Growth Control for Architecture Molecular Conductor of Low

CAS Key Laboratory of Organic Solids, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences,...
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Growth Control for Architecture Molecular Conductor of Low Dimension Nanostructures Nan Chen,†,‡ Changshui Huang,†,‡ Wenlong Yang,†,‡ Songhua Chen,†,‡ Huibiao Liu,† Yongjun Li,† and Yuliang Li*,† CAS Key Laboratory of Organic Solids, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China, and Graduate UniVersity of Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ReceiVed: April 29, 2010; ReVised Manuscript ReceiVed: June 18, 2010

The combination of a modified template technique and an electrochemical polymerization method for synthesizing nanowires and nanotubes of π-conjugated poly[1,4-bis(pyrrol-2-yl)benzene] (PBPB) has been demonstrated. From scanning electron microscopy and transmission electron microscopy photographs, we observed the formation of nanowires with diameters of about 300 nm and lengths of 5-30 µm and nanotubes with wall thicknesses of 25-50 nm by changing the polymerization time and oxidation potential. The photoluminescence and electrical properties of PBPB/ClO4- nanowires have also been studied, which show good photoluminescence and excellent conductivity behavior. Introduction There is currently an intensive effort to develop methods for producing materials with low dimensional structures such as nanowires, nanotubes, and nanorobs which are of great importance and interest, due to their potential applications in the development of optoelectronic and nanoelectronic devices, Organic π-conjugated molecules have interesting electrical, optical, and optoelectronic properties and are extensively applied in fields ranging from field-effect transistors (FET)s,1 lightemitting diodes (LED)s,2 and photovoltaic cells3 to other related devices. Conjugated polymer materials, intrinsically also referred to as organic semiconductors, are polymers with a delocalized ∂-electron system with an “intrinsic” wide band gap that defines their affinity for electrons. They are good candidates for developing potential applications in diverse fields. Past decade has witnessed rapid growth in research on π-conjugated polymer nanostructures, which has been driven by their unique electrochemical and electronic properties, as well as by the processing advantages of polymers relative to inorganic electronic materials, including nanowires, nanotubes, nanoarrays,4-8 etc. Conjugated organic polymers are straightforwardly prepared by chemical or electrochemical methods,9-13 and their electronic states can be reversibly changed between insulating and conducting states by electrochemical redox reactions. Due to the poor solubility in organic solvents and the difficulty in forming huge single crystals, these polymers could hardly selfassemble into ordered nanostructures. To date, conductive polymer nanostructures can be grown by various methods that can be divided in the following categories: hard template,14-16 soft template,17-19 and template-free methods.20-22 The hard template is usually a thin porous aluminum oxide film. It has been performed by methods such as pressure injection,23 vapor deposition,24-27 chemical deposition, electrode position, and other methods.28 The dimensions of the deposited nanostructures can be controlled by regulating the electrochemical conditions. A variety of organic nanowires and nanotubes have been successfully fabricated with precise control over their length

and wall thickness.12,29 We expand the technique for fabricating aligned nanowires and nanotubes with a control over the growth length and wall thickness to meet the needs of future device fabrications. For example, the soft and controllable nanowires and nanotubes probably show significant effects in flexible electronics. Previous studies of conjugated organic polymer nanowires and thin films have shown that their electronic behavior is complex and appears to depend on the structure of the polymer, which in turn depends on the synthetic conditions, doping level,30,31 charge balancing ions,32 etc. Doping of intrinsically conducting polymers are materials that utilize conjugated polymers that can be doped by inorganic or organic molecules. The goal is to produce a new doping material that has distinct properties that were not observed in the pure conjugated polymer. This may result in either new or improved chemical properties that can be exploited for nanoelectronic devices. Fluorescent dipyrroles in which electron-donor pyrrole rings are linked through an aromatic or other electron-acceptor bridge attract interest as precursors of extended π systems that can be used in the field of photoelectric technologies.33 In the present work, we aim to demonstrate that low dimension structured nanowires and nanotubules of conducting polymer PBPB can be fabricated via combination of nanoporous anodic aluminum oxide (AAO) template and electrochemical polymerization methods. The procedure demonstrated here is remarkable for its simplicity, for its effectiveness in forming uniform nanowires and tubules, and for its superior control over the diameter and morphology. The aggregates of polymer dipyrroles are expected to exhibit new size-based properties that are intermediate between PBPB molecules and doping anionic, and incorporate a wide range of elemental and material compositions, including organics, anionic ions, inorganic ions, and hybrid structures. Here, we focus on the combination and of electrochemical polymerization and hard template methods for controllable synthesis of highly conductive nanostructures of PBPB. Experimental Section

* To whom correspondence should be addressed, [email protected]. † Institute of Chemistry, Chinese Academy of Sciences. ‡ Graduate University of Chinese Academy of Sciences.

