1. Art and Diseño

Nano Research
Nano Res
DOI 10.1007/s12274-015-0733-y
Chemically modified STM tips for atomic-resolution
imaging on ultrathin NaCl films
Zhe Li, Koen Schouteden, Violeta Iancu, Ewald Janssens, Peter Lievens, Chris Van Haesendonck ( ),
Jorge I. Cerdá ( )
Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0733-y
http://www.thenanoresearch.com on January 28, 2015
© Tsinghua University Press 2015
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Nano Res.
Chemically modified STM tips for atomic-resolution
imaging on ultrathin NaCl films
Zhe Li, Koen Schouteden, Violeta Iancu, Ewald
Janssens, Peter Lievens, Chris Van Haesendonck*
Solid-State Physics and Magnetism Section, KU
Leuven, BE-3001 Leuven, Belgium
Jorge I. Cerdá†
Instituto de Ciencia de Materiales, ICMM-CSIC,
Cantoblanco, 28049 Madrid, Spain
The chemically modified STM-tip is obtained by picking up a Cl ion
from the NaCl surface. With respect to the bare metal tip, the
Cl-functionalized tip yields an enhanced resolution accompanied by a
contrast reversal in the STM topography image.
Peter Lievens, http://fys.kuleuven.be/vsm/class/
Chris Van Haesondonck, http://fys.kuleuven.be/vsm/spm/
Jorge I. Cerdá, http://www.icmm.csic.es/jcerda/
Nano Research
DOI (automatically inserted by the publisher)
Nano Res.
1
Research Article
Chemically modified STM tips for atomic-resolution
imaging on ultrathin NaCl films
Zhe Li, Koen Schouteden, Violeta Iancu, Ewald Janssens, Peter Lievens, Chris Van Haesendonck ( )
Jorge I. Cerdá ( )
Received: day month year
ABSTRACT
Revised: day month year
Cl-functionalized tips for scanning tunneling microscopy (STM) are obtained by
in situ modifying a tungsten STM-tip on islands of ultrathin NaCl(100) films on
Au(111) surfaces. The functionalized tips achieve a neat atomic resolution
imaging of the NaCl(100) islands. With respect to bare metal tips, the
chemically modified tips yield a drastically enhanced spatial resolution as well
as a contrast reversal in the STM topography images, implying that Na atoms
instead of Cl atoms are imaged as protrusions. STM simulations based on a
Green’s function formalism explain the experimentally observed contrast
reversal in the STM topography images as due to the highly localized character
of the Cl-pz states at the tip apex. An additional remarkable characteristic of the
modified tips is that in dI/dV maps a Na atom appears as a ring with a diameter
that depends crucially on the tip-sample distance.
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
Scanning
tunneling
microscopy,
ultrathin
insulating
films,
functionalized STM-tip,
STM simulation
The chemical termination of the tip apex in
scanning tunneling microscopy (STM) experiments
determines the interaction between the wave
functions of the tip and those of the sample and
hence the resolution that can be achieved in STM
images. For example, it has been demonstrated that
a
molecule-terminated
STM
tip
yields
high-resolution molecular-orbital imaging due to
the p-orbital character of the tip apex, far superior
to what is achieved with a bare metal tip [1, 2].
Atomic resolution imaging is of utmost importance
for the manipulation and investigation of surface
Address correspondence to Chris Van Haesendonck, [email protected]; Jorge I. Cerdá, [email protected]
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Nano Res.
point defects and adatoms, as well as for the
determination of the atomic structures of molecules
and nanoparticles [3-6].
Ultrathin insulating films grown on conductive
substrates effectively reduce the electronic coupling
between deposited nanoparticles and their metallic
support and are therefore ideally suited for local
probe based investigations. This way, the intrinsic
electronic properties of atoms [7], molecules [8, 9],
and clusters [10], as well as charge [11, 12] and spin
[13-15] manipulations of single atoms have been
investigated on different ultrathin insulating films,
including magnesium oxide, sodium chloride, and
copper nitride. Among these insulating materials,
NaCl has the advantage that it can be grown as
atomically flat layers on various metal surfaces
[16-19] and that the thickness of the layers can be
tuned [20]. Previous STM experiments have
reported atomic resolution on ultrathin NaCl films
in STM topography images [16, 19, 21]. Via density
functional theory (DFT) based calculations in the
Tersoff-Hamann approximation it has been found
that the protrusions observed in the topography
images using a bare metal tip are Cl atoms, while
the Na atoms cannot be resolved [16, 21]. Recently,
we showed that simultaneous visualization of both
atomic species of (bilayer) NaCl on Au(111) can be
achieved in the local hcp regions of the Au(111)
surface reconstruction in the dI/dV maps using a
Cl-functionalized tip [22]. We also illustrated that
such tips can be used to probe the surface of
(hemi)spherical nanoparticles [i.e., Co clusters
deposited on NaCl(100)/Au(111)] with atomic
resolution, which could not be achieved with a bare
metal STM tip [23]. In Refs. [22] and [23],
functionalization of the STM tip was only
occasionally obtained by uncontrolled picking up
of a Cl ion during repeated scanning of the NaCl
surface in close proximity, thereby hampering more
challenging systematic investigations.
