- Email: [email protected]
Stable field emission from ZnO nanowires grown on 3D graphene foam
Accepted Manuscript Stable field emission from ZnO nanowires grown on 3D graphene foam Shuyi Ding, Yanhuai Zhou, Miao Ye, Wei Lei PII:
S0042-207X(16)30677-7
DOI:
10.1016/j.vacuum.2017.01.032
Reference:
VAC 7304
To appear in:
Vacuum
Received Date: 9 October 2016 Accepted Date: 31 January 2017
Please cite this article as: Ding S, Zhou Y, Ye M, Lei W, Stable field emission from ZnO nanowires grown on 3D graphene foam, Vacuum (2017), doi: 10.1016/j.vacuum.2017.01.032. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Stable field emission from ZnO nanowires grown on 3D graphene foam
Shuyi Dinga,b, Yanhuai Zhoub, Miao Yeb, Wei Leia,* Display Reseach Center, School of Electronic Science and Engineering, Southeast University, Nanjing
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a
210096, P. R. China b
School of Information Engineering, Nanjing Normal University Taizhou Collage, Taizhou, 225300, P. R.
SC
China
Abstract:
Graphene was grown directly on nickel foam (NF) to form three dimensional graphene foam
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(GF), followed by growth of zinc oxide nanowires (ZNWs) on the surface of GF by a hydrothermal method. In comparison with pristine GF, the ZNW/GF hybrid structure exhibited efficient field emission with a low turn-on field of 1.7 V/µm, a low threshold field of 2.4 V/µm, high emission spot density, a high field enhancement factor of 1878 and excellent emitting stability. We proposed that the introduction of ZNWs on the surface of GF can increase the number of emission points, enhance
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tunneling probability, and lead to optimized field emission for the hybrid emitters. In addition, the graphene buffer layer provided a better electrical contact with ZNW, which also benefit to field emission.
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Keywords：field emission, graphene foam, zinc oxide nanowires, cathode
*
Corresponding author. Tel.: +86 25 83792449; fax: +86 25 83363222. Email: [email protected]
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One-dimensional (1D) nanostructures such as nanotubes, nanowires and nanorods have attracted great interests for their potential application in field emission devices due to their excellent properties.[1][2][3] Among various 1D nanostructured materials, much effort has been devoted to ZNWs because of their excellent field emission (FE) properties.[4][5]
In order to obtain more promising field emitters, several
to the combined field enhancement of the ZNWs and the template substrate.
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groups have proposed enhanced FE from ZNWs grown on micro/nano-template, [6][7][8] which is attributed
Graphene, a two-dimensional monolayer of graphite, has attracted great interest in the past years due to
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its excellent electrical properties, unusual mechanical strength, and large specific surface area.[9] Combining graphene with other functional nano-materials to obtain enhanced effects leads to vast unprecedented possibilities.[15~17] In near recent, some groups have investigated the 3D graphene
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macroscopic structure with a foam-like network graphene, which show extraordinary electrical and mechanical properties. [18, 19]
In this work, controllable growth of ZNWs on the surface of GF was realized by hydro-thermal synthesis.
In comparison with pristine GF, the field emission properties of the hybrid emitter were
significant improved. The field enhancement of GF emitter was magnified by combining with ZNW.
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The graphene film, functioning as an electron buffer layer, provided a better electrical contact with ZNW, which also benefit to field emission. Additionally, the ZNW protect the emitter from ion bombardment during operation, thus a high field emission current stability is sustained. Fig.1a~c shows the procedures for the growth of ZNW/graphene hybrids on nickel substrates. The
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porous nickel substrate used in this research is a foam-like 1.2 mm thick nickel film, a widely used commercial battery material. Few-layer graphene was grown on the NF to form GF using a chemical
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vapor deposition (CVD) method.
[20]
The obtained graphene was characterized by Raman spectroscopy,
demonstrating the few-layered structure of the graphene. Subsequently, ZNWs were grown hydrothermally on the surface of the GF.
