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...

We present spatially resolved vibronic spectroscopy of individual pentacene molecules in a doublebarrier tunneling junction. It is observed that even for this effective single-level system the energy dissipation associated with electron attachment varies spatially by more than a factor of 2. This is in contrast to the usual treatment of electron-vibron coupling in the Franck-Condon picture. Our experiments unambiguously prove that the local symmetry of initial and final wave function determines the dissipation in electron transport. DOI: 10.1103/PhysRevLett.110.136101 PACS numbers: 68.37.Ef, 63.22.Àm, 73.40.Gk, 73.63.Àb In organic and molecular electronics the electrons are much more spatially confined as compared to inorganic semiconductors, leading to a much stronger electronvibron (e-) coupling [1][2][3]. Therefore, e-coupling gives rise to substantial dissipation in such systems, which should be minimized in electronic devices. When an electron tunnels into a given molecule (electron attachment), the nuclei will relax, giving rise to the so-called reorganization energy, a process that is usually treated in the Franck-Condon picture [1]. In the latter, the e-coupling strength for all modes is inferred from projecting the atomic displacements that arise upon electron attachment onto the vibrational eigenmodes [1,4,5]. Whereas the wave functions of the vibrational states are crucial in the Franck-Condon picture, it does not account for the electronic wave functions. In contrast to that, we show in this Letter that the spatial position of the electron injection as well as the local wave function symmetry dramatically affect the e-coupling.Our scanning tunneling microscopy (STM-)based experiments are performed in a double-barrier tunneling junction, enabling spatially resolved vibronic spectroscopy [6,7]. This regime is highly relevant, resembling electron hopping in organic and molecular electronics. Note that vibronic spectroscopy is very different from usual STMbased inelastic electron tunneling spectroscopy [8], for which it has been realized that the symmetries of wave functions play an important role [9][10][11].As a model system we chose pentacene, which is widely used in organic electronics and one of the best studied systems [4,5,12,13]. We consider only transport involving the first molecular resonances, attributed to occupation and depletion of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) level, respectively. Effectively, this renders a single-level system. If several molecular orbitals are closely spaced in energy, tunneling into different orbitals and mixing of vibronic states [6,14] may lead to spatial variations of the e-coupling, which can be ruled out here. In our system, transport involving further orbitals (LUMO þ 1, HOMO À 1) can be excluded due to energy differences of more than 1 eV [12,15].In our low-temperature STM setup we use ultrathin insulating films on copper single crystals as substrates giving rise to a double-barrier tunneling jun...

On metallic and semiconductor surfaces functional nanostructures can be built with atomic scale precision using the tip of an atomic force microscope/scanning tunneling microscope. In contrast, controlled lateral manipulation on insulators has not been reported. The traditional pushing and pulling based manipulation methods cannot be used for molecules adsorbed on insulating films because of the unfavorable ratio between diffusion barrier and desorption energy. Here, we demonstrate that molecules adsorbed on insulating films can be laterally manipulated in a controlled way by injecting inelastically tunneling electrons at well-defined positions in a molecule. The technique was successfully applied to several different molecules.

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.