Supporting Online Material for

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1 Supporting Online Material for Self-Assembly of CdTe Nanocrystals into Free-Floating Sheets Zhiyong Y. Tang, Zhenli L. Zhang, Ying Wang, Sharon C. Glotzer, * Nicholas A. Kotov, * *To whom correspondence should be addressed. kotov@umich.edu (N.A.K.); sglotzer@umich.edu (S.C.G.) This PDF file includes: Materials and Methods Figs. S1 to S12 References Published 13 October 2006, Science 314, 274 (2006) DOI: /science

2 Supplementary Online Materials (SOM) DETAILED MATERIALS AND METHODS Synthesis of DMAET-stabilized NPs. Preparation of 2-(dimethylamino)ethanethiol (DMAET) stabilized CdTe NPs: The synthesis of DMAET-stabilized CdTe NPs followed Rogach-Weller s method (S1, S 2). In brief, H 2 Te gas (generated by the reaction of 0.2 g Al 2 Te 3 lumps with 15 ml of 0.5 M H 2 SO 4 under N 2 atmosphere) was passed to nitrogen-saturated Cd(ClO 4 ) 2 6H 2 O aqueous solution (0.013 M, 125 ml) at ph 5.5 in the presence of DMAET (807 mg) as a stabilizing agent. The CdTe NPs with the desirable sizes were prepared by refluxing the reaction mixture under N 2 gas for various time periods. Caution: Since the intermediate product of H 2 Te was highly toxic, the respirator should be worn during the entire synthesis process to prevent possible inhaling of toxic gas. Cd(ClO 4 ) 2 6H 2 O, DMEAT, NaOH, and H 2 SO 4 were purchased from Aldrich. Al 2 Te 3 powder was purchased from Alfa Aesar. The purity of Cd(ClO 4 ) 2 6H 2 O, DMAET, NaOH, H 2 SO 4 was above 99%. All chemicals were used as received. Because Al 2 Te 3 is easily oxidized in air, it was kept in a glovebox when not in use. Self-assembly process of DMAET-stabilized NPs. Compared to other CdTe NPs stabilized by thiols with functional groups, DMAETstabilized CdTe NPs in water exhibit a large temperature-dependent solubility. CdTe NPs precipitated out from 10 ml solution after one day at 4 C, which was used to separate them from the excess of the stabilizer. After centrifugation at 6000 rpm for 15 minutes, the precipitate was redissolved in 10 ml deionized water (ph 5.5) at room temperature, 1

3 AC BD and then kept in a glove box at 50 C for 1 month. The purpose of using the glove box was to prevent NPs from possible oxidation. After this time, the re-dispersed NP solution became opaque and was taken out for further study. Characterization of DMAET-stabilized NP sheets. The substrates for AFM, TEM and confocal microscopies were quickly immersed in the dispersion of sheets. Excess of the solvent was then removed by filter paper, so that no visible amount of dispersion was left on the substrate. After that the substrates were allowed to dry in air. The perturbation to the solution in the reaction vessel during the assembly and sampling processes was minimized. ADDITIONAL EXPERIMENTAL DATA Confirmation of the free-floating nature of the sheets. To demonstrate that the observed NP assemblies do form the free-floating sheets, we performed confocal microscopy in the vicinity of the interface with the glass slide. The dispersion obtained after 24 hours was deposited as a drop fluid on the hydrophilic glass slide in a form of a very thin layer and then imaged by confocal microscopy (Fig. S1A). The imaging was performed so that the image would be cross-sectioned through the interface with glass (lower part in Fig. S1A) and air (not visible). One can clearly see that the fluid contains fluorescent particles with several microns in diameter dispersed throughout the volume of the fluid. The fact, that the sheets are free-floating and homogeneously distributed in the reaction liquid, can be also inferred from the regular photographs of the DMAET-stabilized CdTe (Fig. S1, B and C). The quality of the 2

4 dispersions can be compared to the solution of Rhodamine B dye, which is similar in color. The uniform dispersion of the adsorbing and luminescent particles in case of forming sheets should be pointed out. The samples in Fig. S1 B and C also display substantially stronger scattering, which is expected from the micron-scale particles. A B C Figure S1. (A) Confocal microscopy image of dispersion obtained from DMAETstabilized CdTe in 24h after the initiation of the self-assembly process. Photographs of the same dispersion (B, C), and solution of Rhodamine B (D, E) under normal illumination (B, D) and UV light (C, E). D E Blank experiments demonstrating that films do not form on the vessel surface. Extensive data using a variety of characterization techniques was collected to show that the films self-organize in the bulk of the solution rather than adsorb on the container s walls and then peel off (see Figs. S5-S12). The blank experiments and observations addressing this point specifically are listed here. We observed no peeling of any materials from the walls, which might explain their transfer to the suspension had they been formed on the solid-liquid interface. Considering the microscopic nature of the film, the presence of a coating on the vessel walls would have been easily observed (see also Fig. S1, B and C). Observation of the glass surface of the vessel under UV illumination, which is a 3

