Ag Colloids Research Articles

N-type single-crystalline Si solar cells: Front side metallization for solar cells reaching 20% efficiency

Screen printed front side contacts were investigated in n-type, high efficiency, single-crystalline Si solar cells with B doped emitters. Cells were processed identically and two different front side pastes were used: paste Ag–Al yielded efficiencies of 20.0%, and paste Ag, without Al, yielded 16.1% efficiency only. Cells contacted with paste Ag–Al yielded a continuous glass layer, a significantly higher density of Ag colloids, and an Al mole fraction of ∼1at%. In contrast, cells contacted with paste Ag revealed a thinner but continuous glass layer as compared to cells processed with paste Ag–Al. It contained a lower density of Ag colloids and Al was below the detection limit of the EDX microanalysis. In n-type cells pyramidal shaped Ag nanocrystals did not appear, whereas for p-type cells they are formed at intersections of differently {111} oriented surface planes at the Si emitter. We did not find Ag/Al spikes at the Si emitter for cells contacted with paste Ag–Al.Cells contacted with paste Ag–Al had a smaller series resistance as well as a significantly lower specific contact resistance (4mΩcm2). A higher series resistance and specific contact resistance >100mΩcm2 was found for cells contacted with paste Ag. It was concluded that the specific contact resistance decreases with increased wetting and increased density of Ag colloids in the glass layer. The series resistance of the Ag–Al alloy contact depended only weakly on temperature, whereas the series resistance linearly increased with temperature for contacts processed with paste Ag.p- and n-type cells with maximum efficiency yield a similar microstructure of the front side contact, similar series and contact resistance, respectively. A high density of Ag colloids in the glass layer was found for cells with low specific contact resistance. This suggests that also for n-type cells a three dimensional percolative charge transport through the glass layer is assisted by the presence of metallic Ag colloids.

Uniform Plasmonic Response of Colloidal Ag Patchy Particles Prepared by Swinging Oblique Angle Deposition.

The plasmonic property of Ag patchy particles fabricated using a colloid monolayer and oblique angle deposition shows significant variations due to the multidomain nature of the monolayer. A swinging oblique angle deposition method is proposed to create uniform patchy particles. Both numerical calculations and experiment show that when the swinging angle is larger than 90°, the resulting plasmonic patchy particles have similar morphology and demonstrate uniform optical response that does not depend on the monolayer domain orientation. These uniform patchy plasmonic particles have great potential for plasmonic-based applications.

Morphological transformations of silver nanoparticles in seedless photochemical synthesis

Photochemical synthesis is an easily controlled and reliable method for the fabrication of silver (Ag) nanoparticles with various morphologies. In this work, we have systematically investigated the seedless photochemical synthesis of anisotropic Ag nanoparticles with and without PVP as surface capping agent. The time evolution of anisotropic Ag nanoparticles during the synthesis process are studied using UV–visible spectra, optical images and transmission electron microscopy. The results show that the light irradiation precisely controls the start and termination of the reaction, and the presence or absence of PVP greatly affects the morphology evolution of anisotropic Ag nanoparticles. With PVP as the surface capping agent, Ag nanoparticles grow into decahedra or prism by the deposition of Ag atoms on {111} or {110} facets through epitaxial growth. However, a different morphology evolution could happen when Ag nanoparticle is synthesized without PVP as surface capping agent. In this case, Ag nanoparticles can fuse into the decahedrons through an edge-selective particle fusion mechanism, which involves attachment, rotation and realignment of Ag nanoparticles. This process was evidenced with HRTEM images at the different stages of the transformation from Ag colloid to decahedra nanoparticles. Oriented attachment and Ostwald ripening also play important role in the transformation process.

