|ANTON VISIKOVSKIY||Last modified date：2019.06.24|
Assistant Professor / Applied Physics / Department of Applied Quantum Physics and Nuclear Engineering / Faculty of Engineering
|1.||Anton Visikovskiy, Takashi Kajiwara, Satoru Tanaka, Atomic and electronic structure of periodically curved graphene on (1-100) m-plane surface of SiC, Physical Society of Japan, 2018.09, It is known that electronic properties of graphene, in particular the existence of band gap in electronic band structure, can be altered by external periodic potential and periodic geometric and atomic modifications of graphene sheet. It has been shown also, that to create a noticeable effect, these modifications has to be very precise and relatively short ranged. The only reliable way to produce such effects is the use of self-organization and specific substrates. SiC(1-100) surface (so called m-plane) is a good candidate as it consists of short-ordered 1D periodic structure (1 nm for 4H- and 1.5 nm for 6H-SiC) of alternating Si- and C-terminated facets with corrugation of ~1.7 Å. Unfortunately, the lattice mismatch does not allow the full utilization of this short-ordered periodicity, but still, different options are available. Here we investigate atomic and electronic structure of various possible configurations of graphene on m-plane of SiC by DFT-based tight-binding calculations (DFTB). One of the investigated configurations is shown in Fig.1. Interestingly, a significant band gap in graphene’s electronic structure is obtained in such a case. .|
|2.||Anton VISIKOVSKIY, Takashi KAJIWARA, Satoru TANAKA , Stability and electronic structure of novel triangular lattice atomic layers of In, Tl, Pb, and Bi on SiC(0001), Physical Society of Japan, 2018.03, Two-dimensional (2D) materials are one of the attention centers of material science due to their fascinating physical properties and potential device application. Most of the efforts are applied towards materials with honeycomb, graphene-like lattice grown on metallic or semiconducting substrates, while 2D overlayers with different lattice, namely triangular ones, may exhibit Dirac-cone like electron dispersion and interesting spin-splitting of states due to larger than carbon spin-orbit interaction. Here, we investigate the possibility of formation of 2D dense triangular atomic lattices layers (TLAL) of different group III, IV, and V elements on SiC(0001) surface [Fig.1(a)] and their electronic structure by means of DFT calculation. We found that In [Fig.1(b)] and Pb triangular structures are stable on SiC(0001) (planar configuration for In and buckled for Pb), while for Tl and Bi it is controversial. The spin-polarized Dirac-cone electron dispersion of In, shown in Fig.1(c), makes it interesting it terms of application in spintronic devices. .|
|3.||Shingo HAYASHI, Takashi KAJIWARA, Anton VISIKOVSKIY, Takushi IIMORI, Tetsuroh SHIRASAWA, Kan NAKATSUJI, Toshio MIYAMACHI, Shuhey NAKASHIMA, Kazuhiko MASE, Fumio KOMORI, Satoru TANAKA, Sn atomic layer by intercalation at graphene/SiC interface, International Symposium on Epitaxial Graphene 2017 (ISEG-2017), 2017.11, Researches on atomic layer materials of group IV elements attracts great attention. Among them, stanene, an atomic layer of Sn, is expected to exhibit such unique characteristics as a two dimensional topological insulator  and quantum anomalous Hall effect . The growth of stanene on Bi2Te3 has been achieved  but no experimental evidence on such characteristics is yet reported. On the other hand, since Sn shows a large spin orbit coupling (SOC), the spin and electronic states in terms of Rashba splitting, antiferromagnetic order and Mott insulator transition etc. are investigated. Adatom Sn (√3×√3) R30° (R3) structure on semiconductor substrates (Ge, SiC) has been reported [4, 5]. Thus, we have been aiming at forming stanene on SiC(0001) surfaces. Here, we tried Sn intercalation at graphene/SiC interfaces, which are advantageous to any ex-situ measurements of a Sn interlayer.
