Ag Monolayer Research Articles

Initial quantum well states in Ag thin films on the In/Si(111)-[formula omitted] surface

Silver thin films have been formed by room temperature deposition of Ag on In/Si(111)-3×3. The Ag films have been investigated using both angle-resolved photoelectron spectroscopy (ARPES) and scanning tunneling microscopy and spectroscopy (STM/STS). This creates a powerful link between the electronic structures and the film morphology. The valence band spectra show a clear evidence of quantum well state (QWS) formation already for a 2 monolayer (ML) film. This QWS moves towards the Fermi level for the 3 ML film, which also reveals a second QWS. The QWSs’ dispersions have been plotted along the Γ¯M¯ and Γ¯K¯ symmetry lines of the 1×1 surface Brillouin zone (SBZ), where the Γ¯M¯ direction shows the umklapp-mediated QWSs. The valence band spectra for the 3 ML Ag film also show a strong Ag sp band close to the edge of the Ag(111) 1×1 SBZ. In the STS spectrum from 2 ML, two peaks are visible below the Shockley surface state. These peaks are compared with the ARPES data and attributed to different features of the QWS, namely the turning part where the QWS band intersects with the bulk Si valence band region and the local binding energy minimum close to the Γ¯ point.

RETRACTED: Tuning the structural and electronic properties of Ag/Au embedded arsenene monolayers and investigation of their adsorption behaviors for various gas molecules

Abstract Motivated by the large number of studies on nanoelectronic and optoelectronic applications of two-dimensional materials, we investigated the structural and electronic properties of Ag and Au-embedded arsenene monolayers, and then examined the interaction of various gas molecules with theses doped structures. The results of adsorption energies indicate that the embedding of Ag and Au atoms into the structure of arsenene is energetically stable. Various adsorption sites of gas molecules on the considered Ag and Au-embedded arsenene nanosheets were examined in this work. The charge accumulation between the Ag/Au and As atoms is an indication of the formation of covalent bonds between these two atoms. Besides, the charge density difference plots show the electron density accumulation on the adsorbed gas molecules. There are large overlaps between the projected density of states of the interacting atoms, suggesting the covalent nature of interaction between them.

Kinetic Monte Carlo simulations of coverage effect on Ag and Au monolayers growth on Cu (1 1 0)

In this work, different coverages (1% ≤ coverage ≤ 20%) are studied during the growth of Ag and Au monolayers on the Cu (1 1 0) surface using kinetic Monte Carlo simulations (kMC). The results have shown that as coverage increases, the likelihood of an isolated adatom connecting to another adatom or island also increases. Thus, the phenomenon by which an adatom disintegrates from one island to another becomes significant. Molecular statics method is used to calculate the activation energy barriers of several diffusion processes in the framework of the Embedded-Atom method (EAM). These diffusion processes are implemented in the kinetic Monte Carlo (kMC) simulations under the Anisotropic Bond-Breaking Model (ABBM) model. The results show that as the coverage increases, 1D clusters are observed at 200 K. However, the transition from compact 1D (at 200 K) to 2D islands has been observed for relatively high temperatures (300 K and 400 K).

Plasmonics in Atomically Thin Crystalline Silver Films

Light-matter interaction at the atomic scale rules fundamental phenomena such as photoemission and lasing while enabling basic everyday technologies, including photovoltaics and optical communications. In this context, plasmons, the collective electron oscillations in conducting materials, are important because they allow the manipulation of optical fields at the nanoscale. The advent of graphene and other two-dimensional crystals has pushed plasmons down to genuinely atomic dimensions, displaying appealing properties such as a large electrical tunability. However, plasmons in these materials are either too broad or lying at low frequencies, well below the technologically relevant near-infrared regime. Here, we demonstrate sharp near-infrared plasmons in lithographically patterned wafer-scale atomically thin silver crystalline films. Our measured optical spectra reveal narrow plasmons (quality factor of ∼4), further supported by a low sheet resistance comparable to bulk metal in few-atomic-layer silver films down to seven Ag(111) monolayers. Good crystal quality and plasmon narrowness are obtained despite the addition of a thin passivating dielectric, which renders our samples resilient to ambient conditions. The observation of spectrally sharp and strongly confined plasmons in atomically thin silver holds great potential for electro-optical modulation and optical sensing applications.

