Surface reconstruction of Au(001): High-resolution real-space and reciprocal-space inspection

The hexagonal reconstruction of Au(001) has been a model system for the understanding of complex metal surface reconstructions, and it has been intensively studied over almost five decades. Nevertheless, details of the reconstruction regarding the top layer to substrate matching and alignment are not unambiguously identified due to restricted resolution of previously available analytical tools as well as nonuniform sample quality. We therefore quantitatively reinvestigated the system by applying high-resolution real-space and reciprocal-space inspection using scanning tunneling microscopy and spot profile analysis low-energy electron diffraction in analyzing a high-quality Au(001) sample. We show that the Au(001) reconstruction consists of two rotational domains of a commensurate $c(28\times 48)$ superstructure. It results from a Moiré-like buckling of a quasihexagonal top layer which is characterized by lattice vectors having a length of $0.9655a$ and $0.9581a$, respectively, where $a$ is the interatomic distance of Au(001). The former vector runs exactly along [110] or [−110], whereas the latter one deviates by 59.75° from those directions. ${\mathrm{Ar}}^{+}$ ion bombardment at elevated temperatures induces a rotation of the top layers up to angles of ±0.83°. Sample annealing yields a turning back into the initial top layer alignment. Rotation and reorientation proceeds continuously, i.e. all angles between 0 and ±0.83° are observed. Using simple hard sphere models, the main characteristics of the reconstruction are explained. This includes even structural details of the reconstruction at step edges for nonrotated and rotated quasihexagonal top layers.

Figure 1

Hard sphere model of the nonrotated Au(001)-hex layer (atoms in black) in (a) top and (b) side views. Lattice vectors ${\mathbf{a}}_{1,2q}$ and ${\mathbf{a}}_{1,2h}$ of the Au(001) substrate square lattice (atoms in yellow) and the hex layer are drawn as black and red arrows, respectively. Due to the structural mismatch between the hex and the Au(001) substrate square lattice a characteristic row pattern along [110] is formed where the hex atoms reside either in top and bridge ($R$) or in hollow and bridge positions ($V$).

Figure 2

Atomically resolved STM images of the Au(001)-hex (500 pA, 1V) displaying the characteristic reconstruction rows along [110]. Image sizes (a) 5 × 5 ${\mathrm{nm}}^2$ and (b) 24 × 34 ${\mathrm{nm}}^2$. Black rectangles indicate (5 × 20) and (5 × 28) unit cells reported in literature. The real dimension of the unit cell present is sketched in red and correspond to a centered $c$$(2m$ $\times 2n)$ structure. The color code for the apparent STM height between 0 and 60 pm is given at the bottom.

Figure 3

(a) and (b) STM images (500 pA, 1.0 V) of a stepped area of the Au(001)-hex showing three terraces 1, 2, and 3. Image contrast is maximized in (a) for terrace 3 (image size $150\times 108{\mathrm{nm}}^2$) and in (b) for terrace 2. Weak depressions appearing in (a) in a darker contrast display a $c$$(2m$ $\times 2n)$ superstructure as indicated by red diamond and rectangle. From the FFT patterns (insets) a $c(28 \times 48)$ structure has been deduced. For the narrow terrace 2, a slight rotation of the reconstruction rows is obvious which indicates that a $\left(\begin{array}{cc}\hfill 14& \hfill 1\\ \hfill -1& \hfill 5\end{array}\right)$ superstructure is possibly present.

Figure 4

(a) SPA-LEED pattern of the Au(001)-hex taken at 220 eV. Reciprocal lattice vectors ${\mathbf{b}}_{1,2q}$ and ${\mathbf{b}}_{1,2h}$ of the Au(001) substrate square lattice and the hex layer on top, respectively, are indicated. (b) and (c) show the surroundings of the (01)${}_h$ and the (00) spots measured at 220 and 190 eV, respectively. In (c), the blue and red diamonds visualize the unit cells of both hex domains. The red diamondlike network demonstrates for one domain the reciprocal Moiré-like lattice of the quasihexagonal Au(001) reconstruction.

