Nano Lett. http://doi.org/w7k (2014)

Unlike graphene, single-layer metal dichalcogenides exhibit an electronic bandgap, which is essential for the realization of electronic devices. Furthermore, because of the bandgap, the interaction of electrons and holes with an electromagnetic field leads to optical absorption and photoluminescence emission, thus, these 2D semiconductors could, in principle, be used in optoelectronic devices such as light-emitting diodes and lasers. A disadvantage of being only one atom thick is, however, that the emission and absorption efficiencies are very low. Stefan Schwarz and colleagues in the UK and Russia have now shown that the photoluminescence emission can be enhanced considerably by placing thin films of metal dichalcogenides in properly designed optical cavities.

The researchers studied two types of configuration. In the first, a film of either MoS2 or GaSe was placed on top of a distributed Bragg reflector, which reflects all of the downward emitted light from the film. This resulted in considerable enhancement of the photoluminescence emitted compared with the free-standing films. In the second experiment, the team placed a second distributed Bragg reflector on top of the film, creating a full optical cavity. The distance between the reflectors was tuned so that the wavelength of the cavity matched that of the film. This resulted in an additional enhancement of the photoluminescence intensity and a sharpening of the emission peak. A time-resolved measurement of the photoluminescence also revealed an increased radiative efficiency, which confirms that with the optical cavities there is a stronger interaction between the carriers in the semiconducting films and the electromagnetic field.