Ag Dots Research Articles

An aerosol-based soft lithography to fabricate nanoscale silver dots and rings for spectroscopic applications.

Site-selective deposition of aerosol Pd nanoparticles on a substrate was employed to fabricate nanoscale Ag dots and rings through a subsequent electroless deposition. The fabricated nanoscale dot and ring arrays respectively showed properties in surface-enhanced Raman (SER, with a 1.8 × 10(5) enhancement factor) and Fourier transform near infrared (FT-NIR, at 6153 cm(-1) absorption band) spectra.

Antibacterial activity of microstructured Ag/Au sacrificial anode thin films.

Ten different Ag dot arrays (16 to 625 microstructured dots per square mm) were fabricated on a continuous Au thin film and for comparison also on Ti film by sputter deposition and photolithographic patterning. To analyze the antibacterial activity of these microstructured films Escherichia coli and Staphylococcus aureus were placed onto the array surfaces and cultivated overnight. To analyze the viability of planktonic as well as surface adherent bacteria, the applied bacterial fluid was subsequently aspirated, plated on blood agar plates and adherent bacteria were detected by fluorescence microscopy. A particular antibacterial effect towards both bacterial strains was induced by Ag dot arrays on fabricated Au thin film (sacrificial anode system for Ag), due to the release of Ag ions from dissolution of Ag dots in contrast to Ag dot arrays fabricated on the Ti thin films (non-sacrificial anode system for Ag) which remained intact to the original dot shape. The required number of Ag dots on gold film to achieve complete bactericidal effects for both bacterial strains was seven times lower than that observed with Ag dot arrays on Ti film.

Prediction of drop-on-demand (DOD) pattern size in pulse voltage-applied electrohydrodynamic (EHD) jet printing of Ag colloid ink

Drop-on-demand printing is receiving a great deal of interest in industrial applications; however, the desired pattern sizes are realized by trial and error, through repeated printing experiments with varied materials (ink and suspended particles), operating conditions (voltage, flow rate, nozzle-to-plate distance, etc.), and substrate wettability. Since this approach requires a great deal of time, cost, and effort, a more convenient and efficient method that will predict pattern sizes with a minimal number of experiments is needed. In this study, we patterned a series of Ag dots and lines using a pulsed voltage-applied electrohydrodynamic jet printing system and measured their sizes with an optical microscope. We then applied a model suggested by Stringer and Derby (J Eur Ceram Soc 29:913–918, 2009) and Gao and Sonin (Proc R Soc Lond Ser A 444:533–554, 1994) to predict the pattern sizes, comparing these predictions with the measured sizes. Finally, we demonstrated our methodology on disconnected line repairing.

Ag dot morphologies printed using electrohydrodynamic (EHD) jet printing based on a drop-on-demand (DOD) operation

Electrohydrodynamic (EHD) jet printing technology is an attractive method for micro-scale electronic device fabrication. The primary advantage of EHD jet printing compared with conventional inkjet printing is the capability to print at resolutions below 10 µm and to eject high-viscosity ink. In this study, by using drop-on-demand (DOD) jetting, we printed silver (Ag) dots onto a silicon (Si)-wafer and evaluated the dot uniformity. Furthermore, we investigated the effects of substrate surface energy and substrate temperature on the dot morphology. We also investigated the effects of overprinting on the dot morphologies. Our results show that we successfully created uniform dot patterns under 10 µm by using EHD jet printing. In addition the dot diameter approached 14 µm while the substrate was heated up to 40 °C. We also found that on the hydrophobic Si-wafer, increasing the substrate temperature and the number of overprinting could be used as an alternative method for increasing the aspect ratio of dot and suppressing the coffee-stain effect.

Preparation and Functions of Hybrid Membranes with Rings of Ag NPs Anchored at the Edges of Highly Ordered Honeycomb‐Patterned Pores

We report a new, simple strategy to apply honeycomb films for the patterning of colloidal particles. By combination of a "bottom-up" breath figure method and the electrochemical properties of the honeycomb films of ferrocenyl-based oligomers, highly ordered hybrid membranes coated with ring-like patterning of 0D- and 1D-Ag nanoparticles (NPs) have been fabricated. One interesting phenomenon is that the nucleation and adsorption of Ag dots occurred preferentially at the edges of the micropores. The hybrid membranes exhibited richly electrochemical activities towards reduction of iodate and enhanced effectively catalytic reduction of organic dyes. We believe that this method can be used to decorate and/or assemble functional metal NPs such as Au, Pd, and Cu on honeycomb-patterned materials for the further applications of photonics, sensors, and catalysis.

