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Low Hysteresis Carbon Nanotube Transistors Constructed via a General Dry-Laminating Encapsulation Method on Diverse Surfaces Yi Yang, Zhongwu Wang, Zeyang Xu, Kunjie Wu, Xiaoqin Yu, Xiaosong Chen, Yancheng Meng, Hongwei Li, Song Qiu, Hehua Jin, Liqiang Li, and Qingwen Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b02684 • Publication Date (Web): 04 Apr 2017 Downloaded from http://pubs.acs.org on April 6, 2017

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ACS Applied Materials & Interfaces

Low Hysteresis Carbon Nanotube Transistors Constructed via a General Dry-Laminating Encapsulation Method on Diverse Surfaces Yi Yanga,b, Zhongwu Wanga, Zeyang Xua,c, Kunjie Wua, Xiaoqin Yua, Xiaosong Chena, Yancheng Menga,c, Hongwei Lia, Song Qiua, Hehua Jina, Liqiang Lia,*, Qingwen Lia,* a

Advanced Nano-materials Division, Key Laboratory of Nano-Devices and Applications,

Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences (CAS), Suzhou 215123, China. b

University of Chinese Academy of Sciences, Beijing 100049, China.

c

Nano Science and Technology Institute, University of Science and Technology of China,

Suzhou 215123, China. KEYWORDS：：carbon nanotube, thin-film transistors, encapsulation, Low hysteresis, Laminating

ABSTRACT：：Electrical hysteresis in carbon nanotube thin-film transistor (CNTTFT) due to surface adsorption of H2O/O2 is a severe obstacle for its practical applications. The conventional encapsulation methods based on vacuum deposited inorganic materials or wet-coated organic

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materials have some limitations. In this work, we develop a general and highly efficient drylaminating encapsulation method to reduce the hysteresis of CNTTFTs, which may simultaneously realize the construction and self-encapsulation of CNTTFT. Furthermore, by virtue of dry procedure and wide compatibility of PMMA, this method is suitable for the construction of CNTTFT on diverse surface including both inorganic and organic dielectric materials. Significantly, the dry-encapsulated CNTTFT exhibits very low or even negligible hysteresis with good repeatability and air stability, which is greatly superior to the nonencapsulated and wet-encapsulated CNTTFT with spin-coated PMMA. The dry-laminating encapsulation strategy, a kind of technological innovation, resolves a significant problem of CNTTFT and therefore will be promising in facile transferring and packaging the CNT films for high-performance optoelectronic devices.

1. INTRODUCTION The superior electrical (high carrier mobility and current-carrying capacity)1-5 and mechanical properties6-9 of semi-conducting single walled carbon nanotubes (s-SWNTs) enable them to be excellent active material for thin film transistors (TFTs).10-14 Carbon nanotube thin-film transistor (CNTTFT) has been considered as a promising candidate for the next-generation electronic products.15-23 Although the tremendous progresses have been achieved in the field of CNTTFT in recent years, they still encounter some severe problems such as hysteresis24 in the electrical characteristic. The hysteresis, a kind of electrical instability, in CNTTFTs poses a significant challenge in realizing reliable and energy-efficient systems,25 because (1) the hysteresis always leads to the variation of the key TFT parameters such as mobility, threshold voltage, on/off ratio, and subthreshold slope between the forward and backward sweeping, which


