Differentielle Phasenkontrastmikroskopie - Entwicklung und Charakterisierung eines Detektors zur direkten Bestimmung der Änderung des lateralen Elektronenimpulses

Die Differentielle Phasenkontrastmikroskopie (DPC) ist eine Messtechnik der Rastertransmissionselektronenmikroskopie, welche es erlaubt magnetische und elektrische Feldverteilungen im Inneren von Proben zu untersuchen und abzubilden. Seit der Entwicklung von sondenkorrigierten Rastertransmissionselektronenmikroskopen mit Auflösungsvermögen im Sub-Ångström-Bereich, wird DPC auch zur Analyse atomarer elektrostatischer Feldverteilungen und Ladungsträgerdichten verwendet. Im Unterschied zu konventionellen DPC-Messungen (cDPC) bei denen üblicherweise Felder gemessen werden, welche als (nahezu) konstant über den Durchmesser der Elektronensonde angenommen werden können, ist dies bei Messungen mit atomarer lateraler Auflösung nicht mehr der Fall. Es ergibt sich eine komplexe Wechselwirkung zwischen den stark inhomogenen Feldverteilungen der Kernpotentiale und der Elektronensonde. Diese resultiert in einer Umverteilung von Intensität innerhalb des Beugungsscheibchens, welche nicht oder nur eingeschränkt mit einem DPC-Ringdetektor vermessen werden kann. In der aktuellen Forschung werden deshalb schnelle CCD- oder CMOS-Kamerasysteme verwendet, um die Verlagerungen des Intensitätsschwerpunktes (COM) im Inneren der Beugungsscheibchen zu detektieren. Den so gemessenen COM-Positionen können mit der Theorie des impulsaufgelösten Phasenkontrasts (morePC) von K. Müller-Caspary und F. F. Krause absolute elektrische Feldstärken und Ladungsträgerdichten zugeordnet werden. Das Ziel dieser Arbeit war es, einen neuartigen Detektor zu entwickeln, welcher sowohl die Anfertigung von quantitativen cDPC-, als auch moreSTEM-Messungen erlaubt. Damit einher geht die Anforderung, dass der Detektor dazu in der Lage sein muss, die absolute COM-Position des Beugungsscheibchens innerhalb weniger Mikrosekunden zu detektieren. Das neue Detektorsystem soll die Anfertigung von DPC-Messungen im gesamten nutzbaren Spektrum aller in einem STEM verfügbaren Vergrößerungen ermöglichen. Im besten Fall sollen damit beispielsweise sowohl quantitative Messungen an ausgedehnten magnetischen Domänen (im Mikrometerbereich), als auch Messungen atomarer elektrischer Feldverteilungen angefertigt werden können. Der neue Detektor soll also die Schnelligkeit eines cDPC-Systems mit der Eigenschaft der COM-Messung von modernen Kamerasystemen kombinieren. Das PSD-Setup basiert auf einer duolateralen positionsempfindlichen Diode (PSD), welche die hohen erreichbaren Messgeschwindigkeiten (~200 kHz) von pin-Dioden mit der Eigenschaft einer absoluten Positionsmessung des COMs des Beugungsscheibchens kombinieren. Das im Zuge dieser Arbeit entwickelte PSD-Setup ermöglicht es (unter Standardbedingungen) die Ablenkung des Elektronenstrahls durch die Probe mit einer Winkelauflösung von circa 0,44 µrad zu detektieren. In dieser Arbeit wird der Aufbau und die Signalentstehung einer duo-lateralen positionsempfindlichen Diode sowie deren Eignung als COM-Detektor in der Elektronenmikroskopie ausführlich diskutiert. Mittels Simulationsrechnungen wurden die Einflüsse von Strahlgröße und -intensität (in der Detektorebene) auf die Positionsempfindlichkeit einer PSD untersucht. Zudem wird ein auf diesen Simulationen basierendes analytisches Modell vorgestellt, welches die Abhängigkeiten des Fehlers der Positionsmessung mit einer PSD beschreibt. Im Anschluss an die theoretischen Untersuchungen wird der – in dieser Arbeit entwickelte – experimentelle Aufbau des PSD-Detektorsystems beschrieben und die Ergebnisse einer umfassenden experimentellen Charakterisierung präsentiert. So wurden beispielsweise die Winkel- beziehungsweise die Feldauflösung und die Linearität des Detektors experimentell bestimmt. Den Höhepunkt dieser Arbeit stellen die ersten Testmessungen an realen Proben dar. Die PSD wurde einerseits zur Untersuchung einer magnetischen Domänenwand in einer polykristallinen Cobalt-Dünnfilm-Probe im LM-STEM-Betrieb, und andererseits zur Abbildung der atomaren Feldverteilung eines Strontiumtitanat-Kristalls im HM-STEM-Betrieb verwendet. Zur Einordnung der Leistungsfähigkeit des neuen Detektors wurden vergleichende Messungen mit einem konventionellen Ringdetektor und mit einem modernen Kamerasystem angefertigt. Abschließend bleibt zu sagen, dass das Ziel dieser Arbeit, einen schnellen Detektor zur Messung des lateralen Elektronenimpulses zu entwickeln, erreicht wurde. Das im Zuge dieser Arbeit entstandene PSD-Detektorsystem hat ein großes Potential, um zukünftig als COM-Detektor in der Transmissionselektronenmikroskopie eingesetzt zu werden

