High-resolution Noncontact Atomic Force Research Articles

On-Surface Synthesis of Anthracene-Fused Zigzag Graphene Nanoribbons from 2,7-Dibromo-9,9'-bianthryl Reveals Unexpected Ring Rearrangements.

On-surface synthesis has emerged as a powerful strategy to fabricate unprecedented forms of atomically precise graphene nanoribbons (GNRs). However, the on-surface synthesis of zigzag GNRs (ZGNR) has met with only limited success. Herein, we report the synthesis and on-surface reactions of 2,7-dibromo-9,9'-bianthryl as the precursor toward π-extended ZGNRs. Characterization by scanning tunneling microscopy and high-resolution noncontact atomic force microscopy clearly demonstrated the formation of anthracene-fused ZGNRs. Unique skeletal rearrangements were also observed, which could be explained by intramolecular Diels-Alder cycloaddition. Theoretical calculations of the electronic properties of the anthracene-fused ZGNRs revealed spin-polarized edge-states and a narrow bandgap of 0.20 eV.

Observation of electron orbital signatures of single atoms within metal-phthalocyanines using atomic force microscopy

Resolving the electronic structure of a single atom within a molecule is of fundamental importance for understanding and predicting chemical and physical properties of functional molecules such as molecular catalysts. However, the observation of the orbital signature of an individual atom is challenging. We report here the direct identification of two adjacent transition-metal atoms, Fe and Co, within phthalocyanine molecules using high-resolution noncontact atomic force microscopy (HR-AFM). HR-AFM imaging reveals that the Co atom is brighter and presents four distinct lobes on the horizontal plane whereas the Fe atom displays a “square” morphology. Pico-force spectroscopy measurements show a larger repulsion force of about 5 pN on the tip exerted by Co in comparison to Fe. Our combined experimental and theoretical results demonstrate that both the distinguishable features in AFM images and the variation in the measured forces arise from Co’s higher electron orbital occupation above the molecular plane. The ability to directly observe orbital signatures using HR-AFM should provide a promising approach to characterizing the electronic structure of an individual atom in a molecular species and to understand mechanisms of certain chemical reactions.

MoS2 oxidative etching caught in the act: formation of single (MoO3) n molecules.

We report the presence of sub-nm MoO x clusters formed on basal planes of the 2H MoS2 crystals during thermal oxidative etching in air at a temperature of 370 °C. Using high resolution non-contact atomic force microscopy (AFM) we provide a histogram of their preferred heights. The AFM results combined with density functional theory (DFT) simulations show remarkably well that the MoO x clusters are predominantly single MoO3 molecules and their dimers at the sulfur vacancies. Additional Raman spectroscopy, and energy and wavelength dispersive X-ray spectroscopies as well as Kelvin probe AFM investigations confirmed the presence of the MoO3/MoO x species covering the MoS2 surface only sparsely. The X-ray absorption near edge spectroscopy data confirm the MoO3 stoichiometry. Taken together, our results show that oxidative etching and removal of Mo atoms at the atomic level follow predominantly via formation of single MoO3 molecules. Such findings confirm the previously only proposed oxidative etching stoichiometry.

Reconstruction of a 2D layer of KBr on Ir(111) and electromechanical alteration by graphene.

A novel reconstruction of a two-dimensional layer of KBr on an Ir(111) surface is observed by high-resolution noncontact atomic force microscopy and verified by density functional theory (DFT). The observed KBr structure is oriented along the main directions of the Ir(111) surface, but forms a characteristic double-line pattern. Comprehensive calculations by DFT, taking into account the observed periodicities, resulted in a new low-energy reconstruction. However, it is fully relaxed into a common cubic structure when a monolayer of graphene is located between substrate and KBr. By using Kelvin probe force microscopy, the work functions of the reconstructed and the cubic configuration of KBr were measured and indicate, in accordance with the DFT calculations, a difference of nearly 900 meV. The difference is due to the strong interaction and local charge displacement of the K+/Br− ions and the Ir(111) surface, which are reduced by the decoupling effect of graphene, thus yielding different electrical and mechanical properties of the top KBr layer.

