Franck Rabilloud

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.

Structure and optical properties of core-shell bimetallic AgnNin clusters: Comparison with pure silver and nickel clusters

We present the structural, electronic, and optical properties of bimetallic Ag(n)Ni(n) (n<or=7) clusters investigated in the framework of the density functional theory (DFT) (DFT and time-dependent DFT). The structure of Ag(n)Ni(n) clusters is found to be governed essentially by the formation of a Ni-core surrounded by silver atoms. The cohesive energies and the ionization potentials are calculated. The UV-visible absorption spectra of bimetallic clusters are compared to those of pure silver and nickel clusters. An interpretation of spectroscopic patterns in terms of contribution from s- and d-type excitations is also given. In particular the d electrons of nickel atoms are found to play a crucial role in the optical transitions in Ni-rich systems.

Assessment of the performance of long-range-corrected density functionals for calculating the absorption spectra of silver clusters.

We assess the accuracy of several long-range-corrected (LC-) density functionals for the prediction of absorption spectra of silver clusters by time-dependent density functional theory. Several types of LC-functionals, with the exact Hartree-Fock exchange at long-range, are used: those applying the long-range correction to a standard GGA-type functional (LC-BP86, LC-ωPBE) or to a local meta-GGA functional (LC-M06L) and two global hybrid functionals (CAM-B3LYP and ωB97x). The spectra calculated with those density functionals are in good agreement with the recent accurate experimental measurements. We show that CAM-B3LYP and ωB97x are some of the most accurate functionals for evaluating the electronic excitation energies, while LC-M06L is more effective in reducing the occurrence of spurious states. The long-range correction appears to be essential in describing the absorption spectra of large clusters. The description of the plasmon-like band with LC functionals as transitions associated with excitations from s orbitals to s + p orbitals is in fairly good agreement with the classical interpretation as a collective excitation of valence s electrons.

Stability of alkali-encapsulating silicon cage clusters.

We report a computational study of the possibility to form alkali-encapsulating Si clusters A@Si(n) with n = 10 - 20. We predict and quantify the stability for lithium, sodium, and potassium atoms encapsulated in silicon cage. The structure and electronic properties are discussed. An electronic charge transfer from the alkali atom to the Si(n) cage is observed. The A@Si(n) cluster is formed of a positive charge located on the alkali surrounded by a negative one distributed on the whole Si cage. For each size the predicted stability of such structure is discussed and compared with that of surface-bound alkali isomers. The alkali-encapsulating Si clusters A@Si(n) are found to be stable but lying much higher in energy as compared to surface-bound alkali isomers.

UV−Visible Absorption Spectra of Small Platinum Carbonyl Complexes and Particles: A Density Functional Theory Study

We investigate the influence of a carbonyl coating on the UV-visible absorption of platinum complexes and particles with the TDDFT method. We first investigate the Chini clusters [Pt(3)(CO)(6)](n)(2-), n = 1-4, for which the absorption spectra are known. We show that PBE is realistic but displays convergence issues and that B3LYP overestimates intertriangle distances and absorption intensities. We discuss the structure of the spectra and the parallel vs perpendicular character of the transition dipole with respect to the stacking axis. We then investigate the spectra of model carbonylated platinum particles: [Pt(13)(CO)(12)](2-) (with only terminal carbonyls) and [Pt(13)(CO)(24)](2-) (with only bridging carbonyls). B3LYP proves much more realistic than in the case of Chini clusters. The influence of the morphology of the platinum particle and of the carbonyl coating on the absorption spectra is discussed. The results suggest an interpretation of a spectrum, recorded in a recent synthesis.

Time-dependent density functional study of UV-visible absorption spectra of small noble metal clusters (Cun, Agn, Aun, n = 2–9, 20)

The absorption UV-visible spectra of noble metal clusters Cun, Agn, Aun, n = 2–9 and 20 are investigated in the framework of the time-dependent density functional theory using the long-range corrected density functionals LC-M06L and CAM-B3LYP and high-quality Gaussian basis sets. Some calculations including the spin–orbit coupling are also presented. The contribution of the d electrons to the optical response was found to be lower than it was when a purely local exchange functional was used. Calculated spectra are compared with experimental ones for clusters embedded in a rare-gas matrix.

Description of plasmon-like band in silver clusters: The importance of the long-range Hartree-Fock exchange in time-dependent density-functional theory simulations

Absorption spectra of Ag20 and Ag55(q) (q = +1, -3) nanoclusters are investigated in the framework of the time-dependent density functional theory in order to analyse the role of the d electrons in plasmon-like band of silver clusters. The description of the plasmon-like band from calculations using density functionals containing an amount of Hartree-Fock exchange at long range, namely, hybrid and range-separated hybrid (RSH) density functionals, is in good agreement with the classical interpretation of the plasmon-like structure as a collective excitation of valence s-electrons. In contrast, using local or semi-local exchange functionals (generalized gradient approximations (GGAs) or meta-GGAs) leads to a strong overestimation of the role of d electrons in the plasmon-like band. The semi-local asymptotically corrected model potentials also describe the plasmon as mainly associated to d electrons, though calculated spectra are in fairly good agreement with those calculated using the RSH scheme. Our analysis shows that a portion of non-local exchange modifies the description of the plasmon-like band.

Structure, Stability, and Electronic and Magnetic Properties of VGen (n = 1–19) Clusters

We systematically study the equilibrium geometries and electronic and magnetic properties of Gen+1 and VGen (n = 1-19) clusters using the density functional theory approach within the generalized gradient approximation. Endohedral structures in which the vanadium atom is encapsulated inside a Gen cage are predicted to be favored for n ≥ 10. The dopant V atom in the Gen clusters has not an immediate effect on the stability of small germanium clusters (n < 6), but it largely contributes to strengthen the stability for n ≥ 7. Our study enhances the large stability of the VGe14 cluster, which presents an Oh symmetry cagelike geometry and a peculiar electronic structure in which the valence electrons of V and Ge atoms are delocalized and exhibit a shell structure associated with the quasi-spherical geometry. Consequently, this cluster is proposed to be a good candidate to be used as the building blocks for developing new materials. The cluster size dependence of the stability, the vertical ionization potentials, and electron affinities of Gen+1 and VGen are presented. Magnetic properties and the partial density of states of the most stable VGen clusters are also discussed.

Growth Behavior and Electronic Structure of Noble Metal-Doped Germanium Clusters

Structures, energetics, and electronic properties of noble metal-doped germanium (MGen with M = Cu, Ag, Au; n = 1-19) clusters are systematically investigated by using the density functional theory (DFT) approach. The endohedral structures in which the metal atom is encapsulated inside of a germanium cage appear at n = 10 when the dopant is Cu and n = 12 for M = Ag and Au. While Cu doping enhances the stability of the corresponding germanium frame, the binding energies of AgGen and AuGen are always lower than those of pure germanium clusters. Our results highlight the great stability of the CuGe10 cluster in a D4d structure and, to a lesser extent, that of AgGe15 and AuGe15, which exhibits a hollow cage-like geometry. The sphere-type geometries obtained for n = 10-15 present a peculiar electronic structure in which the valence electrons of the noble metal and Ge atoms are delocalized and exhibit a shell structure associated with the quasi-spherical geometry. It is found that the coinage metal is able to give both s- and d-type electrons to be reorganized together with the valence electrons of Ge atoms through a pooling of electrons. The cluster size dependence of the stability, the frontier orbital energy gap, the vertical ionization potentials, and electron affinities are given.