Role of Precursor-Conversion Chemistry in the Crystal-Phase Control

Nov 28, 2017 - (4) The wires so obtained are nearly phase-pure WZ QWs that show narrow ... The role of precursor-conversion chemistry (Scheme 1) in th...
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Role of Precursor-Conversion Chemistry in the Crystal-Phase Control of Catalytically Grown Colloidal Semiconductor Quantum Wires Fudong Wang* and William E. Buhro*

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Department of Chemistry and Institute of Materials Science and Engineering, Washington University, St. Louis, Missouri 63130-4899, United States S Supporting Information *

ABSTRACT: Crystal-phase control is one of the most challenging problems in nanowire growth. We demonstrate that, in the solution-phase catalyzed growth of colloidal cadmium telluride (CdTe) quantum wires (QWs), the crystal phase can be controlled by manipulating the reaction chemistry of the Cd precursors and tri-n-octylphosphine telluride (TOPTe) to favor the production of either a CdTe solute or Te, which consequently determines the composition and (liquid or solid) state of the BixCdyTez catalyst nanoparticles. Growth of single-phase (e.g., wurtzite) QWs is achieved only from solid catalysts (y ≪ z) that enable the solution−solid−solid growth of the QWs, whereas the liquid catalysts (y ≈ z) fulfill the solution−liquid−solid growth of the polytypic QWs. Factors that affect the precursor-conversion chemistry are systematically accounted for, which are correlated with a kinetic study of the composition and state of the catalyst nanoparticles to understand the mechanism. This work reveals the role of the precursor-reaction chemistry in the crystalphase control of catalytically grown colloidal QWs, opening the possibility of growing phase-pure QWs of other compositions. KEYWORDS: precursor-conversion chemistry, crystal-phase control, zinc blende, wurtzite, solution−liquid−solid, solution−solid−solid, quantum wire

W

defects such as twinning boundaries, stacking faults, and WZ− ZB phase alternations.3,9,14−19 As these planar defects are deleterious to the optical and transport properties of nanowires,20−24 control of their occurrence in nanowires9,14,25,26 or growth of phase-pure nanowires4,9,17,27−34 has therefore been a major research interest. Methods to produce phase-pure WZ or ZB nanowires are available by the VLS mechanism,9,17,27−34 which include epitaxial nanowire growth in non-[111] (e.g., [111], [110], [001]) orientations,29,30 growing wires thinner than the critical diameters,9,27 varying temperature9,17 or precursor flux ratios,17 impurity doping of the catalysts,28,32 and using non-Au (e.g., Pd, Ag, and Ga) catalysts.31,34−36 However, to our knowledge, successful growth of phase-pure and defect-free II−VI or III−V nanowires has not been achieved by the SLS mechanism, despite that the nanowires were grown at a range of reaction temperatures with various precursor types and ratios and ligands or solvents.3 Recently, we discovered that under conditions close to the SLS synthesis conditions, the catalyst nanoparticles become solid and the wire growth is achieved via

e report herein that the precursor-conversion chemistry of cadmium (Cd) precursors and tri-noctylphosphine telluride (TOPTe) plays a critical role in the crystal-structure control of colloidal semiconductor CdTe quantum wires (QWs) catalytically grown from the initial bismuth (Bi) nanoparticle seeds. Under conditions where the precursors are converted predominantly into a solute of composition CdTe, the catalysts are liquid BixCdyTez (y ≈ z) nanoparticles, so the growth of QWs follows the solution− liquid−solid (SLS) mechanism,1−3 and the wires are polytypic, having wurtzite (WZ) and zinc blende (ZB) alternations. In contrast, the growth of nearly phase-pure, defect-free WZ QWs is achieved under conditions where a significant excess of Te is present in the reaction mixture and incorporated into the Bi nanoparticles, forming solid BixCdyTez (y ≪ z) catalysts. CdTe and Te are supplied and maintained in a way that the growth of WZ QWs is fulfilled by the solid catalysts via the solution− solid−solid (SSS) mechanism.4 As many semiconductor materials (e.g., II−VI and III−V) can grow in multiple (meta)stable crystal phases (e.g., WZ and ZB), selective formation of one phase over another has been one of the most challenging problems in crystal growth.5−12 Catalytically grown semiconductor nanowires by the vapor−liquid− solid (VLS)13 and SLS mechanisms inherit this problem, and as a consequence, they typically contain high densities of planar © 2017 American Chemical Society

