Periodic Reporting for period 1 - CHIMERA (Characterising Heterostructures and Integrated Methodologies for Electronic Real-time Analysis in 2D Materials)
Okres sprawozdawczy: 2024-05-01 do 2026-04-30
Podsumowanie kontekstu i ogólnych celów projektu
The continued scaling of silicon-based electronics is approaching fundamental physical limits in terms of power dissipation, leakage currents, and device miniaturisation. As outlined in the European Chips Act and the broader Digital Europe programme, securing technological sovereignty in advanced semiconductor technologies is a strategic priority for the European Union. Within this landscape, two-dimensional (2D) transition metal dichalcogenides (TMDCs), and MoS2 in particular, have emerged as one of the most promising material classes for next-generation nanoelectronics. Their atomically thin geometry, tunable band gap, and compatibility with flexible substrates make them attractive candidates for ultra-low-power field-effect transistors (FETs), optoelectronic devices, and sensor architectures. Despite remarkable progress in the synthesis and device integration of 2D materials, a critical bottleneck persists: the absence of experimental methodologies capable of probing the electronic structure of a working device in real time. Standard characterisation techniques such as photoemission spectroscopy, transport measurements, and electron microscopy either operate under conditions incompatible with device operation or provide only indirect information on the electronic states that govern charge transport. This gap between materials characterisation and device-level understanding limits our ability to rationally optimise 2D transistors and, ultimately, to translate laboratory-scale results into industrially relevant technology.
The CHIMERA project (Characterising Heterostructures and Integrated Methodologies for Electronic Real-time Analysis in 2D Materials) was conceived to bridge precisely this gap. Its overarching goal was to establish a synchrotron-based, in-operando spectroscopic methodology that would enable the direct correlation between the electronic structure of MoS2 thin films and the electrical performance of FET devices built from them. The project was structured around three interconnected scientific objectives.
Objective O1: addressing the materials foundation of the project. Any meaningful in-operando study requires films whose structural quality and electronic properties are well understood and reproducible. IJD was chosen as the primary deposition technique because of its unique advantages: it operates at room temperature, preserves the stoichiometry of the target material, and is inherently scalable to large areas. These are features that distinguish IJD from conventional chemical vapour deposition (CVD), which requires high substrate temperatures, and from mechanical exfoliation, which yields only micron-scale flakes. The scientific challenge was to demonstrate that IJD-grown MoS2 films, after moderate thermal processing, can attain the electronic quality of their CVD-grown or exfoliated counterparts. Soft X-ray absorption spectroscopy (XAS) at the S L2,₃ and Mo M2,₃ edges was selected as the primary characterisation tool, since these edges directly probe the S 3p–Mo 4d hybridised states that define the conduction band of MoS2 and are exquisitely sensitive to crystallinity, dimensionality, and defect density.
Objective O2: quantifying 2D-FET performance via in-operando synchrotron spectroscopy. The second objective represented the methodological core of CHIMERA. The idea was to apply photon-in / photon-out spectroscopic techniques — specifically X-ray excited optical luminescence (XEOL) and soft X-ray reflectivity — to MoS2-based FETs while the transistors were electrically biased, thereby tracking changes in the electronic structure as the device switches between ON and OFF states. This approach exploits the element specificity of synchrotron radiation (allowing one to isolate, for example, the Mo 4d conduction band contribution) and the non-destructive character of photon-based probes, which unlike electron-based techniques do not induce charging artefacts in insulating device stacks. The experiments were designed for the BEAR beamline at Elettra, which provides tunable soft X-ray radiation combined with an ultra-high-vacuum experimental chamber equipped with reflectometry, luminescence detection, and electrical feedthroughs for simultaneous device biasing.
Objective O3: engineering logic gates with ultra-low energy dissipation. The third objective was planned to build on the knowledge acquired in the first two phases by fabricating and characterising elementary logic circuits (inverters) based on optimised MoS2 FETs, with the ultimate aim of demonstrating measurable reductions in switching energy.
The project sits at the intersection of two major European policy priorities. First, it contributes to the objectives of the European Green Deal by advancing the development of energy-efficient electronic devices based on 2D materials, which promise switching energies orders of magnitude lower than conventional silicon CMOS at equivalent gate lengths. The ultra-thin body of 2D channels virtually eliminates short-channel effects, one of the primary sources of standby power dissipation in scaled transistors. Second, the project aligns with the Europe for the Digital Age strategy and the European Chips Act by strengthening the knowledge base in advanced semiconductor technologies within European research institutions and by developing characterisation tools that are directly applicable to the quality control and process optimisation of next-generation semiconductor materials.
The global market for 2D materials is projected to grow substantially in the coming decade, driven by applications in flexible electronics, sensing, and energy storage. Within this landscape, Europe holds a strong position in fundamental materials science and synchrotron-based characterisation infrastructure (with facilities such as Elettra, ESRF, DESY, Diamond, and SOLEIL), but faces a persistent challenge in translating laboratory discoveries into scalable manufacturing processes. CHIMERA addressed this translation challenge directly: by combining a scalable deposition technique (IJD) with an advanced, non-destructive characterisation methodology (in-operando synchrotron spectroscopy), the project aimed to create a feedback loop between process optimisation and device-level electronic structure understanding that is essential for rational materials engineering.
The collaborative network established during the project, spanning UNIMORE, CNR-IOM (Trieste), Charles University (Prague), Humboldt-Universität zu Berlin, CNR-IMEM (Parma and Trento), the University of Rochester, the Université Libre de Bruxelles, and the University of Genova, further amplifies the potential for long-term impact by embedding the project’s methodological advances within a broad, interdisciplinary European and international research ecosystem. The scientific results are being disseminated through peer-reviewed open-access publications and will continue to generate follow-up studies (notably on in-operando reflectivity and spectroscopic ellipsometry of MoS2 FETs) beyond the formal duration of the fellowship, ensuring a sustained pathway from fundamental insight to technological application.
The CHIMERA project (Characterising Heterostructures and Integrated Methodologies for Electronic Real-time Analysis in 2D Materials) was conceived to bridge precisely this gap. Its overarching goal was to establish a synchrotron-based, in-operando spectroscopic methodology that would enable the direct correlation between the electronic structure of MoS2 thin films and the electrical performance of FET devices built from them. The project was structured around three interconnected scientific objectives.
Objective O1: addressing the materials foundation of the project. Any meaningful in-operando study requires films whose structural quality and electronic properties are well understood and reproducible. IJD was chosen as the primary deposition technique because of its unique advantages: it operates at room temperature, preserves the stoichiometry of the target material, and is inherently scalable to large areas. These are features that distinguish IJD from conventional chemical vapour deposition (CVD), which requires high substrate temperatures, and from mechanical exfoliation, which yields only micron-scale flakes. The scientific challenge was to demonstrate that IJD-grown MoS2 films, after moderate thermal processing, can attain the electronic quality of their CVD-grown or exfoliated counterparts. Soft X-ray absorption spectroscopy (XAS) at the S L2,₃ and Mo M2,₃ edges was selected as the primary characterisation tool, since these edges directly probe the S 3p–Mo 4d hybridised states that define the conduction band of MoS2 and are exquisitely sensitive to crystallinity, dimensionality, and defect density.
Objective O2: quantifying 2D-FET performance via in-operando synchrotron spectroscopy. The second objective represented the methodological core of CHIMERA. The idea was to apply photon-in / photon-out spectroscopic techniques — specifically X-ray excited optical luminescence (XEOL) and soft X-ray reflectivity — to MoS2-based FETs while the transistors were electrically biased, thereby tracking changes in the electronic structure as the device switches between ON and OFF states. This approach exploits the element specificity of synchrotron radiation (allowing one to isolate, for example, the Mo 4d conduction band contribution) and the non-destructive character of photon-based probes, which unlike electron-based techniques do not induce charging artefacts in insulating device stacks. The experiments were designed for the BEAR beamline at Elettra, which provides tunable soft X-ray radiation combined with an ultra-high-vacuum experimental chamber equipped with reflectometry, luminescence detection, and electrical feedthroughs for simultaneous device biasing.
Objective O3: engineering logic gates with ultra-low energy dissipation. The third objective was planned to build on the knowledge acquired in the first two phases by fabricating and characterising elementary logic circuits (inverters) based on optimised MoS2 FETs, with the ultimate aim of demonstrating measurable reductions in switching energy.
The project sits at the intersection of two major European policy priorities. First, it contributes to the objectives of the European Green Deal by advancing the development of energy-efficient electronic devices based on 2D materials, which promise switching energies orders of magnitude lower than conventional silicon CMOS at equivalent gate lengths. The ultra-thin body of 2D channels virtually eliminates short-channel effects, one of the primary sources of standby power dissipation in scaled transistors. Second, the project aligns with the Europe for the Digital Age strategy and the European Chips Act by strengthening the knowledge base in advanced semiconductor technologies within European research institutions and by developing characterisation tools that are directly applicable to the quality control and process optimisation of next-generation semiconductor materials.
The global market for 2D materials is projected to grow substantially in the coming decade, driven by applications in flexible electronics, sensing, and energy storage. Within this landscape, Europe holds a strong position in fundamental materials science and synchrotron-based characterisation infrastructure (with facilities such as Elettra, ESRF, DESY, Diamond, and SOLEIL), but faces a persistent challenge in translating laboratory discoveries into scalable manufacturing processes. CHIMERA addressed this translation challenge directly: by combining a scalable deposition technique (IJD) with an advanced, non-destructive characterisation methodology (in-operando synchrotron spectroscopy), the project aimed to create a feedback loop between process optimisation and device-level electronic structure understanding that is essential for rational materials engineering.
