Radial AlN nanotips on carbon fibers as flexible electron emitters

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Radial AlN nanotips on carbon fibers as flexible electron emitters Fei Chen a b c

a,b,c

, Xiaohong Ji

a,* ,

Qinyuan Zhang

a,b,c

School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510641, China Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510641, China

A R T I C L E I N F O

A B S T R A C T

Article history:

A facile catalyst-free approach using a simple thermal transport method has been devel-

Received 25 April 2014

oped to fabricate high-density AlN nanotips on flexible carbon cloth at large scales for

Accepted 12 September 2014

use as field emission (FE) emitters. The AlN nanotips exhibit good performance as flexible

Available online 19 September 2014

cold-cathode electron emitters, with a very low turn-on electric field of 1.1–2.3 V lm1, a low threshold electric field of 1.5–2.5 V lm1, and a high emission current density. The excellent field emission properties of the AlN nanotips are attributed to the large field enhancement factor of 6895 as well as the combined effect of the tip profile of the AlN nanostructures and the excellent electron transport path of the conductive carbon cloth substrate.  2014 Elsevier Ltd. All rights reserved.

1.

Introduction

High-quality field emitters are desirable for a wide range of field-emission-based devices, such as flat-panel displays [1], X-ray sources [2], microwave-generation devices [3], and high-efficiency lamps [4]. Recently, several strategies, including doping [5], surface decoration [6], patterned growth [7], and the use of conductive substrates [8], have been employed to improve the performance of field emitters. Aluminum nitride (AlN), one of the most important wide-band-gap semiconductors, is highly attractive because of its unique properties, such as its outstanding thermal conductivity, excellent chemical stability, low work function (3.7 eV), and very low electron affinity (60.6 eV). Over the past few years, the morphological design and field emission (FE) properties of AlN nanostructures, especially one-dimensional (1D) nanostructures, have been investigated in depth [9–10] to explore their potential application as promising cold-cathode materials in * Corresponding author: Fax: +86 20 97110149. E-mail address: [email protected] (X. Ji). http://dx.doi.org/10.1016/j.carbon.2014.09.037 0008-6223/ 2014 Elsevier Ltd. All rights reserved.

FE devices. However, the threshold electric fields of most of the reported AlN nanostructures are 10–20 V lm1, which may be insufficient for practical FE-based applications [11]. (In general, the turn-on electric field (Eto) and threshold electric field (Ethr) are defined as the electric fields (E) required to produce current densities of 10 lA cm2 and 1 mA cm2, respectively.) To date, much effort has been devoted to improving the FE properties of AlN nanostructures. Tang et al. have achieved FE current densities as high as 10 mA cm2 at 4.6 V lm1 on Si-doped AlN nanoneedle arrays [5]. Qian et al. have observed enhanced FE from CsI–AlN hybrid nanostructures [6]. Patterned AlN nanocones with an Ethr value of 11.2 V lm1 lower than that of unpatterned AlN nanocones at 1 mA cm2 have been reported by Liu et al. [7]. Considering the many innovative ideas developed to date and the various factors that influence FE behavior, several routes offer the potential to further enhance FE performance. For example, cone-shaped emitters, a good electron transport

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path, and a suitable density of emitters with a rational distribution can guarantee a large available emission area and greatly decrease the screening effect. However, assembling such cold-cathode materials using bottom-up methods remains a formidable challenge. Herein, by employing an effective design for growing 1D AlN nanostructures on flexible carbon cloth through simple control over the gas flow, we realized the large-scale catalyst-free self-assembly of tapered nanowires, nanoflowers, and nanourchins. All of these nanostructures exhibited excellent FE ability, with Eto values of 1.1–2.3 V lm1 and Ethr values of 1.5–2.5 V lm1.

2.