Activation mechanisms of silver toward CO oxidation by tungsten carbide substrate: A density functional theory study

The development of efficient catalysts for low temperature CO oxidation is important to the application of fuel cells. In this work, we report that the Ag monolayer on WC (0001) surface (AgML/WC) could effectively catalyze CO oxidation through the L-H mechanism (CO + O2 → OOCO → CO2 + O). The most sluggish reaction step is suggested to be CO + O → CO2 with a barrier of 0.48 eV, which is 1.21 eV lower than the barrier of O2 dissociation. The electronic structure and d-band center analyses demonstrate that the promoted activity may originate from the synergistic effect between Ag monolayer and WC. The present study is conducive to design new efficient and cost-effective catalysts for CO oxidation without using of the noble platinum.

Low-temperature preferential oxidation of CO over Ag monolayer decorated Mo2C (MXene) for purifying H2

Selective removal of CO from reformate streams at low temperature is an important issue to be solved for anode catalysts of proton exchange membrane fuel cells (PEMFCs). Using density functional theory, we demonstrate that the Ag monolayer decorated Mo2C can be used as an effective low cost catalyst with high activity, selectivity and stability for preferential oxidation of CO. Ag monolayer decorated Mo2C can also be used as a filter membrane for separating H2 and CO. Meanwhile, the high selectivity is achieved because H2 hardly takes part in the reaction, and the good activity is obtained since the CO with moderate binding strength could be smoothly removed at various CO concentration. We propose a facile route for removing CO by injecting low O2 supply because it can resist CO poisoning with promoted activity for CO oxidation at higher CO concentration. We anticipate that Ag monolayer decorated Mo2C can be used as a promising H2 purifying pretreater by preferential oxidation of CO connecting to the anode of PEMFCs. We hope the present results could advance the development of catalysts for purifying H2 and inspire more researches on the application of MXene catalysts.

The Quantum Nature of the Formation Mechanisms of Ag Monolayer Structures on the Monocrystalline Semiconductor Surfaces

The Quantum Nature of the Formation Mechanisms of Ag Monolayer Structures on the Monocrystalline Semiconductor Surfaces

Ag monolayer doped by Pt atom on WC (0001) surface: A good catalyst for H2 dissociation with high sulfur tolerance

H2 dissociation barriers on Ag (111), Ag monolayer on WC (0001) (AgML/WC), Ag monolayer doped by Pt atom on WC (0001) (AgMLPt-d/WC) surface are decreasing from 1.07 to 0.03 eV. The ab initio atomistic thermodynamic data shows that at typical planar SOFC operating temperatures of 923–1073 K, under 100000-ppm pH2S/pH2, AgMLPt-d/WC can be sulfur free clean surface. Therefore, compared with traditional Nickel/Yttria-stabilized zirconia (Ni/YSZ) Solid Oxide Fuel Cells (SOFCs) anode, AgMLPt-d/WC shows high activity towards H2 dissociation and high tolerance of sulfur poisoning.

Growth of Ag(1 1 1) on Si(1 1 1) with nearly flat band and abrupt interface

Growth of Ag films of up to 30 nm thickness on Si(1 1 1) 7 × 7 at room temperature is investigated by low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). LEED revealed the coexistence of Ag and Si spots starting with 1 monolayer (ML) of Ag deposited. The Ag lattice constant, starting with 25 ML, is slightly higher than for bulk Ag and increase linearly with Ag thickness, reaching about 4.2 nm for the thickest films. The average terrace widths detected from LEED spot profile analysis are about 30 nm for clean Si(1 1 1) 7 × 7 and about 5.5 nm for the thickest Ag(1 1 1) film, in agreement with STM observations. The intensity variation of core levels analyzed by XPS is taken into account by a model assuming the initial formation of Ag islands with linear variation of coverage vs. the amount of Ag deposited, followed by growth in a quasi layer-by-layer mode. The interface barrier is in the range of 0.4 eV, lower than all values reported previously. Ag deposited on Si(1 1 1) 7 × 7 at room temperature provides flat Ag(1 1 1) for synthesis of 2D materials, and may be used for low barrier Schottky diodes.

