The ability of light to carry and deliver orbital angular momentum (OAM) in the form of optical vortices has attracted much interest. The physical properties of light with a helical wavefront can be confined onto two-dimensional surfaces with subwavelength dimensions in the form of plasmonic vortices, opening avenues for thus far unknown light-matter interactions. Because of their extreme rotational velocity, the ultrafast dynamics of such vortices remained unexplored. Here we show the detailed spatiotemporal evolution of nanovortices using time-resolved two-photon photoemission electron microscopy. We observe both long- and short-range plasmonic vortices confined to deep subwavelength dimensions on the scale of 100 nanometers with nanometer spatial resolution and subfemtosecond time-step resolution. Finally, by measuring the angular velocity of the vortex, we directly extract the OAM magnitude of light.

We have used Low Energy Electron Microscopy (LEEM) and Photo Emission Electron Microscopy (PEEM) to study and improve the quality of graphene films grown on Ir(111) using chemical vapor deposition (CVD). CVD at elevated temperature already yields graphene sheets that are uniform and of monatomic thickness. Besides domains that are aligned with respect to the substrate, other rotational variants grow. Cyclic growth exploiting the faster growth and etch rates of the rotational variants, yields films that are 99 % composed of aligned domains. Precovering the substrate with a high density of graphene nuclei prior to CVD yields pure films of aligned domains extending over millimeters. Such films can be used to prepare cluster-graphene hybrid materials for catalysis or nanomagnetism and can potentially be combined with lift-off techniques to yield high-quality, graphene based electronic devices

International audienceThe morphology of graphene monolayers on Ir(111) prepared by thermal decomposition of ethylene between 1000 and 1530 K was studied with high resolution low energy electron diffraction. In addition to a well-oriented epitaxial phase, randomly oriented domains are observed for growth temperatures between 1255 and 1460 K. For rotational angles of ±3° around 30° these domains lock-in in a 30° oriented epitaxial phase. Below 1200 K the graphene layer exhibits high disorder and structural disintegrity. Above 1500 K the clear moiré spots reflect graphene in a single orientation epitaxial incommensurate phase

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Like other 2D materials, the boron-based borophene exhibits interesting structural and electronic properties. While borophene is typically prepared by molecular beam epitaxy, we report here on an alternative way of synthesizing large single-phase borophene domains by segregation-enhanced epitaxy. X-ray photoelectron spectroscopy shows that borazine dosing at 1100 °C onto Ir(111) yields a boron-rich surface without traces of nitrogen. At high temperatures, the borazine thermally decomposes, nitrogen desorbs, and boron diffuses into the substrate. Using time-of-flight secondary ion mass spectrometry, we show that during cooldown the subsurface boron segregates back to the surface where it forms borophene. In this case, electron diffraction reveals a (6 × 2) reconstructed borophene χ6-polymorph, and scanning tunneling spectroscopy suggests a Dirac-like behavior. Studying the kinetics of borophene formation in low energy electron microscopy shows that surface steps are bunched during the borophene formation, resulting in elongated and extended borophene domains with exceptional structural order.