Manifestation of charged and strained graphene layers in the Raman response of graphite intercalation compounds

TL;DR

This study uses multi-frequency Raman spectroscopy and ab-initio calculations to analyze how charge transfer and strain affect the vibrational properties of graphite intercalation compounds, revealing insights into electronic decoupling and lattice strain.

Contribution

It provides a detailed understanding of the Raman response of GICs across different stages, linking charge transfer, lattice expansion, and vibrational spectra, with implications for strain measurement in graphene layers.

Findings

Raman G and 2D line positions depend on staging and charge transfer.

Higher stage GICs enable measurement of single to multi-layer graphene vibrations.

Raman spectroscopy can identify internal strain and determine lattice constants.

Abstract

We present detailed multi frequency resonant Raman measurements of potassium graphite intercalation compounds (GICs). From a well controlled and consecutive in-situ intercalation and high temperature de-intercalation approach the response of each stage up to stage VI is identified. The positions of the G and 2D lines as a function of staging depend on the charge transfer from K to the graphite layers and on the lattice expansion. Ab-initio calculations of the density and the electronic band-structure demonstrate that most (but not all) of the transferred charge remains on the graphene sheets adjacent to the intercalant layers. This leads to an electronic decoupling of these "outer" layers from the ones sandwiched between carbon layers and consequently to a decoupling of the corresponding Raman spectra. Thus higher stage GICs offer the possibility to measure the vibrations of single, double, and multi-layer graphene under conditions of biaxial strain. This strain can additionally be correlated to the in-plane lattice constants of GICs determined by x-ray diffraction. The outcome of this study demonstrates that Raman spectroscopy is a very powerful tool to identify local internal strain in pristine and weakly charged single and few-layer graphene and their composites, yielding even absolute lattice constants.