Underpotential Deposition of Silver on Au(111) from an Air‐ and Water‐Stable Ionic Liquid Visualized by In‐Situ STM

AbstractThere is a steadily increasing interest in room‐temperature ionic liquids (ILs) as media for metal electrodeposition. However, the fundamental understanding of the underlying surface processes at the electrode/IL interface and of the initial stages of electrodeposition is still at a very preliminary stage. In this article, we investigate the underpotential deposition (upd) of Ag on a well‐defined Au(111) single crystal surface from an air‐ and water‐stable IL, 1‐butyl‐1‐methylpyrrolidinium dicyanamide [BMP][DCA]. By employing in‐situ scanning tunneling microscopy (STM), we could visualize individual stages of the Ag upd process as a function of the applied electrode potential and correlate voltammetric responses to the obtained STM data. The initial Ag upd process results in the formation of an Ag‐(4×4) lattice, which is further transformed into a pseudomorphic Ag‐(1×1) monolayer at the underpotential of ∼40 mV. The structural stability of the Au(111) surface in [BMP][DCA] is also examined in the absence of Ag+ ions in both the negative and positive potential regions.

Catalytic Activities of Mono- and Bimetallic (Gold/Silver) Nanoshell-Coated Gold Nanocubes toward Catalytic Reduction of Nitroaromatics.

A new class of core-shell metallic nanostructures with tunable near-surface composition and surface morphology with excellent catalytic activity is reported. Very thin shells of metal nanoassemblies such as monolayer (Ag and Au), bilayer of Ag or Au, and AgAu alloy layer with controlled size and morphology were deposited onto a gold nanocube (AuNC) core. UV-vis absorption spectroscopy and high-resolution transmission electron microscopy analyses along with selected-area electron diffraction, energy dispersive X-ray spectroscopy, inductively coupled plasma mass spectrometer, and X-ray diffraction techniques were used to characterize the prepared core-shell nanocubes. High-angle annular dark field scanning transmission electron microscopy-energy dispersive X-ray spectroscopy mapping images were recorded for the bilayer shell and alloy layer shell in the core-shell nanostructures. Reduction of 4-nitroaniline in the presence of sodium borohydride was chosen to validate the catalytic activity of the prepared core-shell metal nanocubes. Interestingly, the AgAu alloy shell layer over the AuNC (AuNC1@Ag0.25Au0.25) showed excellent catalytic activity compared with the pristine AuNC and monolayer and bilayer core-shell nanostructures.

Surface reconstruction induced Co Kondo resonance width modulation on one monolayer Ag covered Cu(111)

We present a systematic study of the Kondo effect of single Co adatom placed on one monolayer (ML) Ag covered Cu(111) surface. The 1-ML Ag/Cu(111) shows apparent local density of states (LDOS) modulation which associated with the surface reconstruction. Interestingly, we find the Kondo resonance width of the Co adatom on 1-ML Ag/Cu(111) is position-dependent. When the Co adatom is placed near the triangular dislocation loop (hcp/fcc boundary), the Kondo resonance width is generally wider than the one measured at fcc site. At the fcc region, the measured Kondo resonance width is closely related to the LDOS variation. In conjunction with the tight binding approximation calculation, we identify that the main contribution of the LDOS modulation is the surface-reconstruction-regulated surface states. With a recently developed model, we further determine the contributions of the bulk and surface states to the Kondo resonance. These findings clarify the role of surface reconstruction on the Kondo effect.

Multi-channel perfect absorber based on a one-dimensional topological photonic crystal heterostructure with graphene.