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may lower the reliability of circuits; (2) dynamic changes in threshold voltage (VT) reduce the noise margin of digital systems, which requires a larger supply voltage and thus more power. It is generally recognized that hysteresis is mainly attributed to the water and oxygen molecules surrounding the surface of nanotube, which may serve as the charge traps for the fieldeffect induced charges.26-33 In view of the origin of hysteresis, many endeavors have been devoted to reducing hysteresis by excluding the influence of H2O/O2.34-45 The commonly-used strategy is to encapsulate the devices with some materials.38-45 Inorganic materials including Al2O3, HfOX, ZrOX grown by Atomic Layer Deposition (ALD) have been successfully used as encapsulation layer and/or top dielectrics for CNTTFTs. However, the inorganic materials usually transform the CNTTFTs from p-type to n-type or bipolar. Furthermore, its encapsulation performance might be problematic for the flexible electronic devices under the multiple bending operation or small bending radius. Organic materials, mainly referring to the polymer, such as Teflon44 and poly(methyl methacrylate) (PMMA)28, are often used for encapsulating CNTTFTs due to their good film-forming ability, compactness against penetration of gas molecules, chemical inertness, and mechanical flexibility. Generally, polymer encapsulation layer for CNTTFTs is prepared by spin-coating44 or dropcasting45, which are called wet process. In such process, the H2O/O2 or solvent that are already adsorbed on the surface of CNTs are embedded under the polymer layer, which would be difficult to be removed thoroughly by the conventional methods such as annealing or vacuumizing. Furthermore, CNTTFTs are generally built on diverse dielectrics including inorganic and organic materials, so the usage of the solvent in wet encapsulation process yields a risk to destroy some dielectrics that are not robust against solvent treatment. Therefore,


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development of an efficient CNTTFT encapsulation method that is suitable for diverse surfaces is necessary for practical applications, which is however a challenging task. Here we report a dry-laminating encapsulation method (without using solvent in the key steps) for CNTTFTs, which is suitable for both inorganic and organic dielectrics. Such dry-laminating encapsulation method may simultaneously transfer CNT and encapsulate CNTTFTs. After encapsulation, the CNTTFTs on inorganic and organic dielectric surface exhibit very low hysteresis. The percentage of hysteresis to applied voltage range in dry-PMMA encapsulated CNTTFT is around 3%, which is among the best performance reported CNTTFTs encapsulated with organic materials (Table S1). This encapsulation method represents a technological progress in the field of CNTTFT, which is not reported before. 2. EXPERIMENTAL SECTION 2.1. Preparation of s-SWNT Solution. 2.5 mg Arc discharged SWNTs (Carbon Solution Inc) and 5 mg 9-(1-octylonoyl)-9H-carbazole-2, 7-diyl (PCz) were dispersed in 10 mg toluene. PCz has a high selectivity (with a purity of 99.9% ) for s-SWNT from mixed metallic and semiconducting CNTs.46 The solution was ultrasonicated with a top-tip dispergator for 1h at a 30 % power, and then was centrifuged at 20000 g (gravity) for 1 h (Allegra X-22R centrifuge) to remove the bundles and insoluble materials. Then the supernatants were collected to obtain the needed s-SWNT solution. The concentration of sorted s-SWNT can be measured to be 0.027 mg/ml through absorption spectra (for details, see Supporting Information, S1). 2.2. Preparation of PMMA/CNTs Film. Silicon wafer modified by Octadecyltrichlorosilane (OTS) (5 µl, 45 min in the vacuum oven) was used as substrate. Uniform random network CNT film was prepared by placing OTS-modified substrate into solution and kept overnight. 9 % concentration of poly(methyl methacrylate) (PMMA) was dissolved with toluene, and magnetic


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stirring was performed to obtain homogeneous solution. Then the PMMA solution was spincoated onto the CNT substrate and then baked at 80 oC to remove the solvent. The thickness of PMMA film usually keep about 1~5 µm due to the considerations about convenience in peeling and adhesion strength to substrate (For details, see Supporting Information, S2). The PMMA film with s-SWNT on the backside was mechanically peeled off by tweeze after the solvent fully volatilized. 2.3. Fabrication of SWNT TFTs. The TFTs have a structure of back-gate and bottom electrodes. Heavily doped p-type Si wafer was used as the substrate, and thermally grown 300 nm SiO2 was used as the dielectric. A 0.5 nm thick Cr and 20 nm thick Au bilayer as source and drain electrodes were fabricated by standard photolithography and thermal evaporation on 300 nm SiO2/Si substrate. The PMMA/CNTs film was laminated onto the devices in a vacuum oven at 90-100 oC for 2 h. 2.4. Characterization Methods. AFM images were obtained by Veeco Dimension 3100 in tapping mode. SEM images were taken by S4800 field-emission instrument from Hitachi, Japan. Raman spectra were gotten by a Lab Ram HR-800 Raman system from Horiba JobinYvon at 532 nm

excitation.

