Combined Scanning Tunneling Microscopy Research Articles

Highly Ordered N-Heterocyclic Carbene Monolayers on Cu(111).

The benzannulated N-heterocyclic carbene, 1,3-dibenzylbenzimidazolylidene (NHCDBZ) forms large, highly ordered domains when adsorbed on Cu(111) under ultrahigh vacuum conditions. A combination of scanning tunnelling microscopy (STM), high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) calculations reveals that the overlayer consists of vertical benzannulated NHC moieties coordinating to Cu adatoms. Long-range order results from the placement of the two benzyl substituents on opposite sides of the benzimidazole moiety, with their aromatic rings approximately parallel to the surface. The organization of three surface-bound benzyl substituents from three different NHCs into a triangular array controls the formation of a highly ordered Kagome-like surface lattice. By comparison with earlier studies of NHCs on Cu(111), we show that the binding geometry and self-assembly of NHCDBZ are influenced by intermolecular and adsorbate–substrate interactions and facilitated by the flexibility of the methylene linkage between the N-heterocycle and the aromatic wingtip substituents.

On‐Surface Synthesis of [3]Radialenes via [1+1+1] Cycloaddition

[3]Radialenes are the smallest carbocyclic structures with unusual topologies and cross-conjugated π-electronic structures. Here, we report a novel [1+1+1] cycloaddition reaction for the synthesis of aza[3]radialenes on the Ag(111) surface, where the steric hindrance of the chlorine substituents guides the selective and orientational assembling of the isocyanide precursors. By combining scanning tunneling microscopy, non-contact atomic force microscopy, and time-of-flight secondary ion mass spectrometry, we determined the atomic structure of the produced aza[3]radialenes. Furthermore, two reaction pathways including synergistic and stepwise are proposed based on density functional theory calculations, which reveal the role of the chlorine substituents in the activation of the isocyano groups via electrostatic interaction.

On-Surface Synthesis of [3]Radialenes via [1+1+1] Cycloaddition.

[3]Radialenes are the smallest carbocyclic structures with unusual topologies and cross-conjugated π-electronic structures. Here, we report a novel [1+1+1] cycloaddition reaction for the synthesis of aza[3]radialenes on the Ag(111) surface, where the steric hindrance of the chlorine substituents guides the selective and orientational assembling of the isocyanide precursors. By combining scanning tunneling microscopy, non-contact atomic force microscopy, and time-of-flight secondary ion mass spectrometry, we determined the atomic structure of the produced aza[3]radialenes. Furthermore, two reaction pathways including synergistic and stepwise are proposed based on density functional theory calculations, which reveal the role of the chlorine substituents in the activation of the isocyano groups via electrostatic interaction.

Harnessing the Quantum Behavior of Spins on Surfaces.

The desire to control and measure individual quantum systems such as atoms and ions in a vacuum has led to significant scientific and engineering developments in the past decades that form the basis of today's quantum information science. Single atoms and molecules on surfaces, on the other hand, are heavily investigated by physicists, chemists, and material scientists in search of novel electronic and magnetic functionalities. These two paths crossed in 2015 when it wasfirst clearly demonstrated that individual spins on a surface can be coherently controlled and read out in an all-electrical fashion. The enabling technique is a combination of scanning tunneling microscopy (STM) and electron spin resonance, which offers unprecedented coherent controllability at the Angstrom length scale. This review aims to illustrate the essential ingredients that allow the quantum operations of single spins on surfaces. Three domains of applications of surface spins, namely quantum sensing, quantum control, and quantum simulation, are discussed with physical principles explained and examples presented. Enabled by the atomically-precise fabrication capability of STM, single spins on surfaces might one day lead to the realization of quantum nanodevices and artificial quantum materials at the atomicscale.