Interaction of a converging laser beam with a Ag colloidal solution during the ablation of a Ag target in water

We studied the nanosecond laser-induced shape modifications of Ag colloids exposed to a converging laser beam during the ablation of a Ag target in water. To this end, we performed a series of laser ablation experiments in which the laser energy was varied while all other parameters were kept constant. In addition to transmission electron microscopy (TEM), the shape distribution of the Ag nanoparticles was determined by modelling the extinction spectra of the final colloidal solutions using theoretical calculations based on shape distributed effective medium theory (SDEMT). From these calculations, two physical parameters named sphericity and dispersity were introduced and used to gauge the evolution of the shape distribution of the particles. As the laser energy on the target was increased from 5 to 20 mJ/pulse, an apparently abrupt modification of the shape distribution of the particles was evidenced by both TEM and SDEMT calculations. This change is explained in terms of competitive fragmentation, growth and reshaping processes. On the basis the heating–melting–vaporization model, we demonstrate how the competition between these processes, occurring at different locations of the converging beam, determines the shape distribution of the final product. We highlight the relevance of the fluence gradient along the beam path and the laser interaction volume on the laser-induced modifications of the suspended particles during the ablation process.

One pot preparation of silver nanoparticles decorated TiO2 mesoporous microspheres with enhanced antibacterial activity

We report a simple “one-pot” solvothermal preparation of silver nanoparticles (Ag NPs) decorated mesoporous titania (TiO2) microspheres as an effective antibacterial agent. TBOT as Ti source was hydrolyzed and crystallized in media composed of acetic acid and ethanol, in which esterification catalyzed by TBOT occurred for in-situ “controlled water release”. AgNO3 as Ag source was reduced by ethanol to form Ag NPs embedded in the TiO2 microspheres. The effect of AgNO3 and HAc on the morphology of Ag/TiO2 was investigated. The Ag/TiO2 with various Ag content showed excellent antibacterial activities with extremely low minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) against Escherichia coli and Staphylococcus aureus when compared with colloidal Ag NPs.

Front Side Metallization of n- and p-Type, High-Efficiency, Single-Crystalline Si Solar Cells: Assessing the Temperature-Dependent Series Resistance

The series resistance of high-quality, single crystalline p-type and n-type solar cells was measured in a temperature range between 80 K and room temperature. Among one cell type (n or p), cells were processed identically. Only the processing of the front side metallization was varied by using different processing conditions and screen printing pastes. High-efficiency n- (η = 20.0%) and p-type (η = 18.0%) cells yielded similar contact and series resistance and common features of the microstructure of the front side contact, i.e. a glass layer containing Ag colloids with typical diameters of 5–200 nm. Temperature-dependent current voltage curves (I–V curves) were acquired and evaluated with respect to the series resistance by using two different methods yielding different results. On average the series resistance follows the trends of the contact resistance of the front side metallization determined at room temperature. Optimally processed cells yielded series resistances of less than 25 mΩ cm2 (method #1) both for n- and p-type cells. It could be shown that the series resistance reflected the processing conditions and paste properties and yielded similar temperature dependence for p- and n-type cells with small contact resistance. Therefore, the relevant current paths of high-efficiency n- and p-type cells appear to be similar in the front side metallization and include the glass layer which contains a high density of Ag colloids.

SERS spectrum of gallic acid obtained from a modified silver colloid

Two different crystals of the gallic acid were microscopically separated from a p.a. commercial product. The Raman spectra analysis allowed distinguishing monomeric and dimeric structures. The vibrational wave numbers were computed using DFT quantum chemical calculations. The data obtained from wave number calculations are used to assign vibrational bands obtained in the Raman spectrum. The dimer, characterized as ellagic acid, involves the carboxyl and hydroxyl moieties. The Raman spectrum in water solution of each species is dominated by the monomeric form. A low negatively charged Ag colloid allowed obtain to the best of our knowledge, the first surface enhanced Raman scattering (SERS) spectrum of the gallic acid. The possible electrophilic attacking sites of the title molecule are identified using MEP surface plot study and the orientation of the analyte on the metal surface is proposed tilted to the surface.

Antibacterial activity of silver nanoparticles obtained by pulsed laser ablation in pure water and in chloride solution.

