等离子体技术制备铁磁性石墨烯及其机理研究

Because of the weak spin orbit interaction, graphene has a long spin diffusion length, which provides favorable medium for the spin transport. Thus, graphene has great potential applications in the fields of spintronics. However, graphene is usually intrinsically non-magnetic and lacks of localized magnetic moments due to a delocalized π bonding network. Furthermore, the spin injection efficiency into graphene is very low, which strongly limits its applications in spintronics devices. The most effective solution is to prepare ferromagnetic graphene. However, the researches and related applications are strongly hindered due to long reaction time, unwanted ferromagnetic impurities and complicated processes involved in the current synthesis methods of ferromagnetic graphene. In this project we will focus on the graphene oxide, which are mainly treated by plasma technology. We will study the magnetic properties of graphene, analyzing the corresponding contribution of the type, amount and distribution of functional groups on graphene to the magnetic moment and spin coupling. We will construct a suitable theoretical model to calculate the spin distribution and coupling by density functional theory (DFT). We will combine the theoretical and experimental results, try to realize the ferromagnetic graphene, which will be applied in building the full-carbon spintronics devices.

由于石墨烯自旋-轨道耦合作用很弱,石墨烯具有很长的自旋弛豫长度,这为电子自旋的输运提供了良好的条件,从而在自旋电子学器件中具有巨大应用前景。但是,完美的石墨烯因为其去局域的二维网络结构因而没有局域磁矩,而自旋的注入效率通常又很低，这极大的限制了其在自旋电子学器件中的应用。最有效的解决办法是获得铁磁性的石墨烯。目前合成铁磁性石墨烯方法通常很复杂，费时间，同时容易带入磁性污染物，针对这些问题，本项目以氧化石墨烯为研究对象，采用等离子体技术手段对氧化石墨烯进行处理，研究其磁性变化。通过分析石墨烯中官能团的种类，数量和分布对磁矩、自旋耦合的贡献对应关系，构建合适的理论模型，用密度泛函理论(DFT)进行自旋分布和耦合规律的计算，与实验相结合，以实现铁磁性石墨烯，并以此为基础构建全碳自旋电子器件。

石墨烯在自旋电子学中具有重要的应用，室温铁磁性石墨烯的制备与研究，成为近年来自旋电子学领域的研究热点。然而石墨烯仅有S和P电子，其磁性来源于部分填充的电子态，如缺陷和边界态等，且磁性非常弱。任何ppm量级的磁性杂质污染都有可能掩盖石墨烯的本征磁性而使其表现出杂质磁性的假象。本项目通过等离子体技术手段制备铁磁性石墨烯同时可以避免磁性杂质的引入。我们将石墨烯直接放在氢等离子体（室温条件）中处理，可以有效地引入氢，得到含氢的还原氧化石墨烯，样品的室温铁磁性明显增强。氢等离子体曝光时间对控制石墨烯的铁磁性起到相当重要的作用。石墨烯在氢等离子体曝光时间从0增加到2小时，样品的铁磁磁化强度(Ms)从0.011 emu/g增加到0.026 emu/g，部分碳原子sp2杂化变成sp3杂化，石墨烯材料中缺陷密度增加，氢化程度增加，样品的磁性明显增强。当我们进一步增加等离子处理时间（2小时以上），我们发现处理后石墨烯样品明显变少了，这可能是随着曝光时间的进一步加长，部分碳原子被刻蚀了，石墨烯样品反而变得不稳定。氢等离子体技术提供了一种便捷快速的方法用于调控石墨烯磁性，具有广泛的应用前景。