Polar EuO(111) on Ir(111): A two-dimensional oxide

Through reactive molecular-beam epitaxy or Eu postoxidation and postannealing, large EuO(111) bilayer islands of high quality and exceptional stability are grown on Ir(111). We use scanning tunneling microscopy, low-energy electron diffraction, magneto-optical Kerr effect measurements, and density functional theory to characterize the properties of this ultrathin polar rare-earth metal oxide in atomistic detail. We analyze the crystallographic properties and their growth dependence, the film morphology including the atomic structure of defects, the mechanisms for reduction in the electrostatic potential related to the film polarity, as well as the work function, thermal stability, and magnetic properties, resulting in a comprehensive picture of this new two-dimensional material.

Figure 1

(a) Top view ball model and (b) cut along the dashed line in (a) of the DFT optimization for a EuO(111) bilayer in 3:4 registry with the Ir(111) substrate. Color scheme: Ir: dark blue; Eu: light blue; O: red. $d_1=0.51\AA ,d_2=3.07\AA$, and $d_3=2.18\AA$ denote the thickness of the EuO bilayer, the height of Eu above Ir, and the Eu-O bond length, respectively. The dense-packed $\langle 1\overline{1}0\rangle$ directions of EuO and Ir are indicated.

Figure 2

Electronic DOS for a EuO(111) bilayer on Ir(111) and bulk EuO, respectively. The total DOS of EuO on Ir is shown in black, the projected DOS of the EuO bilayer is shown in red, and the DOS of bulk EuO is shown in blue.

Figure 3

STM topograph of a 23% BL EuO(111) film grown on Ir(111) under $f_{\mathrm{Eu}}/f_{\mathrm{O}}=1.7$ at $T_{\mathrm{growth}}=720\mathrm{K}$ (image size $320\times 320{\mathrm{nm}}^2$). Terraces partially covered by EuO are labeled (i)–(iv). Inset: area surrounded by the dotted line with a local plane subtraction applied to align the terraces horizontally. The height profile was taken along the path indicated by differently colored arrows.

Figure 4

Schematic side view of EuO(111) bilayer islands at a substrate step edge (light blue: Eu; red: O). The indicated charges correspond to the idealized oxidation states in bulk EuO. The electron density of the substrate (indicated in blue) is smoothed at the step edge, leading to a step edge dipole which attracts the polar EuO bilayer at the ascending step edge and repels it at the descending step edge.

Figure 5

(a) LEED image (76 eV primary electron energy) corresponding to the sample shown in Fig. 3. First-order Ir(111) and EuO(111) spots are exemplarily encircled in blue and green, respectively. Additional superstructure spots are weakly visible and are connected by black lines. (b) Averaged intensity profiles taken in the azimuthal direction over three equivalent spots of Ir (blue dots) and EuO (green squares).

Figure 6

Atomically resolved STM topographs showing EuO bilayer films of different orientations with respect to the substrate. (a) $f_{\mathrm{Eu}}/f_{\mathrm{O}}=1.7$ and $T_{\mathrm{growth}}=620\mathrm{K}$. (b) $f_{\mathrm{Eu}}/f_{\mathrm{O}}=1.7$ and $T_{\mathrm{growth}}=720\mathrm{K}$. (c) $f_{\mathrm{Eu}}/f_{\mathrm{O}}=0.85$ and $T_{\mathrm{growth}}=720\mathrm{K}$. The image size is always $8\times 8{\mathrm{nm}}^2$. The insets show the corresponding Fourier transforms. Spots belonging to the atomic lattice are indicated by green circles and are connected by dashed lines, whereas, the spots highlighted in magenta belong to the superstructure. The partial hexagon in (a) connects the higher-order superstructure spots just as in the LEED image of Fig. 5. The dense-packed $\mathrm{EuO}\langle 1\overline{1}0\rangle$ directions are indicated by white arrows.

Figure 7

Superposing the lattices of EuO and Ir with reciprocal lattice vectors ${\vec{k}}_{\mathrm{EuO}}$ and ${\vec{k}}_{\mathrm{Ir}}$ under an angle ${\varphi }_{\mathrm{EuO},\mathrm{Ir}}$ results in a superstructure with reciprocal lattice vector ${\vec{k}}_{\mathrm{s}}={\vec{k}}_{\mathrm{Ir}}-{\vec{k}}_{\mathrm{EuO}}$, which is rotated by ${\varphi }_{\mathrm{EuO},\mathrm{s}}$ with respect to ${\vec{k}}_{\mathrm{EuO}}$.

