Resolved Scanning Tunneling Microscopy Images Research Articles

Wavefront distortion correction in scanning tunneling microscope image.

We report an algorithm to identify and correct distorted wavefronts in atomic resolution scanning tunneling microscope images. This algorithm can be used to correct nonlinear in-plane distortions without prior knowledge of the physical scanning parameters, the characteristics of the piezoelectric actuator, or individual atom positions. The 2D image is first defined as a sum of sinusoidal plane waves, where a nonlinear distortion renders a curve for an otherwise ideal linear wavefront. Using the Fourier transforms of local areas of the image, the algorithm generates a wavefront vector field. The identified wavefronts are subsequently linearized for each plane wave without changing lattice orders, giving rise to distortion corrections. Our algorithm is complementary to conventional post-processing algorithms that require prior detection of real space features, which can also be used to correct nonlinear distortions in 2D images acquired by other microscopy techniques.

Synthesis and properties of phosphorus nanostructures on Au(110).

Phosphorene, a semiconducting two-dimensional material, has recently attracted huge interest due to its potential applications in opto-electronics. The first attempts to synthesize phosphorene were based mainly on mechanical and chemical exfoliations. A few years later, different groups reported the synthesis of phosphorene using the molecular beam epitaxy process, which opened the way for research on physical properties. In this article, we report phosphorus growth on an Au(110) substrate via molecular beam epitaxy. The atomically resolved scanning tunneling microscopy images exhibit a self-assembled phosphorus-dimer structure, which is converted into phosphorus chains when increasing the phosphorus coverage. The chemical composition of the obtained structures is determined via Auger electron and X-ray photoelectron spectroscopies. Density functional theory calculations support the experimental findings.

Aperiodic Modulation of Graphene Driven by Oxygen-Induced Reconstruction of Rh(110).

Artificial nanostructuring of graphene has served as a platform to induce variations in its structural and electronic properties, fostering the experimental observation of a wide and fascinating phenomenology. Here, we present an approach to graphene tuning, based on Rh(110) surface reconstruction induced by oxygen atoms intercalation. The resulting nanostructured graphene has been characterized by scanning tunneling microscopy (STM) complemented by low-energy electron microscopy (LEEM), micro low-energy electron diffraction (μ-LEED), micro angle-resolved photoemission spectroscopy (μ-ARPES), and micro X-ray photoelectron spectroscopy (μ-XPS) measurements under ultrahigh vacuum (UHV) conditions at room temperature (RT). It is found that by fine-tuning the O2 exposure amount, a mixture of missing row surface reconstructions of the metal surface below the graphene layer can be induced. This atomic rearrangement under the graphene layer results in aperiodic patterning of the two-dimensional (2D) material. The electronic structure of the resulting nanostructured graphene is dominated by a linear dispersion of the Dirac quasiparticles, characteristic of its free-standing state but with a p-doping character. The local effects of the underlying missing rows on the interfacial chemistry and on the quasiparticle scattering processes in graphene are studied using atomically resolved STM images. The possibilities offered by this nanostructuring approach, which consists in inducing surface reconstructions under graphene, could provide a novel tuning strategy for this 2D material.

Magnetic Field-Induced Spin Nematic Phase Up to Room Temperature in Epitaxial Antiferromagnetic FeTe Thin Films Grown by Molecular Beam Epitaxy.

Electronic nematicity, where strong correlations drive electrons to align in a way that lowers the crystal symmetry, is ubiquitous among unconventional superconductors. Understanding the interplay of such a nematic state with other electronic phases underpins the complex behavior of these materials and the potential for tuning their properties through external stimuli. Here, we report magnetic field-induced spin nematicity in a model system tetragonal FeTe, the parent compound of iron chalcogenide superconductors, which exhibits a bicollinear antiferromagnetic order. The studies were conducted on epitaxial FeTe thin films grown on SrTiO3(001) substrates by molecular beam epitaxy, where the bicollinear antiferromagnetic order was confirmed by in situ atomic resolution scanning tunneling microscopy imaging. A 2-fold anisotropy is observed in in-plane angle-dependent magnetoresistance measurements, indicative of magnetic field-induced nematicity. Such 2-fold anisotropy persists up to 300 K, well-above the bicollinear antiferromagnetic ordering temperature of 75 K, indicating a magnetic field-induced spin nematic phase up to room temperature in the antiferromagnet FeTe.

