Crystal-Phase Control of Catalytically Grown Colloidal CdTe Quantum

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Crystal-Phase Control of Catalytically Grown Colloidal CdTe Quantum Wires: Dual Role of n-Tetradecylphosphonic Acid Fudong Wang, and William E. Buhro Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04975 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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Chemistry of Materials

Crystal-Phase Control of Catalytically Grown Colloidal CdTe Quantum Wires: Dual Role of n-Tetradecylphosphonic Acid Fudong Wang* and William E. Buhro* Department of Chemistry and Institute of Materials Science and Engineering, Washington University, St. Louis, Missouri 63130-4899. Corresponding Authors: [email protected], [email protected] ABSTRACT: n-Tetradecylphosphonic acid (TDPA) is an acid and also a source for a strong-binding ligand,

n-tetradecylphosphonate

(TDPT),

for

the

preparation

of

the

cadmium

precursor

Cd(DOPT)x(TDPT)1-0.5x (where DOPT = di-n-octylphosphinate). The reaction chemistry of the cadmium precursor and tri-n-octylphosphine telluride (TOPTe) is manipulated to favor the formation of a CdTe solute if the catalytic role of TDPA as an acid is predominant, otherwise the stabilization of the cadmium precursor by TDPT incorporation results in a significant excess of Te in the reaction mixture. These exert opposite consequences on the crystal-phase purity of colloidal CdTe quantum wires (QWs) grown from initial bismuth (Bi) nanoparticles. Primary dissolution of the available CdTe or Te into Bi nanoparticles affords liquid (y  z) or solid (y « z) BixCdyTez catalysts, respectively. The solid catalysts enable the solution-solid-solid growth of nearly phase-pure wurtzite QWs, whereas the liquid catalysts fulfill the solution-liquid-solid growth of polytypic QWs, having wurtzite and zinc blende alternations. This work reveals the dual role of TDPA and rationalizes its use in the crystal-phase controlled synthesis of catalytically grown colloidal QWs.

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INTRODUCTION We report that the crystal-phase purity of colloidal CdTe quantum wires (QWs) grown from initial bismuth (Bi) nanoparticles is affected by the form of n-tetradecylphosphonic acid (TDPA, Scheme 1) present in the reaction mixture. TDPA may exist as a free acid, or in its deprotonated form, ntetradecylphosphonate (TDPT), incorporated into the cadmium (Cd) precursor Cd(DOPT)x(TDPT)1-0.5x, where DOPT is di-n-octylphosphinate. Nearly phase-pure, defect-free wurtzite (WZ) QWs are grown from pre-synthesized Cd(DOPT)x(TDPT)1-0.5x. In contrast, polytypic QWs result when a catalytic amount of the free acid TDPA is added to the reaction mixture. The mechanistic role of TDPA and TDPT in the crystal-phase control is elucidated herein. Scheme 1. Abbreviations for acids and their deprotonated derivatives.

Semiconductor nanowires present exciting possibilities for applications in electronics,1-2 photonics,2-3 photovoltaics,4 sensors,5-6 and batteries.7 Catalytically grown semiconductor (II-VI and III-V) nanowires by the vapor-liquid-solid (VLS)8 and solution-liquid-solid (SLS)9-11 mechanisms, however, generally contain high densities of planar defects such as twinning boundaries, stacking faults, and wurtzite (WZ)zinc blende (ZB) phase alternations,10,12-18 which are deleterious to their optical and transport properties,19-23 and are thus undesired in many applications. Understanding and controlling the crystal phases of nanowires in growth are necessary for their full potential to be realized, and have therefore received much research interest.12-13,16,24-34 Phase-pure WZ or ZB nanowires have been achieved predominantly for the VLS mechanism,13,16,24-31 by controlling the growth temperature,13,16 precursor-flux ratios,16 compositions of the catalysts,25,2829,31,35-36

wire diameter,13,24 and/or growth orientations,26-27 Successful crystal-phase control of nanowires


