Thin films ofβ-W have attracted much attention due to their fascinating properties for spintronic and magnetic random-access memory but their thermal behavior has still not been well understood. Here we have performed a systematic investigation of their thermal stability and phase transformation behavior using x-ray diffraction, x-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), and thermal desorption spectroscopy (TDS). Our results reveal the formation ofβ-W films, which irreversibly transform into theαphase upon heating above 200 °C. XPS measurements indicate the presence of approximately 12 at.% oxygen in theβphase, decreasing to a few percent after annealing up to 500 °C. Complementary TDS, XPS, and SIMS analyses provide quantitative insights into compositional and structural changes during heat treatment, while XPS elucidates the electronic structure, consistent with ourab initiodensity functional theory calculations and molecular dynamics simulations. Theoretical studies reveal that theαphase is energetically more stable at low oxygen concentrations of at least up to ∼10 at.%, corroborating experimentally observedβtoαphase transformation by oxygen desorption.

Thin films of beta-W have attracted much attention due to their fascinating properties for spintronic and magnetic random-access memory but their thermal behavior has still not been well understood. Here we have performed a systematic investigation of their thermal stability and phase transformation behavior using x-ray diffraction, x-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), and thermal desorption spectroscopy (TDS). Our results reveal the formation of beta-W films, which irreversibly transform into the alpha phase upon heating above 200 degrees C. XPS measurements indicate the presence of approximately 12 at.% oxygen in the beta phase, decreasing to a few percent after annealing up to 500 degrees C. Complementary TDS, XPS, and SIMS analyses provide quantitative insights into compositional and structural changes during heat treatment, while XPS elucidates the electronic structure, consistent with our ab initio density functional theory calculations and molecular dynamics simulations. Theoretical studies reveal that the alpha phase is energetically more stable at low oxygen concentrations of at least up to similar to 10 at.%, corroborating experimentally observed beta to alpha phase transformation by oxygen desorption.

The future growth of alkali metal-based batteries requires an understanding of how ion size affects the exchange mechanisms. In this work, we present a direct, comparative electrochemical study of MXene-based electrodes mechanism vs. lithium (Li+), sodium (Na+), and potassium (K+) ions using the same electrochemical conditions. This controlled method enables an extensive investigation of the size-dependent interactions between the MXene structure and alkali metal ions. X-ray photoelectron spectroscopy and Raman analysis of TMAOH-treated Ti3C2Tx MXene electrodes show that delamination and cycling alter vibrational modes and the surface chemistry. Voltage profile study reveals diverse storage behaviors: Li+ has a prominent intercalation plateau, Na+ shows intermediate properties, and K+ displays sloping profiles, indicating surface-dominated adsorption. The significant correlation between ionic radius and electrochemical reversibility is shown by long-term cycling data over 300 cycles, which show greater capacity retention and stability for Li+ and progressively lower performance for Na+ and K+. These findings provide new mechanistic insights into MXene-ion interactions and build the foundation for developing MXene-based materials for specific alkali-ion chemistries in next-generation energy storage devices.

A spatially resolved angle-resolved photoelectron spectroscopy investigation at the micron scale reveals the effects of the external environment on exfoliated MoS2 multilayers. The electronic band dispersion and related parameters were measured on MoS2 samples that were exfoliated and transferred in a controlled air-free atmosphere. Notably, these samples exhibited a more defined band structure and a higher spectral density of states compared to those prepared and transferred in air. In the latter case, surface contamination resulted in band broadening, particularly in the density of states linked to out-of-plane orbitals in the low binding energy region near the valence band maximum. Slight p-doping is also observed for the non-protected sample, which can be associated with tiny effects of ambient oxygen during transport. These findings underscore the necessity of air-protected exfoliation and transfer to accurately capture the fundamental properties of transition metal dichalcogenides.

