Confined step-flow growth of Cu intercalated between graphene and a Ru(0001) surface
Nicolas Rougemaille, Sergio Vlaic, Lucia Aballe, Michael Foerster, Johann Coraux
CConfined step-flow growth of Cu intercalatedbetween graphene and a Ru(0001) surface
Nicolas Rougemaille , Sergio Vlaic , Lucia Aballe , MichaelFoerster , Johann Coraux Univ. Grenoble Alpes, CNRS, Grenoble INP, Institut NEEL, 38000 Grenoble,France Laboratoire de Physique et d’´Etude des Mat´eriaux, ESPCI Paris, PSLResearch University, CNRS, Sorbonne Universit´es, UPMC Univ Paris 06, 75005Paris, France ALBA Synchrotron Light Source, 08290 Cerdanyola del Valles, SpainE-mail: [email protected]
Abstract.
By comparing the growth of Cu thin films on bare and graphene-coveredRu(0001) surfaces, we demonstrate the role of graphene as a surfactantallowing the formation of flat Cu films. Low-energy electron microscopy, X-rayphotoemission electron microscopy and X-ray absorption spectroscopy reveal thatdepositing Cu at 580 K leads to distinct behaviors on both types of surfaces. Onbare Ru, a Stranski-Krastanov growth is observed, with first the formation ofan atomically flat and monolayer-thick wetting layer, followed by the nucleationof three-dimensional islands. In sharp contrast, when Cu is deposited on agraphene-covered Ru surface under the very same conditions, Cu intercalatesbelow graphene and grows in a step-flow manner: atomically-high growth frontsof intercalated Cu form at the graphene edges, and extend towards the center ofthe flakes. Our findings suggest potential routes in metal heteroepitaxy for thecontrol of thin film morphology.
Accepted for publication in 2D Materials (2019) DOI:10.1088/2053-1583/ab111e
Keywords : graphene, intercalation, surfactant, step-flow growth, heteroepitaxy,low-energy electron microscopy, X-ray photoemission electron microscopy, X-rayabsorption spectroscopy a r X i v : . [ c ond - m a t . m t r l - s c i ] M a r Rougemaille et al. 2D Materials (2019) DOI:10.1088/2053-1583/ab111e Introduction
The way thin films grow epitaxially on a singlecrystal surface strongly depends on the respectivesurface energies, the interaction strength betweenthe deposited adatoms and the surface, and on theepitaxial stress induced by the substrate. Threeprimary growth modes are usually considered in thinfilm epitaxy [1, 2]:– The Frank-van der Merwe growth mode, whenadatoms attach preferentially to the surface ratherthan clustering, leading to a layer-by-layer growth,– The Volmer-Weber growth mode, when adatomspreferentially bond together, resulting in the formationof three-dimensional islands,– The Stranski-Krastanov growth mode, which is anintermediate situation characterized by the formationof three-dimensional islands on top of a two-dimensional layer.These considerations are thermodynamic argu-ments, and (strong) deviations from these growthmodes occur when growth is governed by kinetic limita-tions. Other surface morphologies may be obtained inspecific cases, for example when surface alloying playsa key role [3, 4, 5, 6]. Surfactants can also be em-ployed to influence surface energetics and thus con-trol thin film morphology [7, 8, 9, 10, 11, 12]. Inthat context, graphene has been recently identified asan interesting surfactant, being a floating [13], cova-lent and deformable membrane that allows intercala-tion of adatoms and confined growth under a purelytwo-dimensional (2D) cover [14].Adatoms intercalated between graphene and thesubstrate obviously interact with both materials.Thus, besides affecting the film morphology, onemight also expect graphene to potentially modify thestructural, electronic or magnetic properties of theintercalated layer. For instance, graphene has beenshown to be a promising capping layer in magneticsystems, especially to promote perpendicular magneticanisotropy [15, 16, 17, 18, 19], a redistribution ofthe magnetic moment perpendicular to the surface[20, 21, 22, 23, 18, 24], or chiral magnetic textures[25, 26, 27].Intercalation is usually obtained by post-annealingtreatments [28, 29, 30, 31, 32, 33, 34]. Although adiscussion on the microscopic origin of the intercalationprocess remains often vague, point defects in graphenehave been proposed to be potential channels wherematter can penetrate [30, 35]. Other studies suggestthat besides point defects [36, 37, 38], graphene edges[39, 40, 14] are also efficient intercalation pathways.In this work, we investigate the growth ofCu films on a graphene-covered Ru(0001) surface,using low-energy electron microscopy (LEEM) andX-ray photoemission electron microscopy (PEEM), combined with spatially-resolved electron reflectivityand photoelectron yield spectroscopy. This system isinteresting as graphene is known to strongly interactwith Ru [41], while Cu is a metal of choice to decoupleepitaxial graphene [42, 43, 44, 29] and to modify itselectronic properties [45, 46]. When it is depositedat 580 K, we find that Cu intercalates underneathgraphene and then grows in a step-flow manner. Insharp contrast, on bare Ru(0001) and under thevery same conditions, Cu growth is characteristicof a Stranski-Krastanov mode: randomly distributedthree-dimensional Cu islands form on a wetting layer.These findings highlight the unique surfactant role ofgraphene and suggest potential routes for controlingthe morphology of thin metal films in heteroepitaxy.
