【作者】 王淼；

【导师】 缪峰； 王伯根；

【作者基本信息】 南京大学 ， 物理学， 2018， 博士

【摘要】 忆阻器,是一种可以基于对其施加电压和电流而保留内部电阻状态的电阻开关,在拥有巨大应用前景的同时,其在从科研转向研发的关键阶段也面临着诸多挑战。利用二维材料原子级的厚度以及在电学、光学、热学、机械等方面的优异性能,和相关异质结构的丰富的物理内涵和调控方式,将可能为忆阻器在新材料的运用、器件结构的改进、性能优化等方面实现突破提供重要的机遇。本文将研究石墨烯(graphene)、二硫化钼(MoS2)、二硫化钨(WS2)和二硒化钨(WSe2)等二维材料在忆阻器及其相关电子器件中的应用,并通过透射电子显微镜表征和理论分析等手段研究这些器件的工作机制。首先我们简要介绍了忆阻器的研究历史、工作机制、优异特性以及它所面临的挑战,并对二维材料以及其异质结构进行了介绍。之后我们探讨了二维材料在忆阻器中的应用前景和研究现状。第二章中我们使用石墨烯作为底电极制备了具有超低开关功率的忆阻器。通过与具有类似结构的Pt金属电极忆阻器进行对比,我们发现石墨烯电极能够使忆阻器的开关功率降低约600倍,并且能够很好地保留住TiOx基忆阻器原有的优秀开关性能。结合实验结果与理论拟合分析,我们发现该忆阻器所具有的超低功率来源于石墨烯与氧化物之间的界面。由于界面处的隧穿势垒,该器件的电流-电压关系也表现出了能够解决漏电流问题的非线性特征。通过控制石墨烯电极的质量,我们成功地对该器件的性能进行了调控。第三章中我们利用石墨烯与氧化物界面处的隧穿势垒,制作出了具有全新结构和机制的选择器。我们发现该器件的电流-电压关系具有较强的非线性和对称性,适合作为选择器应用于忆阻器交叉点阵结构中。我们从对照实验中发现该器件表现出的非线性I-V关系来源于石墨烯与氧化物界面处的隧穿势垒。通过测量该器件在不同温度下的I-V表现并对其结果进行了理论拟合分析,我们也进一步地证明了以上结论。我们将该选择器与TaOx基忆阻器串联并进行了测量,证明了该选择器能够在不影响忆阻器开关的前提下,有效地降低交叉点阵中的漏电流。第四章中我们使用MoS2作为电活性层制备了忆阻器,并对该忆阻器的工作机制进行了研究。我们使用压力调制电导显微镜的技术手段成功地定位了器件中的导电通道,并发现该器件中导电通道的形成过程较为剧烈并且会导致器件的破损。我们利用透射电子显微镜对该忆阻器中的导电通道进行了表征,发现导电通道中存在大量的氧原子,表明了氧原子在该器件开关过程中的重要作用。第五章中我们选取了 MoS2-xOx和石墨烯分别作为忆阻器的电活性层和电极材料,制作了基于二维材料异质结的耐高温忆阻器。测试结果显示该忆阻器能够实现非常稳定的开关:可擦写次数超过107次,擦写速度小于100 ns,并且拥有很好的非易失性。我们发现该结构的忆阻器能够在高达340 0C的温度下稳定工作并且保持良好的开关性能,创下了忆阻器工作温度的新记录。我们利用透射电子显微镜进行原位观察,发现该忆阻器的耐热性来源于MoS2-xOx超高的热稳定性,并进一步揭示了该忆阻器中基于氧离子迁移的工作机制。结果显示,该忆阻器中的导电通道在开关过程中一直被具有超高热稳定性的单晶石墨烯和MoS2-xOx很好地保护着,保证了导电通道在高温开关过程中的稳定性。我们将该忆阻器置于柔性衬底聚酰亚胺上,发现器件在大于0.6%的形变应力下伸曲1200次之后同样能够稳定地工作,表明了该忆阻器也具有优秀的抗弯曲性能。第六章中我们基于二维过渡金属硫族化合物制作了具有负微分电阻的垂直结构器件。我们发现 Au/MoS2/Au、Au/WS2/Au、Au/WSe2/Au 以及graphene/MoS2/graphene四种结构均表现出较为相似的负微分电阻现象。通过不同温度下的电流-电压关系表征与文献报道的对比,我们认为器件中的负微分电阻是由过渡金属硫族化合物自加热后电阻的减小导致的。我们利用基本电路元件和负微分电阻器件搭建出电路,成功地实现了交流信号的反向、放大以及倍频。我们还利用负微分电阻器件在RC电路中能够产生自激振荡的原理,成功地产生了神经元脉冲信号。最后一章中,我们总结了本文所有的研究内容,对目前已取得的进展及面临的问题进行了讨论与分析,并且对未来的工作进行了计划和展望。更多还原

