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Enhanced field emission of graphene–ZnO quantum dots hybrid structure
Journal of Alloys and Compounds 632 (2015) 604–608
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Enhanced field emission of graphene–ZnO quantum dots hybrid structure Lei Sun a,b, Xiongtu Zhou b, Yongai Zhang b,⇑, Tailiang Guo b,⇑ a b
Zhicheng College, Fuzhou University, Fuzhou 350002, China National & Local United Engineer Laboratory of Flat Panel Display Technology, Fuzhou University, Fuzhou 350002, China
a r t i c l e
i n f o
Article history: Received 26 December 2014 Accepted 18 January 2015 Available online 24 January 2015 Keywords: ZnO quantum dots Graphene Hybrid composites Field emission
a b s t r a c t The cathode of graphene was prepared by the electrophoretic deposition (EPD) and ZnO quantum dots (QDs) were grown on the surface of graphene sheets by solution method to improve the field emission (FE) properties. The graphene/ZnO QDs hybrid emitters exhibited efficient field emission with lower turn-on field of 0.9 V/lm, lower threshold field of 2.6 V/lm, higher field enhancement factor of 3923 and more stable emission current stability than pristine graphene. The improved field emission performance was attributed to ZnO QDs, which introduce more defects, increase the number of emitting sites and decrease the work function. This investigation proposed that graphene/ZnO QDs composites are promising field cathodes in FE applications. Ó 2015 Published by Elsevier B.V.
1. Introduction Graphene, which has a unique two-dimensional nanostructure, is an attractive material in electric, optoelectronic, and photonic science for its excellent mechanical and electrical properties [1]. In particular, graphene based cold cathodes have attracted intensive attention over the past decades as a promising candidate for field emission applications due to their outstanding properties, including atomic thickness, high aspect ratio, rich edges renders, superior electrical conductivity and good mechanical properties [2]. Several chemical or physical methods have been developed to assemble graphene into functional structure such as chemical vapor deposition, electrophoretic deposition (EPD) [3–5] and inkjet printing. But the most serious problem of the graphene based cathodes prepared by existing methods was that many graphene sheets were horizontally on the substrate [6], which limits FE properties. To overcome this drawback, EPD methods were used in this study, to yield more sheets standing perpendicular to the substrate surface. Zinc oxide is a semiconductor with a wide band gap (3.37 eV) and a large exciton binding energy (60 meV). One-dimensional nanostructured ZnO, such as nanowires [7] and nanorods (ZNRs) [8] have been found to be a good candidate in field emission appli⇑ Corresponding authors. Tel.: +86 591 87893299; fax: +86 591 87892643. E-mail addresses: [email protected] (Y. Zhang), [email protected] (T. Guo). http://dx.doi.org/10.1016/j.jallcom.2015.01.105 0925-8388/Ó 2015 Published by Elsevier B.V.
cations owing to low work function, high aspect ratio, good thermal and chemical stability. Among all the solution-based approaches, the solution method has been extensively studied due to large area deposition, low synthesis temperature, and low production cost. By now, several investigations have been made on graphene/ ZnO hybrid structure [9,10], which show enhanced field emission performance due to the combined effects, including large surface area, improved crystalline quality, better electronic and optical properties, and effective surface passivation. In the previous studies, Lin et al. [11] reported that graphene films are synthesized on the Cu foils by the thermal chemical vapor deposition technique, and transferred onto the ZNRs with various cycles, which exhibit the lowest turn-on electric field of 3.7 V/lm and highest enhancement factor of 8723. Zheng et al. [12] indicated that the ZnO nanowires were synthesized using vapor–liquid–solid growth mechanism, and then the graphene sheets were grown to form hybrids by a combination of sputtering and plasma-enhanced chemical vapor deposition processes, which have lower turn-on voltage of 1.3 V/lm compared to 2.5 V/lm of pure ZnO. Among all of them, few studies were reported on the combination of graphene with ZnO quantum dots (QDs). In this study, a facile and simple method was presented to synthesize graphene/ZnO QDs hybrid structure, and it was found that the field emission performance of the composites was enhanced owing to the presence of ZnO QDs, which not only reduce the contact resistance but also effectively improve the carrier transport of graphene emitters.
L. Sun et al. / Journal of Alloys and Compounds 632 (2015) 604–608 2. Experimental details 2.1. Preparation of graphene cathode The graphene powder in this experiment was prepared by modified Hummers’ method [13]. As the first step, 25 mg graphene powders were dispersed in 500 ml pure isopropyl alcohol (IA) by ultrasonication for 1 h. In order to make Mg2+ absorbed on the surfaces of graphene, same weight ratio of Mg(NO3)26H2O was put into the solution. Mg2+ provides positive charges in electrophoretic deposition process. The patterned Ag electrode (cathode) on glass substrate with a width of 150 lm by screen-printing technique and a graphite plate (anode) were immersed into the above EPD suspension at room temperature. The graphene was deposited on Ag electrode by direct current electrophoretic deposition at a current of 6 mA for 5 min by applying a constant DC voltage of 30 V, and the distance between
605
two electrodes is 3 mm. The diagram of the EPD device was given in Fig. 1(a). After finishing electrophoretic deposition, the graphene emitters were washed with the dilute nitric acid solution and the deionized water, respectively. The samples were annealed in ambient N2 at 300 °C for 1 h to remove the organic impurities on the substrate.
2.2. Decoration of ZnO QDs The growth of ZnO QDs onto the surface of graphene sheets were synthesized in alkaline absolute ethanol solution at mild conditions. First, 0.05 mol/L [Zn(CH3COO)22H2O] was prepared in 20 ml of ethanol under vigorous stirring at 80 °C for 30 min, and 0.1 mol/L LiOHH2O was dissolved in 20 mL of absolute ethanol. Then the pattern electrodes with graphene were placed into the Zn2+ solution.
Fig. 1. Schematic diagram of (a) electrophoretic deposition (EPD) device and (b) the configuration for field emission measurements.
Fig. 2. (a) AFM image of graphene and the bottom is the cross-sectional analysis. (b) SEM image of graphene/ZnO composites. (c) TEM image of composites. (d) HRTEM image of composites.
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L. Sun et al. / Journal of Alloys and Compounds 632 (2015) 604–608
The LiOH/ethanol solution was then added to the zinc acetate/ethanol solution. The reaction solution was kept in ice-bath for 1 h. Finally, the substrate was subsequently rinsed and washed several times by deionized water, then dried in ambient N2 at 200 °C for 1 h. 2.3. Characterization and FE measurement The samples were characterized by scanning electron microscopy (SEM, Hitachi S-3000 N) and transmission electronic microscope (TEM, Tecnai G2 F20 S-TWIN)). The phase structures of graphene were performed by X-ray diffractometer (XRD, Brook D8-advance X). X-ray photoelectron spectroscopy (XPS) spectrum (VG ESCALAB MARK II) was also used to characterize and analyze the graphene/ZnO QDs composites. The field-emission characteristics were measured in a high vacuum chamber with a parallel diode-type configuration at 1.0 104 Pa at room temperature. The graphene-based emitters and the phosphor-coated indium–tin oxide (ITO) glass were used as the cathode and the anode, respectively. The distance between the cathode and anode is 500 lm. Direct voltage was applied to the anode, and the emission current was also measured at the anode. The schematic diagram for the field emission measurements of graphene/ZnO QDs is shown in Fig. 1(b).
3. Results and discussion Atomic force microscopy (AFM) image in Fig. 2(a) indicates that the chemical-reduced graphene has a thickness of around 3 nm and a lateral size at micrometers. The graphene sheets uniformly covered on patterned Ag electrode by EPD method as shown in Fig. 2(b). It is depicted in Fig. 2(b) that most graphenes lay vertically on the electrode surface, and the wrinkled edges with high aspect ratio are beneficial to the active electron field emission. The high-resolution SEM image was shown in the inset of Fig. 2(b), in which ZnO QDs can be indistinctly observed on the graphene because of their small sizes. However, the typical TEM image in Fig. 2(c) obviously illustrates that many ZnO QDs with a spherical shape coated on the surface of graphene sheets, which serve as additional emission sites in the field emission. The high-
