Each discovery of a new nanostructure that promises to revolutionize high-density electronics sparks the question: how will it be possible to quickly and reproducibly assemble large numbers of these structures into a device that is compatible with existing silicon technology? A team of scientists from Korea and the USA1 have now successfully used a scanning tip technique to rapidly grow metal nanoclusters and nanowires that are fully integrated with a silicon surface.

“Such zero- and one-dimensional metal nanocrystals with tailored size, shape and position on semiconductor substrates are well suited to optical, sensing and electronic devices,” says lead researcher Jeong Yong Lee from KAIST in Korea.

Fig. 1: Transmission electron microscopy image showing how a voltage to a tungsten tip (blue, schematic) can draw out zinc from a zinc oxide surface (gray) on a silicon substrate (black) to form metallic zinc nanodots.Adapted from Ref. 1. Reproduced with permission. © 2010 ACS

To make their nanostructures, Lee and his group started with a silicon wafer coated with an ultrathin film of zinc oxide. They then brought an atomically sharp tungsten tip within a few nanometers of the zinc oxide surface and steadily increased the voltage at the tip. In response to the large electric field near the tungsten tip, zinc atoms leapt out from the oxide surface and assembled into tiny ‘pillows’ of metallic zinc (Fig. 1).

Using this technique, a metallic nanostructure a few tens of nanometers in size could be obtained in just seconds. The researchers also used a similar approach to draw out zinc nanopillars that stood vertically on the surface.

The researchers observed the formation of the zinc nanodots in real time using a transmission electron microscope. This is the first time such a rapid growth process has been captured in real time, and the direct observations allowed the team to fabricate dots of almost any shape on the fly. The images also made it possible to calibrate the time needed to grow a structure of a particular size. Based on their findings, it appears that the heat generated by current flowing from the zinc to the tungsten tip is a key factor that promotes the rapid growth of the nanostructures.

Lee expects that the method will lead to the mass production of nanodevices by a bottom-up approach. His team is now working to extend the technique to more materials, such as various semiconductors.