Enhanced Electron Field Emission from Carbon Nanotube Matrices

Mater. Res. Soc. Symp. Proc. Vol. 1283 © 2011 Materials Research Society DOI: 10.1557/opl.2011.546 Enhanced Electron Field Emission from Carbon Nanotube Matrices 2 Archana Pandey1, Abhishek Prasad1, Yoke Khin Yap1, Mark Engelhard , Chongmin Wang 1 2 Department of Physics, Michigan Technological University, 118 Fisher Hall, 1400 Townsend Drive, Houghton, Michigan 49931, U.S.A. 2 EMSL, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, WA 99354, U.S.A. ABSTRACT Field emission from as-grown carbon nanotube (CNTs) films often suffered from high threshold electric field, and low emission site density due to screening effects. These problems can be resolved by patterned growth of CNTs on lithographically prepared catalyst films. However, these approaches are expensive and not applicable for future emitting devices with large display areas. Here we show that as-grown CNTs films can have low emission threshold field and high emission density without using any lithography processes. We have reduced screening effects and work function of as-grown CNTs films and created the novel CNT matrices by addition of vapor- and/or liquid- phase deposition. Furthermore, these CNT matrices can continuous emit electrons for 40 hours without significant degradation. The fabrication of our CNT matrices is described as follows. First, CNT films were grown by plasma-enhanced chemical vapor deposition. These vertically-aligned multiwalled carbon nanotubes (VA-MWCNTs) are having typical length and diameter of 4 microns and 40 nm, respectively. Spacing between these CNTs is ~80 nm in average, leading to poor emission properties due to the screening effect. These asgrown samples were then subjected to the deposition of strontium titanate (SrTiO3) by pulsedlaser deposition to reduce both the work function and screening effect of CNTs. The emission properties of these coated samples can be further improved by fully filled the spaces between VA-MWCNTs by poly-methyl metha acrylate (PMMA). The field emission threshold electric field was decreased from 4.22 V/μm for as-grown VA-MWCNTs to 1.7 V/μm for SrTiO3 coated VA-MWCNTs. The addition filling with PMMA and mechanical polishing can further reduce the threshold to 0.78V/μm for the so called PMMA-STO-CNT matrices. Long term emission stability and emission site density were also enhanced. INTRODUCTION Since their discovery CNTs have gained intensive research interest [1-2]. They are novel nanostructured materials with extraordinarily properties. It has been already demonstrated that they are among strongest nanofibres known till date, with Young’s moduli as high as 1 TPa [34]. They also have astonishing electronic properties [5], chemical stability [6] and superior mechanical properties [7]. Because of their very small radius of curvature and high aspect ratio they are expected to have excellent electronic properties [8]. Numerous efforts have been done to develop a better electron source based on field emission of CNTs that could replace the conventional field emission devices at the industrial scale in future. However, in order to do this, improvements in its field emission properties are still needed. For example, CNTs have very high work function (5eV), and as grown CNTs films suffer from high emission threshold fields (Eth) due to the screening effect from the densely packed CNTs. If one can modify the surface of CNTs by coating a low work function material, then it is possible to surmount this inadequacy of CNTs [9]. Most reported work focuses on coating wide band gap materials to reduce Eth of various types of CNTs [10-12]. In our previous work we showed that etched and bundled CNTs show excellent emission characteristics due to the reduction of screening effect [13]. In this work we suggest the use of strontium titanate (chemical formula SrTiO3) as a coating material on CNTs for lowering Eth and improving the emission stability. Among several coating materials, SrTiO 3 was chosen for its lower work function (2.6 eV) and high dielectric constant (475) [14-15]. SrTiO3 coating can play two major roles during the emission process. First, it will lower the effective work function of the emitter which in turn will increase emission density. Second, it may act as a protective layer for stable interface which will minimize the emission instabilities caused by adsorption or ion bombardment of residual gas molecules [16-17]. Here we report our experimental finding related to these topics. The threshold electric field (Eth), defined as electric field required for emitting electrons to a level of 1 µA/cm2, was 4.22 V/µm for as-grown sample. For SrTiO3 coated CNTs samples, Eth was reduced to 1.7 V/µm. After embedding SrTiO3 coated CNTs into PMMA the Eth was further reduced to 0.78 V/µm. In addition, the SrTiO3 coated and PMMA-embedded, opened-tip sample showed a significant improvement in emission stability as compared to the as-grown CNTs sample. EXPERIMENTAL DETAILS All the samples used in this experiment were prepared