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

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