Research
IRG II
Hierarchically Assembled Molecular and Hybrid Organic-Inorganic Materials
Faculty (* = coordinators): P. Guyot-Sionnest*, M. Hopkins, R. Ismagilov,
H. Jaeger, K. Y. Lee, G. Mazenko, M. Mrksich, S. Rice, N. Scherer, S. Sibener*,
T. Witten, L. Yu
Affiliates: S. Bader, X.-M. Lin
This IRG aims to design, develop, and implement hierarchical materials, composed of molecular assemblies and inorganic building blocks that express novel function. The research exploits convergent advances in chemical synthesis, self-assembly, and surface chemistry, which enable the controlled spatial deployment across a broad range of length scales of molecular and solid-state functional elements. It is made possible by the combined expertise of the collaborators in organic, inorganic, and polymer synthesis, molecular- and meso-scale assembly, physical characterization, and theory, working together to develop “functional molecular materials by design”. Our approach strives to elucidate general routes for hierarchical materials assembly using “bottom-up” techniques, which include the selective placement of functional organic and inorganic moieties positioned in well-defined geometries to optimize function such as chemical activity as well as electronic, optical, or magnetic response.
Two synergistic themes develop functional elements and general routes for proximal assembly: Supramolecular Patterned Overlayers and Hybrid Organic/Inorganic Hierarchical Structures.
Supramolecular Patterned and Functional Overlayers: This theme
develops molecular materials that can be covalently assembled to form ordered
patterns on surfaces, including proximal placement of functional (chemical,
electronic and magnetic) elements.
Self-assembled Coordination Networks. Hopkins
and Sibener are developing new classes of periodic structures (<10 nm)
based on Hopkins’ self-assembled transition-metal coordination networks.
Hopkins and Sibener have examined the formation and properties of 2-D and
3-D assemblies of metalloporphyrin complexes on surfaces. SPM studies of
vacuum-deposited Ni(TPP) on Au(111) revealed that monolayers are arranged
with the porphyrin faces parallel to the Au surface and in a close-packed
edge-to-edge array with interdigitated phenyl groups. Desorption studies
of these monolayers, conducted to establish conditions for the self assembly
of multilayer structures, gave information on molecule-surface and intermolecular
interactions. Vacuum dosing of the Ni(TPP) monolayer with DABCO formed,
in situ, Ni(TPP)(DABCO). This is a rare example of the “synthesis”
of a metal coordination compound on a surface, and is the basis for a novel
method of surface functionalization. The unligated DABCO nitrogen termini
are being functionalized with H-bonded overlayers, which are precursors
to molecular-electronic circuitry, and coordination-complex overlayers,
which have electro-active catalytic properties. Future directions include
utilization of tethering chemistries from Yu and Mrksich to impose patterned
length-scales on the arrays. Yu’s group also developed a new layer-by-layer
surface immobilization approach to immobilize electroactive porphyrin moieties
on ITO surfaces based on selective siloxane formation; immobilization can
be carried out on either hydrophilic glass or ITO. Hopkins has also probed
using IR spectroscopy the CH/π noncovalent interactions through which
1-D polymers of the form [1,4-C6R4{C=W(OBut)3}2(m-4,4'-bpy)]
assemble into 3-D grids, providing the first vibrational data of the fundamental
CH/π interaction under conditions where face/edge separation and π-system
quadrupole moment can be controlled.
SAMs for Polymer Immobilization. Yu and Lee have
carried out detailed studies of multilayer functional polymers prepared
via a new layer-by-layer approach based on the chemoselective ligation between
oxyamine and aldehyde groups. Via impedance spectroscopy, it was demonstrated
that multilayer polymer films chemoselectively immobilized on gold substrates
form highly insulating, defect-free, and smooth nanocoatings. These behave
as an ideal capacitor, the capacitance of which can be controlled by varying
the number of deposition layers. These studies by Lee and Yu complement
ongoing studies by Hopkins, Mrksich, Rice and Sibener on using derivatized
SAMs for functional materials design.
