Spheres Spring 03 - the California NanoSystems Institute

Spring 2003
CALIFORNIA
NANOSYSTEMS
I N S T I T U T E
ExpandingTheFrontiersOfNanosystems
Editors
Paul Bonvallet
Stuart Cantrill
Payam Minoofar
Spheres
contents
Spring 2003
Layout Editor
Rob Ramirez
Published by the CNSI Young Investigators Society at UCLA
Graphic Design
Scott Vignon
Jason Wong
Paul Bonvallet
Stuart Cantrill
Rob Ramirez
2
2 NanoNeighborhood
Membership Officer
Tae H Han
Cover Art
“Expanding the Frontiers of
Nanosystems” by Jason Wong
and Scott Vignon
Contact Address
CNSI
UCLA, 6722 Boelter Hall
Box 957151
Los Angeles, CA 90095-7151
USA
Subscription
Please send requests for a
quarterly subscription along
with your name, title, contact
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State of the CNSI Address
BY J FRASER STODDART
BY CNSI OUTREACH PROGRAM
Materials Discovery at UCLA
BY MATERIALS CREATIONS TRAINING PROGRAM
NanoHour
BY PAUL BONVALLET
Photography
Rob Ramirez
Acknowledgements
Fraser Stoddart and the
CNSI Staff –Wendy Nishikawa,
Roy Doumani, Derrick Boston,
Erick Burchfield, Bonita
Gutierrez
and Natalie Blyumkina
The CNSI Faculty Members
Cari Pentecost
Steve Joiner
Feature Article
Nanoscience in the Community
Contributors
Paul Bonvallet
J Fraser Stoddart
Distribution
Sanhita Dixit
Amar Flood
Volume 1 Issue 3
3 UCLA NanoSystems Seminar Series Reviews
j campbell
3
Devices from Polymer Growth scott
4
Unimolecular Rectifiersmetzger
5
6
BY RICH FAULHABER AND PAUL BONVALLET
robert
BY MICHAEL TOMCSI AND AMAR FLOOD
craig
Nanostructures for Microelectronicshawker
BY EDUARDO FALCAO AND HIEU DUONG
geert-jan
Sweet Protein Interactionsboons
BY ALSHAKIM NELSON AND JASON BELITSKY
paul
7
Single Molecule Electromechanicsmceuen
8
Time & Space Resolved Chemistry de schryver
9
Bio-Inspired Calcite Crystals aizenberg
10
Shell Crosslinked Nanoparticles wooley
11
BY LISA WESOLOSKI AND GREGORY HO
frans
BY JAMES MATTHEWS AND ROB RAMIREZ
joanna
BY ROBERT JOST AND LUIS CAMPOS
karen
BY DEBORA BONTEMPO
stan
World’s Smallest Computer williams
BY HSIAN-RONG TSENG
12 Photographs of Review Authors
SPHERES ¡ Vol. 1 Iss. 3 ¡ www.cnsi-uc.org
Excerpt from the ‘State of the CNSI Address’
T
hese days, when people speculate about the next technological revolution, their minds automatically turn to nanotechnology.
Where might this nanotechnological revolution occur? The answer is here at the doorsteps of the CNSI. It is the goal of the
CNSI to become the global focus of creativity in nanosystems-based research and so set the scene for the nanotechnological revolution
to happen right here in California. Who and what will sustain the creativity, heralding the renaissance and leading to the revolution?
The answer is the CNSI, which will aim to build up its critical mass of talented faculty and students over the next two years and then
to provide them with state-of-the-art facilities to work their small wonders in highly integrated multidisciplinary communities as if by
magic. Why should the CNSI be a focal point for creativity in nanosystems-based research? The answer is that we start from the position
of having extremely strong pools of excellence. It will be the goal of the CNSI to harness and leverage this excellence by creating an
intellectual life that will generate intellectual property in abundance. When will there be a critical mass of creative individuals within
the CNSI to do the fundamental research that will underpin the emergence and growth of an industry built around nanotechnology? The
answer is now, and even more so in the near future. It will be the aim of the CNSI to promote and foster an integrated systems-oriented
approach to fundamental research at the nanometer scale level in order to initiate a sequence of events in the commercial and business
sectors of the State of California that will bring some substantial well-being to all of its citizens.
J Fraser Stoddart
CNSI Director
14 May 2003
NanoNeighborhood
Nanoscience in the Community
T
he CNSI Outreach Committee is a group of UCLA
graduate students and postdoctoral researchers committed
to integrating experiments on nanoscience and nanotechnology
into the 9th and 10th grade science curriculum in the Los Angeles
Unified School District (LAUSD). Under the leadership of
Professor Sarah Tolbert, the committee members have designed
a series of “hands-on” experiments, covering photolithography,
ferrofluid behavior, magnetic self-assembly, simple solar cells, and
a homemade STM, to be introduced to LAUSD teachers in a series
of summer workshops. With funding from a recently awarded
NSF grant, the committee is preparing several thousand kits for
the high school science teachers to conduct these experiments in
their classrooms in the fall of 2003. This joint effort of the CNSI,
the NSF, the Materials Creation Integrated Graduate Education
and Research Training (IGERT) program, Center X at the UCLA
Graduate School of Education and Information Studies, and the
LAUSD, provides an opportunity for graduate students to work
with the community, help improve the quality of science education
in our society, and enhance the public perception of science. To
learn more about this stimulating and innovative project, contact
Professor Tolbert at [email protected].
