Abstract
Field emission properties of a stand-alone MgO-coated carbon nanotube (CNT) have been investigated by a field emission microscopy. A finite element method is used to analyze the electric field distribution on the surface of a CNT and in the layer of MgO. It is found that the dielectric MgO layer decreases the strength of electric field on the CNT surface and then increases the tunnel barrier of field emission. However, it is observed that the MgO coating largely decreases the fluctuation of the emission current from a CNT especially in the current of over 10-7 A.
| Original language | English |
|---|---|
| Pages (from-to) | 1581-1583 |
| Number of pages | 3 |
| Journal | Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures |
| Volume | 25 |
| Issue number | 5 |
| DOIs | |
| Publication status | Published (in print/issue) - 28 Sept 2007 |
Funding
Pan Lujun a) Konishi Yasumoto Tanaka Hiroyuki Department of Physics and Electronics, Osaka Prefecture University , 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan Chakrabarti Supriya Osaka Science and Technology Center , Innovation Plaza Osaka, 3-1-10 Techno Stage, Izumi, Osaka 594-1144, Japan Hokushin Shogo Akita Seiji Department of Physics and Electronics, Osaka Prefecture University , 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan Nakayama Yoshikazu Department of Mechanical Engineering, Graduate School of Engineering, Osaka University , 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan a) Electronic mail: [email protected] 09 2007 25 5 1581 1583 30 01 2007 17 07 2007 15 08 2007 2007 American Vacuum Society 1071-1023/2007/25(5)/1581/3/ $23.00 Field emission properties of a stand-alone MgO-coated carbon nanotube (CNT) have been investigated by a field emission microscopy. A finite element method is used to analyze the electric field distribution on the surface of a CNT and in the layer of MgO. It is found that the dielectric MgO layer decreases the strength of electric field on the CNT surface and then increases the tunnel barrier of field emission. However, it is observed that the MgO coating largely decreases the fluctuation of the emission current from a CNT especially in the current of over 10 − 7 A . carbon nanotube MgO field emission tungsten tip field emission microscopy Carbon nanotubes (CNTs), due to their small radii of apex and high aspect ratios, are expected to be an ideal electron-emission material and have been investigated energetically both in theoretical studies and practical usages during the past decade. 1–3 One of the biggest problems that obstructs the CNTs to be commercially used as a kind of field emission device is the stability of the emission current during the field emission from CNTs. Many methods, such as thermal treatment, 4 laser or ion beam irradiation, 5 plasma treatment, 6 and physical coating, 7–9 have been tried and studied in order to increase the field emission stability and uniformity of CNTs. In these methods, coating CNTs with MgO that has a wide gap, high yield of secondary electrons, and small positive electron affinity ( ∼ 0.85 eV ) , as well as its chemical inertness and mechanical hardness, is a good candidate for the improvement of field emission properties of CNTs including their stability, uniformity, and lifetime. 7,8,10 However, all the MgO-coating concerned experiments were performed on the surface of mass CNTs without the detailed discussion of the mechanism for the effect of MgO coating on CNTs’ field emission. In order to make the system simple and to extract the substance of the effect of MgO coating on the field emission properties, we have used a stand-alone CNT for the study of its field emission properties before and after MgO coating by a field emission microscopy (FEM). It is evidenced that the MgO coating largely decreases the fluctuation of emission current from the CNT. Carbon nanotubes used were prepared by an arc discharge method. These nanotubes have an average diameter of 10 nm and the length of 2 – 5 μ m , and both of their ends are capped. A nanotube was attached to a tungsten tip by using a scanning electron microscope (SEM) manipulator in the same manner as that for preparing nanotube probes and nanotube tweezers. 11,12 The tungsten tip was manipulated to contact nanotubes aligned at the knife-edge (called a nanotube cartridge) and an amorphous carbon layer was formed by an electron induced deposition process on the overlap position of the nanotube and tungsten tip to strengthen the attachment. Removing the nanotube from the knife-edge provides a stand-alone nanotube emitter with a capped tip. The tungsten tip was fabricated by the electrochemical sharpening of one end of a wire of 0.1 mm diameter and 4 mm length. The other end was spot welded in the middle of a staple-shape tungsten wire of 0.2 mm diameter, as illustrated in Fig. 1 . Figure 1 shows an experimental setup for detecting the field emission from a stand-alone CNT. The tungsten tip with the CNT was set as cathode. The distance between the anode plate and the tungsten tip was 28 mm . The chamber was evacuated up to an UHV grade of 10 − 7 Pa order. The CNT connected tungsten tip was heat treated up to the temperature of 700 K by the Joule heating of the staple-shape tungsten wire and then the emission current was measured using an electromultimeter (Keithley 6517A) by applying a positive voltage to the anode at room temperature. After the field emission measurement, the tungsten tip with the CNT was removed to another vacuum chamber and coated with a 15 - nm -thick MgO layer by an electron beam evaporation technique. The MgO powder (Aldrich, 99.9%) was used as a source of MgO, and the substrate temperature was maintained at 623 K . The deposition rate of MgO varied in the range of 0.7 – 0.9 Å ∕ s . The pressure inside the chamber during MgO deposition was maintained at ∼ 1 × 10 − 6 Torr . Figure 2 shows the SEM images of the CNT tip (a) before and (b) after MgO coating. The extrusion length of the CNT is 340 nm . Then the sample was removed to the FEM chamber