Superior B-Doped SiC Nanowire Flexible Field Emitters: Ultra-Low

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Superior B-doped SiC Nanowire Flexible Field Emitters: Ultra-Low Turn-on Fields and Robust Stabilities against Harsh Environments Shanliang Chen, Minghui Shang, Lin Wang, Zuobao Yang, Fengmei Gao, Jinju Zheng, and Weiyou Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07921 • Publication Date (Web): 21 Sep 2017 Downloaded from http://pubs.acs.org on September 22, 2017

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ACS Applied Materials & Interfaces

Superior B-doped SiC Nanowire Flexible Field Emitters: Ultra-Low Turn-on Fields and Robust Stabilities against Harsh Environments Shanliang Chen, Minghui Shang, Lin Wang, Zuobao Yang, Fengmei Gao, Jinju Zheng and Weiyou Yang*

Institute of Material, Ningbo University of Technology, Ningbo 315016, P. R. China

Email: [email protected] (W. Yang)

KEYWORDS: SiC nanowires, B doping, flexible devices, field emitters, field emission properties

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ABSTRACT Low turn-on fields together with boosted stabilities are recognized as two key factors for pushing forward the implementations of the field emitters in electronic units. In current work, we explored superior flexible field emitters based on single-crystalline 3C-SiC nanowires, which had numbers of sharp edges as well as corners surrounding the wire body and B dopants. The as-constructed field emitters behaved exceptional field emission (FE) behaviors with ultra-low turn-on fields (Eto) of 0.94-0.68 V/µm and current emission fluctuations of ±1.0-3.4%, when subjected to harsh working conditions under different bending cycles, various bending configurations as well as elevated temperature environments. The sharp edges together with the edges were able to significantly increase the electron emission sites, and the incorporated B dopants could bring a more localized state close to the Fermi level, which rendered the SiC nanowire emitters with low Eto, large field enhancement factor as well as robust current emission stabilities. Current B-doped SiC nanowires could meet all essential requirements for an ideal flexible emitters, which exhibit their promising prospect to be applied in flexible electronic units.


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1. INTRODUCTION Field emission (FE) is termed as one of the interesting characteristics of nanostructured substances, which has been intensively investigated over the past decades, because of the significance in respect of basic science as well as in nanodevice implementations.1-9 In comparison with the traditional field emitters, the flexible counterparts, in respect of their exclusive lightweight, conformable as well as flexible nature, have aroused considerable attentions, due to their inspired potential applications in the context of roll-up FE displays,10 e-papers,11 together with x-ray tubes.12 Among them, one-dimensional (1D) SiC nanostructures are taken into consideration being an exceptional contestant for flexible field emitters, owing to their premium mechanical attributes, high thermal conductivity, as well as its excellent chemical stability.1,13-18 Encouraged through a wish to attain a perfect field emitter with excellent FE performances, four strategies were established for the improvement of the electron emission from SiC 1D nanostrcutures on the bases of the Fowler-Nordheim (F-N) theory: (i) Developing the nanostructures possessing sharp and clear tips in order to use the local field enhancement effect,13,19,20 (ii) Growing the aligned nanoarrays to limit the shielding effect,21-23 (iii) Customizing the bandgap of developed nanomaterials with the help of doping strategy,13,24,25 in addition to (iv) Augmenting the quantity of the electron emission sites.14,26,27 Accordingly, to date, most of the turn-on fields (Eto, generally defined as the applied external electric fields required to generate a fixed current density of 10 µA/cm2) of SiC 1D nanostructure often fell in just several V/µm, suggesting quite hopeful of being implemented as field emitters.13,15-18

As for increasing the electron emission sites of the SiC nanostructures, one of the effective and often used routes is the surface decoration of nanoparticles. For instance, Cui et al. proposed that the tubular SiC nanostructure decorated with Al2O3 nanoparticle showcases an apparent decrease in Eto (from 8.8 to 2.4 V/µm).28 Zhang et al. reported that amorphous carbon decorated SiC nanowires 3 ACS Paragon Plus Environment


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exhibited low Eto of 0.5 V/µm.14 Chen et al. reported that the Eto was decreased from 2.10 to 1.14 V/µm through the surface decoration of Au nanoparticles.27 However, the numerous interfaces reformed between the SiC nanostructure body and secondly decorated nanoparticles, which is the so-called “dead” layer,3 become the big obstacle to hinder the electron and heat transfer during the FE process, which inevitably cause the fluctuation in current emission.29,30 Furthermore, part of the nanoparticles also tend to be torn away by the electrostatic force due to the weak interface.31 In a brief word, regardless of the surface decoration could be effective to lower the Eto, it is likely to be a bit of contrasting to the current emission steadiness. Herein, the development of advanced SiC nanostructured emitters with both low Eto as well as high current emission stability is truly intended.

