Researchers at the University of California, Santa Cruz, have reported the first observations of ultrafast electronic dynamics in a system of strongly interacting gold nanoparticles. The observations are an important advance in nanoparticle research, because the development of practical devices using metal nanoparticles depends on understanding how they interact.
Jin Zhang, an associate professor of chemistry at UCSC, and a team of graduate students in his laboratory used femtosecond laser spectroscopy to probe the fundamental optical and electronic properties of aggregated gold nanoparticles. Their ultrafast laser system enabled the researchers to observe processes that occur on the timescale of a picosecond (one trillionth of a second).
The researchers measured the absorption of light by gold nanoparticle aggregates, and studied how the absorption changed as electrons in the particles were excited to a higher energy level and then returned to their ground state. This process, called electronic relaxation, takes about one or two picoseconds (the exact time depends on the amount of power in the laser pulses used to excite the sample). Unexpectedly, the electronic relaxation dynamics of the nanoparticle aggregates exhibited periodic oscillations, which Zhang attributed to coherent vibrations of the aggregates.
"This is the first direct observation of vibrational oscillations of nanoparticle aggregates," he said. "Femtosecond spectroscopy is the only way to study this and other fundamental dynamic properties of such aggregates."
Zhang's team reported its findings in a paper published by the Journal of the American Chemical Society. Graduate student Christian Grant is first author of the paper, which was published online on December 5 and will appear in print in an upcoming issue of the journal. The other coauthors are Adam Schwartzberg and Thaddeus Norman, both graduate students at UCSC.
The unique optical properties of gold nanoparticles have been recognized since the Middle Ages, when colloidal gold provided the intense red color in stained glass windows. More recently, metal nanoparticles have attracted attention for their potential uses in optoelectronic devices, sensors, and other applications. So far, however, most studies of the optical properties of metal nanoparticles have focused on isolated particles, Grant said.
"We need to understand how the particles interact and what properties arise from those interactions--this is fundamental knowledge that will help in designing practical devices," he said.
Gold nanoparticles have a characteristic absorption spectrum that is affected by interactions between the particles. A key feature of the spectrum is a peak, known as the "transverse plasmon band," at a wavelength of about 520 nanometers. In a system of weakly or moderately interacting particles, this peak shifts toward longer (redder) wavelengths. When the interactions between particles are very strong, as in the aggregates studied by Zhang's lab, an entirely new absorption band appears in the near-infrared region of the spectrum.
Zhang's group was able to show that this broad absorption band in the near-infrared is made up of many sub-bands that correspond to absorption by aggregates of different sizes and shapes. The periodic oscillations they observed in the electronic relaxation profiles provided a critical clue. These "coherent vibrational oscillations" result from the interaction of the excited electrons with the lattice structure of the aggregates, Grant said.
"When we excite the electrons we are putting in energy, and the electrons relax by dumping that energy mainly into the vibrations of the nanoparticles or, in this case, into the vibrations of the entire aggregate," Grant said.
When they measured the electronic relaxation dynamics at varying wavelengths, the researchers found that the period of the oscillations varied with the probe wavelength. The findings suggested that aggregates of similar size and structure in the sample are all vibrating with the same frequency.
"It's like the strings on a violin," Zhang said. "The thicker the string, the slower the vibrations, and the lower the frequency of the tone it produces."
Previous work by other researchers studying isolated gold nanoparticles showed vibrational oscillations with periods proportional to the size of the particles. Whereas isolated nanoparticles show oscillations at wavelengths corresponding to the transverse plasmon band, the aggregates showed oscillations at wavelengths corresponding to the near-infrared band.
To confirm that the near-infrared band is made up of discrete sub-bands, the researchers performed a "hole-burning" experiment. This involved exciting the sample with an intense laser beam at a single wavelength to disrupt the aggregates that absorb at that wavelength. The absorption spectrum of the sample then showed a distinctive change in the near-infrared band, from one broad peak to a pair of peaks separated by a dip at the "hole-burning" wavelength. "The aggregates that absorbed at that wavelength were probably broken up into smaller aggregates," Grant said.
In future work, the researchers will try to learn more about how the structure of nanoparticle aggregates affects their optical properties.
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Note to reporters: You may contact Zhang at (831) 459-3776 or zhang@hydrogen.ucsc.edu, and Grant at (831) 459-3912 or grant@chemistry.ucsc.edu.
