Article pubs.acs.org/cm Probing Surface Saturation Conditions in Alternating Layer Growth of CdSe/CdS Core/Shell Quantum Dots † ‡ § † ‡ Rui Tan, Douglas A. Blom, Shuguo Ma, and Andrew B. Greytak*, , † ‡ § Department of Chemistry and Biochemistry, USC Nanocenter, and College of Engineering and Computing, University of South Carolina, Columbia, South Carolina 29208, United States *S Supporting Information ABSTRACT: We monitor effective band gap energy shifts and free reagent concentration during the formation of CdS shells on CdSe nanocrystals to test the hypothesis that alternating addition of stoichiometric doses of precursors can effectively saturate surface sites and thereby enforce conformal shell growth. The selective ionic layer addition and reaction (SILAR) mechanism has been proposed to describe growth under such conditions, and the method is attractive because of the opportunity to (1) avoid cross-reaction of precursors in growing binary films in solution and (2) enforce conformal growth, rather than regioselective growth, by saturating all available surface sites in a self-limiting manner in each half-cycle. The strong red shift that takes place when CdS shells are grown on CdSe cores provides a convenient process monitoring tool that complements scanning transmission electron microscopy imaging and analytical measurements of free reagent concentration. We find that, under commonly used conditions, a cadmium oleate precursor reacts incompletely at chalcogenide-saturated nanocrystal surfaces. Although approximately spherical particles are obtained, the growth does not proceed via saturating cycles, as described in the SILAR mechanism. This has implications for the rational control of conformal and regioselective growth of epilayers on nanocrystal quantum dots and higher-dimensional chalcogenide semiconductor nanostructures via solution processes. KEYWORDS: quantum dots, nanocrystals, core/shell, NMR, selective ionic layer adhesion and reaction, SILAR, scanning transmission electron microscopy he surfaces of a roughly spherical nanocrystal necessarily strong requirement of structural homogeneity among particles T include regions that do not share the same symmetry with during shell synthesis if homogeneous electronic properties are respect to the crystal lattice and are, therefore, structurally to be achieved. In the simplest case, this is a requirement for − distinct.1 4 These structurally distinct regions may naturally conformal shell growth on all surfaces and suppression of possess different reaction rates toward ligand exchange homogeneous nucleation of the shell material.22 The develop- reactions or further crystal growth.5 In some cases, ment of a controllable method for growing shells with different regioselective crystal growth, such as the formation of CdSe/ morphologies is necessary, and an understanding of the shell 6−10 CdS rod/tetrapod core/shell structures, CdS − Se nano- growth mechanism at play in existing procedures should aid in − 1 x x rods with axial anisotropy,11 13 multicomponent nanobar- the synthesis of a new generation of core/shell heterostructures. bells,14 or CdSe/CdS core/shell nanoplatelets,15 can be Alternating layer deposition techniques use a sequence of desirable as it leads to properties, such as a highly polarized self-limiting surface reactions to build up conformal thin films − excited state,16 18 high sensitivity to nonisotropic external on surfaces that may be structurally heterogeneous. Atomic stresses,10 multiexcitonic dual emission,19 and the capability to layer deposition (ALD) is an example of such a process using 36−39 engineer charge and energy flows, that are valuable in vapor-phase reagents. Selective ionic layer adhesion and applications.8,20,21 In other cases, enforcing conformal/isotropic reaction (SILAR) is an analogous technique for growing binary 40,41 growth, such as formation of spherical CdSe/CdS or CdSe/ZnS films from solution, in which addition of each ion type is core/shell quantum dots, is desirable as it will lead to increased nominally self-limiting. The goals of such an approach are to − photoluminescence quantum yield (QY),22 26 excitation rate (1) saturate available surface binding sites in each half-cycle in (absorption cross-section),27 and photo- and chemical order to enforce conformalgrowthand(2)avoidthe stability.28,29 Both the synthetic methods and the optical/ simultaneous presence of both precursors in the solution or electrical properties of such core/shell heterostructures have 30−35 been widely studied. While presenting important oppor- Received: July 1, 2013 tunities, the strong influence of the shell morphology on the Revised: August 14, 2013 properties of weakly confining core/shell systems imposes a Published: September 5, 2013 © 2013 American Chemical Society 3724 dx.doi.org/10.1021/cm402148s | Chem. Mater. 