In this work, a report for the development of superior B-doped SiC nanowire flexible field emitters has been provided, which are fabricated based on the typical catalyst-assisted pyrolysis of polymeric precursor. We throw primary focus on the subsequent crucial subjects for offering the SiC field emitters featuring excellent FE performances: (i) Making the SiC nanowires with numbers of sharp edges as well as corners around the wire body, in order to boost the quantity of the emitting sites; (ii) Simultaneously incorporated B dopants into the SiC nanowires in respect of a more localized state close to the Fermi level.22,32 The FE measurements indicate that the Eto of obtained SiC nanowires ranged from 0.94 to 0.68 V/µm accompanied by the temperatures increased from room temperature (RT) to 400 °C. In more of an important manner, they exhibit extensively elevated mechanical as well as electrical steadiness with the typical current emission fluctuations constrained in ±1.0-3.4%, under the harsh working conditions of different bending cycles, various bending scenarios together with elevated temperatures, indicating their inspired potential uses.

2. EXPERIMENTAL PROCEDURE 4 ACS Paragon Plus Environment

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Synthesis of the B-doped SiC nanowires was performed by the pyrolysis of polysilazane precursor (PSN, Institute of Chemistry, Chinese Academic of Science, China) in a graphite-heater furnace (schematic demonstration presented in Figure S1, Supporting Information) that could be availed commonly, in addition to being utilized in a direct way with no additional refinement. Primary solidification of the PSN was performed through the use of heat-treatment at a temperature of 260 °C for a time period of 30 min subjected to Ar environment, followed ball-milled into refined powders. In addition to that, the carbon fabric was positioned on the top of a graphite crucible (purity: 99%). The mixed powders of 0.3 g PSN + 0.06 g B2O3 (Aladdin, Shanghai, China) were put to use in order to grow B-doped SiC nanostructures. Primary immersion of the carbon fabric substrate was done in an ethanol solution containing 0.05 mol/L Co(NO3)2 for the introduction of the catalysts, followed by air-dry in the RT. Thereafter, the crucible that contained the powders together with the substrate was shifted into the graphite-heater furnace. The furnace compartment was primarily pumped to reach 10-4 Pa, proceeding to the introduction of Ar (99.99%, 0.1 Mpa) into the compartment for the purpose of reducing O2 to an insignificant degree. Next to that, the temperature of the system was raised till 1400 °C following a rate of 30 °C/min, followed by a cool down to 1350 °C subjected to a cooling rate of 4 °C/min with a maintaining time of 10 min. In the long run, additional cooling down of the system to a temperature of 1100 °C at a rate of 4 °C/min was performed, proceeding to the cooling of the furnace to the RT. Performance of the entre pyrolysis procedure was done subjected to the argon (99.99% 0.1 Mpa) following a flowing rate of 200 sccm.

Characterization of the attained samples items was done with the use of field emission scanning electron microscopy (FESEM, S-4800, Hitachi, Japan), transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan) supplied with energy dispersive X-ray spectroscopy (EDX, Quantax-STEM, Bruker, Germany), as well as X-ray photoelectron spectroscopy (XPS, AXIS ULTRA DLD, Shimadzu, Japan) and Raman spectra (Raman, inVia, Renishaw, UK). The use of X-ray diffraction (XRD, D8 5 ACS Paragon Plus Environment


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Advance, Bruker, Germany) with Cu Ka radiation (λ=1.5406) was made for the purpose of characterizing the phase structure of the products. The test was performed under common mode with the selected angle in the range of 20°-80°. The applied voltage, current, scanning step size and pace time were fixed at 40 kV, 40 mA, 0.01° and 0.2 s, respectively. The FE properties of B-doped SiC nanowires ranged from RT to 400 °C were performed on a home-built machine with a base vacuum degree of ~1.5×10-7 Pa. The carbon fabric covered with nanowires was cut into small sheets typically sized in 0.4×0.4 cm2 (length×width). Then, the sheet was adhered onto the cathode platform by using the conductive silver paste as the binder, followed by being put into a vacuum oven under 120°C for 30 min. Subsequently, the platform together with carbon fabric was transferred into the vacuum chamber and placed below the anode platform. Recordings of the current-voltage (I-V) curves were taken with the help of a Keithley 248 unit (The test setup, look at Figure S2 in Supporting Information). The space between the surface of the selected SiC nanostructured emitters and the anode was fixed at ~600 µm, which is controlled by a micrometer. The synthesis, characterization, and FE test of the SiC nanowires were presented in Figure S3 in Supporting Information. The current emission stabilities of the emitters under different bending cycles and bending states were carried out by making the emitters into the desired conditions in advance. For instance, the selected five small sheets were firstly bent with different cycles of 0, 50, 100, 150 and 200 times, and then transferred into the vacuum chamber for the FE measurement, respectively. Furthermore, the thermal stability of the B-doped SiC nanowires was measured ranging between 50 and 800 °C (hermogravimetric analyzer, PE Pyris 1). A small piece of carbon fabric covered with SiC nanowires was selected being the standard sample. The weights of sample under different temperatures were recorded. The heating rate was fixed at 15 °C/min and the corresponding weights with an interval of 0.25 °C were collected. The whole measurement process was carried out under the nitrogen atmosphere at a flowing rate of 20 min/ml.


