Scanning Tunneling Microscopy Observations Research Articles

Site-selection and recognition of aromatic carboxylic acid in response to coronene and pyridine derivative

The self-assembled structures of H3BDA molecule with multiple meta-dicarboxylic groups and their stimulus responses to the guest molecules (COR and T4PT) are thoroughly investigated by scanning tunneling microscopy (STM). STM observations display that two kinds of nanostructures are fabricated by H3BDA molecules through intermolecular hydrogen bonds, in which a linear structure is formed at a higher concentration and a flower-like structure is obtained at a lower concentration. After the addition of COR and T4PT, H3BDA appears different responsiveness resulting in different co-assembled structures, respectively. The linear structure is regulated into a flower-like structure by COR and COR molecules are trapped in the cavities. When the pyridine derivative (T4PT) is introduced, a new bicomponent porous structure emerges via the hydrogen bond formed between the carboxyl group and the pyridine. Furthermore, the deposition of additional COR to the H3BDA/T4PT system results in the breakdown of the porous structure and the generation of H3BDA/COR host–guest system. Combined with density functional theory (DFT) calculations and molecular dynamics (MD) simulations, the transformation phenomenon of bi-component nanostructure induced by guest molecules is formulated. The results are expected to understand the modification effect of guest molecules on the host network, which is of great significance for the design and construction of multi-component nanostructures and crystal engineering.

Direct Visualization of CO Interaction on Oxygen Poisoned Co(0001).

The poisoning of catalysts has always been a vital issue in catalytic reactions. In this study, direct observation of the interaction of CO and oxygen-poisoned Co(0001) has been achieved with scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), and density functional theory calculation. A two-stage adsorption process of CO on a well-prepared p(2×2)-O layer covered Co(0001) was directly visualized. With increasing annealing time at a certain temperature after the CO dosage, the ordered (2 × 2) pattern formed in the first stage can be recovered, suggesting the weak interaction of CO with the O-covered Co(0001) surface in the latter stage. Compared to the clean Co(0001) surface, on an oxygen-poisoned surface, no further reaction was observed, illustrating the poisoning of the catalyst. Moreover, TPD results are in good agreement with the STM observation; a desorption energy of 0.35 eV is evaluated with a simple but accurate scheme.

Effects of the substituent position on the structural order, work function change, and thermopower of dichloro-substituted benzenethiolate self-assembled monolayers on Au(1 1 1)

The surface structures, electrochemical behaviors, work function changes, and thermopower of dichloro-substituted benzenethiolate self-assembled monolayers (SAMs) on gold surfaces prepared by vapor deposition were investigated using scanning tunneling microscopy (STM), cyclic voltammetry, Kelvin probe force microscopy, and thermoelectric junction measurements to understand the effects of their substituent positions. STM observations revealed that the adsorption of 2,4-dichlorobenzenethiol (2,4-DCBT) on Au(111) at 363 K for 1 h led to the formation of short-range ordered domains with a (5 × 6√7)R20° structure, while the SAMs formed at 363 K for 4 h consisted of long-range well-ordered domains with a (√3 × 5) structure. 3,4-Dichlorobenzenethiol (3,4-DCBT) SAMs showed coexistence of partial ordered domains and a disordered phase, while 3,4-DCBT SAMs showed fully disordered phases regardless of deposition time. Moreover, the Seebeck coefficient (S) increased in the order of 3,4-DCBT (4.6 ± 0.1 μV/K) < 2,4-DCBT (5.0 ± 0.4 μV/K) < 3,5-DCBT (5.2 ± 0.1 μV/K). Interestingly, the trend of S values was qualitatively consistent with that of the work function change (ΔΦ) of the SAMs. The reductive desorption behavior, work function shifts, and thermopower of aromatic thiolate SAMs were significantly modified by the structural order and direction of molecular dipoles of dichloro-substituted benzenethiols.

Effect of embedded Cr on the structural phase transition of In/Si(111)4 × 1

Using scanning tunneling microscopy and low-energy electron diffraction observations accompanied with the density-functional theory calculations, adsorption of Cr atoms onto the Si(111)4 × 1-In surface and its effect on the 4 × 1-to-8 × 2 structural transition were investigated. It was found that in contrast to other metal adsorbates, such as Na, In and Pb, which remain adatoms, Cr atom becomes embedded below the level of In atoms constituting the In nanowire. But in other respects Cr produces effects similar to those of other metals, namely it also induces distortion in the In rows of its residence resulting in doubling of the periodicity along the rows and also reduces the critical temperature of the 4 × 1-to-8 × 2 transition almost linearly with amount of deposited metal.

