arXiv subjects

Alessandro Ruocco

Publications and source records attributed to Alessandro Ruocco.

10 recordsLinked to original sources

arXiv2026

A depth resolved investigation of hydrogen uptake in carbon based nanostructures by soft-to-hard photoemission spectroscopy

Hydrogen chemisorption on graphitic carbon modifies the carbon orbital hybridization from sp2 to sp3, altering both structural and electronic properties. Understanding not only the lateral extent but also the depth distribution of hydrogen uptake in three-dimensional carbon architectures is essential for both fundamental studies and storage applications. To this end, we investigate here the evolution of the C 1s core-level lineshape in nanoporous graphene (NPG) and vertically aligned carbon nanotubes (CNTs) upon hydrogenation, exploiting soft-to-hard X-ray photoemission spectroscopy to achieve a depth-resolved analysis. Decomposition of the C 1s spectra reveals the formation of an sp3 rich overlayer, indicating hydrogen chemisorption limited to the outermost accessible surfaces in both systems. These results clarify the depth distribution of hydrogen in curved and porous graphitic networks and provide quantitative constraints on its chemisorption for carbon-based hydrogen storage applications.

cond-mat.mtrl-sci↗

arXiv2026

Graphene lattice recoil in hard X-ray photoemission: Experiment and Theory

Hard-x-ray C 1s photoemission from monolayer graphene probes a regime in which nuclear recoil and intrinsic electronic asymmetry contribute on comparable energy scales to the observed spectral line shape. Here we combine experiment and modeling over the photon-energy range 0.8 keV--8 keV to resolve this interplay quantitatively. A graphene-specific implementation of the Fujikawa--Takata cumulant formalism, based on an anisotropic vibrational density of states constrained by first-principles phonon calculations, captures the expected recoil scaling with photon energy and emission geometry but fails to reproduce the pronounced asymmetric tails of the measured spectra. To overcome this limitation, we introduce an explicit electronic convolution model in which an intrinsic, photon-energy-independent electronic line shape extracted from near-recoilless 0.8 keV data is convolved with a phonon recoil kernel carrying the full dependence on photon energy and emission angle. This approach reproduces both the measured line-shape evolution and the observed centroid shifts across the explored energy range without refitting the spectra at higher photon energies. The results show that recoil in graphene cannot be described by a baseline treatment in which the phonon recoil kernel is combined only with symmetric lifetime broadening, but must be treated together with the intrinsic many-body electronic response of the C 1s line.

cond-mat.mtrl-sci↗

arXiv2025

Stability of Highly Hydrogenated Monolayer Graphene in Ultra-High Vacuum and in Air

The stability of hydrogenated monolayer graphene was investigated via X-ray photoemission spectroscopy (XPS) for two different environmental conditions: ultra-high vacuum (UHV) and ambient pressure. The study is carried out by measuring the C 1s line shape evolution for two hydrogenated samples one kept in the UHV chamber and the other progressively exposed to air. In particular, the $sp^3$ relative intensity in the C 1s core-level spectrum, represented by the area ratio $\frac{sp^3}{sp^2+sp^3}$, was used as a marker for the hydrogenation-level. After four months in UHV, it resulted almost unchanged within the experimental uncertainty. Thus, a long-term stability of hydrogenated monolayer graphene was found, that indicates this material as a good candidate for hydrogen (or tritium) storage as long as it is kept in vacuum. On the other hand, the C 1s spectrum of the sample exposed to air shows a significant oxidation. A rapid growth up to saturation of the carbon oxides was observed with a time constant $\tau$ = 2.8 $\pm$ 1.2 hours. Finally, the re-exposure of the oxidised sample to atomic hydrogen was found to be an effective method for the recovery of hydrogenated graphene. The CH stretching mode was measured via electron energy loss spectroscopy as direct footprint of hydrogenated graphene recovery.

cond-mat.mtrl-sci↗

arXiv2025

A Wide Optical-Gap in Fully $sp^3$-Like Hydrogenated Monolayer Graphene

A comprehensive spectroscopic characterisation of two highly hydrogenated monolayer graphene samples transferred onto nickel grids is reported. With X ray photoemission spectroscopy on the C 1s core-level, a 100$\%$ $sp^3$ profile was observed upon hydrogenation of a more $sp^3$-like initially defected graphene, while a flatter, more $sp^2$-arranged, graphene reached a 62$\%$ $sp^3$ saturation. Low-energy reflection electron energy-loss spectroscopy (EELS) corroborates these findings through the $\pi$-plasmon excitation quenching for the fully $sp^3$ sample and a significant reduction for the partially converted one. The extreme surface sensitivity of low-energy reflection EELS enables extraction of the optical band gap of the hydrogenated layer even on a metallic support, yielding values of 6.3 and 6.2 eV for the two samples. The C--H stretching vibrational mode is also resolved, providing a direct fingerprint of graphene--hydrogen bonding. Finally, valence-band measurements of the $62\%$ saturated sample suggest the coexistence of one-sided and two-sided hydrogenation morphologies.

cond-mat.mtrl-sci↗

arXiv2024

Detection of Low-Energy Electrons with Transition-Edge Sensors

We present the first detection of electrons with kinetic energy in the 100 eV range with transition-edge sensors (TESs). This has been achieved with a $(100\times 100)$ $\mu$m$^2$ Ti-Au bilayer TES, with a critical temperature of about 84 mK. The electrons are produced directly in the cryostat by an innovative cold source based on field emission from vertically-aligned multiwall carbon nanotubes. We obtain a Gaussian energy resolution between 0.8 and 1.8 eV for fully-absorbed electrons in the $(90-101)$ eV energy range, which is found to be compatible with the resolution of this same device for photons in the same energy range. This work opens new possibilities for high-precision energy measurements of low-energy electrons.