The topological edge mode, which exists at the interface of a one-dimensional (1D) topological photonic crystal (PhC) heterostructure, provides the possibility to realize perfect absorption for its strong field localization effects. In this Letter, it is found that a huge absorption enhancement appears because of the excitation of topological edge mode, while the graphene is sandwiched between two 1D PhCs. The single peak perfect absorption is realized by means of the strong coupling of incident light and Tamm plasmon polaritons (TPPs) which is excited with Ag-PhC structure. Moreover, we use a heterostructure constructed by two PhCs, a monolayer graphene and Ag mirror to theoretically demonstrate that multi-channel perfect absorption can be achieved based on the effect of topological edge mode, TPPs and critical coupling. The angular selectivity of the proposed absorber is also investigated. Both of the absorption peaks are extremely narrow, and the absorption can be maintained more than 97% with the incident angle varying from 0° to 50°. Hence, our results may have potential applications in optical switches, thermal emissions, and narrowband selective filters.

Electrochemical control of the thermal stability of atomically thin Ag films on Au(111)

Alterations of a surface even on the nanoscale can have a significant impact on the physical properties and subsequent applications of a bulk system. Using Electrochemical Scanning Tunneling Microscopy (EC-STM) to perform an investigation of AgCl and AgBr deposition, evidence suggests multiple chemical mechanisms for the formation of a single, atomically-thin Ag layer on Au(111). Ag deposition has been extensively studied on Au(111). However, few reports provide a detailed mechanism of surface adsorption from Ag solutions. The electrochemical phenomena of Underpotential Deposition (UPD) was used to deposit up to a monolayer of Ag on the Au surface. Cyclic voltammograms (CVs) indicated two UPD peaks of +0.87 V (region 1) and +0.38 V (region 2) vs Ag/AgCl in the AgCl system and one peak at +0.87 V (region 1) in the AgBr system. The presence of two UPD peaks in the AgCl system suggests two different chemical reactions as the deposition mechanism of Ag on Au(111), region 1 representing a reduction of Ag+ as a dissociated cation, which was also seen in the AgBr system, and region 2 representing a reduction of AgCl as a solvated undissociated unit. According to EC-STM images, region 1 deposition resulted in a (4 × 4) packing of the Ag atoms, while deposition in region 2 produced a more closely packed surface with a corresponding (1 × 1) unit cell. A detailed understanding of the nature of these deposition processes is necessary to the fundamental characterization of these atomically thin-metal layers, as well as their subsequent applications. Importantly, relevant applications may include exploiting the thermal stability of the Ag layers, which was drastically affected by the deposition parameters. Deposition of Ag in region 1 resulted in a layer that was thermally unstable after a heat treatment at 1000 K, while the opposite was true from the deposition in region 2. It is proposed that the origin of stability is derived from both the Ag source and the electrochemical deposition conditions.

Substrate‐Driven Electrochemical Stabilization and Activation of Ag Monolayers to Catalyze the Oxygen Reduction Reaction: Beyond Pt‐based Electrocatalysts

AbstractThe search for inexpensive, stable and active electrocatalysts for the oxygen reduction reaction (ORR) on hydrogen fuel cell cathodes is severely limited because only expensive and scarce elements such as gold and platinum‐group elements do not dissolve or oxidize in the reaction environment at acceptable operating cathode potentials. Fortunately, the potential at which dissolution sets in for an element can be increased if a monolayer of such element strongly binds to a suitable substrate. Here, we apply the latter approach to stabilize Ag in the ORR reaction environment by placing a Ag monolayer on Nb, W, Ta, and Ru/Nb substrates. Our first principles calculations and ORR thermodynamics analysis show that all the systems are thermodynamically stable. Ag/Nb has a dissolution potential higher than that of Pt and can be as active toward ORR as Pt. Notably, the cost of Ag/Nb is strikingly lower than that of any Pt‐based ORR catalyst.

