Vicinal Ag Research Articles

Directed growth of quinacridone chains on the vicinal Ag(35 1 1) surface.

The formation of self-assembled domains and chains of monomolecular width of quinacridone (QA) on the vicinal Ag(35 1 1) surface was investigated by scanning tunneling microscopy and low-energy electron diffraction. The focus was on the influence of the steps on the QA structures and their preferential azimuthal orientations with the aim of achieving a selective orientation. After deposition at a sample temperature of 300 K, QA forms the same kind of molecular chains as on the nominally flat Ag(100) surface because of strong intermolecular hydrogen bonds, which we reported in a previous publication [Humberg, N.; Bretel, R.; Eslam, A.; Le Moal, E.; Sokolowski, M. J. Phys. Chem. C 2020, 124, 24861-24873]. The vicinal surface leads to one additional chain orientation, which is parallel to the Ag step edges. However, most chains nucleate on the Ag terraces between steps with four distinct azimuthal orientations that are identical to those on Ag(100), and which are determined by the interactions with the (100) surface. At 300 K, the chains grow across the Ag steps, which do not break the azimuthal chain orientations. In contrast, during the deposition at sample temperatures of 400 and 500 K, the nucleation of the chains takes place at the Ag step edges. Hence, these have a strong influence on the azimuthal orientation of the molecules, resulting in a preferential growth of the chains in two of the four azimuthal orientations. We explain this by the adaptation of favorable adsorption sites, which involve the replacement of Ag atoms by QA molecules with specific azimuthal orientations at the step edges.

Synthesis of Quantum-Confined Borophene Nanoribbons.

Borophene nanoribbons (BNRs) are one-dimensional strips of atomically thin boron expected to exhibit quantum-confined electronic properties that are not present in extended two-dimensional borophene. While the parent material borophene has been experimentally shown to possess anisotropic metallicity and diverse polymorphic structures, the atomically precise synthesis of nanometer-wide BNRs has not yet been achieved. Here, we demonstrate the synthesis of multiple BNR polymorphs with well-defined edge configurations within the nanometer-scale terraces of vicinal Ag(977). Through atomic-scale imaging, spectroscopy, and first-principles calculations, the synthesized BNR polymorphs are characterized and found to possess distinct edge structures and electronic properties. For single-phase BNRs, v1/6-BNRs and v1/5-BNRs adopt reconstructed armchair edges and sawtooth edges, respectively. In addition, the electronic properties of single-phase v1/6-BNRs and v1/5-BNRs are dominated by Friedel oscillations and striped moiré patterns, respectively. On the other hand, mixed-phase BNRs possess quantum-confined states with increasing nodes in the electronic density of states at elevated biases. Overall, the high degree of polymorphism and diverse edge topologies in borophene nanoribbons provide a rich quantum platform for studying one-dimensional electronic states.

Step-guided epitaxial growth of blue phosphorene on vicinal Ag(111)

Blue phosphorene (blue-P) is a promising candidate for developing next-generation nanoelectronics devices due to its unique properties. Therefore, the controllable growth of a two-dimensional (2D) wafer-scale blue-P monolayer on substrates is a fundamental issue. However, the direct growth of blue-P on substrates currently remains a daunting challenge. In this paper, by using first-principles calculations, we propose a feasible route for fabricating blue-P via epitaxial growth. The growth mechanism of one-dimensional (1D) phosphorene chains to finite-sized blue-P clusters on Ag(111) is elucidated, and the reason why blue-P clusters cannot grow into a large-area monolayer on the Ag(111) surface is revealed. Moreover, the growth of blue-P on vicinal Ag(111) is explored and the energetic benefit of 1D zigzag blue-P nanoribbon growth along the $\mathrm{Ag}\ensuremath{\langle}110\ensuremath{\rangle}$ step edge is confirmed. More importantly, we propose that the unidirectionally orientated blue-P nanoribbons can merge into large-area blue-P nanosheets through a step-guide growth mode. The feasibility of this strategy is validated by using molecular dynamics simulations, and a series of candidate substrates is selected. This study not only provides a promising substrate for epitaxial growth of blue-P but also sheds light on the preparation of more 2D materials.

