Toward High Conductivity of Electrospun Indium Tin Oxide Nanofibers

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Towards High Conductivity of Electrospun Indium Tin Oxide Nanofibers with Fiber Morphology Dependent Surface Coverage: Post-Annealing and Polymer Ratio Effects Sangcheol Yoon, Hyebin Kim, Eun-Sol Shin, Jun Nyeong Huh, Yong-Young Noh, Byoung Choo Park, and Inchan Hwang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08987 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 14, 2017

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ACS Applied Materials & Interfaces

Towards High Conductivity of Electrospun Indium Tin Oxide Nanofibers with Fiber Morphology Dependent Surface Coverage: Post-Annealing and Polymer Ratio Effects Sangcheol Yoon,†,∥ Hyebin Kim,†,∥ Eun-Sol Shin,‡ Jun Nyeong Huh,§ Yong-Young Noh,‡ Byoungchoo Park,§,*

and Inchan Hwang†,*

†Department of Electronic Materials Engineering, Kwangwoon University, Seoul 01897, Republic of Korea ‡Department of Energy and Materials Engineering, Dongguk University, Seoul 04620, Republic of Korea §Department of Electrophysics, Kwangwoon University, Seoul 01897, Republic of Korea Corresponding Author *E-mail address: [email protected], [email protected] Author Contributions

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The authors Sangcheol Yoon and Hyebin Kim contributed equally

to this work. Keywords: indium tin oxide; nanofiber; surface coverage; electrical conductivity; electrospinning

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Abstract: High electrical conductivity of metal oxide thin films needs uniform surface coverage, which has been the issue for the thin films based on electrospun nanofibers (NFs) that have advantage over the sputtered/spin-coated films with respect to large surface area and mechanical flexibility. Herein, we investigated a reduction in the sheet resistance of electrospun indium tin oxide (ITO) NF films with improved surface coverage. We found that the surface coverage depends significantly on the electrospinnable polymer concentration in the precursor solutions, especially after post-hot-plate annealing following the infrared radiation furnace treatment. The post-annealing process increases crystallinity and oxygen vacancies. However, with a higher PVP content, it makes the surface of ITO NFs more prominently rough as a result of the formation of larger sphere-shaped ITO particles on the NF surface, which gives rise to poor surface coverage. A less PVP content in ITO NF films by electrospinning for short deposition times was found to improve surface coverage even after post-annealing. The sheet resistance notably decreases, down to as low as 350 Ω/sq, with a high transmittance of over 90%. Our study provides an understanding on how to achieve high electrical conductivity of ITO NF films with high surface coverage, which can be utilized for the optoelectronic and sensing applications.

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1. Introduction Transparent conducting oxide is an important class of materials for the optoelectronic application. Among them, indium-doped tin oxide (ITO) is widely used in optoelectronic devices including solar cells, liquid crystal displays and light-emitting diodes. The fabrication methods of thin ITO layers via electron beam evaporation, DC and RF sputtering techniques12

, nanoparticle synthesis3, and solution-processing4-6 have been introduced toward low sheet

resistance and high transmittance in the visible regime. The sputtered ITO layer has a low sheet resistance of 10 Ω/sq at 90% transmittance. However, the fragility and lack of mechanical flexibility are the big obstacles for use in flexible devices. To resolve such problems, the ITO nanofibers (NFs) that can be fabricated by using electrospinning (ES) techniques

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have been considered for alternative nanostructure in thin ITO layers.

