Invited Speaker - Eddy De Grave (Belgium)

Integral Low-Energy Electron Mössbauer Spectroscopy (ILEEMS):
Methodology and Recent Applications in Material Research
 
E. De Grave1, V.G. de Resende1, C. Van Cromphaut1, Ch. Laurent2, L. Presmanes2 and R.E. Vandenberghe1
1Department of Subatomic and Radiation Physics, University of Gent, Proeftuinstraat 86, B-9000 Gent, Belgium
2CIRIMAT, CNRS/UPS/INPT, LCMIE, Université Paul-Sabatier, Bât. 2R1, 118 route de Narbonne, 31062 Toulouse cedex 9, France
 
Abstract

     ILEEMS is a variant of Mössbauer spectroscopy (MS) in which predominantly the low-energy electrons, E < ~10 eV, emitted by the probe nuclei in the absorber are counted as a function of source velocity. These low-energy electrons are, among others, Auger and “shake-off” electrons that are created in the decay process of the probe nucleus immediately after resonant absorption and excitation by an incident γ-quantum. As a consequence of their low energy, these electrons’ origin lies within a very thin surface layer with thickness of an estimated five nanometers. ILEEMS using 57Fe is as sensitive as transmission MS and the more common emission variants such as CEMS. Consequently, ILEEMS, in combination with conventional transmission MS, is a technique that provides information about the surface of Fe-containing substances. 
     In a first part of this contribution, we describe the design of a home-made ILEEMS instrument allowing the temperature of the investigated sample to be varied continuously between 77 K and room temperature. In essence, the instrument is the same as a spectrometer for transmission, except that detector and “absorber” are contained within a same vacuum (~ 5.10-5 mbar) chamber. The electrons are counted by a single channel electron multiplier (“channeltron”). The efficiency for detecting the low-energy electrons is optimized by applying a bias voltage of ~ 150 V between the sample and the detector input. The ILEEMS instrument has been successfully applied in a few earlier studies reported by the present authors. 
     The second part of this contribution will deal with a selection of results obtained recently. In particular, the following items will be covered: 
• Thin films of hematite, α-Fe2O3, grown by RF sputtering on glass substrates in Ar plasma; ILEEMS measurements, carried out in the temperature range between 80 to 330 K for a number of α-Fe2O3 films with different thickness, will be presented and discussed; this research is focussed on the behaviour of the Morin transition, i.e., the reorientation of the Fe3+ spins from the [111] crystallographic direction at low temperature, to the (111) basal plane at high temperature; the influence of thickness, sputtering parameters and of post-deposition annealing is being examined.
• Carbon nanotubes grown by CCVD method in various Fe-containing oxide matrices; the results have shown that the characteristics of the top surface layers as far as the presence of α-Fe, γ-Fe-C and Fe3C nanoparticles is concerned, are very often significantly different as compared to what is observed for the bulk by transmission MS.
• Freshly synthesized 2 XRD-line and 6 XRD-line ferrihydrite, 5Fe2O3•9H20, to compare the results with those obtained earlier for similar samples that had aged for a very long time.  
     Interesting findings concerning the surface properties of mentioned systems were obtained. ILEEMS studies so far have remained of a rather exploring nature, but the results encourage more systematic research in this and related fields.