Luminescence of nanocrystalline ZnS:Pb 2+
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Luminescence of nanocrystalline ZnS:Pb2+ Ageeth A. Bol, Andries Meijerink Debye Institute, Physics and Chemistry of Condensed Matter, Utrecht University P.O. Box 80 000, 3508 TA Utrecht, The Netherlands Fax: +31 30 253 2403, E-mail: [email protected] ABSTRACT Nanocrystalline ZnS:Pb2+ is synthesized via a precipitation method. The luminescence is studied and the influence of the size of the nanocrystals on the luminescence properties is investigated. Nanocrystalline ZnS:Pb2+ shows a white emission under UV excitation. At least two luminescence centers are involved. One center is identified as a Pb2+ ion located on a regular Zn2+ site and gives a red emission under 480 nm excitation. The luminescence properties of this emission are characteristic for transitions on Pb2+ ions. The other centers are not as well defined and give a broad green emission band under 380 nm excitation and also show luminescence properties typically observed for Pb2+. The green emission probably originates from a chargetransfer like D-band emission of Pb2+ in ZnS close to a defect (e.g. an S2- vacancy or an O2- ion on an S2- site). A relation between the temperature quenching of the emissions and the band gap is observed and indicates that photoionization occurs.
INTRODUCTION Efficiently luminescing undoped and doped semiconductor nanocrystals have received considerable attention during the last decades [1-4]. The changes in the electronic structure as a function of particle size (quantum size effects) are intriguing and have stimulated fundamental research on various types of nanocrystalline semiconductors. Up till now, research of doped nanocrystals has been focussed on the luminescence of nanocrystalline ZnS:Mn2+. Various studies have reported an influence of quantum size effects on the luminescence of luminescing dopants incorporated in the nanocrystals [e.g., 5]. Recent studies have provided convincing evidence against the presence of an influence of quantum size effects on the lifetime and efficiency of the dopant emission in ZnS:Mn2+ [6, 7]. In this article the luminescence of a different quantum sized material, nanocrystalline ZnS:Pb2+, is described. The Pb2+ ion (outer electron configuration: 6s2) belongs to the group of s2 ions. The s2 configuration gives a non-degenerate ground state, 1S0. The first excited state with s-p configuration is split by spin-orbit coupling into three triplet states (3P0, 3P1 and 3P2) and at higher energy there is one singlet state (1P1). In the literature the transitions between the ground state (1S0) and these excited states are labeled with A (3P1), B (3P2) and C (1P1). A transition to a fourth excited state, the so-called D-band, has also been observed for Pb2+ in several compounds, and is considered to be a charge transfer type transition [8]. Charge transfer transitions involve the transfer of an electron from the ligand to the metal ion or vice versa. These transitions are characterized by intense broad absorption and emission bands and large Stokes shifts [9]. The position of this band is strongl
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