Atomically Visualizing Elemental Segregation-Induced Surface

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Atomically Visualizing Elemental Segregation Induced Surface Alloying and Restructuring Lianfeng Zou, Jonathan Li, Dmitri N Zakharov, Wissam A. Saidi, Eric A. Stach, and Guangwen Zhou J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02947 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 3, 2017

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The Journal of Physical Chemistry Letters

Atomically Visualizing Elemental Segregation Induced Surface Alloying and Restructuring Lianfeng Zou1, Jonathan Li2, Dmitri Zakharov3, Wissam A. Saidi4, Eric A. Stach3, Guangwen Zhou1 1

Department of Mechanical Engineering & Materials Science and Engineering Program, State University of New York at Binghamton, NY 13902, USA

2

Department of Physics, Applied Physics and Astronomy & Materials Science and Engineering Program, State University of New York, Binghamton, NY 13902, USA

3

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, USA 4

Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261, USA

1 ACS Paragon Plus Environment

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ABSTRACT: Using in-situ transmission electron microscopy that spatially and temporally resolves the evolution of the atomic structure in the surface and subsurface regions, we find that the surface segregation of Au atoms in a Cu(Au) solid solution results in the nucleation and growth of a (2×1) missing-row reconstructed, half-unit-cell thick L12 Cu3Au(110) surface alloy. Our in-situ electron microscopy observations and atomistic simulations demonstrate that the (2×1) reconstruction of the Cu3Au(110) surface alloy stays as a stable surface structure as a result of the favored Cu-Au diatom configuration.


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Understanding the nature of atomic arrangements and dynamic transformations at surfaces is critical for a wide range of technologically important processes such as corrosion and heterogeneous catalysis. A real crystal surface can drastically differ from a bulk-truncated surface termination as a result of surface relaxation or reconstruction triggered by the adsorption of foreign atoms that either induce local shift of the substrate surface atoms or react with the substrate atoms to form some compounds that self-assemble into long-range ordered patterns 1-6. The representative examples are K, Cs and O adsorption induced (1×1)→(1×n) (n≥2) reconstructions in a wide range of (110) surfaces of 3d, 4d and 5d metals

7-11

. In this work, we

report a surface restructuring phenomenon induced by the interplay between the surface segregation of solute atoms and decay of unstable surface atoms at elevated temperatures, which differs fundamentally from the adsorbate-induced surface reconstruction process. Surface segregation is a ubiquitous phenomenon in multicomponent materials and can have important effects on the overall properties of the material

12-17

. Surface segregation may

induce both composition and structure changes in the surface and subsurface regions. Probing the segregation phenomena has always been a major challenge, mainly because of the experimental difficulties in resolving structure and composition both spatially and temporally in the surface and subsurface regions. While surface spectroscopies can be routinely employed to assess the surface composition, they are not structure sensitive

18-19

. Scanning tunneling microscopy can

provide surface structure information at very high spatial resolution, but it lacks the capability of atomically resolving structure dynamics at the elevated temperature of surface segregation and with the sufficient time resolution to reveal the atomistic detail

20-21

. Surface X-ray or electron

diffraction techniques can provide structure information for samples at elevated temperatures, but lack spatial resolution and are prone to averaging errors.



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Here we describe dynamic, atomic-scale transmission electron microcopy (TEM) observations of the surface restructuring induced by the surface segregation of Au atoms in a Cu(Au) solid solution. Through the use of in-situ atomic-scale TEM, we are able to both spatially and temporally resolve the segregation induced structural evolution in the surface and subsurface regions at elevated temperature (see supporting information for experimental details). The Cu-Au bimetallic alloy is an ideal system for studying surface segregation phenomena because Cu and Au form a stable L10 (CuAu) and L12 (Cu3Au and Au3Cu) structures addition to its technological importance such as for heterogeneous catalysis

