Rapid Stepwise Growth of Water-Dispersive CdS Quantum Dots in

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Rapid Stepwise Growth of Water-Dispersive CdS Quantum Dots in Ethylene Glycol Dong-Won Jeong, and Du-Jeon Jang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00249 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 6, 2018

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Crystal Growth & Design

Rapid Stepwise Growth of Water-Dispersive CdS Quantum Dots in Ethylene Glycol Dong-Won Jeong and Du-Jeon Jang* Department of Chemistry, Seoul National University, Seoul 08826, Korea

* Corresponding author. E-mail address: [email protected]. 1

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract

Water-dispersive CdS quantum dots have been rapidly synthesized in a polyol solvent of ethylene glycol at 160 °C without using nonpolar organic solvents and their stepwise growth mechanisms have been investigated thoroughly. The growth rate of CdS QDs in the second step of ≥70 s is prominently slower than that in the first step of ≤70 s. The dominant growth mechanism of CdS QDs switches from the Ostwald ripening (OR) growth in the first step to the oriented attachment (OA) growth in the second step. The fast decay time of 40 ns and the slow decay time of 300 ns in photoluminescence decay profiles are due to the decay times of excitons in internal defects and OA-induced defects, respectively. In the first step, internal defects are eliminated by the OR growth to reduce photoluminescence and the center of the photoluminescence of the slow decay component shifts from 550 nm to 600 nm. In the second step, the OA growth occurs to enhance the number of OA-induced defects, increasing photoluminescence at 600 nm.

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1. INTRODUCTION

Colloidal quantum dots (QDs), which have size-dependent band-gap energies with remarkably high quantum yields, have received great attention because of their potential as biomarkers1–3 and optoelectronic devices.4–6 Those intriguing properties are related to the surface states of QDs. Therefore, controlling the surface structure of QDs has been of big importance and varying the types of ligands has been often attempted as means of surface control.7–9 Many researchers have focused on gaining high photoluminescence quantum yields (PLQYs) and elucidated that defect-induced additional relaxation pathways of QDs decrease PLQY considerably.4,10 Thus, many groups have tried to reveal the relations of surface states to photoluminescence. It has been shown that using an organic solvent with a high boiling point is the key in the removal of defects from QDs.11–13 The synthesis of QDs usually involves organic ligands. However, it is not possible to functionalize the ligands because an organic ligand contains only one functional group with a long carbon chain which is bound to a quantum-dot crystal surface.14 Therefore, QDs having ligands with additional functional groups have been introduced. For example, because the additional functional groups usually make QDs more polar and increase their solubility in water, the ligands such as mercaptocarboxylic acids have been utilized to synthesize QDs, as reported elsewhere.15,16 Hence, water-dispersive QDs have attracted a lot of interests, particularly in order to functionalize their ligands. Nowadays, diverse functionalization methods of QDs in aqueous solutions are investigated vastly.17–19 In general, ligand-exchange methods have been employed for functionalization, which requires QDs to be brought into an aqueous phase.20–22 It is inefficient to add steps to functionalize as-synthesized QDs. Thus, syntheses via aqueous routes have often been suggested.14,15 Despite having been studied for decades, the synthesis of water-dispersive 3



QDs in an aqueous phase has been rarely reported. For example, although water-dispersive CdS QDs can be functionalized easily and applicable in various fields,14 it takes a lot of time to be synthesized directly in water due to the low boiling point of water.15,23 Furthermore, in previous reports,24–26 the Ostwald ripening (OR) growth and the oriented-attachment (OA) growth of CdS QDs have been known to occur simultaneously in the early stage. It is critical to avoid the OA growth to enhance PLQY, but it is not possible to fully control the defectinduced luminescence of QDs synthesized through aqueous conditions. In this report, we present the rapid stepwise growth of water-dispersive CdS QDs in ethylene glycol. Cadmium bis(3-mercaptopropionate) has been used as a single-sourced precursor to investigate the growth mechanism thoroughly since it dissolves completely at a high concentration in ethylene glycol. In addition, to maintain the solution temperature constant during time-dependent mechanistic study, the hot injection method has been employed to synthesize CdS QDs. The dominant growth mechanism of CdS QDs switches prominently from the OR growth in the first step of ≤70 s to the OA growth in the second step of ≥70 s (Figure 1a). The fast decay time of 40 ns and the slow decay time of 300 ns in photoluminescence decay profiles are due to the decay times of excitons in internal defects (IDs) and OA-induced defects (OADs), respectively (Figure 1b). It is noteworthy that waterdispersive CdS QDs have been quickly synthesized in a polyol solvent having a high boiling temperature without using nonpolar organic solvents.

