Publications

Atomically thin layers of graphene (GR) and other two-dimensional materials (2DM) such as hexagonal Boron Nitride (hBN) possess unique mechanical, electronic and thermal properties making them ideal materials for variety of nanoelectromechanical sensors (NEMS) [1].Exploration of such 2DM NEMS requires both ultimate resolution approaching atomic scale as well as ability to explore short time scale phenomena in such devices. Here we explore 2DM nanostructures using combination of scanning probe microscopy (SPM), ultrasonic vibrations and electrostatic interactions that reveal key nanomechanical and nanoelectromechanical properties of 2DM essential for the systems where the atomically thin layers are subjected to the flexural and normal stresses and electrical fields [2].We use quantitative analysis of the stress field generated by the SPM probe vibrated at the ultrasonic frequencies in the 2DM’s and show that in these elastically transversely isotropic materials, a stress propagation of the 2DM and it heterostructures is directly governed by the ratio of the out-of-plane Young modulus and the in-plane shear modulus. This shows direct proof of “ultrasonic transparency” of few layer graphene and MoS2 observed in ultrasonic force microscope (UFM) and allows to observe defects and structures under immediate surface of such materials [3]. We demonstrate that anisotropic properties of 2DMs allow exploration of local electrostatic interactions between the material and the substrate via nanomechanical actuation, revealing and mapping with nanoscale resolution the charges hidden under the layers of such materials. We then use nonlinear detection of nanoelectromechanical actuation via heterodyne force microscopy (HFM) [4] allowing to port the phase and amplitude of the high frequency vibration to kHz frequencies easily detected by atomic force microscopy. This allowed us to detect vibrations in 2DM NEMS with pm vertical resolution, and ps time-scale sensitivity. Significantly, the charges hidden between the 2D materials and substrate interface can be revealed due to resulting electromechanical actuation of such material.[1] J. S. Bunch et al, Science, 315, 490-493, (2007)[2] Kay N.D et al, Nano Letters. 14(6) :3400, 2014.[3] Dinelli, F., Pingue, P., Kay, N., D. & Kolosov, O., V., Nanotechnology 28, 085706 (2017).[4] M. T. Cuberes et al, Journal of Phys D: App Phys, 33, 2347, (2000)[5] Authors acknowledge the support of EU QUANTIHEAT grant and EPSRC Graphene NowNano CDT.

The availability of accessible fabrication methods based on deterministic transfer of atomically thin crystals has been essential for the rapid expansion of research into van der Waals heterostructures. An inherent issue of these techniques is the deformation of the polymer carrier film during the transfer, which can lead to highly nonuniform strain induced in the transferred two-dimensional material. Here, using a combination of optical spectroscopy, atomic force, and Kelvin probe force microscopy, we show that the presence of nanometer scale wrinkles formed due to transfer-induced stress relaxation can lead to strong changes in the optical properties of MoSe2/WSe2 heterostructures and the emergence of linearly polarized interlayer exciton photoluminescence. We attribute these changes to local breaking of crystal symmetry in the nanowrinkles, which act as efficient accumulation centers for interlayer excitons due to the strain-induced interlayer band gap reduction. Surface potential images of the rippled heterobilayer samples acquired using Kelvin probe force microscopy reveal variations of the local work function consistent with strain-induced band gap modulation, while the potential offset observed at the ridges of the wrinkles shows a clear correlation with the value of the tensile strain estimated from the wrinkle geometry. Our findings highlight the important role of the residual strain in defining optical properties of van der Waals heterostructures and suggest effective approaches for interlayer exciton manipulation by local strain engineering.

For the first time, we demonstrate that monolayer graphene can control surface-plasmon modes in terahertz lasers. Integrating a graphene-coated, multi-band filter, varied in situ by electrochemical gating, individual modes can be switched on and off.

When two-dimensional crystals are brought into close proximity, their interaction results in reconstruction of electronic spectrum and crystal structure. Such reconstruction strongly depends on the twist angle between the crystals, which has received growing attention due to interesting electronic and optical properties that arise in graphene and transitional metal dichalcogenides. Here we study two insulating crystals of hexagonal boron nitride stacked at small twist angle. Using electrostatic force microscopy, we observe ferroelectric-like domains arranged in triangular superlattices with a large surface potential. The observation is attributed to interfacial elastic deformations that result in out-of-plane dipoles formed by pairs of boron and nitrogen atoms belonging to opposite interfacial surfaces. This creates a bilayer-thick ferroelectric with oppositely polarized (BN and NB) dipoles in neighbouring domains, in agreement with our modeling. These findings open up possibilities for designing van der Waals heterostructures and offer an alternative probe to study moiré-superlattice electrostatic potentials.

In this paper, we have sandwiched a single-atom-thick sheet of graphene, a 2-D allotrope of carbon that is exciting tremendous research interest, between two ferromagnetic electrodes. Graphene has already been shown to support spin polarized conduction, when spin polarized electrons were injected into graphene sheets along the plane using magnetic electrodes. Here, we study spin polarized conduction perpendicular the plane of the graphene in conduction perpendicular to plane (CPP) geometry. It was found that graphene was sufficient to reduce the exchange coupling between the magnetic electrodes. We also found that in a NiFe/Au/Graphene/NiFe stack, the graphene channels the spin current perpendicular to the plane and thus produces an enhanced MR effect compared to a simple stack without the graphene. A significant anisotropy was found in the observed magnetoresistance in the cross electrode geometry employed.

No abstract is provided for this article.

