Monday, November 3, 2008

Scaled fabrication of single-nanotube-tipped ends from carbon nanotube micro-yarns and their field emission applications

Yang Wei et al 2008 Nanotechnology 19 475707 (5pp) doi: 10.1088/0957-4484/19/47/475707

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Yang Wei, Liang Liu, Peng Liu, Lin Xiao, Kaili Jiang and Shoushan Fan
Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University, Beijing 100084, People's Republic of China

Abstract. Joule-heating-induced electrical breakdown was applied to break suspended carbon nanotube (CNT) micro-yarns. The yarn ends at the breaking points were well-shaped sharp tips and mostly terminated by a single nanotube. The uppermost CNT was approximately 5 nm in diameter and was firmly compacted with the CNTs below it, yielding better mechanical, electrical and thermal contacts. An individual end could provide an emission current of approximately 25 µA, with potential application as a point electron source. In addition, we developed a pixel structure for a field emission display using oppositely aligned ends as the cathode and gate, respectively.

Print publication: Issue 47 (26 November 2008)
Received 14 July 2008, in final form 27 September 2008
Published 30 October 2008


Figure 5. (a) Sketch of the transfer of a CNT end onto a tungsten probe. (b) Optical micrograph of an as-fabricated CNT emitter. (c) Illustration of the field emission testing method using a tungsten probe as the anode with a 100 μm gap. (d) Schematic illustration of the lateral CNT-based pixel for an FED. (e) A lit pixel of the suggested structure. (f) Field emission properties of the as-fabricated CNT ends. The inset shows the FN curve. (g) I–V curve of the coplanar cathode and gate, demonstrating that the cathode ends start emitting electrons at approximately 60 V. The inset shows the electron emission efficiency, which is >60%.

From the "Full text" noted above:

The gap between two breaking ends of a CNT micro-yarn is approximately 2 μm, as shown in figure 1(e). The small gap between sharp tips suggests that it is possible to initiate field emission at low voltage using one end as the electron emitter and the other as the gate electrode. A specimen was fabricated from a 1 mm wide MWCNT sheet and experiments demonstrated that the field emission threshold voltage was only approximately 60 V, as shown in figure 5(g). On the basis of the low threshold voltage, we suggest a new pixel for FEDs that is schematically illustrated in figure 5(d). The pixel uses the CNT ends on each side of the small gap as the cathode emitter and gate electrode, respectively. The pixel was fabricated from a 1 mm wide MWCNT sheet by electrical breakdown. A piece of ITO glass coated with a layer of green cathode ray phosphor was placed at a distance of 2 mm away as the anode. The pixel could be effectively turned on and off by the gate in the voltage range 80–100 V when the anode voltage was set to 1 kV. The lit pixel (Vg = 100 V) is shown in figure 5(e). The emission efficiency (current collected by the anode divided by the emission current from the cathode, Ia/Ic) of such a structure is plotted in the inset of figure 5(g), and is >60%.
A lateral CNT cathode and gate would greatly simplify the fabrication process for FEDs. CNT emitters in a conventional FED are vertically aligned on the cathode plate, and their alignment requires extra surface treatment processes [23–25]. The gate electrodes are on top of the CNT cathode and separated from it by an insulating layer of precise thickness. Both the vertical alignment and the cathode’s 3D structure are difficult to realize in conventional thin film processes. In the suggested FED, the complicated 3D structures are simplified to a planar structure, which is easy to fabricate. It is also beneficial that both sides of the broken yarn are composed of sharp CNT ends and can be used as electron emitters. The planar cathode is similar to SEDs [8, 26, 27], but the emission efficiency is much higher. In a surface-conduction electron emitter, electron emission arises from discontinuous conducting films that are composed of isolated islands. The electric field between the isolated islands leads to the tunnel effect, and electrons are transferred at the discontinuous thin film surface. Usually, only a small proportion of the electrons can escape from the surface and fly to the anode. Wang et al reported that the electron emission efficiency of their SED was approximately 2% [8]. This low emission efficiency limits practical SED applications owing to large conduction currents and high power consumption. Our suggested FED not only has the planar structure of SEDs, but also has much higher emission efficiency. The latest progress on lateral CNT-based FEDs in our laboratory has demonstrated that both MWCNT sheets and the shrunk yarn can be made into planar FED structures using electrical breakdown and screen-printing technology. This is encouraging, and implies that the suggested FED is feasible and might be realized in the near future.

My comments:
As shown in Fig. 5(d) the left side CNT is the emitter and the right side CNT is the gate - the gap therebetween reminding me of a SED gap and clearly noted as such by the authors in the extract above. And the emission efficiency for the CNT planar FED of 60% compares quite favorably to SED's rather puny 2%.