To minimize parasitic doping effects caused by uncontrolled material adsorption, graphene is often investigated under vacuum. Here we report an entirely unexpected phenomenon occurring in vacuum systems, namely strong n-doping of graphene due to chemical species generated by common ion high-vacuum gauges. The effect --reversible upon exposing graphene to air-- is significant, as doping rates can largely exceed 10^{12} cm^{-2}/hour, depending on pressure and the relative position of the gauge and the graphene device. It is important to be aware of the phenomenon, as its basic manifestation can be mistakenly interpreted as vacuum-induced desorption of p-dopants.
Graphene quality indicators obtained by Raman spectroscopy have been correlated to the structural changes of the graphene/Germanium interface as a function of in-vacuum thermal annealing. Specifically, it is found that graphene becomes markedly defected at 650 °C. By combining scanning tunneling microscopy, x-Ray Photoelectron Spectroscopy and Near Edge x-ray Absorption Fine Structure Spectroscopy, we conclude that these defects are due to the release of H_{2} gas trapped at the graphene/Germanium interface. The H_{2} gas was produced following the transition from the as-grown hydrogen-termina…
Transmission electron microscopy (TEM) and scanning TEM (STEM) are indispensable tools for materials characterization. However, during a typical (S)TEM experiment, the sample is subject to a number of effects that can change its atomic structure. Of these, perhaps the least discussed are chemical modifications due to the non-ideal vacuum around the sample. With single-layer graphene, we show that even at relatively low pressures typical for many instruments, these processes can have a significant impact on the sample structure. For example, pore growth becomes up to two orders of magnitude fas…
The study of vacancies in graphene is a topic of growing interest. A single vacancy induces a localized stable charge of order unity interacting with other charges of the conductor through an unscreened Coulomb potential. It also breaks the symmetry between the two triangular graphene sublattices hence inducing zero energy states at the Dirac points. Here we show the fractional and pseudo-scalar nature of this vacancy charge. A continuous Dirac model is presented which relates zero modes to vacuum fractional charge and to a parity anomaly. This relation constitutes an Index theorem and is achi…
Imaging the vibrations of nanomechanical resonators means measuring their flexural mode shapes from the dependence of their frequency response on in-plane position. Applied to two-dimensional resonators, this technique provides a wealth of information on the mechanical properties of atomically-thin membranes. We present a simple and robust system to image the vibrations of few layer graphene (FLG) resonators at room temperature and in vacuum with an in-plane displacement precision of $\approx0.20$ $μ$m. It consists of a sturdy vacuum enclosure mounted on a three-axis micropositioning stage and…
Quantum Electrodynamics of graphene Landau levels in a deep-subwavelength hyperbolic phonon polariton cavity
JEIP, UAR 3573 CNRS, Collège de France, PSL Research University（法国高等科学研究所、巴黎文理研究大学）；ICFO - Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology（巴塞罗那光子科学研究院、巴塞罗那科学技术研究院）；Dipartimento di Fisica dell’Università di Pisa（比萨大学物理系）；ICREA-Institució Catalana de Recerca i Estudis Avançats（加泰罗尼亚高级研究学院）
This work reports information on the transience of hole doping in epitaxial graphene devices when nitric acid is used as an adsorbent. Under vacuum conditions, desorption processes are monitored by electrical and spectroscopic means to extract the relevant timescales from the corresponding data. It is of vital importance to understand the reversible nature of hole doping because such device processing can be a suitable alternative to large-scale, metallic gating. Most measurements are performed post-exposure at room temperature, and, for some electrical transport measurements, at 1.5 K. Vacuum…
Low Carrier Density Epitaxial Graphene Devices On SiC
在六方Si端SiC衬底上生长的单层外延石墨烯（EG）本质上是电子掺杂的（载流子密度约为10^13 cm^（-2））。我们展示了一个干净的设备制造过程中使用的贵金属保护层，并表明，与王水蚀刻的结果在p型（空穴）分子掺杂我们的非门控，无污染的EG。通过这种简单的工艺制造的器件可以达到10^10 cm^（-2）到10^11 cm^（-2）的载流子密度，迁移率约为8000 cm ^2/V/s或更高。在载流子密度n = 2.4 x 10^11 cm^（-2）、迁移率= 5200 cm ^2/V/s的中度掺杂器件中，我们观察到高度发达的量子化霍尔电阻平台，在磁场强度小于4 T时填充因子为2。掺杂浓度可以通过在Ar中的热处理恢复到更高的水平，而具有p型和n型多数载流子的器件在环境空气中倾向于向较低的载流子浓度漂移。
