Light-Induced Increase of the Local Molecular Coverage on a Surface.

Light is a versatile tool to remotely activate molecules adsorbed on a surface, for example, to trigger their polymerization. Here, we explore the spatial distribution of light-induced chemical reactions on a Au(111) surface. Specifically, the covalent on-surface polymerization of an anthracene derivative in the submonolayer coverage range is studied.

Reversible Photoswitching and Isomer-Dependent Diffusion of Single Azobenzene Tetramers on a Metal Surface.

Azobenzene is a prototypical molecular switch that can be reversibly photoisomerized between the nearly planar and apolar trans form, and the distorted, polar cis form. Most studies related to azobenzene derivatives have focused on planar adsorbed molecules. We present herein the study of a three-dimensional shape-persistent molecular architecture consisting of four tetrahedrally arranged azobenzene units that is adsorbed on a Ag(111) surface.

How Structural Defects Affect the Mechanical and Electrical Properties of Single Molecular Wires.

We report how individual defects affect single graphene nanoribbons by scanning tunneling and atomic force microscopy pulling experiments simultaneously accessing their electrical and mechanical properties. The on-surface polymerization of the graphene nanoribbons is controlled by cooperative effects as theoretically suggested. Further, we find, with the help of atomistic simulations, that defects substantially vary the molecule-substrate coupling and drastically increase the flexibility of the graphene nanoribbons while keeping their desirable electronic properties intact.

Reversible and Efficient Light-Induced Molecular Switching on an Insulator Surface.

Prototypical molecular switches such as azobenzenes exhibit two states, i.e., trans and cis, with different characteristic physical properties.

Covalent Assembly and Characterization of Nonsymmetrical Single-Molecule Nodes.

The covalent linking of molecular building blocks on surfaces enables the construction of specific molecular nanostructures of well-defined shape. Molecular nodes linked to various entities play a key role in such networks, but represent a particular challenge because they require a well-defined arrangement of different building blocks. Herein, we describe the construction of a chemically and geometrically well defined covalent architecture made of one central node and three molecular wires arranged in a nonsymmetrical way and thus encoding different conjugation pathways.

Conductance of a single flexible molecular wire composed of alternating donor and acceptor units.

Molecular-scale electronics is mainly concerned by understanding charge transport through individual molecules. A key issue here is the charge transport capability through a single--typically linear--molecule, characterized by the current decay with increasing length. To improve the conductance of individual polymers, molecular design often either involves the use of rigid ribbon/ladder-type structures, thereby sacrificing for flexibility of the molecular wire, or a zero band gap, typically associated with chemical instability.

Controlled switching within an organic molecule deliberately pinned to a semiconductor surface.

Bistable organic molecules were deposited on a weakly binding III-V semiconductor surface and then pinned into place using individual native adatoms. These pinning atoms, positioned by atomically precise manipulation techniques in a cryogenic scanning tunneling microscope (STM) at 5 K, stabilize the π-conjugated molecule against rotation excited by the tunneling electrons. The pinning allows triggering of the molecule's intrinsic switching mechanism (a hydrogen transfer reaction) by the STM tunnel current.

Emergent multistability in assembled nanostructures.

Scanning tunneling microscopy (STM) at 5 K reveals that native atoms in the surface layer of a semiconductor crystal become bistable in vertical height when a nanostructure is assembled nearby. The binary switching of surface atoms, driven by the STM tip, changes their charge state. Coupling is facilitated by assembling adatom chains, allowing us to explore the emergence of complex multiple switching.

Atom-by-atom quantum state control in adatom chains on a semiconductor.

The vertical manipulation of native adatoms on a III-V semiconductor surface was achieved in a scanning tunneling microscope at 5 K. Reversible repositioning of individual In atoms on InAs(111)A allows us to construct one-atom-wide In chains. Tunneling spectroscopy reveals that these chains host quantum states deriving from an adatom-induced electronic state and substantial substrate-mediated coupling.

Current versus temperature-induced switching in a single-molecule tunnel junction: 1,5 cyclooctadiene on Si(001).

The biconformational switching of single cyclooctadiene molecules chemisorbed on a Si(001) surface was explored by quantum chemical and quantum dynamical calculations and low-temperature scanning tunneling microscopy experiments. The calculations rationalize the experimentally observed switching driven by inelastic electron tunneling (IET) at 5 K. At higher temperatures, they predict a controllable crossover behavior between IET-driven and thermally activated switching, which is fully confirmed by experiment.

Doping of monatomic Cu chains with single Co atoms.

Close-packed Co-Cu chains of various length and composition were assembled from single Co and Cu atoms on Cu(111) by atom manipulation in a low-temperature scanning tunneling microscope. Local spectroscopy reveals significant electronic Co-Cu coupling leading to confined quantum states delocalized along the heteroatomic chain. Composite Co-Cu chains provide a model case in which the quantum state of an atomic-scale host structure can be tuned by the controlled incorporation of foreign atoms.

Microscopic and spectroscopic characterization of paintbrush-like single-walled carbon nanotubes.

Understanding and controlling the chemical reactivity of carbon nanotubes (CNTs) is a fundamental requisite to prepare novel nanoscopic structures with practical uses in materials applications. Here, we present a comprehensive microscopic and spectroscopic characterization of carbon nanotubes which have been chemically modified. Specifically, scanning tunneling microscopy (STM) investigations of short-oxidized single-walled carbon nanotubes (SWNTs) functionalized with aliphatic chains via amide reaction reveal the presence of bright lumps both on the sidewalls and at the tips.