Dynamic Structural Changes of Iron Oxide Nanostructures On Cu(111)

Understanding the structural dynamics of supported iron oxide nanostructures (NSs) and their interfaces with metals is essential for the rational design of iron-oxide-based catalysts for a number of major catalytic reactions. In this study, we have constructed well-defined FeO NSs on Cu(111) to study their surface structures and dynamic changes during the redox treatments using the combination of scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). As deposited FeO NSs were found to exhibit various orientations on Cu(111) with the presence of multilayer FeOx, dislocation, and defect lines on the surface plane of FeO. The oxidation of the FeO/Cu(111) surface in O2 was facilitated by the presence of FeO NSs, causing the formation of the stripe phase of Cu2O1+x surrounding FeO. Meanwhile, the oxidation of FeO NSs also led to the formation of FeOx clusters with predominantly Fe3+ species, which were transformed into 3D FeOx nanoparticles (NPs) upon the annealing at 800 K in UHV while no reduction of Fe3+ species was obvious from XPS. Overall, an irreversible structural dynamics was observed at the atomic level for FeO NSs on Cu(111) during the redox treatments, which furthered our understanding of the FeO–Cu interface.

Edge and Point‐Defect Induced Electronic and Magnetic Properties in Monolayer PtSe2

AbstractEdges and point defects in layered dichalcogenides are important for tuning their electronic and magnetic properties. By combining scanning tunneling microscopy (STM) with density functional theory (DFT), the electronic structure of edges and point defects in 2D‐PtSe2 are investigated where the 1.8 eV bandgap of monolayer PtSe2 facilitates the detailed characterization of defect‐induced gap states by STM. The stoichiometric zigzag edge terminations are found to be energetically favored. STM and DFT show that these edges exhibit metallic 1D states with spin polarized bands. Various native point defects in PtSe2 are also characterized by STM. A comparison of the experiment with simulated images enables identification of Se‐vacancies, Pt‐vacancies, and Se‐antisites as the dominant defects in PtSe2. In contrast to Se‐ or Pt‐vacancies, the Se‐antisites are almost devoid of gap states. Pt‐vacancies exhibit defect induced states that are spin polarized, emphasizing their importance for inducing magnetism in PtSe2. The atomic‐scale insights into defect‐induced electronic states in monolayer PtSe2 provide the fundamental underpinning for defect engineering of PtSe2‐monolayers and the newly identified spin‐polarized edge states offer prospects for engineering magnetic properties in PtSe2 nanoribbons.

Chiral structures of 6,12-dibromochrysene on Au(111) and Cu(111) surfaces

Nanoscale low-dimensional chiral architectures are increasingly receiving scientific interest, because of their potential applications in many fields such as chiral recognition, separation and transformation. Using 6,12-dibromochrysene (DBCh), we successfully constructed and characterized the large-area two-dimensional chiral networks on Au(111) and one-dimensional metal-liganded chiral chains on Cu(111) respectively. The reasons and processes of chiral transformation of chiral networks on Au(111) were analyzed. We used scanning tunneling spectroscopy (STS) to analyze the electronic state information of this chiral structure. This work combines scanning tunneling microscopy (STM) with non-contact atomic force microscopy (nc-AFM) techniques to achieve ultra-high-resolution characterization of chiral structures on low-dimensional surfaces, which may be applied to the bond analysis of functional nanofilms. Density functional theory (DFT) was used to simulate the adsorption behavior of the molecular and energy analysis in order to verify the experimental results.

Epitaxial growth and electronic properties of an antiferromagnetic semiconducting VI2 monolayer.

The van der Waals materials down to the monolayer (ML) limit provide a fertile platform for exploring low-dimensional magnetism and developing the novel applications of spintronics. Among them, due to the absence of the net magnetic moment, antiferromagnetic (AFM) materials have received much less attention than ferromagnetic ones. Here, by combining scanning tunneling microscopy and state-of-the-art first-principles calculations, we investigate the preparation, and electronic and magnetic properties of a vanadium(II) iodide (VI2) ML. Single-layer VI2 has been grown by molecular beam epitaxy on Au(111) surfaces. A band gap of 2.8 eV is revealed, indicating the semiconducting nature of the VI2 ML. Vanadium and iodine vacancy defects give rise to additional feature states within the bandgap. A typical 120° AFM spin ordering is maintained in the ML limit of VI2, as revealed by the first-principles calculations. Besides, the AFM coupling is greatly enhanced by slightly decreasing lattice constants. Our work provides an ideal platform for further studying two-dimensional magnetism with non-collinear AFM ordering and for investigating the possibility of realizing the spin Hall effect in the ML limit.

