In dip-pen nanolithography (DPN), "inks" of organic molecules are applied to an atomic force microscope (AFM), which serves as the writing tool. The molecular ink then is deposited onto an underlying substrate, or "paper," via a tiny capillary in the water droplet that forms naturally at the tip. DPN is a nano-version of the 4,000-year-old quill pen.
When taking the DPN plotter to a parallel process, Mirkin's team made an important scientific discovery. When the writing tips were applied to the substrate using different contact forces, the pens still produced identical dots and lines, with respect to diameter and line width. In other words, with increased pressure, only the water at the AFM tip spreads out, but the width of the nanocapillary, through which the ink flows, remains constant.
This discovery means that only one pen of the multi-pen device needs to be "smart" or have its tip equipped with a feedback system. This pen is called the imaging tip and is used for both imaging and writing. As it patterns an area, sensors in the imaging tip communicate with the customized computer software that drives the nanoplotter. In the case of the eight-pen nanoplotter, the other seven writing tips are passive and follow the lead of the pen with the imaging tip, drawing identical patterns a fixed distance apart.
Mirkin and Hong demonstrated the nanoplotter's parallel writing capability by first drawing two squares using the same ink, then two squares made of two different inks, and finally drawing eight identical patterns -- a set of a dot, a line, an octagon and a square -- made using the same ink. In each demonstration, the patterns were perfectly aligned with respect to each other.
The nanoplotter also can be used in a serial fashion to create nanostructures made up of different inks, one ink being added after another to build the final structure.