Efficient Photoluminescence of Mn²⁺-Doped ZnS Quantum Dots Sensitized by Hypocrellin A

Abstract

Mn2+-doped ZnS semiconductor quantum dots reveal remarkably intense photoluminescence with the4T1(4G) f6A1(6S) transition. In this study, following growth doping technique, Mn2+-doped ZnS quantum dots (ZnS:Mn2+QDs) with high-quality optical properties and narrow size distribution were synthesized successfully. The dopant emission has been optimized with various reaction parameters, and it has been found that the percentage of introduced dopant, reaction temperature, and time as well as the pH of a reaction mixture are key factors for controlling the intensity. Photoluminescence emission (PL) measurements of ZnS:Mn2+QDs show Mn2+d-d orange luminescence along with band-edge blue luminescence. Moreover, the electron transfer from singlet states of hypocrellin A (HA) to colloidal ZnS:Mn2+QDs has been examined by absorption spectra and fluorescence quenching. The absorption spectrum gave an evidence of the increases in the extinction coefficient and the red-shift of the absorption maxima in the absorption spectra of HA in the presence of ZnS:Mn2+QDs, demonstrating the occurrence of surface interactions between the sensitizer and the particle surface. Fluorescence quenching by ZnS:Mn2+QDs also suggested that there were a complex association between HA and ZnS:Mn2+QDs, which was necessary for observing the heterogeneous electron-transfer process at the interface of sensitizer-semiconductor.

Synthesis, optical and fluorescence properties of Mn-doped ZnS Quantum Dots

“…Also, the particle sizes are smaller than the crystallite sizes (D) obtained from the XRD pattern. The results match the data found in references [35,38,42]. The difference found between D and D / values can be attributed to the sensitivity of the XRD analysis to the crystalline size [43], whereas the solvent may affect the particle size determination from the UV-Vis spectra [44].…”

“…Since the two samples were prepared under the same experimental orders, the obtained result demonstrates the presence of a strong interaction between the delectron states of Mn 2+ ions and the s-p states of ZnS host lattice. This, in turn, provides an effective channel for the transfer of electrons and holes, leading to the subsequent emission in ZnS:Mn 2+ QDs [38]. The strong PL emissions indicate that the prepared QDs have large bandgaps compared with their bulk counterpart.…”

“…It was found that the Strain of ZnS and Mn: ZnS is 2.5 x 10 -4 and 3.6x 10 -4 , respectively. [38]. The QDs' absorption edges (λ o ) of the studied samples were blue-shifted from 345 nm [39] (the bulk's value of ZnS) to 325 nm.…”

“…7, Mn:ZnS QDs exhibit a peak at around 570 nm, which is attributed to the transition from 4 T1 excited state of Mn 2+ to the ground state of Mn 2+ ( 6 A1). The value is blue-shifted compared with that of bulk Mn:ZnS [38]. Large density of surface states in the nanoparticles, strong electron-phonon coupling in the nanoparticles, or size-dependent crystal field effect may be the reasons why the peaks of Mn:ZnS QDs are red-shifted [45].…”