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Dive into the research topics where Robert W. Stark is active.

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Featured researches published by Robert W. Stark.


Proceedings of the National Academy of Sciences of the United States of America | 2002

Inverting dynamic force microscopy: From signals to time-resolved interaction forces

Martin Stark; Robert W. Stark; Wolfgang M. Heckl; Reinhard Guckenberger

Transient forces between nanoscale objects on surfaces govern friction, viscous flow, and plastic deformation, occur during manipulation of matter, or mediate the local wetting behavior of thin films. To resolve transient forces on the (sub) microsecond time and nanometer length scale, dynamic atomic force microscopy (AFM) offers largely unexploited potential. Full spectral analysis of the AFM signal completes dynamic AFM. Inverting the signal formation process, we measure the time course of the force effective at the sensing tip. This approach yields rich insight into processes at the tip and dispenses with a priori assumptions about the interaction, as it relies solely on measured data. Force measurements on silicon under ambient conditions demonstrate the distinct signature of the interaction and reveal that peak forces exceeding 200 nN are applied to the sample in a typical imaging situation. These forces are 2 orders of magnitude higher than those in covalent bonds.


Journal of Micromechanics and Microengineering | 2007

Stability of the hydrophilic behavior of oxygen plasma activated SU-8

Ferdinand Walther; Polina Davydovskaya; Stefan Zürcher; Michael Kaiser; Helmut Herberg; Alexander M. Gigler; Robert W. Stark

The effect of O2 plasma treatment on surface energy, topography and surface chemistry of the negative photoresist epoxy novolak SU-8 was investigated by contact angle goniometry, atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS). Directly after plasma treatment, the surfaces were completely wetted by water with a contact angle between water and the SU-8 surface below 5°. The surface free energy can be increased significantly depending on the plasma dose. The surfaces remained hydrophilic for several months showing a moderate hydrophobic recovery. The surface topography of the plasma treated SU-8 showed a formation of nanoscale aggregates. The rms-roughness of the topography was correlated with the plasma dose. An increased plasma dose induced aggregates of up to 200 nm in size. XPS measurements revealed changes in surface chemistry due to the oxygen plasma process and an increased antimony concentration on the surface.


Surface Science | 2000

Fourier transformed atomic force microscopy: tapping mode atomic force microscopy beyond the Hookian approximation

Robert W. Stark; Wolfgang M. Heckl

The periodic impact force induced by the tip‐sample contact in tapping mode atomic force microscopy (TM-AFM ) gives rise to anharmonic oscillations of the sensing cantilever. These anharmonic signals can be understood with a model which goes beyond the common Hookian approximation: the cantilever is described as a multiple degree of freedom system. A theoretical analysis of the anharmonic signals in the light of the extended model shows that these signals contain information on the elastic properties of the specimen surface. In Fourier transformed operation mode of TM-AFM the anharmonic oscillations are analyzed in the frequency domain. This allows for the reconstruction of characteristics of the tip‐sample force, like contact time and maximum contact force.


Ultramicroscopy | 2001

Thermomechanical noise of a free v-shaped cantilever for atomic-force microscopy

Robert W. Stark; Tanja Drobek; Wolfgang M. Heckl

We have calculated the thermal noise of a v-shaped AFM cantilever (Microlever, Type E, Thermomicroscopes) by means of a finite element analysis. The modal shapes of the first 10 eigenmodes are displayed as well as the numerical constants, which are needed for the calibration using the thermal noise method. In the first eigenmode, values for the thermomechanical noise of the z-displacement at 22 degrees C temperature of square root of u2(1) = A/square root of c(cant) and the photodiode signal (normal-force) of S2(1) = A/square root of c(cant) were obtained. The results also indicate a systematic deviation ofthe spectral density of the thermomechanical noise of v-shaped cantilevers as compared to rectangular beam-shaped cantilevers.


Review of Scientific Instruments | 2003

Higher harmonics imaging in tapping-mode atomic-force microscopy

Robert W. Stark; Wolfgang M. Heckl

In tapping-mode atomic-force microscopy usually amplitude and phase of the cantilever motion are acquired. These signals are related to the fundamental oscillation frequency neglecting information at higher frequencies. However, the nonlinear contact between tip and sample induces higher frequency vibrations that are harmonics of the fundamental. In order to recover the available information the full tip motion has to be analyzed. The higher harmonics can be employed for image formation. A setup that consists of two independently operated lock-in amplifiers is used to detect higher harmonics in the dynamic atomic-force microscopy signal. Higher harmonic imaging proves to be useful to monitor the imaging conditions in tapping mode and can be applied for nanoscale imaging with a material contrast.


