J. Phys. Chem. C 2008, 112, 17161–17170
17161
Soft X-ray Absorption and Emission Spectroscopic Investigation of Carbon and Carbon:Transition Metal Composite Films G. Abrasonis,* M. Berndt, M. Krause, K. Kuepper, F. Munnik, A. Kolitsch, and W. Mo¨ller Institute of Ion Beam Physics and Materials Research, Forschungszentrum Dresden-Rossendorf, PF-510119, 01314 Dresden, Germany ReceiVed: June 13, 2008; ReVised Manuscript ReceiVed: September 5, 2008
Carbon and C:V, C:Co, C:Cu nanocomposite films grown by ion beam cosputtering in the temperature range from room temperature (RT) to 500 °C are investigated. Soft X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) have been used to determine electronic structure of the occupied and unoccupied electronic states of the coexisting carbon and transition metal (TM) constituents. The results from the spectroscopy are supplemented by the film composition data and TM inclusion phase structural information obtained by elastic recoil detection analysis and X-ray diffraction, respectively. The TM(2p) XAS shows that V (Cu) is in carbidic (metallic) state over the whole temperature range, while Co shows a transition from a carbidic toward a metallic state when the growth temperature increases from RT to 500 °C. The C(1s) XAS demonstrates that the increase in the growth temperature favors the formation of graphite-like structures in carbon films. On the other hand, the TM metal incorporation strongly promotes the sp3 admixture in the surrounding carbon phase which manifests itself through a significant increase in the intensity of a feature in the C(1s) XAS spectra positioned at ∼291 eV resulting from 1s f σ* transitions. In addition, the codeposition of TM atoms with carbon enhances the formation of carbon structures with the prominent peak between π* and σ* regions in the C(1s) XAS spectra positioned at ∼288.5 eV. The effect is independent of the TM tendency to form carbides or TM state (carbidic metallic) while its magnitude increases concomitantly with the TM content and decreases when the crystallinity degree of the inclusion phase increases. The results are discussed on the basis of the nanoparticle imposed curvature on the surrounding carbon network and interactions at the atomic level at the C-TM interfaces. 1. Introduction Nanocomposites are heterogeneous materials wherein the lateral extension of at least one component is lower than 100 nm.1 They are advanced functional materials whose structural, electronic, and mechanical properties cannot be predicted from the properties of their constituents alone. This is partially due to the presence of interfaces, which occupy a considerable nanocomposite volume fraction. Despite the critical dependence of nanocomposite properties on the nature of the interfaces between constituent phases, there is a lack of understanding on the structure and formation mechanisms of various interface structures, which very often are metastable and stabilized by the pseudomorphic forces due to their epitaxial relationships.2 The nanocurvature of such interfaces renders the microscopic tools less suitable for their analysis. The use of multilayered structures in order to facilitate the determination of the interface structures by the microscopic means2 may not be always justified as in the case of the matrixes consisting of layered materials (C, CNx, BN,...) the presence of curvature may introduce additional structural features.3-5 This strongly motivates the search for the alternatives. A spectroscopic approach is a suitable option provided that the contributions coming from the interface species are considerable to be detected. Among the large variety of spectroscopic techniques, X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) are relevant techniques whose combined use provides information on the conduction and valence bands. The inherent elemental sensitivity allows * To whom correspondence should be addressed. E-mail: g.abrasonis@ fzd.de.
