SUMMARY OF CURRENT RESEARCH ACCOMPLISHMENTS – SARAH H. TOLBERT
Research in my group focuses around two intertwined goals. These are first, to create
complex materials with nanoscale periodicity using self-organization, and second, to produce new
physical properties because of that nanoscale architecture. The specific properties vary dramatically
from project to project and include control of optical, magnetic, electrical, and even structural
behavior. In all cases, however, the goal is to intrinsically tie the physical properties to the nanoscale
structure, and in so doing, to understand the new dimension of control that size and spatial
confinement can bring.
In my group, we use two main methods to produce nanoperiodic structures. Colloidal
assembly is a simple method of producing periodic structures with virtually any length scale.
Monodisperse colloids of almost any size can spontaneously order into close-packed arrays. Because
of this size flexibility, colloidal assembly is the method of choice for producing larger-scale photonic
materials with periodicity on the order of the wavelength of light. A structurally more versatile
method for producing smaller-scale periodicity is inorganic/organic co-assembly. In this approach,
amphiphilic organic surfactants or block co-polymers are co-assembled with inorganic oligomers to
produce periodic inorganic/organic composites or nanoporous inorganics with periodicities similar to
those found in lyotropic liquid crystalline phases. Various research projects in this area involve the
synthesis of both oxide (titania, silica) and non-oxide (Ge, SnTePt, etc.) based materials.
Optical and Magnetic Materials through Host/Guest Interactions – A major research theme
in the group is the use of spatial confinement of semiconducting polymers to produce new optical
materials. Examples of work in this area include our recent accomplishments with highly polarized,
optical quality thin films and our efforts to use pore size to selectively control polymer conformation.
In the first of these experiments, we take advantage of a very fruitful collaboration with Canon basic
research in Japan. In the Canon labs, research scientists have discovered ways to produce hexagonal
honeycomb surfactant-templated porous silicas with uniaxial (in-plane) alignment of the pores. This
is accomplished by growing the silica on a rubbed polyimide substrate. Incorporation of polymer
into these films produces high anisotropic optical materials that show strong polarization dependence
in both absorption and emission.1 Moreover, the well-defined polymer geometry allows us to
address some fundamental questions about polymer photophysics. For example, the angle of the
absorption and emission dipole with respect to the polymer chain axis has been a long debated topic.
Comparison of simulated and measure absorption and emission anisotropies, however, allows us to
conclude that despite calculations to the contrary, this dipole moment lies parallel to the chain axis.
By varying the size of the pores in our hexagonal honeycomb-structured material, we can
also determine how spatial confinement can be used to control polymer conformation. 2 For example,
we find that small pores (~2 nm diameter) produce isolated, straight chains, medium pores (~4 – 5
nm) allow for multiple chains per pore but keep the polymer chains extended and parallel, while
large pores (> 8 nm) allow for multiple polymer chains per pore but now allow these chains to coil up
as they do in a polymer film. This degree of control means that the same polymer can now be placed
in many different conformations and the photophysics of that material can be examined. 3 For
example, we have used CW photoinduced absorption, light-induced ESR, and optically detected
magnetic resonance (ODMR) to examine how polymer conformation controls the ability to produce
free carriers upon photoexcitation. We find that single polymer chains produce free carriers with low
probability; this may be because an interchain exciton that is delocalized across multiple chains is
needed to facilitate the process of charge separation. Free carriers can be produced in samples with
multiple, parallel polymer chains, but these carriers have short lifetimes. Once the chains are
allowed to coil, long lived carriers are produced, indicating that kinks in the polymer chains serve as
trap sites for polarons.
Complementary to these host/guest polymer experiments, we also have a variety of
experiments in collaboration with both the Rubin and Wudl groups here at UCLA to use amphiphilic
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semiconducting polymers to directly template periodic inorganic phases.4 Such direct assembly
removes many of the tedious and inefficient aspects of our polymer host/guest chemistry. Various
experiments make use of both side chain amphiphiles and amphiphilic diblock copolymers.
In unrelated experiments, we have also shown that spatial confinement can be used to control
interactions between nanoscale magnets.5 In this work, superparamagnetic cobalt nanocrystals are
incorporated into the long straight pores of a hexagonal honeycomb-structured silica. By allowing
the magnets to couple only in rows, we find that it is possible to produce pseudo-anisotropic
magnetic nanocrystals which show much harder magnetic behavior than the starting nanocrystals.
The results of all of these experiments show that nanoscale spatial confinement is a power method
for controlling materials properties and producing anisotropic materials behavior.
New Semiconducting Phases through Inorganic/Organic Co-Assembly – Another continuing
area of research in the group is the synthesis and characterization of new semiconducting inorganic
phases through co-assembly of Zintl clusters (reduced, soluble main group clusters) with organic
surfactants. While the work described above nicely shows that optical or magnetic properties can be
introduced into insulating silica phases using host/guest chemistry, the range of materials possibilities
is much larger if the framework itself is a semiconductor. In our recent work in this area, we have
shown that nanoperiodic versions of semiconductors ranging from pure group IV materials (Ge,
SiGe, SnGe, etc) to metal-bridged group IV calcogenides (Pt coupled SnTe44-, Rh coupled Ge4Se104-,
etc) can be synthesized by solution phase self-organization.6,7,8,9 By varying the elemental
composition of the material, composites can be produced with optical band gaps that span the visible.
