Theoretical and experimental investigation of buckypaper: Field emission

Many researchers regard carbon nanotube backlight units (CNT-BLUs) as a potential candidate for the liquid crystal display (LCD) industry. CNT buckypapers were tested as surface luminary sources for CNT-BLU applications. The field emission properties, durability and repeatability of a single-walled Carbon nanotube (SWCNT) buckypaper was studied for developing CNT-BLU. This study reports a laser irradiation process to enhance the field emission properties of buckypaper, which is a thin sheet of high-loading carbon nanotube networks. A scanning laser treated selected regions of the buckypaper to activate CNT emitters. This post-process causes a decrease in turn-on field and increases in the field enhancement factor (beta), luminance intensity and uniformity of buckypaper emitters. The phosphorescence luminance intensity and uniformity of buckypaper emitters were measured and characterized. These exellent properties and performance were achieved by adjusting machining parameters of laser power, laser lens motion speed, laser resolution. Design of Experiment (DOE) methodology provided a method to rapidly search the feasible laser parameter setting for processing buckypaper and improving field emission properties within fewer experimental runs. DOE results indicated the proper laser treatment power density was 0.9 W/cm2. Furthermore, the effects of the laser treated emitter density was investigated under the same laser power density as the DOE results. The CNT emitter's altitude, diameter and spacing were characterized through an optics analysis after laser treatment. The emitter spacing directly impacted emission results when the laser power and treatment time were fixed. The increasing emitter density gave rise to an enhanced field emission current and luminance. However, a continuous and excessive increase of emitter density with spacing reduction generated a screening effect. As a result, the extended screening effect from the smaller spacing eventually crippled the field emission effectiveness. From luminance intensity and uniformity of field emission, the optimal ratio of average emitter altitude to emitter spacing was 3.4. The high effective buckypaper is suggested to have a density of 50 x 50 emitters/cm2, which presents an effective field enhancement factor of 3721 and a moderated screening effect of 0.005. Proper laser treatment appears to be an effective post-treatment process for optimizing field emission and luminance performance for a buckypaper cold cathode.