Chapter 3. Hygroscopic micro/nanolenses along carbon nanotube ion channels
3.2 Decoration of salt micro/nanocrystals along SWNTs via exterior transport of ion
We modified the experimental platform for decorating arrays of salt crystals along the nanotubes. A metal cover was deposited on one side of the SWNTs, and then a droplet of 3M NaCl was placed on the other side, as illustrated in Figure 3.1a (right). Upon application of an electric field across the nanotubes using Pt wires, (+) to the solution and (-) to the metal cover, transport of ions occurred along the exterior of the nanotubes. The procedure decorated the nanotubes with arrays of salt crystals within 10 minutes. Figure 3.1b shows an optical micrograph of the crystals formed along the nanotubes between the metal cover and the solution, which optically visualized over 80% of the total nanotubes.
Figure 3.1 Formation of salt micro/nanocrystals along SWNTs via exterior transport. (a) Experimental platform for forming the array of micro/nanocrystals along the nanotube. (b) Optical image of NaCl micro/nanocrystals formed along nanotubes after application of a bias. Scale bar: 50 µm.
a
Nanotube array
Ionic solution (–) (+)
Metal cover
b
Metal cover
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The result suggested the formation of crystals on both metallic and semiconducting nanotubes. The red arrows in Figure 3.1a-b indicate the direction of the ionic transport. For some samples, we shortened the nanotubes using photolithography and oxygen plasma etching prior to ionic transport. This optional step helped to define the meniscus of the droplet by endowing the etched region with hydrophilicity, but did not affect the efficiency of crystal formation. Our approach was applicable to nanotubes having larger diameters (e.g. double-walled carbon nanotubes) (Figure 3.2) as well as to nanotubes on a bare silicon substrate. The salt crystals can be removed non-destructively by simple rinsing with water (Figure 3.3a), and Raman spectroscopy of the nanotubes before and after crystal formation confirmed that no damage to the nanotubes occurred (Figure 3.3b). The salt crystals formed along the nanotubes confirmed their robustness. The shape and size of the crystals did not change during baking of the substrate at 100 °C, collection of SEM images under a vacuum, or storage in various ambient conditions for 28 days (Figure 3.4).
Figure 3.2 Formation of salt crystals along nanotubes with broad range of diameters. (a) SEM image of four nanotubes before being decorated with salt crystals. Scale bar: 20 μm. (b) AFM height images and corresponding height profile showing that nanotubes #1 and #4 were double-walled carbon nanotubes (DWNT) and that nanotubes #2 and #3 were single-walled nanotubes. Scale bar, 1 μm. (c) Optical image of salt crystals formed along all four nanotubes, suggesting that salt crystals were able to form on a wide range of carbon nanotubes. Scale bar: 20 μm.
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Figure 3.5a show images of the array of NaCl crystals along a nanotube collected in situ under an optical microscope. The images shown were collected at 12 s, 116 s, 222 s, and 365 s. The length of the array increased linearly with time at a rate of about 30 μm/min. When the duration of ionic transport was fixed at 400 s, the use of a higher electric field (10 V/mm) resulted in longer crystal arrays than the use of a lower field (5 V/mm), as shown in Figure 3.5a (inset). The logarithmic normal distribution of the crystal diameters was shifted to the right (i.e. larger crystals) at the stronger electric field (Figure 3.5b). Further increasing the electric field (>12 V/mm) caused formation of continuous narrow bands
a b c d e
Day 4 Day 7 Day 19 Day 28 Day 0
21.2 °C 10% RH
19.2 °C 22% RH
23.4 °C 10% RH
26.9 °C 42% RH
20.5 °C 13% RH
Figure 3.4 The robustness of the NaCl lenses. (a) Optical image of the NaCl lenses formed along a nanotube under ambient conditions (21.1 °C, 10% relative humidity). (b) Baking of the substrate at 100 °C for 2800 s; the shape and size of the lenses did not change. (c) SEM image of the same lens array. (d) Optical micrograph after taking the SEM image. The results of a-d show that the lenses were in the solid-state below the DRH of the salts. Scale bars in a-d: 5 μm. (e) NaCl lenses on a SWNT stored under various ambient conditions (temperature 20–27 °C, relative humidity 10–42%) and observed for 28 days, verifying the stability of the lenses. Scale bar: 5 μm.
a b
1300 1400 1500 1600 1700
0 5000 10000 15000
Ram an shift (cm-1) Pristine After bias & Rinsing After w ashing
Intensity (counts)
Figure 3.3 No damage to the nanotubes (a) Removing salt crystals by simple DI water rinsing before (left) and after (right).
Scale bar: 20 μm. (b) Raman spectra of SWNT before (black) and after the bias, and then water rinsing (blue) show no increase of the disorder mode (D-peak), which suggests that the transport of ionic liquid is not damaging to the nanotubes.
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of NaCl solution by the electrowetting52 of the nanotubes (Figure 3.6). Here, the ionic transport along the nanotubes was driven mainly by the repulsion between the cations accumulated near the edge of the droplet under an electric field. Thus, we expect that the crystals along the nanotubes were enriched with cations, as has been reported in electrospray,76 electrohydrodynamic jet printing,82 and our previous study.106 Also, our approach to form micro/nanoscale salt crystals along horizontally aligned SWNTs is applicable to other 1D nanomaterials such as randomly oriented SWNTs (Figure 3.7a-b) and randomly oriented Ag nanowires (Figure 3.7c-d). Thus, our approach is promising for use the optical visualization of various 1D nanomaterials.
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0 50 100 150 200
Length of crystal array (μm)
Tim e (s)
b
0 20 40
0 1 2 3 4 5 6 7
0 20 40
60 ~5 V/mm
~10 V/mm
Diam eter (μm)
Counts
a
Electric field (V/mm)
5 10
200 400 600
Length (μm)
Figure 3.5 (a) In situ monitoring of the formation of NaCl micro/nanocrystals along a nanotube. Inset shows the length of the NaCl crystal array at two different electric fields. (b) Distributions of the diameter (bars) and logarithmic fits (lines) of salt crystals formed along nanotubes using an electric field of ~10 V/mm (top) and ~5 V/mm (bottom).
3.3 mm
(+)
Metal cover
Figure 3.6 Ionic liquid transport along ultralong nanotubes. Optical image of NaCl liquid formed along the ultralong nanotubes under ambient condition (25.2 °C, 70 % relative humidity). The electrical bias of 40V was applied between metal cover and 3M NaCl solution for 3 min. In this case, oxygen plasma etching does not be performed for efficient liquid transport.
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a b
c
d
Figure 3.7 Formation of salt micro/nanostructures to other 1D nanomaterials. (a) Optical image of NaCl micro/nanostructures along the random networks of SWNTs via exterior transport. Scale bar, 30 µm. (b) SEM image of NaCl nanostructures formed along random networks of nanotube. Scale bar, 5 µm. (c) Optical images of NaCl micro/nanostructures along the random networks of Ag nanowires (d = 115 nm) before (left) and after applying bias (right). Scale bar, 20 µm. Inset shows magnified images. Scale bar, 5 µm. (d) SEM images of random network of Ag nanowires decorated with NaCl micro/nanostructures.
Scale bar, 20 µm (left) and 1 µm (right).
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