We report in situ transmission electron microscopy dynamic observations of the early stage oxidation of Cu(111) surfaces at ∼450 °C, which show that the Cu2O filmmorphology evolves with continued oxidation from initially ramified islands to irregularly connected clusters. The geometrical features of these noncompact oxidefilms are analyzed in terms of the scaling theory of percolation. We show by kinetic Monte Carlo simulations that the percolating oxide film growth is related to a mechanism of neighbor-dependent site percolation

Gallagher, David M.

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Cedarville UniversityDigitalCommons@CedarvilleScience and Mathematics Faculty Publications Department of Science and Mathematics9-2008Percolating Oxide Film Growth During Cu(111)OxidationDavid M. GallagherCedarville University, gallaghd@cedarville.eduFollow this and additional works at: http://digitalcommons.cedarville.edu/science_and_mathematics_publicationsPart of the Engineering Commons, and the Physics CommonsThis Article is brought to you for free and open access byDigitalCommons@Cedarville, a service of the Centennial Library. It hasbeen accepted for inclusion in Science and Mathematics FacultyPublications by an authorized administrator ofDigitalCommons@Cedarville. For more information, please contactdigitalcommons@cedarville.edu.Recommended CitationGallagher, David M., "Percolating Oxide Film Growth During Cu(111) Oxidation" (2008). Science and Mathematics FacultyPublications. 334.http://digitalcommons.cedarville.edu/science_and_mathematics_publications/334Percolating oxide film growth during Cu„111… oxidationGuangwen Zhou,1,a Xidong Chen,2,3 David Gallagher,4 and Judith C. Yang51Department of Mechanical Engineering and Multidisciplinary Program in Materials Scienceand Engineering, State University of New York, Binghamton, New York 13902, USA2Department of Science and Mathematics, Cedarville University, Cedarville, Ohio 45314, USA3Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA4Department of Engineering and Computer Science, Cedarville University, Cedarville, Ohio 45314, USA5Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh,Pennsylvania 15261, USAReceived 25 July 2008; accepted 5 September 2008; published online 23 September 2008We report in situ transmission electron microscopy dynamic observations of the early stageoxidation of Cu111 surfaces at 450 °C, which show that the Cu2O film morphology evolveswith continued oxidation from initially ramified islands to irregularly connected clusters. Thegeometrical features of these noncompact oxide films are analyzed in terms of the scaling theory ofpercolation. We show by kinetic Monte Carlo simulations that the percolating oxide film growth isrelated to a mechanism of neighbor-dependent site percolation. © 2008 American Institute ofPhysics. DOI: 10.1063/1.2990624Understanding the microscopic processes controllingoxidation of metal surfaces has practical importance in manyfields including environmental stability, corrosion, electro-chemistry, gas sensing, heterogeneous catalysis, and dielec-tric film synthesis. Growth of oxide layers has been observedon many metallic systems, and models have been proposedbased on the transport of ionic species through a thick andcontinuous oxide film.1–4 However, such previous studieshave not examined the earliest stages of oxidation beginningwith island nucleation and growth, largely due to the inabil-ity of traditional techniques to perform in situ measurementsof the structure and kinetics at the nanoscale as the reactionprogresses. In situ ultrahigh vacuum UHV environmentaltransmission electron microscopy TEM provides a uniquewindow into understanding the mechanism of nanoscale oxi-dation because of its capabilities for time-resolved imagingand diffraction, as well as the UHV environment necessaryfor atomically clean surfaces.5–7 Here we report in situ UHV-TEM dynamic observations of the nanoscale oxidation ofCu111 surfaces, which reveal the evolution of the oxidefilm morphology from initially ramified islands to irregularlyconnected oxide clusters. The growth morphology of suchdiscontinuous, noncompact oxide films cannot be addressedby thermodynamics, i.e., the interplay