石墨烯边界的动力学行为研究
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摘要
石墨烯从2004年首次在实验中制备出来,已经受到了科学界和工业界的广泛关注,这主要得益于它超高的载流子迁移率,石墨烯也因此被誉为是未来替代硅基半导体的最理想候选材料。此外,石墨烯还具有各种奇特的物理性质,如高热导率,高透光性,高机械强度等。二维石墨烯应用于半导体器件还存在着一个很大的难题,那就是它没有电子能隙。不过,对于狭长的一维石墨烯纳米带,能隙可以因为其边界效应而产生,从而能被应用于具有高开关特性的逻辑器件中。由于石墨烯纳米带的能隙大小还与其边界手性密切相关,也就意味着同一条石墨烯纳米带可以因为边界手性结构组成成分的改变而表现出截然不同的电学性能。所以,石墨烯边界在不同条件下的动力学行为研究是非常迫切且具有重要意义的。但迄今为止,人们对石墨烯的边界动力学行为的认识还非常有限,甚至可以说是空白。本人在上述背景下,通过偏振拉曼光谱对石墨烯边界结构在高温及Ar等离子体处理下的动力学行为进行了研究。
首先,我们在实验中发现单层石墨烯的扶椅型边界结构在热处理过程中相对稳定,即使温度达到500℃,大部分边界扶椅结构成分依然能够稳定存在。这说明扶椅结构是一种热稳定的石墨烯边界结构。
其次,对于与单层石墨烯的锯齿型边界,其在热处理过程中显得很不稳定,主要表现在它非常容易通过原子结构重组转变成更稳定的扶椅型结构。实验发现即使热处理温度仅为200℃,锯齿型边界的原子已经开始重新修正排列成新的扶椅型结构。而到了300℃,原锯齿型边界大部分原子都重新修正排列成扶椅型结构,变成相对稳定的新结构。这一结果对基于石墨烯纳米带的器件制备工艺具有重要的指导意义。
另外,基于前面对单层石墨烯锯齿型和扶椅型边界热动力学行为的研究结果,我们还发现双层石墨烯扶椅型边界的热动力学行为表现形式类似于两个独立的单层扶椅型边界；但同时我们也惊奇的发现AB堆垛形式排列的双层石墨烯锯齿型边界在热处理条件下,上下两层能够形成连续的闭合结构。边界开放结构转变形成闭合结构的实验结果也非常好的与理论计算结果相符,因为新闭合结构可以大大降低整个双层石墨烯体系的系统能量。理论预测该闭合结构会产生能隙且会导致电子和空穴的空间分离。所以这种新结构的发现,对拓展石墨烯在半导体器件中的应用有深刻意义。
最后,本文还研究了石墨烯边界结构在Ar等离子体处理下的变化。研究发现在等离子体环境下,扶椅型边界结构相对稳定,而锯齿型边界结构容易在处理过程中发生结构变化而形成相对稳定的扶椅型结构。这对未来研究石墨烯纳米器件的抗辐照特性提供了一定的参考价值。
Graphene is an allotrope of carbon which consists of one atomic carbon layer attracted tremendous attentions by both scientists and engineers since its first discovery in 2004. This is attributed to its extremely high carrier mobility, and thus it is considered as the ideal candidate toward the next-generation semiconductor devices. Furthermore, graphene has many unique properties, such as high thermal conductivity, high transparency and high Young's modulus, etc. However, two-dimensional graphene cannot be directly applied in semiconductor devices due to the zero-bandgap. One-dimensional graphene nanoribbon exhibit non-zero bandgap and hence can be used to fabricate semiconductor device with good performance owning to its high on-off ratio. But it is theoretically and experimentally proved that the bandgap of graphene nanoribbon is strongly dependent on the chirality of the edges, which means that the electronic behavior can be tuned by modifying its edge structure. Annealing is commonly used in semiconductor manufacturing process, therefore, investigation of thermal dynamic behavior of graphene edges becomes important. However, to date, the understanding of the graphene edges' thermal dynamics is still very limited. Furthermore, the study of structure modification of graphene edges under Ar plasma treatment is also deserved. At this background, I focused on the detailed investigation of thermal dynamics for edges of single- and bi-layer graphene, as well as the edge dynamics of graphene edges under Ar plasma treatment.
