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Temperature-dependent Raman scattering provides valuable information on electron–phonon coupling and phonon anharmonicity of graphene. In this study, we show an enhancement of Joule heating in graphene by confining the current flow in a narrow channel. In addition, we performed a detailed analysis of the anharmonic effect in the hBN/monolayer graphene/hBN heterostructure based on the behaviour of the full-width at half maximum of the G mode with increasing electric power: a non-monotonic trend, leading to the key of approximation of the electronic temperature in graphene. We believe our results could offer a convenient analysis tool to study electron–phonon coupling and anharmonic phonon-decay processes in a high-temperature regime.

We studied the properties of electron transport in mesoscopic GaAs/AlGaAs heterostructure without any intentional dopants in which an external electric field defined the two-dimensional electron gas (2DEG). An electrically formed 2DEG without intentional doping offers many advantages because of the absence of high concentrations of charged scattering centers. We demonstrate that the electron concentration can be easily tuned by varying the gate voltage. This tunability was observed for a high-quality 2DEG with a carrier density ranging from 0.75 to 3.34 × 1011 cm−2, for which the corresponding mobility ranged from 0.26 to 2.93 × 106 cm2V−1s−1. The mobility of the 2DEG is closely followed the experimental power law for high-mobility wafers, μ ∞ n2D0.7.

A layer-engineered exfoliation approach allows us to obtain millimeter-size graphene and control the number of layers., The competition between quality and productivity has been a major issue for large-scale applications of two-dimensional materials (2DMs). Until now, the top-down mechanical cleavage method has guaranteed pure perfect 2DMs, but it has been considered a poor option in terms of manufacturing. Here, we present a layer-engineered exfoliation technique for graphene that not only allows us to obtain large-size graphene, up to a millimeter size, but also allows selective thickness control. A thin metal film evaporated on graphite induces tensile stress such that spalling occurs, resulting in exfoliation of graphene, where the number of exfoliated layers is adjusted by using different metal films. Detailed spectroscopy and electron transport measurement analysis greatly support our proposed spalling mechanism and fine quality of exfoliated graphene. Our layer-engineered exfoliation technique can pave the way for the development of a manufacturing-scale process for graphene and other 2DMs in electronics and optoelectronics.

Of the two stable forms of graphite, hexagonal (HG) and rhombohedral (RG), the former is more common and has been studied extensively. RG is less stable, which so far precluded its detailed investigation, despite many theoretical predictions about the abundance of exotic interaction-induced physics. Advances in van der Waals heterostructure technology have now allowed us to make high-quality RG films up to 50 graphene layers thick and study their transport properties. We find that the bulk electronic states in such RG are gapped and, at low temperatures, electron transport is dominated by surface states. Because of topological protection, the surface states are robust and of high quality, allowing the observation of the quantum Hall effect, where RG exhibits phase transitions between gapless semimetallic phase and gapped quantum spin Hall phase with giant Berry curvature. An energy gap can also be opened in the surface states by breaking their inversion symmetry via applying a perpendicular electric field. Moreover, in RG films thinner than 4 nm, a gap is present even without an external electric field. This spontaneous gap opening shows pronounced hysteresis and other signatures characteristic of electronic phase separation, which we attribute to emergence of strongly-correlated electronic surface states.

We report on the observation of edge electric currents excited in bilayer graphene by terahertz laser radiation. We show that the current generation belongs to the class of second order in electric field phenomena and is controlled by the orientation of the THz electric field polarization plane. Additionally, applying a small magnetic field normal to the graphene plane leads to a phase shift in the polarization dependence. With increasing the magnetic field strength, the current starts to exhibit 1/B-magneto-oscillations with a period consistent with that of the Shubnikov–de Haas effect and amplitude by an order of magnitude larger as compared to the current at zero magnetic field measured under the same conditions. The microscopic theory developed shows that the current is formed in the edge's vicinity limited by the mean-free path of carriers and the screening length of the high-frequency electric field. The current originates from the alignment of the free carrier momenta and dynamic accumulation of charge at the edges, where the P-symmetry is naturally broken. The observed magneto-oscillations of the photocurrent are attributed to the formation of Landau levels.

