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J. Phys. Chem. C 2008, 112, 12628–12637

X-ray Spectroscopic and Magnetic Investigation of C:Ni Nanocomposite Films Grown by Ion Beam Cosputtering G. Abrasonis,*,† A. C. Scheinost,‡,§ S. Zhou,† R. Torres,# R. Gago,| I. Jime´nez,| K. Kuepper,† K. Potzger,† M. Krause,† A. Kolitsch,† W. Mo¨ller,† S. Bartkowski,⊥ M. Neumann,⊥ and R. R. Gareev# Institute of Ion Beam Physics and Materials Research, Forschungszentrum Dresden-Rossendorf, PF-510119, 01314 Dresden, Germany, Institute of Radiochemistry, Forschungszentrum Dresden-Rossendorf, 01314 Dresden, Germany, ROBL-CRG, European Synchrotron Radiation Facility (ESRF), BP 220, 38043 Grenoble, France, Instituto de Ciencia de Materiales de Madrid, Consejo Superior de InVestigaciones Cientı´ficas, E-28049, Madrid, Spain, Department of Physics, UniVersity of Osnabru¨ck, Barbarastrasse 7, D-49069 Osnabru¨ck, Germany, and Department of Physics, Faculty of the Institute of Experimental and Applied Physics, UniVersity of Regensburg, D-93040 Regensburg, Germany ReceiVed: February 7, 2008; ReVised Manuscript ReceiVed: May 7, 2008

The nearest-neighbor coordination and electronic structure in C:Ni(∼30 at.%) nanocomposite films grown by ion beam cosputtering in the temperature range of room temperatue (RT) to 500 °C are investigated by the means of extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS). The obtained results are correlated with the composite nanostructure published elsewhere and magnetic properties determined by the means of X-ray magnetic circular dichroism (XMCD) and superconducting quantum interference device (SQUID) magnetometry. A combined use of EXAFS, XANES, and XPS shows that a carbidic Ni phase exhibiting only local atomic ordering is formed at low growth temperatures (e200 °C), while ordered carbidic Ni phase forms at ∼300 °C. Further increase in growth temperature results in the formation of face-centered cubic (fcc) Ni with a high degree of crystallinity. On the other hand, Ni incorporation strongly promotes the formation of carbon structures with the prominent peak in C K-edge XANES spectra positioned at 288.5 eV in the whole growth temperature range. The magnetic measurements show no magnetic response for the films grown at RT to 200 °C, superparamagnetic behavior for the film grown at 500 °C with >90% of the Ni atoms in metallic state, and a weak magnetic response for the film grown at 300 °C, indicating the presence of Ni-rich regions within carbon containing Ni nanoparticles with ∼3% of Ni atoms in metallic state. 1. Introduction Nanocomposites composed from transition metal (TM) nanoparticles encapsulated in a carbon matrix have attracted considerable attention in the past decade because of their exceptional mechanical, tribological, magnetic, or transport properties.1–9 This combination of the properties comes partially from the corresponding properties of the nanocomposite constituents. However, the fact that those properties cannot only be predicted from the properties of their constituents alone opens a new degree of freedom: even similar material compositions can exhibit different nanocomposite architectures and thus can show completely different mechanical or magnetic behavior. The formation of such nanocomposites is based on the limited C solubility in TMs, which drives an excess of C atoms toward the grain boundaries of TMs or their carbides during the nanocomposite growth.6,10,11 The final structure, and thus the properties, are determined by the kinetic restraints of the growth * To whom correspondence should be addressed. E-mail: g.abrasonis@ fzd.de. † Institute of Ion Beam Physics and Materials Research, Forschungszentrum Dresden-Rossendorf. ‡ Institute of Radiochemistry, Forschungszentrum Dresden-Rossendorf. § European Synchrotron Radiation Facility. # University of Regensburg. | Consejo Superior de Investigaciones Cientı´ficas. ⊥ University of Osnabru ¨ ck.

