Optical Properties of Pt and Ag–Pt Nanoclusters from TDDFT Calculations: Plasmon Suppression by Pt Poisoning

The optical properties of alloyed Ag–Pt nanoclusters are theoretically investigated as a function of composition and chemical ordering via a time-dependent density-functional-theory (TDDFT) approach. Clusters with icosahedral structure ranging in size between 55 and 146 atoms are considered, large enough to start observing strong adsorption peaks related to surface plasmon resonances (SPR) in pure Ag systems. Strikingly it is found that even the modest Pt content here considered, ranging between 14% and 24%, is sufficient to substantially damp the optical response of these clusters. The effect is most disruptive when Pt atoms are scattered at the cluster surface, where the Ag SPR is mostly located, especially at the cluster apex, while the most intense residual peaks occur as Pt 5d → Ag 5p transitions at a Pt­(core)/Ag­(shell) interface and are strongly blue-shifted by 0.7–1.0 eV with respect to the analogous Ag peaks. Smaller Pt₁₃ and Pt₃₈ clusters are also studied for comparison, finding a nonplasmonic behavior but a strong involvement of Pt 5d orbitals in the optical response

Optical Properties of Silver Nanoshells from Time-Dependent Density Functional Theory Calculations

The absorption spectra of Ag monatomic nanoshells of size between 12 and 272 atoms with different shapes (icosahedra, cuboctahedra, and truncated octahedra) are theoretically studied via a time-dependent density functional theory approach and compared with previous results on compact nanoparticles of similar size and shape. Three main findings are drawn from this study: (a) Ag monatomic nanoshells possess absorption spectra exhibiting clear plasmonic features starting already with the 92-atom system; (b) the position of the absorption peaks moves toward lower energy as the radius of the nanoshells increases, with a blue shift of ≈0.2 eV for icosahedral with respect to cuboctahedral shells (also, the peak substantially gains in intensity with increasing size); (c) the absorption maxima are strongly red-shifted by 0.8–1.0 eV with respect to homologous compact arrangements. The red-shift phenomenon (c) is shown to be due not to a combination mode of internal and external surface plasmon resonances, as in thicker nanoshells, but rather to the relief of the charge compression by the underlying metallic layers on the resonating electrons. An analysis of the character of the electronic transitions mostly contributing to the absorption peaks finally provides information on the atomistic character of the corresponding modes

Extension of the Time-Dependent Density Functional Complex Polarizability Algorithm to Circular Dichroism: Implementation and Applications to Ag₈ and Au₃₈(SC₂H₄C₆H₅)₂₄

We detail the calculation of rotatory strengths as an extension of the complex polarizability algorithm of time dependent density functional theory (TDDFT) proposed in a recent publication (O. Baseggio, G. Fronzoni, M. Stener, <i>J. Chem. Phys.</i> <b>2015</b>, <i>143</i>, 024106). To demonstrate the generality and applicability of the proposed algorithm, we calculate the photoabsorption and circular dichroism (CD) spectra of Ag₈ (as a validation case), Au₃₈(SCH₃)₂₄, and Au₃₈(SCH₂CH₂Ph)₂₄ monolayer-protected gold clusters (as a system of great current interest for their optical properties). For Au₃₈(SCH₂CH₂Ph)₂₄, the computed CD spectrum agrees well with the experimental data from the literature. Furthermore, a comparison of the calculated CD spectra of the two thiolate-protected nanoclusters reveals that the most distinctive features of the spectra are rather insensitive to the nature of the thiolate tail groups, which, however, play a significant role in shaping optical and dichroic response of the systems, especially in the higher energy portion of the spectrum

Photoabsorption of Icosahedral Noble Metal Clusters: An Efficient TDDFT Approach to Large-Scale Systems