[21]
A solution of zinc acetate dehydrate (98%, Aldrich) in
1-propanol (spectroscopic grade) was prepared. The solution was then spin coated onto the GF at 2000 rpm for 30 s. The substrates were then annealed at 100 °C for 2 minutes after each spin coating step to
promote adhesion. A uniform seed layer was obtained after three layers of spin coating. Vertical ZnO nanowires were then grown by dipping the substrates in an equimolar mixture of 25 mM zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Sigma Aldrich) and hexamethylenetetramine (HMTA, Sigma Aldrich) in deionized (DI) water heated in an oven at 80°C. ZnO nanowires were also grown on bare nickel foam 2
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substrate to investigate the necessity of graphene as the buffer layer. Scanning electron microscopy (SEM) images of the as-fabricated ZNW-GF are shown in Fig. 2a, b. Fig. 2c shows Raman spectrum of the pristine graphene taken at laser wavelength of 633 nm. The work function of graphene was measured by
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ultraviolet photoelectron spectroscopy (UPS) (Fig. 2c). He I (21.2 eV) was utilized as a photon source for the UPS measurement. Fig. 2d show an SEM micrograph and x-ray diffraction (XRD) pattern of the as-synthesized ZnO nanowires. All diffraction peaks are attributed to the ZnO hexagonal wurtzite crystal
with lattice constants of a = 3.249 Å and c = 5.205 Å – consistent with JCPDS card no. 89-0511. The
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inset of Fig. 2d shows the UPS of ZnO nanowires.
FE properties were determined using a simple diode configuration in a vacuum chamber at a pressure of 5×10−6 mbar. The cathodes with emitter were placed beneath metal plate anode, separated by two
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ceramic spacers with a thickness of 0.25 mm. The measured emission area was 1 cm2. Before testing, all the samples were annealing to 200℃ to remove the residual adsorbate, such as H2O and CO2. The FE properties were determined for three samples: bare GF, ZnO nanowires on GF (ZNW-GF), and ZnO nanowires on NF (ZNW-NF).
The dependencies of the FE current density on the applied electric field (J–E) are shown in Fig.3a.
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It is clear that the ZNW-GF emitter has the lowest macroscopic turn-on field (defined as E which is the field required to produce a current density of 10 µA/cm2) of ~1.7 V/µm, and the lowest threshold field (defined as the field required to produce a current density of 1 mA/cm2) of ~2.4 V/µm. In comparison, the turn-on and threshold fields of the ZNW-NF emitter were ~2.2 V/µm and 3.3 V/µm
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respectively. Unsurprisely, the GF emitter has the highest turn-on and threshold fields, which are 3.3 V/µm and 6 V/µm, respectively.
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The Fowler-Nordheim (FN) plots are shown in Fig. 3b. They exhibit linear behaviour in the low-field measurement ranges. The emission current-voltage characteristics can be analyzed by FN equation for the field emission, J = A (β2V2/Φd2) exp (−BΦ3/2d/βV),
(1)
where J is the current density, A=1.56×10−6 (A V−2eV), B=6.83×109 (V eV−3/2Vm−1), β is a field
enhancement factor, Φ is the work function, E=(V/d) is the applied field, d is the distance between the anode and the cathode, and V is the applied voltage. Here, the effective field enhancement factor β can be calculated from the slope of the FN plot. Using measured values for the work functions of 5.27 eV for ZnO and 4.9 eV for graphene, the field enhancement factors of ZNW-GF, ZNW-NF and GF emitters are 3
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calculated to be about 1878, 1668, and 768, respectively. It is obvious that the β for ZNW-GF emitters is much higher than that of bare GF in our experiment. We believe that the increased field enhancement is due to the combined geometry of the ZNW-GF [14]
Whilst the added
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structure, as shown in Fig. 1b, example of which has reported elsewhere.
enhancement factor is relatively modest (a factor of 2.5) because of field-shielding of ZNWs at the
surface of GF, the ZNW-GF structure significantly increases the number of emission sites resulting in a
higher current density at lower threshold and turn-on fields. This work confirms similar assumptions by
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other group. [9]
It can be seen from Fig. 3a that the FE performance of ZNW-GF emitter was better than that of ZNW-NF emitter. We attribute this phenomenon to the better contact between ZNW and graphene than
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that between ZNW and nickel substrate, which is described as following. ZnO is an ionic semiconductor for which the Schottky–Mott model should describe the barrier formation and height (ΦB). [5] ΦB is given by the difference of the Fermi level of the contact metal and the electron affinity χS of the semiconductor. For a Schottky contact on n-type ZnO (χS=4.1eV), [8] using reported values for the Fermi level of 5.15 eV for nickel [12] and 4.5 eV for graphene, we calculate: ΦB (Graphene) ~0.4 eV, ΦB (Nickel) ~1.05 eV. It is
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obvious that the barrier height at the interface between ZNW and graphene is much smaller than that between ZNW and nickel. It is therefore the electron can transport from graphene into ZNW much easier.
