FINAL PUBLISHABLE SUMMARY REPORT
Scanning Tunneling Microscopy (STM) is one of the most powerful tools for investigating the atomic and
electronic structure of metallic and semiconducting surfaces with extremely high spatial resolution. The
technique uses a sharp tip or probe that ideally ends in a single atom, which is brought close to the surface
being measured, a voltage is placed on the surface and a current flows. The value of this current is
determined by the electronic structure of the interacting tip and sample atoms and the distance between them.
By scanning the tip over the surface, a tunneling current map is produced which reflects the local atomic and
electronic structure.
The major objectives of this Marie Curie project (“Orbital Imaging”) were: the reliable fabrication of
functionalized STM probes with well-defined structures for use in high-resolution STM studies;
investigation of the atom-atom interaction at small tip-surface distances; determination of the optimal
parameters for high-resolution STM imaging using the electron d- and p-orbitals at the tip apex; utilization of
the functionalized probes for atomically resolved STM studies of metallic and semiconducting surfaces with
complicated atomic and electronic structure. The main goals of the project were achieved and the main
results are summarized below.
Fig. 1. (a) and (b) Large area STM images of
graphene nanoribbons synthesized on the
vicinal SiC(001). Inset in panel (a) shows the
Fast Fourier Transform of the STM image
proving one preferential direction of the
boundaries (NBs). (c) and (d) Atomically
resolved STM images showing the system of
graphene domains rotated 17° clockwise
(GrR) and 10° anticlockwise (GrL) relative to
the NB which is rotated 3.5° anticlockwise
from the [110] direction (c) and the atomic
structure of the NB (d). (e) Schematic model
of the NB for the asymmetrically rotated
nanodomains in panels (c) and (d). (f)
Schematic of the nano-gap device for
transport measurements. (g) The dI/dV curve
measured at 10 K demonstrating the existence
of a transport gap. (ACS Nano, 2015)
1. Graphene with Self-Aligned Boundaries on Silicon Carbide
The atomic and electronic structure and transport properties of
trilayer graphene synthesized on thin films of cubic-SiC(001) grown
on vicinal Si(001) wafers with 2°-miscut have been studied. A
simple method to synthesize graphene with self-aligned periodic
nanodomain boundaries on a semiconducting substrate compatible
with silicon technologies have been proposed.
The transport measurements clearly demonstrate that the self-aligned
periodic boundaries induce a charge transport gap which can reflect
charge carriers over a remarkably wide range of energies (0.4–1.3
eV). Moreover, a high on-off current ratio of 104 was achieved with a
voltage of 0.7 V below 50 K and with a voltage of 0.25 V at 100 K.
High-resolution STM experiments with single crystalline W[001]
and W[111] tips and density functional theory (DFT) calculations
demonstrate that the transport properties of graphene on vicinal
SiC(001) can be explained by a slight asymmetric rotation of the
neighbouring nanodomains relative to the <110>-directed domain
boundaries. This asymmetrical rotation leads to the formation of a
periodic atomic structure along the domain boundaries which is
responsible for the effects observed in transport measurements.
The STM experiments on graphene/SiC(001) show that higher tip
stability can be achieved using a W[001] probe, which allowed to
conduct scanning tunneling spectroscopy experiments on this
promising system even at room temperature. At the same time,
extremely high spatial resolution was achieved using a W[111]
probe, which allowed to identify even the minor 3.5° asymmetrical
rotation of the neighbouring graphene nanodomains relative to the
domain boundaries (Fig. 1).
2. Atomically Precise Step Arrays on Si(hhm) Surfaces
Single crystalline W[001] probes have been successfully utilized for
STM studies of the regular stepped Si(557) and Si(556) surfaces
prepared using special sample treatment under ultra-high vacuum
(Fig. 2). Atomically precise triple step staircase with a periodicity of
5.9±0.2 nm have been fabricated on clean Si(557) surfaces. STM
studies show that the number of atomic-scale periodicity breakings in
the Si(557) triple step array can be as low as one per more than one
hundred hill and valley sequences. Atomically resolved STM studies
Fig. 2. (a) Micrometer-scale STM image of
the Si(557) step array and averaged cross- show that, despite the extremely high uniformity of the fabricated
section. (b-e) Atomically resolved STM nanoscale grating, there are at least four possible step and terrace
images of four different triple step configurations maintaining exactly the same groove periodicity.
configurations (ECOSS-30, 2014).
