Orientation-dependent ionization energies and interface dipoles in ordered molecular assemblies

Abstract

Although an isolated individual molecule clearly has only one ionization potential, multiple values are found for molecules in ordered assemblies. Photoelectron spectroscopy of archetypical π-conjugated organic compounds on metal substrates combined with first-principles calculations and electrostatic modelling reveal the existence of a surface dipole built into molecular layers. Conceptually different from the surface dipole at metal surfaces, its origin lies in details of the molecular electronic structure and its magnitude depends on the orientation of molecules relative to the surface of an ordered assembly. Suitable pre-patterning of substrates to induce specific molecular orientations in subsequently grown films thus permits adjusting the ionization potential of one molecular species over up to 0.6 eV via control over monolayer morphology. In addition to providing in-depth understanding of this phenomenon, our study offers design guidelines for improved organic–organic heterojunctions, hole- or electron-blocking layers and reduced barriers for charge-carrier injection in organic electronic devices.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Figure 1: Experimental and theoretical photoelectron spectra of DH6T.
Figure 2: Experimental and theoretical photoelectron spectra of 6T.
Figure 3: Growth model for 6T–DH6T heterostructures on Ag(111).
Figure 4: Electrostatic potential for an isolated 6T molecule and 6T layers.
Figure 5: Electrostatic modelling of the orientation-dependent ionization potential.
Figure 6: Energy-level diagram for the 6T–DH6T heterostructures on Ag(111).

References

  1. 1

    Michaelson, H. B. The work function of the elements and its periodicity. J. Appl. Phys. 48, 4729–4733 (1977).

    CAS  Article  Google Scholar 

  2. 2

    Skriver, H. L. & Rosengaard, N. M. Surface energy and work function of elemental metals. Phys. Rev. B 46, 7157–7168 (1992).

    CAS  Article  Google Scholar 

  3. 3

    Smoluchowski, R. Anisotropy of the electronic work function of metals. Phys. Rev. 60, 661–674 (1941).

    CAS  Article  Google Scholar 

  4. 4

    Lang, N. D. & Kohn, W. Theory of metal surfaces: Work function. Phys. Rev. B 3, 1215–1223 (1971).

    Article  Google Scholar 

  5. 5

    Weinert, M. & Watson, R. E. Contributions to the work function of crystals. Phys. Rev. B 29, 3001–3008 (1984).

    Article  Google Scholar 

  6. 6

    Ishii, H., Sugiyama, K., Ito, E. & Seki, K. Energy level alignment and interfacial electronic structures at organic/metal and organic/organic interfaces. Adv. Mater. 11, 605–625 (1999).

    CAS  Article  Google Scholar 

  7. 7

    Cahen, D. & Kahn, A. Electron energetics at surfaces and interfaces: Concepts and experiments. Adv. Mater. 15, 271–277 (2003).

    CAS  Article  Google Scholar 

  8. 8

    Häkkinen, H. et al. On the electronic and atomic structures of small AuN (N=4–14) clusters: A photoelectron spectroscopy and density-functional study. J. Phys. Chem. A 107, 6168–6175 (2003).

    Article  Google Scholar 

  9. 9

    Li, J., Li, X., Zhai, H. J. & Wang, L. S. Au20: A tetrahedral cluster. Science 299, 864–867 (2003).

    CAS  Article  Google Scholar 

  10. 10

    Friedlein, R. et al. High intercalation levels in lithium perylene stoichiometric compounds. Chem. Phys. Lett. 354, 389–394 (2002).

    CAS  Article  Google Scholar 

  11. 11

    Fukagawa, H. et al. Origin of the highest occupied band position in pentacene films from ultraviolet photoelectron spectroscopy: Hole stabilization versus band dispersion. Phys. Rev. B 73, 245310 (2006).

    Article  Google Scholar 

  12. 12

    Ivanco, J., Winter, B., Netzer, T. R. & Ramsey, M. G. Substrate-mediated electronic structure and properties of sexiphenyl films. Adv. Mater. 15, 1812–1815 (2003).

    CAS  Article  Google Scholar 

  13. 13

    Koch, N. et al. Molecular orientation dependent energy levels at interfaces with pentacene and pentacenequinone. Org. Electron. 7, 537–545 (2006).

    CAS  Article  Google Scholar 

  14. 14

    Ihm, K. et al. Molecular orientation dependence of hole-injection barrier in pentacene thin film on the Au surface in organic thin film transistor. Appl. Phys. Lett. 89, 033504 (2006).

