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Recycling Strategy for Fabricating Low-Cost and High Performance Carbon Nanotube TFT Devices Xiaoqin Yu, Dan Liu, Lixing Kang, Yi Yang, Xiaopin Zhang, Qianjin Lv, Song Qiu, Hehua Jin, Qijun Song, Jin Zhang, and Qingwen Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 20 Apr 2017 Downloaded from http://pubs.acs.org on April 23, 2017

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ACS Applied Materials & Interfaces

Recycling Strategy for Fabricating Low-Cost and High Performance Carbon Nanotube TFT Devices Xiaoqin Yu,a, b Dan Liu,b ,c Lixing Kang,b, c Yi Yang,b Xiaopin Zhang,b Qianjin Lv,b Song Qiu,*b Hehua Jin,b Qijun Song,* a Jin Zhang c and Qingwen Lib a

b

School of Chemical and Material Engineering, Jiangnan university, Wuxi 214122, PR China Key Laboratory of Nanodevices and Applications Suzhou Institute of Nanotech and Nano-

bionics Chinese Academy of Science, Ruoshui Road 398, Suzhou 215123, PR China c

College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR

China. KEYWORDS: semiconducting, single-walled carbon nanotubes, purification, recyclable, thinfilm transistors, contact resistance ABSTACT: High-purity semiconducting single-walled carbon nanotubes (s-SWNTs) can be obtained by conjugated polymer wrapping. However, further purification of sorted s-SWNTs and high costs of raw materials is still a challenge to practical applications. It is inevitable that a lot of polymers still cover the surface of s-SWNTs after separation, and the cost of polymer is relative too high than SWNTs. Here, we demonstrated a facile isolated process to improve the quality of s-SWNTs solutions and films significantly. Compared with the untreated s-SWNTs, the contact resistance between the s-SWNT and the electrode is reduced by 20 times, and the

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thin-film transistors shows 300% enhancement of current density. In this process, most of the polymer can be recycled and can be reused directly without any purification that can greatly decrease the cost for s-SWNTs separation. The results presented herein demonstrate a new scalable and low-cost approach for large-scale application of s-SWNTs in the electronics industry. 1.

INTRODUCTION Semiconducting single-walled carbon nanotubes (s-SWNTs)1 have been supposed to one of

the most promising materials for next-generation electronic devices.2-6 However, as-synthesis SWNTs are mixture of metallic single-walled carbon nanotubes (m-SWNTs) and s-SWNTs.7 There many methods have been developed to separate s-SWNTs from m-SWNTs.8-11 Specially, conjugated polymer wrapping is a powerful and scalable method. This method has simple procedure and good diameter endurance.12-14 Conjugated polymers can selectively extract a specific chirality (n, m) of s-SWNTs by certain molecular design, and it can reach high purity of s-SWNTs with designed polymer structure.13-16 In our previous work, we designed a linear homopolymer poly[9-(1-octylonoyl)-9H-carbazole-2,7-diyl] (PCz) to enrich s-SWNTs with a high selectivity and ultra-high purity up to 99.9%,16 and also studied influence factor at the separated process, such as solvent polarity and ultrasonic temperature.17-19 The high-uniformity large-area films and controllable alignment s-SWNTs films by this s-SWNTs solution with various solution methods. And medium scale integrated circuits and infrared photodetector based on PCz-sorted s-SWNTs have been realized.4,20 However, to become a replacement for silicon electronic materials, there are still some problems restrict the large-scale application of s-SWNTs, focusing on the following aspects. (1)


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It is hard to further purify the s-SWNTs solution after enrichment, such as the removal of the dispersant, the cleanliness of the film, and then improved the tube-tube contacts. There are two main strategies to obtain dispersant-free s-SWNTs sorted by conjugated polymer. One method is controlling dispersion, which changes polymer conformation to weak the interaction between nanotube and polymer;21-24 another method is degrading polymer into small units.15,

