Yu-Yang Zhang

Graphene cover-promoted metal-catalyzed reactions

Significance Carbon deposits have been widely observed on metal surfaces in a variety of catalytic reactions, and the graphitic carbon species are often considered as inhibitors for surface reactions. We demonstrate here that CO adsorption and oxidation can occur on Pt surface covered by monolayer graphene, showing that the space between graphene overlayer and metal surface can act as a two-dimensional (2D) nanoreactor. Inside, CO oxidation happens with lower activation barrier due to the confinement effect of the graphene cover. This finding reminds us to reconsider the role of graphitic carbon in metal-catalyzed surface reactions and further provides a way to design novel catalysts. Graphitic overlayers on metals have commonly been considered as inhibitors for surface reactions due to their chemical inertness and physical blockage of surface active sites. In this work, however, we find that surface reactions, for instance, CO adsorption/desorption and CO oxidation, can take place on Pt(111) surface covered by monolayer graphene sheets. Surface science measurements combined with density functional calculations show that the graphene overlayer weakens the strong interaction between CO and Pt and, consequently, facilitates the CO oxidation with lower apparent activation energy. These results suggest that interfaces between graphitic overlayers and metal surfaces act as 2D confined nanoreactors, in which catalytic reactions are promoted. The finding contrasts with the conventional knowledge that graphitic carbon poisons a catalyst surface but opens up an avenue to enhance catalytic performance through coating of metal catalysts with controlled graphitic covers.

Diffusivity control in molecule-on-metal systems using electric fields

The development of methods for controlling the motion and arrangement of molecules adsorbed on a metal surface would provide a powerful tool for the design of molecular electronic devices. Recently, metal phthalocyanines (MPc) have been extensively considered for use in such devices. Here we show that applied electric fields can be used to turn off the diffusivity of iron phthalocyanine (FePc) on Au(111) at fixed temperature, demonstrating a practical and direct method for controlling and potentially patterning FePc layers. Using scanning tunneling microscopy, we show that the diffusivity of FePc on Au(111) is a strong function of temperature and that applied electric fields can be used to retard or enhance molecular diffusion at fixed temperature. Using spin-dependent density-functional calculations, we then explore the origin of this effect, showing that applied fields modify both the molecule-surface binding energies and the molecular diffusion barriers through an interaction with the dipolar Fe-Au adsorption bond. On the basis of these results FePc on Au(111) is a promising candidate system for the development of adaptive molecular device structures.

The observation of square ice in graphene questioned

Algara-Siller et al.1 reported the observation of a new phase of water— ‘square ice’—sandwiched between two graphene layers at room temperature. Their key evidence consists of transmission electron microscope (TEM) images of a square lattice from small encapsulated crystals, the detection of oxygen from relatively large regions containing such crystals and molecular dynamics (MD) simulations indicating ‘square ice’ formation inside hydrophobic nanochannels. Here we propose that the reported experimental data can be better explained by salt (for example, NaCl) contaminants precipitating as nanocrystals in the dried-out graphene liquid cells2 and common oxide contaminants in graphene. Consequently, we question the observation of roomtemperature ‘square ice’. There is a Reply to this Brief Communication Arising by Algara-Siller, G. et al. Nature 528, http://dx.doi.org/10.1038/ nature16149 (2015) relating to the electron energy-loss spectra (EELS) and a Reply by Wang, F. C. et al. Nature 528, http://dx.doi.org/10.1038/ nature16146 (2015) relating to the MD simulations. The TEM images and the dynamics of the reported ‘square ice’ under electron irradiation bear a considerable resemblance to those we have observed of NaCl nanoplatelets in graphene and dried-out graphene liquid cells. Such NaCl nanoplatelets usually orient along the [100] direction, displaying a square lattice with a spacing of approximately 2.8 Å (Fig. 1a, b, d); the corresponding fast Fourier transform (FFT) matches the reported1 diffraction data. Edge termination, dislocations and grain-boundary structures within the lattice, and the dynamics of

