arXiv subjects

Caterina Cocchi

Publications and source records attributed to Caterina Cocchi.

At least 37 records · Page 2Linked to original sources

arXiv2026

Highly Efficient Exciton Modulation in MoSe$_2$/PdSe$_2$ Heterostructures

Controlling exciton recombination in atomically thin semiconductors is central to their optoelectronic functionality, as the competition between radiative and non-radiative decay channels governs emission efficiency. Existing approaches, such as defect passivation, chemical doping, dielectric engineering, and strain tuning, primarily aim to suppress non-radiative losses. Here, we report a pronounced $\sim$6-fold enhancement of room-temperature A-exciton emission in a type-I MoSe$_2$/PdSe$_2$ van der Waals heterostructure, yielding a photoluminescence quantum yield of 6 %, compared to $\sim$1 % for as-exfoliated monolayer MoSe$_2$. This enhancement is accompanied by strong quenching of the B-exciton, consistent with interlayer electronic coupling that redistributes exciton populations toward the radiative A-exciton channel. Power- and temperature-dependent measurements reveal a suppression of exciton-exciton annihilation and a crossover to quenched emission at low temperature, indicating a redistribution of exciton relaxation pathways. Photoluminescence excitation spectroscopy further reveals a broadband enhancement spanning 450-725 nm, ruling out a resonance-specific mechanism. These results demonstrate that interlayer electronic coupling can be used as an efficient means to redirect exciton populations toward radiative channels, enhancing emission efficiency in two-dimensional semiconductors without chemical modification or strain.

cond-mat.mes-hall↗

arXiv2026

Electronic localization and optical activity of strain-engineered transition-metal dichalcogenide nanobubbles

Strain-engineered transition-metal dichalcogenide nanobubbles are promising platforms for quantum emission, as revealed by recent experimental observations. In this work, we present an \textit{ab initio} investigation of MoS$_2$, WS$_2$, MoSe$_2$, and WSe$_2$ nanobubbles, linking their structural and electronic properties to predictions of their optical activity. Inflating forces yield tunable geometries with non-uniform, apex-concentrated strain, which is sensitive to material rigidity. Strain modifies band gaps and universally induces non-dispersive valence states, exhibiting composition-dependent wave-function character, as revealed by an in-depth analysis of band structures and orbital contributions. Crucially, transitions from these apex-localized valence states are predominantly dark. This characteristic is attributed to their localization at the $Γ$-point, inhibiting transitions to the lowest unoccupied states that reside at the K-valley. While revealing that the herein considered sub-10-nm nanobubbles fall short as single-photon emitters, our findings provide essential understanding of the structure-property relations in emerging quantum materials, providing robust design rules to optimize their characteristics for novel quantum applications.

cond-mat.mes-hall↗

arXiv2026

Janus MoSSe/WSSe Heterobilayers as Selective Photocatalysts for Water Splitting

Identifying materials that simultaneously straddle the water redox potentials and possess an intrinsic electric field is crucial for achieving high solar-to-hydrogen (STH) efficiency. Using state-of-the-art first-principles calculations, including a range-separated hybrid functional and spin-orbit coupling, we investigate MoXY/WXY (X, Y = S, Se) Janus bilayers for overall water splitting. We find a critical competition between the metal-to-metal chemical potential difference and the intrinsic dipoles at the interface between the Janus monolayers. We find that the Se-Se interfaced heterobilayer is intrinsically capable of driving water splitting, while its S-S counterpart can meet the redox requirements through pH modulation. For both configurations, a remarkable STH efficiency of 17.1% is anticipated. Furthermore, we predict a threshold of 1.0 eV for the built-in potential gradient to govern the transition from overall water splitting to band-edge pinning. Compared to homobilayers, heterobilayers benefit from the reciprocity between layer-specific dipoles and the Mo/W chemical potential difference, which promotes spatial separation and suppresses recombination, overall enhancing hydrogen production. Our results establish specific electronic descriptors for Janus heterostructures, providing a rational design rule for maximizing solar-driven hydrogen production in asymmetric two-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2026

Photoluminescence Quenching in WSe$_2$ via p-Doping Induced by Functionalized Rylene Dyes

