Close-Packed Superlattices of Side-by-Side Assembled Au-CdSe

Jul 28, 2009 - The side-by-side assembly of the NRs was triggered by changing the solvent quality for the ligands coating the long side of the nanorod...
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NANO LETTERS

Close-Packed Superlattices of Side-by-Side Assembled Au-CdSe Nanorods

2009 Vol. 9, No. 8 3077-3081

Nana Zhao,†,⊥ Kun Liu,†,⊥ Jesse Greener,† Zhihong Nie,†,‡ and Eugenia Kumacheva*,†,§,| Department of Chemistry, UniVersity of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada, Department of Chemistry and Chemical Biology, HarVard UniVersity, 12 Oxford Street, Cambridge, Massachusetts 02138, Institute of Biomaterials and Biomedical Engineering, UniVersity of Toronto, 4 Taddle Creek Road, Toronto, Ontario M5S 3G9, Canada, and Department of Chemical Engineering and Applied Chemistry, UniVersity of Toronto, 200 College Street, Toronto, Ontario M5S 3E5, Canada Received May 17, 2009; Revised Manuscript Received June 13, 2009

ABSTRACT We report solution-based side-by-side self-assembly of Au-tipped CdSe nanorods (NRs) in large two-dimensional superlattices and the deposition of these lattices on a substrate with NRs aligned perpendicular to the surface. The side-by-side assembly of the NRs was triggered by changing the solvent quality for the ligands coating the long side of the nanorods. The stability of the self-assembled superlattices was enhanced due to the hydrogen bonding between the ligands attached to the Au tips of the nanorods. The reported approach can further facilitate the hierarchical integration of multicomponent NRs into functional devices.

Close-packed arrays of semiconductor nanorods (NRs) aligned with their long axes perpendicular to the substrate have potential applications in photovoltaic, field emission, and data storage devices.1-5 For example, it is predicted that the efficiency of charge separation in a photovoltaic device can be enhanced, compared to randomly orientated NR assemblies, when the NRs are both close-packed and aligned perpendicular to the electrode surface.6 The former feature maximizes the density of NRs per unit area and the latter characteristic provides direct paths for electron and hole transfer to the electrodes.1,5 Currently, fabrication of twodimensional (2D) superlattices of side-by-side aligned semiconductor NRs is realized by evaporating a solvent from the concentrated solution of NRs, which is placed either under electric field, or on a template surface.5-11 For instance, a combination of electric field-induced NR alignment and solvent evaporation7,8 or phase separation in NR-polymer solution6 was used to produce close-packed arrays of * To whom correspondence should be addressed. E-mail: ekumache@ chem.utoronto.ca. † Department of Chemistry, University of Toronto. ‡ Harvard University. § Institute of Biomaterials and Biomedical Engineering, University of Toronto. | Department of Chemical Engineering and Applied Chemistry, University of Toronto. ⊥ These authors contributed equally to this work. 10.1021/nl901567a CCC: $40.75 Published on Web 07/28/2009

 2009 American Chemical Society

vertically oriented semiconductor NRs. Alternatively, in the absence of an external field, self-assembly of perpendicularly aligned NRs was achieved with the assistance of highly oriented pyrolytic graphite substrates and controlled solvent evaporation.5 Evaporation of water from NR-coated droplets, along with horizontally aligned 2D NR structures, yielded a hexagonal array of NRs oriented perpendicular to the substrate.9 Finally, the alignment of CdSe NRs in nematic or smectic liquid crystals has been achieved by gentle evaporation of the solvent from the NR solution.11 Different from the drying methods, the addition of a nonsolvent to the NR solution or controlling solvophobic interactions between NRs paves the way for the formation of 3D lattices of NRs in solutions.12,13 The assembly of superlattices of multicomponent NRs is useful in rendering multiple functions to NR arrays. Synergetic properties may originate from the characteristic properties of individual constituents of NRs or from the coupling of the properties of these components.14,15 For instance, for CdSe NRs with Au tips, the strong mixing of the semiconductor and metal electronic states led to the modified density of states, thereby exhibiting broadened energy levels and a reduced band gap.16,17 The charge-separated state in CdSeAu NRs was also utilized for the photoreduction of an acceptor molecule, methylene blue, which offered a promis-

Figure 1. Schematic illustration of the self-assembly of Au-tipped CdSe NRs, which is mediated by the reduced solvent quality for the side ligands.