【Abstract】 Memristor is a resistive switch which can keep the internal resistance based on the applied voltage and current status.Having great application prospects,it also faces many challenges in the key stages from scientific research to applications.By utilizing the atomic thickness of two-dimensional crystalline materials and their excellent performance in electrical,optical,thermal,mechanical and other aspects,as well as rich physical connotation and regulation of related heterostructures,we can provide an important opportunity to achieve a breakthrough for the memristor in the applications of new materials,improvements of the structure and performance optimization.In this article,we investigated the applications of graphene,molybdenum disulfide(MoS2),tungsten disulfide(WS2)and tungsten diselenide(WSe2)in memristors and related electronic devices,the working mechanism of these devices was also studied through transmission electron microscopy characterization and theoretical analysis.At the beginning,we gave a brief introduction about the research history,switching mechanism,advantages and challenges of the memristor.We also introduced the two-dimensional materials and its heterostructures.Then,we discussed the application prospects and research status of two-dimensional materials in memristors.In Chapter Two,we fabricated a memristor with ultra-low switching power based on graphene bottom electrode.By comparing the device with a Pt-electrode memristor,which has a similar structure,we found that the graphene electrode could reduce the switching power by about 600 times,retaining the excellent switching performances of TiOx-based memristors.The experimental results and theoretical simulations suggest that the interface between graphene and oxide is mainly responsible for the ultra-low switching power of the memristor.Due to the tunnel barrier at the interface,the Ⅰ-Ⅴcurve of the device exhibits nonlinear characteristics,which could potentially solve the leakage current problem.We also demonstrated that the performance of the device could be tuned by the quality of graphene electrode.In Chapter Three,we utilized the tunnel barrier of the interface between graphene and Titanium oxide to fabricate a novel selector.The Ⅰ-Ⅴ curve of the device exhibits both high nonlinearity and symmetry characteristics,which are highly desirable for selector devices.Our control experiments,together with temperature-dependent transport studies and theoretical simulations,suggest that the nonlinearity arises from the graphene/TiOx interface tunneling barriers.By connecting this selector with a TaOx based memristor,we demonstrated that the selector can effectively reduce the leakage current in the cross-bar array without sacrificing the switching performance of the memristor.In Chapter Four,we fabricated a memristor based on MoS2 switching layer and investigated its switching mechanism.The conduction channels in the device were successfully positioned through pressure-modulated conductance microscopy.We found that the formation of conduction channels is related to device damage.We utilized transmission electron microscopy to investigate the conduction channels in the memristor and found that there are a large amount of oxygen atoms in the channel,indicating the important role of oxygen atoms in the switching process of the device.In Chapter Five,we selected MoS2-xOx and graphene as the switching layer and electrode,respectively,and fabricated a memristor with high thermal stability based on heterostructure of two-dimensional materials.The results showed that the memristor could achieve very stable switching:endurance over 107,switching in less than 100 ns,and non-volatile memory.We found that the device could work stably at the temperature up to 340 ℃ and maintain excellent switching performance,which is record-high for memristors.MoS2-xOx layer was found to induce the observed high thermal stability after performing high temperature in situ high-resolution transmission electron microscopy studies.Further in situ scanning transmission electron microscopy investigations revealed a switching mechanism based on the migration of oxygen ions.We found that the conduction channel in the memristor is well protected by the single crystal graphene and MoS2-xOx during the switching process,which ensures the stability of the conductive channel at elevated temperatures.The mechanical flexibility of such structured devices was demonstrated by showing a good endurance against mechanical bending of 1,200 times on polyimide substrate(corresponding to a strain of～0.6%),suggesting possible flexible electronic applications.In Chapter Six,we developed vertical structured devices with negative differential resistance based on two-dimensional transition metal chalcogenides.We found that Au/MoS2/Au,Au/WS2/Au,Au/WSe2/Au,and graphene/MoS2/graphene all exhibit similar negative differential resistance phenomenon.By performing temperature-dependent Ⅰ-Ⅴ characterization,we concluded that the negative differential resistance in these devices is caused by the decrease of the resistance of transition metal chalcogenides after self-heating.With some basic circuit components and the negative differential resistance devices,we built the circuit that could implement the reverse,amplification,and frequency multiplication of AC signal.We also successfully generated neuron spikes with the principle that a negative differential resistance device can generate self-oscillation in RC circuits.In the last chapter,we summarized all the researches in this article,discussed and analyzed the progress we made and the problems we faced,and listed some plans for future investigation.更多还原