![]() Notably, we found that the SCCF sample maintained its ultraflat and pristine surface even after more than a year of air exposure (Fig. We can discriminate this remarkable ultraflat surface structure of the SCCF from that of the conventional polycrystalline Cu thin film (PCCF) and the Cu(111) surface of a bulk Cu single crystal with a notable oxidized copper layer on top of them (Extended Data Fig. Annular dark-field (ADF) and annular bright-field (ABF) STEM images of the SCCF surface are complementary to the HRTEM observation results (Fig. (Fig.1d), 1d), it is evident that the Cu surface is undistorted and ultraflat, and has the same structure as bulk Cu. By comparing the layer spacings of the (111) stacking planes ( d (111) = 0.21 nm) between the simulated and experimental images (Fig. The simulated HRTEM image using an amorphous carbon/flat copper surface model matches well with the experimental HRTEM image (Fig. This means that the SCCF has a nearly perfect atomic structure up to its outermost surface layer without any structural defects, such as vacancies or dislocations. The resulting strain field maps ( E xx and E yy) clearly show that no noticeable changein lattice strain is observed throughout the entire surface region. To examine the local strain behaviour near the surface region, lattice distortions along the in-plane ( x) and out-of-plane ( y) directions relative to the inside of the SCCF were measured by the GPA technique (Fig. It is remarkable that the outermost copper surface layer has the same atomic configuration as the interior copper without evidence of surface relaxation or structural changes by surface oxidation, even at the step-edge positions. Typical multi-atomic step edges and intrinsic defects such as grain boundaries and stacking faults are rarely detected. ![]() 1a, e) show that the copper film grew along the direction, thus creating an exposed surface (111) plane with mono-atomic step-edge structures. ![]() Fig.1 1 for a large-scale characterization) are examined using high-resolution (scanning) transmission electron microscopy (HR(S)TEM) combined with geometrical phase analysis (GPA) 17, 18 (Fig. The surface and structural characteristics of a 110-nm-thick SCCF with an ultraflat surface (see Extended Data Fig. The implication is that the atomically flat surface of the SCCF shows oxidation-resistant properties owing to the high energy barrier for oxygen infiltration and self-regulation owing to high oxygen coverages. Theoretical calculations show the atomic-scale oxidation-resistant mechanism of the flat copper surface from the perspective of the likely pathways for oxygen atoms into the viable structures of the copper surface with a discovery of the self-regulated protection layer at elevated oxygen coverages. In this regard, the close-packed Cu(111) surface is superior to other Cu surfaces 14, 15 and our experimental demonstration thus used a single-crystal Cu(111) film (SCCF) grown by atomic sputtering epitaxy (ASE) 16 to show that a tightly coordinated flat surface can remain semi-permanently stable against oxidation. Given that the step edge is vulnerable to oxidation because surface steps act as the dominant source of Cu adatoms for oxide growth on surface terraces 2, 11, oxidation resistance requires that surface step edges are avoided 6, 12, 13. These combined effects explain the exceptional oxidation resistance of ultraflat Cu surfaces. First-principles calculations confirm that mono-atomic step edges are as impervious to oxygen as flat surfaces and that surface adsorption of O atoms is suppressed once an oxygen face-centred cubic (fcc) surface site coverage of 50% has been reached. Here we report the fabrication of copper thin films that are semi-permanently oxidation resistant because they consist of flat surfaces with only occasional mono-atomic steps. But even though this mechanism explains why single-crystalline copper is more resistant to oxidation than polycrystalline copper, the fact that flat copper surfaces can be free of oxidation has not been explored further. In situ observations have, for example, shown that oxidation involves stepped surfaces: Cu 2O growth occurs on flat surfaces as a result of Cu adatoms detaching from steps and diffusing across terraces 9– 11. ![]() This has prompted numerous studies exploring copper oxidation and possible passivation strategies 8. Oxidation can deteriorate the properties of copper that are critical for its use, particularly in the semiconductor industry and electro-optics applications 1– 7. ![]()
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