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Fabrication and field emission properties of needle-shaped MoO3 nanobelts
Journal of Alloys and Compounds 576 (2013) 332–335
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Fabrication and field emission properties of needle-shaped MoO3 nanobelts Wei-Qing Yang a,b, Zhao-Rong Wei b, Min Gao a, Yin Chen b, Jin Xu b, Chong-Lin Chen c,d, Yuan Lin a,⇑ a
State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China Department of Photoelectric Technology, Chengdu University of Information Technology, Chengdu 610225, China c Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX 78249, USA d The Texas Center for Superconductivity, University of Houston, Houston, TX 77204, USA b
a r t i c l e
i n f o
Article history: Received 22 February 2013 Received in revised form 27 May 2013 Accepted 31 May 2013 Available online 10 June 2013 Keywords: Needle-shaped MoO3 nanobelts RF-magnetron sputtering Field emission properties
a b s t r a c t Slab-sided and needle-shaped MoO3 nanobelts with preferred (0 1 0) orientation normal to the surfaces of Cu coated quartz substrates were fabricated by RF-magnetron sputtering. The threshold fields with the separation of 100 lm and 150 lm between the anode and the sample were successfully controlled to be 6.17 and 5.13 MV/m, respectively. Their enhancement factors were improved to 5928 and 6960, respectively. In addition, the current fluctuation with time at a fixed voltage was limited to be within 4%. These excellent results demonstrated that the as-grown needle-shaped MoO3 nanobelt structure could be a promising candidate for the electronic display devices. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Quasi-one-dimensional materials have become attractive due to their unique optical, electric, and mechanical properties for promising nanodevice applications [1–4]. Among them, the unique field-emission properties from various one-dimensional nanomaterials, such as MoO3 nanobelts and nanowires [1,5–11], ZnO [12], SiC [13], and Cu2S nanowires [14], have been widely studied in the past few years owing to their prospective applications in large-area flat panel displays and other device development. The key efforts for improving the field emission properties in the nanomaterial device performance are to lower down the turn-on and threshold voltages for the field emission and to increase the field enhancement factor. These critical factors are highly related to the small tip radius of curvature for the field emission materials [1,8,13]. Recently, layered oxide, MoO3, has showed its excellent field emission properties for field emission device applications [1,5–9,15]. Normally, MoO3 nanobelts or nanowires were prepared by using the thermal evaporation technique, resulting in misaligned flat tips [1,5,7–9] that are unfavorable to the field emission. Therefore, fabricating needle-shaped MoO3 nanobelts with preferred orientation on the surfaces of the substrates has become a critical issue in enhancing their field emission performance. Recently, we have systematically studied the growth dynamics and ⇑ Corresponding author. Tel.: +86 28 83203901. E-mail address: [email protected] (Y. Lin). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.05.221
optimized their microstructures to obtain the controllable needle-shaped MoO3 nanobelts with preferred orientation, which has shown significantly enhanced field emission rate for field emission device development. Here, we report our achievements on the fabrication of slab-sided and needle-shaped MoO3 nanobelts with preferred orientation on the Cu electrode by the RF-magnetron sputtering technique. It is revealed that their enhancement factor b for the field emission was improved to be 6960 and the current fluctuation with time at a fixed voltage was controlled to be within 4%, suggesting that the slab-sided and needle-shaped MoO3 nanobelts can be a promising candidate for the electronic display devices.
2. Experimental Experimentally, slab-sided and needle-shaped MoO3 nanobelts were deposited directly on the copper electrode by RF-magnetron sputtering technique. The copper thin film was chosen as the electrode because the work function of copper is close to that of MoO3 [17–19], which is good to decrease the Schottky barrier and to increase the current density. Before the preparation of MoO3 nanobelts, the circular Cu thin film electrode was deposited on quartz. The MoO3 nanobelts were deposited directly on the copper thin film electrode by the RF-magnetron sputtering with the MoO3 target (purity 99.99%). The working pressure was maintained at about 1.5 101 Pa with a flowing gas mixed by O2 and Ar throughout the sputtering process. The flow rates of O2 and Ar were, respectively, 5 and 25 sccm. The distance between the target and the substrate was 7.5 cm. MoO3 nanobelts were uniformly deposited directly on the copper/quartz substrate at 200 °C with the sputtering power of 400 W for 10 min. Subsequently, the samples were annealed in air at 500 °C for 1 h to decrease the oxygen vacancies of the as-grown samples and to form the nanobelts with a-MoO3 structure [16].
