Atom-wide Co wires on Cu(775) at room temperature

We report on a surface phase of the Co-vicinal-Cu(111) system which exhibits self-assembled uniform Co quantum wires that are stable at 300 K. Scanning tunneling microscopy (STM)-imaging measurements show that wires will self-assemble within a narrow range of Co coverage and, within this range, the wires increase in length as coverage is increased. The STM images show that the wires form along the leading edge of the step rise, differentiating it from previously theoretically predicted atomic-wire phases. The formation of relatively long laterally unencapsulated one- and two-atom wires also differentiates it from past experimentally observed step-island formation. Furthermore, our experiments also show directly that the Co wires coexist with another Co phase that had been previously predicted for growth on Cu(111). Our observations allow us to comment on the formation kinetics of the atomic-wire phase and on the fit of our data to a recently developed lattice-gas model.

Figure 1

(Color online) STM images of Cu(775) at RT, taken at $V_{\text{sample}}=0.5\text{V}$ and $I=150\text{pA}$; the images are $dz/dx$ (i.e., derivative). (a) $100\times 87{\text{nm}}^2$ scan showing relative uniformity of step spacing. (b) Atomic-resolution scan of steps showing frizz. Inset, topographical closeup of step terrace clearly revealing the (111) surface.

Figure 2

(Color online) STM derivative images of Cu(775) at RT after deposition of Co. Blue-colored (darker colored) circles denote Co atoms and gold-colored (lighter colored) circles denote Cu atoms. The black arrows in the topographical profiles correspond to positions indicated by white arrows in the images. (a) Scan showing one-atom-wide Co wires; frizz is seen on a clean step edge at the left of the image. The inset is a topographical profile along the direction of the corresponding white line in the image. Coverage is 0.09 ML. (b) Scan showing two-atom-wide Co wires, as well as one-atom-wide wires and clean steps. A plateau, denoted by arrows, is seen for the two-atom-wide wire scan; in this image, the plateau is not seen for one-atom-wide wires presumably due to finite tip radius and the narrowness of the wires; however, the lack of frizz is a strong indication of Co at the step edge. Coverage is 0.12 ML. (c) Profile for white line in (b). The gray dashed line serves merely as a step guide for the eyes and does not denote the exact position of the steps. (d) Scan showing terrace-embedded Co. Coverage is 0.12 ML. (e) Embedded Co appears as depressions, as indicated by the arrow, in the step terrace.

Figure 3

Distribution of one-atom-wide Co wire lengths. The smooth gray curves denote a fit based on a one-dimensional lattice-gas model; see discussion in text. At a coverage of 0.09 ML (circles), the average wire length is a $\sim 27$ atoms. As the coverage is increased to 0.12 ML (squares), the average wire length increases to $\sim 40$ atoms, indicating that wire length increases with coverage. This result is in accord with a growth mechanism, in which the Co atoms move facilely along the step edge until encountering a wire end, to which the Co atom attaches. Note that in comparison to the typically reported widths of nanometer-scale islands at step edges (5 nm or $\sim 20$ atoms), the one- and two-atom (not shown above) wires have grown to much longer length scales.

Figure 4

(Color online) Different Co growth phases develop with increasing coverage. Top panel (i): A derivative image shows that at 0.12 ML, four different “phases” exist: (a) frizz, (b) one-atom-wide wire, (c) two-atom-wide wire, and (d) terrace-embedded Co. See details in Fig. 2b for the single-atom-wide wires. Bottom panel (ii): Wire-length-to-step-length ratio versus coverage. “Ideal” refers to calculated ratio for growth of only one-atom-wide wires on a perfect seven-atom-row stepped surface. The falloff in the one-atom-wire ratio above 0.09 ML is due to two-atom-wide wire growth, terrace nucleation, and terrace-width variations; without these components, the one-atom-wire step ratio would follow that of ideal wire growth.

Figure 5

(Color online) Self-assembly at 0.09 ML Co coverage. This STM image ($d^{2}z/d{x}^2$, i.e., second derivative), acquired at RT, clearly shows the presence of self-assembled one-atom-wide Co wires along with bare Cu(111) steps. The one-atom-wide wires appear as straight sharp double lines at the step edge. Frizz, on the other hand, causes the bare Cu(111) step edge to appear irregular. The positions of the self-assembled wires are relatively random, indicating freedom from any substrate-mediated reconstruction or discommensuration. Note: the appearance of broken/shifted wires is merely due to tip change.