In Situ Deformation and Breakage of Silica Particles Inside a SEM

AbstractMechanical properties and particle breakage behavior in the submicron size range are of fundamental importance for many particle related processes and applications. Although many (in situ) studies have been dedicated to materials’ size dependent mechanical characterization, particles as free standing structures have been omitted widely. An important, yet open question is the structure property relationship at small scales. Within this account, the application of a custom built manipulator for particle compression inside a scanning electron microscope (SEM) is presented: Stöber-Fink-Bohn (SFB) particles with mean diameters of 500nm and 1000nm are subjected to heat treatments and their mechanical properties are directly correlated to the internal structure. The as-synthesized SFB particles exhibit a complex and size dependent internal structure. Mechanical properties undermatching the values of fused silica are found and only plastic cracking at large strains is observed: cracks are formed at the surface and propagate in radial direction towards the particle center. Heat treatment leads to densification. The degree of changes is controlled by temperature and treatment time. Starting from initially low values, Young's modulus and hardness are increasing with treatment temperature. Properties of fused silica are approached or even exceeded after a treatment at 1000°C. A significant level of plasticity and high sustained deformations are still found. Whereas small particle show ductile cracking, the heat treated micron sized particles show a brittle behavior. A brittle to ductile transition in the size range of 500 nm to 1000 nm is thus identified

Facile colloidal coating of polystyrene nanospheres with tunable gold dendritic patches

Patchy particles comprise regions of differing material or chemical functionality on otherwise isotropic cores. To meet the great potential of these anisotropic structures in a wide range of application fields, completely new approaches are sought for the scalable and tunable production of patchy particles, particularly those with nanoscale dimensions. In this paper the synthesis of patchy particles via a simple colloidal route is investigated. Using surfactant-free cationic polystyrene nanospheres as core particles, gold patches are produced through the in situ reduction of chloroauric acid with ascorbic acid. The fact that such nanostructured metal patches can be heterogeneously nucleated on polymer nanospheres is related to the electrostatic interaction between core and metal precursor. Furthermore, the lateral expansion of the gold patches over the polystyrene surface is facilitated by an excess of ascorbic acid. The morphology of the patches is highly dendritic and process-induced variations in the structure are related to gold surface mobility using Monte Carlo simulations based on the diffusion limited aggregation principle. Considering the pH dependent behaviour of ascorbic acid it is possible to predict the moiety which most likely adsorbs to the polymer surface and promotes gold surface diffusion. This enables the judicious adjustment of the pH to also obtain non-dendritic patches. On account of the plasmonic behaviour of gold, the patchy particles have morphology-dependent optical properties. The systematic development of the synthetic approach described here is expected to lay a foundation for the development of functional materials based on the self- or directed-assembly of nanoscale building blocks with anisotropic interactions and properties

Synthesis of goethite α-FeOOH particles by air oxidation of ferrous hydroxide Fe(OH)2 suspensions: Insight on the formation mechanism

Iron oxide and iron oxyhydroxide particles, particularly, the goethite α-FeOOH phase, are environmentally friendly materials and are used in various technological applications as adsorbents, precursors of Fe powders for magnetic recording media, and pigments. In this work, the formation process of α-FeOOH by air oxidation of Fe(OH)2 suspensions has been studied. The effects of the air flow rate, as well as of the reactant concentration ratio, R (=[(OH)-]/[Fe(II)]), on the reaction product were analyzed. It has been found that the morphology and the size of the α-FeOOH particles can be modified by means of the air flow rate. Furthermore, by performing a detailed microscopic analysis of the morphology of the initial, intermediate, and final reaction products, we have obtained evidence of epitaxial growth of α-FeOOH on the Fe(OH)2 substrate. It is suggested that the similarity between the anion arrangements in both phases facilitates this process. Based on these results, a pathway for the formation of α-FeOOH in highly alkaline medium is proposed in which the size and shape of the initial Fe(OH)2 particles plays a significant role in the formation the α-FeOOH particles obtained upon completion of the oxidation process.Fil: Encina, Ezequiel Roberto. Universitat Erlangen-Nuremberg; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; ArgentinaFil: Distaso, Monica. Universitat Erlangen-Nuremberg; AlemaniaFil: Klupp Taylor, Robin N.. Universitat Erlangen-Nuremberg; AlemaniaFil: Peukert, Wolfgang. Universitat Erlangen-Nuremberg; Alemani

