In this review, the initial oxidation process of low-index surfaces of single-crystalline platinum group metals (PGMs: Ru, Rh, Pd, Ir, and Pt) is discussed in detail at the atomic level, involving several types of oxygen species: chemisorbed O, subsurface O, dissolved O, oxidic O. The oxidation of PGMs begins only when the surface of the PGM is saturated with chemisorbed O. Oxygen penetration into the metal is a critical next step in surface oxide formation, which can occur either through the step edge or directly through the terrace, depending on the oxidants chosen (O 2 , NO 2 , atomic O, and ozone O 3 ). However, subsurface oxygen (oxygen directly below the top metal layer) does not form a separate phase in PGM. Instead, a surface oxide consisting of a single O-Me-O trilayer nucleates and grows (heterogeneous growth mode). The oxidation process is a nonlinear process with self-acceleration and passivation behavior, where many processes occur in parallel and in sequence, so that patterning can occur on different length scales. For this reason, oxidation studies must be performed at both the atomic and mesoscale using powerful combinations of surface science techniques such as scanning tunneling microscopy (STM) and low-energy electron microscopy (LEEM).

The interest in understanding and controlling the properties of two-dimensional materials (2DMs) has fostered in the last years a significant and multidisciplinary research effort involving condensed matter physics and materials science. Although 2DMs have been investigated with a wide set of different experimental and theoretical methodologies, experiments carried out with surface-science based techniques were essential to elucidate many aspects of the properties of this family of materials. In particular, synchrotron-based X-ray photoelectron spectroscopy (XPS) has been playing a central role in casting light on the properties of 2DMs, providing an in-depth and precise characterization of these materials and helping to elucidate many elusive and intricate aspects related to them. XPS was crucial, for example, in understanding the mechanism of growth of several 2DMs at surfaces and in identifying the parameters governing it. Moreover, the chemical sensitivity of this technique is crucial in obtaining knowledge about functionalized 2DMs and in testing their behavior in several model chemical reactions. The achievements accomplished so far in this field have reached a maturity point for which a recap of the milestones is desirable. In this review, we will showcase relevant examples of studies on 2DMs for which synchrotron-based XPS, in combination with other techniques and state-of-the-art theoretical modeling of the electronic structure and of the growth mechanisms, was essential to unravel many aspects connected to the synthesis and properties of 2DMs at surfaces. The results highlighted herein and the methodologies followed to achieve them will serve as a guidance to researchers in testing and comparing their research outcomes and in stimulating further investigations to expand the knowledge of the broad and versatile 2DMs family.

Many far-reaching applications of graphene require a deep understanding of the interactions between graphene and other surfaces, including the wetting behaviour of graphene. However, its two-dimensional nature does not allow qualifying graphene as simply hydrophobic or hydrophilic, but instead gives rise to a diversity of interfacial phenomena governing the apparent wettability of graphene. As a result, wide disparities in the wetting properties of graphene have been widely reported. In this review we analyse the wettability of graphene with a special focus on the experimental conditions and on discriminating the causes of the reported inconsistencies. The elimination of the environmental factors causing misleading data is a major challenge. Importantly, progresses made in graphene research yielded new experimental insights and tools enabling the minimization of unwanted effects and, ultimately, the achievement of reliable contact angle measurements. Besides the macroscopic wettability studied using contact angle measurements under ambient conditions or by theoretical modelling, we also analysed correlations with the wettability of graphene at the molecular level in supremely pure environment of ultra-high vacuum.