No abstract

Achieving large-area uniform 2D metal-organic frameworks (MOFs) and controlling their electronic properties on inert surfaces is a big step toward future applications in electronic devices. Here a 2D monolayer Cu-dicyanoanthracene MOF with long-range order is successfully fabricated on an epitaxial graphene surface. Its structural and electronic properties are studied by low-temperature scanning tunneling microscopy and spectroscopy complemented by density-functional theory calculations. Access to multiple molecular charge states in the 2D MOF is demonstrated using tip-induced local electric fields. It is expected that a similar strategy could be applied to fabricate and characterize 2D MOFs with exotic, engineered electronic states.

Substituting carbon with silicon in organic molecules and materials has long been an attractive way to modify their electronic structure and properties. Silicon-doped graphene-based materials are known to exhibit exotic properties, yet conjugated organic materials with atomically precise Si substitution have remained difficult to prepare. Here we present the on-surface synthesis of one- and two-dimensional covalent organic frameworks whose backbones contain 1,4-disilabenzene (C4Si2) linkers. Silicon atoms were first deposited on a Au(111) surface, forming a AuSix film on annealing. The subsequent deposition and annealing of a bromo-substituted polyaromatic hydrocarbon precursor (triphenylene or pyrene) on this surface led to the formation of the C4Si2-bridged networks, which were characterized by a combination of high-resolution scanning tunnelling microscopy and photoelectron spectroscopy supported by density functional theory calculations. Each Si in a hexagonal C4Si2 ring was found to be covalently linked to one terminal Br atom. For the linear structure obtained with the pyrene-based precursor, the C4Si2 rings were converted into C4Si pentagonal siloles by further annealing.

The ability to imprint a given material property to another through a proximity effect in layered 2D materials has opened the way to the creation of designer materials. Here, molecular‐beam epitaxy is used for direct synthesis of a superconductor–ferromagnet heterostructure by combining superconducting niobium diselenide (NbSe2) with the monolayer ferromagnetic chromium tribromide (CrBr3). Using different characterization techniques and density‐functional theory calculations, it is confirmed that the CrBr3 monolayer retains its ferromagnetic ordering with a magnetocrystalline anisotropy favoring an out‐of‐plane spin orientation. Low‐temperature scanning tunneling microscopy measurements show a slight reduction of the superconducting gap of NbSe2 and the formation of a vortex lattice on the CrBr3 layer in experiments under an external magnetic field. The results contribute to the broader framework of exploiting proximity effects to realize novel phenomena in 2D heterostructures.

Low symmetry 2D materials offer an alternative for the fabrication of optoelectronic devices which are sensitive to light polarization. The investigation of electron–phonon interactions in these materials is essential since they affect the electrical conductivity. Raman scattering probes light–matter and electron–phonon interactions, and their anisotropies are described by the Raman tensor. The tensor elements can have complex values, but the origin of this behavior in 2D materials is not yet well established. In this work, we studied a single-layer triclinic ReSe2 by angle-dependent polarized Raman spectroscopy. The obtained values of the Raman tensor elements for each mode can be understood by considering a new coordinate system, which determines the physical origin of the complex nature of the Raman tensor elements. Our results are explained in terms of anisotropy of the electron–phonon coupling relevant to the engineering of new optoelectronic devices based on low-symmetry 2D materials.

The combination of two-dimensional (2D) materials into vertical heterostructures has emerged as a promising path to designer quantum materials with exotic properties. Here, we extend this concept from inorganic 2D materials to 2D metal–organic frameworks (MOFs) that offer additional flexibility in realizing designer heterostructures. We successfully fabricate a monolayer 2D Cu-dicyano­anthracene MOF on a 2D van der Waals NbSe2 superconducting substrate. The structural and electronic properties of two different phases of the 2D MOF are characterized by low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS), complemented by density-functional theory (DFT) calculations. These experiments allow us to follow the formation of the kagome band structure from Star of David-shaped building blocks. This work extends the synthesis and electronic tunability of 2D MOFs beyond the electronically less relevant metal and semiconducting surfaces to superconducting substrates, which are needed for the development of emerging quantum materials such as topological superconductors.

We present a tight-binding model that allows a quantitative fitting of the ab initio band structure, near the Fermi energy, of several 2D metal–organic and covalent–organic honeycomb-like frameworks. The model is based on the kagome–honeycomb lattice, defined as the superposition of the named lattices. The full spectrum of the model Hamiltonian is analytically solvable in the case of one orbital per site and nearest-neighbor hopping, and at selected points of the Brillouin zone for hopping up to second neighbors. With proper choices of parameters and band occupation, the model describes five types of electronic structure within this class of materials. All five types are obtained in explicit fittings of the model to first-principles calculations of 2D frameworks. The model also permits the identification of crystallographic point group broken symmetries that lead to band gap openings in Cu3(HITP)2. Spin–orbit effects are also investigated in model and first-principles calculations of Ni3C12S12.

The design of magnetic topological states due to spin polarization in an extended π carbon system has great potential in spintronics application. Although magnetic zigzag edges in graphene nanoribbons (GNRs) have been investigated earlier, real-space observation and manipulation of spin polarization in a heteroatom substituted system remains challenging. Here, we investigate a zero-bias peak at a boron site embedded at the center of an armchair-type GNR on a AuSiX/Au­(111) surface with a combination of low-temperature scanning tunneling microscopy/spectroscopy and density functional theory calculations. After the tip-induced removal of a Si atom connected to two adjacent boron atoms, a clear Kondo resonance peak appeared and was further split by an applied magnetic field of 12 T. This magnetic state can be relayed along the longitudinal axis of the GNR by sequential removal of Si atoms.