conditions and C-TM interactions. Therefore, the knowledge of the atomic and electronic structure is of high importance, as it can not only shed some light on the basic thin film growth processes, but would also significantly contribute to the growthstructure-functionality relationship control in nanotechnology. Such nanocomposite structures are promising candidates for numerous applications such as low-friction, self-lubrication, wear-resistant coatings, high-density recording media, 2D network of tunnel junctions, nanocontainers for foreign materials, or spintronic devices. The C:Ni nanocomposite system is of particular interest: in contrast to the early TMs such as Ti or V, which have strong affinity to C and form stable carbides, or noble metals such as Ag or Pt, which are completely immiscible with C, Ni forms a metastable carbide, namely Ni3C.12 This phase decomposes into its components at ∼700 °C.13 However, during the sputtering growth conditions it becomes already unstable at ∼300 °C.8,11 Additionally, it forms a fine grain structure at e100 °C, while at higher temperatures it develops a columnar morphology with a nanocolumn height as high as the film thickness, and nanocolumn coarsening concomitantly with the temperature.8,11 Thus in a relatively narrow temperature range of room temperature (RT) to 500 °C, a large variety of nanostructures can be formed (spherical-columnar, carbidic-metallic). On the other hand, it is known that bulk face-centered cubice (fcc) Ni is ferromagnetic, while the decrease in size of the metallic phase

10.1021/jp8011415 CCC: $40.75  2008 American Chemical Society Published on Web 07/29/2008

C:Ni Films Grown by Ion Beam Cosputtering down to the nanometer scale as in nanocomposites results in superparamagnetic behavior.7,14 In addition, despite the fact that Ni3C is known to be non-ferromagnetic, the formation of some Ni-enriched regions within the carbidic phase causes the appearance of a certain ferromagnetic response.15 In our recent study,11 we have investigated the morphological and structural dependence on the growth temperature in C:Ni (∼30 at.%) nanocomposite films. The growth regimes and their characteristical nanostructures have been reported.11 Here we present a study of these films by the means of X-ray spectroscopies such as extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge spectroscopy (XANES), and X-ray photoelectron spectroscopy (XPS). Their combination allows the identification of the nearest-neighbor coordination, electronic, and chemical bonding structure. It will be shown that different nanostructures have their characteristic features in the nearest-neighbor coordination and electronic structures, which result in different magnetic properties. 2. Experimental Methods C:Ni (∼30 atom %) films ∼100 nm thick were grown by ion beam cosputtering at temperatures ranging from RT to 500 °C. The experimental details, composition, and structural characterization are reported elsewhere.11 Briefly, the so-grown C:Ni films consist of metal nanoparticles embedded in C medium. The growth at ∼300 °C results in the formation of Ni3C or a hexagonal Ni phase.11 Both phases cannot be discriminated by diffraction because of the similarity of their Ni sublattices and relatively large widths of the X-ray diffraction (XRD) peaks.8,11 At higher growth temperatures, the signature of Ni3C (or hexagonal Ni) decreases while Ni is predominantly in the metallic fcc phase. At e200 °C, the metal nanoparticles exhibit granular shape, while the crystalline structure cannot be determined because of either grain size effects or amorphous structure. At higher temperatures, the metal nanoparticles are columnar with the height close to that of the film thickness. The columnar growth takes place independently of the Ni phase. Furthermore, the nanoparticle diameter increases when the growth temperature increases: at RT it is in the range of 2-4 nm, at 300 °C it is in the range of 3-7 nm, while at 500 °C the diameter is in the range of 3-10 nm. On the other hand, the encapsulating carbon phase in the growth temperature range of 200-500 °C consists of standing graphitic planes, which in the vicinity of metal nanoparticles follow the curved metal nanoparticle boundaries. Below 200 °C, the carbon phase microscopically has featureless amorphous appearance. Despite this, Raman spectroscopy shows that the carbon phase contains a certain amount of 6-fold ring structures. The amount of these structures is larger in comparison to the Ni-free films grown at identical conditions, indicating some Ni effect on the surrounding carbon phase even at low growth temperatures.11 The Ni bonding state and coordination were determined at the Rossendorf Beamline at the European Synchrotron Radiation Facility (Grenoble, France) by Ni K-edge XANES and EXAFS in fluorescence detection mode and normalized by the signal recorded in the ionization chamber placed in front of the sample. During each measurement, the Ni spectrum from a Ni reference foil was recorded simultaneously for energy correction. The use of the fluorescence mode (13-element HPGe detector with digital signal treatment) allows obtaining the information from the bulk of the film. Using a method reported in the literature,16 which is implemented in the ATHENA software,17 and assuming a layer thickness of 100 nm and either Ni or Ni3C as the absorbing solid, self-absorption of the fluorescing photon by the film was found to be negligible in this thickness range.

