materials Article
Eco-Friendly and Biodegradable Biopolymer Chitosan/Y2O3 Composite Materials in Flexible Organic Thin-Film Transistors Bo-Wei Du ID , Shao-Ying Hu, Ranjodh Singh, Tsung-Tso Tsai, Ching-Chang Lin * and Fu-Hsiang Ko * Department of Materials Science and Engineering, National Chiao Tung University, 1001 University Road, Hsinchu 30010, Taiwan; [email protected] (B.-W.D.); [email protected] (S.-Y.H.); [email protected] (R.S.); [email protected] (T.-T.T.) * Correspondence: [email protected] (C.-C.L.); [email protected] (F.-H.K.); Tel.: +886-920-285-453 (C.-C.L.); +886-910-399-192 (F.-H.K.) Received: 15 June 2017; Accepted: 30 August 2017; Published: 3 September 2017
Abstract: The waste from semiconductor manufacturing processes causes serious pollution to the environment. In this work, a non-toxic material was developed under room temperature conditions for the fabrication of green electronics. Flexible organic thin-film transistors (OTFTs) on plastic substrates are increasingly in demand due to their high visible transmission and small size for use as displays and wearable devices. This work investigates and analyzes the structured formation of aqueous solutions of the non-toxic and biodegradable biopolymer, chitosan, blended with high-k-value, non-toxic, and biocompatible Y2 O3 nanoparticles. Chitosan thin films blended with Y2 O3 nanoparticles were adopted as the gate dielectric thin film in OTFTs, and an improvement in the dielectric properties and pinholes was observed. Meanwhile, the on/off current ratio was increased by 100 times, and a low leakage current was observed. In general, the blended chitosan/Y2 O3 thin films used as the gate dielectric of OTFTs are non-toxic, environmentally friendly, and operate at low voltages. These OTFTs can be used on surfaces with different curvature radii because of their flexibility. Keywords: non-toxic; organic thin-film transistors (OTFTs); chitosan; Y2 O3 ; flexible
1. Introduction For two decades, the global developments of the electronics industry have focused on flexible electronic devices, such as curved full-color displays [1,2] integrated sensors [3], flexible solar cells, and the amazing achievement of E-paper [4,5]. Using a solution-based process achieves many advantages that are cost-effective and simple to fabricate, and produces mechanically flexible thin-film transistors compared to conventional semiconductor technologies, which depend on vacuum-based thin film fabrication [6,7]. In the past decade, eco-friendly, biocompatible, and green materials have been the subject of many economic and scientific projects [8–10], and have caused less damage to the environment. Oxide thin films under low annealing temperatures have been fabricated by using an inexpensive “water-inducement” technique [11] that combines a high-k-value YOX dielectric material with an eco-friendly water-inducement process [12]. Because of their superior performance, organic thin-film transistors (OTFTs) can be used to replace conventional thin-film transistors (TFTs). Chitin is a natural amino polysaccharide and is the largest nitrogenous natural organic compound on the planet after protein and the cellulose polysaccharides found in nature [13]. It has many outstanding characteristics, such as biocompatibility, non-toxicity, biodegradability, antimicrobial activity, and excellent mechanical strength, which make it suitable for use in the biomedical Materials 2017, 10, 1026; doi:10.3390/ma10091026
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field [14,15]. Chitosan can be transformed into chitin and has the same excellent characteristics, such as biocompatibility, biodegradability, non-toxicity, antimicrobial activity, and an outstanding film-forming ability [16–18]. Therefore, in previous studies, chitosan has also been used as dielectric 2017, 10, 1026 2 ofthat 9 layerMaterials in organic transistors [19,20]. Yttrium (III) oxide (Y2 O3 ) is a type of rare earth oxide is non-toxic, thermodynamically stabile, stabile at high temperature (Tm = 2430 ◦ C), and has a high biocompatibility, biodegradability, non-toxicity, antimicrobial activity, and an outstanding filmdielectric constant (ε = 15~18), light transparency, and a linear transmittance in the infrared spectra. forming ability [16–18]. Therefore, in previous studies, chitosan has also been used as dielectric layer Y2 O3in is organic commonly known as a high-k canofreplace SiO has a high-k 2 , because transistors [19,20]. Yttriumdielectric (III) oxidematerial (Y2O3) isthat a type rare earth oxide that is it non-toxic, dielectrics value and a phase with cubic Its lattice a =and 10.6has Å, ais high two times as large thermodynamically stabile, stabile at symmetry. high temperature (Tmconstant, = 2430 °C), dielectric as theconstant lattice (ε constant oflight Si (atransparency, = 5.43 Å). Yand be transmittance deposited byindifferent deposition techniques, = 15~18), linear the infrared spectra. Y 2O3 is 2 O3 acan commonly known a high-k dielectric material that can replace SiO2, chemical because it vapor has a high-k including pulsed laserasdeposition (PLD) [21], sputtering metal-organic deposition dielectrics value and a phase with cubic symmetry. Its lattice constant, a = 10.6 Å , is two times as large (MOCVD) [22], and electron beam evaporation [23]. as the lattice constant of Si (a = 5.43 Å ). Y 2O3 can be deposited by different deposition techniques, Therefore, due to its high-k-value, non-toxicity, and biocompatibility, Y2 O3 nanoparticles were including pulsed laser deposition (PLD) [21], sputtering metal-organic chemical vapor deposition blended into the chitosan solution in this study to improve their dielectric properties and pinholes. (MOCVD) [22], and electron beam evaporation [23]. To achieve good electrical performance with the chitosan-based metal-insulator-metal (MIM) structure, Therefore, due to its high-k-value, non-toxicity, and biocompatibility, Y2O3 nanoparticles were various concentrations of Y O3 nanoparticles were blended into the chitosan to decrease the leakage blended into the chitosan2 solution in this study to improve their dielectric properties and pinholes. current improve depthperformance of the pinholes. Chitosan thin filmsmetal-insulator-metal have an electric-double-layer To and achieve good the electrical with the chitosan-based (MIM) effectstructure, that gives OTFTsconcentrations the property of of Ylow-voltage operation. Furthermore, thin films of chitosan various 2O3 nanoparticles were blended into thethe chitosan to decrease blended with Y O nanoparticles were used as the dielectric material in OTFTs, and the performance the leakage2 current and improve the depth of the pinholes. Chitosan thin films have an electric3 double-layer effect that gives OTFTs the property of low-voltage operation. Furthermore, the thin of these OTFTs was enhanced. films of chitosan blended with Y2O3 nanoparticles were used as the dielectric material in OTFTs, and
2. Experimental the performance of these OTFTs was enhanced. Yttrium (III) oxide (Y2 O3 ) was provided by Alfa-Aesar (Heysham, UK). Chitosan, 2. Experimental poly(3-hexylthiophene) (P3HT) and acetic acid were provided by Sigma-Aldrich (St. Louis, MO, Yttrium (III) oxide (Y2O3) was provided by Alfa-Aesar (Heysham, UK). Chitosan, poly(3USA). All other reagents and anhydrous solvents were obtained from local suppliers and used without hexylthiophene) (P3HT) and acetic acid were provided by Sigma-Aldrich (St. Louis, MO, USA). All further purification, unless otherwise noted. other reagents and anhydrous solvents were obtained from local suppliers and used without further The Y2 O3 /chitosan thin film as the dielectric gate of the flexible OTFT, and P-type organic purification, unless otherwise noted. semiconductor, poly(3-hexylthiophene) (P3HT), as the layer on substrate The Y2O3/chitosan thin film as the dielectric gatesemiconductor of the flexible OTFT, andpolyimide P-type organic were semiconductor, demonstratedpoly(3-hexylthiophene) in this study. The basic process flow for the fabrication this flexible device is (P3HT), as the semiconductor layer onof polyimide substrate were demonstrated in this the study. The basic process flow for OTFT the fabrication of this flexible device is shown in Figure 1. Moreover, performance of the flexible during bending tests with different shownradii in Figure 1. Moreover, performance flexible during bending tests with layer curvature was also observed. the In detail, a 5-nmofCrthe metal layerOTFT was deposited as an adhesive curvature was also observed. Inthen detail, a 5-nmAu Cr layer metalwas layerdeposited was deposited an on thedifferent polyimide film byradii thermal deposition and a 30-nm on theasadhesive adhesive layer on the polyimide film by thermal deposition and then a 30-nm Au layer was deposited layer as the bottom gate. The adhesive layer was used to stabilize the Au layer and guarantee that the on the adhesive layer as the bottom gate. The adhesive layer was used to stabilize the Au layer and device would be stable under bending tests. The polyimide substrate was first cleaned before placing it guarantee that the device would be stable under bending tests. The polyimide substrate was first into thermal coater chamber, after which a Cr and Au metal layer was deposited. Before the dielectric cleaned before placing it into thermal coater chamber, after which a Cr and Au metal layer was layerdeposited. was deposited, we dielectric covered layer the adhesive tape on bottom Au electrode, bottom Before the was deposited, we the covered the adhesive tape onfor thethe bottom Au gate can only be exposed the final electrode, for the in bottom gate step. can only be exposed in the final step.
Figure 1. The fabrication process of the bottom-gate top-contact flexible organic thin-film transistor.
Figure 1. The fabrication process of the bottom-gate top-contact flexible organic thin-film transistor.
