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1. Accomplishments during the PRF Grant Funding Period

Presently, the development of efficient ways to utilizing solar energy is one of the forefronts in the scientific community. Both, semiconductor nanocrystals and single wall nanotubes, are widely considered as suitable materials for the construction of photovoltaics.^1,2,3 However, their photophysical properties are still poorly understood and our conducted research has intended to answer the following two fundamental questions: What is the role of exciton-phonon interactions in the dissipation of energy in semiconductor nanocrystals? How can charges be photoinjected into carbon nanotubes? The latter question has turned to be very complex and although we made some progress towards finding an answer, my group will continue to work on this issue.

Energy-related research, funded by the PRF grant, has been carried out in two major thrusts areas:

(i) Optical and electronic properties of functionalized single-walled carbon nanotubes (SWNT)

(ii) Femtosecond probing of optical phonon dynamics in quantum-confined CdTe Nanocrystals  

(i)              Pyrene derivatives attached to the sidewalls of SWNT have been used for the construction of photovoltaic devices,^2,3 but there are rather few experiments studying the photophysical properties of the basic SWNT - pyrene complex. Although, our research has been targeted towards finding suitable pyrene derivatives for efficient charge separation processes, our first step was to understand the photophysical properties of the SWNT - pyrene complex. For this purpose, we dispersed SWNT using water solution of pyrenebutyric acid and removed the non-attached (to SWNT) acid from the dispersion, performing multiple chloroform extraction steps. The absorption spectrum of SWNT-Pyrene is composed solely from the fingerprint of SWNT. However, the fluorescence spectrum resembles the pyrenebutyric acid fluorescence with significant broadening and shifting toward shorter wavelengths (~25 nm). These results suggest that there is a strong electronic interaction/coupling between pyrene and SWNT, which causes change in the photophysical properties of the pyrenebutyric acid. Therefore, we were able to establish that SWNT-pyrene complexes should be described as a new complex molecule and not as separate molecular entities. Producing and examining spectroscopically supramolecular complexes using pyrene derivatives with different electron donating or withdrawing groups will allow us to study the donor acceptor properties of SWNT. Also, we will be able to construct large libraries of molecules with the idea of finding the most suitable pyrene derivatives for potential SWNT “charge separation dependent” applications.

Our studies were not limited to pyrene as a chromophore. Polycyclic aromatic hydrocarbons (PAH) are the building blocks of SWNT. As such, their photophysical and photochemical properties are important for organic chemists attempting to synthesize SWNT, and physical chemists interested in studying the optical and electronic properties of SWNT. In collaboration with Dr. Scott’s group, six PAH molecules have been investigated: Perylenyl fluoranthene; Perylenyl corannulene; Phenanthryl corannulene; Perylene; Fluoranthene; Corannulene. Steady-state and time-resolved measurements were performed. It was observed that in PAH the electron cloud can be localized in different parts of the molecule, and the absorption of light initiates an internal electron transfer process with timescale of several picoseconds.  

(ii)            In addition to the work on carbon nanotubes, we started to work on CdTe/CdSe nanocrystals to explore their usefulness for photovoltaics. One of the central scientific issues in these systems is to understand the interplay between excitons and phonons. One way to observe exciton and phonon dynamics in nanocrystals is through pump-probe spectroscopy.⁴ Specifically, femtosecond laser pulses were used to excite phonon wavepackets in CdTe semiconductor nanocrystals. As a result the wavepackets modulate the pump-probe signal and oscillations are observed. Their frequencies correspond to the longitudinal optical phonons in bulk CdTe (162 cm^-1) and were coupled to 1S-2S_3/2 exciton state. To study how the quantum confinement affects the exciton-phonon interactions, we varied the size of the crystals. The results showed that the energy of the phonons coupled to the exciton and the coupling strength are independent of the particles’ size. However, the phonon-wavepacket dephasing times were size dependent. The loss of coherence in the phonon wavepacket can be explained by hole relaxation from the excited 1S-2S_3/2 to the lower 1S-1S_3/2 state. Normally in bulk semiconductor crystals single phonon emission act is sufficient for hole relaxation between exciton states, while in CdTe nanocrystals multi-phonon processes are needed because of the large energy splitting between states. In fact, smaller particles (stronger quantum confinement) are expected to have slower hole relaxation, because more phonons are needed for the hole to populate the lower exciton state. However, our results show that the hole relaxation is faster for smaller dots, therefore, there is another channel for hole relaxation which most likely is a non-adiabatic transition to the nanocrystals’ surface states.

2.  References

(1) Nozik, A. J. Physica E 2002, 14, 115.

(2) Guldi, D.; Rahman, A.; Jux, N.; Balbinot, D.; Hartnagel, U.; Tagmatarchis, N.; Prato, M. J. Am. Chem. Soc. 2005, 127, 9830

(3) Ehli, C.; Rahman, A.; Jux, N.; Balbinot, D.; Guldi, D.; Paolucci, F.; Marcaccio, M.; Paolucci, D.; Melle-Franco, M.; Zerbetto, F.; Campidelli, S.; Prato M. J. Am. Chem. Soc., 2006, 128, 11222

(4) Sagar, D. M.; Cooney, R. R.; Sewall, S. L.; Dias, E. A.; Barsan, M. M.; Butler, I. S.; Kambhampati, P. Phys. Rev. B 2008, 77, 235321.

3.  Publications

"Femtosecond Probing of Optical Phonon Dynamics in Quantum-Confined CdTe Nanocrystals" 

Dimitrov, S.D; Dooley, C.J.; Trifonov, A.; Fiebig, T. J. Phys. Chem. C  2009, 113, 4198.

"Ultrafast Electron Transfer Dynamics in CdSe/CdTe Donor-Acceptor Nanorods" Dooley, C.J.; Dimitrov, S.D.; Fiebig, T. J. Phys. Chem. C  2008, 112, 12074.