Factors Influencing the Luminescence Quantum Efficiency of Nanocrystalline ZnS:Mn2+

Abstract

The influence of the Mn 2+ concentration, the nature of the passivating polymer and UV irradiation on the luminescence efficiency of nanocrystalline ZnS : Mn 2+ is investigated. With increasing Mn 2+ concentration the quantum efficiency increases, until it reaches a stable value. No drastic decrease in quantum efficiency is observed at high Mn 2+ concentrations, as was found earlier for both bulk and nanocrystalline ZnS : Mn 2+ . By altering the passivating polymer many other factors influencing the quantum efficiency are changed, such as the particle diameter and Mn content. This makes it hard to study the influence of the passivating behaviour of the various polymers on the quantum efficiency. The UV enhancement of the quantum efficiency of the various samples could be explained by two mechanisms: UV curing of the passivating polymer and passivation by a photooxidation of the surface of the nanoparticles during the irradiation. The highest quantum efficiency obtained was about 15%.1. Introduction Efficiently luminescing nanocrystalline ZnS : Mn 2+ particles may be interesting for application in, e.g., electroluminescent devices. Many papers have reported on factors influencing the quantum efficiency of these nanoparticles. Bhargava et al. [1] reported that with decreasing particle size the quantum efficiency increases from 1% for particles of 7 nm to 18% for particles of 3.5 nm due to quantum size effects. Other researchers [2][3][4] have found that the quantum efficiency of nanocrystalline ZnS : Mn 2+ is dependent on the Mn 2+ concentration. Besides, several papers have appeared in which it was reported that the quantum efficiency of nanocrystalline ZnS and ZnS : Mn 2+ was enhanced by irradiation of the samples with UV [5][6][7][8][9].In view of the large surface area and the role of (surface) defects in the luminescence process, it can be expected that the luminescence quantum efficiency is extremely sensitive to the synthesis conditions. If one is studying the role of one parameter one has to be extremely careful in keeping all other conditions the same. It is our impression that this has not always been the case. Furthermore, in the literature often only relative intensities and no absolute quantum efficiencies are mentioned. This makes it difficult to compare the results from different experiments.The purpose of this study is to gain more insight into factors that influence the luminescence efficiency. This was carried out by performing a systematic study on the influence of the Mn 2+ concentration and passivating polymer on the luminescence efficiency. In addition, the UV-irradiation-induced enhancement of the luminescent intensity of various samples was investigated. Absolute quantum efficiencies were measured as well.

Effect of irradiation on the luminescence in ZnS:Mn2+ nanoparticles

“…The radius of the ZnS nanoparticles before the irradiation was calculated to be 2.97 nm and after the irradiation the radius was reduced to 2.36 nm. This indicates that the irradiation resulted in the decreasing of the nanoparticles, as was reported by Bol et al [24], but different from the results of Jin et al [10] that the size of the nanoparticles did not change after irradiation. It also can be seen that the shape of the excitation spectrum also changed.…”

“…The other is that the UV light induced the polymerization of the methacrylic acid stabilizer, leading to a better surface passivation [4,18]. The third is that the UV curing made the surface of the nanoparticles photo-oxidated, which provided a more efficient passivation [24]. In this paper, Mn 2+ -doped ZnS nanoparticles were prepared in AOT micelles and the irradiation-induced luminescence enhancement effect was observed too.…”

Luminescence properties of impurity-doped semiconductor nanoparticles

“…In particular, the report on Mn 2+ -doped ZnS nanoparticles (the high PL efficiency and the short PL lifetime) stimulated considerable efforts [41]. However, almost studies showed that the lifetime shortening of the Mn 2+ luminescence does not occur: The strong sp-d wave function mixing model cannot explain the enhancement of luminescence intensity in ZnS:Mn nanoparticles [36,42,43]. It is likely that the origin of rapid emission in Ref.…”

“…The impurity-doped nanoparticles have usually been fabricated by chemical synthesis methods such as coprecipitation, reverse micelles, and organometallic procedures [35,36,42,43,[57][58][59][60][61][62][63][64]. High-quality undoped II-VI semiconductor nanoparticles have been synthesized by high-temperature organometallic methods [6,13,14].…”

Improved photoluminescence of ZnS:Mn nanocrystals by microwave assisted growth of ZnS shell

“…have been used to modify the nanocrystal surface. UV light irradiation [17,18] was also shown to be effective in reducing surface defects via photooxidation and enhance the photoluminescent efficiency. However, there is still no effective way to control the segregation of the doped Mn ions in the water-soluble nanocrystals.…”