Thiol treatment to enhance photoluminescence and electroluminescence of CdSe/CdS core–shell quantum dots prepared by thermal cycling of single source precursors

Abstract

Stable and high photoluminescence quantum-yield CdSe/CdS core–shell quantum dots (QDs) with a zinc blende (ZB) structure in both the core and the shell are prepared by over-coating CdSe core nanocrystals with a controlled number of CdS monolayers (MLs). The epitaxial growth reaction is induced via the temperature cycling of a single-source precursor, namely cadmium diethyldithiocarbamate (Cd(DEDTC)₂), in oleylamine (OLA) and 1-octadecene (ODE) at temperatures between 140 and 200 °C. The quantum yield of the resulting core–shell quantum dots is then enhanced by means of a ligand exchange process with alkanethiol. The ligand-protected CdSe/CdS core–shell QDs are spin coated on an indium tin oxide (ITO)-glass substrate to form a type-I QD light-emitting device (QD-LED). It is shown that the QD-LED has a current efficiency of 0.22 cd A⁻¹. Notably, the current efficiency is 4.5 times higher than that of a QD-LED incorporating non-thiol-protected QDs, and is comparable to the best reported performance for type-I QD-LED devices in the literature.

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Introduction

Over-coating unstable and low-quantum-yield QD cores with higher bandgap materials enables the synthesis of stable and high-quantum-yield core–shell QDs for practical applications.¹ However, the shell growth process induces lattice mismatch strains in the shell, which can prompt the progressive nucleation of defects that act as non-radiative recombination centers and thus limit the maximum attainable QY.² Accordingly, various synthesis protocols have been proposed for controlling the shell thickness to a single monolayer level so as to obtain the optimal shell thickness which maximizes the QD performance.³ Among these methods, the successive ion layer absorption and reaction (SILAR) method has been widely used for the preparation of core–shell QDs such as CdSe/ZnS and CdSe/CdS.⁴ The SILAR method involves the alternate injection of Zn- (or Cd-) and S-precursors at a high temperature (e.g. 240 °C). However, the synthesis process is time-consuming and complex; particularly for thick-shell QDs.

Due to carrier delocalization, thick-shell CdSe/CdS core–shell QDs exhibit low single-dot photoluminescence (PL) blinking,⁵ low non-radiative Auger recombination,^3d a large Stokes shift,^3c and an enhanced electroluminescence (EL).^3a Consequently, they have attracted significant interest in the recent literature. A TC-SP (thermal cycling with single-source precursors) method has been proposed for the growth of core–shell QDs with a ZnS or CdS shell, in which a single compound such as Zn (or Cd) diethyldithiocarbamate (Zn(DEDTC)₂ or Cd(DEDTC)₂) containing both metal and anionic precursors is alternately adsorbed and decomposed at low temperatures in the presence of a primary amine.⁶ The TC-SP method has several important benefits compared to the SILAR method, including a low reaction temperature, a straightforward precursor manipulation, and products with improved shell crystallinity.⁷ However, the synthesized products frequently have a low QY, and therefore require further processing.⁸

It is known that the photoluminescence (PL) properties of QDs are strongly dependent on their surface treatment.⁹ In the present study, it is speculated that thiol molecules, with a high affinity to nanocrystal (NC) surfaces, remove the surface trap states and therefore improve the QY. Consequently, CdSe/CdS core–shell QDs are synthesized using the TC-SP process and are then protected with thiol ligands by means of a ligand exchange process performed with alkanethiol at room temperature. The ligand-protected QDs are spin coated onto an indium tin oxide (ITO)-glass substrate to form the emissive layer of a type-I QD light-emitting device (QD-LED). It is shown that the current efficiency of the QD-LED is around 0.22 cd A⁻¹. By contrast, that of a QD-LED incorporating non-thiol-protected CdSe/CdS core–shell QDs is just 0.048 cd A⁻¹.

