Domain structures in langmuir-blodgett films investigated by atomic force microscopy

Investigations of phase-separated Langmuir-Blodgett films by atomic force microscopy reveal that on a scale of 30 to 200 micrometers, these images resemble those observed by fluorescence microscopy. Fine structures (less than 1 micrometer) within the stearic acid domains were observed, which cannot be seen by conventional optical microscopic techniques. By applying the force modulation technique, it was found that the elastic properties of the domains in the liquid condensed phase and grains observed within the liquid expanded phase were comparable. Small soft residues in the domains could also be detected. The influence of trace amounts of a fluorescence dye on the micromorphology of monolayers could be detected on transferred films.     

Molecular positional order in langmuir-blodgett films by atomic force microscopy

Langmuir-Blodgett films of barium arachidate have been studied on both macroscopic and microscopic scales by atomic force microscopy. As prepared, the films exhibit a disordered hexagonal structure; molecularly resolved images in direct space establish a connection between the extent of the positional order and the presence of defects such as dislocations. Upon heating, the films reorganize into a more condensed state with a centered rectangular crystallographic arrangement; in this new state the films exhibit long-range positional order and unusual structural features, such as a height modulation of the arachidic acid molecules.     

Unexpected square symmetry seen by atomic force microscopy in bilayer films of disk-like molecules

Thin films of disk-shaped molecules are expected to display anisotropic optical and transport properties, leading to applications in optical display or sensor technologies. Bilayer Langmuir-Blodgett films of monomeric triphenylene mesogens have been studied by atomic force microscopy. The triphenylene cores of the constituent molecules tend to promote the formation of columnar structures in the plane of the substrate and along the direction of deposition of the film. Atomic force microscopy images of bilayer Langmuir-Blodgett films revealed two types of structure, one corresponding to an aligned columnar structure and the other to an unusual square lattice, which may result from the superposition of columnar structures in adjacent layers that intersect at near right angles. Annealing such bilayers near the melting point of the bulk compound improved the structural ordering by reducing the angular spread of orientations associated with the well-developed columnar structure in some areas and by producing a more distinct square lattice in other areas of the sample.     

True atomic resolution by atomic force microscopy through repulsive and attractive forces

The (1014) cleavage plane of calcite has been investigated by atomic force microscopy in water at room temperature. True lateral atomic-scale resolution was achieved; the atomic-scale periodicities as well as the expected relative positions of the atoms within each unit cell were obtained. Along monoatomic step lines, atomic-scale kinks, representing point-like defects, were resolved. Attractive forces on the order of 10(-11) newton acting between single atomic sites on the sample and the front atoms of the tip were directly measured and provided the highest, most reliable resolution on a flat, well-ordered surface.     

Revealing the angular symmetry of chemical bonds by atomic force microscopy

We have measured the angular dependence of chemical bonding forces between a carbon monoxide molecule that is adsorbed to a copper surface and the terminal atom of the metallic tip of a combined scanning tunneling microscope and atomic force microscope. We provide tomographic maps of force and current as a function of distance that revealed the emergence of strongly directional chemical bonds as tip and sample approach. The force maps show pronounced single, dual, or triple minima depending on the orientation of the tip atom, whereas tunneling current maps showed a single minimum for all three tip conditions. We introduce an angular dependent model for the bonding energy that maps the observed experimental data for all observed orientations and distances.     

Complex patterning by vertical interchange atom manipulation using atomic force microscopy

The ability to incorporate individual atoms in a surface following predetermined arrangements may bring future atom-based technological enterprises closer to reality. Here, we report the assembling of complex atomic patterns at room temperature by the vertical interchange of atoms between the tip apex of an atomic force microscope and a semiconductor surface. At variance with previous methods, these manipulations were produced by exploring the repulsive part of the short-range chemical interaction between the closest tip-surface atoms. By using first-principles calculations, we clarified the basic mechanisms behind the vertical interchange of atoms, characterizing the key atomistic processes involved and estimating the magnitude of the energy barriers between the relevant atomic configurations that leads to these manipulations.     

