Nature inspired capillary-driven welding process for boosting metal

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Functional Inorganic Materials and Devices

Nature inspired capillary-driven welding process for boosting metal-oxide nanofiber electronics You Meng, Kaihua Lou, Rui Qi, Zidong Guo, Byoungchul Shin, Guoxia Liu, and Fukai Shan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05104 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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ACS Applied Materials & Interfaces

Nature inspired capillary-driven welding process for boosting metal-oxide nanofiber electronics You Meng,a,b,c Kaihua Lou,a,b,c Rui Qi,a,b,c Zidong Guo,a,b,c Byoungchul Shin,d Guoxia Liu,a,b,c,e* and Fukai Shan a,b,c,e* a b

College of Physics, Qingdao University, Qingdao 266071, China

College of Electronic & Information Engineering, Qingdao University, Qingdao 266071, China c

State Key Laboratory for Biological Polysaccharide Fiber and Ecological Textiles, Qingdao University, Qingdao 266071, China d

Electronic Ceramics Center, DongEui University, Busan 614714, Korea

e

Collaborative Innovation Center for Eco-Textiles of Shandong Province, Qingdao 266071, China

ABSTRACT. Recently, semiconducting nanofibers networks (NFNs) have been considered as one of the most promising platforms for large-area and low-cost electronics applications. However, the high contact resistance among stacking nanofibers remained to be a major challenge, leading to poor device performance and parasitic energy consumption. In this report, a controllable welding technique for NFNs was successfully demonstrated via a bioinspired capillary-driven process. The interfiber connections were well achieved via a cooperative concept combining localized capillary condensation and curvature-induced surface diffusion. With the improvements of the interfiber connections, the welded NFNs exhibited enhanced mechanical property and high electrical performance. The field-effect transistors (FETs) based on the welded Hf-doped In2O3 (InHfO) NFNs were demonstrated for the first time. Meanwhile, the mechanisms involved in the grain-boundary modulation for polycrystalline metal-oxide nanofibers were discussed. When the high-k ZrOx dielectric thin films were integrated into the FETs, the field-effect mobility and operating voltage were further improved to be 25 cm2/Vs and 3 V, respectively. This is one of the best device performances among the reported nanofibers-based FETs. These results demonstrated the potencies of the capillary-driven welding process and grain-boundary modulation mechanism for metal-oxide NFNs, which could be applicable for high-performance, large-scale and lowpower functional electronics.

KEYWORDS. capillary condensation, welding process, nanofibers networks, grainboundary modulation, field-effect transistors

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INTRODUCTION One-dimensional (1D) semiconducting nanostructures with excellent physical and chemical properties are fundamental building blocks for enormous technological applications, such as electronics, sensors, photonics, energy, and so forth.1-4 In particular, 1D metal-oxide semiconducting nanostructures have attracted considerable attentions, because of their high mobility, high stability and high transparency.5,6 Among various 1D nanomaterial fabrication techniques, electrospinning technique is one of the most powerful approaches for fabricating continuous 1D nanomaterial in situ.7,8 During the past decades, electrospinning had been widely used to produce various nanofibers (e.g., organic, inorganic and composite nanofibers) with diameters ranging from tens nanometers to several micrometers. The electrospinning process can be performed in ambient atmosphere without any special precautions, avoiding the expensive synthesis approach. Moreover, electrospinning technique brings high output of nanofibers, which is beneficial to the mass production for consumption electronics.9 These advantages make electrospinning an attractive technique in various fields, such as electronics and biomedical applications. However, the nanofibers networks (NFNs) fabricated by electrospinning technique are usually formed via physical stacking (weak interfiber connections), resulting in inferior mechanical property and high contact resistance.10 This innate drawback potentially restricts the implementation of the NFNs in a number of practical applications, especially in electronic devices in terms of electrical performance and operational stability. Therefore, extra processing steps were usually applied and expected to improve the interfiber connections. Up to now, hot pressing and cross-linking have been the commonly used methods to improve the interfiber connection of the NFNs.11,12 However, hot pressing is difficult to be used for the polymers with high melting point (e.g., polyimide), and most cross-linking agents are detrimental to the human health and environment. Recently, limited success for 1D nanowelding (also known as nano-soldering or nano-joining) had been demonstrated by using electron beam irradiation13, laser irradiation14 or joule heating15. These methods exploited the novel and unique physical phenomenon occurring in 1D nanostructure. Unluckily, they are largely limited in laboratory scale. Most recently, our group proposed a photochemical treatment to enhance the adhesion property between the electrospun nanofibers and substrate. However, the morphological change of interfiber connection was not clearly observed.16 It is highly desirable to develop a facile and universal technique for assembling the stacking NFNs in large area and with strong interfiber connections.

