12 June 2001.
May 2001 �SIGNAL Magazine 2001
Fluorescent Particles Illuminate New Dimensions in Optical Storage
Multicolored emissions could allow storage of more than one bit of information per data point.
By Sharon Berry
What began accidentally could be the foundation for a revolutionary approach to optical data storage. By enhancing and controlling fluorescence exhibited by nanoparticles, scientists can rapidly switch the particle colors on and off, creating robust nanoscopic storage elements that can pack a large amount of data in a small amount of space.
Researchers at the Georgia Institute of Technology, Atlanta, report that they have already successfully demonstrated binary optical storage with the patent-pending system, writing and reading simple images recorded on thin films made of silver oxide nanoparticles. Because of early successes, scientists predict that the approach could lead to multiple levels of storage techniques and even three-dimensional storage--a brand new capability in the data storage trade.
Currently, storage is a two-dimensional medium. Once institute scientists characterize and control the technology's various properties, novel storage capabilities can be gleaned. For example, expanding storage to three dimensions would provide a very dense storage medium that could be written to and read in parallel. A compact disc today contains 650 megabytes of information. That same disk could hold 650 megabytes squared using the fluorescing nanocluster capabilities.
"The technique was discovered accidentally," Lynn A. Peyser, a Georgia Institute of Technology researcher, says. "We were looking for something completely different. We were trying to observe fluorescence from single organic dye molecules on a metal film to see if there were any differences compared to plain cover glass." Instead, researchers found multicolored fluorescence emission from the clusters.
Researchers started by making extremely thin films of silver oxide nanoparticles. They then selectively exposed blue spectrum light--450 to 480 nanometers--to portions of the film, which chemically reduces particles on the surface of the film and partially converts them to clusters of silver atoms. This process photoactivates the clusters, experts say. The photoactivated clusters are then continually exposed to and therefore excited by blue light, causing them to fluoresce strongly.
Robert M. Dickson, assistant professor of chemistry and biochemistry at the institute, is heading the project. "We started looking at silver oxide film because there is all of this wonderful, well-developed photochemistry used in photography," he says. "When you shine lights on the silver halides, you make little silver clusters. Basically, you amplify that signal in the development process to create a negative for photography."
Recently, scientists discovered a new property of the silver nanoclusters. They demonstrated that when light with a wavelength of less than 520 nanometers is trained on the particles, their ability to fluoresce is turned on. The fluorescence can then be read nondestructively by using longer wavelengths of light, which are lower energy. Reading and writing is key to the storage applications of the project.
"It's known that silver clusters at low temperatures fluoresce," Dickson relates. "I wasn't really expecting to see much fluorescence from these at room temperature because fluorescence has only been observed at low temperatures. The only possible interpretation is that the silver clusters are formed because they enter into a similar chemistry as silver halides. The bottom line is that I wasn't expecting to see the fluorescence very easily or for it to be write-capable at room temperature. Photographs are written at room temperature. The regions you shine light on are then fluorescent and, depending on the color, you can imagine using this for high-density storage as well."
When studied under a microscope, the individual silver particles also display a property that may enable an increase in optical storage density. Once photoactivated, silver oxide films exhibit bright multicolored fluorescence under both blue and green excitation. The nanoparticles emit red, yellow and green light. "It is really unusual to see multicolored fluorescence from a single sample when you're exciting the sample with one particular wavelength," Peyser explains. "When I looked through the microscope, it literally looked like a Christmas tree with different colored bulbs blinking everywhere, and it was beautiful."
Dickson notes that color-changing emission relates to alteration of other aspects of the clusters. "Not only are you generating fluorescence, but you are also changing the size and/or geometry of the cluster," he says. By using the correct distribution of particle sizes, this multicolored emission could allow storage of more than one bit of information per data point.
The spectral dynamics of the samples are unique, Peyser shares. "It is somewhat difficult to be able to look at a single particle," she says. "What is even more unique is that we can see emission from the same individual nanoparticle that emits light in yellow, green and red. To a scientist, that's a pretty large spectral range to be able to see from the same tiny nanoparticle. You usually put light of a certain wavelength into a sample, and you get light of a certain wavelength out."
The wide spectral range opens many possibilities for multiple levels of storage techniques, Peyser points out. Currently, a standard storage device, such as a compact disc, is binary storage. "We're looking at what we call N-level storage," Peyser says. "Standard binary storage is N = 2. It's either on or it's off." Because the nanomaterials that are being studied exhibit different colors, researchers may achieve N > 2 or even N = 4. An N-level storage technique is an ability that has not been seen or controlled. "If we could control it, that would open a large range of data storage techniques," Peyser observes. "We've also found that certain wavelengths sent into a sample produce desired results better than others."
At this time, storage is a two-dimensional media. Once scientists gain control over different properties, storage can be expanded. "I can tell you that if we do gain control over the technology, it has great capabilities as well as possibilities for a new storage technique. This could mean expanding storage to three dimensions," she says. This would provide a very dense storage medium that could be written to and read in parallel.
The writing process, also discovered inadvertently, occurred when a mercury lamp projecting blue light was left shining on a sample. After one hour, a pattern was written in bright yellow hexagonal shapes--the outline of a closed aperture--and visible in the field of view. This showed that nanoclusters can be written to as well as read.
Researchers further determined that when a sample is written to with a lower wavelength such as ultraviolet light, it can be read nondestructively with a higher wavelength of light such as blue or green light.
The writing process--which happens when the nanoparticles grow--is easily visible through the 100x objective of the microscope. "I can see them grow, one by one, some faster than others," Peyser notes. "We don't really have complete control over that yet. But we have found that lower wavelengths write much better. If I want to write an image and then look at it, how can I be sure that I am not still writing to it? What I need to do is find another wavelength that happens to be longer that will allow me to look at it without continuing to write to it as efficiently," she says. "All three wavelengths--ultraviolet, blue and green--will write, just some better than others."
Images stored on the film can be read nondestructively by green light for about two days. Scientists are studying how long the effect will continue. Peyser notes that she is characterizing the fluorescence to gain control over the process that makes the films write-capable and distinguish it from the kind of fluorescence that is emitted after the films are written to. "We need to see if those are two separate processes," she explains. "Right now I am leaning toward yes they are. There are two different things going on."
Dickson shares that researchers are still at the characterization and control phase of the project. "We're at a stage of characterizing things photochemically," he says. "We are shining different colors on the samples, looking at photoactivation and the fluorescent spectrum, observing how things change in time and beginning to write patterns. We've done all these things very recently. Once we gain a fundamental understanding of what's really going on, we want to look at changes on the single cluster level, including geometry and size changes. We need to be able to control what we've written--analyze and control it." Other areas of exploration will include determining if information can be optically erased and film rewritten and how to package the technology in later research stages.
"If we are able to expand this, we can produce substantially greater storage than what is out there in two dimensions," Peyser adds. "Three dimensions is the Mecca of where we could get with this project--a lot more information in a lot less space."