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by catalyzed solution-phase methods has just begun to emerge. Recently, we achieved nearly phase-pure, colloidal WZ QWs, and discovered that the initial Bi-nanoparticle seeds transform into solid catalysts, which catalyze the solution-solid-solid (SSS) growth of the wires.32 The catalystwire contact angle in SSS growth stays near invariant,32,37-40 which removes a significant origin of the fluctuating nucleation barriers that alternate between the WZ and the ZB phases in SLS growth, and thus favors the nucleation of the lower-barrier WZ phase and the growth of nearly phase-pure WZ QWs.25,32,41-44 In a subsequent study, we found that the precursor-conversion chemistry (eq 1) of the Cd precursors and tri-n-octylphosphine telluride (TOPTe) plays a critical role in determining the crystal structure of the CdTe QWs.45 The precursor-conversion chemistry is manipulated by the reactivities of the Cd precursors, the Cd:Te precursor ratio, a catalytic additive di-n-octylphosphine (DOP), and/or the temperature. Preferred production of a CdTe solute (process I) or Te (process II) and their subsequent dissolution into the Bi nanoparticles result in liquid or solid BixCdyTez catalyst nanoparticles, respectively. Solid catalysts (y « z) enable the SSS growth of nearly phase-pure WZ QWs, whereas the liquid catalysts (y  z) fulfill the SLS growth of polytypic QWs.

The motivation for the present work was to investigate the mechanistic role of organic acids, particularly TDPA, a commonly used ligand additive in semiconductor nanocrystal syntheses,46-51 in the crystal-phase control of CdTe QWs. Previous catalytic growth of colloidal II-VI nanowires was often conducted by employing excess carboxylic or alkylphosphonic (e.g., TDPA) acids, and those wires were typically polytypic, without phase control.17,52-60 Such polytypism may originate from predominant process I over II in eq 1 under conditions of high Cd-precursor reactivity, high precursor (e.g., Cd:Te) ratio, high DOP concentration, and/or high temperature, as previously found in the growth of CdTe QWs.45 The role of excess acids, however, is unclear. 3 ACS Paragon Plus Environment


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In this study, we identified that TDPA or TDPT influences the precursor-conversion chemistry of Cd(DOPT)2 and TOPTe, and exerts a deleterious or beneficial effect on the crystal-phase control of the CdTe QWs, respectively. A sufficient amount of added TDPA, and hence the presence of acidic protons, catalyzes the formation of CdTe solute (process I),61-66 and subsequent, primary dissolution of CdTe into the Bi nanoparticles affords liquid BixCdyTez (y  z) catalysts for the SLS growth of polytypic QWs.45 In contrast, if TDPA is incorporated as TDPT to form Cd(DOPT)x(TDPT)1-0.5x having no or an insignificant amount of acidic protons present, a decrease in Cd-precursor reactivity and thus a significant excess of Te (process II) is produced. Primary incorporation of Te into Bi nanoparticles affords solid BixCdyTez (y « z) catalysts for the SSS growth of nearly phase-pure, defect-free WZ QWs.45 RESULTS AND DISCUSSION The CdTe-QW growth was initiated by injecting a mixture containing TOPTe and Bi nanoparticles into a hot mixture of Cd precursor and tri-n-octylphosphine oxide (TOPO), which is detailed in the Methods section. All syntheses were conducted under previously optimized conditions,45 with Cd:Te precursor ratios of 2.83.1 and DOP mol% of 0.03%. Cd(DOPT)2, the most useful Cd precursor for WZ CdTe-QW growth,45 was employed as the base Cd precursor to determine how TDPT incorporation in pre-synthesized Cd(DOPT)x(TDPT)1-0.5x or the addition of TDPA to the reaction mixture would affect the crystal phase of the CdTe QWs. Pre-synthesized Cd(DOPT)x(TDPT)1-0.5x. Substituting a small amount of DOPT for TDPT produced Cd(DOPT)x(TDPT)1-0.5x (e.g., Cd(DOPT)1.52(TDPT)0.24), a more ideal precursor for WZ-QW growth (Figures 1 and 2). Although no improvement in WZ% of the QWs was observed at 250 ºC, the formation of multipods possessing Bi core and CdTe arms, having the corresponding lowest-energy absorption feature centered at ~700 nm or 1.77 eV (black trace, Figure 2),32 was quenched, thus improving the yield of wires (Figure 1b vs Figure 1a). This is presumably due to the strong binding of TDPT to the surface 4 ACS Paragon Plus Environment

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of the catalyst nanoparticles that inhibited the growth of CdTe arms.49,62,67 Slightly increased TDPT incorporation (e.g., Cd(DOPT)1.2(TDPT)0.4) produced similar result (Figure S2).

