Structural and Optical Properties of Si-Doped Ag Clusters - The

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Structural and Optical Properties of Si-Doped Ag Clusters Junais Habeeb Mokkath and Udo Schwingenschlögl* Physical Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia ABSTRACT: The structural and optical properties of AgN and AgN−1Si1 (neutral, cationic, and anionic) clusters (N = 5 to 12) are systematically investigated using the density functional based tight binding method and time-dependent density functional theory, providing insight into recent experiments. The gap between the highest occupied and lowest unoccupied molecular orbitals and therefore the optical spectrum vary significantly under Si doping, which enables flexible tuning of the chemical and optical properties of Ag clusters.

I. INTRODUCTION Si is known to possess extraordinary properties on the nanoscale,1 so that it is hard to anticipate how an Si atom will interact with a metallic environment, e.g., an Ag cluster. In general, Ag clusters have been widely investigated due to a range of applications in different fields, such as sensing,2−4 catalysis,5−7 and biomedicine.8,9 The structures of small AgN clusters (N = 2 to 10) have been predicted by several research groups with different methods and accuracy levels, giving rise to a wide variety of results. For instance, the theoretical work by Bonacic-Koutecky and co-workers10 indicates that a 2D → 3D structural transition appears between cluster sizes of 6 and 7 atoms. Specifically, these authors report a 2D planar trapezoidal structure for N = 6 and a 3D pentagonal bipyramidal structure for N = 7. Later, Huda and Ray11 investigated the geometric and electronic structures of neutral, cationic, and anionic AgN clusters (N = 5 to 9), using second order many-body perturbation theory and predicted that neutral Ag6 prefers a 2D planar geometry. On the other hand, Zhao and coworkers12 found a tripyramidal structure for N = 6, using tight binding molecular dynamics simulations. Recent experimental studies find a planar hexagon to be most stable and thus suggest a transition between cluster sizes of 6 and 7 atoms.13 Besides the 2D → 3D structural transition, one of the attractive properties of Ag clusters is their strong optical response in the UV−visible energy range,14 which is related to surface plasmon resonances (i.e., collective resonances of conduction electrons in response to electromagnetic radiation with appropriate wavelength). Although various theoretical and experimental studies have dealt with Ag-doped Si clusters, there is little known about Sidoped Ag clusters, except for a recent experiment carried out by Majer and von Issendorff15 using photoelectron spectroscopy on anionic AgSi clusters. These authors have speculated about the influence of the local geometry on the interaction between Ag and Si, finding some indications for chemisorption of Si © 2014 American Chemical Society

atoms on the Ag cluster surface. However, it has not been possible to identify the most favorable site for the Si adsorption and the type of bonding. In the present work we address these issues by theoretical means. The aim is to explain how the lowest energy structures, bonding characteristics, and UV− visible absorption spectra of Ag clusters are modified by Si doping.

II. METHODOLOGY The lowest energy structures of the AgN, AgN−1Si1, AgN−1Si1+, and AgN−1Si1− (N = 5 to 12) clusters are obtained by a combination of the cluster graph theory global optimization technique16−19 and the self-consistent charge density functional based tight binding method.20 For a systematic and thorough sampling of the energy landscape one has to consider a large, complete, and unbiased set of initial structures that includes not only the representative cluster geometries or topologies but also all inequivalent distributions of the Si dopant for different cluster sizes N. The topologies are generated using cluster graph theory and then optimized by the tight binding approach. The tight binding approach is particularly suitable for describing complex systems at low computational cost and has been demonstrated to provide reliable results on the structure and binding energies for a large variety of systems, ranging from solid state structures to biomolecules.21 Starting from the Kohn−Sham formalism, the molecular orbitals of the noninteracting electron system are generated using a minimal atomic basis set, and a reference density is created as superposition of the atomic densities. The total energy then can be expressed as the sum of a repulsive contribution (depending on the reference density), the band energy, and second order terms. The tight binding approximation allows to Received: November 17, 2013 Revised: February 12, 2014 Published: February 25, 2014 4885

dx.doi.org/10.1021/jp4112958 | J. Phys. Chem. C 2014, 118, 4885−4889

The Journal of Physical Chemistry C

Article

Figure 1. Lowest energy structures and HOMO−LUMO gaps (in eV) of AgN, AgN−1Si1, AgN−1Si1+, and AgN−1Si1− clusters for N = 5 to 12. For the anionic clusters, structures are shown only when they are qualitatively different from the neutral and cationic structures.

