[Show abstract][Hide abstract] ABSTRACT: Monolayers of molybdenum and tungsten dichalcogenides are direct bandgap semiconductors, which makes them promising for opto-electronic applications. In particular, van der Waals heterostructures consisting of monolayers of MoS2 sandwiched between atomically thin hexagonal boron nitride (hBN) and graphene electrodes allows one to obtain light emitting quantum wells (LEQW's) with low-temperature external quantum efficiency (EQE) of 1%. However, the EQE of MoS2 and MoSe2-based LEQW's shows behavior common for many other materials: it decreases fast from cryogenic conditions to room temperature, undermining their practical applications. Here we compare MoSe2 and WSe2 LEQW's. We show that the EQE of WSe2 devices grows with temperature, with room temperature EQE reaching 5%, which is 250x more than the previous best performance of MoS2 and MoSe2 quantum wells in ambient conditions. We attribute such different temperature dependences to the inverted sign of spin-orbit splitting of conduction band states in tungsten and molybdenum dichalcogenides, which makes the lowest-energy exciton in WSe2 dark.
[Show abstract][Hide abstract] ABSTRACT: Strong light-matter interaction in two-dimensional molybdenum diselenide (MoSe2) is observed using a tunable optical microcavity. Polariton states with a Rabi splitting of 20 and 29 meV are observed for a monolayer MoSe2 and a 'double-well' MoSe2/hBN/MoSe2, respectively.
[Show abstract][Hide abstract] ABSTRACT: Layered materials can be assembled vertically to fabricate a new class of van der Waals (VDW) heterostructures a few atomic layers thick, compatible with a wide range of substrates and opto-electronic device geometries, enabling new strategies for control of light-matter coupling. Here, we incorporate molybdenum diselenide/boron nitride (MoSe2/hBN) quantum wells (QWs) in a tun-able optical microcavity. Part-light-part-matter polariton eigenstates are observed as a result of the strong coupling between MoSe2 excitons and cavity photons, evidenced from a clear anticrossing between the neutral exciton and the cavity modes with a splitting of 20 meV for a single MoSe2 monolayer QW, enhanced to 29 meV in MoSe2/hBN/MoSe2 double-QWs. The splitting at resonance provides an estimate of the exciton radiative lifetime of 0.4 ps. Our results pave the way for room temperature polaritonic devices based on multiple-QW VDW heterostructures, where polari-ton condensation and electrical polariton injection through the incorporation of graphene contacts may be realised.
[Show abstract][Hide abstract] ABSTRACT: The advent of graphene and related 2D materials has recently led to a new
technology: heterostructures based on these atomically thin crystals. The
paradigm proved itself extremely versatile and led to rapid demonstration of
tunnelling diodes with negative differential resistance, tunnelling
transistors5, photovoltaic devices, etc. Here we take the complexity and
functionality of such van der Waals heterostructures to the next level by
introducing quantum wells (QWs) engineered with one atomic plane precision. We
describe light emitting diodes (LEDs) made by stacking up metallic graphene,
insulating hexagonal boron nitride (hBN) and various semiconducting monolayers
into complex but carefully designed sequences. Our first devices already
exhibit extrinsic quantum efficiency of nearly 10% and the emission can be
tuned over a wide range of frequencies by appropriately choosing and combining
2D semiconductors (monolayers of transition metal dichalcogenides). By
preparing the heterostructures on elastic and transparent substrates, we show
that they can also provide the basis for flexible and semi-transparent
electronics. The range of functionalities for the demonstrated heterostructures
is expected to grow further with increasing the number of available 2D crystals
and improving their electronic quality.
[Show abstract][Hide abstract] ABSTRACT: In this work we use Raman spectroscopy as a non-destructive and rapid technique for probing the Van der Waals (VdW) forces acting between two atomically thin crystals, where one is a transition metal dichalcogenide (TMDC). In this work, MoS2 is used as a Raman probe: we show that its two Raman active phonon modes can provide information on the interaction between the two crystals. In particular, the in-plane vibration (E2g) provides information on the in-plane strain, while the out-of-plane mode (A1g) gives evidence for the quality of the interfacial contact. We show that a VdW contact with MoS2 is characterized by a blue shift of +2 cm-1 of the A1g peak. In the case of a MoS2 /graphene heterostructure, the VdW contact is also characterized by a shift of +14 cm-1 of the 2D peak of graphene. Our approach offers a very simple, non-destructive and fast method to characterize the quality of the interface of heterostructures containing atomically thick TMDCs crystals.
[Show abstract][Hide abstract] ABSTRACT: Recent developments in the technology of van der Waals heterostructures made from two-dimensional atomic crystals have already led to the observation of new physical phenomena, such as the metal-insulator transition and Coulomb drag, and to the realization of functional devices, such as tunnel diodes, tunnel transistors and photovoltaic sensors. An unprecedented degree of control of the electronic properties is available not only by means of the selection of materials in the stack, but also through the additional fine-tuning achievable by adjusting the built-in strain and relative orientation of the component layers. Here we demonstrate how careful alignment of the crystallographic orientation of two graphene electrodes separated by a layer of hexagonal boron nitride in a transistor device can achieve resonant tunnelling with conservation of electron energy, momentum and, potentially, chirality. We show how the resonance peak and negative differential conductance in the device characteristics induce a tunable radiofrequency oscillatory current that has potential for future high-frequency technology.
