Light-emitting diodes (LEDs) are highly energy-efficient and reliable light sources which have made them popular in handheld devices due to their low power draw on battery systems. With developments in LED optical materials, there are now LEDs available in a huge number of color options, including with different bandwidths.
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A recent collaborative work between the University of Melbourne, the Lawrence Berkeley National Laboratory, the University of California, Berkeley, and the Australian Research Council Centre of Excellence for Transformative Meta-Optical Systems (TMOS) has now found a way to develop infrared LEDs that are tunable to different wavelengths.1
Black phosphorous was used as the starting point for this infrared optoelectronic. The bandgap, the energy difference between the valence and conductive bands, was tuned by applying different amounts of strain. As most LEDs emit light of energy similar to that of the bandgap, the team tuned the LED emission by varying the strain on the black phosphorous to generate light between 2.3 to 5.6 µm.
Using Infrared LEDs in Gas Sensing
Developing tunable LEDs that operate at room temperature is a significant development as most materials will emit at fixed wavelengths and have highly demanding fabrication requirements.
One of the advantages of using black phosphorous, as well as its tunability, is that it only requires a single layer of material to be deposited. This eliminates the need to align and deposit multiple complex crystals layers.
To take advantage of this tunability, the team showed that this IR LED could be used to detect a variety of gases. Many gas sensors for the detection of gases such as carbon dioxide or methane make use of their strong infrared absorptions to create devices with very good lower limits of detection.
However, often these infrared gas sensors use single-wavelength diode lasers. Every chemical species has a unique infrared spectrum that can be used for its identification. To create a device capable of multiplex detection of multiple species, either a dispersive spectrum must be recorded following broadband excitation, or the wavelength of the incident light must be scanned or varied to be resonant with different transitions in different molecules.
The wide tunability of black phosphorous meant the team could achieve the required tunability for the detection of methane, carbon dioxide, and water, all within a single device, simply by varying the strain.
The response times for changing the wavelength emitted from the black phosphorous are relatively quick – on the order of hundreds of seconds. Applying the strain to change the wavelength takes a finite amount of time, as does allowing the sample to relax to a less strained state.
By applying a mixture of water, carbon dioxide, and methane, the team demonstrated that each gas could be selectively detected by strain tuning the emission to be resonant with the correct emission band. No signal was detected by the device when only nitrogen gases were passed through, showing the quick off/on response times.
The Use of Infrared LEDs in Safety Applications
The compact, low-power nature of the LED and its tunability for gases such as carbon dioxide and methane make it ideal for the remote detection of different hazardous gases.
Many industries generate gases that are explosion risks, toxic, or an asphyxiation concern at high concentrations. This includes the potential of methane leaks at liquid petroleum refineries and carbon dioxide build-up at breweries.
While health and safety legislation means that fixed gas monitors should be in any potential risk areas, the information from monitors may not be readily accessible to emergency crews in the event of an incident. With the new device, emergency response personnel could wear a small, unobtrusive device and be warned of the presence of hazardous gases or any risk of asphyxiation. Importantly, the response times of the device are quick enough that personnel would not be exposed to hazardous gases for a long period of time before a measurement could be completed.
Miniature gas sensors might also become part of our homes. A total of 1.3 billion tons of food is wasted by consumers every year.2 One idea to reduce the amount of food wastage is with the use of a smart refrigerator that can help remind you when a particular foodstuff is about to expire.
However, most current smart refrigerator technologies rely on the user manually inputting information about when food items have been added to the fridge. While this information can then be used to estimate when certain products should be used, it is based on typical storage lifetimes for products.
As many food products start to decompose, they start to release gases changing the atmospheric composition in the fridge.3 By monitoring various gas species, real-time information on food spoilage can be retrieved and used to send alerts as part of an Internet of Things (IoT) network.
References and Further Reading
- Kim, H., Uddin, S. Z., Lien, D. H., Yeh, M., Azar, N. S., Balendhran, S., Kim, T., Gupta, N., Rho, Y., Grigoropoulos, C. P., Crozier, K. B., & Javey, A. (2021). Actively variable-spectrum optoelectronics with black phosphorus. Nature, 596(7871), 232–237. https://doi.org/10.1038/s41586-021-03701-1
- Nasir, H., Aziz, W. B. W., Ali, F., Kadir, K., & Khan, S. (2018). The Implementation of IoT Based Smart Refrigerator System. 2018 2nd International Conference on Smart Sensors and Application, ICSSA 2018, 48–52. https://doi.org/10.1109/ICSSA.2018.8535867