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Atomic Resolution of Transmission Electron Microscopy by means of Aberration Correction

Max. Haider1, Heiko Müller1

1CEOS GmbH, Englerstr. 28, D-69126 Heidelberg, Germany

Email: [email protected]

The development of advanced charged particle optical systems has pushed the attainable resolution and the analytical capabilities of electron microscopes to new levels of precision and energy resolution. This process of further advancing the overall performance of electron microscopes is still going on and the current state and future prospects of these developments will be described.

In 1936, just a few years after the invention of the transmission electron microscope (TEM), O. Scherzer showed the impossibility to design an objective lens without spherical and chromatic aberration. In the nineties of the last century commercially available TEMs improved with respect to stability and resolution. As a consequence, the spherical aberration of the objective lens became the dominant limitation of the point resolution which, however, does not limit the information limit. Therefore, the interpretability of the images is strongly reduced due to the increased difference between information limit and point resolution. This resulted in an increased delocalization of information within the images obtained with such high resolution TEMs equipped with high brightness sources.

From 1992 to 1997 a joint project, funded by the Volkswagen Foundation in Germany, brought together scientists from materials science (K. Urban, Juelich), theoretical charged particle optics (H. Rose, Darmstadt), and experimental electron optics (M. Haider). By means of a Hexapole Cs-corrector based on a design idea of H. Rose [1] it was possible to demonstrate an improvement of resolution in a 200kV TEM [2, 3]. It became clear that not only the achieved improvement of resolving power also the vanishing delocalization and, therefore, the enhanced visibility of light elements in a Cs-corrected instrument are important parameters for imaging irregular structures like interfaces, dislocations, stacking faults and etc.

As soon as the Cs-correctors became almost a standard component of modern electron microscopes a further improvement of the resolution was requested. The main limitation which is left is the chromatic aberration Cc. The product of Cc*dE is the limiting parameter with dE, the energy width of the primary beam. This parameter can be tackled by a reduction of either dE or Cc. The energy width dE can be decreased by employing a monochromator and the chromatic aberration can only be reduced substantially by a Cc-correction system. Such a corrector has been developed for the TEAM initiative in the US [4]. For this project an advanced Hexapole-corrector for STEM [5] and a so called Achroplanator (a system which compensates the spherical and the chromatic aberration) has been developed and are installed at the NCEM, Berkeley, USA. The resolution achieved in STEM as well as in TEM mode with this corrector is superior of all other systems before [6].

The full compensation of both limiting aberrations could be demonstrated. Furthermore, this Cc-corrector development for TEM is a first step for a new class of instruments for which the objective lens can be designed most appropriate for the work one would like to carry out. Besides the medium voltage instruments additional projects are currently ongoing which are extending the energy range down to the 20kV regime or up to 1.2 MeV. The current state of developments will be explained and the future prospects of ongoing projects will be described in the presentation.

References [1] H. Rose, Optik 85 (1990) 19. [2] M. Haider, et al., Nature 392 (1998) 768. [3] M. Haider, et al., Adv. Img. Electr. Phys. 153 (Ed. Hawkes, P.) (2008) 43. [4] TEAM: http://ncem.lbl.gov/TEAM-project/index.html [5] H. Müller, et al., Nucl.Instr.andMeth. A(2010), doi: 10.1016/j.nima.2010.12.091 [6] Kisielowski C. et al., Microsc. Microanal. 14, (2008) 469.

http://ncem.lbl.gov/TEAM-project/index.html

Towards 2 resolution

Rudolf M. Tromp

IBM T.J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, NY 10598, USA

Email: [email protected] The spatial resolution of a high quality light microscope is limited to about half the wavelength of the photon, /2. Since the wavelength is long, this resolution is of the order of 0.2 – 0.4 m. The highest resolution transmission electron microscopes do much better: 50 pm resolution is now available with 300 keV electrons. But a 300 keV electron has a wavelength of 2 pm, 25 times smaller than the best available resolution. There are several factors that limit the resolution of the TEM. After correction of the chromatic and third order spherical aberrations there are still numerous other aberrations that prevent further improvement. But even if the fifth order spherical aberration coefficient could be reduced enough to improve the resolution by another factor 2, and if all the other aberrations could be controlled so that they would not spoil that improvement, the required stability of the high tension supplies, and the current supplies for the lenses, the stigmators, and the deflectors would be prohibitive. Already at 50 pm resolution defocus must be controlled to better than 200 pm, the thickness of a single layer of graphene, just at the edge of what is possible. For 25 pm resolution defocus would have to be controlled to better than 50 pm. The high voltage power supply for the 300 keV gun would have to be stable to better than 10 mV. At present, such extraordinary stabilities are not available. In LEEM the electron wavelength at 5 eV is 0.55 nm. Simulations1 for the aberration-corrected LEEM developed at IBM show that sub-nm resolution is possible, at least in principle. Resolution just below 2 nm has already been demonstrated2. While the stability requirements are stringent, spatial resolution at the 2 level appears possible. I will present the various factors that determine and limit resolution in LEEM, from the cathode lens3, to the aberration corrector2, down to the image detector. Recently, we have implemented a number of instrumental upgrades that will bring 2 resolution within reach. Here, I will report on our progress towards that goal. 1. S.M. Schramm, A.B. Pang, M.S. Altman, R.M. Tromp, Ultramicroscopy 115 (2012) 88 2. R.M. Tromp, J.B. Hannon, A.W. Ellis, W. Wan, A. Berghaus, O. Schaff, Ultramicroscopy 110

(2010) 852; R.M. Tromp, J.B. Hannon, W. Wan, A. Berghaus, O. Schaff, Ultramicroscopy in press.

3. R.M. Trop, W. Wan, S.M. Schramm, Ultramicroscopy in press; http://dx.doi.org/10.1016/j.ultramic.2011.09.011

http://dx.doi.org/10.1016/j.ultramic.2011.09.011

Photon and plasmon dynamics in planar waveguides visualized in non-linear PEEM

R. Könenkamp, R. C. Word, J. Fitzgerald, Athavan Nadarajah and S. Saliba

Physics Department, Portland State University, 1719 SW 10th Avenue, Portland, OR 97201, USA


We report the visualization of plasmon and photon dynamics in planar waveguide structures using pulsed photoexcitation in an aberration-corrected photoemission microscope [1]. Using femtosecond light pulses from a Ti-sapphire laser at wavelengths between 400 and 800nm we generate photonic and plasmonic wave propagation which we observe in non-linear photoemission processes [2]. Wave propagation within the waveguides and in the near-field region outside the waveguide is analyzed and discussed. Focusing, retardation and photonic/plasmonic coupling can directly be observed. For some of the waveguide geometries we have developed numerical simulations which are quantitatively compared to the experimental results. Methods to manipulate wave generation and propagation optically, i.e. using the excitation beam as an input were explored with emphasis on applications in plasmonics. Photoemission microscopy can again be utilized to obtain direct images of changes induced by polarization selection and switching. Using single crystalline planar gold structures we explored techniques for plasmonic routing in devices with sub-wavelength feature size. Experimental results with spatial resolution in these images as low as 10nm will be presented and compared to numerical simulations. Finally we will present first results on the visualization of photon/plasmon coupling in non-linear PEEM images.

a) b) Figure 1. Photonic and plasmonic waves observed in a 60nm indium-tin-oxide film on glass using non-linear PEEM at a wavelength of 410 nm. The excitation light is coupled into the ITO film at FIB-milled holes appearing in black color in the image. Illumination occurs at an angle of 60o from the surface normal and a direction from the lower end of the image. Part (a) shows a circular arrangement of holes and a gold nano-wire, part (b) shows details of the in-coupling of the photon energy at the periphery of the holes. Acknowledgement This research was supported by DOE-BES under contract DE-FG02-10ER46406 and by the Oregon Nano and Microtechnologies Institute. References [1] Könenkamp R, Word RC, Rempfer GF, Dixon T, Almaraz L, and Jones T., Ultramicroscopy

110, 899 (2010)

Normal Incidence 2PPE PEEM

Philip Kahl1, Simone Wall1, Christian Witt1, Christian Schneider2, Daniela Bayer2, Alexander Fischer2, Pascal Melchior2, Michael Horn-von Hoegen1, Martin Aeschlimann2,

Frank-J. Meyer zu Heringdorf1

1Faculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Lotharstrasse 1, 47058 Duisburg, Germany

2Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, 67663 Kaiserslautern


Non-linear photoemission microscopy has in the last years been shown to be a successful method for the imaging of surface plasmon polaritons (SPPs) and for the observation of the time-dependent propagation of SPPs across a surface. Using femtosecond laser pulses of a photon energy that is smaller than the work function of the plasmonic material of interest, SPPs are imaged via a plasmon-enhanced two photon photoemission process in which a part of the laser pulse is used to excite SPPs, while the remaining part of the laser pulse is used to generate photoelectrons and probe the SPP state with spatial resolution. 2PPE PEEM then detects the temporal integral of the fourth power of the superposition of all electric fields at the surface. So far, almost all experimental 2 PPE PEEM setups have exploited a grazing incidence geometry in which the fs laser pulses impinge on the surface under an angle of ~70° relative to the surface normal. A moiré pattern can then be used to describe the contrast in two-dimensional plasmonic structures. The periodicity of the observed moiré pattern, however, is different from the wavelength of the SPP. For the moiré contrast the projection of the exciting light field into the surface plane is relevant, and accordingly, the spacing of the moiré maxima depends on the angle of incidence of the laser pulses on the surface. In particular, with increasing incidence angle the moiré pattern is expected to converge into the SPP wavelength until under normal incidence a “direct imaging” of SPP wave packets becomes possible. At the same time the propagation direction of the SPP changes. We present 2PPE PEEM results for Grazing Incidence (GI) and Normal Incidence (NI) 2PPE PEEM and discus the differences between the two situations. The interpretation will be backed up by a simple plane-wave model for the superposition of the plasmon’s electrical field and the femtosecond laser pulse.

Figure 1. Observation of normal incidence 2PPE patterns of surface plasmon polaritons. (a) LEEM image of a self-organized Ag island. (b)-(d) The top corner of the island as seen in 2PPE PEEM under different polarizations. The direction of the electric field is indicated by the arrows on the upper right in each panel. Acknowledgement Financial support by the Deutsche Forschungsgemeinschaft (DFG) through Programs SPP 1391 “Ultrafast Nanooptics” and SFB 616 “Energy Dissipation at Surfaces” is gratefully acknowledged.

Energy Filtered Magnetic Circular Dichroism PEEM for Magnetic Domain Observation

Takeshi Nakagawa1,2, Toshihiko Yokoyama1,2

1Institute for Molecular Science, Okazaki, Japan, 444-8585 2The Graduate University for Advanced Studies, Okazaki, Japan, 444-8585


Magnetic circular dichroism (MCD) in valence band is usually small compared to the core level MCD effect [1]. An angle resolved photoelectron (ARP) measurement enables us to detect sizeable MCD, but the adaption of the microscopic method along with ARP is not straightforward. We have demonstrated that the magnetic circular dichroism near the photoemission threshold has large asymmetry and this finding can be adapted for magnetic domain imaging using PEEM [2]. In order to obtain optimized MCD contrast, it is necessary to adjust both work function and photon energies, which requires samples with the stable work function.

With an energy filter (high pass filter) PEEM and an ultraviolet photon source, we can obtain large MCD asymmetry irrespective of the work function. Figures 1(a) and (b) show the photoelectron intensity and the MCD asymmetry, respectively, by an energy filtered PEEM on a perpendicularly magnetized 15 ML Ni film on Cu(001). The work function is estimated to be 4.8 eV, and the photon energy is 5.9 eV with an incident angle of 65° from the surface normal. Around the photoelectron threshold (defined as the sample bias (Vs) is zero), the MCD asymmetry is high, giving 3 %. As the sample bias decreases, the photoelectron intensity increases and the MCD asymmetry decreases, which is in good agreement with previous results [1,2]. Figures 1 (c) and (d) show the corresponding domain images at Vs = 0 V and -0.8 V, respectively. Figure 1(e) shows a non filtered image. The high pass filtered images measure the electrons closer to the Fermi level. The result demonstrates that the MCD contrast is considerably enhanced using the energy filter despites the reduced intensity.

By taking advantage of its rapid acquisition, we also discuss the change of magnetic domain structures by molecular adsorption.

Figure 1. Laser MCD-PEEM measurements on perpendicularly magnetized 15 ML Ni on Cu(001). (a) total photoelectron intensity in PEEM as a function of the sample bias voltage (Vs). Vs = 0 indicates the photoelectron threshold. (b) The corresponding MCD asymmetry. (c),(d) Magnetic domain images by MCD-PEEM at (c) Vs = 0 V, (d) Vs = - 0.8 V. (e) Magnetic domain image without filtering. References [1] W. Kuch and C.M. Schneider, Rep. Prog. Phys., 64, 147 (2001). [2] T. Nakagawa, and T. Yokoyama, Phys. Rev. Lett. 96, 237402 (2006). T. Nakagawa, T.

Yokoyama, M. Hosaka, and M. Katoh, Rev. Sci. Instrum., 78 , 023907 (2007).

