arXiv subjects

Carlo S. Casari

Publications and source records attributed to Carlo S. Casari.

At least 19 recordsLinked to original sources

arXiv2026

Multiwavelength Raman investigation of mono- and few-layer MoS2 grown by Pulsed Laser Deposition on SiO2

Molybdenum disulfide (MoS$_2$) is a semiconductor whose vibrational and excitonic properties are highly sensitive to layer number and structural disorder. We demonstrate the growth of MoS$_2$ monolayers on inert, electronics-compatible SiO$_2$ substrates using room-temperature pulsed laser deposition (PLD). Control of the process parameters enables tuning from monolayer to multilayer films, which we investigate by multiwavelength Raman spectroscopy. The evolution of the Raman-shift difference between the $E_{2g}^{1}$ and $A_{1g}$ modes, combined with an assessment of defect density, tracks film growth as a function of the number of deposition laser pulses. Although excitonic effects strongly influence the optical response of two-dimensional transition-metal dichalcogenides, experimental reports of symmetry-selective exciton-phonon coupling remain limited. We provide experimental evidence of symmetry-dependent exciton-phonon coupling in PLD-grown monolayer MoS$_2$. Specifically, we observe modulation of the resonant behaviour of the out-of-plane $A_{1g}$ and in-plane $E_{2g}^{1}$ modes, related to their different coupling to A excitons, predominantly derived from Mo $d_{z^2}$ orbitals, and C excitons, characterized by mixed orbital contributions from Mo $d_{z^2}$ and S $p_x$ and $p_y$ states. Comparison with mechanically exfoliated monolayers reveals the role of growth-induced defects in modulating these interactions. These findings establish room-temperature PLD as a viable approach for growing two-dimensional MoS$_2$ on inert, electronics-compatible substrates and provide insight into the interplay between excitonic resonances and growth-induced disorder in two-dimensional MoS$_2$.

cond-mat.mtrl-sci↗

arXiv2026

Iron-mediated on-surface synthesis of substrate-decoupled graphdiyne monolayers

Graphdiynes are emerging two-dimensional sp-sp$^2$ carbon materials with electronic structures complementing those of graphene, yet their on-surface synthesis is limited by the persistence of metalated intermediates or yields disordered covalent networks. Here, we report an iron-assisted route to covalent hydrogenated graphdiyne monolayers on Au(111) from 1,3,5-tris(bromoethynyl)benzene. Low-temperature scanning tunnelling microscopy, X-ray photoelectron spectroscopy and density functional theory show that Fe scavenges chemisorbed Br byproducts forming FeBr$_2$, in turn promoting the removal of Au adatoms from the organometallic network, thus enabling its metalated-to-covalent conversion under mild thermal treatment. Subsequent annealing removes FeBr$_2$ and yields covalent, ordered domains weakly coupled to the substrate. Scanning tunnelling spectroscopy, combined with density functional theory, reveals a semiconducting gap of about 1.6 eV associated with carbon p$_z$ frontier orbitals. This Fe-mediated on-surface synthesis strategy provides a route to atomically precise, weakly substrate-coupled graphdiyne networks and offers a design principle for two-dimensional carbon semiconductors.

cond-mat.mtrl-sci↗

arXiv2026

2D abrupt nano-junctions blending sp-sp2 bonds on atomically precise heterostructures

Two-dimensional heterostructures combining sp-sp2 hybridization,blending graphene with graphyne-based allotropes, offer substantial potential for enhancing the tunability of electronic and transport properties while providing significant structural flexibility. These attributes are desirable for next generation nanoscale electronic applications. Despite such potential, their experimental realization remains elusive, as synthesized carbon heterostructures are limited to doped, graphene-based systems exhibiting exclusively sp2 hybridization. Here, we demonstrate the on-surface synthesis of covalently bonded sp-sp2 lateral heterostructures between graphene nanoribbons and graphdiyne networks on Au(111). Atomic-resolution scanning tunnelling microscopy, combined with density functional theory, reveals the formation mechanism of the covalent interfacial bonds between nanoribbons and graphdiynes, also highlighting the key role of surface chemistry. Bromine atoms deriving from the molecules dehalogenation and chemisorbed along the nanoribbon inhibit the junction formation, but bonding efficiency can be boosted up to 71% by controlled removal of these by-products. Electronic structure and transport calculations show that the 2D heterostructure by itself is characterized by disentangled properties for the two subsystems, forming an atomically narrow junction enabling voltage-tunable spatial current separation in two dimensions. There results define a viable strategy for engineering graphene-based sp-sp2 heterostructures, paving the way for the design and synthesis of all-carbon nanoscale electronic architectures.