Influence of Silver Incorporation on the Structural and Electrical Properties of Diamond-Like Carbon Thin Films

A simple approach is proposed for obtaining low threshold field electron emission from large area diamond-like carbon (DLC) thin films by sandwiching either Ag dots or a thin Ag layer between DLC and nitrogen-containing DLC films. The introduction of silver and nitrogen is found to reduce the threshold field for emission to under 6 V/μm representing a near 46% reduction when compared with unmodified films. The reduction in the threshold field is correlated with the morphology, microstructure, interface, and bonding environment of the films. We find modifications to the structure of the DLC films through promotion of metal-induced sp2 bonding and the introduction of surface asperities, which significantly reduce the value of the threshold field. This can lead to the next-generation, large-area simple and inexpensive field emission devices.

Soft-lithographic patterning of room temperature-sintering Ag nanoparticles on foil

Room temperature-sintering, poly(acrylic acid)-capped silver nanoparticles (Ag-PAA NPs) were used in a wide range of nanofabrication methods to form metallic silver microstructures on flexible poly(ethylene terephthalate) (PET) substrates. Silver wires on top of PET foil were patterned by micromolding in capillaries (MIMIC), and silver wires embedded in SU8 on PET foil were fabricated by wetting-controlled deposition in open microchannels. One hundred μm-wide Ag microwires with lengths of 5–15 mm, heights of 0.6–2.5 μm, and a maximum conductivity of a factor 7.3 lower than bulk Ag were obtained. Methanol was studied as an alternative dispersing solvent. It sped up MIMIC drastically, but the low particle packing quality and pre-coalescence in solution resulted in a higher resistivity. The sintering depth was found to be limited to around 100 nm for HCl-vapor induced sintering. Aqueous NaCl, added in a concentration below 50 mM to the Ag-PAA NP ink, was investigated as self-sintering agent. It resulted mainly in strong particle clustering and formation of numerous non-connected grains upon the evaporation of water. A hydrogel reservoir stamping system was used as an alternative printing technique for the transfer of the Ag-PAA NP ink on a PDMS substrate to yield the repetitive printing of arrays of 144 three-micron-wide Ag dots.

Facile generation of surface structures having opposite tone in metal-assisted chemical etching of Si: pillars vs. holes

Easy control on etched surface structures, either pillars or holes, in metal-assisted chemical etching (MaCE) of Si is presented. Starting with Ag mesh film with holes, which is generated by Ag evaporation in combination with a colloidal monolayer mask, the usual MaCE process has led to the vertical arrays of Si pillars. On the other hand, O2 reactive ion etching (RIE) on such Ag mesh film has led to sputtering and re-deposition of Ag on empty sites. During the following MaCE, the damaged Ag mesh spontaneously delaminates off the substrate and thus the etching with remaining Ag dot patterns results in holes array. Mechanistic studies of these etching behaviours have been performed and thus optimized, including the spontaneous mesh delamination. The obtained Si surfaces, either pillars or holes, were found to have comparable anti-reflection properties to other texturing approaches, which would be useful for photon management in applications such as photovoltaics, photoelectrochemical processes, etc.

Nanostructure Ag dots for improving thermal stability of Ag reflector for GaN-based light-emitting diodes

We report the improved thermal stability of Ag reflectors for GaN-based light-emitting diodes (LEDs) using Ag nano-dots (∼65–∼190 nm in size). The nano-dot Ag samples show much higher reflectance than the Ag only samples. The annealed nano-dot Ag samples exhibit a smoother surface, where the grains contain numerous micro-twins. 〈111〉 texture becomes more dominantly evolved in the nano-dot Ag samples than in the Ag only samples after annealing. LEDs with the 300 °C-annealed nano-dot Ag reflectors exhibit 15%–36% higher output power (at 20 mA) than LEDs with the 300 and 400 °C-annealed Ag only reflectors.

