Surface Step Edges Research Articles

In situ surface evolution dynamics of external-electric-field-triggered structural oscillation on Au(111)

Surface nanostructures serve as an essential role in determining intrinsic physical features and device performance in solid materials. Here, this work systematically investigates the surface dynamical evolution at the nanoscale on Au(111) induced by a “tip-to-surface” external electric field through a scanning tunneling microscope (STM). The Au(111) steps exhibit a “compact to fractal” reconstruction under a relatively high electric field, in which the transition is strengthened with increasing the applied electric field. Inversely, a “fractal to compact” morphological transition enables to be self-triggered at Au(111) surface steps upon a normal STM imaging electric field (very low). These two reversible structural changes are attributed to the diffusion-limited aggregation mechanism where the diffusion barriers were quantified as 0.64–0.75 eV varied with the regularity in step edges. In addition, we further simulate a “terrace-edge-kink” model to trace the effect of atomic coordination on structural transition, determining the surface step edge stability. This study presents insight into morphological and structural transformation at surface steps induced by variant external electric fields, establishing a deep understanding in the nature of surface evolution dynamics.

Imaging Friedel oscillations in rhombohedral trilayer graphene

Quasiparticle interference-induced spatial standing waves in local density of states, i.e., Friedel oscillations, are first visualized in rhombohedral trilayer graphene (rTG) by using scanning tunneling microscopy and spectroscopy. We show that the long-range standing-wave patterns of rTG can be created not only by the scattering off usual potential barriers including defects and step edges, but also by ABC-ABA stacking domain walls. For step edges as scatterers, both the surface step edge and the underlying step edge can effectively generate quasiparticle standing waves on the surface layer. For all observed types of scatterers in rTG, the Friedel oscillations always exhibit a $1/r$ spatial decay. This decay rate of Friedel oscillations is consistent with that in bilayer graphene, while slower than that in monolayer graphene, directly confirming previous theoretical predictions. Our results provide fundamental knowledge of the nature of scatterers in rTG, which would help us to better understand their microscopic scattering mechanisms.

Growth Mechanism of Single-Domain Monolayer MoS2 Nanosheets on Au(111) Revealed by In Situ Microscopy: Implications for Optoelectronics Applications

The nucleation and growth of single-layer molybdenum disulfide single-domain nanosheets is investigated by in situ low-energy electron microscopy. We study the growth of micrometer-sized flakes and the correlated flattening process of the gold surface for three different elevated temperatures. Furthermore, the influence of surface step edges on the molybdenum disulfide growth process is revealed. We show that both nanosheet and underlying terrace grow simultaneously by pushing the surface step in the expansion process. Our findings point to an optimized growth procedure allowing for step-free, single-domain, single-layer islands of several micrometers in size, which is likely transferable to other transition-metal dichalcogenides (TMDs), offering a very fine degree of control over the TMD nanosheet structure and thickness.

Surface step edge-assisted monolayer epitaxy of α-antimonene on SnSe2 substrate

We demonstrate a strategy of van der Waals (vdW) epitaxy assisted by surface step edges. Different from the usual vdW epitaxy where the growth is initiated by the vdW interactions with the substrate, the step edge-assisted epitaxy is most likely initiated by the formation of a strong valence bond at the periphery of surface step edges, thus allowing for the growth of strained vdW monolayers. With this strategy, we have successfully grown the α-antimonene monolayer with a puckered honeycomb lattice on the SnSe2 substrate with a high density of surface step edges, thus forming a horizontal heterostructure. This study paves a way toward tuning the morphology and properties of epitaxial vdW materials via a strong valence bond at the boundary between the epilayer and substrate.

Three-Dimensional Charge Density Wave and Surface-Dependent Vortex-Core States in a Kagome Superconductor CsV3Sb5

The transition-metal-based kagome metals provide a versatile platform for correlated topological phases hosting various electronic instabilities. While superconductivity is rare in layered kagome compounds, its interplay with nontrivial topology could offer an engaging space to realize exotic excitations of quasiparticles. Here, we use scanning tunneling microscopy (STM) to study a newly discovered Z$_2$ topological kagome metal CsV$_3$Sb$_5$ with a superconducting ground state. We observe charge modulation associated with the opening of an energy gap near the Fermi level. When across single-unit-cell surface step edges, the intensity of this charge modulation exhibits a {\pi}-phase shift, suggesting a three-dimensional 2$\times$2$\times$2 charge density wave ordering. Interestingly, a robust zero-bias conductance peak is observed inside the superconducting vortex core on the Cs 2$\times$2 surfaces that does not split in a large distance when moving away from the vortex center, resembling the Majorana bound states arising from the superconducting Dirac surface states in Bi$_2$Te$_3$/NbSe$_2$ heterostructures. Our findings establish CsV$_3$Sb$_5$ as a promising candidate for realizing exotic excitations at the confluence of nontrivial lattice geometry, topology and multiple electronic orders.