cond-mat.mtrl-sci↗

arXiv2025

Wettability and sp2/sp3 ratio effects on supercapacitor performance of N-doped hydrogenated amorphous Carbon Nanofoam

Pulsed laser-deposited amorphous carbon nanofoams are potential candidate for electrochemical energy storage applications due to ultra-light weight, large volumetric void fractions, and co-existence of sp, sp2 and sp3 carbon hybridization. It is known that charge storage in carbon nanostructures containing disordered sp2-domains is determined by their wettability, surface area, and porosity. However, their charge-storage performance is limited to the areal capacitance of the order of a few mF/cm2. We enhanced the supercapacitor performance of nitrogen-doped amorphous carbon nanofoam by engineering its wettability and sp2-C/sp3-C ratio by vacuum annealing. The specific capacitance was enhanced by about fifty times and the device voltage increased from 0.8 to 1.1 V compared to as-grown carbon nanofoam. In addition, we examined for the first time the initial increase in specific capacitance of the aqueous symmetric supercapacitor with respect to the scan rate, employing in-situ measurements coupling Raman spectroscopy and electrochemistry. We attribute this effect, observed but generally not explained in previous works in the literature, to the electrochemical activation induced by structural changes during the charge storage performance. This optimization of pulsed laser deposited carbon nanofoam may open an avenue for fabricating lightweight and porous nanostructures for advanced macro-to-micro-supercapacitor devices.

cond-mat.mtrl-sci↗

arXiv2025

Low-density functionalized amorphous carbon nanofoam as binder-free Supercapacitor electrode

Nanoporous carbon materials containing small domains of sp2-carbon with highly disordered structures are promising for supercapacitor applications. Herein, we synthesize amorphous carbon nanofoam with 98% volumetric void fraction and low mass density of around 30 mg/cm3 by pulsed laser deposition at room temperature. With the unavoidable oxygen functional groups on the nanoporous surface, carbon nanofoam and nitrogen-functionalized carbon nanofoams are directly grown on the desired substrate under different background gases (Ar, N2, N2-H2), and employed as supercapacitor electrodes. Among the background gases used in synthesis, the use of nitrogen yields nanofoam with higher thickness and more N-content with higher graphitic-N. From the test of amorphous carbon nanofoam supercapacitor device, nitrogenated amorphous carbon electrode shows a higher areal capacitance of 4.1 mF/cm2 at 20 mV/s in aqueous electrolyte, a better capacitance retention at higher current, and excellent cycle stability (98%) over 10000 charge-discharge cycles are achieved compared to not-functionalized counterpart prepared under Ar background gas (2.7 mF/cm2 and cycle stability of 88%).

