Investigation of Etching Behavior of Single-Walled Carbon Nanotubes

Subscriber access provided by Gothenburg University Library

Article

Investigation of Etching Behavior of SingleWalled Carbon Nanotubes Using Different Etchants Zequn Wang, Qiuchen Zhao, Lianming Tong, and Jin Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06653 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 16, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Investigation of Etching Behavior of Single-Walled Carbon Nanotubes using Different Etchants Zequn Wang1,2, Qiuchen Zhao1, Lianming Tong1*, Jin Zhang1* 1

Center for Nanochemistry, Beijing Science and Engineering Center for Nanocarbons, College of

Chemistry and Molecular Engineering, Peking University, Beijing 100871, P.R. China 2

Academy for Advanced Interdisciplinary Studies, Peking University, Beijing, 100871, P. R.

China ABSTRACT: As gas-phase etching has become an important method to obtain single-walled carbon nanotubes (SWCNTs) with specific electronic structures, numerous etchants were studied in the past decades. However, the understanding of the dynamic process as well as the etching state is still limited. Herein, we investigated the etching process of SWCNTs at the level of the individual tube in real time using an improved polarized optical microscope equipped with a miniature CVD system. Experiments showed a universal etching behavior in SWCNT oxidation process under different etchants (O2, H2O and CO2), that is, the random appearance of etching sites and the self-terminating phenomenon. We built a new etching model at the level of the individual tubes to describe the etching rate. Based on this model, a new constant R, the ratio of etching rates between metallic and semiconducting SWCNTs, was defined to describe the etching selectivity. Optical spectroscopy was used to identify the chirality of SWCNTs and it was found that the etching selectivities followed the order: H2O (R=17.1, 850oC) > CO2 (R=3.6, 850oC) > O2 (R=1.2, 610oC). Field effect transistors (FETs) performance of the SWCNT arrays also verified the statistical results.

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 21

1. INTRODUCTION The single-walled carbon nanotube (SWCNT) material system is one of the most promising material systems because of high carrier mobility1-2, large current carrying capacity3, and ballistic transport effect4-5. Recently, the successful fabrication of transistors with 5 nm gate lengths based on individual SWCNT further verified its enormous advantages in continuing Moore’s Law6. In order to build large-scale integrated circuits, the pursuit of high purity semiconducting SWCNT (s-SWCNT) array is growing rapidly. Over the past decades, researchers have presented numerous strategies to achieve this goal7-8, including direct growth methods by designing catalysts9-11, adjusting category of carbon sources12, as well as postprocessing methods to eliminate metallic SWCNTs (m-SWCNTs) with covalent modification1314

, radiation15-16, electrophoresis17-18, density-gradient ultracentrifugation19-20, Scotch Tape21, and

SDS assistance ultrasonication22. Among the above methods, gas-phase etching is of special interest as an ex-situ etching method, for its less chemical decoration or defects on SWCNTs. In the past decades, various gases have been used ex-situ as etchants, including O223-25, H226-27, H2O28-30, CO231, O332, SO333 etc., which showed good performance for selective removal for m-SWCNTs or s-SWCNTs. It is noteworthy that gas-phase etching methods decoupled growth and separation process, which allows researchers to obtain SWCNTs with high purity as well as high density. Until now, most of the work only speculated the possible etching mechanisms according to the experimental results, as no direct images could show the SWCNT etching process clearly. Comparing to exsitu measurements, in-situ technologies are more suitable to track the SWCNT etching process in real time, such as in-situ Raman spectroscopy34-36, in-situ environmental transmission electron microscope (ETEM)37 and environmental scanning electron microscope (ESEM)38. However, certain improvements are needed in the above in-situ methods, including the direct correlation of experimental observations to the specific structures of SWCNTs, the ruling out of the external disturbance of electron beam or laser heating, and the improvements of time resolution or scanning area. Recently, optical imaging methods have been utilized to observe SWCNTs with large area39-40. Our previous work showed that the etching of SWCNTs in air exhibited a random mode, different from earlier works41. To further understand the etching behavior of SWCNTs, there are still three questions to be answered: 1) Is the etching behavior universal for all the


2

Page 3 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


etchants? 2) What is the kinetic etching model for SWCNT? 3) How is the selectivity expressed in such an etching model? To answer the above questions, we herein chose air, water and carbon dioxide as three typical oxidative etchants to investigate the SWCNT etching process systematically in real time by using polarized optical microscopy. We used high-contrast reflection absorption spectroscopy to determine the chirality of SWCNTs, and correlated it to the etching processes. During the observation, we found a universal etching behavior in SWCNTs oxidation process with the following characteristics: 1) the etching sites generated randomly on the body of the tubes; 2) the active site would terminate after a certain etching time, and 3) the frequency of sites generation varied significantly with etchants, etching conditions and SWCNT structures. Based on these features, we established an etching rate model, and defined a parameter R to describe the etching selectivity. In different etching conditions, the R varied with different etchants: H2O (R=17.1, 850oC), CO2 (R=3.6, 850oC), O2 (R=1.2, 610oC). Field effect transistors (FETs) performance of the SWCNT arrays was consistent with the statistical results.

