The stoichiometric composition and crystal structure dictates the classification of layered and natural materials into graphene, transition metal di (mono) chalcogenide, sulfosalt, oxide, neo-, phyllo- silicate, and phosphate families. Alternatively, those can be classified based on the feasibility of their exfoliation into separate atomic layers. An energy, which is required for the isolation (or separation) of a singular atomic layer from its bulk would be the quantitative measure of the corresponding feasibility. Within this approach, it is reasonable to distinguish naturally-, potentially- exfoliable and robust materials with threshold exfoliation energies1 of 10 meVÅ-2, 100 meVÅ-2, and 1000 meVÅ-2. Originating within potentially exfoliable ones, the materials lacking the out-of-plane van der Waals bonds in their crystal structure are of particular interest. Those form bonds of a different nature, e.g., covalent, but of comparable strengths and bear the name of non-van der Waals materials.
Non-van der Waals InGaS3
InGaS3 exhibits a hexagonal arrangement of III-III-IV group elements in P65 space group with lattice parameters of a = b = 6.6 Å and c = 17.9 Å, and appears with yellow-to-lustrous grey shades as displayed in Figure 1 (a). It contains various structural bonds, whose strengths can be estimated by first-principle calculations based on the density functional theory. To obtain energies required for the isolation of individual atomic layers, one can estimate the differences among the ground-state energy of relaxed structure and all of its unrelaxed states. Afterwards, seek for planes with minimal binding energies to determine potentially breakable, or in our case, exfoliable directions. Excluding relaxation energies along the c-axis, we found the exfoliation energy of Eexf ≈ 53 meVÅ-2 for planes shown in Figure 1 (b). Our results suggest an emergence of authentically delicate out-of-plane covalent bonds within the unit cell, and, consequently, a generation of artificial layered structure. This value locates alongside the evaluated exfoliation energies of conventional van der Waals materials (see Figure 1 (c)). Notably, the exfoliation energies of known non-van der Waals materials are of larger dispersion.
2D layers of InGaS3
Typically, atomic layers of non-van der Waals materials are produced by means of vigorous sonication-assisted2 and cation-intercalation3 exfoliation methods. Nevertheless, an introduction of slightly elevated temperature treatment to the standard4 scotch-tape exfoliation (see Figure 2(a)) allows obtaining two-dimensional layers of InGaS3. Figure 2 (b-d) demonstrate AFM scans of our pristine flakes with nearly atomically smooth surfaces (RMS roughness of 0.3 nm).
Anisotropic optical properties of non-van der Waals InGaS3
The studies of anisotropic dielectric tensor reveal material’s wide bandgap (2.73 eV), high refractive index (> 2.5), negligible losses, and a birefringence of Δn ~ 0.1 in the visible and infrared spectral ranges (see Figure 3(a)). The non-van der Waals interaction reduces the latter to a relatively small value in contrast to the huge anisotropy of Δn ~ 1.5 observed in transition metal dichalcogenides5 with natural van der Waals bonds. It is also present in our first-principle density functional theory calculations (see inset of Figure 3 (b)). Furthermore, our technique, which combines spectroscopic ellipsometry with density functional calculations unambiguously confirms the hexagonal structure of InGaS3 since the dielectric response is a fingerprint of material's electronic bandstructure.
Check out our manuscript at https://www.nature.com/articles/s41699-022-00359-9.