Synthesis of Goethite α‑FeOOH Particles by Air Oxidation of Ferrous Hydroxide Fe(OH)₂ Suspensions: Insight on the Formation Mechanism

Iron oxide and iron oxyhydroxide particles, particularly, the goethite α-FeOOH phase, are environmentally friendly materials and are used in various technological applications as adsorbents, precursors of Fe powders for magnetic recording media, and pigments. In this work, the formation process of α-FeOOH by air oxidation of Fe­(OH)₂ suspensions has been studied. The effects of the air flow rate, as well as of the reactant concentration ratio, <i>R</i> (=[(OH)⁻]/[Fe­(II)]), on the reaction product were analyzed. It has been found that the morphology and the size of the α-FeOOH particles can be modified by means of the air flow rate. Furthermore, by performing a detailed microscopic analysis of the morphology of the initial, intermediate, and final reaction products, we have obtained evidence of epitaxial growth of α-FeOOH on the Fe­(OH)₂ substrate. It is suggested that the similarity between the anion arrangements in both phases facilitates this process. Based on these results, a pathway for the formation of α-FeOOH in highly alkaline medium is proposed in which the size and shape of the initial Fe­(OH)₂ particles plays a significant role in the formation the α-FeOOH particles obtained upon completion of the oxidation process

Shedding Light on the Growth of Gold Nanoshells

Nanostructured particles containing noble metals can have highly tunable localized surface plasmon resonances and are therefore of particular interest for numerous applications. Nanoshells comprising a dielectric core and gold or silver shell are a widely researched systems because of the strong dependence of their optical properties on the ratio of core diameter to shell thickness. Although seeded-growth procedures have been developed to produce these particles, the many reported studies show significant variation in the nanoshell morphologies and hence optical properties. In order to establish processes that reproducibly synthesize nanoshells with high optical quality, it is necessary to develop techniques that monitor changes at the core particle surface during shell growth. For that purpose, we have carried out <i>in situ</i> nonlinear second-harmonic scattering (SHS) and linear vis–NIR extinction spectroscopy simultaneously during the seeded growth of gold nanoshells on silica core particles. Our SHS measurements show a striking variation in the nonlinear optical properties of the growing gold nanoshells. In comparison with linear optical measurements and with scanning electron microscopy (SEM) images made of gold nanoshells produced with varying shell completenesses, the SHS signal was observed to reach a peak intensity at a stage prior to shell closure. We attribute this high sensitivity of the SHS signal to the incomplete nanoshell surface morphology to the generation and subsequent degeneration of regions of electric field enhancement at gaps between isolated gold islands, which grow and coalesce. This conclusion is corroborated by finite-difference time-domain simulations of incomplete nanoshells. We suggest that the <i>in situ</i> analytical approach demonstrated here offers significant promise for future activities regarding the in-process optimization of the morphology and optical properties of metal nanoshells and other nanostructured plasmonic particles

Quantitative evaluation of electrophoretic deposition kinetics of graphene oxide

\u3cp\u3eThe electrophoretic deposition (EPD) technique is an attractive approach for development of graphene and graphene oxide (GO) films for a variety of applications. However, in order to establish the influence of the EPD parameters on the properties of the deposited films, a deeper investigation of the fundamental GO-EPD kinetics is required. Previous studies have reported a simultaneous anodic reduction of GO flakes during EPD, complicating the kinetics and process control. Therefore, in this study, low voltages were used to prevent significant GO reduction during EPD, as confirmed by XPS and FTIR. Accordingly, the GO-EPD kinetics was established as a function of deposition time and voltage, accompanied by microscopic characterization of the deposited films. The experimental results show that the deposition follows a linear growth law, in good agreement with the predictions of Hamaker's law. Comparisons of optical absorbance and profilometry provide estimates of (reduced) GO deposition rate, extinction coefficient, and density.\u3c/p\u3

Silver-Assisted Colloidal Synthesis of Stable, Plasmon Resonant Gold Patches on Silica Nanospheres

