Grain boundaries (GBs) in polycrystalline graphene scatter charge carriers, which reduces carrier mobility and limits graphene applications in high-speed electronics. Here we report the extraction of the resistivity of GBs and the effect of GBs on carrier mobility by direct four-probe measurements on millimeter-sized graphene bicrystals grown by chemical vapor deposition (CVD). To extract the GB resistivity and carrier mobility from direct four-probe intragrain and intergrain measurements, an electronically equivalent extended 2D GB region is defined based on Ohm’s law. Measurements on seven representative GBs find that the maximum resistivities are in the range of several kΩ·μm to more than 100 kΩ·μm. Furthermore, the mobility in these defective regions is reduced to 0.4–5.9‰ of the mobility of single-crystal, pristine graphene. Similarly, the effect of wrinkles on carrier transport can also be derived. The present approach provides a reliable way to directly probe charge-carrier scattering at GBs and can be further applied to evaluate the GB effect of other two-dimensional polycrystalline materials, such as transition-metal dichalcogenides (TMDCs).Keywords: four-probe measurement; grain boundary; Graphene; mobility; wrinkle;

Silicon-based two-dimensional (2D) materials are uniquely suited for integration in Si-based electronics. Silicene, an analogue of graphene, was recently fabricated on several substrates and was used to make a field-effect transistor. Here, we report that when Ru(0001) is used as a substrate, a range of distinct monolayer silicon structures forms, evolving toward silicene with increasing Si coverage. Low Si coverage produces a herringbone structure, a hitherto undiscovered 2D phase of silicon. With increasing Si coverage, herringbone elbows evolve into silicene-like honeycomb stripes under tension, resulting in a herringbone-honeycomb 2D superlattice. At even higher coverage, the honeycomb stripes widen and merge coherently to form silicene in registry with the substrate. Scanning tunneling microscopy (STM) was used to image the structures. The structural stability and electronic properties of the Si 2D structures, the interaction between the Si 2D structures and the Ru substrate, and the evolution of the distinct monolayer Si structures were elucidated by density functional theory (DFT) calculations. This work paves the way for further investigations of monolayer Si structures, the corresponding growth mechanisms, and possible functionalization by impurities.

The capabilities to tune the conduction properties of materials by doping or electric fields are essential for the design of electronic devices. However, in two-dimensional materials substitutional doping has been achieved in only a few systems, such as Nb substitutional doping in MoS2. Surface charge transfer is still one of the popular ways to control whether the conduction is dominated by holes or electrons. Here, we demonstrate that a capping layer of cross-linked poly(methyl methacrylate) modifies the potential in a black phosphorus (BP) layer so that conduction in the absence of an external electric field is dominated by electrons, rather than holes. Using this technique to form adjoining regions dominated by hole and electron conduction, a family of novel planar devices, such as BP-gated diodes, BP bidirectional rectifier, and BP logic inverters, can be fabricated. The devices are potentially useful for electronic applications, including rectification and switching.Keywords: black phosphorus; field-effect transistor; interfacial charge; logic inverter; p−n diode;

The fate of superconductivity of a nanoscale superconducting film/island relies on the environment; for example, the proximity effect from the substrate plays a crucial role when the film thicknesses is much less than the coherent length. Here, we demonstrate that atomic-scale tuning of the proximity effects can be achieved by one atomically thin graphene layer inserted between the nanoscale Pb islands and the supporting Pt(111) substrate. By using scanning tunneling microscopy and spectroscopy, we show that the coupling between the electron in a normal metal and the Cooper pair in an adjacent superconductor is dampened by 1 order of magnitude via transmission through a single-atom-thick graphene. More interestingly, the superconductivity of the Pb islands is greatly affected by the moiré patterns of graphene, showing the intriguing influence of the graphene–substrate coupling on the superconducting properties of the overlayer.Keywords: graphene; Pb; scanning tunneling microscopy; superconductivity

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.

The intercalation of heteroatoms between graphene and a metal substrate has been studied intensively over the past few years, due to its effect on the graphene properties, and as a method to create vertical heterostructures. Various intercalation processes have been reported with different combinations of heteroatoms and substrates. Here we study Si intercalation between graphene and Ru(0001). We elucidate the role of cooperative interactions between hetero-atoms, graphene, and substrate. By combining scanning tunneling microscopy with density functional theory, the intercalation process is confirmed to consist of four key steps, involving creation of defects, migration of heteroatoms, self-repairing of graphene, and growth of an intercalated monolayer. Both theory and experiments indicate that this mechanism applies also to other combinations of hetero-atoms and substrates.

