Strain physics in graphene - from friction to pseudo magnetic fields

非技术摘要：石墨烯是由碳原子组成的单原子层，具有许多非常极端的特性，例如，即使只有一个原子层厚度，也不渗透气体，具有极高的弹性，具有极高的导热性和导电性。波士顿大学的研究小组正在探索如何在石墨烯上施加应变来操纵这些特性，以实现从机械谐振器、电子和光学器件到热管理器件等新颖而有趣的应用。为了将应变工程用于这些目的，有必要知道有多少摩擦力来固定应变石墨烯。研究人员使用石墨烯覆盖的微型室来测量摩擦，以及如何通过对衬底进行图案化来控制摩擦。石墨烯覆盖的微室通过施加可变的外部压力来调整应变，使悬浮的石墨烯膜偏转，从而在悬浮和支撑区域产生应变。应变响应是用光谱学测量的。某些奇异的应变分布预计会影响石墨烯中的电子，使它们被困住，不再能够导电。波士顿大学的团队正致力于开发腔室形状和摩擦模式，以实现这种状态，这种状态可以通过改变外部压力来开启和关闭。该团队还在研究压力如何改变石墨烯的导热性。石墨烯是应变工程器件的良好候选者，因为它可以承受20%的延伸而不会断裂。因此，巨大的菌株可以诱导和改造为新颖和有趣的应用。应变工程影响许多类型的设备，从机械谐振器到电子和光学设备。应变工程也开辟了奇异物理学和应用的新领域，也许最引人注目的是在室温下将电子和空穴量化到朗道能级的伪磁场。因此，对石墨烯-衬底相互作用和变应变下的摩擦有一个深入的了解是很重要的。波士顿大学的研究小组已经开发出一种方法，通过放置石墨烯来密封具有可变外部压力的微室，从而施加可变应变。由于有限的摩擦，石墨烯膜在腔室上变形并滑动。通过微拉曼测量，研究小组能够绘制出应变分布图，并确定与压力相关的摩擦系数。摩擦依赖于衬底处理的知识允许应变图案化。可变摩擦是通过表面处理和局部摩擦系数的模式化来实现的。不同的摩擦是量身定制的，以产生应变分布，将产生高的局部伪磁场。研究人员将应变产生的高局部伪磁场与等离子体图形相结合，使等离子体热点与高伪磁场区域重叠。然后利用声子和朗道能级激子相互作用通过拉曼光谱读出伪场响应。另一个应用是石墨烯作为高导热性管道。悬浮石墨烯已被证明具有极高的导热性。理论预测携带热量的面外声子比面内声子效率低得多。预计应变将消除面内模态向面外模态的散射，并降低状态密度，因此应变可以大幅增加已经很高的导热系数。研究人员正在使用悬浮石墨烯上的可调应变来实验测量应变对石墨烯导热性的影响。

Nontechnical abstract: Graphene, a single atomic layer of carbon atoms, has a wealth of very extreme properties, e.g. being impermeable to gases even at only one atomic layer thickness, being extremely elastic, and having extremely high heat and electrical conductivity. The research group at Boston University is exploring how applying strain to graphene manipulates these properties for novel and interesting applications from mechanical resonators, and electronic and optical devices, to thermal management devices. In order to use strain engineering for these purposes, it is necessary to know how much friction is there to anchor the strained graphene. The researchers use miniature chambers covered by graphene to measure friction and how to control it by patterning the substrate. Graphene covered microchambers are strain tuned by applying a variable external pressure that deflects the suspended graphene membrane creating strain in both the suspended and supported regions. The strain response is measured using optical spectroscopy. Certain exotic strain distributions are predicted to affect the electrons in graphene in such a way that they get trapped and no longer can conduct electricity. The BU team is working on developing chamber shapes and friction patterning to achieve this state which can be turned on and off by varying the external pressure. The team is also studying how pressure can vary the heat conductivity in graphene. Technical Abstract Graphene is a good candidate for strain engineered devices since it can withstand a 20% extension without breaking. Hence huge strains can be induced and engineered for novel and interesting applications. Strain engineering affects many types of devices, from mechanical resonators to electronic and optical devices. Strain engineering also opens up new areas of exotic physics and applications, perhaps most spectacularly from creating magnetic pseudo fields with quantization of electrons and holes into Landau levels at room temperature. Therefore it is important to have a solid understanding of graphene-substrate interaction and friction under variable strain. The research team at Boston University has developed a method of applying variable strain by placing graphene to seal microchambers with variable external pressure. The graphene membrane deforms over the chamber and slides due to finite friction. With micro-Raman measurements the team is able to map out the strain profile and determine the friction coefficient which is pressure dependent. Knowledge of the friction dependence on substrate treatment allows strain patterning. Variable friction is achieved by patterning the surface treatment and hence local coefficient of friction. The varying friction is tailored to create strain distributions that will create high local pseudo magnetic field. The researchers are combining the strain-created high local pseudo magnetic fields with plasmonic patterning to overlap the plasmonic hotspots with the high pseudo field regions. The pseudo field response is then read out via Raman spectroscopy using phonon and Landau Level exciton interactions. Another application is graphene as high thermal conductivity conduits. Suspended graphene has been shown to have extremely high heat conductivity. Theory predicts that the out-of-plane phonons that carry the heat are much less efficient than the in-plane acoustic modes. Strain is predicted to remove the scattering of the in-plane modes into the out-of-plane modes as well as reducing the density of states so strain could drastically increase the already high thermal conductivity. The researchers are using their tunable strain on suspended graphene to experimentally measure the effect of strain on the thermal conductivity of graphene.