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Chitosan (deacetylated ≥75%) was dissolved in aqueous acetic acid (0.5 wt %) and heated with a hot plate at 50 ◦ C for 24 h, following which its solutions of various concentrations were filtered with a Materials 2017, 10, 1026 of 9 25-mm syringe filter containing a 0.45-µm polyvinylidene difluoride (PVDF) membrane.3The Y2 O3 nanoparticles were blended with deionized water at a ratio of 0.5 wt % and the hybrid solution Chitosan (deacetylated ≥75%) was dissolved in aqueous acetic acid (0.5 wt %) and heated with a was obtained byatmixing aqueous solution with of Y2various O3 nanoparticles aqueous solution, hot plate 50 °C forchitosan 24 h, following which its solutions concentrations were filtered with with a specific volume ratio CS:Y In other words, the difluoride weight percentage of Y2 O3 The in the a 25-mm syringe filter containing a 0.45-μm polyvinylidene (PVDF) membrane. Y2Oblended 3 2 O3 (20:1). nanoparticles werewt blended with deionized at a ratio of 0.5the wt % and wt the% hybrid wasfilm on solutions was 0.023 %. The drop castingwater was used to form 0.023 Y2 O3solution /chitosan by mixing aqueous solution Y2dried O3 nanoparticles with for a 24 h. the obtained bottom-gate as the chitosan dielectric gate, which waswith then in an ovenaqueous at roomsolution, temperature specific volume ratio CS:Y 2O3 (20:1). In other words, the weight percentage of Y 2O3 in the blended After depositing the dielectric gate, a P3HT channel layer was deposited on the Y2 O3 /chitosan solutions was wt %. The was form1500 the 0.023 wt 30 % Ys.2O 3/chitosan film on dielectric film by0.023 spin-coating atdrop 1200casting rpm for 30 used s andtothen rpm for The polyimide substrate the bottom-gate as the dielectric gate, which was then dried in an oven at room temperature for 24 h. ◦ was placed in an oven at 60 C to remove residual solvent in the P3HT active channel layer. Finally, After depositing the dielectric gate, a P3HT channel layer was deposited on the Y2O3/chitosan a 5-nm Cr and a 30-nm Au layer were deposited with a mask to form the source and drain top contacts. dielectric film by spin-coating at 1200 rpm for 30 s and then 1500 rpm for 30 s. The polyimide substrate Cr metal was in used as anatadhesive layer between the P3HT layerchannel and Au contacts was placed an oven 60 °C to remove residual solvent in thechannel P3HT active layer. Finally,asa before. In the meantime, the bottom Au electrode waswith revealed carefully removing the tape. Therefore, as 5-nm Cr and a 30-nm Au layer were deposited a maskby to form the source and drain top contacts. the Cr above procedure was finished, the bottom-gate top-contact flexible organic thin-film transistor metal was used as an adhesive layer between the P3HT channel layer and Au contacts as before. was In the meantime, the bottom Au electrode was revealed by carefully removing the tape. Therefore, as successfully designed. the above procedure was finished, the bottom-gate top-contact flexible organic thin-film transistor
3. Results and Discussion was successfully designed. 3.1.3.Materials andDiscussion Films Characterization Results and Chitosan ≥75%) was dissolved in aqueous acetic acid (0.5 wt %) and heated by 3.1. Materials(deacetylated and Films Characterization using a hot plate at 55 ◦ C for 24 h. The impurities in the chitosan solutions of various concentrations Chitosan (deacetylated ≥75%) was dissolved in aqueous acetic acid (0.5 wt %) and heated by were filtered using a 25-mm syringe filter with a 0.45-µm PVDF membrane. The chitosan solution using a hot plate at 55 °C for 24 h. The impurities in the chitosan solutions of various concentrations with various concentrations was transferred by spin-coating onto separate single silicon substrates were filtered using a 25-mm syringe filter with a 0.45-µ m PVDF membrane. The chitosan solution thatwith were already coated withwas aluminum metal as an electrode by spin-coating. Then, we removed various concentrations transferred by spin-coating onto separate single silicon substrates ◦ C for 1 h. Finally, we deposited the that water in the chitosan thin film by heating on a hot plate at 80 were already coated with aluminum metal as an electrode by spin-coating. Then, we removed the the aluminum metal as the top electrode onon the chitosan, formed a so-called MIM the structure. water in the chitosan thin film by heating a hot plate at which 80 °C for 1 h. Finally, we deposited − 10 as the leakage top electrode on was the chitosan, formed so-calledvoltage MIM structure. We aluminum found thatmetal the lower current 6.827 × which 10 A at anaapplied of 2 V inWe the MIM found the leakage current wasand 6.827observed × 10−10 A atthat an applied of 2 V in the MIM based on that a 1.0 wtlower % chitosan thin film, the 1.0voltage wt % chitosan film hadbased the lowest on a 1.0 wt % chitosan thin film, observed wt % film lowest in leakage leakage current. This result was and attributed tothat the the size1.0and thechitosan number ofhad the the pinholes the surface current. This result was attributed to the size and the number of the pinholes in the surface of of the chitosan, as shown in Figure 2. The thin film with 0.5 wt % chitosan had many smallthe pinholes chitosan, as shown in Figure 2. The thin film with 0.5 wt % chitosan had many small pinholes (10–20 nm), as shown in Figure 2a, and the thin film with 1.5 wt % chitosan had some large pinholes (10–20 nm), as shown in Figure 2a, and the thin film with 1.5 wt % chitosan had some large pinholes (80–100 nm), as shown in Figure 2c. The thin film with 1.0 wt % chitosan had the optimized hole size (80–100 nm), as shown in Figure 2c. The thin film with 1.0 wt % chitosan had the optimized hole size (30–50 nm)nm) andand number of of holes, asas shown Figure (30–50 number holes, showninin Figure2b; 2b;this thisisiswhy whythe the1.0 1.0wt wt%%sample sample had had the the lowest leakage current. lowest leakage current.
Figure 2. (a) 0.5 wt % chitosan shows many small holes (b) 1.0 wt % chitosan shows a few middle-sized
Figure (a) (c) 0.51.5 wtwt %% chitosan holes (b) 1.0 wt % chitosan shows a few middle-sized holes2.and chitosanshows showsmany many small large holes. holes and (c) 1.5 wt % chitosan shows many large holes. The high-k-value Y2O3 nanoparticles were blended into a 1 wt % chitosan water solution with various weight percentages of Y2O3 (from 0.012 wt % to 0.016 wt %, 0.023 wt % and 0.045 wt %). While The high-k-value Y2 O3 nanoparticles were blended into a 1 wt % chitosan water solution with the weight percentage increased, the leakage current decreased. The blended 0.045 wt % Y2O3 in various weight percentages of Y2 O3 (from 0.012 wt % to 0.016 wt−11%, 0.023 wt % and 0.045 wt %). 1 wt % chitosan thin film had the lowest leakage current of 1.81 × 10 A, as shown in Figure 3a. While the weight percentage increased, the leakage current decreased. The blended 0.045 wt % Y O The pure chitosan thin films were almost transparent, the weight percentage of the Y2O3 increased, 2 3
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in 1 wt % chitosan thin film had the lowest leakage current of 1.81 × 10−11 A, as shown in Figure 3a. pure chitosan thin were almost weightinpercentage ofdepth the Y2(from O3 increased, asThe shown in Figure 3b. Infilms the meantime, wetransparent, discovered athe decrease the relative Materials 2017, 10, 1026 41.025 of 9 shown Figure In the meantime, a as decrease relative depth (from the 1.025 to as 0.356 nm)inwhen the3b. concentration of thewe Y2Odiscovered 3 increased, shownin inthe Figure 4a–d. However, to 0.356 nm) when the concentration of the Y O increased, as shown in Figure 4a–d. However, the as shown in Figure 3b. In the meantime, we discovered a decrease in the relative depth (from 1.025 2 3the weight percentage of Y2O3 reached 0.045 wt %, relative depth and roughness were increased when to 0.356 nm) when the concentration of the Y 2 O 3 increased, as shown in Figure 4a–d. However, the relative depth and roughness were increased when the weight percentage of Y O reached 0.045 wt 2 3 as shown in Figure 4e. The purpose of blending the Y2O3 nanoparticles into the chitosan thin film was%, and were increased the ofthe Y2the Osurface. 3 reached %,was asrelative in Figure 4e. The purpose ofbut blending Y2weight O3 nanoparticles into chitosan0.045 thinwt film not toshown only depth reduce theroughness leakage current, alsowhen tothe improve thepercentage pinholes at as shown Figurethe 4e. leakage The purpose of blending 2O3 nanoparticles intoat thethe chitosan thin film was not to onlyinreduce current, but alsothe to Y improve the pinholes surface. not to only reduce the leakage current, but also to improve the pinholes at the surface.
Figure 3. (a) Leakage current of the thin films measured with various concentrations 3 solutions Figure 3. (a) Leakage current of the thin films measured with various concentrations ofof Y2Y O23Osolutions (without Y O , 0.012 wt %, 0.016 wt %, 0.023 wt %, 0.045 wt %) in 1 wt % chitosan solution (b) Pictures 23, 0.012 3 FigureY3. (a) Leakage current the films with of Y2O(b) 3 solutions (without 2O wt %, 0.016ofwt %,thin 0.023 wtmeasured %, 0.045 wt %)various in 1 wt concentrations % chitosan solution Pictures showing various weight percentages Y in the mixed (without Yvarious 2O3, 0.012 wt %, percentages 0.016 wt %, 0.023 %, wt %)solution. in solution. 1 wt % chitosan solution (b) Pictures showing thethe weight of of Y2wt O23O in3 0.045 the mixed showing the various weight percentages of Y2O3 in the mixed solution.