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In nature, some creatures developed amazing ability to collect water from humid air, owing to their special microstructures.17 For instance, some desert creatures (such as cacti, beetles and spiders) have developed special functional structures to collect water. Inspired by these findings, Zheng et al. designed artificial fibers to mimic the structural features of spider silk and the water collecting ability was demonstrated.18 The water-collection ability of the spider silk is proposed to result from the unique fiber structure of the periodic spindle-knots separated by joints.18 Recently, bio-inspired wettability phenomenon has been intensively demonstrated for traditional materials or devices to improve their performances and to extend their practical applications.19 Fisher et al. proposed in both experiments and simulations that the microstructures with small curvatures or nanoscale gaps will be the preferred sites for capillary condensation.20,21 The Kelvin equation forms the basis of capillarity condensation theory, predicting that the undersaturated vapors will condense at position with sufficiently small dimensions.20-22 Impressively, the vapor condensation at the cross points of the nanofibers has been exploited to generate the nanoscale chemical reactors.23 Recent demonstration of the solvent vapor annealing (SVA) exploited the morphological and structural evolution occurring in conjugated polymer.24,25 The SVA has been demonstrated to be an effective approach to improve the photovoltaic efficiency.26 However, the reasons for the morphological and structural evolution in SVA have seldom been investigated. Most recently, dichloromethane (DCM) solvent vapor was introduced to weld polycaprolactone (PCL) nanofibers mats, and significant improvement in mechanical properties was achieved.27 These results imply that the capillary-driven vapor condensation at the cross points of the nanofibers seems to be a feasible approach to enhance the interfiber connections. In this study, a bio-inspired capillary-driven welding process was applied to enhance the interfiber connections. The properties of the NFNs connections before and after welding were studied by various techniques, including optical microscopy, scanning electron microscopy, transmission electron microscopy, taping test and a series of electrical measurements. With the enhanced connections, the welded NFNs exhibited improved mechanical property and high electrical performance. The field-effect transistors (FETs) based on InHfO (Hf-doped In2O3) NFNs, as a function of Hf doping concentration, were investigated and the device performance was further improved by integrating the high-k dielectrics into the InHfO NFNs FETs. The improved electrical performance of the device based on the welded NFNs was confirmed by integrating the FETs into low-voltage inverters.

