Fluorescence and EPR Characteristics of Mn²⁺-Doped ZnS Nanocrystals Prepared by Aqueous Colloidal Method

Abstract

Fluorescence properties have been studied for Mn:ZnS crystallites with average diameter of 4 nm prepared by an aqueous colloidal method under 266 nm light excitation. The intensity ratio of the blue band at ∼430 nm to the orange band at ∼590 nm has decreased after the preparation on a time scale of hours in aqueous solution. On the other hand, hyperfine structures of Mn 2+ in the electron paramagnetic resonance spectrum have increased markedly on the same time scale in solution samples. These phenomena are attributed to the redistribution of defect centers in nanocrystals. Such phenomena have not been observed in samples incorporated into poly(vinyl alcohol). The orange emission is mainly due to the 6 A 1 r 4 T 1 transition of Mn 2+ , while the blue emission is tentatively assigned to the donor-acceptor pair transition in which the acceptor is related to the Zn 2+ vacancy. Fluorescence decay times of the blue and orange bands have been found to be ∼10 ns and ∼1 ms, respectively, the latter being the same as in the bulk samples. A weak fluorescent component with fast kinetics observed in the orange region has been identified as a tail of the blue band. No lifetime shortening of the Mn 2+ emission due to quantum confinement has been observed, contrary to reports in the literature.

Optical Properties of ZnO – CdS Nano Composites

“…This agrees well with the peak value at 428 nm reported by Becker and Bard [30]. The emission peak at 446 nm is attributed to the Zn/Cd vacancies which agree well with the work of Murase et al [31]. The emission peaks observed at 486 and 489 are known as green emission peaks of CdS.…”

Experimental and theoretical studies on the enhanced photoluminescence activity of zinc sulfide with a capping agent

“…Uchida et al 41 made the correlation with the off stoichiometry of the Zn and S, and concluded that samples with both zinc and sulfur excess emitted in the blue range. Murase et al 42 conducted fluorescence and EPR studies of Mn doped ZnS, and the EPR measurements proved the existence of hole centers, such as Zn vacancies, and attributed the blue emission ($450 nm) to Zn vacancies. Zhang et al 19 attributed the blue emission to defects related to sulfur vacancies.…”

Time-resolved luminescence of ZnS:Mn2+ nanocrystals

“…Some groups reported the Mn 2+ luminescence decay times to be in the ns range [10,12], whereas others observed it in the ms range [11,17]. It was later shown by two independent groups [16,18] doped ZnS nanocrystals yielded yet another unexpected observation. In a study of the time-resolved emission spectra of 3.6 nm ZnS:Mn 2+ nanocrystals embedded in polyvinylbutyral, Yu et al [17] reported that, at short delay times, the 4 T 1 -6 A 1 luminescence band of Mn 2+ consisted of two peaks, at the high-and low-energy sides of the expected peak position.…”

“…These claims spurred many other publications, from different groups, reporting on the luminescent properties of Mn 2+ doped nanocrystals [9][10][11][12][13][14][15][16][17][18][19][20][21][22]. Some groups reported the Mn 2+ luminescence decay times to be in the ns range [10,12], whereas others observed it in the ms range [11,17].…”