High Current Emission from Patterned Aligned Carbon Nanotubes Fabricated by Plasma-Enhanced Chemical Vapor Deposition
Vertically, carbon nanotube (CNT) arrays were successfully fabricated on hexagon patterned Si substrates through radio frequency plasma-enhanced chemical vapor deposition using gas mixtures of acetylene (C2H2) and hydrogen (H2) with Fe/Al2O3 catalysts. The CNTs were found to be graphitized with multi-walled structures. Different H2/C2H2 gas flow rate ratio was used to investigate the effect on CNT growth, and the field emission properties were optimized. The CNT emitters exhibited excellent field emission performance (the turn-on and threshold fields were 2.1 and 2.4 V/μm, respectively). The largest emission current could reach 70 mA/cm2. The emission current was stable, and no obvious deterioration was observed during the long-term stability test of 50 h. The results were relevant for practical applications based on CNTs.
KeywordsCarbon nanotube PECVD Field emission Emission stability
Field emission is a quantum mechanical tunneling phenomenon. Electrons in the materials can emit into vacuum from solid surface which is determined by the strength of local electric field and potential barrier to emission. Field emission occurs from a cold cathode at room temperature which is more power efficient than thermionic emission . Field emission is widely used in many kinds of vacuum electronic applications such as flat panel displays, microwave power tubes, electron sources, and electron-beam lithography. However, high local filed is required to obtain useful current. In order to reduce the extraction voltage, field emitters with sharp protruding microstructures can be used such as Spindt tip cathodes [2, 3], silicon tips [4, 5], and carbon-based materials [6, 7, 8, 9]. Carbon nanotube (CNT) has been recognized as an ideal candidate material for field emission applications due to its unique structure and remarkable mechanical, electrical, and chemical stability. Furthermore, the small tip radius and high aspect ratio of CNT can result in electron emission at extraordinary low-threshold electric field and obtain a high-field enhancement factor. Since the first field emission behavior of CNT reported in 1995, many works showed that the CNT emitters exhibited excellent field emission properties [10, 11, 12, 13, 14, 15].
The electron emission of CNTs is originated from the tip of the nanotubes because the electrons located at the tips can easily participate in the field emission [16, 17]. Furthermore, the aligned CNTs with uniform length exhibit better field emission properties than random arrangement ones . The CNT arrays can fulfill the requirements for field emission and manipulated as field emission devices directly. Thus, CNTs had better be vertically aligned and oriented toward an anode. Vertically aligned CNTs can be synthesized by chemical vapor deposition methods (CVD). The CVD methods are ideally suited to prepare CNT films on various substrates, and the process can be assisted by microwave of radio frequency plasma [19, 20, 21, 22].
As CNTs are capable of emitting efficient high currents, they are potential as emitters in various devices [23, 24]. But nevertheless, the emission densities and short emission lifetimes present obstacles for the practically available electron field emitters based on CNTs. The challenge is to improve the field emission properties of CNTs. It is found that the applied external field is strongly screened when the spacing distance is shorter than the length of the carbon nanotubes . In order to reduce the screen effect, patterning CNT is an efficient method. In this work, the CNT emitters were fabricated using radio frequency plasma-enhanced chemical vapor deposition (PECVD) method on patterned Si substrate. The vertically aligned CNT arrays showed good field emission properties with high emission current and ultra-long-term emission stability which were better than other reported patterned vertical CNTs [26, 27, 28].
The surface morphology of samples was observed by FESEM (JSM-6701 F). A transmission electron microscopy (TEM, F-30) with an accelerating voltage of 200 kV was used to characterize the microstructure of CNTs. The microstructure was also investigated by micro-Raman spectroscopy (JY-HR800 spectrometer, the excitation wavelength of 532 nm). The field emission characteristics of the films were measured in a chamber with high vacuum better than 5 × 10−6 Pa using a parallel-plate-electrode configuration. The distance between anode and cathode was adjusted to 300 μm using a spiral micrometer. The current-voltage (I–V) characteristics were obtained by LabVIEW program through a Keithley 248 power source with a computer-controlled data-acquisition card.
Results and Discussion
The emission stability of a field emission electron source is one of the key factors that affect its potential application in vacuum electronic devices. The J-E curves can only show transitory field emission phenomenon in a short time and cannot reflect the field emission behavior sufficiently. Figure 5d showed the looping testing of the CNTs. The anode voltage was increased or decreased by 30 V/step. Seen from loop testing with increased and decreased voltage between 1.5 and 3.5 V μm−1 for 25 loops, there is no obvious deterioration of the maximum current density. Before the looping testing with maximum current of about 20 mA/cm2, the current density of 10 mA/cm2 was also tested with no deterioration. Figure 5e displayed the typical J-E curves for different loops of the loop testing. The current density was relatively stable both in the increased and decreased voltage processes. During the increased or decreased voltage process of the field emission testing, desorption and adsorption of the gas molecules will change the work function of CNT and probably lead to a phenomenon of hysteresis [9, 44]. The hysteresis was unnoticeable in this work indicated that desorption and adsorption may reach an equilibrium state. Furthermore, the long-term test of the sample exhibited good stability for 50 h (Fig. 5f). When ionization vacuum gauge was opened, the emission current increased abruptly during the stability test as shown in the arrow pointed position of Fig. 5f. The conditions in vacuum chamber and the surface state of CNT emitters may change in testing process which resulted in the rising of emission current. All these achievements underlined the potential of CNT emitters in applications.
In summary, the vertically aligned CNTs were synthesized on patterned substrates by PECVD. The field emission properties of CNTs were optimized with different H2/C2H2 mixture ratios. The CNTs exhibited excellent field emission characteristics with high current density and good emission stability. In order to achieve practically available electron field emitters based on CNTs, we should still focus on the enhancement of electron emission density and the structure design.
This work was financially supported by the Natural Science Foundation of China (Nos. 21473153 and 51002161), the Natural Science Foundation of Hebei Province (No. B2013203108), the Science Foundation for the Excellent Youth Scholars from Universities and Colleges of Hebei Province (No. YQ2013026), the Support Program for the Top Young Talents of Hebei Province, the China Postdoctoral Science Foundation (No. 2015 M580214), and the Scientific and Technological Research and Development Program of Qinhuangdao City (No. 201502A006).
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