Higher TDPT

incorporation (e.g., Cd(DOPT)0.4(TDPT)0.8), however, resulted in an insoluble precursor and homogeneously nucleated rods (Figure S2), similar to the Cd(TDPT) precursor previously reported.45

Figure 1. Representative TEM and HRTEM images of nearly defect-free WZ CdTe QWs grown from 9.1-nm diameter Bi nanoparticles at 250 (a,b) and 300 ºC (c,d) using Cd(DOPT)2 (a,c) and Cd(DOPT)1.52(TDPT)0.24 (b,d) as the Cd precursors. The syntheses were conducted at Cd:Te  2.93.0 and DOP mol% = 0.03%. The WZ% of the wires is 98.0  0.6% (a), 96.7  0.8% (b), 88.9  1.6% (c), and 97.4  0.7% (d), respectively. See Figure S1 in the Supporting Information for the diameterdistribution histograms of the QWs.



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Additional benefit was observed at 300 ºC, for which the WZ% of the wires was improved from 88.9% to 97.4% (Figure 1c vs Figure 1d), as a result of the relatively low reactivity (high stability) of Cd(DOPT)x(TDPT)1-0.5x toward CdTe formation (process I) due to TDPT incorporation,59,68-70 favoring process II and the formation of the solid catalysts for the SSS growth of WZ QWs. 45 Moreover, the bimodal diameter distribution (Figure S1c), which was also evident in the extinction spectrum of the wires synthesized using the Cd(DOPT)2 precursor (blue trace, Figure 2), was often absent (Figure S1d), thus resulting in a relatively sharp, lowest-energy excitonic feature in the extinction spectrum (magenta trace, Figure 2).

Figure 2. Extinction spectra of nearly defect-free WZ CdTe QWs shown in Figure 1. We note that the WZ% of 97.4% achieved in this study is not the fundamental limit for the WZ phase purity. We previously reported WZ% as high as 99.9% for CdTe QWs,32 and expect that nearly 100% WZ QWs are obtainable by further finely tuning the composition (x) in the Cd(DOPT)x(TDPT)1-0.5x precursor, the Cd:Te precursor ratio, DOP mol%, and/or temperature. We also found that varying the size 6 ACS Paragon Plus Environment

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of the initial Bi nanoparticles in the range of 615 nm in diameter did not cause noticeable differences in the WZ% of the wires obtained (Figure S3), which all approached nearly 100%. The BixCdyTez catalyst nanoparticles of various sizes obtained are thus expected to be solid, presumably due to the rapid dissolution of Te in Bi and the high diffusion rate of Te in the solid BixCdyTez (y « z) catalysts under the reaction conditions.45,71-76 Effects of Acids. The Cd precursors Cd(DOPT)2 and Cd(DOPT)x(TDPT)1-0.5x employed above were synthesized using stoichiometric equivalents of anionic ligands (DOPT and TDPT) to Cd2+ as indicated in the formula, the reaction mixtures for QW syntheses were thus free of acidic protons (H +) from the respective acids, di-n-octylphosphinic acid (DOPA, Scheme 1) and TDPA. Acids as proton donors were found to catalytically facilitate the cleavage of P=E (E = S, Se, Te) bonds, especially in the presence of secondary phosphines (e.g., DOP), and thus promote the precursor-conversion kinetics in favor of CdE solutes (process I).61-66 A similar mechanism of CdTe-solute formation in the presence of acids is expected in the CdTe-QW synthesis. We thus added TDPA to the reaction mixture containing Cd(DOPT)2, and compared the results below with those of employing Cd(DOPT)2 or Cd(DOPT)1.52(TDPT)0.24 without free TDPA shown in Figure 1. The effect of adding TDPA to the reaction mixture varied with TDPA% and the reaction temperature. At 250 ºC and TDPA% = 0.09 mol% (equivalent to the amount of TDPT used in the synthesis of Cd(DOPT)1.52(TDPT)0.24), the formation of multipods was largely eliminated (Figure 3a), similar to the result of using Cd(DOPT)1.52(TDPT)0.24 (Figure 1b). distribution (Figures S4a, and 3c).

