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Concentration Depth Profiles by ToF-SIMS

One of the key features of foreign ion CAIT is the formation of concentration depth profiles. This is due to the fact that typically one or even more mobile charge carriers in a solid sample are in this case replaced by a foreign ion. Such concentration depth profiles can be ideally analyzed by means of the time-of-flight secondary-ion-mass-spectrometry (ToF-SIMS).

ToF-SIMS is a technique for 3D analysis of the chemical composition in a volume of typically 100µm x 100µm x 2µm viewed from the surface of the sample. Our ToF-SIMS machine employs alternating analysis and sputter beams. Typically a Bi+ or Bi3+ ion beam is scanned across the surface of a sample in an area of 100µm x 100µm. At each spot a complete mass spectrum is recorded. Subsequently a sputter ion beam erodes away a layer of typically 1nm depth. Then the analysis starts again. This procedure is repeated until the required depth of information is reached. Since it is a scanning technique it is inherently 3D resolved. Typically the depth resolution is on the order of 1 or 2nm, the lateral resolution is about 100nm. The mass resolution is on the order of 8000:1.

Figure 1         Scheme of the ToF-SIMS setup (Weitzel 2016)

Often, the lateral distribution is homogeneous, i.e. only the profile in the depth dimension is relevant. This leads to one-dimensional concentration depth profiles. Some typical profiles are shown below. For more details, please refer to the original work or some of the other project pages.

Figure 2         Concentration profiles in a sodium conducting glass after potassium attachment, K+@Ca30Na, as determined by ex-situ ToF-SIMS depth profiling. Taken from Rossrucker et al. (2012).

Figure 3         Concentration depth profile in a mixed sodium-potassium conducting glass after Cesium attachment, Cs+@D263T, as determined by ex-situ ToF-SIMS depth profiling. Taken from Martin et al. (2016)

In studies of Cs+ ion transport through thin polymer films we encountered two different transport pathways. The first pathway is one corresponding to regular bulk transport in a homogeneous polymer film due to ion hopping (see Fig. 4). The second pathway was termed non-intermitted pathway, implying that this pathway is a “highway” without interruption, i.e. for example a channel, but not necessarily a straight channel (see Fig. 5). The latter is characterized by the formation of metal islands between the polymer and the electrode as easily discernible in 3D representation.

Figure 4         3D images of elemental species carbon, cesium, silicon an platinum for a homogeneous polymer sample as well as z-integrated top view and a cut along the yz-plane (along the vertical green line of the top view) through the cesium signal. Taken from Wesp et al. 2016.

Figure 5         3D images of the elemental species carbon, cesium, silicon and platinum observed for the polymer sample with non-intermitted ion-transport pathways (NIP) as well as the z-integrated top view and a cut along the yz-plane (along the vertical green line of the top view) through cesium signal. Taken from Wesp et al. 2016. For a 3D anaimation see the supplementary material of the original paper. 

Literature

L. Rossrucker, P.V. Menezes, J. Zakel, M. Schäfer, B. Roling, K.-M. Weitzel
Bombardment induced potassium ion transport through a sodium ion conductor: conductivities and diffusion profiles
Zeitschrift für Physikalische Chemie, 226, 341-353, (2012)
Open Access Paper:  http://dx.doi.org/10.1524/zpch.2012.0215

V. Wesp, J. Zakel, M. Schäfer, I. Paulus, A. Greiner, K.-M. Weitzel
Highways for ions in polymers - 3D–imaging of electrochemical interphase formation
Electrochimica Acta, 170, 122–130 (2015)
http://dx.doi.org/10.1016/j.electacta.2015.04.117  

K.-M. Weitzel
Bombardment induced Ion Transport through Ion Conducting Glasses
in „Diffusion Foundations: Progress in Ion Transport and Structure of Ion Conducting Compounds and Glasses“,
Volume 6,  H. Mehrer (ed.), Trans Tech Publ. Ltd., p. 107-143, (2016)
http://dx.doi.org/10.4028/www.scientific.net/DF.6.107

J. Martin, S. Mehrwald, M. Schäfer, T. Kramer, C. Jooss, K.-M. Weitzel
Transport of ions in a mixed Na+/K+ ion conducting glass - electrodiffusion profiles and electrochemical interphase formation
Electrochimica Acta, 191, 616–623 (2016)
http://dx.doi.org/10.1016/j.electacta.2016.01.061