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Register a new userSemi-permeability of graphene nanodrums in sucrose solution
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Semi-permeable membranes are important elements in water purification and energy generation applications, for which the atomic thickness and strength of graphene can enhance efficiency and permeation rate while maintaining good selectivity. Here, we show that an osmotic pressure difference forms across a suspended graphene membrane as a response to a sucrose concentration difference, providing evidence for its semi-permeability. This osmotic pressure difference is detected via the deflection of the graphene membrane that is measured by atomic force microscopy. Using this technique, the time dependence of this deflection allows us to measure the water permeation rate of a single 3.4 $mu$m diameter graphene membrane. Its value is close to the expected value of a single nanopore in graphene. The method thus allows one to experimentally study the semi-permeability of graphene membranes at the microscale when the leakage rate is miniscule. It can therefore find use in the development of graphene membranes for filtration, and can enable sensors that measure the concentration and composition of solutions.
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Membranes of suspended two-dimensional materials show a large variability in mechanical properties, in part due to static and dynamic wrinkles. As a consequence, experiments typically show a multitude of nanomechanical resonance peaks, which makes an unambiguous identification of the vibrational modes difficult. Here, we probe the motion of graphene nanodrum resonators with spatial resolution using a phase-sensitive interferometer. By simultaneously visualizing the local phase and amplitude of the driven motion, we show that unexplained spectral features represent split degenerate modes. When taking these into account, the resonance frequencies up to the eighth vibrational mode agree with theory. The corresponding displacement profiles however, are remarkably different from theory, as small imperfections increasingly deform the nodal lines for the higher modes. The Brownian motion, which is used to calibrate the local displacement, exhibits a similar mode pattern. The experiments clarify the complicated dynamic behaviour of suspended two-dimensional materials, which is crucial for reproducible fabrication and applications.
Graphene, a thinnest material in the world, can form moire structures on different substrates, including graphite, h-BN, or metal surfaces. In such systems the structure of graphene, i. e. its corrugation, as well as its electronic and elastic properties are defined by the combination of the system geometry and local interaction strength at the interface. The corrugation in such structures on metals is heavily extracted from diffraction or local probe microscopy experiments and can be obtained only via comparison with theoretical data, which usually simulate the experimental findings. Here we show that graphene corrugation on metals can be measured directly employing atomic force spectroscopy and obtained value coincides with state-of-the-art theoretical results. We also address the elastic reaction of the formed graphene nanodoms on the indentation process by the scanning tip that is important for the modeling and fabrication of graphene-based nanoresonators on the nanoscale.
In this paper, we study the effects of both the amount of open cell walls and their aperture sizes on solid foams permeability. FEM flow simulations are performed at both pore and macroscopic scales. For foams with fully interconnected pores, we obtain a robust power-law relationship between permeability and membrane aperture size. This result owns to the local pressure drop mechanism through the membrane aperture as described by Sampson for fluid flow through a circular orifice in a thin plate. Based on this local law, pore-network simulation of simple flow is used and is shown to reproduce successfully FEM results. This low computational cost method allowed to study in detail the effects of the open wall amount on percolation, percolating porosity and permeability. A model of effective permeability is proposed and shows ability to reproduce the results of network simulations. Finally, an experimental validation of the theoretical model on well controlled solid foam is presented.
The observation of micron size spin relaxation makes graphene a promising material for applications in spintronics requiring long distance spin communication. However, spin dependent scatterings at the contact/graphene interfaces affect the spin injection efficiencies and hence prevent the material from achieving its full potential. While this major issue could be eliminated by nondestructive direct optical spin injection schemes, graphenes intrinsically low spin orbit coupling strength and optical absorption place an obstacle in their realization. We overcome this challenge by creating sharp artificial interfaces between graphene and WSe2 monolayers. Application of a circularly polarized light activates the spin polarized charge carriers in the WSe2 layer due to its spin coupled valley selective absorption. These carriers diffuse into the superjacent graphene layer, transport over a 3.5 um distance, and are finally detected electrically using BN/Co contacts in a non local geometry. Polarization dependent measurements confirm the spin origin of the non local signal.
Two experimental studies reported the spontaneous formation of amorphous and crystalline structures of C60 intercalated between graphene and a substrate. They observed interesting phenomena ranging from reaction between C60 molecules under graphene to graphene sagging between the molecules and control of strain in graphene. Motivated by these works, we performed fully atomistic reactive molecular dynamics simulations to study the formation and thermal stability of graphene wrinkles as well as graphene attachment to and detachment from the substrate when graphene is laid over a previously distributed array of C60 molecules on a copper substrate at different values of temperature. As graphene compresses the C60 molecules against the substrate, and graphene attachment to the substrate between C60s (C60s stands for plural of C60) depends on the height of graphene wrinkles, configurations with both frozen and non-frozen C60s structures were investigated in order to verify the experimental result of stable sagged graphene when the distance between C60s is about 4 nm and height of graphene wrinkles is about 0.8 nm. Below the distance of 4 nm between C60s, graphene becomes locally suspended and less strained. We show that this happens when C60s are allowed to deform under the compressive action of graphene. If we keep the C60s frozen, spontaneous blanketing of graphene happens only when the distance between them are equal or above 7 nm. Both above results for the existence of stable sagged graphene for C60 distances of 4 or 7 nm are shown to agree with a mechanical model relating the rigidity of graphene to the energy of graphene-substrate adhesion. In particular, this study might help the development of 2D confined nanoreactors that are considered in literature to be the next advanced step on chemical reactions.
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