Transcript AaronShojinagaPosterx - Physics

Terahertz Conductivity of Silver Nanoparticles
Aaron Shojinaga with Jie Shan
Department of Physics, Case Western Reserve University
Abstract:
The electrical conductivity for bulk metal is described by the well-known Drude model. As the size of the metal is reduced to the nanometer scale however, the energy levels become
discrete, rather than continuous. The average spacing between adjacent energy levels in a metal nanoparticle is called the Kubo gap, and is related to the Fermi energy of the metal and the
size of the nanoparticle. For instance, in a silver nanoparticle of 3-nm diameter containing ~103 atoms, the Kubo gap is around 5-10 meV. Therefore, at room temperature when the thermal
energy is greater than this gap, the electrical conductivity will be the same as in bulk metal. As the temperature is lowered however, the Kubo gap becomes significant and the nanoparticle
becomes an insulator. Although the DC properties of this metal-to-insulator transition are well understood, the experimental observations and theoretical description for AC conductivity are
much less comprehensive. The AC conductivity of silver nanoparticles will be measured in an interesting frequency range that corresponds with the Kubo gap of the nanoparticles.
Conductivity will be measured using terahertz time-domain spectroscopy based on a mode-locked laser.
The absorption coefficient (k) and refractive index (n) of the
films were calculated from the frequency-dependent
signals.
Introduction:
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Figure 1: Experimental set up.
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Silver nanoparticle composites are created by mixing a
silver nitrate solution with polyvinyl alcohol and
evaporating the mixture until a thin film remains. The
resulting film contains silver nanoparticles suspended in a
polymer matrix.
Results:
The terahertz signals were measured as a function of
time and the FFT was computed to retrieve the frequency
dependence of the signals.
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Signal in time domain
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Methods:
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Figure 3: FFT of terahertz signal.
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Figures 3 and 4: Absorption coefficient and refractive index of
nanoparticle films.
Conclusions and Future Work:
The optical properties of silver nanoparticle films were
measured in a range corresponding to the Kubo gap of the
nanoparticles. Further measurements must be made in
order to extract the conductivity of the nanoparticles
themselves from that of the films. The next step is repeat
the measurements at different temperatures in order to
observe a transition to insulator conductivity.
Further optimizations to the terahertz set-up might be
necessary to achieve a signal-to-noise ratio large enough to
observe the effects of the Kubo gap. In particular, there are
several noticeable spikes in the terahertz spectrum that are
due to absorption by water vapor in the air. The resolution
in the vicinity of these absorption peaks can be increased
by removing water vapor from the air.
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Figure 2: Terahertz signal in time domain.
Signal
Terahertz time-domain spectroscopy is used to
measure the electric field of terahertz radiation as a
function of time. Time-domain spectroscopy allows for
recovery of amplitude and phase information of the
terahertz signal. By measuring the signal after it has passed
through the nanoparticle sample and comparing to a
reference signal, the frequency-dependent optical
properties of the sample can be calculated. Ultrashort laser
pulses generated from a mode-locked Ti:sapphire laser are
used to generate and detect terahertz radiation. Terahertz
radiation is emitted when these laser pulses strike a GaAs
semiconductor. The THz radiation is detected using a
commercially available photoconductive antenna. The
nanoparticle composite sample is placed between one set
of parabolic mirrors, where the terahertz radiation is
focused to a small point.
k
n
Signal
Metal nanoparticles are small clusters of metal, less
than 100 nm in size. Although metal nanoparticles have
been used in various scientific and other fields for some
time, their physical and electronic properties are still not
fully understood. The potential applications for metal
nanoparticles include use in nanoelectronics, electronics
with components of nanometer scale. Detailed
understanding of the electrical properties of these
nanoparticles is crucial in developing nanoelectronics.
If the size of a metal cluster is less than the de Broglie
wavelength of an electron, conduction electrons will be
confined to certain allowed energy levels. The average
spacing between allowed energy levels is called the Kubo
gap. The effect of the Kubo gap will not be apparent at
room temperature, because the thermal energy is greater
than this energy gap. In this case, the nanoparticles have
metallic conductivity. If the thermal energy is less than the
Kubo gap, however, the nanoparticle conductivity should
be like an insulator.
The Kubo gap for a 3 nm diameter silver nanoparticle
is around 5-10 meV, which corresponds to electromagnetic
radiation with frequencies around 1-2 THz. Terahertz timedomain spectroscopy can be used to study the conductivity
in the frequency range of the Kubo gap. By measuring the
frequency-dependent conductivity as the temperature is
varied, the transition from metallic to insulator conductivity
and the effect of the Kubo gap can be observed.
Acknowledgements:
I would like to thank my advisor, Jie Shan for guidance
in the concept and execution of my project. I also thank the
graduate students in my lab, Brian Kubera, Chris Ryan, and
Xia Chen, for providing assistance and technical support.
References:
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27-35 (2000).
3. L.P. Gor’kov, G.M. Eliashberg. JETP 21, 940 (1965).
4. K. Frahm, B. Mühlschlegel, R. Németh. Zeitschrift für Physik B –
Condensed Matter 78, 91-97 (1990).
5. J. Baxter, C. Schmuttenmaer. J. Phys. Chem. B 110, 25229-25239 (2006).