Silver nanoparticles (AgNPs) have increasingly gained importance as antibacterial agents with applications in several fields due to their strong, broad-range antimicrobial properties. AgNP synthesis by pulsed laser ablation in liquid (PLAL) permits the preparation of stable Ag colloids in pure solvents without capping or stabilizing agents, producing AgNPs more suitable for biomedical applications than those prepared with common, wet chemical preparation techniques. To date, only a few investigations into the antimicrobial effect of AgNPs produced by PLAL have been performed. These have mainly been performed by ablation in water with nanosecond pulse widths. We previously observed a strong surface-enhanced Raman scattering (SERS) signal from such AgNPs by “activating” the NP surface by the addition of a small quantity of LiCl to the colloid. Such surface effects could also influence the antimicrobial activity of the NPs. Their activity, on the other hand, could also be affected by other parameters linked to the ablation conditions, such as the pulse width. The antibacterial activity of AgNPs was evaluated for NPs obtained either by nanosecond (ns) or picosecond (ps) PLAL using a 1064 nm ablation wavelength, in pure water or in LiCl aqueous solution, with Escherichia coli and Bacillus subtilis as references for Gram-negative and Gram-positive bacteria, respectively. In all cases, AgNPs with an average diameter less than 10 nm were obtained, which has been shown in previous works to be the most effective size for bactericidal activity. The measured zeta-potential values were very negative, indicating excellent long-term colloidal stability. Antibacterial activity was observed against both microorganisms for the four AgNP formulations, but the ps-ablated nanoparticles were shown to more effectively inhibit the growth of both microorganisms. Moreover, LiCl modified AgNPs were the most effective, showing minimum inhibitory concentration (MIC) values in a restricted range of 1.0–3.7 µg/mL. An explanation is proposed for this result based on the increased surface reactivity of the metal surface due to the presence of positively charged active sites.

The characteristics of novel bimodal Ag–TiO2 nanoparticles generated by hybrid laser-ultrasonic technique

Silver–titania (Ag–TiO2) nanoparticles with smaller Ag nanoparticles attached to larger TiO2 nanoparticles were generated by hybrid ultrasonic vibration and picosecond laser ablation of Ag and Ti bulk targets in deionised water, for the first time. The laser has a wavelength of 1064 nm and a pulse duration of 10 ps. It was observed that without the ultrasonic vibration, Ag and TiO2 nanoparticles did not combine, thus the role of ultrasonic vibration is essential. In addition, colloidal TiO2 and Ag nanoparticles were generated separately for comparison under the same laser beam characteristics and process conditions. The absorption spectra of colloidal Ag–TiO2 cluster nanoparticles were examined by UV–Vis spectroscopy, and size distribution was characterised using transmission electron microscopy. The morphology and composition of Ag–TiO2 nanoparticles were examined using scanning transmission electron microscopy in high-angle annular dark field, and energy-dispersive X-ray spectroscopy. The crystalline structures were investigated by X-ray diffraction. The size of larger TiO2 particles was in the range 30–150 nm, and the smaller-sized Ag nanoparticles attached to the TiO2 was mainly in the range of 10–15 nm. The yield is more than 50 % with the remaining nanoparticles in the form of uncombined Ag and TiO2. The nanoparticles generated had strong antibacterial effects as tested against E. coli. A discussion is given on the role of ultrasonic vibration in the formation of Ag–TiO2 hybrid nanoparticles by picosecond laser ablation.

Sensitization of photoprocesses in colloidal Ag 2 S quantum dots by dye molecules

The effect of photosensitization of IR luminescence excitation (1205 nm) of colloidal Ag2S quantum dots (QDs) with average size of 2.5±0.6 nm in gelatin at 600 to 660 nm by molecules of 3,3’-di-(γ-sulfopropyl)-4,4’,5,5’-dibenzo-9-ethylthiacarbocyanine betaine pyridinium salt (Dye1) and thionine dye (Dye2) was registered. Cis-J-aggregates of Dye1 and cations monomer of Dye2 conjugated with Ag2S QDs take part in this process. The photosensitization of luminescence excitation of colloidal Ag2S QDs was interpreted by resonance nonradiation transfer of electronic excitation energy from cis-J-aggregates of Dye1 and cations of Dye2 to centers of recombination luminescence of Ag2S QDs.