Various
experiments
with
controlled
functionalization of the STM tips have been
reported before [1, 2, 24]. To obtain such tips, it was
required to introduce “impurity” molecules, such
as CO, O2, and H2, on the sample, which can be
picked up by the STM tip to achieve
functionalization and enhanced resolution. When
investigating the properties of nanoparticles,
especially metal nanoparticles, the adsorption of
CO, O2 or H2 molecules on the sample may result
in unwanted reactions with the nanoparticles and
modify their properties. Therefore, it would be
advantageous if the STM tip can be conveniently
functionalized with a species that is available on
the clean substrate surface, without the need to
introduce extra impurity molecules.
Here, we demonstrate that chemically modified
STM tips can be controllably obtained on ultrathin
NaCl(100) films on Au(111) by bringing the tip into
contact with the NaCl surface via current-distance
spectroscopy. Using such Cl-functionalized tips,
atomic resolution of mono-, bi-, and trilayer NaCl
islands is routinely achieved in STM topography
images as well as in constant-current dI/dV maps.
We find that the resolution and the appearance of
the atoms in such dI/dV maps depend crucially on
the tip-sample distance, which can be related to a
different overlap of the tip and sample wave
functions at different tip-sample distances.
Theoretical STM simulations based on a Green’s
function formalism reveal that the observed drastic
enhancement of the contrast as well as the contrast
reversal in the topography images can be explained
by the Cl-termination of the STM tip apex.
NaCl layers are grown using vapor deposition at
800 K in the preparation chamber of the STM setup
(Omicron Nanotechnology) in ultra-high vacuum
(UHV). Monolayer and bilayer NaCl(100) islands
are formed when, during the NaCl deposition, the
Au(111) substrate is kept cold or at room
temperature, respectively. Subsequent annealing of
the sample to 460 K yields trilayer NaCl(100)
islands [20]. The STM measurements are performed
in UHV (10 -11 mbar) and at low temperature (Tsample
= 4.5 K). Tunneling voltages are always given for
the sample, while the STM tip is virtually
grounded. All dI/dV maps are acquired with a
closed feedback loop using tunneling voltage
modulation (amplitude of 50 mV and frequency
around 800 Hz) and lock-in amplification based
detection. Image processing is performed by
Nanotec WSxM [25].
Figure 1(a) illustrates the effect of the modified
STM tip on the resolution of STM topography
images of trilayer NaCl(100) on Au(111).
Modification is controllably achieved by bringing
the tungsten STM tip into contact with the NaCl
surface via current-distance I(z) spectroscopy. It can
be seen in Fig. 1(a) that a drastic enhancement of
the resolution occurred after the I(z) spectrum was
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Nano Res.
recorded near the middle of the image. The lower
part is imaged with the bare W tip and exhibits
rather poor atomic resolution, whereas in the
upper part the atomic structure of the surface can
be clearly resolved with a modified tip [see
Supplementary Material (SM) for more details]. As
evidenced below, we assign the enhancement of the
resolution to picking up a Cl ion by the STM tip
upon contact with the NaCl surface. The transfer of
the Cl ion from the surface to the tip most probably
occurs due to an increasing overlap between the
potential wells associated with Cl adsorption on tip
and sample as they approach each other [26]. For
sufficiently close distances the two wells will merge
into a single one and further retraction of the tip
under an applied bias may then favor the
attachment of the Cl to the tip apex.
Indeed, we find that a surface defect is always
created in the NaCl film after the tip modification.
However, the defect appears to extend to four
neighboring atom sites, as illustrated in Figs.
1(b)-(d). Remarkably, the appearance of the defect
changes drastically after the STM tip has lost its
functionalization, which can spontaneously occur
during scanning. When using a bare W tip, the
defect appears as an atomic size vacancy in the
NaCl film as can be seen in Fig. 1(e). Such vacancies
have previously been reported for the case of
NaCl(100) films on Cu(111) and they were
identified as missing Cl ions in the NaCl film, also
referred to as Cl vacancies [19]. The observed
change in the appearance of the defect indicates
that contrast reversal occurs in STM topography
images that are recorded with the modified tip
when compared to the bare W tip. This contrast
reversal implies that the modified tip images the
Cl － ions as depressions and the Na + ions as
protrusions. Simulations of the STM topography
images (discussed below) confirm that the contrast
reversal is indeed induced by the Cl termination of
the W tip.
Figures 1(b)-(d) presents a series of STM
topography images recorded with increasing
current from 0.1 nA to 0.32 nA at a fixed negative
sample voltage V = －0.8 V. It can be seen that the
Na + ions appear larger with increasing current and
that at the same time the image contrast decreases.
The corrugations are 45 pm in Fig. 1 (b), 42 pm in
Fig. 1 (c) and 35 pm in Fig. 1 (d), where the main
contribution to the corrugation stems from the
Au(111) herringbone reconstruction.
Figure 1. (a) 6.3×6.3 nm2 STM topography image illustrating the influence of the tip modification on the imaging resolution, which
is drastically enhanced after bringing the STM tip into contact with the NaCl surface (V = －1.0 V, I = 0.2 nA). (b)-(d) 8×8 nm2
STM topography image recorded with a modified tip at the same tunneling voltage of －0.8 V, and with different current of 0.1 nA,
0.2 nA, and 0.32 nA, respectively. The surface exhibits a defect that can be assigned to a single Cl vacancy. The color bar indicates
the z amplitudes of 46 pm, 42 pm and 35 pm for the topography images in (b)-(d), respectively. (e) 3.4×3.4 nm2 STM topography
image of two Cl vacancies (indicated by the black arrows) recorded at V = －0.8 V and I = 0.16 nA using a bare W tip. (f)-(h)
Corresponding 8×8 nm2 dI/dV maps of (b)-(d).