Atomic‐Scale Observations of the Immiscible Melted Metals Confined in a Single‐Atom Layer

Elucidating how changing of the dimensionality affects the property of the matter is the key challenge problem of the nanoscience. In particular, it has recently been reported that 2D liquids have fundamentally different dynamic properties to 3D liquids. Herein, the ultimate physical limit is addressed, when the liquid is confined to a single‐atom layer. The case has been realized with the Tl–Pb alloy layer grown on Si(111) substrate terminated by a single‐layer . At room temperature, the alloy behaves as a system of the immiscible melted metals confined in a single‐atom layer. In the scanning tunneling microscopy observation, Tl and Pb arrays have different contrasts that allows a direct visualization of evolution of the forming structures within atomic layer, as a function of the time and layer composition. In particular, it has been found that the structures look much like the Turing patterns. The obtained experimental dataset, especially the recorded STM videos, is believed to provide a hint for the prospective theoretical understanding of the dynamics of the single‐atom‐thick liquids.

Molecular conformation induced 2D self-assembled polymorph of benzothiadiazole-based π-conjugated fluorophore modified by cyano groups

The self-assembly behaviors of a benzothiadiazole-based liquid crystalline molecule (BT-C14) were investigated by scanning tunneling microscopy (STM). STM observations show the coexistence of three kinds of self-assembled patterns without apparent concentration dependence at the heptanoic acid/graphite interface, in which the molecules adopt two conformations due to different orientations of two flanked thiophene groups relative to cyano groups. BT-C14 molecules with one conformation form a linear structure by intermolecular CH∙∙∙NC hydrogen bonds and interchain van der Waals (vdWs) interactions. Meanwhile, the zigzag-like and separated-dimer patterns are observed, in which the molecules adopt another conformation. The CH∙∙∙NC hydrogen bonds and the S∙∙∙N chalcogen bonding in dimers, the dipolar interactions, along with the interchain vdWs interactions are the dominated forces to drive their structural formation. Density functional theory (DFT) approaches are utilized to determine the molecular conformations and the noncovalent interactions. Molecular dynamics (MD) calculations unravel that these three nanostructures have analogous interaction energies and molecular stacking densities, indicating that the coexistence is thermodynamically and kinetically favorable. This study not only instructs the design of liquid crystalline molecules but also provides a strategy for analyzing multi-configuration and multi-structure system in the self-assembly process.

Local hybridized states of adsorbed atomic Sn on WS2 substrate

We study the growth of Sn and the local electronic properties of Sn grown WS2 surfaces by in-situ scanning tunneling microscopy (STM), scanning tunneling spectroscopy, and first principles density functional theory calculations. Our investigations suggest substitutional doping of Sn at the S sites, with Sn atoms occupying a slightly elevated position of 80 pm on the WS2 surface. These results agree well with our STM observations on room temperature growth of atomic Sn, indicating commensurate or nearly commensurate adsorption at the top S sites with a buckling height of 40 ± 10 pm. Pristine WS2 without any S vacancies on the surface exhibits a bandgap of 1.39 eV, while ingap local density of states comprised of W d and S p orbitals are detected when S vacancies are considered. Upon adsorption of Sn atoms, we find the signature of modulated hybridized ingap electronic states of Sn p and W d orbitals with a spin orbit coupling interaction up to 38 meV at the K point of the Brillouin zone. These results throw important light on the future fabrication of Janus like SnWS vertical heterojunctions with atomic level doping accuracy, which will have enormous device applications.

Effect of Metal Coordination on Two-dimensional Assemblies in Curcumin Derivatives

Two-dimensional (2D) structures of Cu(II)-coordinated curcumin derivatives were revealed by scanning tunneling microscopy at the solid/liquid interface. The Cu(II)-coordination resulted in the formation of dimerized structure, which was different from the pristine curcumin derivatives. The appearance and disappearance of 2D structure modulation due to odd-even effect were dependent on the number of alkyl chain substituents. In addition, the appearance of the odd-even was found to be maintained before and after the Cu(II) coordination. Scanning tunneling microscopy observations revealed the two-dimensional (2D) structures of Cu(II)-coordinated curcumin derivatives at the solid/liquid interface. The dimerized structure was formed due to Cu(II) coordination, and the 2D structure of metallated curcumin derivatives were different from the pristine ones. Appearance of odd-even effect was dependent on the alkyl chain substituents, and was maintained before and after the Cu(II) coordination.