    Article  Google Scholar 

  15. 15

    Ivanco, J. et al. Sexithiophene films on ordered and disordered TiO2(110) surfaces: Electronic, structural and morphological properties. Surf. Sci. 601, 178–187 (2007).

    CAS  Article  Google Scholar 

  16. 16

    Ivanco, J., Netzer, F. P. & Ramsey, M. G. On validity of the Schottky–Mott rule in organic semiconductors: Sexithiophene on various substrates. J. Appl. Phys. 101, 103712 (2007).

    Article  Google Scholar 

  17. 17

    Dinelli, F. et al. Spatially correlated charge transport in organic thin film transistors. Phys. Rev. Lett. 92, 116802 (2004).

    Article  Google Scholar 

  18. 18

    Dodabalapur, A., Torsi, L. & Katz, H. E. Organic transistors: Two-dimensional transport and improved electrical characteristics. Science 268, 270–271 (1995).

    CAS  Article  Google Scholar 

  19. 19

    Loi, M. A. et al. Supramolecular organization in ultra-thin films of α-sexithiophene on silicon dioxide. Nature Mater. 4, 81–85 (2005).

    CAS  Article  Google Scholar 

  20. 20

    Facchetti, A. et al. Building blocks for n-type molecular and polymeric electronics. Perfluoroalkyl- versus alkyl-functionalized oligothiophenes (nT;n=2–6). Systematics of thin film microstructure, semiconductor performance, and modeling of majority charge injection in field-effect transistors. J. Am. Chem. Soc. 126, 13859–13874 (2004).

    CAS  Article  Google Scholar 

  21. 21

    Garnier, F et al. Molecular engineering of organic semiconductors: Design of self-assembly properties in conjugated thiophene oligomers. J. Am. Chem. Soc. 115, 8716–8721 (1993).

    CAS  Article  Google Scholar 

  22. 22

    Halik, M. et al. Relationship between molecular structure and electrical performance of oligothiophene organic thin film transistors. Adv. Mater. 15, 917–922 (2003).

    CAS  Article  Google Scholar 

  23. 23

    Dholakia, G. R., Meyyappan, M., Facchetti, A. & Marks, T. J. Monolayer to multilayer nanostructural growth transition in N-type oligothiophenes on Au(111) and implications for organic field-effect transistor performance. Nano Lett. 6, 2447–2455 (2006).

    CAS  Article  Google Scholar 

  24. 24

    Kahn, A., Koch, N. & Gao, W. Y. Electronic structure and electrical properties of interfaces between metals and π-conjugated molecular films. J. Polym. Sci. B 41, 2529–2548 (2003).

    CAS  Article  Google Scholar 

  25. 25

    Koch, N., Elschner, A., Schwartz, J. & Kahn, A. Organic molecular films on gold versus conducting polymer: Influence of injection barrier height and morphology on current–voltage characteristics. Appl. Phys. Lett. 82, 2281–2283 (2003).

    CAS  Article  Google Scholar 

  26. 26

    Hooks, D. E., Fritz, T. & Ward, M. D. Epitaxy and molecular organization on solid substrates. Adv. Mater. 13, 227–241 (2001).

    CAS  Article  Google Scholar 

  27. 27

    Witte, G. & Wöll, C. Growth of aromatic molecules on solid substrates for applications in organic electronics. J. Mater. Res. 19, 1889–1916 (2004).

    CAS  Article  Google Scholar 

  28. 28

    Duhm, S., Glowatzki, H., Rabe, J. P., Koch, N. & Johnson, R. L. Influence of alkyl chain substitution on sexithienyl-metal interface morphology and energetics. Appl. Phys. Lett. 88, 203109 (2006).

    Article  Google Scholar 

  29. 29

    Glowatzki, H., Duhm, S., Braun, K.-F., Rabe, J. P. & Koch, N. Molecular chains and carpets of sexithiophenes on Au(111). Phys. Rev. B 76, 125425 (2007).

    Article  Google Scholar 

  30. 30

    Heiner, C. E. et al. Anisotropy in ordered sexithiophene thin films studied by angle-resolved photoemission using combined laser and synchrotron radiation. Appl. Phys. Lett. 87, 093501 (2005).

    Article  Google Scholar 

  31. 31

    Lang, N. D. & Norskov, J. K. Interaction of helium with a metal surface. Phys. Rev. B 27, 4612–4616 (1983).

    CAS  Article  Google Scholar 

  32. 32

    Yoshikawa, G., Kiguchi, M., Ikeda, S. & Saiki, K. Molecular orientations and adsorption structures of α-sexithienyl thin films grown on Ag(110) and Ag(111) surfaces. Surf. Sci. 559, 77–84 (2004).