Figure 5

(a) Section of the SPA-LEED pattern of the Au(001)-hex as measured over the (00) and (0-1)${}_q$ spot region of the Au(001) substrate square lattice. LEED energy 235 eV. (b) Same section with corrected spot positions given in gray dots. (c) and (d) Diamondlike networks shown in red and blue visualize the reciprocal Moiré-like lattice of both domains of the quasihexagonal Au(001) reconstruction. The corresponding LEED spots are taken from (b) and indicated in red and blue.

Figure 6

(a) Section of the SPA-LEED pattern of the rotated Au(001)-hex (upper right quadrant) with spots of the Au(001) substrate square lattice [(0,1)${}_q$, (1,1)${}_q$, (1,0)${}_q$] and of the hex [(0,1)${}_h$, (1,0)${}_h$]. (b)–(d) show vicinities of (0,0), (0,1)${}_q$, and (0,1)${}_h$, and (1,0)${}_h$ spots, respectively. For comparison, the unit cell of the nonrotated Au(001)-hex is indicated by a blue diamond in (b). In (e), a line scan measured over the blurring of the (1,0)${}_h$ spot is shown. The dotted curves visualize the spot shape of the sharp (0,1) spot. (a)–(c) LEED energy 220 eV, (d) and (e) 235 eV.

Figure 7

(a) Simulated diffraction pattern of one domain of the nonrotated Au(001)-hex (upper right quadrant). Reciprocal lattice vectors ${\mathbf{b}}_{1,2q}$ and ${\mathbf{b}}_{1,2h}$ of the Au(001) substrate square lattice and the hex layer on top are indicated in black and red, respectively. The surrounding of the (0,0) spot is shown in (b)–(e) for the nonrotated hex and for stages of a stepwise hex rotation. Rotation steps 0.05°, (c) rotation angles 0° to 0.4° and (d) 0 to ±0.85°. (e) Superposition of the simulated pattern of (d) with that of the corresponding other hex domain.

Figure 8

(a) SPA-LEED pattern of the vicinity of the (0,0) spot of the rotated Au(001)-hex and its change during annealing to (b) 870 K, (c) 920 K, and (d) 970 K. LEED energy 220 eV.

Figure 9

(a) STM image of the nonrotated Au(001)-hex with a [110] step (25 × 25 ${\mathrm{nm}}^2$, 50 pA, 1.04 V). (b)–(c) Same STM image but with balanced contrast on both terraces. Equidistant lines in (b, bottom) run on the lower (left) terrace between atomic rows and on the higher (right) terrace on atomic rows. In (c), areas of reconstruction row splitting (encircled) are used for defining relative positions of the $c$(28 × 48) unit cell on both terraces.

Figure 10

(a)–(e) Hard sphere models of the $c$(28 × 48) Au(001)-hex structure visualizing different step situations. (a) and (b) hex layer (atoms in red) on Au(001) substrate square lattice (atoms in gray), attached at the lower step side. Black rectangle indicates the $c$(28 × 48) unit cell. (c)–(e) show additionally the hex layer at the upper step side (atoms in magenta) for (c) independent terrace population and for hex layers with (d) A-type and (e) B-type steps. For explanations, see text.

Figure 11

(a)–(c) Hard sphere model of the $c$(28 × 48) Au(001)-hex with B-type step structure. Same situation as shown in Fig. 10. (b) and (c) visualize the structural situation for atoms in step sites (displayed in yellow) in different step areas as indicated by blue and green squares.

Figure 12

(a) and (b) STM images (50 pA, −0.98 V) and (c)–(e) sphere models of the rotated Au(001)-hex in step vicinity. STM image sizes (a) 45 × 45 ${\mathrm{nm}}^2$ and (b) 18 × 18 ${\mathrm{nm}}^2$. For explanation, see text.