Chemical Processes of Ions
Formation - Transport - Reactivity
Nanoionics at
interfaces
In this project we investigate chemical changes induced in the interface region of a material with nanoscale dimensions.
The bombardment ionduced ion transport (BIIT) approach developed in our group allows measuring ionic conductivities of a broad range of materials by attaching an alkali ion of interest to the front surface of a material and measuring currents at a backside electrode. If the material of interest is a solid electrolyte which contains the alkali ion from the beam chemical changes are (most likely) restricted to the interface region. If the material does not contain the alkali ion beforehand, then it must be present in the material afterwards.
To demonstrate this we have performed the following experiments. First we look at the transport of potassium ion through a 1.2 µm thick PPX (poly-para-xylylene) film (PPX is a technically important polymer) deposited on a metal electrode. Here, the transport of potassium ions through the polymer film generates a diffusion profile which can be quantitatively analyzed by means of time-of-flight secondary ion mass spectrometry (ToF-SIMS). The concentration profiles measured can be quantitatively explained by theory assuming that the diffusion coefficient of K+ in PPX depends on the local K+ concentration.
Fig. 1 Upper graph: experimental concentration profiles measured via ToF-SIMS, The C+ signal reflects the bulk properties as well as the thickness of the film. The K+ profiles clearly reach through the entire film. Lower graph: Experimental K+ profile (symbols) together with calculated profiles assuming concentration dependent diffusion coefficients. For further details see ref. [1]
In a second experiment we have investigated ion transport in a sodium ion conductor induced by bombardment with potassium ions! BIIT allows determining the ionic conductivity of the material, which turns out to be determined by the bulk sodium ions. However, depth profiling via ToF-SIMS reveals that the sodium ions are almost (but not entirely) depleted in a region 100 nm below the surface of the sample, and have been replaced by potassium ions.
Fig. 2 Upper graph: experimental concentration profiles as determined by ToF-SIMS. Lower graph: experimental K+ and Na+ concentration profiles (symbols) together with the result of simulations (lines) assuming concentration dependent diffusion coefficients. For further details see ref. [2].
[1] Susanne Schulze, Julia
Zakel, Martin Schäfer and Karl-Michael Weitzel
Potassium ion transport through poly-para-xylylene films
IEEE
– TDEI, 19, 1167-1174 (2012)
[2] Lisa Rossrucker,
Pramod V. Menezes, Julia Zakel, Martin Schäfer, Bernhard Rolingand
Karl-Michael Weitzel,
Bombardment induced potassium ion transport through a sodium ion
conductor: conductivities and diffusion profiles,
Z.
Phys.Chem., 226, 341-353 (2012)