Initially, the graphene buffer layer was grown on SiC (0001)(Si-face) in UHV. Sn irradiation was done at room temperature and annealed at 700℃. The LEED pattern in Fig 1(a) before Sn irradiation shows (6√3×6√3) R30°(6R3) spots due to the buffer layer formation. After Sn irradiation in Fig. 1(b) the 6R3 diffraction is weakened and graphene (1×1) is clearly visible, suggesting the growth of Sn (1×1) structure at the interface. Surface X-ray diffraction analysis also supports Sn (1x1) formation. Thus, possible Sn sites on Si-terminated SiC(0001) surface are T1, where Sn is located above Si, which is further confirmed by XPS, DFT, and ARPES. The structural model of the Sn interlayer is shown in Fig. 2 (a) as compared to the honeycomb lattice. A Sn atom forms in-plane bondings with neighboring Sn atoms due to relatively shorter Sn-Sn distance than in the case of Sn(R3) at T4 sites, resulting in triangular lattice (TL) (1x1). Total energy calculation using DFT indicates TL is most stable configuration under Sn-rich chemical potentials. We investigated the electronic structure by ARPES, which is in good agreement with the DFT calculation shown in Fig. 2(b). Further, the DFT calculation including SOC terms shows Rashba type spin splitting at K points, which was actually observed by spin polarized ARPES, to be discussed in the future.
The Sn triangular lattice atomic layer (TLAL) was formed at the graphene/SiC interface via intercalation. TLAL is a new structure and is expected to show unique properties. Finally, we should emphasize the advantage of the TLAL at the interface is robustness against the atmospheric environment..
|4.||Anton VISIKOVSKIY, Shingo HAYASHI, Fumio KOMORI, Satoru TANAKA, Sn and Pb triangular lattice atomic layers on SiC(0001) and at graphene/SiC(0001) interface, International Symposium on Epitaxial Graphene 2017 (ISEG-2017), 2017.11, The research of 2D Dirac materials has been one of the main topics in recent material science, because of wide possible device application and fascinating physics. Most studies, however, are concentrated on graphene-like honeycomb lattice materials (graphene, silicene, germanene, stanene, etc.)1,2, though other lattice types may possess similar electronic structure features. Dense triangular lattice atomic layers (TLAL) of heavy group IV elements (Sn, Pb) on SiC(0001) surface and at graphene/SiC(0001) interface could be such materials (Fig. 1a,b). Additionally, heavy group IV elements exhibit strong spin-orbit coupling (SOC) which may significantly contribute to electronic structure of 2D layers and result in spin-polarized states. Previous studies did consider sparse triangular systems of group IV adatoms on SiC surface in terms of frustrating magnetic order, spin liquids, and Mott-type insulator state3. However, dense atomic layers where Sn (or Pb) atoms directly interconnected by in-plane bonds are unexplored.
Here we present computational results on such systems. We show that TLALs indeed could be stable on bare SiC(0001) (Fig. 1a). The exotic for group IV elements bonding configuration results in formation of Dirac cone feature in electronic structure, which originates from px and py type orbitals, rather than pz orbital as in graphene. Strong SOC results in spin-polarized electronic states, some of which on SiC substrate at K points of Brillouin zone exhibit Rashba-type splitting with in-plane spin texture, while other demonstrate non-Rashba type splitting with spin components normal to the surface (Fig. 1c). Growth of such triangular layers at graphene-buffer-layer/SiC interface (Fig. 1b) results in complete graphene decoupling from SiC substrate and induce spin-polarization in graphene states. While Sn and Pb layers shows similarities, due to the lattice mismatch the difference in atomic structure is predicted. While Sn TLALs exhibit planar structure with (1x1) periodicity, Pb TLALs buckled resulting in (√3x√3) period on SiC(0001)..
