Letter pubs.acs.org/JPCL
Quantum-Dot-Based (Aero)gels: Control of the Optical Properties André Wolf, Vladimir Lesnyak,* Nikolai Gaponik, and Alexander Eychmüller Physical Chemistry, TU Dresden, Bergstr. 66b, 01062 Dresden, Germany S Supporting Information *
ABSTRACT: In this work, we have developed novel hybrid quantum dot gels based on the controllable and reversible assembly of nanoparticles via metal−tetrazole complexation. Combining in one hybrid network nanocrystals of different semiconductors (ZnSe and CdTe) as well as quantum dots of different sizes (green and red emitting CdTe) with different band gaps, we have examined energy relations within these systems and act out a facile route to the color design of the resulting gels. Efficient energy pumping from donor quantum dots to acceptors leads to a remarkable enhancement of the emission intensity of the gel. Furthermore, by integrating three different quantum dot types into one network, we obtained a white-light-emitting aerogel.
SECTION: Plasmonics, Optical Materials, and Hard Matter
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for the enhancement of emission efficiencies are of great importance for further applications of the QD-based assemblies. In this work, we introduce novel hybrid QD gels obtained by using the method of controllable gelation of nanoparticles via their metal−tetrazole complexation reported in recent publications.15,19 The combination of two different QDs having diverse band gaps (in our case, two differently sized CdTe QD colloids and the ZnSe/CdTe pair) and therefore acting as a donor and an acceptor in a hybrid 3D structure leads to an enhancement of the photoluminescence (PL) intensity of the acceptor as a consequence of efficient energy transfer from the donor. Furthermore, assembling of three different semiconductors such as blue-emitting ZnSe and yellow- and redemitting CdTe particles, we demonstrate a facile method for the fine-tuning of the emission color of a hybrid system, resulting in purely white PL. The tetrazole-capped QDs have been obtained directly from an aqueous synthesis without the need of any ligand exchange procedures. On the basis of the recently developed methodology of the networking of colloidal nanocrystals stabilized by 5-mercaptomethyltetrazole (5-HSCH2Tz),15,19 we extended the range of materials from semiconductor and hybrid semiconductor− metal systems to the assembly of semiconductor particles of the same composition but different sizes (green- and red-emitting CdTe QDs (CdTe-g and CdTe-r, respectively)) and two different semiconductors (ZnSe and CdTe). To obtain blueemitting ZnSe@5-HSCH2Tz QDs, we adopted a procedure of an aqueous synthesis of thioglycolic acid-capped ZnSe nanocrystals. (A scheme of a setup for the synthesis is
he assembly of colloidal semiconductor nanocrystals (or quantum dots (QDs)) provides new opportunities of transferring functions of nanomaterials to the macroscopic scale.1−3 A wide variety of assembly approaches has been developed during the past decades, resulting in the formation of 1D, 2D, and 3D structures with well-defined properties.4−7 Among them the 3D assembly offers the greatest versatility in constructing nanoparticle based composites ranging from small clusters through gel-like structures to macro-scaled solids. The assembly of nanoparticles in gel structures leads to highly porous, lightweight 3D objects, which in many cases preserve the properties of the nanocrystals they consist of,8,9 which makes them promising materials for applications in nanophotonics, electronics, photovoltaics, and catalysis.3,9 QD-based gels are usually obtained in solution by aging, by chemical (via pH tuning10 or oxidation8,11−14) and photochemical treatments,8,11,13,14 and by complexation15 of nanocrystals. Being processed via supercritical drying, hydrogels transform into monolithic aerogels, exhibiting densities hundreds of times less than those of the corresponding bulk materials. Interconnection of semiconductor QDs within a network facilitates an efficient energy and charge transfer from the particles with larger band gaps to those having a narrower band gap in the case of type I aligned systems when the narrower band gap lies in the frame of the wider one, whereas in the case of type II alignment (band gap of one material only partially fits the other), charge separation takes place. At the same time, assemblies of closely packed QDs suffer from a drain of excitation energy through nonradiative deactivation within an ensemble, as in the case of cross-linked16 and aggregated17 CdTe or CdSe/ZnS18 particles. This energy loss results in a strong quenching of their luminescence. Therefore, strategies © 2012 American Chemical Society
Received: June 4, 2012 Accepted: July 27, 2012 Published: July 27, 2012 2188
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presented in Figure SI1 in the Supporting Information.)20,21 Under light irradiation, 5-HSCH2Tz releases sulfur ions, similarly to thioglycolic acid, which react with zinc ions present in solution forming a ZnS enriched shell on the surface of the ZnSe nanocrystals. This treatment leads to a strong enhancement of the emission intensity of the QDs. The PL and absorption spectra of the colloids used for the gel preparations are shown in Figure 1. As follows from their optical properties,
does not involve any destabilization reactions. The equivalent capping of the particles ensures in any proportion the formation of a complete network of all QDs present in the mixture. By this, the level of aggregation depends only on the amount of Zn(OAc)2 solution added. When this reaches a certain limit, large QD aggregates settle down, forming a perfect network. A series of three different gels was prepared: CdTe-g/CdTe-r, ZnSe/CdTe-r, and white-light-emitting ZnSe/CdTe-y/CdTe-r. Transmission electron microscopy (TEM) characterization of the samples displays their highly porous morphologies (see Figure 3), similar to those previously observed in the pure
Figure 1. Absorption (black) and PL (red, λex = 340 nm for ZnSe and 450 nm for CdTe) spectra of the initial ZnSe, CdTe-g, CdTe-y (yellow emitting), and CdTe-r QD colloids.
