Visualizing the Subsurface of Soft Matter: Simultaneous Topographical Imaging, Depth Modulation, and Compositional Mapping with Triple Frequency Atomic Force Microscopy

Characterization of subsurface morphology and mechanical properties with nanoscale resolution and depth control is of significant interest in soft matter fields like biology, polymer science, and even in future applications like nanomanufacturing, where buried structural and compositional features are important to the functionality of the system. However, controllably “feeling” the subsurface is a challenging task for which the available imaging tools are relatively limited. In this paper, we propose a trimodal atomic force microscopy (AFM) imaging scheme, whereby three eigenmodes of the microcantilever probe are used as separate control “knobs” to simultaneously measure the topography, modulate sample indentation by the tip during tip–sample impact, and map compositional contrast, respectively. We illustrate this multifrequency imaging approach through computational simulation and experiments conducted on ultrathin polymer films with embedded glass nanoparticles in ambient air. By actively increasing the tip–sample indentation using a higher eigenmode of the cantilever, we are able to gradually and controllably reveal glass nanoparticles which are buried tens of nanometers deep under the surface, while still being able to refocus on the surface

3‑Dimensional Structure of a Prototypical Ionic Liquid–Solid Interface: Ionic Crystal-Like Behavior Induced by Molecule–Substrate Interactions

The molecular structure of the ionic liquid–electrode interface has been recently investigated by atomic force microscopy (AFM) methods focusing either on the vertical structure of the ion layers or on the lateral structure of the innermost layer. Here, we combine high-resolution AFM imaging with atomic force spectroscopy measurements to elucidate the structure of the interface between the ionic liquid propylammonium nitrate (PAN) and highly ordered pyrolytic graphite (HOPG). The lateral structure of the innermost layer of adsorbed molecules (i.e., the Stern layer) is resolved on the molecular scale by means of amplitude modulation atomic force microscopy (AM-AFM). A quasi (4 × 4)<i>R</i>0° overlayer is formed by the ionic liquid molecules on the HOPG surface. Additional dynamic mode force spectroscopy measurements reveal the existence of a layered structure of the ionic liquid normal to the surface plane and allow for a precise determination of the layer spacing. We are able to infer the <i>three-dimensional</i> structure of the PAN–HOPG interface from a combination of both information, i.e., the experimentally observed lateral and normal structures. The obtained 3D lattice structure is in accordance with a zincblende-type (ZnS) crystal structure

Subsurface-Controlled Angular Rotation: Triphenylene Molecules on Au(111) Substrates

Molecular self-assembly can be used for the bottom-up fabrication of nanoscale electronic devices. For this, polycyclic aromatic hydrocarbons are particularly suited due to their extended π-system. Besides, these compounds can serve as precursors for fabricating graphene-like material. To design future electronic devices it is essential to be able to reproducibly predict the structure formation of these molecules after adsorption on solid surfaces. Here we studied the self-assembly of triphenylene molecules on a reconstructed Au(111) substrate in the submonolayer regime by scanning tunneling microscopy. Only two different orientations of the planar adsorbed molecules are observed. At intermediate coverages self-assembly is mainly determined by repulsive molecule–molecule interactions, leading to a one-to-one ratio of molecular orientations. At 1.0 monolayer coverage, however, a reorientation into close-packed domains occurs, which significantly shifts the ratio of molecular orientations. It is found that this reorientation is controlled by molecule–subsurface interactions, opening new avenues in assembling molecules on surfaces

Precise Monoselective Aromatic C–H Bond Activation by Chemisorption of <i>Meta</i>-Aryne on a Metal Surface

Aromatic C–H bond activation has attracted much attention due to its versatile applications in the synthesis of aryl-containing chemicals. The major challenge lies in the minimization of the activation barrier and maximization of the regioselectivity. Here, we report the highly selective activation of the central aromatic C–H bond in <i>meta</i>-aryne species anchored to a copper surface, which catalyzes the C–H bond dissociation. Two prototype molecules, i.e., 4′,6′-dibromo-<i>meta</i>-terphenyl and 3′,5′-dibromo-<i>ortho</i>-terphenyl, have been employed to perform C–C coupling reactions on Cu(111). The chemical structures of the resulting products have been clarified by a combination of scanning tunneling microscopy and noncontact atomic force microscopy. Both methods demonstrate a remarkable weakening of the targeted C–H bond. Density functional theory calculations reveal that this efficient C–H activation stems from the extraordinary chemisorption of the <i>meta</i>-aryne on the Cu(111) surface, resulting in the close proximity of the targeted C–H group to the Cu(111) surface and the absence of planarity of the phenyl ring. These effects lead to a lowering of the C–H dissociation barrier from 1.80 to 1.12 eV, in agreement with the experimental data

London Dispersion Directs On-Surface Self-Assembly of [121]Tetramantane Molecules

London dispersion (LD) acts between all atoms and molecules in nature, but the role of LD interactions in the self-assembly of molecular layers is still poorly understood. In this study, direct visualization of single molecules using atomic force microscopy with CO-functionalized tips revealed the exact adsorption structures of bulky and highly polarizable [121]­tetramantane molecules on Au(111) and Cu(111) surfaces. We determined the absolute molecular orientations of the completely sp³-hybridized tetramantanes on metal surfaces. Moreover, we demonstrate how LD drives this on-surface self-assembly of [121]­tetramantane hydrocarbons, resulting in the formation of a highly ordered 2D lattice. Our experimental findings were underpinned by a systematic computational study, which allowed us to quantify the energies associated with LD interactions and to analyze intermolecular close contacts and attractions in detail

Hierarchical Dehydrogenation Reactions on a Copper Surface

Hierarchical control of chemical reactions is being considered as one of the most ambitious and challenging topics in modern organic chemistry. In this study, we have realized the one-by-one scission of the X–H bonds (X = N and C) of aromatic amines in a controlled fashion on the Cu(111) surface. Each dehydrogenation reaction leads to certain metal–organic supramolecular structures, which were monitored in single-bond resolution via scanning tunneling microscopy and noncontact atomic force microscopy. Moreover, the reaction pathways were elucidated from X-ray photoelectron spectroscopy measurements and density functional theory calculations. Our insights pave the way for connecting molecules into complex structures in a more reliable and predictable manner, utilizing carefully tuned stepwise on-surface synthesis protocols