A photodetector is at the heart of modern sensing and imaging technology. A very well-known application of photodetectors is an imaging sensor of a digital camera. Compared with the well-established photo-detection methods of visible light, the sensing of near- and mid-infrared light with high sensitivity at room temperature is still challenging. An advanced infrared light photodetector requires cooling of the device below -100oC. Conversion of infrared light-heat-electric signal is a useful way to detect IR light. However, this approach (called as a thermoelectric effect) has several inherent problems which prevent a miniaturization of the device and limit an ability to resolve infrared light with different wavelengths. Also, existing thermoelectric materials exhibit a low efficiency in converting light energy to electric energy at room temperature. It is difficult to address weaknesses of current technology by improving only a single aspect of devices. In this project, multidisciplinary research will be performed for new materials design, theoretical performance evaluation and novel electric device fabrication. The photodetector from this project will efficiently collect infrared light of different wavelengths at room temperature. The nature of the multidisciplinary research will be beneficial in integrating the technical research with education and outreach. Basic science, technology and prototype product of the project will be used in "Nanotechnology" workshop for the Pittsburgh Junior Academy of Science (PJAS) and "Science Research" course for Pittsburgh local high school students. In addition, The PI and co-PIs will integrate outcomes of the project into undergraduate and graduate courses in materials science, mechanical engineering and electrical engineering programs at the University of Pittsburgh.
The objective of this research is to develop a thermoelectric infrared sensor that has a multispectral resolution capability and operates without cryogenic cooling. This will be accomplished using hybrid structures of 2-dimensional materials, such as graphene and molybdenum sulfide, and plasmonic metal Nano-shells. The hypotheses underlying the proposed structure are as follows. First, unlike conventional bulk thermoelectric materials, graphene and molybdenum Nano sheets have an extremely large surface area to volume ratio. This feature can provide an opportunity to improve the Seebeck coefficient through modifying or doping the surface. Second, high and wavelength-dependent absorption of infrared light can be enabled by the surface plasmons of dielectric core - metal Nano-shell particles with tunable characteristic wavelengths. Third, a thin free-standing silicon nitride membrane with low heat capacity and low thermal conductivity can allow faster and larger increase of local temperature upon IR light absorption. This will improve dynamic response and the signal-to-noise ratio of an IR sensor. The low thermal conductivity of silicon nitride can be further reduced by increasing structural and mass disorder. The major intellectual merit of the proposed research lies in the fundamental understanding of the heat and charge transport properties at nanoscale. Through integrated research of simulation and experiment, the principal investigators will develop a model to predict how physical properties (e.g. Seebeck coefficient, thermal conductivity and heat capacity) of a free standing membrane affect important performance factors of the thermoelectric IR sensor, such as output signal, response time and signal-to-noise ratio. Moreover, enhancement of selective light absorption by the metal Nano-shells and subsequent energy dissipation will show a novel way to resolve the wavelength of incident infrared light and create a temperature gradient through local heating.
