An inductive technique for the measurement of dynamical magnetic processes in thin-film materials is described. The technique is demonstrated using 50 nm films of Permalloy (Ni 81 Fe 19). Data are presented for impulse-and step-response experiments with the applied field pulse oriented in the plane of the film and transverse to the anisotropy axis. Rotation times as short as 200 ps and free oscillations of the magnetization after excitation are clearly observed. The oscillation frequency increases as the dc bias field parallel to the anisotropy axis increases as predicted by classical gyromagnetic theory. The data are fitted to the Landau-Lifshitz equation, and damping parameters are determined as a function of dc bias field. Damping for both impulse and step excitations exhibits a strong dependence on bias field. Damping for step excitations is characterized by an anomalous transient damping which rapidly increases at low dc bias field. Transformation of the data to the frequency domain reveals a higher order precessional mode which is also preferentially excited at low dc bias fields. A possible source for both phenomena is precessional mode saturation for large peak rotations. The technique has the potential for 20 ps resolution, although only 120 ps resolution is demonstrated due to the limited bandwidth of the waveguides used.
We have demonstrated the assembly of twodimensional patterns of functional antibodies on a surface. In particular, we have selectively adsorbed micrometer-scale regions of biotinylated immunoglobulin that exhibit specific antigen binding after adsorption. The advantage of this technique is its potential adaptability to adsorbing arbitrary proteins in tightly packed monolayers while retaining functionality. The procedure begins with the formation of a self-assembled monolayer of n-octadecyltrimethoxysilane (OTMS) on a silicon dioxide surface. This monolayer can then be selectively removed by UV photolithography. Under appropriate solution conditions, the OTMS regions will adsorb a monolayer of bovine serum albumin (BSA), while the silicon dioxide regions where the OTMS has been removed by UV light will adsorb less than 2% of a monolayer, thus creating high contrast patterned adsorption of BSA. The attachment of the molecule biotin to the BSA allows the pattern to be replicated in a layer of streptavidin, which bonds to the biotinylated BSA and in turn will bond an additional layer of an arbitrary biotinylated protein. In our test case, functionality of the biotinylated goat antibodies raised against mouse immunoglobulin was demonstrated by the specific binding of fluorescently labeled mouse IgG.We are interested in the development and application of techniques for protein patterning, especially for in vitro studies of protein function and for applications outside of the biological context (e.g., for artificial biomineralization and in biochemical sensors). In this paper we describe a general technique for producing two-dimensional patterns of functional protein on silicon dioxide surfaces by using a combination of self-assembled monolayers (SAMs) of alkyl silanes, albuminalkyl adsorption, and biotin-avidin interactions. We illustrate the technique by generating two-dimensional patterns of functional goat antibodies raised against mouse immunoglobulins (GAM).Our procedure begins with the formation of an noctadecyltrimethoxysilane (OTMS) ¶ SAM on a sio2 substrate. SAMs are increasingly important tools in the micro-and nanolithographic patterning of organic and inorganic materials. They have recently been used for selected area polypeptide synthesis (1), as barriers to chemical etching (2, 3), for the formation of microcrystals (4, 5) or microelectrodes (4), for creation of electrically conducting polypyrrole circuitry (6), for patterned water condensation (4), and for adhesion of cells (7-10) and proteins (8,(11)(12)(13)(14)(15)(16)(17). OTMS is one of the most widely used of the alkyl silanes for producing hydrophobic SAMs on SiO2. The SAM can be selectively removed from the surface by ultraviolet (UV) exposure through a lithographic mask. The difference in adsorption of bovine serum albumin (BSA) to the exposed and unexposed regions of the surface creates a two-dimensional pattern of BSA with a surface coverage "50 times greater on the OTMS than on the exposed sio2. If the BSA is tagged with biotin, the p...
We have studied the low-frequency conductance fluctuations of very small-area metalinsulator-metal tunnel junctions. We find that the spectral power density at high temperature is typical of 1//noise, but is resolved into a small number of Lorentzian spectra for T < 80 K. The thermally activated Lorentzians are described by a two-rate kinetics and a broad distribution of activation energies and attempt rates.
We have studied optical surface second harmonic generation and have determined the magnitude and relative phase of the second-order susceptibility tensor elements for thermally evaporated gold, and sputtered silver, copper, aluminum, and tantalum. The second harmonic data are understood using an isotropic model of the surfaces. The measurements of the parameters from this nonlinear optical characterization, in conjunction with linear optical characterization of the samples, allow us to extract the elements of the tensors. The typical size of χ⊥(2), the tensor element that produces the surface current perpendicular to the surface and is the largest surface element, ranges from 3(±1)×10−12cm2∕statvolt for an aluminum sample with 24(±4)Årms surface roughness to 1.1(±0.1)×10−13cm2∕statvolt for a copper sample with 5(±1)Årms surface roughness. Film preparation and associated surface roughness can reproducibly change the values of χ(2); increasing sample roughness increases the magnitudes of the tensor elements. In agreement with previous aluminum measurements, we again find that the tensor elements associated with creating the second harmonic currents normal to the surface are of roughly the theoretically predicted magnitude, but that the elements associated with creating the second harmonic currents parallel to the surface and in the bulk are an order of magnitude smaller than expected.
We describe an approach to regular triangular arrays of dipolar molecular rotors based on insertion of dipolar rotator carrying shafts as guests into channels of a host, tris(o-phenylenedioxy)cyclotriphosphazene (TPP). The rotor guests can either enter the bulk of the host or stay at or near the surface, if a suitable stopper is installed at the end of the shaft. Differential scanning calorimetry, solid-state NMR, and powder X-ray diffraction were used to examine the insertion of a dipolar rotor synthesized for the purpose, 1-n-hexadecyl-12-(2,3-dichlorophenyl)-p-dicarba-closo-dodecaborane, and it was found that it forms a surface inclusion compound. Rotational barriers from 1.2 to 9 kcal/mol were found by dielectric spectroscopy and were attributed to rotors inserted into the surface to different degrees, some rubbing the surface as they turn.
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