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano
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Nano Res.
To confirm that the modified tip is terminated by
a Cl atom and to gain further insight into the
mechanism of the contrast reversal in topography
images, we carried out simulations of the STM
topography with a Green's function based
formalism that treats the tip on the same footing as
the sample surface, thus allowing to investigate the
effect of different tip terminations. We considered
two differently oriented bare W tips [W(110) and
W(111)], two Cl-functionalized tips of which one
oriented along the W(110) direction [denoted as
W(110)-Cl] and the other one along the (111)
direction of a hypothetical W fcc phase [denoted as
W(111)-Cl] [27], as well as a Na-terminated tip
[W(110)-Na]. The surface was modeled as a
NaCl(100) trilayer on top of the Au(111) surface. As
depicted in Fig. 2(a), to describe the surface we
employed a large c(10×10) NaCl(100) trilayer
commensurate
with
a
11 3 


 3 11
Au(111)
supercell after slightly distorting the Au lattice. All
simulations were performed with the GREEN
package [28, 29], using the extended Hückel theory
(EHT) [30, 31] to describe the electronic structure of
both the sample and tip (details of the calculation
parameters as well as the resulting Au and NaCl
electronic structures are given in the SM).
Figures 2(b)-(d) present topography images
simulated at V = －0.8 V using different tip models
as described above. The bare W [W(110) and
W(111)] tips [see Fig. 2(b) and Fig. S10 in the SM,
respectively] and the Na-terminated tip (see Fig.
S11 in the SM) result in a weaker corrugation with
maxima at the Cl atoms for different tunneling
current. On the other hand, the W(110)-Cl [Fig. 2(c)]
and W(111)-Cl tips [Fig. S13(b)] result in well
resolved bumps on top of the Na atoms at
relatively low currents. Moreover, the W(110)-Cl
tip resolves both species for particular tunneling
parameters [Fig. 2(d)]; however, decreasing the
tip-sample distance (i.e., using larger currents)
shifts the maxima to the Cl atoms, while increasing
the tip-sample distance yields the maxima on the
Na atoms [Fig. 2(c)]. Note that in Fig. 2(d) there is a
clear symmetry breaking with respect to the
expected pmm one (after combining the 4-fold
symmetry of the NaCl and the pmm symmetry of
the tip), which is induced by the underlying Au
substrate. Such asymmetric features only become
visible at tunneling gap resistances where both
species are resolved and for this particular tip [see,
for instance, Fig. S13(b) where the 3-fold W(111)-Cl
tip yields highly symmetric images]. Also note that
although the large size of the supercell used to
model the surface may account to some extent for
the incommensurability between the square
NaCl(100) and hexagonal Au(111) lattices, it is still
too small to accommodate the Au(111) herringbone
reconstruction [32], which causes large-scale
modulations in the images (see for instance the
variations between fcc and hcp regions of the
Au(111) surface in Figs. S3 and S4 and the
corresponding discussion in the SM). We therefore
restrict the theoretical analysis below to the
explanation of the origin of the contrast reversal
induced by the adsorbed Cl in the topographic
images since this effect can be clearly seen in all
regions of the sample.
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Nano Res.
5
Figure 2. (a) Top view of the trilayer NaCl(100) on Au(111) model used in the simulation. Simulated STM topography at V =
－0.8 V, (b) using a bare W(110) tip image at I = 0.1 nA, (c) and (d) using a Cl-terminated W(110) tip at I = 0.02 nA (log10I ≈－1.7)
and I = 0.1 nA (log10I = －1.0), respectively . The dark green and blue circles represent Cl atoms and Na atoms, respectively. (e) I(z)
curves calculated at V = －0.8 V for the bare W(110) (dashed lines) and W(110)-Cl (solid lines) tips placed on top of a Na atom
(blue) and on top of a Cl atom (green), respectively. The points are calculated while the lines are only a guide to the eye.
Figure 2(e) presents simulated I(z) curves for a
bare W(110) tip and a W(110)-Cl tip with the tip
apex placed above a Na and above a Cl atom,
respectively. The simulated I(z) curves for a
W(111)-Cl tip are similar to those for the W(110)-Cl
tip [see Fig. S13(a) in the SM]. For the bare tip
(dashed lines), the current decays faster with the
tip-sample distance z above a Cl atom than above a
Na atom, but is always larger above a Cl atom. For
the Cl-functionalized tip (solid lines), a larger slope
of the I(z) curve is found above a Cl atom but only
at smaller tip-sample distances. At z ≈ 4.2 Å
(corresponding to I ≈ 0.1 nA) the currents above a
Na atom and above a Cl atom become equal and a
contrast reversal occurs as the tip is further
retracted. For z > 4.2 Å Na should then be revealed
in the topography image. Below the contrast
reversal point [i.e. the point where the I(z) curves
above the Na and above the Cl cross], the contrast
between Na and Cl first rather rapidly increases
with decreasing current, which is consistent with
our experimental observations [Figs. 1(b)-(d)],
indicating that the tunneling conditions for Figs.
1(b)-(d) are close to the contrast reversal point.
However, when further decreasing the current, i.e.
at large tip-sample distances, the contrast will start
to gradually decrease with decreasing current and
the I(z) curves above the Na and above the Cl will
ultimately coincide as expected for z >> 4.2 Å . This
decrease of contrast with decreasing current for
low currents is experimentally confirmed in Figs.