Received: September 18, 2017 Accepted: November 28, 2017 Published: November 28, 2017 12526

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ACS Nano the SSS mechanism.4 The wires so obtained are nearly phasepure WZ QWs that show narrow absorption and photoluminescence (PL) spectral line widths superior to the polytypic QWs of like diameter and diameter distribution grown by the SLS mechanism.23 The primary goal of this work is to identify the small variations in the SSS and SLS growth conditions and to understand the associated chemistry that results in such a dramatically different synthetic outcome of the respective WZ and polytypic CdTe QW growth. Experimental evidence shows that there exist two main precursor conversions of TOPTe (Scheme 1): either reaction to form CdTe (via I) or

Reactivity of Cd Precursor and Cd/Te Precursor Ratio. We chose three Cd precursors, cadmium oleate (Cd(OLT)2), cadmium di-n-octylphosphinate (Cd(DOPT)2), and cadmium n-tetradecylphosphonate (Cd(TDPT)), with their chemical formulas shown in eq 1, in order of decreasing reactivity toward CdTe formation (via I in Scheme 1),37,38 and conducted the wire syntheses at 250 °C with optimized DOP mole fraction at ∼0.03 mol % (see next subsection). As shown in Figure 1, the

Scheme 1. Precursor-Conversion Chemistry and the Correlated Liquid and Solid Nature of the Catalyst Nanoparticles and the SLS and SSS Growth of QWs

Figure 1. Dependence of the WZ% of CdTe QWs on the precursor Cd/Te mole ratio for the Cd precursors Cd(DOPT)2, Cd(TDPT), and Cd(OLT)2. The QWs were grown at 250 °C from 9.1 nm diameter Bi nanoparticles with DOP mole fraction of ∼0.03% in the reaction mixtures.

decomposition into Te (via II) in the growth of semiconductor QWs, which are manipulated by the reactivities of the Cd precursors, the Cd/Te precursor ratios, catalytic additives, and the temperature. Kinetic study of the QW syntheses correlates the precursor-conversion chemistry with the composition of the catalyst nanoparticles and consequently the crystal phase of semiconductor QWs. Under conditions where process I is predominant, primary intake of CdTe and the limited solubility of CdTe in the Bi nanoparticles render liquid catalysts that catalyze the SLS growth of QWs, and the resulting QWs are polytypic. In contrast, when a significant excess of Te is present in the wire synthesis mixtures (via II), primary incorporation of Te into the Bi nanoparticles provides solid catalysts having high Te content (BixCdyTez, y ≪ z), from which the QWs are grown by the SSS mechanism and are nearly phase-pure WZ.

average WZ fraction (WZ%) in the nanowires decreased with increasing Cd/Te ratio for all three Cd precursors, with a slower decrease for Cd(TDPT) at high Cd/Te ratios. In these experiments, the amount of Cd precursor was held constant, and the Cd/Te precursor ratio was varied by the amount of Te employed. In the synthetically useful range, the amount of Cd precursor always exceeded the amount of Te, and the Cd/Te ratios studied were all greater than 2.5 (as shown in Figure 1). The most reactive precursor, Cd(OLT)2,37−39 which favored process I, was not a good precursor for crystal-phase control, as the wires contained WZ% of only up to 84% and were kinked and accompanied by ill-defined dots and rods (Figure S1). The catalyst tips of the wires appeared hemispherical and amorphous, consistent with the SLS growth mechanism for polytypic QWs.4 The least reactive Cd(TDPT) was also not a good precursor, as indicated by low yields of wires and the existence of homogeneously nucleated CdTe dots and rods (Figure S2). The wires, however, exhibited WZ% up to 98.8% (Figure 1), and the faceted shape of the catalyst tips and the multiple growth sites for short CdTe arms occasionally present on the catalysts were consistent with the SSS growth of the wires.4 The high WZ% of the wires was attributed to the low solubility of Cd(TDPT) in the reaction mixture and the low reactivity of Cd(TDPT) for TOPTe (in process I) at 250 °C so that the amount of Te (by process II) was significant, favoring the SSS growth of WZ QWs (as is explained below). Cd(DOPT)2 having an intermediate reactivity appeared to be an appropriate precursor for WZ QW growth. QWs with WZ% up to 99.5% were produced at Cd/Te = 2.6−3.6, along with multipods consisting of Bi cores and CdTe arms as was previously found to accompany SSS growth (Figures 2a−c and S3b−d).4 The catalyst tips attached at the end of wires exhibited an epitaxial relationship with the CdTe wires as

