Synchrotron Radiation Research On Tunability Of Cu Nanoclusters’Luminescence Properties

The metal nanoclusters have ultra small size (smaller than2nm), result in unique atom space configuration and discrete electron energy level. The metal nanoclusters exhibit photoluminescence effect, high catalytic activities and other novel physicochemistry properties, made them have a broad application prospect in optics, biology, catalysis, energy and other fields. Metal nanoclusters have good fluorescence properties that make them very suitable for bio-labeling. Meanwhile, the complicated application environments also put forward higher request for the tunability of emission wavelengths, make how to realize the tuning of metal nanoclusters’luminescence properties become an important research direction. The unprecious metal copper is economic and abundant in nature, and both theoretical and experimental studies suggest Cu nanoclusters have unique catalysis and fluorescence properties. However, the difficult preparation of ultra-small clusters and copper element is easily oxidized in air, lead the research of Cu nanoclusters is still in a preliminary stage with respect to the Au and Ag metal clusters. In this paper, on the basis of preparing2-mercapto-5-n-propylpyrimidine (MPP) capping Cu nanoclusters, we realized the regulation of Cu nanoclusters’emission wavelengths through adding different amounts of dodecanethiol (C12SH), then we primarily investigated the internal reasons. The obtained results will provide reference for realizing the tunability of Cu nanoclusters’emission wavelengths and deeply understanding the mechanism of luminescence.Firstly, we synthesized MPP capped copper nanoclusters through a simple one-pot method. The clusters have a tiny size, multiple absorption band, and exhibit red-orange color at623nm when irradiated with a UV lamp (λ=365nm). The mass spectrometry and X-ray absorption fine structure (XAFS) of copper nanoclusters suggested the main components of the clusters is Cus[MPP]4, the overall structure is a tetrahedron. There is a copper atom in the body heart of tetrahedron, which bonds with four external copper atoms that on the vertices of tetrahedron. Each of four external copper atoms connects with two sulfur atoms in a bridge mode. Secondly we studied how ligand-exchange influence the luminescence properties of Cu nanoclusters. Through adding the concentration of dodecanethiol (C12SH) in the MPP capped Cu nanoclusters’solution, the emission wave-length of Cu nanoclusters can be shifted to584nm from623nm when the ratio of MPP/C12SH reduce to4:2from4:0. The further structure studies show that with increasing the concentration of C12SH, the number of Cu atoms and ligands did not change, while C12SH gradually replaced MPP ligand to form Cu5[MPP]2[C12SH]2. After ligand exchange, the metallic Cu-Cu coordination disappeared, and the charge transfer between Cu and S was enhanced. Thus we think the etching effect of C12SH ligands leads to the change of atomic configuration of the Cu cores from the tetrahedron-alike structure to the chain-like structure of-S-Cu-S-. As a result, the electronic features of the Cu clusters show a metallic-to-covalent transition, resulting in the blue-shift of the luminescence and optical absorption wavelengths.