Atomic deuterium adsorption on multi-walled carbon nanotubes (MWCNTs) has been achieved with a high deuteration level (≃70% of deuterated carbon atoms), and studied using complementary spectroscopic techniques, namely, photoelectron spectroscopy and Raman spectroscopy. As a consequence of the deuterium (D) adsorption on the MWCNTs, the sp2 bonds of the C atoms are distorted toward an sp3 configuration, and the π plasmon excitation of the metallic MWCNTs is quenched, suggesting the transition to a semiconducting phase. Such a controlled deuteration in ultra-high vacuum conditions induces the opening of an energy gap in the metallic MWCNTs, with the valence band maximum at about ∼3.1 eV below the Fermi level. The bond distortion and the strain induced by the D uptake is evidenced by the modification of the Raman response. This work shows that the molecular cracking of D2 in ultra-high vacuum is an efficient way to obtain stable, homogeneous, and high uptake of deuterium atoms with minimal presence of defects.

The investigation of intricate alignments in two-dimensional materials has garnered significant attention, fueled by the growing ability to precisely manipulate and engineer electronic properties through controlled stacking and orientation. Recent observations of spontaneous arrangements in multilayer materials with stochastic twist angles offer new opportunities to elucidate the unique physics governing moir & eacute; superstructures, and the critical role of the electronic coupling between layers. In this context, the present study focuses on the atomic-scale characterization of nanoporous graphene, with particular emphasis on regions formed by misoriented graphene layers that assemble into crumpled, continuous patterns. Despite the absence of flat domains at the submicron scale, scanning tunneling microscopy is employed to identify dominant twisted multilayer structures in the samples. To complement the experimental observations, a simplified, yet, effective exponential parametric model is introduced, merging independent analytic electron densities to estimate the number of misaligned layers in the scanned areas. This approach is further refined through density-functional theory calculations of projected electron densities from graphene, which are rotated and stacked at adjusted interlayer distances. The synergy between the two models provides a robust framework for distinguishing between twisted bilayer and twisted trilayer domains, as observed in constant-current imaging. Finally, a full density-functional theory analysis is conducted on simple few-layer graphene structures, assessing the role of interlayer correlations. The present study provides an atomic-level description of porous graphene, while also offering accessible tools for simulating twisted layered materials beyond graphene, which may be beneficial for the STM community.

Graphite, with its van der Waals layered structure, can accommodate a diverse array of intercalant species within its layers. Alkali metals (AMs), a family of donor intercalants, play a pivotal role in technological advancements, notably in lithium-ion batteries. Owing to its structural simplicity and the feasibility of producing high-quality single crystals spanning areas up to hundreds of micrometers squared, few-layer (FL) graphene serves as an exemplary system for investigating the dynamics of AM adsorption and intercalation. This study focuses on the deposition of potassium atoms in ultra-high vacuum onto mechanically exfoliated four- to five-layer FL and multilayer (ML) graphene. Employing spatially resolved Raman spectroscopy, we examine the impact of potassium adsorption, diffusion, and intercalation. Our findings reveal intricate potassium intake spatial patterns in FL graphene. The distinct spatial inhomogeneities of FL graphene are not observed in ML graphene. Density functional theory calculations also confirmed a complex scenario where both charging and steric hindrance introduce local strain in the graphene layers. Furthermore, charge donation from potassium atoms both to adjacent graphene layers and to more distant layers not adjacent to potassium atoms suggest a modification of the structural and vibrational properties of graphene consistent with the Raman experiments. The detection of intercalation fronts and domains with constant charging, and thus constant potassium densities, underscores the complex and collective nature of the potassium diffusion and intercalation in FL graphene, offering valuable insights for potential applications in energy storage systems.

