Strain physics in graphene - from friction to pseudo magnetic fields

非技术性摘要：石墨烯是碳原子的单原子层，具有丰富的非常极端的性质，例如即使在仅一个原子层厚度下也不透气，具有极高的弹性，并且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将应变施加到石墨烯上，以操纵这些特性，用于从机械谐振器、电子和光学设备到热管理设备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锚应变石墨烯。研究人员使用石墨烯覆盖的微型室来测量摩擦力，以及如何通过图案化衬底来控制摩擦力。石墨烯覆盖的微室通过施加可变的外部压力进行应变调节，该外部压力使悬浮的石墨烯膜偏转，从而在悬浮区域和支撑区域两者中产生应变。应变响应是使用光谱学测量的。 预计某些奇异的应变分布会影响石墨烯中的电子，使它们被捕获，不再导电。BU团队正在开发腔室形状和摩擦图案，以实现这种状态，可以通过改变外部压力来打开和关闭。该团队还在研究压力如何改变石墨烯的热导率。 石墨烯是应变工程设备的良好候选者，因为它可以承受20%的延伸而不断裂。因此，巨大的菌株可以被诱导和工程化，用于新的和有趣的应用。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。应变工程还开辟了奇异物理学和应用的新领域，也许最引人注目的是通过在室温下将电子和空穴量子化到朗道能级来创建磁赝场。因此，重要的是要有一个坚实的理解石墨烯基板的相互作用和摩擦下的可变应变。波士顿大学的研究小组开发了一种通过放置石墨烯来密封具有可变外部压力的微室来施加可变应变的方法。石墨烯膜在腔室上变形并由于有限摩擦而滑动。通过显微拉曼测量，该团队能够绘制出应变曲线并确定与压力相关的摩擦系数。摩擦对衬底处理的依赖性的知识允许应变图案化。 通过图案化表面处理实现可变摩擦，从而实现局部摩擦系数。变化的摩擦被定制以产生将产生高局部伪磁场的应变分布。研究人员正在将应变产生的高局部伪磁场与等离子体图案相结合，以使等离子体热点与高伪场区域重叠。然后通过使用声子和朗道能级激子相互作用的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导管。悬浮的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的效率比面内声学模式低得多。应变被预测为去除面内模式到面外模式的散射，以及降低态密度，因此应变可以急剧增加已经很高的热导率。研究人员正在使用他们对悬浮石墨烯的可调应变来实验测量应变对石墨烯热导率的影响。

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.