Materials and Methods. 1,4-Benzenedicarbonyl chloride, tetrabutylammonium perchlorate, triphosgene, and potassium

10.1021/jp103911x  2010 American Chemical Society Published on Web 07/08/2010

Growth Control of Nanostructures tert-butoxide were purchased from from Alfa Aesar Corp. Propargylamine was purchased from Aldrich Corp. Other reagents were purchased from Beijing Chemical Reagent Corporation, China. The AAO templates were purchased from Whattman Co. 1H and 13C NMR spectra were obtained at Bruker ARX400 spectrometer using tetramethylsilane (TMS) as internal standard, and chemical shifts (δ) are given in ppm relative to TMS. Field emission scanning electron microscopy (SEM) images and energy-dispersive X-microanalysis spectrum (EDS) were taken from a Hitachi S-4300 FESEM microscope at an accelerating voltage of 15 kV. Transmission electron microscopy (TEM) images and selective-area electron diffraction pattern (SAED) patterns were taken from a JEOL JEM-1011 microscope at an accelerating voltage of 200 kV. Confocal laser scanning microscopy (CLSM) images were acquired with a WITec CMR200 in the confocal Raman spectra mode using Hg lamp. Current-voltage (I-V) characteristics of devices were recorded with a Keithley 4200 SCS and a Micromanipulator 6150 probe station in a clean and shielded box at room temperature in air, as well as a Lakeshore XTPP6 lowtemperature vacuum probe station. Synthesis of 1,4-Bis(pyrrol-2-yl)benzene (BPB). 1,4Bis(pyrrol-2-yl)benzene (BPB) was synthesized via a three-step route.34 The BPB was obtained in an overall yield of about 8% based on 1,4-benzenedicarbonyl chloride as the starting reagent. Characterization of 1,4-Bis(pyrrol-2-yl)benzene (BPB). 1H NMR spectrum (CDCl3), δ, ppm: 8.43 s (2H,NH), 7.48 s (4H,oH), 6.88 s (2H, 5-H), 6.53 s (2H,4-H), 6.31 s (2H, 3-H). 13C NMR spectrum (CDCl3), δC, ppm: 131.03, 130.88 (C2,Ci), 123.54 (Co), 119.03 (C5), 109.03, 105.14 (C3,C4). HRMS calcd for C14H12N2 208.10 (M+), found 208.00. Electrochemical Template Synthesis Nanowires or Nanotubes. PBPB/ClO4- nanowires were synthesized by a templateassisted electrodeposition method. A layer of Au was evaporated on the one side of the AAO template as conducting layer, and the template was put into a homemade electrolytic cell as a working electrode with a platinum counter electrode and a saturated calomel electrode (SCE) reference electrode. The electropolymerization activity of BPB starts around +0.6 V (vs SCE) (see the cyclic voltammetry scan graph, Figure S1 in Supporting Information). Poly[1,4-bis(pyrrol-2-yl)benzene] (PBPB) nanowires were harvested from a 10 mM acetonitrile solution of BPB and 0.05 M tetrabutylammonium perchlorate (TBAP) by applying a voltage of 0.6 V (vs SCE) for different times. The same procedure as for above was used, the only change was the voltage to synthesized different wall thickness nanotubes. Results and Discussion BPB was electropolymerized in the AAO template (one side coated with Au layer) from a 10 mM BPB and 0.05 M tetrabutylammonium perchlorate (TBAP) acetonitrile solution by applying appropriate voltage. We can get PBPB/ClO4nanostructures (Figure 1). We used the modified template technique by AAO template, which is a well-established selforganizing technique used to develop nanostructures,35,36 and it is a very unique electrochemical technique that forms uniform, well-organized, high-density pores. In this article, we can contribute to form regular low dimension nanostructures by using the technique; the nanostructures are uniform in diameter and length. Initially, nanowires are fabricated by electropolymerization into anodic alumina templates and the approach allows us to both fabricate and organize nanostructures over large areas on

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Figure 1. Model of the PBPB low dimension nanostructures.

