Photochemical Activation of Electrospun In2O3 Nanofibers for High

Mar 6, 2017 - Electrospun metal oxide nanofibers have been regarded as promising blocks for large-area, low-cost, and one-dimensional electronic devic...
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Photochemical Activation of Electrospun In2O3 Nanofibers for HighPerformance Electronic Devices You Meng, Guoxia Liu,* Ao Liu, Zidong Guo, Wenjia Sun, and Fukai Shan* College of Physics and College of Electronic and Information Engineering, Qingdao University, Qingdao 266071, China S Supporting Information *

ABSTRACT: Electrospun metal oxide nanofibers have been regarded as promising blocks for large-area, low-cost, and onedimensional electronic devices. However, the electronic devices based on electrospun nanofibers usually suffer from poor performance and inferior viability. Here, we report an efficient photochemical process using UV light generated by a highpressure mercury lamp to promote the electrical performance of the nanofiber-based electronic devices. Such UV treatment can lead to strong photochemical activation of electrospun nanofibers, and therefore, a stable adherent nanofiber network and electronicclean interface were formed. By use of UV treatment, highperformance indium oxide (In2O3) nanofiber based field-effect transistors (FETs) with highly efficient modulation of electrical characteristics have been successfully fabricated. To reduce the operating voltage and further improve the device performance, the In2O3 nanofiber FETs based on solution-processed high-k AlOx dielectrics were integrated and investigated. The as-fabricated In2O3/AlOx FETs exhibit superior electrical performance, including a high mobility of 19.8 cm2 V−1 s−1, a large on/off current ratio of 106, and high stability over time and cycling. The improved performance of the UV-treated FETs was further confirmed by the integration of the electrospun In2O3/AlOx FETs into inverters. This work presents an important advance toward the practical applications of electrospun nanofibers for functional electronic devices. KEYWORDS: photochemical activation, UV treatment, In2O3, nanofibers, field-effect transistors, inverters

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diameters.12,13 Moreover, the electrospinning technique could also lead to a high output of nanofibers, which is beneficial to the mass production of consumption electronic devices. Metal oxide (MO) semiconducting materials have been actively investigated as possible replacements for Si-based devices for emerging low-cost and disposable electronic applications. Compared to the organic and covalent semiconducting materials, MO semiconducting materials exhibit several advantages, such as high carrier mobility, good environmental stability, and high optical transparency.14 Among MO semiconducting materials, indium oxide (In2O3) is one of the representative wide band gap semiconductors with a direct band gap of 3.75 eV.15,16 The high transmittance in the visible region and intrinsic high electron mobility make In2O3 one of the most important potential candidates for applications in transparent electronics. Even in multicomponent MO systems based on In2O3, such as InZnO, InZrO, and InGaZnO, the high mobility of these composite MO materials mainly originates from the In2O3 matrix because of its edge-sharing structure and ns orbital of indium.17,18 Meanwhile, In2O3-

INTRODUCTION Field-effect transistors (FETs) based on one-dimensional (1D) semiconducting nanostructures are of great interest for future functional electronics such as chemical sensors, logic devices, bioprobes, and active matrix organic light-emitting diodes.1−4 Over the past few years, various techniques have been developed to synthesize the 1D nanostructure, including chemical/physical vapor deposition, electrochemical etching/ deposition, hydrothermal, template-based, and laser ablation methods.5−9 However, all of these methods are cumbersome and time-consuming processes, which significantly increase the manufacturing cost and pose major obstacles for realizing largearea and inexpensive electronic devices. In this consideration, a universal and facile technique is required to fabricate 1D nanostructures for diverse materials. Among various techniques for nanomaterial fabrication, electrospinning is the simplest approach to fabricate 1D nanostructures in situ and can be entirely performed in ambient atmosphere without any special precautions. An electrospinning technique has been widely used to produce polymeric, inorganic, and composite nanofibers with diameters ranging from tens nanometers to several micrometers.10,11 It provides a facile and low-cost route for achieving continuous nanofibers with high specific surface area and uniform © 2017 American Chemical Society

Received: December 12, 2016 Accepted: March 6, 2017 Published: March 6, 2017 10805

