By Yugang Sun and Zhiyong Tang
Free electrons in nanosized metal particles can oscillate collectively to generate resonant plasmons on particle surfaces upon illumination of light with energy matching the electron resonance frequency. The resonant plasmons confined in nanoparticles are usually called localized surface plasmon resonances (LSPRs), which are sensitive towards a number of parameters including composition, size, shape, structure, environment, and so forth. The significant advance in colloidal synthesis enables the successful synthesis and investigation of plasmonic nanoparticles with tailorable LSPRs. Due to the LSPRs, metal nanoparticles made of coinage metals such as gold (Au) and silver (Ag) exhibit strong absorption, scattering, and emission, which are tunable by controlling the physical parameters of the nanoparticles. Such distinctive interactions between plasmonic nanoparticles and light endow the nanoparticles with applications ranging from sensing to energy to medicine. For example, plasmonic nanoparticles can drastically concentrate the electric field under resonant excitation, which can be applied in enhanced near-infrared (NIR) absorption spectroscopy, enhanced photoemission spectroscopy and surface-enhanced Raman spectroscopy. Nonradiative Landau damping of LSPRs in plasmonic nanoparticles creates charge carriers with a significant fraction of the plasmon energy being much higher than thermal energy at ambient temperature, i.e., hot electrons above Fermi energy and hot holes below the Fermi energy of the metal. These energetic hot carriers possess very high chemical potentials to drive chemical transformations on (or near) the surfaces of the plasmonic nanoparticles. If the LSPR frequencies are in the NIR spectral region, the light absorbed by the plasmonic nanoparticles can be efficiently converted to heat, thus benefiting the photothermal treatment of cancers and controlled drug delivery.
The rapid and sustained production of breakthroughs in the exciting field of plasmonics motivated us to put together a special issue specifically dedicated to “Plasmonic Particles”. The issue was conceived as a collection of selected contributions by different researchers who are recognized as experts or rising stars in the field of plasmonic nanoparticles synthesis, characterization, and theoretical modeling. Although the final collection of papers represents only a short list, there is no doubt that these papers provide a unique overview of current research directions. In the following, a brief description of the papers included in this issue is provided to serve as an outline to encourage further reading.
Several contributions are related to the synthesis of plasmonic nanoparticles with complex compositions and hollow structures. Chen et al. (article number 1600384) synthesized trimetallic Au-Cu-Ag nanorods using AuCu nanorods as a precursor template to react with Ag+, in which both nanoscale galvanic replacement reaction and co-reduction process occurred simultaneously. The concentration of Ag+ and reaction temperature determined the spatial distribution of elements in the trimetallic nanorods, and thus their LSPRs. The nanoscale galvanic replacement reaction between cobalt (Co) nanoparticles and chloroauric acid (HAuCl4) is demonstrated by Pu et al. (article number 1600255) to produce hollow Au nanospheres (HGNs). The spheres’ diameters can be varied from 24 to 122 nm by simply controlling the reaction temperature. The tunability in size of the HGNs enabled the LSPR absorption to cover the entire visible and NIR spectral regions. By using galvanic replacement reaction between M@Ag (M = Au, Pd, and Pt) core@shell nanoparticles with HAuCl4, Yang et al. (article number 1600279) synthesized M@Au/Ag nanorattles with ultrathin Au/Ag alloy sheets less than 2.5 nm in thickness. The Au@Au/Ag nanorattles with an edge length of 15 nm exhibited NIR absorption and the corresponding excellent photothermal conversion capability for their use as a transducer in the effective destruction of cancer cells through laser irradiation. Chen et al. (article number 1600358) prepared Ag-Cu hollow nanoshells through co-reduction of silver nitrate and cupric nitrate with sodium borohydride in the presence of sodium thiocyanate. In addition to the coinage metal nanoparticles, the review article by Chen et al. (article number 1600357) overviews the plasmonic nanoparticles made of earth-abundant Al regarding control of their LSPRs and photocatalytic activities, highlighting the promise of cheap plasmonic nanoparticles in artificial solar energy conversion.
The surrounding environment of plasmonic nanoparticles also influences optical properties of the nanoparticles. Kim et al. (article number 1600388) demonstrate that surface thiolate ligands (-SR) play a major role in determining photoluminescence of Au36(SR)24 nanoclusters. The R groups in surface ligands could affect the charge transfer between the ligands and the metal core, which in turn influenced the intensity of luminescence. Fan et al. (article number 1700075) developed a ligand-exchange process to replace polyvinylpyrrolidone and oleylamine, which are strongly adsorbed to Au nanoparticles, with diethylamine. The new nanoparticle surfaces exhibited enhanced catalytic and surface-enhanced Raman scattering (SERS) activities.
Theoretical studies have been widely adopted to help comprehensively understand light-matter interactions in plasmonic nanoparticles and assist the design of appropriate structures in achieving the particular properties. In their progress report, Li et al. (article number 1600380) discuss the efficient enhancement of both fluorescence excitation and emission of dye molecules by the LSPRs in Au nanorods. The interplay of elastic Rayleigh scattering and inelastic Raman scattering is also discussed to describe the plasmonic SERS and tip-enhanced Raman scattering (TERS) nanosystems. Zhou et al. (article number 1600327) report a sandwiched nanofilm, which is composed of a flat Ag layer and two layers of Ag prism arrays on its both sides, to enhance optical transmission (i.e., improve transparency).
At last, we would like to point out that non-plasmonic properties of the plasmonic nanoparticles may not be ignored when the nanoparticles possess interesting structures (for instance, article number 1600358). We want to thank all the authors who contributed to this special issue. Their contributions represent excellent examples of the current research directions in the field of plasmonic nanoparticles. We also want to thank the editorial staff of Particle & Particle Systems Characterization for their help with organizing this issue. We hope that the mixed topics and results presented in this issue will inspire the readers to explore the interesting areas of plasmonic nanoparticles.
Yugang Sun obtained his PhD from University of Science and Technology of China (USTC) in 2001. After postdoctoral training at University of Washington and University of Illinois at Urbana-Champaign, he joined the Center for Nanoscale Materials at Argonne National Laboratory as a staff scientist in 2006. He moved to Chemistry Department of Temple University in January 2016. His research interests include the design/synthesis of functional nanostructures for energy applications.
Zhiyong Tang obtained his PhD degree from Chinese Academy of Sciences in 2000 under the direction of Professor Erkang Wang. After finishing his postdoctoral training at both Swiss Federal Institute of Technology Zurich and University of Michigan, he came back to China and took a professor position at National Center for Nanoscience and Technology at the end of 2006. His main research interests are focused on preparation, assembly, and application of functional nanomaterials.