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Register a new userStructure of Native Two-dimensional Oxides on III--Nitride Surfaces
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When pristine material surfaces are exposed to air, highly reactive broken bonds can promote the formation of surface oxides with structures and properties differing greatly from bulk. Determination of the oxide structure, however, is often elusive through the use of indirect diffraction methods or techniques that probe only the outer most layer. As a result, surface oxides forming on widely used materials, such as group III-nitrides, have not been unambiguously resolved, even though critical properties can depend sensitively on their presence. In this work, aberration corrected scanning transmission electron microscopy reveals directly, and with depth dependence, the structure of native two--dimensional oxides that form on AlN and GaN surfaces. Through atomic resolution imaging and spectroscopy, we show that the oxide layers are comprised of tetrahedra--octahedra cation--oxygen units, similar to bulk $theta$--Al$_2$O$_3$ and $beta$--Ga$_2$O$_3$. By applying density functional theory, we show that the observed structures are more stable than previously proposed surface oxide models. We place the impact of these observations in the context of key III-nitride growth, device issues, and the recent discovery of two-dimnesional nitrides.
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We discuss unreported transitions of oxidized GaAs surfaces between (super)hydrophilic and hydrophobic states when stored in ambient conditions. Contact angles higher than 90deg and high adhesive force were observed for several air-aged epitaxial samples grown under different conditions as well as on epi-ready wafers. Regardless of the morphologies of the surface, superhydrophilicity of oxygen-plasma treated samples was observed, an effect disappearing with storage time. Reproducible hydrophobicity was likewise observed, as expected, after standard HCl surface etching. The relation between surface oxides and hydrophobic/hydrophilic behavior is discussed.
The structure of amorphous materials-continuous random networks (CRN) vs. CRN containing randomly dispersed crystallites-has been debated for decades. In two-dimensional (2D) materials, this question can be addressed more directly. Recently, controlled experimental conditions and atomic-resolution imaging found that monolayer amorphous carbon (MAC) is a CRN containing random graphene nanocrystallites. Here we report Monte Carlo simulations of the structure evolution of monolayer amorphous boron nitride (ma-BN) and demonstrate that it also features distorted sp2-bonding, but it has a purely CRN structure. The key difference is that, at low temperatures, C atoms easily form hexagons, whereas the probability to form canonical B-N-B-N-B-N hexagons is very low. On the other hand, hexagons have lower energy than non-hexagons, which results in hexagonal CRN regions that grow much like nanocrystallites in MAC. The net conclusion is that two distinct forms of amorphous structure are possible in 2D materials. The as-generated ma-BN is stable at room-temperature and insulating.
Chemically stable quantum-confined 2D metals are of interest in next-generation nanoscale quantum devices. Bottom-up design and synthesis of such metals could enable the creation of materials with tailored, on-demand, electronic and optical properties for applications that utilize tunable plasmonic coupling, optical non-linearity, epsilon-near-zero behavior, or wavelength-specific light trapping. In this work, we demonstrate that the electronic, superconducting and optical properties of air-stable two-dimensional metals can be controllably tuned by the formation of alloys. Environmentally robust large-area two-dimensional InxGa1-x alloys are synthesized by Confinement Heteroepitaxy (CHet). Near-complete solid solubility is achieved with no evidence of phase segregation, and the composition is tunable over the full range of x by changing the relative elemental composition of the precursor. The optical and electronic properties directly correlate with alloy composition, wherein the dielectric function, band structure, superconductivity, and charge transfer from the metal to graphene are all controlled by the indium/gallium ratio in the 2D metal layer.
High-performance thermoelectric oxides could offer a great energy solution for integrated and embedded applications in sensing and electronics industries. Oxides, however, often suffer from low Seebeck coefficient when compared with other classes of thermoelectric materials. In search of high-performance thermoelectric oxides, we present a comprehensive density functional investigation, based on GGA$+U$ formalism, surveying the 3d and 4d transition-metal-containing ferrites of the spinel structure. Consequently, we predict MnFe$_2$O$_4$ and RhFe$_2$O$_4$ have Seebeck coefficients of $sim pm 600$ $mu$V K$^{-1}$ at near room temperature, achieved by light hole and electron doping. Furthermore, CrFe$_2$O$_4$ and MoFe$_2$O$_4$ have even higher ambient Seebeck coefficients at $sim pm 700$ $mu$V K$^{-1}$. In the latter compounds, the Seebeck coefficient is approximately a flat function of temperature up to $sim 700$ K, offering a tremendous operational convenience. Additionally, MoFe$_2$O$_4$ doped with $10^{19}$ holes/cm$^3$ has a calculated thermoelectric power factor of $689.81$ $mu$W K$^{-2}$ m$^{-1}$ at $300$ K, and $455.67$ $mu$W K$^{-2}$ m$^{-1}$ at $600$ K. The thermoelectric properties predicted here can bring these thermoelectric oxides to applications at lower temperatures traditionally fulfilled by more toxic and otherwise burdensome materials.
The widely used crystal structures for both heptazine-based and triazine-based two-dimensional (2D) graphitic carbon nitride (g-C$_3$N$_4$) are the flat P-6m2 configurations. However, the experimentally synthesized 2D g-C$_3$N$_4$ possess thickness ranging in 0.2-0.5 nm, indicating that the theoretically used flat P-6m2 configurations are not the correct ground states. In this work, we propose three new corrugated structures P321, P3m1 and Pca21 with energies of 66 (86), 77 (87) and 78 (89) meV/atom lower than that of the corresponding heptazine-based (triazine-based) g-C$_3$N$_4$ in flat P-6m2 configuration, respectively. These corrugated structures have very similar periodic patterns to the flat P-6m2 ones and they are difficult to be distinguished from each other according to their top-views. The optimized thicknesses of the three corrugated structures ranging in 1.347-3.142 {AA} are in good agreement with the experimental results. The first-principles results show that these corrugated structural candidates are also semiconductors with band gaps slightly larger than those of the correspondingly flat P-6m2 ones. Furthermore, they possess also suitable band edge positions for sun-light-driven water-splitting at both $pH=0$ and $pH=7$ environments. Our results show that these three new structures are more promising candidates for the experimentally synthesized g-C$_3$N$_4$.
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