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Transport of ions through ultra-thin films and membranes

While a major part of our transport studies looks at sample in contact with a single metal electrode, there is also interest in the transport through free standing membranes or films.

We have setup an experiment, which allows to investigate the transport of alkali ions through free-standing ultra-thin membranes. The ions have been prepared by thermionic emission. The membranes are polymer films prepared in cooperation with Prof. Greiner.

Originally we have studied the total ion current transmitted through poly-p-Xylylene membranes with thicknesses between 100nm and 3 µm as a function of the impact energy. We observed maxima in the transmission efficiency at specific impact energies depending on the membrane thickness [1].

We have also implemented the time-resolved measurement of the ion transport. Ultimately we aim at a better understanding of scaling effects of transport in both spatial and temporal dimension.

In this context we have pursued two different approaches:

The first approach is one where we irradiate a free-standing membrane, with thickness of 1 µm, with a c.w. ion beam and analyze the time correlation between successive individual transport events (single ion transport!).

Fig. 1              Scheme of the experimental setup for transport of ions through a free-standing membrane with subsequent detection by a micro channel plate (single particle) detector.

The transport appears to be overall stochastic, but carries the signature of a transition from diffusive to space charge zone driven transport [2]. One prominent aspect is a jump in the width of the pulse-pair correlation function (distribution of waiting times) as shown in the figure below. This transition is connected to the macroscopic dielectric breakdown of the material occurring around 1300 V/µm.

Fig. 2   Width of the exponential distribution t derived from the relation

The second approach is one based on irradiating the membrane with very short ion pulses and measuring the delay and the broadening of these ion pulses in the real time domain.

Fig. 3.   Scheme of the experimental setup for time correlated single ion pulse counting after transport through a free standing membrane.

At high impact energies the transport of ions through the membrane occurs with ballistic characteristics, i.e. no energy dispersion. At lower impact energies the transport occurs with diffusive characteristics. This is one of very few direct observations of the transition from diffusive to ballistic transport. As expected the transition energy depends on the membrane thickness.

Figure 4 Relative Ion yields for ballistic (red symbols) and diffusive (black symbols) transport through free standing membranes of thickness indicated. Note, that the cross over energy is shifted to larger energies for the thicker membrane.

References

[1]   T. Kolling, E. Unger, S. Sun, K.-M. Weitzel, Thin solid films, 517, 4583-4586, (2009), http://dx.doi.org/10.1016/j.tsf.2009.03.068 

[2]   K. Schröck, S. Schulze, A. Schlemmer, K.-M. Weitzel, Journal of Physics D, 43 025501, (2010), http://dx.doi.org/10.1088/0022-3727/43/2/025501 

[3]   Susanne Schulze, Karl-Michael Weitzel, PHYSICAL REVIEW E 92, 052602 (2015)