S3(a) and (b) in the SM, which indicates that the
tunneling conditions in that case are considerably
below the contrast reversal point. The trends
described above are the same for a W(111)-Cl tip
(see I(z) curves in Fig. S13 in the SM).
For positive voltages, the behavior of the I(z)
curves is similar to the behavior observed at
negative voltages. On the other hand, the exact
point of contrast reversal is different. Figure S5(a)
in the SM reveals that the contrast in experimental
topography images increases with increasing
current at a fixed positive voltage, which is
corroborated by the simulations (see Fig. S12 in the
SM). All these results consistently explain the
experimental findings and confirm that the
functionalized tips indeed are terminated by a Cl
atom.
A decomposition of the simulated current into
tunneling paths (data not shown) reveals that the
major contributions to the current always involve
the p z orbitals of the Na or Cl atoms at the surface.
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Nano Res.
However, as illustrated in Fig. S8(a) and Table S1 in
the SM, there is a large difference in the level of
localization of these orbitals between both elements,
with those of Na much more extended than the Cl
ones. Hence, the current decays faster at the Cl sites
(i.e., larger I(z) slope) than at the Na sites. For the
Cl-terminated tip, the p-orbitals of the Cl apex
dominate the tunneling current for our
experimental measurement conditions (tunneling
voltage and current range), while other states,
including the W d-orbitals, only have a minor
contribution to the tunneling current. In case the
terminated Cl tip is positioned above a Cl atom, the
overlap between its highly localized pz orbitals
decays so fast when z > 4.2 Å that the signal
becomes smaller than at a Na site [Fig. 2(e)] and
contributions from the more extended pz orbitals of
the neighboring Na atoms start to dominate the
current. This explains both the contrast reversal
and the similar I(z) slopes at the Na and Cl sites
found at large tip-sample distances for the Clterminated tips. A similar analysis for the
W(111)-Cl tip leads to the same conclusions. On the
other hand, for the bare tungsten tip, taking the
W(110) tip for example, the contribution of the
different states to the tunneling current depends on
the precise location of the tip above the NaCl
surface. When the W tip is located on top of a Cl
atom of the NaCl surface, the main contributions
from the W tip are W-dx2-y2 (60%), W-p z (27%), and
W-s (10%), which all interact with the pz orbital of
the Cl atoms of the NaCl surface. However, when
the tip is located on top of the Na atoms of the
NaCl surface, we find a complex interplay between
the different tip and Na states. The largest
contributions to the tunneling current are W-s→
Na-pz (24%) and W-pz→Na-s (15%). The remaining
60% contribution comes from a complex
interference interplay between many different
paths. Overall, the decay of the overlap between
the Cl and W states with z is not sufficiently large
to induce a contrast reversal even at the largest z
values, in accordance with the experiments. We
mention that apart from the pure electronic effects
presented above, dynamical force sensor effects [33]
may also play a role in the contrast reversal,
although their influence should become more
pronounced at small tip-sample distances.
We now turn to our experimentally measured
constant-current dI/dV maps. While we achieved a
good agreement between the measured and the
simulated STM topography images for the high
spatial resolution as well as for the contrast reversal,
the constant-current dI/dV maps, which are
conveniently recorded simultaneously with the
topography images, present an additional
remarkable characteristic of the modified tip that
will be illustrated and discussed below. We would
like to stress that for the constant-current dI/dV
maps we do not aim at any comparison with
simulated maps. First of all, full DFT based
calculations of the local density of states (LDOS)
probed by a Cl-terminated tip cannot be performed
at this point. Also, dI/dV maps acquired under
constant-height or open-feedback-loop conditions
are more appropriate for comparing theory and
experiment when focusing on the LDOS [34]. On
the other hand, for the constant-height or
open-feedback-loop conditions, our Cl-terminated
tips do not survive for a sufficiently long time to
perform a systematic study of the influence of the
tunneling parameters on the spatial resolution. The
spatial
resolution
for
open-feedback-loop
conditions turns out to be prone to fluctuations for
detailed measurements that require longer
measuring times.
Figures 1(f)-(h) present a series of dI/dV maps
recorded at the location corresponding to the STM
topography images of Figs. 1(b)-(d). The dI/dV
maps are recorded at the same tunneling voltage of
－0.8 V, but with different settings of the tunneling
current. While in the corresponding STM
topography images [Figs. 1(b)-(d)] only Na atoms
are resolved as protrusions, independent of the
tunneling current, in the dI/dV maps the
appearance of the atoms depends strongly on the
tunneling current, as illustrated in Figs. 1(f)-(h).
Upon more careful comparison, it can be seen that
the drastic changes in the constant-current dI/dV
maps show a clear correlation with the more subtle
contrast changes observed in the corresponding
topography images. Remarkably, it can be seen that
the brightest dI/dV features evolve from one atomic
species (Na or Cl) to the other when changing the
tunneling current. At lower current each Na atom
appears as a ring-like feature in the dI/dV maps
[Fig. 1(f)]. With increasing current, the diameter of
the rings gradually increases [Fig. 1(g)] and the
neighboring rings start to overlap until, at
sufficiently high currents, the rings can no longer
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Nano Res.
be resolved and the highest dI/dV signal is found
on the other atomic species, i.e., Cl [Fig. 1(h)].