The collaborative network established during the project, spanning UNIMORE, CNR-IOM (Trieste), Charles University (Prague), Humboldt-Universität zu Berlin, CNR-IMEM (Parma and Trento), the University of Rochester, the Université Libre de Bruxelles, and the University of Genova, further amplifies the potential for long-term impact by embedding the project’s methodological advances within a broad, interdisciplinary European and international research ecosystem. The scientific results are being disseminated through peer-reviewed open-access publications and will continue to generate follow-up studies (notably on in-operando reflectivity and spectroscopic ellipsometry of MoS2 FETs) beyond the formal duration of the fellowship, ensuring a sustained pathway from fundamental insight to technological application.
Prace wykonane od początku projektu do końca okresu sprawozdawczego oraz najważniejsze dotychczasowe rezultaty
WP2 — IJD Growth and Structural Optimisation of MoS2
Activities Performed
A systematic campaign of MoS2 thin-film deposition by Ionized Jet Deposition (IJD) was carried out using a laboratory-grade IJD system. Films with nominal thicknesses of 30 nm (thin) and 120 nm (thick) were deposited onto Pt/Si substrates (~240 nm Pt deposited by electron-beam evaporation). A cylindrical MoS2 target (Ø = 5 cm, purity 99.9%, Testbourne Ltd.) was ablated under the following conditions: base pressure 5 × 10−6 mbar, rising to 1 × 10−3 mbar during plasma formation; argon working gas; acceleration voltage 15 kV; discharge frequency 60 Hz. All depositions were performed at room temperature, yielding as-deposited amorphous films. Post-deposition annealing was carried out in ultra-high vacuum (p = 2 × 10−9 mbar) at 250 °C for 120 minutes.
To benchmark the electronic quality of the IJD films, a comprehensive set of reference MoS2 samples was assembled, covering the principal fabrication routes currently employed in the field: (i) bulk single crystals synthesised by flux zone growth (2D Semiconductors), freshly cleaved by adhesive tape immediately before insertion into the UHV measurement chamber; (ii) CVD-grown monolayers synthesised on Si/SiO2 substrates (300 nm thermal oxide) in a two-zone horizontal furnace at atmospheric pressure using MoO2 powder (30 mg, 99%) and sulfur (100 mg, 99.9995%) as precursors, with argon carrier gas and growth temperatures of ~820 °C / ~180 °C in the high/low-temperature zones, respectively; (iii) mechanically exfoliated monolayers obtained via a gold-mediated template-strip approach, involving gold evaporation (100 nm) onto native oxide Si wafers, crystal pressing at 200 °C, polystyrene-assisted membrane transfer, and final cleaning in toluene.
Soft X-ray absorption spectroscopy (XAS) at the S L2,3 and Mo M2,3 edges was performed at the BEAR beamline (CNR-IOM) at the Elettra synchrotron in Trieste. Spectra were acquired in s-polarisation, at an impinging angle of 45° with respect to the sample normal, in total electron yield mode (drain current measurement with –100 V repulsive bias). The energy resolution was 0.1–0.2 eV. All drain currents were normalised to the impinging photon flux measured with a calibrated IRD AXUV-100 photodiode. Complementary bright-field TEM imaging was performed on IJD-MoS2 thick films in both the as-deposited and annealed states to provide direct structural confirmation of the XAS findings.
Main achievements
Spectroscopic fingerprinting of synthesis-dependent electronic quality. The S L2,3 absorption edge proved to be an exceptionally sensitive probe of structural order in MoS2. All spectra exhibit two distinct spectral regions: a main absorption edge above 170 eV, arising from optical transitions from spin-orbit split S 2p3/2 (L3) and 2p1/2 (L2) states into empty S 3d states hybridised with Mo 5p states; and a pre-edge structure below 169 eV, arising from (dipole-disfavoured) transitions from S 2p levels into hybridised S 3p – Mo 4d empty states. The pre-edge region is particularly diagnostic. Bulk, exfoliated, and CVD-grown samples display a rich, well-resolved multiplet structure with features labelled a–e (pre-edge) and f–i (main edge), consistent with the characteristic electronic structure of layered, ordered 2H-MoS2. Fitting of the pre-edge with multiple Voigt peaks reveals two spectral envelopes separated by 1.2 eV, corresponding to the spin-orbit splitting of the S 2p core level, with a branching ratio of 1:2 between L2 and L3 components. Notably, the CVD-grown and exfoliated films show even sharper pre-edge features than the bulk crystal, consistent with their enhanced 2D character. No absorption features attributable to edge corrugations or sulfur vacancies were detected below feature a in any sample, confirming a low defect density across all high-quality references.
IJD films: from amorphous to ordered 2H-MoS2 at 250 °C. As-deposited IJD films (both 30 nm and 120 nm) exhibit broadened, featureless S L2,3 spectra, consistent with an amorphous phase of poorly defined stoichiometry and similar to spectra previously reported for sputtered films. Upon annealing at 250 °C, however, the lineshape undergoes a dramatic transformation: the entire spectrum shifts to lower photon energies, and fine structures characteristic of layered 2H-MoS2 emerge clearly. Bright-field TEM imaging directly confirms the transition from an amorphous film to one containing ordered, crystalline MoS2 nanodomains embedded within the residual amorphous matrix. This result is significant in comparison with conventional sputtered MoS2 films, which require annealing temperatures above 900 °C to develop comparable spectral features. The moderate thermal budget required for IJD films (250 °C, compatible with polymeric and flexible substrates) positions IJD as a particularly attractive route for large-area, low-cost fabrication of electronically ordered 2D MoS2 phases.
Mo M2,3 edge analysis: quantitative tracking of structural order. Complementary analysis at the Mo M2,3 edges provided further quantitative insight. The M3 peak (corresponding to Mo 3p3/2 → Mo 4d transitions into conduction band states) shows a systematic trend: the centroid shifts from 396.7 eV for as-deposited IJD films to 396.5 eV after annealing, then to 396.3 eV for bulk and CVD-grown samples, and 396.1 eV for exfoliated MoS2. In parallel, the full width at half maximum (FWHM) narrows from 3.3 eV (as-deposited IJD) to approximately 3.0 eV (bulk) and 2.9 eV (CVD). These shifts are explained by the linear dichroism of Mo 4d final states: in structurally ordered films with layers aligned parallel to the substrate, in-plane transitions (favoured by the s-polarisation measurement geometry) dominate at lower photon energies. In amorphous films, the absence of preferential orientation renders both in-plane and out-of-plane transitions equally probable, broadening the peak and shifting its centroid to higher energy. The annealed thin film (30 nm) showed a narrower M3 peak than the annealed thick film (120 nm), suggesting a higher proportion of substrate-parallel nanosheet alignment in the thinner film, an observation further supported by the corresponding S L2,3 fine structure.
Outcome: These results directly fulfilled Objective O1 and led to a manuscript entitled “High quality MoS2 layered thin films obtained by Ionized Jet Deposition investigated by X-ray absorption spectroscopy at S L2,3 and Mo M2,3 edges” (Giovanelli, Lodi, Giglia, Mahne, Kesarwani, Vejpravova, Rühl, Ligorio, List-Kratochvil, Nasi, Timpel, Nardi, Pasquali), which has been prepared for submission acknowledging CHIMERA funding.
WP3 — In-Operando Spectroscopic Investigation of MoS2 FETs
Activities performed
Dedicated beamtime allocations at the BEAR beamline (Elettra) were used to perform in-operando spectroscopic measurements on MoS2-based field-effect transistors fabricated on SiO2 substrates. Three complementary experimental approaches were pursued, each exploiting a photon-in / photon-out detection scheme to avoid charging artefacts inherent to electron-based techniques when applied to insulating device stacks.
(i) X-ray Excited Optical Luminescence (XEOL) during transistor operation. The first approach aimed to detect the luminescence emitted by the MoS2 channel under synchrotron X-ray excitation while the transistor was electrically biased. The objective was to exploit the element- and edge-selectivity of the excitation to correlate specific electronic transitions (e.g. at the Mo M2,3 or S L2,3 edges) with changes in luminescence yield induced by the gate-controlled carrier density in the channel.
(ii) In-operando soft X-ray reflectivity at the Mo M2,3 and S L2,3 edges. To circumvent limitations encountered with XEOL (see below), an alternative in-operando methodology was developed. Soft X-ray reflectivity was measured across the Mo M2,3 absorption edge while the FET was biased to different gate voltages, exploiting the sensitivity of the reflectivity lineshape to changes in the imaginary part of the refractive index (which is directly related to the absorption coefficient and, hence, to the electronic occupation of conduction band states).
(iii) Conventional in-operando photoluminescence (ON/OFF states). As a complementary probe, micro-spot photoluminescence was applied to the MoS2 FET channel in both the ON (channel accumulated) and OFF (channel depleted) transistor states, using conventional optical excitation rather than synchrotron X-rays.
(iv) Microscopic Spectroscopic Ellipsometry. In a further extension of the in-operando programme, developed in agreement with the supervisor, microscopic spectroscopic ellipsometry was applied to a MoS2-based FET in collaboration with the Department of Physics of the University of Genova. This technique records spatially resolved maps of the optical constants (refractive index and extinction coefficient) across the FET channel, enabling direct comparison of the dielectric response in the ON and OFF states.