DOI: 10.1021/acsami.6b15916 ACS Appl. Mater. Interfaces 2017, 9, 10805−10812

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Figure 1. (a) Schematic illustration for fabricating In2O3 nanofiber FETs. (b, e) SEM images of the as-electrospun In2O3/PVP composite nanofibers on SiO2/Si substrate. (c, f) SEM images of the In2O3/PVP composite nanofibers after UV treatment for 30 min. (d, g) SEM images of the In2O3 nanofibers after being calcined at 600 °C for 90 min.

nanofiber FETs in terms of electrical performance and operational stability. In this study, a UV treatment process was applied to enhance the adhesion property of the electrospun MO composite nanofibers. With the improved adhesion property, the FET devices exhibit efficient modulations of electrical performance. The electrical performance of the In2O3 nanofiber FETs as a function of fiber density was investigated. To reduce the operating voltage and improve the device performance, the electrospun In2O3 nanofiber FETs based on solution-processed AlOx dielectrics were integrated and investigated for the first time. The improved viability of the UV-treated devices was further confirmed by the integration of electrospun In2O3/AlOx FETs into inverters.

related materials exhibit versatile electrical properties, such as metallic, semiconducting, and insulating characteristics, depending on its stoichiometry and morphology.19,20 When the advantages of electrospinning techniques are combined with versatile In2O3 semiconducting materials, it is promising to fabricate high-performance and low-cost 1D field-effect electronics by optimizing the processing and morphology. In previous reports, the MO nanofiber-based FETs usually exhibited poor electrical performance, especially their low fieldeffect mobilities and the low on/off current ratios, which are far behind the performance requested for real electronic devices.21−24 The unsatisfactory performance is mainly due to its poor interfacial adhesion properties between the nanofibers and the gate insulator. Recently, a thermo-compression step was reported to improve the adhesion property, and the reproducible device performance was demonstrated.25,26 However, the complicated procedure undoubtedly restricts its practical applications. Interestingly, the ultraviolet (UV) irradiation process has been demonstrated to be a simple and efficient method to accelerate the polymer degradation and modify the nanofiber morphology.27,28 During the UV treatments, energetic UV photons would induce photochemical cleavage of alkoxy groups and potentially achieve high degrees of condensation and densification of composite materials.29,30 Meanwhile, the uniformity of the UV irradiation is promising for the realization of large-area manufacturing. In these considerations, the UV treatment is expected to improve the interface adhesion property, which is important for electrospun

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RESULTS AND DISCUSSION

The assembly procedure for In2O3 nanofiber FETs is illustrated in Figure 1a. Parts b−d of Figure 1 exhibit the SEM images of the In2O3 nanofibers without treatment, with UV treatment for 30 min, and the In2O3 nanofibers calcined at 600 °C for 90 min, respectively. The corresponding magnified images of Figure 1b, c, and d are shown in Figure 1e, f, and g, respectively. The as-electrospun In2O3/poly(vinylpyrrolidone) (PVP) composite nanofibers exhibit highly uniform morphology with fiber diameters around 350 nm. Because of the overlapping of the nanofibers, the nanofiber network exhibits poor adhesion properties and tends to peel off easily from the substrate (Figure 1b). It is well-known that the carrier transport in the 10806

DOI: 10.1021/acsami.6b15916 ACS Appl. Mater. Interfaces 2017, 9, 10805−10812

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Figure 2. (a) Light absorption characteristics of In2O3/PVP precursor solution. Inset: chemical structure of PVP. (b) XRD patterns of In2O3 nanofibers annealed at different temperatures. HRTEM images of the In2O3 nanofibers annealed at (c) 400, (d) 500, and (e) 600 °C. Insets of (c−e) show the enlarged high-magnification images. (f) Corresponding average grain size of In2O3 nanofibers annealed at different temperatures.

