The fabrication of ordered metal and semiconductor nanoparticles on solid surfaces with a uniform and controllable size and shape and with a high spatial density is an important challenge as it may find applications in nanoelectronics, ultra-high density recording materials and nanocatalysis. It has been predicted that strain patterned substrates induced by a buried dislocation network (DN) can serve as templates for growing uniform and regularly-spaced nanostructures. We present a new method based on a (001) metal surfaces nanostructured by a misfit dislocation network buried a few nanometres below the surface. We show that the trapping energy of adatoms is large enough to allow an ordering of nanostructures at room temperature. The cobalt/silver interface is chosen because it is a test bed for magnetic nanoparticles, as Co exhibits a three-dimensional (3D) growth on Ag(001) and because it does not alloy at room temperature. To modulate the surface strain field, a Ag film was grown on a MgO(001) substrate. Due to the cube on cube epitaxial relationship and the 3% lattice mismatch between Ag and MgO(001), strain relaxation occurs via a square misfit dislocation network with a period of D ~ 10 nm [1].

The experiments were carried out on the BM32 beamline, using a newly-developed setup allowing GISAXS, GIXD and X-ray Reflectivity (XR) measurements on the same sample, in situ [1], in Ultra High Vacuum (UHV), at different growth stages (here of Ag and Co). A 5 nm-thick 2D Ag(001) epitaxial film on MgO(001) was obtained by growing a much thicker film, annealing it, and then ion bombarding it, with in situ X-rays while monitoring by GISAXS, Ag(110) anti-Bragg GIXD, and XR measurements.

A detailed (nano-)crystallographic study of the strain patterned substrate was first performed by GISAXS. Figures 95a and b display two GISAXS images measured on the Ag/MgO(001) film with the incident X-ray beam respectively parallel to the <110> and <100> MgO(001) crystalline axes. Sharp scattering rods in the Q// direction reveal a periodic nanopattern of four-fold symmetry, which were shown to be due to the buried DN. As expected [1], the in plane rod positions correspond to dislocation lines oriented along the <110> substrate directions, with a periodicity D = 10.95 nm.

Fig. 95: (a) Experimental GISAXS pattern with the incident beam along the MgO[110] direction. (b) Same as (a), but with the incident beam along [100]. (c) Scheme of the scattering geometry of GISAXS (d) Cuts along Q of the dislocation network scattering rods extracted from GISAXS patterns (a) () and (b) () (multiplied by 10 for clarity) and best fits.

GISAXS measurements were then performed during the growth of Co on this nanostructured template for different substrate temperatures and Co growth rate. Co was finally deposited at room temperature and at a very low rate (4x10–3 nm/min), respectively to decrease the thermal energy of the adatoms with respect to the DN nucleation trapping potential, and to increase the diffusion length of Co atoms and thus their probability to find a nucleation site. From the very beginning of the growth (0.04 nm), the subtracted GISAXS images display intensity oscillations along the DN scattering rods with a damped sinusoidal shape (Figure 96a). The oscillation amplitude increases with deposition time, reaches a maximum for an equivalent Co deposited thickness of 0.19 nm, and then decreases (Figure 96b). The period, equals to 5 nm, is a signature of the height difference between the Co clusters and the interfacial DN. Most importantly, these oscillations reveal the organised growth of Co clusters, since an interference effect can only occur if the phase shift between the waves scattered by the Co clusters and those scattered by the DN is well defined, i.e. if the Co clusters are well localised with respect to the dislocations positions.

Fig. 96: (a) Experimental interference pattern with the incident beam along the [110] direction, for a 0.14 nm-thick Co deposition. The intensity is represented on a linear scale. Oscillations along the rods are clearly visible. (b) Intensity of the interference term versus Q , for different deposition times (symbols) with best fits. (c) Schematic representation of the Co clusters position with respect to the dislocation intersection lines.

A quantitative analysis unambiguously showed that the Co dots are located above the dislocation crossing lines, and have a height of 2 atomic layers.

To conclude, we have shown that the periodic surface strain field induced by a misfit dislocation network buried as far as 5 nm below an Ag(001) surface allows control of the growth of Co clusters at room temperature, leading to self-organised growth. We believe that this method could be used for many different systems, metal thin films being favoured with respect to semiconductor ones because of the dislocation mobility necessary to reach the equilibrium state.



[1] G. Renaud et al., Science 300, 1416 (2003).
[2] G. Renaud, P. Guénard and A. Barbier, Phys. Rev. B 58, 7310 (1998).

Principal Publication and Authors

F. Leroy (a,c), G. Renaud (a), A. Letoublon (a), R. Lazzari (b), C. Mottet (c), J. Goniakowski (a), Physical Review Letters 95, 185501 (2005).
(a) CEA-Grenoble, DRFMC (France)
(b) Institut des Nanosciences de Paris (France)
(c) CRMCN-CNRS, Marseille (France)