Results and discussion

Purification and over-coating of CdSe cores

CdSe cores with a ZB structure were prepared using a modified non-trioctylphosphine (non-TOP) method.¹⁰ Previous studies have shown that unreacted precursors from the core synthesis process, e.g., Cd carboxylate and elemental Se, are detrimental to the subsequent TC-SP over-coating process, and hence the addition of TOP is required for purification purposes.⁸ However, TOP is expensive, and thus in the present study, an alternative purification method is used to remove the residual precursors. Specifically, the synthesized CdSe NCs are centrifuged in a mixture of oleylamine and hexane, resulting in a dispersion of the CdSe NCs in the oleylamine and the formation of solid elemental Se precipitates. It is known that Cd(OA)₂, oleylamine, and ethanol form an soluble complex.⁸ Thus, following centrifugation, the CdSe NC suspension is removed and placed in ethanol. The suspension is then centrifuged once again to extract the Cd(OA)₂ and the CdSe NCs are obtained from the precipitation. (Note that full details of this purification procedure are described in the Experimental section.)

Dethlefsen and Døssing have shown in their one-step, one-pot single-source-precursor synthesis of CdSe/ZnS QDs using TOP and oleylamine (OLA) together results in QDs with a higher PL intensity than that obtained when using TOP and OLA alone.^6a Moreover, TOP tends to bind to the Se defect atoms on the surface of the CdSe cores during shell growth,^6a and thus enables ZnS over-coating to be performed at low temperatures (110–120 °C). However, the solvent system used in the present study (i.e., oleylamine and hexane) is TOP-free, and hence a low-temperature over-coating process is infeasible. Thus, a monolayer-by-monolayer growth of a CdS shell on the CdSe core is achieved by injecting a mixture of Cd(DEDTC)₂, oleylamine and ODE into a flask containing QDs at 140 °C and then raising the temperature to 200 °C.^6b,7 It has been reported that a high reaction temperature prompts core–shell interfusion and a phase transformation of the QDs from zinc blende to wurtzite (WZ).¹¹ However, in the present study, no evidence of inter-diffusion between the CdSe core and CdS shell is observed. Similarly, no ZB to WZ transformation occurs (see XRD results, Fig. S1†). Fig. 1(a) presents the PL emission spectra of the CdSe core and the CdSe/CdS core–shell NCs after the growth of each monolayer. It is seen that the PL intensity of the CdSe core (emission wavelength of 564 nm and full-width-at-half-maximum (FWHM) of 27 nm) is significantly lower than that of the CdS over-coated core–shell NCs. In other words, the over-coating process yields an effective improvement in the PL emission. In addition, it is observed that the growth of successive CdS monolayer shells results in a red shift of the PL peak. This phenomenon can be ascribed to that the quantum confinement effect of excitons is weakened as the shell thickness increases, with more exciton wave function distributed in the CdS shell.¹² Notably, the red shift of the PL peak confirms the absence of inter-diffusion between the CdSe core and the CdS shell since such inter-diffusion is known to cause a blue shift in the PL emission.¹³ It is seen in Fig. 1(b) that the FWHM of the PL peaks remains narrow (<33 nm) as the number of monolayers increases from 1 to 5 ML. In other words, the size uniformity of the CdSe/CdS core–shell QDs is maintained as the CdS shell thickness increases. Fig. 1(c) shows the variation of the PL quantum yield (QY) of the core–shell QDs with the shell thickness. It is seen that the maximum QY has a value of 50% and occurs for the core–shell QDs with a three-monolayer shell. It has been reported that the shell thickness associated with the optimal PL performance of multiple-shell CdSe/CdS NCs varies with the synthesis method used.^4,6,8,12 The present results are broadly consistent with those reported in the literature.⁸

Fig. 1 (a) PL spectra, (b) peak wavelength, FWHM, and (c) PL QY of CdSe/CdS core–shell QDs with a shell of precisely controlled number of 1–5 CdS monolayers. (Excitation wavelength = 365 nm.)

Fig. 2 presents typical transmission electron microscope (TEM) micrographs of the original CdSe cores and the CdSe/CdS NCs with a 3-monolayer shell. Fig. 2(a) shows that the CdSe core NCs are monodisperse and have an average size of 4.3 ± 0.2 nm. Meanwhile, Fig. 2(b) shows that the core–shell CdSe/CdS NCs have an average size of around 6.2 ± 0.3 nm. It is noted that these results are consistent with the theoretical value for the epitaxial growth of a 3-monolayer CdS shell (∼0.336 nm/monolayer) on a 4.3 nm CdSe core. Fig. 2(c) presents a high-resolution TEM (HRTEM) image of the CdSe NCs. It is seen that the NCs have a single crystalline phase. The three intense diffraction rings in the selected area electron diffraction (SAED) image shown in Fig. 2(a) can be attributed to the 111, 220 and 311 diffractions of the ZB structure (JCPDF: 88-2346). The HRTEM image presented in Fig. 2(d) and SAED pattern presented in Fig. 2(b) show that the CdSe/CdS core–shell NCs also have a single crystalline phase; suggesting an epitaxial growth of the ZB CdS shell (JCPDF: 89-0440) from the ZB CdSe core. Comparing the SAED patterns in Fig. 2(a) and (b), it is seen that the growth of the CdS shell on the CdSe NC prompts a shift of the diffraction positions from ZB CdSe to ZB CdS. In other words, the outer CdS shell is sufficiently thick to dominate the diffraction results.