FETs is limited in a narrow region between the channel and the dielectric, which means that the interface property plays a critical role in determining the viability and the stability of FETs. The poor adhesion property would undoubtedly degrade the free carrier transport, leading to inferior device performance.20,31,32 To overcome such a bottleneck, a UV treatment process was employed to treat the In2O3/PVP composite nanofibers prior to the calcination. After UV treatment, stable adherent nanofiber networks were achieved with diameters decreased from ∼350 nm to ∼200 nm (Figure 1c). The UV− vis absorption spectrum of the electrospun In2O3/PVP precursor solution reveals strong light absorption at wavelengths shorter than 350 nm (Figure 2a). The photochemical activation of PVP is due to the attack on the α-position (secondary carbon atom A and tertiary carbon atom B) of the amide group.27,30 In addition, the high specific surface area of the nanofibers could offer efficient UV absorption and lead to significant photochemical activation. When In2O3/PVP composite nanofibers are exposed to UV irradiation, low molecular weight photoproducts (pyrrolidone, etc.) will be generated and release to the atmosphere.27 This would result in a decrease of diameter of the nanofibers and the modification of the nanofiber morphology. Subsequent calcination at 600 °C results in a high-quality In2O3 nanofiber network with reduced diameter at around 90 nm (Figure 1d). To help understand the phase formation and photochemical reaction, the thermal behavior and FT-IR analysis are presented in Figures S1 and S2 (Supporting Information). As shown in Figure 2b, when the annealing temperature reaches 400 °C, several diffraction peaks can be clearly observed, demonstrating the crystallization of the In2O3

nanofibers. The orientations of (211), (222), (400), and (440) located at 21.49, 30.58, 35.45, and 51.02° reveals the cubic phase formation of In2O3 nanofibers (corresponding to JCPDS, No. 44-1087).33 No secondary or impurity phases were observed in the spectra. With the increase of the annealing temperature, the diffraction peaks of In2O3 are gradually strengthened and become sharp, suggesting the increment of the crystallite size.34 Crystallite size (D) analysis was carried out using the Scherrer formula34 D=

Kλ β cos θ

where k is the Scherrer constant, λ is the wavelength of the incident X-rays, β is the full-width half-maximum (fwhm), and θ is the diffraction angle. For the annealing temperatures at 400, 500, and 600 °C, the resulting average grain sizes were calculated to be 6.88, 14.46, and 19.05 nm, respectively. Meanwhile, the peak positions are gradually shifted to higher diffraction angles with increasing annealing temperature (Figure S3). This result suggests that contraction of the interplanar spacing in In2O3 nanofibers occurs at higher annealing temperatures, leading to the densification of the nanofiber. Similar phenomena as well as interpretation have been reported previously.34 To further clarify the microstructure evolution of the In2O3 nanofibers annealed at different temperatures, the microstructures of In2O3 nanofibers were also investigated using high-resolution transmission electron microscopy (HRTEM). As shown in Figure 2c−e, the polycrystalline nature of the In2O3 nanofibers is clearly observed. It has been found that the 10807

DOI: 10.1021/acsami.6b15916 ACS Appl. Mater. Interfaces 2017, 9, 10805−10812

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Figure 3. SEM images of the In2O3 nanofibers on SiO2/Si substrate with various densities of (a) 1.2, (b) 1.8, (c) 2.5, (d) 3.8, (e) 5.2, and (f) 10 μm−1, respectively. (g) Representative transfer curves (VDS = 30 V) of the electrospun In2O3/SiO2 FETs with nanofiber densities ranging from 1.2 to 10 μm−1. (h) Output characteristics of the electrospun In2O3/SiO2 FET with nanofiber density of 2.5 μm−1.

average grain size of the In2O3 nanofibers annealed at 400 °C (4.12 ± 0.93 nm) is much smaller than those annealed at higher temperatures (12.34 ± 1.39 nm for 500 °C and 15.67 ± 2.9 nm for 600 °C). This result is consistent with the XRD data. The h(222) interplanar spacings of In2O3 nanofibers annealed at 400, 500, and 600 °C were measured to be 2.95, 2.93, and 2.93 Å, respectively. These values are close to the h(222) interplanar spacing of 2.921 Å for cubic In2O3, indicating the formation of high-quality In2O3 nanofibers.33 To explore the possibility of the UV-treated nanofibers as channel layers, the electrospun In2O3 nanofiber FETs based on thermally grown SiO2 were integrated and investigated. All of the FETs were integrated with the same experimental conditions, except for the variation of the nanofiber density. The SEM images of the In2O3 nanofibers with various nanofiber densities are shown in Figure 3a−f. Figure 3g shows the representative transfer curves of the electrospun In2O3/SiO2 FETs with various nanofiber densities. The corresponding electrical parameters are summarized in Table 1. It was found that the device performance strongly depends

