Abstract

Hydrogen embrittlement in metals and alloys, which occurs following hydrogen adsorption, is a well-documented failure mechanism where hydrogen diffusion leads to material degradation through various processes, such as hydride formation. Conventional microscopy struggles to detect hydrogen due to its low electron density and minimal electron/X-ray cross-section, making direct imaging challenging. In contrast, neutral atom microscopy offers a novel approach to surface imaging. Specifically, hydrogen has a high scattering cross-section for the extremely low-energy helium atom beams used in the scanning helium atom microscope (SHeM). This large cross-section enables highly sensitive, non-destructive imaging of hydrogen-passivated surfaces for the first time. This paper presents the initial SHeM imaging of hydrogen-passivated silicon (H/Si(111)) surfaces, revealing distinct contrast between passivated and non-passivated regions across millimetre-scale lateral dimensions. The resulting images highlight surface defects and wetting-induced structures within the hydrogen passivation layers. Helium atom scattering experiments confirm that passivated areas exhibit ordered surface diffraction aligned with the Si(111) lattice structure, whereas non-passivated regions show no significant diffraction signal. Additionally, thermal desorption spectroscopy (TDS) studies identify desorption peaks corresponding to mono-, di-, and tri-hydride formation, aligning with observed variations in scattered helium intensity. These findings underscore the potential of SHeM as a powerful tool for probing the initiation and evolution of hydrogen adsorption and desorption on surfaces.