(Top) Illustration of a water molecule bonding at a sulfur vacancy in molybdenum disulphide upon exposure to laser light. (Bottom) Photoluminescence increase observed during exposure to laser light in the presence of water molecules; (inset) fluorescence image showing brightened regions spelling out 'NRL'. Image: US Naval Research Laboratory.Scientists at the US Naval Research Laboratory (NRL) have discovered a new method for passivating defects in next generation optical materials, thereby improving optical quality and allowing the miniaturization of light emitting diodes (LEDs) and other optical elements. They report this new method in a paper in ACS Applied Materials & Interfaces.
"From a chemistry standpoint, we have discovered a new photocatalytic reaction using laser light and water molecules, which is new and exciting," said Saujan Sivaram, lead author of the paper. "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."
The NRL scientists developed a versatile 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 100-fold increase in the material's optical emission efficiency in areas ‘written’ with the laser beam.
According to Sivaram, atomically thin layers of transition metal dichalcogenides (TMDs) such as MoS2 are promising components for flexible devices, solar cells and optoelectronic sensors, 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 said. "Unfortunately, their optical properties are often highly variable and non-uniform, 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 non-radiative 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 allow smaller devices to consume less power, which results in a longer operational lifetime for distributed sensors and low-power electronics.
The researchers demonstrated that water molecules passivate the MoS2 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, which exhibit a much weaker emission. This suggest that the laser light drives a chemical reaction between the ambient gas molecules and the MoS2.
"This is a remarkable achievement," said Berend Jonker, senior scientist and principal investigator. "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."
This story is adapted from material from the US Naval Research Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.