Initial growth behavior of bismuth on Ag(111) and Au(111)

The Rashba effect of surface alloys of semimetallic bismuth (Bi) is strongly related to its specific structural properties. The initial growth behaviors of Bi atoms on Ag(111) substrate and Au(111) substrate are systematically investigated by combining scanning tunneling microscopy (STM) and density flooding theory (DFT) in this paper. Continuous Ag<sub>2</sub>Bi alloy films are formed preferentially at the step edge on Ag(111) held at room temperature; Bi atoms replace step edge atoms with low coordination number and are randomly distributed from single atoms to the forming of long-range ordered Ag<sub>2</sub>Bi alloy phase as the coverage increases to 0.33 ML on Ag(111) held at 570 K. With the coverage increasing, Ag<sub>2</sub>Bi is converted into Bi films with a <inline-formula><tex-math id="M3">\begin{document}$ p\times \sqrt{3} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20211360_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20211360_M3.png"/></alternatives></inline-formula> structure by the dealloying process. In contrary to Ag(111), Bi growth behavior on Au(111) held at room temperature and at 570 K are consistent: Bi atoms are adsorbed preferentially on Au atom pairs with coordination 5 and are dispersed as single atoms and clusters in the densely packed region and the corners of the herringbone reconstruction when coverage level is below 0.40 ML; as the coverage level increases to 0.60 ML, the disordered Bi atoms gradually transform into the long-range ordered (<inline-formula><tex-math id="M4">\begin{document}$ \sqrt{37}\times \sqrt{37} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20211360_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20211360_M4.png"/></alternatives></inline-formula>) phase. Moreover, the adsorption of Bi atoms leads the Au(111) surface strain to gradually release. The different growth behaviors of Bi atoms on Ag(111) substrate and Au(111) substrate suggest that the interaction between Bi atoms and the substrate plays a key role. Bi atoms are adsorbed preferentially around atoms with low surface coordination number under low coverage.

Tracking Interfacial Changes of Graphene/Ge(110) During In-Vacuum Annealing

Graphene quality indicators obtained by Raman spectroscopy have been correlated to the structural changes of the graphene/Germanium interface as a function of in-vacuum thermal annealing. Specifically, it is found that graphene becomes markedly defected at 650 {\deg}C. By combining scanning tunneling microscopy, x-Ray Photoelectron Spectroscopy and Near Edge x-ray Absorption Fine Structure Spectroscopy, we conclude that these defects are due to the release of H_{2} gas trapped at the graphene/Germanium interface. The H_{2} gas was produced following the transition from the as-grown hydrogen-termination of the Ge(110) surface to the emergence of surface reconstructions in the substrate. Interestingly, a complete self-healing process is observed in graphene upon annealing to 800 {\deg}C. The subtle interplay revealed between the microscopic changes occurring at the graphene/Germanium interface and graphene's defect density is valuable for advancing graphene growth, controlled 2D-3D heterogeneous materials interfacing and integrated fabrication technology on semiconductors.