Main achievements
XEOL: identification of sensitivity limits. The XEOL signal from the MoS2 channel was found to be extremely weak under synchrotron excitation, preventing quantitative analysis of bias-dependent luminescence modulation. While this constitutes a negative experimental result, it provides valuable methodological information: it establishes the sensitivity floor of XEOL for the specific device geometry employed (thin MoS2 channel on SiO2) and directly motivated the pivot toward reflectivity-based approaches.
In-operando reflectivity: detection of bias-induced electronic structure changes. Subtle but reproducible variations in the Mo M2,3 absorption coefficient were detected as a function of transistor bias. These changes are consistent with partial filling of Mo 4d conduction band states when the FET channel is electrostatically accumulated. This result represents a first demonstration that soft X-ray reflectivity can track real-time changes in the electronic structure of a 2D transistor during operation — a methodological advance with broad applicability to any material system where photon-in / photon-out spectroscopy can be performed at synchrotron facilities.
In-operando photoluminescence: strong ON/OFF modulation. Significant modulation of the photoluminescence signal was observed between the transistor ON and OFF states, confirming that the optical properties of the MoS2 channel respond measurably to gate-controlled carrier injection. Quantitative analysis of the luminescence spectra (peak position, linewidth, intensity ratio) is ongoing and will, together with the reflectivity data, form the basis of a forthcoming publication.
Spectroscopic ellipsometry: ongoing analysis. Ellipsometry experiments were successfully performed on a MoS2 FET device, recording spatially resolved optical constants across the FET channel in the ON and OFF states. The data are currently under analysis.
Outcome: The combined in-operando reflectivity and photoluminescence results substantially address Objective O2 and constitute a significant methodological advance toward real-time electronic structure probing of working 2D devices. A joint publication of these results is planned.
Extension of the spectroscopic methodology: cooperative phase transitions in ditBu-BTBT organic semiconductor crystals
Activities performed
In parallel with the MoS2-focused activities, the photon-in / photon-out spectroscopic methodology developed within CHIMERA was applied to an entirely different material system: single crystals of 2,7-di-tert-butyl-[1]benzothieno[3,2-b]benzothiophene (ditBu-BTBT), a high-mobility organic semiconductor. DitBu-BTBT undergoes a cooperative, diffusionless polymorphic phase transition near 70 °C, driven by the onset of rotational disorder in the lateral tert-butyl side chains, accompanied by a small (~2°) dihedral angle change in the herringbone packing motif of the aromatic BTBT cores.
Single crystals (4–5 mm lateral dimensions, 50–100 μm thickness) were grown by slow evaporation from chloroform solution (4 mg/mL). Two complementary variable-temperature spectroscopic techniques were applied: (i) UV–Visible absorption spectroscopy in transmission mode, recorded from room temperature to above 100 °C using an Ocean Optics DH-2000-BAL spectrophotometer with temperature monitoring via K-type thermocouple; (ii) Resonant soft X-ray reflectivity (RSXRR) at the carbon K-edge (~280–320 eV, resolution 0.1 eV) at the BEAR beamline (Elettra, proposal 20240135), measured at a grazing incidence of 3° in both s- and p-polarisations, with temperature varied by resistive heating of the sample holder.
The experimental reflectivity was interpreted through simulations based on the Parratt recursive formalism, using optical constants derived from first-principles calculations of the absorption cross section of the individual ditBu-BTBT molecule. The DFT calculations were performed using the StoBe code, with the revised Perdew–Burke–Ernzerhof (RPBE) exchange-correlation functional and triple zeta valence polarisation (TZVP) basis sets. The C K-edge absorption cross sections for all nonequivalent carbon atoms were calculated with the transition-potential method and merged after applying a ΔKohn–Sham correction for energy-scale alignment. The crystal was treated as an isotropic aggregate of non-interacting molecules — justified by the alternating herringbone orientation of the molecules and the absence of experimentally observable angular effects on the reflectivity lineshape between s- and p-polarisation.
Main achievements
Identification of intramolecular disorder at the phase transition. Variable-temperature RSXRR spectra reveal two principal temperature-dependent effects. First, a progressive reduction in overall reflected intensity, attributed to increased diffuse scattering from cracks and defects that develop in the crystal during heating (directly confirmed by optical observation). Second, and more significantly, a sudden shift of approximately 1 eV to lower energies in the centroid of the broad C1s → σ* structure (305–310 eV region) upon crossing the phase-transition temperature. DFT simulations of the absorption cross section, decomposed into contributions from the BTBT core and the lateral tert-butyl side chains, attribute this shift specifically to carbon atoms in the side chains (simulated features at 310–315 eV). The shift is interpreted as evidence of bond distortions and local structural rearrangement within the molecule, reflecting the increased intramolecular disorder that sets in as the tert-butyl groups become rotationally disordered at high temperature. This finding supports the interpretation that structural disorder inside the molecule, in the lateral chains, plays a key role in triggering the cooperative phase transition.
Band gap widening at the phase transition. UV–Vis absorption spectroscopy in the 250–1000 nm range reveals that the optical band gap initially decreases with temperature, as expected for a conventional semiconductor (due to enhanced vibrational motion and lattice expansion). However, this monotonic trend is interrupted by a sudden increase in band gap at the phase-transition temperature, before the gap begins to decrease again above ~100 °C. The anomalous widening is attributed to reduced intermolecular interactions and diminished band dispersion in the disordered phase — essentially, the cooperative reorganisation weakens the solid-state coupling between neighbouring molecules, leading to less band-like and more localised electronic states. This behaviour is analogous to band gap increases reported in hybrid perovskite semiconductors undergoing structural phase transitions.
Quantitative reflectivity simulations. Despite being based on single-molecule DFT calculations (neglecting intermolecular and so
lid-state effects), the Parratt-formalism simulations achieve remarkable quantitative agreement with the experimental reflectivity at room temperature, both in overall intensity and in the position and relative weight of the principal π* and σ* features. This validates the computational approach and enables precise spectral assignment of the different contributions from core and side-chain carbon atoms.
Outcome: This work resulted in a manuscript entitled “Electronic Structure Effects During Cooperative Phase Transitions in Organic Semiconductor Single Crystals” (Giovanelli, Lodi, Mery Duarte, Giglia, Mahne, West, Hastings, Geerts, Ruggiero, Catalano, Pasquali), which has been submitted for publication with CHIMERA acknowledgement. It demonstrates the broad applicability of the photon-in / photon-out spectroscopic methodology beyond 2D inorganic semiconductors.
Alternative dielectric strategies: epitaxial ionic fluoride films
In the context of optimising the gate dielectric interface in 2D FET devices — a critical parameter for transistor performance that directly affects threshold voltage stability, hysteresis, and interface trap density — a review article was prepared on the application of thin epitaxial ionic fluoride films (such as CaF2 and LaF3) as dielectrics for electronics applications. Ionic fluorides offer potential advantages over conventional SiO2 gate oxides for 2D devices, including atomically sharp interfaces, high dielectric constant, and compatibility with van der Waals-type bonding at the channel–dielectric interface. This work was published as: Giovanelli, G.; Borghi, M.; Lodi, A.; Grasser, T.; Pasquali, L. “Thin Epitaxial Ionic Fluoride Films for Electronics Applications.” Surfaces, 2025, 8, 22.
WP4 — Logic Gate Engineering
This work package, which foresaw the fabrication and characterisation of elementary logic circuits (inverters) based on optimised MoS2 FETs with the aim of demonstrating reduced switching energy, was planned for the second year of the fellowship and was not initiated due to the early termination of the action.
Summary of scientific outcomes
The scientific work carried out during the effective duration of the CHIMERA fellowship produced the following principal outcomes:
(1) A comprehensive, comparative XAS study of MoS2 prepared by four different fabrication methods (bulk, exfoliation, CVD, IJD), demonstrating that the S L2,3 and Mo M2,3 edges provide quantitative fingerprints of structural order and electronic quality, and establishing IJD as a viable low-temperature route to electronically ordered 2D MoS2 phases (manuscript ready for submission).
(2) First demonstration of in-operando soft X-ray reflectivity as a tool for tracking bias-induced electronic structure changes in a working MoS2 FET, complemented by strong photoluminescence ON/OFF modulation (publication in preparation).
(3) Experimental evidence that cooperative phase transitions in organic semiconductor crystals (ditBu-BTBT) induce measurable changes in both the distribution of electronic states (probed by RSXRR) and the optical band gap (probed by UV–Vis), providing new insight into the interplay between lattice dynamics and electronic structure in molecular semiconductors (manuscript submitted).
Activities Performed
A systematic campaign of MoS2 thin-film deposition by Ionized Jet Deposition (IJD) was carried out using a laboratory-grade IJD system. Films with nominal thicknesses of 30 nm (thin) and 120 nm (thick) were deposited onto Pt/Si substrates (~240 nm Pt deposited by electron-beam evaporation). A cylindrical MoS2 target (Ø = 5 cm, purity 99.9%, Testbourne Ltd.) was ablated under the following conditions: base pressure 5 × 10−6 mbar, rising to 1 × 10−3 mbar during plasma formation; argon working gas; acceleration voltage 15 kV; discharge frequency 60 Hz. All depositions were performed at room temperature, yielding as-deposited amorphous films. Post-deposition annealing was carried out in ultra-high vacuum (p = 2 × 10−9 mbar) at 250 °C for 120 minutes.
