By Stephanie Laird

Summary: Paul Weiss, a plenary lecturer at the recent joint NSS5/SP-STM2 international conference, discussed his research in understanding, measuring and controlling molecular functions and inter-molecular interactions at the atomic scale. During his presentation, Weiss discussed the progress and results of his research into various molecular properties and functions in three sections: self- and direct-assembly concerning the placement of molecules; molecular electronics and driven motion; and complex assemblies of molecules. The research Weiss has accomplished thus far shows promise for the precise assembly of functional parts and for understanding and designing molecular interactions, which will have vast implications in the nanoscience realm.

Paul S. Weiss, a prominent figure in the nanoscience community, delivered the first plenary lecture at the recent Nanoscale Spectroscopy & Nanotechnology 5 and Spin-Polarized Scanning Tunneling Microscopy 2 (NSS5/SP-STM2) conference. His scientific presentation entitled, “Designing, Measuring and Controlling Molecular- and Supermolecular-Scale Properties for Molecular Devices” addressed numerous advances in the understanding of molecular functions, interactions, assemblies, switching, measurements, and driven motion generated by his research.

During his college years, Weiss became interested in how electrons could be used to control chemical reactions. This led to his work in manipulating electrons on surfaces and using that process to understand and to control the chemical dynamics of the molecules, he said. Around this time, the scanning tunneling microscope (STM) was invented, which was an ideal tool for him since it allowed scientists to look simultaneously at the atomic and electron structure of the surface for the first time. Today, Weiss is a distinguished professor from the Departments of Chemistry and Physics at Pennsylvania State University, where his research group focuses on atomic-scale measurements and control in chemistry, physics, electronics and biology.

The scientific investigations conducted by Weiss and his group “use molecular design, tailored syntheses, intermolecular interactions and selective chemistry to direct molecules into desired positions to create nanostructures, to connect functional molecules to the outside world, and to serve as test structures for measurements of single or bundled molecules,” according to an abstract by Weiss.

It is known that “interactions within and between molecules can be designed, directed, measured, understood and exploited at unprecedented scales,” said Weiss. With this scientific precedent, Weiss is “looking at how these interactions influence the chemistry, dynamics, structure, electronic function and other properties of molecules, since such molecular interactions are advantageous in the formation of precise molecular assemblies, nanostructures, and patterns, and to control and to stabilize function,” according to his abstract.

Weiss' current research involves selecting and tailoring molecules to determine the intermolecular interaction strengths and the structures formed with the film. In addition, Weiss and his associates are “employing some of these approaches in directed assembly to enable bioselective and biospecific binding.” According to Weiss, his research is also focused on the selective testing of hypothesized mechanisms for molecular function such as electronic switching by varying molecular design, chemical environment, and measurement conditions in individual molecules and assemblies in order to enable or to disable the proposed mechanisms of function.

In order to understand the variations of molecular function and control stemming from their extensive research, the development of a system capable of making tens to hundreds of thousands of independent single-molecule measurements was essential since this is the means for developing sufficiently significant statistical distributions that can be compared to those of ensemble-averaging measurements, according to Weiss.

Additional nanoscience forays in which he is involved include: “quantitatively comparing the conductance of molecule-substrate junctions and demonstrating the importance of these junctions in conductance switching of single molecules,” according to Weiss. The preceding strategies are now being applied to photo-driven, electrochemically-driven, and chemically-driven motion in both single molecules and assemblies of molecules.

This molecularly based research “enables us to address how concerted nanoscale motions can be used to drive motion at larger scales, which we hope will be the key to technological innovations in motion in coming years,” said Weiss. His findings at the atomic scale provide surprising details and a glimpse into the limitations and potential applications of cooperative motion.

During his presentation, Weiss discussed the progress and results of his research into various molecular properties and functions in three sections: self- and direct-assembly concerning the placement of molecules; molecular electronics and driven motion; and complex assemblies of molecules.

In the first part of his lecture, Weiss noted some of the themes and challenges associated with self-assembly, including: controlled assembly via intermolecular interactions; controlling type and density of defects to control structure at the nanometer scale; and controlling and patterning chemical functionality. For single-molecule measurements, one of the challenges posed is recording substantial data sets on single molecules and/or particles.

Weiss learned to control the positions of molecules by controlling their chemistry, interactions, motion and defects. In addition, he is able to “select interactions and interaction strengths to guide and to stabilize structures,” said Weiss, adding that being able to control the type and density of defects is advantageous in controlling molecules.