by dual RF-plasma-enhanced chemical vapor deposition (PECVD). The detailed experimental procedures are reported elsewhere [13]. In brief, Ni films (10 nm thick) were first deposited on p-type Si substrates (1–10 Ω cm) by RF magnetron sputtering. These substrates were then used for the growth of VAMWCNTs at 450 oC by using pure methane gas. Our VA-MWCNTs were grown within a circular area (0.385 cm2 in area). Three identical samples can be prepared in each growth process. In this case we are comparing three samples. Figure 1 shows the schematic of the processing techniques. The first sample is as grown CNTs (figure 1a) with Ni catalytic nanoparticles on tips. In the second method we coated a high dielectric constant material strontium titanate (SrTiO 3) using pulsed laser deposition technique (figure 1b). The coating was confirmed using XPS (because of page limitation the data is not shown here). Next we dipped the SrTiO3 coated CNTs in PMMA matrix (figure 1c). The PMMA solution was prepared by adding a developer solution to a volume ratio of 1:1, and a VAMWCNT sample was dipped into the solution for 15 minutes before removal. This was followed by annealing the sample at ~100°C for 2-3 minutes to dry the sample. Finally, the sample was mechanically polished by using fiber-free lapping cloth and a colloidal silica (0.02 µm) solution to expose the tips of carbon nanotubes. SrTiO3 coated CNTs with the exposed tips, separated by PMMA are schematically shown in figure 1(d). These were then characterized for field emission measurements. (a) (b) (c) (d) Figure 1. Fabrication steps of PMMA-STO-CNT field emitters. (a) As grown VA-MWNTs on Si substrates (b) SrTiO3 coated CNT (c) SrTiO3-PMMA coated CNTs and (d) Mechanical polishing to open the tips of SrTiO3-PMMA CNTs DISCUSSION Figure 2a shows the field emission characteristic curve for the as grown VA-MWCNTs. This is the curve of current density (J) versus electric field (E) for as grown VA-MWCNTs. The Fowler–Nordheim (FN) equation [16], J = Aβ2E2 exp(-BΦ3/2/βE) is used to describe field emission, where A, B are constants, E is the applied electric field in V cm-1, and Φ is the work function in eV, β is the field enhancement factor. A linear F-N plot (inset 1 of figure 2a) verified that the detected currents are due to quantum tunneling. The threshold electric field, Eth (applied electric field for generating a current density of 1 µA/cm2) is 4.22 V/µm for the as grown CNTs. Figure 2b describes the field emission stability of as grown sample (SEM image shown in inset 2 of figure 2b). The stability plot indicates that as grown CNTs are not stable after long emission duration. Inset 3 of figure 2b shows the images of emission sites of as grown CNTs. Then these as grown CNTs were coated with SrTiO3. As shown in figure 2c, Eth = 1.7 V/µm are detected from the SrTiO3 coated CNTs. The linear FN relations (inset 4 of figure 2c) were also revealed. Current saturation at high applied fields is detected in these samples. We think that electron supply is limited by the impedance (mostly resistance, but may include some capacitance and inductance) present especially along the CNTs and at the contacts between the CNTs and the substrate. These limiting factors become obvious at high current density probably due to Joule heating and/or current-induced dislocation [17]. As shown in inset of figure 2b, the distances between the as grown VA-MWCNTs are small (50–300 nm) and will initiate screening effect that reduces the β factors. This means not all as grown VA-MWCNTs will contribute to the collected current except those are longer in lengths or located at the edges of the larger spacing. For the SrTiO3 coated CNTs shown in inset 5 of figure 2d, the emission site density is improved. This might be because of low work function as well as high dielectric property of SrTiO 3. Since as grown CNTs are coated with high dielectric constant material then it will reduce the screening effect between as grown CNTs. We have compared these samples for their emission stability. As shown in figure 2d, the SrTiO3 coated CNTs is stabilized at a current density >200 µA/cm2 after continuous 40-hours operation while the as grown sample has reduced its current density to <175 µA/cm2. As shown in the insets of figure 2d, the emission density for SrTiO3 coated CNTs (inset 5) is higher than that of the as grown sample (inset 2). Apparently, lower screening effects on the SrTiO3 coated CNTs offers more emission sites. SrTiO3 coating acts as a protective layer and prevent from current stabilities as well. Also, the emission loads (heat and mechanical stress from Joule heating) is now shared by more CNTs, the emission stability is thus improved. As indicated by the FN equation, a lower local field on each emitter will lead to the emission of (a) Eth = 4.22 V/m 1 10 0 10 -1 10 1 ln(I/V2) A / V2 -28 -2 10 -35 -3 10 -4 10 -42 1.5 4 3.5 4 10 3 10 Eth = 1.7 V/m (c) 2 10 1 10 0 10 -20 -1 -2 10 -3 10 -4 -25 -30 -35 10 0 4 1 2 3 4 1/ 5 6 7 V (10-4V-1) 8 9 3 4 Electric field (V/m) 4 10 3 10 Eth = 0.78 V/m (e) -20 1 7 ln(I/V2) A / V2 10 0 10 -24 -1 10 100 0.5 -3 10 0.5 1.0 Electric 2 1.0 1/ V (10-3V-1) 1.5 2.0 field (V/m) 1.5 2.5 3 50 0 0 600 1200 1800 Time (min) 2400 250 200 (d) STO coated 150 100 5 6 50 0 0 600 1200 Time (min) 1800 2400 250 PMMA-STO coated (f) 150 100 -28 -2 10 0.0 150 200 2 10 (b) 200 8 Electric field (V/m) 10 Current Density (A/cm2) 2.5 3.0 -1 1/ V (10-4V ) 6 ln(I/V2) A / V2 Current Density (A/cm2) 2 2.0 250 Current Density (A/cm2) 3 10 2 10 Current Density (A/cm2) 4 10 Current Density (A/cm2) Current Density (A/cm2) lower current density per emitter. This will reduced Joule heating and stresses on these emitters and thus produce stable emission. 