Abstract
Electronic and optical properties of silver clusters were calculated using two different \textit{ab initio} approaches: 1) based on all-electron full-potential linearized-augmented plane-wave method and 2) local basis function pseudopotential approach. Agreement is found between the two methods for small and intermediate sized clusters for which the former method is limited due to its all-electron formulation. The latter, due to non-periodic boundary conditions, is the more natural approach to simulate small clusters. The effect of cluster size is then explored using the local basis function approach. We find that as the cluster size increases, the electronic structure undergoes a transition from molecular behavior to nanoparticle behavior at a cluster size of 140 atoms (diameter $\sim 1.7$\,nm). Above this cluster size the step-like electronic structure, evident as several features in the imaginary part of the polarizability of all clusters smaller than Ag$_\mathrm{147}$, gives way to a dominant plasmon peak localized at wavelengths 350\,nm$\le\lambda\le$ 600\,nm. It is, thus, at this length-scale that the conduction electrons' collective oscillations that are responsible for plasmonic resonances begin to dominate the opto-electronic properties of silver nanoclusters.
Highlights
3.1 Comparison between local basis function and plane wave methods. He and Zeng [46] used the SIESTA code [33,34] to calculate the optical properties of Ag clusters with sizes ranging from 13 to 586 atoms and noted the need to scissor-shift their spectra by a fixed energy of 1.28 eV to align with experimental results; when comparing different computational results, it is important to notice that the magnitude of the shift needed to align with experiments depends on the exchange correlation functional
Since one of the aims here is to compare the applicabilities of the two different approaches, the results presented in this work do not include any shift except for ∼0.3 eV blue-shift to the WIEN2k data to align with the SIESTA results for easier comparison
For N > 140, the molecular transitions give way to nanometallic single optical transition energy which is characteristic of s-electrons excitations of the noble metal. This finding is in excellent agreement with time-dependent density functional theory (TDDFT) computations of Weissker and Mottet who found a cross-over from multi-feature optical spectrum of Ag clusters as the size increases above 140 atoms [62]
Summary
The readiness with which noble metal nanoparticles create and support surface plasmons makes them useful in a wide range of applications, e.g., in photonic devices, as chemical sensors [1,2,3,4,5], in bio-imaging [6], in drug delivery [7], cancer therapy [7,8,9], optical manipulation [2,10,11,12], electrical conduits in microelectronic industry [13,14,15], and even as ‘nanoears’ in optical readout of acoustic waves generated in liquid media [16] and in channeling photon energy in vortex nanogear transmission [17]. We used the WIEN2k all-electron-full-potential linearizedaugmented plane-wave (LAPW) code [32], with the generalized gradient approximation (GGA) [37] for the exchange and correlation potential, to calculate the electronic and optical properties of Ag clusters In this method, the system is described by a 3D periodically repeated unit cell which is partitioned into non-overlapping muffin-tin spheres centered on each atom and interstitials. Two parameters govern the accuracy of the calculations: (1) the product of the plane-wave cut-off and the smallest muffin-tin radius in the system (RKM ); and (2) the number of k-points in the irreducible Brillouin zone (IBZ) For these clusters, RKM -value of 5.5 is sufficient to yield converged optical properties. The DFT fine-tuning was continued until forces on all atoms became smaller than 40 meV/ ̊A
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