Experimental details

In this work, AlN nanostructures were grown on carbon cloth using a catalyst-free chemical vapor deposition method based on the vapor–solid mechanism in a horizontal multi-temperature-zone tube furnace. AlCl3 powder and NH3 were used as the Al and N sources, respectively. Carbon cloth (1 cm · 4 cm) was placed in a small quartz tube located 15 cm from the source material. The carbon cloth and AlCl3 were each located at the center of a different temperature zone. After evacuating and purging the tube with a constant Ar flow of 500 standard cubic centimeters per minute (sccm), the temperature zones for the AlCl3 and carbon cloth in the furnace were heated to 280 C at a rate of 5 C/min and to 850 C at a rate of 15 C/ min, respectively. The furnace was maintained at these temperatures for 120 min under a mixed gas flow of Ar and NH3. AlN nanowires, nanoflowers, and nanourchins (designated samples A, B, and C, respectively) were obtained by adjusting the Ar/NH3 flow rates to 100 sccm/30 sccm, 150 sccm/30 sccm, and 175 sccm/30 sccm, respectively. AlN nanostructures were also grown on a Si(100) substrate and a graphite sheet for comparison. The morphologies, compositions, and crystal structures of the as-prepared products were characterized using fieldemission scanning electron microscopy (FESEM, FEI Nova NanoSEM 430), X-ray energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM, FEI Tecnai G2 F30), and X-ray diffraction analysis (XRD, PANalytical X’pert PRO with a standard Cu-Ka radiation source, k = 0.15418 nm). The conductance was measured using a homemade probe table with two polished Cu plates as the electrodes. The current–voltage characteristics of the AlN nanostructures grown on the carbon cloth under different bending curvatures were measured by sweeping the voltage from 1.0 to 1.0 V using a Keithley 2400 source meter. The conductance of the AlN nanostructures was also determined by measuring the I–V characteristics using a carbon cloth-AlN-Cu plate sandwich structure. The FE measurements were performed using a two-parallel-plate configuration in a vacuum chamber with a base pressure of 3.0 · 105 Pa. Indium tin oxide (ITO) glass was used as the anode. The distance between the anode and the sample surface was maintained at 400 lm using a Teflon-film spacer. High voltage was supplied by a power source (Keithley 248), and the current was collected by an electrometer (Keithley 6485) with picoampere sensitivity.

3.

125

Results and discussion

Fig. 1a presents optical images of the carbon cloth before and after AlN nanowires were grown on its surface. The color of the carbon cloth changed from glossy black to gray–black after the growth of the AlN nanowires. The carbon fibers in the cloth were approximately 7 lm in diameter, as shown in the FESEM image in Fig. 1b. After the CVD process, uniform AlN nanostructures covered the entire surface of the carbon fibers (Fig. 1c). The inset in Fig. 1c presents an optical image of the bent sample, which demonstrates the flexibility of the AlN nanostructures grown on the carbon cloth. Fig. 1d–f shows representative FESEM images of the AlN nanowires (sample A), nanoflowers (sample B), and nanourchins (sample C) obtained in this work. The insets in Fig. 1d–f present schematic diagrams of the nanowires, nanoflowers, and nanourchins. The tip end of sample A was several nanometers in diameter, and the length of this sample was 300–500 nm. The diameter of the nanoflowers in sample B was approximately 0.8–1.2 lm. Each AlN nanoflower consisted of hundreds of tapered nanowires with sharp tips similar in size to those of sample A. In comparison, the diameter of the nanourchins in sample C, approximately 1.2–1.5 lm, was greater than the diameter of the nanoflowers. Individually, the AlN nanourchins were composed of more tapered nanowires than the nanoflowers. The densities of the nanowires in samples A, B, and C were estimated to be 2.8 · 108 cm2, 6.5 · 107 cm2, and 5.5 · 107 cm2, respectively.

Fig. 1 – (a) Digital photographs of the carbon cloth before and after deposition. (b, c) FESEM images of the carbon cloth before and after the CVD process, where the inset in (c) is the bent carbon cloth with AlN nanostructures. FESEM images of the as-synthesized AlN nanostructures: (d) AlN nanowires, (e) AlN nanoflowers, and (f) AlN nanourchins. The insets in (d)–(f) are the corresponding schematic diagrams of the nanostructures. (A color version of this figure can be viewed online.)

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Fig. 2 – (a) XRD pattern and (b) EDS spectrum of the AlN nanourchins grown on the carbon cloth. The insets in (b) are the FESEM image and the corresponding EDS elemental mapping images. (A color version of this figure can be viewed online.)

