nfei Li

Buckled Silicene Formation on Ir(111)

Silicene, a two-dimensional (2D) honeycomb structure similar to graphene, has been successfully fabricated on an Ir(111) substrate. It is characterized as a (√7×√7) superstructure with respect to the substrate lattice, as revealed by low energy electron diffraction and scanning tunneling microscopy. Such a superstructure coincides with the (√3×√3) superlattice of silicene. First-principles calculations confirm that this is a (√3×√3)silicene/(√7×√7)Ir(111) configuration and that it has a buckled conformation. Importantly, the calculated electron localization function shows that the silicon adlayer on the Ir(111) substrate has 2D continuity. This work provides a method to fabricate high-quality silicene and an explanation for the formation of the buckled silicene sheet.

Buckled Germanene Formation on Pt(111)

Germanene, a 2D honeycomb lattice analogous to graphene, is fabricated on a Pt(111) surface. It exhibits a buckled configuration with a (3 × 3) superlattice coinciding with the substrates (√19 × √19) superstructure. Covalent bonds exist throughout the germanene layer. The resulting high-quality germanene enables researchers to explore the fundamentals of germanene and its potential applications.

Monolayer PtSe2, a New Semiconducting Transition-Metal-Dichalcogenide, Epitaxially Grown by Direct Selenization of Pt

Single-layer transition-metal dichalcogenides (TMDs) receive significant attention due to their intriguing physical properties for both fundamental research and potential applications in electronics, optoelectronics, spintronics, catalysis, and so on. Here, we demonstrate the epitaxial growth of high-quality single-crystal, monolayer platinum diselenide (PtSe2), a new member of the layered TMDs family, by a single step of direct selenization of a Pt(111) substrate. A combination of atomic-resolution experimental characterizations and first-principle theoretic calculations reveals the atomic structure of the monolayer PtSe2/Pt(111). Angle-resolved photoemission spectroscopy measurements confirm for the first time the semiconducting electronic structure of monolayer PtSe2 (in contrast to its semimetallic bulk counterpart). The photocatalytic activity of monolayer PtSe2 film is evaluated by a methylene-blue photodegradation experiment, demonstrating its practical application as a promising photocatalyst. Moreover, circular polarization calculations predict that monolayer PtSe2 has also potential applications in valleytronics.

Reversible Single Spin Control of Individual Magnetic Molecule by Hydrogen Atom Adsorption

The reversible control of a single spin of an atom or a molecule is of great interest in Kondo physics and a potential application in spin based electronics. Here we demonstrate that the Kondo resonance of manganese phthalocyanine molecules on a Au(111) substrate have been reversibly switched off and on via a robust route through attachment and detachment of single hydrogen atom to the magnetic core of the molecule. As further revealed by density functional theory calculations, even though the total number of electrons of the Mn ion remains almost the same in the process, gaining one single hydrogen atom leads to redistribution of charges within 3d orbitals with a reduction of the molecular spin state from S = 3/2 to S = 1 that directly contributes to the Kondo resonance disappearance. This process is reversed by a local voltage pulse or thermal annealing to desorb the hydrogen atom.

Two-dimensional transition metal honeycomb realized: Hf on Ir(111).

Two-dimensional (2D) honeycomb systems made of elements with d electrons are rare. Here, we report the fabrication of a transition metal (TM) 2D layer, namely, hafnium crystalline layers on Ir(111). Experimental characterization reveals that the Hf layer has its own honeycomb lattice, morphologically identical to graphene. First-principles calculations provide evidence for directional bonding between adjacent Hf atoms, analogous to carbon atoms in graphene. Calculations further suggest that the freestanding Hf honeycomb could be ferromagnetic with magnetic moment μ/Hf = 1.46 μ(B). The realization and investigation of TM honeycomb layers extend the scope of 2D structures and could bring about novel properties for technological applications.