The cross-sectional images the blended films shown Figure and The cross-sectional images ofof the blended Y2YO23O /chitosan thinthin films areare shown in in Figure 5, 5, and 3 /chitosan The cross-sectional images offilms the blended Y2O3/chitosan120 thin films are shown inalso Figure 5, and thicknesses blended thin films were approximately 120 nm–200 nm. analyzed thethe thicknesses of of thethe blended thin were approximately nm–200 nm. WeWe also analyzed thethe the thicknesses of the thin filmsinwere approximately 120 nm–200 nm. Weby analyzed the distribution surface blended thin films energy-dispersive distribution of of thethe Y2Y O23Oblended nanoparticles in thethe surface of of thethe blended thin films by also energy-dispersive 3 nanoparticles distribution of the Y2O3 nanoparticles in the surface of the blended thin films by energy-dispersive X-ray spectroscopy (EDX, JEOL, Freising, Germany). discovered that 0.012 0.016 X-ray spectroscopy (EDX, JEOL, Freising, Germany). WeWe discovered that thethe 0.012 wtwt %,%, 0.016 wtwt %,%, X-ray spectroscopy (EDX, JEOL, Freising, Germany). We discovered that the 0.012 wt %, 0.016 wt %, and 0.023 % blended showed a uniform dispersion Y2 O3 nanoparticles, and 0.023 wt wt % blended thin thin filmsfilms showed a uniform dispersion of the Yof 2Othe 3 nanoparticles, as shownas and 0.023 wt % blended thin films showed a uniform dispersion of the Y2O3 nanoparticles, as shown Figure Y2 O3 nanoparticles attracted andformed clusterswhen formed in shown Figure in 6a–c. The6a–c. Y2O3 The nanoparticles attracted each othereach and other clusters the when weightthe in Figure 6a–c. The Y2O3 nanoparticles attracted each other and clusters formed when the weight weight percentage of Y2 O30.045 reached 0.045 wt %6d). (Figure 6d). In4e, Figure 4e, we measured the relatively percentage of Y2O3 reached wt % (Figure In Figure we measured the relatively large percentage of Y2O3 reached 0.045 wt % (Figure 6d). In Figure 4e, we measured the relatively large large profile depth of nm, 1.051 nm, and we attributed this phenomenon to the clustering of the profile depth of 1.051 to the the clustering the 2Y O2 3O3 profile depth of 1.051 nm,and andwe weattributed attributed this this phenomenon phenomenon to clustering ofof the YY 2O3 nanoparticles. It was observed that the 0.023 wt % thin film had smoothest surface, and its pinholes nanoparticles. It was observed that the 0.023 wt % thin film had smoothest surface, and its pinholes nanoparticles. It was observed that the 0.023 wt % thin film had smoothest surface, and its pinholes were much improved. were much improved. were much improved.
Figure4.4.AFM AFM morphology morphology and profile depth of the pinholes in the surface ofof the Y2Y O3O /chitosan thinthin Figure and profile depth the pinholes thesurface surface the 2 3/chitosan 3 /chitosan Figure 4. AFM morphology and profile depth ofof the pinholes ininthe of the Y2O thin film with various Y 2O3 concentrations: (a) Without Y2O3; (b) 0.012 wt %; (c) 0.016 wt %; (d) 0.023 wt %; film with various Without Y23;O(b) 0.012 0.016 0.023 3 concentrations: 3 ; (b) film with various Y2Y O23O concentrations: (a)(a) Without Y2O 0.012 wt wt %; %; (c) (c) 0.016 wt wt %; %; (d)(d) 0.023 wt wt %; %; and (e) 0.045 wt %. and (e) 0.045 wt %. and (e) 0.045 wt %.
The FTIR spectra analysis (PerkinElmer, Waltham, MA, USA) of the blended Y2O3/chitosan thin The FTIRused spectra analysis (PerkinElmer, Waltham, USA) as of shown the blended Y2O37a. /chitosan thin films was to obtain information on the chemicalMA, bonding, in Figure The pure films was used obtain information on the chemical bonding, as in Figure 7a. The pure −1 that were chitosan thin to film showed broad absorption peaks at 3000–5000 cmshown attributed to NH 2 chitosan thin film showed broad absorption peaks at 3000–5000 cm−1 that were attributed to NH2
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The FTIR spectra analysis (PerkinElmer, Waltham, MA, USA) of the blended Y2 O3 /chitosan thin films was used to obtain information on the chemical bonding, as shown in Figure 7a. The pure Materials 2017, 10, 1026 5 of 9 Materials 2017, 10,film 1026 showed broad absorption peaks at 3000–5000 cm−1 that were attributed to NH 5 of 92 chitosan thin −1 asymmetric asymmetric stretching stretching and and the the hydrogen-bonded hydrogen-bonded OH. OH. The Thepeak peakatatapproximately approximately2879 2879cm cm−1 was was asymmetric stretching and the hydrogen-bonded OH. The peak at approximately 2879 cm cm−−11 was attributed attributed to to the the CH CH33 asymmetric asymmetric stretching stretching vibrations, vibrations, and andthe theabsorption absorptionpeak peakatat1541 1541 cm−1 was was attributed to the the CH3 asymmetric stretching [24–26] vibrations,NH and+ the absorption peak at 1541 cm−1 was attributed attributed to to the asymmetric asymmetricbending bendingmodes modes [24–26]of of NH33+.. Figure Figure 7a 7a also also shows shows the the FTIR FTIR spectra spectra + + attributed thethe asymmetric bending modesthin [24–26] of which NH3 . Figure 7a also shows the+ FTIR spectra analysis oftoall all blendedY2YO films, are listed in 1. Table NH 23O 3 /chitosan 3 was analysis of the blended /chitosan thin films, which are listed in Table The 1. NHThe 3 was assigned + −1 forwhich analysis oftoall blended Y2O3/chitosan are listedwhich in Table 1. The NH 3 wasfrequency assigned assigned thethe bending frequency 1541 cmfilms, pure chitosan, shifted a higher −1thin to the bending frequency at 1541atcm for pure chitosan, which shifted to atohigher frequency at −1 for pure chitosan, which shifted to a higher frequency at − 1 − 1 to the bending frequency at 1541 cm at 1559 O3 = wt 0.012 wt %0.016 and wt 0.016 wt %, respectively, then shifted to a −1 andand 1559 cmcm 15801580 cm−1cm for Yfor 2O3 Y = 20.012 % and %, respectively, then shifted to a higher −1 and 1580 cm−1 for Y −21Ofor −1 1559 cmfrequency 3 =Y 0.012 wt % and 0.016 wt %, respectively, then shifted to a higher higher at 1598 cm O = 0.023 wt % and shifted to a lower frequency as 1578 cm −1 −1 2 3wt % and shifted to a lower frequency as 1578 cm for Y2O3 = frequency at 1598 cm for Y2O3 = 0.023 −1 −1 frequency at 1598wt cm%. for Y2O3 = 0.023 wt %toand to abetween lower frequency asof 1578 cm for Yoxide 2O3 = for Y2 O The theshifted H-bonds theof oxygen yttrium (III) 0.045 wt3 =%.0.045 The shifts wereshifts due were to thedue H-bonds between the oxygen yttrium (III) oxide and the 0.045 wt %. The shifts were due to the H-bonds between the oxygen of yttrium (III) oxide and the and the amine of chitosan, as shown in Figure 7b. The AFM morphology (Veeco, Plainview, NY, USA) amine of chitosan, as shown in Figure 7b. The AFM morphology (Veeco, Plainview, NY, USA) also amine of chitosan, as shown in Figure 7b. Thepinholes AFM morphology (Veeco, Plainview, USA) also also proved a dense structure fewer formed by blending O3NY, nanoparticles proved that that a dense structure and and fewer pinholes werewere formed by blending Y 2OY 32 nanoparticles into proved that a dense structure and fewer pinholes were formed by blending Y 2 O 3 nanoparticles into into the thin as shown in Figure the thin film,film, as shown in Figure 4. 4. the thin film, as shown in Figure 4.
Figure 5. Cross-section images of the blended Y2O3/chitosan thin films: (a) Without Y2O3; (b) 0.012 wt %; Figure 5.5.Cross-section Cross-section images the blended Y2 O3 /chitosan films: Y(a) Y2 O Figure images of theof blended Y2O3/chitosan thin films: thin (a) Without 2O3;Without (b) 0.012 wt %;3 ; (c) 0.016 wt %; (d) 0.023 wt %; and (e) 0.045 wt %. (b)0.016 0.012wt wt%; %; (d) (c) 0.023 0.016 wt wt %; %; and (d) 0.023 wt %; (c) (e) 0.045 wtand %. (e) 0.045 wt %.
Figure 6. EDX images showing the distribution of yttrium: (a) 0.012 wt %; (b) 0.016 wt %; (c) 0.023 wt %; Figure 6. EDX images showing the distribution of yttrium: (a) 0.012 wt %; (b) 0.016 wt %; (c) 0.023 wt %; and (d)6.0.045 %. showing the distribution of yttrium: (a) 0.012 wt %; (b) 0.016 wt %; (c) 0.023 wt %; Figure EDXwt images and (d) 0.045 wt %. and (d) 0.045 wt %.
