Field emission microscopy of carbon nanotube fibers: evaluating and interpreting spatial emission

In this work, we quantify field emission properties of cathodes made from carbon nanotube (CNT) fibers. The cathodes were arranged in different configurations to determine the effect of cathode geometry on the emission properties. Various geometries were investigated including: 1) flat cut fiber tip, 2) folded fiber, 3) looped fiber and 4) and fibers wound around a cylinder. We employ a custom field emission microscope to quantify I-V characteristics in combination with laterally-resolved field-dependent electron emission area. Additionally we look at the very early emission stages, first when a CNT fiber is turned on for the first time which is then followed by multiple ramp-up/down. Upon the first turn on, all fibers demonstrated limited and discrete emission area. During ramping runs, all CNT fibers underwent multiple (minor and/or major) breakdowns which improved emission properties in that turn-on field decreased, field enhancement factor and emission area both increased. It is proposed that breakdowns are responsible for removing initially undesirable emission sites caused by stray fibers higher than average. This initial breakdown process gives way to a larger emission area that is created when the CNT fiber sub components unfold and align with the electric field. Our results form the basis for careful evaluation of CNT fiber cathodes for dc or low frequency pulsed power systems in which large uniform area emission is required, or for narrow beam high frequency applications in which high brightness is a must.