Single Crystal Ag Research Articles

A twin-free single-crystal Ag nanoplate plasmonic platform: hybridization of the optical nano-antenna and surface plasmon active surface

Surface plasmons based on metallic nanostructures enable light manipulation beyond the optical diffraction limit. We have epitaxially synthesized twin-free single-crystal Ag nanoplates on SrTiO3 substrates. Unlike the nanoplates synthesized in a solution phase, these nanoplates have perfectly clean surfaces as well as a quite large size of tens of micrometers. As-synthesized defect-free single-crystal Ag nanoplates have an atomically flat surface and sides with well-defined angles, allowing long distance propagation of surface plasmons and highly reliable plasmonic integration. By spatially separating receiving and transmitting antennas and plasmonically interfacing them, the signal quality of transmission/reception can be largely improved. Furthermore, by combining sub-dimensional nanostructures onto the two-dimensional space effective hierarchical plasmonic nano-complexes can be built up. Theoretical simulations well reproduced unique experimental results of coupling between SPPs and free-space radiation by the nanoplate antenna sides, low-loss long-range SPP propagation, and tunneling or scattering of SPPs at a nano-gap as well as a nano-structure introduced on the nanoplate. The single-crystal Ag nanoplate will find superb applications in plasmonic nano-circuitry and lab-on-a-chip for biochemical sensing.

Evidence for graphite-like hexagonal AlN nanosheets epitaxially grown on single crystal Ag(111)

Ultrathin (sub-monolayer to 12 monolayers) AlN nanosheets are grown epitaxially by plasma assisted molecular beam epitaxy on Ag(111) single crystals. Electron diffraction and scanning tunneling microscopy provide evidence that AlN on Ag adopts a graphite-like hexagonal structure with a larger lattice constant compared to bulk-like wurtzite AlN. This claim is further supported by ultraviolet photoelectron spectroscopy indicating a reduced energy bandgap as expected for hexagonal AlN.

Solid-source growth and atomic-scale characterization of graphene on Ag(111)

Silver is a desirable platform for graphene growth because of the potential for hybrid graphene plasmonics and its emerging role as a preferred growth substrate for other two-dimensional materials, such as silicene. Here we demonstrate the direct growth of monolayer graphene on a single-crystal Ag(111) substrate. The inert nature of Ag has made it difficult to use for graphene synthesis using standard chemical vapour deposition techniques, which we have overcome by using an elemental carbon source. Atomic-scale scanning tunnelling microscopy reveals that the atomically clean graphene–silver substrate is free of organic residue and other contaminants. The dendritic graphene possesses a variety of edge terminations, many of which give rise to quantum interferences previously seen only on insulating substrates. This scattering supports spectroscopic evidence that the graphene electronic structure is minimally perturbed by the underlying silver, providing a new system in which graphene is decoupled from its growth substrate.

Surface electronic bands of submonolayer Ge on Ag(111)

Germanium deposited on single-crystal Ag(111) substrates at about 1/3-monolayer coverage forms a stable $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R{30}^{\ensuremath{\circ}}$ superstructure as seen by reflection high-energy electron diffraction. In situ angle resolved photoelectron spectroscopy reveals a rich band structure consisting of conelike features that cross the Fermi surface at the center ($\overline{\ensuremath{\Gamma}}$ points) of the superstructure surface Brillouin zone (SBZ), exhibiting a high dispersion with the surface parallel wave vector. With the help of first-principles calculations, it can be inferred that most of the features are explained in terms of an Ag${}_{2}$Ge-ordered surface alloy corresponding to surface states localized in the first two layers. One of the bands has a saddle point near the $\overline{\mathrm{M}}$ point of the SBZ with a characteristic split-band behavior distinguishing Ge surface alloy from other similar systems.