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Figure7.7.(a) (a)FTIR FTIRspectra spectraofofthe theblended blendedY2YO2O 3/chitosan thin films; (b) Schematic diagram of hydrogen Figure 3 /chitosan thin films; (b) Schematic diagram of hydrogen bondingbetween betweenthe theYY 2O3 nanoparticles and the amine group of chitosan. bonding 2 O3 nanoparticles and the amine group of chitosan. Table FTIR spectra analysis blended 3/chitosan thin film. Table 1. 1. FTIR spectra analysis of of thethe blended Y2Y O23O/chitosan thin film. Wave Number (cm−1) 1) Y2O3 wt % CH Stretching Wave NumberBroad (cm−Absorption O–H and N–H Stretching Peaks CH Stretching Y2 O3 wt % O–H and N–H Stretching3368 Broad Absorption Peaks n/a 2879 0.012 3368 2880 n/a 3368 2879 0.016 3368 2880 0.012 3368 2880 0.023 3367 2880 0.016 3368 2880 0.045 3367 2879 0.023 3367 2880 0.045 3367 2879
NH3+ Bending NH3 + Bending 1541 1559 1541 1580 1559 1598 1580 1578 1598 1578
3.2. Electric Characteristics of the Flexible Organic Thin Film Transistor 3.2. Electric Characteristics of the Flexible Organic Thin Film Transistor We investigated the electrical properties of the flexible P3HT-based OTFTs with a 0.023 wt % blended Y2O3/chitosan on the polyimide substrate. Figure OTFTs 8 shows the atransfer plots We investigated thedielectric electricalgate properties of the flexible P3HT-based with 0.023 wt % for concave convexdielectric bending gate of the with concave bending radii, R, of the 3.5 cm and 2.8 cm blended Y2 O3and /chitosan onOTFTs the polyimide substrate. Figure 8 shows transfer plots and convexand bending radii, R, of 3.5 cm and 2.8 with cm. In this figure, the electrical the for concave convex bending of the OTFTs concave bending radii, R, characterization of 3.5 cm and 2.8ofcm OTFTs on polyimide substrates without bending to thatthe of electrical the OTFTs on silicon wafers. and convex bending radii, R, of 3.5 cm and 2.8 cm.isInsimilar this figure, characterization of −10 A to 5.740 × 10−11 A due to extrusion, After concave bending, substrates the Ioff value decreased fromis7.421 × 10to the OTFTs on polyimide without bending similar that of the OTFTs on silicon wafers. 10 Aconvex causing a decrease in the size the pinholes. Onfrom the other bending, off value After concave bending, the Ioffofvalue decreased 7.421hand, × 10−after to 5.740 × 10−11the A Idue to −9 A. No matter the direction of bending, the Ion value would decrease. In Figure 8, increased causing to 1.722 ×a10 extrusion, decrease in the size of the pinholes. On other hand, after convex bending, weIcompared the electrical characterization OTFTsthe with differentofbending radii and would 2.8 cm), the to 1.722 × 10−9 A. of Nothe matter direction bending, the(3.5 Ioncm value off value increased and the comparison chart is listedthe in Table 2. The output characterization (IDS -VDSdifferent ) of this device, decrease. In Figure 8, we compared electrical characterization of the OTFTs with bendingas shown Figure S1.cm), and the comparison chart is listed in Table 2. The output characterization radii (3.5incm and 2.8 (IDS -VDS ) of this device, as shown in Figure S1.
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Figure Figure 8. 8. Comparison of the electrical characterization of the P3HT-based organic organic thin-film thin-film transistors transistors (OTFTs) dielectric gate for bending tests at different different bending (OTFTs)with withaa0.023 0.023wt wt% %blended blendedYY22O O33/chitosan /chitosan dielectric bending radii radii and and without without bending. bending. Table Table2.2.Electrical Electricalcharacterization characterizationof ofthe theP3HT-based P3HT-basedOTFTs OTFTswith witha a0.023 0.023wt wt%%blended blendedYY 2O 3/chitosan 2O 3 /chitosan dielectric dielectric gate gate on on the the polyimide polyimide substrate substrate for for bending bending tests. tests.
Condition Vth (V) Vth (V) Flat −2.0 Flat −2.0 Concave bending 3.50 (cm) −2.5 Concave bending 3.50 (cm) −2.5 Concave bending 2.85 (cm) −2.7 Concave bending 2.85 (cm) −2.7 Flat −2.1 Flat −2.1 Convex bending 3.50 (cm) −3.0 −3.0 Convex bending 3.50 (cm) Convex bending 2.85 (cm) −3.3 −3.3 Convex bending 2.85 (cm) Flat −1.5 Flat −1.5 Condition
Ion (A) −5 1.268 × 10 1.268 × 10−5−6 4.592 × 10 4.592 × 10−6−5 1.298 −5 1.298 ××1010 −5 − 1.294 × 10 1.294 × 10 5 −6−6 9.899 9.899 ××1010 −6−6 9.480 9.480 ××1010 − 1.059 ××10105−5 1.059 Ion (A)
Ioff (A) 7.421 × 10−−10 10 7.421 × 10 −11 5.740 × 10 5.740 × 10−11 1.913××10 10−−9 9 1.913 −10 − 10 4.571 × 10 4.571 × 10 9 1.722××10 10−−9 1.722 10 3.296××10 10−−10 3.296 9 2.539 2.539××10 10−−9 Ioff (A)
Ioff/Ion Ratio 1055 10 1055 104 10 1045 10 105 3 10 103 3 10 104
Ioff /Ion Ratio
Mobility (cm22/Vs) Mobility (cm /Vs) 2.50 × 10−2 2.50 × 10−2 3.33 × 10−2 −2 3.33 × 10−2 2.70 × 10 −2 2.70 × 10−4 1.87 × 10 1.87 × 10−4 −2 8.53 8.53× ×10 10−2 −2 4.80 4.80× ×10 10−2 −2 3.33× ×10 10−2 3.33
4. Conclusions Conclusions 4. A solution-based solution-based processed processedand andlow-voltage low-voltageoperating operatingP3HT-based P3HT-basedOTFT OTFTwith witha aYY 2O/chitosan A 2O 3 3/chitosan gate dielectric dielectric layer layer was was demonstrated demonstrated in in this this study. study. To To improve improve the the electrical electrical performance performance of of the the gate chitosan-based MIM, various concentrations of Y 2 O 3 nanoparticles were blended into the chitosan, chitosan-based MIM, various concentrations of Y2 O3 nanoparticles were blended into the chitosan, whichachieved achievedaadecreased decreasedleakage leakagecurrent current and improved depth of the pinholes. Furthermore, which and improved thethe depth of the pinholes. Furthermore, the the P3HT-based with wt a 0.023 wt % blended Y 2O 3/chitosan gate dielectric layer was P3HT-based OTFT OTFT with a 0.023 % blended Y2 O /chitosan gate dielectric layer was manufactured 3 manufactured onbending polyimide bending tests. The electrical ofincurred the flexible device on polyimide for tests.for The electrical performance of theperformance flexible device no obvious incurred except no obvious except for slight increase in theand leakage current and current. and The changes for achanges slight increase in athe leakage current off current. Theoff non-toxic non-toxic and eco-friendly biopolymer chitosan, with Y2O3was nanoparticles, successfully eco-friendly biopolymer chitosan, blended with Y2blended O3 nanoparticles, successfullywas used in flexible used inasflexible as the gate dielectric, the OTFT to voltages, operate under low voltages, and OTFTs the gateOTFTs dielectric, enabling the OTFTenabling to operate under low and producing a Ion /I off 5 5 producing off ratio of 10ofat voltage of −10 V and a drain ratio of 10 aatIon a /I gate voltage −a10gate V and a drain voltage of − 1 V. voltage of −1 V. Supplementary Materials: The following are available online at www.mdpi.com/1996-1944/10/9/1026/s1. Supplementary Materials: The following are available online at www.mdpi.com/1996-1944/10/9/xx/s1. Figure Figure S1: Output characterization of P3HT-based OTFTs with 0.023 wt % blended Y2 O3 /chitosan dielectric gate. S1. Output characterization of P3HT-based OTFTs withthe 0.023 wt% blended Y2O3/chitosan dielectric gate. for Acknowledgments: The authors would like to thank Ministry of Science and Technology of Taiwan supporting this work The under Contract MOST MOST 105-2622-M-009-004-CC3. Acknowledgments: authors would like104-2113-M-009-008-MY3 to thank the Ministry ofand Science and Technology of Taiwan for supporting this work under Contract Ko, MOST 104-2113-M-009-008-MY3 and MOST Author Contributions: Fu-Hsiang Ranjohn Singh and Ching-chang Lin 105-2622-M-009-004-CC3. designed the experiments. Shao-Ying Hu and Tsung-Tso Tsai carried out the experimental work. Bo-Wei Du wrote the manuscript. All authors Author Contributions: Fu-Hsiang Ko, Ranjohn Singh and Ching-chang Lin designed the experiments. contributed to the discussions of this work. Shao-Ying Hu and Tsung-Tso Tsai carried out the experimental work. Bo-Wei Du wrote the manuscript. Conflicts of Interest: The authors declare no conflicts of interest. All authors contributed to the discussions of this work.
Conflicts of Interest: The authors declare no conflicts of interest.