The spin polarizing electron mirror: Efficient spin resolved photoelectron microscopy and bandstructure mapping

Christian Tusche1, Martin Ellguth1, Aimo Winkelmann1, Alexander Krasyuk1,

Dima Kutnyakhov2, Pavel Lushchyk2, Katerina Medjanik2, Gerd Schönhense2, and Jürgen Kirschner1

1Max-Planck-Institut für Mikrostrukturphysik, 06120 Halle, Germany 2Institut für Physik, Johannes Gutenberg-Universitat Mainz, 55128 Mainz, Germany


We measure the spin-polarization of photoelectrons emitted from several atomic layers thick Co films grown on Cu(100) using a momentum microscope. This instrument, consisting of a photoemission electron microscope (PEEM) and an aberration corrected electrostatic energy analyzer directly records the parallel momentum component, k||, of photoelectrons emitted from the sample. Spin-filtering is based on the diffraction of low energy electrons in the (00)-LEED beam of a W(100) crystal, installed at the exit of the energy filter. Spin contrast is obtained due to the spin-dependent reflectivity of low energy electrons, while the image information is conserved in the outgoing elastic (00) diffraction spot. Figure 1a) shows a spin filtered PEEM image of the magnetic domain structure of the Co film, recorded in the spatial imaging mode. The results show that 3800 image points can be recorded simultaneously [1]. Compared to spin-resolved single-channel electron spectroscopy, an increase in detection efficiency by 4 orders of magnitude can be obtained [2]. A series of constant-energy cuts through the Brillouin zone gives direct access to the valence band electronic structure of the ultra-thin Co-film. By inserting the W(100) electron mirror into the electron optical path, the spin-resolved distribution of photoelectrons as a function of k||(x,y) is recorded simultaneously with unprecedented efficiency. Figure 1b) shows the spin-resolved photoelectron intensities, measured at the Fermi energy of a 12ML Co film. The measurements allow to assign the electronic states in the Co Fermi surface to the majority or minority spin channel. The results open a way to map the complete spin-resolved band-structure of correlated electron systems, and test recent theoretical concepts of the electronic structure beyond the single-particle picture.

Figure 1. (a) Spin filtered PEEM image of the magnetic domain structure of a 8ML Co film measured using two-photon photoemission. The inset shows the intensity variation over the domain wall. (b) Cut

rough the Fermi surface of 12ML fct-cobalt separated by majority (↑) and minority (↓) spin channel.

e[1] A. Krasyuk, M. Hahn,

[2] G. Schönhense, A. Oelsner, C. Tusche, J. Kirschner: Phys. Rev. Lett. 107, 207601 (2011)

th Ref rences

C. Tusche, M. Ellguth, A. Ünal, C.-T. Chiang, A. Winkelmann, G. Schönhense and J. Kirschner: Appl. Phys. Lett. 99, 032505 (2011) M. Kolbe, P. Lushchyk, B. Petereit, H.J. Elmers,

High performance spin-polarized photocathode using strain-compensated superlattice

X.G. Jin1,2, A. Mano3, F. Ichihashi2, N. Yamamoto2,3, Y. Takeda3

1Institute for Advanced Research, Nagoya University, Nagoya 464-8603, Japan 2Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan

3Synchrotron Radiation Research Center, Nagoya University, Nagoya 464-8602, Japan


We have successfully developed a transmission-type GaAs/GaAsP strained superlattice (SL) photocathode, and a high spin-polarization (90%) with a super-high brightness (1.3107 Acmsr) of electron beam was achieved [1, 2]. In this study, we report the design and fabrication of an optimized transmission-type photocathode with strain-compensated SL for high quantum efficiency (QE). In the GaAs/GaAsP strained SL, a compressive strain introduced in the GaAs well layers for large band splittings of heavy-hole and light-hole mini-bands. Therefore, the spin-polarized electrons are selectively excited by circularly polarized photons only from the heavy-hole mini-band to the conduction mini-band. Because of the accumulation of the strain, the increase of the SL thickness should cause the strain relaxation, which lowers spin-polarization [3]. To overcome this problem, the use of strain-compensated SL was proposed. In this structure, a strain is introduced in the SL barrier layers in the opposite direction to compensate the strain in the SL well layers so that no critical thickness limitation exists on overall thickness of the SL structure. Figure 1 shows the GaAs/GaAsP strain-compensated SL structure. Figure 2 shows the change of maximal spin-polarization and QE at the maximum spin-polarization with increase of SL pair number. The maximum values of the spin-polarization were close to 90% in all photocathodes. The maximal spin-polarization of 92% was observed in 24-pair of SL. The values of QE increase steadily with increasing SL layers. In the photocathode with 36-pair of SL, the QE is as high as 0.5%. A lattice-matched structure all through the substrate and strain-compensated SL will be proposed and a preliminary result will also be reported.

GaP (001) SubstrateZn 1.31018 cm3

Al0.1Ga0.9As0.81P0.19 500-nm

GaAs Cap layer

GaAs 4-nm/GaAs0.62P0.38 4-nm

superlattice

12、24、36 pairs

Compressivestrain

Tensile strain

Figure 1. Strain-compensated superlattice structure

Figure 2. Maximal spin-polarization and quantum efficiency with increase of super lattice thickness.

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Acknowledgement This work was supported by Grants-in-Aid for Scientific Research (A) and (S) and Grant-in-Aid for Young Scientists (B) from Japan Society for Promotion of Science (JSPS). References [1] X.G. Jin et al., Appl. Phys. Express, 1 (2008) #045002. [2] X.G. Jin et al., J. Cryst. Growth, 310 (2008) 5039. [3] T. Maruyama et al., Appl. Phys. Lett., 85 (2004) 2640.

Recent Advances in Biological Microscopy

Mark H. Ellisman, Ph.D.

Professor of Neurosciences and Bioengineering, Director, National Center for Microscopy and Imaging Research and

Center for Research in Biological Systems, University of California at San Diego, La Jolla, CA, USA 92093


New methods for extreme multi-scale 3D EM applied to biological samples are enabling new syntheses of information about natural systems, including new revelations advancing our understanding of the structure and function of nervous systems. Brain researchers are now beginning to be able to explore this important organ across the full range of scales, from genomics and molecular structure to networks of neuronal systems. Gaps in knowledge and limited abilities to span scales in this and other tissues highlight the need for new tools and methods that will allow the acquisition of high fidelity 3D image information at high resolution, but over very large expanses. Several ongoing projects related to the technological challenges of working across scales will be highlighted in this presentation. These include work on advanced extreme-scale and high quality image recording systems for electron tomography; development of specimens and procedures to increase the image quality, resolution and field of view for 3D volumes acquired by automated serial block face scanning electron microscopy or multibeam SEM.

A Monochromatic, Aberration-Corrected, Dual-Beam LEEM

Marian Mankos and Khashayar Shadman Electron Optica, 1000 Elwell Court, Palo Alto, CA 94303, USA


The monochromatic, aberration-corrected, dual-beam low energy electron microscope (MAD-LEEM, Fig.1) is a novel instrument aimed at imaging of nanostructures and surfaces at sub-nanometer resolution that utilizes electrons with landing energies in the range of 0 to a few 100 eV for imaging. The monochromator [1] reduces the energy spread of the illuminating electron beam, which significantly improves spectro-scopic and spatial resolution. The aberration corrector [2] is needed to improve the spatial resolution in order to achieve sub-nm resolution. Dual flood illumination [3] eliminates charging generated when a conventional LEEM is used to image insulating specimens. The electron-optical properties of the objective lens combined with an electron mirror aberration corrector have been analyzed up to 5th order for electron energies ranging from 1 to 1000 eV. The spherical and chromatic aberration coefficients of the electron mirror are fine-tuned iteratively to cancel the aberrations of the objective for a range of electron energies, thus providing a path for sub-nm resolution. MAD-LEEM is in particular aimed at imaging of biological and insulating specimens, which are difficult to image with conventional LEEM, Low-voltage SEM, and TEM instruments. The low energy of electrons is critical for avoiding beam damage, as high energy electrons with keV kinetic energies used in SEMs and TEMs cause irreversible damage to many specimens, in particular biological materials. A potential application for MAD-LEEM is in DNA sequencing which demands imaging techniques that enable DNA sequencing at high resolution and speed, and low cost [4]. The key advantages of the MAD-LEEM approach are long read length, the absence of heavy-atom DNA labeling, and use of low electron energies. Image contrast simulations of the detectability of individual nucleotides in a DNA strand have been developed in order to refine the optics blur and nucleotide contrast requirements. The MAD-LEEM approach promises to significantly improve the performance of a LEEM for a wide range of applications in the biosciences, material sciences, and nanotechnology where nanometer scale resolution and analytical capabilities are required.

AcknowledgementsThe authors would like to thank A.T. N’Diaye and A.K. Schmid at the NCEM, Lawrence Berkeley National Laboratory in Berkeley, CA, and H. H. Persson and Prof. Ron Davis at the Stanford Genome Technology Center in Palo Alto, CA for their support, encouragement and invaluable advice. This project was supported by Grant Number R43HG006303 from the National Human Genome Research Institute (NHGRI). The content is solely the responsibility of the authors and does not necessarily represent the official views of the NHGRI or the National Institutes of Health.

References[1] M. Mankos, U.S. Patent No. 8,870,172; (May 22, 2012). [2] D. Preikszas and H. Rose, Journal of Electron Microscopy 1, 1 (1997). [3] M. Mankos, Nucl. Instr. Meth. Phys. Res. A, 645, 35 (2011).[4] M. Mankos et al., submitted to JVST B (2012).

Screen(CCD)

Specimen/Stage

Objectivelens

Projection optics

Electron source(Imaging beam)

Illumination optics

Electron source(Charging beam)Illumination opticsMain beamseparator

Symmetrymirror

Aberration corrector

Monochromator

Figure 1. Electron-optical layout of MAD-LEEM.

mailto:[email protected]


A flange-on electron spectromicroscope

Krzysztof Grzelakowski

OPTICON Nanotechnology, Muchoborska 18, PL54-424 Wroclaw, Poland


A flange-on double imaging electron spectromicroscope based on the concept of newly developed [1] spherical deflector energy analyzer α-SDA is reported. Its compact design and flange-on construction enables simple adaption to UHV systems. Two independent imaging columns allow quasi-simultaneous observation of real and reciprocal images by quickly switching one of the spherical deflectors on and off.

Figure 1. Horizontally mounted flange- on electron spectromicroscope. 1: real image, 2: reziprocal image

References [1] K. Grzelakowski, Ultramicroscopy, 116 (2012) 95

Probing Buried Layers with Hard X-Ray Spectromicroscopy

C. Wiemann1, M. Patt1, M. Escher2, N.B. Weber2, M. Merkel2, A. Gloskovskii3,

W. Drube3 and C.M. Schneider1

1Peter Grünberg Institute (PGI-6) and JARA-FIT, FZ Jülich, D-52425 Jülich, Germany 2Focus GmbH, D-65510 Hünstetten, Germany

3DESY Photon Science, Deutsches Elektronen-Synchrotron, D-22603 Hamburg, Germany


Hard X-ray photoelectron spectroscopy (HAXPES) has proven to be a valuable tool to retrieve spectroscopic data from bulk regions and buried layers [1]. On the other hand, photoemission electron microscopy (PEEM) is a well-established technique for high-resolution imaging with soft x-rays. Various contrast mechanisms provide valuable insight into electronic and magnetic properties of microstructured systems, among them elemental contrast due to absorption at characteristic absorption edges, magnetic contrast due to magnetic dichroism, and spectroscopic imaging using energy-selected photoelectrons [2]. Combining both methods into a spectro-microscopy tool that offers both high spatial and energy resolution together with a high information depth is a rewarding task. So far, PEEM has been combined with hard x-rays as an imaging detector for local x-ray-absorption (NanoXAFS), using the secondary electron yield for image formation [3].

Figure 1. (a) Image of the Au electrodes on SrTiO3 in threshold photoemission. (b) Same sample region imaged using Sr 2p3/2 electrons at 4.56keV. (c) Sr2p3/2 photoelectron spectra for hard x-ray excitation (hν=6.5keV) integrated over the green and blue areas in (b).