CNSI Outreach Committee
Materials Discovery at UCLA
T
he Materials Creation Training Program (MCTP) is one
of three Integrated Graduate Education and Research
Training (IGERT) Programs at UCLA funded by the National
Science Foundation. This multidisciplinary program focuses
upon training the next generation of scientists and engineers in the
synthesis and development of new materials as well as the design,
construction, and characterization of electronic and photonic
devices based upon those materials. Students from Physics &
Astronomy, Chemistry & Biochemistry, Chemical Engineering,
Materials Science & Engineering, Mechanical & Aerospace
Engineering, and Electrical Engineering are selected for the
MCTP at the end of their first year of graduate study. Further
details regarding this pioneering program can be obtained online
at http://mctp.chem.ucla.edu.
NanoHour
N
anoHour is a cross-disciplinary gathering of students,
faculty, and postdoctoral researchers interested in a
“sneak preview” of the research to be presented by the next
UCLA NanoSystems Seminar speaker. The discussion is held in
an informal setting (with refreshments!) and led by the student
host of the upcoming guest. NanoHour presenters are also
welcome to introduce their own research interests with an eye
towards developing new modes of collaboration which transcend
the traditional boundaries between physics, chemistry, the life
sciences, and engineering. All are welcome to join; contact Paul
Bonvallet at [email protected] for details.
Paul Bonvallet
NanoHour Chair
From left to right: Alshakim Nelson, Jason Belitsky, Robin Hayes, Sarah Tolbert, Branden
Brough, Paul Bonvallet, Shabnam Virji, and Rob Ramirez. Not Pictured: Adam Braunschweig,
Erica DeIonno, Amar Flood, and Bob Jost.
2
SPHERES ¡ Vol. 1 Iss. 3 ¡ www.cnsi-uc.org
Scott
Devices from Polymer Growth
By Rich Faulhaber and Paul Bonvallet
F
or more than a decade, the IBM Research DiviStandard “top down” manufacturing methods such as
sion has maintained a document called the Globoptical, electron-beam, and nanoimprint lithography are
al Technology Outlook which identifies emerging trends
well-established and robust, though not suited for conin information technology. In step with this philosophy,
structing structures smaller than 50 nm. The “bottom-up”
J Campbell Scott of the IBM Almaden Research Center
approach, which utilizes molecular processes like selfprovided an overview of the strengths and limitations of
assembly and polymerization, is appropriate for making
semiconductor-based technology, as well as an analysis
features below 10 nm but relies upon developing techof the promise held by nanoscale electronic devices.
nologies that are unable to achieve exact feature positionThe future of silicon-based microprocessors is often
ing over large surface areas. Researchers at IBM have
discussed in terms of Moore’s Law, the observation that
developed a novel approach for nanofabrication which
the number of transistors on planar integrated circuits
combines the utility of conventional semiconductor lidoubles approximately every two years. The exponential
thography with the tunable power of polymerization.
performance increase of silicon cannot continue forever,
The key to the IBM method is imbedding an “inhowever, because of inherent
imer”
(initiator/monomer)
physical limitations. For excompound into a curable phoample, as the feature size on
topolymer used to make the
semiconductor chips contininitial pattern (Box). This patues to shrink, the development
tern is made from a nanoimof new optical lithographic
print lithographic technique.
techniques for their fabricaA second polymer is then
tion presents increasingly difgrown onto the inimer sites via
ficult technical and economic
atom transfer radical polymerchallenges, and the leakage
ization (ATRP). Feature sizes
of electrical power between
down to 5 nm can be made
electronic components within
with this technology. The next
such a small circuit becomes
step to making a transistor
problematic. While a handful
– and the one which is currentof design improvements have
ly proving to be more difficult
helped to alleviate some of the
than expected – is the linking
problems associated with elecof the two terminals (source
tronic miniaturization, it has
and drain) via a conducting
become clear that new techchannel (gate) whose conducnologies must be developed if
tivity can be controlled by an
nanoscale electronic devices
applied voltage.
Although
are to become a reality. Nathere are many challenges facscent systems, however, face
ing this new technology, Scott
the “innovator’s dilemma” in
and his group are confident
that their technical specificathat a working, viable alternations are dictated by currently
tive to silicon transistors is on
existing technology. For a new Box. Nanoscopic surface patterning by controlled graft the horizon.
product to be economically polymerization.
As a physicist curviable, Scott suggests that it
rently working as a chemist
should complement or extend the incumbent technoland businessman, Scott was well-suited to address the
ogy, while seeking to secure a well-defined niche in the
broad array of disciplines represented by the California
market.
