(Invited) Camera-Based Strain Visualization Using Carbon Nanotube Fluorescence

It is well known that each semiconducting species of single-wall carbon nanotube (SWCNT) gives distinct near-infrared fluorescence following excitation with visible light. Fundamental band gap modulation makes the peak emission wavelengths shift predictably when the nanotubes are deformed by axial stretching or compression. This effect forms the basis for a new method of mechanical strain measurement in which nanotubes are dilutely dispersed in thin polymer films applied to surfaces of interest. When the substrate is subsequently strained, load transfer through the film to the nanotubes causes small spectral shifts that are monitored through non-contact spectral measurements. We have demonstrated this method through successful strain mapping on specimens of aluminum, copper, steel, acrylic, polycarbonate, cement, and concrete, using point-by-point scanning. To improve the speed and convenience of SWCNT-based strain mapping, we are adapting the method to camera-based measurements. A macroscopic region of a coated specimen surface is illuminated with an excitation laser and the resulting SWCNT emission is imaged from a stand-off distance of ca. 50 cm by a near-IR sensitive camera. Multiple images are acquired through a tunable spectral filter to span the emission band of interest. The image data are then analyzed by spectrally fitting intensities at each pixel to find the local peak emission wavelength, from which the local strain value is deduced. The set of local strains is converted into a color-coded strain map showing detailed deformations of the specimen. We expect that the introduction of camera-based spectral strain measurement technology will lead to an important industrial application of SWCNTs.