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Figure 1. Schematic diagrams for the growth (a) and relaxation (b) mechanisms of our prepared CdS QDs.

2. EXPERIMENTAL SECTION

Chemicals. Cadmium oxide (CdO, ≥99.99%), and 3-mercaptopropionic acid (3-MPA, HSC2H4COOH, ≥90%) were purchased from Sigma Aldrich. Acetone (CH3COCH3, 99.5%), ethylene glycol (HOC2H4OH, ≥99.0%), and sodium hydroxide (NaOH, ≥97%) were purchased from Daejung Chemicals & Metals. All the chemicals were used without further purification. Water (>15 MΩ cm) deionized by using an Elga PURELAB option-S system was used throughout the experiments. Synthesis of the Precursor of CdS QDs. CdO powder (5.0 mmol) was loaded in a vial having 5.0 mL of water. 3-MPA (10 mmol) was added in the vial, and NaOH, which had been stored in a desiccator for more than one day, was added to adjust the pH of the solution as 6; pH was set as 6 in order to make CdO react with 3-mercaptopropionate ions.15 After being vigorously shaked and sonicated for 10 min, the reddish turbid solution slowly faded out and turned into a completely transparent solution in 24 h. The as-prepared precursor of CdS QDs has been assigned as cadmium bis(3-mercaptopropionate) (see below). 5



Synthesis of CdS QDs. Ethylene glycol (10 mL) was transferred into a 50 mL 3-neck round-bottom flask and heated to be 160 °C under a reflux system. The pressure in the reflux condenser was maintained by a balloon. The as-prepared precursor (0.50 mL) was mixed with ethylene glycol (0.50 mL) in a vial to prevent the precursor from boiling over. After bubbling N2 into the ethylene glycol solvent of the flask for degassing, the solution of the vial was injected into the flask at 160 °C. The concentration of the precursor in the final reaction mixture was 50 mM. The reaction was terminated by placing the flask in cool water at a desired time. The reaction mixture was precipitated with addition of acetone (30 mL) and centrifuged. Then, the precipitate was redispersed into 2.0 mL of water. As-synthesized CdS QDs could be roughly figured out under UV light irradiation (Figure S1). Characterization. High-resolution transmission electron microscopy (HRTEM) images and selected-area electron diffraction (SAED) patterns were obtained with a Tecnai F20 microscope. FFT patterns were processed from HRTEM images using a Gatan microscopy suite 3 (GMS3) program. X-ray diffraction (XRD) patterns were measured with a Rigaku Smartlab powder X-ray diffractometer. X-ray photoelectron spectroscopy (XPS) was conducted with a KRATOS AXIS Ultra DLD spectrometer (Binding energies were calibrated with the C 1s peak at 294.88 eV). UV-vis absorption spectra of CdS colloidal solutions diluted 150 times were obtained with a Scinco S3100 spectrophotometer. To obtain photoluminescence (PL) spectra and kinetic profiles, the optical densities of CdS colloidal solutions at the first-excitonic peaks were adjusted to 1.00. PL spectra were acquired with an Ocean Optics USB2000+ detector after excitation with 355 nm pulses from a 6 ns Quantel Brilliant Nd:YAG laser. Photoluminescence quantum yields (PLQYs) were obtained by comparing the PL intensities of CdS QDs with PL intensities of primary standard rhodamine B (PLQY in ethanol = 65%) solutions. The PL kinetic spectra were collected by a Hamamatsu R928 PMT with a Schoeffel GM 200 double monochromator and recorded with 6


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a 1 GHz Lecroy Wavepro 950 oscilloscope. All the optical measurements were conducted using a UV-graded fused-silica cuvette having a path length of 1.0 cm.