[Show abstract][Hide abstract] ABSTRACT: Topological materials may exhibit Hall-like currents flowing transversely to the applied electric field even in the absence
of a magnetic field. In graphene superlattices, which have broken inversion symmetry, topological currents originating from
graphene’s two valleys are predicted to flow in opposite directions and combine to produce long-range charge neutral flow.
We observed this effect as a nonlocal voltage at zero magnetic field in a narrow energy range near Dirac points at distances
as large as several micrometers away from the nominal current path. Locally, topological currents are comparable in strength
with the applied current, indicating large valley-Hall angles. The long-range character of topological currents and their
transistor-like control by means of gate voltage can be exploited for information processing based on valley degrees of freedom.
[Show abstract][Hide abstract] ABSTRACT: Integration of quasi-two-dimensional
(2D) films of metal–chalcogenides
in optical microcavities permits new photonic applications of these
materials. Here we present tunable microcavities with monolayer MoS2 or few monolayer GaSe films. We observe significant modification
of spectral and temporal properties of photoluminescence (PL): PL
is emitted in spectrally narrow and wavelength-tunable cavity modes
with quality factors up to 7400; a 10-fold PL lifetime shortening
is achieved, a consequence of Purcell enhancement of the spontaneous
[Show abstract][Hide abstract] ABSTRACT: Plasmonics has established itself as a branch of physics which promises to revolutionize data processing, improve photovoltaics, and increase sensitivity of bio-detection. A widespread use of plasmonic devices is notably hindered by high losses and the absence of stable and inexpensive metal films suitable for plasmonic applications. To this end, there has been a continuous search for alternative plasmonic materials that are also compatible with complementary metal oxide semiconductor technology. Here we show that copper and silver protected by graphene are viable candidates. Copper films covered with one to a few graphene layers show excellent plasmonic characteristics. They can be used to fabricate plasmonic devices and survive for at least a year, even in wet and corroding conditions. As a proof of concept, we use the graphene-protected copper to demonstrate dielectric loaded plasmonic waveguides and test sensitivity of surface plasmon resonances. Our results are likely to initiate wide use of graphene-protected plasmonics.
[Show abstract][Hide abstract] ABSTRACT: One of the challenges associated with the development of next-generation electronics is to find alternatives to silicon oxide caused by the size-reduction constraints of the devices. The dielectric properties of two-dimensional (2D) crystals, added to their excellent chemical stability, mechanical and thermal properties, make them promising dielectrics. Here we show that liquid-phase exfoliation (LPE) in water by using low-cost commercial organic dyes as dispersant agents can efficiently produce defect-free 2D nanosheets, including mono-layers, in suspensions. We further show that these suspensions can be easily incorporated into current practical graphene-based devices. In particular, it is found that boron nitride thin films made by LPE are excellent dielectrics that are highly compatible with graphene-based electronics.
[Show abstract][Hide abstract] ABSTRACT: In graphene placed on hexagonal boron nitride, replicas of the original Dirac
spectrum appear near edges of superlattice minibands. More such replicas
develop in high magnetic fields, and their quantization gives rise to a fractal
pattern of Landau levels, referred to as the Hofstadter butterfly. Some
evidence for the butterfly has recently been reported by using transport
measurements. Here we employ capacitance spectroscopy to probe directly the
density of states and energy gaps in graphene superlattices. Without magnetic
field, replica spectra are seen as pronounced minima in the density of states
surrounded by van Hove singularities. The Hofstadter butterfly shows up in
magnetocapacitance clearer than in transport measurements and, near one flux
quantum per superlattice unit cell, we observe Landau fan diagrams related to
quantization of Dirac replicas in a reduced magnetic field. Electron-electron
interaction strongly modifies the superlattice spectrum. In particular, we find
that graphene's quantum Hall ferromagnetism, due to lifted spin and valley
degeneracies, exhibits a reverse Stoner transition at commensurable fluxes and
that Landau levels of Dirac replicas support their own ferromagnetic states.
[Show abstract][Hide abstract] ABSTRACT: The new paradigm of heterostructures based on two-dimensional (2D) atomic crystals has already led to the observation of exciting physical phenomena and creation of novel devices. The possibility of combining layers of different 2D materials in one stack allows unprecedented control over the electronic and optical properties of the resulting material. Still, the current method of mechanical transfer of individual 2D crystals, though allowing exceptional control over the quality of such structures and interfaces, is not scalable. Here we show that such heterostructures can be assembled from chemically exfoliated 2D crystals, allowing for low-cost and scalable methods to be used in the device fabrication.