W.-Q. Yang et al. / Journal of Alloys and Compounds 576 (2013) 332–335
Fig. 1. The XRD pattern of the MoO3 nanowires/Cu/quartz.
The microstructures and surface morphologies of the samples were then characterized by an X’Pert Pro MPD (Holland) X-ray diffractometer (XRD) with 2.2 kW Cu Ka radiation and a field-emission scanning electron microscope (SEM) (S4800, Japan). To measure the field emission properties of the needle-shaped MoO3 nanobelts, the samples were placed in a vacuum chamber with a pressure of about 4.5 105 Pa at room temperature. The MoO3 nanobelts/Cu/quartz was acted as the cathode emitter. A quartz plate coated with conducting indium tin oxide (ITO), as an anode, was placed parallel to the cathode emitter.
3. Results and discussion 3.1. Structure properties MoO3 has the orthorhombic structure with the lattice constants of a = 3.962 Å, b = 13.858 Å, c = 3.697 Å and Pbnm (62) space group (PDF# 05-0508) [20]. Fig. 1a and b are, respectively, the typical XRD patterns of our post-annealing and pre-annealing MoO3 nanobelt samples which can be indexed with the orthorhombic structure mentioned above. It is worth noting that the structure of the sample becomes a-MoO3 phase from polycrystalline a–b mixed MoO3 phase after annealing [16]. Moreover, the a-MoO3 nanobelts have a preferred orientation with the b-axis normal to the Cu electrode surface [16,21,22]. Besides, the Cu electrode also has a preferred orientation (2 0 0) after annealing. Fig. 2a is the SEM image showing the morphology of the samples, revealing that most of
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the MoO3 nanobelts are basically straight and up with needleshaped tips. The lengths of the nanobelts are close to 2 lm and their shapes are slab-sided and straightforward. In general, the tips of the nanobelts are needle-shaped and the radii of their tips are in the range of 50 to 90 nm, which are remarkably in favor of the field emission application. It is interesting to find a broken tip at the low left corner of Fig. 2a and its enlarged picture is shown at the up left corner of the figure. The needle-shaped tip is normally cleaved in a flat surface parallel to the bottom of slab-sided MoO3 nanobelt. Fig. 2b shows the schematic view of the crystal structures of MoO3 nanobelts that the MoO6 octahedras have the corners shared with each other to form a layer of MoO3 in the ac plane. Thus, along the b-axis direction, the layers are stacked in a staggered arrangement with van der Waals’ force [23], which are much weaker than the interactions between the atoms within each layer. So, the nanobelt should be more easily to break off along the transversal ac plane, further confirming that as-grown MoO3 nanobelts should have preferred b-axis orientation. 3.2. Growth mechanism To understand the growth mechanism of the needle-shaped MoO3 nanowires, the crystal structure of the MoO3 needs to be considered. As shown in Fig. 2b, the MoO3 layers are stacked in a staggered arrangement along the b-axis direction and are held together by van der Waals’ forces. Since the nanowires were grown at a low temperature with no metallic catalysts in the original growth process, a modified vapor–solid (VS) mechanism [3,6,9,24,25] should be dominant for the growth process of MoO3 nanowires rather than the vapor–liquid–solid (VLS) growth mechanism [26,27]. In a typical VS process, the molybdenum oxides (MoOx) are sputtered from a solid source by bombarding with high energetic ions, and directly deposited onto the very thin CuO film, which is the oxidation layer on the surface of Cu electrode during the sputtering process. When the sputtered molybdenum oxide molecules with high mobility reach the vicinity of the substrate, they have enough energy to diffuse and nucleate on CuO surface at the deposition temperature [28,29]. These foreign atoms and molecules normally take the atomic hopping mechanism rather than the atomic exchange diffusion mechanism [30] to hop to
Fig. 2. (a) The SEM micrograph of the MoO3 nanobelts; (b) the crystal structure of MoO3 nanowires (silver gray lozenge representing MoO6 octahedra); (c) the schematic of field emission measurement.
