The structure of water films on mica was locally modified by contact with the tip of an atomic force microscope (AFM) in a humid environment. The subsequent evolution of the film was studied by noncontact scanning polarization force microscopy. At high relative humidity (>20%), capillary condensation caused water to form droplets and two-dimensional islands around the contact point. The droplets evaporated in a short period of time, but the islands remained for much longer periods (hours). At low relative humidity (>20%), the tip contact produced a circular depression in the local polarizability. None of these structures could be observed in contact AFM images which revealed only the usual atomically flat mica surface.

Atomic force microscopy (AFM) has been used to study the effect of pressure on the structural and frictional properties of self-assembled monolayers of n-octadecanethiol on Au(111). Sharp microfabricated silicon nitride tips (tip radii 100-300 Angstrom) were used. At low load, the periodicity of the thiol layer is imaged. At higher load, the layer is observed to become disordered. At a critical contact pressure of similar to 2.3 GPa, a transition from the thiol overlayer to the Au(111) substrate periodicity is observed in the lattice resolution images. This transition is gradual and reversible. During the transition, frictional forces first increase and then decrease as the tip-sample separation decreases by a distance approximately equivalent to the thickness of the thiol layer.

Polymerized thin films based on polydiacetylenes (PDAs) undergo distinct color transitions that lend themselves to applications in biosensing, surface modification, nonlinear optics, and molecular electronics. The mechanism of the thermochromic blue to red color transition of PDA thin films was investigated at the molecular level using atomic force microscopy and at the macroscopic level with visible absorption and Fourier transform infrared spectroscopy. The thermochromic transition temperature is found to be between 70 and 90 degrees C. At the molecular level, the ordering of the film increases at the thermochromic transition and remains ordered up to temperatures well above the transition (e.g., 130 degrees C). No evidence for previously suggested entanglement or disordering of the alkyl side chains is observed. The pendant side chains rearrange from a partially disordered configuration characteristic of the blue film, to a well-ordered close-packed hexagonal arrangement in the red form. The rearrangment of the pendant side chains is linked to the formation of the red phase PDA.

A comparative study of the frictional properties of alkanethiols and alkylsilanes as a function of chain length is presented. The monolayers were produced by self-assembly on Au(lll) and mica, respectively. The same tip was used for all the experiments, and freshly cleaved mica was used as a reference. For both types of films, the frictional forces depend strongly on the number of carbon atoms in the alkane chain (CH3-(CH2)(n-1)-R). Thiols and silanes give rise to similar frictional force for the same It when n > 11, while for n < 11 the behavior is markedly different; the silanes exhibit higher friction, larger than that for the thiols by a factor of similar to 3 for n = 6. The increase in friction is attributed to the increased disorder that occurs when going from a thiol to a silane anchor or when decreasing n. It is proposed that disorder favors the increase of the number and type of low-energy modes (kinks, bending, distortions) that are available for excitation and energy dissipation.

The structure and tribological properties (friction, adhesion and wear) of boundary layer lubricants has been studied using Atomic Force Microscopy. The influence of molecular parameters such as chain length and terminal group identity on the film structure and properties has been investigated. It is found that in the case of alkylthiols and silanes, friction depends on the chain length, a result that is interpreted as due to variations in the packing of the monolayers. Long range order does not appear to be a dominant factor. The terminal groups determine the adhesive properties of the layers, both to the substrate and to the tip. We have also studied the influence of external elements such as water found in humid environments. Water is found to penetrate the layers even in the case of hydrophobic silanes, due to residual defects. Its presence affects both the structure and the wear properties of the layers.

Despite decades of study, the exact mechanism for chromatic transitions in polydiacetylene materials remains somewhat elusive. The mechanism of the blue to red color transition for overcompressed mixed lipid thin films of 10,12 pentacosadiynoic acid has been investigated by atomic force microscopy. Blue layers were prepared by the horizontal Langmuir-Schaefer deposition method. The red form was attained by thermal annealing of blue films !thermochromism). The blue films reveal micro-sized domains. Each domain shows a stripelike morphology on top of a nearly complete layer. The polymer backbone direction shows a high degree of order. A partial disorder is shown to exist along the interbackbone direction. Our study of the red form films indicates that changes occur in the micron-scale morphology of the films. Interestingly the alkyl side chains rearrange into a completely ordered structure and remain ordered well beyond the transition temperature (70 degrees C < T < 90 degrees C). Our results strongly suggest that the reorganization of the side chains is the origin of the thermochromic effect for these films. (C) 1996 American Vacuum Society.

Biological matrices can direct the absolute alignment of inorganic crystals such as calcite. Cooperative effects at an organic-inorganic interface resulted in similar co-alignment of calcite at polymeric Langmuir-Schaefer films of 10,12-pentacosadiynoic acid (p-PDA). The films nucleated calcite at the (012) face, and the crystals were co-aligned with respect to the polymer's conjugated backbone. At the same time, the p-PDA alkyl side chains reorganized to optimize the stereochemical fit to the calcite structure, as visualized by changes in the optical spectrum of the polymer. These results indicate the kinds of interactions that may occur in biological systems where large arrays of crystals are co-aligned.

A scanning probe microscope operating in air with interchangeable atomic force-friction force (AFM-FFM) and electronic-tunnelling (STM) heads is presented. Our AFM operates in the so-called contact mode and utilizes the optical-lever detection method which allows simultaneous measurement of the topography as well as the lateral force. The set-up also contains an optical microscope to control both the sample and the probe laser spot on the cantilever. The experimental method to change from AFM to STM operation is based on the use of the probe laser beam and the optical microscope. The maximum scanning area is (24×24) μm2 and it is well embraced in the optical-microscope visual field. The microscope attains atomic resolution in air in both AFM and STM configuration. Its performance is demonstrated on the surface of different samples.

The effects of laser radiation on cantilevers used in atomic force microscopes have investigated. Optical parameters of the cantilevers, deflection due to radiation pressure and thermal effects have been measured on cantilevers freely moving in air. For cantilevers with a gold coating we have found thermal effects at least two orders of magnitude larger than for uncoated cantilevers.