Increasing/decreasing the current at a constant
voltage [Figs. 1(f)-(h)] in fact decreases/increases
the tip-sample distance. The above results therefore
indicate that the appearance of the atoms in the
dI/dV maps recorded with the modified tip
depends mainly on the height of the STM tip above
the NaCl surface or, equivalently, on the tunneling
gap resistance. In particular, a contrast reversal in
the dI/dV maps is found to occur at a specific
tip-sample distance which in general varies from
tip to tip due to changes in the apex geometries
and/or the Cl adsorption sites. These trends in the
dI/dV maps are the same for positive and negative
sample voltages and in fact allow to identify if one
is close to or far away from the contrast reversal
point. At larger tip-sample distances (well below
the contrast reversal point) Na is revealed as a dot
[Figs. 1(f) and Fig. S3(d)], while at smaller
tip-sample distances (near the contrast reversal
point) it is revealed as a ring [Figs. 1(g)]. When
approaching the tip further towards the NaCl
surface (very close to the contrast reversal point),
the ring-like features overlap and form a dot on the
Cl sites [Fig. 1(h)].
Notably, the enhanced resolution and the
reversal processes do not depend on the sign of the
applied voltage, since similar results are obtained
under positive biases (Fig. S5 in the SM). Regarding
the film thickness, bilayer NaCl presents the same
behavior (Fig. S6 in the SM), while for monolayer
NaCl we only observe the enhancement in the
topography images after the tip modification [35],
but no ring structures in the dI/dV maps. We assign
this absence to larger tip-sample distances when
measuring with similar tunneling setpoints, since
the monolayer film presents a much smaller
electrical resistance than the bi- and trilayer films.
Bare W tips, on the other hand, yield dI/dV maps
with no or only very weak atomic resolutions (see
Fig. S7 in SM), and hence a detailed comparison of
the bias/current dependence with the modified tip
is not feasible.
Occasionally, the modified tip is able to
simultaneously image both the Na atoms and the
Cl atoms in the dI/dV maps (Fig. S3 in the SM) as
well as in the topography images (Fig. S4).
Although the resolution obtained with the
modified STM tips varies somewhat from tip to tip
(see Fig. 1 and also Figs. S2-S4 in the SM), the main
features are consistent: Na atoms are observed as
ring-like features in the dI/dV maps with their
diameter depending on the tip-sample distance.
In summary, tungsten STM tips were chemically
modified on ultrathin NaCl(100) films, resulting in
Cl-functionalized tips that are used to demonstrate
atomic resolution imaging of NaCl(100) islands on
Au(111). It was demonstrated that the modified
STM tips enhance and reverse the contrast in STM
topography images compared to a bare metal STM
tip. Simulated STM images, which take into
account the specific termination of the tip apex,
demonstrate that the modified STM tips are indeed
functionalized by a Cl atom. Cl-functionalized tips
can be used for systematic high-resolution
investigations of adsorbates, such as adatoms,
molecules, and nanoclusters, on thin NaCl
insulating films. The reported approach may be
generalized
to
other
thin
films
or
semiconductor/oxide surfaces. We believe the key
issue is to have an electronegative atom exposed at
the surface. This should not necessarily be Cl as
used in this study, but S or P also should improve
the resolution. On the other hand, lighter
electronegative elements such as oxygen or carbon
present too highly localized orbitals and thus
require smaller tip-sample distances in order to
achieve the contrast reversal. In such regime,
however, dynamical tip-sample interactions may
become predominant and the interpretation of the
data becomes less straightforward.
Acknowledgements
The research in Leuven has been supported by the
Research Foundation – Flanders (FWO, Belgium)
and the Flemish Concerted Action research
program (BOF KU Leuven, GOA/14/007). Z. L.
thanks the China Scholarship Council for financial
support (No. 2011624021). K. S. and V. I. are
postdoctoral researchers of the FWO. J. C.
acknowledges financial support from the Spanish
Ministry of Innovation and Science under contract
NOs. MAT2010-18432 and MAT2013-47878-C2-R.
Electronic
Supplementary
Material:
Supplementary Material (more details of the STM
experiments and simulations) is available in the
online
version
of
this
article
at
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Nano Res.
http://dx.doi.org/10.1007/s12274-***-****-*
(automatically inserted by the publisher).
States in Two-Dimensional Gold Clusters on MgO Thin
Films. Phys. Rev. Lett. 2009, 102, 206801.
[11] Repp, J.; Meyer, G.; Olsson, F. E.; Persson, M.
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Electronic Supplementary Material
Chemically modified STM tips for atomic-resolution
imaging on ultrathin NaCl films
Zhe Li, Koen Schouteden, Violeta Iancu, Ewald Janssens, Peter Lievens, Chris Van Haesendonck (  )
Jorge I. Cerdá ( )
Content:
1. Description of the STM tip functionalization on NaCl(100)/Au(111) films
2. Topography and dI/dV maps using a functionalized STM tip
3. Topography and dI/dV maps using a bare W STM tip
4. Methodological details of the extended Hückel theory and band structure calculations
5. Simulated STM topography images of NaCl(100)/Au(111) using a bare W(111) tip, a Na-terminated
W(110) tip, and a Cl-terminated W(111) tip
Address correspondence to Chris Van Haesendonck, [email protected]; Jorge I. Cerdá, [email protected]
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1. STM tip functionalization on NaCl(100)/Au(111) films
The termination of the tungsten tip is controllably modified by bringing the tip into contact with the
NaCl surface via current-distance I(z) spectroscopy. Typically, the tip is approached by 0.2 − 0.5 nm
depending on the tip and measurement settings. As can be seen in the scanning tunneling microscopy
(STM) topography image of trilayer NaCl(100) on Au(111) before [Fig. S1(a)] and a fter Fourier filtering [Fig.
