Yeliang Wang

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.

Metal-organic coordination interactions in Fe-Terephthalic acid networks on Cu(100)

Metal-organic coordination interactions are prime candidates for the formation of self-assembled, nanometer-scale periodic networks with room-temperature structural stability. We present X-ray photoelectron spectroscopy measurements of such networks at the Cu(100) surface which provide clear evidence for genuine metal-organic coordination. This is evident as binding energy shifts in the O 1s and Fe 3p photoelectron peaks, corresponding to O and Fe atoms involved in the coordination. Our results provide the first clear evidence for charge-transfer coordination in metal-organic networks at surfaces and demonstrate a well-defined oxidation state for the coordinated Fe ions.

Silicon layer intercalation of centimeter-scale, epitaxially grown monolayer graphene on Ru(0001)

We develop a strategy for graphene growth on Ru(0001) followed by silicon-layer intercalation that not only weakens the interaction of graphene with the metal substrate but also retains its superlative properties. This G/Si/Ru architecture, produced by silicon-layer intercalation approach (SIA), was characterized by scanning tunneling microscopy/spectroscopy and angle resolved electron photoemission spectroscopy. These experiments show high structural and electronic qualities of this new composite. The SIA allows for an atomic control of the distance between the graphene and the metal substrate that can be used as a top gate. Our results show potential for the next generation of graphene-based materials with tailored properties.

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.

Intercalation of metal islands and films at the interface of epitaxially grown graphene and Ru(0001) surfaces

We report on intercalation of seven kinds of metals—Pt, Pd, Ni, Co, Au, In, and Ce—at the interface between an epitaxially grown graphene layer and a Ru(0001) substrate. Atomic resolution scanning tunneling microscopy images of perfect graphene lattice are obtained on top of these intercalated metals, showing that the high quality of the original graphene is, in the end, undisturbed by the intercalation. A model based theoretical calculation is proposed for the intercalation mechanism: metal atom-aided defect formation and self-healing of C–C bonds at high temperature. These intercalated materials include noble metals, magnetic metals, a IIIA group metal, and a rare earth metal, which indicates that intercalation through epitaxial graphene on Ru(0001) is a universal approach for metals.

Epitaxial Growth and Air-Stability of Monolayer Antimonene on PdTe2

Monolayer antimonene is fabricated on PdTe2 by an epitaxial method. Monolayer antimonene is theoretically predicted to have a large bandgap for nanoelectronic devices. Air-exposure experiments indicate amazing chemical stability, which is great for device fabrication. A method to fabricate high-quality monolayer antimonene with several great properties for novel electronic and optoelectronic applications is provided.

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.

Direct imaging of intrinsic molecular orbitals using two-dimensional, epitaxially-grown, nanostructured graphene for study of single molecule and interactions

Using epitaxially grown graphene on Ru(0001) as a buffer layer, the intrinsic molecular orbitals of perylene-3,4,9,10-tetracarboxylic dianhydride, pentacene, and C60 molecules were imaged by means of scanning tunneling microscope (STM). Combined with density functional theory calculations, our high resolution STM images of the molecules reveal that the graphene layer decouples the individual molecules electronically from the metallic substrate. Our results show that graphene-based moire pattern can be used as a unique way to probe the intrinsic electronic structures of molecular adsorbates and their interactions.