Centre for Graphene Science, College of Engineering, Mathematics and Physical Sciences , University of Exeter , Exeter EX4 4QF , United Kingdom.
University of Duhok , Duhok 42001 Kurdistan Region , Iraq.
Nano Lett. 2018 Dec 12;18(12):7919-7926. doi: 10.1021/acs.nanolett.8b03854. Epub 2018 Nov 30.
The observation of novel physical phenomena such as Hofstadter's butterfly, topological currents, and unconventional superconductivity in graphene has been enabled by the replacement of SiO with hexagonal boron nitride (hBN) as a substrate and by the ability to form superlattices in graphene/hBN heterostructures. These devices are commonly made by etching the graphene into a Hall-bar shape with metal contacts. The deposition of metal electrodes, the design, and specific configuration of contacts can have profound effects on the electronic properties of the devices possibly even affecting the alignment of graphene/hBN superlattices. In this work, we probe the strain configuration of graphene on hBN in contact with two types of metal contacts, two-dimensional (2D) top-contacts and one-dimensional edge-contacts. We show that top-contacts induce strain in the graphene layer along two opposing leads, leading to a complex strain pattern across the device channel. Edge-contacts, on the contrary, do not show such strain pattern. A finite-elements modeling simulation is used to confirm that the observed strain pattern is generated by the mechanical action of the metal contacts clamped to the graphene. Thermal annealing is shown to reduce the overall doping while increasing the overall strain, indicating an increased interaction between graphene and hBN. Surprisingly, we find that the two contact configurations lead to different twist-angles in graphene/hBN superlattices, which converge to the same value after thermal annealing. This observation confirms the self-locking mechanism of graphene/hBN superlattices also in the presence of strain gradients. Our experiments may have profound implications in the development of future electronic devices based on heterostructures and provide a new mechanism to induce complex strain patterns in 2D materials.
新型物理现象的观察，如霍夫斯塔特蝴蝶、拓扑电流和非常规超导电性，在石墨烯中已经通过用六方氮化硼（hBN）替代 SiO2 作为衬底以及在石墨烯/hBN 异质结构中形成超晶格的能力得以实现。这些器件通常通过用金属接触将石墨烯刻蚀成霍尔条形状来制造。金属电极的沉积、设计和特定的接触配置对器件的电子性质有深远的影响，甚至可能影响石墨烯/hBN 超晶格的排列。在这项工作中，我们研究了与两种类型的金属接触（二维（2D）顶接触和一维边缘接触）接触的 hBN 上石墨烯的应变配置。我们表明，顶接触会在石墨烯层中沿两个相对的引线引起应变，导致器件通道中出现复杂的应变模式。相比之下，边缘接触不会显示这种应变模式。使用有限元建模模拟来证实观察到的应变模式是由夹在石墨烯上的金属接触的机械作用产生的。热退火显示出整体掺杂减少而整体应变增加，这表明石墨烯和 hBN 之间的相互作用增加。令人惊讶的是，我们发现两种接触配置导致石墨烯/hBN 超晶格的扭转角不同，但在热退火后会收敛到相同的值。这一观察结果证实了即使存在应变梯度，石墨烯/hBN 超晶格也存在自锁定机制。我们的实验可能对基于异质结构的未来电子器件的发展具有深远的意义，并为在 2D 材料中诱导复杂应变模式提供了一种新的机制。