5 reliable method of visualizing the emissive NP deposits, did not reveal any luminescent coating. Since the films are robust enough to withstand considerable mechanical stress during rinsing and drying (Fig. 1), such a coating should be easily identifiable. Lastly, the sheets can be obtained in vessels made from any material, e.g. hydrophilic glass, hydrophobic glass modified with silane, and hydrophobic plastic such as Teflon. Thus, we conclude that the NP sheets form in the bulk solution. Kinetics of the Self-Assembly Reaction and the Intermediate State Kinetics of the self-organization was followed by the experiments of dynamic light scattering using Nanosizer instrument from Malvern. We observed that the hydrodynamic diameter of the scattering agents gradually increasing from a diameter of a few nanometers to a few micrometers (Fig S2, A-E) as expected from the suggested mechanism. The particle distribution is always bimodal or even trimodal (Fig. S2E). The origin of this phenomenon is unclear and has to be investigated further. As a plausible hypothesis, one can speculate that multimodal distribution could be an indication that the smaller sheets can self-assemble into larger ones along with the attachment of the elementary NPs to the elementary sheets. To get a snapshot of the sheet growth, the intermediate state of the self-assembly product was analyzed. The product forming in 24 hours as opposed to the typical 1 month incubation time was imaged by TEM (Fig. S2F). Besides observing many free CdTe NPs, there are many fairly small 2D CdTe NP sheets, which range in size from several tens of nanometers to several hundred nanometers. An average diameter can be estimated to be 4

6 about 200 nm, which is very close to the data obtained by light scattering (Fig. S2D). In comparison, the final self-assembly products are 2D NP sheets with lateral dimensions up to 50 microns (Fig. 1). These observations are consistent with the growth of the sheets from the initial seeds followed by the spontaneous attachment of larger number of particles in the 2D pattern as described by the computer simulation and give the reader some appreciation of the growth kinetics. A F B C D E Figure S2. Kinetics of the selforganization monitored by particle size distribution data obtained from the light scattering experiments. From (A) to (E): 0 min: 15 ± 0.4 nm (peak 1), 84 ± 8 nm (peak 2); 1 hour: 148 ± 13 nm (peak); 12 hours: 61 ± 3 nm (peak 1), 285 ± 10 nm (peak 2); 24 hours: 93 ± 17 nm (peak 1), 498 ± 23 nm (peak 2); 6 days: 221 ±287 nm (peak 1), 1120 ± 40 nm (peak 2), 4830 ± 430 nm (peak 3). (F) TEM of the intermediate state of the self-assembly product after the redissoved DMAETstabilized CdTe NPs solution was kept at 50 C for 24 hours. Preparation of DMAET-stabilized CdSe NPs and self-assembled sheets formed from them. 5

7 DMAET-stabilized CdSe NPs were made using the same synthetic procedure as for CdTe with the exception that, instead of Al 2 Te 3, we took the equivalent amount of Al 2 Se 3. DMAET-stabilized CdSe NPs were precipitated out from 10 ml solution after one day at 4 C, which was used to separate them from the excess stabilizer. After centrifugation at 6000 rpm for 15 minutes, the formed solid was redissolved in 10 ml deionized water (ph 5.5) at room temperature, and then kept in a glove box at 50 C for 2 weeks. TEM investigation demonstrates (Fig. S3) that the sheets still form in large numbers. Figure S3. TEM image of DMAET-stabilized CdSe NP sheets. Preparation of CdTe NPs with longer stabilizing chains and the self-assembled sheets formed from them. The synthesis of 2-(dimethylamino)buthanethiol and 2-(dimethylamino)hexanethiol is described in references (S3, S4). Briefly, 4-dimethylaminobutyl chloride or 6- dimethylaminohexyl chloride was prepared by the addition of thionyl chloride into commercially-available 4-dimethylaminobutanol or 6-dimethylaminohexanol, receptively. Then, 2-(dimethylamino)buthanethiol or 2-(dimethylamino)hexanethiol was prepared from the above chloro-compound with the thiuronium salt as intermediate, according to the method of Clinton et al.(4) The synthesis procedure of (dimethylamino)buthanethiol and 2-(dimethylamino)hexanethiol stabilized CdTe NPs was the same as that of 6