Behavior under magnetic field of resonance at the edge of the upper Hubbard band in 1T−TaS2

Recent theoretical investigations of quantum spin liquids have described phenomenology amenable to experimental observation using scanning tunneling microscopy. This includes characteristic resonances found at the edge of the upper Hubbard band of the host Mott insulator, that under certain conditions shift into the Mott gap under external magnetic field [W.-Y. He and P. A. Lee, arXiv:2212.08767]. In light of this we report scanning tunneling microscopy observations, in samples of the quantum spin liquid candidate 1T-TaS2, of a conductance peak at the upper Hubbard band edge and its magnetic field dependent behavior. These observations potentially represent evidence for the existence of a quantum spin liquid in 1T TaS2. We also observe samples in which such field dependence is absent, but with no observed correlate for the presence or absence of field dependence. This suggests one or more material properties controlling electronic behavior that are yet to be understood, and should help to motivate renewed investigation of the microscopic degrees of freedom in play in 1T-TaS2, as well as the possible realization of a quantum spin liquid phase.

Dynamics of Poly(3-hexylthiophene) Monolayers at Solution/Graphite Interfaces.

We have studied the dynamics of poly(3-hexylthiophene) (P3HT) monolayers at the interfaces between highly oriented pyrolytic graphite (HOPG) and P3HT solutions in 1,2,4-trichlorobenzene, using scanning tunneling microscopy (STM). Real-time STM observation at room temperature reveals that P3HT molecules adsorbed on graphite are substantially mobile even in densely packed conditions, causing significant fluctuations of the self-organized monolayer of P3HT. We find that in the monolayers, the orientational order is limited to a short range comparable to the polymer chain length. We show that our observations can be understood based on a 2D semiflexible lattice polymer model by performing kinetic Monte Carlo simulations.

Influence of core size on self-assembled molecular networks composed of C3h-symmetric building blocks through hydrogen bonding interactions: structural features and chirality.

The effect of the core size on the structure and chirality of self-assembled molecular networks was investigated using two aromatic carboxylic acid derivatives with frameworks displaying C3h symmetry, triphenylene derivative H3TTCA and dehydrobenzo[12]annulene (DBA) derivative DBACOOH, each having three carboxy groups per molecule. Scanning tunneling microscopy observations at the 1-heptanoic acid/graphite interface revealed H3TTCA exclusively forming a chiral honeycomb structure, and DBACOOH forming three structures (type I, II, and III structures) depending on its concentration and whether the system is subjected to annealing treatment. Hydrogen bonding interaction patterns and chirality were carefully analyzed based on a modeling study using molecular mechanics simulations. Moreover, DBACOOH forms chiral honeycomb structures through the co-adsorption of guest molecules. Structural diversity observed for DBACOOH is attributed to its relatively large core size, with this feature modulating the balance between molecule-molecule and molecule-substrate interactions.

Epitaxial Growth of High-Quality Monolayer MoS2 Single Crystals on Low-Symmetry Vicinal Au(101) Facets with Different Miller Indices

Epitaxial growth of wafer-scale monolayer semiconducting transition metal dichalcogenide single crystals is essential for advancing their applications in next-generation transistors and highly integrated circuits. Several efforts have been made for the growth of monolayer MoS2 single crystals on high-symmetry Au(111) and sapphire substrates, while more prototype growth systems still need to be discovered for clarifying the internal mechanisms. Herein, we report the epitaxial growth of unidirectionally aligned monolayer MoS2 domains and single-crystal films on low-symmetry Au(101) vicinal facets via a facile chemical vapor deposition method. On-site scanning tunneling microscopy observations reveal the formation of a specific rectangular Moiré pattern along the [101̅] step edge of Au(101) and along its perpendicular direction. The perfect lattice constant matching of MoS2/Au(101) along the substrate high-symmetry directions (i.e., Au[101̅], Au [010]) as well as the step-edge-guiding effect are proposed to facilitate the robust epitaxy. Multiscale characterizations further confirm the domain-boundary-free feature of the monolayer MoS2 films merged by unidirectionally aligned monolayer domains. This work hereby puts forward a symmetry mismatched epitaxial system for the direct synthesis of monolayer MoS2 single crystals, which should deepen our understanding about the epitaxy of 2D layered materials and propel their applications in various fields.

Atomic-Scale Mechanism of Platinum Catalyst Restructuring under a Pressure of Reactant Gas.

