The DYNASTY Workshop and Summer School on 2D Materials was held in the framework of the 3rd International Conference on Nanotechnologies and Bionanoscience (NanoBio 2025), which took place on 8–12 September 2025 in Heraklion, Crete, Greece.
Tittle Exciton Formation in 2D Semiconductors (Invited Tutorial Lecture)
Invited Speaker: Dr. X. Marie (INSA)
Abstract:
Robust excitons dominate the optical properties of atomically thin semiconductors based on transition-metal dichalcogenides (TMDs). The use of fully encapsulated hexagonal boron nitride (hBN) and charge-tunable (CT) TMD monolayers (MLs) allow the electrostatic doping of the MLs, thereby substantially improving the control of exciton complexes. Exciton relaxation and formation dynamics in TMDs have been extensively studied by time-resolved optical spectroscopies. Nonetheless, a crucial question persists: What is the exciton formation mechanism, and how does this process occur in two-dimensional semiconductor systems? This study addresses this fundamental problem through polarization-dependent micro-photoluminescence (PL) studies performed at cryogenic temperatures (4K) on fully hBN-encapsulated and CT TMD monolayers close to the neutrality point.
The results of our experiments performed on both WSe2 and MoS2 MLs clarify the role played by the two potential formation mechanisms: a) geminate and b) bimolecular. The geminate exciton formation process corresponds to the monomolecular annihilation of the photogenerated correlated electron-hole pair. In contrast the non-geminate formation results from the random bimolecular binding of two free charges, losing all correlation between the excitation photon and the electron-hole pair of the exciton.
For a laser excitation energy below the band gap, we show that the geminate mechanism prevails as expected, whereas above the band gap, both geminate and bimolecular phenomena coexist. These results bring precious information on the exciton formation mechanism in 2D semiconductors, which is crucial for the optoelectronic applications of these materials.
Tittle Elastic Screening of Pseudogauge Fields in Graphene
Invited Speaker: Dr. Lucian Covaci (University of Antwerp)
Abstract:
Lattice deformations in graphene couple to the low-energy electronic degrees of freedom as effective scalar and gauge fields. Using molecular dynamics simulations, we show that the optical component of the displacement field, i.e., the relative motion of different sublattices, contributes at equal order as the acoustic component and effectively screens the pseudogauge fields. In particular, we illustrate this effect for twisted bilayer graphene and corrugated monolayer graphene [1]. In both cases, optical lattice displacements significantly reduce the overall magnitude of the pseudomagnetic fields. For corrugated graphene, optical contributions also reshape the pseudomagnetic field and significantly modify the electronic bands near charge neutrality.
Previous studies based on continuum elasticity, which ignores this effect, have therefore systematically overestimated the strength of the strain-induced pseudomagnetic field. Our results have important consequences for the interpretation of experiments and design of straintronic applications. Furthermore, similar considerations must be considered for other two-dimensional materials where both acoustic and optical lattice displacements are expected to contribute significantly to the optoelectronic properties of these materials.
Tittle Topochemical reactions from monoelemental Xenes to MXenes
Invited Speaker: Dr. Zdenek Sofer (University of Chemistry and Technology Prague)
Abstract:
The Xenes represent a rapidly developing family of monoelemental two-dimensional (2D) materials. The chemistry of monoelemental materials from the tetrel group will be presented, with a detailed discussion of various synthesis strategies and chemical exfoliation techniques based on topochemical deintercalation and exfoliation. The differences between the exfoliation of pnictogens and tetrels will be highlighted, comparing chemical and mechanical exfoliation approaches [1].
In addition, methods for synthesizing 2D compounds across all main group elements, as well as techniques for crystal growth, will be discussed. MXene synthesis is closely related to approaches used for tetrel-based Xenes, involving selective etching of elements from parent layered carbides through topochemical deintercalation and exfoliation. The surface chemistry of MXenes and their functionalization strategies will be presented. In particular, siloxane chemistry enables the effective introduction of various organic groups onto the MXene surface, expanding their range of applications. Furthermore, direct reactions with chalcogens enable modification of surface functional groups and, at elevated temperatures, the formation of composite systems through the direct synthesis of transition metal dichalcogenides. Such direct topochemical conversion of MXenes yields composite materials and chalcogen-terminated MXene surfaces.
Tittle Alloy-Driven Tuning of Bandgap, Spin-Orbit Splitting and Phonon Energy in 2D Mo-Based TMDs
Invited Speaker: Dr. Ioannis Paradisanos (FORTH)
Abstract:
Two-dimensional transition metal dichalcogenides (TMDs) provide a highly tunable platform for exploring excitonic and vibrational phenomena in the atomically thin limit[1]. This work presents a systematic investigation of the evolution of optical properties in a series of Mo-based TMD monolayers with varying chalcogen composition, in the alloy system MoS2xSe2(1-x), where x ranges from 0 to 1.
