Optical properties of copper clusters embedded in alumina: An experimental and theoretical study of size dependence

Abstract

Optical properties of copper clusters of 3 to 5 nm in diameter, produced by laser vaporization and embedded in alumina, are investigated and compared to a semi-quantal model [based on time-dependent local-density approximation (TDLDA) and density functional theory (DFT)], including the absorption and screening properties of the ionic core background and the surrounding matrix. To begin with, the experiments show that the alumina trapped clusters are oxidized if codeposited at room temperature whereas they do remain metallic by holding the substrate temperature at 400°C. On the other hand, reducing under H 2 -N 2 atmosphere of samples made at room temperature is an alternative to elaborate embedded metallic copper clusters. Consequently, the alumina matrix evaporated on a substrate at 400°C is optically characterized and the oxidization level of the copper clusters is carefully investigated through x-ray photoelectron spectroscopy (XPS). Concerning the optical properties of the copper clusters, a strong damping and a broadening of the surface plasmon resonance (SPR) with decreasing size is observed, in good agreement with theoretical calculations. This damping, much more important than in gold clusters, is correlated to a strong coupling of the collective excitation with interband transitions. These results allow us to validate the semiquantal model for the three systems Au, Ag, and Cu, in which the size effects are ruled by the electronic spillout phenomenon, the surface of ineffective ionic core polarization and the local porosity at the metal/matrix interface. Lastly, the inhomogeneous effects such as size dispersion are shown to be of less importance compared to intrinsic size effects in the optical properties.

Instability of Ag nanoparticles in SiO2at ambient conditions

“…the alumina films used in ref. [20]. Consequently no silica defects other than a small fluorescence signal from the substrate attributed to an oxygen deficiency center (ODC(II) [23]) were optically detectable.…”

“…The authors show a comparison with denser matrices such as ion-exchanged soda-lime glasses, where only a minor reduction of the plasmon is observed. The work of Celep et al [20] is the only study of oxidation influences on the preparation of dielectric matrices with pre-formed clusters we know of, in this case Cu n in alumina. They show that only deposition at elevated temperatures (400 • C as opposed to room temperature) allows the fabrication of unoxidized clusters with the typical plasmon resonance.…”

Optical properties and refractive index sensitivity of reactive sputtered oxide coatings with embedded Au clusters

“…(24) is employed, for a Au thickness of 3 nm, the resulting volume fraction would be 0.04. Such an arrangement of clusters on a surface can be achieved, for instance, by applying substrate heating on conventional sputtering 52 or by using a cluster source for depositing the metal nanoparticles with the desired size and distribution 53 during the Au clusters deposition step. The major factor influencing S is, however, the depolarization factor L; when decreasing L, S increases exponentially.…”

Optical absorption of small copper clusters in neon: Cun, (n = 1–9)

“…3 While silver develops a strong and narrow plasmon absorption centered at 4 eV, followed by interband transitions at higher energies, the surface plasmon in copper is strongly damped by s-d hybridization effects and interband transitions. 3,4 The concentration of oscillator strength (OS) in the energy range of the surface plasmon in silver is visible down to the smallest cluster sizes, 5 and explains why the experimental study of the optical absorption of neutral noble metal clusters beyond the trimer was limited to Ag clusters, which have, in the UV-visible range, a cross section considerably larger than that of gold or copper clusters. 6 Measurements on free mass selected species are limited by the low density of particles.…”