Nanofiber- and nanoweb-structured ITO thin layers have shown that they have improved mechanical flexibility by demonstrating almost unchanged sheet resistance after multipletime bending tests.14-17 In addition to improved mechanical flexibility, the device performance can be improved, taking advantage of large surface area of the fiber-based thin layers. For example, the electrospun TiO2 NFs were utilized for various types of optoelectronic devices including solar cells18-20, photocatalytic applications21-23 and ultraviolet (UV)-photodetectors.24-26 A larger surface area of ITO NFs and the CdS-coaxial structure enhances the power conversion efficiency of dye sensitized solar cells (DSSCs), compared to flat-structured CdS DSSCs.27 Such an improvement arise from the fact that large interfacial area between the active layer and electrodes facilitate charge collection. For gas sensors, large surface area can improve the sensibility to analyte gases as the interaction with chemical species occurs at the surfaces.28-32

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Despite such advantages of NF structures, ES techniques contains some technical issues when the fibers are deposited directly onto the glass substrates. It is often observed that the electrospun fibers are easily peeled off from the glass substrate right after finishing electrospinning or after calcination at high temperatures.33-36 This gives poor surface coverage and is very detrimental for electronic devices, as it causes poor reliability and also poor electrical properties of the electrospun NF films. A low sheet resistance has to be achieved for better electronic performances. It has been reported that the sheet resistance of ITO NF films is as high as a few of kΩ/sq with reasonably high transmittance 37-38, or is fairly low but with low transmittance.39 Recently, air-blown electrospinning gives better electronic properties of ITO NF films. The sheet resistance is a few hundreds of Ω/sq with about 80% transmittance at 550 nm wavelength.16 In this article, we investigated the enhancement in the surface coverage of ITO NF film. The two-step calcination process, infrared radiation (IR) furnace annealing followed by hotplate annealing under nitrogen atmosphere, was conducted to reduce the sheet resistance. We discuss the origin of a reduction in sheet resistance upon the two-step calcination process. In addition, the relationship between the surface coverage and ITO NF morphologies that depend on the PVP solution concentrations is discussed. We improved the surface coverage and thus the sheet resistance is significantly reduced to as low as 350 Ω/sq with a transmittance of over 90%.

2. Experimental methods 2.1 Materials Indium nitrate hydrate (In(NO3)3, 99.99%), indium chloride (InCl3, 99.999%), tin chloride (SnCl2, 99.99%), poly(vinylpyrrolidone) (PVP, Mw= 1,300,000) ethylene glycol (EG, 99%), -4-


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acetic acid (AA, 99.7%), anhydrous ethanol (EtOH), and ethanolamine (EA, 99%) were purchased from Sigma Aldrich. All materials were used without further purification.

2.2 Nanofiber fabrication All processes were conducted under ambient conditions. Glass substrates (25 mm x 25 mm) were cleaned by sonication in acetone and isopropyl alcohol (IPA) for 10 min, respectively. Subsequently, UV ozone treatment was carried out for 10 min. The ITO solution for thin under-layer (UL solution) was prepared by 0.166 M of In(NO3)3 and SnCl2 with the In:Sn atomic ratio of 9:1 in an EG:AA (1:1 vol) co-solvent, and was then stirred for 2 h at room temperature. UL solutions were spin-coated at 3,000 rpm for 45 s on a clean glass substrate, and then the substrate was annealed at 300 °C on a hot plate for 5 min. The ITO underlying films were heat-treated by infrared (IR) furnace (DFI-4GF, Dae Heung Scientific) at 450 °C with 10 °C/min for an hour. For ES, InCl3 and SnCl2 (In:Sn = 9:1 atomic ratio) were dissolved with 2 M in an EtOH:DMF (1.2:1 vol) co-solvent, and then the solution (ITO solution) was stirred for 3 h at room temperature. EA was added into the ITO solution with a molar ratio of 400:1 (ITO:EA). PVP solutions were prepared in 1 ml of the EtOH:DMF solvent with a concentration of 7, 8, and 9 wt%. The PVP solutions were stirred for an hour at room temperature. The ITO solution was mixed with the PVP solution with a volume ratio of 2:9. The mixed solution (ES solution) was loaded into a syringe with a needle (Gauge no.23, inner diameter: 0.33 mm), and then the syringe was loaded to a syringe pump (NE-1000, New Era Pump Systems Inc.). A stainless-steel collector was horizontally placed with the tip of the needle, and the distance between the collector and the needle was 20 cm. The ES solution was supplied at 0.4 ml/h of flow rate. Applied voltage between the needle (positive) and the collector (ground) was 20

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kV through a power supply (FC50P2.4, Glassman High Voltage Research Inc.). The glass substrates with a thin ITO UL were attached on the flat collector. The electrospun NFs were heat-treated by IR furnace at 450 °C for 2 h. Subsequently, some of the ITO NF films were transferred to a nitrogen-filled glove box, and thermally annealed on a hot plate at 450 °C for an hour.

