A quantitative and detailed understanding of the optical and electrical (perturbed) properties of molecular building blocks of dimensions from 0.3 to 10 nm (in contact with nanostructured substrates) is crucial for the scientific underpinning of atomically precise functional devices based on individual molecules. In project C3, optical and electrical properties are measured experimentally and modeled theoretically for potential applications of molecule-based nanostructures as light emitters, electrical conductors, or optical/electrical switches in the areas of molecular electronics and nanophotonics. Experimental (spectroscopic) studies are complemented by quantum-chemical ab initio computations and/or molecular-dynamics simulations, and project C3 focuses on the (further) development of computational and experimental methods as well as their joint application.
Developing Computational Methods
Electronic-structure approaches beyond conventional density-functional theory (DFT) (e.g., a many-particle Green’s functions approach) are developed to obtain more accurate binding energies and geometries at the molecule–metal interface and to obtain improved quasiparticle energies for charge-transport studies. Force-field-based methods are combined with quantum-mechanical methods, and electronic transport is studied in low-band-gap polymers for organic solar cells (see subproject C3.6: Theory of Transport Through Single Molecules).
Subsystem quantum-chemical (embedding) methods are enhanced to better describe extended structures such as DNA assemblies (e.g., with carbon-based materials), metal-organic frameworks, and molecular metamaterials. Furthermore, subsystem methods for molecules are connected to ab initio methods for systems with periodic boundary conditions (e.g., substrates, solids). See subproject C3.13: Subsystem Quantum Chemistry for Nanostructures.
Concepts borrowed from predictive-level, molecular ab initio wave-function theory (WFT) are explored for use in DFT, for molecules as well as solids. For example, the problem of slow basis-set convergence of the correlation energy obtained from the random-phase approximation in DFT may be ameliorated by using explicitly-correlated wave-function methods (see subproject C3.3: Computation of Electronic and Intermolecular Interactions).
Developing Experimental Methods
Spectromicroscopic studies of individual single-walled carbon nanotubes (SWNTs, both free-standing and on surfaces) focus on understanding and controlling changes to optical and vibrational properties due to local perturbations such as uniaxial strain and torsion, electric fields, covalent functionalization, exohedral molecular chemi- or physisorption (e.g., for carrier doping), and cuts or defects patterned onto SWNTs via electron-beam methods. Corresponding method development comprises the implementation of Rayleigh-scattering- and rapid-nIR-photoluminescence imaging microscopy as well as the setup of a UHV cryocell for studies of individual SWNTs (see subproject C3.14: Frequency-Domain Electronic Spectroscopy of Single-Walled Carbon Nanotubes).
Novel luminescent materials for blue-light excitation are developed further in terms of rare-earth-doped materials as well as organic–inorganic hybrid materials. Luminescent nanoparticles are incorporated into biological systems and polymers, and particle–polymer interactions are studied (see subproject C3.12: Nanoscale Luminescent Materials for Blue Light Excitation).
Two- and three-pulse time-resolved femtosecond absorption spectroscopy is applied to molecules of various sizes ranging from small conjugated π-bonded systems to SWNTs — with emphasis on exploring the size-dependent dynamics of dark states effecting photoluminescence yields (see subproject C3.15: Time-Resolved Electronic Spectroscopy of Size-Selected Molecular Nanostructures).
Using modular building blocks, a library of copolymers (with alternating electron-poor and -rich subunits) is set up to investigate the correlation between polymer structure and SWNT-dispersion selectivity (see subproject C3.8: Self-Assembled Molecules as Pre-Organized Building Blocks of Nanoscale Networks and Structures).