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.