again and the field emission properties of the MgO-coated CNT were measured for a comparison with that without MgO coating. Figure 3 shows the emission currents as a function of applied voltage for the CNT tip before and after MgO coating. It is observed that the turn-on voltage of the field emission from the CNT tip is 340 V and this value is increased to 500 V after the MgO coating. The increase of the turn-on voltage is due to the dielectric layer of MgO, which blocks the electron emission from CNT to vacuum. The electric field distributions on a CNT with and without MgO coating were calculated by a finite element method. The model used for the calculation is shown in Fig. 4 , where a single CNT with a diameter of 10 nm and a length of 300 nm is set on a truncated cone which acts as the cathode (grounded) and a metal plate as the anode. The distane between the anode and the CNT is set to be 50 μ m and the applied voltage is 100 V . This setting corresponds to our experimental case of applying a voltage of 750 V between a CNT-containing cathode and an anode plate separated by 28 mm . The thickness of MgO coating layer is set to be 15 nm . Figure 5 is the simulation result showing the distributions of electric field and potential near the surface of a CNT with and without MgO coating. In the case of the CNT without MgO coating, the electric field is concentrated strongly on the CNT surface and decreased sharply along the direction apart from the CNT tip. In the case of the MgO-coated CNT, the electric field on the surface of the CNT is relatively weaker than that of the bare CNT and is decreased markedly through the MgO layer. The field is concentrated again on the surface of MgO layer and then decreased moderately along the direction apart from the tip. It is an opposite trend in the case of potential distribution. From the potential distributions for a CNT with and without MgO coating based on the results of overall simulations, the electrons must penetrate the MgO layer before being released to the vacuum. The tunneling barrier for MgO is thicker than that for the vacuum surrounding the CNT, which is the reason why the turn-on voltage for MgO-coated CNT is larger than that for the CNT without MgO coating. Figure 6 shows the FEM patterns for the CNT (a) before and (b) after MgO coating around the level of emission current of 1 × 10 − 6 A . It is clearly observed that the emission pattern is enlarged and the number of bright spot is increased by the MgO coating. This is speculated to have resulted from the scattering of electrons by the defects in MgO crystalline structure and the smaller electric field in the MgO film and the larger curvature of MgO surface. Figure 7 shows the temporal fluctuations of the emission current for the CNT before and after MgO coating at the emission levels of (a) 10 − 7 and (b) 10 − 6 A , respectively. It is found that the current fluctuation is largely decreased by MgO coating, which is consistent with the result from the MgO-coated vertically aligned mass CNTs. 10 In the level of emission current of 10 − 6 A , the fluctuation (rms value) is decreased from 25% to 9% and in the 10 − 7 A level, the fluctuation is decreased from 39% to 4%. The possible reasons causing the stabilization of the emission current by MgO coating are considered as the follows: (1) The MgO layer simply acts as a compensation resistance. In the field emission circuit, when the emission current passing through the coated MgO film increases without changing of the overall applied voltage, e.g., during a transient current spike, the resistance of the MgO possibly decreases, perhaps similar to the reactance of a dielectric containing capacitor decreasing for high frequency spikes. Therefore, the potential across the MgO layer decreases. As a consequence, the tunnel barrier in the MgO film becomes thicker, resulting in the decrease of the emission current. It is opposite in the reverse case. In other words, the MgO layer plays a compensation role during the field emission to stabilize the emission current. (2) The interaction between a residual gas molecule and MgO is different from that between a gas molecule and CNT. The adsorption/desorption of residual gases may be suppressed by the MgO coating. (3) The enlarged emission area averages out the local temporal instabilities, resulting in a more stable total emission current. The detailed mechanism for the MgO effect can be an interesting subject for further study. In summary, the effect of MgO thin layer coated on a CNT has been investigated by a FEM and a finite element simulation. It is observed that the MgO coating increases the turn-on voltage due to the dielectric layer, which increases the tunneling barrier for the electrons, and increases the emission area. The other excellent effect of MgO coating is that it largely decreases the fluctuation of the emission current from a CNT, which is desired and would be utilized in fabrication of the CNT or the other kinds of emitters. This work was partially supported by New Energy and Industrial Technology Development Organization (NEDO) and by the Grant-in-Aid for Scientific Research from Japan Society for the Promotion of Science. FIG. 1. Schematic configuration of FEM apparatus. FIG. 2. SEM images of a CNT attached on a tungsten tip (a) before and (b) after MgO coating. FIG. 3. Voltage-current ( I - V ) curves of field emission from the CNT with and without MgO coating, respectively. The arrows show the turn-on and turn-off directions of the current during a voltage scanning cycle. FIG. 4. Model of the simulation using finite element method. FIG. 5. Distributions of the electric field and potential on the CNT before and after MgO coating calculated by a finite element method. FIG. 6. FEM images of the emission patterns for the CNT (a) before and (b) after MgO coating. FIG. 7. Fluctuations of emission current for the CNT before and after MgO coating at the emission level of (a) 10 − 6 and (b) 10 − 7 A , respectively.
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