physics.ins-det↗

arXiv2023

Disclosing the nature of asymmetric interface magnetism in Co/Pt multilayers

Nowadays a wide number of applications based on magnetic materials relies on the properties arising at the interface between different layers in complex heterostructures engineered at the nanoscale. In ferromagnetic/heavy metal multilayers, such as the [Co/Pt]$_N$ and [Co/Pd]$_N$ systems, the magnetic proximity effect was demonstrated to be asymmetric, thus inducing a magnetic moment on the Pt(Pd) layer that is typically higher at the top Co/Pt(Pd) interface. In this work, advanced spectroscopic and imaging techniques were combined with theoretical approaches to clarify the origin of this asymmetry both in Co/Pt trilayers and, for the first time, in multilayer systems that are more relevant for practical applications. The different magnetic moment induced at the Co/Pt interfaces was correlated to the microstructural features, which are in turn affected by the growth processes that induce a different intermixing during the film deposition, thus influencing the interface magnetic profile.

cond-mat.mtrl-sci↗

arXiv2020

Response of Windowless Silicon Avalanche Photo-Diodes to Electrons in the 90-900 eV Range

We report on the characterization of the response of windowless silicon avalanche photo-diodes to electrons in the 90-900 eV energy range. The electrons were provided by a monoenergetic electron gun present in the LASEC laboratories of University of Roma Tre. We find that the avalanche photo-diode generates a current proportional to the current of electrons hitting its active surface. The gain is found to depend on the electron energy $E_e$, and varies from $2.147 \pm 0.027$ (for $E_e = 90$ eV) to $385.8 \pm 3.3$ (for $E_e = 900$ eV), when operating the diode at a bias of $V_{apd} = 350$ V.} This is the first time silicon avalanche photo-diodes are employed to measure electrons with $E_e < 1$ keV.

physics.ins-det↗

arXiv2020

Electron correlation effects in the exchange coupling at the Fe/CoO/Ag(001) ferro-/antiferro-magnetic interface

Angle resolved-Auger-photoelectron coincidence spectroscopy (AR-APECS) has been exploited to investigate the role that electron correlation plays in the exchange-coupling at the ferromagnetic/antiferromagnetic interface of a Fe/CoO bilayer growth on Ag(001). The effective correlation energy U$_{\textrm{eff}}$, usually employed to assess the energy distribution of core-valence-valence Auger spectra, has been experimentally determined for each possible combination of the orbital (e$_g$ or t$_{2g}$) and the spin (majority or minority) of the two valence electrons involved in the Auger decay. Coulomb and exchange interactions have been identified and compared with the result obtained on the Fe/Ag system. The presented analysis reveals in the Fe/CoO interface an enhancement of the Coulomb interaction for the e$_g$ orbital and of the exchange interaction for the t$_{2g}$ orbital with respect to the Fe/Ag case that can be associated to the stronger electron confinement and to the exchange coupling between the two layers, respectively.

cond-mat.str-el↗

arXiv2013

Interplay of the volume and surface plasmons in the electron energy loss spectra of C$_{60}$

The results of a joint experimental and theoretical investigation of the C60 collective excitations in the process of inelastic scattering of electrons are presented. The shape of the electron energy loss spectrum is observed to vary when the scattering angle increases. This variation arising due to the electron diffraction of the fullerene shell is described by a new theoretical model which treats the fullerene as a spherical shell of a finite width and accounts for the two modes of the surface plasmon and for the volume plasmon as well. It is shown that at small angles, the inelastic scattering cross section is determined mostly by the symmetric mode of the surface plasmon, while at larger angles, the contributions of the antisymmetric surface plasmon and the volume plasmon become prominent.

physics.atm-clus↗

arXiv2011

Insight on Hole-Hole Interaction and Magnetic Order from Dichroic Auger-Photoelectron Coincidence Spectra

The absence of sharp structures in the core-valence-valence Auger line shapes of partially filled bands has severely limited the use of electron spectroscopy in magnetic crystals and other correlated materials. Here by a novel interplay of experimental and theoretical techniques we achieve a combined understanding of the Photoelectron, Auger %$M_{23}M_{45}M_{45}$ and Auger-Photoelectron Coincidence Spectra (APECS) of CoO. This is a prototype antiferromagnetic material in which the recently discovered Dichroic Effect in Angle Resolved (DEAR) APECS reveals a complex pattern in the strongly correlated Auger line shape. A calculation of the \textit{unrelaxed} spectral features explains the pattern in detail, labeling the final states by the total spin. The present theoretical analysis shows that the dichroic effect arises from a spin-dependence of the angular distribution of the photoelectron-Auger electron pair detected in coincidence, and from the selective power of the dichroic technique in assigning different weights to the various spin components. Since the spin-dependence of the angular distribution exists in the antiferromagnetic state but vanishes at the Néel temperature, the DEAR-APECS technique detects the phase transition from its local effects, thus providing a unique tool to observe and understand magnetic correlations in such circumstances, where the usual methods (neutron diffraction, specific heat measurements) are not applicable.

cond-mat.str-el↗