A new technique for growing single-crystal nanorods and
controlling their shape using biomolecules could enable the
development of smaller, more powerful heat pumps and devices
that harvest electricity from heat.
Researchers at Rensselaer Polytechnic Institute have
discovered how to direct the growth of nanorods made up of two
single crystals using a biomolecular surfactant. The
researchers were also able to create "branched" structures by
carefully controlling the temperature, time, and amount of
surfactant used during synthesis.
"Our work is the first to demonstrate the synthesis of
composite nanorods with branching, wherein each nanorod
consists of two materials — a single-crystal bismuth telluride
nanorod core encased in a hollow cylindrical shell of
single-crystal bismuth sulfide," said G. Ramanath, professor of materials science and engineering at
Rensselaer and director of the university’s Center for
Future Energy Systems, who led the research project. "Branching
and core-shell architectures have been independently
demonstrated, but this is the first time that both features
have been simultaneously realized through the use of a
biomolecular surfactant."
Most nanostructures comprised of a core and a shell
generally require more than one step to synthesize, but these
new research results demonstrate how to synthesize such
nanorods in only one step.
"Our single-step synthesis is an important development
toward realizing large-scale synthesis of composite
nanomaterials in general," said Arup Purkayastha, who worked on
the project as a postdoctoral researcher at Rensselaer and is
now a scientist with Laird Technologies in Bangalore,
India.
Because of their attractive properties, core-shell nanorods
are expected to one day enable the development of new nanoscale
thermoelectric devices for power generation, as well as
nanoscale heat pumps for cooling hot spots in nanoelectronics
devices.
"Our discovery enables the realization of two very important
attributes for heat dissipation and power generation from
heat," Ramanath said. "First, the core-shell junctions in the
nanorods are conducive for heat removal upon application of an
electrical voltage, or generating electrical power from heat.
Second, the branched structures open up the possibility of
fabricating miniaturized conduits for heat removal alongside
nanowire interconnects in future device architectures."
The researchers discovered that synthesis at high
temperatures or with low amounts of the biomolecular surfactant
L-glutathonic acid (LGTA) yields branched nanorod structures in
highly regulated patterns. In contrast, synthesis at low
temperatures or with high levels of LGTA results in straight
nanorods without any branching. It is interesting to note that
at the point of branching, atoms in the branch resemble a
mirror image of the parent crystal – a finding that reinforces
Ramanath’s conclusion that LGTA is able to induce branching
through atomic-level sculpture.
"Since LGTA is similar to biological molecules, our
discovery could be conceivably used as a starting point to
explore the use of proteins and enzymes to atomically sculpt
such nanorod architectures through biological processes," said
Ramanath.
Results of the study, titled "Surfactant-Directed Synthesis
of Branched Bismuth Telluride/Sulfide Core/Shell Nanorods,"
were recently published online and will be featured in an
upcoming issue of the journal Advanced Materials.
The full study may be viewed at: http://www.sciencenet.cn/upload/blog/file/2008/6/200863175450831473.pdf
Along with Ramanath and Purkayastha, co-authors of the paper
include: Theodorian Borca-Tasciuc, associate professor of
mechanical, aerospace and nuclear engineering at Rensselaer;
Rensselaer materials science and engineering postdoctoral
researcher Huafang Li; Rensselaer graduate students Makala S.
Raghuveer and Darshan D. Gandhi; as well as materials science
and engineering professor Raju V. Ramanujan, assistant
professor Qingyu Yan, and postdoctoral researcher Zhong W. Liu
of Nanyang Technological University in Singapore.
The research project was supported by the Interconnect Focus
Center New York through MARCO, DARPA and New York state. The
National Science Foundation and Honda Motor Co. also supported
this project through research grants.
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