Field emission (FE) from carbon nanotubes (CNTs) grown on a cathode substrate is difficult to characterize using simple formulae or data fitting, which is due to (1) electron-phonon interaction (2) electromechanical force field leading to stretching of CNTs (3) ballistic transport induced thermal spikes, coupled with high dynamic stress, leading to degradation of emission performance at the device scale. Fairly detailed physics-based models of CNTs considering the aspects (1) and (2) above have already been developed by these authors. However, design optimization issues aimed at better FE devices to reduce the extent of electro-mechanical fatigues and improved spatio-temporal localization of emitted electrons remain open and important areas of research. With due success in designing such devices, various applications such as in-situ biomedical x-rays probes and thin film pixel based imaging technology etc., to name just a few, are of great significance. In this paper we analyze a new design concept, wherein (a) the electrodynamic force field leading to strong electron-phonon interaction during ballistic transport and also (b) the usually observed reorientation of the CNT tips and instability due to coulomb repulsions, can be harnessed optimally. In the proposed design, we introduce two additional gates on the edges of the cathode substrate. An array of stacked CNTs is considered on the cathode substrate. The height of the CNTs is such that a symmetric force field is maintained in each pixel with respect to the central axis parallel to vertical axis . As a result, it is expected that a maximum current density as well as well-shaped beam can be produced under DC voltage across the cathode-anode structure. In the present design, the anode is assumed to be simply a uniform conducting slab. However, such an anode can be replaced with a porous thin film along with MEMS-based beam control mechanism. The present design objective is to estimate the field emission current as function of the side-wise gate height dc with respect to the CNT height h and the slope of stacking of the CNT tips. Results show the transverse electric field distribution in the pixel, which directly influences the field emission current. Details of a coupled model involving ballistic transport of electrons, electron-phonon interaction and nonlinear deformation of the CNTs due to electrodynamics forces will be discussed, followed by optimization studies involving the parameters dc/h and Vg/VDC, where Vg denotes the side-wise gate voltage and VDC denotes the DC voltage across the anode-cathode structure.
Journal: TechConnect Briefs
Volume: 1, Nanotechnology 2008: Materials, Fabrication, Particles, and Characterization – Technical Proceedings of the 2008 NSTI Nanotechnology Conference and Trade Show, Volume 1
Published: June 1, 2008
Pages: 55 - 58
Industry sector: Advanced Materials & Manufacturing
Topic: Carbon Nano Structures & Devices