Electron energetics at surfaces and interfaces: Concepts and experiments

Advanced Materials

Volumn 15, Issue 4, 2003, Pages 271-277

  Cahen, David ^a   Kahn, Antoine ^b  

^a WEIZMANN INSTITUTE OF SCIENCE   (Israel)

^b PRINCETON UNIVERSITY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ELECTROCHEMISTRY; ELECTRONIC EQUIPMENT; EMISSION SPECTROSCOPY; INTERFACES (MATERIALS); IONIZATION; MOLECULES; ORGANIC COMPOUNDS; SOLID STATE PHYSICS; SURFACES;

ABSOLUTE VACUUM LEVEL; ELECTRON AFFINITY; MOLECULAR FILMS; ORGANIC MOLECULES; PHOTOEMISSION SPECTROSCOPY; SURFACE DIPOLE;

ELECTRON ENERGY LEVELS;

EID: 0037450228     PISSN: 09359648     EISSN: None     Source Type: Journal    
DOI: 10.1002/adma.200390065     Document Type: Article

References (39)

1. A. G. Milnes, D. L. Feucht, Heterojunctions and Metal-Semiconductor Junctions, Academic, New York 1972.
2. W. Mönch, Semiconductors surfaces and interfaces, 2nd ed., Springer, Berlin 1995.
3. H. Lüth, Surfaces and Interfaces of Solid Materials, 3rd ed., Springer, Heidelberg 1998.
4. H. Ishii, K. Sugiyama, E. Ito, K. Seki, Adv. Mater. 1999, 11, 605.
5. G. Ashkenasy, D. Cahen, R. Cohen, A. Shanzer, A. Vilan, Acc. Chem. Res. 202, 35, 121.
6. C. D. Dimitrakopoulos, D. J. Mascaro, IBM J. Res. Dev. 2001, 45, 11.
7. S. Forrest, P. Burrows, M. Thompson, IEEE Spectrum 2000, 37, 29.
8. R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley, D. A Dos Santos J. L. Bredas, M. Logdlung, W. R. Salaneck, Nature 1999, 397, 121.
9. J. Jortner, M. Ratner, Molecular Electronics, Blackwell Science, Oxford 1997.
10. D. Cahen, G. Hodes, Adv. Mater. 2002, 14, 789.
11. S. Seker, K. Meeker, T. F. Keuch, A. B. Ellis, Chem. Rev. 2000, 100, 2505.
12. We recall here that "just outside" the solid means that the distance between the electron and the surface is larger than the interatomic distance smaller than the size of the crystal surface.
13. All energies here are given on the so-called solid-state scale; to convert them to the electrochemical scale the values need to be multiplied by -1 (as the solid state scale is for electrons and the electrochemical one for positive unit charges) and 4.5-4.75 eV subtracted [14]. The uncertainty is due to the uncertainty in the value of the standard hydrogen electrode's potential on the solid-state scale.
14. E. M. Stuve, A. Krasnopoler, D. E. Sauer, Surf. Sci. 1995, 335, 177.
15. We consider here the following methods: ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and two photon photoemission spectroscopy (2PPES).
16. This is the normal case in photoelectron emission expirements where the detector is biased negatively with respect to the sample to assure this situation.
17. K. Besocke, S. Berger, Rev. Sci. Instrum. 1976, 47, 840.
18. N. A. Surplice, R. J. D'Arcy, J. Phys. E: Sci. Instrum. 1970, 3, 477.
19. L. Kronik, Y. Shapira, Surf. Sci. Rep. 1999, 37, 1.
20. K. Wandelt, Appl. Surf. Sci. 1997, 111, 1.
21. Scanning Kelvin microscopy also provides laterally resolved work function data, but not on an atomic scale.
22. A. H. Marshak, IEEE Trans. Electron Devices 1989, ED-36, 1764.
23. A. R. Plummer, in The Electrochemistry of Semiconductors (Ed: P. J. Holmes), Academic, London 1962.
24. e is generally very small, at least for metals, the two terms are often used interchangeably at finite temperature. Following the customary definition of the Fermi energy as the energy which separates the filled from the empty energy levels, no single Fermi level can be defin for intrinsic (and most extrinsic) semiconductors. In such a case, whenever the term Fermi level or Fermi energy is used, chemical potential of electron (electrochemical, if there is an electrostatic potential contribution to the electron partial free energy) is meant. For further details, for example. N. W. Ashcroft and N. D. Mermin, Solid State Physics, Saunders, Philadelphia, PA 1976.