3. RESULTS AND DISCUSSIONS

Although reactions between metal oxides and thiol ligands have already been reported,27,28 reactions between CdO and 3-MPA have not been reported yet. Thus, the reaction product between CdO and 3-MPA, which is the precursor of CdS QDs, has been characterized. Whereas reddish CdO powder cannot be dissolved in water, the as-prepared precursor dissolves completely into water without absorbing light over 290 nm (Figure S2). Thus, we suggest that a formula unit of CdO and two 3-mercaptopropionate ions react to form a cadmium thiolate complex (eq 1). CdO(s) + 2 HS(CH2)2COO−(aq) → Cd(S(CH2)2COO−)2(aq) + H2O(l)

(1)

Note that the predominant prototropic species of 3-MPA at pH 6 is a 3-mercaptopropionate ion. According to calculations, cadmium bis(3-mercaptopropionate), which is displayed as the inset of Figure S2, is the most stable structure among the reaction products expected from CdO and 3-MPA.29,30 The CdO 3d XPS peaks of Figure S3a suggest that the precursor has Cd-S bonds,31 while the S 2p XPS peaks designate that the precursor contains C-S-Cd bonds.32 In addition, the O 1s peak at 531 eV indicates that metal oxide bonds do not exist in the precursor; the oxygen atoms are due to carboxylate groups originated from 3-MPA molecules.33,34 Considering our results, we thus can conclude that the precursor of CdS QDs is cadmium bis(3-mercaptopropionate).

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Figure 2. Absorption (red) and photoluminescence (green) spectra of CdS QDs synthesized in ethylene glycol for 110 s and dispersed in water. The photoluminescence spectrum was obtained with excitation at 355 nm.

The absorption and photoluminescence spectra of Figure 2 indicate that water-dispersive CdS QDs have been well synthesized in a polyol solvent of ethylene glycol. The firstexcitonic peak, which is the lowest excitonic transition of QD, has been observed at 380 nm for CdS QDs synthesized for 110 s, suggesting that the QDs have a band-gap energy of 3.27 eV having an average diameter of 2.79 nm (Table 1). Because of a quantum confinement effect, the observed band-gap energy of as-synthesized QDs is larger than that of bulk CdS (2.42 eV). As-prepared CdS QDs show a broad photoluminescence spectrum at 600±64 nm without displaying band-edge luminescence. It has been reported22,35 that thiol ligands acting as hole traps quench the band-edge luminescence of QDs drastically. The broad photoluminescence band of CdS QDs reveals information on the surface states of CdS QDs (see below).

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Table 1. Particle Sizes, Band-Gap Energies, Stokes Shifts, and PLQYs of CdS QDs Dispersed in Water

synthesis time (s)

da (nm)

db (nm)

dc (nm)

Egd (eV)

∆λSe (nm)

PLQYf (%)

30

2.56

-

-

3.47

191

1.71

50

2.70

-

-

3.34

192

1.58

70

2.76

-

-

3.30

205

1.14

90

2.77

-

-

3.28

208

1.40

110

2.79

3.67

2.52

3.27

209

1.56

a

Diameter calculated from an absorption spectrum using the Brus equation. bAverage value of the long and short axes measured from HRTEM images. cAverage diameter calculated from an XRD pattern. dBand-gap energy measured from an absorption spectrum. eStokes shift. fPhotoluminescence quantum yield.

