Thin-Film LiNbO 3 Pyroelectric Detectors Coated with Multiwalled CNTs Enhance Sensitivity
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Thin-Film LiNbO3 Pyroelectric Detectors Coated with Multiwalled CNTs Enhance Sensitivity The current generated by a pyroelectric detector is inversely proportional to its thermal mass. Therefore, a practical limit for building more sensitive pyroelectric detectors is imposed by conventional lapping and polishing of the detector plate. Thin-film pyroelectric detectors fabricated by crystal ion slicing show higher sensitivity than those prepared by conventional lapping and polishing while maintaining the pyroelectric properties of the bulk material. By coating the detectors with a low mass thermal absorber, such as carbon nanotubes (CNTs), the sensitivity of the devices can be further enhanced, with a uniform spectral responsivity. J.H. Lehman and K.E. Hurst from the National Institute of Standards and Technology, Colorado, A.M. Radojevic from The Charles Stark Draper Laboratory, Mass., A.C. Dillon from National Renewable Energy Laboratory, Colorado, and R.M. Osgood Jr. from Columbia University, New York, reported such improvements in this type of pyroelectric detector when compared to a nickel-coated detector, in the April 1 issue of Optics Letters (p. 772). The detector consisted of a 10-µm thick freestanding film of LiNbO3, fabricated by crystal ion slicing by bombardment of the parent material with high-energy helium ions. The detector film is separated by acid etch or thermal shock at a depth where the ions run out of momentum. The film is then coated with 250-nm thick nickel electrodes on each face and packaged as a freestanding detector. The researchers coated the freestanding detector with a 5–10-µm thick layer of commercially available multiwalled CNTs (MWNTs) by spraying it with a dispersion of 0.33 g of these nanotubes in 13.4 ml of chloroform by an airbush technique. MWNTs were chosen over singlewall CNTs (SWNTs) to avoid the spectral features characteristic of interband transi-
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tions of metallic and semiconducting materials, which SWNTs readily exhibit. The MWNT-coated detector, black due to the optical properties of the individual tubes and the topology of the bulk, appeared like “a mat of bundled ropes with various clumps interspersed” under the scanning electron microscope. The clumps are believed to be catalyst metals and non-nanotube carbons. The MWNT coating is attached to the electrode surface by van der Waals forces and by interactions with pi electrons in the orbital perpendicular to the axis of the tubes. The coating is robust: it can be scratched with a rigid stylus, but it is not removed by forced air or water. The increase in spectral responsivity of the MWNT-coated detector is about four times over the spectral region from 600 nm to 1800 nm compared to a Ni-coated detector, with minimal penalty to the frequency response. According to the researchers, the enhanced performance, along with the facile and inexpensive application of the coating, is immediately achievable and is promising for other types of radiometric detectors and thermal detector arrays. JOAN J. CARVAJAL
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