Sensing gas optically with nanocomposite thin films
Thin films can be used in a variety of devices. For example, some studies used tin-oxide (SnO2) films as detectors, including odor sensors.1–3 For the best gas detectors, a thin film should be porous. Such films can be created with sol-gel technology, in which a sol—a colloid composed of a solid suspended in a liquid—changes to a gel or a solid phase. Researchers have used sol-gel techniques to create porous versions of silicon-dioxide (SiO2) that have valuable properties, such as the ability to sense humidity.4,5 Here, we describe a combination of thin films and optical reader that, together, make sensitive gas detectors.
Our research shows that improved gas sensors can be made by combining SnO2 particles with porous SiO2 materials. These nanocomposites absorb gases very well. Moreover, doping a nanocomposite with antimony (Sb) enhances a thin-film gas detector's sensitivity and selectivity.
To study the effect of thin-film composition, we made three: SnO2, Sb:SnO2, and Sb:SnO2/SiO2.6Figure 1 shows x-ray diffraction (XRD) spectra of the Sb-doped films on a silicon substrate. At about 28°, both samples produced a diffraction peak caused by the Si(111)-crystal face in the silicon substrate. Moreover, XRD revealed strong diffraction peaks at 27°, 33.4°, and 51.8°, which were generated by the crystal faces (110), (101), and (211) of the SnO2. Those peaks indicate that the SnO2 changed to a polycrystalline structure. Furthermore, the Sb:SnO2/SiO2 film showed no peaks that were not in the XRD of the Sb:SnO2 film, which reveals that the SiO2 particles are amorphous.
Figure 2 provides atomic-force microscopy (AFM) images of the Sb:SnO2 and Sb:SnO2/SiO2 composite films. These show that the crystal-particle size of the Sb:SnO2/SiO2 composite films is smaller than that of the Sb:SnO2 films. Specifically, the Sb:SnO2/SiO2 particles have a diameter of about 34nm, and they exist homogeneously in the films as nanometer-sized crystals. The Sb:SiO2 particles, on the other hand, are distributed inhomogeneously, and they tend to conglomerate. Furthermore, the crystal particles of Sb:SnO2/SiO2 have larger specific surface areas and duty porosity, which improves gas-sensing capabilitied.
Figure 3 depicts the arrangement for our gas-sensing experiments. A p-polarized, helium-neon (He-Ne) laser beam (λ=632.8nm) hits the sample surface at θi, and a charge-coupled device camera detects two reflected beams—with intensities Ia and Ib—from the front and the rear surface of the sample. The intensity ratio (γ Ia/Ib) is closely related to the overall optical parameters.6 To select the laser's optimum angle of incidence, we filled the chamber with fresh air and measured the intensity ratio, γair, of a sample thin film. Then, we filled the gas chamber with a test gas and, again, measured the intensity ratio, γgas. The gas sensitivity, Sg , is defined as:
This procedure was used to measure the sensitivities of SnO2, Sb:SnO2, and Sb:SnO2/SiO2 films to various gases: propane (C3H8), gaseous ethanol (C2H5OH), and ammonia (NH3), as shown in Figure 4. This data shows that Sb doping increased the gas sensitivity of the thin films, especially the Sb:SnO2/SiO2 film, because it possesses smaller particles, larger specific surface area, and higher duty porosity. Moreover, the Sb:SnO2/SiO2 film's surface has many active centers, which tend to adsorb gases that react on the surface.
Further calculations show that—under optimal conditions—these optical-thin film sensors can detect gases down to 10-1ppm. To improve the gas sensitivity of these nanocomposite thin films even more, they can be doped with different transition metals. For example, platinum (Pt) and palladium (Pd) doping can improve the gas sensitivity to C2H5OH and C3H8, respectively. In the future, we will write more about these modifications.
Zhengtian Gu received his MS in physical electronics and optoelectronics from Southeast University, Nanjing, China, in 1995. In 2000, he received his PhD from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. He now works as a professor of physics and optics at the University of Shanghai for Science and Technology. His main research interests are optical thin films, opto-chemical sensors, and optoelectronic engineering. He has published about 50 articles, and he has written numerous papers for SPIE conferences.
