Difference work function

If two metals with different work functions are placed m contact there will be a flow of electrons from the metal with the lower work function to that with the higher work fimction. This will continue until the electrochemical potentials of the electrons in the two phases are equal. This change gives rise to a measurable potential difference between the two metals, temied the contact potential or Volta potential difference. Clearly... 

Friend et at. studied the influence of electrodes with different work-functions on the performance of PPV photodiodes 143). For ITO/PPV/Mg devices the fully saturated open circuit voltage was 1.2 V and 1.7 V for an ITO/PPV/Ca device. These values for the V c are almost equal to the difference in the work-function of Mg and Ca with respect to 1TO. The open circuit voltage of the ITO/PPV/A1 device observed at 1.2 V, however, is considerably higher than the difference of the work-function between ITO and Al. The Cambridge group references its PPV with a very low dark carrier concentration and consequently the formation of Schottky barriers at the PPV/Al interface is not expected. The mobility of the holes was measured at KT4 cm2 V-1 s l [62] and that for the electrons is expected to be clearly lower. 

In the presence of a potential difference particles of different work functions are brought into contact by a force of attraction defined as follows ... 

To avoid having different-sized Schottky barriers at the two interfaces, the same metal (or metals with almost the same work functions) should be used for both electrodes. For example, the different work functions of Pt and Mg made studies of glass I Pt I molecule I Mg I Ag sandwiches hard to interpret [34]. In that case, Mg probably reacted with the end of the molecule containing the strong acceptor TCNQ to form a TCNQ-salt Schottky barrier that dominated the electrical asymmetry [34], With a different molecule lacking TCNQ, the dominating Schottky barrier effect was eliminated [35, 36],... 

In reality, as the barrier becomes narrower, it deviates from the square shape. One often used model is the parabolic barrier (dashed line in Fig. 1). When the barrier is composed of molecules, not only is the barrier shape difficult to predict, but the effective mass of the electron can deviate significantly from the free-electron mass. In order to take these differences into account, a more sophisticated treatment of the tunneling problem, based on the WKB method, can be used [21, 29-31]. Even if the metals are the same, differences in deposition methods, surface crystallographic orientation, and interaction with the active layer generally result in slightly different work functions on either side of the barrier. 

The standard electrode potential [1] of an electrochemical reaction is commonly measured with respect to the standard hydrogen electrode (SHE) [2], and the corresponding values have been compiled in tables. The choice of this reference is completely arbitrary, and it is natural to look for an absolute standard such as the vacuum level, which is commonly used in other branches of physics and chemistry. To see how this can be done, let us first consider two metals, I and II, of different chemical composition and different work functions 4>i and 4>ii-When the two metals are brought into contact, their Fermi levels must become equal. Hence electrons flow from the metal with the lower work function to that with the higher one, so that a small dipole layer is established at the contact, which gives rise to a difference in the outer potentials of the two phases (see Fig. 2.2). No work is required to transfer an electron from metal I to metal II, since the two systems are in equilibrium. This enables us calculate the outer potential difference between the two metals in the following way. We first take an electron from the Fermi level Ep of metal I to a point in the vacuum just outside metal I. The work required for this is the work function i of metal I. 

Figure 2.2 Two metals of different work functions before (a) and after (b) contact (schematic).

Several ways exist to image these regions of different work function. We have already discussed scanning electron and field emission microscopy in this chapter. Scanning photoemission microscopy (SPM) is carried out by scanning a focussed UV beam (beam diameter of 0.5 pm) over the surface and recording the photoemis-... 

Figure 7.23 Ordering of adsorbates on a surface into islands gives rise to regions of different work function, which can be imaged because of the associated differences in photoelectron intensity. The principle forms the basis of photoemission electron microscopy (PEEM). The same principle underlies the imaging of single molecules in the field electron microscope (FEM) (see also Fig. 7.9).

To understand the role of the noble metal in modifying the photocatalysts we have to consider that the interaction between two different materials with different work functions can occur because of their different chemical potentials (see [200] and references therein). The electrons can transfer from a material with a high Fermi level to another with a lower Fermi level when they contact each other. The Fermi level of an n-type semiconductor is higher than that of the metal. Hence, the electrons can transfer from the semiconductor to the metal until thermodynamic equilibrium is established between the two when they contact each other, that is, the Fermi level of the semiconductor and metal at the interface is the same, which results in the formation of an electron-depletion region and surface upward-bent band in the semiconductor. On the contrary, the Fermi level of a p-type semiconductor is lower than that of the metal. Thus, the electrons can transfer from the metal to the semiconductor until thermodynamic equilibrium is established between the two when they contact each other, which results in the formation of a hole depletion region and surface downward-bent band in the semiconductor. Figure 12.6 shows the formation of semiconductor surface band bending when a semiconductor contacts a metal. 

