PhotochemistryAnd Photophysics Of Nanoparticles

Brian Ellis

Discrete Electronic Structure: Quantum Confinement

•Bulk: properties depend on composition, independent of size; large overlapping bands

•Nanometer regime: electronic structure altered: discrete energy levels

•Physical properties become size-dependant

Photochemical Processes in Nanoparticles

•Just a reminder: Physical properties size-dependant

•Morphology changes: experimental observations

•Optical properties arise due to electron transitions between these discrete levels

•Quantum levels of an isolated spherical nanocrystal: E α 1/a2 (a = crystal radius)

Absorption Spectra

Clusters of CuCl nanoparticles

1) 450 nm

2) 250 nm

3) 175 nm

As cluster size decreases, blue shift occurs (since E α 1/a2)

Transient Bleaching

Plasmon: a collective excitation of quantised oscillations of the electrons in a metal.

Phonon absorption: absorption of light energy by its conversion to vibrational energy

•Excitation by a short laser pulse

•Metal colloids experience a prompt “bleaching” of the plasmon band during excitation (Au: 520 nm)

•Electrons above the Fermi level are excited by the laser, oscillate at a different frequency than those in the ground state, resulting in lower absorption

•Recovery is on the picosecond time scale, results from phonon, electron relaxation

Photofragmentation

•Facetted culsters of silver nanoparticles (40-60 nm) exhibit a plasmon absorption band at 420 nm (A)

•Laser pulse (355 nm for 3 minutes) induces blue-shift in absorption (to 400 nm), decrease in particles size (B)

•Transient state (600 nm) involves localized agglomeration of clusters, dissociates after 14.2 ns

Photofragmentation

•Facetted culsters of silver nanoparticles (40-60 nm) exhibit a plasmon absorption band at 420 nm (A)

•Laser pulse (355 nm for 3 minutes) induces blue-shift in absorption (to 400 nm) decrease in particles size (B)

•Transient state (600 nm) involves localized agglomeration of clusters, has 1.5 ns growth time

Photofusion

•If metal nano-particles occur as aggregates, fusion may occur with less energetic laser excitation

•Thionicotinamide-stabilized Au nanoparticles:

•(A) Native gold colloids (15-20 nm diameter), nanoparticles in close contact

•(B) 1 minute laser pulse (532 nm), large size spherical particles

•(C) 30 minute laser pulse: fragmentation

Conclusions

•Photochemical properties are size dependant: quantum energy levels increase as size decreases

•Fusion of aggregated nanoparticles is possible by excitation with short pulses

•Fragmentation of clusters and aggregates can be achieved by long laser pulses

References

•P. Kamat, J. Phys. Chem. B. 106 (2002), 7729.

•C. Burda, et al. Chem. Rev. 105 (2005), 1025.

•M. Gratzel, et al. Chem. Rev. 95 (1995), 49.

•D. Astruc et al. Chem. Rev. 104 (2004), 293.