Highly conductive vertically aligned molybdenum nanowalls and their field emission property
© Shen et al.; licensee Springer. 2012
Received: 19 July 2012
Accepted: 8 August 2012
Published: 17 August 2012
We report that vertically aligned molybdenum (Mo) nanowalls can grow on various substrates by simple thermal vapor deposition. Individual nanowalls have a typical thickness of about 50 nm and very good conductivity with a typical average value of about 1.97 × 104 Ω−1 cm−1, i.e., only an order of magnitude less than the value of bulk Mo. The formation process is characterized in detail, and it is found that Mo nanowalls grow from nanorods through nanotrees. The atomic arrangement, lattice mismatch relationship, and competition growth are all believed to contribute to the growth mechanism. The field emission performance is attractive, typically with a very low fluctuation of about approximately 1.18% at a high current density level of 10 mA/cm2, and a sustainably stable very large current density of approximately 57.5 mA/cm2 was recorded. These indicate that the Mo nanowall is a potential candidate as a cold cathode for application in vacuum electron devices, which demand both a high current and high current density.
KeywordsMolybdenum nanowalls Thermal vapor deposition Growth mechanism Field emission Electrical conductivity
Field emission cold cathode has many important applications. There are typically two areas which have attracted the attention of researchers, i.e., the field emission flat panel display (FED) and the vacuum electron devices, such as X-ray sources[2, 3] and microwave sources[4, 5]. FED requires large-area cold cathode arrays, which are addressable and thus need low-field emitters. The vacuum electron devices generally require both a high current and high current density. Now, it has been shown that nanostructured materials can give low-field cold electron emission. However, one has come across a number of problems in pursuing a high-current and high-current density cold cathode. The major problem is that there are high-resistance regions in the individual nanostructure using nanowire as an emitter, which can lead to vacuum breakdown when it undergoes a high-current density operation. Even more, so far, the conductivity of many nanotubes and nanowires is not high enough, except that for carbon nanotubes. Carbon nanotubes, however, are easily oxidized during the vacuum sealing process and thus lose its good properties.
We have been trying two other approaches aimed at overcoming the above difficulties. One is to grow nanowires or nanowalls with a high electrical conductivity[7–16]. In the recent years, one has seen a large number of studies focused on the preparation of one-dimensional molybdenum oxide nanostructures, such as MoO2 nanorods, pinaster-like MoO2 nanoarrays, and MoO2 nanostars, with a few cases on two-dimensional (2D) molybdenum oxide nanostructures such as MoO3 nanobelts. Our group has been developing a thermal vapor deposition method for synthesizing vertically well-aligned molybdenum oxide nanowires[11–14] and for a large-area preparation of 2D MoO3 microbelts[15, 16]. In this paper, we report that uniform Mo nanowalls can be grown on various substrates by simple thermal vapor deposition. The morphology and crystalline structure of the nanowalls are characterized by scanning electron microscopy, X-ray diffraction, and transmission electron microscopy techniques. The growth mechanism is discussed. The field emission performance and electrical transport property of the nanowalls are shown to be promising in meeting the requirements of high current and high current density of some vacuum electron devices.
Mo atoms of the Mo boat react with residual oxygen in the vacuum chamber spontaneously to give rise to MoO2 (Equation 1). Then, Mo atoms coming from the decomposition of MoO2 deposit during a condensation process (Equation 2). Here, we should note that when the substrate temperature is over 1,428 K, the condensation process of MoO2 will be difficult. So, the key to synthesize the molybdenum nanostructures rather than oxide ones is the high-enough reaction temperature. Silicon, stainless steel, or silicon carbide was used as substrates in this study. They were first washed with acetone and then with alcohol, respectively, for more than 15 min in an ultrasonic bath. The substrate and Mo boat were then placed in the center of vacuum chamber, keeping them about 5 mm away from each other. The Mo boat as evaporation source was heated by electric current. The vacuum chamber was evacuated using a mechanical pump. When the vacuum reached below approximately 6 × 10−2 Torr, high-purity argon (Ar) gas (99.99%) was introduced into the system at a flow rate of approximately 200 sccm, accompanied by high-purity hydrogen (H2) gas (99.99%) at a flow rate of approximately 80 sccm. The boat temperature was then increased gradually at a rate of 50 K/min to above 1,623 K, in order to ensure the growth of the metallic molybdenum nanowall rather than the oxide one. After deposition for more than 10 min, the preparation of molybdenum nanowalls was completed. No catalysts were used in the whole process of growth.
