Optical Properties of Monolayer-Protected Aluminum Clusters: Time-Dependent Density Functional Theory Study

Abstract

We examine the electronic and optical properties of experimentally known monolayer-protected aluminum clusters Al 4 (C 5 H 5 ) 4 , Al 50 (C 5 Me 5 ) 12 , and Al 69 (N(SiMe 3 ) 2 ) 18 3− using time-dependent density functional theory. By comparing Al 4 (C 5 H 5 ) 4 and the theoretical Al 4 (N(SiMe 3 ) 2 ) 4 cluster, we observe significant changes in the optical absorption spectra caused by different hybridization between metal core and ligands. Using these initial observations, we explain the calculated spectra of Al 50 (C 5 Me 5 ) 12 and Al 69 (N(SiMe 3 ) 2 ) 18 3− . Al 50 (C 5 Me 5 ) 12 shows a structured spectrum with clear regions of low-intensity core-to-core transitions followed by high-intensity ligand-to-core transitions due to its high symmetry and π-bonding to the Cp ligands. The spectrum of Al 69 (N(SiMe 3 ) 2 ) 18 3− is rather featureless as the core-to-core and ligand-to-core regions partially merge because of the lower symmetry found in the metal core and differences in the ligand−core hybridization. Though there are minimal features in the spectra, the most intense features are identified as excitations from ligand states to metal core states. We show that our calculated absorption spectrum for Al 69 (N(SiMe 3 ) 2 ) 18 3− agrees with recent experimental results for N(SiMe 3 ) 2 -protected Al nanoclusters with an average diameter of 1.5 nm. ■ INTRODUCTIONAtomically precise, monolayer-protected (MP) noble metal clusters with diameters of <2 nm have received a great deal of attention because of their unique chemical and physical properties with potential applications, for example, in catalysis, 1−3 fuel cells, 4 and biosensors. 5 Specifically, monolayer-protected clusters (MPCs) of gold have been extensively studied both experimentally and theoretically (for reviews, see refs 6 and 7). For many thiolate-protected gold systems, the electronic structure has been explained in the framework of the so-called superatom complex (SAC) model. 8 The SAC model, first used to explain the stability of the Au 102 (p-MBA) 44 cluster, 8−10 takes into account the number of delocalized ("itinerant") electrons, the effect of the ligands, and the charge of the cluster by using the simple formulawhere N A is the total number of metal atoms, ν A is the valence of the metal atom, M L is the number of electron-withdrawing ligands, w L is the number of electrons withdrawn per ligand, and z is the charge of the cluster. 11 The resultant electronic shell, similar to those found within the jellium model for metallic systems, 12 provides a series of magic numbers (i.e., 2, 8, 18, 20, 34, 40, ...) corresponding to electronic shells (i.e., 1S 2 1P 6 1D 10 2S 2 1F 14 2P 6 ...) similar to those of atoms on the periodic table. Following Au 102 (p-MBA) 44 , many thiolate monolayer-protected systems have used this model, such as Au 67 (2-PET) 35 2− (n e = 34) 13 and Au 25 (2-PET) 18 − (n e = 8). 14 Further, various studies have illustrated that the electronic properties can be tuned through either doping the metal core with other metals...

Ligand-Dependent Colloidal Stability Controls the Growth of Aluminum Nanocrystals

“…Time-dependent density functional theory (TDDFT) calculations of the electronic absorption spectrum of Ti 3+ (OiPr) 4 -AlH 3 in dioxane have an absorption edge at ∼350 nm (Figure S9), while TDDFT calculations of the optical properties of monolayer-protected Al clusters indicate they have absorption edges around ∼350 nm that increase as the Al clusters become larger. 50 From these calculations, we conclude the initially observed broad spectra reflect the dynamic mixture of Ti 3+ (OiPr) 4 -AlH 3 complexes and Ti− Al−H clusters present at the beginning of the reaction. After ∼10 min, production of plasmonic Al NCs was evident from the increase in extinction at ∼300 nm due to the growing number of light-scattering particles.…”

“…After an aliquot of DMEAA in toluene was injected into the cuvette, the reaction immediately turned golden brown and displayed broad absorbance across the visible and UV with a shoulder at ∼350 nm. Time-dependent density functional theory (TDDFT) calculations of the electronic absorption spectrum of Ti 3+ (OiPr) 4 -AlH 3 in dioxane have an absorption edge at ∼350 nm (Figure S9), while TDDFT calculations of the optical properties of monolayer-protected Al clusters indicate they have absorption edges around ∼350 nm that increase as the Al clusters become larger . From these calculations, we conclude the initially observed broad spectra reflect the dynamic mixture of Ti 3+ (OiPr) 4 -AlH 3 complexes and Ti–Al–H clusters present at the beginning of the reaction.…”

Theoretical Study of Tetrahydrofuran-Stabilized Al13 Superatom Cluster

“…Moreover, the stabilization energy derived from 1P superatomic orbital is larger than that from 1S (Figure 6B), which is quite different from EP-bonding Al 13 cluster where the stabilization energies dominantly derive from 1S or 1D orbitals. 29 Similarly, by checking the energy diagram and features of MOs, we recognized that the stability of Al 13 (THF) 2 also originates from overlaps between superatomic orbitals (1S, 1P) of Al 13 and MOs (19,11) of THF (Figure 7 and Figures S10− 8d). Analogy to bonding and antibonding molecular orbitals, the bonding-type donor−acceptor overlap is probably more favorable for the charge transfer between moieties than the antibonding overlap.…”

“…As the most abundant metal in the Earth’s crust, aluminum is remarkable for its low density and ability to resist corrosion due to surface passivation, serving as vital component materials in aerospace industry, transportation, and building industry. Nanoscale aluminum structures have been found being able to expand the metal’s performance and applications in such fields as superconducting spintronics, − plasmonics, − optics, − and so forth. Obtaining stable clusters of aluminum is critical to tune its performance at the atomic-precise level.…”

[PtZn2Ge18(Hyp)8] (Hyp = Si(SiMe3)3): A Neutral Polynuclear Chain Compound with Ge9(Hyp)3 Units

“…It is interesting to note the electron density for various electron transitions from occupied to unoccupied KS-states. While it may be intuitive to describe transitions being from metal-to-ligand (MLCT) or even (metal) core-to-ligand as in the cases of organometallics and larger metalloid nanoparticles, 18 this description may not fit the current system. Compound 1 can be viewed as a metalloid assembly with three distinct parts: (1) Ge 9 Hyp 3 , (2) Zn−R, and (3) a linking atom (Pt).…”