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

Abstract

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.

Raman and Far-Infrared Synchrotron Nanospectroscopy of Layered Crystalline Talc: Vibrational Properties, Interlayer Coupling, and Symmetry Crossover

“…Layered materials (LMs) possess exquisite electrical, optical, and mechanical properties that have been predicted theoretically − and measured experimentally by Raman spectroscopy, − infrared spectroscopy, − and other analytical techniques. − The potential use of LMs as building blocks for future ultrathin and flexible devices , has created interest from both scientific and economic perspectives to search for and characterize LMs that are easily obtained in nature. − Talc, also known as soapstone, is an abundant, naturally occurring magnesium hydrosilicate mineral from the phyllosilicate group and is the softest known mineral . It is an electrical insulator (bandgap of ∼5 eV) , and allows for excellent basal cleavage, with layers held together by van der Waals forces, , making it an excellent target for future low-cost optoelectronic applications.…”

Phyllosilicates as earth-abundant layered materials for electronics and optoelectronics: Prospects and challenges in their ultrathin limit

“…[17][18][19] Phyllosilicate minerals are wide bandgap insulators, stable in different environments, and abundant in nature. [17][18][19][20][21][22][23][24][25][26][27] They present a layered structure that enables their exfoliation down to 1L. 20,23,28,29 The variety of phyllosilicate specimens arises from ionic substitutions with different geometric arrangements of atoms within the crystal.…”

“…[17][18][19][20][21][22][23][24][25][26][27] They present a layered structure that enables their exfoliation down to 1L. 20,23,28,29 The variety of phyllosilicate specimens arises from ionic substitutions with different geometric arrangements of atoms within the crystal. 29,30 In this sense, the understanding of the general properties of phyllosilicates is required for the optimized applicability of these nanomaterials in future applications.…”

High throughput investigation of an emergent and naturally abundant 2D material: Clinochlore

“…Synthetic materials can be produced in controlled environments, but some processes required for specific material classes are complex and consequently expensive. In an effort to increase the list of naturally occurring LMs that are abundant in nature and could potentially become low-cost sources of 2D materials, recent research has been carried out in the promising group of phyllosilicates minerals, which are wide bandgap insulators that can be mechanically exfoliated down to 1L and FL-flakes [8][9][10][11][12][13][14][15][16], and and can be embedded in van der Waals heterostructures with other LMs [17][18][19][20][21][22][23][24]. The challenge in using naturally occurring van der Waals materials for 2D applications is related to the existence of impurities and defects in their crystal lattice [25].…”

“…Phyllosilicates are a subgroup of silicates that include clays, micas, and chlorites [31]. Most of the investigations of their 2D form have focused on muscovite mica or talc [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]26,27], and barely explored other minerals of this family, such as chlorites [10]. Within the chlorite group, clinochlore is one of the most abundant mineral.…”