The wires, however, had a broad diameter

Their WZ% decreased from 96.7% (Figure 1b, using

Cd(DOPT)1.52(TDPT)0.24) to 86.3% (Figure 3a), consistent with the increase in the rate of process I and thus the partial loss of crystal-phase control, as expected in the presence of acids (see the kinetic study below).61-66 Process I was dominant after adding a larger amount of TDPA (0.36 mol%), as evident by



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the formation of magic-size CdTe clusters having an absorption peak at 435 nm (Figure S5c).45,77-78 No wire growth, however, was observed in this case.

Figure 3. Representative TEM and HRTEM images (a,b) and extinction and PL (dashed lines) spectra (c) of WZ-rich CdTe QWs synthesized with added 0.09 mol% TDPA at 250 (a) and 300 ºC (b). The wires were grown from 9.1-nm diameter Bi nanoparticles using Cd(DOPT)2 as the Cd precursor at Cd:Te = 2.82.9 with DOP mol% of 0.03%. The WZ% of the wires are 86.3  1.4% (a) and 95.7  1.5% (b). See Figure S4 for the diameter-distribution histograms of the QWs. In contrast, at 300 ºC with added TDPA% = 0.09 mol%, we observed a slight decrease of WZ% from 97.4% (Figure 1d, using Cd(DOPT)1.52(TDPT)0.24) to 95.7% (Figure 3b), which was still a significant improvement over 88.9% obtained from Cd(DOPT)2 (Figure 1c). High WZ-phase purity and a narrow diameter distribution (Figure S4b) resulted in a richly structured extinction spectrum and a narrow PL spectrum (Figure 3c). Higher TDPA% (0.2 mol%), however, resulted in polytypic wires having WZ% of 66.1% (Figure S5a), consistent with enhanced process I at increasing TDPA%. The deleterious effect of added TDPA at 250 ºC and at higher TDPA% at 300 ºC was consistent with a sufficient amount of acidic protons present (from TDPA) that facilitated the formation of CdTe (process 8 ACS Paragon Plus Environment

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I),61-63 thus favoring the liquid catalysts for the SLS growth of polytypic QWs.45 The near non-detrimental effect at 300 ºC at lower TDPA% (Figure 3b), however, suggested a reduced amount of acidic protons present that would not sufficiently promote process I, thus improving the WZ% of the wires. This is confirmed below by the 31P NMR study of TDPA in a mixture containing Cd(DOPT)2 and purified TOPO. Figure S6 showed that a significant portion of TDPA was incorporated as TDPT and TDPA anhydride into Cd(DOPT)2, which was 70% at 250 ºC and 92% at 300 ºC, due to stronger binding of TDPT or TDPA anhydride to Cd than that of DOPT to Cd.59,69 Water resulted as byproduct of TDPA-anhydride formation, the amount of which was significantly different at 250 and 300 ºC, and accounted for 8.1% and 23.5% of TDPA added, respectively. Most of the water was presumably removed from the wire synthesis, as indicated by the tiny water droplets formed on the upper inner wall of the reaction tube at both temperatures prior to the injection of Te precursor, which disappeared gradually with the nitrogen flow. Thus, less acidic protons present at 300 ºC and the improved stability of the Cd precursor by partial TDPT substitution59,68-70 hindered process I, favoring process II and the formation of the solid catalysts for the SSS growth of WZ QWs.45 Whereas at 250 ºC, relatively more acidic protons present corresponded to the enhancement of process I, resulting in partial loss of crystal-phase control (see the kinetic study below). We also added DOPA to the reaction mixture and found it deleterious to the wire synthesis. The WZ% of the wires decreased from 98.0% to 64.0%, and the multipods remained (Figure S5b,c). We did not observe water formation via condensation of DOPA even at higher temperatures (Figure S6),69 so the concentration of the acidic protons would remain the same throughout the QW synthesis, unlike the syntheses involving TDPA shown above. As Cd(DOPT)2 is a good Cd precursor for WZ CdTe-QW growth,45 the dual role of DOPA as an acid to catalyze CdTe formation or as an appropriate stabilizing ligand (DOPT) for the Cd precursor is also established.



