Shear-Induced Detachment of Polystyrene Beads from SAM-Coated Surfaces.

In this work we experimentally and theoretically analyze the detachment of microscopic polystyrene beads from different self-assembled monolayer (SAM) surfaces in a shear flow in order to develop a mechanistic model for the removal of cells from surfaces. The detachment of the beads from the surface is treated as a thermally activated process applying an Arrhenius Ansatz to determine the activation barrier and attempt frequency of the rate determing step in bead removal. The statistical analysis of the experimental shear detachment data obtained in phosphate-buffered saline buffer results in an activation energy around 20 kJ/mol, which is orders of magnitude lower than the adhesion energy measured by atomic force microscopy (AFM). The same order of magnitude for the adhesion energy measured by AFM is derived from ab initio calculations of the van der Waals interaction energy between the polystyrene beads and the SAM-covered gold surface. We conclude that the rate determing step for detachment of the beads is the initiation of rolling on the surface (overcoming static friction) and not physical detachment, i.e., lifting the particle off the surface.

Field evaporation of insulators and semiconductors: Theoretical insights for ZnO.

We look at the new challenges associated with Atom Probe Tomography of insulators and semiconductors with regard to local fields inside and on the surface of such materials. The theoretical discovery that in high fields the band gap in these materials is drastically reduced to the point where at the evaporation field strength it vanishes will be crucial in our discussion. To understand Atom Probe results on the field evaporation of insulators and semiconductors we use density functional theory on ZnO clusters to follow the structural and electronic changes during field evaporation and to obtain potential energy curves, HOMO-LUMO gaps, field distributions, desorption pathways and fragments, dielectric constants, and polarizabilities. We also examine the effects of electric field reversal on the evaporation of ZnO and compare the results with Si.

Disintegration and field evaporation of thiolate polymers in high electric fields.

High electrostatic fields cause major changes in polymers, structural (e.g. electrostriction) and electronic (e.g. reduction of the "band gap" with final metallization). Using density functional theory we have studied field effects on amino-alkane-thiols and perfluoro-alkane-thiols adsorbed on a metal substrate. Our results agree well with the APT fragmentation spectra obtained by Stoffers, Oberdorfer and Schmitz and shed light on disintegration pathways. We demonstrate that in SAMs the HOMO/LUMO gap is again reduced as a function of the field strength and vanishes at evaporation. We also follow the field dependence of the dielectric constant and polarizability.

Do dielectric nanostructures turn metallic in high-electric dc fields?

Three-dimensional dielectric nanostructures have been analyzed using field ion microscopy (FIM) to study the electric dc field penetration inside these structures. The field is proved to be screened within a few nanometers as theoretically calculated taking into account the high-field impact ionization process. Moreover, the strong dc field of the order of 0.1 V/Å at the surface inside a dielectric nanostructure modifies its band structure leading to a strong band gap shrinkage and thus to a strong metal-like optical absorption near the surface. This metal-like behavior was theoretically predicted using first-principle calculations and experimentally proved using laser-assisted atom probe tomography (APT). This work opens up interesting perspectives for the study of the performance of all field-effect nanodevices, such as nanotransistor or super capacitor, and for the understanding of the physical mechanisms of field evaporation of dielectric nanotips in APT.

Disintegration of graphene nanoribbons in large electrostatic fields.

The deformation and disintegration of a graphene nanoribbon under external electrostatic fields are investigated by first principle quantum mechanical calculations to establish its stability range. Zigzag edges terminated by various functional groups are considered. By analyzing the phonon spectrum, the critical fracture field for each edge structure is obtained. It is found that different terminal groups on the zigzag graphene nanoribbons lead to different fracture patterns at different fracture fields. The failure mechanism is demonstrated to involve both the carbon bond alternation feature across the ribbon and the terminal group electronegativity.

Field evaporation of oxides: a theoretical study.

To understand atom probe results on the field evaporation of oxides we use density functional theory on MgO clusters to follow the structural changes during field evaporation and toobtain potential energy curves, partial charges and desorption pathways. It is straightforward to understand that Mg evaporates doubly charged. We also show that MgO(+), MgO2(+), MgO(2+) and O(+) ions leave the surface. Two questions are however new for oxides. (1) Where do the electrons go? When the oxides are deposited on a metal tip it can be assumed that the electrons are used to complete the electrical circuit. However this leaves the second question unanswered, namely (2) what happens to the oxygen? We will argue that there are two channels for the oxygen, namely (a) To travel down the (metallic) surface of the tip and eventually to desorb either as atoms or molecules. (b) The oxygen can recombine within the oxide layer itself and desorbs as a neutral molecule accelerated in the inhomogeneous field due to its induced dipole.