Structure and electronic states of vicinal Ag(111) surfaces with densely kinked steps

Vicinal surfaces exhibiting arrays of atomic steps are frequently investigated due to their diverse physical-chemical properties and their use as growth templates. However, surfaces featuring steps with a large number of low-coordinated kink-atoms have been widely ignored, despite their higher potential for chemistry and catalysis. Here, the equilibrium structure and the electronic states of vicinal Ag(111) surfaces with densely kinked steps are investigated in a systematic way using a curved crystal. With scanning tunneling microscopy we observe an exceptional structural homogeneity of this class of vicinals, reflected in the smooth probability distribution of terrace sizes at all vicinal angles. This allows us to observe, first, a subtle evolution of the terrace-size distribution as a function of the terrace-width that challenges statistical models of step lattices, and second, lattice fluctuations around resonant modes of surface states. As shown in angle resolved photoemission experiments, surface states undergo stronger scattering by fully-kinked step-edges, which triggers the full depletion of the two-dimensional band at surfaces with relatively small vicinal angles.

Functionalized Graphdiyne Nanowires: On-Surface Synthesis and Assessment of Band Structure, Flexibility, and Information Storage Potential.

Carbon nanomaterials exhibit extraordinary mechanical and electronic properties desirable for future technologies. Beyond the popular sp2 -scaffolds, there is growing interest in their graphdiyne-related counterparts incorporating both sp2 and sp bonding in a regular scheme. Herein, we introduce carbonitrile-functionalized graphdiyne nanowires, as a novel conjugated, one-dimensional (1D) carbon nanomaterial systematically combining the virtues of covalent coupling and supramolecular concepts that are fabricated by on-surface synthesis. Specifically, a terphenylene backbone is extended with reactive terminal alkyne and polar carbonitrile (CN) moieties providing the required functionalities. It is demonstrated that the CN functionalization enables highly selective alkyne homocoupling forming polymer strands and gives rise to mutual lateral attraction entailing room-temperature stable double-stranded assemblies. By exploiting the templating effect of the vicinal Ag(455) surface, 40 nm long semiconducting nanowires are obtained and the first experimental assessment of their electronic band structure is achieved by angle-resolved photoemission spectroscopy indicating an effective mass below 0.1m0 for the top of the highest occupied band. Via molecular manipulation it is showcased that the novel oligomer exhibits extreme mechanical flexibility and opens unexplored ways of information encoding in clearly distinguishable CN-phenyl trans-cis species. Thus, conformational data storage with density of 0.36 bit nm-2 and temperature stability beyond 150 K comes in reach.

Role of step edges on the structure formation of α-6T on Ag(441)

Controlling the orientation of organic molecules on surfaces is important in order to tune the physical properties of the organic thin films and, thereby, increase the performance of organic thin film devices. Here, we present a scanning tunneling microscopy (STM) and photoelectron emission microscopy (PEEM) study of the deposition of the organic dye pigment α-sexithiophene (α-6T) on the vicinal Ag(441) surface. In the presence of the steps on the Ag(441) surface, the α-6T molecules exclusively align parallel to the step edges oriented along the [11¯0]-direction of the substrate. The STM results further reveal that the adsorption of the α-6T molecules is accompanied by various restructuring of the substrate surface: Initially, the molecules prefer the Ag(551) building blocks of the Ag(441) surface. The Ag(551) termination of the terraces is then changed to a predominately Ag(331) one upon completion of the first α-6T monolayer. When closing the two layer thick wetting layer, the original ratio of Ag(331) and Ag(551) building blocks ( ≈ 1:1) is recovered, but a phase separation into microfacets, which are composed either of Ag(331) or of Ag(551) building blocks, is found.

Multitude of PTCDA Superstructures on Ag(111) and Vicinal Surfaces

Organic molecules that covalently bond to metal surfaces, like perylene tetracarboxylic acid dianhydride (PTCDA) on Ag, may cause large-scale reconstruction of the interface, in particular if the adsorption takes place on a “stepped” (i.e., vicinal) surface. Under certain conditions, such surfaces develop a large variety of high-index facets that are stabilized by superstructures of the PTCDA adsorbate. These superstructures can be commensurate, point-on-line, or incommensurate with respect to the substrate facets depending on facet orientation and step direction. The present scanning tunneling microscopy and low-energy electron diffraction data were taken from two different vicinal surfaces, Ag(775) and Ag(10 8 7), both inclined by 8.5° with respect to the (111) direction but in different azimuthal directions. Altogether, we identify and evaluate 18 new superstructures and correlate their data with the properties of the corresponding facets. The observed richness of superstructures and the occurrence of ...

Thermodynamics is not enough — The case of the Volmer reaction on silver

The energy of activation for the Volmer step of the hydrogen evolution reaction on silver electrodes is evaluated using our own theory of electrocatalysis. The reactivity at the hollow sites of the flat Ag(1 0 0) surface and the vicinal Ag(1 1 5) surface is compared. Although the adsorption energy calculated by Density Functional Theory is similar for both systems, we found a substantially higher value for the activation energy in the case of the (1 1 5) crystal orientation. Our analysis shows that other important factors, such as the electronic interactions between reactant and substrate at the transition state, play a decisive role. Thermodynamics is not enough to explain reactivity.

