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Time- and space- resolved ultrafast dynamics in molecular-plasmonic hybrid systems

Periodic Reporting for period 2 - QUEM-CHEM (Time- and space- resolved ultrafast dynamics in molecular-plasmonic hybrid systems)

Reporting period: 2019-11-01 to 2021-04-30

The aim of the project is to develop and apply methods and techniques to describe and investigate the complex time- and space-resolved dynamics of molecular-plasmonic hybrid systems interacting with external light fields. Interaction of such systems with external light fields leads to a complex interplay of physical processes: for the plasmonic system, the excitation of collective electron dynamics inside the metallic nanoparticles leads to strongly modified electromagnetic near-fields with complex polarization and spatial structure. Of course, also the molecular system could be independently polarized and excited, with induced electronic and/or vibrational dynamics. However, the combined system cannot necessarily be partitioned into two individual subsystems (molecule and plasmonic nanoparticle) as it features additional astonishing phenomena which only occur when both systems are in close proximity and may interact: the nanoparticle’s enhanced near field interacts with the molecular system and contributes to its polarisation and/or excitation. The induced dipole of the molecular system, in turn, interacts with a mirror dipole in the plasmonic nanoparticle, and this interaction will self-consistently modify the electromagnetic nearfield. Besides these electromagnetic (EM) aspects, the close proximity of the metal near the molecule modifies the molecular electronic structure at the quantum level (QU): new, charge-transfer states between the molecular and the metallic system may arise. These ingredients (QU+EM) lead to a plethora of new and exciting physical and chemical phenomena, such as the formation of new quasi-particles, new mechanisms for chemical reactions or the ultra-high spatial resolution and selectivity in molecular detection.

In order to model and investigate this exciting physics and chemistry, a combined description of the (static) quantum mechanical electronic structure of the molecular system in close vicinity of the plasmonic system (QU), the interaction with the electromagnetic field (EM) and also the induced complex dynamics of the hybrid system is necessary. Existing approaches to describe and investigate these complex physics and chemistry commonly focus on either an accurate description of the electronic structure of the hybrid systems or the electromagnetic properties of these systems. Only few approaches and works aim at a description of both, the quantum mechanical electronic structure and the simulation of the spatially inhomogeneous, time-dependent near field. And almost nothing is known about the field-induced complex time- and space-resolved dynamics of the hybrid systems – which lies at the heart of this project (QUEM-CHEM).

Besides being of fundamental interest, this interplay between near-fields and molecules is very promising for applications, potentially enabling revolutionary breakthrough in new emerging technologies in a broad range of research fields, such as nanophotonics, energy and environmental research, biophotonics, light-harvesting energy sources, highly sensitive nano-sensors etc. Thus, within this project, new approaches and methods beyond the state of the art are being developed, aiming at a synergistic description of existing but typically independently applied approaches and employ these to certain prototype applications and scientific questions in sensing/spectroscopy, plasmon-induced catalysis and the dynamics of new quasiparticles.
The central aim of QUEM-CHEM is to investigate the physics and chemistry of molecular-plasmonic hybrid systems and their dynamics in external light fields. The majority of existing work focuses on either the quantum chemical contributions or the electromagnetic simulation of these hybrid systems. We aim at closing the bridge and combining both approaches, including a description of the induced ultrafast dynamics.

In order to reach this goal, novel multi-scale multi-physics methods need to be developed (part I). In part II, these methods are applied to pre-selected application questions. Concerning the methods section, part I, in the initial phase, we have parallelly investigated different “ingredients”: as planned, we have compared different existing methods to describe the electronic and/or vibrational dynamics induced by the interaction of the molecular system with strong(er) light fields: the whole spectrum of dynamical methods, purely quantum dynamical, classical and mixed quantum-classical dynamical methods has been employed to describe and simulate electronic and/or vibrational dynamics.

In parallel, we have investigated and compared different existing program packages providing access to the calculation of the quantum chemical electronic structure of the combined molecular-plasmonic hybrid system. For comparison, we have compared different a large variety of quantum chemical methods and applied them already to some of the planned and envisioned application cases.

At the same time, we have started to investigate the first three exemplary science cases (part II), the first one being the dynamics of new quasi-particles occurring due to the strong coupling of the plasmonic moiety and an excitonic system (the so-called “plexcitons”). Here, we have started with a very simple and intuitive model system consisting of coupled (harmonic) oscillators (the excitonic part) interacting with the combined action of an external laser field and the temporally inhomogeneous near field of a plasmonic nanoparticle (which is in or around resonance with the incident laser field).