Colorimetric Evaluation of the Viability of the Microalga Dunaliella Salina as a Test Tool for Nanomaterial Toxicity.

A diagnostic test system was developed to determine the toxicity of nanomaterials to the saltwater microalga Dunaliella salina through evaluation of cell death and changes in the culture growth rate at various toxicant concentrations, providing LC50 and other toxicological metrics. The viability of cells was shown to decrease with decreasing chlorophyll absorption of red light by damaged cells. This correlation was confirmed by independent fluorescence microscopic measurements of live and dead cells in the population. Two standard colorless pollutants, hydrogen peroxide and formaldehyde, were used to validate the colorimetric method. The method's performance is exemplified with three Ag-containing preparations (Ag nitrate, Ag proteinate, and 20-nm Ag nanoparticles) and with cetyltrimethylammonium bromide (CTAB) mixed with colloidal 15-nm Au and 20-nm Ag nanoparticles. The toxicity of the Ag-containing preparations to D. salina decreased in the order Ag nitrate ≥ Ag proteinate ≫ colloidal Ag. The toxicity of colloidal Au-CTAB mixtures was found to depend mostly on the content of free CTAB. The toxicity of colloidal Ag increased substantially in the presence of CTAB. The results suggest that our D. salina-based colorimetric test system can be used for simple and rapid preliminary screening of the toxicity of different nanomaterials.

Surface-enhanced Raman scattering based on colloidal Ag nanoparticles for quantitative detection of cetyltrimethyl ammonium bromide

Surface-enhanced Raman scattering based on colloidal Ag nanoparticles for quantitative detection of cetyltrimethyl ammonium bromide

Preparation of gold and gold–silver alloy nanoparticles for enhancement of plasmonic dye-sensitized solar cells performance

A two-step method was applied to obtain gold–silver alloy nanoparticles (Au–Ag alloy NPs). First, Au and Ag colloids with average particle sizes of 14.80 and 8.47nm, respectively, were synthesized by pulsed laser ablation of pure metallic targets immersed in ethanol, subsequent mixing, and re-irradiation of individual Au and Ag colloids (volume ratio of 4:1). In this study, a dense layer of porous titanium dioxide (TiO2) film with a thickness of ∼11μm was deposited directly on the central area of the conductive face of a fluorine tin oxide (FTO) glass by blade doctor method. A specific amount of N719 dye molecules with and without the plasmonic-metal nanoparticles (NPs) was mixed, incorporated into the porous TiO2 film, and used as photoanode to fabricate dye-sensitized solar cells (DSSCs). The optical and structural properties of the prepared photoanodes before their application to DSSCs were investigated. The photocurrent density–voltage and external quantum efficiency characteristics of the DSSCs fabricated with these photoanodes were also analyzed and discussed. The experimental results confirmed the plasmonic enhancement effects on the performance of DSSCs, which indicated that cells loaded with the plasmonic-metal NPs yielded larger enhancement. The plasmonic effect of the Au–Ag alloy NPs significantly enhanced the broadband light absorption of dye molecules. The best cell was achieved using the photoanode loaded with Au–Ag alloy NPs, exhibiting a power conversion efficiency of 5.81%, which was ∼8.4% and 52.1% higher than that of the DSSCs with Au NPs and the reference cell, respectively.

