Intramolecular Dipole Coupling and Depolarization in Self-Assembled Monolayers

Authors

  • Maria L. Sushko,

    Corresponding author
    1. London Centre for Nanotechnology, University College London, 17-19 Gordon Street, London, WC1H 0AH (UK)
    2. Department of Physics and Astronomy, University College London, 17-19 Gordon Street, London, WC1H 0AH (UK)
    • London Centre for Nanotechnology, University College London, 17-19 Gordon Street, London, WC1H 0AH (UK).
    Search for more papers by this author
  • Alexander L. Shluger

    1. London Centre for Nanotechnology, University College London, 17-19 Gordon Street, London, WC1H 0AH (UK)
    2. Department of Physics and Astronomy, University College London, 17-19 Gordon Street, London, WC1H 0AH (UK)
    Search for more papers by this author

  • This work has been funded by EPSRC under “Materials Modeling Initiative” programme, grant GR/S80103/01. The HPCx computer time was funded by EPSRC through the same grant. We thank Leeor Kronik, Amir Natan, Franco Cacialli, Gianluca Latini and Matt Watkins for useful discussions and critical reading the manuscript. Supporting Information is available online from Wiley InterScience or from the authors.

Abstract

Quantum mechanical and classical atomistic computational methods are used to simulate the chain-length dependence of depolarization effects in S(CH2)n−1CH3 and S(CH2)n−1COOH self-assembled monolayers on gold (111) surface. These calculations show that due to weak cooperative effects, the electrostatic properties of alkanethiol monolayers are well described by the gas phase dipole moments of the molecules. However, depolarization in monolayers with the molecules carrying head- and tail-group dipoles, such as COOH-terminated monolayers, strongly depends on the degree of intramolecular dipole coupling. Thus the electrostatic properties of self-assembled monolayers can be engineered by changing the length of the aliphatic spacer between the polar groups. The transition from strong to weak coupling regime was found to be accompanied by the change in the sign of the asymptotic value of electrostatic potential above the surface of the monolayers and hence in the sign of the metal work function change. Therefore, the use of weakly polarizable spacers between the polar groups inside the molecules forming the SAM is beneficial for accessing a wider range of work-function changes.

Ancillary