Carbon allotropes built from rings of two-coordinate atoms, known as cyclo
carbons, have fascinated chemists for many years, but until now they could not be isolated or structurally characterized ...because of their high reactivity. We generated cyclo18carbon (C
) using atom manipulation on bilayer NaCl on Cu(111) at 5 kelvin by eliminating carbon monoxide from a cyclocarbon oxide molecule, C
O
Characterization of cyclo18carbon by high-resolution atomic force microscopy revealed a polyynic structure with defined positions of alternating triple and single bonds. The high reactivity of cyclocarbon and cyclocarbon oxides allows covalent coupling between molecules to be induced by atom manipulation, opening an avenue for the synthesis of other carbon allotropes and carbon-rich materials from the coalescence of cyclocarbon molecules.
Scanning tunnelling microscopy and atomic force microscopy can be used to study the electronic and structural properties of surfaces, as well as molecules and nanostructures adsorbed on surfaces, ...with atomic precision, but they cannot directly probe the distribution of charge in these systems. However, another form of scanning probe microscopy, Kelvin probe force microscopy, can be used to measure the local contact potential difference between the scanning probe tip and the surface, a quantity that is closely related to the charge distribution on the surface. Here, we use a combination of scanning tunnelling microscopy, atomic force microscopy and Kelvin probe force microscopy to examine naphthalocyanine molecules (which have been used as molecular switches) on a thin insulating layer of NaCl on Cu(111). We show that Kelvin probe force microscopy can map the local contact potential difference of this system with submolecular resolution, and we use density functional theory calculations to verify that these maps reflect the intramolecular distribution of charge. This approach could help to provide fundamental insights into single-molecule switching and bond formation, processes that are usually accompanied by the redistribution of charge within or between molecules.
Triangulene, the smallest triplet-ground-state polybenzenoid (also known as Clar's hydrocarbon), has been an enigmatic molecule ever since its existence was first hypothesized. Despite containing an ...even number of carbons (22, in six fused benzene rings), it is not possible to draw Kekulé-style resonant structures for the whole molecule: any attempt results in two unpaired valence electrons. Synthesis and characterization of unsubstituted triangulene has not been achieved because of its extreme reactivity, although the addition of substituents has allowed the stabilization and synthesis of the triangulene core and verification of the triplet ground state via electron paramagnetic resonance measurements. Here we show the on-surface generation of unsubstituted triangulene that consists of six fused benzene rings. The tip of a combined scanning tunnelling and atomic force microscope (STM/AFM) was used to dehydrogenate precursor molecules. STM measurements in combination with density functional theory (DFT) calculations confirmed that triangulene keeps its free-molecule properties on the surface, whereas AFM measurements resolved its planar, threefold symmetric molecular structure. The unique topology of such non-Kekulé hydrocarbons results in open-shell π-conjugated graphene fragments that give rise to high-spin ground states, potentially useful in organic spintronic devices. Our generation method renders manifold experiments possible to investigate triangulene and related open-shell fragments at the single-molecule level.
Petroleum is one of the most precious and complex molecular mixtures existing. Because of its chemical complexity, the solid component of crude oil, the asphaltenes, poses an exceptional challenge ...for structure analysis, with tremendous economic relevance. Here, we combine atomic-resolution imaging using atomic force microscopy and molecular orbital imaging using scanning tunnelling microscopy to study more than 100 asphaltene molecules. The complexity and range of asphaltene polycyclic aromatic hydrocarbons are established in detail. Identifying molecular structures provides a foundation to understand all aspects of petroleum science from colloidal structure and interfacial interactions to petroleum thermodynamics, enabling a first-principles approach to optimize resource utilization. Particularly, the findings contribute to a long-standing debate about asphaltene molecular architecture. Our technique constitutes a paradigm shift for the analysis of complex molecular mixtures, with possible applications in molecular electronics, organic light emitting diodes, and photovoltaic devices.
Resolving individual atoms has always been the ultimate goal of surface microscopy. The scanning tunneling microscope images atomic-scale features on surfaces, but resolving single atoms within an ...adsorbed molecule remains a great challenge because the tunneling current is primarily sensitive to the local electron density of states close to the Fermi level. We demonstrate imaging of molecules with unprecedented atomic resolution by probing the short-range chemical forces with use of noncontact atomic force microscopy. The key step is functionalizing the microscope's tip apex with suitable, atomically well-defined terminations, such as CO molecules. Our experimental findings are corroborated by ab initio density functional theory calculations. Comparison with theory shows that Pauli repulsion is the source of the atomic resolution, whereas van der Waals and electrostatic forces only add a diffuse attractive background.