The effects of uniaxial strain on the local band structure modification in a single wrinkle in a trilayer (3L) molybdenum disulfide (MoS2) flake have been enlightened by combining complementary atomic force microscopy, Raman and photoluminescence microspectroscopies. Controlled wrinkles were introduced in 3L MoS2 flakes by using buckling instability, inducing local tensile strains of up to 0.07%. The ability to induce and fabricate stable wrinkles in 3L MoS2, which is a nanoscale system whose thickness is smaller than 2 nm, arises from the material's reduced thickness. Submicron-scale spatial mapping of the isolated wrinkle revealed a reduction of the direct bandgap by 10 meV, accompanied by a quenching of the radiative recombination of excitons in the wrinkle compatible with an antifunneling effect, where excitons drift away from lower-bandgap areas before recombination. Finally, angle-resolved photoemission microspectroscopy further demonstrated a shift of the valence band toward higher binding energies in the isolated MoS2 wrinkle. Combining these results with the ones from optical spectroscopies results in a type-II band alignment between the flat and the strained regions, with downward shifts in both the conduction and valence bands. These new insights into the local electronic structure in locally strained 3L MoS2 nanosheets may help the design and performance of nanoscale optoelectronic and photonic devices by enabling precise control over the excitonic properties and the energetic spatial landscape.

Alkali metal doping of multi-walled carbon nanotubes is of great interest, both fundamentally to explore the effect of dopants on quasi-one-dimensional electrical systems and for energy applications such as alkali metal storage. We present an investigation with complementary photoemission and Raman spectroscopies, fully carried out in an ultra-high vacuum, to unveil the electronic and vibrational response of a forest of highly aligned multi-walled carbon nanotubes by in situ potassium doping. The charge donation by the alkali adatoms induces a plasmon mode, and the density of states undergoes an energy shift consistent with electron donation and band filling of the multi-walled carbon nanotube band structure. The π-states in the valence band and the Raman peaks unveil an evolution that can be ascribed to charge donation and partially to a tensile strain exerted by the K adatoms on the carbon lattice. All these effects are thermally reversible, fostering these materials as a potential system for electronic charge harvesting.

The effects of optical excitation on fully hydrogenated free-standing nanoporous graphene have been characterized by pump-probe X-ray photoemission spectroscopy. Hydrogenated graphene, known as graphane, is characterized by a sp3 hybridization, which induces a sp3 component in the C1s core level whose intensity can be used to monitor the hydrogen content. Under optical excitation we observe a partial dehydrogenation of graphane, which we attribute to local laser-induced heating; such result allows us to estimate the thermal conductivity of the material, for which we found an upper limit of 0.2 W/m K, four orders of magnitude smaller than that of graphene. Such stark difference, combined with the possibility of dehydrogenating the graphane substrate via laser exposure, may be exploited to engineer nanostructured heat conduction channels in organic and hybrid organic-inorganic devices. We then explored the sub-nanosecond dynamics of the C 1s core level, which displays a kinetic energy shift and a peak broadening with two different decay constants, 210 ps and 130 ps, respectively. We assign the former to surface photovoltage, and the latter to transient lattice heating.

Herewith, we propose a comprehensive study of the vibrational response of chemical doping of free-standing graphene (Gr). Complementary insights on the increased metallicity have been demonstrated by the emerging plasmon excitation in the upper Dirac cone, observed by inelastic electron scattering and core-level photoemission. The electron migration in the pi* upper Dirac band unveils an electron-phonon coupling of contaminant-free K-doped Gr, as evidenced by advanced micro-Raman spectroscopy in ultrahigh vacuum ambient. The vibrational response of potassium-doped Gr correlated with the charge injected in the upper Dirac cone, and the Fermi level shift unravel a notable electron-phonon coupling, which is stronger than that observed for gate voltage-doped Gr.

The doping mechanism of fully suspended nanoporous graphene with alkali metals unveils the charge redistribution between the donors and graphene. The evolution of the electronic charge induced in the graphene upper Dirac cone has been correlated with the electron-phonon interaction by employing complementary electronic and vibrational spectroscopic techniques in ultra high vacuum ambient. K and Cs doping induces a significant rigid band shift and Dirac cone filling correlated with the neat formation of a pi*-plasmon mode and a clear signature of electron-phonon interaction in the Raman spectra, while Na doping slightly influences the graphene electronic and vibrational response, fostered by adatom Na clustering.