the ZnSe QDs possess both a wider band gap and a type-I alignment of their bands relative to all types of CdTe QDs used here and consequently can act as energy donors. In the pair CdTe-g/CdTe-r, the first is a potential donor and the second is an acceptor. Furthermore, overlapping of the emission spectra of the donors and the absorption of the acceptors enables Förster resonance energy transfer (FRET). In addition, the interdot surface-to-surface distance of ∼1.6 nm calculated for the bridge consisting of two zinc ions complexed with two tetrazole molecules (see Figure 2) increases the probability of
Figure 3. TEM images of CdTe-g/CdTe-r (left) and ZnSe/CdTe-r (right) aerogel fragments. Below are high-resolution images of the corresponding samples.
CdTe and hybrid CdTe/Au aerogels.15 We note, due to similar sizes of the particles and their similar contrast in the TEM, that it is rather difficult to distinguish between different QDs. Nevertheless, we expect a random distribution of the nanocrystals within the network, as was confirmed for the case of the hybrid CdTe/Au structures where gold nanocrystals were well-distinguishable from the semiconductor particles owing to their higher contrast in the TEM images.15 The gelation process of the individual QD colloids as well as of their mixtures has been monitored by means of absorption, PL, and time-resolved PL spectroscopy measurements during the stepwise addition of Zn(OAc)2 solution. Spectra acquired from the individual colloids are presented in Figure SI2. (See the Supporting Information.) All samples show similar changes in their optical spectra in the course of gelation: strong emission quenching accompanied by its lifetime decrease, a shift of the PL maxima to lower energies, and the appearance of a scattering part in the absorption spectra due to the formation and growth of QD-aggregates. At the same time, the full width at half-maximum of the PL spectra was unaltered after gelation (not shown). The addition of the first portions of the zinc acetate solution results in a remarkable enhancement of the PL intensity in the case of the ZnSe as well as the CdTe QD colloids, which most probably is due to healing of surface defects responsible for surface trapping of charge carriers. As seen from Figure SI2 and Table SI1 of the Supporting Information, upon further addition of zinc salt solution, the strongest quenching is observed for the CdTe-g QDs, which lost ca. 90% of their initial emission intensity. The
Figure 2. Scheme of the gelation of two different types of QDs by the addition of zinc ions.
energy transfer between adjacent QDs. Taking into account the mean sizes of the QDs estimated from TEM images for ZnSe and obtained using the sizing curve22 for CdTe nanocrystals (3 nm for ZnSe and CdTe-r, 2 nm for CdTe-g), the center-tocenter distances in the donor−acceptor pairs range from 3.7 to 4.2 nm. The interconnection of the QDs in the network occurs via complexation of the tetrazole ligands on the surface of the particles by Zn2+ ions, as is schematically presented in Figure 2. The gelation process is rapid and highly reproducible, and it 2189
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Figure 4. Stepwise gelation of CdTe-g/CdTe-r and ZnSe/CdTe-r QD mixtures monitored by PL, absorption, and time-resolved PL spectroscopy. (Light gray traces in all of the time-resolved spectra series depict a prompt signal as an instrument response function.)
