Photoinduced Ultrafast Structural Dynamics of Nanomaterials

1
Photoinduced Ultrafast Structural Dynamics of
Nanomaterials
.

• Pyng Yua, Po-Tse Tai, Sheng-Hsien Linb and Jau
  Tangc,
• aDepartment of Chemistry, National Taiwan
  University, Taipei, Taiwan
• bInstitute of Molecular Science and Department of
  Applied Chemistry, National Chiao-Tung
  University, Hsinchu, Taiwan
• cResearch Center for Applied Sciences, Academia
  Sinica, Taipei, Taiwan

ABSTRACT We examined photoinduced ultrafast
structural dynamics such as coherent acoustic
waves in many shapes of fcc metallic
nanomaterials. Experimental data of nano-sized
thin films, prisms, spheres, rods, discs and
pyramids from transient optical
absorption/reflectance measurements were analyzed
based on a combined Fermi-Pasta-Ulam model and
two-temperature model. This work elucidates the
structural dynamics induced by femtosecond laser
heating, its size and shape effects on the period
and phase of the excited acoustic phonon modes.
3.2 Transient Optical Absorption
1. Synthesis and Characterizations
The Ag nanoprisms were synthesized using the
method prescribed earlier by others with minor
modifications. PVP with a molecular weight of
about 55 kg/mol was by means of preparation of
stable blue solutions containing Ag nanoprisms.
The solution becomes light in color within the
first 20 minutes of NaBH4 injection, indicating
the formation of silver nanoparticles through
polyol reduction. After which the silver triangle
nanoprisms were synthesized with the indication
that the yellow mixtures became blue suddenly.
(a) 3-D atomic array in fcc structure, showing
100, 111 and 110 planes. (b) Vrious
shapes of laser-heated nanomaterials to be
analyzed in this work.
To characterize the Ag NPs, UV-visible absorption
measurements (JASCO V-570) and a transmission
electron microscope (JEOL, JEM-1200EX II)
operated at 80 kV are employed.
(c)
(c-e) The change of transient absorption of
silver nanoprisms (open circle) with a bisector
height of 31.4 nm. (f-g) Transient absorption
profile (open circle) for gold nanorods with a
length L 55 15 nm and a radius R 13 1 nm.
4. A Schematic Diagram Showing a Triangular Plate
(A) A model of interaction and orientation of the
poly-vinylpyrrolidone (PVP) and citrate monolayer
at the surface. (B) silver nanoprisms are
immobilized on a silicon film coated copper
grids. (C) Absorption spectra of triangular Ag
NPs and spherical Ag nanoparticles.
2. Ultrafast Laser System
3. Fermi-Pasta-Ulam Model TTM
(a) Excitation of acoustic oscillations of a
triangular plate with 49.8 nm bisector and (b)
the oscillation periods versus triangle bisector.
5. Conclusions
In this work we examined photoinduced coherent
phonon excitations of metal nanomaterials by
time-resolved electron diffraction and transient
optical absorption/reflectance. We have
systematically investigated experimental data of
our own and those published in literature by
others for six different shapes and various sizes
of gold or silver composition. The FPU-TTM model
provides a better physical picture than the usual
but crude phenomenological model assuming a
simple damped oscillator to describe the dynamics
of the whole film or nanoparticle. We have found
out that the phase for the acoustic phonon mode
along the direction of an impulse and the phase
for the other mode with sound wave propagating
perpendicular to the impulse indeed have a
90-degree phase shift.
3.1 Time-Resolved Electron Diffraction
6. References

1. J. Tang, P. Yu, P. T. Tai and S. H. Lin, SPIE
   Proceeding (in press, 2009),"Photoinduced
   ultrafast structural dynamics of nanomaterials".
2. P. T. Tai, P. Yu, S. H. Lin and J. Tang, (to be
   submitted),"The period and phase of coherent
   phonons in nanoprisms by femtosecond laser
   heating".
3. P. T. Tai and J. Tang, Journal of Scientific
   Conference Proceedings (in press,
   2009),"Coherent Acoustic Phonon Excitation in
   Nanoprisms by a Femtosecond Impulse".
4. P. T. Tai and J. Tang, J. Chem. Phys.
   (submitted),"Excitation of Different Acoustic
   Phonon Modes in Triangular-Shaped Nanoplates".
5. P. Yu, J. Tang, and S. H. Lin, J. Phys. Chem. C
   112, 17133 (2008),Photoinduced Structural
   Dynamics in Laser-Heated Nanomaterials of Various
   Shapes and Sizes.
6. J. Tang, Appl. Phys. Lett. 92, 011901
   (2008),"Nano-Scale Heat Transfer in a Thin
   Aluminum Film and Femtosecond Time-Resolved
   Electron Diffraction",

The experimental data of time-resolved electron
diffraction by Nie at al., for a polycrystalline
aluminum thin film of about 20 nm in thickness
(open circle). The data curve for the changes of
the diffraction ring position can be fitted well
by using the FPU-TTM model (blue line).