    CAS  Article  Google Scholar 

  33. 33

    Koch, N. et al. Influence of molecular conformation on organic/metal interface energetics. Chem. Phys. Lett. 413, 390–395 (2005).

    CAS  Article  Google Scholar 

  34. 34

    Hill, I. G., Mäkinen, A. J. & Kafafi, Z. H. Initial stages of metal/organic semiconductor interface formation. J. Appl. Phys. 88, 889–895 (2000).

    CAS  Article  Google Scholar 

  35. 35

    Salaneck, W. R. Intermolecular relaxation energies in anthracene. Phys. Rev. Lett. 40, 60–63 (1978).

    CAS  Article  Google Scholar 

  36. 36

    France, C. B., Schroeder, P. G., Forsythe, J. C. & Parkinson, B. A. Scanning tunneling microscopy study of the coverage-dependent structures of pentacene on Au(111). Langmuir 19, 1274–1281 (2003).

    CAS  Article  Google Scholar 

  37. 37

    Horowitz, G. et al. Growth and characterization of sexithiophene single crystals. Chem. Mater. 7, 1337–1341 (1995).

    CAS  Article  Google Scholar 

  38. 38

    Mattheus, C. C. et al. Polymorphism in pentacene. Acta Crystallogr. C 57, 939–941 (2001).

    CAS  Article  Google Scholar 

  39. 39

    Coropceanu, V. et al. Hole- and electron-vibrational couplings in oligoacene crystals: Intramolecular contributions. Phys. Rev. Lett. 89, 275501 (2002).

    Article  Google Scholar 

  40. 40

    Ito, E. et al. Interfacial electronic structure of long-chain alkane/metal systems studied by UV-photoelectron and metastable atom electron spectroscopies. Chem. Phys. Lett. 287, 137–142 (1998).

    CAS  Article  Google Scholar 

  41. 41

    Witte, G., Lukas, S., Bagus, P. S. & Wöll, C. Vacuum level alignment at organic/metal junctions: “Cushion” effect and the interface dipole. Appl. Phys. Lett. 87, 263502 (2005).

    Article  Google Scholar 

  42. 42

    Sreearunothai, P. et al. Influence of copolymer interface orientation on the optical emission of polymeric semiconductor heterojunctions. Phys. Rev. Lett. 96, 117403 (2006).

    CAS  Article  Google Scholar 

  43. 43

    Sirringhaus, H. et al. Two-dimensional charge transport in self-organized, high-mobility conjugated polymers. Nature 401, 685–688 (1999).

    CAS  Article  Google Scholar 

  44. 44

    Johnson, R. L. & Reichardt, J. FLIPPER II—a new photoemission system in HASYLAB. Nucl. Instrum. Methods Phys. Res. 208, 791–796 (1983).

    CAS  Article  Google Scholar 

  45. 45

    Vollmer, A. et al. The effect of oxygen exposure on pentacene electronic structure. Eur. J. Phys. E 17, 339–343 (2005).

    CAS  Article  Google Scholar 

  46. 46

    Kresse, G. & Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 59, 1758–1775 (1999).

    CAS  Article  Google Scholar 

  47. 47

    Kresse, G. & Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 6, 15–50 (1996).

    CAS  Article  Google Scholar 

  48. 48

    Kresse, G. & Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 54, 11169–11186 (1996).

    CAS  Article  Google Scholar 

  49. 49

    Kokalj, A. Computer graphics and graphical user interfaces as tools in simulations of matter at the atomic scale. Comput. Mater. Sci. 28, 155–168 (2003).

    CAS  Article  Google Scholar 

Download references

Acknowledgements

The authors thank H. C. Starck GmbH for providing DH6T. N.K. acknowledges financial support by the Emmy Noether Program (DFG). G.H. is a Marie-Curie Fellow under the INSANE project (contract no. 021511). We thank L. Romaner for helpful discussions and the SFB 488 ‘Mesoscopically Organized Composites’ for financial support.

Author information

Affiliations

Authors

Contributions

S.D. and G.H. contributed equally to this work.

Corresponding authors

Correspondence to Steffen Duhm or Georg Heimel.

Supplementary information

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Duhm, S., Heimel, G., Salzmann, I. et al. Orientation-dependent ionization energies and interface dipoles in ordered molecular assemblies. Nature Mater 7, 326–332 (2008). https://doi.org/10.1038/nmat2119

Download citation

Further reading

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing