High-resolution Scanning Tunneling Microscopy Research Articles

Twisted charge-density-wave patterns in bilayer 2D crystals and modulated electronic states

The twistronics of the atomic-thick two-dimensional (2D) van der Waals materials has opened a new venue to investigate the interlayer coupling. Till now, most studies focus on the twist of atomic lattices and the resulted moiré superstructures, while the reports about the twist of charge density waves (CDWs), the superstructures of which are from individual layers are limited. Here, using molecular beam epitaxy, we construct bilayer (BL) 1T-NbSe2 vertical structures. With high resolution scanning tunneling microscopy observations, we identify two cases of CDW twisted stacking with atomic precision. The typical twist angles are 0° and 60° between the 1st and the 2nd layer, although the top Se atomic lattices of these two layers are parallel. Compared to the single layer case, the dI/dV at BL shows an insulator-to-metal transition, with the Hubbard bands shrinking towards the Fermi level (E F). More intriguingly, interlayer coupling states rise near E F, which are related to the CDW twist angles. These findings give fresh insight into the engineering of 2D materials by CDW twisting and are potentially applicable for future nanoelectronic devices.

Molecular-Level Insights on Reactive Arrangement in On-Surface Photocatalytic Coupling Reactions Using Tip-Enhanced Raman Spectroscopy.

Plasmon-enhanced photocatalytic coupling reactions have been used as model systems in surface-enhanced Raman spectroscopy and tip-enhanced Raman spectroscopy (TERS) research for decades. However, the role of reactive arrangement on efficiency of these model reactions has remained largely unknown to date often leading to conflicting interpretations of experimental results. Herein, we use an interdisciplinary toolbox of nanoscale TERS imaging in combination with molecular-resolution ambient scanning tunnelling microscopy (STM) and density functional theory (DFT) modeling to investigate the role of reactive arrangement in photocatalytic coupling of 4-nitrobenzenethiol (4-NTP) to p,p'-dimercaptoazobisbenzene on single-crystal and polycrystalline Au surfaces for the first time. TERS imaging with 3 nm resolution clearly revealed a significantly higher catalytic efficiency inside a kinetically driven disordered phase of the 4-NTP adlayer on Au compared to the thermodynamically stable ordered phase. Furthermore, molecular level details of the self-assembled structures in the disordered and ordered phases obtained using ambient high-resolution STM enabled an unambiguous structure-reactivity correlation of photocatalytic coupling. Finally, quantitative mechanistic insights obtained from DFT modeling based on the accurate parameters determined from STM imaging emphatically confirmed that a combination of steric hindrance effect and energetic barrier leads to a lower reaction efficiency in the ordered phase of the 4-NTP adlayer. This fundamental study establishes the first direct structure-reactivity correlation in photocatalytic coupling and highlights the critical role of reactive arrangement in the efficiency of on-surface coupling reactions in heterogeneous catalysis at large.

Se-concentration dependent superstructure transformations of CuSe monolayer on Cu(111) substrate

As one of the most distinctive members of the monolayer transition metal monochalcogenides (TMM) family, the CuSe monolayer with a honeycomb structure has drawn much attention in the past few years. Depending on the Se concentration, the CuSe monolayer has two distinct superstructures on a Cu(111) substrate, a one dimensional (1D) moiré pattern, and two dimensional (2D) periodic nanopores. Here, we devise a strategy for simultaneous fabrication of the two superstructures of the CuSe monolayer on a Cu(111) substrate via artificially creating a density gradient of the Se concentration by an off-centered deposition method. At the boundary of the two superstructures, an intermediate state of the CuSe monolayer with a 2D hexagonal moiré pattern connected by six twisted petal-like stripes is observed. High-resolution scanning tunneling microscopy characterizations of three distinct CuSe monolayer superstructures demonstrate that the Se density can effectively modulate the stress in the CuSe monolayer formed by the lattice mismatch, driving the superstructure transformation from 1D moiré pattern through 2D intermediate states to 2D periodic nanopores. In addition, scanning tunneling spectroscopy measurements show that the intermediate state features a semiconducting behavior with a band gap of ∼2.0 eV. Our findings open up a new route for superstructure transformation control of 2D materials.

Compressed sensing for scanning tunnel microscopy imaging of defects and disorder

Compressed sensing (CS) is a valuable technique for reconstructing measurements in numerous domains. CS has not yet gained widespread adoption in scanning tunneling microscopy (STM), despite potentially offering the advantages of lower acquisition time and enhanced tolerance to noise. Here we applied a simple CS framework, using a weighted iterative thresholding algorithm for CS reconstruction, to representative high-resolution STM images of superconducting surfaces and adsorbed molecules. We calculated reconstruction diagrams for a range of scanning patterns, sampling densities, and noise intensities, evaluating reconstruction quality for the whole image and chosen defects. Overall we find that typical STM images can be satisfactorily reconstructed down to 30\% sampling - already a strong improvement. We furthermore outline limitations of this method, such as sampling pattern artifacts, which become particularly pronounced for images with intrinsic long-range disorder, and propose ways to mitigate some of them. Finally we investigate compressibility of STM images as a measure of intrinsic noise in the image and a precursor to CS reconstruction, enabling a priori estimation of the effectiveness of CS reconstruction with minimal computational cost.

Comparison of 2D crystals formed by dissociative adsorption of fluorinated and nonfluorinated alkyl iodides on Cu(111)

We report the comparison of a series of 2D molecular crystals formed from the intermediates of the dehalogenation reaction of iodoethane versus various fluorinated iodoalkanes on Cu(111). High-resolution scanning tunneling microscopy enables us to distinguish the alkyl groups from the iodine atoms, and we find that the ethyl groups and iodine atoms formed from the dissociation of ethyl iodide are well mixed. However, fluorination of the alkyl tail changes this behavior and leads to local segregation of the two species on the surface. We postulate that the low-polarizability and relatively large dipole moment of the fluorinated species drive the ordered assemblies of the fluorinated alkyl species on the surface and discuss this in the context of how solvophobicity can drive the clustering of fluorinated groups and, hence, phase separation.

Low-Defect, High Molecular Weight Indacenodithiophene (IDT) Polymers Via a C–H Activation: Evaluation of a Simpler and Greener Approach to Organic Electronic Materials

The development, optimization, and assessment of new methods for the preparation of conjugated materials is key to the continued progress of organic electronics. Direct C–H activation methods have emerged and developed over the last 10 years to become an invaluable synthetic tool for the preparation of conjugated polymers for both redox-active and solid-state applications. Here, we evaluate direct (hetero)arylation polymerization (DHAP) methods for the synthesis of indaceno[1,2-b:5,6-b′]dithiophene-based polymers. We demonstrate, using a range of techniques, including direct visualization of individual polymer chains via high-resolution scanning tunneling microscopy, that DHAP can produce polymers with a high degree of regularity and purity that subsequently perform in organic thin-film transistors comparably to those made by other cross-coupling polymerizations that require increased synthetic complexity. Ultimately, this work results in an improved atom economy by reducing the number of synthetic steps to access high-performance molecular and polymeric materials.

Self-assemblies of TTF derivatives with fluorinated phenyls and pyridine group

The self-assembly characteristics of tetrathiafulvalene (TTF) derivatives molecules 1–3 at the 1-phenyloctane/HOPG (HOPG = highly oriented pyrolytic graphite) interface had been carefully studied by scanning tunneling microscopy (STM) method. The number of F atoms on the phenyl group had significantly affected the self-assembly structures. High-resolution STM images make clear the different assembly structures between the molecules 1–3, which attribute to the different F atom numbers and pyridine group in the molecule. Density functional theory (DFT) calculations have been performed to reveal the formation mechanism.

Unconventional chiral charge order in kagome superconductor KV3Sb5.