2. METHODS 2.1 Synthesis of gas-flow directed SWCNT arrays. A 3 mm×4 mm Si/SiO2 with 90 nm oxide layer (Hefei Kejing Materials Technology Company, China) was used as substrate for gas-flow aligned growth of SWCNTs in chemical vapor deposition (CVD) system. CuCl2 was purchased from Sinopharm Chemical Reagent as catalysts. The Si/SiO2 substrate with 0.05 mmol L-1 CuCl2/ ethanol catalyst was placed in a quartz tube and heated to 980 oC in air. Then, the tube was purged with 300 standard cubic centimeters per minutes (sccm) argon to exclude air for 10 minutes. After argon was switched off, 200 sccm H2 was introduced to reduce CuxOy to Cu nanoparticles, and 50 sccm argon flow through ethanol bubbler was introduced to grow SWCNTs. After growth for 30 minutes, carbon source was turned off, and extra 50 sccm H2 was added to maintain the gas pressure. Finally, the furnace was cooled down to 800 oC for 1h. Figure S1a, b shows a typical scanning electron microscopy (SEM) image of ultra-long SWCNTs. The density of gas-oriented SWCNTs was one tube in per tens of microns, and the length was about a few millimeters. The major parts of as-grown SWCNTs were single-walled, with minor parts being double-walled or multi-walled. Given that the etching mechanisms of multi-walled carbon nanotubes (MWCNTs) might be different from SWCNTs as reported in


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 21

previous works42, we assigned the chirality of the nanotubes before the etching process to ensure that each carbon nanotube selected was single-walled. 2.2 Synthesis of lattice oriented SWCNT arrays The synthesis of horizontal lattice aligned SWCNT arrays utilized a 4 mm×6 mm quartz (Hefei Kejing Materials Technology Company, China) as substrate. First, the substrate with patterned catalyst precursor (FeCl3 ethanol solution) was heated up to 820 oC for 10 min, and purged with 300 sccm Ar for 10 min. Then, 200 sccm H2 was supplied to reduce catalyst nanoparticles for 10 min. Finally, 40 sccm Ar through ethanol bubbler was introduced as carbon source, and was kept on for 20 min. The average density of the horizontally aligned SWCNTs was 5 tubes/µm and the length was about 200 µm. 2.3 In-situ optical observation system. In-situ optical observation was performed using an optical system equipped with a miniature CVD system. Optical system was composed of a polarized optical microscope (Olympus BX51), a monochrome camera (Andor Zyla 4.2)41, an objective lens (50×), and two polarizers. One polarizer (Daheng optics, GCL-050003) was positioned in the incident light (tungsten–halogen lamp) path, and the other analyzer polarization in the detection light path was near-perpendicular to the incident one with a small angle. A shortwave pass filter (Thorlabs, FGB 37) was placed before the camera to cut-off most of the blackbody radiation from the substrate under high temperature. A mini-CVD system including heating chamber, gas auxiliary, cooling water was used for in-situ etching of the carbon nanotubes. The diameter of the center heating stage was 4 mm and heating rate could reach 100 oC/min. A 200 quartz plate was fabricated as a lid on the top of the heating chamber, and the direction of SWCNTs in the chamber had an angle of 45o with the incident polarized light. Besides, series of gases could be introduced into the CVD system. This improved apparatus (as shown in Figure 1a) made it possible not only to observe an SWCNT, but to track its dynamic etching process with different etchants (O2, H2O and CO2) under atmospheric pressure and high temperature. 2.4 Optimization of Gas-phase etching parameters of as-grown SWCNTs. Too many or too few gaps would be difficult to analyze, so etching temperatures, flow rates and etching times were chosen to yield a statistically usable number of gaps . The experimental results showed that the appropriate etching conditions for as-grown SWCNTs on Si/SiO2 were 610-690 oC for 10 min in air (21% O2 and 78%N2), 750-850 oC for 60 min in H2O (purged by 200 sccm argon >99.99%), and 830-870 oC for 30 min in CO2 (>99.99%,200 sccm).


4

Page 5 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The statistical analysis of etching behavior of the SWCNTs on Si/SiO2 was measured. In case of O2, the sample was placed 10 cm away from a moveable furnace (OTF-1200X). When the center temperature reached to 610 oC, 650 oC, or 690 oC, we moved the furnace to the sample position and heated SWCNTs for 10 min in the room air. After reaction, the furnace was moved back to the previous position, and the sample was cooled down to the room temperature. In case of H2O, a quartz boat with water was placed 15 cm in the opposite direction, close to the intake. Then, the tube was purged with 200 sccm argon to exclude air and carried water vapor into the furnace for 10 minutes. After moving the furnace, the sample was heated at 750 oC, 800 oC, or 850 oC for 60 min. For the part of CO2, argon was supplied for 10 min to exclude the air. After the argon was turned off, in the same move, 200 sccm pure CO2 was introduced to oxidize the SWCNTs at 830 oC, 850 oC, or 870 oC for 30 min. The etching process of preparing the samples on quartz for FET measurement was similar to the preceding methods. We choose the same etching temperature and gas flow according to the statistics of the frequency of gap length ,that is, 610oC for air, 850oC for water and carbon dioxide, The etching time was controlled to ensure the similar number of gaps on s-SWCNTs using different etchants. 2.5 Morphology characterization of SWCNTs. Morphology of SWCNTs were characterized by atomic force microscope (AFM, Dimension icon ScanAsyst, tapping mode) and scanning electron microscope (SEM, Hitachi S4800 field emission, Japan, operated at 1.0kV). 2.6 Chirality indication of SWCNTs. The assignment of the chirality of SWCNTs was accomplished

by

a

home-built

high-contrast

reflection/absorption

spectra40

with

a

supercontinuum laser as light source and a spectrometer as photon energy analyzer. By comparing optical resonances with atlas table established before43, we identified the chirality of the SWCNTs (table S1). 2.7 FET Devices Fabrication. The as-grown SWCNTs were transferred from quartz to Si/SiO2 substrate with 300 nm oxide layer after etching for FET devices fabrication. A channel with the width of 200 µm and length of 20 µm was patterned by photolithography. Thin films of 6 nm Cr and 50 nm Au were deposited by vacuum evaporation followed by a lift-off process, where Cr acted as a contact layer and Au as the pad of source and drain. The Keithely 4200-SCS semiconductor characterization system was utilized to measure the electrical properties.


