Science at the Surface Diffraction Beamline / ID03

Here some examples of scientific studies performed at the ID03 beamline:

Roughness as a motor for reaction oscillations

Spotlight 2010

Oscillatory chemical reactions are often called chemical ‘clocks’ due to their periodic nature. A well-known system that shows periodic oscillation of reaction rate is the catalytic oxidation of carbon monoxide over platinum and palladium surfaces. A team of researchers from Leiden University and the ESRF have pinpointed a new mechanism that makes this chemical clock ‘tick’.

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A palladium catalyst sample being heated in the UHV chamber of beamline ID03 (image courtesy R. van Rijn).

A catalyst is a substance that speeds up a chemical reaction without being consumed by the reaction. A well-know example is the catalyst in the exhaust of a car, which facilitates the oxidation of poisonous CO gas with oxygen to produce the less-harmful carbon dioxide. The rate of conversion of CO to CO2 can spontaneously oscillate in time. A complex interplay of the reactants, CO and O2, modifies the surface structure of the catalyst and thereby its catalytic activity. This causes the rate to oscillate for reactions at low pressures in ultrahigh vacuum systems, as has been beautifully demonstrated by Nobel laureate G. Ertl and co-workers [1]. However, at atmospheric pressures the structure of a catalyst surface is much harder to study because most analytical tools require low-pressure vacuum conditions.

The Leiden University and ESRF team has developed a novel setup at beamline ID03 that allows them to study the surface structure of the single crystal Pd catalyst inside a flow reactor during the catalytic reaction by means of surface X-ray diffraction (SXRD) at atmospheric pressure [2]. With these SXRD experiments they show that during the rate oscillations the Pd catalyst spontaneously switches in time between a metallic surface, with a low CO2 production rate, and an oxidised Pd surface, which exhibits a much higher rate, as shown in Figure 1. These observations are in full agreement with earlier studies of the catalytic activity of ultrathin surface oxides [3,4].

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Figure 1. Top panel: Inverse full width at half maximum (FWHM) of the diffracted intensity at a surface sensitive (anti-bragg) position as a function of time. The inverse FWHM is proportional to the smoothness of the truncation of the Pd(100) crystal. Bottom panel: partial pressure of CO and CO2 in the reactor. The partial O2 pressure was 500 mbar. The sample temperature was kept constant at 447 K. The colours indicate whether the SXRD intensities identify the Pd(100) surface as a metallic structure (light blue) or as being covered by a thin oxide film (light red).

Further SXRD experiments show that a smooth metal surface oxidises easier, i.e. at the less-oxidising conditions of a higher CO partial pressure, than a rough metal surface. The authors suggest that this is due to the favourable adsorption of CO at the steps of a rough metallic Pd surface, which thermodynamically stabilises the metal surface. Only when the level of roughness is sufficiently low does the surface becomes oxidised.

The newly discovered mechanism responsible for the oscillatory behaviour, depicted schematically in Figure 2, was found by studying the evolution of the roughness of the metallic and oxidised surface during the oscillations. In the oxide phase of an oscillation cycle, the thin oxide layer becomes gradually roughened by reaction of CO with the oxygen atoms from the oxide layer. This is shown by the broadening of the diffraction peaks. At a certain level of roughness the oxide layer becomes thermodynamically unstable and disappears, leaving a less reactive metallic surface. This metallic surface becomes smooth again by metal atom diffusion, as shown by the narrowing of the diffraction peak, see Figure 1. The smoothening continues until the roughness is low enough to make the oxide surface thermodynamically stable again, at which point the surface immediately forms a highly reactive oxide film. This completes the cycle: the process starts all over again from the beginning. Based on these observations the authors conclude that the evolution of the roughness is responsible for the periodic switching between the metallic and oxidised surface and is at the heart of the mechanism that makes this chemical clock tick.

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Figure 2. Metal-oxide stability diagram. Each cycle takes the surface through stages (1) smooth oxide, (2) rough oxide, (3) rough metal, and (4) smooth metal, after which the next cycle starts again at (1). The phase boundary is determined by the roughness and the CO partial pressure.

