Andrea Locatelli

Recent advances in chemical and magnetic imaging of surfaces and interfaces by XPEEM

Synchrotron-based photoemission electron microscopy (XPEEM) is one of the most powerful spectro-microscopic techniques for the investigation of surfaces, interfaces, thin films and buried layers. By exploiting the tunability and polarizability of x-ray sources as well as progress in electron optics design, modern XPEEM instruments can perform several x-ray spectroscopic investigations with a lateral resolution of a few tens of nanometres. We review here the latest developments in XPEEM, illustrating the state of the art capabilities of the technique. The usefulness of chemical and magnetic imaging XPEEM methods is demonstrated by examples of fundamental and applied studies in surface and material sciences, as well as other fields of application ranging from magnetism to biology and geology.

Work functions of individual single-walled carbon nanotubes

Work functions of individual single-walled carbon nanotubes (SWNTs) were studied by means of photoemission electron microscopy. Work function differences between the nanotubes were clearly observed in secondary electron images. The work functions of 93 SWNTs were found to range within 0.6eV, but most distributed in a much narrower energy range of 0.2eV. The work functions of single-walled nanotubes do not seem to have large structural dependence.

Spectromicroscopy of single and multilayer graphene supported by a weakly interacting substrate

We report measurements of the electronic structure and surface morphology of exfoliated graphene on an insulating substrate using angle-resolved photoemission and low-energy electron diffraction. Our results show that, although exfoliated graphene is microscopically corrugated, the valence band retains a massless fermionic dispersion with a Fermi velocity of

Coexistence of multiple silicene phases in silicon grown on Ag(1 1 1)

ensuremath{sim}{10}^{6}text{ }text{m}/text{s}

Image blur and energy broadening effects in XPEEM

. We observe a close relationship between the morphology and electronic structure, which suggests that controlling the interaction between graphene and the supporting substrate is essential for graphene device applications.

Composition of Ge — Si – islands in the growth of Ge on Si — 111 – by x-ray spectromicroscopy

Silicene, the silicon equivalent of graphene, is attracting increasing scientific and technological attention in view of the exploitation of its exotic electronic properties. This novel material has been theoretically predicted to exist as a free-standing layer in a low-buckled, stable form, and can be synthesized by the deposition of Si on appropriate crystalline substrates. By employing low-energy electron diffraction and microscopy, we have studied the growth of Si on Ag(1u20091u20091) and observed a rich variety of rotationally non-equivalent silicene structures. Our results highlight a very complex formation diagram, reflecting the coexistence of different and nearly degenerate silicene phases, whose relative abundance can be controlled by varying the Si coverage and growth temperature. At variance with other studies, we find that the formation of single-phase silicene monolayers cannot be achieved on Ag(1u20091u20091).

Composition of Ge(Si) islands in the growth of Ge on Si(111)

We report image blurring and energy broadening effects in energy-filtered XPEEM when illuminating the specimen with soft X-rays at high flux densities. With a flux of 2 × 10(13)photons/s, the lateral resolution in XPEEM imaging with either core level or secondary electrons is degraded to more than 50 nm. Fermi level broadening up to several hundred meV and spectral shift to higher kinetic energies are also systematically observed. Simple considerations suggest that these artifacts result from Boersch and Loeffler effects, and that the electron-electron interactions are strongest in the initial part of the microscope optical path. Implications for aberration corrected instruments are discussed.

One-dimensional Au on TiO2

The stoichiometry of Ge∕Si islands grown on Si(111) substrates at temperatures ranging from 460to560°C was investigated by x-ray photoemission electron microscopy (XPEEM). By developing a specific analytical framework, quantitative information on the surface Ge∕Si stoichiometry was extracted from laterally resolved XPEEM Si 2p and Ge 3d spectra, exploiting the chemical sensitivity of the technique. Our data show the existence of a correlation between the base area of the self-assembled islands and their average surface Si content: the larger the lateral dimensions of the 3D structures, the higher their relative Si concentration. The deposition temperature determines the characteristics of this relation, pointing to the thermal activation of kinetic diffusion processes.

Multipurpose modular experimental station for the DiProI beamline of Fermi@Elettra free electron laser.

X-ray photoemission electron microscopy (XPEEM) is used to investigate the chemical composition of Ge/Si individual islands obtained by depositing Ge on Si(111) substrates in the temperature range 460–560u200a°C. We are able to correlate specific island shapes with a definite chemical contrast in XPEEM images, at each given temperature. In particular, strained triangular islands exhibit a Si surface content of 5%–20%, whereas it grows up to 30%–40% for “atoll-like” structures. The island’s stage of evolution is shown to be correlated with its surface composition. Finally, by plotting intensity contour maps, we find that island centers are rich in Si.

Tuning the domain wall orientation in thin magnetic strips using induced anisotropy

One-dimensional Au rows are produced on the TiO2(110) surface by controlled photon-stimulated desorption followed by Au deposition at 750?K. In the resulting (1 ? 2) structure, O vacancies are replaced by Au atoms, as demonstrated by combined laterally resolved x-ray spectroscopy and micro-LEED (low-energy electron diffraction) measurements. The experimental results are confirmed by first-principles calculations, which indicate strong Au?Ti bonding and a very stable configuration of the Au rows. The calculations determine the detailed atomic structure of the Au rows, with an inter-row spacing of 13???and an intra-row Au distance of 2.95??.