Firstly, it is discovered that the armchair edges of single layer graphene is thermally stable. Even the annealing temperature is as high as 500℃, large portion of armchair segments can still keep remained. Meanwhile, the center region of single layer graphene is also thermally stable as no obvious appearance of defects.
Secondly, it is found that the zigzag edges of single layer graphene is not as thermally stable as armchair edges because it is the atoms of zigzag edges prefer to transform to armchair segments during annealing. Even for annealing temperature as low as 200℃. small portion of zigzag edges is starting to rearrange/modify to form armchair segments along±30°with respect to the original edge direction. When the annealing temperature is 300℃, large amount of atoms of zigzag edges rearrange/modify to form armchair segments which is relatively thermal stable. Therefore, the modifications of edge structures by thermal annealing (zigzag segments rearrange in form of armchair segments) provide a flexible way to control the electronic properties of graphene and graphene nanostructures.
Thirdly, based on the aforementioned thermal dynamic behaviour of single layer graphene edges, we found that the armchair edges of AB-stacked bilayer graphene present thermal dynamic behavior similar to that of single layer graphene, which means that the armchair edges of top and bottom graphene can be considered as two independent individual armchair edges. However, for zigzag edges of AB-stacked bilayer grpahene. the thermal dynamic behaviour shows obviously difference compare to that of single layer. In stead of rearrange to form armchair segments during annealing for zigzag edges of single layer graphene. the top and bottom zigzag edges of AB-stacked bilayer graphene prefer to form perfect closed structure hybridized by sp2 bonds. This is also supported by the theoretical calculation results very well as there is no energy barrier from the initial open edge state to closed edge state, moreover. the system energy undergoes more than 2 eV drop when the atoms on the top and bottom zigzag edges firstly bonded. Furthermore, it is theoretically predicted that AB-stacked bilayer graphene with closed zigzag edges present exotic electronic properties, such as pseudospin repulsion induced bandgap opening and charge separation. Therefore, the discovery of this new graphene-based form may illuminate a simple and easy way to engineer graphene electronics.
Finally, the graphene edge structure evolution after Ar plasma treatment is investigated. It is found that the armchair edges present relatively good stability under Ar plasma treatment. Meanwhile, the zigzag edge atoms prefer to transform to armchair segments after Ar plasma treatment. This result indicates that the armchair segments are more stable under plasma radiation. This finding provides useful information for studying anti-radiation properties of graphene based nano-electronic devices.
首先,我们在实验中发现单层石墨烯的扶椅型边界结构在热处理过程中相对稳定,即使温度达到500℃,大部分边界扶椅结构成分依然能够稳定存在。这说明扶椅结构是一种热稳定的石墨烯边界结构。
其次,对于与单层石墨烯的锯齿型边界,其在热处理过程中显得很不稳定,主要表现在它非常容易通过原子结构重组转变成更稳定的扶椅型结构。实验发现即使热处理温度仅为200℃,锯齿型边界的原子已经开始重新修正排列成新的扶椅型结构。而到了300℃,原锯齿型边界大部分原子都重新修正排列成扶椅型结构,变成相对稳定的新结构。这一结果对基于石墨烯纳米带的器件制备工艺具有重要的指导意义。
另外,基于前面对单层石墨烯锯齿型和扶椅型边界热动力学行为的研究结果,我们还发现双层石墨烯扶椅型边界的热动力学行为表现形式类似于两个独立的单层扶椅型边界；但同时我们也惊奇的发现AB堆垛形式排列的双层石墨烯锯齿型边界在热处理条件下,上下两层能够形成连续的闭合结构。边界开放结构转变形成闭合结构的实验结果也非常好的与理论计算结果相符,因为新闭合结构可以大大降低整个双层石墨烯体系的系统能量。理论预测该闭合结构会产生能隙且会导致电子和空穴的空间分离。所以这种新结构的发现,对拓展石墨烯在半导体器件中的应用有深刻意义。
最后,本文还研究了石墨烯边界结构在Ar等离子体处理下的变化。研究发现在等离子体环境下,扶椅型边界结构相对稳定,而锯齿型边界结构容易在处理过程中发生结构变化而形成相对稳定的扶椅型结构。这对未来研究石墨烯纳米器件的抗辐照特性提供了一定的参考价值。
