Analysis of nanostructural layers using low frequency impedance spectroscopy

1. Analysis of nanostructural layers using low frequency impedance spectroscopy Part 2: Dielectric Structure Refinement Hans G. L. Coster

2. We will examine actual impedance data for a tetradecane film on Silicon Electrolyte Silicon wafer The equivalent 2 layer circuit model

3. Self Assembled Alkane layers on Si Presentation of actual data and fitting of equivalent circuit layers using the INPHAZE Dielectric Structure Refinement software

4. Tetradecane SAM on Silicon Set the area of the sample Initiate fitting Model and data plot View the model structure Set the minimum systematic error Start fitting with 1 layer and fit up to 2 layers

5. Structure of the SAM Note the large differences in the conductance Characteristic Frequency typical of a thin, insulating layer Characteristic Frequency typical of electrolyte Dimensions and conductivity of layers Known dielectric constants Alkane layer is 1.7 nm thick

6. Zooming to reveal details of fitting Left click-drag to outline zoom area

7. 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

8. Examples of other representations of the data Dielectric plot (real vs imaginary admittance) Impedance vs frequency Not very sensitive to model parameters

9. Refining the model Reduce the minimum systematic error Zoom to reveal detail Old Fit with 2 layers Force the software to fit with 3 layers Fit with 3 layers

10. Refined structure View the new model

11. The model structure New layer Frequency constant of additional layer Additional layer is at the SAM-electrolyte interface and might have a slightly elevated dielectric constant Slightly modified dielectric parameters for the main SAM layer New layer is ~ 0.2 nm

12. The structure of the molecular layer 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 Silicon wafer Deduced from the 2 layer model 1.70 nm Deduced from the 3 layer model 0.1-0.2 nm 1.68 nm

13. Impedance spectra for several alkane layers on Si C 10 C 12 C 14 C 16 C 18 The dimensions of these layers can be deduced to within atomic resolution!

14. Si-SiO2 Presentation of actual data and fitting of equivalent circuit layers using the INPHAZE Dielectric Structure Refinement software

15. Electrolyte Solution SiO2 Silicon – highly conducting SiO2 on Si Data fitted: yields a 2 layer circuit model Data for the Si-SiO2- electrolyte system

16. Dielectric properties of the glass layer Note the large differences in the conductance Characteristic Frequency typical of electrolyte Characteristic Frequency typical of a thin, insulating layer Known dielectric constants SiO2 layer is 3.1 nm

17. 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

18. Hybrid Bilayer Lipid Membrane formed by adsorption of lipid vesicles on hydrophobic alkyl monolayers Monolayer of lipids Attached by hydrophobic forces Alkyl monolayer covalently bonded to Si Silicon substrate (111 surface)

19. Impedance spectroscopy of Hybrid Bilayers  Alkyl monolayer Hybrid Bilayer Fitting the data yields the individual dielectric parameters of the layers

20. Detailed dielectric structure of Lipid Bilayers

21. Lipid Bi-Molecular Membranes: the core matrix of cell membranes Lecithin Lecithin + Cholesterol

22. The effect of cholesterol using the INPHAZE Dielectric Modeling software

23. O O O O O O O O O O O O O O O O O O O O O O O O P P P P P P P P O O O O O O O O O O O O O O O O N N N N N N N N + + + + + + + + O O O O O O O O O O O O O O O O - - - - - - - - H H O O Locating the cholesterol molecule 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.

24. The Spectrometer Impedance range: 0.1 -1010W Frequency: < 10-2 – 106 Hz Impedance precision: 0.002% Phase resolution: 0.001 o Inphaze.com.au

25. inphaze.com.au