physics.app-ph↗

arXiv2024

Cumulenic sp-carbon Atomic Wires Wrapped Polymers for Supercapacitor Application

Carbon atomic wires, a linear atomic chain of sp-carbon, is theoretically predicted to have around five times higher surface area than graphene, notable charge mobilities, as well as excellent optical and thermal properties. Despite these impressive properties, the properties of sp-carbon as an electrochemical energy-storage electrode have not been reported so far. Herein, we prepare solution processed thin films of tetraphenyl[3]cumulenic sp-carbon atomic wires embedded in a polymer matrix, in which sp-carbon atomic wires feature three cumulated carbon-carbon double bonds terminated at each end by two phenyl groups. Raman and UV-visible spectroscopy are used to confirm the presence and possible degradation of sp-carbons inside the polymeric matrix. Finally, we investigate the supercapacitor performance of cumulenic sp-carbon atomic wires embedded polymer in three aqueous mediums, namely 1M Na2SO4 (neutral), 1M H2SO4 (acidic), and 6M KOH (basic). The results suggest 6M KOH is the best electrolyte to obtain high charge-storage performance of device with areal capacitance of 2.4 mF/cm2 at 20 mV/s, 85% cycle stability after 10000 charge-discharge cycles, and excellent frequency response.

physics.app-ph↗

arXiv2024

One-step Pulsed Laser Deposition of Metal oxynitride/Carbon Composites for Supercapacitor Application

Advanced material composite of nanocarbons and metal-based materials provides a synergistic effect to obtain excellent electrochemical charge-storage performance and other properties. Herein, 3D porous carbon-metal oxynitride nanocomposites with tunable carbon/metal and oxygen/nitrogen ratio are synthesized uniquely by simultaneous ablation from two different targets by single-step pulsed laser deposition at room temperature. Co-ablation of titanium and vanadium nitride targets together with graphite allowed us to synthesize carbon-metal oxynitride porous nanocomposite and exploit them as a binder-free thin film supercapacitor electrode in aqueous electrolyte. We show that the elemental composition ratio and hence the structural properties can be tuned by selecting target configuration and by manipulating the ablation position. We investigate how this tuning capability impacts their charge-storage performances. We anticipate the utilization of as-synthesized various composites in a single PLD production run as next-generation active materials for flexible energy storage and optoelectronic applications.

physics.app-ph↗

arXiv2024

Ballistic-aggregated Carbon Nanofoam in Target-side of Pulsed Laser Deposition for Energy Storage Applications

In pulsed laser deposition, along the traditionally exploited deposition on the front-side of the plasma-plume, a coating forms on the surface of the target as well. For reproducibility, this residue is usually cleaned and discarded. Here we instead investigate the target-side coated materials and employ them as a binder-free supercapacitor electrode. The ballistic-aggregated, target-side nanofoam is compact and features a larger fraction of sp2-carbon, higher nitrogen content with higher graphitic-N and lower oxygen content with fewer COOH groups than that of diffusive-aggregated conventional nanofoams. They are highly hydrogenated graphite-like amorphous carbon and superhydrophilic. The resulting symmetric micro-supercapacitor delivers higher volumetric capacitance of 522 mF/cm3 at 100 mV/s and 104% retention after 10000 charge-discharge cycles over conventional nanofoam (215 mF/cm3 and 85% retention) with an areal capacitance of 134 μF/cm2 at 120 Hz and ultrafast frequency response. Utilizing the normally discarded target-side material can therefore enable high performing devices while reducing waste, cost and energy input per usable product. leading towards a greater sustainability on nanomaterials synthesis and deposition techniques.

physics.app-ph↗

arXiv2024

Unravelling the Band Structure and Orbital Character of a $π$-Conjugated 2D Graphdiyne-Based Organometallic Network

Graphdiyne-based carbon systems generate intriguing layered sp-sp$^2$ organometallic lattices, characterized by flexible acetylenic groups connecting planar carbon units through metal centers. At their thinnest limit, they can result in two-dimensional (2D) organometallic networks exhibiting unique quantum properties and even confining the surface states of the substrate, which is of great importance for fundamental studies. In this work, we present the on-surface synthesis of a highly crystalline 2D organometallic network grown on Ag(111). The electronic structure of this mixed honeycomb-kagome arrangement - investigated by angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy - reveals a strong electronic conjugation within the network, leading to the formation of two intense electronic band-manifolds. In comparison to theoretical density functional theory calculations, we observe that these bands exhibit a well-defined orbital character that can be associated with distinct regions of the sp-sp$^2$ monomers. Moreover, we find that the halogen by-products resulting from the network formation locally affect the pore-confined states, causing a significant energy shift. This work contributes to the understanding of the growth and electronic structure of graphdiyne-like 2D networks, providing insights into the development of novel carbon materials beyond graphene with tailored properties.

