Luminescence in colloidal Mn2+-doped semiconductor nanocrystals

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Abstract

Recent advances in nanocrystal doping chemistries have substantially broadened the variety of photophysical properties that can be observed in colloidal Mn2+-doped semiconductor nanocrystals. A brief overview is provided, focusing on Mn2+-doped II–VI semiconductor nanocrystals prepared by direct chemical synthesis and capped with coordinating surface ligands. These Mn2+-doped semiconductor nanocrystals are organized into three major groups according to the location of various Mn2+-related excited states relative to the energy gap of the host semiconductor nanocrystals. The positioning of these excited states gives rise to three distinct relaxation scenarios following photoexcitation. A brief outlook on future research directions is provided.

Graphical abstract

Mn2+-doped semiconductor nanocrystals are organized into three major groups according to the location of various Mn2+-related excited states relative to the energy gap of the host semiconductor nanocrystals. The positioning of these excited states gives rise to three distinct relaxation scenarios following photoexcitation.

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Introduction

Colloidal doped semiconductor nanocrystals (doped quantum dots) have drawn considerable attention in recent years. Such materials have been shown to display efficient sensitized impurity luminescence [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], large magneto-optical effects [7], [12], [13], [14], [15], [16], and interesting quantum size effects on impurity-carrier binding energies [17]. They have been proposed for use as biological labels [2], [18], [19] and as recombination centers in hybrid organic/inorganic electroluminescent devices [20], [21]. They could potentially even find application as spin filters in future spin-based information processing devices [22], [23]. Dopants within nanocrystals have been used as probes of microscopic structural parameters, [24], [25], [26] and charged/doped colloidal nanocrystals have been examined experimentally [27] and theoretically [23], [28], [29], [30], [31] as models for magnetic polarons. Colloidal doped oxide semiconductor nanocrystals have played an important role in revealing the significance of grain-boundary defects in activating the high-temperature ferromagnetism sometimes observed in these materials [13], [32], [33], [34], [35], [36], [37]. The synthetic challenges of doping semiconductor nanocrystals have also provided fertile grounds for investigation of the basic chemistries of homogeneous nucleation and crystal growth in the presence of impurities [38]. Dopant exclusion from critical nuclei has been suggested to be a general phenomenon [13], [15], [38], [39], and the modes of dopant binding to crystallite surfaces have been investigated experimentally [40] and theoretically [41] in the context of impurity incorporation. For related reviews containing more description of sample syntheses, the reader is referred to Refs. [1], [2], [3], [38], [42] and references therein.

This short review focuses on one of the most fundamental physical properties of colloidal doped semiconductor nanocrystals, namely the nanocrystal photoluminescence (PL). The discussion focuses on Mn2+ as a dopant, and on II–VI semiconductors as hosts. Although the luminescence of doped semiconductors has attracted a great deal of interest for many years, the emergence of synthetic methodologies for preparing high-quality colloidal doped semiconductor nanocrystals has sparked renewed interest in this class of materials. Even within the narrow range of materials covered in this review (Mn2+-doped II–VI semiconductor nanocrystals), a rich variety of photophysical properties is already found, and large qualitative changes in these physical properties (and hence in potential applications) can be achieved through changes in the host semiconductor lattice or through quantum confinement effects. This wealth of physical properties is attractive for future applications of such materials.

Section snippets

Case studies of luminescence in Mn2+-doped II–VI semiconductor nanocrystals

In this section, Mn2+-doped II–VI semiconductor nanocrystals are organized into three distinct categories according to the nature of their lowest energy excited state, which determines their resulting photophysical properties.

Overview and outlook

Fig. 10 summarizes schematically the electronic structural features of colloidal Mn2+-doped II–VI semiconductor nanocrystals that lead to the three distinct photophysical scenarios described above. These three scenarios reflect three distinct relationships between the energies of the magnetic ions’ excited states and those of the host semiconductor nanocrystals. Scenario III PL has only very recently been demonstrated, in Mn2+-doped CdSe semiconductor nanocrystals [62]. In that case, the size

Acknowledgments

The authors thank their coworkers, collaborators and collegues who have contributed to research described in this manuscript, including: X. Liu, S. Lee, G.M. Salley, M. Dobrowolska, J.K. Furdyna, A. Meijerink, J. van Rijssel, W.K. Liu, S.A. Santangelo, N.S. Norberg, and K.R. Kittilstved. Financial support from the US NSF (PECASE DMR-0239325 to D.G.), Research Corporation, Dreyfus Foundation, Sloan Foundation, and the NSERC Postdoctoral Fellowship program (to R.B.) is gratefully acknowledged.

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