Preparation and properties of highly stable quantum dot-based flexible silica films

Abstract

Highly luminescent hydrophobic CdSe and CdSe/Cd_xZn_1−xS quantum dots (QDs) were synthesized via an organic route. The phase transfer of the QDs was carried out through a ligand exchange from 3-aminopropyltrimethoxysilane (APS) instead of an organic capping agent to get aqueous QDs. A functional sol–gel SiO₂ sol with a high QD concentration was obtained from the aqueous QD colloidal solution with APS through the hydrolysis and condensation which subsequently occurred. Flexible inorganic SiO₂ films with QDs were fabricated via various methods. The photodegradation experiments of the resulting films were completed. It is surprising that the QDs in films were revealed to be highly stable. Especially, the PL intensity of the films increased dramatically after irradiation by 365 nm UV light. By integrating a thin CdSe QD–silica film on a solar cell, the enhanced current demonstrated that a thin film can facilitate the continuous development of solar cells. Because of their high PL brightness, multicolor emission, flexibility and stability, these films will have great potential applications.

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Introduction

Colloidal semiconductor quantum dots (QDs), with unique size-dependent optical and electronic properties such as size-tunable optical characteristics, high quantum yield, narrow, symmetric and stable fluorescence,^1–4 have attracted tremendous interest in recent years. Compared with organic fluorophores, the QDs reveal several advantages including flexible photoexcitation and superb resistance against photobleaching,^5–7 and have been shown to have promising applications in solar cells,^8–10 biological imaging,¹¹ biosensors,¹² and light-emitting diodes (LEDs).¹³ To apply these QDs to optical devices, much effort has been undertaken to incorporate them into an appropriate matrix with the retention of their initial photoluminescence (PL) properties. Compared with traditional devices, functional thin films with QDs can be more appropriate for various applications including photodetectors, solar cells coatings, and LEDs because of their advantages of excellent foldability, crack-resistance, and resistance against photobleaching.^14–16 To extend the application of QDs in thin film devices, it is of paramount importance to develop novel strategies for fabricating QD-containing films with excellent quality by optimizing preparation processes and selecting perfect linkers.¹⁷ However, there are some challenges to fabricate flexible luminescent materials involving solid state QDs, especially for inorganic films. Firstly, it is likely that the hydrodynamic diameter of the original QDs was significantly increased via an amphiphilic polymer encapsulating transfer step which can be involved in fabrication procedures. Generally, QDs synthesized via organic solvents exhibited few defects, well-controlled sizes, and high crystallinity. The transfer of QDs from an organic solvent to an aqueous solution can be required in various potential applications. An amphiphilic polymer encapsulating process capping the initial ligands of QDs surface was widely used leading to a certain degree of increment in hydrodynamic diameter. Moreover, both the optical absorption and emission spectra can exhibit a red shift because of QD aggregation during the solidification process.^18–21

To solve the hydrodynamic diameter increment problem generated via an amphiphilic polymer encapsulating, some solutions have been developed which are based on exchanging the initial surface ligands, instead of the addition of a second layer. QDs have been exchanged the surface ligands that ensure water solubility of the QDs with their hydrophilic groups.²² However, it is challenging to fabricate QD-based luminescent inorganic films with crack-resistance, high flexibility, and stability.

Several fabrication techniques are widely used to produce thin film materials combined with QDs including melt glasses, polymers and sol–gel glasses which have all been doped with different QDs.^23–25 Thin films fabricated via the precipitation of organic polymers on substrates have potential applications in biomedical and optical devices. Organic polymers are chemically compatible with QDs synthesized via organic solvents, but suffer from UV sensitivity and limited thermal range because of the poor thermal conductivity of such organic films.^26,27 In many of fabrication techniques on QD based thin films, sol–gel based inorganic composites have been primarily employed.²⁸ Sol–gel methods can be used to assemble metal chalcogenide QDs into gels, xerogels, and aerogels that interconnected networks of QDs.¹⁵ The sol–gel method is desired for assembling QDs in thin film materials as the structural nature of the sol is easily controlled through acid, catalyst and water. Moreover, compared with QD–polymer composite films, the sol–gel QD multilayer films show excellent stability owing to their physical rigidity, chemical inertness, and negligible swelling.¹⁷ And silica films with the stability have advantages in various applications. Martucci's group have transferred highly luminescent semiconductor QDs produced by organometallic approaches into ZrO₂–SiO₂ hybrid sol–gel glass films.²⁵ The sol–gel process makes functional QDs homogeneous dispersion in the films, and the absorption and the emission properties of QDs are slightly affected by the incorporation into the sol–gel matrix. More recently, Eunjoo Jang's group prepared homogeneously doped QDs–silica monolith substance by preliminary surface exchange of the QDs and base-catalyzed sol–gel condensation of silica while retaining the original PL efficiency and set out their applications to LEDs.²⁹ It is reasonable to believe that the combination of QDs within functional building blocks composed of inorganic materials by sol–gel method provides a powerful approach for achieving well-stabilized and flexible luminescent films. Traditional sol–gel matrix is fabricated using tetraethyl orthosilicate (TEOS) by the hydrolysis of TEOS in an acid or alkaline as a catalyst. These catalysts lead to a certain degree of decrease of PL efficiency of the QDs. 3-Aminopropyltrimethoxysilane (APS) was found to be an excellent precursor because it has slow hydrolysis and gentle condensation and results in a smaller shrinkage. For example, Murase's group reported the assembly of aqueous CdTe nanoparticles with APS by layer-by-layer self-assembled method. Such film is feasible to prepare the composite films with high transparency because the hydrolyzed APS can be dissolved in water.³⁰ Liu and co-workers reported a novel method for preparing luminescent sol–gel SiO₂–CdTe films for getting unicolor and multicolor emission using APS as a precursor.¹⁷

In this article, we developed a method for preparing luminescent sol–gel QD–SiO₂ films using APS as a precursor. The functional films may own high photostability, crack-resistance, and mechanical property, as a result of the unique rigid QD–SiO₂ films architecture. These films revealed with green, yellow, and orange luminescence by adjusting the composition and size of QDs. The aqueous APS-coated QD colloidal solution was prepared by a ligand exchange from APS followed with a sol–gel reaction. The solution retained stable optical properties under ambient conditions for a week. Flexible silica films with high QD concentrations were fabricated from aqueous APS-coated QD colloidal solution. These films revealed enhanced PL after irradiation by 365 nm UV light. This is the first time to find out the PL enhancement of the QDs in such films both for CdSe cores and CdSe/Cd_xZn_1−xS core/shell QDs. Furthermore, we incorporated the film containing CdSe QDs with the commercialized solar cells, and obtained largely improved current intensity by means of photo-electrochemical measurements.

Experimental

Cadmium oxide (CdO, 99.5%) was supplied by Shanghai Chemical Reagent Company. Stearic acid (SA, Ar), cadmium acetate dihydrate (Cd(Ac)₂·2H₂O, 98%), and zinc acetate dihydrate (Zn(AC)₂·2H₂O, 99.99%) were purchased from Sinopharm Chemical Reagent Company. Trioctyphosphine (TOP, 97%), hexadecylamine (HDA, technical grade of 90%), octadecene (ODE, technical grade of 90%), trioctylphosphine oxide (TOPO, technical grade of 90%) and 3-aminopropyltrimethoxysilane (APS, 98%) were obtained from Sigma-Aldrich. Hexane, chloroform, sulfur (S, 99.98%), and ethanol were taken from Tianjin Chemical Reagent Company. Selenium powder (Se, 99.99%) was gotten from Aladdin. Toluene (99.5%) was purchased from Laiyang Chemical Reagent Company. All the chemicals were used directly without any further purification. The deionized water was obtained from a Milli-Q synthesis system (18.2 Ωcm).