another cluster size. The Ag7 and Ag6Si clusters relax to pentagonal bipyramidal and capped octahedral structures, respectively. The first excited Ag7 cluster has a bicapped trigonal bipyramidal shape, which is 0.50 eV less stable than the pentagonal bipyramid, and the first excited Ag6Si cluster is a pentagonal bipyramid, which is 0.20 eV less stable than the capped octahedron. We observe that the neutral and cationic Ag6Si clusters relax to the same geometry but not the anionic cluster; see Figure 1. For N = 8, all clusters have a bicapped octahedral shape, though with different Si adsorption sites. The first exited Ag8, Ag7Si/Ag7Si1+, and Ag7Si1− clusters have tricapped trigonal bipyramidal, bicapped octahedral (with different Si adsorption site), and capped pentagonal bipyramidal structures, respectively, which are 0.58, 0.33/0.17, and 0.08 eV higher in energy than the corresponding ground states. For N = 9, the doped clusters maintain the original shape, whereas for N = 10 to 12 remarkable differences are found (where the geometries of the pure clusters are qualitatively in agreement with the first principles study of ref 27). Figure 1 demonstrates that the charge state of the doped clusters has significant influence on the lowest energy structure, changing the geometry and/or the preferred Si adsorption site, which defines the type of interaction between the Si and Ag atoms. This is an important observation since the Si absorption sites experimentally had

write the repulsive contribution as the sum of atomic pair interactions, and the Kohn−Sham matrix elements are expressed in the atomic basis. Moreover, the second order terms can be related to Mulliken charges. UV−visible absorption spectra are calculated within time-dependent density functional theory using the B3LYP22,23 exchange correlation functional together with the def-TZVP basis24 [5s3p3d|7s6p5d] for Ag and [5s4p1d|14s9p1d] for Si, as implemented in the TURBOMOLE package.25 Finally, the core electrons are described by effective core potentials and the quadrature grids are taken of m5 quality.26

III. RESULTS AND DISCUSSION Figure 1 summarizes the favorable structures and the energy gap between the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals of the AgN, AgN−1Si1, AgN−1Si1+, and AgN−1Si1− clusters for N = 5 to 12. The pure structures show agreement with earlier first-principles results27 and recent experiments by Lecoultre and co-workers13 on AgN clusters (N = 1 to 9) embedded in a solid neon matrix, whereas remarkable structural changes affect the clusters upon Si doping. We observe a 2D → 3D structural transition between the Ag6 (C2v symmetry) and Ag7 (D3h symmetry) clusters, in agreement with experiment13 and supporting the theoretical results of refs 10 and 11, while ref 12 finds the transition at 4886



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remained an open question.15 It can be expected that the behavior of larger clusters in this respect is not different from that of the Si-doped Ag clusters investigated here. An electronic charge transfer analysis is performed in the Voronoi deformation density scheme28 on all the optimized AgSi clusters. We do not observe any charge transfer between the Si and Ag atoms in accordance with the similar Pauling electronegativities of 1.90 (Si) and 1.93 (Ag). To analyze how the electronic structure depends on the Si doping, we report in Figure 2, as an example, the orbitally resolved density of states

Figure 2. Partial electronic densities of states of the Ag12Si cluster. A Gaussian width of 0.05 eV is used for broadening the energy levels. The dashed line indicates the position of the Fermi energy.

Figure 3. Evolution of the absorption spectra of AgN (blue), AgN−1Si1 (red), AgN−1Si1+ (green), and AgN−1Si1− (orange) clusters for N = 5 to 12. Experimental spectra (black) of pure Ag clusters are given for comparison.13,29

of the Ag12Si1 cluster obtained by the tight binding approach. The energy region between about −5.5 and −2.5 eV is dominated by the Ag 4d states, whereas around the Fermi energy, between −2.5 and 2 eV, we find Si 3p, Ag 4d, and Ag 5s states. Moreover, the Si 3s states are localized deep inside the valence band below −7 eV and thus do not participate in chemical bonding. Note that the HOMO shows almost equal contributions of the Si and Ag states, which points to chemisorption. We next discuss the evolution of the HOMO−LUMO gap as a function of the cluster size, finding for the AuN and AgNSi1 clusters pronounced even−odd oscillations; see Figure 1. However, the AgN−1Si1+ and AgN−1Si1− clusters do not exhibit such effects. In the case of small pure Ag clusters we obtain for even N larger gaps than for odd N (both N − 1 and N + 1), due to pairing of the valence s electrons, where the value varies between 0.42 and 3.21 eV (Ag6). In the case of the AgNSi1 clusters, the HOMO−LUMO gap varies between 0.50 and 2.47 eV. Si doping in clusters with an even number of Ag atoms lowers the HOMO−LUMO gap significantly, while the opposite happens for clusters with an odd number of Ag atoms. In general, the sizable HOMO−LUMO gaps point to high stability of the various clusters. The UV−visible absorption spectra of the lowest energy structures of the AgN, AgN−1Si1, AgN−1Si1+, and AgN−1Si1− clusters (N = 5 to 12) are calculated in the energy range from 1.5 to 5 eV, see Figure 3. Let us first focus on the positions and oscillator strengths of the main peaks. In general, we notice substantial differences in the spectra for different cluster sizes. Even the addition of a single Ag atom in the case