NanoSystems Seminar audience. He moved seamlessly
The quest for nanoscale electronic devices focuses
between the technical aspects of his research and the
on the development of a working nanoscale field effect
broader economic and philosophical issues that affect
transistor that can be produced quickly and economically.
scientists who endeavor to develop the next generation of
SPHERES ¡ Vol. 1 Iss. 3 ¡ www.cnsi-uc.org
jcampbellSCOTT
IBM Almaden
3
Metzger
Unimolecular Rectifiers
By Michael Tomcsi and Amar Flood
W
robertMETZGER
University of
Alabama
hile much of the focus of the drive for smaller
electronics has been in the form of shrinking
wires and switches, Robert Metzger of the University of
Alabama has taken to shrinking other components. Specifically, Metzger and his coworkers have tackled the
problem of shrinking rectifiers.
The largest class of the diode family, rectifiers are
mainly used to block electrical conduction in one direction while allowing it in another. While there are a variety
of inorganic systems used for this purpose, little headway
has been made in finding alternatives that could shrink
these systems down further.
In 1974, Aviram and Ratner proposed the notion that
a properly designed molecule could be used for rectification. The system that they proposed consisted of three
major parts. There is an electron-rich portion (electron
Box. (a) Chemical structure of a quinolinium-based molecular
rectifier. (b) Schematic diagram of the molecular rectifier
device. (c) Current-voltage curve illustrating rectification.
4
donating), a non-conducting spacer, and an electron-poor
portion (electron accepting). In theory, with appropriate
metals, such devices should show rectification of current
when the various energy levels of all the components are
appropriately tuned.
While Aviram and Ratner’s proposed “Gedankenmolekül” (thought molecule) has never been synthesized,
Metzger and coworkers have shown that a variety of
other molecules can be used as organic molecular rectifiers. Such systems include pyridine-based organic dyes,
fullerene-based compounds, and quinolinium salts. All
of these systems are based on the Aviram-Ratner ansatz
of unimolecular rectification, namely a donor-spacer-acceptor motif, with the quinolinium salts being the primary
focus.
Devices based on quinolinium salts were demonstrated (Box) in 1990 to possess some rectifying behavior.
These initial studies showed that although the molecule
existed in a charged zwitterionic ground state, it was believed to conduct via an uncharged excited state. This
highly polar molecule was, however, only assembled in
multilayer systems.
Metzger studied the rectification of a monolayer of
one quinolinium salt between two aluminum electrodes.
They discovered rectification ratios of 26 at ±1.5 V with
no temperature dependence in the system. He also demonstrated that the rectification is derived from the molecular monolayer as opposed to the oxide layer that usually
forms on aluminum electrodes (Schottky barrier formation). In particular, Metzger replaced the aluminum with
gold, upon which oxides are not formed. The deposition
of the top electrode necessitated the development of an
ingenious technique, one he termed “cold gold”. The
use of gold led to a dramatic increase in the rectifying
behavior for most cases. Metzger pointed out that that
there is a loss of the rectification over multiple cycles,
with a dramatic decrease often occurring after six cycles.
Although this work represents a demonstration of Aviram
and Ratner’s theory, the poor cycling behavior, coupled
with the occurrence of system saturation, makes these
systems far from ideal.
Professor Metzger’s pioneering work in the area of
unimolecular rectification, his appreciation of the original
theories, and his frank admission of device shortcomings,
are key elements in the healthy development of the relatively new field of molecular electronics.
SPHERES ¡ Vol. 1 Iss. 3 ¡ www.cnsi-uc.org
Hawker
Nanostructures for Microelectronics
By Eduardo Falcao and Hieu Duong
A
s we demand higher capacity and reliability
Applications in information storage, optical devices,
of electronic devices and smaller features in
and catalysis have contributed to the development of new
circuitry, new procedures to prepare well-defined nanopatterning techniques. Hawker and coworkers developed
structured materials are required. At the IBM Almaden
a technique to produce ordered arrays of metallic nanodots
Research Center, Craig Hawker and his colleagues are deand nanoporous metallic films. Using block copolymers
veloping techniques based mainly on macro- and suprathat self-segregate, they could create well-defined, hexmolecular chemistry to create such materials. Hawker’s
agonally packed cylinders of the polymer with lower
previous achievements include the convergent synthesis
mole fraction embedded in the second polymer. They
of dendrimers, and he has made important contributions
forced the cylinders to orient perpendicular to a substrate
to the field of living radical polyby coating a layer of a random comerization.
polymer on the substrate surface.
