Publications

Graphene-based Josephson junctions have attracted significant interest as a novel system to study the proximity effect due to graphene's unique electronic spectrum and the possibility to tune junction properties by gate voltage. Here we describe graphene junctions with the mean free path of several micrometers, low contact resistance and large supercurrents. Such devices exhibit pronounced Fabry-Perot oscillations not only in the normal-state resistance but also in the critical current. The proximity effect is mostly suppressed in magnetic fields of <10 mT, showing the conventional Fraunhofer pattern. Unexpectedly, some proximity survives even in fields as high as 1 T. Superconducting states randomly appear and disappear as a function of field and carrier concentration, and each of them exhibits a supercurrent carrying capacity close to the universal limit of eD/h where D is the superconducting gap, e the electron charge and h Planck's constant. We attribute the high-field Josephson effect to individual Andreev bound states that persist near graphene edges. Our work reveals new proximity regimes that can be controlled by quantum confinement and cyclotron motion.

We show that the width of the longitudinal magnetoconductivity peaks in graphene related to the $N=1$ Landau level displays a power-law type temperature dependence, $\ensuremath{\Delta}\ensuremath{\nu}\ensuremath{\propto}{T}^{\ensuremath{\kappa}}$, with $\ensuremath{\kappa}=0.37\ifmmode\pm\else\textpm\fi{}0.05$. Similarly, the derivative of the Hall conductivity at the plateau transition, $(d{\ensuremath{\sigma}}_{xy}/d\ensuremath{\nu})$, scales as ${T}^{\ensuremath{-}\ensuremath{\kappa}}$ with $\ensuremath{\kappa}=0.41\ifmmode\pm\else\textpm\fi{}0.04$ for both the first and second Landau levels of electrons and holes. These results confirm the universality of a critical quantum Hall scaling in the higher Landau levels of graphene. In the zeroth Landau level, however, $\ensuremath{\Delta}\ensuremath{\nu}$ and $d{\ensuremath{\sigma}}_{xy}/d\ensuremath{\nu}$ are essentially temperature independent, pointing toward a different type of scaling that is possibly governed by a temperature independent intrinsic length.

We report intriguing and hitherto overlooked low-field room temperature extremely large magnetoresistance (XMR) patterns in graphene/hexagonal boron nitride (h-BN) superlattices that emerge due to the existence of open orbits within each miniband. This finding is set against the backdrop of the experimental discovery of the Hofstadter butterfly in moir superlattices, which has sparked considerable interest in the fractal quantum Hall regime. To cope with the challenge of deciphering the low magnetic field dynamics of moir minibands, we utilize a novel semi-classical calculation method, grounded in zero-field Fermi contours, to predict the nontrivial behavior of the Landau-level spectrum. This is compared with fully quantum simulations, enabling an in-depth and contrasted analysis of transport measurements in high-quality graphene-hBN superlattices. Our results not only highlight the primary observation of the open-orbit induced XMR in this system but also shed new light on other intricate phenomena. These include the nuances of single miniband dynamics, evident through Lifshitz transitions, and the complex interplay of semiclassical and quantum effects between these minibands. Specifically, we document transport anomalies linked to trigonal warping, a semiclassical deviation from the expected linear characteristics of Landau levels, and magnetic breakdown phenomena indicative of quantum tunneling, all effects jointly contributing to the intricacies of a rich electronic landscape uncovered at low magnetic fields.

We demonstrate a fabrication procedure of hybrid devices that consist of reduced graphene oxide films supported by porous polymer membranes that host ionic solutions. We find that we can control the thermal radiation from the surface of reduced graphene oxide through a process of electrically driven reversible ionic intercalation. Through a comparative analysis of the structural, chemical, and optical properties of our reduced graphene oxide films, we identify that the dominant mechanism leading to the intercalation-induced reduction of light emission is Pauli blocking of the interband recombination of charge carriers. We inspect the capabilities of our devices to act as a platform for the electrical control of mid-infrared photonics by observing a bias-induced reduction of apparent temperature of hot surfaces visualized through an infrared thermal camera.