Multi-oriented moire superstructures of graphene on Ir(111): experimental observations and theoretical models

Six types of moiré superstructures of graphene on Ir(111) with different orientations (labeled as R0, R14, R19, R23, R26 and R30) are investigated by low-energy electron diffraction, scanning tunneling microscopy and first-principles calculations. The moiré superstructure of R0 graphene has remarkable diffraction spots and deeper corrugation than that of the other superstructures. A high-order commensurate (HOC) method is applied to produce a list of all possible graphene moiré superstructures on Ir(111). Several useful structural data including the precise matrices of the moiré patterns are revealed. Density functional theory based first-principles calculations that include van der Waals interactions reveal the differences of the geometric environment and electronic structures of carbon atoms with respect to the underlying Ir(111) lattices for all the observed moiré patterns. The further calculations of electronic properties at the graphene-Ir interfaces show that the electron transfers for all superstructures are small and of the same order of magnitude, which demonstrates a weak interaction between graphene and the Ir(111) substrate, leading to the coexistence of multi-oriented moiré superstructures.

Silicon intercalation at the interface of graphene and Ir(111)

We report on the structural and electronic properties in the heterostructure of graphene/silicon/Ir(111). A (√19 × √19)R23.41° superstructure is confirmed by low energy electron diffraction and scanning tunneling microscopy and its formation is ascribed to silicon intercalation at the interface between the graphene and the Ir(111) substrate. The dI/dV measurements indicate that the interaction between graphene and Ir is effectively decoupled after silicon intercalation. Raman spectroscopy also reveals the vibrational states of graphene, G peak and 2D peak, which further demonstrates that the silicon-buffered graphene behaves more like intrinsic graphene.

Construction of 2D Atomic Crystals on Transition Metal Surfaces: Graphene, Silicene, and Hafnene

The synthesis and structures of graphene on Ru(0001) and Pt(111), silicene on Ag(111) and Ir(111) and the honeycomb hafnium lattice on Ir(111) are reviewed. Epitaxy on a transition metal (TM) substrate is a pro-mising method to produce a variety of two dimensional (2D) atomic crystals which potentially can be used in next generation electronic devices. This method is particularly valuable in the case of producing 2D materials that do not exist in 3D forms, for instance, silicene. Based on the intensive investigations of epitaxial graphene on TM in recent years, it is known that the quality of graphene is affected by many factors, including the interaction between the 2D material overlayer and the substrate, the lattice mismatch, the nucleation density at the early stage of growth. It is found that these factors also apply to many other epitaxial 2D crystals on TM. The knowledge from the reviewed systems will shine light on the design and synthesis of new 2D crystals with novel properties.

Direct observation of spin-layer locking by local Rashba effect in monolayer semiconducting PtSe 2 film

The generally accepted view that spin polarization in non-magnetic solids is induced by the asymmetry of the global crystal space group has limited the search for spintronics materials mainly to non-centrosymmetric materials. In recent times it has been suggested that spin polarization originates fundamentally from local atomic site asymmetries and therefore centrosymmetric materials may exhibit previously overlooked spin polarizations. Here, by using spin- and angle-resolved photoemission spectroscopy, we report the observation of helical spin texture in monolayer, centrosymmetric and semiconducting PtSe2 film without the characteristic spin splitting in conventional Rashba effect (R-1). First-principles calculations and effective analytical model analysis suggest local dipole induced Rashba effect (R-2) with spin-layer locking: opposite spins are degenerate in energy, while spatially separated in the top and bottom Se layers. These results not only enrich our understanding of the spin polarization physics but also may find applications in electrically tunable spintronics.

Hafnium intercalation between epitaxial graphene and Ir(111) substrate

We report on the change of structural and electronic properties while depositing Hf atoms onto the graphene epitaxially grown on Ir(111) substrate. We find that the Hf atoms intercalate between the graphene and its iridium host. This intercalation induces a new interface superstructure, as confirmed by scanning tunneling microscopy and low energy electron diffraction. Raman spectra reveal that the Hf-intercalated graphene shows the prominent features of intrinsic graphene. Our study suggests that the Hf intercalation acts as a buffer layer between the graphene and the Ir(111) substrate, restoring the graphenes intrinsic electronic properties.