Evidence of a topological edge state in a superconducting nonsymmorphic nodal-line semimetal

Topological materials host fascinating low dimensional gapless states at the boundary. As a prominent example, helical topological edge states (TESs) of two-dimensional topological insulators (2DTIs) and their stacked three-dimensional (3D) equivalent, weak topological insulators (WTIs), have sparked research enthusiasm due to their potential application in the next generation of electronics/spintronics with low dissipation. Here, we propose layered superconducting material CaSn as a WTI with nontrivial Z2 as well as nodal line semimetal protected by crystalline non-symmorphic symmetry. Our systematic angle-resolved photoemission spectroscopy (ARPES) investigation on the electronic structure exhibits excellent agreement with the calculation. Furthermore, scanning tunnelling microscopy/spectroscopy (STM/STS) at the surface step edge shows signatures of the expected TES. These integrated evidences from ARPES, STM/STS measurement and corresponding ab initio calculation strongly support the existence of TES in the non-symmorphic nodal line semimetal CaSn, which may become a versatile material platform to realize multiple exotic electronic states as well as topological superconductivity.

Directly correlated microscopy of trench defects in InGaN quantum wells

Directly correlated measurements of the surface morphology, light emission and subsurface structure and composition were carried out on the exact same nanoscale trench defects in InGaN quantum well (QW) structures. Multiple scanning probe, scanning electron and transmission electron microscopy techniques were used to explain the origin of their unusual emission behaviour and the relationship between surface morphology and cathodoluminescence (CL) redshift. Trench defects comprise of an open trench partially or fully enclosing material in InGaN QWs with different CL emission properties to their surroundings. The CL redshift was shown to typically vary with the width of the trench and the prominence of the material enclosed by the trench above its surroundings. Three defects, encompassing typical and atypical features, were prepared into lamellae for transmission electron microscopy (TEM). A cross marker technique was used in the focused ion beam-scanning electron microscope (FIB-SEM) to centre the previously characterised defects in each lamella for further analysis. The defects with wider trenches and strong redshifts in CL emission had their initiating basal-plane stacking fault (BSF) towards the bottom of the QW stack, while the BSF formed near the top of the QW stack for a defect with a narrow trench and minimal redshift. The raised-centre, prominent defect showed a slight increase in QW thickness moving up the QW stack while QW widths in the level-centred defect remained broadly constant. The indium content of the enclosed QWs increased above the BSF positions up to a maximum, with an increase of approximately 4% relative to the surroundings seen for one defect examined. Gross fluctuations in QW width (GWWFs) were present in the surrounding material in this sample but were not seen in QWs enclosed by the defect volumes. These GWWFs have been linked with indium loss from surface step edges two or more monolayers high, and many surface step edges appear pinned by the open trenches, suggesting another reason for the higher indium content seen in QWs enclosed by trench defects.

Anchoring single Pt atoms and black phosphorene dual co-catalysts on CdS nanospheres to boost visible-light photocatalytic H2 evolution

• Decorate black phosphorene onto CdS nanospheres to create BP/CdS heterostructures by a grinding and sonication method. • Anchor Pt single-atoms on BP/CdS heterostructures via a photo-reduction deposition procedure. • Achieve a hydrogen evolution rate that is 96 times greater than that of CdS nanospheres, alongside an AQE of 46% at 420 nm. • Unravel interfacial electronic interactions, photoexcited charge-carrier dynamics and photocatalytic mechanism. • Pave a path to the rational design of viable co-catalysts at the nanoscale and atomic level. Co-catalysts play a crucial role in semiconductor-based artificial photosynthesis and stabilizing co-catalysts on heterojunction photocatalysts is essential for achieving high photocatalytic efficiency. Herein, we report novel dual co-catalysts of black phosphorene (BP) and single Pt atoms on CdS nanospheres. BP/CdS heterostructures are prepared by grinding and sonication and then single Pt atoms are deposited onto BP/CdS through a photo-reduction procedure. In addition to being anchored on the surface step edges of CdS nanospheres, Pt single-atoms with positive charge are embedded on Cd vacancies and stabilized by Pt-S covalent bonds. Single Pt atoms are immobilized on the surface of BP as well. The as-prepared Pt-BP/CdS composites are evaluated toward visible-light-driven hydrogen generation. In a range of Pt and BP loading contents, 0.5 wt% Pt-5 wt% BP/CdS composites display the greatest photoactivity and outperform pristine CdS nanospheres by a factor of 96 in terms of hydrogen evolution rate, alongside a remarkable apparent quantum efficiency of 46% at 420 nm. In-depth analyses on interfacial electronic interactions and photoexcited charge-carrier dynamics demonstrate that both single Pt atoms and BP strongly interact with CdS and synergistically steer spatial charge separation, thereby boosting photocatalytic performance. This study may pave a path to the rational design of co-catalysts at the nanoscale and atomic level for solar-to-chemical energy conversion and beyond.

