The fate of the 2 √ 3 × 2 √ 3 R ( 30 ° ) silicene phase on Ag ( 111 )

Silicon atoms deposited on Ag(111) produce various single layer silicene sheets with different buckling patterns and periodicities. Low temperature scanning tunneling microscopy reveals that one of the silicene sheets, the hypothetical √7 × √7 silicene structure, on 2√3 × 2√3 Ag(111), is inherently highly defective and displays no long-range order. Moreover, Auger and photoelectron spectroscopy measurements reveal its sudden death, to end, in a dynamic fating process at ∼300 °C. This result clarifies the real nature of the 2√3 × 2√3R(30°) silicene phase and thus helps to understand the diversity of the silicene sheets grown on Ag(111).

Silicon atoms deposited on Ag(111) produce various single layer silicene sheets with different buckling patterns and periodicities.Low temperature scanning tunneling microscopy reveals that one of the silicene sheets, the hypothetical √ 7 × √ 7 silicene structure, on 2 √ 3 × 2 √ 3 Ag(111), is inherently highly defective and displays no long-range order.Moreover, Auger and photoelectron spectroscopy measurements reveal its sudden death, to end, in a dynamic fating process at ∼300 • C.This result clarifies the real nature of the 2 √ 3 × 2 √ 3R(30 • ) silicene phase and thus helps to understand the diversity of the silicene sheets grown on Ag(111).© 2014 Author(s).All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.[http://dx.doi.org/10.1063/1.4894871]There is a boom in research on silicene, 1 graphene's silicon based cousin, that is, a monolayer of silicon atoms arranged in a buckled two-dimensional (2D) honeycomb lattice, 2 since its first artificial creation on silver (111) 3,4 and zirconium (0001) templates. 5Indeed, this boom is related to the fact that silicene is expected to share the same exotic properties as graphene, and, hence, be similarly a new wonder material, with the advantage of being more easily compatible with the current electronics industry.
Single layer silicene grown on Ag(111), presently the favored substrate, can form various atomic structures with different buckling patterns and periodicities, 6 however, two main ordered phases dominate.The first one is a 3 × 3 reconstructed silicene sheet coinciding with a 4 × 4 Ag(111) supercell, hereafter named 3 × 3/4 × 4, which is synthesized at ∼220 • C and forms a beautiful "flower pattern" in scanning tunneling microscopy (STM) imaging (see Fig. 1(a)), 3 while possibly covering 95% of the total silver single crystal surface area. 7Its nominal coverage ratio in Si atoms, θ Si is 18/16 = 1.125 and the in-plane Si-Si nearest neighbor distance d Si-Si is 0.225 nm, comparable to the calculated value for free standing silicene of 0.21 nm. 2 The second one is a When Si is deposited onto the Ag(111) surface at higher temperatures, typically >250 • C, STM images with a highly defective appearance are observed at room temperature; accompanying (rather poor) low energy electron diffraction (LEED) patterns of a "quasi-pure" 2 √ 3 × 2 √ 3R(30 • ) at 300 • C led Jamgotchian et al. to assign this to a new "2 √ 3 × 2 √ 3R(30 • ) silicene phase" in terms of the silver coincidence supercell. 13Ideally, if really existing, this phase would correspond to a √ 7 × √ 7R(±19.1 • ) reconstructed silicene sheet, which should be described as 3, using our labeling.The nominal Si coverage ratio would be 1.17, a bit higher than those of the 3 × 3/4 × 4 and √ 7 × √ 7/ √ 13 × √ 13 phases, while the Si-Si in-plane distance would be 0.218 nm, that is, somewhat lower than in the previous cases.
However, as mentioned above, only highly defective STM images have been shown until now, [13][14][15] bearing strong similarity with the one we have recorded at low temperature, which is displayed in Fig. 2 along with the inserted image simulated by Guo et al., for a hypothetical ideal 3 silicene structure on Ag(111). 16As indicated by the encircled regions, such a configuration does exist, but only just very locally, and, overall, non periodically, in a disordered way.
As a matter of fact, the assignment to a 2 √ 3 × 2 √ 3R(30 • ) superstructure with respect to the silver substrate surface has been questioned: 17 such a superstructure was not observed by Arafune et al., in a thorough detailed LEED and STM study. 8We note in passing that a highly perfect, "2 √ 3 × 2 √ 3R(30 • ) silicene phase," prematurely claimed to have been grown essentially without any defect between 220 and 250 • C, 18,19 has been proved to be just a delusive phase, i.e., a contrast reversed appearance of the bare Ag(111) surface. 20,21 e 2 We have followed this type of evolution using spectroscopy measurement to give a quantitative evaluation of the peculiar behavior.Typically, we plotted the evolution of the relative intensities of characteristic signals (Si LVV/Ag MNN) in Auger electron spectroscopy measurements during growth of the 2 √ 3 superstructure at 303 • C, as shown in Fig. 3 (we have obtained the same trend upon photoelectron spectroscopy measurements shown as an inset at ∼300 • C).
We first stress that the maximum Si LVV/Ag MNN intensity ratio is obtained after a deposition time of ∼100 min, while it takes just ∼35 min upon growth at 220 • C, using the same impinging flux.This indicates in-diffusion of Si atoms at this too high temperature.Obviously, two kinetic processes compete: the spreading of the 2 √ 3 superstructure and the incorporation of silicon into the silver sample.Next, we emphasize the sudden drop during growth of the intensity ratio, after ∼100 min deposition time, which we relate to the dewetting process mentioned above and illustrate in the top left inset.
At this stage we stress that not only the same periodicity points to the uniqueness of this 2 √ 3 superstructure but also the similar appearance of the STM images, the practically identical growth conditions under which the structure is observed, and, moreover, the same type of fating decay determined in our quantitative AES intensity measurements as the one typically observed in LEED by Moras et al. 17 (top left inset in Fig. 3).
The decay of the intensity ratio was followed during ∼100 additional minutes with the Si cell turned off.Following a rapid drop, the ratio levels off to a small, but non-zero, value, while the 2 √ 3like LEED pattern, as shown in the right down inset of Fig. 3, is still present.This peculiar behavior results again from the competing kinetic processes at stake, namely, dewetting/islanding and indiffusion.However, we believe that in-diffusion does not go far below the surface; instead, a confined sub-surface alloy yielding the 2 √ 3-like LEED pattern, is probably formed, as is, for example, the case of the c(2 × 2) superstructure formed upon deposition of manganese on a Pd(100) surface. 23his scenario explains the fating tendency of the 2 √ 3 superstructure, as expressed by the authors of Ref. 22, and why it appears so highly defective in STM imaging.As a matter of fact it is totally unstable, while in a struggle during growth with the competing formation of the confined sub-surface alloy.

FIG. 3 .
FIG. 3. Relative Auger intensities plotted during growth at 303 • C. A typical Auger spectrum as well as a LEED pattern (taken at 66 eV) after decay are displayed (right insets), along with intensity variations of relevant LEED spots recorded 296 • C (top left inset, after Fig. 4 of Ref. 17).The middle inset shows the evolution of the Si 2p/Ag 4d intensity ratio at ∼300 • C.