= Solution
\b[Use conventional <bright-field transmission electron microscopy>, with nearly parallel illumination, a suitably small <objective aperture> and low magnification covering the whole cross-section.] More strongly scattering material sends more intensity outside the selected central beam and therefore looks darker. Tilt crystalline layers away from strong <Bragg diffraction> conditions to reduce <diffraction contrast>, and use a near-focus image so that <electron phase contrast> does not dominate.
The nominally uniform $150\,\mathrm{nm}$ thickness helps separate composition from thickness variations, but does not make the contrast purely compositional: density, atomic number and multiple scattering still matter. Choose an accelerating voltage sufficient to transmit this thickness, commonly $200$–$300\,\mathrm{kV}$ for a robust inorganic film, subject to its damage limit. <High-angle annular dark-field imaging> in <scanning transmission electron microscopy> is a useful alternative for approximately monotonic mass-thickness contrast, but is a different acquisition mode and still requires thickness and detector-angle control.
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