Christian Félix

Controlled Deposition of Size-Selected Silver Nanoclusters

Variable-temperature scanning tunneling microscopy was used to study the effect of kinetic cluster energy and rare-gas buffer layers on the deposition process of size-selected silver nanoclusters on a platinum(111) surface. Clusters with impact energies of ≤1 electron volt per atom could be landed nondestructively on the bare substrate, whereas at higher kinetic energies fragmentation and substrate damage were observed. Clusters with elevated impact energy could be soft-landed via an argon buffer layer on the platinum substrate, which efficiently dissipated the kinetic energy. Nondestructive cluster deposition represents a promising method to produce monodispersed nanostructures at surfaces.

Optical absorption of small silver clusters: Agn, (n=4–22)

We present a joint theoretical and experimental investigation of the absorption spectra of silver clusters Ag(n) (4<or=n<or=22). The experimental spectra of clusters isolated in an Ar matrix are compared with the calculated ones in the framework of the time-dependent density functional theory. The analysis of the molecular transitions indicates that the s-electrons are responsible for the optical response of small clusters (n<or=8) while the d-electrons play a crucial role in the optical excitations for larger n values.

Fluorescence and excitation spectra of Ag4 in an argon matrix

We report the fluorescence and excitation spectra of size selected Ag4 deposited in an argon matrix. The main fluorescence peak for Ag4 is observed at 458 nm. The excitation spectrum for this emission is in good agreement with recent theoretical calculations and photodepletion experiments.

Ultraviolet-visible absorption of small silver clusters in neon: Agn (n = 1–9)

We present optical absorption and fluorescence spectra in the UV-visible range of size selected neutral Ag(n) clusters (n = 1-9) in solid neon. Rich and detailed optical spectra are found with linewidths as small as 50 meV. These spectra are compared to time dependent density functional theory implemented in the TURBOMOLE package. Excellent agreement between theory and experiment is achieved in particular for the dominant spectroscopic features at photon energies below 4.5 eV. This allows a clear attribution of the observed electronic transitions to specific isomers. Optical transitions associated to the s-electrons are concentrated in the energy range between 3 and 4 eV and well separated from transitions of the d-electrons. This is in contrast to the other coinage metals (Au and Cu) which show a strong coupling of the d-electrons.

UV–visible absorption of small gold clusters in neon: Aun (n = 1–5 and 7–9)

We present optical absorption spectra in the UV-visible range (1.5 eV < E < 6 eV) for mass selected neutral gold clusters Au(n) (n = 1-5 and 7-9) embedded in solid Ne at 7 K. The experimental spectra are compared with time-dependent density functional calculations. Electronic transitions are distributed over the whole energy range without any concentration of the oscillator strength in a small energy window, characteristic for the more s-like metals such as the alkalis or silver. Contrary to the case of silver and partly copper clusters, transitions issued from mainly d-type states are significantly involved in low energy transitions. The measured integrated cross section is smaller (<20%) than expected from a free-electron system, manifesting the strong screening of the s electrons due to the proximity of the s and d levels in gold.

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

We present optical absorption spectra in the UV-visible range (1.6 eV < ℏω < 5.5 eV) of mass selected neutral copper clusters Cu(n)(n = 1-9) embedded in a solid neon matrix at 7 K. The atom and the dimer have already been measured in neon matrices, while the absorption spectra for sizes between Cu(3) and Cu(9) are entirely (n = 6-9) or in great part new. They show a higher complexity and a larger number of transitions distributed over the whole energy range compared to similar sizes of silver clusters. The experimental spectra are compared to the time dependent density functional theory (TD-DFT) implemented in the TURBOMOLE package. The analysis indicates that for energies larger than 3 eV the transitions are mainly issued from d-type states; however, the TD-DFT scheme does not reproduce well the detailed structure of the absorption spectra. Below 3 eV the agreement for transitions issued from s-type states is better.

Hard and soft landing of mass selected Ag clusters on Pt(111)

Mass selected Ag/sub n/ clusters (n=1,7,19) from a secondary ion source have been deposited onto a Pt(111) substrate at low temperature. The surface and resulting cluster morphology have subsequently been characterized within the same UHV chamber by variable temperature STM as a function of cluster size, kinetic impact energy, and substrate temperature. The kinetic energy per cluster atom was found to be the decisive parameter for a controlled deposition. Noble gas buffer layers ( approximately=10 ML Ar), which were pre-adsorbed onto the surface at low temperatures, were found to efficiently dissipate the impact energy opening up the possibility of soft landing clusters with elevated kinetic energy.

Instability of Ag nanoparticles in SiO2 at ambient conditions

The room-temperature stability of nanometre-sized silver clusters in silica matrices has been investigated by following the temporal evolution of their surface plasmon absorption. Ag clusters in SiO2 were prepared by either annealing silica samples doped with atomic silver or by co-deposition of preformed clusters of defined size. Clusters were found to be unstable at ambient conditions on a timescale of days to weeks, independent of preparation conditions. The disappearance of the plasmonic resonance is explained by successive oxidation of the clusters even inside the matrix.

High sensitivity absorption measurement of small metal clusters embedded in an argon matrix

We describe a new UV-Vis absorption setup designed to measure with improved sensitivity the absorption spectra of size-selected small metal clusters embedded in rare gas matrices. We aim at studying the electronic structure of clusters of different materials and their interaction with the matrix. The design, construction, and performance of the setup are discussed and demonstrated on the absorption spectrum of Ag1 in argon. The absorption spectra for the two least abundant silver cluster ions produced by our cluster source in the size range of 1–39 atoms/cluster, namely, Ag6 and Ag10, are reported. A sensitivity increase of about 20 is demonstrated.

Matrix effects on the optical response of silver nanoclusters

We report absorption spectra for Ag(7), Ag(9), and Ag(11) in an argon matrix grown at a temperature of 28 K and compare them with previous spectra of the same species measured in matrices of argon grown at lower temperatures as well as in neon matrices. We discuss the discrepancies in the light of the matrix crystallinity and show that this leads to an understanding of the influence of the matrix on the optical response of small clusters.