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Thermally Induced Transfiguration of Polymer Nanowires under Irradiation of Electron Beams Hai Peng Xu,† Yu Mao,† Jian Wang,† Bo Yu Xie,† Jia Ke Jin,† Jing Zhi Sun,*,† Wang Zhang Yuan,†,‡ Anjun Qin,†,‡ Mang Wang,† and Ben Zhong Tang*,†,‡ Institute of Biomedical Macromolecules, Department of Polymer Science & Engineering, Key Laboratory of Macromolecular Synthesis and Functionalization of the Ministry of Education, Zhejiang UniVersity, Hangzhou 310027, China, and Department of Chemistry, William Mong Institute of Nano Science and Technology, The Hong Kong UniVersity of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China ReceiVed: March 23, 2009; ReVised Manuscript ReceiVed: July 10, 2009
Polymer nanowires were found to respond to external perturbations and to undergo transfiguration upon exposure to the irradiation of high-flux electron beams. The phenomenon is general, observable in the nanowires of conjugated and nonconjugated polymers with linear and hyperbranched structures. The transfiguration ceased to occur after the nanowires had been annealed at temperatures higher than the glass transition temperatures of the polymers. The phenomenon is rationalized to be associated with the residual internal stress in the nanowires that is relieved through the electron-beam irradiation and/or thermal annealing. This work thus offers cautionary advice that a proper annealing treatment should be exercised, if one wishes to fabricate polymer nanostructures and miniature devices with reliable stability and durable performance. Introduction One-dimensional nanowires are one of the most important nanostructures and have been widely used in the fabrication of nanoscale electronic and photonic devices.1-8 The modulation of the nanowires by external stimuli has attracted much interest among scientists and technologists because it helps develop useful methods and processes for controlling and optimizing the performances of the devices based on the nanowires.9-16 While the prospect looks bright, the actual work has been difficult. So far there have been only some scattered reports about the responses of inorganic semiconducting and metallic nanostructures to external perturbations, with different operating mechanisms being proposed. Wang and co-workers, for example, reported a novel selfattraction phenomenon in vertically aligned Au/ZnO nanorods under illumination of electron beams.12 The bending motion was proposed to be caused by the interaction between the accumulated charges near the metal-semiconductor junctions for two nanorods with different lengths. Lin et al. observed bending and bundling in the vertically aligned arrays of ZnO nanowires with flat (0001) top surfaces.17 The authors excluded the involvement of the electron-beam bombardment effect in the scanning electron microscopy (SEM) measurements and attributed the nanowire movements to the electrostatic interactions due to the charged (0001) surfaces. Shik and co-workers reported that a high-frequency electric field caused the freestanding nanowires with one end clamped to the substrate to bend.13 The bending was found to be the result of a superposition of static bending and induced mechanical oscillations. Zhang et al. observed reversible bending of Si3N4 nanowires under irradiation of electron beams during the transmission electron microscope (TEM) measurements, driven by electrostatic interaction.14 Ruoff * To whom correspondence should be addressed. Phone: +86-571-87953797, fax: +86-571-8795- 3734, e-mail:
[email protected] (J.Z.S.); phone: +852-2358-7375, fax: +852-2358-1594, e-mail:
[email protected] (B.Z.T.). † Zhejiang University. ‡ The Hong Kong University of Science & Technology.
CHART 1: Chemical Structures of Conjugated Polyacetylene Derivative P1, Nonconjugated Poly(methyl methacrylate) (PMMA), and Hyperbranched Poly(1,3,5-phenoylphenylene) hb-P2 Studied in This Work
et al. compared the resonance vibrations of amorphous SiO2 nanowires driven by mechanical and ac electrical field loading.15 Under irradiation of a high flux of electron beams in an SEM chamber, significant charge trapping in the nanowires was observed. Nanowires of π-conjugated organic polymers are promising candidate materials for the construction of nanoscopic photoelectronic devices, such as photovoltaic cells, light-emitting diodes, and field-effect transistors.16-30 The operation of these devices is often accompanied with profound physical changes, such as significant increase in temperature due to the generation of large amounts of heat. However, it is virtually unknown how the polymer nanowires behave under the working conditions in these devices, for example, how they respond to sudden rises in temperature due to the passages of large currents through the devices.31-33 In our previous work, we found that the single-layer photoreceptors with disubstituted polyacetylene derivative P 1 (Chart 1) as photogeneration component exhibited high photoconductivity.34 In an effort to understand the origin of the high photoconductivity, we investigated the surface morphologies of
10.1021/jp902624k CCC: $40.75 2009 American Chemical Society Published on Web 07/27/2009
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Figure 1. Stretching deformation of nanowires of P 1 after being exposed to high-flux electron beams; SEM images taken at exposure times of (A) 0 s, (B) 5 s, (C) 10 s, and (D) 25 s.