Ultrasensitive and Ultrahigh Resolution Optical Spectroscopy by Combining Scanning Tunneling Microscopy with Plasmonics

Ultrasensitive and Ultrahigh Resolution Optical Spectroscopy by Combining Scanning Tunneling Microscopy with Plasmonics

Real-space subfemtosecond imaging of quantum electronic coherences in molecules

Tracking electron motion in molecules is the key to understanding and controlling chemical transformations. Contemporary techniques in attosecond science are able to generate and trace the consequences of this motion in real time, but not in real space. Scanning tunnelling microscopy, on the other hand, can locally probe the valence electron density in molecules, but cannot alone provide dynamical information at this ultrafast timescale. Here we show that, by combining scanning tunnelling microscopy and attosecond technologies, quantum electronic coherences induced in molecules by <6-fs-long carrier-envelope-phase-stable near-infrared laser pulses can be directly visualized at ångström-scale spatial and subfemtosecond temporal resolutions. We demonstrate concurrent real-space and -time imaging of coherences involving the valence orbitals of perylenetetracarboxylic dianhydride molecules, and full control over the population of the involved orbitals. This approach opens the way to the unambiguous observation and manipulation of electron dynamics in complex molecular systems.

Abiotic Formation of an Amide Bond via Surface-Supported Direct Carboxyl-Amine Coupling.

Amide bond formation is one of the most important reactions in biochemistry, notably being of crucial importance for the origin of life. Herein, we combine scanning tunneling microscopy and X-ray photoelectron spectroscopy studies to provide evidence for thermally activated abiotic formation of amide bonds between adsorbed precursors through direct carboxyl-amine coupling under ultrahigh-vacuum conditions by means of on-surface synthesis. Complementary insights from temperature-programmed desorption measurements and density functional theory calculations reveal the competition between cross-coupling amide formation and decarboxylation reactions on the Au(111) surface. Furthermore, we demonstrate the critical influence of the employed metal support: whereas on Au(111) the coupling readily occurs, different reaction scenarios prevail on Ag(111) and Cu(111). The systematic experiments signal that archetypical bio-related molecules can be abiotically synthesized in clean environments without water or oxygen.

Abiotic Formation of an Amide Bond via Surface‐Supported Direct Carboxyl–Amine Coupling

AbstractAmide bond formation is one of the most important reactions in biochemistry, notably being of crucial importance for the origin of life. Herein, we combine scanning tunneling microscopy and X‐ray photoelectron spectroscopy studies to provide evidence for thermally activated abiotic formation of amide bonds between adsorbed precursors through direct carboxyl–amine coupling under ultrahigh‐vacuum conditions by means of on‐surface synthesis. Complementary insights from temperature‐programmed desorption measurements and density functional theory calculations reveal the competition between cross‐coupling amide formation and decarboxylation reactions on the Au(111) surface. Furthermore, we demonstrate the critical influence of the employed metal support: whereas on Au(111) the coupling readily occurs, different reaction scenarios prevail on Ag(111) and Cu(111). The systematic experiments signal that archetypical bio‐related molecules can be abiotically synthesized in clean environments without water or oxygen.

Imaging the Single-Electron Ln-Ln Bonding Orbital in a Dimetallofullerene Molecular Magnet.

Chemically robust single-molecule magnets (SMMs) with sufficiently high blocking temperatures TB are among the key building blocks for the realization of molecular spintronic or quantum computing devices. Such device applications require access to the magnetic system of a SMM molecule by means of electronic transport, which primarily depends on the interaction of magnetic orbitals with the electronic states of the metallic electrodes. Scanning tunneling microscopy in combination with ab initio calculations allows to directly address the unoccupied component of the single-electron molecular orbital that mediates the ferromagnetic exchange coupling between two 4f ions within a lanthanide endohedral dimetallofullerene deposited on a graphene surface. The single-electron metal-metal bond provides a direct access to the molecule's magnetic system in the transport experiments, paving the way for investigation and controlled manipulation of the spin system of individual dimetallofullerene SMMs, essential for molecularspintronics.