between surface/interface energies and relaxation of the elastic energy due tothe metal-oxide lattice mismatch, as adopted in the epitaxialgrowth of three-dimensional 3D compact oxide islands.8–10In contrast, we show by both scaling analysis of the geo-metrical features of these oxide films and kinetic MonteCarlo KMC simulations of the oxide film growth that thenoncompact morphologies of these discontinuous oxide filmsare of kinetic origin.Our experiments were carried out in a modified JEOL200CX TEM equipped to allow observation of oxidation un-der controlled gas environments.11 An UHV chamber is at-tached to the middle of the TEM column, where the basepressure is 10−8 Torr. The microscope was operated at 100keV to minimize the possibility of irradiation induced ef-fects. Cu111 single crystal films were grown on irradiatedNaCl111 by sputter deposition. The Cu films were removedfrom the substrate by floatation in de-ionized water, washedand mounted on a specially prepared TEM specimen holderthat allows for resistive heating of the specimen up to1000 °C. Any native Cu oxide is removed by annealing thefilms in the TEM under vacuum conditions at 750 °C,12resulting in clean copper surfaces. Oxygen gas can be admit-ted into the microscope through the leak valve at a partialpressure pO2 between 510−5 and 760 Torr. Real timeobservations of oxidation can be made at pressures of 510−4 Torr.Figure 1 shows representative images from in situ TEMobservations of the morphological evolution of the oxideaAuthor to whom correspondence should be addressed. Electronic mail:gzhou@binghamton.edu.FIG. 1. In situ TEM images of the oxide islands formed from the oxidationof a Cu111 surface at 450 °C and an oxygen pressure of pO2=510−4 Torr upper panel and the corresponding white and black matriceswhite=occupied, black=unoccupied. The images are 10241024 latticeunits lower panel.APPLIED PHYSICS LETTERS 93, 123104 20080003-6951/2008/9312/123104/3/$23.00 © 2008 American Institute of Physics93, 123104-1film during the oxidation of Cu111 at 450 °C under pO2=510−4 Torr. When oxygen is introduced, Cu2O islandsnucleate rapidly and form a “discontinuous-branched” shape.Further oxidation results in growth of these Cu2O islandswhere they coalesce to create irregularly connected oxideclusters. Figure 1 reveals that these oxide islands have rami-fied morphologies and grow into each other and form a per-colating network as the oxidation progresses. These discon-tinuous, noncompact oxide films are irregularly shaped. Theylook like tree branches with stringy boundaries and certainlyare not describable in terms of the familiar Euclidean shapes.In order to quantitatively understand the features of thegrowth morphologies of the oxide films, the TEM images aredigitized to a binary contrast with 1 and 0 for occupiedwhite and vacant black sites, respectively, as shown in thelower-panel images in Fig. 1. This allows us to analyze theTEM images as a site-percolation problem.With use of a connectivity-checking algorithm,13 the in-dividual Cu2O clusters can be isolated for statistical analysis,as shown in Figs. 2a and 2b. The largest cluster connect-ing all sides of the image is the infinite spanning cluster.We have found the threshold oxide coverage of 45% for theformation of the infinite cluster. The finite clusters are firsteliminated leaving the infinite cluster alone Fig. 2c. Un-der the scaling hypothesis, the infinite cluster at the percola-tion threshold pc has the property of statistical self-similarity,which can be checked by measuring the mass ML of theinfinite cluster and that of the backbone with different lengthscale L. At the percolation threshold, p= pc, the mass of thespanning cluster scales with L as14,15ML = LDfL/→ LD for L LE for L  . 