Growth of germanium-silver surface alloys followed by in situ scanning tunneling microscopy: Absence of germanene formation

Theoretical studies have shown that new physical properties such as tunable gap openings or quantum spin Hall effects could be expected from group-IV graphene analogs (silicene, germanene, stanene). While there have been numerous studies of growth of such Si, Ge, Sn monolayers, the demonstration of their hexagonal organization has been often based on postgrowth characterization, and their analogy to graphene has remained controversial. Our real-time scanning tunneling microscopy (STM) observation during Ge deposition on Ag(111) in the 380–430 K temperature range reveals that Ag atoms are involved in all the structures observed before the formation of a second layer, rejecting the possible formation of germanene on this substrate within these experimental conditions. The observation by STM of Ge atomic diffusion shows that easy exchange between Ag and Ge atoms is responsible for the Ge-Ag surface alloying at such temperatures.

Figure 1

Evolution of the Ag(111) surface during Ge deposition at 380 K for (a) 0 min, (b) 100 min (${\theta }_{\mathrm{Ge}}=0.08$ ML), (c) 190 min (${\theta }_{\mathrm{Ge}}=0.16$ ML), and (d) 360 min (${\theta }_{\mathrm{Ge}}=0.30$ ML). In (b), the white arrows indicate the outgrowths that have grown during Ge deposition. Size of the images $580\times 580\mathrm{n}{\mathrm{m}}^2$. Tunneling conditions $V_{\mathrm{S}}=1.4\mathrm{V}$, $I=30\mathrm{pA}$. See movie in the Supplemental Material [32] for the complete film.

Figure 2

Detailed view of the Ge/Ag(111) surface for increasing coverage: (a) diluted phase for ${\theta }_{\mathrm{Ge}}=0.0025\mathrm{ML}$, (b) diluted phase at saturation, i.e., just before the formation of twiglike dendrites, (c) boundary between the diluted phase and a dendrite, (d) triangle phase, (e), (f) striped phase, and (g), (h) disordered hexagonal phase. Deposition temperatures are 380 K (a), (e), 400 K (d), 414 K (f)–(h), 420 K (b), (c). Images (a) and (e) are acquired during deposition, and the other images are acquired at 300 K after deposition. In (e), the image is displayed modulo the step height. Tunneling conditions: (a), (g) $V_{\mathrm{S}}=1.5\mathrm{V}$, $I=30\mathrm{pA}$; (b) $V_{\mathrm{S}}=0.4\mathrm{V}$, $I=20\mathrm{pA}$; (c) $V_{\mathrm{S}}=-0.3\mathrm{V}$, $I=0.5\mathrm{nA}$; (d) $V_{\mathrm{S}}=0.4\mathrm{V}$, $I=0.2\mathrm{nA}$; (e) $V_{\mathrm{S}}=1.0\mathrm{V}$, $I=50\mathrm{pA}$; (f) $V_{\mathrm{S}}=0.3\mathrm{V}$, $I=0.2\mathrm{nA}$; (h) $V_{\mathrm{S}}=0.4\mathrm{V}$, $I=0.1\mathrm{nA}$.

Figure 3

Fraction of the different phases as a function of Ge coverage during growth at 380 K. Red circles: diluted phase; blue crosses: twiglike dendrites; green triangles: triangles; black squares: striped-phase SP; and pink hexagons: disordered hexagonal (DH) phase. Lines are guides for the eyes. Note that although consisting in the mixing of two phases, the triangle phase is counted as a whole.

Figure 4

Evolution of the Ag concentration in the surface structures, as a function of Ge coverage, during deposition at 380 K. Red circles correspond to the experiment reported in Fig. 1. The line is a linear fit with slope −0.9. The black square corresponds to another deposit at the same temperature on an area not affected by the shadowing effect.

Figure 5

Histograms of Ge atomic displacements, normalized to the to the Ag-Ag interatomic distance: (a) measured between successive images acquired at 13-min interval and at $T=380$ K, (b) best fit within the framework of single diffusion mechanism, and (c) best fit within the framework of diffusion through deinsertion, jumps, and reinsertion.