To benchmark the electronic quality of the IJD films, a comprehensive set of reference MoS2 samples was assembled, covering the principal fabrication routes currently employed in the field: (i) bulk single crystals synthesised by flux zone growth (2D Semiconductors), freshly cleaved by adhesive tape immediately before insertion into the UHV measurement chamber; (ii) CVD-grown monolayers synthesised on Si/SiO2 substrates (300 nm thermal oxide) in a two-zone horizontal furnace at atmospheric pressure using MoO2 powder (30 mg, 99%) and sulfur (100 mg, 99.9995%) as precursors, with argon carrier gas and growth temperatures of ~820 °C / ~180 °C in the high/low-temperature zones, respectively; (iii) mechanically exfoliated monolayers obtained via a gold-mediated template-strip approach, involving gold evaporation (100 nm) onto native oxide Si wafers, crystal pressing at 200 °C, polystyrene-assisted membrane transfer, and final cleaning in toluene.
Soft X-ray absorption spectroscopy (XAS) at the S L2,3 and Mo M2,3 edges was performed at the BEAR beamline (CNR-IOM) at the Elettra synchrotron in Trieste. Spectra were acquired in s-polarisation, at an impinging angle of 45° with respect to the sample normal, in total electron yield mode (drain current measurement with –100 V repulsive bias). The energy resolution was 0.1–0.2 eV. All drain currents were normalised to the impinging photon flux measured with a calibrated IRD AXUV-100 photodiode. Complementary bright-field TEM imaging was performed on IJD-MoS2 thick films in both the as-deposited and annealed states to provide direct structural confirmation of the XAS findings.
Main achievements
Spectroscopic fingerprinting of synthesis-dependent electronic quality. The S L2,3 absorption edge proved to be an exceptionally sensitive probe of structural order in MoS2. All spectra exhibit two distinct spectral regions: a main absorption edge above 170 eV, arising from optical transitions from spin-orbit split S 2p3/2 (L3) and 2p1/2 (L2) states into empty S 3d states hybridised with Mo 5p states; and a pre-edge structure below 169 eV, arising from (dipole-disfavoured) transitions from S 2p levels into hybridised S 3p – Mo 4d empty states. The pre-edge region is particularly diagnostic. Bulk, exfoliated, and CVD-grown samples display a rich, well-resolved multiplet structure with features labelled a–e (pre-edge) and f–i (main edge), consistent with the characteristic electronic structure of layered, ordered 2H-MoS2. Fitting of the pre-edge with multiple Voigt peaks reveals two spectral envelopes separated by 1.2 eV, corresponding to the spin-orbit splitting of the S 2p core level, with a branching ratio of 1:2 between L2 and L3 components. Notably, the CVD-grown and exfoliated films show even sharper pre-edge features than the bulk crystal, consistent with their enhanced 2D character. No absorption features attributable to edge corrugations or sulfur vacancies were detected below feature a in any sample, confirming a low defect density across all high-quality references.
IJD films: from amorphous to ordered 2H-MoS2 at 250 °C. As-deposited IJD films (both 30 nm and 120 nm) exhibit broadened, featureless S L2,3 spectra, consistent with an amorphous phase of poorly defined stoichiometry and similar to spectra previously reported for sputtered films. Upon annealing at 250 °C, however, the lineshape undergoes a dramatic transformation: the entire spectrum shifts to lower photon energies, and fine structures characteristic of layered 2H-MoS2 emerge clearly. Bright-field TEM imaging directly confirms the transition from an amorphous film to one containing ordered, crystalline MoS2 nanodomains embedded within the residual amorphous matrix. This result is significant in comparison with conventional sputtered MoS2 films, which require annealing temperatures above 900 °C to develop comparable spectral features. The moderate thermal budget required for IJD films (250 °C, compatible with polymeric and flexible substrates) positions IJD as a particularly attractive route for large-area, low-cost fabrication of electronically ordered 2D MoS2 phases.
Mo M2,3 edge analysis: quantitative tracking of structural order. Complementary analysis at the Mo M2,3 edges provided further quantitative insight. The M3 peak (corresponding to Mo 3p3/2 → Mo 4d transitions into conduction band states) shows a systematic trend: the centroid shifts from 396.7 eV for as-deposited IJD films to 396.5 eV after annealing, then to 396.3 eV for bulk and CVD-grown samples, and 396.1 eV for exfoliated MoS2. In parallel, the full width at half maximum (FWHM) narrows from 3.3 eV (as-deposited IJD) to approximately 3.0 eV (bulk) and 2.9 eV (CVD). These shifts are explained by the linear dichroism of Mo 4d final states: in structurally ordered films with layers aligned parallel to the substrate, in-plane transitions (favoured by the s-polarisation measurement geometry) dominate at lower photon energies. In amorphous films, the absence of preferential orientation renders both in-plane and out-of-plane transitions equally probable, broadening the peak and shifting its centroid to higher energy. The annealed thin film (30 nm) showed a narrower M3 peak than the annealed thick film (120 nm), suggesting a higher proportion of substrate-parallel nanosheet alignment in the thinner film, an observation further supported by the corresponding S L2,3 fine structure.
Outcome: These results directly fulfilled Objective O1 and led to a manuscript entitled “High quality MoS2 layered thin films obtained by Ionized Jet Deposition investigated by X-ray absorption spectroscopy at S L2,3 and Mo M2,3 edges” (Giovanelli, Lodi, Giglia, Mahne, Kesarwani, Vejpravova, Rühl, Ligorio, List-Kratochvil, Nasi, Timpel, Nardi, Pasquali), which has been prepared for submission acknowledging CHIMERA funding.
WP3 — In-Operando Spectroscopic Investigation of MoS2 FETs
Activities performed
Dedicated beamtime allocations at the BEAR beamline (Elettra) were used to perform in-operando spectroscopic measurements on MoS2-based field-effect transistors fabricated on SiO2 substrates. Three complementary experimental approaches were pursued, each exploiting a photon-in / photon-out detection scheme to avoid charging artefacts inherent to electron-based techniques when applied to insulating device stacks.
(i) X-ray Excited Optical Luminescence (XEOL) during transistor operation. The first approach aimed to detect the luminescence emitted by the MoS2 channel under synchrotron X-ray excitation while the transistor was electrically biased. The objective was to exploit the element- and edge-selectivity of the excitation to correlate specific electronic transitions (e.g. at the Mo M2,3 or S L2,3 edges) with changes in luminescence yield induced by the gate-controlled carrier density in the channel.
(ii) In-operando soft X-ray reflectivity at the Mo M2,3 and S L2,3 edges. To circumvent limitations encountered with XEOL (see below), an alternative in-operando methodology was developed. Soft X-ray reflectivity was measured across the Mo M2,3 absorption edge while the FET was biased to different gate voltages, exploiting the sensitivity of the reflectivity lineshape to changes in the imaginary part of the refractive index (which is directly related to the absorption coefficient and, hence, to the electronic occupation of conduction band states).
(iii) Conventional in-operando photoluminescence (ON/OFF states). As a complementary probe, micro-spot photoluminescence was applied to the MoS2 FET channel in both the ON (channel accumulated) and OFF (channel depleted) transistor states, using conventional optical excitation rather than synchrotron X-rays.
(iv) Microscopic Spectroscopic Ellipsometry. In a further extension of the in-operando programme, developed in agreement with the supervisor, microscopic spectroscopic ellipsometry was applied to a MoS2-based FET in collaboration with the Department of Physics of the University of Genova. This technique records spatially resolved maps of the optical constants (refractive index and extinction coefficient) across the FET channel, enabling direct comparison of the dielectric response in the ON and OFF states.
Main achievements
XEOL: identification of sensitivity limits. The XEOL signal from the MoS2 channel was found to be extremely weak under synchrotron excitation, preventing quantitative analysis of bias-dependent luminescence modulation. While this constitutes a negative experimental result, it provides valuable methodological information: it establishes the sensitivity floor of XEOL for the specific device geometry employed (thin MoS2 channel on SiO2) and directly motivated the pivot toward reflectivity-based approaches.
In-operando reflectivity: detection of bias-induced electronic structure changes. Subtle but reproducible variations in the Mo M2,3 absorption coefficient were detected as a function of transistor bias. These changes are consistent with partial filling of Mo 4d conduction band states when the FET channel is electrostatically accumulated. This result represents a first demonstration that soft X-ray reflectivity can track real-time changes in the electronic structure of a 2D transistor during operation — a methodological advance with broad applicability to any material system where photon-in / photon-out spectroscopy can be performed at synchrotron facilities.
In-operando photoluminescence: strong ON/OFF modulation. Significant modulation of the photoluminescence signal was observed between the transistor ON and OFF states, confirming that the optical properties of the MoS2 channel respond measurably to gate-controlled carrier injection. Quantitative analysis of the luminescence spectra (peak position, linewidth, intensity ratio) is ongoing and will, together with the reflectivity data, form the basis of a forthcoming publication.
Spectroscopic ellipsometry: ongoing analysis. Ellipsometry experiments were successfully performed on a MoS2 FET device, recording spatially resolved optical constants across the FET channel in the ON and OFF states. The data are currently under analysis.
Outcome: The combined in-operando reflectivity and photoluminescence results substantially address Objective O2 and constitute a significant methodological advance toward real-time electronic structure probing of working 2D devices. A joint publication of these results is planned.