Temperature effects on the nano-friction across exposed atomic step edges

Despite the practical and fundamental importance of friction, there are many issues that are still needed to be explored and revealed; one of them is how friction changes across an atomic surface step edge at different temperatures. In this article, a friction force microscope under ultrahigh vacuum conditions has been used to study the temperature dependence of nanoscale friction between a silicon tip and a freshly cleaved HOPG surface with exposed single- and double-layer step edges. For the upward scanning from the lower terrace to the upper terrace, a large resistive force which is linearly dependent on the normal force is observed. A similar resistive force but with a smaller magnitude together with an assistive force is observed for the downward scanning; however, the resistive force is found to be independent of the normal force while the assistive force increases with the normal force. Besides, the resistive force for the double-layer step edge is found to be twice as high as that of the single-step edge, while the assistive force seems to be less influenced by the height of the step. Finally, the experimental results reveal that temperature has a negligible effect on the friction coefficients at the step edges, which is inconsistent with the thermal activated process where friction should decrease with temperature. Based on the theoretical studies, this observation can be explained by a process where the temperature effect is very small compared with the edge Schwoebel–Ehrlich barrier. These findings may help in understanding the temperature effects on macroscopic friction having a lot of step edges at the interface.

Robust A -Type Order and Spin-Flop Transition on the Surface of the Antiferromagnetic Topological Insulator MnBi2Te4

Here, we present microscopic evidence of the persistence of uniaxial A-type antiferromagnetic order to the surface layers of MnBi_{2}Te_{4} single crystals using magnetic force microscopy. Our results reveal termination-dependent magnetic contrast across both surface step edges and domain walls, which can be screened by thin layers of soft magnetism. The robust surface A-type order is further corroborated by the observation of termination-dependent surface spin-flop transitions, which have been theoretically proposed decades ago. Our results not only provide key ingredients for understanding the electronic properties of the antiferromagnetic topological insulator MnBi_{2}Te_{4}, but also open a new paradigm for exploring intrinsic surface metamagnetic transitions in natural antiferromagnets.

Emerging edge states on the surface of the epitaxial semimetal CuMnAs thin film

Epitaxial thin films of CuMnAs have recently attracted attention due to their potential to host relativistic antiferromagnetic spintronics and exotic topological physics. Here, we report on the structural and electronic properties of a tetragonal CuMnAs thin film studied using scanning tunneling microscopy (STM) and density functional theory (DFT). STM reveals a surface terminated by As atoms, with the expected semi-metallic behavior. An unexpected zigzag step edge surface reconstruction is observed with emerging electronic states below the Fermi energy. DFT calculations indicate that the step edge reconstruction can be attributed to an As deficiency that results in changes in the density of states of the remaining As atoms at the step edge. This understanding of the surface structure and step edges on the CuMnAs thin film will enable in-depth studies of its topological properties and magnetism.

Quantifying Capacitive‐Like and Battery‐Like Charge Storage Contributions Using Single‐Nanoparticle Electro‐optical Imaging

AbstractPseudocapacitors promise to combine the high‐rate capability of electrochemical capacitors with the high energy‐density of batteries. A major challenge in the field is to demonstrate that pseudocapacitors behave electrochemically like a capacitor and the charge storage process is faradaic in nature. It is challenging to do so because pseudocapacitive charging has the same electrical signatures as non‐faradaic electrical double‐layer charging. Here we use electro‐optical imaging to measure Li‐ion insertion reactions in single WO3 nanoparticles. On average, these WO3 particles exhibit a hybrid charge storage mechanism: both diffusion‐limited (battery‐like) and pseudocapacitive (capacitor‐like) mechanisms contribute to the total charge stored. Individual particles exhibit different charge storage mechanisms at the same applied potential. Longer nanorods store more pseudocapacitive charge than shorter nanorods, presumably due to 1) a surface step edge gradient that exposes large hexagonal window Li‐ion binding sites along the nanorod length and/or 2) higher structural water content that influences the Li‐ion binding energetics and diffusion behavior. Importantly, we quantified the penetration depth of Li‐ion insertion and concluded that Li ions insert as deep as two‐unit cells below the surface. The methodology presented herein can be applied to a wide range of solid‐state ion‐insertion materials.