A large emission current is important for realizing high brightness FEDs and other FE-based devices. As shown in Fig.3a, the maximum emission current of the ZNW-GF hybrid is much larger
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than that of GF under the same vacuum condition. This can be explained as following. Due to unavoidable geometrical variation in different emission points, there is a significant variation in β
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from point to point. Because FE current increases exponentially with applied field, the emission point in GF with large β will emit current at lower fields and, as the field increases, will burn out before other emission points reach their threshold field. The current density is therefore limited by the number of emission points switched on at any given time. However, in the ZNW-GF emitters, the ZnO is a semi-conductor of relatively high resistance which can then function as a ballast resistance, which makes the emission more uniform. Consequently, more emitters will emit electrons at higher macroscopic fields, leading to a higher maximum emission current. Stability is another important issue in carbon-based emitters. We have conducted stability measurements at a pressure of 5×10-6 mBar with an initial emission current density of 1 mA/cm2, 4
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which are shown in Fig. 4a. It is clear that the ZNW-GF and ZNW-NF emitters have much more stable field emission than that of bare GF. We firstly attribute this behavior to the ballasting function of the ZNWs. In addition, ZnO is a large band-gap semiconductor, which by its nature will have a
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limited electron supply. Consequently, when a molecule is adsorbed onto the emission area, a change in the work function will have a negligible effect on the emitted current. Therefore, emission from ZNWs is much more stable at higher base pressures. Whilst semiconducting field emitters would not normally be considered for FE devices because of the limited current density which can be extracted
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from them, their combination with a graphene buffer layer significantly increases the density of emission sites and enhances the macroscopic field, compensating for the lack of current emitted from each ZNW. Fig. 4b depicts the emission behavior of the emission current in response to
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time-varied pulsed anode voltage for three samples at pulsed emission currents of 1mA/cm2. Fast field emission response is noted for all three samples, which indicates that the coating of ZNW doesn't affect the response time of the emitter.
To summarize, ZNWs were grown on 3D graphene foam by a simple and cost effective hydrothermal method. Efficient field emission with low turn-on field, low threshold field, high maximum emission
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current and excellent stability was obtained for ZNW-GFs. The introduction of ZNWs on the surface of GF can increase the number of emission points and tunneling probability, leading to optimized field emission for the hybrid emitters. The graphene buffer layer also provided a better electrical contact with ZNW, which enhance the FE as well. Such a ZNW-GF hybrid structure is a promising field emitter for
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FE applications. Acknowledgment
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This work is supported by Jiangsu College Natural Science project (No. 16KJB140007). Reference
[1] X. W. Sun, J. Z. Huang, and J. X. Wang, and Z. Xu, “A ZnO Nanorod Inorganic/Organic Heterostructure Light-Emitting Diode Emitting at 342 nm” Nano. Lett. vol. 8, no. 4, pp. 1219-1223, 2008.
[2] K. B. K. Teo, E. Minoux, L. Hudanski, F. Peauger, J. P. Schnell, L. Gangloff, P. Legagneux, D. Dieumegard, G. A. J. Amaratunga and W. I. Milne, “Microwave devices: Carbon nanotubes as cold cathodes”, Nature. vol. 437, pp. 968, 2005.
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[3] J. Zhang, G. Yang, Y. Z. Lee, S. Chang, J. P. Lu and O. Zhou, “Multiplexing radiography using a carbon nanotube based x-ray source”, Appl. Phys. Lett. vol. 89, no. 6, pp. 064106, 2006. [4] C. X. Xu, X. W. Sun, and B. J. Chen, “Field emission from gallium-doped zinc oxide nanofiber
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array”, Appl. Phys. Lett. vol. 84, no. 9, pp. 1540-1542, 2004. [5] C. Li, K. Hou, W. Lei, X. Zhang, B. Wang, and X. W. Sun, “Efficient surface-conducted field emission from ZnO nanotetrapods”, Appl. Phys. Lett. vol. 91, no. 16, pp.163502, 2007.