Using nanometer-scale phase separation to produce patterns at the few-nanometer scale in self-assembled monolayers, Weiss demonstrated that molecules remain mobile even after absorption. The defects in self-assembled monolayers play a crucial function in both patterning and pattern dissolution, he said. One can selectively study molecules at or away from step edges on the surface, said Weiss, using research on an Au substrate as an example for how molecules can be inserted after vacancy islands and step edges are largely eliminated. Domain boundaries in the molecular layers are additional defects that result in sites where single molecules can be selectively inserted.

In discussing directed assembly and placement of molecules, Weiss said he uses self-assembly, intermolecular interactions, deposition and processing to select film structure. Moving on to a soft lithography called microcontact insertion printing, Weiss discussed how molecules can be isolated across the surface of a substrate. “We can make it so two molecules are rarely close to each other and we can also capture large molecules such as proteins on the basis of their function,” said Weiss, who is interested in determining the structures of single molecules and molecular complexes. In order to do so, he fashions a matrix and then uses the known defects to insert single molecules in it.

According to Weiss, defects mediate absorption, exchange and insertion. Some ways to control defect densities and types by processing films include: varying chain length; varying chain functionality; varying terminal functional group; and varying head group (S, Se for different purposes), he said. Another defect Weiss discussed involved the structures of multi-component films. Codeposition is used to create a mixed film, said Weiss, utilizing “sequential deposition to insert a small amount of a second component at defect sites; use/enhance soft lithography to pattern; and remove/displace part of film and replace with a second component.” This is done to probe the diffusion of marker molecules, study phase behavior, and enhance pattern precision to the molecular scale, he added.

The remaining portion of Weiss' lecture focused on molecular electrons and driven motion and the function of molecules in electronics and motion. In Weiss' research on molecular switches and other devices, he “measures isolated and bundled molecules in known and controlled environments.” He found that “switching can be controlled and stabilized via tailored interactions, and that key information can be lost when one cannot control the environment of what they're measuring.” In following and operating molecular switches, Weiss discovered that “molecules change their apparent protrusion from the matrix, which can also be driven, and this can be used to determine the state of the switch.”

In addition, many isolated switches can be monitored simultaneously, by using the matrix to control and to insert more isolated switches, which can be tracked automatically, said Weiss. The switching rate depends strongly on local matrix density and order. Using an electrical field generated by a scanning tunneling microscope tip, Weiss is able to switch single molecules on and off, he reported, and that motion is in part responsible for controlling switching activity.

The hypotheses tested by Weiss and his fellow researchers on what functionality is required for molecular switching include: chemical functionality; coordinated motion of pairs or larger clusters of molecules; internal molecular rotations; molecular tilts and associated bonding changes to the substrate.

Weiss' molecular research also concerns bias-dependent switching mechanisms, in which the one electric field direction switches mostly on and the other switches mostly off. For this mechanism, switching is correlated to the interactions between the electric field from the STM tip and the molecular dipole; these states are made stable by hydrogen bonding, both aspects are handles for molecular design, said Weiss.

While they study controlled switching with electric field polarity and intermolecular interactions, there is not yet a good way to measure the tilts of molecules, according to Weiss. It is known, however, that a significant dipole magnitude is required to drive switching and that inverting molecular dipole inverts the polarity of the switch. Also, Weiss discussed hybridization changes via molecular tilt as a potential switching mechanism, which can produce quite a significant redistribution of electrons between the molecules and the surface to which they are attached.

According to Weiss, there are “lots of things we want to measure at the same time, which means we need new methods to understand the events happening at the atomic scale.” Also, there is a statistical problem in understanding these molecular systems since it is so complicated due to all the variations in the assemblies measured.

The intended applications for Weiss' research include gaining control over single molecular components and understanding the entire systems, he said. “There are unique advantages in studying motion, electronics, and patterning down to those scales,” said Weiss about molecules' chemical and electronic interactions. In addition, they are trying to make assemblies in a direct way and to organize their data in such a way as to increase both function and the measurements of function so that this research can be applied throughout science and engineering fields.

The research Weiss has accomplished thus far shows promise for the precise assembly of functional parts and for understanding and designing molecular interactions. His continued commitment to understanding these fundamental processes and advancing this emerging technology are imperative to the expanding realm of nanoscience.