 

References
[1] R. Imbihl, G. Ertl, Chem. Rev. 95, 697-733 (1995).
[2] R. van Rijn, M.D. Ackermann, O. Balmes, T. Dufrane, A. Geluk, H. Gonzalez, H. Isern, E. de Kuyper, L. Petit, V.A. Sole, D. Wermeille, R. Felici, and J.W.M. Frenken, Rev. Sci. Instrum. 81, 014101 (2010).
[3] H. Over, Y.D. Kim, A.P. Seitsonen, S. Wendt, E. Lundgren, M. Schmid, P. Varga, A. Morgante, G. Ertl, Science 287, 1474 (2000).
[4] M.D. Ackermann, T.M. Pedersen, B.L.M. Hendriksen, O. Robach, S.C. Bobaru, I. Popa, C. Quiros, H. Kim, B. Hammer, S. Ferrer, and J.W.M. Frenken, Phys. Rev. Lett. 95, 255505 (2005).

 

Principal publication and authors

B.L.M. Hendriksen (a), M.D. Ackermann (a,b), R. van Rijn (a,b), D. Stoltz (a), I. Popa (b), O. Balmes (b), A. Resta (b), D. Wermeille (b), R. Felici (b), S. Ferrer (b,c), and J.W. M. Frenken (a), A new role for steps in catalysis and reaction oscillations, Nature Chemistry, DOI: 10.1038/NCHEM.728.

(a) Kamerlingh Onnes Laboratory, Leiden University (The Netherlands)
(b) ESRF
(c)  Present address: CELLS - ALBA, Universita Autónoma de Barcelona (Spain)

 

 

Structural and electronic reconstruction at the interface between LaAlO3 and SrTiO3 band insulators revealed by X-rays

Spotlight 2013

The interface between two of the most popular band insulating oxides, LaAlO3 and SrTiO3, is conducting under certain conditions due to the formation of a high mobility 2D-electron gas, a discovery made in 2004 by Ohtomo and Hwang [1]. Here, by using a combination of advanced X-ray synchrotron-based spectroscopic and structural measurements, we show that this phenomenon is linked to a structural and electronic reconstruction of the interface which precede the appearance of the 2D-electron gas. The results challenge some of the most accredited theoretical models describing this system.

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When two-band insulating oxides form a 2D-metal at their interface: X-rays reveal the nature of the electronic and structural reconstruction at the LaAlO3/SrTiO3 interface.

Nowadays, the latest advances in the atomic control of epitaxial heterostructures means that interfaces with perfection down to the atomic scale can be obtained even in complex oxides [1]. As a consequence of these technological achievements, new functionalities, not present in any of the constituent elements in isolation, were discovered in heterostructures made of transition metal oxides (TMO). The most spectacular example is the formation of a 2D-electron gas in LaAlO3/SrTiO3 (LAO/STO) bilayers, composed by thin epitaxial LaAlO3 (001) films deposited on a SrTiO3 single crystal [2-4]. In view of their large band gaps, the realisation of a conducting system at the LaAlO3/SrTiO3 interface was unexpected. The phenomenon is attributed to the transfer of electrons from the polar LaAlO3 (001) surface to the SrTiO3 conduction band, and simultaneous change of the valence of titanium ions, a mechanism which is common to many other transition metal oxide heterostructures [5]. This charge transfer can avoid a 'polarisation catastrophe' as the thickness of the polar LaAlO3 (001) film exceeds a threshold of four unit cells. If this mechanism is proven, it could lead to a general method for creating devices where functionalities could be added at will by interface-engineering.