[6] C. J. Park, D. K. Choi, J. Yoo, G. C. Yi, and C. J. Lee, “Enhanced field emission properties from Ti
n - Si substrate”, Appl. Phys. Lett. vol.
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well-aligned zinc oxide nanoneedles grown on the Au 90, no. 8, pp. 083107, 2007.
[7] Q. H. Li, Q. Wan, Y. J. Chen, T. H. Wang, H. B. Jia, and D. P. Yu, “Stable field emission from
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tetrapod-like ZnO nanostructures”, Appl. Phys. Lett. vol. 85, no. 4, pp. 636-638, 2004. [8] J. H. Jo, D. Banerjee, and Z. F. Ren, Appl. Phys. Lett. vol. 85, no. 8, pp. 1407-1409, 2004. [9] A. K. Geim, K. S. Novoselov, “The Rise of Graphene,” Nat.Mater. 6, 183–191. 2007, [10] Xu, H.; Zhang, Z.; Wang, Z. X.; Wang, S.; Liang, X.; Peng, L. M.Quantum Capacitance Limited Vertical Scaling of Graphene Field-Effect Transistor. ACS Nano 2011, 5, 2340–2347.
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[11] X. C.Dong,; Shi, Y. M.; Huang, W.; Chen, P.; Li, L. J. Electrical Detection of DNA Hybridization with Single-Base Specific Using Transistors Based on CVD-Grown Graphene Sheets. Adv. Mater. 2010, 22, 1649–1653.
[12] Wu, Q.; Xu, Y. X.; Yao, Z. Y.; Liu, A. R.; Shi, G. Q. Supercapacitors Based on Flexible
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Graphene/Polyaniline Nanofiber Composite Films. ACS Nano 2010, 4, 1963–1970. [13] Yu, D.; Park, K.; Durstock, M.; Dai, L. M. Fullerence-Grafted Graphene for Efficient Bulk
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Heterojunction Polymer Photovoltaic Devices. J. Phys. Chem. Lett. 2011, 2, 1113–1118. [14] 14. Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J.; Kim, P.; Choi, J.; Hong, B. H. Large-Scale Pattern Growth of Graphene Films for Stretchable Transparent Electrodes. Nature 2009, 457, 706–710.
[15] Tung, V. C.; Huang, J.; Tevis, L.; Kim, F.; Kim, J.; Chu, C.; Stupp, S. I.; Huang, J. X. Surfactant-Free Water-Processable Photoconductive All-Carbon Composite. J. Am. Chem. Soc. 2011, 133, 4940–4947.
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[16] Mukherji, A.; Seger, B.; Lu, G. Q.; Wang, L. Z. Nitrogen Doped Sr2Ta2O7 Coupled with Graphene Sheets as Photocatalysts for Increased Photocatalytic Hydrogen Production. ACS Nano 2011, 5, 3483–3492.
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[17] Huang, X.; Qi, X. Y.; Zhang, H. Graphene-Based Composites. Chem. Soc. Rev. 2012, 41, 666–686. [18] X.-C. Dong, H. Xu, X.-W. Wang, Y.-X. Huang, M. B. Chan-Park, H. Zhang, L.-H. Wang, W. Huang and P. Chen, ACS Nano, 2012, 6, 3206 [19]
Z. Yan, L. L. Ma, Y. Zhu, I. Lahiri, M. G. Hahm, Z. Liu, S. B. Yang, C. S. Xiang, W. Lu, Z. W.
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Peng, Z. Z. Sun, C. Kittrell, J. Lou, W. B. Choi, P. M. Ajayan and J. M. Tour, ACS Nano, 2013, 7,
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Figure captions Fig.1 fabrication process of the ZNW-GF hybrid emitter.
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Fig.2 (a) Low magnification SEM image of ZNW-GF hybrid structure, (b) High magnification SEM image of ZNW grow on GF. (c) Raman spectrum of the as-grown graphene film, while the inset shows its UPS. (d) An XRD pattern of the as-grown ZNW, while the inset shows its UPS.
corresponding Fowler-Nordheim (FN) plots
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Fig.3 (a) The dependencies of the FE current density on the applied electric field (J–E). (b) The
Fig.4 (a) Emission current density versus time for the bare GF, ZNW-NF and ZNW-GF emitters,
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respectively. (b) pulsed measurements of all the three emitters.