In this contribution, we present for the first time imaging spectroscopy at high kinetic energies using hard x-rays (HAXPEEM). The measurements were performed at beamline P09 (PETRA III, DESY, Hamburg), using a NanoEsca-type photoemission electron microscope with double hemisphere energy filter [4,5]. We will show the feasability of imaging at high kinetic energies at a reasonable spatial resolution as well as spatially resolved spectra from buried layers, demonstrating the power of this promising new experimental method. The studies on a sample comprising Au electrodes on a Fe:SrTiO3 resistive switching film (Fig. 1) show a clear spectroscopic signature in the Sr2p3/2 states from the SrTiO3/Au interface. References [1] K. Kobayashi, Nucl. Instr. Methods A 601, 32 (2009) [2] A. Locatelli, E. Bauer, J. Phys. Condens. Matter 20, 093002 (2008) [3] M. Kotsugi, T. Wakita, T. Taniuchi, et al., e-J. Surf. Sci. Nanotech. 4, 490 (2006) [4] M. Escher, N. Weber, M. Merkel, et al., J. Electron Spectrosc. 144, 1179 (2005) [5] C. Wiemann et a. e-J. Surf. Sci. Nanotech. 9 (2011) 395-399 [6] C. Wiemann et al., Appl. Phys. Lett. 100, 223106 (2012)

Depth-resolved soft x-ray photoelectron emission microscopy

Florian Kronast1, Alexander Gray8, Christian Papp4, See-Hun Yang5, Stephan Cramm6, Ingo Krug6, Farhad Salmassi7, Dawn Hilken7, Eric Anderson7,

Claus M. Schneider6, Charles. S. Fadley2,3

1Helmholtz-Zentrum Berlin, Albert-Einstein-Strasse 15, D-12489 Berlin, Germany 2Department of Physics, University of California, Davis, CA, USA

3Material Sciences Division, Lawrence Berkeley National Laboratory, CA, USA 4Physical Chemistry II, University of Erlangen, Germany

5IBM Almaden Research Center, San Jose, CA, USA 6Jűlich Research Center, Germany

7Center for X-Ray Optics, Lawrence Berkeley National Laboratory, CA, USA 8Stanford University, SSRL, USA


Photoelectron microscopy in combination with synchrotron light (XPEEM) is a well established imaging technique. Its specific contrast is widely used for magnetic imaging and microspectroscopy with quantitative analysis. We extend conventional laterally-resolved soft x-ray photoelectron emission microscopy to provide depth resolution along the surface normal. Depth resolution in the range of Angstroms can be achieved by using standing-wave (SW) excitation[1]. The SW is generated by reflection of soft x-rays from a synthetic multilayer mirror which is used as the substrate on which the sample is grown. Tuning the incident x-ray to the mirrors Bragg angle sets up a standing x-ray wave field in the multilayer and the sample structure. The standing wave is moved vertically through the sample by means of varying the photon energy or the incidence angle. The technique was applied to Co nanodots, 4 nm in thickness and 1000 nm in diameter, grown on a Si/Mo (23.6Å/15.8Å)×40 multilayer substrate, and capped with a 2 nm-thick aluminum layer. Element- and depth-selective microscopy images were obtained for the constituent elements. The photoemission intensities as functions of photon energy were compared to x-ray optical theoretical calculations in order to quantitatively derive depth-resolved film structure of the sample, complimentary to the lateral picture provided by the PEEM[2].

References [1] F. Kronast, R. Ovsyannikov, A. Kaiser, C. Wiemann, S.-H. Yang, D. E. Bürgler, R. Schreiber,

F. Salmassi, P. Fischer, H.A. Dűrr, C. M. Schneider, W. Eberhardt, C. S. Fadley, Appl. Phys. Lett. 93, 243116 (2008)

[2] A. X. Gray , F. Kronast, C. Papp, S.-H. Yang, S. Cramm, I. Krug, F. Salmassi, E. Gullikson, D. Hilken, E. Anderson, P. Fischer, C.M. Schneider, C.S. Fadley, Appl. Phys. Lett. 97, 062503 (2010)


Signature of n-doping in Gallium Nitride microwires studied by spectroscopic XPEEM

J. Morin1, O. Renault1, P. Tchoulfian1, N. Chevalier1, V. Feyer2, M. Patt2,

C. M. Schneider2

1CEA, LETI, MINATEC Campus, 17 rue des Martyrs, F-38054 Grenoble Cedex 09, France 2Peter Grünberg Institute (PGI-6), Research Center Jülich, D-52425 Jülich, Germany


We investigate by spectroscopic XPEEM the doping signature of intentionally n-doped GaN microwires. We present spectromicroscopic results at threshold (Fig.1a) and Ga3d core-level emission (Fig. 1b), c)) excited with soft x-ray radiation (NanoESCA Beamline, Sincrotrone Trieste, Italy, [1]). Fig. 1a) shows the threshold spectra of the Si-doped and undoped region of the wire. The two regions have a 134meV work function difference, the negative shift in the doped region (as seen from the corresponding images) expected to have a lower work function than the un-doped one, is consistent with n-doping. We cross-checked the work function measurements with Ga3d core-level spectromicroscopy (Fig. 1b)). Fig. 1c) highlights a 160 meV binding energy shift of the Ga3d peak as a function of position along the wire. The positive binding energy shift in the doped region is qualitatively in agreement with n-doping and a lower work function, due to the shift of the Fermi level closer to the conduction band. This shift is also quantitatively consistent with the 134 meV work function difference, providing evidence that the work function changes are dominated by shift of the Fermi level upon doping. Based on complementary laboratory experiments, we will discuss on the influence of the band bending effect and the surface photovoltage [2] on the measured energy level shifts.

(a) (c) (b)

Figure 1. (a) Threshold spectra of doped (solid line) and undoped (dash line) wire region highlighted in the insets: top image taken at 4,88 eV shows the doped region, the bottom inset image (5.02 eV) shows the entire wire. (b) Ga3d core-level (21.05 eV) image of the GaN wire. (c) Ga3d binding energy variations over 7 areas along the wire represented in (b). All results recorded at hν=227.4 eV. The work was supported by the French National Research Agency through the Recherche Technologique de Base program and was performed in the Nanocharacterization Centre of MINATEC. We thank Elettra for providing high quality synchrotron light. References [1] C. M. Schneider et al., J. Electron. Spectrosc. Relat. Phenom. (2012), accepted. [2] J.P. Long, V.M. Bermudez, Phys. Rev. B.66 (2012), 121308.

Organic Light Emitting Diode for Display Applications

Ching Tang

Department of Chemical Engineering University of Rochester Rochester, NY 14627

USA

Email: [email protected] After nearly three decades of research and development, organic light emitting diode (OLED) has emerged as a key display technology with the potential of displacing liquid crystal displays. In this talk, the pathways from early discoveries to commercialization of OLED displays will be traced with highlights on materials, device architectures, color patterning methods, and backplane electronics.

In-situ Microscopic Study of the Energetics at the Organic-Metal Interface: ZnPc on Ag(100)

Abdullah Al-Mahboob and Jerzy T. Sadowski

Center for Functional Nanomaterials, Brookhaven National Laboratory Upton, NY 11973, USA


Metal phthalocyanines have attracted significant attention in the past decades, having good potential for applications in chemical sensors, solar cells and organic magnets. As the electronic properties of molecular films are related to their crystallinity and molecular packing, the optimization of film quality is important for improving the performance of organic devices. In this work, as a model system, we studied the structural evolution of zinc phthalocyanine (ZnPc) films on a Ag(100) single crystal, in order to elucidate the relation between the balance between molecule-molecule and molecule-substrate interactions, and the resulting structure of the molecular film. ZnPc has been thermally deposited on a clean Ag(100) surface, at substrate temperatures varied from 360K to 460K. The nucleation and growth of the ZnPc film has been observed in-situ in the low-energy electron microscope (LEEM). Micro-spot LEED obtained from film grown at lower temperatures revealed presence of a double domain R33.69o epitaxial structure (Fig.1a), while at higher temperatures a very sharp 5x5 pattern was observed (Fig.1b). Most interestingly, we have observed a long delay in nucleation for both phases – nucleation begins at nominal coverage of 0.48 ML at 410K, while almost 0.75ML is required for the onset of nucleation at 460K. We show that the large equilibrium concentration for onset of nucleation is not caused by molecule desorption, but it rather results from combination of molecule diffusion energy barrier, energy barrier for molecule reorientation and the balance between detachment/attachment energy barriers (Fig.1c). Figure 1. (a) -LEED diffraction patterns obtained from 1ML of ZnPc grown on Ag(100) surface at substrate temperature of 360K (a) and 460K (b), respectively; (c) Arrhenius plots showing the relation between the equilibrium concentration of molecules required for onset of nucleation and the growth temperature for both epitaxial structures; The differences between molecule attachment and detachment energy barriers E for both structures, extracted from the experimental data, are shown in the plots. Acknowledgement Research carried out at the Center for Functional Nanomaterials and National Synchrotron Light Source, Brookhaven National Laboratory, which are supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.

Initial growth and topography of 4,4’-biphenyldicarboxylic acid on Cu(001)

Daniel Schwarz, Raoul van Gastel, Harold J.W. Zandvliet and Bene Poelsema

Physics of Interfaces and Nanomaterials MESA+ Institute for Nanotechnology, University of Twente,

P.O.Box 217, 7500AE Enschede, The Netherlands

Email: [email protected] We have investigated the nucleation and initial growth of 4,4’-biphenyldicarboxylic acid (BDA) on Cu(001) at temperatures ranging from 300 – 410K, using Low Energy Electron Microscopy (LEEM) and Selected Area Low Energy Electron Diffraction (μLEED). The BDA condenses in a two-dimensional (2D) supramolecular c(8 × 8) network composed of (almost) flat lying molecules. The dehydrogenated molecules form hydrogen bonds with their perpendicularly oriented direct neighbors. The resulting open structure was suggested to provide a suitable template for nanostructures.

First, the adsorbed BDA molecules form a disordered dilute phase. LEEM enables us to directly monitor the density in this phase by a careful analysis of the bright-field image intensity. Once this phase reaches a sufficiently high density, the c(8x8) crystalline phase nucleates. From the respective equilibrium densities at different temperatures, we determine the 2D phase diagram. The position and shape of the phase coexistence line provides the cohesive energy, which amounts to 0.35 eV.

The unique properties of LEEM to probe the dilute phase led us to a detailed study of nucleation and growth of BDA on Cu(001) at low supersaturation. The real time microscopic information allows a direct visualization of near-critical nuclei. At a substrate temperature of 332 K and a deposition rate of 1.4 x 10-6 monolayers per seconds we find a critical nucleus size of about 600 nm2. With the exerted access to the temporal behavior of the dilute phase density during nucleation and the previously determined line tension of the c(8x8) nuclei (from the cohesive energy) we estimate the Gibbs free energy for nucleation under these conditions at 4 eV. The corresponding critical nucleus size obtained from classic nucleation theory, using these numbers, corresponds with about 680 nm2 nicely with the direct result. We find that nuclei with a size 6 times larger than the critical value still have a finite decay probability, implying that the size fluctuations are an order of magnitude stronger than expected.

At the relatively high temperature of 410 K the influence of substrate steps on the growth process becomes evident: domain growth is blocked by steps even when they are highly transparent for diffusing individual molecules. This leads to a Mullins-Sekerka type of growth instability of a novel kind: the growth is very fast along the steps and less fast perpendicular to the steps. The large solid angle at the advancing edge of the condensate dictates the high growth rate along the step. The observed non-wetting of the steps is in line with these findings and gives rise to spectacular rearrangements of the BDA condensates immediately after closing the shutter of the Knudsen cell.

Morphology and electronic structure of graphene on a square lattice

Andrea Locatelli1, T. Onur Menteş1, Cristina Africh2, G. Comelli1,2,3,

C. Wang4, N. Stojić4, N.Binggeli4

1Sincrotrone Trieste S.C.p.A., Trieste, Italy. 2IOM-CNR Laboratorio TASC, Trieste, Italy.

3University of Trieste, Department of Physics, Trieste, Italy. 4Abdus Salam International Centre for Theoretical Physics, Trieste, Italy.


Graphene on metals has evolved into a paradigm for understanding how the substrate interactions influence the electronic properties of two-dimensional sp2 hybridized atomic networks [1]. Aiming at characterizing its band structure, recent research has focused on high quality graphene layers grown on transition metals, investigating both intrinsic and extrinsic electronic effects. Known since the early days of surface science [2], the growth of few-layer graphene can nowadays be accurately controlled on most fcc(111) or hcp(0001) transition metal surfaces. As demonstrated by atomic scale characterization methods, graphene on metals shows a great variety of rotational structures and moiré patterns originating from the small lattice mismatch with the substrate [3]. Such models turned out to be ideally suited to study the band structure of graphene, allowing also to access electronic effects induced by the interaction with the support. Despite the unprecedented number of studies that the rise of graphene has triggered, the properties of graphene on crystal faces showing different symmetry than the threefold are still largely unexplored, few exceptions being provided by LEEM studies [4,5]. We report here on graphene grown on a prototypical Iridium square symmetry substrate, illustrating how the complex interplay between lattices with different symmetries affects both the morphology and the electronic structure of the film [6]. A remarkable aspect of this system is a temperature-driven reversible transformation between physisorbed incommensurate graphene and a more strongly bound commensurate type of graphene showing regularly spaced one-dimensional nanoripples. By combined use of advanced microscopy methods (spectroscopic photoemission and low energy electron microscopy, microprobe angle-resolved photoelectron spectroscopy, scanning tunneling microscopy) and ab initio calculations, we will show how graphene reacts to elastic forces, adapting its morphology to efficiently release the strain at the interface. By following the film evolution across and above the phase transition, we demonstrate how the graphene morphology and electronic structure are intimately connected to the occurring substrate interactions, which involve the formation and rupture of a surprisingly small number of chemical bonds. References [1] A.K. Geim and K.S. Novoselov, Nat. Mat. 6, 183-191 (2007). [2] H.P. Boehm et al., Z. Naturforsch. B 17, 150-153 (1962). [3] M. Batzill, Surf. Sci. Rep 67, 83-115 (2012). [4] J.W Wofford et al., Nano Lett. 10, 4890-4896 (2010). [5] A.L. Walter et al., Phys. Rev. B 84, 195443 (2011). [6] These authors, in preparation.

