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
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ELSEVIER
Contents lists available at ScienceDirect
Materials Letters
journal homepage: www.elsevier.com/locate/matlet
materials letters
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
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>) 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, (the silicon source) was then introduced into a
* 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
CVD reactor containing a silicon substrate at 850 °C using Hz (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 cured at 200 °C for 30 min in a nitrogen stream, which resulted in a solid polyimide membrane over the embedded nanowires The membrane was then slightly etched with Oz plasma (20 min at 100 W in a 50 sccm QO; stream) to expose the silicon nanowire tips Au nanoparticles at the tips of the nanowires were then etched using potassium iodide and iodine (KI/Iz) solution, and the substrate was thoroughly rinsed with deionized (DI) water To selectively etch the silicon nanowires and leave the polyimide membrane intact, XeF> etching was carried out over 50-200 cycles of exposure to 4 Torr of XeF, and 2 Torr of Nz for 60 s Polyimide membranes were either detached from silicon substrates after this stage or were detached by dipping substrates in buffered hydrofluoric acid (BHF) solution For scanning electron microscopy (SEM) characterizations, about 5 nm of Au was sputtered onto the polyimide membranes produced
3 Results and discussion
The procedure used to fabricate the porous polyimide membranes
is summarized in Fig 1 As a first step, silicon nanowires were epitaxially grown on Si (111) substrates by CVD Since nanowires grow preferentially in the<111>direction 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)
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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, plasma, and (d) silicon etching with XeF A schematic of a cross sectional view is shown under each SEM image
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V x1S0k 2COnm
WD20F4 mmy2S)
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
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 ul 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
QO» plasma Etching conditions, e.g., power, duration, and O> 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/I2) solution [10]
Finally, the silicon component of nanowires was selectively removed using XeF2 as etchant Etch rate of polyimide is negligible
ý x200k 200nr
Fig 4 SEM images of porous membranes with various pore sizes; (a)~800 nm, (b) 1 um, and (c) 1.5 pm Pore sizes are dependent nanowire diameter, which in the present study were
Trang 4compared to that of silicon [10] Fiz 4b shows the top surface of a pore-
containing polyimide membrane after this selective etching process
As mentioned earher, mernbrane 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 relation between
nanowire and gald nanoparticle numbers [J1} Fie 2 shows two
membranes with different pore densities A membrane with a pore
density of ~1/100 uwm* is shown in SEM images with different
magnifications (Fig 2a and b), and a membrane with density of ~10
pores/100 pm? is shown in Fig 2c and ở
The pore size can also be controlled, as itis dependent on two factors
only; namely, template-nanowire diameter and the duration of XeF>
etching On the other hand, wire diameter is determined by gold
nanoparticle size and is slightly larger than the AUNP seeds, Fig, 3 shows
nanowires with various diameters of ~50, 90, 150 and 220 nm syn-
thesized from Au nanoparticles with diameter of ~30, 50 100 and
156 nm, respectively Fic 4 a, b, and c show polyimide membranes
fabricated using silicon nanowires with diameter of ~90, 150, and
400 nm, respectively After exposure to XeF, for 200 cycles, the
resulting pore diameter were ~800 nm, 1 um and 1.5 um, respectively
Etching time also affected pore size For example, $0 nm silicon
nanowires resulted in pore diarneters of ~170 nm when membranes
were exposed to XeF, 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 described process is expected to
increase the ranges of the pore diameters and densities formed
4, Conclusions
Our studies indicate that silicon nanowires can be used as
sacrificial templates for the fabrication of porous polyimide mern-
branes Moreover, since the densities and diameters of silicon nanc-
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 um and densities ranging from 0.1 to 1 pore/10 wm 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)
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