Single Crystal Ag Research Articles

Microstructure evolution and SERS performance of self-formed Ag nanoparticles/Ag-27.9 at% Co-8.9 at% Zr alloy films

Ag-27.9 at% Co-8.9 at% Zr alloy films were deposited on flexible polyimide (PI) by magnetron sputtering and annealed in argon atmosphere. Results show that many fine nanoparticles are formed on the surface of as-deposited Ag–Co–Zr alloy films. Furthermore, the average size of self-formed Ag nanoparticles gradually increases while the number of Ag nanoparticles decreases as alloy film thickness increases. In addition, it is worth noting that flake-like and polyhedral nanoparticles are formed spontaneously on the surface of annealed alloy films and are proved to be single crystal Ag nanoparticles. Moreover, the morphology of flake-like Ag nanoparticles is dependent on annealing temperature. In order to improve “hotspots” activity of the alloy films, a 12 nm pure Ag layer was deposited on the annealed alloy film surface to obtain Ag layer/Ag nanoparticles/Ag–Co–Zr alloy films SERS substrates which can detect 5 × 10−14 M R6G. This paper provides a potential candidate for preparation of SERS substrates with ultra-high sensitivity and stability.

Enhanced CO oxidation by reversible structural variation of supported Ag nanoparticle catalyst from single to twin by CO treatment

Twinned structure of metal nanoparticles can strongly affect catalysis because of its unique atomic alignment and bond strain in metal nanoparticles. Here, the twin and single crystal structures of supported Ag nanoparticles are controlled reversibly by gas treatment. The single crystal Ag nanoparticles were formed in Ag/Al2O3 by H2 treatment, and subsequent CO gas treatment changed the Ag particle structure to twin. The change in the Ag particle structure from single to twin caused enhancement of CO oxidation activity. In addition, the catalytic activity was reversibly changed from low to high by applying H2 and CO gas treatment to Ag/Al2O3. The kinetic analysis for CO oxidation revealed that the twin Ag nanoparticles have stronger CO adsorption energy than the single crystal ones, which is responsible for the enhanced CO oxidation activity of twin Ag nanoparticles.

Abnormal microstructure evolution behaviors of annealed Ag-18.9 at% Co alloy film on flexible polyimide substrates

Ag-Co alloy films were fabricated at room temperature by magnetron sputtering. The results revealed that a large amount of polyhedral single-crystal Ag nanoparticles grow on the surface of Ag-Co alloy films, which is significantly different from the microstructure of Ag-Co alloy films on rigid substrates. Furthermore, after annealing, Ag atoms get enough energy to move toward the film surface and thus the mass and average size of Ag particles both increase with the increase of annealing temperatures. In addition, the average size of Ag particles decreases with the increase of annealing time but the particles per square micron increases. Amorphous Co is observed in the Ag-Co films and Ag grains in Ag-Co films become smaller, which indicates the growth of Ag grains is inhibited by Co. A new type of Ag particles / Ag-Co films with a convex structure is obtained and applied as SERS substrates, where the characteristic Raman spectra of 5 × 10−12 M R6G could be detected.

Molecular dynamics simulation of microstructure evolution during the fracture process of nano-twinned Ag

In this paper, the evolution of microstructure and the role of twin boundary during the fracture process of nano-twinned Ag are studied through molecular dynamics simulation. The fracture process of nano-twinned Ag with multiple twin boundaries is compared with both the fracture process of single crystal Ag and nano-twinned Ag with single twin boundary. Results demonstrate that multiple twin boundaries can significantly improve the strength of material by influence the propagation of crack and dislocation as observed by experiments. By comparing the stress–strain relationship and evolution of microstructure, we found that the elastoplastic transition in single crystal Ag is characterized by dislocation emission from the crack tip, while the elastoplastic transition in nano-twinned Ag is characterized by dislocation crossing the twin boundary. According to crack-resistance curves and observation results of microstructure evolution, multiple twin boundaries can significantly improve the fracture toughness of nano-twinned Ag compared to single twin boundary. The role of twin boundary in the fracture process is mainly to influence the propagation of crack and dislocation. The crack tip in single crystal Ag gradually gets blunt with the nucleation and emission of dislocations, while the crack in nano-twinned Ag propagates by cleavage. To explain this phenomenon, the generalized stacking fault energies and surface energies of both nano-twinned Ag and single crystal Ag are calculated and compared. It is shown that twin boundary can influence the energy barrier for atomic slip while have no influence on the generation of fracture surface. Twin boundary can also change the propagation direction of dislocations and stacking faults. Stacking faults propagate along (111) planes in matrix grain and propagate along (111¯) planes in twin grain, showing a tortuous shape. In this process, dislocations accumulate on the twin boundary, leading to high local dislocation density and high strength of nano-twinned material, which is agreement with experiments and simulations. Our study is helpful for understanding the relationship between microscopic deformation and macroscopic properties of nano-twinned crystal material.

