Chemistry at confined interfaces is governed by the same forces as in the bulk, but these forces are manifested at different, often highly nonlinear, scales. Understanding and controlling these local forces are thus critical to the success or failure of bulk processes ranging from separations to corrosion to energy storage. It is necessary to correlate nanoscale structural heterogeneity with confinement‐induced changes in mass and charge transport, local electric fields, and steric effects under in operando conditions. The goal of this NSF‐DFG project is to utilize single-particle dark‐field scattering and surface enhanced Raman microscopy to optically read out nanoscale details about the interfacial chemistry and physics governing mass, charge and energy transport in individual metal nanoparticle/conductive polymer hybrids. The project’s hypothesis is that electrical‐to‐optical signal transduction and in operando analysis can be achieved by exploiting the charge transfer plasmon resonance that has a distinct optical signature and only exists when two metal nanoparticles are brought into electrical contact. The team will pursue three objectives: 1) Synthetically control the electronic coupling between metal core and polymer shell, tuned through their chemical linkage, by rational design of conductive polymer coated plasmonic nanoparticles of different size, shape, and interfacial chemistry. 2) Understand the underlying heterogeneity in mass, charge, and energy transport in single nanoparticle/conductive polymer hybrids using custom dark‐field scattering and surface-enhanced Raman scattering. 3) Determine the conductance in different nanoscale assembly geometries by controlling the interfacial coupling and modulating the chemical environment through the emergence of charge transfer plasmons, which are highly sensitive to nano‐ and Angstrom‐scale distances.

Polymer semiconductors (PSCs) combining high charge carrier mobility with superior mechanical properties and solution-processability are ideal candidates to achieve large-area electronic applications on flexible substrates, e.g. displays, sensors or photovoltaics. The electronic performance of PSCs improved considerably in the recent years, and state-of-the-art donor-acceptor (D-A) polymers now routinely achieve charge carrier mobilities exceeding 1 cm2 V-1s-1, the classic benchmark of amorphous silicon. Yet the control over the physical properties of the materials did not progress at the same rate, and current high performance PSCs suffer from a range of shortcomings closely related to the conjugated molecular structure and well-stacked morphology which bestow record-setting charge carrier mobilities: They are strongly aggregated, leading to brittle films and limiting flexible applications, and are furthermore poorly soluble, causing batch-to-batch variations in polymer synthesis and poor reproducibility of device fabrication. Solution-deposition of these materials usually requires elevated temperatures, dilute solutions, and halogenated solvents, conditions which are hampering cost-efficient printing processes. An even more interesting fabrication approach would be the solvent-free processing from polymer melt, which would eliminate toxic solvents from the process and grant higher reproducibility and morphology control. Yet while melt-processing is a standard technique in many fields of polymer research and processing, only few melt-processable D-A PSCs were obtained and investigated so far. So far, the tuning of the physical properties of PSCs, such as solubility, melting point and tensile modulus, is explored by either modifying the backbone or the side chains. Endcap engineering could be a new strategic pathway to improve physical properties, e.g. solubility, tune mechanical characteristics, e.g. stretchability, and enable the melt-processing of PSCs. In this project, we will endcap state-of-the-art diketopyrrolopyrrole (DPP) based PSCs with elastomeric polydimethylsiloxane (PDMS) chains, and investigate the resulting second order block co-polymers physico-chemically, electrically and with regards to their morphology and phase separation. The elastomer-endcapped PSCs are expected to exhibit enhanced solubility and, for higher PDMS-to-PSC ratios, melt-processability.

Single molecule machines on a surface can convert external stimuli into motion. In the last years, several examples of rotations, translations, or conformational changes of molecules on a surface under the tip of a scanning tunneling microscope have been reported. However, the next step, i.e., developing mechanical molecular devices able to produce work or store energy, requires controlling the movement. For this, a detailed understanding of the underlying physical mechanisms is needed, which is still lacking. Thermal excitation can provide energy to the ground state of a molecule, yet according to the microscopic reversibility principle, unidirectional rotation is impossible in this case. On the other hand, tunneling electrons interact with the electronic excited states of the molecule on each electron transfer event and can allow directed, i.e., controlled motion. Both energy sources are available under the tip of a scanning tunneling microscope. In this project, we will combine molecular design and synthesis with scanning tunneling microscope experiments (imaging, spectroscopy, and manipulation) at variable temperatures to investigate molecular machines on a Au(111) surface and elucidate the physical mechanisms inducing controlled movements and conformational changes. Based on the established collaboration between the two participating groups, we will start with the synthesis of specifically designed vertical rotors and switches. These molecular machines contain two structural elements, i.e., an anchoring group for stable binding to the surface, and a switching or rotating part spatially decoupled from the surface. For the anchoring, we will employ N-heterocyclic carbenes, and combine them with molecular switches and rotors designed for having different energetic barrier heights for rotation. The vertically anchored molecular machines will then be tested using scanning tunneling microscopy. Specifically, we will combine surface thermal heating with tunneling electron excitation. The latter allows involving the electronic excited states of the molecule, a possibility which is absent in a classical case. The conformational changes of the switches, i.e., the toggeling between different states, will be induced by inelastic tunneling electrons and electric fields. The molecular rotors will be kept at a fixed temperature (varying from 5 K to RT) to investigate how thermal energy can be transferred to molecular mechanical degrees of freedom. Determining how the interplay between thermal energy and electron tunneling excitation contributes to movement will fundamentally advance the understanding of chemical reactions and mechanics on surfaces and provide unique information for the design of innovative molecular machines able to store thermal energy or produce work. In terms of quantum engineering, this opens fascinating perspectives in the direction of mono-thermal motors.

Chemistry at confined interfaces is governed by the same forces as in the bulk, but these forces are manifested at different, often highly nonlinear, scales. Understanding and controlling these local forces are thus critical to the success or failure of bulk processes ranging from separations to corrosion to energy storage. It is necessary to correlate nanoscale structural heterogeneity with confinement‐induced changes in mass and charge transport, local electric fields, and steric effects under in operando conditions. The goal of this NSF‐DFG project is to utilize single-particle dark‐field scattering and surface enhanced Raman microscopy to optically read out nanoscale details about the interfacial chemistry and physics governing mass, charge and energy transport in individual metal nanoparticle/conductive polymer hybrids. The project’s hypothesis is that electrical‐to‐optical signal transduction and in operando analysis can be achieved by exploiting the charge transfer plasmon resonance that has a distinct optical signature and only exists when two metal nanoparticles are brought into electrical contact. The team will pursue three objectives: 1) Synthetically control the electronic coupling between metal core and polymer shell, tuned through their chemical linkage, by rational design of conductive polymer coated plasmonic nanoparticles of different size, shape, and interfacial chemistry. 2) Understand the underlying heterogeneity in mass, charge, and energy transport in single nanoparticle/conductive polymer hybrids using custom dark‐field scattering and surface-enhanced Raman scattering. 3) Determine the conductance in different nanoscale assembly geometries by controlling the interfacial coupling and modulating the chemical environment through the emergence of charge transfer plasmons, which are highly sensitive to nano‐ and Angstrom‐scale distances.