Probing image potential states on the surface of the topological semimetal antimony

A point charge near the surface of a topological insulator (TI) with broken time-reversal symmetry is predicted to generate an image magnetic charge in addition to an image electric charge. We use scanning tunneling spectroscopy to study the image potential states (IPS) of the topological semimetal Sb(111) surface. We observe five IPS with discrete energy levels that are well described by a one-dimensional model. The spatial variation of the IPS energies and lifetimes near surface step edges shows the first local signature of resonant interband scattering between IPS, which suggests that image charges too may interact. Our work motivates the exploration of the TI surface geometry necessary to realize and manipulate a magnetic charge.

Atomic-Level Observation of Electrochemical Platinum Dissolution and Redeposition.

An understanding of electrochemical dynamics at solid-liquid interfaces is essential to develop advanced batteries and fuel cells and so on. For example, an atomic-level understanding of electrochemical Pt dissolution and redeposition behavior is crucial for optimizing the material design and operating conditions of polymer electrolyte fuel cells (PEFCs). This understanding enables the prevention of the degradation of Pt nanoparticles used as electrocatalysts. However, the mechanisms of Pt dissolution and redeposition are still not fully understood due to the lack of spatial resolution available with current observation techniques. Here, we have revealed for the first time atomic-level electrochemical Pt dissolution and redeposition behavior using in-house-developed observation techniques. We achieved atomic-level observations of closed-cell type liquid electrochemical transmission electron microscopy (TEM) by combining in-house-developed microelectromechanical system (MEMS) chips as an electrochemical cell, an aberration-corrected TEM apparatus, and an energy filter. Furthermore, accurate and stable potential control was achieved using an in-house-developed reversible hydrogen electrode (RHE) with a liquid junction connected to the outside of a TEM specimen holder. Our observation results confirmed that Pt dissolves from surface step edges layer-by-layer, as previously predicted by the density functional theory (DFT). The observation techniques developed are also applicable to other research fields concerning electrochemistry.

Molecular-Scale Mechanistic Investigation of Oxygen Dissociation and Adsorption on Metal Surface-Supported Cobalt Phthalocyanine.

Ultrahigh vacuum scanning tunneling microscopy and density functional theory are used to investigate adsorption of oxygen on cobalt phthalocyanine (CoPc), a promising nonprecious metal oxygen reduction catalyst, supported on Ag(111), Cu(111), and Au(111) surfaces at the molecular scale. Four distinct molecular and atomic oxygen adsorption configurations are observed for CoPc supported on Ag(111) surfaces, which are assigned as O2/CoPc/Ag(111), O/CoPc/Ag(111), CoPc/(O)2/Ag(111), and (O)2/CoPc/Ag(111). In contrast, no oxygen adsorption is observed for CoPc supported on Cu(111) and Au(111) surfaces. The results show that for Ag(111), atomic O that is predominantly catalytically produced from the dissociation of molecular O2 at metal surface step edges is responsible for the observed adsorption configurations. However, Cu(111) binds atomic O too strongly, and Au(111) does not produce atomic O. These results show the active role of the supporting metal surface in facilitating oxygen adsorption on CoPc.

Airborne contamination of graphite as analyzed by ultra-violet photoelectron spectroscopy

This paper reports the surface chemistry of highly ordered pyrolytic graphite (HOPG) upon exposure to ambient air. Ultra-violet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) measurements were collected from freshly-cleaved HOPG in ultra-high vacuum and after exposure to ambient air. We observed an immediate surface contamination during exfoliation of HOPG in ambient air, which we attribute to functionalization of surface defects and step edges. Additional air exposures result in further growth of the contamination layer, as well as dynamic composition changes from more weakly bound to more strongly bound species over time. This work is the first to show the immediate contamination of graphite upon exfoliation in air and the dynamic exchange of surface contaminants over long exposure times.