of the pure clusters or of a single Si atom in the case of the doped clusters has huge effects on the optical response. The absorption spectrum of Ag5 is characterized by two pronounced peaks at 3.36 and 3.83 eV with oscillator strengths of 0.15 and 0.13, respectively. The first peak can be assigned to a linear combination of the transitions HOMO − 5 → LUMO (34%), HOMO − 2 → LUMO + 1 (32%), and HOMO − 1 → LUMO + 2 (26%). The numbers in brackets here and in the following denote the contribution of specific transitions to the linear combination. The second peak accordingly can be assigned to a linear combination of the transitions HOMO − 1 → LUMO + 5 (24%), HOMO − 3 → LUMO (22%), HOMO − 2 → LUMO + 3 (21%), and HOMO → LUMO + 7 (20%). The peak locations agree with the experimental values in ref 13 (3.27 and 3.69 eV). Finally, a Mulliken population analysis indicates that transitions below/above 4.5 eV are largely due to the s/d states. While the d states do not actively contribute below 4.5 eV, they are still important for the exact locations of the transition peaks.10 Figure 3 demonstrates that the spectra of the AgN clusters are governed by plasmonic peaks with intense and focused excitations. This fact can be ascribed to the special electronic structure of Ag: Since the 4d−5s gap is 3.2 eV in bulk Ag, the 4d shell is too deep to participate in excitations. For this reason, the Ag 5s electrons behave similar to a free electron gas, giving rise to well-defined intense and sharp plasmonic resonances. This is prominently reflected by the absorption spectrum of Ag8, for example, via two pronounced peaks at 3.78 and 4.16 eV with oscillator strengths of 0.11 and 0.23, respectively. The first peak traces back to a linear combination of the transitions 4887



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HOMO − 2 → LUMO (64%), HOMO − 1 → LUMO + 2 (12%), and HOMO − 1 → LUMO + 1 (12%), whereas the second peak can be assigned to a linear combination of the transitions HOMO − 1 → LUMO + 6 (37%), HOMO − 1 → LUMO + 5 (20%), and HOMO − 2 → LUMO + 1 (13%). In fact, the Ag8 spectrum exhibits clear similarities with the spectra of alkali metal clusters, for instance, Na8 with 8 valence electrons in a filled shell. There are significant differences in the absorption spectra for doped clusters with different charge states, due to the fact that the charge modifies the HOMO−LUMO gap; see Figure 1. For instance, the gaps obtained for the Ag4Si1, Ag4Si1+, and Ag4Si1− clusters are 2.47, 2.04, and 1.11 eV, respectively. As the Ag4Si1− cluster has the smallest HOMO−LUMO gap, it shows the earliest onset and highest intensity of the absorption at low energy; see Figure 3. In the case of the neutral and cationic clusters, the main effect of Si doping can be summarized as follows: The spectral intensity is distributed over a wider energy range, combined with a significant reduction of the oscillator strength. Since this behavior is also seen for the Ag9 and Ag8Si clusters with very similar geometries, we can conclude that the effect of Si doping is similar for all sizes. The anionic clusters, however, exhibit rather different spectra because of different HOMO−LUMO gaps and/or structures.

REFERENCES

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IV. CONCLUSIONS The physical and chemical properties of binary nanoalloys have potential in many applications as they can be tuned by varying the size, composition, and atomic arrangement. We therefore have analyzed the structure and optical response of AgN, AgN−1Si1, AgN−1Si1+, and AgN−1Si1− gas phase clusters for N = 5 to 12 in the framework of the density functional based tight binding method and time-dependent density functional theory. We have studied the evolution of the HOMO−LUMO gap and UV−visible absorption spectrum as a function of the cluster size and Si doping. Our results demonstrate that Si bonds covalently to the Ag cluster surface, which could not be clarified by the experimental data of Majer and von Issendorff.15 It turns out that Si doping leads to a broadening and damping of the absorption peaks, i.e., scattering of the weight in the UV−visible region. The appearance of plasmonic peaks for increasing Ag cluster sizes is in excellent agreement with experiment.13 The dominating features of the UV−visible absorption spectra have been used to elucidate the electronic origin of the transitions and their dependence on the constituents. All calculated spectra show characteristic features so that they can be utilized as fingerprints for identifying the structure. Si-doped Ag clusters are promising building blocks of stable bifunctional nanoparticles since one can systematically tune the plasmonic response and chemical reactivity of pure Ag clusters.

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AUTHOR INFORMATION

Corresponding Author

*(U.S.) E-mail: [email protected]. Phone: +966(0)544700080. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Computational resources provided by KAUST IT are gratefully acknowledged. 4888



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Structural and Optical Properties of Si-Doped Ag Clusters - The

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