Vibrant Colors Created in Si Nanowires
CAMBRIDGE, Mass., April 19, 2011 — Individual vertical silicon nanowires that shine in all colors of the spectrum have been developed that could increase the efficiency of nanoscale image sensors and give them the ability to detect color without the use of filters.
“It is surprising,” said Kenneth B. Crozier, who led the group that developed the nanometer-scale structures. “A lot of people are making nanowires, and you really don’t think of the color so much. In this vertical configuration, you can get very strong color effects, and you can tune them over a range of wavelengths of the visible region. The strong effects can be seen right down to the level of the individual wire.”
The finding, published in the March 17 online edition of Nano Letters by Crozier and his colleagues from Harvard University and Zena Technologies, may be the first experimental report that silicon nanowires can take on a variety of colors, depending on their diameter and whether they are under bright-field illumination. Previous work has shown that nanowires can take on different colors; however, only scattered, rather than directly reflected, light has been studied.
To demonstrate the ease of controlling and positioning colorful nanowires, researchers created a nanoscale-size tribute to Harvard, designing a pattern resembling the engineering school’s Veritas seal and spelling out the acronym SEAS. (Image: Ken Crozier and Kwanyong Seo, Harvard School of Engineering and Applied Sciences)
To create the multicolored array of vertical silicon nanowires, the investigators used a combination of electron beam lithography and inductively coupled plasma reactive ion etching. They etched a smooth wafer of silicon until all that remained were the vertically protruding nanowires, resembling bristles on a toothbrush. While the nanowires were created in arrays of thousands for convenience, the colors they exhibited resulted from the properties of the individual wires, not from the way light was scattered or diffracted in the group.
“Each nanowire acts as a waveguide,” Crozier said. “At short wavelengths, there is not much optical coupling to the nanowire. At long wavelengths, the coupling is better, but the properties of the waveguide are such that there is not much absorption. In between, there is a range of wavelengths where the light is coupled to the nanowire and absorbed. This range is determined by the nanowire diameter. We made nanowires with diameters of 90, 100 and 130 nm that appeared red, blue and green, respectively.”
To demonstrate the remarkable phenomenon and the relative ease of controlling and positioning the colorful nanowires, the researchers created a nanoscale-size tribute to Harvard, designing a pattern resembling the engineering school’s Veritas seal and spelling out the acronym SEAS (for School of Engineering and Applied Sciences) in a rainbow of colors.
While the Harvard image closely matched the school’s seal, the desired color eluded the engineers. Small changes in the radius of a wire can alter the resulting color.
“We actually wanted to make the seal red rather than blue, but it turned out that the diameter was a little bit wrong,” Crozier said.
The researchers’ eventual aim is to use the wires in image sensors. Traditional photodetectors can gauge the intensity of light but not determine its color without the use of an additional filter, which throws away much of the light, limiting the device’s sensitivity. The group hopes to address this by fabricating vertical nanowires containing photodetectors above standard photodetectors formed on a silicon wafer. The nanowire and standard photodetectors could each detect a different part of the spectrum of the incident light. By comparing the signals from each, the color could be determined without losing so much of the light.
“With image sensors, every little bit of efficiency counts. Moreover, we even imagine using the colored wires to encode data in a read-only type of information storage,” Crozier said.
For more information, visit: www.seas.harvard.edu
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