Ultrasensitive Calorimetry Enabled by Suspended Semiconductor Nanostructures

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In mesoscopic systems at low temperatures, heat transport and thermal equilibration occur in a very different manner from macroscopic systems at room temperature. This is due to the small heat capacities involved, and very long thermal relaxation times to reach equilibrium with a heat reservoir, the environment. At the ultimate limit, thermal transport involves exchange of a single energy channel between a system and the environment. During the preceding phase of this project, investigators observed, for the first time, this predicted quantization of thermal conductance. This places an important, hard upper bound on the thermal conductance available through future molecular electronic devices. The current project continues the investigation of heat capacities of nanomachined mesoscopic systems: Suspended semiconductor nanostructures that are thermally-isolated and have integral transducers that permit the localized introduction of heat and local temperature measurements. Heat capacity measurements on minute samples with unprecedented sensitivity should be possible. This should provide data relevant to the engineering of miniaturized thermal detectors, and will provide crucial information relating to limits of power dissipation in molecular-scale and ultrasmall electronic devices. With this level of sensitivity, calorimetry experiments that elucidate processes involving individual atoms and molecules should also become possible for the first time. The effort will introduce undergraduates, graduate students, and postdoctoral researchers to advanced techniques in nanofabrication and in techniques and principles of ultrasensitive measurements.%%%Future electronics will likely be based upon molecular scale devices. Active electronic devices, at any scale, require power to operate and this must ultimately be dissipated to their surroundings. However at the molecular scale the processes that govern power dissipation become very weak; hence it can be problematic. This domain had remained largely unexplored until 1999, when, in a previous NSF-funded research program, investigators observed the quantization of thermal conductance -- a fundamental limit to the rate at which power can be conducted from a small system to its surroundings. In their current proposal, the authors propose to continue with research in this realm, turning now to the heat capacity of very small systems, i.e. their ability to "store" energy. Their approach involves suspended semiconductor nanostructures, fabricated by new surface nanomachining processes they have developed. These enable the construction of complex exploratory devices at the nanometer-scale, with internal components allowing quantitative and precise measurements on their properties to be carried out. In the proposed research program these will be utilized to obtain a more complete understanding of heat transport and the heat capacity of nanometer-scale structures. They should also prove to be extremely useful for the engineering of miniaturized thermal detectors, and will provide crucial information relating to limits of power dissipation in molecular-scale and ultrasmall electronic devices. With this level of sensitivity, experiments that elucidate processes involving heat flow between individual atoms and molecules should also become possible for the first time. The effort will introduce undergraduates, graduate students, and postdoctoral researchers to advanced techniques in nanofabrication and in techniques and principles of ultrasensitive measurements.