Graphene waves on Fe(110): LEEM and XPEEM studies

A.A. Zakharov, N.V. Vinogradov and A.B. Preobrajenski

MAX-lab, Lund University, Box 118, 22100, Lund, Sweden

E-mail: [email protected]

Strongly corrugated monolayers of sp2-bonded materials, such as hexagonal boron nitride and graphene, on lattice mismatched transition metals are promising self-organized templates for ordering nano-scale objects. Symmetry and periodicity of the corrugation are dictated by the lattice mismatch while the corrugation amplitude is defined by the strength of metal d – carbon 2p chemical interaction. Different structures were found for graphene adsorbed on 3d, 4d and 5d metal surfaces [1-3]. Surprisingly, the interface between graphene and iron has not been studied in detail yet. The main reason for this is a challenging procedure of graphene growth on iron because of preferential nucleation of the carbide Fe3C phase. Using LEEM and XPEEM we demonstrate that the growth of graphene on epitaxial iron films can be realized by chemical vapor deposition (CVD) if it occurs beyond the thermodynamic equilibrium conditions to avoid formation of carbide phases. The resulting graphene monolayer on Fe(110) creates a novel periodically corrugated pattern with the modulation in one dimension because of the partial match between distorted hexagonal symmetry of the Fe(110) face and graphene. Fig.1 shows two LEEM images of the C/Fe(110) surface as a result of the CVD process (a) at equilibrium growth conditions, when an iron carbide phase forms, and (b) at highly non-equilibrium gas pressure which is essential to suppress the formation of iron carbide in favor of graphene growth. Graphene on Fe(110) is corrugated in a periodic wavy pattern which creates the superstructure in the LEED pattern and can be seen in real-space by STM. This new corrugation pattern with a period of ~4nm parallel to the [001] direction is a promising platform for ordering nano-objects in one dimensional chain and possible applications will be discussed.

b a

Figure 1. (a) a LEEM image of iron carbide Fe3C on Fe(110) (E=3.5eV, FoV=15m). On the right side of the image are -LEED patterns (E = 30eV) from two different domains of the carbide phase (white and black areas in (a)). (b) aLEEM image of 1ML graphene (E=4.6eV, FoV=15m) on Fe(110); On the right side of the image are -LEED pattern (top, E = 50eV; red and blue are principal spots from graphene and iron, correspondingly) and a 30x30nm2 STM image of graphene waves (bottom). References 1. A. Varykhalov et al. PRL, 101, 157601 (2008). 2. D. Eom et al. Nano Lett 9, 2844 (2009). 3. A.B. Preobrajenski et al. PRB78, 073401 (2008).


Chemistry under graphene cover: intercalation reactions and nano-confinement effects

Qiang Fu, Rentao Mu, Li Jin, Hui Zhang, Yi Cui, Xinhe Bao

Dalian Institute of Chemical Physics, the Chinese Academy of Sciences, Dalian 116023, P.R. China


The recent world-wide research efforts on two-dimensional (2D) graphene have been stimulated by its unique electronic structure and, more importantly, many extraordinary physical properties. So far, the focus has been mainly, although not exclusively, laid on the physics and material science. The research of the 2D graphene structure in the field of chemistry, for example, catalysis, is much less explored, compared to its allotropes, 0D fullerenes and 1D carbon nanotubes. We show that an extended nanoscale environment forms between graphene sheets and solid surfaces, where guest atoms or molecules can be accommodated at the interfaces. It has been demonstrated that oxygen (O), lead (Pb), and silicon (Si) atoms can penetrate underneath graphene at elevated temperatures, decoupling the graphene layers from the Ru(0001) substrate [1, 2]. Even at room temperature, the intercalation of CO molecules at graphene/Pt(111) interfaces was also observed. The formed 2D nanospace presents as an extended confined environment, which exhibits nano-confinement effect on the chemistry of adsorbates on the solid surface. In-situ low energy electron microscopy (LEEM)/photoemission electron microscopy (PEEM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations reveal the nano-confinement effect from the graphene cover, which destablizes adsorption of CO on Pt(111) surfaces. The adsorption of oxygen on Ru(0001) was also shown to be weakened by the grahene cover [2,3]. The graphene sheet shows itself to function as an imaging agent to visualize reactons under its cover, and, furthermore, it can tune adsorbate-substrate interaction via the confinement in the nanospace. References [1] H. Zhang, Q. Fu, Y. Cui, D.L. Tan, X.H. Bao, J. Phys. Chem. C 2009, 113, 8296; Y. Cui, Q.

Fu, H. Zhang, D.L. Tan, X.H. Bao, J. Phys. Chem. C 2009, 113, 20365. [2] L. Jin, Q. Fu, R.T. Mu, D.L. Tan, X.H. Bao, Phys. Chem. Chem. Phys. 2011, 13, 16655; L. Jin,

Q. Fu, H. Zhang, R.T. Mu, Y.H. Zhang, D.L. Tan, X.H. Bao, J. Phys. Chem. C, 2012, 116, 2988; Yi Cui, Junfeng Gao, Li Jin, Jijun Zhao, Dali Tan, Qiang Fu, Xinhe Bao, Nano Research, 2012, 5, 352.

[3] Rentao Mu, Qiang Fu, Li Jin, Liang Yu, Guangzong Fang, Dali Tan, Xinhe Bao, Angew. Chem. Int. Ed. 2012, 51, 4856.

Long-range atomic ordering and variable interlayer interactions in two overlapping graphene lattices with stacking misorientations

Taisuke Ohta1, Jeremy T. Robinson2, Thomas E. Beechem1, Peter J. Feibelman1,

C. Bogdan Diaconescu1, Aaron Bostwick3, Eli Rotenberg3, G. L. Kellogg1

1 Sandia National Laboratories, Albuquerque, New Mexico 87185, USA 2 Naval Research Laboratory, Washington, D.C. 20375, USA

3 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA


The low energy electronic dispersion of graphene is extremely sensitive to the nearest layer interaction and thus the stacking sequence [1]. Here, we report a method to examine the effect of stacking misorientation in bilayer graphene by transferring chemical vapor deposited (CVD) graphene onto monolithic graphene epitaxially grown on silicon carbide (SiC) (0001). The resulting hybrid bilayer graphene displays long-range Moiré diffraction patterns having various misorientations even as it exhibits electron reflectivity spectra nearly identical to epitaxial bilayer graphene grown directly on SiC [2]. These varying twist angles induce the changes in the valence and conduction electronic states. Except at very small twist angles, we observe two non-interacting cones near the Dirac crossing energy, and the emergence of van Hove singularities where the cones overlap, using angle-resolved photoemission spectroscopy and ab initio calculations. This interlayer interaction is also reflected in the 2D (G’)-band shape of the Raman spectrum. The hybrid bilayer graphene fabricated via a transfer process therefore offers a means to systematically study the electronic properties of bilayer graphene films as a function of stacking misorientation angle. Acknowledgement We are grateful to N. Bartelt and S. K. Lyo for fruitful discussions, and R. Guild Copeland and Anthony McDonald for sample preparation and characterization. J.T.R. is grateful for experimental assistance from F. Keith Perkins on sample growth. The work at SNL was supported by the US DOE Office of Basic Energy Sciences, Division of Materials Science and Engineering (DE-AC04-94AL85000) and by Sandia LDRD, and was performed in part at CINT (DE-AC04-94AL85000). SNL is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US DOE NNSA under contract DE-AC04-94AL85000. A portion of this work was performed at Advanced Light Source, LBNL, supported by the U.S. DOE, Office of Basic Sciences under Contract No.DE-AC02-05CH11231. The work at NRL was funded by the Office of Naval Research and NRL’s NanoScience Institute. References [1] A. H. MacDonald & R. Bistritzer, Materials science: Graphene moiré mystery solved? Nature,

474, 453 (2011). [2] T. Ohta, T. E. Beechem, J. T. Robinson, and G. L. Kellogg, Long-range atomic ordering and

variable interlayer interactions in two overlapping graphene lattices with stacking misorientations, Phys. Rev. B 85, 075415 (2012).

The Equilibrium Shape of Graphene Domains on Ni(111)

Meifang Li1, J.B. Hannon2, R.M. Tromp2, J. Sun3, and E. Chason2

1School of Engineering, Brown University, Providence, RI 2IBM T.J. Watson Research Center, Yorktown Heights, NY

3Department of Physics and Astronomy, Michigan State University, East Lansing, MI


The potential technological applications of graphene have driven the search for efficient and inexpensive synthesis methods. Graphene synthesis via CVD on metal surfaces, and on copper in particular, is arguably the most widely-used method. Copper substrates are attractive because it is relatively easy to limit the graphene thickness to a single layer. However, due to the weak interaction between graphene and copper, graphene domains are randomly oriented. Growth on nickel foils has also been extensively explored [1]. In particular, Ni(111) has the advantage that graphene growth is epitaxial. In principle, isolated graphene domains will coalesce without forming a domain boundary. We grew graphene domains via ethylene CVD on ultra-thin Ni(111) foils [1]. Using in situ LEEM and PEEM we find that, due to the strong interaction of graphene with Ni(111), the equilibrium domain shape is triangular (Fig. 1a), in contrast to the hexagonal shape observed for graphene domains on copper (Fig. 1b). The domains have “zig-zag” edges, but only one particular type of zig-zag edge is observed (Fig. 1c). On copper the graphene is randomly oriented and the six possible zig-zag orientations are equivalent. We determine the orientation of the graphene relative to the Ni(111) substrate using a novel “real space” LEED technique that exploits the spherical aberrations of the cathode lens. In the absence of spherical aberrations, the images formed from the individual diffraction beams are coincident. Spherical aberrations cause the images from each diffracted beam to be displaced in the image plane. If the beam diameter is small, the displacements can be easily measured, resulting in a real-space diffraction pattern. This pattern can be used to determine the orientation of the image relative to the crystal structure.

Figure 1. (a) PEEM image of a triangular graphene domain on Ni(111). (b) 9 eV bright-field LEEM image of monolayer (dark) and bilayer (bright) graphene grown on a polycrystalline copper foil. (c) The orientation of a graphene domain with respect to the Ni(111) lattice [top layer atoms white, second layer atoms gray]. The domain exhibits only one of the two possible “zig-zag” edge terminations.

References [1] A. Reina et al, Nano Letters, 9, 30 (2009) [2] J.W. Shin, A. Standley, and E. Chason, Appl. Phys. Lett. 90, 261909 (2007)

Lights, action, camera: Making movies of molecules and materials with ultrafast electron diffraction and microscopy

Bradley J. Siwick, Robert P. Chatelain, Vance R. Morrison and Chris Godbout

Departments of Physics and Chemistry, Center for the Physics of Materials, McGill University, 801 Sherbrooke St. W, Montreal, QC Canada H3A 2K6

Email: [email protected] Recent developments in time-resolved diffraction techniques -- both x-ray and electron -- have made it possible to observe matter at unprecedented spatio-temporal resolution. These developments have been enabled by so-called 'ultrafast' xray and electron sources, which are capable of taking (essentially) instantaneous snapshots of the atomic structure of matter. Studies that make use of these sources have begun to open up a new window on the time-evolving atomic configuration of molecules and solids during structural transformations, since by piecing together a sequence of such snapshots we can obtain an atomic resolution "movie" of the process. My talk will focus on ultrafast electron diffraction (UED), and will provide a very accessible introduction to this field. The goals and methods of UED will be described, as well as the technical challenges associated with the development of ultrafast electron sources. As part of this overview I will demonstrate that we have recently been able to improve our diffractometer performance by several orders of magnitude by implementing a radio-frequency pulse compression strategy borrowed from the particle accelerator community [1,2]. Several experiments that highlight the power of ultrafast diffraction for studying structural dynamics at (or near) atomic resolution will then be described. Moving beyond diffraction experiments, ultrafast transmission electron microscopy will also be discussed (time permitting).

A B

Figure 1. UED with radio-frequency compressed electron pulses. A) Structural dynamics following femtosecond laser excitation of single crystal gold (UED pattern shown in inset). A coherent oscillation of the atomic positions (only ~0.001 Angstroms in amplitude) following the excitation is evident from changes in the position of the Bragg peaks. Data is shown for the peak indicated in the inset with a white circle B) A direct measurement of impulse response function of the ultrafast diffractometer; the time resolution of the instrument is better than 350 fs. References [1] T. van Oudheusden et al, Electron source concept for single-shot sub-100 fs electron

diffraction in the 100 keV range J. Appl. Phys. 102 (2007), Art. No. 093501. [2] R. P. Chatelain et al, Ultrafast electron diffraction with radio-frequency compressed electron

pulses, Appl. Phys. Lett. (In Press, 2012).

Coherent imaging of surface plasmon dynamics by time-resolved photoelectron emission microscopy

Hrvoje Petek

Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh PA 15260 USA

E-mail: [email protected]

We study surface plasmon polariton (SPP) generation, propagation, diffraction, interference, focusing, and decay by femtosecond time-resolved photoemission electron microscopy (PEEM). Equal-pulse pump-probe pulses with interferometrically defined delay excite two-photon photoemission from Ag surfaces. The imaging of the spatial distribution of photoemitted electrons by PEEM reveals a nonlinear map of the total surface electromagnetic fields impressed on the sample. The images reveal coherent polarization gratings consisting of superposition of the incoming excitation pulses and propagating SPP wave packets that are generated at nanofabricated coupling structures. By changing the delay between the pump and probe plusses in steps of ~330 as we record movies of the evolving coherent polarization at the Ag/interface, which reflects the evolution of the surface electromagnetic fields. Through the combination of femtosecond laser excited photoemission and imaging of photoelectrons we can record <10 fs time scale coherent polarization dynamics with ~50 nm spatial resolution.1-3 The SPP fields are generated by specifically designed coupling structures formed by lithographic techniques in Ag films. The physical properties of the coupling structures and the geometry of the excitation define the subsequent SPP dynamics. To obtain a quantitative understanding of the SPP generation and PEEM imaging we perform FDTD calculations on the coupling of the external field into the SPP mode and compare them to experiments for slit coupling structures with different geometries.4 Using more complicated coupling structures, we demonstrate SPP interference and focusing (Fig. 1).5 Through time-resolved PEEM measurements on nanostructured metal films we plan to develop techniques for the coherent control of electromagnetic fields on the femtosecond temporal and nanometer spatial scales.

