PHILADELPHIA â When semiconductor nanorods are exposed to light, they blink in a seemingly random pattern. By clustering nanorods together, physicists at the University of Pennsylvania have shown that their combined âonâ time is increased dramatically providing new insight into this mysterious blinking behavior.
The research was conducted by associate professor Marija Drndicâs group, including graduate student Siying Wang and postdoctorial fellows Claudia Querner and Tali Dadosh, all of the Department of Physics and Astronomy in Pennâs School of Arts and Sciences. They collaborated with Catherine Crouch of Swarthmore College and Dmitry Novikov of New York Universityâs School of Medicine.
Their research was published in the journal Nature Communications.
When provided with energy, whether in the form of light, electricity or certain chemicals, many semiconductors emit light. This principle is at work in light-emitting diodes, or LEDs, which are found in any number of consumer electronics.
At the macro scale, this electroluminescence is consistent; LED light bulbs, for example, can shine for years with a fraction of the energy used by even compact-fluorescent bulbs. But when semiconductors are shrunk down to nanometer size, instead of shining steadily, they turn âonâ and âoffâ in an unpredictable fashion, switching between emitting light and being dark for variable lengths of time. For the decade since this was observed, many research groups around the world have sought to uncover the mechanism of this phenomenon, which is still not completely understood.
âBlinking has been studied in many different nanoscale materials for over a decade, as it is surprising and intriguing, but itâs the statistics of the blinking that are so unusual,â Drndic said. âThese nanorods can be âonâ and âoffâ for all scales of time, from a microsecond to hours. Thatâs why we worked with Dmitry Novikov, who studies stochastic phenomena in physical and biological systems. These unusual Levi statistics arise when many factors compete with each other at different time scales, resulting in a rather complex behavior, with examples ranging from earthquakes to biological processes to stock market fluctuations.â
Drndic and her research team, through a combination of imaging techniques, have shown that clustering these nanorod semiconductors greatly increases their total âonâ time in a kind of âcampfire effect.â Adding a rod to the cluster has a multiplying effect on the âonâ period of the group.
âIf you put nanorods together, if each one blinks in rare short bursts, you would think the maximum âonâ time for the group will not be much bigger than that for one nanorod, since their bursts mostly donât overlap,â Novikov said. âWhat we see are greatly prolonged âonâ bursts when nanorods are very close together, as if they help each other to keep shining, or âburning.ââ
Drndicâs group demonstrated this by depositing cadmium selenide nanorods onto a substrate, shining a blue laser on them, then taking video under an optical microscope to observe the red light the nanorods then emitted. While that technique provided data on how long each cluster was âon,â the team needed to use transmission electron microscopy, or TEM, to distinguish each individual, 5-nanometer rod and measure the size of each cluster.
A set of gold gridlines allowed the researchers to label and locate individual nanorod clusters. Wang then accurately overlaid about a thousand stitched-together TEM images with the luminescence data that she took with the optical microscope. The researchers observed the âcampfire effectâ in clusters as small as two and as large as 110, when the cluster effectively took on macroscale properties and stopped blinking entirely.
While the exact mechanism that causes this prolonged luminescence canât yet be pinpointed, Drndicâs teamâs findings support the idea that interactions between electrons in the cluster are at the root of the effect.
âBy moving from one end of a nanorod to the other, or otherwise changing position, we hypothesize that electrons in one rod can influence those in neighboring rods in ways that enhance the other rodsâ ability to give off light,â Crouch said. âWe hope our findings will give insight into these nanoscale interactions, as well as helping guide future work to understand blinking in single nanoparticles.â
As nanorods can be an order of magnitude smaller than a cell, but can emit a signal that can be relatively easily seen under a microscope, they have been long considered as potential biomarkers. Their inconsistent pattern of illumination, however, has limited their usefulness.
âBiologists use semiconductor nanocrystals as fluorescent labels. One significant disadvantage is that they blink,â Drndic said. âIf the emission time could be extended to many minutes it makes them much more usable. With further development of the synthesis, perhaps clusters could be designed as improved labels.â
Future research will use more ordered nanorod assemblies and controlled inter-particle separations to further study the details of particle interactions.
This research was supported by the National Science Foundation.