Methods
Experiments were performed under ultrahigh vacuumconditions (base pressure of 4 × − mbar). Aruthenium single-crystal cut with a (0001) surface wasused as a substrate. The surface was prepared byrepeated cycles of 820 eV Ar + ion sputtering andflash-annealing to 1720 K. To reduce the concentrationof dissolved carbon close to the surface region, thesample was occasionally annealed at 870 K under a10 − mbar partial pressure of O for 10 min. Justafter this treatment, and in any case prior to graphenegrowth, the sample was flash-annealed to 1510 K. Thisprocedure yielded a clean surface, with no apparenttrace of surface contamination or residual grapheneflakes as assessed by low-energy electron microscopy(LEEM) and low-energy electron diffraction (LEED).Graphene growth was then performed by slowlydecreasing the substrate temperature in order topromote surface segregation of carbon dissolved intothe bulk [47, 48]. We observed that graphene islandsstart to form below 1120 K, while they rapidlydecompose above 1170 K as bulk dissolution of carbonis activated. We chose a temperature of about 1100 Kfor which the typical distance between graphene nucleiis a few to several 10 µ m. After a ∼ Rougemaille et al. 2D Materials (2019) DOI:10.1088/2053-1583/ab111e (a)(b) N o r m a li s ed e l e c t r on r e f l e c t i v i t y Energy (eV) with graphenewithout grapheneRuGraphene E F W VacuumSample electronsEnergyDOSRu
Figure 1.
Ru(0001) surface partly covered with graphene. (a)LEEM image of single-layer graphene flakes grown on Ru(0001)(electron energy: 2.5 eV). Protruding Ru mesas imprintingholes in graphene are schematized in the bottom-left inset.Micro-diffraction patterns with 150 eV electrons, taken onbare Ru(0001) (black-circled) and on graphene/Ru(0001) (blue-circled), are shown. (b) Electron reflectivity as function of theelectron kinetic energy extracted from the two colored framesin (a). The sudden decrease of reflectivity at about 0 eVcorresponds to the work function ( W ) of the surface (see inset). amount of Cu deposited, and to the ”ML” notation todescribe the actual Cu thickness.Copper deposition and graphene growth wereperformed under ultrahigh vacuum conditions, inoperando , in front of the objective lens of theElmitec LEEM-III end-station at the CIRCE beamlineof the ALBA Synchrotron [50]. Both soft X-raysynchrotron light and low-energy electrons were usedas sources for imaging, yielding PEEM and LEEMimages, respectively. Electron reflectivity spectroscopywas performed by varying the potential between theelectron gun of the microscope and the sample surface( i.e. by varying the kinetic energy of the electrons).In addition, X-ray absorption spectroscopy (XAS) in partial electron yield mode was performed byscanning the photon energy across the Cu L and L absorption edges, while detecting the emittedsecondary photoelectrons. LEEM observations
Before addressing the growth of copper, we brieflydescribe the sample surface after graphene deposition.In the LEEM image of figure 1(a), graphene partiallycovers the Ru(0001) surface (in the imaging conditionsused here, graphene appears brighter than bare Ru).We note the presence of holes in the graphene flakes,having sizes of few 100 nm, typically. These grapheneholes are presumably a consequence of graphenegrowth proceeding from the ascending step edges ofthe Ru(0001) substrate [47], revealing the presence ofmesas in the Ru surface (see inset in figure 1(a)). Thus,the graphene flakes are not continuous, although singlecrystalline.The micro-diffraction patterns [figure 1(a)] on bareand graphene-covered Ru(0001) show the expected(1 ×