determining the electronic structure of the different constituents of a material. The latter feature is very relevant to study nanocomposite materials as one can assess structural features of coexisting phases and track their changes as a function of synthesis or modification conditions. Among the nanocomposites, carbon:transition metal (TM) nanocomposite films exhibit a unique combination of properties which make them promising candidates for high density magnetic media, spintronic devices, low-friction solid lubricants, or 2D networks of tunnel junctions.6-14 In order to understand the properties of these materials, a detailed analysis of their electronic structure is of major importance. The assessment of the electronic structure requires the use of appropriate spectroscopic tools such as soft X-ray absorption and emission spectroscopy. XAS of early transition metal L2,3 edges is due to 2p f 3d electronic transitions and consists of the so-called white lines above the absorption background whose position, shape, and intensity are extremely sensitive to the changes in local symmetry owing to the localized nature of 3d electronic orbitals.15,16 On the other hand, XAS of the C K edge is due to 1s f 2p electronic transitions and usually consists of two photon energy regions associated with 1s f π* (∼284-288 eV) and 1s f σ* (>289 eV) transitions.17 For graphite as a prototype material, these transition edges are accompanied by π* and σ* exciton peaks positioned at 285.4 and 291.7 eV, respectively.18-21 In addition, a peak between the π* and σ* resonances is often observed for different carbon modifications.19,22-27 The origin of this peak is still disputable: while Fischer et al. attributed it to the free interlayer states in the conduction band,19 some of
10.1021/jp805209r CCC: $40.75 2008 American Chemical Society Published on Web 10/15/2008
17162 J. Phys. Chem. C, Vol. 112, No. 44, 2008 the more recent investigators have attributed it to the C-O bonds.23,24,28 However, this is still controversial.25,26,29,30 In the presence of TMs, additional states may occur in the same range due to C 2p and TM 3d-4sp orbital overlap,15,16 respectively. Besides, theoretical calculations predict additional changes in the electronic structure of carbon brought in a direct contact with TM surfaces.31,32 XES spectra arise from X-ray emission as a result of transitions from the valence band to the core hole produced by X-ray absorption. For carbon, they are consequently due to 2p f 1s transitions and consist of two regions resulting from transitions from the states of π and σ symmetries.29,33 XAS measurements of different carbon modifications are often performed in a total electron yield (TEY) mode whose signal probes the first 5-10 nm of the material34,35 and is therefore strongly affected by the presence of surface contaminations. To avoid this limitation, in this study, we report our findings on the electronic structure of C:TM nanocomposite films of a thickness of ∼100 nm measured by XAS in surface sensitive TEY and bulk sensitive (∼100 nm) total fluorescence yield (TFY)34,36 modes. Vanadium, cobalt, and copper have been chosen for introduction into the carbon medium to test the influence of the affinity to carbon. Vanadium forms very stable carbides, and Cu is practically immiscible with carbon, while Co exhibits moderate but nevertheless considerable affinity to C. This affinity results in the formation of metastable Co carbides37 and is one of the reasons for its well-known catalytic activity in carbon nanotube (CNT) synthesis. XES results obtained from selected samples are also presented to complement the XAS measurements findings. The results are corroborated with the findings of the film composition and phase structure. 2. Experimental Details C:TM nanocomposite films were grown by ion beam cosputtering on SiO2 (∼500 nm)/Si substrates. The setup used for the film deposition was similar to that in our previous work.38 Briefly, a 3 cm Kaufman ion source generates Ar+ ion beam of an ion energy of 1 keV and an ion current of 40 mA which is directed toward a 6 in. (or 4 in.) pyrolytic graphite target partially covered with a V, Co, or Cu strip. The Co strip widths of 2.8 and 5.5 mm result in a metal content of ∼15 and ∼30 at % in the films, respectively. For other TMs, a strip width of 7.3 and 3.3 mm results in a TM content of ∼25 at % for V and Cu, respectively. The substrates were located on a heated substrate holder facing the graphite-TM target at a distance of ∼14 cm. The substrate temperatures were room temperature (RT), 300, and 500 °C. Before each deposition, the target was presputtered for 15 min, then a shutter placed in front of the substrate was removed without interruption of the sputtering process, and the depositions were performed for 60 min resulting in the film thickness of ∼100 nm.38 The VC reference was grown at 500 °C employing a V strip of the thickness of 14.3 mm. For the sake of comparison, some films, hereafter termed carbon reference films, were grown under identical conditions but with no metal strip on the graphite target. As a supplementary reference, a tetrahedral amorphous carbon (ta-C) film was grown on Si substrate employing pulsed filtered cathodic vacuum arc (for the details, see ref 39). The film areal density and composition were obtained by elastic recoil detection analysis (ERDA). The measurements were performed with 35 MeV Cl7 + ions impinging at an angle of 15° relative to the film surface. The backscattered ions and the recoils were detected with a Bragg ionization chamber placed at a scattering angle of 30°. Additionally, a standard Si-detector
