Theoretical study of the stability, structure, and optical spectra of small silver clusters and their formation using density functional theory

DFT Study of the Adsorption and SERS of Pyridine on M10N10 (M, N = Cu, Ag) Tetrahedral Clusters

This work presents a theoretical detailed analysis of the surface-enhanced Raman spectroscopy (SERS) of the pyridine-M10N10 (M, N = Ag, Cu) tetrahedral (Td) clusters considering two binding positions: vertex (V) and surface (S). In addition to the well-known monometallic Td structure, we added two different bimetallic Ag-Cu compositions, named Td1 and Td2 geometries. Density functional methodology with the use of BP86 and CAM-B3LYP exchange-correlation functionals (XCs) and LANL2DZ pseudopotential has been employed for analyzing the electronic structure and geometries, the chemical static (CHEM), and resonant Raman mechanisms (RR): charge transfer RR-CT and intracluster excitation RR-CR. The static CHEM mechanism shows an increase in the enhancement factors (EFs) of Py-V concerning Py-S positions, which can also be distinguished by the averaged adsorption energies and bond polarizabilities. The static SERS response for Cu-Py-V junction is from 5 to 10 times greater than Ag-Py-V EFs and up to 28 times greater than Py-S complexes. For the static Raman, we found that the analyses of ν8a and ν1 normal modes are related to the EF changes and allow us to distinguish V from S complexes. The TDDFT calculations show striking differences between BP86 and CAM-B3LYP XCs analyzed spectra, and CAM-B3LYP granted a clear distinction between V and S for the location of CT-type transitions. In addition, important differences were obtained from the analysis of the charge transfer excitations between both XCs. Resonant Raman calculations evidenced significant enhancements for RR-CT and RR-CR as compared to the static enhancements, and RR-CT can be distinguished from the RR-CR mechanism, while specific normal modes help to differentiate the vertex from the surface Py-junction. Bimetallic Ag-Cu nanostructures represent promising choices for SERS substrates, showing EFs higher than those of monometallic Ag.

Properties of Naked Silver Clusters with Up to 100 Atoms as Found with Embedded-Atom and Density-Functional Calculations

The structural and energetic properties of small silver clusters Agn with n = 2-100 atoms are reported. For n = 2-100 the embedded atom model for the calculation of the total energy of a given structure in combination with the basin-hopping search strategy for an unbiased structure optimization has been used to identify the energies and structures of the three energetically lowest-lying isomers. These optimized structures for n = 2-11 were subsequently studied further through density-functional-theory calculations. These calculations provide additional information on the electronic properties of the clusters that is lacking in the embedded-atom calculations. Thereby, also quantities related to the catalytic performance of the clusters are studied. The calculated properties in comparison to other available theoretical and experimental data show a good agreement. Previously unidentified magic (i.e., particularly stable) clusters have been found for n>80. In order to obtain a more detailed understanding of the structural properties of the clusters, various descriptors are used. Thereby, the silver clusters are compared to other noble metals and show some similarities to both copper and nickel systems, and also growth patterns have been identified. All vibrational frequencies of all the clusters have been calculated for the first time, and here we focus on the highest and lowest frequencies. Structural effects on the calculated frequencies were considered.

Amphiphilic Silver Nanoparticles for Inkjet-Printable Conductive Inks

The large-scale manufacturing of flexible electronics is nowadays based on inkjet printing technology using specially formulated conductive inks, but achieving adequate wetting of different surfaces remains a challenge. In this work, the development of a silver nanoparticle-based functional ink for printing on flexible paper and plastic substrates is demonstrated. Amphiphilic silver nanoparticles with narrow particle size distribution and good dispersibility were prepared via a two-step wet chemical synthesis procedure. First, silver nanoparticles capped with poly(acrylic acid) were prepared, followed by an amidation reaction with 3-morpholynopropylamine (MPA) to increase their lipophilicity. Density functional theory (DFT) calculations were performed to study the interactions between the particles and the dispersion medium in detail. The amphiphilic nanoparticles were dispersed in solvents of different polarity and their physicochemical and rheological properties were determined. A stable ink containing 10 wt% amphiphilic silver nanoparticles was formulated and inkjet-printed on different surfaces, followed by intense pulsed light (IPL) sintering. Low sheet resistances of 3.85 Ω sq^-1, 0.57 Ω sq^-1 and 19.7 Ω sq^-1 were obtained for the paper, coated poly(ethylene terephthalate) (PET) and uncoated polyimide (PI) flexible substrates, respectively. Application of the nanoparticle ink for printed electronics was demonstrated via a simple flexible LED circuit.

Thin Films of Nonlinear Metallic Amorphous Composites

We studied the nonlinear optical response of metallic amorphous composite layers in terms of a self-phase-modulated, third-order Kerr nonlinearity. A nonlinear effective medium theory was used to describe low densities of gold and iridium nanoparticles embedded in an equally nonlinear host material. The fill fraction strongly influences the effective nonlinear susceptibility of the materials, increasing it by orders of magnitude in the case of gold due to localized surface plasmonic resonances. The enhancement of the nonlinear strength in amorphous composites with respect to the bulk material has an upper limit in metallic composites as dominating absorption effects take over at higher fill factors. Both saturated and induced absorption in the thin films of amorphous composites were observed depending on the selected frequency and relative position to the resonant frequency of electron excitation in the metallic inclusions. We demonstrated the depths to which thin films are affected by nonlinear enhancement effects.

An Algorithmic Approach Based on Data Trees and Genetic Algorithms to Understanding Charged and Neutral Metal Nanocluster Growth

Metal nanocluster growth pathways are challenging to study due to the number of possible species involved and the branching nature of possible mechanisms. We present a data tree-based approach to mapping these reaction pathways based on structures and energies obtained from Density Functional Theory (DFT) computations and including positive, negative, and neutral clusters in a continuum solvent of water. We also develop a genetic algorithm to study the relative stability of the clusters, which we combine with the data tree method to determine the effects that favorable decomposition reactions have on the reaction pathways. Introducing data tree pruning based on the exothermicity of each reaction, and including the first and second most important paths for each cluster up to five atoms including positive, neutral, and negative clusters, is then implemented to determine the cluster growth paths. These most favorable Ag-Ag reaction pathways are in agreement with more limited prior theoretical and experimental results, but they provide more systematic results that include predictions about the importance of clusters not previously identified. A key feature of the data tree approach is that it provides a comprehensive sampling of possible clusters, but without needing to generate the entire reaction tree. Additionally, the data tree-based approach allows for flexibility in the analysis based on restricting reactant charge or size, selecting a starting species to mimic an experimental precursor, and choosing a maximum allowable final cluster size or charge of interest.