8 9 50 0 0 600 1200 1800 Time (min) 2400 Figure 2. (a) The field emission characteristic curves for the as grown VA-MWCNTs and the inset 1 shows the corresponding F-N plot. (b) The emission current stability curve at a current density of 0.2mA/cm2. Inset 2 and 3 shows the corresponding SEM image of as grown VA-MWCNTs and images of emission sites. (c) The field emission characteristic curves for the etched and bundled MWCNTs and the inset 4 shows the corresponding F-N plot. (d) The emission current stability curve at a current density of 0.3mA/cm2. Inset 5 and 6 shows the corresponding SEM image of Etched and bundled MWCNTs and images of emission sites. (e) The field emission characteristic curves for the PMMA coated VAMWCNTs and the inset 7 shows the corresponding F-N plot. (f) The emission current stability curve at a current density of 0.3mA/cm2. Inset 8 and 9 shows the corresponding SEM image of PMMA coated VAMWCNTs and images of emission sites. Next we created SrTiO3-PMMA coated CNT matrices for further testing. As can be seen from figure 2e that Eth for PMMA coated samples were much reduced (Eth = 0.78V/µm) as compared to SrTiO3 coated and as grown VA-MWCNTs. The threshold electric field depends mainly on two parameters: the aspect ratio of the CNTs and their inter-tube spacing. High aspect ratio of carbon nanotubes enables large electric field enhancement at the tips and initiates electron emission at a relatively lower applied electric field. All of our samples, as-grown as well as PMMA-coated samples, were grown simultaneously under the same growth conditions, so they have the same length and diameter and, therefore, same aspect ratio. This was confirmed by SEM observation. So in our case, the lower threshold field and better emission of the SrTiO3PMMA-coated CNTs cannot be due to variations in the CNT aspect ratios and graphitic order. Again Figure 2f shows the emission stability graph for the PMMA-coated CNT sample. It can be clearly seen from the figure that stability after PMMA coating improved considerably than that for the as-grown and SrTiO3 coated CNTs. As both the samples have CNTs with same aspect ratio, same structural order, and same intertube spacing, we think that the improvement in the emission stability of the SrTiO3-PMMA-coated CNTs sample is due to the reduction in the screening effect. As the screening effect from surrounding CNTs is less, CNTs in SrTiO3PMMA coated CNTs can emit electrons more easily and at lower electric field. So, SrTiO3PMMA coated CNTs emit electrons under low applied field. While for the as-grown sample, higher electric field is required to emit the same amount of current density, and so is under the higher electric field. Also field emission is more stable at lower electric field for longer time due to lesser Joule heating effect. We also proposed a model to explain this which is discussed elsewhere [19]. PMMA (3.4) has lower dielectric constant than SrTiO3 (475). Therefore, SrTiO3PMMA coating as well as removal of potential resistance barrier due to Ni catalyst particles by polishing improved the field emission characteristics. Thus lower emission threshold and considerably improved life time stability was observed in case of SrTiO 3-PMMA coated CNTs. CONCLUSIONS In summary, we found that SrTiO3 coated CNTs can produce more stable emission. This coating can further reduce the screening effects, increase the emission density, and improve the emission stability. We also demonstrated a significant improvement in the field emission and the emission stability in the CNT film by PMMA-SrTiO3 coating and polishing techniques. The threshold field to emit the electrons at a level of 1 µA/cm2 was reduced to 0.78 V/µm for PMMA-SrTiO3 coated CNTs from 4.22 V/µm for as grown CNTs. We discussed the possible reasoning behind this field emission improvement and attributed this to the reduction in the screening effect from the neighboring CNTs due to combined dielectric properties of PMMA and SrTiO3. After polishing the number of emission sites also increases. This was due to polishing as it opens more sites (remove catalyst particles from tip of as grown CNTs) for emitting electrons. ACKNOWLEDGMENTS Yoke Khin Yap acknowledges support from the U.S. Army Research Laboratory and the Defense Advanced Research Projects Agency (Contract number DAAD17-03-C-0115). 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