Metal overlayers

Madey and co-workers followed the reduction of titanium with XPS during the deposition of metal overlayers on TiOi [87]. This shows the reduction of surface TiOj molecules on adsorption of reactive metals. Film growth is readily monitored by the disappearance of the XPS signal from the underlying surface [88, 89]. This approach can be applied to polymer surfaces [90] and to determine the thickness of polymer layers on metals [91]. Because it is often used for chemical analysis, the method is sometimes referred to as electron spectroscopy for chemical analysis (ESCA). Since x-rays are very penetrating, a grazing incidence angle is often used to emphasize the contribution from the surface atoms. 

Other topics recently studied by XPS include the effects of thermal treatment on the morphology and adhesion of the interface between Au and the polymer trimethylcy-clohexane-polycarbonate [2.72] the composition of the surfaces and interfaces of plasma-modified Cu-PTFE and Au-PTFE, and the surface structure and the improvement of adhesion [2.73] the influence of excimer laser irradiation of the polymer on the adhesion of metallic overlayers [2.74] and the behavior of the Co-rich binder phase of WC-Co hard metal and diamond deposition on it [2.75]. 

In Figure 5-12 is a set of core level spectra shown, which have been recorded between successive steps during the growth of an aluminum metallic overlayer on top of PPV [59]. The thickness of the aluminum layer for the lowest and highest coverage correspond to 2 and 20 A, respectively. 

The UPS spectra (not shown here) recorded upon Al deposition onto the conjugated thiophene systems shows only small visible changes in the positions of the peaks in the spectra [84]. The main effect is a rapid decrease in intensity, which indicates that a metallic overlayer is formed since the cross-sections for the Al(3p) or Al(3s) are much lower than for the C(2p) or S(3p) orbitals. This is consistent with the Al(2p) XPS spectra discussed above. 

The tltanla-based thin film catalyst models were constructed by first oxidizing the titanium surface In 5 x 10 torr of O2 for approximately 30 minutes at 775 K. This produced an AES llneshape consistent with fully oxidized TIO2. The metal was then vapor deposited onto the oxide support with the latter held at 130 K. The thickness of the metal overlayer and Its cleanliness were verified by AES. After various annealing and adsorption procedures, these thin films were further characterized using SSIMS, AES and TDS. For comparison, some work was done with Pt on Al20s. In this case a Mo foil covered with AI2O3 replaced the Tl(OOOl) substrate. 

Similar SSIMS and TDS results were obtained for rhodium on tltanla and fiir hydrogen chemisorption on both substrates. In a blank experiment Involving i o metal over layer, temperature programming while following the T1 and TIO SIMS signals (Fig. 4) shows that the tltanla thin film does not begin to change until the temperature reaches about 760 K, well beyond the 615 K where Tl was first noted to Increase on the systems with thin metal overlayers. ... 

Figure 4. TPSSIMS of the oxidized Ti(OOOl) substrate without a metal overlayer.

Further studies were carried out on the Pd/Mo(l 1 0), Pd/Ru(0001), and Cu/Mo(l 10) systems. The shifts in core-level binding energies indicate that adatoms in a monolayer of Cu or Pd are electronically perturbed with respect to surface atoms of Cu(lOO) or Pd(lOO). By comparing these results with those previously presented in the literature for adlayers of Pd or Cu, a simple theory is developed that explains the nature of electron donor-electron acceptor interactions in metal overlayer formation of surface metal-metal bonds leads to a gain in electrons by the element initially having the larger fraction of empty states in its valence band. This behavior indicates that the electro-negativities of the surface atoms are substantially different from those of the bulk [65]. 

Figure 3.16 Volcano plot for the hydrogen evolution reaction (HER) for various pure metals and metal overlayers. Values are calculated at 1 barof H2 (298K) and at a surface hydrogen coverage of either 0.25 or 0.33 ML. The two curved lines correspond to the model (3.24), (3.25) transfer coefficients (not included in the indicated equations) of 0.5 and 1.0, respectively, have also been added to the model predictions in the figure. The current values for specific metals are taken from experimental data on polycrystalline pure metals, single-crystal pure metals, and single-crystal Pd overlayers on various substrates. Adapted from [Greeley et al., 2006a] see this reference for more details.

