SCIENCE CHINA Physics, Mechanics & Astronomy • Article •
November 2013 Vol.56 No.11: 2081–2084 doi: 10.1007/s11433-013-5220-4
Uniform arrays of carbon nanotubes applied in the field emission devices LI DeTian1, CHENG YongJun1, CAI Min1, YAO JinLi2 & CHANG Peng2* 2
1 Lanzhou Institute of Physics, Lanzhou 730000, China; Key Laboratory for Magnetism and Magnetic Materials of MOE, Lanzhou University, Lanzhou 730000, China
Received December 7, 2012; accepted January 18, 2013; published online September 5, 2013
Carbon nanotubes (CNTs) were grown into anodic aluminum oxide (AAO) channels by chemical vapor deposition (CVD) using C2H2/N2 mixtures as feeding gas, which can be used as field emitters. The bottom surface of AAO template was etched slightly and the tips of CNTs were explored as the field emission arrays which were uniform and vertical. Field emission characterization showed a low turn-on field about 3.25 V/m and high emission current about 30 mA/cm2 with the electric field about 4 V/m. These superior field emission characteristics could be attributed to low density of vertical CNTs and higher conductivity of the substrate. carbon nanotubes, anodic aluminum oxide template, field emission PACS number(s): 61.46.Fg, 79.70.+q, 85.45.Fd Citation:
Li D T, Cheng Y J, Cai M, et al. Uniform arrays of carbon nanotubes applied in the field emission devices. Sci China-Phys Mech Astron, 2013, 56: 20812084, doi: 10.1007/s11433-013-5220-4
1 Introduction A number of sensitive applications, such as electron microscopes [1], X-ray tubes [2], microwave power amplifiers [3] and field emission displays [4], largely depend on the better understanding of cold cathodes related to the electron field emission process. For carbon nanotubes (CNTs) to possess the unique properties [5,6] and have a critical role in the designed nanostructure devices, it is necessary to investigate the field emission effect [7]. However, the controlled uniform CNTs arrays are too difficult to be obtained by using the conventional preparation methods, which usually affects the subsequent application of the devices. There are two critical issues of CNTs arrays that required to be noted before practical application. Firstly, the random distribution of CNTs is significant. Large height-to-radius ratio can be
*Corresponding author (email:
[email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2013
overwhelmed because of the random distribution. It results directly in a relatively weak field enhancement factor. Secondly, the density of vertical distribution of CNTs can be critical when the field enhancement factor is much smaller than that of an individual CNT because of the alleged screening effect which results in the electrostatic interaction between CNTs [8]. Therefore, a controlled approach to prepare uniform and vertical CNTs is required in the relevant research fields, particularly in field emission devices. The anodic aluminum oxide (AAO) template has the advantage of large area, high aspect-ratio, and simple processing capability can be obtained by electrochemical anodization of aluminum in an acidic electrolyte. By adjusting the anodizing conditions, the diameter, the length, and the density of nanopores can be controlled, making AAO an ideal template for fabricating ordered arrays of nanostructure materials [9,10]. CNTs fabricated grown into AAO templates prepared by a two-step anodization process are uniform in diameter, highly ordered, and vertical with rephys.scichina.com
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spect to the plane of the template [11,12]. Therefore, CNTs fabricated in AAO templates are an ideal substance with which to study field emissions.
2 Experimental In order to obtain the uniform CNTs arrays, the AAO template method was used. The fabrication scheme of the AAO-CNTs arrays is shown in Figure 1. The detailed preparation processes were as follows. After the preparation of the AAO-CNTs, Ag was sputtered at the top surface and then connected the electrode Mn via the common thermal treatment. Next, the bottom surface of AAO template was etched slightly and the uniform tips of CNTs were exposed (Figure 1(d)). Based on the above simple techniques, the suitable AAO-CNTs field emission arrays were designed (Figure 1(e)). 2.1
Preparation of AAO template
The two-step anodization was introduced to produce ordered pore channel arrays of AAO [13–15]. Experiments were performed on 99.999% pure Al foils with 1.0 cm× 2.0 cm area and 0.5 mm thickness, previously degreased and electro-polished in order to remove defects and improve their planarity. Anodization was first carried out in a 0.3 M oxalicacid solution at 20°C under a constant polarization voltage of 40 V for 30 min. After removing the resultant aluminum oxide film formed by the first anodization by wet chemical etching at 60°C for 50 min with a mixed solution of H3PO4 and CrO3 formed by the first anodization, a second anodization was performed for 8 h under identical conditions as in the first case. The template was dipped into saturated HgCl2 to dissolve aluminum existing in the template.
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2.2
CNTs growth with AAO templates
The CNTs fabricated by chemical vapor deposition (CVD) using AAO as template [16,17] were prepared using the following processes. The AAO temple was dipped into saturated NiCl2 solution for 0.5 h to obtain the catalytic and then it was placed into a horizontal CVD facility stove after sufficient rinse with deionized water and drying at 70°C in atmosphere. Acetylene (ultrahigh purity, >99.99%), argon (ultrahigh purity, >99.99%) and hydrogen (ultrahigh purity, >99.99%) were used to grow the CNTs. The growth of CNTs was carried out at 750°C and 102 Torr for 0.5 h under argon, acetylene, hydrogen flow ratio of 1:1:3 after the reduction in a gas mixture of 50% H2 and 50% Ar at 500°C, 103 Torr for 1 h. After the growth of CNTs, the temperature was cooled to room temperature in Ar atmosphere. After the preparation of the AAO-CNT arrays, Ag was sputtered at the top surface and then connected the electrode Mn via the common thermal treatment at 300°C for 1 h. The uniform tips of CNTs were embedded in AAO in that pores bottom opening by chemical etching (in H3PO4 at 30°C, for about 50 min) were used to obtain the CNT arrays. 2.3
Sample characterization
The morphology of CNTs was examined using FE-SEM (Hitachi, S4800, Japan) and TEM (EM-400T). Raman (Jobin Yvon HR800) spectra were obtained using a laser of 532 nm as an excitation source. It provided a typical spatial resolution