Figure 2. A model of the growth processes of PBPB nanowires by electropolymerization. The scheme shows methods to fabricate nanowire or nanotube arrays onto nanoporous anodic alumina template films, and then the alumina is removed.

substrates in the same process. Electropolymerization is a versatile method for fabricating nanostructures because the morphology and thickness of the deposition can be easily controlled. Polymerization time is a key factor to control, when growth time was 1500 s, the nanowires can reach 5 µm. If the growth time was 21600 s (6 h), the length of nanowires was about 30 µm, as shown in Figure 2. Figure 2e shows nanowire arrays when the alumina template is removed by NaOH solution (6 M). The observation of results is summarized in Figure 3. Figure 3a shows well-ordered nanowire arrays with the alumina template dissolved. The diameter and length of the nanowires are generally uniform, and the surface is smooth and clean. We can also observe the nanowire structure from a side view (Figure 3b). We speculated that the conducting polymer formation was possibly nanotube-like by observation of SEM images from Figure 3c; however, taking into account the transmission electron microscope image, we can easily conclude that the nanowires form with an open-ended tip, because the polymer grows faster along the pore wall than in the middle, as shown in the yellow dotted line (Figure 3f). In addition, EDS measurements on PBPB/ClO4- nanowires had only C, N, O, and Cl elements. A branched structure of the nanowires was also observed (Figure 3d, marked with “1”) attributed to the branched structure of the alumina pores. In Figure 3d, the bright parts of the nanowires (marked with “2”) were parallel growth by two independent nanowires on the Au layer. Finally, according to the analysis above, we describe the model of PBPB/ClO4- nanowire in Figure 4a. It should be mentioned that these PBPB/ClO4- nanowires possess excellent flexibility; the single nanowire shows a soft characteristic (Figure 4b). When the nanowires were manipulated with mechanical probes under an optical microscope, it was found that the nanowires did not fracture even when they were bent at any

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Figure 5. Model of the growth processes of PBPB/ClO4- nanowires (top) or nanotubes (below) by different potentials: low overpotentials ) 0.6 and 0.65 V; high overpotentials ) 0.7, 0.75, and 0.8 V.

Figure 3. Images of PBPB/ClO4- nanowires by electropolymerization. (a, c) Top view SEM images of PBPB/ClO4- nanowires. (b) Side view SEM images of PBPB/ClO4- nanowires. (d) EDS of PBPB/ClO4nanowires on silicon substrates. Only elements C (wt % 92.61), N (5.02), O (2.21), and Cl (0.15) were detected. (e, f) TEM images of the PBPB/ClO4- nanowires removed from the substrate (e) and (f). (e, inset) SAED taken from a segment in the single PBPB nanowire shows its noncrystalline characteristic.

Figure 4. (a) Model of nanowire and its section. (b) Nanowires show a soft characteristic. (c) Fluorescence of PBPB nanowires (excitation at 330 nm). (d) A bright-field image of a single nanowire. (e) A fluorescence (visible blue-green light) image of the same nanowire, removed from the alumina membrane (excitation from 330 to 380 nm).

angle, such as nanowires bent in U-like, S-like, and O-like shapes. The PBPB/ClO4- nanowires can be well dispersed in the ethanol without dissolution. CLSM was used to investigate the photoluminescence of the nanowires. Photoluminescence of the nanowires was collected from the excitation at 330-380 nm. Importantly, fluorescence studies clearly show a broad

emission centered around 400 nm (Figure 4c, black line); the fluorescence is caused by the extended π conjugated systems. The sample was prepared by naturally drying drops of the PBPB/ ClO4- nanowire solution on a glass slide. The optical micrograph of a selected single nanowire is shown in panels d and e of Figure 4. In Figure 4e the nanowire shows a visible blue-green light consistent with the fluorescence map. The corresponding fluorescence images confirmed that the fluorescence indeed originated from the single nanowire. Martin’s group proposed a mechanism based on electrostatic and solvophobic interactions between the nascent polymer and pore wall to explain the nanotubes growth in template pores.38 The polymer grows preferentially along the pore wall to form tubular structures. As polymerization proceeds further, the polymer grows inwardly to form nanowires. Figure 6d presents SEM images of PBPB/ClO4- nanowires and nanotubes obtained by 0.60, 0.65, 0.70, 0.75, and 0.80 V, respectively. At low overpotentials (0.6 and 0.65 V), the monomers have enough time to diffuse into the pore bottom under the slow reaction rate (Figure 5a), this leads to rigid, dense nanowires (Figure 6d, 0.6 and 0.65 V). The other is for high overpotentials (g0.7 V). The polymerization reaction is very fast in this situation. The diffused monomers are immediately consumed to elongate the polymer chain. The thickness is uniform from the top to the bottom of the nanotubes, showing that the formation of the PBPB/ClO4- is uniform along the pores of the template, as shown in Figure 5b. Because the reaction initiates along the electrode surface at the pore bottom, the conductive polymers are continuously deposited along the pore wall.37 This produces long, porous, thin-walled nanotubes, and the higher the oxidation potentials (from 0.70 to 0.80 V), the thinner the walls of the nanotubes (Figure 6d). The nanotubes are different in wall thickness, which is 50, 33, and 25 nm, respectively. Taking advantage of this phenomenon, we can precisely control the tube size and the wall thickness simply by adjusting the oxidation potential. While the oxidation potential is too high (>0.8 V), there is no complete nanotube in the template (see Supporting Information, Figure S3). These results show that the wall thickness decreases with increasing oxidation potential. In order to measure the conductive properties of the PBPB/ ClO4- low dimension nanostructures, we use the nanowires as a model (oxidation potentials was 0.6 V), the PBPB/ClO4nanowires aligned between two gold stripes (lighter color in SEM images, Figure 7c-g) on silica (darker color in SEM images, Figure 7c-g), and two conductive probes were contacted with the gold stripes. Schematic of the experimental setup