Hybrid heterostructures combining transition metal dichalcogenides (TMDs) with light-harvesting dyes are promising materials for next-generation optoelectronics. Yet, controlling and understanding interfacial charge transfer mechanisms in these complex systems remains a major challenge. Here, we investigate the microscopic origin of photoluminescence (PL) quenching in $\text{WSe}_2$ functionalized with a novel, strongly electron-deficient perylene monoimide dye, $\text{CN}_4\text{PMI}$. Experimentally, the hybridization induces a $\sim$97\% PL quenching in $\text{WSe}_2$, confirming substantial static charge transfer and increased $p$-doping from the dye. To isolate the dominant electronic mechanism, we investigate from first principles various interface morphologies, including differing molecular orientations and layer thicknesses. Our density-functional theory results confirm that $\text{CN}_4\text{PMI}$ acts as a strong electron acceptor, inducing $p$-doping and forming a type-II level alignment with all considered configurations, giving rise to a small or vanishing band gap. Based on these findings, we attribute the observed PL suppression in $\text{WSe}_2$ to these strong electronic interactions with the dye. Our study provides a clear and validated strategy for tailoring the electronic structure of TMDs through targeted, electron-deficient organic functionalization.

cond-mat.mtrl-sci↗

arXiv2026

Atomistic Origin of Photoluminescence Quenching in Colloidal MoS2 and WS2 Nanoplatelets

Large chemical tunability and strong light-matter interactions make colloidal transition metal dichalcogenide (TMD) nanostructures particularly suitable for light-emitting applications. However, ultrafast exciton decay and quenched photoluminescence (PL) limit their potential. Combining femtosecond transient absorption spectroscopy with first-principles calculations on MoS2 and WS2 nanoplatelets, we reveal that the observed sub-picosecond exciton decay originates from edge-located optically bright hole traps. These intrinsic trap states stem from the metal d-orbitals and persist even when the sulfur-terminated edges are hydrogen-passivated. Notably, WS2 nanostructures show more localized and optically active edge states than their MoS2 counterparts, and zigzag edges exhibit a higher trap density than armchair edges. The nanoplatelet size dictates the competition between ultrafast edge-trapping and slower core-exciton recombination, and the states responsible for exciton quenching enhance catalytic activity. Our work represents an important step forward in understanding exciton quenching in TMD nanoplatelets and stimulates additional research to refine physicochemical protocols for enhanced PL.

cond-mat.mtrl-sci↗

arXiv2026

Optical properties of Fermi polarons in a GaInP/MoSe2 monolayer heterostructure

Engineering optical properties, such as luminescence purity and charge transfer, is crucial for harnessing the application potential of atomically thin transition metal dichalcogenides (TMDCs). While electrostatic gating is widely applied to gain charge control in TMDC monolayers, charge transfer can also be engineered via coupling of TMDC monolayers at semiconductor III/V, organic, or van der Waals interfaces. This confers great advantages, such as ease in implementation and compatibility in device integration. Here, we shed light on the optical properties of many-particle complexes emerging at the GaInP/MoSe2 interface as a highly relevant material combination to manipulate the optical properties of TMDCs in integrated photonic devices. Our study verifies its nature as a type II hetero-interface, which bears the feasibility to display disorder-free photoluminescence. Through optical absorption measurements, we verify that the charged complexes acquire substantial oscillator strength. Furthermore, temperature-dependent photoluminescence, supported by a microscopic theory framework, evidences the suppression of the characteristic carrier recoil effect that was previously observed in the photoluminescence of trions in TMDCs. These phenomena allow us to identify the optical signatures at the TMDC-GaInP interface as Fermi polaron quasiparticle resonances, which are of high importance in researching Bose-Fermi mixtures in condensed matter systems.

cond-mat.mtrl-sci↗

arXiv2025

Edge Magnetism in Colloidal MoS2 Triangular Nanoflakes

The control of localized magnetic domains at the nanoscale holds great promise for next-generation spintronic applications. Colloidal transition metal dichalcogenides nanostructures are experimentally accessible and chemically tunable platforms for spintronics, deserving dedicated research to assess their potential. Here, we investigate from first principles free-standing triangular MoS2 nanoflakes with sulfur-terminated, hydrogen-passivated edges, to probe intrinsic spin behavior at varying side lengths. We find a critical edge length of approximately 1.5 nm separating nonmagnetic nanoflakes from larger ones with a magnetic ground state emerging from several, energetically competing spin configurations. In these systems, the magnetic activity is not uniformly distributed along the edges but localized on specific "magnetic islands" around molybdenum edge atoms. The localization of magnetic moments is robust even in non-equilateral nanoflake geometries, highlighting their intrinsic stability regardless of the (high) symmetry of the hosting structure. These findings establish that the S-terminated, H-passivated triangular MoS2 nanoflakes are a stable and experimentally accessible platform via colloidal synthesis for low-dimensional, next-generation spintronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Unveiling the Role of Solvents in DBTTF:HATCN Ternary Cocrystals

