Single-Cycle-PLL Detection for Real-Time FM-AFM Applications

In this paper we present a novel architecture for phase-locked loop (PLL) based high-speed demodulation of fre- quency-modulated (FM) atomic force microscopy (AFM) signals. In our approach, we use single-sideband (SSB) frequency upcon- version to translate the AFM signal from the position sensitive detector to a fixed intermediate frequency (IF) of 10 MHz. In this way, we fully benefit from the excellent noise performance of PLL-based FM demodulators still avoiding the intrinsic band- width limitation of such systems. In addition, the upconversion to a fixed IF renders the PLL demodulator independent of the cantilever’s resonance frequency, allowing the system to work with a large range of cantilever frequencies. To investigate if the additional noise introduced by the SSB upconverter degrades the system noise figure we present a model of the AM-to-FM noise conversion in PLLs incorporating a phase-frequency detector. Using this model, we can predict an upper corner frequency for the demodulation bandwidth above which the converted noise from the single-sideband upconverter becomes the dominant noise source and therefore begins to deteriorate the overall system performance. The approach is validated by both electrical and AFM measurements obtained with a PCB-based prototype imple- menting the proposed demodulator architecture

DEVELOPMENT AND APPLICATIONS OF MULTIFREQUENCY IMAGING AND SPECTROSCOPY METHODS IN DYNAMIC ATOMIC FORCE MICROSCOPY

Force spectroscopy and surface dissipation mapping are two of the most important applications of dynamic atomic force microscopy (AFM), in addition to topographical imaging. These measurements are commonly performed using the conventional amplitude-modulation and frequency-modulation dynamic imaging modes. However, the acquisition of the tip-sample interaction force curves using these methods can generally be performed only at selected horizontal positions on the sample, which means that a 3-dimensional representation of the tip-sample forces requires fine-grid scanning of a volume above the surface, making the process lengthy and prone to instrument drift. This dissertation contains the development of two novel atomic force spectroscopy methods that could enable acquisition of 3-dimensional tip-sample force representations through a single 2-dimensional scan of the surface. The force curve reconstruction approach in the first method is based on 3-pass scanning of the surface using the recently proposed single-frequency imaging mode called frequency and force modulation AFM. A second, more versatile method based on bimodal AFM operation is introduced, wherein the fundamental eigenmode of the cantilever is excited to perform the topographical scan and a simultaneously excited higher eigenmode is used to perform force spectroscopy. The dissertation further presents the development of a trimodal AFM characterization method for ambient air operation, wherein three eigenmodes of the cantilever are simultaneously excited with the objective of rapidly and quantitatively mapping the variations in conservative and dissipative surface properties. The new methods have been evaluated within numerical simulations using a multiscale simulation methodology, and experimental implementation has been accomplished for two multifrequency variants that can provide 2-dimensional surface property contrast

Characterization of a home-built low temperature scanning probe microscopy system

