A new approach to second-order nonlinear optical (NLO) materials is reported, in which chirality and supramolecular organization play key roles. Langmuir-Blodgett films of a chiral helicene are composed of supramolecular arrays of the molecules. The chiral supramolecular organization makes the second-order NLO susceptibility about 30 times larger for the nonracemic material than for the racemic material with the same chemical structure. The susceptibility of the nonracemic films is a respectable 50 picometers per volt, even though the helicene structure lacks features commonly associated with high nonlinearity. Susceptibility components that are allowed only by chirality dominate the second-order NLO response.
For many years, glass-polyalkenoate cements have been described as possessing the unique properties of self-adherence to human hard tissues, such as bones or teeth. However, direct experimental evidence to prove the existence of chemical bonding has not been advanced. X-ray Photoelectron Spectroscopy (XPS) was used to analyze the chemical interaction of a synthesized polyalkenoic acid with enamel and synthetic hydroxyapatite. For both enamel and hydroxyapatite, the peak representing the carboxyl groups of the polyalkenoic acid was detected to have significantly shifted to a lower binding energy. De-convolution of this shifted peak disclosed two components with a peak representing unreacted carboxyl groups and a peak suggesting chemical bonding to hydroxyapatite. On average, 67.5% of the carboxyl groups of the polyalkenoic acid were measured to have bonded to hydroxyapatite. XPS of hydroxyapatite also disclosed its surface to be enriched in calcium and decreased in phosphorus, indicating that phosphorus was extracted at a relatively higher rate than calcium. Analysis of these data supports the mechanism in which carboxylic groups replace phosphate ions (PO4(3-)) of the substrate and make ionic bonds with calcium ions of hydroxyapatite. It is concluded that an ultrathin layer of a polyalkenoic acid can be prepared on a hydroxyapatite-based substrate by careful removal of non-bonded molecules. With this specimen-processing method, XPS not only provided direct evidence of chemical bonding, but also enabled us to quantify the percentages of functional groups of the polyalkenoic acids that bonded to calcium of hydroxyapatite.
We present the first atomic-resolution image of a surface obtained with an optical implementation of the atomic-force microscope (AFM). The native oxide on silicon was imaged with atomic resolution, and ≊5-nm resolution images of aluminum, mechanically ground iron, and corroded stainless steel were obtained. The relative merits of an optical implementation of the AFM as opposed to a tunneling implementation are discussed.
A planar microcavity has been inserted into a self-assembled colloidal crystal by a combination of convective self-assembly in a vertical geometry and the Langmuir−Blodgett technique. This planar cavity, which is parallel to the {111} planes, induces the appearance of a localized state, a pass band, into the forbidden band gap. We have experimentally determined the optical properties of this defect. The dependence of the position of the localized state is investigated as a function of the thickness of the defect layer. It is found that the defect behaves as a donor impurity, as it originates in the conduction band. Analogously, the crystal surrounding a constant defect layer can be varied. It is also found that the pass band depends on the thickness of the surrounding crystal of the planar defect.
Colloidal crystals with three-dimensional periodicities in the refractive index have a photonic band gap (PBG) in which electromagnetic waves are forbidden. We present a method to fabricate stacked colloidal crystals containing a two-dimensional defect as a middle layer by combining vertical deposition method with the Langmuir–Blodgett (LB) technique. The defect layer introduces an impurity mode within the optical stop band, which is observed as a defect peak (pass band) in the optical density spectrum. The result shows that the combination of vertical deposition with LB technique provides a way for introducing defect modes in PBG materials.
Thermoplastic vulcanizates (TPVs), as prepared by dynamic vulcanization, are blends in which cross-linked rubber particles are finely dispersed in a thermoplastic matrix. The reported nylon-6/EPDM TPVs show significant strain recovery behavior, even though the matrix consists of semicrystalline nylon-6, which deforms plastically via shear yielding. Atomic force microscopy and transmission electron microscopy experiments revealed an inhomogeneous plastic deformation of the matrix phase. During straining, the plastic deformation is initiated in those zones where the nylon matrix between the rubber particles is the thinnest. Even at high strains, the thick ligaments of the nylon matrix remain almost undeformed and act as adhesion points holding the rubber particles together. When the external force is removed, the elastic force of the stretched, dispersed rubber phase pulls back the plastically deformed nylon parts by either buckling or bending. This is considered to be the key mechanism for the elastic behavior of the investigated TPVs.
We have prepared face centered close packed arrays of polystyrene spheres using convective self-assembly. The optical properties of the arrays of spheres were studied and revealed high quality crystals that could be prepared using this method. Optical transmission measurements over a wide spectral range reveal the presence of multiple diffraction peaks. Their use for checking the quality of the arrays is proposed. The orientation of the arrays formed was studied using optical diffraction and atomic force microscopy. Both indicate the formation of oriented crystals. This was until now only reported for epitaxial growth of colloidal crystals. The oriented crystals were used to measure their photonic band structure.
A conductive atomic force microscope (AFM) tip based on B-implanted diamond has been developed for the determination of the spatial distribution of charge carriers in semiconducting structures. The characteristics of this tip have been determined by studying the current–voltage behavior as a function of substrate resistivity and tip load. From this work a model of the electrical properties of the microcontact is emerging. It includes an Ohmic contribution to the overall resistance, which is related to the plastically deformed area, and contributions from a barrier. The tip imprints have been imaged with AFM and their physical dimensions are seen to match the requirements of the model. From resistance measurements on uniformly doped silicon a calibration curve has been established which can be used as a standard to convert measured resistance into resistivity.
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