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Flexible, Transparent, Thickness-Controllable SWCNT/ PEDOT:PSS Hybrid Films based on Coffee-Ring Lithography for Functional Noncontact Sensing Device Yan Long Tai, and Zhen-Guo Yang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03449 • Publication Date (Web): 09 Nov 2015 Downloaded from http://pubs.acs.org on November 10, 2015

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Flexible,

Transparent,

Thickness-Controllable

SWCNT/PEDOT:PSS Hybrid Films based on Coffee-Ring Lithography for Functional Noncontact Sensing Device Yan-Long Tai, 1,2 Zhen-Guo Yang 1*

1. Department of Materials Science, Fudan University, Shanghai 200433, China. 2. Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia

ACS Paragon Plus Environment

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ABSTRACT Flexible transparent conductive films (FTCFs) as the essential components of the next generation of functional circuits and devices have attracted more attentions at present. Here, a new strategy has been demonstrated to fabricate thickness-controllable FTCFs through coffee ring lithography (CRL) of single wall carbon nanotube (SWCNT)/poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS) hybrid ink. The influence of ink concentration and volume on the thickness and size of hybrid film has been investigated systematically. Results show that the final FTCFs present a high performance, including a homogeneous thickness of 60 - 65 nm, a sheet resistance of 1.8 kohm/sq, a visible/infrared-range transmittance (79 %, PET = 90 %), and a dynamic mechanical property (>1000 cycle, much better than ITO film), respectively, when SWCNT concentration is 0.2 mg/ml, ink volume is 0.4 µl, drying at room temperature. Moreover, the benefits of these kinds of FTCFs have been verified through a full transparent, flexible noncontact sensing panel (3×4 sensing pixels) and a flexible battery-free wireless sensor based on humidity sensing mechanism, showing excellent human/machine interaction with high sensitivity, well stability and fast response/recovery ability.

KEYWORDS Flexible transparent conductive films (FTCFs), coffee ring lithography (CRL), single wall carbon

nanotube

(SWCNT)/poly(3,4-ethylenedioxythiophene)-polystyrene

sulfonate

(PEDOT:PSS) hybrid ink, flexible sensing device, human/machine interaction.


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1. Introduction Flexible electronics will be the main characteristic of the next generation of functional circuits and devices, such as flexible field-effect transistors, flexible sensor, stretchable actuator, bendable solar cell, and elastic bionic devices.

1-3

As its important component, flexible and

transparent conductive film (FTCF) is still the largest obstacle despite nearly two decades of research.

At present, TCFs based on indium tin oxide (ITO) have presented many disadvantages, such as high cost due to the limited reserve of indium, poor durability due to the brittle nature of ITO, and the complex manufacturing process which includes photolithography and etching where patterning is required. 4-6

Therefore, more interests were attracted to develop alternatives for the present ITO, including various nanomaterials, especially carbon nanotube which combines high transparency and conductivity, chemical and thermal stability, stretchability, and low contact resistance with organic materials. 7-9 Then, TCFs are fabricated through the methods of dip/spin/spray coating, 10 drop casting,

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chemical exfoliation,

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layer-by-layer (LbL) assembly

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and inkjet printing.

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However, for dip/spin/spray coating and drop casting, the thickness and homogeneity are not easier to control, along with serous pollution and waste generates usually. exfoliation results in huge pollution and critical process.

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Chemical

LbL assembly leaves residual

dispersion agents in the film and leads a higher contact resistance.

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Inkjet printing brings high


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cost and low efficiency.

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Therefore, it is still crucial to find a convenient way to assemble

transparent films with controlled thickness and high quality. Coffee ring effect (CRE) is a very ubiquitous phenomenon, in which when a drop of liquid dries on a solid surface, its suspended particulate matter is deposited in ring-like pattern.

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This is

resulted from the capillary flow and marangoni flow in the liquid drop during drying process. 24 Normally, it can be an obstacle in numerous applications that require homogenous solute deposition for film or hump accumulation for track, motivating researchers to intensively seek ways to control the CRE. The main contribution contains: the particle shape (sphere to ellipsoids),

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solution formula (solvent with high boil point and low surface tension),

photosensitive surfactants as template, CRE).

