[en] Full text: The electronic properties of single-walled carbon nanotubes (SWCNTs) can be very sensitive to and efficiently controlled by chemical doping, as obtained through the interaction with gas molecules or alkali metals. The influence that molecular adsorbates have on the SWCNT electronic properties appears very promising in the ld of gas sensing, whereas the enhanced reactivity that tubes gain upon alkali metal doping makes them able to bind molecules with otherwise low chemisorption energies, which is essential, for example, for H₂ storage. Synchrotron radiation photoemission spectroscopy was used to study the interaction of SWCNTs with gases and with Li atoms. Our sample was a commercial purified bucky-paper. The measurements demonstrate that the presence of contaminants in purified SWCNT bundles, mainly chemical residues of the purification, dispersion and filtration processes, is responsible for a strong sensitivity to oxygen. After removal of these contaminants, the exposure to gas molecules present in air, such as O₂, N₂, H₂O and CO, has no influence on the tube electronic properties, showing the lack of even weak chemical bonds. Instead, a sizeable effect on the electronic spectra is observed for NH₃, SO₂, NO and NO₂ adsorption, confirming the possible application of SWCNTs as powerful sensors capable of measuring environmentally significant levels of toxic gases (i.e. 100 ppb). After accurate cleaning, SWCNTs exhibit core level and valence band photoemission features similar to graphite, apart the presence of peaks due to van Hove singularities near the Fermi level (EF). Changes of the density of states at EF are induced by Li doping because of an efficient charge transfer from the alkali metal to the tubes and the partial filling of the conduction band

[en] Here we review the possible application of carbon nanotubes (CNTs) as chemiresistor and field-effect transistor chemical sensors. The endeavor of this paper is to understand the key facts emerging from the literature that seem to demonstrate the high sensitivity of CNTs to several molecular species, with the effort to catch the results in a correct manner. (topical review)

[en] Full text: Here we report on the temperature dependent phonon-broadening of the HOMO and LUMO photoemission features in alkali metal intercalated fullerides, including C₆₀, K₆C₆₀, Cs₆C₆₀, K₄C₆₀ and Cs₄C₆₀. Compounds with different alkali metal stoichiometry (different C₆₀ electron filling) show different temperature behaviours that may be related to differences in the electron phonon coupling constant (λ). In particular we have estimated a ratio λ(C₆₀⁺)/ λ(C₆₀^-) 1.7 ± 0.3, in agreement with theoretical predictions.We will also show angle resolved photoemission measurements of the band dispersion in bcc K₆C₆₀ and fcc K3C₆₀. While the dispersion of the filled LUMO-derived bands near the Fermi level in K3C₆₀, as measured at 30 K, is less than 100 meV along the two main symmetry directions of the (111) surface (i.e. about a factor of 2 less than expected), the band dispersion of both LUMO and HOMO derived bands in K₆C₆₀ compares well with calculations. Possible reasons for this difference will be discussed and the band dispersion of these 'bulk' intercalated compounds will be compared with the band dispersion measured for C₆₀ monolayers on Ag surfaces

[en] The control of the graphene electronic structure is one of the most important problems in modern condensed matter physics. The graphene monolayer synthesized on the Re(0001) surface and then subjected to the intercalation of Pb atoms is studied by angle-resolved photoelectron spectroscopy and low-energy electron diffraction. The intercalation of Pb atoms under graphene takes place when the substrate is annealed above 500°C. As a result of the intercalation of Pb atoms, graphene becomes quasi-free-standing and a local band gap appears at the Dirac point. The band gap changes with the substrate temperature during the formation of the graphene/Pb/Re(0001) system. The band gap is 0.3 eV at an annealing temperature of 620°C and it increases up to 0.4 eV upon annealing at 830°C. Based on our data, we conclude that the band gap is mainly caused by the hybridization of the graphene π state with the rhenium 5d states located near the Dirac point of the graphene π state.