The 'polarisation catastrophe' picture predicts simultaneous electronic and structural reconstruction of the interface SrTiO3 layers as a consequence of the formation of a mobile 2D-electron gas. With the aim to definitively verify these theoretical predictions, we have performed a series of high resolution synchrotron radiation based X-ray diffraction and X-ray spectroscopy experiments on LAO/STO bilayers deposited by pulsed laser deposition at the University of Geneva (group of Prof. J.-M. Triscone) and at the University of Augsburg (group of Prof. J. Mannhart). X-ray absorption spectroscopy and related X-ray linear dichroism measurements were performed at ID08, the ESRF’s soft X-ray beamline for polarisation-dependent studies. Grazing incidence X-ray diffraction (GXID) experiments were conducted at ID03, the surface diffraction beamline. In Figure 1 we present X-ray linear dichroism (XLD) measurements at the L2,3 edge of titanium ions on insulating (LAO thickness lower than 4 unit cells) and conducting (t >= 4 unit cells) LAO/STO bilayers. The XLD data, acquired in the total electron yield mode, provide information about the splitting of the 3d levels at the interface, and in particular it evidences an inversion of the hierarchy of in-plane and out-of plane orbitals in the case of LAO/STO bilayers compared to an insulating SrTiO3 surface, which is a signature of an electronic (orbital) reconstruction taking place within the SrTiO3 layers close to the interface. As we can see from Figure 1, both insulating and conducting LAO/STO show this electronic reconstruction, demonstrating that it precedes the formation of the 2D-electron gas.

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Figure 1. X-ray linear dichroism (XLD) spectra around the titanium L2,3 absorption edge of SrTiO3 (green line) and LaAlO3/SrTiO3 bilayers characterised by a LaAlO3 thickness below (red lines) and above (blue lines) the critical thickness of four unit cells. Data from two sample sets are shown. Black lines are calculations which reproduce the data on STO (bottom) and LAO/STO (top) using multiplet atomic model calculations with point charge crystal field. On the right a schematic of the orbital splitting needed to reproduce the data is depicted, showing the inversion of hierarchy between in-plane and out-of-plane t2g orbitals at the LaAlO3/SrTiO3 interface.

In Figure 2, on the other hand, we show the outcome of the structural refinement obtained from the fit of the crystal truncation rods (CTR) measured by GIXD. The main result concerns the evolution of the rumpling of the AO (A=La, Sr) and BO2 (B=Al, Ti) planes as function of the nominal LAO thickness. This rumpling can be interpreted as a polar response of the STO layers to an internal electric field. The data show that large rumpling of the planes are observed below the critical thickness in both LAO and STO layers, while above the structural distortions are reduced. While this behaviour is qualitatively in line with the predictions of the 'polar catastrophe' picture for the LaAlO3 layers, the SrTiO3 planes are distorted before the formation of a mobile 2D-electron gas and not after, at variance with the models [6]. Overall, the results indicate that both electronic and structural reconstructions, taken as signatures of the validity of the "polar catastrophe" scenario, occur below the nominal critical thickness of 4 unit cells, i.e. before the appearance of a conducting state. In particular, we find that the main structural and electronic changes in STO take place already when one complete LaAlO3 layer is deposited. The results show that the orbital reconstruction, and the appearance of simultaneous polar distortions in STO, are due to a combination of interface symmetry breaking and of the transfer of localised electrons to interface states before the realisation of a mobile 2D-electron gas.

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Figure 2. Rumpling of the AO (A=La,Sr) and BO2 (B=Al, Ti) planes of LAO/STO bilayers with different LAO thicknesses: bare insulating STO surface (green open squares), 2-uc (red triangles), 4-uc (blue open circles) and 6-uc (black open squares) set-B LAO/STO samples. On the right LAO/STO structures of 2-uc and 4-uc samples (set-B) are shown as a result of the structural refinement. Blue and green spheres are La and Al ions, while red and orange spheres are Ti and Sr ions. Oxygen ions are the small cyan spheres.

To conclude, it appears that the interface polar discontinuity is only one of the ingredients ruling the physics of the LAO/STO system. Furthermore, it is necessary to consider the surface and interface properties of the constituent materials in the heterostructures, and in this specific case, the interesting properties of STO surfaces.