Atomically resolved low-temperature scanning tunneling microscope operating in a 22 T water-cooled magnet

We present the design and construction of a nonmetallic tip-sample mechanical loop featured Scanning Tunneling Microscope (STM) that operates in a 22 T water-cooled magnet at a low temperature of l.8 K. The STM head mainly consists of a spider-drive motor, stand-alone scanner, moveable sapphire sample holder, and sapphire frame. All parts exist in the tip-sample mechanical loop are made of sapphire to reduce the interference from high magnetic fields. Except for the necessary movement of the tip and scanner, all STM parts are stationary. More importantly, the tip-sample mechanical loop is separate from the motor after detecting the tunneling current, which helps prevent the high voltage signal interference from entering the tip-sample junction, leading to a high stable imaging. A Janis liquid helium cryostat is used to obtain a variable temperature range from 1.8 K to 300 K, and the STM head is cooled down via helium exchange gas. The STM head hangs at the bottom of a probe with a two-stage spring suspension to prevent the huge vibration generated by the water-cooled magnet from entering the tip-sample junction. The performance is demonstrated by atomically resolved STM images of graphite surface at 0 T and 22.8 T under room temperature. Furthermore, the obtained atomic-resolution images of NbSe2 at 1.8 K and 22 T, as well as high-resolution dI/dV spectrums at temperatures from 1.8 K to 8.5 K and magnetic fields from 0 T to 22 T are displayed. This is the first STM capable of atomic-resolution imaging and dI/dV measurement at 1.8 K in a 22 T water-cooled magnet. The high immunity to the magnetic field makes the nonmetallic tip-sample mechanical loop widely useable for atomic-resolution STM imaging in ultra-high magnetic field conditions.

Framework for Donor-Qubit Spatial Metrology in Silicon with Depths Approaching the Bulk Limit

Impurities in silicon are fundamental to a variety of modern nanoscale technologies working in both classical and quantum regimes of operation. The aggressive miniaturization of electronic devices has reduced their size to the nanometer scale, where the exact count and positioning of a few impurity atoms dictates their overall operation and performance. In the emerging area of quantum hardware development, single-impurity spins in silicon form excellent qubits and identifying their exact locations is important to engineer two-qubit interactions for high-fidelity quantum operations and the associated quantum control systems. This work reports a theoretical framework for the spatial metrology of single-impurity atoms in silicon with exact atomic precision for impurity depths approaching the bulk limit. The application of a carefully designed electric field pulls the impurity wave functions toward the surface, leading to spatially resolved scanning tunneling microscope images of electronic states exhibiting features that distinctly depend on the exact locations of the impurity atoms beneath the silicon surface. After verification of the developed metrology technique for individual atom positions, we train a machine-learning algorithm that can autonomously perform the metrology with high throughput in the presence of noise commensurate with experimental measurements. A future experimental implementation of the established capability for impurity-atom characterization is anticipated to play an important role in the design of a wide range of electronic and quantum devices.

Valley-protected one-dimensional states in small-angle twisted bilayer graphene

Theory predicts that the application of an electric field breaks the inversion symmetry of AB and BA stacked domains in twisted bilayer graphene, resulting in the formation of a triangular network of one-dimensional valley-protected helical states. This two-dimensional network of one-dimensional states has been observed in several studies, but direct experimental evidence that the electronic transport in these one-dimensional states is valley-protected is still lacking. In this study, we report the existence of the network in small-angle twisted bilayer graphene at room temperature. Moreover, by analyzing Fourier transforms of atomically resolved scanning tunnelling microscopy images of minimally twisted bilayer graphene, we provide convincing experimental evidence that the electronic transport in the counter-propagating one-dimensional states is indeed valley protected.