2.3 Characterization The transmittance was observed by UV-visible spectrophotometer (HP8453, HP). The sheet resistance was measured from a 4-point probe set-up (FPP-3, TNP). The crystallization and structure ordering of ITO NF films were investigated using X-ray diffractometer (XRD) with Cu Kα radiation (Ultima IV, Rigaku). The morphologies of the electrospun films were acquired by scanning electron microscopy (SEM, JSM-7610, JEOL) at an accelerating voltage of 5 kV. The elemental analysis of the electrospun films was conducted using energydispersive X-ray spectroscopy (EDS, AZTEC, Oxford Instruments) in SEM. X-ray photoemission spectroscopy (XPS) measurements were performed in an ultra-high vacuum surface analysis system, equipped with a monochromatic Al Kα X-ray (1486.6 eV) at a base pressure of 1 x 10-6 Pa. Thermal behavior of the composite nanofibers containing indium/tin precursors and PVP was monitored with Thermogravimetry/differential thermal analyzer (Seiko Exstar 6000, TG/DTA 6100) under nitrogen atmosphere with a heating rate of 5 °C/min.

3. Results and discussion

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First, we investigate the effects of a ITO UL on the surface coverage of electrospun ITO NFs on glass substrates. The ITO UL was used to enhance the adhesion of electrospun ITO NFs to the substrates. To minimize the contribution of the spin-coated ITO UL to the optoelectrical properties, the minimum thickness of the UL is desired. The ITO UL was spincoated from the ITO precursor solutions with different concentrations (0.042, 0.083, and 0.166 M) on glass substrates. These ITO ULs have almost 100 % transmittance above 500 nm wavelength, indicating that they are very thin (Figure S1). The sheet resistance of the ITO UL prepared from the 0.166 M ITO solution was measurable (35 k Ω/sq), and the other films have a sheet resistance greater than hundreds of MΩ/sq.

Figure 1. Photographic images of the electrospun ITO NF films (a) without an ITO UL, or with an ITO UL spin-coated from (b) 0.042, (c) 0.083, and (d) 0.166 M precursor solutions, after the furnace annealing process. The glass substrate size is 2.5 × 2.5 cm2.

Figure 1 shows the photographs of the IR furnace-treated ITO NF film electrospun from the solution blend of the ITO precursors and 9 wt% PVP onto the glass substrates spin-coated with the ITO ULs with different thicknesses. The NF films without an ITO UL (Figure 1 (a)) show serious peel-off of the electrospun ITO NFs from the glass substrates after calcination, especially near the edge of the substrate. The electrospun ITO NF films with an ITO UL spincoated from 0.042 M and 0.083 ITO precursor solutions have better film coverage than the films without an UL, but there is still some film-removal near the edge, with some cracks

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being created. As the ITO precursor solution increases up to 0.166 M, we could successfully coat the ITO NF films without any serious detachment after the IR furnace annealing process. This is attributed to the fact that the surface of the glass substrates is well covered by the spin-coated ITO UL. The ITO UL has a strong adhesion to the ITO NFs, like electrospun titanium oxide (TiO2) NFs and the substrate composed of TiO2.36 Low concentrations of ITO precursor solution used for the UL have dewetting issues, especially near the edge of the substrate, which explains why electrospun ITO NFs were peeled off while the center has good surface coverage. The sheet resistance of the electrospun ITO NFs without an ITO UL was not measurable, due to poor surface coverage caused by a low adhesion of the ITO NF film. As the solution concentration for ULs increases, 0.042, 0.085 and 0.166 M, the sheet resistance of the electrospun ITO NF films decreases, 342, 55 and 1.6 k Ω/sq, respectively. Such a high sheet resistance of the electrospun ITO NF films with ITO ULs prepared from 0.042 M and 0.083 M ITO solutions arises from the poor surface coverage of the electrospun ITO NFs.