cond-mat.mes-hall↗

arXiv2023

Tailoring the Dimensionality of Tellurium Nanostructures via Vapor Transport Growth

The interest in tellurium nanostructures is on the rise due to their outstanding physical properties including high carrier mobility, anisotropic charge conduction, photoconductivity, thermoelectricity, and piezoelectricity. Applications in related technologies require tailoring the synthesis of tellurium from its preferred vertical growth toward the lateral growth. Here, the synthesis of pillar-like and pennette-like structures of tellurium through a powder vapor deposition technique has been discussed. It has been shown that exploiting salt additives such as NaCl in vapor deposition technique can enhance tellurium pillar dimensionality toward large planar grains. Further, we report on the synthesis of hexagonal ultrathin tellurium nanoflakes, namely tellurene, from few-layer to monolayer thickness by optimizing the growth kinetics without the usage of any additives. In addition, we explore the surface quality and physical properties of as-grown two-dimensional (2D) tellurium, using a variety of characterization techniques, including Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and Kelvin probe force microscopy. This study provides a pivotal scheme for enabling scalable 2D tellurium integration in numerous potential applications for electronics and optoelectronic devices.

physics.app-ph↗

arXiv2023

Disclosing Early Excited State Relaxation Events in Prototypical Linear Carbon Chains

One-dimensional (1D) linear nanostructures comprising sp-hybridized carbon atoms, as derivatives of the prototypical allotrope known as carbyne, are predicted to possess outstanding mechanical, thermal, and electronic properties. Despite recent advances in the synthesis, their chemical and physical properties are still poorly understood. Here, we investigate the photophysics of a prototypical polyyne (i.e., 1D chain with alternating single and triple carbon bonds), as the simplest model of finite carbon wire and as a prototype of sp-carbon-based chains. We perform transient absorption experiments with high temporal resolution (<30 fs) on monodispersed hydrogen-capped hexayne H$-$(C$\equiv$C)$_6-$H synthesized by laser ablation in liquid. With the support of detailed computational studies based on ground state density functional theory (DFT) and excited state time-dependent (TD)-DFT calculations, we provide a comprehensive description of the excited state relaxation processes at early times following photoexcitation. We show that the internal conversion from a bright high-energy singlet excited state to a low-lying singlet dark state is ultrafast and takes place with a 200-fs time constant, followed by thermalization on the picosecond timescale and decay of the low-energy singlet state with hundreds of picoseconds time constant. We also show that the timescale of these processes does not depend on the end groups capping the sp-carbon chain. The understanding of the primary photo-induced events in polyynes is of key importance both for fundamental knowledge and for potential optoelectronic and light-harvesting applications of low dimensional nanostructured carbon-based materials.

cond-mat.mes-hall↗

arXiv2022

A single liquid chromatography procedure to concentrate, separate and collect size-selected polyynes produced by pulsed laser ablation in water

Polyynes are linear carbon chains characterized by alternated single and triple bonds and terminated by hydrogen or other terminal substituents. They can be synthesized by pulsed laser ablation in liquid (PLAL) as a scalable, cost-effective, and fast physical technique. Water can be employed as a solvent for PLAL to avoid toxicity problems and to reduce costs compared to organic solvents. However, in this case, the production yield of polyynes reached is extremely low and prevents further characterization and implementation in new functional materials. In this work, we synthesized polyynes by pulsed laser ablation in water and we optimized the process parameters to improve the yield of polyynes by PLAL. Then, we developed a procedure entirely based on reversed-phase high-performance liquid chromatography (RP-HPLC) which effectively enables the concentration, separation and collection of polyynes according to their length. Since the polyynes sample is an aqueous solution, we could inject it directly into the RP-HPLC column without the dilution step required in the case of a sample in an organic solvent. Thanks to our single RP-HPLC procedure, it is possible to highly increase the concentration and separately characterize different size-selected polyynes for further use in functional materials.