The Kondo effect, a widely studied phenomenon in which the scattering of conduction electrons by magnetic impurities increases as the temperature T is lowered, depends strongly on the density of states at the Fermi energy. It has been predicted by theory that magnetic impurities on free-standing monolayer graphene exhibit the Kondo effect and that control of the density of states at the Fermi level by external means can be used to switch the effect on and off. However, though transport data for Co adatoms on graphene monolayers on several substrates have been reported, there exists no evidence for a Kondo effect. Here we probe the role of the substrate on the Kondo effect of Co on graphene by combining low-temperature scanning tunneling microscopy and spectroscopy measurements with density functional theory calculations. We use a Ru(0001) substrate that is known to cause graphene to ripple, yielding a moiré superlattice. The experimental data show a sharp Kondo resonance peak near the Fermi energy from only Co adatoms at the edge of atop regions of the moiré pattern. The theoretical results show that the variation of the distance from the graphene to the Ru substrate, which controls the spin polarization and local density of states at the Fermi energy, is the key factor for the appearance of the Kondo resonance. The results suggest that rippling of graphene by suitable substrates is an additional lever for tuning and selectively switching the appearance of the Kondo effect.

Cyclotrimerization of alkynes to aromatics represents a promising approach to two-dimensional conjugated networks due to its single-reactant and atom-economy attributes, in comparison with other multicomponent coupling reactions. However, the reaction mechanism of alkyne cyclotrimerization has not yet been well understood due to characterization challenges. In this work, we take a surface reaction approach to study fundamental polymerization mechanism by using a diyne monomer named 4,4′-diethynyl-1,1′-biphenyl as a test bed. We have succeeded in directly characterizing reactants, intermediates, and their reaction products with the aid of scanning tunneling microscope, which allows us to gain mechanistic insights into the reaction pathways. By combining with density functional theory calculation, our result has revealed for the first time that the polycyclotrimerization is a two-step [2+2+2] cyclization reaction. This work provides an in-depth understanding of polycyclotrimerization process at the atomic level, offering a new avenue to design and construct of single-atom-thick conjugated networks.

Induction of chirality in planar adsorbates by hydrogenation of phthalocyanine molecules on a gold surface is demonstrated. This process merely lowers the molecular symmetry from 4- to 2-fold, but also breaks the mirror symmetry of the entire adsorbate complex (molecule and surface), thus rendering it chiral without any realignment at the surface. Repositioning of single molecules by manipulation with the scanning tunneling microscope (STM) causes interconversion of enantiomers. Dehydrogenation of the adsorbate by means of inelastic electron tunneling restores the mirror symmetry of the adsorbate complex. STM as well as density functional theory (DFT) calculations show that chirality is actually imprinted into the electronic molecular system by the surface, i.e., the lowest unoccupied orbital is devoid of mirror symmetry.Keywords: chirality; inelastic electron tunneling; phthalocyanine; scanning tunneling microscopy; single-molecule manipulation;

The theory of adenine dimer chain assembly on Cu(110) surface is controversial, due in large to the lack of an adequate description of the van der Waals (vdW) interactions. Here, we show by a combined first-principles calculation and experiment that the role of the vdW interactions is to tilt the energy balance at different levels of chain hierarchy. We find that the stable chains are made of metastable dimers, whereas the metastable chains, close in energy to the stable ones, are made of stable dimers. As such, at room temperate deposition, adenine dimers exist primarily in their stable form. This leads to the formation of metastable chains. By annealing at elevated temperature, however, more dimers can exist in the metastable form. This leads to the nucleation of the stable chains at a different orientation. The thermally controlled chain rotation is expected to be of general importance to the understanding of amino acids assembly and functionalization at the most elemental level.

Low-dimensional H2 aggregates have been successfully fabricated on Au(111) surfaces and investigated by means of low temperature scanning tunneling microscopy. We use manganese phthalocyanine (MnPc) molecules anchored on the Au(111) surface to efficiently collect and pin hydrogen molecules. A two-dimensional (2D) molecular hydrogen cluster is formed around the MnPc. The hydrogen cluster exhibits bias-dependent topography and spatial-dependent conductance spectra, which are rationalized by the exponentially decreasing threshold energy with distance from the central MnPc to activate the motion of the H2 molecules. This exponential drop reveals an interfacial phase behavior in the 2D cluster.

