Analysis of nanostructural layers using low frequency impedance spectroscopy

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Analysis of nanostructural layers using low
frequency impedance spectroscopy
Part 1 Background Physics
Hans G. L. Coster
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Response of materials to AC currents
voltage
Phase shift
Current
Shift in relative magnitude
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Electrical Circuit Equivalents
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Impedance of capacitance elements
C Q/V
Sinusoidal currents
Same form as
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Impedance of a capacitance element
(Ohmss Law)
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Impedance of capacitance elements
C Q/V
The voltage lags behind (phase lags) the current
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Parallel Circuits
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Parallel impedances
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Circuit elements to represent Layers
Real Part
Imaginary Part
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Phase relationships
We measure ZT and q
But it is simpler to work in terms of
Admittances (Y 1/Z)
For a single layer containing parallel R and C
elements
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Detecting substructure with Impedance Spectroscopy
Homogeneous film
Film with two homogeneous sub-structural layers
Consider an example with the following dielectric
parameters
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Impedance Spectroscopy Dispersion of Impedance
with frequency
Conclusion The impedance as a function of
frequency does not allow us to discriminate
between the single and 2 layer structures.
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Impedance Spectroscopy Capacitance dispersion
with frequency
0.0060 0.0059 0.0058 0.0057 0.0056 0.0055
Capacitance mF m-2
0.1 1 10
100 1000
Frequency - Hz
The capacitance as a function of frequency allows
the single and 2 layered structure to be readily
distinguished.
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Impedance Spectroscopy Conductance dispersion
with frequency
0.040 0.035 0.030 0.025 0.020 0.015 0.010 0.005
Conductance S m-2
0.1 1 10
100 1000
Frequency - Hz
The conductance as a function of frequency also
allows the single and 2 layered structure to be
readily distinguished.
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A two layer sandwich
Whilst C1, G1, C2, G2 are frequency independent,
C and G for the combination is frequency
dependent
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Alkane layers on silicon
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Self Assembled Organic Films
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Organic films on Silicon substartes
Decane layer attached to Si
The Born energy for ion partitioning into the
alkane layer is very high. Therefore the
electrical conductance of this layer will be very
low.
More info on Born Energy
Silicon 111
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The Spectrometer
Impedance range 0.1 -1010 W
Frequency lt 10-2 106 Hz
Impedance precision 0.002 Phase resolution
0.001 o
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Ion Partitioning into an organic film
Organic film
External solution
Ion in water
Image of ion in dielectric
em 2 -3 for hydrophobic films
ew 80
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Ion partitioning into molecular films
The Born Energy arising from image forces on the
ions is
For a K ion in a alkane layer, WB 3 eV
Recall that kT 0.025 eV at room temperature So
that the partitioning goes as e-120 !!
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It will be a very poor electrical conductor!