Figure 2. Thermal analysis of as-electrospun ITO@PVP NFs for a 9 wt% PVP concentration.

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The TG-DTA curves of the as-electrospun NFs containing indium/tin precursors in the PVP fiber matrix (ITO@PVP NFs) are shown in Figure 2. A broad endothermic reaction accompanying a weight loss of 12.86% until 180 °C was observed near 90 °C, which is attributed to the removal of adsorbed water on the fiber surface and organic residues including residual solvents. A small weight loss of 7.67%, followed by the exothermic peak at 312 °C, might be due to the decomposition of indium/tin chlorides and the degradation of PVP by dehydration in the polymer side chain. Subsequently, the sample weight begins to steeply decrease at 350 °C, accompanied by a broad endothermic peak at 366 °C, which we attribute to the melting of the PVP chains inside NFs40. The decomposition of PVP continues until 420 °C, with the corresponding exothermic peak at 394 °C.41-45 The subsquent exothermic reaction peaked at 433 °C would be attributed to the crystallization of ITO.5, 46-47 The further weight loss beyond the ITO crystallization temperature might be due to the removal of chlorides, probably from the decomposition of indium chlorides,48 which could be detected in the IR furnace annealed samples at 450 °C also by the EDS analysis (Table S1). It should be noted that as the crystallization and decomposition temperatures can be influenced by the annealing method, the actual temperatures at the those processes occur might be lower for IR furnace annealing.46 To investigate the dependence of the PVP weight ratio on the electrospun ITO NF deposition, the PVP solutions were prepared at different concentrations of 7, 8, and 9 wt%. All substrates included the ITO UL spin-coated from the 0.166 M ITO solution prior to electrospinning with the deposition time of 20 min. The electrospun deposition area on the collector decreases with increasing PVP solution concentration. The deposition area for the 7 wt% PVP solution is ~2.4 times larger than the area for the 9 wt% PVP solution (Figure S2). The small deposition area implies a high fiber density on the collector, and more importantly, a high gradient of fiber density. In addition, the deposition center on the flat collector was

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observed to be different for each time, because of the unstable Taylor cone due to the high viscosity. These two factors give rise to poor reproducibility of the ITO NF films in thickness, and of the film uniformity. To ensure a reasonable level of reproducibility, the substrate was placed on the center of the deposition area.

Figure 3. SEM images of the electrospun ITO NF films with the different PVP solution concentrations, (a, d) 7, (b, d)8, and (c, f) 9 wt%, (upper panel) before and (lower panel) after the furnace heat treatment. Inset: high magnification SEM images.

Figure 3 shows SEM images of the electrospun NFs successfully formed for the range of PVP solution concentrations studied here. If the PVP concentration is higher than 9 wt%, the surface coverage was found to be too poor even after IR annealing. The average diameter of ITO@PVP NFs (Figure 3a-c) was 188.0 ± 29.6 (7 wt%), 243.3 ± 47.8 (8 wt%), and 289.7 ± 41.5 nm (9 wt%). The corresponding fiber-diameter distribution histograms are exhibited in Figure S3. The high viscosity of the PVP solutions leads to the formation of thicker NFs.49

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The furnace annealing effectively evaporates PVP that acts as an insulator, which was confirmed by the fact that the nitrogen atom contained in PVP, (C6H9NO)n, is not detected by the EDS measurement (see Table S1). It was found that upon IR furnace annealing, the electrospun pure ITO NFs (Figure 2d-f) become discontinuous and have rough surface compared to the ITO@PVP fibers, because of PVP evapration. These pure electrospun ITO NFs obtained by furnace annealing have an average diameter of 85.5 ± 11.1, 138.5 ± 35.0, and 128.7 ± 26.1 nm for the PVP solution concentrations of 7, 8, and 9 wt%, respectively.

