Realizing a SnO2-based ultraviolet light-emitting diode via breaking the dipole-forbidden rule

Although many oxide semiconductors possess wide bandgaps in the ultraviolet (UV) regime, currently the majority of them cannot efficiently emit UV light because the band-edge optical transition is forbidden in a perfect lattice as a result of the symmetry of the band-edge states. This quantum mechanical rule severely constrains the optical applications of wide-bandgap oxides, which is also the reason why so few oxides enjoy the success of ZnO. Here, using SnO2 as an example, we demonstrate both theoretically and experimentally that UV photoluminescence and electroluminescence can be recovered and enhanced in wide-bandgap oxide thin films with ‘forbidden’ energy gaps by engineering their nanocrystalline structures. In our experiments, the tailored low-temperature annealing process results in a hybrid structure containing SnO2 nanocrystals in an amorphous matrix, and UV emission is observed in such hybrid SnO2 thin films, indicating that the quantum mechanical dipole-forbidden rule has been effectively overcome. Using this approach, we demonstrate the first prototypical electrically pumped UV-light-emitting diode based on nanostructured SnO2 thin films. Oxide semiconductors typically possess wide energy gaps between the conduction and valence bands in their electronic structure. This predisposes them to be efficient ultraviolet light emitters, and thus promising components in lighting, display and photonic devices. Yet, in most cases, the optical transition between electronic states that is responsible for this emission is a forbidden one in quantum mechanics. A research team led by Tom Wu and Su-Huai Wei has now circumvented this issue with tin dioxide by altering the material's nanoscale structure, in particular its effective surface. Calculations were carried out that subsequently guided the preparation of hybrid nanocrystalline-amorphous thin films through an annealing step. The tin dioxide thin films were then used to construct an efficient UV light-emitting diode. These findings suggest that engineering the nanostructure of other oxides might also render them optically active. It is commonly believed that bulk SnO2 is not a suitable ultraviolet (UV) light emitter due to the dipole-forbidden nature of its band-edge states, which has hindered its potential use in optical applications. Here, we demonstrate both theoretically and experimentally an effective method to break the dipole-forbidden rule in SnO2 via nano-engineering its crystalline structure. Furthermore, we designed and fabricated a prototypical UV-light-emitting diode (LED) based on SnO2 thin films. Our methodology is transferable to other semiconductors with ‘forbidden’ energy gaps, offering a promising route toward adding new members to the family of light-emitting materials.