A detailed inverse photoemission study unveils the unoccupied electronic structure induced by the adsorption of CuPc and CoPc phthalocyanines on Au(110) reconstructed channels. The different behavior in the two systems is related to the different intermixing of orbitals with the underlying gold states. Broadening of the density of states at the Fermi level is detected after CoPc adsorption, absent in the case CuPc. A detailed comparison with the element-selective X-ray absorption spectroscopy enlightens and complements the IPES results and confirms a surface-driven intermixing of the CoPc orbitals involved in the interaction, with the out-of-plane Co 3dz2 orbital strongly hybridized with the gold electronic states. Moreover, the contribution of the 3d empty states to the IPES data is reported for FePc, CoPc, and CuPc thin films.

Titanium trisulfide (TiS3) nanoribbons, when coated with titanium dioxide (TiO2), can be used for water splitting in the KOH electrolyte. TiO2 shells can be prepared through thermal annealing to regulate the response of TiS3/TiO2 heterostructures by controlling the oxidation time and growth atmosphere. The thickness and structure of the TiO2 layers significantly influence the photoelectrocatalytic properties of the TiS3/TiO2 photoanodes, with amorphous layers showing better performance than crystalline ones. The oxide layers should be thin enough to transfer photogenerated charge through the electrode-electrolyte interface while protecting TiS3 from KOH corrosion. Finally, the performance of TiS3/TiO2 heterostructures has been improved by coating them with various electrocatalysts, NiSx being the most effective. This research presents new opportunities to create efficient semiconductor heterostructures to be used as photoanodes in corrosive alkaline aqueous solutions.

Highly aligned multi-wall carbon nanotubes were investigated with scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) before and after bombardment performed using noble gas ions of different masses (argon, neon and helium), in an ultra-high-vacuum (UHV) environment. Ion irradiation leads to change in morphology, deformation of the carbon (C) honeycomb lattice and different structural defects in multi-wall carbon nanotubes. One of the major effects is the production of bond distortions, as determined by micro-Raman and micro-X-ray photoelectron spectroscopy. We observe an increase in sp3 distorted bonds at higher binding energy with respect to the expected sp2 associated signal of the carbon 1s core level, and increase in dangling bonds. Furthermore, the surface damage as determined by the X-ray photoelectron spectroscopy carbon 1s core level is equivalent upon bombarding with ions of different masses, while the impact and density of defects in the lattice of the MWCNTs as determined by micro-Raman are dependent on the bombarding ion mass; heavier for helium ions, lighter for argon ions. These results on the controlled increase in sp3 distorted bonds, as created on the multi-wall carbon nanotubes, open new functionalization prospects to improve and increase atomic hydrogen uptake on ion-bombarded multi-wall carbon nanotubes.

The growing demand for improved electrochemical performance in energy storage systems has stimulated research into advanced two-dimensional (2D) materials for electrodes. In this work, we obtain a layered MXene compound by exfoliating a titanium aluminum carbide precursor using tetramethylammonium hydroxide (TMAOH) ions in a full room temperature process followed by manual shaking. The hexagonal crystal structure and composition of the layered materials are characterized using different techniques. X-Ray diffraction shows the formation of 2D nano-sheets before and after the TMAOH treatment via its characteristic (002) diffraction peak, bringing to light an increase in the interlayer spacing after treatment. Scanning electron microscopy images confirm the layered morphology, whose composition is determined by energy dispersive x-ray analysis for the bulk material and by x-ray photoelectron spectroscopy for the surface of the obtained compounds. This study demonstrates a promising route to enhance delamination of this MXene 2D material in a low-cost room-temperature approach.

Free-standing nanoporous graphene was hydrogenated at about 60 at.% H uptake, as determined by the emerging of the sp3 bonding component in the C 1s core level investigated by high-resolution X-ray photoelectron spectroscopy (XPS). Fully unsupported graphane was investigated by XPS under optical excitation at 2.4 eV. At a laser fluence of 1.6 mJ/cm2, a partial irreversible dehydrogenation of the graphane was observed, which could be attributed either to the local temperature increase or to a photo-induced softening of the H-to-C stretching mode. The sub-ns dynamics of the energy shift and peak broadening of the C 1s core level revealed two different decay constants: 210 ps and 130 ps, respectively, the former associated with photovoltage dynamics and the latter with thermal heating on a time scale comparable with the synchrotron temporal resolution.

