1998 MRS Fall Meeting & Exhibit

Chairs

Dan Feldheim, North Carolina State Univ 
Chad Mirkin, Northwestern Univ
Michael Natan, Pennsylvania State Univ

Symposium Support 

• Air Force Office of Scientific Research
• Army Research Office
• National Science Foundation

* Invited paper

SESSION BB1: 
Chair: Dan L. Feldheim 
Monday Morning, November 30, 1998 
Essex South (W)

8:30 AM BB1.1 ASSEMBLING NANOCOMPOSITE MATERIALS WITH HOLOGRAPHIC OPTICAL TWEEZER ARRAYS. M.T. Dearing , G.C. Spalding, Illinois Wesleyan University, Dept. of Physics, Bloomington, IL; E.R. Dufresne, D.G. Grier, University of Chicago, Dept. of Physics, Chicago, IL. Optical tweezers exploit field gradients in focused laser light to trap and manipulate dielectric particles. Holographically generated arrays of optical traps can manipulate large numbers of microparticles simultaneously [1]. We will discuss methods for producing large arrays of such traps in arbitrary configurations using diffractive optical elements. In addition to addressing issues encountered in designing and fabricating optical tweezer arrays, we will introduce practical applications. 

8:45 AM BB1.2 
STRUCTURE OF SINGLE-AND DOUBLE-STRANDED DNA MONOLAYERS ON GOLD FROM NEUTRON REFLECTIVITY. R. Levicky, T.M. Herne, M.J. Tarlov , S.K. Satija, National Institute of Standards and Technology, Gaithersburg, MD. 

Single-stranded DNA probes immobilized on surfaces form the operational heart of a new generation of genomic diagnostic devices. There is surprisingly little information, however, about the molecular scale structure of these technologically important systems. We report here the determination of in situ concentration profiles of DNA monolayers on gold in aqueous salt solutions using neutron reflection. The DNA used in these studies was derivatized with a thiol functional group and a hexamethylene spacer arm attached to the 5' phosphate end. Concentration profiles indicate that adsorbed layers of single-stranded DNA on bare gold are compact, suggesting the presence of multiple adsorption points between DNA strands and the surface. After treatment with mercaptohexanol, a short alkanethiol with a terminal hydroxy group, the DNA strands stand-up and extend further into the solvent phase. These changes are consistent with the DNA remaining attached solely through its thiol endgroup with minimal interaction between DNA and the mercaptohexanol monolayer. The resulting end-tethered DNA layer undergoes hybridization specifically with approximately 100% efficiency to its complementary sequence, resulting in DNA helices oriented nearly perpendicular to the substrate plane. The impact of probe and alkanethiol monolayer structure on hybridization performance will be discussed. 

9:00 AM *BB1.3 
DNA-DRIVEN ASSEMBLY OF NANOPARTICLES INTO FUNCTIONAL PERIODIC MATERIALS. Chad A. Mirkin , Robert Elghanian, Robert L. Letsinger, Gregory Mitchell, Robert C. Mucic, R. Andrew Reynolds, James J. Storhoff. 

Strategies for synthesizing and characterizing extended periodic structures from oligonucleotide modified nanoparticles and DNA interconnects will be discussed. The novel collective properties of these periodic structures will be described, and the application of these new particle assembly strategies and materials in the areas of biodiagnostics, chemical lithography, and colloidal crystallization will be addressed. 

9:30 AM *BB1.4 
SOFT LITHOGRAPHY. George M. Whitesides , Harvard University, Cambridge, MA. 