Figure 4. Photographic images of the ITO NFs films electrospun from (a),(d) 7 wt%, (b),(e) 8 wt%, and (c),(f) 9 wt% PVP solutions onto the ITO UL/glass substrates for 20 min. Upper and lower panels show the ITO NFs upon the furnace heat treatment only, and further hot plate annealing following the furnace treatment, respectively. The glass substrate size is 2.5 × 2.5 cm2.

Having found the minimum precursor concentration for the spin-coated ITO UL and the range of PVP solution concentrations that ensures good surface coverage of ITO NF films upon IR furnace annealing, the next step was to investigate the dependence of the PVP solution concentration on the surface coverage of electrospun ITO NFs upon the furnace heat

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treatment followed by an additional heat treatment on a hot plate under nitrogen atmosphere. The upper panels of Figure 4 show the photography of electrospun ITO NF films annealed only in the furnace. As expected, none of the films shows serious detachments or cracks from after the furnace heat treatment. The images in the lower panels shows electrospun ITO NF films additionally annealed on a hot plate at 450 °C for an hour in a glove box filled with nitrogen gas. After hot plate annealing, the samples prepared from the 9 wt% PVP solution clearly show some serious cracks on the film. We also found that the ITO NF films peeled off like fluttering, even when weak air was blown to the sample. This suggests that the adhesion of ITO NFs to the substrate is weakened, possibly due to some changes in microstructure upon post-annealing, which will be discussed later.

Figure 5. Sheet resistance and the transmittance at 550 nm wavelength of the ITO NFs film electronspun from 7 (black square), 8 (red circle), and 9 (blue triangle) wt% PVP solutions for a 20 min deposition time, before (open symbol), and after hot plate annealing (solid symbol).

Figure 5 shows the sheet resistance and the transmittance at 550 nm wavelength of the best electrospun ITO NF films electrospun with different PVP solution concentrations for the

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deposition time of 20 min. The ITO NFs were annealed either in the IR furnace only or on the hot plate following the furnace treatment. The sheet resistance without any heat treatment was not measurable, due to the presence of PVP in the fibers. We found that the sheet resistance of the electrospun ITO NF films was slightly increased with increasing the PVP solution concentrations (1.6 ~ 1.8 k Ω/sq), but the transmittance at 550 nm wavelength notably decreased to 94.9, 81.8, and 67.1 %, as the PVP concentration was 7, 8, and 9 wt%, respectively. This results from the fact that more fibers are deposited on the collector for a higher PVP concentration, and the electrical conductivity is almost saturated with the film thickness. The change in the sheet resistance upon hot plate annealing differs, depending on the PVP concentration. After the ITO NF film from the 9 wt% PVP solution is thermal-annealed on the hot plate, its sheet resistance increases from 1.6 to 2.4 k Ω/sq, and its transmittance increases from 68% to 85%, which clearly indicates a reduction in the density of ITO NFs on the substrate that contribute to the electrical conductivity. This results from poor surface coverage of ITO NF films after the two-step annealing, as discussed above. For the 8 wt% PVP solution, the sheet resistance was almost identical by hot plate annealing. The sheet resistance of the electrospun ITO NF film prepared by the 7 wt% PVP solution decreased after hot plate thermal annealing (1.8 to 1.3 k Ω/sq). These results indicate that hot plate annealing effectively increases the electrical conductivity of the films as long as ITO NFs are stuck onto the substrates.

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Figure 6. XRD spectra of the electrospun ITO NF films from different PVP solution concentrations (a) for the entire range and (b) for the 27°–35° range.