Charge Transfer Kinetics at Ag(111) Single Crystal Electrode/Ionic Liquid Interfaces: Dependence on the Cation Alkyl Side Chain Length

AbstractThe influences of the electrode/ionic liquids interfaces on the charge transfer kinetics are addressed with the example of the oxidation of chromocene at a single crystal Ag(111) electrode. We considered three different imidazolium‐based room temperature ionic liquids (RTILs) with alkyl chains of different lengths but with a common anion. The standard charge transfer rate constants k° are extracted from cyclic voltammograms (CVs) by analyzing the peak potential variations with the scan rate and/or performing semi‐integral electroanalysis with a careful treatment of the ohmic drop. Linear variations between the logarithms of k° and the longitudinal relaxation times τL or the dynamic viscosity η of the ionic liquids are obtained. However, the amplitude of the decrease is much larger than it could be expected from the sole bulk properties of the ionic liquid indicating a strong effect of the interface on the kinetics. These observations agree with a well‐organized electrode/RTIL interface that behaves as a sort of self‐assembled monolayer that controls the charge transfer kinetics with an exponential tunneling coefficient β of 5.51 nm−1.

Graphene-assisted preparation of large-scale single-crystal Ag(111) nanoparticle arrays for surface-enhanced Raman scattering

Surface plasmon resonance (SPR) of metal nanostructures has broad application prospects in the fields of sensing, energy, catalysis and optics. This paper reports a graphene-assisted method for preparing large-scale single-crystal Ag(111) nanoparticle (NP) arrays based on the ion implantation technique. By surface periodic patterning treatment and annealing of the implanted sample, regularly arranged Ag NPs can be prepared on the sample surface. A new application for graphene is proposed, that is, as a perfect barrier layer to prevent metal atoms from evaporating or diffusing. All the Ag NPs show (111) crystal orientation. Besides, the Ag atoms are covered by graphene immediately when they precipitate from the substrate, which can prevent them from being oxidized. On the basis of this structure, as one of the applications of the metal SPR, we have measured the surface-enhanced Raman scattering effect and found that the G peak of the Raman spectrum of the graphene achieved about 20 times enhancement.

Room temperature self-assembled Ag nanoparticles/Mo-37.5% Ag film as efficient flexible SERS substrate

Mo-Ag films with different thicknesses were deposited on flexible polyimide (PI) substrate by DC magnetron sputtering. The phase, surface morphology, composition and thickness of the Mo-Ag films were characterized by XRD, FESEM, EDS and TEM, respectively. The most outstanding result indicates that many monodisperse single crystal Ag nanoparticles are spontaneously formed on the surface of the deposited Mo-Ag films at room temperature. The Ag nanoparticles/Mo-37.5% Ag film has rich active “hot spots”, which can be used as an effective surface-enhanced Raman scatting (SERS) substrate by covering a thin layer of silver on the surface, and the detection limit of Rhodamine 6G solution can reach 10-12 mol/L. The Ag nanoparticles/Mo-Ag films prepared at room temperature provided a new idea for the preparation of novel particle films and high-sensitivity flexible SERS substrates.

Fabrication of ZnO@Ag@Ag3PO4 Ternary Heterojunction: Superhydrophilic Properties, Antireflection and Photocatalytic Properties.

A ZnO seed layer was formed on the fluorine-doped tin oxide substrate by magnetron sputtering, and then a ZnO nanorod was grown on the ZnO seed layer by a hydrothermal method. Next, we prepared a single-crystal Ag seed layer by magnetron sputtering to form a ZnO@Ag composite heterostructure. Finally, Ag3PO4 crystals were grown on the Ag seed layer by a stepwise deposition method to obtain a ZnO@Ag@Ag3PO4 ternary heterojunction. The composite heterostructure of the material has super strong hydrophilicity and can be combined with water-soluble pollutants very well. Besides, it has excellent anti-reflection performance, which can absorb light from all angles. When Ag exists in the heterojunction, it can effectively improve the separation of photo-generated electrons and holes, and improve the photoelectric conversion performance. Based on the above characteristics, this nano-heterostructure can be used in the fields of solar cells, sensors, light-emitting devices, and photocatalysis.

