Strain-engineered graphene: growth, modification and electronic properties

我们最近已经证明，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，可以在固体表面生长石墨烯的结晶层。该系统于2013年使用EPSRC Graphene Engineering Call提供的设备资金购买，并于2014年成功安装，此后在世界上首次用于展示拉伸的石墨烯可以生长。据认为，拉伸是由于生长过程中使用的高温--当石墨烯在生长后冷却时，它试图收缩，但无法做到，因为它被钉在生长表面的几个锚定点上。应变的存在完全出乎人们的意料，并导致了许多新的性质，例如，石墨烯可以被纳米级的机械触针刺穿，然后恢复到松弛的形式--有点像爆裂的气球。此外，众所周知，拉伸石墨烯可以强烈地修饰其电学性质，使其更适合于技术应用，如制造晶体管。在这项提案中，我们请求支持，以在我们最初的成功基础上再接再厉，以便我们能够探索这种新型石墨烯的前景，更好地了解它是如何生长的，研究它的新物理性质，并尝试利用应变石墨烯来制造简单的原型设备。从历史上看，2004年Geim、Novoselov和他的同事发现石墨烯及其非凡的电子特性，为科学家和工程师提供了一种革命性的电子学和光电子学材料体系。石墨烯具有许多非凡的特性--它非常灵活，非常坚固，是一种优秀的电导体和热导体。然而，目前的石墨烯研究存在一些局限性。首先，它不能直接用于许多电子应用中，因为电流在石墨烯中不能被切断，这是制造晶体管的基本要求，晶体管是现代电子产品的核心部件。其原因可能可以追溯到石墨烯中电子的量子力学性质，特别是对于所有能量，都有可用的量子力学状态供电子占据-换句话说，材料缺乏半导体中存在的能隙。自2004年以来，世界各地一直在努力开发控制石墨烯电子性质的方法，特别是引入禁带，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非常理想的特性。实现这一目标最有希望的途径之一是通过引入在分子束外延生长的石墨烯中自发产生的菌株。此外，原始石墨烯工作的第二个缺点是依赖于剥离，即从材料块中剥离石墨烯。尽管这在研究石墨烯的基本性质方面取得了非凡的成功，但剥离在石墨烯的技术开发方面存在重大限制。特别是，希望在大面积上形成层。诺丁汉集团采用的方法是使用分子束外延生长石墨烯，利用一种在工业上广泛使用的技术来生长其他材料。然而，在诺丁汉小组的工作之前，通过分子束外延生长石墨烯的尝试基本上没有成功。分子束外延是通过在合适的表面燃烧碳原子来实现生长的。我们的系统在世界范围内是独一无二的，它允许在比以前使用的温度高得多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.