23-27

22

in

. We show that

the interplay of two simultaneous processes, namely thermodynamically driven surface segregation of Au atoms and the decay of unstable surface atoms, results in the formation of a half-unit-cell thick L12 ordered Cu3Au(110) surface alloy that is terminated with a (2×1) reconstruction, a phenomenon that has not been recognized before due to the difficulty of probing the fast dynamics of the local atomic configurations using traditional experimental surface science and bulk materials science techniques. For the (110) surface of the Cu-10at.%Au, both the (2×1) reconstruction and unreconstructed (1×1) are frequently observed at 350°C and 10-3 Torr of H2 gas flow. Fig. 1(a) is a high-resolution (HR) TEM image of the (2×1) reconstructed (110) surface, which shows a periodic hill-and-valley structure with every other [001] atom row absent. The atomic columns in the topmost layer and every other [001] columns in the third layer show much darker contrast, indicating the enrichment of Au atoms in these columns. Meanwhile, the periodic arrangement of the dark columns suggests the formation of an ordered surface alloy. Inset in Fig. 1(a) is a simulated HRTEM image, which further confirms that the (110) Au-rich surface alloy is a halfunit-cell thick layer of L12 Cu3Au, as shown schematically in Fig. 1(a) (right). By contrast, the


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unreconstructed surface shows relatively uniform lattice contrast, suggesting that the surface region still maintains the disorder Cu-Au solid solution without significant Au segregation. As a result, the surface is terminated with the (1×1) structure of the FCC lattice, as shown schematically in Fig. 1(b) (right), which resembles a pure Cu(110) surface that does not reconstruct under UHV or a reducing condition

28-29

. Fig. 1(c) shows a HRTEM image of a

typical (110) facet of a pure Cu film at 350°C and 10-3 Torr of H2 flow, which displays the unreconstructed (1×1) surface structure, similar to the non-Au segregated (110) surface of the Cu-10at.%Au solution shown in Fig. 1(b)). These comparative observations between pure Cu and Cu-10at.%Au illustrate that the (2×1) reconstruction is related to the Au-segregation induced formation of the L12 Cu3Au surface alloy. Fig. 2 shows time-sequence HRTEM images revealing the nucleation and growth of the half-unit-cell-thick L12 Cu3Au(110)-(2×1) surface alloy during the annealing of the Cu10at.%Au solid solution at 350°C and 10-3 Torr of H2 gas flow. The (100) and (110) facets intersect at a corner area (Fig. 2(a)). The blurred contrast in Fig. 2(a) is induced by the thermal drift of the sample. The HRTEM image nevertheless clearly shows that both the (100) and (110) facets are initially of a non-reconstructed (1×1) structure and the (100) and (110) facets have a flat morphology except for the presence of several atomic steps in their intersecting corner area (pointed by a green arrow). After ~ 30.5 s, a small high-index facet begins to form at the corner area via the lateral decay of the (100) facet, accompanied with the segregation of Au atoms to the surface and sub-surface regions of the high-index facet in the corner. Au atomic columns appearing in dark contrast are marked with white dashed rings and numbered 1, 2, 3, and 4, as shown in Fig. 2(b). With the decay of the surface atomic columns, the sub-surface Au columns become exposed to the surface, as shown in columns 1, 2 and 3. A stable Cu3Au nucleus with the 5 ACS Paragon Plus Environment


