Mo tips 50-100 Si tips 50-100 p-type diamond 160 Defective CVD diamond 30-120 Amorphic diamond 20-40 Cesium-coated diamond 20-30 Graphite powders 10-20 Nano-diamond 3-5 (unstable > 30 mA/cm2) Carbon Nanotubes 1-2 (stable >4000mA/cm 2) Table 1. Threshold fields for various cathode materials and some CNT emission characteristics.