[en] Phase-pure MgB₂ thin films have been grown in ultrahigh-vacuum conditions on Mg(0001) and Mg(1010) at low-temperature (185-220 deg. C) via molecular beam epitaxy. By depositing B on the Mg substrate a maximum film thickness of ∼4 ML can be achieved, while thicker layers can be obtained by codeposition of Mg and B. Thick films grown on Mg(0001) show a sharp hexagonal low energy electron diffraction pattern, as well as the angle resolved photoemission band structure and x-ray absorption features of MgB₂(0001) surface. Poor long-range order can be achieved by codeposition on Mg(1010). This method allows important in situ measurements, may enable the controlled doping of MgB₂, and could be used to realize MgB₂-based superconducting devices

[en] Full text: The discovery of superconductivity at 39 K in MgB₂, a simple inter- metallic compound, has generated great scientific interest. A critical step for studying both intrinsic superconducting properties and the possibility to realize superconducting devices based on MgB₂ is the controlled growth of high quality epitaxial MgB₂ thin film heterostructures. MgB₂ ordered films have been successfully grown on Mg(0001) by means of ultra-high-vacuum molecular beam epitaxy. The films presented hexagonal low energy electron diffraction patterns and allow angle-resolved photoemission experiments. The measured band structure is in good agreement with theory and previous experiments on single-crystals. For the first time, we report the band mapping along the Γ-A direction perpendicular to the Mg (B) planes. The present in - situ growth method has obvious advantages in terms of purity, size and stability of the sample and can be extended to insulating substrates, as SiC, permitting other important investigations like transport measurements or may allow the controlled growth of other systems, like LiBC or MgB₂ doped with impurity

[en] We have investigated the electronic properties of clean and Li-doped single-walled carbon nanotubes (SWCNTs) using several spectroscopic techniques. For the clean sample the overall shape of the valence band photoemission and X-ray absorption spectra show the characteristic features of C atoms in sp² hybridization, as in graphite. C 1s photoemission spectrum shows instead a broader peak, with a full width at half maximum of 0.43 ± 0.02 eV. Close to the Fermi level and at low loss energy, peaks due to van Hove singularities have been observed in the photoemission and electron energy loss spectra of SWCNT mats, respectively. At 150 K, the Li doping saturates at a Li/C concentration of about 0.06, causing a shift of the photoemission peaks towards higher binding energy by about 0.25 eV, and an increase in the spectral intensity near the Fermi level. This result is consistent with an electron charge transfer from Li atoms and the partial filling of the SWCNT conduction band

[en] Photoelectron emission spectra in a photon energy range between 7.5 and 21 eV are measured for in situ grown polycrystalline Yb films. By comparing bulk and surface core level shifted 4f components we give an estimation of the effective attenuation length (EAL) for low energy (6-20 eV) electrons in Yb, establishing a moderate increase of the EAL upon electron energy decrease. The experimental EAL data are found to be a factor of four smaller than those predicted from the so-called 'universal curve'.

[en] High resolution photoemission spectroscopy was used to follow the modification of the electronic properties of single wall carbon nanotubes determined by the interaction with adsorbed electron donor and acceptor species. Valence band, C1s and adsorbate core level spectra were monitored to reveal the adsorption and to follow the Fermi level shift induced by the charge exchange between C lattice and dopants. In the presence of nitrogen dioxide, a considerable charge withdrawal from C atoms takes place, which is revealed by the binding energy shift of the C1s peak. On the contrary, adsorbed H₂O does not appear to interact with nanotubes. For Li doping, the occurrence of an efficient electron injection into the tube structure is demonstrated by the evidence of Li ionisation, by the enhancement of the density of states near the Fermi level and by the partial conduction band filling, which originates a rigid shift of all C spectral structures

[en] Highlights: • Angle resolved photoemission spectroscopy (ARPES). • Charge density wave (CDW). • TiSe_2- Splitting of the conduction bands. Horizontal and vertical polarizations. Temperature dependence. - Abstract: Very high resolution angular resolved photoemission (ARPES) spectra on TiSe_2 in two distinct polarization geometries (vertical and horizontal) at temperatures between 300 K and 22 K enabled the observation of details of bands near the Fermi level not reported so far. Calculations of the electronic band structure based on density functional theory (DFT) using B3LYP hybrid functional and MBJ potential (with and without spin-orbit coupling) were performed to obtain the orbital symmetry and dispersion. Two degenerate conduction bands (CB’s) were observed at the Γ-point, a weak CB- emission at the A-point, and two non degenerate CB’s (i.e. splitting of CB) at the M/L-point of the Brillouin Zone (BZ). The splitting was detected at L for both polarizations, while at M remarkably only for horizontal polarization. These results cannot be fully accounted for by current theories for the charge density wave (CDW) and point to a reduced symmetry of the electronic states, possibly due to the chiral CDW.