Photolithography is an enormously powerful technique in microfabrication. It also has limitations: extending the range of feature sizes below 100 nm is complicated; preparing patterns on non-planar surfaces is not practical; many classes of materials cannot be fabricated using photolithography; it is intrinsically expensive; it is not ideal for large-area patterning. This talk will discuss an alternative set of techniques for micro- and nanofabrication. This set-known collectively as soft lithography-includes microprinting and micromolding using elastomeric stamps/molds, and near-field photolithography in which the optical elements are, or are fabricated by, these elastomeric elements. These techniques are especially promising for microfabrication of relatively simple structures with function in sensors, microanalytical systems, physical optics, MEMS, and related areas. They may also be useful for preparing simple nanostructures conveniently.  
Microcontact printing (µCP) allows the in-plane dimensions of structures in SAMs to be controlled routinely at the scale of 500 nm, and with some difficulty at dimensions down to 100 nm. Micromolding provides a route to quasi-three dimensional structures fabricated of polymers, and to planar structures with dimensions as small as 30 nm. Near-field photolithography using soft optical elements generates features in the 50 - 200 nm range. A combination of these techniques offers approaches to certain fabrication problems that are complementary to conventional photolithography. 
Soft lithography is early in its development, and its strengths and limitations (especially in registration and pattern distortion) are just beginning to be defined. It does not compete with photolithography for the core businesses in microelectronics fabrication, but it offers new solutions to problems where photolithography has limitations. 

10:30 AM *BB1.5 
SELF-ASSEMBLED COLLOIDAL GOLD SPHERES AND RODS: TOWARD CONTROLLED 3-D METAL ARCHITECTURES. Michael J. Natan , Pennsylvania State Univ, University Park, PA. 

We will describe recent results on self-assembly of colloidal Au spheres and rods, both in solution and on surfaces. The focus of these efforts is on construction of hierarchical materials using simple organic and/or biological crosslinkers, and on characterization of the resulting novel optical and electrical properties. 

11:00 AM *BB1.6 
ENABLING CRYSTALLINE COLLOIDAL MATERIALS FOR OPTICS, CHEMICAL SEPARATIONS AND CHEMICAL SENSING. Sanford A. Asher , Lei Liu, Jesse Weissman, Gushing Pan, University of Pittsburgh, Department of Chemistry, Pittsburgh, PA. 

A number of enabling materials are fabricated through the crystalline colloidal array self assembling of cubic arrays of colloidal particles. These materials efficiently diffract light which meet the Bragg condition. We will discuss the development of nsec non linear optical switching devices. In addition, we have developed chemical sensing materials which utilize chemical recognition agents. In addition, we have fabricated cubic arrays of water pockets in polyacrylamide hydrogels. These water arrays serve as entropic traps for macromolecules and can be used as macromolecular separation materials. 

11:30 AM BB1.7 
USING SELF ASSEMBLED MONOLAYERS AND SCANNING PROBE MICROSCOPY IN NANOMANIPULATIONS. Christopher B. Gorman , Igor Touzov, Richard L. Carroll, Dept. of Chemistry, North Carolina State University, Raleigh, NC. 

Organothiolate self-assembled monolayers (SAMs) on an atomically flat Au(111) surface seem to be the ideal starting point for chemically well-defined nanomanipulations. Several questions emerge as to how to elegantly perform such manipulations. Results will be presented that indicate ideal procedures for preparing SAMs that are defect free on the molecular scale over large regions of a surface. Moreover, minimization of the setpoint tunneling current into the femtoampere range will be shown to have several advantages in both imaging and lithography. The effect of tail group structure on SAM organization and stability will also be illustrated. 

11:45 AM BB1.8 
SYNTHESIS OF HIGHLY ORDERED MACROPOROUS METAL OXIDES. Brian T. Holland , Christopher F. Blanford, Andreas Stein, University of Minnesota, Department of Chemistry, Minneapolis, MN. 

Macroporous oxides of Zr, Ti, Al, Si, Fe, Sb and W have been synthesized from readily available alkoxide precursors and polystyrene spheres as templates. The products consist of highly ordered three-dimensional arrays of close-packed, interconnected pores, presently at a size range of 320-360 nm. The inorganic walls surrounding the macropores form a diamond-type lattice. Depending on sample treatment, the walls can be amorphous or crystalline, mesoporous or nonporous. We will discuss the relationships between synthesis parameters and product structures. It is expected that the synthesis can easily be adapted to many other metal oxides, phosphates, chalcogenides, or hybrid organic/inorganic compositions. As a result one can foresee an impact on numerous diverse applications that would benefit from the porosity, low density, and order of these novel ceramic structures, such as quantum optics, chromatographic stationary supports, large molecule catalysts, host materials for biological molecules, porous electrodes, metal-ceramic composites, or thermal insulators. 

