C. Wiemann

Bulk mixed ion electron conduction in amorphous gallium oxide causes memristive behaviour

In thin films of mixed ionic electronic conductors sandwiched by two ion-blocking electrodes, the homogeneous migration of ions and their polarization will modify the electronic carrier distribution across the conductor, thereby enabling homogeneous resistive switching. Here we report non-filamentary memristive switching based on the bulk oxide ion conductivity of amorphous GaOx (x~1.1) thin films. We directly observe reversible enrichment and depletion of oxygen ions at the blocking electrodes responding to the bias polarity by using photoemission and transmission electron microscopies, thus proving that oxygen ion mobility at room temperature causes memristive behaviour. The shape of the hysteresis I-V curves is tunable by the bias history, ranging from narrow counter figure-eight loops to wide hysteresis, triangle loops as found in the mathematically derived memristor model. This dynamical behaviour can be attributed to the coupled ion drift and diffusion motion and the oxygen concentration profile acting as a state function of the memristor.

Time-Resolved 2PPE and Time-Resolved PEEM as a Probe of LSP's in Silver Nanoparticles

The time-resolved two-photon photoemission technique (TR-2PPE) has been applied to study static and dynamic properties of localized surface plasmons (LSP) in silver nanoparticles. Laterally, integrated measurements show the difference between LSP excitation and nonresonant single electron-hole pair creation. Studies below the optical diffraction limit were performed with the detection method of time-resolved photoemission electron microscopy (TR-PEEM). This microscopy technique with a resolution down to 40 nm enables a systematic study of retardation effects across single nanoparticles. In addition, as will be shown in this paper, it is a highly sensitive sensor for coupling effects between nanoparticles.

The lateral photoemission distribution from a defined cluster/substrate system as probed by photoemission electron microscopy

We used photoemission electron microscopy (PEEM) to investigate the lateral distribution of the photoemission yield from a defined system of silver clusters supported by a highly oriented pyrolytic graphite (HOPG) substrate. For threshold photoemission using conventional photoemission (PE) and two-photon photoemission (2PPE) we find that distinct, well-separated emitters are responsible for the measured integral photoemission yield. Complementary characterization of the surface using STM shows that the emitter density as probed by PEEM is reduced by about three orders of magnitude in comparison to the actual cluster density. Wavelength and light polarization scans in combination with two-photon-PEEM clearly show that the origin of the 2PPE signal is related to small silver particles. Furthermore, the PEEM differentiates between inhomogeneous and homogeneous broadening effects in the 2PPE signal. This observation allows one to assign the origin of the local photoemission signal to either a distinct single silver particle or a number of coherently coupled silver particles. We conclude that the 2PPE-yield is highly selective with respect to specific properties of the supported silver particles. Our results show that in future experiments, PEEM as a highly local field probe, may be a key tool in the identification of these properties.

Depth-resolved soft x-ray photoelectron emission microscopy in nanostructures via standing-wave excited photoemission

Depth-resolved soft x-ray photoelectron emission microscopy in nanostructures via standing-wave excited photoemission F. Kronast 1 , R. Ovsyannikov 1 , A. Kaiser 2 , C. Wiemann 2 , S.-H. Yang 3 , A. Locatelli 4 , D. E. Burgler 3 , R. Schreiber 3 , F. Salmassi 5 , P. Fischer 5, H.A. Dűrr 1 , C. M. Schneider 2 ,W. Eberhardt 1 and C. S. Fadley 2,5,6 BESSY mbH, Albert-Einstein-Str. 15, 12489 Berlin, Germany Forschungzentrum Julich GmbH, IFF-9, 52425 Julich, Germany Almaden Research Center, San Jose, CA 95120 USA Elettra, Sincrotrone Trieste S.C.p.A., 34012 Basovizza, Trieste, Italy Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA University of California, Davis, CA 95616, USA Corresponding author: Florian Kronast Address: BESSY mbH, Albert-Einstein-Str. 15, 12489 Berlin, Germany e-mail : kronast@bessy.de Phone: (+49) 30 6392 4620 Fax: (+49) 30 63924980 Abstract We present an extension of conventional laterally resolved soft x-ray photoelectron emission microscopy. A depth resolution along the surface normal down to a few A can be achieved by setting up standing x-ray wave fields in a multilayer substrate. The sample is an Ag/Co/Au trilayer, whose first layer has a wedge profile, grown on a Si/MoSi2 multilayer mirror. Tuning the incident x-ray to the mirror Bragg angle we set up standing x-ray wave fields. We

Adsorption geometry and electronic structure of iron phthalocyanine on Ag surfaces: A LEED and photoelectron momentum mapping study

Abstract We present a comprehensive study of the adsorption behavior of iron phthalocyanine on the low-index crystal faces of silver. By combining measurements of the reciprocal space by means of photoelectron momentum mapping and low energy electron diffraction, the real space adsorption geometries are reconstructed. At monolayer coverage ordered superstructures exist on all studied surfaces containing one molecule in the unit cell in case of Ag(100) and Ag(111), and two molecules per unit cell for Ag(110). The azimuthal tilt angle of the molecules against the high symmetry directions of the substrate is derived from the photoelectron momentum maps. A comparative analysis of the momentum patterns on the substrates with different symmetry indicates that both constituents of the twofold degenerate FePc lowest unoccupied molecular orbital are occupied by charge transfer from the substrate at the interface.

