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Winner of the IUPAC Prize
for Young Chemists - 2001

 

 

Stephan Link wins one of the five IUPAC Prize for Young Chemists, for his Ph.D. thesis work entitled "Spectral Properties and relaxation dynamics of surface plasmon electronic oscillations in gold and silver nanodots and nanorods"

Current address (at the time of application)

Laser Dynamics Laboratory
Georgia Institute of Technology
School of Chemistry & Biochemistry
Atlanta, GA 30332-0400 USA

Tel: +1 404 8944009
Fax: +1 404 8940294
E-mail: [email protected]

Academic degrees

  • Ph.D.: Georgia Tech, December 2000, Chemistry.
  • Diplom (Masters): Technical University of Braunschweig (Germany) July 1996.

Ph.D. Thesis

Title Spectral Properties and Relaxation Dynamics of Surface Plasmon Electronic Oscillations in Gold and Silver Nanodots and Nanorods
Adviser Professor Mostafa A. El-Sayed
Thesis Committee Z. L. Wang, School of Materials Science & Engineering; R. L. Whetten, School of Chemistry and Biochemistry; J. Zhang, School of Chemistry and Biochemistry; R. Hernandez, School of Chemistry and Biochemistry, Georgia Tech.

Essay

Noble metals and especially gold in colloidal form have fascinated scientists since the Middle Ages because of its intense red color originating from the surface plasmon absorption. The red color of stained glass windows as used in old churches and cathedrals originates from nanoparticles of metallic gold in its neutral valence state, which was first recognized by Faraday. Recently, the study of small metal particles of nanometer size has found tremendous interest because of interesting new material properties due to the reduction of the particle size and because of possible applications of nanotechnology in such diverse fields as (photo)catalysis, (opto)electronics, magnetic devices and sensors.

A strong visible absorption appears when the size of the gold decreases to the nanometer length scale. It results from the coherent oscillation of he free electrons in the conduction band. This is called the surface plasmon oscillation and the resulting absorption is called the surface plasmon absorption. Both the radiative and nonradiative relaxation properties of this unique motion were focus of the thesis research. 16 publications with two invited review articles have resulted from this research. [1,2, and references therein]

Spherical gold nanoparticles in aqueous solution having different sizes with mean diameters between 10 and 100 nm have been synthesized and the size and temperature dependence of the surface plasmon absorption has been studied. The band width of the surface plasmon absorption is related to the dephasing time (T2) of the coherent plasmon electronic oscillation which is found to be on the order of a few femtoseconds. From its weak temperature dependence it is concluded that the dephasing results from mainly electron-electron repulsion and not electron phonon coupling.

Gold-silver nanoparticle alloys with varying compositions have been prepared similar to pure gold nanoparticles. The formation of homogeneously mixed alloy nanoparticles is concluded from high resolution transmission electron microscopy (TEM), energy dispersive spectrometry (EDS) and optical absorption spectroscopy. The latter reveals the presence of only one plasmon absorption band whose maximum shifts linearly with the gold mole fraction between the values of the pure silver and gold nanoparticles enabling an easy spectral tuning of the surface plasmon band.

For gold nanorods, the surface plasmon absorption splits into two bands corresponding to the electron oscillations along (longitudinal mode) and perpendicular (transverse mode) to the long axis of the nanorods. The wavelength of the maximum absorption intensity of the longitudinal plasmon absorption increases linearly with increasing nanorod aspect ratio. This relationship is examined theoretically by using the Gans theory.

The same gold nanorods encapsulated in micelles in aqueous solution fluoresces with a quantum yield, which is over a million times larger than that of the bulk gold films. The origin of this enhancement is treated theoretically and is found to be caused by the local field effect ("lightning" gold nanorods) similar to the previously proposed fluorescence and Raman enhancement on noble metal rough surfaces.

Enhancement factors of 107 are calculated, in agreement with experimental results. Simulations can furthermore reproduce the linear dependence of the fluorescence maximum on the gold nanorod aspect ratio and the quadratic dependence of the fluorescence quantum yield on the aspect ratio. Enhanced electron-surface scattering in nanoparticles smaller than the electron mean free path (~ 50 nm in gold) has been suggested to influence not only the dephasing time of the coherent plasmon oscillation (T2) but also the energy relaxation (T1) of photoexcited hot electrons into lattice excitation. Excitation with femtosecond laser pulses leads to a transient broadening of the surface plasmon band(s), which results in a transient bleach signal caused by a hot electron gas. It is found that the excited electrons couple to the lattice vibrations by electron-phonon coupling establishing a thermal equilibrium between electron and phonon subsystem within a few picoseconds. The deposited laser energy is then released to the surrounding medium which is found to take place in about 100 ps. An enhanced electron-surface scattering should cause a decrease in the electron-phonon relaxation time. However, no size and shape dependence of the electron-phonon relaxation dynamics are found for particles in the size range between 10 and 100 nm.

The effect of the surrounding medium on the cooling dynamics of spherical gold nanoparticles is examined by comparing the dynamics of gold nanoparticles in solution with the same particles embedded in MgSO4 powder. A slower electron-phonon relaxation time is found and the results are attributed to a hindered heat exchange between the gold nanoparticles and the surrounding medium for the particles in the MgSO4 powder.

Depending on the femtosecond laser pulse energy, irradiation of gold nanorods in colloidal solution is found to lead to a shape transformation into spherical nanoparticles of comparable volume or into smaller nanodots by fragmentation. Femtosecond pulses are found to be more gentle and effective in transforming the gold nanorods into nanodots. High energy nanosecond laser pulses cause fragmentation of the nanorods into smaller nanodots by the absorption of additional photons while the nanoparticle lattice is still hot.

The time required to melt a gold nanorod in solution is investigated by femtosecond transient absorption spectroscopy. This allows one to follow the rise of the permanent bleach of the longitudinal surface plasmon absorption due to the transformation of the nanorods into spheres of comparable volume. The dynamics of this shape transformation is found to occur in 35 ps for nanorods with aspect ratios between 1.9 and 3.7, at laser pump power at the energy threshold for the complete melting of the gold nanorods. The minimum absorbed energy necessary for this shape transformation is further determined to be about 60 femtojoule per nanorod.

The mechanism of the nanorod-to-nanodot shape transformation is examined by high resolution TEM. The as-prepared gold nanorods are single crystals and are found to be defect-free. However, after excitation with femtosecond and nanosecond laser pulses with energies below the energy required for the complete melting different internal defect structures appear in the middle of the rod. It is therefore concluded that the initial step in the shape photothermal transformation involves the creation of defect structures inside the rods followed by surface diffusion and reconstruction of the unstable {110} surface of the gold nanorods.

The knowledge of the cooling dynamics is also important for any possible future optical applications since the heat absorbed by laser light needs to be dissipated without structural damage to the nanostructural device itself on a time scale faster than its duty cycle. On the other hand, the controlled shape control of chemically prepared metallic nanoparticles can be achieved with pulsed laser light of the appropriate pulse width and energy.

 

[1] S. Link, M. A. El-Sayed, Steady-state and time-resolved optical properties of metallic nanoparticles: The surface plasmon absorption as an analytical tool to investigate particle properties. Int. Rev. Phys. Chem. 19, 409 (2000).

[2] S. Link, M. A. El-Sayed, Spectral properties and relaxation dynamics of surface plasmon electronic oscillations in gold and silver nanodots and nanorods. J. Phys. Chem. B 103, 8410 (1999).

 


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