Helical Nanofibrils of Block Copolymer for High-Performance

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Organic Electronic Devices

Helical Nanofibrils of Block Copolymer for High-Performance Ammonia Sensors Shiyu Wei, Fengshou Tian, Feng Ge, Xiaohong Wang, Guobing Zhang, Hongbo Lu, Jun Yin, Zongquan Wu, and Longzhen Qiu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06458 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 14, 2018

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ACS Applied Materials & Interfaces

Helical Nanofibrils of Block Copolymer for High-Performance Ammonia Sensors

Shiyu Wei,† Fengshou Tian,† Feng Ge,† Xiaohong Wang,† Guobing Zhang,†‡ Hongbo Lu,†‡ Jun Yin,*‡ Zongquan Wu, *‡and Longzhen Qiu*†‡ †

National Engineering Lab of Special Display Technology, State Key Lab of Advanced Display Technology, Academy of Opto-Electronic Technology, Hefei University of Technology, Hefei 230009, China ‡ Key Laboratory of Advanced Functional Materials and Devices, Anhui Province School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, China Abstract Conjugated polymers with helical structure have been in rapid development in recent years because of their potential applications in chemical and biological sensors. We demonstrate the fabrication and characterization of helical nanofibrils of block copolymer poly(4-iso-cyano-benzoic acid

5-(2-dimethylamino-ethoxy)-2-nitro-benzylester)-b-poly(3-hexylthiophene)

(PPI(-DMAENBA)-b-P3HT) via a transfer-etching method. The density and lateral length of nanofibrils can be facilely controlled by regulating the process conditions, which in turn, directly determines the electronic property. Organic field effect transistors based on helical nanofibrils was successfully fabricated with the highest mobility of 9.1×10-3 cm2/Vs, an on/off ratio of 3.4×105, and high bias stability. The helical nanofibrils were proved to be beneficial for obtaining a highly sensitive and selective chemical sensor. And the transistor based on helical nanofibrils exhibit a relative response of 28.6 % to 100 ppb ammonia, which is even much higher than the responses to 1 ppm ammonia for homo Poly(3-hexylthiophene) nanofibrils (7 %) and block copolymer nanofibrils without helical structure (0.9 %). The combination of helical structure with nanofibrils may provide a new strategy to fabricate high performance chemical sensors suitable for use in environmental monitoring, industrial and agricultural production, health care, and food safety. Keywords: helical structure, nanofibrils, morphology control, ammonia sensor, OFET, phase separation

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1. INTRODUCTION The detection of toxic gases like ammonia is of great importance for industries, laboratories, food storage, health monitoring and security purposes.1 Gas sensors based on organic field-effect transistors (OFETs) attract attention from numerous fronts since the gate-induced amplifying effect of transistor device makes OFETs more scalable and sensitive than the organic chemiresistors.2, 3 To make sure the sensors can be practically applied, it is essential to reduce the detection limit and improve selectivity. There are many strategies to improve the sensing property of a gas-sensor based on OFETs including implanting sensory layers, optimizing device configuration, modifying organic semiconductor (OSCs) molecules, constructing certain nanostructure, and so on.4-7 Compared to other methods, constructing a nanostructure is advanced in terms of high performance, simple process and low-cost.8, 9 The application of ultrathin films or special nanostructure of OSCs can achieve high-performance sensors for expeditious response/recovery and high selectivity substantially desired for practical circumstances.10-13 To achieve the ideal performance for either charge carrier or gas sensing, it is essential to control the morphology of nanostructures.14, 15 Conjugated/insulation block copolymers (BCPs) are of great significance in organic devices since these provide a simple route towards achieving a series of nanostructures via self-assembly and they combine advantages of semiconducting and insulating polymers with expected functions such as rigid rod and flexible coil backbone into the same polymer chain, allowing to meet the requirements of different applications.16-19 Poly(3-hexylthiophene)-insulation block copolymers (P3HT-BCPs) have been synthesized and investigated a lot due to the relatively simple synthetic route and their ability of self-organization and micro phase separation into nanostructure such as spheres, hexagonal close-packed cylinders, lamellae, nanofibrils, nanoribbons etc., which makes them applied extensively in organic photovoltaics.20-23 Theoretically, P3HT-BCPs with special nanostructure have a potential application in gas sensing, because the insulation part can be designed to be sensitive to a certain analyte,24-26 and the morphology demanding nano/micro-structure can be tailored by changing the molecule structure and preparation process in favor of gas sensing.1, 27 However, the P3HT-BCPs have not been reported to be applied in gas sensor till date. The probable reason may lie on the


