Research Article www.acsami.org
Morphology-Driven High-Performance Polymer Transistor-based Ammonia Gas Sensor Seong Hoon Yu,† Jangwhan Cho,† Kyu Min Sim,† Jae Un Ha,† and Dae Sung Chung*,† †
School of Chemical Engineering and Material Science, Chung-Ang University, Seoul 156-756, Korea S Supporting Information *
ABSTRACT: Developing high-performance gas sensors based on polymer field-effect transistors (PFETs) requires enhancing gas-capture abilities of polymer semiconductors without compromising their high charge carrier mobility. In this work, cohesive energies of polymer semiconductors were tuned by strategically inserting buffer layers, which resulted in dramatically different semiconductor surface morphologies. Elucidating morphological and structural properties of polymer semiconductor films in conjunction with FET studies revealed that surface morphologies containing large two-dimensional crystalline domains were optimal for achieving high surface areas and creating percolation pathways for charge carriers. Ammonia molecules with electron lone pairs adsorbed on the surface of conjugated semiconductors can serve as efficient trapping centers, which negatively shift transfer curves for p-type PFETs. Therefore, morphology optimization of polymer semiconductors enhances their gas sensing abilities toward ammonia, leading to a facile method of manufacturing high-performance gas sensors. KEYWORDS: ammonia sensor, high sensitivity, morphology control, field-effect transistor, buffer layer
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INTRODUCTION Recent advances in industrial gas sensors have made the development of efficient and low-cost gas leak detectors for combustible, flammable, and toxic gases a very important research topic.1,2 In particular, volatile organic compounds with high toxicity need to be constantly monitored in order to protect the environment and human health.3−5 Ammonia is one of the major industrial chemical products that is both caustic and hazardous.6 Therefore, significant research efforts have been devoted to developing low-cost ammonia gas sensors with a simple device structure.7−10 Organic field-effect transistors (OFETs) possess many attractive properties and can be particularly suitable for this purpose because of their lightweight, flexibility, and low material/process costs.11−14 The ammonia sensing mechanism of organic semiconductors is arguably related either to the base dedoping effect, which accompanies various chemical interactions, or to simple physical adsorption.15,16 In any case, increasing the surface area of active layers without compromising percolation pathways for charge carriers is essential for the realization of highly sensitive ammonia gas sensors. Therefore, early versions of OFET-based ammonia gas sensors used derivatives of linear acenes as active layers, presumably due to their good semiconducting properties and, more importantly, the ease of morphology tuning.8,16−20 For example, Li et al. demonstrated highly sensitive OFET-based ammonia sensors with ultrathin microstrip morphology manufactured from linear acene derivatives.16 Although the reported results are impressive, in © XXXX American Chemical Society
order to take full advantage of the organic electronics, small molecular semiconductors have to be replaced with polymer semiconductors because of their high compatibility with plastic substrates and outstanding mechanical robustness.11−14 Until now, very few research studies on polymer field-effect transistor (PFET)-based ammonia sensors were reported because it was difficult to manufacture submicron-sized patterned structures on PFET semiconductor surfaces without sacrificing high charge carrier mobility, which is essential for achieving high sensitivity toward ammonia. For example, Klug et al. have reported highly sensitive PFET-based ammonia sensors containing pH-sensitive dielectric layers.21 However, such polar dielectric layers have a detrimental effect on charge transport by acting as trap sites, which leads to a poor PFET performance as indicated by low charge carrier mobility and significant hysteresis.22 External doping methods such as using graphene oxide with functional groups that facilitate gas capture by polymer semiconductors also decrease the PFET performance.23,24 Maintaining high charge carrier mobility is very important for gas sensors even when the semiconductor device is not used for integrated circuits, because it directly determines the signal transfer speed. For example, in the case of hopping transport, the transit time between two electrodes installed in a FET can be determined by using the following equation:25 Received: January 13, 2016 Accepted: February 29, 2016
A
DOI: 10.1021/acsami.6b00471 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic device structure with proposed ammonia adsorption mechanism. (b) Schematic energy diagram demonstrating the formation of locally different energy level of PBTTT, enabled by ammonia adsorption. (c) Measured transfer behaviors of PFET-based ammonia sensor without buffer layer under the programmed ammonia gas exposure.
t=
molecules, the prepared films were annealed at 175 °C for 30 min and then slowly cooled. Finally, the PFET geometry was completed by depositing Au source-drain electrodes onto the annealed films. Measurement. The electrical characteristics of the transistors were measured by HP4156A Precision semiconductor parameter analyzers (Agilent Technologies) and LabView-controlled Keithley 2400. Ammonia (99.99% purity) exposure was conducted at room temperature in closed chamber with vacuum and gas inlet/outlet valve controlled by home-built LabView program. Characterization. Grazing incident X-ray diffraction (GIXD) measurements were performed using the PLS-II 3C, 9A U-SAXS beamline at the Pohang Accelerator Laboratory (PAL) in Korea. The X-rays coming from the in-vacuum undulator (IVU) were monochromated (E = 11.06 keV) using Si(111) double crystals and then focused at the detector position by using a K−B focusing mirror system. The horizontal and vertical beam sizes were 300 (H) μm and 30 (V) μm, respectively. The incidence angle (αi) was adjusted to 0.13°, which is above the critical angle. GIXD patterns were recorded with a 2D CCD detector (Rayonix. SX-165). The diffraction angles were calibrated using precalibrated sucrose (Monoclinic, P21), and the sample-to-detector distance was approximately 225 mm. All the morphological images were obtained by using an atomic force microscope (Park systems, NX20). To calculate the surface free energy of OTS and Cytop as well as their adhesion energy with PBTTT, contact-angle measurements were conducted using homebuilt setup. Contact angles of water and diiodomethane were measured and used to calculate the surface free energy (γs) based on the following equation.26
L2 Vμ
where L is the channel distance, V is the applied voltage, and μ is the charge carrier mobility. Although a semiconductor layer response to ammonia exposure can be very quick, if the transit time is too long then the timely response of the semiconductor is not reflected in the final signal extracted from the FET device. Therefore, PFET-based ammonia sensors with high gascapture abilities must be developed without compromising high charge carrier mobility. In this work, we suggest that strategically introduced nanostructures of polymer semiconductors can be very efficient candidates for highly sensitive ammonia sensor applications. To achieve this purpose, the surface energy difference between the dielectric layer surface and the semiconductor layer had to be effectively controlled, so that the cohesive interaction between semiconductor atoms could be optimized. As a result, we managed to fabricate optimal nanostructures of polymer semiconductors with large surface area and well-defined percolation pathways for charge carriers. The resulting PEFTs successfully maintained the high charge carrier mobility of 0.12 cm2/(V s), while preserving high sensitivity and timely response to ammonia exposure at concentrations as low as 10 ppm.
