Controllable Synthesis Quadratic-Dependent Unsaturated Magnetoresistance of Two-Dimensional Nonlayered Fe₇S₈ with Robust Environmental Stability

Abstract

Two-dimensional (2D) iron chalcogenides (FeX, X = S, Se, Te) are emerging as an appealing class of materials for a wide range of research topics, including electronics, spintronics, and catalysis. However, the controlled syntheses and intrinsic property explorations of such fascinating materials still remain daunting challenges, especially for 2D nonlayered Fe7S8 with mixed-valence states and high conductivity. Herein, we design a general and temperature-mediated chemical vapor deposition (CVD) approach to synthesize ultrathin and large-domain Fe7S8 nanosheets on mica substrates, with the thickness down to ∼4.4 nm (2 unit-cell). Significantly, we uncover a quadratic-dependent unsaturated magnetoresistance (MR) with out-of-plane anisotropy in 2D Fe7S8, thanks to its ultrahigh crystalline quality and high conductivity (∼2.7 × 105 S m–1 at room temperature and ∼1.7 × 106 S m–1 at 2 K). More interestingly, the CVD-synthesized 2D Fe7S8 nanosheets maintain robust environmental stability for more than 8 months. These results hereby lay solid foundations for synthesizing 2D nonlayered iron chalcogenides with mixed-valence states and exploring fascinating quantum phenomena.

Coexistence of Anisotropic Large Magnetoresistance and Ferroelectricity in Two-Dimensional Narrow-Bandgap Bi2O2Te

“…Besides, the angular dependence of the MR obtained under a magnetic field of 9 T at 2 and 10 K both followed a cos 2θ dependence (Figure f and Figure S12). This kind of anisotropic MR is attributed to the dimensionality effect, showing 3D conduction properties in the Bi 2 O 2 Te nanosheets . Besides, the magnetoresistance measurement had been performed on another Bi 2 O 2 Te nanosheet, which also exhibited similar large magnetoresistance and SdH quantum oscillations (Figure S13).…”

“…This kind of anisotropic MR is attributed to the dimensionality effect, showing 3D conduction properties in the Bi 2 O 2 Te nanosheets. 23 Besides, the magnetoresistance measurement had been performed on another Bi 2 O 2 Te nanosheet, which also exhibited similar large magnetoresistance and SdH quantum oscillations (Figure S13). These results imply a similarity to the Bi 2 O 2 Se nanosheet reported, which can be attributed to the fact that they share the same lattice symmetry (I4/mmm) and similar characteristics of a valence electron between Se and Te.…”

Advanced Strategies in Synthesis of Two‐Dimensional Materials with Different Compositions and Phases

“…For example, as displayed in Figure 3c-e, the iron sulfide and selenide possess complicated valence states, which are easier to form abundant compositions, such as Fe 3 Se 4 , Fe 5 Se 8 , Fe 7 Se 8 , and Fe 7 S 8 nanoflakes with mixed-valence states of Fe 3+ and Fe 2+ . [58][59][60] All these were synthesized by accurately adjusting the Fe/Se precursor ratio. [58] The supplement of the Se precursors during preparation results in the formation of different Fe-intercalated crystals.…”

“…Panel (e) reproduced with permission. [ 59 ] Copyright 2022, American Chemical Society. f) Schematic of the experimental setup to synthesize CrTe nanoflakes.…”

“…Additionally, by cleverly selecting Fe 2 O 3 and FeCl 2 as precursors with the assistance of NaCl, Fe 7 S 8 with a mixed valence of Fe 3+ and Fe 2+ can also be synthesized. [ 59 ] Particularly, a universal dual‐metal precursors method was also developed to grow non‐layered Fe 1‐ x S flakes in which the mixture of a metal and chlorides serves as the source. [ 61 ] Similar to iron‐based compounds, chromium tellurides are a novel group of 2D magnetic materials with multiple stoichiometric ratios, such as CrTe, Cr 2 Te 3 , Cr 5 Te 8 , CrTe 2 , and Cr 3 Te 4 .…”