Chemically Exfoliated VSe₂ Monolayers with Room‐Temperature Ferromagnetism

Abstract

Among van der Waals layered ferromagnets, monolayer vanadium diselenide (VSe2) stands out due to its robust ferromagnetism. However, the exfoliation of monolayer VSe2 is challenging, not least because the monolayer flake is extremely unstable in air. Using an electrochemical exfoliation approach with organic cations as the intercalants, monolayer 1T‐VSe2 flakes are successfully obtained from the bulk crystal at high yield. Thiol molecules are further introduced onto the VSe2 surface to passivate the exfoliated flakes, which improves the air stability of the flakes for subsequent characterizations. Room‐temperature ferromagnetism is confirmed on the exfoliated 2D VSe2 flakes using a superconducting quantum interference device (SQUID), X‐ray magnetic circular dichroism (XMCD), and magnetic force microscopy (MFM), where the monolayer flake displays the strongest ferromagnetic properties. Se vacancies, which can be ubiquitous in such materials, also contribute to the ferromagnetism of VSe2, although density functional theory (DFT) calculations show that such effect can be minimized by physisorbed oxygen molecules or covalently bound thiol molecules.

Emerging 2D Materials Produced via Electrochemistry

“…This method was also versatile for the production of other 2D materials such as WSe 2 , NbSe 2 , and Bi 2 Se 3 . Recently, Yu et al utilized TPA + cations with C 3 alkyl groups for the rapid synthesis of ultrathin vanadium diselenide (VSe 2 ) sheets in PC . A high fraction (93.9%) of the exfoliated flakes were thinner than five layers, in which 43% were monolayers with an average lateral dimension of 40 µm (Figure e).…”

“…f) Magnetization‐magnetic field hysteresis loop of bare and passivated VSe 2 flakes at 300 K. g) Element specific X‐ray magnetic circular dichroism spectra on V L‐edge of bare and passivated VSe 2 flakes on HOPG substrate at room temperature ( µH = ±2 T). Reproduced with permission . Copyright 2019, Wiley‐VCH.…”

Can Reconstructed Se‐Deficient Line Defects in Monolayer VSe2 Induce Magnetism?

“…It was previously shown by X‐ray magnetic circular dichroism (XMCD) the absence of intrinsic ferromagnetism in 2D VSe 2 and postulated that the system is magnetically frustrated. [ 19 ] Experimentally, magnetism in 2D TMDs can be induced by extrinsic defects, e.g., Se vacancies can enhance the ferromagnetism of solution‐processed VSe 2 , [ 16 ] and Pt vacancies can induce magnetism in ultrathin metallic PtSe 2 . [ 21 ] Thus, to fully understand the mechanisms contributing to the contradictory magnetism in VSe 2 monolayer, it is essential to explore its surface inhomogeneities (e.g., defects [ 22 ] and patterns) [ 23 ] that manifest exotic properties which are absent in the pristine crystal.…”

Tunable Ferromagnetism and Thermally Induced Spin Flip in Vanadium‐Doped Tungsten Diselenide Monolayers at Room Temperature

“…[ 9–13 ] Since these discoveries, a large body of work has been devoted to the realization of long‐range ferromagnetic order in monolayer forms of bulk van der Waals materials (e.g., Cr 2 Ge 2 Te 6 , Fe 3 GeTe 2 , Fe 5 GeTe 2 , VI 3 ). [ 14–17 ] While these systems hold enormous potential for sophisticated magnetoelectronics, as well as for the combination of logic and memory for high‐performance computing, they are restricted to low temperature, well below 300 K. [ 9–17 ] 2D vanadium‐based dichalcogenides such as VSe 2 monolayers have recently been reported to show room‐temperature ferromagnetism; [ 18–20 ] however, the origin of the observed ferromagnetism has remained a subject of debate. [ 21–24 ] While decoupling the intrinsic magnetism from the defect‐induced magnetism is still a challenging task, [ 20 ] the latter has been shown to contribute dominantly to the magnetism of this 2D system.…”

“…Since the most important technological applications demand magnetic ordering above 300 K, current research is driven by the creation of air‐stable room temperature 2D magnets. [ 18,20,25 ]…”