Figure 8

Top view ball model for a EuO(111) bilayer island on Ir(111) rotationally misaligned by ${\varphi }_{\mathrm{EuO},\mathrm{Ir}}=1.3^{\circ }$ as in the topograph of Fig. 6. The experimentally determined lattice parameter $a_{\mathrm{EuO}}=3.67\AA$ is used for EuO(111). Color scheme: Ir: dark blue; Eu: light blue; O: red. In the right part, the resulting moiré is emphasized by overlaying the EuO and Ir lattices as black dots. The white arrow shows the dense-packed direction of the moiré superstructure.

Figure 9

(a) STM topograph taken at 35 K of a partial EuO(111) bilayer grown under 70% Eu excess ($130\times 130{\mathrm{nm}}^2$). The upper left corner consists of EuO(111), whereas, the rest of the image is Ir(111) covered by Eu adatom phases of different densities. (b) Zoom into the area marked by a white square in (a) ($50\times 50{\mathrm{nm}}^2$).

Figure 10

(a)–(f) STM topographs of a partial EuO(111) film grown under Eu excess (image size always $40\times 40{\mathrm{nm}}^2$). Brightness and contrast are adjusted for the EuO layers on the lower and upper Ir terraces. During tunneling, the sample was exposed to $1\times {10}^{-8}\mathrm{mbar}{\mathrm{O}}_2$ at 300 K. The ${\mathrm{O}}_2$ exposure (given in langmuirs) increases from (a) 0 L via (b) 2 L, (c) 4 L, (d) 6 L, and (e) 8 L to (f) 10 L. The apparent height of Ir continuously decreases from $+0.4\AA$ to $-1.8\AA$ with respect to the EuO level. (g) and (h) show schematics taken along the lines in (a) and (f), respectively. Color scheme: Ir: dark blue; Eu: light blue; O: red.

Figure 11

$I\left(z\right)$ measurements on Eu-covered Ir (light blue) and EuO (dark green) measured on a 23% BL EuO(111) film grown under $f_{\mathrm{Eu}}/f_{\mathrm{O}}=1.7$ at $T_{\mathrm{growth}}=720\mathrm{K}$. The additional data for O-covered Ir (red) and EuO (light green) are taken on the same sample after 10 L ${\mathrm{O}}_2$ exposure. The solid lines are exponential fits to the data points.

Figure 12

STM topographs showing different types of defects in EuO bilayer films. (a) $f_{\mathrm{Eu}}/f_{\mathrm{O}}=1.7$ and $T_{\mathrm{growth}}=620\mathrm{K}(37\times 37{\mathrm{nm}}^2)$. Inset: point defects (indicated by arrows in the large-scale image) appear as missing superstructure protrusions which are indicated by the white dots ($5\times 5{\mathrm{nm}}^2$). (b) $f_{\mathrm{Eu}}/f_{\mathrm{O}}=0.85$ and $T_{\mathrm{growth}}=720\mathrm{K}(79\times 79{\mathrm{nm}}^2)$. Inset: Zoom into a EuO island ($29\times 29{\mathrm{nm}}^2$). (c) Same growth conditions as for (b). Green and black lines indicate dense-packed superstructure rows on the right- and left-hand sides of the bright stripe, respectively ($8\times 8{\mathrm{nm}}^2$). The $\mathrm{EuO}\langle 1\overline{1}0\rangle$ directions are indicated.

Figure 13

(a) LEED image (54 eV primary electron energy) of a EuO film (23% BL, $f_{\mathrm{Eu}}/f_{\mathrm{O}}=1.7$ and $T_{\mathrm{growth}}=720\mathrm{K}$) after annealing to 1420 K for 60 s. Ir and EuO first-order spots are highlighted in blue and green, respectively. (b) Corresponding STM topograph ($40\times 40{\mathrm{nm}}^2$). The whole image is covered by a closed EuO film except for the small rim close to the descending substrate step edge. The inset ($30\times 30{\mathrm{nm}}^2$) shows the dilute Eu adatom phase on Ir at 35 K.

Figure 14

STM topographs showing a EuO film with 51% BL nominal coverage grown under $f_{\mathrm{Eu}}/f_{\mathrm{O}}=1.7$ at $T_{\mathrm{growth}}=720\mathrm{K}$. (a) Overview over the occurring structures ($160\times 160{\mathrm{nm}}^2$). (b) Eu structure of (a) ($20\times 20{\mathrm{nm}}^2$). (c) EuO(111) structure of (a) ($70\times 70{\mathrm{nm}}^2$). Inset: Self-correlation. (d) Same as (c) ($20\times 20{\mathrm{nm}}^2$). One of the most common triangles with edges parallel to the $\mathrm{EuO}\langle 1\overline{1}0\rangle$ directions is indicated.