Scanning probe microscopy of the quantum Hall effect and charge pumping in graphene for meterological applications

人们在铅笔核心发现的块状石墨是由数百层碳原子相互堆叠而成的。正是这种简单的原子结构使石墨在轻轻摩擦另一个表面时很容易沉积，因为这些层可以自由地相互滑动。最近发现，这一过程甚至会产生单原子层，即只有一个原子厚度的微小碳片。这种扁平的碳同素异形体被称为石墨烯，自从发现以来就引起了巨大的兴奋。它展示了与广泛的设备应用和基础研究问题相关的大量新的电子、机械和光学特性。电子界对石墨烯特别感兴趣，因为它结合了高流动性、高透明度和携带非常高的电流密度的能力。最近，英国的计量标准机构，国家物理实验室(NPL)表明，石墨烯可以在低温和高磁场下用作电阻标准，因为它在霍尔电阻中显示出非常精确的平台的量子霍尔效应。然而，石墨烯还不能与目前用于校准电阻的半导体材料竞争，这可能是由于无序程度的原因。这个项目的第一个目标是使用低温扫描探针显微镜和化学功能化来表征并减少这些层中的无序，从而提高量化的精度。此外，我们的表征结果应该有助于那些生长石墨烯层的人开发生产更高质量材料的技术。石墨烯的导电能力无法开启和关闭，除非它的图案宽度小于5纳米，因此目前它不适合数字电子中的晶体管等应用。然而，双层石墨烯由两层组成，一层在另一层之上，可以使用垂直电场使其绝缘。我们项目的第二部分旨在利用这种行为来控制电子通过石墨烯时的路径。特别是，我们的目标是引导电子沿着小的传导路径进入电子陷阱，即所谓的量子点，在那里它们是局域化的。然后，使用高频，我们将为通过圆点的单电子计时，一次一个。其效果是产生的电流等于电子上的电荷乘以我们对它们通过点的时钟频率的乘积。这样就有可能产生定义明确的电流，作为校准科学仪器和非常精确地测量自然基本常量的标准。此外，因为我们是使用金属电极的电场来定义量子点，所以限制势应该非常平滑，载流子在这个势上的散射应该是镜面反射的。因此，电子将在没有背向散射的情况下通过窄通道。这种行为还没有在石墨烯中看到，可能是因为到目前为止设计的设备具有粗糙的边缘和大量具有复杂散射特性的无序。通过使用双层门控器件，我们应该能够消除这种散射，并增加石墨烯量子点的自旋寿命，从而开辟了将铅笔作为量子计算机基础的诱人前景。

The bulk graphite which one finds at the core of a pencil is composed of many hundreds of layers of carbon atoms stacked on top of one another. It is this simple atomic architecture which makes graphite so easy to deposit when gently rubbed against another surface because the layers are free to slide over one another. It was discovered recently that this process even produces single atomic layers, i.e., tiny flakes of carbon which are only one atom thick. This flat allotrope of carbon is called graphene and has created enormous excitement since its discovery. It exhibits a remarkable number of new electronic, mechanical, and optical properties relevant to a wide range of device applications and fundamental research questions. The electronics community is particularly attracted to graphene because it combines high mobility, high transparency, and the ability to carry very high current densities. Recently the UK's meterological standards agency, the National Physical Laboratory (NPL), has shown that graphene can be used at low temperatures and at high magnetic fields as resistance standard as it shows the quantum Hall effect with very accurate plateau in the Hall resistance. Graphene is not yet competitive with the semiconducting material currently used to calibrate resistors, however, probably due to the level of disorder. The first objective of this project is to use low temperature scanning probe microscopy and chemical functionalisation to characterise and then reduce the disorder in these layers, thus improving the precision of the quantisation. In addition, the results of our characterisation should help those who grow the graphene layers to develop techniques for producing better quality material. Graphene's ability to conduct electricity cannot be switched on and off unless it is patterned so as to have widths less than 5 nm, so at the moment it is unsuitable for applications such as transistors in digital electronics. However, bilayer graphene, which consists of two layers one above the other, can be made insulating using a vertical electric field. The second part of our project aims to exploit this behaviour to control the path taken by electrons as they travel through graphene. In particular our aim is to channel electrons down small conducting pathways and into electron traps, known as quantum dots , where they are localised. Then, using high frequencies we will clock single electrons through the dot one at a time. The effect is to produce a current that is equal to the charge on the electron times the frequency that we clock them through the dot. This opens up the possibility of producing a well defined current that could be used as a standard for calibrating scientific instruments and for making very precise measurements of the fundamental constants of nature. In addition, because we are defining our quantum dots using the electric field from metal electrodes, the confinement potential should be very smooth and the scattering of the charge carriers off this potential should be specular. As a result, electrons will go through narrow channels without back scattering. This behaviour has not been seen in graphene yet, probably because devices designed so far have rough edges and a great deal of disorder with complex scattering properties. By using the bilayer gated devices we should be able to get rid of this scattering and increase the spin lifetime in graphene quantum dots, thereby opening up the tantalising prospect of using pencil lead as the basis for a quantum computer.