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However, s-SWNTs tend to aggregate when residual polymers are completely removed,33 which reduces the stability of the solution and is detrimental to the later electronic application. (2) The cost of dispersing process is still too high though it has been reported that recycling the monomers after s-SWNTs enrichment can reduce the costs.15 (3) The technology to fabricate large-area film is lacking. At present, large-area film can be fabricated by inkjet printing, immersed-coating and lift-dipping. But a lot of polymers will remain in the film while using the printing and lift-dipping process, which requires additional post-processing steps, such as washing34, 35 or high-temperature annealing36, 37 to remove it. This complex process will seriously interfere the later circuit preparation process. For the immersed-coating method, too long time which is 4-10 hours has been taken at the once. Here, we developed a complete technological process to significantly improve the preparation of s-SWNTs solutions and films. Firstly, the content of remaining polymer is continuously decreased until it reaches minimum, which can still ensure the stability of the sSWNTs dispersion after careful cleaning procedures. Secondly, most of the polymer in the washing process is separated from the s-SWNTs and can be recycled, which drastically reduced the cost of the final s-SWNT. Thirdly, a variety of rapid and large-scale preparation of s-SWNTs film processes have been realized without further post-treatment, such as washing or high temperature annealing. At last, the contact resistance between the carbon tube and the electrode


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is reduced by 20 times, and the thin-film transistors fabricated with this s-SWNTs shows 300% enhancement of current density compared with untreated s-SWNTs. In short, we developed a facile, low-cost and scalable process to obtain high-quality solution and film of s-SWNTs, which laid a foundation for large-scale application s-SWNTs in the electronics industry. 2.

RESULTS AND DISCUSSIONS In order to remove free and loosely polymer in sorted SWNTs solution, the experiment

process is designed and shown in Figure 1. During the filtration and washing processes, there are two factor should be considered. One is that the washing solvent, which could have good solubility with selected polymer and poor solubility with s-SWNTs. THF is moderate polar solvent, which is suitable to removing most loosely polymer since our selected polymer is linear molecular with weak interaction. Another is that the solvent, which is used to re-dispersed sSWNTs after washing, must has good ability to disperse SWNTs. N-methyl-2-pyrrolidone (NMP) and Dimethyl Formamide (DMF) are the most common solvent used to disperse SWNT after sonic.38-40 But the two solvents have high boiling point and viscosity, which is harmful to fabricated film and limit the range of application. On contrast, chloroform has good solubility with s-SWNTs, lower boiling-point and viscosity, which is suitable for low temperature processes and fabricating film rapidly. Based on above analysis, the chloroform is the proper solvent for the s-SWNTs after THF washing. As Figure 1 shows, firstly, high-purity s-SWNTs dispersion is obtained by ultrasonic dispersing and centrifugation with the help of PCz. Then sSWNTs dispersion is divided into two equal parts. One part is filtered and washed by THF. The obtained s-SWNTs is re-dispersed in chloroform. And the filtrate is collected to rotary evaporation for polymer and solvent recycle. The other part is taken as control. For better


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explanation, the s-SWNTs untreated and washed by THF are denoted as untreated s-SWNTs and CLEAN s-SWNTs.

Figure 1. The diagram of a low-cost and stable method to collect CLEAN s-SWNTs dispersion. The purity of sorted s-SWNTs is demonstrated by the UV-vis-NIR absorption and Raman spectra (Figure S1).41 The M11 absorption of metallic SWNT is from 600 to 800 nm. As Figure 2a shows, it displays a deep valley which means the purity of sorted s-SWNTs is very high.6, 15 Moreover, As the S22 (800-1200 nm) region shows no shift for s-SWNTs washed by THF, it can be inferred that THF has weak interaction with s-SWNTs and no charge transfer happens between s-SWNTs and THF. The absorption peak of PCz is 394 nm and its absorption value decreased very much. This indicates that most of PCz have been effectively removed by this simple method. Raman spectra excited by 2.41 eV (532 nm) is shown in Figure 2b. The peak at 1624 cm-1 belongs to the PCz and disappears after washing, which further demonstrates that most of PCz are removed. The G peaks of CLEAN s-SWNTs show blue shift compared with untreated s-SWNTs because of the different dielectric environment around s-SWNTs.20


