Preparation Of Metal Nanoparticles By Laser Ablation And Its Spectral Properties Study

In the past decade, there has been widespread interest in the metal nanoparticles because they have unique physical and chemical properties. The nanosize of these materials is responsible for novel electro-optical properties, which are important for applications in materials research, analytical chemistry, catalytic applications and magnetic, etc. It has been found that their peculiar physical and chemical properties depends not only on the particle size, but also the particle shape. Therefore, it is a challenge to seek approaches capable of well controlling both the shape and size. Up to now, metal nanoparticles can be prepared by many methods. A novel and highly promising one of these procedures is the preparation of chemically pure metal by laser ablation of metals in aqueous media. In this method, the metal plate in water or other solvents is irradiated with a pulsed Nd:YAG laser for a period of time. As a result, finely dispersed metal particles form a stable colloid, which is free from chemical reagents or ions associated with their surface. Advantages of this method include the simplicity of the procedure, its versatility with respect to metals or solvents and the absence of chemical reagents or ions in the final preparation.Recently, the applications of surface enhanced Raman scattering (SERS) and surface enhanced infrared absorption (SEIRA) technique have attracted extensive interest. They are very attractive technique for the detection of various organic and inorganic molecules due to its sensitivity and selectivity. However, the most critical aspect for performing a SERS/ SEIRA experiment is the choice and fabrication of the substrates. Surface roughness plays a key role in optimizing the SERS/SEIRA template synthesis method.In the present work, metal nanoparticles were prepared with the laser ablation of Ag, Au and Cu metals in an aqueous medium. Since no chemistry was involved in the preparation, the laser-ablated metal nanoparticles were free from extraneous ions or other chemicals. After preparation, metal nanoparticles were deposited on an aluminium plate to form a layer of metal islands. We testing of the metal islands as substrates for SERS/ SEIRA spectroscopy by both qualitative and quantitative evaluation of the SERS/ SEIRA spectra of selected probe adsorbates.In this paper, the following chapters have been presented:Chapter One. The research history of metal colloids, their preparation as well as their optical characteristic were mainly introduced. Basic concepts of Surface Enhanced Raman Spectra and Surface Enhanced Infrared Absorption Spectroscopy were also presented.Chapter Two. Silver colloids were produced by laser ablation using radiation of 1064 nm excitation from an Nd: YAG laser from a solid silver foil in water with different laser power. After the laser ablation, one drop of the colloid was deposited on the aluminum plate and dried naturally to form the silver island films. The silver colloids and silver island films were studied by scanning electron microscopy and UV/VIS spectroscopy. The experimental results showed that variation of laser power leads to solutions of metal clusters of different sizes in water;all samples of silver island films were surface-enhanced Raman scattering (SERS) active but had different effect.Chapter Three. Metal nanoparticles are prepared with the laser ablation of Au and Cu metals in an aqueous medium. Gold island films and copper island films were formed using the same method as in Chapter Two. Their SEM images and SERS properties were obtained and analysised. Experimental results revealed that either the Gold and copper island films could be used as an efficient and convenience SERS-active substrate.Chapter Four."Chemically pure"Pd nanoparticles were obtained via laser ablation in pure water. There is no characteristic peak in UV/VIS spectrum of Pd colloids in measuring range of 200 nm～800 nm. According to the SEM measurement, the average diameter of Pd nanoparticles was～200 nm. After the laser ablation, Pd nanoparticles was deposited on the aluminum plate and dried naturally to form the Pd island films. Pd island composed of Pd nanoparticles exhibited fairly strong surface-enhance Raman scattering. The enhancement factor for 4-MPY could reach values as large as 8.7×104 with 632.8 nm excitation.Chapter Five. This part combined the SERS and SEIRA technique to study the adsorption behaviors of thymine on silver island films. The SERS and SEIRA results indicated that thymine adsorbed almost perpendicularly via the N1-end and O4-end on silver nanoparticles.