Various methods have been used to create the electric field necessary for DEP nanofabrication, but all are limited in some way. As Nan Xiang, a researcher at Southeast University, Nanjing, explains: "the biggest drawback of classical microelectrode DEP is its low flexibility, while the alternatives are complicated and costly."

To improve on the classical method, Xiang and colleagues tried a dot-matrix electrode pattern, but found that the leads between the electrodes interfered with the manipulation of the nanowires. While searching for the solution, the team chanced upon the idea of the multilayer printed circuit board (PCB).

Xiang continues: "We figured we could imitate the structure of the PCB to design the chip, with only 12 individual dot electrodes on the upper surface, and the lead buried in the bottom layer of the chip." The result is a "leadless" dielectrophoresis chip with dot-matrix electrodes, which the researchers have called an LDME-DEP chip.

The bottom layer of the chip, on which the leads are etched, is a piece of indium tin-oxide glass. Above this, another layer of glass forms the penetrating electrode layer, in which a 3×4 pattern of holes is drilled for the 12 silver electrodes. On top of the chip, with the electrode dot matrix in its centre, sits a cross-shaped flow channel layer. As numerical simulations confirmed, the 150 µm thickness of the middle penetrating electrode layer shields the top surface of the chip from interference caused by the leads below, resulting in a much-improved electric field distribution.

A pump in each flow channel allowed the researchers to pass a suspension of silver nanowires over the electrodes in any of four directions, while activating the electrodes caused lengths of nanowire to be caught between them. Hydrodynamic forces in the fluid meant that nanowires parallel to the direction of flow were more likely to be adsorbed at a given voltage, and nanowires perpendicular to the flow were more likely to be washed away.

Imperfectly aligned

In principle, the nanowires would be expected to settle on the electrode centrelines, where the electric field is strongest, but the researchers found that lengths of nanowire would frequently be caught at an angle, and often together with a number of other nanowires. Xiang explained why this is a problem: "Single-nanowire-based devices such as nanowire sensors and nanowire transistors require a single nanowire positioned precisely between the electrodes to obtain stable performance and accurate parameters. Therefore, it is preferred to avoid having multiple nanowires between the electrodes, or nanowires that do not pass through the electrodes' centrelines."

Since nanowires settling at undesirable angles and in large numbers could not be avoided, the team needed a way to remove all of the lengths that were caught at less preferable orientations. They achieved this by altering the pattern of electrode activation together with the flow direction and velocity. Lowering the voltage across the affected electrodes, and pumping the fluid in a direction perpendicular to the unsatisfactory nanowires, enabled those lengths of nanowire to be flushed away. Repeating this process allowed the researchers to produce a custom pattern of single nanowires aligned with the electrodes' centrelines (see video above).

This version of the LDME-DEP chip was created with a 3×4 matrix of electrodes, but any number and arrangement could be used, making the new approach a very versatile one. "Therefore," claims Xiang, "our LDME-DEP chip is potentially useful for fabricating bigger and complicated nanostructures in an easier way. And the manufacturing process of nanowire transistors, nanowire batteries and molecular sensors will become more simple and inexpensive."

The research is reported in full in Nanotechnology.