Second-Order Nonlinear Optical Scattering Properties of Phosphine-Protected Au20 Clusters

Insights into the nonlinear optical (NLO) response of pure Aum (2 ≥ m ≤ 7) and copper-doped Au m -xCu x clusters

A series of small pure Au m (2 ≥ m ≤ 7) and copper-doped Au m-x Cu x clusters was evaluated by density functional theory (DFT) at the CAM-B3LYP/LANL2DZ level for their geometric, electronic, and nonlinear optical (NLO) properties. The charge transfer for the Au cluster significantly improved by reducing the HOMO-LUMO energy gap from 3.67 eV to 0.91 eV after doping with Cu atoms. The doping of Cu also showed noteworthy impacts on other optical and NLO properties, including a decrease in the excitation energy and increase in the dipole moment and oscillator strength. Furthermore, changes in the linear isotropic and anisotropic polarizabilities (α iso and α aniso) and first and second NLO hyperpolarizabilities (β static, γ static) were also observed in the pure and Cu-doped clusters, which enhanced the NLO response. The nonlinear optical properties of the clusters were evaluated by calculating the static and frequency dependent second- and third-order NLO polarizabilities at 1064 nm wavelength. Among all the doped structures, the Au3Cu1 cluster showed the largest static first hyperpolarizability of β (total) = 4.73 × 103 au, while the Au1Cu6 cluster showed frequency dependent first hyperpolarizability of β (-2w;w,w) = 1.26 × 106 au. Besides this, large static and frequency-dependent second hyperpolarizability values of 6.30 × 105 au and 1.05 × 10 au were exhibited by Cu7 and Au1Cu6, respectively. This study offers an effective approach to design high-performance NLO materials utilizing mixed metal clusters which might have broad applications in the fields of optoelectronics and electronics.

Toward Solution Syntheses of the Tetrahedral Au20 Pyramid and Atomically Precise Gold Nanoclusters with Uncoordinated Sites

A long-standing objective of cluster science is to discover highly stable clusters and to use them as models for catalysts and building blocks for cluster-assembled materials. The discovery of catalytic properties of gold nanoparticles (AuNPs) has stimulated wide interests in gaseous size-selected gold clusters. Ligand-protected AuNPs have also been extensively investigated to probe their size-dependent catalytic and optical properties. However, the need to remove ligands can introduce uncertainties in both the structures and sizes of ligand-protected AuNPs for catalytic applications. Ideal model catalysts should be atomically precise AuNPs with well-defined structures and uncoordinated surface sites as in situ active centers. The tetrahedral ( T_d) Au₂₀ pyramidal cluster, discovered to be highly stable in the gas phase, provided a unique opportunity for such an ideal model system. The T_d-Au₂₀ consists of four Au(111) faces with all its atoms on the surface. Bulk synthesis of T_d-Au₂₀ with appropriate ligands would allow its catalytic and optical properties to be investigated and harnessed. The different types of its surface atoms would allow site-specific chemistry to be exploited. It was hypothesized that if the four corner atoms of T_d-Au₂₀ were coordinated by ligands the cluster would still contain 16 uncoordinated surface sites as potential in situ catalytically active centers. Phosphine ligands were deemed to be suitable for the synthesis of T_d-Au₂₀ to maintain the integrity of its pyramidal structure. Triphenyl-phosphine-protected T_d-Au₂₀ was first observed in solution, and its stability was confirmed both experimentally and theoretically. To enhance the synthetic yield, bidentate diphosphine ligands [(Ph)₂P(CH₂) _nP(Ph)₂ or L ⁿ] with different chain lengths were explored. It was hypothesized that diphosphine ligands with the right chain length might preferentially coordinate to the T_d-Au₂₀. Promising evidence was initially obtained by the formation of the undecagold by the L³ ligand. When the L⁸ diphosphine ligand was used, a remarkable Au₂₂ nanocluster with eight uncoordinated Au sites, Au₂₂(L⁸)₆, was synthesized. With a tetraphosphine-ligand (PP₃), a new Au₂₀ nanocluster, [Au₂₀(PP₃)₄]Cl₄, was isolated with high yield. The crystal structure of the new Au₂₀ core did not reveal the expected pyramid but rather an intrinsically chiral gold core. The surface of the new chiral-Au₂₀ was fully coordinated, and it was found to be highly stable chemically. The Au₂₂(L⁸)₆ nanocluster represents the first and only gold core with uncoordinated gold atoms, providing potentially eight in situ catalytically active sites. The Au₂₂ nanoclusters dispersed on oxide supports were found to catalyze CO oxidation and activate H₂ without ligand removal. With further understanding about the formation mechanisms of gold nanoclusters in solution, it is conceivable that T_d-Au₂₀ can be eventually synthesized, allowing its novel catalytic and optical properties to be explored. More excitingly, it is possible that a whole family of new atomically precise gold nanoclusters can be created with different phosphine ligands.

Observation of self-polarization in BSA protected Au20 clusters

Bovine serum albumin (BSA)-protected gold clusters (atomicity ∼ 20), prepared using a wet chemical route, show strong dipolar radiative transition with a gap energy of 1.93 eV due to the high oscillator strength, as confirmed by the emission studies. Self-arrangement of the clusters with fixed atomicity yields a low dispersive dielectric and electric self-polarization nature. The electrical hysteresis loop measurements returned a remanent polarization of 0.05 μC cm^-2, which can be correlated with the dipolar orientation (activation energy ∼ 45.32 meV), originating from the structure-dependent deformation of the charge density.

Resonance Enhancement of Nonlinear Optical Scattering in Monolayer-Protected Gold Clusters

Monolayer-protected metal clusters (MPCs) have recently gained significant research interest, since they are promising candidates for various applications in bioimaging and catalysis. Besides this, MPCs promise to aid in understanding the evolution of the metallic state from bottom-up principles. MPCs can be prepared with atomic precision, and their nonscalable properties (indicating molecule-like behavior) have been studied with a variety of techniques both theoretically and experimentally. Here, we present spectrally resolved second-order nonlinear optical scattering experiments on thiolate-protected gold clusters (Au₁₃₀(SR)₅₀, Au₁₄₄(SR)₆₀, and Au₅₀₀(SR)₁₂₀). The three clusters share common resonance enhancement around 490 nm, which is ascribed to an interband transition. This indicates emerging metal-like properties, and we tentatively assign the onset of metal-like behavior somewhere between 102 and 130 gold atoms.

Au10(SG)10: A Chiral Gold Catenane Nanocluster with Zero Confined Electrons. Optical Properties and First-Principles Theoretical Analysis

We report facile synthesis of the Au₁₀(SG)₁₀ nanoclusters, where SG stands for glutathione, found to be promising as a new class of radiosensitizers for cancer radiotherapy. The homoleptic catenane structure with two Au₅SG₅ interconnected rings, among different isomer structures, gives the best agreement between theoretical and experimental optical spectra and XRD patterns. This catenane structure exhibits a centrosymmetry-broken structure, resulting in enhanced second harmonic response and new characteristic circular dichroism signals in the spectral region of 250-400 nm. This is the first determination of the nonlinear optical properties of a ligated cluster with an equal Au-to-ligand ratio, thus without a metallic core and therefore zero confined electrons. Insight into the nonlinear and chiroptical efficiencies arising from interplay between structural and electronic properties is provided by the TD-DFT approach.