On-surface activation of benzylic C-H bonds for the synthesis of pentagon-fused graphene nanoribbons

Graphene nanoribbons (GNRs) have potential for applications in electronic devices. A key issue, thereby, is the fine-tuning of their electronic characteristics, which can be achieved through subtle structural modifications. These are not limited to the conventional armchair, zigzag, and cove edges, but also possible through incorporation of non-hexagonal rings. On-surface synthesis enables the fabrication and visualization of GNRs with atomically precise chemical structures, but strategies for the incorporation of non-hexagonal rings have been underexplored. Herein, we describe the on-surface synthesis of armchair-edged GNRs with incorporated five-membered rings through the C-H activation and cyclization of benzylic methyl groups. Ortho-Tolyl-substituted dibromobianthryl was employed as the precursor monomer, and visualization of the resulting structures after annealing at 300 °C on a gold surface by high-resolution noncontact atomic force microscopy clearly revealed the formation of methylene-bridged pentagons at the GNR edges. These persisted after annealing at 340 °C, along with a few fully conjugated pentagons having singly-hydrogenated apexes. The benzylic methyl groups could also migrate or cleave-off, resulting in defects lacking the five-membered rings. Moreover, unexpected and unique structural rearrangements, including the formation of embedded heptagons, were observed. Despite the coexistence of different reaction pathways that hamper selective synthesis of a uniform structure, our results provide novel insights into on-surface reactions en route to functional, non-benzenoid carbon nanomaterials.

Hydrogen bonded trimesic acid networks on Cu(111) reveal how basic chemical properties are imprinted in HR-AFM images.

High resolution non-contact atomic force microscopy measurements characterize assemblies of trimesic acid molecules on Cu(111) and the link group interactions, providing the first fingerprints utilizing CO-based probes for this widely studied paradigm for hydrogen bond driven molecular self assembly. The enhanced submolecular resolution offered by this technique uniquely reveals key aspects of the competing interactions. Accurate comparison between full-density-based modeled images and experiment allows to identify key structural elements in the assembly in terms of the electron-withdrawing character of the carboxylic groups, interactions of those groups with Cu atoms in the surface, and the valence electron density in the intermolecular region of the hydrogen bonds. This study of trimesic acid assemblies on Cu(111) combining high resolution atomic force microscopy measurements with theory and simulation forges clear connections between fundamental chemical properties of molecules and key features imprinted in force images with submolecular resolution.

High resolution noncontact atomic force microscopy imaging with oxygen-terminated copper tips at 78 K.

Functionalizing atomic force microscopy (AFM) tips by picking up single inert probe particles like CO or Xe from the surface drastically increase the resolution. In particular, this approach allows imaging organic molecules with submolecular resolution revealing their internal bonding structure. However, due to the weak coupling of these probe particles to both, the surface they are picked up from and the tip apex, these experiments require liquid helium temperatures (i.e.≈5 K). In the present study we demonstrate that functionalizing an AFM tip with an atomically defined O-terminated copper tip (CuOx tip) allows performing such experiments at liquid nitrogen temperatures (i.e.≈78 K) with outstanding quality. We show that it is possible to utilize CuOx tips for chemically selective imaging of a copper oxide nanodomain on a partially oxidized Cu(110) surface in the repulsive force regime at elevated temperatures. Moreover, the high structural and chemical stability of CuOx tips allow even ex situ investigations where these tips are used to perform experiments on other, non-Cu, non-oxidized, substrates. In particular, we present results obtained from a dicoronylene (DCLN) molecule with submolecular resolution. An analysis of inner and peripheral bond lengths of the DCLN molecule shows excellent agreement with theoretical gas phase simulations emphasizing the exceptional imaging properties of CuOx tips also at elevated temperatures.

Resolving the Unpaired-Electron Orbital Distribution in a Stable Organic Radical by Kondo Resonance Mapping.

The adsorption geometry and the electronic structure of a Blatter radical derivative on a gold surface were investigated by a combination of high-resolution noncontact atomic force microscopy and scanning tunneling microscopy. While the hybridization with the substrate hinders direct access to the molecular states, we show that the unpaired-electron orbital can be probed with Ångström resolution by mapping the spatial distribution of the Kondo resonance. The Blatter derivative features a peculiar delocalization of the unpaired-electron orbital over some but not all moieties of the molecule, such that the Kondo signature can be related to the spatial fingerprint of the orbital. We observe a direct correspondence between these two quantities, including a pronounced nodal plane structure. Finally, we demonstrate that the spatial signature of the Kondo resonance also persists upon noncovalent dimerization of molecules.

