Surface characterization of amorphous hydrogenated carbon thin films containing nanoclusters of noble metals

摘要

In this work nanocomposite thin films of amorphous hydrogenated carbonud(a-C:H) doped with noble transition metals of 1B group (gold, silver, and copper) areudstudied. The composite materials are obtained by combined magnetron sputteringud(MS) of a metal target by argon, and plasma-assisted chemical vapor depositionud(PACVD) of methane under vacuum conditions. Particular attention is devoted to theudlow metal-content a-C:H samples, in which metallic inclusions have a form of isolatedudnanoclusters. Our aim was to reveal surface cluster arrangement, i.e. to figure outudwhether topmost metallic nanoclusters are covered with a layer of a-C:H or are baldudon the surface and hence exposed to the surrounding environment. We depositedudour samples onto substrates kept on the ground potential and on �150 V dc biasudvoltage. The differences encountered in the surface structure and nanocluster arrangementudbetween samples, which differed in the deposition process in this parameterudonly, provided an answer to the question of surface clusters coverage.udThe experimental techniques used to reach this goal comprised in vacuo andudex vacuo photoelectron spectroscopy (PES), direct imaging by atomic force microscopyud(AFM) and scanning electron microscopy (SEM), and grazing incidence smallangleudx-ray scattering (GISAXS). The majority of work is made in several photoemissionudexperiments, using both x-ray and UV-excited photoelectron spectroscopy (XPSudand UPS, respectively). In the course of the work, series of (grounded only) a-C:H/Au, a-C:H/Ag, anduda-C:H/Cu samples, with metallic content varying from zero to 100 at.%, have beenuddeposited and studied in vacuo by XPS and UPS (Section 3.1). The XPS results ofudthese series show that, with decreasing metal content below the percolation thresholdud(at about 40-50 at.% of metal concentration), a shift in binding energy (BE) ofudmetal core levels towards higher BEs is observed. With non-carbidic metals like theudones we used in our work, these shifts can be related to their isolated cluster structureudin the host matrix. Decrease of the total metal content in the sample is followedudby the decrease in the cluster size, which is reflected in increased binding energy ofudelectrons escaping from them (Paragraph 2.2.2 and Section 3.1). At the same time,udcarbon C 1s core level is shifted in the opposite direction, towards lower binding energies,udand this shift rises as metal content increases in the sample. Negative shift inudC 1s binding energy reveals increased relative content of sp2-coordinated carbon inudthe a-C:H matrix, which, we believe, should be attributed to the compressive stressudthat metallic inclusions introduce in the host a-C:H, being higher with higher metaludcontent in the sample. Another possible reason is catalytic reduction of hydrogen inudthe a-C:H matrix with increasing metallic content. UPS of the same series (Fig. 3.2)udshowed increased relative contribution of metal valence band features with increasedudmetal content. The Fermi edge evolution from monocrystalline metal reference samplesudto 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 metallicudinclusions and to the same direction-shifts in metal core levels.udOur further attention was focused on low metal-content a-C:H that is, typicallyudbelow 10 at.%, and to the differences between grounded and biased samples. Theudreprint of the communication on these effects encountered in a-C:H/Au and publishedudin the Applied Physics Letters (Vol. 80, 2002, p.2863) is given in Section 3.2.udDirect imaging techniques (Sections 3.2 and 3.3) reveal that even smalludamount of metal included in the a-C:H significantly changes the surface morphologyudand increases roughness. For all three nanocomposite materials, biased samplesudshow similar surface morphology, characterized by relatively flat basis and isolatedudbump structures, of about 30 nm in diameter and up to 15 nm in height. Theseudstructures are attributed to the altered morphology of a-C:H component of a nanocomposite,udwith metallic clusters concentrated on them. Grounded samples characterizeudlow roughness in a-C:H/Au and increased in a-C:H/Ag and a-C:H/Cu. The increaseudof roughness in the latter two materials is explained by enhanced surface diffusionudof metal atoms and clusters coalescence into bigger islands.udGISAXS patterns (Section 3.4) showed isotropic cluster size and interclusteruddistance distribution in grounded a-C:H/Au and a-C:H/Cu samples. With grounded a-udC:H/Ag sample, no spatial correlation could be revealed, probably again due to theudsurface agglomeration of Ag clusters. These analysis of GISAXS patterns showedudthat biased samples contain bigger clusters than grounded ones, and slightly flattenedudin the grow direction. The differences between grounded and biased samples were first detected byudin vacuo XPS of a-C:H/Au in systematically lower, in average about 50%, Au contentudin the biased than in the grounded case. This decrease is followed by higher positiveudshift of the Au 4f7/2 core level binding energy in the biased case (Section 3.2 andudParagraph 3.5.1). The similar situation was encountered in a-C:H/Ag samples, whileuda-C:H/Cu