ing route for converting solar energy into chemical energy.18 Currently, the only method used for producing arrays of vertically aligned arrays of Au-tipped semiconductor NRs involves an instantaneous reduction of spin-cast AuCl3 solution on superlattices of CdS NRs, which were aligned by using an electric field or pyrolytic graphite substrates.19 Solution-based side-by-side assembly of multicomponent NRs in 2D sheets, followed by their subsequent planar deposition on a substrate offers a straightforward approach to the formation of close-packed vertically aligned NR arrays. Here, we report solution-based side-by-side self-assembly of Au-tipped CdSe NRs in 2D superlattices with micrometersize lateral dimensions. The assembly of the NRs was triggered by changing the quality of the solvent for the ligands coating the long side of CdSe NRs. The formation of stable NR superlattices were enhanced by conjugating the 11-mercaptoundecanoic acid to the Au-tips, owing to the hydrogen bonding between the carboxylic groups of this ligand. The 2D NR superlattices deposited onto various substrates with the long axis of the NRs aligned perpendicular to the substrate. CdSe NRs stabilized with octadecylphosphonic acid (ODPA) and hexylphosphonic acid (HPA) were synthesized using the method described elsewhere.20-22 The selective growth of Au onto the tips of CdSe NRs was achieved in the procedure reported elsewhere16 (AuCl3 was replaced with HAuCl4). The reduction of HAuCl4 on the tips of the CdSe NRs was carried out by using dodecylamine (DDA) in the presence of didodecyldimethylammonium bromide (DDAB). After the synthesis, Au-tipped CdSe NRs (later in the text referred to as “nanorods” or NRs) were precipitated by adding isopropanol to the reaction solution, separated by centrifugation, and redispersed in toluene. The approach to NR self-assembly is schematically illustrated in Figure 1. Following NR synthesis, the CdSe segment was coated with a mixture of ODPA and HPA (side ligands, SLs)20 and the Au tips were stabilized with DDAB and DDA (tip ligands, TLs) (Figure 1a).16 To achieve the self-assembly of the NRs in a side-by-side fashion, we exploited anisotropic attraction forces acting between the NRs in a selective solvent. Since both SLs and TLs carry alkyl chains, the as-synthesized NRs had a good solubility in nonpolar solvents but a poor solubility in polar media. To render distinct solubilities to the SLs and TLs, the TLs were replaced with 11-mercaptoundecanoic acid (MUA) (Figure 1b).18,23 Possible replacement of SLs with MUA24 3078

Figure 2. (a) Dark-field TEM image of Au-tipped CdSe NRs in toluene, taken 1 h after the NRs preparation. (b) Dark field TEM image (top view) of the close-packed NR array, inset shows the magnified image and the scale bar represents 50 nm. (c,d) Side view SEM images of the NR arrays. TEM and SEM images in (b-d) were taken 1 day after the addition of dimethylformamide solution of MUA.

was suppressed by introducing MUA in the system in a low concentration. The ligand exchange and the self-assembly of the NRs were mediated by adding a solution of MUA in a polar solvent dimethylformamide to the concentrated solution of NRs in toluene, so that in the mixed solvent, the concentration of toluene was approximately 1 vol %. The NRs underwent side-by-side assembly in order to minimize interactions between the nonpolar SLs and the polar solvent (Figure 1c). In addition, the replacement of TLs with MUA allowed us to exploit the ability of carboxylic groups of MUA molecules attached to neighboring NRs to form hydrogen bonds and thereby, to stabilize self-assembled structures.25-28 Figure 2a shows a typical dark-field transmission electron microscopy (TEM) image of the individual NRs in toluene. The CdSe segment in the NRs had an average length and width of 21 and 4.5 nm, respectively, and the average diameter of the Au tip was approximately 3 nm. The standard deviation in the lengths of nanorods was 1.9 nm. For the time intervals shorter than ca. 20 h, the solution of NRs in toluene remained stable; however, at longer time periods the NRs underwent weak aggregation via gold tips (Figure S1, Supporting Information). After the addition of MUA and dimethylformamide to the solution of NRs in toluene and the incubation of the system for 1 day, a sediment formed at the bottom of solution. Electron microscopy images Nano Lett., Vol. 9, No. 8, 2009

Figure 3. Dark-field TEM images of the superlattices of NRs, which were taken 5 min (a) and 19 h (b) after the beginning of self-assembly.

