Optical Activity of Metal Nanoclusters Deposited on Regular and Doped Oxide Supports from First-Principles Simulations

We report a computational study and analysis of the optical absorption processes of Ag20 and Au20 clusters deposited on the magnesium oxide (100) facet, both regular and including point defects. Ag20 and Au20 are taken as models of metal nanoparticles and their plasmonic response, MgO as a model of a simple oxide support. We consider oxide defects both on the oxygen anion framework (i.e., a neutral oxygen vacancy) and in the magnesium cation framework (i.e., replacing Mg++ with a transition metal: Cu++ or Co++). We relax the clusters’ geometries via Density-Functional Theory (DFT) and calculate the photo-absorption spectra via Time-Dependent DFT (TDDFT) simulations on the relaxed geometries. We find that the substrate/cluster interaction induces a broadening and a red-shift of the excited states of the clusters, phenomena that are enhanced by the presence of an oxygen vacancy and its localized excitations. The presence of a transition-metal dopant does not qualitatively affect the spectral profile. However, when it lies next to an oxygen vacancy for Ag20, it can strongly enhance th

Catalytic activity of Pt_(38) in the oxygen reduction reaction from first-principles simulations

The activity of truncated octahedral Pt_(38) clusters as a catalyst in the oxygen reduction reaction (ORR) is investigated via first-principles simulations. Three catalytic steps: O_2 dissociation (O_(2ads) → 2_O_(ads)), O hydration (O_(ads) + H_2O_(ads) → 2OH_(ads)), and H_2O formation (OH_(ads) + H_(ads) → H_2O_(ads)) are considered, in which all reactant species are co-adsorbed on the Pt_(38) cluster according to a Langmuir–Hinshelwood mechanism. The minimum structures and saddle points for these different steps are then calculated at the density-functional theory (DFT) level using a gradient-corrected exchange–correlation (xc-)functional and taking into account the effect of the solvent via a self-consistent continuum solvation model. Moreover, first-principles molecular dynamics (AIMD) simulations in which the H_2O solvent is explicitly described are performed to explore dynamic phenomena such as fast hydrogen transfer via meta-stable hydronium-type configurations and their possible role in ORR reaction paths. By comparing the present findings with previous results on the Pt(111) surface, it is shown that in such a nanometer-size cluster the rate-determining-step (rds) corresponds to H_2O formation, at variance with the extended surface in which O hydration was rate-determining, and that the overall reaction barrier is actually increased with respect to the extended system. This is in agreement with and rationalizes experimental results showing a decrease of ORR catalytic activity in the nanometer-size cluster range

Theoretical Analysis of a 2D Metallic/Semiconducting Transition-Metal Dichalcogenide NbS2//WSe2 Hybrid Interface

A first-principles theoretical study of a monolayer-thick lateral heterostructure (LH) joining two different transition metal dichalcogenides, NbS2 and WSe2, is reported. The NbS2//WSe2 LH can be considered a prototypical example of a metal (NbS2)/semiconductor (WSe2) 2D hybrid heterojunction. First, realistic atomistic models of the NbS2//WSe2 LH are generated and validated, its band structure is derived, and it is subjected to a fragment decomposition and electrostatic potential analysis to extract a simple but quantitative model of this interfacial system. Stoichiometric fluctuations models are also investigated and found not to alter the qualitative picture. Then, electron transport simulations are conducted and they are analyzed via band alignment analysis. It is concluded that the NbS2//WSe2 LH appears as a robust seamless in-plane 2D modular junction for potential use in optoelectronic devices going beyond the present miniaturization technology

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

Vertical Heterostructures between Transition-Metal Dichalcogenides -- A Theoretical Analysis of the NbS2_2/WSe2_2 junction

