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In this work nanocomposite thin films of amorphous hydrogenated carbon (a-C:H) doped with noble transition metals of 1B group (gold, silver, and copper) are studied. The composite materials are obtained by combined magnetron sputtering (MS) of a metal target by argon, and plasma-assisted chemical vapor deposition (PACVD) of methane under vacuum conditions. Particular attention is devoted to the low metal-content a-C:H samples, in which metallic inclusions have a form of isolated nanoclusters. Our aim was to reveal surface cluster arrangement, i.e. to figure out whether topmost metallic nanoclusters are covered with a layer of a-C:H or are bald on the surface and hence exposed to the surrounding environment. We deposited our samples onto substrates kept on the ground potential and on �150 V dc bias voltage. The differences encountered in the surface structure and nanocluster arrangement between samples, which differed in the deposition process in this parameter only, provided an answer to the question of surface clusters coverage. The experimental techniques used to reach this goal comprised in vacuo and ex vacuo photoelectron spectroscopy (PES), direct imaging by atomic force microscopy (AFM) and scanning electron microscopy (SEM), and grazing incidence smallangle x-ray scattering (GISAXS). The majority of work is made in several photoemission experiments, using both x-ray and UV-excited photoelectron spectroscopy (XPS and UPS, respectively). In the course of the work, series of (grounded only) a-C:H/Au, a-C:H/Ag, and a-C:H/Cu samples, with metallic content varying from zero to 100 at.%, have been deposited and studied in vacuo by XPS and UPS (Section 3.1). The XPS results of these series show that, with decreasing metal content below the percolation threshold (at about 40-50 at.% of metal concentration), a shift in binding energy (BE) of metal core levels towards higher BEs is observed. With non-carbidic metals like the ones we used in our work, these shifts can be related to their isolated cluster structure in the host matrix. Decrease of the total metal content in the sample is followed by the decrease in the cluster size, which is reflected in increased binding energy of electrons escaping from them (Paragraph 2.2.2 and Section 3.1). At the same time, carbon C 1s core level is shifted in the opposite direction, towards lower binding energies, and this shift rises as metal content increases in the sample. Negative shift in C 1s binding energy reveals increased relative content of sp2-coordinated carbon in the a-C:H matrix, which, we believe, should be attributed to the compressive stress that metallic inclusions introduce in the host a-C:H, being higher with higher metal content in the sample. Another possible reason is catalytic reduction of hydrogen in the a-C:H matrix with increasing metallic content. UPS of the same series (Fig. 3.2) showed increased relative contribution of metal valence band features with increased metal content. The Fermi edge evolution from monocrystalline metal reference samples to low metal-content a-C:H showed both decrease in the density of states near the Fermi level and its shift to the higher BEs, related to the cluster structure of metallic inclusions and to the same direction-shifts in metal core levels. Our further attention was focused on low metal-content a-C:H that is, typically below 10 at.%, and to the differences between grounded and biased samples. The reprint of the communication on these effects encountered in a-C:H/Au and published in the Applied Physics Letters (Vol. 80, 2002, p.2863) is given in Section 3.2. Direct imaging techniques (Sections 3.2 and 3.3) reveal that even small amount of metal included in the a-C:H significantly changes the surface morphology and increases roughness. For all three nanocomposite materials, biased samples show similar surface morphology, characterized by relatively flat basis and isolated bump structures, of about 30 nm in diameter and up to 15 nm in height. These structures are attributed to the altered morphology of a-C:H component of a nanocomposite, with metallic clusters concentrated on them. Grounded samples characterize low roughness in a-C:H/Au and increased in a-C:H/Ag and a-C:H/Cu. The increase of roughness in the latter two materials is explained by enhanced surface diffusion of metal atoms and clusters coalescence into bigger islands. GISAXS patterns (Section 3.4) showed isotropic cluster size and intercluster distance distribution in grounded a-C:H/Au and a-C:H/Cu samples. With grounded a- C:H/Ag sample, no spatial correlation could be revealed, probably again due to the surface agglomeration of Ag clusters. These analysis of GISAXS patterns showed that biased samples contain bigger clusters than grounded ones, and slightly flattened in the grow direction. The differences between grounded and biased samples were first detected by in vacuo XPS of a-C:H/Au in systematically lower, in average about 50%, Au content in the biased than in the grounded case. This decrease is followed by higher positive shift of the Au 4f7/2 core level binding energy in the biased case (Section 3.2 and Paragraph 3.5.1). The similar situation was encountered in a-C:H/Ag samples, while a-C:H/Cu showed higher Cu content in the biased than in the grounded