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EXPERIMENTAL SECTION
1 + cos θ =
Device Fabrication. A Si wafer with a 100 nm thick SiO2 dielectric layer on heavily n-doped silicon was used as a substrate. The substrate was cleaned in a piranha solution. The semiconducting polymer, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) was dissolved at 10 mg/mL in anhydrous 1,2dichlorobenzene, followed by agitation at 100 °C for 1 h. Octyltrichlorosilane (OTS)-modified SiO2 surface was obtained by immersing the substrate in 1 mM OTS (in toluene) at 40 °C for 30 min, followed by thermal annealing at 120 °C for 30 min. To formulate a Cytop solution, a solute of Cytop (CTL-809M, Asahi Glass) was mixed with its corresponding solvent (CT-solv.180, Asahi Glass) at 1:40 volume ratio. Cytop-modified SiO2 surface was obtained by spin-coating the formulated Cytop solution onto the substrate at 4000 rpm for 40 s, followed by thermal annealing at 200 °C for 30 min. The Ci values per unit area of pristine SiO2 (100 nm), OTS/SiO2 and Cytop (20 nm)/SiO2 layer were obtained as 33.5, 32.5, and 29.5 nF/cm2, respectively. The capacitance values for the dielectric layers were measured using Keysight E4981 at 1 kHz. The active layer was completed by spin-coating the PBTTT solution in 1,2-dichlorobenzene onto OTS-treated, Cytop-treated and pristine substrate at 2000 rpm for 90 s. To further enhance the molecular order between PBTTT
2(γsd)0.5 (γlvd)0.5 γlv
+
2(γsp)0.5 (γlvp)0.5 γlv
where γlv is the surface energy of the test liquid, and superscripts d and p refer to the dispersive and polar components, respectively. The adhesion energy was calculated following the previous method.27 Fourier transform infrared spectroscopy (FT-IR) absorbance peaks were acquired using an FTIR 4700 device (JASCO).
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RESULTS AND DISCUSSION Figure 1(a) shows the proposed interaction mechanism between a semiconducting polymer (PBTTT) and ammonia molecules. Although it is not clear whether this mechanism involves chemical base dedoping or physical dipole-charge interactions, ammonia molecules with lone electron pairs can be briefly captured by conjugated polymer surface molecules. As indicated by the FTIR analysis (Figure S1), no permanent changes in the PBTTT chemical structure due to ammonia exposure were detected. Thus, while captured ammonia molecules remain on the surface of PBTTT, their lone electron pairs can serve as effective charge trapping centers, especially B
DOI: 10.1021/acsami.6b00471 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 2. AFM images of surface of PBTTT films deposited onto (a) pristine SiO2, (b) OTS, and (c) Cytop. The rms roughness values are 0.9, 1.9, and 6.1 nm for a, b, and c, respectively. All the films were annealed at 175 °C for 30 min.
Figure 3. GIXD images measured from (a) SiO2-, (b) OTS-, and (c) Cytop-based PBTTT films and their corresponding extracted profiles along with (d) out-of-plane direction and (e) in-plane direction. (f) The pole figures for each GIXD data.
same time, the trap density can be expressed by the relation CV Ntr = ie th where Ci is the capacitance of a dielectric layer, e is the electron charge, and Vth is the threshold voltage obtained from transfer characteristics.29 Thus, in the case of p-type FETs, ammonia-originated trap generation should shift the Vth (or the turn-on voltage Vturn‑on) parameters of transfer curves in the negative direction. A representative example of such a trap effect on transfer characteristics is shown in Figure 1(c). Here, the PFET structure is a conventional one with a Au electrode on the top and a SiO2 (100 nm)/Si2+ substrate at the bottom. Upon ammonia exposure, the transfer curve of the PBTTTbased PFET shifts in the negative direction, while the oncurrent decreases, indicating trap generation. Furthermore, because ammonia molecules do not remain permanently on the surface of PBTTT, the transfer curve completely returns to its original position after the ammonia stream is stopped. At the same time, the observed changes in Ion and Vturn‑on values are not very significant for highly sensitive ammonia sensors. These results indicate that although PBTTT interacts with ammonia gas molecules to a certain degree, the surface morphology of PBTTT films needs to be strategically tuned to further enhance the PFET sensitivity, so that a greater number of PBTTT surface sites can be exposed to ammonia gas.
for semiconductor positive charge carriers, which can be directly related to the PFET performance. Here one needs to note that chemical impurities in conjugated organic semiconductors often result in diagonal disorder (energetic disorder) due to the formation of locally different HOMO/ LUMO energy levels for charge carrier pathways. In the case of ammonia molecules, they can induce polarization effects on the original conjugated structure of PBTTT, which lead to fluctuations of local HOMO/LUMO energy levels. According to the widely accepted multiple trapping and release model, such localized energy states are associated with trap states as schematically depicted in Figure 1(b). In this case, the charge carrier mobility can be expressed as ⎛ E ⎞ μ = μ0 α exp⎜ − t ⎟ ⎝ kT ⎠
where μ0 is the trap-free mobility, Et is the trapping energy, and α represents the ratio between the density of delocalized levels available for transport and the density of traps.28 According to this formula, if the trapping energy and the number of trap sites increase, the charge carrier mobility exponentially decreases. For a FET, the charge carrier mobility is directly related to the slope of the transfer curve, and thus, high charge carrier mobility always yields higher on-current (Ion) values. At the C
DOI: 10.1021/acsami.6b00471 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 4. Transistor behaviors under the programmed ammonia gas exposure. Transfer curves of (a) SiO2-, (b) OTS-, and (c) Cytop-based PFETs, Their corresponding shifts of Von and Ion are summarized in panels d−f.