It is significant that to obtain molybdenum nanowalls, the substrates should be placed below the Mo boat with a certain distance away, instead of being directly placed inside the boat, as we did when growing Mo nanowires. In the present study, the growth orientation and the driving force of nucleation can be controlled by adjusting the spacing between the Mo boat and substrates.
Field emission scanning electron microscopy (XL-SFEG SEM, FEI Co., Hillsboro, OR, USA) was employed to investigate the morphology of molybdenum nanowalls. Transmission electron microscopy (Tecnai-20 TEM, Philips, FEI Co., Hillsboro, OR, USA), and X-ray diffraction (XRD; Rigaku RINT 2400, Rigaku Corporation, Tokyo, Japan) were applied to study their crystalline structures. The field emission properties of molybdenum nanowall films were measured in the field emission analysis system. In addition, the electrical transport measurement of the individual nanowalls was carried out using a micro-point anode in situ in a modified SEM system (JEOL-6380, JEOL Ltd., Akishima, Tokyo, Japan); the details of which are described in our previous work.
Results and discussion
Taking MoO2 as the reference crystal, the value of m between Mo and is only 7.23%. This explain how the trunk of the molybdenum nanotree grows along the  direction of bcc Mo. Furthermore, there exist other two kinds of relationship: and. The corresponding values of misfit are also as small as 8.37% and 8.64%, respectively. For this reason, the condensing Mo grows along the [121̄] and [11̄2] directions as the branches, on both sides of the  trunk. The angle between the  and [121̄] and the angle between the  and [11̄2] are both 60°, which are consistent with the observation from Figure8a. Here, it should be pointed out that in the bcc structure of Mo, <211 > shows six geometrically equivalent directions (Figure8d), and in theory, nanowires may grow along all these directions as Zhou et al. suggested. However, we did not observe these in the present work, and this is attributed to the limitation due to the spatial confinement.
Values of turn-on field and threshold field of molybdenum nanowall films at different vacuum gaps ( d )
Type of materials
Threshold field (V/μm)
Molybdenum nanowall films
Figure11b shows the corresponding F-N plots. One can see that these curves may be seen to compose of some discrete regions. One may artificially define the regions so that the regions are nearly straight lines. However, they have different slopes.
where Ф is the work function of the molybdenum nanowall, which adopts the work function (4.3 eV) of bulk molybdenum materials. SFN is the slope of the F-N plot. According to Equation 4, we obtained the values of FE enhancement factor β for the sample with different vacuum gaps. As shown in Figure11b, the β values in low-field emission regions are calculated, and they are 181, 395, and 817 for different d values of 100, 200, and 350 μm, respectively. It is clear that as vacuum gap increases, the β value increases. On the other hand, in each curve, the absolute value of the slope at the high-field region is lower than that at the low-field region; thus, the corresponding value of β at the high-field region is higher than that at the low-field region. In order to understand what causes the non-linearity in the F-N plots, we consider the following: First, the whole emission process may be affected by the complexity of the material property of the nanowall films. We have known that the molybdenum nanowall films consist of a nonmetallic part. The individual nanowall is covered with a very thin layer of amorphous molybdenum oxide, and there is a layer of nanorods containing molybdenum oxides existing between the nanowalls and the substrates. Both of them may have an effect on the conductivity of the material. When the applied field is very high and, thus, the emission current is very high, the FE enhancement factor β would be suppressed because the increase of the voltage drop in the nanowall film itself can result in a decrease of the local field on the surface of emitters. However, the calculated value of β at the high-field region is higher, so the above reason does not suit our case. In fact, we will show that the mid-layer and the molybdenum nanowalls both have very good conductivity, while the oxide layer is too thin to have such effect. The higher value of β at the high-field region may be explained by both the increasing effect of the thermionic emission and the increasing number of emission sites. Firstly, thermionic emission will gradually increase its effect in the high fields when the current density becomes high. The temperature of the emitters will increase due to the Joule heating. More and more hot electrons will take part in the emission, and thus, the emission will be enhanced. Secondly, the number of emission sites will increase in the high fields. We suggest that the sharp edges of the nanowalls serve as main electron emitters in the low fields, although it is not yet clear without observing the emission pattern of an individual nanowall. At higher fields, however, the nanoprotrusions adhere to the nanowall bodies, and/or the nanorods may be expected to contribute to the total emission current. The increasing emission sites with different values of β may enhance the emission, thus leading to the non-linearity in the F-N plots. The detailed explanation awaits to be given after more theoretical and experimental studies.