EXPERIMENTAL SECTION 3



Precursor Preparation. In a typical procedure, 0.1 g anhydrous indium (III) chloride (InCl3, 99.99%) was mixed with 0.9 g of poly(vinyl pyrrolidone) (PVP, Mw=1300000) and 5 ml of N, N-dimethylformamide (DMF, 99.9%). InHfO precursor solutions were prepared by adding hafnium chloride (HfCl4, 99%) with appropriate amount into In2O3 precursor solution to achieve Hf molar ratios of 5%, 10%, and 15 % in the mixed solution. After vigorous stirring for 12 h, a clear viscous solution was obtained. To analyze the thermal behavior of the dried electrospun precursor solutions, thermogravimetric analysis (TGA, Pyris1) was carried out with a heating rate of 5 oC min-1 from room temperature to 700 oC. Device Fabrication. Typical electrospinning procedure can be found in our previous report.16 The channel with various nanofiber densities were prepared by varying electrospinning time. The nanofiber density (µm-1) was calculated from the number of nanofibers bridging the source and drain electrodes divided by the width of electrodes. In order to weld the nanofibers, the samples were simply hung facing downward on a beaker containing chemicals (i.e., DMF, DI water, H2O2, alcohol or acetone) for specific time duration at room temperature. The distance between liquid level and samples was kept at ~ 0.5 cm. Subsequently the samples were calcined at 550 oC in atmosphere for 2 h to remove PVP matrix and form oxidation state. Finally, Al source and drain electrodes were deposited by thermal evaporation using shadow mask. For field-effect measurement, the channel width and length were 1000 and 100 µm, respectively. To integrate the electrospun InHfO NFNs FETs based on high-k dielectric, thermallygrown SiO2 dielectrics were replaced by solution-processed ZrOx thin films. The 0.2 M ZrOx precursor solution was prepared by dissolving zirconium acetylacetonate [Zr(C5H7O2)4, 99.9%] in 2-Methoxyethanol (2-ME, 99.99 %). After vigorous stirring for 6 h, a clear solution was obtained. The procedure details for spin coating had been reported somewhere else.16 To obtain high-quality ZrOx thin films, a sequential process, using UV irradiation for 0.5 h and thermal annealing at 600 °C for 2 h, was used to treat the samples. Characterization. The microstructure of the nanofibers was investigated by scanning electron microscopy (SEM, S-4800, Hitachi). The grain size and interplanar spacing of the InHfO nanofibers were investigated by high resolution transmission electron microscopy (HRTEM, JEM-2100Plus, JEOL). The crystal structures of InHfO NFNs were investigated by X-ray diffractometry (XRD, X’Pert-PRO, PANalytical, Holland). To examine the dielectric properties of the gate insulator layers, the capacitors with a structure of Al/ZrOx/p+-Si were investigated using an impedance analyzer (4294A, Agilent). The areal capacitance (i.e., per unit area capacitance) was calculated from the measured capacitance (F) divided by the 4


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electrode area (1.5 mm2). The electrical properties of ZrOx capacitors and the electrical performance of the integrated FETs were measured using a semiconductor parameter analyzer (2634B, Keithley) in a dark box.

Figure 1. Bio-inspired capillary-driven welding process. (a) Design concept of capillary condensation in nature. (b) Schematic of electrospinning process for fabricating nanofibers. (c) SEM image of as-spun nanofibers. Scale bar: 2 µm. (d-f) Schematic illustration of capillary-driven welding process. TEM image of (g) pristine interfiber junction and welded interfiber junction with Tw of (h) 5 s and (i) 10 s. Scale bars: 200 nm.

RESULTS AND DISCUSSION The bio-inspired mimicking microstructures with unusual micro features have attracted considerable interest in various fields such as machines, clothing and electronics.28,29 An intriguing example is provided by spider webs, which is usually adorned with dew drops in the morning (Figure 1a). Based on this unique phenomenon, remarkable progress has been achieved towards the realization of directional water collection30 and controllable water transport31. This inspires one to explore the feasible application of such wetting abilities in the welding of the NFNs. As two microstructures are closely contacted with each other, a spontaneous capillary condensation occurs followed by a liquid bridge formation at contact area.32 Compared to the traditional vapor condensation, the capillary condensation could occur even at the condition that equilibrium vapor pressures (Pv) is much lower than the 5



saturation vapor pressure (Psat).20 In thermodynamics, the capillary condensation is explained by the Kelvin equation:22

1 1 1 RT  PV   = + = ln rm r1 r2 γ lV  Psat 

(1)

where, 1/rm is the mean curvature, r1 and r2 are the principal radius of the curvatures, R is the gas constant, T is the absolute temperature, V is the molar volume of the condensed liquid, γl is the liquid surface tension, and Pv/Psat is the relative vapor pressure. This equation predicts an almost linear relation between In(Pv/Psat) and 1/rm, which means that the larger the curvature, the easier the vapor condensation.21,22 As shown in Figure 1c, the as-spun nanofibers exhibit a stacking structure with poor interfiber connections (more SEM images are shown in Figure S1), which is further confirmed by the TEM investigation (Figure 1g). The poor physical contacts between nanofibers form nanogaps, where large mean curvature forms, making the vapor condensation preferentially occur at such nanogaps (Figure 1e).21 To verify the capillary-driven welding process, N,N-dimethylformamide (DMF) vapor was used to treat nanofibers (Figures 1h and1i). The samples were simply hung facing downward on a beaker containing DMF solvent for specific time duration at room temperature. The distance between liquid level and samples was kept at ~ 0.5 cm. With insufficient welding time (Tw) of 5 s, partial surface diffusion at the junction area produced an immature interfiber welding (Figure 1h). With Tw of 10 s, the junction was completely welded while the morphology of the other part of the nanofibers remained almost intact (Figure 1i; more TEM images are shown in Figure S2). Such distinct morphology variation of the nanofibers was clearly observed by the optical microscope (Figure S3). The nanofiber morphology reorganization could be explained by a surface diffusion process, which is well known in spherical and cylindrical nanostructures.33,34 The surface diffusion flux (Js) is defined as:33  D γΩv  J s = − S ∇ S K  kT 

