Elsevier

Optical Materials

Volume 37, November 2014, Pages 641-645
Optical Materials

Cu-doped CdS and ZnS nanocrystals grown onto thiolated silica-gel

https://doi.org/10.1016/j.optmat.2014.08.008Get rights and content

Highlights

  • CdS and ZnS nanocrystals were prepared onto a thiolated silica-gel.

  • Nanocrystals’ surfaces were partially passivated.

  • Nanocrystals were doped with copper.

  • Semiconductor defect emissions were enhanced by copper ions.

Abstract

CdS and ZnS nanocrystals were grown over specific binding sites onto a thiolated silica-gel aiming to favor defect emission processes. This strategy was found to be effective in yielding ZnS nanocrystals with simultaneous blue and blue–green emissions owing to different types of defects. The effects of doping with copper ions have been observed on the photoluminescence properties. The intensity of defect-related emissions from both semiconductor nanocrystals increased with increasing dopant concentration from 0.25% to 1.5% copper, consistent with the presence of sulfur vacancies. Higher dopant concentrations lead to concentration quenching.

Introduction

Transition metal-doped semiconductor nanocrystals form an interesting class of nanomaterials that can be designed with specific characteristics, including tunable emissions over the visible range depending on the dopant metal [1]. Moreover, the doping process provides a means to probe and interfere in the electronic structures of the semiconductors since the dopant ions create energy levels in the band gap [2]. In contrast to Mn2+ for which the doping behavior is better understood, several other ions are being studied currently, but in the specific case of copper some questions remain open [3]. In this context Pradhan and co-workers solved some of the questions related to doping with copper ions such as the origins of Cu dopant emission and reasons for its emission tunability [4]. ZnS/ZnS1xCdxS zinc blend surface alloyed nanocrystals were designed with the band gap energy being dependent on the composition of the surface alloy. Authors observed that photoluminescence emission of the copper-doped nanocrystals was tunable over the visible region at different stages of the doping process. This observation clarified the participation of the copper levels as acceptor of an electron from the conduction band, which is consistent with the proposition that copper(I) d10 is formed during the doping process [4].

Recently Kumar and co-workers reported copper-doped ZnS films for which the emission of defect levels from ZnS is enhanced by copper doping, pointing out that the defects in the structure were related to sulfur vacancies [5]. The incorporation of additional cations in the structure increases the defect concentration and consequently the intensity of emissions arising from defect states to the valence band, in addition to the presence of emissions from the conduction band to the copper state. Thus, in order to contribute to the understanding of the effect of copper doping on CdS and ZnS nanocrystals photoluminescence, here CdS and ZnS nanocrystals were grown directly onto the surface thiolated silica-gel. It is well known that organic thiol groups have a high chemical affinity for both metallic and semiconducting nanoparticles, thus this has been used as a strategy for nanoparticle growth onto specific binding sites [6]. In this context Akins and co-workers reported that significantly higher amounts of CdS nanocrystals can be impregnated within the pores of thiol-modified mesoporous silicas in comparison to unmodified silicas [7]. We showed recently [8] that the use of thiol-modified silica matrices provides specific binding sites for semiconductor nanocrystals, which grow with surface defects favored by insufficient surface passivation by the matrix. This is particularly interesting here since it may open the possibility to study the effect of doping on defect emissions. In the opposite case, Kher and co-workers evidenced that an effective passivation of nanocrystal surface is observed upon the simultaneous preparation of silica particles embedded with copper-doped ZnS [9]. Here the effect of doping was studied by spectroscopic techniques, aiming to both understand the nature of the defects generated in different semiconductors and to obtain materials with interesting optical properties.

Section snippets

Preparation of mercaptopropyl-silica (MPS)

The functionalization of silica gel used here was reported previously [8] and was based on the procedure reported by Walcarius and co-workers [10]. Briefly 5 mL of 3-(mercaptopropyl)trimethoxysilane were mixed with 50 mL of a suspension of 5 g silica gel in toluene. After 24 h of reflux under stirring the solid was filtered, washed with toluene and dried at 45 °C for 12 h.

Growth of semiconductors onto MPS

  • (a)

    Undoped semiconductor nanocrystals: 25 mL of a 20 mmol L−1 cadmium acetate (or zinc acetate) and 37 mmol L−1 of thiourea solution in

Results and discussion

The effective silylation of silica gel with 3-(mercaptopropyl)trimethoxysilane to yield the thiolated silica was evidenced by infrared spectroscopy, Fig. 1. After silylation, the band assigned to Si–O stretching from silanol groups (Si–OH) originally present at 973 cm−1 weakens and shifts to 956 cm−1 concomitant to an increase of the band at 1068 cm−1 assigned to asymmetric stretching of siloxane groups (Si–O–Si). This is consistent with the silylation of the silica surface as it tends to both

Conclusions

The emissions of undoped samples presented significant contribution of defect states, ensured by the anchoring only at specific sites onto a thiolated matrix. In the case of ZnS this strategy lead to the presence of both blue and blue–green emissions, which is not very common at room temperature. Semiconductor emissions were shifted and enhanced by the presence of copper ions, which is consistent with the presence of defects related to sulfur vacancies in both cases.

Acknowledgments

Authors are grateful to Brazilian funding agencies CNPq, Fapitec, Capes, from financial support. Contributions from LNNano-LME (Campinas-SP, Brazil) for TEM analysis (TEM-MSC-13242) are also gratefully acknowledged.

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