The wide-view HRTEM image of Figure 3a designates that CdS QDs synthesized for 110 s have an average diameter of 3.67 nm (Table 1), while the SAED pattern of Figure 3b suggests that ring patterns match excellently with the respective standard d-spacing values of the cubic CdS structure. The detailed structure of a CdS QD has been provided further by monitoring an enlarged HRTEM image (Figure 3c) and an FFT pattern (Figure 3d). The observed latticefringe distance of 0.331 nm estimated from HRTEM images agrees well with the standard spacing 0.334 nm between the adjacent (111) lattice planes of cubic CdS (JCPDS No. 01080-4441). The observed d-spacing values of 0.332, 0.208, and 0.174 nm from the FFT pattern match reasonably with the respective standard spacing values of 0.334, 0.204, and 0.174 nm for the adjacent (111), (220), and (311) lattice planes of cubic CdS.

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Figure 3. HRTEM image (a) and SAED pattern (b) of CdS QDs. Enlarged HRTEM image (c) with dspacing value of (111) plane, FFT pattern (d) of a CdS QD.

The XRD pattern of Figure S4 also display that as-synthesized CdS QDs are composed of the cubic CdS structure; note that the XRD pattern of as-synthesized CdS QDs still contains peaks originated from the precursor. The average crystallite size of CdS QDs synthesized for 110 s, calculated from the 2Ɵ value of the (111) planes with the Scherrer’s equation, has been observed as 2.52 nm (Table 1).

Figure 4. HRTEM images of CdS QDs synthesized for 110 s. Each scale bar indicates 2 nm. 10


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Table 1 indicates that for CdS QDs synthesized for 110 s, the average diameter of 3.67 nm measured from HRTEM images is much larger than that of 2.79 nm from absorption spectra or that of 2.52 nm from XRD spectra. The HRTEM images of Figure 4 indicate that two CdS nanospheres have been attached to each other to form a dumbbell-shaped CdS QD, causing CdS QDs to be seen particularly large in HRTEM images. Overall, Figure 4 designates apparently that the OA growth process has taken place during our synthesis of CdS QDs in ethylene glycol (see below).

Figure 5. Relative (a) and peak-normalized (b) absorption spectra of CdS QDs synthesized for times indicated in the units of s in ethylene glycol and dispersed in water. Diameters (circles) and band-gap energies (squares) of CdS quantum dots dependent on the synthesis time (c).

The absorption spectra of Figure 5a show that the optical density of CdS QDs increases 11



gradually with time, suggesting that precursor molecules transform little by little into CdS QDs because only the activated precursor complexes that can overcome the activation energy of transformation participate in the formation of CdS QDs.13 The peak-normalized absorption spectra of Figure 5b indicate that the wavelength of the first-excitonic peak increases step by step with the synthesis time, as presented in Figure S5a. Consequently, band-gap energies converted from the wavelengths of first-excitonic peaks dwindle steadily with the increase of the synthesis time. Subsequently, the average diameters of CdS QDs, calculated from bandgap energies according to the Brus equation,36 increase gradually with the synthesis time (Figure 5c). A close examination of the volume change with time (Figure S5b) reveals that the volume growth rate (Table S1) of CdS QDs in the first step taking place until 70 s (6.7×10-2 nm3/s) is noticeably faster than that in the second step occurring subsequently (1.2×10-2 nm3/s). This suggests that the dominant growth mechanism of CdS QDs switches from the OR growth in the first step to the OA growth in the second step (see below). The photoluminescence spectra and PLQYs also show that the growth of CdS QDs can be divided into two steps. Figure 6a indicates that the wavelength as well as the intensity of the photoluminescence of CdS QDs varies with the synthesis time. The peak-normalized photoluminescence spectra of Figure 6b designate that the wavelength of the photoluminescence maximum (λmax) increases gradually with the synthesis time. Figure S6 displays in detail that the increase of λmax is also faster in the first step of ≤70 s than in the second step of ≥70 s. In particular, the half width at the half maximum (∆λ1/2) of photoluminescence increases with the synthesis time until 70 s. Since then, ∆λ1/2 decreases with the synthesis time. These indicate apparently that the growth mechanism switches around 70 s from the first step of the OR growth to the second step of the OA growth. Figure 6c also verifies the change of the dominant growth mechanism at 70 s. The Stokes shift (∆λS) of CdS QDs increases suddenly at 70 s; internal defects (IDs), which emit photoluminescence 12


