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Topic Name: U. of I. Researchers has Developed a Process that Makes Nanofibers in Complex Shapes and Infinite Lengths
Category: Nanofabrication
Research persons: Min-Feng Yu
Location: University of Illinois, United States
Details
The continuous fabrication of complex, three-dimensional nanoscale structures
and the ability to grow individual nanowires of unlimited length are now
possible with a process developed by researchers at the University
of Illinois.
Based on the rapid evaporation of solvent from simple “inks,” the process
has been used to fabricate freestanding nanofibers, stacked arrays of nanofibers
and continuously wound spools of nanowires. Potential applications include
electronic interconnects, biocompatible scaffolds and nanofluidic networks.
“The process is like drawing with a fountain pen – the ink comes out and
quickly dries or ‘solidifies,’ ” said Min-Feng Yu, a professor of
mechanical science and engineering, and an affiliate of the Beckman
Institute. “But, unlike drawing with a fountain pen, we can draw objects
in three dimensions.”
Yu and graduate students Abhijit Suryavanshi and Jie Hu describe the drawing
process in a paper accepted for publication in the journal Advanced Materials,
and posted on its Web site.
To use the new process, the researchers begin with a reservoir of ink
connected to a glass micropipette that has an aperture as small as 100
nanometers. The micropipette is brought close to a substrate until a liquid
meniscus forms between the two. As the micropipette is then smoothly pulled
away, ink is drawn from the reservoir. Within the tiny meniscus, the solute
nucleates and precipitates as the solvent quickly evaporates.
So far, the scientists have fabricated freestanding nanofibers approximately
25 nanometers in diameter and 20 microns long, and straight nanofibers
approximately 100 nanometers in diameter and 16 millimeters long (limited only
by the travel range of the device that moves the micropipette).
To draw longer nanowires, the researchers developed a precision spinning
process that simultaneously draws and winds a nanofiber on a spool that is
millimeters in diameter. Using this technique, Yu and his students wound a coil
of microfiber. The microfiber was approximately 850 nanometers in diameter and
40 centimeters long.
To further demonstrate the versatility of the drawing process, for which the
U. of I. has applied for a patent, the researchers drew nanofibers out of sugar,
out of potassium hydroxide (a major industrial chemical) and out of densely
packed quantum dots. While the nanofibers are currently fabricated from
water-based inks, the process is readily extendable to inks made with volatile
organic solvents, Yu said.
“Our procedure offers an economically viable alternative for the
direct-write manufacture of nanofibers made from many materials,” Yu said.
“In addition, the process can be used to integrate nanoscale and microscale
components.”
Note for Nanowire
A nanowire is a wire of diameter of the order of a nanometer (10−9 meters). Alternatively, nanowires can be defined as structures that have a lateral size constrained to tens of nanometers or less and an unconstrained longitudinal size. At these scales, quantum mechanical effects are important — hence such wires are also known as "quantum wires". Many different types of nanowires exist, including metallic (e.g., Ni, Pt, Au), semiconducting (e.g., Si, InP, GaN, etc.), and insulating (e.g., SiO2,TiO2). Molecular nanowires are composed of repeating molecular units either organic (e.g. DNA) or inorganic (e.g. Mo6S9-xIx).
The nanowires could be used, in the near future, to link tiny components into extremely small circuits. Using nanotechnology, such components could be created out of chemical compounds.
The nanowires can show peculiar shapes. Sometimes they can show noncrystalline order, assuming e.g. a pentagonal symmetry or a helicoidal (spiral) shape. Electrons zigzag along pentagonal tubes and spiral along helicoidal tubes.
The lack of crystalline order is due to the fact that a nanowire is periodic only in one dimension (along its axis). Hence it can assume any order in the other directions (in plane) if this is energetically favorable.
E.g., in some cases nanowires can show a fivefold symmetry, usually not observed in nature, but for clusters of few atoms. The fivefold symmetry is equivalent to the icosahedral symmetry of (small) atomic clusters: the icosahedron is often an energetically favorable shape for cluster of few atoms, but icosahedral ordering is not observed in crystals since it is not possible to stack together icosahedra (repeating infinite copies of them in each direction) and tile the whole space (fill it without holes).
Note for Nanofibers
Nanofibers are defined as fibers with diameters less than 100 nanometers. They can be produced by interfacial polymerization and electrospinning. Carbon nanofibers are graphitized fibers produced by catalytic synthesis.
Applications
In one study, combined neural stem cells with carbon nanofibers triggered neural tissue regeneration in the brains of rats that had suffered a simulated
stroke. On their own, neither nanofibers nor stem cells could heal the rats.
Napkins with nanofibers contain antibodies against numerous biohazards and chemicals that signal by changing color (potentially useful in identifying bacteria in kitchens).
In wound healing nanofibers assemble at the injury site and stay put, drawing the body's own growth factors to the injury site.
Donaldson develops nanofiber filter media for new air and liquid filtration applications, such as vacuum cleaners.
Other applications include industrial and high-tech applications for aerospace, capacitors, transistors, battery separators, energy storage, fuel cells and information technology.
Note for Quantum Dot
A quantum dot is a semiconductor nanostructure that confines the motion of conduction band electrons, valence band holes, or excitons (bound pairs of conduction band electrons and valence band holes) in all three spatial directions. The confinement can be due to electrostatic potentials (generated by external electrodes, doping, strain, impurities), the presence of an interface between different semiconductor materials (e.g. in core-shell nanocrystal systems), the presence of the semiconductor surface (e.g. semiconductor nanocrystal), or a combination of these. A quantum dot has a discrete quantized energy spectrum. The corresponding wave functions are spatially localized within the quantum dot, but extend over many periods of the crystal lattice. A quantum dot contains a small finite number (of the order of 1–100) of conduction band electrons, valence band holes, or excitons, i.e., a finite number of elementary electric charges.
Quantum dots can be contrasted to other semiconductor nanostructures:
quantum wires, which confine the motion of electrons or holes in two spatial dimensions and allow free propagation in the third.
quantum wells, which confine the motion of electrons or holes in one dimension and allow free propagation in two dimensions.
Note for Micropipette
Micropipettes are tools constructed from glass tubing for microinjection, micromanipulation and measuring purposes. Many types and sizes of glass tubing are available, mainly in three different compositions: borosilicate, aluminosilicate and quartz. Each composition has its own unique properties and the right selection is determined by the application it is used for. Micropipettes are mainly used in biological and chemistry experiments. Normal glass pipettes which are used in chemical labs are not highly accurate for volumes less than 2ml, but micropipettes (autopipettes) are both accurate and precise. Various sizes of micropipettes allow for accurate measurements of volumes less than 1µl, or as large as 1000µl (1ml).
The Grainger
Foundation, the National
Science Foundation and the Office
of Naval Research provided funding. Part of the work was carried out in the
university’s Center for
Microanalysis of Materials, which is partially supported by the U.S.
Department of Energy.
In figure 1, Min-Feng Yu, a professor of mechanical science and engineering, and an affiliate of the Beckman Institute, has developed a new process for creating complex, three-dimensional nanoscale structures.
In figure 2, A Gilson multichannel micropipette. This one has only four channels in use.
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