Electric performance

was

tested

by Keithley 4200

Semiconductor

Characterization System. Device mobility was calculated using the equation: =

∙

Where dId/dVg is the transconductance according to the slop of the forward and backward transfer curves, L and W are the channel length and width respectively, and Ci is the specific capacitance per unit area of the active channel. Capacitance of 300 nm SiO2 is 11.05 ×10-5 F/m2, and the channel length and width for device with SiO2 dielectrics are 20 µm and 400 µm respectively. Capacitance of 680 nm PMMA was measured to be 5.6×10-5 F/m2 at the frequency


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of 1000 Hz, and the channel length and width are 50 µm and 1000 µm, respectively. For all devices Vd is -1 V. 3. RESULTS AND DISCUSSION 3.1. Fabrication Procedures of the Dry-Laminating Encapsulation Method. Figure 1 illustrates the procedures of dry-laminating encapsulation method for CNTTFTs, which consists of four main steps: firstly, PMMA film is prepared on CNT network by spin-coating; Secondly, mechanical peeling-off yields PMMA film with CNTs on the backside; Subsequently, transferring and laminating PMMA/CNTs film on the target substrate with pre-patterned electrodes to produce PMMA-covered CNTTFTs; Finally, vacuumizing and annealing the PMMA-covered CNTTFTs are used to complete the dry-laminating encapsulation, and this step may remove the trace water, oxygen, and solvent at the buried interface and increase the interface adhesion between PMMA and substrate.

Figure 1. Schematic fabrication procedures of dry-laminating encapsulation method for CNTTFTs with PMMA film. (a) (b) The formation of the dry PMMA/CNTs film by spincoating. (c) (d) Transferring and laminating PMMA/CNTs film onto the pre-patterned electrodes.


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(e) Completing the encapsulation by annealing and vacuumizing. (f) Photograph of the lamination process. In the first step, a uniform random semiconducting single-walled carbon nanotube (s-SWNT) network film is deposited on the octadecyltrichlorosilane (OTS) modified silicon wafer substrate by immersing the substrate in s-SWNT solution (for details, see Experimental Section). AFM measurements (Figure 2a) confirm the existence of s-SWNT network film on the substrate surface and it also reveals the high density as well as good uniformity and connectivity of sSWNT network film. PMMA film is prepared on s-SWNT network by spin-coating. In this work, PMMA is selected to transfer and encapsulate s-SWNT since it has good interface adhesion and a low water absorptivity of 0.3%.47 After the evaporation of solvent, PMMA layer is mechanically peeled off from the OTS-modified substrate in the second step. It should be noted that OTS modification is a key trick for the peeling treatment, since OTS layer has low surface energy and weak adhesion with PMMA (for details, see Supporting Information S4). We have tried to peel PMMA from bare silicon substrate, but fail to get a whole PMMA film (Figure S3). On the contrary, it is quite easy to peel PMMA off over large area (4 inch wafer size) from the OTS modified substrate, as shown in Figure 2b. After peeling off PMMA, the silicon wafer surface become quite clean without detectable CNTs confirmed by AFM measurement (Figure 2c). To verify the existence of the CNTs on the backside of PMMA film, we measured the Raman spectra and SEM image of this film. As shown in Figure 2d, the characteristic RBM peak centered at 169 cm-1 is clearly observed, which confirms that the s-SWNT network has been transferred from SiO2/Si substrate to the PMMA film. From Figure 2e, the transferred s-SWNT network is also of high density as well as good uniformity and connectivity, which is meaningful for the construction of TFT devices in the following steps.


