Alessandro Sala

First experimental proof for aberration correction in XPEEM: Resolution, transmission enhancement, and limitation by space charge effects

The positive effect of double aberration correction in x-ray induced Photoelectron Emission Microscopy (XPEEM) has been successfully demonstrated for both, the lateral resolution and the transmission, using the Au 4f XPS peak for element specific imaging at a kinetic energy of 113 eV. The lateral resolution is improved by a factor of four, compared to a non-corrected system, whereas the transmission is enhanced by a factor of 5 at a moderate resolution of 80 nm. With an optimized system setting, a lateral resolution of 18 nm could be achieved, which is up to now the best value reported for energy filtered XPEEM imaging. However, the absolute resolution does not yet reach the theoretical limit of 2 nm, which is due to space charge limitation. This occurs along the entire optical axis up to the contrast aperture. In XPEEM the pulsed time structure of the exciting soft x-ray light source causes a short and highly intense electron pulse, which results in an image blurring. In contrast, the imaging with elastically reflected electrons in the low energy electron microscopy (LEEM) mode yields a resolution clearly below 5 nm. Technical solutions to reduce the space charge effect in an aberration-corrected spectro-microscope are discussed.

Tuning the electronic structure of monolayer graphene/ Mo S 2 van der Waals heterostructures via interlayer twist

We directly measure the electronic structure of twisted graphene/MoS2 van der Waals heterostructures, in which both graphene and MoS2 are monolayers. We use cathode lens microscopy and microprobe angle-resolved photoemission spectroscopy measurements to image the surface, determine twist angle, and map the electronic structure of these artificial heterostructures. For monolayer graphene on monolayer MoS2, the resulting band structure reveals the absence of hybridization between the graphene and MoS2 electronic states. Further, the graphene-derived electronic structure in the heterostructures remains intact, irrespective of the twist angle between the two materials. In contrast, however, the electronic structure associated with the MoS2 layer is found to be twist-angle dependent; in particular, the relative difference in the energy of the valence band maximum at {\Gamma} and K of the MoS2 layer varies from approximately 0 to 0.2 eV. Our results suggest that monolayer MoS2 within the heterostructure becomes predominantly an indirect bandgap system for all twist angles except in the proximity of 30 degrees. This result enables potential bandgap engineering in van der Waals heterostructures comprised of monolayer structures.

Nanobubbles at GPa Pressure under Graphene.

We provide direct evidence that irradiation of a graphene membrane on Ir with low-energy Ar ions induces formation of solid noble-gas nanobubbles. Their size can be controlled by thermal treatment, reaching tens of nanometers laterally and height of 1.5 nm upon annealing at 1080 °C. Ab initio calculations show that Ar nanobubbles are subject to pressures reaching tens of GPa, their formation being driven by minimization of the energy cost of film distortion and loss of adhesion.

Subfilamentary Networks Cause Cycle-to-Cycle Variability in Memristive Devices

A major obstacle for the implementation of redox-based memristive memory or logic technology is the large cycle-to-cycle and device-to-device variability. Here, we use spectromicroscopic photoemission threshold analysis and operando XAS analysis to experimentally investigate the microscopic origin of the variability. We find that some devices exhibit variations in the shape of the conductive filament or in the oxygen vacancy distribution at and around the filament. In other cases, even the location of the active filament changes from one cycle to the next. We propose that both effects originate from the coexistence of multiple (sub)filaments and that the active, current-carrying filament may change from cycle to cycle. These findings account for the observed variability in device performance and represent the scientific basis, rather than prior purely empirical engineering approaches, for developing stable memristive devices.

Switchable graphene-substrate coupling through formation/dissolution of an intercalated Ni-carbide layer.

Control over the film-substrate interaction is key to the exploitation of graphene’s unique electronic properties. Typically, a buffer layer is irreversibly intercalated “from above” to ensure decoupling. For graphene/Ni(111) we instead tune the film interaction “from below”. By temperature controlling the formation/dissolution of a carbide layer under rotated graphene domains, we reversibly switch graphene’s electronic structure from semi-metallic to metallic. Our results are relevant for the design of controllable graphene/metal interfaces in functional devices.

Fabrication of 2D Heterojunction in Graphene via Low Energy N2+ Irradiation

Substitutional doping in graphene is locally induced with very low energy nitrogen ions. Irradiated and nonirradiated areas exhibit different charge carrier densities and are separated by a sharp boundary, stable up to 750 °C. The way towards lithographic control of the electronic properties of graphene by ion irradiation is paved, providing a proof of principle for the fabrication of 2D graphene-based heterojunctions.

Corrigendum: Room-temperature chiral magnetic skyrmions in ultrathin magnetic nanostructures

This corrects the article DOI: 10.1038/nnano.2015.315.