Tailoring Magnetically Active Defect Sites in MoS2 Nanosheets for Spintronics Applications,ACS Applied Nano Materials

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Tailoring Magnetically Active Defect Sites in MoS2 Nanosheets for Spintronics Applications
ACS Applied Nano Materials ( IF 5.3 ) Pub Date : 2020-01-10 , DOI: 10.1021/acsanm.9b02121
Rohini Sanikop ₁ , C. Sudakar ₁

Affiliation

We show that it is possible to tune the magnetic properties in MoS₂ without any intentional magnetic dopants but by simply controlling the magnetically active defect sites. The layer thickness and defect-density-controlled MoS₂ (2H-phase) nanosheets are obtained by annealing a wet-chemical precursor from 500 to 900 °C. High-resolution transmission electron microscopy images show the presence of planar and edge defects arising from abruptly terminating edges, bends, tears, and folding of nanosheets. In addition, Mo⁵⁺ and sulfur vacancy point defects are predominant, as evidenced from the electron paramagnetic resonance and X-ray photoelectron spectra. Nanosheets gradually become more crystalline and devoid of defects upon annealing, with MoS₂ (900 °C) becoming >70% defect-free. The interlayer spacing decreases monotonically from ∼6.351 to 6.231 Å with annealing; however, the lattice parameter a increases from 3.107 to 3.172 Å. These structural changes induce a large local strain (∼9%) in MoS₂ (500 °C) nanosheets compared to the small strain (∼3%) present in MoS₂ (900 °C). Raman spectroscopy further manifests control on the defect density, with the intensity ratio of the defect-induced LA(M) mode (∼227 cm^–1) relative to the E_2g¹ and A_1g modes decreasing monotonically with the annealing temperature. We demonstrate that magnetically active defect sites can be tuned to enhance the ferromagnetism. Magnetization (at 5 K) systematically varies with 1 order higher M_S (=0.10 emu/g) found in MoS₂ (500 °C) nanosheets compared to MoS₂ (900 °C). Interestingly, temperature-dependent magnetization shows a ferromagnetic-like transition around 120 K, which becomes more pronounced in defect-rich MoS₂. The ferromagnetic interaction is more likely due to a bound magnetic polaron made up of a spin ¹/₂ Mo⁵⁺ ion with trapped carriers present at the sulfur vacancies. These findings open a new way of exploring spintronic devices such as spin-field-effect switches, spin valves, magnetic sensors, and ultrathin high-density data storage devices using MoS₂ 2D nanosheets by controlling magnetically active defect sites.

中文翻译：

在用于自旋电子学的MoS ₂纳米片中定制磁性活性缺陷位点

我们表明，有可能在没有任何故意的磁性掺杂剂的情况下，通过简单地控制磁活性缺陷位点来调整MoS _2中的磁性能。通过将湿化学前体从500到900°C进行退火，可以获得层厚和缺陷密度受控的MoS ₂（2H相）纳米片。高分辨率透射电子显微镜图像显示，由于纳米片的突然终止边缘，弯曲，撕裂和折叠而引起的平面和边缘缺陷的存在。另外，从电子顺磁共振和X射线光电子能谱证明，Mo ⁵⁺和硫的空位点缺陷是主要的。使用MoS ₂时，纳米片逐渐变得更加结晶，并且在退火后没有缺陷（900°C）达到> 70％的无缺陷。随着退火，层间间距从约6.351到6.231Å单调减小；但是，晶格参数a从3.107增大到3.172Å。与MoS ₂（900°C）中存在的小应变（〜3％）相比，这些结构变化在MoS ₂（500°C）纳米片中引起较大的局部应变（〜9％）。拉曼光谱进一步证明了缺陷密度的控制，缺陷诱导的LA（M）模式的强度比（〜227 cm ^–1）相对于E _2g¹和A _1g模态随退火温度单调降低。我们证明，可以调整磁性活动缺陷部位以增强铁磁性。与MoS ₂（900°C）相比，在MoS ₂（500°C）纳米片中发现的磁化强度（在5 K时）系统变化，其M _S（= 0.10 emu / g）高1个数量级。有趣的是，随温度变化的磁化强度在120 K附近显示出类似铁磁的转变，这种转变在富含缺陷的MoS _2中更为明显。铁磁相互作用是更可能是由于自旋的由一个束缚磁极化子¹ / ₂的Mo ⁵⁺离子与硫空位处存在的被捕获的载流子结合。这些发现为控制自旋电子器件（如自旋场效应开关，自旋阀，磁传感器和超薄高密度数据存储设备）打开了一种新的途径，该方法使用MoS ₂ 2D纳米片通过控制磁活动缺陷部位来实现。

更新日期：2020-01-10

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