25. Similar concepts exist in electrochemistry, where electrode potentials are relative values with respect to a reference and there has been a long quest to determine "absolute" electrode potentials experimentally (cf. Samec et al., [26]; Stuve et al., [14]) and to understand it conceptually. The issue has been discussed often (cf. Trasatti [28,29], and recently, for a solid state electrochemical system, by Tsiplakides and Vayenas [27]). Trasatti also compares the use of an absolute vacuum level at infinity to that of a local one. In general we reach a similar conclusion to him. However, while he states that the absolute vacuum level at infinity is in principle measurable, we claim that this level is NOT amenable to experimental determination. The importance of the comparison with concepts from electrochemistry for the "organic electronics" community lies in the frequent use of electrochemical experiments, mostly in a solvent, such as cyclic voltammetry, to judge suitability of a material or compare between materials (cf. for example, Brabec et al. [30]). A. lucid didactic discussion can be found in the book of Bockris and Reddy [31], Vol. 2, Ch. 7.2. In electrochemistry the outer (or Volta or psi, ψ potential of a solid electrode is defined as the work done to bring a unit (positive) test charge from infinity to a point just outside the range of the image forces of the electrode (i.e., to what we call here the local vacuum level). The surface (or dipole or χ) potential is defined for a condensed phase, with no extra electric charge but surrounded by a dipole layer, as the work done to bring the test charge from infinity to a point just across the dipole layer into the condensed phase. In our definitions here this potential is part of the work function and electron affinity of the condensed phase. The combination of both is called the inner (or Galvani or φ) potential. Bockris and Reddy argue that of these only the outer potential and its diferences can be measured. For the others, only changes are experimentally accessible.
26. Z. Samec, B. W. Johnson, K. Doblhofer, Surf. Sci. 1992, 264, 440.
27. D. Tsiplakides, C. G. Vayenas, J. Electrochem. Soc. 2001, 148, E189.
28. S. Trasatti, Electrochim. Acta 1990, 35, 269.
29. S. Trasatti, Electrochim. Acta 1991, 36, 1659.
30. C. J. Brabec, A. Cravino, D. Meissner N. S. Saricifci, T. Fromherz, M. T. Rispens, L. Sanchez, J. C. Hummelen, Adv. Funct. Mater. 2001, 11, 374.
31. J. O'M. Bockris, K. N. Reddy, Modern Electrochemistry, Vol. 2, Plenum. New York 1970.
32. J. H. Weaver, D. M. Poirier, in Solid State Physics (Eds: H. Ehrenreich, F. Spaepen), Vol. 48, Academic, New York 1994.
33. C. I. Wu, Y. Hirose, H. Sirringhaus, A. Kahn, Chem. Phys. Lett. 1997, 272, 43.
34. I. G. Hill, A. Kahn, Z. G. Soos, R. A. Pascal, Chem. Phys. Lett. 2000, 327, 181.
35. X. Y. Zhu, Ann. Rev. Phys. Chem. 2002, 53, 221.
36. http://www.fkp.uni-erlangen.de/methoden/2ppetutor/start.html
37. P. A. Cox, The Electronic Structure and Chemistry of Solids, Oxford University Press, Oxford 1987.
38. R. Hoffman, Solids and Surfaces, VCH, Weinheim 1988.
39. R. Cohen, S. Bastide, D. Cahen, J. Libman, A. Shanzer, Y. Rosenwaks, Adv. Mater. 1997, 9, 746.