Understanding Crystal Growth

Selim Elhadj

After receiving a Ph.D. in chemical engineering from Virginia Polytechnic Institute and State University (Virginia Tech), Selim Elhadj has spent the last three years at Lawrence Livermore National Laboratory contributing to such diverse programs as NIF and biosciences.

“I was brought to the Laboratory because of my knowledge of crystal growth processes, which are essential for understanding thestructural dynamics of nonlinear optical materials and, surprisingly, biological membranes,” says Elhadj. Using atomic force microscopy (AFM), he helped develop a method for mitigating damage on laser optics and characterized the dynamics of surface processes in spores and bacterial membranes exposed to different environmental perturbations.

In AFM, a nanometer-size pointed tip is moved across a substrate in a raster pattern. The amount of force on the tip changes as it passes over variations, such as scratches or elevated areas on the substrate’s surface, which then deflect the tip. AFM records the deflections and reconstructs a complete topography of the surface.

When a substrate is exposed to a solventcontaining atmosphere, a nanometer-thin layer of solvent and a meniscus form where the tip contacts the substrate surface. Elhadj and his colleagues discovered that this approach provides a mechanism by which ions within the material can be transported and redistributed to dissolve mounds and fill in grooves on a material’s surface.

In an LDRD-funded study, Elhadj, Vaughn Draggoo, Alex Chernov, and Jim De Yoreo placed a laser-damaged potassium–dihydrogen–phosphate (KDP) crystal substrate into a tightly controlled atmosphere. As the meniscus passed over imperfections in the crystal, KDP molecules were dissolved from convex features and precipitated in concave ones. This redistribution of material was thermodynamically driven and well predicted by a form of the Gibbs–Thomson law, which relates surface curvature to vapor pressure and chemical potential. Says Elhadj, “The mitigation method relies on the shape-dependent solubility of the features, the contrast in their local solubility, and the molecular fluxes within the solvent layer.”

Elhadj has also worked with Ibo Matthews and Steven Yang to research methods that will mitigate defects in silica optics by using lasers to melt and vaporize silica. “We used thermographic techniques to measure the temperature of the laserexposed surfaces,” he says. “We then included these measurements in odels to predict how the optical materials change and to build diagnostic tools for process control. Measuring the temperature is essential because it represents the driving force of the observed changes relevant to laser-based mitigation.”

In a project funded by the Department of Energy and other government agencies, Elhadj and Alex Malkin used AFM to characterize bacterial spores and study structural dynamics of cell surfaces at subnanometer resolution. “Bacteria derive many of their characteristics from their environment,” says Elhadj. “We can measure their structures to deduce formulation signatures, for example, to determine if bacteria grew in their natural environment or were manufactured.”

AFM is an excellent tool for studying extremely small organisms because spore formulation can be observed in vivo using high-resolution images. “AFM is the only technique that can provide structural information at the scales we are interested in and within relevant environments,” says Elhadj. As a result of his efforts, Elhadj has helped expand AFM as a tool for mission-related applications. In doing so, he has deepened the Laboratory’s understanding of complex biological crystal growth processes.

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