Abrasonis et al. was located at a scattering angle of 38° for hydrogen detection. In this case, an aluminum foil was employed in front of the detector to stop heavier recoils and backscattered Cl7+ ions. The phase structure of TMs was derived by XRD by employing a D-5000 (Bruker-AXS) diffractometer with Cu KR radiation (8048 eV) used in grazing incidence mode with the incident angle of 1°. The electronic structure of the above films was ex-situ investigated by XAS and XES using synchrotron radiation from an undulator, at the beamline 8.0.1, Advanced Light Source, Berkeley, CA, in the X-ray fluorescence end station.40 It should be noted that in order to maintain the proper data collection and to avoid the interference of the carbon contamination on the signal comming from the gold mesh which monitors the incident photon flux, the regular maintenance procedure employing gold evaporation on the gold mesh has been applied. 3. Results and Discussion 3.1. Film Depth Profiles and Composition. Typical ERDA depth profile examples are presented in Figure 1 for the carbon reference, C:Co (∼15 at %), C:Co (∼30 at %), C:Cu (∼25 at %), and C:V (∼25 at %) films grown at 300 °C. It should be noted that natural ERDA depth units are 1015 atoms · cm- 2. Two regions can be identified consisting of the C or C:TM film followed by the SiO2 substrate. The apparent smeared elemental distribution near the surface and the nonsharp interface between the films and SiO2 substrates is due to the finite ERDA depth resolution. Besides, the ERDA depth resolution decreases when the probing depth increases which results in an apparently broader interface layer in comparison to the surface. It can be seen that both major film constituentsscarbon and TMssare homogeneously distributed over the film thickness. The amount of impurities such as oxygen or hydrogen is low in comparison to carbon or TMs. Their origin should be related to the adsorption of residual gas species during the film growth. For oxygen, a low intensity peak can be indentified near the surface for the metal containing films, which most probably is related to the oxidation of metal species after exposure of the film surfaces to the air. Some Ar impurities have been detected for the carbon reference films. Besides, it can be seen that at the interface TM and C atomic ratios decrease rapidly to zero at the same depth which indicates no preferential TM or C inward diffusion into the substrate. This is observed for all the TM containing films. The total film areal density t or the total amount of atoms deposited per squared centimeter was calculated by integrating the atomic ratios for every film constituent over the film depth and then summing all the integrals of each constituent. The atomic ratio of a particular constituent of the thin film was obtained by dividing the depth integral of the constituent over t. The results on so-determined film areal densities t, atomic ratios of principal constituents, and impurities are summarized in Table 1. Besides, the ratio of partial TM film areal density tTM over the sum of partial film areal densities of carbon tC and TM tTM are presented. This value directly reflects the C and TM ratio in the films. By calculating the statistical average and deviation, it can be deduced from the data presented in Table 1 that the oxygen and hydrogen contents are 1.6 ( 1.3 and 3.6 ( 1.6 at %, respectively. The ratio is close to that of water, whose presence in the residual gas would naturally explain the largest fraction of these impurities. This conclusion is further supported by the fact that the maximum hydrogen and oxygen content are found in the films deposited at RT. 3.2. X-ray Diffraction. Figure 2 shows the XRD patterns of the VC reference film and C:TM nanocomposite films grown
XAS and XES of C and C:TM Composite Films
Figure 1. ERDA depth profiles of the carbon reference (a), C:Co (∼15 at %) (b), C:Co (∼30 at %) (c), C:Cu (∼25 at %) (d), C:V (∼25 at %), and (e) films grown at 300 °C.
at RT and 500 °C, respectively. The VC reference film, as expected, shows the XRD pattern typical for the rock salt structure with the peak positions corresponding to those of the VC from the database International Center for Diffraction Data (card 73-0476). The peaks are sharp indicating a relatively high degree of crystallinity. It should be noted that preliminary Raman spectroscopy measurements do not show any features characteristic for amorphous or ordered carbons coexisting with the metal carbide suggesting that the observed VC phase present in this film is indeed a single phase.41 This is in contrast to the rest of the TM containing films where Raman spectroscopy shows the presence of a separate carbon phase indicating about the nanocomposite structure.41 The C:V (∼25 at %) nanocomposites show broad peaks at the positions of those of VC reference film and independently of the growth temperature. This is a strong indication about the low degree of crystallinity in the nanocomposite C:V films films grown in the temperature range RT-500 °C. This also demonstrates that the excess of carbon hinders the VC grain growth during the codeposition process.
