Performing in situ closed gas cell transmission electron microscopy (TEM)—based on silicon nitride (SiNx) membranes as electron transparent encapsulation material—has many technological advantages over the differential pumping environmental TEM (ETEM), such as realistic appliable pressure and large collection angle in scanning TEM (STEM) mode [1]. However, additional electron scattering from the top and bottom membranes with a general total thickness of> 80 nm will impose many adverse effects on the post electron optics system leading to significant degeneration of signal quality, such as loss of spatiotemporal resolution, diffused electron diffraction, and convoluted electron energy loss spectra (EELS). To implement many other state-of-the-art STEM techniques such as quantitative EELS analysis to resolve electronic structure and 4D-STEM, researchers have to reduce intrinsic electron scattering, which mostly stem from the thick SiNx windows (typically 30–50 nm). For example, the log-ratio of scattered electron over zero-loss electron (ln (It/I0)= t/λi) already exceeds one for two-50 nm SiNx encapsulation without specimen. However, adopting a low-scattering membrane by only reducing the thickness can be risky since the mechanical robustness will be compromised. The pressure gradient across the membrane during the operation process can be as high as 1 atm, which can rupture the thin membrane. Thus, novel design strategies for fabrication of stable ultrathin SiNx membrane are needed. Here, we propose a robust and scalable backing support strategy to enable the thinnest possible (< 10 nm) SiNx gas encapsulation material (Fig. 1)[2]. For the mechanical performance, stress intensity on this ultrathin membrane is reduced to 30% at 1 atm pressure gradient compared to the conventional 50 nm SiNx membrane. However, it can still withstand up to 6 atm pressure with 50% less bulging. Unlike graphene-based encapsulations [3], stability under the electron beam is comparable to a 50 nm SiNx membrane, which is sufficient for most high-resolution S/TEM applications on non-electron sensitive materials. The contrast of 10 nm gold nanoparticle in 1 atm Ar gas is increased by 70% and the accessible information limit is increased by 130% compared to the conventional encapsulation. More importantly, the t/λi is reduced from 1.0 to 0.3 using a 1 atm gas cell, implying that acquisition of the fine electron structure of materials using EELS is now possible within the gas cell. Using this technique, spatiotemporal detection of gas species, which are unachievable with an integrated residual gas analyser (RGA) for monitoring, is thus enabled. For example, Fig. 2 is spectrum data excerpted from one pixel in the spectrum imaging (SI) map at the corner of a Au nanocube, which reflects the surface plasmon at 2.21–2.25 eV (λ= 550–560 nm) and Ar-M2, 3 core-loss at 11.75, 14.25 eV. Both technical details of the ultrathin SiNx membrane and the advantages of reduced thickness for in situ analytical measurements will be covered [4].