Growth Control of Nanostructures

Figure 6. (a) SEM image of PBPB/ClO4- nanotubes (0.75 V). (b, c) TEM images of PBPB/ClO4- nanotubes (0.75 V). (d) PBPB/ClO4nanotube wall thickness variation with applied potential (TEM images). When the applied potential is 0.6 and 0.65 V, we used the radius of nanowires as the wall thickness under this condition. Scale bar, 100 nm.

J. Phys. Chem. C, Vol. 114, No. 30, 2010 12985 ing typical current-voltage (I-V) curves (Figure 7a). As shown, Figure 7a displays the current-voltage (I-V) traces of a single PBPB nanowire doped with different ClO4-. The conductivity of nanowires increased dramatically as the tetrabutylammonium perchlorate (TBAP) concentration was increased (before 0.2 M). For example, the conductivity of BPB/ClO4- nanowires synthesized at 0.6 V with 0.05, 0.1, 0.2, 0.3, and 0.4 M TBAP were 8.64 × 10-7, 5.07 × 10-6, 6.79 × 10-4, 1.22 × 10-4, and 4.07 × 10-4 S/cm, respectively. The blue line in Figure 7a, the I-V characteristic curve of PBPB/ClO4- nanowires made with an electrolyte concentration at 0.2 M TBAP and measured at room temperature, shows good conductivity behavior and was much higher than that of 0.05 and 0.1 M TBAP, the electron flow within the doped nanowires enhanced by increasing the amount of TBAP. When the electrolyte concentration increased (from 0.2 to 0.4 M), the conductivity generally remained at the same level. EDS experiments also have been carried out, when the electrolyte was more than 0.2 M and the concentration of Cl element was a constant value (see Supporting Information, Table S1). The results indicated that the conductivity increased rapidly with the doping level increased, when the doping level was lower. The change of conductivity was slight with an increase of electrolyte concentration, when electrolyte concentration was between 0.2 and 0.4 M, suggesting the conductivity of a single PBPB/ClO4- nanowire depends strongly on the doping level. Conclusion We have successfully demonstrated the strategy of a combination of the modified template technique and electrochemical polymerization method for synthesizing nanowires and nanotubes. The nanowires exhibited very interesting flexibility with formation of U-like, S-like, and O-like structures and showed excellent properties of solid photoluminescence and conductivity. Our results confirmed that the conductivity property of nanowires can be controlled by tuning the doping level. We believe that the association technique for direct construction of low dimension nanostructures precisely controls the nanomaterial size and shape. Controllable electrical properties of the nanostructures will guide the development of organic intelligent devices and circuits on the nanoscale. Acknowledgment. This work was supported by the National Nature Science Foundation of China (20831160507, 10874187, 20873155, and 20721061), the National Basic Research 973 Program of China, and the NSFC-DFG joint fund (TRR 61). Supporting Information Available: Figures showing cyclic voltammogram of electropolymerization of BPB and SEM images of PBPB nanowires and nanotubes and a table of EDS of PBPB nanowires with different electrolyte concentrations. This material is available free of charge via the Internet at http:// pubs.acs.org.

Figure 7. Typical current-voltage (I-V) curves of a single PBPB/ ClO4- nanowire measured at five different electrolyte concentrations: (c) 0.05 M TBAP, (d) 0.1 M TBAP, (e) 0.2 M TBAP, (f) 0.3 M TBAP, (g) 0.4 M TBAP. (a) The inset shows the field dependence of the nanowire resistance at room temperature. (b) Schematic of the experimental setup. Nanowire devices with lengths of 8.0 (c), 4.3 (d), 19.0 (e), 7.5 (f), and 13.3 µm (g). Panels c, d, e, f, and g in (b) are SEM images of each device.

is shown in Figure 7b. The electrical characteristics of the individual PBPB/ClO4- nanowires were investigated by record-

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