Theory of luminescent emission in nanocrystal ZnS:Mn with an extra electron

Abstract

We consider the effect of an extra electron in a doped quantum dot ZnS : M n 2+ . The Coulomb interaction and exchange interaction between the extra electron and the states of the Mn ion will mix the wavefunctions, split the impurity energy levels, break the previous selection rules and change the transition probabilities. Using this model of an extra electron in the doped quantum dot, we calculate the energy and the wave functions, the luminescence efficiency and the transition lifetime and compare with the experiments. Our calculation shows that two orders of magnitude of lifetime shortening can occur in the transition 4 T 1 − 6 A 1 , when an extra electron is present.PACS numbers: 73.20. Dx, 78.60.J, 42.65, 71.35 1 I.Introduction. In contrast to undoped materials, the impurity states in a doped nanocrystal play an important role in the electronic structure, transition probabilities and the optical properties. In recent years, attempts to understand more about these zero-dimensional nanocrystal effects have been made in several labs by doping an impurity in a nanocrystal, searching for novel materials and new properties, and among them Mn-doped ZnS nanoparticles have been intensively studied [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Among many bulk wide band gap compounds, manganese is well known as an activator for photoluminescence (PL) and electroluminescence (EL) and the Mn 2+ ion d-electrons states act as efficient luminescent centers while doped into a semiconductors.In 1994 Bhargava and Gallagher [1,2] reported the first realization of a ZnS semiconductor nanocrystal doped with Mn isoelectronic impurities and claimed that Mn-doped ZnS nanocrystal can yield both high luminescence efficiency and significant lifetime shortening. The yellow emission characterized for Mn 2+ in bulk ZnS [16][17][18], which is associated with the transition 4 T 1 − 6 A 1 , was reported to be observed in photoluminescence (PL) spectra for the Mn 2+ in nanocrystal ZnS. In nanocrystals, however, the PL peak for the yellow emission is reported slightly shifted toward a lower energy (in bulk ZnS:Mn it peaks arount 2.12 ev, in nanocrystal ZnS:Mn it peaks at 2.10 eV). Also, the reported linewidth of the yellow emision in the PL spectrum for a nanocrystal is larger than for the bulk. Most strikingly, the luminescence lifetime of the Mn 2+ 4 T 1 − 6 A 1 transition was reported to decrease by 5 orders of magnitude, from 1.8 ms in bulk to 3.7 ns and 20.5 ns in nanocrystals while maintaining the high (18%) quantum efficiency.In ref.[6] the authors suggested that the increase in quantum efficiency as well as the lifetime shortening is the result of strong hybridization of s-p electrons of the ZnS host and d-electrons of the Mn impurity due to confinement, and also of the modification of the crystal field near the surface of the nanocrystals. Stimulated by this dramatic result, many other laboratories are trying to synthesize the Mn-doped ZnS nanocrystals and considerable attention has been paid to optical properties o...

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