Figure 1. a) A PEEM image of a 60° ‘V”-shaped coupling structure with Hg lamp excitation. b) A PEEM image of the same structure with 10 fs, 400 nm laser pulse excitation using a phase-locked pulse pair with 17.0 optical cycles delay (22.6 fs). The interference pattern between the external excitation and SPP wave packet fields causes the polarization grating of the total field. The expanded region of b) shows the complementary interference pattern when the phase is advanced by 0.5 cycles (0.65 fs).

References [1] A. Kubo, K. Onda, H. Petek, Z. Sun, Y. S. Jung, and H. K. Kim, Nano Lett. 5, 1123 (2005). [2] A. Kubo, N. Pontius, and H. Petek, Nano Lett. 7, 470 (2007). [3] A. Kubo, Y. S. Jung, H. K. Kim, and H. Petek, J. Phys. B: 40, S259 (2007). [4] L. Zhang, A. Kubo, L. Wang, H. Petek, and T. Seideman, Phys. Rev. B 84, 245442 (2011). [5] H. Petek and A. Kubo, in Handbook of Instrumentation and Techniques for Semiconductor

Nanostructure Characterization, edited by R. Haight, F. Ross and J. Hannon (World Scientific Publishing/Imperial College Press, 2011).

PEEM using femtosecond and attosecond optical and extreme ultraviolet laser pulses

Erik Mårsell1, Cord L. Arnold1, Eleonora Lorek1, Diego Guenot1, Thomas Fordell1, Johan

Mauritsson1, Hongxing Xu1, Anne L’Huillier1, Anders Mikkelsen1

1Department of Physics, Lund University, Box 118, 221 00 Lund, Sweden


Recent advances in ultrafast optics and nanostructure fabrication have opened up possibilities for new types of PEEM studies. Two fields that have experienced particularly rapid growth during the last decade are plasmonics and attosecond physics. In view of these advances, we present PEEM studies of metal and semiconductor nanostructures using advanced short-pulse laser systems as light sources. The general setup is depicted in figure 1, showing how we can use a variety of different laser pulses in a pump-probe PEEM setup. Using femtosecond optical pulses, we study several different phenomena, such as surface plasmons bound to metal nanostructures and resonant electromagnetic modes in III/V semiconductor nanowires [1]. Among other results, we find an interesting polarization dependence of the photoemission intensity from single InAs nanowires under illumination of 800 nm femtosecond laser pulses.

We also use extreme ultraviolet (XUV) attosecond pulse trains produced via high-order harmonic generation as the excitation source for PEEM. This setup has the possibility to combine the nanometre spatial resolution of PEEM with the attosecond temporal resolution of today’s laser spectroscopy techniques [2,3]. The experiment is, however, difficult to realize because of the unusual properties of the radiation. The very large bandwidth (~10 eV) needed to produce attosecond pulses results in increased chromatic aberration, and the low repetition rate of today’s attosecond laser systems (~1 kHz) leads to extremely low average photocurrents from the sample in order to avoid space charge effects. Further, we show how image contrast becomes an important issue for studies of noble metal systems with this type of radiation. We present how we explore these issues and how secondary electron imaging can help us take one more step towards PEEM imaging with attosecond temporal resolution.

Finally, we discuss the upgrades currently being made to both our PEEM and our laser system, and how electron energy filtering and a high-power, high-repetition rate laser system will help us in our future research.

Figure 1.(a) Schematic drawing of our atto-PEEM setup. High-order harmonics are generated in argon and separated by the fundamental by an Al filter. Part of the IR beam is sent through a delay stage for pump-probe experiments. If desired, a frequency doubling crystal can be inserted into one of the arms. (b) Typical spectrum of the HHG radiation. (c) Temporal structure of an attosecond pulse train.

References [1] L. Cao et al., Nature Mater.8, 643-647 (2009). [2] A. Mikkelsen et al., Rev. Sci. Instrum. 80, 123703 (2009). [3] M. I. Stockman et al., Nature Photon.1, 539-544 (2007).

Magnetic Nanodots induced Novel Magnetic Phenomena

Jian Shen

Department of Physics, Fudan University, Shanghai 200433, China


Study of Magnetic nanodots is at central in the field of nanomagnetism. Besides the interesting properties caused by dimensionality effect, magnetic nanodots can induce many novel phenomena when forming heterostructures with other materials. In this work, I will use several examples to demonstrate their effect. These examples include collective ferromagnetism of nanodot arrays on 2-dimensional electron gas, colossal Coulomb blockade magnetoresistance in tri-layers, and dramatic enhancement of metal-insulator transition temperature in manganites. All these fascinating phenomena originate directly from the presence of magnetic nanodots. Their underlying mechanisms, while somewhat understood, need further theoretical studies.

References [1] T.Z. Ward, Z. Gai, X.Y. Xu, H.W.Guo, L.F. Yin, J. Shen, Phys. Rev. Lett. 106, 157207

(2011). [2] Dali Sun, Lifeng Yin, Chengjun Sun, Hangwen Guo, Zheng Gai, X.-G. Zhang, T. Z. Ward,

Zhaohua Cheng, and Jian Shen, Phys. Rev. Lett. 104, 236602 (2010). [3] Lifeng Yin, Di Xiao, Zheng Gai, Thomas Z. Ward, Noppi Widjaja, G. Malcolm Stocks, Zhao-

hua Cheng, E. Ward Plummer, Zhenyu Zhang, and Jian Shen, Phys. Rev. Lett. 104, 167202 (2010).

Structural and magnetic studies on self-organized metal films Tevfik Onur Menteş1, Andrea Locatelli1, Nataša Stojić2, Nadia Binggeli2, Miguel Ángel

Niño3, Lucia Aballe4, Ernst Bauer5

1Sincrotrone Trieste S.C.p.A., Trieste 34149, Italy 2Abdus Salam International Center for Theoretical Physics, Trieste 34151, Italy

3IMDEA-Nanociencia, Cantoblanco, Madrid 28049, Spain 4CELLS-ALBA, Cerdanyola del Vallés, Barcelona 08290, Spain

5Arizona State University, Tempe, AZ 85287-1504, USA


Self-organized periodic patterns under thermal equilibrium have been studied since the late eighties. The main idea, both for magnetic and for structural patterns, is based on competing interactions at different length scales, which minimize the free energy at a particular length scale depending on temperature [1]. In recent studies, we have focused on examples of stress-driven structural patterns of metal adlayers on W(110) surface. In particular the Pd monolayer was shown to order into a mesoscopic stripe phase [1], which can be modified dramatically through adsorbing oxygen as a second adspecies [2]. Here, we focus on two new aspects regarding these stripe phases on the tungsten surface. First, we describe the internal structure of the Pd layer within the mesoscopic stripe phases. We show that there is yet another periodic structure with a few nanometer period within the Pd layer (Fig. 1a), which is identified as periodic vacancy-lines by density-functional theory calculations [3]. This temperature-dependent (Fig. 1b) internal phase is shown to be distinctly different than a surface reconstruction. The result is the coexistence of two periodic structures at two different length scales, which are crucially interrelated. In the second part of the talk, we discuss the possibilities in using the mesoscopic stripes as templates for growing magnetic wires. The idea is illustrated by Fe growth on Pd-O stripes. We show that Fe preferentially sticks to the Pd covered regions, thus forming ferromagnetic wires (Fig. 1c).

Figure 1. (a) LEEM image of Pd/W(110) stripes. The DFT-modeled vacancy-line structure internal to Pd stripes is sketched below. (b) Temperature dependence of the vacancy-line period. (c) XMCD-PEEM image of Fe wires grown on a Pd-O stripe template. References [1] T. O. Menteş, A. Locatelli, L. Aballe, E. Bauer, Phys Rev. Lett. 101, 085701 (2008). [2] T. O. Menteş, N. Stojić, A. Locatelli, L. Aballe, N. Binggeli, M. Á. Niño, M. Kiskinova, E.

Bauer, EPL 94, 38003 (2011). [3] N. Stojić et al., in preparation.

Magnetic properties of Fe nanostructures on W(110) studied with X-PEEM

M. Ślęzak1, T. Giela2, D. Wilgocka-Ślęzak2, A. Kozioł-Rachwał1, T. Ślęzak1, R. Zdyb3,

N. Spiridis2, C. Quitmann4, J. Raabe4, J. Korecki1,2

1Faculty of Physics and Applied Computer Science, AGH, Kraków, Poland

2Jerzy Haber Institute of Catalysis and Surface Chemistry, PAN, ul. Niezapominajek 8, Kraków, Poland

3Institute of Physics, Maria Curie-Sklodowska University, 20-031 Lublin, Poland 4Swiss Light Source, Paul Scherrer Institut, 5232 Villigen-PSI, Switzerland


A PEEM III microscope with the energy analyzer from Elmitec, intended for the Polish synchrotron source SOLARIS [1], was successfully installed and tested for several weeks at the NanoXAS beamline (SLS) in cooperation between PSI and several Polish Laboratories. The microscope is equipped with a preparation chamber enabling also in situ MBE growth of metal and oxide films. Despite the fact that the NanoXAS beamline doesn’t perfectly fit to PEEM measurements, most of the X-PEEM features could be tested and verified, including chemical and magnetic sensitivity given by XAS, XPS, XMCD and XMLD methods combined with the high spatial resolution and the real time imaging. During five weeks long beamtime a wide range of materials, including bulk crystal surfaces, ultrathin films and self-organized as well as patterned nanostructures was studied and this contribution will review some selected results. Thickness and temperature induced SRT in thin Fe films on W(110) will be discussed in view of previously reported non-collinear magnetic structure during SRT [2]. For both transitions, X-PEEM movies and images showed that SRT proceeds via magnetic domain wall movement. Also X-PEEM studies of magnetic domain structures in self-organized Fe islands on W(110) will be presented. Our previous Nuclear Resonant Scattering (NRS) studies showed that for the particular nominal coverage of 1.5 ML, islands formed by annealing above 600ºC exhibit strange magnetic properties. NRS spectra display enhanced magnetism with increasing temperature. To shed more light on this effect, various iron nanostructures formed by annealing on the W(110) surface were studied in a broad temperature range. High quality X-PEEM images (lateral spatial resolution ~ 25nm) revealed rich variety of islands magnetic domain structure and its correlation with their shape and size. In conclusion some technical limitations and problems that were met during the experiment, as well as future Polish-Swiss plans, namely the idea to install aberration corrected LEEM on NanoXAS beamline (scheduled for 2013 year) will be discussed. Acknowledgement This work was supported in part by the SPINLAB project financed by the EU European Regional Development Fund and by the Team Program of the Foundation for Polish Science co-financed by the EU European Regional Development Fund. References [1] http://www.synchrotron.uj.edu.pl/ [2] T. Ślęzak et al., Phys. Rev. Lett. 105 (2010) 027206

Real-space observation of chiral magnetic order in metallic thin films at room temperature

G. Chen1,2, J. Zhu1, A. Quesada2, J. Li1, A. N'Diaye2, Y. Huo1, T.P. Ma1, Y. Chen1, H. Y.

Kwon3, C. Won3, Z.Q. Qiu4, A. K. Schmid2*, Y. Z. Wu1*

1Department of Physics, State Key Laboratory of Surface Physics and Advanced Materials Laboratory, Fudan University, Shanghai 200433, People’s Republic of China

2NCEM, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 3Department of Physics, Kyung Hee University, Seoul 130-701, Korea

4Department of Physics, University of California at Berkeley, Berkeley, California 94720, USA

Email: [email protected], [email protected]

Magnetic domains, usually considered as the result of competition of exchange interaction, dipolar interaction and magnetic anisotropy, will not contain chiral order. However, the Dzyaloshinskii-Moriya (DM) interaction, which arises from the spin-orbit scattering of electrons in a system with broken inversion symmetry, will cause chiral magnetic order. Chiral spin structures were observed in helimagnets such as Fe0.5Co0.5Si with a non-centrosymmetric crystal structure (Uchida et. al, Science, 311, 359(2006), Yu et. al, Nature, 465, 901(2010)), and in a Mn atomic layer on a tungsten substrate with inversion symmetry broken at the interface (Bode et. al, Nature, 447,190(2007)). Chirality in nanoscale magnets may play a crucial role in spintronic devices, but this should be performed at room temperature for real applications.

In this talk, we will present our results on chiral magnetic order observed directly in real space at room temperature. The experiments were performed with spin polarized low-energy electron microscopy (SPLEEM) at the Lawrence Berkeley national laboratory. An Fe/Ni bilayer grown on Cu(001) exhibits a magnetic stripe domain phase. The domain wall of the magnetic stripe is Néel-type, and the in-plane components of the neighboring domain walls are always antiparallel. These results clearly demonstrate the existence of a cycloidal chiral magnetic order when the magnetic spins rotate. The chiral order in the Fe/Ni bilayer is independent of the orientation and the width of the magnetic stripes, but will disappear for a Ni layer thicker than 7ML. The chirality can switch from the right-hand cycloid in Fe/Ni/Cu(001) to the left-hand cycloid in Ni/Fe/Cu(001), which indicates that the chirality is caused by the DM interaction mainly located at the Fe/Ni interface. A Monte-Carlo simulation can fully explain our results, and also predicts a new type of the skyrmion spin structure. Our results provide a new way to enhance the DMI at room temperature, which will benefit the application of spintronic devices.

