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.
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.
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