SESSION BB2: 
Chair: Michael J. Natan 
Monday Afternoon, November 30, 1998 
Essex South (W)

1:30 PM BB2.1 
NANOCRYSTAL SUPERLATTICE ASSEMBLY AND SELF-ORGANIZATION: THERMODYNAMICALLY ORDERED ``SOFT SPHERES''. Brian A. Korgel , Department of Chemistry, University College Dublin, Dublin, IRELAND. 

The ability to manipulate matter on the nanometer length-scale is an important goal, due in part to the significant technological challenges facing future reductions in electronic device size. One approach is to organize nanocrystals (i.e. particles ranging between 20 and 100 A in diameter) like ``building blocks'' into well-defined mesoscopic architectures. Such an approach provides the possibility of employing the unique size-dependent properties of nanocrystals in a device; and the possibility of creating materials with new optoelectronic properties resulting from collective interactions between neighboring nanocrystals. Therefore, a fundamental understanding of how to order nanocrystals could potentially be very useful. 
Synthetic techniques that have been developed during the last few years now enable the production of sterically stabilized nanocrystals with tight size distributions. These very small colloids behave essentially like hard spheres which order into a face-center cubic lattice above a critical volume fraction. Although hard sphere behavior is encountered in many instances, arrays with more complex structures can be formed by manipulating the ratio of capping ligand chain length to core radius, particle shape and/or the interparticle interaction potential through the solvent polarity, capping ligand coverage, or the adsorption of DNA base-pair molecular analogs to the nanocrystal surface. These organized nanocrystal assemblies can be treated as thermodynamic phases and understanding the disorder-order (or fluid-solid) phase transition is essentially the goal to understanding superlattice formation. 
Therefore, this presentation will focus on the relationship between our experimental transmission electron microscopy and small angle X-ray scattering measurements of silver nanocrystal superlattices, and the thermodynamics of nanocrystal superlattice formation. 

1:45 PM BB2.2 
PERMEABILITY AND pH-STABILITY OF LAYERED POLYIONIC FILMS. Jeremy Harris, Merlin Bruening , Michigan State University, Department of Chemistry, East Lansing, MI. 

We are investigating the permeation properties and stability of ultrathin, layered polyelectrolyte films. These films are especially attractive because of their simple synthesis, which involves alternating, short immersions of a charged surface in solutions containing polyanionic or polycationic polymers.¹ Cyclic voltammetry and ac-impedance studies with films containing alternating layers of poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate) (PSS) deposited on self-assembled monolayers on gold show that electrode blocking increases nonlinearly with the number of layers in the film. For example, the charge transfer resistances of a gold electrode covered with one and ten layers of PAH and PSS are 1 and 10⁴cm², respectively. Passivation due to polyelectrolyte films is similar in pH 3- and pH 6.3-buffered solutions, but most of the blocking is irreversibly lost upon exposure to a pH 10-buffered solution. Ellipsometry shows that the dry thickness of films does not change significantly upon exposure to a pH 10 solution. Time-resolved in situ ellipsometry, however, reveals that upon immersion in pH 10 buffer these films expand for a few minutes and then contract to their original thickness. Exposure to pH 10-buffered solution does cause a structural change in the films that we detect electrochemically. The peak current in cyclic voltammograms (pH 10) increases as the film swells, but does not decrease when the film contracts. The blocking behavior of these films is similar for positively and negatively charged redox species. We are currently investigating passivation due to other types of polyelectrolyte films. In addition to electrochemical studies, we plan to discuss work on composite membranes containing polyelectrolyte films. 

2:00 PM *BB2.3 
pH-GATED SINGLE ELECTRON TUNNELING IN CHEMICALLY MODIFIED GOLD NANOCLUSTERS AND 1-DIMENSIONAL CLUSTER ARRAYS. Dan Feldheim , Louis C. Brousseau, III, James P. Novak and Stella M. Marinakos, Department of Chemistry, North Carolina State University, Raleigh, NC. 

