Strain Forms Striped Nanorods

Strain engineering, when applied to certain particles in a solution, spontaneously creates superlatticed or "striped" nanorods containing evenly spaced quantum dots, a research team has found. The approach is less expensive and exacting than current processes and suggests new uses for the crystalline materials, such as in tiny optoelectronic devices, LEDs and biological applications.

Researchers with Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) at Berkeley found a way to make striped nanorods in a colloid -- a suspension of particles in solution. Previously, striped nanorods, which are only a few molecules thick and made up of two or more semiconductors, were made through epitaxial processes, in which the rods were attached to or embedded within a solid medium.

A team at Lawrence Berkeley National Laboratory led by chemist Paul Alivisatos found a way to make superlatticed nanorods in a solution rather than through epitaxial processes by using strain engineering. The approach could make the nanorods easier and less expensive to make and open up new uses for the crystalline materials, such as in tiny optoelectronic devices and LEDs. (Images courtesy Berkeley Lab)

"We have demonstrated the application of strain engineering in a colloidal quantum-dot system by introducing a method that spontaneously creates a regularly spaced arrangement of quantum dots within a colloidal quantum rod," said chemist Paul Alivisatos, the Larry and Diane Bock Professor of Nanotechnology at UC Berkeley, who led the research. "A linear array of quantum dots within a nanorod effectively creates a one-dimensional superlattice, or striped nanorod."

Alivisatos, an internationally recognized authority on colloidal nanocrystal research, is also director of the Materials Sciences Div. and associate laboratory director for physical sciences at Berkeley Lab. He's the lead author on a paper about the research published in the July 20 edition of the journal Science.

Today’s electronics industry is built on two-dimensional semiconductor materials that feature carefully controlled doping and interfaces. Tomorrow’s industry will be built upon one-dimensional materials, in which controlled doping and interfaces are achieved through superlatticed structures. Formed from alternating layers of semiconductor materials with wide and narrow bandgaps, superlatticed structures, such as striped nanorods, not only can display outstanding electronic properties, but photonic properties as well.

"A target of colloidal nanocrystal research has been to create superlatticed structures while leveraging the advantages of solution-phase fabrication, such as low-cost synthesis and compatibility in disparate environments," Alivisatos said. "A colloidal approach to making striped nanorods opens up the possibility of using them in biological labeling, and in solution-processed LEDs and solar cells."

Previous research by Alivisatos and his group had shown that the exchange of cations (an atom or a group of atoms carrying a positive charge) could be used to vary the proportion of two semiconductors within a single nanocrystal without changing the crystal's size and shape, so long as the crystal's minimum dimension exceeded 4 nm. This led the group to investigate the possibility of using a partial exchange of cations between two semiconductors in a colloid to form a superlattice.

Working with previously formed cadmium-sulfide nanorods, they engineered a cation exchange with free-standing quantum dots of the semiconductor silver-sulfide.

"We found that a linear arrangement of regularly spaced silver-sulfide contained within a cadmium-sulfide nanorod forms spontaneously at a cation exchange rate of approximately 36 percent," said Alivisatos. "The resulting striped nanorods display properties expected of an epitaxially prepared array of silver-sulfide quantum dots separated by confining regions of cadmium-sulfide. This includes the ability to emit near-infrared light, which opens up potential applications such as nanometer-scale optoelectronic devices.”

In these transmission electron microscope images of superlatticed or "striped" nanorods formed through partial cation exchange, (A) shows the original cadmium-sulfide nanorods; (B and C) show cadmium-sulfide nanorods striped with silver-sulfide (quantum dots). The inset is a histogram showing the pattern spacing of the silver-sulfide stripes.

One of the key differences between quantum dots epitaxially grown on a substrate and free-standing colloidal quantum dots is the presence of strain. The use of temperature, pressure and other forms of stress to place a strain on material structures that can alter certain properties is called "strain engineering." This technique is used to enhance the performance of electronic devices and has recently been used to spatially pattern epitaxially grown striped nanorods.

However, strain engineering in epitaxially produced striped nanorods requires clever tricks, whereas Alivisatos and his colleagues discovered -- through energy calculations and computer modeling -- that naturally occurring strain in the colloidal process would be the driving force that induced the spontaneous formation of the superlattice structures.

Even though the colloidal striped nanorods form spontaneously, Alivisatos said it should be possible to control their superlatticed pattern -- hence their properties -- by adjusting the length, width, composition, etc., of the original nanocrystals. However, much more work remains to be done before the colloidal method of fabricating striped nanorods can match some of the "spectacular results" that have been obtained from epitaxial fabrication.

"For now, the value of our work lies in the unification of concepts between epitaxial and colloidal fabrication methods," he said.

The paper, "Spontaneous Superlattice Formation in Nanorods Through Partial Cation Exchange," is co-authored by Richard Robinson of Berkeley Lab’s Materials Sciences Div.; Denis Demchenko and Lin-Wang Wang of Berkeley Lab’s Computational Research Div.; and Bryce Sadtler and Can Erdonmez of the UC Berkeley Department of Chemistry.

For more information, visit: www.lbl.gov