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28, 29

26, 27

and baking condition (low temperature to avoid

Recently, some researches also have tried their efforts to utilize the CRE, and the

successful instance is to fabricate TCF with micro-scale stacked conductive ring.

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More

interesting, parallel conductive lines also can be generated by CRE and was used to fabricate TCF with its net grid structure.

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It also should be noted that, between the totally

homogeneous film and total coffee-ring pattern for TCFs, there should exist ultrathin nanofilm within the coffee-ring pattern. However, seldom attention was attracted, up to now. Motivated by above, we try to reveal this ultrathin nanofilm with the controlled thickness via single wall carbon nanotube (SWCNT)/PEDOT:PSS ink, and further to discuss its application to flexible, transparent noncontacting-sensing device. First,

stable

and

monodispersed

SWCNT

ink

was

prepared

through

poly(3,4-

ethylenedioxythiophene)/polystyrene sulfonate (PEDOT:PSS) as dispersant. Due to the π–π intermolecular interaction between SWCNTs and the conjugated thiophene chains in


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PEDOT:PSS, PEDOT:PSS can inhibit the aggregation of SWCNT effectively. This kind of ink will provide the material guarantee for the following fabrication of transparent ultrathin film (Figure 1a). Second, the previous SWCNT/PEDTO:PSS ink was dropped on various substrates, a transparent homogeneous ultrathin film generates along with the emerge of conductive ring during the evaporation process at room temperature. The formation mechanism of these TCFs based on CRE was analyzed systematically (Figure 1b), demonstrating out by the TCF fabrication with controlled thickness through different drop volumes and ink concentrations (Figure 1c and 1d). Third, the performance of those transparent ultrathin nanofilms were also evaluated through the analysis of transmittance, sheet resistance, mechanical properties, and the performance stability in detail. In addition, to verify the efficiency of the prepared transparent ultrathin films, these coffee rings were integrated into ITO based pattern, fabricating a full transparent, flexible noncontact sensing panel with 3×4 sensing pixels and flexible battery-free wireless sensor. These devices will exhibit excellent human/machine interaction with high sensitivity, well stability and fast response/recovery ability based on humidity sensing mechanism. It also should be point out that this kind of FTCFs not only can be fabricated effectively, but also is fully compatible with the industrial-scale manufacturing of flexible devices, which permits scalable production at a significantly lower unit-cost.


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Figure 1. a) Schematic illustration of the preparation of SWCNT/PEDOT:PSS hybrid films through CRL. Insets are SWCNT/PEDOT:PSS hybrid inks with different concentrations (0.1 mg/ml, 0.2 mg.ml, 0.3 mg/ml, from right to left) (i), and their TEM image (ii), contact angle on PET substrate (iii); Scale bar is 50 nm. b) Formation Mechanism of TCFs through coffee ring lithograph. c) Optical images of the prepared SWCNT/PEDOT:PSS hybrid films with different volumes (0.2 µl, 0.4 µl, 0.6 µl, from left to right) and concentrations (i, 0.1 mg/ml; ii, 0.2 mg/ml; iii, 0.3 mg/ml) drying at room temperature, along with their surface profiles; Scale bar is 1 mm. d) Relationship between the heights of the hybrid film/contact line and the ink volume/ concentration.


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2.

