Jahangirov Research Group

Silicene and germanene

Upon synthesis of graphene, we wondered whether the same atomic structure could be supported by silicon or germanium atoms, which are below carbon in the periodic table. Planar structure of graphene is supported by the pi-bonds, which are strong and effective due to a relatively smaller core of carbon atoms. Calculating phonon dispersions, we have shown that the planar structure of silicene and germanene is unstable but can gain stability upon buckling. Three years later, silicene was synthesized on the Ag(111) surface. This development inspired theoretical studies searching for other 2D materials without a bulk parent material. Silicene quickly became a hot topic, and many groups reported its synthesis on various substrates. However, results seen in the experiments required explanation in three aspects: (i) ARPES measurements taken from the 3×3 silicene structure grown on the Ag(111) surface revealed a linear band as expected, but the Fermi velocity of this band was three times what was expected (ii) when deposition was continued, a √3×√3 silicene structure was formed but it acquired a mysterious 5 % contraction in the lattice constant thereby becoming mismatched with the substrate (iii) when growth continued even further, a multilayer silicene preserving the shrunk √3×√3 lattice constant (not matching any known bulk structure of silicon) appeared. We have made important contributions to these three scientific discussions with our studies. First, we projected the bands of the 3×3 structure onto the unit cell using the band-unfolding technique and showed that the observed linear band is due to hybridization between Ag(111) surface atoms and silicon atoms. Next, we showed that when new silicon atoms are deposited on the formed silicene, a dumbbell structure forms, and, when surface interactions are taken into account, these dumbbells form a 5% shrunken √3×√3 structure, as observed in the experiment. Finally, we showed that a novel layered silicon structure is formed when silicon atoms are continued to be added to the resulting √3×√3 structure. In the meantime, we also investigated the thermoelectric properties of silicene and germanene. We collaborated with experimentalists and provided crucial theoretical insight in a study that reported the first-ever synthesis of germanene on Au(111) surfaces. We continued our collaboration with experimentalists in two recent studies investigating the vibrational properties of pristine and hydrogenated silicene on Ag(111). We have written a book and a chapter summarizing the literature on silicene and other 2D materials.

Other 2D materials

Following our original study on silicene and germanene, we employed phonon dispersion analysis to predict the stability of group III-V and group IV binary honeycomb polymorphs. Some of these structures were realized experimentally. Our work on 2D materials continued with a detailed analysis of ZnO and SiC in a honeycomb structure. We revealed the mechanical and electronic properties of graphane under strain, which inspired theoretical and experimental studies on strain engineering of 2D materials. We investigated Coulomb blockade and other confinement effects in core/shell structures of graphene and boron nitride. We studied the frictional properties of layered nanostructures and developed the frictional figure of merit, which defines whether a material will slide in a superlubric or stick-slip regime. We also studied the superlubricity of graphene multilayers sandwiched between Ni(111) surfaces and attributed our findings to charge transfer between the layers. Our studies on friction appeared as a chapter in the new edition of a classic book in this field. We revealed the stages of epitaxial graphene growth and the role of the substrate in defect healing. We investigated the effect of uniform biaxial strain on the vibrational and electronic properties of MoSe2, which shows a direct-to-indirect bandgap transition at low strains. We proposed a novel single-layer silica structure and attributed its auxetic properties to reentrant bonds. We predicted stanene with dumbbell units (DBs) and demonstrated that it is a two-dimensional topological insulator with an inverted band gap, which can be tuned by compressive strain. We investigated the mechanical, electronic, and optical properties of WTe2, which showed significant anisotropy. We showed that the stacking of bilayer SnS2 can be tuned by electrostatic charging and loading pressure. We proposed a monolayer structure of RuS2 and RuSe2 stabilized by Peierls distortion. In our recent influential study, we investigated all possible binary compounds of group III monochalcogenides. We showed that the structures have significantly higher bending rigidity than graphene, and some of them possess a Mexican hat dispersion at the valence band edge. We have shown that this kind of dispersion is also present in a novel square-octagon monolayer of GaN and AlN. Following this study, we wrote a review on the progress and future directions of nitride-based 2D semiconductors. We proposed a semiconducting allotrope of borophene that is formed by Peierls distortion in two dimensions. We worked on the characterization of Janus monolayers and a novel structure of ruthenium carbide. Recently, we proposed a novel anisotropic structure of boron nitride that has a direct band gap at around 1.7 eV, which is highly tunable via strain engineering. We speculated that this structure can be synthesized by a bottom-up approach involving two molecules that intrinsically possess B-B and N-N bonds. We have summarized our work as a chapter in a definitive book on 2D materials.

1D materials

Our studies on 1D materials started with an investigation of silicon and germanium nanowire superlattices. We have shown that band lineups of Si and Ge zones result in multiple quantum wells, where specific states at the band edges and in band continua are confined. Next, we investigated structural, electronic, and magnetic properties of both finite and infinite 3d transition metal atomic chains. We studied GaAs nanowires in zinc-blende and wurtzite stacking and revealed the crucial changes of states near the Fermi level depending on the nanowire radius and hydrogen passivation. We showed that quantum well structures can be engineered by periodic modulation of hydrogen passivated silicon nanowires. We investigated armchair nanoribbons of silicene and germanene, revealing the family behavior in the dependence of the band gap on the nanoribbon width. Using DFT data, we constructed a simple tight-binding model to investigate large nanoribbons with periodic width modulation. We studied long-range interactions in carbon atomic chains, revealing that a displacement in one carbon atom creates a non-negligible force on its 35th neighbor. We attributed this finding to Friedel oscillations carried by free-electron-like orbitals. We had a great opportunity to work with the experimentalists who created 6000 atom long carbon chains encapsulated in carbon nanotubes. Our calculations showed that there is a charge transfer between the nanotube and the carbon chain, which has a crucial effect on structural and vibrational properties. We followed this study with a more detailed theoretical study of the same system using various hybrid functionals. Recently, we studied phase transitions in selenium and tellurium, which are composed of atomic chains held together by a mixture of weakly covalent and dispersion forces. We showed that depending on the sign of the electrostatic charging, the structure transforms into a cubic or layered phase.

Complex systems

Our interest in interdisciplinary studies has led us to various subjects that fall under the umbrella term complex systems. We introduced an extension to binary cellular automata whereby the transition pace between states can be tuned by a parameter. This enabled the investigation of the logistic versions of famous cellular automata like the Game of Life and Rule 90. We showed that by tuning the aforementioned parameter, it is possible to induce a deterministic phase transition from non-active to active asymptotic state without making too many changes to the behavioral complexity of the original Game of Life. Furthermore, we revealed the emergence of complex propagators as a result of the stage-by-stage construction of autocatalytic loops. Finally, we showed that there is a plethora of Class 4 (life-like) behavior in the logistic version of elementary cellular automata. In collaboration with the Blue Brain Project, we engineered a reduced model of hippocampal circuits. We developed a model of spiking neural networks and showed that using evolutionary pressure and dopamine-modulated circuitry, it can be trained to do simple visual tasks. We are collaborating with Turkish Aerospace Industries (TAI) in the development of nature-inspired computational morphogenesis algorithms to be used in the generative design of micro-satellites and other space systems.