Fig. 2 TEM and HRTEM micrographs of (a and c) CdSe NCs and (b and d) CdSe NCs with three CdS monolayers.

Surface modification

The Cd/S ratio in the coating precursor used in the present study (i.e., Cd(DEDTC)₂) is 1/4. Thus, the decomposed products contain not only CdS monomers, but also excess S-containing byproducts. It is known that such byproducts tend to adsorb on the QD surface and form surface trap states, which cause a reduction in the QY.^8,14,15 Dethlefsen and Døssing showed that for a given amount of TOP addition, an excessive amount of Zn(DEDTC)₂ is detrimental to the optical properties.^6a Thus, it is inferred that the S-containing byproducts have a constraining effect on the QY of the core–shell CdSe/CdS QDs. Nan et al. reported that the addition of Cd(OA)₂ after the over-coating process prompts a reaction with the excess S-containing byproducts adsorbed on the surface of the NCs, and therefore improves the PL QY.⁸ The core–shell QDs synthesized in the present study using a TOP-free TC-SP method have a very low QY (see Fig. 1(c)). It is presumed that this low QY stems from the adsorption of S-containing byproducts during the coating process. Previous studies have demonstrated the potential for using high affinity ligands to replace the S-containing byproducts on the surface, thereby enhancing both the stability and the optical performance. Thus, several commonly-used ligands, namely tri-octylphosphine (TOP), tri-octylphosphine oxide (TOPO), oleic acid (OAH), and octanethiol (OT), were selected in the present study for ligand exchange with the core–shell QDs. Fig. 3(a) shows the variation over time of the normalized PL emission of the resulting core–shell QDs under room temperature conditions. It is seen that OT and TOP improve the PL performance of the QDs, while TOPO and OAH have no effect and an adverse effect, respectively. Notably, OT yields a faster PL enhancement rate (time to reach equilibrium: 500 s; the final PLQY of 80%) and a higher PL enhancement ratio (60%) than TOP (3600 s, 28%). The results presented in Fig. 3(b) and (c) show that none of the ligands change the band edge energy or peak width. In other words, in every case, the integrity of the core–shell NCs is maintained during the ligand exchange process. As shown in the inset in Fig. 3(c), the sample with OT addition has a greater intensity under UV radiation than the other samples. Previous studies have shown that alkanethiol ligands have a stronger affinity to metal sulfide surfaces than OLA or TOPO, but a weaker affinity than OAH.^10,16 The enhanced PL of the OT sample thus suggests that the OT efficiently replaces the protecting ligands (OLA or OA in the present case) and S-containing byproducts on the CdSe/CdS core–shell NCs. However, the addition of OAH to the as-prepared core–shell QDs or OT-modified core–shell QDs results in precipitation. The dynamic nature of the ligand exchange process on NC surfaces results in the emergence of various behaviors, depending on factors such as the surface composition, the solvent composition, the ligand concentration, and the temperature. The present results contradict those reported by Munro for the effects of thiol addition on the PL performance of CdSe/CdS NCs prepared using the SILAR method.¹⁷ According to the energy-dispersive X-ray spectroscopy (EDS) spectrum presented in Fig. S2† for the CdSe/CdS QDs with a 3-monolayer shell, the atomic percentages of Cd, Se and S are 32.45%, 5.17%, and 62.38%, respectively. Moreover, the ratio of anions to cations, i.e., (Se + S)/Cd, is 2.08. The high S ratio confirms the attachment of the thiol ligands to the core–shell NC surface. For comparison purposes, alkanethiols with chain lengths other than that of OT (C₈), namely DDT (C₁₂) and HDT (C₁₆), were also tested in order to explore their PL enhancement effect. The results showed no significant difference in the PL enhancement compared to that obtained using the original thiol ligands (see Fig. S3 and S4†). The ligand length is an important consideration for QDs when deposited in a NC film for practice applications. For example, in EL applications, the inter-particle distance in the QD emissive layer can be controlled to achieve the optimal balance between carrier injection and radiative exciton recombination.¹⁸

