Dielectrophoretically Modulated Optical Spectroscopy of Colloidal Nanoparticle Solutions in Microfluidic Channels

Optical spectroscopic techniques (e.g., extinction, scattering, and fluorescence spectroscopies) are important for the analysis of colloidal solutions of nanoparticles (NPs). They are routinely applied to plasmonic and quantum-dot NP samples assuming that these contain a single population of particles with modest size and shape dispersity. However, these spectroscopic techniques become less effective when the sample is a mixture of particles with different sizes, shapes, or composition. Here, an original microfluidic method is proposed for the optical spectroscopic analysis of colloidal NP solutions that combines periodic trapping of NPs by dielectrophoresis (DEP) with in situ optical extinction spectroscopy. The periodic trapping leads to modulation of the continuously monitored optical spectrum depending on the DEP properties of the NPs. DEP-modulated spectroscopy is demonstrated using colloidal gold NPs as small as 40 nm diameter. It is found that the DEP modulation is significantly enhanced when employing suitable microfluidic flow over a multielectrode array. Finally, it is shown that the method can identify and characterize the NP species simultaneously present in a mixture of 40 and 80 nm gold NPs, opening the way toward optical spectroscopic analysis of higher complexity NP mixtures through the combination of the DEP-modulated spectroscopy with chemometric methods.     

Nanostructured Deposition of Nanoparticles from the Gas Phase

For many applications, nanoparticles from the gas phase are of interest due to their physical properties. Especially for electronic or optoelectronic applications, the transfer from their random distribution in the gas phase onto flat substrate surfaces has to be controlled because the particles are needed in exactly defined areas on the substrate. We demonstrate a parallel process for the transfer of charge patterns on oxidized silicon surfaces followed by the deposition of monodisperse singly charged nanoparticles, which allows the creation of particle arrangements reaching from 100 nm resolution up to structures in the upper micrometer range. The charge patterns are transferred using a polydimethylsiloxane (PDMS) stamp, which is covered with a metal layer. By applying different voltages to the stamp, negative or positive charges can be transferred. Thus, nanoparticles of different polarities can be guided to certain places.     

2D MoSe2 Structures Prepared by Atomic Layer Deposition

Here, we demonstrate the preparation of 2D MoSe₂ structures by the atomic layer deposition technique. In this work, we use ((CH₃)₃Si)₂Se as the Se precursor and Mo(CO)₆ or MoCl₅ as the Mo precursors. The X‐ray photoelectron spectroscopy (XPS) analyses of the prepared samples have revealed that using the MoCl₅ precursor the obtained structure of MoSe₂ is nearly identical to the reference powder MoSe₂ sample while the composition of the sample prepared from Mo(CO)₆ contains a significant amount of oxygen atoms. Further inspection of as‐deposited samples via scanning electron microscopy (SEM), X‐ray diffraction (XRD), and Raman spectroscopy has disclosed that the MoSe₂ structure based on MoCl₅ is formed from randomly oriented well crystalline flakes with their size ≈100 nm in contrast to the Mo–Se–O compact film originating from Mo(CO)₆.     

Prediction of Spherule Size in Gas Phase Nanoparticle Synthesis

Surprisingly, there is still no rational yet practical method to reliably predict absolute primary nanospherule sizes and, hence, specific surface areas, in gas phase flame nanoparticle synthesis. The present paper summarizes our approach to this important problem, using a plausible and tractable coagulation–coalescence (two-rate process) model, but with an important modification to the rate of nanoparticle coalescence. The Smoluchowski equation is used to describe the particle Brownian coagulation rate process (free-molecule regime), together with the assumption that the particle population follows a self-preserving size distribution. The decisive coalescence process, driven by the minimization of surface energy of the coalescing nanoparticles, is presumed to occur via the mechanism of surface diffusion. However, a curvature-dependent energy barrier for surface-diffusion is proposed, taking into account the extended surface-melting behavior of nanoparticles. This is shown here to have the effect of accelerating the coalescence rate of touching nanoparticles, leading to absolute sizes (at the predicted onset of aggregate formation) in encouraging agreement with available experiments. It was found that the coalescence rate, especially with a curvature-augmented surface diffusivity, is far more sensitive to particle size than is the Brownian coagulation rate. As a result, when cast in terms of characteristic process times, a distinct crossover generally exists, allowing the determination of observed primary spherule sizes within larger aggregates. This approach is successfully applied here to several published synthesis examples of vapor-derived nanosized alumina and titania. Its broader implications for nanoparticle synthesis in non-isothermal reactors, including our own counterflow diffusion flame reactor, are also briefly summarized.     