of the sediment revealed 2D superlattices of close-packed side-by-side aligned NRs (Figure 2b-d). On the TEM grids the NRs were aligned with their long axis perpendicular to the substrate. The lateral dimensions of NR arrays were up to several micrometers. Inspection of the superlattices under high magnification revealed that the NRs formed a hexagonal close-packed structure (inset to Figure 2b). The average internanorod distance was ca. 2 nm, which correlated with the thickness of the double layer of the SLs. The standard deviation in the length of nanorods assembled in sheets was approximately 1 nm, that is, smaller than for as-synthesized NRs (thus the side-by-side assembly of NRs “in registry” fractionated them by their length). Generally, on the substrate the superlattices formed a monolayer, as shown in the side-view scanning electron microscopy (SEM) image (Figure 2c). Occasionally, doublelayer NR structures were observed (Figure 2d), which formed either in the solution, or during consecutive deposition of NR sheets on the substrate. We verified that the NR superlattices formed in solution (and not due to solvent evaporation) in a series of control experiments. First, no notable effect of the nature of the substrate was observed when carbon-coated copper TEM grids, silicon wafers, or mica substrates were used to image the assembled structures. Second, in the dynamic light scattering experiments an increase in scattering occurred following the addition of the solution of MUA in dimethylformamide into the solution of NRs in toluene (Figure S2, Supporting Information), which suggested the formation of NR assemblies. Finally, imaging of NR superlattices was carried out following different time intervals after the beginning of self-assembly process. The TEM images in Figure 3 show the increase in lateral dimensions of NR arrays and the enhanced vertical NR orientation with increasing the self-assembly time. Based on these experiments, we conclude that the superlattices of NRs formed in solution and subsequently, due to gravity, deposited on the substrate in the most stable configuration, that is, with the larger face of the 2D NR sheets facing the substrate. The concentration of MUA in the system, CMUA, had a profound effect on the NR self-assembly. Without addition of MUA, only small-area (ca. 100 nm in diameter) NR lattices formed after the addition of dimethylformamide. These 2D sheets coexisted with individual NRs oriented parallel to the substrate (Figure 4a). Following the addition of MUA at 0.05 < CMUA < 0.5 g/L, it replaced TLs and thereby facilitated the formation of close-packed micrometerNano Lett., Vol. 9, No. 8, 2009

Figure 4. TEM image of assemblies of Au-tipped CdSe nanorods formed at different concentrations of MUA: (a) 0, (b) 0.05 g/L, (c) 10 g/L.

size superlattices in which the NRs were aligned perpendicular to the substrate (Figure 4b). The superlattices formed due to the association of SLs and further enhanced by hydrogen bonding between the MUA molecules attached to adjacent NRs (see below).10,27 For CMUA > 0.5 g/L, the NR arrays gradually disintegrated, and for the MUA concentration exceeding 10 g/L, the size-by-side assembly of NRs was completely suppressed (Figure 4c). This effect was caused by ligand exchange between MUA and the SLs, thereby rendering NR solubility in dimethylformamide.29 The suggested mechanism of the organization of NRs in a selective solvent was supported by the results of the following two control experiments: by dissolving the selfassembled NR structures in toluene (a good solvent for SLs) and by the side-by-side assembly of CdSe NRs (without Au tips) in toluene-dimethylformamide mixture (Figures S3 and S4, respectively, Supporting Information). In the first series of experiments, following the addition of toluene, the NR sheets assembled into the absence of MUA rearranged in networks, in which the NRs were linked via gold tips (Figure 3079

Figure 5. FT-IR absorption spectra of the self-assembled structures of NRs formed at different concentrations of MUA: (a) 0 g/L, (b), 0.5 g/L, and (c) 10 g/L. (d) A reference spectrum acquired upon addition of MUA at CMUA ) 10 g/L to the solution of CdSe NRs (no Au tips). The peak at 1645 cm-1 in (a) corresponds to residual dimethylformamide, and it was subtracted from spectra b-d. The spectra were acquired from the dried film of the NRs or their superlattices, both deposited on the ATR crystal.

S3a). The superlattices formed in the presence of MUA at 0.05 g/L e CMUA e 0.5 g/L, did not disassemble upon the addition of toluene due to the formation of hydrogen bonds between MUA molecules. When MUA was introduced at CMUA > 0.5 g/L, the NRs aggregated through side faces, due to the poor MUA solubility in toluene (Figure S3b). These results proved the stepwise exchange of ligands on the NR surface with MUA. In the second experiment, we assembled CdSe NRs without Au tips into side-by-side aligned structures, which confirmed that the driving force for lattice formation was the poor solubility of SLs on the CdSe surface. The stability of these superlattices was substantially lower than of those formed from Au-CdSe NRs. Attachment of MUA to the NR surface was examined by analyzing the changes in the attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra of the NRs at different concentrations of MUA. Figure 5a-c shows the IR spectra of the NRs prior to, and following the addition of MUA at intermediate and high concentrations. Figure 5d shows the IR spectrum of CdSe NRs at high CMUA, used as a control system. Two distinctive features follow from the IR spectra: the replacement of SLs with MUA at high CMUA and the formation of hydrogen bonds between the carboxylic groups of MUA attached to the NR surface. The spectra of the NRs in the absence of MUA and at CMUA e 0.5 g/L were identical in the spectral range 650-600 and 1300-900 cm-1 (Figure 5a,b, respectively), suggesting that no SL replacement occurred at the intermediate concentration of MUA. At CMUA ) 10 g/L, the exchange of SLs with MUA was supported by the disappearance of the vibration bands (1300-900 cm-1) of phosphonic acid attached to the NR side surface and the subsequent appearance of the bands at 1140 and 640 cm-1 (Figure 5c). The latter band corresponded to deprotonated thiol groups attached to the NR.30 The spectrum of CdSe NRs (in the absence of gold tips), acquired at CMUA) 10 g/L (Figure 3080

