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It has previously been demonstrated that organic semiconductor lasers can be used as chemical sensors for the detection of nitroaromatic-based explosive vapors (for example TNT). But what has not been clear is over what timescale such detectors respond to these vapors, a crucial factor for the practical suitability of the detectors. Now, a new study by Yang, Turnbull and Samuel investigates this issue by studying the time-varying response of a polyfluorene laser to the presence of explosive vapors.

An organic semiconductor laser functions as a sensor of nitroaromatic vapors due to chemical interaction between the semiconductor and the vapors. Nitroaromatic molecules are electron deficient and the transfer of electrons from the semiconductor to the vapors reduces the laser’s emission of light. This change in the emission can then be observed and used to infer the presence of the vapors.

In this study, a polyfluorene laser was exposed to vapors of 1,4-dinitribenzene (DNB) at a concentration of 9.8 ppb, and the time-varying effect on the laser was monitored. After exposure to the DNB vapors, the input energy threshold, which needs to be exceeded for the laser to function, increased by 1.8 times, due to the aforementioned electron-transfer effect. In addition, the ratio of output energy to input energy for the laser was observed to reduce by a factor of 3. For a future detection system, both of these effects could be used together to detect explosive vapors.

However, measurement of the response and recovery times for the detector is the main result of this study. After being exposed to the vapors, the laser’s light emission first decreased rapidly, making vapor detection possible within tens of seconds after exposure. But then this decrease slowed, flattening off after around 4-5 minutes. The authors suggest that the first rapid decrease is due to the DNB molecules interacting with the surface of the polyfluorene semiconductor, whilst the slower decay that follows is caused by the slower diffusion of the DNB deeper into the semiconductor. The timescale of recovery was also measured, as this is also important for the practicality of the sensor. Full recovery took 3.5 hours when the laser was left in air and 3 minutes when nitrogen gas was flushed through the laser, but only a mere 20 seconds when the laser was purged under a vacuum.

The authors also developed a simple theoretical model based on diffusion of the DNB vapors into the semiconductor, which supported their experimental findings and enabled them to calculate the depth of diffusion.

All of this comes together to show that polyfluorene lasers can detect tiny amounts of explosive vapors within a matter of seconds after exposure. This makes them an ideal tool for the detection of explosive devices, and it is surely only a matter of time until these devices are developed for practical use.

Image: http://www.flickr.com/photos/hellochris/ / CC BY 2.0

Y. Yang et al., Adv. Funct. Mater. ; DOI: 10.1002/adfm.200901904