Fig. 2a and b presents the XRD pattern and EDS spectrum of sample C, respectively. With the exception of the diffraction peaks arising from the carbon cloth (marked with an asterisk), all the other diffraction peaks could be indexed to the hexagonal wurtzite AlN structure with lattice constants ˚ and c = 4.986 A ˚ (JCPDS Card No. 08-0262). As of a = 3.114 A indicated by the EDS spectrum (Fig. 2b), the AlN nanourchins consisted primarily of Al and N elements. The atomic ratio of Al/N was approximately 1/0.94. The inset in Fig. 2b presents the EDS elemental mapping images, which reveal a uniform distribution of Al and N in the AlN nanourchins. Fig. 3 shows representative TEM results for the AlN nanowires from sample C. This figure clearly shows that each individual AlN nanowire exhibited a smooth surface with a

Fig. 3 – (a) Typical TEM image of the AlN nanowires. (b) Highmagnification TEM image, where the inset in (b) is the corresponding SAED pattern. (c) EDS pattern of a single AlN nanowire. (e, f) Two-dimensional elemental mapping images of Al and N corresponding to the AlN nanowire shown in (d), respectively. (A color version of this figure can be viewed online.)

tapered profile. The tip diameter of the nanowire was 10 nm, as shown in Fig. 3a. A high-resolution TEM (HRTEM) image of an individual AlN nanowire and the corresponding selected area electron diffraction (SAED) pattern are shown in Fig. 3b, which clearly shows that the AlN nanowire was perfectly single-crystalline. The spacing of 0.25 nm between two adjacent lattice fringes was consistent with the (0 0 0 2) planes of hexagonal AlN, indicating that the AlN nanowires grew along the [0 0 0 1] direction. The corresponding EDS spectrum recorded during the TEM observation is shown in Fig. 3c. Fig. 3d–f presents elemental mapping images of the sample, which further confirm that the Al and N elements were uniformly distributed throughout the AlN nanowire. To obtain a flexible and stable cold cathode, the substrate should be tough and highly conductive. Therefore, the electrical conductivity of sample C was measured without stress and with severe stress, as shown in Fig. 4a. The linear behavior of the I–V curves indicated that the Cu plates and carbon cloth possessed good Ohmic contact. The stress clearly did not significantly affect the overall conductance of the sample, even after bending at different curvatures for a large number of bending cycles. The conductivity of the cold-cathode material plays an important role in its FE property [12]. The conductance of the carbon cloth and the carbon cloth-AlN-Cu plate sandwich configuration were thoroughly studied, as shown in Fig. 4b. Schematic diagrams and photographs of the measurement setup are shown in the insets in Fig. 4b. The resistance of the carbon cloth-AlN-Cu plate sandwich configuration was ten times greater than that of the carbon cloth. The resistance of the carbon cloth-AlN-Cu plate sandwich configuration was calculated to be 250 X, which is sufficiently low for good FE performance. Fig. 5a presents the FE current density versus electric field (J–E) plots of samples A, B, and C. The J–E curve of the carbon cloth is also shown for comparison. The Eto and corresponding Ethr values of samples A, B, and C were 2.3 and 2.5 V lm1, 1.8 and 2.3 V lm1, and 1.1 and 1.5 V lm1, respectively. The values of Eto and Ethr depended on the morphology of the AlN nanostructures. The lowest Eto and Ethr values were obtained for sample C, and these values were much lower than the majority of values reported for AlN nanostructures, as listed in Table 1. The FE current density for sample C reached 7 mA cm2 under an electric field of 2.2 V lm1. Such

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Fig. 4 – (a) Current–voltage characteristics of sample C without (blue curve) and with (red curve) bending. The upper and lower insets show images of sample C without and with bending, respectively. (b) I–V characterization of the bare carbon cloth and AlN nanostructures deposited on the carbon cloth under the carbon cloth-AlN-Cu plate sandwich configuration. The upper and lower insets present a schematic diagram and image of the corresponding heterojunction configuration, respectively. (A color version of this figure can be viewed online.)

Fig. 5 – (a) Plots of the field emission current density (J) vs. electric field (E) of samples A, B, and C, where d is 400 lm; (b) the corresponding FN plots; and (c) the field emission current stability of sample C over 180 min at room temperature. (A color version of this figure can be viewed online.)

