Quantum Transport of the 2D Surface State in a Nonsymmorphic Semimetal.

In a topological semimetal with Dirac or Weyl points, the bulk-boundary correspondence principle predicts a gapless edge mode if the essential symmetry is still preserved at the surface. The detection of such topological surface state has been considered as the fingerprint prove for crystals with nontrivial topological bulk band. On the contrary, it has been proposed that even with symmetry broken at the surface, a new surface band can emerge in nonsymmorphic topological semimetals. The symmetry reduction at the surface lifts the bulk band degeneracies and produces an unusual "floating" surface band with trivial topology. Here, we first report quantum transport probing to ZrSiSe thin flakes and directly reveal transport signatures of this new surface state. Remarkably, though topologically trivial, such a surface band exhibits substantial two-dimensional Shubnikov-de Haas quantum oscillations with high mobility, which signifies a new protection mechanism and may open applications for quantum computing and spintronic devices.

Nano-Thermodynamics of Chemically Induced Graphene-Diamond Transformation.

Nearly 2D diamond, or diamane, is coveted as an ultrathin sp3 -carbon film with unique mechanics and electro-optics. The very thinness (≈h) makes it possible for the surface chemistry, for example, adsorbed atoms, to shift the bulk phase thermodynamics in favor of diamond, from multilayer graphene. Thermodynamic theory coupled with atomistic first principles computations predicts not only the reduction of required pressure (p/p∞  > 1 - h0 /h) but also the nucleation barriers, definitive for the kinetic feasibility of diamane formation. Moreover, the optimal adsorbent chair-pattern on a bilayer graphene results in a cubic diamond lattice, while for thicker precursors the adsorbent boat-structure tends to produce hexagonal diamond (lonsdaleite), if graphene is in AA' stacking to start with. As adsorbents, H and F are conducive to diamond formation, while Cl appears sterically hindered.

Phase diagram of carbon and the factors limiting the quantity and size of natural diamonds.

Phase diagrams of carbon, and those focusing on the graphite-to-diamond transitional conditions in particular, are of great interest for fundamental and applied research. The present study introduces a number of experiments carried out to convert graphite under high-pressure conditions, showing a formation of stable phase of fullerene-type onions cross-linked by sp3-bonds in the 55-115 GPa pressure range instead of diamonds formation (even at temperature 2000-3000 K) and the already formed diamonds turn into carbon onions. Our results refute the widespread idea that diamonds can form at any pressure from 2.2 to 1000 GPa. The phase diagram built within this study allows us not only to explain the existing numerous experimental data on the formation of diamond from graphite, but also to make assumptions about the conditions of its growth in Earth's crust.

Resistive Switching in Bigraphene/Diamane Nanostructures Formed on a La3Ga5SiO14 Substrate Using Electron Beam Irradiation.

Memristors, resistive switching memory devices, play a crucial role in the energy-efficient implementation of artificial intelligence. This study investigates resistive switching behavior in a lateral 2D composite structure composed of bilayer graphene and 2D diamond (diamane) nanostructures formed using electron beam irradiation. The resulting bigraphene/diamane structure exhibits nonlinear charge carrier transport behavior and a significant increase in resistance. It is shown that the resistive switching of the nanostructure is well controlled using bias voltage. The impact of an electrical field on the bonding of diamane-stabilizing functional groups is investigated. By subjecting the lateral bigraphene/diamane/bigraphene nanostructure to a sufficiently strong electric field, the migration of hydrogen ions and/or oxygen-related groups located on one or both sides of the nanostructure can occur. This process leads to the disruption of sp3 carbon bonds, restoring the high conductivity of bigraphene.

The Role of Structural Defects in the Growth of Two-Dimensional Diamond from Graphene.

The presented work is devoted to the study of the formation of the thinnest diamond film (diamane). We investigate the initial stages of diamond nucleation in imperfect bilayer graphene exposed by the deposition of H atoms (chemically induced phase transition). We show that defects serve as nucleation centers, their hydrogenation is energy favorable and depends on the defect type. Hydrogenation of vacancies facilitates the binding of graphene layers, but the impact wanes already at the second coordination sphere. Defects influence of 5|7 is lower but promotes diamondization. The grain boundary role is similar but can lead to the final formation of a diamond film consisting of chemically connected grains with different surfaces. Interestingly, even hexagonal and cubic two-dimensional diamonds can coexist together in the same film, which suggests the possibility of obtaining a new two-dimensional polycrystal unexplored before.