|5.||Anton VISIKOVSKIY, Shingo HAYASHI Takashi KAJIWARA, Fumio KOMORI, Satoru TANAKA, 2D triangular Sn and Pb metallic layers on SiC(0001) and at graphene/SiC(0001) interface, Physical Society of Japan, 2017.09, Two-dimensional (2D) materials are one of the attention centers of material science due to their fascinating physical properties and potential device application. Most of the efforts are applied towards materials with honeycomb lattice such as graphene, silicene, germanene and similar. 2D material with other lattices could be of Dirac type. One of such possible atomic lattices is a triangular one. Our experimental results and calculations points out on possibility to form dense (1x1) triangular lattice atomic layers (TLAL) of heavy group IV elements (Sn, Pb) that has not been reported before. Here we present our ab initio calculations showing properties of such TLALs on bare SiC(0001) and at graphene/SiC(0001) interface. Non-trivial bonding configuration in this 2D layers results in formation of perfect 2D metal. It also exhibits Dirac-cone-like electron dispersion features. Owing to large spin-orbit interaction (SOI), these layers show both Rashba- and non-Rashba-type of spin-polarization of electron bands at high symmetry points and also may induce spin-polarization in electronic structure of graphene layer..|
|6.||Anton VISIKOVSKIY, Shingo HAYASHI, Takashi KAJIWARA, Fumio KOMORI, Satoru TANAKA, DFT study of a triangular lattice atomic layer of group IV elements (Ge, Sn, Pb) on SiC(0001), Japanese Society of Applied Physics, 2017.09, Two-dimensional (2D) materials are one of the attention centers of material science due to their fascinating physical properties and potential device application. Most of the efforts are applied towards materials with honeycomb lattice such as graphene, silicene, germanene and similar. This is because of interesting physics of Dirac fermions. It was shown, however, that Dirac-like electron dispersion is possible in non-honeycomb lattices [1, 2], however the studies of non-honeycomb lattice 2D materials are rather rare up to date. One of such possible atomic lattices is a triangular one. Triangular lattices of heavy group IV elements has been addressed previously in a context of Mott-insulator and topological-insulator state of sparse (√3x√3) adatom reconstruction on hexagonal surfaces of SiC(0001) and Si(111). Dense (1x1)-type lattices, where metal atoms are directly interacting through in-plane bonding (Fig. 1a), are not reported.
Here we present our ab initio calculations and experimental results showing that such triangular atomic lattices of Ge, Sn and Pb could be stable on SiC(0001), and especially at graphene/SiC(0001) interface. Non-trivial bonding configuration in this 2D layers results in formation of perfect 2D metal. It also exhibits Dirac-cone-like electron dispersion features. Owing to large spin-orbit interaction (SOI), these layers show both Rashba- and non-Rashba-type of spin-polarization of electron bands at high symmetry points in the Brillouin zone simultaneously (Fig. 1b) which has not been observed before and represents an interesting case both for science and possible application in spintronic device application.
|7.||ANTON VISIKOVSKIY, Kohei FUKUMA, Takashi KAJIWARA, Fumio KOMORI, Tanaka Satoru, STM study of graphene nanoribbon arrays formed on large facets of vicinal SiC(0001), Physical Society of Japan, 2017.03, Graphene's nanostructures such as nanoribbons (GNRs) attract a great interest due to peculiar quantum effects and possibility to open a band gap in graphene’s electronic structure - a condition required for graphene utilization in switching logic electronic devices. It is challenging task to produce GNRs with good control over width and orientation on semiconducting substrate. Previous reports have shown that GNRs could be selectively grown on high-index facets of SiC(0001) surface, albeit the width of such GNRs was relatively wide.
Here, we report a development of a method to grow periodic arrays of very narrow GNRs (width ~1.6 nm) with armchair edge structure and sizable band gap (several hundred meV) and 1D structurally modulated graphene layers on large facets (induced by step bunching) of vicinal SiC(0001) surfaces by high-temperature annealing in Ar atmosphere. We present observations and analysis of atomic and electronic structures of these nanomaterials by means of scanning tunneling microscopy (STM) and spectroscopy (STS).
