Functional metal surfaces with

Using Equation (1.7) calculate the velocity of photoelectrons ejected from a sodium metal surface, with a work function of 2.46 eV, by ultraviolet light of wavelength 250 nm. 

For simple metal surfaces with fundamental periodicity a, the corrugation amplitude of the Fermi-level LDOS as a function of tip-sample distance can be estimated with reasonable accuracy (Tersoff and Hamann, 1985) ... 

Fig. 5.1. A metal surface with one-dimensional periodicity. The lowest Fourier components of the charge-density distribution are determined by the Bloch functions at the r and the K points in reciprocal space.

De Boer (130) first drew attention to the contribution of the work-function effect to the heat of chemisorption of alkali atoms on a metal surface. With Cs on W, for example, the heat of adsorption is described by the equation,... 

The effect of the particular functional group on adsorption, however, is not always the same as seen on oxide surfaces. For example, carboxylic acids adsorb strongly on metal oxide surfaces, and the heat of adsorption of stearic acid is higher than that of the corresponding ester, methyl stearate (Hironaka et al., 1978). For adsorption on the nascent surface, however, propionic acid is a poor adsorbate, and the adsorption activity of propionic acid is lower than that of methyl propionate. Although propyl amine adsorbs easily on metal oxide surfaces, the adsorption activity of propyl amine is low on the fresh steel surfaces. Also, the heat of adsorption of organic sulfides on iron oxide is less than that of esters (Forbes et al., 1970b), but the results of adsorption activity on the nascent surface were the opposite. It is thus noteworthy that the chemical nature of the nascent surface of steel is often opposite to that of oxide-covered metal surfaces, with respect to adsorption. 

The energy required to eject an electron from a metal is deternuned by its work function . For example, sodium has 4> = 1.82 eV. The electron-volt is a convenient unit of energy on the atomic scale 1 eV = 1.602 x 10 J, corresponding to the energy which an electron picks up when accelerated across a potential difference of 1 volt. The classical expectation would be that radiation of sufficient intensity should cause ejection of electrons from a metal surface, with their kinetic energies increasing with the radiation intensity. Moreover, a time delay would be expected between the absorption of radiation and the ejection of electrons. The experimental facts are quite different. It is found that no electrons are ejected, no... 

If two or more electrochemical half-cell reactions can occur simultaneously at a metal surface, the metal acts as a mixed electrode and exhibits a potential relative to a reference electrode that is a function of the interaction of the several electrochemical reactions. If the metal can be considered inert, the interaction will be between species in the solution that can be oxidized by other species, which, in turn, will be reduced. For example, ferrous ions can be oxidized to ferric ions by dissolved oxygen and the oxygen reduced to water, the two processes occurring at different positions on the inert metal surface with electron transfer through the metal. If the metal is reactive, oxidation (corrosion) to convert metal to ions or reduction of ions in solution to the neutral metal introduces additional electrochemical reactions that contribute to the mixed electrode. 

These potential limitations, however, have been largely eliminated. Early attempts describe modifications of the metal surface with thin insoluble layers, such as silica with subsequent functionalization via silane compounds [16]. The SAM approach created a tool for convenient introduction of various surface functionalities that can be used for immobilization [6j. Examples include SAMs that are cw-terminated with hydroxyl or carboxyl groups, which can be activated for covalent coupling via nucleophilic reactions. In addition, this modification with epoxy groups leads to activated surfaces that can be directly used for nucleophilic linkage. 

The strong bond formed between the thiol endgroups and gold and silver surfaces allows the possibility of forming molecules that have a wide variety of different functional groups at the opposite end and thus of coating a noble metal surface with a variety of differently fimctionalized molecules and mixtures. 

The individual elements of this potential energy diagram are readily explained When an atom or molecule approaches a metal surface with free electrons, image charges will be introduced through polarization. This leads to an attractive interaction, the so-called Van der Waals interaction, which is proportional to dr, where d is the distance between the atom/molecule and the surface. However, if the electrons in the surface and the molecule do not adjust there will be a strong repulsion proportional to e since this is the manner in which the electronic wave-functions decay far from the surface and the atom/molecule. The dotted line going very steeply upwards indicates this repulsive behavior. 

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