RESULTS The reaction chemistry for the CdTe QW syntheses is shown in eq 1. We demonstrate below that the reactivity of the Cd precursor, the Cd/Te ratio in the precursor mixture, the mole fraction of the reaction catalyst di-n-octylphosphine (DOP), and the reaction temperature all play essential roles and are intercorrelated in the crystal-phase control of the CdTe QWs. The role of precursor-conversion chemistry (Scheme 1) in the control of the catalyst−nanoparticle compositions and the subsequent crystal phase of the QWs is unraveled by kinetic studies of the QW syntheses under several representative conditions. Bi nanoparticles

CdX n + TOPTe ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CdTe QWs DOP mol % temperature

(1) −

where X = oleate (OLT = R(O)CO ), di-n-octylphosphinate (DOPT = R′2(O)PO−), or n-tetradecylphosphonate (TDPT = R″(O)PO22−). 12527

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Figure 2. Representative transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of CdTe QWs grown from 9.1 nm diameter Bi nanoparticles using Cd(DOPT)2 as the Cd precursor at Cd/Te precursor ratios of 2.6 (a), 2.9 (b), 3.4 (c), and 5.5 (d), corresponding to Figure 1. The syntheses were conducted at 250 °C with DOP mole fraction of ∼0.03% in the reaction mixtures. White dashed lines highlight the crystalline region of the catalysts, which exhibited an epitaxial relationship with the CdTe. Detailed characterization of such relationships was reported previously.4 The QWs in the HRTEM images are viewed in the ZB [110] or WZ [112̅0] zone or their equivalents, normal to the growth axis of ZB [11̅1] and WZ [0001]. The ZB (blue) and WZ (gray) segments are labeled as indicated, with red dashed lines denoting twin boundaries or stacking faults. Multipods having CdTe arms and Bi cores coexisted with the QWs (a−c), and their detailed characterization was reported previously.4 Additional TEM images are provided in Figure S3 in the Supporting Information.

Table 1. Compositions and Liquid or Solid States of the Catalyst Nanoparticles Prior to Wire Growth and the WZ% and Growth Rates of the Resulting CdTe QWsa BixCdyTez catalyst nanoparticles T (°C)

precursor Cd/Te

DOP (mol %)

reaction time (s)b

Te (mol %)

Cd (mol %)

mol % of solid (S)

QW WZ%

250 250 250 250 300 300 300

2.88 5.54 2.92 2.85 2.02 2.90 2.96

0.031 0.031 0.058 0.089 0.029 0.032 0.085

120 25 40 8 2 8 3

47 13 21 14 28 26 13

19 13 17 9 8 3 12

100 26 48 34 85 84 26

97 30 66 22 97 87 18

a

QW growth rate (nm/s) 4 163 40 183 882 403 895

± ± ± ± ± ± ±

1 16 1 17 167 99 359

Data taken from ref 4. bAlso referred to as induction period.

previously established4 and were often found to have CdTe arms arising from multiple nucleation sites on the catalysts, consistent with the SSS mechanism by which the CdTe grew from the solid catalysts (Figures 2 and S3b−d).4 The multipods were diminished at Cd/Te > 4.0, as the wire growth was switched to the SLS mechanism. The wires lost phase control and were polytypic, and the catalyst tips appeared hemispherical and amorphous or polycrystalline (Figures 2d and S3e), typical for the SLS wire growth from liquid catalysts.4 We focus below on the wire growth using Cd(DOPT)2. To gain insight into the role of the Cd/Te precursor ratio in the crystal-phase control, we monitored and compared the wire growth by taking aliquots at various time intervals for the syntheses conducted at various Cd/Te ratios. The synthetic conditions and results are summarized in Table 1. We focus here on the results achieved at Cd/Te = 2.88 and 5.54, which are shown in Figure 3. TEM images in Figure 3c,d reveal a period prior to which major wire growth was initiated, which extended at least 2 min