8 DMAET-stabilized CdTe NPs described in the part of Materials and Methods in SOM. Longer molecules such as 2-(dimethylamino)buthanethiol (DMABT) and 2- (dimethylamino)hexanethiol (DMAHT) were used as the stabilizers of CdTe NPs. Interestingly, both DMABT- and DMAHT-CdTe NPs exhibit a remarkable solubility in water: they do not precipitate at 4 C. Thus we precipitated out the DMABT and DMAHT stabilized CdTe NPs by changing the ph value of the media to Subsequently, the obtained solids were redissolved into pure water. As shown in Fig. S4, only 3D disordered structures can be observed for the products of selfassembly in these systems. This proves that shortranged, directional, hydrophobic attraction is crucial for the formation of the 2D floating sheets. Figure S4. TEM image of the self-assembly product of 2- (dimethylamino)buthanethiol stabilized CdTe NPs. Effect of Electrostatic Screening Additional control experiments will be useful in order to verify the proposed requirements for the dipole moment and charge-charge repulsion for 2D self-organization. As a part of this effort, DMAET-stabilized CdTe NPs were redissolved into a solution of 1M NaCl. High ionic strength of the salt solution can be expected to screen the electrostatic repulsion between NPs. TEM and corresponding elemental analysis (Fig. S5) indicate the CdTe NPs self-assembled into 3D spheres rather than 2D floating sheets. 7

9 Note that the Cu signal arises from the TEM grids. Theses observations are consistent with the simulation result that a decrease of the electrostatic repulsion between NPs will lead to the formation of 3D spherical aggregates (Fig. S11). Figure S5. (A) TEM image and (B) corresponding energy dispersion X-ray spectroscopy of the selfassembly product of DMAET-stabilized CdTe NPs in the solution of 1M NaCl. Effect of Surface Charge The charge density on the surface of CdTe NPs also determines the electrostatic forces between the NPs. It can be controlled by the ph value of the solution since the protonation of DMAET stabilizer decreases in media with higher ph. If DMAETstabilized CdTe NPs are redissolved in ph 9.5 solution as opposed to ph 5.5 as described in the text, TEM images (Fig. S6) clearly reveal that the morphology of 2D floating sheet become considerably rougher, with many holes and random aggregates inside the sheets. Moreover, besides 2D sheets, free, odd-shaped aggregates are observed on the TEM grids. The combination of experiments on electrostatic screening and ph variation explicitly 8

10 demonstrates that the electrostatic repulsion is an essential parameter for film selforganization. Figure S6. (A) and (B) TEM images of the self-assembly product of DMAET-stabilized CdTe NPs in ph 9.5 solution. Assembly of Au NPs stabilized by DMAET In order to demonstrate that the dipole moment is one of the key requirements for the described self-assembly process, the self-organization of DMAET-stabilized Au NPs was tested. The dipole moment in Ag or Au nanoparticles will be much smaller (but most likely still non-zero) than that for CdTe due to a shift of electron density. If they assemble into a similar sheet, the structure-directing role of the dipole moment in the case of CdTe NPs may be questioned. DMAET-stabilized Au NPs prepared by standard reduction technique were found to be 4-6 nm in diameter (Fig. S7B). DMAET-stabilized Au NPs were prepared by ligand exchange reactions (S5). In brief, tetraoctylammonium bromide (TOAB) stabilized Au NPs were prepared in toluene following the Brust two-phase 9

11 method (S6). Subsequently, the gold nanoparticles were transferred to water phase, which containing the DMAET stabilizers. The water-soluble Au NPs were precipitated by centrifugation at 15,000 RPM, and then redissolved in the deionized water to induce potential self-organization reactions. The Au nanocolloids were subjected to the same sequence of operations as DMAET-CdTe NPs. TEM results (Fig. S7) suggest that there are no 2D floating sheets forming in solutions or otherwise and most Au NPs retain their individuality in solution. Indeed, a similar result was also found by Hutchison and coworkers for DMAET-stabilized Au NPs with a diameter of 1.4 nm (S5). This experiment demonstrates (1) the importance of the dipole moment for the formation of the sheets, and (2) the absence of sheet formation on the walls of the vessel. Figure S7. TEM (A) and HRTEM (B) of the self-assembly product of DMAET-stabilized Au NPs. Effect of different media on the assembly CdTe NPs were observed to self-assemble into two-dimensional sheets in water as well as in some aqueous mixtures with polar solvent such as methanol, ethanol, isopropanol 10