|8.||ANTON VISIKOVSKIY, Takashi KAJIWARA, Masamichi YOSHIMURA, Tanaka Satoru, Tuning graphene electronic properties with substrate interface structure, Physical Society of Japan, 2016.09, Large scale graphene production for device application preferably has to be done directly on substrates it will be used on (such as SiC). The strong coupling between graphene and substrate surface results in significant degrading of graphene's properties. One of the possible ways to overcome this problem and acquire additional control over electronic properties of graphene is to modify the interface atomic structure between graphene and substrate. As seen from calculations graphene’s band structure has crucial dependence on periodicity and location of influential interface atoms. On the other hand, STM shows large variety of possible Si-induced interface structures produced by intercalation in graphene/SiC(0001) system. Although many of the interface structures are still unknown, here we tried to track what influence they may have on graphene electronic bands by means of calculation of simplified models based on STM observations..|
|9.||ANTON VISIKOVSKIY, Shin-Ichi KIMOTO, Takashi KAJIWARA, Masamichi YOSHIMURA, Fumio KOMORI, Tanaka Satoru, Graphene/SiC(0001) interfaces induced by Si intercalation, 18th International Conference on Crystal Growth and Epitaxy (ICCGE-18), 2016.08, [URL], Graphene is an important 2D material with extremely useful properties for device applications. Growth of graphene on substrates faces the problem of interface influence on graphene's properties. In particular, graphene forms buffer layer when growing on SiC(0001) which does not possess characteristic graphene band dispersion and affect, often negatively, subsequent graphene layers. The interface between graphene and SiC can be modified by different atoms intercalation. In the present work, we consider one of the simplest cases of Si intercalation. Various amounts of Si atoms were intercalated into graphene/SiC(0001) interface and studied by means of electron diffraction (LEED, RHEED), scanning tunneling microscopy (STM), angle-resolved photoemission (ARPES) and first principals calculations. We have shown that interface structures are similar to those formed by Si adsorption on clean SiC(0001) surface. The formation of these interface structures decouples graphene layer from substrate and restore it's linear electronic band dispersion.
The experiments were carried out on off-axis (4o-off) SiC(0001) substrates prepared by hydrogen etching and high temperature annealing in UHV to obtain uniform buffer layer coverage exhibiting well-known 6√3 reconstruction. Vicinal samples were used to promote intercalation process near the step edges, where graphene layer is believed to have a lot of defects. Intercalation was performed by Si deposition on hot (700-800oC) surface, as well as room temperature deposition with subsequent annealing. Both methods give identical results. As observed by diffraction and STM, the interface reconstruction changes as follows 6√3 → “2 x 2” → (2√3 x 4) → (3 x 3). Low-bias STM scans have revealed decoupled graphene layer being on top of these structures. The structure of (3 x 3) phase of Si on Si(0001) is well-established. We used the same model with graphene on-top to calculate electronic properties of such system by means of density functional theory (DFT) and DFT-based tight-binding methods. The results are in good agreement with experimental data. Such ability to engineer different interfaces between graphene and the substrate may allow to tune graphene's electronic properties to the ones desired by applications..
|10.||ANTON VISIKOVSKIY, TAKASHI KAJIWARA, MASAMICHI YOSHIMURA, FUMIO KOMORI, Tanaka Satoru, STM and computational study of graphene nanodots formed on SiC(0001), Physical Society of Japan, 2016.03.|
|11.||ANTON VISIKOVSKIY, TAKASHI KAJIWARA, Tanaka Satoru, MASAMICHI YOSHIMURA, STM study of graphene nanodots array on SiC, 23rd International Colloquium on Scanning Probe Microscopy (ICSPM23), 2015.12, [URL].|
|12.||ANTON VISIKOVSKIY, TAKASHI KAJIWARA, MASAMICHI YOSHIMURA, FUMIO KOMORI, SATORU TANAKA, Modification of graphene electronic properties by periodic potential of a substrate, Physical Society of Japan, 2015.09, It is well known that graphene is a zero-gap semiconductor, thus it is difficult to apply it directly as an alternative to classical semiconductors in technological application despite its outstanding mechanical and carrier mobility characteristics. However, the introduction of even weak periodic potential may change graphene electronic properties significantly (see Fig.1 for example). It may influence graphene sheet doping level as well as open a band-gap in its electronic structure. Such potential may be represented by underlying atoms of the interface structure between graphene and substrate bonded to graphene or step structure of the substrate itself. Here we show that electronic structure changes caused by interface atoms influence are strongly dependent on periodicity of interface structure with respect to graphene lattice and site of influence. We discuss the possible interface structures between graphene and SiC substrate caused by atoms intercalation based on theoretical calculations and experimental observations. .|
