Hydrogen on silicene: like or unlike graphene?

Hydrogenation of free-standing silicene, the two-dimensional allotrope of silicon, is investigated in detail using first-principles methods and compared with the adsorption of H atoms on graphene. Similarly to graphene, chemisorption of a single H atom on silicene induces the formation of a semilocalized state around the adatom, a sharp peak in the density of states at the Fermi level which acts as a strong resonant scatterer for charge carriers. This state hosts an unpaired electron, the itinerant electron of the resonating valence bond picture which primarily resides on the ``majority'' sublattice and biases the reactivity towards specific lattice positions. Contrary to graphene, sticking of hydrogen atoms is barrierless, on both the pristine and the hydrogenated surface. As a consequence, hydrogen adsorption on silicene is expected to proceed randomly under typical laboratory conditions, and preferential binding to form balanced dimers (or clusters) only occurs when thermodynamic equilibrium conditions prevail. The absence of clustering can be experimentally confirmed using scanning tunneling microscopy techniques since simulated imaging shows that the investigated structures provide distinguishable features that should allow their easy identification, if present on the surface. Overall, our findings can be rationalized by the fact that in silicene p bonds are weaker and the lattice is softer than in graphene and suggest that in silicene adatoms may severely limit carrier mobility.

Surface Reactions are Crucial for Energy Storage

Elsa Callini, Shunsuke Kato, Philippe Mauron

Reactions between gas molecules, e.g. H-2 and CO2 and solids take place at the surface. The electronic states and the local geometry of the atomic arrangement determine the energy of the adsorbate, i.e. the initial molecule and the transition state. Here we review our research to identify the surface species, their chemical state and orientation, the interaction with the neighbouring molecules and the mobility of the adsorbed species and complement the experimental results with thermodynamic modelling. The role of the Ti was found to be a bridge between the charged species preventing the individual movement of the ions including charge separation. The Ti has no catalytic effect on the hydrogen sorption reaction in borohydrides. The physisorption of molecular hydrogen is too weak at ambient temperature to reach a significant hydrogen storage density. The addition of a hydrogen dissociation catalyst to a nanoporous material with a large specific surface area may potentially enable the spillover of hydrogen atoms from the metal catalyst to the surface of the porous material and chemisorb on specific sites with a much higher binding energy compared to physisorption. The intercalation of alkali metals in C-60 fullerenes increases the interaction energy of hydrogen with the so-called metal fullerides significantly. Sterical diffusion barriers by partial oxidation of the surface of borohydrides turned out to redirect the reaction path towards pure hydrogen desorption and suppress the formation of diborane, a by-product of the hydrogen evolution reaction from borohydrides previously undetected. The combination of a newly developed gas controlling system with microreactors allows us to investigate complex reactions with small quantities of nano designed new catalytic materials. Furthermore, tip-enhanced Raman spectroscopy (TERS) will allow the investigation of the reactions locally on the surface of the catalyst and the near ambient pressure photoelectron spectroscopy enables analysis of the surfaces in ultra-high vacuum and in situ interaction with the adsorbates i.e. while the reaction takes place. This brings us in a unique position for the investigation of the heterogeneous reactive systems. The mechanism of the Ti catalysed hydrogen sorption reactions in alanates was recently established based on spectroscopic investigations combined with thermodynamic analysis of the transition states.

Schweizerische Chemische Gesellschaft2015

Magnetism of Single Adatoms and Small Adsorbed Clusters Investigated by Means of Low-Temperature STM

