Combined Scanning Tunneling Microscopy and Noncontact Atomic Force
Microscopy Imaging of Silicene Layers on Ag(111)
–- Supporting Information –-
Andrea Resta1*, Thomas Leoni1, Clemens Barth1, Alain Ranguis1, Conrad Becker1,
Thomas Bruhn2, Patrick Vogt2, Guy Le Lay1
1
Aix-Marseille Université, CNRS, CINaM UMR 7325, 13288 Marseille, France.
Technische Universität Berlin, Institut für Festkörperphysik, Hardenbergstrasse 36,
10623 Berlin, Germany.
2
Abstract
For sake of the article flow some experimental results were not included into the main
text. The intent of this supporting information is to round up the information
mentioned in the main article.
Atomic scale contrasts of STM and nc-AFM images
Supp. 1 All the three ncAFM images (a-c) were
obtained in the constant
mode with the following
scanning parameters: Δf =
-0.9 Hz ; Ap-p = 4.5 nm, f0
= 26150 Hz, Ubias = -1 V
(filled states). The sample
was at liquid nitrogen
temperature.
In the main article we mention typical features in the atomic scale contrast that can
be seen in nc-AFM topography images. In Supp. 1 we show three other examples of
atomically resolved nc-AFM images with the flower like pattern, which are typical for
the silicene 3x3 structure, also regularly found in STM images 1,2,3.
Supp. 2: (a) STM topography image (filled states Ubias = -0.8 V, I = 0.4 nA, Ap-p = 10.5 nm), (b)
simultaneously recorded current image and (c) Δf image, (d) nc-AFM topography image (Δf=-1.45 Hz,
Ap-p = 10.5 nm, Ubias = -0.8 V). (e) Simultaneously recorded current and (f) Δf image. All images were
acquired on the sample hold at liquid nitrogen temperature.
Image Supp 2(a) is a STM topography image obtained with the Qplus4 sensor
oscillating while regulating on the tunneling current. The current set point was at I
=0.4 nA, and the current of the current image (Supp 2(b)) ranges from 0.29 nA to
0.39 nA. The detuning image (Supp 2(c)) barely shows any contrast, and to extract
the weak contrast here we had to filter part of the noise by averaging over 4
neighbors’ data points. The detuning f ranges from -0.15 Hz to -0.35 Hz. Image
Supp 2(d) is a nc-AFM topography image obtained by regulating on the detuning Δf.
Image supp. 2(e) contains the current recorded simultaneously while regulating on Δf
(range in I: from 0.9 nA to 2 nA). Image Supp. 2(f) contains the Δf signal also
recorded together with Supp 2(d) and (e). The detuning ranges between -1 Hz to -3.2
Hz.
The images in Supp. 2 support the article’s assumption that the nc-AFM contrast is
obtained for tip-surface distances smaller than during STM operation: no atomic
contrast in the Δf image in Supp 2(c) can be seen despite the rather high current
setting point during the STM operation. Additionally when regulating on Δf = -1.45 Hz
and Ubias = 0.8 V in the nc-AFM mode, the current is two to five times larger than the
current used to obtain atomic resolution in STM mode at the same bias voltage.
As indicated in the main article we report in the insert Figure 2(d) a better contrast on
the lower silver terrace (blue region). This region presents the same structure as on
the main terrace (red region). A similar situation can also be found in the current
image Supp 2(e) where the area corresponding to the lower terrace clearly exhibits
the same periodicity as the main terrace.
Domains in silicene films
Supp. 3 Large scale atomic resolution STM image (Filled states Ubias = 0.4 V, 0.1 nA, Ap-p= 4.5 nm, T
= liquid nitrogen temperature). Therein are visible as in figure 4 of the text the two family appearances
for the Ag(111)(√13x√13)R± 13.9° superstructure, domains of (3x3) silicene and some islands of the
second layer. On the right side LEED patterns from the same surface. The signals frm the Ag(111)
substrate are marked in red, the signals from the Ag(111)(√13x√13)R±13.9° are in magenta and those
from the Ag(111)(4x4) structure are in green.
Supp. 3 contains a large scale STM image with the two families of supercells at ±5.2°
and ±33° from the Ag[10] surface vector belonging to the Ag(111)(√13x√13)R±13.9°
structure: an hexagonal like moiré pattern and hexagonal array of protrusions. It is
also visible that a large part of the surface is covered with a silicene(3x3) structure.
Therein are also included a few second layer islands with a silicene(√3x√3)R±30°5,6
structure, which appear as an hexagonal array of protrusion 7. The image here clearly
shows the silicene(√3x√3)R±30° as an ad-island. Despite this discrepancy with other
works5,6,7, this article confirms and emphasizes the major importance of this phase in
the second layer. Here as in the main article we made use of the Wood notation 8 that
uses surface unit vectors to describe the supercells. Such vectors are related to the
bulk unit vectors: surface Ag[10]= √2/2[1-10]a0, surface Ag[01]= √2/2[0-11] a0 and
the vector normal to the surface plane for this new cell become √3[111] a0, were a0
is the bulk lattice constant.
In Supp. 3 we also show two Low Energy Electron Diffraction (LEED) patterns.
Ag(10) spot of the substrate is marked by the red circle (LEED at 43 eV).
magenta
circles
mark
some
of
the
contributions
arising
from
Ag(111)(√13x√13)R±13.9° structure. The green circles mark some of the spots
belong to the silicene(3x3) structure.
The
The
the
that
1
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2
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5
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8
Wood E. A. Vocabulary of Surface Crystallography. J. Appl. Phys. 35, 1306 (1964)