the mixed gels than in the individual (pure) gel (cf. Figures 4 and Figure SI2 of the Supporting Information, the main changes in the emission properties of the colloids are summarized in Tables SI1 and SI2 of the Supporting Information). Opposed to this, the acceptor in the gel state preserves ca. 80% of its initial value, whereas its emission lifetime is dramatically shortened in the pure gel. These observations suggest efficient energy pumping from the donor QDs to the acceptors. Varying the content of donor and acceptor particles results in an effective color tuning, which enables practically an entire suppression of the donor emission with an enhancement of that of the acceptor keeping its intense pure color. (See Figure 5.) In the case of the mixed colloids, the red shift of the PL maxima is smaller than that of the individual particles (cf. the corresponding data in Tables SI1 and SI2 of the Supporting Information), which further facilitates a color design of the resulting gels. Although we observed in the pair ZnSe/CdTe-r quite similar tendencies (see Figure 4), we also monitored minor alterations.
luminescence lifetime decrease correlates well with its intensity drop for all samples: the larger the decrease in the emission intensity, the shorter the corresponding emission decay becomes. In this scenario, owing to the formation of a ZnS shell by the photochemical treatment, the ZnSe QDs still retain half of their initial PL lifetime in the gel state. Figure SI2 of the Supporting Information reveals that an increase in the aggregation level leads to a shortening of the PL lifetime in all pure gelating colloids. Likely, this shortening is the result of various processes leading to emission quenching like energy transfer, charge transfer, and enhanced radiationless transitions due to structural impact of the particle surfaces. The combination of two semiconductors with different band gaps in one network fosters energy transfer within the system. To demonstrate this process reflected in the optical properties of the hybrid aerogels, we used two different pairs of QDs: CdTe-g/CdTe-r whose emissions are relatively close to each other and ZnSe/CdTe-r with more distant PL maxima. The optical spectra acquired during their gelation are presented in Figure 4. In both systems, the absorption represents the superimposed spectra of the initial individual QD colloids with almost unaltered positions of their absorption maxima, revealing the retention of the quantum confinement of the initial nanocrystals during gelation. Therefore, for the excitation of both components, wavelengths significantly shorter than the positions of the first absorption maxima of the donors have been used. As is seen from Figure 4 and Table SI2 (see the Supporting Information), in the case of the CdTe-g/CdTe-r gel, the emission intensity of the donor (CdTe-g) drops down to 6% of its initial value. Although the PL of the acceptor (CdTe-r) is also quenched, its decrease is lower than that in the case of individual QD colloid: 52% of the initial intensity is retained in the mixture versus 18% in the individual gel. The overall decrease in the acceptor emission can be attributed to radiationless transitions, which are favored in the gelated sample due to the large number of contacts between the nanocrystals. Further confirmation of an energy transfer provides timeresolved PL spectroscopy. From the comparison of the lifetimes, it follows that the donor emission decays faster in
Figure 5. (a) PL spectra of mixed CdTe-g/CdTe-r QD colloids (at QD molar ratios of 1.7/1, 5.2/1, and 14/1) before and after their gelation. (b) Photographs of CdTe-g, CdTe-g/CdTe-r mixed (14/1, 5.2/1, and 1.7/1), and CdTe-r QD colloids before and after their gelation under UV light (365 nm) highlighting the color-tuning opportunities. 2190
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During the first gelation steps, the lifetime of the acceptor slightly increased owing to an efficient energy pumping from the donor (cf. the black (initial) and red (intermediate gelation step) curves in the second panel lower right). Its subsequent remarkable decrease differs from that of the CdTe-r QDs in the previous pair. An apparent fast component of the decay, which gains weight in the course of the gelation, is caused by reflection and scattering of the second-order wave of the excitation beam (generated at 590 nm) by the growing QD aggregates. As is clearly seen from Figure 4, its rate is comparable to the prompt signal (light gray trace). To prove this assumption, we monitored the gelation of the ZnSe/CdTeg QDs pair also at the acceptor emission being distant enough from the position of the second-order excitation line. The acceptor decay traces shown in Figure SI3 of the Supporting