The team members constitute a cross-disciplinary research group with long-standing experience on the optical, vibrational and electronic properties of materials such as graphene, other 2D materials and molecular nanomaterials (e.g. fullerenes, carbon nanotubes). Moreover, we are interesting on the synthesis and manipulation of 2D materials as well as the fabrication of polymer nanocomposites based on either 2D materials or carbon nanotubes.
The team have participated in several funded EU and national projects and have strong links with industry and SMEs (e.g. Nanonics (Israel), CealTech (Norway), BiC-Violex, AXONAS, tipografio).
Upon request, our facilities and know-how can be available to other researchers and industries to investigate any materials of interest.
By using spectroscopic techniques such as Raman (both in frequency and time domain), Photoluminescence, absorption, reflectance, and transmission in UV-vis-IR) and the synergy of theoretical calculations (phenomenological models, first-principles calculations, group theory) the optical properties of a wide range of materials (semiconductors, fullerenes, carbon nanotubes, garnets, low-dimensional materials, etc.) have been studied.
In the last decade, our efforts have been devoted to the optical properties of two-dimensional materials such as single and multilayer graphene, transition metal dichalcogenides (e.g. MoS2, WS2, Mo1-xWxS2, MoSe2, SnSexS1-x), BN, lateral and vertical heterostructures. Optical spectroscopy provides the unique opportunity to deeply investigate temperature, doping, and mechanical strain effects, 2D material-substrate interactions, edges, structural disorder, oxidation, hydrogenation or chemical functionalization, carrier mobility, thermal conductivity, electron-phonon coupling, interlayer coupling, quantum interference effects, interband transitions, and excitonic effects.
Moreover, the optical properties are quite sensitive to changes in many external perturbations such as temperature, uniaxial, biaxial, or shear mechanical deformation, hydrostatic pressure, the polarization of the incident beam (valley polarization), electrochemical and chemical doping, gate voltage, electric and magnetic field providing, thus, insight on the physical phenomena and allowing the investigation of structural stability and pressure/doping induced structural or electronic phase transitions in nanomaterials.
– Mechanical exfoliation of 2D crystals. Production of large-size flakes.
– CVD production of 2D materials. We have recently developed a method for the production of large-area triangular monolayer MoS2 and WS2 crystals via the reaction between sodium salts of molybdenum and tungsten as well as sulfur vapors at elevated temperatures and inert nitrogen atmosphere. By alternating growth parameters such as the sulfur temperature and the inert gas flow, the fabrication of large-area triangular TMDC monolayer crystals or even continuous single layer TMDC films with few-layered domains can be achieved. Furthermore, by this method, the growth of lateral or vertical single-layer MoS2-WS2 heterostructures and ternary alloys Mo1-xWxS2 can be achieved by mixing the Mo and W precursors.
– Application of several transfer methods (e.g. stamping, a sacrificial polymer layer, electrochemical, lamination) to deposit high quality, wrinkle-free 2D sheets onto target substrates.
– Intense characterization of the fabricated materials such as Raman and Photoluminescence mapping using automated stages, AFM using various modules, and AFM/Raman. Also, a wide range of instrumentation including SEM, TEM, XPS/AES/UPS/EELS, XRD, FTIR, UV-Vis-NIR, Hg-porosimetry, particle size analyzer, DSC/μ-DSC, and DMA are available to our group at AUTh and FORTH/ICE-HT.
We assess the mechanical behavior of a variety of 2D materials (e.g. single and multi-layer graphene, BN, selected TMDCs) under uniaxial/biaxial/shear tensile and compressive loadings, using cantilever-beam arrangements with PMMA bars. Essentially, we fabricate model composite materials by embedded flakes (mechanical exfoliated or CVD) supported on a plastic bar within a layer of PMMA and a layer of SU8 (~200 nm) photoresist. By monitoring the shift of selected Raman active phonon modes with strain, unique information about the mechanical response of 2D crystals at the nanoscale can be delivered, such as failure and deformation characteristics, stress transfer efficiency as a function of strain sign, critical buckling strain, the influence of wrinkles on the mechanical performance, etc.
An important aspect of this activity is strain engineering giving rise to the possibility of inducing local strains by pocking, bending, folding, deforming homogenously or inhomogenously the 2D materials to tune their electronic and photonic properties, dramatically.
The response of 2D materials at various temperatures (4 -800 K) is another interesting research topic. Our aim is to understand the temperature induced changes of first order, overtone and combination phonon modes, phonon-phonon interactions, the adhesion of 2D materials with the substrate, the level of thermally induced strain on 2D materials (thermal strain engineering), thermal conductivity and possible temperature induced phase transitions.
Finally, we conduct in-situ Raman spectroelectrochemistry measurements which is an alternative method to controllably doped 2D materials and at the same time follow the doping induced changes of phonon and electronic properties. The samples were also investigated by means of electrochemical impedance spectroscopy.
Polymer nanocomposites
Another line of research is the fabrication of novel polymer composites with various nanofillers such as carbon nanotubes (buckypapers) and 2D materials using the melt-mixing, buckypaper and electrospinning approaches, having exceptional mechanical, electrical and thermal properties.
PTFE/graphene in razor blade technology
In collaboration with BIC Violex S. A. (Appl. Patent US0251641 A1 (2018) we have developed a novel functional PTFE-based nanocomposite coating, incorporating nanoplatelets of graphene, graphene-oxide or other 2D materials, exhibiting improved lubricity. Incorporation of the coating onto BIC’s razor blades, could result in a significantly improved product
Graphene based PTFE inks
We have fabricated aqueous graphene-based inks using aqueous solutions of various types of graphene and a binder (PTFE/detergent). Aqueous graphene inks in various concentrations of graphene (w/w 1-4%) have been used in spray coatings on paper and metal substrates with absolute coating success. Our effort is to use the inks in screen printing to develop among others RF-ID antennas, wearable electronics, and conductive textiles
In order to address key questions in relevant topics of solid-state physics the group uses large research facilities such as ESRF (Grenoble (France)), ILL (Grenoble (France)), ISIS Rutherford Appleton Laboratory (Oxford (UK)), LANSCE (Los Alamos (USA)) to conduct complementary: i) inelastic X-ray and neutron scattering measurements to study the dispersion curves and the density of states of crystalline solids, and ii) powder diffraction X-ray measurements at ambient conditions and under variable temperature (2-1200 K) or high pressure (0- 30 GPa) to investigate the structure and possible phase transitons of structural or electronic origin.
We deeply investigate the lattice dynamics of crystalline solids (3D and 2D) using phenomenological force field models, molecular dynamics and first-principles calculations. Application of group theory for the interpretation of Raman and Infrared spectra. The experimental findings supported by numerical simulations.