Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học
Fabrication of a porous polyimide membrane using a silicon nanowire array as a template Woong Kim ⁎ , Myung-Ki Lee Department of Materials Science and Engineering, Korea University, Seoul 136-713, South Korea abstractarticle info Article history: Received 10 October 2008 Accepted 17 January 2009 Available online 26 January 2009 Keywords: Nanomaterials Polymers Porosity Membranes Nanowires Polyimide We demonstrate that a porous polyimide membrane can be fabricated by curing liquid polyimide on a vertically oriented silicon nanowire array and selectively etching away the nanowire-array-template using xenon difluoride (XeF 2 ). Pore size and density using the described technique are controllable. The former is dependent on nanowire diameter and the duration of etching, whereas pore density is determined by silicon nanowire density. We believe that the described porous membrane fabrication method can be applied to various polymer and nanowire systems. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Porous polymer membranes have many applications in biotech- nology and electronics for the separation and/or filtration of gases, biomolecules, and environmentally hazardous materials, and as in- sulating materials with low dielectric constants [1–4]. It has also been recently demonstrated that porous polymer membranes can be used as templates to synthesize silicon nanowires electrochemically [5]. Among the many polymers available, polyimide (PI) has been widely used in academia and industrially due to its high thermal stability, good chemical resistance, and excellent mechanical properties [6]. Porous polyimide membranes have been prepared via the phase inversion of cast films [7], by irradiation with energetic heavy ions and subsequent oxidization [8], and by the decomposition of thermally labile domains in phase-separated block copolymers [4]. In this letter, we describe the preparation of a porous polyimide membrane using a vertically oriented silicon nanowire (SiNW) array as a template. Since the diameters and densities of silicon nanowires can be controlled, polyimide membranes with predetermined pore densities and diameters can be readily produced using the described technique. 2. Experimental procedures Silicon nanowires were synthesized on silicon (111) substrates by chemical vapor deposition (CVD) as described elsewhere [9]. Briefly, gold (Au) nanoparticles were deposited on silicon substrates as catalytic seeds. SiCl 4 (the silicon source) was then introduced into a CVD reactor containing a silicon substrate at 850 °C using H 2 (10%) in Ar as a carrier gas. Silicon nanowires grew vertically from substrates. Polyimide solution (PI-2556) was purchased from HD Microsystems. A drop of this solution was applied onto silicon substrates with nanowires. The solution was cur ed at 200 °C for 30 min in a nitrogen stream, which resulted in a solid polyimide membrane over the embedded nanowires. The membra ne was then slightly etched with O 2 plasma (20 min at 1 00 W in a 50 sccm O 2 stream) to expose the silicon nanowire tips. A u nanoparticles at the tips of the nanowires were then etched using potassium iodide and iodine (KI/I 2 ) solution, and the substrate was thoroughly rinsed with deionized (DI) water. To selectively etch the silicon nanowires and lea ve the polyimide membr ane intact, XeF 2 etching was c arried out over 5 0–200 cy cles of exposure t o 4 Torr of XeF 2 and 2 Torr of N 2 for 60 s . Polyimide membr anes were e ither detached from silicon substr at es aft er this stage o r were detached by dipping substr at es in buffered hydrofluoric acid (BHF) so lution. F or scanning ele ctron microscopy (S EM) characterizations, a bout 5 nm of Au was sputter ed onto the poly imide membranes produced. 3. Results and discussion The procedure used to fabricate the porous polyimide membranes is summarized in Fig. 1.Asafirst step, silicon nanowires were epitaxially grown on Si (111) substrates by CVD. Since nanowires grow preferentially in theb111Ndirection under the conditions used, they were oriented vertically to the substrate. An SEM image of an angled view of the vertically aligned silicon nanowires is shown in Fig. 1a. Since nanowires grow via a vapor liquid solid (VLS) mechanism, gold nanoparticles (AuNPs) are retained at the tips of the nanowires; these appeared as bright dots in SEM images (Fig. 1a). Materials Letters 63 (2009) 933–936 ⁎ Corresponding author. Tel.: +82 2 3290 3266; fax: +82 2 928 3584. E-mail address: woongkim@korea.ac.kr (W. Kim). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.01.060 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet Fig. 1. Fabrication process used to produce porous polyimide membranes using silicon nanowire arrays as templates; (a) synthesis of silicon nanowires, (b) polyimide coating, (c) polyimide etching with O 2 plasma, and (d) silicon etching with XeF 2 . A schematic of a cross sectional view is shown under each SEM image. Fig. 2. SEM images of porous polyimide membranes with pore density of (a, b) ~1 pore/100 μm 2 , and (c, d) ~10 pores/100 μm 2 . 