New Microprinting Technique Improves Nanoscale Fabrication

The new microdisplacement technique is based on a widelyused patterning method known as microcontact printing--a simple way offabricating chemical patterns that does not require clean rooms andother kinds of special and expensive environments. Both methods involve"inking" a patterned rubber-like stamp with a solution of molecules,then applying the inked stamp to a surface.

"Microdisplacementgives us more control over the precision with which the patterns areplaced and retained, and also allows us to use a wider range ofmolecules," Weiss says.

One of the limitations of microcontactprinting is that its precision is limited at the edges of a stampedpattern by the tendency of the applied molecules to skitter across thestamped surface, blurring or obliterating the applied pattern anddestroying its usefulness. Weiss's improved microdisplacement techniquesolves this problem by applying a self-assembled-monolayer film--asingle ordered layer of spherical adamantanethiolate molecules--to keepthe stamped molecules in place on the surface. "We specificallyengineered the adamantanethiol molecule to have a very weak chemicalbond with the surface so that it would detach easily when bumped by astronger-bonding molecule," Weiss explains. The molecules inked on thestamp replace the adamantanethiolate molecules wherever they touch themonolayer film, but the surrounding molecules in the film remainattached to the surface to prevent the applied molecules from wandering.

"Microdisplacementprinting uses many of the same procedures as microcontact printingexcept one first prepares the substrate by coating it with aself-assembled monolayer of adamantanethiolate, which is inexpensiveand easy to apply," Weiss explains. "You dip the substrate in asolution of these molecules, pull it out, and they assemble themselvesinto an ordered film one molecule thick."

In addition toproviding more control over the precision of stamped patterns, the newmicrodisplacement technique also relaxes the requirements in preciselypositioning a series of stamps used to apply consecutive patterns withdifferent molecular inks. "You don't have to be extremely precise aboutthe exact placement of the stamps as long as you apply the molecularinks in order of their bonding strengths," Weiss explains. Eachsuccessive layer of molecules either will displace or will not displacethe already-applied molecules, depending on their relative bondingstrengths with the underlying surface.

The research was aided bythe Weiss lab's unusual collection of microscopes, which enable thescientists to get a clear picture of the results of their experiments,both at the broad scale of a stamped pattern and at the narrow scale ofjust a single molecule. One scanning tunneling microscope that Weissand his group designed and built themselves, for example, has 1,000times more resolution than is needed to image an individual atom.

Adamantanethiolis related to the family of alkanethiol molecules, which have beenstudied extensively as a model systems for their ability to formwell-ordered monolayer films on gold. Weiss and his team were studyingthe adamantanethiolate-on-gold system when graduate student ArrelaineDameron discovered that stronger-bonding molecules easily displaced theadamantanethiolate molecules. Her discovery has led to further studiesof this system by the Weiss team, including how the displacement can beapplied in a broad range of applications using a variety of materials.

"Wehave mapped out strategies in this model system and are nowinvestigating how we can apply these strategies more broadly as thechemistry is developed for self-assembled monolayers on othersubstrates, especially semiconductors," Weiss says. "Our goals are tosee how far we can take these kinds of simple techniques, along withour knowledge of intermolecular interactions, to bridge the1-to-100-nanometer length scale in nanofabrication, which even at thehigh end currently requires very difficult, slow, and expensivetechniques."

In addition to Weiss and Dameron, the Penn Stateresearch team includes postdoctoral fellows Jennifer Hampton and SusanGillmor and graduate students Rachel Smith and T. J. Mullen. Theresearch was supported by the Air Force Office of Scientific Research,the Army Research Office, the Defense Advanced Research ProjectsAgency, the National Science Foundation, the Office of Naval Research,and the Semiconductor Research Corporation. The work was performed as apart of both the Center for Nanoscale Science and the NationalNanofabrication Infrastructure Network.