Laser writing offers route to microscale gas sensors

05 Jul 2022

Penn State project uses laser-induced thermal voxels to create customizable device.

A project at Penn State University has taken a step towards environmental sensors better able to detect multiple gas species simultaneously.

The new approach exploits advances in laser writing techniques to create what the team describes as the first highly customizable microscale gas sensing devices.

"The detection of gases is of critical importance to various fields, including pollution monitoring, public safety assurance and personal health care," said Huanyu Cheng of Penn State.

"To fill these needs, sensing devices must be small, lightweight, inexpensive and easy to use, and apply to various environments and substrates, such as clothing or piping."

As described in Applied Materials & Interfaces, the new device's fabrication relies upon laser-induced thermal voxels (LITV). It builds on previous Penn State research into LITV as a patterning approach, whereby an absorbing material converts the photonic energy of a high-power laser into thermal energy at a finely focused area.

This method promotes the conversion of material directly from a precursor solution, according to Penn State, and diverse materials can be patterned with features nearing 1 micron in size using the LITV technique. In the new project, a LITV process enabled the simultaneous creation and integration of metal oxides directly into sensor platforms, with the oxides then reacting to various compounds and gases to test the sensing mechanism.

Fabrication involves dissolving the metal salts in water, then focusing a laser into the solution. The high temperatures generated decompose the solution, leaving behind metal oxide nanoparticles that can be sintered onto the sensor platform. This streamlines previous methods, which required a pre-defined mask of the planned pattern and a new mask for any changes or adjustments, noted Penn State. Laser writing using LITVs is maskless and allows for the rapid iteration and testing of multiple designs or materials, to find the most effective combinations.

Electronic nose prototypes

"Precise patterning is a necessary component for the creation of 'electronic noses,' or arrays of sensors that act like a nose and can precisely detect multiple gases at the same time," said Alexander Castonguay of Penn State.

"Such precise detection requires the patterning of different materials in close proximity, at the thinnest microscale. Few patterning techniques have the resolution to do this, but the approach detailed in this study does. We plan to use the techniques and materials described here to develop electronic nose prototypes."

In trials, the team used LITV and laser writing to investigate the sensing performance of a representative set of n-type and p-type LITV-deposited metal oxides and their mixtures, including CuO, NiO, and CuO/ZnO. The response to a range of reducing and oxidizing gases was then monitored.

The heterojunction where different metal oxides touch is known to enhance the response of gas sensors, and the project found that a heterojunction of copper oxide and zinc oxide had a 5- to 20-fold enhanced response to ethanol, acetone, nitrogen dioxide, ammonia and hydrogen sulfide, compared to just copper oxide alone.

"This finding supports other reports that the creation of mixed oxide systems can lead to significant increases in sensor response, and demonstrates the efficacy of the LITV technique for mixed-oxide gas sensor fabrication," commented Castonguay.

"We hope by pooling the laser writing knowledge of Penn State's Zarzar group with the wearable sensor expertise of the Cheng group, we will be able to expand our capabilities to create novel, customizable sensors."