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relative poor electronic property of many P3HT-BCPs which makes the devices based on those unsuitable for sensor application. Although some workers have focused on improving the electronic property of P3HT-BCPs, the stratified structures which favor charge transport, also obstruct the contact of analyte with P3HT part in the multiphase films.19, 28-30 Here, we demonstrate a high-performance ammonia sensor based on P3HT-BCPs nanofibrils with helical structure for the first time. The insulation block of the P3HT-BCP is a derivate of poly(phenylisocyanide) (PPI).31, 32 The strong π-π interactions of the side groups on PPI drive the main chains to self-assemble towards helical configuration.33-35 P3HT-BCPs nanofibrils were fabricated by blending P3HT-BCPs with an insulation polymer36-40 and used as the active layer, on the basis of their high crystallinity,41, 42 durable electronic property,43 and high surface-to-volume ratio to enhance the sensitivity.44, 45 The details of helical structure have been investigated through high-resolution atomic force microscope (AFM) and transmission electron microscope (TEM). The density and lateral length of the nanofibrils was controlled by adjusting the solvents and their concentrations during the process. Furthermore, the bottom gate/top contact (BG/TC) OFETs based on P3HT-BCPs nanofibrils were fabricated to investigate the electrical performance and the influence of helical nanofibrils on ammonia sensing property. Highly sensitive and selective chemical sensor could be obtained by the helical structures contained in semiconducting nanofibrils. 2. EXPERIMENTAL SECTION 2.1. Materials. PPI(-DMAENBA)-b-P3HT (Mn ~26.1 kDa; Mw/Mn = 1.22) and P3HT-b-PHA(Mn = 34.56 kDa; Mw/Mn = 1.25) were synthesized by our research group46,34. P3HT (P100, Mw ~41.9 kDa) were purchased from Rieke Metals, Inc. PMMA (Mw ∼996 kDa), chlorobenzene, and dichlorobenzene were purchased from Sigma Aldrich. Cytop was purchased from AGC Asahi Glass. 2.2. Preparation of PPI(-DMAENBA)-b-P3HT helical nanofibrils. PPI(-DMAENBA)-b-P3HT and PMMA were separately dissolved in chlorobenzene and dichlorobenzene respectively, and mixed at optimum concentrations of PPI(-DMAENBA)-b-P3HT (0.50, 0.75, 1.00 mg/ml) and PMMA (3 wt. %). The blend solutions were spin coated on silicon substrate (dealt with piranha solution, washed in deionized water, blown through in N2 flow and heated to complete dryness) at 2000 rpm for 60 s in a glovebox, and the films were dried overnight in vacuum oven to remove the solvent without thermal treatment. Then the films were immersed in KOH solution (5 wt. %) to strip



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it off from the silicon substrates. The floated films were rinsed in deionized water 3 times and transferred to Cytop treated SiO2/Si substrates (300 nm-thick SiO2), and ethyl acetate was used to remove PMMA. 2.3. Preparation of P3HT nanofibrils. P3HT (Mn ~ 9.52 KDa, Mw/Mn = 1.21) was dissolved in dichloromethane at 70 °C and then cooled to room temperature, leading to the formation of P3HT NWs (0.5 mg/ml). The solution was spin-casted on SiO2/Si substrates (300 nm-thick SiO2) treated with Cytop. 2.4. Preparation of P3HT-b-PHA nanofibrils. The preparation process of P3HT-b-PHA nanofibrils followed the same preparation of PPI(-DMAENBA)-b-P3HT nanofibrils. 2.5. Nanofibrils OFETs fabrication. Gold source/drain electrodes (30 nm) were thermally evaporated on nanofibrils with patterned shadow mask (W: L = 1000 µm : 100 µm). 2.6. Nanofibrils characterization. The surface morphologies and the thickness of the polymer thin films were investigated using tapping-mode atomic force microscopy (AFM) (Nanoscope, Veeco Instrument Inc.). Field-Emission transmission electron microscopy (TEM) observations were conducted on a high-resolution JEM-2100F field-emission transmission electron microscopy. The samples for TEM characterization were obtained by peeling off the spin-cast films on silica wafers substrates and subsequently transferring those onto 200-mesh copper grids. The UV-Visible Spectroscopy (UV-vis) measurements were performed on a UNIC 4802UV/VIS Double beam spectrophotometer. 2.6. Electrical measurement. The FET electrical characteristics were measured by a Keithley 4200-SCS instrument in an ambient atmosphere. The field-effect mobility of the devices was obtained in the saturation regime from the highest slope of |IDS|1/2 vs. VG plots by using Eq. (1): IDS =