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 21

3. RESULTS AND DISCUSSION 3.1 Observation of the etching behavior of SWCNTs. Figure 1b,d shows representative snapshots of optical images of the etching process with different etchants for the exposure time of 3s (O2), 7s (H2O) and 7s (CO2) respectively. The SWCNTs were found as the dark thin lines in the images. To highlight the etching process, we schematically illustrated the SWCNTs by white lines in the blank area. From the figure, a process of “active site generation – gap elongation – active site termination – new site generation” could be observed clearly. For instance, the first two sites appeared on the right tube at 5’01’’ after CO2 injection and the gap length enlarged at 5’08’’ (Figure 1d). Afterwards, a third site had appeared at 5’15’’. The fourth image was taken at one minute later, indicating that the length of the three terminated gaps no longer changed afterwards. The other two etchants (O2, H2O) produced similar results (Figure 1b shows SWCNTs etched by O2, and Figure 1c shows SWCNTs etched by H2O). Raman mapping of the SWCNT sample after etching showed a homogeneous and very low signal of D-band, as shown in Figure S1d and S1e. This indicated that there are very few defects along the body of the tube after etching process. Figure 1e illustrates atomic models of etching process of the SWCNTs under different etchants. As SWCNTs were visualized as dark lines in the image, the brightness of pixels with SWCNTs was significantly lower than those without SWCNTs. Figure 2a,c shows the brightness signal of SWCNT before (x-marks in red) and after (x-marks in black) the etching process. We could find a significant brighter region, which referred to the gap area, and the length of this region was defined as the gap length at this time. Therefore, we further measured the change of the gap length over time (Figure 2d,f), from which the elongation and the termination stage could be monitored during the formation of one gap. We measured and statistically analyzed the distance between the centers of the adjoining gaps, which reflected a random position distribution on the SWCNTs (Figure S2). It is probably caused by the uniform structure along SWCNTs, of which the structure can remain unchanged in tens of millimeters44. It makes the probability of the occurrence of etching cites random along the whole nanotube. Unlike previous conjecture, the etching process extended to both sides from a random site and would terminate after a certain etching time, which was called ‘self-termination phenomenon’. This phenomenon was verified by O2, H2O, CO2, and we claimed that the self-termination phenomenon was probably a universal behavior in the SWCNTs etching process.


6

Page 7 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.2 Dynamic model of SWCNT etching process. SEM images showed a typical SWCNT etched randomly into several small fragments by H2O (Figure S1c). As a small gap would play a key role in electronic device in practical application, this characteristic was very important for further analysis. Compared with previous tube-by-tube statistical method, the focus on gap-bygap analysis could be more applicable and reliable. Therefore, we defined three parameters, (), and () to further describe the etching process: ( ∙ ), ∆ = = ∆ × ×

(1)

In which referres to average etching rate ( ∙ ), referres to the length of tube at represents to the average gap length; is the frequency that a , is the length of tube at ; ∆ gap appears per unit time and length; is the total length of an individual tube. It’s expected that the neighboring gaps may mix together with etching time and temperature extending, which would result in the increase of ∆ , and the decrease of . On the other hand, topological structure38

45-46

, electronic structure24 of SWCNTs and temperature had been reported to be

can be expressed in terms of the functional form: related to the etching rate. Thus, and ∆ = () × ∆ (, , , ⋯ ) (2) ∆ = "() × γ (, , , ⋯ )

(3)

In the above formula, () and "() are parameters only related to the etching time , is the etching temperature, and (, ) is the structure of an SWCNT expressed in polar coordinates. In the initial period of etching process, the density of gaps on the etched SWCNTs was relatively low. For example, the length of tube etched by CO2 at 850oC for 30 min only contributed 0.4% of the total length of the SWCNT, thus gaps had quite low odds to mix with each other, which indicated that () ≈ "() ≈ 1. Further, the etching rate could be described approximately as: ≈ ∆ × ×

(4)

Another interesting fact was that most of the active etching site terminated after a certain etching time. The probability P of the gaps in a state of termination was calculated by: P = 1 – & m/ & n

(5)