1Here  is the correlation length defined as the average rootmean square distance between occupied sites that belong tothe same cluster, D is the fractal dimension of the cluster,and E is the Euclidean dimension where E=2 is for a two-dimensional surface. According to Eq. 1, for length scaleL, the largest cluster is self-similar and spans the wholebox and MLLD; for L, the cluster looks homoge-neous and MLLE. As shown in Fig. 2d, the mass den-sities for the infinite cluster are scale dependent and followthe power law in Eq. 1 with L, and have the fractaldimension of D=1.71.The fractal dimension D=1.71 of the infinite oxide clus-ter is inconsistent with the prediction by random percolation,which gives D=1.9. The theory of random percolation dealswith the formation of clusters by random occupation of sites,i.e., the probability of a site being occupied is independent ofits neighbors. We speculate such a random site occupationmechanism does not apply to metal oxidation, where the ox-ide nuclei can act as capture centers to attract oxygen atomsfor the oxide growth. This implies that the probability of siteoccupation is neighbor dependent for surface oxidation, andsites adjacent to the periphery of existing oxide have largerprobability to be occupied.We use KMC simulations to verify the above specula-tion. The basic processes in KMC simulations include ran-dom deposition of oxygen atoms onto a Cu surface and cap-ture of the deposited oxygen atoms either by independent Cusurface sites e.g., the process corresponding to nucleation ofoxide islands or by the surface sites adjacent to existingoxide islands if any, leading to the oxide growth. The prob-ability for oxygen atoms being captured by existing oxideislands is larger than the nucleation of new islands due to thesmaller activation energy for an oxygen atom sticking to anexisting oxide island. The formation of percolating oxidestructure is observed from our KMC simulations and Fig.3a shows the infinite cluster from simulations at the perco-lation threshold. Analysis of the mass densities of the infinitecluster reveals the power law of Eq. 1 and a fractal dimen-sion of D=1.75 with L Fig. 3b, which is very close tothe fractal dimension D=1.71 of the infinite oxide clusterobserved experimentally. Their agreement supports ourspeculation that the percolating oxide film growth duringFIG. 2. Color online The oxide film at the percolation threshold. a Iden-tification of oxide clusters by a connectivity analysis of the discontinuousoxide film. Different clusters are shown by different colors and the largestone infinite cluster has the brightest contrast and spans over the wholeimage. b The infinite cluster singled out from a. c Scaling analysis ofthe infinite cluster. d Log-log plot of ML /L2 vs L for the infinite cluster.The fraction dimension D for the infinite cluster is determined to be 1.71.Note that L is the number of pixels of the image and can be converted tolength scale by Lnm=Lpixel number image sizenm /1024.FIG. 3. Color online a The infinite cluster formed at the percolationthreshold from the KMC simulations based on the neighbor-dependent siteoccupation mechanism. b Log-log plot of ML /L2 vs L for the infinitecluster. The fraction dimension D for the infinite cluster is determined to be1.75 L is the number of pixels.123104-2 Zhou et al. Appl. Phys. Lett. 93, 123104 2008Cu111 oxidation is related to the processes of neighbor-dependent site occupation of oxygen.One might expect that the mechanism of neighbor-dependent site occupation is typical for oxide growth duringmetal oxidation. This implies that percolating oxide filmgrowth could be a general phenomenon for metal oxidation.However, our in situ TEM observations of the oxidation ofCu100, Cu110, and Cu111 under similar oxidation con-ditions reveal that the oxidation of Cu100 and 110 leadsto the formation of 3D compact Cu2O islands,8–10,16,17 whichobviously deviates from the fashion of percolating oxidegrowth. To address these seemingly contradictory oxidationbehaviors on the different surface orientations, we start withconsideration of the effect of oxygen surface chemisorptionon oxide formation during metal oxidation. Many investiga-tions have elegantly demonstrated that Cu100, 110, and111 surfaces show different reconstructions due to oxygenchemisorption. Oxidation of Cu100 surfaces leads to theformation of an ordered 222R45° O–Cu100 struc-ture, where every fourth 100 row of the uppermost Cu at-oms is missing e.g., missing-row structure with a corruga-tion of 0.35 Å.18 The oxygen chemisorption on Cu110surface results in the formation of a 21O–Cu110 struc-ture added-row structure, where every second 100 row ofthe outermost Cu atoms is displaced