The differential phase contrast uncertainty relation: Connection between electron dose and field resolution

Differential phase contrast (DPC) microscopy is a STEM imaging technique, which is used to measure magnetic and electric fields of mesoscopic and nanoscopic dimensions, i.e. interatomic distances (Chapman et al. 1978; Chapman et al. 1981; Chapman, 1984; Chapman et al. 1985; Chapman et al. 1997; Lohr et al. 2012; Shibata et al. 2015; Bauer et al. 2014; Carvalho et al. 2016; Lohr et al. 2016; Mueller-Caspary et al. 2019a,2019b; Mueller-Caspary et al. 2018; Mueller-Caspary et al. 2017; Mueller-Caspary et al. 2014; Winkler et al. 2020; Toyama et al. 2020). In this paper we will demonstrate that the electron dose per pixel deposited on the specimen is decisive to the precision and resolution of measurements of a field's local strength. Relations are given which connect a given electron dose per pixel to the fundamentally achievable precision to which the specimen's interaction with the electrons may be determined, taking into account quantum mechanical considerations. Vice versa, given a certain required precision, the required dose per pixel can be easily predicted for reliable measurements of a desired property. First, these relations are given for the case of a continuous, i.e. non-pixelated, detector followed by simulations which show that the same relations hold for pixelated detectors. Then, the achievable precision for detectors with different pixel counts in combination with different camera lengths is discussed and the maximum measurable field amplitude per set-up is determined. Finally, the effect of inhomogeneities within the diffraction disk is discussed and possible deviations from the derived relations are considered. We also demonstrate that Heisenberg's uncertainty relation determines the possible field resolution in differential phase contrast microscopy, and that the achievable local field resolution is a function of the applied electron dose per pixel

On the achievable field sensitivity of a segmented annular detector for differential phase contrast measurements

Differential phase contrast microscopy measures minute deflections of the electron probe due to electric and/or magnetic fields, using a position sensitive device. Although recently, pixelated detectors have become available which.also serve as a position sensitive device, the most frequently used detector is a four-segmented annular semiconducting detector ring (or variations thereof), where the difference signals of opposing detector elements represent the components of the deflection vector. This deflection vector can be used directly to quantitatively determine the deflecting field, provided the specimen's thickness is known. While there exist many measurements of both electric and magnetic fields, even at an atomic level, until now the question of the smallest clearly resolvable field value for this detector has not yet been answered. This paper treats the problem theoretically first, leading to a calibration factor K which depends solely on simple, experimentally accessible parameters and relates the deflecting field to the measured deflection vector. In a second step, the calibration factor for our combination of microscope and detector is determined experimentally for various combinations of camera length, condenser aperture and spot size to determine the optimum setup. From this optimized condition we determine the minimum change in field which leads to a clearly measurable signal change for both HMSTEM and LMSTEM operation. A strategy is described which allows the experimenter to choose the setup giving the highest field sensitivity. Quantification problems due to scattering processes in the specimen are addressed and ways are shown to choose a setup which is less sensitive to these artefacts. (C) 2017 Elsevier B.V. All rights reserved

On detector linearity and precision of beam shift detection for quantitative differential phase contrast applications

Differential phase contrast is a STEM imaging mode where minute sideways deflections of the electron probe are monitored, usually by using a position sensitive device (Chapman, 1984 [1]; Lohr et al., 2012 [2]) or, alternatively in some cases, a fast camera (Muller et al., 2012 [3,4]; Yang et al., 2015 [5]; Pennycook et al., 2015 [6]) as a pixelated detector. While traditionally differential phase contrast electron microscopy was mainly focused on investigations of micro-magnetic domain structures and their specific features, such as domain wall widths, etc. (Chapman, 1984 [1]; Chapman et al., 1978, 1981, 1985 [7-9]; Sannomiya et al., 2004 [10]), its usage has recently been extended to mesoscopic (Lohr et al., 2012, 2016 [2,12]; Bauer et al., 2014 1111; Shibata et al., 2015 [13]) and nano-scale electric fields (Shibata et al., 2012 [14]; Mueller et al., 2014 [15]). In this paper, the various interactions which can cause a beam deflection are reviewed and expanded by two so far undiscussed mechanisms which may be important for biological applications. As differential phase contrast microscopy strongly depends on the ability to detect minute beam deflections we first treat the linearity problem for an annular four quadrant detector and then determine the factors which limit the minimum measurable deflection angle, such as S/N ratio, current density, dwell time and detector geometry. Knowing these factors enables the experimenter to optimize the set-up for optimum performance of the microscope and to get a clear figure for the achievable field resolution error margins. (C) 2016 Elsevier B.V. All rights reserved

Introducing a non-pixelated and fast centre of mass detector for differential phase contrast microscopy