Donor-acceptor (D:A) cocrystals offer a promising platform for next-generation optoelectronic applications, but the impact of residual solvent molecules on their properties remains an open question. We investigate six novel D:A cocrystals of dibenzotetrathiafulvalene (DBTTF) and 1,4,5,8,9,11-hexaazatriphenylenehexacarbo-nitrile (HATCN), prepared via solvent evaporation, yielding 1:1 molar ratios, and horizontal vapor deposition, resulting in solvent-free 3:2 cocrystals. Combining spectroscopy and density-functional theory (DFT) calculations, we find that, while the electronic and optical properties of the cocrystals are largely unaffected by solvent inclusion, the charge transfer mechanism is surprisingly complex. Raman spectroscopy reveals a consistent charge transfer of 0.11 $e$ across all considered structures, corroborated by DFT calculations on solvent-free systems. Partial charge analysis reveals that in solvated cocrystals, solvent molecules actively participate in the charge transfer process as primary electron acceptors. This involvement can perturb the expected D:A behavior, revealing a faceted charge-transfer mechanism in HATCN even beyond the established involvement of its cyano group. Overall, our study demonstrates that while solution-based methods preserve the intrinsic D:A characteristics, solvents can be leveraged as active electronic components, opening new avenues for material design.

cond-mat.mtrl-sci↗

arXiv2025

Excited State Absorption Drives Low-Energy Optical Limiting in Oligothiophenes

Optical limiting (OL), a crucial mechanism for protecting human eyes and sensitive sensors from intense radiation, relies on understanding the optical nonlinearities acting on the systems. Assessing and disentangling the effects at play is crucial to predict and control the nonlinear optical response in real materials. In this ab initio study based on real-time time-dependent density-functional theory, we investigate non-perturbatively the absorption spectra of a set of thiophene oligomers, the building blocks of technologically relevant organic semiconductors, excited by broadband radiation of increasing intensity. Under strong electric fields, the absorption cross section grows significantly below the onset of linear excitations, exhibiting saturation typical of OL. By exciting the oligothiophenes with a train of pulses targeting the first and second excited states of each moiety and analyzing the resulting population dynamics, we reveal excited-state absorption (ESA) in the near-infrared to visible region. Our results indicate ESA as the driving mechanism for OL in oligothiophene molecules, thereby providing important insight to design novel compounds with optimized nonlinear optical characteristics.

physics.optics↗

arXiv2025

Ab initio X-ray Near-Edge Spectroscopy of Sodium-Based Multi-Alkali Antimonides

Multi-alkali antimonides (MAAs) are promising materials for vacuum electron sources. While sodium-based MAAs have demonstrated superior characteristics for ultrabright electron sources, their synthesis remains challenging, often resulting in mixed stoichiometries and polycrystalline domains. To address this complexity and guide the characterization of experimentally grown photocathodes, we present a comprehensive theoretical study of the X-ray near-edge spectroscopy (XANES) of four ternary MAAs: cubic Na$_2$KSb and hexagonal NaK$_2$Sb, representing the experimentally known phase of each stoichiometry, as well as hexagonal Na$_2$KSb and cubic NaK$_2$Sb, two computationally predicted polymorphs. Employing state-of-the-art ab initio methods based on all-electron density-functional theory and the solution of the Bethe-Salpeter equation (BSE), we compute and analyze the XANES at the sodium and potassium K-edges, potassium L$_{2,3}$-edge, and antimony K and L$_2$-edges. Our analysis reveals distinct spectral fingerprints for the experimentally known phases, cubic Na$_2$KSb and hexagonal NaK$_2$Sb, particularly at the sodium K-edge and potassium L$_{2,3}$-edge, providing useful indications for their identification in complex samples. We further investigate the role of excitonic effects by comparing BSE spectra and their counterparts obtained in the independent-particle approximation, highlighting their significant influence on the near-edge features, especially for shallower core levels. Our findings offer a useful theoretical benchmark for the experimental characterization and diagnostics of sodium-based MAA photocathodes, complementing experiments on resolved phases and providing the spectral fingerprints of computationally predicted phases that could emerge in polycrystalline samples.

