Optical Properties of Pt and AgâPt Nanoclusters from TDDFT

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Optical Properties of Pt and Ag-Pt Nanoclusters from TDDFT Calculations: Plasmon Suppression by Pt Poisoning Giovanni Barcaro, Luca Sementa, Alessandro Fortunelli, and Mauro Stener J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp508824w • Publication Date (Web): 10 Nov 2014 Downloaded from http://pubs.acs.org on November 16, 2014

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The Journal of Physical Chemistry

Optical properties of Pt and Ag-Pt nanoclusters from TDDFT calculations: plasmon suppression by Pt poisoning Giovanni Barcaro1, Luca Sementa1, Alessandro Fortunelli1,* and Mauro Stener2,3,*

1

CNR-ICCOM & IPCF, Consiglio Nazionale delle Ricerche, via G. Moruzzi 1, 56124, Pisa, Italy

2

Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Trieste, via L. Giorgieri 1, I34127, Trieste, Italy 3

Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali, INSTM, Unità di Trieste

Abstract 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 non-plasmonic behaviour but a strong involvement of Pt 5d orbitals in the optical response.

* Corresponding authors’ email: [email protected]; [email protected]

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Introduction

The interest for the optical properties of metal nanoparticles has recently experienced a fast growing, both experimentally and theoretically.1 The reason is that such systems are characterized by unique optical properties, with potential applications in several fields, such as opto-electronic devices, thermal treatment of cancer, “cell imaging”, i.e. colorimetric probes for DNA detection, catalysis, and magnetism. Such properties are dominated by a strong absorption band which, for sufficiently large size of the nanoparticle, falls in the visible region.1 This feature is very sensitive to specific characteristics of the sample, namely the particle size, its shape, the nature of the stabilizing ligand and the nature of the solvent or the medium in which the nanoparticles are dispersed. For bimetallic systems, also the chemical composition and the chemical ordering play an important role with respect to photoabsorption properties.2-4 The strong absorption band is generally referred as the Surface Plasmon Resonance5 (SPR) and is interpreted as a collective excitation of the conduction electrons of the metal particle. Due to the importance of the SPR as a leading nanoparticle optical property, various theoretical models have been proposed with the aim to simulate SPR and rationalize the experimental findings, with the ambitious ultimate goal of performing a computational design of novel nanostructured materials with specific optical properties. In this context, the very first and widely used approach is the Mie theory, which treats the light scattering by a conducting sphere in a classical way. If nanoparticles with generic shapes are considered the Mie theory can be extended via numerical techniques such as electromagnetic Finite Differences Time Domain (FTTD)6 or Discrete Dipole Approximation (DDA)7. A first improvement beyond a classical electrodynamics theory is represented by the Time Dependent Local Density Approximation (TDLDA) implemented within the jellium model,8,9 in which the metal nanoparticle is modelled as a background positive charge density describing the average nuclei positions. Also in this case the jellium model neglects the discrete atomic structure of the cluster, but the TDLDA method gives a quantum mechanical description of the electron excitations of the model system: the SPR is properly described as a collective excitation of the electrons. Moreover, the TDLDA jellium model offers a practical tool to consider more specific effects which can be included in the method, namely the effect of an embedding medium to simulate, for example, solvent effects.10,11 To further improve the theoretical description, it is necessary to describe explicitly the atomic structure of the cluster, and one natural choice is represented by the Time Dependent Density Functional Theory (TDDFT) formalism implemented in the conventional quantum chemical Linear ACS Paragon Plus Environment

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Combination of Atomic Orbital (LCAO) method. This choice joins the reliability of modern TDDFT approaches to describe electron excitations, with the accuracy of the LCAO implementation which avoids the drastic approximation of the effective potential in the jellium approach. In fact the jellium model is quite crude and it has been thanks to more accurate numerical schemes where the discrete atomic structure of the clusters is explicitly considered, like in the LCAO of quantum chemistry and plane waves methods in solid state physics, that has become possible to obtain more accurate results. Very recently optical spectra of metal clusters protected by ligands have been calculated by the TDDFT method using finite basis set of contracted Gaussiantype orbitals with clusters size up to 314 gold atoms.12 Of course the LCAO-TDDFT method is computationally much more demanding than jellium methods, however it allows one to understand in detail the electron excitation process and thus represents a valid instrument to help the design of new nanoparticles with specific optical properties. Computational limitations for large clusters can be overcome e.g. 13-15 by considering high-symmetry (octahedral, icosahedral, etc.) “magic” clusters: since the computer code employed in present work (the ADF16,17 program) is able to exploit point group symmetry in calculations, it becomes possible to routinely treat clusters up to 200 atoms. Alternative strategies are based e.g. on real-time propagation or the Sternheimer method.18,19 An atomistic description of the clusters opens a number of appealing opportunities. For example, one can investigate binary particles, and understand in detail how the SPR is modified when two or more elements are mixed as a function of the distribution of the atomic species within the structural framework.19-21 This is especially important when the two components have quite different optical characteristics such as when mixing a noble and a transition metal (Au-Fe, Ag-Pt, Ag-Ni, etc.) in magneto-optics applications,22 so that a linear interpolation of the optical response of the separate components represents a poor approximation. However to the best of our knowledge investigations in this field have been very limited, with the exception of Ag-Ni and sizes up to 13 atoms.23 In present work we deal with one of such cases, and employ a TDDFT atomistic description16,17 combined with the exploitation of point-group-symmetry in high-symmetry clusters13-15 to study the adsorption spectra of Ag-Pt clusters in the size range 55-146 atoms, which is large enough to start observing SPR peaks. We are then able to analyze in detail how the occupied and virtual electronic states of the alloy particle change depending on the position of Ag and Pt in the cluster, and to generalize this analysis to draw principles which can be useful to interpret experimental results. Experimentally it has been found in fact that while pure Ag clusters are known to display strong SPR, bimetallic Ag-Pt nanoclusters do not show any SPR band, unless very large cluster size (with cluster diameter around 10 nm) is reached,24 but the true origin of this ACS Paragon Plus Environment