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3. RESULTS AND DISCUSSION Figure 1(a) and (b) depict those digital photographs of the carbon fabric substrate after the growth of SiC nanostructures via the pyrolysis process. It seems that they can be strongly bended and twisted periodically without obvious damage of the structure, which suggests their highly flexible nature. Figure 1(c-d) show the SEM images of obtained SiC nanostructures under low magnifications, showing that extensive curved wire-like nanostructures are covered homogenously on the whole carbon fabric substrate. The nanostructures are up to several hundred micrometres in length (which is showed as a highlight with the help of red color tracing a single nanowire in Figure 1(d)), which suggests a high aspect ratios in their nature. The thorough observations of those products that are as-grown under high magnifications show, in Figure 1(e-f), that the wires’ average diameter is about 540 nm. Figure 1(g-h) show the closer observations on the wire body. It seems like that there is exists a rough configuration in the nature of nanowires with numerous periodically decorated knots and necks throughout the entire wire body. Especially, each of the knots presented high-quality sharp edges and corners uniformly distributed surrounding the surface. The FE behavior can be highly improved by these particular structures because both sharp edges and corners are considered good for electron emission, due to the improvement effect of local field.1,14,28 Moreover, the catalyst droplet particle could be obviously found on the top of nanowire (as shown in Figure 1(i)), clarifying that the development of SiC nanowires is ruled by the conventional vapor-liquid-solid (VLS) mechanism,17 which is discussed in Figure S4 in Supporting Information.

Figure 2(a) is a typical TEM image of an individual wire which is under a low magnification, suggesting that the obtained nanowires are averagely diameter in ~540 nm with a rough appearance, which is constant with the SEM findings. Figure 2(b) depicts the knot of the SiC nanowire noted from the marked area of A in Figure 2(a) under a high magnification, presenting that it is composed of high 7 ACS Paragon Plus Environment


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qualified sharp edge and corner. Notably, no interface between the edge and corner can be observed, implying both of them are well-attached. Figure 2(c) provides the high-magnification TEM image recorded from the binding section between the knot and neck (from the noted area of B in Figure 2(a)), revealing that different atom arrangements existed on the two sides of such section. Their corresponding HRTEM images of the knot and neck of the nanowire (recorded from the selected areas that marked with C and D in Figure 2(c)) are depicted in Figure 2(d) and 2(e), in respective order. It suggests that the knot of the nanowire is structurally uniform without detectable defects such as stacking faults (Figure 2(d)). While for the neck, there are high-density twinned stacking faults. The interplanar spacing, which is denoted by d, of two neighbored lattice fringes is measured to be about 0.25 nm, which fit the {111} plane distance of 3C-SiC, which is the favored one of 3C-SiC to maintain the least amount of surface energy. The corresponding SAED patterns are inset in Figure 2(d-e), which respond to the knot and neck, respectively. The SAED pattern from the knot depicts just the perky spots whereas those from the neck exhibits perky spots as well as streaks, further showing that the stack faults are present in the neck part. Both the patterns have the identical orientation, proposing that the nanowire is single-crystalline SiC with a cubic arrangement grown along [111] direction, as labeled in Figure 2(a). Figure 2(f) depicts an element mapping B within the wire, which suggests its uniform spatial dispersal.

To further confirm the phase composition and structure of the obtained products, the fabricated nanowires were examined by the XRD, XPS and Raman, respectively. Figure 3(a) and 3(b) show the XRD patterns recorded from the B-doped SiC nanowires which were grown on the carbon fabric substrate. Each peak can be assigned to the diffractions from the planes of (111), (200), (220) as well as (311), which confirms that the resultant products are of only 3C-SiC phase (JCPDS Card No. 29-1129). The high intensity peak which is denoted by “C” is assigned to carbon fabric substrate. The low intensity peak denoted by “S.F.” can be accredited to the defects, such as the stacking faults and twinning structures, within the 3C-SiC nanostructures.33-35 Figure 3(b) shows a refined inspection of the 8 ACS Paragon Plus Environment

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(111) peak of the typical XRD patterns. In comparison with the standard data (JCPDS Card No. 29-1129), which is denoted by a solid line, the (111) peak of the products moves to increased angle with a 2θ of 0.142°. The increased 2θ could be ascribed to the crystalline lattice distortion induced by the substitution of Si atoms (radius: 0.134 nm) by the smaller B atoms (radius: 0.095 nm), implying that the B dopants have been combined into the 3C-SiC nanostructure though the creation of substitutional solid solutions.32 Further evidence for the dopants is provided by the XPS data, which is depicted in Figure 3(c-e). The binding energies attained in the XPS spectra are standardized for sample charging with the help of C 1s at 284.6 eV. Figure 3(c) and 3(d) respond to the XPS spectra of Si and C, respectively. The peak fixed at 100.9 is given rise to the binding energies of Si 2p of SiC (Figure 3(c)).36 Through a peak fitting procedure, as depicted in Figure 3(d), the binding energies of C 1s are confirmed to be 282.8, 284.6 and 285.3 eV, respectively. The peak at 284.6 eV is ascribed to the carbon fabric substrate and contaminated carbon, which are unavoidable for the XPS test.37,38 Generally, the peak at 282.8 eV responds to the Si-C bond.36 The binding energy corresponding to 285.3 eV implies the existence of C-O bonding,39,40 Notably, the B 1s peak at ~190.5 eV is detected (Figure 3(e)), verifying the B dopants incorporated into the SiC nanowires with a doping level of ca. ~8.47 at.%. Furthermore, the typical Raman spectrum of the B-doped SiC nanowire is recorded by using the laser with a value of wavelength i.e. 633 nm as the source of excitation, that is depicted in Figure 3(f). The two peaks at ~792 and 969 cm-1 are the dominate characteristics of the crystalline structure of 3C-SiC, which are given to the modes of transverse (TO) as well as longitudinal optical (LO) phonons, correspondingly.41-43 Compared to those of the pure SiC nanostructures,1 these two peaks of B-doped SiC nanowires display small blue shifts, which can be ascribed to the existence of lattice stresses induced by the B dopants.