at Cd/Te = 2.88 and at least 25 s at Cd/Te = 5.54. We refer to this period as the induction period (prior to wire growth). During the induction period, the initial, spherical Bi nanoparticles (Figure 3a) were transformed to larger, faceted, pseudorhomboidal morphologies. Analysis of these nanoparticles by energy-dispersive X-ray spectroscopy (EDS) indicated that they contained different amounts of Te and Cd, which depended on the Cd/Te precursor ratio and the time of reaction (Figure 3b). At the higher Cd/Te ratio of 5.54, significant and equal amounts of Te and Cd (13 mol %) were incorporated into the catalyst nanoparticles within 25 s (Figure 3b, triangles, and Table 1). The overall composition of the catalysts (Bi/Cd/Te = 74:13:13) induced their liquid state at 250 °C (Figure S7) and provided sufficient solute, due to the limited solubility of CdTe in Bi, to initiate QW growth by the SLS mechanism, after a short (25 s) induction period. In contrast, at the lower Cd/Te ratio of 2.88, the catalyst nanoparticles acquired a significant Te content (21 mol %) 12528

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Thus, the more reactive Cd precursor and higher Cd/Te precursor ratios (smaller amount of Te) inducing significant, early CdTe dissolution in the catalyst particles favored process I (Scheme 1) and thus the growth of polytypic QWs from liquid catalysts by the SLS mechanism. Alternatively, less reactive Cd precursors and lower Cd/Te precursor ratios (larger amount of Te) resulting in preferential early incorporation of Te into the catalyst nanoparticles favored process II and thus the growth of WZ QWs by the SSS mechanism. Evidence for the equilibrium in process II was provided by 31P NMR of a mixture of TOPTe and TOP, for which a single exchange-averaged resonance was observed at an intermediate chemical shift of those of TOP and TOPTe, indicating rapid exchange of Te between TOPTe and TOP (Figure S10). DOP Concentration. DOP, an impurity in commercial TOP,37,41−43 was found to be a catalyst in the II−VI soluteforming chemical reactions (process I).43,44 DOP was essential to the growth of CdTe QWs and the mole fraction of it in the reaction mixture played a critical role in the crystal-phase control. Figure 4 clearly shows that high WZ selectivity was

Figure 4. Dependence of WZ% of CdTe QWs on the mole fraction of DOP at 250 °C. The QWs were grown from 9.1 nm diameter Bi nanoparticles using Cd(DOPT)2 as the Cd precursor with the Cd/ Te precursor ratio = ∼2.9.

achieved at low DOP fractions of ≤0.031 mol %, above which the WZ% decreased monotonously with increasing DOP fraction. Along with this trend was an increasing yield of QWs, from only ill-defined Te-rich morphologies initiated from Bi nanoparticles to a mixture of CdTe QWs and CdTe multipods and to only CdTe QWs (Figures S11 and S12). Kinetic studies were conducted to understand the role of DOP in the crystal-phase control (Table 1). The results were compared at three DOP mole fractions, 0.031% (Figure 3b,c), 0.058% (Figure 5), and 0.089% (Figure S12), all acquired at Cd/Te = 2.9 and 250 °C. The induction period was shortened from 2 min to 8 s as the DOP mole fraction increased from 0.031 to 0.089%. Meanwhile the difference between the Te and Cd contents for the BixCdyTez catalyst nanoparticles just prior to QW growth decreased from 28 to 5 mol %, constituting a transition from solid catalysts to liquid catalysts (Table 1). These results were consistent with the increasing dominance of CdTe-solute-forming process I with increasing DOP concentration. At low DOP concentration, a significant excess of Te over Cd initially dissolved in the Bi catalyst nanoparticles, producing a long induction period, and then the SSS growth of WZ QWs from solid catalysts (Figure 3b,c). At higher DOP concentration, process I became sufficiently rapid to achieve the transient appearance of CdTe magic-size nanoclusters, evidenced by the sharp absorption at 435 nm45,46 (Figure 5f).