physics.chem-ph↗

arXiv2022

Nanoparticle-enhanced Multifunctional Nanocarbons as Metal-ion Battery and Capacitor Anodes and Supercapacitor Electrodes -- Review

As renewable energy is becoming a critical energy source to meet the global demand, electrochemical energy storage devices become indispensable for the efficient energy storage and reliable supply. The electrode material is the key factor determining the energy storage capacity and the power delivery of the devices. Carbon-based materials are emerging as a viable candidate for electrodes, yet their low energy densities impede the development of advanced energy storage materials. Nanoparticle decoration of the carbon structures is one of the most promising and easy-to-implement a strategy to enhance the charge-storage performance of carbon-based electrodes. Decoration by nanoparticles of metals, metal oxides, nitrides, carbides, phosphides, chalcogenides, and bimetallic components lead to significant enhancements in the structural and electronic properties, pore refinement, charge storage, and charge transfer kinetics of both pristine and doped carbon structures, thereby making their performance promising for next-generation energy storage devices. This review covers the state-of-art nanoparticle decorated nanocarbons for battery, supercapacitor, and metal-ion capacitor applications. A critical analysis of the elemental composition, structure, associated physico-chemical properties and performance relationships of nanoparticle-decorated nanocarbon electrodes is provided as well to inform the future development of the next generation of advanced energy storage materials, devices, and systems.

physics.app-ph↗

arXiv2022

Stable and Solution-Processable Cumulenic sp-Carbon Wires: A New Paradigm for Organic Electronics

Solution-processed, large-area, and flexible electronics largely relies on the excellent electronic properties of sp$^2$-hybridized carbon molecules, either in the form of $π$-conjugated small molecules and polymers or graphene and carbon nanotubes. Carbon with sp-hybridization, the foundation of the elusive allotrope carbyne, offers vast opportunities for functionalized molecules in the form of linear carbon atomic wires (CAWs), with intriguing and even superior predicted electronic properties. While CAWs represent a vibrant field of research, to date, they have only been applied sparingly to molecular devices. The recent observation of the field-effect in microcrystalline cumulenes suggests their potential applications in solution-processed thin-film transistors but concerns surrounding the stability and electronic performance have precluded developments in this direction. In the present study, ideal field-effect characteristics are demonstrated for solution-processed thin films of tetraphenyl[3]cumulene, the shortest semiconducting CAW. Films are deposited through a scalable, large-area, meniscus-coating technique, providing transistors with hole mobilities in excess of 0.1 cm$^2$ V$^{-1}$ s$^{-1}$, as well as promising operational stability under dark conditions. These results offer a solid foundation for the exploitation of a vast class of molecular semiconductors for organic electronics based on sp-hybridized carbon systems and create a previously unexplored paradigm.

cond-mat.mtrl-sci↗

arXiv2021

Graphdiynes interacting with metal surfaces: first-principles electronic and vibrational properties

Graphdiynes (GDYs) represent a class of 2D carbon materials based on sp-sp$^2$ hybridization with appealing properties and potential applications. Recent advances have demonstrated the experimental self-assembly of GDYs on metal substrates. Here we focus on $α$- and $β$-GDYs on Au(111) and Pt(111), and investigate how their electronic and vibrational properties are affected by the interaction with a metal substrate. We adopt hydrogenated GDY, previously characterized experimentally, as a benchmark for density functional theory simulations, that we apply to show that Au and Pt substrates impose a different degree of distortion on both $α$- and $β$-GDY. By comparing the adsorbed and the freestanding structures, we evaluate the effect of the surface interaction on the bandstructure and the simulated Raman spectra. Different charge transfers result in different energy shift of the Dirac cone in semi-metallic $α$-GDY and changes from semiconducting to metallic behavior for $β$-GDY. These changes in electronic properties are accompanied by characteristic frequency shifts and modifications of Raman active modes. Our results contribute in the understanding of the metal-interaction effects on GDYs and can open a route to the design of novel 2D materials with tailored properties.

