Growth of Fe on Au(111)
Considerable interest exists in the physical properties of ultra-thin metal
films. Particular interest in magnetic systems has resulted from novel magnetic
behavior predicted and observed in the few monolayer regime. In analogy to
band-gap engineering in strained layer epitaxy in semiconductors, one can
exploit the dependence of the exchange energy on lattice expansion in epitaxial
metal films to produce new magnetic phases not found to occur naturally. Such
atomic engineering of low dimensional magnetic systems has only recently been
realized.
In the growth of Fe on Au(111) nucleation of triangular Fe islands is observed
to occur at the corners of the herringbone reconstruction of Au(111) surface,
which are imaged with atomic resolution. STM measurements show that the first
layer of Fe on the Au(111) surface is a metastable close-packed fcc phase of
Fe with a 0.288 nm in-plane nearest neighbor distance. The fcc film
growth proceeds in a nonideal layer-by-layer growth mode in which new layers
start nucleating before preceding layers are complete.
Figure 1 shows the initial nucleation of Fe on the Au(111) surface. Fe
islands of about 1-2 nm in diameter are observed to nucleate at the
corners of the herringbone reconstruction of the Au(111) surface. The
superlattice of the reconstruction causes the Fe to nucleate in an ordered
array of islands with a spacing of about 12.5 nm between rows of islands
running along the [121] direction (see arrows in Fig. 1) and 6.3 nm
between islands in these rows. The preferential nucleation results from the
structural imperfections at the bulged corners of the herringbone
reconstruction.
Figure 1: STM images of 0.1 ML of Fe on Au(111). The grey scale cycles
twice through the image to enhance contrast. (a) 80x56 nm image,
(b) 29x29 nm image. The tunneling current was 0.5 nA and the
sample bias was -1.0 and -1.5 V for (a) and (b), respectively.
The Au(111) reconstruction consists of partial dislocation ridges resulting from the uniaxial 4.2% contraction along the <110> directions, where there are 23 atoms for every 22 sites resulting in a 23x3 unit cell. These ridges result from atoms in bridge sites (at elevated positions) and separate regions where atoms are in the hollows sites of fcc (ABC) and hcp (ABA) stacking. The double ridges are periodic with a period of 6.3 nm; the larger spacing between individual ridges corresponds to regions of fcc stacking (more energetically favored) while the narrower regions correspond to hcp stacking (see Fig. 2). On large enough terraces the reconstruction forms a superstructure where the ridges alternate by 120° along <112> directions, as shown in Fig. 1. The elbows in one of the ridges are bulged. These special corners result from strain causing a surface dislocation at the corner (Fig. 2), which can be inferred by examining the Burgers vectors and surface topology when two ridges made up of different orientation join.
Figure 2: STM images of the clean Au(111) surface showing a closeup
of the bulged herringbone corner. (a) 11x7 nm image showing the
transition between fcc and hcp stacking. The average corrugation is 3 pm
obtained at a tunneling current of 0.5 nA and sample bias of -0.9 V.
(b) A zoomed-in portion of the image in (a), 6.6x5.2 nm. The white
lines align the atomic rows across the herringbone corner with one extra column
appearing in the hcp region indicated by the arrow.
Figure 3 shows the atomic structure of a 1 nm triangular Fe island, which intersects multiple dislocation ridges. Fig. 3(a) shows the normal STM topograph, where maxima associated with individual Fe atoms are seen. The edge atoms surrounding the island have a lower contour; a lower edge charge density may result from the lower coordination of these atoms. Figure 3(b) shows an image of the same Fe island, enhanced by displaying the local curvature of the image. The complete structure of the island is observed overlaid with an fcc(111) lattice net with a nearest neighbor distance of 0.288 nm. The agreement of the atomic positions with this lattice shows that the first layer of Fe is pseudomorphic with the Au(111) lattice with a nearest neighbor distance of 0.288 nm. The close-packed structure for Fe represents a metastable phase of room temperature Fe that does not occur naturally; normally the room temperature structure is the a-Fe bcc phase. By analyzing the rows of atoms in data at higher coverages where three layers are imaged, the stacking is determined to be fcc. The structure in Fig. 3(b) is expanded by 12% compared to the 0.257 nm in-plane nearest neighbor spacing of g-Fe, which exists in bulk at higher temperature (910-1400° C). Larger atom spacing favors ferromagnetism and increased magnetic moments suggesting that novel magnetic properties might be expected from such films of Fe on Au(111).
Figure 3: STM image of a single monolayer Fe island on the Au(111)
surface. (a) STM topograph displayed as a 3D rendered solid obtained at a
tunneling current of 0.5 nA and sample bias of -0.2 V. (b) Same
image as (a) but with grey scale keyed to the negative of the surface curvature.
The fcc(111) lattice net is shown overlaid on the image with a nearest neighbor
lattice spacing of 0.288 nm.
Microscopic Aspects of the Initial Growth of
Metastable fcc Iron on Au(111)
The Growth of Iron on Iron Whiskers
Homoepitaxial Growth of Iron and a Real Space
View of Reflection-High-Energy-Electron Diffraction
Joseph A. Stroscio
Daniel T. Pierce
Robert J. Celotta
Robert Dragoset - NIST
Phillip First - Georgia Institute of Technology
Supported in part by the Office of Naval Research
Online: May 1996
Last Updated: February 2008
egpwebmaster@nist.gov