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To get insight into the THF treatment process, the effect of THF washing volume is investigated. As Figure 2c shows, the ratio of PCz and s-SWNTs absorption value decreases with the increase of THF washing volume and reaches the lowest level after eight times of THF volume. In the first filtering without THF washing, free polymers are nearly all removed.42,43 As the increasing of washing solvent, remained free polymers and loosely wrapped polymers on sSWNTs continue to decrease until all the remained polymers are the tightly wrapped polymers. In our systems, eight times washing solvent are enough. The surface of s-SWNTs are still wrapped by a small amount polymer because the π-π stacking interaction between the polymer and the nanotube is very strong. Though these tightly adsorbed polymers cannot be removed, they show another advantage to play an important role on preventing the re-aggregation of mono-dispersed SWNTs.23 To estimate the content of residual polymer on s-SWNTs after washing, the absorption spectra of PCz with different concentration in toluene and chloroform are investigated and shown in Figure S2. The absorption value is linear with the concentration of PCz and the relationship curves are built and then analogous to the familiar extinction coefficient of Beer’s Law. As Figure 2d shows, the concentrations of PCz are 2098.37 µg/ml for untreated sSWNTs dispersion and 8.68 µg/ml for CLEAN s-SWNTs dispersion treated by THF, respectively. Near 99% of PCz with free state and loosely wrapped on s-SWNTs are removed after treatment by vacuum filter and THF washing. In order to evaluate residual polymers in this solution more accurately, UV-vis-IR spectra and TG (Figure S3-4) are used and it is deduced that the content of PCz-polymer remained in final product is 45%. The detail of calculation process can be found in the SI.


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Figure 2. (a) The UV-Vis-IR absorption of untreated s-SWNTs and CLEAN s-SWNTs; (b) Raman of untreated s-SWNTs and CLEAN s-SWNTs, and PCz polymer by 532 nm laser; (c) The ratio of A(PCz)/A(SWNT) with different volume of THF washing, each washing with an equal volume of THF; (d) Concentration of PCz in s-SWNTs solution before and after washing calculated by Lambert-Beer's law. According to above analysis, most polymers are removed by THF washing. To make it clear, the diameters of untreated s-SWNTs and CLEAN s-SWNTs are compared. The atomic force microscope (AFM) is used to confirm the diameter distribution of different s-SWNTs by tapping mode. As Figure 3a and 3b shows, the diameter distributions are 1.5-2.8 nm for untreated s-SWNTs and 1.3-2.1 nm for CLEAN s-SWNTs. The average diameters are 2.08 nm and 1.56 nm, respectively.31 The decrease of diameter for CLEAN s-SWNTs further demonstrates that the loosely wrapped polymers are removed. The stability of s-SWNTs


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dispersion after THF washing is also investigated. As Figure 3c shows, the UV-vis-IR absorption spectra of newly prepared and after six months CLEAN s-SWNTs in chloroform are measured and almost the same, which indicates that the tightly wrapped polymers on s-SWNTs are enough to maintain the mono-disperse of s-SWNTs. And the inset picture is the solution after six months, it is stable with good transparency and no precipitation is observed. The stability of CLEAN s-SWNTs solution after six months was further testified by AFM and there is no change for the diameters of s-SWNTs (Figure 3d). This means that the s-SWNTs in re-dispersed solution still remain mono-dispersed after six months. And THF washing solution in chloroform with no precipitation after 20000g centrifugation, it can also elucidate that well stable of CLEAN sSWNTs re-disperse in chloroform. The mono-dispersed and good stability of the obtained polymer-less s-SWNTs is the premise for high-quality thin films.

Figure 3. The diameters of PCz-sorted untreated s-SWNTs (a) and CLEAN s-SWNTs (b). Totally 120 tubes are counted; (c) The UV-Vis-IR absorption of CLEAN s-SWNTs solution new


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and after six months (the inset figure is photograph of solution after six months); (d) The AFM image of CLEAN s-SWNTs solution washing after six months and the film is fabricated by drop method. There are little tightly wrapped polymers in CLEAN s-SWNTs solution, so the s-SWNTs show narrow diameter distribution and good stability. In order to illustrate difference of the two kind of s-SWNTs dispersion. There are three different methods to fabricate thin film. Figure 4a and 4b provide the AFM images of s-SWNTs films fabricated by drop-casting method. There are many large particles and the residual polymers near the s-SWNTs in the untreated solution, which are harmful to related application without further treatments. The s-SWNTs films fabricated by lift-dipping method are shown in Figure 4c and 4d. Higher density and uniform nanotube network film can obtain with the CLEAN s-SWNTs compared with untreated sSWNTs at the same condition. As Figure 4e and 4f show, these two dispersions can form high density film by immersed-coating method. The untreated s-SWNTs solution obtained highquality film just fitting by immersed-coating method. All of three methods, obtained CLEAN sSWNTs shows much cleaner and lower diameter than untreated s-SWNTs, which is in accordance with Figure 3a and 3b. Drop-casting is similar to ink-jet printing, which means this CLEAN s-SWNTs dispersion can be directly used in ink-jet printing without additional washing, and suitable for large area and quickly fabrication film application. And lift-dipping method can achieve large area film rapidly. It’s obvious that CLEAN s-SWNTs dispersion with broader applicability. These mean that the CLEAN s-SWNTs can form high quality thin films than untreated s-SWNTs. Above all, we can obtain a new system of polymer-less s-SWNTs dispersion and can get high performance thin film.


