Light emitting diodes (LEDs) that emit ultraviolet light are important for applications in medicine and photolithography. Now, Xiaowei Sun and colleagues from Nanyang Technological University, Singapore and Beijing Jiaotong University, China have produced LEDs made from ZnO nanorods that emit light further into the ultraviolet than reported previously.1 The secret, they say, is in the appropriate choice of an organic material they use as the ‘hole’ carrier.

The heart of an LED is a junction between an electron conducting material and a material that conducts holes – empty states that can accept electrons. When a voltage forces electrons to move from the electron carrying material across the junction, the electrons lose energy and emit light in the process.

ZnO is a semiconductor that is often used for blue and ultraviolet LEDs. In their devices, Sun and colleagues grew single crystal ZnO nanorods on indium tin oxide (ITO), a transparent and conducting material. They coated the (ITO) with ZnO nanoparticles, dipped it into a solution of ammonia, zinc acetate and water and then heated the solution to just below 100oC to initiate the formation of ZnO.

Fig. 1: A scanning electron micrograph of the ZnO nanorods after the space between them has been filled with PMMA. The white spots are the sharp tips of the ZnO nanorods. The inset shows the light emitted from the ZnO LED under a forward bias voltage.

The ZnO nanorods were about 2.5 microns long and 150 nm in diameter (Fig. 1). Filling the space between the nanorods with a polymer (PMMA) provided structural stability and a smooth top surface for the next step in the processing, which was to deposit a layer of the hole-transporting material N,N’-di(naphtha-2-yl)-N,N’-diphenyl-benzidine (NPB for short) and a thin layer of silver.

When a positive voltage of ~ 20 V was applied to the silver layer, the structure emitted light with a maximum intensity at a wavelength of 342 nm—nearly 40 nm deeper into the UV than reported in other ZnO devices. The researchers suggest that both the choice of NPD as the hole-carrier and the quantum confinement the electrons experience at the sharp tips of the ZnO nanorods contributed to the observed shift to shorter wavelengths.

An additional factor that makes these LEDs attractive is that they are produced at low temperatures, which is usually less expensive than high temperature methods.