Biophysical Journal | 2012

Label-Free Live-Cell Imaging with Confocal Raman Microscopy

Katharina Klein; Alexander M. Gigler; Thomas Aschenbrenner; Roberto Monetti; Wolfram Bunk; Ferdinand Jamitzky; Gregor E. Morfill; Robert W. Stark; Juergen Schlegel

Confocal Raman spectroscopy is a noninvasive alternative to established cell imaging methods because it does not require chemical fixation, the use of fluorescent markers, or genetic engineering. In particular, single live-cell, high-resolution imaging by confocal Raman microscopy is desirable because it allows further experiments concerning the individually investigated cells. However, to derive meaningful images from the spectroscopic data, one must identify cell components within the dataset. Using immunofluorescence images as a reference, we derive Raman spectral signatures by means of information measures to identify cell components such as the nucleus, the endoplasmic reticulum, the Golgi apparatus, and mitochondria. The extracted signatures allow us to generate representations equivalent to conventional (immuno)fluorescence images with more than three cell components at a time, exploiting the Raman spectral information alone.


Applied Physics Letters | 1999

Tapping-mode atomic force microscopy and phase-imaging in higher eigenmodes

Robert W. Stark; Tanja Drobek; Wolfgang M. Heckl

Tapping-mode atomic force microscopy (TM-AFM) is a powerful tool to study soft biological samples. Higher eigenmodes of the vibrating cantilever offer enhanced signal and smaller time constants increasing the sensitivity of the tapping probe as compared to conventional TM-AFM. The first five eigenmodes of a v-shaped silicon cantilever were investigated with respect to their suitability for imaging. Stable imaging was possible in the first and third modes. Phase imaging in the third mode was extremely sensitive to surface inhomogeneities and surface contamination particles not visible in standard TM-AFM.


Journal of Applied Physics | 2012

Synthesis of diamond fine particles on levitated seed particles in a rf CH4/H2 plasma chamber equipped with a hot filament

Satoshi Shimizu; Tetsuji Shimizu; H. M. Thomas; G. Matern; Robert W. Stark; M. Balden; S. Lindig; Y. Watanabe; W. Jacob; Noriyoshi Sato; G. E. Morfill

The first successful growth of diamond layers on levitated seed particles in CH4/H2 plasma is presented. The particles were grown in a rf CH4/H2 plasma chamber equipped with a tungsten hot filament. The seed diamond particles injected in a plasma are negatively charged and levitated under the balance of several forces, and diamond chemical vapor deposition takes place on them. The SEM images show that the crystalline structures are formed after the coagulation of islands. The micro-Raman spectroscopy of the particle grown after several hours shows the clear peak assigned to diamond.


Journal of Materials Chemistry | 2012

Non-leaching antimicrobial surfaces through polydopamine bio-inspired coating of quaternary ammonium salts or an ultrashort antimicrobial lipopeptide

Tal Shalev; Anna Gopin; Michael Bauer; Robert W. Stark; Shai Rahimipour

Bacterial fouling on surfaces significantly increases the resistance of bacteria toward antibiotics, which leads to medical complications and a corresponding financial burden. Here, we report on a general and robust technique for facile modification of various surfaces with different antibacterial agents. Our approach in this study was inspired by the strong adhesion of mussel adhesion proteins (MAPs) to many types of surfaces, including metals, polymers, and inorganic materials. Thus, glass and polymeric slides were dip-coated with dopamine, as a MAP mimic, and the resulting surfaces were characterized. The reactivity of dopamine-coated surfaces toward nucleophilic addition was then confirmed by reacting them with fluorescent probes containing either a free amino or a free thiol group. Laser scanning confocal microscopy (LSCM), X-ray photoelectron spectroscopy (XPS), confocal Raman microscopy, matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectroscopy, and cyclic voltammetry studies collectively suggested that the probes had covalently attached to the surfaces. Fabrication of dopamine-coated surfaces with an antibacterial quaternary amine or an ultrashort lipopeptide analog generated surfaces that effectively kill Escherichia coli and Staphylococcus aureuscells on contact. Moreover, minimal leaching of the fabricated agent was detected after prolonged incubation. This technique could be further developed to a “paint-like” or self-assembling monolayer-like procedure for the preparation of antibacterial surfaces on various materials.


Nanotechnology | 2004

Spectroscopy of higher harmonics in dynamic atomic force microscopy

Robert W. Stark

Dynamic atomic force microscopy (AFM) is a standard technique for imaging and the analysis of surfaces at the nanometre scale. In order to estimate material properties from the microscope data it is important to understand the nonlinear dynamics in the tip–sample interaction. Here, the system response of a tapping-mode atomic force microscope is investigated with numerical simulations. In the numerical model, the AFM cantilever is treated as a distributed parameter system that is capable of higher eigenmode excitation. With this multiple-degree-of-freedom (MDOF) approach the generation of higher harmonics as well as the generation of subharmonics is analysed. Under typical imaging conditions higher harmonics are generated whereas a closer approach to the specimen surface can lead to a complicated dynamics.

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Christian Dietz

Technische Universität Darmstadt

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Georg Schitter

Vienna University of Technology

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Ioanna Martinaiou

Technische Universität Darmstadt

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Marcus Schulze

Technische Universität Darmstadt

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Na Liu

Technische Universität Darmstadt

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