resolution TEM image which presented in Fig. 2(d) shows that small ZnO QDs uniformly attach on the surface of graphene sheets. The average diameter of ZnO QDs is estimated to be around 6 nm, and the lattice spacing has been measured to be 0.29 nm, which agrees well with the interlayer spacing of ZnO QDs [14]. The XRD pattern of graphene/ZnO QDs composites collected from the Ag substrate is presented in Fig. 3(a). The broad diffraction peak, which is centered at around 24.6°, can be indexed to the (0 0 2) plane of the graphene. The rest shallow peaks are clearly observed, which are assigned to the reflections from (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) planes of typical Wurzite hexagonal ZnO (JCPDS No. 36-1451). As shown in the pattern, all samples are highly crystallized. The simultaneous presence of the diffraction peaks of graphene and ZnO confirm that ZnO QDs were successfully grown on the graphene. And no other peaks of any impurities were detected confirming that the high purity of the synthesized products. To further study the surface chemical states of the graphene composites, XPS analysis was performed in Fig. 3(b–d). The XPS spectrum confirms C, Zn, O appeared in the synthesized composites. The binding energies at 284.6 eV corresponded to the C 1s core level from the graphene, which can be fitted by the SP3-hybridized C (centered at 285.3 eV) and the planar SP2-hybridized C (centered at 284.3 eV). The peaks of Zn 2p3/2 and Zn 2p1/2 identified at 1021.7 eV and 1044.8 eV, respectively. The peak of O 1s appeared at 531.8 eV indicated that O is from ZnO and the contamination of graphene. XPS measurements verify that ZnO QDs were coating on the graphene emitters. To characterize the field emission behaviors of graphene/ZnO composites, the field emission current density versus the applied electric field (J–E) and the corresponding Fowler–Nordheim (F–N) plot from both the pristine graphene and the ZnO QDs-decorated graphene were shown in Fig. 4(a). The turn-on field (Eon) and threshold field (Ethr) are defined as the electric fields required to
Fig. 3. (a) XRD spectrum and (b–d) XPS spectra of graphene/ZnO composites.
L. Sun et al. / Journal of Alloys and Compounds 632 (2015) 604–608
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Fig. 4. (a) Emission current density versus electric field curves of pristine graphene and graphene/ZnO emitters. The inset is the F–N plot. (b) Emission stability of pristine graphene and graphene/ZnO composites. Insets are the emission images of (left) graphene and (right) graphene/ZnO hybrid emitters.
produce a current density of 10 lA/cm2 and 1 mA/cm2, respectively. Eon for the pristine graphene and graphene/ZnO QDs composite cathodes are 1.2 V/lm and 0.7 V/lm, respectively. And Ethr for the pristine graphene and graphene/ZnO QDs emitters are 3.5 V/lm and 2.6 V/lm, respectively. The field emission performance for graphene/ZnO QDs composites improved obviously in comparison to that of pristine graphene, as seen from the left-shift of the J–E curves. To elucidate the electron-emission mechanism in the field emission behaviors of the synthesized cathode, ln(J/E2) 1/E curves were plotted in the inset of Fig. 4(a), which indicated that the emitting electrons were resulted from barrier tunneling electrons extracted by the electric field. The F–N plots in the inset of Fig. 4(a) agreed well with F–N equation [15]:
lnðJ=E2 Þ ¼
lnðAb2 =/Þ B/3=2 bE
where A and B are constants with the value of 1.56 1010 A V2 eV and 6.83 103 V eV3/2 m1, respectively. E is the applied electric field, b is the field enhancement factor, and / is the work function of 5 eV and 5.3 eV for graphene [2] and ZnO [16], respectively. The enhancement factor b of pristine graphene and ZnO QDsdecorated graphene emitters can be calculated from the slope of the F–N plots. The values are estimated to be around 2166 for pristine graphene emitters and about 3923 for graphene/ZnO QDs hybrid emitters. The field emission properties of the graphene emitters are effectively enhanced after coating with quantum dots. The field emission stability of pristine graphene