Nanomaterials can be produced using a wide variety of methods but not all synthesis routes are compatible with all target applications. For example, high temperatures are often required for the synthesis of nanowires and rods, but elevated temperatures are incompatible with growth on polymer substrates.

Moon-Ho Jo and colleagues from Pohang University of Science and Technology in Korea1 have developed a method that circumvents this problem by enabling the synthesis of nanowires at only 200°C. Moreover, their method shows other advantages for nanometer scale applications, including the finding that the growth is one-to-one deterministic between catalysts and nanowires in size and position in a controllable manner."

The well known vapour-liquid-solid (VLS) approach for making nanomaterials uses a combination of catalysts and target materials at temperatures high enough to melt these components. The method described here requires much lower temperatures than VLS, and is governed by a ‘vapour-solid-solid’ growth mechanism.

Fig. 1: Images at different scales showing the germanium nanowires (NWs) grown on polymer films.

The researchers deposited a copper catalyst on substrates before initiating synthesis. Then, chemical vapour deposition of germanium hydride produced arrays of germanium nanowires on the substrate (Fig.1). This method was used for growth on substrates ranging from silicon to polymers.

The nanowires were single crystal from tip to stem, although the stem was pure germanium and the tip was composed of a copper/germanium compound. The authors found that the diameters of the nanowires were extremely uniform because they were ‘templated’ from the solid copper catalysts—the diameters being narrowly distributed around 7nm.

The semiconducting properties of the nanowires were confirmed the fabrication of field-effect transistors. The researchers expect their approach to be applicable for the synthesis of nanowires using other materials.

“We showed that germanium nanowires can be grown using ‘solid-phase’ copper catalysts instead of liquid ones,” says Jo. “The lower limit of the growth temperature is then set by diffusion kinetics, providing a wider growth parameter space, and a lower temperature growth pathway.”

Jo adds that, “the size and position of nanowires within arrays are predictable and controllable.” These two advantages combine to give a method ideal for producing large-area integrated nanowire systems, even on polymer substrates.