Work function metal

Fig. 9. Schottky barrier band diagrams (a) a rare situation where the metal work function is less than the semiconductor electron work affinity resulting in an ohmic contact (b) normal Schottky barrier with barrier height When the depletion width Wis <10 nm, an ohmic contact forms.

Schematic energy level diagrams of a metal/polymer/metal structure before and after the layers are in contact are shown in the top two drawings of Figure 11-6. Before contact, the metals and the polymer have relative energies determined by the metal work functions and the electron affinity and ionization potential of the polymer. After contact there is a built-in electric field in the structure due to the different Schottky energy barriers of the asymmetric metal contacts. Capacitance-voltage measurements demonstrate that the metal/polymer/metal structures are fully depleted and therefore the electric field is constant throughout the bulk of the structure [31, 35]. The built-in potential, Vhh i.e. the product of the constant built-in electric field and the layer thickness may be written...

Table 11-2 shows the built-in potential in metal/MEH-PPV/metal structures measured by either electroabsorption [15] or photocurrenl techniques [37] for a variety of contact metals. The uncertainty in both the work function differences and the built-in potential measurements is about 0.1 eV. For all of the structures except the Pt-Ca and Al-Sm devices there is good agreement between the metal work function difference, AW, and the built-in potential, Vhi. This indicates that for a wide range of metal contacts the Schottky energy barrier between the metal and MEH-PPV is well approximated by the ideal Schottky model and that state chaiging, which pins the Schottky energy barrier, is not significant. A built-in potential smaller than the difference between the contact work functions implies that... 

Alkali metals are strongly electropositive elements with low (2-3 eV) work function and low ionization potential. Upon adsorption on other metal surfaces they cause a severe (up to 3 eV) lowering of the metal work function, as already established by Langmuir in the early 1920 s. 

Figure 4.20. Schematic of an electron acceptor (left) and an electron donor (right) adsorbate on a metal surface. The former increases the metal work function,

Figure4.17 Ionization potentials (IP) and electron affinities (EA) of Group 11 clusters M up ton = 23 (in eV). The bulk metal work-functions for the (1 00) plane are also shown on the left hand side in open symbols. Experimental values from Refs. [370-374].

Similar results were obtained with SrTiOj but not with CdS In the case of p-InP the opposite effect was found, i.e. an increase of the barrier height upon admittance of H2 to the metal. The nature of the ambient gas-induced changes were interpreted by a change in the surface dipole component of the metal work function . The results obtained with CdS did not fit into this scheme probably because the surfaw chemistry of this material is rather complex . ... 

Underpotential deposition of heavy metals on H2 evolving electrodes is a well known problem [133], The existence of a direct correlation between H2 evolution activity and metal work function, makes UPD very likely on high work function electrodes like Pt or Ni. Cathode poisoning for H2 evolution is aggravated by UPD for two reasons. First, deposition potentials of UPD metals are shifted to more anodic values (by definition), and second, UPD favors a monolayer by monolayer growth causing a complete coverage of the cathode [100]. Thus H2 evolution may be poisoned by one monolayer of cadmium for example, the reversible bulk deposition potential of which is cathodic to the H2 evolution potential. 

Fig. 10 Aviram-Ratner rectification via HOMO and LUMO. (a) A D-o-A molecule is sandwiched between two metal electrodes. MD is the electrode proximal to the donor, MA is the electrode proximal to the acceptor, is the electrode metal work function, IPD is the ionization potential of the donor, EAa is the electron affinity of the acceptor, (b) No pathway for current exists when a voltage is applied in the reverse bias direction, (c) Under a comparable voltage to (b) but in the forward bias direction, rectification results from electrons flowing from MA to LUMO to HOMO to MD...

The opposite occurs for atoms with a high electron affinity that is on the order of the metal work function or higher. Here the broadened level 2 falls partly below the Fermi level and becomes partially occupied (Fig. A. 10c). In this case the adatom is negatively charged. Examples are the adsorption of electronegative species such as F and Cl. Table A.3 gives ionization potentials and electron affinities of some catalytically relevant atoms. 

Chen C-Y, Wu K-Y, Chao Y-C, Zan H-W, Meng H-F, Tao Y-T (2011) Concomitant tuning of metal work function and wetting property with mixed self-assembled monolayers. Org Electron 12 148-153... 

Engelkes VB, Beebe JM, Frisbie CD (2004) Length-dependent transport in molecular junctions based on SAMs of alkanethiols and alkanedithiols effect of metal work function and applied bias on tunneling efficiency and contact resistance. J Am Chem Soc 126 (43) 14287-14296... 

The transfer of electrons to the anti-bonding molecular orbitals is in general most facile when the metal work function is small. Low work functions are found for the most open surfaces. The work function can also be lowered locally by adsorbed electropositive species like alkali atoms. 

Fig. 3.10 Variation of potential of zero charge with metal work function for sp ( ) and d (A) metals (------) line of unit slope (from Ref. 18 with permission).

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