S1(b)], there occurs a drastic enhancement of the resolution near the middle of the image where an I(z)
spectrum was recorded [Fig. S1(c)]. The lower part is imaged with the bare W tip and reveals only a very
weak atomic resolution, while after tip modification the atomic structure of the surface can be much more
clearly resolved in the upper part. The drastic improvement of the resolution (i.e., the measured
corrugation becomes much more pronounced) can also be seen in the height profile in Fig. S1(d).
Figure S1. (a) 6.3×6.3 nm2 STM topography image of trilayer NaCl(100) on Au(111) (V = －1 V, I = 0.2 nA). The resolution
becomes drastically enhanced after bringing the STM tip into contact with the NaCl surface. (b) Same as (a) after Fourier filtering.
(c) A typical I(z) spectrum recorded during the tip modification. Note that the current became saturated around z = －0.18 nm due
to the limitations of the control electronics. (d) Height profiles taken along the arrows in (a).
Upon more careful inspection of Fig. S1(b), one can observe a “shift” of the atomic rows before and after
tip modification. The dotted line is located in between two atomic rows in the lower part of the image,
while it becomes located on an atomic row in the upper part of the image. This indicates that “contrast
reversal” has occurred, which can be related to picking up a Cl atom with the tip during contact with the
NaCl surface. Since a bare W tip only resolves the Cl atoms as protrusions in STM topography images [1],
the contrast reversal implies that the modified tip images the Cl locations as depressions and the Na
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locations as protrusions.
2. Topography and dI/dV maps using a functionalized STM tip
Figure S2 presents a series of STM images of trilayer NaCl(100) on Au(111) recorded with a modified
STM tip. For these images the ratio between the tunneling voltage and the tunneling current is kept
constant, which implies that the tip-sample distance is (approximately) the same for the presented images.
The STM topography images are similar at the settings used here, as illustrated in Fig. S2(a). In the different
dI/dV maps the Na atoms are observed as ring-like features, except at the highest voltages [Figs. S2(h) and
(i)]. This can be explained by taking into account the surface projected band structure, i.e., there is a higher
contribution of the electrons above the onset of the Au(111) surface state and the trilayer NaCl(100)/Au(111)
interface state around －0.5 V and －0.2 V, respectively. The tip-sample distances are therefore more or less
the same for Figs. S2(b)-(f) and the dI/dV maps have a similar resolution. Below the onset of the
surface/interface state, the STM tip has to be located closer to the surface in order to keep the tunneling
current constant and, as a result, the highest dI/dV signal is probed on the other atomic species (Cl) [Fig.
S2(h) and Fig. S2(i)]. This further confirms that the tip-sample distance is the main parameter that
determines the observed resolution in the dI/dV maps.
Figure S2. (a) 8×8 nm2 STM topography image of trilayer NaCl(100) on Au(111). The three defects in the NaCl film can be
assigned to Cl vacancies. Corresponding dI/dV maps at (b) －0.1 V, 0.04 nA (c) －0.2 V, 0.08 nA [the inset in (c) is a close-up
view of the region enclosed by the black square], (d) －0.3 V, 0.12 nA, (e) －0.4 V, 0.16 nA, (f) －0.5 V, 0.2 nA, (g) －0.6 V,
0.24 nA, (h) －0.8 V, 0.32 nA, (i) －1.0 V, 0.4 nA.
Remarkably, it can be seen in Fig. S2(c) that both the atomic Na and Cl species are resolved as bright
protrusions and ring-like features on the entire surface at －0.2 V, while for the dI/dV map in Fig. S2(d),
which is obtained at －0.3 V, both atomic species are resolved only on the hcp regions. This is illustrated in
more detail in Fig. S3. At －0.4 V, by lowering the tunneling current, i.e., from 0.16 nA [Fig. S2(e)] to 0.06
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nA [Fig. S3(d)] or 0.08 nA [Fig. S3(c)], we can also resolve both species in the dI/dV map. At 0.16 nA only Na
is imaged [Fig. S2(e)]. Upon lowering the current to 0.08 nA both atomic species are resolved on the hcp
regions, while only Na is resolved on the fcc regions and the herringbone ridges [Fig. S3(c)]. This
observation is similar to previously reported results on bilayer NaCl using a functionalized STM tip (of
which the precise termination was uncertain at the time), where it was demonstrated that the different
atomic resolution on hcp and fcc regions is related to local differences of the electronic properties in these
regions [2]. Following our present findings, local differences of the tip-sample distance resulting from the
local differences of the electronic properties may explain as well the different atomic resolution on hcp and
fcc regions reported in Ref. [2]. Upon further reducing the current to 0.06 nA, both atomic species are
resolved everywhere on the surface [Fig. S3(d)].
Figures S3(a) and (b) show that the contrast in the topography images decreases with decreasing curr ent,
which indicates that the tunneling conditions for Fig. S3(a) and (b) are far from the contrast reversal point
[where the I(z) curves above the Na and above the Cl cross as illustrated in Fig. 2(e)].