The preparation of SA-capped CdSe QDs was carried out via an organic phase synthesis. Briefly, a cadmium precursor was firstly prepared by mixing 16 mg of CdO, 0.25 g of SA, 2 g of TOPO and 1 g of HDA at 140 °C for 1 h in N₂ to remove adsorbed H₂O. Then, the mixture was heat-treated to 260 °C until the CdO powder completely dissolved and the solution became clear and colourless. A selenium precursor (TOPSe) was prepared by dissolving 9.87 mg of Se in 0.5 mL of TOP using an ultrasonic bath at room temperature. The TOPSe solution was injected into the cadmium precursor solution. The temperature remained at 260 °C for 1 min throughout the course of growth. The as-prepared samples were dissolved in toluene, washed with copious ethanol and centrifuged at 9000 rpm for 4 min to remove the impurity. Finally, samples were precipitated with ethanol again and re-dispersed in 5 mL of hexane for further application.

For a shell coating of Cd_xZn_1−xS, typically, 11 mg of Zn(AC)₂, 18 mg of Cd(Ac)₂·2H₂O, 0.5 g of SA, and 5 mL of ODE were mixed in a four-neck flask and heat-treated to 240 °C until the powder was completely dissolved in a nitrogen atmosphere. 6 mg of S powder was dissolved in 1 mL of TOP. Then a 2 mL solution of CdSe QDs in hexane was directly injected followed by the injection of a TOPS solution. The solution was taken out at intervals. Then the products were precipitated with copious ethanol, separated by centrifugation, and re-dissolved in hexane for further characterization.

To fabricate sol–gel silica films with QDs, the SiO₂ precursor sol was firstly prepared by mixing APS with ethanol and H₂O through the partly hydrolysis and condensation of APS. Briefly, the molar ratio of ethanol/H₂O/APS is 10/1/1. The mixture of APS with ethanol and H₂O was stirred for 24 h at room temperature and then heat-treated at 60 °C to evaporate ethanol. The hexane solution of CdSe or CdSe/Cd_xZn_1−xS QDs (1 mL) was mixed with the precursor APS sol of 1 mL with stirring for 12 min. The pure water of 300 μL was added to the mixture with vigorous stirring. Being stirred for several minutes, the solution was created accompanied by a phase separation. Hexane was remained on the upper part because of its low density. All QDs and APS sol was transferred into the water phase, which was then kept in the clean room for the removal of organic solvents to obtain silica–QDs sol. Glass sides were cut into 2.5 × 2.5 cm as substrates. Slide glass substrates were cleaned with acetone, ethanol and deionized water. The film samples were prepared by spinning coating (3000 rpm for 30 s) on a slide glass. Thick films were fabricated by coating silica sol (500 μL) on a free-standing Teflon substrate, the sample was kept in a clean room for a day to evaporate the water and then taken out. The APS-capped CdSe QDs sol was spin-coated (3000 rpm for 30 s) on the surface of solar cells for I–V curve characterization.

The measurement of photo-degradation of films and solutions were carried out by irradiating for different time periods using a light of 365 nm from a 250 W xenon lamp. Following the irradiation, the sample was removed, and the optical absorption and emission spectra were measured at room temperature as a function of irradiation time. The absorption and PL spectra of solution and film samples were recorded using conventional spectrometers, Hitachi U-4100 and F-4600, respectively. The Fourier transform infrared (FTIR) spectra were measured by Nicolet 380. Current density–voltage (I–V) characteristics of solar cells were measured with a Keithley 2612A sourch/meter. The APS sol and the CdSe QD–silica sol were directly spinning coated on top of the solar cells at 3000 rpm for 30 s, preliminarily cleaned with solvent, and dried. The surface morphologies of APS-capped CdSe QDs thin films were examined by atomic force microscope (AFM) in air (Bruker, Germany). The PL efficiencies of QDs were calculated by comparison with Rhodamine 6G with known PL efficiency of 95%.