Table 1 – Eto, Ethr, and b values of various AlN nanostructures: previously reported studies and current work. AlN nanostructure

Eto @10 lAcm2 (V lm1)

Ethr @1 mAcm2 (V lm1)

Aligned nanotips Nanoneedle arrays Nanorod array Quasi-aligned nanotips Si-doped nanoneedle array Patterned nanocones CsI-coated nanocones Nanowires Nanoflowers Nanourchins

10.8 3.1 8.8 6.0 1.8 4.8 7.0 2.3 1.8 1.1

13.6 >8 >12.5 10 @0.22 Acm2 4.6 @10 mAcm2 11.2 13.6 2.5 2.3 1.5

b 367 748 565 – 3271 1561 – 847 1061 6895

References [9] [14] [15] [10] [5] [7] [6] This work This work This work

low values of Eto and Ethr have rarely been reported, and these values were lower than those of many cold-cathode 1D nanostructures [5,13]. Fowler–Nordheim (FN) theory is generally used to verify FE behavior, as expressed in the following equation [16].

where A = 1.54 · 106 A eV V2, B = 6.83 · 103 eV3/2 V lm1, / is the work function, E is the electric field, and b is the field-enhancement factor. b is inversely proportional to the slope (k) of the linear fit determined from the FN equation:

lnðJ=E2 Þ ¼ lnðAb2 =/Þ  B/3=2 =bE

b ¼ B/3=2 =k

ð1Þ

ð2Þ

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Therefore, the value of b could be easily obtained from the slope k of the FN plot. The corresponding FN plots of the AlN nanostructures, which were obtained by plotting ln (J/E2) versus 1/E, are presented in Fig. 5b. The FN plot of sample C shows an approximately linear relationship, which was in agreement with FN theory and indicated that the electron emission occurs through a tunneling process. Note that the FN plots of samples A and B exhibited a nonlinear relationship between ln (J/E2) versus (1/E), exhibiting a downward bend with two distinct slopes. The slope in the high-field region was much smaller than that in the low-field region. Similar two-slope characteristics have also been observed for other cold-cathode materials [17,18]. One possible mechanism for the observed two-slope phenomenon is the saturation of the emission current due to an intrinsic resistance or depletion of electrons in AlN semiconductor emitters. Under a high electric field, a high current density is obtained. The above FESEM results revealed that the density of emission sites in sample C was much lower than that in samples A and B. A higher density of nanostructures increases the probability of the field screening effect because the neighboring nanostructures become much closer to each other, which may prevent the macroscopic electrical field from penetrating into the nanostructure film, thereby reducing the current density. Similar results have been observed with injector-like ZnO nanostructures and carbon nanotubes [19,20]. The local electric field (Elocal) at the tip of a single nanowire is related to b such that Elocal = bE. The b values of samples A, B, and C were estimated to be 847 (at lower field), 1061 (at lower field), and 6895, respectively. The electron emission ability of nanostructures, in terms of b, depends on such factors as the density, uniformity of emitters, radius of curvature of a single emitter, and resistive potential drop across the nanostructure body. Furthermore, the electrical properties of the substrate essentially control the electron injection across the substrate-nanostructure junction, and the micro-geometry of the substrate has a significant impact on the electron emission characteristics [21,22]. Substrates with an intrinsic cylindrical geometry may improve the FE performance due to the field distribution modification around this geometry. Therefore, the woven-like geometry of the carbon cloth substrate favors a high field enhancement factor. The FE stability of materials is of considerable importance for practical device applications. Fig. 5c shows the FE current stability of sample C over 180 min. The emission current fluctuation was as low as 4%. The slight fluctuation observed in the emission current density may be due to the absorption and desorption of molecules on the tips of the nanostructures in the vacuum chamber during electron emission [23]. The high emission current density obtained at relatively low applied fields and the good long-term stability achieved confirm that the AlN nanostructures on carbon cloth are promising candidates for practical FE devices. To further verify that the carbon cloth enhanced the FE performance of the AlN nanowires, FE measurements were also performed on AlN nanowires grown on a graphite sheet and on a Si(100) substrate. The conditions used to grow the AlN nanostructures on the graphite sheet and Si substrate were identical to those used to grow the AlN nanourchins on the carbon cloth. The corresponding surface morphology, J–E