The FE performance of the fabricated B-doped SiC flexible emitters are investigated systematically. The dependence of the emission current density (J) on the functional electric field (E) is depicted in Figure 4a (see Figure S2 in Supporting Information). The J-E curves are attained after 9 ACS Paragon Plus Environment


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extending the voltage many times for removal of contaminants and degassing the sample. The comparatively smooth and constant curve indicates their steady emission of electrons. The Eto is ca. ~0.94 V/µm, which can be compared to the lowest one of SiC nanostructures1,13,14,16,18,21,28,44-46 and other inorganic nanostructured emitters (e.g., carbon nanotubes (0.4-1.1 V/µm),47 ZnO nanobelts (1.3 V/µm),48 AlN nanoneedle array (1.8 V/µm),49 ZnS nanobelts (3.47 V/µm),50 and rare-earth hexaborides (LaB6 nanowires: 1.06-1.82 V/µm,51 PrB6 nanorods: 0.95-2.80 V/µm,52 CeB6 nanorods: 1.8 V/µm,53 SmB6 nanowires: 2.7-4.2 V/µm54)), which are summarized in Table 1.

Table 1. Turn-on fieldsa, field enhancement factors (β) and field emission current stability (current density, testing time and current fluctuation) for SiC nanostructured field emitters, other nanostructure flexible emitters along with other commonly used emitters.

6244 --

Current stability (current density, time, fluctuations) 1.762 mA/cm2, 20 h, ±1.0-3.7% --, --, -2.0 mA/cm2, 2 h, ＜5%

This work 27 14

2.4

--

--, --, --

28

P-doped SiC nanoparticles

0.73-1.03

5508

1

N-doped SiC nanoneedles B-doped SiC nanoneedle arrays n-type β-SiC nanoarrays Well-aligned SiC nanowires arrays B-doped 3C-SiC nanowires N-doped SiC nanoneedles N-doped nanoporous SiC N-doped SiC nanoarrays N-doped 3C-SiC nanoneedles Tapered SiC nanowires Tubular β-SiC Aligned SiC porous nanowires β-SiC nanowires β-SiC nanoarchitectures SiC nanowires/nanorods Nonaligned SiC nanowires Vertical carbon nanotubes

1.11 0.98-1.92

-3643

1.57-1.95 1.50

3217-3340 4482

1.35 0.67-1.37 4.4-9.6 1.9-2.65 ~1.1 1.2 5 2.3-2.9 -12 3.33 3.1-3.5 0.4-1.1

4895 2486 936-3636 1710 6500 3368 -5241 2000 ---900014500 --

2.65 mA/cm2, 20 h, ±2.1-3.4% 1.138 mA/cm2, 1 h, 8.1% 0.5129 mA/cm2, 8 h, 6.5-7.8% --, --, -0.6933 mA/cm2, 4 h, ＜ 3.8% --, 10 h, 11-14% --, 1 h, 7.7%-14.1% --, --, ---, --, -1.7 mA/cm2, 0.5 h, ---, --, ---, --, -0.57 mA/cm2, 20 h, ---, --, ---, --, ---, --, -0.06 mA/cm2, 2 h, ±15% ~0.5 mA/cm2, 158 h, --

32 15 44 17 16 18 55 56 57 58 59 60 47

0.5 mA/cm2, 20 h, --

61

SiC Field emitters B-doped SiC nanowires SiC nanostructure emitters with sharp rough surface

Other typical SiC nanostructure emitters

Au-decorated SiC nanowires Carbon decorated SiC nanowires Al2O3-decorated tubular SiC

Carbon nanotubes arrays

Eto (V/µm)

β

0.68-0.94

5464

1.14 0.5

-10

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Ref.