In his presentation, Hawker
Decomposing and washing off one
addressed many significant chalof the polymers – in his example,
lenges facing the process of propoly(methylmethacrylate) – furgressively miniaturizing integrated
nished a polystyrene template for
electronic circuits. A couple of
metal vapor deposition. After metexamples focused on the requireal coverage and elimination of the
ment of a practical material with
remaining polymer, the substrate
dielectric constants lower than
was left with the arrays of metal
2.0 and the need to controllably
nanodots or porous metallic films
pattern ordered arrays in the low(Box 2). The fabrication process
nanometer scale. He eloquently
afforded arrays of metal nanodots
demonstrated that one could
(arrays of Au, Ti, AuPd, Co, SiO2
circumvent formidable problems
were obtained) whose memory
by ultimately employing polymer/
storage capacity is projected to be
dendrimer self-assembly and phase
much higher than that capable with
separation.
today’s conventional technology.
Increase in feature and wir- Box 1. Procedure for generating nanopores in an
The lecture culminated with
ing densities requires insulating SSQ matrix using well-defined dendrimers.
the presentation of the IBM “Milmaterials with dielectric constants
lipede”, an array of thousands of
(k) much lower than that of the traparallel AFM tips that can indiditionally used silicon oxide (k ~ 4.0). Organosilicates,
vidually write to or read from a surface, and, in time,
such as poly(silsesquioxane) (SSQ, k ~ 2.8), have the
may revolutionize information storage technology. Apphysical properties necessary for those applications, beplication of this powerful device, however, requires very
ing a potential substitute for SiO2. Introduction of porosspecific surface media. The ideal material would be one
ity has been studied as a means of decreasing and controlthat is soft for writing, yet hard for reading. With this
ling the dielectric constant of SSQ (as well as other matecondition in mind, Hawker and his team are currently
rials), but the precise control of the size and distribution
developing materials possessing this important interplay
of the pores is critical. Hawker’s approach consists of
of properties.
using macromolecular architectures that self-assemble in
the SSQ matrix, forming well-defined nanoparticles that
act as porogens. Porogens comprised of dendrimers and
self-crosslinking polymers (Box 1) were amongst those
studied. Thermal decomposition of these nanoparticles
ultimately produces a composite material of SSQ and air
(k = 1.01). By varying the number of nanoparticles, the
material’s dielectric constant can be conveniently tuned
Box 2. Patterning process using a polymer template and an
down to 1.5.
craigHAWKER
IBM Almaden
array of nanorods.
SPHERES ¡ Vol. 1 Iss. 3 ¡ www.cnsi-uc.org
5
Boons
Sweet Protein Interactions
By Alshakim Nelson and Jason Belitsky
A
geert-janBOONS
University of
Georgia, Athens
6
s a member of the Complex Carbohydrate Research Center and a Professor of Chemistry at
the University of Georgia at Athens, Professor Geert-Jan
Boons works at the interface of chemistry and biology
where he utilizes his understanding of organic chemistry
to build the tools necessary for probing biological events.
One clear message that stemmed from his presentation
concerned the vital role that carbohydrates play in regulating seemingly diverse physiological processes in the
human body, ranging from fertilization to immunological
responses. Indeed, it is the desire to understand exactly
how carbohydrates mediate such biological events that
has driven scientists, such as Geert-Jan Boons, to study
them in a systematic manner.
One area of Boons’ research combines synthetic
organic chemistry with molecular biology to probe just
how sugars are involved in maintaining the structure of
proteins, such as the Fc (constant chain domain) of the
Another interesting aspect of Boons’ research
involves the notion that viral sialidases, enzymes that
remove sialic acid from the terminal ends of oligosaccharides, are comprised of a binding domain and a catalytic
domain (Box). The catalytic domain is responsible for
cleaving the sialic acid, whereas the binding domains,
which are specific for galactose, introduce a multivalent
aspect into the system that improves the efficiency of
the enzyme. Interestingly, Boons found that the rate of
cleavage by the enzyme is much faster for a polymeric
substrate displaying multiple numbers of sialosides relative to the monovalent substrate. The rate was faster for
the multivalent species because the binding domain of
the enzyme could effectively act as a “handle” – guiding the catalytic domain to other uncleaved sialic acid
residues. Applying this concept of multivalency inhibitor
design, the Boons group subsequently synthesized large
polymers bearing many galactose units. As perhaps one
may have predicted,
these carbohydratepresenting polymers
are potent inhibitors
of viral sialidases,
and may lead to the
development of antiviral therapeutics
of nanoscale dimensions.
The final topic
discussed by Boons
also demonstrated
potential implicaBox. Multivalent interactions in the binding domain aids the removal of sialic acid by viral sialidase.
tions for the development of nanoscale
human IgG protein, a critical part of our immune syshuman therapeutics, particularly for the treatment of
tem. Using site-directed mutagenesis, where specific
septic shock. He has used carbohydrate-bearing polyDNA base sequences can be modified in order to alter
mers to study and potentially inhibit the events that occur
the corresponding amino acid sequences expressed in
when human immune system cells called monocytes are
the resulting protein, a single asparagine amino acid
exposed to a component of bacterial cell walls, an event
residue was changed to a cysteine. The thiol group of
which can lead to septic shock. The visual highlight of the
the cysteine residue can then be used as an anchor point
seminar was a fascinating animation which showed what
for the introduction of any desired sugar, even complex
has been learned about this pathway. Boons hopes that his
nanoscale carbohydrates, via a disulfide linkage. This
contributions to the fields of carbohydrate chemistry and
technique will allow scientists to probe the functions of
nanotechnology will lead to a better fundamental underspecific sugars in relation to the activity of the protein to
standing of multivalency as well as the development of
which it is attached. Boons noted that an added benefit of
nanoscale pharmaceuticals for treating disease.
this approach is that it creates the possibility of appending
both natural and unnatural sugars to the protein.
