Tagged Metrology Assists CNT Self-Assembly

It was obvious to Professor John Hartley of the University at Albany’s College of Nanoscale Science and Engineering (CSNE) that the information content needed to do any sort of a bottom-up self-assembly strategy for a complicated real-world device was absent.

Hartley realized that to do self-assembly, the process must include the information necessary to build complex structures. He and his groups came upon the idea of doing what they call lock-and-key lithography. The basic concept is that to have molecular-scale elements self-assemble, they require some sort of chemical lock and key. “Initially, this was looked at in an abstract sense, referring to coded information, looking at it as binary bits,” Hartley said. “With complementary sets of bits, some would be locks, others keys, and thermodynamics would drive them together to self-assemble into a complex structure.”

Hartley worked with Professor Mihri Ozkan, University of California at Riverside , who was binding DNA and peptide nucleic acid (PNA) onto the ends of carbon nanotubes (CNTs). This provided the requisite chemistry for binding locks and keys onto electronically interesting molecular structures, and a method was developed of designing locks and keys for assembling specific types of structures. The group designed a set of genetic locks and keys — DNA sequences — with a given lock binding only to its designated key, without interfering with one another. This allows multiple locks and keys to exist simultaneously in a solution, with lock 1 going only go to key 1 and lock 2 to key 2 without cross talk.

Initially, the group used a number of short genetic sequences and, instead of binding them to CNTs, they tagged them with fluorophores — later, quantum dots will be used. A matrix experiment was then done, where they deposited columns of four different versions of the genetic locks and passed four sets of genetic keys in solution over them. The result was a high degree of selectivity, and the locks and keys mated as intended. The intensity of fluorophore signals, imaged optically, was used to track the degree of success and give a measure of the relative selectivity.

This unusual metrology provided a means of measuring what was taking place at the molecular scale (Figure ). Because of the nature of the molecules and the aqueous environment that was being worked, it was not possible to do this using atomic force microscopy (AFM), SEM or TEM technologies. The fluorophores provided a method to perform highly parallel measurements of what was happening in the experiments by simply using an optical microscope and video camera. The experimenters could see what was taking place quantitatively at the molecular scale; in this case, they were looking at large numbers of simultaneous pair bindings. A variation of this method will be applicable to techniques to detect single pairing events, because today this is only being done in relatively large aggregates.

The metrology test developed for lock-and-key binding uses fluorophores to tag self-assembly processes at the molecular scale, which could not be done by more conventional means. (Source: College of Nanoscale Science and Engineering)

Hartley and his colleagues are planning to next look at the controlled deposition of carbon nanowires. “Many are experimenting with individual nanowires with methods suited for the lab, but unsatisfactory for manufacturing — they throw a plate of nanowires at some metal wires hoping that one will land on two electrodes where they can measure it and do a bit of FIB trimming, but this is only good to build one or two devices, and is far from being a manufacturing solution,” he said. “Suppose you were considering a CNT FET. You'd like to put down a single carbon nanotube in a controlled fashion between two electrodes followed by a metallization step to form a gate electrode. With our locks and keys, we believe we can deposit locks on the substrate with a conventional lithographically derived step, and then wash the solution of keyed CNTs over the substrate. Instead of going down in random distribution, the nanotubes line up with the keys mated to the locks, resulting in a large array of ordered CNT structures with which to build a device.”

This, of course, could not be done without adequate metrology. Integrated test structures carrying fluorophores or quantum dots can serve as a highly parallel means of sampling the overall success rate of the process.

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