|13.||ANTON VISIKOVSKIY, SHIN-ICHI KIMOTO, TAKASHI KAJIWARA, SATORU TANAKA, MASAMICHI YOSHIMURA, DFT and STM study of Si-rich graphene/SiC interface, Physical Society of Japan, 2015.03, Epitaxial growth on SiC substrate is a promising method of large-scale graphene production as graphene can be relatively easy formed on this surface by thermal decomposition, and there is no need for transfer process. The first graphitic layer formed on SiC, however, strongly interacts with the substrate. This can be used to tune graphene properties by creating suitable interface structures on SiC surfaces by means of atoms intercalation. Here, we have studied Si intercalation into graphene/SiC interface. It turns out, that variety of Si structures can be formed depending on Si coverage. The STM appearance of these structures is strikingly similar to that of Si overlayers formed on clean SiC. Unfortunately, most of the structures Si form on clean SiC surface are metastable (except for √3 and 3x3) and not well studied. Moreover, partially decoupled graphene exhibit structural shift with respect to the underlying interface structure, which may result in local differences in graphene electronic structure (Fig.1b,c). In the present work we have studied the interaction of graphene with Si interface structures by means of STM and DFT calculations..|
|14.||ANTON VISIKOVSKIY, Shin-Ichi Kimoto, Takashi Kajiwara, Masamichi Yoshimura, SATORU TANAKA, Si intercalation in graphene/SiC interface, 1st Asia-Pacific Symposium on Solid Surfaces, 2014.09, [URL], Large-scale graphene epitaxy on SiC substrates is considered to be very promising for future graphene-based electronic devices production. The first graphitic layer on-top of SiC substrate takes form of a buffer layer with periodicity of (6√3x6√3) and loses its characteristic linear energy dispersion as well as advantageous extremely high carrier mobility. This emphasizes the influence of a substrate on graphene properties. In order to eliminate undesired substrate effects and tune graphene layer properties to our needs the interface between graphene and SiC can be modified by different elements intercalation. The most popular, as of today, is hydrogen intercalation which decouples graphene layer from the SiC substrate. Data on other elements intercalations are reported mainly based on theoretical predictions.
Here, we present the experimental as well as computational results on Si intercalation into graphene/SiC interface. We have found that, depending on the amount of intercalated Si, different structures are formed at the interface. Most interestingly, all the structures are similar to those stable and metastable ones formed by excess of Si on clean SiC surfaces. In most cases the graphene layer decouples from the substrate and show characteristic linear band dispersion. The interface structures are analyzed by STM and modelled using DFT calculations..
|15.||ANTON VISIKOVSKIY, Shin-Ichi Kimoto, Takashi Kajiwara, SATORU TANAKA, Masamichi Yoshimura, Reconstructions of Si intercalated graphene/SiC interface: atomic and electronic structure, Physical Society of Japan, 2014.09, Epitaxial graphene on SiC substrate looks very promising in terms of technological application. Unfortunately, first graphitic layer grown on SiC takes form of buffer layer and looses graphene-specific electronic properties owing to strong graphene-substrate interaction. Intercalating different materials into graphene/SiC interface may decouple graphene layer and tune its electronic properties. Nowadays, hydrogen intercalation is widely used for this purpose. There are some reports (mainly computational studies) on the other elements intercalation. Here, we present the experimental as well as computational results on Si intercalation into graphene/SiC interface. We have found that, depending on the amount of intercalated Si, different structures are formed at the interface. The structures are similar to those stable and metastable ones formed by excess of Si on clean SiC surfaces. In most cases the graphene layer decouples from the substrate and show characteristic linear band dispersion. The interface structures are analyzed by STM and modelled using DFT calculations..|
|16.||Yuzuru Nakamori, Takashi Kajiwara, ANTON VISIKOVSKIY, SATORU TANAKA, D-band Raman characteristics of graphene nanoribbons on SiC, IUMRS-ICA, 2014.08, [URL], Graphene nanoribbons (GNRs) attract a lot of attention not only because of possible applications to future electronic devices but also edge induced unique electronic characteristics. We have fabricated GNRs by epitaxial growth on self-ordered vicinal SiC surfaces (1). In this presentation we show the results of micro-Raman spectroscopy study of such GNRs in terms of electron-phonon interactions at nanoribbon edges.
