Picosecond Heat Transport through Molecular Layers Zhaohui Wang, Nak-Hyun Seong, Alexei S. Lagoutchev, Dana D. Dlott School of Chemical Sciences University of Illinois at Urbana-Champaign

Heat conduction through monolayer S (CH 2 ) n CH 3 h Ultrafast thermal conductance Heat flow along chain Alkanethiols: SH-(CH 2 ) n -CH 3 on Au surface Heat conduction through interface Heat Pulse To follow this process, we need: (1) Ultrafast T jump (2) Ultrafast time resolution (3) High spatial resolution

Au Glass SFG signal IR 3400 nm120 fs Visible 800 nm 1 ps Heat Pulse 800 nm 0.5 ps Alkanethiols HS-(CH2-………-CH2)-CH wavenumber (cm -1 ) s CH3 a CH3 2  CH3 Picosecond Heat Transport through Molecular Layers Cr 0.8 nm 50 nm 2 mm

Ultrafast thermal reflectance measurements delay time (ps) temperature (arb) 80% 99% artifact Ultrafast subtrate heat up: ps T jump

wavenumber (cm -1 ) with heat pulse long time a CH 3 s CH 3 a CH 2 s CH 2  CH 3 no heat pulse S (CH 2 ) n CH 3 h SFG spectra of SAM with n = 17 (C18) Optical thermometer approximately one atom thick

SFG spectra: C8 vs. C18 20 ps 10 ps 5 ps -2 ps wavenumber (cm -1 ) SFG intensity C8 -2 ps 5 ps 10 ps 20 ps C18 wavenumber (cm -1 )

Data analysis Vibrational Response Function VRF = [I(T cold )-I(t)]/I(T cold )-I(T hot ) I(T cold ) is the SFG intensity at ambient T I(T hot ) is the SFG intensity at long delay time wavenumber (cm -1 ) I(T cold ) I(T hot )

Vibrational response function C8 Vs. C C8 C18 Vibrational response function time (ps) (1) Coherent artifact at t = 0 (2) Delayed build up: t 0 (3) Exponential rise time constant: 

Vibrational response function for C8 (n=7) ln(1-VRF) time (ps) VRF t0t0

Vibrational response function for C18 (n=17) ln(1-VRF) time (ps) VRF t0t0

VRF for C8 (n=7), C12 (n=11), and C18 (n=17) time (ps) ln(1-VRF)

dependence on chain length of the delay time (t 0 ) chain length (nm) delay (ps) y = * x delay Linear Fit h(nm) = 0.127n + 0.4, ( J. Am. Chem. Soc., 111, 321 *1989) t 0 is the time for the leading edge of the heat burst launched from hot Au surface to arrive at the terminal CH 3

dependence on chain length of time constant  (1)Molecular simulation of a C16 SAM shows that orientational disorder can be created in 2 ps with infinitely fast heating (2)heat transfer dominated by interface thermal conductancey

Summary: An ultrafast thermal conductance apparatus with an optical thermometer approximately one atom thick was used to study heat conduction through SAM on gold substrate The linear dependence of t 0 on chain length indicates that the heat burst propagates ballistically along the chain with a speed of ~1 km/s Interface thermal conductance G =  hC p /  G = 720(±100) MWm -2 K -1 corresponds to molecular conductance per chain of 1.6 x W K -1 = 1 eV ns -1 K -1

Acknowledges: Jeffrey A. Carter Yee Kan Koh David G. Cahill (MRL, UIUC) Sponsor: DOE, NSF, AFOSR

Vibrational response decay curves Peak amplitude ratio vs. delay time

Decay of C18 with different heating power Intensity (heated/noheat) time (ps) 120  J 90  J wavenumber (cm -1 )

Decay of C18 with different heating power Vibrational response function time (ps) Normalized to unit