Quantum dots (QDs) play a fundamental role in nanotechnology because of their unique optical properties, especially photoluminescence (PL). Quantum confinement effects combined with tailor-made materials make QDs extremely versatile for understanding basic physical phenomena intrinsic to them as well as defining their use in a vast range of applications. With the advent of graphene in 2004, and the discovery of numerous other two-dimensional (2D) materials subsequently, it became possible to develop novel 2D quantum dots (2DQDs). Intensive research of the properties of 2DQDs over the last decade have revealed their outstanding properties and grabbed the attention of researchers from different fields: from photonics and electronics to catalysis and medicine. In this review, we explore several aspects of 2DQDs from their synthesis, functionalization, and characterization to applications, focusing on their bioimaging, biosensing, and theranostic solutions

Defect centers in insulators play a critical role in creating important functionalities in materials: prototype qubits, single-photon sources, magnetic field probes, and pressure sensors. These functionalities are highly dependent on their mid-gap electronic structure and orbital/spin wave-function contributions. However, in most cases, these fundamental properties remain unknown or speculative due to the defects being deeply embedded beneath the surface of highly resistive host crystals, thus impeding access through surface probes. Here, we directly inspected the atomic and electronic structures of defects in thin carbon-doped hexagonal boron nitride (hBN:C) using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Such investigation adds direct information about the electronic mid-gap states to the well-established photoluminescence response (including single photon emission) of intentionally created carbon defects in the most commonly investigated van der Waals insulator. Our joint atomic-scale experimental and theoretical investigations reveal two main categories of defects: 1) single-site defects manifesting as donor-like states with atomically resolved structures observable via STM, and 2) multi-site defect complexes exhibiting a ladder of empty and occupied mid-gap states characterized by distinct spatial geometries. Combining direct probing of mid-gap states through tunneling spectroscopy with the inspection of the optical response of insulators hosting specific defect structures holds promise for creating and enhancing functionalities realized with individual defects in the quantum limit. These findings underscore not only the versatility of hBN:C as a platform for quantum defect engineering but also its potential to drive advancements in atomic-scale optoelectronics.

A new approach to clean the surface of graphene is reported by Hailin Peng, Zhongfan Liu, and co-workers in article number 1902978, who use a force-engineered “lint roller” to selectively removing the graphene's intrinsic surface contaminants. The as-obtained super-clean graphene can be transferred to dielectric substrates with significantly reduced polymer residues, and it exhibits superior electronic and optical properties such as ultrahigh carrier mobility and low contact resistance.

Abstract Most AI-for-Materials research to date has focused on ideal crystals, whereas real-world materials inevitably contain defects that play a critical role in modern functional technologies. The defects break geometric symmetry and increase interaction complexity, posing particular challenges for traditional ML models. Here, we introduce Defect-Informed Equivariant Graph Neural Network (DefiNet), a model specifically designed to accurately capture defect-related interactions and geometric configurations in point-defect structures. DefiNet achieves near-DFT-level structural predictions in milliseconds using a single GPU. To validate its accuracy, we perform DFT relaxations using DefiNet-predicted structures as initial configurations and measure the residual ionic steps. For most defect structures, regardless of defect complexity or system size, only 3 ionic steps are required to reach the DFT-level ground state. Finally, comparisons with scanning transmission electron microscopy (STEM) images confirm DefiNet’s scalability and extrapolation beyond point defects, positioning it as a valuable tool for defect-focused materials research.

The mechanical behavior of embedded monolayer graphene in a polymer matrix under axial compression is examined here by monitoring the shifts of the 2D Raman phonons under an incremental applied strain. In order to establish the effect of aspect ratio upon the critical strain to failure a wide range of length-to-width ratios of almost rectangular 1LG flakes were tested up to an external compression strain of approximately -1 %. Care was taken to define the position of zero strain due to the presence-in some cases-of a residual stress and to assess the effect of transfer length upon the efficiency of stress transfer. The obtained critical strain values for first failure- after transfer length correction in short flakes- were found to be independent of flake size with a mean value of - 0.60 + 0.11%. By combining Euler mechanics with a Winkler type of approach, both the modulus of interaction between graphene and polymer, as well as, the buckling wavelength could be established. The results show clearly that unlike buckling in air, the presence of a constraint such as a polymer matrix induces graphene buckling of very short wavelength of the order of 1-2 nm. Finally, by conducting calculations of the interaction between an analogue of monolayer graphene (coronene) and PMMA oligomers within the framework of density functional theory (DFT), the effect of lateral constrain provided by the polymer, upon the out-of-plane buckling of graphene has been assessed.