13 22 23 21

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Carbon nanotubes Multiwall carbon nanotubes

Other typical inorganic nanostructure emitters

Aligned ZnO nanobelts Ultrathin ZnO nanobelts Vertical ZnO nanowires/graphene Single-layer graphene films Graphene nanosheets Si-doped AlN nanoneedle array Aligned AlN nanorods AlN nanorod arrays Ultrafine ZnS nanobelts ZnS nanobelts arrays Single-crystalline PrB6 nanorods Single-crystalline LaB6 nanowires CeB6 nanorods

3.6 2.05 (1 µA/cm2) 1.3 -2.0-2.8

1112-1546 1023, 16434

--, --, ---, 10 h, --

62 10

14000 700 3834-6473

--, --, -7.4 mA/cm2, 16 h, ~14% --, --, --

48 63 64

2.3 2.04 1.8 3.8 4.7 3.47 3.8 0.95-2.8

3700 -3271 950 1175.5-1888.7 2000 1839 823-1390

65 66 49 67 68 50 69 52

1.06-1.82

1072

1.8

1035-3863

11.46 mA/cm2, 12 h, 4% ~0.7 mA/cm2, 12 h, -10 mA/cm2, 5 h, ＜5% --,--,---, 4 h, 0.74% --,--,---,--,-0.025 mA/cm2, ~17 h, ＜10% ~0.5 mA/cm2, ~17 h, ＜ 6.0% 0.0128 mA/cm2, 3 h, 1.41-1.51% --, 500 min, 10% ~1 mAcm-2, 40 h, -~0.2 mA/cm2, 75 h, 5%

51 53

SmB6 nanowires 2.7-4.2 2207-4741 54 SnO2 nanowires 3.5 1225 70 Single-crystalline CdS 3.7 1298 71 nanobelts Tungsten oxide nanowires -1657 5.25 mA/cm2, 1 h, ~5% 8 Single-crystalline GaN 2.5 -0.003 mA/cm2, 1 h, ＜ 72 nanocolumns 7.4% Oriented CuO nanoknife arrays 0.9 2400-5400 1.15 mA/cm2, ~1 h, ~5% 73 a 2 Eto required generating an emission current density of 10 µA/cm . If other values are used, it is separately mentioned.

The developed field emission J-E data behaviors are additionally examined based on the Fowler-Nordheim (F-N) equation:74 J = (ηAβ 2 E 2 / Φ ) ⋅ exp(− BΦ 3 / 2 / β E ) Where A = 1.54×10-6 AeV/V2, B = 6.83×103 e/V3/2 V/µm1, E is the applied electric field, Φ is refer to the work function of the emitter, and β is the field enhancement factor. The F-N plot attained by plotting 1/E versus ln(J/E2) is showed in Figure 4b. The linear relationships of F-N plots show that the emitted electron from the SiC nanowires obeys the conventional FE mechanism. Through consideration of the work function of 4.0 eV for SiC under RT,75 the field enhancement factor β is computed to be of ~4929 by considering the slope of F-N plots. The attractive β can be compared to the finest value of the SiC nanostructures as well as other representatively inorganic system emitters (see Table 1).



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The superior FE performance of the novel SiC nanowires with ultra-low Eto and large β could be mostly accredited to the subsequent points: (i) The large number of sharp edges as well as corners all over the surface of the SiC nanowires. Owing to local enhanced field effect, the sharp edges and corners can be employed as the efficient electron emission sites (as depicted schematically in Figure 4(c)), which bring the profound increase of the densities of electron emission sites, which leads to the significant improvement of the FE properties with a low Eto. Furthermore, the β value is related to the emitter configuration as well as vacuum gaps, which is a vital reason of merit for the emission of electrons. In case of a flat surface emitter, the β value is 1, that improves for any emitter having a tip-like structure.76 In current case, in respect to the impressive feature of the as-synthesized SiC nanowire with high aspect ratios, together with the quite many sharp corners existed around the SiC nanowires surface, the obtained field emitters presented large β values. (ii) The incorporated B dopants. The structures of electronic band of the intrinsic as well as B-doped SiC are computed (see the details in Supporting Information), as shown in Figure 4(d and e), on the basis of the VASP code77 and density functional theory (DFT) with the exchange-correlation functional of Perdew, Burke, and Ernzerhof revised for solids (PBEsol).78 It looks like, in comparison with the intrinsic counterpart, a more localized state close to the Fermi level would be formed caused by the B dopants, which facilitates a larger probability of electron movement from the top of valence band to the bottom of conduction band (as depicted in Figure S5 in Supporting Information), consequently leading to the creation of added electron-hole pairs for the enhanced FE performance.79

To examine the mechanical and electrical sturdiness of the flexible cathode under harsh working conditions, the FE characteristics of the B-doped SiC nanowires emitters exposed to different bending cycles and bending states were examined, respectively. Figure 5(a) and 5(b) depict the current emission properties of flexible emitters after the given bending cycles of 0, 50, 100, 150 and 200 times along with a bending radio of about 1.2 cm and their corresponding F-N plots, respectively. The attained J-E curves 12 ACS Paragon Plus Environment