Edges in bilayered h-BN: insights into the atomic structure.

This study is devoted to the study of the edges of bilayered h-BN, whose atomic structure was previously generally unknown. It is shown that the edges tend to connect regardless of the edge cut. A defectless connection can be expected only in the case of a zigzag edge, while in other cases a series of tetragonal and octagonal defects will be formed. This result was obtained by carrying out an analogy between the edge of bilayered h-BN and the interface of monolayer h-BN. Information on the structure and energetics of closed edges allowed us to predict the shape of holes in h-BN, which agreed with the reference experimental data. Finally, it is shown that the closed edges do not create electronic states in the band gap, thus not changing the dielectricity of h-BN.

Diamane Oxide. Two-Dimensional Film with Mixed Coverage and a Variety of Electronic Properties.

Here, we investigate stability of the diamane oxide films and show that various compositions can be realized depending on the precursors, temperature, and pressure. We demonstrate that the commonly used oxygen source in the H2O form requires pressures of GPa order to fabricate the film, which is in full agreement with the experimental data. We show that different types of functional groups can tailor electronic properties of bilayer diamane. Finally, we study electronic property dependence on the film thickness, elucidating its connection with surface states.

Formation of Diamane Nanostructures in Bilayer Graphene on Langasite under Irradiation with a Focused Electron Beam.

In the presented paper, we studied bilayer CVD graphene transferred to a langasite substrate and irradiated with a focused electron beam through a layer of polymethyl methacrylate (PMMA). Changes in the Raman spectra and an increase in the electrical resistance of bigraphene after irradiation indicate a local phase transition associated with graphene diamondization. The results are explained in the framework of the theory of a chemically induced phase transition of bilayer graphene to diamane, which can be associated with the release of hydrogen and oxygen atoms from PMMA and langasite due to the "knock-on" effect, respectively, upon irradiation of the structure with an electron beam. Theoretical calculations of the modified structure of bigraphene on langasite and the experimental evaluation of sp3-hybridized carbon fraction indicate the formation of diamane nanoclusters in the bigraphene irradiated regions. This result can be considered as the first realization of local tunable bilayer graphene diamondization.

Semiconductor nanochannels in metallic carbon nanotubes by thermomechanical chirality alteration.

Carbon nanotubes have a helical structure wherein the chirality determines whether they are metallic or semiconducting. Using in situ transmission electron microscopy, we applied heating and mechanical strain to alter the local chirality and thereby control the electronic properties of individual single-wall carbon nanotubes. A transition trend toward a larger chiral angle region was observed and explained in terms of orientation-dependent dislocation formation energy. A controlled metal-to-semiconductor transition was realized to create nanotube transistors with a semiconducting nanotube channel covalently bonded between a metallic nanotube source and drain. Additionally, quantum transport at room temperature was demonstrated for the fabricated nanotube transistors with a channel length as short as 2.8 nanometers.

The Features of Phase Stability of GaN and AlN Films at Nanolevel.

Recently, two-dimensional gallium and aluminum nitrides have triggered a vast interest in their tunable optical and electronic properties. Continuation of this research requires a detailed understanding of their atomic structure. Here, by using first-principles calculations we reported a systematic study of phase stability of 2D-GaN and 2D-AlN. We showed that the films undergo a phase transition from a graphene-like to a wurtzite structure with a thickness increase, whereas the early reported body-centered-tetragonal phase requires specific conditions for stabilization. Additionally, we studied how the functionalization of the surface can modify the film structure as exemplified by hydrogenation.

On the Edge of Bilayered Graphene: Unexpected Atomic Geometry and Specific Electronic Properties.

The tendency of bilayered graphene edges to connect with each other allows to create hollow sp2-hybridized material with specific electronic properties. However, the unknown geometry of the formed edges hinders further investigation. Here we show that a closed bigraphene edge can be represented as a connection of generally misoriented graphene domains with topological defects and can be further described by grain boundary theory. The energy dependence of closed edges of commensurate twisted bilayered graphene is derived for any twist angle and edge orientation. Our findings allow to predict what particular edge types appear in the bilayered graphene holes and explain the structure of the connected bilayered graphene edges, which are often observed in the experiment.