Wafer-scale high-quality Ag thin film using a ZnO buffer layer for plasmonic applications

Realizing laterally continuous, ultraflat silver (Ag) single-crystal films is a significant technological challenge. Ag thin film grown on various hetero-substrates has been used in numerous applications, due to its superior electrical and optical properties. To exploit these properties without degradation and apply these films to high-precision patterning, surface plasmonics, and so on, a high-quality thin film having an ultraflat surface and few grain boundaries is needed. A zinc oxide (ZnO) buffer layer can be used to facilitate the growth of a single-crystalline Ag thin film on a sapphire (Al2O3) substrate. ZnO films deposited on Al2O3 substrates have grain boundaries; however, Ag films grown on ZnO are nearly grain-free and close to single-crystalline quality. This can be explained by the exceptionally small extended atomic distance mismatch, of ~0.08%, between Ag and Al2O3, in which the ninth Ag atom and the eighth Al atom are matched in terms of coherence and periodicity. A modified radio frequency sputtering system with a single-crystal Ag target enabled wafer-scale growth of ultraflat, grain-free Ag films. The proposed approach using a ZnO buffer provides a new method for fabricating Ag films with high adhesion, anti-oxidative stability, and superior optical properties, and allows for easy nano-patterning.

Abnormal Microstructure Evolution Behaviors of Annealed Ag-Co Alloy Film on Flexible Polyimide Substrates

Ag-Co alloy films were deposited on polyimide by magnetron sputtering. XRD, HRTEM and FESEM were used to characterize phase and surface morphology of Ag-Co alloy films. Surprisingly, the results showed that a large amount of polyhedral single crystal Ag nanoparticles were self-grown on the surface of Ag-Co alloy films, which is significantly different from the microstructure of Ag-Co alloy film on rigid substrates. Moreover, the average number and size of Ag particles are dependent on annealing temperature. These analysis results indicated that Co inhibits the growth of Ag grains and leads to the formation of Ag particles on the surface of the film. These nano-sized Ag particles and Ag-Co films are assembled into a new type of particles/ films with a convex structure.

Facile synthesis of ultra-large, single-crystal Ag nanosheet-assembled films at chloroform-water interface

The ultra-large, single-crystal Ag nanosheets and their assembled films can be easily one-pot prepared at room temperature via a chloroform-water interfacial reaction route with o-ethoxyaniline (OEA) in chloroform as reductant and AgNO3 in water as precursor. The as-obtained single-crystal Ag nanosheets can reach as large as 16 μm in side length and as thin as 22 nm in thickness. The results show that reagent concentrations, temperature and organic phase in current liquid-liquid reaction have significant influences on the formation of Ag sheet-like nanostructures. The possible mechanism was proposed for the growth and formation of Ag nanosheets and their assembled films. Furthermore, using p-aminothiophenol (PATP) as a model molecular probe, the Ag nanosheet-assembled films exhibit outstanding surface-enhanced Raman scattering (SERS) response and high reproducibility.

Surface Lattice Resonances in Self-Assembled Arrays of Monodisperse Ag Cuboctahedra.

Plasmonic metal nanoparticles arranged in periodic arrays can generate surface lattice plasmon resonances (SLRs) with high Q-factors. These collective resonances are interesting because the associated electromagnetic field is delocalized throughout the plane of the array, enabling applications such as biosensing and nanolasing. In most cases such periodic nanostructures are created via top-down nanofabrication processes. Here we describe a capillary-force-assisted particle assembly method (CAPA) to assemble monodisperse single-crystal colloidal Ag cuboctahedra into nearly defect-free >1 cm2 hexagonal lattices. These arrays are large enough to be measured with conventional ultraviolet-visible spectroscopy, which revealed an extinction peak with a Q-factor of 30 at orthogonal illumination and up to 80 at oblique illumination angles. We explain how the experimental extinction changes with different light polarizations and angles of incidence, and compare the evolution of the peaks with computational models based on the coupled dipole approximation and the finite element method. These arrays can support high Q-factors even when exposed to air, because of the high aspect ratio of the single-crystal nanoparticles. The observation of SLRs in a self-assembled system demonstrates that a high level of long-range positional control can be achieved at the single-particle level.

