Search

We present a direct observation of carbon-atom tunneling in the flipping reaction of formaldehyde between its two mirror-reflected states on a Cu(110) surface using low-temperature scanning tunneling microscopy (STM). The flipping reaction was monitored in real time, and the reaction rate was found to be temperature independent below 10 K. This indicates that this reaction is governed by quantum mechanical tunneling, albeit involving a substantial motion of the carbon atom (∼1 Å). In addition, deuteration of the formaldehyde molecule resulted in a significant kinetic isotope effect (RCH2O/RCD2O ≈ 10). The adsorption structure, reaction pathway, and tunneling probability were examined by density functional theory calculations, which corroborate the experimental observations.

After growth of graphene by chemical vapour deposition on copper foil, Cu(4 1 0) facets were distinguished from Cu(1 0 0) facets using the low energy electron diffraction technique. Evolution of the diffraction patterns was studied after oxidation of the Cu substrate and subsequent annealing in ultrahigh vacuum. An ordered Cu(4 1 0)–O phase was then identified. The distinction between Cu(4 1 0) and Cu(4 1 0)–O is relevant in determination of the coupling between graphene and the substrate.

Significant changes in the Raman spectrum of single-layer graphene grown on a copper film were observed after the spontaneous oxidation of the underlying substrate that occurred under ambient conditions. The frequencies of the graphene G and 2D Raman modes were found to undergo red shifts, while the intensities of the two bands change by more than an order of magnitude. To understand the origin of these effects, we further characterized the samples by scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and atomic force microscopy (AFM). The oxidation of the substrate produced an appreciable corrugation in the substrate without disrupting the crystalline order of the graphene overlayer and/or changing the carrier doping level. We explain the red shifts of the Raman frequencies in terms of tensile strain induced by corrugation of the graphene layer. The changes in Raman intensity with oxidation arise from the influence of the thin cuprous oxide film on the efficiency of light coupling with the graphene layer in the Raman scattering process.

Dissociation of molecular oxygen is an important elementary process in heterogeneous catalysis. Here, we report on a real-space observation of oxygen photolysis on the Ag(110) surface at 78 K by far- and near-field excitation in the ultraviolet–near-infrared range using a low-temperature scanning tunneling microscope (STM) combined with wavelength-tunable laser excitation. The photolysis of isolated oxygen molecules on the surface occurs even by visible light with the cross section of ∼10−19 cm2. Time-dependent density functional theory calculations reveal optical absorption of the hybridized O2–Ag(110) complex in the visible and the near-infrared range which is associated with the oxygen photolysis. We suggest that the photolysis mechanism involves a direct charge transfer process. We also demonstrate that the photolysis can be largely enhanced in plasmonic STM junctions, and the cross section is estimated to be ∼10−17 cm-2in the visible and the near-infrared range, which appears to be an interesting feature of plasmon-induced reactions from the perspective of photochemical conversion with the aid of solar energy.

We report preparation of large area quasi-one-dimensional (1D) monolayer graphene superlattices on a prototypical high-index surface Cu(410)-O and characterization by Raman spectroscopy, Auger electron spectroscopy, low-energy electron diffraction, scanning tunneling microscopy, and scanning tunneling spectroscopy. The periodically stepped substrate gives a 1D modulation to graphene, forming a superlattice of the same superperiodicity. Consequently, the moir\'e pattern is also quasi-1D, with a different periodicity. Scanning tunneling spectroscopy measurements revealed new Dirac points formed at the superlattice Brillouin zone boundary as predicted by theories.

Ordered configurations of hydrogen adatoms on graphene have long been proposed, calculated, and searched for. Here, we report direct observation of several ordered configurations of H adatoms on graphene by scanning tunneling microscopy. On the top side of the graphene plane, H atoms in the configurations appear to stick to carbon atoms in the same sublattice. Scanning tunneling spectroscopy measurements revealed a substantial gap in the local density of states in H-contained regions as well as in-gap states below the conduction band due to the incompleteness of H ordering. These findings can be well explained by density functional theory calculations based on double-sided H configurations. In addition, factors that may influence H ordering are discussed.

Selective growth of either monolayer or bilayer graphene is of great importance. We developed a method to readily tune large area graphene growth from complete monolayer to complete bilayer. In an ambient pressure chemical vapor deposition process, we used the sample temperature at which to start the H2 flow as the control parameter and realized the change from monolayer to bilayer growth of graphene on Cu foil. When the H2 starting temperature was above 700°C, continuous monolayer graphene films were obtained. When the H2 starting temperature was below 350°C, continuous bilayer films were obtained. Detailed characterization of the samples treated under various conditions revealed that heating without the H2 flow caused Cu oxidation. The more the Cu substrate oxidized, the less graphene bilayer could form.

The antenna-mediated coupling of light into Ag nanowire is investigated both in experiments and simulations. The coupling efficiency is strongly depended on the architecture of the metallic particles related to the Ag nanowire. Different incident angles of excitation laser are also tested for the maximum coupling effiecieny. The results demonstrate three-arm triangle antenna group fabricated at the incident end can effectively enhance the surface plasmon polariton (SPP) coupling and propagation. The SPP resonance and the Fabry-Perot cavity theory are used to explain the plasmon enhancement and propagation phenomena. The suggested structure can be served as an enhanced plasmonic waveguide for the nanophotonic and plasmonic circuits in the future.

Planar plasmonic focusing of surface plasmon polaritons (SPPs) by an in-plane nanostructure consisting of Ag-column arrays and an in-plane Fresnel zone plate (FZP) with a Cu grating underneath for energy compensation was demonstrated. The CdS-based hybrid plasmonic waveguide generated in the Ag-column arrays was characterized with a scanning near-field optical microscope. By using the FZP focusing structure, the SPP modes were separated from the CdS photoluminescence background and focused at the FZP focus area, and in this way, were used as the source for the SPP waveguide. Finite-difference time-domain simulations correspond with the experimental observations, suggesting that this is indeed an effective approach to control SPP coupling within the dielectric nanoribbon waveguide.

Few-layer graphene grown on Ni thin films has been studied by scanning tunneling microscopy. In most areas on the surfaces, moire patterns resulted from rotational stacking faults were observed. At a bias lower than 200 mV, only one sublattice shows up in regions without moire patterns while both sublattices are seen in regions with moire pattens. This phenomenon can be used to identify AB stacked regions. The scattering characteristics at various types of step edges are different from those of monolayer graphene edges, either armchair or zigzag.