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are comparatively smooth and constant, and the differences of Eto are almost insignificant (Figure 5(c)), suggesting the highly electrical constancy of the attained SiC emitters. The linear relationships of the F-N plots show that the electron emission afterwards bending again follows the conventional FE mechanism. Notably, associated with the SiC emitters in Figure 1(c-i) beforehand the repeated bending, there are almost no structures damaged once 200 bending cycles are performed (Figure S6 in Supporting Information), further representing the mechanical sturdiness of the obtained B-doped SiC nanowire emitters. The J-E curves of flexible cathode were also measured in concave, flat as well as convex states, these are depicted in Figure 5(d). The bending radii are secured at ~1.2 cm for concave and convex I configurations, and ~0.4 cm for concave and convex II counterparts, respectively. With respect to the fact that all the SiC nanowire emitters covered homogenously on the whole carbon fabric substrate could acted as effective emission sites at various bending states, the current densities estimated from the convex and concave states are based on the entire surface of the cathodes, which are similar to that of the flat states. It reveals that the Eto of flexible cathode configured in concave I, concave II, flat, convex I and convex II are typically ~1.01, 0.98, 0.94, 0.89 and 0.86 V/µm, respectively, as showed in Figure 5(c). As compared to their concave and flat geometries, a little decline of Eto of SiC nanowire emitters in convex configuration can be accredited to the weakened screening effect from the neighbored emission sites.13,64,80 Figure 5(e) gives the matching F-N plots, and all of them exhibit the linear relationships. These results as mentioned above verify that the as-constructed SiC flexible cathode is mechanically as well as electrically sturdiness.

To additionally examine the FE behavior of B-doped SiC nanowire flexible field emitters under harsh conditions, their electron emission characteristic are measured at the temperatures ranged from RT to 400 °C with an interval of 100 °C under a constant pressure of ~1.5×10-7 Pa, which are shown in Figure 6(a). All the curves have the similar behaviors: As soon as the given electric field is outside a 13 ACS Paragon Plus Environment


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specific value, the detected current density increases almost exponentially with the proliferation of electric fields. The value of Eto decreases from 0.94 to 0.68 V/µm with a raise in temperatures ranged from RT to 400 °C, as depicted in Figure 6(b), suggesting that the FE is heavily reliant on the temperatures. For instance, at an immobile electric field of E at 1.13 V/µm, the J increased significantly from 0.05 to 2.47 mA/cm2 upon the temperature increase from RT to 400 °C that has value two-order-of-magnitudes increased than that at RT, as shown in Figure 6(b). The slopes of all F-N plots present an almost linear relationship (Figure 6(c)), which suggests that the FE of the nanowires from RT to 400 °C follows the F-N rule, and the FE input from the thermionic electron emission can be ignored. With respect to no important effect of the temperatures below 400 °C on the crystal structure of SiC,20,81,82 the β could be recognized as a constant, irrespective of the temperature disparities in current case. By outlining the F-N plots and noting the wok function of 4.0 eV for SiC at RT,75 the calculation of Φ can be done by Equation

k slope = − BΦ 3 / 2 / β It is revealed that the Φ shows a decline from 4.0 to 3.31 eV with the increase in temperature ranged from RT to 400 °C (Figure 6(d)). The reduced Φ is accountable for the experimental thermo-enhanced FE behaviors. The mechanism could be mainly attributed to the increment of the electron-hole pairs induced by the increase in the temperature levels.15,83 The electrons present at the top of valence band can start to move towards the bottom of conduction band with an increased energy as well as larger chance prompted by the increment in temperature levels, which results in the creation of more electron-hole pairs. The additional electrons could take part in the current transference during the FE process, which resultantly leads to the declined resistance as well as enhanced FE properties.79 Moreover, it looks as if the loss in weight can be ignored which ranges from 50 to 800 °C (Figure S7 in Supporting Information), proving the increased thermal steadiness of our flexible field emitters. These results demonstrate that the attained B-doped SiC nanowires can be an excellent candidate for 14 ACS Paragon Plus Environment

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nanostructured emitters with the capacity to be well serviced in the high-temperature harsh conditions.

As compared to the bulk materials, the FE properties of the one-dimensional (1D) are often significantly enhanced in 1D nanostructures because of their character featuring elevated aspect ratios. However, their larger surface area, smaller mass, larger scale bending as well as lower melting point would turn them be susceptible to the risk of the chemical and physical infringements as well as the structure destruction.84 Thereby, the current emission stability of the field emitters poses to be one other quintessential parameter associated with practical uses.4,70 As presented in Figure 7(a), all through 20 h of continued measurement at a current density of about 1.762 mA/cm2 subjected to RT, the current fluctuation of nanowire amounted to be merely ±2.8% without any hint of current density degradation. Comparison of this kind of a low fluctuation can be made with the best one of SiC nanostructured field emitters and other typical semiconductor emitters ever reported (see Table 1). Meanwhile, no any structure damages associated with the emitters, subsequent to the long work duration of 20 h, as presented in Figure 7(b) and Figure S8 in Supporting Information. Figure 7(c) shows the current density stabilities subsequent to bending cycles of 0, 50, 100, 150 as well as 200 times using a bending radio of about 1.2 cm over 4 h in ~1.762 mA/cm2. No degradation can be observed for different cycles, and the current fluctuations are ~±2.8, ±2.6, ±3.0, ±2.7, and ±2.8 %, respectively, suggesting the periodic bending does not exert any considerable impact on the current emission stabilities. Figure 7(d) shed light on the current emissions stabilities of the obtained nanowires under different bending states over 4 h. Observation cannot be made for any degradation in current densities in respect of every state. Moreover, the current emission fluctuations amount to be about ±2.9, ±2.6, ±2.8, ±2.7, as well as ±2.8 %, corresponding to the emitters adjusted in the flat condition, concave bending having the radius of ~0.4 and 1.2 cm, convex bending having the radius amounting to be 0.4 and 1.2 cm, correspondingly, suggesting the pressures of the substrate resulted by the bending, don’t make any profound impact on the current emissions stabilities as well. The findings make strong verifications that our B-doped 15 ACS Paragon Plus Environment


