The Surface Structure of Cu 2 O(100)

Transcription

1 The Surface Structure of Cu 2 O(100) Supplementary (Supporting) information Markus Soldemo 1, Joakim Halldin Stenlid 2, Zahra Besharat 1, Milad Ghadami Yazdi 1, Anneli Önsten 1, Christofer Leygraf 3, Mats Göthelid 1, Tore Brinck 2,*, Jonas Weissenrieder 1,* 1 KTH Royal Institute of Technology, Material Physics, Kista, Sweden 2 KTH Royal Institute of Technology, Applied Physical Chemistry, Stockholm, Sweden 3 KTH Royal Institute of Technology, Division of Surface and Corrosion Science, Stockholm, Sweden * Corresponding authors: jonas@kth.se and tore@physchem.kth.se S1

2 1. Identification of the (1 1)-spots in the LEED-pattern To identify the Cu 2 O(100) (1 1) spots of the LEED-pattern, the reconstructed (3,0;1,1) surface of Cu 2 O(100) was gently sputtered to remove the long range atomic order on the surface. The preparation results in a (1 1)-pattern reflecting the bulk periodicity of Cu 2 O, shown in Figure S1 (a). The corresponding (3,0;1,1) pattern of Cu 2 O(100) is shown Figure S1(b). To calibrate the LEED we used the well-known Cu 2 O(111) surface as a reference. For Cu 2 O(111), two terminations have been reported: (1 1) and ( 3 3)R The preparation of the Cu 2 O(111) surface to achive the ( 3 3)R30 termination follows the recipe by Önsten et al. 1. Once the ( 3 3)R30 termination had been achieved, the surface was gently sputtered in order to remove the long range atomic order, in the same way as for the Cu 2 O(100) surface. The LEED-patterns for the (1 1) and ( 3 3)R30 terminations of Cu 2 O(111) are shown in Figure S1(c) and Figure S1(d), respectiviely. S2

3 Figure S1: LEED-images taken at electron beam energy of 27.6 ev. The (0,0) spot is positioned behind the electron gun at the center of the screen. (a) Cu 2 O(100) (1 1). (b) Cu 2 O(100) reconstructed. (c) Cu 2 O(111) (1 1). (d) Cu 2 O(111) ( 3 3)R30. The relative distances in the patterns can now be compared since the same sample position and diffractometer settings were used in the recording of all LEED-patterns in Figure S1. The relative unit lengths in reciprocal space for the (100) and (111) unit cells is (111):(100) = 2/ 3. The same ratio is found in the analysis of the LEED-images, confirming that the correct (1 1)-spots on Cu 2 O(100) are indeed identified. 2. Scanning tunneling microscopy Figure S2 show large area STM images of two differently prepared surfaces. In Figure S2(a), the sample was annealed at a temperature of 580 C, resulting in many small 90 rotational domains of the (3,0;1,1) termination on the same terrace separated by domain S3

4 boundaries. In Figure S2(b), the sample was annealed at a temperature of 630 C, resulting in a surface where the two rotational domains are located on separate terraces. To maintain charge neutrality and stoichiometry in the 100 -direction of Cu 2 O, a repeating unit consisting of one oxygen plane and one copper plane is necessary. By STM the step height is measured to 2.3 Å, this is reasonable close to the corresponding bulk distance of such repeating bilayer unit of ~2.14 Å or half the lattice parameter of 4.27 Å 3. Furthermore, it is noticed that the one-dimensional crevices penetrating in upper terraces or wires extending over lower terraces (Figure S2(b)) follow the directions of the reconstruction. Figure S2: Two 90 rotational domains of the reconstructed surface. (a) Different rotational domains on the same terrace with many domain boundaries. V = 2.59 V, I = 0.28 na. (b) Different rotational domains on every second terrace. The step height is measured to 2.3 Å. V = V, I = 0.97 na. From LEED it was shown that the c(2 2)-structure and the (3,0;1,1)-structure can coexist on the same surface, see Figure 1(c) in the article. STM also observe this coexistence, see Figure S3. The two structures may form adjacent on the same terrace and not only at macroscopically separated parts of the surface. The domain boundary between the two structures runs along the 011 -direction (or the equivalent perpendicular 011 -direction). The line profile in Figure S3 extending from the c(2 2)-structure in the left part of the image S4

