Optical Properties of Metal Clusters from First Principles Calculations.
Abstract
Ground State structures of neutral Copper clusters CuN=3−6 were generated and opti- mised within the framework of the Density Functional Theory (DFT) using Generalised Gradient Approximation (GGA) and ultrasoft pseudopotential.
The shapes and binding energies of the clusters obtained showed good agreement with other theoretical works, except for the trimer cluster which has the shape of an equilateral triangle rather than the isosceles triangle as reported in previous works.
The optical absorption strength func- tion of the neutral Copper clusters CuN=3−6 in vacuum were then computed within the frame work of Time Dependent Density Functional Theory (TDDFPT). The spectrum were computed for the energy range of 1.5eV < kω < 5.5eV .
The clusters that had odd numbers of atoms showed metallic character and could not be implemented in the turboTDDFT package.
Whilst the clusters with even numbers of atoms showed semi- conductor property with the Highest Occupied Molecular Orbital-Lowest Unoccupied Molecular Orbital (HOMO-LUMO) gap increasing as the number of atoms change from four to six , and full implementation with turboTDDFT was achieved.
In both clusters with even number of atoms four absorption peaks were obtained with that of six atoms higher in intensity.
The shift in the direction of the peaks as the cluster size increases shows no unique trend. However, there was a slight broadening of the peaks as the cluster size increased.
Most of the peaks were observed to be from the electron transiting from the s-orbital in the occupied state to the orbitals in the unoccupied state.
Introduction
Background of Study
Metal clusters are particles composed of a certain number N of atoms with 3 < N < 107 . Early studies on metal clusters are traceable to the work of Faraday on a gold colloidal particle that is responsible for the red stain in glass .
Mie in 1908 gave a mathematical and theoretical framework for studying the properties of small particles when he gave the exact solution to Maxwell’s equations for spherical particles with sizes smaller than the wavelength of incident radiation .
Since then, the field of metal clusters (clusters in general) have attracted a lot of attention both at the theoretical and experimental level of investigations.
These studies have led to the development of more refined methods and theories for investigating the properties of metal clusters and thus their viability for diverse areas of applications as witnessed today.
Current Methods and Challenges
In the past, people have paid a lot of attention to metal clusters . This is largely attributed to their chemical and physical properties being size, shape, composition and chemical ordering dependent on their large surface to volume ratio.
The applications of these cut across the search for efficient materials for solar energy conver- sion in photo-voltaic and photo-catalysis, biomedical nano materials for cancer detection and treatment, and optoelectronic devices .
For solar energy conversion, the emphasis has been on the optical properties of the metal clusters.
For example, dis- cussed novel methods of enhancing the transport property of haematite using plasmonic nanoparticles such gold and silver for photocatalytic systems;
conducted DFT stud- ies on Ag-Cu nano-alloy and found a ferroelectric and ferromagnetic effect that can be applied to non-linear optical devices, conducted TDDFT studies on the dependence of the surface plasmon resonance of gold nanorods on its aspect-ratio and size.
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