Recent progress in surface NMR-electrochemistry

NMR spectroscopy is one of the newer spectroscopic techniques for investigating the static, dynamic and electronic structures of molecules adsorbed onto metal catalyst surfaces. We review recent progress in the application of solid-state NMR methods to the investigation of molecules adsorbed onto metal surfaces in an electrochemical environment: in the presence of electrolyte, and at an electrified interface under external potentiostatic control. While at a very early stage of development, the NMR-electrochemistry approach has considerable potential for investigating otherwise inaccessible aspects of electrode and adsorbate structure, and should enable a comparison of results obtained from different spectroscopies, in particular from IR spectroscopy. We present a brief review of the development of the subject, followed by details of the instrumentation necessary for NMR-electrochemistry studies. We show how spin-spin relaxation can give information on surface structure and surface diffusion, how spin-lattice relaxation can give information on the presence of conduction electron spillover onto the adsorbate, and how the NMR of surface species responds to an externally applied electric field. The C-13-NMR of CO on Pt in an electrochemical environment is compared with the C-13-NMR of CO on Pt catalysts in vacuum, which are well characterized. In the case of CN on Pt, we show large spectral shifts of the resonance as the electrode potential is varied, providing an independent measurement of the effects of the electrified interface on the chemisorption bond. Spectral sensitivity is also now adequate to observe nuclei which produce even weaker signals than C-13, such as N-15. The NMR-electrochemistry method thus opens up a broad new array of possibilities for probing static structures (from T-2), surface diffusion (from the temperature dependence of T-2) as well as electronic properties of the chemisorption bond (from T-1, and from electrode potential effects) at electrochemical interfaces, and for studying reactive intermediates and poisons on high-surface-area catalysts, such as those utilized in hydrogen and organic fuel cells.

Monitoring Charge Carrier Dynamics at Atomic Length Scales by STM-induced Luminescence

Christoph Grosse

The goal of this thesis is the combination of the high spatial resolution of scanning tunneling microscopy (STM) with the high temporal resolution of optical spectroscopy, by monitoring the light emitted from the tunnel junction. Two complementary techniques are presented, which allow (a) access to the local photon statistics and (b) following the luminescence response to nanosecond voltage pulses at atomic length scales. The versatility of these methods is demonstrated by using two different model systems: pure C60 films on Ag(111) and Au(111) substrates, as well as single fac-tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) molecules adsorbed on them. These two systems reveal the possibility to investigate both the local charge carrier dynamics of mesoscopic systems as well as individually and selectively addressable quantum systems. Furthermore, these methods are not limited to pure emission processes such as the recombination of electron hole pairs; rather, it is also possible to study the dynamics of non-emitting processes via the excitation and radiation of surface plasmon polaritons (SPPs) from the tunnel junction. To achieve high time resolutions in the luminescence response of an investigated system, a further technique is developed that permits a quantitative mapping of the voltage at the tunnel junction with millivolt and nanosecond accuracy. Knowing the precise shape of the voltage pulses arriving at the tunnel junction offers compensation for ubiquitous pulse distortions arising from reflections and attenuations of the pulses on their way to the tunnel junction, thus enabling pulse rise and falling times of a few nanoseconds. Investigations of the electronic structure of pure C60 films show that the electronic states of C60 multilayers experience a band bending in STM due to the electric field between the STM tip and the metal substrate. As the band bending is sufficiently strong to align the lowest unoccupied molecular orbitals with the Fermi energy of the substrate, electrons can be injected by the substrate. At structural defects, these electrons can recombine with holes in the highest occupied molecular orbitals injected by the STM tip. At such defects, local deviations from the highly ordered structure of the C60 film result in localized electronic states within the band gap. These states act as traps for the injected electrons and holes and increase their lifetime such that they can recombine with each other. The underlying injection dynamics are accessible by the luminescence time response of these defects to nanosecond voltage pulses. Measurements of the second-order intensity correlation function prove that these defects act as single photon sources with exciton lifetimes of

EPFL2015

Controlling Single Molecule Conductance by a Locally Induced Chemical Reaction on Individual Thiophene Units

Klaus Kern, Hagen Klauk, Ivan Pentegov, Verena Katharina Schendel, Uta Schlickum, Peter Wahl

Among the prerequisites for the progress of single-molecule-based electronic devices are a better understanding of the electronic properties at the individual molecular level and the development of methods to tune the charge transport through molecular junctions. Scanning tunneling microscopy (STM) is an ideal tool not only for the characterization, but also for the manipulation of single atoms and molecules on surfaces. The conductance through a single molecule can be measured by contacting the molecule with atomic precision and forming a molecular bridge between the metallic STM tip electrode and the metallic surface electrode. The parameters affecting the conductance are mainly related to their electronic structure and to the coupling to the metallic electrodes. Here, the experimental and theoretical analyses are focused on single tetracenothiophene molecules and demonstrate that an in situ-induced direct desulfurization reaction of the thiophene moiety strongly improves the molecular anchoring by forming covalent bonds between molecular carbon and copper surface atoms. This bond formation leads to an increase of the conductance by about 50 % compared to the initial state.

WILEY-V C H VERLAG GMBH2020

Efficient ab initio semiclassical dynamics for linear and nonlinear electronic spectroscopy

Tomislav Begusic

Molecular spectroscopy is an essential experimental technique in both fundamental physical sciences and applied research. For example, electronic spectroscopy, in which light induces a change in the electronic state of a molecule, is a powerful tool for studying light-matter interactions appearing in solar energy conversion and light-emitting devices. To explain experimental findings and compare them with theoretical predictions, scientists often compute properties that can only be inferred indirectly, rather than simulating the observed spectroscopic signals. This is partly due to challenges associated with the simulation of electronic spectra, which requires both an accurate description of the electronic structure and the quantum-dynamical treatment of the motion of atomic nuclei. As the exact methods to solve the underlying quantum-mechanical problem scale exponentially with the number of atoms, this task must be performed approximately for all but the smallest molecules. In this thesis, we develop a set of methods to systematically study the effects of different approximations on the computed spectra. As our starting point, we consider a semiclassical method called the thawed Gaussian approximation, which has resurfaced in recent years as an efficient and robust alternative to simple but crude models or highly accurate but costly quantum dynamics methods. First, we present several practical improvements of the method, such as generalization to non-Condon effects or the efficient single-Hessian version, which enable a broader set of molecules to be studied. Second, we introduce a more fundamental modification of the methods - the inclusion of non-zero temperature effects. The underlying theory, which is inspired by the concept of so-called thermo-field dynamics, is exact and applies to any quantum dynamics method. Remarkably, within the thawed Gaussian approximation, the inclusion of non-zero temperature comes at nearly no additional computational cost. These methods, developed originally for steady-state linear spectra, are then employed to compute state-of-the-art, time-resolved spectra, which are typically measured with ultrashort laser pulses. More precisely, we study how the anharmonicity of the potential energy surfaces and mode-mode (Duschinsky) couplings affect pump-probe and two-dimensional electronic spectra. This approach, based on the thawed Gaussian wavepacket propagation, is the first method that can exactly evaluate the nonlinear electronic spectra of many-dimensional, shifted, distorted, and Duschinsky-rotated harmonic potentials. As in the case of linear absorption and emission spectra, we again use the concept of thermo-field dynamics to include non-zero temperature effects in the simulation of two-dimensional electronic spectra. The resulting wavepacket picture of two-dimensional spectroscopy at arbitrary temperature could strongly impact how we interpret and simulate multidimensional spectra in the future.