58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

This project’s goal is to experimentally probe the coupling of electrons to vibrations and photons in current-carrying hydrocarbon-based molecular junctions using measurements of Raman Spectra and photoconductance.  Molecular junctions are formed with an organic molecule is bound between two inorganic electrodes.  In order to understand how vibrations couple to electrons and photons, we have initially focused on understanding how vibrations within the molecule couple to phonons and electrons within the inorganic electrodes.  To do so, we have formed self assembled monolayer (SAM) junctions and measured their thermal conductance using a thermoreflectance technique, in parallel to making first principles predictions of the thermal conductance with molecular dynamics.

In semi-conductors and insulators, phonons are the dominant heat carriers, while in metals electrons carry the heat.  Prototypical molecular junctions and SAM junctions are based on insulating molecules that hence carry heat by vibrations.  The scientific question that our research initially addresses is how do phonons and electrons in the inorganic contacts of a SAM junction couple and transmit through the vibrational levels of the SAM.   Despite a plethora of work on electronic properties in SAMs, there have been limited developments on thermal transport in SAM junctions.   Interfacial thermal conductance (G) defines the temperature difference ΔT for a specified heat flux q” across an interface as G=ΔT/q”.  Experimentally a handful of measurements have considered the G of a SAM junction with asymmetric electrodes including GaAs-alkanethiol-Au [1] and Quartz-alkanesilane-Au [2].  Computational work has instead focused on Au-alkanthiol-Au junctions and clear agreement with experiments has not been shown [3, 4].

In collaboration with Alan McGaughey (Professor of Mechanical Engineering at CMU), I am coadvising a PhD. student in MechE named Shubhaditya Majumdar, to study jointly make experimental measurements and computational predictions of SAM thermal conductance. Other students have also benefited from the ACS-PRF-DNI funds including Keith Regner, Zonghui Su, and Wee-Liat Ong, whom have made minor contributions to the experimental setup.  The award has given me a unique ability to measure the thermal conductance of SAMs. Our focus has been to create junctions that could be readily simulated and had novelty relative to what has already been measured.  We decided that metal-alkanethiol-metal junctions made the most sense since they can be modeled using well known molecular dynamics potentials and their electronic properties have been extensively studied.

 

Experimental Measurements of SAM Thermal Conductance

            We have used the frequency domain thermoreflectance technique to measure metal-SAM-metal junctions formed using a unique transfer printing technique.  Initially we templated metal (Au, Pt, Pd) films to create very flat surfaces to grow the SAMs on top of.  To do so, we sputtered 500 nm of Au, Pt, or Pd onto a Si wafer,  then epoxied a tab wafer on top of the metal.   After the epoxy hardened we pulled off the tab wafer, which picked up the metal, such that the exposed surface had sub-nm roughness as templated by the Si wafer.  We formed decanedithiol [SH(CH₂)₁₀SH] and dodecanemonothiol [CH₃(CH₂)₁₁SH] SAMs on this layer from both ethanol and toluene solvents based on literature preps. To probe heat transport through a SAM junction it is necessary to apply a top contact.  Electronic measurements have shown that forming a top contact by physical vapor deposition destroys the SAM.  We have used the transfer printing process, pioneered by John Rogers lab and used for similar measruments by Losego et al. to transfer a 100nm Au top contact to the SAM [2]. The final sample is (100nm metal top contact)-(decanethiol SAM)-(500nm metal bottom contact)-(Epoxy). 

The SAM junction was measured using an optical technique called frequency domain thermoreflectance.  The FDTR technique employs two continuous wave lasers to heat the sample and measure its thermal response to identify the unknown SAM thermal conductance.  A 488 nm laser (“the pump”) is intensity-modulated by an electro-optic modulator and periodically heats the gold surface while the 532 nm laser (“the probe”) continuously monitors the resultant thermal response through the thermoreflectance of the gold layer.  The pump is modulated sinusoidally from 100 kHz to 10 MHz.  The resulting probe signal is measured using a radio frequency lock-in amplifier, producing a set of frequency-dependent phase data, related to the thermal properties of the sample.  These data are fit by a thermal conduction model to determine the unknown SAM thermal conductance [5].

Our results to date are shown in the TOC image and indicate that all of the metal-SAM-metal junctions have G of similar magnitude to SAM junctions measured before which have G = 25-60 MW/m²-K [1, 2].  Three preliminary conclusions are: 

(1) G of SAM junctions is strongly influenced by the head group.  The decanedithiol, which makes covalent thiol bonds with both electrodes, has a much higher overall G than the dodecanemonothiol, which makes a Van der Waals bond with the electrode on one side.  This result agrees with Losego et al. [2] 

(2) G of an asymmetric Au-SAM-Pt and Au-SAM-Pd junctions is lower than the symmetric Au-SAM-Au junction.  This effect is more pronounced for decanedithiol.  This may result for two reasons: (i) As the vibrational mismatch between Au and the second contact increases, vibrations have more trouble transmitting. Based on Debye temperatures, Au is more mismatched with Pd than Pt, consistent with the trend in G.  (ii) The thiol-metal have variable strength with different metals.

(3) G of the symmetric Au-SAM-Au is still much lower than molecular dynamics based predictions, which range from 250-400 MW/m²-K [3, 4]. 

References.

[1]       R. Y. Wang, R. A. Segalman, and A. Majumdar, Applied Physics Letters 89 (2006).

[2]       M. D. Losego, M. E. Grady, N. R. Sottos, D. G. Cahill, and P. V. Braun, Nat Mater 11 (2012).

[3]       J. C. Duda, C. B. Saltonstall, P. M. Norris, and P. E. Hopkins, J Chem Phys 134 (2011).

[4]       T. F. Luo, and J. R. Lloyd, International Journal of Heat and Mass Transfer 53 (2010).

[5]       D. G. Cahill, Review of Scientific Instruments 75 (2004).