ARTICLE IN PRESS Ultramicroscopy 108 (2008) 1224– 1227
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Electrochromic nanostructures grown on a silicon nanowire template Yuna Kim a, Jehoon Baek a, Myoung-Ha Kim b, Heon-Jin Choi b,, Eunkyoung Kim a, a b
Department of Chemical Engineering, Yonsei University, 134 Shinchon-dong, Seodaemun-gu, 120-749 Seoul, Republic of Korea Department of Materials Science and Engineering, Yonsei University, 134 Shinchon-dong, Seodaemun-gu, 120-749 Seoul, Republic of Korea
a r t i c l e in f o
PACS: 73.63.Rt 79.60.Jv 79.60.Fr Keywords: Si nanowires Nanostructures PEDOT Electrochromic device
a b s t r a c t Vertically grown Si nanowires were prepared as a nanotemplate for conducting polymers. Electrochromic (EC) PEDOT (poly(3,4-ethylenedioxythiophene)) layer was successfully grown on Si nanowires by electrochemical polymerization method to form PEDOT nanowires having average wall thickness of 60 nm. As-prepared conductive nanowire electrode was applied to a low voltage working EC device by fabricating an all solid state EC device. The EC properties of the device were enhanced in the nanowire structure, showing reversible fast optical transition by applying 72 V. The response time (tR) of the EC device from the PEDOT grown on Si nanowires was 0.7 s, which was much faster than that from PEDOT film coated on ITO glass electrochemically (tR ¼ 1.9 s). & 2008 Elsevier B.V. All rights reserved.
1. Introduction As the faster and greater response of electrical or optical signal is getting important, modification of functional surface by nanostructures becomes an essential factor in electronic, switching or sensing devices [1–4]. Thus several methods such as vapor–liquid–solid process [5], self-assembly [6], and template fabrication [7,8] have been suggested to assemble desired nanostructures. A variety of materials including metals, nanoparticles, conductive polymers can be deposited onto the microtemplates physically or chemically [9,10]. In the electrochromic (EC) process, electrons are injected or extracted under an applied voltage, and at the same time, charge balancing counter ions are transported into or out of the EC layer [11]. Accordingly, the transport of electrons and ions are directly related to the EC properties including coloration efficiency and response time. Recently, EC devices fabricated using a nanostructured EC electrode have been reported to achieve fast response and high color contrast since the nanostructures can provide large surface area [12], which could facilitate ion transport and provide more accessible reacting sites for charge/ discharge process [13–18]. Such a nanostructured EC electrode could be prepared by the layer by layer (LBL) deposition [13,14] or nanotemplate fabrication using such as anodized aluminum oxide (AAO) template, [15] polycarbonate membrane [16] and block copolymer assisted sol-gel methods [17,18]. Although several lateral type of nanostructures in EC electrode has been reported
Corresponding authors. Tel.: +82 2 2123 5752; fax: +82 2 312 6401.
E-mail addresses:
[email protected] (H.-J. Choi),
[email protected] (E. Kim). 0304-3991/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2008.04.054
and found, vertical types of EC nanostructures are rare [15,16,19]. As the charge transport in EC device occurs vertically between the working and counter electrode, it is a challenge to explore a vertical type of EC nanostructure. As a vertical template we adopt Si nanowires, which can be coated with gold to afford conductive nanowires. Si nanowires have been considered as a nanotemplate for interconnector or the channels of field-effect transistors and sensors [20–22]. Herein we report EC device prepared from a vertically grown nanostructured EC electrode, which was prepared from direct electrochemical polymerization of 3,4-ethylenedioxythiophene (EDOT) on silicon nanowires grown on Si wafer.
2. Experiment 2.1. Synthesis of nanowires Vertically aligned Si nanowires were synthesized from silicon tetrachloride (SiCl4, Alfa, 99.999%) as the silicon source on Si (111) substrates, and Au catalyst layer by CVD process. Carrier gas was transferred from the source precursor through a bubbler to the quartz reactor, and H2 was then introduced onto the system at a flow rate of 20 sccm. H2 (100 sccm) and Ar (100 sccm) gas were used as diluent gases, which regulate the concentration of the mixture containing SiCl4 vapor and carrier gas. Typically, the system was heated to 900 1C and maintained for 30 min as SiCl4 was supplied. The flow was then cooled to room temperature. The nanowires uniformly covered the entire substrate and the diameter and length were 100–140 nm and tens of micrometers, respectively.