Molecular Electronics. We aim to design and synthesize
new classes of molecular electronic materials, to measure both ensemble
and single molecule transport, and to explore their self- or guided assembly
into devices. Yu’s group has succeeded in synthesizing a new series
of diode molecules based on diblock oligomers with electron-rich biphenyl
covalently connected with electron-deficient dipyrimidine. Sequential deprotection
of these thiols enables the assembly of diode molecules between two gold
electrodes with controlled directionality. STM/STS studies revealed rectifying
effect as well as a dramatic effect due to protonation, which can reversibly
alter the rectifying direction. The origin of the rectification has been
studied with Prof. Oleynick (U. of S. Florida); it is due to the resonant
electron transfer in the presence of an electric field. Yu's group, collaborating
with IBM/Zurich, has investigated rectification effect of their molecular
diodes with a break-junction technique, confirming conclusions drawn from
their STS studies.
Hopkins prepared the first well characterized examples of conjugated organometallic
wires on surfaces, which is a key step in demonstrating their potential
in molecular-electronic devices. The family of metallo(phenyleneethynylene)s
of the form [ClL4W{CC6H4(CCC6H4)nR}]
(R = H, SH) was studied. The electron-transfer kinetics as a function of
chain length were established via cyclic voltammetry. Time-resolved resonance
Raman studies of the H terminated molecules showed that excited-state dynamics
may enable them to function as photo-activated switching elements. Hopkins
& Rosenbaum also examined the magnetic properties of open-shell congeners
of these materials to evaluate their potential for building 1-D spin chains.
The magnetic susceptibility of d1d1 di(metalloethynyl)benzene
ions indicate super-exchange coupling between metal centers. A quantitative
description of the susceptibility revealed a robust antiferromagnetic coupling
between spins. Synthetic routes to 1-D spin chains are under development.
Hybrid Organic/Inorganic Hierarchical Structures: This theme seeks
to develop and generally implement strategies for the guided assembly of
functional thin-film structures extending methods pioneered by our MRSEC
in templated materials assembly. These assembly strategies are based on
the utilization of phase-separated and spatially-controlled copolymers,
as well as vicinal surfaces acting as templates for hierarchical assembly.
Microdomain Polymer Films. We seek the ability to fabricate
hierarchical structures on a variety of length-scales, spanning the range
from that of individual molecular constituents to more macroscopic dimensions.
Our MRSEC, with key contributions from Jaeger, Sibener, Mazenko and Witten,
has been a pioneer in controlling and exploiting the spontaneous nanoscale
phase-separation patterns of block copolymer films. We have developed general
new methods involving polymer flow and confinement, discovered by Sibener’s
group, and now successfully modeled by Witten and Mazenko, for guiding the
spontaneous spatial alignment of polymeric cylindrical nanodomains on Si
gratings. This work has been extended to include phase-separated pattern
formation in complex, lithographically generated structures, with notable
agreement between AFM imagery from Sibener and Mazenko’s numerical
simulations. Conformal patterns in which polymer nanostructures are guided
into intentionally interconnected patterns by top-down lithography can now
be generated. Mazenko and his group are working to understand both the constrained
growth experiments studied in Sibener's group and the work on surface ordering
of diblocks in Jaeger's group. They are concentrating on the identification
of defects which disrupt the ordering on a free surface or during confined
growth. A key aspect of connecting theory and experiment is to understand
the role of boundary conditions. Recent work has emphasized the importance
of noise (impurities) in the boundary conditions. The intensity of the noise
affects the system dynamics. The simulations are in close agreement with
the experiments under different geometric constraints. Moreover, Mazenko
numerically investigated the development of order in the Swift-Hohenberg
equation. Witten and Lee developed a potential explanation of a distinctive
25nm spiral stripe pattern seen with mixed polymer-lipid monolayers deposited
on a surface. The explanation makes a correspondence between Lee’s
monolayer system and the spiral cracks that form in a drying layer of brittle
gel elucidated by other authors.
Yu’s group has also synthesized novel conjugated polymers containing
first row transition metal complexes for studies of their magnetic properties.
Metal complexes of high spin state, including Mn(II), Fe(II), Co(II), Ni(II),
and Cu(II), were incorporated into the bipyridine coordination sites of
the conjugated polymer backbone. In collaboration with Rosenbaum’s
group, the magnetic properties of these polymers were studied and showed
that they all had high paramagnetic susceptibility; where applicable they
were all stable in the high-spin state. Their large free spins and stability
makes them promising as magnetic materials.