Extension of the spectroscopic methodology: cooperative phase transitions in ditBu-BTBT organic semiconductor crystals
Activities performed
In parallel with the MoS2-focused activities, the photon-in / photon-out spectroscopic methodology developed within CHIMERA was applied to an entirely different material system: single crystals of 2,7-di-tert-butyl-[1]benzothieno[3,2-b]benzothiophene (ditBu-BTBT), a high-mobility organic semiconductor. DitBu-BTBT undergoes a cooperative, diffusionless polymorphic phase transition near 70 °C, driven by the onset of rotational disorder in the lateral tert-butyl side chains, accompanied by a small (~2°) dihedral angle change in the herringbone packing motif of the aromatic BTBT cores.
Single crystals (4–5 mm lateral dimensions, 50–100 μm thickness) were grown by slow evaporation from chloroform solution (4 mg/mL). Two complementary variable-temperature spectroscopic techniques were applied: (i) UV–Visible absorption spectroscopy in transmission mode, recorded from room temperature to above 100 °C using an Ocean Optics DH-2000-BAL spectrophotometer with temperature monitoring via K-type thermocouple; (ii) Resonant soft X-ray reflectivity (RSXRR) at the carbon K-edge (~280–320 eV, resolution 0.1 eV) at the BEAR beamline (Elettra, proposal 20240135), measured at a grazing incidence of 3° in both s- and p-polarisations, with temperature varied by resistive heating of the sample holder.
The experimental reflectivity was interpreted through simulations based on the Parratt recursive formalism, using optical constants derived from first-principles calculations of the absorption cross section of the individual ditBu-BTBT molecule. The DFT calculations were performed using the StoBe code, with the revised Perdew–Burke–Ernzerhof (RPBE) exchange-correlation functional and triple zeta valence polarisation (TZVP) basis sets. The C K-edge absorption cross sections for all nonequivalent carbon atoms were calculated with the transition-potential method and merged after applying a ΔKohn–Sham correction for energy-scale alignment. The crystal was treated as an isotropic aggregate of non-interacting molecules — justified by the alternating herringbone orientation of the molecules and the absence of experimentally observable angular effects on the reflectivity lineshape between s- and p-polarisation.
Main achievements
Identification of intramolecular disorder at the phase transition. Variable-temperature RSXRR spectra reveal two principal temperature-dependent effects. First, a progressive reduction in overall reflected intensity, attributed to increased diffuse scattering from cracks and defects that develop in the crystal during heating (directly confirmed by optical observation). Second, and more significantly, a sudden shift of approximately 1 eV to lower energies in the centroid of the broad C1s → σ* structure (305–310 eV region) upon crossing the phase-transition temperature. DFT simulations of the absorption cross section, decomposed into contributions from the BTBT core and the lateral tert-butyl side chains, attribute this shift specifically to carbon atoms in the side chains (simulated features at 310–315 eV). The shift is interpreted as evidence of bond distortions and local structural rearrangement within the molecule, reflecting the increased intramolecular disorder that sets in as the tert-butyl groups become rotationally disordered at high temperature. This finding supports the interpretation that structural disorder inside the molecule, in the lateral chains, plays a key role in triggering the cooperative phase transition.
Band gap widening at the phase transition. UV–Vis absorption spectroscopy in the 250–1000 nm range reveals that the optical band gap initially decreases with temperature, as expected for a conventional semiconductor (due to enhanced vibrational motion and lattice expansion). However, this monotonic trend is interrupted by a sudden increase in band gap at the phase-transition temperature, before the gap begins to decrease again above ~100 °C. The anomalous widening is attributed to reduced intermolecular interactions and diminished band dispersion in the disordered phase — essentially, the cooperative reorganisation weakens the solid-state coupling between neighbouring molecules, leading to less band-like and more localised electronic states. This behaviour is analogous to band gap increases reported in hybrid perovskite semiconductors undergoing structural phase transitions.
Quantitative reflectivity simulations. Despite being based on single-molecule DFT calculations (neglecting intermolecular and so
lid-state effects), the Parratt-formalism simulations achieve remarkable quantitative agreement with the experimental reflectivity at room temperature, both in overall intensity and in the position and relative weight of the principal π* and σ* features. This validates the computational approach and enables precise spectral assignment of the different contributions from core and side-chain carbon atoms.
Outcome: This work resulted in a manuscript entitled “Electronic Structure Effects During Cooperative Phase Transitions in Organic Semiconductor Single Crystals” (Giovanelli, Lodi, Mery Duarte, Giglia, Mahne, West, Hastings, Geerts, Ruggiero, Catalano, Pasquali), which has been submitted for publication with CHIMERA acknowledgement. It demonstrates the broad applicability of the photon-in / photon-out spectroscopic methodology beyond 2D inorganic semiconductors.
Alternative dielectric strategies: epitaxial ionic fluoride films
In the context of optimising the gate dielectric interface in 2D FET devices — a critical parameter for transistor performance that directly affects threshold voltage stability, hysteresis, and interface trap density — a review article was prepared on the application of thin epitaxial ionic fluoride films (such as CaF2 and LaF3) as dielectrics for electronics applications. Ionic fluorides offer potential advantages over conventional SiO2 gate oxides for 2D devices, including atomically sharp interfaces, high dielectric constant, and compatibility with van der Waals-type bonding at the channel–dielectric interface. This work was published as: Giovanelli, G.; Borghi, M.; Lodi, A.; Grasser, T.; Pasquali, L. “Thin Epitaxial Ionic Fluoride Films for Electronics Applications.” Surfaces, 2025, 8, 22.
WP4 — Logic Gate Engineering
This work package, which foresaw the fabrication and characterisation of elementary logic circuits (inverters) based on optimised MoS2 FETs with the aim of demonstrating reduced switching energy, was planned for the second year of the fellowship and was not initiated due to the early termination of the action.
Summary of scientific outcomes
The scientific work carried out during the effective duration of the CHIMERA fellowship produced the following principal outcomes:
(1) A comprehensive, comparative XAS study of MoS2 prepared by four different fabrication methods (bulk, exfoliation, CVD, IJD), demonstrating that the S L2,3 and Mo M2,3 edges provide quantitative fingerprints of structural order and electronic quality, and establishing IJD as a viable low-temperature route to electronically ordered 2D MoS2 phases (manuscript ready for submission).
(2) First demonstration of in-operando soft X-ray reflectivity as a tool for tracking bias-induced electronic structure changes in a working MoS2 FET, complemented by strong photoluminescence ON/OFF modulation (publication in preparation).
(3) Experimental evidence that cooperative phase transitions in organic semiconductor crystals (ditBu-BTBT) induce measurable changes in both the distribution of electronic states (probed by RSXRR) and the optical band gap (probed by UV–Vis), providing new insight into the interplay between lattice dynamics and electronic structure in molecular semiconductors (manuscript submitted).
Innowacyjność oraz oczekiwany potencjalny wpływ (w tym dotychczasowe znaczenie społeczno-gospodarcze i szersze implikacje społeczne projektu)
Overview of results
Despite the early termination of the fellowship (effective research duration: 12.5 months out of 24 planned), the CHIMERA project generated a coherent body of scientific results across three interconnected research lines. These results are embodied in four publications (one published, one submitted, one ready for submission, one in preparation) and in ongoing experimental datasets that will produce further outputs beyond the fellowship framework.
Result 1: Comprehensive XAS benchmarking of IJD-grown MoS2 films
The principal scientific result of the project is a comparative soft X-ray absorption spectroscopy (XAS) study of MoS2 prepared by four different fabrication routes: bulk crystallisation, mechanical exfoliation, chemical vapour deposition (CVD), and ionized jet deposition (IJD). Analysis of the S L2,3 and Mo M2,3 absorption edges, performed at the BEAR beamline of Elettra, established a quantitative spectroscopic framework for assessing the electronic quality of MoS2 films as a function of synthesis method and post-deposition treatment.
The key finding is that IJD-grown MoS2 films, which are amorphous and electronically disordered in the as-deposited state, develop the spectroscopic fingerprint of ordered, layered 2H-MoS2 after annealing at just 250 °C. Quantitatively, the Mo M3 peak centroid shifts from 396.7 eV (as-deposited) to 396.5 eV (annealed), approaching the values of 396.3 eV (bulk, CVD) and 396.1 eV (exfoliated); in parallel, the FWHM decreases from 3.3 eV toward ~3.0 eV. TEM imaging confirms the formation of crystalline 2H-MoS2 nanodomains within the residual amorphous matrix. Crucially, conventional sputtered films require annealing temperatures above 900 °C to achieve comparable structural ordering, making the IJD result highly significant from a manufacturing perspective. The manuscript reporting these findings (Giovanelli, Lodi, Giglia, Mahne, Kesarwani, Vejpravova, Rühl, Ligorio, List-Kratochvil, Nasi, Timpel, Nardi, Pasquali) is ready for submission.
Result 2: In-operando electronic structure probing of MoS2 FETs
Three complementary in-operando techniques were applied to MoS2-based field-effect transistors during electrical biasing. Soft X-ray reflectivity at the Mo M2,3 edge detected subtle but reproducible variations in the absorption coefficient as a function of gate voltage, consistent with partial filling of Mo 4d conduction band states under channel accumulation. Conventional micro-spot photoluminescence showed significant signal modulation between the transistor ON and OFF states. These results constitute a first proof-of-concept that photon-in / photon-out synchrotron spectroscopy can track real-time electronic structure changes in an operating 2D device. Additionally, the attempted XEOL measurements, while yielding insufficient signal for quantitative analysis, provided valuable methodological information by establishing the sensitivity floor of this technique for thin-channel device geometries on SiO2 substrates. The combined reflectivity and luminescence results will form the basis of a forthcoming publication. Microscopic spectroscopic ellipsometry data, obtained in collaboration with the University of Genova, are currently under analysis and will generate an additional publication.
