AFM Applied in Molecular-Scale Flattening of DAST Crystals
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a group at the Institut für Theoretische und Angewandte Physik (ITAP) of Stuttgart University carried out a demonstration run with over five billion particles. With 512 processing nodes and a total memory of 256 GB, the program IMD (ITAP Molecular Dynamics) performed a simulation with 5,180,116,000 particles. The atoms, interacting via LennardJones potentials, were placed in a cubic fcc lattice cell with an edge length of 1540 atom distances or 0.42 µm (for Al). This means that the simulations are arriving at scales comparable to the wavelength of visible light and to the width of the conductor paths of the computer. Six time steps were carried out and took a computer time of 2,328 s. According to the researchers, 223 time steps would require about a day, a typical simulation more than a month. The real challenge, they said, is the output data which cannot be stored directly but has to be analyzed parallel to the simulation. IMD is a software package designed to perform classical molecular dynamics simulations on massively parallel computers. The interactions are not limited to pair potentials. The program permits embedded atom potentials, three-body potentials, and anisotropic Gay-Berne-potentials. In addition to the common thermodynamic ensembles, a number of boundary conditions are implemented enabling the program to deform a sample, to stress load it, or to simulate shock waves.
AFM Applied in Molecular-Scale Flattening of DAST Crystals Due to their nonlinear optical properties, certain organic crystals are useful in applications such as ultrahigh-speed signal detection and integrated circuit diagnosis. While the polishing of these soft organic crystals is required for optimizing light transmittance, the usual polishing and cutting techniques are often insufficient for flattening these materials. A research group from the Institute for Chemical Reaction Science at Tohoku University and the Intelligent Cosmos Research Institute has invented a molecular-scale polishing method utilizing atomic-force microscopy (AFM). As reported in the August 1 issue of Optics Letters, ion pairs of the 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal may be removed by applying some force to the AFM cantilever tip. Single DAST crystals were grown from seed crystals by lowering the temperature of a solution saturated with DAST. The flattening of the (001) surface was performed by the scanning action of the AFM. “When the force applied to an AFM tip is larger than that of the interlayer bond, the MRS BULLETIN/SEPTEMBER 2000
layer(s) that are exposed on the surface will be removed,” the scientists reported. An approximately 10 nN force was applied to the tip (with a tip curvature of 20 nm) during the scan. Three stepped terraces appeared after the first five scans, and another five after eight scans. The average step height was approximately 0.9 nm. This is a good agreement with the interlayer distance of 0.893 nm. The scientists generated a terrace area of 250,000 nm2. A few existing terrace steps are not expected to
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