Silver-Based Superlattices Create Molecular Machines With Varied Uses In Nanotechnology
Research conducted by scientists at the Georgia Institute of Technology and University of Toledo has found unique properties in self-assembled silver-based structures known as superlattices - there is a range of gear-like molecular-scale machines, or nanomachines, that rotate in unison when pressure is applied to them.
Superlattices are artificial structures consisting of alternate layers of two different materials nanometers in thickness. Computational and experimental studies showed that disordered silver nanoparticles formed an organized pattern - a process called self-assembly - and then formed superlattices with alternate layers of organic protecting molecules. Hydrogen molecules create bonds, or "hinges," between the molecules, to facilitate their rotation. Controlling the rotation involves applying pressure to the superlattice, which softens it.
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Materials like these, which contain about 500 atoms each, might be useful for molecular-scale switching, sensing, and even energy absorption. In a first of its kind, this complex superlattice has been mapped in detail using a combination of X-ray and computational techniques.
"As we squeeze on this material, it gets softer and softer and suddenly experiences a dramatic change," said Uzi Landman, from the Georgia Institute of Technology, in a statement. "When we look at the orientation of the microscopic structure of the crystal in the region of this transition, we see that something very unusual happens. The structures start to rotate with respect to one another, creating a molecular machine with some of the smallest moving elements ever observed."
When pressure is applied, the gears rotate up to 23 degrees and return to their original positions when pressure is released. Alternating layers of gears move in opposite directions. The researchers studied superlattice structures comprising of clusters with cores of 44 silver atoms each. The silver clusters were protected by 30 ligand molecules of an organic material -
mercaptobenzoic acid (p-MBA) - that include an acid group. These were attached to the silver by sulfur atoms.
"It's not the individual atoms that form the superlattice," Landman said in the statement. "You actually make the larger structure from clusters that are already crystallized. You can make an ordered array from those." The clusters attach themselves to the superlattice only when guided by the hydrogen bonds at certain angles between the organic material, p-MBA. "The self-assembly process is guided by the desire to form hydrogen bonds," Landman said. "These bonds are directional and cannot vary significantly, which restricts the orientation that the molecules can have."
Quantum-mechanical molecular dynamics simulations were first used to study the superlattice in Landman's lab. Hydrostatic techniques were applied to compress the material. After the structure was compressed by about six percent, the researchers noticed that further compression did not require much pressure. This unusual behavior occurred because the nanocrystal components rotated, layer-by-layer, in opposite directions. Hydrogen bonds determine not only the structure but also the movement of the superlattice under pressure.
"The hydrogen bond likes to have directionality in its orientation," Landman said. "When you press on the superlattice, it wants to maintain the hydrogen bonds. In the process of trying to maintain the hydrogen bonds, all the organic ligands bend the silver cores in one layer one way, and those in the next layer bend and rotate the other way."
The hydrogen bonds act as axles between the nanoclusters. When they move, the hydrogen bonds act as "molecular hinges," giving the system motion. The material can be compressed because the crystalline structure has a half-open structure.
The movement of the silver nanocrystallites may allow the superlattice material to serve as an energy-absorbing structure, converting force to mechanical motion. By changing the conductive properties of the silver superlattice, compressing the material could also lead to using it as molecular-scale sensors and switches. "We now have complete control over a unique material that, by its composition, has a diversity of molecules," Landman said. "It has metal, it has organic materials, and it has a stiff metallic core surrounded by a soft material."
The researchers plan to study more superlattice systems, and to look into how their unique properties can be exploited by combining molecular-scale machines with many other small-scale units. "We make the small particles, and they are different because small is different. When you put them together, having more of them is different because that allows them to behave collectively, and that collective activity makes the difference."
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