Surface impurities and contamination often seriously degrade the properties of two-dimensional materials such as graphene. To remove contamination, thermal annealing is commonly used. We present a comparative analysis of annealing treatments in air and in vacuum, both ex situ and "pre-situ", where an ultra-high vacuum treatment chamber is directly connected to an aberration-corrected scanning transmission electron microscope. While ex situ treatments do remove contamination, it is challenging to obtain atomically clean surfaces after ambient transfer. However, pre-situ cleaning with radiative or laser heating appears reliable and well suited to clean graphene without undue damage to its structure.
Realization of high-quality van der Waals (vdW) heterostructures with tailored properties by stacking two-dimensional (2D) layers requires atomically clean interfaces. Because of strong adhesion between the constituent layers, the vdW forces could drive trapped contaminants together into submicron-size bubbles, which leaves large interfacial areas atomically clean. Such a phenomenon is dubbed self-cleaning mechanism in 2D systems. Here, we demonstrate the breakdown of self-cleaning mechanism for nanoscale interfacial bubbles in tiny-angle twisted bilayer graphene (TBG). In the tiny-angle TBG, …
We report on the electronic properties of GaN$(1\bar{1}00)$ and $(0001)$ surfaces after three different and subsequent device processing compatible cleaning steps: HCl etching, annealing at $400$ $^\circ$C in N$_2$ atmosphere, and O$_2$ plasma exposure. The surface electronic properties are quantified, in the dark and under ultraviolet illumination, using X-ray photoelectron spectroscopy and a Kelvin probe. We find that the cleaning steps largely affect the work function and the band bending of both GaN orientations. These modifications are attributed to the presence of different surface state…
Graphene, consisting of an inert, thermally stable material with an atomically flat, dangling bond-free surface is by essence an ideal template layer for van der Waals heteroepitaxy of two-dimensional materials such as silicene. However, depending on the synthesis method and growth parameters, graphene (Gr) substrates could exhibit, on a single sample, various surface structures, thicknesses, defects, and step heights. These structures noticeably affect the growth mode of epitaxial layers, e.g. turning the layer-by-layer growth into the Volmer-Weber growth promoted by defect-assisted nucleatio…
Effects of annealing on chemical vapor deposited graphene are investigated via a weak localization magnetoresistance measurement. Annealing at \SI{300}{\celsius} in inert gases, a common cleaning procedure for graphene devices, is found to raise the dephasing rate significantly above the rate from electron-electron interactions, which would otherwise be expected to dominate dephasing at 4 K and below. This extra dephasing is apparently induced by local magnetic moments activated by the annealing process, and depends strongly on the backgate voltage applied.
Sarin is a very lethal synthetic organophosphorated compound that inhibits the nervous system muscle control. Although not used as a chemical weapon anymore, it still worries the authorities regarding possible use by terrorists. Most of the studies about sarin are theoretical/computational due to its high lethality and are concentrated in its detection and degradation. Few studies are about air filtration from sarin gas. Here, the potential of graphene oxide-based membranes to filter air from sarin/air mixtures is investigated by classical molecular dynamics simulations. Membranes formed by on…
Manganese oxide nano flakes onto simultaneously activated defective CNTs for the creation of CNTs-graphene supported oxygen reduction composite catalysts applied in air fuel cell
非贵金属氧化物与经济炭材料的合理组合是创造高性能空气电极材料的关键。在稀H $_2$ SO $_4$中，通过取消常规水热处理或添加表面活性剂模板剂，将商品化的碳纳米管（CNTs）与MnO $_4^-$反应，制备了CNTs负载的锰氧化物复合催化剂（CMnCs）。然后将CMnC组装成装备在金属空气燃料电池中的商用空气阴极。碳纳米管表现出结构上的缺陷。在合成过程中，外部CNT壁破裂成石墨烯纳米片，但仍保持部分附着于内部碳管。石墨烯平面的这种暴露表现出额外的氧还原反应性。在碳纳米管表面均匀生长了纳米级的氧化锰薄片（γ-MnOOH）。其沉积密度与KMnO $_4$对CNTs的添加量密切相关。在相同条件下，单独的CMnCs的ORR活性超过了原料CNT和合格的市场催化剂，并且在当前的生产工艺下，CMnCs添加的空气燃料电池能够在100 mA/cm $^2$下输出约15%以上的功率。
Topological surface state under graphene for two-dimensional spintronics in air