The Pentagonal Nature of Self-Assembled Silicon Chains and Magic Clusters on Ag(110)

The atomic structures of self-assembled silicon nanoribbons and magic clusters on Ag(110) substrate have been studied by high-resolution noncontact atomic force microscopy (nc-AFM) and tip-enhanced Raman spectroscopy (TERS). Pentagon-ring structures in Si nanoribbons and clusters have been directly visualized. Moreover, the vibrational fingerprints of individual Si nanoribbon and cluster retrieved by subnanometer resolution TERS confirm the pentagonal nature of both Si nanoribbons and clusters. This work demonstrates that Si pentagon can be an important element in building silicon nanostructures, which may find important applications for future nanoelectronic devices based on silicon.

Formation routes and structural details of the CaF1 layer on Si(111) from high-resolution noncontact atomic force microscopy data

We investigate the ${\mathrm{CaF}}_{1}/\mathrm{Si}(111)$ interface using a combination of high-resolution scanning tunneling and noncontact atomic force microscopy operated at cryogenic temperature as well as x-ray photoelectron spectroscopy. Submonolayer ${\mathrm{CaF}}_{1}$ films grown at substrate temperatures between 550 and 600 ${}^{\ensuremath{\circ}}\mathrm{C}$ on $\mathrm{Si}(111)$ surfaces reveal the existence of two island types that are distinguished by their edge topology, nucleation position, measured height, and inner defect structure. Our data suggest a growth model where the two island types are the result of two reaction pathways during ${\mathrm{CaF}}_{1}$ interface formation. A key difference between these two pathways is identified to arise from the excess species during the growth process, which can be either fluorine or silicon. Structural details as a result of this difference are identified by means of high-resolution noncontact atomic force microscopy and add insights into the growth mode of this heteroepitaxial insulator-on-semiconductor system.

Corrugation in the Weakly Interacting Hexagonal-BN/Cu(111) System: Structure Determination by Combining Noncontact Atomic Force Microscopy and X-ray Standing Waves.

Atomically thin hexagonal boron nitride (h-BN) layers on metallic supports represent a promising platform for the selective adsorption of atoms, clusters, and molecular nanostructures. Specifically, scanning tunneling microscopy (STM) studies revealed an electronic corrugation of h-BN/Cu(111), guiding the self-assembly of molecules and their energy level alignment. A detailed characterization of the h-BN/Cu(111) interface including the spacing between the h-BN sheet and its support-elusive to STM measurements-is crucial to rationalize the interfacial interactions within these systems. To this end, we employ complementary techniques including high-resolution noncontact atomic force microscopy, STM, low-energy electron diffraction, X-ray photoelectron spectroscopy, the X-ray standing wave method, and density functional theory. Our multimethod study yields a comprehensive, quantitative structure determination including the adsorption height and the corrugation of the sp2 bonded h-BN layer on Cu(111). Based on the atomic contrast in atomic force microscopy measurements, we derive a measurable-hitherto unrecognized-geometric corrugation of the h-BN monolayer. This experimental approach allows us to spatially resolve minute height variations in low-dimensional nanostructures, thus providing a benchmark for theoretical modeling. Regarding potential applications, e.g., as a template or catalytically active support, the recognition of h-BN on Cu(111) as a weakly bonded and moderately corrugated overlayer is highly relevant.

Strain-induced skeletal rearrangement of a polycyclic aromatic hydrocarbon on a copper surface

Controlling the structural deformation of organic molecules can drive unique reactions that cannot be induced only by thermal, optical or electrochemical procedures. However, in conventional organic synthesis, including mechanochemical procedures, it is difficult to control skeletal rearrangement in polycyclic aromatic hydrocarbons (PAHs). Here, we demonstrate a reaction scheme for the skeletal rearrangement of PAHs on a metal surface using high-resolution noncontact atomic force microscopy. By a combination of organic synthesis and on-surface cyclodehydrogenation, we produce a well-designed PAH—diazuleno[1,2,3-cd:1′,2′,3′-fg]pyrene—adsorbed flatly onto Cu(001), in which two azuleno moieties are highly strained by their mutual proximity. This local strain drives the rearrangement of one of the azuleno moieties into a fulvaleno moiety, which has never been reported so far. Our proposed thermally driven, strain-induced synthesis on surfaces will pave the way for the production of a new class of nanocarbon materials that conventional synthetic techniques cannot attain.