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We expect the same deleterious effect if a carboxylic acid (e.g., oleic acid) is added. Due to the weak binding strength of oleate,68,70 cadmium oleate is highly reactive and not a good precursor for WZ CdTe QW growth.45 As such, no dual role should be found for oleic acid. Kinetic Study. To confirm the role of added TDPA in the crystal-phase control, we monitored and compared the wire growth using the Cd(DOPT)2 precursor by taking aliquots at various time intervals for the syntheses conducted at 250 and 300 ºC. The synthetic conditions and results are summarized in Table 1. Results without TDPA addition from a previous study45 are also provided for comparison. Table 1. Compositions and Liquid or Solid States of the Catalyst Nanoparticles and the WZ% of the Resulting CdTe QWs.

a b

T (C)

DOP (mol%)

TDPA (mol%)

Precursor Cd:Te

250 250 300 300

0.031 0.032 0.032 0.031

0.09 0.09

2.88 2.84 2.90 2.96

BixCdyTez catalyst nanoparticles @ reaction time (s)a 120 40 8 8

Te (mol%) 47 22 26 32

Cd (mol%) 19 16 3 5

mol% of solid (S) 100 52 84 100

QW WZ%

QW growth rate (nm/s)

97b 86

4  1b 184  83 403  99b 364  217

87b 96

Also referred to as induction period. Data taken from ref 32. TEM images (Figure 4a-d) reveal, upon addition of 0.09 mol% TDPA at 250 ºC, a period of ~40 s

prior to which predominant wire growth was initiated. We refer to this period as the induction period. During this induction period, a magic-size CdTe cluster intermediate (λmax ~ 435 nm, Figure 4e)45,77-78 was transiently present in the solution and consumed upon predominant wire growth. The catalyst nanoparticles acquired nearly equal amounts of Te (22 mol%) and Cd (16 mol%) at 40 s (Figure 4f), and were only partially solidified (according to the phase diagram in Figure S12), thus resulting in partial loss of crystal-phase control of the QWs (WZ% = 86%). In contrast, we observed a longer (2 min) induction period under the same condition without TDPA addition (Table 1), for which no magic-size clusters were evidently present.45 The catalyst nanoparticles possessed comparatively higher Te (26 mol%) and lower Cd (11 mol%) contents at 40 s (Figure 4f), were solid at 2 min, and thus initiated the SSS growth of WZ 10 ACS Paragon Plus Environment

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QWs.45 The results above thus confirmed the enhancement of process I for the formation of CdTe in the presence of free TDPA.61-63

Figure 4. CdTe-QW growth kinetics at 250 C in the presence of added 0.09 mol% TDPA. The synthesis was conducted at Cd:Te = 2.84 and DOP mol% = 0.03% using Cd(DOPT)2 as the Cd precursor. (a-d) TEM images of QW growth at 8 s, 25 s, 40 s, and 1 min of reaction, respectively. (e) Extinction spectra showing the presence of magic-size CdTe cluster (~435 nm) at 840 s prior to predominant wire growth. (f) Temporal composition evolution (as mol% of Cd or Te) of the catalyst nanoparticles determined by energy-dispersive X-ray spectroscopy (EDS) (triangles). The data obtained under the same condition without TDPA addition (squares) are provided for comparison.45 The near 50-fold increase in the QW-growth rate (Table 1) in the presence of free TDPA is consistent with the availability of magic-size CdTe clusters nutrients, and the partially liquefied catalyst nanoparticles suggested by the phase diagram (Figure S12), which we imagine to have solid cores and liquid surfaces.79 More efficient CdTe accommodation (adsorption) from the reaction mixture into the liquid catalyst surface than that into the solid catalyst surface is expected to be the primary origin of the difference in QW-growth rate, because the solid (y « z) BixCdyTez catalyst nanoparticles, containing van 11 ACS Paragon Plus Environment


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der Waals layers and antisite defects and vacancies, are effective material-transport media, comparable to the liquid (y  z) BixCdyTez catalyst nanoparticles.45,70-75 A similar kinetic study conducted at 300 ºC revealed a significant Te content (32 mol%) and a small Cd content (5 mol%) in the catalyst nanoparticles prior to wire growth (8 s) (Table 1 and Figure 5). The catalysts were solid (Figure S13) and thus catalyzed the SSS growth of WZ QWs. In comparison, the catalyst nanoparticles obtained without TDPA addition possessed a smaller Te content (26 mol%) and nearly unchanged Cd content (3 mol%) prior to wire growth (8 s) (Table 1 and Figure 5), and were determined to be primarily (84%) solid.32 The results thus confirmed the enhancement of process II due to the stabilization of the Cd precursor by TDPT, as suggested above. The slower QW-growth rate upon TDPA addition (Table 1) is consistent with the less-efficient CdTe accommodation (adsorption) into the solid catalyst surface as noted above.45

