Fig.1: (left) XPEEM images (Fov 25 μm) of SrTiO3 as the KE is scanned through the average photoemission threshold. (right): chematic of the three mode operation of the Omicron NanoESCA.
Photoelectron emission microscopy combined with intense photon sources is becoming a powerful analytical tool with both spatial resolution down to a few nanometers and spectroscopic capabilities. One can laterally resolve surface domains according to their magnetization, chemical composition, electronic structure, and even follow the dynamics of domain movement, catalysis or laser emissions using time resolved detection systems.
The lateral resolution varies from 1 mm to 10 nm, placing this technique at the heart of the mesoscopic physics, and corresponds to the need for spectroscopic information on the intermediate scale between atomic and macroscopic, so lacking in multiscale modeling. Furthermore, the possibility of studying nanostructured materials and objects on the 100 nm scale corresponds perfectly to the technologically interesting length scale in electronics, bioelectronics.
CEA has acquired the first production model of the NanoESCA (OMICRON GmbH), a spectromicroscope designed for high spatial and energy resolution over a wide range of photoelectron kinetic energies (Δx ~ 100 nm; ΔE ~ 100 meV). This is done thanks to the combination of a simple purely electrostatic lens column allowing accurate focus tracking over several hundred eV with the use of a double hemispherical analyzer, eliminating spherical aberrations usually present in energy analyzers for photoelectron microscopes.
Fig.2(Left) : SEM micrographs of pMAN grafted on Au-Si heterogeneous surface. (Right): XPEEM energy filtered images: O 1s coordinated Ti (turquoise) and Si (blue); Grafting on gold strips and nanoparticles Au 4f7/2 (yellow), C 1s (red), Ti 2p3/2 (green)
The instrument is also unique in the world because it can operate with both synchrotron radiation sources and laboratory sources. It has a three mode operation: direct PEEM, small spot spectroscopy and energy filtered XPEEM. The instrument is at the heart of a close collaboration between the LETI and the DRECAM and constitutes the spectroscopy section of the joint DSM-DRT nanocharacterization centre within the MINATEC complex. The instrument was delivered in August 2006 and successfully passed the acceptance tests using the synchrotron radiation of the ID08 beamline at the ESRF and subsequently with the laboratory Hg, D2 and Al Kα sources at MINATEC. We have studied grafting of Polymethacrylonitrile on gold motifs deposited on a native oxide silicon substrate. The SEM images on the right of Fig.3 below show the area imaged and furthermore reveal the polymer grafted onto the gold motifs. The third cliché with a field of view of 127μm is a superposition of the XPEEM images taken on the O 1s levels shown in the inset, the blue corresponds to oxygen bound to Si in the native oxide, the turquoise to oxygen bound to Ti in the TiO2 interface layer necessary for Au adhesion. On the right the same FoV superimposes XPEEM images on the Au 4f7/2, C 1s and Ti 2p3/2 levels. Clearly no grafting occurs on the TiO2, however the 10-15 nm particles observed on the SiO2 (inset second from left) are shown to be Au, with a core level shift due to their small size (see inset).
Fig.3:612 nm spatial resolution from a line scan through cross etched into Au overlayer (green square).
Finally the instrument has demonstrated the best ever lateral resolution obtained with a laboratory Xray source: 650 nm measured on the step of a slit in a gold overlayer on a CdTe substrate. This has been achieved thanks to an optimized X-ray source combining higher thermal resistance to the electron beam with improved monochromator focusing. Within the framework of MINATEC, the CEA and the ESRF collaborate on a project for a dedicated spectromicroscopy beamline at the ESRF. Linked to this is the preparation of NanoSPECS, in reply to the call FP7 in order to constitute a network of expert European users of quantitative XPEEM.
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