Complex Band Structures and Decay Length in Polyethylene Chains

TL;DR

This study calculates the complex band structure of polyethylene chains using DFT, compares methods, and discusses implications for tunneling conduction, revealing free-electron-like states and chain interactions.

Contribution

It provides detailed DFT-based complex band structures of polyethylene and compares plane wave and local basis results, highlighting free-electron-like states and chain interaction effects.

Findings

HOMO-LUMO gap of 9.3 eV in DFT

Non-resonant tunneling β parameter of 0.9 per monomer

Presence of free-electron-like parabolic bands in complex bands

Abstract

The complex band structure of an isolated polyethylene chain is calculated within Density Functional Theory (DFT). A plane wave basis and ultrasoft pseudopotentials are used. The results are compared with those obtained via a local basis set. We obtain a gap between the highest occupied molecular orbilar (HOMO) and the antibonding unoccupied molecular orbitals of 9.3 eV and a non-resonant tunneling $\beta$ parameter of 0.9 per monomer, in reasonable agreement with experiment and with results obtained via local basis. Polyethylene is a negative electron affinity material and the actual gap should be the energy of the HOMO with respect to the vacuum level (in DFT approximation only about 5.14 eV). The Bloch states at imaginary k are mainly free-electron-like parabolic bands which are missing in the local basis. We present also the complex bands of the bulk polyethylene in order to estimate the effects of the chain-chain interactions on the complex band structure. The relevance of these results for the tunnelling conduction of n-alkane chains is discussed.