Figure11c shows the field emission stability curve, revealing that the molybdenum nanowall films have a relatively stable field emission at a current density of 10 mA/cm2 with a fluctuation of about approximately 1.18% in an hour of testing time. The distribution of field electron emission sites was also recorded (bottom right inset in Figure11c), showing that over 70.01% of the area has bright spots and that the distribution of emission sites is relatively homogeneous, with a value of 66.7%.
The field emission properties of various emitter materials
Reported largest current density (mA/cm2)
Fluctuation of emission current density
1.18%, 10 mA/cm2
2.5 to 3.5
3%, 5 mA/cm2
2%, 0.4 mA/cm2
5%, 10 mA/cm2
2.5%, 10 mA/cm2
10%, 10 mA/cm2
4 to 18
Pinaster-like MoO2 nanoarrays
9.2%, 2.8 mA/cm2
15%, 260 mA/cm2
2%, 0.94 mA/cm2
10%, 0.01 mA/cm2
Mo oxide nanostars
13%, 0.35 mA/cm2
2%, 0.16 mA/cm2
The calculated conductivities of different individual molybdenum nanowalls compared with other nanomaterials
Conductivity (×104Ω−1 cm−1)
Molybdenum bulk material
Agave-like 1D ZnO nanostructures
1D molybdenum nanotips
Vertically aligned molybdenum nanowalls can be prepared on varied substrates by thermal vapor deposition. Their formation undergoes through a four-stage process associated with the production of molybdenum and its dioxide nanorods, molybdenum nanotrees, molybdenum nanoflakes, and molybdenum nanowalls. Atomic arrangement, lattice mismatch relationship, and competition growth all together contribute to the growth mechanism. The nanowalls can sustain a very much larger field emission current density and better emission stability as compared to most of other nanostructures. Moreover, the single nanowall is shown to have a remarkably high electrical conductivity. The results suggest that molybdenum nanowalls should have promising futures in the application of vacuum electron devices that demand both high current and high current density.
The authors gratefully acknowledge the financial support of the project from the National Natural Science Foundation of China (Grant no.U1134006, 50725206), National Basic Research Program of China: Grant no. 2010CB327703, the Fundamental Research Funds for the Central Universities, the Science and Technology Department of Guangdong Province, the Economic and Information Industry Commission of Guangdong Province, and the Science & Technology and Information Department of Guangzhou City.
1State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yatsen University, Guangzhou 510275, People's Republic of China. 2Guangdong Province Key Laboratory of Display Material and Technology, Sun Yat-sen University, Guangzhou 510275, People's Republic of China. 3School of Physics and Engineering, Sun Yat-sen University, Guangzhou 510275, People's Republic of China.
