"You might want to use the AFM like a phonograph stylus to feel the bumps on a surface, but if you couldn't turn the ink off you'd be leaving a trace of ink as you moved the tip across the surface," said Paul Sheehan of the Naval Research Laboratory. "With the ability to turn the ink on and off, you can feel the surface without depositing material and then turn the heat on and put material down only where you want it."

To perform the tDPN technique, the team employed a silicon cantilever that contained a resistive heater and had a radius of curvature at its tip of about 100 nm. As the ink they used octadecylphosphonic acid (OPA), a material that has a melting point of 99 °C and self-assembles into monolayers on mica, stainless steel, aluminium and oxides such as titania and alumina. Sheehan and colleagues coated the cantilever with OPA before heating it to 122 °C to melt the ink. Scanning the tip across a mica substrate laid down 98 nm wide lines of OPA.

The scientists were able to stop depositing molecules from the cantilever by turning off the current supply to the resistive heater. That said, it took around two minutes for the deposition process to stop, perhaps because of the low thermal conductivity of the mica substrate.

The researchers believe that optimizing the technique, for example by decreasing the radius of curvature of the cantilever tip, should enable them to deposit features around 10 nm in size. So tDPN could find applications in producing features too small to be formed by photolithography, as a nanoscale soldering iron for repairing circuits on semiconductor chips, or for making bioanalytical arrays.

"This technique extends DPN into new sets of materials and provides a higher degree of control," said Lloyd Whitman of the Naval Research Laboratory. "We also believe it will extend DPN into new environments, such as the vacuum environments that would be more compatible with conventional semiconductor device fabrication."

The researchers reported their work in Applied Physics Letters.