Nanoparticle synthesis: In addition to the colloidal
nanoparticle synthesis routes established by Guyot-Sionnest and X.-M. Lin
at Argonne, Ismagilov developed a novel approach to the control of nucleation
and growth time steps in the formation of colloidal nanoparticles. This
approach uses microfluidic cells to inject the reagents and control the
synthesis, semiconducting particles were shown to be monodisperse, characterized
using absorption and fluorescence spectroscopies, and by TEM. Jaeger and
Lin initiated an effort to synthesize superconducting nanoparticles. First
results on Pb produced a range of shapes, including ultrathin prisms that
behave like type-II superconductors in their response to applied magnetic
fields.
Patterned Decoration of Nanoparticles: Controlled placement
of the sub-10 nm particles in an ordered but not necessarily close-packed
array has been achieved using the selective decoration of metals on diblocks.
We now seek to refine our understanding/control of such systems to fabricate
functional hierarchical assemblies for physical and chemical applications,
such as chemical sensors and fundamental studies of e-- transport in confined
systems. A photochemical approach of great generality has been developed.
Here, a selective VUV etch of the surface of the diblock patterns has been
observed to vastly increase the selectivity of the decoration of colloidal
nanoparticles on the domains, both for magnetic particles (Bader, Lin, Sibener
& Darling (ANL) and semiconducting colloidal nanoparticles (Guyot-Sionnest,
Sibener). The focus of this work is now on improving the intradomain ordering
of the nanoparticles. Darling & Hoffmann (ANL) are building on this
work using patterned electron beam radiation to alter the relative selectivity
of polymer blocks to metallic adsorption and to spatially tune the diffusion
behavior of metal nanoclusters. Sibener is exploiting the potential of using
the striped copolymer substrates to align and register rodlike nanostructures
such as single walled carbon nanotubes. He is developing strategies to deposit
rods in registry with the polymer domains, encouraged by preliminary estimates
of feasibility from Witten. Strategies include contact-line combing, evaporative
flows, “squeegeeing” with PDMS brushes and polymer domain alignment.
Ordered Nanocluster Arrays. Recent experiments in
this MRSEC by Jaeger and Lin with theory support by Witten found a new approach
for the fabrication of highly-uniform, long-range-ordered nanocrystal monolayers.
This approach is based on drop casting and produces nanoparticle monolayers
at the liquid-air interface of a drying drop of colloidal solution. The
advantage of this approach is that the monolayer drapes itself over the
substrate during the final drying stage and, in this way, can accommodate
surface roughness and features such as prefabricated electrodes. Graduate
student Klara Elteto, working with Jaeger and Lin, now discovered that these
monolayers can also drape themselves over holes in the substrate, producing
free-standing, close-packed nanoparticle membranes. From controlled AFM
indentation measurements we estimate a Young’s modulus of several
GPa. This strength is remarkable for ultrathin free-standing arrays, spanning
holes several hundred particles across, given that the particles are linked
not by long entangled polymers but instead by short ligands.
Transport studies: We are now exploring the interactions
and functional properties that can be obtained using different particle
types (metallic, magnetic, semiconducting, superconducting) in ordered geometries.
This new endeavor involves colloidal particle synthesis (Guyot-Sionnest,
Lin), self-assembly (Jaeger, Lin), low-temperature transport measurements
(Guyot-Sionnest, Jaeger, Kang, Rosenbaum) and theoretical modeling (Witten,
Gruzberg). Results from the previous year showed that 3D films of charged
semiconductor quantum dot arrays at low temperature (0.3-4K), exhibited
large magneto-resistive effects (150%). This effect is due to the wavefunction
shrinking due to the magnetic field and corresponding reduced interactions
between particles. It was explained quantitatively by the theory of variable
range hopping. In the absence of paramagnetic impurities and at low fields
(between 0 T and 0.1T), a novel narrow 10-15% magneto-resistance has been
observed which is also strongly bias dependent. It has now been attributed
to a spin-blockade effect, mediated by the hyperfine interaction.
From annual report to the NSF, March 2007