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Fig.1
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Fig.2
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Fig.3
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Fig.4
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ZnO nanowires were hydrothermally grown on the surface of 3D graphene foam.
An enhanced field emission current was obtained from hybrid structure.
The field emission stability of the hybrid emitter was greatly improved.
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S0042-207X(16)30677-7
DOI:
10.1016/j.vacuum.2017.01.032
Reference:
VAC 7304
To appear in:
Vacuum
Received Date: 9 October 2016 Accepted Date: 31 January 2017
Please cite this article as: Ding S, Zhou Y, Ye M, Lei W, Stable field emission from ZnO nanowires grown on 3D graphene foam, Vacuum (2017), doi: 10.1016/j.vacuum.2017.01.032. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Stable field emission from ZnO nanowires grown on 3D graphene foam
Shuyi Dinga,b, Yanhuai Zhoub, Miao Yeb, Wei Leia,* Display Reseach Center, School of Electronic Science and Engineering, Southeast University, Nanjing
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a
210096, P. R. China b
School of Information Engineering, Nanjing Normal University Taizhou Collage, Taizhou, 225300, P. R.
SC
China
Abstract:
Graphene was grown directly on nickel foam (NF) to form three dimensional graphene foam
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(GF), followed by growth of zinc oxide nanowires (ZNWs) on the surface of GF by a hydrothermal method. In comparison with pristine GF, the ZNW/GF hybrid structure exhibited efficient field emission with a low turn-on field of 1.7 V/µm, a low threshold field of 2.4 V/µm, high emission spot density, a high field enhancement factor of 1878 and excellent emitting stability. We proposed that the introduction of ZNWs on the surface of GF can increase the number of emission points, enhance
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tunneling probability, and lead to optimized field emission for the hybrid emitters. In addition, the graphene buffer layer provided a better electrical contact with ZNW, which also benefit to field emission.
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Keywords：field emission, graphene foam, zinc oxide nanowires, cathode
*
Corresponding author. Tel.: +86 25 83792449; fax: +86 25 83363222. Email: [email protected]
1
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One-dimensional (1D) nanostructures such as nanotubes, nanowires and nanorods have attracted great interests for their potential application in field emission devices due to their excellent properties.[1][2][3] Among various 1D nanostructured materials, much effort has been devoted to ZNWs because of their excellent field emission (FE) properties.[4][5]
In order to obtain more promising field emitters, several
to the combined field enhancement of the ZNWs and the template substrate.
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groups have proposed enhanced FE from ZNWs grown on micro/nano-template, [6][7][8] which is attributed
Graphene, a two-dimensional monolayer of graphite, has attracted great interest in the past years due to
SC
its excellent electrical properties, unusual mechanical strength, and large specific surface area.[9] Combining graphene with other functional nano-materials to obtain enhanced effects leads to vast unprecedented possibilities.[15~17] In near recent, some groups have investigated the 3D graphene
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macroscopic structure with a foam-like network graphene, which show extraordinary electrical and mechanical properties. [18, 19]
In this work, controllable growth of ZNWs on the surface of GF was realized by hydro-thermal synthesis.
In comparison with pristine GF, the field emission properties of the hybrid emitter were
significant improved. The field enhancement of GF emitter was magnified by combining with ZNW.
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The graphene film, functioning as an electron buffer layer, provided a better electrical contact with ZNW, which also benefit to field emission. Additionally, the ZNW protect the emitter from ion bombardment during operation, thus a high field emission current stability is sustained. Fig.1a~c shows the procedures for the growth of ZNW/graphene hybrids on nickel substrates. The
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porous nickel substrate used in this research is a foam-like 1.2 mm thick nickel film, a widely used commercial battery material. Few-layer graphene was grown on the NF to form GF using a chemical
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vapor deposition (CVD) method.
[20]
The obtained graphene was characterized by Raman spectroscopy,
demonstrating the few-layered structure of the graphene. Subsequently, ZNWs were grown hydrothermally on the surface of the GF.
