Zhihai Cheng

Real-Space Identification of Intermolecular Bonding with Atomic Force Microscopy

Imaging Hydrogen Bonds The decoration of atomic force microscope tips with terminal CO molecules has afforded much higher resolution of the bonding of adsorbed molecules. Zhang et al. (p. 611, published online 26 September) show that this method, in combination with density function theory calculations, can image and characterize hydrogen-bonding contacts formed between 8-hydroxyquinoline molecules adsorbed on the (111) surface of copper under cryogenic conditions. At room temperature, a different bonding configuration was revealed that was the result of the molecules dehydrogenating on the copper surface and coordinating with surface copper atoms. An atomic force microscope tip bearing a single carbon monoxide molecule was used to resolve hydrogen-bonding contacts between molecules. We report a real-space visualization of the formation of hydrogen bonding in 8-hydroxyquinoline (8-hq) molecular assemblies on a Cu(111) substrate, using noncontact atomic force microscopy (NC-AFM). The atomically resolved molecular structures enable a precise determination of the characteristics of hydrogen bonding networks, including the bonding sites, orientations, and lengths. The observation of bond contrast was interpreted by ab initio density functional calculations, which indicated the electron density contribution from the hybridized electronic state of the hydrogen bond. Intermolecular coordination between the dehydrogenated 8-hq and Cu adatoms was also revealed by the submolecular resolution AFM characterization. The direct identification of local bonding configurations by NC-AFM would facilitate detailed investigations of intermolecular interactions in complex molecules with multiple active sites.

Toward single-layer uniform hexagonal boron nitride-graphene patchworks with zigzag linking edges.

The atomic layer of hybridized hexagonal boron nitride (h-BN) and graphene has attracted a great deal of attention after the pioneering work of P. M. Ajayan et al. on Cu foils because of their unusual electronic properties (Ci, L. J.; et al. Nat. Mater. 2010, 9, 430-435). However, many fundamental issues are still not clear, including the in-plane atomic continuity as well as the edge type at the boundary of hybridized h-BN and graphene domains. To clarify these issues, we have successfully grown a perfect single-layer h-BN-graphene (BNC) patchwork on a selected Rh(111) substrate, via a two-step patching growth approach. With the ideal sample, we convinced that at the in-plane linking interface, graphene and h-BN can be linked perfectly at an atomic scale. More importantly, we found that zigzag linking edges were preferably formed, as demonstrated by atomic-scale scanning tunneling microscopy images, which was also theoretically verified using density functional theory calculations. We believe the experimental and theoretical works are of particular importance to obtain a fundamental understanding of the BNC hybrid and to establish a deliberate structural control targeting high-performance electronic and spintronic devices.

Toward the growth of an aligned single-layer MoS2 film.

Molybdenum disulfide (molybdenite) monolayer islands and flakes have been grown on a copper surface at comparatively low temperature and mild conditions through sulfur loading of the substrate using thiophenol (benzenethiol) followed by the evaporation of Mo atoms and annealing. The MoS(2) islands show a regular Moiré pattern in scanning tunneling microscopy, attesting to their atomic ordering and high quality. They are all aligned with the substrate high-symmetry directions providing for rotational-domain-free monolayer growth.

Quasi-Freestanding Monolayer Heterostructure of Graphene and Hexagonal Boron Nitride on Ir(111) with a Zigzag Boundary

In-plane heterostructure of hexagonal boron nitride and graphene (h-BN-G) has become a focus of graphene research owing to its tunable bandgap and intriguing properties. We report herein the synthesis of a quasi-freestanding h-BN-G monolayer heterostructure on a weakly coupled Ir(111) substrate, where graphene and h-BN possess distinctly different heights and surface corrugations. An atomically sharp zigzag type boundary has been found to dominate the patching interface between graphene and h-BN, as evidenced by high-resolution Scanning tunneling microscopy investigation as well as density functional theory calculation. Scanning tunneling spectroscopy studies indicate that the graphene and h-BN tend to exhibit their own intrinsic electronic features near the patching boundary. The present work offers a deep insight into the h-BN-graphene boundary structures both geometrically and electronically together with the effect of adlayer-substrate coupling.