Moreover, by changing variables as subtle as the oxidation state of the bridging ion, semiconductors
can be produced with systematically varying band gaps and conductivities.
In an exciting new area for us, we have also begun using ultraviolet photoemission
spectroscopy (UPS) to determine absolute energy levels for these materials. 10 We find that in
addition to tuning band gaps, we can also tune absolute valence and conduction band energies by
changing the composition of the composites. We are currently in the process of examining periodic
trends in absolute energy levels to understand how elemental composition systematically controls
band energies.
While the Zintl systems are optimal for producing moderate and narrow band gap amorphous
semiconductors, one oxide system also shows promise as a wide band gap semiconductor: titania.
Mesoporous amorphous titania can be readily synthesized using surfactant or polymer templating.
Unfortunately, the mobility of amorphous titania (which is a hydrated phase) is very low and so the
walls must be carefully crystallized to produce a mesoporous crystalline anatase material. Once
crystalline grain growth begins, however, it frequently proceeds unchecked and results in destruction
of the nanometer-scale periodicity. Random trial-and-error synthesis has shown that under some
circumstances, the titania framework can be crystallized without destroying the nanoporous
architecture, but there has been no systematic understanding of the process. In recent experiments,
we used in-situ X-ray diffraction to determine activation energies for both titania crystallization and
for destruction of the nanoscale pores in order to determine the types of thermal treatments that
should most effectively crystallize the titania framework without destruction of the nanoscale pores.11
The results both help provide a route to periodic nanocrystalline, nanoporous semiconductors, and
provide fundamental insight into the links between atomic and nanometer scale restructuring.
Now that a broad range of periodic semiconducting frameworks have been produced, we are
working to combine our host/guest chemistry with these new materials. For example, incorporation
of a semiconducting polymer into the pores of a semiconducting framework will produce a
semiconductor-semiconductor heterojunction. If the energy levels of the two semiconductors are
appropriately aligned, this junction can be used to separate optically-produced electron hole pairs,
thus forming the basis for a photovoltaic. Experiments involving semiconducting polymers
incorporated into both Zintl and titania-based frameworks show that photoinduced charge separation
is possible and that the composites can show a strong photoresponse.
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Mechanical Properties of Bulk and Nanophase Materials – Another major effort in the group
over the past years has been the investigation of mechanical properties of composite materials. In the
past year and a half, however, we have taken this effort to a new level by undertaking a collaboration
with the group of Vijay Gupta in the Department of Mechanical and Aerospace Engineering here at
UCLA. In this work, we have been examining how nanoscale architecture can control mechanical
properties. The work involves applying tension to periodic silica/surfactant composite films to
measure Young’s moduli and various elastic limits and failure points. The results are quite dramatic.
Producing a nanoscale composite lowers the stiffness of a composite by approximately the expected
amount – in our case to about 1/3 the original value for a material that is approximately 1/3
inorganic. The failure strain of the composite, however, is dramatically increased: from 0.08% for
the bulk material to over 3% for the nanoscale composite. This remarkable increase in elasticity can
be understood by three factors. First, the nanoscale architecture produces a new length scale for
deformations that is not present in the bulk material. Second, the nanoscale periodicity prevents
crack growth and formation of critical cracks. Finally, the periodic nature of the structure means that
unlike disordered porous materials, the composites have no weak points that can cause materials
failure. Our current results are an important step toward understanding how nanometer-scale
architecture can be used to tune the mechanical properties of materials.
In collaboration with the Kaner group, we also have a project aimed at establishing a new
paradigm for ultra-hard materials.12 In this work, materials are synthesized according to a
prescription of optimized electron density and maximal covalency. Our initial experiments with
OsB2 resulted in a material with a bulk modulus approaching that of diamond, and a hardness greater
than sapphire. We are now working to use physical measurements of elastic moduli, and elastic and
plastic deformation limits to refine these synthetic criteria in an effort to produce a new class of
boride based ultra-hard materials.
Colloid-Based Photonic Materials – We have utilized a variety of methods to produce
interesting new photonic materials. For example, we have shown that core-shell polymer colloids
can be used to produce environmentally responsive photonic materials.13 If the shell is reasonably
inert, the core can be reversibly swollen with solvent, resulting in a shift in the photonic stop-band of
a crystal made up of these colloids. Such materials may find applications as solvent sensors. Upon
extreme swelling, the colloidal array can restructure from a simple colloidal array to a robust
polymeric material consisting of cores of one material embedded in a matrix of the other.
Restructuring photonic materials such as these show the diverse phase stability that can be created
using mixed polymer colloids.