Results and discussion

2.1 Theoretical consideration As described above, when a drop of liquid dries on a solid surface, CRE plays a dominant role. Usually, there will generate three different surface profiles finally, incl. hump accumulation (i), homogenous solute deposition (ii), ring like deposition (iii), respectively, depending on the liquid physical parameters and baking environments, as shown in Figure 1b. 19, 25, 36 In mechanism, these characteristic profiles can be ascribed to the interaction outcomes of the pinned contact line, the outward capillary flow and the inward marangoni flow in the drying liquid drop, which are independent of the particular substrate, carrier fluid or deposited solids, and have been really observed in previous research. In specific, for the pinned contact line, it has a major impact on the formation of coffee ring pattern, and is mainly up to the contact angle (ω) of solution on substrate. Higher or lower ω will lead the slide of contact line to form a close-packed hump structure or spreading film, only under the appropriate ω, the contact line can be pinned, coffee ring structure can be obtain and stable. For outward capillary flow, it is driven by thermal gradients between the edge (higher) and the interior (lower) of the drying drop, or the unbalance evaporation. The resulting outward flow can carry all the dispersed material to the edge to promote the formation of a coffee ring structure. For inward marangoni flow, it is driven by surface tension gradients between the edge (lower) and the center (higher) of the drying drop. The resulting inward flow can carry the dispersed material back to the center from the edge, showing total inversely performance with capillary flow to generate a homogeneous deposition.


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Therefore, it can be found that, under the pinned contact line, the two flows discussed above can lead to circulating flows. The stronger inward marangoni flow will bring the surface profile of hump accumulation, inversely, the surface profile of ring like deposition is generated from the stronger outward capillary flow. With the well balance of both flows, surface profile of homogenous solute deposition can be obtained. Conclusively, these findings provide a theoretical basis to control the CRE to fabricate the desired surface profile. However, the real situation is more complex, and almost all the experimental conditions will affect this formation process of coffee ring pattern.

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According to Equation (1), it summarize

the deposition mass in the ring within the time (t) required for a point starting at rt to move to the contact line R. R

t = ∫ dr / v ∝( R − rt )1+λ (1) rt

In which ῡ is the flow velocity determined by solvent evaporative flux, λ is from contact angle (θc) between drop and substrate via the relationship of λ = (∏-2θc)/(2∏-2θc). It can be found that the influence factors include environment temperature/pressure, solvent evaporation rate, surface tension or wetting contact angle of solution, as well as solute concentration/micromorphology and the dosage and type of dispersant. The above parameters can be used to restrain or strength the above three actions to obtain the desired structure normally. In our case, the transparent ultrathin film generated during the mutual conversion between of (ii) and (iii) in Figure 1b marked with red frame, will be revealed with the controlled thickness through different ink concentrations and volumes as demonstration.


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Figure 2. SEM images of the generated SWCNT/PEDOT:PSS coffee ring patterns through CRL with different ink concentrations. a) 0.1 mg/ml; b) 0.2 mg/ml; c) 0.3 mg/ml. The scale bars are 1 mm, 100 µm, 1 µm, 1 µm, respectively from left to right.

2.2 Preparation of SWCNT/PEDOT:PSS hybrid film To confirm the above analysis, SWCNT/PEDOT:PSS hybrid inks with different concentrations were prepared and used to fabricated transparent ultrathin film through CRL. Noted that SWCNT with the outer diameter of 1-2 nm and the length of 5-30 µm was chose as transparent conductive material mainly because of its high aspect ratio (up to 250030000). This properties can lead a low percolation threshold value for a continuous and steady conductive network. Especially, after surface modification via PEDOT:PSS, every single


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SWCNT can be identified with clear boundaries from it TEM images (Figure. S1), which was coated with a very thin and unsmooth PEDOT/PSS layer (approximate 2 - 4 nm) via ∏–∏ interactions. Thus, the resulting inks with different concentrations were very stable for at least 1 month without apparent precipitation, as shown in the inset image of Figure 1a. The above SWCNT/PEDOT:PSS hybrid inks were dropped on PET substrate, showing the contact angles between 67 ° - 71 °, as shown in Figure 1a. After dried at room temperature, the desired coffee ring patterns with relevant surface profiles can be seen in Figure 1c. Results show that 1) coffee ring patterns with transparent uniform films can be generated effectively. 2) with the increase of solution volume from 0.2 ul, 0.4 ul, to 0.6 ul with the same ink concentration, the diameter of the final coffee ring patterns increases from 1.6 mm, 2.3 mm, to 2.6 mm. Meanwhile, the height of contact lines and hybrid films also shows the same phenomenon though it is not evidently. So, the gradually distinct contact line is due to the increase of its width. Whereas, the transmittance of the hybrid film almost doesn’t change. 3) with the increase of solution concentration from 0.1 mg/ml, 0.2 mg/ml, to 0.3 mg/ml with the same ink volume, the height of contact lines and hybrid films increase sharply from 210 -350 nm, 520 - 650 nm, to 950 - 1200 nm, from 30 - 50 nm, 65 - 85 nm, to 120 - 140 nm, respectively, indicating more SWCNT deposition. These can be confirmed by the color variation of the relevant contact line and the transmittance change of hybrid film. Whereas the diameter of the final coffee ring pattern are almost the same. This can be explained that these ink drops (only different concentration) show the similar contact angle, which means the same pinned contact line, as well as the same inward/out flows.


