Probing Thermal Transport on the Nanoscale

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Probing nanoscale thermal processes and carrier interactions is generally a difficult tasks due to contact resistance and substrate interactions. In the Nanoscale Energy Transport (NET) laboratory at University of Virginia, we have developed the following series of optical-based pump-probe ther-mometry techniques to probe ultrafast and nanoscale carrier interactions in nanosystems. The experiments are supplemented with molecular dynam-ics (MD) simulations that target the atomistic origin of the observed thermal transport mechanisms. Our experimental capabilities include:

1. Time domain/frequency domain thermoreflectance (TDTR/FDTR) – Oscillator-based set up that modulates a train of fs laser pulses at high fre-quencies (MHz) giving 10’s – 100’s of nm spatial resolution.

2. Ultrashort pulsed transient thermoreflectance (UPTTR) – Amplifier-based set up that monitors the transient decay of a single pulse (10 – 200 fs pulse width) over picoseconds to nanoseconds. Our capability allows us to span photon energies from the near UV to near IR and pulse pow-ers from millijoules to nanojoules

3.  Phase sensitive thermoreflectance (PSTR) – Continuous wave based set-up that monitors the temperature change on the surface of a sample as a function of frequency. The low frequencies (kHz) used to modulate the pump allow thermomemetry on the microscale, bridging the gap between the nanoscale measurements.

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Figure 1: Typical pump-probe experimental setup. This same basic geometry is used for our TDTR/FDTR, UPTTR, and PSTR experiments. In the PSTR experiments, the delay line is not present.  Figure 2: Sample TDTR data as a function of pump-probe delay time used to extract electronic scattering processes such as electron-electron relaxation and electron-phonon thermalization times of a thin nickel film.  Figure 3: Sample TDTR data as a function of pump-probe delay time used to extract the thermal conductivity and heat capacity of a thin silicon nitride film. Figure 4: MD results showing the effect of an interstitial layer of varying mass on the overall conductance across an effective interface. An increase of ~50% is obtained through the addition of an interstitial layer.   Figure 5a & 5b: Typical geometry used for molecular dynamics simulations of interfacial thermal transfer.

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