An Experimental Setup for Heterogeneous Catalysis on Atomically Defined Metal Nanostructures

The main objective of this PhD thesis is to link the atomic scale structure to the catalytic properties of self-assembled nanostructures. The growth and characterization of these nanostructures was carried out in an ultra-high vacuum (UHV) chamber initially designed for the study of the kinetics of epitaxial growth of thin films and nanostructures by variable temperature scanning tunneling microscopy. During this PhD thesis, we built a very sensitive and selective gas detector based on a quadrupole mass spectrometer and adapted to the geometry of this UHV system. The proper functioning of the gas detector is confirmed by temperature programmed desorptions of Xe on Ag(100) which demonstrate the very high sensitivity of the device of about $10^{-6}$ ~monolayer (ML). The zero-order desorption parameters obtained for the first monolayer of Xe adsorbed on Ag(100) are $E_d=0.22-0.23$ ~eV with a prefactor $\nu_0$ between $4\times10^{12}$ ~s $^{-1}$ and $1 \times10^{13}$ ~s $^{-1}$ .

An exchange process between Fe adatoms and Ag atoms from the Ag(100) surface is identified by means of temperature programmed desorption of N $_2$ . Desorption spectra suggest that the exchange is complete from an annealing temperature of 140~K.

Our study of the catalytic activity of metal nanostructures focuses on the ability of passivated Fe clusters to adsorb N $_2$ molecules without dissociating them, which is a fundamental step in the bio-inspired heterogeneous catalysis of ammonia. The first system investigated for the growth of Fe clusters is MgO/Ag(100). Diffusion of Fe adatoms deposited on a MgO monolayer is observed from an annealing temperature of 280~K onwards. Fe clusters with an average size of $12.1\pm 0.5$ ~atoms are obtained on a MgO monolayer by thermal ripening of 0.072~ML of Fe deposited at 40~K. No sign of N $_2$ adsorption was observed on these clusters by thermal programmed desorption investigation.

The second system chosen for the growth of Fe clusters is graphene/Ir(111) obtained by chemical vapor deposition. The deposition of 0.3 and 0.4 ML of Fe on a graphene/Ir(111) sample at 50~K, followed by annealing at 300~K, generates a superlattice of Fe clusters which matches the periodicity of the moiré formed by graphene/Ir(111). The theoretically expected mean cluster size is 26 atoms for 0.3~ML of Fe, and 35 atoms for 0.4~ML of Fe. The temperature stability of the clusters obtained with 0.4~ML Fe is investigated from 300~K to 600~K. It is established that Smoluchowski ripening takes place as the annealing temperature increases and the superlattice structure has completely disappeared after annealing at 600~K. Temperature programmed desorption of molecularly adsorbed N $_2$ at 50~K on a sample with 0.4~ML of Fe deposited on graphene/Ir(111) reveals 3 desorption peaks at 120~K, 92~K, and 60~K. In addition, slow dissociation of nitrogen molecules at 50~K adsorbed on the higher energy adsorption sites is demonstrated.