Structure of one-dimensional monolayer Si nanoribbons on Ag(111)

One-dimensional silicon nanoribbons (SiNRs) have the potential for application to future electronic devices because of their compatibility with current silicon-based electronic devices and their theoretical outstanding electronic properties, such as the size-dependent band gap. Here we grew SiNRs by the deposition of Si on a Ag(111) surface. We investigated SiNRs on the Ag(111) surface using a combination of scanning tunneling microscopy (STM), atomic force microscopy (AFM), and density functional theory (DFT) calculations. The atomic resolution STM and AFM images revealed that SiNRs have the same width and align along the equivalent orientations of Ag(111). The main body of SiNRs was found to be symmetric about the long axes of the ribbons, but the terminals of SiNRs break the symmetry. The DFT calculation results revealed a buckled single-layer structure of SiNRs, which is composed of Si hexagonal and tetragonal rings.

On-Surface Growth of Single-Layered Homochiral 2D Covalent Organic Frameworks by Steric Hindrance Strategy

Investigation of chirality in on-surface synthesis is of significance not only for fabricating atomically precise covalently bonded chiral species but also for unveiling chiral phenomena involving chemical reactions. In this contribution, we present the growth of single-layered homochiral 2D covalent organic frameworks (COFs) on surfaces based on a steric hindrance strategy, by which both the chiral expression of the prochiral precursor and the newly formed C═N bonds are successfully steered. When coupling a tritopic monomer with the prochiral ditopic molecule with phenyl substituents, two enantiomers of the precursor are randomly integrated in the product via variable C═N linkages, resulting in distorted hexagonal frameworks without chiral expression. After equipping the prochiral precursor with more hindered bulky substituents, highly regular homochiral 2D COFs are fabricated, in which only one of the enantiomers of the prochiral precursor is integrated, and all C═N linkages possess the same configuration. Structural analysis based on high resolution scanning tunneling microscopy images and theoretical simulations indicate that the homochiral 2D COFs are generated through an enantioselective on-surface polymerization driven by the steric hindrance effect. This result not only benefits understanding and controlling chirality in on-surface synthesis but also provides a new approach for the growth of highly regular COFs on surfaces.

Atomic insights into single-layer and bilayer germanene on Al(111) surface

The artificial postgraphene two-dimensional (2D) materials termed 2D-Xenes, which consist of atoms arranged in atomically thin sheets with a honeycomb lattice, have received extensive attention because of their unique electronic properties. Insights into the growth mechanism at the atomic scale constitute a major challenge for realizing high-quality 2D materials, as often the substrate and deposited atoms tend to form alloys. Here, we present evidence for intrinsic growth of germanene on Al (111), i.e. a postgraphene elemental 2D atomically thin sheet. The atomic-scale extra-high resolution scanning tunneling microscopy (STM) characterization supported by density functional theory–based calculations provides deeper insight into the true atomic arrangement for the (2 × 2)/Al (111) (3 × 3) and (√3 × √3)R (30°)/Al (111) (√7 × √7)R (±19.1°) germanene phases and unambiguously confirms their real 2D honeycomb nature, which till now lacks convincing experimental proof. First-principles calculations suggest atomic models with strongly buckled (2 × 2) and (√3 × √3)R (30°) germanene, with one of eight and one of six Ge atoms protruding upward, respectively, providing perfect atom-by-atom agreement with the true atomically resolved STM images for both germanene phases. Moreover, the experimentally observed phenomena of local removal of the buckling is consistent with the formation of a flat germanene structure. Furthermore, the formation of bilayer germanene with AB-stacking is demonstrated by means of re-deposition of germanene flakes from the STM tip on top of single layer germanene, indicating that germanene flakes can be easily torn off from the aluminum substrate and attached to the STM tip, retaining their 2D configuration. On the other hand, the formation of bilayer germanene suggests an increased electronic decoupling of the bottom germanene layer from the substrate, in contrast to single layer germanene.

Observation of Simplest Water Chains on Surface Stabilized by a Hydroxyl Group at One End**Supported by the National Natural Science Foundation of China under Grant Nos 11774395 and 91753136, the Beijing Natural Science Foundation under Grant No 4181003, the Strategic Priority Research Program (B) of the Chinese Academy of Sciences under Grant Nos XDB30201000 and XDB28000000.