Quentin Dubout

This thesis works reports on the magnetic properties of single transition metal atoms and hydrogenated compounds adsorbed on crystal surfaces. It is an experimental study, based on scanning tunneling microscopy (STM), initiated by the project of demonstrating the existence of magnetic remanence for single adsorbed atoms (adatoms). All the measurements have been performed with a scanning tunneling microscope operating at 0.4 K and equipped with 8.5 T superconducting coils. The investigated systems are cobalt atoms, pure and hydrogenated (cobalt hydrides), adsorbed on Pt(111) and graphene on Pt(111) surfaces. In the first case, we have immediately been confronted to the presence of cobalt hydrides on the surface. Three types of hydrides (CoH, CoH2, and CoH3) have been identified, quite different from the clean adatoms, and readily distinguished by their tunnel spectrum (STS). Twoofthesehydridesdisplaylow-energyvibrationalmodes(1−30meV),identifiedbytheir absence of response to a magnetic field, the effect of an isotopic substitution, and by comparison with known systems. The three hydrides can be deprotonated applying a sufficient voltage between tip and sample. CoH2 on Pt(111) presents a Kondo resonance, due to a half-integer spin (S = 3/2) and a peculiar magnetic anisotropy. This system corresponds to the first demonstration of the appearance of the Kondo effect upon hydrogen adsorption. The results of several attempts to observe remanence for single cobalt adatoms on Pt(111) with spin-polarized scanning tunneling microscopy (SP-STM) are also presented and discussed. The second system differs from the first one in the presence of a graphene layer on the Pt surface. The first experimental determination of the magnetic moment and magnetic anisotropy of adatoms on graphene is reported in this thesis. A magnetic moment of (2.2 ± 0.4) μB was measured by spin-excitation spectroscopy (SES), as well as a hard magnetization axis with an anisotropy energy of 8.1 ± 0.4 meV. This value, comparable to the record value of single cobalt atoms on Pt(111), has been mainly attributed to the strong hybridization between cobalt and graphene. Ab initio calculations confirmed our observations. Graphene thus represents an extremely promising substrate for future applications in nanoscale magnetism and spintronics. This system also exhibits three different cobalt hydrides in addition to the clean atoms. Their respective chemical composition, CoH, CoH2, and CoH3, was determined by direct comparison between high resolution STM images and simulated images obtained from ab initio calculated atomic structures. A controlled deprotonation method is presented. CoH3 is the only hydride to display spin excitations. Its magnetic moment is comparable to that of clean cobalt, it presents a hard magnetization axis as well, however, its magnetic anisotropy energy is sensibly lower, 1.7 ± 0.05 meV. The other two hydrides display vibrational excitations only. The effect of hydrogen on the magnetic properties of metal adatoms is therefore very strong in this system as well, and must be taken into account in future studies, hydrogen being a very common contaminant in ultra-high vacuum set-ups. Finally, a detailed study of the structure graphene mono- and bilayers on Ru(0001) was realized. Two different stackings, AB and AA, have been identified for the bilayers, the last one not observed to date for this system. The moiré structure of monolayer graphene and of this last type of bilayer has been determined: (11.57×11.57)R4.3◦ and (10.54×10.54)R4.7◦, respectively. Moreover, C-C bond distortions of more than 10%, observed in the STM images, have been studied in detail. A quantitative comparison with ab initio calculations allowed to identify their origin as the tilt of the graphene π-orbitals. These distortions are therefore purely artificial. This is an important result for future scanning tunneling microscopy studies of graphene.

EPFL2013

On-Surface Assembly and Reactions of Organic Molecules in Ultra-High Vacuum

Claudius Morchutt

This thesis comprises the study of two types of 2D materials, those stabilized by non-covalent and those by covalent interactions. They are synthesized and studied on well-defined metallic surfaces in ultra-high vacuum (UHV). The self-assembly of terephthalic acid (TPA) on Ag(100) is explored. TPA stays intact upon deposition at room temperature (RT) and forms islands stabilized by hydrogen bonds. TPA islands influence the homoepitaxial growth of silver and reduce the sticking coefficient of Ag atoms. At RT Ag atoms intercalate TPA islands which are not disrupted. In contrast, TPA molecules in conjunction with Mg atoms on Ag(100) results in tip-induced altering of the surface. The electric field between tip and sample interacts with Mg atoms and TPA, which leads to a restructuring of the step-edges during scanning. Moreover, the self-assembly of the organic semiconductor 2,7-dicyano[1]benzothiene[3,2-b]benzothiophene (cBTBT) on Ag(111) at RT is presented. The pro-chiral molecules form compact islands with a chevron-like structure containing both enantiomers. Deposition of Fe atoms leads to an amorphous metal-organic coordination network (MOCN). Statistical analysis reveals that conformational entropy plays a critical part in its stabilization. Phase segregation into spatially homogeneously crystalline domains of molecules and metal atoms upon annealing suggests that the amorphous network is kinetically trapped at RT during the preparation process. The second part presents the on-surface synthesis of 2D polymers via different coupling schemes. First, the Ullmann coupling on Au(111) is explored. The molecule 1,3,5-tris(4-bromophenyl)benzene (TBPB) debrominates upon annealing and polyphenylene is formed. To investigate the influence of metal substrates on the dehalogenation, a single layer of hexagonal boron nitride as well as graphene grown on Ni(111) is introduced. On both surfaces TBPB forms self-assembly islands stabilized by halogen bonds, while on bare Ni(111) the substrate-molecule interaction dominates resulting in structures without long-range order. Upon annealing on both surfaces dehalogenation is induced and 2D nanostructures are formed. On bare Ni(111) the precursor molecules merely decompose. The experimental annealing temperatures are consistent with debromination barriers calculated by DFT. An example of the synthesis of tailor-made 2D structures by using particularly designed precursor molecules is also given. A terminal alkyne with a triazine core undergoes on-surface Glaser coupling and cyclotrimerization on Au(111) resulting in nitrogen doped 2D polymers. Finally, a comprehensive study of the on-surface decarboxylation reaction of 1,3,5-tris(4-carboxyphenyl)benzene (TCPB) on Cu(111) is presented. TCPB deprotonates upon deposition on Cu(111) at RT and self-assembles in compact islands. The self-assembly is stabilized by ionic hydrogen bonds between deprotonated and negatively charged oxygen atoms and hydrogen atoms of neighboring molecules. Annealing leads to decarboxylation and formation of 2D nanostructures. The reaction occurs in a clean fashion because CO2 leaves the surface. In addition, STS reveals that the lowest unoccupied molecular orbital (LUMO) of the 2D polymer is destabilized compared to the LUMO of the monomer although the pi-electron system is more extended in the polymer. The decarboxylation impacts the energy position of the LUMO to a greater extend which is corroborated by DFT calculations.

EPFL2016