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theory calculations, and these show that the Pt8 cluster on alumina withdraws electrons from the surface and becomes negatively charged.

Once the nanocatalysts had been characterized, catalysis tests were carried out under atmospheric pressure, and the turnover frequencies were measured at a temperature around 500 °C. The surprising result is that the activity of the Pt8–10 catalyst is far higher than that of any other reported platinum-based catalyst for oxidative dehydrogenation of propane. Clearly, the special activity is associated with the tiny dimension of the catalyst particles. To rationalize these findings Vajda and colleagues performed density functional theory calculations on the reaction mechanism using a Pt8 cluster on an Al2O3 model. The calculations show the existence of a relatively small energy barrier for the breaking of the first C–H bond, which is

likely to be the rate-limiting step of the reaction because the rest of the pathway to propylene formation is thermodynamically downhill. Furthermore, there are no important differences between the computed reaction profiles on a gas-phase or supported Pt8 nanocatalyst, providing evidence that the support has little effect on the reaction barrier in this case. The conclusion is that the undercoordinated sites in small platinum clusters are much more active than a platinum surface for propane dehydrogenation.

The work of Vajda and collaborators represents an important example of catalytic activity of mass-selected metal clusters under realistic conditions of temperature and pressure. The fact that the clusters are both active and stable is a sizeable step towards the design of nanocatalysts of practical use. Although challenging, the development of nanometre and

subnanometre clusters stabilized on specific supports could result in new catalysts for a variety of industrially relevant processes. ❐

Gianfranco Pacchioni is in the Dipartimento di Scienza dei Materiali, Università degli Studi Milano-Bicocca, via R. Cozzi 53, 20125 Milano, Italy. e-mail: gianfranco.pacchioni@unimib.it

References1. Bell, A. T. Science 299, 1688–1691 (2003).2. Heiz, U. & Landman, U. (eds) Nanocatalysis (Springer, 2006).3. Freund, H. J. & Pacchioni, G. Chem. Soc. Rev.

37, 2224–2242 (2008).4. Vajda, S. et al. Nature Mater. 8, 213–216 (2009). 5. Socaciu, L. D. et al. J. Chem. Phys. 120, 2078–2081 (2004).6. Sanchez, A. et al. J. Phys. Chem. A 103, 9573–9578 (1999).7. Abbet, S. et al. J. Am. Chem. Soc. 122, 3453–3457 (2001).8. Benz, L. et al. J. Chem. Phys. 122, 081102 (2005).9. Fan, C. Y., Wu, T. P., Kaden, W. E. & Anderson, S. L.

Surf. Sci. 600, 461–467 (2006).10. Schaub, R. et al. Phys. Rev. Lett. 86, 3590–3593 (2001).11. Fallace, W. T. & Whetten, R. L. J. Am. Chem. Soc.

124, 7499–7505 (2002).

MultifeRRoics

A way forward along domain wallsThe discovery that domain walls in insulating thin films of the multiferroic compound BiFeO3 are electrically conducting opens the door for a number of possible applications.

Hélène Béa and Patrycja Paruch

correlated oxides are an expanding field of study, with a richness of fundamental physics

and phenomenal possibilities for multifunctional applications1. Despite their relatively simple structure, these systems present a great complexity of different properties, including superconducting as well as ferroic orders such as magnetism or ferroelectricity, sometimes within the same material2. Multiferroic materials that display ferroelectricity and magnetism are good candidates for spintronic applications relying on magnetoelectric coupling3.

Interfaces in such correlated oxides, intrinsically nanoscopic because of their small thickness, are of particular interest because specific strain or polar boundary conditions can lead to additional functionality absent from the already multifunctional parent material4–6. Domain walls in ferroic compounds are one such type of interface, separating regions with different orientations of the magnetic, ferroelectric or ferroelastic order characterizing the material7,8.

Ph

[001]

[010]

[100]

Potential

Bandgap

Pv

Figure 1 | Schematic representation of a 109° domain wall in BiFeO3. The orientation of polarization orientation across the domain wall is depicted within the material. Shown below are the horizontal (Ph) and vertical (Pv) components of the polarization projected in the (010) plane, along with the resulting potential step, and the calculated bandgap.

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Many studies of ferroic domain walls have focused on their behaviour in terms of the basic properties of the parent compound, and on their increasingly important (and often negative) effect on the performance of ever-smaller electronic devices9,10. However, identifying their additional functionalities and incorporating them in the design of applications could in fact allow domain walls to be used as active, nanoscale device components. The report by Jan Seidel and his colleagues on page 229 of this issue11 is an important advance in this direction, demonstrating electrical conductivity across two kinds of ferroelectric domain wall in the insulating room-temperature multiferroic BiFeO3.

In BiFeO3 the ferroelectric polarization is oriented along the diagonals of the monoclinically distorted tetragonal unit cell, giving rise to eight different possible polarization states and three types of domain wall separating regions with polarization orientations differing by 180°, 109° and 71° (ref. 12). Seidel et al. report that out of these three, the first two show a clear signature of electric conductance in local probe measurements. Using high-resolution transmission electron microscopy and density functional theory calculations, the authors correlate this unusual conducting behaviour in an otherwise insulating material with a local structural distortion. This distortion reduces the electronic bandgap in the region of the domain wall, but also increases the polarization component normal to the domain wall, resulting in an electrostatic potential step (Fig. 1). Either effect may explain the increase in conductivity. The first decreases the band edge relative to the local probe tip used in the conductivity measurements, whereas the second leads to an accumulation of free charge carriers at the domain wall in order to provide screening for the polar discontinuity caused by the increase in polarization.

