Growth of ultrathin cobalt oxide films on Pt(111)

Cobalt surface oxides were grown on Pt(111) by depositing Co and dosing with molecular oxygen at temperatures ranging between 300 and 740 K. Oxidation of 1 monolayer (ML) Co results in a two-dimensional (2D) moiré structure, observed using both low-energy electron diffraction and scanning tunneling microscopy and interpreted as a polar (oxygen terminated) CoO(111) atomic bilayer. It is expanded by 2.7 ± 0.6% in the surface plane with respect to bulk CoO. An almost-flawless moiré pattern is obtained after a final step of annealing at 740 K in oxygen. Insufficient oxidation leads to defects in the moiré pattern, consisting of triangular dislocation loops of different sizes; the smaller ones occupy half of the moiré cell. Low-temperature annealing (450 K) can be used to create a zigzag phase, which is mainly observed in 1-ML-thick areas after several cycles of Co deposition (1 ML each) and oxidation at 10−7 mbar. The CoO films obtained by deposition/oxidation cycles exhibit Stranski-Krastanov growth; the structure of the 2D layer between the islands depends on the thermal treatment. It exhibits the moiré pattern after annealing at 740 K, whereas the zigzag phase was observed after low-temperature annealing. The second monolayer consists of a moiré pattern different from that of the first layer, presumably a wurtzite-like structure. Above the third layer, we observe only small three-dimensional islands, which exhibit a bandgap. We have also studied oxidation of surface alloys obtained by depositing Co and annealing. On these surfaces, we found a quasi-(3 × 3) reconstruction. Structure models are presented for all phases observed, and we argue that some of the moirélike structures might be useful as templates for metal cluster growth.

Figure 1

STM images ($V_s$ $=$ $+$0.5 V, $I_t$ $=$ 0.1 nA) of 1 ML Co/Pt(111) deposited at RT and: (a) dosed with 50 L O${}_2$; (b) annealed and dosed with O${}_2$ at 570 K and then annealed under oxygen at 740 K. The arrows in (a) indicate two dendrites. Image processing was used in (b) to reduce the step height, making the corrugation on the terraces more apparent.

Figure 2

LEED pattern of the moiré shown in Fig. 1b ($E$ $=$ 120 eV).

Figure 3

STM images of the CoO/Pt(111) moiré structure. (a)–(e) The same region is imaged with different sample bias ($I_t$ $=$ 1 nA for all). Regions with Co in fcc, hcp, and on-top sites of the Pt substrate atoms are identified by comparison with images of the triangular stacking faults. (a)–(e) were obtained on the 2D layer of a sample obtained with three cycles of 1 ML Co deposition and O${}_2$ exposure at 570 K, followed by annealing at 740 K in 1 × 10${}^{-8}$ mbar O${}_2$ (see Sec. III D); circles mark the same three defects in (a)–(e). (f) Dislocation triangles (1 ML Co, 50 L O${}_2$ at 620 K, and then annealing at 740 K; $I_t$ $=$ 0.1 nA).

Figure 4

(a) 18 × 6 nm${}^2$ STM image of 1 ML Co/Pt(111) annealed and exposed to O${}_2$ at 640 K ($V_s$ $=$ −1 mV, $I_t$ $=$ 2 nA). (b)–(e) Ball models of the unperturbed moiré structure, and different types of triangular dislocation loops; we argue that (c) is the correct model. Oxygen atoms at the dislocations that are in asymmetric twofold or fourfold hollow sites are shown as dark (brown) filled circles.

Figure 5

(a) STM image (28 × 12 nm${}^2$, $V_s$ $=$ 10 mV, $I_t$ $=$ 1 nA) of the zigzag structure, obtained in a region between the islands after two cycles of Co deposition (1 ML) and O${}_2$ dosing (50 L) at 450 K. (b) Ball model of the structure. Oxygen atoms at the zigzag dislocation line (shown darker) are in asymmetric fourfold hollow sites.

Figure 6

STM images of thicker films. (a) Three cycles of Co deposition (1 ML) and O${}_2$ dosing (50 L) at 570 K, followed by annealing in oxygen at 740 K at the end ($V_s$ $=$ 1.2 V, $I_t$ $=$ 0.1 nA). (b) The same procedure at $T$ $=$ 450 K ($V_s$ $=$ −1 V, $I_t$ $=$ 0.1 nA). In both cases, the first monolayer is deposited at RT. (c) The second-layer moiré structure after depositing 1 ML Co at RT and annealing and dosing O${}_2$ at 570 K and then 740 K. (d) Ball model of wurtzite-terminated cobalt oxide.

Figure 7

STS measured on the two-layer oxide and the 3D islands of the preparation shown in Fig. 6b.

Figure 8

(a) STM images of 6 ML Co/Pt(111) annealed at 740 K and then oxidized at 640 K ($V_s$ $=$ $+$1 V, $I_t$ $=$ 0.1 nA). The insets on the left and right sides show a zoom in the dark triangles ($V_s$ $=$ $+$50 mV, $I_t$ $=$ 0.1 nA) and in the matrix ($V_s$ $=$ −1 mV, $I_t$ $=$ 2.5 nA), respectively. (b) STM images of 3 ML Co/Pt(111) annealed and dosed at 640 K ($V_s$ $=$ −1 V, $I_t$ $=$ 0.1 nA). Image processing was used for better visibility of low-corrugation features on terraces, causing the noisy appearance of the steps. The inset shows a high-resolution image ($V_s$ $=$ 2 mV, $I_t$ $=$ 1 nA).

Figure 9

The quasi-(3 × 3) phase: (a) STM image ($V_s$ $=$ −2 mV, $I_t$ $=$ 0.1 nA). (b) LEED pattern at 65 eV with the substrate spots marked by circles and a large hexagon showing the expansion in the [0$\overline{1}$1] direction. (c) Structure model. The superstructure cell is marked in all frames.