"Although conventional DPN has been used previously to deposit polymer nanostructures, these structures took much longer to write and were disordered compared with our tDPN structures," Lloyd Whitman of the Naval Research Laboratory told nanotechweb.org. "As the inventors of tDPN, and to our knowledge still the sole practitioners, we are working to demonstrate that tDPN can be used to directly 'write' a wide range of electronic and optical materials."

Thermal dip-pen nanolithography employs an atomic force microscope (AFM) cantilever with a built-in tip heater, along with an ink that is solid at room temperature. Heating the tip melts the ink and allows its deposition onto a surface with great accuracy. For the case of polymers, the method can control both the physical dimensions and orientation of the material.

Whitman and colleagues Minchul Yang, Paul Sheehan and Bill King deposited layers of the conducting polymer poly(3-dodecylthiophene) (PDDT) onto silicon oxide surfaces. They produced nanostructures with lateral dimensions of less than 80 nm and achieved monolayer-by-monolayer thickness control – a monolayer of the molecules was around 2.6 nm thick. The researchers were also able to control the orientation of the polymer chains.

PDDT has promise in the field of organic electronics and could have applications in areas such as transistors, photovoltaic devices and video displays. "The performance of these devices depends critically on the degree of molecular ordering and orientation within the polymer film, a property that has been difficult to control," said Whitman. "We have succeeded in directly writing polymer nanostructures with monolayer-by-monolayer thickness control using tDPN. The deposition process employs highly local heating to produce this polymer ordering and orientation."

The researchers would like to make DPN a viable technology for depositing metals and electronic and optoelectronic materials without the need for high vapour pressure 'inks', volatile solvents or humidity (for a water meniscus).

"By using an AFM tip with an integrated heater, custom fabricated by Bill King at the Georgia Institute of Technology, we can deposit solid 'inks' with precise control over the deposition in a wide range of environments, including ultra-high vacuum," said Whitman. "We have also recently demonstrated metal (indium) deposition."

Now the scientists are completing a study of polymer deposition on clean Si(001) in ultra-high vacuum. "Our goal is to use tDPN to directly write all the materials needed to prototype and study functional nanoelectronic devices," said Whitman.

The researchers reported their work in the Journal of the American Chemical Society.