J. Phys. Chem. C, Vol. 112, No. 33, 2008 12629 Additional information on the Ni bonding structure and the C bonding state was obtained by XANES at the OPTICS beamline (D-08-1B2) of the synchrotron facility BESSY-II in Berlin, Germany. The data were acquired in the total electron yield (TEY) mode by recording the total current drained to the ground from the sample and normalized to the signal recorded simultaneously from a gold-covered grid located upstream in the X-ray path. The information on the electronic structure using this method comes from the first ∼5-10 nm.18 The surfacesensitive information (∼1 nm) on the bonding structure was obtained by XPS. The XPS measurements were obtained using a Perkin-Elmer PHI 5600 ci Multitechnique System with monochromatized Al KR radiation (full width at half-maximum of 0.3 eV). The energy resolution of the spherical capacitor analyzer was adjusted to approximately ∆E ) 0.45 eV. The samples were measured in their as-received state and after successive steps of in situ sputter cleaning with Ar+ ions with an energy of 1.5 keV. For the highest cleaning time of 210 s used in this study, the sputtered depth is in the range of 5-10 nm. In order to gain element-selective data on possible ferro- or superparamagnetic properties of the Ni nanoparticles, X-ray magnetic circular dichroism (XMCD) spectra have been recorded at beamline 6.3.1 at the Advanced Light Source in Berkeley, CA. In order to maximize magnetization, the minimum achievable measurement temperature of 23 K and the maximum magnetic field of ( 2 kOe parallel to the incident beam direction was applied. The fixed measurement geometry yields an angle of 30° between the field and the sample surface. Additionally, the integral magnetic properties were measured with a superconducting quantum interference device (SQUID, Quantum Design MPMS XL) magnetometry. Both the temperature dependence of the magnetization at a constant field and the field dependence at a constant temperature have been investigated. The temperature-dependent magnetization measurement has been carried out in the following way: a sample was cooled in zero field from above RT to 2 K. Then a 50 Oe field was applied, and the zero-field-cooled magnetization curve (ZFC curve) was measured with increasing temperature from 2 to 300 K, after which the field-cooled magnetization curve (FC curve) was measured in the same field from 300 to 2 K with decreasing temperature. Magnetotransport experiments have been performed using the van der Pauw method. The resistance of the sample of quadratic form was determined from the voltage drop between different pairs of four contacts. The sample was isotropic. The magnetic field H was applied in the Hall geometry and varied in a wide range between -50 kOe and 50 kOe. 3. Experimental Results and Discussion 3.1. Nickel Nearest-Neighbor Coordination. Figure 1 a presents the nonweighted normalized Ni K-edge EXAFS oscillations χ as a function of wave vector k and the corresponding k3 -weighted Fourier transforms of the C:Ni (∼30 atom %) films grown at different substrate temperatures. The Ni EXAFS of the film grown at 500 °C is similar to that of an fcc Ni reference foil (not shown) and thus is attributed to metallic fcc Ni. A similar pattern is also obtained for the film grown at 400 °C, except that the oscillation amplitude decreases. When the growth temperature decreases to 300 °C and below, the oscillation patterns change significantly, indicating that the Ni short-range structure starts to deviate from that of fcc Ni. From further decrease in temperature down to 200 °C, further changes of the EXAFS oscillations occur, which then remain down to RT. These changes in the atomic environment are highlighted in

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Figure 1. EXAFS spectra treatment of C:Ni films grown between RT and 500 °C: (a) Nonweighted EXAFS oscillations χ(k). (b) Magnitude of FT(k3χ(k)). (c) Isolated in the FT(k3χ(k)) region of 0-2.8 Å, back transformed weighted EXAFS oscillations k3χ(k) (circles) and their fit (lines). (d) Magnitude and imaginary part of FT of isolated EXAFS oscillations k3χ(k) (circles) and their fit (lines). The experimental magnitude and imaginary part are represented by filled and open circles, respectively. The fit of the magnitude and imaginary part are represented by solid and dashed lines, respectively.