Figure 2. SEM images showing sequential displacement of nanowires of PMMA with increasing time of exposure to irradiation of high-flux electron beams.
the photoreceptors and found that the polymer chains had assembled into nanowires in the devices. We attributed the high photosensitivity in the photoreceptors to the fast charge transport bestowed by the polymer nanowires penetrating into the chargetransport layers in the devices. Intriguingly, we found that the nanowires responded to the irradiation of electron beams during the morphology examinations using electron microscopy techniques. Specifically, the nanowires moved when they were scanned by high-flux electron beams and when their SEM images were taken in high magnifications. To gain an insight into the mechanism of the transfiguration process of P 1 nanowires under electron beam irradiation, we conducted similar experiments using nanowires of polymers with very different electronic and topological structures. Thus, besides the conjugated linear polymer (P 1), nonconjugated linear polymer [poly(methyl methacrylate) or PMMA] and nonlinear (hyperbranched) polymer (hb-P 2) were used to fabricate nanowires, and their behaviors under electron-beam illumination were carefully monitored. These studies reveal that the electron beam-induced transfiguration is a general phenomenon for all the nanowires. The nanowires cease to undergo transfiguration after they have been thermally annealed. In this paper, we report our experiment results and present our understanding on the nanowire transfiguration process. Experimental Section Materials and Instrumentation. All the chemicals and solvents used in this study were purchased from Acros and Aldrich, unless otherwise specified. Anodic aluminum oxide (AAO) membranes were purchased from Whatman Inc. Polymers P 1 and hb-P 2 were prepared in our laboratories according to the synthetic procedures reported in our previous papers.34-36 SEM images were taken on a JSM-5510 microscope. Most of the SEM photomicrographs shown in the figures below were extracted from the video clips at the specific times. Differential scanning calorimetry (DSC) thermograms were recorded on a Perkin-Elmer Pyris 1 DSC under nitrogen at a heating rate of 20 °C/min. Fabrication of Nanowires of P 1. The nanowires of P 1 were spontaneously formed during the preparation of its singlelayer photoreceptors. The procedure for the photoreceptor fabrication was reported in detail in our previous papers34,37,38 and is briefly described as follows. Charge-generation material P 1, charge-transport material R-naphthylphenyl-N,N-diethylbenzylhydrozone, polycarbonate binder, and DMF solvent were
sequentially added into a nitrogen-flushed flask at room temperature. A typical ratio for the charge-generation material, charge-transport material, and binder was 4:40:40 by weight. A brown suspension was obtained after stirring the mixture for 6 h. An interface layer with a thickness of ∼1 µm was cast from a 5% ethanol solution of a polyamide onto a freshly cleaned surface of aluminum substrate. After the interface layer was completely dried, the brown suspension was cast on top of the interface layer to form a functional layer about 20 µm in thickness. The devices were put into a vacuum oven and dried at 50-70 °C for prolonged periods of time. Preparation of Nanowires of PMMA. The AAO membranes were used as templates to grow the PMMA nanowires.39,40 The average diameter of the nanochannels in the AAO templates was ∼200 nm. Before being used for growing the polymer nanowires, the templates were washed with acetone several times. Unfortunately, however, the PMMA solutions could not be directly used to fabricate nanowires, because it was difficult to fill the nanochannels in the AAO template by concentrated polymer solutions. When dilute polymer solutions were used, the resultant structures were nanotubes rather than nanowires, because of the volume shrinkage accompanying the solvent evaporation. Alternatively, we adapted an indirect method, that is, postpolymerization of prepolymers of methyl methacrylate (MMA) in the nanochannels. A typical experimental procedure for the indirect process is described below. Into a clean glass tube were added MMA monomer and a catalytic amount of benzoic peroxide. The tube was put into an oil bath preheated to 90 °C. Heating at the temperature for ∼1 h yielded a prepolymer of MMA with an appropriate viscosity. In the bottom of a three-necked flask was placed a plate of the AAO template in a flat-lying manner, and the flask was evacuated with a vacuum pump. Through a rubber septum, the prepolymer of MMA was injected into the flask and the AAO template was immersed in the liquid. The flask was then filled with pure nitrogen to bring the pressure back to 1 atm. Heating the flask on an oil bath at 120 °C for 6 h completed the polymerization reaction, and the AAO template containing the PMMA nanowires inside its nanochannels was obtained. After cooling to room temperature, the template was treated with a KOH aqueous solution (1 M) to erode the AAO template. Preparation of Nanowires of hb-P 2. In the bottom of a three-necked flask was placed a plate of the AAO template in a flat-lying manner, and the flask was evacuated under vacuum for 2 h. Into a separate tube (20 mL) were sequentially added
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Figure 3. SEM images showing sequential displacement of nanowires of hb-P 2 with increasing time of exposure to irradiation of high-flux electron beams.
