Growth of two-dimensional metals has eluded materials scientists since the discovery of the atomically thin graphene and other covalently bound 2D materials. Here, we report a two-atom-thick hexagonal copper-gold alloy, grown through thermal evaporation on freestanding graphene and hexagonal boron nitride. The structures are imaged at atomic resolution with scanning transmission electron microscopy and further characterized with spectroscopic techniques. Electron irradiation in the microscope provides sufficient energy for a phase transformation of the 2D structure--atoms are released from their lattice sites with the gold atoms eventually forming face-centered cubic nanoclusters on top of 2D regions during observation. The presence of copper in the alloy enhances sticking of gold to the substrate, which has clear implications for creating atomically thin electrodes for applications utilizing 2D materials. Its practically infinite surface-to-bulk ratio also makes the 2D CuAu particularly interesting for catalysis applications.
The graphene-graphite relationship in structural geometry is a basic principle to predict novel two-dimensional (2D) materials. Here, we demonstrate that this is not the case in binary metallic systems. We use the Bayesian optimization framework combined with the density-functional theory approach to determine the stable configuration of atomic species on a hexagonal plane. We show that the optimized structure of 2D Cu-Au exhibits the hexagonal lattice of a hexagonal ring of Cu atoms containing one Au atom, where the number of the Cu atoms is larger than that of the Au atoms in the unit cell, …
Honeycomb structure has been frequently observed in two-dimensional (2D) materials. CuAu in the buckled honeycomb (BHC) structure has been synthesized recently, which is the first case of 2D intermetallic compounds. Here, the dynamical stability of 2D $AX$ in the BHC structure, where $A=$ Cu, Ag, and Au and $X$ is a metallic element in the periodic table, is systematically studied by calculating phonon dispersions from first-principles. Among 135 $AX$, more than 50 $AX$ are identified to be dynamically stable. In addition, (i) a relationship between the dynamical stability and the formation en…
We predict a new three-dimensional (3D) boron allotrope based on systematic first-principles electronic structure calculations. This allotrope can be derived by substituting each carbon atom in a hexagonal diamond lattice with a B$_{4}$ tetrahedron and possesses the same space group $P6_{3}/mmc$ as hexagonal diamond, hence it is termed as H-boron. We show that H-boron has good stability and excellent mechanical property. Remarkably, we find that H-boron is a topological metal with rich types of spin-orbit-free emergent fermions, including semi-Dirac fermion, quadratic and linear triple-point f…
In this paper, we perform theoretical study on the physical properties of two-dimensional transition metal chalcogenides MX$_{2}$ and M$_{2}$X$_{3}$ (M= Ni, Pd; X= S, Se, Te). These studied materials are classified in three stable phases according to their lattice structures: hexagonal MX$_{2}$, orthorhombic MX$_{2}$ and orthorhombic M$_{2}$X$_{3}$. They have either isotropic or anisotropic in-plane properties depending on their symmetries. In particular, the orthorhombic MX$_{2}$ and M$_{2}$X$_{3}$ have low lattice symmetry and present highly anisotropic properties. The orthorhombic MX$_{2}$ …
Hexagonal boron nitride (h-BN) is an important insulating substrate for two-dimensional (2D) heterostructure devices and possesses high dielectric strength comparable to SiO2. Here, we report two clear differences in their physical properties. The first one is the occurrence of Fermi level pinning at the metal/h-BN interface, unlike that at the metal/SiO2 interface. The second one is that the carrier of Fowler-Nordheim (F-N) tunneling through h-BN is a hole, which is opposite to an electron in the case of SiO2. These unique characteristics are verified by I-V measurements in the graphene/h-BN/…
The production of high-quality hexagonal boron nitride (h-BN) is essential for the ultimate performance of two-dimensional (2D) materials-based devices, since it is the key 2D encapsulation material. Here, a decisive guideline is reported for fabricating high-quality h-BN on transition metals: It is crucial to exclude carbon from h-BN related process. Otherwise carbon prevails over boron and nitrogen due to its larger binding energy, thereupon forming graphene on metals after high-temperature annealing. We demonstrate the surface reaction-assisted conversion from h-BN to graphene with high-tem…
We use dispersion-corrected density-functional theory to determine the relative energies of competing polytypes of bulk layered hexagonal post-transition-metal chalcogenides, to search for the most stable structures of these potentially technologically important semiconductors. We show that there is some degree of consensus among dispersion-corrected exchange-correlation functionals regarding the energetic orderings of polytypes, but we find that for each material there are multiple stacking orders with relative energies of less than 1 meV per monolayer unit cell, implying that stacking faults…