SPHERES ¡ Vol. 1 Iss. 3 ¡ www.cnsi-uc.org
McEuen
Single Molecule Electromechanics
By Lisa Wesoloski and Gregory Ho
R
emarkable and unexpected physical phenomena await discovery in the world of single
molecules. NanoElectroMechanical devices (NEMs), in
conjunction with advanced probe microscopy techniques,
have the potential to uncover these phenomena. In Paul
McEuen’s lab at Cornell University, they have the ability
to do just that – to understand the unique electrical and
mechanical properties of single molecules.
Nanotubes are among the strongest materials
known, possess a high aspect ratio, and display versatile
electronic properties. In addition to their other work, the
McEuen group also specializes in the study and manipulation of single-walled carbon nanotubes. For instance,
the McEuen group constructs field-effect transistors from
semiconducting nanotubes. By gating a nanotube at different voltages, they can cause it to exhibit n-type, metallic, or p-type properties. Another
project studies the effects of mechanical strain on nanotubes. Specifically,
a nanotube is suspended over a trench
and prodded with an AFM tip (Box
2). The mechanical stress alters the
conductive properties of the nanotube, perhaps as a consequence of
localized strain around the AFM tip
modifying the nanotube bandgap.
This result potentially introduces a
novel technique by which one may
engineer bandgaps mechanically.
Box 1. Schematic diagram of a single electron transistor. Inset: An AFM image of the
Other avenues of nanotube work
device (100 nm scale bar).
in the McEuen laboratory include
McEuen and his group build single molecule transisstudying tube-tube interactions through crossed nanotube
tors. These transistors, being orders of magnitude smaller
junctions, using nanotubes as chemical or biological senthan the tiniest silicon transistor, work by transporting
sors, using nanotubes as microfluidic components, createlectrons one at a time. One particular type of transising and studying nanotube quantum dots, and inventing
tor utilizes as the active element a cobalt ion bonded to
advanced scanning probe microscopy techniques to study,
polypyridyl ligands that are appended with insulating
cut, and manipulate nanotubes.
tethers of differing lengths. The device is constructed
Underby placing a monolayer of these molecules upon a gold
standing
the
nanowire through which a large current is passed in order
physics of single
to create an approximately nanometer-sized gap. A molmolecules is esecule subsequently falls into the gap to create the transissential for the
tor (Box 1). When the applied gate voltage is above a
rational creation
certain threshold, electrons are able to tunnel through the
of future techmolecule single-file, using the cobalt ion as a “stepping
nological applistone.”
cations through
Another type of transistor utilizes a fullerene (C60)
intelligent deBox 2. Experimental setup for variable
molecule as the active element. The fabrication of this
signs and archi- conductance measurements of a carbon
device begins with the stretching of a gold nanowire until
tectures. From nanotube (black) perturbed by an AFM
tip (red).
it cracks. Once formed, a buckyball slips into the crack
single molecule
and completes the transistor circuit. Subsequent transtransistors
to
port measurements through this device have led to the
novel scanning probe techniques, from strain-induced
discovery of a completely new type of electromechanibandgap engineering to nanotube microfluidics, Paul
cal coupling – as current runs through the transistor, the
McEuen will no doubt continue to contribute exciting
buckyball appears to bounce continually, in a quantized
discoveries to the field of nanotechnology.
manner, upon the surface of the nanowire.
SPHERES ¡ Vol. 1 Iss. 3 ¡ www.cnsi-uc.org
paulMcEUEN
Cornell University
7
De Schryver
Time & Space Resolved Chemistry
By James Matthews and Rob Ramirez
W
fransDe SCHRYVER
Catholic University
of Leuven, Belgium
hereas traditional photochemical methods
probe the properties of ensembles of molecules, Professor De Schryver and his research group
have developed a number of techniques that investigate
the photochemical properties of single molecules.
Observation and spectroscopic investigation of single molecules involves the use of confocal laser microscopy. However, in order to make observations of single
molecules, they must first be isolated from one another.
This requirement may be achieved by diluting the sample
into a polymer matrix, which also effectively restricts
molecular motion. Any motion of the matrix-bound
molecule reorients its transition dipole as a function of
time and leads to detectable fluctuations in the molecule’s
absorbance and fluorescence spectra.
Box.
Photobleaching of the receptor chromophore in a
chromophoric system.
