Nanoscale Heat Flow Mapped with 50 Nanosecond Precision Using New Technique

Scientists are increasingly focused on understanding heat dissipation in nanoscale materials and devices, a challenge hindered by the limited availability of direct, local measurements of thermal conductivity and heat capacity. Mairi McCauley from Humboldt-Universität zu Berlin, Joel Martis and Ondrej L Krivanek from Bruker AXS LLC, et al., have addressed this issue by developing a novel platform and framework for time-resolved nanoscale thermal transport measurements within a scanning transmission electron microscope (STEM). Their innovative system integrates a laser-excitation source with ultra-high-resolution electron energy-loss spectroscopy, achieving approximately 50ns temporal resolution and enabling the determination of local temperatures via detailed balance. This work represents a significant advance as it allows for direct probing of thermal properties at the nanoscale, validated through measurements of amorphous carbon films, and promises to accelerate progress in thermal management strategies for future technologies.

Nanoscale thermal characterisation via synchronised laser-excited electron energy-loss spectroscopy

Scientists have developed a new platform for measuring heat transport at the nanometer scale, overcoming a longstanding challenge in materials science and semiconductor research. This breakthrough centers on a laser-excitation system seamlessly integrated into a scanning transmission electron microscope (STEM), enabling direct, local measurements of thermal conductivity and heat capacity with unprecedented precision.
The research introduces a method for synchronizing pulsed laser excitation with an externally gated direct electron detector, achieving temporal resolution of approximately 50 nanoseconds at energy resolutions below 10 milli-electron volts. By employing ultra-high-resolution electron energy-loss spectroscopy (EELS), researchers can now probe thermal properties of materials with both high spatial and temporal accuracy.

A key innovation lies in the system’s design, utilizing a fiber-coupled laser introduced via a modified aperture mechanism. This approach avoids placing optical components within the critical polepiece gap of the STEM, maintaining compatibility with diverse sample holders and allowing for large tilt angles essential for advanced experiments.

Local temperatures are determined through the principle of detailed balance, analyzing the ratio of energy-loss and energy-gain phonon excitations. Thermal transport parameters are then extracted by fitting a forward-time central-space heat diffusion model, which accounts for radiative losses and provides a comprehensive analysis of heat flow.

Demonstrating the capabilities of this new framework, the team successfully measured the thermal conductivity of amorphous carbon films, obtaining a value of 1.24W m·K and a heat capacity of 821 J kg·K, consistent with established literature values. This validation confirms the accuracy and reliability of the technique. The presented instrumentation and analysis methodology provide a versatile platform for investigating nanoscale thermal transport in a wide range of materials and devices, opening avenues for optimizing thermal management in future technologies.

Laser-excitation STEM setup for nanoscale thermal measurements

A 72-qubit superconducting processor forms the foundation of this work, integrating a laser-excitation system directly into a scanning transmission electron microscope (STEM) for nanoscale thermal transport measurements. A modified aperture mechanism facilitates the introduction of a fiber-coupled laser, enabling flexible holder geometries and large tilt angles without obstructing the optical path.

This innovative design utilises a hollow rod inserted in place of a standard aperture holder, allowing light to reach the sample and subsequently focusing the beam onto the specimen with a lens positioned 25mm upstream. The optical system employs a parallel light beam, approximately 3mm in diameter, generated by a pigtailed laser diode coupled to a single-mode fiber and collimator.

A flat mirror redirects the focused beam onto the sample at an incidence angle of roughly 20° relative to the sample plane, while the optic rod’s nm-level precision translation capabilities allow for precise positioning of the illuminated spot relative to the electron beam. Approximately 70% of the initial laser output power reaches the sample, accounting for combined losses within the coupler, fiber, and optical components.

Synchronized laser excitation and electron detection are achieved using a Rigol DG5252 Pro signal generator to power the laser and trigger the externally gated mode of a Dectris ELA direct detector, providing a temporal resolution of approximately 50ns at below 10meV energy resolution. This time resolution was verified by characterizing the detector response time using a pre-characterized in-situ biased capacitor, aligning with data from a corresponding x-ray detector, the Dectris Eiger.

Multiple detector frames are acquired, aligned, and integrated to enhance the signal-to-noise ratio without compromising energy resolution, ensuring sufficient exposure time for accurate data collection. Local temperatures are determined via the principle of detailed balance, and thermal conductivity and heat capacity are extracted by fitting a forward-time central-space heat diffusion model incorporating radiative losses.

Nanoscale thermal properties of amorphous carbon films determined by laser-excited electron microscopy

Thermal conductivity of 1.24mm and a heat capacity of 821 were obtained for amorphous carbon films using a newly developed laser-excitation system integrated into a scanning transmission electron microscope. This system facilitates nanoscale thermal transport measurements with ultra-high-resolution electron energy-loss spectroscopy.

The integrated setup introduces a fiber-coupled laser via a modified aperture mechanism, enabling flexible holder geometries and large tilt angles without compromising the imaging system. Synchronization of pulsed laser excitation with an externally gated direct electron detector achieves temporal resolution of approximately 50ns at below 10 meV energy resolution.

Local temperatures are determined by applying the principle of detailed balance, and thermal transport parameters are extracted through fitting a forward-time central-space heat diffusion model that incorporates radiative losses. The laser injection system allows for nm-level precision in positioning the illuminated spot on the sample, aligning it with the electron beam traverse.

Experiments utilized a red laser with a 637nm wavelength and a peak power of 80mW, controlled by a ThorLabs CLD1010LP laser controller. The combined optical losses result in approximately 70% of the laser output power impinging on the sample. A silicon photodetector was used to characterize the laser pulses, with power scaling applied to account for detector limitations.

Analysis of electron energy-loss and energy-gain spectra, employing a Pearson VII function for zero-loss peak subtraction, ensures accurate temperature determination via the principle of detailed balance. Measurements conducted on a 16nm thick Quantifoil R1.2/1.3 film and an 8nm continuous amorphous carbon film were used to validate the methodology.

A Python script was developed to simulate heat conduction within the continuous amorphous carbon sample, incorporating both heat conduction and radiative losses to improve the agreement between experimental data and modeling. The research demonstrates a framework for time-resolved nanoscale measurements of thermal transport in materials and devices.

Nanoscale thermal conductivity mapping using fibre-coupled laser-stimulated electron energy-loss spectroscopy

Scientists have developed a new experimental framework for measuring thermal transport at the nanoscale within a scanning transmission electron microscope. The system integrates laser excitation with vibrational electron energy-loss spectroscopy to determine local temperatures and extract thermal conductivity and heat capacity parameters.

A key innovation is the fibre-coupled laser injection system, which allows for flexible sample geometries and tilt angles without requiring optical components within the microscope’s polepiece gap. Measurements performed on amorphous carbon films yielded thermal conductivity of 1.24 and heat capacity of 821, aligning with previously published values and confirming the quantitative accuracy of the technique.

This approach enables time-resolved measurements with approximately 50 nanosecond temporal resolution and sub-10 meV energy resolution, facilitating the study of nanoscale temperature gradients without complex microfabrication. The authors acknowledge that the method currently functions optimally with samples transparent to the laser wavelength, potentially limiting its application to certain materials.

Future research will focus on enhancing spatial localization of heating through the use of resonant absorbers or nanostructures. Combining the system with momentum-resolved vibrational spectroscopy promises to reveal the behaviour of individual phonon modes across defects and interfaces. This instrumentation and analytical method establishes a versatile platform for investigating thermal transport in nanoscale materials and devices, with potential applications in areas such as densely integrated electronics, plasmonics, and quantum materials.