Quantum confinement can dramatically slow down electron-phonon relaxation in nanoclusters. Known as the phonon bottleneck, the effect remains elusive. Using a state-of-the-art time-domain ab initio approach, we model the observed bottleneck in CdSe quantum dots and show that it occurs under quantum Zeno conditions. Decoherence in the electronic subsystem, induced by elastic electron-phonon scattering, should be significantly faster than inelastic scattering. Achieved with multiphonon relaxation, the phonon bottleneck is broken by Auger processes and structural defects, rationalizing experimental difficulties.

As shown experimentally, strong nonradiative decay channels exist in carbon nanotubes (CNT) and are responsible for low fluorescence yields. The decay of the electronic excitation to its ground state is simulated in the (6,4) semiconducting CNT with surface hopping in the Kohn-Sham representation, providing a unique time-domain atomistic description of fluorescence quenching. The decay in the ideal CNT is estimated to occur on a 150 ps time scale and is only weakly dependent on temperature. Vibrationally induced decoherence strongly influences the electronic relaxation. Defects decrease the excited state lifetime to tens of picoseconds, rationalizing the multiple decay time scales seen in experiments.

The implementation of fewest-switches surface-hopping (FSSH) within time-dependent Kohn-Sham (TDKS) theory [Phys. Rev. Lett. 95, 163001 (2005)] has allowed us to study successfully excited state dynamics involving many electronic states in a variety of molecular and nanoscale systems, including chromophore-semiconductor interfaces, semiconductor and metallic quantum dots, carbon nanotubes and graphene nanoribbons, etc. At the same time, a concern has been raised that the KS orbital basis used in the calculation provides only approximate potential energy surfaces [J. Chem. Phys. 125, 014110 (2006)]. While this approximation does exist in our method, we show here that FSSH-TDKS is a viable option for computationally efficient calculations in large systems with straightforward excited state dynamics. We demonstrate that the potential energy surfaces and nonadiabatic transition probabilities obtained within the TDKS and linear response (LR) time-dependent density functional theories (TDDFT) agree semiquantitatively for three different systems, including an organic chromophore ligating a transition metal, a quantum dot, and a small molecule. Further, in the latter case the FSSH-TDKS procedure generates results that are in line with FSSH implemented within LR-TDDFT. The FSSH-TDKS approach is successful for several reasons. First, single-particle KS excitations often give a good representation of LR excitations. In this regard, DFT compares favorably with the Hartree-Fock theory, for which LR excitations are typically combinations of multiple single-particle excitations. Second, the majority of the FSSH-TDKS applications have been performed with large systems involving simple excitations types. Excitation of a single electron in such systems creates a relatively small perturbation to the total electron density summed over all electrons, and it has a small effect on the nuclear dynamics compared, for instance, with thermal nuclear fluctuations. In such cases an additional, classical-path approximation can be made. Third, typical observables measured in time-resolved experiments involve averaging over many initial conditions. Such averaging tends to cancel out random errors that may be encountered in individual simulated trajectories. Finally, if the flow of energy between electronic and nuclear subsystems is insignificant, the ad hoc FSSH procedure is not required, and a straightforward mean-field, Ehrenfest approach is sufficient. Then, the KS representation provides rigorously a convenient and efficient basis for numerically solving the TDDFT equations of motion.

Phonon-induced dephasing of electronic transitions in semiconducting single-wall carbon nanotubes (CNT) is investigated by ab initio molecular dynamics. Pure-dephasing is shown to be the source of the photoluminescence linewidths observed experimentally in isolated CNTs at low and room temperatures. In ideal tubes, the dephasing is found to occur by coupling to optical phonons. The dephasing proceeds notably faster in the presence of some defects due to stronger coupling to local modes, suggesting that the defects can be identified in CNTs by broadened optical bands.

Model systems of perfluorosulfonic acid (PFSA) polymers exhibiting regular shaped and idealized channel morphologies were constructed by functionalizing single walled carbon nanotubes (CNTs) with -CF 2 SO 3 H groups and adding from 1 to 3H 2 O/SO 3 H to investigate structural and chemical factors affecting proton dissociation and transport. No a priori assumptions about either the dissociation or hydration of the protons were assumed and extensive ab initio molecular dynamics (AIMD) simulations were performed and subject to analysis. The importance of the hydrophobic environment was assessed by comparing the hydration of the protons both with and without fluorine atoms attached to the CNT walls. The AIMD trajectories showed that dissociation of the acidic proton increased with increasing density of sulfonic acid groups; however, greater densities also brought about trapping of the dissociated proton. The fluorine atoms accepted hydrogen bonds from the water molecules, stabilized hydrogen bonding, and enhanced proton dissociation. The CNT systems without fluorination of the walls exhibited a propensity for the formation of Zundel cations (H 5 O 2 + ), while the fluorinated systems favoured hydrated structures involving hydronium ions or hydrated H 3 O + species depending on the amount of water in the system.

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Proton dissociation and transfer were examined with ab initio molecular dynamics (AIMD) simulations of carbon nanotubes (CNT) functionalized with perfluorosulfonic acid (-CF(2)SO(3)H) groups with 1-3 H(2)O/SO(3)H. The CNT systems were constructed both with and without fluorine atoms covalently bound to the walls to elucidate the effects of the presence of a strongly hydrophobic environment, the fluorine, on proton dissociation, hydration, and stabilization. The simulations revealed that the dissociated proton was preferentially stabilized as a hydrated hydronium cation (i.e., Eigen like) in the fluorinated CNTs but as a Zundel (H(5)O(2)(+)) cation in the nonfluorinated CNTs. This feature is attributed to the fluorine atoms forming hydrogen bonds with the water molecules coordinated to the central hydronium ion.