The long-distance quantum transfer between electron-spin qubits in semiconductors is important for realising large-scale quantum computing circuits. Electron-spin to photon-polarisation conversion is a promising technology for achieving free-space or fibre-coupled quantum transfer. In this work, using only regular lithography techniques on a conventional 15 nm GaAs quantum well, we demonstrate acoustically-driven generation of single photons from single electrons, without the need for a self-assembled quantum dot. In this device, a single electron is carried in a potential minimum of a surface acoustic wave (SAW) and is transported to a region of holes to form an exciton. The exciton then decays and creates a single optical photon within 100 ps. This SAW-driven electroluminescence, without optimisation, yields photon antibunching with g(2)(0) = 0.39 ± 0.05 in the single-electron limit (g(2)(0) = 0.63 ± 0.03 in the raw histogram). Our work marks the first step towards electron-to-photon (spin-to-polarisation) qubit conversion for scaleable quantum computing architectures., Electron-spin to photon-polarisation conversion is a promising technology for achieving free-space or fibre coupled quantum transfer. Here, the authors demonstrate acoustically-driven single photons from single electrons, without the need for self-assembled quantum dots, using a SAW-driven lateral n-i-p junction.

We report the observation of the resonant excitation of edge photocurrents in bilayer graphene subjected to terahertz radiation and a magnetic field. The resonantly excited edge photocurrent is observed for both inter-band (at low carrier densities) and intra-band (at high densities) transitions between Landau levels (LL). While the intra-band LL transitions can be traced to the classical cyclotron resonance (CR) and produce strong resonant features, the inter-band-LL resonances have quantum nature and lead to the weaker features in the measured photocurrent spectra. The magnitude and polarization properties of the observed features agree with the semiclassical theory of the intra-band edge photogalvanic effect, including its Shubnikov-de-Haas oscillations at low temperatures., Comment: 11 pages, 11 figures

The quantum Hall effect (QHE) originates from discrete Landau levels forming in a two-dimensional (2D) electron system in a magnetic field. In three dimensions (3D), the QHE is forbidden because the third dimension spreads Landau levels into multiple overlapping bands, destroying the quantisation. Here we report the QHE in graphite crystals that are up to hundreds of atomic layers thick - thickness at which graphite was believed to behave as a 3D bulk semimetal. We attribute the observation to a dimensional reduction of electron dynamics in high magnetic fields, such that the electron spectrum remains continuous only in the direction of the magnetic field, and only the last two quasi-one-dimensional (1D) Landau bands cross the Fermi level. In sufficiently thin graphite films, the formation of standing waves breaks these 1D bands into a discrete spectrum, giving rise to a multitude of quantum Hall plateaux. Despite a large number of layers, we observe a profound difference between films with even and odd numbers of graphene layers. For odd numbers, the absence of inversion symmetry causes valley polarisation of the standing-wave states within 1D Landau bands. This reduces QHE gaps, as compared to films of similar thicknesses but with even layer numbers because the latter retain the inversion symmetry characteristic of bilayer graphene. High-quality graphite films present a novel QHE system with a parity-controlled valley polarisation and intricate interplay between orbital, spin and valley states, and clear signatures of electron-electron interactions including the fractional QHE below 0.5 K.

Surface acoustic waves (SAWs) have been used to transport single electrons across long distances of several hundreds of microns. They can potentially be instrumental in the implementation of scalable quantum processors and quantum repeaters, by facilitating interaction between distant qubits. While most of the work thus far has focused on SAW devices in doped GaAs/AlGaAs heterostructures, we have developed a method of creating lateral p-n junctions in an undoped heterostructure containing a quantum well, with the expected advantages of having reduced charge noise and increased spin-coherence lifetimes due to the lack of dopant scattering centres. We present experimental observations of SAW-driven single-electron quantised current in an undoped GaAs/AlGaAs heterostructure, where single electrons were transported between regions of induced electrons. We also demonstrate pumping of electrons by a SAW across the sub-micron depleted channel between regions of electrons and holes, and observe light emission at such a lateral p-n junction. Improving the lateral confinement in the junction should make it possible to produce a quantised electron-to-hole current and hence SAW-driven emission of single photons.