and graphene/ ZnO QDs hybrid emitters were shown in Fig. 4(b). The field emission current density was monitored for 120 min with an initial emission current density of 5 mA/cm2. It is indicated that graphene/ZnO QDs hybrid emitters exhibit excellent emission stability without evident degradation at high current density. The current density has a fluctuation of less than 5% during the 120min observation. The images in the inset of Fig. 4(b) indicate that the hybrid of graphene/ZnO QDs have higher density of emitting sites and more uniform luminance intensity than that of pristine graphene. The low turn-on field and the high field enhancement actor of graphene/ZnO QDs emitters can be attributed to the tunneling effect of electrons through the graphene/ZnO heterojunction. For the small energy barrier of ZnO (ZnO at 3.4 eV and graphene at 4.5 eV), the electrons can more easily inject from the conduction band into vacuum than from graphene due to the low energy threshold. And the decrease in the potential barrier between graphene and ZnO, which is caused by the reduction of energy gaps
and the change of the electron density distributions of the states in the vicinity of the Fermi level, allows the electrons transfer from the Fermi level of graphene to ZnO QDs into the conduction band of ZnO more efficiently [17]. C-atoms of graphene directly bind to the Zn-atoms of ZnO by the first principles calculation [18]. More defects were further introduced to the graphene by ZnO QDs, which were formed during the solution growth process and provide channels of electron transport. In comparison with the planar SP2-hybridized defects in the graphene, the distorted SP3-hybridized defects introduced new active emission sites thus the field emission properties of emitters were improved [19]. In addition, the high field enhancement factor of graphene/ZnO emitters is also a result of the low work function of graphene/ZnO, comparing with the work functions of both pristine ZnO and pristine graphene sheets [18]. 4. Conclusions Graphene/ZnO QDs hybrid composites with low turn-on field, low threshold field, high field enhancement factor, improved emitting uniformity and high emission stability are successfully fabricated on patterned Ag-substrates by solution process. Graphene/ ZnO QDs hybrids exhibit higher levels of field emission properties than pristine graphenes ascribed to the presence of ZnO QDs. ZnO QDs not only introduce more defects to the synthesized hybrids, but also increase the number of additional emission sites. The composition of graphene and ZnO QDs also decrease the work function in the hybrid emitters, which may be another reason for its good FE performance. The present investigations indicate that graphene/ ZnO QDs hybrid emitters are promising materials in electron sources and field emission devices. Acknowledgments This work was supported by the Natural Science Foundation of China (Grant Nos. 61306071 and 61474024) and the Natural Science Foundation of Fujian Province, China (Grant No. 2013J01236). References [1] Q. Wang, J. Zhao, W.F. Shan, X.B. Xia, L.L. Xing, X.Y. Xue, J. Alloys Comp. 590 (2014) 424–427. [2] I. Lahiri, V.P. Verma, W. Choi, Carbon 49 (2011) 1614–1619. [3] Z.-S. Wu, S. Pei, W. Ren, D. Tang, L. Gao, B. Liu, F. Li, C. Liu, H.-M. Cheng, Adv. Mater. 21 (2009) 1756.
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[4] L.F. Chen, H. He, H. Yu, Y.Q. Cao, D. Lei, Q.Q.G. Menggen, C.X. Wu, L.Q. Hu, J. Alloys Comp. 610 (2014) 659–664. [5] J. Liu, B. Zeng, Z. Wu, H. Sun, ACS Appl. Mater. Interfaces 4 (2012) 1219–1224. [6] G. Eda, H. Emrah Unalan, N. Rupesinghe, G.A. Amaratunga, M. Chhowalla, Appl. Phys. Lett. 93 (2008) 233502. [7] S. Jo, Z. Ren, D. Banerjee, Appl. Phys. Lett. 85 (2004) 1407–1408. [8] L. Wang, A. Huang, G.N. Panin, T. Kang, D.J. Fu, Mater. Lett. 112 (2013) 183– 186. [9] C.-X. Wu, F.-S. Li, Y.-A. Zhang, T.-L. Guo, Carbon 50 (2012) 3622–3626. [10] J. Ding, X. Yan, Q. Xue, Mater. Chem. Phys. 133 (2012) 405–409. [11] J.-C. Lin, B.-R. Huang, T.-C. Lin, Appl. Surf. Sci. 289 (2014) 384–387. [12] W.T. Zheng, Y.M. Ho, H.W. Tian, M. Wen, J.L. Qi, Y.A. Li, J. Phys. Chem. C 113 (2009) 9164–9168.
