Optical properties of Cu nanoclusters supported on MgO(100)

Abstract

The vertical transitions of Cu atoms, dimers, and tetramers deposited on the MgO surface have been investigated by means of ab initio calculations based either on complete active space second-order perturbation theory or on time-dependent density functional theory. Three adsorption sites have been considered as representative of the complexity of the MgO surface: regular sites at flat (100) terraces, extended defects such as monoatomic steps, and point defects such as neutral oxygen vacancies (F or color centers). The optical properties of the supported Cu clusters have been compared with those of the corresponding gas-phase units. Upon deposition a substantial modification of the energy levels of the supported cluster is induced by the Pauli repulsion with the substrate. This causes shifts in the optical transitions going from free to supported clusters. The changes in cluster geometry induced by the substrate have a much smaller effect on the optical absorption bands. On F centers the presence of filled impurity levels in the band gap of MgO results in a strong mixing with the empty levels of the Cu atoms and clusters with consequent deep changes in the optical properties of the color centers. The results allow to interpret electron energy loss spectra of Cu atoms deposited on MgO thin films.

First-principles absorption spectra of Cun(n=2–20) clusters

“…59 The TDLDA overestimates the s → p transition energy by ∼ 10%, similar to those observed for Ag and Au. Our values are in reasonable agreement with the computed values reported by Del Vitto et al 43 who find 4s → 4p transition energies of 3.89 eV (B3LYP-TDDFT) and 4.03 eV (CASTP2) with an OS of f = 0.27 for the latter, and by Bosko et al 44 who find a transition energy of 4.12 eV with f = 0.15. We find the next strong TDLDA transition at 5.17 eV with f = 0.38.…”

“…41 More recently, Wang et al performed a detailed study of Cu 2 within the time-dependent density functional theory (TDDFT) using the hybrid B3LYP functional. 42 Optical properties of free and MgO(100)-supported Cu n clusters have been investigated by Del Vitto et al 43 for n = 1, 2, 4 using B3LYP-TDDFT and complete active space second-order perturbation theory (CASPT2); by Bosko et al 44 for n = 1 − 2, and by Huix-Rotllant et al 45 for n = 2 using TDDFT with the Becke and Perdew (BP86) functional. Recently, Zhou and Zeng performed a TDDFT study of Cu n (n = 3 − 9) clusters using the Amsterdam Density Functional code.…”

Density Functional Embedded Cluster Study of Cu4, Ag4 and Au4 Species Interacting with Oxygen Vacancies on the MgO(001) Surface

“…(There is no contradiction between the anionic character of the adsorbed metal tetramers and their rather limited distortion with respect to the gasphase structure because the bond lengths of free M 4 and M 4 À species are very close. [27] ) These charges of the adsorbates are even larger by absolute value than those which we calculated for the dimers of the same metals, bound to F s sites: À0.80, À0.83 and À0.73 e, respectively. [14] The difference seems to reflect the more limited capacity of smaller closed-shell metal entities to accommodate extra electron density.…”

“…The adsorption energies of the complexes Cu 4 /F s and Cu 4 /F s + were calculated at À305 and À220 kJ mol À1 , respectively; these values are to be compared with À191 kJ mol À1 for the system Cu 4 /O 2À . [15] Using the hybrid B3LYP method, Del Vitto et al [27] recently computed a similar geometry for Cu 4 adsorbed on an F s center at the MgOA C H T U N G T R E N N U N G (001) terrace. They obtained slightly longer Cu-Cu distances: 257 pm for the edge directly involved in the bonding with the surface, 246 pm for the opposite Cu-Cu edge.…”

“…However, various DF results showed that the energetic preference of two atomic species to form homonuclear dimers (reactions 1) on isolated neutral F s defect sites of the MgOA C H T U N G T R E N N U N G (001) surface (Cu, [14,27,31] Ag, [14] Au, [14,32] Pd [33] ) is not significantly larger or even smaller than that for the dimerization involving only regular MgOA C H T U N G T R E N N U N G (001) sites (Table 5). For M 1 /O 2À + M 1 /F s , we calculated reaction energies of À138 (Cu), À152 (Ag) and À204 kJ mol À1 (Au).…”