(b)
(a)
hcp
hcp
fcc
fcc
hcp
hcp
(c)
(d)
Figure S3. 8×8 nm2 STM images of trilayer NaCl(100) on Au(111) with a functionalized tip. (a) Topography image and (c)
corresponding dI/dV map recorded at V = －0.4 V and I = 0.08 nA. (b) Topography image and (d) corresponding dI/dV map
recorded at V = －0.4 V and I = 0.06 nA. Insets in (c) and (d) are the Fourier transform images of the dI/dV maps.
As described above, the functionalized tips reveal only one species (Na atoms) in the STM topography
images. However, occasionally the modified tip is able to simultaneously image both the Na atoms and the
Cl atoms in the topography images. This is illustrated in Figs. S4(a)-(d) at different voltages, which are
recorded with another (different from the tips used for Figs. S1-S3) modified tip. One species is observed as
a small dot, while the other species is observed as a larger sphere. At voltages below －0.8 V, the two
species are clearly resolved, while at voltages above －0.6 V the dot-like atoms tend to fade away. In the
corresponding dI/dV maps presented in Figs. S4(e)-(h) small dot-like atoms are resolved as ring-like
features. As discussed in the main text the Na atoms in the dI/dV maps have a ring-like feature, while the
small dot and the large sphere resolved in the topography images in Figs. S4(a)-(d) can be assigned to be
Na atoms and Cl atoms, respectively.
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a
b
c
e
f
g
d
h
Figure S4. 6.3×6.3 nm2 STM images of trilayer NaCl(100) on Au(111) (I = 0.2 nA). (a)-(d) Topography images recorded at V = －
1.0 V, －0.8 V, －0.6 V, －0.4 V, respectively. (e)-(h) Corresponding dI/dV maps of (a)-(d).
Figure S5 shows the current dependent (at fixed voltage) and voltage dependent (at fixed current) dI/dV
maps for trilayer NaCl at positive sample voltages. Similar to the images at negative bias, the appearance of
the ring-like dI/dV maps is also dependent on the tip-sample distance.
In addition, the topography image [Fig. S5(a)] illustrates that Na sites appear as protrusions and the
contrast decreases for decreasing current, which is well supported by our simulations presented in Fig. S12
below. This implies that the tunneling conditions for Fig. S5(a) are far from the contrast reversal point, i.e.,
in the region below the horizontal dashed line in the I(z) curve in Fig. S12.
Figure S5. STM images of trilayer NaCl(100) on Au(111). (a) Topography image and (b) the corresponding dI/dV map recorded at
different currents at a fixed tunneling voltage of +1 V; (c) Topography image and (d) the corresponding dI/dV map at a fixed
tunneling current of 0.07 nA.
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For bilayer NaCl, the STM images (see Fig. S6 below) are similar to the trilayer NaCl, whereas the current
settings are typically higher than those used for trilayer NaCl.
Figure S6. 3.2×2.7 nm2 (a) and (b) STM topography images of bilayer NaCl(100) on Au(111); (c) and (d) corresponding dI/dV
maps.
3. Topography and dI/dV maps using a bare W STM tip
Figure S7. 6×6 nm2 (a) and (b) STM topography images of trilayer NaCl(100) on Au(111); (c) and (d) the corresponding dI/dV
maps obtained using a bare W tip.
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4. Methodological details of the extended Hückel theory and band structure calculations
All the theoretical results shown in this work, including the Hamiltonian parameterization, the electronic
structure of the semi-infinite surfaces and the STM topographic simulations, have been performed with the
GREEN package [3-5]. Since use of a large unit cell size is required to properly match the NaCl(100) trilayer
to the Au(111) substrate, ab initio calculations are computationally highly demanding. Therefore, we relied
on the simplified extended Hückel theory (EHT) which, if correctly parameterized, can accurately
reproduce the electronic structure of the system [6] while its adequacy for the calculation of the tunneling
currents has been long recognized [7]. Here, to describe the Au and W tip atoms we employed the spd basis
already parameterized from their respective bulk phases [6]. For both the Na and Cl atoms we defined a sp
basis and fitted their corresponding EHT parameters to reproduce the band structure of bulk NaCl as
calculated from density functional theory (DFT) under the generalized gradient approximation (GGA).
Prior to the fit, and in order to reproduce the experimental NaCl band gap of 9 eV, the c onduction bands
were shifted by 4 eV (DFT yields an underestimated gap value of 5 eV). Bands above 14 eV with respect to
the Fermi level (fixed here to －10 eV [6]) were excluded from the fits. The EHT constant was set to kEHT =
2.3 for all interactions.
Figure S8. Comparison between the DFT-GGA and EHT derived electronic structure of bulk NaCl. (a) DOS(E) projected onto the s
and p orbitals of Na and Cl, and (b) the bulk band structure along high symmetry lines. Bands above E = 14 eV (upper horizontal
line) are not included for fitting the EHT parameters.
In Fig. S8 the GGA and EHT derived band structures and density of states (DOS) projected on each
species are compared, with the optimized EHT parameters given in Table S1. The DOS plots show well
defined peaks for the Cl states in the lower energy region (occupied states) while those of Na present a
larger dispersion above the Fermi level (empty states). As reflected from the Slater orbital exponents given
in Table S1, the larger dispersion of the Na bands implies a larger extension of the Na atomic orbitals (AOs),
while those of Cl are more localized. This may be rationalized by recalling that the Cl AOs contract as the
valence shell becomes filled, while the opposite holds for Na.
