1. Nanoscale Science with Functional Molecules|
The research activities in the group of Leonhard Grill focus on the investigation of inorganic and organic matter, in particular functional molecules, on surfaces by low temperature scanning tunnelling microscopy (STM), preferentially at 5 K. This method makes it possible to image single molecules with a spatial resolution below 100 pm and is especially used for molecular manipulation by the Nanoscale Science group: Lateral or vertical displacement of single molecules or chemical reactions are induced with the STM tip, taking advantage of chemical forces, the tunnelling electrons or the electric field in the junction.
While the manipulation of atoms and molecules by STM is a well-known technique, the manipulation of inorganic crystallites has hardly been attempted in the past. By using an ultrathin crystalline film of NaCl on a Cu(111) surface, the research group could show that such a nanostructure can react in various ways on the stimulus of the STM tip . Depending on the crystallite size, NaCl nanostructures can be moved laterally or, if the island is large and the interaction with the metal surface is strong, it can be cut. Additionally, a cracking process can be induced for crystallites with a cantilever shape. Furthermore, an elastic regime of NaCl crystallites was found, where Hookes law is valid as proven by theoretical simulations.
Beside the fundamental understanding of physical and chemical processes at the atomic scale, the research group is interested in the formation of molecular assemblies that are stabilized by a well-defined intermolecular interaction. Depending on the chemical structure of the molecules, rather weak van der Waals interaction, hydrogen bonds or strong interactions such as metal-ligand or covalent carbon-carbon bonds are achieved. In the latter case, polymerization occurs directly on the surface, leading to macromolecules, which are more promising than supramolecular structures, in view of stability and potential charge transport.
Fig. 7: On-surface polymerization of nano-trains on a Cu(111) surface . (a) shows the scheme of connecting individual molecules in a linear fashion. The chemical structure of a molecular building block is presented in (b) with an STM image in (c). After connection by metal ligand bonds, chains of molecules are visible in the STM image (d).
The group has recently attempted to connect so-called wagon molecules, molecular building blocks with carborane side groups that should act as wheels and roll on the surface during diffusion (see Fig.7 a-c). While this rolling motion could not be proven, due to a rather strong interaction with the substrate, the molecular building blocks could be connected successfully to so-called nano-trains (Fig. 7 d) . A detailed analysis of the formed structures revealed that metal-ligand bonds stabilize these structures, including a copper atom from the substrate, which is even visible in the STM images (indicated by an arrow in the inset of Fig. 7 d).
An important class of molecules in terms of functionality are molecular switches that exist in at least two stable states. Recently, the group has also found evidence for an inverted thermal switching behaviour in imine-based molecules . As typically the trans state is energetically favoured, such molecules relax from cis to trans upon heating. On a gold surface however, the inverse behaviour is found, probably induced by the interaction with the substrate, leading to an increasing number of cis isomers after heating procedures.
2. Tip-enhanced Raman Spectroscopy
||C. Bombis, et al., Phys. Rev. Lett. 104, 185502 (2010).|
||C.J. Villagómez, T. Sasaki, J. M. Tour, and L. Grill, J. Am. Chem. Soc. 132, 16848 (2010)..|
||J. Mielke, et al., ACS Nano 5, 2090-2097 (2011).|
Tip-enhanced Raman spectroscopy (TERS) represents a promising tool to investigate interfaces topographically and with chemical and molecular sensitivity on a nanometer scale. The TERS approach bears a great potential for local identification and characterization of adsorbates by optical vibrational spectroscopy with high sensitivity and nanometer resolution.
The group of Bruno Pettinger has implemented TERS in UHV by employing a unique concept: (i) an adjustable high numerical aperture parabolic mirror is placed in between the STM scanner and the sample and (ii) all other necessary optics are mounted on a common platform together with the scanning tunnelling microscope (STM). In 2007 first very promising results were obtained, however, the instrument developed permitted only the proof of principle. More recently, the instrument was improved along various lines in order to make UHV-TERS more easily applicable and complementary for advanced UHV studies. This concerns, for example, the optical alignment for which piezo-driven mirrors have been installed permitting a re-alignment of the optical path without opening the UHV chamber. In addition, a preparation chamber has been added, which enables the sputtering and annealing of a single crystal sample, the evaporation of molecules and the transfer to the TERS unit in UHV for vibrational investigations.