Intertwining quantum order and non-trivial topology is at the frontier of condensed matter physics1-4. A charge-density-wave-like order with orbital currents has been proposed for achieving the quantum anomalous Hall effect5,6 in topological materials and for the hidden phase in cuprate high-temperature superconductors7,8. However, the experimental realization of such an order is challenging. Here we use high-resolution scanning tunnelling microscopy to discover an unconventional chiral charge order in a kagome material, KV3Sb5, with both a topological band structure and a superconducting ground state. Through both topography and spectroscopic imaging, we observe a robust 2 × 2 superlattice. Spectroscopically, an energy gap opens at the Fermi level, across which the 2 × 2 charge modulation exhibits an intensity reversal in real space, signalling charge ordering. At the impurity-pinning-free region, the strength of intrinsic charge modulations further exhibits chiral anisotropy with unusual magnetic field response. Theoretical analysis of our experiments suggests a tantalizing unconventional chiral charge density wave in the frustrated kagome lattice, which can not only lead to a large anomalous Hall effect with orbital magnetism, but also be a precursor of unconventional superconductivity.

Atomic mechanism of the phase transition in monolayer bismuthene on copper oxide

Problems associated with large-scale growth of high-quality single-layer bismuthene constitute one of the main obstacles to the applications of bismuthene in electronic devices. Here we report the direct synthesis of high-quality bismuthene using molecular-beam epitaxy on dielectric copper oxide thin films. Scanning tunneling microscopy (STM) reveals the generation of dimer pair arrays and black-phosphorus-like bismuthene (BKP-Bi, \ensuremath{\alpha} phase) at the initial stage on ${\mathrm{Cu}}_{3}{\mathrm{O}}_{2}/\mathrm{Cu}(111)$ surface. A phase transition from BKP-Bi to blue-phosphorus-like bismuthene (BLP-Bi, \ensuremath{\beta} phase) is characterized by high-resolution STM, complemented with first-principles calculations based on density-functional theory (DFT) showing a bond-breaking path. Employing the phase transition, we also demonstrate the construction of an atomically sharp in-plane homojunction along the phase boundary. DFT calculations reveal a one-dimensional edge state along the homojunction.

Single Molecular Adsorption of Terbium(III) Bis-phthalocyaninato (TbPc2) Governed by Two Surface Reconstructions of Perovskite Type SrVO3 Epitaxial Ultrathin Film

Using low temperature scanning tunneling microscopy (STM), we observed a single molecule magnet, Tb3+ double decker molecule TbPc2 (Pc = phthalocyaninato), adsorbed on a perovskite type transition metal oxide SrVO3 ultrathin film. TbPc2 molecules adsorbed intact and flat on two surface reconstructions of SrVO3, -R45° and -R26.6°, with different configurations. High resolution STM images revealed that the adsorption configurations were governed by the adatom structure of each surface reconstruction according to the proposed adsorption configurations.

On-Surface Fabrication of Bimetallic Metal-Organic Frameworks through the Synergy and Competition among Noncovalent Interactions.

Two-dimensional metal-organic frameworks (2D-MOFs) are attracting more attention due to their unique properties. Various 2D-MOF structures have been fabricated on surfaces in which either only one kind of metal was incorporated or only one kind of noncovalent interaction was involved in a bimetallic network. However, 2D-MOFs involving different kinds of noncovalent interactions and multiple metal components are more complex and less predictable. Here, we choose the uracil (U) molecule together with alkali metals [sodium (Na) and cesium (Cs)] and a transition metal [iron (Fe)] as model systems and successfully construct two kinds of bimetallic 2D-MOFs through the synergy and competition among noncovalent interactions, which is revealed by the high-resolution scanning tunneling microscopy imaging and density functional theory calculations. Such a systematic study may help to improve our fundamental understanding of the interaction mechanism of noncovalent bonds and, moreover, lead to further investigations of the unprecedented functions of surface-supported 2D-MOF structures.

Binding and stability of MgO monomers on anatase TiO2(101).

In catalysis, MgO is often used to modify the acid-base properties of support oxides and to stabilize supported metal atoms and particles on oxides. In this study, we show how the sublimation of MgO powder can be used to deposit MgO monomers, hither on anatase TiO2(101). A combination of x-ray electron spectroscopy, high-resolution scanning tunneling microscopy, and density functional theory is employed to gain insight into the MgO monomer binding, electronic and vibrational properties, and thermal stability. In the most stable configuration, the Mg and O of the MgO monomer bind to two surface oxygens and one undercoordinated surface titanium, respectively. The additional binding weakens the Mg-O monomer bond and makes Mg more ionic. The monomers are thermally stable up to 600K, where the onset of diffusion into the TiO2 bulk is observed. The monomeric MgO species on TiO2(101) represent an ideal atomically precise system with modified acid-base properties and will be employed in our future catalytic studies.