outward by 0.8 Å.19The O chemisorption on Cu111 results in ordered 29− 13R46.1°7R21.8° and 44− 73R5.8°21R−10.9° lattice structures, which comprise distorted hexago-nal arrays of O atoms with unit cell areas 29 and 44 timeslarger than that of the substrate Cu111.20–22 The oxygenatoms in the 29 and 44 structures have more than one well-defined height with corrugation up to 3.1 Å with respectto the Cu surface.20,21,23 Compared to the chemisorbed struc-tures of 222R45° O–Cu100 and 21O–Cu110,the oxygen-chemisorbed ‘29’ and ‘44’ structures on Cu111surfaces have much larger surface corrugation, and such en-hanced surface roughness can greatly inhibit surface diffu-sion of oxygen atoms. It has been widely observed and mod-eled that the morphologies of islands formed during thin filmdeposition closely depend on surface mobilities of adatoms,i.e., efficient surface and edge diffusions usually lead to com-pact island growth, while sluggish surface diffusion causesramified island morphologies.24,25 Our observations suggestthese principles apply as well to the growth morphologies ofoxide films during transient oxidation of metals. The perco-lating oxide growth during Cu111 oxidation is related tothe restricted surface diffusion of oxygen due to the highlycorrugated O–Cu111 surface structure, and the compact ox-ide island growth observed during the oxidation ofO–Cu100 and O–Cu110 surfaces is related to the efficientoxygen surface diffusion due to their smooth surfacestructures.In summary, we have observed percolating oxide growthduring Cu111 oxidation. The fractal dimension extractedfrom scaling analysis of the oxide films is consistent with amechanism of neighbor-dependent site occupation of oxy-gen. The insights obtained from this study have broader im-plications to understanding transient oxidation of metals,where oxygen surface chemisorption generally occurs, buttheir effect on the oxide growth still remains poorly under-stood.1C. Wagner, Z. Phys. Chem. B21, 25 1933.2W. W. Smeltzer, R. R. Haering, and J. S. Kirkaldy, Acta Metall. 9, 8801961.3J. Stringer, in Point Defects and Transport in Oxides, edited by M. S.Seltzer and R. J. Jaffee Plenum, New York, 1974, pp. 495–517.4J. Atkinson, Rev. Mod. Phys. 57, 437 1985.5J. C. Yang, B. Kolasa, J. M. Gibson, and M. Yeadon, Appl. Phys. Lett. 73,2841 1998.6J. C. Yang, M. Yeadon, B. Kolasa, and J. M. Gibson, Appl. Phys. Lett. 70,3522 1997.7J. C. Yang, M. Yeadon, B. Kolasa, and J. M. Gibson, Scr. Mater. 38, 12371998.8G. W. Zhou and J. C. Yang, Phys. Rev. Lett. 89, 106101 2002.9G. W. Zhou and J. C. Yang, Appl. Surf. Sci. 210, 165 2003.10G. W. Zhou and J. C. Yang, Appl. Surf. Sci. 222, 357 2004.11M. L. McDonald, J. M. Gibson, and F. C. Unterwald, Rev. Sci. Instrum.60, 700 1989.12G. W. Zhou and J. C. Yang, Phys. Rev. Lett. 93, 226101 2004.13A cluster is defined as a set of occupied sites all of which are connected byneighboring occupied sites. The connectivity checking algorithm in thisstudy is developed based on examination of eight neighbors of each oc-cupied pixel site. Any occupied sites on neighboring sites belong to thesame cluster.14J. Fender, Fractals Plenum, New York, 1988.15D. Stauffer, Phys. Rep. 54, 1 1979.16G. W. Zhou and J. C. Yang, Surf. Sci. 531, 359 2003.17G. W. Zhou, L. Wang, and J. C. Yang, J. Appl. Phys. 97, 063509 2005.18F. Jensen, F. Besenbacher, E. Lagsgaard, and I. Stensgaard, Phys. Rev. B42, 9206 1990.19F. M. Chua, Y. Kuk, and P. J. Silverman, Phys. Rev. Lett. 63, 386 1989.20F. Jensen, F. Besenbacher, E. Lagsgaard, and I. Stensgaard, Surf. Sci. 259,L774 1991.21F. Jensen, F. Besenbacher, and I. Stensgaard, Surf. Sci. 269–270, 4001992.22T. Matsumoto, R. A. Bennett, P. Stone, T. Yamada, K. Domen, and M.Bowker, Surf. Sci. 471, 225 2001.23S. M. Johnston, A. Mulligan, V. Dhanak, and M. Kadodwala, Surf. Sci.519, 57 2002.24M. Z. Li and J. W. Evans, Phys. Rev. B 69, 035410 2004.25R. Q. Hwang, J. Schroder, C. Gunther, and R. J. Behm, Phys. Rev. Lett.67, 3279 1991.123104-3 Zhou et al. Appl. Phys. Lett. 93, 123104 2008

Percolating Oxide Film Growth During Cu(111) Oxidation

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Percolating Oxide Film Growth During Cu(111) Oxidation

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