Shaping Carbon Nanotube Forests for Field Emission Ben Pound and T.-C. Shen Department of Physics Background Elastocapillary Self-Assembly Method to Make CNT Needle Circular patches of 10 µm in diameter coated with a layer of 3-nm Al will be defined by photolithography on a Si wafer. Fig. 3. CNT needles from Ref. 5. Scale bar,100 µm. Inset scale bar, 60 um. Fig 1. After methanol was dropped onto the CNT forest (left), it formed random voids, as seen on the right. Fig. 2. After putting methanol on defined CNT pillars (left), the capillary force densified the walls of the pillars, as seen on the right. Preliminary SIMION Study SIMION was used to investigate electron emission as a function of gate voltage, gate distance, and tip sharpness. Acknowledgements We thank the Space Dynamics Lab and the Department of Physics for their financial support. Citations Conclusions The elastocapillary force allows CNTs to be predictably densified into useful configurations. [1] J.-M. Bonard, et al. Phys. Rev. Lett. 89, (2002). [2] T.-W. W., et al. Appl. Surf. Sci. 254, 7755 (2008.) [3] R. Padmnabh, et al. Appl. Phys. Lett. 93, (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). Gate Fabrication Fig. 6. Basic design of field emitter. The cathode is the CNT needle. 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. A Brief Description of Fabrication Process: SOI wafer Oxide Si BOX Si 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. (A) (B) (C) (D) 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. 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. 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. 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]. Sharp conducting tips can emit electrons by cold field emission.  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. Single carbon nanotubes have achieved high emission current at low voltages but are not very robust. [1] Carbon nanotubes (CNTs) have a similar work function to that of metals. Carbon nanotube bundles of 5 µm in diameter have achieved field emission current density of 1 mA/cm 2 at a threshold electric field of 1 V/µm. [2] Sharpening the CNT bundles by plasma can generate a current density of 100 mA/cm 2 at a field of 0.6 V/µm. [3] Liquid can be introduced to CNTs by direct, top, or side immersion, or vapor exposure. 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 Sib Sib-65 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. Carbon nanotube bundles will be grown selectively on the Al-coated patches. Nanoscale Device Laboratory 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. 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. 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. Tip size plays an important role in field emission – smaller tips allow for smaller emission areas and more concentrated electric fields in those areas. Studies will be focused on the current density, threshold voltage, and the lifetime of the emitter. GateCNT emitter Oxide