Atomic and electronic structures of Si/Ge(100) interfaces studied by high-resolution photoelectron spectroscopy and scanning tunneling microscopy

The close similarity of silicon and germanium, isoelectronic group-IV elements, makes the integration of Ge layers on Si substrates suitable for technology development, but the atomic and electronic structures of ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ surfaces are still an open issue, in particular, for the alloy systems where Si is deposited on the Ge substrate. In this study, utilizing low-energy electron diffraction, scanning tunneling microscopy, and photoelectron spectroscopy using synchrotron radiation, we demonstrate that the formation mechanisms of the Si-on-Ge structures are controlled by two interface phenomena, namely Si indiffusion and Ge segregation on top of this surface. Employing these phenomena and controlling the Si quantity, one can synthesize the well-defined crystalline Ge-(2 \ifmmode\times\else\texttimes\fi{} 1)/${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$/Ge(100) stacks where the number of Si atoms at the host Ge lattice sites can be tuned. Using the obtained data on the atomic and electronic structures of such systems, we also propose a method for interface engineering of Ge/Si/Ge stacks with tailored properties as promising templates for growing the device junctions.

Self-Assembled Borophene/Graphene Nanoribbon Mixed-Dimensional Heterostructures.

Atomically thin metal-semiconductor heterojunctions are highly desirable for nanoelectronic applications. However, coherent lateral stitching of distinct two-dimensional (2D) materials has traditionally required interfacial lattice matching and compatible growth conditions, which remains challenging for most systems. On the other hand, these constraints are relaxed in 2D/1D mixed-dimensional lateral heterostructures due to the increased structural degree of freedom. Here, we report the self-assembly of mixed-dimensional lateral heterostructures consisting of 2D metallic borophene and 1D semiconducting armchair-oriented graphene nanoribbons (aGNRs). With the sequential ultrahigh vacuum deposition of boron and 4,4″-dibromo-p-terphenyl as precursors on Ag(111) substrates, an on-surface polymerization process is systematically studied and refined including the transition from monomers to organometallic intermediates and finally demetallization that results in borophene/aGNR lateral heterostructures. High-resolution scanning tunneling microscopy and spectroscopy resolve the structurally and electronically abrupt interfaces in borophene/aGNR heterojunctions, thus providing insight that will inform ongoing efforts in pursuit of atomically precise nanoelectronics.

Exploring Intramolecular Methyl–Methyl Coupling on a Metal Surface for Edge-Extended Graphene Nanoribbons

Intramolecular methyl–methyl coupling on Au (111) is explored as a new on-surface protocol for edge extension in graphene nanoribbons (GNRs). Characterized by high-resolution scanning tunneling microscopy, noncontact atomic force microscopy, and Raman spectroscopy, the methyl–methyl coupling is proven to indeed proceed at the armchair edges of the GNRs, forming six-membered rings with sp3- or sp2-hybridized carbons.

Directional observations of guanine and cytosine pairing structures on HOPG

Abstract The molecular self-assembly of deoxyribonucleic acid (DNA) plays an important role in understanding how nucleic acids form and can be exploited to construct nanostructures. In this study, scanning tunneling microscopy (STM) was used to observe nanostructures that form from co-adsorption of the guanine (G) and cytosine (C) complementary DNA bases on a highly oriented pyrolytic graphite surface. The high-resolution STM images show that the well-ordered co-adsorption structures are attributed to the rows formed from Watson–Crick G–C pairs, which are distinct from the structures observed for individual G–C components. The observed co-adsorption structure was modeled by density functional theory calculations, which provide information on the intermolecular interactions underlying its formation.