LEEM and LEED observations of ultra-thin iron oxide film growth on oxygen-deficient YSZ(001)

Ivan Ermanoski and G. L. Kellogg

Sandia National Laboratories, Albuquerque, NM 87185, USA


We use LEEM and LEED to study in real time the growth of iron oxide thin films on the (001) surface of yttria-stabilized zirconia (YSZ). Our initial results, reported at the 7th International LEEM-PEEM conference, showed that growth by Fe deposition in a background of ~10-6 Torr O2 at temperatures of 1000°C and above produces a FeO(111) film (wüstite) with four non-equivalent domains arising from two rotational domains and two stacking sequences. Here we report on subsequent research showing that uniform spreading of 2-dimensional islands can be observed in LEEM by initiating growth at ~1000°C and raising the temperature to 1110-1145°C during Fe deposition (Fig. 1). The growth is anisotropic with the fast growth direction depending strongly on both the rotational and stacking domain structure. This anisotropic growth is attributed to preferential O2 dissociation at specific island edge configurations. LEEM-IV spectra from the films are quite similar to those taken from FeO(111) films grown on Ru(0001), where the films are identified as bilayer structures (two Fe and two O layers) [1]. Island coarsening at temperatures above 1160°C rotates the film orientation by 15° with respect to the substrate (seen in LEED) and reduces the coverage by about one half (seen in LEEM) consistent with a dewetting process at the higher temperatures. Second-layer Fe oxide grows as Fe3O4(111) (magnetite) and has a LEEM-IV fingerprint different from both FeO(111) and the YSZ(001) substrate. Figure 1. LEEM images (20 m field of view, start voltage = 3.0V) recorded during deposition of Fe on YSZ(001) in 9x10-6 Torr O2. (a) after 181 sec. at 893ºC and 344 sec. at 1040ºC (b) after an additional 245 sec. at 1040ºC (c) after an additional 420 sec. at 1040º C. The bright regions are FeO. Acknowledgement This work was supported by the U. S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering and by the LDRD program at Sandia National Laboratories. Sandia is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. References [1] M. Monti, B. Santos, A. Mascaraque, O.R. de la Fuente, M.A. Nino, T.O. Mentes, A.

Locatelli, K.F. McCarty, J.F. Marco, J. de la Figuera, Phys. Rev. B, 85 (2012).

Antiphase boundaries in the layer-by-layer growth of Fe3O4(111)/Pt(111) thin films

Alessandro Sala, Thomas Schmidt, Hans-Joachim Freund

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany


Fe3O4 (magnetite) is a transition-metal oxide with a wide range of applications. Magnetite films have shown several interesting properties in the fields of heterogeneous catalysis [1] and magnetism [2]. Most of them are connected with the film preparation methods and the substrate features. In particular, antiphase boundaries (APBs, i.e. dislocations and rotational domain boundaries) play an active role for the production of anomalous film resistivity and magnetic susceptibility, and could be a considerable factor in the determination of surface electronic properties. A new preparation recipe has been applied to study in-situ and in-real-time the layer-by-layer growth of a closed Fe3O4(111) film on Pt(111) with the SMART, the aberration-corrected energy-filtered LEEM/PEEM operating at the synchrotron light source BESSY II in Berlin. The magnetite film grown layer-by-layer shows morphological properties that differ from the film grown by iterations of room temperature Fe deposition and oxidation at 900 K. In addition to the usual atomic steps, new line defects appear on the surface. Their behavior during the layer-by-layer growth indicates that they are line dislocations produced by a shift which is a fraction of the unit cell height in [111] direction. Dark field imaging of the Fe3O4 surface for different film thickness show interesting behavior of the twin domains rotated by 180° during the layer-by-layer growth. It is known [3] that Fe3O4/Pt(111) presents an asymmetry in the domain distribution, with the predominance of one domain. The origin for the rotation domains is a stacking fault process at the interface that is triggered by the substrate morphology: while stacking faults are not produced on large terraces, the process is highly enhanced on step bunches. For a larger film thickness, the rotational domains modify their shape during the growth: small domains are fully converted to the predominant orientation, while the boundaries of large domains assume a polygonal shape. The consideration about the formation and dynamic behavior of the APBs will be discussed.

Figure 1. (a) bright field image and (b) dark field image of the Fe3O4(111)/Pt(111) thin film. During the layer-by-layer growth atomic steps and line dislocations flow independently on the surface, while the rotational domains change their shape and disappear if too small. References [1] H.-J. Freund, in Oxide Ultrathin Films, Science and Technology, ed. G. Pacchioni and S.

Valeri (Wiley-VCH, Weinheim, 2012) p. 145 [2] A. A. Mills, Ann. Science 61, 273 (2004) [3] A. Sala, H. Marchetto, Z.-H. Qin, Sh. Shaikhutdinov, Th. Schmidt, H.-J. Freund, submitted

Low pressure formation of α-Fe2O3(0001) on Pt(111)

Francesca Genuzio, Alessandro Sala, Thomas Schmidt, Hans-Joachim Freund

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany


α-Fe2O3 (hematite) films are important for both, catalytical and technological applications. A standard procedure to produce thin crystalline films is Fe deposition on a suitable metal crystal like Pt(111) and subsequent oxidation in a “high pressure cell” at about 1 mbar at an elevated temperature of about 1100 K [1]. In this work we present a recipe to grow hematite at a lower pressure range of less than 10-5 mbar of oxygen. This film growth and the surface properties are investigated in situ and in real time by the spectro-microscope SMART, combing microscopy (LEEM, X-PEEM, UV-PEEM) with diffraction (LEED) and spectroscopy (XPS, UPS). This is directly compared with the Fe3O4 (magnetite) results, grown under similar conditions. A closed α-Fe2O3 film can be obtained from a complete Fe3O4 film by Fe deposition and subsequent oxidation. This process requires a higher O2 pressure and a lower annealing temperature compared to the initial Fe3O4 film growth. As found by LEED the resulting α-Fe2O3

(0001) surface exhibits always the so-called “bi-phase structure”, which is assumed to be an oxygen depleted phase [2]. The oxidation process has been varied, but no low pressure condition could be found to avoid this termination. By Fe deposition and annealing in UHV the α-Fe2O3

(0001) film can be reduced and transformed back into a Fe3O4(111) film. A special situation is the co-existence of α-Fe2O3(0001) and Fe3O4(111), which allows a direct comparison in X-PEEM under identical conditions. The different ionic states Fe2+ and Fe3+ for both phases will be discussed.

1µm

(a) (b) (c)

(d) (e) (f)

Figure 1. Transformation of an α-Fe2O3 (0001) film ((a) LEEM and (d) LEED) into a Fe3O4(111) film by Fe deposition and subsequent short annealing ((b) and (e)). A longer annealing results in coexisting regimes of α-Fe2O3 (0001) and Fe3O4(111) ((c) and (f)). The oxide film is slightly dewetted and shows therefore additional FeO patches. The satellite spots in the LEED are due to the bi-phase structure of α-Fe2O3 (0001). References [1] W. Weiss, W. Ranke, Prog. Surf. Sci 79, 1 (2002) [2] N. G. Condon, F.M. Leibsle, A. R. Lennie, P. W. Murray, D. J. Vaughan, G. Thornton, Phys.

Rev. Lett. 75, 1961 (1995)

Micromagnetism in (001) Magnetite by Spin-Polarized LEEM

J. de la Figuera1, L. Vergara1, A. T. N'Diaye2, A. Quesada3, G. Chen2, M. Monti1, and A. K. Schmid2

1 Instituto de Quimica-Fisica “Rocasolano”, CSIC, Madrid 28006, Spain 2 Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

3 Instituto de Cerámica y Vidrio, CSIC, Calle Kelsen 5, 28049, Madrid, Spain


Magnetite has played an important role throughout the history of science and technology [1], e.g., the discipline of paleomagnetism relies to a large extent on its properties. Interest in magnetite as a candidate material for spintronic applications [2] is driven by the predicted half-metal character [3], high conductivity, chemical stability and high-Curie temperature. Promising recent results include the use of magnetite as a spin-injector source [4] as well as observations suggesting that the magnetic properties of magnetite are robust even in the nanometer thickness limit [5]. Using spin-polarized low-energy electron microscopy, we find that domains are magnetized along the surface [110] directions, and domain wall structures include 90° and 180° walls. A type of unusually curved domain walls are interpreted as Néel-capped surface terminations of 180° Bloch walls.

Figure 1. Details of particular magnetization configurations observed on (100) magnetite. Panels (a) and (b): SPLEEM images acquired with spin polarization along the [011] direction, (d) and (e) are acquired with [01-1] spin alignment. Small red arrows show the in-plane magnetization directions as measured from pairs of SPLEEM images acquired with orthogonal beam polarizations, large red arrows indicate average magnetization in each domain. (a) 90° wall, image size 2.7 μm. (b) Wavy 180° wall, image size 5.3 μm. (c) Intensity profile along the line indicated in previous panel suggests that the structure is a 180° Néel-capped Bloch wall. (d) Bloch line separating two sections of opposite chirality within Néel-capped 180° Bloch wall; image size 2.7 μm. (e) Periodic array of 90° walls, image size is 5.3 μm. (f) Image intensity profile across array of 90° walls (along the line indicated in previous panel).

Work was supported by the Spanish Ministry of Science and Technology, No. MAT2009-14578-C03-01 and by the Office of Basic Energy Sciences, U.S. DOE, No. DE-AC02—05CH11231. A.T.N. acknowledges the Alexander von Humboldt Foundation for a Feodor Lynen fellowship.

References [1] Allan A. Mills. The lodestone: History, physics, and formation. Ann. Sci. 61, 273, 2004. [2] M. Bibes and A. Barthelemy. Oxide spintronics. IEEE Trans. Elec. Dev. 54, 1003, 2007. [3] M. I. Katsnelsonet, et al., Rev. Mod. Phys. 80, 315, 2008. [4] E. Wada, et al., Appl. Phys. Lett. 96,102510, 2010. [5] M. Monti, et al., Physical Review B, 85, 020404, 2012.

Linear and Circular Dichroism in threshold PEEM: Domain Imaging at Multiferroic Perovskites

Anke Höfer1, Mario Kiel1, Stefan Förster1, Cheng-Tien Chiang2,1, Wolf Widdra1,2

1Martin-Luther Universität Halle-Wittenberg, 06120 Halle, Germany 2Max-Planck-Institute for Microstructure Physics, 06120 Halle, Germany


The surface domain structures of ferroelectric perovskite surfaces can be mapped with high contrast by Laser-excited PEEM in photoemission threshold [1]. By this technique it has been demonstrated that the surface domain structure of ferroelectric BaTiO3(100) persists above the bulk critical temperatures [2]. Whereas these PEEM studies exploit the workfunction contrast between different ferroelectric domains, one additionally demands for investigation of multiferroic materials access to the ferromagnetic and/or antiferromagnetic domain structure. Here we present the domain imaging at multiferroic BiFeO3(001) surfaces by Laser-excited PEEM. BiFeO3 is one of the very rare single-phase magnetoelectric multiferroics and shows ferroelectric and antiferromagnetic behavior at room temperature with a complicated domain structure. By their different photoemission yields PEEM discriminates two different ferroelectric domain types of BiFeO3. Furthermore, the photoemission yield depends differently on the light polarization as shown in Fig. 1 for different domains. This is used for a linear-dichroism and circular-dichroism threshold imaging in contrast to the known magnetic XPEEM which requires synchrotron radiation. Based on workfunction contrast as well as on linear and circular dichroism we are able to discriminate 8 different multiferroic domains.

Figure 1. PEEM photoemission yield of two different BiFeO3(001) domains as function of the linear light polarization. The sum, the difference and the asymmetry of the photoemission yield are display from top to bottom. Right panel: Linear dichroism contrast of the BiFeO3(001) surface for a photon energy of 4.3 eV and 150 µm field-of-view. This work was supported by the Deutsche Forschungsgemeinschaft through the Sonderforschungs-bereich SFB-762 “Functional Oxidic Interfaces”. References [1] A. Höfer, K. Duncker, M. Kiel, and W. Widdra, IBM J. Res. Dev. 55, 4 (2011). [2] A. Höfer, M. Fechner, K. Duncker, M. Hölzer, I. Mertig, and W. Widdra, Phys. Rev. Lett.

108, 087602 (2012).