Determination of Complete Melting and Surface Premelting Points of Silver Nanoparticles by Molecular Dynamics Simulation

A molecular dynamics simulation based on the embedded-atom method was conducted at different sizes of single-crystal Ag nanoparticles (NPs) with diameters of 4 to 20 nm to find complete melting and surface premelting points. Unlike the previous theoretical models, our model can predict both complete melting and surface premelting points for a wider size range of NPs. Programmed heating at an equal rate was applied to all sizes of NPs. Melting kinetics showed three different trends that are, respectively, associated with NPs in the size ranges of 4 to 7 nm, 8 to 10 nm, and 12 to 20 nm. NPs in the first range melted at a single temperature without passing through a surface premelting stage. Melting of the second range started by forming a quasi-liquid layer that expanded to the core, followed by the formation of a liquid layer of 1.8 nm thickness that also subsequently expanded to the core with increasing temperature and completed the melting process. For particles in the third range, the 1.8 nm liquid laye...

Synthesis of Silver Octahedra with Controlled Sizes and Optical Properties via Seed-Mediated Growth

Silver octahedra with edge lengths controlled in the range of 20-72 nm were synthesized via seed-mediated growth. The key to the success of this synthesis is the use of single-crystal Ag seeds with uniform and precisely controlled sizes to direct the growth and the use of citrate as a selective capping agent for the {111} facets. Our mechanistic studies demonstrated that Ag seeds with both cubic and quasi-spherical shapes could evolve into octahedra. For the first time, we were able to precisely control the edge lengths of Ag octahedra below 100 nm, and the lower limit of size could even be pushed down to 20 nm. Using the as-obtained Ag octahedra as sacrificial templates, Au nanocages with an octahedral shape and precisely tunable optical properties were synthesized through a galvanic replacement reaction. Such hollow nanostructures are promising candidates for a broad range of applications related to optics, catalysis, and biomedicine.

Simultaneous preparation of Ag/Fe3O4 core–shell nanocomposites with enhanced magnetic moment and strong antibacterial and catalytic properties

In this study, core–shell Ag/Fe3O4 nanocomposites have been successfully prepared by modified co-precipitation route followed by facile hydrothermal treatment in one-pot synthesis under various (Ag(NH3)2+) ion concentrations without adding any reducing agents. A single crystal Ag core and polycrystallized Fe3O4 shell nanostructures could be obtained when high amount of AgNO3 precursor are used in the presence of Fe2+ ions source, i.e., ferrous salt alone. A mechanism leading to the formation of such morphologies was proposed and samples were characterized with several analytical techniques including field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FT-IR) spectroscopy. Significant increase of saturation magnetic moment was observed for the Ag/Fe3O4 nanocomposite. The results showed enhanced magnetic moment and strong catalytic and antibacterial activities of the present Ag/Fe3O4 nanocomposite, demonstrating its importance for potential application in water treatment and biomedical application, with the ability of subsequent removal by means of an external magnetic field.

Double-sided ZnO nanorod arrays on single-crystal Ag holed microdisks with enhanced photocataltytic efficiency

Novel hierarchical heterostructures of double-sided ZnO nanorod (NR) arrays grown on single-crystal Ag holed microdisks (HMDs) have been prepared through a two-step aqueous strategy including ZnO seed loading and the subsequent heteroepitaxial growth of ZnO NRs on Ag HMDs. By simply adjusting the synthetic parameters, ZnO NRs with variable NR diameters (20-200 nm), lengths (100-1.8 μm) and unusual shapes (concave, tubular and sharp tips) on Ag HMDs have been realized, which endows the Ag/ZnO heterostructures with versatile morphologies. The novel Ag/ZnO heterostructures consisting of integrated 1D semiconductor/2D metal nanostructured blocks with high specific surface area (SSA) and opened spatial architectures may promise important applications related to photoelectric fields. As expected, in photocatalytic measurements, the typical Ag HMD/ZnO NR heterostructure exhibits superior catalytic activity over other catalysts of bare ZnO NRs, ZnO NR arrays or heterostructured Ag nanowires (NWs)/ZnO NRs. The synergistic effect of the unique Ag HMD/ZnO NR heterostructures contributing to the high catalytic performance has been discussed in detail.

