Energy work function

Building a successful D-a-A molecule requires knowledge of the appropriate HOMO and UUMO energies and the work function energies of the electrodes. A donor s ionization potential IPD corresponds to its HOMO energy, and an... 

A direct experimental measure of the density of states in the bands is by optical spectroscopy, particularly photoemission (Ley 1984). In this experiment, illustrated in Fig. 3.5, a photon of energy h(o excites an electron from the valence to the conduction band. Provided that the electron energy is high enough to overcome the work function energy, T, and no inelastic scattering takes place, the electrons are ejected from the surface with kinetic energy. 

The probability of electron emission into the vacuum jumps abruptly at the work function energy, vao J l d is approximately constant above this energy, so that the valence band density of states is given by dT(A[Pg.71]

Another electrical characteristic of semiconducting solids is the Fermi level. This level, which describes the thermodynamic potential of the valence electrons, is central to any discussion of potentials of electron transfer. The work function is for solids what the electronegativity is for molecules. Potentials in metals are schematized in Figure 6.3. The work function (energy to get a valence electron out of the solid) of two different facets is (t>i and 02, the inner potential is the result of net charge on the metal lattice, is the chemical potential of the electrons, Ep is the Fermi level, Xi and X2 are the surface potentials of the two facets, and is the potential difference of an electron between a position just outside of the solid and infinity, where the potential is There is a contact potential between two different planes, which is equal to the difference between the work functions of those planes." ... 

Work function Energy to pull an electron off of a bulk metal. 

On the other hand, an ultrafast liquid-jet UPS [33], shown in Fig. 35.2, resolved the bound (vertical binding as an equivalent of work function) energies of 1.6 and 3.3 eV for solvated electrons at the water skin and in the bulk solution, respectively. The bound energy decreases with the number of water molecules, indicating the size-induced strong polarization [34]. 

Fig. V-14. Energy level diagram and energy scales for an n-type semiconductor pho-toelectrochemical cell Eg, band gap E, electron affinity work function Vb, band bending Vh, Helmholtz layer potential drop 0ei. electrolyte work function U/b, flat-band potential. (See Section V-9 for discussion of some of these quantities. (From Ref. 181.)...

Fig. VIII-5. Schematic potential energy diagram for electrons in a metal with and without an applied field , work function Ep, depth of the Fermi level. (From Ref. 62.)...

The final technique addressed in this chapter is the measurement of the surface work function, the energy required to remove an electron from a solid. This is one of the oldest surface characterization methods, and certainly the oldest carried out in vacuo since it was first measured by Millikan using the photoelectric effect [4]. The observation of this effect led to the proposal of the Einstein equation ... 

The work function (p is the energy necessary to just remove an electron from the metal surface in thermoelectric or photoelectric emission. Values are dependent upon the experimental technique (vacua of 10 or torr, clean surfaces, and surface conditions including the crystal face identification). 

Similarly, adsorption of ions (n+) onto a metal surface leads to a heat of adsorption of Q,. Generally, Q is about 2-3 eV and is greater than Q, which itself is about 1 eV. The difference between Q, and is the energy required to ionize neutrals (n ) on a metal surface so as to give ions (n+) or vice versa. This difference, Q - Q, can be equal to, greater than, or less than the difference, I - ( ), between the ionization energy (1) of the neutral and the ease with which a metal can donate or accept an electron (the work function, ( )). Where Q, - Q, > I - ( ), the adsorbed... 

Therefore, the ratio of the number of ions to the number of neutrals desorbing from a heated filament depends not only on the absolute temperature but also on the actual surface coverage of ions and neutrals on the filament (C, C ) and crucially on the difference between the ionization energy and work function terms, I and (j). This effect is explored in greater detail in the following illustrations. 

Clearly, the lower the ionization energy with respect to the work function, the greater is the proportion of ions to neutrals produced and the more sensitive the method. For this reason, the filaments used in analyses are those whose work functions provide the best yields of ions. The evaporated neutrals are lost to the vacuum system. With continued evaporation of ions and neutrals, eventually no more material remains on the filament and the ion current falls to zero. 

This thermal ionization process requires fiiament temperatures of about 1000-2000°C. At these temperatures, many substances, such as most organic compounds, are quickiy broken down, so the ions produced are not representative of the structure of the original sample substance placed on the filament. Ionization energies (1) for most organic substances are substantially greater than the filament work function (( )) therefore 1 - ( ) is positive (endothermic) and few positive ions are produced. 

Thermal ionization has three distinct advantages the ability to produce mass spectra free from background interference, the ability to regulate the flow of ions by altering the filament temperature, and the possibility of changing the filament material to obtain a work function matching ionization energies. This flexibility makes thermal ionization a useful technique for the precise measurement of isotope ratios in a variety of substrates. 

Surface ionization. Takes place when an atom or molecule is ionized when it interacts with a solid surface. Ionization occurs only when the work function of the surface, the temperature of the surface, and the ionization energy of the atom or molecule have an appropriate relationship. 

Photoelectron spectroscopy involves the ejection of electrons from atoms or molecules following bombardment by monochromatic photons. The ejected electrons are called photoelectrons and were mentioned, in the context of the photoelectric effect, in Section 1.2. The effect was observed originally on surfaces of easily ionizable metals, such as the alkali metals. Bombardment of the surface with photons of tunable frequency does not produce any photoelectrons until the threshold frequency is reached (see Figure 1.2). At this frequency, v, the photon energy is just sufficient to overcome the work function

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Work functions of alkali mefal surfaces are only a few electronvolfs so fhaf fhe energy of near ultraviolef radiation is sufticienf to produce ionization. 

Phofoelectron spectroscopy is a simple extension of the photoelectric effect involving the use of higher-energy incident photons and applied to the study not only of solid surfaces but also of samples in the gas phase. Equations (8.1) and (8.2) still apply buf, for gas-phase measuremenfs in particular, fhe work function is usually replaced by fhe ionization energy l so fhaf Equation (8.2) becomes... 

In order for the primary photoelectron, which is bound to the surface atom with binding energy to be detected ia xps, the electron must have sufficient kinetic energy to overcome, ia addition to E the overall attractive potential of the spectrometer described by its work function, Thus, the measured kinetic energy of this photoelectron, Ej is given by... 

If the energy of the iacident x-rays and the spectrometer work function are known, the measured kinetic energy can be used to determine the binding energy E from... 

Trigonal selenium is a -type semiconductor with an energy gap of 1.85 eV (104) and a work function of about 6 eV (105), which is the largest value reported for all the elements. Accordingly, a Schottky barrier should be created at the contact of selenium with any metal. This is consistent with the... 

The simple picture of the MOS capacitor presented in the last section is compHcated by two factors, work function differences between the metal and semiconductor and excess charge in the oxide. The difference in work functions, the energies required to remove an electron from a metal or semiconductor, is = —25 meV for an aluminum metal plate over a 50-nm thermally grown oxide on n-ty e siUcon with n = 10 cm . This work... 

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