Electroluminescence is a phenomenon whereby a material emits light in response to an electrical current passing through it. From the glowing screens of our smartphones and ceiling lights to sensors and communication devices, electroluminescence is found in various aspects of modern technology. Today, the miniaturisation of digital devices is fuelling the demand for compact, low-power, and high-speed electroluminescent devices that can be seamlessly integrated into semiconductor chips for next-generation information technologies.
Now, NUS researchers, led by Professor Goki Eda from NUS Physics and Chemistry, have taken a multidisciplinary approach that relies on the interplay between quantum mechanics, condensed matter physics and materials chemistry. Through their work, which was reported in
Nature Nanotechnology in April 2024, the team have revealed how the interaction between electrons and ultrathin, or 2D, semiconductors can be harnessed to produce light in a way previously considered unfeasible.
Observation of upconversion electroluminescence
In their study, Prof Eda and his team created a heterostructure consisting of gold (Au) and graphene electrodes, with an ultrathin layer of hexagonal boron nitride (hBN) and 2D semiconductor (WS2).
When a voltage was applied between the graphene and the Au electrodes, an electrical current was generated, and light emission was observed. While analysing the wavelength of the emitted light, the team noticed that there was something unusual about this electroluminescence.
The team found that the energy of the emitted light is greater than the supplied electrical energy. This phenomenon, sometimes referred to as upconversion electroluminescence, is rarely found in conventional devices where the upper limit of the energy of emitted light is defined by the quantum cutoff i.e. the energy of the input electrical potential.
Upon investigation, the team found that none of the previously proposed models explains the experimental observations and that this unusual phenomenon has a complex origin involving tunnelling electrons that occasionally transfer their energy to electrons in the graphene electrode.
Role of inelastic tunnelling electrons
Schematic diagram showing energy changes in an energy barrier for inelastic electron tunnelling and elastic electron tunnelling
As a voltage is applied, electrons in the Au electrode undergo quantum mechanical tunnelling through the hBN layer to the graphene electrode, causing an electrical current. While most electrons tunnel through without any loss in energy (elastic tunnelling), a small fraction of them lose energy. The latter is known as inelastic electron tunnelling (IET). The team hypothesised that the manner in which the energy is lost during IET is key to understanding the observed phenomenon.
According to theory, the dissipated energy can be transferred to electrons in the graphene layer, resulting in the formation of energetic “hot” electrons and holes, which are quasi-particles denoting the absence of an electron. With sufficient voltage applied, “hot” holes in the graphene layer can be propelled into the WS2 layer, where they interact with electrons that were injected via elastic tunnelling. The annihilation of an electron and a hole in the WS2 layer subsequently results in the emission of light.
Excitation of hot carriers in the graphene layer enabled by IET, and the formation of excitons in WS2 layer
The discovery of IET-enabled upconversion electroluminescence from the 2D semiconductor heterostructure reveals the intricate and unique physics underlying what initially seems to be a straightforward simple electrical system. This breakthrough underscores its potential for unconventional photonic functionalities, showcasing a promising avenue for innovation. With continued research, the team is optimistic that integrating these light sources onto semiconductor chips will catalyse the development of novel devices across telecommunications, sensing, and medical diagnostic technologies.
References
Wang, Z., Kalathingal, V., Trushin, M., Liu, J., Wang, J., Guo, Y., ... & Eda, G. (2024). Upconversion electroluminescence in 2D semiconductors integrated with plasmonic tunnel junctions. Nature Nanotechnology, 1-7.
