Abstract
Annealing or growth at high temperatures for an extended period of time is considered detrimental for most synthetic strategies for high-quality Mn-doped II-VI semiconductor nanocrystals. It can lead to the broadening of size distribution and, more importantly, to the loss of dopants. Here, we examine how ripening can be beneficial to doping in a simple “heat-up” approach, where high dopant concentrations can be achieved. We discuss the interplay of the loss of dopants, Ostwald ripening, and the clustering of Mn near the surface during nanocrystal growth. Smaller nanocrystals in a reaction batch, on average, exhibit higher undesirable band-edge photoluminescence (PL) and lower desirable dopant PL. The optimization of dopant loss and the removal of such smaller undesirable nanocrystals through Ostwald ripening along with surface exchange/passivation to remove Mn clustering lead to high Mn PL quantum yields (45 to 55 %) for ZnSxSe1−x, ZnS, CdS, and CdSxSe1−x host nanocrystals. These results provide an improved understanding of the doping process in a simple and potentially scalable synthetic strategy for achieving “pure” and bright dopant emission.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-015-1123-9) contains supplementary material, which is available to authorized users.
Highlights
Doping semiconductors with transition metals can impart new and useful optical, electronic, and/or magnetic properties [1]
While there are currently several different approaches to synthesizing Mn-doped II-VI semiconductor NCs, we focus here on the heat-up synthesis because high-quality/ high-PL materials can be achieved with simplicity and because of its potential for scale up and extension to anisotropic structures [41, 44, 45]
An understanding of how dopant incorporation and loss are affected by high temperature growth/annealing is of critical importance in order to take advantage of potential benefits of the heat-up synthesis
Summary
Doping semiconductors with transition metals can impart new and useful optical, electronic, and/or magnetic properties [1]. Additional prospects arising from combining quantum size effect with dopant-enabled capabilities have led to the exploration of various dopants in several different types of semiconductor nanocrystals (NCs) [1,2,3,4,5,6,7,8,9,10], with Mn-doped II-VI NCs garnering much attention [2, 4, 6,7,8,9,10,11,12,13,14,15,16,17,18,19]. In addition to intriguing magneto-optical properties [20, 21], energy transfer from the photo-excited host to the dopant can lead to high quantum yield (QY) photoluminescence (PL) from relaxed spin-forbidden 4T1 → 6A1 transition [22]. Mn-doped Zn-based II-VI NCs are often considered as heavymetal-free, low-toxicity alternatives to CdSe and related NCs
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