Results and discussion

Fig. 1 shows the absorption and PL spectra of CdSe QDs. The optical absorption spectrum displays clearly the first excitonic absorption peak wavelength of CdSe QDs with 525 nm. The first sharp PL peak from the CdSe QDs appears at around 532.7 nm after injecting a TOPSe solution for 1 min. Subsequently, CdSe QDs were coated with a Cd_xZn_1−xS shell by an epitaxial growth. The preparation conditions and PL properties of CdSe QDs and CdSe/Cd_xZn_1−xS core/shell QDs are summarized in Table 1. The temporal evolution of PL and optical absorption spectra of the CdSe/Cd_xZn_1−xS QDs are shown in Fig. 2. With prolonging reaction time from 5 min to 60 min, the following PL peaks reveal a typical red shift in wavelength from 553.6 nm to 599.4 nm, indicating that the QDs enlarge according to the quantum size effect. After coating with Cd_xZn_1−xS shells, a large red shift was observed in both the absorption and PL spectra of CdSe/Cd_xZn_1−xS core–shell QDs, demonstrating the formation of a Cd_yZn_1−yS shell layer on CdSe cores. The high PL efficiency of the core–shell QDs were caused by the higher crystallinity and lower inter-diffusion. Being coated with a Cd_xZn_1−xS shell, the PL efficiency was significantly improved as shown in Table 1.

Fig. 1 Optical absorption and PL spectra of CdSe QDs.

Table 1 Preparation conditions and properties of CdSe and CdSe/Cd_xZn_1−xS QDs

Sample	Composition	Reaction time (min)	FWHM (nm)	PL peak wavelength (nm)	PL efficiency (%)
1	CdSe	N/A	29.2	532.7	5.2
2	CdSe/CdxZn1−xS	5	27.5	553.6	32.1
3	CdSe/CdxZn1−xS	20	28.2	578.4	39.2
4	CdSe/CdxZn1−xS	60	28.8	599.4	45.1

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Fig. 2 Optical absorption and PL spectra of CdSe/Cd_xZn_1−xS core/shell QDs.

For the application of hydrophobic QDs in a sol–gel film, phase transfer is necessary. In our experimental, a ligand exchange process was first performed for CdSe and CdSe/Cd_xZn_1−xS core/shell QDs. A pre-hydrolysis process made APS high hydrophilization. Pre-hydrolyzed APS sol was added in a QD solution with stirring to produce APS-capped QDs. Pre-hydrolyzed APS molecules were attached with the QDs instead of organic ligands. To reveal the successful ligand exchange process, Fourier transform infrared (FTIR) spectra of the initial oil-soluble CdSe/Cd_xZn_1−xS QDs and APS-capped CdSe/Cd_xZn_1−xS QDs were measured (Fig. 3). It can be noted the –CH stretching vibration bands (2800–3000 cm⁻¹) due to the presence of long hydrocarbon chain portion in the surface ligand SA.³² The peak at 2800–3000 cm⁻¹ disappeared in APS-capped QDs, indicating that SA molecules originally attached on the QDs surface were replaced after phase transfer. The peak of initial CdSe/Cd_xZn_1−xS QDs at 1500–1650 cm⁻¹ belonged to the –CH bending vibration. Still weak vibrations at the same location can be observed, which may be attributed to the small portion of hydrocarbon chain in APS molecules. The prominent difference of two spectra was the new peaks around 1100 cm⁻¹ due to siloxane groups (Si–O–Si) from hydrolysed APS, indicating that silane agent had been grafted onto the surface of QDs, while no apparent peak appeared for initial QDs.³³ With regard to the FTIR spectra of CdSe/Cd_xZn_1−xS QDs and APS-capped QDs, both showed a broad band around 3500 cm⁻¹, corresponding to the O–H stretching bands of hydrogen-bonded water molecules,³⁴ which may be attributed to adsorbed water during the test process. The FTIR spectroscopy analysis confirms that APS molecules were introduced on the QD surface after ligand exchange.

Fig. 3 FTIR spectra of CdSe/Cd_xZn_1−xS QDs and corresponding.