100 mg of the diyne monomer,36 1.9 mL of dioxane, and 0.6 mL of piperidine solution in dioxane. After being stirred for 10 min, the mixture was transferred to the AAO templatecontaining flask under vacuum using a hypodermic syringe. After the template was completely immersed in the solution, the flask was flushed with dry nitrogen. The excess solution was removed, and the system was heated to 80 °C and reacted under nitrogen for 24 h. A large amount of methanol was poured into the flask, and the precipitate was allowed to stand overnight. The template was taken out of the flask and washed with methanol several times. After being dried in a vacuum oven, the hb-P 2 nanowires embedded in the AAO template were obtained. Results and Discussion Figure 1 shows the nanowires of P 1 extruded out of the surface of a photoreceptor. Although some of the dangling nanowires are straight, most of them take a bending configuration before being subjected to high-flux irradiation of electron beams. With commencement of high-resolution SEM scanning, the nanowires start to stretch. The nanowires become more stretched with lengthening the exposure time. After 25 s exposure, the bending nanowires become straight, while the originally straight nanowires are elongated in length (Figure 1D). Extension of the exposure time to more than 25 s does not result in further changes in the nanowires, suggesting that the transfiguration process is completed. Evidently, the nanowires of P 1 undergo transfiguration upon being illuminated by the high-flux electron beams. Is this an isolated phenomenon for P 1 nanowires or a general phenomenon observable in other polymer nanowire systems? Furthermore, what is the mechanism for the nanowire transfiguration process? To answer these questions, we prepared nanowires of PMMA, a nonconjugated polymer. The PMMA nanowires were prepared by radical polymerization in the nanochannels of the porous AAO template.39 After exfoliation from the aluminum substrate and partially erosion with alkaline solution, many dangling PMMA nanowires were observed. The high-flux electron-beam irradiation caused dramatic changes in the configuration of the dangling polymer nanowires. As can be seen from Figure 2, the nanowires marked by the arrows change both positions and lengths after the electron-beam irradiation. Evidently, the transfiguration process occurs in the nanowires of both conjugated and nonconjugated polymers. PMMA is known to decompose under the bombardment of electron beams. Is the structural damage induced by the electronbeam irradiation responsible for the transfiguration of the PMMA nanowires? The disubstituted polyacetylene derivative P 1 is very stable,41,42 which makes the structural damage an unlikely cause for the transfiguration process. In an effort to collect more information on the nanowire transfiguration, we prepared nanowires of hb-P 2, a stable hyperbranched polymer.36 As can be seen from Figure 3, the relative positions of the polymer nanowires obviously change after the electron-beam
Figure 4. DSC thermograms of P 1, PMMA, and hb-P 2 measured under nitrogen at a scanning rate of 20 °C/min.