The conversion of semimetallic suspended graphene (Gr) to a large-gap semiconducting phase is here realized by controlled adsorption of atomic hydrogen (deuterium) on free-standing nanoporous Gr veils. This approach allows to achieve a very clean and neat adsorption, overcoming any spurious influence associated to the presence of substrates. The effects of local rehybridization from sp(2) to sp(3) chemical bonding are investigated by combining X-ray photoelectron spectroscopy and high-resolution electron energy-loss spectroscopy (HREELS) with ab-initio based modelling. We find that the hydrogen adatoms on the C sites induce a stretching frequency, clearly iden-tified in the vibrational spectra thanks to the use of the D isotope. Overall, the results are compatible with the predicted fingerprints of adsorption on both sides of Gr corresponding to the graphane configuration. Moreover, HREELS of the deuterated samples shows a sizeable opening of the optical band gap, i.e. 3.25 eV, consistent with the modified spectral density observed in the valence band photoemission. The results are in agreement with ab-initio calculations by GW and Bethe-Salpeter equation approaches, predicting a large quasiparticle gap opening and huge exciton binding energy.

Tuning the electrocatalytic properties of MoS2 layers can be achieved through different paths, such as reducing their thickness, creating edges in the MoS2 flakes, and introducing S-vacancies. We combine these three approaches by growing MoS2 electrodes by using a special salt-assisted chemical vapor deposition (CVD) method. This procedure allows the growth of ultrathin MoS2 nanocrystals (1-3 layers thick and a few nanometers wide), as evidenced by atomic force microscopy and scanning tunneling microscopy. This morphology of the MoS2 layers at the nanoscale induces some specific features in the Raman and photoluminescence spectra compared to exfoliated or microcrystalline MoS2 layers. Moreover, the S-vacancy content in the layers can be tuned during CVD growth by using Ar/H2 mixtures as a carrier gas. Detailed optical microtransmittance and microreflectance spectroscopies, micro-Raman, and X-ray photoelectron spectroscopy measurements with sub-millimeter spatial resolution show that the obtained samples present an excellent homogeneity over areas in the cm2 range. The electrochemical and photoelectrochemical properties of these MoS2 layers were investigated using electrodes with relatively large areas (0.8 cm2). The prepared MoS2 cathodes show outstanding Faradaic efficiencies as well as long-term stability in acidic solutions. In addition, we demonstrate that there is an optimal number of S-vacancies to improve the electrochemical and photoelectrochemical performances of MoS2.

The PTOLEMY transverse drift filter is a new concept to enable precision analysis of the energy spectrum of electrons near the tritium β -decay endpoint. This paper details the implementation and optimization methods for successful operation of the filter for electrons with a known pitch angle. We present the first demonstrator that produces the required magnetic field properties with an iron return-flux magnet. Two methods for the setting of filter electrode voltages are detailed. The challenges of low-energy electron transport in cases of low field are discussed, such as the growth of the cyclotron radius with decreasing magnetic field, which puts a ceiling on filter performance relative to fixed filter dimensions. Additionally, low pitch angle trajectories are dominated by motion parallel to the magnetic field lines and introduce non-adiabatic conditions and curvature drift. To minimize these effects and maximize electron acceptance into the filter, we present a three-potential-well design to simultaneously drain the parallel and transverse kinetic energies throughout the length of the filter. These optimizations are shown, in simulation, to achieve low-energy electron transport from a 1 T iron core (or 3 T superconducting) starting field with initial kinetic energy of 18.6 keV drained to < 10 eV (< 1 eV) in about 80 cm. This result for low field operation paves the way for the first demonstrator of the PTOLEMY spectrometer for measurement of electrons near the tritium endpoint to be constructed at the Gran Sasso National Laboratory (LNGS) in Italy.