Hexagonal boron nitride (h-BN), a layered material similar to graphite, is a promising dielectric. Monolayer h-BN, so-called "white graphene", has been isolated from bulk BN and could be useful as a complementary two-dimensional dielectric substrate for graphene electronics. Here we report the large area synthesis of h-BN films consisting of two to five atomic layers, using chemical vapor deposition. These atomic films show a large optical energy band gap of 5.5 eV and are highly transparent over a broad wavelength range. The mechanical properties of the h-BN films, measured by nanoindentation, show 2D elastic modulus in the range of 200-500 N/m, which is corroborated by corresponding theoretical calculations.

Pressure‐stabilized hydrides are a new rapidly growing class of high‐temperature superconductors, which is believed to be described within the conventional phonon‐mediated mechanism of coupling. Here, the synthesis of one of the best‐known high‐TC superconductors—yttrium hexahydride Im3¯m‐YH6 is reported, which displays a superconducting transition at ≈224 K at 166 GPa. The extrapolated upper critical magnetic field Bc2(0) of YH6 is surprisingly high: 116–158 T, which is 2–2.5 times larger than the calculated value. A pronounced shift of TC in yttrium deuteride YD6 with the isotope coefficient 0.4 supports the phonon‐assisted superconductivity. Current–voltage measurements show that the critical current IC and its density JC may exceed 1.75 A and 3500 A mm−2 at 4 K, respectively, which is higher than that of the commercial superconductors, such as NbTi and YBCO. The results of superconducting density functional theory (SCDFT) and anharmonic calculations, together with anomalously high critical magnetic field, suggest notable departures of the superconducting properties from the conventional Migdal–Eliashberg and Bardeen–Cooper–Schrieffer theories, and presence of an additional mechanism of superconductivity.

Highlights• Superconductivity in fcc-ThH10 at 159-161 K at the pressure 174 Gigapascals • Very wide interval of stability of fcc-ThH10 from 85 to 185 GPa. • Upper critical magnetic field ThH10 ~45 Т. • Novel discovered superhydride hcp-ThH9 with TC of 146 K (170 GPa) and upper critical field ~38 Т • Newly discovered thorium hydrides: I4/mmm-ThH4 and Cmc21-ThH6 Abstract Here we report targeted high-pressure synthesis of two novel high-TC hydride superconductors, P63/mmc-ThH9 and 3 ̅ -ThH10, with the experimental critical temperatures (TC) of 146 K and 159-161 K and upper critical magnetic fields (μHC) 38 and 45 Tesla at pressures 170-175 Gigapascals, respectively. Superconductivity was evidenced by the observation of zero resistance and a decrease of TC under external magnetic field up to 16 Tesla. This is one of the highest critical temperatures that has been achieved experimentally in any compounds, along with such materials as LaH10, H3S and HgBa2CaxCu2O6+z. Our experiments show that fcc-ThH10 has stabilization pressure of 85 GPa, making this material unique among all known high-TC metal polyhydrides. Two recently predicted Th-H compounds, I4/mmm-ThH4 (> 86 GPa) and Cmc21-ThH6 (86-104 GPa), were also synthesized. Equations of state of obtained thorium polyhydrides were measured and found to perfectly agree with the theoretical calculations. New phases were examined theoretically and their electronic, phonon, and superconducting properties were calculated.Graphical Abstract

We consider a new C 2 H nanostructure based on bilayer graphene transformed under the covalent bond of hydrogen atoms adsorbed on its external surface, as well as compounds of carbon atoms located opposite each other in neighboring layers. They constitute a "film" of the 111 diamond with a thickness of less than 1 nm, which is called diamane. The energy characteristics and electron spectra of diamane, graphene, and diamond are calculated using the density functional theory and are compared with each other. The effective Young's moduli and destruction thresholds of diamane and graphene membranes are determined by the molecular dynamics method. It is shown that C 2 H diamane is more stable than CH graphane, its dielectric "gap" is narrower than the band gap of bulk diamond (by 0.8 eV) and graphane (by 0.3 eV), and is harder and more brittle than the latter.

Distribution of superconducting properties among metal hydrides was investigated using stateof-the-art computational simulation techniques. We proposed a search rule for high-T C metalhydrogen systems based on analysis of electronic structure of atomic s, d, f-orbitals. Results of actinides and lanthanides study show that they form highly symmetric superhydrides XH 7-9 at relatively low pressures. However, actinides do not exhibit high-temperature superconductivity (except for Th-H system) and should not be considered as materials appropriate for experimental studies, as well as all d m -elements with m > 4 including metal hydrides of the precious elements. A refinement rule based on monotonic behavior of the maximum achievable critical temperature as a function of d+f electrons, maxT C (N d+f ), was proposed for already known materials. Using this rule, the reported T C values for the higher hydrides in K-H, Zr-H, Hf-H and Ti-H systems were corrected. The dependences of maxT C on the group number, period, pressure, and phase composition of hydrides were investigated. Developed model enables to make new targeted predictions relating to existence of new superconducting compounds. For Mg-H, Sr-H, Ba-H, Cs-H, Rb-H, we predict the existence of new high-T C phases XH n with n ≥ 10. Electron doping of H-sublattice by pressure-driven delocalization of d,f-electrons is suggested as the key factor for determining superconductive properties of polyhydrides. Graphical abstract:

We explore how a few-layer graphene can undergo phase transformation into thin diamond film under reduced or no pressure, if the process is facilitated by hydrogenation of the surfaces. Such a "chemically induced phase transition" is inherently nanoscale phenomenon, when the surface conditions directly affect thermodynamics, and the transition pressure depends greatly on film thickness. For the first time we obtain, by ab initio computations of the Gibbs free energy, a phase diagram (P, T, h) of quasi-two-dimensional carbon-diamond film versus multilayered graphene. It describes accurately the role of film thickness h and shows the feasibility of creating novel quasi-two-dimensional materials. Further, the role of finite diameter of graphene flakes and possible formation of the diamond films with the (110) surface are described as well.

1The atomic structure and physical properties of few-layered 111 oriented diamond nanocrystals (diamanes), covered by hydrogen atoms from both sides are studied using electronic band structure calculations. It was shown that energy stability linear increases upon increasing of the thickness of proposed structures. All 2D carbon films display direct dielectric band gaps with nonlinear quantum confinement response upon the thickness. Elastic properties of diamanes reveal complex dependence upon increasing of the number of 111 layers. All theoretical results were compared with available experimental data.

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