The key to fully understanding water-solid interfaces relies on the microscopic nature of hydrogen bond networks, including their atomic structures, interfacial interactions, and dynamic behaviors. Here, we report the observation of two types of simplest water chains on Au(111) surface which is expected unstable according to the rules of hydrogen network on noble metal surfaces. A common feature at the end of chain structures is revealed in high resolution scanning tunneling microscopy images. To explain the stability in observed hydrogen bond networks, we propose a structure model of the water chains terminated with a hydroxyl group. The model is consistent with detailed image analysis and molecular manipulation. The observation of simplest water chains suggests a new platform for exploring fundamental physics in hydrogen bond networks on surfaces.

Fabricating Fe nanocrystals via encapsulation at the graphite surface

In this paper, the authors describe the conditions under which Fe forms encapsulated nanocrystals beneath the surface of graphite, and they characterize these islands (graphite + Fe) thoroughly. The authors use the experimental techniques of scanning tunneling microscopy (STM) plus x-ray photoelectron spectroscopy (XPS) and the computational technique of density functional theory (DFT). Necessary conditions for encapsulation are preexisting ion-induced defects in the graphite substrate and elevated deposition temperature of 875–900 K. Evidence of encapsulation consists of atomically resolved STM images of a carbon lattice, both on top of the islands and on the sloping sides. The nature of the images indicates that this carbon lattice corresponds to a graphene blanket consisting of more than one graphene sheet that drapes continuously from the top of the island to the graphite substrate. The formation of iron carbide is not observed based on XPS. Shapes of the island footprints are consistent with metallic Fe, predominantly in the hcp or fcc form, though larger islands tend toward bcc. Island structures with hexagonally close-packed lateral hcp or fcc planes are stabilized by their excellent lattice match with the graphite substrate. Evolution of island density with prolonged deposition time provides evidence of coarsening, perhaps via Smoluchowski ripening. The encapsulated Fe clusters are stable in air at room temperature, protected by smaller Fe clusters that decorate defect sites and block permeation of gases. DFT shows that two configurations of Fe are more stable within the gallery than adsorbed on top of the surface: a single atom of Fe and a film (slab) of metallic Fe. Comparison with other metals shows that encapsulated Fe is similar to Cu but dissimilar to Ru or Dy, leading the authors to conclude that carbon dissolution in the metal does not play a role in encapsulation.

Temperature-Triggered Self-Assembled Structural Transformation: From Pure Hydrogen-Bonding Quadrilateral Nanonetwork to Trihexagonal Structures.

Adequate control over the structures of molecular building blocks plays an important role in the fabrication of desired supramolecular nanostructures at interfaces. In this study, the formation of a pure hydrogen-bonding co-assembly supramolecular nanonetwork on a highly oriented pyrolytic graphite surface was demonstrated by means of a scanning tunneling microscope. The thermal annealing process was conducted to monitor the temperature-triggered structural transformation of the self-assembled nanonetwork. On the basis of the single-molecule-level resolution scanning tunneling microscopy images, together with the density functional theory calculations, the formation mechanisms of the formed nanoarrays were proposed. The results have great significance with regard to controlled construction of complex nanostructures on the surface.

Coexistence of Elastic Modulations in the Charge Density Wave State of 2 H-NbSe2.

Bulk and single-layer 2 H-NbSe2 exhibit identical charge density wave order (CDW) with a quasi-commensurate 3 × 3 superlattice periodicity. Here we combine scanning tunnelling microscopy (STM) imaging at T = 1 K of 2 H-NbSe2 with first-principles density functional theory (DFT) calculations to investigate the structural atomic rearrangement of this CDW phase. Our calculations for single-layers reveal that six different atomic structures are compatible with the 3 × 3 CDW distortion, although all of them lie on a very narrow energy range of at most 3 meV per formula unit, suggesting the coexistence of such structures. Our atomically resolved STM images of bulk 2 H-NbSe2 unambiguously confirm this by identifying two of these structures. Remarkably, these structures differ from the X-ray crystal structure reported for the bulk 3 × 3 CDW which in fact is also one of the six DFT structures located for the single-layer. Our calculations also show that due to the minute energy difference between the different phases, the ground state of the 3 × 3 CDW could be extremely sensitive to doping, external strain or internal pressure within the crystal. The presence of multiphase CDW order in 2 H-NbSe2 may provide further understanding of its low temperature state and the competition between different instabilities.