7noWe apply a recently developed time-dependent density functional theory (TDDFT) algorithm based on the complex dynamical polarizability to calculate the photoabsorption spectrum of the following series of closed-shell icosahedral clusters of increasing size (namely, [M13]5+, [M55]3−, [M147]−, and [M309]3+ with M = Ag, Au), focusing in particular on their plasmonic response. The new method is shown to be computationally very efficient: it simultaneously retains information on the excited-state wave function and provides a detailed analysis of the optical resonances, e.g., by employing the transition contribution map scheme. For silver clusters, a very intense plasmon resonance is found for [Ag55]3−, with strong coupling among low-energy single-particle configurations. At variance, for gold clusters we do not find a single strong plasmonic peak but rather many features of comparable intensity, with partial plasmonic behavior present only for the lowest-energy transitions. Notably, we also find a much greater sensitivity of the optical response of Ag clusters with respect to Au clusters to cluster charge, the exchange-correlation (xc) functional, and the basis set, as demonstrated via a detailed comparison between [Ag55]q and [Au55]q. The results of the TDDFT algorithm obtained with the complex dynamical polarizability are finally compared with those produced by alternative (real-time evolution or Lanczos) approaches, showing that, upon proper choice of numerical parameters, overall nearly quantitative agreement is achieved among all of the considered approaches, in keeping with their fundamental equivalence.partially_openopenBaseggio, Oscar; De Vetta, Martina; Fronzoni, Giovanna; Stener, Mauro; Sementa, Luca; Fortunelli, Alessandro; Calzolari, ArrigoBaseggio, Oscar; De Vetta, Martina; Fronzoni, Giovanna; Stener, Mauro; Sementa, Luca; Fortunelli, Alessandro; Calzolari, Arrig

Au₂₄(SAdm)₁₆ Nanomolecules: X‑ray Crystal Structure, Theoretical Analysis, Adaptability of Adamantane Ligands to Form Au₂₃(SAdm)₁₆ and Au₂₅(SAdm)₁₆, and Its Relation to Au₂₅(SR)₁₈

Here we present the crystal structure, experimental and theoretical characterization of a Au₂₄(SAdm)₁₆ nanomolecule. The composition was verified by X-ray crystallography and mass spectrometry, and its optical and electronic properties were investigated via experiments and first-principles calculations. Most importantly, the focus of this work is to demonstrate how the use of bulky thiolate ligands, such as adamantanethiol, versus the commonly studied phenylethanethiolate ligands leads to a great structural flexibility, where the metal core changes its shape from five-fold to crystalline-like motifs and can adapt to the formation of Au_24±1(SAdm)₁₆, namely, Au₂₃(SAdm)₁₆, Au₂₄(SAdm)₁₆, and Au₂₅(SAdm)₁₆. The basis for the construction of a thermodynamic phase diagram of Au nanomolecules in terms of ligands and solvent features is also outlined

Fe L‑Edge X‑ray Absorption Spectra of Fe(II) Polypyridyl Spin Crossover Complexes from Time-Dependent Density Functional Theory

L-edge near-edge X-ray fine structure spectroscopy (NEXAFS) has become a powerful tool to study the electronic structure and dynamics of metallo-organic and biological compounds in solution. Here, we present a series of density functional theory calculations of Fe L-edge NEXAFS for spin crossover (SCO) complexes within the time-dependent framework. Several key factors that control the L-edge excitations have been carefully examined using an Fe­(II) polypyridyl complex [Fe­(tren­(py)₃)]²⁺ (where tren­(py)₃ = tris­(2-pyridylmethyliminoethyl)­amine) as a model system. It is found that the electronic spectra of the low-spin (LS, singlet), intermediate-spin (IS, triplet), and high-spin (HS, quintet) states have distinct profiles. The relative energy positions, but not the spectral profiles, of different spin states are sensitive to the choice of the functionals. The inclusion of the vibronic coupling leads to almost no visible change in the resulting NEXAFS spectra because it is governed only by low-frequency modes of less than 500 cm^–1. With the help of the molecular dynamics sampling in acetonitrile at 300 K, our calculations reveal that the thermal motion can lead to a noticeable broadening of the spectra. The main peak position is strongly associated with the length of the Fe–N bond

Au₂₇₉(SR)₈₄: The Smallest Gold Thiolate Nanocrystal That Is Metallic and the Birth of Plasmon

We report a detailed study on the optical properties of Au₂₇₉(SR)₈₄ using steady-state and transient absorption measurements to probe its metallic nature, time-dependent density functional theory (TDDFT) studies to correlate the optical spectra, and density of states (DOS) to reveal the factors governing the origin of the collective surface plasmon resonance (SPR) oscillation. Au₂₇₉ is the smallest identified gold nanocrystal to exhibit SPR. Its optical absorption exhibits SPR at 510 nm. Power-dependent bleach recovery kinetics of Au₂₇₉ suggests that electron dynamics dominates its relaxation and it can support plasmon oscillations. Interestingly, TDDFT and DOS studies with different tail group residues (−CH₃ and −Ph) revealed the important role played by the tail groups of ligands in collective oscillation. Also, steady-state and time-resolved absorption for Au₃₆, Au₄₄, and Au₁₃₃ were studied to reveal the <i>molecule-to-metal</i> evolution of aromatic AuNMs. The optical gap and transient decay lifetimes decrease as the size increases