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However, it should be emphasized that the expected structure (coffee ring pattern with a homogeneous TCF) can be fabricated easily under the present physical properties of our inks and drying conditions. Moreover, the contact line doesn’t shrink during baking process, which has been reported before. These performance have to owe to the excellent homogeneity of SWCNT/PEDOT:PSS inks, which can ensure that the deposition of SWCNT is uniform, and ink evaporating from the edge is replenished by ink from the interior stably and continuously. The free PEDOT:PSS and the coating on SWCNT also provided some contribution due to its well film-forming ability, which can be confirmed in TEM images in Figure S1. In addition, SEM was also used to further exhibit these variations of the prepared coffee ring patterns intuitively, incl. the whole coffee-ring pattern, the contact line, and the microstructure of the contact line and the hybrid film, as presented in Figure 2. It can be found that hybrid films can be easily distinguished by a strong color contrast. The diameters of the coffee ring pattern from the same drop volume (0.2 µl) are almost the same no matter the how the ink concentrations are. The width of contact line also increase obviously along with the concentration from 0.1 mg/ml to 0.3 mg/ml. These results are consistent with the described above. More interesting, the stack density of SWCNT/PEDOT:PSS in the contact line is higher than that in ultrathin film. This microstructure difference indicates that the deposited speed of SWCNT is quicker than that of free PEDOT:PSS in drop solution. This is because PEDOT:PSS has better solubility than PEDOT:PSS coated SWCNT. To further verify the efficient of this CRL, the larger or smaller coffee ring patterns were fabricated on PET substrate, as well as aluminum foil and silicon wafer, as shown in Figure S2. Results shows that the coffee ring pattern with uniform transparent film can be prepared from 5


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cm to 300 µm (low to 10 µm in theory), regardless of the drop volumes and substrates. 37, 38 This phenomenon is consistent with the analysis in theoretical consideration. Moreover, the homogeneous of the hybrid film generated within the ring pattern was also investigated through its surface morphology. Results in Figure S3 demonstrated that the very similar micro morphologies of the nine points in a sample were presented, indicating the uniform deposition of SWCNTs during the drying process. Generally speaking, TCFs can be fabricated by CRL without substrate and size dependency, showing the controlled TCF thickness through different ink concentrations and the controlled TCF size through different ink volumes effectively.


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Figure 3. Performance of the SWCNT/PEDOT:PSS hybrid films. a) Raman spectra, inset shows the shift G peak; b) Sheet resistance; c) Optical transmittance of hybrid films generated by the volume of 3 ml with different concentrations; d) Mechanical properties, inset shows the test method (long cyclic tests under a dynamic force of 0.1 N with the preload of 0.1 N, the contact line was cut with a scalpel). The default ink concentration is 0.2 mg/ml.