Low-dimensional metal-semiconductor vertical heterostructures (VH) are promising candidates in the search of electronic devices at the extreme limits of miniaturization. Within this line of research, here we present a theoretical/computational study of the NbS$_2$/WSe$_2$ metal-semiconductor vertical hetero-junction using density functional theory (DFT) and conductance simulations. We first construct atomistic models of the NbS$_2$/WSe$_2$ VH considering all the five possible stacking orientations at the interface, and we conduct DFT and quantum-mechanical (QM) scattering simulations to obtain information on band structure and transmission coefficients. We then carry out an analysis of the QM results in terms of electrostatic potential, fragment decomposition, and band alignment. The behavior of transmission expected from this analysis is in excellent agreement with, and thus fully rationalizes, the DFT results, and the peculiar double-peak profile of transmission. Finally, we use maximally localized Wannier functions, projected density of states (PDOS), and a simple analytic formula to predict and explain quantitatively the differences in transport in the case of epitaxial misorientation. Within the class of Transition-Metal Dichalcogenide systems, the NbS$_2$/WSe$_2$ vertical heterostructure exhibits a wide interval of finite transmission and a double-peak profile, features that could be exploited in applications.Comment: 22 pages main text, 11 pages supplementar

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, timedependent 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. Powerdependent 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 ( 12CH3 and 12Ph) revealed the important role played by the tail groups of ligands in collective oscillation. Also, steady-state and timeresolved absorption for Au36, Au44, and Au133 were studied to reveal the molecule-to-metal evolution of aromatic AuNMs. The optical gap and transient decay lifetimes decrease as the size increases

Peptide-Based Supramolecular Systems Chemistry

Peptide-based supramolecular systems chemistry seeks to mimic the ability of life forms to use conserved sets of building blocks and chemical reactions to achieve a bewildering array of functions. Building on the design principles for short peptide-based nanomaterials with properties, such as self-assembly, recognition, catalysis, and actuation, are increasingly available. Peptide-based supramolecular systems chemistry is starting to address the far greater challenge of systems-level design to access complex functions that emerge when multiple reactions and interactions are coordinated and integrated. We discuss key features relevant to systems-level design, including regulating supramolecular order and disorder, development of active and adaptive systems by considering kinetic and thermodynamic design aspects and combinatorial dynamic covalent and noncovalent interactions. Finally, we discuss how structural and dynamic design concepts, including preorganization and induced fit, are critical to the ability to develop adaptive materials with adaptive and tunable photonic, electronic, and catalytic properties. Finally, we highlight examples where multiple features are combined, resulting in chemical systems and materials that display adaptive properties that cannot be achieved without this level of integration

The atomistic origin of the extraordinary oxygen reduction activity of Pt_3Ni_7 fuel cell catalysts

Recently Debe et al. reported that Pt_3Ni_7 leads to extraordinary Oxygen Reduction Reaction (ORR) activity. However, several reports show that hardly any Ni remains in the layers of the catalysts close to the surface (“Pt-skin effect”). This paradox that Ni is essential to the high catalytic activity with the peak ORR activity at Pt_3Ni_7 while little or no Ni remains close to the surface is explained here using large-scale first-principles-based simulations. We make the radical assumption that processing Pt–Ni catalysts under ORR conditions would leach out all Ni accessible to the solvent. To simulate this process we use the ReaxFF reactive force field, starting with random alloy particles ranging from 50% Ni to 90% Ni and containing up to ~300 000 atoms, deleting the Ni atoms, and equilibrating the resulting structures. We find that the Pt_3Ni_7 case and a final particle radius around 7.5 nm lead to internal voids in communication with the exterior, doubling the external surface footprint, in fair agreement with experiment. Then we examine the surface character of these nanoporous systems and find that a prominent feature in the surface of the de-alloyed particles is a rhombic structure involving 4 surface atoms which is crystalline-like but under-coordinated. Using density-functional theory, we calculate the energy barriers of ORR steps on Pt nanoporous catalysts, focusing on the O_(ad)-hydration reaction (O_(ad) + H_2O_(ad) → OH_(ad) + OH_(ad)) but including the barriers of O_2 dissociation (O_(2ad) → O_(ad) + O_(ad)) and water formation (OH_(ad) + H_(ad) → H_2O_(ad)). We find that the reaction barrier for the O_(ad)-hydration rate-determining-step is reduced significantly on the de-alloyed surface sites compared to Pt(111). Moreover we find that these active sites are prevalent on the surface of particles de-alloyed from a Pt–Ni 30 : 70 initial composition. These simulations explain the peak in surface reactivity at Pt_3Ni_7, and provide a rational guide to use for further optimization of improved catalytic and nanoporous materials