Building a Nanostructure Array One Slice at a Time
Researchers at Harvard University in Cambridge, Mass., have employed a nanoscale version of skiving — cutting a material into thin layers — to create large-area arrays of patterned metallic structures. By producing frequency-selective surfaces, the nanoskiving technique might yield negative-index-of-refraction and three-dimensional metamaterials, with applications in optical imaging, polarization, filtration and other areas.
A bright-field optical image shows an epoxy slab containing U-shaped nanostructures sitting on top of a hole in a copper sheet (A). The inset depicts a scanning electron microscope image of a single structure with a wall thickness of 50 nm and a height of 100 nm. The transmission spectrum of the array, obtained using unpolarized incident light, shows three distinct resonant peaks in the mid-IR region (B). Images courtesy of Qiaobing Xu, Harvard University.
The scientists manufactured the nanostructures by depositing a 15-nm-thick layer of gold on a silicon wafer covered by a thin layer of native silicon dioxide. They used a mold of the transparent rubber polydimethylsiloxane to create an array of 2 × 2-μm posts in an epoxy layer on the gold-coated silicon.
They then deposited a 40-nm-thick layer of gold onto the epoxy posts. By varying the angle of deposition, they could cover the posts on two, three or all sides. They embedded the gold-coated posts in epoxy and peeled the structure free — an easy task because of the poor adhesion of the bottom layer of gold to the silicon.
Next, they performed rough and fine sectioning of the embedded gold nanostructures. The fine sectioning, they reported, required painstaking alignment of an ultramicrotome from Leica Microsystems of Wetzler, Germany. Thus, they produced epoxy slabs of thin and uniform thickness covering an area of about 3 mm2. As they cut the slabs, they collected them in a water-filled sample trough mounted on the back side of the ultramicrotome’s diamond knife. Because of capillary forces, the floating slabs self-assembled into larger arrays measuring more than 9 mm2.
A dark-field optical microscopy image shows an array of L-shaped nanostructures patterned over a 3-mm2 area (A). The nanostructures, shown in a high-magnification image in the inset, have a wall thickness of 50 nm and are 100 nm high. The IR transmission spectrum of an array of L-shaped structures supported on a calcium fluoride substrate shows two dominant transmission band stops centered at 8.4 and 4.8 μm (B).
The researchers transferred the arrays onto a solid substrate, choosing various substrates as needed. They removed the epoxy in an oxygen plasma, leaving behind an array of gold nanostructures composed of a series of closed loops, of U-shaped or L-shaped components with metal wall thicknesses of about 50 nm and metal heights of 100 nm.
They characterized the samples using a Fourier transform infrared spectrometer from Thermo Fisher Scientific of Waltham, Mass., in transmission mode. They found that the sample transmittance had a notch in it, with the closed loop dipping from greater than 90 percent transmission at a 2-μm wavelength to around 45 percent at about 11 μm. The polarization of the light had no effect on the location of the notch, which was not the case for either the U- or L-shaped structures. These results were in agreement with calculations.
The fabrication technique could be used to make nanostructure devices that would act like true plasmonic oscillators in the infrared and possibly in the visible range. That capability would make producing negative refraction index materials possible.
Structures that might be built could include functional nanostructures on highly curved optical surfaces such as microlenses, small optical arrays for intracellular plasmonic probes for cell biology, and optical systems with new properties. Citing these examples, chemistry professor George M. Whitesides said that future research will involve developing nanoskiving for types of fabrication that are difficult or impossible to accomplish using current technology.
Nano Letters, September 2007, pp. 2800-2805.
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