The nature of the polymer matrix used for the isolation of molecules is important. Oligomers formed from
chromophores behave very differently depending on how
well the polymer matrix solvates them. If well-solvated,
they tend to adopt an extended conformation and the
chromophores behave as isolated systems. If they are
poorly solvated, the oligomers tend to coil and the chromophores are brought into contact with each other and
may interact.
The properties of an ensemble of molecules are
necessarily an average of its constituent parts. By investigating a series of single molecules, however, a more
detailed understanding of their photophysical behavior
and properties may be obtained. For example, single
molecule spectroscopy may help to elucidate mechanistic
pathways for Förster energy transfer in dendritic systems
or to reveal a process to be bi- or multimodal in nature.
8
Such crucial information may be lost in an ensemble investigation. An example of an observation that can be
made on the single molecule scale, but which is missed
when observing an ensemble, is the detection of the interactions between spatially close chromophores in a dendritic multi-chromophoric system (Box). One can even
observe selective bleaching of different chromophores in
a single molecule. A small proportion of these molecules
have a different spatial arrangement of the attached chromophores. The proximity and alignment of the transition
dipoles in this minor product lead to an extended fluorescence lifetime for this molecule relative to that of the
major isomer. Therefore, since only a small proportion
of the molecules have an extended fluorescence lifetime,
this chromophoric interaction is missed when the ensemble is studied as a whole, but can be clearly
distinguished when observing a series of
individual molecules.
Dendritic molecules have been developed which in aqueous acidic conditions, collapse to form vesicles that are
approximately 400-500 nm in diameter.
Moreover, these vesicles may be manipulated to form more complex structures.
Single vesicles may be trapped and held
against Brownian motion using a laser. If
a surface-bound vesicle is forcibly passed
into contact with a laser-trapped vesicle,
the two superstructures deform but do not
dendritic multimerge. However, if the same vesicles are
irradiated with the correct wavelength of
light, the dendritic structure undergoes a conformational
change which allows the colliding vesicles to merge.
This process may be repeated multiple times, and the De
Schryver group has exploited this technique to grow long
structures (µm) from these vesicles.
In addition to his work on single molecule photochemistry, De Schryver is also interested in developing
surface arrays as templates for the generation of welldefined three-dimensional structures. This strategy was
realized through the self-assembly of ligands onto a
graphite surface, followed by the addition of palladium to
this coordinating layer. This platform is the foundation of
subsequent layers of coordinating molecules, allowing for
the growth of three-dimensional structures.
By studying both isolated single molecules and
highly organized multi-molecular arrays, De Schryver is
making valuable contributions to nanoscience.
SPHERES ¡ Vol. 1 Iss. 3 ¡ www.cnsi-uc.org
Aizenberg
Bio-Inspired Calcite Crystals
By Robert Jost and Luis Campos
L
essons from nature in the context of crystal
science was the main theme of Dr. Aizenberg’s
lecture. Mollusks, coccoliths, and brittlestars have the
ability to grow perfect calcite (CaCO3) crystals as part of
their exoskeletons. Laboratory crystallization techniques,
however, are severely limited by the lack of control and
understanding of the nucleation mechanism at the onset of
crystal growth. Aizenberg studies biologically-generated
crystals that exhibit superior physical properties in order
to gain insight on the growth of perfect inorganic calcite
and to reproduce the fantastic architectural, mechanical,
and optical properties of natural materials.
a dorsal arm plate made of calcite. This calcite is mechanically strong and displays extraordinary optical properties in some cases. The dorsal arm plate is composed
of an array of microlenses with nerve bundles situated 5
µm below each lens that receive a 50-fold amplification
of light. Because of the discovery of this arrangement of
lenses and photoreceptors, brittlestars are now believed to
have a compound eye capability.
These biological systems have inspired the detailed
study of mineral growth with control over crystallographic
orientation and the densities of nucleation. Using stamps
fabricated by conventional lithographic techniques, selfassembled monolayers of various alkyl thiols were transferred onto metal
substrates. Monolayers of compounds
containing various functional groups
(-COOH, -OH, -SO3H) showed highly
active calcite crystal growth. Interestingly, the functional group determined
the nucleating plane of the crystal that
Box 1. Left to right: calcite crystals grown from HS(CH2)15CO2H, HS(CH2)22OH, and
HS(CH2)11SO2H monolayers on gold with a nucleating plane of 015, 014, and 001, was grown on top of the monolayer.
respectively.
