Exploring the structural and optoelectronic properties of natural insulating phlogopite in van der Waals heterostructures

Naturally occurring van der Waals crystals have brought unprecedented interest to nanomaterial researchers in recent years. So far, more than 1800 layered materials (LMs) have been identified but only a few insulating and naturally occurring LMs were deeply investigated. Phyllosilicate minerals, which are a class of natural and abundant LMs, have been recently considered as a low-cost source of insulating nanomaterials. Within this family an almost barely explored material emerges: phlogopite (KMg3(AlSi3)O-10(OH)(2)). Here we carry out a high throughput characterization of this LM by employing several experimental techniques, corroborating the major findings with first-principles calculations. We show that monolayers (1L) and few-layers of this material are air and temperature stable, as well as easily obtained by the standard mechanical exfoliation technique, have an atomically flat surface, and lower bandgap than its bulk counterpart, an unusual trend in LMs. We also systematically study the basic properties of ultrathin phlogopite and demonstrate that natural phlogopite presents iron impurities in its crystal lattice, which decreases its bandgap from about 7 eV to 3.6 eV. Finally, we combine phlogopite crystals with 1L-WS2 in ultrathin van der Waals heterostructures and present a photoluminescence study, revealing a significant enhancement on the 1L-WS2 optical quality (i.e. higher recombination efficiency through neutral excitons) similarly to that obtained on 1L-WS2/hexagonal boron nitride heterostructures. Our proof-of-concept study shows that phlogopite should be regarded as a good and promising candidate for LM-based applications as a low-cost layered nanomaterial.

Automated summary

Naturally occurring van der Waals crystals, such as the phlogopite mineral explored in this study, have attracted significant interest in the nanomaterial research field. The researchers conducted a high throughput characterization of phlogopite and discovered its stable nature, easy obtainability, atomically flat surface, and lower bandgap in monolayers and few-layers. They also investigated the presence of iron impurities in natural phlogopite, which affected its bandgap. Furthermore, the researchers successfully combined phlogopite crystals with 1L-WS2 to create ultrathin van der Waals heterostructures, showing enhanced optical quality.