arXiv subjects

Alice Apponi

Publications and source records attributed to Alice Apponi.

6 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↗

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↗