molecules when exposed to ammonia stream due to their higher surface area. However, as was previously indicated, not only good gas-capture abilities but also well-developed percolation pathways for charge transport (and, thus, high charge carrier mobility) are required for good gas sensors. Because charge carrier mobility is dependent on the crystalline nature of the film (which cannot be fully investigated by a morphological analysis), the structural properties of the PBTTT films deposited onto buffer layers were studied by means of 2-D grazing-incidence X-ray diffraction (GIXD). As shown in Figures 3(a−c), all the PBTTT films reveal traditional lamellar stacking patterns along the out-of-plane direction, which can be obtained from the well-developed Bragg diffraction peaks up to (400) (Figure 3(d)). At the same time, close π−π interactions were also observed along the inplane direction, resulting in a π−π stacking distance of 3.71 Å (Figure 3(e)). Thus, no qualitative differences in crystalline nature between the PBTTT films deposited onto different buffer layers were observed. To quantitatively elucidate the crystalline orientation of the PBTTT films, we conducted a pole figure analysis that examines the orientational distribution of the diffraction peaks as a function of all possible crystalline orientations.30 It was performed by analyzing the distribution of possible (200) orientations to avoid surface scattering effects resulting from the substrate. For a quantitative comparison, the relative degree of crystallinity (rDoC) of PBTTT was further calculated by integrating the relative population distribution, taking into account the factor of X-ray exposure time, beam footprint, sample thickness, and polymer volume fraction.30 The calculated rDoC values were 1.00, 5.02, and 3.09 for the pristine, OTS-based, and Cytop-based films, respectively. The resulting pole figures are presented in Figure 3(f) and indicate that all the PBTTT films adopt a preferential edge-on orientation. In particular, most crystallites in the Cytop-based
Unlike small molecular semiconductors, it is very difficult to create nanosized patterns on the surface of polymers with high molecular weight due to their entanglement nature. Thus, we strategically introduced an energy difference between the PBTTT and dielectric surfaces, so that stable nanosized morphological features can be created on the PBTTT surface. When the adhesion energy of the semiconductor/substrate interface is smaller than the cohesive energy of semiconductor molecules, the relatively stronger cohesive energy induces the formation of large crystalline domains, resulting in nanosized features along the vertical direction. In the case of pristine SiO2 substrate, the adhesion energy of PBTTT/SiO2 can be as high as 76.4 mN m−1 due to its hydrophilic nature, which is much greater than the cohesive energy of PBTTT molecules (62.2 mN m−1). Therefore, only featureless morphology of the PBTTT film with a low root-mean-square (rms) roughness of 0.9 nm can be observed in Figure 2(a). After that, the OTS molecules were self-assembled on the SiO2 substrate to make the substrate surface more hydrophobic. In this case, the adhesion energy of the PBTTT/substrate decreases to 48.8 mN m−1, which is certainly lower than the cohesive energy of PBTTT. As a result, large crystalline domains with an increased rms roughness of around 1.9 nm are observed, as shown in Figure 2(b). When the SiO2 substrate was modified by using a Cytop thin interlayer to make it more hydrophobic, the adhesion energy further decreased to 36.2 mN m−1, which is much lower than the PBTTT cohesive energy. In this case, very well developed three-dimensional morphological structures with a high rms roughness of 6.1 nm were created. In other words, as the relative cohesive energy of the PBTTT molecule increases compared to the adhesion energy of the PBTTT/ substrate, three-dimensional morphological features are developed, leading to higher surface roughness. Therefore, Cytopbased PBTTT PFETs may capture larger amounts of gas D
DOI: 10.1021/acsami.6b00471 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Table 1. Summary of PFET Performances with Various Buffer Layers μ (cm2/(V·s))a off SiO2 OTS Cytop
b
0.007 0.122 0.013
Ion/Ioff
Vturn‑on (V)
Vth (V)
Ss‑th
onc
offd
off
on
off
off
on
off
off
on
off
off
on
off
0.004 0.065 0.003
0.006 0.118 0.010
> 104 > 105 > 103
> 103 > 105 > 103
> 104 > 105 > 103
5 0 8
3 −7 −1
5 0 8
5 −1 8
2 −9 −2
5 −1 8
4.02 0.92 2.06
4.05 1.02 2.15
4.02 0.93 2.08
Mobility values were summarized from more than 10 devices for each fabrication condition, and all the films were annealed at 175 °C for 30 min and then slowly cooled. bThis corresponds to “Pristine” stage in the Figure 4 cThis corresponds to “Gas exposure” stage in the Figure 4 dThis corresponds to “Recovery” stage in the Figure 4
a
Figure 5. (a) Transient responses of Si-, OTS-, and Cytop-based PFET ammonia gas sensor under programmed ammonia gas exposure. (b) Transient responses of OTS-based PFET ammonia sensor under various ammonia concentrations from 10 to 100 ppm. (c) Transient responses of OTS-based PFET ammonia sensor at off-regime.