(2)

where DS is the surface self-diffusion coefficient, γ is the surface tension, Ω is the atomic volume, v is the number of diffusing atoms per unit surface area, k is the Boltzmann's constant, and ∇SK is the surface curvature gradient. The surface diffusion flux is proportional to the surface curvature gradient on the basis of the fact that all other parameters are considered to be constants. This means that the surface diffusion process is governed by the surface curvature. In the surface diffusion process, the curvature-induced free energy gradient drives

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atoms away from high-curvature areas, leading to the localized geometrical rearrangement.34 As the welding process was completed (Tw~10s), the curvature difference disappeared. Subsequently, the vapor did not condense preferentially at the junctions but occurred everywhere. With the extension of the Tw to 20 s, the NFNs seemed to be flattened and sunk, as can be seen in Figure 2a. With the further extension of the Tw to 30 s, the NFNs were broken into small dots and short segments. In a typical electrospinning process, the rapid solvent evaporation and the fiber solidification lead to the nonequilibrium polymer matrix in nanofibers. The degree of chain entanglement of such metastable nanofibers is lower than the bulk polymer, giving rise to the significantly morphological change in the samples with longer welding time. These results reveal that the capillary-driven welding process is an efficient method to assemble the overlapped nanofibers into joint NFNs, which can be controlled by Tw.

Figure 2. (a) Optical images of NFNs treated with DMF vapor as a function of Tw. Scale bars: 20 µm. (b) Tapping tests of NFNs with/without welding process. Scale bars: 200 µm. (c) (Top) Photograph of welded In2O3 NFNs on 4-inch SiO2/Si wafer. (Bottom) TEM image of welded In2O3 NFNs annealed at 550 oC. Scale bar: 500 nm. (d) (Top) I-V characteristics and corresponding resistance of welded In2O3 NFNs with different Tw. (Bottom) Light emission modulation of an LED connected with In2O3 NFNs treated with various Tw.

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To demonstrate the universality of the capillary-driven welding process, chemicals other than DMF (e.g. DI water, H2O2, alcohol or acetone) were also applied (Figure S4). The polyvinyl pyrrolidone (PVP) was used as precursor polymer in the electrospinning process, the vapors of various soluble solvent (i.e., DMF, DI water and alcohol) were selected to treat NFNs. Obvious morphology changes of the NFNs were clearly observed, indicating that the vapors of DMF, DI water and alcohol could effectively tune the interfiber connections. Strong chemical vapor (i.e., H2O2) was applied to the welding process and the preferential jointing of nanofibers at interfiber junctions was also observed. On the contrary, there was negligible influence of acetone vapor on the morphologies of the nanofibers, which is mainly due to the poor solubility of PVP nanofibers in acetone. These results indicate that the soluble-solvent vapors or strong chemical vapors are capable for the nanofibers welding via capillary-driven welding process at room temperature. Meanwhile, the Tw required for the capillary-driven welding process in different chemical vapors were different. The welding capabilities of the tested chemical vapors were found in such order: DMF ≈ H2O2 ≈ water > alcohol >> acetone. The Tw can be reduced to sub-second by using higher vapor pressure, which can be easily realized by heating the chemicals in higher temperatures (Figure S5). Compared to the conventional welding techniques based on the mechanical, optical, and thermal processes, the capillary-driven welding process presented here is more efficient, much simpler and free of expensive reagents or facilities. The mechanisms discussed above are potentially applicable to the welding for other polymer-based nanofibers. To be demonstrations, the capillary-driven welding process was successfully applied to weld several polymer-based nanofibers, including polyvinyl alcohol (PVA), polyethylene oxide (PEO), polylactic acid (PLA) and PCL (Figure S6). The capillarydriven welding process is controlled by the kinetics of the capillary condensation in nanoscale rather than high-energy radiation or specific hot press.35,36 Therefore, the application of the capillary-driven welding process is not limited to the small area and the flat substrates, it can be generalized for scalable and non-planar substrates. The unique welding mechanism enables the capillary-driven welding process to avoid the shadow effect.32 As shown in Figure S7, the welding process is easy to weld NFNs over wafer-scale area, exhibiting considerable potentials in large-area applications. The high spatial uniformity of the capillary-driven welding process over wafer scale was confirmed and shown in Figure S8. It is believed that the capillary-driven welding process is a facile, universal and readily scalable approach, and thus may find a wide range of applications.