WCi µ (VG − VT ) 2L

(1)

where IDS is the drain-source current, VGS is the gate voltage, VDS is the drain-source voltage, Ci is the capacitance per unit area, and L and W are the channel length and width, respectively. 2.7. Sensor test. The NH3 concentrations of 1 ppm were obtained by adjusting the volume ratios between standard 10 ppm NH3 and pure N2 from a gas cylinder. To obtain the sensing parameters, the sensitivity (S) was defined as follows:


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S=

Igas − off − Igas − on Igas − off

(2)

where Igas-off is the drain current of the sensor in timing mode when the gas is off, Igas-on is the drain current of the sensor in timing mode when the gas is on. In a sensor cycle testing, the response time (Tres) corresponds to the period in which the sensor output signal falls from 100% to 10% of the steady value from the time of switching the gas on. 3. RESULTS AND DISCUSSION 3.1. Morphology Control and Characteristics of Helical Nanofibrils The semiconducting block polymer being investigated is a recently reported derivative of poly(phenylisocyanide)-b-P3HT

diblock

polymer,

i.e.

poly(4-iso-cyano-benzoic

acid

5-(2-dimethylamino-ethoxy)-2-nitro-benzylester)-b-poly(3-hexylthiophene) (PPI(-DMAENBA)-b-P3HT) (Figure 1a), which is synthesized following a previously reported procedure.46 The thickness of nanofibrils formed by this common method is at least dozens of nanometers.47, 48

According to the former report, the thickness of polymer semiconductor can be controlled by

vertical phase separation in blending solution. Thus, we developed a transfer-etching method to achieve a nanofibrils semiconductor layer with ultrathin thickness. The samples were prepared by the following process: First, PPI(-DMAENBA)-b-P3HT and PMMA were dissolved in dichlorobenzene solvent separately and this two solution were blended together. The vertical phase separation in film-casting process induces the PPI(-DMAENBA)-b-P3HT aggregate on the top of the blend film.49 Second, the as-casted film was immersed in KOH solution to strip it off from the original substrate. The third step involved the transfer of the floated film on to the target substrate and etching PMMA by soaking the substrate orthogonally into the solvent ethyl acetate. Through the transfer-etching process (Figure 1b, the detailed process is shown in Figure S1), an evenly dispersed semiconductor layer with ultrathin thickness can be obtained. AFM measurement is used to investigate the morphology and nanostructure evolution during the transfer-etching process. Figure 1c-e show AFM images of surface morphology of as-casted films with dichlorobenzene solvent. According to the phase-model images (Figure 1c-e), it is obvious that there are nanofibrils formed in the as-casted film with lateral dimensions of 30-50 nm and lengths of



several

micrometers.

The

PPI(-DMAENBA)-b-P3HT

is

morphology

is

changed.