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Figure 2.Characterizations of the CNTs before and after peeling-off. (a) AFM image of a random CNTs network film on the silicon substrate. (b) Optical image of peeling off the PMMA/CNTs film from 4 inch silicon wafer. (c) AFM image of the silicon substrate after PMMA/CNTs film being peeled off, showing no CNTs left. (d) Raman spectrum and (e) SEM image of the backside of the PMMA film, confirming the existence of CNTs. The first and second steps realize the efficient transfer of uniform CNT network from rigid silicon substrate to flexible PMMA film. In fact, the preparation of uniform CNT network on arbitrary substrate is very important for diverse applications,48,49 but it is challenging because the high growth temperature in CVD process or the solvent effect in solution process may limit the selection of substrate materials. Accordingly, the preparation of uniform CNT film on certain substrate and then transferring CNT film to the target substrate would be another feasible


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strategy. The free-standing PMMA film with CNTs on one side provides the possibility to transfer CNTs to another target substrate by simple dry-laminating process. As the first test, the PMMA/CNTs film is laminated onto Si/SiO2 substrate (Figure 1c, d) on which the source and drain electrodes (0.5 nm Cr and 20 nm Au, Figure 1d) are pre-patterned by standard photolithography and thermal evaporation. After lamination, the PMMA covered CNTTFTs are obtained (Figure 1e). At this stage, the PMMA/CNTs film can still be re-peeled off from the substrate, indicating the weak interface adhesion. To complete the encapsulation, the as-prepared device is annealed at 80 oC under vacuum for about 30 min (Figure 1e). The vacuumization process is used to remove the residual water, oxygen, and solvent on the surface of PMMA, CNTs and SiO2, and the annealing process may render the PMMA to occur glass transition, and thus to adhere to the SiO2 surface tightly. The above processes indicate that our method may simultaneously realize the transferring of s-SWNT film and the construction and encapsulation of CNTTFTs. After annealing, we have tried to re-peeling off PMMA layer, but fail to get a whole PMMA film (Figure S5). This result indicates PMMA film and substrate are tightly adhered each other, which is a favorable factor for effective encapsulation.


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a

b

10

-8

-10

Au

s-SWNT

SiO2 Si

1V

Au

Backward

Vd=-1V VH= 1V μF=16.28 μB=15.52

(mA)

0.0

PMMA

ds

I (A) ds

10-6 10

0.4 Forward

I

10

-4

-0.4

20V -0.8

c

-20V

-1.2

-30 -20 -10 0 10 20 30 V (V) gs

-40

-30

d -4

10-8 10-10

33.4V

0

10

Vd=-1V VH= 33.4V μF=19.77 μB= 47.33

-30 -20 -10 0 10 20 30 V (V) gs

10-6

I (A) ds

10-6

-20 -10 V (V) ds

10-4

10 I (A) ds

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10-8 10-10

5.5V Vd=-1V VH= 5.5V μF=1.35 μB=1.48

-30 -20 -10 0 10 20 30 V (V) gs

Figure 3.(a) Transfer curves and (b) output curves of dry-PMMA encapsulated CNTTFT with a negligible hysteresis of 1 V. Inset in a denotes the section structure of CNTTFT. (c) Transfer curves of non-encapsulated CNTTFT showing obviously large hysteresis of 33.4 V. (d) Transfer curves of encapsulated CNTTFT with spin-coated PMMA showing relatively large hysteresis of 5.5 V. The channel length and width for all the devices are 20 and 400 µm, respectively. 3.2. Electrical Characterization of CNTTFTs with and without Encapsulation. To verify the performance of the dry-laminating encapsulation method, the electrical properties of nonencapsulated and dry-PMMA encapsulated CNTTFT are measured (inset of Figure 3a). Typical transfer and output characteristics of dry PMMA-encapsulated CNTTFT are given in Figure 3a and 3b, respectively. Output curves demonstrate the good Au/CNT contact in this transfer process, which is achieved through the following strategies: annealing under vacuum ensures the


