I deposited a 2-5 nm layer of a certain polymer, and after that I deposited a layer of PCBM using sping-coating. I want to ensure the presence of the polymer after the deposition. Could I measure that using impedance spectroscopy?
Such kind of measurements have actually be done a lot in the past. As mentioned by Ioannis Samaras, you can deduce the thickness right away from the capacitance, provided you know the effective area A and the permittivity of the dielectric (the polymer layer).
The challenge is actually to provide an electrical contact on the top side of the polymer WITHOUT shortening the very thin capacitor.
Using a liquid electrode is one option as already discussed.
Personally I prefer an aluminum electrode (about 100nm thick) prepared by fast vapor physical deposition (Flash PVD). This technique is very robust and works in most cases even down to 3-4nm, since little shorts by pinholes in the polymer film will be burned away when you apply a voltage to the capacitor for the first time. This configuration is so to say self-healing – a property that is widely used in rolled thin film capacitors for a long time.
We have studied dielectric properties of ultrathin polymer films over the years very extensively, you find all references in my profile. Just a few recommendations:
1. V. Lupascu, H. Huth, C. Schick, and M. Wübbenhorst, "Specific heat and dielectric relaxations in ultra-thin PS layers," Thermochimica Acta, vol. 432, pp. 222-228, 2005.
2. V. Lupascu, S. J. Picken, and M. Wübbenhorst, "Cooperative and non-cooperative dynamics in ultra-thin films of polystyrene studied by dielectric spectroscopy and capacitive dilatometry," J. Non-Cryst. Solids, vol. 352, pp. 5594-5600, 2006.
3. C. Rotella, S. Napolitano, S. Vandendriessche, V. K. Valev, T. Verbiest, M. Larkowska, S. Kucharski, and M. Wübbenhorst, "Adsorption Kinetics of Ultrathin Polymer Films in the Melt Probed by Dielectric Spectroscopy and Second-Harmonic Generation," Langmuir, vol. 27, pp. 13533-13538, September 27, 2011 2011.
If you form a cell using a (suitable) strong electrolyte, then the (Capacitive[1]) impedance (amplitude) Za[2,3] scales with the thickness[3,1] of your (insulating, dielectric[1]) layer[4,5].
1. If there are holes in your (insulating) layer, the phase of the impedance (Zp) decline from the ideal (-)90o.
2. At the higher frequencies (>100kHz), Z is, more, easy to measure.
3. Z scale with the 1/C. According to the formula C=ε × S/d, (ε=dielectric constant, S=electrode area, d=thickness of the dielectric), then Za scales with the d, the thickness of the dielectric.
4. If there is a partly formed layer, the phase (Zp) is near 45o (~1MHz), or below.
5. If there is no dielectric layer, the phase (Zp) should be near 0o (~1MHz).
Perhaps with such a thin layer you should better use ellipsometry. In the case of the capacitance test you must know the dielectric constant from independent test - and you must be sure that the permittivity does not change with thickness (especially in the nanometer range, where orientation, defects etc. may have an effect). It is true, that the same can be said of ellipsometry - there you must take into account the change of refractive index with thickness. One more possibility is to scratch the surface and perform AFM or Talystep step--height measurement.
Such kind of measurements have actually be done a lot in the past. As mentioned by Ioannis Samaras, you can deduce the thickness right away from the capacitance, provided you know the effective area A and the permittivity of the dielectric (the polymer layer).
The challenge is actually to provide an electrical contact on the top side of the polymer WITHOUT shortening the very thin capacitor.
Using a liquid electrode is one option as already discussed.
Personally I prefer an aluminum electrode (about 100nm thick) prepared by fast vapor physical deposition (Flash PVD). This technique is very robust and works in most cases even down to 3-4nm, since little shorts by pinholes in the polymer film will be burned away when you apply a voltage to the capacitor for the first time. This configuration is so to say self-healing – a property that is widely used in rolled thin film capacitors for a long time.
We have studied dielectric properties of ultrathin polymer films over the years very extensively, you find all references in my profile. Just a few recommendations:
1. V. Lupascu, H. Huth, C. Schick, and M. Wübbenhorst, "Specific heat and dielectric relaxations in ultra-thin PS layers," Thermochimica Acta, vol. 432, pp. 222-228, 2005.
2. V. Lupascu, S. J. Picken, and M. Wübbenhorst, "Cooperative and non-cooperative dynamics in ultra-thin films of polystyrene studied by dielectric spectroscopy and capacitive dilatometry," J. Non-Cryst. Solids, vol. 352, pp. 5594-5600, 2006.
3. C. Rotella, S. Napolitano, S. Vandendriessche, V. K. Valev, T. Verbiest, M. Larkowska, S. Kucharski, and M. Wübbenhorst, "Adsorption Kinetics of Ultrathin Polymer Films in the Melt Probed by Dielectric Spectroscopy and Second-Harmonic Generation," Langmuir, vol. 27, pp. 13533-13538, September 27, 2011 2011.
- Badges
- Science topic
- Similar topics
- Chemistry
- Nanotechnology
- Nanostructures
- Thin Films