[13] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Carbon 45 (2007) 1558–1565. [14] S.Y. Bae, H.C. Choi, C.W. Na, J.H. Park, Appl. Phys. Lett. 86 (2005) 033102. [15] J.Z. Xu, R. Pan, Y.W. Chen, X.Q. Piao, M. Qian, T. Feng, Z. Sun, J. Alloys Comp. 551 (2013) 348–351. [16] F. Xu, K. Yu, G. Li, Q. Li, Z. Zhu, Nanotechnology 17 (2006) 2855–2859. [17] J. Hwang, D. Lee, J. Kim, T. Han, B. Kim, M. Park, J. Mater. Chem. 24 (2011) 3432–3437. [18] S. Zhang, Y. Zhang, S. Huang, H. Liu, P. Wang, H. Tian, J. Phys. Chem. C 114 (2010) 19284. [19] W. Gu, Appl. Phys. Lett. 89 (2006) 143111.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Enhanced field emission of graphene–ZnO quantum dots hybrid structure Lei Sun a,b, Xiongtu Zhou b, Yongai Zhang b,⇑, Tailiang Guo b,⇑ a b
Zhicheng College, Fuzhou University, Fuzhou 350002, China National & Local United Engineer Laboratory of Flat Panel Display Technology, Fuzhou University, Fuzhou 350002, China
a r t i c l e
i n f o
Article history: Received 26 December 2014 Accepted 18 January 2015 Available online 24 January 2015 Keywords: ZnO quantum dots Graphene Hybrid composites Field emission
a b s t r a c t The cathode of graphene was prepared by the electrophoretic deposition (EPD) and ZnO quantum dots (QDs) were grown on the surface of graphene sheets by solution method to improve the field emission (FE) properties. The graphene/ZnO QDs hybrid emitters exhibited efficient field emission with lower turn-on field of 0.9 V/lm, lower threshold field of 2.6 V/lm, higher field enhancement factor of 3923 and more stable emission current stability than pristine graphene. The improved field emission performance was attributed to ZnO QDs, which introduce more defects, increase the number of emitting sites and decrease the work function. This investigation proposed that graphene/ZnO QDs composites are promising field cathodes in FE applications. Ó 2015 Published by Elsevier B.V.
1. Introduction Graphene, which has a unique two-dimensional nanostructure, is an attractive material in electric, optoelectronic, and photonic science for its excellent mechanical and electrical properties [1]. In particular, graphene based cold cathodes have attracted intensive attention over the past decades as a promising candidate for field emission applications due to their outstanding properties, including atomic thickness, high aspect ratio, rich edges renders, superior electrical conductivity and good mechanical properties [2]. Several chemical or physical methods have been developed to assemble graphene into functional structure such as chemical vapor deposition, electrophoretic deposition (EPD) [3–5] and inkjet printing. But the most serious problem of the graphene based cathodes prepared by existing methods was that many graphene sheets were horizontally on the substrate [6], which limits FE properties. To overcome this drawback, EPD methods were used in this study, to yield more sheets standing perpendicular to the substrate surface. Zinc oxide is a semiconductor with a wide band gap (3.37 eV) and a large exciton binding energy (60 meV). One-dimensional nanostructured ZnO, such as nanowires [7] and nanorods (ZNRs) [8] have been found to be a good candidate in field emission appli⇑ Corresponding authors. Tel.: +86 591 87893299; fax: +86 591 87892643. E-mail addresses: [email protected] (Y. Zhang), [email protected] (T. Guo). http://dx.doi.org/10.1016/j.jallcom.2015.01.105 0925-8388/Ó 2015 Published by Elsevier B.V.
cations owing to low work function, high aspect ratio, good thermal and chemical stability. Among all the solution-based approaches, the solution method has been extensively studied due to large area deposition, low synthesis temperature, and low production cost. By now, several investigations have been made on graphene/ ZnO hybrid structure [9,10], which show enhanced field emission performance due to the combined effects, including large surface area, improved crystalline quality, better electronic and optical properties, and effective surface passivation. In the previous studies, Lin et al. [11] reported that graphene films are synthesized on the Cu foils by the thermal chemical vapor deposition technique, and transferred onto the ZNRs with various cycles, which exhibit the lowest turn-on electric field of 3.7 V/lm and highest enhancement factor of 8723. Zheng et al. [12] indicated that the ZnO nanowires were synthesized using vapor–liquid–solid growth mechanism, and then the graphene sheets were grown to form hybrids by a combination of sputtering and plasma-enhanced chemical vapor deposition processes, which have lower turn-on voltage of 1.3 V/lm compared to 2.5 V/lm of pure ZnO. Among all of them, few studies were reported on the combination of graphene with ZnO quantum dots (QDs). In this study, a facile and simple method was presented to synthesize graphene/ZnO QDs hybrid structure, and it was found that the field emission performance of the composites was enhanced owing to the presence of ZnO QDs, which not only reduce the contact resistance but also effectively improve the carrier transport of graphene emitters.