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thickness of a half unit cell and the lateral length of a single unit cell starts to appear, as pointed by the red arrow in Fig. 2(c). Meanwhile, Au atoms continue to segregate in the corner area adjacent to the Cu3Au nucleus. Fig. 2(c) shows that these newly segregated Au columns (numbered 5, 6, and 7) form a small (100)-type facet. The surface segregation shows some fluctuations that lead to less Au in the segregated atomic columns, as discerned from the contrast in column 5 that changes from dark (Fig. 2(c)) to brighter (Fig. 2(d)). Meanwhile, Au atoms are seen occasionally to segregate and form an Au enriched atomic column (numbered 0 in Fig. 2(d)) at the other end of the Cu3Au segment, thereby resulting in the lateral growth of the Cu3Au nucleus by one additional unit cell towards the bottom-left corner direction (Fig. 2(d)). However, the lateral growth of the Cu3Au(110)-(2×1) surface alloy occurs dominantly toward the upperright corner direction through the decay of the (100) planes, as seen in Figs. 2(e, f). Fig. 2(e) shows that column 5 becomes Au-rich again, as indicated by its increased dark contrast. Fig. 2(e) also shows that the decay of atomic column 7 results in the lateral growth of the Cu3Au(110)(2×1) segment by another unit-cell length (formed by atom columns 3, 5, and 6). In the same way as seen in Fig. 2(c), Au atoms continue to segregate and form a new small (100)-type facet (numbered 8, 9, and 10 in Fig. 2(e)) adjacent to the growth front of the Cu3Au(110)-(2×1) segment. The (100) facet evolves into a new Cu3Au(110)-(2×1) unit cell with the decay of column 10. This process of Au segregation in the corner area to form the (100)-type terrace and the subsequent decay of the (100) terrace repeats itself, leading to the lateral growth of the Cu3Au(110)-(2×1) segment with the constant thickness of the half-unit-cell L12 Cu3Au lattice (Fig. 2(f)). Meanwhile, it is worth noting that the Cu3Au surface alloy grows unidirectionally along the (110) surface toward the corner direction via the layer-by-layer decay of the (100) planes. In contrast, the other end of the Cu3Au(110)-(2×1) segment stays relatively stationary


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and does not propagate over the bulk-terminated (110) surface of the Cu-Au solid solution. This is clearly shown in Fig. 2(f), where the unreconstructed (1×1) surface initially seen in Fig. 2(a) is still visible in the bottom-left corner (see supplementary in-situ TEM movie S1). The stable Cu3Au(110)-(2×1) configuration is always Au terminated, suggesting the essential role of Au in stabilizing the surface structure. Fig. 3 presents in-situ HRTEM images showing surface evolution to the stable Cu3Au(110)-(2×1) configuration out of the Cu-10at.%Au solid solution. Fig. 3(a) corresponds to the beginning of the sequence, where the Cu3Au(110)-(2×1) segment borders with a high-index (410) facet (the blue arrow points to the location where they meet). The blue arrows in Figs. 3(af) point to the same location and serve as a marker to monitor the lateral propagation of the Cu3Au(110)-(2×1) segment. The (410) facet does not present as a stable structure but encounters changes induced by fast attachment and detachment of atoms at the step edges. The blue and red rings in Fig. 3(a) circle the atoms that stay and disappear, respectively, in the next 1.5 s, as shown in Fig. 3(b). The (100) terraces within the (410) facet are observed to decay via the stepedge detachment of the surface atoms (marked by the red rings shown in Fig. 3(a)), similar as shown in Fig. 2. As a result, the (410) unit that borders with the Cu3Au(110)-(2×1) segment transits to a (420) configuration (Fig. 3(b)). The (420)/(410) stepped facet shows structural fluctuations as additional atoms diffuse in and out, and Fig. 3(c) shows an intermediate structure in which the occupation of the atom columns circled by the orange rings results in the formation of a single-unit-cell Cu3Au(110)-(2×1) segment between the (410) and (420) facets (i.e., the (420)/(410) stepped facet evolves into a (420)/(110)/(410) stepped facet). However, the singleunit-cell wide Cu3Au(110)-(2×1) facet is unstable, where the newly attached atoms (circled by the orange rings in Fig. 3(c)) diffuse away, thereby reverting the (420)/(110)/(410) facet to the 7 ACS Paragon Plus Environment