The second application science case concentrates on the more chemical question of plasmon catalysis. Two different plasmon-induced catalytic reactions have been investigated; the selection of which has been motivated by our collaboration partners. As observable, the systems’ characteristic Raman frequencies including the enhancement effect have been calculated. Part of the work has been done together with our collaboration partners in Tomsk, Russia (Prof. Raul Rodriguez).

The third, already running application science case investigates the question concerning the resolution in tip-enhanced Raman spectroscopy (TERS). Based on our previous work, we have investigated in detail the chemical effects contributing to the resolution in TERS, which suggest sub-nanometre spatial resolution, and extended it towards resonance excitation. We could show that resonance excitation not only further enhances the Raman signal by several orders of magnitude compared to non-resonant case. Additionally, for non-resonant excitation, we have developed an approach to mimic the effect of the spatially inhomogeneous near field as calculated by standard finite element methods (FEM). For this, we make use of partial charges, parameterized such that their resulting field distribution resembles closely the afore calculated electromagnetic nearfield, obtained from FEM. These charges then enter the quantum chemical electronic structure calculations additionally. This way, the influence of the electromagnetic field directly enters the quantum chemical calculations of the combined metal and molecule.
Next steps will involve a self-consistent treatment of both, the electromagnetic effects and the quantum chemical contributions for the case of resonant excitation.
Most of the work sketched above constitutes progress beyond the state of the art. Specifically, concerning the application part, a few highlights are selected in what follows:
Experimental evidence suggests an extremely high, possibly even sub-molecular, spatial resolution of tip-enhanced Raman spectroscopy (TERS). Theoretically, two different contributions are discussed: an electromagnetic effect, leading to a spatially confined near field due to plasmonic excitations; and the so-called chemical effect originating from the locally modified electronic structure of the molecule due to the close proximity of the plasmonic system. Most of the theoretical efforts have concentrated on the electromagnetic contribution or the chemical effect in case of non-resonant excitation. In a recent publication, we have presented a fully quantum mechanical description including non-resonant, resonant and charge-transfer chemical contributions as well as charge-transfer phenomena of these molecular-plasmonic hybrid systems at the density functional and the time-dependent density functional level of theory. We have considered a surface-immobilized tin(II) phthalocyanine molecule as the molecular system, which is minutely scanned by a plasmonic tip, modelled by a single silver atom. These different relative positions of the Ag atom to the molecule lead to pronounced alterations of the Raman spectra. These Raman spectra vary substantially, both in peak positions and several orders of magnitude in the intensity patterns under non-resonant and resonant conditions, and also, depending on, which electronic states are addressed. Our computational approach reveals that unique – non-resonant and resonant – chemical interactions among the tip and the molecule significantly alter the TERS spectra and are mainly responsible for the high, possibly sub-Angstrom spatial resolution. The performed quantum chemical simulations reveal a pronounced enhancement of the Raman intensity under non-resonant and resonant conditions with respect to the uncharged reference system, while the contribution of charge transfer phenomena and of locally excited states of the molecule are highly dependent on the tip’s charge. In addition, the introduction of both a negative and positive charge at the tip’s apex alters the region of strongest chemical interactions among the tip mimic and the molecule. In the case of the present hybrid systems, this was observed particularly for SnPc–Ag+—favoring much shorter bonding distances and considerably higher binding energies in comparison to SnPc–Ag. The incorporation of the electromagnetic effect in the form of more complex fields is the point of interest of ongoing studies under non-resonant and resonant conditions, as is the addition of the immobilizing surface.

We expect the methods being currently developed and partially already applied to be of high importance for a broader science community. These methods and approaches will be generally applicable for a variety of scientific questions and may enable detailed investigation of astonishing physical and chemical phenomena, such as the plasmon-induced catalysis or the formation of new quasi-particles. The approaches and methods to be developed in this project will enable for the first time a self-consistent description of both, the quantum nature of the molecule-nanoparticle moiety and the complexity of near-field electrodynamics. The simulation of these effects is essential not only for the fundamental research but also in application to develop highly sensitive nano-shaped detection schemes and for broad range of applications in such fast growing and critically important research fields like nanophotonics, biophysics, light harvesting energy sources etc. Thus, the possible outcome of this truly interdisciplinary project will provide new knowledge both in physics and chemistry, and might have impact on large variety of new arising critical technologies.
Visualization of the molecular-plasmonic hybrid systems and their interaction with external light fi