12 (Fig. S8A). The DMAET-stabilized CdTe NPs are not soluble in pure ethanol and spontaneously aggregate into 3D disordered structures (Fig. S8B). Figure S8. TEM of the self-assembly product of DMAET-stabilized CdTe NPs in the mixture of ethanol and water (A) or pure ethanol (B). Effect of the temperature on self-assembly The self-assembly process of DMAETstabilized CdTe NPs was performed at different temperatures. Similarly to the product at 50 C, DMAET-stabilized CdTe NPs self-assembled into 2D sheets at 20 C (Fig. S9). Actually, DMAET-stabilized CdTe NPs were observed to self-assemble into 2D sheet over a range of temperatures from 10 C to 80 C. Figure S9. TEM of self-assembly product of DMAET-stabilized CdTe NPs at 20 C. The arrow represents the wrinkled part of the sheet upon drying on the TEM grid. 11

13 ADDITIONAL SIMULATION RESULTS Simulations of two other systems. Except the simulation of N = 60 at φ = 0.13 as we showed in Fig. 3 we also performed simulations of two other systems, N = 20 at φ = 0.13 and N = 30 at φ = All other parameters are the same as those for N = 60 at φ = 0.13 in the paper. The change of order parameter as a function of attraction strength is shown in Fig. S10. A B Figure S10 The order parameter S as a function of hydrophobic attraction strength ξ. (A) N = 20 and φ = The critical hydrophobic attraction is about 56 ~ 58.5kJ/mol. (B) N = 30 and φ = The critical hydrophobic attraction is around 58~61kJ/mol. Both figures are obtained by averaging over 20 independent runs. Simulations of NPs without net charge. To test the effect of net charge we performed simulations of self-assembly of NPs without net charge. N = 33, φ = independent runs are performed. All other parameters are the same as those in simulations of NPs with three truncated corners and with charge q = +3e in the main text. The results are shown in Fig. S11. 12

14 A B Figure S11 (A) The three-dimensional aggregates spontaneously formed from NPs with three truncated corners and without charge. (B) The distribution of NPs with specific relative orientations in aggregates in A. 62.3% for Bottom-Side orientation, 37.7% for Side-Side orientation, 0% for Bottom-Bottom orientation. This figure indicates the Bottom-Side orientation is the majority, which agrees with the energy analysis in Table 1. Simulations of NPs with two corners truncated. We also performed simulations of self-assembly of NPs with two corners truncated. N = 33, φ = independent runs are performed. All other parameters are the same as those in simulations of NPs with three truncated corners and with charge q = +3e in the main text. The results in Fig. S12 demonstrate that, instead of sheets, they form particle chains. This confirms the NPs form chain-like structures. The calculation of the percentage of nanoparticles with neighbours substantiates the chain formation (Fig. S12C). 13

15 A B C C Figure S12. (A) Semi-empirical quantum calculation of a single CdTe NP by Spartan V4. Colors: Cd green, Te orange, S orange, O red and H white. The faces are coated by SH and the corners are coated by H 2 O molecules. The edge between truncated corners is called Short edge and the edge between untruncated corners is called Long edge. The calculated dipole moment μ = Debye. The dipolar direction is from Long edge to Short edge as illustrated by the blue arrow. If the corners are coated by SH, μ = Debye and the dipolar direction does not change. (B) Example of a chain-like structure formed via self-assembly of NPs with two truncated corners taken from simulation cell containing multiple chains. (C) The distribution of NPs with N neighbors. 5% for N = 0, 7.5% for N = 1, 90.5% for N = 2, 1.5% for N = 3, 0% for N = 4. 14

16 Stability of the self-assembled CdTe sheets. To confirm the stability of the sheets we doubled the system size (N = 160, φ = 0.13) and tested whether they spontaneously decompose due to particle repulsion. We do not see the break up of films during 60 million MC steps and the order parameter is 0.85 with a standard derivation 0.06, which is within the error bar in Fig. 3F and S10. These results support that the film is indeed a stable state. Reference List for Supplementary Information S1. A. L. Rogach et al., Ber. Bunsen-Ges. Phys. Chem. 100, 1772 (1996). S2. N. Gaponik et al., J. Phys. Chem. B 106, 7177 (2002). S3. J. N. Ashley, S. S. Berg, J. Chem. Soc (1959). S4. R. O. Clinton, U. J. Salvador, S. C. Laskowski, C. M. Suter, J. Amer. Chem. Soc. 70, 950 (1948). S5. M. G. Warner, S. M. Reed, J. E. Hutchison, Chem. Mater. 12, 3316 (2000). S6. M. Brust, M. Walker, D. Bethell, D. J. Schiffrin, R. Whyman, J. Chem. Soc., Chem. Commun. 801 (1994). 15