Long-range surface faceting induced by chemisorption of PTCDA on stepped Ag(111) surfaces

The organic molecule PTCDA preferentially adsorbs on steps of vicinal Ag(111) surfaces and bunches them to well defined facet planes. These depend on coverage and annealing temperature and are independent of the nominal step direction and angle of inclination of the unreconstructed initial surface. We study the development of the facets and present a map of all 16 types of facets in a stereographic triangle of 35° off the [111]-direction. The faceting mechanism is interpreted as orientational phase separation originating from different bonding strengths of PTCDA on various facets. The faceting drives the system to the minimum of its surface free energy.

Noncollinearity of the canted spins across ultrathin Fe films on vicinal Ag surfaces

We investigate the depth dependence of the canted magnetization in a bcc Fe ultrathin film grown on a vicinal Ag substrate. This study is performed for different Fe thicknesses, in the vicinity of the spin reorientation transition, by soft x-ray resonant magnetic reflectivity (SXRMR). Above 5.5 monolayers, the high spatial resolution of SXRMR allows us to describe a gradient of the canted magnetization. The tilt angle is larger near the Ag interface and decreases towards the surface. This noncollinearity is ascribed to a different balance of various anisotropies across the layer thickness. Below that value, a large increase of the tilt angle is observed and ascribed to the spin reorientation transition. Unlike the previous situation, the tilt angle is found to be homogeneous throughout the film. This can be related either to the out-of-plane spin reorientation associated to a reduced number of monolayers or to the $q$-space limitation of the SXRMR technique at the Fe ${L}_{3}$ edge.

Metallic thin films on stepped surfaces: lateral scattering of quantum well states

Quantum well states of Ag films grown on stepped Au(111) surfaces are shown to undergo lateral scattering, in analogy with surface states of vicinal Ag(111). Applying angle resolved photoemission spectroscopy we observe quantum well bands with zone-folding and gap openings driven by surface/interface step lattice scattering. Experiments performed on a curved Au(111) substrate allow us to determine a subtle terrace-size effect, i.e., a fine step-density-dependent upward shift of quantum well bands. This energy shift is explained as mainly due to the periodically stepped crystal potential offset at the interface side of the film. Finally, the surface state of the stepped Ag film is analyzed with both photoemission and scanning tunneling microscopy. We observe that the stepped film interface also affects the surface state energy, which exhibits a larger terrace-size effect compared to surface states of bulk vicinal Ag(111) crystals.

First insights of the electrocatalytical properties of stepped silver electrodes for the hydrogen evolution reaction

The electrocatalytic properties of vicinal Ag(11n) surfaces for the hydrogen evolution reaction are investigated. An analysis of the reactivity as a function of the step density is performed. The rate constants for Volmer and Heyrovsky steps are evaluated from potentiostatic transients. A correlation between the anisotropy of hydrogen adsorption energy for different sites with the non-linear dependence on the step density is established.

Electronic Anisotropy at Vicinal Ag(11n) Surfaces: Work Function Changes Induced by Steps and Hydrogen Adsorption

We have systematically analyzed both the electrical dipole originated by the presence of steps and that produced by the H–Ag bond at different sites of the vicinal Ag(11n) surfaces. We have considered both basal planes as reference systems. We have taken into account relaxation, coverage, and structural effects. An anisotropy of the electronic properties has been found along the terraces between steps. Long-range interactions and localization effects have been identified.

Influence of Steps on the Tilting and Adsorption Dynamics of Ordered Pentacene Films on Vicinal Ag(111) Surfaces

Here we present a structural study of pentacene (Pn) thin films on vicinal Ag(111) surfaces by He atom diffraction measurements and density functional theory (DFT) calculations supplemented with van der Waals (vdW) interactions. Our He atom diffraction results suggest initial adsorption at the step edges evidenced by initial slow specular reflection intensity decay rate as a function of Pn deposition time. In parallel with the experimental findings, our DFT+vdW calculations predict the step edges as the most stable adsorption site on the surface. An isolated molecule adsorbs as tilted on the step edge with a binding energy of 1.4 eV. In addition, a complete monolayer (ML) with pentacenes flat on the terraces and tilted only at the step edges is found to be more stable than one with all lying flat or tilted molecules, which in turn influences multilayers. Hence our results suggest that step edges can trap Pn molecules and act as nucleation sites for the growth of ordered thin films with a crystal structure similar to that of bulk Pn.