The controlling of the ligand exchange process plays an important role for the QDs remained high PL efficiencies. The PL efficiencies of APS-capped QDs are 1.28%, 28%, 32% and 35%, respectively. Normally, quick ligand exchange resulted in increased surface defects which decreased the PL efficiencies of the QDs. A pre-hydrolyzed APS molecule layer has advantages because of its small size and high hydrophilization. This is different from the phase transfer of hydrophobic QDs from organic polymer. For example, we completed the aqueous phase transfer of CdTe_xSe_1−x/Cd_yZn_1−yS and CdSe/Cd_yZn_1−yS QDs from amphiphilic poly(styrene-co-maleic) anhydride and ethanolamine polymers.^35,36 This organic layer on the QDs is thick and the QDs are not stable after encapsulating. In addition, a silica film as an inorganic film has advantages for further applications.

Fig. 4 displays the PL spectra of APS-capped CdSe/Cd_xZn_1−xS QD–silica films with green to orange emission color. The change of local surface microenvironment of QDs arising from ligand exchange can cause spectrum fluctuations. The consistent but respectable blue shifts of PL spectra are observed for green-, yellow-, and orange-emitting films. The maximum blue shift is 9.4 nm with emission decreasing from 599.4 nm to 590 nm, meanwhile the minimum blue shift is 1.6 nm with emission decreasing from 553.6 nm to 552 nm. The blue shift indicated that the hydrodynamic diameter of QDs in the films was small compared with that in solution because of the hydrolysed APS instead of the original organic long hydrocarbon chain ligand. Table 2 indicates the PL properties of these fluorescent QD films. Compared with hydrophobic CdSe/Cd_xZn_1−xS QDs in solutions, the FWHM of PL spectra of these QDs in the films which is controlled by the surface ligands is widening, indicating that the surface state of the QDs in the films changed. The inset in Fig. 4 shows the colour images of green-, yellow-, and orange-emitting films under 365 nm UV light. Fig. 5 shows the TEM image of the orange film prepared using sample 3. The QDs is monodispersed in the film. The result confirmed that the perfect procedure described in experiments resulted in the formation of homogenous, crack-resistance, and colour emitting sol–gel silica–QDs films.

Fig. 4 PL spectra of CdSe/Cd_xZn_1−xS QD–silica films with green-, yellow-, and orange-emission. Their PL peak wavelengths were 552, 575 and 590 nm, respectively. Insets show the colour images of green-, yellow- and orange-emitting films under 365 nm UV light.

Table 2 PL properties of CdSe/Cd_xZn_1−xS QD–silica films

Sample	QDs used	FWHM (nm)	PL peak wavelength (nm)
Green film	Sample 2	31.7	552
Yellow film	Sample 3	33.4	575
Orange film	Sample 4	30.6	590

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Fig. 5 TEM image of orange-emitting film prepared using sample 3.

To appreciate the thickness and uniformity of APS-capped CdSe films, Fig. 6 shows the AFM image of thin APS-capped CdSe QD film created using sample 3. The result indicates the film with a smooth and uniform surface morphology. To measure the thickness of the thin films obtained by spin-coating, the film was cut with the blade and the camber was corresponding to fracture surface. The thickness of the thin films was measured about 15 nm.

Fig. 6 AFM image of APS-capped CdSe QDs thin film prepared using sample 1 (CdSe QDs).

As shown in the inset in Fig. 7c, silica films with QDs revealed excellent flexible property. This inorganic film has the advantages of foldability and crack-resistance, has attracted extensive interest because of their broad application in collapsible optoelectronic devices compared with organic ones. This differs from the flexible film reported in literature.³¹ In that case, organic flexible films were fabricated highly luminescent and flexible films via self-assembly of triple building blocks: layered double hydroxide nanoplatelets, polyvinyl alcohol and QDs, which show 2D ordered structure and finely tunable fluorescence (green, yellow, orange and red).¹⁶ In our case, the film was facile prepared and the concentration of the QDs can be adjusted according to the request of applications.

Fig. 7 Comparison of photostability of initial APS-capped QD colloidal solutions and corresponding QD–silica films irradiated under 365 nm UV. (a) CdSe–silica film. (b) Green film. (c) Yellow film. (d) Orange film.

