Platform and Framework for Time-Resolved Nanoscale Thermal Transport Measurements in STEM

TL;DR

This work addresses the challenge of measuring nanoscale thermal transport by coupling laser heating to ultra-high-resolution vibrational EELS thermometry in STEM, enabling time-resolved local temperature measurements. A fiber-coupled laser injected through a modified aperture provides optical excitation without obstructing the polepiece gap, while an externally gated detector achieves ~50 ns time resolution. Local temperatures are inferred from the principle of detailed balance, and thermal parameters are extracted by fitting a forward-time central-space diffusion model that includes radiative losses, validated by quantitative measurements on amorphous carbon (k ≈ 1.24 W/m·K, Cp ≈ 821 J/kg·K). The approach is broadly applicable and compatible with in-situ biasing, tilt-series, and momentum-resolved vibrational studies, offering a general platform for time-resolved nanoscale thermal transport in materials and devices.

Abstract

Understanding heat transport at the nanometer scale is critical for semiconductor devices, quantum materials, and thermal management of nanostructures, yet direct local measurements of thermal conductivity and heat capacity remain scarce. We developed a laser-excitation system integrated into a scanning transmission electron microscope (STEM) for nanoscale thermal transport measurements using ultra-high-resolution electron energy-loss spectroscopy (EELS). A fiber-coupled laser is introduced via a modified aperture mechanism, enabling flexible holder geometries and large tilt angles without optical elements in the polepiece gap. Synchronization of pulsed laser excitation with an externally gated direct electron detector provides temporal resolution about 50 ns at <10 meV energy resolution. Local temperatures are determined via the principle of detailed balance, and thermal transport parameters are extracted by fitting a forward-time central-space heat diffusion model including radiative losses. For amorphous carbon films, we obtain a thermal conductivity of 1.24 $\frac{W}{m\cdot K}$ and a heat capacity of 821 $\frac{J}{kg\cdot K}$, consistent with literature. This framework enables time-resolved nanoscale measurements of thermal transport in materials and devices.

Figures (4)

• Figure 1: Experimental setup. a) Sketch of the laser injection system with a modified aperture mechanism attached to a port of the objective lens. b) Photograph of the laser controller, signal generator and oscilloscope setup (left) and of the microscope column with the laser fiber leading to the light insertion mechanism (right). c) Schematic of the microscope and time-resolved triggering system where a function generator is used to send synchronized time-delayed pulses to the laser and triggers to the detector. d) Screenshot of the laser plugin (motor and laser power control) integrated into the microscope control software (Nion Swift) and the homemade graphical user interface for the arbitrary wave generator (AWG).
• Figure 2: Temperature analysis from EEGS/EELS measurements. a) EEGS/EELS spectrum with fitted ZLP and fit window indications. b) Plot of the natural logarithm of the loss/gain ratio against energy across the gain/loss intensity window indicated in a, showing a linear trend and corresponding fit of T = 1630 K. c) Measured temperature of a carbon film with continuous wave laser excitation of increasing optical power, where the pink data point is the assumed temperature with no laser stimulation (room temperature). The temperature increases linearly with increasing power (blue data points) from no optical power (pink) but deviates at high laser powers (purple) as the carbon structure is modified (diffraction patterns shown as insets).
• Figure 3: Temperature and laser power distributions. a) Plot of temperature against the relative laser position along both laser motor axes (X and Y), fitted with Gaussian functions. The position axis is calibrated relative to the center of the profile as determined afterwards and shown in b. b) 2D Gaussian fit of the temperature, where solid lines indicate the measured axes in panel a and dashed lines indicate the center of the spot. The FWHM of the long axis (vertical) was 57.2 $\mu$m and 38.6 $\mu$m for the short axis (horizontal). c) Defocused ronchigram images of the Quantifoil grid before and after evaporation due to the laser, with no tilt and a 17$^\circ$ tilt angle of the sample stage. d) Profile of the laser beam power at the sample plane considering losses for the calculated spot size from simulations with a FWHM of 28.5 $\mu$m along the long axis (vertical) and 20.2 $\mu$m along the short axis (horizontal). e) Temperature gradients over the center of the laser spot (dashed lines in b) along both axes.
• Figure 4: Time resolved temperature measurements of amorphous carbon and comparison to simulations. a) Specimen temperature measurements (blue) during the 50 $\mu$s period between successive laser pulses with the simulated temperature evolution in red. b) Cumulative energy supplied by the laser pulse and specimen temperature during 5 $\mu$s time window of laser stimulation, where the red trace is the temperature simulation as in a. The inset shows the shape of the laser pulse scaled to the laser output power across the same time range.