Dynamics of graphene growth on a metal surface: A time-dependent photoemission study

Applying time-dependent photoemission we unravel the graphene growth process on a metallic surface by chemical vapor deposition (CVD). Graphene CVD growth is in stark contrast to the standard growth process of two-dimensional films because it is self-limiting and stops as soon as a monolayer of graphene has been synthesized. Most importantly, a novel phase of metastable graphene was discovered that is characterized by permanent and simultaneous construction and deconstruction. The high quality and large area graphene flakes are characterized by angle-resolved photoemission, proving that they are indeed monolayer and cover the whole 1×1 cm Ni(111) substrate. These findings are of high relevance to the intensive search for reliable synthesis methods for large graphene flakes of controlled layer number

Using a dual plasma process to produce cobalt--polypyrrole catalysts for the oxygen reduction reaction in fuel cells -- part II: analysing the chemical structure of the films

The chemical structure of cobalt--polypyrrole -- produced by a dual plasma process -- is analysed by means of X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption spectroscopy (NEXAFS), X-ray diffraction (XRD), energy-dispersive X-Ray spectroscopy (EDX) and extended x-ray absorption spectroscopy (EXAFS).It is shown that only nanoparticles of a size of 3\,nm with the low temperature crystal structure of cobalt are present within the compound. Besides that, cobalt--nitrogen and carbon--oxygen structures are observed. Furthermore, more and more cobalt--nitrogen structures are produced when increasing the magnetron power. Linking the information on the chemical structure to the results about the catalytic activity of the films -- which are presented in part I of this contribution -- it is concluded that the cobalt--nitrogen structures are the probable catalytically active sites. The cobalt--nitrogen bond length is calculated as 2.09\,\AA\ and the carbon--nitrogen bond length as 1.38\,\AA

Dynamics of graphene growth on a metal surface: a time-dependent photoemission study

Applying time-dependent photoemission we unravel the graphene growth process on a metallic surface by chemical vapor deposition (CVD). Graphene CVD growth is in stark contrast to the standard growth process of two--dimensional films because it is self-limiting and stops as soon as a monolayer graphene has been synthesized. Most importantly, a novel phase of metastable graphene was discovered that is characterized by permanent and simultaneous construction and deconstruction. The high quality and large area graphene flakes are characterized by angle-resolved photoemission proofing that they are indeed monolayer and cover the whole 1$\times$1 cm Nickel substrate. These findings are of high relevance to the intensive search for reliable synthesis methods for large graphene flakes of controlled layer number

Moment canting and domain effects in antiferromagnetic DyRh2_2Si2_2

A combined experimental and theoretical study of the layered antiferromagnetic compound DyRh$_2$Si$_2$ in the ThCr$_2$Si$_2$-type structure is presented. The heat capacity shows two transitions upon cooling, the first one at the N{\'e}el temperature $T_{\rm N}=55\,\rm K$ and a second one at $T_{\rm N2}=12\,\rm K$. Using magnetization measurements, we study the canting process of the Dy moments upon changing the temperature and can assign $T_{\rm N2}$ to the onset of the canting of the magnetic moments towards the $[100]$ direction away from the $c$ axis. Furthermore, we found that the field dependence of the magnetization is highly anisotropic and shows a two-step process for $H\parallel 001$. We used a mean-field model to determine the crystalline electric field as well as the exchange interaction parameters. Our magnetization data together with the calculations reveal a moment orientation close to the $[101]$ direction in the tetragonal structure at low temperatures and fields. Applying photoemission electron microscopy, we explore the (001) surface of the cleaved DyRh$_2$Si$_2$ single crystal and visualize Si- and Dy-terminated surfaces. Our results indicate that the Si-Rh-Si surface protects the deeper lying magnetically active Dy layers and is thus attractive for investigation of magnetic domains and their properties in the large family of LnT$_2$Si$_2$ materials

Simulating high-pressure surface reactions with molecular beams

Using a reactive molecular beam with high kinetic energy ($E_{kin}$) it is possible to speed gas-surface reactions involving high activation barriers ($E_{act}$), which would require elevated pressures ($P_0$) if a random gas with a Maxwell-Boltzmann distribution is used. By simply computing the number of molecules that overcome the activation barrier in a random gas at $P_0$ and in a molecular beam at $E_{kin}$=$E_{act}$, we establish an $E_{kin}$-$P_0$ equivalence curve, through which we postulate that molecular beams are ideal tools to investigate gas-surface reactions that involve high activation energies. In particular, we foresee the use of molecular beams to simulate gas surface reactions within the industrial-range ($>$ 10 bar) using surface-sensitive Ultra-High Vacuum (UHV) techniques, such as X-ray photoemission spectroscopy (XPS). To test this idea, we revisit the oxidation of the Cu(111) surface combining O$_2$ molecular beams and XPS experiments. By tuning the kinetic energy of the O$_2$ beam in the range 0.24-1 eV we achieve the same sequence of surface oxides obtained in Ambient Pressure Photoemission (AP-XPS) experiments, in which the Cu(111) surface was exposed to a random O$_2$ gas up to 1 mbar. We observe the same surface oxidation kinetics as in the random gas, but with a much lower dose, close to the expected value derived from the equivalence curve

Simulating high-pressure surface reactions with molecular beams

Using a reactive molecular beam with high kinetic energy (Ekin), it is possible to speed gas-surface reactions involving high activation barriers (Eact), which would require elevated pressures (P0) if a random gas with a Maxwell-Boltzmann distribution is used. By simply computing the number of molecules that overcome the activation barrier in a random gas at P0 and in a molecular beam at Ekin = Eact, we establish an Ekin-P0 equivalence curve, through which we postulate that molecular beams are ideal tools to investigate gas-surface reactions that involve high activation energies. In particular, we foresee the use of molecular beams to simulate gas surface reactions within the industrial-range (>10 bar) using surface-sensitive ultra-high vacuum (UHV) techniques, such as X-ray photoemission spectroscopy (XPS). To test this idea, we revisit the oxidation of the Cu(111) surface combining O2 molecular beams and XPS experiments. By tuning the kinetic energy of the O2 beam in the range of 0.24-1 eV, we achieve the same sequence of surface oxides obtained in ambient pressure photoemission (AP-XPS) experiments, in which the Cu(111) surface was exposed to a random O2 gas up to 1 mbar. We observe the same surface oxidation kinetics as in the random gas, but with a much lower dose, close to the expected value derived from the equivalence curveTED2021-130446B-I00, PID2020-116093RBC4