Growth and Magnetism of Nanostructures Investigated by STM, MOKE, and XMCD

Alberto Cavallin

This thesis presents our studies on nanostructures growth and magnetism, mainly based on scanning tunneling microscopy (STM), magneto-optical Kerr effect (MOKE), and x-ray magnetic circular dichroism (XMCD) measurements. In most of these experiments, nanostructures were grown by molecular beam epitaxy on single crystal substrates under ultra high vacuum conditions. Our combined morphological and magnetic investigations aim at an atomic-level description of the nanostructures magnetic properties. In particular, we are searching for strategies to design the smallest ferromagnet with stable magnetization at room temperature to be used in devices such as magnetic random access memories, storage media, and sensors. A critical factor in pursuing this aim is the determination and control of the magnetization reversal mechanism. Different reversal mechanisms may occur in the nanostructures we considered: either all the spins quasi-coherently rotate or a domain wall is nucleated and propagated along the particle. We prepared monolayer-high Co islands on Pt(111) with compact and ramified shape by adjusting the deposition temperatures and coverages for a sequence of deposition and annealing steps. Combining field-dependent magnetization curves and temperature dependent zero-field susceptibility measurements we identify a size- and shape-dependent transition from quasi-coherent to domain wall propagation reversal regimes. In a second study we investigated a system which according to a recent publication presents many of the characteristics desired in bit patterned magnetic media for ultra-high density storage devices. In particular, it was claimed that Co nanoclusters, growing in registry with the GdAu2/Au(111) moiré periodic substrate, and featuring a density of 52 Tera/in2, present out-of-plane remanent magnetization at T = 300 K. At odds with this claim, no perpendicular magnetic anisotropy was found in our combined MOKE and STM study, which shows an in-plane remanent signal appearing only for coalesced islands. A third study is devoted to FeRh, an alloy material which, thanks to its peculiar magnetic phase diagram, has been proposed coupled to FePt for advanced magnetic recording schemes. We present the first observation of low temperature stabilization of the ferromagnetic (FM) phase in FeRh chemically ordered crystals. Nanocrystals measuring 3.3 nm in diameter have B2 structure with alternating atomic Fe and Rh layers. XMCD demonstrates FM alignment of the Fe and Rh magnetic moments of 3 and 1 μB, respectively. In sharp contrast to FeRh bulk and thin films, which show antiferromagnetic iii Abstract (AFM) order below 300 K, the low-temperature FM 3.3 nm nanocrystals are the hallmark of a size-dependent AFM-FM transition temperature. Finally we present our results towards creation by chemical vapor deposition of a “mille-feuille” structure composed of graphene and magnetic nanoclusters. To realize this objective we optimized the cluster size, composition, and annealing temperature to obtain a compromise between graphene syntesis and cluster coalescence. We demonstrate by STM that annealing to T = 720 K in ethylene stabilizes Ir-seeded clusters against diffusion and coalescence, and by Auger Electron Spectroscopy (AES) that Ni-coated clusters can be covered with up to half of the Carbon contained in a graphene layer.

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

Rare Earth and Bimetallic Transition Metal Islands at Surfaces

Dimitrios Mousadakos

This thesis reports on the growth and on the magnetic characterization of nanostructures made of 3d-4d transition metals and rare earths. In all the experiments the nanostructures have been grown by atomic beam epitaxy (ABE) in ultra high vacuum conditions (UHV) by performing growth steps with the precise selection of external parameters as: the deposition flux F, the sample deposition temperature Tdep and the sample annealing temperature Tann . The morphological and magnetic properties of the nanostructures have been characterized by performing two in situ measurement techniques in our experimental setup, namely: scanning tunneling microscopy (STM) and magneto-optical Kerr effect (MOKE). The first aims of the thesis consisted in the formation of a supperlattice of rare earth on the graphene moiré pattern induced by the lattice mismatch of a single graphene layer grown on Ir(111) crystal surface. We report the first cluster supperlattice made of rare earths, namely Sm, where clusters nucleate in registry with the moiré pattern, forming a superlattice for deposition temperatures between 80 K and 110 K. Within this temperature range, the Sm supperlattice shows long-range order extending over several tens of nanometers and a cluster size distribution competing with the finest superlattices grown by ABE. Sm cluster spatial order is preserved, up to a coverage of 0.5 ML yielding a mean cluster size of 50 atoms, while for higher coverages coalescence starts. Moreover, Sm clusters with a mean size of 9 atoms are thermally stable up to Tann = 130 K from where on the order is lost and the density reduces progressively by cluster coalescence. Similar experiments carried out for Dy in the same deposition temperature range show the absence of cluster arrays. The second part focused on the enhancement of the MAE of bimetallic nanostructures, grown on Pt(111), having a Co-core by forming atomically sharp interlines and interfaces with three 4d elements, namely Ag, Pd and Rh. Our approach is based on the experimental measurement of the island magnetic susceptibility x(T) and morphology by means of MOKE and STM, respectively, combined with magnetic simulation analysis in order to quantify the different contributions to the island magnetic anisotropy. The monolayer capping for all studied elements contributes positively to the out-of-plane MAE of Co islands, with Pd giving the largest increase in the magnetic hardness. In addition, the capping with two Pd monolayers of pure Co islands maximizes the blocking temperature Tb . However, the lateral decoration of Co islands with all elements contributes negatively to MAE, with the Co/Pd shell giving the smallest effect and Rh the largest. Morphologically, Ag was found to produce a partial decoration of the Co island edges while Rh completely decorates laterally Co islands forming a rim of irregular shape without nucleating on top of Co islands for the used deposition parameters. Simulations of the magnetization reversal process by using coherent rotation (CR) and/or domain wall nucleation and propagation (DW) reversal models including different contributions to the MAE for atoms located at the interface (Ks) and at interline (Kp) and an exchange interaction between Co and 4d transition metals have reproduced the experimental x'(T) and x''(T) curves.

EPFL2017