Directed Assembly of Functional Nanoscale Materials

Our goal is to develop a platform for in situ measurements of the directed assembly of complex solutions of nanoscale building blocks (NBB)s into functional materials and devices. These measurement methods will enable new methods of nanofabrication for applications orthogonal to traditional semiconductor devices.

While top-down lithography has been the workhorse for commercial manufacturing of functional materials and devices, barriers including limited materials sets, two-dimensional patterning processes, and increasing cost of next-generation tools are apparent. The spectacular variety of nanoscale building blocks offers the potential of three-dimensional structures and patterns, large area and high volume manufacturing.To meet the needs we will:

• Provide critical information about the dynamic evolution of NBB structures, such as mechanisms of nanoparticle growth, nanoparticle assembly in solution, and complex fluid transport properties at nanometer length scales.

• Develop unique measurements including quartz crystal microbalance-dissipation, microfluidic channels with trapping and adjustable flow fields, and evanescent wave and confocal fluorescence correlation spectroscopy.

In situ Measurements of Nanoparticle Growth:

The inorganic synthetic approaches that prepare well-defined nanoparticles of varying size and shape have attracted attention as functional materials. However, in many cases these nanoscale building blocks may have broad size distribution and or little control on the aqueous or organic solution phase behavior and stability.In this project we have investigated the nucleation and growth kinetics of gold nanoparticles mediated by a model peptide.Complementary small-angle X-ray scattering and UV-Vis spectroscopic approaches are used to measure the degree of conversion and size of gold nanocrystals.From these measurements rate laws for nucleation and growth were proposed that quantify the kinetics of peptide-mediated nanocrystal biomineralization. We also compare kinetic rate laws for this peptide, citrate, and various other polymer ligands and find that the peptide belongs to a unique class of non-reducing inhibitor ligands. The peptide regulates the surface-reaction-limited growth of nanocrystals. Growth inhibition is due solely to the strong surface binding of peptides to gold and not diffusion limited or steric effects. Finally, we propose that control over the size and shape of nanocrystals arises from the formation of a full shell of AYSSGAPPMPPF peptides that lie flat on the surface.

Nanoparticle Assembly:

Nanoparticles often assemble collectively into closed packed lattices, or fractal aggregates. We provide a physical handle on directing nanoparticle assembly in solution or to interfaces by introducing physical or chemical anisotropy to break the spherical symmetry. Methods to produce anisotropic particles include cross-linking partially mixed streams of photosensitive monomers in microfluidic channels and lithographic templating of adsorbed particles on substrates. We have developed and characterized particles that self-assemble into desired anisotropic structures by performing an in situparticle lithography.By adsorbing particles to surfaces a well-defined patch is prepared through a layer-by-layer patterning with polyelectrolytes. The tailored patch associates in aqueous solution with single or multiple oppositely charged particles resulting in the controlled formation of colloidal doublets which are excellent candidates as model anisotropic particles. Size, shape, and charge anisotropy are characterized by electron microscopy and zeta potential measurements.

Complex Fluid Transport:

The directed assembly of nanoparticles is expected to be achieved through an ensemble of particle size, shape and function. Therefore in a fluid mixture the transport properties are not representative of single component solutions, but involve complex intermediate configurations. Measuring the transport properties of a single NBB-type in multicomponent mixtures cannot be achieved through conventional dynamic light scattering or particle tracking methods, especially as particles are reduced to sub 50 nm. We have designed a fluorescence correlation spectroscopy (FCS)instrument that measures transport properties of dye-labeled nanoparticles and polymers in complx fluids mixtures. Model systems of charged nanoparticle/polyelectrolyte complexes have been studied as a model electrostatic-driven assembling system.FCS provides the capability of measuring the diffusion constant from dilute to concentrated mixtures and as a function of changing medium properties such as pH and ionic strength.

• For nanoscale electronics, the markets for nanomaterials, tools and equipment totaled $1.827 billion in 2005 and are forecasted to reach $4.219 billion by the year 2010, according to Global Nanoelectronics Markets and Opportunities.
• Emerging nanotechnology applications that employ directed assembly such as photovoltaics, organic and flexible electronics,and applications that incorporate biological functionality, are poised to accelerate innovation in the US.
• The development of directed assembly processes with industrial level quality control will require the development of new metrology systems that describe all facets of the new manufacturing processes: the formation of NBBs, their transport through the manufacturing steps and their final assembly.
• NIST is leading a Workshop to identify and authenticate measurement needs facing directed assembly of functional nanoscale materials.Participants and invited speakers include recognized scientific leaders and representatives from industry, government, and academia.