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thorough exclusion of air at the interface; high density CNT network increases the Au/CNT contact area; clean environment minimizes the contaminations. The transfer curves reveal that dry-PMMA encapsulated TFT exhibits good performance with very low hysteresis of 1 V (the hysteresis is defined as the gate voltage difference at which the logarithm of Ids takes the half maximum Idmax/2-Idmin/2 between the forward and backward sweeping50). This hysteresis value makes a 1.67 % percentage of the applied voltage, which is smaller than that (4-20 %) of CNTTFT encapsulated with organic materials (Table S1). Until now, there are some reports with 1 V hysteresis or less than 2 % of power supply,51-54 most of which use inorganic oxide as encapsulation layer. They hold great potential for high-end applications such as nano-scale transistor and circuits.55 Although the encapsulation performance in this work is a little bit inferior to the best values with inorganic materials, polymer encapsulation materials may be applicable for the low-cost and flexible CNTTFT devices due to their intrinsic features. The mobility in the forward and backward sweeping of encapsulated TFT is calculated to be 16.28 and 15.52 cm2 V-1 s-1. The threshold voltage (~ 6 V) also shows negligible difference. On the contrary, the non-encapsulated TFT (Figure 3c) shows large hysteresis of 33.4 V, which is about 55.67 % of applied voltage. As a result of large hysteresis, the mobility of nonencapsulated CNTTFT in the forward and backward sweeping is 19.77 and 47.33 cm2 V-1 s-1, respectively, and the difference (27.56 cm2 V-1 s-1) is significantly larger than that (0.76cm2 V-1 s1

) of encapsulated TFT. Moreover, the threshold voltage (26.0 V and -10.1 V) also shows big

difference. In the non-encapsulated CNTTFT, the traps caused by H2O/O2 surrounding the CNT may trap the charge, limit the charge transport, and lower the mobility during the forward sweeping. If the traps are deep enough, the trapped charge could not be released immediately, which means the traps are filled. Then, during the backward sweeping, the charged (or filled)


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traps would exert little effect the charge transporting, which may therefore enhance the mobility. However, the trapped charged may screen part of gate voltage and thus results in a big threshold voltage shift. In fact, TFT is generally used as switch or amplifier in the circuits, and is frequently switched between off-state and on-state under the driving of operation voltage. The different mobility between the low-to-high and high-to-low bias operation may result in distinct switching speed. Moreover, the different threshold voltage may reduce the noise margin and thus lower the immunity of circuits toward the noise. All these phenomena of non-encapsulated CNTTFTs will greatly deteriorate the reliability of the circuits. In contrast, the dry-laminating encapsulation method may significantly improve the reliability of CNTTFT. Previous researches have reported the encapsulation of CNTTFTs with PMMA by solution method. Lorraine RISPAL et al38 used a novel self-aligned fabrication process to produce PMMA-passivated CNTFETs. This method improves the hysteresis, but the percentage of hysteresis to supply voltage is still as high as 9.67 %, which suggests that the encapsulation does not reach the ideal level. There are also researches reporting the encapsulation of CNTTFTs by spin-coating PMMA31, but clear hysteresis can still be observed after encapsulation, indicating the unsatisfactory effect of spin-coating encapsulation method. For comparison, we also prepared PMMA encapsulation layer on the CNTTFTs by spin-coating. As shown in Figure 3d, after spincoating PMMA and annealing under vacuum, hysteresis decreased to 5.5 V, which is smaller than that (33.4 V) of non-encapsulated TFT, but is still larger than that (1 V) of dry PMMA encapsulated TFT. Furthermore, the mobility of wet-encapsulated CNTTFT is lower than dryencapsulated and non-encapsulated devices. During the spin-coating process, PMMA solution can flow into the space among CNTs network and separate CNTs so that the conducting channel


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may partly be cut off. Therefore, the effective CNTs conductive paths decrease, which reduces the mobility and results in lower on-current. The reason for the better encapsulation performance of dry-encapsulation method is that it ensures

the

thorough

exclusion

of

water,

oxygen

and

solvent

residue

at

the

PMMA/CNT/dielectric interface, as schematically illustrated in Figure S6. Quantitative analysis54,

56-59

indicates that the interface traps caused by H2O/O2 for dry-capsulated, non-

encapsulated, and wet-encapsulated CNTFT are 6.9 × 1010/cm2, 2.31 × 1012/cm2, and 3.81 × 1011/cm2, respectively (for details, see Supporting Information, S7). From these data, it can be clearly deduced that dry encapsulation may significantly reduce the density of deep traps compared with non-encapsulation and wet-encapsulation.

b 10-3

10-5

10-5 I (A) ds

a 10-3

10-7

26V 43V

I

ds

(A)