Magnetic entropy change and its temperature variation in compounds La(Fe1−xCox)11.2Si1.8

Magnetic entropy change ΔS of compounds La(Fe1−xCox)11.2Si1.8 with the cubic NaZn13-type structure was investigated around their Curie temperature TC. It is found that the phase transition is completely reversible, indicating a nature of second order phase transition. The maximum value of |ΔS|∼13.0 J/kg K under a field of 5 T was achieved in compound LaFe11.2Si1.8 at its TC of ∼222 K, which exceeds that of most materials involving a second order transition at the corresponding temperature. With increasing substitution of Co for Fe from x=0 to x=0.8, TC shifts from 222 to 307 K and entropy change decreases. However, |ΔS| still has a considerable magnitude near room temperature. The large magnetic entropy change is believed to be due to the abrupt change of magnetization at TC, which is associated with the strong structural and magnetic interplay in the compounds.

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.

Do Two-Dimensional “Noble Gas Atoms” Produce Molecular Honeycombs at a Metal Surface?

Anthraquinone self-assembles on Cu(111) into a giant honeycomb network with exactly three molecules on each side. Here we propose that the exceptional degree of order achieved in this system can be explained as a consequence of the confinement of substrate electrons in the pores, with the pore size tailored so that the confined electrons can adopt a noble-gas-like two-dimensional quasi-atom configuration with two filled shells. Formation of identical pores in a related adsorption system (at different overall periodicity due to the different molecule size) corroborates this concept. A combination of photoemission spectroscopy with density functional theory computations (including van der Waals interactions) of adsorbate-substrate interactions allows quantum mechanical modeling of the spectra of the resultant quasi-atoms and their energetics.

Determination of the critical interspacing for the noninteracting magnetic nanoparticle system

The dipole–dipole interactions of monodisperse Fe3O4 nanoparticles (NPs) can be directly controlled by a uniform SiO2 shell with different thickness, i.e., different interspacings. Thus, the interacting strength of a serial of Fe3O4–SiO2 NPs system can be revealed by fitting the blocking temperature TB measured at ac fields to the Vogel–Fulcher law. The interspacing over five times of diameter for less than 8.0 nm Fe3O4 NPs is the critical value to achieve a noninteracting system. Furthermore, a general equation to evaluate critical interspacing for noninteracting magnetic NPs systems with different sizes and saturation magnetizations was calculated by Monte Carlo method.

Power of Confinement: Adsorbate Dynamics on Nanometer-Scale Exposed Facets

The diffusion and arrangements of CO adsorbates within nanometer-scale pores on a copper surface are investigated by low-temperature scanning tunneling microscopy. In contrast to extended terraces, confinement stabilizes dislocation lines that expose more than one-fourth of the adsorbate population to potentially more reactive adsorption configurations. Confinement allows correlation between adsorbate diffusivity and the number of adsorbates in the pore. A marked increase is found that coincides with the absence of dense films on the exposed facets. In combination, we find that in confinement CO molecules are much more likely to be at adsorption sites that allow lateral access, in contrast to the dense and uniform films on extended terraces.

Nearly constant magnetic entropy change involving two closely spaced transitions in the compound LaFe11.375Al1.625

Al argechange in the magnetic entropy, |� S| ,w asobserved in the Febased NaZn13-type compound LaFe11.375Al1.625 ,w hich was nearly temperature independent over a wide temperature range (an about 70 K span from ∼140 to 210 K). This behaviour of the magnetic entropy change is associated with two closely spaced magnetic transitions. X-ray diffraction investigation at different temperatures indicates that the crystal structure remains cubic, of NaZn13 type, when the magnetic state changes with temperature, but the cell parameter changes dramatically at the first-order transition point.