Fig. 3. (a,b) STM images of the GdSi2x/Si(111) measured with clean and oxygenterminated tungsten tips and cross-sections
showing three non-equivalent sublattices of
silicon atoms. (c) Model of the Gd3Si5 with a
120°-rotated vacancy network in the
subsurface layers. (d) Comparison of the
STM experiments and theoretical simulations.
(ECOSS-30, 2014; DPG Meeting, 2015)
Fig. 4. (a-c) Atomic chains on the Gd/Si(557)
at 0.2 monolayer coverage of Gd atoms. (d-f)
STM images of the anisotropic silver islands
on Si(557) at one (d), two (e), and three (f)
monolayers. (Semiconductors, 2015)
3. Distance-Dependent STM Studies of GdSi2-x/Si(111)
Distance- and voltage-dependent STM studies have been conducted
on the GdSi2-x/Si(111) system using clean and functionalized,
oxygen-terminated polycrystalline ([011]-oriented) tungsten tips
(Fig. 3). The experiments show that atomic-resolution STM images
reflect the positions of buried vacancies in the subsurface layers of
gadolinium silicide grown on Si(111) with both types of tips. The
results show that the STM depth sensitivity may reach down to the
fifth layer at a metallic surface (1 nm below the top layer), which is
hardly accessible by other experimental techniques. In order to
confirm this picture, the total energy DFT calculations and STM
image simulations beyond the Tersoff-Hamann approach have been
carried out.
The comparison of the high-resolution STM data obtained with clean
(d-orbital imaging) and oxygen-terminated tungsten probes (p-orbital
imaging) show that atomic resolution can be achieved at larger gap
resistances (larger tip-sample distances) using oxygen-terminated
probes. The observed atomic corrugations are approximately two
times larger in the case of the oxygen-terminated tungsten probe. The
STM image simulations suggest that the difference in atomic
corrugations in the experiments with clean and functionalized
tungsten tips is related to different atomic relaxations of the tungsten
and oxygen tip atoms interacting with the surface atoms. Despite
some advantages of the oxygen-terminated probes, at small gap
resistances they can produce asymmetric features, which do not
correspond to the atomic positions of the surface atoms, as STM
experiments and theoretical calculations demonstrate.
4. Low-Dimensional Metallic Structures on Si(557)
High-resolution STM studies demonstrate the possibility of
fabricating quasi-one-dimensional (atomic chains) and anisotropic
quasi-two-dimensional
structures
(nanometer-sized
islands
elongated along the steps) of gadolinium and silver atoms,
respectively, on the regular Si(557) surface (Fig. 4). At
submonolayer Gd coverages (0.1–0.2 monolayers) it was possible to
fabricate an ordered system of atomic chains aligned with one of the
<110> directions of the silicon single crystal (step direction). The
utilization of the vicinal (stepped) substrate allowed to select one of
three equivalent crystallographic directions on the (111)-oriented
terraces and avoid the formation of a multi-domain structure.
The results obtained during the project are novel and correspond to STM research of the highest quality. The
picoscale lateral and vertical resolution achieved in STM experiments with tungsten probes on
graphene/SiC(001) and GdSi2-x/Si(111) systems is comparable to the best resolution obtained in scanning
probe microscopy (SPM) studies so far. The results show the prospects of different types of probes for
application in high-resolution SPM studies. Therefore, they represent an important step in the development
of high-resolution SPM methods and precise instruments for the examination of surface atomic structures.
They are especially important for the improvement of the spatial resolution of SPM, fabrication of welldefined and stable probes for SPM studies of complex surfaces, atomically resolved chemical and spinsensitive STM imaging. Surface analysis with extremely high spatial resolution is crucial for many different
fields of science (physics, chemistry, biology) and technology since a large number of prospective materials
consist of just one or several atomic layers (e.g., graphene, topological insulators, etc.). The results obtained
on vicinal SiC(001) substrates represent a good example of utilizing high-resolution SPM data for
explanation of the physical properties of technologically relevant low-dimensional systems. The results of
the studies illustrate the prospects of graphene grown on the technologically relevant, vicinal cubic-SiC(001)
wafers for potential applications in nanoelectronics.
Alexander Chaika ([email protected]), “Orbital Imaging”, FP7 MC IIFR 911027