Information exhibit a behavior quite similar to that of CdTe-r acceptor QDs in the CdTe-g/CdTe-r pair with no fast component. The donor ZnSe QDs in the mixture are characterized by an approximately two times smaller lifetime than in the individual colloid. This observation suggests the occurrence of some energy transfer already in the initial mixed colloid. It is further proven by the increase in the CdTe-r lifetime in the mixture as compared with that in the individual colloid. We suppose that in this case, the formation of small ZnSe/CdTe-r QD clusters takes place immediately upon their mixing owing to the interaction of the CdTe@5-HSCH2Tz with the ZnSe@5-HSCH2Tz bearing on the surface some amount of incompletely complexed Zn2+ ions absorbed during the photochemical treatment. Interestingly, by excitation at 403 nm (see the third panel lower right in Figure 4), where the ZnSe QDs do not absorb, there is still a sufficient enhancement of the CdTe-r lifetime in the gel owing to an effective separation of CdTe nanocrystals by ZnSe, which was added in a large excess to reach a comparable PL intensity. In this case, the CdTe-r dots behave as individual entities, similar to the initial colloid. The combination of different QDs into one network provides an opportunity to tune the color of the resulting system in a wide range. As an example of such kind of color design, we depict the preparation of a white-emitting gel integrating ZnSe, CdTe-y, and CdTe-r QDs in one hybrid system. (See Figure 6.) In this scheme, ZnSe dots can act as energy donors for both CdTe-y and CdTe-r particles and CdTe-y plays the dual role of
a donor and an acceptor mediating the energy transfer from ZnSe to CdTe-r depending on the excitation energy. These three different emitters (see Figure 1 for their PL spectra) form a color space in the CIE 1931 standard diagram, as shown in Figure 6a. Any color within this space is achievable by varying the content of each emitter. For comparison, the color space of the Society of Motion Picture and Television Engineers (SMPTE) with its white day light spot D65 is also displayed.23 We note that the color coordinates of the white-emitting gel are stable and do not change after supercritical drying. Employing other QDs makes feasible a sufficient extension of the achieved color space, for example, by combination of UV blue- (PL max at ∼380 nm), green- (520 nm), and red (680 nm)-emitting materials. By this, effects of energy transfer should be taken into account to compensate for the quenching of the donor emission. We note that energy-transfer phenomena have already been successfully applied to the fabrication of color conversion QD-based layered light-emitting devices.24 In conclusion, we demonstrated the successful extension of the recently developed method of networking of tetrazolecapped nanocrystals via the complexation by metal ions to prepare hybrid gels containing semiconductor particles with different band gaps: ZnSe and CdTe as well as two differently sized sorts of CdTe quantum dots. Energy relations between donors and acceptors within these systems have been investigated by means of absorption, steady-state, and timeresolved PL spectroscopy. The addition of energy donors compensates the emission quenching of the 3D assembly and results in a fine control of its optical properties. The approach developed allows for a facile preparation of a white-lightemitting aerogel via the combination of UV-blue-emitting ZnSe and yellow- and red-emitting CdTe QDs. This experiment enlightens the great potential of the method for the fabrication of light-emitting devices possessing a wide variety of colors. Aerogels can be infiltrated with polymers using a technique recently reported to improve mechanical properties.8 Furthermore, they can be hybridized with hole-conducting conjugated polymers to ameliorate the electrical-transport properties of the material. Another possible application of emitting QD-based aerogels is photocatalysis.
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EXPERIMENTAL SECTION Synthesis of CdTe@5-HSCH2Tz QDs. The synthesis of the stabilizer 5-HSCH2Tz and the aqueous synthesis of 5HSCH2Tz-capped CdTe QD colloids have been performed according to ref 19. Synthesis of ZnSe@5-HSCH2Tz QDs. The preparation of the tetrazole-capped ZnSe QDs and their subsequent photochemical treatment has been accomplished similarly to the recently reported synthesis of thioglycolic acid-capped ZnSe particles.20,21 In brief, 1.52 g of Zn(ClO4)2 × 6H2O and 1.18 g of 5-HSCH2Tz were dissolved in 250 mL of water, followed by a pH adjustment to 12 by the addition of 1 M NaOH solution. Under stirring, H2Se gas (generated by the reaction of 0.25 g of Al2Se3 lumps with 0.5 M H2SO4 solution) was bubbled into the solution together with a slow argon flow. The molar ratio of Zn2+/Se2−/5-HSCH2Tz was 1/0.625/2.5. The further nucleation and growth of the nanocrystals proceeded under reflux at 100 °C under open air conditions for ∼20 h. The setup for the synthesis is shown in Figure SI1 (in the Supporting Information). Because as-synthesized ZnSe nanocrystals show only a weak emission, their photochemical treatment is necessary to grow a
