Laser Processing Method Could Increase Efficiency in Optoelectronic Devices

A new way to passivate defects in next-generation optical materials could improve quality in optoelectronic applications and enable the miniaturization of LEDs and other optical elements. The advance uses a laser processing technique developed by scientists at the U.S. Naval Research Laboratory (NRL).

The researchers used a laser processing method to improve the optical properties of monolayer molybdenum disulphide (MoS₂), a direct gap semiconductor with high spatial resolution. Monolayer MoS₂ was synthesized by chemical vapor deposition (CVD) and by controlled exposure to laser light in ambient conditions. This spatially resolved passivation treatment was air- and vacuum-stable. Regions that were not exposed to laser light remained dark in fluorescence despite continuous impingement of ambient gas molecules, while treated regions maintained a strong light emission. According to the researchers, up to a two-hundredfold increase in the material’s optical emission efficiency is possible in the areas exposed to the laser processing method.

(Top) Illustration of a water molecule bonding at a sulfur vacancy in the MoS₂ upon laser light exposure. (Bottom) Photoluminescence (PL) increase observed during laser light exposure in ambient. (Inset) Fluorescence image showing brightened regions spelling out NRL. Courtesy of U.S. Naval Research Laboratory.

A wavelength-dependent study confirmed that photoluminescence brightening was concomitant with exciton generation in the MoS₂. Laser light below the optical band gap did not enhance photoluminescence. The researchers highlighted the photo-sensitive nature of their process by successfully brightening with a low-power broadband white light source. Through a series of gas exposure studies, the researchers demonstrated a correlation between photoluminescence brightening and the presence of water.

According to researcher Saujan Sivaram, atomically thin layers of transition metal dichalcogenides (TMDs), such as MoS₂, are promising materials for use in flexible devices, solar cells, and optoelectronic sensors due to their high optical absorption and direct bandgap.

“These semiconducting materials are particularly advantageous in applications where weight and flexibility are a premium,” Sivaram said. “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.”

Sivaram said that defects in the material can interfere with its ability 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,” he said.

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 can result in a longer operational lifetime for distributed sensors and low-power electronics. The results of the NRL study could increase the successful use of TMD materials such as MoS₂ in optoelectronic devices.

The research was published in ACS Applied Materials & Interfaces (http://dx.doi.org/10.1021/acsami.9b00390).