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Posted: Jun 30, 2015

Trapping individual metal nanoparticles in air

(Nanowerk Spotlight) While there is a great deal of knowledge on optical manipulation of metallic nanoparticles in liquids, aerosol trapping of metallic nanoparticles is essentially unexplored. In general, very little is known about optical manipulation of any type of particle in air, where the physics appear to be rather different than in water. For instance, the relation between laser power and trapping strength is found to be linear in water but not in air.
"The ability to manipulate and study individual metallic or semiconductor nanostructures in air or vacuum would open up many exciting opportunities," Lene Broeng Oddershede a professor at the Niels Bohr Institute, University of Copenhagen, tells Nanowerk. "This includes, for example, the study of catalytic processes, of heat transfer at the solid-gas interface at the nanoscale, or of the construction of advanced nanostructures away from a surface where electron-beam lithography cannot be used."
Oddershede led a team that has, for the first time, trapped individual gold nanoparticles in air. The researchers reported their findings in Nano Letters ("Optical Trapping of Gold Nanoparticles in Air").
"Our results show how to realize optical control of aerosol metallic nanoparticles and give the first hints on the physics involved, for instance by showing that the linear relation between laser power and trapping strength is reversed in a certain size regime," says Liselotte Jauffred, a postdoc in Oddershede's Optical Tweezer Group and first author of the paper. "We also show how to obtain the positions visited by the metallic nanoparticle in the trap, and this information can be used to shed light on Brownian motion in air with temperature gradients."
Analysis of the positions visited by an 80 nm gold nanoparticle trapped in air
Analysis of the positions visited by an 80 nm gold nanoparticle trapped in air 12 µm from the surface. (a) Position versus time in one of the lateral directions. (b) Power spectrum of the positions visited. The lines denote Lorentzian fits to data using eqs 2 (gray line) and 3 (dashed black line). The two fits appear equally good and return the corner frequencies fc,Eq2 = 5622 ± 167 Hz and fc,Eq3 = 5629 ± 169 Hz, which within the error bars are indistinguishable. The inset is a confocal image of the backscattered light, accumulated for 5 s, from a trapped 80 nm gold sphere, and the scale bar is 1 µm. (Reprinted with permission by American Chemical Society)
The researchers' results were made possible because they carefully minimized turbulence in their custom-made trapping chamber and because they precisely counteracted spherical aberrations.
As they describe in their paper, they were able to demonstrate stable aerosol trapping of individual metallic nanoparticles with diameters from 80 to 200 nanometers and quantified the corresponding trapping strengths.
"This is exciting because it opens for the possibility to build nanostructures in air, away from any surfaces that might restrict the geometry of the structure or chemically affect the structure," notes Oddershede. "As metallic nanoparticles have plasmonic properties and as the thermal conductance of air is much lower than that of water, the heating of a laser-trapped airborne metallic nanoparticles is significant and can easily exceed hundreds of degrees, maybe even thousand degrees."
"This combined manipulation and heating of metallic nanoparticles makes it possible to perform aerotaxy, i.e. controlled growth of nanostructures by 'soldering' one nanoparticle to another in air," she adds.
Aerotaxy is expected to be a highly efficient method for mass-production of well-defined nanostructures that cannot be obtained by other means (see: "Continuous gas-phase synthesis of nanowires with tunable properties").
These results pave the way for fabricating nanoscale architectures in air, either by means of simply placing particles in a certain structure, or by means of aerotaxy.
Also, metallic nanoparticles are known to increase electromagnetic field strength and therefore can be used to amplify weak signals, for instance in different types of microscopy. With these new results, electromagnetic signal amplification is also possible in air.
Furthermore, these findings can help understand the physical mechanisms behind aerotaxy. As the results provide the positions of the particle as a function of time, these time series shed light on how metallic nanoparticles move in air, and in particular for achieving a fundamental understanding of 'hot Brownian motion', the science of how a particle moves in a thermal gradient.
Since it is not exactly known how much a trapped airborne metallic nanoparticle heats, it would not only be of scientific interest to measure the temperature profile of an irradiated airborne nanoparticle but also a requirement for potential technological applications, e.g. aerotaxy.
"It may be worth noticing" says Oddershede, "that the reason quantification of heating is not trivial – it cannot be directly theoretically predicted – is that the focal intensity distribution in air is highly aberrated at the nanoscale and the exact distribution a priori unknown. Hence, it is also of interest to map out the intensity distribution at the nanoscale of a focused laser beam in air.–
The method could probably as well be used for optical manipulation of droplets, potentially from the atmosphere, with the goal of studying their size, composition, gas uptake, dynamics and coagulation. In addition, each droplet could be taken in a controlled fashion through a spectroscopic beam with the goal of analyzing the content.
The motivation for conducting this work came from Oddershede's previous work on optical manipulation of metallic nanoparticles in liquid (see for instance "Efficient Optical Trapping and Visualization of Silver Nanoparticles") and from her work on how to minimize spherical aberration ("Optimizing immersion media refractive index improves optical trapping by compensating spherical aberrations") – "which we knew would be crucial in order to realize aerosol trapping," she says.
The researchers conclude that the future scientific directions of the field of aerosol optical manipulation will probably go towards achieving control of other types of nanoparticles; controlling a larger range of particle sizes; and towards gaining control over particle orientation and possibly particle growth, too.
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