Strain-engineered graphene: growth, modification and electronic properties

我们最近证明了石墨烯晶体层可以在固体表面生长，使用一种新安装的基于分子束外延（MBE）技术的高温生长系统。该系统于2013年使用EPSRC石墨烯工程项目的设备资金购买，并于2014年成功安装，此后在世界上首次用于证明拉伸石墨烯可以生长。人们认为拉伸是由生长过程中使用的高温引起的——石墨烯在生长后冷却，它试图收缩，但由于它被固定在生长表面的几个固定点上，因此无法收缩。应变的存在是完全出乎意料的，并导致了许多新颖的特性，例如石墨烯可以被纳米级机械触针刺穿，并弹回到松弛的形式，就像一个破裂的气球。此外，众所周知，拉伸石墨烯可以强烈地改变其电性能，使其更适合于技术应用，如晶体管的制造。在这个提案中，我们请求支持，以我们最初的成功为基础，这样我们就可以探索这种新型石墨烯的前景，更好地了解它是如何生长的，研究它的新物理特性，并尝试利用应变石墨烯制造简单的原型设备。从历史上看，2004年Geim、Novoselov及其同事发现石墨烯及其卓越的电子特性，为科学家和工程师提供了一种革命性的电子和光电子材料系统。石墨烯具有许多显著的特性——它具有高度的柔韧性，非常强，是一种优秀的导电体和导热体。然而，目前石墨烯的研究也存在一些局限性。首先，石墨烯不能直接用于许多电子应用，因为石墨烯不能切断电流，而这是制造晶体管（现代电子产品的核心部件）的基本要求。其原因可以追溯到石墨烯中电子的量子力学特性，特别是对于所有能量，电子都可以占据可用的量子力学状态——换句话说，这种材料缺乏半导体中存在的能隙。自2004年以来，全世界都在努力开发控制石墨烯电子特性的方法，特别关注引入带隙，以提供一种半导体模拟材料，其中保留了石墨烯的许多其他高度理想的品质。实现这一目标的最有希望的途径之一是通过引入在MBE生长的石墨烯中自发发生的应变。此外，原始石墨烯工作的第二个缺点是依赖于剥离，或从材料块上剥离石墨烯层。尽管这在研究石墨烯的基本性质方面取得了巨大的成功，但在石墨烯的技术开发中，剥离有很大的局限性。特别是，希望在大面积上形成层。诺丁汉研究小组采用的方法是利用MBE生长石墨烯，这种方法利用了一种广泛用于工业生产其他材料的技术。然而，在诺丁汉小组的工作之前，通过MBE生长石墨烯的尝试在很大程度上是不成功的，MBE是通过在合适的表面上发射碳原子来实现生长的。我们的系统是世界上独一无二的，可以在比以前使用的更高的温度下生长石墨烯，我们已经证明，使用这种技术可以生长高质量的石墨烯，并为新的科学和技术方向提供了令人兴奋的机会。

We have recently demonstrated that crystalline layers of graphene can be grown on a solid surface using a newly installed high temperature growth system based on a technique called molecular beam epitaxy (MBE). This system was purchased in 2013 using equipment funding from the EPSRC Graphene Engineering Call and was successfully installed in 2014 and has since been used to demonstrate, for the first time in the world, that graphene which is strained, i.e. stretched, can be grown. It is thought that the stretching arises from the high temperatures used during growth - as the graphene cools after growth it tries to contract but cannot do so since it is pinned at several anchoring points on the surface on which it grows. The presence of strain was completely unexpected and results in many novel properties, for example the graphene can be punctured by a nanoscale mechanical stylus and snap back into a relaxed form - rather like a burst balloon. In addition, it is known that stretching graphene can modify strongly its electrical properties making it more compatible with technological applications such as the fabrication of transistors. In this proposal we are requesting support to build on our initial success so that we can explore the promise of this new type of graphene, to gain a much better understanding of how it grows, to investigate its novel physical properties and also to try and exploit strained graphene to make simple prototype devices. Historically, the discovery of graphene and its remarkable electronic properties by Geim, Novoselov and colleagues in 2004 has provided scientists and engineers with a material system for revolutionising electronics and opto-electronics. Graphene has many remarkable properties - it is highly flexible, very strong and is an excellent electrical and thermal conductor. However, there are some limitations of current graphene research. Firstly, it cannot be used directly in many electronic applications because the flow of electrical current cannot be switched off in graphene, an essential requirement for the fabrication of a transistor, the central component of modern electronics. The reason for this may be traced back to the quantum mechanical properties of electrons within graphene, in particular the fact that for all energies there are available quantum mechanical states which electrons can occupy - in other words the material lacks an energy gap which is present in semiconductors. Since 2004 there has been an enormous effort worldwide to develop methods to control the electronic properties of graphene with a particular focus on introducing a band-gap to provide a semiconducting analogue material in which many of the other, highly desirable qualities of graphene, are retained. One of the most promising routes towards this goal is through the introduction of strain which occurs spontaneously in the MBE grown graphene.In addition, a second drawback of the original graphene work was the reliance on exfoliation, or peeling off layers of graphene from a block of material. Although this has been extraordinarily successful in terms of investigating the fundamental properties of graphene, exfoliation has significant limitations in the technological exploitation of graphene. In particular, it is desirable to form layers over large areas. The approach adopted by the Nottingham group, to use MBE to grow graphene, makes use of a technique which is used widely in industry to grow other materials. However, before the work of the Nottingham group, attempts to grow graphene by MBE, in which growth is achieved by firing carbon atoms at a suitable surface, had been largely unsuccessful. Our system, which is unique worldwide, allows growth of graphene at much higher temperatures than have been used previously and we have already demonstrated that growth of high quality graphene is possible using this technique and offers exciting opportunities for new scientific and technological directions.