Quantitative phase and refractive index measurements with point-source digital in-line holographic microscopy.

Point-source digital in-line holographic microscopy with numerical reconstruction is ideally suited for quantitative phase measurements to determine optical path lengths and to extract changes in refractive index within accuracy close to 0.001 on the submicrometer length scale. This is demonstrated with simulated holograms and with detailed measurements on a number of different micrometer-sized samples such as suspended drops, optical fibers, as well as organisms of biological interest such as E. coli bacteria, HeLa cells, and fibroblast cells.

A capillary absorption spectrometer for stable carbon isotope ratio (13C∕12C) analysis in very small samples.

A capillary absorption spectrometer (CAS) suitable for IR laser isotope analysis of small CO(2) samples is presented. The system employs a continuous-wave (cw) quantum cascade laser to study nearly adjacent rovibrational transitions of different isotopologues of CO(2) near 2307 cm(-1) (4.34 µm). This initial CAS system can achieve relative isotopic precision of about 10 ppm (13)C, or ∼1 per thousand (per mil in delta notation relative to Vienna Pee Dee Belemnite) with 20-100 picomoles of entrained sample within the hollow waveguide for CO(2) concentrations ∼400-750 ppm. Isotopic analyses of such gas fills in a 1-mm ID hollow waveguide of 0.8 m overall physical path length can be carried out down to ∼2 Torr. Overall (13)C∕(12)C ratios can be calibrated to ∼2 per thousand accuracy with diluted CO(2) standards. A novel, low-cost method to reduce cw-fringing noise resulting from multipath distortions in the hollow waveguide is presented, which allows weak absorbance features to be studied at the few ppm level (peak-to-rms) after 1000 scans are co-added in ∼10 s. The CAS is meant to work directly with converted CO(2) samples from a laser ablation-catalytic combustion micro-sampler to provide (13)C∕(12)C ratios of small biological isolates currently operating with spatial resolutions ∼50 µm.

Water whiskers in high electric fields.

In electrostatic fields of the order of volts per Angstrom long whiskers of up to 12 water molecules form that have been observed in the field ion microscope. Here we present a detailed analysis on the basis of the density functional theory that substantiates the earlier claims. We present whisker structures and energetics, lower and upper threshold fields, and fragmentation patterns.

Reconstruction of high-resolution holographic microscopic images.

In in-line holographic microscopy a pinhole illuminates an object and a CCD-detector directly records the hologram in a pixel-pitch-dependent distance. A rapidly calculating exact reconstruction technique using a reorganized hologram with a low number of pixels, the tile superposition technique, is presented. The algorithm is applied on imaging of a 2 microm bead cluster, and it is compared with other reconstruction techniques. The high-contrast image corresponds to an NA of 0.7. A full 4 megapixel reconstruction with a resolution approaching the diffraction limit is possible in less than a minute. The technique is a base for high-resolution wide-field imaging by multispot illumination.

Collective diffusion in two-dimensional systems: exact analysis based on the kinetic lattice gas model.

Using a gradient expansion of the local microscopic particle current, in the long-time and large-scale limit, we derive an exact analytical expression for the density and temperature dependence of the diffusivity in terms of the density dependence of correlation functions on a rectangular lattice. The latter are calculated by transfer matrix methods, generalized to an adsorbate with lateral interactions and subjected to an external field. This allows an examination of the effects of generalized hopping kinetics. Where both initial and final state interactions are involved, the Reed-Ehrlich factorization, commonly assumed, cannot apply.

High resolution Talbot self-imaging applied to structural characterization of self-assembled monolayers of microspheres.

We report the observation of the Talbot self-imaging effect in high resolution digital in-line holographic microscopy (DIHM) and its application to structural characterization of periodic samples. Holograms of self-assembled monolayers of micron-sized polystyrene spheres are reconstructed at different image planes. The point-source method of DIHM and the consequent high lateral resolution allows the true image (object) plane to be identified. The Talbot effect is then exploited to improve the evaluation of the pitch of the assembly and to examine defects in its periodicity.

Dimerization boosts one-dimensional mobility of conformationally adapted porphyrins on a hexagonal surface atomic lattice.

We employed temperature-controlled fast-scanning tunneling microscopy to monitor the diffusion of tetrapyridylporphyrin molecules on the Cu(111) surface. The data reveal unidirectional thermal migration of conformationally adapted monomers in the 300-360 K temperature range. Surprisingly equally oriented molecules spontaneously form dimers that feature a drastically increased one-dimensional diffusivity. The analysis of the bonding and mobility characteristics indicates that this boost is driven by a collective transport mechanism of a metallosupramolecular complex.