cond-mat.mtrl-sci↗

arXiv2025

Ultrafast Photoexcitation of Semiconducting Photocathode Materials

Cs-based semiconductors like $\mathrm{Cs_3Sb}$ and $\mathrm{Cs_2Te}$ are currently used as photocathodes in particle accelerators. Their performance as electron sources critically depends on their interaction with intense laser sources. In this work, we investigate from first principles the time-dependent response of $\mathrm{Cs_3Sb}$ and $\mathrm{Cs_2Te}$ to ultrafast pulses of varying intensities, ranging from $1~\mathrm{GW/cm^2}$ to $1~\mathrm{PW/cm^2}$. Nonlinear effects, including high harmonic generation, emerge starting from $100~\mathrm{GW/cm^2}$ in $\mathrm{Cs_3Sb}$ and $200~\mathrm{GW/cm^2}$ in $\mathrm{Cs_2Te}$. Above these intensities, the numbers of absorbed photons and excited electrons saturate due to the depletion of one-photon absorption channels, with renewed increases beyond $1~\mathrm{TW/cm^2}$ for $\mathrm{Cs_3Sb}$ and $5~\mathrm{TW/cm^2}$ for $\mathrm{Cs_2Te}$ where multi-photon absorption appears. Finally, the analysis of the occupation density in $\mathrm{Cs_3Sb}$ reveals the onset of tunnel ionization at intensities above $10~\mathrm{TW/cm^2}$. Our findings provide new insights into the nonlinear optical properties of $\mathrm{Cs_3Sb}$ and $\mathrm{Cs_2Te}$, contributing to the optimization of these materials for the development of next-generation photoinjectors.

cond-mat.mtrl-sci↗

arXiv2025

Benchmarking Selected Density Functionals and Dispersion Corrections for MOF-5 and its Derivatives

Accurate computational predictions of metal-organic frameworks (MOFs) and their properties is crucial for discovering optimal compositions and applying them in relevant technological areas. This work benchmarks density functional theory (DFT) approaches, including semi-local, meta-GGA, and hybrid functionals with various dispersion corrections, on MOF-5 and three of its computationally predicted derivatives, analyzing structural, electronic, and vibrational properties. Our results underline the importance of explicitly treating van der Waals interactions for an accurate description of structural and vibrational properties, and indicate the meta-GGA functional R2SCAN as the best balance between accuracy and efficiency for characterizing the electronic structure of these systems, in view of future high-throughput screening studies on MOFs.

cond-mat.mtrl-sci↗

arXiv2025

Electronic and optical excitations of K-Sb and Na-Sb crystals

Recent advances in experimental techniques and computational methods have significantly expanded the family of alkali antimonides, a class of semiconducting materials used as photocathodes in particle accelerators, unveiling new crystal structures and stoichiometries with improved stability and quantum efficiency. This work investigates the electronic and optical properties of eight Na- and K-based alkali antimonide binary crystals with 3:1 and 1:1 alkali-to-antimony ratios, which were predicted to be stable in a recent high-throughput screening study. Employing density functional theory and many-body perturbation theory, we find that all systems exhibit direct band gaps, except for monoclinic Na$_8$Sb$_8$, which has a nearly degenerate indirect gap. Optical spectra are characterized by near-infrared absorption onsets and intense visible excitations. Our analysis highlights the significant role of electron-hole correlations, particularly in K-based compounds, leading to exciton binding energies above 100~meV and sharper absorption peaks. An in-depth analysis of the electronic contributions to the excited states provides additional insight into the role of excitonic effects. By shedding light on the fundamental properties of alkali antimonide binary crystals, our results are relevant for the design and optimization of next-generation electron sources for particle accelerators.

cond-mat.mtrl-sci↗

arXiv2024

Electronic and optical properties of computationally predicted Na-K-Sb crystals

Thanks to their favorable electronic and optical properties, sodium-potassium-antimonides are an emerging class of crystals used as photocathodes in particle accelerators. The persisting challenges related to the synthesis and characterization of these materials demand support from theory and make the study of computationally predicted polymorphs particularly relevant to identifying the structure and composition of the samples. Using first-principles methods based on density-functional theory and many-body perturbation theory, the electronic and optical properties of cubic NaK$_{2}$Sb and hexagonal Na$_{2}$KSb are studied. Both systems, most commonly found in the hexagonal and cubic phase, respectively, exhibit an indirect fundamental gap that is energetically very close to the direct band gap at $Γ$ of magnitude 0.81 eV for NaK$_{2}$Sb and 0.70 eV for Na$_{2}$KSb. In the band structure of both materials, Sb $p$-states dominate the valence region with minor contributions from the alkali $p$-states, while the alkali $s$-states mainly contribute at lower energies. The optical spectra of both crystals are not subject to sizeable excitonic effects, except for a redshift of the excitation energies of the 50-100 meV and some redistribution of the oscillator strength beyond the lowest-energy peak in the near-infrared region. Our results indicate that computationally predicted cubic NaK$_{2}$Sb and hexagonal Na$_{2}$KSb have favorable characteristics as photocathodes and, as such, their presence in polycrystalline samples is not detrimental for these applications.

