Titration experiments: Complete monolayer

The indication of free Cd present after 1.0 ML equivalent dose, together with evidence for high-yielding shell growth from Cd(oleate)2 and (TMS)2S when Cd is added

first, prompted us to examine how the effective bandgap, as well as the solution Cd concentration, varies over the course of a complete SILAR cycle. Figure 2.8 presents a titration experiment in which CdSe cores were treated first with the of 1 ML of Cd(oleate)2, followed by the addition of excess (TMS)2S, at a constant temperature of

200°C (details are in Table 2.3). The absorption and emission spectra that recorded for each aliquot as well as the effective bandgap shifts and emission energy shifts were shown in Figure 2.9. Each aliquot was drawn at the end of each 0.1 ML eq. dose addition step. For a subset of these aliquots, the solution Cd concentration was determined by precipitating the QDs with acetone and analyzing the supernatant for Cd via ICP-MS.

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Figure 2.8 Titration of one complete ML eq. of CdS shell growth. (A) Selected absorption spectra, normalized by the height of the lowest energy exciton peak and vertically offset for clarity. (B) Shift of effective band gap during first ML of CdS shell growth. A selection of 12 aliquots was analyzed further with ICP-MS; these are indicated by the numbers and arrows at the bottom of the plot. (C) Free Cd concentration ([Cd]) measured by ICP-MS. The red squares indicate the measured [Cd] in the aqueous digest of the supernatant of the selected aliquots after QDs have been removed by precipitation; blue circles indicate the values that would be expected if all of the added Cd remained in free solution (i.e. none consumed). Free Cd concentration continuously increasing over the addition of Cd(oleate)2 and decreasing followed the addition of (TMS)2S, finally

completely consumed after the addition of 1 ML eq. dose of (TMS)2S (D) Increase in

surface coverage of Cd as a function of [Cd] during the Cd(oleate)2 addition cycle shown

in (A-C). Surface coverage is expressed as a percentage of 1 ML eq. dose and is calculated from the difference between total added Cd and the amount remaining in free solution. The surface coverage only reaches ~60% even after 1 ML eq. dose of Cd(oleate)2 were added. Experimental details are provided in Table 2.3. Copyright 2013

Figure 2.9 (A), Absorption and (B), emission spectra for titration with complete monolayer of CdS shell. (C), Band gap energy shift and (D), emission energy shift for titration with complete monolayer of CdS shell. Copyright 2013 American Chemical Society.

The resulting changes in spectrum and effective bandgap are shown in Figure 2.8A, B. As before, two different red-shifting rates steps were observed over the course

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of the 1 ML Cd(oleate)2 half-cycle, followed by a strong and continuous red-shift when

(TMS)2S was added. The addition of (TMS)2S was continued to excess (past 1 ML) in

order to investigate saturation effects. Indeed, the redshift of the effective bandgap abruptly stops, and in fact reverses direction, at 1 ML – the point at which precisely equal amounts of Cd and S precursor have been introduced. The reason for the blue-shift at excess (TMS)2S is not presently clear, but the fact that it happened right after a full 1 ML dose of Cd and S precursors suggests that the change in behavior is associated with depletion of the reactivity provided by the added Cd.

Figure 2.8C shows the total Cd concentration ([Cd]) detected by ICP-MS in aliquots sampled throughout the complete 1 ML SILAR cycle (red squares). A small but measureable [Cd] was found after bringing the CdSe core sample to temperature in the shell growth solvent; a much larger concentration builds in as Cd(oleate)2 is added. It is

clear that while [Cd] continuously increased during titration of Cd(oleate)2, [Cd] was

lower than would be expected if all of the added Cd remained in solution (i.e. if none were consumed by reaction with the QD surface: blue circles).

This indicates that some, but not all, of the added Cd reacts with the initial CdSe surface, in agreement with our interpretation of the results shown in Figure 2.6. With our knowledge of the total amount of Cd added, the amount that remains in solution, and the estimated dose corresponding to 1 ML, it is possible to construct a plot of the fractional occupancy of surface sites by Cd as a function of the solution concentration (Figure 2.8D). Interestingly the amount of bound Cd is seen to increase late in the Cd cycle, even though the effective bandgap experiences only a small redshift in this region. And the surface coverage only reaches ~60% even after 1 ML eq. dose of Cd(oleate)2.

In the subsequent half-cycle in which (TMS)2S is titrated in, the free Cd is

continuously consumed, consistent with the formation of a CdS, and after 1 ML eq. of (TMS)2S, [Cd] has nearly returned to its starting value. Exhaustion of the free Cd

coincides with the endpoint observed in the redshift data shown in Figure 2.8B.

The high concentration of free Cd at the conclusion of the Cd addition cycle indicates incomplete saturation of the QD surface and would appear to increase the risk of nucleation of CdS particles by cross-reaction between Cd(oleate)2 and (TMS)2S. These

conditions are contrary to the SILAR mechanism and potentially detrimental to the conformal and high-yielding shell growth process that the SILAR procedure is designed to provide. At the same time, these observations imply that a benefit may be obtained by reducing the dose per cycle from a full monolayer to a sub-monolayer dose in order to suppress nucleation32,126 and increase synthetic yield, while retaining a largely spherical shell.