Functional groups affect monolayer
morphology, resulting in a nucleating
Biological architectural motifs are important to
plane specific for each functional group (Box 1). Usunderstand because they provide stiffness to skeletal
ing photolithography to make a specific micropattern,
components. The skeleton of a sea urchin, for example,
Aizenberg synthesized a millimeter-sized single calcite
is composed of a single crystal of “glassy” calcite. Syncrystal with sub-10-µm features. Crystal nucleation was
thetic inorganic calcite, on the other hand, is not glassy
initiated from a self-assembled monolayer deposited onto
and contains many cleavage planes, making it a poor
a gold sulfate by an atomic force microscope tip. Under
structural material. Hence, the sea urchin provides much
crossed polarizers the crystals were shown to be birefrinstiffer structural support for itself by controlling calcite
gent, demonstrating that they were single crystals (Box
crystallization. This lack of control over synthetic crystal
2). Thus, the experiments showed that crystal nucleation
growth – and thus over detailed patterning – has been a
and growth can be controlled, and provided clues as to
major obstacle in the field of crystal science. In a breakhow the proteins that control crystallization in nature may
through discovery, Aizenberg found that 0.01 - 0.1 weight
be exerting their influence.
percent of biologically formed
Dr. Aizenberg clearly demcalcite contains proteins that act
onstrated the value of the lessons
as scaffolds which direct crystal
that one can learn from nature.
growth. These macromolecules
The biologically inspired strateimprove the mechanical constigies have made it possible to contution of the calcite and increase
trol various aspects of inorganic
the toughness 100-fold. Interestcrystallization with high preciingly, these proteins can be used
sion. Nature remains a superior
to synthesize biogenic crystals,
micro- and nano-designer of opsynthetic calcite templated with
tical and mechanical materials,
biological macromolecules.
however. Therefore, Aizenberg
Brittlestars, relatives of Box 2. Schematic calcite microcrystal under crossed will likely continue to look to
starfish and sea urchins, contain polarizers.
nature for future discoveries.
SPHERES ¡ Vol. 1 Iss. 3 ¡ www.cnsi-uc.org
joannaAIZENBERG
Bell Laboratories
9
Wooley
Shell Crosslinked Nanoparticles
By Debora Bontempo
K
karenWOOLEY
Washington University
St Louis
aren L. Wooley, professor of chemistry at
Washington University in St. Louis, has developed a powerful method for the production of welldefined nanostructured materials. These polymers are
called shell crosslinked knedel-like nanoparticles (SCKs)
due to their similarity to the popular Polish dumplings.
Professor Wooley’s long-term goal is to synthesize a
material that mimics lipoproteins and viruses in structure,
function, and size (10-100 nm in diameter) capable of
circulating in the body for a long period of time without
being destroyed by the immune system and without much
toxic effect. Therefore, the ability to further engineer
these materials by tailoring their surface properties would
make the SCKs truly intelligent drug and gene therapy
delivery systems.
ous regions within the core, interface, or shell of the SCK
nanocarrier was investigated.
After describing the general synthetic strategy,
Wooley demonstrated that the flexibility of the approach
and the robustness of the method allow modification of
the properties of the SCKs for the intended application.
For example, peptide sequences attached to the surface
of the nanoparticles act as intermediates in biological
processes, facilitating cell binding and entry. The local
delivery of small antibiotics was achieved with saccharide-coated SCKs.
Wooley also discussed the possibility of synthesizing multifunctional agents by appending different ligands
onto the nanoparticles. Possible extracellular ligands
include molecules that interact with overexpressed re-
Box. Hollow nanocages are fabricated by (a) formation of micelles from an amphiphilic block copolymer, (b) regioselective
crosslinking, c) cleavage of core chains, and d) extraction of the core.
The straightforward synthetic approach, for which
Wooley has received numerous awards, employs a stepwise combination of self-assembly and covalent bonding.
First, polymeric micelles are formed from amphiphilic
block copolymers in aqueous solution. The subsequent
regioselective crosslinking of the shell layer yields
discrete nanoscale macromolecules with a core-shell
morphology. While the crosslinked shell confers stability
to the particles, the core exhibits different behavior (fluidlike or crystalline) depending on its chemical composition. Hollow nanocages were also obtained by cleavage
of the core chains followed by extraction (Box).
Wooley’s presentation emphasized the enormous
potential of the SCKs in the delivery or capture of biomedically, environmentally, and agriculturally active
agents. She showed how genes can be packed inside
the SCK nanocages just as they are in viral structures.
She also demonstrated that positively-charged SCKs can
behave as synthetic histones and encapsulate DNA. Additionally, nanoparticles which mimic lipoproteins were
loaded with guest molecules whose partitioning to vari-
10
ceptors on cancer cells, antigens for immune response,
and therapeutic agents. Nanostructures targeted intracellularly can also carry peptides to gain entry into cancer
cells; these include complementary sequences to bind to
the overexpressed mRNA and nuclear targeting ligands.
Radionuclear coated SCKs were also studied for in vivo
imaging.
Karen Wooley’s seminar provided a fascinating
journey from the conception of a brilliant idea to the
production of new materials with a wide range of applications. She developed an easy route for the synthesis of a
new class of materials, optimized the chemistry toward
a robust method, and exploited many possible variations
and applications of the SCKs. Further variation of the
three-dimensional shape of polymer structures, such as
the preparation of cylindrical or needle-shaped particles,
is expected to generate materials with unique properties.
Considering their wide-ranging applications, it is certain
that we will hear more about SCKs in the upcoming
years.
