Chain Length Dependence of the Dielectric Constant and Polarizability in Conjugated Organic Thin Films

Colin Van Dyck, Tobin J Marks, Mark A Ratner

Research output: Contribution to journalArticle

17 Citations (Scopus)

Abstract

Dielectric materials are ubiquitous in optics, electronics, and materials science. Recently, there have been new efforts to characterize the dielectric performance of thin films composed of molecular assemblies. In this context, we investigate here the relationship between the polarizability of the constituent molecules and the film dielectric constant, using periodic density functional theory (DFT) calculations, for polyyne and saturated alkane chains. In particular, we explore the implication of the superlinear chain length dependence of the polarizability, a specific feature of conjugated molecules. We show and explain from DFT calculations and a simple depolarization model that this superlinearity is attenuated by the collective polarization. However, it is not completely suppressed. This confers a very high sensitivity of the dielectric constant to the thin film thickness. This latter can increase by a factor of 3-4 at reasonable coverages, by extending the molecular length. This significantly limits the decline of the thin film capacitance with the film thickness. Therefore, the conventional fit of the capacitance versus thickness is not appropriate to determine the dielectric constant of the film. Finally, we show that the failures of semilocal approximations of the exchange-correlation functional lead to a very significant overestimation of this effect.

Original languageEnglish
Pages (from-to)5970-5981
Number of pages12
JournalACS Nano
Volume11
Issue number6
DOIs
Publication statusPublished - Jun 27 2017

Keywords

  • chain length
  • conjugation
  • depolarization
  • dielectric constant
  • permittivity
  • thickness
  • thin film

ASJC Scopus subject areas

  • Materials Science(all)
  • Engineering(all)
  • Physics and Astronomy(all)

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