The continuing advancement of technology is the driving force behind science and fundamental research. Scanning probe instruments still have a major impact in nanoscience and technology, because they provide a link between the macroscopic world and the atomic scale. The key to a reliable performance of experiments at the nanometer scale is the instrumentation, that allows probe positioning ranging from micrometers to Ångstroms with sub atomic precisions. A new type of scanning probe microscopy (SPM) system operating in ultra high vacuum (UHV) and at liquid Helium (LHe) temperature was developed. This offers the advantages that even reactive surfaces remain clean over time periods of several days, permitting long time experiments. Moreover, these experiments this low temperature scanning probe microscopy (LTSPM) system is the implementation of a focussing Fabry Perot interferometer (fFPi) that allows the following features: - Small amplitude operations and stiff cantilevers require sensors with high deflection sensitivity. With the fFPi in this low temperature SPM system, a deflection sensitivity of 4fm/ sqrt(Hz) at 1MHz can be obtained. - Wide detection bandwidth (DC-10MHz) enables the operation of higher flexural oscillation modes as well as the torsional modes of the cantilever. - A laser spot size of 3µm allows the use of ultra small cantilevers with the dimensions 1/10 of conventional cantilevers. - Photothermal excitation of cantilevers avoids undesirable mechanical vibrations near the cantilever resonance frequency. - Simultaneous flexural and torsional force detection provides quantitative studies of frictions and thus, atom manipulations by atomic force microscopy (AFM). - The combination of both types of microscopes (simultaneous AFM/STM) reveals more information than a scanning tunneling microscopy (STM) or AFM alone. A series of measurements on Si(111)7x7, herringbone superstructure of Au(111) and highly oriented pyrolytic graphite (HOPG) provides information regarding imaging performance of the system. Among these performance tests are atomically resolved scans at three different operating temperatures in STM mode. In non-contact atomic force microscopy (nc-AFM) mode, imaging was performed with the cantilever driven at the fundamental and 2nd oscillation mode. Additional measurements were performed with the fFPi in order to quantify the impact of the laser cooling effects (radiation pressure and photothermal effects) on the oscillating cantilever at three different operating temperatures. The aim of this work is the development, implementation and characterization of a new low temperature scanning probe microscope with an ultra sensitive and high bandwidth fFPi deflection sensor, suitable for nc-AFM operations with small, simultaneous flexural and torsional cantilever oscillation modes. Furthermore, expected upgrades will allow simultaneous nc-AFM/STM operations. Keywords: low temperature home-built simultaneous STM/ nc-AFM, tip-sample gap stability, PLL and self-excitation, highly oriented pyrolytic graphite (HOPG), reconstructed Si(111)7x7, herringbone superstructure, focussing Fabry-Perot interferometer, cantilever cooling, radiation pressure and photothermal effects. Der kontinuierliche, technologische Fortschritt ist die treibende Kraft hinter Wissenschaft und Grundlagenforschung. Rasterkraft und -tunnel Instrumente haben immer noch einen bedeutenden Einfluss auf die Nanotechnologie und -wissenschaft, weil sie eine Verbindung zwischen der makroskopischen Welt und den atomaren Massstäben darstellen. Der Schlüssel für eine zuverlässige Ausführung von Experimenten mit Nanometer Massstäben ist die Instrumentierung, die eine Spitzenpositionierung von Mikrometer bis Ångstroms mit subatomarer Präzision erlaubt. Ein neuartiges Rasterspitzen Mikroskop (SPM) System wurde entwickelt, das im Ultra Hoch Vakuum (UHV) und bei flüssig Helium Temperaturen arbeitet. Dies bietet Vorteile weil sogar reaktive Oberflächen über eine Dauer von einigen Tagen sauber bleiben, was eine längere Experimentierphase zulässt. Zusätzlich zeigen diese Experimente bei tiefen Temperaturen weitere Vorteile wie kleine Driftwerte und tiefe Piezo Kriechraten. Der Ansatz bei diesem Tieftemperatur Rasterspitzen Mikroskop System ist die Implementierung eines fokussierenden Fabry Perot Interferometers das die folgenden Eigenschaften vorweist: - Der Betrieb bei kleinen Amplituden und mit steifen Cantilever setzt Sensoren mit einer hohen Ablenkempfindlichkeit voraus. Mit diesem fokussierenden Fabry Perot Interferometer (fFPi) kann eine Ablenkempfindlichkeit von 4fm/ sqrt(Hz) bei 1MHz erreicht werden. - Detektion mit einer grossen Bandbreite (DC-10MHz) erlauben einen Betrieb von Cantilever mit flexuralen und torsionalen Oszillation Modi. - Ein Laser mit einem Brennpunkt von 3µm lässt einen Betrieb mit einem ultra kleinen Cantilever zu, der 1/10 so gross ist wie ein konventioneller Cantilever. - Photothermische Anregung eines Cantilevers vermeidet unerwünschte mechanische Vibrationen rund um die Resonanzfrequenz. - Gleichzeitige flexural und torsional Kraftdetektion erlauben quantitative Untersuchungen von Reibungen und daher atomare Manipulationen mit Rasterkraft Mikroskopie (AFM). - Die Kombination und simultanen Betrieb von beiden Rasterspitzen Mikroskopen (AFM/STM) zeigen mehr Information als ein Raster Tunnel Mikroskop (STM) alleine. Eine Serie von Messungen mit Si(111)7x7, Herringbone Superstrukturen auf Au(111) und Highly Oriented Pyrolytic Graphite (HOPG) geben Information bezüglich der Leistungen des Systems preis. Einige dieser Leistungstests sind atomar aufgelöste Abbildungen bei drei unterschiedlichen Betriebstemperaturen im STM Betriebsart. Im nicht-Kontakt AFM (nc-AFM) Betriebsart, Abbildungen sind ausgeführt worden auf der Grundschwingung und der zweiten Oberschwingung. Zusätzliche Messungen wurden mit dem fFPi ausgeführt um den Einfluss der Laserkühlung auf den oszillierenden Cantilever bei drei unterschiedlichen Betriebstemperaturen zu quantifizieren. Das Ziel dieser Arbeit ist die Entwicklung, Implementation und Charakterisierung eines neuen Tieftemperatur Rasterspitzen Mikroskops mit einem ultra-empfindlichen und Breitband fokussierenden Fabry Perot Interferometer Ablenk Sensor, geeignet für den nicht-Kontakt AFM Betrieb mit kleinen, simultanen flexural und torsional Cantilever Schwingungsmodi. Naheliegende Erweiterungen des Systems gewährleisten einen simultan nc-AFM/STM Betrieb. Schlüsselwörter: Tieftemperatur simultan nc-AFM/STM aus Eigenbau, Spitzen-Probe Spalt Stabilität, PLL und Eigenanregungsbetrieb, Highly Oriented Pyrolytic Graphite (HOPG), reconstrukturiertes Si(111)7x7, Herringbone Superstruktur, fokussierenden Fabry Perot Interferometer, Cantilever Kühlung, Strahlendruck und photothermische Effekte

