Intricate, curving 3-D nanostructures created using capillary action forces

October 19, 2010

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Twisting spires are one of the 3-D shapes researchers at the University of Michigan were able to develop using a new manufacturing process. Credit: A. John Hart

ANN ARBOR, Mich.---Twisting spires, concentric rings, and gracefully bending petals are a few of the new three-dimensional shapes that University of Michigan engineers can make from carbon nanotubes using a new manufacturing process.

The process is called "capillary forming," and it takes advantage of capillary action, the phenomenon at work when liquids seem to defy gravity and travel up a drinking straw of their own accord.

The new miniature shapes, which are difficult if not impossible to build using any material, have the potential to harness the exceptional mechanical, thermal, electrical, and chemical properties of carbon nanotubes in a scalable fashion, said A. John Hart, an assistant professor in the Department of Mechanical Engineering and in the School of Art & Design.

They could lead to probes that can interface with individual cells and tissues, novel microfluidic devices, and new materials with a custom patchwork of surface textures and properties.

A paper on the research is published in the October edition of Advanced Materials, and is featured on the cover.

"It's easy to make carbon nanotubes straight and vertical like buildings," Hart said. "It hasn't been possible to make them into more complex shapes. Assembling nanostructures into three-dimensional shapes is one of the major goals of nanotechnology. The method of capillary forming could be applied to many types of nanotubes and nanowires, and its scalability is very attractive for manufacturing."

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By using unique two-dimensional templates, researchers at the University of Michigan could coax carbon nanotubes to grow in intricate, curving three-dimensional structures. Credit: A. John Hart

Hart's method starts by stamping patterns on a silicon wafer. His ink in this case is the iron catalyst that facilitates the vertical growth of the carbon nanotubes in the patterned shapes. Rather than stamp a traditional, uniform grid of circles, Hart stamp hollow circles, half circles and circles with smaller ones cut from their centers. The shapes are arranged in different orientations and groupings. One such grouping is a pentagon of half circles with their flat sides facing outward.

He uses the traditional "chemical vapor deposition" process to grow the nanotubes in the prescribed patterns. Then he suspends the silicon wafer with its nanotube forest over a beaker of a boiling solvent, such as acetone. He lets the acetone condense on the nanotubes, and then lets the acetone evaporate.

As the liquid condenses, capillary action forces kick in and transform the vertical nanotubes into the intricate three-dimensional structures. For example, tall half-cylinders of nanotubes bend backwards to form a shape resembling a three-dimensional flower.

"We program the formation of 3D shapes with these 2D patterns," Hart said. "We've discovered that the starting shape influences how the capillary forces change the structures' geometry. Some bend, others twist, and we can combine them any way we want."

The capillary forming process allows the researchers to create large batches of 3D microstructures---all much smaller than a cubic millimeter---over essentially limitless areas, Hart said. In addition, the researchers show that their 3D structures are up to 10 times stiffer than typical polymers used in microfabrication. Thus, they can be used as molds for manufacturing of the same 3D shapes in other materials.

"We'd like to think this opens up the idea of creating custom nanostructured surfaces and materials with locally varying geometries and properties, " Hart said. "Now, we think of materials as having the same properties everywhere, but with this new technique we can dream of designing the structure and properties of a material together."

Multi-component nano-structures with tunable optical properties

 BNL scientists used DNA linkers with three binding sites (black ?strings?) to connect gold nanoparticles (orange and red spheres) and fluorescent dye molecules (blue spheres) tagged with complementary DNA sequences. These units are self-assembled to form a body-center cubic lattice with nanoparticles at the corners and in the center, and fluorescent dye molecules in between.

(PhysOrg.com) -- Scientists at the U.S. Department of Energy's Brookhaven National Laboratory report the first successful assembly of 3-D multi-component nanoscale structures with tunable optical properties that incorporate light-absorbing and -emitting particles. This work, using synthetic DNA as a programmable component to link the nanoparticles, demonstrates the versatility of DNA-based nanotechnology for the fabrication of functional classes of materials, particularly optical ones, with possible applications in solar-energy conversion devices, sensors, and nanoscale circuits. The research was published online September 29, 2010, in the journal Nano Letters.

?For the first time we have demonstrated a strategy for the assembly of 3-D, well-defined, optically active structures using DNA encoded components of different types,? said lead author Oleg Gang of Brookhaven?s Center for Functional Nanomaterials (CFN). Like earlier work by Gang and his colleagues, this technique makes use of the high specificity of binding between complementary strands of DNA to link particles together in a precise way.