PBTTT films are preferentially edge-on oriented within a polar angle (ω) of 15°, and such tendency gradually decreases when the buffer layer is changed to OTS, while the overall rDoC is significantly enhanced. In the case of pristine PBTTT without a buffer layer, the worst preferential edge-on orientations as well as the lowest rDoC values were observed. The results of the GIXD analysis show that because of the insertion of an OTS buffer layer, the PBTTT atoms have a higher probability of intermolecular interactions leading to larger crystalline domains with preferential edge-on orientations, which is beneficial for inplane charge transport. In the case of a Cytop buffer layer, the overall crystallinity of the upper-deposited PBTTT was much worse than in the case of OTS presumably because of a very rough three-dimensional surface morphology. To investigate the effect of the observed morphological/ structural differences on the charge transport behavior, various FET performances were compared, and the resulting transfer curves were summarized in Figure S2. In general, charge carrier mobility increases owing to the insertion of a hydrophobic buffer layer between the semiconductor and the SiO2 layers. The OTS interlayer yielded the highest mobility increase from 7 × 10−3 cm2/(V s) to 1.2 × 10−1 cm2/(V s), while the Cytop interlayer resulted in a marginal mobility of 1.3 × 10−2 cm2/(V s). These results indicate that in contrast to featureless and large three-dimensional featured morphologies, two-dimensional morphology with well-defined crystalline domains can results in the most effective percolation pathways for charge carriers, which is consistent with the GIXD analysis. Thus, both high surface area and efficient percolation pathways for charge carriers required for effective gas sensing have been established for the OTS-based PBTTT PFETs. Figure 4 summarizes the transfer behavior of the PBTTT PFETs with various buffer layers under ammonia exposure. Additionally, all the important PFET parameters obtained from Figure 4 are summarized in Table 1. Notably, both the OTS- and Cytop-based devices show dramatically better ammonia-sensing abilities compared to the pristine device. The change in Vturn‑on against ammonia exposure increases from 2 V (pristine) to 7 V (OTS) and 9 V
(Cytop), indicating a good agreement with the morphological observations characterized by gradual increases in surface roughness in the same sequence. The decrease in charge carrier mobility against ammonia exposure shows a similar tendency: from 7 × 10−3 cm2/(V s) to 4 × 10−3 cm2/(V s) (pristine), from 1.2 × 10−1 cm2/(V s) to 6.0 × 10−2 cm2/(V s) (OTS), and from 1.3 × 10−2 cm2/(V s) to 3 × 10−3 cm2/(V s) (Cytop). Despite the obviously different degrees of change in the FET behavior upon ammonia exposure, all the PBTTT PFETs show a very prompt recovery after blocking the ammonia stream indicating that irreversible deformation of the polymer semiconductor chemical structure does not occur even in the case of enlarged PBTTT surfaces. An analysis of the resulting transfer curves presented in Figure 4 suggests that the “turn-off” regime can yield better sensing abilities than the “turn-on” regime if the sensitivity toward ammonia exposure is defined as Igas‑on/Igas‑off. In the case of OTS-based PFETs, the calculated sensitivity is greater than 105 at VG = −7 V, while it becomes less than 10 at VG = −30 V. The subthreshold swing of FET, S, is known to be related with trap density by ⎡ qS log(e) ⎤C Ntrap ≈ ⎢ − 1⎥ i ⎣ kT ⎦q
where q is the electronic charge, k is Boltzmann’s constant, and T is the temperature.31 Based on this equation, we could estimate the interface trap densities of pristine, OTS- and Cytop- based PFET as 1.48 × 1013, 2.85 × 1012, and 6.01 × 1012 cm−2, respectively. The increasing trap densities as a sequence OTS-, Cytop-, and pristine- PFETs are in good agreement with other characterization results such as GIXD, AFM, and mobility measurement. Notably, in each PFET, gas exposure always accompanied increase in the value of S, again confirming the local energetic disorder driven by ammonia adsorption. Transient measurements of ammonia-detecting characteristics (at 100 ppm ammonia exposures) were also performed for PBTTT PFETs, as summarized in Figure 5(a). After E
DOI: 10.1021/acsami.6b00471 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces ammonia exposure, the drain current ID (measured at VG = −30 V) rapidly decreases by more than 4 μA within a few seconds. Turning off the ammonia stream results in a quick recovery (within 10 s) of the ID value to its original level without any external manipulations. The sensing test was repeated more than 320 times during 220 min, which points to excellent device reliability without any protecting layers at ambient conditions (Figure 5(a)). In the next step, OTS-based PFETs were exposed to various concentrations of ammonia gas for 40 s. As shown in Figure 5(b), the PFETs reveal reasonably good sensing characteristics at ammonia concentrations as low as 10 ppm. Roughly, it can be estimated that each 10 ppm increase of ammonia gas concentration enabled 130 nA increase in output current level. As was noted earlier, the sensitivity (Igas‑on/Igas‑off) parameter can be enhanced at lower VG values. As shown in Figure 5(c), at VG = −7 V, the same transient measurements were repeated. In this case, the sensitivity magnitude was increased to 12, while maintaining similar temporal response values.
OFET OTS GIXD IVU rms rDoC
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CONCLUSION Until now, most surface-engineered OFETs for gas sensor applications have utilized small molecular semiconductors due to their ease of morphology control. However, to fully exploit the operational advantages of OFETs, polymer semiconductors characterized by difficulties in realizing nanosized surface morphological features should be used. In this work, PBTTTbased PFETs with strategically designed surface morphologies were demonstrated as high-performance ammonia gas sensors. Elucidating the morphological and structural parameters of PBTTT films with various surface morphologies in conjunction with FET studies revealed that the surface morphology containing large two-dimensional crystalline domains is optimal for achieving high surface area and creating percolation pathways for charge carriers. By optimizing the morphology of polymer semiconductor surfaces effectively contaminated with ammonia molecules, good gas sensing characteristics, and prompt response times can be obtained. ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00471. FTIR spectra of the PBTTT films before and after ammonia gas exposure and Transfer curves of SiO2-, OTS-, and Cytop-based PFETs without gas exposure. (PDF)
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organic field effect transistor Octyltrichlorosilane Grazing incident X-ray diffraction in-vacuum undulator root-mean-square relative degree of crystallinity
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant number: NRF-2015R1C1A1A02037219).
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ABBREVIATIONS PFET polymer field effect transistor F
DOI: 10.1021/acsami.6b00471 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces TIPS-pentacene Organic Thin-film Transistors as Ammonia Gas Sensors. J. Mater. Chem. C 2013, 1, 6532−6535. (18) Yu, J.; Yu, X.; Zhang, L.; Zeng, H. Ammonia Gas Sensor based on Pentacene Organic Field-Effect Transistor. Sens. Actuators, B 2012, 173, 133−138. (19) Zan, H. W.; Tsai, W. W.; Lo, Y. R.; Wu, Y. M.; Yang, Y. S. Pentacene-based Organic Thin Film Transistors for Ammonia Sensing. IEEE Sens. J. 2012, 12, 594−601. (20) Huang, W.; Yu, J.; Yu, X.; Shi, W. Polymer Dielectric Layer Functionality in Organic Field-Effect Transistor based Ammonia Gas Sensor. Org. Electron. 2013, 14, 3453−3459. (21) Klug, A.; Denk, M.; Bauer, T.; Sandholzer, M.; Scherf, U.; Slugovc, C.; List, E. J. Organic Field-effect Transistor based Sensors with Sensitive Gate Dielectrics used for Low-concentration Ammonia Detection. Org. Electron. 2013, 14, 500−504. (22) Noh, Y. H.; Park, S. Y.; Seo, S. M.; Lee, H. H. Root Cause of Hysteresis in Organic Thin Film Transistor with Polymer Dielectric. Org. Electron. 2006, 7, 271−275. (23) Cheon, K. H.; Cho, J.; Kim, Y. H.; Chung, D. S. Thin Film Transistor Gas Sensors Incorporating High-Mobility Diketopyrrolopyrole-Based Polymeric Semiconductor Doped with Graphene Oxide. ACS Appl. Mater. Interfaces 2015, 7, 14004−14010. (24) Kaake, L. G.; Barbara, P. F.; Zhu, X. Y. Intrinsic Charge Trapping in Organic and Polymeric Semiconductors: a Physical Chemistry Perspective. J. Phys. Chem. Lett. 2010, 1, 628−635. (25) Pfister, G. Hopping Transport in a Molecularly Doped Organic Polymer. Phys. Rev. B 1977, 16, 3676. (26) Kinloch, A. Adhesion and Adhesives: Science and Technology; Springer Science & Business Media: Berlin, 2012. (27) Effertz, C.; Lahme, S.; Schulz, P.; Segger, I.; Wuttig, M.; Classen, A.; Bolm, C. Design of Novel Dielectric Surface Modifications for Perylene Thin-Film Transistors. Adv. Funct. Mater. 2012, 22, 415−420. (28) Coropceanu, V.; Cornil, J.; da Silva Filho, D. A.; Olivier, Y.; Silbey, R.; Brédas, J. L. Charge Transport in Organic Semiconductors. Chem. Rev. 2007, 107, 926−952. (29) Park, S.; Lim, B. T.; Kim, B.; Son, H. J.; Chung, D. S. High Mobility Polymer based on a π-Extended Benzodithiophene and its Application for Fast Switching Transistor and High Gain Photoconductor. Sci. Rep. 2014, 4, 5482. (30) Rivnay, J.; Mannsfeld, S. C.; Miller, C. E.; Salleo, A.; Toney, M. F. Quantitative Determination of Organic Semiconductor Microstructure from the Molecular to Device Scale. Chem. Rev. 2012, 112, 5488−5519. (31) Nam, S.; Chung, D. S.; Jang, J.; Kim, S. H.; Yang, C.; Kwon, S. K.; Park, C. E. Effects of Poor Solvent for Solution-processing Passivation of Organic Field Effect Transistors. J. Electrochem. Soc. 2010, 157, H90−H93.