the χ(k) FT spectra (Figure 1b): Ni atoms in the film deposited at 500 °C exhibit a well defined peak at ∼2.1 Å (uncorrected for phase shift) due to single scattering of the photoelectron. This peak is attributed to the first coordination shell of Ni atoms. At lower temperatures an additional peak at ∼1.5 Å appears. This indicates that an additional coordination shell is present in the films deposited at temperatures of e400 °C positioned at distances significantly smaller than those of nearest Ni atoms. In the presence of a high C amount and the absence of a considerable amount of impurities,11 this shell should be attributed to C. Similarly, the presence of C in the first coordination shell has been reported for C:Fe nanocomposite films also grown by ion beam cosputtering.6 The observation of the carbon atoms in the first coordination shell can be due to several factors. When the size of metal nanoparticles decreases, the ratio of the number of surface atoms to the number of bulk atoms increases, which results in an apparent carbon coordina-

tion shell, as the nanoparticle surface atoms are in direct contact with the surrounding carbon phase. This could be observed in the case of carbon-free metal nanoparticles as reported for the deactivated Ni catalyst obtained after long exposures to methane gas.19 The intensity of the Ni-C should increase when the average size of metal nanoparticles decreases as observed for Cr-C nanocomposites grown with different Cr contents.20 The absence of such effect at least down to 100 °C for the C:Ni films of the present study while the size of the nanoparticles significantly decreases11 indicates that the size effects are not the major reason. Following this, the Ni-C shell should be attributed to the structure within the nanoparticles, i.e., to the presence of carbides, which can be expected in the conditions far away from equillibrium as in the present case.12 The XRD pattern11 of the film grown at 300 °C is compatible with a crystalline hexagonal Ni phase. The presence of a different lattice structure in comparison to the metallic fcc present in the

C:Ni Films Grown by Ion Beam Cosputtering films grown at higher temperatures is reflected in the different distribution of higher order coordination shells (see Figure 1b). This phase can be attributed to pure Ni hexagonal phase or Ni3C. In both phases, the structure of the Ni sublattice is almost identical. However, C atoms are added into one-third of the octahedral interstices in Ni3C, which only slightly changes the positions of the Ni atoms. Because of the relatively large widths of the peaks, it is difficult to determine the nature of this Ni phase on the basis of XRD alone. On the basis of the present EXAFS data, this phase should be unambiguosly attributed to the carbidic Ni phase. Further decrease in temperature does not change significantly the peak height of the first two Ni-C and Ni-Ni coordination shells, while the intensity corresponding to the higher coordination shells decreases drastically. As mentioned above, this cannot be solely attributed to the size effects, thus it should be associated with a high degree of disorder or the formation of an amoprhous structure. In order to get some deeper insight in the changes of the local atomic structure, a fitting of the experimental results is necessary. The EXAFS data fitting was performed in the following way. The |FT(k3χ(k))| were isolated in the distance R range of 0-2.8 Å corresponding to the first Ni-C and Ni-Ni shells by back Fourier transformation. The so-treated EXAFS oscillations are presented in Figure 1c, while their subsequent FT (magnitude and imaginary parts) is presented in Figure 1d. No attempt was made to fit higher coordination shells due to strong overlapping of single and multiple scattering paths, which introduce a high degree of complexity. The fitting was performed on the basis of an fcc Ni lattice for the films grown at 400-500 °C and on the basis of Ni3C for the other samples. Amplitudes and phase shifts were generated by the FEFF-8.20 code21 using the crystalline structures from the literature22,23 for Ni3C and fcc Ni, respectively. In fcc Ni, the Ni-Ni nearest-neighbor distance is 2.49 Å, and the shell consists of 12 atoms. In Ni3C, the Ni-C distance is 1.86 Å, while the Ni-Ni distance is 2.63 Å. The surrounding C shell consists of two atoms, while the surrounding Ni shell consists of 12 atoms. It should be noted that what is considered as a single shell for Ni-Ni in Ni3C actually consists of two Ni-Ni shells containing six atoms, each with distances 2.6287 Å and 2.6344 Å. However, the distance resolution of EXAFS (0.13 Å for the given k-range) cannot resolve these two shells. The amplitude reduction factor S02 was obtained by fitting the reference Ni foil and fixing the nearest neighbor number to 12. The so-obtained S02 value (0.858) was fixed and used as a constant for other fits. The isolated and back-transformed EXAFS data were fit by applying one-shell (Ni-Ni) or twoshell (Ni-C and Ni-Ni) models, allowing the coordination numbers, interatomic distances, and Debye-Waller (DW) factors σ2 to vary freely. The same energy shift E0 was used for both C and Ni atoms. The use of uncorrelated E0 was found not to considerably influence the fitting parameters. The fitting results together with the error bars obtained from the fitting are summarized in Table 1. The error bars represent the so-called asymptotic standard deviation of the nonlinear least-squared fitting algorithm and reflect the degree of correlation among the free parameters in the fitting. These values are significantly lower than the global error bars because of the accuracy degree of EXAFS, which are (25%, (0.01 Å, (25%, and (1 eV for the coordination number, neighbor distance R, DW, and E0, respectively. From the fitting results of the film grown at 500 °C it can be seen that the structure resembles closely that of fcc Ni. The slightly lower coordination number than that of the reference Ni foil is most likely related to the higher degree of disorder