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To be applicable for the practical devices, the NFNs should meet certain mechanical requirements to ensure structural stability. As shown in Figure 2b, the mechanical properties of the NFNs with/without the welding process were evaluated qualitatively by a simple taping tests.37 A piece of Scotch tape (3M) was pressed against the film with finger pressure and pulled back, an optical microscope was used to check the adhesion properties of the nanofibers on the substrates. For the pristine NFNs without welding process, the NFNs were easily peeled off from the substrate, and only a few nanofibers were left on the substrate after taping. By contrast, for the welded NFNs, almost all the nanofibers were kept in the original positions after taping, and the strong adhesion behavior of the welded nanofibers with the substrate was confirmed. The welded NFNs exhibit the much improved mechanical properties compared to the pristine NFNs without welding, which agrees well with previous reports.27 This ensures that the welded NFNs can survive during the device manufacturing process. To obtain high-quality metal-oxide nanofibers, the composite nanofibers with metal ions (e.g., In3+/PVP nanofibers) were calcined to remove the polymer components. In the thermogravimetry analysis (TGA), no apparent weight loss was observed at temperatures above ~520 °C, implying a complete conversion from composite nanofibers into metal oxide nanofibers. Therefore, the annealing temperature of 550 oC was chosen in this study (Figure S9). The welded connections remained during the transformation from composite nanofibers to In2O3 nanofibers (Figure 2c; more TEM images are shown in Figure S10 and Figure S11). Due to the overlapping of the individual nanofibers, the welded In2O3 NFNs exhibit threedimensional (3D) network architecture (Figure S12). As shown in Figure 2d, the sheet resistances of the welded In2O3 NFNs were examined as a function of Tw. The I-V characteristics of the In2O3 NFNs exhibited perfect linear behavior with Tw, implying excellent ohmic transport in the welded In2O3 NFNs. With increasing Tw, the sheet resistance of the In2O3 NFNs was decreased from 180 Ω/sq (pristine) to 33 Ω/sq (Tw~10 s). The resistance decrease of the welded In2O3 NFNs is due to the elimination of the contact resistance among the interfiber junctions. The sheet resistance of the welded In2O3 NFNs reported here is comparable to those metal nanofibers webs38, metal nanotrough network39, and tin-doped In2O3 (ITO) transparent nanofibers electrodes40, exhibiting great potentials in high performance transparent conducting electrodes. The further enhancement of the conductivity could be achieved by the optimization of the nanofibers density or post annealing in hydrogen atmosphere. Figure 2d (Bottom) demonstrates the light emission modulation ability of an LED connected with In2O3 NFNs welded with various Tw. It can be

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clearly observed that the light intensity of the LED can be modulated by changing welding time Tw of the In2O3 NFNs.

Figure 3. Electrical performances of the welded InHfO NFNs FETs. (a) Bottom-gate topcontact FET architecture in this study. (b) Transfer curves and (c) corresponding electrical parameters of the InHfO NFNs FETs with various Hf doping concentrations. (d) Output curves of the InHfO NFNs FETs with various Hf doping concentrations. Three dimensional histogram of field-effect mobility of InHfO NFNs FETs (5×5 array) (e) without and (f) with welding process. (g) XRD patterns of InHfO NFNs as a function of Hf doping concentration. (h) (222) diffraction peaks of InHfO NFNs. (i) Average grain size of InHfO NFNs with various Hf doping concentrations. For practical device applications, the Hf doped In2O3 (InHfO) NFNs were welded by the capillary-driven welding process and were integrated into FETs as semiconducting channel layers (Figure 3a and Figure S13). The optimized InHfO nanofibers density of ~2 µm-1 was used in this study (Figure S14). The nanofibers density (µm-1) was calculated from the