Basal slip acts as a secondary deformation mode in hexagonal close-packed titanium and becomes one of the primary mechanisms in titanium alloyed with simple metals. As these solute elements also lead to a pronounced reduction of the energy of the basal stacking fault, one can hypothesize that they promote basal dissociation of dislocations which can then easily glide in the basal planes. Here, we verify the validity of this hypothesis using ab initio calculations to model the interaction of a screw dislocation with indium (In) and tin (Sn). These calculations confirm that these simple metals a…
Elastic and electronic properties of hexagonal rhenium sub-nitrides Re3N and Re2N in comparison with hcp-Re and wurtzite-like rhenium mononitride ReN
最近，两个新的六方晶系亚氮化物Re 3 N和Re 2N，它们属于已知的4d，5d重金属的富金属（M/N > 1）氮化物的相当罕见的组，已经被成功地合成，并且它们作为超不可压缩材料的潜在的技术应用已经被提出。在这项工作中，我们提出了一个详细的从头算研究的新的六方亚氮化物相比，六方Re和纤锌矿状的六方单氮化物ReN，与这些六方系统的弹性，电子性能和化学键的Re/N的化学计量比的函数的趋势进行评估：Re \rightarrow Re 3 N \rightarrow Re 2 N \rightarrow Re N。
Hexagonal oxide perovskites, in contrast to the more familiar perovskites, allow for face-sharing of metal-oxygen octahedra or trigonal prisms within their structural frameworks. This results in dimers, trimers, tetramers, or longer fragments of chains of face-sharing octahedra in the crystal structures, and consequently in much shorter metal-metal distances and lower metal-oxygen-metal bond angles than are seen in the more familiar perovskites. The presence of the face-sharing octahedra can have a dramatic impact on magnetic properties of these compounds, and dimer-based materials, in particu…
Phosphorene, a single layer of black phosphorous (BLK-P), has a significant potential for flexible and tunable electronics, but attempts to grow it epitaxially have been unsuccessful to date. Meanwhile, hexagonal blue phoshorous (BL-P) has been achieved on closed-packed (111) metal surfaces in special growth conditions of high vapor pressure and high reactivity of phosphorous. The (111) surfaces favors BL-P over BLK-P due to its hexagonal symmetry. Here, we investigate computationally the alternative offered by stepped substrates. Using the Cu(311) surface as a model, we find that surface step…
A Kagome lattice, formed by triangles of two different directions, is known to have many emergent quantum phenomena. Under the breathing anisotropy of bond strengths, this lattice can become a higher-order topological insulator (HOTI), which hosts topologically protected corner states. Experimental realizations of HOTI on breathing Kagome lattices have been reported for various artificial systems, but not for simple natural materials with an electronic breathing Kagome lattice. Here we prove that a breathing Kagome lattice and HOTI are hidden inside the electronic structure of hexagonal transi…
Magnetic hardness of hexagonal and orthorhombic Fe$_{3}$C, Co$_{3}$C, (Fe-Co)$_{3}$C, and their alloys with boron, nitrogen, and transition metals: A first-principles study
本研究利用 FPLO 密度泛函计算系统考察六方和正交 Fe-Co 碳化物及其 B、N、过渡金属合金的磁性质，识别出硬磁候选材料并首次揭示 MAE 的近等电子规律。
Nanocrystalline Ni can adopt a hexagonal close-packed (hcp) structure,which does not appear in a bulk form under any conditions.Structural studies found its $>$ 20% expansion of volume per atom compared with face-centered cubic (fcc) Ni.Hard x-ray photoemission spectroscopy of the hcp phase revealed a $>$ 0.14 valence electron transfer from 3$d$ to 4$s$ and 4$p$ orbitals and a 0.25 eV reduction in band energy per electron compared with the fcc phase.Its nonmagnetic nature was attributed to a low density of states at the Fermi level.Thermodynamic stability of the hcp phase can be explained by t…
We describe a technique for making one-dimensional ohmic contacts to individual graphene layers encapsulated in hexagonal boron nitride (h-BN) using CF4 and O2 plasmas. The high etch selectivity of h-BN against graphene (>1000) is achieved by increasing the plasma pressure, which enables etching of h-BN, while graphene acts as an etch stop to protect underlying h-BN. A low-pressure O2 plasma anisotropically etches graphene in the vertical direction, which exposes graphene edges at h-BN sidewalls. Despite the O2 plasma bombardment, the lower h-BN layer functions as an insulating layer. Thus, th…
In van der Waals heterostructures, the relative alignment of bands between layers, and the resulting band hybridisation, are key factors in determining a range of electronic properties. This work examines these effects for heterostructures of transition metal dichalcogenides (TMDs) and hexagonal boron nitride (hBN), an ubiquitous combination given the role of hBN as an encapsulating material. By comparing results of density functional calculations with experimental angle-resolved photoemission spectroscopy (ARPES) results, we explore the hybridisation between the valence states of the TMD and …