networks with low nanofiber density. It should be noted that the parallel-plate capacitance model overestimates the actual gate capacitance and underestimates the mobility.35 Therefore, the extracted μFE values in this study represent a lower boundary. When the nanofiber density was 1.25 μm−1, the device exhibited poor performance, including a small oncurrent (Ion) of 0.4 μA and an inferior μFE of 0.02 cm−1 V−1 s−1. The poor performance was mainly due to the insufficient channel coverage, which leads to the limited electron pathway. With the formation of adequate nanofiber network and the increase of the free carriers, Ion and μFE gradually increased. Although the FET with nanofiber density over 3.8 μm−1 exhibited higher μFE value, an inferior switching characteristic was simultaneously observed. This would undoubtedly limit its practical applications. With the trade-off between Ion/off and μFE, the electrospun In2O3 nanofiber FET with nanofiber density of 2.5 μm−1 exhibited optimal device performance, including a μFE of 2.8 cm−1 V−1 s−1and a large Ion/off of 107. Figure 3h shows the typical output characteristics of the electrospun In2O3 nanofiber FET with density of 2.5 μm−1. The device exhibits a typical nchannel behavior with clear pinch-off and current saturation behavior, and no current crowding phenomenon was observed. It is plausible that the formation of stable adherent nanofiber network by using UV treatment is promising for controllable high-performance devices. In order to further verify the feasibility of the UV treatment, the electrical performances of the In2O3 nanofiber FETs with/ without UV treatment were compared, and the results are shown in Figure 4a, b. The corresponding performance distribution (i.e., μFE and Ion/off) and statistical parameters are shown in Figure 4c, d and Table S1 (Supporting Information). For the fabricated devices without UV treatment, the poor electrical performance and inferior device uniformity were observed. On the contrary, the In2O3 nanofiber FETs with UV treatment exhibited enhanced device uniformity including an average μFE of 2.83 cm−1 V−1 s−1 (σ = 0.35) and an excellent Ion/off around 107, which is comparable to the solutionprocessed thin-film transistors.31 Apparently, the improved interfacial adhesion property induced by UV treatment significantly enhances the viability of the electrospun nanofiber field-effect devices. To reduce the operating voltage and further improve the device performance, the electrospun In2O3 nanofiber FET based on solution-processed AlOx dielectric thin film was integrated, and the typical transfer curves are shown in Figure 5a. The physical properties of the solution-processed AlOx

Table 1. Electrical Parameters of Electrospun In2O3/SiO2 FETs as a Function of Nanofiber Density fiber density (μm−1) 1.2 1.8 2.5 3.8 5.2 10

SS (V dec−1)

μFE (cm2 V−1 s−1)

VTH (V)

Ion/off

−3 −10 −12 −25

∼104 4.4 ± 0.5 0.02 ± 0.01 ∼104 3.9 ± 0.4 0.1 ± 0.05 ∼107 1.7 ± 0.2 2.8 ± 0.35 ∼104 3.8 ± 0.6 3.5 ± 0.42 ∼102 4.8 ± 0.76 conductive (always on)

± ± ± ±

5 4 3 5

on the nanofiber density. The field-effect mobility (μFE) in the saturation region was extracted from the equation 1W IDS = μ Ci(VGS − VTH)2 2 L FE where VGS is the gate voltage and W and L are the channel width and length, respectively. The threshold voltage (VTH) was calculated from the linear portion of (IDS)1/2 vs VGS. The areal capacitance (Ci) was directly measured from a parallelplate capacitance model, which was widely used in calculating the field-effect mobility of conventional thin-film transistors. In the case of nanofiber FETs, the nanofibers are sparsely distributed in the channel, particularly for the nanofiber 10808

