We have been developing an original technique, STEM-based Thermal Analytical Microscopy (STAM), for evaluating thermal transport properties at the nanoscale within a scanning transmission electron microscope. In conventional STAM, a focused electron beam is used as a localized heat source, while the resulting temperature change is detected using a specially developed nanoscale thermocouple placed in contact with the specimen. By scanning the electron-beam irradiation position and recording the corresponding thermocouple response, the spatial distribution of heat transport under steady-state conditions can be visualized as a STAM image.

For example, in heat-dissipation composites consisting of highly thermally conductive fillers embedded in a low-thermal-conductivity polymer matrix, STAM enables the analysis of heat-transport pathways through the fillers, polymer, and their interfaces in direct correlation with electron microscopy images. This approach therefore allows the effects of microstructure, composition, grain boundaries, and heterointerfaces on local thermal transport to be evaluated within the same specimen and observation area.

A limitation of conventional STAM is that the amount of heat deposited by electron-beam irradiation depends on factors such as specimen thickness, composition, density, and accelerating voltage. Quantitative analysis of thermal properties therefore requires careful consideration of the deposited heat input. To overcome this limitation, we are currently developing pulsed STAM, or p-STAM, in which an Electrostatic Dose Modulator equipped with an electrostatic shutter is installed in a transmission electron microscope to generate a pulsed electron beam at a controlled frequency. This enables periodic thermal waves to be produced within a TEM specimen.

In p-STAM, minute temperature variations generated by the pulsed electron beam are detected with a nanoscale thermocouple, and the amplitude and phase of the thermal waves are measured with high sensitivity using a lock-in amplifier. In particular, because the phase response is relatively insensitive to the absolute magnitude of the heat input, the thermal diffusivity of TEM specimens can be quantitatively evaluated while reducing the influence of uncertainties in electron-beam heating.

The measurement principle has been validated using model materials such as sapphire, and the technique is being applied to ceramics, thermoelectric materials, metals, and composite materials. Current studies focus on the evaluation of thermal diffusivity, crystallographic anisotropy in thermal transport, changes in heat conduction across grain boundaries, and thermal resistance at heterointerfaces. Another important feature of this method is its compatibility with HAADF-STEM imaging, electron diffraction, and elemental analysis, enabling direct correlation between local structure, composition, and thermal transport properties.

By integrating high-spatial-resolution structural and compositional characterization with quantitative thermal-property measurements based on thermal waves, this research aims to clarify nanoscale heat-transport mechanisms in semiconductors, thermoelectric materials, ceramics, and heat-dissipation composites, and ultimately to provide design guidelines for advanced thermal-management materials and devices.