Figure 3. CdTe QW growth kinetics at Cd/Te precursor ratios of 2.88 and 5.54. The syntheses were conducted at 250 °C using Cd(DOPT)2 as the Cd precursor with DOP mol % = 0.03%. (a) Initial 9.1 nm diameter Bi nanoparticles. Inset: HRTEM image of a Bi nanoparticle viewed in the [022̅1] zone, with the fast Fourier transform indexed to the rhombohedral structure.40 (b) Temporal composition evolution of the catalyst nanoparticles at Cd/Te precursor ratios of 2.88 and 5.54, determined by energy-dispersive X-ray spectroscopy and shown as mol %. A temporal size evolution of the catalyst nanoparticles is provided in Figure S4. Their representative TEM images at selected reaction-time intervals are shown in (c) and (d), respectively. The circles indicate the initiation of CdTe QW growth. Additional images and the extinction spectra of the kinetics are provided in Figures S5 and S6.

within 8 s (Figure 3b, squares). However, the Cd content in the catalyst nanoparticles increased much more slowly, requiring 1 min to reach 18 mol %. The early composition of the catalysts (Bi/Cd/Te = 74:5:21) rendered them to be primarily solid nanoparticles (Figure S8), and when the Cd content grew sufficiently, the catalyst nanoparticles initiated the growth of WZ QWs by the SSS mechanism, after a much longer induction period (of 2 min). 12529

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Figure 6. Dependence of WZ% of CdTe QWs on the mole fraction of DOP (a) and the Cd/Te precursor ratio (b) in the reaction mixtures at 250, 300, 350, and 400 °C. The QWs were grown from 9.1 nm diameter Bi nanoparticles using Cd(DOPT)2 as the Cd precursor with the Cd/Te precursor ratio = 2.8−2.9 (a) and DOP mole fraction of ∼0.03% (b), respectively. A 3D plot containing more data points is provided in Figure S13.

Figure 5. CdTe QW growth kinetics at DOP mol % = 0.058%. The synthesis was conducted at 250 °C and Cd/Te = 2.9 using Cd(DOPT)2 as the Cd precursor. (a−d) TEM images of QW growth at 25 s, 40 s, 1 min, and 2 min of reaction, respectively. Inset in (a): HRTEM image of a BixCdyTez catalyst nanoparticle (viewed in the [022̅1] zone of the rhombohedral structure) that exhibited similar lattice fringes, fast Fourier transform patterns, and measured d spacings (0.34 nm) as those of Bi nanoparticles (Figure 3a). Insets in (b,c) show the CdTe rods/wires present. (e) Temporal composition evolution of the catalyst nanoparticles determined by EDS. (f) Extinction spectra showing the magic-size clusters (435 nm) appeared (disappeared) prior to (after) major wire growth.

Although higher reaction temperatures were in general deleterious, the synthetic parameters could be balanced at 300 °C to produce nearly defect-free WZ CdTe QWs in high yields (Figures 6, 7 and S18b,c). These WZ QWs possessed faceted catalyst tips, which exhibited epitaxial interfaces with the QWs (Figures 7d and S18b,c), typical for SSS wire growth.4 The QWs showed sharp PL and extinction spectra with wellresolved excitonic features (Figure 7e), characteristic of phasepure QW specimens having narrow diameter distributions.23 Thus, at the higher reaction temperature of 300 °C, a very low Cd/Te precursor ratio (of 2.0) sufficiently hindered process I to maintain primarily solid catalyst nanoparticles (Table 1) and the growth of WZ QWs by the SSS mechanism. Under these conditions, the higher reaction temperature promoted a significantly higher yield of QWs relative to other morphologies (Figures 7, S14, S18, and S19).

At the highest DOP concentration, nearly equal amounts of Cd and Te were initially dissolved in the catalyst nanoparticles, the induction period nearly disappeared, and polytypic QWs grew by the SLS mechanism from liquid catalysts (Table 1 and Figure S12). Thus, a strong relationship was demonstrated between the rate of process I, the presence of liquid catalyst nanoparticles, and the SLS growth of polytypic CdTe QWs. Temperature. Over a range of reaction conditions, the primary effect of increasing reaction temperature was a decrease in the WZ% of the CdTe QWs and thus a decrease in crystalphase control (Figures 6 and S14). Kinetic studies conducted at 300 °C indicated that the induction periods were all shortened as compared with those performed at 250 °C under similar conditions (Table 1 and Figures S15−S17). A lowered Te content (26% at 300 °C vs 47% at 250 °C) was determined in the catalyst nanoparticles prior to QW growth (at Cd/Te = 2.90) (Table 1 and Figure S15). These results suggested a relative increase in the rate of process I with increasing temperature and thus the progressive loss of crystal-phase control.