Controlling molecular order in "hairy-rod" langmuir-blodgett films: a polarization-modulation microscopy study

The interplay of molecular weight, layer thickness, and thermal annealing in controlling molecular order in ultrathin Langmuir-Blodgett films is characterized with the use of polarization-modulation laser-scanning microscopy. The degree and direction of molecular alignment can be imaged rapidly and sensitively through the magnitude and orientation of linear dichroism in Langmuir-Blodgett films of rodlike poly(phthalocyaninatosiloxane) (PcPS). Images are presented for films as thin as two molecular layers ( approximately 44 angstroms). Molecular alignment along the transfer direction is much stronger for films of PcPS with approximately 25 repeat units ( approximately 10 nanometers long) than for those with approximately 50 repeat units ( approximately 20 nanometers long). Enhancement of alignment by thermal annealing is also much greater for PcPS-25 than PcPS-50. Intimate interaction with the substrate suppresses improvement in alignment by annealing, evident by an anomalously small increase in anisotropic absorption of the first two layers.     

Surface order and stability of langmuir-blodgett films

Angstrom-resolution atomic force microscope images of Langmuir-Blodgett monolayers and multilayers of cadmium arachidate in air and under water show a dramatic change from a disordered arrangement to a crystalline lattice by the addition or removal of a single layer of molecules. The disordered surface is less stable than the ordered one to mechanical stresses such as atomic force microscopy tip forces or at the air-water contact line during contact angle measurements. The difference in the degree of order in the alkyl chains is attributed to the strong attractive interaction between headgroups in the presence of the divalent cation.     

Scanning tunneling microscopy and atomic force microscopy: application to biology and technology

The scanning tunneling microscope (STM) and the atomic force microscope (AFM) are scanning probe microscopes capable of resolving surface detail down to the atomic level. The potential of these microscopes for revealing subtle details of structure is illustrated by atomic resolution images including graphite, an organic conductor, an insulating layered compound, and individual adsorbed oxygen atoms on a semiconductor. Application of the STM for imaging biological materials directly has been hampered by the poor electron conductivity of most biological samples. The use of thin conductive metal coatings and replicas has made it possible to image some biological samples, as indicated by recently obtained images of a recA-DNA complex, a phospholipid bilayer, and an enzyme crystal. The potential of the AFM, which does not require a conductive sample, is shown with molecular resolution images of a nonconducting organic monolayer and an amino acid crystal that reveals individual methyl groups on the ends of the amino acids. Applications of these new microscopes to technology are demonstrated with images of an optical disk stamper, a diffraction grating, a thin-film magnetic recording head, and a diamond cutting tool. The STM has even been used to improve the quality of diffraction gratings and magnetic recording heads.     

毛竹纤维微纤丝取向的原子力显微镜观察

为了观察到竹纤维不同壁层微纤丝的取向,该文利用原子力显微镜对毛竹纤维的微纤丝取向进行了高分辨观察。采用了两种样品制备方式, 一是化学离析后纤维,用于观察竹纤维表层的微纤丝取向;二是经过脱木素处理后的弦切片（厚度为30 μm）,用于观察竹纤维细胞腔内壁的微纤丝取向。结果表明,毛竹纤维初生壁微纤丝呈无序排列,但其细胞腔内壁的微纤丝相对纤维长轴则几乎垂直排列,这种排列模式与木材细胞对应壁层微纤丝的排列模式相似。同时,还观察到某些壁层的微纤丝呈高度定向排列,但拍摄到这类图像的几率较小。该研究证实,利用原子力显微镜可以实现对竹纤维微纤丝取向的高分辩观察,并且样品制备远较透射电镜简单,可操作性强。      

Impact of molecular order in langmuir-blodgett films on catalysis

Catalytically active Langmuir-Blodgett films of a rhodium complex were prepared and characterized to determine the possible effect of the molecular order of metal complexes on catalytic activity. The hydrogenation of carbon-oxygen double bonds was used as a model reaction. The complex in solution exhibited low catalytic activity, whereas it was highly active in the film. The catalytic activity was found to be highly dependent on the orientation of the complex within the film. The reactions were also highly selective with regard to the substrate. These observations and the observed rate dependence on temperature strongly implicate the molecular order of a metal complex as an important dimension in catalysis.     