Result 3: Electronic structure effects across cooperative phase transitions in organic semiconductors
The photon-in / photon-out methodology was extended to single crystals of ditBu-BTBT, a high-mobility organic semiconductor undergoing a cooperative polymorphic transition near 70 °C. Variable-temperature resonant soft X-ray reflectivity at the carbon K-edge revealed a ~1 eV shift in C1s → σ* transitions at the phase transition, attributed by DFT simulations (StoBe code, RPBE/TZVP) to increased intramolecular disorder in the tert-butyl side chains. Concurrently, UV–Vis absorption spectroscopy showed an anomalous increase in the optical band gap at the transition temperature, reflecting reduced intermolecular coupling and diminished band dispersion in the disordered phase. These results establish a direct experimental link between cooperative lattice dynamics and the electronic structure of organic semiconductors, with implications for the rational design of phase-change-responsive organic devices. The manuscript (Giovanelli, Lodi, Mery Duarte, Giglia, Mahne, West, Hastings, Geerts, Ruggiero, Catalano, Pasquali) has been submitted for publication.
Result 4: Review of ionic fluoride dielectrics for 2D electronics
A review article surveying the state of the art on thin epitaxial ionic fluoride films for electronics applications was published open access (Giovanelli, Borghi, Lodi, Grasser, Pasquali, Surfaces, 2025, 8, 22; doi:10.3390/surfaces8020022). This work contextualises ionic fluorides (e.g. CaF2, LaF3) as promising gate dielectric candidates for 2D-material-based FETs, where the quality of the channel–dielectric interface critically governs threshold voltage stability, hysteresis, and trap density. The review complements the experimental activities of WP2 and WP3 by mapping the broader landscape of dielectric solutions for 2D transistor technology.
Potential impacts
Scientific and methodological impact
The most far-reaching scientific outcome of CHIMERA is the demonstration that photon-in / photon-out synchrotron spectroscopy — encompassing soft X-ray reflectivity, resonant reflectivity at absorption edges, and luminescence-based techniques — constitutes a versatile, non-destructive platform for probing the electronic structure of functional materials and devices under realistic operating conditions. The significance of this result lies in its generality. The methodology is not limited to MoS2 or to any single material class: the project demonstrated its applicability to inorganic 2D semiconductors (MoS2 FETs, via in-operando reflectivity and photoluminescence), to the quality assessment of thin films produced by different growth techniques (via systematic XAS at the S L2,3 and Mo M2,3 edges), and to organic semiconductors (ditBu-BTBT single crystals, via variable-temperature reflectivity at the C K-edge). This cross-material versatility positions the approach as a potential standard characterisation tool for the broader nanoelectronics community, applicable wherever element-specific, non-contact, operando probing of electronic states is needed.
At the level of fundamental understanding, the XAS study of IJD-grown MoS2 provides the research community with a spectroscopic reference library for evaluating film quality across fabrication methods. The S L2,3 pre-edge region, in particular, emerges as an exceptionally sensitive probe of crystallinity, stoichiometry, dimensionality, and defect density — information that is often inaccessible to conventional structural techniques such as X-ray diffraction alone. Similarly, the ditBu-BTBT study breaks new ground by establishing a direct experimental link between cooperative phase transitions and electronic structure modulations in organic semiconductors, providing a quantitative test case for theoretical models of electron–phonon coupling in molecular crystals.
Technological and industrial impact
The project’s results carry direct technological implications along two complementary directions.
Validation of IJD as a scalable deposition route. Ionized Jet Deposition operates at room temperature, preserves the stoichiometry of the target material, and requires only simple single-target sources — features that translate into lower equipment costs, reduced process complexity, and compatibility with large-area substrates, including polymeric and flexible materials. The CHIMERA results demonstrate that the electronic quality gap between IJD films and established high-quality references (bulk crystals, CVD monolayers) can be closed by a moderate annealing step at 250 °C — a thermal budget that is dramatically lower than the 500–900 °C required for sputtered films and fully compatible with industrial roll-to-roll processing on plastic foils. This finding directly addresses one of the principal barriers to the commercialisation of 2D semiconductor devices: the need for scalable growth methods that deliver electronic-grade material without requiring high-temperature or ultrahigh-vacuum post-processing. If these results are confirmed at wafer scale and integrated into device fabrication workflows, IJD could enable the manufacturing of flexible MoS2-based transistors, sensors, and optoelectronic components at costs significantly below those achievable with current CVD-based approaches.
In-operando diagnostics for process control. The demonstration that soft X-ray reflectivity can detect gate-voltage-induced changes in the Mo 4d conduction band occupation during transistor operation opens a path toward synchrotron-based inline diagnostics for semiconductor device development. While synchrotron measurements are not themselves compatible with high-volume manufacturing environments, they can serve as high-accuracy benchmarking tools during the development phase of new device architectures, providing the kind of direct electronic structure feedback that is needed to optimise gate stack engineering, contact interfaces, and channel doping strategies for 2D FETs. This is particularly relevant in the context of the European Chips Act, which emphasises the need for advanced metrology and characterisation capabilities to support next-generation semiconductor technologies.
Impact on the organic electronics sector
The ditBu-BTBT study opens a new direction with practical relevance to the organic electronics industry. The finding that cooperative phase transitions in amphidynamic molecular crystals produce measurable and abrupt changes in the optical band gap and in the distribution of electronic states has direct implications for the design of phase-change-responsive organic devices. It has already been shown in the literature that OFETs based on ditBu-BTBT thin films exploit the cooperative transition to achieve a functional memory effect, enabling reversible, thermally controlled modulation of charge carrier mobility. The CHIMERA results provide the first spectroscopic evidence of the underlying electronic mechanism — reduced intermolecular coupling and diminished band dispersion in the disordered phase — thereby supplying the physical picture that is needed to rationally engineer the next generation of multifunctional, stimuli-responsive organic semiconductors. This knowledge is transferable to the broader class of BTBT derivatives and other amphidynamic crystals that are actively being explored for applications in organic transistors, thermoelectric generators, and neuromorphic computing elements.
Alignment with European policy priorities
The project’s results are aligned with two overarching European strategic objectives. Under the European Green Deal, the advancement of 2D semiconductor technology contributes to the development of energy-efficient electronic devices. MoS2-based transistors promise ultra-low switching energies owing to their atomically thin channel geometry, which virtually eliminates short-channel effects and enables aggressive gate length scaling with minimal leakage. Under Europe for the Digital Age and the European Chips Act, the project strengthens Europe’s knowledge base in advanced semiconductor characterisation and builds new capabilities at European synchrotron facilities (Elettra) that can be leveraged by the wider research and industrial ecosystem. The published review on ionic fluoride dielectrics additionally contributes to the European materials science knowledge base for beyond-silicon technologies.
Impact on the researcher’s career
The fellowship provided the researcher with a unique combination of skills spanning synchrotron spectroscopy, thin-film growth, device fabrication and characterisation, advanced data analysis, and interdisciplinary project management. The integration within the host department at UNIMORE — including involvement in teaching (tutoring in the Master’s course ‘Physics of Materials’, assistance during examinations, supervision of a Master thesis student), active participation in internal collaborations (probe-station transport measurements with electronic engineers, low-temperature physics laboratory setup), and sustained interaction with an international network of collaborators — resulted in a significantly broadened scientific and professional profile. The researcher subsequently secured a permanent position outside academia, demonstrating the transferability of the acquired competences to the private sector and positive career progression consistent with the objectives of the MSCA fellowship programme.
Key needs for further uptake
To ensure that the scientific results of CHIMERA achieve their full potential impact, the following needs have been identified:
Further research
Completion of ongoing analyses. The in-operando reflectivity and photoluminescence datasets on MoS2 FETs, as well as the spectroscopic ellipsometry data, are currently under analysis. Completing these analyses and publishing the results is the immediate priority. The host group (Prof. Pasquali, UNIMORE) is committed to pursuing this work, ensuring continuity beyond the fellowship.
Systematic optimisation of IJD parameters. While the CHIMERA results demonstrate that IJD can produce electronically ordered MoS2 at 250 °C, a systematic study mapping the IJD parameter space (target composition, pulse frequency, gas pressure, substrate temperature, annealing atmosphere) against the resulting film quality — using the XAS spectroscopic framework established in this project as the primary diagnostic — is needed to identify optimal conditions for device-grade films. Extension to other TMDCs (WS2, MoSe2, WSe2) would multiply the technological applicability of the approach.
In-operando methodology at higher spectral throughput. The bias-induced changes detected in the Mo M2,3 reflectivity were subtle, operating near the sensitivity limit of the current experimental configuration. Future beamtime proposals should target higher-brilliance insertion-device beamlines or free-electron laser facilities to improve the signal-to-noise ratio and enable time-resolved measurements that could capture transient electronic states during device switching.
Extension to heterostructures and novel dielectrics. The review on ionic fluoride films identified promising dielectric candidates (CaF2, LaF3) for 2D FETs. A natural next step is to apply the in-operando spectroscopic methodology to MoS2 FETs incorporating ionic fluoride gate dielectrics, to assess whether the improved interface quality translates into detectable differences in the electronic structure response under biasing. This would close the loop between the dielectric engineering and operando spectroscopy threads of CHIMERA.