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Eco-Friendly and Biodegradable Biopolymer Chitosan/Y2O3 Composite Materials in Flexible Organic Thin-Film Transistors Bo-Wei Du ID , Shao-Ying Hu, Ranjodh Singh, Tsung-Tso Tsai, Ching-Chang Lin * and Fu-Hsiang Ko * Department of Materials Science and Engineering, National Chiao Tung University, 1001 University Road, Hsinchu 30010, Taiwan; [email protected] (B.-W.D.); [email protected] (S.-Y.H.); [email protected] (R.S.); [email protected] (T.-T.T.) * Correspondence: [email protected] (C.-C.L.); [email protected] (F.-H.K.); Tel.: +886-920-285-453 (C.-C.L.); +886-910-399-192 (F.-H.K.) Received: 15 June 2017; Accepted: 30 August 2017; Published: 3 September 2017
Abstract: The waste from semiconductor manufacturing processes causes serious pollution to the environment. In this work, a non-toxic material was developed under room temperature conditions for the fabrication of green electronics. Flexible organic thin-film transistors (OTFTs) on plastic substrates are increasingly in demand due to their high visible transmission and small size for use as displays and wearable devices. This work investigates and analyzes the structured formation of aqueous solutions of the non-toxic and biodegradable biopolymer, chitosan, blended with high-k-value, non-toxic, and biocompatible Y2 O3 nanoparticles. Chitosan thin films blended with Y2 O3 nanoparticles were adopted as the gate dielectric thin film in OTFTs, and an improvement in the dielectric properties and pinholes was observed. Meanwhile, the on/off current ratio was increased by 100 times, and a low leakage current was observed. In general, the blended chitosan/Y2 O3 thin films used as the gate dielectric of OTFTs are non-toxic, environmentally friendly, and operate at low voltages. These OTFTs can be used on surfaces with different curvature radii because of their flexibility. Keywords: non-toxic; organic thin-film transistors (OTFTs); chitosan; Y2 O3 ; flexible
1. Introduction For two decades, the global developments of the electronics industry have focused on flexible electronic devices, such as curved full-color displays [1,2] integrated sensors [3], flexible solar cells, and the amazing achievement of E-paper [4,5]. Using a solution-based process achieves many advantages that are cost-effective and simple to fabricate, and produces mechanically flexible thin-film transistors compared to conventional semiconductor technologies, which depend on vacuum-based thin film fabrication [6,7]. In the past decade, eco-friendly, biocompatible, and green materials have been the subject of many economic and scientific projects [8–10], and have caused less damage to the environment. Oxide thin films under low annealing temperatures have been fabricated by using an inexpensive “water-inducement” technique [11] that combines a high-k-value YOX dielectric material with an eco-friendly water-inducement process [12]. Because of their superior performance, organic thin-film transistors (OTFTs) can be used to replace conventional thin-film transistors (TFTs). Chitin is a natural amino polysaccharide and is the largest nitrogenous natural organic compound on the planet after protein and the cellulose polysaccharides found in nature [13]. It has many outstanding characteristics, such as biocompatibility, non-toxicity, biodegradability, antimicrobial activity, and excellent mechanical strength, which make it suitable for use in the biomedical Materials 2017, 10, 1026; doi:10.3390/ma10091026
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field [14,15]. Chitosan can be transformed into chitin and has the same excellent characteristics, such as biocompatibility, biodegradability, non-toxicity, antimicrobial activity, and an outstanding film-forming ability [16–18]. Therefore, in previous studies, chitosan has also been used as dielectric 2017, 10, 1026 2 ofthat 9 layerMaterials in organic transistors [19,20]. Yttrium (III) oxide (Y2 O3 ) is a type of rare earth oxide is non-toxic, thermodynamically stabile, stabile at high temperature (Tm = 2430 ◦ C), and has a high biocompatibility, biodegradability, non-toxicity, antimicrobial activity, and an outstanding filmdielectric constant (ε = 15~18), light transparency, and a linear transmittance in the infrared spectra. forming ability [16–18]. Therefore, in previous studies, chitosan has also been used as dielectric layer Y2 O3in is organic commonly known as a high-k canofreplace SiO has a high-k 2 , because transistors [19,20]. Yttriumdielectric (III) oxidematerial (Y2O3) isthat a type rare earth oxide that is it non-toxic, dielectrics value and a phase with cubic Its lattice a =and 10.6has Å, ais high two times as large thermodynamically stabile, stabile at symmetry. high temperature (Tmconstant, = 2430 °C), dielectric as theconstant lattice (ε constant oflight Si (atransparency, = 5.43 Å). Yand be transmittance deposited byindifferent deposition techniques, = 15~18), linear the infrared spectra. Y 2O3 is 2 O3 acan commonly known a high-k dielectric material that can replace SiO2, chemical because it vapor has a high-k including pulsed laserasdeposition (PLD) [21], sputtering metal-organic deposition dielectrics value and a phase with cubic symmetry. Its lattice constant, a = 10.6 Å , is two times as large (MOCVD) [22], and electron beam evaporation [23]. as the lattice constant of Si (a = 5.43 Å ). Y 2O3 can be deposited by different deposition techniques, Therefore, due to its high-k-value, non-toxicity, and biocompatibility, Y2 O3 nanoparticles were including pulsed laser deposition (PLD) [21], sputtering metal-organic chemical vapor deposition blended into the chitosan solution in this study to improve their dielectric properties and pinholes. (MOCVD) [22], and electron beam evaporation [23]. To achieve good electrical performance with the chitosan-based metal-insulator-metal (MIM) structure, Therefore, due to its high-k-value, non-toxicity, and biocompatibility, Y2O3 nanoparticles were various concentrations of Y O3 nanoparticles were blended into the chitosan to decrease the leakage blended into the chitosan2 solution in this study to improve their dielectric properties and pinholes. current improve depthperformance of the pinholes. Chitosan thin filmsmetal-insulator-metal have an electric-double-layer To and achieve good the electrical with the chitosan-based (MIM) effectstructure, that gives OTFTsconcentrations the property of of Ylow-voltage operation. Furthermore, thin films of chitosan various 2O3 nanoparticles were blended into thethe chitosan to decrease blended with Y O nanoparticles were used as the dielectric material in OTFTs, and the performance the leakage2 current and improve the depth of the pinholes. Chitosan thin films have an electric3 double-layer effect that gives OTFTs the property of low-voltage operation. Furthermore, the thin of these OTFTs was enhanced. films of chitosan blended with Y2O3 nanoparticles were used as the dielectric material in OTFTs, and
2. Experimental the performance of these OTFTs was enhanced. Yttrium (III) oxide (Y2 O3 ) was provided by Alfa-Aesar (Heysham, UK). Chitosan, 2. Experimental poly(3-hexylthiophene) (P3HT) and acetic acid were provided by Sigma-Aldrich (St. Louis, MO, Yttrium (III) oxide (Y2O3) was provided by Alfa-Aesar (Heysham, UK). Chitosan, poly(3USA). All other reagents and anhydrous solvents were obtained from local suppliers and used without hexylthiophene) (P3HT) and acetic acid were provided by Sigma-Aldrich (St. Louis, MO, USA). All further purification, unless otherwise noted. other reagents and anhydrous solvents were obtained from local suppliers and used without further The Y2 O3 /chitosan thin film as the dielectric gate of the flexible OTFT, and P-type organic purification, unless otherwise noted. semiconductor, poly(3-hexylthiophene) (P3HT), as the layer on substrate The Y2O3/chitosan thin film as the dielectric gatesemiconductor of the flexible OTFT, andpolyimide P-type organic were semiconductor, demonstratedpoly(3-hexylthiophene) in this study. The basic process flow for the fabrication this flexible device is (P3HT), as the semiconductor layer onof polyimide substrate were demonstrated in this the study. The basic process flow for OTFT the fabrication of this flexible device is shown in Figure 1. Moreover, performance of the flexible during bending tests with different shownradii in Figure 1. Moreover, performance flexible during bending tests with layer curvature was also observed. the In detail, a 5-nmofCrthe metal layerOTFT was deposited as an adhesive curvature was also observed. Inthen detail, a 5-nmAu Cr layer metalwas layerdeposited was deposited an on thedifferent polyimide film byradii thermal deposition and a 30-nm on theasadhesive adhesive layer on the polyimide film by thermal deposition and then a 30-nm Au layer was deposited layer as the bottom gate. The adhesive layer was used to stabilize the Au layer and guarantee that the on the adhesive layer as the bottom gate. The adhesive layer was used to stabilize the Au layer and device would be stable under bending tests. The polyimide substrate was first cleaned before placing it guarantee that the device would be stable under bending tests. The polyimide substrate was first into thermal coater chamber, after which a Cr and Au metal layer was deposited. Before the dielectric cleaned before placing it into thermal coater chamber, after which a Cr and Au metal layer was layerdeposited. was deposited, we dielectric covered layer the adhesive tape on bottom Au electrode, bottom Before the was deposited, we the covered the adhesive tape onfor thethe bottom Au gate can only be exposed the final electrode, for the in bottom gate step. can only be exposed in the final step.
Figure 1. The fabrication process of the bottom-gate top-contact flexible organic thin-film transistor.
Figure 1. The fabrication process of the bottom-gate top-contact flexible organic thin-film transistor.
