Precursor Conversion Kinetics and the Nucleation of Cadmium Selenide Nanocrystals Jonathan S

Home , Cadmium selenide

Precursor conversion kinetics and the nucleation of cadmium selenide nanocrystals Jonathan S. Owen, Emory M. Chan, Haitao Liu, and A. Paul Alivisatos* Department of Chemistry, University of California, Berkeley, The Materials Science Division, and The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720. Email: [email protected]

Uncertainty in Qo and nf. ……. S2

Figure S1. TEM of CdSe particles prepared using our optimized conditions. ……. S3

Figure S2. Example high-throught absorption data used in the determination of ……. S4 Figures 3 and 5.

Figure S3. Straight line fit used to determine Qo from [CdSe] versus time points ……. S4 after the induction period.

Figure S4. Concentration of nanocrystals in reactions with 1 - 3. ……. S5

Figure S5. The final number of nanocrystals versus the initial rate (Qo) of the ……. S6 precusor conversion (1 - 3).

Figure S6. Appearance of CdSe (solid symbols) and conversion of 1 (empty ……. S6 symbols) for reactions conducted with preformed Cd-ODPA (orange symbols) versus those conducted by simultaneous injection of CdMe2 and 1 (red symbols).

Figure S7. Appearance of CdSe across four concentrations of added 1 (0.25, 0.5, ……. S7 1.0, 2.0 equiv.) and three concentrations of ODPA (1.67, 1.72, 1.80 equiv.).

Figure S8. Diameter of NCs in aliquots taken from reactions using several ratios ……. S8 of ODPA:Cd and a range of tri-n-butylphosphine selenide (1).

Table S1. Experimental Results under standard conditions. ……. S9

Dissolution of CdSe in TOPO and ODPA. ……. S10

Scheme S1. Proposed equilibrium expression describing cadmium selenide ……. S10 dissolution and crystallization.

S1 Figure S9. Temporal evolution of the [CdSe] upon injection of nanocrystals into ……. S11 TOPO and ODPA, as measured from the nanocrystal absorbance at 350 nm.

Figure S10. Final [CdSe] measuered after equilibration with a range of ODPA ……. S11 concentrations.

Figure S11. Photoluminescence spectra from a series of aliquots (30 – 3930 sec.) taken from a study of NC ripening in the presence of ODPA. ……. S12

Figure S12. The average size of the nanocrsytals estimated from the ……. S12 fluorescence maximum in TOPO solution with ODPA (Red: 10%, Orange 5%, Green 0% ODPA by weight.).

Figure S13. Absorption (blue) and luminescence (orange) spectra from reaction ……. S13 between hydrogen selenide and cadmium n-octadecylphosphonate in TOPO at 100 ˚C.

Uncertainty in Qo and nf. Errors in Qo are predominantly caused by uncertainty in the slope of the straight line fit to early time points, particularly in reactions with substantial lag times where [CdSe] = 0 at early times. In order to determine Qo from each kinetic trace, a line is fit to the first ~6 non-zero points, and the slope used as a measure of Qo (Figure S2). By excluding the early time points during the lag time, the measured slope may be a slight overestimate of the initial reaction rate. We also determined a lower bound to the initial reaction rate, Qmin, by including the time points obtained during the lag and fitting a straight line through the intercept. The range of rate estimates Qo to Qmin were used to estimate the error as ± (Qo - Qmin)/2. This method gave us uncertainties as large as ±16% in the slowest reactions with the greatest lag times, and as little as ±5% where there was very little lag time and much faster reaction rates. Error bars shown in Figures 5, 6, and 8 show these range of rates, and make it clear that despite the inherent uncertainty in our measurement, we can easily distinguish between reaction rates as the concentration of the reagents is changed over a factor of eight. Uncertainty in our measure of nf is more significant. Error bars shown in Figure 8 reflect our estimated precision of ±5% in the mean diameter (davg) of nanocrystals that we estimate from the wavelength of their exciton absorption. The error in the davg of approximately one CdSe lattice plane (~0.2 nm) is inherent to the empirical relationship published by Yu et al., which was constructed using nanocrystal diameters obtained with TEM. Since the mean diameter of the nanocrystal samples averaged for each nf data point is 4 nm, the relative error in mean diameters and radii is 5%, which propagates to a 15% error in the volume. Although the CdSe concentration also contributes to the error in nf, the larger error in the volume dominates the overall uncertainty. Random error in [CdSe] can arise from fluctuations in liquid dispensing and absorbance detection, which we have determined to be ±3% by measuring the statistical variation in replicate samples.1 This small error, however, is insignificant relative to our estimate of 15% error in the volume.

Figure S1. TEM of CdSe particles prepared using our optimized conditions.

Figure S2. Example high-throughput absorption data used to construct Figures 3, 5 and S2.

!"#$ !%#&

$" !%#$ !$#& !$#"&!'()*+

&'()*+,-./0 %"

" 1"" %""" %1"" $""" $1""

23.*,-40

Figure S3. Straight line fit to the [CdSe] versus time points after the induction period used to determine Qo.

Figure S4. Concentration of nanocrystals in reactions with 1 - 3.

Figure S5. The final number of nanocrystals versus the initial rate (Qo) of the precusor conversion (1 - 3).

Figure S6. Appearance of CdSe (solid symbols) and conversion of 1 (empty symbols) for reactions conducted with preformed Cd-ODPA (orange symbols) versus those conducted by simultaneous injection of CdMe2 and 1 (red symbols).