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at 550 nm, disappear while OA-induced defects (OADs), which emit photoluminescence at 600 nm, come out rapidly. On the other hand, the PLQY of CdS QDs decreases with the synthesis time until 70 s and then increases with the time; the number of IDs diminishes during the OR growth, decreasing the overall PLQY of CdS QDs in the first step of ≤70s. However, the PLQY of CdS QDs increases in the second step of ≥70 s because the number of OADs increases during the second step of the OA growth. Overall, the photoluminescence of CdS QDs has also indicated manifestly that the transformation of the precursor into CdS QDs in ethylene glycol takes place via two easily distinguishable steps.

Figure 6. Relative (a) and peak-normalized (b) photoluminescence spectra of CdS QDs synthesized for times indicated in the units of s. Stokes shifts (circles) and PLQYs (squares) of CdS QDs dependent on the synthesis time (c). 13



Figure 7. Photoluminescence decay profiles of CdS QDs synthesized for times indicated in the units of s. Samples were dispersed in water, excited at 355 nm, and monitored at 500 nm (a) and 650 nm (b). Solid lines are best-fitted curves to extract kinetic constants.

Although being broad, the photoluminescence spectrum of QDs is rarely affected by the heterogeneous ensembles of nanocrystals.10 Instead, defects uncoupled with core excitons are critical factors of broadening, and they give generally slower excitonic decay kinetics compared to core excitons.35,37 Therefore, we have monitored the decay constants to understand the characteristic properties of defects in CdS QDs deeply. Figure 7 and Table 2 indicate that the photoluminescence decay profiles of CdS QDs can be well fitted biexponentially with the fast component of 40 ns (τf) arising from shallow traps and the slow component of 300 ns (τs) arising from deep traps.15,38 As the initial total amplitude (I0) and the mean lifetime decrease at 500 nm but increase at 650 nm, the initial amplitude fraction of the slow component (As) also decreases at 500 nm but increases at 650 nm while the initial 14


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amplitude fraction of the fast component (Af) increases at 500 nm but decreases at 650 nm. It has been reported24 that the number of IDs decreases during the OR growth and that the number of OADs increases during the OA growth. Thus, we attribute the fast component to the decay of excitons in IDs, which are coupled with core excitons. On the other hand, the slow component arises from the decay of excitons in OADs, which are relatively uncoupled with core excitons.10

Table 2. Emission Decay Constants of CdS QDs Observed at 500 and 650 nm

synthesis time (s)

λmon (nm)

I0a

decay time (ns)

mean lifetime (ns)

30

500

0.98

40 (0.63) + 300 (0.37)b

140

50

0.62

40 (0.72) + 300 (0.28)

110

70

0.24

40 (0.80) + 300 (0.20)

93

90

0.27

40 (0.81) + 300 (0.19)

89

110

0.24

40 (0.85) + 300 (0.15)

78

0.53

40 (0.72) + 300 (0.28)

110

50

0.65

40 (0.58) + 300 (0.42)

150

70

0.66

40 (0.38) + 300 (0.62)

200

90

0.85

40 (0.40) + 300 (0.60)

200

110

1.00

40 (0.38) + 300 (0.62)

200

30

a

650

Initial total amplitude. b Initial amplitude fraction of each component.