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behavior is so far unknown. Apart from clearly demonstrating that the optical behavior of a bimetallic alloy is not correlated in a simple way with that of its pure constituents, these findings show that there is still much to understand in the field of the optical properties of bimetallic nanoparticles, despite the substantial amount of experimental work devoted especially to the Ag-Au system25 or systems involving silver or gold26 and indium.27 We hope that the present analysis will contribute to make progress in this direction. The article is organized as follows. In Sec. 2, the theoretical approach is described in detail. Results and discussion are presented in Sec. 3, while conclusions are summarized in Sec. 4.


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2. Theoretical method

A Scalar Relativistic Self-Consistent Field (SCF) Kohn-Sham (KS) formalism has been employed to describe the cluster electronic structure at the Density Functional Theory (DFT) level. Relativistic effects have been included at the Scalar-Relativistic Zeroth-Order Regular Approximation (ZORA) level28 using the ADF suite of programs16,17. Ag, Pt, and mixed Ag–Pt bimetallic nanoclusters with optimized geometries have been considered. For geometry optimization, the double zeta (DZ) ZORA basis set of Slater type orbitals (STO) included in the ADF database was employed, with frozen core (FC) up to 4f and 4p shells for Pt and Ag, respectively. The basis set for Pt contains two STO functions for each of the 5s, 6s, 5d, and 5p shells as well as one STO function for the 6p shell, while for Ag the basis set consists of two STO functions for each of the 5s and 4d shells as well as one STO function for the 5p shell. The local density approximation (LDA) was used as the exchange-correlation (xc-) functional in the geometry optimizations, with the VWN parametrization.29 Optical spectra have been calculated at the TDDFT level, which involves solving the following eigenvalue equation30:

ΩFI = ω I2 FI

(1)

where Ω is a four index matrix with elements Ω iaσ , jbτ , the indices consisting of products of occupied-virtual (ia and jb) KS orbitals, while σ and τ refer to the spin variable. The eigenvalues

ω I2 correspond to squared excitation energies while the oscillator strengths are extracted from the eigenvectors FI . The Ω-matrix elements can be formulated in terms of KS eigenvalues (ε) and the coupling matrix K (built employing the KS orbitals), in the present work the Adiabatic Local Density Approximation (ALDA)31 for the xc-kernel has been employed. The TDDFT calculations were performed at the optimized geometries, the scalar relativistic ZORA formalism was used, with the all electron Double Zeta (DZ) ZORA basis set of Slater Type Orbitals (STO) included in the ADF database for both Ag and Pt. In the TDDFT calculations, the SAOP xc- functional32 has been employed, which has proven to be more suitable than LB9433 for Pt clusters. In preliminary test calculations with LB94 we have experienced severe SCF convergence problems for Pt clusters, in fact, due to the overlap in energy between the d and the s bands, near the Fermi energy we observed the presence of many states changing their order in successive SCF cycles rendering the whole procedure unstable. The SAOP


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functional, on the other hand, being more accurate than LB94 and having the same correct asymptotic behaviour, allows to reach convergence safely. SAOP has also the advantage of being more accurate than LB94 in terms of the description of the optical response of Ag nanoclusters.14 Therefore we have employed SAOP in all present calculations. Unfortunately SAOP as implemented in ADF does not allow to use frozen-core basis sets, and all-electron basis sets had to be employed instead. The charge of the clusters was chosen so to have a closed shell electronic structure, except for Pt13 which is treated at the spin-polarized level of theory. Pt nanoclusters in this size range can be magnetic,34,35 so that we do not claim that the closed shell configuration is the true ground state – we choose it for computational convenience and assume that the optical properties of these clusters are not strongly dependent upon the electron configuration of the initial state and on the charge employed in the calculations. This has been actually demonstrated for the Au147 Oh cluster,15 whose absorption spectrum is in practice invariant with respect to the cluster charge. Indeed we limited our optical study of pure Pt clusters to small sizes (i.e. Pt13 and Pt38) because we were unable to find larger clusters (e.g, from Pt116 to Pt147) with closed-shell electronic configurations and a sufficiently large HOMO-LUMO gap to be dealt with using TDDFT. Investigation of magneto-optic effects is in principle possible but is deferred to future work. For an easier comparison, all the calculated discrete spectra have been broadened with Gaussian functions of FWHM = 0.12 eV.


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3. Results and discussion Ptn clusters Although the focus of the present work is the optical properties of bimetallic AgPt nanoclusters, we start the analysis with a few pure Pt clusters. In fact whereas on pure silver clusters there are many theoretical14,36-40 as well as experimental41 studies about their optical properties, at the moment experimental and theoretical studies on small Ptn clusters (with n

Optical Properties of Pt and AgâPt Nanoclusters from TDDFT

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Optical Properties of Pt and AgâPt Nanoclusters from TDDFT