We have investigated single electron tunneling (SET) in chemically modified Au clusters in-situ with scanning tunneling spectroscopy. Coulomb staircase behavior was observed in individual surface-confined clusters which could be modulated predictably via solution composition. For example, using a pH-active cluster capping ligand allowed the position of the staircase along the potential axis to be controlled with solution pH. Applications of this Œchemically gated¹ SET to new single molecule detection schemes will be highlighted. In addition, by attaching Au clusters to an STM tip the electronic properties of Au/semiconductor quantum dot and other molecular heterojunctions were characterized. The extension of these methods to the characterization of electron transport in longer 1D nanocrystal arrays will also be discussed. 

2:30 PM *BB2.4 
HETEROSUPRAMOLECULAR CHEMISTRY: THE PROGRAMMED ASSEMBLY OF METAL NANOCRYSTALS. Stephen Connolly, Stephen Fullam, Hugh Doyle, Brian Korgel, Donald Fitzmaurice , University College Dublin, Department of Chemistry, Belfield, Dublin, IRELAND. 

Recent years have seen the development of powerful techniques and methodologies for the self-assembly and self-organisation of supermolecules and for the preparation of metal and semiconductor nanocrystals. Together, these developments allow us explore the possibility of self-assembling and self-organising complex structures containing both molecules and nanocrystals, referred to as heterosupermolecules. 
The first part of this talk will describe the self-assembly and self-organisation of metal and semiconductor nanocrystals in solution. Also described will be new experimental methods which have been used to study nanocrystal self-assembly and self-organisation in real-time and in real-space. 
The second part of this talk will describe strategies that have been developed to control the self-assembly and self-organisation of metal and semiconductor nanocrystals in solution. These strategies rely on adsorbing high information content molecules on the surface of the nanocrystal which programme the location of a nanocrystal within an assembly. 
Finally, some implications of these and related findings for the bottom-up assembly of nanometer scale devices in solution will be considered. 

3:30 PM *BB2.5 
DEPOSITION OF METALLIC NANOCLUSTERS WITHIN DENDRIMER TEMPLATES. Richard M. Crooks , Mingqi Zhao, Li Sun, Texas A&M University, Department of Chemistry, College Station, TX. 

Here we introduce a new template-synthesis strategy for preparing metal nanoclusters within dendrimer ënanoreactorsí. Proof of concept is demonstrated by preparation of Cu clusters because analysis is simplified, but the approach is applicable to any transition metal ions that can be extracted into the interior of a dendrimer and subsequently reduced. Clusters ranging in size from 4-64 atoms are prepared by partitioning of Cu(II) into the interior of poly(amidoamine) (PAMAM) Starburst dendrimers and subsequent chemical reduction. By controlling the chemical structure and size of the dendrimers, different-size clusters can be prepared. 
The number of Cu(II) ions extracted into each generation of dendrimer was quantitatively assessed by spectrophotometric titration. The data indicate that second, fourth, and sixth generation dendrimers sorb 4, 16, and 64 Cu(II) ions. Reduction of dendrimer-encapsulated Cu(II) with borohydride results in intradendrimer Cu clusters. Evidence for this comes from the immediate change in solution color from blue to golden brown: the absorbance bands originally present at 605 nm and 300 nm disappear and are replaced with a monotonically increasing spectrum of nearly exponential slope towards shorter wavelengths. This behavior results from the appearance of a new interband transition arising from the formation of intradendrimer Cu clusters. The measured onset of this transitions at 590 nm agrees with literature values and the exponential shape is characteristic of a band-like electronic structure, which strongly suggests that the reduced Cu does not exist as isolated atoms, but rather as clusters. In addition to Cu clusters, results relating to Pt, Pd, and Ru, as well as bimetallics prepared from these elements, will be described. 

4:00 PM *BB2.6 
DEVELOPMENT OF METALLIZED LIPID TUBULES FOR FUNCTIONAL APPLICATIONS. Joel M. Schnur , Mark Spector, Jonathan Selinger, Ronald Price, Paul Schoen, and Dan Zabetakis; Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, Washington, DC. 