Surface Reconstruction of Germanium: Hydrogen Intercalation and Graphene Protection

Understanding the interfacial properties of functional nanomaterials on semiconductor surfaces is crucial for developing electronics, optoelectronics, and other devices. By using graphene on the germanium (110) surface as a model, we performed scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) to examine the surface reconstructions of Ge (110) under graphene. Two reconstructions, the (1 × 1) phase (R2) which was previously proposed to survive only at high temperature and a [225−1] superstructure (R1 phase), were determined based on atomically resolved STM images. The R2 phase will transform to R1 after annealing in UHV above 300 °C, while the R1 phase can reversibly change to the (1 × 1) phase after heating in hydrogen at 700 °C. Finally, we confirmed the presence of interfacial hydrogen that stabilized the (1 × 1) phase at the initial stage of graphene growth based on control experiments. The zigzag edge of graphene is perpendicular to the close-packed [1–10] direction of Ge(1...

The Growth and Crystalline Structure of Ultrathin Mn Films on Ni(111)

The growth and crystalline structure of ultrathin Mn films are studied in terms of thickness and temperature by scanning tunneling microscopy (STM) as well as medium‐ and low‐energy electron diffraction. We show that the Mn films deposited at room temperature exhibit a thickness‐dependent growth behavior with a layer‐by‐layer growth mode up to 5.3 monolayers. Two transitions in the growth mode can be observed in this range. Atomically resolved STM images of a submonolayer of Mn on Ni(111) reveal a lateral lattice distortion which becomes less significant after the islands form a wetting layer at room temperature. The low‐temperature‐deposited wetting layer shows a metastable lattice distortion which vanishes after annealing to room or higher temperatures. This phenomenon may be attributed to a temperature‐dependent metastable arrangement of the Mn adlayer as well as the distinct interaction strength between Mn–Mn and Mn–Ni atoms. Low‐temperature‐grown thick Mn films exhibit a Stranski–Krastanov growth mode with a stable 30° reconstruction, which remains stable up to room temperature.

Phosphorus Nanostripe Arrays on Cu(110): A Case Study to Understand the Substrate Effect on the Phosphorus thin Film Growth

Black phosphorus has attracted significant attention as a new emerging 2D material in recent years. Due to its novel and unique properties, new 2D phosphorus nanostructures have also been predicted by theoretical calculations. However, experimental realization of new phosphorus structures has been rarely reported and remains one major challenge. Therefore, it is crucial to investigate phosphorus growth behaviors on different substrates to unravel the growth mechanism and provides useful guidelines for the selection of suitable substrates to synthesis the desired structure. This study reports a molecular‐beam‐epitaxial growth of phosphorus on Cu(110) by using bulk black phosphorous crystals as precursor, through the combination of in situ low temperature scanning tunneling microscopy (STM), low energy electron diffraction, X‐ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. Atomically resolved STM image and the corresponding DFT simulations reveal the formation of phosphorus monomers and dimmers locating in the valley of the Cu(110) surface. XPS and Bader charge analysis confirm a significant electron transfer from the Cu(110) substrate to P atoms on top.

Structure and Dynamics of Individual Diastereomeric Complexes on Platinum: Surface Studies Related to Heterogeneous Enantioselective Catalysis.