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DOI: 10.1021/acsami.6b00471 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Morphology-Driven High-Performance Polymer Transistor-based Ammonia Gas Sensor Seong Hoon Yu,† Jangwhan Cho,† Kyu Min Sim,† Jae Un Ha,† and Dae Sung Chung*,† †
School of Chemical Engineering and Material Science, Chung-Ang University, Seoul 156-756, Korea S Supporting Information *
ABSTRACT: Developing high-performance gas sensors based on polymer field-effect transistors (PFETs) requires enhancing gas-capture abilities of polymer semiconductors without compromising their high charge carrier mobility. In this work, cohesive energies of polymer semiconductors were tuned by strategically inserting buffer layers, which resulted in dramatically different semiconductor surface morphologies. Elucidating morphological and structural properties of polymer semiconductor films in conjunction with FET studies revealed that surface morphologies containing large two-dimensional crystalline domains were optimal for achieving high surface areas and creating percolation pathways for charge carriers. Ammonia molecules with electron lone pairs adsorbed on the surface of conjugated semiconductors can serve as efficient trapping centers, which negatively shift transfer curves for p-type PFETs. Therefore, morphology optimization of polymer semiconductors enhances their gas sensing abilities toward ammonia, leading to a facile method of manufacturing high-performance gas sensors. KEYWORDS: ammonia sensor, high sensitivity, morphology control, field-effect transistor, buffer layer
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INTRODUCTION Recent advances in industrial gas sensors have made the development of efficient and low-cost gas leak detectors for combustible, flammable, and toxic gases a very important research topic.1,2 In particular, volatile organic compounds with high toxicity need to be constantly monitored in order to protect the environment and human health.3−5 Ammonia is one of the major industrial chemical products that is both caustic and hazardous.6 Therefore, significant research efforts have been devoted to developing low-cost ammonia gas sensors with a simple device structure.7−10 Organic field-effect transistors (OFETs) possess many attractive properties and can be particularly suitable for this purpose because of their lightweight, flexibility, and low material/process costs.11−14 The ammonia sensing mechanism of organic semiconductors is arguably related either to the base dedoping effect, which accompanies various chemical interactions, or to simple physical adsorption.15,16 In any case, increasing the surface area of active layers without compromising percolation pathways for charge carriers is essential for the realization of highly sensitive ammonia gas sensors. Therefore, early versions of OFET-based ammonia gas sensors used derivatives of linear acenes as active layers, presumably due to their good semiconducting properties and, more importantly, the ease of morphology tuning.8,16−20 For example, Li et al. demonstrated highly sensitive OFET-based ammonia sensors with ultrathin microstrip morphology manufactured from linear acene derivatives.16 Although the reported results are impressive, in © XXXX American Chemical Society
order to take full advantage of the organic electronics, small molecular semiconductors have to be replaced with polymer semiconductors because of their high compatibility with plastic substrates and outstanding mechanical robustness.11−14 Until now, very few research studies on polymer field-effect transistor (PFET)-based ammonia sensors were reported because it was difficult to manufacture submicron-sized patterned structures on PFET semiconductor surfaces without sacrificing high charge carrier mobility, which is essential for achieving high sensitivity toward ammonia. For example, Klug et al. have reported highly sensitive PFET-based ammonia sensors containing pH-sensitive dielectric layers.21 However, such polar dielectric layers have a detrimental effect on charge transport by acting as trap sites, which leads to a poor PFET performance as indicated by low charge carrier mobility and significant hysteresis.22 External doping methods such as using graphene oxide with functional groups that facilitate gas capture by polymer semiconductors also decrease the PFET performance.23,24 Maintaining high charge carrier mobility is very important for gas sensors even when the semiconductor device is not used for integrated circuits, because it directly determines the signal transfer speed. For example, in the case of hopping transport, the transit time between two electrodes installed in a FET can be determined by using the following equation:25 Received: January 13, 2016 Accepted: February 29, 2016
A
DOI: 10.1021/acsami.6b00471 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 1. (a) Schematic device structure with proposed ammonia adsorption mechanism. (b) Schematic energy diagram demonstrating the formation of locally different energy level of PBTTT, enabled by ammonia adsorption. (c) Measured transfer behaviors of PFET-based ammonia sensor without buffer layer under the programmed ammonia gas exposure.
