© 2018 Elsevier B.V. Triacetylcellulose (TAC) was subjected to surface modification by low-energy nitrogen ion irradiation for use as the diaphragm of an environmental cell transmission electron microscope (E-TEM). TAC films formed by the dipping method were placed on Cu grids for E-TEM and then irradiated by nitrogen ions with acceleration voltages of 1.0 and 1.5 kV and with ion doses between 5.9 × 1016 and 1.2 × 1017 ions/cm2. The TAC films were removed by acetone after irradiation and the surface-modified layers at the top were kept for assessment as diaphragms. The thicknesses of the surface-modified layers, which varied depending on the acceleration voltage and dose of the nitrogen ions, ranged from around 12 to 29 nm. X-ray photoelectron spectroscopy revealed that the surface-modified layers mainly consisted amorphous carbon with a nitrogen-to‑carbon atomic ratio of around 1%. These films were transparent to an electron beam accelerated to 300 kV. The pressure resistances of all samples were maintained at around 0.1 MPa.