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Single-domain Ca-Si(111)3×1 Reconstruction

3×1 or 3×2 Structure?

Basic considerations suggest that Ca-Si(111)3×1 reconstruction is an anisotropic surface structure that could exhibit one-dimensional (1D) behavior. The quotation marks around "3×1" assignment, however, indicate a controversy regarding the actual atomic arrangement. Almost all studies with diffraction techniques (LEED, RHEED) produce a typical 3×1 pattern. STM observations, on the other hand, indicate a 3×2 arrangement, which agrees with photoemission results. In fact, a structural model of a 3×2 reconstruction in a similar Ba-induced system suggests the most elegant way to reconsile the various experimental observations.

STM Image of Atomic Chains

An STM image of the rows of the Ca-Si(111)3×1 reconstruction shows a "zig-zag" pattern typical for other 3×1 reconstructions on Si(111), most of which are induced by alkali metals.

Atomic resolution STM image of Ca-Si(111)3×1.

9×3 nm2 filled-states STM image, experiment in collaboration with F. M. Leibsle (UMKC).

Structural Models of Zig-zag Chains

Two types of structures are proposed for these zig-zag chains. First model (left image below) associates the observed bright protrusions with actual atoms (or atomic orbitals) arranged in a zig-zag manner. Second type of models (right image below) assumes a chain of ring-like structures instead, and associates the bright spots with two edges of the rings. Zig-zag atomic chain model. Model with ring structures.

Grey circles in the above images represent the protrusions seen in STM images, unit cells for the 3×1 structures are outlined with thick dashed lines.

Chains of Atoms or Rings?

Even basic considerations, e.g., those of typical bond lengths, are not very consistent with a simple zig-zag chain of atoms. In contrast, the ring-like model tends to explain the observed STM images well. Another strong supporting evidence for the ring-like structural element is the observation of the edge row features. Basically, rows on the boundaries of the 3×1 domains are of bright-dark-bright structure consistent with the ring-like structural units, but inconsistent with a double-wide-bright-dark structure expected for a chain-like arrangement.

Diagram of the edge-row features.

Single-domain 3×1 Reconstruction

Before one sets out to do an angle-resolved PES measurement, uniformity of the sample structure across a macroscopic scale needs to be ensured. Looking for 1D signatures is also much easier if the reconstruction is single-domain, i.e., all the rows are along the same direction. As seen below, using vicinal silicon substrates produces the desired high-quality single-domain Ca-Si(111)3×1 samples.

Single-domain clean Si(111)7×7. Single-domain Ca-Si(111)3×1.

Photoelectron Spectroscopy

Of course the prize in the hunt for 1D surface structures is to find a metallic surface state that has 1D character. Alkali-induced 3×1 reconstructions have been always found to be semiconducting. If the structure of a 3×1 unit cell remains the same, then from the simple principles of electron counting, the exchange of a monovalent alkali atom by a divalent Ca should change the count from even to odd and the system from semiconducting to metallic.

Unfortunately, excellent samples notwithstanding, no 1D or even just a metallic state is observed for this system, as shown in the photoemission data below. The absence of a metallic state was a real mystery initially, because the electron count principles can not be violated.

Ca-Si(111)3×1 PES Results.

Of course, the electron count is not violated in Ca-Si(111)3×1. Rather, the structure of a "3×1" unit cell is changed. In fact, a metal atom is only present in every one of the 3×1 cells, making a 3×2 structure and producing an even electron count. More details can be found in Petrovykh et al., Surface Science 512, 269 (2002) and in other publications.

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