Probing buried layers by photoelectron spectromicroscopy with hard x-ray excitation

We report about a proof-of-principle experiment which explores the perspectives of performing hard x-ray photoemission spectromicroscopy with high lateral resolution. Our results obtained with an energy-filtered photoemission microscope at the PETRA III storage ring facility using hard x-ray excitation up to 6.5 keV photon energy demonstrate that it is possible to obtain selected-area x-ray photoemission spectra from regions less than 500 nm in diameter.

Bulk sensitive hard x-ray photoemission electron microscopy

Hard x-ray photoelectron spectroscopy (HAXPES) has now matured into a well-established technique as a bulk sensitive probe of the electronic structure due to the larger escape depth of the highly energetic electrons. In order to enable HAXPES studies with high lateral resolution, we have set up a dedicated energy-filtered hard x-ray photoemission electron microscope (HAXPEEM) working with electron kinetic energies up to 10 keV. It is based on the NanoESCA design and also preserves the performance of the instrument in the low and medium energy range. In this way, spectromicroscopy can be performed from threshold to hard x-ray photoemission. The high potential of the HAXPEEM approach for the investigation of buried layers and structures has been shown already on a layered and structured SrTiO3 sample. Here, we present results of experiments with test structures to elaborate the imaging and spectroscopic performance of the instrument and show the capabilities of the method to image bulk properties. Additionally, we introduce a method to determine the effective attenuation length of photoelectrons in a direct photoemission experiment.

Probing femtosecond plasmon dynamics with nanometer resolution

In combining time-resolved two-photon photoemission (TR-2PPE) and photoemission electron microscopy (PEEM) the ultrafast dynamics of collective electron excitations in silver nanoparticles (localized surface plasmons - LSP) is probed at femtosecond and nanometer resolution. In two examples we illustrate that a phase-resolved (interferometric) sampling of the LSP-dynamics enables detailed insight into dephasing and propagation processes associated with these excitations. For two close-lying silver nano-dots (diameter 200 nm) we are able to distinguish small particle to particle variations in the plasmon eigenfrequency, which typically give rise to inhomogeneous line-broadening of the plasmon resonance in lateral integrating frequency domain measurements. The observed spatio-temporal modulations in the photoemission yield from a single nanoparticle can be interpreted as local variation in the electric near-field and result from the phase propagation of the plasmon through the particle. Furthermore, we show that the control of the phase between the used femtosecond pump and probe laser pulses used for these experiments can be utilized for an external manipulation of the nanoscale electric near-field distribution at these particles.

Quantitative spectromicroscopy from inelastically scattered photoelectrons in the hard X-ray range

We demonstrate quantitative, highly bulk-sensitive x-ray photoelectron emission microscopy by analysis of inelastically scattered photoelectrons in the hard X-ray range, enabling elemental depth distribution analysis in deeply buried layers. We show results on patterned structures used in electrical testing of high electron mobility power transistor devices with an epitaxial Al0.25Ga0.75N channel and a Ti/Al metal contact. From the image series taken over an energy range of up to 120 eV in the Ti 1s loss feature region and over a typical 100 μm field of view, one can accurately retrieve, using background analysis together with an optimized scattering cross-section, the Ti depth distribution from 14 nm up to 25 nm below the surface. The method paves the way to multi-elemental, bulk-sensitive 3D imaging and investigation of phenomena at deeply buried interfaces and microscopic scales by photoemission.

Deflection gating for time-resolved x-ray magnetic circular dichroism-photoemission electron microscopy using synchrotron radiation

In this paper, we present a newly developed gating technique for a time-resolving photoemission microscope. The technique makes use of an electrostatic deflector within the microscopes electron optical system for fast switching between two electron-optical paths, one of which is used for imaging, while the other is blocked by an aperture stop. The system can be operated with a switching time of 20 ns and shows superior dark current rejection. We report on the application of this new gating technique to exploit the time structure in the injection bunch pattern of the synchrotron radiation source BESSY II at Helmholtz-Zentrum Berlin for time-resolved measurements in the picosecond regime.