Graphene is an allotrope of carbon which consists of one atomic carbon layer attracted tremendous attentions by both scientists and engineers since its first discovery in 2004. This is attributed to its extremely high carrier mobility, and thus it is considered as the ideal candidate toward the next-generation semiconductor devices. Furthermore, graphene has many unique properties, such as high thermal conductivity, high transparency and high Young's modulus, etc. However, two-dimensional graphene cannot be directly applied in semiconductor devices due to the zero-bandgap. One-dimensional graphene nanoribbon exhibit non-zero bandgap and hence can be used to fabricate semiconductor device with good performance owning to its high on-off ratio. But it is theoretically and experimentally proved that the bandgap of graphene nanoribbon is strongly dependent on the chirality of the edges, which means that the electronic behavior can be tuned by modifying its edge structure. Annealing is commonly used in semiconductor manufacturing process, therefore, investigation of thermal dynamic behavior of graphene edges becomes important. However, to date, the understanding of the graphene edges' thermal dynamics is still very limited. Furthermore, the study of structure modification of graphene edges under Ar plasma treatment is also deserved. At this background, I focused on the detailed investigation of thermal dynamics for edges of single- and bi-layer graphene, as well as the edge dynamics of graphene edges under Ar plasma treatment.
Firstly, it is discovered that the armchair edges of single layer graphene is thermally stable. Even the annealing temperature is as high as 500℃, large portion of armchair segments can still keep remained. Meanwhile, the center region of single layer graphene is also thermally stable as no obvious appearance of defects.
Secondly, it is found that the zigzag edges of single layer graphene is not as thermally stable as armchair edges because it is the atoms of zigzag edges prefer to transform to armchair segments during annealing. Even for annealing temperature as low as 200℃. small portion of zigzag edges is starting to rearrange/modify to form armchair segments along±30°with respect to the original edge direction. When the annealing temperature is 300℃, large amount of atoms of zigzag edges rearrange/modify to form armchair segments which is relatively thermal stable. Therefore, the modifications of edge structures by thermal annealing (zigzag segments rearrange in form of armchair segments) provide a flexible way to control the electronic properties of graphene and graphene nanostructures.
Thirdly, based on the aforementioned thermal dynamic behaviour of single layer graphene edges, we found that the armchair edges of AB-stacked bilayer graphene present thermal dynamic behavior similar to that of single layer graphene, which means that the armchair edges of top and bottom graphene can be considered as two independent individual armchair edges. However, for zigzag edges of AB-stacked bilayer grpahene. the thermal dynamic behaviour shows obviously difference compare to that of single layer. In stead of rearrange to form armchair segments during annealing for zigzag edges of single layer graphene. the top and bottom zigzag edges of AB-stacked bilayer graphene prefer to form perfect closed structure hybridized by sp2 bonds. This is also supported by the theoretical calculation results very well as there is no energy barrier from the initial open edge state to closed edge state, moreover. the system energy undergoes more than 2 eV drop when the atoms on the top and bottom zigzag edges firstly bonded. Furthermore, it is theoretically predicted that AB-stacked bilayer graphene with closed zigzag edges present exotic electronic properties, such as pseudospin repulsion induced bandgap opening and charge separation. Therefore, the discovery of this new graphene-based form may illuminate a simple and easy way to engineer graphene electronics.
Finally, the graphene edge structure evolution after Ar plasma treatment is investigated. It is found that the armchair edges present relatively good stability under Ar plasma treatment. Meanwhile, the zigzag edge atoms prefer to transform to armchair segments after Ar plasma treatment. This result indicates that the armchair segments are more stable under plasma radiation. This finding provides useful information for studying anti-radiation properties of graphene based nano-electronic devices.
引文
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