1) and surface superstructure [47]. The lattercorresponds to the coincidence lattice (so-called moir´epattern) resulting from the lattice mismatch betweengraphene and Ru(0001) [51]. Finally, electronreflectivity (figure 1(b)) reveals typical features forgraphene-free and graphene-covered regions [47, 52].The onset of total reflection, which corresponds toelectrons having a kinetic energy higher than thesurface work function ( W ), is decreased by 1.3 eV inpresence of graphene, due to charge transfer at themetal-graphene interface [53].This surface, partly covered with graphene,has been used to investigate the growth of Cu at580 K. Growth modes on bare Ru(0001) and ongraphene/Ru(0001) can be then studied simultane-ously. The growth temperature was 580 K. Followingprevious reports [54, 55], this temperature was chosento observe a Stranski-Krastanov growth of Cu on bareRu(0001). We also used a 700 K growth temperatureto speed up the intercalation process. In these condi-tions (deposition at 580 and 700 K), the pressure inthe system did not exceed the high 10 − mbar-range. Copper growth on bare Ru(0001)
We first analyse Cu growth on bare Ru based on theLEEM image sequence reported in the supplementaryinformation (SI) figure S1. After the deposition ofabout 1.2 eq. ML, the LEEM contrast coming fromthe atomic step structure of the substrate is recovered(see zoomed-in region in figure S1), indicating that acomplete 1 ML Cu is formed (table S1, movie S1).This threshold amount of 1.2 eq. ML is substantiallylarger than 1 ML. The difference may be related to the
Rougemaille et al. 2D Materials (2019) DOI:10.1088/2053-1583/ab111e i Cu i Figure 2.
Copper on bare Ru(0001) and intercalated (Cu i )below graphene. LEEM image after the deposition of 9 eq.ML (electron energy: 0.5 eV; same region as the one shownin figure 1(a)). Underneath graphene, growth proceeds in astep-flow-like fashion, from the edges of the graphene flake. Onbare Ru(0001), a characteristic Stranski-Krastanov morphologydevelops, with a critical thickness for the 2D-3D transitionbetween 1 and 2 ML of Cu. calibration of the Cu deposition rate (we estimate theuncertainty to be of the order of 10-15%). In addition,part of the Cu adatoms might be in a dilute phase onthe surface at the 580 K deposition temperature.Further Cu deposition (see zoomed-in region infigure S1 for 1.5 eq. ML and movie S1) leads to aroughening of the surface, suggesting that the growthof the second atomic layer in the form of one-extra-layer-thick islands, separated by few 10 nm distances.Further increasing the amount of deposited Cu neverleads to the completion of the second Cu ML. Instead, asudden formation of three-dimensional (3D) Cu islandsis observed (see zoomed-in region in figure S1, figure 2,movie S1). The 3D islands have a much lower densitythan the above-discussed 2 ML-thick islands and aretypically separated by a few micrometers. We notethat the location of these 3D islands is not correlatedto the position of the mesas on the Ru(0001) surface.The low nucleation density of the islands signalsa transition from a 2D growth mode to a 3D growthmode: above 1.9 eq. ML, 3D islands coexist with a 2Dwetting layer, which is typical for a Stranski-Krastanovgrowth. The critical thickness for the 2D/3D transitionis here between 1 and 2 ML of Cu. Althoughconsistent, this value is different from those reportedwhen deposition is made at slightly lower temperaturesof 520 K (3 ML [55]) or 540 K (1 ML [54]). Thedriving force for this transition is presumably notthe distinct surface energies of Ru(0001) and Cu(111) ( ∼ and ∼ respectively [56, 57]), asthe larger value for Ru(0001) should rather promotelayer-by-layer growth (Frank-van der Merwe growth).However, the relief of tensile elastic energy in Cu, whichresults from the 5.5% lattice mismatch between thetwo materials, could be a natural driving force for theformation of 3D Cu islands.Consistent with previous literature, we find thatthe first Cu atomic layer is in fact pseudomorphicto Ru(0001), hence strongly stretched (see figure S2).Above this thickness, a variety of stress relief patternshas been documented as a function of post-depositionannealing temperatures [58, 59, 60]. The wetting layerthat we observe between the 3D Cu islands consistsof a full single layer plus a fraction of a double layer,and its diffraction pattern (see figure S2) indicates (atleast) partial stress relief. N o r m a li s ed e l e c t r on r e f l e c t i v i t y Energy (eV)graphene/Ru(0001) 1-2 ML Cu 1 ML Cu i i i i Figure 3.