L. Sun et al. / Journal of Alloys and Compounds 632 (2015) 604–608 2. Experimental details 2.1. Preparation of graphene cathode The graphene powder in this experiment was prepared by modified Hummers’ method [13]. As the first step, 25 mg graphene powders were dispersed in 500 ml pure isopropyl alcohol (IA) by ultrasonication for 1 h. In order to make Mg2+ absorbed on the surfaces of graphene, same weight ratio of Mg(NO3)26H2O was put into the solution. Mg2+ provides positive charges in electrophoretic deposition process. The patterned Ag electrode (cathode) on glass substrate with a width of 150 lm by screen-printing technique and a graphite plate (anode) were immersed into the above EPD suspension at room temperature. The graphene was deposited on Ag electrode by direct current electrophoretic deposition at a current of 6 mA for 5 min by applying a constant DC voltage of 30 V, and the distance between
605
two electrodes is 3 mm. The diagram of the EPD device was given in Fig. 1(a). After finishing electrophoretic deposition, the graphene emitters were washed with the dilute nitric acid solution and the deionized water, respectively. The samples were annealed in ambient N2 at 300 °C for 1 h to remove the organic impurities on the substrate.
2.2. Decoration of ZnO QDs The growth of ZnO QDs onto the surface of graphene sheets were synthesized in alkaline absolute ethanol solution at mild conditions. First, 0.05 mol/L [Zn(CH3COO)22H2O] was prepared in 20 ml of ethanol under vigorous stirring at 80 °C for 30 min, and 0.1 mol/L LiOHH2O was dissolved in 20 mL of absolute ethanol. Then the pattern electrodes with graphene were placed into the Zn2+ solution.
Fig. 1. Schematic diagram of (a) electrophoretic deposition (EPD) device and (b) the configuration for field emission measurements.
Fig. 2. (a) AFM image of graphene and the bottom is the cross-sectional analysis. (b) SEM image of graphene/ZnO composites. (c) TEM image of composites. (d) HRTEM image of composites.
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L. Sun et al. / Journal of Alloys and Compounds 632 (2015) 604–608
The LiOH/ethanol solution was then added to the zinc acetate/ethanol solution. The reaction solution was kept in ice-bath for 1 h. Finally, the substrate was subsequently rinsed and washed several times by deionized water, then dried in ambient N2 at 200 °C for 1 h. 2.3. Characterization and FE measurement The samples were characterized by scanning electron microscopy (SEM, Hitachi S-3000 N) and transmission electronic microscope (TEM, Tecnai G2 F20 S-TWIN)). The phase structures of graphene were performed by X-ray diffractometer (XRD, Brook D8-advance X). X-ray photoelectron spectroscopy (XPS) spectrum (VG ESCALAB MARK II) was also used to characterize and analyze the graphene/ZnO QDs composites. The field-emission characteristics were measured in a high vacuum chamber with a parallel diode-type configuration at 1.0 104 Pa at room temperature. The graphene-based emitters and the phosphor-coated indium–tin oxide (ITO) glass were used as the cathode and the anode, respectively. The distance between the cathode and anode is 500 lm. Direct voltage was applied to the anode, and the emission current was also measured at the anode. The schematic diagram for the field emission measurements of graphene/ZnO QDs is shown in Fig. 1(b).
3. Results and discussion Atomic force microscopy (AFM) image in Fig. 2(a) indicates that the chemical-reduced graphene has a thickness of around 3 nm and a lateral size at micrometers. The graphene sheets uniformly covered on patterned Ag electrode by EPD method as shown in Fig. 2(b). It is depicted in Fig. 2(b) that most graphenes lay vertically on the electrode surface, and the wrinkled edges with high aspect ratio are beneficial to the active electron field emission. The high-resolution SEM image was shown in the inset of Fig. 2(b), in which ZnO QDs can be indistinctly observed on the graphene because of their small sizes. However, the typical TEM image in Fig. 2(c) obviously illustrates that many ZnO QDs with a spherical shape coated on the surface of graphene sheets, which serve as additional emission sites in the field emission. The high-