DISCUSSION Roles of Reaction Parameters in QW Crystal-Phase Control. The results above demonstrate that the key to crystalphase control and the growth of nearly phase-pure WZ CdTe QWs is achieving a proper balance in the relative rates of processes I and II in Scheme 1. Conditions that favor the dissolution of excess Te (over Cd) into the Bi catalyst nanoparticles prior to QW growth ensure solid catalyst nanoparticles and the operation of the SSS growth mechanism, which produces WZ QWs. In contrast, conditions that promote the near equal dissolution of Cd and Te into the catalyst nanoparticles prior to QW growth produce liquid catalysts that induce the SLS growth of polytypic CdTe QWs. Thus, the 12530

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Cd precursor reactivity and/or Cd/Te precursor ratio favor process I and the loss of crystal-phase control, producing polytypic QWs. Similarly, increasing concentration of DOP, a catalyst for process I, and/or increasing reaction temperature have very similar effects. However, process I becomes sufficiently hindered with intermediate Cd precursor reactivity, with low Cd/Te precursor ratios, and at low concentrations of DOP to allow the formation of solid catalyst nanoparticles via process II, and therefore the maintenance of crystal-phase control affording WZ QWs. Material Accommodation and Transport in Solid BixCdyTez (y ≪ z) Catalyst Nanoparticles. We now consider the rate-limiting process in the SSS growth of WZ CdTe QWs. We begin with the assumption that CdTe solute supply via process I is not rate limiting. As evidence in support of this assumption, we note the simultaneous growth of multipods and WZ QWs under conditions of slow QW growth (4 ± 1 nm/s, Table 1 and Figures 2a−c and 3c). Such homogeneous growth of multipods usually requires a high CdTe solute concentration.47,48 Under conditions of rapid WZ QW growth, process I operates even more rapidly. Thus, the slow growth rate of 4 ± 1 nm/s for the WZ CdTe QWs grown at 250 °C suggests a low CdTe accommodation from the reaction mixture into the solid catalyst surface, a slow CdTe transport through the solid catalyst nanoparticles to the wire-growth interface, or both. Weaker surface reactivity and/or lower nutrient diffusivity through the solid Au−Ge catalyst were suggested by Ross and co-workers based on the 10 to 100 times slower Ge nanowire growth rates in the vapor−solid− solid (VSS) mode than in the VLS mode at the same temperature and digermane (Ge2H2) pressure.49 Our analysis below suggests that the slow WZ QW growth may be attributed to the low CdTe accommodation into the solid catalyst surface. Interestingly, the growth rate increases more than 2 orders of magnitude to 882 ± 167 nm/s for the CdTe WZ QWs grown from the primarily solid catalysts at 300 °C, which is surprisingly comparable to that (859 ± 359 nm/s) of polytypic

Figure 7. Representative TEM (a,b) and HRTEM images (c,d) and the extinction and PL spectra (e) of CdTe QWs grown from 9.1 nm diameter Bi nanoparticles using Cd(DOPT)2 as the Cd precursor at Cd/Te precursor ratio of 2.0. The synthesis was conducted at 300 °C with DOP mole fraction of ∼0.03%. The QWs in the HRTEM images were viewed in the ZB [110] or WZ [112̅0] zone, and the crystal phases are labeled the same as in Figure 2. The QWs had an average WZ% of 96.8% and an average diameter of 7.7 ± 1.0 nm.

reaction parameters must be adjusted to sufficiently hinder process I relative to process II. Therefore, the reaction parameters may be analyzed for their relative contributions to process I versus process II. Increasing