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 21

where & m was the lifetime of an active site, and & n was the total etching time in the process. According to the real-time observation, & m of a gap was about 10 s for all the etchants, but & n were 5 min for O2, 30 min for CO2, 60 min for H2O, respectively. Calculation showed that more than 96.7%, 99.4% and 99.7% active sites had been terminated by using O2, CO2, and H2O, and and could be measured by ex-situ measurements. thus ∆ ∆ , firstly, we gave the assignment of the 3.3 Analysis of ∆'( . For a quantitative analysis of chirality of 30 SWCNTs (Table S1). Then, we etched the sample under 850oC for 30 min in CO2, and analyzed the distribution of average gap length of these tubes. Statistics show that the average gap length on an individual tube tended to be a fixed value at certain temperature, and no obvious correlation was found between ∆L0 and the structure of SWCNTs (Figure S4). Besides, H2O and O2 showed the same phenomenon. The average gap length of 10 identified SWCNTs etched under different etchants were shown in the Figure 3a,c (a, O2, 690oC; b, H2O, 850oC; c, CO2, 850oC). Figure 3d,f shows an obvious increase of ∆ with the increase of temperature, suggesting that average gap length was a function of etching temperature. Meanwhile, the individual length tends to follow Gaussian distribution (Figure 3g,i). 3.4 Analysis of )( . According to Equation (4), the difference of etching rate might be showed a constant value at certain temperature. To unravel in depth determined by , since ∆ the correlation between and the SWCNT structure, the optical spectrum was used to characterize the structure of SWCNTs before etching43. Figure 4a shows the morphology of an SWCNT before gas-phase etching, and Figure 4b displays a typical optical spectrum of this SWCNT. The SWCNT was indicated as a (28, 24) semiconducting tube, which was consistent with the previous report40. After the etching process, two sharp-edged gaps were found in the AFM image (Figure 4c) with the length of ~ 500 and ~800 nm. No catalyst nanoparticles could be observed nearby, suggesting that catalyst nanoparticles did not have an effect on the etching rate. Based on the sufficient number of characterized gaps ( > 400) on each condition, we measured and statistically analyzed the frequency that a gap appears per unit time and length on the individual tube. In Equation (3), etching temperature T and the structure of an SWCNT (d, θ) may affect . First, we studied how could be influenced by the diameter d and chiral angle θ, which were two key factors to define the SWCNT structures, and the results were shown in Figure S5 and Figure S6. Figure S5 shows the scatter plots of to d, in which d ranged from


8

Page 9 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0 nm to 3.5 nm. From the statistics derived from the data, the smaller diameter SWCNTs showed both the low and high frequency of gaps, meanwhile, the larger diameter SWCNTs also had the same results. Then, to test whether the chiral angle played a key role or not, we compared with different chiral angle of SWCNTs, ranging from 0o to 30o (Figure S6). Similarly, both the bigger and smaller chiral angle SWCNTs displayed the low and high frequency of gaps. The random distribution suggested that average frequency of gaps could not be determined by diameter or chiral angle individually. However, figure 4d,f reveals that the average frequency of gaps had a strong correlation with electronic structure of SWCNT (mSWCNTs were marked in red and s-SWCNTs were marked in black). For O2, the frequency of gap generation of m-SWCNTs (2.38 ∙ ) was almost the same as s-SWCNTs (1.92 ∙ ); when H2O was used as etchants, m-SWCNTs (1.07 ∙ ) showed a much higher frequency than s-SWCNTs (0.06 ∙ ); And for CO2, m-SWCNTs (0.40 ∙ ) was a little higher than s-SWCNTs (0.11 ∙ ) in Figure 4g. Overall, statistics showed that the frequency of gap creation per unit time and length reflected the etching rate of SWCNTs of specific electronic structures. 3.5 Re-definition and illustration of the selectivity in gas-phase etching To quantitatively analyze the differences in the etching process, the selectivity R is redefined as × - × ∆ - ⁄∆ ≈ ∆

(6)

× / × ∆ / ⁄∆ ≈ ∆

(7)

R=

∆0 ⁄∆ ∆1 ⁄∆

≈

)2 )3

(8)

where - and / are average fracture frequency per unit time and length of m- and sSWCNTs, respectively. As shown in Figure 4h, R is 1.2 for O2, 17.1 for H2O, 3.6 for CO2 in histogram, indicating that the etching selectivity follows the order: H2O > CO2 > O2. The ranking of the three etchants could be illustrated by the different doping of SWCNTs under various etchants. Many published works have paid attention to the density of states (DOS) of SWCNTs near Fermi level, which results in different chemical reactivity of SWCNTs29, 47. As shown in Figure 4i, there are three possible degrees of doping. First, the Fermi level of SWCNTs locates between the first and second singularities, and the DOS of s-SWCNTs is higher than that


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 21

of m-SWCNTs. Thus, the chemical reactivity of s-SWCNTs is higher than that of m-SWCNTs, which is the case for H2O247. Second, the Fermi level shifts to the top of the valence band of mSWCNTs due to the doping, resulting in a higher DOS of m-SWCNTs than that of s-SWCNTs at the Fermi Level. This is the case for CO2 and H2O, and m-SWCNTs are more reactive than sSWCNTs. Third, if the doping effect is strong enough, the DOS of s-SWCNTs and m-SWCNTs are almost equal, which leads to a very weak selectivity during etching process. This is the case for O2. 3.6 FET measurement. Experimental and theoretical works have demonstrated that the semiconducting SWCNT arrays are important and desirable for the applications of nanotubebased electronics, such as field-effect transistors (FETs)6, 48, complementary metal-oxide semiconductor (CMOS)49, and infrared sensors (IR)50-51. The performance of FETs is usually regarded as a measurement of proportion of the s-SWCNTs. To verify our defined selectivity, bottom-gated FETs based on the etched SWCNT arrays were fabricated as showed in Figure 5a. The gaps of SWCNTs between the two electrodes could be seen clearly in SEM image (Figure 5b). Figure 5c presents the typical transfer characteristic curve (I-V curves) of s-SWCNTs, where Vds = -1 V, and Vgs ranged from -40 V to 40 V. The output characteristic curve of the etched SWCNT arrays was displayed in the inset, where Vtg varied from -30V to 30V with steps of 15V, which indicated a good ohmic contact. To compare the different selectivity of etchants, it was reasonable to control the same number of gaps creation per unit length of s-SWCNTs, which means, to make 4(5 ) × (5 ) = 4(6 7) × (6 7) = 4(87) × (87 ). Then, we calculated the percentage of the total number of devices with an Ion/Ioff higher than 10, which suggested that the content of s-SWCNTs was higher than 90%. As shown in Figure 5d, we could find that the percentage was 6.9%, 22.4% and 15.5% corresponding to O2, H2O and CO2 respectively. The results showed that semiconducting selectivity of SWCNTs ranked in the following order: H2O > CO2 >O2, which was consistent with the tendency of the statistical results using our model. On the other hand, the reasons that the percentage of s-SWCNTs was not high enough in the FETs might be: a) the carrying current of the s-SWCNTs and m-SWCNTs was not the same, so that the percentage of s-SWCNTs was underestimated; b) the etching conditions were not optimized. Experiments showed that longer etching time and lower etching temperature helped improve the selectivity of s-SWCNTs (Figure S7 and Figure S8). The Ion/Ioff (>10) was 6.5% (10


