In general, lattice-expanded states are often found near solid/solid boundaries, like in thin films and nanostructures, where the atomic environment abruptly changes. A classical example is the interface between the Fe and the W(110) surface, for which it is well known that the first iron layer grows in pseudomorphic manner, i.e. the Fe film adopts the periodicity of the substrate [1]. This results in a relative stretching of the Fe lattice by about 10% compared to its bulk value, implying significant changes of the elastic properties in this and the adjacent layers. Increasing the number of iron layers relaxes the lattice strain via a network of misfit dislocations and re-establishes the bulk Fe-Fe coordination distances. Consequently the vibrational properties of the Fe films are expected to differ remarkably from the corresponding bulk behaviour.

In order to study systematically the lattice vibrations of the iron films deposited on W(110) upon transition from the bulk to a single iron monolayer we have measured the density of phonon states (DOS) as a function of the film thickness applying nuclear inelastic scattering (NIS) of synchrotron radiation. From the measured DOS, a number of relevant parameters (mean specific heat, vibrational entropy, average force constant, sound velocity, etc.) can be calculated.

The experiment was performed at the Nuclear Resonance Beamline ID18 using a recently commissioned ultra-high vacuum (UHV) system, which allows for sample preparation by molecular beam epitaxy, characterisation by low energy electron diffraction and Auger electron spectroscopy, and in situ NIS experiments.

The DOS extracted from the measured NIS spectra are shown in Figure 7 in comparison with the DOS of polycrystalline - Fe foil. The DOS of the 40 ML film resembles closely the DOS of the bulk Fe but the spectral features are shifted by about 1 meV to lower values. This is a consequence of the expanded state of the film that results from the mismatch of the W and Fe lattices and goes along with a lattice volume expansion of about 2%. Relating the observed energy shift to the pressure dependence of the Fe DOS and its spectral features [2], one arrives at a negative pressure of 2 GPa to which the film is effectively subjected. For the 10 ML film one observes a pronounced suppression of the peak at 34 meV and an increase of the modes at low energies. Decreasing the coverage results in a further enhancement of the low-energy modes while spectral features at high energies are significantly reduced. Qualitatively, the observed softening is caused by a decrease in the coordination number at the surface and is typical for surface phonons, but certainly also mirrors the coupling of the adsorbed iron atoms to the softer crystal lattice of the tungsten substrate.

Fig. 7: The DOS of single-crystalline Fe films on W(110) for coverages ranging from 1 ML to a 40 ML thick film (1 ML = 0.22 nm). The DOS of polycrystalline bulk - iron foil is shown for comparison.

The presented results could be obtained because the high penetration depth of the X-rays is combined with the isotopic selectivity of the nuclear resonant absorption process. While the former aspect renders the technique sensitive to the full volume of the film, the latter aspect assures that the experimental data are practically free of contributions from non-relevant elements, like for example the substrate itself or the residual gases, always present in the UHV system. The method can be extended to many other isotopes. Moreover, the experiment has shown that nuclear inelastic scattering at third-generation synchrotron radiation sources such as the ESRF is well suited to extremely small quantities of material. The lattice dynamics of epitaxial nanostructures on surfaces can be studied via this “state-of-the-art” technique, correlating structural, electronic and dynamical properties of low-dimensional systems. In particular, the use of ultrathin isotopic probe layers allows one to establish ‘phonon microscopy’ for the spatially resolved study of vibrational properties with atomic resolution.

 

 

References

[1] U. Gradmann, G. Waller, Surf. Sci. 116, 539 (1982).
[2] H.K. Mao et al., Science 292, 914 (2001).

Principal Publication and Authors

S. Stankov (a,b), R. Röhlsberger (c), T. Slezak (d,e), M. Sladecek (b), B. Sepiol (b), G. Vogl (b), A. I. Chumakov (a), R. Rüffer (a), N. Spiridis (e), J. Lazewski (f), K. Parlinski (f,g), and J. Korecki (d,e), to be submitted.
(a) ESRF
(b) Faculty of Physics, Institute of Materials Physics, University of Vienna (Austria)
(c) DESY, Hamburg (Germany)
(d) Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Krakow (Poland)
(e) Institute of Catalysis and Surface Chemistry, Polish Academy of Science, Krakow (Poland)
(f) Institute of Nuclear Physics, Polish Academy of Science, Krakow (Poland)
(g) Institute of Techniques, Pedagogical University, Krakow (Poland)