showed higher Cu content in the biased than in the grounded case and nouddifference in Cu 2p3/2 shifts from the reference BE. The decrease of the total metaludcontent in biased a-C:H/Au and a-C:H/Ag samples most probably results from increaseduddistance among surface clusters and their concentration on isolated bumpudstructures. The reason for the increased BE shift of Au 4f7/2 and Ag 3d5/2 core levelsudin the biased samples should be searched in cluster baldness at the sample surface.udThe violation of both above conclusions in a-C:H/Cu, i.e. higher Cu content in theudbiased sample and equal shift of the Cu 2p3/2 core level in grounded and biasedudsample is probably due to high relative increase of copper cluster size upon biasing,udwhich compensates the effect of their baldness at the surface. The topmost clustersudof grounded nanocomposite samples are, therefore, most probably covered with audlayer of a-C:H. The thickness of this layer must be below escape depth of metaloriginatedudphotoelectrons, i.e. less than about 2 nm. The negative shift of the C 1sudcore level in the biased samples is induced by increased sp2/sp3 coordinated carbonudratio 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 theudbiased samples.udThese conclusions are supported by several following PES experiments, otherudthan XPS of as-deposited samples. First of them was in vacuo UPS of as-depositedudsamples. In the grounded samples, the He I spectra mostly reproduce the characteristicudshape of a-C:H valence band, and only higher sensitivity He II UPS reveal theudpresence of metallic inclusions. Upon biasing, even when total measured metal contentudwas lower than in the grounded case (a-C:H/Au and a-C:H/Ag), all spectraudclearly showed increased metallic features, evidencing on higher metal exposure atudthe surface.udXPS at off-normal take-off angle of escaping electrons also confirmed our conclusionsudon the surface clusters coverage. Increasing the tilting angle of a sample,udmeasured intensity ratio of a metal core level to appropriate C 1s showed in mostudgrounded samples monotonous decrease, and regular and steady increase in theudbiased ones. These results support our conclusion on the surface clusters coverageudin grounded samples and their baldness in the biased ones. The higher metalcontentudgrounded samples of a-C:H/Ag, however, did not show the expected decreaseudin the intensity ratio, and that was the first indication that the effect of coverageudmay be a particularity of small clusters only, i.e. low-metal content groundedudsamples. This suspicious is confirmed in the next test experiment that we have undertaken,udby subsequent in situ low-energy Ar+ ion etching and PES analysis of a sample.udThe same metal to carbon core level intensity ratio curves were measuredudagainst the sputtering time. In the grounded samples, at the beginning of the sputtering,udan increase in the intensity ratio is observed, related to the thinning of the topuda-C:H layer. In most cases, after some time of sputtering, the maximum is reachedudrelated to the total removal of the cover layer, and from that point onwards, Ar+ ionudetching erodes the metallic clusters as well. In biased samples, a monotonous decreaseudof intensity ratio curves was observed throughout the experiment and isudclearly related to the bald surface clusters that are sputtered together with the a-C:Hudmatrix. The grounded samples intensity ratio curves showed one more importantudregularity: in higher metal-content samples the maximum is reached after shorterudtime, i.e. these samples need less time to be fully uncovered. As a special case, a-udC:H/Ag 32.3 at.% did not show any increase in the intensity ratio curve, but monotonousuddecrease throughout the measurement. That encouraged the conclusion thatudthe coverage of the topmost metallic clusters of grounded samples with a-C:H is anudeffect that is characteristics of small clusters in the host matrix, i.e. low metal-contentudsamples. With higher metal contents, there is no observable difference betweenudgrounded and biased samples regarding surface clusters coverage.udApart from the core level intensity ratio curves, the evolution of our samplesudwith in situ Ar+ ion etching is described in XPS and UPS spectra recorded at eachudpoint of the sputtering time scale. Metal core levels in these figures remained eitherudunchanged 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 toudthe decrease of the cluster size by Ar+ ion sputtering. Carbon C 1s core level in alludsamples shows shift with sputtering time towards lower binding energies, which isudrelated to the further sp2-coordinated carbon favoring by the in situ Ar+ ion bombardment.udThe UPS spectra evolution generally follows the trend described by core leveludintensity ratio curves. That is, in grounded samples, the metal features in valenceudband spectra rise to the point of total removal of the cover layer, and decrease furtherudto the end of the sputtering experiment. Biased samples, on the other hand, showudcontinuous decrease of the metal features. In both grounded and biased valenceudband spectra, the development of the carbon π-states is observed throughout theudexperiment, evidencing on the increase of sp2-coordinated carbon content with sputteringudtime. udPointed