Molecular dynamics and thermodynamic simulations of segregation phenomena in binary metal nanoparticles

An approach combining atomistic molecular dynamics (MD) and thermodynamic simulations has been applied to predict the distribution of components in binary Ni–Cu and Au–Ag nanoparticles consisting of 2000 atoms (of about 4 nm in size). The term ‘thermodynamic simulation’ has referred to solving, in some approximations, the Butler equation for a core–shell particle model. Both atomistic and thermodynamic approaches predict the surface segregation of Cu atoms in Ni–Cu nanoparticles and segregation of Ag to the surface of Au–Ag nanoalloys. Then, contrary to the Ni–Cu systems, some Au–Ag nanoparticles demonstrated an onion-like structure with the outer Ag monolayer. The results of MD and thermodynamic simulations agree with each other and with some available direct and indirect experimental data.

Work Function Dependence of Vibrational Relaxation Probabilities: NO(v = 2) Scattering from Ultrathin Metallic Films of Ag/Au(111)

We present measurements on the vibrational relaxation of NO(v = 2) scattered from atomically defined thin films of Ag on Au(111). The vibrational relaxation probability is strongly dependent on film thickness, increasing with each of the first three Ag monolayers. We interpret this as the influence of work function which changes with layer thickness. This supports the postulated mechanism of NO vibrational relaxation involving a transient NO– anion.

Charge Transfer in an Ordered Ag/Cu2S/4-MBA System Based on Surface-Enhanced Raman Scattering

In this study, we introduced an ordered metal–semiconductor–molecular system and investigated the surface-enhanced Raman scattering (SERS) effect. The carrier distribution and density can be easily monitored by changing the thickness of the Cu2S film on the surface of the Ag monolayer under different laser excitations. Meanwhile, different laser wavelengths were selected for investigating the enhancement mechanism of the system based on the viewpoint of the number of carriers and changes in the excitation energy. During this process, we found that the SERS peaks located at 1586 cm–1 changed significantly because the intensity can be “borrowed” from exited transition and the molecule-to-semiconductor transition. In combination with the theories of Albrecht and Lombardi, this phenomenon was analyzed in detail, and the experimental results and theoretical research match well. This research regarding the semiconductor-based SERS substrates will expand the basic theory of SERS enhancement and provide a theoret...

A FirstPrinciple Study on the Magnetic Properties of Ag, Al, Li, Mg, and NaDoped ReS2 Monolayers

Magnetic properties of ReS2 monolayer doped with nonmagnetic metals (Ag, Al, Li, Mg, and Na) are studied by the first-principle method Different dopants and doping sites are considered. Similar to transition metal (TM) atoms, magnetic behavior appears in Al and Mg-doped systems. On the other hand, the calculated binding energies show that the Al-doped system has a more stable-formed system than the Mg-doped system. Hence, we study the ferromagnetic interaction in two Al-doped ReS2 monolayers. Interestingly, as the Al–Al distance increases, both ferromagnetic (FM) and nonmagnetic (NM) states are found. Our results deem that the FM coupling originates from a p-d exchange-like p-p coupling interaction.

Alloyed monolayers of Cu, Ag, Au and Pt in hexagonal phase: A comprehensive first principles study

We present density functional theory (DFT) based comprehensive study of two-dimensional (2D) alloyed monolayers of noble metals (AgCu, AgPt, AgAu, AuCu, AuPt and CuPt) in hexagonal phase within numerical atomic orbitals and plane wave basis sets methods. The monolayers considered exhibit positive phonon frequencies suggesting them to be dynamically stable. The Pt containing alloyed monolayers have superior structural stability (binding energy and tensile strength) and exhibit metallic and ferromagnetic character amongst all the alloyed monolayers. Interestingly, alloying of Au monolayer with Cu and Ag show semiconducting behavior whereas alloyed AgCu monolayer posseses Dirac-cone like features at high symmetry points. These distinct features in electronic structures of alloyed 2D monolayers have been captured in STM like set up. An anisotropic behavior has been observed in dielectric spectra for all the considered structures. Tunneling characteristics show NDR region for Pt containing alloyed monolayers. The considered alloyed monolayers may potentially be useful as a building blocks for the applications in nano- and opto-electronics.