Figure 6. (a) CIE (Commission internationale de l′éclairage) diagram (1931) with SMTPE color space and ZnSe (x = 0.17, y = 0.07)/CdTey(x = 0.41, y = 0.58)/CdTe-r(x = 0.6, y = 0.4) color space, D65 daylight spot (x = 0.3127, y = 0.329), and white-emitting ZnSe/CdTey/CdTe-r gel (x = 0.31, y = 0.33). (b) PL spectrum of the white emitting hybrid gel. The inset is a photograph of the white aerogel under UV light (365 nm). 2191
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ZnS enriched shell on their surface. The treatment of ZnSe@5HSCH2Tz QD colloid was carried out with a 1000 W xenon lamp. In a typical procedure, a portion of 80 mL of the as prepared colloid was mixed with 25 mL of an aqueous solution containing 0.25 g of Zn(ClO4)2 and 0.39 g of 5-HSCH2Tz, followed by the pH adjustment to 11.5 by the addition of 0.1 M NaOH solution. The mixture obtained was irradiated while stirring and bubbling with argon for ∼20 h until a maximal PL intensity had been reached. For further gelation, the treated ZnSe QD colloid was concentrated on a rotary evaporator, then precipitated by adding 2-propanol (nonsolvent), separated by centrifugation, and subsequently dissolved in pure water. Preparation of Hybrid CdTe-g/CdTe-r, ZnSe/CdTe-r, and ZnSe/CdTe-y/CdTe-r Hydro- and Aerogels. For the gel preparations, green-, yellow-, and red-emitting CdTe fractions (CdTe-g, CdTe-y, and CdTe-r, respectively) and blue-emitting ZnSe QD colloid were used. Gelation of both the individual and the mixed colloids was achieved by the stepwise addition of aqueous Zn(OAc)2 solution (10−2 M) until a flocculation of the nanocrystals became visible. The gelation process has been monitored by means of absorption and PL steady-state and time-resolved spectroscopy directly in a 1 cm quartz cuvette using diluted QD colloids. After reaching the flocculation point, the samples were centrifuged at 800 rpm for 2 h to accomplish the formation and strengthening of the network. The thuscompacted hydrogels have been washed three times by discarding the liquid layer above the gel and adding a portion of pure water with subsequent centrifugation for 20 min. Prior to supercritical drying, the purified hydrogels underwent solvent exchange with acetone according to the procedure described in ref 8. Further water withdrawing has been implemented in a desiccator in an acetone-saturated atmosphere, above CaCl2 for 1−3 days. The thus-obtained gels were subjected to supercritical drying using an autoclave 13200J-AB (Spi Supplies) with supercritical CO2.8 Characterization. UV−vis absorption measurements were performed on a Cary 50 spectrophotometer (Varian). Fluorescence spectra were measured on a FluoroMax-4 spectrofluorimeter (HORIBA Jobin Yvon). By using 200 ps pulsed laser diodes emitting at 295 and 403 nm, time-resolved PL traces were recorded on a Fluorlog-3 spectrofluorimeter (HORIBA Jobin Yvon). All measurements were performed at room temperature. For TEM, a drop of diluted aerogel dispersions, obtained by their quick sonication in methanol, was placed onto copper grids coated with a thin Formvar-carbon film with subsequent evaporation of the solvent. TEM imaging was carried out on a Tecnai T20 microscope operating at 200 kV (FEI). ArgusLab software (Mark Thompson & Planaria Software) has been used for the calculation of the −SCH2Tz−Zn−Tz CH2S− bond length. GoCIE 1931 CIE coordinates plotting utility (JTSoft) has been employed for the estimation of the CIE coordinates using PL spectra.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge Christine Mickel (IFW Dresden e.V.) for assistance in performing the TEM imaging and Sergei V. Voitekhovich (Research Institute for Physical Chemical Problems, Belarusian State University, Minsk, Belarus) for assistance in the synthesis of 5-mercaptomethyltetrazole. This work was supported by the EU FP7 project INNOVASOL and the NoE Nanophotonics4Energy and by the DFG through the project EY16/10-2.
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ASSOCIATED CONTENT
S Supporting Information *
Scheme of the synthetic setup and results of monitoring of stepwise gelation of ZnSe, CdTe-g, and CdTe-r QDs as well as their mixtures by PL, absorption, and time-resolved PL spectroscopy. This material is available free of charge via the Internet at http://pubs.acs.org. 2192
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