To investigate the origin of the reduction in sheet resistance upon post-annealing, we conducted the XRD and XPS measurements. Enhancement in the electrical conductivity of ITO is largely due to better crystallinity and/or more oxygen vacancy defects.50-51 Figure 6 shows the XRD spectra of the electrospun ITO NF films prepared with different PVP solution concentrations before, and after hot plate annealing. In Figure 6(a), the broad signal between 15° and 35° arises from the amorphous phase of a glass substrate. A strong peak at around 30.6° indicates a (222) plane crystalline orientation of In2O3, based on the XRD pattern of a polycrystalline In2O3 cubic phase in JCPDS no. 06-0416. The XRD patterns in our study do - 14 -


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not show the SnO and SnO2 major peaks, which indicates that the tin was completely doped into In2O3.37 The strong (222) peak only in XRD patterns without other In2O3 major peaks, (211), (400), (440), and (622), might be attributed to the fact that the ITO films are crystallized in a low oxygen-pressured atmosphere. This preferential growth related to the (222) planes might result from the low surface energy of the planes.52 Previous research on ITO crystallization under different oxygen pressures reported that oxygen induces the (211) plane orientation of the ITO crystalline structure during ITO growth.53 The (222) diffraction peak of In2O3 and the suppression of other major peaks implies that the ITO crystallites are grown along the same direction, leading to the highest mobility and the lowest resistivity compared to the ITO films that show the other peaks together.54 We found that the (222) peak intensity increases after hot plate annealing, which suggests that the post-annealing process makes the ITO nanofiber more crystalline (Figure 6 (b)). Comparison of the (222) peaks for the ITO NFs electrospun with different PVP solution concentrations indicates that there is almost no dependence of PVP solution concentration on the ITO crystallinity. The ITO crystallite size determined by the Scherrer equation ranges from 20 to 26 nm and there is no distinct PVP ratio and annealing dependence (Table S2).

Figure 7. XPS O 1s core level spectra of ITO nanofibers (a) before and (b) after postannealing. - 15 -



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Figure 7 exhibits the XPS O 1s core spectra of ITO nanofiber before and after postannealing. The XPS spectra was fitted with two distinct Gaussian peaks centered at 530.3 and 531.4 eV binding energies. It is known that the first peak at 530.3 eV is assigned to oxygen in crystalline ITO, and the second peak at 531.4 eV arises from oxygen vacancies.55-57 The XPS spectra clearly show that the relative peak intensity (or peak area) is changed upon postannealing. The relative increase in the second peak indicates that post-annealing creates more oxygen vacancies, and consequently enhances electrical conductivity.

Figure 8. SEM images of the post-hot-plate annealed ITO NF films electrospun with different PVP solution concentrations, (a) 7, (b) 8, and (c) 9 wt%.

Although the factors such as crystallinity and oxygen deficiencies are not much different depending on the PVP solution concentrations, the post-hot-plate annealing process dramatically changes the fiber morphology. Figure 8 shows the SEM images of the postannealed ITO NFs electrospun with different PVP solution concentrations. Low magnification SEM images for post-annealed ITO NF films are presented in the Supporting Information, Figure S4. Upon post-annealing, the ITO nanofibers are fused and interconnected to each other, which also in part contributes to the enhancement in electrical