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Chitosan (deacetylated ≥75%) was dissolved in aqueous acetic acid (0.5 wt %) and heated with a hot plate at 50 ◦ C for 24 h, following which its solutions of various concentrations were filtered with a Materials 2017, 10, 1026 of 9 25-mm syringe filter containing a 0.45-µm polyvinylidene difluoride (PVDF) membrane.3The Y2 O3 nanoparticles were blended with deionized water at a ratio of 0.5 wt % and the hybrid solution Chitosan (deacetylated ≥75%) was dissolved in aqueous acetic acid (0.5 wt %) and heated with a was obtained byatmixing aqueous solution with of Y2various O3 nanoparticles aqueous solution, hot plate 50 °C forchitosan 24 h, following which its solutions concentrations were filtered with with a specific volume ratio CS:Y In other words, the difluoride weight percentage of Y2 O3 The in the a 25-mm syringe filter containing a 0.45-μm polyvinylidene (PVDF) membrane. Y2Oblended 3 2 O3 (20:1). nanoparticles werewt blended with deionized at a ratio of 0.5the wt % and wt the% hybrid wasfilm on solutions was 0.023 %. The drop castingwater was used to form 0.023 Y2 O3solution /chitosan by mixing aqueous solution Y2dried O3 nanoparticles with for a 24 h. the obtained bottom-gate as the chitosan dielectric gate, which waswith then in an ovenaqueous at roomsolution, temperature specific volume ratio CS:Y 2O3 (20:1). In other words, the weight percentage of Y 2O3 in the blended After depositing the dielectric gate, a P3HT channel layer was deposited on the Y2 O3 /chitosan solutions was wt %. The was form1500 the 0.023 wt 30 % Ys.2O 3/chitosan film on dielectric film by0.023 spin-coating atdrop 1200casting rpm for 30 used s andtothen rpm for The polyimide substrate the bottom-gate as the dielectric gate, which was then dried in an oven at room temperature for 24 h. ◦ was placed in an oven at 60 C to remove residual solvent in the P3HT active channel layer. Finally, After depositing the dielectric gate, a P3HT channel layer was deposited on the Y2O3/chitosan a 5-nm Cr and a 30-nm Au layer were deposited with a mask to form the source and drain top contacts. dielectric film by spin-coating at 1200 rpm for 30 s and then 1500 rpm for 30 s. The polyimide substrate Cr metal was in used as anatadhesive layer between the P3HT layerchannel and Au contacts was placed an oven 60 °C to remove residual solvent in thechannel P3HT active layer. Finally,asa before. In the meantime, the bottom Au electrode waswith revealed carefully removing the tape. Therefore, as 5-nm Cr and a 30-nm Au layer were deposited a maskby to form the source and drain top contacts. the Cr above procedure was finished, the bottom-gate top-contact flexible organic thin-film transistor metal was used as an adhesive layer between the P3HT channel layer and Au contacts as before. was In the meantime, the bottom Au electrode was revealed by carefully removing the tape. Therefore, as successfully designed. the above procedure was finished, the bottom-gate top-contact flexible organic thin-film transistor
3. Results and Discussion was successfully designed. 3.1.3.Materials andDiscussion Films Characterization Results and Chitosan ≥75%) was dissolved in aqueous acetic acid (0.5 wt %) and heated by 3.1. Materials(deacetylated and Films Characterization using a hot plate at 55 ◦ C for 24 h. The impurities in the chitosan solutions of various concentrations Chitosan (deacetylated ≥75%) was dissolved in aqueous acetic acid (0.5 wt %) and heated by were filtered using a 25-mm syringe filter with a 0.45-µm PVDF membrane. The chitosan solution using a hot plate at 55 °C for 24 h. The impurities in the chitosan solutions of various concentrations with various concentrations was transferred by spin-coating onto separate single silicon substrates were filtered using a 25-mm syringe filter with a 0.45-µ m PVDF membrane. The chitosan solution thatwith were already coated withwas aluminum metal as an electrode by spin-coating. Then, we removed various concentrations transferred by spin-coating onto separate single silicon substrates ◦ C for 1 h. Finally, we deposited the that water in the chitosan thin film by heating on a hot plate at 80 were already coated with aluminum metal as an electrode by spin-coating. Then, we removed the the aluminum metal as the top electrode onon the chitosan, formed a so-called MIM the structure. water in the chitosan thin film by heating a hot plate at which 80 °C for 1 h. Finally, we deposited − 10 as the leakage top electrode on was the chitosan, formed so-calledvoltage MIM structure. We aluminum found thatmetal the lower current 6.827 × which 10 A at anaapplied of 2 V inWe the MIM found the leakage current wasand 6.827observed × 10−10 A atthat an applied of 2 V in the MIM based on that a 1.0 wtlower % chitosan thin film, the 1.0voltage wt % chitosan film hadbased the lowest on a 1.0 wt % chitosan thin film, observed wt % film lowest in leakage leakage current. This result was and attributed tothat the the size1.0and thechitosan number ofhad the the pinholes the surface current. This result was attributed to the size and the number of the pinholes in the surface of of the chitosan, as shown in Figure 2. The thin film with 0.5 wt % chitosan had many smallthe pinholes chitosan, as shown in Figure 2. The thin film with 0.5 wt % chitosan had many small pinholes (10–20 nm), as shown in Figure 2a, and the thin film with 1.5 wt % chitosan had some large pinholes (10–20 nm), as shown in Figure 2a, and the thin film with 1.5 wt % chitosan had some large pinholes (80–100 nm), as shown in Figure 2c. The thin film with 1.0 wt % chitosan had the optimized hole size (80–100 nm), as shown in Figure 2c. The thin film with 1.0 wt % chitosan had the optimized hole size (30–50 nm)nm) andand number of of holes, asas shown Figure (30–50 number holes, showninin Figure2b; 2b;this thisisiswhy whythe the1.0 1.0wt wt%%sample sample had had the the lowest leakage current. lowest leakage current.
Figure 2. (a) 0.5 wt % chitosan shows many small holes (b) 1.0 wt % chitosan shows a few middle-sized
Figure (a) (c) 0.51.5 wtwt %% chitosan holes (b) 1.0 wt % chitosan shows a few middle-sized holes2.and chitosanshows showsmany many small large holes. holes and (c) 1.5 wt % chitosan shows many large holes. The high-k-value Y2O3 nanoparticles were blended into a 1 wt % chitosan water solution with various weight percentages of Y2O3 (from 0.012 wt % to 0.016 wt %, 0.023 wt % and 0.045 wt %). While The high-k-value Y2 O3 nanoparticles were blended into a 1 wt % chitosan water solution with the weight percentage increased, the leakage current decreased. The blended 0.045 wt % Y2O3 in various weight percentages of Y2 O3 (from 0.012 wt % to 0.016 wt−11%, 0.023 wt % and 0.045 wt %). 1 wt % chitosan thin film had the lowest leakage current of 1.81 × 10 A, as shown in Figure 3a. While the weight percentage increased, the leakage current decreased. The blended 0.045 wt % Y O The pure chitosan thin films were almost transparent, the weight percentage of the Y2O3 increased, 2 3
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in 1 wt % chitosan thin film had the lowest leakage current of 1.81 × 10−11 A, as shown in Figure 3a. pure chitosan thin were almost weightinpercentage ofdepth the Y2(from O3 increased, asThe shown in Figure 3b. Infilms the meantime, wetransparent, discovered athe decrease the relative Materials 2017, 10, 1026 41.025 of 9 shown Figure In the meantime, a as decrease relative depth (from the 1.025 to as 0.356 nm)inwhen the3b. concentration of thewe Y2Odiscovered 3 increased, shownin inthe Figure 4a–d. However, to 0.356 nm) when the concentration of the Y O increased, as shown in Figure 4a–d. However, the as shown in Figure 3b. In the meantime, we discovered a decrease in the relative depth (from 1.025 2 3the weight percentage of Y2O3 reached 0.045 wt %, relative depth and roughness were increased when to 0.356 nm) when the concentration of the Y 2 O 3 increased, as shown in Figure 4a–d. However, the relative depth and roughness were increased when the weight percentage of Y O reached 0.045 wt 2 3 as shown in Figure 4e. The purpose of blending the Y2O3 nanoparticles into the chitosan thin film was%, and were increased the ofthe Y2the Osurface. 3 reached %,was asrelative in Figure 4e. The purpose ofbut blending Y2weight O3 nanoparticles into chitosan0.045 thinwt film not toshown only depth reduce theroughness leakage current, alsowhen tothe improve thepercentage pinholes at as shown Figurethe 4e. leakage The purpose of blending 2O3 nanoparticles intoat thethe chitosan thin film was not to onlyinreduce current, but alsothe to Y improve the pinholes surface. not to only reduce the leakage current, but also to improve the pinholes at the surface.
Figure 3. (a) Leakage current of the thin films measured with various concentrations 3 solutions Figure 3. (a) Leakage current of the thin films measured with various concentrations ofof Y2Y O23Osolutions (without Y O , 0.012 wt %, 0.016 wt %, 0.023 wt %, 0.045 wt %) in 1 wt % chitosan solution (b) Pictures 23, 0.012 3 FigureY3. (a) Leakage current the films with of Y2O(b) 3 solutions (without 2O wt %, 0.016ofwt %,thin 0.023 wtmeasured %, 0.045 wt %)various in 1 wt concentrations % chitosan solution Pictures showing various weight percentages Y in the mixed (without Yvarious 2O3, 0.012 wt %, percentages 0.016 wt %, 0.023 %, wt %)solution. in solution. 1 wt % chitosan solution (b) Pictures showing thethe weight of of Y2wt O23O in3 0.045 the mixed showing the various weight percentages of Y2O3 in the mixed solution.