SPHERES ¡ Vol. 1 Iss. 3 ¡ www.cnsi-uc.org
Williams
World’s Smallest Computer
By Hsian-Rong Tseng
D
ue to fundamental physical limitations and cost
considerations, the conventional lithographic
manufacturing of solid-state devices faces formidable
technological and economic hurdles over the next decade.
One remedy may be the creation of molecular electronic
devices that can, in principle, operate on the scale of
single-molecule junctions of a few cubic nanometers.
These devices, based upon the switching of bistable molecules, provide a “bottom-up” alternative to the present
“top-down” silicon-based construction of memory and
logic circuits.
Former UCLA professor Stan Williams and his coworkers at Hewlett-Packard Laboratories have developed
the world’s smallest memory devices that utilize amphiphilic bistable rotaxanes and simple amphiphiles as molecular switches. Similar to the rotaxane-based memory
(“HPinvent”) were stored into this 64-bit memory device
as standard ASCII code. Williams predicts that a rewritable nonvolatile molecular memory device with a density
of 6.4 Gbits cm-2 can be built and brought to market in
the near future. In addition, by setting the resistance at
specific cross-points, two 4 × 4 subarrays of the memory
device were configured as a nanoscale demultiplexer and
multiplexer that could read and write memory bits into
a third subarray. This design allows nanoscale circuits
to communicate with other systems in the macroscopic
word.
While the devices from HP and UCLA both incorporate rotaxane molecules as the functional elements,
they also display some key differences. The switching
voltages for HP’s devices, for example, are not stable and
move to higher values as they are cycled. By contrast,
the devices fabricated at UCLA
operate at a constant ± 2 V, even
after many switching cycles. The
switching amplitude, the ratio of
current flow between the “on” and
“off” states, ranges between 1000
and 10,000 for the HP devices
but only between 2 and 10 for the
UCLA devices. Researchers at HP
have determined that their devices
operate independent of temperature,
indicating that there is no substantial activation barrier to switching,
whereas the UCLA switches are
thermally activated and do not operate below 200 K. While the HP
devices are almost completely nonvolatile, meaning that the closed
Box. Micrographs of a silicon wafer containing 625 memory devices based upon molecular and open states persist permanently
switching. Each image in the series represents a tenfold magnification of the previous
until they are changed, the UCLA
image. In the upper right is a single 64-bit memory chip.
switches are quite volatile and relax
devices that have been demonstrated by Professors Heath
from the closed to the open state over 10-60 minutes.
and Stoddart at UCLA, HP’s approach is based upon a
Most interestingly, Williams reported that practically any
crossbar architecture to connect molecular switches along
amphiphilic molecule, including degenerate catenanes,
a two-dimensional grid. Williams used imprint lithoglone dumbbell components of rotaxanes, and even simple
raphy – a new nanoscale processing technique that can
amphiphiles, exhibit switching behavior. Researchers at
produce sub-10 nm features with high throughput and low
UCLA, on the other hand, have observed switching becost – to construct the crossbar. A prototype 64-bit nonhavior only when bistable, mechanically interlocked molvolatile memory device, consisting of an 8 × 8 array of
ecules are incorporated into their devices. These many
crossed metal nanowire electrodes, has been successfully
parallels and stark differences in device characteristics,
constructed (Box). Each 1 µm2 cross-section accomas well as Dr. Williams’ close ties to UCLA, produced an
modates roughly 1,000 rotaxane molecules. Eight letters
engaging and unforgettable seminar.
SPHERES ¡ Vol. 1 Iss. 3 ¡ www.cnsi-uc.org
stanWILLIAMS
Hewlett-Packard
11
Review Authors
Rich
Faulhaber
Paul
Bonvallet
Hieu
Duong
J Cambell
Scott
Michael
Tomcsi
Craig
Hawker
Eduardo
Falcao
Amar
Flood
Robert
Metzger
James
Matthews
Frans
De Schryver
Rob
Ramirez
Barbara
Bontempo
Robert
Jost
Karen
Wooley
Luis
Campos
Joanna
Aizenberg
Stan
Williams
Hsian-Rong
Tseng
Geert-Jan
Boons
Jason
Belitsky
Gregory
Ho
12
Paul
McEuen
Alshakim
Nelson
Lisa
Wesoloski
SPHERES ¡ Vol. 1 Iss. 3 ¡ www.cnsi-uc.org
CNSI Poster Day 14 May 2003
Contact Details
The California NanoSystems Institute
Wendy Nishikawa – [email protected]
NanoHour
Paul Bonvallet – [email protected]
Education and Outreach
Professor Sarah Tolbert – [email protected]
CNSI’s Young Entrepreneurs
Stephane Wong – [email protected]
CNSI Young Investigators Society
Amar Flood – [email protected]
CALIFORNIA
NANOSYSTEMS
I N S T I T U T E
The California NanoSystems Institute
UCLA, 6722 Boelter Hall
Box 957151
Los Angeles, CA 90095-7151
USA
www.cnsi-uc.org