ARTICLE IN PRESS Y. Kim et al. / Ultramicroscopy 108 (2008) 1224–1227
2.2. Electro-deposition of PEDOT
3. Results and discussion
The 3,4-ethylenedioxythiophene, EDOT (0.5 M, Aldrich) was dissolved into electrolyte solution containing acetonitrile and 0.1 M of lithium perchlorate. All other chemicals were also purchased from Aldrich. The growth of PEDOT on nanotemplate was carried out through potential scanning between 0 and 2 V vs. Ag/AgCl introducing silicon nanowire template as a working electrode and stainless steel as a counter electrode.
3.1. FE-SEM image of the vertically growth of Si nanowires
2.3. EC device fabrication Ion permeable polymer electrolyte was made as previously reported [13,14]. Si nanowire template contained EC device was assembled by introducing the photopolymerizable electrolyte solution between PEDOT deposited Si nanowire electrode and a counter electrode (indium tin oxide (ITO) glass). Then the EC device was irradiated for 15 min with UV lamp for photocuring of the electrolyte, to afford an all solid state EC device.
2.4. Instrumentation The nanostructures of Si nanowires and PEDOT layer were examined by FE-SEM (JSM-6701F JEOL Inc.). Cyclic voltammogram (CV) was carried on a potentiostat from CH Instruments (model: CHI624B). In situ spectroelectrochemical setup for measuring EC properties was consisted of UV spectrometer (Avaspec-2048 fiber optic spectrometer) and CHI624B (CH Instruments Inc.).
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Vertically grown Si nanowires were prepared as a nanotemplate to grow nanostructured conducting polymers. Fig. 1 shows FE-SEM image of the Si nanowire template coated with gold. The Si wire was grown vertically with homogeneous surface. The average diameter and length of the Si nanowires were 138 nm and 20 mm, respectively. The Au tip on top of the Si nanowires was appeared as bright tip. The average diameter and length of the gold tip were determined as 145 and 277 nm, respectively, from the FE-SEM image.
3.2. Growth of PEDOT layer on Si nanowire Electrochemical polymerization of EDOT was performed on Au-coated Si nanowires by successive potential cycling from 0 to 2 V (vs. Ag/AgCl) using a cyclic voltammetry [23]. Fig. 2 shows CV of the electrochemical polymerization of EDOT in a supporting electrolyte of LiClO4 in CH3CN, with a scan rate of 100 mV/s. The oxidation potential of EDOT was 1.23 V and new broad peak appeared at 0.6 V which is the characteristic of PEDOT (Fig. 2(a)). The electrochemical polymerization behaviour of PEDOT on Si nanowires was well matched to that of PEDOT deposited on ITO glass (Fig. 2(b)). The key variables in polymer growth on Si nanowires were applied potential, potential cycles, monomer concentration and electropolymerization scan rates. In particular, potential cycles were adjusted as three cycles to control the size of
Fig. 1. FE-SEM image of vertically grown Si nanowires (a) and gold tip coated on Si nanowires (b).
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Fig. 2. Electrochemical polymerization of PEDOT on Si nanowire template (a) and ITO glass (b) in a supporting electrolyte of LiClO4 in CH3CN with scanning rate at 0.1 V/s.
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Fig. 3. FE-SEM image of the growth of poly(3,4-ethylenedioxythiophene) on the surface of Si nanowires (a) and its magnified image (b).
ITO glass electrode Polymer electrolyte Spacer PEDOT-Si nanowire electrode Fig. 4. Structure of the PEDOT–Si nanowire based electrochromic device.
PEDOT wires. Upon polymerization, the color of the Si nanowire electrode was changed to blue, indicating the formation of PEDOT. The growth of PEDOT was further confirmed by FE-SEM. Fig. 3 shows FE-SEM image of the Si nanowire templates coated with gold, which become thicker upon additional coating with PEDOT as compared to the bare Si nanowire in Fig. 1. Upon electropolymerization of EDOT, the nanowires became aggregated as the conductive polymer was grown from the gold tip. To our surprise, PEDOT layer was coated even on the bare silicon nanowires wrapping whole nanowires as shown in Fig. 3(b) in the magnified image. The average diameter of the nanowires was determined as 250 nm after electro-deposition, suggesting that average wall thickness of the PEDOT layer is 60 nm. This result indicates that the gold tip serves as the seed for the polymerization and the initially formed cation radical of the EDOT macromers formed on the gold tip are grown by coupling with new EDOT molecules in solution to cover entire Si nanowires.