Demonstration and access to facilities
Beamtime access. Continued access to the BEAR beamline at Elettra (or equivalent soft X-ray facilities) is essential. The in-operando measurements require dedicated beamtime with specialised sample environments (electrical feedthroughs, temperature control, reflectometry and luminescence detection in the same UHV chamber). Sustained beamtime allocation through peer-reviewed proposals, and the development of standardised sample holders compatible with in-operando biasing, would lower the barrier for adoption by other research groups.
Scale-up demonstration. The IJD results were obtained on laboratory-scale substrates (Pt/Si, ~1 cm2). A critical next step is to demonstrate that the same electronic quality can be achieved on industrially relevant substrate sizes (4– or 6-inch wafers) and on flexible substrates (polyimide, PEN). This requires access to IJD systems with larger deposition areas and collaboration with industrial partners who can provid
e real-world substrate and integration constraints.
Internationalisation and collaborative network
The collaborative network established during CHIMERA — encompassing UNIMORE, CNR-IOM (Trieste), Charles University (Prague), Humboldt-Universität zu Berlin, CNR-IMEM (Parma and Trento), the University of Rochester, the Université Libre de Bruxelles, and the University of Genova — provides a strong foundation for sustained international cooperation. Maintaining and expanding this network is important for several reasons: it ensures access to complementary expertise (DFT simulations from Rochester, exfoliation from Berlin, device engineering from UNIMORE), it facilitates multi-access beamtime proposals at European synchrotron facilities, and it provides the critical mass needed for larger collaborative funding applications (e.g. ERC, HORIZON-CL4 calls on advanced materials and semiconductor technologies). The involvement of researchers across multiple European countries also reinforces the ERA (European Research Area) dimension of the project.
Commercialisation, IPR, and market access
No patents were filed during the project, and no intellectual property requiring formal protection was generated. The primary outputs are scientific publications in open-access journals, which maximise dissemination and do not restrict downstream use. Should the IJD optimisation pathway described above lead to a reproducible, device-grade deposition process, intellectual property considerations would become relevant. In that scenario, the key steps would include: (i) filing process patents covering the specific IJD parameter windows that yield electronic-grade 2D films at low annealing temperatures; (ii) engaging with IJD equipment manufacturers (the technique is already marketed by Italian companies for biomedical coatings) to explore technology licensing or co-development agreements; (iii) establishing contacts with the European semiconductor industry (e.g. through the ECSEL Joint Undertaking, now Chips Joint Undertaking, or the pilot lines of the Graphene Flagship) to identify integration opportunities. At the present stage, however, the priority remains on completing the scientific characterisation and publishing the results, which will establish the credibility and visibility needed to attract commercial interest.
Regulatory and standardisation framework
The XAS spectroscopic framework established by CHIMERA for assessing the electronic quality of MoS2 films could, if validated across a wider range of laboratories and sample types, contribute to the development of standardised quality metrics for 2D semiconductor materials. Currently, no widely accepted standard exists for benchmarking the electronic quality of TMDC films; the community relies on a patchwork of Raman spectroscopy, photoluminescence peak ratios, and transport measurements, each of which captures only a partial picture. The quantitative correlation between XAS lineshape (S L2,3 fine structure, Mo M3 peak position and FWHM) and structural order demonstrated in this project suggests that XAS-based metrics could serve as a more comprehensive and element-specific quality indicator. Engagement with standardisation bodies (e.g. ISO TC 229 on nanotechnologies, or the emerging efforts within the EU Nanoelectronics Strategy) would be a valuable long-term step, though it lies beyond the immediate scope of the current results.
Despite the early termination of the fellowship (effective research duration: 12.5 months out of 24 planned), the CHIMERA project generated a coherent body of scientific results across three interconnected research lines. These results are embodied in four publications (one published, one submitted, one ready for submission, one in preparation) and in ongoing experimental datasets that will produce further outputs beyond the fellowship framework.
Result 1: Comprehensive XAS benchmarking of IJD-grown MoS2 films
The principal scientific result of the project is a comparative soft X-ray absorption spectroscopy (XAS) study of MoS2 prepared by four different fabrication routes: bulk crystallisation, mechanical exfoliation, chemical vapour deposition (CVD), and ionized jet deposition (IJD). Analysis of the S L2,3 and Mo M2,3 absorption edges, performed at the BEAR beamline of Elettra, established a quantitative spectroscopic framework for assessing the electronic quality of MoS2 films as a function of synthesis method and post-deposition treatment.
The key finding is that IJD-grown MoS2 films, which are amorphous and electronically disordered in the as-deposited state, develop the spectroscopic fingerprint of ordered, layered 2H-MoS2 after annealing at just 250 °C. Quantitatively, the Mo M3 peak centroid shifts from 396.7 eV (as-deposited) to 396.5 eV (annealed), approaching the values of 396.3 eV (bulk, CVD) and 396.1 eV (exfoliated); in parallel, the FWHM decreases from 3.3 eV toward ~3.0 eV. TEM imaging confirms the formation of crystalline 2H-MoS2 nanodomains within the residual amorphous matrix. Crucially, conventional sputtered films require annealing temperatures above 900 °C to achieve comparable structural ordering, making the IJD result highly significant from a manufacturing perspective. The manuscript reporting these findings (Giovanelli, Lodi, Giglia, Mahne, Kesarwani, Vejpravova, Rühl, Ligorio, List-Kratochvil, Nasi, Timpel, Nardi, Pasquali) is ready for submission.
Result 2: In-operando electronic structure probing of MoS2 FETs
Three complementary in-operando techniques were applied to MoS2-based field-effect transistors during electrical biasing. Soft X-ray reflectivity at the Mo M2,3 edge detected subtle but reproducible variations in the absorption coefficient as a function of gate voltage, consistent with partial filling of Mo 4d conduction band states under channel accumulation. Conventional micro-spot photoluminescence showed significant signal modulation between the transistor ON and OFF states. These results constitute a first proof-of-concept that photon-in / photon-out synchrotron spectroscopy can track real-time electronic structure changes in an operating 2D device. Additionally, the attempted XEOL measurements, while yielding insufficient signal for quantitative analysis, provided valuable methodological information by establishing the sensitivity floor of this technique for thin-channel device geometries on SiO2 substrates. The combined reflectivity and luminescence results will form the basis of a forthcoming publication. Microscopic spectroscopic ellipsometry data, obtained in collaboration with the University of Genova, are currently under analysis and will generate an additional publication.
Result 3: Electronic structure effects across cooperative phase transitions in organic semiconductors
The photon-in / photon-out methodology was extended to single crystals of ditBu-BTBT, a high-mobility organic semiconductor undergoing a cooperative polymorphic transition near 70 °C. Variable-temperature resonant soft X-ray reflectivity at the carbon K-edge revealed a ~1 eV shift in C1s → σ* transitions at the phase transition, attributed by DFT simulations (StoBe code, RPBE/TZVP) to increased intramolecular disorder in the tert-butyl side chains. Concurrently, UV–Vis absorption spectroscopy showed an anomalous increase in the optical band gap at the transition temperature, reflecting reduced intermolecular coupling and diminished band dispersion in the disordered phase. These results establish a direct experimental link between cooperative lattice dynamics and the electronic structure of organic semiconductors, with implications for the rational design of phase-change-responsive organic devices. The manuscript (Giovanelli, Lodi, Mery Duarte, Giglia, Mahne, West, Hastings, Geerts, Ruggiero, Catalano, Pasquali) has been submitted for publication.
Result 4: Review of ionic fluoride dielectrics for 2D electronics
A review article surveying the state of the art on thin epitaxial ionic fluoride films for electronics applications was published open access (Giovanelli, Borghi, Lodi, Grasser, Pasquali, Surfaces, 2025, 8, 22; doi:10.3390/surfaces8020022). This work contextualises ionic fluorides (e.g. CaF2, LaF3) as promising gate dielectric candidates for 2D-material-based FETs, where the quality of the channel–dielectric interface critically governs threshold voltage stability, hysteresis, and trap density. The review complements the experimental activities of WP2 and WP3 by mapping the broader landscape of dielectric solutions for 2D transistor technology.
Potential impacts
Scientific and methodological impact
The most far-reaching scientific outcome of CHIMERA is the demonstration that photon-in / photon-out synchrotron spectroscopy — encompassing soft X-ray reflectivity, resonant reflectivity at absorption edges, and luminescence-based techniques — constitutes a versatile, non-destructive platform for probing the electronic structure of functional materials and devices under realistic operating conditions. The significance of this result lies in its generality. The methodology is not limited to MoS2 or to any single material class: the project demonstrated its applicability to inorganic 2D semiconductors (MoS2 FETs, via in-operando reflectivity and photoluminescence), to the quality assessment of thin films produced by different growth techniques (via systematic XAS at the S L2,3 and Mo M2,3 edges), and to organic semiconductors (ditBu-BTBT single crystals, via variable-temperature reflectivity at the C K-edge). This cross-material versatility positions the approach as a potential standard characterisation tool for the broader nanoelectronics community, applicable wherever element-specific, non-contact, operando probing of electronic states is needed.
At the level of fundamental understanding, the XAS study of IJD-grown MoS2 provides the research community with a spectroscopic reference library for evaluating film quality across fabrication methods. The S L2,3 pre-edge region, in particular, emerges as an exceptionally sensitive probe of crystallinity, stoichiometry, dimensionality, and defect density — information that is often inaccessible to conventional structural techniques such as X-ray diffraction alone. Similarly, the ditBu-BTBT study breaks new ground by establishing a direct experimental link between cooperative phase transitions and electronic structure modulations in organic semiconductors, providing a quantitative test case for theoretical models of electron–phonon coupling in molecular crystals.