10

Page 11 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


min), 7.1% (20 min), 11.1% (40 min), 23.2% (60 min). In case of temperature, Ion/Ioff (>10) was 21.4% (600 oC), 17.6% (650 oC), 13.3% (700 oC), 11.1% (750 oC). Theoretically, the etching rate of the m-SWCNTs was higher than s-SWCNTs in O2, H2O and CO2, thus, there might be an optimized condition to obtain high purity of s-SWCNTs and get better device performance in the future.

4. CONCLUSION In summary, we confirmed the ‘self-termination’ to be a universal phenomenon in SWCNT etching process with different etchants (O2, H2O and CO2) by in-situ polarized optical microscopy. Based on the gap-by-gap statistical method, a new definition of etching rate is proposed, which is more appropriate to describe the dynamic process of selectively etching SWCNTs. Statistical results showed that selectivity of each etchants followed the order: H2O (R=17.1, 850 oC) > CO2 (R=3.6, 850 oC) > O2 (R=1.2, 610 oC) under specific etching condition, which was consistent with the performance of FET devices. This newly established etching model could be helpful in understanding the etching mechanism of SWCNTs, and provide advices in searching more efficient etchants.


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 21

ASSOCIATED CONTENT Supporting Information Available The Supporting information is available free of charge on the ACS Publications websites at DOI, including: Morphology characterization of SWCNTs etched; Position distribution of the adjoining gaps on the SWCNTs; Chirality indication of 30 SWCNTs by optical spectroscopy; The statistical relationship between the average length of gaps and diameter, chiral angle, S/M properties of SWCNTs. The statistical relationship between frequency of gap creation and diameter, chiral angle of SWCNTs; the statistics of the percentage of the Ion/Ioff of SWCNTs in FET devices with a series of time and temperature.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; [email protected] Notes: The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of China (2016YFA0200101 and 2016YFA0200104),the National Natural Science Foundation of China (grant No. 21233001, 21790052 and 51720105003) and the Beijing Municipal Science and Technology Planning Project (No. Z161100002116026). REFERENCES (1) Dürkop, T.; Getty, S.; Cobas, E.; Fuhrer, M. Extraordinary Mobility in Semiconducting Carbon Nanotubes. Nano Lett. 2004, 4, 35-39. (2) Perebeinos, V.; Tersoff, J.; Avouris, P. Mobility in Semiconducting Carbon Nanotubes at Finite Carrier Density. Nano Lett. 2006, 6, 205-208.