 

Principal publication and authors
M. Salluzzo (a), S. Gariglio (b), X. Torrelles (c), Z. Ristic (a,d), R. Di Capua (a), J. Drnec (f), M. Moretti Sala (f), G. Ghiringhelli (e), R. Felici (f), N.B. Brookes (f), Advanced Materials (2013).
(a) CNR-SPIN and Department of Physics, Complesso Monte Sant'angelo, Napoli (Italy)
(b) Département de Physique de la Matière Condensée, University of Geneva (Switzerland)
(c) Institut de Ciencia de Materials de Barcelona (CSIC) (Spain)
(d) Laboratory of Atomic Physics, Institute of Nuclear Sciences VinĨa, University of Belgrade (Serbia)
(e) CNR-SPIN and Department of Physics Politecnico di Milano (Italy)
(f) ESRF

 

References
[1] A. Ohtomo, D.A. Muller, J.L. Grazul, & H.Y. Hwang, Nature 419, 378 (2002).
[2] A. Ohtomo & H.Y. Hwang, Nature 427, 423–426 (2004).
[3] N. Reyren S. Thiel, A.D. Caviglia, L. Fitting Kourkoutis, G. Hammerl, C. Richter, C.W. Schneider, T. Kopp, A.-S. Rüetschi, D. Jaccard, M. Gabay, D.A. Muller, J.-M. Triscone, J. Mannhart, Science 317, 1196 (2007).
[4] S. Thiel, G. Hammerl, A. Schmehl, C.W. Schneider & J. Mannhart, Science 313, 1942 (2006).
[5] S. Okamoto and A.J. Millis, Nature 428, 630 (2004).
[6] S.A. Pauli, S.J. Leake, B. Delley, M. Björck, C.W. Schneider, C.M. Schlepütz, D. Martoccia, S. Paetel, J. Mannhart, P.R. Willmott, Phys. Rev. Lett. 106, 036101 (2011).

 

 

 

Beauty of science: isolated core-shell silver/gold nanowire

Spotlight 2013

The image shows the coherent X-ray diffraction pattern of an isolated, single-crystalline core-shell Ag/Au nanowire measured in Bragg condition at the ID03 beamline.

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Coherent X-ray diffraction pattern of an isolated, single-crystalline core-shell Ag/Au nanowire (Credit: M.-I. Richard, ID01/ESRF, Aix-Marseille Université, IM2NP-CNRS).

Characterisation of nano-objects is a technological challenge, in particular for atomic level details of their interfacial structures, spatial elemental distribution and strain gradients. Recent studies within a collaboration between IM2NP-CNRS, MPI Stuttgart, CEA-Grenoble and ID03/ESRF have demonstrated that anomalous coherent X-ray diffraction imaging is a promising and attractive method to map the shape, concentration and deformation fields simultaneously inside coherent and coherently diffracting nanostructures. The technique even permits in situ experiments owing to its non-destructive nature. During an in situ annealing experiment core-shell morphology was preserved in Ag/Au nanowire at temperatures that are reported to lead to significant intermixing by volume diffusion in bulk material. Under these conditions, the rate of intermixing in the nanowire was lower than expected for bulk diffusion.

 

S. Haag et al., Phys. Rev. B 87, 035408 (2013); S. Haag et al., NanoLetters accepted (2013).

 

 

BaTiO3 (001) (2x1): Relation between structure and magnetism

ESRF Highlights 2013

BaTiO3 (BTO) is an archetype ferroelectric whose physical properties have been well known for decades and have been thoroughly documented in solid state physics textbooks. Bulk BTO is a perovskite type ferroelectric (transition temperature TC = 408 K) insulator (bulk band gap = 3.2 eV). It is used in technological applications as a piezoceramic capacitor and in nonlinear optics. In contrast, nothing is known about the structure and physical properties of the surface of a BTO crystal, which has become a focus for research owing to possible applications in nanoscale oxide spintronic devices [1]. Although the (001) surfaces of STO and BTO are known to reconstruct to from a (2x1) and a (2x2) superstructure [2], a structure model exits only for STO [3]. We have carried out a combined surface X-ray diffraction (SXRD) and theoretical study of the atomic structure and the corresponding physical properties for BTO.