The GNRs were grown on vicinal SiC surfaces (miscut toward [1-100]) by molecular-beam epitaxy (MBE) using solid carbon source (2). Figure 1 shows AFM images (phase-mode) of the surface after (a) 20 and (b) 100 min growth. Graphene islands selectively grow on (0001) terraces and evolve along [11-20] direction to form GNRs. Figures 2(a) and (b) show polarized Raman spectra of D- and G-band region after each growth shown in Fig. 1(a) and (b), respectively. 0 (90) degree corresponds to laser polarization angle with respect to the step edge direction. Figure 2(a) indicates almost no polarization dependence whereas strong reduction of the D-band intensity is observed in Fig. 2(b). This is attributed to the dominant armchair edge character of our GNRs (3). Note the presence of ~ 4cm-1 shift in the D-band peak position. It is speculated that this shift is caused by a new type of phonon mode, softened and localized at the arm-chair edges (edge phonons).
|17.||ANTON VISIKOVSKIY, Kei Mitsuhara, Masayuki Hazama, Yoshiaki Kido, Experimental and computational study of NiO(001)/Au(001) interface, IUMRS-ICA, 2014.08, [URL], The electronic contact between metal and oxide is an important issue in terms of oxide electronics, which has recently attracted much attention. It has been also revealed that electronic charge transfers between metal and oxide supports play a crucial role in catalytic activities, in particular, for Au nano-particles on oxide supports. NiO is one of the most frequently utilized supports in catalysis and a model transition oxide. Indeed, Au nano-particles grown on NiO supports work well as catalyst in CO oxidation and other reactions. Although there are a number of reports on NiO/Ag interface, Au interaction with NiO stays much out of focus, in spite of its importancy. In the present work the atomic and electronic structures of NiO(001)/Au(001) interface were analyzed by medium-energy ion scattering spectroscopy, (MEIS), photoelectron spectroscopy (PES) and DFT calculations.
We first revealed by MEIS that the O atoms were located above Au atoms at the NiO(001)/Au(001) interface (Fig. 1). This was supported by the DFT calculations. Concerning the electronic properties of the NiO/Au interface, we observed metallic features for NiO(001)/Au(001) for NiO thickness up to 3 ML. Relevant to the metallic features, the bandgap decreased with decreasing the NiO thickness from 10 down to 5 ML. We also observed the interface component for Au 4f core level, which shifted by 0.35 eV to higher binding energy side compared with that for bulk Au. Despite that, the DFT calculations demonstrated no significant charge transfer. We, however, found out the presence of image charge beneath the Ni atoms at the NiO(001)/Au(001) interface (Fig. 2), which may be a key issue to explain the core level shift and band structure.
|18.||ANTON VISIKOVSKIY, SHIN-ICHI KIMOTO, TAKASHI KAJIWARA, SATORU TANAKA, MASAMICHI YOSHIMURA, Silicon layer at graphene/SiC(0001) interface, structural and electronic properties by calculation and scanning tunneling microscopy, Physical Society of Japan , 2014.03, Graphene proved to be one of the most exciting materials in terms of scientific and technological importance. Unfortunately, first graphitic layer grown on SiC takes form of buffer layer and looses graphene-specific electronic properties owing to strong graphene substrate interaction. Hydrogen intercalation may decouple graphene from the substrate . It is, however, interesting to check if other elements intercalation takes place in graphene/SiC interface and how it affects graphene properties. Silicon is one of such possible materials. Moreover, there are reports of graphene-like silicone 2D structure, named silicene . And confined region of the graphene/SiC interface is a possible place to grow such 2D material.
Accessing the properties of such interfacial layer in experiments is difficult, so in the present work we investigate its atomic and electronic structures by means of first-principals and classical molecular dynamics calculations. We have also performed initial experiments with silicon intercalation in graphene/SiC interface and study the resultant structures by STM.
|19.||ANTON VISIKOVSKIY, Kei Mitsuhara, T. Matsuda, K. Tominaga, P.L. Grande, G. Schiwietz, Yoshiaki Kido, Skimming-trajectory effect for energy losses of medium energy He ions passing along major crystal axes of KI(001) and RbI(001), 2013.08, [URL].|
|20.||ANTON VISIKOVSKIY, Yusuke Kurisu, Takashi Kajiwara, Tanaka Satoru, Masamichi Yoshimura, Fumio Komori, Growth of graphene nanoribbons on vicinal SiC(0001) studied by STM, Physical Society of Japan, 2013.03.|