nanowires were matched to that of PEDOT deposited on ITO glass as compared in Fig. 5(b). However, it was noteworthy that the reduction peak of PEDOT on Si nanowires from the three electrode system was shifted to 0.1 V, indicating that the reduction was easier when PEDOT was coated on the Si nanowires as compared to that on an ITO glass (Ered 0.5 V). Such a difference may arise from the nanostructure of Si wire, which can facilitate electron transfer more efficiently. The color of the device was stable even after the electricity on the device was off. On the other hand, the dark blue color was faded when the polarity of the potential was reversed. The bleached state was also stable when the electricity was off. Such a bistable color change was apparent in the visible spectra resulting from the absorbance increase at visible range when oxidized and reduced (Fig. 6). The EC response was reversible. The EC response of the nanostructured EC device using the vertically grown Si nanowire templates was monitored at 592 nm under the applied potential step of 72 V (Fig. 7). The color of the EC device was reversibly switched between the bleached and colored state under the potential step. The coloration efficiency was determined as 12 cm2/C from the absorbance change against charge consumption [24]. The response time (tR) of the EC device from the vertically grown Si nanowires was (Fig. 7(a)) 0.7 s, which is much faster than that from PEDOT film coated on ITO glass electrochemically (Fig. 7(b), tR ¼ 1.9 s). This result suggests that electron/ion transport in the EC device from the PEDOT on Si nanowires become fast, possibly due to the large surface area of the Si nanowires. Further effects of Si nanowires on electrochemical and optical properties of conducting polymer electrode are in progress.
3.3. EC properties of PEDOT on Si nanowires An electrochemical cell was constructed as shown in Fig. 4. The PEDOT on Si nanowire was used as a working electrode and polymer electrolyte containing lithium salt was introduced as an ion conducing polymer layer for an all solid state EC device. EC properties were determined from an in situ spectroelectrochemical setup equipped with a UV–Vis spectrometer and an electrochemical analyzer. Fig. 5(a) shows CV of the all solid state EC device. In the two electrode device, the CV shows much wider potential range compared to that of the three electrode system in liquid electrolyte system (Fig. 5(b)). When the applied potential was 2 V, the solid state two-electrode EC device showed a dark blue color. In the three electrode system, the color change occurred at the potential below 0 V. Therefore, the peak below 2 V in the solid state CV could be assigned as the reduction of PEDOT on Si nanowire. The redox potentials of PEDOT on Si
4. Conclusion Si nanowires having diameter of 138 nm were vertically grown on a silicon wafer and coated with gold to prepare a nanotemplate for conducting polymers. Electrochromic PEDOT layer was grown on the Si nanowires from gold tip and extended to the entire wires by electropolymerization, to afford PEDOT nanowires having average wall thickness of 60 nm. The redox potentials of PEDOT on Si nanowires were well matched to that of PEDOT deposited on ITO glass. An all solid state EC device fabricated from the PEDOT on Si nanowires showed much faster electrochromic response within 0.7 s compared to that on ITO glass, suggesting that Si nanowire template is appropriate for the fine tuning of the electronic properties of conductive films.
ARTICLE IN PRESS Y. Kim et al. / Ultramicroscopy 108 (2008) 1224–1227
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Fig. 5. Cyclic voltammogram of Si nanowire based EC device (a) and three electrode system in monomer free liquid electrolyte system (Si nanowire based; solid line, ITO glass based; dashed line) (b).
Acknowledgments
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This work was supported by MOCIE (Ministry of Commerce, Industry, and Energy), Korea and Seoul R&DB and H.-J. Choi also acknowledges the partial support from the NRL.
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Fig. 6. UV–Vis absorbance change of EC device according to the applied potentials at (a) 2.7 V, (b) 2.4 V, (c) 2.1 V, (d) 1.8 V, (e) 1.5 V, (f) 1.2 V, (g) 0.8 V, (h) 0.5 V, (i) 0.3 V, (j) 0.7 V, (k) 1.0 V, (l) 1.3 V, (m) 1.7 V, (n) 2 V, (o) 2.3 V, and (p) 2.7 V.
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Fig. 7. Electrochromic response of PEDOT–Si nanowire based EC device under step potential of 72 V monitored at 592 nm (a) compared to that of PEDOT on ITO glass (b).
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