High-level first-principles computations predict blue phosphorene bilayer to be a two-dimensional metal. This structure has not been considered before and was identified by employing a block-diagram scheme that yields the complete set of five high-symmetry stacking configurations of buckled honeycomb layers, and allows their unambiguous classification. We show that all of these stacking configurations are stable or at least metastable configurations both for blue phosphorene and gray arsenene bilayers. For blue phosphorene, the most stable stacking configuration has not yet been reported, and …
Based on crystal chemistry considerations and quantum density functional theory ground state calculations, rhombohedral rh-C3 and hexagonal h-C6 carbon allotropes are proposed and energetically calculated as new stable ultra-hard phases likewise lonsdaleite. Along the two kinds of carbon in linear C2-C1-C2 lattice, distorted tetrahedra C2(sp3) with an angle of 106.17° (smaller than ideal 109.4°) and C1(sp)-like hybridizations are inferred from charge density projections. The calculated elastic constants point to a strong anisotropy of mechanical properties of rh-C3 and h-C6 with an exceptional…
Hexagonal boron nitride (h-BN) is unique among two-dimensional materials, with a large band gap (~6 eV) and high thermal conductivity (>400 W/m/K), second only to diamond among electrical insulators. Most electronic studies to date have relied on h-BN exfoliated from bulk crystals; however, for scalable applications the material must be synthesized by methods such as chemical vapor deposition (CVD). Here, we demonstrate single- and few-layer h-BN synthesized by CVD on single crystal platinum and on carbon nanotube (CNT) substrates, also comparing these films with h-BN deposited on the more com…
Conversion of mechanical forces to electric signal is possible in non-centrosymmetric materials due to linear piezoelectricity. The extraordinary mechanical properties of two-dimensional materials and their high crystallinity make them exceptional platforms to study and exploit the piezoelectric effect. Here, the piezoelectric response of non-centrosymmetric hexagonal two-dimensional crystals is studied using the modern theory of polarization and ${\bm k} \cdot {\bm p}$ model Hamiltonians. An analytical expression for the piezoelectric constant is obtained in terms of topological quantities su…
Polar metals are an intriguing class of materials that simultaneously host free carriers and polar structural distortions. Despite the name "polar metal," however, most well-studied polar metals are poor electrical conductors. Here, we demonstrate the molecular beam epitaxial (MBE) growth of LaPtSb and LaAuGe, two polar metal compounds whose electrical resistivity is an order of magnitude lower than the well studied oxide polar metals. These materials belong to a broad family of $ABC$ intermetallics adopting the stuffed wurtzite structure, also known as hexagonal Heusler compounds. Scanning tr…
I search for the ground state structures of the kagome metals KV$_3$Sb$_5$, RbV$_3$Sb$_5$, and CsV$_3$Sb$_5$ using first principles calculations. Group-theoretical analysis shows that there are seventeen different distortions that are possible due to the phonon instabilities at the $M$ $(\frac{1}{2},0,0)$ and $L$ $(\frac{1}{2},0,\frac{1}{2})$ points in the Brilouin zone of the parent $P6/mmm$ phase of these materials. I generated these structures for the three compounds and performed full structural relaxations that minimize the atomic forces and lattice stresses. I find that the $Fmmm$ phase …
The production of high-quality two-dimensional (2D) materials is essential for the ultimate performance of single layers and their hybrids. Hexagonal boron nitride (h-BN) is foreseen to become the key 2D hybrid and packaging material since it is insulating, tight, flat, transparent and chemically inert, though it is difficult to attain in ultimate quality. Here, a new scheme is reported for producing single layer h-BN that shows higher quality and much more uniformity than material from chemical vapor deposition (CVD). We delaminate CVD h-BN from Rh(111) and transfer it to a clean metal surfac…
Anisotropic core-shell model of a nano-grained polycrystal is extended to estimate the effective elastic stiffness of several metals of hexagonal crystal lattice symmetry. In the approach the bulk nanocrystalline material is described as a two-phase medium with different properties for a grain boundary zone and a grain core. While the grain core is anisotropic, the boundary zone is isotropic and has a thickness defined by the cutoff radius of a corresponding atomistic potential for the considered metal. The predictions of the proposed meanfield model are verified with respect to simulations pe…
Carbon(C) doped hexagonal boron nitride(hBN) has been experimentally reported in recent years to be a possible catalytic host to oxygen reduction reaction(ORR), as well as a possible ferromagnet at room temperature. Substitution by C in hBN has been also reported to form islands of graphene. In this work, we explore from first principles, the connection between these different aspects of C doped hBN. We find formation of graphene islands covering unequal number of B and N sites in hBN to be energetically plausible. They possess a net non-zero magnetic moment and are also found to be substantia…