These observations open intriguing possibilities for potential device applications. The authors mention that their theoretical analysis suggests very high sensitivity of the conductive response to unit cell size, which can easily be changed by applying strain to a thin film. Further detailed investigations looking at the conductivity of domain walls in BiFeO3 films on different substrates, with different lattice mismatch and hence strain, as well as in relaxed versus strained samples would therefore be of interest.

A possible application could be as a local strain sensor incorporated on devices such as an atomic force microscopy (AFM) probe, as shown in Fig. 2a. As the electrical

resistance measurement is sensitive to the strain state, the bending of the cantilever to which the probe tip is attached may be quantified. Such device geometry could potentially avoid the precision alignment and heating that accompanies the use of a standard AFM probe in laser beam deflection mode.

An important prerequisite for using domain walls for such applications is the control of their nucleation and position. In their prototype device, Seidel et al. use an AFM tip to write domain walls in the ferroelectric film. In future devices, such writing could be done using a multi-tip probe on an array of device elements. However, a faster approach, requiring no mechanical motion, would be to use previously defined domain wall patterns, moved uniformly, as in a racetrack memory13, so that the desired domain wall pattern defining a fixed conductive state lies between the measurement electrodes (Fig. 2b). Although ferroelectric wall motion could be induced by the application of an electric field, care has to be taken as such fields could lead to a change of domain size, and thus non-uniform motion of the domain wall patterns, and their possible erasure.

Furthermore, as BiFeO3 is a multiferroic material, in which ferroelectric and antiferromagnetic domains are strongly magnetoelectrically coupled, one could envisage a device geometry based on a BiFeO3 layer adjacent to a ferromagnetic film (Fig. 2c). Spin-polarized currents in the ferromagnetic layer could then be used to drive its magnetic domain walls by means of the spin transfer torque effect13,14. The exchange coupling between the ferromagnetic layer and BiFeO3 means that

the domain walls of BiFeO3 would follow this movement, making it possible to create a multilevel resistance-state device that is written by an electrical current.

Obviously, such functional application of these conductive ferroelectric domain walls requires that practical obstacles be overcome, for example the motion of domain walls in BiFeO3 through an underlying ferromagnet. Indeed, only the reverse effect — that is, the manipulation of a ferromagnet domain configuration by an electric field applied on BiFeO3 — has so far been demonstrated15. Nonetheless, Seidel and colleagues have taken an important step, identifying and understanding a new property of ferroic domain walls that may eventually lead to their incorporation into ever-smaller nanoscale technological devices. ❐

Hélène Béa and Patrycja Paruch are in the Département de la Matière Condensée, Université de Genève, 1211 Geneva 4, Switzerland. e-mail: Helene.Bea@unige.ch, Patrycja.Paruch@unige.ch

References1. Dagotto, E. Science 318, 1076–1077 (2007).2. Bibes, M. & Barthelemy, A. IEEE Trans. Electron. Devices

54, 1003–1023 (2007).3. Spaldin, N. A. & Fiebig, M. Science 309, 391–392 (2005).4. Ohtomo, A. & Hwang, H. Y. Nature 427, 423–426 (2004).5. Reyren, N. et al. Science 317, 1196–1199 (2007).6. Bousquet, E. et al. Nature 452, 732–737 (2008).7. Zubko, P. et al. Phys. Rev. Lett. 99, 167601 (2007).8. Přívratská, J. & Janovec, V. Ferroelectrics 222, 23–32 (1999).9. Paruch, P., Tybell, T., Giamarchi, T. & Triscone, J.-M.

J. Appl. Phys. 100, 051608 (2006).10. Scott, J. F. J. Phys.: Condens. Matter 18, R361–R386 (2006).11. Seidel, J. et al. Nature Mater. 8, 229–234 (2009).12. Lebeugle, D. et al. Phys. Rev. B 76, 024116 (2007).13. Parkin, S. S. P. et al. Science 320, 190–194 (2008).14. Ralph, D. C. & Stiles, M. D. J. Magn. Magn. Mater.

320, 1190–1216 (2008).15. Zhao, T. et al. Nature Mater. 5, 823–829 (2006).

a b c

Current

Magnet

BiFeO3

DW

BiFeO3

R

Figure 2 | Potential applications for conductive domain walls (DW). a, Variations in the conductivity of domain walls (purple) as a function of strain could be envisaged for sensing applications (R is resistance). b, Alternatively, devices could be created in which the conductive domain walls are uniformly moved along the film (shown in top view), thus changing the conducting state between two electrodes (yellow). These could be useful as memory devices or as switches. c, One possibility for controlling the domain wall motion would be to use an underlying ferromagnet in which domain walls are moved by a spin-polarized current.

© 2009 Macmillan Publishers Limited. All rights reserved.

In the print version of this News & Views article the reference list should have included the following:

3. Spaldin, N. A. & Fiebig, M. Science 309, 391–392 (2005).

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A way forward along domain wallsHélène Béa and Patrycja Paruch

Nature Materials 8, 168–169 (2009); published online: 20 February 2009; corrected after print: 20 February 2009.

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