How a few atoms can influence materials' properties

(Nanowerk News) A closer look at gold nanoparticles, which were welded together by laser excitation, revealed interesting insights on the atomic structure and how it influences the physical properties of the system.
A team of researchers from the University of Antwerp, Complutense University of Madrid and the Centre for Cooperative Research in Biomaterials CIC biomaGUNE investigated the 3D atomic structure and optical properties of these novel nanosystems and published their work in ACS Nano ("3D Characterization and Plasmon Mapping of Gold Nanorods Welded by Femtosecond Laser Irradiation").
Visualization of the measured interface area of two welded gold nanorods. The discrete dots are single atoms and the distribution of them gives the researchers insight about how perfect the crystallinity is.
Gold behaves very differently at the nanoscale compared to the macroscale. Macroscale gold has its well-known shining yellow color, but when confined to the nanosize, it is rather purple-red. The reason for the different color of nanogold are so-called localized surface plasmons, which are collective excitations of electrons, similar to a stadium wave, which arises from the single movement of all the people participating.
When the motion of the electron movement is in resonance with the wavelength of an exciting laser pulse, a gold nanoparticle strongly absorbs or scatters light, which lead to its distinct color This resonance can be tuned by modifying the size and shape of the gold nanoparticle.
Due to its unique optical properties and non-toxicity, gold nanoparticles are part of many emerging technologies, such as photocatalysis, optical data storage, localized hyperthermic treatment and sensing technologies.
However, nanoparticles are normally not perfect and can exhibit rough surfaces, where atoms are not positioned at their ideal locations, or internal defects of the atomic lattice. Such imperfections influence the properties of the nanoparticles.
For example, it hampers the strength of the plasmon resonance and consequently reduces the absorption and scattering properties of gold nanoparticles. Understanding these imperfections, and even more link them to changes in physical properties, is important to improve existing technologies.
“Welding nanoparticles together by laser pulses is an interesting approach to fabricate novel nanostructures but we also need to understand what happens to the atomic lattice upon welding” says Dr. Wiebke Albrecht of the University of Antwerp. In fact, by making use of a novel 3D electron microscopy technique with atomic resolution, the researchers found out that welding of the nanoparticles introduces defects at the interface area between them. “For most welded systems these defects were single defects. For this reason, they are excellent model systems to study the influence of a single atomic lattice defect on the optical properties” continues Dr. Albrecht.
By using spectroscopic single particle techniques inside the electron microscope, the researchers found out that a single crystal defect can lead to a significant plasmon resonance broadening. These results give important insights for chemists, who develop novel nanoparticles and reveal that it will be important to strive for perfect crystallinity, when plasmonic applications are targeted. Furthermore, this information can lead to improve existing and emerging technologies, which make use of metal nanoparticles.
Source: University of Antwerp
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