Fig. 3 (a) Time-dependent luminescence (605 nm) intensity variation following addition of various ligands (sample prepared: 3.5 mL hexane, 0.05 mL as-prepared QD suspension, 0.03 mL ligands; conditions: room temperature, λ_exc = 365 nm.). (b) Absorption spectra, and (c) PL spectra of CdSe/CdS core/shell QDs in as-synthesized condition and following ligand modification. (Note that the inset shows the QDs modified using various ligands under UV light.)

PL decay kinetics

To further explore the PL properties of the thiol-modified core–shell QDs, the luminescence decay of CdSe/CdS core–shell QDs (3 MLs) was measured at the emission peak maximum both before and after OT-modification. The resulting decay curves are shown in Fig. 4. For both samples, the decay curves were fitted using a bi-exponential model with the form F(t) = A₁exp(t/τ₁) + A₂exp(t/τ₂), where τ₁ and τ₂ are time constants and A₁ and A₂ are the amplitudes of the respective components. The average lifetime (τ) was then calculated as τ = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂).¹⁹ The fitting parameters and lifetime results are summarized in Table 1. In general, a shorter lifetime can be attributed to the intrinsic recombination of the initially-populated core states of the CdSe QDs,²⁰ while a longer lifetime is generally related to surface-related emission or the emission of dark excitons.²¹ It has been reported that the distribution of the radiative lifetime components in PL decay dynamics is mainly determined by the surface quality of the QDs.²⁰ However, the PL decay cannot clearly identify this difference in present study, probably due to the fast non-radiative decay that is not revealed in these experiments. Nevertheless, the non-modified QDs still show a tendency toward a longer average lifetime, while modified QD samples show a tendency toward a shorter average lifetime; shedding some light on the importance of the surface-protection effect.

Fig. 4 PL decay kinetics of CdSe/CdS core–shell QDs with 3 CdS monolayers (a) with and (b) without thiol modification.

Table 1 Time constants τ₁ and τ₂, components A₁ and A₂, and average lifetimes τ of CdSe/CdS core/shell QDs with and without thiol modification

Sample	A1 (%)	A2 (%)	τ1 (ns)	τ2 (ns)	τ (ns)
3 ML	65.7	34.3	15.5	31.6	23.80
3 ML + OT	44.7	55.3	13.7	26.7	22.88

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Electroluminescence properties of QD-LEDs

It is known that QDs experience a significant QY quenching effect following the purification process due to the loss of ligands during washing.^8a While this process cannot be avoided due to the existence ligand is a negative effect for charge injection in QD-LED device. In a typical purification process in the present study, the QDs were dispersed in a hexane–ethanol solution and precipitated with acetone. Fig. 5 shows the PL spectra of the thiol-modified CdSe/CdS core/shell NCs (3 ML) before and after the purification process, respectively. Note that the PL spectra of the non-thiol-modified CdSe/CdS NCs are also shown for comparison purposes. From inspection, the emission intensity of the non-modified QDs reduces by 68% following the purification process. By contrast, that of the thiol-modified QDs reduces by just 32%. The dispersion of purified thiol-capped QDs in hexane was found to remain clear for more than 1 week. However, the non-modified QDs formed precipitates within 1 day. These findings indicate that the thiol modification process is beneficial for both the QY of the CdSe/CdS core–shell QDs and their colloidal stability.

Fig. 5 PL spectra of CdSe/CdS core–shell QDs with 3 CdS monolayers with and without thiol modification before (a and b) and after (c and d) washing. (Note that the inset shows the washed QDs under UV irradiation.)