It is noteworthy that excitons in deep traps decay on the time scale of excitonic lifetimes in OADs.35,38 In general, the deep traps are found from the CdS QDs passivated by thiolate ligands.24,39 A sulfur atom from the ligand, which is bound to a cadmium atom surface, acts as a dangling bond, introducing an additional state in the mid gap. The formation mechanism of OADs in the second step could be explained as the surface stabilization of CdS QDs; 3-MPA ligands bound to CdS QDs are decomposed to leave sulfur atoms on the surfaces of QDs 15



during the thermolysis process. Left sulfur atoms introduce energy states in the middle of the band gap,40 which become deep traps yielding the slow decay component of photoluminescence. As most of the stabilizing ligand molecules disappear via decomposition, two CdS QDs are attached to each other to stabilize the surfaces of the QDs. This mechanism is supported by the observation that the initial amplitude fraction of the slow component at 650 nm increases with the synthesis time. Note that in this work, we have used the terms of IDs and OADs rather than shallow traps and deep traps, respectively, which are more often used in other kinetic studies, because the growth mechanism of CdS QDs can be described well with IDs and OADs.

Figure 8. Photoluminescence spectra of the fast (open) and the slow (closed) decay components of CdS QDs synthesized in ethylene glycol for 30 s (squares), 50 s (circles), 70 s (triangles), 90 s (inverted triangles) and 110 s (diamonds) and dispersed in water.

To understand the growth mechanism of CdS QDs profoundly, we have separated the total photoluminescence intensity (I) of CdS QDs at each wavelength into the photoluminescence

16


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intensity of the fast component (If) and that of the slow component (Is) using eq 2:

= exp − + exp −

= + = +

(2)

Figure 8 displays that Is centered at 550 nm decreases in the first step of ≤70 s. This indicates that the number of IDs decreases in the first step, suggesting that the OR growth to eliminate IDs takes place dominantly in the early step. Figure 8 also shows that the center of Is shifts from 550 nm to 600 nm with the synthesis time during the first step of ≤70 s, implying that ligands dissociate to leave sulfur atoms on the surfaces of CdS QDs in the first step. As the density of ligands on the surfaces of CdS QDs becomes low, two CdS QDs start to bind to each other to lower the surface energy of the QDs.24,25 This is known as the OA growth, which has been observed by the increase of photoluminescence at 600 nm and the increase of the initial amplitude fraction of the slow component in the photoluminescence decay kinetics. The rise of Is around 600 nm provides the evidence for the occurrence of the OA growth. Since the rate of precursor activation to transform into CdS QDs at 160 °C in ethylene glycol is faster than that at 100 °C in water, decrease in the number of IDs is well distinguished in time from increase in the number of OADs in ethylene glycol. In summary, the formation of CdS QDs from precursor complexes of cadmium thiolate in ethylene glycol at a high temperature of 160 °C takes place via two easily distinguishable steps.13 The growth rate of CdS QDs in the first step occurring until 70 s is noticeably faster than that in the second step, suggesting the dominant growth mechanism of CdS QDs switches from the OR growth in the first step to the OA growth in the second step. As 17



suggested in Figure S7, in the first step of ≤70 s, IDs are eliminated by the OR growth to decrease PLQY and the center of Is shifts from 550 nm to 600 nm as ligands dissociate to leave sulfur atoms on the surfaces of CdS QDs. In the second step of ≥70 s, the OA growth takes place to increase the number of OADs, increasing photoluminescence at 600 nm and the amplitude fraction of the slow decay component in photoluminescence decay kinetic profiles.

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4. CONCLUSIONS

The time-dependent growth mechanisms of water-dispersive CdS QDs have been investigated thoroughly. CdO and 3-mercaptopropionate ions react in water to form cadmium thiolate complexes, which can dissolve in ethylene glycol at a high concentration and transform gradually into CdS QDs at 160 °C. Absorption spectra reveal that the growth rate of CdS QDs in the first step of ≤70 s (6.7×10-2 nm3/s) is noticeably faster than that in the second step of ≥70 s (1.2×10-2 nm3/s). Static and time-resolved photoluminescence spectra suggest that the dominant growth mechanism of CdS QDs switches from the Ostwald ripening (OR) growth in the first step to the oriented attachment (OA) growth in the second step. Photoluminescence decay profiles have suggested that the fast decay time of 40 ns and the slow decay time of 300 ns are due to the decay times of excitons in internal defects (IDs) and OA-induced defects (OADs), respectively. In the first step, IDs are eliminated by the OR growth to reduce photoluminescence and the center of the photoluminescence of the slow decay component shifts from 550 nm to 600 nm as ligands dissociate to leave sulfur atoms on the surfaces of CdS QDs. In the second step, the OA growth occurs to enhance the number of OADs, increasing photoluminescence at 600 nm. Overall, the two stepwise mechanisms of the OR growth and the OA growth can be well distinguishable in a polyol solvent of ethylene glycol at a high reaction temperature of 160 °C.