Synthetic phospholipids with diacetylenic moieties in the acyl chain self-assemble to form hollow, cylindrical tubules with diameters of approximately 0.5 microns and lengths of 50-200 microns. These tubules can then be coated with metal to make rugged metallic cylinders for applications in controlled release and in composite dielectric materials. In our recent research using electron microscopy, differential scanning calorimetry and circular dichroism spectroscopy we have found that the length and wall thickness of lipid tubules depend on the length of the acyl chain, the lipid concentration, and on the alcohol/water solvent. By controlling these parameters, we have optimized the lipid tubules for both function and cost. These micro-cylinders are used to fabricate composite polymer coatings. These tubule containing coatings have been shown to provide effective approaches for the controlled release of medical and non-medical compounds, as well as the fabrication of composite dielectric coatings. 

4:30 PM BB2.7 
IRON OXIDE NANOROD ARRAYS. Y.Jin, C.L. Dennis, and S.A. Majetich , Carnegie Mellon Univ, Dept of Physics, Pittsburgh, PA. 

Compared to other self-assembling systems, magnetic nanoparticles have an additional magnetostatic force, which can be either attractive or repulsive. This favors the formation of magnetically aligned chains of magnetic dipoles, rather than two- or three-dimensional structures. Here this problem is surmounted by forming a nonmagnetic liquid crystal first and transforming it to a magnetic following matrix solidification. 
Schiller layers are liquid crystal arrays of akaganeite nanorods formed by slow hydrolysis of iron trichloride. Characteristic iridescence with colors ranging from pink to blue occurs because the separation between layers of rods is on the order of a wavelength of light. Schiller layers are prepared by centrifuging the solution until iridescence is seen in the deposited sediment. Following removal of most of the supernatant, the acid catalyzed reaction of tetraethylorthosilicate (TEOS) in a mixture of ethanol and water at 50 degrees forms a silica matrix around the particles. To make ferrimagnetic magnetite, the antiferromagnetic akaganeite arrays are slowly heated in an inert atmosphere and exposed to hydrogen gas. Without the silica coating this reaction does not preserve the rod-like shape of the original particles, but generates spherical hematite particles. 
Optical microscopy and TEM are used to characterize the degree of ordering of the nanoparticle arrays. The ordered domain size decreases with heating, but micron-sized domains are still present. The magnetization directions of individual particles and direct evidence of interparticle coupling are obtained from the Foucault method of Lorentz microscopy. If particles within a layer interact, and their moments are normal to the plane, magnetostatic coupling favors antiferromagnetic alignment of magnetic moments. The magnetic results will be related to industry requirements for inexpensive but high density magnetic recording media. 

4:45 PM BB2.8 
ION IMPLANTED ORDERED-ARRAY OF SILICA SPHERES. Y. Zhang , S. Vijayalakshmi, H. Grebel, Optical Waveguide Laboratory, New Jersey Institute Of Technology, Newark, NJ; J. Federici, Physics Dept., New Jersey Institute of Technology, Newark, NJ;C. W. White, Oak-Ridge National Lab, Oak Ridge, TN. 

We report on the optical properties of thin films made of ordered-array of silica spheres (opals) implanted with Si nanoclusters. The synthetic silica opal films were prepared by known chemical extraction techniques that produce a narrow dispersion of opal sizes. The thin opal films were fabricated on quartz substrates and annealed at 600-700C. Three different types of films were prepared with particulate sizes of 200, 240, and 300 nm. The films were found to have highly periodic structure and SEM images showed perfect hexagonal packing of particles which corresponds to <111> oriented fcc lattice of a colloidal crystal. The scattering from these films was seen to be polarization dependent which is a further indication of periodic structure of the spheres. 
The opal films were implanted with Si ions (400 KeV, dose 6.E17 1/cm² with a peak excess Si of 2E22 1/cm^-3). SEM images indicated that the opal films retained their structure. The samples were further annealed at 1100C. Our studies showed that the fluorescence curve shape as well as its intensity have changed as a result of the confining environment of the opal structure; the implanted films exhibited a larger and narrower fluorescence spectra. In this paper, Si implanted opal will be compared to Si implanted, plane fused-silica samples under similar ion dosages. For example, the implanted plane fused-silica samples showed a strong non-linear effects at blue and green wavelengths depending on the size of the resultant nano- Si cluster. Preliminary results on Si implanted opal showed that they too possess large non-linear optical properties. 