In the current study, the DNA linker molecules had three binding sites. The two ends of the strands were designed to bind to complementary strands on ?plasmonic? gold nanoparticles ? particles in which a particular wavelength of light induces a collective oscillation of the conductive electrons, leading to strong absorption of light at that wavelength. The internal part of each DNA linker was coded to recognize a complementary strand chemically bound to a fluorescent dye molecule. This setup resulted in the self-assembly of 3-D body centered cubic crystalline structures with gold nanoparticles located at each corner of the cube and in the center, with dye molecules at defined positions in between.

The scientists also demonstrated that the assembled structures can be dynamically tuned by altering the salt concentration of the solution in which they are formed. Changes in salinity alter the length of the negatively charged DNA molecules, leading to reversible contraction and expansion of the whole lattice by about 30 percent in length.

?It has long been understood that the distance between metal nanoparticles and paired dye molecules can affect the optical properties of the latter,? said Matthew Sfeir, coauthor and an optical scientist at the CFN. In this experiment, the expansion and contraction of the crystal lattice triggered by the changes in salt concentration allowed for a dramatic modulation of an optical response: a three-fold increase in the emission rate of the fluorescent molecules was observed.

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The resulting 3-D structures could be tuned by adjusting the salt concentration. As salt concentration increased, the crystals contracted by about 30 percent, decreasing the distance (D) between the particles. This contraction in interparticle distance had a dramatic effect on the fluorescence of the dye molecules, making them cycle photons faster, as indicated by the color scale at the left of the crystal images (see image below), which ranges from nearly 2 nanoseconds per cycle for the free dye (A), to about 0.7 nanoseconds per cycle in larger lattices (C), to just above 0.3 nanoseconds per cycle for the contracted crystals (E).

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These results were determined using a combination of small angle x-ray scattering at Brookhaven?s National Synchrotron Light Source (NSLS) and time-resolved fluorescent methods at the CFN. ?This combination of synchrotron-based structural methods and time-resolved optical imaging techniques provided invaluable direct insight into the relationship between the structure and fluorescent properties of these light emitting arrays,? Gang said.

?Our study tackles important questions about the self-assembly of systems from components of multiple types. Such systems potentially allow for the modulation of properties of individual components, and might lead to the emergence of new behavior due to collective effects. This assembly approach can be applied to explore such collective behavior of three-dimensional nano-optical arrays ? for example, the influence of the plasmonic lattice on quantum dots.

?An understanding of these interactions would be relevant for developing novel optical materials for photovoltaic, photocatalysis, computing, and light-emitting applications. We now have an approach to make these structures and further study these effects.?

Nanospheres made of aromatic amino acids: The most rigid organic nanostructures to date

 (PhysOrg.com) -- Organic nanostructures are key elements of nanotechnology because these building blocks can be made with tailored chemical properties. Their disadvantage has been that their mechanical properties have so far been significantly inferior to those of metallic nanostructures.

Ehud Gazit, Itay Rousso, and a team from the Tel Aviv University, the Weizmann Institute of Science and the Ben-Gurion University of the Negev (Israel) have now introduced organic nanospheres that are as rigid as metal. As the scientists report in the journal Angewandte Chemie, they are interesting components for ultrarigid biocomposite materials.

Nanoscale biological structures often exhibit unique mechanical properties; for example spider silk is 25 times as strong as steel by weight. The most rigid synthetic organic materials known to date are aramids, such as Kevlar. Their secret is a special spatial arrangement of their aromatic ring systems and the network of interactions between their planar amide bonds. The new nanospheres are based on a similar construction principle. However, unlike the large polymeric chains, they are formed in a self-organization process from very simple molecules based on aromatic dipeptides of the amino acid phenylalanine.

Using an atomic force microscope, the scientists examined the mechanical properties of their nanospheres. This device uses a nanotip (cantilever), a tiny flexible lever arm with a very fine tip at the end. When this tip is pressed against a sample, the deflection of the lever indicates whether the tip of the needle can press into the sample object and how far in it can go. A metal needle was not able to make any impression on the nanospheres; only a needle made of diamond was able to do it. The researchers used these measurements to calculate the elasticity modulus (Young?s modulus) for the nanospheres. This value is a measure of the stiffness of a material. The larger the value, the more resistance a material has to its deformation. By using a high-resolution scanning electron microscope equipped with a nanomanipulator, it was possible to directly observe the deformation of the spheres.

For the nanospheres, the team measured a remarkably high elasticity modulus (275 GPa), which is higher than many metals and similar to the values found for steel. This makes these nanostructures the stiffest organic molecules to date; they may even eclipse aramids. In addition to having outstanding mechanical properties, the nanospheres are also transparent. This makes them ideal elements for the reinforcement of ultrarigid biocomposite materials, such as reinforced plastics for implants or materials for tooth replacement, aerospace, and other applications that require inexpensive, lightweight materials with high stiffness and unusual stability.