PHILADELPHIA –- Collaboration by chemists, physicists and
materials scientists at the University of Pennsylvania has created
a simple and inexpensive method to rapidly grow centimeter-scale
membranes of binary nanocrystal superlattices, or BNSLs, by
crystallizing a mixture of nanocrystals on a liquid surface.
The study demonstrates a new and spontaneous way to grow
long-range-ordered BNSL membranes with rigorous control of
nanocrystal size, shape and concentration by combining two types of
nanocrystals and assembling them during a drying stage at the
surface of a liquid under normal conditions.
The method overcomes several limitations of the existing
assembly strategies and produces large, free-standing membranes
that can be transferred to any desired substrate such as silicon
wafers, glass slides and plastic substrates, allowing the
nanocryatalline films to be introduced at any stage in the device
fabrication process.
The team demonstrated the potential for integrating these novel
materials by growing millimeter-scale superlattice membranes
containing iron oxide nanocrystals of two different sizes and
incorporating the membranes into magnetoresistive devices.
Measurements showed that the magnetoresistance of the resulting
device was dependent on the structure of the BNSL and therefore
controllable.
The physical properties intrinsic in these nanocrystals --
nanometer sized crystalline building blocks - offer a modern twist
on the studies of interfacial assembly that reach as far back as
Penn founder Benjamin Franklin and his studies of oil spreading on
water in the 1770s.
Single and multi-component nanocrystal films are already under
intense investigation by researchers as enablers of novel optical
technologies that range from low-cost solar cells, light-emitting
diodes and photo detectors and also in electronic systems that
include field-effect transistors and solid-state thermoelectric
coolers and generators and magnetic technologies that include
magnetic recording materials and magnetic sensors and even as
tailored electrocatalytic and photocatalytic films.
Co-assembly of two types of nanocrytals into BNSLs provides a
low-cost, modular route to program the self assembly of materials
with a precisely controlled combinations of properties. Advances in
these complex interfacial assemblies and improvements in the
transfer of single-component nanocrystal membranes in the past few
years have heightened anticipation that this control could be
extended to much more complex systems.
This Penn study establishes a route to free-standing large-area
BNSLs membranes with the added ability to laminate them on any
arbitrary substrate.
"Fundamentally, growing BNSLs on a liquid surface will shed
light on the mechanisms of multi-component nanocrystal assembly,
which are critical to new concepts in self-assembly based
nanomanufacturing," said Christopher B. Murray, the Richard Perry
University Professor of Chemistry and Material Science and
Engineering at Penn.
The research, funded by the U.S. Army Research Office and a
National Science Foundation Materials Research Science and
Engineering Centers Award, is published in this week's
Nature.
Existing strategies for growing BNSLs involve a more complex
process of evaporating a two-nanocrystal solution on a solid
substrate under carefully regulated temperature and pressure that
influence BNSL formation. The method suffers from several
limitations, most notably a limited choice of substrate, nucleation
of irregular micrometer-sized, isolated islands of BNSLs on the
substrates and an inability to transfer them once formed.
"Given the fact that this novel assembly strategy is general for
different nanocrystal combinations, we anticipate that membranes of
quasicrystalline BNSLs and ternary nanocrystal superlattices will
also be grown by this method, greatly expanding the systems that
can be explored" Murray said. "Our dream is to program the
organization of materials on all lengths scales for nanometers to
millimeters combining the desirable physical properties multiple
nanoscale systems. Fundamentally we are focused on identifying,
understanding and optimizing new synergistic interactions in
nanomaterials and in exploiting these emergent properties in new
devices and systems."
SOURCE