t=
molecules, the prepared films were annealed at 175 °C for 30 min and then slowly cooled. Finally, the PFET geometry was completed by depositing Au source-drain electrodes onto the annealed films. Measurement. The electrical characteristics of the transistors were measured by HP4156A Precision semiconductor parameter analyzers (Agilent Technologies) and LabView-controlled Keithley 2400. Ammonia (99.99% purity) exposure was conducted at room temperature in closed chamber with vacuum and gas inlet/outlet valve controlled by home-built LabView program. Characterization. Grazing incident X-ray diffraction (GIXD) measurements were performed using the PLS-II 3C, 9A U-SAXS beamline at the Pohang Accelerator Laboratory (PAL) in Korea. The X-rays coming from the in-vacuum undulator (IVU) were monochromated (E = 11.06 keV) using Si(111) double crystals and then focused at the detector position by using a K−B focusing mirror system. The horizontal and vertical beam sizes were 300 (H) μm and 30 (V) μm, respectively. The incidence angle (αi) was adjusted to 0.13°, which is above the critical angle. GIXD patterns were recorded with a 2D CCD detector (Rayonix. SX-165). The diffraction angles were calibrated using precalibrated sucrose (Monoclinic, P21), and the sample-to-detector distance was approximately 225 mm. All the morphological images were obtained by using an atomic force microscope (Park systems, NX20). To calculate the surface free energy of OTS and Cytop as well as their adhesion energy with PBTTT, contact-angle measurements were conducted using homebuilt setup. Contact angles of water and diiodomethane were measured and used to calculate the surface free energy (γs) based on the following equation.26
L2 Vμ
where L is the channel distance, V is the applied voltage, and μ is the charge carrier mobility. Although a semiconductor layer response to ammonia exposure can be very quick, if the transit time is too long then the timely response of the semiconductor is not reflected in the final signal extracted from the FET device. Therefore, PFET-based ammonia sensors with high gascapture abilities must be developed without compromising high charge carrier mobility. In this work, we suggest that strategically introduced nanostructures of polymer semiconductors can be very efficient candidates for highly sensitive ammonia sensor applications. To achieve this purpose, the surface energy difference between the dielectric layer surface and the semiconductor layer had to be effectively controlled, so that the cohesive interaction between semiconductor atoms could be optimized. As a result, we managed to fabricate optimal nanostructures of polymer semiconductors with large surface area and well-defined percolation pathways for charge carriers. The resulting PEFTs successfully maintained the high charge carrier mobility of 0.12 cm2/(V s), while preserving high sensitivity and timely response to ammonia exposure at concentrations as low as 10 ppm.
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EXPERIMENTAL SECTION
1 + cos θ =
Device Fabrication. A Si wafer with a 100 nm thick SiO2 dielectric layer on heavily n-doped silicon was used as a substrate. The substrate was cleaned in a piranha solution. The semiconducting polymer, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) was dissolved at 10 mg/mL in anhydrous 1,2dichlorobenzene, followed by agitation at 100 °C for 1 h. Octyltrichlorosilane (OTS)-modified SiO2 surface was obtained by immersing the substrate in 1 mM OTS (in toluene) at 40 °C for 30 min, followed by thermal annealing at 120 °C for 30 min. To formulate a Cytop solution, a solute of Cytop (CTL-809M, Asahi Glass) was mixed with its corresponding solvent (CT-solv.180, Asahi Glass) at 1:40 volume ratio. Cytop-modified SiO2 surface was obtained by spin-coating the formulated Cytop solution onto the substrate at 4000 rpm for 40 s, followed by thermal annealing at 200 °C for 30 min. The Ci values per unit area of pristine SiO2 (100 nm), OTS/SiO2 and Cytop (20 nm)/SiO2 layer were obtained as 33.5, 32.5, and 29.5 nF/cm2, respectively. The capacitance values for the dielectric layers were measured using Keysight E4981 at 1 kHz. The active layer was completed by spin-coating the PBTTT solution in 1,2-dichlorobenzene onto OTS-treated, Cytop-treated and pristine substrate at 2000 rpm for 90 s. To further enhance the molecular order between PBTTT
2(γsd)0.5 (γlvd)0.5 γlv
+
2(γsp)0.5 (γlvp)0.5 γlv
where γlv is the surface energy of the test liquid, and superscripts d and p refer to the dispersive and polar components, respectively. The adhesion energy was calculated following the previous method.27 Fourier transform infrared spectroscopy (FT-IR) absorbance peaks were acquired using an FTIR 4700 device (JASCO).
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RESULTS AND DISCUSSION Figure 1(a) shows the proposed interaction mechanism between a semiconducting polymer (PBTTT) and ammonia molecules. Although it is not clear whether this mechanism involves chemical base dedoping or physical dipole-charge interactions, ammonia molecules with lone electron pairs can be briefly captured by conjugated polymer surface molecules. As indicated by the FTIR analysis (Figure S1), no permanent changes in the PBTTT chemical structure due to ammonia exposure were detected. Thus, while captured ammonia molecules remain on the surface of PBTTT, their lone electron pairs can serve as effective charge trapping centers, especially B
DOI: 10.1021/acsami.6b00471 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 2. AFM images of surface of PBTTT films deposited onto (a) pristine SiO2, (b) OTS, and (c) Cytop. The rms roughness values are 0.9, 1.9, and 6.1 nm for a, b, and c, respectively. All the films were annealed at 175 °C for 30 min.
Figure 3. GIXD images measured from (a) SiO2-, (b) OTS-, and (c) Cytop-based PBTTT films and their corresponding extracted profiles along with (d) out-of-plane direction and (e) in-plane direction. (f) The pole figures for each GIXD data.
same time, the trap density can be expressed by the relation CV Ntr = ie th where Ci is the capacitance of a dielectric layer, e is the electron charge, and Vth is the threshold voltage obtained from transfer characteristics.29 Thus, in the case of p-type FETs, ammonia-originated trap generation should shift the Vth (or the turn-on voltage Vturn‑on) parameters of transfer curves in the negative direction. A representative example of such a trap effect on transfer characteristics is shown in Figure 1(c). Here, the PFET structure is a conventional one with a Au electrode on the top and a SiO2 (100 nm)/Si2+ substrate at the bottom. Upon ammonia exposure, the transfer curve of the PBTTTbased PFET shifts in the negative direction, while the oncurrent decreases, indicating trap generation. Furthermore, because ammonia molecules do not remain permanently on the surface of PBTTT, the transfer curve completely returns to its original position after the ammonia stream is stopped. At the same time, the observed changes in Ion and Vturn‑on values are not very significant for highly sensitive ammonia sensors. These results indicate that although PBTTT interacts with ammonia gas molecules to a certain degree, the surface morphology of PBTTT films needs to be strategically tuned to further enhance the PFET sensitivity, so that a greater number of PBTTT surface sites can be exposed to ammonia gas.
