IMR: Acquisition and Development of an Ultrafast Thermal Conductance Apparatus for Materials Research and Student Training

美国国家科学基金会被要求资助购买和开发一台超快热导装置，该装置由一台高重复频率飞秒激光器和一台拉曼光谱仪组成，将用于研究热在时间分辨率和埃空间分辨率的界面上通过分子的流动。这种类型的测量技术包括将一个分子或一层分子放置在金属表面或两个金属接触之间以测量热流。这些方法没有揭示导致热流的详细机制。在我们的方法中，组装在金属表面或具有非常薄的氧化物涂层的金属表面的分子受到飞秒激光脉冲的1ps温度跳跃的影响。当热从金属表面流入分子时，它到达不同位置，以及携带热的单个振动的瞬时布居，将用反斯托克斯拉曼(ASR)光谱来探测。通过使用表面增强拉曼(SERS)衬底，尽管拉曼截面很小，ASR探测仍是可能的。超短脉冲对SERS衬底上界面分子的低激光损伤阈值，以及产生大的T-跳跃的需要，需要一种直到最近才现成的飞秒激光。闪热ASR概念可以用来研究各种材料结构，其结构可以在原子的基础上系统地改变。这将创建从第一原理设计分子所需的知识库，这些第一原理表现出有效设计分子电子器件和分子纳米机械所需的增强或抑制热流特性。所有的机械和电气设备的运行都涉及到热的运动，并且对热传导的原理和工程有很好的理解。从事纳米技术的科学家试图制造极其微小的设备和电路，其中一些甚至只有一个分子那么小。然而，我们对热流的理解目前还不能扩展到这些微小设备的操作所涉及的微小长度和短时间尺度。通过在单分子水平上了解和控制热，我们可以更好地设计纳米机械，我们可以开发出新的设备，如热敏二极管，让热只向一个方向流动。国家科学基金会提供的资金将用于购买和开发测量分子中热量流动的仪器。放置了一层分子的金属表面会突然受到持续时间仅为十万亿分之一秒(10-13秒)的激光脉冲的加热。当热量从熔化的金属流入分子时，分子就会迅速运动。第二个激光脉冲将被用来详细探测这种运动，使用一种被称为“反斯托克斯拉曼散射”的效应，即光脉冲中的某些频率被分子运动选择性地放大。这些信号极其微弱，直到最近才能被检测到。纳米技术和先进的激光技术相结合，使这些测量成为可能。纳米技术允许我们对金属表面进行纹理处理，从而增强了某些光的频率，就像音叉一样。这种对分子特征运动的测量将在任何时刻告诉我们热的位置和移动的速度。我们将第一次能够实时观察分子装置和分子导线中的热量流动。对不同大小和形状的分子的系统研究将创建设计高效和多功能分子机械所需的知识库。

NSF is requested to fund the acquisition and development of an ultrafast thermal conductance apparatus, consisting of a high repetition rate femtosecond laser and a Raman spectrometer, that will be used to study the flow of heat through molecules at interfaces with picosecond time resolution and angstrom space resolution. The state of the art of measurements of this type involves placing a molecule or a layer of molecules on a metal surface or between two metal contacts to measure the heat flow. Such methods do not reveal the detailed mechanisms responsible for the heat flow. In our approach, molecules assembled on metal surface, or a metal surface with a very thin oxide coating, are subjected to a 1 ps temperature jump by a femtosecond laser pulse. As heat flows into the molecules from the metal surface, its arrival at different locations, as well as the instantaneous populations of the individual vibrations that carry the heat, will be probed with anti-Stokes Raman (ASR) spectroscopy. ASR probing is made possible despite the small Raman cross-section by the use of surface-enhanced Raman (SERS) substrates. The low laser damage threshold with ultrashort pulses for interfacial molecules on SERS substrates, and the need to produce a large T-jump, necessitates a femtosecond laser that has not been readily available until recently. The flash-heating ASR concept can be used to study a vast range of material architectures whose structures can be systematically varied on an atom-by-atom basis. This will create the knowledge base required to design molecules from first principles that exhibit enhanced or inhibited heat flow characteristics needed to effectively engineer molecular electronic devices and molecular nanomachinery. The operation of all mechanical and electrical devices involves the movement of heat, and the principles and engineering of heat conduction are understood very well. Scientists involved in nanotechnology seek to produce extremely tiny devices and electric circuits, some as small as a single molecule. However our understanding of heat flow does not presently extend to the tiny length and short time scales involved in the operation of such tiny devices. By understanding and controlling heat at the level of single molecules we can better engineer nanomachinery and we can develop new devices such as thermal diodes that let heat flow in one direction only. Funds provided by the National Science Foundation will be used to purchase and develop instrumentation to measure the flow of heat through molecules. A metal surface on which a layer of molecules has been placed will be suddenly heated by a laser pulse lasting only one ten-trillionth (10-13) of a second. As heat flows into the molecules from the hot metal, the molecules will be set into rapid motion. A second laser pulse will be used to probe this motion in detail using an effect called "anti-Stokes Raman scattering" where certain frequencies within the optical pulse are selectively amplified by the molecular motion. These signals are extremely weak and could not have been detected until recently. A combination of nanotechnology, which allows us to texture the metal surface so it enhances certain frequencies of light like a tuning fork, and advanced laser engineering makes these measurements possible. This measurement of the characteristic motions of the molecules will tell us at any instant where the heat is located and how fast it is moving. For the first time we will be able to watch the flow of heat through molecular devices and molecular wires in real time. Systematic studies on molecules of different sizes and shapes will create the knowledge base needed to design efficient and versatile molecular machinery.