12

Page 13 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(3) Wei, B. Q.; Vajtai, R.; Ajayan, P. M. Reliability and Current Carrying Capacity of Carbon Nanotubes. Appl. Phys. Lett. 2001, 79, 1172-1174. (4) Javey, A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H. Ballistic Carbon Nanotube Field-Effect Transistors. Nature. 2003, 424, 654-657. (5) Avouris, P.; Chen, Z.; Perebeinos, V. Carbon-Based Electronics. Nat. Nanotechnol. 2007, 2, 605-615. (6) Qiu, C.; Zhang, Z.; Xiao, M.; Yang, Y.; Zhong, D.; Peng, L.-M. Scaling Carbon Nanotube Complementary Transistors to 5-Nm Gate Lengths. Science. 2017, 355, 271-276. (7) Islam, A. E.; Rogers, J. A.; Alam, M. A. Recent Progress in Obtaining Semiconducting Single‐Walled Carbon Nanotubes for Transistor Applications. Adv. Mater. 2015, 27, 79087937. (8) Liu, C.; Cheng, H.-M. Controlled Growth of Semiconducting and Metallic Single-Wall Carbon Nanotubes. J. Am. Chem. Soc. 2016, 138, 6690-6698. (9) Zhang, S.; Hu, Y.; Wu, J.; Liu, D.; Kang, L.; Zhao, Q.; Zhang, J. Selective Scission of C-O and C-C Bonds in Ethanol Using Bimetal Catalysts for the Preferential Growth of Semiconducting Swnt Arrays. J. Am. Chem. Soc. 2015, 137, 1012-1015. (10) Qin, X.; Peng, F.; Yang, F.; He, X.; Huang, H.; Luo, D.; Wang, S.; Liu, H.; Peng, L.; Li, Y.et al. Growth of Semiconducting Single-Walled Carbon Nanotubes by Using Ceria as Catalyst Supports. Nano Lett. 2014, 14, 512-517. (11) Li, X.; Tu, X.; Zaric, S.; Welsher, K.; Seo, W. S.; Zhao, W.; Dai, H. Selective Synthesis Combined with Chemical Separation of Single-Walled Carbon Nanotubes for Chirality Selection. J. Am. Chem. Soc. 2007, 129, 15770-15771. (12) Che, Y.; Wang, C.; Liu, J.; Liu, B.; Lin, X.; Parker, J.; Beasley, C.; Wong, H. S. P.; Zhou, C. Selective Synthesis and Device Applications of Semiconducting Single-Walled Carbon Nanotubes Using Isopropyl Alcohol as Feedstock. ACS Nano 2012, 6, 7454-7462. (13) Strano, M. S.; Dyke, C. A.; Usrey, M. L.; Barone, P. W.; Allen, M. J.; Shan, H. W.; Kittrell, C.; Hauge, R. H.; Tour, J. M.; Smalley, R. E. Electronic Structure Control of Single-Walled Carbon Nanotube Functionalization. Science. 2003, 301, 1519-1522. (14) Yoon, S.-M.; S. J.; Shin, H. J.; Benayad, A.; Choi, S. J.; Kim, K. K; Kim, S. M; Park, Y. J.; Kim, G.; Choi, J. Y. et al. Selective Oxidation on Metallic Carbon Nanotubes by Halogen Oxoanions. J. Am. Chem. Soc. 2008, 130, 2610-2616. (15) Huang, H. J.; Maruyama, R.; Noda, K.; Kajiura, H.; Kadono, K. Preferential Destruction of Metallic Single-Walled Carbon Nanotubes by Laser Irradiation. J. Phys. Chem. B 2006, 110, 7316-7320. (16) Tsuchiya, K.; Kumazawa, A.; Tajima, I.; Uchida, K.; Ishii, T.; Yajima, H. Laser-IrradiationInduced Enrichment of Metallic Single-Walled Carbon Nanotubes from as-Synthesized Nanotubes Individually Dispersed in Aqueous Solution. Jpn. J. Appl. Phys. 2012, 51,105101. (17) Tanaka, T.; Jin, H.; Miyata, Y.; Kataura, H. High-Yield Separation of Metallic and Semiconducting Single-Wall Carbon Nanotubes by Agarose Gel Electrophoresis. Appl. Phys. Express 2008, 1,114001 (18) Kim, W.-J.; Usrey, M. L.; Strano, M. S. Selective Functionalization and Free Solution Electrophoresis of Single-Walled Carbon Nanotubes: Separate Enrichment of Metallic and Semiconducting Swnt. Chem. Mater. 2007, 19, 1571-1576. (19) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C. Sorting Carbon Nanotubes by Electronic Structure Using Density Differentiation. Nat. Nanotechnol. 2006, 1, 6065.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 21

