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

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

Organosoluble Au₁₀₂(SPh)₄₄ Nanomolecules: Synthesis, Isolation, Compositional Assignment, Core Conversion, Optical Spectroscopy, Electrochemistry, and Theoretical Analysis

Characterization of <i>p</i>-mercaptobenzoic acid (p-MBA) protected Au₁₀₂(p-MBA)₄₄ nanomolecules has been so far limited by its water-soluble ligand system. In this work we report the first synthesis and isolation of thiolate-protected organosoluble Au₁₀₂(SPh-X)₄₄ nanomolecules via one-phase synthesis. Monodispersity of the nanomolecules was confirmed from matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS), and composition was determined from high-resolution electrospray ionization mass spectrometry (ESI-MS). For the first time we report the electrochemical behavior and temperature-dependent optical spectra of Au₁₀₂(SPh)₄₄. Theoretical simulations on the titled nanomolecules fully validate experimental data and demonstrate the role of electronic conjugation on optical properties

Atomistic Quantum Plasmonics of Gold Nanowire Arrays

The dielectric properties of a regular 2D array of Au nanowires are investigated using time-dependent density-functional theory employing a fully atomistic quantum description. Longitudinal modes produce a Drude-like peak in the infrared that is rather insensitive to geometrical parameters. Transverse modes, instead, give rise to a plasmonic peak in the optical region, which exhibits a nontrivial dependence on the spatial separation between the wires: a strong resonant enhancement and a shift from the optical to the far-infrared region is observed as the interwire distance is decreased, with the formation of “hot spots” in which induced field and charge distributions exhibit nondipolar shape and rapidly alternating quantum phase. The general character of this phenomenon is confirmed by its occurrence in Au nanoparticle arrays. Addition of ligand species in the hot spot region can lead to the appearance of new resonances due to strong coupling between plasmonic and molecular modes, as exemplified in a proof-of-concept case. This shows the possibilities of atomistic quantum plasmonics effects and subwavelength control of electromagnetic field intensity in properly engineered nanogaps

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

Structural Motifs of Bimetallic Pt_{101–<i>x</i>}Au_<i>x</i> Nanoclusters

The evolution of the structure of bimetallic Pt_{101–<i>x</i>}Au_<i>x</i> (<i>x</i> = 0–101) clusters is theoretically studied as a function of composition. The basin hopping method using the Gupta empirical potential (EP) is used to perform an exhaustive sampling of the potential energy surface (PES). Several highly symmetric morphologies such as Marks decahedra, incomplete icosahedra, two types of anti-Mackay-covered 5-fold structures, Leary tetrahedra and close-packed structures are identified and reoptimized at the first-principles density functional theory (DFT) level to take into account electronic effects. Alloyed configurations at very low Pt content and ubiquitous Pt­(core)­Au­(shell) segregated motifs with different morphology and core shape are found as the lowest energy structural motifs at the empirical potential level, with an appreciable influence of Pt concentration on the nanocluster structure and a strong competition between different structural motifs, especially in the region of the lowest values of mixing energy. At variance with these predictions, at the DFT level a core–shell crystalline motif (which is only marginally present as a global minimum at the Gupta level) becomes dominant over a broad range of compositions including pure particles. This shows the importance of adopting a combined DFT/empirical–potential investigation for third-row transition metal clusters, also in connection with the prediction of the catalytic properties of these systems

Global Minimum Pt₁₃M₂₀ (M = Ag, Au, Cu, Pd) Dodecahedral Core–Shell Clusters

In this work, we report finding dodecahedral core–shell structures as the putative global minima of Pt₁₃M₂₀ (M = Ag, Au, Cu, Pd) clusters by using the basin hopping method and the many-body Gupta model potential to model interatomic interactions. These nanoparticles consist of an icosahedral 13-atom platinum core encapsulated by a 20 metal-atom shell exhibiting a dodecahedral geometry (and <i>I</i>_<i>h</i> symmetry). The interaction between the icosahedral platinum core and the dodecahedral shell is analyzed in terms of the increase in volume of the icosahedral core, and the strength and stickiness of M–Pt and M–M interactions. Low-lying metastable isomers are also obtained. Local relaxations at the DFT level are performed to verify the energetic ordering and stability of the structures predicted by the Gupta potential finding that dodecahedral core–shell structures are indeed the putative global minima for Pt₁₃Ag₂₀ and Pt₁₃Pd₂₀, whereas decahedral structures are obtained as the minimum energy configurations for Pt₁₃Au₂₀ and Pt₁₃Cu₂₀ clusters

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

Work Function of Oxide Ultrathin Films on the Ag(100) Surface

Theoretical calculations of the work function of monolayer (ML) and bilayer (BL) oxide films on the Ag(100) surface are reported and analyzed as a function of the nature of the oxide for first-row transition metals. The contributions due to charge compression, charge transfer and rumpling are singled out. It is found that the presence of empty d-orbitals in the oxide metal can entail a charge flow from the Ag(100) surface to the oxide film which counteracts the decrease in the work function due to charge compression. This flow can also depend on the thickness of the film and be reduced in passing from ML to BL systems. A regular trend is observed along first-row transition metals, exhibiting a maximum for CuO, in which the charge flow to the oxide is so strong as to reverse the direction of rumpling. A simple protocol to estimate separately the contribution due to charge compression is discussed, and the difference between the work function of the bare metal surface and a Pauling-like electronegativity of the free oxide slabs is used as a descriptor quantity to predict the direction of charge transfer

Metal Tungstates at the Ultimate Two-Dimensional Limit: Fabrication of a CuWO₄ Nanophase

Metal tungstates (with general formula MWO₄) are functional materials with a high potential for a diverse set of applications ranging from low-dimensional magnetism to chemical sensing and photoelectrocatalytic water oxidation. For high level applications, nanoscale control of film growth is necessary, as well as a deeper understanding and characterization of materials properties at reduced dimensionality. We succeeded in fabricating and characterizing a two-dimensional (2-D) copper tungstate (CuWO₄). For the first time, the atomic structure of an ultrathin ternary oxide is fully unveiled. It corresponds to a CuWO₄ monolayer arranged in three sublayers with stacking O–W–O/Cu from the interface. The resulting bidimensional structure forms a robust framework with localized regions of anisotropic flexibility. Electronically it displays a reduced band gap and increased density of states close to the Fermi level with respect to the bulk compound. These unique features open a way for new applications in the field of photo- and electrocatalysis, while the proposed synthesis method represents a radically new and general approach toward the fabrication of 2-D ternary oxides