Showing papers by "Irina V. Grigorieva published in 2016"

Negative local resistance caused by viscous electron backflow in graphene

Abstract: Graphene hosts a unique electron system in which electron-phonon scattering is extremely weak but electron-electron collisions are sufficiently frequent to provide local equilibrium above the temperature of liquid nitrogen. Under these conditions, electrons can behave as a viscous liquid and exhibit hydrodynamic phenomena similar to classical liquids. Here we report strong evidence for this transport regime. We found that doped graphene exhibits an anomalous (negative) voltage drop near current-injection contacts, which is attributed to the formation of submicrometer-size whirlpools in the electron flow. The viscosity of graphene’s electron liquid is found to be ~0.1 square meters per second, an order of magnitude higher than that of honey, in agreement with many-body theory. Our work demonstrates the possibility of studying electron hydrodynamics using high-quality graphene.

Molecular transport through capillaries made with atomic-scale precision

Abstract: Nanometre-scale graphitic capillaries with atomically flat walls are engineered and studied, revealing unexpectedly fast transport of liquid water through channels that accommodate only a few layers of water. Artificial nanometre-sized capillaries have enabled new research and led to the emergence of nanofluidics, but surface roughness in particular makes it very challenging to exactly control their dimensions. Andre Geim and colleagues now show that van der Waals assembly can produce narrow and smooth capillaries that have atomically flat top and bottom graphite sheets, separated by spacers made from a precisely controlled number of graphene layers. Water transport through the channels, which range in height from a single atomic plane to dozens of them, is unexpectedly fast and speeds up further in channels that accommodate only a few layers of water. The fabrication method is expected to give access to a wide range of capillaries with atomically precise sizes, and with permeation properties that are tunable by the choice of two-dimensional material used for creating the channel walls. Nanometre-scale pores and capillaries have long been studied because of their importance in many natural phenomena and their use in numerous applications1. A more recent development is the ability to fabricate artificial capillaries with nanometre dimensions, which has enabled new research on molecular transport and led to the emergence of nanofluidics2,3,4. But surface roughness in particular makes it challenging to produce capillaries with precisely controlled dimensions at this spatial scale. Here we report the fabrication of narrow and smooth capillaries through van der Waals assembly5, with atomically flat sheets at the top and bottom separated by spacers made of two-dimensional crystals6 with a precisely controlled number of layers. We use graphene and its multilayers as archetypal two-dimensional materials to demonstrate this technology, which produces structures that can be viewed as if individual atomic planes had been removed from a bulk crystal to leave behind flat voids of a height chosen with atomic-scale precision. Water transport through the channels, ranging in height from one to several dozen atomic planes, is characterized by unexpectedly fast flow (up to 1 metre per second) that we attribute to high capillary pressures (about 1,000 bar) and large slip lengths. For channels that accommodate only a few layers of water, the flow exhibits a marked enhancement that we associate with an increased structural order in nanoconfined water. Our work opens up an avenue to making capillaries and cavities with sizes tunable to angstrom precision, and with permeation properties further controlled through a wide choice of atomically flat materials available for channel walls.

Universal shape and pressure inside bubbles appearing in van der Waals heterostructures.

Abstract: The interface between vertically stacked 2D materials can host contaminants trapped within bubbles. Here, the authors show that such nano-bubbles can be used as a platform to explore the van der Waals pressure and elasticity in atomically thin films, in a previously inaccessible confined environment.

Sieving hydrogen isotopes through two-dimensional crystals.

Abstract: One-atom-thick crystals are impermeable to atoms and molecules, but hydrogen ions (thermal protons) penetrate through them. We show that monolayers of graphene and boron nitride can be used to separate hydrogen ion isotopes. Using electrical measurements and mass spectrometry, we found that deuterons permeate through these crystals much slower than protons, resulting in a separation factor of ≈10 at room temperature. The isotope effect is attributed to a difference of ≈60 milli–electron volts between zero-point energies of incident protons and deuterons, which translates into the equivalent difference in the activation barriers posed by two-dimensional crystals. In addition to providing insight into the proton transport mechanism, the demonstrated approach offers a competitive and scalable way for hydrogen isotope enrichment.

Commensurability Effects in Viscosity of Nanoconfined Water

Abstract: The rate of water flow through hydrophobic nanocapillaries is greatly enhanced as compared to that expected from macroscopic hydrodynamics. This phenomenon is usually described in terms of a relatively large slip length, which is in turn defined by such microscopic properties as the friction between water and capillary surfaces and the viscosity of water. We show that the viscosity of water and, therefore, its flow rate are profoundly affected by the layered structure of confined water if the capillary size becomes less than 2 nm. To this end, we study the structure and dynamics of water confined between two parallel graphene layers using equilibrium molecular dynamics simulations. We find that the shear viscosity is not only greatly enhanced for subnanometer capillaries, but also exhibits large oscillations that originate from commensurability between the capillary size and the size of water molecules. Such oscillating behavior of viscosity and, consequently, the slip length should be taken into account in designing and studying graphene-based and similar membranes for desalination and filtration.