17V

-9

10

10-11

-40

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0 20 V (V) gs

10-7 10-9

40

10-11

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10-5 I (A) ds

-3 d 10

10-7

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10-7

0 20 (V) gs

40

1day 30days

10-9

10-9 10-11

-40

V

c 10-3

I (A) ds

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-30 -20 -10 0 10 20 30 V (V) gs

10-11

-30 -20 -10 0 10 20 30 V (V) gs


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Figure 4.(a) Transfer curves of non-encapsulated CNTTFT and (b) dry-PMMA encapsulated CNTTFT under different voltage range. (c) 10 times cyclic test and (d) storage stability of the dry-PMMA encapsulated CNTTFT. The channel length and width for all the devices are 20 and 400 µm, respectively. It is reported that hysteresis may vary with the applied bias.60 Therefore, we test the electrical property of non-encapsulated and dry-PMMA encapsulated CNTTFT under different voltage range. In Figure 4a, the hysteresis of non-encapsulated TFT is 43 V when the applied bias ranges from -40 V to +40 V. The corresponding hysteresis is 26 V and 17 V for the applied voltage range of ± 30 V and ± 25 V, respectively. In contrast, the dry-PMMA encapsulated TFT exhibits little difference along the various applied bias (Figure 4b), which verified the excellent encapsulation performance of this dry method. The above dry-encapsulation method simultaneously realizes the transferring of CNT and construction of encapsulated CNTTFT. We have also tried to encapsulate CNTTFTs with peeled pure PMMA film, which shows good TFT performance with negligible hysteresis (Figure S7). This attempt indicates our dry method would be applicable for the direct encapsulation of asfabricated CNTTFTs. 3.3. Operational and Storage Stability of Dry-PMMA Encapsulated CNTTFT. We also test the operational and storage stability of the dry-PMMA encapsulated devices. Figure 4c shows the transfer characteristic of a TFT through cyclic test for 10 times. These curves coincide very well and there is almost no shift with the increase of the sweeping times, indicating the good operational stability by the encapsulation. The storage stability is shown in Figure 4d, from which it can be found that electrical performance of CNTTFT does not change much after stored in ambient air for 30 days. This result suggests that, even stored in atmosphere, water and


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oxygen molecules hardly reenter the CNT/dielectric contact interface from either the PMMA film or the PMMA/substrate interface because of the hydrophobic property, low gas permeability, and strong adhesion of PMMA.

Figure 5.Statistical data of 30 CNTTFTs. (a) Transfer curves and (b) enlarged panel of 30 dryPMMA encapsulated CNTTFTs. Histograms of the (c) hysteresis, (d) mobility, (e) log of the


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ON/OFF current ratio, and (f) threshold voltage for 30 dry-PMMA encapsulated CNTTFTs. The channel length and width for all the devices are 20 and 400 µm, respectively. 3.4. Reproducibility of the Dry-Laminating Encapsulation Method for CNTTFTs. To test the reproducibility of this dry encapsulation method, several batches of TFTs are fabricated, and almost all the devices work very well in TFT mode with small hysteresis, which denotes the high success rate for device fabrication and encapsulation. The transfer curves and enlarged panel of 30 TFTs are displayed in Figure 5a, and 5b, respectively. All the TFTs exhibit hysteresis lower than 3 V (Figure 5c), among which there are two hysteresis-free TFTs. This result strongly confirms the good reproducibility of the dry encapsulation method. The distribution of mobility of 30 CNTTFTs is shown in Figure 5d. About 56 % TFTs have mobility of 1-5 cm2 V-1 s-1, 27 % ranges from 5 cm2 V-1 s-1 to 10 cm2 V-1 s-1, and 17 % is higher than 10 cm2 V-1 s-1. The maximum mobility is 16.28 cm2 V-1 s-1 with a hysteresis of 1 V which makes 1.67 % percentage of the supply voltage. In fact, the transfer and encapsulation processes exert small effect on the mobility, while the mobility range of encapsulated TFT mainly arises from the variation of quality and uniformity of CNT network film prepared via solution process. The distribution of log (on/off current ratio) and threshold voltage are shown in Figure 5e and 5f, respectively, both of which are in small range as well. All these results suggest the dry encapsulation method is effective and repeatable. 3.5. Construction of dry-PMMA encapsulated CNTTFTs on Organic Dielectric Surface. The above work constructs encapsulated CNTTFTs on rigid silicon oxide dielectrics. Until now, most of high mobility CNTFTs are based on inorganic dielectrics61-63 that are generally deposited under vacuum and/or high temperature. In contrast, organic dielectrics generally yield low or moderate performance. Therefore, organic dielectrics might be unsuitable for high-end


