Layered materials that can be exfoliated into two dimensional materials are currently limited to van der Waals solids with weak interlayer interactions. One material, called a layered electride, defies this rule by being exfoliable despite its large binding energy. Electrides are similar to ionic materials, except the anion is instead an electron with no nucleus. In layered electrides, these electrons manifest as a 2D electron gas contained between cationic slabs. The electride [Sr2N]+e- contains one electron per formula unit, and the energy of these electrons is near the Fermi level, resulting in a low workfunction (3.2 eV). While calculations show that [Sr2N]+e- may be exfoliable, this has not been demonstrated experimentally. In this work, [Sr2N]+e- is exfoliated into thin flakes which retain the structural and electronic properties of the bulk, providing evidence for a 2D material with the lowest workfunction known among exfoliable materials.
The exfoliation potential of layered materials can be estimated by calculating the binding energy using density functional theory (DFT). The total energy of a bilayer is calculated at varying interlayer distances and the lowest energy confirmation is extracted. For the electride [Sr2N]+e-, the binding energy is only five times larger than graphite, a material which is well known to exfoliate in a variety of solvents. Expectedly, [Sr2N]+e- was able to be suspended in propylene carbonate upon sonication, producing thin flakes observable by transmission electron microscopy. The flakes with lateral sizes of 0.5-3 µm showed clear electron diffraction patterns which matched structurally to [Sr2N]+e-, confirmed also by X-ray diffraction. From these results, we can conclude that the structure of [Sr2N]+e- is retained after exfoliation.
In addition to comparing structural characteristics, it is also important to observe changes in the electronic properties. UV-Vis-NIR spectroscopy was performed on the exfoliated [Sr2N]+e- sample to visualize band transitions occurring within the material. A Drude-Lorentz response typical of metals was observed with two peaks at 375 nm and 625 nm and a linear tail in the infrared region, which was attributed to reflection by the electron gas. The band structure confirms the metallicity of [Sr2N]+e- since the electron gas band crosses the Fermi level, and the two peaks observed in the spectrum correlate to peaks in the joint density of states, which is an integration of the band transitions near the Fermi level. From these results, we can conclude that the electronic properties of [Sr2N]+e- are also retained in the electrene form. Given the low workfunction for bulk [Sr2N]+e-, the 2D form of this layered electride could act as a strong electron donor to other two-dimensional materials, defining a new kind of heterostructure not demonstrated previously. Future work for electrene [Sr2N]+e- will involve further quantifying the electron donation ability of this new 2D material.