5 to the (3,0;1,1)-structure on the right shows that the corrugation is significantly larger on the reconstructed (3,0;1,1)-structure than on the c(2 2)-structure. Figure S3: V = 2.1 V, I = 96 pa. c(2 2) on the left and (3,0;1,1) on the right. The line profile shows that the reconstructed (3,0;1,1) is more corrugated than c(2 2). In addition to the ordered surface structures reported above, formation of an unordered surface structure at specific sample preparation conditions is observed (see Figure S4). The unordered structure primarily coexist with the (3,0;1,1) structure and was only observed after annealing in oxygen at a temperature of 580 C. Increased annealing temperatures in oxygen gas after sputtering did not result in the unordered structure, nor did multiple cycles of annealing in UHV at 580 C. This brings two possible explanations, either the presence of oxygen gas during anneal disturbs the surface and destroys the ordering or the oxygen gas prevents the surface to get ordered during annealing at lower temperatures. S5

6 Figure S4: Wide area STM-images. (a) Multiple domains of the (3,0;1,1)-structure and the non-ordered structure. V= 2.32 V, I = na. (b) The non-ordered structure dominates the surface and initial formation of the ordered (3,0;1,1)-structure. (V = 1.58 V, I = na) 3. Photoelectron spectroscopy The Cu 2p photoelectron spectrum for the (3,0;1,1)-structure, shown in Figure S5, was measured using a photon energy of 1100 ev. The overall line profile and spin-orbit split of 19.8 ev agrees well with earlier reported spectra from Cu 2 O surfaces 4. From DFT, surface core level shift for the (3,0;1,1) surface of 0.1 ev 0.6 ev is suggested and indicated by blue bars in Figure S5. S6

7 Figure S5: Photoelectron spectra of the Cu 2p-region for the (3,0;1,1)-termination of Cu 2 O(100). The measured spin orbit split is 19.8 ev. The blue colored bars correspond to calculated surface core level shifts. 4. Additional reconstruction structures Atomic structure figures of the dimer (1 1) and ridge-dimer c(2 2) structures (Figure S6) and corresponding simulated STM images (Figure S7). Included in Figure S7 is also a simulated STM figure for the Cu-terminated (1x1) structure. Figure S6. Atomic structures of (a) the dimer (1x1) and (b) ridge-dimer c(2x2) surface reconstructions found by DFT. The green areas mark the surface unit cells. S7

8 Figure S7. Simulated STM figures for (a) the copper terminated (1x1) surface structure, (b) dimer (1x1) and (c) ridge-dimer c(2x2). The images were generated by (a) a bias V= -3 V and a tip-surface distance d= 1.9 Å, (b) V=2 V, d= 3.5 Å, and (c) V= 2 V, d= 3.5 Å. 5. Density functional theory coordinates The Cartesian coordinates for the slabs of the optimized structures are supplied below: Low-energy (3,0;1,1) Lattice=" ; ; " Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu S8

9 Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu O O O O S9

10 O O O O O O O O O O O O O O Low-energy c(2 2) Lattice=" ; ; " Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu S10

11 Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu O O O O O O O O O O O O Ridge-dimer c(2 2) Lattice=" ; ; " S11

12 Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu O O O O S12

13 O O O O O O O O O-terminated (1 1) 19 Lattice=" ; ; " Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu O O O O O S13

14 O O Dimer (1 1) Lattice=" ; ; " Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu O O O O O O Cu-terminated (1 1) Lattice=" ; ; " S14

15 Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu Cu O O O O O O References 1. Önsten, A.; Göthelid, M.; Karlsson, U. O., Atomic structure of Cu 2 O(1 1 1). Surf Sci 2009, 603, Schulz, K. H.; Cox, D. F., Photoemission and low-energy-electron-diffraction study of clean and oxygen-dosed Cu 2 O (111) and (100) Surfaces. Phys Rev B 1991, 43, Cuprous Oxide (Cu 2 O) Crystal Structure, Lattice Parameters. In Non-Tetrahedrally Bonded Elements and Binary Compounds I O. Madelung, U. R., M. Schulz, Ed. SpringerMaterials 1998; Vol. 41C S15

16 4. Pauly, N.; Tougaard, S.; Yubero, F., Determination of the Cu 2p primary excitation spectra for Cu, Cu 2 O and CuO. Surf Sci 2014, 620, S16