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The most reported method of removing the wrapped polymer on surface of s-SWNTs to obtain a clean dispersion is adding protonic acid to change the electronic environment of the dispersion, such as trifluoroacetic acid (TFA),25,

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so THF and TFA is compared to further

demonstrate the advantage of THF, specific process in Figure S5-7. The experiment results show that this method not an ideal choice for this polymer sorted system. THF washed most loosely polymers and CLEAN s-SWNTs dispersion can be obtained by re-dispersing s-SWNTs in chloroform, which is suitable for various methods of making high-quality film.

Figure 4. The AFM images of s-SWNTs solution before (a) and after (b) THF washing by drop method; (c) and (d) The AFM images of s-SWNTs solution before and after THF washing by lift-dipping method; (e) and (f) The AFM images of s-SWNTs solution before and after THF washing by immersed-coating method.


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In order to further characterize and verify the film by THF washing. Thin-film transistors (TFTs) are applied. Since the drop-casting is not a suitable method for fabricating the film of untreated solution, and immersed-coating method cost too long time. We choose the lift-dipping method to fabricate the thin film. TFTs are the most important application for sorted s-SWNTs. There are still two factors affecting its performance. One is the remaining polymer on the surface of s-SWNTs; the other is enough uniformity and density of the s-SWNTs film. These two problems can be well solved through washing the s-SWNTs with THF from the above analysis. TFTs have been fabricated with these two kinds of s-SWNTs. The structure of TFTs devices are schematically shown in Figure 5a. Top contact (30 nm Pd electrode) and bottom gate TFTs device on 100 nm SiO2 dielectric layer with 2-20 µm channel length and 4-40 µm channel width were fabricated. The source/drain electrode patterns were prepared by lithographic process and Pd electrodes44 were deposited on the s-SWNTs network through thermal evaporation method. Typical transfer curves of TFTs (L=3 µm, W=6 µm) are shown in Figure 5b. The current density of TFTs fabricated with CLEAN s-SWNTs are improved almost 3 times, which demonstrate that the density of CLEAN s-SWNTs dense and higher than the untreated s-SWNTs. The typical transfer curves with logarithmic coordinates are shown in Figure S8, there is 10-20 V hysteresis in transfer curves since no further encapsulation in our devices. The further experiment in our lab demonstrated that the hysteresis can be reduced through package. 45 Figure 5c show the current density and on/off ratio of TFTs fabricated by s-SWNTs before and after THF washing. There are 10 devices with channel length of 3 µm and channel width of 6 µm is counted. The improved current density of TFTs indicates that the contact become better after loosely wrapped polymers are removed. These demonstrate that this polymer removing method can form more clean-surface nanotube-nanotube junctions. The output curves (Figure


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S9) show that the contacts between the s-SWNTs and Pd electrodes are ohmic contact, all the tests are in the linear region. Current densities of different channel length are shown in Figure 5d. It can demonstrate the uniform of s-SWNTs film. The change of contact resistance between the s-SWNTs and source/drain surface are analyzed with Y function method (YFM).3, 46, 47 The detail of calculation process shown in SI. As Figure 5e and 5f shown, the 2Rc of s-SWNTs washed by THF has 20 times lower than untreated s-SWNTs. And we calculated the 2Rc for ten TFTs with untreated s-SWNTs and CLEAN s-SWNTs, the 2Rc of untreated s-SWNTs TFTs range from 5.2 KΩ to 9.9 KΩ, the 2Rc of CLEAN s-SWNTs TFTs range from 0.3 KΩ to 1.7 KΩ. This result further indicates that there are a small amount polymers remained at electrode-SWNTs interface. The stability of s-SWNTs dispersion after THF washing is also important for its application. TFTs fabricated with that s-SWNTs still show high performance, the average mobility is 65 cm2V-1s-1 and the on/off ratio is 105 ~ 106.48 Table S1 shows the comparison of before and after THF washing s-SWNTs solution, which carefully compared difference of two kinds of dispersions. It indicates that THF washing method can obtain new s-SWNTs dispersion with broader application.


