Formation of dysprosium carbide on the graphite (0001) surface

Using scanning tunneling microscopy, we characterize a surface carbide that forms when Dy is deposited on the basal plane of graphite. To form carbide islands on terraces, Dy is first deposited at 650–800 K, which forms large metallic islands. Upon annealing at 1000 K, these clusters convert to carbide. Deposition directly at 1000 K is ineffective because nucleation on terraces is inhibited. Reaction is signaled by the fact that each carbide cluster is partially or totally surrounded by an etch pit. The etch pit is one carbon layer deep for most carbide clusters. Carbide clusters are also identifiable by striations on their surfaces. Based on mass balance, and assuming that only the surface layer of carbon is involved in the reaction, the carbide has stoichiometry $\mathrm{D}{\mathrm{y}}_2\mathrm{C}$. This is Dy-rich compared with the most common bulk carbide $\mathrm{Dy}{\mathrm{C}}_2$, which may reflect limited surface carbon transport to the carbide.

Figure 1

Representative STM images of Dy after deposition on pristine graphite at various temperatures, and line profiles associated with the horizontal bars in the STM images. (a)–(d) 300 K and $2.55\pm 0.17\mathrm{ML}$. (e)–(h) 650 K and $2.50\pm 0.51\mathrm{ML}$. (g) An area that is dense with graphite step edges. (i)–(l) 1000 K; all areas are dense with graphite step edges. Inset in (j) shows enlargement of the white square on the step.

Figure 2

STM images of Dy deposited at 650 K on pristine graphite (a)–(f), Dy carbide islands that form after annealing at 1000 K for 10 min (g)–(l), and their respective derivatized images (m)–(r). These images are obtained in two separate experiments, represented in (a)–(d), (g)–(j), and (m)–(p) and (e) and (f), (k) and (l), and (q) and (r), respectively. Note that images in the second experiment are at twofold higher magnification.

Figure 3

Height histograms showing the depth of rings around islands after annealing at 1000 K. Height histograms are generated from the zoomed-in areas of the ring and island as shown by the white boxes. The two peaks marked by red lines correspond to one layer of graphene, which is 0.335 nm in bulk graphite. Line profiles of carbide islands along the black lines are shown in the right panel.

Figure 4

(a) and (b) Dy carbide islands with small regions that are two C layers deep in the depressed ring as outlined by white rectangles. (a′) and (b′) Enlargement of the areas in the white rectangles, with line profiles (c) and (d) indicating a single C layer (solid lines) and double C layers (dashed lines). Regions such as these are observed around a small fraction of carbide islands $(\sim 10\%)$.

Figure 5

Height histograms of (a) Dy metal islands (56 islands, average height is $2.30\pm 0.33\mathrm{nm})$ and (b) Dy carbide islands (58 islands, average height is $2.81\pm 0.33\mathrm{nm})$. Annealing time is 70 min total for this experiment. Bin size for both histograms is 0.15 nm.

Figure 6

Derivatized STM images of carbide islands. (a) A carbide island at low magnification. The white box is enlarged and shown in (a′) to reveal the fine structure. (b) Another carbide island. The white box is enlarged in (b′) and the black box in (b″).

Figure 7

Bulk structure and two bulk terminations of $\mathrm{D}{\mathrm{y}}_2\mathrm{C}$. C atoms are shown as black dots. Dy atoms are shown in multicolor, with atoms closest to the viewer colored bright yellow, and deeper atoms colored progressively darker. (a) Bulk unit cell. (b) Dy-terminated (001) surface. Black rhombus is the surface unit cell. (c) Dy-terminated (1/2,0,$\overline{2})$ surface, which exhibits a row structure parallel to the $b$ axis. Black rectangle is the surface unit cell.