Fig. 7 shows the evolution of PL intensity of APS-capped QD colloidal solutions and corresponding QD–silica films with irradiation time under 365 nm UV light. The insets in Fig. 7c and d shows the optical images of the films under 365 UV and room light, respectively. For APS-coated QDs in APS sol, APS-coated CdSe QD sol and orange sol revealed huge enhanced PL with time. In contrast, green and yellow sol revealed a small increase of PL intensity. Normally, photo degradation of QDs occurs in solutions. This is ascribed to the photo oxide of QDs and the removal of ligands during irradiation.³⁷ In current experiments, photo degradation did not occur. Most possibly this is related to the properties of pre-hydrolyzed APS molecules. In the one hand, the ligand exchange was controlled by adjusting the molar ratio of APS and water. Slow ligand exchange make QDs bright PL in APS sol. On the other, pre-hydrolyzed APS molecules have high stability under UV light, in which, the surface state of QDs in the sol was kept in their initial situation.

To investigate the stability of QD–silica films, Fig. 7 shows the evolution of PL intensity of samples with irradiation time under 365 nm UV light. All samples revealed increased PL with time. The PL intensity of PL spectra is two times more after irradiation for 400 min compared their initial ones. This result indicates the QDs with high stability in the films. This is also ascribed to the pre-hydrolyzed APS as ligands attached on the surface of the QDs. The enhanced PL of the QDs in sol solution solutions and the films may be ascribed to the photo-etching for the surface defects of the QDs. Hydrolyzed APS as ligands in the films play important role to keep high stability because of the stability of silica against physical and chemical environment. To further indicate the high stability of the QDs in the films, Fig. 8 shows the evolution of PL spectra of the film with orange-emitting QDs (QD 3) after keeping it in an air atmosphere for 3 days. After irradiating for 200 min, the PL intensity of the film increased 2 times more. This high stability makes the film critical applications in various fields.

Fig. 8 Evolution of PL spectra of orange-emitting QD–silica film prepared from QD 3 with irradiation time using 365 nm UV light after preparation for 3 days and storing in air atmosphere.

To expand the application of QD–silica films, a solar cell was coated with a CdSe QD–silica film. We obtained an increase in cell efficiency of solar cells deposited with APS-capped CdSe QDs films compared to that APS films, as shown in Fig. 9. The increase in cell efficiency is ascribed to APS-capped CdSe QDs films as a luminescent down-shifting layer, which allows for conversion of high-energy photos to photons with energy that can be efficiently converted to electricity. CdSe QDs with tunable band gaps and high intrinsic charge carrier mobilities, can offer new opportunities for harvesting light energy efficiently in the visible region of the solar spectrum. In addition, these QDs can generate multiple charge carriers with a single photon which has been successfully use for tuning photoresponse and improving the efficiency of solar cells. The APS films deposited at the surface of solar cell may resist a part of light leading to relative low cell efficiency.

Fig. 9 I–V characteristics of solar cells with pure silica film and CdSe QD–silica film.

Conclusions

A ligand exchange technique has been developed to transfer highly luminescent hydrophobic CdSe and CdSe/Cd_xZn_1−xS QDs into water phase using pre-hydrolyzed APS molecules instead of organic capping agents. The concentration of the QDs was easily adjusted by controlling the amount of silica sol. Flexible inorganic SiO₂ films with QDs were fabricated from the functional silica sol by spinning and dipping coating or others. A flexible film was prepared by coating film on a free-standing Teflon substrate. The QDs revealed high PL in the sol and the films. The photodegradation experiments indicated the QDs with high stability in silica sol and in the films. Moreover, the PL intensity of the QDs in the films increased more than 2 times after irradiation by 365 nm UV light. Thus, the QD–silica film was coated on a solar cell. In which, enhanced current was observed. High PL brightness, multicolor emission, flexibility and stability make these films potential applications in the future.

Acknowledgements

This work was supported in part by the program for Taishan Scholars, the projects from National Natural Science Foundation of China (51202090) and Outstanding Young Scientists Foundation Grant of Shandong Province (BS2012CL004 and BS2012CL006).

Notes and references

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