The optical bandgap is found to decrease continuously from 1.9 eV in MoS2 to 1.55 eV in MoSe2, tracking the substitution of sulfur with selenium. At the same time, the A-B exciton splitting increases from 150 meV to 200 meV, reflecting the influence of chalcogen composition on spin–orbit coupling. Additionally, the average phonon energy varies from approximately 24 meV (MoS2) to 19 meV (MoSe2), indicating systematic modifications in lattice vibrational properties across the alloy series.
These findings demonstrate that chalcogen composition offers an effective means to tune the optical response of Mo-based TMD monolayers. The observed trends provide valuable insights into the excitonic and vibrational behavior of 2D alloys, with direct implications for the development of tunable photonic, optoelectronic, and valleytronic devices.
Tittle Tuning the optoelectronic properties of 2D-TMDs via dielectric engineering
Invited Speaker: Prof. George Kioseoglou (University of Crete, FORTH)
Abstract:
Nanoscale-engineered surfaces induce regulated strain in atomic layers of 2-dimensional (2D) materials that could be useful for unprecedented photonics applications and for storing and processing quantum information. This work presents textured induced strain distribution in single layers of WS2 (1L-WS2) transferred over Si/SiO2 (285nm) substrates [1]. The detailed nanoscale landscapes and their optical detection are carried out through AFM, SEM and optical spectroscopy. Remarkable differences have been observed in the WS2 sheet localized in the confined well and at the periphery of the cylindrical geometry of the capped engineered surface. Raman spectroscopy independently maps the whole landscape of the samples. Temperature dependent helicity-resolved Photoluminescence (PL) experiments (off-resonance excitation), show that suspended areas sustain circular polarization from 150 K up to 300 K, in contrast to supported (on un-patterned area of Si/SiO2) and strained 1L-WS2.
These findings highlight the impact of dielectric environment on the optical properties of 2D materials, providing valuable insights into the selection of appropriate substrates for implementing atomically thin materials in advanced optoelectronic devices.
Acknowledgement: This work was supported by the EU-funded DYNANSTY project, ID:101079179, under the Horizon Europe framework programme
Tittle Engineering carrier density and exciton polarization in WSe2 monolayers via photochlorination
Invited Speaker: MSc. Eirini Katsipoulaki (FORTH)
Abstract:
Transition Metal Dichalcogenides (TMDs) of the form MX2 (M = Mo, W and X=S, Se, Te) represent a special class of 2D van der Waals materials. Unlike their 3D-counterparts, which are indirect gap semiconductors, MX2 monolayers exhibit a direct bandgap, leading to a significant enhancement in photoluminescence quantum yield. Moreover, TMDs feature valley dependent optical selection rules, establishing them as promising candidates for atomically thin optoelectronic devices and spin-valley memory applications. A key factor influencing the performance of TMDs in these applications is the carrier density. To address this, we demonstrate the modulation of the Fermi level in WSe2 monolayers using an ultraviolet-assisted photochlorination method.
Systematic shifts and relative intensities between charged and neutral excitons indicate a progressive and controllable decrease of the electron density and switch WSe2 from n- to a p-type semiconductor. Density functional theory predicts Cl2 adsorption at Se vacancies drives p-type doping, while X-ray photoelectron spectroscopy confirms the incorporation of chlorine [1]. Furthermore, this method can strongly impact the circular polarization degree of excitons, demonstrating its potential to control exciton-carrier scattering processes [2]. These findings indicate that photochemical techniques can be utilized to tailor nanopatterned lateral p-n junctions, while also highlighting their potential for engineering valley relaxation phenomena.
Tittle Low dose electron microscopy imaging, one electron at a time
Invited Speaker: Dr. Johan Verbeeck
Abstract:
Transmission electron microscopy is an indispensable tool providing a direct image of matter down to its atomic structure. Nanotechnology relies entirely on it and in life science imaging, it is responsible for most of what we know about the structure of proteins, viruses and cell constituents. Imaging with electrons however also has the drawback that the strong interaction can cause damage through structural changes. In this talk, we will look into how modern detectors and computational methods are reshaping this field. We now can build up images literally one electron at a time allowing to strike the most optimal balance between information gained and damage created.
Tittle Atomic-Scale Imaging of Moiré Superlattices in Twisted Transition Metal Oxide Membranes
Invited Speaker: Dr. N. Gauquelin (University of Antwerp)
Abstract:
Heterostructures composed of dissimilar materials have been pivotal in advancing nanoscale science, particularly through the study of two-dimensional (2D) materials and van der Waals interfaces. Recent breakthroughs in fabrication now allow the creation of freestanding complex oxide films approaching monolayer thickness [1] — opening new directions to explore the rich functional landscape of transition metal oxides (TMOs).
