Peter Bøggild is making a toolbox—one with very small tools. As leader of the Nanohand project at Mikroelektronik Centret (MIC) in the Technical University of Denmark (Lyngby), he has directed the construction of a pair of tweezers that can pick up and move nanoparticles and a soldering device that can fasten them to just about anything. ”[It’s] similar to the tools used in making electronics in a workshop,” he says. The Nanohand could be used to build experimental nanometer-scale research devices, such as transistors made with semiconducting nanowires. But Bøggild’s goal is more fundamental. ”We’re taking the concept of the hand as a basic human tool” and shrinking it down, he says. ”We want to know how far it can go.”
Handy in a pinch: the Nanohand is a silicon set of tweezers for picking up nanometer-scale objects. Electrostatic forces caused by voltages at five electrodes open and close the tweezers. Gold extensions [inset] decrease the gap between the tips to just 100 nm, and gaps as small as 25 nm have been produced.
The Nanohand starts as a pair of silicon microcantilevers that hang over the edge of a microchip, built using standard silicon processing [see above micrograph]. Adjusting the voltages on a set of five electrodes electrostatically opens them as wide as 700 nm or closes them. But the best one could do working only with silicon was not quite fine-fingered enough. To make the tweezers hold something of nanometer size, the MIC group had to find a way to add nanoscale prongs to the tweezer tips. The Nanohand was put inside an electron microscope, whose chamber contains a gold- and carbon-based gas. Focusing the microscope’s electron beam on a tweezer tip and then slowly sweeping the focal point in toward the other tip catalyzes a reaction in the gas and literally draws a solid wire of carbon-encrusted gold along the focal point’s path. Doing the same with the other tip produces tweezers that, at rest, have a gap as small as 25 nm.
Bøggild likens the elongated tips on the Nanohand to fingernails. The device can easily pick up and manipulate silicon wires 100 nm thick, and Bøggild says the group will go for 10 nm this summer.
The same electron-beam technique that draws the fingernails can also solder things—a handy trick when building nanodevices. The electron beam is focused on the spot to be soldered, and the gold-carbon gas mix reacts to form a solder joint, this time of pure gold.
Bøggild and his colleagues have soldered carbon nanotubes across the tweezers’ tips and stretched them to study their electromechanical properties. The hope is that nanotubes will make good force sensors, since their electrical properties change when they are stretched. In the end, Bøggild would like to use one Nanohand to solder such sensors between the moving parts of another, and then use the sensor output to provide a sense of touch to the hand’s user.