Showing papers by "K. S. Novoselov published in 2013"

Strong light-matter interactions in heterostructures of atomically thin films.

Abstract: The isolation of various two-dimensional (2D) materials, and the possibility to combine them in vertical stacks, has created a new paradigm in materials science: heterostructures based on 2D crystals. Such a concept has already proven fruitful for a number of electronic applications in the area of ultrathin and flexible devices. Here, we expand the range of such structures to photoactive ones by using semiconducting transition metal dichalcogenides (TMDCs)/graphene stacks. Van Hove singularities in the electronic density of states of TMDC guarantees enhanced light-matter interactions, leading to enhanced photon absorption and electron-hole creation (which are collected in transparent graphene electrodes). This allows development of extremely efficient flexible photovoltaic devices with photoresponsivity above 0.1 ampere per watt (corresponding to an external quantum efficiency of above 30%).

Cloning of Dirac fermions in graphene superlattices

Abstract: Placing graphene on a boron nitride substrate and accurately aligning their crystallographic axes, to form a moire superlattice, leads to profound changes in the graphene’s electronic spectrum. In 1976 Douglas Hofstadter predicted that electrons in a lattice subjected to electrostatic and magnetic fields would show a characteristic energy spectrum determined by the interplay between two quantizing fields. The expected spectrum would feature a repeating butterfly-shaped motif, known as Hofstadter's butterfly. The experimental realization of the phenomenon has proved difficult because of the problem of producing a sufficiently disorder-free superlattice where the length scales for magnetic and electric field can truly compete with each other. Now that goal has been achieved — twice. Two groups working independently produced superlattices by placing ultraclean graphene (Ponomarenko et al.) or bilayer graphene (Kim et al.) on a hexagonal boron nitride substrate and crystallographically aligning the films at a precise angle to produce moire pattern superstructures. Electronic transport measurements on the moire superlattices provide clear evidence for Hofstadter's spectrum. The demonstrated experimental access to a fractal spectrum offers opportunities for the study of complex chaotic effects in a tunable quantum system. Superlattices have attracted great interest because their use may make it possible to modify the spectra of two-dimensional electron systems and, ultimately, create materials with tailored electronic properties1,2,3,4,5,6,7,8. In previous studies (see, for example, refs 1, 2, 3, 4, 5, 6, 7, 8), it proved difficult to realize superlattices with short periodicities and weak disorder, and most of their observed features could be explained in terms of cyclotron orbits commensurate with the superlattice1,2,3,4. Evidence for the formation of superlattice minibands (forming a fractal spectrum known as Hofstadter’s butterfly9) has been limited to the observation of new low-field oscillations5 and an internal structure within Landau levels6,7,8. Here we report transport properties of graphene placed on a boron nitride substrate and accurately aligned along its crystallographic directions. The substrate’s moire potential10,11,12 acts as a superlattice and leads to profound changes in the graphene’s electronic spectrum. Second-generation Dirac points13,14,15,16,17,18,19,20,21,22 appear as pronounced peaks in resistivity, accompanied by reversal of the Hall effect. The latter indicates that the effective sign of the charge carriers changes within graphene’s conduction and valence bands. Strong magnetic fields lead to Zak-type cloning23 of the third generation of Dirac points, which are observed as numerous neutrality points in fields where a unit fraction of the flux quantum pierces the superlattice unit cell. Graphene superlattices such as this one provide a way of studying the rich physics expected in incommensurable quantum systems7,8,9,22,23,24 and illustrate the possibility of controllably modifying the electronic spectra of two-dimensional atomic crystals by varying their crystallographic alignment within van der Waals heterostuctures25.

Resonant tunnelling and negative differential conductance in graphene transistors

Abstract: The chemical stability of graphene and other free-standing two-dimensional crystals means that they can be stacked in different combinations to produce a new class of functional materials, designed for specific device applications. Here we report resonant tunnelling of Dirac fermions through a boron nitride barrier, a few atomic layers thick, sandwiched between two graphene electrodes. The resonance occurs when the electronic spectra of the two electrodes are aligned. The resulting negative differential conductance in the device characteristics persists up to room temperature and is gate voltage-tuneable due to graphene’s unique Dirac-like spectrum. Although conventional resonant tunnelling devices comprising a quantum well sandwiched between two tunnel barriers are tens of nanometres thick, the tunnelling carriers in our devices cross only a few atomic layers, offering the prospect of ultra-fast transit times. This feature, combined with the multi-valued form of the device characteristics, has potential for applications in high-frequency and logic devices.

