"Switchable" nanomaterials – that is, those that can change their properties and/or function in response to external stimuli – could be used as electronic components, sensors and catalysts, to name but a few applications. To date, most researchers have focused on molecular switches in which changes in structure can modulate the optical, redox, magnetic or electronic properties of the material. Now, Bartosz Grzybowski and colleagues have discovered a new class of nanomaterial whose electrical conductance can be varied by changing the flow of "counterions" around a core of charged metal nanoparticles.

This "steering" technology, as it has been dubbed, allows current flow to be directed through a piece of continuous bulk material, explains Grzybowski. "Like redirecting a river, 'steams' of electrons can be steered in multiple directions through a block of the material."

"Rewiring"
The reversible nature of the new material could allow a computer chip, for example, to redirect and adapt its own circuitry to specific needs at a given moment in time, adds team member David Walker. In short, a device made of the material could reconfigure (or "rewire") itself and become something else altogether – for example, it could rearrange itself into a resistor, rectifier, diode or transistor in response to external electrical pulses.

The new material is a hybrid made up of electrically conducting gold particles (or "nanoions") that are around 6 nm wide, coated (or functionalized) with the positively charged chemical N,N,N-trimethyl(11-mercaptoundecyl) ammonium chloride. The gold particles are in turn surrounded by negatively charged atoms (the "counterions"). When an electrical charge is applied to the material, the counterions move but the relatively larger positively charged nanoions stay fixed in position. Moving the counterions around the central gold particle core allows regions of low and high conductance to be modulated throughout the material, forging out current "paths" that can be altered at will.

One component, many devices
The hybrid system can be made into electronic devices, similar to those familiar to us from silicon technology, say the researchers. Indeed, the team has already fabricated structures that act as conductive wires, resistors with varying degrees of resistivity, devices that block and rectify current (rectifiers), and diodes. "And, since you only need to wire two diodes back to back make a transistor, we have also made a transistor and logic gates that we will talk about in another paper," revealed Walker.

The nanoparticles can be cast from solution, unlike "hard" silicon, which cannot, he adds and form flexible films too. "This is similar to what people have been doing with plastic electronics but the nanoparticle system we have made offers much higher electron transport rates," said Walker. "As a case in point, our nanoparticle diode (the first ever of its kind) is already faster than commercially available polymeric devices and we have only invested around 1000 USD, compared with the billions that Phillips and other electronics giants have poured into plastics technology," he told nanotechweb.org.

Higher density
The team would ultimately like to make switches that are just one nanionic particle across – around 5 nm or so. "By then moving counterions around the nanoparticles, we could direct currents over length scales that are not achievable with conventional electronics," explained Walker. "This would naturally translate into much higher density of elements that one can place on a single computer chip – something that is important as electronic devices become ever smaller."

Another direction the researchers are taking is to build all-electronic components from nanoparticles. They say that they have actually completed their first fully fledged logical gate circuit recently – another result that will be published soon. "The key point here is that instead of using hard silicon materials, we can have n- and p-like materials made of positively and negatively charged nanoparticles," said Walker.

The current work is detailed in Nature Nanotechnology.