[Show abstract][Hide abstract] ABSTRACT: Hexagonal boron nitride is the only substrate that has so far allowed graphene devices exhibiting micron-scale ballistic transport. Can other atomically flat crystals be used as substrates for making quality graphene heterostructures? Here we report on our search for alternative substrates. The devices fabricated by encapsulating graphene with molybdenum or tungsten disulphides and hBN are found to exhibit consistently high carrier mobilities of about 60,000 cm2V-1s-1. In contrast, encapsulation with atomically flat layered oxides such as mica, bismuth strontium calcium copper oxide and vanadium pentoxide results in exceptionally low quality of graphene devices with mobilities of ~1,000 cm2V-1s-1. We attribute the difference mainly to self-cleansing that takes place at interfaces between graphene, hBN and transition metal dichalcogenides. Surface contamination assembles into large pockets allowing the rest of the interface to become atomically clean. The cleansing process does not occur for graphene on atomically flat oxide substrates.
[Show abstract][Hide abstract] ABSTRACT: We present the first study of the intrinsic electrical properties of WS$_2$
transistors fabricated with two different dielectric environments WS$_2$ on
SiO$_2$ and WS$_2$ on h-BN/SiO$_2$, respectively. A comparative analysis of the
electrical characteristics of multiple transistors fabricated from natural and
synthetic WS$_2$ with various thicknesses from single- up to four-layers and
over a wide temperature range from 300K down to 4.2 K shows that disorder
intrinsic to WS$_2$ is currently the limiting factor of the electrical
properties of this material. These results shed light on the role played by
extrinsic factors such as charge traps in the oxide dielectric thought to be
the cause for the commonly observed small values of charge carrier mobility in
transition metal dichalcogenides.
Full-text · Article · May 2014 · Scientific Reports
[Show abstract][Hide abstract] ABSTRACT: The next-nearest-neighbor hopping term t′ determines a magnitude, and, hence, the importance of several phenomena in graphene that include self-doping due to broken bonds and the Klein tunneling, which in the presence of t′, is no longer perfect. Theoretical estimates for t′ vary widely, whereas a few existing measurements by using polarization-resolved magnetospectroscopy have found surprisingly large t′, close to or even exceeding 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 theory estimates. We discuss the role of electron-electron interactions in determining t′ and overview phenomena, which can be influenced by such a large value of t′.
Full-text · Article · Oct 2013 · Physical Review B
[Show abstract][Hide abstract] ABSTRACT: We investigate the optoelectronic properties of novel graphene/FeCl3-intercalated few-layer graphene (FeCl3-FLG, dubbed graphexeter) heterostructures using photovoltage spectroscopy. We observe a prominent photovoltage signal generated at the graphene/FeCl3-FLG and graphene/Au interfaces, whereas the photovoltage at the FeCl3-FLG/Au interface is negligible. The sign of the photovoltage changes upon sweeping the chemical potential of the pristine graphene through the charge neutrality point, and we show that this is due to the photothermoelectric effect. Our results are a first step toward all-graphene-based photodetectors and photovoltaics.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate a novel method to tune the energy gap 1 between the localized states and the mobility edge of the valence band in chemically functionalized graphene by changing the coverage of fluorine adatoms via electron-beam irradiation. From the temperature dependence of the electrical transport properties we show that 1 in partially fluorinated graphene CF0.28 decreases upon electron irradiation up to a dose of 0.08 C cm−2. For low irradiation doses (<0.1 C cm−2) partially fluorinated graphene behaves as a lightly doped semiconductor with impurity bands close to the conduction and valence band edges, whereas for high irradiation doses (>0.2 C cm−2) the electrical conduction takes place via Mott variable range hopping.
Full-text · Article · Mar 2013 · New Journal of Physics
[Show abstract][Hide abstract] ABSTRACT: The development of future flexible and transparent electronics relies on
novel materials, which are mechanically flexible, lightweight and
low-cost, in addition to being electrically conductive and optically
transparent. Currently, tin doped indium oxide (ITO) is the most wide
spread transparent conductor in consumer electronics. The mechanical
rigidity of this material limits its use for future flexible electronic
applications. We report novel graphene-based transparent conductors
obtained by intercalating few-layer graphene (FLG) with ferric chloride
(FeCl3). Through a combined study of electrical transport and optical
transmission measurements we demonstrate that FeCl3 enhances the
electrical conductivity of FLG by two orders of magnitude while leaving
these materials highly transparent . We find that the optical
transmittance in the visible range of FeCl3-FLG is typically between 88%
and 84%, whereas the resistivity is as low as 8.8 φ. These
parameters outperform the best values found in ITO (i.e. resistivity of
10 φ at an optical transmittance of 85%), making therefore
FeCl3-FLG the best candidate for flexible and transparent electronics.
[4pt]  I. Khrapach, F. Withers, T. H. Bointon, D. K. Pplyushkin, W.
L. Barnes, S. Russo, M. F. Craciun, Adv. Mater. 24, 2844 (2012).