Publications

The Mermin-Wagner theorem states that long-range magnetic order does not exist in one- (1D) or two-dimensional (2D) isotropic magnets with short-ranged interactions. Here we show that in finite-size 2D van der Waals magnets typically found in lab setups (within millimetres), short-range interactions can be large enough to allow the stabilisation of magnetic order at finite temperatures without any magnetic anisotropy. We demonstrate that magnetic ordering can be created in 2D flakes independent of the lattice symmetry due to the intrinsic nature of the spin exchange interactions and finite-size effects. Surprisingly we find that the crossover temperature, where the intrinsic magnetisation changes from superparamagnetic to a completely disordered paramagnetic regime, is weakly dependent on the system length, requiring giant sizes (e.g., of the order of the observable universe ~ 1026 m) to observe the vanishing of the magnetic order as expected from the Mermin-Wagner theorem. Our findings indicate exchange interactions as the main ingredient for 2D magnetism.

We perform femtosecond pump-probe spectroscopy to in situ investigate the ultrafast photocarrier dynamics in bilayer graphene and observe an acceleration of energy relaxation under pressure. In combination with in situ Raman spectroscopy and ab initio molecular dynamics simulations, we reveal that interlayer shear and breathing modes have significant contributions to the faster hot-carrier relaxations by coupling with the in-plane vibration modes under pressure. Our work suggests that further understanding the effect of interlayer interaction on the behaviors of electrons and phonons would be critical to tailor the photocarrier dynamic properties of bilayer graphene.

The transfer of electronic charge across the interface of two van der Waals crystals can underpin the operation of a new class of functional devices. Among van der Waals semiconductors, an exciting and rapidly growing development involves the “post‐transition” metal chalcogenide InSe. Here, field effect phototransistors are reported where single layer graphene is capped with n‐type InSe. These device structures combine the photosensitivity of InSe with the unique electrical properties of graphene. It is shown that the light‐induced transfer of charge between InSe and graphene offers an effective method to increase or decrease the carrier density in graphene, causing a change in its resistance that is gate‐controllable and only weakly dependent on temperature. The charge transfer at the InSe/graphene interface is probed by Hall effect and photoconductivity measurmentes and it is demonstrated that light can induce a sign reversal of the quantum Hall voltage and photovoltaic effects in the graphene layer. These findings demonstrate the potential of light‐induced charge transfer in gate‐tunable InSe/graphene phototransistors for optoelectronics and quantum metrology.

The knowledge of the mechanism of stress transfer from a polymer matrix to a 2-dimensional nano-inclusion such as a graphene flake is of paramount importance for the design and the production of effective nanocomposites. For efficient reinforcement the shape of the inclusion must be accurately controlled since the axial stress transfer from matrix to the inclusion is affected by the axial-shear coupling observed upon loading of a flake of irregular geometry. Herein, we study true axial phenomena on regular- exfoliated-graphene micro-ribbons which are perfectly aligned to the loading direction. We exploit the strain sensitivity of vibrational wave numbers in order to map point-by-point the strain built up along the length of graphene. By considering the balance of shear-to-axial forces, we identify the shear stress at the interface and develop a universal inverse-length parameter that governs the stress transfer process at the nanoscale. An important parameter that has come out of this approach is the prediction and measurement of the transfer length that is required for efficient stress in these systems.

No abstract is provided for this article.

Since the first isolation of graphene monolayer via mechanical exfoliation in 2004, many other production methods have emerged in the following years. These methods include chemical vapour deposition, liquid phase graphite exfoliation, graphite oxide reduction and epitaxial growth, each having its pros and cons. The unique properties of graphene, such as high electron mobility, large specific surface area, mechanical strength and high transparency, ignited an enormous research interest to explore the electrochemical properties of graphene sheets, with potential uses in many immediate applications such as energy storage, solar cell technology and corrosion protection. Understanding how the electron transfer kinetics of a redox reaction between the graphene surface and a molecule compares to graphite or other carbon based materials is crucial to future use of graphene as a ‘supreme’ electrode material. 1 Herein, the electrochemical response of mono-layer and multi-layer graphene electrodes, prepared using both chemical vapour deposition and mechanical exfoliation, is presented. Experiments were carried out using a microinjector, micromanipulator and optical microscopy, enabling precise deposition of size-controlled droplets on the flake surface. 1 The insulating substrates used for flake preparation include inorganic materials and organic polymers. Graphene/graphite flakes were characterised using optical microscopy, Raman spectroscopy and atomic force microscopy. The electron transfer rate at the basal planes, edge planes and boundaries between graphene flakes was determined using voltammetric techniques. The redox couple, number of graphene layers, presence of defects and nature of the substrate are all shown to have a significant effect on the electrochemical activity. 2-3 Furthermore, the stability of the droplets on the surface depends on various factors, such as the solvent choice, electrolyte content, number of graphene layers, underlying substrate and method of flake preparation. 1. Valota, A. T.; Toth, P. S.; Kim, Y. J.; Hong, B. H.; Kinloch, I. A.; Novoselov, K. S.; Hill, E. W.; Dryfe, R. A. W., Electrochimica Acta , 2013 , 110 , 9-15. 2. Valota, A. T.; Kinloch, I. A.; Novoselov, K. S.; Casiraghi, C.; Eckmann, A.; Hill, E. W.; Dryfe, R. A. W., ACS Nano , 2011 , 5 (11), 8809-8815. 3. P. S. Toth; A. Valota; M. Velicky; I. Kinloch; K. Novoselov; E. W. Hill; R. A. W. Dryfe, Chemical Science, 2013 , DOI:10.1039/C3SC52026A .