cond-mat.mtrl-sci↗

arXiv2021

Topology-dependent conjugation effects in graphdiyne molecular fragments

Graphdiynes (GDYs) as two-dimensional carbon structures based on sp2 hybridized aromatic rings connected by sp-hybridized acetylenic linear links are gathering an increasing popularity, both for their peculiar properties and for the promising applications. In these materials, structural features affect the degree of pi-electron conjugation resulting in different electronic, optical and vibrational properties. In particular, how topology, connectivity between sp and sp2 domains and system size are related with the final properties is fundamental to understand structure-property relationships and to tailor the properties by proper structure design. By using a computational approach based on density functional theory calculations, we here investigate structure-property relations in a class of 1D and 2D GDY molecular fragments as building block models of extended structures. By analysing how the structure can modulate the pi-electron conjugation in these systems, HOMO-LUMO gap is found to depend on the peculiar topology and connections between linear sp domains and aromatic units. A topological indicator is computed, showing a trend with the gap and with the frequency of the main vibrational mode occurring in Raman spectra. Our findings can contribute to guide the molecular design of new GDY-based sp-sp2 carbon materials, aiming at tuning their properties by precise control of the structure.

cond-mat.mtrl-sci↗

arXiv2021

Designing all graphdiyne materials as graphene derivatives: topologically driven modulation of electronic properties

Designing new 2D systems with tunable properties is an important subject for science and technology. Starting from graphene, we developed an algorithm to systematically generate 2D carbon crystals belonging to the family of graphdiynes (GDYs) and having different structures and sp/sp2 carbon ratio. We analyze how structural and topological effects can tune the relative stability and the electronic behavior, to propose a rationale for the development of new systems with tailored properties. A total of 26 structures have been generated, including the already known polymorphs such as α-, \b{eta}- and γ-GDY. Periodic density functional theory calculations have been employed to optimize the 2D crystal structures and to compute the total energy, the band structure, and the density of states. Relative energies with respect to graphene have been found to increase when the values of carbon sp/sp2 ratio increase, following however different trends based on the peculiar topologies present in the crystals. These topologies also influence the band structure giving rise to semiconductors with a finite bandgap, zero-gap semiconductors displaying Dirac cones, or metallic systems. The different trends allow identifying some topological effects as possible guidelines in the design of new 2D carbon materials beyond graphene.

cond-mat.mtrl-sci↗

arXiv2020

Structural, Electronic, and Vibrational Properties of 2D Graphdiyne-Like Carbon Nanonetwork Synthesized on Au(111): Implications for the Engineering of sp-sp2 Carbon Nanostructures

Graphdiyne, atomically-thin 2D carbon nanostructure based on sp-sp2 hybridization, is an appealing system potentially showing outstanding mechanical and optoelectronic properties. Surface-catalyzed coupling of halogenated sp-carbon-based molecular precursors represents a promising bottom-up strategy to fabricate extended 2D carbon systems with engineered structure on metallic substrates. Here, we investigate the atomic-scale structure and electronic and vibrational properties of an extended graphdiyne-like sp-sp2 carbon nanonetwork grown on Au(111) by means of on-surface synthesis. The formation of such 2D nanonetwork at its different stages as a function of the annealing temperature after the deposition is monitored by scanning tunneling microscopy (STM), Raman spectroscopy and combined with density functional theory (DFT) calculations. High-resolution STM imaging and the high sensitivity of Raman spectroscopy to the bond nature provide a unique strategy to unravel the atomic-scale properties of sp-sp2 carbon nanostructures. We show that hybridization between the 2D carbon nanonetwork and the underlying substrate states strongly affects its electronic and vibrational properties, modifying substantially the density of states and the Raman spectrum compared to the free standing system. This opens the way to the modulation of the electronic properties with significant prospects in future applications as active nanomaterials for catalysis, photoconversion and carbon-based nanoelectronics.

cond-mat.mtrl-sci↗