J. Phys. Chem. C, Vol. 112, No. 33, 2008 12631 TABLE 1: Isolated EXAFS Spectra Fitting Parameters of C:Ni Nanocomposite Films Grown at RT to 500 °C and Ni Reference Foila temperature number/type distance R to DW factor energy shift E0 (eV) (°C) layer of neighbors neighbors (Å) (Å2) RT 100 200 300 400 500 Ni foil

1 2 1 2 1 2 1 2 1 1 1

1.6(0)/C 5.4(0)/Ni 1.0(0)/C 4.9(0)/Ni 1.1(0)/C 6.3(0)/Ni 0.9(0)/C 13.7(0)/Ni 8.0(0)/Ni 10.3(0)/Ni 12/Ni

1.89(0) 2.46(0) 1.92(0) 2.46(0) 1.88(0) 2.46(0) 1.91(0) 2.57(0) 2.47(0) 2.48(0) 2.48(0)

0.0044(0) 0.0114(0) 0.0004(1) 0.0105(1) 0.0045(0) 0.0120(0) 0.0012(0) 0.0197(0) 0.0084(0) 0.0069(0) 0.0057(0)

4.5(0) 4.5(0) 4.6(1) 4.6(1) 3.9(0) 3.9(0) 11.1(0) 11.1(0) 8.2(0) 9.5(0) 10.4(0)

a The corresponding error bars presented in parenthesis and refering to the last digit of the fitting values represent the asymptotic standard deviation of the non-linear least-squared fitting algorithm. The amplitude reduction factor S02 of 0.858 was determined by fitting the isolated EXAFS spectra of Ni foil and kept fixed for all the other fits.

