These new-generation humidity sensors are periodic multilayered nanostructures that comprise a stimuli-responsive 2D-nanosheet material with an ultrahigh refractive index (RI) as active component. Three new sensing devices for humidity sensing consist of either nanosheets (NSs) and nanoparticles (NPs), or two different nanosheets, and were developed by Professor Bettina Lotsch and her colleagues.
The resulting photonic crystals possess excellent optical quality caused by the excellent swelling capacity and the ultrahigh RI contrast due to the application of these lithium tin sulfide (LTS) nanosheets. “It is amazing that we are able to build functional humidity sensors with very good optical and sensing properties, even comparable to commercial systems, after stacking of only five or seven layers of nanosheets and nanoparticles on the glass substrate!” explains Katalin Szendrei-Temesi, first author of the publication in Advanced Functional Materials, excited. “Such ultrathin set-ups reduce material costs and production time compared to other sensing devices with higher thicknesses.”
Upon water uptake, the integrated nanosheets swell, and hence the structural color of the photonic crystal changes according to the changed layer thicknesses. The system is comparable to puff pastry, the more water molecules integrate in the LTS nanosheet layers, the more their thickness increases.
Part of the incoming light is reflected by the photonic crystal. The layers consisting of nanoparticles such as titanium dioxide (TiO2) or silicon dioxide (SiO2) NPs, or other nanosheet materials like layered phosphate antimonate (H3Sb3P2O14), respectively, serve as contrast material in the layered structure. In addition, they help to create the structural color and the high reflectance through interference and wavelength-selective reflection. The wavelength of the reflected light depends on layer thickness and RI of the NSs.
The sensing mechanism was clarified and approved also by theoretical calculations: the color change arises mainly from the swelling of the nanosheets upon water uptake. Using simulations, the sensors thickness, and thus the structural color, could be predicted accurately.
With below 10 seconds (7.0 sec NS-NP crystals / 9.2 sec NS-NS crystals) response time, and only 1.1 seconds (NS-NP crystals) / 2.4 seconds (NS-NS crystals) recovery time, these colorimetric humidity sensors are very fast compared to other sensor systems.
Sensors from nanosheets and nanoparticles
Novelty of these photonic crystals is that the TiO2 nanoparticles, widely used as high refractive index material, are instead assigned as the low RI compound, as the LTS 2D-nanosheets exhibit an even higher refractive index.
The use of titanium dioxide nanoparticles in photonic crystals is well established due to their high refractive index and the characteristic transparency as thin films. So these photonic crystals, for the first time using porous titania nanoparticles as the low RI material and LTS nanosheets as high refraction index compound, resulted in high transparency thin films with a potential application in light managing devices.
The application of earth-abundant and easy-to-process silica nanoparticles as the low refractive index material in photonic crystals is a standard procedure in this research field, but the combination with the ultrahigh refractive index LTS nanosheets was seminal. „Despite their unusual composition, these photonic crystals have a fantastic optical quality, the best ever realized, due to the highest ever realized refractive index contrast.“ explains Lotsch. „We could reduce the sensor thickness, as five layer photonic crystals comprising two layers of SiO2 nanoparticles and three layers of LTS nanosheets delivered comparable results to the standardly used SiO2-TiO2-systems, composed at least of 12 layers usually, from the literature.“
Ultrathin and robust photonic crystals composed only of 2D materials
The two different types of 2D-nanosheet materials, LTS and H3Sb3P2O14 nanosheets, are both capable of swelling when exposed to humidity. With this exceptional capability, photonic crystals combining both are promising candidates for optical humidity sensors with exceptionally high sensitivity to ambient humidity as the swelling of both materials results in ultrahigh structural color changes.
As an additional feature, with increasing humidity, the reflectance intensity of the sensor drastically decreases, because the refractive indices become similar to each other due to the water accumulation. At 100 % relative humidity, the RI contrast completely vanishes and the device turns transparent because of the equalization of the refractive indices. This transparency switching by an analyte is a rarely observed phenomenon as a careful optical and material design is needed.
In addition, the combination of the two robust NS materials in ultrathin photonic crystals ensures besides the flexibility a high stability compared to organic material-based devices. With well below one micron and a few layers only, the thickness of such sensing devices is reduced by 60% compared to systems with similar optical qualities.
Lithium tin sulfide – a high-refractive-index 2D material for humidity-responsive Photonic Crystals. Szendrei-Temesi K, Sanchez-Sobrado O, Betzler S, Durner KM, Holzmann T, Lotsch BV. doi: 10.1002/adfm.201705740