Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore, 117543, Singapore.
Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore, 117546, Singapore.
Adv Mater. 2019 Oct;31(40):e1903779. doi: 10.1002/adma.201903779. Epub 2019 Aug 18.
Among van der Waals layered ferromagnets, monolayer vanadium diselenide (VSe ) stands out due to its robust ferromagnetism. However, the exfoliation of monolayer VSe 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-VSe flakes are successfully obtained from the bulk crystal at high yield. Thiol molecules are further introduced onto the VSe 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 VSe 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 VSe , although density functional theory (DFT) calculations show that such effect can be minimized by physisorbed oxygen molecules or covalently bound thiol molecules.
在范德瓦尔斯层状铁磁体中，单层二硒化钒（VSe ）因其稳定的铁磁性而脱颖而出。然而，单层 VSe 的剥离颇具挑战性，尤其是因为单层薄片在空气中极其不稳定。通过使用有机阳离子作为插层剂的电化学剥离方法，从块状晶体中以高产率成功获得了单层 1T-VSe 薄片。巯基分子进一步被引入到 VSe 表面，以钝化剥离的薄片，从而提高了薄片在后续特性分析中的空气稳定性。使用超导量子干涉装置（SQUID）、X 射线磁圆二色性（XMCD）和磁力显微镜（MFM）在剥离的二维 VSe 薄片上证实了室温铁磁性，其中单层薄片显示出最强的铁磁性质。尽管密度泛函理论（DFT）计算表明，通过物理吸附的氧分子或共价结合的巯基分子可以最小化这种效应，但这种材料中普遍存在的硒空位也有助于 VSe 的铁磁性。