Heterogeneous catalysis is key for chemical transformations. Understanding how catalysts' active sites dynamically evolve at the atomic scale under reaction conditions is a prerequisite for accurately determining catalytic mechanisms and predictably developing catalysts. We combine in situ time-dependent scanning tunneling microscopy observations and machine-learning-accelerated first-principles atomistic simulations to uncover the mechanism of restructuring of Pt catalysts under a pressure of carbon monoxide (CO). We show that a high CO coverage at a Pt step edge triggers the formation of atomic protrusions of low-coordination Pt atoms, which then detach from the step edge to create sub-nano-islands on the terraces, where under-coordinated sites are stabilized by the CO adsorbates. The fast and accurate machine-learning potential is key to enabling the exploration of tens of thousands of configurations for the CO-covered restructuring catalyst. These studies open an avenue to achieve an atomic-scale understanding of the structural dynamics of more complex metal nanoparticle catalysts under reaction conditions.

In Situ Observation of C-C Coupling and Step Poisoning During the Growth of Hydrocarbon Chains on Ni(111).

The synthesis of high-value fuels and plastics starting from small hydrocarbon molecules plays a central role in the current transition towards renewable energy. However, the detailed mechanisms driving the growth of hydrocarbon chains remain to a large extent unknown. Here we investigated the formation of hydrocarbon chains resulting from acetylene polymerization on a Ni(111) model catalyst surface. Exploiting X-ray photoelectron spectroscopy up to near-ambient pressures, the intermediate species and reaction products have been identified. Complementary in situ scanning tunneling microscopy observations shed light onto the C-C coupling mechanism. While the step edges of the metal catalyst are commonly assumed to be the active sites for the C-C coupling, we showed that the polymerization occurs instead on the flat terraces of the metallic surface.

In Situ Observation of C−C Coupling and Step Poisoning During the Growth of Hydrocarbon Chains on Ni(111)

AbstractThe synthesis of high‐value fuels and plastics starting from small hydrocarbon molecules plays a central role in the current transition towards renewable energy. However, the detailed mechanisms driving the growth of hydrocarbon chains remain to a large extent unknown. Here we investigated the formation of hydrocarbon chains resulting from acetylene polymerization on a Ni(111) model catalyst surface. Exploiting X‐ray photoelectron spectroscopy up to near‐ambient pressures, the intermediate species and reaction products have been identified. Complementary in situ scanning tunneling microscopy observations shed light onto the C−C coupling mechanism. While the step edges of the metal catalyst are commonly assumed to be the active sites for the C−C coupling, we showed that the polymerization occurs instead on the flat terraces of the metallic surface.

Influence of Molecular Configurations on the Desulfonylation Reactions on Metal Surfaces.

On-surface synthesis is a powerful methodology for the fabrication of low-dimensional functional materials. The precursor molecules usually anchor on different metal surfaces via similar configurations. The activation energies are therefore solely determined by the chemical activity of the respective metal surfaces. Here, we studied the influence of the detailed adsorption configuration on the activation energy on different metal surfaces. We systematically studied the desulfonylation homocoupling for a molecular precursor on Au(111) and Ag(111) and found that the activation energy is lower on inert Au(111) than on Ag(111). Combining scanning tunneling microscopy observations, synchrotron radiation photoemission spectroscopy measurements, and density functional theory calculations, we elucidate that the phenomenon arises from different molecule-substrate interactions. The molecular precursors anchor on Au(111) via Au-S interactions, which lead to weakening of the phenyl-S bonds. On the other hand, the molecular precursors anchor on Ag(111) via Ag-O interactions, resulting in the lifting of the S atoms. As a consequence, the activation barrier of the desulfonylation reactions is higher on Ag(111), although silver is generally more chemically active than gold. Our study not only reports a new type of on-surface chemical reaction but also clarifies the influence of detailed adsorption configurations on specific on-surface chemical reactions.

Thermally Driven Structural Order of Oligo(Ethylene Glycol)-Terminated Alkanethiol Monolayers on Au(111) Prepared by Vapor Deposition.

To probe the effects of deposition temperature on the formation and structural order of self-assembled monolayers (SAMs) on Au(111) prepared by vapor deposition of 2-(2-methoxyethoxy)ethanethiol (CH3O(CH2)2O(CH2)2SH, EG2) for 24 h, we examined the surface structure and electrochemical behavior of the resulting EG2 SAMs using scanning tunneling microscopy (STM) and cyclic voltammetry (CV). STM observations clearly revealed that EG2 SAMs vapor-deposited on Au(111) at 298 K were composed of a disordered phase on the entire Au surface, whereas those formed at 323 K showed improved structural order, showing a mixed phase of ordered and disordered phases. Moreover, at 348 K, uniform and highly ordered EG2 SAMs on Au(111) were formed with a (2 × 3√3) packing structure. CV measurements showed sharp reductive desorption (RD) peaks at −0.818, −0.861, and −0.880 V for EG2 SAM-modified Au electrodes formed at 298, 323, and 348 K, respectively. More negative potential shifts of RD peaks with increasing deposition temperature are attributed to an increase in van der Waals interactions between EG2 molecular backbones resulting from the improved structural quality of EG2 SAMs. Our results obtained herein provide new insights into the formation and thermally driven structural order of oligo(ethylene glycol)-terminated SAMs vapor-deposited on Au(111).

