Self-Assembling Branched and Hyperbranched Nanostructures of

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Self-Assembling Branched and Hyperbranched Nanostructures of Poly(3-hexylthiophene) by a Solution Process Han Yan,†,‡ Yong Yan,†,‡ Zai Yu,†,‡ and Zhixiang Wei*,† † ‡

National Center for Nanoscience and Technology, Beijing 100190, People's Republic of China The Graduate School of the Chinese Academy of Science, Beijing 100039, People's Republic of China ABSTRACT: As a typical linear conjugated polymer, poly (3-hexylthiophene) (P3HT) has been successfully used in organic electronic and optoelectronic devices. Strong anisotropic π-π interactions between planar molecular backbones and weak van der Waals interactions between their alkyl side groups make P3HT favorable for the formation of long nanowires. Herein, we propose a facile solution strategy to prepare branched and hyperbranched P3HT nanostructures. A P3HT nanowire was first generated as the trunk, and then branches formed by a heterogeneous nucleation process. The process was realized by a slow evaporation of P3HT in a blend solvent, in which 1,2-dichlorobenzene acted as a good solvent and trimethanolamine acted as a poor solvent. Interestingly, the hyperbranched P3HT nanostructures can be produced using the same principle by changing the evaporation conditions. The molecular direction of P3HT in the trunks and branches was determined clearly by selected area electron diffraction. Due to its good crystallinity and unique morphology, branched and hyperbranched nanostructures might have potential opportunities for organic electronic and optoelectronic devices.

1. INTRODUCTION Construction of micro- and nanostructures with a well-defined shape and size plays a key role in improving properties of electronic and optoelectronic functional materials.1-4 Up to now, numerous micro- and nanostructures with unique electrical and optical properties have been prepared by using a self-assembly method.5-7 Compared with one-dimensional nanowires, assembly of these nanometer scale building blocks to form an ordered branched structure or more complicated architectures offers great opportunities for fabricating nanodevices and exploring their novel properties.8,9 Although various branched nanostructures of inorganic semiconductors, such as cadmium selenide and lead sulfide, have been successfully fabricated and applied for the construction of devices,10,11 preparation of organic nanowires with clearly defined superstructures is not an easy task due to their soft properties.12 On the basis of the understanding and adjustments for the noncovalent interactions, poly(3hexylthiophene) (P3HT) branched and hyperbranched nanostructures are successfully prepared by a self-assembly process. This might lead to programmable control of the functional superstructures for a further improvement of material properties and device performance. Poly(3-hexylthiophene) (P3HT) is a typical linear conjugated polymer, which is successfully used in organic electronic and optoelectronic devices, such as light-emitting diodes,13,14 field effect transistors,15-18 memories,19,20 and solar cells.21 Strong anisotropic π-π interactions between planar molecular backbones and weak r 2011 American Chemical Society

van der Waals interactions between their alkyl side groups make P3HT favorable for the formation of long nanowires. Until now, there are three main methods to fabricate P3HT nanowires: slow solvent evaporation,22,23 recrystallization,24 and blend solvent deposition.25 Recently, a shish-kebab structure of P3HT was prepared by an epitaxial method, and it exhibited polarized optical absorption and photoluminescence characteristics.12 However, more complicated hyperbranched nanostructures are difficult to obtain by this strategy. In this article, we propose a solvent evaporation strategy to prepare branched and hyperbranched P3HT nanostructures. As is commonly known, in polymer chemistry branching occurs with the replacement of a substituent by another covalently bonded chain of that polymer. Herein a similar concept for supramolecular chemistry is applied to generate the branched and hyperbranched nanostructures. A P3HT nanowire as the trunk was first self-assembled by a homogeneous nucleation process, and then branches were produced by a heterogeneous nucleation process. These two processes are realized by a slow evaporation of a blend solvent, i.e., 1,2-dichlorobenzene (DCB) as a good solvent and trimethanolamine (TEA) a poor solvent. The length of branches can be controlled by varying the content of poor solvent. More interestedly, we can also produce the

Received: October 12, 2010 Revised: January 12, 2011 Published: February 09, 2011 3257

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Figure 1. SEM images of P3HT nanostructures obtained by adding different volumes of TEA solution to a 1 mL P3HT solution: (a) 0; (b) 30 μL; (c) 55 μL; (e) 65 μL. (g) Diagram of branch length of nanostructure vs TEA solution volume.

hyperbranched P3HT structure using the same principle by changing the evaporation conditions.