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conductivity. In addition, the sphere-shaped ITO particles (or clusters) are observed on the outer of ITO NFs, and we found that the ITO particle size markedly differs with different PVP concentrations. For all cases, post-annealing enlarges the ITO particles, making the surface of ITO NFs rough. However, interestingly, the enlargement of ITO particle is found to be more prominent for high PVP solution concentrations. The average size of ITO particles placed on the surface of ITO NFs electrospun with 8 wt% and 9 wt% PVP solution concentrations is 76.7 ± 13.9 and 93.2 ± 12.8 nm, respectively (Figure S5). These ITO particle sizes are greater than the crystallite size, ~20-26 nm, determined by the XRD analysis. It indicates that the ITO particles are polycrystalline and might also contain some amorphous phase. It seems to be surprising that upon post-annealing, despite the absence of PVP inside NFs, the growth/agglomeration of ITO particles differs depending on the composition ratio of PVP which existed in the NFs before furnace annealing. For high PVP concentrations, the furnace-treated ITO fibers would possess more defect sites, dislocation, and the number of small ITO particles with greater interfacial boundaries due to PVP evaporation. Therefore, we infer that with more PVP weight in NFs, the evaporation of PVP through furnace annealing might produce defect sites, which enables for ITO crystallites to form bigger particles through post-annealing. The enlargement of ITO particles causes both positive and negative influences on the electrical properties. On one hand, the larger ITO particles would enable electrons to transport with fewer boundaries which act as the barrier of electron transport between particles, leading to higher electrical conductivity.1, 58-59

On the other hand, we reason that

the weak adhesion of ITO NF film is attributed to the fact that the increase in the interfacial separation between rough NFs and relatively smooth surface of the substrate reduces the van der Waals interactions while the van der Waals interactions between rough ITO NFs may increase forming fiber aggregates, which results in more cracks or film-detachment.60-61

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Indeed, as discussed above, we found that the surface coverage becomes poor and the entire films often are peeled-off from the substrate upon post-annealing, if the PVP solution concentration is high and the deposition time is long. In contrast, a low PVP concentration gives a rise to smooth surface of ITO NFs and any peel-off phenomena do not occur, leading to an enhancement in electrical conductivity. The positive contributions of the post-annealing treatment revealed so far in our study are better crystallinity, increased oxygen vacancies and larger ITO particles that may enhance charge transport. The high PVP content is expected to give low electrical conductivity, as long as post-annealing does not cause any cracks and peeling-off. Therefore, instead of using a low PVP concentration solution (< 7 wt%) that is unable to form the continuous fiber structure of ITO@PVP, the ITO NFs were electrospun for shorter deposition times, in an attempt to enhance electrical conductivity upon hot plate annealing with little sacrificing surface coverage owing to less aggregations of ITO NFs.

Figure 9. Sheet resistance and transmittance at 550 nm wavelength of the ITO NFs film electrospun for a 5 min deposition time. ITO NF films were annealed either in the IR furnace only (open symbol) or on the hot plate subsequent to the furnace treatment.

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Figure 9 and Table 1 shows the sheet resistance and the transmittance at 550 nm wavelength of the pure ITO NF films electrospun for 5 min deposition time with different PVP solution concentrations before and after post-annealing. Although the ITO NF films before hot plate annealing have high sheet resistances, 2.11 (7 wt%), 2.36 (8 wt%), and 2.08 (9 wt%) k Ω/sq, it is worth noting that these values are still better, given such high transmittances, than those of the electrospun ITO NF films demonstrated the most of previous studies, perhaps because of the IR furnace treatment that can homogenously transfer heat into the inside of the fibers. The thickness of IR-furnace annealed ITO films electrospun for 5 min was measured by the AFM depth profiles around the center of the films, which are exhibited in Figure S6. It is difficult to determine the NF film thickness because the NF film surface is too rough and not so uniform as the sputtered or spin-coated films. We assigned the average height of the AFM profile to be the thickness of the film. The thickness increases with increasing PVP solution concentration, as discussed with the deposition area. For 7, 8, and 9 wt% cases, the IR furnace annealed ITO NF films have an average thickness of 75.7, 105.1, and 131.6 nm, which is lower than the lateral fiber diameter determined by the SEM analysis, Figure 2 (see Figure S3 for histograms). It can be explained by some deformation of the ITO@PVP NFs through deposition onto the substrate during electrospinning and IR furnace treatment. In addition, there is little thickness-dependence of the sheet resistance, which might be attributed to the comparable size of the ITO crystallites/particles.62

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Table 1. The average sheet resistance and the transmittance at 550 nm wavelength for the ITO NF films electrospun for 5 min, after hot plate annealing following the IR furnace heat treatment. 7 wt% 2.11 k Ω/sq (95.0%) 0.54 k Ω/sq (97.1%)