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W.-Q. Yang et al. / Journal of Alloys and Compounds 576 (2013) 332–335
Fig. 3. A proposed growth model of needle-shape MoO3 nanowires.
the low energy surface sites such as surface step terrace edge, ledge, etc. to form into small clusters or nucleation seeds. Generally, according to the kinetics of whisker growth [24,31,32], CuO has much lower surface energy than MoOx. Thus, the MoOx molecules/particles will favor to nucleate in the flat MoO3 islands along (0 1 0) crystallographic plane on the CuO surfaces, as seen in Fig. 3a. The newly arriving MoOx molecules will accumulate on the new formed MoO3 island surfaces, resulting in the fast growth along the b-axis direction on the MoO3 surface area, as seen in Fig. 3b. The continual accumulation of the incoming MoO3 molecules on the MoO3 island surfaces will enable the fast stacking of MoO3 layers to form nanobelts (Fig. 3c). With the belts continually growing, the top surface areas are gradually reduced due to the EhrlichSchwoebel potential at the surface step terrace edges [33–35]. Therefore, a long nanobelt with needle shaped tip is formed along the b-axis direction, as seen in Fig. 3d. But at this moment, the structure of the sample is a polycrystalline a–b mixed MoO3 phase (Fig. 1b). After annealing in air at 500 °C for one hour, the final asgrown nanobelts (Fig. 3e) are with a-MoO3 structure (Fig. 1a) and have a preferred orientation with the b-axis normal to the Cu electrode surface. This mechanism is in good agreement with the previous observation as seen in Fig. 2a, where the tips of MoO3 nanowires have the needle shape.
3.3. Field emission properties Fig. 2c shows a schematic to demonstrate the setup for the measurements of field emission performance. Fig. 4a is the typical plots of field-emission current density versus the applied electrical field. Generally, the field emission turn-on field (Eto) and threshold field (Ethr) are defined to be the electrical fields required to produce a current density of 10 lA/cm2 and 10 mA/cm2 [2,5,6], respectively. As seen in Table 1, the threshold fields, Ethr, of the MoO3 nanobelts with cathode–anode separations for 100 lm and 150 lm are 6.17 MV/m and 5.13 MV/m, respectively. Obviously, the Eto and Ethr values of our MoO3 nanobelts are lower than the early reported values [1,5,6,9], which may result from the sharp tips of the needle-shaped MoO3 nanobelts with preferred (0 1 0) orientation normal to the surface of the substrate. Furthermore, the field-emission current density–electric field properties are analyzed using Fowler–Nordheim (FN) theory [17]:
ln
J E
2
Aab2 ¼ ln /
!
B/3=2 bE
ð1Þ
where A is 1.54 106 A eV V2, B is 6.83 103 eV3/2 V lm1, b is the field-enhancement factor, a is the effective emission area, and /
Table 1 The emission turn-on fields Eto (MV/m), threshold fields Ethr (MV/m) and the field enhancement factor b of various molybdenum and molybdenum oxides for different anode–cathode separations d (lm).
a
Fig. 4. (a) Field emission J–V curves measured at different sample-anode separations (inset: corresponding FN plots) and (b) a curve of emission current density versus time at a fixed field.
b c d
Materials
d
Eto
Ethr
b
Mo nanowiresa MoO2 nanowiresa MoO3 nanowiresa MoO3 nanobeltsb MoO3 nanobeltsb MoO3 nanostarsc MoO3 nanoflakesd MoO3 nanowires MoO3 nanowires
300 300 300 50 80 150
2.2 2.4 3.5 13.2 8.7 4.2
4400
100 150
2.30 1.86
6.24 5.6 7.65 19.5 12.9 5.6 13 6.17 5.13
[6]. [5]. [1]. [9].
