Rewritable photonic paper

Rewritable photonic paper

Photonic crystals are a new kind of material. Also known as photonic band gap material, they are similar to semiconductors, where the electrons are replaced by photons (i.e. light). By creating periodic structures out of materials with contrast in their dielectric constants, it becomes possible to guide the flow of light through the photonic crystals in a way similar to how electrons are directed through doped regions of semiconductors. The photonic band gap (that forbids propagation of a certain frequency range of light) gives rise to distinct optical phenomena and enables one to control light with amazing facility and produce effects that are impossible with conventional optics.

Photonic crystals are very suitable for fabricating sensor devices because the optical signals of these responsive materials are tightly coupled with various external stimuli and modulations. They could also play a significant role on the way to all-optical devices in communication and information technology – they form a basis material for the future realization of optical components and circuits, and maybe even complex optical circuits or optical computers.

One application area that has seen a lot of activity recently is photonic paper and ink. One advantage of photonic papers and displays is that they are brilliant and free of glare in sunlight, which are superior to the characteristics of other emissive display technologies for outdoor usage such as advertising billboards. Another advantage is that, for recording purposes, the structural colors of photonic paper are usually more durable than traditional pigments and dyes.

However, there are still a number of issues and problems that prevent the practical applications of the photonic crystal based materials. Researchers in California have now addressed some of them by reporting a new type of rewritable photonic paper on which a durable ink mark can be written or erased by introducing or removing a hygroscopic salt in the surface layer of the paper.
The key point of their work is the use of hygroscopic salt solutions as 'ink' to swell the polymer matrix of photonic crystals and create a very durable contrast of diffraction colors on the photonic paper. The ink marks can be erased by rinsing the paper in water and drying thereafter. The rewritability of the photonic paper and low toxicity of the materials involved make this paper/ink system environmentally friendly, inexpensive, and useful for general applications involve color writing and display.

Digital photo images of flexible photonic paper (3 cm x 4 cm) on the plastic substrate with and without letters printed. The color gradient in the photos is caused by the flash lamp illuminating as a point light source.

Yadong Yin, an assistant professor in the Department of Chemistry at the University of California, Riverside, explains to Nanowerk that his lab's new invention shows several major improvements compared to existing photonic paper fabrication technologies:

1) Conventionally, the fabrication for the photonic-crystal based paper typically involves very slow processes, such as colloidal sedimentation or solvent evaporation. It usually takes days or even months to grow photonic crystals with a reasonable size, therefore limiting their practical use.

In Yin's work, the photonic paper can be produced instantaneously by assembling magnetic particles into periodic arrays using an external field. The periodic photonic structures can be immediately fixed in a polymer matrix. This allows for future mass production.

2) Conventionally, writing the color information in the photonic paper is achieved by applying a volatile solvent (ink), which swells the polymer matrix, changes the periodicity of the photonic structure, and eventually changes the color of inked region. The color disappears when the solvent evaporates. So the color writing is not permanent.

"In our work, the ink is a water-ethanol solution of inorganic hygroscopic salt" says Yin. "This salt solution does not evaporate at room temperature and keeps the polymer matrix in the swelled state so that the color information stays for a long time. Erasing of the written information can be simply achieved by removing the salt solution using additional water."

3) Unlike conventional methods which typically involve toxic organic solvent, the salt ink and rinsing water (and the photonic paper itself) are nontoxic and environmentally benign. Yin's team prepared their new type of photonic paper through the fast magnetic field-induced self-assembly of Fe3O4@SiO2 colloids, followed by a simultaneous UV curing process to fix the photonic structures inside the PEGDA matrix.

The photonic paper can be fabricated onto a glass substrate or flexible plastic substrate. The findings have been published in a paper in the July 17, 2009 online edition of Advanced Materials Schematic illustrations of the structure of the unit cell and the mechanism of writing or erasing realized by infiltrating or removing the hygroscopic salt.

Yin says that, in principle, the technique developed in his lab can potentially be used to replace conventional paper/ink systems in many areas. Also, compared to existing techniques, his team's rewritable photonic systems have several advantages including the easiness for large-area fabrication, the versatility in pattern generation, and brilliant color in bright sunlight illumination (in contrast to liquid-crystal-type displays). "We believe that more elaborate features, such as multicolor printing, may be realized in the future with the use of multiple inks and modifications to the polymeric matrix of the photonic paper"

There are still some kinks to iron out, though. For instance, the researchers experienced a problem when they write on the photonic paper manually using a small glass capillary 'pen'. As the capillary is not specially designed for writing, the delivery of 'ink' seems to be not very uniform. Yin says that he would expect a better design of a 'pen' or the integration with ink-jet printing process would solve the problem.

The immediate future work for Yin's team is to find out the ultimate resolution of this photonic paper, which is an important factor for practical applications. "One interesting question that we will keep working on is the realization of multi-color writing/printing, which might be possible if multiple ink salts are used," says Yin. "We will also explore the integration of this writing mechanism with ink-jet printing techniques in order to achieve a highly controllable delivery of inks."

