Final Report Summary - ORBITAL IMAGING (Electron orbital resolution in scanning tunneling microscopy)
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.
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 show that, despite the extremely high uniformity of the fabricated nanoscale grating, there are at least four possible step and terrace configurations maintaining exactly the same groove periodicity.
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 well-defined and stable probes for SPM studies of complex surfaces, atomically resolved chemical and spin-sensitive 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.
Please see attachment for the publishable summary
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.
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 show that, despite the extremely high uniformity of the fabricated nanoscale grating, there are at least four possible step and terrace configurations maintaining exactly the same groove periodicity.
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 well-defined and stable probes for SPM studies of complex surfaces, atomically resolved chemical and spin-sensitive 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.
Please see attachment for the publishable summary