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(420)/(410) configuration, as shown in Fig. 3(d). The lateral growth of the Cu3Au(110)-(2×1) segment occurs on the basis of the (420) facet. As shown in Fig. 3(e), the departure of the four atomic columns (circled by yellow rings in Fig. 3(e)) at the step-edge area of the (420) facet, together with the decay of the (100) terrace (atomic columns circled by red rings in Fig. 3(e)) of the (410) facet, results in the lateral growth of the Cu3Au(110)-(2×1) segment by one-unit-cell length, with the concomitant formation of a new (420) facet to border with the front of the Cu3Au(110)-(2×1) segment. By following the same transformation path that involves the structural fluctuations of the (420)/(410) stepped facets and the decay of the (100) terraces within the stepped facets, the Cu3Au surface alloy is seen to propagate toward the bottom-right corner direction. Fig. 3(f) shows the lateral growth by another unit-cell length of the Cu3Au(110)-(2×1) lattice, where the green arrow points to the new location of the growth front. While the Cu(110) surface does not reconstruct under UHV or reducing conditions, here we find that the L12 Cu3Au(110) surface alloy formed out of the Cu-10%Au solution develops into the (2×1) reconstruction. To understand the role of Au segregation in the (2×1) surface restructuring of the Cu3Au surface alloy, we employ density functional theory (DFT) to evaluate the energetics for the experimentally observed structural transformation (see supporting information for computational details). As shown in Fig. 3, the lateral growth of the Cu3Au(110)(2×1) segment involves the formation of an intermediate (420) configuration despite the complex structural fluctuations in the stepped facets. Therefore, the behavior of the structural evolution of the (420) facet is critical for arriving the final configuration. Fig. 4(a) shows the structure model based on the TEM results of the Cu3Au(110)-(2×1) surface alloy that intersects with a (420) facet, as seen in Fig. 3. The arrangement of Cu and Au atoms within the (420) facet shown in Fig. 4(a) is determined by HRTEM results and image simulations, which show that the (420) 8 ACS Paragon Plus Environment

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facet with alternate columns of Au and Cu atoms at the surface provides the best match with the experimental HRTEM images (see supporting information). As shown in Fig. 3(e), the growth of the Cu3Au surface alloy involves the transformation of the (420) facet to the Cu3Au(110)-(2×1) surface via the departure of four atomic columns (numbered 1-4 in Fig. 4) from the (420) facet. We compare the relative stability of the surface atoms by calculating the vacancy formation energies of the sites located at the (420) facet. The results show that the energy costs for removing atoms 1 and 5 are relatively higher than other atoms (see supporting information), suggesting that the (420) facet is one of the metastable configurations toward transforming to the stable (110)-(2×1) structure. This is consistent with the TEM results (Fig. 3), which show that the (420) surface is indeed one of the intermediate states and is frequently observed during the Cu3Au(110)-(2×1) growth. After the departure of atom 1, further detachment for atoms at position 2, 3, and 4 is facile (the energy cost is relatively low, i.e., 0.02, -0.3, 0.15 eV, respectively), and the resulting surface configurations formed by the departure of any of these atomic columns are not sufficiently stable to be directly captured by the TEM imaging. However, the surface can stabilize again at the next high-energy cost site (i.e., site 5) and this gives rise to the formation of the (2×1) configuration of the Cu3Au(110) surface alloy, which corroborates well with the in-situ TEM observations (Figs. 2 and 3). Although the higher energy cost for removing atom 5 lowers the probability of further structural decay, it is still likely for these surface atoms (e.g., atoms 5 and 7 in Fig. 4(a)) to detach from the Cu3Au(110)-(2×1) surface at the elevated temperature, which would lead to the formation of a local (1×1) segment. However, no such (1×1) segments form out of the (2×1) reconstructed Cu3Au(110) surface, as shown in Figs. 2 and 3. By contrast, this is different from the surface of the disorder Cu-Au solid solution, which is terminated with the (1×1) structure, as