curves, and I–V results are shown in Fig. 6. Typical FESEM images of AlN nanostructures grown on a Si(100) substrate, graphite sheet, and carbon cloth are shown in Fig. 6a–c. The morphologies, densities, and aspect ratios of the nanostructures on the Si (100) substrate and graphite sheet were clearly quite similar to those of the nanostructures on the carbon cloth. The relationship for the aspect ratio (S) for the AlN nanostructures on different substrates was SSi > SGS > SCC. However, as shown in Fig. 6d, the turn-on and threshold fields of the AlN nanostructures on carbon cloth are much lower than those on the Si and graphite substrates. To confirm the effect of the conductance of the substrates on the FE performance, the conductivity of AlN nanostructures on the Si substrate, graphite sheet, and carbon cloth using the same sandwich configuration were studied. As indicated by the I-V curves shown in Fig. 6e, all of the samples exhibited a low contact resistance between the nanostructures and substrate; the relative resistances (R) of the sandwich structure for the AlN nanostructures on different substrates were RSi > RCarbon > RGraphite. It is generally accepted that FE performance is improved by a lower resistance in the sandwich structure. However, the FE performance of the AlN nanostructures on carbon cloth was better than that of the AlN nanostructures on graphite, contradicting the relationship RCarbon > RGraphite. To investigate the divergence between the relationship RCarbon > RGraphite and the FE performances, the microstructures of the carbon cloth and graphite sheet were investigated using FESEM and a schematic atomic model. Fig. 7 shows the (a, b) top-view and (c, d) cross-section FESEM images of the graphite sheet and carbon cloth, respectively. The graphite clearly possessed a layered structure, and the graphene layer appears to be easily torn off. In contrast, the morphology of the carbon cloth was completely different. The inset in Fig. 7d presents a low-magnification cross-section FESEM image of the carbon cloth, which shows a round cross-section of a single fiber. Fig. 7c and f shows the schematic structures of the graphite sheet and carbon cloth, respectively. From the perspective of the microstructure shown in Fig. 7c, the graphite sheet is composed of isotropic graphite layers, which are a two-dimensional (2D) system with carbon atoms arranged in a hexagonal honeycomb lattice. The binding between the layers was weak as a result of the characteristics of regular van der Waals bonds. As shown in the inset of Fig. 7f, the carbon cloth was woven by carbon fibers. The carbon fibers also consisted of carbon atoms, which were found in anisotropic graphite layers (interlocked graphene sheets) and amorphous carbon, and the binding between various layers was irregular (Fig. 7f). Consequently, the conductivity of the graphite sheet was better than that of the carbon cloth. However, the carbon cloth featured a multistage structure and provided more open space for neighboring nanowires due to the cylinder-like structure of the carbon fabric and bundled carbon fiber, which not only enhanced the FE performance but also reduced the field screening effect. Therefore, the use of the woven-like carbon cloth substrate yielded better FE properties than the use of the graphite sheet or Si substrate. According to Jo et al. [24], the total field enhancement factor can be written as btotal ¼ bAlN bcarbon

ð3Þ

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Fig. 6 – Typical FESEM images of the as-synthesized AlN nanostructures on different substrates: (a) Si(100) substrate, (b) graphite sheet, and (c) carbon cloth. (d) J–E plots corresponding to AlN nanostructures grown on a graphite sheet and on a Si(100) substrate using the same experimental conditions as those for growing the structures on carbon cloth. (e) I–V characteristics of the AlN nanostructures on the three substrates. (A color version of this figure can be viewed online.)

Fig. 7 – (a, b) Top-view and cross-sectional FESEM images of the graphite sheet. (d, e) Top-view and cross-sectional FESEM images of the carbon cloth, where the inset in (e) shows a low-magnification cross-sectional FESEM image of the carbon cloth. (c, f) Schematic microstructure of the graphite sheet and carbon cloth. (A color version of this figure can be viewed online.)

