Structural and optical properties evaluation of gold nanoparticles via synthesis, characterization modeling and simulation

Abstract

Gold nanoparticles (AuNPs) with customized morphologies, structures, optical and electronic properties for varied functional applications require an accurate synthesis and characterization technique. Furthermore, the basic understanding of these properties and validation of the experimental results depends on the precise modelling and first-principle density functional theory (DFT)-based simulations. In view of this, some AuNPs were prepared using the eco-friendly pulse laser ablation in liquid (PLAL) technique. As-grown AuNPs were characterized via diverse analytical tools including the ultraviolet-visible (UV-Vis) absorption, attenuated total reflectance (ATR), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and photoluminescence (PL) spectroscopy. The influence of various laser parameters (laser energies, repetition rates, liquid environments, and laser wavelengths) on the structure, morphology, and optical traits of these AuNPs was determined. In addition, the first-principle DFT simulation was performed using WIEN2k software to complement the experimental results and explain the electronic structure properties of the produced AuNPs. For the first time, the spin-orbit coupling with the modified Becke-Johnson exchange potential (TB-mBJ) was included in the DFT framework for the band structure calculations of the AuNPs. A phenomenological model was also developed by integrating the effects of surface states and quantum confinement to describe the PL and absorption mechanism of AuNPs. The MATLAB code based on Mie-Gans theory was used to fit the experimental absorption data of AuNPs. By optimizing the laser parameters (especially the low laser energy ranging from 96.6 mJ up to 318 mJ, short pulse duration time of 3 min and low repetition rate of 1 Hz), the sizes of the spherical AuNPs were controlled in the deionized water (mean diameter of 7 to 30 nm) and ethanol (diameter of 3 to 6 nm) liquid medium. The strong UV-Vis absorption and surface plasmon resonance (SPR) peaks in the range of 521 to 529 nm accompanied by a blue-shift revealed by these AuNPs clearly indicated their effectiveness for sundry applications. The observed intense PL spectra of the studied AuNPs with optical band gap in the range of 2.95 to 3.9 eV were attributed to the effect of quantum-confinement. The obtained absorption characteristics, PL peak shifts, phonon energy dispersion in the Raman spectra, widening and broadening of the spectral peak due to the quantum size effects of AuNPs were validated using the model. The TEM images disclosed the formation of the colloidal AuNPs of average size ranged from 1 to 50 nm. Based on the WIEN2K simulation and experimental outcome of AuNPs, a structural and optical correlation was developed. The inclusion of spin-orbit coupling with the modified TB-mBJ potential in the DFT framework could more accurately predict the band structure (band gap energy) and shifts in the optical spectra of the proposed colloidal AuNPs compared to the existing reports. The achieved excellent fit of the experimental data with the model and simulation outcome in terms of bandgap and PL energy indicated the accuracy of the present method. It is established that the good quality colloidal AuNPs with tailored attributes can be produced by tuning the laser parameters of the PLAL technique. In short, the present study improved the prediction accuracy over the existing art-of-the techniques. This disclosure may contribute towards the development of spherical colloidal AuNPs useful for various applications.