Exciton Formation and Quenching in a Au/CdS Core/Shell Nanostructure

An atomistic description is presented of the excited state dynamics in spherical Au/CdS core/shell nanocrystals up to a diameter of 15 nm. Au-core excited states are considered in a multipole plasmon scheme, whereas a tight-binding description combined with a configuration interaction approach is used to compute single electron–hole pair excitations in the CdS-shell. The electron–hole pair energy-shift and screening due to an Au-core polarization is found of minor importance. For the studied system, the energy transfer coupling can be identified as the essential core–shell interaction. Characterizing the CdS-shell excitons by atomic centered transition charges and the Au-core by its multipole plasmon moments, an energy transfer coupling can be introduced that gives a complete microscopic description beyond any dipole–dipole approximation and with values around 10 meV. Together with a considerable plasmon–exciton energy mismatch, these coupling values explain the measured 300 ps lifetime of shell excitons due to energy transfer to the Au-core

Control of Intermolecular Electronic Excitation Energy Transfer: Application of Metal Nanoparticle Plasmons

Control of intermolecular electronic excitation energy transfer via metal nanoparticles is analyzed. Different control scenarios are presented which all utilize the effect of field-enhancement close to an optically excited metal nanoparticle. Due to this field-enhancement that part of a molecular chain or a molecular cluster gets excited which is in close proximity to the nanoparticle. Various simulation results are discussed related to the energy transfer kinetics in a uniform chain of 50 molecules in the vicinity of a single or two spherical gold nanoparticles. Interesting changes of the energy transfer pathways are found if the spatial and energetic arrangement between the molecular chain and the nanoparticles is altered

Atomistic Simulations of Charge Separation at a Nanohybrid Interface: Relevance of Photoinduced Initial State Preparation

Charge separation kinetics at a nanohybrid interface are investigated in their dependence on ultrafast optical excitation. A prototypical organic/inorganic interface is considered. It is formed by a vertical stacking of 20 <i>para</i>-sexiphenyl molecules physisorbed on a ZnO nanocluster of 3783 atoms. A first principle parametrized Hamiltonian is employed, and the photoinduced subpicosecond evolution of Frenkel-excitons in the organic part is analyzed besides the formation of charge separated states across the interface. The interface absorption spectrum is calculated. Together, the data indicate that the charge separation is based on the direct excitation of the charge separated states but also on the migration of created Frenkel excitons to the interface with subsequent decay. Further, the photoinduced interface dynamics are compared with data resulting from direct set-ups of an initially excited state. Mostly such set-ups lead to substantially different charge separation processes

Frenkel to Wannier–Mott Exciton Transition: Calculation of FRET Rates for a Tubular Dye Aggregate Coupled to a CdSe Nanocrystal

The coupling is investigated of Frenkel-like exciton states formed in a tubular dye aggregate (TDA) to Wannier–Mott-like excitations of a semiconductor nanocrystal (NC). A double well TDA of the cyanine dye C8S3 with a length of 63.4 nm and a diameter of 14.7 nm is considered. The TDA interacts with a spherical Cd₈₁₉Te₆₃₀ NC of 4.5 nm diameter. Electronic excitations of the latter are described in a tight-binding model of the electrons and holes combined with a configuration interaction scheme to consider their mutual Coulomb coupling. To achieve a proper description of TDA excitons, a recently determined structure has been used, the energy transfer coupling has been defined as a screened interaction of atomic centered transition charges, and the site energies of the dye molecules have been the subject of a polarization correction. Even if both nanoparticles are in direct contact, the energy transfer coupling between the exciton levels of the TDA and of the NC stays below 1 meV. It results in FRET-type energy transfer with rates somewhat larger than 10⁹/s. They coincide rather well with recent preliminary experiments

Calculating Optical Absorption Spectra of Thin Polycrystalline Organic Films: Structural Disorder and Site-Dependent van der Waals Interaction

We propose a new approach for calculating the change of the absorption spectrum of a molecule when moved from the gas phase to a crystalline morphology. The so-called gas-to-crystal shift ΔE<i>_m</i> is mainly caused by dispersion effects and depends sensitively on the molecule’s specific position in the nanoscopic setting. Using an extended dipole approximation, we are able to divide ΔE_<i>m</i>= −<i>QW</i>_<i>m</i> in two factors, where <i>Q</i> depends only on the molecular species and accounts for all nonresonant electronic transitions contributing to the dispersion while <i>W</i>_m is a geometry factor expressing the site dependence of the shift in a given molecular structure. The ability of our approach to predict absorption spectra is demonstrated using the example of polycrystalline films of 3,4,9,10-perylenetetracarboxylic diimide (PTCDI)