Main Content

New mass spectrometer for chemical microscopy installed

A new mass spectrometer with improved resolution and extended dynamic range has been installed for secondary ion mass spectrometry (SIMS). SIMS enables chemical microscopy of interfaces and the bulk of solid state materials with nm resolution.

The energy landscape and structure of solid state materials are intimately related in determining the function, e.g. the transport characteristics in materials for energy storage and conversion.

Current research efforts in the DFG research unit ELSICS (Energy Landscapes and Structure in Ion Conducting Solids) aims at the quantification of concentration depth profiles as a result of transport processes – amongst other things. Such concentration depth profiles are the result of the charge attachment induced transport (CAIT) experiments developed by the Weitzel group in Marburg. Here, an alkali ion beam is attached to the front side of an alkali ion conductor inducing charge carrier transport inside the bulk of the material.

The spatial distribution of native and foreign ions in the material is subsequently analyzed by chemical microscopy, which allows looking deep into the material with 1nm resolution in the transport direction. Chemical microscopy is enabled by means of time-of-flight secondary-ion-mass-spectrometry (ToF-SIMS). So far the analysis of the data was hampered by the unprecedented sensitivity of ToF-SIMS towards all alkali ions leading to the situation that the conventional detectors were easily running into saturation. Consequently intricate tricks, some based on assumptions, had to be applied in the analysis in the past.

Now these sometimes problematic assumptions and tricks have become obsolete by the purchase and installation of a new high resolution reflecting ToF-mass spectrometer employing a so called extended dynamic range (EDR) detector (c.f. Figure 1). This detector allows to attenuate the ion signal by a well-defined factor extending the available dynamic range to 1 000 000 : 1. That means, one ion of one chemical kind can be observed while ions of another chemical kind may have a signal level of 1 million counts. This is illustrated in Figure 2 by a CAIT concentration depth profile of K+ in a Na⁺ conductor. At the same time the new MS provides a mass resolution of up to 15 000 : 1, meaning that e.g. ²⁸SiH (m/z=28.9848) can be distinguished from ²⁹Si (m/z=28.9765) (c.f. Figure 3). Ultimately, it appears right now, that the saturation of alkali ions signals has been completely solved. The new device will make a series of experiments possible, which were not possible in that way before. As an example, we mention the transport of isotope selected ⁶Li⁺ into a Lithium ion conductor containing a natural mixture of ⁶Li and ⁷Li or even in a pure ⁷Li⁺ ion conductor. So far the ⁷Li signal was always in saturation and we either had to analyze the ⁶Li⁺ signal, making the ⁶Li⁺-CAIT impossible or a LiO⁺ signal (m/z=23.0109) making the Na⁺ (m/z=22.9898) experiment difficult. All these problems are solved, such that we can kick-off now.

The acquisition of this new ToF-MS has been made possible by a grant to project P1 of the DFG research unit FOR_5065 (ELSICS) (100% DFG funding!).

For further details contact: Prof. Karl-Michael Weitzel (weitzel@chemie.uni-marburg.de)

Figure 1:     ToF-SIMS machine with new mass spectrometer and EDR detector.

Figure 2:    Concentration depth profiles of K⁺ in a Na⁺ conductor ( Na_1.5Al_0.5Ge_1.5PO₄)₃ )

Figure 3:    Mass spectrum of a technical Borosilicate glass (D263T, Schott) in the region of m/z = 29.