for semiconductor positive charge carriers, which can be directly related to the PFET performance. Here one needs to note that chemical impurities in conjugated organic semiconductors often result in diagonal disorder (energetic disorder) due to the formation of locally different HOMO/ LUMO energy levels for charge carrier pathways. In the case of ammonia molecules, they can induce polarization effects on the original conjugated structure of PBTTT, which lead to fluctuations of local HOMO/LUMO energy levels. According to the widely accepted multiple trapping and release model, such localized energy states are associated with trap states as schematically depicted in Figure 1(b). In this case, the charge carrier mobility can be expressed as ⎛ E ⎞ μ = μ0 α exp⎜ − t ⎟ ⎝ kT ⎠
where μ0 is the trap-free mobility, Et is the trapping energy, and α represents the ratio between the density of delocalized levels available for transport and the density of traps.28 According to this formula, if the trapping energy and the number of trap sites increase, the charge carrier mobility exponentially decreases. For a FET, the charge carrier mobility is directly related to the slope of the transfer curve, and thus, high charge carrier mobility always yields higher on-current (Ion) values. At the C
DOI: 10.1021/acsami.6b00471 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 4. Transistor behaviors under the programmed ammonia gas exposure. Transfer curves of (a) SiO2-, (b) OTS-, and (c) Cytop-based PFETs, Their corresponding shifts of Von and Ion are summarized in panels d−f.
molecules when exposed to ammonia stream due to their higher surface area. However, as was previously indicated, not only good gas-capture abilities but also well-developed percolation pathways for charge transport (and, thus, high charge carrier mobility) are required for good gas sensors. Because charge carrier mobility is dependent on the crystalline nature of the film (which cannot be fully investigated by a morphological analysis), the structural properties of the PBTTT films deposited onto buffer layers were studied by means of 2-D grazing-incidence X-ray diffraction (GIXD). As shown in Figures 3(a−c), all the PBTTT films reveal traditional lamellar stacking patterns along the out-of-plane direction, which can be obtained from the well-developed Bragg diffraction peaks up to (400) (Figure 3(d)). At the same time, close π−π interactions were also observed along the inplane direction, resulting in a π−π stacking distance of 3.71 Å (Figure 3(e)). Thus, no qualitative differences in crystalline nature between the PBTTT films deposited onto different buffer layers were observed. To quantitatively elucidate the crystalline orientation of the PBTTT films, we conducted a pole figure analysis that examines the orientational distribution of the diffraction peaks as a function of all possible crystalline orientations.30 It was performed by analyzing the distribution of possible (200) orientations to avoid surface scattering effects resulting from the substrate. For a quantitative comparison, the relative degree of crystallinity (rDoC) of PBTTT was further calculated by integrating the relative population distribution, taking into account the factor of X-ray exposure time, beam footprint, sample thickness, and polymer volume fraction.30 The calculated rDoC values were 1.00, 5.02, and 3.09 for the pristine, OTS-based, and Cytop-based films, respectively. The resulting pole figures are presented in Figure 3(f) and indicate that all the PBTTT films adopt a preferential edge-on orientation. In particular, most crystallites in the Cytop-based
Unlike small molecular semiconductors, it is very difficult to create nanosized patterns on the surface of polymers with high molecular weight due to their entanglement nature. Thus, we strategically introduced an energy difference between the PBTTT and dielectric surfaces, so that stable nanosized morphological features can be created on the PBTTT surface. When the adhesion energy of the semiconductor/substrate interface is smaller than the cohesive energy of semiconductor molecules, the relatively stronger cohesive energy induces the formation of large crystalline domains, resulting in nanosized features along the vertical direction. In the case of pristine SiO2 substrate, the adhesion energy of PBTTT/SiO2 can be as high as 76.4 mN m−1 due to its hydrophilic nature, which is much greater than the cohesive energy of PBTTT molecules (62.2 mN m−1). Therefore, only featureless morphology of the PBTTT film with a low root-mean-square (rms) roughness of 0.9 nm can be observed in Figure 2(a). After that, the OTS molecules were self-assembled on the SiO2 substrate to make the substrate surface more hydrophobic. In this case, the adhesion energy of the PBTTT/substrate decreases to 48.8 mN m−1, which is certainly lower than the cohesive energy of PBTTT. As a result, large crystalline domains with an increased rms roughness of around 1.9 nm are observed, as shown in Figure 2(b). When the SiO2 substrate was modified by using a Cytop thin interlayer to make it more hydrophobic, the adhesion energy further decreased to 36.2 mN m−1, which is much lower than the PBTTT cohesive energy. In this case, very well developed three-dimensional morphological structures with a high rms roughness of 6.1 nm were created. In other words, as the relative cohesive energy of the PBTTT molecule increases compared to the adhesion energy of the PBTTT/ substrate, three-dimensional morphological features are developed, leading to higher surface roughness. Therefore, Cytopbased PBTTT PFETs may capture larger amounts of gas D
DOI: 10.1021/acsami.6b00471 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Table 1. Summary of PFET Performances with Various Buffer Layers μ (cm2/(V·s))a off SiO2 OTS Cytop
b
0.007 0.122 0.013
Ion/Ioff
Vturn‑on (V)
Vth (V)
Ss‑th
onc
offd
off
on
off
off
on
off
off
on
off
off
on
off
0.004 0.065 0.003
0.006 0.118 0.010
> 104 > 105 > 103
> 103 > 105 > 103
> 104 > 105 > 103
5 0 8
3 −7 −1
5 0 8
5 −1 8
2 −9 −2
5 −1 8
4.02 0.92 2.06
4.05 1.02 2.15
4.02 0.93 2.08
Mobility values were summarized from more than 10 devices for each fabrication condition, and all the films were annealed at 175 °C for 30 min and then slowly cooled. bThis corresponds to “Pristine” stage in the Figure 4 cThis corresponds to “Gas exposure” stage in the Figure 4 dThis corresponds to “Recovery” stage in the Figure 4
a
Figure 5. (a) Transient responses of Si-, OTS-, and Cytop-based PFET ammonia gas sensor under programmed ammonia gas exposure. (b) Transient responses of OTS-based PFET ammonia sensor under various ammonia concentrations from 10 to 100 ppm. (c) Transient responses of OTS-based PFET ammonia sensor at off-regime.