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number of fibers bridging the source and drain electrodes divided by the width of electrodes. The transfer curves and the corresponding electrical parameters of the welded InHfO NFNs FETs were shown in Figures 3b and 3c (more transfer curves can be found in Figure S15). With the increase of the Hf doping concentrations from 0 to 15 mol%, the threshold voltage (VTH) was shifted from -5 to 21 V and the field-effect mobility (µFE) was decreased from 4.5 to 2.3 cm2V-1s-1. For the InHfO NFNs with 5 mol% Hf doping concentration, the FET exhibited optimized device performance, including a high on current (Ion) of 180 µA, a large on/off current ratio (Ion/Ioff) of ~108 and a reasonable µFE of 4.4 cm2V-1s-1. All the InHfO NFNs FETs were operated in enhancement modes with VTH ˃ 0 V, avoiding the high power consumption and the complicated integration design.41,42 The output curves of the InHfO NFNs FETs with various Hf doping concentrations are shown in Figure 3d. It is found that the FETs based on InHfO NFNs exhibit n-type characteristics with clear pinch-off and currentsaturation behaviors. Meanwhile, the electrical performances of the InHfO NFNs FETs (5×5 array) with/without welding processes were compared (Figures 3e and 3f). For the InHfO NFNs FETs without welding process, the poor electrical performance and the inferior device uniformity (0.62±0.48 cm2V-1s-1) were observed. The poor device performance is mainly due to the stacking structure together with the inferior interfiber connections in the non-welded InHfO NFNs (Figure S16). For comparison, the InHfO NFNs FETs with welding process exhibited enhanced electrical performances and the improved device uniformity with µFE of 4.49±0.26 cm2V-1s-1. In previous reports, the MO nanofiber-based FETs usually exhibited poor electrical performance (e.g., low µFE and the low Ion/Ioff), which are far behind the performance requirements requested for real applications (Table S1). The unsatisfactory performance is mainly due to its poor interfacial adhesion properties between the nanofibers and the gate insulator as well as the high contact resistance between the individual nanofibers. Apparently, the improved interfiber connections and the interface adhesion to the substrate induced by capillary-driven welding process significantly enhance the overall electrical performance of the InHfO NFNs FETs. X-ray diffraction (XRD) was used to clarify the crystallinity of the InHfO nanofibers. As shown in Figure 3g, the XRD patterns demonstrated the formation of cubic bixbyite In2O3. No secondary or impurity phases were observed in the XRD patterns even with high Hf doping concentration of 15 mol%. With the increase of the Hf doping concentration, the diffraction peaks of the InHfO NFNs were gradually broadened and lowered. The average grain sizes (D),

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evaluated by the Scherrer formula, were calculated to be 18.7±2.5, 15.6±1.5, 13.8±1.2, and 10.9±1.8 nm for InHfO nanofibers with Hf doping concentration of 0, 5, 10, and 15 mol%, respectively. Meanwhile, the peak position was gradually shifted to higher diffraction angles with increasing doping concentration, as shown in Figure 3h. This is mainly because that the smaller-radius Hf4+ ions (0.71 Å) replaced the In3+ ions (0.79 Å) of the host In2O3 lattice as substitutional dopants. As the TEM images shown in Figure 4a to 4d, the polycrystalline natures of the InHfO nanofibers were clearly observed. The h(222) interplanar spacing was measured to be 2.93, 2.93, 2.92 and 2.91 Å for the InHfO nanofibers with Hf doping concentration of 0, 5, 10, and 15 mol%, respectively. These values are close to the typical h(222) interplanar spacing of 2.92 Å for cubic In2O3. The grain size was found to be 18.1±2.7, 15.2±1.8, 12.8±1.5 and 9.7±2.0 nm for the InHfO nanofibers with Hf doping concentration of 0, 5, 10 and 15 mol%, respectively. The grain size measured by TEM was consistent with that derived from XRD measurements (Figure 3i). It is believed that the grain size distribution as well as the number of grain boundaries (GBs) in the polycrystalline metal-oxide nanofibers is highly dependent on the doping concentration. Up to now, the mechanisms of the carrier transport and the carrier trapping in the polycrystalline nanofibers have rarely been explored, particularly for those doped metal-oxide nanofibers. FETs provide a versatile platform for studying the transport properties of the polycrystalline semiconductors in a controlled manner. By using the capillary-driven welding process, the reliable electrical contacts and the reproducible device performance were achieved. This allows us to evaluate charge transport properties of the doped semiconductors across a wide range of doping concentration. The polycrystalline metal-oxide nanofibers were composed of small crystallites joined together by GBs (Figure 5a; more TEM images are shown in Figure S17). Inside each crystallite, the atoms are periodically arranged, which could be considered as a small single crystal. However, the GB exhibits complex structure, usually consisting of a few atomic layers with disordered arrangement. Since the atoms at the GB are disordered, there exist a large number of traps due to the incomplete bonding. It is reasonable to assume that a wide distribution of defect states existed in the GB. Spectroscopic analyses for the typical metal-oxide material (e.g., ZnO) revealed that the trap energy levels (E1) are located as deep states with discrete energy levels (E2).43,44 The trapping states are capable of trapping free carriers and thereby immobilizing them, reducing the number of the free carriers. Once the mobile carriers are trapped, the traps become electrically charged, creating a double Schottky potential barrier on both sides of the GB under thermal equilibrium conditions. This double Schottky potential barrier is the main obstacle to impede 12