Table S1. Optimized EHT parameters (on-site energy E, Slater exponents ζ1 and ζ2 , and the coefficient c1 for the former) for Na and
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Cl obtained from the fits shown in Fig. S8. See Ref. [5] for further details.
Element
Na
Cl
AO (nl)
E (eV)
ζ1 (bohr -1)
c1
3s
－4.420
1.09861
0.83506
3p
－3.668
0.84913
0.81922
3s
－24.529
2.76215
0.83813
3p
－12.964
1.91016
0.87033
ζ2 (bohr -1)
3.66857
In order to address the accuracy of the EHT for the combined trilayer NaCl(100)/Au(111 ) system, we
present in Fig. S9(a) the k-resolved DOS projected on the NaCl trilayer calculated assuming a semi-infinite
geometry. For comparison, we present in Fig. S9(b) equivalent DOS projected on the first Au layer of the
clean Au(111) surface calculated for the same supercell. Despite the backfolding effect, the highly
dispersive Au surface bands can be clearly resolved in the NaCl film and should therefore be accessible to
the STM tip. Due to the adsorption of the NaCl trilayer, the Au surface bands are shifted by about 0.2 eV,
leading to surface band onsets at －0.2, －0.4, and －0.6 eV below the Fermi level, in nice agreement with
the experiment and previous calculations using the “embedding method” [2].
Figure S9. k-resolved DOS(E) graph projected on (a) the trilayer NaCl(100) on the Au(111) substrate and (b) the first atomic layer
on a clean Au(111) surface. Both graphs are obtained for the large supercell depicted in Fig. 2(a) and assuming a semi-infinite
geometry. The energies are given in eV. The surface BZ and the high-symmetry points are indicated in the sketch on the right.
For the STM simulations, we first computed the scattering states at the sample surface and the tip
independently assuming a semi-infinite geometry for both blocks [3, 4]. Next, given a relative tip-sample
position, the tip and surface scattering states are coupled up to first order to calculate the elastic
transmission coefficient. The current I is then obtained after integrating over the energy window fixed by
the bias voltage V. For the topographic maps the unit cell is divided into a grid with 0.4×0.4 Å 2 size
elements and the tip-sample normal distance, z, is adjusted at each pixel until the desired current value I is
reached. A (8×8) k-supercell was employed to sample the surface Brillouin zone (BZ), comprising over 7100
k-points of the Au(111) BZ, while the energy resolution and imaginary part of the energy entering the
Green's functions was fixed to 20 meV.
It should be noted that despite current image tunneling spectroscopy (CITS) maps were simulated for
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different tips and tunneling parameters, the ring structure around the Na atoms was not reproduced (data
not shown). At present, we are not certain about the reason for the lack of agreement with the experimental
dI/dV maps, which may be due to "special" tips or inaccuracies in the EHT.
5. Simulated STM topography images of NaCl(100)/Au(111) using a bare W(111) tip, a Na-terminated
W(110) tip, and a Cl-terminated W(111) tip
We performed STM simulations of NaCl films on Au(111) using a bare W(111) tip and found that it yields
a very similar resolution in STM topography images as a bare W(110) tip, i.e., without the contrast reversal,
which is only observed after functionalizing the tip with Cl. A simulated STM topography image using a
bare W(111) tip is presented in Fig. S10.
Figure S10. Simulated STM topography image of trilayer NaCl(100) on Au(111) at V = －0.8 V, I = 0.1 nA using a bare W(111)
tip.
We also performed STM simulations of NaCl films on Au(111) using a Na-terminated W(110) tip at
different voltages. Similar to bare W tip, only Cl ions can be revealed in the topography images, as shown
in Fig. S11 below.
Figure S11. Simulated STM topography images of trilayer NaCl(100) on Au(111) using a W(110)-Na tip at I = 0.1 nA , (a) V = －
1.0 V, (b) V = －0.6 V, and (c) V = －0.2 V.
Figure S12 presents the calculated I(z) curves and simulated STM topography images at positive voltage.
The Na ions are resolved, which indicates the contrast reversal also occurs for positive bias. Moreover, the
corrugation (D) of the simulated topography, i.e., the contrast, decreases with decreasing current, which
very well agrees with the experimental observation in Fig. S5(a). This implies that the tunneling conditions
for Fig. S5(a) are in the region below the horizontal dashed line indicated in the I(z) curves in Fig. S12.
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Figure S12. I(z) curves and a series of simulated STM topography images of trilayer NaCl(100) on Au(111) using a Cl-terminated
W(111) tip and calculated at V = +0.8 V. According to the I(z) curves, in the region between the “contrast reversal point” and the
horizontal dashed line, the contrast between the Na and Cl rather rapidly increases with decreasing current, while in the region
below the horizontal dashed line the contrast gradually decreases with decreasing current. The corrugation (D) and the tunneling
settings are given in the simulated STM images.
Figure S13. (a) I(z) curves calculated at V = －0.8 V for a W(111)-Cl tip placed on top of a Na atom (blue dots) and on top of a Cl
atom (green dots) for trilayer NaCl(100) on Au(111). In the region between the “contrast reversal point” and the horizontal dashed
line the contrast between the Na and Cl rather rapidly increases with decreasing current, while in the region below the horizontal
dashed line the contrast gradually decreases with decreasing current. (b) Simulated STM topography image using a bare W(111)-C l
tip at V = －0.8 V and I = 0.05 nA (log10I ≈－1.3).
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