- Xu NS, Ejaz Huq S: Novel cold cathode materials and applications. Mater Sci Eng R 2005, 48: 47. 10.1016/j.mser.2004.12.001View Article
- Yue GZ, Qiu Q, Gao B, Cheng Y, Zhang Z, Shimoda H, Chang S, Lu JP, Zhou O: Generation of continuous and pulsed diagnostic imaging X-ray radiation using a carbon-nanotube-based field-emission cathode. Appl Phys Lett 2002, 81: 355. 10.1063/1.1492305View Article
- Park JS, Kim JP, Noh YR, Jo KC, Lee SY, Choi HY, Kim JU: X-ray images obtained from cold cathodes using carbon nanotubes coated with gallium-doped zinc oxide thin films. Thin Solid Films 2010, 519: 1743. 10.1016/j.tsf.2010.08.154View Article
- Whaley DR, Bellew CL: 100 W operation of a cold cathode TWT. IEEE Trans Electro Dev 2009, 56: 896.View Article
- Legagneux P, Le Sech N, Guiset P, Gangloff L, Cojocaru C, Schnell JP, Pribat D, Teo KBK, Robertson J, Milne WI, Andre F, Rozier Y, Dieumegard D: Carbon nanotube based cathodes for microwave amplifiers. In IVEC'09: International Vacuum Electronics Conference (IVEC): April 28–30 2009; Rome. IEEE, Washington, DC; 2009:80–81.View Article
- She JC, Xiao ZM, Yang YH, Deng SZ, Chen J, Yang GW, Xu NS: Correlation between resistance and field emission performance of individual ZnO one-dimensional nanostructures. ACS Nano 2015, 2008: 2.
- Liu JG, Zhang ZJ, Pan CY, Zhao Y, Su X, Zhou Y, Yu DP: Enhanced field emission properties of MoO2 nanorods with controllable shape and orientation. Mater Lett 2004, 58: 3812. 10.1016/j.matlet.2004.07.034View Article
- Zhang JY, Liu YG, Shi SL, Wang YG, Wang TH: Low-field electron emission from pinaster-like MoO2 nanoarrays as two-stage emitters. Chin Phys B 2008, 17: 4333. 10.1088/1674-1056/17/11/062View Article
- Khademi A, Azimirad R, Zavarian AA, Moshfegh AZ: Growth and field emission study of molybdenum oxide nanostars. J Phys Chem C 2009, 113: 19298. 10.1021/jp9056237View Article
- Li YB, Bando YS, Golberg D, Kurashima K: Field emission from MoO3 nanobelts. Appl Phys Lett 2002, 81: 5048. 10.1063/1.1532104View Article
- Zhou J, Xu NS, Deng SZ, Chen J, She JC, Wang ZL: Large-area nanowire arrays of molybdenum and molybdenum oxides: synthesis and field emission properties. Adv Mater 2003, 15: 1835. 10.1002/adma.200305528View Article
- Zhou J, Xu NS, Deng SZ, Chen J, She JC: Synthesis of large-scaled MoO2 nanowire arrays. Chem Phys Lett 2003, 382: 443. 10.1016/j.cplett.2003.10.002View Article
- Zhou J, Deng SZ, Xu NS, Chen J, She JC: Synthesis and field-emission properties of aligned MoO3 nanowires. Appl Phys Lett 2003, 83: 2653. 10.1063/1.1613992View Article
- Zhou J, Deng SZ, Gong L, Ding Y, Chen J, Huang JX, Chen J, Xu NS, Wang ZL: Growth of large-area aligned molybdenum nanowires by high temperature chemical vapor deposition: synthesis, growth mechanism, and device application. J Phys Chem B 2006, 110: 10296.View Article
- Ban DM, Deng SZ, Xu NS, Chen J, She JC, Liu F: Improved field emission characteristics of large-area films of molybdenum trioxide microbelt. J Nanomater 2010, 1155: 136860.
- Ban DM, Xu NS, Deng SZ, Chen J, She JC: Low-temperature synthesis of large-area films of molybdenum trioxide microbelts in air and the dependence of their field emission performance on growth conditions. J Mater Sci Technol 2010, 26: 584. 10.1016/S1005-0302(10)60089-8View Article
- Vaddiraju S, Chandrasekaran H, Sunkara MK: Vapor phase synthesis of tungsten nanowires. J Am Chem Soc 2003, 125: 10792. 10.1021/ja035868eView Article
- Dai ZR, Pan ZW, Wang ZL: Ultra-long single crystalline nanoribbons of tin oxide. Solid State Commun 2001, 118: 351. 10.1016/S0038-1098(01)00122-3View Article
- Matthews JW: Dislocations in Solids. Amsterdam, North-Holland; 1979:2.
