Synthesis of efficient near-Infrared-emitting $CuInS_2/ZnS$ quantum dots by inhibiting cation-exchange for bio application

Abstract

NIR-emitting $CuInS_2/ZnS$ QDs were synthesized by off-stoichiometry effect, and the tuned emission wavelength is 850nm which is proper to deep tissue imaging. Due to the cation-exchange phenomena, however, the emission wavelength of synthesized CIS core was blue-shifted after ZnS shell coating. In order to inhibit this blue-shift of photoluminescence, controlling the synthesis temperature during ZnS shell coating was required, and the selected synthesis temperature were $180^\circ C$, $210^\circ C$, $230^\circ C$ and $250^\circ C$. Temperature effect on CIS/ZnS QDs could be related to substitutional diffusion mechanism expressed by Fick’s law. The least blue-shifted emission wavelength was 810nm and degree of blue-shift is about 40nm. Also, not only blue-shift but also the different evolution of PL peak components was observed as ZnS coated on the surface of CIS core. These PL peak components were related to defect-level emission of CIS/ZnS QDs, and could be confirmed by low-temp PL and proposed defect diagram of Cu-rich CIS. Four PL peak components were contained and the change of longer energy peaks could be related to cation-exchange. The degree of passivation by ZnS shell for different synthesis temperatures, results in QY (26-38%). To confirm the possibility of applying CIS/ZnS QDs to bio-imaging, the experiment for exchange hydrophobic QDs to hydrophilic QDs was progressed using HSA/PEG encapsulation method. The 6-armend PEG-NHS was used as an amphiphilic ligand, and the fluorescent intensity was decreased to 57% of initial QDs after fabrication of QD-HSA/PEG nanocapsule and the size of nanocapsule was 140 $\pm$ 21.6nm which measured by SEM and TEM. According to these results, this synthesized NIR-emitting CIS.ZnS QDs using inhibiting cation-exchange, are suitable for bio-application, especially in vivo-imaging.