Molecular gear machines

Dr. Neil Canter, Contributing Editor | TLT Tech Beat July 2014

Researchers compressed a crystalline superlattice forcing the nanoparticles to rotate in a gear-like manner.

KEY CONCEPTS
• Nanoparticles based on a core of silver atoms surrounded by para-mercaptobenzoic acid molecules self-assemble into an ordered crystalline superlattice that is very stable.
• Hydrogen bonding among the p-mercaptobenzoic acid ligands is the key to the stability of the superlattice.
• Compression-property analysis shows that the material becomes softer as its volume is reduced and the nanoparticles rotate in a machine-like cooperative motion resembling gears.

AS RESEARCH CONTINUES TO GENERATE NEW TYPES OF STRUCTURES on the nanoscale, it is very interesting to see how these technologies relate to machinery that the lubricant industry must deal with on the macroscale on a daily basis. A case in point is gears.

One challenging application for gears is in large wind turbines. A similar type of device was developed that operates on the nanoscale. Recent work in a previous TLT article describes the development of much smaller micro-windmills with blades that are approximately 0.9 millimeters in length (1).

One important difference between the two devices is the micro-windmill does not have to stand up and can be placed flat on a surface. Potential applications include using the micro windmills as wireless sensors to protect infrastructure such as bridges.

Molecular self-assembly is an interesting area of study where molecules will form specific structures without guidance or influence from an outside source. This effect can occur at the macroscale and at the nanoscale. One example is the formation of micelles by surfactants in aqueous solution.

Uzi Landman, a Regents’ and F.E. Callaway professor in the School of Physics, and the director of the center for computational materials science at the Georgia Institute of Technology in Atlanta, Ga., says, “Together with our colleagues, a group of experimentalists led by professor Terry Bigioni at the University of Toledo in Toledo, Ohio, have developed and analyzed an ultrastable nanomaterial that is prepared by assembling nanoparticles, each comprised of close to 500 atoms—a core of 44 silver atoms surrounded by 30 para-mercaptobenzoic acid (p-MBA) molecules. These nanoparticles are characterized as stable superatoms that self-assemble into a macroscopic superlattice.”

The analogy that Landman makes is that “these superatom units may be regarded as prefabricated 500- atom building blocks that, when self assembled, form an ordered crystalline superlattice a fraction of a centimeter in length and containing millions of these superatoms.”

The synthesis of the superatoms is relatively simple using standard wet chemistry. Landman says, “Aqueous silver nitrate is combined with ethanolic p-MBA and reduced with sodium borohydride [converting silver (I) to silver (0)]. The resulting material self-assembles to form nanoclusters which are separated from the reaction mixture by precipitation with dimethylformamide.”

In contrast to essentially all other preparations of nanoparticles, the present straightforward synthesis protocol produces a pure molecular material without the need for size separations, and the preparation achieves near quantitative yield in large (kilogram) quantities. “That is nature’s way,” says Landsman, who characterizes this crystalline material as highly stable because of the particular organization of the electrons into a closed-shell superatom structure and the specific geometric arrangement of both the metal atoms and the organic ligands. “These factors combined to place the material in a deep basin of the free-energy landscape and, hence, its exceptional stability,” he adds.

A detailed analysis of the superlattice structure has now been made and more has been learned about the mechanical properties of this unique nanoparticle superlattice assembly.

HYDROGEN BONDING
The researchers crystallized the superlattice nanoparticles from dimethylformamide solution to form rhombus-shaped crystals at room temperature over a one- to three-day period. The resulting x-ray structure reveals a hierarchical material containing a variety of elements.

Landman says, “The nanocluster contains an organic component with carbon, hydrogen, oxygen that hooks through a sulfur (thiol) bond into a core of silver atoms. It combines the cohesion, toughness and strength of metallic bonding with the covalent bonding of the organic, soft-matter component.”

The key to the stability of the superlattice is that hydrogen bonding network that links the nanoparticles. The p-MBA ligands are the source of the hydrogen bonding. At the same time, the electronic spectrum of the individual nanoparticles has been found—through firstprinciples, density-functional theory quantum mechanical calculations—to exhibit the enhanced superatom stability of a closed shell structure.

To gain further insight, the researchers evaluated the hydrostatic compression properties of the superlattice through the use of a large scale quantum mechanical computational analysis. Landman says, “We were rather surprised to find that the nanoparticle superlattice becomes softer as the volume of the material was reduced, which is an anomalous behavior because most substances become harder as they are compressed. Moreover, we were taken totally off guard when we discovered that in response to increasing compression the individual nanoparticles in this superlattice assembly rotate in a machine-like cooperative motion resembling gears.”

In the superlattice crystal structure, the nanoparticles in each layer are the mirror image reflection of those in an adjacent layer. Under compression, the theoretical study predicts that when nanoparticles in a given nanocrystalline layer rotate in a clockwise manner, then the nanoparticles in an adjacent layer rotate counter-clockwise. Landman notes that this constitutes a chiral mechanical response to the pressure.

Figure 1 shows, on the left side, images of two neighboring layers of the superlattice in the equilibrium state and on the right side, the rotating images after being subjected to volume compression. Landman says, “The bulk modulus which measures the resistance to compression is about one-sixth that of solid silver at ambient conditions. This notable value originates from the hydrogen bond network that links the nanoparticles together; whereas each individual hydrogen bond is relatively weak, the assembly of many of them results in a framework material of increased mechanical strength and toughness.”


Figure 1. On the left is shown images of two neighboring layers of the superlattice in the equilibrium state. Upon compression, the images rotate in a similar manner to gears as shown on the right. (Courtesy of the Georgia Institute of Technology)

Underlying the layers’ rotation is the fact that 50 percent of the superlattice is empty space or free volume. Landman says, “The emergent molecular rotation is in direct response to the nanoclusters seeking to fill up the empty voids, which is in effect the path of least resistance. The organic ligands flex about the hydrogen bonds between them that act as molecular ‘hinges,’ and the resulting torque on the silver cores brings about their machine-like chiral rotations acting as molecular gears.”

Once the pressure is released, the superlattice returns to its original position. Landman notes that because the process does not involve breaking any bonds, the large amount of energy absorbed by the superlattice can be released once the compressive load is removed. Future work includes the experimental exploration of the compressive properties of the silver nanoparticle superlattice.

Landman remarked that this superlattice is self-lubricating. He says, “The organic ligands act as a lubricant enabling the gear rotation to take place without any wear.”

Potential applications for the superlattice are as sensors, switches and piezo generators. Landman also believes they can be used to absorb and release energy. He adds, “In future work, one may wish to dope the nanoparticles, for example with magnetic, or optically active elements. This may lead to the emergence of novel material properties brought about by the superlattice’s peculiar machine-like mechanical response.”

The theoretical research at Georgia Tech has been supported by grants from the Air Force Office of Scientific Research and the Office of Basic Energy Sciences of the U.S. Department of Energy. The experimental work at the University of Toledo has been supported by the National Science Foundation.

Additional information can be found in a recent article (2) or by contacting Landman at Uzi.Landman@physics.gatech.edu.

REFERENCES
1. Canter. N. (2014), “Potential for windmills at the microscale,” TLT, 70 (4), pp. 8-9
2. Yoon, B., Luedtke, W., Barnett, R., Gao, J., Desireddy, A., Conn. B., Bigioni, T. and Landman, U. (2014), “Hydrogen-bonded structure and mechanical chiral response of a silver nanoparticle superlattice,” Nature Materials, doi:10.1038/nmat3923.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.