present in the grown film, which manifests itself through a larger DW factor. The DW factor σ2 is the mean-square relative displacement of the absorber and backscatterer atoms. This deviation from the mean value is due to thermal vibrations and static disorder. As all the samples were measured at RT, the vibration contribution to the DW factor should be similar for all the samples. Thus, the higher value of σ2 of the C:Ni 500 °C film indicates a higher degree of static disorder. When a high degree of disorder is approached (σ2 ≈ 0.01 Å2), the integral under the coordination shell peak is not conserved anymore.24 This is related to the fact that |FT(k3χ)| is not truly the radial distribution function, even if its major peaks represent the shells of neighboring atoms.24 The origin of this is the finite range of k values (3-15 Å-1 in the present work). The region of k e 3 Å-1 of EXAFS oscillations is usually cut off as it is intermixed with the density-of-states features near the absorption edge. The region k > 15 Å-1 had to be discarded because of a low intensity-to-noise ratio. The former region is related to the slowly varying parts of the neighbor distribution function. For highly ordered materials, the distributions are sharp, thus |FT(k3χ)| is close to the real distribution function as slowly varying tails of the pair distribution function contribute only very little. However, for higher degrees of disorder the contributions from slowly varying parts to the pair distribution function increase. As these contributions are cut off from EXAFS spectra, this results in an apparent decrease in coordination number. This tendency is further observed when the growth temperature decreases to 400 °C. The contribution coming from nearest C atoms is very low, and the fit assuming only Ni shell atoms reproduces well the features of isolated χ, |FT(k3χ(k))|, and Im (FT(k3χ(k))) (see Figure 1c,d). At 300 °C, the Ni coordination number exceeds 12, which is the nearest Ni neighbor number in Ni3C. This could be due to several reasons. First, the amplitude reduction factor S02 determined from Ni foil and used for the fit might slightly depend on the Ni chemical environment. This parameter is directly correlated to the coordination number. Additionally, despite the FT isolation, the multiple scattering peaks might also contribute in this region, taking into account the broad pair distribution, which can be already seen in the FT spectrum (Figure 1b) and is represented by the DW factor of 0.0197 from the fit (see Table 1). In addition, the Ni-Ni coordination number might partially correlate with the DW factor, as fixing the DW factor values to 0.015 or 0.013 results in nearest-neighbor

12632 J. Phys. Chem. C, Vol. 112, No. 33, 2008 numbers of 8.7 and 7.0, respectively. However, the fit employing lower DW values reproduces rather poorly the k regions of k < ∼4 Å-1 and k > 9 Å-1 of the isolated experimental k3χ, as it is shown in Figure 2. The deviation is lower in the intermediate k range. It should be noted that restricting the initial k range before the Fourier transform to 3 < k < 11 Å-1 yields similar coordination numbers and DW values. Thus the obtained fitting values for this sample is not an artifact of the contributions coming from the high k region in the initial experimental spectrum which usually exhibits low signal-to-noise ratio. The large DW factor value implies a high degree of disorder, which is surprising as the contributions from higher coordination shells in the |FT(k3χ(k))| spectrum clearly point toward a certain degree of crystallinity. This is confirmed by XRD and transmission electron microscopy (TEM) observations reported elsewhere.11 On the other hand, the Ni-Ni distance R is 2.57 Å (see Table 1), which is significantly lower than that of Ni3C or hexagonal Ni (2.63 Å). This together with a high DW value might indicate that within the metal nanoparticles there are regions enriched with Ni with R closer to Ni-Ni distances in fcc nickel and the regions with distances close to Ni3C, their superposition resulting in the high DW value obtained from the fit. The further decrease in temperature results in strong decrease of the nearest Ni shell coordination number down to ∼5-6 and Ni-Ni distance down to 2.46 Å. This distance is no longer close to that of Ni3C (2.63 Å) but is close that of fcc Ni. This can be due to several reasons. One can expect that size effects start to become considerable. The Ni-Ni coordination number of a single fcc Ni cell is 5.1, which is close to the value obtained from the fit. However, the TEM results mentioned above11 clearly show that the particle sizes are on the order of several nanometers. Assuming a spherical fcc metal nanoparticle of a radius of ∼2 nm and calculating the total number of atoms in a nanoparticle assuming the method presented by Greegor and Lytle,25 it yields ∼395 atoms per nanoparticle. The ratio of first nearest neighbors in a particle in comparison to that of the bulk is >0.8,25 which should give a Ni-Ni coordination number of >9.6. Similar values can be expected for hexagonal Ni. Thus the decrease in Ni-Ni coordination number cannot be associated with the size effects. This supports the observation that there is no significant increase in Ni-C coordination number except for the sample grown at RT. Thus most probably the decrease in coordination number should be associated with an increase in disorder. The almost complete absence of the signal from higher coordination shells supports this statement and points to only locally ordered or amorphous structures of carbidic nanoparticles. Besides, for disordered systems, the contributions coming from slowly varying parts to the pair distribution function become significant. As these contributions are cut off from EXAFS spectra because of the limited k range, this results in an apparent decrease in coordination number.24 Additionally, if the pair distribution is asymmetrical with slowly varying parts contributing mostly at the distances higher than the average nearest-neighbor distance, this results not only in a decrease of the nearest coordination number but, more importantly, in a “shrinkage” of the interatomic distance, as the slowly varying parts of the asymmetric distribution are lost from the EXAFS spectra. The so-determined mean Ni-Ni distance represents the distance of the nearest approach more than the real mean Ni-Ni distance.24 At present, it is not possible to unambiguously determine whether the Ni-Ni distance of 2.46 Å obtained from the fit represents the average Ni-Ni distance. However, the significant decrease in Ni-Ni distance and coordination number when the growth temperature decreases from 300 to 200 °C