Conformational transitions in single polymer molecules modeled with a complete energy landscape: continuous two-state model.

An extension of the two-state Freely Jointed Chain model is presented in which the discrete energies of the two conformers are replaced by continuous functions of the conformer length. The statistical mechanics is initially developed in the Gibbs ensemble and leads to a conformational multi-state model. This is used to fit the equilibrium force-extension curve for Dextran. The continuous model also allows the use of Transfer Matrix methods to calculate all statistical properties in the Helmholtz ensemble, including thermal fluctuations. The latter are obtained with near perfect agreement to experiment.

Theoretical modeling and interpretation of X-ray absorption spectra of liquid water.

We report extensive calculations to examine the capability of theory to explain the XAS spectra of liquid water. Several aspects that enter the theoretical model are addressed, such as the quantum mechanical methods, the statistics and the XAS model. As input into our quantum mechanical calculations we will use structural information on liquid water obtained from first principles and from classical molecular dynamics simulations. As XAS models, we will examine the full core hole and the half core hole approximations to transition state theory. The quantum mechanics is performed on the basis of density functional theory. We conclude from this study that recent experimental results are fully consistent with, and can be completely explained by, present day theory, in particular, the pre-edge peak is reproduced. We also find that the average bond coordination in liquid water is 3.1 and that the assertion in a recent paper that the hydrogen bond number is much less than that cannot be substantiated. Our calculations emphasize that further advances in our understanding of water can only be made by more sophisticated spectroscopy with significantly increased resolution.

Oxygen adsorption on gold nanofacets and model clusters.

We have studied oxygen interaction with Au crystals (field emitter tips) using time-resolved (atom-probe) field desorption mass spectrometry. The results demonstrate no adsorption to take place on clean Au facets under chosen conditions of pressures (p < 10(-4) m/bar) and temperatures (T = 300-350 K). Steady electric fields of 6 V/nm do not allow dissociating the oxygen molecule. The measured O2+ intensities rather reflect ionization of O2 molecules at critical distances above the Au tip surface. Certain amounts of Au-O2 complex ions can be found at the onset of Au field evaporation. Calculations by density functional theory (DFT) show weak oxygen end-on interaction with Au10 clusters (Delta E = 0.023 eV) and comparatively stronger interaction with Au1/Au(100) model surfaces (Delta E = 0.25 eV). No binding is found on {210} facets. Including (positive) electric fields in the DFT calculations leads to an increase of the activation energy for oxygen dissociation thus providing an explanation for the absence of atomic oxygen ions from the field desorption mass spectra.

Field-induced CO adsorption and formation of carbonyl waves on gold nanotips.

We report a study of the adsorption and reaction of CO on a gold nanotip in high electrostatic fields. Field ion microscopy is used to investigate the emergence of a Au-carbonyl wave that is made visible with oxygen as the imaging gas. We set up a simple kinetic model that reproduces the adsorption wave and confirms that the presence of oxygen merely serves as an imaging gas and does not lead to field-induced oxidation of CO.

Comment on "Reconstruction algorithm for high-numerical-aperture holograms with diffraction-limited resolution".

We comment on a recent Letter by Zhang et al. [Opt. Lett. 31, 1633 (2006)] in which the authors proposed a reconstruction algorithm for high-numerical-aperture (NA) holograms. Such an algorithm has been available for in-line holography for more than a decade. The authors' "achievement" of high NA for digital in-line holography, NA=0.17, is below what was reported already several years ago (NA=0.30) and is considerably lower than what is routinely achieved now. We present reconstructions of holograms acquired with NAs above 0.4 in which we show maximal achievable resolution.

Immersion digital in-line holographic microscopy.

Digital in-line holographic microscopy is a promising new tool for high resolution imaging. We demonstrate, by using latex beads, that a considerable increase in numerical aperture, and, therefore, resolution can be achieved if the space between a source and a CCD camera chip is filled with a high refractive index medium. The high refractive index medium implies a shorter effective wavelength so that submicrometer resolution can be obtained with laser light in the visible range.

Digital in-line holographic microscopy.

We first briefly review the state of the art of digital in-line holographic microscopy (DIHM) with numerical reconstruction and then discuss some technical issues, such as lateral and depth resolution, depth of field, twin image, four-dimensional tracking, and reconstruction algorithm. We then present a host of examples from microfluidics and biology of tracking the motion of spheres, algae, and bacteria. Finally, we introduce an underwater version of DIHM that is suitable for in situ studies in an ocean environment that show the motion of various plankton species.