The

quite

different

sample

Page 6 of 20

when

obtained

concentration

from

solution

of with

PPI(-DMAENBA)-b-P3HT concentration of 0.5 mg/ml (DCB-0.5m) shows sparse nanofibrils. The nanofibrils become much denser and uniformly dispersed as the concentration increases to 0.75 mg/ml (DCB-0.75m). As the concentration is further increased to 1 mg/ml (DCB-1m), spheres connected by nanofibrils appear. Figure 1f-h illustrate the corresponding morphology of the nanofibrils obtaining by the transfer-etching method. The morphologies are consistent with that of the as-casted films which means the transfer-etching process doesn’t change the structure of the formed nanofibrils. For the samples using chlorobenzene as a solvent, the morphology of the nanofibrils obtained with 0.5 mg/ml PPI(-DMAENBA)-b-P3HT (CB-0.5m) is almost identical to that of the sample obtained from the 1 mg/ml dichlorobenzene solution (DCB-1m). As the concentration increases, the nanofibrils disappear and there are only isolated spheres remaining as evident from Figure 1i-k. The top morphology of the as-casted films is also consistent with the film after completion of the transfer-etching process, which further indicates that this process does not alter the nanostructure of blend film (Figure S2). These results demonstrate that both the solvent, and concentration of PPI(-DMAENBA)-b-P3HT have a great influence on morphology of the films. Based on the results above, the conditions for achieving the most ideal morphology are that the concentration is 0.75 mg/ml and the solvent is dichlorobenzene. In this condition, the nanofibrils almost fully cover the substrate and no spherical structure is formed. The dramatic change in morphology is probably caused by the different mechanism of phase separation.50-53 When the solubility of PPI(-DMAENBA)-b-P3HT is good in a solvent, the polymer chains are fully extended and self-assemble into well-ordered nanofibrils structures. As the concentration increases, the starting composition is changed and the spinodal decomposition turns into exhibition of nucleation and growth which transforms nanofibrils to spheres. For a relatively poorer solvent, solvent-vaporing time is shortened and movement of molecular chain is not efficient and prone to aggregate. Obviously, the transfer-etching process presents a simple and efficient way to prepare PPI(-DMAENBA)-b-P3HT nanofibrils with controllable density and microstructure. To examine the universality of this method, P3HT and another kind of P3HT-BCP, namely


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poly(3-hexylthiophene)-block-poly(hexadecyloxyallene) (P3HT-b-PHA) (Mn = 34.56 kDa; Mw/Mn = 1.25) were prepared by transfer-etching process. The P3HT-b-PHA can form well-defined nanofibrils through this process. However, P3HT forms continuous ultrathin films by this process (Figure S3), and the P3HT nanofibrils should be prepared by the other method. These results indicate that the transfer-etching process can be applied for P3HT-BCPs to form nanofibrils.

Figure 1. (a) Chemical structure of PPI(-DMAENBA)-b-P3HT. (b) Schematic illustration of transfer etching process. (c-k) AFM images of the PPI(-DMAENBA)-b-P3HT (c-e) the as-casted film before transfer-etching process using different concentrations (c) 0.5 mg/ml; (d) 0.75 mg/ml; (e) 1 mg/ml of dichlorobenzene solvent, respectively. (f-h) are obtained from transfer-etching the films (c-e) respectively. (i-k) are obtained with the same process using chlorobenzene solvent and concentrations (i) 0.5 mg/ml; (j) 0.75 mg/ml; (k) 1 mg/ml, respectively.

According to previous works, the molecular chains of PPIs have a helical conformation leading to the formation of nanofibrils with helical structures.31, measurement

were

used

to

further

investigate

33

the

TEM and high resolution AFM detailed

microstructure

of

the

PPI(-DMAENBA)-b-P3HT nanofibrils (Figure 2a, b). The TEM image shows that there are well-defined nanofibrils formed by our method and these nanofibrils have regularly repeating



Page 8 of 20

structural units. The light and dark area alternately appears in nanofibrils indicating that the nanofibrils are helically twisted. The AFM image also shows twisted nanofibrils with similar diameter and helical pitch of about 45 nm, which confirms the transfer-etching process to be capable of

producing

nanofibrils

with

helical

structures.

Figure

2c

proposes

a

model

for

PPI(-DMAENBA)-b-P3HT self-assembly into helical nanofibrils. For comparison purposes, P3HT nanofibrils, which can not be obtained by transfer-etching process, were also prepared by heating a solution of P3HT in dichloromethane at 70 ºC for 2 hours followed by cooling down to room temperature.54 The morphology of P3HT nanofibrils as established from characterization confirms a smooth

surface

and

the

diameter

(10-20

nm)

to

be

much

smaller

than

that

of

PPI(-DMAENBA)-b-P3HT (Figure 2d).38, 55 UV spectrum is used to characterize the effect of the helical structure on the packing of the P3HT chains (Figure 2e). The spectrum shows a weak 0−0 peak of the PPI(-DMAENBA)-b-P3HT nanofibrils and the peaks position (0−0) and (0−1) are blue shifted comparing to P3HT. The molecular weight of an isolated P3HT is close to the P3HT part of PPI(-DMAENBA)-b-P3HT, implying that the impact of molecular weight can be excluded, and the existence of insulation block does harm the π-electron delocalization and molecular packing. The free exciton bandwidth (W) can be calculated by the equation below:

I 0 − 0 1 − 0.24W / E p 2 =( ) I 0 − 1 1 + 0.37W / E p

(3)

I0−0 and I0−1 represent the intensities of the (0−0) and (0−1) transitions, respectively and EP is the vibrational energy of the symmetric vinyl stretch (taken as 0.18 eV). The free exciton bandwidth (W) of PPI(-DMAENBA)-b-P3HT is 108 meV which is much larger than P3HT nanofibrils (W = 79 meV) indicating that the effective conjugated length is shorter than P3HT nanofibrils. 56, 57 The stable rigid rod-helical conformation arises from intramolecular π-π interaction between the adjacent amide groups in the side chain of PPIs, which makes the P3HT backbone twisted and, thus leads to a relatively poor molecular packing and π-stacking of P3HT part of this block polymer.


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Figure 2. (a, b) TEM image and AFM height modal images of self-assembled helical nanofibrils, respectively. (c) AFM height modal images of P3HT nanofibrils. (d) Schematic representation of the assembled molecules into helical nanofibrils structures. (e) UV-vis absorption spectra of PPI(-DMAENBA)-b-P3HT and P3HT nanofibrils.

3.2. Electrical Characteristics of Helical Nanofibrils OFETs Bottom-gate/top-contact OFETs having Si/SiO2/PPI(-DMAENBA)-b-P3HT/Au (source-drain contacts) have been fabricated to investigate the electrical properties of these films, along with the charge transport property of the traditional as-cast film of PPI(-DMAENBA)-b-P3HT. There is no field effect of the spin-coating film of neat PPI(-DMAENBA)-b-P3HT (Figure S4), which highlights the necessity of the transfer-etching process. Figure 3a, b shows the transfer and output curves of PPI(-DMAENBA)-b-P3HT nanofibrils processed with different concentrations and solvents. According to the transfer curve, the DCB-0.75m film achieved the best electronic property wherein the mobility and on/off ratio reached up to 2.9×10-3 cm2 V−1 s−1and 3.4×105, respectively. These are quite advanced performance compared to the other P3HT-BCPs with similar molecule of P3HT block part that the mobility of them is usually distributed in 0.001 ~ 0.1 cm2 V−1 s−1 especially considering the on/off ratio.28, 29, 58 Moreover, this phenomenon is consistent with its morphology that the nanofibrils density is highest and most ordered. This property makes it most favorable for charge



transport. Compared to other samples, the nanofibrils of DCB-0.5m (maximum mobility µmax = 4.6×10-4 cm2 V−1 s−1) and DCB-1m (µmax = 2.3×10-3 cm2 V−1 s−1) were found to be too dilute to be able to provide any substantially sufficient transport channel. However, only CB-0.5m exhibited field-effect properties in a solution in chlorobenzene, while the sample with higher concentration did not show any electronic property (Figure 3c). This can be explained by the AFM images (Figure 1i-k) where it can be observed that at the concentration of 0.5 mg/ml, the spheres are connected by nanofibrils,

while

the

spheres

get

detached

with

increase

in

the

concentration

of

PPI(-DMAENBA)-b-P3HT. The bias stability is characterized by applying constant voltages to the drain and gate electrodes over a period of time (Figure 3d). Bias stability of the devices is the basis for suitability in sensing. The DCB-0.75m appears the most stable drain current since the decrease rate is 25 % after 500 seconds of bias-stress time, owing to the optimized morphology. Table 1 exhibits the detailed electrical data (mobility µ, threshold voltage VTH, on/off ratio, sub-threshold swing) of all the transistors based on PPI(-DMAENBA)-b-P3HT nanofibrils obtained by the transfer-etch method. (b)

(a)1E-6 DCB-1m DCB-0.75m DCB-0.5m

0.0

|ID|/A

|ID|/A

1/2

0.0006

-8

-8.0x10

-50V -60V

√

1E-10

-8

-4.0x10

ID/A

1E-8

-70V

1E-12

-7

0.0000

-80

-60

-40

-20

0

-80

1E-11

-60

-40

-20

VG/V

-60

-40

0

20

-20

0

VG/V 1.0

DCB-0.75m DCB-1m CB-0.5m DCB-0.5m

0.8

I(t)/I(0)