3881 2500 5928 6960
W.-Q. Yang et al. / Journal of Alloys and Compounds 576 (2013) 332–335
is the work function of the emitting materials which is about 5.7 for molybdenum oxides [17]. By plotting ln (J/E2) versus 1/E (see the inset of Fig. 4a), the linear relationships of different anode–cathode separations can be obtained and the slopes for both 100 lm and 150 lm separations are found to be 15.678 ± 0.487 and 13.355 ± 0.195, respectively. Their field enhancement factors b, can be estimated to be 5928 and 6960, respectively. These values are higher than the previous reported results [1,6,9], suggesting that the sharp tips of the needle-shaped MoO3 nanobelts are favorable for the field emission properties of MoO3 nanobelts. To study the field emission stability of the MoO3 nanobelts, the current fluctuation with time was measured at a fixed voltage (dc mode). As seen in Fig. 4b, our nanobelts show excellent current stability with their fluctuations less than ±4%, which is much lower than the previous reported values of ±15% and ±10% [5,8]. No obvious degradation of current density is observed after three hours. Therefore, the slab-sided and needle-shaped MoO3 nanobelts are stable enough for various field emission applications. 4. Conclusions In summary, slab-sided and needle-shaped MoO3 nanobelts with preferred (0 1 0) orientation normal to the surfaces of Cu coated quartz substrates were deposited by the RF-magnetron sputtering technique and corresponding field emission properties were investigated in details. The results showed that the Eto and Ethr values of the MoO3 nanobelts were obviously reduced and their enhancement factors b were improved. In addition, the current fluctuation with time at a fixed voltage was controlled to be within 4%. The investigation demonstrated the slab-sided and needleshaped MoO3 nanobelts could be a promising candidate for the electronic display devices. Acknowledgments This work is supported by the National Basic Research Program of China (973 Program) under Grant No. 2011CB301705, National Natural Science Foundation of China (No. 51202023), the Scientific Research Foundation of CUIT (No. KYTZ201208), and the Guangdong Innovative Research Team Program (No. 201001D0104713329).
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Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Fabrication and field emission properties of needle-shaped MoO3 nanobelts Wei-Qing Yang a,b, Zhao-Rong Wei b, Min Gao a, Yin Chen b, Jin Xu b, Chong-Lin Chen c,d, Yuan Lin a,⇑ a
State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China Department of Photoelectric Technology, Chengdu University of Information Technology, Chengdu 610225, China c Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX 78249, USA d The Texas Center for Superconductivity, University of Houston, Houston, TX 77204, USA b
a r t i c l e
i n f o
Article history: Received 22 February 2013 Received in revised form 27 May 2013 Accepted 31 May 2013 Available online 10 June 2013 Keywords: Needle-shaped MoO3 nanobelts RF-magnetron sputtering Field emission properties
a b s t r a c t Slab-sided and needle-shaped MoO3 nanobelts with preferred (0 1 0) orientation normal to the surfaces of Cu coated quartz substrates were fabricated by RF-magnetron sputtering. The threshold fields with the separation of 100 lm and 150 lm between the anode and the sample were successfully controlled to be 6.17 and 5.13 MV/m, respectively. Their enhancement factors were improved to 5928 and 6960, respectively. In addition, the current fluctuation with time at a fixed voltage was limited to be within 4%. These excellent results demonstrated that the as-grown needle-shaped MoO3 nanobelt structure could be a promising candidate for the electronic display devices. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Quasi-one-dimensional materials have become attractive due to their unique optical, electric, and mechanical properties for promising nanodevice applications [1–4]. Among them, the unique field-emission properties from various one-dimensional nanomaterials, such as MoO3 nanobelts and nanowires [1,5–11], ZnO [12], SiC [13], and Cu2S nanowires [14], have been widely studied in the past few years owing to their prospective applications in large-area flat panel displays and other device development. The key efforts for improving the field emission properties in the nanomaterial device performance are to lower down the turn-on and threshold voltages for the field emission and to increase the field enhancement factor. These critical factors are highly related to the small tip radius of curvature for the field emission materials [1,8,13]. Recently, layered oxide, MoO3, has showed its excellent field emission properties for field emission device applications [1,5–9,15]. Normally, MoO3 nanobelts or nanowires were prepared by using the thermal evaporation technique, resulting in misaligned flat tips [1,5,7–9] that are unfavorable to the field emission. Therefore, fabricating needle-shaped MoO3 nanobelts with preferred orientation on the surfaces of the substrates has become a critical issue in enhancing their field emission performance. Recently, we have systematically studied the growth dynamics and ⇑ Corresponding author. Tel.: +86 28 83203901. E-mail address: [email protected] (Y. Lin). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.05.221
optimized their microstructures to obtain the controllable needle-shaped MoO3 nanobelts with preferred orientation, which has shown significantly enhanced field emission rate for field emission device development. Here, we report our achievements on the fabrication of slab-sided and needle-shaped MoO3 nanobelts with preferred orientation on the Cu electrode by the RF-magnetron sputtering technique. It is revealed that their enhancement factor b for the field emission was improved to be 6960 and the current fluctuation with time at a fixed voltage was controlled to be within 4%, suggesting that the slab-sided and needle-shaped MoO3 nanobelts can be a promising candidate for the electronic display devices.
2. Experimental Experimentally, slab-sided and needle-shaped MoO3 nanobelts were deposited directly on the copper electrode by RF-magnetron sputtering technique. The copper thin film was chosen as the electrode because the work function of copper is close to that of MoO3 [17–19], which is good to decrease the Schottky barrier and to increase the current density. Before the preparation of MoO3 nanobelts, the circular Cu thin film electrode was deposited on quartz. The MoO3 nanobelts were deposited directly on the copper thin film electrode by the RF-magnetron sputtering with the MoO3 target (purity 99.99%). The working pressure was maintained at about 1.5 101 Pa with a flowing gas mixed by O2 and Ar throughout the sputtering process. The flow rates of O2 and Ar were, respectively, 5 and 25 sccm. The distance between the target and the substrate was 7.5 cm. MoO3 nanobelts were uniformly deposited directly on the copper/quartz substrate at 200 °C with the sputtering power of 400 W for 10 min. Subsequently, the samples were annealed in air at 500 °C for 1 h to decrease the oxygen vacancies of the as-grown samples and to form the nanobelts with a-MoO3 structure [16].
W.-Q. Yang et al. / Journal of Alloys and Compounds 576 (2013) 332–335
Fig. 1. The XRD pattern of the MoO3 nanowires/Cu/quartz.
The microstructures and surface morphologies of the samples were then characterized by an X’Pert Pro MPD (Holland) X-ray diffractometer (XRD) with 2.2 kW Cu Ka radiation and a field-emission scanning electron microscope (SEM) (S4800, Japan). To measure the field emission properties of the needle-shaped MoO3 nanobelts, the samples were placed in a vacuum chamber with a pressure of about 4.5 105 Pa at room temperature. The MoO3 nanobelts/Cu/quartz was acted as the cathode emitter. A quartz plate coated with conducting indium tin oxide (ITO), as an anode, was placed parallel to the cathode emitter.