Before After

8 wt% 2.36 k Ω/sq (94.2%) 0.49 k Ω/sq (95.3%)

9 wt% 2.08 k Ω/sq (89.5%) 0.44 k Ω/sq (93.1%)

The dramatic reduction in the sheet resistance of ITO NF films was observed upon postannealing. The sheet resistance of the hot plate annealed ITO NF films is decreased by ~79%, for the 9 wt% PVP concentration case, compared to that of the ITO NF films before hot plate annealing. Although ITO NFs electrospun from the PVP solutions with a high PVP concentrations for a deposition time of 5 min still sometimes show some peeling-off near the substrate edges (Figure S7), the average sheet resistance is decreased to 536 (7 wt%), 492 (8 wt%), and 439 Ω/sq (9 wt%), and the average transmittance at 550 nm wavelength is 97.1, 95.3, and 93.1% after hot plate annealing, respectively. The full transmittance spectra of ITO nanofiber films are exhibited in the Supporting Information, Figure S8. Post-annealing leads to an enhancement in optical transmittance and steeper curve, which can be seen more clearly for the 9 wt% PVP concentration case. This indicates a better crystallinity with lowering the defect density near the band edge, consistent with the XRD analysis. The furnace-treated ITO NFs contain more defect sites when ITO@PVP NFs contains a higher PVP content. The defect sites are removed during the ITO growth upon post-annealing, which in turn increases the transmittance.47,

63-67

It is questionable why the ITO NF films fabricated for shorter

electrospinning times and then post-annealed show lower sheet resistance. We speculate that for longer electrospun ITO NF films, there might be some micro-cracks due to evaporation of PVP and weak adhesion to the substrate. The best ITO NF film achieved has a sheet

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resistance of 350 Ω/sq with a transmittance of 92% at 550 nm wavelength. To the best of our knowledge, this is the lowest sheet resistance with a transmittance above 90%.

4. Conclusions In summary, we demonstrated that the surface coverage of ITO NF film depends on the presence of an ITO underlying layer (UL) and PVP solution concentrations. The two-step calcination process, infrared radiation (IR) furnace annealing followed by hot-plate annealing under nitrogen atmosphere, was conducted to significantly reduce the sheet resistance. This process improves the crystallinity of ITO, creates more oxygen vacancies and enlarges ITO, evidenced by the XRD, XPS and SEM analysis. However, the surface coverage was found to become seriously poor after hot-plate annealing, for the high PVP concentration cases. It is attributed to the reduction in interactions between the rough surface of ITO NFs and the flat substrates. Based on understanding on the factors, such as crystallinity, oxygen vacancies, particle size and surface coverage, contributing an enhancement/reduction in electrical conductivity of ITO NF films, low sheet resistances (a few hundreds of Ω/sq) with high transmittance above 90% were attained. Our results show the control of surface coverage associated with fiber morphologies is possible. This in turn enhances the electrical conductivity of ITO electrospun fibers, the advantages of which can be utilized for optoelectronic and sensing applications.

Supporting Information Transmittance spectra of the ITO underlying layers only and ITO NF films, the photographic images of the electrospun deposition area on the flat collector, and the ITO NF films before and after post-annealing, tables for atomic composition in percentage of the as- 21 -



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electrospun and furnace-treated NFs and the ITO crystallite size, histograms corresponding to the distribution of fiber diameters and ITO particle sizes, low magnification SEM images of post-annealed ITO NF films, AFM depth profiles.

Acknowledgments. This research was supported by the Basic Science Research Program through the National Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (2014R1A1A1002217),

and

the

Ministry

of

Education

(2014R1A2A1A10054643,

2016R1D1A1B03932615). I.H. acknowledges a Research Grant from Kwangwoon University in 2017.

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Toward High Conductivity of Electrospun Indium Tin Oxide Nanofibers

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