Quantifying residual hydrogen adsorption in low-temperature STMs

a r t i c l e i n f o Keywords: Hydrogen adsorption Residual gas Low-temperature scanning tunneling microscope Kondo effect Hexagonal boron nitride Titanium We report on low-temperature scanning tunneling microscopy observations demonstrating that individual Ti atoms on hexagonal boron nitride dissociate and adsorb hydrogen without measurable reaction barrier. The clean and hydrogenated states of the adatoms are clearly discerned by their apparent height and their differential conductance revealing the Kondo effect upon hydrogenation. Measurements at 50 K and 5 × 10 −11 mbar indicate a sizable hydrogenation within only 1 h originating from the residual gas pressure, whereas measurements at 4.7 K can be carried out for days without H 2 contamination problems. However, heating up a low-T STM to operate it at variable temperature results in very sudden hydrogenation at around 17 K that correlates with a sharp peak in the total chamber pressure. From a quantitative analysis we derive the desorption energies of H 2 on the cryostat walls. We find evidence for hydrogen contamination also during Ti evaporation and propose a strategy on how to dose transition metal atoms in the cleanliest fashion. The present contribution raises awareness of hydrogenation under seemingly ideal ultra-high vacuum conditions, it quantifies the H 2 uptake by isolated transition metal atoms and its thermal desorption from the gold plated cryostat walls. The adsorption of hydrogen is the ongoing focus of intense studies in chemisorption/physisorption [1] in the context of heterogeneous catalysis and in the field of hydrogen storage. 1 Concerning investigations with scanning tunneling microscopy (STM), modifications of the sample's electronic and magnetic properties through an interaction with hydrogen have been reported. Examples are the coverage dependent inelastic excitations detected in the differential conductance (dI/dV) of physisorbed hydrogen on a copper surface [2], the emergence of vibrational excitations in dI/dV for hydro-genated La, Cr, and Ce adatoms on Ag(100) [3], as well as the alteration of the magnetic characteristics (Kondo effect) of Co adatoms on Pt(111) upon hydrogen uptake [4]. Furthermore, molecular hydrogen was also studied for its role in providing ultra-high geometrical resolution in STM topographs [5,6]. The above investigations were carried out under ultra high vacuum (UHV), and some of the H-induced effects showed up unintentionally, before they were confirmed by deliberately dosing molecular hydrogen. This reflects the frequently encountered assumption of an ideal UHV environment in surface science studies with the consequence of being oblivious …