[21]
A solution of zinc acetate dehydrate (98%, Aldrich) in
1-propanol (spectroscopic grade) was prepared. The solution was then spin coated onto the GF at 2000 rpm for 30 s. The substrates were then annealed at 100 °C for 2 minutes after each spin coating step to
promote adhesion. A uniform seed layer was obtained after three layers of spin coating. Vertical ZnO nanowires were then grown by dipping the substrates in an equimolar mixture of 25 mM zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Sigma Aldrich) and hexamethylenetetramine (HMTA, Sigma Aldrich) in deionized (DI) water heated in an oven at 80°C. ZnO nanowires were also grown on bare nickel foam 2
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substrate to investigate the necessity of graphene as the buffer layer. Scanning electron microscopy (SEM) images of the as-fabricated ZNW-GF are shown in Fig. 2a, b. Fig. 2c shows Raman spectrum of the pristine graphene taken at laser wavelength of 633 nm. The work function of graphene was measured by
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ultraviolet photoelectron spectroscopy (UPS) (Fig. 2c). He I (21.2 eV) was utilized as a photon source for the UPS measurement. Fig. 2d show an SEM micrograph and x-ray diffraction (XRD) pattern of the as-synthesized ZnO nanowires. All diffraction peaks are attributed to the ZnO hexagonal wurtzite crystal
with lattice constants of a = 3.249 Å and c = 5.205 Å – consistent with JCPDS card no. 89-0511. The
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inset of Fig. 2d shows the UPS of ZnO nanowires.
FE properties were determined using a simple diode configuration in a vacuum chamber at a pressure of 5×10−6 mbar. The cathodes with emitter were placed beneath metal plate anode, separated by two
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ceramic spacers with a thickness of 0.25 mm. The measured emission area was 1 cm2. Before testing, all the samples were annealing to 200℃ to remove the residual adsorbate, such as H2O and CO2. The FE properties were determined for three samples: bare GF, ZnO nanowires on GF (ZNW-GF), and ZnO nanowires on NF (ZNW-NF).
The dependencies of the FE current density on the applied electric field (J–E) are shown in Fig.3a.
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It is clear that the ZNW-GF emitter has the lowest macroscopic turn-on field (defined as E which is the field required to produce a current density of 10 µA/cm2) of ~1.7 V/µm, and the lowest threshold field (defined as the field required to produce a current density of 1 mA/cm2) of ~2.4 V/µm. In comparison, the turn-on and threshold fields of the ZNW-NF emitter were ~2.2 V/µm and 3.3 V/µm
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respectively. Unsurprisely, the GF emitter has the highest turn-on and threshold fields, which are 3.3 V/µm and 6 V/µm, respectively.
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The Fowler-Nordheim (FN) plots are shown in Fig. 3b. They exhibit linear behaviour in the low-field measurement ranges. The emission current-voltage characteristics can be analyzed by FN equation for the field emission, J = A (β2V2/Φd2) exp (−BΦ3/2d/βV),
(1)
where J is the current density, A=1.56×10−6 (A V−2eV), B=6.83×109 (V eV−3/2Vm−1), β is a field
enhancement factor, Φ is the work function, E=(V/d) is the applied field, d is the distance between the anode and the cathode, and V is the applied voltage. Here, the effective field enhancement factor β can be calculated from the slope of the FN plot. Using measured values for the work functions of 5.27 eV for ZnO and 4.9 eV for graphene, the field enhancement factors of ZNW-GF, ZNW-NF and GF emitters are 3
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calculated to be about 1878, 1668, and 768, respectively. It is obvious that the β for ZNW-GF emitters is much higher than that of bare GF in our experiment. We believe that the increased field enhancement is due to the combined geometry of the ZNW-GF [14]
Whilst the added
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structure, as shown in Fig. 1b, example of which has reported elsewhere.