Technological and industrial impact
The project’s results carry direct technological implications along two complementary directions.
Validation of IJD as a scalable deposition route. Ionized Jet Deposition operates at room temperature, preserves the stoichiometry of the target material, and requires only simple single-target sources — features that translate into lower equipment costs, reduced process complexity, and compatibility with large-area substrates, including polymeric and flexible materials. The CHIMERA results demonstrate that the electronic quality gap between IJD films and established high-quality references (bulk crystals, CVD monolayers) can be closed by a moderate annealing step at 250 °C — a thermal budget that is dramatically lower than the 500–900 °C required for sputtered films and fully compatible with industrial roll-to-roll processing on plastic foils. This finding directly addresses one of the principal barriers to the commercialisation of 2D semiconductor devices: the need for scalable growth methods that deliver electronic-grade material without requiring high-temperature or ultrahigh-vacuum post-processing. If these results are confirmed at wafer scale and integrated into device fabrication workflows, IJD could enable the manufacturing of flexible MoS2-based transistors, sensors, and optoelectronic components at costs significantly below those achievable with current CVD-based approaches.
In-operando diagnostics for process control. The demonstration that soft X-ray reflectivity can detect gate-voltage-induced changes in the Mo 4d conduction band occupation during transistor operation opens a path toward synchrotron-based inline diagnostics for semiconductor device development. While synchrotron measurements are not themselves compatible with high-volume manufacturing environments, they can serve as high-accuracy benchmarking tools during the development phase of new device architectures, providing the kind of direct electronic structure feedback that is needed to optimise gate stack engineering, contact interfaces, and channel doping strategies for 2D FETs. This is particularly relevant in the context of the European Chips Act, which emphasises the need for advanced metrology and characterisation capabilities to support next-generation semiconductor technologies.
Impact on the organic electronics sector
The ditBu-BTBT study opens a new direction with practical relevance to the organic electronics industry. The finding that cooperative phase transitions in amphidynamic molecular crystals produce measurable and abrupt changes in the optical band gap and in the distribution of electronic states has direct implications for the design of phase-change-responsive organic devices. It has already been shown in the literature that OFETs based on ditBu-BTBT thin films exploit the cooperative transition to achieve a functional memory effect, enabling reversible, thermally controlled modulation of charge carrier mobility. The CHIMERA results provide the first spectroscopic evidence of the underlying electronic mechanism — reduced intermolecular coupling and diminished band dispersion in the disordered phase — thereby supplying the physical picture that is needed to rationally engineer the next generation of multifunctional, stimuli-responsive organic semiconductors. This knowledge is transferable to the broader class of BTBT derivatives and other amphidynamic crystals that are actively being explored for applications in organic transistors, thermoelectric generators, and neuromorphic computing elements.
Alignment with European policy priorities
The project’s results are aligned with two overarching European strategic objectives. Under the European Green Deal, the advancement of 2D semiconductor technology contributes to the development of energy-efficient electronic devices. MoS2-based transistors promise ultra-low switching energies owing to their atomically thin channel geometry, which virtually eliminates short-channel effects and enables aggressive gate length scaling with minimal leakage. Under Europe for the Digital Age and the European Chips Act, the project strengthens Europe’s knowledge base in advanced semiconductor characterisation and builds new capabilities at European synchrotron facilities (Elettra) that can be leveraged by the wider research and industrial ecosystem. The published review on ionic fluoride dielectrics additionally contributes to the European materials science knowledge base for beyond-silicon technologies.
Impact on the researcher’s career
The fellowship provided the researcher with a unique combination of skills spanning synchrotron spectroscopy, thin-film growth, device fabrication and characterisation, advanced data analysis, and interdisciplinary project management. The integration within the host department at UNIMORE — including involvement in teaching (tutoring in the Master’s course ‘Physics of Materials’, assistance during examinations, supervision of a Master thesis student), active participation in internal collaborations (probe-station transport measurements with electronic engineers, low-temperature physics laboratory setup), and sustained interaction with an international network of collaborators — resulted in a significantly broadened scientific and professional profile. The researcher subsequently secured a permanent position outside academia, demonstrating the transferability of the acquired competences to the private sector and positive career progression consistent with the objectives of the MSCA fellowship programme.
Key needs for further uptake
To ensure that the scientific results of CHIMERA achieve their full potential impact, the following needs have been identified:
Further research
Completion of ongoing analyses. The in-operando reflectivity and photoluminescence datasets on MoS2 FETs, as well as the spectroscopic ellipsometry data, are currently under analysis. Completing these analyses and publishing the results is the immediate priority. The host group (Prof. Pasquali, UNIMORE) is committed to pursuing this work, ensuring continuity beyond the fellowship.
Systematic optimisation of IJD parameters. While the CHIMERA results demonstrate that IJD can produce electronically ordered MoS2 at 250 °C, a systematic study mapping the IJD parameter space (target composition, pulse frequency, gas pressure, substrate temperature, annealing atmosphere) against the resulting film quality — using the XAS spectroscopic framework established in this project as the primary diagnostic — is needed to identify optimal conditions for device-grade films. Extension to other TMDCs (WS2, MoSe2, WSe2) would multiply the technological applicability of the approach.
In-operando methodology at higher spectral throughput. The bias-induced changes detected in the Mo M2,3 reflectivity were subtle, operating near the sensitivity limit of the current experimental configuration. Future beamtime proposals should target higher-brilliance insertion-device beamlines or free-electron laser facilities to improve the signal-to-noise ratio and enable time-resolved measurements that could capture transient electronic states during device switching.
Extension to heterostructures and novel dielectrics. The review on ionic fluoride films identified promising dielectric candidates (CaF2, LaF3) for 2D FETs. A natural next step is to apply the in-operando spectroscopic methodology to MoS2 FETs incorporating ionic fluoride gate dielectrics, to assess whether the improved interface quality translates into detectable differences in the electronic structure response under biasing. This would close the loop between the dielectric engineering and operando spectroscopy threads of CHIMERA.
Demonstration and access to facilities
Beamtime access. Continued access to the BEAR beamline at Elettra (or equivalent soft X-ray facilities) is essential. The in-operando measurements require dedicated beamtime with specialised sample environments (electrical feedthroughs, temperature control, reflectometry and luminescence detection in the same UHV chamber). Sustained beamtime allocation through peer-reviewed proposals, and the development of standardised sample holders compatible with in-operando biasing, would lower the barrier for adoption by other research groups.
Scale-up demonstration. The IJD results were obtained on laboratory-scale substrates (Pt/Si, ~1 cm2). A critical next step is to demonstrate that the same electronic quality can be achieved on industrially relevant substrate sizes (4– or 6-inch wafers) and on flexible substrates (polyimide, PEN). This requires access to IJD systems with larger deposition areas and collaboration with industrial partners who can provid
e real-world substrate and integration constraints.
Internationalisation and collaborative network
The collaborative network established during CHIMERA — encompassing UNIMORE, CNR-IOM (Trieste), Charles University (Prague), Humboldt-Universität zu Berlin, CNR-IMEM (Parma and Trento), the University of Rochester, the Université Libre de Bruxelles, and the University of Genova — provides a strong foundation for sustained international cooperation. Maintaining and expanding this network is important for several reasons: it ensures access to complementary expertise (DFT simulations from Rochester, exfoliation from Berlin, device engineering from UNIMORE), it facilitates multi-access beamtime proposals at European synchrotron facilities, and it provides the critical mass needed for larger collaborative funding applications (e.g. ERC, HORIZON-CL4 calls on advanced materials and semiconductor technologies). The involvement of researchers across multiple European countries also reinforces the ERA (European Research Area) dimension of the project.
Commercialisation, IPR, and market access
No patents were filed during the project, and no intellectual property requiring formal protection was generated. The primary outputs are scientific publications in open-access journals, which maximise dissemination and do not restrict downstream use. Should the IJD optimisation pathway described above lead to a reproducible, device-grade deposition process, intellectual property considerations would become relevant. In that scenario, the key steps would include: (i) filing process patents covering the specific IJD parameter windows that yield electronic-grade 2D films at low annealing temperatures; (ii) engaging with IJD equipment manufacturers (the technique is already marketed by Italian companies for biomedical coatings) to explore technology licensing or co-development agreements; (iii) establishing contacts with the European semiconductor industry (e.g. through the ECSEL Joint Undertaking, now Chips Joint Undertaking, or the pilot lines of the Graphene Flagship) to identify integration opportunities. At the present stage, however, the priority remains on completing the scientific characterisation and publishing the results, which will establish the credibility and visibility needed to attract commercial interest.
Regulatory and standardisation framework
The XAS spectroscopic framework established by CHIMERA for assessing the electronic quality of MoS2 films could, if validated across a wider range of laboratories and sample types, contribute to the development of standardised quality metrics for 2D semiconductor materials. Currently, no widely accepted standard exists for benchmarking the electronic quality of TMDC films; the community relies on a patchwork of Raman spectroscopy, photoluminescence peak ratios, and transport measurements, each of which captures only a partial picture. The quantitative correlation between XAS lineshape (S L2,3 fine structure, Mo M3 peak position and FWHM) and structural order demonstrated in this project suggests that XAS-based metrics could serve as a more comprehensive and element-specific quality indicator. Engagement with standardisation bodies (e.g. ISO TC 229 on nanotechnologies, or the emerging efforts within the EU Nanoelectronics Strategy) would be a valuable long-term step, though it lies beyond the immediate scope of the current results.