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 μB. The realization and investigation of TM honeycomb layers extend the scope of 2D structures and could bring about novel properties for technological applications.

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.

Structural properties of Pb islands grown on graphene/Ru(0001) at various deposition temperatures (TD) and annealing temperatures (TA) are investigated by a low-temperature scanning tunneling microscope. Single-layer Pb islands with a 2 × 2 reconstruction are only formed at TD of 80 K and disappear with post-annealing to room temperature (RT). It is revealed that a morphological transition of the Pb islands takes place, from irregular shapes to a hexagonal equilibrium shape, with increasing TD or TA to RT. Moreover, Pb islands grown at TD of RT are larger than those grown at a TD of 80 K and annealed to RT. All Pb islands with a TA or TD of RT are (111)-faceted with thicknesses of even-numbered atomic layers and exhibit a weak interaction between Pb and graphene.

The template-directed assembly of planar pentacene molecules on epitaxial graphene grown on Ru(0001) (G/Ru) has been investigated by means of low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. STM experiments find that pentacene adopts a highly selective and dispersed growth mode in the initial stage. By using DFT calculations including van der Waals interactions, we find that the configuration with pentacene adsorbed on face-centered cubic (fcc) regions of G/Ru is the most stable one, which accounts for the selective adsorption at low coverage. Moreover, at high coverage, we have successfully controlled the molecular assembly from amorphous, local ordering, to long-range order by optimizing the deposition rate and substrate temperature.

We report electrical conductance and thermopower measurements on InAs nanowires synthesized by chemical vapor deposition. Gate modulation of the thermopower of individual InAs nanowires with a diameter around 20 nm is obtained over T = 40–300 K. At low temperatures (T < ∼100 K), oscillations in the thermopower and power factor concomitant with the stepwise conductance increases are observed as the gate voltage shifts the chemical potential of electrons in InAs nanowire through quasi-one-dimensional (1D) subbands. This work experimentally shows the possibility to modulate semiconductor nanowire’s thermoelectric properties through 1D subband formation in the diffusive transport regime for electron, a long-sought goal in nanostructured thermoelectrics research. Moreover, we point out the scattering (or disorder) induced energy level broadening as the limiting factor in smearing out the 1D confinement enhanced thermoelectric power factor.

The Kagome lattice of iron phthalocyanine (FePc) on the graphene moiré pattern is employed as host template for two kinds of guest molecules, FePc and tert-butyl zinc phthalocyanine ((t-Bu)4–ZnPc), to fabricate stable host–guest molecular superstructures. Both FePc and (t-Bu)4–ZnPc molecules prefer to occupy the nanoscale pores of the Kagome lattice. Ordered superstructures with alternate rows of FePc and (t-Bu)4–ZnPc are formed after coadsorption of these two species with a ratio of 1:1 on the Kagome lattice. We elucidate that formation of ordered superstructures of guest FePc and (t-Bu)4–ZnPc are controlled by long-range interaction between the guest molecules mediated by the host Kagome lattice with additional contribution from the graphene/Ru(0001) substrate.

Using first-principles calculations based on density functional theory, we studied the possible geometric configurations and electronic structures of three types of monolayer boron sheets (BSs) on different metal (Mg, Al, Ti, Au, and Ag) surfaces. We find that, when adsorbed on metal surfaces, hexagonal BS (h-BS) is more energy-favorable than triangular BS or mixed hexagonal-triangular BS, and the atop-site adsorption configuration is the most favored. For all h-BS/metal configurations, electrons are observed to transfer from metal to BS, due to the intrinsic electron deficiency of h-BS. Electronic structure analyses show that the substrates could be classified into two types according to the interactions between boron and metal: (1) h-BS on Mg(0001), Al(111), or Ti(0001) shows a relatively larger charge transfer and stronger BS–metal interactions, and the σ (in-plane) bands have the same profile as freestanding h-BS, except for a Fermi level shift caused by the charge transfer. (2) h-BS on Au(111) or Ag(111) surfaces has in-plane bands split into several subbands. A model of coronene on h-BS/Mg(0001) is also investigated, which shows that it is possible to decouple the molecular electronic structure from the metal surface by a buffer of a single sheet of boron.