Structure-guided shape-preserving mesh texture smoothing via joint low-rank matrix recovery

Mesh texture smoothing (MTS) seeks to smooth out the detailed appearances in noise-free surfaces, while preserving the intrinsic geometric properties. However, the intricacy of mesh texture patterns commonly results in side-effects, including remnant textures, improperly-smoothed structures and distorted shapes. We propose a new joint low-rank matrix recovery (J-LRMR) model to the problem of MTS with the properties of texture smoothing, structure preservation, and shape fitting. J-LRMR benefits from our three key observations: (1) local surface patches sharing approximate geometric properties always exist within an original surface (OS); (2) the OS comprises three geometric properties, i.e., details, structures, and the smooth-varying shape, where the structures construct the step-edge surface (SS), but more importantly the later two components contribute to forming the underlying surface (US); (3) the US normals can be defined as signals over both the OS and SS. By exploring the three observations, we first collect similar patches together by the local iterative closest point technique on the OS, and concurrently optimize the OS to produce the SS by ℓ0 normal minimization. Second, we pack the normals on the similar patches of both the OS and SS into the patch-group matrix (PGM) for rank analysis. The rank of sub-PGM from the OS is high due to suffering from details (textures), while the rank of sub-PGM from the SS is low but does not have a smooth shape. Third, we formulate a structure-guided shape-preserving optimization framework by non-local low-rank matrix recovery to produce the normals of US containing both the structures and the smooth-varying shape. Finally, we update the vertex positions of OS to fit the recovered normals of US by Poisson reconstruction. We have verified our method on a variety of surfaces with abundant details, and compared it with several state-of-the-art methods. Both visual and quantitative comparisons show that our method can better remove mesh textures with a minimal disturbance of the underlying surface.

Evidence of nematic order and nodal superconducting gap along [110] direction in RbFe2As2

Unconventional superconductivity often intertwines with various forms of order, such as the nematic order which breaks the rotational symmetry of the lattice. Here we report a scanning tunneling microscopy study on RbFe2As2, a heavily hole-doped Fe-based superconductor (FeSC). We observe significant symmetry breaking in its electronic structure and magnetic vortex which differentiates the (π, π) and (π, -π) directions of the unfolded Brillouin zone. It is thus a novel nematic state, distinct from the nematicity of undoped/lightly-doped FeSCs which breaks the (π, 0)/(0, π) equivalence. Moreover, we observe a clear V-shaped superconducting gap. The gap is suppressed on surface Rb vacancies and step edges, and the suppression is particularly strong at the [110]-oriented edges. This is possibly due to a {{d}}_{{{x}}^2 - {{y}}^2} like pairing component with nodes along the [110] directions. Our results thus highlight the intimate connection between nematicity and superconducting pairing in iron-based superconductors.

Tuning electronic properties by oxidation-reduction reactions at graphene-ruthenium interfaces

Mass production of graphene is associated with the growth on catalysts used also in other chemical reactions. We exploit the oxidation-reduction to tailor the properties of single layer graphene domains with incorporated bi-layer patches on ruthenium. Using photoelectron spectromicroscopy techniques, we find that oxygen, intercalating under single layer and making it p-doped by the formation of Ru-Ox, does not intercalate under the bilayer patches with n-doped upper layer, but decorates them under single layer surrounding creating lateral p-n junctions with chemical potential difference of 1.2 eV. O-reduction by thermal treatment in vacuum results in C-vacancy defects enhancing electronic coupling of remained graphene to Ru, whereas in H2, vacancy formation is suppressed. For the domains below 15–25 μm size, after O-reduction in H2, graphene/Ru coupling is restored, while wrinkle pattern produced by O-intercalation is irreversible and can trap reaction products between the wrinkles and Ru surface step edges. In fact, in certain regions of bigger domains, the products, containing H2O and/or its fragments, remain at the interface, making graphene decoupled and undoped.

Crystal step edges can trap electrons on the surfaces of n-type organic semiconductors

Understanding relationships between microstructure and electrical transport is an important goal for the materials science of organic semiconductors. Combining high-resolution surface potential mapping by scanning Kelvin probe microscopy (SKPM) with systematic field effect transport measurements, we show that step edges can trap electrons on the surfaces of single crystal organic semiconductors. n-type organic semiconductor crystals exhibiting positive step edge surface potentials display threshold voltages that increase and carrier mobilities that decrease with increasing step density, characteristic of trapping, whereas crystals that do not have positive step edge surface potentials do not have strongly step density dependent transport. A device model and microelectrostatics calculations suggest that trapping can be intrinsic to step edges for crystals of molecules with polar substituents. The results provide a unique example of a specific microstructure–charge trapping relationship and highlight the utility of surface potential imaging in combination with transport measurements as a productive strategy for uncovering microscopic structure–property relationships in organic semiconductors.