Unraveling the Highly Complex Nature of Antimony on Pt(111)

AbstractUnderstanding the growth behavior of group‐V elements on metal surfaces provides valuable information that can shed light on the feasibility of tailoring atomically thin monoelemental 2D polymorphs composed of pnictogens on these metallic substrates. Here, by combining scanning tunneling microscopy (STM), low energy electron diffraction and Auger electron spectroscopy measurements under ultra‐high vacuum conditions, a wide variety of Sb reconstructions on single‐crystal Pt(111) are identified and characterized. At Sb coverage of ≈0.2 ML, STM data are compatible with a scenario in which Sb atoms are randomly embedded into the topmost layer of Pt, in a substitutional configuration and without establishing a periodic structure. This disorder is robust against thermal annealing and quenching. Increasing the surface coverage and whether or not the sample is annealed, different well‐ordered Sb phases are formed. The Sb structures synthesized at room temperature without any heating process are best interpreted as surface alloys that involve only the first atomic layer. In contrast, experimental evidence points towards the development of multilayer alloy phases for the annealed samples.

On-Surface Synthesis of Boroxine-Based Molecules

The on-surface synthesis of boroxine-containing molecules can be a convenient method of introducing specific functionalities. Here, we show the validity of a previously described synthesis protocol on the Au (111) surface by applying it to a different molecular precursor. We study in detail the assembly of the precursor, highlighting possible intermediate stages of the condensation process. We combine scanning tunneling microscopy and X-ray spectroscopies to fully characterize both the morphology and the electronic properties of the system. DFT calculations are presented to assign the main electronic transitions originating the B K-edge absorption spectrum. The study paves the way to a facile strategy for functionalizing a surface with molecules of tailored sizes and compositions.

Intrinsically Honeycomb-Patterned Hydrogenated Graphene.

Since the advent of graphene ushered the era of 2D materials, many forms of hydrogenated graphene have been reported, exhibiting diverse properties ranging from a tunable bandgap to ferromagnetic ordering. Patterned hydrogenated graphene with micron-scale patterns has been fabricated by lithographic means. Here, successful millimeter-scale synthesis of an intrinsically honeycomb-patterned form of hydrogenated graphene on Ru(0001) by epitaxial growth followed by hydrogenation is reported. Combining scanning tunneling microscopy observations with density-functional-theory (DFT) calculations, it is revealed that an atomic-hydrogen layer intercalates between graphene and Ru(0001). The result is a hydrogen honeycomb structure that serves as a template for the final hydrogenation, which converts the graphene into graphane only over the template, yielding honeycomb-patterned hydrogenated graphene (HPHG). In effect, HPHG is a form of patterned graphane. DFT calculations find that the unhydrogenated graphene regions embedded in the patterned graphane exhibit spin-polarized edge states. This type of growth mechanism provides a new pathway for the fabrication of intrinsically patterned graphene-based materials.

Electron spin resonance of single iron phthalocyanine molecules and role of their non-localized spins in magnetic interactions.

Electron spin resonance (ESR) spectroscopy is a crucial tool, through spin labelling, in investigations of the chemical structure of materials and of the electronic structure of materials associated with unpaired spins. ESR spectra measured in molecular systems, however, are established on large ensembles of spins and usually require a complicated structural analysis. Recently, the combination of scanning tunnelling microscopy with ESR has proved to be a powerful tool to image and coherently control individual atomic spins on surfaces. Here we extend this technique to single coordination complexes-iron phthalocyanines (FePc)-and investigate the magnetic interactions between their molecular spin with either another molecular spin (in FePc-FePc dimers) or an atomic spin (in FePc-Ti pairs). We show that the molecular spin density of FePc is both localized at the central Fe atom and also distributed to the ligands (Pc), which yields a strongly molecular-geometry-dependent exchange coupling.

Hydration of iodine adsorbed on the Au(111) surface

The hydration of halogens has been widely researched because of its close relation with the water desalination and biochemical reactions. In this work, by a combination of scanning tunneling microscopy and X-ray photoelectron spectroscopy, we have explored the hydration process of iodine via the Eley-Rideal process on the Au(111) surface. Moreover, the hydration process of iodine with the presence of the NiPc self-assembled network as a template has also been investigated, where the stepwise hydration of iodine at room temperature can be visualized on Au(111).