Therefore, the carbon cloth is confirmed to have an indispensable role in increasing the field enhancement factor of AlN nanostructures. As shown in Fig. 5a, the bare carbon cloth showed almost negligible FE, and bcarbon can be considered the same for samples A–C. Thus, the observed variation in btotal should be due solely to the variation in the AlN nanostructures (aspect ratio and density) over bAlN. The sharp tip and low density of AlN nanowires in sample C greatly enhanced the emission of electrons because of the strong local electric field at the tip profile and decreased field screening effect. On the other hand, the excellent conductivity of the carbon cloth guaranteed an abundance of electrons, which further increased the emission current density. These observations demonstrate the strong influence of the carbon cloth with a woven geometry on the FE characteristics. It is well known that the FE current density is determined by both the density of the injected electrons from the substrate to emitters and the tunneling barrier at the emitter/

Fig. 8 – Schematic energy-band diagram of AlN nanostructures on a substrate. As shown, / is the work function of the carbon fiber, VL is the vacuum level, EF is the Fermi level, and CB and VB are the conduction band and valence band, respectively. (A color version of this figure can be viewed online.)

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vacuum surface. The injected electrons usually induce large band bending near the emitter/vacuum interface [25]. A schematic of the energy-band diagram for the electron emission from the AlN/substrate heterojunction is presented in Fig. 8. The carbon cloth-AlN junction can be considered a metal– semiconductor junction, and the interface is a Schottky barrier, which induces band bending due to the equilibrium of the Fermi level [26]. It is clear that the superior FE performance of the AlN nanostructures on the carbon cloth resulted from not only the morphology of the AlN nanostructures but also the conductive carbon fabric, which supplied an abundance of electrons. When a high external electric field is applied, the electron emission mechanism of the AlN nanostructures on carbon cloths might be considered a two-barrier mechanism [27]. In the first barrier, numerous electrons accumulate around the metal–semiconductor heterojunction, and electrons are easily injected from the Fermi level of the carbon cloth to the conduction band of AlN through a tunneling process under an applied electric field [21]. In the second barrier, the electrons are emitted by FE at the AlN and vacuum interface. The electron affinity of AlN (0.6 eV) is much lower than the work function of carbon (5 eV), which reduces the barrier and helps electrons escape from the conduction band of AlN to vacuum. This process effectively lowers the energy barrier between the carbon cloth and AlN, resulting in a high emission current density and low values of Eto and Ethr. From the perspective of conductivity, AlN nanostructures on carbon cloth and graphite sheets exhibit better FE properties than those on Si substrates. The carbon fabric is graphitic in nature, but the FE performance of AlN nanostructures on carbon cloth is better than that of AlN nanostructures on graphite sheets due to the woven-like geometry of the carbon cloth, as discussed above.

4.

Conclusions

AlN nanowires, nanoflowers, and nanourchins were successfully fabricated on carbon cloth. The AlN nanourchins exhibited the lowest turn-on and threshold electric fields of 1.1 and 1.5 V lm1, respectively. The significantly enhanced FE properties originated from the combined effect of the intrinsic tip morphology of the AlN nanostructures and the carbon cloth, which provided electrically conductive pathways for electron conduction. The results indicate that the AlN nanostructures grown on carbon cloth are promising for the future development of nano-based flexible electronics.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 51102098 and 51125005) and the Fundamental Research Funds for the Central Universities, SCUT.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2014.09.037.