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nanowire flexible emitters are fully strong showing exceptional mechanically as well as electrically sturdiness.

Currently, we aim at figuring out the stabilities of B-doped SiC nanowires under harsh environment of high temperatures. Figure 7(e) brings forth the current emission stabilities of the nanowire emitters subjected to a given current density of ~1.762 mA/cm2 in the range of RT-400 °C. No degradation in the current density exists subjected to the work temperature at RT, 100, 200, 300, and 400 °C, moreover, their respective current emission fluctuations amount to be about ±2.8, ±2.9, ±2.8, ±3.0 and ±3.1 %, correspondingly. The low as well as slightly changed current fluctuations (i.e., ~±0.3 %) in the range of RT and 400 °C sturdily imply that the current B-doped SiC nanowires were able to be extensively steady field emitters with the capability of being well serviced subjected to the stern condition of elevated temperatures. Meanwhile, the nanowires also exhibit the best comprehensive electron emission stabilities among those of other SiC 1D nanostructures ever reported under high temperature (seen Table 1). For examples, Chen et al. brought forth the report that the current emission fluctuation of N-doped SiC nanoneedles is ~14.1% at 200 °C.15 Yang et al. confirmed that the current emission fluctuations of pure as well as B-doped SiC nanowires at J≈0.09 mA/cm2 are ~22 % and ~11% at RT as well as 200 °C, correspondingly.32 Wang et al. demonstrated that the current fluctuation of B-doped SiC nanoarrays at 0.5129 mA/cm2 is 7.8 %.22 Figure 7(f) throws light on the change in emission current density of nanowires at a fixed E =1.13 V/µm under different temperatures. The current density are taken into observation for fluctuating merely ±1.0, ±1.7, ±2.2, ±3.0, and ±3.4 % at RT, 100 °C, 200 °C, 300 °C and 400 °C, respectively. The excellent current emission stabilities as mentioned above represent that our B-doped SiC nanowires could meet all essential requirements for an ideal flexible emitters, which are quite hopeful of being put to application in field-emission-based flexible electronics, particularly in respect of those served subjected to harsh working situations.


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The failure of emitters during the FE process is primarily considerable being a compound effect of two important impacts: the Joule heating together with electrostatic force.29,31,85 The Joule heating is likely to lead to the emitter primarily dilapidated at the topmost temperature, proceeding to component collapse from the 1D emitter’s tip.86 In the meantime, the electrostatic force on the emitter is expected to produce the Maxwell stress that is able to additionally segregated into axial stress as well as radial stress.86 The radial stress is likely to lead to the emitter’s tip to be split.87,88 On the other hand, the axial stress is expected to be breaking, together with tearing away the nanostructured emitter from the substrate.30,31 In this work, the elevated electron emission stabilities of the B-doped SiC nanowire emitters is able to be primarily ascribed to the points hereunder: (i) The extensively steady morphology of the high-qualified sharp edges and corners emitters. Compared with the 1D nanostructured emitters, the sharp edges as well as corners feature low aspect ratios (Figure 1(g and h)) that makes sure the emitters for being strong against the tip collapse resulted by the Joule heat impact, in addition to splitting stimulated by the radial stress of electrostatic force impact;1,85,89 (ii) The strong connection between the emitter tips and nanowires body. The emitter tips present solid connection on the nanowire body without any interfaces (both of the body and the dips are indeed single-crystalline (Figure 2(b) and (d)), which is capable of guaranteeing the tips are not to be deteriorated and/or tear away by the axial stress, leading to small changes of emission sites.90 Furthermore, the well connection between the emission tips and the wire body with a whole single crystal is capable of facilitating the shift of the Joule heat from the emission sites to the body. In the same way the tips are able to be proficiently safeguarded against the destruction caused by the superheat, resulting into robust thermal stabilities of the emitters; (iii) The incorporated B dopants. The existed B dopants are able to bring forth steadier B-C atom pairs in comparison with Si-C classes in interstitial places, resulting into an improved solubility as well as enhanced heat spread, together with electrical conductivity.22,32 In a brief word, the desired sharp edges and corners with an ideal connection to the wire body by forming a single crystal as well as the incorporated B dopants bring a robust current emission stabilities synergistically to the SiC nanowire 17 ACS Paragon Plus Environment


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flexible field emitters.