Uric acid detection by means of SERS spectroscopy on dried Ag colloidal drops

Uric acid concentration in human bodily fluids is an important marker for disorders such as gout, pre‐eclamsia or cardiovascular disease. However, currently used methods for its detection either lack sensitivity or require sophisticated, bulky and expensive equipment. In this work, we show that by using surface‐enhanced Raman scattering spectroscopy (SERS) on dried Ag colloidal drops and with 1064 nm excitation, concentrations of uric acid in aqueous solutions down to 10−6 M can be detected. Such sensitivity is sufficient for medical applications as concentration of uric acid in various bodily fluids are in the range of 10−3–10−4 M. Drying of the colloidal drops is known to result in the formation of ‘coffee‐ring’ structures that allow obtaining high enhancements but poor reproducibility. Here, the formation of the structures was avoided by choosing aluminum oxide as a base substrate and by controlling environment conditions. Despite the fact that variations of signal enhancement from sample to sample prevent quantitative analysis from being performed, the results of this work imply that strict control of sample preparation conditions could lead to obtaining reproducible SERS enhancements. Results of density functional theory calculations of uric acid tautomer – five‐atom silver cluster complexes performed for the first time show that the differences between Raman and SERS spectra of uric acid can be mainly explained by tautomerization of the molecule and its bonding to the silver surface. Assignment of spectral bands is important for correct SERS signal interpretation and detection of uric acid in biological fluids in the future studies. (Less)

High‐throughput screening of measuring conditions for an optimized SERS detection

Surface‐enhanced Raman spectroscopy (SERS) is a perfect method for analytical applications because of delivering specific molecular information combined with a high sensitivity when applying rational design metallic nanostructures. However, the pH level and the activation of ctheir mutual influence. Besidesolloidal SERS substrates influence the sensitivity and comparability of the SERS analysis. In this study, a routine is presented that allows for a fast and effective recognition of the influences of pH and activating agents on the SERS signal of an analyte molecule. To do so, a citrate‐reduced Ag colloid served as SERS substrate. The influence of four activating agents as well as five pH levels on the SERS signal of the model analyte melamine was investigated. Here, not only the influence of the anion but also that of the cation of the activating agents was examined. In addition to different signal intensities, a shift of the dominant melamine signal occurred depending on the adjusted conditions. At pH 11, the lowest intensities of the SERS signal from the model analyte melamine were detected. The maximal enhancement of the ring breathing II mode in combination with this specific colloid was achieved using sodium chloride as an activating agent at a pH level of 5. Thus, the study introduces a routine, which can be applied on various analyte molecules to optimize the SERS intensity by changing the activation agent as well as the pH. Copyright © 2015 John Wiley & Sons, Ltd.

Third-order nonlinear optical responses of colloidal Ag nanoparticles dispersed in BMIBF_4 ionic liquid

The third-order nonlinear optical responses of tetrafluoroborate 1-n-butyl-methylimidazolium (BMI.BF4) and colloidal Ag nanoparticles dispersed in this medium were investigated. Hybrid organic-metallic colloids consisting of silver nanoparticles dispersed in BMI.BF4 were synthesized. Using the thermally managed Z-scan technique, the electronic contribution to the nonlinear refractive indexes of the BMI.BF4 and colloids with two different nanoparticles filling factors were measured, as well as the media's nonlinear absorption. The performance of these systems for all-optical switching was also evaluated. The obtained results indicate that these are promising candidates for the development of nonlinear optical applications.

Rich Capping Ligand–Ag Colloid Interactions

Ag cubes, plates, and irregular particles were acquired by the reduction of AgNO3 respectively in poly(vinylpyrrolidone) (PVP) and ethylene glycol, PVP and dimethylformamide (DMF), and DMF. The surface adsorbates on various Ag colloids were studied with X-ray photoelectron spectroscopy, infrared spectroscopy, and Raman spectroscopy. PVP adsorbs on Ag cubes via its N and O atoms with charge transferring from PVP to Ag. DMF adsorbs on irregular Ag particles in two configurations with charge transferring from Ag to DMF. One adsorbs via the N atom, and the other adsorbs via the N and O atoms. PVP adsorbs on Ag plates via the N atoms with charge transferring from PVP to Ag, whereas DMF adsorbs on Ag plates via the N and O atoms with charge transferring from Ag to DMF. These results reveal rich capping ligand–Ag colloid interactions that will strongly affect the surface chemistry and surface-mediated properties of Ag colloids.

