Scientists at the U.S. Naval Research Laboratory (NRL; Washington, DC) have discovered a new method to passivate defects in next-generation monolayer optical materials to improve optical quality and enable the miniaturization of monolayer-based LEDs and other optical elements.1
The NRL scientists developed a laser-processing technique to significantly improve the optical properties of monolayer molybdenum disulphide (MoS2), a direct gap semiconductor, with high spatial resolution. Their process produces a 200-fold increase in the material's optical-emission efficiency in the areas written with the laser beam. The resulting passivation is stable in air and vacuum.
"From a chemistry standpoint, we have discovered a new photocatalytic reaction using laser light and water molecules, which is new and exciting," says Saujan Sivaram, one of the researchers. "From a general perspective, this work enables the integration of high quality, optically active, atomically thin material in a variety of applications, such as electronics, electro-catalysts, memory, and quantum computing applications."
According to Sivaram, atomically thin layers of transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS2), are promising components for flexible devices, solar cells, and photosensors due to their high optical absorption and direct band gap.
"These semiconducting materials are particularly advantageous in applications where weight and flexibility are a premium," he says. "Unfortunately, their optical properties are often highly variable and nonuniform, making it critical to improve and control the optical properties of these TMD materials to realize reliable high-efficiency devices. Defects are often detrimental to the ability of these monolayer semiconductors to emit light. These defects act as nonradiative trap states, producing heat instead of light; therefore, removing or passivating these defects is an important step towards high-efficiency optoelectronic devices."
In a traditional LED, approximately 90% of the device is a heat sink to improve cooling. Reduced defects enable smaller devices to consume less power, which results in a longer operational lifetime for distributed sensors and low-power electronics.
Passivation via water molecules
The researchers demonstrated that water molecules passivate the MoS2 only when exposed to laser light with an energy above the band gap of the TMD. The result is an increase in photoluminescence with no spectral shift. Treated regions maintain a strong light emission compared to the untreated regions that exhibit much a weaker emission. This suggests that the laser light drives a chemical reaction between the ambient gas molecules and the MoS2.
"The results of this study pave the way for the use of TMD materials critical to the success of optoelectronic devices and relevant to the Department of Defense mission," says Berend Jonker, senior scientist and principal investigator.
1. Saujan V. Sivaram et al., ACS Applied Materials & Interfaces (2019); doi: 10.1021/acsami.9b00390.