Figure 5. CdTe-QW growth kinetics at 300 C in the presence of added 0.09 mol% TDPA. The synthesis was conducted at Cd:Te = 2.90 and DOP mol% = 0.03% using Cd(DOPT)2 as the Cd precursor. (a,b) TEM images of QW growth at 8 s and 25 s of reaction, respectively. (c) Extinction spectra of the kinetics. (d) Temporal composition evolution (as mol% of Cd or Te) of the catalyst nanoparticles determined by


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EDS (triangles). The data obtained under the same condition without TDPA addition (squares) were provided for comparison.45 TDPA incorporation as TDPT and TDPA anhydride into Cd(DOPT)2 stabilized the Cd precursor, thus hindering process I, which was beneficial to the SSS growth of WZ QWs. Such a beneficial effect, however, may be overridden if sufficient acidic protons (originally from TDPA) are present to facilitate process I for the SLS growth of polytypic QWs. We ruled out the possible role of TDPA or TDPT as a ligand in selectively binding to and stabilizing the nascent WZ CdTe phase80-81 at the triple phase line where the catalyst, the wire, and the reaction mixture meet, thus favoring the WZ phase, as no TDPA or its derivatives were found as the surface ligands in similar CdTe-QW syntheses.55 CONCLUSIONS We have demonstrated that TDPA (and its derivative TDPT) played a dual role in controlling precursor-conversion chemistry, and thus the composition of the catalyst nanoparticles, and consequently the crystal phase of semiconductor QWs. The beneficial role of TDPT in further stabilizing Cd(DOPT)2 due to its stronger binding to Cd affords Cd(DOPT)x(TDPT)1-0.5x (e.g., Cd(DOPT)1.52(TDPT)0.24) as an ideal precursor for the SSS growth of WZ CdTe QWs. TDPA catalytically promotes the formation of CdTe (process I)61-66 and the SLS-wire growth, and thus exerts a deleterious effect on the crystal-phase control of QWs. DOPA and its derivative DOPT demonstrate a similar dual role. Thus the use of TDPA and DOPA and other organic-acid additives complicates the synthesis of phase-pure QWs. METHODS Materials. Cadmium di-n-octylphosphinate (Cd(DOPT)2),32 di-n-octylphosphine (DOP),69 tri-noctylphosphine telluride (TOPTe) stock solutions (0.025 mmol/g solution), di-n-octylphosphinic acid (DOPA),59,82 and bismuth (Bi) nanoparticle stock solutions (0.04 mmol Bi atoms g-1 solution)58,83 were prepared using previously reported procedures. Cd(DOPT)1.52(TDPT)0.24 was prepared similarly as 13 ACS Paragon Plus Environment


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Cd(DOPT)2, and its characterizations was provided in the Supporting Information. Tri-n-octylphosphine (TOP, 97%, Strem, batch no. 18647600) was used as received, which contained 0.02 mol% DOP as determined by 31P NMR following a previously reported procedure.69 A DOP stock solution (1.01 wt%, 1.45 mol%) was made by diluting DOP (100 mg, 0.387 mmol) into a portion (9.900 g) of the 0.025 mmol/g TOPTe stock solution prepared from the Strem TOP.