The electroluminescence (EL) properties of the thiol-modified CdSe/CdS core–shell QDs (3 ML) were evaluated by constructing a type-I QD-LED device^1a with the structure shown in Fig. 6(a). For comparison purposes, a QD-LED with an active layer consisting of unmodified CdSe/CdS core–shell QDs was also fabricated. For convenience, the two QD-LEDs are referred to as Device A and Device B in the following discussions. Fig. 6(a) shows the PL spectrum of the QD/hexane suspension and the EL spectrum of the QD-LED. A red-shift of around 10 nm is observed in the EL spectrum relative to the PL spectrum due to the dielectric function of the surrounding medium²² and/or energy transfer from the smaller to larger QDs in the ensemble.²³ However, emission from the transporting layers in the QD-LED is not observed, which implies that the electrons and holes recombine in the QD layer only. Based on the reported energy level of the conduction band edge of CdS (4.2 eV)²⁴ and the work function of Al (4.3 eV),²⁵ the electrons injected from the Al cathode should have a low voltage threshold (i.e., less than 1 V) for both devices. In Fig. 6(b), the voltage threshold of electron injection of Device A and Device B start at around 1.5 V and 2 V, respectively. Since the holes encounter the ligands before the QDs, the difference in the turn-on voltage of the two devices indicates that the thiol ligands serve as a good hole transport medium. Fig. 6(c) shows that Device A achieves a maximum luminance of around 1200 cd m⁻² under a bias voltage of approximately 8 V. By contrast, Device B yields a luminance of less than 100 cd m⁻² irrespective of the bias voltage. Finally, Fig. 6(d) shows that the maximum current efficiency of Device A (∼0.22 cd A⁻¹) is around 4.5 times higher than that of Device B (0.048 cd A⁻¹). Notably, the luminance output of the thiol-modified QD-LED is comparable to the best reported luminance value in the literature for type-I QD-LEDs based on CdSe/CdS core–shell QDs with a thick shell (>10 ML).^3a,26 Thus, the present results suggest that for QD-LEDs based on a thin-shell CdSe/CdS emissive layer, robust thiol ligands facilitate matched carrier injection and therefore enhance the EL performance. Fig. 7 presents atomic force microscope (AFM) images of the CdSe/CdS core–shell QD emissive films in the two QD-LEDs. The surface roughness (R_a) of the non-thiol-modified QD layer is equal to 1.09 nm (Fig. 7(a)), while that of the modified QD layer is equal to 0.76 nm (Fig. 7(b)). In addition, it is seen that the thiol-modified QD film has a uniform QD arrangement and fewer defects as a result of its improved colloidal stability, Overall, the PL and AFM results imply that the improved current efficiency of Device A stems from an improved radiative recombination of the QDs and a lower leakage current as a result of the controlled inter-dot distance effect induced by the thiol ligands.^19,27

Fig. 6 (a) PL and EL spectra of thiol-modified CdSe/CdS core–shell QDs with 3 CdS monolayers. (b) Current density–voltage (in log scale), (c) luminance–voltage, and (d) current efficiency–current density characteristics of QD-LEDs containing thiol-modified QD emission layer (Device A) and unmodified QD emission layer (Device B).

Fig. 7 AFM images of QD films prepared by spin-coating (a) non-thiol-modified and (b) thiol-modified CdSe/CdS core–shell QDs with 3 CdS monolayers onto poly-TPD transporting layer.

Conclusions

Monodisperse, stable, and high PL QY (up to ∼80%) CdSe/CdS core–shell quantum dots with a zinc blende structure in both the core and the shell have been prepared by over-coating CdSe cores with a controlled number of CdS monolayers. It has been shown that the optimal PL is obtained using a CdS shell consisting of 3 MLs. In addition, it has been shown that adding alkanethiol to the reaction solution at room temperature following the TC-SP over-coating process improves the QY of the core–shell QDs from 50% to 80%. Finally, the PL lifetime results have confirmed the robustness of the thiol ligands on the CdSe/CdS core–shell QDs.

Experimental section

Chemical

Cadmium oxide (CdO, 99%, Showa), cadmium acetate dihydrate (Cd(Ac)₂·2H₂O, 99%, J. T. Baker), selenium(IV) oxide (SeO₂, 99.8%, Strem), oleic acid (OAH, 90%, Showa), sodium diethyldithiocarbamate (NaDDTC·3H₂O, 99%, Aldrich), hexadecylamine (HDA, 98%, Acros), oleylamine (OLA, 70%, Aldrich), oleic acid (OAH, 90%, Showa), 1-octadecene (ODE, 90%, Acros), 1-dodecanthiol (DDT, 98%, Acros), 1-octanethiol (OT, 98%, Acros), 1-hexanethiol (HDT, 95%, Acros), trioctylphosphine (TOP, 97%, Aldrich), and trioctylphosphine oxide (TOPO, 97%, Aldrich) were used in an as-received condition without further purification.