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shown in Fig. 1(a). This suggests that the (2×1) reconstruction is indeed a stable surface configuration for the L12 Cu3Au(110) surface alloy, which is further confirmed from the in-situ TEM imaging of the evolution of the (2×1) reconstructed Cu3Au(110) surface alloy in Figs. 4(be). Instead of detaching by single atoms at the surface, the in-situ TEM observation shown in Figs. 4(b-e) demonstrates that surface atoms detach from the Cu3Au(110)-(2×1) surface in pairs of Cu and Au atoms, i.e., the pair of atoms 7 and 8 shown schematically in Fig. 4(a). The atom columns marked by the colored dots in Figs. 4(b-e) represent the Cu-Au pairs that detach at the different moments, i.e., the red pairs diffuse away at 0 s, the white pairs diffuse away between 0 s and 2.5 s, etc. It can be clearly seen from Figs. 4(b-e) that the surface decays by strictly following the pattern of diatom detachment. As a result of the sequential detachment of Cu and Au atoms in pairs, the (2×1) reconstructed surface is constantly maintained without the formation of the (1×1) configuration. The experimentally observed diatom detachment can be also confirmed from DFT calculations. Using the structure model in Fig. 4(a), the DFT results show that the energy barrier for removing atom 6 from the (1×1) surface (i.e., after atoms 1-5 have been removed) is only 0.07 eV. This is a strong indication that the Cu atom (i.e., atom 6) at the (1×1) surface of the Cu3Au surface alloy becomes highly unstable once losing the protection from the surface Au atom (i.e., atom 5), and is thus prone to diffuse away together with atom 5. The neighboring (2×1) unit, i.e., atoms 7 and 8 shown in Fig. 4, possess the similar energetics as atoms 5 and 6, and tends to decay in pair. For comparison, DFT calculations are also performed with Cu(110) with the same configuration shown in Fig. 4(a). The results show that the removal of atom 5 from the Cu(110)-(2×1) surface is a spontaneous process (the energy cost is -0.33 eV), whereas the further removal of atom 6 is more energetically costly (the energy cost is 0.29 eV) (see 10 ACS Paragon Plus Environment

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supporting information). This confirms that the (1×1) surface is more favored than the (2×1) for Cu(110), consistent with the experimental results as shown in Fig. 1(c). By contrast, the (2×1) reconstructed surface structure develops for the L12 Cu3Au surface alloy because of the favored Cu-Au diatom configuration. In conclusion, we have demonstrated the formation of a half-unit-cell thick L12 Cu3Au(110) surface alloy terminated with the (2×1) surface restructuring. We show that the nucleation and growth of the Cu3Au(110)-(2×1) surface occur via Au surface segregation with the favored Cu-Au diatom configuration. The lateral growth of the Cu3Au(110)-(2×1) surface alloy proceeds via the formation and structural fluctuations of the (420)/(410) stepped facets along with the decay of step-edge atoms of the (100) terraces. The present type of surface restructuring may hold more generally because the prototypes of the basic processes, i.e., thermodynamically driven surface segregation and decay of unstable surface atoms, can be anticipated in multicomponent materials.



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Figures: FIG. 1. HRTEM images of the (110) surface of Cu-10at.%Au and pure Cu at 350°C and 1×10-3 Torr of H2 gas flow. (a) Cu3Au(110)-(2×1) reconstructed surface formed out of the Cu-10at.%Au solid solution (left) and schematic structure model (right), inset is a simulated HRTEM image using the structure model of the (2×1) reconstructed L12 ordered Cu3Au surface alloy. (b) FCC (110)-(1×1) non-reconstructed surface of Cu-10at.%Au solid solution (left) and the schematic structure model (right). (c) (110)-(1×1) surface and structure model of pure Cu.

FIG. 2. Time-sequence HRTEM images (Supplementary in-situ TEM Movie S1) showing the nucleation and growth of the half-unit-cell-thick L12 ordered Cu3Au(110)-(2×1) surface alloy out of the Cu-10at.%Au solution at 350°C and 1×10-3 Torr of H2 gas flow. (a) A corner area at the intersection of the (100) and (110) surfaces, where the (110) surface show a (1×1) structure. (b) Segregation of Au atoms to the corner area, where the dashed white rings mark the Au segregated sites. (c) Formation of a Cu3Au(110)-(2×1) reconstructed surface unit by exposed Aurich atomic columns 1, 2, and 3, meanwhile, Au atoms segregate onto a newly formed (100) terrace (marked by atomic columns 5, 6, and 7). (d) Growth of the Cu3Au(110)-(2×1) segment at the other end via the segregation of an Au column (column 0) on the (1×1) surface. (e, f) Growth of the Cu3Au(110)-(2×1) segment via the preferential Au segregation to the corner area of (100)type terraces and the layer-by-layer decay of the (100) atomic planes toward the corner. The red arrows in (d-f) point to the first nucleated Cu3Au unit shown (c). FIG. 3. Time-sequence HRTEM images (Supplementary in-situ TEM Movie S2) showing the structural transition pathway leading to the growth of the L12 ordered Cu3Au(110)-(2×1) reconstruction at 350°C and 1×10-3 Torr of H2 gas flow. Red and blue rings denote the atoms that remain and diffuse away, respectively, in each next sequence. The blue arrows point to the same position in all the images, which is the end of the (110)-(2×1) segment at the beginning of the sequence. (a) A Cu3Au(110)-(2×1) reconstructed segment intersecting with a (410) segment. (b) Departure of the atomic columns marked by the red circles in (a) results in the formation of a small (420) facet bordered with the (2×1) segment. (c, d) Structure fluctuations in the (420)/(410) region as the atoms denoted by the orange circles in (c) diffuse in and then away. (e, f) Lateral growth of the Cu3Au(110)-(2×1) segment toward the (420)/(410) region with the departure of the atoms denoted by the yellow and red rings. The red and green arrows point to the new front of the (2×1) segment shown in (e) and (f).

