Transcript Fig. 6. Basic design of field emitter . The cathode is the CNT needle.

Shaping Carbon Nanotube Forests for Field Emission
Ben Pound and T.-C. Shen
Department of Physics
Background
Method to Make CNT Needle
• Sharp conducting tips can emit electrons by cold field emission.
Gate Fabrication
• Circular patches of 10 µm in diameter coated with a
layer of 3-nm Al will be defined by photolithography
on a Si wafer.
 Less power consumption.
 Sharper electron energy distribution - no thermal spread.
 Less temperature-induced outgassing from the emitter.
• Extraction voltage depends on the work function and the curvature of the tip.
• Carbon nanotubes (CNTs) have a similar work function to that of metals.
CNT
emitter
Fig. 3. CNT needles from Ref.
5. Scale bar,100 µm. Inset
scale bar, 60 um.
A Brief Description of Fabrication Process:
SOI wafer
Oxide
Si
• Sharpening the CNT bundles by plasma can generate a current density of 100
mA/cm2 at a field of 0.6 V/µm. [3]
Elastocapillary Self-Assembly
•
Oxide
Elastocapillary self-assembly occurs when a liquid is introduced into a CNT
forest. Because of the capillary force between the liquid and CNTs, the liquid
draws the CNTs together as it evaporates, densifying the forest. The CNTs stay in
the densified state because of the Van Der Waals force between CNTs [4].
Liquid can be introduced to CNTs by direct, top, or side immersion, or vapor
exposure.
Si
Preliminary SIMION Study
• SIMION was used to investigate electron emission as a function of gate voltage, gate
distance, and tip sharpness.
• SIMION does not support the inclusion of work function parameters in its
calculations, so electrons were placed at the tip of the field emitter with near zero
velocity at a range of initial angles (which were constant throughout) to account for
possible emission irregularities.
• In order to investigate the electric field lines that the electrons will follow,
equipotential curves were plotted for three tip sizes using SIMION in Fig. 4, 5, and
6.: large (radius of 1 μm), intermediate (radius of 500 nm), and sharp (radius of 2
nm). The electric field lines can be drawn perpendicular to the equipotential lines.
• Direct Immersion: If the CNT forest has no ordered form, the densification
happens randomly, as shown in Fig. 1. However, if the CNTs have been
lithographically defined, densification occurs predictably, as shown in Fig. 2.
A schematic of a cold field emitter is
shown in Fig. 6. The cathode will be the
CNT needle, and the anode will be the
fabricated gate structure. The substrate is
a silicon-on-insulator (SOI) wafer, which
has a buried oxide (BOX) layer.
Fig. 6. Basic design of field emitter .
The cathode is the CNT needle.
• Carbon nanotube bundles of 5 µm in diameter have achieved field emission
current density of 1 mA/cm2 at a threshold electric field of 1 V/µm. [2]
•
Gate
• Carbon nanotube bundles will be grown selectively
on the Al-coated patches.
• Dipping the top of CNT bundles in liquid first and
drawing the bundle out slowly by a piezomotor
system, the bottom should densify first and the tip
last, creating a needle shape similar to that of Fig. 3.
• Single carbon nanotubes have achieved high emission current at low
voltages but are not very robust. [1]
Nanoscale
Device
Laboratory
(A)
(B)
BOX
This is the color code for the wafer. In addition, black
signifies CNT growth.
• The wafer has a layer of oxide on top (A). Mask #1 is used with
positive photoresist (PR), and after development and removal of
oxide and PR, (B) is reached.
• The wafer is then etched with KOH and then, using negative PR,
mask #1 is then used again. After development and gold
deposition/liftoff, (C) is obtained.
(C)
(D)
• Using mask #2 and positive PR, Al is deposited, upon which CNTs
are grown. The entire device is dipped, and (D) is obtained,
completing the fabrication process.
Conclusions
• The elastocapillary force allows CNTs to be predictably densified into useful
configurations.
• Tip size plays an important role in field emission – smaller tips allow for smaller
emission areas and more concentrated electric fields in those areas.
Fig. 4. Tip radius of 1 μm. The equipotential lines are of low density near the tip. As a
consequence, the electrons are subject to a small force and only three electrons are emitted.
• Studies will be focused on the current density, threshold voltage, and the lifetime
of the emitter.
Citations
Fig 1. After methanol was dropped onto the CNT forest (left), it formed random voids, as seen
on the right.
Fig. 5. Tip radius of 500 nm. The equipotential lines are slightly denser near the tip,
corresponding to a slightly stronger electric field. However, only three electrons are emitted,
though two others nearly are emitted.
[1] J.-M. Bonard, et al. Phys. Rev. Lett. 89, 197602 (2002).
[2] T.-W. W., et al. Appl. Surf. Sci. 254, 7755 (2008.)
[3] R. Padmnabh, et al. Appl. Phys. Lett. 93, 131921 (2008).
[4] M. Volder and J. Hart. Angew. Chem. Int. Ed. 52, 2412 (2013).
[5] D.N. Futaba, et al. Nature Materials 5, 987 (2006).
Acknowledgements
111Sib-40
111Sib-65
Fig. 2. After putting methanol on defined CNT pillars (left), the capillary force densified the
walls of the pillars, as seen on the right.
Fig. 6. Tip radius of 2 nm. The equipotential lines are quite dense near the tip; therefore, the
electrons feel a strong focusing force. All 11 electrons are emitted in a tight beam. The scale
of these figures is different from Figs. 4 and 5 in order to see the tip.
We thank the Space Dynamics Lab and the Department of Physics for their financial
support.