Electron reflectivity as function of the kinetic energyafter the deposition of 2.5 (full lines) and 14 (dotted lines) eq.ML of Cu, extracted from different regions on the sample surface.Cu and Cu i stand for Cu on bare Ru and intercalated belowgraphene, respectively. Copper growth on graphene-covered Ru(0001)
We now describe the real-time growth of Cu ongraphene-covered Ru(0001). As Cu is deposited, novisible change in the LEEM contrast is observed onthe inner regions of the graphene flakes, contrary towhat is found on bare Ru (see figure S1 for lowcoverages and table S1). The only discernable changeon graphene-covered regions is the appearance of a rimextending over several 10 nm, located at the edgesof the flakes (see figure 2, figure S1 for 1.2 eq. ML,and movie S1). Consistent with other works [14], thissuggests that Cu adatoms have long-range mobility ongraphene at the deposition temperature, presumablybeyond the few 10 µ m size of the graphene flakes. This Rougemaille et al. 2D Materials (2019) DOI:10.1088/2053-1583/ab111e
PEEM observations
Our LEEM observations reveal that Cu is inhomo-geneously distributed on the Ru surface, with well-defined thicknesses at the edges of the graphene flakesand thick 3D islands on bare Ru. To confirm this find-ing, we used chemically-resolved microscopy, combin-ing XAS with PEEM. A typical PEEM image acquiredat an energy below the absorption edge is reported infigure 4(a), after the deposition of ∼
14 eq. ML of Cu(the last 5 eq. ML have been deposited at 700 K tospeed up the intercalation process). The inset identifiesthe graphene-covered regions.Recording a series of PEEM images while scanningthe photon energy allows extracting local XAS spectradown to the single pixel level (figure 4(b)). Asexpected, Cu is found everywhere on the surface, onbare Ru and on graphene-covered regions as well,where Cu is intercalated. Except for the Cu wettinglayer on bare Ru, all spectra are rather similar. Thelarger absorption signal is obtained on the thick Cuislands on graphene-free regions and is similar to theone of bulk Cu [65]. For the Cu wetting layer onbare Ru (orange curve in figure 4(b)), the oscillationsafter the L edge are not located at the same energiesas in the other spectra, suggesting a different localenvironment for Cu atoms. This difference may arisefrom the strain present in the wetting layer, which ispseudomorphic to Ru(0001).In figure 4(c) and figure S3, the L absorption edgeheight is plotted as a function of the number of Culayers. Although these two quantities are not directlyproportional [66], the step height monotonouslyincreases with the number of Cu layers, so that a mapof the Cu thickness can be extracted from the series ofPEEM images. Complementary to what we found withLEEM, the rims of intercalated Cu have a 3 to 4 MLthickness, while the remaining of the graphene flake isfully intercalated with 2 ML of Cu. However, thickerintercalated deposits are found. In particular, 5 ML-thick intercalated regions are prominently found at theedges of the graphene flake. Others are also found inthe center of the flake, at the location of a graphenehole, which acts as an effective edge.Noteworthy, intercalated regions thicker than2 ML are not distributed equally all around thegraphene flake shown in figure 4(a). In particular, Rougemaille et al. 2D Materials (2019) DOI:10.1088/2053-1583/ab111e N o r m a li s ed e l e c t r on y i e l d X-ray energy (eV)925 965930 940 945 950 955 9605 µm (a) (b)(c) Y i e l d s t ep he i gh t ( a r b . u . ) i i i i graphene electronsCu i Ru graphenen ML Cu Y i e l d s t ep he i gh t Cu/Ru Graphene X-rays
Figure 4.