Superconductivity in Potassium-Doped Metallic Polymorphs of MoS2

Abstract: Superconducting layered transition metal dichalcogenides (TMDs) stand out among other superconductors due to the tunable nature of the superconducting transition, coexistence with other collective electronic excitations (charge density waves), and strong intrinsic spin–orbit coupling. Molybdenum disulfide (MoS2) is the most studied representative of this family of materials, especially since the recent demonstration of the possibility to tune its critical temperature, Tc, by electric-field doping. However, just one of its polymorphs, band-insulator 2H-MoS2, has so far been explored for its potential to host superconductivity. We have investigated the possibility to induce superconductivity in metallic polytypes, 1T- and 1T′-MoS2, by potassium (K) intercalation. We demonstrate that at doping levels significantly higher than that required to induce superconductivity in 2H-MoS2, both 1T and 1T′ phases become superconducting with Tc = 2.8 and 4.6 K, respectively. Unusually, K intercalation in this case is res...

Superconductivity in Ca-doped graphene laminates.

Abstract: Despite graphene’s long list of exceptional electronic properties and many theoretical predictions regarding the possibility of superconductivity in graphene, its direct and unambiguous experimental observation has not been achieved. We searched for superconductivity in weakly interacting, metal decorated graphene crystals assembled into so-called graphene laminates, consisting of well separated and electronically decoupled graphene crystallites. We report robust superconductivity in all Ca-doped graphene laminates. They become superconducting at temperatures (Tc) between ≈4 and ≈6 K, with Tc’s strongly dependent on the confinement of the Ca layer and the induced charge carrier concentration in graphene. We find that Ca is the only dopant that induces superconductivity in graphene laminates above 1.8 K among several dopants used in our experiments, such as potassium, caesium and lithium. By revealing the tunability of the superconducting response through doping and confinement of the metal layer, our work shows that achieving superconductivity in free-standing, metal decorated monolayer graphene is conditional on an optimum confinement of the metal layer and sufficient doping, thereby bringing its experimental realization within grasp.

Raman spectroscopy of highly pressurized graphene membranes

Abstract: Raman spectroscopy is an ideal tool for the characterization of strained graphene. Biaxial strain, in particular, allows for more reliable calculation of the Gruneisen parameters than uniaxial strain. However, the application of biaxial strain is rather difficult to achieve experimentally, so all previous studies reported on graphene subjected to relatively small biaxial strains (0.1%–1%), in contrast to uniaxial strain above 10%. Here, we report a simple fabrication technique to produce pressurized and stable graphene membranes that can support differential pressures up to 14 bar, corresponding to a reversible strain up to ∼2%. We find that the Gruneisen parameters remain constant even for the largest strains achieved, in agreement with the theoretical predictions. However, for strains above 1%, a distinctive broadening of both the G and 2D peaks was observed for biaxial strain. We attribute this to the nanoscale variations of strain in the membrane within an area comparable with the laser spot size.

Magnetoresistance of vertical Co-graphene-NiFe junctions controlled by charge transfer and proximity-induced spin splitting in graphene

Abstract: Graphene is hailed as an ideal material for spintronics due to weak intrinsic spin-orbit interaction that facilitates lateral spin transport and tunability of its electronic properties, including a possibility to induce magnetism in graphene. Another promising application of graphene is related to its use as a spacer separating ferromagnetic metals (FMs) in vertical magnetoresistive devices, the most prominent class of spintronic devices widely used as magnetic sensors. In particular, few-layer graphene was predicted to act as a perfect spin filter. Here we show that the role of graphene in such devices (at least in the absence of epitaxial alignment between graphene and the FMs) is different and determined by proximity-induced spin splitting and charge transfer with adjacent ferromagnetic metals, making graphene a weak FM electrode rather than a spin filter. To this end, we report observations of magnetoresistance (MR) in vertical Co-graphene-NiFe junctions with 1 to 4 graphene layers separating the ferromagnets, and demonstrate that the dependence of the MR sign on the number of layers and its inversion at relatively small bias voltages is consistent with spin transport between weakly doped and differently spin-polarized layers of graphene. The proposed interpretation is supported by the observation of an MR sign reversal in biased Co-graphene-hBN-NiFe devices and by comprehensive structural characterization. Our results suggest a new architecture for vertical devices with electrically controlled MR.

Fluorination Clusters on Graphene Resolved by Conductive AFM

Abstract: Partially fluorinated graphene samples were studied by conductive AFM. The conductivity is found to be strongly non-uniform with low conductivity patches of about 10 nm. We interpret this observation as formation of low conducting clusters of fluorinated graphene separated by areas of pristine material. This is the first direct observation of clustering of covalently bonded impurities on graphene surface.