Improvement of method for preparing electron microscope samples in high-pressure research using diamond anvil cell

The sealing limitations of the diamond anvil cell (DAC) prevent electron beams from penetrating the device during high-pressure experiments, restricting in-situ high-pressure characterization. To investigate the nanoscale microstructural changes of materials under pressure, only indirect analysis through comparison of post-decompression samples at different pressures using multiple transmission electron microscope (TEM) characterizations is feasible. This imposes stringent requirements on the thickness and reproducibility of TEM samples prepared from decompressed DAC specimens. Notably, DAC samples are extremely scarce and minute in size, rendering collection and manipulation highly challenging. Moreover, after DAC decompression, the phase state, content, and dimensions of substances in the sample cavity vary significantly depending on the sample itself and the pressure-transmitting medium. Conventional methods of preparing powder and bulk samples for TEM analysis, as well as commercial equipment for preparing TEM samples, are generally inapplicable, resulting in limited research progress on the electron microscopic characterization of high-pressure decompressed samples. Thus, improving TEM sample preparation techniques is crucial for enhancing the quality of high-pressure samples and the reliability of microstructural characterization.

In this study, titanium dioxide compressed at 10–30 GPa by DAC and then decompressed to ambient pressure was used as the sample for analysis. Decompressed samples obtained from high-pressure experiments employing methanol–ethanol as the pressure-transmitting medium were prepared for TEM analysis by applying the rinse-dispersion method. Decompressed samples obtained from high-pressure experiments employing the sample itself as the pressure-transmitting medium were prepared for TEM analysis using the puncture-dispersion and focused ion beam (FIB) methods. The recovery rate of the decompressed samples was comprehensively analyzed using scanning electron microscopy (SEM) and TEM to determine the quality of the TEM specimens and their hydrostatic pressure performance. The advantages and disadvantages of these methods were summarized, and an innovative approach was proposed in which ultraviolet (UV)-curable resin is used as both the pressure-transmitting medium and embedding agent. UV-curable resin is a colorless liquid at room temperature, capable of providing excellent hydrostatic pressure in high-pressure experiments. After decompression, it can be cured by UV light, enabling complete sample recovery and integration with the gasket, facilitating further sample preparation using existing commercial equipment such as ion thinning machines and dual-beam microscopes.

The experimental results demonstrate that 1) uncured UV resin exhibits no distinct characteristic peaks in the high-pressure Raman spectra and delivers superior hydrostatic pressure, making it suitable as a pressure-transmitting medium for DAC in high-pressure experiments. 2) UV light of a specific wavelength can penetrate the diamond windows of the DAC to cure the UV resin in the sample cavity. 3) The cured UV resin integrates the sample with the gasket, improving the stability of the decompressed samples and the ease of preparing the TEM samples, thereby enhancing the compatibility of commercial TEM sample preparation equipment with high-pressure decompressed specimens. 4) TEM samples prepared from decompressed and cured UV resin can be effectively utilized for nanoscale microstructural characterization, including TEM morphology observation and high-resolution TEM (HRTEM) analysis.

These studies indicate that the use of UV-curable resin as a pressure-transmitting medium enables the simultaneous achievement of efficient recovery of decompressed samples and excellent hydrostatic pressure in high-pressure experiments. This method can improve the quality and stability of TEM samples derived from high-pressure decompressed materials, thereby providing technical support for advancing research on pressure-induced nanoscale microstructural changes.