3. Results and discussion 3.1. Structure properties MoO3 has the orthorhombic structure with the lattice constants of a = 3.962 Å, b = 13.858 Å, c = 3.697 Å and Pbnm (62) space group (PDF# 05-0508) [20]. Fig. 1a and b are, respectively, the typical XRD patterns of our post-annealing and pre-annealing MoO3 nanobelt samples which can be indexed with the orthorhombic structure mentioned above. It is worth noting that the structure of the sample becomes a-MoO3 phase from polycrystalline a–b mixed MoO3 phase after annealing [16]. Moreover, the a-MoO3 nanobelts have a preferred orientation with the b-axis normal to the Cu electrode surface [16,21,22]. Besides, the Cu electrode also has a preferred orientation (2 0 0) after annealing. Fig. 2a is the SEM image showing the morphology of the samples, revealing that most of
333
the MoO3 nanobelts are basically straight and up with needleshaped tips. The lengths of the nanobelts are close to 2 lm and their shapes are slab-sided and straightforward. In general, the tips of the nanobelts are needle-shaped and the radii of their tips are in the range of 50 to 90 nm, which are remarkably in favor of the field emission application. It is interesting to find a broken tip at the low left corner of Fig. 2a and its enlarged picture is shown at the up left corner of the figure. The needle-shaped tip is normally cleaved in a flat surface parallel to the bottom of slab-sided MoO3 nanobelt. Fig. 2b shows the schematic view of the crystal structures of MoO3 nanobelts that the MoO6 octahedras have the corners shared with each other to form a layer of MoO3 in the ac plane. Thus, along the b-axis direction, the layers are stacked in a staggered arrangement with van der Waals’ force [23], which are much weaker than the interactions between the atoms within each layer. So, the nanobelt should be more easily to break off along the transversal ac plane, further confirming that as-grown MoO3 nanobelts should have preferred b-axis orientation. 3.2. Growth mechanism To understand the growth mechanism of the needle-shaped MoO3 nanowires, the crystal structure of the MoO3 needs to be considered. As shown in Fig. 2b, the MoO3 layers are stacked in a staggered arrangement along the b-axis direction and are held together by van der Waals’ forces. Since the nanowires were grown at a low temperature with no metallic catalysts in the original growth process, a modified vapor–solid (VS) mechanism [3,6,9,24,25] should be dominant for the growth process of MoO3 nanowires rather than the vapor–liquid–solid (VLS) growth mechanism [26,27]. In a typical VS process, the molybdenum oxides (MoOx) are sputtered from a solid source by bombarding with high energetic ions, and directly deposited onto the very thin CuO film, which is the oxidation layer on the surface of Cu electrode during the sputtering process. When the sputtered molybdenum oxide molecules with high mobility reach the vicinity of the substrate, they have enough energy to diffuse and nucleate on CuO surface at the deposition temperature [28,29]. These foreign atoms and molecules normally take the atomic hopping mechanism rather than the atomic exchange diffusion mechanism [30] to hop to
Fig. 2. (a) The SEM micrograph of the MoO3 nanobelts; (b) the crystal structure of MoO3 nanowires (silver gray lozenge representing MoO6 octahedra); (c) the schematic of field emission measurement.
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W.-Q. Yang et al. / Journal of Alloys and Compounds 576 (2013) 332–335
Fig. 3. A proposed growth model of needle-shape MoO3 nanowires.
the low energy surface sites such as surface step terrace edge, ledge, etc. to form into small clusters or nucleation seeds. Generally, according to the kinetics of whisker growth [24,31,32], CuO has much lower surface energy than MoOx. Thus, the MoOx molecules/particles will favor to nucleate in the flat MoO3 islands along (0 1 0) crystallographic plane on the CuO surfaces, as seen in Fig. 3a. The newly arriving MoOx molecules will accumulate on the new formed MoO3 island surfaces, resulting in the fast growth along the b-axis direction on the MoO3 surface area, as seen in Fig. 3b. The continual accumulation of the incoming MoO3 molecules on the MoO3 island surfaces will enable the fast stacking of MoO3 layers to form nanobelts (Fig. 3c). With the belts continually growing, the top surface areas are gradually reduced due to the EhrlichSchwoebel potential at the surface step terrace edges [33–35]. Therefore, a long nanobelt with needle shaped tip is formed along the b-axis direction, as seen in Fig. 3d. But at this moment, the structure of the sample is a polycrystalline a–b mixed MoO3 phase (Fig. 1b). After annealing in air at 500 °C for one hour, the final asgrown nanobelts (Fig. 3e) are with a-MoO3 structure (Fig. 1a) and have a preferred orientation with the b-axis normal to the Cu electrode surface. This mechanism is in good agreement with the previous observation as seen in Fig. 2a, where the tips of MoO3 nanowires have the needle shape.
