Ultrasensitive Calorimetry Enabled by Suspended Semiconductor Nanostructures

在低温下的介观系统中，热传输和热平衡以与室温下的宏观系统非常不同的方式发生。 这是由于所涉及的热容小，以及与热源（环境）达到平衡的热弛豫时间很长。 在极限情况下，热传输涉及系统与环境之间的单个能量通道的交换。 在该项目的前一阶段，研究人员首次观察到这种预测热导的量化。 这就为未来的分子电子器件提供了一个重要的、严格的热导上限。 目前的项目继续研究纳米机械介观系统的热容：热隔离的悬浮半导体纳米结构，具有允许局部引入热量和局部温度测量的集成换能器。 应该能够以前所未有的灵敏度对微小样品进行热容测量。这将提供与小型化热探测器的工程相关的数据，并将提供与分子尺度和超小型电子设备的功耗限制有关的关键信息。 有了这种灵敏度，阐明涉及单个原子和分子的过程的量热实验也将首次成为可能。 这项工作将向本科生、研究生和博士后研究人员介绍纳米纤维的先进技术以及超灵敏测量的技术和原理。未来的电子学将可能基于分子尺度的器件。 任何规模的有源电子设备都需要功率来运行，并且最终必须耗散到周围环境中。 然而，在分子尺度上，控制功率耗散的过程变得非常弱;因此它可能是有问题的。 直到1999年，这个领域在很大程度上仍未被探索，当时，在之前的NSF资助的研究项目中，研究人员观察到了热导的量化-这是功率从小型系统传导到其周围环境的速率的基本限制。 在他们目前的提案中，作者建议继续在这一领域进行研究，现在转向非常小的系统的热容，即它们“储存”能量的能力。他们的方法涉及悬浮的半导体纳米结构，通过他们开发的新表面纳米加工工艺制造。 这使得能够在纳米尺度上构建复杂的探索性设备，内部组件允许对其属性进行定量和精确的测量。 在拟议的研究计划中，这些将用于更全面地了解纳米尺度结构的热传输和热容。 它们也应该被证明是非常有用的小型化热探测器的工程，并将提供有关的限制在分子尺度和超小型电子设备的功耗的关键信息。 有了这种灵敏度，阐明涉及单个原子和分子之间热流过程的实验也将首次成为可能。 这项工作将向本科生、研究生和博士后研究人员介绍纳米纤维的先进技术以及超灵敏测量的技术和原理。

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.