Carbon nanotubes (CNTs) are finding a variety of applications because of their high electrical and thermal conductivities, high aspect ratios and robust mechanical properties. They have been particularly successful in application to field-emission displays and composites.

Fig. 1: The brightness of phosphors can be improved by mixing them with carbon nanotubes.

Now, Shang Hyeun Park from the Samsung Advanced Institute of Technology in Korea and colleagues1 demonstrate that CNTs can be used to enhance the performance of phosphors in electroluminescent displays. They show that the addition of CNTs to display phosphor at concentrations of less than one percent reduces the driving voltage needed for the display to less than a fifth of that required for the same brightness when no CNTs are added.

In their devices, the researchers used a standard cathodoluminescent phosphor that emits green light. The phosphor, consisting of zinc sulfide with copper and aluminum, was sandwiched between two electrodes, and an alternating current was then passed through the device to make it glow. They found that when no CNTs were included, the driving voltage needed to turn the device on was higher than 1000 V. In contrast, with the addition of CNTs, the device turned on at only 150 V and became brighter much quicker.

The addition of CNTs improved the device's characteristics for concentrations up to 1% in the phosphor, and the authors showed that CNTs were uniformly distributed throughout the phosphor material when mixed at this ratio. At concentrations over 1%, it was difficult to uniformly disperse the nanotubes in the material, and consequently the performance could not be further improved.

CNTs are known to be able to support high electric fields because of their high aspect ratio and high conductivity. The researchers explained that adding the CNTs to the phosphor allows a high electric field to be reached at localized points in the material, leading to emission at lower applied voltage. The nanotubes also improve device operation by lowering the barrier to electron injection at the electrodes, which allows electron injection to occur at lower voltage.

The authors hope that in the future the idea of simply mixing CNTs into conventional phosphors could lead to improved electroluminescent devices that operate at low voltage.