Full field electron spectromicroscopy of ferroelectrics

N. Barrett1, J. Rault1, D. Sando2, S. Fusil2,3, M. Bibes2,4, A. Barthélémy2,4, W. Ren5, S. Prosandeev5, L. Bellaiche5, S. Lisenkov6, A. Locatelli7, T.O. Mentes7, G. Geneste8

1CEA, DSM/IRAMIS/SPCSI, F- 91191 Gif sur Yvette cedex, France 2UMPhy CNRS/Thales, 1 Av. Augustin Fresnel, 91767 Palaiseau, France

3Université d’Evry-Val d’Essonne, Boulevard François Mitterrand, 91025 Evry cedex, France, 4Université Paris-Sud, 91405 Orsay, France

5Physics Department and Institute for Nanoscience and Engineering, University of Arkansas, Fayetteville, Arkansas 72701, USA

6Department of Physics, University of South Florida, 4202 East Fowler Avenue, Tampa, FL 33620-5700, USA

7Sincrotrone Trieste S.C.p.A., S.S. 14 Km 163.5, I-34149 Basovizza, Italy 8CEA, DAM, DIF, F-91297 Arpajon, France


We present two examples of ferroelectric domain order studied by full field electron spectromicroscopy. Standard electrical characterization of ultra-thin films is impossible due to the high leakage current in the tunnel regime. However, surface charge and hence FE polarization can be estimated from the electrostatic surface potential and the work function measured in low energy electron microscopy (LEEM) and energy filtered photoelectron emission microscopy (PEEM), respectively. There is a critical film thickness in BiFeO3 below which the film polarization drops to zero despite constant tetragonality. The results are interpreted using first principles based effective Hamiltonian calculations suggesting the formation of stripe domains in ultra-thin films. The second example presents a study of a micron scale region in BaTiO3(001). Spatially resolved reciprocal space images of the electron dispersion relations in the first Brillouin zone is obtained [3] and compared with first principles calculations, confirming the in-plane polarization. It is possible to switch the polarization direction using the electron beam of the LEEM. Until now, domain switching has been accomplished using electrical or chemical potentials, or mechanical or temperature gradients. Here we show reversible switching using e-beam induced charge. These examples demonstrate that the combination of real and reciprocal space imaging of ferroelectric surfaces using full field electron spectromicroscopy is a powerful new tool for unraveling the electronic structure underpinning ferroelectric order.

Figure 1. (left) work function map of oppositely polarized FE domains in a 70 nm BiFeO3 film. (center) in-plane electron beam induced domain switching in BaTiO3(001) showing the characteristic advance of needle like cones across 90° domain walls. (right) model of e-beam induced switching. ANR projects "Meloc" and "Nomilops". DoE contracts ER-46612, ONR Grants N00014-11-1-0384, N00014-07-1-0825 (DURIP) and N00014-08-1-0915, NSF grants DMR-1066158 and DMR-0701558, ARO Grant W911NF-12-1-0085, MRI grant 0722625. E. Jacquet, C. Carrétero and H. Béa for sample preparation, K. Winkler, B. Krömker, (both Omicron Nanotechnology), C. Mathieu, D. Martinotti for help with the PEEM/LEEM experiments and P. Jégou for XPS measurements.

Ferroelectrically induced valency change in multiferroic PbTiO3/La0.7Sr0.3MnO3 nanostructure arrays

Ingo P. Krug1, Ionela Vrejoiu2,3, Alessio Morelli2, Florian Nickel1, Daniel Gottlob1,

Hatice Doganay1, Nick Barrett4, Jiale Wang4, and C.M. Schneider1,5,6

1 Peter Grünberg Institut (PGI-6), Forschungszentrum Jülich, DE-52425 Jülich, Germany 2 Max Planck Institut für Mikrostrukturphysik, DE-06120 Halle, Germany 3 Max-Planck-Institut für Metallforschung, DE-70569 Stuttgart, Germany

4 CEA, DSM/IRAMIS/SPCSI/LENSIS, Bâtiment 462, F-91191, Gif-sur-Yvette cedex, France 5 JARA Jülich-Aachen Research Alliance, Forschungszentrum Jülich, DE-52425 Jülich

6 Fakultät für Physik and Center for Nanointegration Duisburg-Essen (CeNIDE), DE-47048 Duisburg, Germany


Multiferroic nanostructures could represent one of the roads towards a revolution in storage technology. The physical limits imposed on both the timescale and size of the storage elements have already been touched in some mainstream applications like magnetic recording and random access memory. One possibility to circumvent these restrictions is to go to emergent technologies unifying one or several information-carrying properties of the underlying systems, like spin- and charge-degrees of freedom [1]. In this way, multilevel logical elements become possible, boasting storage-capacity by possibly an order of magnitude. At the same time the enhanced control of the information state via cross-interactions between different degrees of freedom leads to new ways to manipulate the information state, possibly under maximum energy-efficiency. In this work, we investigated the possibility to use ferroelectric PbTiO3 (PTO) layers to manipulate the charge and spin-degrees of freedom in magnetic La0.7Sr0.3MnO3 nanostructures (LSMO). By X-PEEM we found evidence for a correlation between the valence state of the LSMO and the polarization state of the ferroelectric PTO. The interaction is mutual and can be used for electrical manipulation of the magnetization state of LSMO. a)

b)

Figure 1. a) sample structure: PLD-grown layer stack with Nb:STO substrate and dot array deposited by stencil-mask. A continuous PTO layer on top forms a self-organizing polarity-pattern with up and down polarized areas. b) PEEM and PFM contrast of the dot sample. The PTO polarization manifests in the Ti L-edge fine structure. Acknowledgements We thank O. Schaff and A. Kaiser for technical support concerning the LEEM-PEEM endstation at BESSY UE56-1 SGM. We acknowledge the help of S. Cramm and the PGI-6 technical staff.

References [1] Martin et al. Journal of Physics: Condensed Matter 20 (2008) 434220.

Quantum size effect driven allotropism of Bi films

Raoul van Gastel, Tjeerd R.J. Bollmann, Harold J.W. Zandvliet and Bene Poelsema

Physics of Interfaces and Nanomaterials MESA+ Institute for Nanotechnology, University of Twente,

P.O.Box 217, 7500AE Enschede, The Netherlands


We have investigated the initial growth of Bi on Ni(111) using Low Energy Electron Microscopy (LEEM) and Selected Area Low Energy Electron Diffraction (μLEED). Bismuth 1) has a tendency for allotropism, probably related to its low Young’s modulus, 2) it forms several ordered alloys with Ni and 3) with Bi being a neighbor of Pb in the periodic system, one may find evidence for quantum size effects in ultrathin Bi layers. Indeed, we obtain ample evidence that Bi/Ni(111) is feature rich system, even at a fixed substrate temperature of 474 K. We find first that the deposition of Bi leads to the formation of a surface alloy with a (√3x√3)-R30° structure at a Bi-coverage of 1/3. Continued Bi deposition leads to the formation of an incommensurate wetting layer with a continuously decreasing lattice parameter, finally ending in a (7x7) structure. From the variation of step positions at the buried interface, nicely accessible with LEEM, we conclude that the dealloying of the root3 phase is incomplete and that the (7x7) wetting layer in fact spans two layers with a small, but finite bismuth content in the lower layer. Upon further Bi deposition elongated, 3-4 layers high nanowires emerge, with a (5x2) structure and a width of 50-100 nm. Further deposition of Bi leads to a sequence of different structures: first (3x3)-patches develop with a thickness of three atomic layers, followed by patches with a (3 -1 1 2) matrix structure and a thickness of five atomic layers. This accurate height assignment is uniquely enabled by the analysis of LEEM-IV data. For Bi/Ni(111) the results are fully consistent with a quantum size effect driven thin film morphology: the different film structures and their thicknesses fit with integer numbers of nodes in the Fermi wave function, even for the seven layers thick (7x7) structure obtained at a lower temperature of 422 K. Tensor LEED calculations of the interlayer spacing of the different structures are consistent with this assignment. The influence of the structure and morphology on electronic properties of various materials is well known. The interaction between electronic and crystal structure should be reciprocal. Bi/Ni(111) provides a nice and we think first illustration: electronic properties, in particular quantum size effects (QSE’s), actually drive the structure of the thin metal films.

Low-energy Electron Reflectivity of Graphene on various Substrates

R. M. Feenstra1, N. Srivastava1, P. C. Mende1, M. Widom1, I. V. Vlassiouk2

1Dept. Physics, Carnegie Mellon University, Pittsburgh, PA, USA 15213 2Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN, USA 37831

Email: [email protected] We have developed a self-consistent description of low-energy electron reflectivity (LEER) spectra, yielding results that compare well with experimental data that we have obtained for graphene on SiC and on Cu substrates. Our approach utilizes the wavefunctions of a thin slab of material obtained from the Vienna Ab-initio Simulation Package (VASP), a well known parameter-free density-functional method for obtaining self-consistent electronic structure. For free-standing graphene we obtain the reflectivity curves shown in Fig. 1(a), for slabs consisting of 1 – 6 graphene layers. For an n-layer graphene slab, we find 1n minima in the spectra. This result is consistent with experimental data for graphene on various surfaces, so long as the graphene layer closest to (i.e. bonded to) the substrate is included in the n-layer count. Our result is, however, inconsistent with the prior interpretation of these curves put forward by Hibino et al. [1]. Those workers proposed a tight-binding model in which, for n-layers of graphene, the LEER spectra would contain n-minima, with each minimum corresponding to a particular standing wave having peaks localized on the graphene layers. We find, rather, that the wavefunctions for the relevant scattering states are localized in between the graphene layers, not on them. Prior computations employing a conventional multi-scattering formalism were also found to be inadequate to explain the graphene LEER spectra [2], presumably because such computations do not provide a fully self-consistent description of the electronic structure. We have also made explicit computations for graphene on substrates, with the electronic structure of the substrate also described by VASP and the wavefunctions for the graphene on the substrate then matched to those of the bulk material. Results are shown in Fig. 1(b) for graphene on Cu(111), compared to experimental results in Fig. 1(c). Here again, we find, for n-layers of graphene that 1n distinct minima are found in the spectra, although an additional, shallow minimum is seen for single- and bilayer graphene at about 3.5 eV, associated with the space between the graphene and the Cu substrate. Our method can predict LEER spectra for thin layers of any material on any substrate, so long wavefunctions of the system are available. Additional examples will be provided for graphene on other metal substrates and for stacking faults associated with graphene on SiC.

Figure 1. (a) and (b) Theoretical LEER spectra for (a) free-standing graphene and (b) graphene on Cu. The n values refer to the number of graphene layers. Simulations with various vacuum widths are shown by different colored symbols. (c) Experimental results for graphene on Cu, shifted down in energy by 1.5 eV to match the theory.

References [1] H. Hibino et al., Phys. Rev. B 77, 075413 (2008). [2] H. Hibino et al., Phys. Rev. B 80, 085406 (2009).

Recent Progress in Spintronics Materials

Fumihiro Matsukura1,2

1WPI Advanced Institute for Materials Research, Tohoku University 2Center for Spintronics Integrated Systems, Tohoku University, Sendai 980-8577, Japan


Ferromagnetic semiconductors, such as (Ga,Mn)As and (In,Mn)As, shows a carrier-induced ferromagnetism, and thus their magnetic properties are carrier-concentration dependent [1]. These materials offer a number of opportunities to observe novel spin-related phenomena and to demonstrate new schemes of device operation [2]. I will discuss about the control of magnetism of (Ga,Mn)As by the application of electric field E that changes the hole concentration p. Particular focus will be given on E control of the Curie temperature TC and magnetic anisotropy. The p dependence of TC and magnetic moments can be described by the adapted p-d Zener model taking into account the nonuniform distribution of holes [3, 4]. The E dependent magnetic anisotropy is expected to provide a new scheme of magnetization direction control [5, 6]. Therefore, the studies on (Ga,Mn)As triggered extensive studies on E effects in a variety of ferromagnetic materials. The recent topics on E effects on ferromagnetic metals, such as CoFeB, will be also presented [7-9]. If time allows I will touch upon the ferromagnetism of transition-metal doped wide gap materials, which may not be related to carrier-induced mechanism [10]. I acknowledge fruitful collaboration with H. Ohno and T. Dietl. The work was supported in part by the FIRST program of JSPS. References [1] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, Science 287, 1019 (2000). [2] F. Matsukura, D. Chiba, and H. Ohno, Semiconductors and Semimetals 82, 207 (2008). [3] M. Sawicki, D. Chiba, A. Korbecka, Y. Nishitani, J. A. Majewski, F. Matsukura, T. Dietl,

and H. Ohno, Nature Phys. 6, 22 (2010). [4] Y. Nishitani, D. Chiba, M. Endo, M. Sawicki, F. Matsukura, T. Dietl, and H. Ohno, Phys.

Rev. B 81, 045208 (2010). [5] D. Chiba, M. Sawicki, Y. Nishitani, Y. Nakatani, F. Matsukura, and H. Ohno, Nature 455,

515 (2008). [6] D. Chiba, Y. Nakatani, F. Matsukura, and H. Ohno, Appl. Phys. Lett. 96, 192506 (2010). [7] T. Maruyama, Y. Shiota, T. Nozaki, K. Ohta, N. Toda, M. Mizuguchi, A. A. Tulapurlar, T.

Shinjo, M. Shiraishi, S. Mizukami, Y. Ando, and Y. Suzuki, Nature Nanotech. 4, 158 (2009). [8] M. Endo, S. Kanai, S. Ikeda, F. Matsukura, and H. Ohno, Appl. Phys. Lett. 96, 212503

(2010). [9] Y. Shiota, T. Nozaki, F. Bonell, S. Murakami, T. Shinjo, and Y. Suzuki, Nature Mater. 11,

39 (2012). [10] L. Li, Y. Guo, X. Y. Cui, R. Zheng, K. Ohtani, C. Kong, A. V. Ceguerra, M. P. Moody, J. D.

Ye, H. H. Tan, C. Jagadish, H. Liu, C. Stampfl, H. Ohno, S. P. Ringer, and F. Matsukura, Phys. Rev. B 85, 174430 (2012).

