  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)O10(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/hBN
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.