Organic salts as super-high rate capability materials for lithium-ion batteries

First-principles calculation reveals that organic salts could be super-high rate capability electrode materials for Li-ion batteries. We show that di-lithium terephthalate, an anode material demonstrated recently by experiment, has low Li diffusion barrier (EA). A resonant bonding model for the low EA is developed, which leads to the prediction that di-potassium terephthalate (K2TPA) has even lower EA (150 meV), with diffusion rate orders of magnitude higher than that in Li-intercalated graphite. The calculated anode voltage (0.62 V), specific energy density (209 mA·h/g), and volume change upon lithiation (5%) make K2TPA a promising anode material for power-intensive applications such as electric-vehicles.

Polymorphism and chiral expression in two-dimensional subphthalocyanine crystals on Au(111).

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.

Kondo Effect of Cobalt Adatoms on a Graphene Monolayer Controlled by Substrate-Induced Ripples

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.

Intrinsically patterned two-dimensional materials for selective adsorption of molecules and nanoclusters

Two-dimensional (2D) materials have been studied extensively as monolayers, vertical or lateral heterostructures. To achieve functionalization, monolayers are often patterned using soft lithography and selectively decorated with molecules. Here we demonstrate the growth of a family of 2D materials that are intrinsically patterned. We demonstrate that a monolayer of PtSe2 can be grown on a Pt substrate in the form of a triangular pattern of alternating 1T and 1H phases. Moreover, we show that, in a monolayer of CuSe grown on a Cu substrate, strain relaxation leads to periodic patterns of triangular nanopores with uniform size. Adsorption of different species at preferred pattern sites is also achieved, demonstrating that these materials can serve as templates for selective self-assembly of molecules or nanoclusters, as well as for the functionalization of the same substrate with two different species.

Sequence of Silicon Monolayer Structures Grown on a Ru Surface: from a Herringbone Structure to Silicene

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.

Mo-Terminated Edge Reconstructions in Nanoporous Molybdenum Disulfide Film

The catalytic and magnetic properties of molybdenum disulfide (MoS2) are significantly enhanced by the presence of edge sites. One way to obtain a high density of edge sites in a two-dimensional (2D) film is by introducing porosity. However, the large-scale bottom-up synthesis of a porous 2D MoS2 film remains challenging and the correlation of growth conditions to the atomic structures of the edges is not well understood. Here, using molecular beam epitaxy, we prepare wafer-scale nanoporous MoS2 films under conditions of high Mo flux and study their catalytic and magnetic properties. Atomic-resolution electron microscopy imaging of the pores reveals two new types of reconstructed Mo-terminated edges, namely, a distorted 1T (DT) edge and the Mo-Klein edge. Nanoporous MoS2 films are magnetic up to 400 K, which is attributed to the presence of Mo-terminated edges with unpaired electrons, as confirmed by density functional theory calculation. The small hydrogen adsorption free energy at these Mo-terminated edges leads to excellent activity for the hydrogen evolution reaction.

Molecular Beam Epitaxy of Highly Crystalline Monolayer Molybdenum Disulfide on Hexagonal Boron Nitride

Atomically thin molybdenum disulfide (MoS2), a direct-band-gap semiconductor, is promising for applications in electronics and optoelectronics, but the scalable synthesis of highly crystalline film remains challenging. Here we report the successful epitaxial growth of a continuous, uniform, highly crystalline monolayer MoS2 film on hexagonal boron nitride (h-BN) by molecular beam epitaxy. Atomic force microscopy and electron microscopy studies reveal that MoS2 grown on h-BN primarily consists of two types of nucleation grains (0° aligned and 60° antialigned domains). By adopting a high growth temperature and ultralow precursor flux, the formation of 60° antialigned grains is largely suppressed. The resulting perfectly aligned grains merge seamlessly into a highly crystalline film. Large-scale monolayer MoS2 film can be grown on a 2 in. h-BN/sapphire wafer, for which surface morphology and Raman mapping confirm good spatial uniformity. Our study represents a significant step in the scalable synthesis of highly crystalline MoS2 films on atomically flat surfaces and paves the way to large-scale applications.