enhancement factor is relatively modest (a factor of 2.5) because of field-shielding of ZNWs at the
surface of GF, the ZNW-GF structure significantly increases the number of emission sites resulting in a
higher current density at lower threshold and turn-on fields. This work confirms similar assumptions by
SC
other group. [9]
It can be seen from Fig. 3a that the FE performance of ZNW-GF emitter was better than that of ZNW-NF emitter. We attribute this phenomenon to the better contact between ZNW and graphene than
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that between ZNW and nickel substrate, which is described as following. ZnO is an ionic semiconductor for which the Schottky–Mott model should describe the barrier formation and height (ΦB). [5] ΦB is given by the difference of the Fermi level of the contact metal and the electron affinity χS of the semiconductor. For a Schottky contact on n-type ZnO (χS=4.1eV), [8] using reported values for the Fermi level of 5.15 eV for nickel [12] and 4.5 eV for graphene, we calculate: ΦB (Graphene) ~0.4 eV, ΦB (Nickel) ~1.05 eV. It is
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obvious that the barrier height at the interface between ZNW and graphene is much smaller than that between ZNW and nickel. It is therefore the electron can transport from graphene into ZNW much easier.
A large emission current is important for realizing high brightness FEDs and other FE-based devices. As shown in Fig.3a, the maximum emission current of the ZNW-GF hybrid is much larger
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than that of GF under the same vacuum condition. This can be explained as following. Due to unavoidable geometrical variation in different emission points, there is a significant variation in β
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from point to point. Because FE current increases exponentially with applied field, the emission point in GF with large β will emit current at lower fields and, as the field increases, will burn out before other emission points reach their threshold field. The current density is therefore limited by the number of emission points switched on at any given time. However, in the ZNW-GF emitters, the ZnO is a semi-conductor of relatively high resistance which can then function as a ballast resistance, which makes the emission more uniform. Consequently, more emitters will emit electrons at higher macroscopic fields, leading to a higher maximum emission current. Stability is another important issue in carbon-based emitters. We have conducted stability measurements at a pressure of 5×10-6 mBar with an initial emission current density of 1 mA/cm2, 4
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which are shown in Fig. 4a. It is clear that the ZNW-GF and ZNW-NF emitters have much more stable field emission than that of bare GF. We firstly attribute this behavior to the ballasting function of the ZNWs. In addition, ZnO is a large band-gap semiconductor, which by its nature will have a
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limited electron supply. Consequently, when a molecule is adsorbed onto the emission area, a change in the work function will have a negligible effect on the emitted current. Therefore, emission from ZNWs is much more stable at higher base pressures. Whilst semiconducting field emitters would not normally be considered for FE devices because of the limited current density which can be extracted
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from them, their combination with a graphene buffer layer significantly increases the density of emission sites and enhances the macroscopic field, compensating for the lack of current emitted from each ZNW. Fig. 4b depicts the emission behavior of the emission current in response to
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time-varied pulsed anode voltage for three samples at pulsed emission currents of 1mA/cm2. Fast field emission response is noted for all three samples, which indicates that the coating of ZNW doesn't affect the response time of the emitter.
To summarize, ZNWs were grown on 3D graphene foam by a simple and cost effective hydrothermal method. Efficient field emission with low turn-on field, low threshold field, high maximum emission
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current and excellent stability was obtained for ZNW-GFs. The introduction of ZNWs on the surface of GF can increase the number of emission points and tunneling probability, leading to optimized field emission for the hybrid emitters. The graphene buffer layer also provided a better electrical contact with ZNW, which enhance the FE as well. Such a ZNW-GF hybrid structure is a promising field emitter for
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FE applications. Acknowledgment
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This work is supported by Jiangsu College Natural Science project (No. 16KJB140007). Reference
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Figure captions Fig.1 fabrication process of the ZNW-GF hybrid emitter.
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Fig.2 (a) Low magnification SEM image of ZNW-GF hybrid structure, (b) High magnification SEM image of ZNW grow on GF. (c) Raman spectrum of the as-grown graphene film, while the inset shows its UPS. (d) An XRD pattern of the as-grown ZNW, while the inset shows its UPS.
corresponding Fowler-Nordheim (FN) plots
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Fig.3 (a) The dependencies of the FE current density on the applied electric field (J–E). (b) The
Fig.4 (a) Emission current density versus time for the bare GF, ZNW-NF and ZNW-GF emitters,
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respectively. (b) pulsed measurements of all the three emitters.
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ZnO nanowires were hydrothermally grown on the surface of 3D graphene foam.
An enhanced field emission current was obtained from hybrid structure.
The field emission stability of the hybrid emitter was greatly improved.
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