FIG. 4. (a) Structure model used for calculating the energy cost for removing surface atoms from the (2×1) reconstructed (110) terrace and the (420) facet of the Cu3Au surface alloy. (b-e) Timesequence HRTEM images (Supplementary in-situ TEM Movie S3) showing the decay of surface atoms as pairs of atomic columns from the Cu3Au(110)-(2×1) reconstructed surface at 350°C and 1×10-3 Torr of H2 gas flow. Red, white, green and purple dots mark the surface atoms that diffuse away within the time interval between 0 and 2.5 s, 2.5 s and 6 s, and 6 s and 8.5 s, respectively. 12 ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information. Supporting movies, the sample preparation and the in-situ TEM experimental procedures, HRTEM simulation details and the DFT calculation methods are available free of charge in the supporting information document. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-SC0001135. J.L. was supported by National Science Foundation via Award CBET-1264940 for the computational work. The authors thank Zhenyu Liu for constructing the atomic model in Fig. 4(a). This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. This work used the computational resources from the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number OCI-1053575.



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19. Lausmaa, J.; Kasemo, B.; Mattsson, H. Surface Spectroscopic Characterization of Titanium Implant Materials. Appl. Surf. Sci. 1990, 44, 133-146. 20. Binnig, G.; Rohrer, H. Scanning Tunneling Microscopy. Surf. Sci. 1983, 126, 236-244. 21. Binnig, G.; Rohrer, H. Scanning Tunneling Microscopy. IBM. J. Res. Dev. 2000, 44, 279. 22. Hansen, M.; Anderko, K.; Salzberg, H. Constitution of Binary Alloys. J. Electrochem.Soc. 1958, 105, 260C-261C. 23. Bracey, C. L.; Ellis, P. R.; Hutchings, G. J. Application of Copper–Gold Alloys in Catalysis: Current Status and Future Perspectives. Chem. Soc. Rev. 2009, 38, 2231-2243. 24. Della Pina, C.; Falletta, E.; Rossi, M. Highly Selective Oxidation of Benzyl Alcohol to Benzaldehyde Catalyzed by Bimetallic Gold–Copper Catalyst. J. Catal. 2008, 260, 384-386. 25. Ferrando, R.; Jellinek, J.; Johnston, R. L. Nanoalloys: from Theory to Applications of Alloy Clusters and Nanoparticles. Chem. Rev. 2008, 108, 845-910. 26. Petkov, V.; Shastri, S.; Shan, S.; Joseph, P.; Luo, J.; Zhong, C.-J.; Nakamura, T.; Herbani, Y.; Sato, S. Resolving Atomic Ordering Differences in Group 11 Nanosized Metals and Binary Alloy Catalysts by Resonant High-Energy X-Ray Diffraction and Computer Simulations. J. Phys. Chem. C 2013, 117, 22131-22141. 27. Vaughan, O. P.; Kyriakou, G.; Macleod, N.; Tikhov, M.; Lambert, R. M. Copper as a Selective Catalyst for the Epoxidation of Propene. J. Catal. 2005, 236, 401-404. 28. Baddorf, A.; Lyo, I. W.; Plummer, E.; Davis, H. Removal of Surface Relaxation of Cu(110) by Hydrogen Adsorption. J. Vac. Sci. Technol. A 1987, 5, 782-786. 29. Jensen, F.; Besenbacher, F.; Lægsgaard, E.; Stensgaard, I. Surface Reconstruction of Cu(110) Induced by Oxygen Chemisorption. Phys. Rev. B 1990, 41, 10233.