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the motion of carriers from one crystallite to another. The relevant energy level diagram of a charged grain boundary in polycrystalline metal-oxide nanofiber is given in Figure 5b.

Figure 4. The TEM images, enlarged high-magnification images and grain size distribution of InHfO nanofibers with Hf doping concentration of (a) 0, (b) 5, (c) 10 and (d) 15 mol%. Scale bars: 20 nm. The single GB effect mentioned above could be extended to the nanofibers consisting of multiple GBs. In the pristine In2O3 nanofibers, there are insufficient GBs to act as potential barriers for blocking the carrier transport from the source to the drain electrode. Excessive electrons flow relatively free through the nanofibers and result in the conductive nanofibers. In this regard, the poor current modulation behavior will be achieved. With the increase of the Hf doping concentration, the grain size gradually decreased and more GBs are generated, leading to the rise of the thermal activation energy and thus high resistivity of polycrystalline InHfO nanofibers is expected.45 Consequently, a low off current is achieved and a positive 13



gate bias is required to induce carriers for operating the nanofibers FETs. The trapping states at the GBs are able to capture the free carriers, and a smaller µFE is achieved at higher Hf doping concentration. Therefore, the GBs play a critical role in determining the electrical performance of the polycrystalline InHfO nanofibers FETs.

Figure 5. (a) HRTEM images of InHfO nanofiber with Hf doping concentration of 5 mol%. Scale bar: 2 nm. (b) One-dimensional band diagram of a charged grain boundary in polycrystalline metal-oxide nanofiber. To improve the electrical performance of the FETs, the InHfO NFNs FETs based on solution-processed ZrOx dielectric layer (thickness~20 nm) were integrated. The dielectric properties of the ZrOx dielectric layer are shown in Figure S18, including an areal capacitance of ~250 nF/cm2 at 20 Hz and a leakage current density of 1 nA at 2 MV/cm. Figure 6a exhibits the transfer characteristics of the welded InHfO NFNs FETs based on ZrOx dielectric layer, and the corresponding performance distribution (20 devices) is shown in Figure 6b (4×5 array; all the transfer characteristics are shown in Figure S19). The InHfO NFNs FETs based on ZrOx dielectrics exhibit an average µFE of 25 cm2V-1s-1 (standard deviation of ~ 1.6 cm2Vs for 20 devices). It is worth noting that the µFE of the InHfO NFNs FETs based on ZrOx

1 -1

dielectrics is around 6 times larger than that based on SiO2 dielectrics. All the devices are operated in enhancement mode with VTH of 0.82±0.18 V and exhibit excellent current modulation ability of ~108. The operating voltage of the FETs based on ZrOx is 3 V, which is one order of magnitude smaller than that based on SiO2 dielectrics (Figure S20). Small operating voltage is important for low-power electronics applications, especially in the field of mobile electronics. The SS values of 180±34 mV/decade for InHfO/ZrOx NFNs FETs are achieved, which is acceptable compared to previously reported devices. Generally, the subthreshold swing (SS) value directly correlates with the switching speed and the power consumption of the TFT devices.16,46 The employment of the high-k ZrOx dielectric together with the high-quality InHfO NFNs channel and the electronically clean interface of InHfO