Figure 6. UV-vis absorption (a) and photoluminescence emission (b) spectra of (1) CdSe NRs and (2) Au-tipped CdSe NRs in toluene and (3, 4) the self-assembled superlattices of NRs in the toluenedimethylformamide mixture. In panel a, spectra (3) and (4) were taken 1 and 24 h, respectively, after the beginning of self-assembly. In panel b, spectrum (3) was acquired 24 h after the beginning of self-assembly. Photoluminescence spectra (1-3) were acquired at the same concentration of NRs in the solution. Spectra (2) and (3) were multiplied by a factor of 100.

5d), showed similar features at 650-600 and 1300-900 cm-1, confirming that the replacement of SLs was followed by the attachment of MUA to the CdSe segment of NRs. The band at 1140 cm-1 could correspond to coordinated MUA, or the residual SLs coordinated to CdSe in the presence of coadsorbed MUA.31,32 (We note that the band associated with C-S attached to Au tips was likely obscured by broad absorption bands at 720 cm-1 from either P-C bond in SLs, or alkyl skeletal vibrations, or a combination thereof.33,34) Hydrogen bonding between the carboxylic groups of MUA attached to the Au tips of NRs was identified in the spectral range 1750-1650 cm-1. (Free MUA was removed by repeatedly washing the NRs with dimethylformamide). For the NRs assembled in superlattices at CMUA ) 0.5 g/L, the bands at 1712 and 1665 cm-1 corresponded to head-to-head and polymeric hydrogen bonding, respectively (Figure 5b).27 At CMUA ) 10 g/L, the high frequency shoulder at 1712 cm-1 had shifted to 1735 cm-1, indicating the existence of free carboxylic groups. This change in the spectrum correlated with the disassembly of NR superlattices at high values of CMUA.27 Figure 6a shows the UV-vis absorption spectra of the solutions of NRs in toluene (prior to and after deposition of Au tips) and of the NR superlattices in the tolueneNano Lett., Vol. 9, No. 8, 2009

dimethylformamide mixture, following different time intervals after the beginning of self-assembly. The spectrum of the original CdSe NRs showed two peaks centered at 503 and 616 nm, which were characteristic of the excitonic band transitions.35 The absorption spectrum of the Au-tipped CdSe NRs showed the peak at 616 nm and a shoulder at 503 nm. The latter corresponded to the overlapped absorption peaks of CdSe NRs and Au tips.16 No significant change in absorption properties of the NR superlattices was observed in comparison with those of individual NRs. The intensity of the absorption peaks decreased with time, owing to the precipitation of the NR sheets. Figure 6b shows photoluminescence spectra of the individual NRs and their self-assembled arrays. Considerable decrease in fluorescence emission of CdSe NRs measured at ca. 630 nm was observed after the attachment of Au tips, due to the electron transfer from CdSe to Au.16 A small blue shift was observed in photoluminescence of NR arrays in comparison with individual NRs. To our best knowledge, this work is the first report on solution-based side-by-side NR self-assembly into 2D superlattices that, upon their deposition on a substrate, form close-packed arrays of vertically aligned NRs. The formation of self-assembled close-packed arrays of Au-terminated semiconductor NRs can further facilitate their hierarchical integration into functional devices. In addition to potential optoelectronic applications of the NR lattices, the Au tips provide an anchor point for a variety of thiolated organic and biological molecules, thereby paving the way for sensing applications of the NR arrays.36,37 The stepwise ligand exchange can be exploited in further studies of the selfassembly of asymmetric, chemically heterogeneous nanoparticles and this method can be extended to the selfassembly of other multicomponent NRs. Acknowledgment. This work was supported by the Canada Research Chair Grant (NSERC Canada). The authors thank Professors Mitchell Winnik and Gregory D. Scholes for fruitful discussions and Dr. Gerald Guerin for dynamic light scattering measurements. Supporting Information Available: TEM images of NR superlattices, acquired in control experiments and dynamic light scattering data. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Lieber, C. M. MRS Bull. 2003, 28, 486–491. (2) Fan, S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. Science 1999, 283, 512–514. (3) Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Science 2005, 310, 462–465. (4) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425–2427.

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