The SXRD experiments were carried out at beamline ID03 using a bulk crystal, which after mild Ar+ sputtering and annealing up to about 1000°C exhibits a (2x1) reconstruction with no traces of a (2x2) reconstruction.

The structure model is shown in Figure 110 in perspective side view. The BTO crystal is terminated by two stoichiometric TiO2 layers, similar to the STO(001) surface [3], but significant differences exist with respect to the position of the top layer structure. Here, the most important characteristic is that one titanium atom (5) resides in a fivefold coordination to oxygen atoms (7, 7’, 8, 8’ and 4) in the centre of a pyramid. In this way, the titanium atom (5) shifts inward thereby binding to oxygen atom (4) and to the second layer oxygen atoms at a distance of 2.30 ± 0.15 Å.

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Fig. 110: Model of the BaTiO3(001)-(2x1) structure in perspective side view. Two TiO2 layers (atoms #1-8) are located above the bulk-like BaO layer (primed labels correspond to symmetrically equivalent atoms).

Based on this structure model the electronic and magnetic properties of the (2x1) BTO surface were calculated within the density functional theory (DFT) in the local density approximation using a Korringa-Kohn-Rostoker Green-function method, which is specially designed for semi-infinite layered systems. According to the DFT calculations, the BTO(001)-(2x1) surface is metallic and magnetic. Figure 111 shows the density of states (DOS) for spin up (↑) in red which is different from the spin down DOS (↓) shown in blue.

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Fig. 111: Spin resolved DOS of BTO(001)-(2x1). The contributions of the different atoms are indicated. Dark colour (red, blue) corresponds to Ti (5). The light (red) and blue profiles correspond to the total DOS.

The DOS is large at the Fermi level (EF), this is related to the (↑) contribution of titanium atom (5) and oxygen atom (3). The filling of the titanium 3d states is mainly a consequence of the charge transfer from oxygen to titanium. Moreover, due to the low coordination and reduced symmetry, the different DOS contributions are narrow and involve partially unsaturated 2p states in the case the oxygen atoms (3) and (4). In turn this leads to high local magnetic moments up to 1.3 μB and -2.0 μB for the titanium atom (5) and oxygen atom (4), respectively.

In summary, our X-ray diffraction analysis of the BTO(001)-(2x1) reconstruction has identified an atomic arrangement that had not been considered before for (001) oriented perovskite surfaces. The most remarkable unit is a titanium atom in the centre of a tetragonal pyramid. This unique motif causes symmetry breaking, localisation of the electronic states, and charge transfer to the central titanium atom from surrounding oxygen atoms. This leads to metallisation and magnetisation which is now identified as an intrinsic property of the surface. We infer that this metallisation might also contribute to the stabilisation of the reconstruction related to the depolarisation of the surface.

 

Principal publication and authors

H.L. Meyerheim (a), A. Ernst (a), K. Mohseni (a), I.V. Maznichenko (b), S. Ostanin (a), F. Klimenta (a), N. Jedrecy (c), W. Feng (a), I. Mertig (a,b), R. Felici (d) and J. Kirschner (a,b), Phys. Rev. Lett. 108, 215502 (2012).

(a) Max-Planck-Institut f. Mikrostrukturphysik, Halle (Germany)

(b) Institut für Physik, Martin-Luther-Universität Halle-Wittenberg, Halle (Germany)

(c) Institut des NanoSciences de Paris, UPMC-Sorbonne Univ. CNRS-UMR7588, Paris (France)

(d) ESRF

 

References

[1] E.Y. Tsymbal and H. Kohlstedt, Science 313, 181 (2006).

[2] R. Courths, Phys. Stat. Solidi B 100, 135 (1980).

[3] R. Herger, P.R. Willmott, O. Bunk, C.M. Schlepütz, B.D. Patterson and B. Delley, Phys. Rev. Lett. 98, 076102 (2007).