934 W. Kim, M K. Lee / Materials Letters 63 (2009) 933–936 The nanowire array substrates were covered with polyimide solution, which was dropped onto substrates. Final membrane thickness can be adjusted by simply altering the amount of solution applied. About 5–10 μl of polyimide solution was found to be appropriate for a silicon substrate of 0.5 by 0.5 cm. Substrates treated with polymer solution were cured at 200 °C. This process results in the formation of polyimide membranes on nanowires. The film becomes extremely thin at the nanowire tips as shown in Fig. 2b. To expose nanowires, the polyimide membrane was etched with O 2 plasma. Etching conditions, e.g., power, duration, and O 2 flow rate, were optimized to expose only the upper portions of nanowires without appreciably damaging the supporting polyimide. An SEM image of the exposed portion is shown in Fig. 1c. Having exposed the nanowires, the Au nanoparticles at the nanowire tips were removed using gold etchant (KI/I 2 ) solution [10]. Finally, the silicon component of nanowires was selectively removed using XeF 2 as etchant. Etch rate of polyimide is negligible Fig. 3. Silicon nanowires with various diameters; (a) ~50, (b) 90, (c) 150, and (d) 220 nm. Nanowires were synthesized from gold nanoparticles with diameter of ~ 30, 50, 100, and 150 nm, respectively. Fig. 4. SEM images of porous membranes with various pore sizes; (a) ~800 nm, (b) 1 μm, and (c) 1.5 μm. Pore sizes are dependent nanowire diameter, which in the present study were 90, 150, and 400 nm, respectively. Pore size is dependent on etching time, e.g., (a) ~800 nm at 200 cycles vs. (d) ~170 nm at 50 cycles. 935W. Kim, M K. Lee / Materials Letters 63 (2009) 933–936 compared to that of silicon [10]. Fig. 4b shows the top surface of a pore- containing polyimide membrane after this selective etching process. As mentioned earlier, membrane pore density is wholly dependent on nanowire density, which is in turn determined by gold nanoparticle density . Moreover , there is usually a near one to one r elation between nanowire and gold nanoparticle numbers [11]. Fig. 2 shows two membranes with different pore densities. A membrane with a pore density o f ~1/100 μm 2 is shown in SEM images with d ifferent magnifications (Fig. 2aandb),andamembranewithdensityof~10 pores/100 μm 2 is shown in Fig. 2candd. The por e s ize c a n a lso be contr olled, as it is dependent on two fact ors only; namely , template-nanowire diameter and the duration of XeF 2 etching. On t he other hand, wire diameter is determined by gold nanoparticle size and i s s lightly larger than t he AuNP seeds. Fi g. 3 shows nanowires with various diameters of ~50, 90, 1 50 and 220 nm syn- thesized from Au nanoparticles with diameter of ~30, 50 100 and 150 nm, respectively. Fig. 4 a, b, and c show polyimide membranes fab ricated using silicon nan owires with diameter of ~90, 150, and 400 nm, respectively. After exp osure to XeF 2 for 200 cycles, the resulting pore diameter were ~800 nm, 1 μmand1.5μm, respectively. Etching time also affected pore siz e. For exam ple, 90 nm silicon nan owires resulted in pore diameters of ~170 nm when membranes were exposed to XeF 2 for 50 cycles (Fig. 4d), while the final pore diameter was ~800 nm when 200 cycles were used (Fig. 4a). Interestingly, when etching time was reduced, the original hexagonal cross sections of the nanowires were transferred to the pores (Fig. 4d). Further optimization of the descri bed process is expected to inc rease the ranges of the pore diameters and densitie s formed. 4. Conclusions Our studies indicate that silicon nanowires can be used as sacrificial templates for the fabrication of porous polyimide mem- branes. Moreover, since the densities and diameters of silicon nano- wires can be easily adjusted, membranes can be fabricated with pre- determined pore densities and sizes. In the present study, membranes were successfully fabricated with pore diameters ranging from 170 nm to 1.5 μm and densities ranging from 0.1 to 1 pore/10 μm 2 . Furthermore, the novel fabrication technique described can be applied to the fabrication of porous membranes from different polymers and template nanowires comprising different materials. Acknowledgement This work was supported in part by the Korea Science and Engineering Foundation through the Pioneer Converging Technology Program (No. M10711160001-08M1116-00110). References [1] Pandey P, Chauhan RS. Prog Polym Sci 2001;26:853–93. [2] Ulbricht M, Yang H. Chem Mater 2005;17:2622–31. [3] Mendelsohn JD, Barrett CJ, Chan VV, Pal AJ, Mayes AM, Rubner MF. Langmuir 2000;16:5017–23. [4] Hedrick JL, Miller RD, Hawker CJ, Carter KR, Volksen W, Yoon DY, et al. Adv Mater 1998;10:1049–53. [5] J. Mallet, M. Molinari, F. Martineau, F. Delavoie, P. Fricoteaux, M. Troyon, Nano Lett 2008;8:3468–74. [6] Wilson AM. Thin Solid Films 1981;83:145–63. [7] Echigo Y, Iwaya Y, Saito M, Tomioka I. Macromolecules 1995;28:6684–6. [8] Trautmann C, Bruchle W, Spohr R, Vetter J, Angert N. Nucl Instrum Meth Phys Res Sect B-Beam Interact Mater Atoms 1996;111:70–4. [9] Hochbaum AI, Fan R, He RR, Yang PD. Nano Lett 2005;5:457–60. [10] Williams KR, Gupta K, Wasilik M. J Microelectromech Syst 2003;12:761–78. [11] Wang DW, Tu R, Zhang L, Dai HJ. Angew Chem Int Ed 2005;44:2925–9. 936 W. Kim, M K. Lee / Materials Letters 63 (2009) 933–936 . Fabrication of a porous polyimide membrane using a silicon nanowire array as a template Woong Kim ⁎ , Myung-Ki Lee Department of Materials Science and. polyimide membrane using a vertically oriented silicon nanowire (SiNW) array as a template. Since the diameters and densities of silicon nanowires can be controlled, polyimide