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Sublimation of Anthracene Single Crystal

This movie shows the sublimation of a single crystal of anthracene in air. Anthracene is an organic molecule consisting of three fused benzene rings; it is solid at room temperature, but volatile enough that it sublimates directly into gas. It is similar in nature to naphthalene, which is composed of two fused benzene rings. Naphthalene was commonly used in mothballs, where the sublimation of centimeter-sized crystals over a few months creates the gas that kills the insects. Anthracene is a model organic semiconductor and is used as a scintillator for the detection of high energy photons and particles. It is also a precursor for the synthesis of pharmaceuticals and dyes.

The steps on the (001) face of anthracene imaged here are 0.9 nm high. The fastest horizontal step velocities are on the order of 20 nm/s. For fast scanning, people often quote the tip velocity in the fast scan direction (260 µm/s in this case). However, for imaging many dynamic processes, the velocity in the slow scan direction (over 540 nm/s here) determines whether you are capturing a snapshot of the process or whether there is time for the sample to change significantly during the image. Because the slow scan velocity is much faster than the step velocities, we are capturing an accurate picture of the terrace shapes. With traditional AFM scan rates there would be significant distortion of the steps. For example, at a slow enough scan rate, the retreating steps could even appear to be advancing--much like the wheels on a car can appear to spin backwards in a movie.

Many AFM images are image-processed using a "flatten" algorithm, i.e. subtraction of a different fitted function from each line of the image. This is often necessary to eliminate streaks from the image, especially in images with a small Z scale. However, the images in this movie were not flattened or even planefit individually. A planefit function (1st order in Y, 2nd order in X) was calculated once and subtracted identically from every frame in the movie. Only the overall Z offset was corrected individually for each image to keep the features in range. The ability to construct a 3-hour movie with nanometer features, without flattening, is a testament to the stability of the Cypher AFM.

Some interesting physics is visible in the movie. As the steps retreat, they get pinned, or hung up, on several defects. Just as the surface tension of a three-dimensional soap bubble acts to minimize its area, the step edges have a line tension that acts to minimize their length. The line tension combines with the pinning and sublimation to determine the curved shape of the step in two dimensions. As the crystal continues to sublimate, a pinned step bows out farther until it breaks free of the defect and races across the terrace.

The defects persist as many steps pass across them, so they must extend down perpendicular to the crystal face, into the body of the crystal. Because no step edges terminate at the defects, they do not appear to be screw dislocations. They may be edge dislocations with a Burgers vector in the plane of the image. Note that in images 999 to 1003, near the image center, a defect moves to the right and then disappears. It may have joined with an adjacent defect. Then, in image 1153, a defect (possibly the same one) becomes visible another 300 nm to the right. The sublimation of the crystal is exposing, layer-by-layer, the three-dimensional arrangement of the defects.

 


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