analysis of nanostructural layers using low frequency impedance spectroscopy hans g. l. coster part...
TRANSCRIPT
Analysis of nanostructural layers using low frequency impedance spectroscopy
Hans G. L. Coster
Part 2: Dielectric Structure Refinement
We will examine actual impedance data for a tetradecane film on Silicon
Sil
ico
n w
afer
Ele
ctro
lyte
The equivalent 2 layer circuit model
Self Assembled Alkane layers on Si
Presentation of actual data and fitting of equivalent
circuit layers using the INPHAZE Dielectric Structure
Refinement software
Tetradecane SAM on Silicon
Set the area of the sample
Set the minimum systematic error
Start fitting with 1 layer and fit up to 2 layers
Initiate fitting
View the model structure
Model and data plot
Structure of the SAM
Note the large differences in the conductance
Characteristic Frequency typical of electrolyte
Characteristic Frequency typical of a thin, insulating layer
Known dielectric constants
Dimensions and conductivity of layers
Alkane layer is 1.7 nm thick
Zooming to reveal details of fitting
Left click-drag to outline zoom area
Expanded view reveals additional “structure”
Overall good fit but towards the characteristic frequency of the electrolyte an additional layer may be required to fit detail
Examples of other representations of the data
Impedance vs frequencyNot very sensitive to model parameters
Dielectric plot (real vs imaginary admittance)
Refining the modelReduce the minimum systematic error
Force the software to fit with 3 layers
Fit with 3 layers
Old Fit with 2 layers
Zoom to reveal detail
Refined structure
View the new model
The model structureNew layer
Slightly modified dielectric parameters for the main SAM layer
Frequency constant of additional layer
Additional layer is at the SAM-electrolyte interface and might have a slightly elevated dielectric constant
New layer is ~ 0.2 nm
The structure of the molecular layer
1.70 nm
1.68 nm
0.1
-0.2
nm
Very thin (< 0.2 nm) interfacial layer
The cruder 2 layer model (alkane SAM + solution) yields the same overall dimensions for the SAM as the 3 layer model
Deduced from the 2 layer model
Deduced from the 3 layer model
Sil
ico
n w
afer
Impedance spectra for several alkane layers on Si
10-2
10-1
1 101
102
103
104
105
Frequency (Hz)
Cap
acit
ance
Fm
-2
10-4
10-3
10-2
10-2 10-1 1 101 102 103Cap
acit
ance
Fm
-2
10-2
4 x 10-3
6 x 10-3
8 x 10-3
4 x 10-2
Frequency Hz
C 10C 12
C 14C 16
C 18
10 12 14 16 18
Chain length of alkane (No of carbon atoms)
12
14
16
18
20
22
Th
ick
ne
ss
(A
)
Ca
pa
cit
an
ce
at
0.5
58
Hz
(mF
m
)-2
8
9
10
11
12
13
14
15
O
Cd or
The dimensions of these layers can be deduced to within atomic resolution!
Si-SiO2
Presentation of actual data and fitting of equivalent
circuit layers using the INPHAZE Dielectric Structure
Refinement software
SiO2 on Si
SiO
2
Ele
ctr
oly
te
So
luti
on
Sili
co
n –
hig
hly
co
nd
uct
ing
Data for the Si-SiO2- electrolyte system
Data fitted: yields a 2 layer circuit model
Dielectric properties of the glass layer
SiO2 layer is 3.1 nm
Note the large differences in the conductance
Characteristic Frequency typical of electrolyte
Characteristic Frequency typical of a thin, insulating layer
Known dielectric constants
Multilayered structures
Multilayered structures manifest a more complicated dispersion of capacitance and conductance with frequency
The individual layers can be determined from data over a sufficiently large frequency range
Hybrid Bilayer Lipid Membrane
formed by adsorption of lipid vesicles on hydrophobic alkyl monolayers
Silicon substrate (111 surface)Alkyl monolayer covalently bonded to Si
Monolayer of lipids
Attached by hydrophobic forces
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
0.001 0.1 10 1000 100000Frequency (Hz)
Ca
pa
cit
an
ce
(m
F m
-2)
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
1.E+01
1.E+02
1.E+03
0.001 0.1 10 1000 100000Frequency (Hz)
Co
nd
uc
tan
ce
(S
m-2
)
Alkyl monolayer
Hybrid Bilayer
m onolayerC
solutionG
m onolayerG
top lipid leafletCm onolayerC
solutionG
top leafletG
m onolayerG
Impedance spectroscopy of Hybrid Bilayers
Fitting the data yields the individual dielectric parameters of the layers
Detailed dielectric structure of Lipid Bilayers
N N
CurrentElectrode
Electrolyte
Electrolyte layer betw een m em brane andthe planes containingpotential electrodes
P olar H eadR egion
P olar H eadR egion
H2O and ionsH O and ions2
H ydrophobicR egion
AcetylRegion
AcetylRegion
Electrolyte
CurrentElectrode
G E G P GPG AG A G H
CP CPCACA C H
GE
Lecithin
Lecithin + Cholesterol
Lipid Bi-Molecular Membranes: the core matrix of cell membranes
The effect of cholesterol using the INPHAZE Dielectric Modeling software
0
2
4
10
8
6
600
400
200
0
800
Acyl Carbonyl Glycerol Phosphatidylcholine
LecithinLec/CholLec/OxChol
Ca
pac
itan
ce (
mF
m)
(Ac
yl c
hai
n r
gio
n)-2
Cap
aci
tan
ce
(mF
m)
(Oth
er r
egio
ns
)-2
Acyl Carbonyl Glycerol Phosphatidylcholine
LecithinLec/CholLec/OxChol10 4
10 3
10 2
10 1
10 0
Co
nd
uc
tan
ce
(mS
m)
-2
Locating the cholesterol molecule
OPOO -
O
N+ O
OO
OH
OPOO -
O
N+ O
OO
OPOO -
O
N+ O
OO
OPOO -
O
N+ O
OO
OPOO-
O
N+O
OO
OPOO-
O
N+O
OO
OPOO-
O
N+O
OO
OPOO-
O
N+O
OO
From the changes in the capacitance and conductances of the various layers, it is possible locate the cholesterol molecule in the lipid bilayer.
As the cholesterol is oxidised (either in situ or by using the oxidised form) the molecule moves out towards the surface.
OH
The Spectrometer
Impedance range: 0.1 -1010 Frequency: < 10-2 – 106 Hz
Impedance precision: 0.002% Phase resolution: 0.001 o
Inphaze.com.au
inphaze.com.au