FIG. 1. HRTEM images of the (110) surface of Cu-10at.%Au and pure Cu at 350°C and 1×10-3 Torr of H2 gas flow. (a) Cu3Au(110)-(2×1) reconstructed surface formed out of the Cu-10at.%Au solid solution (left) and schematic structure model (right), inset is a simulated HRTEM image using the structure model of the (2×1) reconstructed L12 ordered Cu3Au surface alloy. (b) FCC (110)-(1×1) non-reconstructed surface of Cu-10at.%Au solid solution (left) and the schematic structure model (right). (c) (110)-(1×1) surface and structure model of pure Cu. 284x183mm (200 x 200 DPI)

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FIG. 2. Time-sequence HRTEM images (Supplementary in-situ TEM Movie S1) showing the nucleation and growth of the half-unit-cell-thick L12 ordered Cu3Au(110)-(2×1) surface alloy out of the Cu-10at.%Au solution at 350°C and 1×10-3 Torr of H2 gas flow. (a) A corner area at the intersection of the (100) and (110) surfaces, where the (110) surface show a (1×1) structure. (b) Segregation of Au atoms to the corner area, where the dashed white rings mark the Au segregated sites. (c) Formation of a Cu3Au(110)-(2×1) reconstructed surface unit by exposed Au-rich atomic columns 1, 2, and 3, meanwhile, Au atoms segregate onto a newly formed (100) terrace (marked by atomic columns 5, 6, and 7). (d) Growth of the Cu3Au(110)(2×1) segment at the other end via the segregation of an Au column (column 0) on the (1×1) surface. (e, f) Growth of the Cu3Au(110)-(2×1) segment via the preferential Au segregation to the corner area of (100)type terraces and the layer-by-layer decay of the (100) atomic planes toward the corner. The red arrows in (d-f) point to the first nucleated Cu3Au unit shown (c). 323x190mm (200 x 200 DPI)



FIG. 3. Time-sequence HRTEM images (Supplementary in-situ TEM Movie S2) showing the structural transition pathway leading to the growth of the L12 ordered Cu3Au(110)-(2×1) reconstruction at 350°C and 1×10-3 Torr of H2 gas flow. Red and blue rings denote the atoms that remain and diffuse away, respectively, in each next sequence. The blue arrows point to the same position in all the images, which is the end of the (110)-(2×1) segment at the beginning of the sequence. (a) A Cu3Au(110)-(2×1) reconstructed segment intersecting with a (410) segment. (b) Departure of the atomic columns marked by the red circles in (a) results in the formation of a small (420) facet bordered with the (2×1) segment. (c, d) Structure fluctuations in the (420)/(410) region as the atoms denoted by the orange circles in (c) diffuse in and then away. (e, f) Lateral growth of the Cu3Au(110)-(2×1) segment toward the (420)/(410) region with the departure of the atoms denoted by the yellow and red rings. The red and green arrows point to the new front of the (2×1) segment shown in (e) and (f). 333x216mm (200 x 200 DPI)


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FIG. 4. (a) Structure model used for calculating the energy cost for removing surface atoms from the (2×1) reconstructed (110) terrace and the (420) facet of the Cu3Au surface alloy. (b-e) Time-sequence HRTEM images (Supplementary in-situ TEM Movie S3) showing the decay of surface atoms as pairs of atomic columns from the Cu3Au(110)-(2×1) reconstructed surface at 350°C and 1×10-3 Torr of H2 gas flow. Red, white, green and purple dots mark the surface atoms that diffuse away within the time interval between 0 and 2.5 s, 2.5 s and 6 s, and 6 s and 8.5 s, respectively. 167x234mm (200 x 200 DPI)


Atomically Visualizing Elemental Segregation-Induced Surface

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