In an effort to extend the ideas of a photonic crystal to a material with a full photonic band
gap, we have also been collaborating with the Gordon group at Harvard to develop methods whereby
atomic layer deposition (a layer-by-layer CVD technique) can be used to convert simply polymer or
silica colloids to inverse opal structures.14 We have used WN (a metallic nitride) as a proof of
principle system, and we are now moving on to the high-dielectric insulator Ta2N5 in an effort to
produce photonic materials with a 3-dimensional photonic band gap.
Nanoscale Electrochemical Systems – A final related project that we have been exploring is
the use of self-organized nanoscale materials for the production of microscale batteries. In this work,
the same ideas of surfactant templating, discussed above, are used to produce colloidal versions of
layered vanadia materials for incorporation into lithographically-produced microscale batteries.15
After synthesis, the surfactant is exchanged for an alkali cation to produce the final colloidal cathode
material. The layered vanadia nanorolls are produced by a hydrothermally driven structural
rearrangement in which layered vanadates exfoliate and then roll up into scroll like structures.
Because we have extensively studied the process of nanoscale restructuring in related surfactant
templated materials, we have been able to logically tune the nanoscale architecture of these vanadia
nanorolls for optimal cathode performance.
3
W. Molenkamp, M. Watanabe, H. Miyata, and S.H. Tolbert, “Highly Polarized Luminescence from
Optical Quality Films of a Semiconducting Polymer Aligned within Oriented Mesoporous Silica.” J.
Am. Chem. Soc. in press.
1
A. Cadby and S.H. Tolbert, “Control Of Optical Polaron Production In Semiconducting Polymers
Using Host-Guest Chemistry In Hexagonal Nanoporous Silica.” Polym. Preprints, in press.
2
A. Cadby and SH. Tolbert, “Pore Size Dependant Optical Properties Of Periodic Polymer–
Mesoporous Silica Composites.” To be submitted to Phys. Rev. B.
3
A.P.Z. Clark, A.J. Cadby, K.-F. Shen, Y.F. Rubin, and S.H. Tolbert, “Amphilphilic Poly(phenylene
ethynylene) as the Structure-Directing Agent for Nanostructured Silica Composite Materials.” To be
submitted to Nano. Lett.
4
A.F. Gross, M.R. Diehl, K.C. Beverly, E.K. Richman, and S.H. Tolbert, “Controlling Magnetic
Coupling between Cobalt Nanoparticles through Nanoscale Confinement in Hexagonal Mesoporous
Silica.” J. Phys. Chem. B, 107, 5475-5482 (2003).
5
D. Sun, A.E. Riley, A. Cadby, and S.H. Tolbert, “Hexagonal Nanoporous Germanium through
Surfactant-Driven Self-Organization of Soluble Zintl Clusters.” Submitted to Nature.
6
D. Sun, A.E. Riley, and S.H. Tolbert, “From Germanium Zintl Clusters to Periodic Nanoporous
Germanium: Synthesis and Characterization of a New Type of Mesoporous Material.” To be
submitted to J. Am. Chem. Soc.
7
A.E. Riley and S.H. Tolbert, “Synthesis of Periodic Hexagonal Surfactant Templated Platinum Tin
Tellurides: Narrow Band Gap Inorganic/Organic Composites.” J. Am. Chem. Soc. 125, 4551-4559
(2003).
8
A.E. Riley and S.H. Tolbert, “Synthesis and Characterization of Tin Telluride Inorganic/Organic
Composite materials with Nanoscale Periodicity through Solution Phase Self-Assembly: A New
Class of Composite Materials Based on Zintl Cluster Self-Oligomerization.” Res. Chem. Intermed.,
in press.
9
Andrew E. Riley, Scott D. Korlann, Bradley L. Kirsch, Sarah H. Tolbert, “Chemical Tuning of the
Electronic Properties of a Hexagonal Surfactant Templated Nanostructured Semiconductor.” To be
submitted to J. Am. Chem. Soc.
10
B.F. Kirsch, E.K. Richman, A.E. Riley, and S.H. Tolbert, “In-Situ X-Ray Diffraction Study Of The
Crystallization Kinetics Of Mesoporous Titania Films.” Submitted to J. Phys. Chem. B.
11
R. Cumberland, M. Weinberger, J. Gilman, S. Clark, S.H. Tolbert, and R. Kaner, “Osmium
Diboride: An Incompressible and Superhard Material,” To be submitted to Adv. Mater.
12
A. Rugge, W.T. Ford, and S.H. Tolbert, “From a Colloidal Crystal to an Interconnected Colloidal
Array: A Mechanism for a Spontaneous Rearrangement.” Langmuir, 19, 7852-7861, (2003).
13
A. Rugge, J.S. Becker, R.G. Gordon, and S.H. Tolbert, “Tungsten Nitride Inverse Opals by Atomic
Layer Deposition.” Nano Lett., 3, 1293-1297 (2003).
14
D. Sun, C.W. Kwon, G. Baure, E. Richman, J. MacLean, B. Dunn, and S.H. Tolbert, “Vanadium
Oxide Nanorolls as Cathode Materials for Rechargeable Batteries: The Relationship between
Nanoscale Structure and Electrochemical Performance.” Submitted to Adv. Func. Mater.
15
March 25, 2004
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