Two-dimensional semiconductors - atomic layers of materials with covalent intra-layer bonding and weak (van der Waals or quadrupole) coupling between the layers - are a new class of materials with great potential for optoelectronic applications. Among those, a special position is now being taken by post-transition metal chalcogenides (PTMC), InSe and GaSe. It has recently been found that the band gap in 2D crystals of InSe more than doubles in the monolayer compared to thick multilayer crystals, while the high mobility of conduction band electrons is promoted by their light in-plane mass. Here, we use Raman and PL measurements of encapsulated few layer samples, coupled with accurate atomic force and transmission electron microscope structural characterisation to reveal new optical properties of atomically thin GaSe preserved by hBN encapsulation. The band gaps we observe complement the spectral range provided by InSe films, so that optical activity of these two almost lattice-matched PTMC films and their heterostructures densely cover the spectrum of photons from violet to infrared. We demonstrate the realisation of the latter by the first observation of interlayer excitonic photoluminescence in few-layer InSe-GaSe heterostructures. The spatially indirect transition is direct in k-space and therefore is bright, while its energy can be tuned in a broad range by the number of layers.

Optical materials with colour-changing abilities have been explored for display devices, smart windows, or modulation of visual appearance. The efficiency of these materials, however, has strong wavelength dependence, which limits their functionality to a narrow spectral range. Here, we report graphene-based electro-optical devices with unprecedented optical tunability covering the entire electromagnetic spectrum from the visible to microwave. We achieve this non-volatile and reversible tunability by electro-intercalation of lithium into graphene layers in an optically accessible device structure. This unique colour-changing capability, together with area-selective intercalation, inspires fabrication of new multispectral devices, including display devices and electro-optical camouflage coating. We anticipate that these results provide realistic approaches for programmable smart optical surfaces with a potential utility in many scientific and engineering fields.

No abstract is provided for this article.

The most promising way to address modern environmental and energy supply challenges is via rapid implementation of decarbonization and hydrogen production technologies. Development of gas separation membranes with high selectivity and permeability is essential for these processes. Traditional polymeric membranes often struggle to strike the right balance between these properties due to their structural and chemical limitations. Our research focuses on achieving precise control of gas diffusion pathways through on-demand regulation of material interactions in nanometer-thick composite membranes. We combine covalent organic frameworks (COFs) and graphene oxide (GO) to create COF-GO composite membranes. These materials offer enhanced gas separation performance and tailor-made selectivity. By pH-assisted self-assembly, we fine-tune material interactions, achieving a balance between selectivity and permeability by regulating the surface charge density of COF and GO. Our work presents a novel approach to gas separation by manipulating inter-layer interactions between COF and GO, paving the way for tailored gas separation. Through these efforts, we fabricate COF-GO composite membranes with controlled thickness and gas pathways. Compared to bare GO and COF membranes, the composite structure offers an optimal transport performance, enhanced selectivity in thicker membranes without losing permeability, and superior long-term stability. This enables the unique 2D transport mechanism to be utilized under real practical conditions. Our research offers a strategy for the design of composite membranes from two-dimensional (2D) materials for gas separation technologies. It contributes to sustainable decarbonization and hydrogen production solutions, bringing us closer to a greener, more environmentally friendly future.

Dataset and codes for the manuscript "Topological engineering of terahertz light using electrically tuneable exceptional point singularities".

The interplay between dimensionality and electronic correlations in van der Waals (vdW) materials offers a powerful toolkit for engineering light-matter interactions at the nanoscale. Excitons, bound electron-hole pairs, are central to this endeavor, yet maximizing their oscillator strength, which dictates the interaction cross-section, remains a challenge. Conventional wisdom suggests a trade-off, where the observable oscillator strength often decreases in strongly bound systems due to population dynamics. Here, we unveil a colossal oscillator strength associated with the quasi-one-dimensional (quasi-1D) excitons in the layered magnetic semiconductor CrSBr, which fundamentally defies this established scaling law. Through comprehensive optical characterization and ab initio calculations, we establish that this anomalous enhancement originates directly from the reduced dimensionality, which enforces an increased electron-hole wavefunction overlap. Moreover, we find a close connection between fundamental exciton and local spin fluctuations that contribute to the opening of the gap in the electronic spectrum. The resulting optical anisotropy shows a giant in-plane birefringence (Delta_n = 1.45) and profoundly anisotropic waveguiding, which we directly visualize using nano-optical imaging. Leveraging this extreme response, we realize a true zero-order quarter-wave plate with an unprecedented wavelength-to-thickness ratio (lambda/t) exceeding 3.4, surpassing the limits of current miniaturization technologies, including state-of-the-art metasurfaces. Our findings underscore the profound impact of dimensionality engineering in magnetic vdW materials for realizing novel regimes of light-matter coupling and developing next-generation ultracompact photonic architectures.

Cyclotron motion of charge carriers in metals and semiconductors leads to Landau quantization and magneto-oscillatory behavior in their properties. Cryogenic temperatures are usually required to observe these oscillations. We show that graphene superlattices support a different type of quantum oscillations that do not rely on Landau quantization. The oscillations are extremely robust and persist well above room temperature in magnetic fields of only a few T. We attribute this phenomenon to repetitive changes in the electronic structure of superlattices such that charge carriers experience effectively no magnetic field at simple fractions of the flux quantum per superlattice unit cell. Our work points at unexplored physics in Hofstadter butterfly systems at high temperatures.

Stacking different two-dimensional (2D) atomic layers is a feasible approach to create unique multilayered van der Waals heterostructures with desired properties. 2D materials, graphene, hexagonal boron nitride (h-BN), molybdenum disulphate (MoS2) and graphene based van der Waals heterostructures, such as h-BN/graphene and MoS2/graphene have been investigated by means of Scanning Transmission Electron Microscopy (STEM).