Self-assembly of disk-shaped molecules to coiled-coil aggregates with tunable helicity

A disk-shaped molecule with chiral tails is shown to form long fibers of molecular diameter and micrometer length by self-assembly in chloroform. The molecules are derived from crown ethers and contain a phthalocyanine ring. In the fibers, they have a clockwise, staggered orientation that leads to an overall right-handed helical structure. These structures, in turn, self-assemble to form coiled-coil aggregates with left-handed helicity. Addition of potassium ions to the fibers leaves their structure intact but blocks the transfer of the chirality from the tails to the cores, leading to loss of the helicity of the fibers. These tunable chiral materials have potential in optoelectronic applications and as components in sensor devices.     

Imaging and manipulating molecules on a zeolite surface with an atomic force microscope

The adsorption of neutral molecules and ions on the surfaces of zeolites was observed in real time with an atomic force microscope (AFM). Direct imaging of the surface of the zeolite clinoptilolite was possible by using a diluted tert-butyl ammonium chloride solution as a medium. Images of the crystal in different liquids revealed that molecules could be bound to the surface in different ways; neutral molecules of tert-butanol formed an ordered array, whereas tert-butyl ammonium ions formed clusters. These absorbed molecules were not rearranged by the AFM tip when used in an imaging mode. However, when a sufficiently large force was applied, the tip of the AFM could rearrange the tert-butyl ammonium ions on the zeolite surface. This demonstration of molecular manipulation suggests new applications, including biosensors and lithography.     

Functional group imaging by chemical force microscopy

Mapping the spatial arrangement of chemical functional groups and their interactions is of significant importance to problems ranging from lubrication and adhesion to recognition in biological systems. A force microscope has been used to measure the adhesive and friction forces between molecularly modified probe tips and organic monolayers terminating in a lithographically defined pattern of distinct functional groups. The adhesive interactions between simple CH(3)/CH(3), CH(3)/COOH, and COOH/COOH functional groups correlate directly with friction images of sample surfaces patterned with these groups. Thus, by monitoring the friction between a specifically functionalized tip and sample, one can produce friction images that display predictable contrast and correspond to the spatial distribution of functional groups on the sample surface. Applications of this chemically sensitive imaging technique are discussed.     

Atomic resolution of the silicon (111)-(7x7) surface by atomic force microscopy

Achieving high resolution under ultrahigh-vacuum conditions with the force microscope can be difficult for reactive surfaces, where the interaction forces between the tip and the samples can be relatively large. A force detection scheme that makes use of a modified cantilever beam and senses the force gradient through frequency modulation is described. The reconstructed silicon (111)-(7x7) surface was imaged in a noncontact mode by force microscopy with atomic resolution (6 angstroms lateral, 0.1 angstrom vertical).     

Studying atomic structures by aberration-corrected transmission electron microscopy

Seventy-five years after its invention, transmission electron microscopy has taken a great step forward with the introduction of aberration-corrected electron optics. An entirely new generation of instruments enables studies in condensed-matter physics and materials science to be performed at atomic-scale resolution. These new possibilities are meeting the growing demand of nanosciences and nanotechnology for the atomic-scale characterization of materials, nanosynthesized products and devices, and the validation of expected functions. Equipped with electron-energy filters and electron-energy-loss spectrometers, the new instruments allow studies not only of structure but also of elemental composition and chemical bonding. The energy resolution is about 100 milli-electron volts, and the accuracy of spatial measurements has reached a few picometers. However, understanding the results is generally not straightforward and only possible with extensive quantum-mechanical computer calculations.     

Single molecules observed by near-field scanning optical microscopy

Individual carbocyanine dye molecules in a sub-monolayer spread have been imaged with near-field scanning optical microscopy. Molecules can be repeatedly detected and spatially localized (to approximately lambda/50 where lambda is the wavelength of light) with a sensitivity of at least 0.005 molecules/(Hz)(1/2) and the orientation of each molecular dipole can be determined. This information is exploited to map the electric field distribution in the near-field aperture with molecular spatial resolution.