Quantum Drops

Materials that normally don't mix can now be blended within single microparticles and nanoparticles, thanks to a droplet technique discovered at ORNL for producing arbitrarily sized particles. The technique may lead to new materials with tunable properties with applications to drug delivery systems, improved coatings, and faster components for electronic devices.

In this molecular dynamics simulation, a nanosized polymer droplet is formed in a solvent and forced through a micron-sized orifice. Submicron polymer particles exhibit unique properties that could make them extremely useful for optical displays and industrial coatings.

In 1998 Don W. Noid, Bobby Sumpter, and Michael D. Barnes, all CASD researchers, developed a novel way to prepare electrically charged, submicron-diameter, spherical composite particles of organic polymers of nearly uniform size. They started by forming droplets of a dilute solution of two polymers that do not ordinarily mix together. For droplets less than 10 microns in diameter, the solvent evaporates faster than the polymer molecules could disentangle, resulting in homogeneous mixed-polymer particles. Optical probes were used to determine the material homogeneity, size, and dielectric constant of the particles. The work showed that material properties of the particles could be tuned simply by adjusting the mixture of polymers in solution.

Barnes, Noid, Sumpter, Thomas Thundat of the Life Sciences Division (LSD), and M. Alfred Akerman of ETD have received LDRD funding to refine this technique to make "quantum drops," clusters of one or more different types of charged polymer molecules predicted to act like "artificial atoms" with tunable electronic properties. Electrons, like photons, possess both "wave" and "particle" properties; the characteristic (DeBroglie) wavelength of a particle (usually very nearly zero for macroscopic objects) is inversely proportional to its momentum. When electrons are confined to a "box" whose dimensions are comparable to their wavelength, discrete, or "quantized," energy levels are observed whose spacing increases with decreasing "box" size. Thus, such systems are termed artificial atoms because the colors of light they absorb or emit depend on the size of the particle. The researchers will investigate whether these spherical 2- to 10-NM drops (which may contain more than 10,000 atoms) exhibit size quantization behavior similar to that of more familiar semiconductor quantum dots, as predicted by computational simulations.

"When we model quantum drops," Noid says, "we can predict their electron energy levels and their chemical potential, or electron affinity - that is, whether they have the correct symmetry to make it easy to attach electrons. We can predict the effects of electric and magnetic fields on the drops. We can predict the melting points of the particles in the drops, which become lower as particle size gets smaller."

"We think our drop technology will enable us to tune, or select, the size of the drop particles and the number of electrons on each particle," Barnes says. "The particle size is determined by the droplet size and the concentration of polymers in the droplet solution. The material properties of the quantum drops are determined by the polymer composition.

"The electronic properties of quantum drops, such as their ability to emit light of a particular color, can be tuned by adjusting or selecting the particle size or the number of excess electrons. The resulting quantum drops could be a new class of luminescent particle."

In this computer simulation, electron orbits sit on a polymer droplet. Such computations could guide the development of new materials for possible use in flat panel displays and new storage media.

Sumpter notes that his simulations predict that the hardness of a polymer can be tuned by mixing the hard material with a squishy polymer using the droplet technique. Mixing polymers to make particles can also alter the index of refraction or strength and compressibility of each material.

Quantum drops could be used for applications ranging from catalysis to quantum computing. In the gas phase these particles could speed up reactions because their high surface area increases the opportunity for contact and electron exchange between the particles and the reactant species diffusing throughout the reaction chamber. Quantum dots theoretically could be tuned to make each electron represent a bit of information: Each electron with an up spin could be a "1" and each electron with a down spin could be a "0." "We propose to use the droplet technique to encode several memory elements on each nanoparticle," Barnes says. "If we are successful, this technique could be useful for the proposed development of a quantum computer."

"We have to learn how to make a variety of mixed-polymer particles and sort them by size," Noid says. "Then we must use spectroscopic tools to measure their electron energy levels experimentally and verify that the mixed-polymer particles behave like artificial atoms, as predicted by our simulations."