Spatially Resolved Vibrational Energy Transfer in Molecular Monolayers

Abstract

We have shown that it is possible to input heat to one location of a molecule and simultaneously measure its arrival in real time at two other locations, using an ultrafast flash-thermal conductance technique. A femtosecond laser pulse heats an Au layer to approximately 800 degrees C, while vibrational sum-frequency generation spectroscopy (SFG) monitors heat flow into self-assembled monolayers (SAMs) of organic thiolates. Heat flow into the SAM creates thermally induced disorder, which decreases the coherent SFG signal from the CH-stretching transitions. Recent improvements in the technique are described, including the use of nonresonant background-suppressed SFG. The improved apparatus was characterized using alkanethiolate and benzenethiolate SAMs. In the asymmetric 2-methyl benzenethiolate SAM, SFG can simultaneously monitor CH-stretching transitions of both phenyl and methyl groups. The phenyl response to flash-heating occurs at least as fast as the 1 ps time for the Au surface to heat. The methyl response has a faster portion similar to the phenyl response and a slower portion characterized by an 8 ps time constant. The faster portions are attributed to disordering of the methyl-substituted phenyl rings due to thermal excitation of the Au-S adbonds. The slower portion, seen only in the methyl SFG signal, is attributed to heat flow from the metal surface into the phenyl rings and then to the methyl groups.

Electron transfer across a thermal gradient

“…An important driving factor in this growing interest is the development of experimental capabilities that greatly improve on the ability to gauge temperatures (and "effective" temperatures in nonequilibrium systems) with high spatial and thermal resolutions (29-43) and to infer from such measurement the underlying heat transport processes. In particular, vibrational energy transport/ heat conduction in molecular layers and junctions has recently been characterized using different probes (6,19,(44)(45)(46)(47)(48)(49)(50)(51)(52).The interplay between charge and energy (electronic and nuclear) transport (53-60) is of particular interest as it pertains to the performance of energy-conversion devices, such as thermoelectric, photovoltaic, and electromechanical devices. In particular, the thermoelectric response of molecular junctions, mostly focusing on the junction linear response as reflected by its Seebeck coefficient, has been recently observed (61-65) and theoretically analyzed (2,20,64,(66)(67)(68)(69)(70)(71)(72)(73)(74)(75)(76)(77).…”

Probing of molecular adsorbates on Au surfaces with large-amplitude temperature jumps

“…Figure 6(a) shows nonresonance-suppressed s (NO 2 ) spectra as a function of delay. We analyzed those spectra using the method of moments, where the first three moments M (0) (t), M (1) (t) and M (2) …”

“…9(a), creating disorder in the nitro group ensemble. The SFG signal is proportional to the square of a second-order susceptibility v (2) , which itself is the ensemble average of individual molecular hyperpolarizabilities. Disorder reduces this average, thereby reducing s (NO 2 ) SFG signals.…”

“…5 We showed it was possible to use SFG to probe hot electron-and phonon-excited vibrations at multiple locations on the adsorbate molecules, 2 and it was shown that the hot electrons excited atomic groups most effectively if they were no farther than 4-5 carbon atoms away from the surface. 5 Flash-heating of Au surfaces by short pulses of light has been studied extensively over the past 25 years using transient reflectance probes.…”

“…1,5 Some of the measurements here were made using 800 nm flash-heating pulses to produce DT ¼ 35 K as used in previous works. [1][2][3][4][5] In addition, we used 400 nm (blue) pulses, which are more strongly absorbed by Au, to make the same DT ¼ 35 K and also much larger-amplitude T-jumps where DT ¼ 175 K.…”