Review pubs.acs.org/cm
Chemistry of Doped Colloidal Nanocrystals Raffaella Buonsanti* and Delia J. Milliron* The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ABSTRACT: Synthetic control over inorganic nanocrystals has made dramatic strides so that a great number of binary and a few ternary or more complex compounds can now be prepared with good control over size and physical properties. Recently, chemists have tackled the long-standing challenge of introducing dopant atoms into nanocrystals, and strategies that apply across diverse compositions are beginning to emerge. In this review, we first briefly summarize the array of characterization methods used to assess doping efficacy for reference throughout the discussion. We then enumerate chemical strategies for doping with illustrative examples from the literature. A key concept is that the reactions leading to growth of the host crystal and to deposition of dopant ions must be balanced to succeed in incorporating dopants during crystal growth. This challenge has been met through various chemical strategies, and new methods, such as postsynthetic diffusion of dopant ions, continue to be developed. The opportunity to deliver new functionality by doping nanocrystals is great, particularly as characterization methods and synthetic control over introduction of multiple dopants advance. KEYWORDS: nanoparticle, single source precursor, defects, photoluminescence, core/shell, semiconductor
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aluminum-doped zinc oxide (AZO) NCs by applying an electrochemical potential.12,13 Such modulation offers the unique opportunity of using these NCs as window coating material to dynamically modulate the transmittance of solar infrared radiation. The introduction of dopants can also broaden the range of luminescence properties achievable beyond what is possible in pure materials. Cu and Mn-doped ZnSe NCs can be, for example, alternative less toxic light emitters compared to CdSe NCs.18 Tunable dual-color emission can be achieved in Mndoped ZnS nanorods as well as in more complicated doped heterostructures such as Zn1‑xMnxSe/ZnCdSe core/shell and Zn1−x−yCdxMnySe alloyed nanocrystals.19−21 Upconverting NCs represent an interesting class of luminescent doped NCs.22−28 They can be used as probes for single-molecule imaging in cells and other complex biological environment, applications inaccessible with bulk materials. Interesting magneto-optical phenomena have been observed when magnetic impurities are incorporated in semiconductor NCs.29−34 Compared to their bulk counterparts, more exotic behavior has been revealed in diluted magnetic semiconductor NCs since spin−spin exchange interactions are strongly influenced by confinement of the electrons and holes. Predicted but not previously experimentally proven, carrier mediated ferromagnetic interactions were observed in Mn-doped ZnO NCs by Gamelin’s group.32−34 The same group demonstrated
t the nanoscale, unique effects, such as variation in electronic state density and magnetic moments, surfaceand strain-driven lattice distortion, and charge redistribution, systematically evolve with size and impact the structural stability, the magneto-optoelectronic response, and the chemical reactivity.1 When dopants are introduced in such systems, the emergence of new properties and the development of new advances for applications can be easily anticipated. Chemists have developed excellent control over size and shape by colloidal synthesis, facilitating systematic studies.2 Recently, the chemistry to control dopant incorporation has progressed, opening new opportunities and posing new challenges. In heterovalent doping, impurities in a different valence state than the host cations are intentionally incorporated to provide either extra electrons (n-type) or extra holes (p-type). These carriers can introduce extrinsic conductivity to an otherwise poorly conducting material. Relatively few examples of heterovalent doping of chalcogenide and pnictide colloidal nanocrystals (NCs) have been reported in the literature.3−9 Recent results suggest that the prototypical semiconductor NCs, CdSe quantum dots, have now been doped n-type and ptype through the use of indium, tin, aluminum, or silver.3,5,6 Meanwhile, n- and p-type doping has been definitively shown to occur through noble metal incorporation in InAs NCs.4 Heterovalent doping is perhaps the best developed, and the potential applications are most compelling, among wide band gap oxide semiconductors.10−17 When degenerately doped, they become transparent conductive oxides (TCOs). Localized surface plasmon resonance absorption has been revealed by others and us in n-type TCO NCs as a result of their small size compared to the plasmon wavelength within the material.10−14 We also demonstrated the possibility of dynamically tuning the infrared absorption of tin-doped indium oxide (ITO) and © 2013 American Chemical Society
Special Issue: Synthetic and Mechanistic Advances in Nanocrystal Growth Received: December 23, 2012 Revised: March 6, 2013 Published: March 19, 2013 1305
dx.doi.org/10.1021/cm304104m | Chem. Mater. 2013, 25, 1305−1317
Chemistry of Materials
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the spontaneous photoinduced polarization of Mn2+ spins in colloidal doped CdSe NCs.35 Interesting size-dependent magnetic exchange for a fixed dopant concentration in Mndoped CdSe NCs was revealed by Zheng and Strouse.36 It is important to emphasize that only effective doping incorporation within the crystalline lattice would give rise to the aforementioned properties. Effective doping incorporation means that the dopants are actually substituting host atoms in the NC core rather than just being adsorbed on the nanocrystal surface. This has not been trivial to achieve to the point that NCs were earlier deemed “undopable”.37−40 One of the recurring explanations has been that NCs undergo selfpurification due to the higher formation energy of defects and impurity incorporation compared to bulk materials.38,40 For this reason, NCs will tend to anneal out dopants while growing to minimize the overall free energy with the assumption that NCs are grown under thermodynamic equilibrium. However, kinetics rather than thermodynamics govern NC growth at the relatively low-temperature (