Non-monotonic changes in conductance of Bi(111) films induced by Cs adsorption

Effects of Cs adsorption on the conductance of Bi(111) films were studied using scanning tunneling microscopy (spectroscopy) observations and in situ transport measurements at low temperatures. Based on the obtained results and the known data on the Bi(111) electronic band structure, Cs-induced modifications of the band structure were shown to control the changes in the Bi(111) film conductance. Adsorbed Cs atoms donate electrons to Bi(111), causing the shifting of the system Fermi level upward along the energy scale. Due to the peculiarities of the Bi(111) band structure, the density of states at the Fermi level, which is directly related to the carrier density, varies non-monotonically with shifting of the Fermi level. Conductance changes in the same way with Cs coverage, namely, decreases almost twice at 0.011 ML of Cs, restores to the initial value at 0.025 ML of Cs, and grows gradually up to 0.167 ML; the maximal coverage when adsorbed Cs remains an assembly of the individual adatoms. Thus, Cs adsorption shows up as an effective tool to tune the electronic and transport properties of the Bi(111) films. The results also prove an effective surface characteristic of the electron transport in the Bi films.

Surface structure and work function change of pentafluorobenzeneselenolate self-assembled monolayers on Au (111)

The formation and surface structure of pentafluorobenzeneselenolate (PFB-Se) self-assembled monolayers (SAMs) on Au(111) prepared by solution and vapor depositions were examined by scanning tunneling microscopy (STM). STM observations showed that solution-deposited PFB-Se SAMs on Au(111) had only a disordered phase containing many dark areas with irregular shapes, regardless of the deposition time. However, it was found that the degree of structural order of PFB-Se SAMs was markedly improved by the vapor deposition method at 348 K. After short deposition for 5 min, PFB-Se SAMs on Au(111) consisted of two distinct regions: a bright protruding region and a dark region with partial ordered domains. Interestingly, PFB-Se SAMs after deposition for 1 h showed various structural features with three different contrasts: an ordered dark region, an ordered and disordered bright region, and a very bright region. The ordered domain is assigned to a (5 × √3) structure. The relatively uniform and nicely ordered PFB-Se SAMs with a (14 × √3) structure were formed after longer deposition for 2 h, resulting from further adsorption and structural rearrangements. Furthermore, Kelvin probe force microscopy measurements (KPFM) exhibited a significant difference in the work function of pentafluorobenzenethiolate and the PFB-Se SAM-modified gold surface by 0.36 eV due to the large difference in the domain and packing structures of both SAMs. Herein, STM and KPFM results will give new insights into the formation and structures of PFB-Se SAMs on Au(111) and further the fundamental understanding of the interfacial electronic properties of fluorinated aromatic backbone-based thiolate and selenolate SAMs.

On-Site Synthesis and Characterizations of Atomically-Thin Nickel Tellurides with Versatile Stoichiometric Phases through Self-Intercalation.

Self-intercalation of native metal atoms in two-dimensional (2D) transition metal dichalcogenides has received rapidly increasing interest, due to the generation of intriguing structures and exotic physical properties, however, only reported in limited materials systems. An emerging type-II Dirac semimetal, NiTe2, has inspired great attention at the 2D thickness region, but has been rarely achieved so far. Herein, we report the direct synthesis of mono- to few-layer Ni-tellurides including 1T-NiTe2 and Ni-rich stoichiometric phases on graphene/SiC(0001) substrates under ultra-high-vacuum conditions. Differing from previous chemical vapor deposition growth accompanied with transmission electron microscopy imaging, this work combines precisely tailored synthesis with on-site atomic-scale scanning tunneling microscopy observations, offering us visual information about the phase modulations of Ni-tellurides from 1T-phase NiTe2 to self-intercalated Ni3Te4 and Ni5Te6. The synthesis of Ni self-intercalated NixTey compounds is explained to be mediated by the high metal chemical potential under Ni-rich conditions, according to density functional theory calculations. More intriguingly, the emergence of superconductivity in bilayer NiTe2 intercalated with 50% Ni is also predicted, arising from the enhanced electron-phonon coupling strength after the self-intercalation. This work provides insight into the direct synthesis and stoichiometric phase modulation of 2D layered materials, enriching the family of self-intercalated materials and propelling their property explorations.