DNA nanotechnology in computers knocks down another roadblock

DNA nanotechnology in computers knocks down another roadblock

DNA origami, tiny shapes and patterns self-assembled from DNA, have been heralded as a potential breakthrough for the creation of nanoscale circuits and devices. One roadblock to their use has been that they are made in solution, and they stick randomly to surfaces – like a deck of playing cards thrown onto a floor. Random arrangements of DNA origami are not very useful – if they carry electronic circuits for example, they are difficult to find and 'wire-up' into larger circuits. A collaboration between Caltech and IBM research Almaden has found a way to position and orient DNA origami on surfaces by creating sticky patches in the shape of origami – as a demonstration they positioned and aligned triangular DNA origami on triangular sticky patches. This success knocks down one of the major roadblocks for the use of DNA origami in computer nanotechnology

Reporting their work in the August 16, 2009 online edition of Nature Nanotechnology (Placement and orientation of individual DNA shapes on lithographically patterned surfaces), the scientists demonstrate a way to put DNA origami exactly where they want it on a surface, to line them up like little ducks in a row. They do this by using electron-beam lithography, to make sticky patches that have the same shape as the outline of the DNA origami.

DNA origami is a method for folding long strands of DNA into whatever very small shape or pattern you desire. Using a computer-aided design program a scientist can design the desired nanoscale shape (typically about 100 nanometers across) and the computer designs a set of short DNA strands that can be ordered from a company that specializes in synthesizing DNA strands. The short DNA strands get mixed with the long DNA strands, heated up to nearly boiling, and cooled to room temperature over the course of a couple hours. In a single drop of water one then has 100 billion copies of the desired shape, or shape with a pattern on top. In the first DNA origami made were shapes like triangles and smiley faces, and patterns like maps of the western hemisphere, snowflakes, etc.

"The interesting thing about DNA origami is that it allows you to make such nanoscale shapes and patterns by pure self-assembly, Paul W. K. Rothemund explains to Nanowerk. "It gives you the power to place little tiny things, whether they are nanometer-sized electronic components or chemical entities into arbitrary arrangements without touching them or using some other man-made machine."

Rothemund, a senior research associate at Caltech, has been conducting groundbreaking research on using DNA origami to develop bottom-up nanotechnology fabrication techniques. Read his 2006 Nature paper Folding DNA to create nanoscale shapes and patterns or a more recent paper on bottom-up construction approaches ("DNA origami seeds to direct bottom-up fabrication processes").

One problem today with DNA origami structures is that they are created in saltwater solution and they adhere randomly to surfaces, which means that "if you just pour DNA origami over a surface to which they stick, they attach everywhere," says Rothemund. "It's a little like taking a deck of playing cards and throwing it on the floor; they are scattered willy-nilly all over the place. Such random arrangements of DNA origami are not very useful. If they carry electronic circuits, for example, they are difficult to find and wire up into larger circuits."

Typically, though, nanoelectronics researchers would like their devices to be on a surface, like a silicon wafer, so that they can wire them up and integrate them with other technologies. "If the origami carry a device, you have to go find them, with some kind of ultramicroscopy like TEM or AFM, and then you have to use the very finest lithography we have to write special fine wires to them" says Rothemund. "It kind of defeats the purpose of using self-assembly in the first place."

In Rothemund's new work, conducted with a 10-strong research team at IBM's Almaden Research Center, led by Gregory M. Wallraff, the scientists used sticky patches of triangular DNA origami shape.

In a process developed by IBM scientists, electron-beam lithography and oxygen plasma etching, conventional semiconductor techniques, are used to make patterns on silicon wafers, creating lithographic templates of the proper size and shape to match those of individual triangular DNA origami structures created by Rothemund. The etched patches are negatively charged, as are DNA origami structures, and are therefore 'sticky'.

IBM scientists are using DNA scaffolding origami to build tiny circuit boards; in this image, low concentrations of triangular DNA origami are binding to wide lines on a lithographically patterned surface.

To connect the origami to the templates, magnesium ions are added to the saltwater solution containing the origami. The positively charged magnesium ions can stick to both the DNA origami and the negatively charged patches on the template. Thus, when the solution is poured over the template, a negative-positive-negative 'sandwich' is formed, with the magnesium atoms acting as a glue to hold the origami to the sticky patches.

"So not only can we put origami where we want them, but they can be oriented in the direction we want them" Rothemund points out.

The positioned DNA nanostructures can then serve as scaffolds or miniature circuit boards for the precise assembly of components such as carbon nanotubes, nanowires, and nanoparticles at dimensions significantly smaller than possible with conventional semiconductor fabrication techniques. This opens up the possibility of creating functional devices that can be integrated into larger structures as well as enabling studies of arrays of nanostructures with known coordinates.

"The spacing between the components can be 6 nanometers, so the resolution of the process is roughly 10 times higher than the process we currently use to make computer chips," says Rothemund. "Then, if you want to design a really small electronic device, say, you just design DNA strands to create the pattern you want, attach little chemical 'fastening posts' to those DNA strands, assemble the pattern, and then assemble the components onto the pattern," he explains.

Further development of the technique is at the top of the team's to-do list. First will be new shapes that go down with an absolute orientation. "Triangles are symmetric and while we can orient them and control the direction they point, we can't control which of the three tip of the triangle points which way, or whether they go upside down or not," says Rothemund. "Making asymmetric shapes that line up and all point 'north' will be important. So to will be making multiple shapes that can all be used at once without interfering with each other so we can construct more complex patterns on surfaces."

The researchers pint out that there are still several roadblocks before we can see the use of DNA origami in computers – even after the 'placement and orientation problem' has been solved. For example, what is the best material to act as an electronic component on DNA origami? Should one use silicon nanowires, carbon nanotubes, graphene, gold nanocrystals, what? How can they be stuck to the origami in the highest yield with the best results?

"It really is difficult to say how long the remaining roadblocks will take –at least 5 years, more likely at least 10" says Rothemund. "But the solution to the current problem came much faster than expected."