Browsing by Author "Aikens, Christine M."

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TD-DFTB study of optical properties of silver nanoparticle homodimers and heterodimers
(AMER INST PHYSICS, 1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA, 2020) Liu, Zhen; Alkan, Fahri; Aikens, Christine M.; 0000-0002-0854-7997; AGÜ, Mühendislik Fakültesi, Malzeme Bilimi ve Nanoteknoloji Mühendisliği Bölümü
The absorption spectra for face-centered cubic nanoparticle dimers at various interparticle distances are investigated using time-dependent density functional tight binding. Both homodimers and heterodimers are investigated in this work. By studying nanoparticles at various interparticle distances and analyzing their vertical excitations, we found that as the interparticle distance decreases, a red shift arises from contributions of the transition dipole moment that are aligned along the z-axis with nondegenerate features; blue shifts occur for peaks that originate from transition dipole moment components in the x and y directions with double degeneracy. When the nanoparticles are similar in size, the features in the absorption spectra become more sensitive to the interparticle distances. The best-fit curves from vertical excitation energy in the form of AR(-b) for Delta E-redshift/Delta E-blueshift vs R are determined. In this way, we determined trends for absorption peak shifts and how these depend on the interparticle distance.
An Ultrastable 155-Nuclei Silver Nanocluster Protected by Thiacalix[4]arene and Cyclohexanethiol for Photothermal Conversion
(WILEY-V C H VERLAG GMBH, 2022) Wang, Zhi; Alkan, Fahri; Aikens, Christine M.; Kurmoo, Mohamedally; Zhang, Zhen-Yi; Song, Ke-Peng; Tung, Chen-Ho; Sun, Di; 0000-0002-4046-9044; AGÜ, Mühendislik Fakültesi, Malzeme Bilimi ve Nanoteknoloji Mühendisliği Bölümü; Alkan, Fahri
Thiacalix[4]arenes have emerged as a family of macrocyclic ligands to protect metal nanoparticles, but it remains a great challenge to solve the mystery of their structures at the atomic level, especially for those larger than 2 nm. Here, we report the largest known mixed-valence silver nanocluster [Ag-155(CyS)(40)(TC4A)(5)Cl-2] (Ag155) protected by deprotonated cyclohexanethiol (CySH) and macrocyclic ligand p-tert-butylthiacalix[4]arene (H(4)TC4A). Its single-crystal structure consists of a metallic core of four concentric shells, Ag-13@Ag-42@Ag-30@Ag-70, lined with a organic skin of 40CyS(-) and 5TC4A(4-) and 2Cl(-). Ag155 manifests an unusual pseudo-5-fold symmetry dictated by the intrinsic metal atom packing and the regioselective distribution of mixed protective ligands. This work not only reveals a macrocyclic ligand effect on the formation of a large silver nanocluster, but also provides a new structural archetype for comprehensively perceiving their interface and metal kernel structures.
Understanding plasmon coupling in nanoparticle dimers using molecular orbitals and configuration interaction
(ROYAL SOC CHEMISTRY, THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND, 2019) Alkan, Fahri; Aikens, Christine M.; 0000-0002-0854-7997; AGÜ, Mühendislik Fakültesi, Malzeme Bilimi ve Nanoteknoloji Mühendisliği Bölümü
We perform a theoretical investigation of the electronic structure and optical properties of atomic nanowire and nanorod dimers using DFT and TDDFT. In both systems at separation distances larger than 0.75 nm, optical spectra show a single feature that resembles the bonding dipole plasmon (BDP) mode. A configuration interaction (CI) analysis shows that the BDP mode arises from constructive coupling of transitions, whereas the destructive coupling does not produce significant oscillator strength for such separation distances. At shorter separation distances, both constructive and destructive coupling produce oscillator strength due to wave-function overlap, which results in multiple features in the calculated spectra. Our analysis shows that a charge-transfer plasmon (CTP) mode arises from destructive coupling of transitions, whereas the BDP results from constructive coupling of the same transitions at shorter separation distances. Furthermore, the coupling elements between these transitions are shown to depend heavily on the amount of exact Hartree-Fock exchange (HFX) in the functional, which affects the splitting of CTP and BDP modes. With 50% HFX or more, the CTP and BDP modes mainly merge into a single feature in the spectra. These findings suggest that the effects of exact exchange must be assessed during the prediction of CTP modes in plasmonic systems.
Understanding the Effect of Symmetry Breaking on Plasmon Coupling from TDDFT
(AMER CHEMICAL SOC1155 16TH ST, NW, WASHINGTON, DC 20036, 2021) Alkan, Fahri; Aikens, Christine M.; AGÜ, Mühendislik Fakültesi, Malzeme Bilimi ve Nanoteknoloji Mühendisliği Bölümü; Alkan, Fahri
We perform a time-dependent density functional theory (TDDFT) investigation for the optical properties of nanorod assemblies for different sizes (Ag-10, Ag-59, and Ag-139), interparticle distances, and orientations with a focus on the effect of symmetry breaking via an angle on plasmon coupling. For the model systems, the angle (theta) between the particles is varied between 0 and 180 degrees, where theta = 0 degrees and theta = 180 degrees correspond to symmetric side-by-side and end-to-end orientations of the nanorods, respectively. Our analysis reveals that for a sufficiently large interparticle distance (r > 0.7 nm), where the wave-function overlap between monomers is negligible, TDDFT results agree quite well with the predictions of the dipole-dipole interaction model for the intensity of the different modes of coupled plasmons. For smaller gap distances (0.4-0.5 nm), a charge-transfer plasmon (CTP) mode occurs for the symmetry broken case of the Ag-10 dimer. For the assemblies of larger nanorods, however, the CTP mode is predicted to be less pronounced, especially for the cases where the deviation from the end-to-end geometry is larger than 30 degrees. The orbital overlap and configuration-interaction analyses show that these results are related to the fact that the relative overlap strength between monomeric energy levels is significantly reduced for symmetry-broken orientations of larger nanorods.