3.3. Field emission properties Fig. 2c shows a schematic to demonstrate the setup for the measurements of field emission performance. Fig. 4a is the typical plots of field-emission current density versus the applied electrical field. Generally, the field emission turn-on field (Eto) and threshold field (Ethr) are defined to be the electrical fields required to produce a current density of 10 lA/cm2 and 10 mA/cm2 [2,5,6], respectively. As seen in Table 1, the threshold fields, Ethr, of the MoO3 nanobelts with cathode–anode separations for 100 lm and 150 lm are 6.17 MV/m and 5.13 MV/m, respectively. Obviously, the Eto and Ethr values of our MoO3 nanobelts are lower than the early reported values [1,5,6,9], which may result from the sharp tips of the needle-shaped MoO3 nanobelts with preferred (0 1 0) orientation normal to the surface of the substrate. Furthermore, the field-emission current density–electric field properties are analyzed using Fowler–Nordheim (FN) theory [17]:
ln
J E
2
Aab2 ¼ ln /
!
B/3=2 bE
ð1Þ
where A is 1.54 106 A eV V2, B is 6.83 103 eV3/2 V lm1, b is the field-enhancement factor, a is the effective emission area, and /
Table 1 The emission turn-on fields Eto (MV/m), threshold fields Ethr (MV/m) and the field enhancement factor b of various molybdenum and molybdenum oxides for different anode–cathode separations d (lm).
a
Fig. 4. (a) Field emission J–V curves measured at different sample-anode separations (inset: corresponding FN plots) and (b) a curve of emission current density versus time at a fixed field.
b c d
Materials
d
Eto
Ethr
b
Mo nanowiresa MoO2 nanowiresa MoO3 nanowiresa MoO3 nanobeltsb MoO3 nanobeltsb MoO3 nanostarsc MoO3 nanoflakesd MoO3 nanowires MoO3 nanowires
300 300 300 50 80 150
2.2 2.4 3.5 13.2 8.7 4.2
4400
100 150
2.30 1.86
6.24 5.6 7.65 19.5 12.9 5.6 13 6.17 5.13
[6]. [5]. [1]. [9].
3881 2500 5928 6960
W.-Q. Yang et al. / Journal of Alloys and Compounds 576 (2013) 332–335
is the work function of the emitting materials which is about 5.7 for molybdenum oxides [17]. By plotting ln (J/E2) versus 1/E (see the inset of Fig. 4a), the linear relationships of different anode–cathode separations can be obtained and the slopes for both 100 lm and 150 lm separations are found to be 15.678 ± 0.487 and 13.355 ± 0.195, respectively. Their field enhancement factors b, can be estimated to be 5928 and 6960, respectively. These values are higher than the previous reported results [1,6,9], suggesting that the sharp tips of the needle-shaped MoO3 nanobelts are favorable for the field emission properties of MoO3 nanobelts. To study the field emission stability of the MoO3 nanobelts, the current fluctuation with time was measured at a fixed voltage (dc mode). As seen in Fig. 4b, our nanobelts show excellent current stability with their fluctuations less than ±4%, which is much lower than the previous reported values of ±15% and ±10% [5,8]. No obvious degradation of current density is observed after three hours. Therefore, the slab-sided and needle-shaped MoO3 nanobelts are stable enough for various field emission applications. 4. Conclusions In summary, slab-sided and needle-shaped MoO3 nanobelts with preferred (0 1 0) orientation normal to the surfaces of Cu coated quartz substrates were deposited by the RF-magnetron sputtering technique and corresponding field emission properties were investigated in details. The results showed that the Eto and Ethr values of the MoO3 nanobelts were obviously reduced and their enhancement factors b were improved. In addition, the current fluctuation with time at a fixed voltage was controlled to be within 4%. The investigation demonstrated the slab-sided and needleshaped MoO3 nanobelts could be a promising candidate for the electronic display devices. Acknowledgments This work is supported by the National Basic Research Program of China (973 Program) under Grant No. 2011CB301705, National Natural Science Foundation of China (No. 51202023), the Scientific Research Foundation of CUIT (No. KYTZ201208), and the Guangdong Innovative Research Team Program (No. 201001D0104713329).
335
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