resolution TEM image which presented in Fig. 2(d) shows that small ZnO QDs uniformly attach on the surface of graphene sheets. The average diameter of ZnO QDs is estimated to be around 6 nm, and the lattice spacing has been measured to be 0.29 nm, which agrees well with the interlayer spacing of ZnO QDs [14]. The XRD pattern of graphene/ZnO QDs composites collected from the Ag substrate is presented in Fig. 3(a). The broad diffraction peak, which is centered at around 24.6°, can be indexed to the (0 0 2) plane of the graphene. The rest shallow peaks are clearly observed, which are assigned to the reflections from (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) planes of typical Wurzite hexagonal ZnO (JCPDS No. 36-1451). As shown in the pattern, all samples are highly crystallized. The simultaneous presence of the diffraction peaks of graphene and ZnO confirm that ZnO QDs were successfully grown on the graphene. And no other peaks of any impurities were detected confirming that the high purity of the synthesized products. To further study the surface chemical states of the graphene composites, XPS analysis was performed in Fig. 3(b–d). The XPS spectrum confirms C, Zn, O appeared in the synthesized composites. The binding energies at 284.6 eV corresponded to the C 1s core level from the graphene, which can be fitted by the SP3-hybridized C (centered at 285.3 eV) and the planar SP2-hybridized C (centered at 284.3 eV). The peaks of Zn 2p3/2 and Zn 2p1/2 identified at 1021.7 eV and 1044.8 eV, respectively. The peak of O 1s appeared at 531.8 eV indicated that O is from ZnO and the contamination of graphene. XPS measurements verify that ZnO QDs were coating on the graphene emitters. To characterize the field emission behaviors of graphene/ZnO composites, the field emission current density versus the applied electric field (J–E) and the corresponding Fowler–Nordheim (F–N) plot from both the pristine graphene and the ZnO QDs-decorated graphene were shown in Fig. 4(a). The turn-on field (Eon) and threshold field (Ethr) are defined as the electric fields required to
Fig. 3. (a) XRD spectrum and (b–d) XPS spectra of graphene/ZnO composites.
L. Sun et al. / Journal of Alloys and Compounds 632 (2015) 604–608
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Fig. 4. (a) Emission current density versus electric field curves of pristine graphene and graphene/ZnO emitters. The inset is the F–N plot. (b) Emission stability of pristine graphene and graphene/ZnO composites. Insets are the emission images of (left) graphene and (right) graphene/ZnO hybrid emitters.
produce a current density of 10 lA/cm2 and 1 mA/cm2, respectively. Eon for the pristine graphene and graphene/ZnO QDs composite cathodes are 1.2 V/lm and 0.7 V/lm, respectively. And Ethr for the pristine graphene and graphene/ZnO QDs emitters are 3.5 V/lm and 2.6 V/lm, respectively. The field emission performance for graphene/ZnO QDs composites improved obviously in comparison to that of pristine graphene, as seen from the left-shift of the J–E curves. To elucidate the electron-emission mechanism in the field emission behaviors of the synthesized cathode, ln(J/E2) 1/E curves were plotted in the inset of Fig. 4(a), which indicated that the emitting electrons were resulted from barrier tunneling electrons extracted by the electric field. The F–N plots in the inset of Fig. 4(a) agreed well with F–N equation [15]:
lnðJ=E2 Þ ¼
lnðAb2 =/Þ B/3=2 bE
where A and B are constants with the value of 1.56 1010 A V2 eV and 6.83 103 V eV3/2 m1, respectively. E is the applied electric field, b is the field enhancement factor, and / is the work function of 5 eV and 5.3 eV for graphene [2] and ZnO [16], respectively. The enhancement factor b of pristine graphene and ZnO QDsdecorated graphene emitters can be calculated from the slope of the F–N plots. The values are estimated to be around 2166 for pristine graphene emitters and about 3923 for graphene/ZnO QDs hybrid emitters. The field emission properties of the graphene emitters are effectively enhanced after coating with quantum dots. The field emission stability of pristine graphene and graphene/ ZnO QDs hybrid emitters were shown in Fig. 4(b). The field emission current density was monitored for 120 min with an initial emission current density of 5 mA/cm2. It is indicated that graphene/ZnO QDs hybrid emitters exhibit excellent emission stability without evident degradation at high current density. The current density has a fluctuation of less than 5% during the 120min observation. The