cond-mat.mtrl-sci↗

arXiv2024

Thermodynamic stability and vibrational properties of multi-alkali antimonides

Modern advances in generating ultrabright electron beams have unlocked unprecedented experimental advances based on synchrotron radiation. Current challenges lie in improving the quality of electron sources with novel photocathode materials such as alkali-based semiconductors. To unleash their potential, a detailed characterization and prediction of their fundamental properties is essential. In this work, we employ density functional theory combined with machine learning techniques to probe the thermodynamic stability of various alkali antimonide crystals, emphasizing the role of the approximations taken for the exchange-correlation potential. Our results reveal that the SCAN functional offers an optimal trade-off between accuracy and computational costs to describe the vibrational properties of these materials. Furthermore, it is found that systems with a higher concentration of Cs atoms exhibit enhanced anharmonicities, which are accurately predicted and characterized with the employed methodology.

cond-mat.mtrl-sci↗

arXiv2024

First-Principle Characterization of Structural, Electronic, and Optical Properties of Tin-Halide Monomers

The growing interest in tin-halide semiconductors for photovoltaic applications demands an in-depth knowledge of the fundamental properties of its constituents, starting from the smallest monomers entering the initial stages of formation. In this first-principles work based on time-dependent density-functional theory, we investigate the structural, electronic, and optical properties of tin-halide molecules SnX$_n^{2-n}$, with $n=1,2,3,4$ and X = Cl, Br, I, simulating these compounds in vacuo as well as in an implicit solvent. We find that structural properties are very sensitive to the halogen species while the charge distribution is also affected by stoichiometry. The ionicity of the Sn-X bond is confirmed by the Bader charge analysis albeit charge displacement plots point to more complex metal-halide coordination. Particular focus is posed on the neutral molecules SnX$_2$, for which electronic and optical properties are discussed in detail. Band gaps and absorption onset decrease with increasing size of the halogen species and despite general common features, each molecule displays peculiar optical signatures. Our results are elaborated in the context of experimental and theoretical literature, including the more widely studied lead-halide analogs, aiming to contribute with microscopic insight to a better understanding of tin-halide perovskites.

cond-mat.mtrl-sci↗

arXiv2024

Electronic properties of MoSe$_2$ nanowrinkles

Mechanical deformations, either spontaneously occurring during sample preparation or purposely induced in their nanoscale manipulation, drastically affect the electronic and optical properties of transition metal dichalcogenide monolayers. In this first-principles work based on density-functional theory, we shed light on the interplay among strain, curvature, and electronic structure of MoSe$_2$ nanowrinkles. We analyze their structural properties highlighting the effects of coexisting local domains of tensile and compressive strain in the same system. By contrasting the band structures of the nanowrinkles against counterparts obtained for flat monolayers subject to the same amount of strain, we clarify that the specific features of the former, such as the moderate variation of the band-gap size and its persisting direct nature, are ruled by curvature rather than strain. The analysis of the wave-function distribution indicates strain-dependent localization of the frontier states in the conduction region while in the valence the sensitivity to strain is much less pronounced. The discussion about transport properties, based on the inspection of the effective masses, reveals excellent perspectives for these systems as active components for (opto)electronic devices.

cond-mat.mtrl-sci↗

arXiv2024

Tuning Structural and Electronic Properties of Metal-Organic Framework 5 by Metal Substitution and Linker Functionalization

The chemical flexibility of metal-organic frameworks (MOFs) offers an ideal platform to tune structure and composition for specific applications, from gas sensing to catalysis and from photoelectric conversion to energy storage. This variability gives rise to a large configurational space that can be efficiently explored using high-throughput computational methods. In this work, we investigate from first principles the structural and electronic properties of MOF-5 variants obtained by replacing Zn with Be, Mg, Cd, Ca, Sr, and Ba, and by functionalizing the originally H-passivated linkers with CH$_3$, NO$_2$, Cl, Br, NH$_2$, OH, and COOH groups. To build and analyze the resulting 56 structures, we employ density-functional theory calculations embedded in an in-house developed library for automatized calculations. Our findings reveal that structural properties are mainly defined by metal atoms and large functional groups which distort the lattice and modify coordination. Stability is largely influenced by functionalization and enhanced by COOH and OH groups which promote the formation of hydrogen bonds. The charge distribution within the linker is especially influenced by functional groups with electron-withdrawing character while the metal nodes play a minor role. Likewise, the band-gap size is crucially determined by ligand functionalization. The smallest gaps are found with NH$_2$ and OH groups which introduce localized orbitals at the top of the valence band. This characteristic makes these functionalizations particularly promising for the design of MOF-5 variants with enhanced gas uptake and sensing properties.

cond-mat.mtrl-sci↗