Atomic force microscopy (AFM) as well as scanning tunneling microscopy induced light emission (STM-LE) are, each on their own, powerful tools used to investigate a large variety of properties of ...single molecules adsorbed on a surface. However, accessing both structural information by AFM as well as optical information by STM-LE on the same molecule so far remains elusive. We present a combined high-resolution AFM and STM-LE study on single metal-oxide phthalocyanines. Using atomic manipulation, the molecules can be deliberately reduced. We demonstrate structure elucidation and adsorption geometry determination of single molecules with atomic resolution combined with optical characterization by STM-LE and the possibility of investigating the change in a molecule’s exciton emission intensity by a chemical reaction.
Using scanning probe microscopy techniques, at low temperatures and in ultrahigh vacuum, individual molecules adsorbed on surfaces can be probed with ultrahigh resolution to determine their structure ...and details of their conformation, configuration, charge states, aromaticity, and the contributions of resonance structures. Functionalizing the tip of an atomic force microscope with a CO molecule enabled atomic‐resolution imaging of single molecules, and measurement of their adsorption geometry and bond‐order relations. In addition, by using scanning tunneling microscopy and Kelvin probe force microscopy, the density of the molecular frontier orbitals and the electric charge distribution within molecules can be mapped. Combining these techniques yields a high‐resolution tool for the identification and characterization of individual molecules. The single‐molecule sensitivity and the possibility of atom manipulation to induce chemical reactions with the tip of the microscope open up unique applications in chemistry, and differentiate scanning probe microscopy from conventional methods for molecular structure elucidation. Besides being an aid for challenging cases in natural product identification, atomic force microscopy has been shown to be a powerful tool for the investigation of on‐surface reactions and the characterization of radicals and molecular mixtures. Herein we review the progress that high‐resolution scanning probe microscopy with functionalized tips has made for molecular structure identification and characterization, and discuss the challenges it will face in the years to come.
The tip of the iceberg? Functionalizing the tip of an atomic force microscope enabled atomic‐resolution imaging of single molecules and offers unique applications in chemistry for the investigation of on‐surface reactions and the characterization of elusive molecules and complex molecular mixtures. This Review covers the recent progress in high‐resolution scanning probe microscopy with functionalized tips.
The field of molecular electronics aims at using single molecules as functional building blocks for electronics components, such as switches, rectifiers or transistors. A key challenge is to perform ...measurements with atomistic control over the alignment of the molecule and its contacting electrodes. Here we use atomic force microscopy to examine charge transfer between weakly coupled pentacene molecules on insulating films with single-electron sensitivity and control over the atomistic details. We show that, in addition to the imaging capability, the probe tip can be used to control the charge state of individual molecules and to detect charge transfers to/from the tip, as well as between individual molecules. Our approach represents a novel route for molecular charge transfer studies with a host of opportunities, especially in combination with single atom/molecule manipulation and nanopatterning techniques.
Using atomic manipulation, one can dissociate, form and rearrange bonds, as well as alter the conformation or charge state of molecules. The molecular structures of reactants, intermediates and ...products are revealed at unprecedented resolution by using atomic force microscopy (AFM) and a suitably functionalized tip. Our present capabilities of manipulation and imaging of molecules by AFM approach the level of control predicted by Richard P. Feynman in his famous lecture ‘There's plenty of room at the bottom’, in which he described how molecules and materials might be formed by attaching and detaching individual atoms at will. In this Review, we discuss recent progress and the future prospects of molecule generation by atom manipulation and molecular characterization by AFM.Atomic force microscopy (AFM) enables the imaging and manipulation of individual molecules at atomic resolution. This Review addresses experimental considerations, including operating modes and choices for tips and substrates. Examples are presented in which AFM is used to image molecules and induce bond formation or dissociation.
Controlling selectivity of reactions is an ongoing quest in chemistry. In this work, we demonstrate reversible and selective bond formation and dissociation promoted by tip-induced ...reduction-oxidation reactions on a surface. Molecular rearrangements leading to different constitutional isomers are selected by the polarity and magnitude of applied voltage pulses from the tip of a combined scanning tunneling and atomic force microscope. Characterization of voltage dependence of the reactions and determination of reaction rates demonstrate selectivity in constitutional isomerization reactions and provide insight into the underlying mechanisms. With support of density functional theory calculations, we find that the energy landscape of the isomers in different charge states is important to rationalize the selectivity. Tip-induced selective single-molecule reactions increase our understanding of redox chemistry and could lead to novel molecular machines.
Tip-induced organic reactions
Control over the reaction products of a unimolecular transformation on a surface have been induced and visualized with a scanning tunneling microscope (STM) tip. Albrecht
et al
. synthesized a tetrachlorotetracene molecule and absorbed it on a thin salt layer grown on copper (see the Perspective by Alabugin and Hu). Under cryogenic conditions, voltage pulses from the STM tip led to the elimination of the chlorine atoms and produced intermediates with a large central ring. Subsequent voltage pulses created other isomers of this molecule, a diyne and a chrysene-based bisaryne, in reactions that could be reversed with opposite polarity pulses. —PDS
Different bonds are formed selectively in a molecule by atom manipulation with the voltages applied using a scanning probe tip.
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