DOI: 10.1021/acsami.6b15916 ACS Appl. Mater. Interfaces 2017, 9, 10805−10812

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V−1 s−1 (σ = 1.2), a large Ion/off of ∼106, a small SS value of 100 mV/decade, and a hysteresis of 0.3 V. The improved features are undoubtedly related to the employment of the high-k AlOx dielectric, high-quality In2O3 nanofiber channel, and the electronically clean interface of In2O3/AlOx.36−40 The maximum areal density of states (Nsmax), calculated from the SS value, was 1.27 × 1012 for the electrospun In2O3/AlOx FET. Such a small Nsmax value is quite acceptable compared with previously reported thin film devices based on solutionprocessed high-k MO dielectrics.41,42 It can be seen that the operating voltage of the FET is only 3 V, which is one order of magnitude smaller than the FET based on conventional SiO2 dielectric. This is important for low power consumption electronic devices. These performance parameters are much more superior than previous electrospun nanofiber field-effect devices (summarized in Table 2), which represents a great step toward the achievement of low-cost, low-power, and highperformance 1D electronics. Generally, device operational stability is a challenging and significant task for electrospun nanofiber FETs. However, there have been few reports that demonstrate the electrical stability of nanofiber field-effect devices. In this study, the UV treatment apparently improved the adhesion property, and the operational stability of the UV-treated electrospun In2O3/AlOx FETs was demonstrated. As shown in Figure 5b, a negligible VTH shift was observed with no obvious degradation in device performance measured 300 days after fabrication. The cycling stability over 70 electrical measurement cycles was also tested, and the results are shown in Figure 5c. The electrical parameters were extracted and are shown in Figure 5d−f. The electrical parameters, such as μFE, VTH, and Ion/off, remained nearly unchanged after 70 measurement cycles. The negligible change of the electrical parameters, especially for VTH, reveals that there are a small number of defects at the interface, which is consistent with the calculated Nsmax data.

Figure 4. Transfer curves at VDS = 30 V of In2O3 nanofibers FETs (4 × 5 array) (a) with and (b) without UV treatment. (c, d) Corresponding performance distribution of electrospun In2O3 nanofibers FETs fabricated with/without UV treatments. The nanofiber density of all the devices was ∼2.5 μm−1.

dielectric thin films are shown in Figure S7 (Supporting Information). The electrospun In2O3/AlOx FET exhibits excellent device performance, such as a high μFE of 19.8 cm−1

Figure 5. (a) Transfer curves of the electrospun In2O3/AlOx FETs measured with forward and reverse sweep. (b) Transfer curves of the electrospun In2O3/AlOx FETs measured immediately after fabrication and measured 300 days later. (c) Transfer curves of the electrospun In2O3/AlOx FETs measured from first to 70th cycles. (d) μFE, (e) Ion/off, and (f) VTH as a function of measurement cycle. 10809

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ACS Applied Materials & Interfaces Table 2. Performance Summary of the Electrospun Nanofiber Field-Effect Devices channel materials

dielectric materials

In2O3 In2O3 ZnO Al-doped ZnO Li-doped NiO InGaZnO InGaZnO Ce-doped ZnO P3HT/PEO P3HT P3HT P3HT ZnO CuO P3HT PANi/PEO

a

AlOx (SC ) SiO2 SiO2 SiO2 SiO2 MgBiZnNbOb (RFc) SiO2 SiO2 ion gel SiO2 ion gel SiO2 SiO2 SiO2 SiO2 SiO2

mobility (cm2 V−1 s−1)

Ion/off

19.8 2.8 0.018 3.3 × 10−3

106 107

7.04 2.04

3.13 × 105 6.82 × 106

3.8d/9.7e 0.192 2 0.017

1.70 × 106 4.45 × 104 105 102

0.03

103

notes UV-treated UV-treated aligned

103 p-type hot-pressing hot-pressing p-type near-field ESf coaxial ES periodic array

p-type

year

ref

2017 2017 2016 2016 2014 2014 2014 2014 2013 2011 2010 2009 2008 2006 2005 2003

this work this work 51 43 22 26 26 44 45 46 47 48 49 50 24 23

a

Spin-coating. bHigh-K (MgO)0.3-(Bi1.5Zn1.0Nb1.5O7)0.7 gate insulator layer. cRadio frequency magnetron sputtering. dAverage value. eMaximum value of P3HT/PEO nanowire FET array. fNear-field electrospinning.

