As an elementary type of collective excitation, plasmon has been found to dominate the optical properties of metals. The collective behavior of electrons in plasmons reflects the important difference between condensed matter and molecule-like ones. It is of great significance to study the evolution of plasmonic response and find out the boundary.
Controversy exists on such interesting questions as the division between the nanoparticle and molecules, and the physics of mesoscopic and microscopic plasmonic evolution. A unified understanding covering the small and large size limit, namely macro / meso / micro scales with sufficiently atomic precision is thus required. Clusters, as the transition from atomic molecules to condensed matter, are the ideal candidate for studying the evolution of plasmons.
In a new overview published in the Beijing-based National Science Review, A joint team from Nanjing University, Southern University of Science and Technology, National University of Defense Technology, Institute of Physics, Chinese Academy of Sciences and National Center for Nanoscience and Technology present a research on the evolution of plasmon through mass-selected gold clusters. In this work, scientists push the limit to atomic scale and present a complete evolution picture of size-dependent plasmon physics.
Gold clusters with precise atomic number ranging from 70000 to 100 were prepared using time-of-flight mass-selected magnetron sputtering gas-phase condensation cluster beam source in Fengqi Song's group. The mass resolution M / Δ M was about 50. Scientists then successfully measured the plasmonic responses of a series of atomically precise individual gold particles with atom number (N) of 100-70000 through the well collected high-resolution electron energy loss spectroscopy with the great help of Jiaqing He's group and Pico Center at SUSTech.
Scientists found three characteristic regimes separated by the two grey bold vertical dashed lines, as shown below. In regime 3 (N~887-70000), the positions of the peak of surface plasmon (SP) exhibit a very slight red shift with decreasing N, while the peak of bulk plasmon (BP) remains unchanged and the full width at half maximum (FWHM) remains at a high value of about 0.36 eV. In regime 2 (N~300-887), the position of the peak of SP exhibits a steady blueshift with decreasing N, while the peak of BP disappears completely and the FWHM stays at a high value of about 0.26 eV. In regime 1 (N~100-300), the peak of SP is replaced by 3 fine features with a much smaller FWHM, which is close to the FWHM of the EELS zero-loss peak.
Based on the basic physical model, the physics of the three regimes is explained below. For large clusters with similar electronic structures like bulk, the redshift of SP in regime 3 is explained by electron-boundary scattering modified classical plasmon (Nanoscale. 2017; 9: 3188-95). The reduction in the size only introduces extra boundary scattering for free electrons in the metal, in addition to the Coulomb scattering between electrons. The classical model calculations performed by Jianing Chen's group show good agreement with the experimental results. With the continuous decrease of cluster size, the monotonic blueshift of SP in regime 2 is caused by the well-studied quantum confinement effects (Nature. 2012; 483: 421-7; Nat Phys. 2019; 15: 275-80). The classical Drude model for the dielectric function becomes invalid and gives its way to the quantum-descripted one. Here the total permittivity ε is the sum of the permittivity of free electron transitions in the quantized conduction band and the frequency-dependent permittivity εinter of interband transitions between the d bands and the higher conduction bands. When the bulk-like electronic structure is finally destroyed with even fewer atom numbers in clusters (Nat Commun. 2016; 7: 13240; J Am Chem Soc. 2018; 140: 5691-5), superimposed transitions between quantized molecule-like electronic structures happens, and the traditional plasmonic peak degenerated into fine structures (molecular plasmon, regime 1). With the help of Jiayu Dai's group, the rt-TDDFT calculations show that after strong laser action, the collective charge density oscillation could be found at the core of the cluster, which is quite different from the case for a weak laser field. The collective behavior of electrons is some superimposition of single electron transitions between quantized molecular energy levels (Nat Commun. 2015; 6: 10107). Thus 3 regimes in the plasmonic evolution are observed with distinct physics, namely classical plasmon (N=887-70000), quantum confinement corrected plasmon (N=300-887) and molecule related plasmon (NThis work paves the way for new developments in physics and for future applications of nanoplasmonics as their noting: "Au887 is very small, indicating hardly any quantum effect for most gold nanoparticles since they are larger than Au887, and most nanoplasmonic designs in industrial nanofabrication can currently be tackled based on classical electromagnetism and the dielectric function."