SESSION BB3: 
Chair: Chad A. Mirkin 
Tuesday Morning, December 1, 1998 
Essex South (W)

8:30 AM BB3.1 
ASSEMBLY OF METAL NANOSTRUCTURES ON DIBLOCK COPOLYMER TEMPLATES. Robert W. Zehner , Lawrence R. Sita, The University of Chicago, Department of Chemistry, Chicago, IL; Ward A. Lopes, Heinrich Jaeger, The University of Chicago, Department of Physics and James Franck Institute, Chicago, IL. 

Phase-separated diblock copolymers have been employed as templates for the construction of nanoscale aggregates of passivated metal nanocrystals. Selective adsorption of the nanocrystals on only one block of the polymer produces patterns with widths on the order of 25 nm. This process is postulated to occur through surface free energy interactions between the organic passivants and the polymers, and thus can be generalized to a variety of passivated nanoparticles. Significantly, the resulting colloidal assemblies lie on the surface of the polymer, leaving them accessible to solution-phase chemical reactions. As an application of this technique, patterns of palladium nanocrystals have been metallized by electroless copper deposition to produce nanometer-scale continuous copper structures. 

8:45 AM BB3.2 
BUNDLING AND INTERDIGITATION OF SURFACE ADSORBED MOLECULES ON SURFACES OF SELF-ASSEMBLED NANOCRYSTALS. Z.L. Wang , S.A. Harfenist¹ and R.L. Whetten¹, J. Bentley² and N.D. Evans², School of Material Science and Engineering, ¹School of Physics, Georgia Institute of Technology, Atlanta GA, ²Metals and Ceramics Div., Oak Ridge National Laboratory, Oak Ridge TN. 

Self-assembling of nanocrystals involves organization of nanocrystals encapsulated by protective compact organic molecules into a crystalline material. The adsorbed molecules serve not only as the protection layer for the nanocrystals but provide the dominant cohesive interactions (or 'bonding') sustaining the nanocrystal superlattices. In this paper, Ag nanocrystals dominated by truncated octahedral shape and packed into a face-centered cubic structure are taken as a model system for studying the distribution of adsorbed molecules on the nanocrystal surfaces [1]. Direct imaging of the passivation thiolates has been performed using the energy-filtered transmission electron microscopy (EF-TEM). By selecting the electrons which have excited the carbon K ionization edge in transmission electron imaging, the distribution of thiolates around nanocrystals is imaged at a resolution 2 nm. For Ag nanocrystals with truncated octahedral shape, the density of thiolates passivated on (100) faces is significantly higher than that on the (111) faces. The results support the conclusion of interdigitation and bundling of thiolates on the nanocrystal surface [2,3]. 

9:00 AM *BB3.3 
NEW PROPERTIES OF METALLIC AND SEMICONDUCTOR NANOPARTICLES. M. A. El-Sayed , C. Burda, T. Green, S. Link, R. Little, M. Mohamed, J. Petroski, Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA; Z. L. Wang, School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, GA. 

Metallic nanostructures are synthesized having different shapes. The shape dependence of their physical, photochemical and chemical (catalytic) properties are studied and will be discussed. The femtosecond electron and hole dynamics in different quantum dots (Q.D.) and quantum dot-quantum wells (Q.D.Q.W.) are examined. The effect of charge separated states present in the Q.D.Q.W. of heterostructured materials; the effect of surface modifications and adsorped electron acceptors in the Q.D. on these dynamics will be illustrated and discussed. 

9:30 AM *BB3.4 
Abstract Withdrawn. 

10:30 AM *BB3.5 
ELECTROCHEMICAL BEHAVIOR OF ELECTRICAL CONNECTIONS BUILT FROM ORGANIC CHARGE TRANSFER SALTS. Michael J. Sailor , Christian Gurtner, Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla CA; Andrew S. Katz and Robert C. Dynes, Department of Physics, University of California at San Diego, La Jolla, CA. 

A non-lithographic technique for the formation of microscopic electronic connections has been developed using electrochemical deposition techniques. The method involves construction of electrical connections using conducting organic charge- transfer salts (CT-salts) with highly anisotropic morphologies. Electrical contacts have been made between two macroscopic electrodes (0.08 mm diameter) separated by 0.25 mm using the CT-salt TTFBrx (x=0.74-0.79).