Highly ordered Pd, Fe, and Co clusters on alumina on ${\text{Ni}}_3\text{Al}\left(111\right)$

Template-mediated growth of metals has attracted much interest due to the remarkable magnetic and catalytic properties of clusters in the nanometer range and provides the opportunity to grow clusters with narrow size distributions. We have grown well-ordered Fe and Co clusters on the ultrathin aluminum oxide on ${\text{Ni}}_3\text{Al}\left(111\right)$, a template with a 4.1 nm lattice. The structure of the $\approx 0.5\text{-nm}$-thick oxide film exhibits holes reaching down to the metal substrate at the corners of the $\left(\sqrt{67}\times \sqrt{67}\right)R12.2°$ unit cell. Pd atoms trapped in these corner holes create metallic nucleation sites where Fe as well as Co clusters can nucleate and form a well-ordered hexagonal arrangement on the oxide nanomesh. We have studied these Fe and Co clusters and applied different methods such as scanning tunneling microscopy and surface x-ray diffraction to determine the morphology and crystallography of the clusters. For Fe, we found cluster growth in either bcc[110] or bcc[100] direction, depending on the deposition temperature and for Co we found close-packed planes on top of the clusters and random stacking of close-packed planes. Pd clusters grow with fcc(111) orientation.

Figure 1

STM images taken (a) at 3.1 V/0.1 nA where the net structure appears and (b) at 2.3 V/0.1 nA showing the dot structure. The unit cell is drawn in black with the corner holes as hexagons, triangles mark the threefold sites, and circles mark the defects.

Figure 2

STM images (3.1 V/0.1 nA) of (a) 0.1 nm Fe and (d) 0.06 nm Co deposited at RT, (b) STM image (2.3 V/0.1 nA) of 0.01 nm Fe where Fe clusters are marked with white arrows, and (c) a Fourier transform of the Fe cluster centers in image (a). The cross points of the white grids in (b) and (d) mark the corner holes.

Figure 3

(Color online) (a) Side view of the corner hole of the oxide with a nearby Fe or Co atom on top. (b) Schematic drawing of the potential-energy surface for Fe as well as Co atoms. Empty hexagons symbolize the corner holes, filled hexagons the corner holes filled by three Pd atoms each, triangles are the threefold sites of the unit cell, and circles are the defects (see also Fig. 1).

Figure 4

STM images (3.1 V/0.1 nA) of (a) 0.1 nm Fe and (b) 0.06 nm Co deposited on the Pd-seeded oxide at RT. Cross points of the white grids mark the corner holes. Inset shows an STM image (3.1 V/0.1 nA) of the Pd-seeded oxide. The brightest spots ($\approx 30\mathrm{\%}$ of all corner holes) are attributed to holes with at least four Pd atoms; holes filled by two or three Pd atoms appear comparable to or slightly brighter than the surrounding oxide (Ref. 43).

Figure 5

STM images (3.1 V/0.1 nA) of (a) 0.1 nm Fe and (b) 0.06 nm Co deposited on the Pd-seeded oxide at 470 K. Cross points of the white grids mark the corner holes. In (a), the long-range order is disturbed by domain boundaries of the oxide (e.g., at the right edge of the white grid).

Figure 6

(Color online) (a) Sketch of the beam alignment with respect to the cluster arrangement in real space. In situ GISAXS measurements taken at coverages of (b) $\Theta =0.18\text{nm}$ Co, (c) $\Theta =0.54\text{nm}$ Co, and (d) $\Theta =1\text{nm}$ Co, deposited at 470 K (Pd seeded). The background measured on the Pd-seeded oxide surface prior to deposition has been subtracted leading to some artifacts near the specular beam. Insets are STM images (3.1 V/0.1 nA) taken at equal experimental conditions (50-nm wide each).

Figure 7

(a) Apparent average height vs tunneling voltage of 0.06 nm Fe clusters deposited at RT. Height histograms of (b) the same preparation taken at 3.1 V and (c) of 0.1 nm Fe deposited at 570 K. STM images were taken at 0.1 nA; all clusters grown on the Pd-seeded oxide.

Figure 8

(a) Apparent average height vs tunneling voltage of 0.06 nm Co clusters deposited at 470 K. Height histograms of (b) the same preparation taken at 3.1 V and (c) of 0.1 nm Co deposited at 570 K. STM images were taken at 0.1 nA; all clusters grown on the Pd-seeded oxide.

Figure 9

STM images (3.1 V/0.1 nA) of (a) 2.8 nm Fe and (b) 2.8 nm Co deposited at 470 K (Pd seeded). Images are high-pass filtered to reveal the morphology of the clusters. Insets show STM images (3-nm wide) with atomic resolution from facets on top of flat clusters visible in the large STM images.

Figure 10

(a) Map of the reciprocal-space projected onto the surface plane, with Miller indices of the ${\text{Ni}}_3\text{Al}\left(111\right)$ surface cell. The oxide cell and the Pd as well as the Co hexagonal unit cells are also shown. The radial short thick lines at the Pd ${\left(11\right)}_{\text{hex}}$ and the Co ${\left(10\right)}_{\text{hex}}$ reciprocal-lattice points mark the positions of the scans shown below. (b) Radial XRD scan across the Pd ${\left(11\right)}_{\text{hex}}$ Bragg peak (at $L=0.06$) and (c) radial scan across the Co ${\left(10\right)}_{\text{hex}}$ Bragg peak (at $L=1.5$).

Figure 11

Processed STM image of Pd clusters deposited at RT $\left(\Theta =0.23\text{nm}\right)$. A close-packed direction of the substrate has been identified on a large scale STM image from the same experiment where both oxide orientations were present and labeled $\left[\overline{1}01\right]$. The typical orientation of the Pd clusters is indicated.

Figure 12

(2 0) rod of the surface after deposition of 2 nm Co clusters deposited at 470 K (Pd seeded). (a) With the Bragg peak of ${\text{Ni}}_3\text{Al}\left(111\right)$ at $q_{\perp }=10.2{\text{nm}}^{-1}$ $\left(L=1\right)$ fitted by a pseudo-Voigt function (dashed) and (b) after Bragg peak subtraction. This corresponds to the ${\left(10\right)}_{\text{hex}}$ rod of a Co hcp (0001) or fcc(111) surface. The peak is fit for random stacking of close-packed planes.