Worm-Like Ag/ZnO Core–Shell Heterostructural Composites: Fabrication, Characterization, and Photocatalysis

Novel worm-like Ag/ZnO core–shell heterostructural composites were fabricated using a two-step chemical method. As-prepared silver nanowires were soaked in a solution of zinc acetate and triethanolamine to form worm-like Ag/ZnO core–shell composites under ultrasonic irradiation. Samples were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy, and UV–vis spectrophotometer. The results show that the core–shell composites are composed of single-crystal Ag nanowires serving as the core, on which dense ZnO particles grow as the shell. The surface plasmon absorption band of Ag/ZnO composites is distinctly broadened and red shifted to monometallic Ag nanowires. The PL intensity of Ag/ZnO heterostructural composites varies and has the minimum intensity for the sample prepared with Ag of 2.8 atom %. Moreover, photocatalytic tests show that the Ag/ZnO composites ...

Selective Supramolecular Fullerene–Porphyrin Interactions and Switching in Surface-Confined C60–Ce(TPP)2 Dyads

The control of organic molecules, supramolecular complexes and donor-acceptor systems at interfaces is a key issue in the development of novel hybrid architectures for regulation of charge-carrier transport pathways in nanoelectronics or organic photovoltaics. However, at present little is known regarding the intricate features of stacked molecular nanostructures stabilized by noncovalent interactions. Here we explore at the single molecule level the geometry and electronic properties of model donor-acceptor dyads stabilized by van der Waals interactions on a single crystal Ag(111) support. Our combined scanning tunneling microscopy/spectroscopy (STM/STS) and first-principles computational modeling study reveals site-selective positioning of C(60) molecules on Ce(TPP)(2) porphyrin double-decker arrays with the fullerene centered on the π-system of the top bowl-shaped tetrapyrrole macrocycle. Three specific orientations of the C(60) cage in the van der Waals complex are identified that can be reversibly switched by STM manipulation protocols. Each configuration presents a distinct conductivity, which accounts for a tristable molecular switch and the tunability of the intradyad coupling. In addition, STS data evidence electronic decoupling of the hovering C(60) units from the metal substrate, a prerequisite for photophysical applications.

Aerobic epoxidation of propene over silver (1 1 1) and (1 0 0) facet catalysts

Catalytic performance of single-crystal Ag(100) and Ag(111) catalysts for propene oxidation has been investigated by means of Raman spectroscopy and mass spectrometry, and the energy profile has been obtained at the DFT-D level. It has been theoretically found and confirmed by Raman spectroscopy that Ag(100) surface is more reactive toward O2 dissociation than Ag(111) and that adsorbed oxo-species are more strongly bonded to the Ag(100) facet. The higher selectivity toward propylene oxide (PO) observed for the Ag(100) catalyst can be rationalized from theoretical calculations indicating that the activation barriers for formation of the allylic intermediate – precursor of combustion products – are rather similar on both silver surfaces, whereas energy barriers on the reaction pathways for formation of PO and carbonylic products (acetone and propanal) are systematically smaller on the Ag(100) surface.

Rapid, green synthesis and surface-enhanced Raman scattering effect of single-crystal silver nanocubes

Single-crystal silver (Ag) nanocubes have been synthesized by a rapid and green method at room temperature by adding sodium hydroxide solution to the mixed solutions of silver nitrate, glucose and polyvinylpyrrolidone (PVP). The X-ray diffraction (XRD), ultraviolet–visible (UV–visible) and transmission electron microscopy (TEM) were used to characterize the phase composition and morphology. The results showed that the as-prepared particles were single-crystal Ag nanocubes with edge lengths of around 77nm and a growing direction along {100} facets. As substrates for surface-enhanced Raman scattering (SERS) experiment on crystal violet (CV), the SERS enhancement factor of the as-prepared Ag nanocubes were measured to be 5.5×104, indicating potential applications in chemical and biological analysis.