Surface plasmon-waveguide hybrid polymer light-emitting devices using hexagonal Ag dots

We investigated surface plasmon-waveguide hybrid resonances for enhancement of light emission in polymer light-emitting diodes (PLEDs). Hybrid waveguide-plasmon resonances in the visible range for waveguide mode and near IR range for surface plasmons were observed by incorporation of hexagonal Ag dot arrays. Considerable overlap between the emission wavelength of the PLEDs and the waveguide mode by an Ag dot array with a lattice constant of 500 nm was observed. Because of enhanced light extraction by Bragg scattering of waveguide modes, photoluminescence (PL) and electroluminescence (EL) were increased by 70% and 50%, respectively.

Simulation of Surface Plasmon Coupled Conjugate Polymer for Polymer Light-Emitting Diodes

Since 1977, conjugated polymers have received attention as materials for display devices with a low-cost solution process, but the low efficiency of these materials has been considered as a drawback which should be overcome. Nowadays metal nanoparticles are inserted on the display device's cathode to overcome the low efficiency of the materials through the enhanced coupling between the Localized surface plasmon resonance (LSPR) and exciton in emitting material . In our previous work, conjugated polymer with an imprinted regular Ag-dot-array structure showed a 2.7-fold improvement of integrated photoluminescence (PL) intensity , but the result was not optimized. Therefore, in this study, we calculated the Ag-dot-array absorbance-peak shift in detail using finite-difference time-domain (FDTD) simulation and found the absorbance peak location which maximized photoluminescence (PL) intensity, depending on various Ag dot condition. The resulting information was applied to the previous structure . Thus, we reduced the trial and error of finding the optimized absorbance peak location and the imprint processing costs. The most important parameter of the Ag-dot-array absorbance peak was the lattice constant. Furthermore, we proved the indium tin oxide (ITO) waveguide effect in our structure using FDTD.

Solvent-free nanoparticle fluids with highly collective functionalities for layer-by-layer assembly

Here we demonstrate the successful preparation of solvent-free inorganic nanoparticle (NP) fluids with high functionalities that allow mass production. The inorganic NPs (Ag, Au, and quantum dots [QDs]) used in this study were directly synthesized using hydrophobic stabilizers such as palmitic acid, oleic acid, or tetraoctylammonium bromide in a nonpolar solvent (chloroform or toluene), and these NPs were directly phase-transferred to solvent-free low molecular weight liquid media (i.e., thiol-containing imidazolium-type ionic liquid). The NP fluids formed at room temperature showed excellent functionalities (i.e., long-term dispersion stability, high electrical conductivity, strong optical energy transfer, and high photoluminescent intensity) without requiring any additional process such as anion exchange or thermal processing. Furthermore, layer-by-layer deposition of QDs stabilized by the ionic liquid induced highly fluorescent properties compared to those of layer-by-layer QD multilayers prepared using conventional ligands such as mercaptoacetic acid or cysteamine, which mainly originated from the high packing density and the relatively high quantum efficiency of ionic liquid-stabilized QDs.

Monte Carlo Simulation of X-Ray Photoemission Electron Microscopic Image for Ag/Si Nano-Structure

With the development of the third generation synchrotron radiation sources the X-ray photoemission electron microscopy (XPEEM) is playing as a powerful tool for analyzing specimens with spatial resolution about tens of nanometer. It has been used to observe the chemical reaction process, and to investigate the surfaces, interfaces, thin films, buried layers and nano-structures. XPEEM image signals are contributed from photoelectrons, Auger electrons, backscattering electrons and secondary electrons excited by X-rays of certain energy. In this work, a Monte Carlo model for XPEEM image simulation is built. XPEEM images in total electron yield (TEY) mode are simulated with this model for several nanostructures (spheres and cylinders) of silver deposited on/in a silicon substrate (Ag/Si). The simulated XPEEM image in TEY mode for Ag dot array on a Si substrate is quite close to an experimental image. Furthermore, the XPEEM image simulation has been performed for different incident X-rays and for different geometric structures.

