Gerhard Meyer

Novel optical approach to atomic force microscopy

A sensitive and simple optical method for detecting the cantilever deflection in atomic force microscopy is described. The method was incorporated in an atomic force microscope, and imaging and force measurements, in ultrahigh vacuum, were successfully performed.

The Chemical Structure of a Molecule Resolved by Atomic Force Microscopy

Atomic Imaging Within Adsorbed Molecules Scanning tunneling microscopy provides atomic resolution images of surfaces and adsorbed atoms, but imaging atoms within an organic molecule adsorbed on a surface is difficult because contrast is lacking in the states that determine the tunneling current. Atomic force microscopy should be able to resolve atoms through changes in short-range chemical forces, but resolution is lost if the scanning tip undergoes modifications or if it moves the molecule. Gross et al. (p. 1110) show that in situ functionalization of the tip—for example, with CO—can dramatically improve the resolution of images of pentacene molecules adsorbed on conducting surfaces, like copper, and nonconductors, like NaCl. Derivitization of atomic force microscope tips with carbon monoxide molecules allows atoms to be resolved within adsorbed molecules. 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.

Simultaneous measurement of lateral and normal forces with an optical‐beam‐deflection atomic force microscope

An atomic force microscope capable of measuring, simultaneously yet separately, lateral (‘‘frictional’’) and normal forces is described. A direction‐dependent feature, absent in topological images, is found when scanning stepped surfaces of NaCl (001) in ultrahigh vacuum. A simple model is presented to account for this observation.

Current-induced hydrogen tautomerization and conductance switching of naphthalocyanine molecules

The bistability in the position of the two hydrogen atoms in the inner cavity of single free-base naphthalocyanine molecules constitutes a two-level system that was manipulated and probed by low-temperature scanning tunneling microscopy. When adsorbed on an ultrathin insulating film, the molecules can be switched in a controlled fashion between the two states by excitation induced by the inelastic tunneling current. The tautomerization reaction can be probed by resonant tunneling through the molecule and is expressed as considerable changes in the conductivity of the molecule. We also demonstrated a coupling of the switching process so that the charge injection in one molecule induced tautomerization in an adjacent molecule.

Bond-order discrimination by atomic force microscopy.

Visualizing Bond Order Bond lengths in conjugated molecules closely reflect individual bond order and are usually determined by diffraction methods. It is valuable to know bond order for rationalizing aromaticity, and reactivity and for chemical structure determination. Gross et al. (p. 1326; see the Perspective by Perez and the cover) differentiated the bond orders in individual molecules in the fullerene C60 and in polyaromatic hydrocarbons by imaging with noncontact atomic force microscopy (AFM). The molecules were adsorbed onto a copper surface, and the AFM tip was decorated with a CO molecule, which was used to measure tip frequency shifts above the bonds and their apparent lengths. Multiple bonds appeared brighter in the images because of stronger Pauli repulsion, and their shorter length was amplified by bending of the CO at the tip apex. Images detected with an atomic force microscope tip decorated with a carbon monoxide molecule could distinguish Pauling bond order. We show that the different bond orders of individual carbon-carbon bonds in polycyclic aromatic hydrocarbons and fullerenes can be distinguished by noncontact atomic force microscopy (AFM) with a carbon monoxide (CO)–functionalized tip. We found two different contrast mechanisms, which were corroborated by density functional theory calculations: The greater electron density in bonds of higher bond order led to a stronger Pauli repulsion, which enhanced the brightness of these bonds in high-resolution AFM images. The apparent bond length in the AFM images decreased with increasing bond order because of tilting of the CO molecule at the tip apex.

Optical‐beam‐deflection atomic force microscopy: The NaCl (001) surface

We have imaged, in ultrahigh vacuum, the (001) surface of NaCl using an optical‐beam‐deflectin force microscope operating in the short‐range repulsive regime. The design and performance characteristics of the microscope are given, and the observed atomic corrugations are compared with those deduced from He‐atom scattering experiments.

Imaging Bond Formation Between a Gold Atom and Pentacene on an Insulating Surface

A covalent bond between an individual pentacene molecule and a gold atom was formed by means of single-molecule chemistry inside a scanning tunneling microscope junction. The bond formation is reversible, and different structural isomers can be produced. The single-molecule synthesis was done on ultrathin insulating films that electronically isolated the reactants and products from their environment. Direct imaging of the orbital hybridization upon bond formation provides insight into the energetic shifts and occupation of the molecular resonances.

Controlled vertical manipulation of single CO molecules with the scanning tunneling microscope: A route to chemical contrast

A reliable procedure for controlled vertical transfer of single CO molecules between a Cu(111) surface and a scanning tunneling microscope tip and vice versa is demonstrated. It is shown that with a tip having a single CO molecule at its apex, chemical contrast is achieved allowing distinction of adsorbed CO molecules and oxgen atoms, which look very similar to the bare metal tip.

Measuring the Charge State of an Adatom with Noncontact Atomic Force Microscopy

Spotting Charges Many nanoscale physical systems are sensitive to the position of isolated charges, such as single-electron transistors and molecular assemblies that separate charges with energy from photons. In order to probe the location of a charged atom, the most general methods would work on insulating surfaces. Gross et al. (p. 1428; see the Perspective by Meyer and Glatzel) show that a tuning-fork atomic force microscope (AFM) operating in a noncontact mode at cryogenic temperatures can resolve the charge state of gold and silver atoms absorbed on a sodium chloride film. Charged atoms set up image charges in the AFM tip, which creates an electrostatic force not present with a neutral atom. Charging of gold and silver atoms on salt films changes the force detected by the tip of a scanning probe microscope. Charge states of atoms can be investigated with scanning tunneling microscopy, but this method requires a conducting substrate. We investigated the charge-switching of individual adsorbed gold and silver atoms (adatoms) on ultrathin NaCl films on Cu(111) using a qPlus tuning fork atomic force microscope (AFM) operated at 5 kelvin with oscillation amplitudes in the subangstrom regime. Charging of a gold atom by one electron charge increases the force on the AFM tip by a few piconewtons. Moreover, the local contact potential difference is shifted depending on the sign of the charge and allows the discrimination of positively charged, neutral, and negatively charged atoms. The combination of single-electron charge sensitivity and atomic lateral resolution should foster investigations of molecular electronics, photonics, catalysis, and solar photoconversion.

Imaging the charge distribution within a single molecule

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.