Epitaxial Ag(001) grown on MgO(001) and TiN(001): Twinning, surface morphology, and electron surface scattering

Epitaxial Ag(001) layers were deposited on MgO(001) in order to study electron surface scattering. X-ray reflection indicates 3D layer nucleation with a high rms surface roughness of 1.0 nm for a layer thickness d = 3.5 nm. X-ray diffraction shows that {111} twins form at d < 11 nm, followed by 2nd generation twinning for 11 nm < d < 120 nm. Increasing the growth temperature from 25 to 150 °C suppresses 2nd generation twinning and reduces the twin density by 2 orders of magnitude. In situ deposition of epitaxial 2.5-nm-thick TiN(001) underlayers prior to Ag deposition results in twin-free single-crystal Ag(001) with 10 × smoother surfaces for d = 3.5 nm. This is attributed to a better wetting on the higher energy TiN(001) than MgO(001) surface, resulting in the absence of 3D nuclei with exposed {111} facets, which facilitate twin nucleation. The twinned Ag/MgO layers have a higher resistivity ρ than the single crystal Ag/TiN layers at both 298 and 77 K, due to electron scattering at grain and twin boundaries. The ρ for single-crystal Ag layers increases with decreasing d, which is well explained with known surface scattering models and provides specularity parameters for the Ag-vacuum and the Ag-air interfaces of p = 0.8 ± 0.1 and 0.4 ± 0.1, respectively. A comparison with corresponding epitaxial Cu(001) layers shows that ρAg < ρCu for d > 50 nm, consistent with known bulk values. However, ρAg > ρCu for d < 40 nm. This is attributed to the larger electron mean free path for electron-phonon scattering and a correspondingly higher resistivity contribution from surface scattering in Ag than Cu. In contrast, air exposure causes ρAg < ρCu for all d, due to diffuse scattering at the oxidized Cu surface and the correspondingly higher Cu resistivity.

Electrochemical layer by layer growth and characterization of copper sulfur thin films on Ag(1 1 1)

Copper sulfide (CuS) thin films were grown on a single crystal Ag(1 1 1) substrate by Electrochemical Atomic Layer Deposition (ECALD) method, i.e., by alternated surface limited deposition of copper and sulfur. A detailed investigation of deposition of Cu on S allowed to find the best conditions for copper deposition. The electrochemical characterization of deposits obtained with different deposition cycles suggests a 1:1 stoichiometric ratio between Cu and S corresponding to Cu monosulfide. The compositional analysis was performed by X-rays Photoelectron Spectroscopy (XPS), and the morphological was investigated by Atomic Force Microscopy (AFM) for deposits formed with 20 ECALD cycles.

A novel shape-controlled synthesis of dispersed silver nanoparticles by combined bioaffinity adsorption and TiO 2 photocatalysis

Shape-controlled silver nanoparticles (Ag NPs) were prepared in a well-dispersed mode on the active imprinting sites of chitosan-TiO 2 adsorbent (CTA) by means of bioaffinity adsorption and TiO 2 photocatalysis. Nontoxic hydrogen peroxide (H 2O 2) was used as a suitable etching reagent in our production of shape-controlled Ag NPs, since it could regulate the TiO 2 photocatalysis and accelerate the generation of O 2. With the same amount of H 2O 2 addition, silver nanocubes, nanospheres and truncated triangular nanoplates were individually obtained on the surface of CTA under UV irradiation by facilely adjusting the initial Ag + concentration. The FE-SEM, XRD and UV–visible characterizations confirmed single crystal Ag NPs with different shapes loaded on CTA. The mechanism for the formation of shape-controlled Ag NPs was discussed based on a photocatalytic reaction system. As an example of applications of the Ag NPs, we tested the biocidal properties, and silver nanocubes exhibited the highest antibacterial activity. Our research provided a simple synthesis for shape-regulated Ag NPs steadily loaded on CTA. It might moreover be a guide in preparing metal nanocrystals monodispersely immobilized on chemical substrates.

