We investigated dibenzo [a,h]thianthrene molecules adsorbed on ultrathin layers of NaCl using a combined low-temperature scanning tunneling and atomic force microscope. Two stable configurations exist corresponding to different isomers of free nonplanar molecules. By means of excitations from inelastic electron tunneling we can switch between both configurations. Atomic force microscopy with submolecular resolution allows unambiguous determination of the molecular geometry, and the pathway of the interconversion of the isomers. Our investigations also shed new light on contrast mechanisms in scanning tunneling microscopy. DOI: 10.1103/PhysRevLett.108.086101 PACS numbers: 68.43.Fg, 68.37.Ef, 68.37.Ps, 82.37.Gk Recently, the chemical structure of a pentacene molecule has been visualized by means of noncontact atomic force microscopy (AFM) [1]. Shortly after, this method assisted in identifying the structure of an organic molecule [2]. In conjunction with the capability of scanning tunneling microscopy (STM) to perform orbital imaging on ultrathin insulating films [3], it is possible to gain independent and complementary information of the molecular as well as of the adsorption geometry, but also of the electronic structure of individual molecules.Unambiguous identification of configurational changes of adsorbed molecules is a challenging task by means of STM alone [4], probing the local density of states rather than geometry. Usually, additional techniques such as nearedge x-ray adsorption fine structure measurements have to be employed [5,6].In this Letter, we present combined STM and AFM experiments of dibenzo[a,h]thianthrene (DBTH) molecules adsorbed on ultrathin layers of sodium chloride. We demonstrate controlled switching between two different molecular configurations by means of inelastic excitations. AFM images with submolecular resolution directly reveal the configurational changes. Stereochemistry could be utilized to determine their interconversion pathway in detail.All AFM measurements were carried out in a homebuilt combined STM and AFM operating in an ultrahigh vacuum (p < 10 À10 mbar) at T ¼ 5 K. The AFM, based on the qPlus tuning fork design (spring constant k 0 % 1:8 Â 10 3 N m À1 , resonance frequency f 0 ¼ 26 057 Hz, quality factor Q % 10 4 ) [7], was operated in the frequency modulation mode [8]. Sub-Å ngstrom oscillation amplitudes have been used to maximize the lateral resolution [9]. Some of the STM measurements ( Figs. 1 and 2) were performed in a similar modified commercial STM from SPS-CreaTec. The bias voltage V was applied to the sample.Sodium chloride was evaporated onto clean Cu(111) single crystals at sample temperatures of about 280 K [10]. All experiments were carried out on a double layer, and we denote this substrate system as NaClð2MLÞ= Cuð111Þ. The DBTH molecules were synthesized as described previously [11].Low coverages of CO (for tip functionalization) and DBTH molecules were adsorbed at sample temperatures below 10 K. Following a recently developed technique, the tip ha...

The underlying mechanisms of image distortions in atomic force microscopy (AFM) with CO-terminated tips are identified and studied in detail. AFM measurements of a partially fluorinated hydrocarbon molecule recorded with a CO-terminated tip are compared with state-of-the-art ab initio calculations. The hydrogenated and fluorinated carbon rings in the molecule appear different in size, which primarily originates from the different extents of their π-electrons. Further, tilting of the CO at the tip, induced by van der Waals forces, enlarges the apparent size of parts of the molecule by up to 50%. Moreover, the CO tilting in response to local Pauli repulsion causes a significant sharpening of the molecule bonds in AFM imaging.

Metal-organic complexes were formed by means of inelastic excitations in a scanning tunneling microscope (STM). The electronic structure of the complex was characterized using STM imaging and spectroscopy. By exploiting the symmetry of the complex, its electronic structure can be rationalized from linear combinations of molecular orbitals. The actual bonding geometry, which cannot be inferred from STM alone, was determined from atomic force microscopy images with atomic resolution. Our study demonstrates that the combination of these techniques enables a direct quantification of the interplay of geometry and electronic coupling in metal-organic complexes in real space.

It has been demonstrated that atomic force microscopy imaging with CO-functionalized tips provides unprecedented resolution, yet it is subject to strong image distortions. Here we propose a method to correct for these distortions. The lateral force acting on the tip apex is calculated from three-dimensional maps of the frequency shift signal. Assuming a linear relationship between lateral distortion and force, atomic force microscopy images could be deskewed for different substrate systems.

Recently, we reported on the bistable configurational switching of dibenzo[a,h]thianthrene (DBTH) molecules adsorbed on NaCl using combined low‐temperature scanning tunneling and atomic force microscopy (STM/AFM). Here, we discuss the intra‐molecular contrast in AFM images of the molecules as a function of the tip–molecule distance. Our experiments show that ridges in the frequency shift do not necessarily correlate with chemical bonds in this case of a non‐planar molecule. To explain this finding we compare images acquired at different tip–molecule distances to the calculated electron density of the molecules obtained from density functional theory calculations (DFT). In addition, we analyze the probability of finding different configurations after adsorption onto the surface. DBTH molecules in two configurations probed by a CO‐functionalized tip. Insets show AFM (left) and STM (right) images of a U molecule.