19



ACKNOWLEDGMENTS We thank Dr. Yeonho Kim for measuring XPS spectra. This work was financially supported by a research grant from National Research Foundation of Korea (2017-006153).

ASSOCIATED CONTENT Supporting Information is available: the picture of CdS samples under UV light irradiation; the absorption and XPS spectra of cadmium bis(3-mercaptopropionate); the XRD spectra, the first-excitonic wavelengths, and the volumes of CdS quantum dots; the growth rates, the band widths, and the defect states of CdS quantum dots.

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For Table of Contents Use Only

Rapid Stepwise Growth of Water-Dispersive CdS Quantum Dots in Ethylene Glycol

Dong-Won Jeong and Du-Jeon Jang*

Synopsis

Water-dispersive CdS quantum dots have been rapidly synthesized in ethylene glycol at 160 °C, and their stepwise growth mechanisms have been investigated with time-resolved photoluminescence spectroscopy. The dominant growth mechanism of CdS quantum dots switches prominently from the Ostwald ripening growth to the oriented attachment growth. This work helps us to understand the direct synthesis of water-dispersive quantum dots.

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Figure 1. Schematic diagrams for the growth (a) and relaxation (b) mechanisms of our prepared CdS QDs. 82x62mm (300 x 300 DPI)



Figure 2. Absorption (red) and photoluminescence (green) spectra of CdS QDs synthesized in ethylene glycol for 110 s and dispersed in water. The photoluminescence spectrum was obtained with excitation at 355 nm. 75x54mm (300 x 300 DPI)


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Figure 3. HRTEM image (a) and SAED pattern (b) of CdS QDs. Enlarged HRTEM image (c) with d-spacing value of (111) plane, FFT pattern (d) of a CdS QD. 75x75mm (300 x 300 DPI)



Figure 4. HRTEM images of CdS QDs synthesized for 110 s. Each scale bar indicates 2 nm. 75x75mm (300 x 300 DPI)


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Figure 5. Relative (a) and peak-normalized (b) absorption spectra of CdS QDs synthesized for times indicated in the units of s in ethylene glycol and dispersed in water. Diameters (circles) and band-gap energies (squares) of CdS quantum dots dependent on the synthesis time (c). 65x123mm (300 x 300 DPI)



Figure 6. Relative (a) and peak-normalized (b) photoluminescence spectra of CdS QDs synthesized for times indicated in the units of s. Stokes shifts (circles) and PLQYs (squares) of CdS QDs dependent on the synthesis time (c). 65x123mm (300 x 300 DPI)


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Figure 7. Photoluminescence decay profiles of CdS QDs synthesized for times indicated in the units of s. Samples were dispersed in water, excited at 355 nm, and monitored at 500 nm (a) and 650 nm (b). Solid lines are best-fitted curves to extract kinetic constants. 82x101mm (300 x 300 DPI)



Figure 8. Photoluminescence spectra of the fast (open) and the slow (closed) decay components of CdS QDs synthesized in ethylene glycol for 30 s (squares), 50 s (circles), 70 s (triangles), 90 s (inverted triangles) and 110 s (diamonds) and dispersed in water. 79x79mm (300 x 300 DPI)


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Rapid Stepwise Growth of Water-Dispersive CdS Quantum Dots in

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