Scanning probe microscopy studies of the deposition of molecular magnets onto surfaces

Die vorliegende Arbeit befasst sich mit der Deposition molekularer Magnete auf Oberflächen. Neben der Präsentation und Diskussion eigener experimenteller Ergebnisse, die mittels Rastertunnelmikroskopie im Konstantstrommodus und dynamischer Rasterkraftmikroskopie im frequenzmodulierten Modus und im TappingModeTM gewonnen worden sind, enthält die Arbeit unter anderem auch ein Kapitel, in welchem andere als die für die Probenpräparation gewählten Möglichkeiten zur Deposition molekularer Magnete auf Oberflächen zusammengestellt sind. Da diese Präparationsalternativen für das Aufbringen der molekularen Magnete, die für die Experimente zur Verfügung gestanden hatten, nicht anwendbar waren, und da man auf die Verwendung thermischer Depositionstechniken, wie zum Beispiel der Molekularstrahldeposition (MBD), verzichten musste, weil diese Verfahren die aufzubringenden Moleküle zerstört hätten, wurden die Moleküle, deren Adsorption auf Oberflächen man herbeiführen wollte, in geeigneten Lösungsmitteln gelöst und aus Lösung auf die ausgewählten Substrate aufgebracht. Für die Deposition von Mo72Fe30 auf HOPG(0001) bot es sich an, die molekularen Magnete in Milli-Q-Wasser zu lösen und die Präparationslösung unter Umgebungsbedingung auf das Substrat zu tropfen. Für unterschiedliche Konzentrationen der Lösungen konnten nach dem Eintrocknen des Lösungsmittels interessante Modifikationen der Graphitoberfläche beobachtet werden, die im Falle niedriger Konzentrationen anhand eines auf geometrischen Betrachtungen von Molekül und Oberfläche beruhenden Erklärungsmodells verstanden werden können. Die Deposition von Galvinoxyl auf den Isolatoroberflächen KBr(100) und CaF2(111) erfolgte mittels eines Magnetventils, welches im Pulsbetrieb geöffnet und geschlossen werden kann, im Ultrahochvakuum (UHV). Das verwendete Lösungsmittel war Ethanol. Weil dieser Alkohol Kaliumbromidoberflächen, wie in der vorliegenden Arbeit ausführlich beschrieben, nachhaltig schädigt, musste die Interpretation der Ergebnisse zur Pulsinjektion von Galvinoxyl auf KBr(100) unter Berücksichtigung des ethanolinduzierten Lösungsätzens der Alkalihalogenidoberfläche erfolgen. Demnach stellen die durch das Lösungsmittel hervorgerufenen monoatomar tiefen Ätzlöcher in der Oberfläche bevorzugte Adsorptionsplätze für die molekularen Magnete dar, weswegen sich nur im Bereich dieser Vertiefungen und an Stufenkanten des Substrats Galvinoxylmoleküle rastersondenmikroskopisch nachweisen ließen. This thesis deals with the deposition of molecular magnets onto surfaces. Experimental results obtained by means of scanning tunneling microscopy in constant current mode and dynamic force microscopy in frequency modulation mode and TappingModeTM are presented and discussed. In addition, a review of sample preparation techniques different from those which have been used for our own experiments is provided. As those alternatives could not be used for the deposition of the molecular magnets which were at our disposal for the experiments, and as thermal deposition techniques, e. g. molecular beam deposition (MBD), were not applicable because such methods would have caused molecular decomposition, the molecules to be deposited were dissolved in suitable solvents and put onto the chosen substrates from solution. For the deposition of Mo72Fe30 onto HOPG(0001) it was possible to dissolve the molecular magnets in Milli-Q water and to drop the solution onto the substrate under ambient conditions. After the solvent had evaporated interesting modifications of the graphite surface could be observed for different concentrations of the solution. The results obtained for low concentrations are explained by a model which is based on the correspondence of molecular geometry with surface geometry. The deposition of galvinoxyl onto the insulating surfaces KBr(100) and CaF2(111) was carried out under ultrahigh vacuum (UHV) conditions by means of a magnetic valve which can be opened and closed in pulse operation. Ethanol was used as a solvent. As described in detail in the thesis, this alcohol has a very damaging impact on surfaces of potassium bromide single crystals. Therefore in the interpretation of the results on the pulse injection of galvinoxyl onto KBr(100), the solution etching induced by ethanol has to be taken into consideration. Monatomic deep holes which have been etched into the surface by the solvent represent preferential adsorption sites for the molecular magnets so that in scanning probe microscopy studies galvinoxyl molecules could only be found in the etched pits and at the steps of the substrate. </p

Creation of multiple nanodots by single ions

In the challenging search for tools that are able to modify surfaces on the nanometer scale, heavy ions with energies of several 10 MeV are becoming more and more attractive. In contrast to slow ions where nuclear stopping is important and the energy is dissipated into a large volume in the crystal, in the high energy regime the stopping is due to electronic excitations only. Because of the extremely local (< 1 nm) energy deposition with densities of up to 10E19 W/cm^2, nanoscaled hillocks can be created under normal incidence. Usually, each nanodot is due to the impact of a single ion and the dots are randomly distributed. We demonstrate that multiple periodically spaced dots separated by a few 10 nanometers can be created by a single ion if the sample is irradiated under grazing angles of incidence. By varying this angle the number of dots can be controlled.Comment: 12 pages, 6 figure

First results on DEPFET Active Pixel Sensors fabricated in a CMOS foundry - a promising approach for new detector development and scientific instrumentation

DEPFET Active Pixel Sensors (APS) have been introduced as focal plane detectors for X-ray astronomy already in 1996. Fabricated on high resistivity, fully depleted silicon and back-illuminated they can provide high quantum efficiency and low noise operation even at very high read rates. In 2009 a new type of DEPFET APS, the DSSC (DEPFET Sensor with Signal Compression) was developed, which is dedicated to high-speed X-ray imaging at the European X-ray free electron laser facility (EuXFEL) in Hamburg. In order to resolve the enormous contrasts occurring in Free Electron Laser (FEL) experiments, this new DSSC-DEPFET sensor has the capability of nonlinear amplification, that is, high gain for low intensities in order to obtain single-photon detection capability, and reduced gain for high intensities to achieve high dynamic range for several thousand photons per pixel and frame. We call this property "signal compression". Starting in 2015, we have been fabricating DEPFET sensors in an industrial scale CMOS foundry maintaining the outstanding proven DEPFET properties and adding new capabilities due to the industrial-scale CMOS process. We will highlight these additional features and describe the progress achieved so far. In a first attempt on double-sided polished 725 μm thick 200 mm high resistivity float zone silicon wafers all relevant device related properties have been measured, such as leakage current, depletion voltage, transistor characteristics, noise and energy resolution for X-rays and the nonlinear response. The smaller feature size provided by the new technology allows for an advanced design and significant improvements in device performance. A brief summary of the present status will be given as well as an outlook on next steps and future perspectives