Figure S7. Appearance of CdSe across four concentrations of added tri-n-octylphosphine selenide (0.25, 0.5, 1.0, 2.0 equiv.) and three concentrations of ODPA (1.67, 1.72, 1.80 equiv.).

Figure S8. Diameter of NCs in aliquots taken from reactions using three ratios of ODPA:Cd and a range of equivalents of tri-n-butylphosphine selenide (1).

Table S1. Experimental results under standard conditions.

[Cd-ODPA] Q [n ] va va ODPA:Cd Se : Cd o f (mM) (µM/sec) (µM) (CdSe/sec) (nm3/sec) 52 1.67 0.25 3.4(6) 5.2(5) 0.65 0.037 0.50 7.3(8) 8.2(7) 0.89 0.050 1.0 16(1) 11.5(7) 1.39 0.0779 1.5 24(1) 14(5) 1.71 0.0960 2.0 33(2) 15.9(2) 2.08 0.116

52 1.72 0.25 3.6(5) 2.9(5) 1.2 0.070 0.50 8(1) 5.1(4) 1.6 0.088 1.0 17(1) 8(4) 2.1 0.12 2.0 32(2) 11.8(9) 2.71 0.152

52 1.8 0.25 3.2(5) 2.2(2) 1.5 0.081 0.50 7.5(8) 4.2(3) 1.8 0.10 1.0 18(2) 6.7(3) 2.7 0.15 2.0 34(2) 11(1) 3.09 0.173

24 2.0 1.0 3(2) 1.8(2) 1.7 0.093 2.0 7.4(4) 3.2(2) 2.3 0.13 4.0 16.1(6) 5.2(2) 3.1 0.17 6.0 27.2(1) 8.9(3) 3.1 0.17 8.0 37(1) 13.4(1) 2.76 0.155

a Nucleus growth rates were calculated according to Equation 2 in the main text assuming Vm = 0.056 nm3/CdSe.

S9 Dissolution of CdSe in TOPO and ODPA. We investigated the effect of ODPA on the ripening of nanocrystals in TOPO solution. Ripening experiments were conducted by injecting an octadecene solution of purified nanocrystals into a TOPO and ODPA mixture at 300 ˚C. Aliquots of known mass were diluted to a known concentration as described in the experimental section and monitored with absorption and fluorescence spectroscopy. Upon mixing, the absorbance at 350 nm gradually drops and stabilizes at a value that is proportional to [ODPA] (Figure S9 and S10). Fluorescence spectra of the same aliquots reveal a continual increase in the average size and size dispersity of the nanocrystals (Figure S11). The evolution of these changes is fastest at high [ODPA] while little change is recorded if the nanocrystals are heated in TOPO alone (Figure S11). In addition to these changes the fluorescence spectra developed a broad emission band at wavelengths higher in energy than the nanocrystals band gap. (λ = 400 - 550 nm). At high [ODPA] (> 200 : 1, ODPA : CdSe) the color of the nanocrystals completely disappears, as does their fluorescence, and no diffraction from the cadmium selenide lattice could be observed from a dried sample of the reaction mixture. To test for the formation of hydrogen selenide under these conditions, dissolution of the cadmium selenide was performed under vacuum in a distillation apparatus with a liquid nitrogen cooled receiving flask. Over the course of ~15 minutes as the red-brown color of the reaction mixture disappeared and was replaced by a colorless solution, hydrogen selenide condensed in the receiving flask (1H NMR, d 1 77 1 = -1.3 ppm, CDCl3, J( Se - H) = 50 Hz). We were curious to see whether cadmium selenide could be prepared at low temperatures by conducting this reaction in reverse. For this purpose we added hydrogen selenide to a molten solution of Cd-ODPA (2:1 ODPA:Cd) in TOPO at 100 ˚C, which caused an immediate change to a yellow-orange color. An aliquot of this solution showed a UV-Visible absorption spectrum reminiscent of cadmium selenide clusters smaller than 2 nm in diameter and broad photoluminescence (See Figure S13). The powder X-ray diffraction spectrum of this material gave broad scattering and no reflections characteristic of the CdSe lattice. Rapid heating of this mixture to 275 ˚C caused the color to quickly darken, (< 1 min.) reminiscent of particle growth, and the broad fluorescence was quenched. These dissolution experiments suggest that a solute form of CdSe is in equilibrium with both nanoparticles as well as hydrogen selenide and cadmium n-octadecylphosphonate (Scheme S1). Based on these measurements we estimate that 200 moles/kg of ODPA is sufficient to dissolve 1 mole/kg of CdSe.

m H -ODPA (CdSe) 2 n+1

(CdSe) (H2-ODPA) (CdSe) m n n

Cd(ODPA) (H -ODPA) H Se 2 m-1 2 n Scheme S1. Proposed Solution equilibria responsible for cadmium selenide crystallization.

S10

Figure S9. Temporal evolution of the [CdSe] upon injection of nanocrystals into TOPO and ODPA, as measured from the nanocrystal absorbance at 350 nm.

Figure S10. Final [CdSe] measuered after equilibration with a range of ODPA concentrations.

S11

Figure S11. Photoluminescence spectra from a series of aliquots (30 – 3930 sec.) taken from a study of NC ripening in the presence of ODPA. The peak maximum of the growing nanocrystals was used to make Figure S12.

Figure S12. The average size of the nanocrsytals estimated from the fluorescence maximum in TOPO solution with ODPA (Red: 10%, Orange 5%, Green 0% ODPA by weight.).

S12

Figure S13. Absorption (blue) and luminescence (orange) spectra from reaction between hydrogen selenide and cadmium n-octadecylphosphonate in TOPO at 100 ˚C.

S13