This research explores the crystallization of needle-like whiskers of KCl on the surface of porous silica nanoparticle coatings. In a typical whisker growth experiment, a substrate with a porous coating is partially immersed in an aqueous KCl solution and the assembly placed in a controlled humidity chamber.  The aqueous solution is pulled into the coating by capillary action, evaporation from the pore space leads to supersaturation, and then KCl crystals nucleate and grow.  Whisker growth dominates when the relative humidity is between 30 and 80%, the solution concentration is in the range of 0.03 – 0.15 g/ml and nanoporous silica coatings are used.  The anisotropic morphology of the resulting crystals is unexpected based on KCl's cubic crystal structure.  Uncovering the mechanism of whisker formation and controlling the whisker microstructure are the primary goals of the research.  In the second year of the project, the focus was on the use patterned porous coatings on Si substrates as platforms for growth and continued exploration of whisker growth mechanisms.

Patterned silica coatings were prepared by dip coating aqueous suspensions of silica particles onto Si substrates that had a pattern of lines of hydrophilic oxidized silica separated by spaces of hydrophobic fluorinated self assembled monolayers. The hydrophobic/hydrophilic pattern was created by a photolithographic method.  During dip coating, porous coatings formed only on the hydrophilic lines.  The patterned substrates were immersed in KCl solution such that the solution was transported up into the coated lines.  KCl whiskers grew selectively onto these lines, demonstrating that the crystal growth can be confined.  Thus far, whisker growth has only occurred on the larger line widths (e.g, 50 µm).

Studies of whisker growth mechanism included experiments to characterize dislocations in the whiskers and in situ monitoring of whisker growth.  After the first year of the project, we postulated that whisker growth occurs by ion addition to an axial dislocation at the tip of the whisker.  This mechanism, originally proposed by Sears (J. Chem. Phys., 26, 1957, p. 1549), requires the presence of a dislocation and transport of solution up the whisker by a surface tension gradient.  In the second year, we searched for evidence for this "tip-growth" mechanism and considered other mechanisms.  Etching methods were used in an attempt to reveal dislocations in the whiskers via etch pits.  A broad range of etchant solutions and conditions were explored, but no etch pits were found.  Fluorescent dyes were added during whisker growth to document a thin liquid layer on the whisker using confocal microscopy.  In spite of preliminary evidence for the liquid layer, no evidence for this layer was found during this set of experiments, which were performed under more controlled relative humidity conditions than the preliminary study. In an attempt to gather more direct evidence on mechanisms, whisker growth was monitored in situ using a video camera focused on the actual experimental set-up and an optical microscope coupled with a modified whisker growth set-up.  Both in situ techniques, which were performed on the lower end of the relative humidity range (30 - 55%) showed evidence of an alternative mechanism – "base-growth".  In this mechanism, the whisker grows by ion addition to the bottom face of the crystal, which is in contact with a layer of solution from the porous coating.  In this mechanism, the whisker is essentially pushed up from beneath.  In the experiments, a visible feature on the whisker, such a large kink or attached crystallite, is tracked with time.  Upward motion of the feature indicates the base-growth.  Amelinckx (J. Chem. Phys., 31, 1959, p. 1687) first proposed this mechanism.  He speculated that a dislocation on the growing crystal face was necessary, but two-dimensional nucleation and growth is also plausible.  Two other observations are consistent with the base-growth mechanism.  First, when whiskers fell onto the coating surface, they grew upward as sheets.  Second, SEM micrographs of some whiskers show a silica particle layer on their tips, indicating that the KCl crystal nucleated below the coating and then pushed it upward.  The substrate beneath the coating appears to play a role in the mechanism as the latter observation only occurred for whisker growth on coatings prepared on Si substrates.

The goals for the final segment of the research are to control whisker growth through the use of more finely patterned coatings, to study the very early stages of nucleation and crystal growth, and to explore at least one application for the whiskers.

This research has allowed the PI to learn more about the transport phenomena and crystallization, and to use her experience with coatings in a new research area.  The graduate student researcher has developed experimental skills and acquired a broad knowledge base in chemical engineering and materials science principles.