A key feature of these systems is the formation of Moiré superlattices, emerging from twisted bilayer configurations. These nanoscale interference patterns can induce unprecedented electronic, optical, magnetic, and mechanical properties—absent in the untwisted or bulk forms of the same materials. [2]
In this study, we apply advanced Scanning Transmission Electron Microscopy (STEM), including annular dark field (ADF) imaging and four-dimensional STEM (4DSTEM), to directly visualize the local atomic structure of twisted TMO membranes. While such techniques are well-established in the 2D materials field [3], this marks their first application to freestanding oxide heterostructures.
Complemented by simulations and Density Functional Theory (DFT) calculations, our findings reveal chiral lattice distortions and structural modulations directly linked to the twist angle.
Tittle Silicon nanoantennas for tailoring the optical properties of MoS2 monolayers
Invited Speaker: Ms Danae Katrisioti
Abstract:
Silicon-based dielectric nanoantennas provide an effective platform for engineering light-matter interactions in van der Waals semiconductors [1]. We demonstrate nearfield coupling between monolayer MoS₂ and silicon-based dielectric nanoantennas arranged in hexagonal lattices with tunable geometries. This interaction leads to a three-fold enhancement in photoluminescence and excitation-wavelength-dependent emission aligned with Mie-resonant modes. Raman spectroscopy reveals up to an 8- fold enhancement in the vibrational modes of MoS₂, while second-harmonic generation exhibits a 20–30-fold increase in efficiency, tightly correlated with the nanoantenna resonances.
Through a combination of photoluminescence excitation (PLE) spectroscopy, polarization-resolved photoluminescence, atomic force microscopy, and numerical simulations, we decouple the roles of strain, thin-film interference, and Purcell enhancement in modifying the optical response. These results highlight the tunable nature of near-field interactions in 2D materials and establish dielectric Mie-resonant structures as a scalable, CMOS-compatible platform for engineering both linear and nonlinear optical properties at the nanoscale.
Tittle Exploring 2D materials with theory and simulation
Invited Speaker: Prof. Georgios Kopidakis
Abstract:
Two-dimensional (2D) materials of atomic thickness exhibit interesting physics and show great promise in electronics, optoelectronics, quantum technologies, catalysis, clean energy and environment applications. Beyond graphene, many 2D materials with diverse electronic properties are being intensively explored. Layer by layer stacking of monolayers gives rise to van der Waals heterostructures of nanometer thickness and clean interfaces. Electronic properties are strongly affected by strain, nanostructuring, structural and chemical defects, disorder, generating a wide range of 2D nanostructures with novel features, possibilities and challenges.
Theory and simulation play a crucial role in answering emerging fundamental questions and identifying candidate materials with properties tailored for specific applications. First-principles calculations for the solution of the quantum mechanical problem are the main tool for atomic-scale understanding of these materials but face serious challenges in non-periodic and multi-component systems. We will present theoretical and computational approaches used by our research group for overcoming such challenges, with results that often explain observations and make useful predictions, thus assisting and, sometimes, guiding experiments. Finally, we will briefly discuss efforts to combine theory, multi-scale modeling/simulation, and machine learning to accelerate progress in fundamental and applied aspects of 2D materials research.
Tittle Twist-angle tuned second harmonic generation in 2D transition metal dichalcogenide homo- and heterobilayers
Invited Speaker: Dr. Sotiris Psilodimitrakopoulos (FORTH)
Abstract:
Two-dimensional (2D) transition metal dichalcogenide (TMD) monolayers (MLs) exhibit second harmonic generation (SHG) due to their non-centrosymmetric crystal structure along their armchair direction [1]. The 2D TMDs can be vertically stacked via van der Waals forces, enabling interlayer coupling between the MLs [2]. Such coupling gives rise to emergent physical phenomena that are highly sensitive to the twist-angle between the MLs [3]. Furthermore, the stacked MLs generate interference SHG, which is also strongly dependent on their twist-angle [4-6].
Here, we show that tuning the twist-angle in 2D TMD homo- and hetero-bilayers allows accurate control over both the intensity and the polarization of the SHG signals. Moreover, we demonstrate that these SHG signals can be used to precisely determine the twist-angle between the stacked MLs.
The ability to tailor the intensity and the polarization of SHG signals by adjusting the twist-angle between MLs, represents a significant advancement for next-generation 2D nonlinear optical materials. Additionally, the produced SHG signals enable rapid, non-invasive and large-area mapping of the twist-angle, which is invaluable for the characterization of twisted 2D devices.
Acknowledgements: This work has been supported by the EU-funded project DYNASTY under the Horizon Europe framework programme (Grant Agreement number: GA 101079179]). SP acknowledges DemosAxia project under the Horizon Europe framework programme (Grant Agreement number: GA 101160387)