Surface dipole layer, formation

The region of contact of two different materials, generally of differing chemical potentials or work functions will give rise to an interfacial potential and therefore a dipolar surface layer. In the event that one or both of the materials has a high dielectric constant or is perhaps easily dissociated, then the formation of ionic species at this surface dipole layer can occur. 

The work function of a solid is also sensitive to the presence of adsorbates. In fact, in virtually all cases of adsorption the work function of the substrate either increases or decreases the change being due to a modification of the surface dipole layer. The formation of a chemisorption bond is associated with a partial electron transfer between substrate and adsorbate and the work function will change. Two extreme cases are (i) the adsorbate may only be polarized by the attractive interaction with the surface giving rise to the build up of a dipole layer, as in the physisorption of rare gases on metal surfaces and (ii) the adsorbate may be ionized by the substrate, as in the case of alkali metal adsorption on transition metal surfaces. If the adsorbate is polarized with the negative pole toward the vacuum the consequent electric fields will cause an increase in work function. Conversely, if the positive pole is toward the vacuum then the work function of the substrate will decrease. 

According to this figure, the experimental values agree well with the theory. Butler and Ginley neglected any fraction of the Helmholtz layer due to the formation of a surface dipole layer which is not determined by zeta potential measurements. In any case they claim that the flatband potential can be predicted from electronegativities with an accuracy of about 0.3 V. The authors also applied the same method successfully to CdS. ... 

The presence of charge on the surface of a material causes the formation of a surface dipole layer. Thus further work is needed to take the test charge from just outside the surface to it. The potential change resulting from the presence of the dipole layer is the surface potential x-The inner, or Qalvani, potential is defined as (4),... 

The fact of a transfer of an electron from an absorbed particle to adsorbent [25] is widely considered as a criterion to differentiate between various forms of adsorption. Yet, as it has been already mentioned in previous section, there is a neutral form of chemisorption, i.e. weak binding formed without changing the surface charge state which only affects the dipole component of the work function. On the other hand, in several cases the physical adsorption can result in electron transitions in solids. Indeed, apart from formation of a double layer, changing the work function of adsorbent [26] the formation of surface dipoles accompanying physical adsorption can bring free charge carriers to substan-... 

Unfortunately, the work function is a rather complicated (and not fully understood) function of the surface composition and geometry. The work function change is usually attributed to the formation of a dipole layer on the surface, such as occurs when charge flows from a substrate to an adsorbate, or vice versa. If a is the dipolar charge density, d the dipole length (perpendicular to the surface) and e the electronic charge, then one can write... 

Rg. 6.41. A schematic representation of the origin of the surface, or %, potential of a metal. Because of the finite probability of an electron s being found outside the metal surface, the electron density decays to zero outside the metal. This phenomenon is equivalent to the formation of a dipole layer across the metal surface. 

We assumed in Fig. 4.2 that no surface charge or surface dipole is present in the semiconductor. In general, however, both surface charges and surface dipoles are present in the semiconductor owing to adsorption equilibria for various ions between the electrolyte and the semiconductor surface as well as formation of polar bonds at the semiconductor surface. Such surface charges and surface dipoles change the potential difference in the (outer) Helmholtz layer and thus cause shifts in the surface band positions, as shown schematically in Fig. 4.3. The shifts can be expressed as changes in 0(0) or in the above equations, with the... 

The image potential is a specific surface contribution to W, and a second surface contribution is the existence of a surface double layer or dipole layer. Surface atoms are in an unbalanced environment, they have other atoms on one side of them but not on the other thus, the electron distribution around them will be unsymmetrical with respect to the positive ion cores. This leads to the formation of a double layer. Two important effects emanate from this the work function is sensitive to both the crystallographic plane exposed and to the presence of adsorbates. 

Fig. 12 Schematic energy level diagram of the impact of the formation of an interface dipole at an ITO/organic semiconductor (OS) interface (a) the barrier in the absence of the dipole layer (b) and (c) reduction of the hole injection and the electron injection barrier, respectively, in the presence of the interface dipole layer on the surfaces of ITO. Here, the interface dipoles with opposite sign are directed toward the ITO surfaces (b) increasing and (c) decreasing the work function of ITO.

To achieve equilibrium electrons cross the interface until a corresponding potential difference occurs. The latter depends on the difference of the chemical potential of the two phases, i.e., on the difference of the two Fermi levels before contact is made. Neglecting any specific adsorption on and OH ions or the formation of a dipole layer on the surface, the potential difference is given by... 

The double electric layer formation occurs at the air-water interfaces owing to different reasons. First of all, the double electric layer may arise as a result of specific orientation of water molecules inside the boundary layer. So, it may be formed due to dipole-ion interactions. Second, the double electric layer may arise as a result of adsorption to the surface of hydroxyl ions, halide ions, surfactants, and others. So, it may be formed dne to ion-ion interactions [34]. For example, the hydroxyl ions, which are present in water dne to its dissociation, are hydrophobic when compared with protons. And their concentration at the interface is mnch greater than that in the water bulk. 

At this point, it becomes also evident how adsorbates can influence . Adsorbates at a condensed-matter surface do certainly not influence p but may influence the dipole layer at the surface. As it was outlined already by Bonzel in the introduction, adsorbed species may develop a dipole moment during bond formation with the surface atoms or its permanent dipole may get oriented in the electric field at the surface. The work function change AO is given by the classic Helmholtz equation... 

A type of molecular resonance scattering can also occur from the formation of short-lived negative ions due to electron capture by molecules on surfrices. While this is frequently observed for molecules in the gas phase, it is not so important for chemisorbed molecules on metal surfaces because of extremely rapid quenching (electron transfer to the substrate) of the negative ion. Observations have been made for this scattering mechanism in several chemisorbed systems and in phys-isorbed layers, with the effects usually observed as smaU deviations of the cross section for inelastic scattering from that predicted from dipole scattering theory. 

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