Tri-n-octylphosphine oxide (TOPO) was

recrystallized from 99% TOPO (Aldrich) before use.59,82 n-tetradecylphosphonic acid (TDPA, >99%, PCI Synthesis), methanol (99.8%, Aldrich), and toluene (99.5%, Aldrich) were used as received. CdTe-QW Synthesis and Kinetic Studies. All QW specimens were synthesized by the SLS or SSS mechanism using the same amounts of Cd precursors (~0.0434 mmol), TOPTe stock solution (e.g., 590 mg, 0.0148 mmol TOPTe, intrinsically containing 0.00032 mmol DOP), DOP stock solution (e.g., 100 mg, 1.01 mg/0.00391 mmol DOP, 0.0025 mmol TOPTe), Bi-nanoparticle stock solution (20 mg, 0.00080 mmol Bi atoms), and TOPO (4.0 g, 10.4 mmol). In a general procedure, Cd precursor, TDPA (3.2 mg, 0.011 mmol if applicable), and purified TOPO were loaded into a 50-mL Schlenk reaction tube in air. The reaction mixture was degassed under vacuum (0.010.1 torr) at ~100 C for 1 h, during which it was backfilled with N2(g) at least five times. The reaction mixture filled with N2(g) flow was then inserted into a salt bath (NaNO3:KNO3:Ca(NO3)2, 21:54:25 mol%) at a desired temperature to achieve a clear colorless solution. The 9.1-nm Bi-NP stock solution, TOPTe stock solution, and DOP stock solution were combined in a separate vial and septum capped in a N2(g)-filled glove box. This mixture was brought out of the glove box, loaded into a 3-mL syringe, and then quickly injected into the Schlenk tube that had been equilibrated in the salt bath for 4 min. The tube was withdrawn from the bath after 37 min of total reaction time and allowed to cool to room temperature. The amounts of TOPTe and DOP injected were calculated by measuring the difference between masses of the syringe before and after the injection. The quantities of the reagents employed above generated Cd:Te ratios of 2.93.0 and TOP mol% of 0.03%. 14 ACS Paragon Plus Environment

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For kinetic studies, small aliquots (0.1 mL) were taken from the reaction mixture at various time intervals and immediately quenched in toluene (3 mL, room temperature) for subsequent optical, TEM and Energy-dispersive X-ray spectroscopy (EDS) analyses (see below). TEM and EDS Analyses. The QWs and/or nanoparticles were isolated in the ambient atmosphere as a brownish black precipitate from the toluene mixture (ca. 1.5 mL from the kinetic runs or by dispersing the as-synthesized QW mixture (ca. 0.3 g) into toluene (ca. 2 mL)) and methanol (ca. 1 mL), followed by centrifugation (1150 g) and decanting of the colorless supernatant. The precipitate was redispersed in toluene (ca. 1 mL), and obtained again by adding methanol (ca. 1 mL) and by centrifugation and decanting of the supernatant. The precipitate was ultimately redispersed in toluene. The precipitate isolated from kinetic study was redispersed in ca. 0.5 mL of toluene for EDS analysis and further diluted to ca. 5 mL for wire length determination. Carbon-coated copper grids were dipped in the toluene solution and then immediately taken out to evaporate the solvent. TEM analysis was performed within 24 h. TEM images were collected using a JEOL 2000 FX microscope with an acceleration voltage of 200 kV. High-resolution TEM (HRTEM) was carried out on a JEOL JEM-2100F TEM at 200 kV. The statistics of the WZ% were determined by using a previously reported procedure.32 The length of the wires at a specific time of reaction for the kinetic study was measured using a previously reported procedure,32 and is detailed in the Supporting Information. EDS analysis of the catalyst nanoparticles isolated from the kinetic study was performed using JEOL 2000 FX TEM at 200 kV. The electron beam was spotted on areas containing several free catalyst nanoparticles that had not initiated the CdTe-QW growth (with no CdTe arms). Such acquisition was repeated 35 times at various locations, and the acquired values were finally averaged. Optical Spectroscopic Analyses. Small aliquots (0.1 mL) of the as-synthesized QW mixtures before solidification were withdrawn and dispersed in toluene (3 mL) to form optically clear suspensions. The



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QW-toluene mixtures obtained from kinetic studies were analyzed directly. The extinction spectra were collected at room temperature using a Varian Cary 100 Bio UV-visible spectrophotometer, and the PL spectra were obtained at room temperature using a Varian Cary Eclipse fluorescence spectrophotometer at the excitation wavelength of 528 nm (2.35 eV). ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional TEM and HRTEM images and extinction spectra of CdTe QWs synthesized at various TDPA mol% and DOPA mol%, 31P{1H} NMR spectra of heat-treated mixtures of Cd(DOPT)2, TDPA, and purified TOPO at 250 and 300 ºC, and determinations of the liquid or solid state of the catalyst nanoparticles. AUTHOR INFORMATION Corresponding Authors [email protected], [email protected] ORCID Fudong Wang: 0000-0003-2914-1360 William E. Buhro: 0000-0002-7622-4145 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Science Foundation (CHE-1607862). We acknowledge financial support from Washington University and the Institute of Materials Science and Engineering for the use of instruments and staff assistance. 16 ACS Paragon Plus Environment

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Crystal-Phase Control of Catalytically Grown Colloidal CdTe Quantum

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