Singular phase nano-optics in plasmonic metamaterials for label-free single-molecule detection

Abstract: The non-trivial behaviour of phase is crucial for many important physical phenomena, such as, for example, the Aharonov-Bohm effect and the Berry phase. By manipulating the phase of light one can create 'twisted' photons, vortex knots and dislocations which has led to the emergence of the field of singular optics relying on abrupt phase changes. Here we demonstrate the feasibility of singular visible-light nano-optics which exploits the benefits of both plasmonic field enhancement and the peculiarities of the phase of light. We show that properly designed plasmonic metamaterials exhibit topologically protected zero reflection yielding to sharp phase changes nearby, which can be employed to radically improve the sensitivity of detectors based on plasmon resonances. By using reversible hydrogenation of graphene and binding of streptavidin-biotin, we demonstrate an areal mass sensitivity at a level of fg mm(-2) and detection of individual biomolecules, respectively. Our proof-of-concept results offer a route towards simple and scalable single-molecule label-free biosensing technologies.

Ferromagnetic two-dimensional crystals: Single layers of k2cuf4

Abstract: The successful isolation of graphene ten years ago has evoked a rapidly growing scientific interest in the nature of two-dimensional (2D) crystals. A number of different 2D crystals has been produced since then, with properties ranging from superconductivity to insulating behavior. Here, we predict the possibility for realizing ferromagnetic 2D crystals by exfoliating atomically thin films of K2CuF4. From a first-principles theoretical analysis, we find that single layers of K2CuF4 form exactly 2D Kosterlitz-Thouless systems. The 2D crystal can form a free-standing membrane, and exhibits an experimentally accessible transition temperature and robust magnetic moments of 1 Bohr magneton per formula unit. 2D K2CuF4 unites ferromagnetic and insulating properties and is a demonstration of principles for nanoelectronics such as novel 2D-based heterostructures.

Quantum capacitance measurements of electron-hole asymmetry and next-nearest-neighbor hopping in graphene

Abstract: the highest theoretical values. Here, we report dedicated measurements of the density of states in graphene by using high-quality capacitance devices. The density of states exhibits a pronounced electron-hole asymmetry that increases linearly with energy. This behavior yields t � ≈− 0.3 eV ±15%, in agreement with the high end of

Giant magnetodrag in graphene at charge neutrality

Abstract: We report experimental data and theoretical analysis of Coulomb drag between two closely positioned graphene monolayers in a weak magnetic field. Close enough to the neutrality point, the coexistence of electrons and holes in each layer leads to a dramatic increase of the drag resistivity. Away from charge neutrality, we observe nonzero Hall drag. The observed phenomena are explained by decoupling of electric and quasiparticle currents which are orthogonal at charge neutrality. The sign of magnetodrag depends on the energy relaxation rate and geometry of the sample.

Cross-sectional imaging of graphene-based devices

Abstract: The 2D structure and excellent electronic properties of graphene have generated worldwide interest, and offer the potential to overcome the limitations of silicon electronics.1 Early graphenebased devices consisted of a single atomic layer, but the possibility of tailoring electronic properties by stacking several different 2D crystals atop each other has since been discovered.2–5 In the assembly of these multilayer heterostructures, unwanted materials can become trapped between layers, adversely affecting the device quality. Understanding the cause of performance variations and thereby optimizing the properties of these devices is an extremely challenging task. Using high-resolution transmission electron microscopy imaging, a top view of the atomic structure of free-standing graphene sheets has been revealed.6–12 However, as the number of layers increases, these images become difficult or impossible to interpret due to the uppermost layers obscuring the view of lower layers. Characterizing the uppermost layer of a device is possible using a wide range of surface-sensitive analytical techniques, but these approaches are insensitive to buried interfaces. To characterize the structure of graphene devices, we have pioneered an unusual approach that involves the extraction of cross sections.13, 14 Focused ion beam milling is a technique in which a beam of ions (we use gallium) is focused on the sample, locally eroding the surface in a highly controlled fashion. This method has been used to image a wide range of devices, including 3D nanostructures, and has also found application in the preparation of thin sections of material suitable for transmission electron microscopy imaging. The major benefit of this approach, compared with conventional sample preparation routes, is the ability to Figure 1. (a) Schematic image showing the device structure and intended cross-sectional slice geometry. The heterostructure incorporates layers of boron (BN) and graphene (G) with gold (Au) contacts on an oxidized silicon (Si) substrate. (b) Optical image of actual device structure, with different layers indicated. Scanning electron microscopy images show (c) top-view image of device and (d) the process of sample preparation. (d)(i) The surface of the coated sample, (ii) after depositing protective platinum strip, (iii) after etching around all sides of the slice with a gallium ion beam, and (iv) a slice removed from the device using a nanomanipulator needle. (v) The slice is ready for transmission electron microscopy imaging after being attached to the grid and polished to reach a thickness of 20nm.