(20) Ghosh, S.; Bachilo, S. M.; Weisman, R. B., Advanced Sorting of Single-Walled Carbon Nanotubes by Nonlinear Density-Gradient Ultracentrifugation. Nat. Nanotechnol. 2010, 5, 443450. (21) Hong, G.; Zhou, M.; Zhang, R.; Hou, S.; Choi, W.; Woo, Y. S.; Choi, J.-Y.; Liu, Z.; Zhang, J. Separation of Metallic and Semiconducting Single-Walled Carbon Nanotube Arrays by "Scotch Tape". Angew. Chem. Int. Ed. 2011, 50, 6819-6823. (22) Hu, Y.; Chen, Y.; Li, P.; Zhang, J. Sorting out Semiconducting Single-Walled Carbon Nanotube Arrays by Washing Off Metallic Tubes Using Sds Aqueous Solution. Small 2013, 9, 1306-1311. (23) Moon, C.-Y.; Kim, Y.-S.; Lee, E.-C.; Jin, Y.-G.; Chang, K. J., Mechanism for Oxidative Etching in Carbon Nanotubes. Phys. Rev. B 2002, 65,154401. (24) Yu, B.; Hou, P.-X.; Li, F.; Liu, B.; Liu, C.; Cheng, H.-M. Selective Removal of Metallic Single-Walled Carbon Nanotubes by Combined in Situ and Post-Synthesis Oxidation. Carbon 2010, 48, 2941-2947. (25) Yu, B.; Liu, C.; Hou, P. X.; Tian, Y.; Li, S.; Liu, B.; Li, F.; Kauppinen, E. I.; Cheng, H. M. Bulk Synthesis of Large Diameter Semiconducting Single-Walled Carbon Nanotubes by Oxygen-Assisted Floating Catalyst Chemical Vapor Deposition. J. Am. Chem. Soc. 2011, 133, 5232-5235. (26) Hou, P.-X.; Li, W.-S.; Zhao, S.-Y.; Li, G.-X.; Shi, C.; Liu, C.; Cheng, H.-M. Preparation of Metallic Single-Wall Carbon Nanotubes by Selective Etching. ACS Nano 2014, 8, 7156-7162. (27) Li, W.-S.; Hou, P.-X.; Liu, C.; Sun, D.-M.; Yuan, J.; Zhao, S.-Y.; Yin, L.-C.; Cong, H.; Cheng, H.-M. High-Quality, Highly Concentrated Semiconducting Single-Wall Carbon Nanotubes for Use in Field Effect Transistors and Biosensors. ACS Nano 2013, 7, 6831-6839. (28) Li, J.; Liu, K.; Liang, S.; Zhou, W.; Pierce, M.; Wang, F.; Peng, L.; Liu, J. Growth of HighDensity-Aligned and Semiconducting-Enriched Single-Walled Carbon Nanotubes: Decoupling the Conflict between Density and Selectivity. ACS Nano 2014, 8, 554-562. (29) Li, P.; Zhang, J. Sorting out Semiconducting Single-Walled Carbon Nanotube Arrays by Preferential Destruction of Metallic Tubes Using Water. J. Mater. Chem. 2011, 21, 1181511821. (30) Zhou, W.; Zhan, S.; Ding, L.; Liu, J. General Rules for Selective Growth of Enriched Semiconducting Single Walled Carbon Nanotubes with Water Vapor as in Situ Etchant. J. Am. Chem. Soc. 2012, 134, 14019-14026. (31) Huang, J.; Zhang, Q.; Zhao, M.; Wei, F. Process Intensification by Co2 for High Quality Carbon Nanotube Forest Growth: Double-Walled Carbon Nanotube Convexity or Single-Walled Carbon Nanotube Bowls? Nano Res. 2009, 2, 872-881. (32) Byl, O.; Liu, J.; Yates, J. T. Etching of Carbon Nanotubes by Ozone a Surface Area Study. Langmuir 2005, 21, 4200-4204. (33) Zhang, H.; Liu, Y.; Cao, L.; Wei, D.; Wang, Y.; Kajiura, H.; Li, Y; Noda, K.; Luo, G.; Wang, L.et al.A Facile, Low-Cost, and Scalable Method of Selective Etching of Semiconducting Single-Walled Carbon Nanotubes by a Gas Reaction. Adv. Mater. 2009, 21, 813-816. (34) Rao, R.; Liptak, D.; Cherukuri, T.; Yakobson, B. I.; Maruyama, B. In Situ Evidence for Chirality-Dependent Growth Rates of Individual Carbon Nanotubes. Nat. Mater. 2012, 11, 213216. (35) Li-Pook-Than, A.; Lefebvre, J.; Finnie, P. Type-and Species-Selective Air Etching of Single-Walled Carbon Nanotubes Tracked with in Situ Raman Spectroscopy. ACS Nano 2013, 7, 6507-6521.


14

Page 15 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(36) Li-Pook-Than, A.; Finnie, P. Observation of the Metallic-Type Selective Etching of Single Walled Carbon Nanotubes by Real-Time in Situ Two-Laser Raman Spectroscopy. Carbon 2015, 89, 232-241. (37) Koh, A. L.; Gidcumb, E.; Zhou, O.; Sinclair, R. Observations of Carbon Nanotube Oxidation in an Aberration-Corrected Environmental Transmission Electron Microscope. ACS Nano 2013, 7, 2566-2572. (38) Mehedi, H.-a.; Ravaux, J.; Yazda, K.; Michel, T.; Tahir, S.; Odorico, M.; Podor, R.; Jourdain, V. Increased Chemical Reactivity of Single-Walled Carbon Nanotubes on Oxide Substrates: In Situ Imaging and Effect of Electron and Laser Irradiations. Nano Res. 2015, 9, 517-529. (39) Liu, K.; Hong, X.; Choi, S.; Jin, C.; Capaz, R. B.; Kim, J.; Wang, W.; Bai, X.; Louie, S. G.; Wang, E.; et al. Systematic Determination of Absolute Absorption Cross-Section of Individual Carbon Nanotubes. P. Natl. Acad. Sci. USA. 2014, 111, 7564-7569. (40) Liu, K.; Hong, X.; Zhou, Q.; Jin, C.; Li, J.; Zhou, W.; Liu, J.; Wang, E.; Zettl, A.; Wang, F. High-Throughput Optical Imaging and Spectroscopy of Individual Carbon Nanotubes in Devices. Nat. Nanotechnol. 2013, 8, 917-22. (41) Zhao, Q.; Yao, F.; Wang, Z.; Deng, S.; Tong, L.; Liu, K.; Zhang, J. Real‐Time Observation of Carbon Nanotube Etching Process Using Polarized Optical Microscope. Adv. Mater. 2017.1701959. (42) Tsang, S.; Harris, P.; Green, M. Thinning and Opening of Carbon Nanotubes by Oxidation Using Carbon Dioxide. Nature 1993, 362, 520-522. (43) Liu, K.; Deslippe, J.; Xiao, F.; Capaz, R. B.; Hong, X.; Aloni, S.; Zettl, A.; Wang, W.; Bai, X.; Louie, S. G. An Atlas of Carbon Nanotube Optical Transitions. Nat. Nanotechnol. 2012, 7, 325-329. (44) Zhu, Z.; Wei, N.; Xie, H.; Zhang, R.; Bai, Y.; Wang, Q.; Zhang, C.; Wang, S.; Peng, L.; Dai, L. Acoustic-Assisted Assembly of an Individual Monochromatic Ultralong Carbon Nanotube for High on-Current Transistors. Sci. Adv. 2016, 2, e1601572. (45) Yudasaka, M.; Zhang, M.; Iijima, S. Diameter-Selective Removal of Single-Wall Carbon Nanotubes through Light-Assisted Oxidation. Chem. Phys. Lett. 2003, 374, 132-136. (46) Moon, C.-Y.; Kim, Y.-S.; Lee, E.-C.; Jin, Y.-G.; Chang, K. Mechanism for Oxidative Etching in Carbon Nanotubes. Phys. Rev. B 2002, 65, 155401. (47) Miyata, Y.; Maniwa, Y.; Kataura, H. Selective Oxidation of Semiconducting Single-Wall Carbon Nanotubes by Hydrogen Peroxide. J. Phys. Chem. B 2006, 110, 25-29. (48) Franklin, A. D.; Luisier, M.; Han, S. J.; Tulevski, G.; Breslin, C. M.; Gignac, L.; Lundstrom, M. S.; Haensch, W. Sub-10 Nm Carbon Nanotube Transistor. Nano Lett. 2012, 12, 758-762. (49) Zhang, Z. Y.; Liang, X.; Wang, S.; Yao, K.; Hu, Y.; Zhu, Y.; Chen, Q.; Zhou, W. W.;Yao,Y. G.; Zhang, J. et al. Doping-Free Fabrication of Carbon Nanotube Based Ballistic Cmos Devices and Circuits. Nano Lett. 2007, 7, 3603-3607. (50) Liu, Y.; Wei, N.; Zhao, Q.; Zhang, D.; Wang, S.; Peng, L.-M. Room Temperature Infrared Imaging Sensors Based on Highly Purified Semiconducting Carbon Nanotubes. Nanoscale 2015, 7, 6805-6812. (51) Liu, Y.; Wei, N.; Zeng, Q.; Han, J.; Huang, H.; Zhong, D; Wang, F. L.; Ding, Q. W.; Xia, J.Y.; Xu, H. T. et al. Room Temperature Broadband Infrared Carbon Nanotube Photodetector with High Detectivity and Stability. Adv. Opt. Mater. 2016, 4, 238-245.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 21

