Researchers at the National Institute of Technology Rourkela (NIT Rourkela), in collaboration with IIT Guwahati, IIT Bombay and NISER Bhubaneswar, have developed lead-free light-emitting materials that respond to heat and moisture, opening up potential applications in anti-counterfeiting, humidity sensors and lighting technologies.
The bismuth-based materials can change their light-emission properties in response to environmental conditions, offering a potential alternative to lead-based photoluminescent compounds that raise health and environmental concerns.
The findings have been published in the Journal of Materials Chemistry C. The research was co-authored by Saroj L. Samal, Associate Professor in the Department of Chemistry at NIT Rourkela, and research scholar Subhasish Sahoo, along with researchers from the three collaborating institutions.
Heat and moisture trigger reversible light emission
The researchers found that the developed bismuth-based organic-inorganic hybrid halide compound undergoes reversible structural changes in response to heat and moisture.
When heated, the material loses water trapped within its structure and emits blue light under ultraviolet (UV) exposure. When moisture returns, it absorbs water again, causing the blue emission to disappear.
The reversible response could enable the development of materials that signal changes in environmental conditions through visible changes in light emission.
“The Bi-based organic-inorganic hybrid halide compound is synthesized using cheaper and environmentally friendly constituents utilising a simple synthesis technique. The most interesting aspect of this material is its ability to undergo reversible structural transformation in response to heat and moisture, accompanied by change in its light emission properties,” said NIT Rourkela, Associate Professor, Saroj L. Sama.
Potential for anti-counterfeiting and humidity sensors
One potential application is the authentication of important documents and products. Special inks incorporating the material could be used on certificates, signatures and security labels. Their response to controlled heating and subsequent UV exposure could provide an additional means of checking authenticity.
The material’s sensitivity to moisture could also be used in humidity-sensing applications. Potential use cases include food packaging, storage facilities and other environments where moisture levels need to be monitored.
However, these applications will require further development to establish the material’s sensitivity, reliability and durability under practical operating conditions.
Lead-free materials open up lighting opportunities
The research team has also developed related lead-free materials that emit yellow light under UV exposure.
By combining the yellow-light-emitting material with a commercially available blue-light-emitting material, the researchers produced a white LED with a cool, daylight-like appearance. The white-light system maintained stable emission during continuous operation, indicating potential for further development in solid-state lighting and light-conversion technologies.
“The synthesised compound shows promising potential for reversible luminescence switching, moisture-sensing, solid state lighting and anti-counterfeiting applications,” Samal said.
Next step: Improve efficiency and stability
The researchers plan to improve the light-emission efficiency and long-term stability of the materials and investigate their integration into practical devices.
Further work will focus on developing more efficient luminescent compounds, improving sensor sensitivity and reliability, and optimising the performance of white-light-emitting diodes.
The research could contribute to the development of lead-free materials for optical security, environmental monitoring and lighting. However, commercial applications will depend on further validation of their performance, durability and manufacturing suitability.