The initial stages, notably the formation of a monolayer on a foreign substrate at underpotentials, were mainly studied by classical electrochemical techniques, such as cyclic voltammetry [8, 9], potential-step experiments or impedance spectroscopy [10], and by optical spectroscopies, e.g., by differential reflectance [11-13] or electroreflectance [14] spectroscopy, in an attempt to evaluate the optical and electronic properties of thin metal overlayers as function of their thickness. Competently written reviews on the classic approach to metal deposition, which laid the basis of our present understanding and which still is indispensable for a thorough investigation of plating processes, are found in the literature [15-17]. 

The first deals with small islands of silver on a ruthenium substrate. One may look at this sample as a, perhaps somewhat far-fetched, model of a supported catalyst or a bimetallic surface. As metal layers are almost never in perfect registry with the substrate, they possess a certain amount of strain. Goodman and coworkers [46] used these strained metal overlayers as model systems for bimetallic catalysts. Here we look first at the electronic properties of the Ag/Ru(001) system as studied by UPS. 

That CO chemisorption is perturbed on strained-layer Ni is not surprising in view of CO chemisorption behavior on other metal overlayer systems. For example, on Cu/Ru it has been proposed that charge transfer from Cu to Ru results in decreased occupancy of the Cu 4s level. This electronic modification makes Cu more nickel-like , and results in an increase in the binding energy... 

The catalytic activity of strained-layer Ni on W(llO) for methanation and ethane hydrogenolysis has been studied as a function of Ni coverage. The activity per Ni atom site for methanation, a structure-insensitive reaction, is independent of the Ni coverage and similar to the activity found for bulk Ni. The activation energy for this reaction is lower on the strained-metal overlayer, however, very likely reflecting the lower binding strength of CO on the bimetallic system. 

We have studied several other metal overlayers on W(110), W(100), and Ru(0001) substrates . Table 1 Usts properties of the metal overlayers, and the effect of the substrate on CO chemisorption. In general only the first monolayer grows pseudomorphically, though more than one monolayer may... 

Herdt GC, Jung DR, Czandema AW (1995) Weak interactions between deposited metal overlayers and organic functional groups of self-assembled monolayers. Prog Surf Sci 50 103-129... 

Herdt GC, Czandema AW (1997) Metal overlayers on organic functional groups of self-organized molecular assemblies. 7. Ion scattering spectroscopy and X-ray photoelectron spectroscopy of Cu/CHs and CU/COOCH3. J Vac Sci Technol A 15 513-519... 

Similar effects can be found for metal overlayers, where a monolayer of one metal is deposited on top of another metal. Here there is an additional effect relating to the fact that the overlayer usually takes the lattice constant of the substrate. For metal overlayers we therefore find a combination of ligand and strain effects. 

There are several major areas of interfacial phenomena to which infrared spectroscopy has been applied that are not treated extensively in this volume. Most of these areas have established bodies of literature of their own. In many of these areas, the replacement of dispersive spectrometers by FT instruments has resulted in continued improvement in sensitivity, and in the interpretation of phenomena at the molecular level. Among these areas are the characterization of polymer surfaces with ATR (127-129) and diffuse reflectance (130) sampling techniques transmission IR studies of the surfaces of powdered samples with adsorbed gases (131-136) alumina(137.138). silica (139). and catalyst (140) surfaces diffuse reflectance studies of organo- modified mineral and glass fiber surfaces (141-143) metal overlayer enhanced ATR (144) and spectroelectrochemistry (145-149). 

The purpose of performing calculations of physical properties parallel to experimental studies is twofold. First, since calculations by necessity involve approximations, the results have to be compared with experimental data in order to test the validity of these approximations. If the comparison turns out to be favourable, the second step in the evaluation of the theoretical data is to make predictions of physical properties that are inaccessible to experimental investigations. This second step can result in new understanding of material properties and make it possible to tune these properties for specific purposes. In the context of this book, theoretical calculations are aimed at understanding of the basic interfacial chemistry of metal-conjugated polymer interfaces. This understanding should be related to structural properties such as stability of the interface and adhesion of the metallic overlayer to the polymer surface. Problems related to the electronic properties of the interface are also addressed. Such properties include, for instance, the formation of localized interfacial states, charge transfer between the metal and the polymer, and electron mobility across the interface. 

---