J. Phys. Chem. C, Vol. 112, No. 44, 2008 17163 On the other hand, the C:Cu (∼25 at %) films show the features characteristic for the fcc copper in the whole temperature range. This indicates that even at lowest growth temperature used in our study copper is immiscible with carbon and forms metallis fcc grains. This is consistent with the observations in the literature where a complete demixing has been observed at similar Cu contents and growth temperatures.42 In contrast to C:V nanocomposites, the increase in temperature results in a drastic narrowing of the peaks which should be attributed to the grain coarsening at higher deposition temperatures. This indirectly indicates that mobility of copper is relatively high and that high amount of the coexisting carbon phase does not prevent the formation of the metallic copper grains with a high degree of crystallinity. C:Co (∼30 at %) nanocomposite film grown at RT shows very broad peaks which are compatible with the amorphous structure as reported in the literature for C:Co or C:Ni nanocomposite films.38,43-47 As discussed in the literature,43 there is a lack of driving forces for the cystallization in C:Co alloys grown at low temperature in contrast to the C:TM systems with a strong tendency to form carbides as for the C:V nanocomposites. Besides, it has been shown that in the C:Ni films grown at similar TM contents and similar temperatures Ni is in carbidic phase which becomes unstable once the growth temperature reaches 400 °C.47 As Co shows a similar tendency to form carbides as Ni,37,43 similar effect can be expeted for the C:Co samples grown at lower temperatures. The cobalt carbide becomes unstable in the temperature range of 300-450 °C,43 thus at higher temperatures the formation of metallic cobalt embedded in the carbon matrix can be expected. This can be seen in Figure 2 where the C:Co (∼30 at %) grown at 500 °C shows the signatures compatible with hcp cobalt. Due to the large widths of the peaks it is difficult to identify whether a coexisting fcc phase is present or not in the sample. As the hcp f fcc phase transformation occurs at ∼450 °C, one can expect that the cobalt phase in the nanocomposite consists of the mixture of both hcp and fcc phases. One should note that the width of the peaks for the film grown at 500 °C is intermediate between those observed for C:V and C:Cu nanocomposites grown at the same temperature suggesting an intermediate degree of crystallinity. In summary, the C:V films show a presence of vanadium carbide phase in the whole temperature range adressed in this study, while crystalline metallic copper tends to form in C:Cu nanocomposite films. C:Co films show the presence of an amorphous structure at low temperatures which transforms into a metallic phase at 500 °C. At the highest growth temperature, the degree of crystallinity decreases in the sequence Cu > Co > V. 3.3. Transition Metal L Edge XAS. The TM XAS L edge spectra are presented in Figure 3. The Co white-lines N1 and N2 measured in more surface sensitive TEY mode exhibit a shoulder at lower photon energy side which is absent in TFY mode. This should be attributed to the oxidation of the cobalt in the near surface region. Besides, this shoulder is significantly weaker for C:Co (∼15 at %) films. On the other hand, for both Co contents the features N1 and N2 tend to narrow with increasing growth temperature and approach the shape characteristic for metallic Co as in ref 48. This tendency is displayed in Figure 3b where the XAS TFY Co L edge spectra are compared separately for C:Co (∼15 at %) and C:Co (∼30 at %) films. The white-lines of metallic Co represent the density of unoccupied states. The low temperature phase might be attributed to cobalt carbides which form in C-Co system at