Growth of nanosize Ag dots with uniform height on a Si(111)-7×7-C2H5OH surface, and their electronic properties

C2H5OH adsorbs by dissociating on Si-adatom/Si-rest atom pair sites on Si(111)-7×7 surfaces. A half of six Si adatoms and three Si rest atoms are changed to Si-OC2H5 and Si-H in every half unit cell at the saturation. When an Ag atom was deposited on this surface, it was stabilized on an intact Si adatom remained in the half unit cell and it did not migrate by hopping. With the increasing number of deposited atoms, uniform height with ca. 5-nm size Ag dots were grown in wide area. A similar growth mode was observed by depositing Ga and Zn on this surface. We deduced that the uniform height growth of 5-nm dots may be given by a layer-by-layer growth of dots in the natural templates composed of six half unit cells. Scanning tunneling spectroscopy indicated that one-monolayer Ag dots had nonmetallic energy gap of ca. 2.2 V at the Fermi level, but the energy gap became narrower with the increasing number of layers and became metallic at eight or nine layers.

Nano-Size Surface Materials Stabilized by Weak Interaction

Scanning tunneling microscopy (STM) proved the existence of quasi-compounds on solid surfaces. A typical example is (-Ag-O-) or (-Cu-O-) chains grown on Ag(110) or Cu(110) surface by exposing to O2. The (-Ag-O-) chains on a Ag(110) reacts with Cu atoms to form a new quasi-compound of (-Cu-O-) chains on the Ag(110) surface. The (-Cu-O-) on the Ag(110) readily decomposes at ca. 570ºK to form Cu6 dots, and a reversible reaction of (Cu2)3 + O2. ↔ (-Cu-O-) takes place by exposing to O2. Deposited Zn, Sn and Ag atoms on a Si(111)-7x7 surface stabilize by forming Zn3, Sn2 and Sn, and Ag in a half unit cell. Layer-by-layer growth of Zn3 clusters occurs in a half unit cell, which results in the growth of a semi-conductive honeycomb layer of Zn3 clusters on the Si(111)-7x7 surface. By prohibiting hopping migration of Ag atoms on the Si(111)-7x7 surface by the adsorption of C2H5OH, nano-size Ag dots grow layer-by-layer in a limited mold spacing. The band gap of Ag-dots becomes narrower and narrower and becomes metallic at higher than 6 layers.

Ag dots array fabricated using laser interference technique for biosensing

Abstract An interference lithography technique was adopted to fabricate Ag dots array. It has advantages of mask free, cost effective, and good controllability compared to the conventional nanosphere lithography (NSL) method. One-dimensional (1D) grating structures, two-dimensional (2D) grid patterns, and 2D dots array were obtained on the photoresist film by means of direct exposing the resist using the interference patterns. Then the photoresist patterns were transferred to the quartz substrate by wet etching and reactive ion etching. Atomic force microscope (AFM) was employed for geometrical characterization of the patterns generated in the steps from pattern generation to pattern transfer, respectively. The Ag dots array was formed by depositing an Ag thin film of ∼25 nm in thickness on the quartz. Our experimental results showed that wavelength shifts in the extinction spectra is acceptable for the applications of localized surface plasmon resonance (LSPR)-based immunoassay and binding detection.

Analysis of Potential Radiosensitizing Materials for X-Ray-Induced Photodynamic Therapy

For a development of deep tumor treatment in photodynamic therapy, a feasibility of novel radiosensitizers induced by x-ray was investigated. The sensitizers are designed to generate reactive oxygen species (ROS) inside or outside the cell, possibly leading to damage exclusively on tumor cells and reservation of normal cells along the x-ray path. Taking note of the similarity in energy transfer mechanism in photocatalysts, scintillators, and particulate semiconductors, we chose TiO2, ZnS:Ag, CeF3, and quantum dots (CdTe and CdSe) in particulate form, which contain heavy atoms for efficient absorption of x-rays. A parameter study for x-ray operating conditions showed that in a typical scenario, photons with 20 to 170 keV energy are attenuated by 90% through the region of particle dispersed aqueous solution at varying concentration between 0.01 and 100 wt%. The amount of ROS generation under the exposure of polychromatic x-ray was measured using dihydroethidium reagent which detects an integrated amount of several species. Proportional increase in ROS generation to x-ray dose was observed for varying concentrations of TiO2, ZnS:Ag, CeF3, and CdSe quantum dot dispersions. Then, HeLa cells were mixed with aqueous solutions dispersed with sensitizing materials at a concentration of 3.0 mg/ml and were exposed to x-ray. Their survival fraction obtained by a cell proliferation reagent WST-1 immediately after the irradiation showed insignificant effects of sensitizing materials except at large doses. To enhance the sensitization effect, bio-conjugated CdSe quantum dots were internalized in the cytoplasm up to a concentration of 1.0 ng/ml. The cells were irradiated by x-ray up to 5 Gy, and their survival fraction was measured by the colony forming ability 9 days after irradiation. Survival fraction of the cells treated with quantum dots were less than those without quantum dots for all doses, suggesting that the colony forming ability is impaired by the internalized quantum dots.

