Article pubs.acs.org/JACS
Colloidal Atomic Layer Deposition (c-ALD) using Self-Limiting Reactions at Nanocrystal Surface Coupled to Phase Transfer between Polar and Nonpolar Media Sandrine Ithurria† and Dmitri V. Talapin*,†,‡ †
Department of Chemistry and James Frank Institute, University of Chicago, Chicago, Illinois 60637, United States Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States
‡
S Supporting Information *
ABSTRACT: Atomic layer deposition (ALD) is widely used for gas-phase deposition of high-quality dielectric, semiconducting, or metallic films on various substrates. In this contribution we propose the concept of colloidal ALD (cALD) for synthesis of colloidal nanostructures. During the cALD process, either nanoparticles or molecular precursors are sequentially transferred between polar and nonpolar phases to prevent accumulation of unreacted precursors and byproducts in the reaction mixture. We show that binding of inorganic ligands (e.g., S2−) to the nanocrystal surface can be used as a half-reaction in c-ALD process. The utility of this approach has been demonstrated by growing CdS layers on colloidal CdSe nanocrystals, nanoplatelets, and CdS nanorods. The CdS/CdSe/CdS nanoplatelets represent a new example of colloidal nanoheterostructures with mixed confinement regimes for electrons and holes. In these materials holes are confined to a thin (∼1.8 nm) two-dimensional CdSe quantum well, while the electron confinement can be gradually relaxed in all three dimensions by growing epitaxial CdS layers on both sides of the quantum well. The relaxation of the electron confinement energy caused a shift of the emission band from 510 to 665 nm with unusually small inhomogeneous broadening of the emission spectra.
1. INTRODUCTION Atomic layer deposition (ALD) is a powerful technique for deposition of high-quality thin films of dielectric oxides (e.g., Al2O3, HfO2, etc), semiconductors (ZnO, ZnS, InGaAs), and metals (Pt, Ir, etc) on various substrates.1,2 Compared to other gas-phase deposition techniques, ALD enables precise control over the layer thickness on the atomic scale and allows conformal deposition on porous and high-aspect ratio substrates. Most ALD processes rely on binary reaction sequences where two surface half-reactions are used to deposit a binary compound.1,2 Self-limiting surface reactions deliver gaseous precursors to all available surface sites but prevent material deposition beyond single atomic layer. In this work we discuss the concept of ALD in application to colloidal nanomaterials. The syntheses of functional nanostructures typically involve growth of new inorganic phase at the surface of colloidal “cores”.3 For example, the growth of a widegap shell around a semiconductor nanocrystal (NC) is routinely used to obtain core-shells with high quantum efficiency and stability.4 The growth of an epitaxial shell requires careful control over reactivity and concentrations of the shell precursors and the reaction temperature. If these requirements are not met, then the shell material can nucleate in form of separate nanoparticles (so-called secondary nucleation).5,6 Secondary nucleation can be significantly suppressed by employing successive ionic layer adsorption © XXXX American Chemical Society
and reaction (SILAR) technique that uses successive additions of metal and chalcogen precursors to grow shells of II−VI semiconductors in a layer-by-layer fashion.7 SILAR is conceptually related to ALD, but there is an important difference between these techniques. In SILAR no free precursors are allowed to be present after the completion of half-reaction. The amounts of the precursors used for each half-reaction are calculated to match one monolayer coverage for all cores.6,7 This requires precise knowledge of the total surface area for all cores present in the reaction mixture, which is a very challenging requirement, especially in the case of polydisperse or nonspherical cores. Moreover, SILAR assumes quantitative reaction yields for both half-reactions. Any inaccuracies in estimations of the above parameters accumulate with each SILAR cycle and lead to the lack of control.5 On the other hand, in traditional gas-phase ALD, precursors are efficiently separated throughout the deposition process by pulsing a purge gas after each half-reaction to remove excess precursor from the chamber. The ALD of a binary compound AB requires a four step sequence: (i) deposition of A layer; (ii) purge to remove all unreacted precursors for A; (iii) deposition of B layer that reacts with A on the surface; and (iv) purge of B precursors before staring new cycle.1 Steps (i) and (iii) are selfReceived: August 17, 2012
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dx.doi.org/10.1021/ja308088d | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Journal of the American Chemical Society
Article
limiting, once all the surface sites have reacted. Excessive reagents are purged off to prevent their reaction with the following reagent. The purge stages (ii) and (iv) permit using an excess of precursor in each half-cycle to drive saturation of surface binding sites. To design similar processes in a colloidal solution (further referred to as colloidal ALD or c-ALD), a similar sequence of steps should be realized in a liquid phase without compromising colloidal stability of NCs. We propose a c-ALD process based on self-limiting halfreactions coupled to the phase transfer of NCs or molecular precursors between immiscible nonpolar and polar phases. The phase transfer allows facile removal of unreacted molecular precursors and prevents accumulation of the reaction byproducts. During the past decade, colloidal syntheses of highquality nanomaterials have been dominated by the use of nonpolar solvents and long-chain hydrocarbon ligands sterically stabilizing NCs in nonpolar media.8 Our approach builds upon recent progress in stabilization of colloidal nanomaterials in highly polar solvents such as formamide (FA), N-methylformamide (NMF), hydrazine, etc using inorganic ligands, such as S2−, Se2−, Sn2S64−, etc.9−11 We show that the combination of these new surface ligands with traditional approach significantly expands the synthetic toolbox for colloidal nanomaterials.
repeated at least two times. It required ∼10 min for growing one CdS layer. Method B2. In a 6 mL vial, 1 mL FA, 2 μL of 40% aqueous solution (NH4)2S, 1 mL of toluene, 200 μL of 3.8 nm CdSe NCs in toluene (20 mg/mL), and 15 μL oleylamine (OAm) were combined and stirred at room temperature for 5 min. The NCs stayed in toluene phase stabilized by OAm. The nonpolar phase containing NCs was washed twice with pure formamide. Then 1 mL of formamide and 30 μL of 0.1 M Cd(Ac)2 solution in formamide were added to the polar phase and stirred for 5 min. The NCs stayed in toluene phase stabilized with OAm, while a layer of Cd2+ formed on the surface of CdSe/S2− NCs. The nonpolar phase was rinsed two times with fresh formamide. The successive monolayers were grown in the same way, without precipitation of NCs after each monolayer. It required less than 15 min for growing one CdS layer. c-ALD Growth in Polar Phase (e.g., NPLs). Method C. In a 6 mL vial, 1 mL NMF, 2 μL of 40% aqueous solution (NH4)2S, 1 mL hexane, 200 μL of CdSe NPLs with the absorption maximum at 510 nm dissolved in hexane (20 mg/mL) were stirred until complete phase transfer of NPLs from hexane to NMF. The polar phase containing CdSe NPLs was rinsed two times with hexane, followed by precipitation with acetonitrile and redispersion in fresh NMF. Then 30 μL of 0.1 M Cd(Ac)2 solution in NMF was added, and the mixture was stirred for 30 s. The NPLs were precipitated with toluene and redispersed in NMF. It required
