Intercalation—the insertion of guest species into the interlayer space of a layered host material—is a platform to drastically alter properties.By inserting either an electron donor or acceptor species into a material, the fermi energy of the host shifts, resulting in different electronic and optical properties. While layered materials have been intercalated since 1841, their two-dimensional (2D) counterparts have only recently been intercalated with the first demonstration in 2009. As the host dimensions are reduced, the energetic penalty for intercalation, and therefore the rate and mechanism, are altered. Furthermore, since 2D materials are known to exhibit different electronic and optical properties than bulk, intercalation offers a unique platform to further alter and tune the properties of thin material.
Iron trichloride (FeCl3) intercalates both bulk and few-layer graphite, resulting in a hole-doped graphite host, with the stage determining the extent of doping. In a stage I intercalation compound, each host layer is adjacent to an intercalant layer, while in lower stages, the stage number dictates the number of host layers between intercalant layers. Ultimately, the carrier concentration and resulting properties of the compound are dependent on the stage of intercalation.
FeCl3 has previously been intercalated in bulk and few-layer graphite thermally, and in bulk graphite electrochemically in water (Carbon N. Y.35,285–290 (1997)) or propylene carbonate (Carbon N. Y.36,383–390 (1998)).Thermal methods produced stage I FeCl3-graphite, while electrochemical methods yielded stage II in the bulk material. We hypothesize that reducing the dimensions of the graphite host will reduce energetic barriers and allow for stage 1 compounds to be produced electrochemically. Furthermore, we hypothesize that FeCl3 might be able to be deintercalated from few-layer graphite, enabling this compound to be studied for reversible optoelectronic applications.
Here, we electrochemically intercalate bulk and few-layer graphite with 10M FeCl3 in propylene carbonate under inert environments (< 1 ppm O2, < 0.1 ppm H2O). We monitor the intercalation through in-situ techniques, including cyclic voltammetry, optical microscopy, and Raman-spectroscopy, as well as ex-situ techniques, including X-ray photoemission spectroscopy (XPS). Characterization of the resulting compound via XPS indicate the presence of FeCl3 in the interlayer space. Raman spectroscopy further supports the electrochemical intercalation of bulk and few-layer material, resulting in a hole-doped host. Interestingly, we find that the thin material results in mixed stage I and stage II compounds, verifying our initial hypothesis that reduced dimensions enable access to higher stages and carrier concentrations, which was unattainable in bulk material by electrochemical methods. Furthermore, we investigate the reversibility of this intercalation through calculations and experiments. Calculations using density functional theory indicate that this intercalation should be reversible. We use in-situ characterization techniques to investigate this experimentally.
In this work, we employ in-situ and ex-situ techniques to demonstrate the first electrochemical intercalation of few-layer graphite with FeCl3, allowing the role of dimensions in the intercalation to be investigated. Furthermore, this work is the first to demonstrate stage I FeCl3-graphite through electrochemical techniques. Finally, we address the reversibility of this system and its promise for reversible optoelectronic technologies, including optical switches and batteries.