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Figure 5. (a) The schematic cross section, optical and SEM images of CLEAN s-SWNTs-TFTs device; (b) The linear transfer curves of s-SWNTs device before and after washing; (c) The current density and on-off ratio of different treatment; (d) current density of different channel by THF washing solution; (e) YFM fits for representative TFTs before and after treatment, respectively; and (f) is 2Rc from (e). 3.

CONCLUSIONS


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In summary, we demonstrated the treatment of PCz sorted s-SWNTs with THF is better to control the content of residual polymer and with good stability. The UV-Vis-NIR and Raman spectra demonstrate that most of PCz are removed. The results of AFM further confirmed that there are no s-SWNTs bundle formed after treatment. The proper content of residual polymer on s-SWNTs can prevent the aggregation of individual s-SWNTs and facilitate the formation of high density and uniform film. And this CLEAN s-SWNTs solution is suitable for various depositing method. TFTs devices fabricated by this CLEAN s-SWNTs show high performance. The contact resistance between the carbon tube and the electrode is reduced by 20 times, and the thin-film transistors fabricated with this s-SWNTs shows 300% enhancement of current density compared with untreated s-SWNTs. 4.

EXPERIMENTAL SECTION

4.1. Materials and Instruments. Raw Arc-discharged SWNTs (AP-SWNTs, AP-A204) were purchased from Carbon Solution Inc. The 9-(1-octylonoyl)-9H-carbazole-2, 7-diyl (PCz, Mw=45 KDa) was prepared by Suzuki polycondensation in relatively high yield. Optical absorption spectra were measured on a UV-Vis-NIR spectrophotometer (Lambda 750). Raman spectra were measured by a LabRAM HR Raman spectrometer from HORIBA Jobin Yvon, equipped with 785 and 532 nm lasers were used for Raman measurements. Atomic force microscope images were recorded on a Veeco Dimension 3100 AFM. The SEM images were using S4800 fieldemission instrument from Hitachi, Japan. The ultrasonic process was completed with a tipsonicator (SONICS VCX500). The Pd electrodes were prepared by Thermal Metal Evaporator. All electrical measurements were carried out in air conditions using a Keithley 4200 semiconductor parameter analyzer.


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4.2. Enrichment of s-SWNTs solution. Dispersants PCz (200 mg) and SWNTs (100 mg) were mixed in toluene (100 mL). Then the solutions were ultrasonic with a top-tip sonicator for 30 min with 40% amplitude and then centrifuged at 20,000 g for 3 h to remove insoluble materials. The upper supernatants were collected after centrifugation. 4.3. Preparation of s-SWNTs film for AFM. The commercially purchased silica wafers were ultrasonic for 5 min with acetone, ethanol, and distilled water successively. Then the wafers were dried by stream of nitrogen and heated at 120 P for 30 min. One method is that the s-SWNTs solutions were diluted by o-xylene for several times according to the concentration of s-SWNTs. The clean silicon substrates were immersed into s-SWNT solution for different time at 50 P. The other is by drop and lift-coating method to fabricate thin film. Finally, drying by nitrogen and heating at 120 P for 60 min. 4.4. Preparation of s-SWNTs film for Raman spectra characterization. The silica was treated above similarly. Two different solutions of before, after THF washing were drop-cast on the substrates. Then the wafers were heating at 120 P for 60 min after nitrogen-drying. 4.5. Fabrication of TFTs. TFTs devices were fabricated on silicon wafer with a 100nm thick thermal oxide layer. The wafers were washed by acetone, ethanol, distilled water for 5 min successively. After thoroughly baking, the chips were lift-dipping in diluted s-SWNT dispersion and obtaining large-area uniform s-SWNTs films. The coated chips were annealed at 120 P for 1 h before lithographic process. Top contact (30 nm thick Pd layer) and bottom gate TFTs device with 100 nm SiO2 based on as-fabrication the source/drain patterns were deposited through thermal evaporation and followed by lift-off process. Finally, a second lithographic process was


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used to define the channel region and the exposed s-SWNTs were removed with oxygen plasma etching. This step also removed undesirable current paths outside the designated channel region. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: *** Additional results (Raman spectra; Calculation of residual polymer in CLEAN s-SWNTs; The results of removing PCz polymer from s-SWNTs by adding TFA into solution; Calculation of the mobility of s-SWNTs TFTs with the parallel plate model and the 2Rc of TFTs with Y function) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was generously supported by National Key Research and Development Program of China (2016YFB0401104), Key Research Program of Frontier Science of Chinese Academy of


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