Fig. 8: Time dependence of TERS on a C60 island at Au(111) at RT. Over 100 original spectra were recorded subsequently with 0.5 s acquisition time and grouped into nine average spectra, separated in time by 5s; Color code: from red to blue. Two bands marked by x represent artifacts. Laser power: 100 mW. Exciting line: 632.8 nm. Insert: STM image of a section of a C60 island.
Currently, new experiments are in progress. These include investigations on C60 molecules in form of well ordered islands deposited at room temperature on Au(111) samples (see Fig. 8). First results on such samples yield TER spectra with high signal-to-noise ratio that show much more Raman lines (>25) than allowed for isolated C60 molecules (where only 10 modes are Raman active). This is attributed to the symmetry reduction caused by adsorption and intermolecular interactions, which may lift the degeneracy. Thus four former IR-active modes as well as some other silent modes (among 32) may become not only Raman active but also relatively intense under TERS conditions. Most strikingly is a significant time dependence of the C60 TER spectra, not so much in observed vibrational frequencies, but in the relative band intensities. This could point to structural and chemical changes within the adsorbed C60 adlayer. These changes in the C60 TER spectra make this system an interesting test case for molecule-substrate and intermolecular interactions.
Sub-monolayers of dye-molecules (e.g. isotope substituted two-analyte systems) will be investigated next, in order to determine the spectroscopic properties of individual adsorbates. The measurements may be extended also toward optically non-resonant molecules adsorbed at single crystalline surfaces as well as individual nanoclusters.
Furthermore, we plan to study in close cooperation with the department of Chemical Physics supported nanoparticles on thin oxide films by TERS. For the preparation of such more samples a new preparation chamber has been constructed and will soon be implemented in the UHV system.
3. Photoinduced Surface Reactions and Vibrational Spectroscopy
The group headed by Christian Frischkorn has studied photoinduced chemical processes at surface in various directions over the last years. Besides femtosecond laser-induced surface reactions like associative desorption reactions of diatomic molecules (e.g. H+HH2; C+OCO), electron solvation dynamics in D2O ice on Ru(001) have been investigated using vibrational spectroscopy based on sum-frequency generation (SFG). In particular, the group performed experiments on the vibrational response of the polar D2O solvent molecules to excess electrons injected after UV laser excitation. As a result, in crystalline D2O ice layers a giant SF signal enhancement by 3 to 4 orders of magnitude was observed (see Fig. 9).
The explanation for this phenomenon is based on ferroelectric ordering in the ice crystallites resulting in a net dipole moment of the ice through molecular reorientation. This breaks inversion symmetry by proton flipping also inside crystallites (Fig. 9 right) and thus the bulk of the crystalline D2O layer contributes to the SFG process, leading to the tremendous SF signal enhancement.
Fig. 9: (left) SFG spectra (linear plot and inset logarithmic) of 8 BL crystalline D2O ice on Ru(001) (red trace). Upon irradiation with 4.66 eV photons, the SF signal is enhanced by a factor of ~103 (blue). If both UV and IR pulse temporally overlap, a further increase is observed (green). (right) Temporal evolution of the resonance amplitude centered at 2285 cm-1 and 2435 cm-1, respectively, which involves a structural transformation from ice Ih to ice XI (ferroelectric ice) via proton flipping (inset) as a proposed mechanism which leads to the observed SF intensity enhancement.
In a second project, the group has studied the defect-mediated chemistry on metal ox-ides, in particular the UV-photoinduced dissociation of N2O on thin MgO films on Ag(100) . Using postirradiation thermal desorption spectroscopy (TDS), the UV wavelength and photon dose-dependent formation of N2 in conjunction with recombina-tive desorption of atomic oxygen is found, while the coverage of the parent N2O molecules is depleted. If the atomic oxygen is not completely removed by high temperature desorption, the reactive sites for subsequent N2O photoreduction cycles are blocked. On the contrary, investigating the reactivity of these photogenerated atomic oxygen species on MgO showed that CO oxidation can be achieved using UV photoexcitation of the prepared CO+O/MgO/Ag(100) system. Currently, electron energy loss spectroscopy is used in the vibrational fingerprint region to identify the reaction products directly on the surface and not only indirectly with TDS, where photo- and thermally induced reaction products cannot be distinguished.
|| P. Giese et.al., J. Phys. Chem. C 115, 10012 (2011).|
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