Another potential advantage of the printing method is that the nanowires could be printed not only onto silicon, but also onto paper or plastics, says Javey. He foresees such applications as “sensor tapes”–long roles of printed sensors used to test air quality or detect minute concentrations of chemicals. “Our next challenge is to develop a wireless component” that would relay the signals from the circuit to a central processing unit, he says.
But for now, the researchers have demonstrated the technique as a way to create an image sensor. They patterned the nanowires onto the substrate to make a 13-by-20 array of circuits, in which each circuit acts as a single pixel. The cadmium selenide nanowires convert incoming photons into electrons, and two different layers of germanium-silicon nanowire transistors amplify the resulting electrical signal by up to five orders of magnitude. “This demonstrates an outstanding application of nanowires in integrated electronics,” says Zhong Lin Wang, director of the Center for Nanostructure Characterization at Georgia Tech.
After putting the device under a halogen light and measuring the output current from each circuit, the group found that about 80 percent of the circuits successfully registered the intensity of the light shining on them. Javey attributes the failure of the other 20 percent to such fabrication defects as shorted electrodes and misprints that resulted in poor nanowire alignment. He notes that all of these issues can be resolved with refined manufacturing methods.
The researchers also plan to work toward shrinking the circuit to improve resolution and sensitivity. Eventually, says Javey, they want everything on the circuit to be printable, including the electrodes and contacts, which could help further reduce costs.