Transparent field emitters fabricated with ultra-thin single walled carbon nanotubes

Carbon nanotubes (CNTs) with superior chemical, electrical, and mechanical properties have been actively studied for a wide range of applications.[1] In particular, their applications to field emission devices including backlight units of liquid crystal display, lighting lamps, X-ray source, microwave amplifiers, electron microscopes, etc., have been strongly pursued because of their geometric merits of allowing low-voltage electron emission such as a large aspect ratio and a small radius of curvature at tip. [2] In this study, we fabricated a transparent cathode back plate by depositing an ultra-thin film of single walled CNTs (SWCNTs) on an indium tin oxide (ITO)-coated glass substrate. First, an aqueous CNT solution was prepared by ultrasonically dispersing purified SWCNTs in deionized water with sodium dodecyl sulfate (SDS). After centrifugation, several milliliters or even a hundred of microliters of the well-dispersed CNT solution was deposited onto a porous alumina membrane through vacuum filtration. Thereafter, the alumina membrane was solvated with the 3 M NaOH solution and the floating CNT film was easily transferred to an ITO glass substrate in an area of 1 cm diameter defined by using a film mask. The CNT film was subjected to an activation process with an adhesive roller, standing the CNTs up to serve as electron emitters. Figures 1 and 2 show that the turn-on voltages of the CNT emitters were lowered, and their field enhancement factors became higher, respectively, as increasing the volume of the CNT solution used to prepare the CNT emitter samples. For display applications, field emission devices are configured in such a way that the phosphor anode (front plate) is placed against the CNT emitters (back plate). In most cases, light, which is generated from the phosphor, passes through the anode front plate to reach observers.[3] When electrons bombard onto the phosphor particles whose diameters are usually in a range of several micrometers, light is produced only in a narrow depth of the surface of the phosphor particles due to a limited penetration depth of electrons. During the transmission through the phosphor layer, a light intensity considerably decreases by scattering. In this case, we observe only a portion of light emitted from the phosphor. First, two types of phosphor anode plates were engaged: a phosphor layer screened on an ITO glass substrate and a phosphor layer screened on a Crcoated glass substrate. For the former type of a phosphor plate, light was observed on both the front side (case 1) and the back side (case 2), where brightness on the back was ~13% higher than that on the front in our experiments. For the latter type of a phosphor plate, however, light was observed only on the cathode back side as the Cr layer on the anode glass served as a reflecting mirror (case 3), improving the light brightness as much as ~56% compared with that of the case 1. Secondly, we formed an Al reflecting layer on a cathode plate by which all lights pass through only the anode front plate (case 4), showing an increase of luminance but smaller than the case 3. Luminance variations for these four cases of device configurations are given in Fig. 3. This study demonstrated the lamps emitting light on the both sides by using transparent CNT films, but showed that reflecting all lights through the cathode back side exhibited the highest brightness.