Exploring Molecules beyond CO as Tip Functionalizations in High-Resolution Noncontact Atomic Force Microscopy: A First Principles Approach.

Atomic resolution of molecules has been achieved using noncontact atomic force microscopy (AFM) with the key step to functionalize the tip apex by attaching suitable molecules so as to achieve high spatial resolution through a sharper tip. A few molecular terminations have been explored theoretically and experimentally, and they exhibit various imaging behaviors. Here, we explore the influence of the structures and chemical compositions of various molecular candidates as tips on the contrast of AFM images by a first principles approach. Our results reveal that the two end atoms of a linear molecule terminating nearest the sample dominate the imaging behaviors, for example, atomic resolution, sharpness, distortion, and so forth, whereas the symmetry of the termination plays an important role in the distortion of AFM images. These findings suggest that new tip terminations can be engineered by decoupling the three end atoms responsible for imaging behaviors from the tip structure behind them, which is attached to the macro tip apex.

Increasing the Templating Effect on a Bulk Insulator Surface: From a Kinetically Trapped to a Thermodynamically More Stable Structure

Molecular self-assembly, governed by the subtle balance between intermolecular and molecule–surface interactions, is generally associated with the thermodynamic ground state, while the competition between kinetics and thermodynamics during its formation is often neglected. Here, we present a simple model system of a benzoic acid derivative on a bulk insulator surface. Combining high-resolution noncontact atomic force microscopy experiments and density functional theory, we characterize the structure and the thermodynamic stability of a set of temperature-dependent molecular phases formed by 2,5-dihydroxybenzoic acid molecules, self-assembled on the insulating calcite (10.4) surface. We demonstrate that a striped phase forms before the thermodynamically favored dense phase, indicating a kinetically trapped state. Our theoretical analysis elucidates that this striped-to-dense phase transition is associated with a distinct change in the chemical interactions involved in the two phases. The striped phase is c...

Charge density wave order in 1D mirror twin boundaries of single-layer MoSe2

Properties of two-dimensional transition metal dichalcogenides are highly sensitive to the presence of defects in the crystal structure. A detailed understanding of defect structure may lead to control of material properties through defect engineering. Here we provide direct evidence for the existence of isolated, one-dimensional charge density waves at mirror twin boundaries in single-layer MoSe2. Our low-temperature scanning tunneling microscopy/spectroscopy measurements reveal a substantial bandgap of 60 - 140 meV opening at the Fermi level in the otherwise one dimensional metallic structure. We find an energy-dependent periodic modulation in the density of states along the mirror twin boundary, with a wavelength of approximately three lattice constants. The modulations in the density of states above and below the Fermi level are spatially out of phase, consistent with charge density wave order. In addition to the electronic characterization, we determine the atomic structure and bonding configuration of the one-dimensional mirror twin boundary by means of high-resolution non-contact atomic force microscopy. Density functional theory calculations reproduce both the gap opening and the modulations of the density of states.

Molecular self-assembly on an insulating surface: interplay between substrate templating and intermolecular interactions

We report on molecular self-assembly of biphenyl-4,4′-dicarboxylic acid (BPDCA) on under ultra-high vacuum conditions. Two-dimensional, ordered islands are obtained upon deposition at room temperature, coexisting with a streaky structure that is ascribed to individual, mobile molecules forming a two-dimensional gas-like phase. High-resolution non-contact atomic force microscopy (NC-AFM) images of the molecular islands reveal an ordered inner structure that is dominated by rows of molecules aligned side by side running along the crystallographic direction. A detailed analysis of these rows exhibits inter-row distances that are multiples of the calcite unit cell dimension along the direction, clearly demonstrating the templating effect of the substrate. Our results indicate that an excellent size match of the molecular structure with respect to the underlying substrate results in an increased binding of the BPDCA molecules to the surface. In between the rows, a different molecular structure is coexisting with the molecules aligning head to tail. This structure is explained by intermolecular hydrogen bond formation very similar to the BPDCA bulk structure. The coexistence of the bulk-like structure with the row structure suggests a close balance of intermolecular and molecule–surface interactions to be responsible for the observed structure formation.