Figure 6. (a) Voltage transfer characteristics and (b) corresponding gains of the resistor-loaded inverter based on electrospun In2O3/AlOx FETs. Inset of (b): circuit schematic of the resistor-loaded inverter. (c, d) Dynamic responses of resistor-loaded inverter at VDD = 2 V when Vin was pulsed at 1 Hz.

of Figure 6 display the dynamic responses of the resistor-loaded inverter at VDD = 2 V when Vin was pulsed at 1 Hz. The inverter based on In2O3/AlOx FETs demonstrates good action and responses close to the square-wave input signal, implying good operational stability and high reliability of the UV-treated nanofibers FETs. To the best of our knowledge, the resistorloaded inverters presented here are the first logic devices based on electrospun MO nanofibers, indicating the great potential of UV-treated electrospun MO nanofibers for applications in consumption logic devices.

After the successful fabrication of high-performance, lowvoltage In2O3 nanofiber FETs based on high-k AlOx dielectrics, we attempted to demonstrate the feasible integration of electrospun In2O3/AlOx FETs into resistor-loaded inverters. The typical voltage-transfer characteristics of the inverter are shown in Figure 6a, where the output voltage (Vout) is clearly transformed to the inverse with different supply voltages (VDD). The corresponding gains (defined as −dVout/dVin) and the schematic circuit are shown in Figure 6b. The inverter exhibits a maximum gain of 8.4 at a supply voltage of 4 V. Note that the noise margin is very important in the design and application of multistage digital circuits. It can be shown visually by overlaying the voltage transfer characteristic and its mirror image at each VDD, as highlighted by the orange rectangles in Figure S9 (Supporting Information). The noise margin increases from 230 mV at VDD = 2 V to 1200 mV at VDD = 4 V. Parts c and d

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CONCLUSIONS In conclusion, we demonstrated a simple and efficient UV treatment process for the fabrication of high-performance In2O3 nanofiber FETs. The unique photochemical function of the UV treatment enables the stable adhesion of the nanofiber network 10810

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and the formation of an electronic-clean interface. With the improved adhesion property, the devices exhibit efficient modulations of electrical performance. The device performance is strongly dependent on the nanofiber density. The optimized electrospun In2O3/SiO2 FETs, with nanofiber density of 2.5 μm−1, exhibit high device performance, including a μFE of 2.8 cm−1 V−1 s−1 and large Ion/off of ∼107. To decrease the operating voltage and further improve the device performance, the electrospun In2O3 nanofiber FETs based on solutionprocessed AlOx dielectrics were integrated and investigated. As the advantages, including high-k AlOx dielectric, the highquality In2O3 nanofiber channel, and the electronically clean interface of In2O3/AlOx, are combined, the electrospun In2O3/ AlOx FETs exhibit superior device performance, such as a high μFE of 19.8 cm−1 V−1 s−1 and large Ion/off of ∼106. Apart from these direct performance metrics, the operational stability over time and cycles was also examined. The device parameters remain nearly unchanged even measured 300 days after fabrication and with continuous 70 measurement cycles. In addition, the improved viability of the present UV-treated devices was further confirmed by the integration of electrospun In2O3/Al2O3 FETs into inverters. Typical voltage transfer characteristics with a maximum gain of 8.4 and good dynamic responses were achieved. This work presents an important advancement toward the practical application of electrospun nanofibers for functional electronic devices.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15916. Detailed experimental section and instrumental analysis; thermal behavior of dried electrospun In2O3 precursor solution; FT-IR analysis of electrospun In2O3 nanofibers; HRTEM image of In2O3 nanofiber annealed at 300 °C; photographic image of calcined In2O3 nanofiber channels with/without UV treatment; output characteristics of the UV-treated electrospun In2O3/SiO2 FETs with various nanofiber densities; statistical parameters of respective 20 devices with and without UV treatment; physical properties of solution-processed AlOx dielectric thin films; output characteristics of the UV-treated electrospun In2O3/AlOx FETs; noise margin of the resistorloaded inverter based on electrospun In2O3/AlOx FETs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Fukai Shan: 0000-0002-7158-9559 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 51472130, 51572135, and 51672142). 10811

DOI: 10.1021/acsami.6b15916 ACS Appl. Mater. Interfaces 2017, 9, 10805−10812

Research Article

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DOI: 10.1021/acsami.6b15916 ACS Appl. Mater. Interfaces 2017, 9, 10805−10812