With the advent of probe corrected STEM machines it became possible to probe specimens on a scale of less than 50 pm resolution. This opens completely new horizons for research, as it is e.g. possible to probe the electrostatic fields between individual rows of atoms, using differential phase contrast (DPC). However, in contrast to conventional DPC, where one deals with extended fields which can be assumed constant across the electron probe, this is not possible for sub-atomic probes in DPC. For the latter case it was shown [1,2], that the strongly inhomogeneous field distribution within the probe diameter, which usually is caused by the nuclear potentials of an atomic column, leads to a complicated intensity redistribution within the diffraction disk. The task is then to determine the intensity weighted centre of the diffraction disk pattern (frequently also called centre of mass, COM), which is proportional to the average lateral momentum gained by the average electron, transmitted through the probe diameter. In first reported measurements, the determination of this COM was achieved using a pixelated detector in combination with a software-based evaluation of the COM. This suffers from two disadvantages: first, the nowadays available pixelated detectors are still not very fast (approximately 1000 fps) and quite expensive, and second, the amount of data to be processed after acquisition is comparatively huge. In this paper we report on an alternative to a pixelated detector, which is able to directly deliver the COM of a diffraction disk's intensity distribution with frequencies up to 200 kHz. We present measurements on the sensitivity of this detector as well as first results from DPC imaging. From these results we expect the detector also to serve well in sub-atomic DPC field sensing, possibly replacing today's segmented or pixelated detectors. (C) 2018 Elsevier B.V. All rights reserved

Olefin-Stabilized Cobalt Nanoparticles for C=C, C=O, and C=N Hydrogenations

The development of cobalt catalysts that combine easy accessibility and high selectivity constitutes a promising approach to the replacement of noble-metal catalysts in hydrogenation reactions. This report introduces a user-friendly protocol that avoids complex ligands, hazardous reductants, special reaction conditions, and the formation of highly unstable pre-catalysts. Reduction of CoBr2 with LiEt3BH in the presence of alkenes led to the formation of hydrogenation catalysts that effected clean conversions of alkenes, carbonyls, imines, and heteroarenes at mild conditions (3 mol% cat., 2-10 bar H-2, 20-80 degrees C). Poisoning studies and nanoparticle characterization by TEM, EDX, and DLS supported the notion of a heterotopic catalysis mechanism

Influence of combinatory effects of STEM setups on the sensitivity of differential phase contrast imaging

Differential phase-contrast (DPC) imaging in the scanning transmission electron microscopy (STEM) mode has been suggested as a new method to visualize the nanoscale electromagnetic features of materials. However, the quality of the DPC image is very sensitive to the electron-beam alignment, microscope setup, and specimen conditions. Unlike normal STEM imaging, the microscope setup variables in the DPC mode are not independent; rather, they are correlated factors decisive for field sensitivity. Here, we systematically investigated the independent and combinatory effects of microscope setups on the sensitivity of the DPC image in a hard magnet, Nd2Fe14B alloy. To improve sensitivity, a smaller overlap of the electron beam with annular detectors and a greater camera length were required. However, these factors cannot be controlled independently in the two-condenser-lens system. In this linked system, the effect of the camera length on the DPC sensitivity was slightly more predominant than the overlap. Furthermore, the DPC signal was noisy and scattered at a small overlap of less than 11%. The electron-beam current does not evidently affect the sensitivity. In addition, the DPC sensitivity was examined with respect to the sample thickness, and the optimum thickness for high sensitivity was approximately 65 nm for the hard magnetic material Nd2Fe14B. This practical approach to the STEM setup and sample thickness may provide experimental guidelines for further application of the DPC analysis method

Olefin-Stabilized Cobalt Nanoparticles for C=C, C=O, and C=N Hydrogenations

The development of cobalt catalysts that combine easy accessibility and high selectivity constitutes a promising approach to the replacement of noble-metal catalysts in hydrogenation reactions. This report introduces a user-friendly protocol that avoids complex ligands, hazardous reductants, special reaction conditions, and the formation of highly unstable pre-catalysts. Reduction of CoBr2 with LiEt3BH in the presence of alkenes led to the formation of hydrogenation catalysts that effected clean conversions of alkenes, carbonyls, imines, and heteroarenes at mild conditions (3 mol% cat., 2–10 bar H2, 20– 808C). Poisoning studies and nanoparticle characterization by TEM, EDX, and DLS supported the notion of a heterotopic catalysis mechanism

Stereoselective Alkyne Hydrogenation by using a Simple Iron Catalyst

The stereoselective hydrogenation of alkynes constitutes one of the key approaches for the construction of stereodefined alkenes. The majority of conventional methods utilize noble and toxic metal catalysts. This study concerns a simple catalyst comprised of the commercial chemicals iron(II) acetylacetonate and diisobutylaluminum hydride, which enables the Z-selective semihydrogenation of alkynes under near ambient conditions (1-3 bar H-2, 30 degrees C, 5 mol % [Fe]). Neither an elaborate catalyst preparation nor addition of ligands is required. Mechanistic studies (kinetic poisoning, X-ray absorption spectroscopy, TEM) strongly indicate the operation of small iron clusters and particle catalysts