The cross-sectional images the blended films shown Figure and The cross-sectional images ofof the blended Y2YO23O /chitosan thinthin films areare shown in in Figure 5, 5, and 3 /chitosan The cross-sectional images offilms the blended Y2O3/chitosan120 thin films are shown inalso Figure 5, and thicknesses blended thin films were approximately 120 nm–200 nm. analyzed thethe thicknesses of of thethe blended thin were approximately nm–200 nm. WeWe also analyzed thethe the thicknesses of the thin filmsinwere approximately 120 nm–200 nm. Weby analyzed the distribution surface blended thin films energy-dispersive distribution of of thethe Y2Y O23Oblended nanoparticles in thethe surface of of thethe blended thin films by also energy-dispersive 3 nanoparticles distribution of the Y2O3 nanoparticles in the surface of the blended thin films by energy-dispersive X-ray spectroscopy (EDX, JEOL, Freising, Germany). discovered that 0.012 0.016 X-ray spectroscopy (EDX, JEOL, Freising, Germany). WeWe discovered that thethe 0.012 wtwt %,%, 0.016 wtwt %,%, X-ray spectroscopy (EDX, JEOL, Freising, Germany). We discovered that the 0.012 wt %, 0.016 wt %, and 0.023 % blended showed a uniform dispersion Y2 O3 nanoparticles, and 0.023 wt wt % blended thin thin filmsfilms showed a uniform dispersion of the Yof 2Othe 3 nanoparticles, as shownas and 0.023 wt % blended thin films showed a uniform dispersion of the Y2O3 nanoparticles, as shown Figure Y2 O3 nanoparticles attracted andformed clusterswhen formed in shown Figure in 6a–c. The6a–c. Y2O3 The nanoparticles attracted each othereach and other clusters the when weightthe in Figure 6a–c. The Y2O3 nanoparticles attracted each other and clusters formed when the weight weight percentage of Y2 O30.045 reached 0.045 wt %6d). (Figure 6d). In4e, Figure 4e, we measured the relatively percentage of Y2O3 reached wt % (Figure In Figure we measured the relatively large percentage of Y2O3 reached 0.045 wt % (Figure 6d). In Figure 4e, we measured the relatively large large profile depth of nm, 1.051 nm, and we attributed this phenomenon to the clustering of the profile depth of 1.051 to the the clustering the 2Y O2 3O3 profile depth of 1.051 nm,and andwe weattributed attributed this this phenomenon phenomenon to clustering ofof the YY 2O3 nanoparticles. It was observed that the 0.023 wt % thin film had smoothest surface, and its pinholes nanoparticles. It was observed that the 0.023 wt % thin film had smoothest surface, and its pinholes nanoparticles. It was observed that the 0.023 wt % thin film had smoothest surface, and its pinholes were much improved. were much improved. were much improved.
Figure4.4.AFM AFM morphology morphology and profile depth of the pinholes in the surface ofof the Y2Y O3O /chitosan thinthin Figure and profile depth the pinholes thesurface surface the 2 3/chitosan 3 /chitosan Figure 4. AFM morphology and profile depth ofof the pinholes ininthe of the Y2O thin film with various Y 2O3 concentrations: (a) Without Y2O3; (b) 0.012 wt %; (c) 0.016 wt %; (d) 0.023 wt %; film with various Without Y23;O(b) 0.012 0.016 0.023 3 concentrations: 3 ; (b) film with various Y2Y O23O concentrations: (a)(a) Without Y2O 0.012 wt wt %; %; (c) (c) 0.016 wt wt %; %; (d)(d) 0.023 wt wt %; %; and (e) 0.045 wt %. and (e) 0.045 wt %. and (e) 0.045 wt %.
The FTIR spectra analysis (PerkinElmer, Waltham, MA, USA) of the blended Y2O3/chitosan thin The FTIRused spectra analysis (PerkinElmer, Waltham, USA) as of shown the blended Y2O37a. /chitosan thin films was to obtain information on the chemicalMA, bonding, in Figure The pure films was used obtain information on the chemical bonding, as in Figure 7a. The pure −1 that were chitosan thin to film showed broad absorption peaks at 3000–5000 cmshown attributed to NH 2 chitosan thin film showed broad absorption peaks at 3000–5000 cm−1 that were attributed to NH2
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The FTIR spectra analysis (PerkinElmer, Waltham, MA, USA) of the blended Y2 O3 /chitosan thin films was used to obtain information on the chemical bonding, as shown in Figure 7a. The pure Materials 2017, 10, 1026 5 of 9 Materials 2017, 10,film 1026 showed broad absorption peaks at 3000–5000 cm−1 that were attributed to NH 5 of 92 chitosan thin −1 asymmetric asymmetric stretching stretching and and the the hydrogen-bonded hydrogen-bonded OH. OH. The Thepeak peakatatapproximately approximately2879 2879cm cm−1 was was asymmetric stretching and the hydrogen-bonded OH. The peak at approximately 2879 cm cm−−11 was attributed attributed to to the the CH CH33 asymmetric asymmetric stretching stretching vibrations, vibrations, and andthe theabsorption absorptionpeak peakatat1541 1541 cm−1 was was attributed to the the CH3 asymmetric stretching [24–26] vibrations,NH and+ the absorption peak at 1541 cm−1 was attributed attributed to to the asymmetric asymmetricbending bendingmodes modes [24–26]of of NH33+.. Figure Figure 7a 7a also also shows shows the the FTIR FTIR spectra spectra + + attributed thethe asymmetric bending modesthin [24–26] of which NH3 . Figure 7a also shows the+ FTIR spectra analysis oftoall all blendedY2YO films, are listed in 1. Table NH 23O 3 /chitosan 3 was analysis of the blended /chitosan thin films, which are listed in Table The 1. NHThe 3 was assigned + −1 forwhich analysis oftoall blended Y2O3/chitosan are listedwhich in Table 1. The NH 3 wasfrequency assigned assigned thethe bending frequency 1541 cmfilms, pure chitosan, shifted a higher −1thin to the bending frequency at 1541atcm for pure chitosan, which shifted to atohigher frequency at −1 for pure chitosan, which shifted to a higher frequency at − 1 − 1 to the bending frequency at 1541 cm at 1559 O3 = wt 0.012 wt %0.016 and wt 0.016 wt %, respectively, then shifted to a −1 andand 1559 cmcm 15801580 cm−1cm for Yfor 2O3 Y = 20.012 % and %, respectively, then shifted to a higher −1 and 1580 cm−1 for Y −21Ofor −1 1559 cmfrequency 3 =Y 0.012 wt % and 0.016 wt %, respectively, then shifted to a higher higher at 1598 cm O = 0.023 wt % and shifted to a lower frequency as 1578 cm −1 −1 2 3wt % and shifted to a lower frequency as 1578 cm for Y2O3 = frequency at 1598 cm for Y2O3 = 0.023 −1 −1 frequency at 1598wt cm%. for Y2O3 = 0.023 wt %toand to abetween lower frequency asof 1578 cm for Yoxide 2O3 = for Y2 O The theshifted H-bonds theof oxygen yttrium (III) 0.045 wt3 =%.0.045 The shifts wereshifts due were to thedue H-bonds between the oxygen yttrium (III) oxide and the 0.045 wt %. The shifts were due to the H-bonds between the oxygen of yttrium (III) oxide and the and the amine of chitosan, as shown in Figure 7b. The AFM morphology (Veeco, Plainview, NY, USA) amine of chitosan, as shown in Figure 7b. The AFM morphology (Veeco, Plainview, NY, USA) also amine of chitosan, as shown in Figure 7b. Thepinholes AFM morphology (Veeco, Plainview, USA) also also proved a dense structure fewer formed by blending O3NY, nanoparticles proved that that a dense structure and and fewer pinholes werewere formed by blending Y 2OY 32 nanoparticles into proved that a dense structure and fewer pinholes were formed by blending Y 2 O 3 nanoparticles into into the thin as shown in Figure the thin film,film, as shown in Figure 4. 4. the thin film, as shown in Figure 4.
Figure 5. Cross-section images of the blended Y2O3/chitosan thin films: (a) Without Y2O3; (b) 0.012 wt %; Figure 5.5.Cross-section Cross-section images the blended Y2 O3 /chitosan films: Y(a) Y2 O Figure images of theof blended Y2O3/chitosan thin films: thin (a) Without 2O3;Without (b) 0.012 wt %;3 ; (c) 0.016 wt %; (d) 0.023 wt %; and (e) 0.045 wt %. (b)0.016 0.012wt wt%; %; (d) (c) 0.023 0.016 wt wt %; %; and (d) 0.023 wt %; (c) (e) 0.045 wtand %. (e) 0.045 wt %.
Figure 6. EDX images showing the distribution of yttrium: (a) 0.012 wt %; (b) 0.016 wt %; (c) 0.023 wt %; Figure 6. EDX images showing the distribution of yttrium: (a) 0.012 wt %; (b) 0.016 wt %; (c) 0.023 wt %; and (d)6.0.045 %. showing the distribution of yttrium: (a) 0.012 wt %; (b) 0.016 wt %; (c) 0.023 wt %; Figure EDXwt images and (d) 0.045 wt %. and (d) 0.045 wt %.