Figures

Figure 1. (a) Scheme of SWCNT etching process by in-situ polarized optical microscope equipped with a high temperature stage. Snapshots of optical images showing starting and ending of an etching fracture of SWCNTs. The exposure time for each frames was (b) 3s, O2 etching, (c) 7s, H2O etching, (d) 7s, CO2 etching; the fourth image taken at one minute later. (e) Atomic models of the mechanism of SWCNTs in etching process using O2, H2O, and CO2.


16

Page 17 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Measurement of the gap length changed with time for various etchants. Representation of the brightness value taken along an SWCNT before etching (plots in red) and after etching (plots in black) by using (a) O2, (b) H2O and (c) CO2 as etchants. Variation of the gap length over time with the detailed conditions: (d) air, 610 oC, atmospheric pressure; (e) H2O, 900 oC, purged by 200 sccm argon flow and (f) CO2, 900 oC, 200 sccm.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 21

Figure 3. Ex-situ statistics of the average etching length of gaps of ten SWCNTs (a) under O2 etching 690 oC, (b) H2O etching 850 oC, and (c) CO2 etching 850 oC. Average etching length distribution of SWCNTs at temperature series. (d) O2 etching, 610oC (L=0.68±0.50 ), 650 oC (L=0.85±0.65 ) and 690 oC (L=1.41±0.38), (e) H2O etching, 750oC (L=0.72±0.49), 800 oC (L=4.40±2.99 ) and 850 oC (L=7.13±3.40 ), (f) CO2 etching, 830 oC (L=0.36±0.13), 850oC (L=0.79±0.06) and 870 oC (L=1.69±1.17). Gap etching Length distributions (bars) and Gaussian fits (lines) of SWCNTs at different temperatures. (g) O2 etching, 610 oC (orange), 650 oC (purple) and 690 oC (pink), (h) H2O etching, 750 oC (orange), 800 oC (purple) and 850 oC (pink), (i) CO2 etching, 830 oC (orange), 850 oC (purple) and 870 oC (pink).


18

Page 19 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. The structure-independent etching process of SWCNTs using different etchants. (a) SEM image of an SWCNT before etching. (b) Optical spectrum of an SWCNT identified as (28, 24). (c) AFM image of an SWCNT after etching. Scatter plots of gap frequency with individual SWCNT (m-SWCNTs in red and s-SWCNTs in black) under different etchants, respectively. (d) O2 (610 oC), (e) H2O (850 oC), (f) CO2 (850 oC). (g) The statistical graphs of gap frequency with of m-or s-SWCNTs under O2 (Red), H2O (Green) and CO2 (Blue), respectively. (h) Etching selectivity R of three kinds of etchants. (i) 1D density of states plots of m-SWCNTs and s-SWCNTs.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 21

Figure 5. The performance of field-effect transistors (FETs) devices of SWCNTs. (a) SEM image showing FET devices fabricated on etched SWCNT arrays with S, D electrodes. (b) SEM image of the etching gaps of SWCNT etched arrays. (c) Typical transfer characteristics of etched SWCNT arrays device, where Vds = -1V, Vgs ranged from -40V to 40V, and Ion/Ioff = 116. Inset: Output characteristics of the SWCNT etched arrays devices, where Vtg varies from -30V to 30V with steps of 15V. W = 200 , Lch = 20 . (d) Statistics of Ion/Ioff ratio devices of SWCNTs etched array under O2, H2O and CO2, respectively.


20

Page 21 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic


21

Investigation of Etching Behavior of Single-Walled Carbon Nanotubes

Recommend Documents