The excess electron at polyethylene interfaces.

Fernan Saiz,Nick Quirke,David Cubero

doi:10.1039/c8cp01330f

Abstract

This work investigates the energy and spatial properties of excess electrons in polyethylene in bulk phases and, for the first time, at amorphous vacuum interfaces using a pseudopotential single-electron method (Lanczos diagonalisation) and density functional theory (DFT). DFT calculations are made employing two approaches: with pseudopotentials/plane waves and the local-density approximation; and with all-electron Gaussian basis functions at the B3LYP level of theory, supplemented with a lattice of ghost atoms. All three methods predict similar spatial localisation of the excess electron, but a reliable comparison of its energy can only be made between the Lanczos and DFT using Gaussian bases. While Lanczos predicts that an excess electron would preferentially localise in nanovoids with diameters smaller than 1 nm, DFT suggests that it would localise on surfaces in nanovoids larger than 1 nm. Overall we conclude that in DFT studies of polyethylene/vacuum interfaces at the current level of theory, orbital-based methods provide a useful representation of excess electron properties.

Highlights

As a first step, this work studies the energy and localisation of excess electrons in polyethylene in bulk, and at interfaces with vacuum
The band gap in our crystalline polyethylene system decreases to 6.22 eV using local-density approximation (LDA) + BP and plane waves (PW) in the Quantum Espresso package
This value is in agreement with that of 6.46 eV reported by Serra and colleagues[36,52] who employed LDA (BP and BLYP) and plane waves

Summary

Introduction

This work studies the energy and localisation of excess electrons in polyethylene in bulk, and at interfaces with vacuum. The Lanczos method has been used to study pure systems such as alkanes,[21,22] rare gases,[21,23] and water.[24] In previous work we have used this pseudopotential approach to predict the density of states for excess electrons in bulk polyethylene, including the mobility edge between delocalised states and those states representing electrons localised (trapped) at nanovoids in bulk polyethylene with radii of up to 0.4 nm.[25] The Lanczos polyethylene pseudopotential was fitted to experimental data for the energy of excess electrons in alkanes (from a difference in work functions of electrons in a metal in vacuum and the same metal in the alkane fluid,[26] so with respect to a zero of potential in vacuum away from the metal surface) and ab initio data.[22] For materials with complex chemistries the Lanczos method is more difficult to apply since we would need a semi-empirical pseudopotential for each component This limitation could be overcome by using, for 25186 | Phys.

Methods

Results

Conclusion