Ion irradiation is a versatile tool to introduce controlled defects into two-dimensional (2D) MoS2 on account of its unique spatial resolution and plethora of ion types and energies available. In order to fully realise the potential of this technique, a holistic understanding of ion-induced defect production in 2D MoS2 crystals of different thicknesses is mandatory. X-ray photoelectron spectroscopy, electron diffraction and Raman spectroscopy show that thinner MoS2 crystals are more susceptible to radiation damage caused by 225 keV Xe+ ions. However, the rate of defect production in quadrilayer and bulk crystals is not significantly different under our experimental conditions. The rate at which S atoms are sputtered as a function of radiation exposure is considerably higher for monolayer MoS2, compared to bulk crystals, leading to MoO3 formation. P-doping of MoS2 is observed and attributed to the acceptor states introduced by vacancies and charge transfer interactions with adsorbed species. Moreover, the out-of-plane vibrational properties of irradiated MoS2 crystals are shown to be strongly thickness-dependent: in mono- and bilayer MoS2, the confinement of phonons by defects results in a blueshift of the A 1 g mode. Whereas, a redshift is observed in bulk crystals due to attenuation of the effective restoring forces acting on S atoms caused by vacancies in adjacent MoS2 layers. Consequently, the A 1 g frequency of tri- and quadrilayer crystals is statistically invariant on account oft competition between phonon confinement effects and interlayer interactions. The A 1 g linewidth is observed to decrease in bi- and trilayer crystals after low dose irradiation and is attributed to layer decoupling. This work shows that there is a complex interplay between defect production, crystal thickness and interlayer interactions in MoS2. Our results demonstrate that ion irradiation is an effective tool to modulate the electronic, vibrational and structural properties of MoS2, which may prove beneficial for practical applications.

The excellent electronic and mechanical properties of graphene allow it to sustain very large currents, enabling its incandescence through Joule heating in suspended devices. Although interesting scientifically and promising technologically, this process is unattainable in ambient environment, because graphene quickly oxidises at high temperatures. Here, we take the performance of graphene-based incandescent devices to the next level by encapsulating graphene with hexagonal boron nitride (hBN). Remarkably, we found that the hBN encapsulation provides an excellent protection for hot graphene filaments even at temperatures well above 2000 K. Unrivalled oxidation resistance of hBN combined with atomically clean graphene/hBN interface allows for a stable light emission from our devices in atmosphere for many hours of continuous operation. Furthermore, when confined in a simple photonic cavity, the thermal emission spectrum is modified by a cavity mode, shifting the emission to the visible range spectrum. We believe our results demonstrate that hBN/graphene heterostructures can be used to conveniently explore the technologically important high-temperature regime and to pave the way for future optoelectronic applications of graphene-based systems.

To realize graphene-encapsulated semiconductor nanocrystals (NCs), an additional graphene coating process, which causes shape destruction and chemical contamination, has so far been inevitable. We report herein one-pot growth of uniform graphene-germanium core–shell nanocrystals (Ge@G NCs) in gram scale by the addition of methane as a carbon source during the thermal pyrolysis of germane. The methane plays a critical role in the growth of the graphene shell, as well as in the determination of the nucleation density and diameter of the NCs, similar to a surfactant in the liquid-phase growth of monodisperse NCs. By adjusting the gas ratio of precursors, a mixture of germane and methane, we can control the size of the Ge@G NCs in the range of ∼5–180 nm. The Ge@G NCs were characterized by various microscopic and spectroscopic tools, which indicated that the Ge core is single crystalline, and is completely covered by the graphene shell. We further investigated the merits of the graphene shell, which can enhance the electrical conductivity of nanocrystalline materials.