Principles of Optical Spectroscopy of Aromatic Alloy Nanomolecules: Au_{36–<i>x</i>}Ag<i>_x</i>(SPh‑<i>t</i>Bu)₂₄

Here, we report the synthesis and experimental and theoretical characterizations of Au_{36–<i>x</i>}Ag<i>_x</i>(SPh-<i>t</i>Bu)₂₄ alloy nanomolecule to atomic precision. By changing the incoming gold-to-silver metal ratio during the synthesis of crude mixture, up to eight silver atoms can be incorporated into Au₃₆(SPh-<i>t</i>Bu)₂₄, as theoretically confirmed and rationalized in terms of its core and staple structure. Tuning of optical response by Ag doping is strongly affected by aromatic conjugation and qualitatively different with respect to the aliphatic case, with a strikingly nonmonotonic behavior of absorption intensity in the low- and high-energy regions, in fair agreement with theoretical predictions, as rationalized via an original analysis tool: independent component mapping of oscillatory strength plots

Au₃₈(SPh)₂₄: Au₃₈ Protected with Aromatic Thiolate Ligands

Au₃₈(SR)₂₄ is one of the most extensively investigated gold nanomolecules along with Au₂₅(SR)₁₈ and Au₁₄₄(SR)₆₀. However, so far it has only been prepared using aliphatic-like ligands, where <i>R</i> = −SC₆H₁₃, −SC₁₂H₂₅ and −SCH₂CH₂Ph. Au₃₈(SCH₂CH₂Ph)₂₄ when reacted with HSPh undergoes core-size conversion to Au₃₆(SPh)₂₄, and existing literature suggests that Au₃₈(SPh)₂₄ cannot be synthesized. Here, contrary to prevailing knowledge, we demonstrate that Au₃₈(SPh)₂₄ can be prepared if the ligand exchanged conditions are optimized, under delicate conditions, without any formation of Au₃₆(SPh)₂₄. Conclusive evidence is presented in the form of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), electrospray ionization mass spectra (ESI-MS) characterization, and optical spectra of Au₃₈(SPh)₂₄ in a solid glass form showing distinct differences from that of Au₃₈(S-aliphatic)₂₄. Theoretical analysis confirms experimental assignment of the optical spectrum and shows that the stability of Au₃₈(SPh)₂₄ is not negligible with respect to that of its aliphatic analogous, and contains a significant component of ligand−ligand attractive interactions. Thus, while Au₃₈(SPh)₂₄ is stable at RT, it converts to Au₃₆(SPh)₂₄ either on prolonged etching (longer than 2 hours) at RT or when etched at 80 °C

Ligand-Enhanced Optical Response of Gold Nanomolecules and Its Fragment Projection Analysis: The Case of Au₃₀(SR)₁₈

Here we investigate via first-principles simulations the optical absorption spectra of three different Au₃₀(SR)₁₈ monolayer-protected clusters (MPC): Au₃₀(S^tBu)₁₈, Au₃₀(SPh)₁₈, and Au₃₀(SPh-<i>p</i>NO₂)₁₈. Au₃₀(S^tBu)₁₈ is known in the literature, and its crystal structure is available. In contrast, Au₃₀(SPh)₁₈ and Au₃₀(SPh-<i>p</i>NO₂)₁₈ are two species that have been designed by replacing the <i>tert</i>-butyl organic residues of Au₃₀(S^tBu)₁₈ with aromatic ones so as to investigate the effects of ligand replacement on the optical response of Au nanomolecules. By analogy to a previously studied Au₂₃(SR)₁₆^– anionic species, despite distinct differences in charge and chemical composition, a substantial ligand enhancement of the absorption intensity in the optical region is also obtained for the Au₃₀(SPh-<i>p</i>NO₂)₁₈ MPC. The use of conjugated aromatic ligands with properly chosen electron-withdrawing substituents and exhibiting steric hindrance so as to also achieve charge decompression at the surface is therefore demonstrated as a general approach to enhancing the MPC photoabsorption intensity in the optical region. Additionally, we here subject the ligand-enhancement phenomenon to a detailed analysis based on the fragment projection of electronic excited states and on induced transition densities, leading to a better understanding of the physical origin of this phenomenon, thus opening avenues to its more precise control and exploitation