2.3 Characterization of SWCNT/PEDOT:PSS hybrid film To further confirm the existence of the SWCNT/PEDOT:PSS hybrid film within the contact line, Raman spectra was used to detect the characteristic band of SWCNT as shown in Figure 3a,


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compared with pure SWCNT. This coffee ring pattern was deposited on silicon wafer to remove the disturbance of substrate. Results show that the spectra of hybrid film present three similar characteristic bands with that of pure SWCNT from1000 cm-1 to 3000 cm-1, which is corresponding to mode D, mode G, and mode G’ of SWCNT, respectively, signifying the existence of hybrid film. Meanwhile, it also can be found that the peak value of mode G shift slightly from 1581 to 1585 cm-1. The red shift phenomena in mode G can be contribute to the π– π intermolecular interaction between SWCNTs and the conjugated thiophene chains in PEDOT:PSS. This interaction enhanced the phonon excitation energy with the increasing coherence length and electron mobility under the electron-phonon scattering and binding. Figure 3b exhibits the electrical properties of the prepared hybrid film. To avoid the influence of contact line, it was cut only the continuous hybrid film. It can be found that with the increase of ink concentration from 0.1 mg/ml to 0.3 mg/ml, the sheet resistance of SWCNT-PTCFs decreases from 4.9 kohm/sq to 0.8 kohm/sq due to the gradually deposited SWCNT coated with PEDOT:PSS. Figure 3c presents the optical properties of the prepared hybrid film. First, it can be seen that all the films exhibit stable and uniform transmittance in the visible/infrared range. Second, the similar phenomena with electrical properties also takes place here. In specific, the transmittance of hybrid film decreased from 86 %, 79 %, to 70 % with increased ink concentration from 0.1 mg/ml, 0.2 mg/ml, to 0.3 mg/ml. It is easy to understand that with the thicker film, its transmission decreases. Moreover, it should be noted that the leaps at 800 nm were caused by the conversion of visible light and UV light during measurement by UV-vis instrument.


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In addition to the electrical properties and optical properties, mechanical flexibility and durability of the prepared hybrid film were also investigated in Figure 3d. The resistance variations of the films on PET substrate were recorded as a function of cycle numbers under a fixed external compressing stress. Meanwhile, ITO film (130 nm in thickness) on PET substrate was also constructed as comparison. It can be found that sheet resistance of hybrid films almost doesn’t change after a continuous compressing test of one thousand cycles. These results are much better than that of ITO film with a drastic resistance variation. Generally, our SWCNTPTCFs exhibited well mechanical flexibility and durability under compressing stress, specifying its feasibility in practical application to replace ITO film. Moreover, the performance stability of the prepared hybrid films via CRL process is very crucial in practical applications. Thus, sheet resistance and transmittance was employed to access this through a series of samples. Results, summarized in Figure S4, indicated that the physical properties of the prepared hybrid film were very stable, varying within the scope of 1.6 - 1.9 kohm/sq for sheet resistance, 75 - 80 % for transmittance, respectively, when SWCNT concentration is 0.2 mg/ml, ink volume is 3 ml, drying at room temperature.


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Figure 4. a) Digital image of a typical full transparent flexible noncontact sensing panel (3×4 sensing pixels); b) Schematic equivalent electric circuit of this panel; c) Calibration of relative humidity (RH) gradient of human index fingertip with different distance by an RH meter and resistance response of different SWCNT-PTCFs to human finger with different distances; Inset


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image is the humidity-sensing mechanism of SWCNT network; d) Short cyclic tests of humidityresistance response with different RH value or distance; e) Digital image of the human finger distance/resistance response detection capability of the flexible noncontact sensing panel and f) the relevant human finger distance and distribution presented by resistance variation of every detecting unit. All the scale bar is 1 cm.

2.4 Flexible noncontact sensing device To demonstrate the benefit of the SWCNT/PEDOT:PSS hybrid film, they were integrated into a full transparent, flexible sensing panel with 3×4 sensing pixels. The fabrication process is described in the Experimental Section, as well as in Supporting Information Figure S5. The typical panel can be seen in Figure 4a with its schematic equivalent electric circuit in Figure 4b. Accordingly, one of the sensing pixel from the above panel was selected, and its relative humidity (RH)-resistance response was evaluated through a changing distance to human finger. 39, 40

From Figure 4c, the humidity gradient (to the distance) around human finger was defined via humidity meter (TM325, Dickson), showing that the RH decreases from 72.8 RH % to 42.1 RH % with distance from 1 mm to 1 cm, compared with the room humidity of 41.5 RH %. Fig. 4c also demonstrated that with the increasing distance from 1 mm to 10 mm between hybrid film and human finger, the resistance variation of the hybrid film decreased sharply from 7.7 % to 0 %, closing to be stable after 8 mm. These performance revealed its high distance-dependent humidity sensitivity (about 1.1 %/mm, defined as Delta R/R0), as well as the effective detectable distance of 7-9 mm.