The electrical and optical properties of semiconducting transition metal dichalcogenides (TMDs) can be tuned by controlling their composition and the number of layers they have. Among various TMDs, the monolayer WSe2 has a direct bandgap of 1.65 eV and exhibits p-type or bipolar behavior, depending on the type of contact metal. Despite these promising properties, a lack of efficient large-area production methods for high-quality, uniform WSe2 hinders its practical device applications. Various methods have been investigated for the synthesis of large-area monolayer WSe2, but the difficulty of precisely controlling solid-state TMD precursors (WO3, MoO3, Se, and S powders) is a major obstacle to the synthesis of uniform TMD layers. In this work, we outline our success in growing large-area, high-quality, monolayered WSe2 by utilizing methane (CH4) gas with precisely controlled pressure as a promoter. When compared to the catalytic growth of monolayered WSe2 without a gas-phase promoter, the catalytic growth of the monolayered WSe2 with a CH4 promoter reduced the nucleation density to 1/1000 and increased the grain size of monolayer WSe2 up to 100 μm. The significant improvement in the optical properties of the resulting WSe2 indicates that CH4 is a suitable candidate as a promoter for the synthesis of TMD materials, because it allows accurate gas control.

Crystallographic alignment between two-dimensional crystals in van der Waals heterostructures brought a number of profound physical phenomena, including observation of Hofstadter butterfly and topological currents, and promising novel applications, such as resonant tunnelling transistors. Here, by probing the electronic density of states in graphene using graphene-hexagonal boron nitride tunnelling transistors, we demonstrate a structural transition of bilayer graphene from incommensurate twisted stacking state into a commensurate AB stacking due to a macroscopic graphene self-rotation. This structural transition is accompanied by a topological transition in the reciprocal space and by pseudospin texturing. The stacking transition is driven by van der Waals interaction energy of the two graphene layers and is thermally activated by unpinning the microscopic chemical adsorbents which are then removed by the self-cleaning of graphene., 13 pages, 6 figures

Exploiting the properties of two-dimensional crystals requires a mass production method able to produce heterostructures of arbitrary complexity on any substrate. Solution processing of graphene allows simple and low-cost techniques such as inkjet printing to be used for device fabrication. However, the available printable formulations are still far from ideal as they are either based on toxic solvents, have low concentration, or require time-consuming and expensive processing. In addition, none is suitable for thin-film heterostructure fabrication due to the re-mixing of different two-dimensional crystals leading to uncontrolled interfaces and poor device performance. Here, we show a general approach to achieve inkjet-printable, water-based, two-dimensional crystal formulations, which also provide optimal film formation for multi-stack fabrication. We show examples of all-inkjet-printed heterostructures, such as large-area arrays of photosensors on plastic and paper and programmable logic memory devices. Finally, in vitro dose-escalation cytotoxicity assays confirm the biocompatibility of the inks, extending their possible use to biomedical applications. Device fabrication can be realized via inkjet printing of water-based 2D crystals.

Few-layer graphene (FLG) has recently been intensively investigated for its variable electronic properties, which are defined by a local atomic arrangement. While the most natural arrangement of layers in FLG is ABA (Bernal) stacking, a metastable ABC (rhombohedral) stacking, characterized by a relatively high-energy barrier, can also occur. When both types of stacking occur in one FLG device, the arrangement results in an in-plane heterostructure with a domain wall (DW). In this paper, we present two approaches to demonstrate that the ABC stacking in FLG can be controllably and locally turned into the ABA stacking. In the first approach, we introduced Joule heating, and the transition was characterized by 2D peak Raman spectra at a submicron spatial resolution. The transition was initiated in a small region, and then the DW was controllably shifted until the entire device became ABA stacked. In the second approach, the transition was achieved by illuminating the ABC region with a train of 790-nm-wavelength laser pulses, and the transition was visualized by transmission electron microscopy in both diffraction and dark-field imaging modes. Further, using this approach, the DW was visualized at a nanoscale spatial resolution in the dark-field imaging mode.

Numerous applications call for electronics capable of operation at high temperatures where conventional Si-based electrical devices fail. In this work, we show that graphene-based devices are capable of performing in an extended temperature range up to 500 °C without noticeable thermally induced degradation when encapsulated by hexagonal boron nitride (hBN). The performance of these devices near the neutrality point is dominated by thermal excitations at elevated temperatures. Non-linearity pronounced in electric field-mediated resistance of the aligned graphene/hBN allowed us to realize heterodyne signal mixing at temperatures comparable to that of the Venus atmosphere (∼460 °C).