2. EXPERIMENTAL SECTION Preparation of P3HT Nanostructures. The regioregular P3HT was obtained from Sigma-Aldrich (Mn ∼ 64000). 1,2Dichlorobenzene (DCB) and acetone were distilled before being used. Triethanolamine (TEA) was used without further purification. Silicon substrates were cleaned by ultrasonication in ethanol, acetone, and deionized water for 15 min, respectively. For the preparation of branched P3HT nanostructures, P3HT was first dissolved in DCB with a concentration of 0.1 mg/mL. TEA was also dissolved in DCB with a concentration of 0.01 mg/ mL. Different volumes of TEA solution (0-75 μL) were added into a 1 mL P3HT solution to fabricate various P3HT nanostructures, which were obtained on silicon substrates by a slow solvent evaporation in a closed jar. The samples were washed by acetone before characterization. For the preparation of hyperbranched nanostructures, the only difference is adding 5 μL of DCB into the closed jar at the beginning of the assembly process to provide a saturated vapor environment. Characterization of P3HT Nanostructures. The morphologies of P3HT naostructures were characterized by SEM

(scanning electron microscopy, model S4800, Hitachi, Japan). The structure and molecular orientation were identified by TEM (transmission, electron microscopy, Tecnai G2 F20 U-TWIN, FEI Co., USA). The structure of P3HT was tested by a Renishaw Micro-Raman Spectroscopy System with a polarized transmitted light kit. UV-vis absorption spectra and PL spectra of P3HT solution were recorded using a UV-vis spectrophotometer (Lambda 950, Perkin-Elmer, USA) and photoluminescence spectrophotometer (LS 55 Perkin-Elmer, USA), respectively.

3. RESULTS AND DISCUSSION 3.1. Preparation of Branched P3HT Nanostructures. Linear P3HT nanowires were obtained directly by slow-solvent evaporation process using DCB as a solvent (Figure 1a). For designing branched structures, a second solvent, triethanolamine (TEA), is introduced into this system. The P3HT solution in a mixed solvent of DCB and TEA was dropped onto a silicon substrate and then evaporated slowly in a sealed jar. After 4 h, acetone was used to wash away the residual TEA. As-prepared samples were characterized by SEM, demonstrating branched nanostructures of P3HT formed (Figure 1b-d). Interestedly, morphologies of branched nanostructures changed with the content of TEA solution. As shown in Figure.1a, linear P3HT nanowires were observed without any poor solvent. 3258

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Figure 2. UV-vis and PL spectrum with different ratios of TEA to DCB (the concentration of P3HT is 0.05 mg/mL in all the samples).

Figure 4. SEM of P3HT nanostructures: (a) branched nanostructures and (b) hyperbranched nanostructures obtained by adding 5 μL of DCB at the beginning of the evaporation process.

Figure 3. Time-dependent evolution morphology of adding a 30 μL TEA solution: (a) 2 h; (b) 3 h; (c) 6 h. (d) Growth mechanism of P3HT branched nanostructures.

Upon the addition of TEA, we successfully obtained the branched nanostructure, a thick nanowire trunk with thin nanowire branches. Increasing the contents of TEA, the length of branches varies from tens of nanometers to above 1 μm (Figure 1b-d). The tendency is branches becoming longer with higher TEA concentrations, which is attributed to more P3HT molecules being used for branches growth than nucleation. Further increasing TEA concentration induces a decrease of branch length (Figure 1d), and finally, only bundles of nanowires are observed (Figure 1e). The variations of the branch length with the increase of TEA contents are graphed in Figure 1g. From this diagram, a clear morphology evolution tendency was observed, and there exists a maximum branch length corresponding to the volume of TEA solution at ca. 55 μL. In the pure DCB, a homogeneous nucleation-growth process was dominated with the evaporation of DCB; therefore linear nanowires were produced.22,23 With a small amount of TEA, blend solvent deposition25 will play an important role for the formation of P3HT trunks. The formation of branched P3HT nanostructures can then be divided into two processes: (i) homogeneous nucleation-growth process to produce trunks by blend solvent deposition; (ii) heterogeneous nucleation-growth process to produce branches by slow evaporation 3259

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Figure 5. (a) TEM image of oriented P3HT branches. (b) SAED pattern of oriented P3HT branches. (c) TEM image of a P3HT branched nanostructure. (d) SAED pattern of P3HT branched nanostructure. (e) Arrangement of P3Ht molecules in the trunck and branches.

