Characterization of Charge States in Conducting Organic Nanoparticles by X-ray Photoemission Spectroscopy.

Jordi Fraxedas,Dominique De Caro,Norbert Koch,Kane Jacob,Christophe Faulmann,Antje Vollmer,Lydie Valade

doi:10.3390/ma14082058

Abstract

The metallic and semiconducting character of a large family of organic materials based on the electron donor molecule tetrathiafulvalene (TTF) is rooted in the partial oxidation (charge transfer or mixed valency) of TTF derivatives leading to partially filled molecular orbital-based electronic bands. The intrinsic structure of such complexes, with segregated donor and acceptor molecular chains or planes, leads to anisotropic electronic properties (quasi one-dimensional or two-dimensional) and morphology (needle-like or platelet-like crystals). Recently, such materials have been synthesized as nanoparticles by intentionally frustrating the intrinsic anisotropic growth. X-ray photoemission spectroscopy (XPS) has emerged as a valuable technique to characterize the transfer of charge due to its ability to discriminate the different chemical environments or electronic configurations manifested by chemical shifts of core level lines in high-resolution spectra. Since the photoemission process is inherently fast (well below the femtosecond time scale), dynamic processes can be efficiently explored. We determine here the fingerprint of partial oxidation on the photoemission lines of nanoparticles of selected TTF-based conductors.

Highlights

Published: 19 April 2021Among the myriad of techniques used for the characterization of nanoparticles (NPs) we will put the emphasis on X-ray photoelectron spectroscopy (XPS), a well-known and well-established surface-science technique that provides relevant information on the electronic structure of materials including the chemical composition, stoichiometry, and chemical state and environment [1,2,3,4]
The hole left behind in the solid is screened differently depending on the electronic structure of the materials, giving rise to finite lifetimes that define the intrinsic widths of the features, quantified by the full-width at half maximum (FWHM)
For TTF-TCNQ NPs, both neutral and charged states were clearly distinguishable but extra contributions appeared at higher binding energies that we associated with surface charging

Summary

Introduction

Published: 19 April 2021Among the myriad of techniques used for the characterization of nanoparticles (NPs) we will put the emphasis on X-ray photoelectron spectroscopy (XPS), a well-known and well-established surface-science technique that provides relevant information on the electronic structure of materials including the chemical composition, stoichiometry, and chemical state and environment (e.g., formal oxidation state and bonding) [1,2,3,4]. Once the energy references (work function φ and Fermi level EF ) are known, the binding energy (EB ) of the electrons can be obtained typically within an error of ±0.1 eV using the well-known Einstein’s equation hν = EB + K + φ [5]. The inelastic mean free path of electrons in solids is small, in the 1 nm range, and it is a function of the kinetic energy so that XPS is intrinsically a surface-sensitive technique. XPS has been traditionally limited to the ultrahigh vacuum (UHV) environment (below 10−9 mbar) in order to strongly reduce surface contamination and to allow the photoelectrons to reach the analyzer within several cm The cut-off was obtained by applying a −10 V bias to the sample in order to clear the analyzer work function. Within the 0.08–6 ML range, the binding energy of the most intense feature was

Methods

Results

Conclusion