Synthesis of Well-Defined Nanosize Particles and Their Role in the
Formation of Simple and Composite Monodispersed Colloids

Research Findings

  The goal of our project has been to develop the understanding of the formation of uniform nanosize and colloidal particles of various shapes and morphologies. This knowledge is essential in numerous areas of modern technology, medicine, and national security, because many properties of materials (such as optical, magnetic, catalytic, etc.) are strongly dependent on the particle size and shape.
  In order to produce materials of desired properties predictably and reproducibly, it is essential to develop techniques for preparation of monodispersed particles and to devise theoretical models to explain the mechanisms of their formation. We have made several major advances in the project. First, using an improved version of our new two-stage aggregation model, we were able to explain the size selection in CdS and Au systems, identify key parameters responsible for the dynamics of the process, and thus fit the size distribution of the growing particles at different stages of the process for different experimental protocols with a single set of two adjustable parameters.
  The two-stage aggregation model identifies two coupled growth processes. The first process involves nucleation of nanosize crystalline precursors (primary particles) from a supersaturated solution. The second process involves primary particle aggregation into larger polycrystalline colloidal particles (secondary particles). Earlier versions of the two-stage model have been successful in predicting semiquantitatively the position of the peak-shaped experimental size distributions and their widths for early times, for the gold and cadmium sulfide systems. Recently, we have developed an improvement of the model, which leads to better, quantitative agreement between experimental data and theory, and further clarifies the physical mechanisms of particle formation. We have collected extensive experimental data and were able to test the validity of the model for different experimental regimes and different time scales.
  In the new model, we have included particle-particle aggregation, of secondary particles up to a cutoff size, which is a new adjustable parameter, replacing the earlier dimer-suppression factor. Allowing for cluster-cluster aggregation, in addition to the cluster-singlet aggregation, yields broader size distributions of the final colloidal particles. This mechanism yields the width of the distribution much closer to that experimentally observed, than did earlier modeling. The model yields very reasonable quantitative agreement with all the experimental data sets available.
  For few-micron-size metal copper particles, the figures beside present our recent results on the variety of shapes obtained at different controlled conditions for making obscurant smokes for national security applications.

Research and Education Activities

  The long-term objective of our project has been to elucidate mechanisms of controlled colloidal particle synthesis, explain size and shape selection, and extend this know-how to nano-size particle formation. We have set up a comprehensive program of study of different systems, both experimentally and theoretically, utilizing the new two-stage aggregation model devised by our group. Our aim has been to develop means for effective experimental characterization of structure and morphology of obtained solids, model the growth process for better understanding of fundamental mechanisms and apply collected knowledge for improving existing and producing new commercial products for industry, medical applications, and national security applications.
  The educational impact has included research training of undergraduate students, graduate students, postdoctoral researchers, and development of a new course to introduce novel scientific concepts to graduate and undergraduate students.
  Our effort has been interdisciplinary, and we have collaborated with industrial partners from many fields of technology, such as catalysts with OMG Inc., electronic materials with Nanodynamics, Ferro, Phoenix Electronic Displays, coatings and fluorescent particles for medical diagnostics with Beckman Coulter, nanosize drugs with Elan technologies, nanolithography with Lincoln MIT Lab., fuel cells with Umicore, slurries for chemical mechanical polishing with Ferro, IBM, and Intel, and obscurant smokes for US Army.
  Our Outreach Program has included numerous lectures and presentations by the PI, co-PIs, post-doctoral researchers and students. Our research results were published in several papers in refereed journals.