Patchy particles possessing heterogeneous surface composition show great promise as self-organizing building blocks for new classes of hierarchical functional structures. A major hurdle is the scalable synthesis of stable patches on nanosized core particles with arbitrarily defined patch number and coverage. So far, few methods have been reported which could be expected to meet these challenges. Recently, we described the heterogeneous nucleation and growth of silver patches on silica nanospheres via a template free colloidal route. The patches produced, although tunable in size and number and showing interesting plasmon resonant properties, were rather unstable and degraded rapidly during attempts to process them further. In the present work, therefore, we set out to explore if related approaches can be employed to produce patchy particles involving gold, which is known to be more stable. The differences between typical patch precursors Ag⁺ and [AuCl_<i>x</i>(OH)_4–<i>x</i>]⁻ and their respective interactions with amorphous silica make this a significant challenge. We show that preformed small silver patches in addition to the presence of a reducing agent are necessary for the formation of gold patches conformal to the silica nanosphere surface. Systematic study of the process parameters and their influence on the patchy particle morphology as well as in-depth analytical transmission electron microscopy investigation of the patch composition reveal that patches spread over the silica surface via a cycle of galvanic dissolution and redeposition of silver. The resulting gold patchy particles remain stable during subsequent storage or washing and display tunable plasmon resonances within the visible and near-IR spectrum

Determination of the Quantum Dot Band Gap Dependence on Particle Size from Optical Absorbance and Transmission Electron Microscopy Measurements

This work addresses the determination of arbitrarily shaped particle size distributions (PSDs) from PbS and PbSe quantum dot (QD) optical absorbance spectra in order to arrive at a relationship between band gap energy and particle size over a large size range. Using a modified algorithm which was previously developed for ZnO, we take only bulk absorption data from the literature and match the PSDs derived from QD absorbance spectra with those from transmission electron microscopical (TEM) image analysis in order to arrive at the functional dependence of the band gap on particle size. Additional samples sized solely from their absorbance spectra with our algorithm show excellent agreement with TEM results. We investigate the influence of parameters of the TEM image analysis such as threshold value on the final result. The band gap <i>versus</i> size relationship developed from analysis of just two samples lies well within the bounds of a number of published data sets. We believe that our methodology provides an attractive shortcut for the study of various novel quantum-confined direct band gap semiconductor systems as it permits the band gap energies of a broad size range of QDs to be probed with relatively few synthetic experiments and without quantum mechanical simulations

Determination of the Quantum Dot Band Gap Dependence on Particle Size from Optical Absorbance and Transmission Electron Microscopy Measurements

This work addresses the determination of arbitrarily shaped particle size distributions (PSDs) from PbS and PbSe quantum dot (QD) optical absorbance spectra in order to arrive at a relationship between band gap energy and particle size over a large size range. Using a modified algorithm which was previously developed for ZnO, we take only bulk absorption data from the literature and match the PSDs derived from QD absorbance spectra with those from transmission electron microscopical (TEM) image analysis in order to arrive at the functional dependence of the band gap on particle size. Additional samples sized solely from their absorbance spectra with our algorithm show excellent agreement with TEM results. We investigate the influence of parameters of the TEM image analysis such as threshold value on the final result. The band gap <i>versus</i> size relationship developed from analysis of just two samples lies well within the bounds of a number of published data sets. We believe that our methodology provides an attractive shortcut for the study of various novel quantum-confined direct band gap semiconductor systems as it permits the band gap energies of a broad size range of QDs to be probed with relatively few synthetic experiments and without quantum mechanical simulations

On-Demand Coupling of Electrically Generated Excitons with Surface Plasmons via Voltage-Controlled Emission Zone Position

The ability to confine and manipulate light below the diffraction limit is a major goal of future multifunctional optoelectronic/plasmonic systems. Here, we demonstrate the design and realization of a tunable and localized electrical source of excitons coupled to surface plasmons based on a polymer light-emitting field-effect transistor (LEFET). Gold nanorods that are integrated into the channel support localized surface plasmons and serve as nanoantennas for enhanced electroluminescence. By precise spatial control of the near-infrared emission zone in the LEFET via the applied voltages the near-field coupling between electrically generated excitons and the nanorods can be turned on or off as visualized by a change of electroluminescence intensity. Numerical calculations and spectroscopic measurements corroborate significant local electroluminescence enhancement due to the high local density of photonic states in the vicinity of the gold nanorods. Importantly, the integration of plasmonic nanostructures hardly influences the electrical performance of the LEFETs, thus, highlighting their mutual compatibility in novel active plasmonic devices