Molecular Foundry Seminar

"Manipulation of Mineral Crystallization and Nanocrystal Properties with Small Organic Molecules,"

Dr. Jonathan Lee, Lawrence Livermore National Laboratory,

Tuesday, March 30th at 1:30 pm, Room 67-3111

Abstract:

Organic molecules are known to modify crystallization pathways and the properties of nanocrystalline materials. In this presentation, synchrotron-based studies into the effects of organic monomers on two distinct materials systems will be discussed:

(1) Directed crystallization on organothiol self assembled monolayer (SAM) templates

In nature, bio-organisms utilize surface matrices of organic molecules to exert remarkable control over the mode of mineral crystallization from solution. Such precise levels of control over crystal structure (and material properties) are unparalleled in current laboratory synthesis. Hence, an improved understanding of the underlying physical processes is of paramount importance in developing new material growth technologies. SAMs of organothiol monomers act as templates for patterned crystallization, thereby mimicking the natural processes of biomineralization. As a result, the SAMs represent suitable models from which to characterize the interactions between organic and inorganic phases during crystal nucleation and growth, which reside at the heart of controlled biomineralization. X-ray absorption spectroscopy (XAS) and photoemission spectroscopy (PES) have been applied in the study of CaCO3 crystallization on SAMs prepared from the isomers of mercaptophenol and mercaptodecylbenzoic acid. Analysis of the XAS/PES data enables assignment of the SAM structure at various stages of the nucleation and growth and indicates that the monolayers of each isomer follow distinct pathways during crystallization. Meanwhile, the SAMs exhibit vastly different abilities to direct the growth of CaCO3. The implications of these results regarding the mechanisms of ‘templated’ crystallization will be addressed.

(2) Semiconductor nanocrystals (NCs) overcoated with different organic surfactants
Semiconductor NCs are of considerable technological interest due to their size-dependent optical and electronic properties. While changes in the optical properties of semiconductor NCs as a function of size can be explained in terms of quantum confinement effects, further understanding is essential in order to optimize their performance in new and existing technologies (including photovoltaic devices). Addressing the effects of the organic molecules used to passivate the NC surfaces represents a central component of this endeavor: surface interactions can play a particularly significant role in determining the physical properties of the NCs because they have a large proportion of surface atoms. Synchrotron based techniques (XAS, PES and x-ray magnetic circular dichroism) have been used to investigate the effects different surface passivants on CdSe NCs, a canonical nanoscale material among the group II-VI semiconductors. The experimental data indicates profound, ligand-induced, changes in the electronic structure and magnetic susceptibility of the NCs, which will be compared with theoretical treatments and a range of physical properties reported for nanoscale CdSe.
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344