2013, 25, 3724−3736 Chemistry of Materials Article vapor so as to prevent uncontrolled surface growth or of scanning transmission electron microscopy (STEM). With homogeneous nucleation of film material. While SILAR was these tools in hand, we vary the order and amounts of the Cd 40,42 originally developed for use on planar substrates, over the and S precursor doses. Cd(oleate)2 is used as the Cd precursor past decade, alternating-layer methods have been extensively and bis(trimethylsilyl) sulfide ((TMS) S) is used as the S − 2 applied to growth of shells on colloidal nanocrystals.23,27,43 47 precursor for the formation of the CdS shell. In typical thin film growth by SILAR (or ALD), an excess of First, we are able to identify the native surface condition reagent is used in each half-cycle step to drive the surface (reactivity) of the CdSe cores by varying the order of Cd and S reaction to completion at all available sites. The excess is easily addition. Despite XPS and 31P NMR data indicating a Cd/Se removed before the next step. However, it is tedious to separate elemental ratio slightly greater than unity, the initial surface colloidal particles from excess reagent in each half-cycle. shows reactivity toward addition of Cd, but not toward the Ithurria and co-workers recently described a SILAR procedure addition of S. Second, we perform titration experiments in (termed colloidal ALD)48 that couples QD surface reactions to which we identify two regimes of response to Cd addition and solubility changes in order to achieve such separation in a find that addition of the Cd precursor beyond 1 ML equivalent biphasic solvent environment, and SILAR has also been applied continues to cause red shifting, consistent with an incomplete to supported nanocrystal QDs.49,50 However, more typically, surface reaction subject to equilibrium with dissolved species. reagents are added in doses that are calculated to provide The presence of unbound Cd is confirmed by ICP-MS. We also exactly 1 monolayer (ML) of coverage per nanocrystal in the show that the reactivity of the nanocrystals toward S addition is sample, and they are not removed between steps.43,46 If the entirely determined by the total amount of Cd added. Third, dose is too small, some surface sites will remain vacant, we conduct a series of alternating-addition shell growths using whereas, if the dose is too large, reagent will remain at the end submonolayer equivalent doses and find that lower doses favor of the step, which could lead to nucleation or non-self-limited greater red shifts, consistent with an overall higher synthetic growth when the complementary reagent is introduced in the yield for shell growth at small doses due to loss of precursors to subsequent half-step. Importantly, even if 1 ML equivalent (ML cross-reaction when larger doses are used. Analysis of STEM eq.) of reagent is added precisely, the surface reaction may not images supports the finding of higher synthetic yield for run to completion in the absence of excess reagent, instead submonolayer equivalent doses without loss of structural reaching an equilibrium state with dissolved species. control. Despite these concerns, a great deal of success has been achieved in terms of forming isotropic shells by SILAR-based ■ EXPERIMENTAL SECTION methods, notably in the case of CdSe/CdS core/shell quantum Materials. The following chemicals were used as received. − dots.23,27,43 46 These achievements have spawned increasing Cadmium oxide (CdO; 99.999%), trioctylphosphine (TOP; 97%), interest in the mechanisms of growth and intermediates that are and trioctylphosphine oxide (TOPO; 99%) were purchased from formed in these alternating layer addition procedures. Strem Chemicals. Oleic acid (OA; 99%), 1-octadecene (ODE; 90% Mulvaney et al. have examined changes in surface enrichment technical grade), 1-tetradecylphosphonic acid (TDPA; 98%), and Se ff (99.999%) were purchased from Alfa Aesar. Decylamine (95%) was and e ective band gap under single reagent addition to CdSe purchased from Sigma Aldrich. Oleylamine (80−90%) and bis(tri- QDs.51 Krauss et al. have reported significant changes in surface fi methylsilyl) sul de ((TMS)2S; 95%) were purchased from Acros enrichment and photoluminescence associated with alternating Organics. Toluene-d8 (D, 99.5%) was purchased from Cambridge layer addition to CdS QDs.52 Vela and co-workers have Isotope Laboratories, Inc. 200 proof ethyl alcohol (ethanol) was recently examined limitations of the SILAR procedure in obtained from Decon Laboratories, Inc. Acetone (99.9%) was controlling the growth of very thick CdS shells on CdSe QDs.46 purchased from VWR. Methanol (99.9%) was purchased from Fisher fi These studies