Solid State Chemistry: from Thermoelectric to Nonlinear Optical Materials

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Thermoelectric Energy Conversion My NSERC-funded research is focused on the thermoelectric energy conversion. This sustainable energy creation method is becoming increasingly important, as our natural resources continue to decline and mankind's need for electricity increases. Thermoelectric (TE) materials can create electricity via the Seebeck effect from a temperature gradient, and thus from the abundant waste heat, or in turn create a temperature gradient from electricity via the Peltier effect. Most notably, this method of energy generation has been in continuous use in spacecrafts since the early 1960s. Since the better of two decades, TEs have been at the forefront of research into utilizing the waste heat in automotives to reduce the load on the alternator and thereby enhance fuel economy, and for waste heat utilization in stationary applications such as photovoltaics and wood-stoves as well. More widespread applications of the TE effect are still hindered by the comparatively low energy conversion efficiency despite recent success with a number of new materials and strategies. The TE efficiency of a material is evaluated by the dimensionless figure-of-merit, zT, which depends on the Seebeck coefficient, the electrical conductivity, and the thermal conductivity (as well as the temperature). To obtain high efficiency, the materials must exhibit high Seebeck coefficient (high thermopower), high electrical conductivity but low thermal conductivity. Unfortunately all these physical properties depend on the charge carrier concentration, and can therefore not be independently optimized. We will focus on four promising materials classes, with each having different challenges to overcome.  Nonlinear Optics A second, new research area is in nonlinear optical (NLO) materials, after our search for new TEs resulted in discovering potential NLO materials. The demand for intense light at various frequencies exceeds what current light sources on the market can deliver. This may be solved by using NLO materials that can modulate the frequencies of light via up-conversion or down-conversion. A classical example is frequency doubling (halving the wavelength) of a Nd:YAG laser from 1064 nm to 532 nm through an NLO material via the second harmonic generation (SHG). Currently NLO materials are being used in communication systems, remote sensing, tissue imaging, environmental monitoring, and minimally invasive surgeries. The optimization of IR NLO materials is complex, for one needs - in addition to a large second-order harmonic generation (SGH) response - also a high laser damage threshold (LDT), a wide IR transmission range, a wide band gap, and phase matching behavior. Specifically, a larger band gap generally results in a higher LDT but smaller SHG. On top of that, a necessary criterion for the existence of NLO behavior is a noncentrosymmetric space group. Here we will explore various main group chalcogenides with promising structural features.