Polymorphic Pairing Configurations of Guanine and Cytosine at the Water–HOPG Interface

A series of nucleobases guanine (G) and cytosine (C) pairing configurations have been fabricated on highly oriented pyrolytic graphite (HOPG) surface by controlling the molar ratio of G and C in water solution. Watson-Crick (WC) base pairing governs the association of C and G nucleobases when the molar ratio of C/G is adjusted to 1:1. Nucleobase-rich is preferentially hydrogen-bonded to the sites exposed around WC motifs with the adjustment of the C/G molar ratio. At a higher C/G molar ratio imbalance, the pairing configurations depend on the combination of interspace and sites of hydrogen binding between G and C bases. The systematic analysis of the high-resolution STM images and DFT calculations reveal that hydrogen bonding plays a dominant role in the formation of these pairing configurations and that the competition between the priority and diversity of hydrogen-bonded configurations bonding between G and C is the key for the pairing structural polymorphism.

Construction of poly-naphthalocyanine linked by [4]-radialene-like structures on silver surfaces

Cyclic-conjugated linkages between planar-macrocydic molecules contribute to the robustness of the two-dimensional (2D) polymerization and extension of π-interactions. The fabrication of such linkages in 2D polymers remains challenging. Combining scanning tunneling microscope (STM) measurements and density functional theory (DFT) calculations, we demonstrate a linear polymerization of metal-free naphthalocyanine (NPc) molecules with [4]-radialene-like linkages on silver surfaces. Experimentally, by depositing NPc molecules on the Ag(110) surface and subsequent annealing up to 750 K, one-dimensional polymers are constructed along the [ $$1\overline 1 0$$ ] direction. High-resolution STM images show a stem-leaf-like feature. STM simulations based on a linear polymer of NPc molecules linked by four-membered carbon rings, [4]-radialene-like structure, agree well with the experimental observations. DFT calculations reveal that the polymerization process includes detaching two-terminal H atoms of NPc molecules along [ $$1\overline 1 0$$ ] direction, then bonding with a neighboring dehydrogenated NPc molecule by forming a four-membered ring. The dehydrogenation process can be promoted by on-surface impurities such as additional H atoms. Similar polymerizations have been achieved on Ag(111) surfaces in an amorphous way. Moreover, the energy gap of the NPc molecule decreases after linear polymerization, suggesting a red-shift for its optical absorption/scattering spectrum. Our study offers a new route to polymerize conjugated molecules with extended planar π-interactions.

Imaging Vacancy Defects in Single-Layer Chromium Triiodide.

As a van der Waals magnetic semiconductor, chromium triiodide (CrI3) is widely considered for its high research value and potential applications. Defects in CrI3 are inevitably present and significantly alter the material properties. However, experimental identification of defects of CrI3 at the atomic level is still lacking. Here for the first time, we carried out a scanning tunneling microscopy (STM) study and density functional theory calculations to explore the intrinsic defects in monolayer CrI3 grown by molecular beam epitaxy. The three most common types of intrinsic point defects, i.e., I vacancy (VI), Cr vacancy (VCr), and multiatom CrI3 vacancy (VCrI3) with distinct spatial distributions of the localized defect states, are identified and characterized by high-resolution STM. Moreover, defect concentrations are estimated based on our experiments, which agree with the calculated formation energies. Our findings provide vital knowledge on the types, concentrations, electronic structures, and migration mechanism of the intrinsic point defects in monolayer CrI3 for future defect engineering of this novel 2D magnet.

Reassessing Alkyne Coupling Reactions While Studying the Electronic Properties of Diverse Pyrene Linkages at Surfaces.

The combination of alkyne and halogen functional groups in the same molecule allows for the possibility of many different reactions when utilized in on-surface synthesis. Here, we use a pyrene-based precursor with both functionalities to examine the preferential reaction pathway when it is heated on an Au(111) surface. Using high-resolution bond-resolving scanning tunneling microscopy, we identify multiple stable intermediates along the prevailing reaction pathway that initiate with a clearly dominant Glaser coupling, together with a multitude of other side products. Importantly, control experiments with reactants lacking the halogen functionalization reveal the Glaser coupling to be absent and instead show the prevalence of non-dehydrogenative head-to-head alkyne coupling. We perform scanning tunneling spectroscopy on a rich variety of the product structures obtained in these experiments, providing key insights into the strong dependence of their HOMO–LUMO gaps on the nature of the intramolecular coupling. A clear trend is found of a decreasing gap that is correlated with the conversion of triple bonds to double bonds via hydrogenation and to higher levels of cyclization, particularly with nonbenzenoid product structures. We rationalize each of the studied cases.