Engineers Create Vibrant Colors İn Vertical Silicon Nanowires

Engineers may soon be singing, "I'm going to wash that gray right out of my nanowires," thanks to a colorful discovery by a team of researchers from Harvard University and Zena Technologies. In contrast to the somber gray hue of silicon wafers, Kenneth B. Crozier and colleagues demonstrated that individual, vertical silicon nanowires can shine in all colors of the spectrum. The vibrant display, dependent on the diameter of the individual wires, is even visible to the naked eye. In addition to adding a splash of color to the lab, the finding has potential for use in nanoscale image sensor devices, offering increased efficiency and the ability to detect color without the use of filters.

"It is surprising," says Crozier, John L. Loeb Associate Professor of the Natural Sciences at the Harvard School of Engineering and Applied Science (SEAS). "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, 2011, online edition of Nano Letters, may be the first experimental report that silicon nanowires can take on a variety of colors depending on their diameter and under bright-field illumination. Previous work has shown that nanowires can take on different colors but only by looking at scattered, rather than directly reflected, light.

To create the multicolored array of vertical silicon nanowires, the engineers at Harvard and Zena Technologies used a combination of electron beam lithography and inductively coupled plasma reactive ion etching.

A smooth wafer of silicon was plasma etched 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 were due to the properties of the individual wires, not by the way light was scattered or diffracted in the group.

"Each nanowire acts as a waveguide, like a nano-sized optical fiber—but an optically absorbing one," explains Crozier. "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."