images in the inset of Fig. 4(b) indicate that the hybrid of graphene/ZnO QDs have higher density of emitting sites and more uniform luminance intensity than that of pristine graphene. The low turn-on field and the high field enhancement actor of graphene/ZnO QDs emitters can be attributed to the tunneling effect of electrons through the graphene/ZnO heterojunction. For the small energy barrier of ZnO (ZnO at 3.4 eV and graphene at 4.5 eV), the electrons can more easily inject from the conduction band into vacuum than from graphene due to the low energy threshold. And the decrease in the potential barrier between graphene and ZnO, which is caused by the reduction of energy gaps
and the change of the electron density distributions of the states in the vicinity of the Fermi level, allows the electrons transfer from the Fermi level of graphene to ZnO QDs into the conduction band of ZnO more efficiently [17]. C-atoms of graphene directly bind to the Zn-atoms of ZnO by the first principles calculation [18]. More defects were further introduced to the graphene by ZnO QDs, which were formed during the solution growth process and provide channels of electron transport. In comparison with the planar SP2-hybridized defects in the graphene, the distorted SP3-hybridized defects introduced new active emission sites thus the field emission properties of emitters were improved [19]. In addition, the high field enhancement factor of graphene/ZnO emitters is also a result of the low work function of graphene/ZnO, comparing with the work functions of both pristine ZnO and pristine graphene sheets [18]. 4. Conclusions Graphene/ZnO QDs hybrid composites with low turn-on field, low threshold field, high field enhancement factor, improved emitting uniformity and high emission stability are successfully fabricated on patterned Ag-substrates by solution process. Graphene/ ZnO QDs hybrids exhibit higher levels of field emission properties than pristine graphenes ascribed to the presence of ZnO QDs. ZnO QDs not only introduce more defects to the synthesized hybrids, but also increase the number of additional emission sites. The composition of graphene and ZnO QDs also decrease the work function in the hybrid emitters, which may be another reason for its good FE performance. The present investigations indicate that graphene/ ZnO QDs hybrid emitters are promising materials in electron sources and field emission devices. Acknowledgments This work was supported by the Natural Science Foundation of China (Grant Nos. 61306071 and 61474024) and the Natural Science Foundation of Fujian Province, China (Grant No. 2013J01236). References [1] Q. Wang, J. Zhao, W.F. Shan, X.B. Xia, L.L. Xing, X.Y. Xue, J. Alloys Comp. 590 (2014) 424–427. [2] I. Lahiri, V.P. Verma, W. Choi, Carbon 49 (2011) 1614–1619. [3] Z.-S. Wu, S. Pei, W. Ren, D. Tang, L. Gao, B. Liu, F. Li, C. Liu, H.-M. Cheng, Adv. Mater. 21 (2009) 1756.
608
L. Sun et al. / Journal of Alloys and Compounds 632 (2015) 604–608
[4] L.F. Chen, H. He, H. Yu, Y.Q. Cao, D. Lei, Q.Q.G. Menggen, C.X. Wu, L.Q. Hu, J. Alloys Comp. 610 (2014) 659–664. [5] J. Liu, B. Zeng, Z. Wu, H. Sun, ACS Appl. Mater. Interfaces 4 (2012) 1219–1224. [6] G. Eda, H. Emrah Unalan, N. Rupesinghe, G.A. Amaratunga, M. Chhowalla, Appl. Phys. Lett. 93 (2008) 233502. [7] S. Jo, Z. Ren, D. Banerjee, Appl. Phys. Lett. 85 (2004) 1407–1408. [8] L. Wang, A. Huang, G.N. Panin, T. Kang, D.J. Fu, Mater. Lett. 112 (2013) 183– 186. [9] C.-X. Wu, F.-S. Li, Y.-A. Zhang, T.-L. Guo, Carbon 50 (2012) 3622–3626. [10] J. Ding, X. Yan, Q. Xue, Mater. Chem. Phys. 133 (2012) 405–409. [11] J.-C. Lin, B.-R. Huang, T.-C. Lin, Appl. Surf. Sci. 289 (2014) 384–387. [12] W.T. Zheng, Y.M. Ho, H.W. Tian, M. Wen, J.L. Qi, Y.A. Li, J. Phys. Chem. C 113 (2009) 9164–9168.
[13] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Carbon 45 (2007) 1558–1565. [14] S.Y. Bae, H.C. Choi, C.W. Na, J.H. Park, Appl. Phys. Lett. 86 (2005) 033102. [15] J.Z. Xu, R. Pan, Y.W. Chen, X.Q. Piao, M. Qian, T. Feng, Z. Sun, J. Alloys Comp. 551 (2013) 348–351. [16] F. Xu, K. Yu, G. Li, Q. Li, Z. Zhu, Nanotechnology 17 (2006) 2855–2859. [17] J. Hwang, D. Lee, J. Kim, T. Han, B. Kim, M. Park, J. Mater. Chem. 24 (2011) 3432–3437. [18] S. Zhang, Y. Zhang, S. Huang, H. Liu, P. Wang, H. Tian, J. Phys. Chem. C 114 (2010) 19284. [19] W. Gu, Appl. Phys. Lett. 89 (2006) 143111.