在存在Rashba型自旋-轨道耦合的情况下，半导体异质结构中的电场可以产生允许无耗散信息传输的自旋电流。最大的Rashba效应发生在金属的电子表面态，但这些不能存在，但在真空条件下。在这里，我们揭示了Ir（111）表面态的巨大Rashba效应（α R ~ 1.5E-10 eVm）。我们证明，它的自旋分裂和自旋极化保持不受影响时，铱覆盖石墨烯。石墨烯保护又足以使自旋分裂表面状态在环境大气中存活。我们讨论了这一结果与证据的拓扑保护的表面状态。
The technique of passive daytime radiative cooling (PDRC) is used to cool an object down by simultaneously reflecting sunlight and thermally radiating heat to the cold outer space through the Earth's atmospheric window. However, for practical applications, current PDRC materials are facing unprecedented challenges such as complicated and expensive fabrication approaches and performance degradation arising from surface contamination. Here, we develop a scalable paper-based material with excellent self-cleaning and self-cooling capabilities, through air-spraying ethanolic polytetrafluoroethylene…
Two-dimensional layered and atomically thin elemental superconductors may be key ingredients in next-generation quantum technologies, if they can be stabilized and integrated into heterostructured devices under ambient conditions. However, atomically thin elemental superconductors are largely unexplored outside ultra-high vacuum due to rapid oxidation, and even 2D layered superconductors require complex encapsulation strategies to maintain material quality. Here we demonstrate environmentally stable, single-crystal, few-atom-thick superconducting gallium, 2D-Ga, produced by confinement heteroe…
The aim of this study is to assess femtosecond laser patterning of graphene in air and in vacuum for the application as source and drain electrodes in thin-film transistors (TFTs). The analysis of the laser-patterned graphene with scanning electron microscopy, atomic force microscopy and Raman spectroscopy showed that processing in vacuum leads to less debris formation and thus re-deposited carbonaceous material on the sample compared to laser processing in air. It was found that the debris reduction due to patterning in vacuum improves the TFT characteristics significantly. Hysteresis disappe…
The role of surface chemical reactivity in the stability of electronic nanodevices based on two-dimensional materials "beyond graphene" and topological insulators
Graphene is a 2D material that displays excellent electronic transport properties with prospective applications in many fields. Inducing and controlling magnetism in the graphene layer, for instance by proximity of magnetic materials, may enable its utilization in spintronic devices. This paper presents fabrication and detailed characterization of single-layer graphene formed on the surface of epitaxial FeRh thin films. The magnetic state of the FeRh surface can be controlled by temperature, magnetic field or strain due to interconnected order parameters. Characterization of graphene layers by…
The surface structure of Few-Layer Graphene (FLG) epitaxially grown on the C-face of SiC has been investigated by TM-AFM in ambient air and upon interaction with diluted aqueous solutions of bio-organic molecules (dimethyl sulfoxide, DMSO, and L-Methionine). On pristine FLG we observe nicely ordered, three-fold oriented rippled domains, with a 4.7+/-0.2 nm periodicity (small periodicity, SP) and a peak-to-valley distance in the range 0.1-0.2 nm. Upon mild interaction of the FLG surface with the molecular solution, the ripple periodicity relaxes to 6.2+/-0.2 nm (large periodicity, LP), while th…
Carbon nanostructures with zigzag edges exhibit unique properties with exciting potential applications. Such nanostructures are generally synthesized under vacuum because their zigzag edges are unstable under ambient conditions: a barrier that must be surmounted to achieve their scalable exploitation. Here, we prove the viability of chemical protection/deprotection strategies for this aim, demonstrated on labile chiral graphene nanoribbons (chGNRs). Upon hydrogenation, the chGNRs survive an exposure to air, after which they are easily converted back to their original structure via annealing. W…
Ultra-high vacuum Raman platform for in situ characterization of graphene nanoribbons
Department of Information Technology and Electrical Engineering, ETH Zürich - Swiss Federal Institute of Technology Zürich（苏黎世联邦理工学院信息与电气工程系）；Karlsruhe Institute of Technology（卡尔斯鲁厄理工学院）；Max Planck Institute for Polymer Research（马克斯·普朗克聚合物研究所）；Faculty of Chemistry and Food Chemistry & Center for Advanced Electronics Dresden, TUD - Dresden University of Technology（德累斯顿工业大学化学与食品化学学院及先进电子中心）；Max Planck Institute of Microstructure Physics（马克斯·普朗克微观结构物理研究所）