Abrasonis et al.

Figure 2. Experimental (dots) and theoretical (lines) isolated EXAFS k3χ(k) oscillations for C:Ni film grown at 300 °C. The theoretical curves correspond to the fitting of the experimental results by fixing the Debye-Waller term σ2 to 0.0013, 0.0015, and 0.00197 Å2.

while Ni-C distance remains relatively unchanged points in favor of the second hypothesis, i.e., that this “shrinkage” is due to the asymmetrical broad distribution whose slowly varying parts at higher distances are cut off as a result of the limited k range. At 300 °C, the contribution coming from nearest C atoms becomes significant. The C shell coordination number remains ∼1 down to ∼100 °C. This number is a factor of 2 lower than that in Ni3C. While the error of EXAFS in the coordination number can be as high as 25%, the observed deviation is much larger than this error. This strongly indicates that the carbidic Ni3C phase is substochiometric or that the stochiometric Ni3C nanoparticles contain Ni-enriched regions. At RT, there is a significant increase in the nearest C shell coordination number up to 1.6. This might indicate an increase of C within Ni nanoparticles or the Ni nanoparticle edge effects. TEM shows a fine grain structure with a grain size of 2-4 nm.11 There is a strong increase in the fraction of the surface atoms for the metal particle sizes 90%) in the metallic fcc phase with a high degree of crystallinity manifesting itself through the well-defined first and higher coordination shells. The film shows superparamagnetic behavior with broad particle size distribution. Despite the complete separation of C and Ni phases, isolation of Ni nanoparticles, and columnar shape, the C:Ni film still shows in-plane magnetization, which indicates that the C “tissue” phase does not magnetically isolate them. A decrease in the film growth temperature to 300 °C results in the formation of a carbidic Ni phase evidenced by the presence of carbon in the first coordination shell. The Ni-C coordination number of ∼1 is a factor of 2 below that for the stechiometric Ni3C. The carbon-containing Ni nanoparticles exhibit a certain degree of crystallinity with well-defined first and higher coordination shells but with a relatively broad Ni-Ni pair distribution function yielding a DW value of ∼0.02 Å2. The carbon presence in the first coordination shell strongly affects the electronic structure of 3d orbitals, and consequently the sample shows no dichroic signal at the Ni L X-ray absorption edge at 23 K. However, a weak isotropic magnetic response at lower temperatures shows that ∼3% of Ni is in metallic state and indicates that Ni-enriched zones with blocking temperatures below 2 K are present within the carbon-containing Ni nanoparticles. Further decrease in the growth temperature results in the formation of carbon-containing Ni nanoparticles exhibiting only local atomic ordering with the well-defined first Ni-C and Ni-Ni shells and no considerable signal from higher coordination shells. This phase does not show any magnetic response for temperatures greater than 2 K. The presence of Ni strongly enhances the intensity of the peak at 288.5 eV in C K-edge XANES spectra, which is a dominant feature in the film growth temperature range of RT to 500 °C. The origin of this peak remains to be revealed. The C electronic structure could not be unambiguously revealed because of interference of the Ni absorption in the third-order light as well as surface adsorbates, which give a considerable contribution in the XPS spectra. The metal nanoparticles might act as anchoring sites of carboxylic or carbonate groups, which might have a positive effect as their presence is expected to enhance the field emission properties,37 which may be revealed in future studies. Acknowledgment. This work has been carried out as a part of the integrated EU project “Fullerene-Based Opportunities for Robust Engineering: Making Optimised Surfaces for Tribology”

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