Electron beam analysis induces Cl vacancy defects in a NaCl thin film

This work reports on the electron-induced modification of NaCl thin film grown on Ag(110). We show using low energy electron diffraction (LEED) that electron beam bombardment leads to desorption and formation of Cl vacancy defects on NaCl surface. The topographic structure of these defects is studied using scanning tunneling microscopy (STM) showing the Cl defects as depressions on the NaCl surface. Most of the observed defects are mono-atomic vacancies and are located on flat NaCl terraces. Auger electron spectroscopy confirms the effect of electron exposure on NaCl thin films showing Cl atoms desorption from the surface. Using density functional theory (DFT) taken into account the van der Waals dispersion interactions, we confirm the observed experimental STM measurements with STM simulation. Furthermore, Comparing the adsorption of defect free NaCl and defective NaCl monolayer on Ag(110) surfaces, we found an increase of the adhesion energy and the charge transfer between the NaCl film and the substrate due to the Cl vacancy. In details, the adhesion energy increases between the NaCl film and the metallic Ag substrate from 30.4 〖meVÅ〗^(-2) for the NaCl film without Cl vacancy and from 39.5 〖meVÅ〗^(-2) for NaCl film with a single Cl vacancy. The charge transfer from the NaCl film to the Ag substrate is enhanced when the vacancy is created, from 0.63e- to 1.25e-.

Initiating Ullmann-like coupling of Br2Py by a semimetal surface

Intensive efforts have been devoted to surface Ullmann-like coupling in recent years, due to its appealing success towards on-surface synthesis of tailor-made nanostructures. While attentions were mostly drawn on metallic substrates, however, Ullmann dehalogenation and coupling reaction on semimetal surfaces has been seldom addressed. Herein, we demonstrate the self-assembly of 2, 7-dibromopyrene (Br2Py) and the well controllable dehalogenation reaction of Br2Py on the Bi(111)–Ag substrate with a combination of scanning tunnelling microscopy (STM) and density functional theory calculations (DFT). By elaborately investigating the reaction path and formed organic nanostructures, it is revealed that the pristinely inert bismuth layer supported on the silver substrate can initiate Ullmann-like coupling in a desired manner by getting alloyed with Ag atoms underneath, while side products have not been discovered. By clarifying the pristine nature of Bi–Ag(111) and Ullmann-like reaction mechanisms, our report proposes an ideal template for thoroughly exploring dehalogenative coupling reaction mechanisms with atomic insights and on-surface synthesis of carbon-based architectures.

Ultrathin Films of Pentacene on Ag(111): Charge-Transfer Bonding and Inter-Adsorbate Interactions

Temperature programmed desorption (TPD) was used to examine the surface binding and intermolecular interactions of mono- and multi-layer thin films of the polycyclic aromatic acene, pentacene, deposited on a Ag(111) surface. The TPD spectra of sub-monolayer cov- erages revealed the presence of three distinct phases (denoted as α1, α2, and α3). The α1 phase was attributed to adsorption on step sites, while the α2 and α3 phases were assigned to adsorption on terrace sites under different local molecular densities. A physical model was constructed to describe the desorption kinetics from each of the three monolayer phases, including intermolecular repulsion from interfacial dipoles produced as a result of charge transfer bonding between pentacene and the Ag substrate. Fit analysis of the sub-monolayer spectra revealed desorption energies in the zero-coverage limit of 218±8, 166±8, and 162±9 kJ/mol for the α1, α2, and α3 phases, respectively. The interface dipoles of the α2 and α3 terrace adsorption sites were found to be effectively invariant (within error) and deduced as 18±7 and 23±10 D, respectively. These values suggest a partial charge transfer of 0.6 to 0.7 electrons from each pentacene molecule to the Ag substrate and is equivalent to 0.13 electrons per aromatic ring. The TPD spectra from the multilayer films also exhibited three phases. Leading edge analysis of the lowest temperature multilayer peak yielded a desorption energy of 121±15 kJ/mole, while simulations predicted desorption energies ca. 10-15 kJ/mole higher for the higher temperature phases. The three multilayer phases were assigned, from lowest to highest temperature, as an amorphous bulk film, a thin film, and polycrystalline structures.

Ultrathin Films of Pentacene on Ag(111): Charge-Transfer Bonding and Inter-Adsorbate Interactions

Temperature programmed desorption (TPD) was used to examine the surface binding and intermolecular interactions of mono- and multi-layer thin films of the polycyclic aromatic acene, pentacene, deposited on a Ag(111) surface. The TPD spectra of sub-monolayer cov- erages revealed the presence of three distinct phases (denoted as α1, α2, and α3). The α1 phase was attributed to adsorption on step sites, while the α2 and α3 phases were assigned to adsorption on terrace sites under different local molecular densities. A physical model was constructed to describe the desorption kinetics from each of the three monolayer phases, including intermolecular repulsion from interfacial dipoles produced as a result of charge transfer bonding between pentacene and the Ag substrate. Fit analysis of the sub-monolayer spectra revealed desorption energies in the zero-coverage limit of 218±8, 166±8, and 162±9 kJ/mol for the α1, α2, and α3 phases, respectively. The interface dipoles of the α2 and α3 terrace adsorption sites were found to be effectively invariant (within error) and deduced as 18±7 and 23±10 D, respectively. These values suggest a partial charge transfer of 0.6 to 0.7 electrons from each pentacene molecule to the Ag substrate and is equivalent to 0.13 electrons per aromatic ring. The TPD spectra from the multilayer films also exhibited three phases. Leading edge analysis of the lowest temperature multilayer peak yielded a desorption energy of 121±15 kJ/mole, while simulations predicted desorption energies ca. 10-15 kJ/mole higher for the higher temperature phases. The three multilayer phases were assigned, from lowest to highest temperature, as an amorphous bulk film, a thin film, and polycrystalline structures.

Tunable narrowband metamaterial perfect absorber with single absorption peak

A tunable narrowband metamaterial perfect absorber (NMPA) with single absorption peak at both normal incidence and oblique incidence is proposed. The spectral responses of the designed NMPA are calculated using finite-difference time-domain (FDTD) method. By using an Ag guided-mode resonance grating layer with great fill factor, the designed NMPA exhibits single resonance peak, which is converted to heat due to the strong surface plasmon resonance around the Ag grating and the Ag substrate. An NMPA sample is fabricated and the spectral responses are measured to verify the theoretical results. The experimental results are consistent with the theoretical ones.