Magnetic spin reorientation transition in graphene covered Cobalt on Iridium(111)

Alpha T. N'Diaye1, Johann Coraux2, Nicolas Rougemaille2,

Chi Võ Văn2, Andreas K. Schmid1

1National Center for Electron Microscopy, 1 Cyclotron Rd MS-72-150, Berkeley, Ca-94720, USA 2Institut NÉEL, CNRS & Université Joseph Fourier,

BP166, 25 rue des Martyrs, Grenoble Cedex 9, 38042 France


One of graphene's promises is to be relevant for spintronic applications. While the influence of a magnet on graphene is under intense investigation by many groups less attention is given to the influence of graphene on a magnet. Graphene on Cobalt on Ir(111) can be prepared in a three step process: First, defect rich graphene is grown by the pyrolytic decomposition of ethylene (C2H4) on Ir(111) at 600°C. Second, Cobalt is deposited at room temperature and as a third step it is intercalated by annealing to 300°C. With spin polarized low energy electron microscopy (SPLEEM) we studied the thickness dependent spin reorientation transition on this system and compare with Co/Ir(111) without graphene. Monitoring the spin orientation in three dimensions while increasing the film thickness by one ML at a time, we find that the presence of graphene on the film at least doubles the thickness at which the spin reorientation from out-of-plane to in-plane occurs from 6ML Co to transition to 12ML-13ML at 300°C and to between 15ML and 20ML at room temperature. Figure 1. SPLEEM asymmetry images of graphene/Co/Ir(111), FOV 14µm. Domains are exclusively magnetized out of plane for 8LM and Co thickness, for 16ML a faint in plane contribution is measurable and a 20 ML thick film the magnetization is almost exclusively magnetized out of plane. Also note the varying domain sizes for in plane and out of plane domains. Acknowledgement This work was supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, by the French ANR contract ANR-2010-BLAN-1019-NMGEM and by the Alexander von Humboldt Foundation.

LEEM Image Phase Contrast Calculations of MnAs Stripes

A. B. Pang1, 2, A. Pavlovska3, L. Daeweritz4, A. Locatelli5, E. Bauer3, M. S. Altman2

1School of Physics and Electronic Information, Huaibei Normal University, Huaibei, Anhui, 235000, PR China

2Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, PR China

3Department of Physics, Arizona State University, Tempe, Arizona 85287-1504, USA 4Paul-Drude-Institut für Festkörperelektronik,

Hausvogteiplatz 5-7, D-10117 Berlin, Germany 5Sincrotrone Trieste,S.C.p.a., Basovizza, Trieste 34012 Italy


The Fourier optics approach that was developed to evaluate the image formation in LEEM has been adapted to explore the contrast in LEEM image of MnAs stripes [1], as well as its evolution as a function of positive and negative defocus. MnAs ridge-groove structure being responsible for these stripes is recognized to have a height difference. The intensity profiles extracted from the LEEM images of three periods of stripes as a function of position for minus defocus are shown in Fig.1a. Between each pair of adjacent stripes there is a strong bright dark contrast with a weaker contrast on top of it that looks like fringes. The contrast which describes the relative intensity difference between bright and dark areas drops continuously as the absolute value of the defocus decrease, for both positive and minus defocus. In order to examine the source of the duplex contrast that appears on this periodically varying surface of one dimensional MnAs ridge-groove structure, the Fourier optics calculation was employed by treating that structure as a standard phase object with the phase varying correspondingly. The intensity distribution across the ridge-groove under minus defocus condition exported from the theoretical calculation is thereby shown in Fig.1b, where the smearing effect of the detector has been considered properly [2], in addition to the previous Fourier optics model. By comparing these two figures, we can directly see that the main features from the experimental observations were reproduced nicely. 400nm (b)(a) Figure 1. The intensity profiles (in arbitrary unit) across three periods of MnAs ridge-groove structure with different value of minus defocus, (a) extracted from the experimental images as shown on top, and (b) exported from the model calculation, respectively. Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 11104098), and by the Anhui Provincial Foundation of Higher Education Institutions (No. 2012SQRL080 and No. KJ2012B167). References [1] A. B. Pang, Th. Müller, M. S. Altman, E. Bauer, Journal of Physics: Condensed Matter 21

(2009) 314006. [2] S. M. Schramm, A. B. Pang, M. S. Altman, R. M. Tromp, Ultramicroscopy 115 (2012) 88.

Nanoscale redox chemistry of cerium oxide islands grown on Ru(0001) J. I. Flege1, B. Kaemena1, S. D. Senanayake2, A. Meyer1, J. Sadowski3, E. E. Krasovskii4,

J. Falta1

1Institute of Solid State Physics, University of Bremen, Bremen, Germany 2Department of Chemistry, Brookhaven National Laboratory, Brookhaven (NY), USA

3Center for Functional Nanomaterials, Brookhaven National Laboratory, Brookhaven (NY), USA 4Donostia International Physics Center, San Sebastian, Spain


Cerium oxide is an enormously versatile material system and has attracted tremendous interest due to its wide range of already existing and potential applications in, e.g., catalysis [1], energy harvesting and storage, sensing, and microelectronics. Its inherent structural variability combined with its electronic complexity arises from the unfilled shell of highly-localized 4f electrons in the valence band, making it a particularly attractive model system in fundamental studies of electron correlation and chemical bonding in binary metal oxides. Here, we characterize the growth of cerium oxide on Ru(0001) (Fig. 1), its temperature and pressure dependence as well as its oxidation state using a combination of intensity-voltage (I(V)) LEEM, LEED, resonant photoemission spectroscopy (RPES), and ab initio scattering theory [2]. We demonstrate that major differences for fully-oxidized and reduced ceria, as confirmed by quantitative RPES, in the k||=0 projected bandstructure, which determines the electron reflectivity, are not due to the Ce-4f states, but the Ce-5d states. Furthermore, we illustrate these results by performing in-situ LEEM during methanol oxidation on nanoscale cerium oxide islands grown on Ru(0001). Using distinct I(V) fingerprints for Ce3+ and Ce4+ oxidation states, we are able to image and identify local reduction of the cerium oxide islands upon methanol decomposition and subsequent thermal annealing.

Figure 1. Temperature dependent island density and shape of cerium oxide islands on Ru(0001). Acknowledgement Research carried out in part at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U. S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. The authors acknowledge partial support from the Spanish Ministerio de Ciencia e Innovación (Grant No. FIS2010-19609-C02-02). References [1] A. Trovarelli, ed., Catalysis by Ceria Related Materials (Imperial College Press, London,

2001). [2] J. I. Flege, A. Meyer, J. Falta, and E. E. Krasovskii, Phys. Rev. B 84, 115441 (2011).

Si nanoparticles motion onto SiO2 driven by a chemical reaction : a real time study

F. Leroy, F. Cheynis, Y. Saito, E. Bussmann, T. Passanante, P. Müller

CINaM-CNRS, UMR 7325,Campus de Luminy Case 913, 13288 Marseille cedex 9, France

corresponding author: F. Leroy, e-mail:[email protected]

In the literature, recent examples of spontaneous motion of droplets have been provided, especially for liquid droplets on solid surfaces [1-6]. In particular it has been shown that a chemical reaction between a nanoparticle and its underlying substrate can modify locally the interface free energy. The induced asymmetry of the solid/liquid contact can generate a self-propulsion [2]. From a fundamental respect, the motion of a reactive triple-line and its interaction with a time-evolving substrate surface is of high interest.

Here we report on the motion of solid state 3D Si nanoparticles onto SiO2 substrate. The Si nanoparticles are obtained by dewetting of a thin Si film (20nm thick) onto an amorphous SiO2 layer [7,8] when annealed at high temperature (T> 750°C). Increasing the temperature in the range 950-1100°C we have measured in situ and in real time, by Low Energy Electron Microscopy (LEEM), the motion of Si nanoparticles. This process is concomitant with the chemical reaction Si(3D) + SiO2(substrate) → 2 SiO(g) resulting in a progressive shrinking of the nanoparticles [9] and consumption of the SiO2 substrate. Following the centre of mass of each nanoparticle we put in evidence that the motion in the small timescale limit is random whereas at late time nanoparticles get trapped inside the hole formed in the SiO2 substrate induced the chemical reaction. At the very end the Si nanoparticles have completely disappeared giving rise to a conic hole into the SiO2 layer [9].

From the kinetics of the Si nanoparticles motion an effective diffusion coefficient Deff could be extracted as function of temperature and nanoparticle size. The first stage of Brownian motion is thermally activated with an activation energy Ea=4.1 eV. A clear size-dependent behaviour is also put in evidence Deff ~R-1/2. The shrinking of the nanoparticules is analysed assuming a chemical reaction rate occurring at the interface with gas expelled at the triple line. These experimental results are highlighted by Monte Carlo simulations based on a solid on solid model including surface diffusion processes and chemical reaction kinetics. We acknowledge the support of ANR PNANO for funding (ANR 08-Nano-036). References [1] J. Tersoff et al., Science 324, 236 (2009). [2] A. K. Schmid, N. C. Bartelt, R. Q. Hwang, Science 290, 1561 (2000). [3] C. D. Bain, G. D. Burnett-Hall, R. R. Montgomerie, Nature 372, 414 (1994). [4] F. D. Dos Santos, T. Ondarçuhu, Phys. Rev. Lett. 75, 2972 (1995). [5] Y. Sumino, N. Magome, T. Hamada, K. Yoshikawa, Phys. Rev. Lett. 94, 068301 (2005). [6] K. Ichimura, S.-K. Oh, M. Nakagawa, Science 288, 1624 (2000). [7] E. Bussmann, et al., New Journal of Physics 13, 043017 (2011). [8] F. Cheynis et al., Phys. Rev. B 84, 245439 (2011). [9] K. Sudoh et al., J. Appl. Phys. 108, 083520 (2010).

Tuning velocity of Ga droplet by Arsenic flux

Changxi Zheng1, Wen-Xin Tang1, Zhenyu Zhou1, and David E. Jesson1

1School of Physics, Monash University, Australia Victoria 3800


Spontaneous droplet motion on surface has been intensively studied over the years due both to its scientific and technological importance. Recently, an intriguing self-motion of Ga droplet is found on GaAs(001) surface during Langmuir evaporation, in which the surface is heated in a vacuum and evaporates by decomposing into Ga and As [1]. Central to understanding the running Ga droplets is the congruent evaporation temperature , near this characteristic temperature, with the

droplet motion speed increasing both below and above [1]. cT

cT Here we show the ability to tune the velocity of Ga droplet motion by using Arsenic flux. When the surface is exposed to external As flux, we have demonstrated that one can directly control

by varying As deposition flux [2]. More interestingly, we found, by fixing As deposition flux, the parabolic shape Ga droplet motion velocity-temperature curve shift to the new (Fig. 1). The

result provides further information toward the understanding of driven force for the droplet motion and open up new way to tune the droplet motion.

cT

cT

Figure 1. Average velocities of Ga droplet as a function of temperature with and without As flux, respectively (Arsenic flux is 4.8×10−6 Torr). References [1] J. Tersoff, D. E. Jesson, and W. X. Tang, Science 324 (2009) 236. [2] Z. Y. Zhou, C. X. Zheng, W. X. Tang, D. E. Jesson, and J. Tersoff, Appl. Phys. Lett. 97

(2010) 121912.

Silicon lattice gas on Si(111) (1x1)

S. M. Schramm1, J. B. Hannon2, S. J. van der Molen1, R. M. Tromp1,2

1Leiden University, Kamerlingh Onnes Laboratorium, P.O. Box 9504, NL-2300 RA Leiden, The Netherlands

2IBM T.J. Watson Research Center, 1101 Kitchawan Road, P.O. Box 218, Yorktown Heights, NY 10598, USA

Email: [email protected], [email protected]

We present a low energy electron diffraction (LEED) study of Si(111) performed with a low energy electron microscopy (LEEM) instrument. It is well established that Si(111) shows a phase transition of its surface reconstruction from (7x7) to ‘(1x1)’ at a temperature of about TC≈1100 K [1]. We find that above TC, the LEED pattern of clean Si(111) show clear but weak and broad √3 LEED spots (see Fig. 1a). The spots weaken and broaden with increasing sample temperature in the range of about 1130 K – 1400 K as illustrated by line profiles of the experimental data in Fig. 1b. The profiles are taken along lines similar to the yellow dashed line in Fig. 1a. In dedicated LEED instruments the weak √3 spots are buried in the inelastic background. The beam separator in LEEM, however, disperses the inelastic electrons and the √3 signal becomes detectable. These √3 spots are believed to be caused by a lattice gas of Si atoms on top of the Si(111) surface. The lattice gas concentration decreases with increasing sample temperature. We use Monte Carlo type simulations to model the lattice gas on a Si(111) surface. Theoretical LEED patterns of different lattice gas concentrations are obtained from the Monte Carlo simulations and are plotted in Fig. 1c in units of a perfect ‘(1x1)’ monolayer (ML). The simulations suggest that the lattice gas atoms are located at energetically equivalent sites of one type - presumably T4 sites – while next-nearest neighbors are excluded. Comparison of simulated and experimental LEED data yields insight into lattice gas energetics.

Figure 1. (a) Low energy electron diffraction pattern of Si(111) just above TC, colored for clarity. The (0,0) spot is visible at the bottom. Two distinct but weak and broad spots are visible at the center surrounded by bright integer spots. Profile plots of experimental data taken along the yellow dashed line in (a) are shown in (b) for sample temperatures from 1169 – 1407 K. (c) Theoretical LEED pattern of different lattice gas concentrations in units of a perfect ‘(1x1)’ monolayer (ML) obtained from Monte Carlo type simulations. References [1] N. Osakabe, Y. Tanishiro, K. Yagi, G. Honjo, Surface Science 97 (1980) 393.



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