Stock solution of Se

A stock solution of SeO₂ (8 mmol, 0.88 g) and ODE (10 mL) was loaded into a 100 mL three-neck flask and heated at 200 °C for 2 hours under ambient conditions to obtain a clear solution. DDT (0.8 mmol, 0.19 mL) was then added to the hot ODE-Se solution for subsequent hot injection.

Synthesis of Cd(OA)₂

CdO (50 mmol, 6.42 g) and OAH (100 mmol, 36 mL) were mixed in a 100 mL three-neck flask and then heated at 240 °C for 30 min to obtain a clear solution. The solution was cooled to room temperature to obtain a yellow-white solid product. The product was ground into powder for subsequent use without further purification.

Stock solution of Cd(DEDTC)₂

A stock solution of Cd(DEDTC)₂ was prepared through the reaction of Cd(Ac)₂·2H₂O and NaDDTC·3H₂O in an aqueous solution.^6,8 Specifically, Cd(Ac)₂·2H₂O (10 mmol) was dissolved with 100 mL of distilled water in a 400 mL beaker. NaDDTC·3H₂O (20 mmol) dissolved in 60 mL of distilled water was added dropwise to the beaker under vigorous stirring; resulting in the rapid formation of white Cd(DDTC)₂ precipitates. The mixture was stirred for an hour to ensure a full reaction and the white precipitates were then separated from the solution phase by filtration and washed three times with distilled water. The final product was obtained in white powder form by drying at 60 °C under vacuum conditions for 1 day. Stock solution of Cd(DDTC)₂ (1 mmol), OLA (5 mL) and ODE (5 mL) were loaded into a 20 mL flask and sonicated for 10 min under ambient conditions to obtain a clear solution.

Synthesis of CdSe cores

CdSe NCs were synthesized using a modified literature method.¹⁰ In a typical reaction, Cd(OA)₂ (4 mmol, 2.7 g), ODE (10 mL) and HDA (1 mmol, 0.241 g) were mixed in a 100 mL three-neck flask and degassed at 100 °C for 30 min under a N₂ flow. The mixture was further heated to 240 °C and 10 mL of hot Se stock solution was injected into the flask. The temperature was then reduced and maintained at 220 °C. Following the injection of the Se stock solution, the color of the reactant changed from clear colorless to green, yellow, and finally dark orange as the reaction proceeded. After 20 min, the heating mantle was removed and the flask was cooled in a cold-water bath. The resulting product (5 mL) was diluted with hexane (5 mL), ethanol (15 mL) and acetone (25 mL) and was then centrifuged at a speed of 5000 rpm. The ODE solvent and HDA separated to form a liquid part, while the CdSe NCs and unreacted precursors remained in the form of precipitates. The precipitates were diluted with hexane (15 mL) and oleylamine (2 mL) and were then centrifuged at a speed of 5000 rpm; resulting in the formation of solid elemental Se precipitates and a suspension of CdSe NCs in oleylamine. The suspension was separated from the solid precipitates using a syringe, diluted with ethanol (30 mL) and then centrifuged once again at a speed of 5000 rpm. The Cd(OA)₂, oleylamine and ethanol combined to form a soluble complex which remained in the liquid part; yielding purified CdSe NCs free of elemental Se and Cd(OA)₂ precursors.

The purification procedures described above were repeated 2–3 times to ensure the complete removal of any elemental Se or Cd(OA)₂. The phase of the precipitates was then checked by XRD. TEM samples were prepared by dropping a hexane suspension of CdSe NCs onto a copper grid (200 mesh) coated with a carbon film. The size of the products was determined by averaging the lengths of the major and minor axes of a minimum of 500 different particles using Sigmascan Pro 5 software.