Figure 5. SEM images of hb-P 2 nanowires under exposure to electron beams after the nanowires were annealed for 6 h at (A-C) 60 °C, (D-F) 100 °C, and (G-I) 150 °C.
irradiation, offering further evidence that stable nanowires can readily undergo transfiguration. The experimental results shown in Figures 1-3 above corroborate that the nanowire transfiguration is a general phenomenon observable in the nanowires fabricated from the polymers with both conjugated and nonconjugated electronic structures and both linear and nonlinear topological structures. The electron beam-induced structural damage should not be the main cause for the nanowire transfiguration because no evident decomposition or breakage of the nanowires have been observed in all the cases. Is the transfiguration caused by the electrostatic interactions between the nanowires and/ or between the nanowires and the substrates, as proposed for the nanowire motions in the inorganic semiconducting and metallic systems?12-17,32,33 This seems plausible: organic
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Figure 6. SEM images of P 1 nanowires under exposure to electron beams after the nanowires were annealed at 150 °C for 24 h.
Figure 7. SEM images of PMMA nanowires under exposure to electron beams after the nanowires were annealed for 6 h at (A, B) 60 °C and (C, D) 110 °C.
materials usually have low charge transport capability and the incident electrons may be trapped by the polymer nanowires. If the nanowires were indeed polarized by the electron-beam irradiation, they should carry the same negative charges and the electrostatic repulsion should drive the nanowires away from each other. However, the nanowires were found to move in random directions. In the special case shown in Figures 2, the two nanowires marked by the arrows actually are getting closer to each other. In addition, the transfiguration of the polymer nanowires occurs only when the high-resolution images were taken or when the polymers were shone with high-flux electron beams. Thus, the mechanism proposed for the inorganic nanostructure systems cannot be simply extended to the organic system: the electrostatic interaction cannot be the driving force for the transfiguration of the polymer nanowires. It is well-known that the structures of organic polymers are more prone to thermal deformation than those of inorganic materials. Products made of polymeric materials generally need to be annealed at temperatures higher than their glass transition temperatures (Tg) before use, in order to relieve the internal stress generated during the thermal and mechanical production processes. We thus turned our attention to the heating effect of the electron beams, because a high flux of incident electrons can lead to fast elevation in temperature. The configuration changes in the polymer nanowires might be associated with thermally induced relaxation of the residue stress. To verify our assumption, we measured the DSC thermograms of the polymers. The Tg values of PMMA and hb-P 2 were found to be 104 and 140 °C, respectively, consistent with the values given in the literature,34-36,43 although that of P 1 could not be clearly determined from its DSC curve (Figure 4).41 Figure 5 shows the SEM images of the nanowires of hb-P 2 annealed at different temperatures for a long period of time (6 h) and exposed to electron-beam irradiation for different
periods of time (0-25 s). After being annealed at 60 °C, the nanowires still undergo active transfiguration, as can be understood from the changes in the relative positions of the nanowires labeled with numbers 1-4 shown in panels A-C of Figure 5. The large movement of nanowire 2 is particularly noteworthy. When the annealing temperature is raised to 100 °C, the situation is hardly changed: the nanowires labeled by numbers 5 and 6 become more stretched with lengthening the exposure time. After the nanowires are annealed at a temperature (150 °C) higher than the Tg of hb-P 2 (140 °C), all the nanowires cease to respond to the electron beam irradiation (Figure 5, panels G-I). Similar experiments were carried out on the nanowires of P 1, with the aim of further confirming the thermal effect. Because the Tg of P 1 is unclear, we annealed its nanowires at 150 °C for a very long period of time (24 h), to make sure that the residue stress is completely relieved. The thus-treated nanowires stopped undergoing the electron beam-induced