A facile route to well-dispersed single-crystal silver nanoparticles from [AgSO 3] − in water

Well-dispersed single-crystal Ag nanoparticles (NPs) were synthesized through a simple hydrothermal strategy in an aqueous solution of [AgSO 3] −. The obtained Ag NPs are quasispherical in shape and the diameter of particles is ranging from 35 to 50 nm with a narrow size distribution. The proper choices of [AgSO 3] − as precursor and PVP as stabilizer are the keys to the formation of Ag NPs. A possible formation mechanism was proposed. And the optical absorption property of the obtained Ag NPs was also investigated. The as-prepared Ag NPs exhibit an absorption band at ca. 430 nm, which is the characteristic surface plasmon resonance of spherical/quasispherical Ag NPs.

Structural stability and theoretical strength of the single crystal Ag under uniaxial loading

The structural stability and theoretical strength of FCC crystal Ag under uniaxial loading have been investigated by combining MAEAM with Wang modified Born stability criteria. The results reveal that, under sufficient compression, there exists a stress-free BCC phase, which is unstable and slips spontaneously to a stress-free mBCT phase by consuming internal energy. The stable region ranges from −2.076 to 4.390 GPa in the theoretical strength or from −7.972% to 8.721% in the strain correspondingly. The calculated structural energy difference between the BCC and FCC phase is in good agreement with the experimental value.

Seed-Mediated Synthesis of Ag Nanocubes with Controllable Edge Lengths in the Range of 30−200 nm and Comparison of Their Optical Properties

Silver nanocubes with edge lengths controllable in the range of 30-200 nm were synthesized using an approach based on seeded growth. The keys to the success of this synthesis are the use of single-crystal Ag seeds to direct the growth and the use of AgNO(3) as a precursor to elemental Ag, where the byproduct HNO(3) can block both the homogeneous nucleation and evolution of single-crystal seeds into twinned nanoparticles. Either spherical (in the shape of a cuboctahedron) or cubic seeds could be employed for this growth process. The edge length of the resultant Ag nanocubes can be readily controlled by varying the amount of Ag seeds used, the amount of AgNO(3) added, or both. For the first time, we could obtain Ag nanocubes with uniform edge lengths controllable in the range of 30-200 nm and then compare their localized surface plasmon resonance and surface-enhanced Raman scattering properties.

Visualizing the Electron Scattering Force in Nanostructures

In nanoscale metal wires, electrical current can cause structural changes through electromigration, in which the momentum of electrons biases atomic motion, but the microscopic details are complex. Using in situ scanning tunneling microscopy, we examined the effects of thermally excited defects on the current-biased displacement of monatomic islands of radius 2 to 50 nanometers on single-crystal Ag(111). The islands move opposite to the current direction, with velocity varying inversely with radius. The force is thus in the same direction as electron flow and acts on atomic defect sites at the island edge. The unexpectedly large force on the boundary atoms can be decreased by over a factor of 10 by adding a mildly electron-withdrawing adsorbate, C60, which also modifies the step geometry. The low coordination of the identified scattering sites is the likely origin of the large force.

Synthesis of small silver nanocubes in a hydrophobic solvent by introducing oxidative etching with Fe(iii) species

The role of oxidative etching in the synthesis of noble-metal nanocrystals with a hydrophilic solvent (in particular, ethylene glycol) has been explored in our previous work. In this paper, we demonstrate that the mechanism of oxidative etching could be extended to the synthesis of single-crystal Ag nanocrystals in a hydrophobic solvent such as isoamyl ether. In this case, Fe(III) species in the form of FeCl3 or Fe(acac)3 had to be introduced into the hydrophobic system as an effective etching agent. The final product contained single-crystal Ag nanocubes (slightly truncated) with an edge length as small as ∼13.5 nm, a size that has been difficult to achieve using a hydrophilic system.