Plasmonic nanoparticles scatter light, which is considered to be very useful; however, some of this light becomes lost at the surface. Now, researchers are beginning to understand why this phenomenon occurs.
In innovative experiments performed at the Johannes Gutenberg University of Mainz and Rice University, together with hypothetical work carried out at Princeton University, a research team discovered that when molecules are positioned on the surface of a single gold nanorod, its plasmonic response is affected by the alteration of the particle’s electronic structure itself. These findings can possibly improve applications like catalysis involving plasmon-driven chemistry.
Plasmons are basically electron ripples that resonate across a metal nanoparticle’s surface when activated by light. Here, the light received by them at one color, or wavelength, is radiated at the same wavelength, informing scientists regarding the particle as well as its environment.
Through surface plasmons, the presence of chemicals can be sensed, photochemistry can be realized, and chemical reactions can be selectively catalyzed. However, the light lost between the surface of the particle and the eye of the researcher can contain more data that were not considered in the past.
It was believed that signal loss caused by plasmon damping was because of the chemicals adsorbed to the surface of the nanoparticle, possibly through the transfer of charge from the metal to the chemical substances. However, Stephan Link, a professor of chemistry and of electrical and computer engineering at Rice University, had misgivings that just a single explanation would fit all researches.
That led Link; Benjamin Förster, lead author; and their colleagues to the discovery of a fully different mechanism, recently reported in Science Advances.
The team’s approach was to place two kinds of molecules with identical size but varied atomic arrangements onto single gold nanorods for investigation. These molecules were cage-like carborane thiols, which caused surface dipoles in the metal that consecutively scattered sufficient energy of the plasmons to damp their signal.
That allowed the investigators to view and determine the damping directly without any disturbance from other nanorods or other molecules. The nearness of the thiols, which are analogous except for the placement of a single carbon atom, to the nanorod induced unusual dipole moments—that is, the negative and positive poles of the molecules that alter the strength and move like a compass needle—on the surface of the metal.
In-depth quantum mechanical calculations were carried out by Emily Carter, a theoretical-computational scientist and dean of the School of Engineering and Applied Science at Princeton, in order to test the mechanisms that can possibly shed light on the experiments.
“Plasmonic resonances have a spectral width that, together with resonance wavelengths, gives specific colors. A narrow line gives you a truer color. So we looked at how the width of this resonance changes when we put molecules on the particle.”
Stephan Link, Professor, Departments of Chemistry and Electrical and Computer Engineering, Rice University
However, any kind of molecules would not work. The carborane thiols are molecules that have the same, exact size; while these molecules adhere to gold nanoparticles in equal measure, they are also exceptionally chemically different to alter the spectral width of the plasmons. These carborane thiols allowed the team to determine plasmon damping by each kind of molecule without any disturbance from other damping mechanisms. The plasmons flowing over a surface depend on the shape and size of the particle to such an extent that not much attention was paid to the impact of chemicals adsorbed to the surface, stated Förster.
“If you change the surface of the nanorod, the energy gets lost in different ways,” he stated. “We didn’t understand this at all. But if something loses energy, it’s not functioning as you want it to function.”
The signal can also be affected by the refractive traits of the surrounding medium as well as the averaging of signals from numerous particles of various shape and size. That had also made it challenging to examine the effect of adsorbed chemicals.
Several contributions determine the plasmon resonance width. But there’s a fudge factor everybody invokes that nobody had really tackled in a quantitative way. A lot of people blamed charge transfer, meaning excited hot electrons moved from the metal to the molecule. We are saying that’s not the case here. It may not be the same every time you put a molecule on a metal particle, but this gives us, for the first time, a complete quantitative study that also doesn’t turn a blind eye to the chemistry at the interface. It lets us understand that the chemistry is important. The work is fundamental and I think it’s pretty because it’s so simple. We combined the right sample, the experiment and single-particle spectroscopy with advanced theory, and we put it all together.
Stephan Link, Professor, of Departments of Chemistry and Electrical and Computer Engineering, Rice University.
A former graduate student at the University of Mainz, Förster is currently a research scientist at BASF who came to Rice University through the Toulouse-Mainz Scholar Exchange Program for applied physics students. A collaboration between the Link lab and Förster and his mentor and co-author Carsten Sönnichsen—a professor of physical chemistry at the University of Mainz—resulted in the publication of three papers.
Vincent Spata is the paper’s co-author and a former postdoctoral research associate of Carter’s at Princeton University.
The European Research Council, the Excellence Initiative by the Graduate School of Materials Science in Mainz, the Robert A. Welch Foundation, and the Air Force Office of Scientific Research through the Department of Defense Multidisciplinary University Research Initiative supported the study.