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Figure7.7.(a) (a)FTIR FTIRspectra spectraofofthe theblended blendedY2YO2O 3/chitosan thin films; (b) Schematic diagram of hydrogen Figure 3 /chitosan thin films; (b) Schematic diagram of hydrogen bondingbetween betweenthe theYY 2O3 nanoparticles and the amine group of chitosan. bonding 2 O3 nanoparticles and the amine group of chitosan. Table FTIR spectra analysis blended 3/chitosan thin film. Table 1. 1. FTIR spectra analysis of of thethe blended Y2Y O23O/chitosan thin film. Wave Number (cm−1) 1) Y2O3 wt % CH Stretching Wave NumberBroad (cm−Absorption O–H and N–H Stretching Peaks CH Stretching Y2 O3 wt % O–H and N–H Stretching3368 Broad Absorption Peaks n/a 2879 0.012 3368 2880 n/a 3368 2879 0.016 3368 2880 0.012 3368 2880 0.023 3367 2880 0.016 3368 2880 0.045 3367 2879 0.023 3367 2880 0.045 3367 2879
NH3+ Bending NH3 + Bending 1541 1559 1541 1580 1559 1598 1580 1578 1598 1578
3.2. Electric Characteristics of the Flexible Organic Thin Film Transistor 3.2. Electric Characteristics of the Flexible Organic Thin Film Transistor We investigated the electrical properties of the flexible P3HT-based OTFTs with a 0.023 wt % blended Y2O3/chitosan on the polyimide substrate. Figure OTFTs 8 shows the atransfer plots We investigated thedielectric electricalgate properties of the flexible P3HT-based with 0.023 wt % for concave convexdielectric bending gate of the with concave bending radii, R, of the 3.5 cm and 2.8 cm blended Y2 O3and /chitosan onOTFTs the polyimide substrate. Figure 8 shows transfer plots and convexand bending radii, R, of 3.5 cm and 2.8 with cm. In this figure, the electrical the for concave convex bending of the OTFTs concave bending radii, R, characterization of 3.5 cm and 2.8ofcm OTFTs on polyimide substrates without bending to thatthe of electrical the OTFTs on silicon wafers. and convex bending radii, R, of 3.5 cm and 2.8 cm.isInsimilar this figure, characterization of −10 A to 5.740 × 10−11 A due to extrusion, After concave bending, substrates the Ioff value decreased fromis7.421 × 10to the OTFTs on polyimide without bending similar that of the OTFTs on silicon wafers. 10 Aconvex causing a decrease in the size the pinholes. Onfrom the other bending, off value After concave bending, the Ioffofvalue decreased 7.421hand, × 10−after to 5.740 × 10−11the A Idue to −9 A. No matter the direction of bending, the Ion value would decrease. In Figure 8, increased causing to 1.722 ×a10 extrusion, decrease in the size of the pinholes. On other hand, after convex bending, weIcompared the electrical characterization OTFTsthe with differentofbending radii and would 2.8 cm), the to 1.722 × 10−9 A. of Nothe matter direction bending, the(3.5 Ioncm value off value increased and the comparison chart is listedthe in Table 2. The output characterization (IDS -VDSdifferent ) of this device, decrease. In Figure 8, we compared electrical characterization of the OTFTs with bendingas shown Figure S1.cm), and the comparison chart is listed in Table 2. The output characterization radii (3.5incm and 2.8 (IDS -VDS ) of this device, as shown in Figure S1.
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Figure Figure 8. 8. Comparison of the electrical characterization of the P3HT-based organic organic thin-film thin-film transistors transistors (OTFTs) dielectric gate for bending tests at different different bending (OTFTs)with withaa0.023 0.023wt wt% %blended blendedYY22O O33/chitosan /chitosan dielectric bending radii radii and and without without bending. bending. Table Table2.2.Electrical Electricalcharacterization characterizationof ofthe theP3HT-based P3HT-basedOTFTs OTFTswith witha a0.023 0.023wt wt%%blended blendedYY 2O 3/chitosan 2O 3 /chitosan dielectric dielectric gate gate on on the the polyimide polyimide substrate substrate for for bending bending tests. tests.
Condition Vth (V) Vth (V) Flat −2.0 Flat −2.0 Concave bending 3.50 (cm) −2.5 Concave bending 3.50 (cm) −2.5 Concave bending 2.85 (cm) −2.7 Concave bending 2.85 (cm) −2.7 Flat −2.1 Flat −2.1 Convex bending 3.50 (cm) −3.0 −3.0 Convex bending 3.50 (cm) Convex bending 2.85 (cm) −3.3 −3.3 Convex bending 2.85 (cm) Flat −1.5 Flat −1.5 Condition
Ion (A) −5 1.268 × 10 1.268 × 10−5−6 4.592 × 10 4.592 × 10−6−5 1.298 −5 1.298 ××1010 −5 − 1.294 × 10 1.294 × 10 5 −6−6 9.899 9.899 ××1010 −6−6 9.480 9.480 ××1010 − 1.059 ××10105−5 1.059 Ion (A)
Ioff (A) 7.421 × 10−−10 10 7.421 × 10 −11 5.740 × 10 5.740 × 10−11 1.913××10 10−−9 9 1.913 −10 − 10 4.571 × 10 4.571 × 10 9 1.722××10 10−−9 1.722 10 3.296××10 10−−10 3.296 9 2.539 2.539××10 10−−9 Ioff (A)
Ioff/Ion Ratio 1055 10 1055 104 10 1045 10 105 3 10 103 3 10 104
Ioff /Ion Ratio
Mobility (cm22/Vs) Mobility (cm /Vs) 2.50 × 10−2 2.50 × 10−2 3.33 × 10−2 −2 3.33 × 10−2 2.70 × 10 −2 2.70 × 10−4 1.87 × 10 1.87 × 10−4 −2 8.53 8.53× ×10 10−2 −2 4.80 4.80× ×10 10−2 −2 3.33× ×10 10−2 3.33
4. Conclusions Conclusions 4. A solution-based solution-based processed processedand andlow-voltage low-voltageoperating operatingP3HT-based P3HT-basedOTFT OTFTwith witha aYY 2O/chitosan A 2O 3 3/chitosan gate dielectric dielectric layer layer was was demonstrated demonstrated in in this this study. study. To To improve improve the the electrical electrical performance performance of of the the gate chitosan-based MIM, various concentrations of Y 2 O 3 nanoparticles were blended into the chitosan, chitosan-based MIM, various concentrations of Y2 O3 nanoparticles were blended into the chitosan, whichachieved achievedaadecreased decreasedleakage leakagecurrent current and improved depth of the pinholes. Furthermore, which and improved thethe depth of the pinholes. Furthermore, the the P3HT-based with wt a 0.023 wt % blended Y 2O 3/chitosan gate dielectric layer was P3HT-based OTFT OTFT with a 0.023 % blended Y2 O /chitosan gate dielectric layer was manufactured 3 manufactured onbending polyimide bending tests. The electrical ofincurred the flexible device on polyimide for tests.for The electrical performance of theperformance flexible device no obvious incurred except no obvious except for slight increase in theand leakage current and current. and The changes for achanges slight increase in athe leakage current off current. Theoff non-toxic non-toxic and eco-friendly biopolymer chitosan, with Y2O3was nanoparticles, successfully eco-friendly biopolymer chitosan, blended with Y2blended O3 nanoparticles, successfullywas used in flexible used inasflexible as the gate dielectric, the OTFT to voltages, operate under low voltages, and OTFTs the gateOTFTs dielectric, enabling the OTFTenabling to operate under low and producing a Ion /I off 5 5 producing off ratio of 10ofat voltage of −10 V and a drain ratio of 10 aatIon a /I gate voltage −a10gate V and a drain voltage of − 1 V. voltage of −1 V. Supplementary Materials: The following are available online at www.mdpi.com/1996-1944/10/9/1026/s1. Supplementary Materials: The following are available online at www.mdpi.com/1996-1944/10/9/xx/s1. Figure Figure S1: Output characterization of P3HT-based OTFTs with 0.023 wt % blended Y2 O3 /chitosan dielectric gate. S1. Output characterization of P3HT-based OTFTs withthe 0.023 wt% blended Y2O3/chitosan dielectric gate. for Acknowledgments: The authors would like to thank Ministry of Science and Technology of Taiwan supporting this work The under Contract MOST MOST 105-2622-M-009-004-CC3. Acknowledgments: authors would like104-2113-M-009-008-MY3 to thank the Ministry ofand Science and Technology of Taiwan for supporting this work under Contract Ko, MOST 104-2113-M-009-008-MY3 and MOST Author Contributions: Fu-Hsiang Ranjohn Singh and Ching-chang Lin 105-2622-M-009-004-CC3. designed the experiments. Shao-Ying Hu and Tsung-Tso Tsai carried out the experimental work. Bo-Wei Du wrote the manuscript. All authors Author Contributions: Fu-Hsiang Ko, Ranjohn Singh and Ching-chang Lin designed the experiments. contributed to the discussions of this work. Shao-Ying Hu and Tsung-Tso Tsai carried out the experimental work. Bo-Wei Du wrote the manuscript. Conflicts of Interest: The authors declare no conflicts of interest. All authors contributed to the discussions of this work.
Conflicts of Interest: The authors declare no conflicts of interest.
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