NANO LETTERS
Efficient Phase Transfer of Luminescent Thiol-Capped Nanocrystals: From Water to Nonpolar Organic Solvents
2002 Vol. 2, No. 8 803-806
Nikolai Gaponik,* Dmitri V. Talapin, Andrey L. Rogach, Alexander Eychmu1 ller, and Horst Weller Institute of Physical Chemistry, UniVersity of Hamburg, Bundesstr. 45, 20146 Hamburg, Germany Received June 22, 2002
ABSTRACT Highly luminescent thiol-capped CdTe and HgTe nanocrystals synthesized in aqueous solutions were subject to a partial exchange of capping ligands with 1-dodecanethiol and transferred into different nonpolar organic solvents. It was found that acetone plays an important role in an efficient phase transfer of the nanocrystals. Both CdTe and HgTe nanocrystals retain their luminescence properties after being transferred to organic solvents, thus providing a new source of easily processable luminescent materials for possible applications in photovoltaics and optoelectronics.
Advances in the chemical synthesis of highly luminescent semiconductor nanocrystals (NCs), often referred to as colloidal quantum dots (QDs), currently allow their extensive applications in different fields, ranging from optoelectronics1-6 to biolabeling.7-10 In general, the luminescent NCs can be prepared either in an aqueous medium11 or by an organometallic route,12 while both strategies have their own advantages and drawbacks.13 Usually, NCs grown in an aqueous medium do not possess the degree of crystallinity of the organometallically prepared QDs where high annealing temperatures are used during the synthesis. On the other hand, aqueous synthetic approaches generally are simpler, less expensive, more reproducible, and can easily be scaled up. Among numerous recipes reported in the literature during the past decade, we like to set off the aqueous syntheses of thiol-capped CdTe and HgTe NCs11,13,14 possessing luminescence properties comparable with organometallically synthesized QDs. Thus, the reported values of roomtemperature photoluminescence quantum yields (PL QY) are about 40% and 50% for aqueous CdTe and HgTe NCs, respectively.13,14 Alloying these materials within a nanocrystal results in mixed CdxHg1-xTe particles,15 opening up an opportunity for further band-gap engineering of QDs. The control of the CdTe and HgTe NC sizes and the composition of alloyed CdxHg1-xTe nanoparticles allowed us to tune the luminescence band in the wide spectral region from green to near-IR (Figure 1). * Corresponding author: fax +49-40-42838-3452; tel. +49-40-428387069, e-mail
[email protected]. 10.1021/nl025662w CCC: $22.00 Published on Web 07/12/2002
© 2002 American Chemical Society
Figure 1. Tunability of the PL spectra of CdTe, CdxHg1-xTe, and HgTe NCs prepared in water.
For further use of the NCs, the peculiarities of their postpreparative processing become a key issue. For biological applications water-soluble NCs are required, and a variety of methods, often sufficiently complex, have been developed for making metal16,17 or semiconductor7-9,18,19 QDs originally synthesized in organic solvents water-compatible. For optoelectronic applications, however, a compatibility of NCs with common organic solvents, monomers, and polymers is required, thus preventing the use of QDs synthesized in water. A possibility to overcome this problem would be a simple and sufficiently general method allowing the transfer
Figure 2. Microphotographs of the water/DDT interface before (a) and after (b) addition of acetone. Magnification 280×.
of water-synthesized NCs to organic solvents while preserving the high luminescence quantum yields. In this communication we present such a simple approach allowing almost complete (∼90%) phase transfer of thiolcapped NCs synthesized in water to nonpolar organic solvents. The optical properties of the transferred QDs exhibit only a moderate alternation in comparison to the particles in water. The phase transfer is demonstrated below on CdTe NCs stabilized with thioglycolic acid. The transfer procedure is, however, a general one for a variety of II-VI NCs synthesized in water and capped with different short-chain thiols (CdS, CdSe, HgTe, etc.). The proposed transfer procedure is based on the partial exchange of the stabilizing short-chain thiol molecules by dodecanethiol (DDT) promoted by the use of acetone. Long chain aliphatic thiols were widely used for extraction of metal20 and semiconductor21 NCs into the organic phase after their synthesis in aqueous nanocontainers provided by reverse micelles. However, the direct extraction of thiol-capped semiconductor NCs from aqueous solutions by the use of DDT was found to be impossible. An explanation for this may be the relatively low interface between the aqueous and organic phases as compared to that in reverse micellar solutions. In the absence of a surfactant the surface tension reduces the area of the water/DDT interface, making the NC phase transfer inefficient. As seen from the optical microscopy image (Figure 2a), the DDT phase in this case is separated from water in the form of large bubbles, even after intensive shaking. To facilitate the transfer of the NCs through the phase boundary we introduced an additional component (acetone) to reduce the surface tension at the water/DDT interface. The addition of a common surfactant such as sodium bis(2-ethylhexyl) sulfosuccinate (AOT) provided very stable emulsions, thus hindering the separation of the aqueous and organic phases after the phase transfer. In contrast, acetone was found to be suitable as a substance being soluble in both DDT and aqueous phases, resulting in moderately stable emulsions and effectively promoting the phase transfer of thiol-capped NCs. In addition, acetone did not cause an irreversible flocculation of the NCs and did not quench their luminescence. Figure 2b shows a milky emulsion consisting of fine droplets of DDT in water, which is formed after shaking of the water-acetone-DDT mixture. This emulsion is moderately stable and converts easily into a two-layered structure allowing the decantation of the organic phase. 804
Figure 3. Phase transfer of CdTe NCs from water into DDT demostrated by photographs made under daylight (a,c,e) and under an UV lamp (b,d,f). Pictures (a) and (b): CdTe NCs of two different sizes in water emitting green or red light, DDT and acetone are added on top. Pictures (c) and (d): shaking and heating results in the transfer of CdTe NCs from water into the DDT phase. After precipitation from DDT, CdTe NCs can be redissolved in a nonpolar solvent (e.g., toluene) as shown in pictures (e) and (f), providing stable and optically clear colloidal solutions. Luminescent composites of polylauryl methacrylate and toluene-transferred CdTe NCs under daylight (g) and under a UV lamp (h) are also shown.
In a typical phase transfer procedure, 1 mL of an aqueous solution of CdTe NCs with a concentration of ∼ 10-2 M referring to Te was placed in a vessel, and 1 mL of Nano Lett., Vol. 2, No. 8, 2002
Figure 4. TEM overview image of CdTe NCs transferred into toluene demonstrates a good spatial separation of particles capped by DDT.
1-dodecanethiol was added on top followed by the addition of 2-3 mL of acetone (Figure 3a,b). To initiate the phase transfer, the vessel was vigorously shaken and heated to the boiling point of acetone. The transfer of the NCs to the organic phase occurred on the time scale of minutes and was easily detectable by its color change (Figure 3c). Simultaneously, the volume of the organic phase decreased to ∼1 mL indicating almost quantitative transfer of acetone into the aqueous phase. The efficiency of the phase transfer process was found to be directly dependent on the amount of acetone present in the mixture. Exceeding a critical amount of acetone (known to be a nonsolvent for thiol-capped NCs) led to the flocculation of the NCs both in the aqueous and organic phases. Thus, the exact amount of acetone has to be determined experimentally for each type of NC to be transferred. After completion of the phase transfer, the organic phase containing the luminescent NCs (Figure 3c,d) was isolated by decantation, diluted 1:1 by volume with toluene, and the NCs were precipitated with methanol. The isolated precipitate consists of CdTe NCs partially capped with dodecanethiol which are readily soluble in nonpolar organic solvents such as toluene, hexane, chloroform, etc., yielding an optically clear solution (Figure 3e,f). Transmission electron microscopy (TEM) images of the CdTe NCs transferred into the organic phase show that the quality of the particles in terms of their crystallinity and monodispersity generally remains unaltered as compared to the initial aqueous solution. Remarkably, the NCs transferred into the organic phase appear to be separated on the TEM grids (Figure 4), which is hardly possible for original aqueous samples. Reasons for this might be both the modification of their surface with a long-chain stabilizer (DDT), providing a better spatial separation in comparison with short-chain thiols, and the different affinity of the two solvents (toluene and water) to Nano Lett., Vol. 2, No. 8, 2002
Figure 5. Normalized room-temperature absorption and emission spectra of CdTe NCs before (solid lines) and after (dashed lines) phase transfer from water into toluene. Inset shows relative photoluminescence spectra of CdTe NCs just transferred into organic phase (dashed line) and after 1 h of heating at 100 °C in inert atmosphere (dotted line).
the TEM grids causing the aggregation of thiol-capped NCs when deposited on TEM grids from aqueous solutions. The absorption and emission spectra of CdTe NCs transferred into toluene were very similar to those of the initial aqueous solutions (Figure 5). The room temperature PL QY of the transferred NCs generally decreased by 2 times in comparison to those in the initial aqueous solutions being thus in the range of 12-15% when using aqueous solutions of CdTe NCs emitting with an efficiency of 25-30%, which are routinely available up to date.13 This partial decrease of PL QY is probably due to the formation of surface traps during the exchange of the capping agents. On the other hand, heating of the DDT solution of transferred NCs at 100 °C during 1 h was found to be a useful method to restore the luminescence efficiency up to initial values (inset to Figure 5). Considerable difference in the shape of PL spectra of HgTe nanocrystals before and after phase transfer is obviously due to the transmittance of the solvents used (Figure 6). As it seen from the figure, organic solvents such as toluene allow to measure emission from the HgTe NCs in the wavelength regions where it was “invisible” in water. The colloidal solutions of luminescent NCs in organic solvents can further be used for spreading high quality films on different substrates, fabrication of composites with polymers, etc. As an example, Figure 3g,h shows a luminescent polylauryl methacrylate (PLMA)-CdTe NC com805
NATO Collaborative Linkage Grant CLG 976365 and the DFG SPP “Photonic Crystals”. References
Figure 6. Room temperature emission spectra of HgTe NCs before (solid line) and after (dashed line) phase transfer from water into toluene. Transmission spectra of corresponding solvents are additionally shown in the top panel.
posite prepared according to ref 22 with the use of CdTe NCs transferred from water to toluene. Summarizing, a novel and simple approach allowing an efficient phase transfer of thiol-capped NCs from water to nonpolar organic solvents is presented in this communication. The procedure utilizes the surface stabilizer exchange and is promoted by acetone. NCs modified by dodecanthiol become soluble in different nonpolar organic solvents while preserving the strong luminescence which sufficiently broaden their possible applications in photovoltaics and optoelectronics. Acknowledgment. We thank Andreas Kornowski for the TEM images. Partial financial support was provided by the
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