4. CONCLUSIONS In summary, we report the superior B-doped SiC nanowire flexible field emitters. The SiC nanowires possess numbers of sharp edges as well as corners surrounding the wire body with ~8.47 at.% doping level, which are synthesized via pyrolysis of polymeric precursors assisted by the carbon fabric substrates. The as-constructed field emitters exhibit ultra-low Eto of 0.94-0.68 V/µm with the temperatures elevated from RT to 400 °C. Moreover, their current emission fluctuations are typically of ±1.0-3.4%, when subjected to the harsh working conditions of different bending circles, various bending states and high temperature environments. The sharp edges together with the corners are capable of significantly increasing the electron emission sites, and the incorporated B dopants could bring a more localized state close to the Fermi level, which synergistically cause the SiC nanowire emitters with excellent FE performances such as low Eto, large field enhancement factor and robust current emission stabilities. Current work represents that our B-doped SiC nanowires with desired rough morphology and B dopants could meet all essential requirements for exploring superior field-emission-based flexible electronic units, particularly in respect to those serviced under high-temperature working environments.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (W. Yang). Tel: +86-574-87080966, Fax: +86-574-87081221. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The work received the support from the National Natural Science Foundation of China (NSFC, Grant Nos. 51372122, 51372123, 51572133, 51702174 and 51702175), Zhejiang Provincial Natural Science 18 ACS Paragon Plus Environment

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Foundation (Grant No. LQ17E020002) and Natural Science Foundation of Ningbo Municipal Government (Grant Nos. 2016A610103 and 2017A610002).

SUPPORTING INFORMATION AVAILABLE Schematic diagram of the graphite-heater furnace (S1), Schematic diagram for the FE measurements (S2), Schematic diagram of the synthesis, characterization and FE test of B-doped SiC nanowires (S3), Proposed mechanism for the growth of B-doped SiC nanowires (S4), Schematic illustration of energy band structures of intrinsic and B-doped 3C-SiC (S5), typical SEM images of SiC flexible emitters after bending for 200 cycles (S6), thermal stability of B-doped SiC nanowires (S7), and representative SEM images of SiC flexible emitters after working for 20 h (S8). The Supporting Information is available free of charge via the internet at http://pubs.acs.org.



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FIGURES

Figure 1. (a, b) Digital photos disclosing the highly flexibility of obtained SiC nanowire field emitters. (c-h) Representative SEM images of the nanowires under different magnifications. (i) A typical SEM image of the wire tip.


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Figure 2. (a) Typical TEM image of a single B-doped SiC nanowires under low magnification. (b) A typical TEM image of the sharp edge and corner around the SiC nanowire body recorded from the marked area of A in (a). (c) Representative TEM image of the transition section of the stem and knot recorded from the marked area of B in (a). (d-e) The HRTEM images recorded from the marked area of C and D in (c). (f) A typical element mapping of the B dopants within the wire.



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Figure 3. (a) A typical XRD pattern recorded from the B-doped SiC nanowires fabricated on the carbon fabric substrate. (b) The enlarged XRD pattern. (c-e) the typical binding energy spectra of Si 2p, C 1s and B 1s recorded from the as-synthesized nanowires, respectively. (f) The Raman scattering spectrum of the B-doped SiC nanowires.


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Figure 4. (a-b) Typical J-E curve of the B-doped SiC nanowires and the F-N plot, respectively. (c) Schematic illustration of the electron emission from the B-doped SiC nanowire with sharp edges and corners. (d-e) The calculated energy band structures of intrinsic as well as B-doped 3C-SiC, respectively.



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Figure 5. (a-b) The J-E curves of SiC nanowires after different bending cycles and their F-N plots, respectively. (c) The variations of Eto under different bending cycles and different bending states. (d-e) The J-E curves of SiC nanowires under different bending configurations and their F-N plots, respectively.


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Figure 6. (a) Typical J-E curves of B-doped SiC nanowires under various temperatures. (b) The variations of Eto and emission current densities at 1.13 V/µm with the variations of the temperatures. (c) The corresponding F-N plots. (d) The variation of work functions with the change of the temperatures.



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Figure 7. (a) The current stability of B-doped SiC nanowires over 20 h at a current density of ~1.762 mA/cm2. (b) Representative SEM images of B-doped SiC nanowires before and after 20 h FE measurement. (c) The current stabilities of SiC flexible emitters after different bending cycles. (d) The current stabilities of SiC flexible emitters under different bending geometries (Concave I and II: in concave configurations with radii of ~0.4 and 1.2 cm, respectively; Convex I and II: in convex configurations with radii of ~0.4 and 1.2 cm, respectively). (e) The current stabilities of SiC flexible emitters under various temperatures. (f) The current stabilities of SiC flexible field emitters at 1.13 V/µm with the change of temperatures.


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Superior B-doped SiC nanowire flexible field emitters were explored, which had ultra-low turn-on fields of 0.94-0.68 V/µm and high current emission stabilities of ±1.0-3.4%, when subjected to harsh working conditions under different bending cycles, various bending states and high temperature evironments.


Superior B-Doped SiC Nanowire Flexible Field Emitters: Ultra-Low

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