Enhanced thermoelectric properties of Ag-doped MnO2 single crystal nanowires for room-temperature application

Based on colossal Seebeck coefficient [Nanotechnology, 2012, 23, 085401], here we focused on electrical transportation of single crystal nanowire structure and Ag dopant on improving the figure of merit (ZT) for MnO2. In this work, a series of Mn1−xAgxO2 (x = 0, 0.01 wt%, 0.02 wt%, 0.05 wt%, 0.1 wt%) single crystal nanowires have been firstly synthesized by a simple and green chemical vapor deposition (CVD) method where 0.01 wt% doped MnO2 is considered as a good candidate due to its promising electrical transport properties. Although there is at room-temperature, the maximum ZT is achievable 0.0075 for the 0.01 wt% Ag-doped sample at 300 K, which may be attributed to single crystal nanowire structure, together with effective number of charge carrier concentration caused by the Ag dopant. Ag-doped MnO2 single crystal nanowires provide a new possibility for green collecting energy at room-temperature.

Single crystalline silicene consist of various superstructures using a flexible ultrathin Ag(111) template on Si(111)

Using scanning tunneling microscopy, we studied the formation of silicene on an ultrathin Ag(111) film with a thickness of 6–12 monolayers, which was prepared on a Si(111) substrate. A low-energy electron diffraction pattern with an oval spot indicated that the ultrathin Ag(111) film is more disordered than the single-crystal Ag(111). After Si epitaxy growth, we still measured the classical 4 × 4, √13 × √13, and 2√3 × 2√3 silicene superstructures, which are the same as the silicene superstructure on single-crystal Ag(111). Growing silicene on a single-crystal Ag(111) bulk usually results in the formation of a defect boundary due to the inconsistent orientation of various superstructures. By comparing the angles and boundary conditions between various silicene superstructures on the ultrathin film and single-crystal Ag(111), we discovered that a consistent orientation of various superstructures without obvious boundary defects formed on the ultrathin Ag(111) film. The results indicated single crystalline silicene formation, which was attributed to the domain rotation and lateral shift of the disordered ultrathin Ag(111) film.

Shape Control of Electrochemically Deposited Metal Films and Nanostructures through Additive Effects

The use of electrolyte additives to affect nanocrystallite shape and film morphology in electroless and electrodeposited metal films is presented. Copper electrodeposited onto gold results in preferential square pyramidal copper nanostructure morphologies in the presence of the organic additive malachite green (MG). We have employed in-situ and ex situ methods to investigate the additive effects, including nonlinear optical spectroscopy, electrochemical scanning tunneling microscopy, linear sweep and cyclic voltammetry, atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). These shape-controlled additive effects are supported by additive adsorption energy calculations, which indicate preferential interactions, and differential growth kinetics on different facets of the film's growing nanostructures during electrodeposition. Facet-specific interactions have also been exploited in the epitaxial electroless deposition of noble metals onto single crystal Ag(100) substrates. Here, we examine the role of specific anions in determining the resulting film and nanostructure morphologies via SEM and high resolution transmission electron microscopy (HRTEM). These effects have been exploited to yield large area patterned, and shape-controlled nanoarrays of single crystal metal nanostructures for plasmonic and metamaterial applications. These approaches offer new and cost effective routes to achieve shape-controlled surface nanostructure.

Spontaneous and Selective Nanowelding of Silver Nanowires by Electrochemical Ostwald Ripening and High Electrostatic Potential at the Junctions for High-Performance Stretchable Transparent Electrodes.

Metal nanowires have been gaining increasing attention as the most promising stretchable transparent electrodes for emerging field of stretchable optoelectronic devices. Nanowelding technology is a major challenge in the fabrication of metal nanowire networks because the optoelectronic performances of metal nanowire networks are mostly limited by the high junction resistance between nanowires. We demonstrate the spontaneous and selective welding of Ag nanowires (AgNWs) by Ag solders via an electrochemical Ostwald ripening process and high electrostatic potential at the junctions of AgNWs. The AgNWs were welded by depositing Ag nanoparticles (AgNPs) on the conducting substrate and then exposing them to water at room temperature. The AgNPs were spontaneously dissolved in water to form Ag+ ions, which were then reduced to single-crystal Ag solders selectively at the junctions of the AgNWs. Hence, the welded AgNWs showed higher optoelectronic and stretchable performance compared to that of as-formed AgNWs. These results indicate that electrochemical Ostwald ripening-based welding can be used as a promising method for high-performance metal nanowire electrodes in various next-generation devices such as stretchable solar cells, stretchable displays, organic light-emitting diodes, and skin sensors.