Self-assembly of metal phthalocyanine (MPc) molecules on monolayer graphene (MG) epitaxially grown on Ru(0001) and Pt(111) is investigated by means of low-temperature scanning tunneling microscopy. At low coverage, dispersive single molecules, dispersive molecular chains, and small patches of Kagome lattice are observed for iron phthalocyanine (FePc), manganese phthalocyanine (MnPc), nickel phthalocyanine (NiPc), and phthalocyanine (H2Pc) on MG/Ru(0001). In contrast, although MG/Pt(111) exhibits various domains with different moiré patterns and corrugations, FePc molecules always form densely packed two-dimensional islands with a square lattice on MG/Pt(111) at submonolayer coverage. The different self-assembling behaviors of MPc molecules on MG/Ru(0001) and MG/Pt(111) originate from a subtle balance between molecule–molecule and molecule–substrate interactions tuned by central metal ions of the MPc molecules and the metal substrates.

Large-area patterned boron carbide nanowires (B4C NWs) have been synthesized using chemical vapor deposition (CVD). The average diameter of B4C NWs is about 50 nm, with a mean length of 20 μm. The B4C NWs have a single-crystal structure and conductivities around 5.1 × 10−2 Ω−1·cm−1. Field emission measurements of patterned B4C NWs films show that their turn-on electric field is 2.7 V/μm, lower than that of continuous B4C NWs films. A single nanowire also exhibits excellent flexibility under high-strain bending cycles without deformation or failure. All together, this suggests that B4C NWs are a promising candidate for flexible cold cathode materials.

Ultrathin Iron(II) Phthalocyanine (FePc) monolayers have been employed to decouple individual FePc molecules electronically from the metallic substrate, which allows the intrinsic electronic structure of the free molecule to be preserved and imaged by means of low-temperature scanning tunneling microscopy (STM). High-resolution images reveal the standard “cross” structure for molecules adsorbed at the metal surface, but a detailed electronic structure, corresponding to the highest occupied molecular state (HOMO), for molecules adsorbed at the buffer layer. Different STM images of the molecules in the first and second layer are referred to different interactions between molecules and substrate. This interpretation is verified by control experiments on a second molecule, tetra-tert-butyl zinc Phthalocyanine ((t-Bu)4ZnPc). We therefore conclude that this effect is generic and can be used to investigate a molecule's electronic structure in detail.Research Highlights► Ultrathin FePc and (t-Bu)4ZnPc monolayers as buffer layers. ► The surface charge spilled out into the vacuum vanishes for larger distances. ► The buffer layer decouples free molecules electronically from the metal surface. ► Intrinsic electronic structure of free molecule can be preserved on buffer layer. ► We directly observed detailed electronic structures of the molecule by STM.

Adsorption behavior of iron–phthalocyanine (FePc) at low submonolayer coverage on a reconstructed Au(111) single crystalline surface was investigated by a combination of low temperature scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. A site- and orientation-selective adsorption was found at different temperatures and molecular coverages by means of STM. Further DFT calculations demonstrate that the energy difference between different adsorption configurations leads to the selectivity, and thus the formation of one-dimensional molecular chains on the monatomic step edges in the fcc surface reconstruction domains. The exact adsorption site and configuration of the FePc molecule as well as the simulated STM images are obtained on the basis of DFT calculations, which is in good agreement with experimental observations.

Self-assembly of manganese phthalocyanine (MnPc) and iron phthalocyanine (FePc) molecules on Pb(111) and Au(111) surfaces is investigated by means of low-temperature scanning tunneling microscopy and density functional theory calculations. Both metal phthalocyanine (MPc) molecules form ordered close-packed islands on Pb(111) with different detail superstructures. In contrast, dispersive single molecules are observed for both MPc molecules on Au(111). The different self-assembling behaviors of MPc molecules on Pb(111) and Au(111) originate from a subtle balance between molecule–molecule and molecule–substrate interactions tuned by the substrate based on our theoretical calculation results.