1E-9

-80

-80V

(d) CB-0.5m CB-0.75m CB-1m

1E-7

1E-13

-1.2x10

20

VG/V

(c)

|ID|/A

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 20

0.6 0.4 0.2 0

100

200

300

400

500

Time/s

Figure 3. Electrical characteristics of PPI(-DMAENBA)-b-P3HT OFETs. (a) Transfer characteristics of the devices fabricated with DCB-0.5m DCB-0.75m and DCB-1m. (b) Output characteristics of the device comprising DCB-0.75m. (c) Transfer characteristics of the devices based on CB-0.5m


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CB-0.75m and CB-1m. (d) In situ drain current measurements of a DCB-0.5m, DCB-0.75m, DCB-1m and CB-0.5m transistor as a function of the bias-stress time for fixed values of VG = −80 V and VD = −80V. Given the smaller effective channel area in the PPI(-DMAENBA)-b-P3HT nanofibrils, the effective mobility (µeff) normalized to the effective channel coverage is expected to be higher than the usually measured devices. Thus, µeff will be a more relevant, intrinsic value, and it’s correlation with the charge-pathway structure created in the discontinuous nanofibrils will be more useful for practical applications as a reference.37 Note that although this image analysis may not provide the exact value of the effective channel area, nonetheless, it enables us to qualitatively investigate the approximate channel coverage of the nanofibrils. According to the AFM images (Figure S5, processed by Adobe Photoshop),37 the coverage of DCB-0.5m, DCB-0.75m, DCB-1m, and CB-0.5m are 19.13%, 41.5%, 23.19%, 31.90% respectively. The effective-channel-area-normalized µeff is shown in Table1 wherein, the maximum µeff is 9.1×10-3 cm2 V−1 s−1, achieved by DCB-0.75m.

Table1. OFET Characteristics of PPI(-DMAENBA)-b-P3HT prepared by using different concentrations and solvents Concentration

SS µavg ( cm2 V−1 s−1)

µmax ( cm2 V−1 s−1)

µeff ( cm2 V−1 s−1)

On/off ratio

Vth(V)

(mg.ml-1)

(V/dec)

CB-0.5m

6.7×10-4 ± 1×10-4

8.4×10-4

6.7×10-4 ± 1×10-4

-28.9

6.3

2.3×105

DCB-0.5m

4.4×10-4 ± 8×10-5

4.6×10-4

2.3×10-3 ± 4×10-4

-16.1

5.0

1.9×104

DCB-0.75m

2.1×10-3 ± 7×10-4

2.9×10-3

7.1×10-3 ± 2×10-3

-22.5

3.4

3.4×105

DCB-1m

1.6×10-3 ± 6×10-4

2.3×10-3

6.9×10-3 ± 2×10-3

-19.7

3.7

2.2×104

3.3. Ammonia Sensing Performance of Helical Nanofibrils OFETs and the Investigation of Sensing Mechanism. The characteristics of chemical gas sensors based on the nanofibrils of DCB-0.75m were investigated by fabricated OFETs. To evaluate the sensing properties of the device towards ammonia, the transfer curves were tested in the saturation regime upon exposure to NH3 over a series of concentration gradients between 0.1–100 ppm in ambient conditions (Figure 4a). The NH3 gas was