The Role of Nonlinear Dynamics in Quantitative Atomic Force Microscopy

Various methods of force measurement with the Atomic Force Microscope (AFM) are compared for their ability to accurately determine the tip-surface force from analysis of the nonlinear cantilever motion. It is explained how intermodulation, or the frequency mixing of multiple drive tones by the nonlinear tip-surface force, can be used to concentrate the nonlinear motion in a narrow band of frequency near the cantilevers fundamental resonance, where accuracy and sensitivity of force measurement are greatest. Two different methods for reconstructing tip-surface forces from intermodulation spectra are explained. The reconstruction of both conservative and dissipative tip-surface interactions from intermodulation spectra are demonstrated on simulated data.Comment: 25 pages (preprint, double space) 7 figure

Advances in atomic force microscopy

This article reviews the progress of atomic force microscopy (AFM) in ultra-high vacuum, starting with its invention and covering most of the recent developments. Today, dynamic force microscopy allows to image surfaces of conductors \emph{and} insulators in vacuum with atomic resolution. The mostly used technique for atomic resolution AFM in vacuum is frequency modulation AFM (FM-AFM). This technique, as well as other dynamic AFM methods, are explained in detail in this article. In the last few years many groups have expanded the empirical knowledge and deepened the theoretical understanding of FM-AFM. Consequently, the spatial resolution and ease of use have been increased dramatically. Vacuum AFM opens up new classes of experiments, ranging from imaging of insulators with true atomic resolution to the measurement of forces between individual atoms.Comment: In press (Reviews of Modern Physics, scheduled for July 2003), 86 pages, 44 figure

Development of a Biocompatible Layer-by-Layer Film System Using Aptamer Technology for Smart Material Applications

Aptamers are short, single-stranded nucleic acids that fold into well-defined three dimensional (3D) structures that allow for binding to a target molecule with affinities and specificities that can rival or in some cases exceed those of antibodies. The compatibility of aptamers with nanostructures such as thin films, in combination with their affinity, selectivity, and conformational changes upon target interaction, could set the foundation for the development of novel smart materials. In this study, the development of a biocompatible aptamer-polyelectrolyte film system was investigated using a layer-by-layer approach. Using fluorescence microscopy, we demonstrated the ability of the sulforhodamine B aptamer to bind its cognate target while sequestered in a chitosan-hyaluronan film matrix. Studies using Ultraviolet-visible (UV-Vis) spectrophotometry also suggest that deposition conditions such as rinsing time and volume play a strong role in the internal film interactions and growth mechanisms of chitosan-hyaluronan films. The continued study and development of aptamer-functionalized thin films provides endless new opportunities for novel smart materials and has the potential to revolutionize the field of controlled release

Doctor of Philosophy

dissertationDynamic Tunneling Force Microscopy (DTFM) is an Atomic Force Microscopy (AFM) technique used for imaging and characterizing trap states on nonconducting surfaces. In this thesis, DTFM images are acquired under Kelvin Probe Force Microscopy (KPFM) feedback and height feedback control. Simultaneous acquisition of DTFM, surface potential, and topographic images is realized, and correlation between trap states, surface potential, and surface topography can be extracted. The methodology for obtaining three-dimensional location and energy of individual atomic scale electronic trap states is described. The energy and depth of states accessible by a DTFM experiment are calculated using tunneling and electrostatic models. The DTFM signal amplitude is derived using a one-dimensional electrostatic model. Comparison between simulated DTFM signal and experimental results show a good consistency, verifying the single electron tunneling model. DTFM is demonstrated on interlayer dielectric materials. Density, spatial distribution, energy, and depth distribution of trap states in these materials are measured by DTFM. An atomic scale study of electrical stressing effects using the DTFM method is performed showing both state appearance and disappearance after electrical stressing