The modification of heterogeneous catalysts through the chemisorption of chiral molecules is a method to create catalytic sites for enantioselective surface reactions. The chiral molecule is called a chiral modifier by analogy to the terms chiral auxiliary or chiral ligand used in homogeneous asymmetric catalysis. While there has been progress in understanding how chirality transfer occurs, the intrinsic difficulties in determining enantioselective reaction mechanisms are compounded by the multisite nature of heterogeneous catalysts and by the challenges facing stereospecific surface analysis. However, molecular descriptions have now emerged that are sufficiently detailed to herald rapid advances in the area. The driving force for the development of heterogeneous enantioselective catalysts stems, at the minimum, from the practical advantages they might offer over their homogeneous counterparts in terms of process scalability and catalyst reusability. The broader rewards from their study lie in the insights gained on factors controlling selectivity in heterogeneous catalysis. Reactions on surfaces to produce a desired enantiomer in high excess are particularly challenging since at room temperature, barrier differences as low as ∼2 kcal/mol between pathways to R and S products are sufficient to yield an enantiomeric ratio (er) of 90:10. Such small energy differences are comparable to weak interadsorbate interaction energies and are much smaller than chemisorption or even most physisorption energies. In this Account, we describe combined experimental and theoretical surface studies of individual diastereomeric complexes formed between chiral modifiers and prochiral reactants on the Pt(111) surface. Our work is inspired by the catalysis literature on the enantioselective hydrogenation of activated ketones on cinchona-modified Pt catalysts. Using scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations, we probe the structures and relative abundances of non-covalently bonded complexes formed between three representative prochiral molecules and (R)-(+)-1-(1-naphthyl)ethylamine ((R)-NEA). All three prochiral molecules, 2,2,2-trifluoroacetophenone (TFAP), ketopantolactone (KPL), and methyl 3,3,3-trifluoropyruvate (MTFP), are found to form multiple complexation configurations around the ethylamine group of chemisorbed (R)-NEA. The principal intermolecular interaction is NH···O H-bonding. In each case, submolecularly resolved STM images permit the determination of the prochiral ratio (pr), pro-R to pro-S, proper to specific locations around the ethylamine group. The overall pr observed in experiments on large ensembles of KPL-(R)-NEA complexes is close to the er reported in the literature for the hydrogenation of KPL to pantolactone on (R)-NEA-modified Pt catalysts at 1 bar H2. The results of independent DFT and STM studies are merged to determine the geometries of the most abundant complexation configurations. The structures reveal the hierarchy of chemisorption and sometimes multiple H-bonding interactions operating in complexes. In particular, privileged complexes formed by KPL and MTFP reveal the participation of secondary CH···O interactions in stereocontrol. State-specific STM measurements on individual TFAP-(R)-NEA complexes show that complexation states interconvert through processes including prochiral inversion. The state-specific information on structure, prochirality, dynamics, and energy barriers delivered by the combination of DFT and STM provides insight on how to design better chiral modifiers.

Imaging structural transitions in organometallic molecules on Ag(100) for solar thermal energy storage

The use of opto-thermal molecular energy storage at the nanoscale creates new opportunities for powering future microdevices with flexible synthetic tailorability. Practical application of these molecular materials, however, requires a deeper microscopic understanding of how their behavior is altered by the presence of different types of substrates. Here we present single-molecule resolved scanning tunneling microscopy imaging of thermally- and optically-induced structural transitions in (fulvalene)tetracarbonyldiruthenium molecules adsorbed onto a Ag(100) surface as a prototype system. Both the parent complex and the photoisomer display distinct thermally-driven phase transformations when they are in contact with a Ag(100) surface. This behavior is consistent with the loss of carbonyl ligands due to strong molecule-surface coupling. Ultraviolet radiation induces marked structural changes only in the intact parent complex, thus indicating a photoisomerization reaction. These results demonstrate how stimuli-induced structural transitions in this class of molecule depend on the nature of the underlying substrate.

PTCDA growth on Ge(111)- surfaces: a scanning tunneling microscopy study

The initial stages of growth of PTCDA (3,4,9,10 perylene tetracarboxylic dianhydride) at room temperature (RT) on Ge(111)- surfaces have been studied by means of scanning tunneling microscopy (STM) under ultrahigh vacuum conditions. The results show that PTCDA molecules have a high mobility at RT on the well ordered areas of the semiconductor substrate, since nucleation is only observed in domain walls, steps and surface defects. However, no molecular ordering has been detected at submonolayer coverage. For higher coverages, the formation of three-dimensional (3D) molecular islands has been observed. These 3D islands present a crystalline nature as demostrated by molecularly resolved STM images. According to these STM measurements, PTCDA molecules are ordered in a herringbone structure, similar to the one observed in PTCDA bulk crystals. Moreover, the 3D crystallites are grown on top of a disordered molecular layer, which acts as a passivating layer.