d band, 9 E) the total density of states and Ep the Fermi energy. 

We will limit ourselves to the surface segregation energy of an impurity of atomic number Z + 1 in a BCC matrix of atomic number Z and study the variation of this energy as a function of the number Nj of d electrons per atom in the d band of e transition metal Z. 

The surface have been assumed, unrelaxed and unreconstructed. The d band filling has been varied in the range (3 - 4.6)e per atom which includes the BCC transition metals and, in particular, the case of Ta and W. The results are displayed in Fig. 2. As often assumed, we have taken Nd(Z + 1) - Nd(Z) = 1.1. However, as shown in Fig. 2, changing this difference to 1 modifies only slightly the numerical results. 

Figure 2. Segregation energy in layer Sp (p = 0 surface layer...) of a transition metal impurity of atomic number Z + 1 (d band-filling (Nj + l.l)e /atom, full curves (Nj + l)e /atom, dashed curve) in a BCC transition metal matrix of atomic number Z (d band-filling Nje" /atom) for various crystallographic orientations of the surface...

Here, we address the more general question of the relative stability of monomers, dimers and triangular trimers on the (111) surface of FCC transition metals of the same chemical species as a function of the d band filling Nd. All possible atomic configurations of the systems are considered monomers and dimers at sites N and F, triangles with A and B borders at sites N and F (Fig. 4). The d band-filling includes the range of stability of the FCC phase (Nd > 7.5e /atom). The densities of states are obtained from... 

It is well known that in bulk crystals there are inversions of relative stability between the HCP and the FCC structure as a fxmction of the d band filling which follow from the equality of the first four moments (po - ps) of the total density of states in both structures. A similar behaviour is also expected in the present problem since the total densities of states of two adislands with the same shape and number of atoms, but adsorbed in different geometries, have again the same po, pi, P2/ P3 when the renormalization of atomic levels and the relaxation are neglected. This behaviour is still found when the latter effects are taken into account as shown in Fig. 5 where our results are summarized. 

Figure 5. Diagram giving the relative stability of the various atomic configurations shown in Fig. 4 as a function of the d band-filling Nj- From the second to the fifth line relative stability of the F and N sites for the monomer, dimer, A trimer and B trimer. On the sixth and seventh lines relative stability of A and B triangles at N and F sites. The relative stability of HCP and FCC bulk phases is given for comparison in the first line.

It is seen that there exists a domain of d band-fillings Nchemical species. This domain narrows very rapidly when the cluster size increases. Consequently, outside this domain and in the range of stability of bulk FCC, i.e., when N 8.2e /atom, we predict that cluster adatoms sit always at normal sites irrespective of the size of the... 

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