Copper thickness-dependent XAS spectra. (a) Pre-edge PEEM image acquired with 927.5 eV X-rays), after the depositionof 14 eq. ML of Cu. In the explored range of X-ray energies, the escape length of the photoelectrons is of the order of 1 nm, significantlysmaller than the X-ray penetration depth. A cartoon identifies graphene-covered regions in (a,c). (b) Cu absorption across the L , edges extracted from the different regions highlighted in (a). The curves are vertically shifted (reference at 965 eV) for clarity. (c) L absorption edge step height (see (b)) extracted point-by-point (with 4 pixel-binning) across the surface, and represented in threedimensions (viewpoint indicated with an arrow an the symbol of an eye in (a)). The height increases as the number of Cu layerincreases, although not linearly. intercalation seems to be more efficient at the loweredge in the image. It could be that the upper edge inthe image is located along a bunch of ascending atomicsteps of Ru(0001). Such graphene edges are knownto be strongly bound to the substrate and remainessentially immobile during graphene growth. There,Ru(0001) steps effectively act as walls preventinggraphene growth [47]. In contrast, the bottom edgein the image would be weakly bound and mobileduring graphene growth [47]. We expect that thesetype of mobile edges are preferred pathways for Cuintercalation. Coming back to the discussion onthe growth mode of the intercalated Cu film, thesePEEM measurements indicate that the Ru atomicstep structure has in fact partial influence on theintercalation mechanism. But this influence seemsmore related to the energy barrier involved to initiateintercalation than the structure of the Ru surfaceatomic steps.The variability in the thickness of intercalatedCu at the vicinity of graphene edges may be seenas a tendency to surface roughening. But thedynamics of this roughening is different from the one usually observed in standard layer-by-layer growth.In fact, in absence of a graphene capping layer,surface roughening is inevitable, even in the case ofa Frank-van der Merwe growth, due to the small,but non-zero, probability for two (or more) metaladatoms to meet on a terrasse. Atomic dimers (ortrimers, tetramers, etc) are then seeds for the furtherattachment of adatoms, hence nucleating a new atom-thick layer. The formation of few atom clusters canalso occur on these newly created atom-thick islands,and roughening develops accordingly. This clustermechanism should be less probable when the surfaceis graphene-covered. This is so because two (or more)adatoms first need to climb up a one-atom-thick islandbefore nucleating a new island. In other words, thefact that growth is confined under graphene and thatadditional matter can only enter from the grapheneedges should significantly lower the probability ofroughening. This expected low probability is infact consistent with the very low density of > > Rougemaille et al. 2D Materials (2019) DOI:10.1088/2053-1583/ab111e
Summary and concluding remarks
Copper deposition at 580 K on a Ru(0001) surfacepartly covered with a graphene layer reveals twodistinct growth modes. On bare Ru, a Stranski-Krastanov growth is observed and the morphology ofthe Cu film is characterized by large micron-size, three-dimensional islands, coexisting with a Cu wetting layerof thickness between 1 and 2 ML. On graphene-coveredRu, Cu intercalates at the edges of the graphene flakesand a confined step-flow regime takes place, bringingCu towards the center of the flakes. The main resultof this work is the observation that graphene allowsto bypass the formation of three-dimensional islandsby modifying the surface energetics and the kineticsof mass transport on the surface, and imposes a step-flow growth. The steps that matter in this confinedgrowth mode are not those of the Ru substrate butthose defined by the graphene edges. Interestingly,oxygen has been used also to promote the formation oftwo-dimensional Cu films on Ru(0001), but followinga layer-by-layer growth mode [67, 68]. Here instead,graphene forces a step-flow regime.A natural question is whether the growth modewe revealed can be implemented, beyond the scale of afew 10 µ m set by the size of the graphene flakes, at amacroscopic length scale. In other words, one mightwonder whether confined step-flow growth can beachieved underneath a full graphene layer. Obviously,this question deserves more work, but a prerequisite isthat intercalation channels exist for Cu. We note that,unlike the case of Co intercalation between grapheneand Ir(111), which comes with a variety of intercalationchannels (edges [40, 14], bent graphene regions [38],point defects [30]), here we did not observe otherintercalation channels than edges (at 580 and 700 K).Besides, preliminary attempts to intercalate a thin Cufilm (6 eq. ML) deposited at room temperature ontop of polycrystalline graphene/Ru(0001) (prepared bycatalytic decomposition of ethylene at 870 K; datanot shown) and subsequently annealed up to 700 K,were unsuccessful. This is also in contrast withthe Co intercalation between polycrystalline grapheneand Ir(111) [30] or Pt(111) [26]. Point defects inpolycrystalline graphene on Ru(0001) do not seem tofavor intercalation. Even though we cannot excludethat point defects or other kinds of intercalationchannels become active at temperatures beyond therange we explored (580, 700 K), a confined step- flow growth might be induced by generating defects,artificially. These artificial defects could be obtainedfor example after graphene islands have coalesced, butbefore a 100% coverage is achieved, or could be created a posteriori by an etching process. After intercalation,a full protection of the surface could be achieved bya second chemical vapor deposition of graphene ontothe bare Cu regions (those covering the initially bareRu(0001) regions, at the location of the graphenedefects).Our work opens new prospects to revisit epitaxialgrowth with a new ingredient, namely a covalent,deformable graphene layer. By altering both thegrowth kinetics and the thermodynamics of the system,graphene offers the appealing opportunity to controlthin film morphology through confinement under anatomically-thin cover. Besides, graphene is also anexcellent protective cover against oxidation in air andother environments. Acknowledgments
These experiments were performed at the CIRCEbeamline of the ALBA Synchrotron. We thank JordiPrat for his great technical support.
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