Figure 8. Crystal structures of bulk Bi (a) and Bi2Te3 (b) and the structural model of solid BixCdyTez-catalyst and CdTe QW (arm) interfaces (c), viewed in their respective [1̅21̅0] and [112̅0] zones as indicated. Bi contains layers of Bi2 blocks perpendicular to the [0001̅] direction, and the interaction between Bi2 blocks is of weak covalent nature (a).50 Bi2Te3 contains layers of [Te1−Bi−Te2−Bi−Te1] blocks, and the interaction between neighboring [Te1−Bi−Te2−Bi−Te1] blocks is of van der Waals type (b).50 A 56.5 or 52.1° angle is formed between the epitaxial CdTe {0001} planes and the solid BixCdyTez-catalyst {101̅2̅} planes (c), as suggested by experimental observations (Figure S20). Material (Te and Cd) transport across the catalyst and diffusion into the catalyst-wire interface enable the nucleation of a CdTe monolayer at the triple phase line12,49,51 (where the reaction mixture, catalyst, and wire meet) and the completion of the monolayer by ledge flow.12,51 12531

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catalyst−wire contact angle in SSS growth removes an important origin of the fluctuating nucleation barriers that switch between the WZ phase and the ZB phase in SLS growth, and thus favors the nucleation of the lower barrier WZ phase and the growth of nearly phase-pure, defect-free WZ QWs.4

QWs grown from liquid catalysts at the same temperature (Table 1). This suggests similar CdTe accommodation and/or transport rates through the liquid and solid catalyst nanoparticles. Since the primarily solid catalysts at 300 °C contain a small amount (∼15%) of liquid Bi that presumably resides on the surface of the catalysts,4 similar accommodation coefficients of CdTe into the liquid or liquefied surface of the catalysts are thus expected in the two cases above. As suggested below by the unique structure of the solid BixCdyTez catalyst nanoparticles, the CdTe solute may transport through the solid catalysts at a rate comparable to that in the liquid catalysts. The BixCdyTez catalyst nanoparticles are nearly crystallographically indistinguishable from rhombohedral Bi nanoparticles on the basis of HRTEM (Figures 3a and 5a).4 In addition, the solid BixCdyTez (y ≪ z) catalyst nanoparticles are compositionally related to Bi2Te3 nanoparticles also of rhombohedral structure, which were previously shown to initiate the SSS growth of WZ CdTe QWs.4 Furthermore, both Bi and Bi2Te3 have layered structures perpendicular to the [0001] direction.50 Bi contains Bi2 blocks of weak covalent nature (Figure 8a), and Bi2Te3 contains [Te1−Bi−Te2−Bi− Te1] blocks of van der Waals type (Figure 8b). As such, BixCdyTez should exhibit a layered structure of similar characteristics. Bi2Te3 exhibits high anisotropic diffusion coefficients on the order of 104−1010 nm2/s in directions parallel to the {0001} plane at 25−500 °C for metals such as Cu, Ag, Au, Ni, and Sn, which are up to 4 orders of magnitude higher than in the [0001] direction (Figure 8b).52−56 This is attributed to the weak van der Waals forces and the relatively large spacings between adjacent [Te1−Bi−Te2−Bi−Te1] blocks that allow the Cu, Ag, Au, Ni, and Sn atoms to diffuse interstitially.53,56 Substitutional diffusion aided by high concentrations of antisite defects and thermal VBi and VTe vacancies and interstitial diffusion through Bi−Te bonds are responsible for the significant (although slower) diffusion occurring across the {0001} planes.56−58 Additional defects due to Cd incorporation in solid BixCdyTez catalyst nanoparticles may further facilitate substitutional diffusion.56 The fast WZ QW growth rate (882 ± 167 nm/s) at 300 °C agrees well with the high diffusion coefficients exhibited for the metal atoms in Bi2Te3. As the Cd/ Te diffusion coefficient is not expected to be low at 250 °C,52−56 the slow WZ QW growth rate of 4 ± 1 nm/s should thus result from a low CdTe accommodation from the reaction mixture into the solid catalyst surface. Material transport through the solid catalyst and the SSS growth of the CdTe QW are schematically shown in Figure 8c. Although the detailed mechanism is unknown, we speculate that the accommodated CdTe solute on the catalyst surface may decompose into individual atoms or ions to initiate dissolution into the solid catalyst. The small amount of liquid Bi on the surface of the solid catalysts should facilitate such decomposition. Adsorbed CdTe may also migrate on the surface of the CdTe QW and catalyst to the triple phase line, where the reaction mixture, catalyst, and wire meet.59 Under the conditions of SSS growth, elemental Te is incorporated simultaneously with greater ease into the catalysts (see Results) such that the catalysts maintain their solid form. Once CdTe reaches supersaturation, a solid CdTe monolayer nucleates at the triple phase line, followed by ledge flow to complete the growth of the monolayer (Figure 8c).12,49,51 Such nucleation and growth of the monolayer are repeated so the wire grows, as in the VSS growth of nanowires.49,51,60 The near invariant solid-