PBTTT films are preferentially edge-on oriented within a polar angle (ω) of 15°, and such tendency gradually decreases when the buffer layer is changed to OTS, while the overall rDoC is significantly enhanced. In the case of pristine PBTTT without a buffer layer, the worst preferential edge-on orientations as well as the lowest rDoC values were observed. The results of the GIXD analysis show that because of the insertion of an OTS buffer layer, the PBTTT atoms have a higher probability of intermolecular interactions leading to larger crystalline domains with preferential edge-on orientations, which is beneficial for inplane charge transport. In the case of a Cytop buffer layer, the overall crystallinity of the upper-deposited PBTTT was much worse than in the case of OTS presumably because of a very rough three-dimensional surface morphology. To investigate the effect of the observed morphological/ structural differences on the charge transport behavior, various FET performances were compared, and the resulting transfer curves were summarized in Figure S2. In general, charge carrier mobility increases owing to the insertion of a hydrophobic buffer layer between the semiconductor and the SiO2 layers. The OTS interlayer yielded the highest mobility increase from 7 × 10−3 cm2/(V s) to 1.2 × 10−1 cm2/(V s), while the Cytop interlayer resulted in a marginal mobility of 1.3 × 10−2 cm2/(V s). These results indicate that in contrast to featureless and large three-dimensional featured morphologies, two-dimensional morphology with well-defined crystalline domains can results in the most effective percolation pathways for charge carriers, which is consistent with the GIXD analysis. Thus, both high surface area and efficient percolation pathways for charge carriers required for effective gas sensing have been established for the OTS-based PBTTT PFETs. Figure 4 summarizes the transfer behavior of the PBTTT PFETs with various buffer layers under ammonia exposure. Additionally, all the important PFET parameters obtained from Figure 4 are summarized in Table 1. Notably, both the OTS- and Cytop-based devices show dramatically better ammonia-sensing abilities compared to the pristine device. The change in Vturn‑on against ammonia exposure increases from 2 V (pristine) to 7 V (OTS) and 9 V
(Cytop), indicating a good agreement with the morphological observations characterized by gradual increases in surface roughness in the same sequence. The decrease in charge carrier mobility against ammonia exposure shows a similar tendency: from 7 × 10−3 cm2/(V s) to 4 × 10−3 cm2/(V s) (pristine), from 1.2 × 10−1 cm2/(V s) to 6.0 × 10−2 cm2/(V s) (OTS), and from 1.3 × 10−2 cm2/(V s) to 3 × 10−3 cm2/(V s) (Cytop). Despite the obviously different degrees of change in the FET behavior upon ammonia exposure, all the PBTTT PFETs show a very prompt recovery after blocking the ammonia stream indicating that irreversible deformation of the polymer semiconductor chemical structure does not occur even in the case of enlarged PBTTT surfaces. An analysis of the resulting transfer curves presented in Figure 4 suggests that the “turn-off” regime can yield better sensing abilities than the “turn-on” regime if the sensitivity toward ammonia exposure is defined as Igas‑on/Igas‑off. In the case of OTS-based PFETs, the calculated sensitivity is greater than 105 at VG = −7 V, while it becomes less than 10 at VG = −30 V. The subthreshold swing of FET, S, is known to be related with trap density by ⎡ qS log(e) ⎤C Ntrap ≈ ⎢ − 1⎥ i ⎣ kT ⎦q
where q is the electronic charge, k is Boltzmann’s constant, and T is the temperature.31 Based on this equation, we could estimate the interface trap densities of pristine, OTS- and Cytop- based PFET as 1.48 × 1013, 2.85 × 1012, and 6.01 × 1012 cm−2, respectively. The increasing trap densities as a sequence OTS-, Cytop-, and pristine- PFETs are in good agreement with other characterization results such as GIXD, AFM, and mobility measurement. Notably, in each PFET, gas exposure always accompanied increase in the value of S, again confirming the local energetic disorder driven by ammonia adsorption. Transient measurements of ammonia-detecting characteristics (at 100 ppm ammonia exposures) were also performed for PBTTT PFETs, as summarized in Figure 5(a). After E
DOI: 10.1021/acsami.6b00471 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces ammonia exposure, the drain current ID (measured at VG = −30 V) rapidly decreases by more than 4 μA within a few seconds. Turning off the ammonia stream results in a quick recovery (within 10 s) of the ID value to its original level without any external manipulations. The sensing test was repeated more than 320 times during 220 min, which points to excellent device reliability without any protecting layers at ambient conditions (Figure 5(a)). In the next step, OTS-based PFETs were exposed to various concentrations of ammonia gas for 40 s. As shown in Figure 5(b), the PFETs reveal reasonably good sensing characteristics at ammonia concentrations as low as 10 ppm. Roughly, it can be estimated that each 10 ppm increase of ammonia gas concentration enabled 130 nA increase in output current level. As was noted earlier, the sensitivity (Igas‑on/Igas‑off) parameter can be enhanced at lower VG values. As shown in Figure 5(c), at VG = −7 V, the same transient measurements were repeated. In this case, the sensitivity magnitude was increased to 12, while maintaining similar temporal response values.
OFET OTS GIXD IVU rms rDoC
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CONCLUSION Until now, most surface-engineered OFETs for gas sensor applications have utilized small molecular semiconductors due to their ease of morphology control. However, to fully exploit the operational advantages of OFETs, polymer semiconductors characterized by difficulties in realizing nanosized surface morphological features should be used. In this work, PBTTTbased PFETs with strategically designed surface morphologies were demonstrated as high-performance ammonia gas sensors. Elucidating the morphological and structural parameters of PBTTT films with various surface morphologies in conjunction with FET studies revealed that the surface morphology containing large two-dimensional crystalline domains is optimal for achieving high surface area and creating percolation pathways for charge carriers. By optimizing the morphology of polymer semiconductor surfaces effectively contaminated with ammonia molecules, good gas sensing characteristics, and prompt response times can be obtained. ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00471. FTIR spectra of the PBTTT films before and after ammonia gas exposure and Transfer curves of SiO2-, OTS-, and Cytop-based PFETs without gas exposure. (PDF)
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organic field effect transistor Octyltrichlorosilane Grazing incident X-ray diffraction in-vacuum undulator root-mean-square relative degree of crystallinity
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant number: NRF-2015R1C1A1A02037219).
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ABBREVIATIONS PFET polymer field effect transistor F
DOI: 10.1021/acsami.6b00471 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
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DOI: 10.1021/acsami.6b00471 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