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NFNs/ZrOx result in high device performance. The InHfO NFNs FET based on ZrOx dielectric demonstrated here is proved to be one of the best devices among those reported nanofibers-based ones (summarized in Table S1). After the successful fabrication of the high-performance InHfO NFNs/ZrOx FETs, the resistor-loaded inverter based on the FET was further integrated (Figure S21).47 The typical voltage transfer characteristics (VTCs) are shown in Figure 6c, where the output voltages (Vout) are clearly transferred to the inverse with different supply voltages (VDD). The gain values of the inverters were calculated from the negative slope (defined as -dVout/dVin) in the VTCs and are shown in Figure 6d.48 The gain values as a function of VDD are shown in Figure 6e, the inverters exhibit a maximum gain value of 20 under VDD = 4 V. The acceptable voltage gain is mainly attributed to the large mobility and small SS value of the NFNs FETs. The inverters presented here further confirm that the capillary-driven welding process possesses great potency in consumption logic devices.

Figure 6. Welded InHfO NFNs FETs based on solution-processed ZrOx dielectric layers. (a) Transfer characteristics and (b) corresponding performance distribution of the welded InHfO NFNs/ZrOx FETs. (c) Voltage transfer characteristics and (d) corresponding gains of the resistor-loaded inverter based on InHfO NFNs/ZrOx FETs. (e) Maximum gain values as a function of input voltages.

CONCLUSIONS 15



In conclusion, a bio-inspired capillary-driven welding process was introduced to improve the mechanical/electrical properties of the NFNs. The synergistic effect combining the localized capillary condensation and the curvature-induced surface diffusion not only improves the interaction among the individual nanofibers, but also improves the interaction between the nanofibers and the substrate. The capillary-driven welding process was proved to be timedependent and compatible with various solvent vapors (e.g. DMF, DI water, alcohol and H2O2) as well as various polymer materials (e.g., PVA, PEO, PLA and PCL). By using the welded InHfO NFNs as the channel component, the NFNs FETs exhibited the efficient current modulation abilities. The electrical performances of the FETs based on InHfO NFNs with various Hf doping concentrations were investigated. It is found that µFE decreases and VTH increases with increasing Hf doping concentration. In this report, the FETs based on the InHfO NFN with Hf doping concentration of 5 mol% exhibited high device performance, including an Ion of 180 µA, a VTH of 3.2 V, a µFE of 4.4 cm2V-1s-1 and an Ion/Ioff of 108. The GBs were found to be the dominant factor in determining the electrical properties of the polycrystalline InHfO nanofibers, which certainly decided the device performance of the FETs. Furthermore, the welded InHfO NFNs FETs based on high-k ZrOx dielectrics were integrated and exhibited much improved performance, including a µFE of 25 cm2V-1s-1, an Ion/Ioff of ~108, an SS of 180 mV/decade, and an operating voltage of 3 V. For the inverters based on the low-voltage InHfO NFNs/ZrOx FETs, typical VTCs with a maximum gain of 20 were achieved. The capillary-driven welding process and the grain-boundary modulation mechanism offer a simple and versatile approach for fabricating the high-quality NFNs and exhibit great potency in fabricating the high-performance, large-scale and low-power electronics.

ASSOCIATED CONTENT Spatial uniformity of the capillary-driven welding process over wafer scale. Transfer curves and corresponding performance parameters of the InHfO NFNs FETs with various Hf doping concentrations. Physical properties of solution-processed ZrOx dielectrics. Output characteristics of the InHfO NFNs/ZrOx FETs. Schematic of the resistor-loaded inverter based on InHfO NFNs/ZrOx FETs.

AUTHOR INFORMATION Corresponding Authors

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

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51572135, 51672142 and 51472130). The authors thank Yang Yang at University of California, Los Angeles for the results discussion and comments on this manuscript.

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Graphical table of content

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Nature inspired capillary-driven welding process for boosting metal

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