Si Nanoribbons on Ag(110) Studied by Grazing-Incidence X-Ray Diffraction, Scanning Tunneling Microscopy, and Density-Functional Theory: Evidence of a Pentamer Chain Structure

We report a combined grazing incidence x-ray diffraction (GIXD), scanning tunneling microscopy (STM), and density-functional theory (DFT) study which clearly elucidates the atomic structure of the Si nanoribbons grown on the missing-row reconstructed Ag(110) surface. Our study allows us to discriminate between the theoretical models published in the literature, including the most stable atomic configurations and those based on a missing-row reconstructed Ag(110) surface. GIXD measurements unambiguously validate the pentamer model grown on the reconstructed surface, obtained from DFT. This pentamer atomistic model accurately matches the high-resolution STM images of the Si nanoribbons adsorbed on Ag(110). Our study closes the long-debated atomic structure of the Si nanoribbons grown on Ag(110) and definitively excludes a honeycomb structure similar to that of freestanding silicene.

Figure 1

Schematic models of 1.6-nm-wide $\mathrm{Ag}(110)\text{−}\mathrm{Si}$ nanoribbons and their simulated STM images.

Figure 2

Computed DFT-LDA formation energies for the structural models shown in Fig. 1, plotted with respect to the deviation of the Si chemical potential from its bulk value. Corresponding results using generalized gradient approximation-van der Waals are shown in Fig. S1 [31].

Figure 3

STM image ($9\times 4.5{\mathrm{nm}}^2$) of Si DNRs below 1 ML. The high resolution allows us to visualize both the Ag atoms (left part) of the (110) substrate and the protrusion of the DNRs (right part). Both pentamer and zigzag B models match the STM image. Si adatoms (yellow circles) appear as protrusions in the STM images whereas Si atoms in the missing rows (green circles) are not imaged. $I=50\mathrm{nA}$, $V_{\text{sample}}=100\mathrm{mV}$. The high tunneling current used to resolve the atomic structure of the Ag(110) substrate induced a partial alteration of the Si nanoribbons. The inset in the bottom-left corner shows two unaltered Si DNRs imaged at a lower tunneling current [($7.5\times 4.5{\mathrm{nm}}^2$), $I=480\mathrm{pA}$, $V_{\text{sample}}=40\mathrm{mV}$]. See also Fig. S2(b) [31].

Figure 4

Comparison between experimental (blue dots) and simulated structure factors along various rods, and for in-plane structure factors. Red dotted line: raw DFT results (PBE, 13-layer thick slabs) for the zigzag $B$ model. Green dashed line: raw DFT results for the pentamer model. Black continuous line and black crosses: after refinement of the atomic positions for the pentamer model.