Predicted dimensions of nano-blocks achieved by growing individual (PbS)32 baby crystals. STM images confirmed these dimensions. (Illustration courtesy Bowen Lab)

How
small can a chemical compound be and still retain the properties of
that same compound in bulk? With computer models and laboratory
experiments, researchers at Johns Hopkins University, collaborating with
those at McNeese State University in Lake Charles, La., and the
University of Konstanz in Germany, have determined the smallest crystal
configuration, or as they call it, a “baby crystal,” of lead sulfide.

The
team first determined the structure theoretically with computer
modeling. They then proved their model experimentally in the laboratory
by carefully depositing clusters of (PbS)32 onto a graphite surface,
where the clusters could migrate together into larger nanoscale units.

“By
using scanning tunneling microscope (STM) images to measure the
dimensions of the resultant lead sulfide nano-blocks, we confirmed that
(PbS)32 baby crystals had indeed stacked together as predicted by
theory,” said Kit Bowen Jr., the E. Emmet Reid Professor in the
Department of Chemistry at Johns Hopkins. Bowen worked on the project
with, Howard Fairbrother, also a professor of chemistry. Both are
affiliated faculty members of the Institute for NanoBioTechnology.

Bowen
explained that the baby crystal needed just 32 units of lead sulfide to
“exhibit the same structural coordination properties” of the same
material at macroscale. Nanoblocks this small would have photovoltaic
(solar power) applications.

“Determining
the size of nano and sub-nano scale assemblies of atoms or molecules at
which they first take-on recognizable properties of the same substance
in the macroscopic world is an important goal in nanoscience,” Bowen
said.

Their
research can be found in the Journal of Chemical Physics and The
Virtual Journal of Nanoscale Science & Technology. A Department of
Energy grant funded this research.

Source: Johns Hopkins University