Direct epitaxial growth of subsurface Co nanoclusters

A new subsurface growth mode in the Co-Cu system is reported. This mode provides a direct subsurface growth of Co nanoclusters by depositing Co atoms on the Cu(001) surface in a single stage. The resulting subsurface Co nanoclusters are located 2 monolayers (ML) deep below the atomically flat surface of Cu(001). Although these hidden nanoclusters cannot be directly accessed by a scanning tunneling microscopy/spectroscopy (STM/STS) probe, their shape could be deduced using STM/STS via a careful analysis of the local deformation of the Cu(001) surface as well as local variations of surface electron density induced by the subsurface clusters. A strongly asymmetric shape of the nanoclusters is deduced: they are typically 5-10 nm in lateral size but only 2 to 3 ML in thickness. The thickness of the nanoclusters does not evolve significantly under a heat treatment. A simple model is implemented to describe the growth kinetics. The results in this study reveal that intense processes of diffusion, nucleation, and growth take place in a region 1 nm deep, thus defining the near-surface region.

Figure 1

(a) Morphology of the Cu(001) surface ($70\times 70n{m}^2$) after 0.85-ML Co deposition at 650 K. The tunneling set point is (0.4 V, 1 nA). The bright and dark areas are two neighboring atomically flat terraces. Surface depressions are visible as spots on the terraces. (b) The surface cross section along the line shown in (a). The ellipse indicates the same surface depression indicated by the white ellipse in (a). (c) A typical STM image ($18\times 18n{m}^2$) of a surface with a surface depression. The tunneling set point is (0.2 V, 1 nA). (d) The surface differential conductance map corresponding to the area shown in (c) at the same tunneling set point. (e) The surface cross section along the line shown in (c). (f) STM image of an area ($11.25\times 11.25n{m}^2$) with the ringlike ripples in between the depressions. The arrow shows one of the ringlike ripples. The tunneling set point is ($-0.2$ V, 1 nA). (g) A result of an ex situ room temperature magneto-optic Kerr effect measurement.

Figure 2

The surface differential conductance spectra $dI/dV\left(V\right)$ measured with a tunneling set point of ($-1$ V, 1 nA). The spectra are measured on (a) the clean Cu surface (black), (b) depressions (magenta), and Co islands that are (c) 1 ML (red) and (d) 2 ML (blue) thick. The inset shows the corresponding $I\left(V\right)$ spectra. The schematic of the structure of the reference sample is given in the bottom left corner.

Figure 3

A schematic side view of a Co cluster embedded below a Cu substrate, resulting in a depression at the surface with a depth $\delta$. The blue circles represent Co atoms, while the orange ones represent Cu atoms.

Figure 4

Time dependence of the (a) average and (b) maximum depth of the depressions and (c) their surface coverage under a heat treatment at 650 K. The sample was fabricated by deposition of 0.7 ML of Co on a hot Cu(001) surface. The vertical line divides the plots into regions I and II, which correspond to treatment times less than and more than 10 min, respectively. The error bars in (a) and (c) show the standard deviations of the data distribution.

Figure 5

(a) Cu-Normalized [$dI/dV\left(V\right)]/[I\left(V\right)/V$] spectra from various depressions obtained after 0.6- and 0.8- ML Co depositions. The averaged spectrum is shown by black squares. (b) Spectrum (i) is the averaged spectrum from the depressions, the same as in (a). Spectra (ii) and (iii) are two selected spectra from the calibration sample. The schematic picture of the calibration sample is in the top left corner. All spectra were measured with a tunneling set point of ($-1$ V, 1 nA).

Figure 6

(a) The surface coverage and (b) average depth of the depressions vs the nominal coverages $\theta$. Straight lines in (a) show the expected surface-nominal coverage dependencies based on the simplified assumptions given by Eq. (1) for clusters thicknesses of 1 (red), 2 (green), and 3 (blue) ML. The solid squares represent the experimental results. The solid fitted curve in (a) is obtained from the model equation (2) with $n=2$. The dashed line in (b) is a guide to the eyes showing the data tendency. The gray area indicates the experimental regime where no surface depressions were found. The error bars show the standard deviation of the data distribution.

Figure 7

A schematic illustration of the main processes considered in the model described by Eq. (2). The black circles represent the Co atoms, while the orange ones represent the Cu atoms. (a) Start of Co deposition on a hot Cu(001) substrate. (b) Incorporation and accumulation of Co in the near-surface region. (c) Formation of an initially $n$-ML-thick Co cluster. (d) Further growth of the initial clusters, accompanied by the formation of the next layer as well as Co segregation from the initial cluster (blue arrow) and the thicker cluster (red arrow).