case and no difference in Cu 2p3/2 shifts from the reference BE. The decrease of the total metal content in biased a-C:H/Au and a-C:H/Ag samples most probably results from increased distance among surface clusters and their concentration on isolated bump structures. The reason for the increased BE shift of Au 4f7/2 and Ag 3d5/2 core levels in the biased samples should be searched in cluster baldness at the sample surface. The violation of both above conclusions in a-C:H/Cu, i.e. higher Cu content in the biased sample and equal shift of the Cu 2p3/2 core level in grounded and biased sample is probably due to high relative increase of copper cluster size upon biasing, which compensates the effect of their baldness at the surface. The topmost clusters of grounded nanocomposite samples are, therefore, most probably covered with a layer of a-C:H. The thickness of this layer must be below escape depth of metaloriginated photoelectrons, i.e. less than about 2 nm. The negative shift of the C 1s core level in the biased samples is induced by increased sp2/sp3 coordinated carbon ratio due to the sample bombardment by Ar+ ions during the deposition process. To our belief, the same effect is responsible for surface metallic clusters baldness in the biased samples. These conclusions are supported by several following PES experiments, other than XPS of as-deposited samples. First of them was in vacuo UPS of as-deposited samples. In the grounded samples, the He I spectra mostly reproduce the characteristic shape of a-C:H valence band, and only higher sensitivity He II UPS reveal the presence of metallic inclusions. Upon biasing, even when total measured metal content was lower than in the grounded case (a-C:H/Au and a-C:H/Ag), all spectra clearly showed increased metallic features, evidencing on higher metal exposure at the surface. XPS at off-normal take-off angle of escaping electrons also confirmed our conclusions on the surface clusters coverage. Increasing the tilting angle of a sample, measured intensity ratio of a metal core level to appropriate C 1s showed in most grounded samples monotonous decrease, and regular and steady increase in the biased ones. These results support our conclusion on the surface clusters coverage in grounded samples and their baldness in the biased ones. The higher metalcontent grounded samples of a-C:H/Ag, however, did not show the expected decrease in the intensity ratio, and that was the first indication that the effect of coverage may be a particularity of small clusters only, i.e. low-metal content grounded samples. This suspicious is confirmed in the next test experiment that we have undertaken, by subsequent in situ low-energy Ar+ ion etching and PES analysis of a sample. The same metal to carbon core level intensity ratio curves were measured against the sputtering time. In the grounded samples, at the beginning of the sputtering, an increase in the intensity ratio is observed, related to the thinning of the top a-C:H layer. In most cases, after some time of sputtering, the maximum is reached related to the total removal of the cover layer, and from that point onwards, Ar+ ion etching erodes the metallic clusters as well. In biased samples, a monotonous decrease of intensity ratio curves was observed throughout the experiment and is clearly related to the bald surface clusters that are sputtered together with the a-C:H matrix. The grounded samples intensity ratio curves showed one more important regularity: in higher metal-content samples the maximum is reached after shorter time, i.e. these samples need less time to be fully uncovered. As a special case, a- C:H/Ag 32.3 at.% did not show any increase in the intensity ratio curve, but monotonous decrease throughout the measurement. That encouraged the conclusion that the coverage of the topmost metallic clusters of grounded samples with a-C:H is an effect that is characteristics of small clusters in the host matrix, i.e. low metal-content samples. With higher metal contents, there is no observable difference between grounded and biased samples regarding surface clusters coverage. Apart from the core level intensity ratio curves, the evolution of our samples with in situ Ar+ ion etching is described in XPS and UPS spectra recorded at each point of the sputtering time scale. Metal core levels in these figures remained either unchanged or are slightly shifted towards higher binding energies. In the grounded samples, this is related to the thinning of the cover layer, and in the biased ones to the decrease of the cluster size by Ar+ ion sputtering. Carbon C 1s core level in all samples shows shift with sputtering time towards lower binding energies, which is related to the further sp2-coordinated carbon favoring by the in situ Ar+ ion bombardment. The UPS spectra evolution generally follows the trend described by core level intensity ratio curves. That is, in grounded samples, the metal features in valence band spectra rise to the point of total removal of the cover layer, and decrease further to the end of the sputtering experiment. Biased samples, on the other hand, show continuous decrease of the metal features. In both grounded and biased valence band spectra, the development of the carbon π-states is observed throughout the experiment, evidencing on the increase of sp2-coordinated carbon content