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July 16th, 2009

Group designs inverse photoconductors

Published on July 16th, 2009 | By: pwray@ceramics.org

Credit: Nature, Hideyuki Nakanishi

The Grzybowski Group at Northwestern University has been working for some time on understanding and working with self-assembling and self-organizing nanomaterials. They particularly have been focused on the field of “dynamic self-assembly” whereby they develop “rules that allow ‘synthesis’ of self-assembling systems from various types of interactions and/or phenomena.”

One new product of their work is a method to turn a material into an “inverse” photoconductor. Generally speaking, photoconductors are materials in which the conductivity of the material can be modified by shining a light on it. More accurately – until now – the conductivity could only be increased via light. But a letter just published in Nature indicates that Bartosz Grzybowski and his colleagues have figured out a way to engineer a self-assembling monolayer in such a way that a beam of light decreases conductivity.

From the letter:

“The remarkable feature of these plasmonic materials is that the sign of the conductivity change and the nature of the electron transport between the nanoparticles depend on the molecules comprising the self-assembled monolayers (SAMs) stabilizing the nanoparticles. For SAMs made of electrically neutral (polar and non-polar) molecules, conductivity increases on irradiation. If, however, the SAMs contain electrically charged (either negatively or positively) groups, conductivity decreases. The optical and electrical characteristics of these previously undescribed inverse photoconductors can be engineered flexibly by adjusting the material properties of the nanoparticles and of the coating SAMs. In particular, in films comprising mixtures of different nanoparticles or nanoparticles coated with mixed SAMs, the overall photoconductance is a weighted average of the changes induced by the individual components.”

Interestingly, Grzybowski has made a strategic decision to forge his research group as a multidisciplinary group composed of people with expertise in physical, inorganic, and analytical chemistry, statistical physics and thermodynamics, cell biology and biological chemistry. (A personal observation: It seems like I hear a lot about multidisciplinary work going on at Northwestern.)

Another interesting aspect of the group is there work to apply principles and techniques of SA and SO at the macro level. From their website:

“[We] are acutely interested in the societal and global aspects of self-assembly and self-organization. One example here is the study of networks of chemical reactions (cf. Angew. Chem. 2005, 2006), where our group discovered how apparently autonomous agents (here, chemists) give rise to a well defined, higher-order structure of the chemical universe (i.e., the network of all known reactions). By representing this universe as a directed graph and by analyzing it using stochastic modeling and graph theory, we were able to identify a set of statistical laws that govern all synthetic transformations carried out to date or to be carried out in the future. The amazing regularity embodied in these laws allows identification of most useful chemicals, prediction of the efficiencies of new chemical transformations, the properties of most likely products, and more.”

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