Twisted bilayer graphene (TBG), in which two monolayer graphene are stacked with an in-plane rotation angle, has recently become a hot topic due to unique electronic structures. TBG is normally produced in air by the tear-and-stack method of mechanical exfoliation and transferring graphene flakes, by which a sizable, millimeter-order area, and importantly clean interface between layers are hard to obtain. In this study, we resolved these problems by directly transferring the easy-to-exfoliate CVD-grown graphene on SiC substrate to graphene in a high vacuum without using any transfer assisting …
We have investigated the surface of lithium metal using x-ray photoemission spectroscopy and optical spectroscopic ellipsometry. Even if we prepare the surface of lithium metal rigorously by chemical cleaning and mechanical polishing inside a glovebox, both spectroscopic investigations show the existence of a few tens of nanometer-thick surface layers, consisting of lithium oxides and lithium carbonates. When lithium metal is exposed to room air (~50% moisture), in situ real-time monitoring of optical spectra indicates that the surface layer grows at a rate of approximately 24 nm/min, presumab…
Graphene is an ultrathin material, which allows us to control surface phenomena by means of field-effect gating. Among various surface phenomena, photo-oxidation is known to be a facile method to largely control the electronic structure of graphene. In this study, gate controllability of photo-oxidation of graphene is thoroughly examined using a field-effect-transistor configuration. The presence of water molecules enhances gate controllability, which can be explained using water-oxygen co-adsorption picture. In addition, the photo-oxidation reaction evolves from the edge and proceeds towards …
Photon-phonon coupling holds strong potential for sound and temperature control with light, opening new horizons in detector technology, remote sound generation and signal broadcasting. Here, we report on a novel stereoscopic ultralight converter based on a three dimensional graphene structure 3G-sponge, which exhibits very high absorption, near-to-air density, low inertia, and negligible effective heat capacity. We studied the heat and sound generation under the excitation of electromagnetic waves. 3G-sponge shows exceptional photon to heat and sound transduction efficiency over an enormous f…
We measured Drude absorption of gated CVD graphene using far-infrared transmission spectroscopy, and determined carrier scattering rate (g) as function of the varied carrier density (n). The n-dependent g(n) was obtained for a series of conditions systematically changed as (10 K, vacuum) -> (300 K, vacuum) -> (300 K, ambient pressure), which reveals that (1) at low-T, charged impurity (=A/sqrt(n)) and short-range defect (=B*sqrt(n)) are the major scattering sources which constitute the total scattering g=A/sqrt(n)+B*sqrt(n), (2) among various kinds of phonons populated at room-T, surface polar…
While the basal plane of graphene is inert, defects in it are centers of chemical activity. An attractive application of such defects is towards controlled functionalization of graphene with foreign molecules. However, the interaction of the defects with reactive environment, such as ambient, decreases the efficiency of functionalization and makes it poorly controlled. Here, we report a novel approach to generate, monitor with time resolution, and functionalize the defects $\textit{in situ}$ without ever exposing them to the ambient. The defects are generated by an energetic Argon plasma and t…
We performed Scanning Thermal Microscopy measurements on single layers of chemical-vapor-deposited -CVD- graphene supported by different substrates, namely SiO2, Al2O3 and PET using a double-scan technique to remove the contribution to the heat flux through the air and the cantilever. Then, by adopting a simple lumped-elements model, we developed a new method that allows determining, through a multi-step numerical analysis, the equivalent thermal properties of thermally conductive coatings of nanometric thickness. In this specific case we found that our CVD graphene is thermally equivalent, fo…