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The real-time dynamic response curves of this sensing panel with repeated distance (2 mm or 6 mm to hybrid film, respectively) of human finger were compared in Figure 4d. The results indicated that these films can detect RH with high sensitivity, good stability and fast response/recovery ability, reproducing with stable signal output. The response and recovery time was defined as the time required for attaining 90 % of the steady state value. They are generally only 0.4 s and 1.0 s, respectively, which is better than previous reports.

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These performances

have to contribute to the homogeneous deposition of PEDOT:PSS coated SWCNTs. Moreover, this sensing panel also can demonstrate the capability to sensing the distance and distribution of multi-fingers at the same time, showing with high accuracy, as shown in Fig. 4e and 4f. This result further confirm its promising future for human/machine interactive activities. For the working mechanism, according to the previous report, water molecules were absorbed on the surface of carbon nanotube via physisorption, and a weak bond take place between a H atom of a H2O molecules and a C atom on the surfaces of the nanotubes. This change will essentially reduce the conduction in nanotube-based materials according to Quantum conductance calculation.

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Moreover, the donation from the junctions between SWCNTs

should not be ignored, in which the absorption of water molecules will produce hydronium ions (H3O+) or free hydrogen ion (H+) from carboxyl, resulting in the emerge of electrical double layer (CEDL). 45, 46 Therefore, those junctions are not simply ohmic contact. The above variations result in the increase of total resistance. Generally, our full transparent, flexible sensing panel integrated the hybrid film through CRL can exhibit excellent human/machine interaction via noncontact sensing mode, with high


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sensitivity, well stability and fast response/recovery ability. This ability shows a promising practical application to replace the present touch panels for crystal/light-emitting diode display through touch mode. In addition, a flexible battery-free wireless sensor also was employed to give further demonstration for the benefits of the hybrid film through CRL, as shown in S1 and Fig. S6.

3. Conclusion In conclusion, we reported a new strategy to fabricate thickness controllable FTCFs through CRL of SWCNT/ PEDOT:PSS hybrid ink successfully. The influence of ink concentration and volume on the thickness and size of hybrid film was investigated systematically. The final FTCFs present a homogeneous thickness, an acceptable sheet resistance, a high visible/infraredrange transmittance, and an excellent dynamic mechanical property. Moreover, the fabricated full transparent, flexible noncontact sensing panel (3×4 sensing pixels) based on these hybrid films also shows excellent human/machine interaction with high sensitivity (up to 1.1 %/mm), well stability and fast response/recovery ability, through humidity sensitive mechanism. In this fashion, it can be inferred that CRL provided a facile, low-cost process to prepare FTCFs, which can be well applied to the next generation of functional circuits and devices.

4. Experimental Section 5.1 Materials PEDOT/PSS aqueous dispersion (1.3 wt. %, Clevios™ P and PH1000) was purchased from HC Starck, Inc. Carboxyl group (−COOH) functionalized SWCNT was purchased