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TABLE 1: Film Areal Densities t, Compositions, and tTM/(tTM + tC) Ratios of the Carbon Reference, C:Co (∼15 at %), C:Co (∼30 at %), C:Cu (∼25 at %), C:V (∼25 at %) Films Grown at RT, 300, and 500 °C Film type
film areal density t 1018 cm-2
C
O
C RT C 300 °C C 500 °C C:Co (∼15 at %) RT C:Co (∼15 at %) 300 °C C:Co (∼15 at %) 500 °C C:Co (∼30 at %) RT C:Co (∼30 at %) 300 °C C:Co (∼30 at %) 500 °C C:Cu (∼25 at %) RT C:Cu (∼25 at %) 300 °C C:Cu (∼25 at %) 500 °C C:V (∼25 at %) RT C:V (∼25 at %) 300 °C C:V (∼25 at %) 500 °C
0.91 0.92 0.89 1.14 1.05 1.07 1.22 1.04 1.09 0.97 1.03 1.07 0.96 1.06 1.11
93.0 95.8 95.4 73.9 77.6 79.3 58.7 63.2 63.8 70.3 69.3 65.0 64.6 69.2 70.2
1.4 0.0 0.0 3.6 1.3 1.1 1.5 1.3 1.0 1.5 0.0 2.8 4.9 1.5 1.9
atomic ratio (at %) H N 4.6 3.9 4.6 5.7 3.3 2.3 6.0 1.7 0.8 4.4 3.4 2.9 6.2 2.6 1.9
0.7 0.0 0.0 0.5 1.6 0.0 3.6 0.6 0.3 1.0 0.4 0.3 1.1 0.5 1.0
Ar
TM
tTM/(tTM + tC)
0.3 0.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
16.3 16.2 17.3 30.1 33.3 34.2 22.8 26.9 29.0 23.2 26.2 25.0
0.18 0.17 0.18 0.34 0.35 0.35 0.25 0.28 0.31 0.26 0.27 0.26
the conditions far away from equillibrium.37 The presence of Co carbides in sputtered C-Co films at low substrate temperatures have been reported in the literature.43,45,49 These carbides are metastable and transform into metallic Co upon heating at 380-420 °C.43,45,49 The presence of carbon within the Co phase may significantly modify the white-line shape and position due to crystal field splitting multiplet or charge transfer effects15,16 as reported for the Fe catalyst after CNT growth.22 Following this, the presence of carbidic species in the C:Co nanocomposites at low growth temperatures is the most probable explanation. The exact nature of these states is out of the scope of the present study. On the other hand, the present results clearly identify a transition from carbidic to metallic Co when the growth temperature increases from RT to 500 °C. Cu(2p) XAS spectra show strong features, namely P1 and P5 (almost absent in TFY mode), which are related to the
Figure 2. XRD patterns of the VC reference film and C:V (∼25 at %), C:Co (∼30 at %), and C:Cu (∼25 at %) films grown at RT and 500 °C (lines). The symbols represent the positions of the XRD peaks of the VC, fcc Cu, and hcp and fcc Co taken from the database International Center for Diffraction Data (cards 73-0476, 04-0836, 040836, 05-0727, and 15-0806 for VC, Cu, hexagonal Co, and fcc Co, respectively).
Figure 3. Co(2p) (a and b), Cu(2p) (c), and V(2p) (d) XAS spectra measured in TEY and TFY modes of the C:Co (∼15 at %), C:Co (∼30 at %), C:Cu (∼25 at %), VC reference, and C:V (∼25 at %) films, respectively. The TEY spectra are normalized to unity. In panels a, c, and d, the TFY spectra for the comparison purposes are normalized in order to make similar the absorption background between L3 and L2 edges for each sample of the spectra measured in TEY and TFY modes. The TEY and TFY spectra of the same sample are shifted vertically by the same amount. In panel b, the TFY spectra of the C:Co (∼15 at %) and C:Co (∼30 at %) films grown at 500 °C are normalized to unity, while the normalization of the XAS TFY spectra of the rest of the C:Co films is performed in order to make similar the absorption background between L3 and L2 edges of the given film with that of the C:Co film grown at 500 °C with the same Co content. The spectra for a given Co concentration are shifted vertically by the same amount.
XAS and XES of C and C:TM Composite Films surface oxidation, the characteristic P2-P4 triplet for metallic fcc Cu50 following the LIII absrotpion edge and the peak P6 corresponding to the LII absorption edge. It should be noted that the P4 feature is not seen in the film deposited at RT. It has been reported in the literature that decrease in the copper cluster size (