of blend solvent. The homogeneous and heterogeneous nucleations are two competive processes during DCB evaporation, and their relative contribution determined the lengths and densities of branches. At a high TEA contents, the blend solvent deposition process becomes dominant, and thus linear nanowires are produced again.25 3.2. Growth Mechanism of Branched Nanowires. To illustrate the function of TEA in the blend solvent, P3HT solutions with different ratios of blend solvent were measured. The evolution of the absorption and emission spectroscopy of P3HT solutions in different DCB to TEA ratios are shown in Figure 2. When the ratio of TEA to DCB is 1:3, only one P3HT absorption peak at about 460 nm is observed, indicating that the P3HT remains in a well-dissolved state. On further increase of the ratio from 1:3 to 3:1, additional low-energy peaks emerged and increased gradually, while the peak at 460 nm decreased. The low-energy peaks at 515, 560, and 610 nm are identical to those

observed for P3HT nanowires.24 The shape of the UV-vis spectroscopy at the end of this evolution process clearly indicates that adding TEA induced P3HT aggregating into a highly ordered stacking form. Moreover, fluorescence extinction is observed from the PL spectroscopy with an increase of TEA contents, which is also originated from the P3HT aggregation. From a series of UV-vis and PL spectra obtained different ratios of TEA to DCB, we prove that at a certain point TEA acted as a bad solvent to induce the aggregation of P3HT, and this played a key role to form P3HT trunks during DCB evaporation process. To understand the detailed formation process of the branched structure, investigations of time-dependent morphology evolution are carried out. The P3HT solution in blend solvent was dropped onto the silicon substrate in a closed jar. After certain time intervals (e.g., 2, 3, and 6 h), unevaporated solution was removed by a high-speed spin-coating method. This ensured that only assembled nanostructures were left on the substrate. 3260

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Figure 6. (a) SEM image of an oriented P3HT branched structure. The inset shows the polarized direction relative to the observed object. The scale bar represents 500 nm. (b) Polarized Raman spectroscopy at different angles.

Panels a-c of Figure 3 show a series of SEM images of P3HT nanostructures produced by the above-mentioned method. After 2 h only short P3HT nanowires assembled on the substrate with the evaporation of DCB (Figure 3a). After 3 h, P3HT nanowires grew from hundreds of nanometers to longer than 1 μm (Figure 3b), but no branches could be observed. These nanowires can act as trunks to grow P3HT branches. After 6 h, a branched P3HT nanostructure was observed (Figure 3c). On the basis of the results shown above, the formation of a branched structure containing several continuous steps as illustrated in Figure 3d is proven. P3HT short nanowires were first deposited by homogeneous nucleation process, and then form long P3HT nanowires as trunks due to strong π-π stacking interactions. With the increase of nanowire concentration, the trunks acted as the nucleation template for the growth of branches with a slow evaporation of DCB solvent. It is reasonable to suggest that the gradient of P3HT concentration around the trunks and π-π stacking interactions between P3HT molecules induce unique oriented branches. Finally, the trunks in accompany with the branches constructed the branched P3HT nanowires. 3.3. Preparation of Hyperbranched P3HT Nanostructures. We should emphasize that the formation of branches is mainly controlled by the heterogeneous nucleation process. This nucleation