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from CheapTubes, Inc., with an outer diameter of 1-2 nm, a length of 5-30 µm, over 95 wt. % purity and 2.56 wt. % of COOH groups. Polyethylene terephthalate (PET) films were purchased from Teonex® Inc. with the thickness of 125 um and the size of 210×297 mm. ITO coated PET films were purchased from Sigma-Aldrich (sheet resistance = 60 ohm/sq, PET thickness = 125 µm, ITO thickness = 130 nm, transmittance > 79% at 550 nm of wavelength). Deionized water was used in all experimental processes. 5.2 Preparation of SWCNT/PEDOT:PSS ink PEDOT/PSS aqueous dispersion, DI water and SWCNT (0.05 g) were added into a glass bottle, to realize weight ratio of 1: 0.75 (SWCNT vs. PEDOT:PSS) and different solid contents (0.1 mg/ml, 0.2 mg/ml, 0.3 mg/ml, respectively) of SWCNT in the prepared suspensions. Then, the homogenization process of SWCNTs was performed using a Brason 8510 bath sonicator (Thomas Scientific) for one hour, followed by the exfoliation of the SWCNTs through an ultrasonic processor (Cole-Parmer) at 20 kHz, 500 W for 15 min in an ice bath to prevent extensive heating and damage to the SWCNTs. The typical SWCNT/PEDOT:PSS inks can be seen in Figure 1a. For information, the surface tension of the inks with different concentrations is 66 - 69 mN/m, with the relevant contact angle on PET substrate between 67° - 71°. 5.3 Preparation of SWCNT/PEDOT:PSS hybrid film The as-prepared SWCNT-PEDOT:PSS inks were dropped onto the PET substrate using Thermo Scientific Finnpipette with different volumes. Then, dried at room temperature for 30 minutes, the typical coffee ring patterns with ultrathin transparent SWCNT/PEDOT:PSS films were shown in Figure 1b. 5.4 Fabrication of flexible noncontact sensing panel


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First, a 3×4 ITO electrode array was patterned on PET film by shadow mask lithography; Second, the as-prepared SWCNT/PEDOT:PSS inks were dropped between electrodes using Thermo Scientific Finnpipette (1 - 10 µl), then dried at room temperature for 1 hour. Finally, copper wires were bonded on the surface of every ITO electrode using silver paste that is then cured on a hot plate at 100 ℃ for 1 h. More details are presented in Section 3.4. 5.5 Characterization and measurements The prepared SWCNT/PEDOT:PSS inks were examined by transmission electron microscopy (TEM, Titan G2 80-300 CT,

FEI company) at an accelerating voltage of 300 kV for

homogenous analysis of SWCNT; Contact angle measuring system (Easy Drop Standard, Kruss) with the droplet volume 0.5 µL and the standard drop-shape analysis methods on PET substrate and surface tension meter (K100MK2, Kruss) with multi-mode for the analysis of ink physical properties. The fabricated TCFs were investigated by scanning electron microscopy (SEM, Quanta 600, FEI Company) and Zygo 3D Optical Surface Profilers (Profilometers, NewView™ 7300)for micromorphology analysis; Raman spectrometer (LabRAM Aramis, Horiba, Ltd.) using a 473 nm laser for structure analysis; 4-point probe system (Pro4-440N, Lucas Labs) for sheet resistance; ultra-vilate-vis instrument for transmittance; a PC-controlled universal test machine (UTM, Instron 5944, 5 N load cell) with a PC-recordable multimeter (Agilent 34401A) for mechanical fatigue properties. Noted that the samples for UV-vis characterization or four-probe measurement were fabricated with the volume of 3 ml (0.2 mg/ml) on PET substrate, and the typical sample with the average diameter of 2.6 cm is shown in Fig. S3. The full transparent flexible noncontact sensing device was characterized by a PC-recordable multimeter (Agilent 34401A) for the evaluation of real-time humidity-resistance response with


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various distances to human finger. Humidity gradient of human finger was defined by a humidity meter (TM325, Dickson).

ASSOCIATED CONTENT Supporting Information. Supporting Information Available: Materials, additional methods, and figures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Y-L Tai); [email protected] (Z-G Yang)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The key discipline fund of Shanghai (B117) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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We express gratitude to the key discipline fund of Shanghai (B117) for financial support and AKM industrial Ltd for helpful discussions. We would like to thank Prof. Gilles Lubineau for important advice for this manuscript.

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Flexible, Transparent, Thickness-Controllable ... - ACS Publications

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