No abstract is provided for this article.

We demonstrated experimentally that graphene–Cu–graphene heterogeneous films reveal strongly enhanced thermal conductivity as compared to the reference Cu and annealed Cu films. Chemical vapor deposition of a single atomic plane of graphene on both sides of 9 μm thick Cu films increases their thermal conductivity by up to 24% near room temperature. Interestingly, the observed improvement of thermal properties of graphene–Cu–graphene heterofilms results primarily from the changes in Cu morphology during graphene deposition rather than from graphene's action as an additional heat conducting channel. Enhancement of thermal properties of graphene-capped Cu films is important for thermal management of advanced electronic chips and proposed applications of graphene in the hybrid graphene–Cu interconnect hierarchies.

We present a novel approach to address the challenges of variable occupation numbers in direct optimization of density functional theory (DFT). By parametrizing both the eigenfunctions and the occupation matrix, our method minimizes the free energy with respect to these parameters. As the stationary conditions require the occupation matrix and the Kohn–Sham Hamiltonian to be simultaneously diagonalizable, this leads to the concept of “self-diagonalization,” where, by assuming a diagonal occupation matrix without loss of generality, the Hamiltonian matrix naturally becomes diagonal at stationary points. Our method incorporates physical constraints on both the eigenfunctions and the occupations into the parametrization, transforming the constrained optimization into an fully differentiable unconstrained problem, which is solvable via gradient descent. Implemented in JAX, our method was tested on aluminum and silicon, confirming that it achieves efficient self-diagonalization, produces the correct Fermi-Dirac distribution of the occupation numbers and yields band structures consistent with those obtained with SCF eigensolver methods in Quantum Espresso.

Transport measurements have been a powerful tool for uncovering new electronic phenomena in graphene. We report nonlocal measurements performed in the Hall bar geometry with voltage probes far away from the classical path of charge flow. We observe a large nonlocal response near the Dirac point in fields as low as 0.1T, which persists up to room temperature. The nonlocality is consistent with the long-range flavor currents induced by lifting of spin/valley degeneracy. The effect is expected to contribute strongly to all magnetotransport phenomena near the neutrality point.

The morphology and chemical composition of native gold from the Murtykty deposit (South Urals) are discussed in the paper. Native gold is subdivided into fve types: 1) inclusions in sulfdes, 2) grains in fractures and interstices of pyrite and other sulfdes, 3) grains at the boundaries of sulfdes and gangue minerals, 4) inclusions in gangue minerals and their interstices and 5) «free¬ grains from heavy concentrates. The chemical composition of 128 gold grains is studied. In accordance with Ag content (from 6.49 to 30.8 wt. %), native gold from the deposit varies from high- to low-fneness (rare grains). Native gold in sphalerite contains Cu (0.85–2.27 wt. %)., The morphological features of 264 gold grains are quantifed using Image Tool software. The morphological characteristics of native gold from types 1–4 are similar. The data on morphology and size of native gold grains can be used for improvement of technology of gold extraction.

The emergence of van der Waals (vdW) materials resulted in the discovery of their giant optical, mechanical, and electronic anisotropic properties, immediately enabling countless novel phenomena and applications. Such success inspired an intensive search for the highest possible anisotropic properties among vdW materials. Furthermore, the identification of the most promising among the huge family of vdW materials is a challenging quest requiring innovative approaches. Here, we suggest an easy-to-use method for such a survey based on the crystallographic geometrical perspective of vdW materials followed by their optical characterization. Using our approach, we found As2S3 as a highly anisotropic vdW material. It demonstrates rare giant in-plane optical anisotropy, high refractive index and transparency in the visible range, overcoming the century-long record set by rutile. Given these benefits, As2S3 opens a pathway towards next-generation nanophotonics as demonstrated by an ultrathin true zero-order quarter-waveplate that combines classical and the Fabry-Perot optical phase accumulations. Hence, our approach provides an effective and easy-to-use method to find vdW materials with the utmost anisotropic properties.

Interactive clothing requires sensing and display functionalities to be embedded on textiles. Despite the significant progress of electronic textiles, the integration of optoelectronic materials on fabrics remains as an outstanding challenge. In this Letter, using the electro-optical tunability of graphene, we report adaptive optical textiles with electrically controlled reflectivity and emissivity covering the infrared and near-infrared wavelengths. We achieve electro-optical modulation by reversible intercalation of ions into graphene layers laminated on fabrics. We demonstrate a new class of infrared textile devices including display, yarn, and stretchable devices using natural and synthetic textiles. To show the promise of our approach, we fabricated an active device directly onto a t-shirt, which enables long-wavelength infrared communication via modulation of the thermal radiation from the human body. The results presented here provide complementary technologies which could leverage the ubiquitous use of functional textiles.