out several times, the Ag surface clusters coalescence is confirmed inudexperiment in which we compared XPS and UPS spectra of as-deposited samples,udafter 20 hours residence in the ultra-high vacuum (UHV) conditions and after additionalud20 hours in the air. Generally, all as-deposited spectra and after 20 hours in theudUHV were almost identical. After exposure to the air, in all samples carbon C 1s coreudlevel is shifted towards lower binding energies. The most interesting differences afterudresidence in the air show metal core levels. The Au 4f7/2 core level remained practicallyudidentical to the one measured in the UHV, revealing that air conditions do notudaffect Au clusters, and that their size and arrangement remain fully determined by theuddeposition process. That is not the case, however, with Ag clusters. The Ag 3d5/2udcore levels of both grounded and biased a-C:H/Ag samples shift towards lower bindingudenergies. From the differences in UHV- and air-residence binding energy positionsudof the Ag 3d5/2, it is estimated that the increase factor of cluster volume inudgrounded samples is about 170, and the one of biased samples clusters � about 12.udIn these rough figures one may find the cause of the specific behavior of theuda-C:H/Ag sample that we encountered in several occasions and assigned to the Agudsurface clusters coalescence: roughness revealed by AFM, lack of correlation in theudGISAXS patterns, pronounced Ag 4d features even in low metal content valenceudband spectra, and negative shift of the Ag 3d5/2 in the Ar+ ion in situ in-depth profiling.udThe last of our nanocomposites subjected to UHV- and air-dwell comparison was a-udC:H/Cu. Copper, however, oxidizes in the air, but nevertheless it provided in this experimentudone of most elegant evidences on the surface clusters coverage. The deconvolutionudprocedure applied to copper- and CuO-originated Cu 2p3/2 revealed thatudrelative content of oxidized copper is higher on the surface of biased sample (withudbald surface clusters), than on the grounded one (where surface clusters are coveredudby a-C:H). The last in the series of experiments aimed to check our conclusions on theudsurface clusters coverage was based on the prospective sulfur binding to noble metaludatoms. Our samples, together with appropriate monocrystalline reference samples,udwere covered with a layer of liquid thiophene (C4H4S) and, after evaporation, subjectedudagain 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 levelsudregion, in spite of increased measurement statistics. The S 2p spectra adsorbed onudthe reference samples were fitted with three S 2p1/2 � S 2p3/2 doublets assigned to Sudbonds to a noble metal, S in C4H4S, and to S�O bonds (Figs. 3.32-3.34). Intercomparisonudof our nanocomposite samples with reference ones showed that sulfur adsorbedudon surfaces originates predominantly from C4H4S itself. However, in biasedudcases a higher relative contribution of the shoulder related to sulfur bonds to a nobleudmetal is observed in spectra, evidencing on higher metal exposure at the biasedudsamples surfaces. The exception of a-C:H/Cu is due to the oxidation of copper.udIn conclusion, in several different PES experiments, by direct imaging of samples,udand using GISAXS technique, we have revealed that MS/PACVD-obtained lowudnoble metal-content amorphous hydrogenated carbon nanocomposites are characterizedudwith topmost metallic clusters covered with a tiny layer of a-C:H when depositedudon a grounded substrate, and bald surface clusters when substrate is biased withud�150 V dc. Beside this main result, we encountered few other effects, like e.g. increasedudsp2/sp3 coordinated carbon ratio in the a-C:H matrix in the biased samplesudand surface clusters coalescence in a-C:H/Ag (and to some extent in a-C:H/Cu)udnanocomposites. By changing one parameter only � the substrate bias voltage in deposition ofudour grounded and biased �counterparts�, we have shown that surface clusters coverageudeffect has an origin in the plasma deposition process itself. We believe that oneudshould look for its cause in the plasma afterglow, the state established in the ionizedudgas immediately after switching off the plasma power supply.udFrom the applicative point of view, we have described, in principle, the mechanismudthat may be employed to tailor the coverage of topmost metallic clusters embeddedudin the a-C:H matrix. Metal inclusions in the a-C:H showed to improve theudwear resistance of the coatings, so one can also envisage the applications when theudcoverage of surface metal clusters with a-C:H would be useful. In tribology, theseudwould be cases when incorporated metal reduces the lubricating properties, i.e. increasesudthe friction coefficient. In biocompatible materials the same would be necessaryudwhen incorporated metals are toxic, like e.g. silver or copper. Vice versa, oneudmay also envisage applications when topmost cluster baldness would be desirable,udlike e.g. with low-friction MoS2 and WS2 inclusions in a-C:H for tribological purposes.udIn addition, surface clusters exposure to the surrounding environment probably influencesudthe optical and aging properties of solar selective coatings based on metal- orudmetal carbide-containing amorphous hydrogenated carbon nanocomposites.