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J. Phys. Chem. C 2010, 114, 13168–13175
Alloying Effect in CoPt Nanoparticles Probed by X-ray Photoemission Spectroscopy: Validity of the Bulk Phase Diagram Juliette Tuaillon-Combes,* Estela Bernstein, Olivier Boisron, and Patrice Melinon Laboratoire de Physique la Matie`re Condense´e et Nanostructures UMR 5586, UniVersite´ de Lyon, UniVersite´ Lyon 1 et CNRS, Baˆtiment Le´on Brillouin, 6 rue ampe`re, Domaine de la Doua, F 69622 Villeurbanne, France ReceiVed: March 11, 2010; ReVised Manuscript ReceiVed: June 29, 2010
CoPt organized cluster assemblies are developed to produce ultra-high-density magnetic media that need the smallest sizes for recording particles. At the nanometer scale (3-4 nm in diameter in our case), surface atoms represent around 50% of the total number of atoms and the segregation effect could dominate the alloying effect. Our purpose is to describe the competition between segregation and alloying effects in asdeposited or embedded bimetallic CoPt clusters preformed in the gas phase. The segregation is studied by X-ray photoemission spectroscopy. First of all, it was found that satellite peaks at high energy disappear when particles are fully embedded in a matrix. Moreover, CoPt clusters exhibit a partial core-shell structure when the clusters are not chemically bonded to the matrix. Conversely, an alloying effect is reported when CoPt particles are embedded in a silicon matrix. These effects are analyzed on the basis of both the phase diagrams and the enthalpy formation in the bulk phase. The bulk phase diagram remains valid for the nanometric scale, taking into account a scaling factor. Furthermore, the asymmetry of the photoemission peaks is carefully analyzed in terms of intrinsic and extrinsic energy losses. Introduction 1
Metallic and bimetallic clusters (A and B elements) in interaction with a substrate or an embedding matrix play an increasingly important role that is directly related to the large number of potential applications in various fields, such as catalysis and 2-4 organized arrays of clusters5-8 and spintronic,9,10 nanophotonic,11,12 and magnetic properties.13-15 The interest is due to the flexibility in the set of parameters, including the size, the nature of the two elements A and B, and the stoichiometry. The inner structure of the bimetallic structure depends on several factors that are not completely understood. In the bulk phase, the phase diagram shows four cases: a mixture of two immiscible phases, a single phase in which atoms of both sorts occupy positions on the same lattice (i.e., a solid solution), a phase with a structure different from that of either one of the constituents (i.e., an intermetallic compound), and an intimate mixture of two phases (i.e., a two-phase alloy). In the size range, around 2 or 3 nm, the surface-to-volume ratio and then the radius of the clusters are essential of the present investigations. It is, therefore, relevant to question the validity of the phase diagram at the nanoscale. Alloying effects and/or segregation effects in mixed clusters are at the origin of modifications of the electronic structures, which can be investigated using X-ray photoemission spectroscopy (XPS).16 Information is given by two distinctive photoemission peak characteristics: the core level energy and the asymmetric tail toward high binding energy (BE). The chemical bond is characterized by the core level position, whereas the high binding energy asymmetry comes from an energy loss by the emitted photoelectron. We can distinguish a core level shift without an asymmetric tail due to a new chemical bond, an asymmetric tail toward high binding energy without a core level shift due to a cluster environment evolution, and a mixture of * To whom correspondence should
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these two behaviors. The position and the shape of the photoemission peak are a subtle balance between the initial state due to a chemical bond and the final state (relaxation). In the atomic gas phase, the situation is considerably simplified because molecules do not have a channel for extra-atomic relaxation. In supported or embedded clusters, extra-atomic relaxation might occur due to the environment, such as the substrate, the matrix, or the contamination. The asymmetry of the photoemission peak toward high binding energy is due to complex many-body mechanisms17-21 but is necessarily due to a loss of the kinetic energy of the emitted photoelectron. Two origins are to be considered, the intrinsic and extrinsic energy losses.22-24 The intrinsic part (IEL) is related to the evolution of the electronic structure of the emitting atom itself. In metallic materials, this effect comes from the more or less efficient screening of the created core hole by the free valence electrons,18,25 which are very mobile and which can easily flow to the site of the core hole. Another cause for the asymmetry is the variation of the density of states at the Fermi level, which influences this screening.26 Another possible cause is relaxation through the rearrangement of the orbital to an intermediate state when the excited state after the photoelectron ejection can return to the ground state (this includes shake-up and shake-off processes). The second part of the energy loss is the extrinsic part (EEL). It is related to the electronic structure of the environment of this emitting atom. This effect corresponds to the energy losses by the photoelectron during the transport from the emitting atom to the surface of the sample.27 In smaller clusters, this effect is essential as the interface between clusters and matrix is larger. Other effects may also be considered: a very low chemical shift or low spin-orbit splitting, less than 1 eV, can also induce a broadening on the photoemission peak.28 Also, a broadening is often reported in the various contributions from surface and volume atoms in an ultrathin layer or successive multilayers,29 but this
10.1021/jp1022086 2010 American Chemical Society Published on Web 07/19/2010
Alloying Effect in CoPt Nanoparticles Probed by XPS
Figure 1. Synopsis of the experimental setup. XPS spectra are stored in situ to avoid any contamination. Additional devices (gas analyzer, Auger, and time-of-flight mass spectrometers) are not shown for clarity.
specific surface core level shift, less than 1 eV, is lower than the broadening observed in our case. In this study, we report the interaction and the alloying effect in CoPt clusters with a matrix and/or a substrate through the analysis of the core level shift and of the asymmetric tail. The validity of the bulk phase diagram at the nanoscale will be discussed in light of our results. For this purpose, isolated clusters with different compositions were prepared in a gas phase and subsequently deposited on a surface and either left as such as-deposited samples or further embedded in several matrices. The cluster-substrate set was selected as a function of the possibility for alloying regarding the bulk phase diagram. Our results clearly indicate that the validity of the bulk phase diagram only holds for clusters that are fully embedded into a matrix rather than barely deposited on a substrate. Experimental Setup Cluster assembled films are prepared by deposition of lowenergy neutral clusters preformed in the gas phase (LECBD technique).30,31 A cluster generator based on a combined Nd:YAG laser vaporization-rare gas (He) condensation source is used to produce supersonic jets of clusters with sizes ranging from a few tens to a few thousands of atoms (diameter ) 2 to a few nm). A synopsis is displayed in Figure 1. The target is formed by Co, Pt, or the mixing of Co and Pt atoms in the suited stoichiometry. The very high cooling rate (1010 K/s) characteristic of this type of cluster source is at the origin of the formation of original nanoscale systems in nonequilibrium conditions. Because clusters are prepared in a nonsteady state, the sticking coefficient for all of the Co-Co, Pt-Pt, and Pt-Co pairwise collisions is 1: bimetallic clusters grow and keep the initial atomic stoichiometry of the target. The atomic composition of the deposited clusters is checked by the Rutherford backscattering technique. The measured mean composition in the collection of clusters is CoxPt1-x, with x ) 0.5 ( 0.05.32 All cluster depositions are performed in ultrahigh vacuum, (8 × 10-7 Pa). Except for the buffer helium gas, the main contaminant detected by a gas quadrupole analyzer is carbon monoxide. Fortunately, this contaminant does not significantly affect the core level shifts. For transmission electron microscopy (TEM) observations, the deposition is done with a low coverage rate (