Synthesis of CdSe/CdS core–shell nanocrystals

CdS shells were over-coated on CdSe NCs using a modified TC-SP method.^6,7 Specifically, red-emitting CdSe/CdS QDs were synthesized by loading a mixture of purified CdSe NCs (0.1 mmol, 0.02 g), OLA (2 mL) and ODE (4 mL) into a 100 mL three-neck flask and degassing at 100 °C for 30 min under a flow of N₂. The reaction mixture was further heated to 140 °C and Cd(DDTC)₂ solution (0.65 mL) was then added. As in the conventional TC-SP method, the reaction temperature was maintained at 140 °C for 10 min. However, in the present study, the temperature was then raised to 200 °C for a further 10 min. In general, the amount of Cd(DDTC)₂ added to the reaction solution for shell growth depends on the size of the particles to be grown.⁸ In the present study, the amount of Cd(DDTC)₂ solution was set as 0.85, 0.108, 0.134 and 1.69 mL in order to grow shells with 2, 3, 4 and 5 monolayers, respectively. The amount of precursor solution for each injection was estimated using a procedure prescribed in the literature⁷ and was calibrated by means of TEM measurements.

Thiol modification

In a typical modification reaction, the ligand (e.g., OT) was added to the as-prepared core–shell QD solution (thiol to QD molar ratio ∼1 : 1000) at room temperature. The mixture was sonicated for 30 min and then left for 1 h under ambient conditions. The resulting solution (0.2 mL) was diluted with hexane (3 mL) for subsequent UV-vis and PL spectrum measurements.

Purification of core–shell QDs

QD solution (2 mL) was mixed with hexane (10 mL) and ethanol (10 mL), and was then centrifuged at a speed of 9000 rpm for 30 min. Purified core–shell QDs were precipitated by adding hexane (1 mL) and acetone (5 mL) and then centrifuging at a speed of 9000 rpm for a further 15 min.

Fabrication of QD-LEDs

Patterned indium tin oxide (ITO)-glass substrates were sequentially cleaned by detergent, acetone, isopropyl alcohol, and de-ionized water, respectively. The substrates were then UV-ozone treated for 25 minutes to remove any cleaning agent residues and to modify the work function of ITO. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Baytron 4083) and poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)diphenylamine) (TFB, ADS259BE) were subsequently spin-coated on the substrates at 3000 rpm for 40 seconds. CdSe/CdS QDs (with and without thiol-treatment, respectively) were dissolved in toluene (10 mg mL⁻¹) and then spin-coated on the transporting layers to a thickness of several monolayers to form an emissive layer. Finally, the substrates were baked at 150 °C in a nitrogen-filled oven and then transferred to a thermal evaporator to deposit Al cathodes with a thickness of 120 nm.

Characterization

The phases of the various products were characterized by X-ray powder diffraction (Shimadzu XRD-6000) using Cu K_α radiation. Moreover, imaging was performed using a transmission electron microscope (JEOL JEM 1200EX) with an accelerating voltage of 80 kV and a high-resolution transmission electron microscope (JEOL JEM 2100) with an accelerating voltage of 200 kV. The mass percentage of inorganic NP in the dried precipitates was determined using a thermogravimetric analyzer (TGA) under a constant flow of N₂ gas and a heating rate of 20 °C min⁻¹. The temperature UV-vis absorption spectra and PL spectra of the CdSe cores and CdSe/CdS core–shell QDs were obtained using a UV-vis spectrophotometer (MPC-2200, SHIMADZU) and fluorescence spectrophotometer (FluoroMax-3, HORIBA JOBIN YVON), respectively. The PL QY properties of the core QDs and core–shell QDs were determined via known QY standards (counmarin 540, QY = 78% in ethanol, Aldrich 99%; rhodamine 6G, QY = 95% in ethanol, Aldrich 99%) with an identical optical density (OD). The OD values of the samples were all set in the range of 0.04–0.06 in order to avoid possible inter-dot energy transfer. The QY data of the QDs and dye standards were collected using a fluorescence spectrophotometer fitted with an integrating sphere (IS module, 4′′ diameter, HORIBA JOBIN YVON). The current–voltage characteristics of the QD-LEDs were measured using a Keithley 2400 source meter. Moreover, the luminance properties of the LEDs under different voltages were obtained by calibrating the photocurrent of a silicon detector using a luminance meter (Minolta LS-110), while the EL spectra were recorded by a fiber-coupled spectrometer (Ocean Optics USB4000). Both LED devices were tested in air in the as-made condition without additional environmental packaging. Moreover, the devices were stored under atmospheric conditions between experiments.

Acknowledgements

The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under contract no. NSC 103-2113-M-269-001.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra14280b

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