Growth of nano-crystalline metal dots on the Si(1 1 1)-7 × 7 surface saturated with C 2H 5OH

Metal atom on the Si(1 1 1)-7 × 7 surface undergoes migration by hopping among Si-adatom and Si-rest atom. If the hopping migration is prohibited, how change the deposited metals? In this paper, we studied the deposition of metals on the Si(1 1 1)-7 × 7 surface saturated with C 2H 5OH, on which the whole Si-rest atoms are changed to Si–H so that the hoping migration of metals will be prohibited. We found the growth of ca. 5 nm of crystalline dots by the deposition of Sn, Zn and Ag. Interestingly, Ag dots undergo layer-by-layer growth so that the surface is covered with 5 nm size dots with uniform height. When the hopping migration is prohibited, growth of dots is controlled by the kinetics of precursor state atoms instead of the lattice energy relating to lattice matching or strain.

Control of near-infrared optical response of metal nano-structured film on glass substrate for intense Raman scattering

Near-infrared SERS activity of the Ag film under electric polarization was evaluated in aqueous solution containing 1 mM glutamic acid. Spectra were obtained in situ from the near infrared laser Raman microscope system with an excitation wavelength of 785 nm. Intensity of the SERS increased significantly upon application of an external electric field to the film. Empirical signal enhancement factor, which was determined from the peak integration ratio of the SERS vibration to the unenhanced signal from the solution of a defined sample concentration, was estimated to be in the range between 10(5) and 10(9). The evolution of the scattering signal was not observed in the absence of an applied external field. Under the present conditions, the SERS intensity was fully controlled by the applied field and the time. Relatively strong enhancement observed at the present system could be attributable to closed-packed particulate structure characterized by the diameters of approximately 20-90 nm on the Ag film. Raman images prove that the scattering signals are highly localized at the specific sites on the films showing possible achievement of relatively larger enhancement more than 10(12). Importance of the control of the size and inter-particle distance for intense Raman scattering was proved by the preparation of the well-ordered chained Ag dot array showing stronger SERS signals than those at the Ag films.

Transient charging of copper phthalocyanine: model and experiment

Thin films of copper phthalocyanine (CuPc) were deposited on indium tin oxide (ITO) substrates in ultra-high vacuum. Without breaking the vacuum, Ag dots were evaporated through a shadow mask. The present contribution is aimed at comparing experimental and simulated data on feedback-charge capacitance/voltage ( C/ V) and isothermal charge deep-level transient spectroscopy (Q-DLTS) of Ag/CuPc/ITO devices kept at ambient temperature. Two types of charge relaxation of different origins in response to a bias step (pulse) Δ U were resolved: (a) dielectric relaxation within the bulk of the CuPc semi-insulating layer; (b) emission from traps of charges injected and captured at the traps during the pulse. The dielectric relaxation is characterized by a well-defined time constant τ D ≈10 −4 s and is observable for both polarities of Δ U. The onset bias for observing the emission from traps corresponds roughly to one of the forward current. A positive Δ U is to be applied to ITO (hole injecting anode) in order to fill at least a fraction of the traps. Realizing that the traps are spatially distributed, the model previously applied to C/ V of a Si:H based devices has been updated to obtain at least a qualitative picture about the spatial distribution of the density of traps N T ( x) in CuPc. N T ( x) seems to rise when approaching the CuPc/ITO interface. These states are classified as acceptor-like ones, a situation analogous to that reported earlier for poly-( p-phenylene-vinylene). Protonation of the CuPc/ITO interface is discussed as the source of the acceptor-like states.