Contact formation of front side metallization in p-type, single crystalline Si solar cells: Microstructure, temperature dependent series resistance and percolation model

Screen printed front side contacts were investigated in single-crystalline (planar and textured) Si solar cells with n-type emitters, yielding maximum efficiencies of 18.0%. The crystallographic orientation of the Si surface and the paste strongly affect the contact formation as well as the contact resistance of the cells. For textured cells a continuous glass layer together with the formation of Ag colloids yielded a small contact resistance. Planar 〈111〉oriented Si yielded specifically lower contact resistance (〈5mΩcm2) as compared to planar 〈100〉 orientation (〉10–40mΩcm2) for different pastes. Pyramidal Ag crystals are formed only on 〈100〉 oriented Si, whereas lens shaped Ag crystals are grown on 〈111〉surfaces. From this it was concluded that the shape of the Ag nanocrystals determines the contact resistance, pyramidal Ag crystals formed on 〈100〉 planar surfaces yielded cells with large contact resistance and are, therefore, not considered to be necessary for a low contact resistance.Temperature dependent series resistance measurements yielded metallic behavior for cells with the lowest contact resistance bound to a certain paste. For other pastes and processing conditions a semiconducting behavior of the series resistance was found. However, cells with significant density of colloids in the glass layer yielded a small series and contact resistance. By considering the above arguments, a percolation model has been introduced in which metallic Ag colloids generate current filaments across the glass layer. This reduces the resistivity of the glass layer and thereby introduces a percolative nature of the current via Ag nanocolloids. The percolation limit for the 2d case was calculated for periodically arranged colloids with equal size and yields a minimum volume fraction of 15% for the Ag colloids in the glass layer.

Photobiological synthesis of noble metal nanoparticles using Hydrocotyle asiatica and application as catalyst for the photodegradation of cationic dyes

Solar light induced photo catalysis by plasmonic nanoparticles such as Au and Ag is an important field in green chemistry. In this study an environmental benign method was investigated for the rapid synthesis of colloidal Ag and AuNPs using the extract of Hydrocotyle asiatica, as a reducing and stabilizing agent under sunlight irradiation. The nanoparticles were formed in few seconds and were characterized by UV–Vis., FT-IR, TEM, EDAX, XRD, DLS and Zetasizer. The nanoparticles were stable in aqueous solution for more than 6 months. TEM analysis established that the Ag and AuNPs were predominantly spherical with average size of 21 and 8 nm, respectively. The flavonoids and glycosides from the extract of H. asiatica were proved to be responsible for the reduction and capping through FT-IR analysis. The antimicrobial studies of AgNPs showed effective inhibitory activity against the clinical strains of gram-negative and positive bacteria. The localized surface plasmon resonance of AgNPs was used for the photo-driven degradation of cationic dyes (malachite green and methylene blue). Thus, this green technique can be used for bulk production of AgNPs, and thus prepared nanoparticles may be used for removal of dyes from effluent.

Identification of Natural Dyes in Ancient Textiles by Time-of-Flight Secondary Ion Mass Spectrometry and Surface-Enhanced Raman Spectroscopy.

The identification of dyes in archaeological remains is a long standing challenge. Major problems include contamination by environmental conditions over long periods of time, small amounts and limited availability of excavated samples, and low concentrations of dyestuff in the obtained samples. To address these issues, highly sensitive and non-destructive techniques are required. In response, in this work, two non-destructive analytical techniques, Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) and Surface-Enhanced Raman Spectroscopy (SERS), were used for dye detection and the analysis results are compared. TOF-SIMS provides high detection efficiency for the analysis of organic materials whereas SERS is a useful technique for the detection of dyes in ancient textiles. An Ag colloid was employed to surmount the limitations of normal Raman measurement such as background fluorescence and weak Raman signals in small amounts of components. To identify the dyes used in ancient textiles, standard samples prepared using various dyestuffs and historical samples were analyzed with TOF-SIMS and Raman techniques. From the TOF-SIMS and the SERS spectra, dyestuffs such as alizarin, berberine, an indigo were identified in ancient textiles. The results suggest that TOF-SIMS and SERS are efficient non-destructive techniques for the characterization of archaeological textiles.