CONCLUSIONS We have achieved an in-depth understanding of the mechanistic role of precursor-conversion chemistry in controlling the composition of the catalyst nanoparticles and consequently the crystal phase of semiconductor QWs. We have shown that under conditions in which Te dissolution into the catalyst nanoparticles surpasses Cd dissolution, solid BixCdyTez (y ≪ z) catalyst nanoparticles are formed and catalyze the SSS growth of WZ CdTe QWs. The solid catalysts are effective materialtransport media for CdTe, which enable the continuous, catalytic growth of QWs. Under conditions in which the rates of Cd and Te dissolution are similar, liquid BixCdyTez (y ≈ z) catalyst nanoparticles are generated and catalyze the SLS growth of polytypic QWs. This work represents a significant advance in the field of solution-phase, catalytic growth of semiconductor QWs. The findings and knowledge gained may potentially benefit the growth of phase-pure QWs of other compositions. For example, we have recently achieved the growth of WZ-rich CdS and CdSe QWs using similar but not identical strategies. Mechanistic investigation for these two types of wires will further our understanding of the crystalphase control in catalytically grown, colloidal semiconductor QWs. METHODS Materials. Cadmium di-n-octylphosphinate (Cd(DOPT)2),4 di-noctylphosphine (DOP),37 tri-n-octylphosphine telluride (TOPTe) stock solutions (0.025 mmol/g solution), and bismuth (Bi) nanoparticle stock solutions (0.04 mmol Bi atoms g−1 solution)40,61 were prepared using previously reported procedures. Cadmium oleate (Cd(OLT)2) and cadmium n-tetradecylphosphonate (Cd(TDPT)) were prepared similarly as Cd(DOPT)2, and their characterizations are provided in the Supporting Information. Two different batches of trin-octylphosphine (TOP, 97%, Aldrich, batch no. 11996DM; 97%, Strem, batch no. 18647600) were used as received, which contained 0 or 0.02 mol % DOP, respectively, as determined by 31P NMR following a previously reported procedure.37 The TOPTe stock solution prepared from the Strem TOP was primarily employed in this study, if not otherwise specified. 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.39,62 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), Bi nanoparticle stock solution (20 mg, 0.00080 mmol Bi atoms), and TOPO (4.0 g, 10.35 mmol). The amounts of TOPTe and DOP stock solutions and the temperatures may vary as required. In a general procedure, Cd precursor 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 and then backfilled with N2(g). After repeating this degassing and N2(g) backfilling cycle at least four additional times and degassing for 1 h, the reaction mixture was 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 (e.g., 590 mg, 0.0148 mmol TOPTe, intrinsically containing 0.00032 mmol DOP), and 12532

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ACS Nano DOP stock solution (e.g., 100 mg, 1.01 mg/0.00391 mmol DOP, 0.0025 mmol TOPTe) were combined in a separate vial and septum capped in a N2(g)-filled glovebox. This mixture was brought out of the glovebox, 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 ratio of 2.9−3.0 and TOP mol % of 0.03%. 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, transmission electron microscopy, and energy-dispersive spectroscopy analyses (see below). TEM and EDS Analyses. The QWs 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 (1150g) 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 QWs ultimately were redispersed in toluene. 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.4 EDS was performed using either JEOL 2000 FX TEM or JEM-2100F TEM at 200 kV. 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 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 photoluminescence spectra were obtained at room temperature using a Varian Cary Eclipse fluorescence spectrophotometer at the excitation wavelength of 528 nm (2.35 eV).

Washington University and the Institute of Materials Science and Engineering for the use of instruments and staff assistance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b06639. Syntheses and characterizations of Cd precursors, additional TEM and HRTEM images and extinction spectra of CdTe QWs synthesized at various conditions, kinetic studies, and 31P{1H} NMR spectra of TOPTe in TOP (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [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 12533

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