Quantum Well States and Oscillatory Magnetic Anisotropy in Ultrathin Fe Films

The magnetic anisotropy of Fe film grown on vicinal Ag(1,1,10) surfaces studied with the in situ magneto-optic Kerr effect shows oscillatory behavior. We found that both the in-plane step-induced uniaxial anisotropy and the perpendicular anisotropy measured at low temperature show strong oscillations as a function of Fe thickness with a period of ~ 5.7 monolayers. Such novel oscillation of the anisotropy is attributed to the quantum well states (QWS) of d-band electrons at the Fermi level in the Fe film. These QWS have been observed by angle-resolved photoemission spectroscopy from a Fe wedge films grown on Ag(001) surface.

Element-specific study of epitaxial NiO/Ag/CoO/Fe films grown on vicinal Ag(001) using photoemission electron microscopy

NiO/Ag/CoO/Fe single crystalline films are grown epitaxially on a vicinal Ag(001) substrate using molecular beam epitaxy and investigated by photoemission electron microscopy. We find that after zero-field cooling, the in-plane Fe magnetization switches from parallel to perpendicular direction of the atomic steps of the vicinal surface at thinner CoO thickness but remains in its original direction parallel to the steps at thicker CoO thickness. CoO and NiO domain imaging result shows that both CoO/Fe and NiO/CoO spins are perpendicularly coupled, suggesting that the Fe magnetization switching may be associated with the rotatable-frozen spin transition of the CoO film.

Rotatable magnetic anisotropy of CoO/Fe/Ag(001) in ultrathin regime of the CoO layer

CoO/Fe thin films were grown epitaxially onto vicinal Ag(001) and investigated using magneto-optic Kerr effect, x-ray magnetic circular dichroism (XMCD), and x-ray magnetic linear dichroism (XMLD) techniques. We show that the CoO film in the ultrathin regime does not induce a uniaxial magnetic anisotropy but a coercivity enhancement. This result provides a mechanism for the microscopic origin of the rotatable magnetic anisotropy. XMLD measurement further reveals that the underlying mechanism is that the CoO spins are totally rotatable in the ultrathin regime to follow the Fe magnetization.

Oscillatory Magnetic Anisotropy Originating from Quantum Well States in Fe Films

The magnetic anisotropy of Fe film grown on vicinal Ag(1,1,10) surfaces was studied with the in situ magneto-optic Kerr effect. Below 200 K, strong oscillations of the uniaxial magnetic anisotropy as a function of Fe thickness with a period of 5.7 monolayers are found, which can even cause the easy magnetization axis to oscillate between perpendicular and parallel to the steps. Such novel oscillation of the anisotropy is attributed to the quantum well states of d-band electrons at the Fermi level in the Fe film. This is unlike the previously observed oscillatory behaviors of ferromagnetic films caused by the quantum well states in nonmagnetic interfacing layers.

First-Principles Study of Carbon Monoxide Oxidation on Ag(111) in Presence of Subsurface Oxygen and Stepped Ag(221)

The role of subsurface oxygen in Ag(111) and the step edge of vicinal Ag(221) on CO oxidation were studied by using density functional theory calculations. For high oxygen coverage, the formation of subsurface oxygen is not only energetically favorable but also kinetically likely. CO oxidation with on-surface atomic oxygen is facile with a barrier of about 0.16 eV, whereas the reaction with subsurface oxygen is hindered because of the significant barrier for diffusion of the subsurface oxygen to the surface from the subsurface region. It is found that the adsorption of molecules (CO, atomic O and O2) is stabilized in the presence of subsurface oxygen, and surface reactivity is enhanced. An energetically favorable reaction pathway for CO oxidation with O2 is identified, with a reaction barrier of 0.23 eV via a unique four-center Ο2···CΟ intermediate. A catalytic cycle for CO oxidation on Ag(111) in the presence of subsurface oxygen is proposed. On Ag(221) surfaces, our calculations show that both the adsor...

Analysis of x-ray linear dichroism spectra for NiO thin films grown on vicinal Ag(001)

Antiferromagnetic (AFM) NiO thin films are grown epitaxially on vicinal Ag(118) substrate and investigated by x-ray linear dichroism (XLD). We find that the NiO AFM spins exhibit an in-plane spin-reorientation transition from parallel to perpendicular to the step edge direction with increasing the NiO film thickness. In addition to the conventional ${L}_{2}$ absorption edge, XLD effect at the $\text{Ni}\text{ }{L}_{3}$ absorption edge is also measured and analyzed. The results identify a small energy shift of the ${L}_{3}$ peak. Temperature-dependent measurement confirms that the observed XLD effect in this system at the normal incidence of the x rays originates entirely from the NiO magnetic ordering.