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applications. However, organic dielectrics possess some unique features including low-cost, mechanical flexibility, low temperature processing, and optical transparency. Therefore, CNTTFT with organic dielectrics may have great application potential in low-cost and flexible electronics, which is currently a hot research topic. However, due to the low robustness against solvent of some organic dielectrics, the deposition and the encapsulation of CNT through solution need the selection of orthogonal solvent, which may limit their application. For example, in this work, the solvent for CNT and PMMA is toluene, so it would be impossible to deposit CNT on PMMA film directly with solution method. As demonstrated above, the dry-laminating method may simultaneously realize the transferring of CNTs and encapsulation of CNTTFTs without usage of solvent, so it may have great potential for the organic dielectrics. As a test, we fabricated CNTTFTs on PMMA dielectrics (inset of Figure 6) through the similar procedures (Figure 1). Since no solvent is used in the key steps, polymer dielectrics would have more choices. After encapsulation, the CNTTFT with PMMA dielectrics show good performance with negligible hysteresis (4 V), which yields a percent of 2.5 % to the applied voltage range. High operation voltage is due to the thick PMMA dielectric layer (680 nm) with smaller capacitance. The demonstration of dry-encapsulation strategy on organic dielectric provides great potential for the construction of ultra-flexible CNTTFT and circuits with organic dielectric and encapsulation materials.


17


a

b

10

10-7

-80 -60 -40 -20 0 20 40 60 80 V (V) gs

(mA)

-6

PMMA Au s-SWNT Au 4V PMMA Si Vd= -1 V，， VH= 4 V μF= 18.78 μB= 19.64

ds

10-5

I

(A) ds

10-4

I

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0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -40

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20V 10V 0V -10V -20V

-30

-20 -10 V (V) ds

0

10

Figure 6.(a) Transfer curves and (b) output curves of dry-PMMA encapsulated CNTTFT on organic dielectric with a very small hysteresis of 4V. Inset in a denotes the section structure of CNTTFT. The channel length and width for all the devices are 50 and 1000 µm, respectively. 4. CONCLUSION In summary, we develop a facile and efficient dry-laminating encapsulation method to realize by laminating low hysteresis CNTTFTs with excellent repeatability and air stability, which shows good compatibility with both inorganic and organic dielectrics. The comparative experiments confirm that the hysteresis of dry-encapsulated CNTTFT is remarkably smaller than that of non-encapsulated and wet-encapsulated CNTTFT. Such encapsulation strategy provides a novel remedy for the hysteresis, a significant problem, of CNTTFT. The excellent encapsulation performance, low cost, low processing temperature, no solvents usage, wide applicability for diverse substrate, transparency, flexibility, and large-area coverage enable the dry-laminating encapsulation method to be highly promising in the field of CNT-based electronics as well as other electronics based on air-sensitive materials such as 2D materials and organic semiconductors.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interests. Author Contributions L.L. designed the experiments. Y.Y. performed the experiments. Z.W. Z.X., K.W., X. C., Y.M., H.L., J. H., S. Q. assisted in the device fabrication and characterization. L.L. Y.Y. and Q.L. cowrote the manuscript. L.L. and Q.L. supervised the project. All authors discussed the results and commented on the manuscript. ACKNOWLEDGMENT The

authors

are

grateful

to

National

Key

Research

and

Development

Program

(2016YFB0401104), Key Research Program of Frontier Sciences of Chinese Academy of Sciences (QYZDB-SSW-SLH031),National Natural Science Foundation of China (21573277,


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