Direct bandgap 2D semiconductor materials such as monolayers of transition metal dichalcogenides (TMDCs), show great promise in optoelectronic devices enabling exciting new technologies such as ultra-thin quantum light LED’s [1]. These structures can have incredible advantages, enabling almost seamless integration into conventional silicon structures. However, extracting light out of these structures can be a challenge, often requiring costly and time consuming processing e.g. engineered waveguides or cavities [2]. Furthermore none of these methods allow you to observe the light directly, therefore are unhelpful in certain applications, such as an optical version of a quantum unique device [3]. We have previously demonstrated that epoxy based solid immersion lenses can be used to increase light out of semiconductor nanostructures. We furthered this idea to see if they could be used to increase the light out of monolayer TMDC materials; and investigate how the epoxy-2D material interface affects the emission. Our studies revealed that a SIL can greatly enhance the photoluminescence of WSe2 by up to 6x (more than theory predicts for a SIL of this shape), without effecting the wavelength (figure 1). However we also found that the epoxy appears to reduce the emission of the MoS2, suggesting that there could be doping effects due to the epoxy. Overall this method shows great promise as a cheap, and scalable method for enhancing the efficiency of low intensity WSe2 based devices.

The rapid development of infrared spectroscopy, observational astronomy, and scanning near-field microscopy has been enabled by the emergence of sensitive mid- and far-infrared photodetectors. Superconducting hot-electron bolometers (HEBs), known for their exceptional signal-to-noise ratio and fast photoresponse, play a crucial role in these applications. While superconducting HEBs are traditionally crafted from sputtered thin films such as NbN, the potential of layered van der Waals (vdW) superconductors is untapped at THz frequencies. Here, we introduce superconducting HEBs made from few-layer NbSe2 microwires. By improving the interface between NbSe2 and metal leads, we overcome impedance mismatch with RF readout, enabling large responsivity THz detection (0.13 to 2.5 THz) with a minimal noise equivalent power of 7 pW/ Hz and nanosecond-range response time. Our work highlights NbSe2 as a promising platform for HEB technology and presents a reliable vdW assembly protocol for custom bolometer production.

The stress transfer between the internal layers of multilayer graphene within polymer-based nanocomposites has been investigated from the stress-induced shifts of the 2D Raman band. This has been undertaken through the study of the deformation of an ideal composite system where the graphene flakes were placed upon the surface of a polymer beam and then coated with an epoxy polymer. It is found that the rate of band shift per unit strain for a monolayer graphene flake is virtually independent of whether it has one or two polymer interfaces (i.e., with or without an epoxy top coating). In contrast, the rate of band shift is lower for an uncoated bilayer specimen than a coated one, indicating relatively poor stress transfer between the graphene layers. Mapping of the strain in the coated bilayer regions has shown that there is strain continuity between adjacent monolayer and bilayer regions, indicating that they give rise to similar levels of reinforcement. Strain-induced Raman band shifts have also been evaluated for separate flakes of graphene with different numbers of layers, and it is found that the band shift rate tends to decrease with an increase in the number of layers, indicating poor stress transfer between the inner graphene layers. This behavior has been modeled in terms of the efficiency of stress transfer between the inner graphene layers. Taking into account the packing geometry of polymer-based graphene nanocomposites and the need to accommodate the polymer coils, these findings enable the optimum number of graphene layers for the best reinforcement to be determined. It is demonstrated that, in general, multilayer graphene will give rise to higher levels of reinforcement than monolayer material, with the optimum number of layers depending upon the separation of the graphene flakes in the nanocomposite.

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.

No abstract is provided for this article.

Insights into the fundamental properties of graphene’s Dirac-Weyl fermions have emerged from studies of electron tunnelling transistors in which an atomically thin layer of hexagonal boron nitride (hBN) is sandwiched between two layers of high purity graphene. Here, we show that when a single defect is present within the hBN tunnel barrier, it can inject electrons into the graphene layers and its sharply defined energy level acts as a high resolution spectroscopic probe of electron-electron interactions in graphene. We report a magnetic field dependent suppression of the tunnel current flowing through a single defect below temperatures of ~2 K. This is attributed to the formation of a magnetically-induced Coulomb gap in the spectral density of electrons tunnelling into graphene due to electron-electron interactions.

A scientific session of the Physical Sciences Division of the Russian Academy of Sciences (RAS) was held on February 27, 2008 in the conference hall of the Lebedev Physical Institute, RAS. The following reports were presented at the session: (1) Lozovik Yu E, Merkulova S P, Sokolik A A (RAS Institute for Spectroscopy, Troitsk, Moscow Region) electron phenomena in graphene; (2) Morozov S V (RAS Institute for Problems in Micro-electronics Technology, Chernogolovka, Moscow Region), Novoselov K S, Geim A K (University of Manchester, Manchester, UK) transport in graphene. An abridged version of these reports is given below. • Collective electron phenomena in graphene, Yu E Lozovik, S P Merkulova, A A Sokolik Physics-Uspekhi, 2008, Volume 51, Number 7, Pages 727-744 • Electron transport in graphene, S V Morozov, K S Novoselov, A K Geim Physics-Uspekhi, 2008, Volume 51, Number 7, Pages 744-748