R E F E R E N C E S

[1] Kim D-H, Kim C-D, Lee HR. Effects of the ion irradiation of screen-printed carbon nanotubes for use in field emission display applications. Carbon 2004;42(8):1807–12. [2] Saito Y, Uemura S. Field emission from carbon nanotubes and its application to electron sources. Carbon 2000;38(2):169–82. [3] Ma H, Pan LJ, Zhao Q, Zhao ZB, Qiu JS. Thermal conductivity of a single carbon nanocoil measured by field-emission induced thermal radiation. Carbon 2012;50(3):778–83. [4] Lee D-J, Moon S-I, Lee Y-H, Yoo J-E, Park J-H, Jang J, et al. The vacuum packaging of a flat lamp using thermally grown carbon nano tubes. Vacuum 2004;74(1):105–11. [5] Tang YB, Cong HT, Wang ZM, Cheng HM. Catalyst-seeded synthesis and field emission properties of flowerlike Si-doped AlN nanoneedle array. Appl Phys Lett 2006;89(25):253112 (3pp). [6] Qian WJ, Lai HW, Pei XZ, Jiang J, Wu Q, Zhang YL, et al. Improving field emission by constructing CsI–AlN hybrid nanostructures. J Mater Chem 2012;22(35):18578–82. [7] Liu N, Wu Q, He CY, Tao HS, Wang XZ, Lei W, et al. Patterned growth and field-emission properties of AlN nanocones. ACS Appl Mater Interfaces 2009;1(9):1927–30. [8] Zhang F, Wu Q, Wang XZ, Liu N, Yang J, Hu YM, et al. Vertically aligned one-dimensional AlN nanostructures on conductive substrates: synthesis and field emission. Vacuum 2012;86(7):833–7. [9] Chen Z, Cao CB, Zhu HS. Controlled growth of aluminum nitride nanostructures: aligned tips, brushes, and complex structures. J Phys Chem C 2007;111(5):1895–9. [10] Shi S-C, Chen C-F, Chattopadhyay S, Chen K-H, Chen L-C. Field emission from quasi-aligned aluminum nitride nanotips. Appl Phys Lett 2005;87(7):073109 (3pp). [11] Baughman RH, Zakhidov AA, de Heer WA. Carbon nanotubes-the route toward applications. Science 2002;297(5582):787–92. [12] She JC, Xiao ZM, Yang YH, Deng SZ, Chen J, Yang GW, et al. Correlation between resistance and field emission performance of individual ZnO one-dimensional nanostructures. ACS Nano 2008;2(10):2015–22. [13] Fang XS, Bando Y, Gautam UK, Ye CH, Golberg D. Inorganic semiconductor nanostructures and their field-emission applications. J Mater Chem 2008;18(5):509–22. [14] Zhao Q, Xu J, Xu XY, Wang Z, Yu DP. Field emission from AlN nanoneedle arrays. Appl Phys Lett 2004;85(22):5331–3. [15] Tang YB, Cong HT, Zhao ZG, Cheng HM. Field emission from AlN nanorod array. Appl Phys Lett 2005;86(15):153104 (3pp). [16] Fowler RH, Nordheim LW. Electron emission in intense electric fields. Proc R Soc London Ser A 1928;119(781):173–81. [17] Choi YC, Shin YM, Bae DJ, Lim SC, Lee YH, Lee BS. Patterned growth and field emission properties of vertically aligned carbon nanotubes. Diamond Relat Mater 2001;10(8):1457–64. [18] Pandey S, Rai P, Patole S, Gunes F, Kwon G-D, Yoo J-B, et al. Improved electron field emission from morphologically disordered monolayer graphene. Appl Phys Lett 2012;100(4):043104 (3pp). [19] Li C, Yang Y, Sun XW, Lei W, Zhang XB, Wang BP, et al. Enhanced field emission from injector-like ZnO nanostructures with minimized screening effect. Nanotechnology 2007;18(13):135604 (3pp). [20] Nilsson L, Groening O, Emmenegger C, Kuettel O, Schaller E, Schlapbach L, et al. Scanning field emission from patterned carbon nanotube films. Appl Phys Lett 2000;76(15):2071–3. [21] Park CJ, Choi D-K, Yoo J, Yi G-C, Lee CJ. Enhanced field emission properties from well-aligned zinc oxide nanoneedles grown on the Au/Ti/n-Si substrate. Appl Phys Lett 2007;90(8):083107 (3pp).

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8 1 (2 0 1 5) 1 2 4–13 1

[22] Maiti UN, Maiti S, Thapa R, Chattopadhyay KK. Flexible cold cathode with ultralow threshold field designed through wet chemical route. Nanotechnology 2010;21(50):505701 (3pp). [23] Lim SH, Kim HS, Lee CH, Pietruszko SM, Jang J. High stability of emission current for a new carbon nanostructure film. J Non-Cryst Solids 2002;299–302: 864–7. [24] Jo SH, Banerjee D, Ren ZF. Field emission of zinc oxide nanowires grown on carbon cloth. Appl Phys Lett 2004;85(8):1407–9.

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[25] Tsong T. Field penetration and band bending near semiconductor surfaces in high electric fields. Surf Sci 1979;81(1):28–42. [26] Dayeh SA, Soci C, Paul KL, Edward TY, Wang DL. Influence of surface states on the extraction of transport parameters from InAs nanowire field effect transistors. Appl Phys Lett 2007;90(16):162112 (3pp). [27] Givargizov EI, Zhirnov VV, Stepanova AN, Rakova EV, Kiselev AN, Plekhanov PS. Microstructure and field emission of diamond particles on silicon tips. Appl Surf Sci 1995;87– 88:24–30.