with sputtering time. Pointed out several times, the Ag surface clusters coalescence is confirmed in experiment in which we compared XPS and UPS spectra of as-deposited samples, after 20 hours residence in the ultra-high vacuum (UHV) conditions and after additional 20 hours in the air. Generally, all as-deposited spectra and after 20 hours in the UHV were almost identical. After exposure to the air, in all samples carbon C 1s core level is shifted towards lower binding energies. The most interesting differences after residence in the air show metal core levels. The Au 4f7/2 core level remained practically identical to the one measured in the UHV, revealing that air conditions do not affect Au clusters, and that their size and arrangement remain fully determined by the deposition process. That is not the case, however, with Ag clusters. The Ag 3d5/2 core levels of both grounded and biased a-C:H/Ag samples shift towards lower binding energies. From the differences in UHV- and air-residence binding energy positions of the Ag 3d5/2, it is estimated that the increase factor of cluster volume in grounded samples is about 170, and the one of biased samples clusters � about 12. In these rough figures one may find the cause of the specific behavior of the a-C:H/Ag sample that we encountered in several occasions and assigned to the Ag surface clusters coalescence: roughness revealed by AFM, lack of correlation in the GISAXS patterns, pronounced Ag 4d features even in low metal content valence band spectra, and negative shift of the Ag 3d5/2 in the Ar+ ion in situ in-depth profiling. The last of our nanocomposites subjected to UHV- and air-dwell comparison was a- C:H/Cu. Copper, however, oxidizes in the air, but nevertheless it provided in this experiment one of most elegant evidences on the surface clusters coverage. The deconvolution procedure applied to copper- and CuO-originated Cu 2p3/2 revealed that relative content of oxidized copper is higher on the surface of biased sample (with bald surface clusters), than on the grounded one (where surface clusters are covered by a-C:H). The last in the series of experiments aimed to check our conclusions on the surface clusters coverage was based on the prospective sulfur binding to noble metal atoms. Our samples, together with appropriate monocrystalline reference samples, were covered with a layer of liquid thiophene (C4H4S) and, after evaporation, subjected again to the XPS analysis. The total amount of adsorbed sulfur was generally low, about 5 at.% or less. That results in noisy XPS spectra of the S 2p core levels region, in spite of increased measurement statistics. The S 2p spectra adsorbed on the reference samples were fitted with three S 2p1/2 � S 2p3/2 doublets assigned to S bonds to a noble metal, S in C4H4S, and to S�O bonds (Figs. 3.32-3.34). Intercomparison of our nanocomposite samples with reference ones showed that sulfur adsorbed on surfaces originates predominantly from C4H4S itself. However, in biased cases a higher relative contribution of the shoulder related to sulfur bonds to a noble metal is observed in spectra, evidencing on higher metal exposure at the biased samples surfaces. The exception of a-C:H/Cu is due to the oxidation of copper. In conclusion, in several different PES experiments, by direct imaging of samples, and using GISAXS technique, we have revealed that MS/PACVD-obtained low noble metal-content amorphous hydrogenated carbon nanocomposites are characterized with topmost metallic clusters covered with a tiny layer of a-C:H when deposited on a grounded substrate, and bald surface clusters when substrate is biased with �150 V dc. Beside this main result, we encountered few other effects, like e.g. increased sp2/sp3 coordinated carbon ratio in the a-C:H matrix in the biased samples and surface clusters coalescence in a-C:H/Ag (and to some extent in a-C:H/Cu) nanocomposites. By changing one parameter only � the substrate bias voltage in deposition of our grounded and biased �counterparts�, we have shown that surface clusters coverage effect has an origin in the plasma deposition process itself. We believe that one should look for its cause in the plasma afterglow, the state established in the ionized gas immediately after switching off the plasma power supply. From the applicative point of view, we have described, in principle, the mechanism that may be employed to tailor the coverage of topmost metallic clusters embedded in the a-C:H matrix. Metal inclusions in the a-C:H showed to improve the wear resistance of the coatings, so one can also envisage the applications when the coverage of surface metal clusters with a-C:H would be useful. In tribology, these would be cases when incorporated metal reduces the lubricating properties, i.e. increases the friction coefficient. In biocompatible materials the same would be necessary when incorporated metals are toxic, like e.g. silver or copper. Vice versa, one may also envisage applications when topmost cluster baldness would be desirable, like e.g. with low-friction MoS2 and WS2 inclusions in a-C:H for tribological purposes. In addition, surface clusters exposure to the surrounding environment probably influences the optical and aging properties of solar selective coatings based on metal- or metal carbide-containing amorphous hydrogenated carbon nanocomposites. |