Probing Intermolecular Vibrational Symmetry Breaking in Self-Assembled Monolayers with Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy.

Ultrahigh vacuum tip-enhanced Raman spectroscopy (UHV-TERS) combines the atomic-scale imaging capability of scanning probe microscopy with the single-molecule chemical sensitivity and structural specificity of surface-enhanced Raman spectroscopy. Here, we use these techniques in combination with theory to reveal insights into the influence of intermolecular interactions on the vibrational spectra of a N-N'-bis(2,6-diisopropylphenyl)-perylene-3,4:9,10-bis(dicarboximide) (PDI) self-assembled monolayer adsorbed on single-crystal Ag substrates at room temperature. In particular, we have revealed the lifting of a vibrational degeneracy of a mode of PDI on Ag(111) and Ag(100) surfaces, with the most strongly perturbed mode being that associated with the largest vibrational amplitude on the periphery of the molecule. This work demonstrates that UHV-TERS enables direct measurement of molecule-molecule interaction at nanoscale. We anticipate that this information will advance the fundamental understanding of the most important effect of intermolecular interactions on the vibrational modes of surface-bound molecules.

Bonding technology based on solid porous Ag for large area chips

A bonding technology is introduced by using surface polished porous Ag in die-attachment structure. Bonding strength did not change much as the chip size varied from 3×3mm2 to 15×15mm2. This confirms that the technology was not influenced by the chip size, and thus can be used in large area bonding. Bonding mechanism based on stress-induced migration was discussed with the three dimensional finite element analyses. Transmission electron microscopy (TEM) observation further confirmed that single crystal hillocks and Ag particles formed at the bonding interface, bridging the interface together.

Electrochemical Cycling of Polycrystalline Silver Nanoparticles Produces Single-Crystal Silver Nanocrystals.

Electrochemically driven phase transformations in redox-active nanoparticles (NPs) are important in a number of areas, including batteries and sensors. We use high-resolution electron microscopy in conjunction with ex situ electrochemical experiments on TEM grids to study the oxidative conversion of polycrystalline silver NPs to amorphous silver oxide nanoparticles and their reductive conversion back to single-crystal silver nanocrystals (NCs). Results show that during oxidation nucleation occurs uniformly at the NP surface, producing a Ag@Ag2O core@shell structure during growth. The images reveal polycrystalline Ag cores and amorphous Ag2O shells for these structures. Electron microscopy also showed that the electrochemical reduction of Ag2O NPs can produce single-crystal Ag nanocrystals, suggesting that point nucleation at the NP-electrode interface during reduction enables a growth mechanism favoring the formation of single-crystal nanoparticles.

Adsorption and charge transfer interactions of bi-isonicotinic acid on Ag(111).

The adsorption and charge transfer dynamics of the organic molecule bi-isonicotinic acid (4,4-dicarboxy-2,2-bipyridine) on single crystal Ag(111) has been studied using synchrotron radiation-based photoemission, x-ray absorption, and resonant core spectroscopies. Measurements for multilayer and monolayer coverage are used to determine the nature of the molecule-surface interactions and the molecular orientation. An experimental density of states for the monolayer with respect to the underlying metal surface is obtained by combining x-ray absorption spectroscopy at the N 1s edge and valence photoemission to measure the unoccupied and occupied valence states, respectively. This shows that the lowest unoccupied molecular orbital in the core-excited state lies energetically below the Fermi level of the surface allowing charge transfer from the metal into this orbital. Resonant photoelectron spectroscopy was used to probe this charge transfer in the context of super-spectator and super-Auger electron transitions. The results presented provide a novel interpretation of resonant core-level spectroscopy to explore ultra-fast charge transfer between an adsorbed organic molecule and a metal surface through the observation of electrons from the metal surface playing a direct role in the core-hole decay of the core-excited molecule.