- Khademi A, Azimirad R, Nien YT, Moshfegh AZ: Field-emission enhancement of molybdenum oxide nanowires with nanoprotrusions. J Nanopart Res 2010, 13: 115.View Article
- Cao MH, Liu TF, Gao S, Sun GB, Wu XL, Hu CW, Wang ZL: Single-crystal dendritic micro-pines of magnetic α-Fe2O3: large-scale synthesis, formation mechanism, and properties. Angew Chem Int Ed 2005, 44: 4197. 10.1002/anie.200500448View Article
- Meng XP, Deng SZ, Xu NS: Mo nanotip arrays: synthesis and field emission properties. In IVNC'08: 21st International Vacuum Nanoelectronics Conference (IVNC): Jul 13–17 2008; Poland. IEEE, Washington, DC; 2008.
- Xu NS: The physical origin of prebreakdown electron “pin-holes”. In High Voltage Vacuum Insulator: Basic Concepts and Technological Practice. Edited by: Latham RV. Academic; UK, London; 1995.
- Rao AM, Jacques D, Haddon RC, Zhu W, Bower C, Jin S: In situ-grown carbon nanotube array with excellent field emission characteristics. Appl Phys Lett 2000, 76: 3813. 10.1063/1.126790View Article
- Pan ZW, Lai HL, Au FCK, Duan XF, Zhou WY, Shi WS, Wang N, Lee CS, Wong NB, Lee ST, Xie SS: Oriented silicon carbide nanowires: synthesis and field emission properties. Adv Mater 2000, 12: 1186. 10.1002/1521-4095(200008)12:16<1186::AID-ADMA1186>3.0.CO;2-FView Article
- Liu F, Li L, Mo FY, Chen J, Deng SZ, Xu NS: A catalyzed-growth route to directly form micropatterned WO2 and WO3 nanowire arrays with excellent field emission behaviors at low temperature. Cryst Growth Des 2010, 10: 5193. 10.1021/cg100995fView Article
- Zhou J, Gong L, Deng SZ, Chen J, She JC, Xu NS, Yang RS, Wang ZL: Growth and field-emission property of tungsten oxide nanotip arrays. Appl Phys Lett 2005, 87: 223108. 10.1063/1.2136006View Article
- Chen J, Deng SZ, Xu NS, Wang SH, Wen XG, Yang SH, Yang CL, Wang JN, Ge WK: Field emission from crystalline copper sulphide nanowire arrays. Appl Phys Lett 2002, 80: 3620. 10.1063/1.1478149View Article
- Lee CJ, Lee TJ, Lyu SC, Zhang Y, Ruh H, Lee HJ: Field emission from well-aligned zinc oxide nanowires grown at low temperature. Appl Phys Lett 2002, 81: 3648. 10.1063/1.1518810View Article
- Wei GD, Qin WP, Zhang DS, Wang GF, Kim RJ, Zheng KZ, Wang LL: Synthesis and field emission of MoO3 nanoflowers by a microwave hydrothermal route. J Alloy Compd 2009, 481: 417. 10.1016/j.jallcom.2009.03.007View Article
- Liu F, Shen CM, Su ZJ, Ding XL, Deng SZ, Chen J, Xu NS, Gao HJ: Metal-like single crystalline boron nanotubes: synthesis and in situ study on electric transport and field emission properties. J Mater Chem 2010, 20: 2197. 10.1039/b919260cView Article
- Shao PR, Deng SZ, Chen J, Chen J, Xu NS: Study of field emission, electrical transport, and their correlation of individual single CuO nanowires. J Appl Phys 2011, 109: 023710. 10.1063/1.3536478View Article
- Shi SL, Xue XY, Feng P, Liu YG, Zhao H, Wang TH: Low-temperature synthesis and electrical transport properties of W18O49 nanowires. J Cryst Growth 2008, 310: 462. 10.1016/j.jcrysgro.2007.10.038View Article
- He JH, Deng SZ, Liu F, Xu NS: Characteristics of field emission from individual molybdenum nanotip. In IVESC'10: 8th International Vacuum Electron Sources Conference and Nanocarbon: Oct 14–16 2010; Nanjing, China. IEEE, Washington, DC; 2010:504–505.View Article
- Worthing AG: Physical properties of well seasoned molybdenum and tantalum as a function of temperature. Phys Rev 1926, 28: 190. 10.1103/PhysRev.28.190View Article
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