If the work functions of every portion of the crystal constituting the tip were identical, one would simply observe a uniformly bright circular patch on the fluorescent screen. Different crystal faces have slightly different work functions, however, so that one sees a pattern showing different intensities for different faces. In general, closely packed faces have higher work functions than loosely packed ones. In fee cubic crystals like Ni the (111), (100), and (110) faces are the most closely packed and appear darker than the rest of the pattern (Plate IA and B). In bee cubic metals like tungsten, the (110), (100), and (211) faces are most closely packed and appear darkest (Plate IC). 

This double layer may also form in systems as, for example, the interface between two metals of different nature (with different work functions) or between two immiscible electrolytes and even when one of the two phases is an insulator or a semiconductor [7, 10]. 

Figure 15.9 presents the results of calculations of the charge state of Ni nanostructures on metallic substrates with different work functions. It can... 

Several ways exist to image these regions of different work function. SEM and FEM have been discussed earlier in this chapter. As an alternative, scanning photoemission microscopy is carried out by scanning a focused UV beam (beam diameter 0.5 pm) over the surface and recording the photoemission intensity point by point. This is of course a slow procedure, but much faster imaging in real time becomes available if the electrons are collected from the entire surface in parallel, as is carried out in photoemission electron microscopy (PEEM). The lateral resolution of this technique is presently around 200 nm, but by using... 

The work functions and ionization potentials of sputter-deposited ZnO and ZnO Al films are shown in Fig. 4.13. The different Fermi level positions of ZnO and ZnO Al for deposition at room temperature in pure Ar are also observed in the work function. The undoped films prepared under these conditions have a work function of 4.1eV, while the Al-doped films show values of 3.2eV. The difference is almost of the same magnitude as for the Fermi level position and, therefore, explained by the different doping level. Also the ionization potentials are almost the same under these preparation conditions. The work function of the undoped material is close to the value reported by Moormann et al. for the vacuum-cleaved Zn-terminated (0001) surface [20], The same authors report a work function of 4.95 eV for the oxygen terminated ZnO(OOOl) surface, which is in good agreement with the values obtained for films deposited with >5% oxygen in the sputter gas. Since the Fermi level position of the undoped ZnO films does not depend on the oxygen content in the sputter gas (Fig. 4.12), the different work functions correspond to different ionization potentials. 

Fig. 5.2. Energy diagram of a metal/semiconductor/metal Schottky barrier under open-circuit conditions, when the metals have different work functions. work function, Xs electron affinity, IP ionization potential, Eg energy gap, W depletion...

Fig. 5.10. Temperature dependent I/V characteristics of a p-type diode (ITO/ PEDOT/MDMO-PPV/LiF-Al), in which the different work functions of the electrodes guarantee ambipolar charge injection (electrons at the LiF-Al electrode, holes at the ITO/PEDOT electrode)...

Fig. 9.1 shows a schematic diagram of a metal Schottky contact on a semiconductor. In isolation, the metal and the semiconductor generally have different work functions and Og. (The work function is the energy needed to remove an electron from the Fermi energy to the vacuum.) When electrical contact is made between... 

Interest in the electronic properties of interfaces centers around a-Si H/Si3N4, because this combination is used in multilayers (Section 9.4) and field effect transistors (Section 10.1.2). The electronic structure of the interface is illustrated in Fig. 9.18. Apart from the band offset which confines carriers to the a-Si H layer, the distribution of localized interface states and the band bending are the main factors which govern the electronic properties of the interface. The large bulk defect density of the SijN also has an effect on the electronic properties near the interface. Band bending near the interface may result from the different work functions of the two materials or from an extrinsic source of interface charge - for example, interface states. 

The condition of thermodynamic equilibrium requires that the electrochemical potential should be the same throughout the metal. In a polycrystalline sample with many crystal planes exposed (which have, in general, different work functions), the condition of equilibrium requires the development of net charge in each plane. 

The kinetics of adsorption can be studied from the rate of change of 0 provided H is constant. However, this is not necessarily the case except over a limited range. It must also be remembered that each crystal plane will have a different work function in the clean state. If more than one crystal plane is exposed, an average value will be obtained. Added to this is the fact that many gases adsorb differently on different crystal planes which makes the interpretation of results rather complicated. 

If two metals of differing work function are connected electrically, at the same temperature and without a source of emf, the electrostatic potentials just outside the two surfaces are different. This potential difference V12 is known as the contact potential difference and is equal to the difference in the work functions of the two metals and < 2- compensating potential, equal and opposite, is applied... 

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