High-resolution scanning tunneling microscope images of iron phthalocyanine and zinc phthalocyanine molecules on Au(111) have been obtained using a functionalized tip of a scanning tunneling microscope (STM), and show rich intramolecular features that are not observed using clean tips. Ab initio density functional theory calculations and extended Hückel theory calculations revealed that the imaging of detailed electronic states is due specifically to the decoration of the STM tip with O2. The detailed structures are differentiated only when interacting with the highly directional orbitals of the oxygen molecules adsorbed on a truncated, [111]-oriented tungsten tip. Our results indicate a method for increasing the resolution in generic scans and thus, have potential applications in fundamental research based on high-resolution electronic states of molecules on metals, concerning, for example, chemical reactions, and catalysis mechanisms.

How nature uses water molecules to create fascinating patterns ranging from snowflakes to ice cubes has intrigued mankind for centuries. Here we use ZnO to mimic nature's versatility in creating microscopic patterns with tunable morphology. During growth of ZnO on Zn-dominant spheres via chemical vapor deposition, highly regular and symmetric dendritic snowflake patterns and smooth compact islands can be obtained at different growth conditions. We reproduce the dendritic patterns using atomistic Monte Carlo simulations. These findings not only improve understanding of how water molecules form various patterns, but may also be instrumental in tailoring ZnO nanostructures for desirable functionality.

The manipulation, self-assembly, and application of functional nanostructures on solid surfaces are fundamental issues for the development of electronics and optoelectronics. For a future molecular electronics the fabrication of high-quality organic thin films on metal surfaces is crucial, which can be achieved by thermal evaporation for various organic/metal systems. The switching property of single molecules can be manipulated and measured, revealing a possibility to realize single molecular devices. Manipulation of a local conductance transition in organic thin films, used for ultra-high density data storage, has also been achieved based on several different mechanisms. The stability, reversibility, and repeatability of the local conductance transition have been improved by molecular design. In this article, we will summarize our recent scanning tunneling microscopy studies on these issues and discuss their perspectives.

La0.67Ca0.33MnO3 thin films were epitaxially grown on miscut MgO(001) substrates by pulsed laser ablation. Electrical transport properties were studied by using an ultra high vacuum, four-probe STM system at different temperatures. Anomalous resistivity behavior and metal−insulator transition temperatures were found, both of which are highly dependent upon the miscut angle (1, 3, and 5°). These phenomena are attributed to the difference in residual strain that results from the difference in terrace widths of the vicinal surfaces.Keywords: epitaxial thin film; mangnite; metal−insulator transition; miscut; strain

The adsorption of subphthalocyanine (SubPc) on the Au(111) surface has been studied by scanning tunnelling microscopy (STM). Depending on coverage and deposition temperature, four different phases have been observed, of which two are coexisting. Spontaneous symmetry breaking inducing mirror domains is observed for all structures. Supramolecular chirality is expressed at different levels and length scale. Our detailed STM study allows conclusions on the origin of polymorphism due to changing coverage and temperature.

We report on the structural transitions of molecules on metal surfaces by external electrostatic field. An electrode–molecule–electrode model is considered to quantify the effect of electrostatic forces at the molecule–electrode interface. Within a quasi-parallel-plate capacitor approach, this model reveals how external electrostatic fields change the delicate balance between molecule–substrate and molecule–molecule interactions, leading to substantial changes in the molecular conformation. The predictions are validated by scanning tunneling microscopy (STM) observations of four different molecules and electrode facets. In addition, first-principles simulations verify the results of our model calculations.

The self-assembly behavior of the mixture of tetra-tert-butyl zinc(II) phthalocyanine (TB-ZnPc) isomers on a Au(111) surface has been investigated by means of scanning tunneling microscopy (STM). Four tert-butyl groups are added to four peripheral benzene rings of zinc(II) phthalocyanine, which gives the planar molecules a three-dimensional structure. Two possible substituting locations for each tert-butyl group result in a mixture of structural isomers, which are easily identified by STM. Two different structures have been distinguished in the monolayer regime depending on the preparation procedure. One is a quasi-ordered structure whereby the molecules are arranged in the closest-packed way, and the second is an ordered structure in which the molecules are arranged side by side. The more ordered molecular network in the latter is considered to be a consequence of unification of TB-ZnPc isomers.

The adsorption of subphthalocyanine (SubPc) on the Au(111) surface has been studied by scanning tunnelling microscopy (STM). Depending on coverage and deposition temperature, four different phases have been observed, of which two are coexisting. Spontaneous symmetry breaking inducing mirror domains is observed for all structures. Supramolecular chirality is expressed at different levels and length scale. Our detailed STM study allows conclusions on the origin of polymorphism due to changing coverage and temperature.