Direct Visualization of Molecule Deprotonation on an Insulating Surface

Elucidating molecular-scale details of basic reaction steps on surfaces is decisive for a fundamental understanding of molecular reactivity within many fields, including catalysis and on-surface synthesis. Here, the deprotonation of 2,5-dihydroxybenzoic acid (DHBA) deposited onto calcite (101;4) held at room temperature is followed in situ by noncontact atomic force microscopy. After deposition, the molecules form two coexisting phases, a transient striped phase and a stable dense phase. A detailed analysis of high-resolution noncontact atomic force microscopy images indicates the transient striped phase being a bulk-like phase, which requires hydrogen bonds between the carboxylic acid moieties to be formed. With time, the striped phase transforms into the dense phase, which is explained by the deprotonation of the molecules. In the deprotonated state, the molecules can no longer form hydrogen bonds, but anchor to the surface calcium cations with their negatively charged carboxylate group. The deprotonation step is directly confirmed by Kelvin probe force microscopy images that unravel the change in the molecular charge.

Contrast inversion of the h-BN nanomesh investigated by nc-AFM and Kelvin probe force microscopy

Single sheets of hexagonal boron nitride (h-BN) on transition metals provide a model system for layered insulating materials as well as a functional substrate for molecules and metal clusters. The progress in the understanding of h-BN layers on transition metals was mainly driven by scanning tunnelling microscopy (STM) and photoelectron spectroscopy (PES) measurements within the last decade, while direct measurements of mechanical and electrical properties are still rare. Our investigations of the two-dimensional (2D) h-BN nanomesh on a Rh(111) substrate by high-resolution noncontact atomic force microscopy (nc-AFM) reveal a complex surface structure including a frequently observed contrast inversion. Detailed 2D force spectroscopy measurements are revealing towards a mechanical elastic deformation of the h-BN monolayer caused by the tip–sample interaction. Furthermore, Kelvin probe force microscopy (KPFM) and spectroscopy measurements show local work function variations of the nanomesh, proving the results obtained by PES but additionally providing detailed local information.

Noncontact atomic force microscopy study of the spinel MgAl2O4(111) surface

Based on high-resolution noncontact atomic force microscopy (NC-AFM) experiments we reveal a detailed structural model of the polar (111) surface of the insulating ternary metal oxide, MgAl2O4 (spinel). NC-AFM images reveal a 6√3×6√3R30° superstructure on the surface consisting of patches with the original oxygen-terminated MgAl2O4(111) surface interrupted by oxygen-deficient areas. These observations are in accordance with previous theoretical studies, which predict that the polarity of the surface can be compensated by removal of a certain fraction of oxygen atoms. However, instead of isolated O vacancies, it is observed that O is removed in a distinct pattern of line vacancies reflected by the underlying lattice structure. Consequently, by the creation of triangular patches in a 6√3×6√3R30° superstructure, the polar-stabilization requirements are met.

Mechanical properties of H2Pc self-assembled monolayers at the single molecule level by noncontact atomic force microscopy

The mechanical properties of molecular self-assembled monolayers (SAMs) play an important role in understanding the interactions between molecules in the self-assembly, the interactions between molecules and substrate, and thus the formation mechanism of SAMs. Using a high-resolution noncontact atomic force microscope (NC-AFM) combined with a scanning tunneling microscope (STM), we have successfully obtained the sub-molecular resolution of a H2Pc self-assembled monolayer grown on a Pb(111) surface. A 2 × 2 superstructure was observed in both AFM and STM topographic images. The lateral critical force of removing a H2Pcmolecule from its SAM and moving a single H2Pc molecule on Pb(111) were measured. An oscillation of the critical force along the edge of the H2Pc SAM with a period of two molecular sites was observed, which can be attributed to the 2 × 2 superstructure. The lateral critical force caused by intermolecular interaction was found to be 25 pN on average and is typically two times larger than the molecule–substrate interaction.