transfiguration process (Figure 6), consistent with the result obtained above from the hb-P 2 system. When the nanowires of PMMA were annealed for 6 h at a temperature (60 °C) lower than its Tg (104 °C), the transfiguration phenomenon was clearly observed (Figure 7A). In sharp contrast, when annealed at a temperature (110 °C) higher than its Tg, the nanowires kept still even after being irradiated with the high-flux electron beams for as long as 25 s. The picture now becomes clear. During the fabrication processes of the nanowires of the polymers (or plastics) at room temperature, the slow relaxation of the glassy polymer chains cannot follow the fast evaporation of the volatile solvents or the quick solidification of the polymerization solutions, thereby creating great stress in the nanowires formed in the confined nanochannels of the rigid AAO templates. The huge amounts of heat generated by the high-flux electron-beam irradiation during high-resolution SEM scanning boost the temperatures of the nanowires, which greatly accelerates the relaxation process of the strained polymer chains. It is the relief of the internal residue stress in the polymers that caused the nanowires to undergo the electron beam-induced transfiguration process. Concluding Remarks In summary, we have discovered a novel transfiguration phenomenon of polymer nanowires induced by electron-beam irradiation. The phenomenon is general because it is observable in the nanowires of conjugated and nonconjugated linear and nonlinear polymers. The transfiguration is banished when the nanowires are annealed at temperatures higher than the Tgs of the polymers. The Tg dependence of the phenomenon indicates that the transfiguration is associated with the residual stress in the nanowires. The internal stress imposed on the nanowires during their fabrication process is relieved by the high-flux electron-beam irradiation unless it was relaxed through thermal annealing treatment. Considering that even a small change in position or configuration can lead to dysfunction of a nanostructured device, polymer-based nanostructures must be thermally treated before they are put into practical applications in
Transfiguration of Polymer Nanowires real devices. Our work thus not only presents a rational explanation to the nanowire transfiguration process but also offers a helpful guideline to the fabrication of polymer nanostructures and devices with reliable stability and durable performance. Acknowledgment. The work reported in this paper was partially supported by the National Science Foundation of China (20634020, 50740460164, and 50573065), the Ministry of Science & Technology of China (2009CB623605), and the Research Grants Council of Hong Kong (603509, CUHK2/CRF/ 08, and 602707). B.Z.T. appreciates the support from the Cao Guangbiao Foundation of Zhejiang University. References and Notes (1) Tian, B.; Kempa, T. J.; Lieber, C. M. Chem. Soc. ReV. 2009, 38, 16. (2) Heath, J. R. Acc. Chem. Res. 2008, 41, 1609. (3) Hayden, O.; Agarwal, R.; Lu, W. Nano Today 2008, 3, 12. (4) Sun, Y. G.; Rogers, J. A. AdV. Mater. 2007, 19, 1897. (5) Lu, W.; Lieber, C. M. J. Phys. D: Appl. Phys. 2006, 29, 387. (6) Gall, K.; Diao, J. K.; Dunn, M. L. Nano Lett. 2004, 4, 2431. (7) Xia, Y. N.; Yang, P. D. AdV. Mater. 2003, 15, 351. (8) McAlpine, M. C.; Friedman, R. S.; Jin, S.; Lin, K. H.; Wang, W. U.; Lieber, C. M. Nano Lett. 2003, 3, 1531. (9) Jie, J.; Zhang, W.; Peng, K.; Yuan, G.; Lee, C. S.; Lee, S.-T. AdV. Funct. Mater. 2008, 18, 3251. (10) Pearton, S. J.; Kang, B. S.; Tien, L. C.; Norton, D. P.; Heo, Y. W.; Ren, F. Nano Lett. 2007, 2, 201. (11) Zhou, J.; Wang, Z. L.; Grots, A.; He, X. L. Solid State Commun. 2007, 144, 118. (12) Wang, X. D.; Summers, C. J.; Wang, Z. L. Appl. Phys. Lett. 2005, 86, 013111. (13) Shik, A.; Ruda, H. E. J. Appl. Phys. 2005, 98, 094306. (14) Zhang, Y. J.; Wang, N. L.; He, R. R.; Zhu, J.; Yan, Y. J. J. Mater. Res. 2000, 15, 1048. (15) Dikin, D. A.; Chen, X.; Ding, W.; Wagner, G.; Ruoff, R. S. J. Appl. Phys. 2003, 93, 226. (16) Martin, J.; Mijangos, C. Langmuir 2009, 25, 1181. (17) Zhu, D. F.; He, Q. G.; Cao, H. M.; Cheng, J. G.; Feng, S. L.; Xu, Y. S. Lin, T. Appl. Phys. Lett. 2008, 93, 261909. (18) Cao, Y.; Kovalev, A. E.; Xiao, R.; Kim, J.; Mayer, T. S.; Mallouk, T. E. Nano Lett. 2008, 8, 4653. (19) Jung, Y. S.; Jung, W.; Tuller, H. L.; Ross, C. A. Nano Lett. 2008, 8, 3776.
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