introduced into the devices about 5 seconds before starting the test.12 The exposure of the devices to NH3 gas induces a steady decrease in the drain current (IDS). When the device is exposed to NH3, the gas molecules act as electron donors to the p-channels and a charge transfer reaction takes place between ammonia molecules and the polymer surface resulting in a change in the charge carrier density, leading to a decrease of the source-drain current. From the transfer curves (Figure 4a), it can be observed that the drain current of the device shows a dramatic relative response (∆I/I0) for NH3 even when the concentration is 100 ppb (∆I/I0)% = 28.6%). According to the response and ammonia concentration fitting function extrapolation, the theoretical detection limit of DCB-0.75m reaches up to 10 ppb. The reproducibility and operational/storage stability of a sensor are important criteria for practical applications in information technology. The reproducibility of the DCB-0.75m was investigated for a total of 5 cycles exposed to different concentrations between 1-100 ppm of NH3 in air, at a constant gate voltage (VGS) of -80V and drain voltage (VDS) of -80 V (Figure 4b). During the whole sensing period, the IDS produced a stable large response and recoverability to its initial level after a period about 40-50 seconds. The sensor exhibited good reproducibility and operational stability for almost 5 cycles. Also, the response time was found to be relatively short (3-7 seconds) compared with other organic semiconductors3 and it decreased with the increase in the concentration of NH3. The short response time and high sensitivity are owing to the large surface-to-volume ratios favoring sufficient contact between the analyte and the active layer, and also to the thickness of the nanofibrils which were under 10 nm. The majority of the charge transport occurs in the interface of dielectric layer and semiconductor layer, which makes it almost exposed to the ambient conditions benefitting the target molecule to approach to the charge transport directly. To prove the specification of sensing performance, six different organic solvents (ethyl acetate, ether, ethanol, toluene, acetone, and cyclohexane) in the form of vapor at elevated concentrations (1000 ppm) were exposed to the devices respectively. In comparison with ammonia, the response of the devices to these organic solvent vapors was quite low, indicating the specificity of the material towards ammonia. For investigating the sensing mechanism, the OFETs based on P3HT nanofibrils (P3HT-NWs) and P3HT-b-PHA nanofibrils were fabricated to test the ammonia sensing property as control groups (Figure 4d,e the transfer curve shown in Figure S6). Results show that the relative response of both


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P3HT-NWs (∆I/I0)% = 8.4-18.5%) and P3HT-b-PHA (∆I/I0)% = 0.8-4.3%) for 1-100 ppm NH3 to be much weaker than that of DCB-0.75m. Also, the response time of P3HT-NWs (10-12s) and P3HT-b-PHA (6-10s) is longer than that of DCB-0.75m. According to the charge-transfer reaction reported by Diao and co-workers, the sensing process is a first-order charge-transfer reaction between the hole charge carrier (P3HT+) and the analyte (NH3). 44 Experimentally obtained dynamic monitoring curves fit the device response following the equation as given below: ∆I/I 0 = −

1 (1 − exp(−(k 1CNH 3 + k − 1))) k −1 1+ k 1CNH 3

(4)

Where, CNH3 represents the concentration of NH3, and the forward and backward reaction rate constants (k1 and k−1) can be obtained. The k1 of DCB-0.75m (2.13×105) is an order of magnitude larger than that of P3HT-NWs (4.94×104) and P3HT-b-PHA (1.58×104) (Table S2), which means the reaction rate of DCB-0.75m and NH3 are much faster than others. These results indicate that the existence of PPIs elevate the ammonia sensing performance of DCB-0.75m. increase NH3 concentration 0ppm 0.1ppm 1ppm 10ppm 50ppm 100ppm

-7

-8

5.0x10

-40

-20

0

0.4

0

20

0.4 0.2 0

100

150

200

250

300

40

20

e e e ia te ther anol E Eth ToluenAceton hexan mmon ceta lo A yl a Eth Cyc

Selectvity

(f)

100

1.0 0.8 increase NH3 concentration C6 H 13 S

n

1ppm 10ppm 50ppm 100ppm

∆ I/I(0)

0.6

50

(e)

1.0 0.8


Time/s

VG/V

(d)

DCB-0.75m P3HT-NW P3HT-b-PHA

60

0.6

0.2 -60

(c)

0.8

0.0 -80

increase NH3 concentration

P3HT 50 100 150 200 250 300 350

Time/s

increase NH3concentration

0.6 0.4 0.2 0

C6 H 13 S


50

OC 16H 33 m

n

∆ I/I(0)%

| ID| /A

1.0x10

(b)1.0

∆ I/I(0)%

-7

∆ I/I(0)

(a) 1.5x10

∆ I/I(0)

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


P3HT-b-PHA 100

150

200

250

10

DCB-0.75m P3HT-NW P3HT-b-PHA

1 300

Time/s

0

20

40

60

80

100

NH3 Concentration

(g) P3HT PPIs e-

Figure 4. (a) The saturation regime of current-transfer characteristics of the ultrathin PPI(-DMAENBA)-b-P3HT film based sensor exposed to 0–100 ppm NH3 gas, VDS = −80V. (b, d, e) The cycle test for sensing the performance of DCB-0.75m P3HT-NWs and P3HT-b-PHA



respectively with different NH3 concentrations. (c, f) Selectivity and relative response of DCB-0.75m P3HT-NWs and P3HT-b-PHA respectively. (g) Schematic diagram of the ammonia sensor based on helical nanofibrils OFETs.