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mode can generally be adjusted by the thermodynamic and dynamic factors. Adding the poor solvent could separate the growth process into different stages, and the first formed P3HT nanowires would act as the templates for heterogeneous nucleation. This approach can be simply extended to produce more complicated nanowire architectures by just adding an additional tuning factor. To illustrate this point, hyperbranched P3HT nanostructures are synthesized by a three-stage growth procedure, i.e., trunk formation and growth of first generation and second generation branches, respectively. A drop of DCB is added into the closed jar at the beginning of the assembly process. The added good solvent provides a saturated vapor environment to slow the initial homogeneous nucleation speed, and this changes the dynamic condition of nucleation. When the nucleation speed is slowed down, the density of nuclei is also decreased in unit time, favoring the heterogeneous process to form the branched structure. Along with the effect of poor solvent (thermodynamic condition), the residual solute molecules would grow as the hyperbranches in the complicated structure as shown in Figure 4. 3.4. Structural Characterization of P3HT Nanostructures. The arrangement of P3HT in their nanostructures is characterized by selected area electron diffraction. Nanostructures are prepared on carbon-coated copper grid from the blend solvent (DCB/TEA). Branched P3HT nanostructures are distributed on the copper grid uniformally after solvent evaporation (Figure 5a,c). The SAED pattern obtained for oriented P3HT branches is indexed by using the structure model in the literature.22 The SAED pattern shows arc-shaped reflections from branches, which are indexed as (020), (012), and (004) reflection, respectively (Figure 5b). The relatively oriented SAED pattern with respect to the morphology obtained in TEM image implies that P3HT molecules are perpendicular to the nanowire long axis (i.e., π-stacking directions is along nanowire long axis). In Figure 5c, we selected an integrated branched nanostructure with uniformly oriented branches for SAED investigation. Interestedly, a pair of perpendicular arc-shaped (004) reflections, demonstrating that the polythiophene backbones arranged in two perpendicular directions. Combining with the TEM image we infer that these two (004) reflections may separately represent the trunk and branches in the integrated branched-structure. Therefore, the P3HT in the trunks are also arranged perpendicular to the nanowire direction. These results are different from the shish-kebab structure of P3HT reported in the literature,12 in which P3HT nanowires are parallel arranged in the trunks. We proposed that branches grow epitaxially along the trunk, taking the edge-to-face molecular arrangement. From the ED pattern in Figure 5b, the nanowire grows along the (020) direction, perpendicular to the (004) direction. We can calculate that d(020) = 1.809 Å, and d(004) = 2.054 Å. The misfit of the interface is calculated by δ = (d(004) - d(020))/d(004), almost equals 0.1193. This value is quite small, indicating energy favorable for epitaxial growth. To match the lattice parameters, dislocations exist through deformation of P3HT molecules. In Figure 5d, we did not observe any diffraction pattern between the two couples of diffraction arcs; this also supports our assumption. The orientation of the P3HT nanowires in their superstructures can induce anisotropic properties. Herein, polarized Raman spectroscopy, i.e., the variation of the scattering intensity versus the direction of polarized incident light is measured using a single superstructure as shown in Figure 6a. The incident light is rotated at four different angles corresponding to the orientation of 3261

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The Journal of Physical Chemistry C branches, including parallel and perpendicular directions. The intensity of Raman spectra is determined by two main factors: the molecular polarized direction and the quantity of materials. For the spectra of P3HT, the characteristic peaks at 1380 and 1440 cm-1 separately represent the C—C and CdC stretching vibration, and they are much more intense along the backbone than the other directions. So the intensity of these two characteristic peaks could reflect the P3HT mainchain direction. Figure 6b depicts the variation of Raman scattering signal versus the angle of polarization of the incident light. The position of the two characteristic peaks is the same at the four incident angles, but the intensity is quite different. The minimum value appears when the polarized direction is parallel to the branches. The maximum value is obtained when the polarized direction rotates 90°. Since P3HT molecular direction is perpendicular to the nanowire, the intensity of signal at 90° is mainly determined by the molecular orientation of branches, while the signal at 0° is mainly contributed by the trunk. For polarized Raman spectroscopy, we can roughly know the quantities of P3HT molecules for the construction of trunks and branches.

4. CONCLUSIONS In conclusion, we report a facile method that enables the designed preparation of branched P3HT nanostructures. We choose a good solvent and a poor solvent in this process, which have large differences of the boiling point. The trunk formed first by homogeneous nucleation, and the branches grew through the heterogeneous nucleation process. The length and density of branches can be varied by tuning the ratio of good solvent to bad solvent. This method can be extended to produce a hyperbranched P3HT nanostructure. TEM studies of the branched P3HT nanostructures demonstrate the molecular orientations in trunks and branches are the same. The unidirectional molecular arrangement induced anisotropic properties, such as anisotropic polarized Raman spectroscopy. These unique structures have great potential application in organic electronic and optoelectronic devices, including the three-dimensional interconnected active layer in polymer solar cells. ’ AUTHOR INFORMATION

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Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We acknowledge the financial support of the National Natural Science Foundation of China (Nos. 20974029, 91027031), The Ministry of Science and Technology of China (Nos. 2006CB932100, 2009CB930400), and the Chinese Academy of Sciences (KJCX2-YW-M13). ’ REFERENCES (1) Briseno, A. L.; Mannsfeld, S. C. B.; Ling, M. M.; Liu, S. H.; Tseng, R. J.; Reese, C.; Roberts, M. E.; Yang, Y.; Wudl, F.; Bao, Z. N. Nature 2006, 444, 913–917. (2) Yim, K. H.; Zheng, Z. J.; Liang, Z. Q.; Friend, R. H.; Huck, W. T. S.; Kim, J. S. Adv. Funct. Mater. 2008, 18, 1012–1019. (3) Liu, B. Q.; Zhao, X. P.; Zhu, W. R.; Luo, W.; Cheng, X. C. Adv. Funct. Mater. 2008, 18, 3523–3528. (4) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111– 1114. 3262

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