For OFETs based on P3HT-BCPs, the P3HT backbone serves as the charge transport channel and the response of IDS current is caused by charge density changes in the P3HT backbone. Hence, the different sensing properties of PPI(-DMAENBA)-b-P3HT, P3HT-b-PHA and P3HT-NWs arise from different interaction between the P3HT component and the chemical analytes. Although all of these polymers form nanofibrils, the P3HT backbone is helical in PPI(-DMAENBA)-b-P3HT nanofibrils, resulting in the larger specific surface area and the higher grain boundary density. The P3HT-b-PHA nanofibrils show no regularly repeated structures which indicate that the nanofibrils with different insulation chains do not contain helical structures like the P3HT-NWs (Figure 1e and Figure S6). It is known that the sensing response of P3HT OFET to ammonia is due to the interactions of electron-rich ammonia molecules with the electron-deficient holes in the P3HT conduction channel. However, the conduction channel of an OFET is usually concentrated within a very thin layer at the dielectric/semiconductor interface, so that the diffusion of the ammonia molecules to interact with charge carriers in the OFET conduction channel is a critical factor for their sensing performance. From the dynamic calculations, the reaction rate of PPI(-DMAENBA)-b-P3HT nanofibrils with NH3 is much higher than those of P3HT-b-PHA and P3HT-NWs. The reason behind this property is the helical structure which lets the P3HT backbone to expose π-electrons to the highly reactive sites in air which originally packed in nanofibrils tightly. With the twisted backbone of P3HT, PPI(-DMAENBA)-b-P3HT can achieve chemical sensing ability with high sensitivity and selectivity towards ammonia (Figure 4g). According to the above results and analysis, it is obvious that the PPI(-DMAENBA)-b-P3HT nanofibrils achieve the desirable performance and the sensing corresponds is promising enough. 4. CONCLUSION. This work is the first-time demonstration of fabricating nanofibrils of conjugated/insulation block copolymer with helical structure and the application as an excellent NH3 sensor. The structure


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of helical nanofibrils prepared from transfer-etching method was clearly characterized by AFM and TEM. The electronic property and morphology of helical nanofibrils could be controlled by adjusting the concentration and solvent of blending solution. And the mobility reached up to 9.1×10-3 cm2 V−1 s−1 after correction and the largest on/off ratio was found to be 3.4×105. The OFETs sensor based on helical nanofibrils show highly sensitivity (∆I/I0) of 28.6% for 100 ppb NH3 and selectivity to ammonia in ambient condition. Through comparative tests and dynamic calculations, we have proven that it is the helical structure that exposes highly reactive sites of P3HT and enhances the sensing property. Moreover, the helical nanofibrils composed by conjugated/insulation block copolymer have high potential in field of chemical sensing. Associate Content

Supporting Information. Schematic illustration of transfer etching process, Morphology of as-casted PPI(-DMAENBA)-b-P3HT film with chlorobenzene solvent, Morphology of P3HT-b-PHA and P3HT formed by transfer etching process, transfer characteristics of the OFETs based on the spin-coated film of PPI(-DMAENBA)-b-P3HT chlorobenzene solvent, AFM images of the PPI(-DMAENBA)-b-P3HT nanofibrils processed by Adobe Photoshop, the sensing performance of P3HT and P3HT-b-PHA nanofibers, Calculated reaction constants obtained from fitting the dynamic monitoring curves.

Author Information Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] Acknowledgements This study was supported by National Natural Science Foundation of China (NSFC, Grant no. 51703047, 51573036 and 51673058), the Distinguished Youth Foundation of Anhui Province (1808085J03), and the Fundamental Research Funds for the Central Universities (JZ2018HGPB0276, JZ2017HGBZ0919 and JZ2017HGBH0952).



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Helical Nanofibrils of Block Copolymer for High-Performance

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