Metal Nanoparticle Coating of Oxide Nanospheres for Core‐Shell Structures

A low‐temperature route for coating oxide nanospheres with metal nanoparticles to achieve core‐shell structures is introduced. First results indicating a dense coverage of silica nanospheres of about 300 nm size with regularly arranged Ag and Au nanoparticles deposited by a modified incipient wetness impregnation procedure are presented. This synthesis works completely without external reducing agents or media, adhesive aids or functionalizing agents. Metal particles of only a few nanometers in size may serve as seeds for continuous metal coating of the oxide spheres by complementary processes. Structural characterization of the materials by transmission electron microscopy reveals a nearly spherical shape of the metal particles, the structure of which ranges from single crystalline to single twinned and multiply twinned configurations.     

基于钯/碳纳米粒子复合薄膜的双氧水电化学传感性能研究

本文采用团簇束流沉积方法制备了一种复合纳米粒子电化学催化剂,在碳纳米粒子支撑层上沉积钯纳米粒子薄膜,发现其在双氧水电化学传感中具有较高的灵敏度.碳纳米粒子的覆盖率对钯纳米粒子薄膜的双氧水电化学催化活性有明显的影响.当碳纳米粒子覆盖满一个单层的时候,钯/碳纳米粒子复合薄膜对双氧水的检测灵敏度达到了最高值,是没有碳纳米粒子支撑层时的两倍之多.     

Hematite/Polystyrene Raspberry-Like Microcomposites as Stable Support for Silver Nanoparticle Immobilization

Among the methods utilized for the preparation of raspberry-like microcomposites, due to its simplicity and universality, the electrostatically-driven deposition of nanoparticles at the surface of microparticles is especially attractive. This process, leading to the formation of a single nanoparticle monolayer, is widely reported. On the other hand, no data concerning the electrostatically-driven formation of nanoparticle bilayers at the surface of microparticles are reported. To fill this gap, the detailed investigation of the formation of silver/hematite nanoparticle bilayers at the surface of polystyrene microparticles is reported. First, the hematite/polystyrene raspberry-like microcomposites are obtained by immobilization of hematite nanoparticles under an electrostatically-driven process. The stability of hematite/polystyrene microcomposites at elevated temperatures up to 323 K is monitored using microelectrophoresis. No significant changes in the hematite nanoparticle layer coverage are observed after prolonged time of incubation. This proves the irreversible character of nanoparticle immobilization. Next, the hematite/polystyrene microcomposites are utilized as the interfaces for immobilization of negatively charged silver nanoparticles. This process is quantitatively described using electrokinetic measurements. The changes in the hydrodynamic diameter of microcomposites are also determined. Finally, the validity of the electrokinetic model used in this work for predicting the zeta potential of the silver/hematite/polystyrene microcomposites is confirmed.     

Review on Atomic Layer Deposition and Applications of Oxide Thin Films

Atomic layer deposition technique is able to grow conformal thin films over high aspect ratio structures. This article reviews the various aspects of oxides grown by this method including applications in photovoltaics and memristors. The main focus of this review is to concentrate on the oxides grown by atomic layer deposition and their growth mechanisms. The oxides deposited using atomic layer deposition are also likely to find application in memristor, an emerging field in the non volatile memories design with the ability to retain data and memory states even in power-off condition. The use of this technique to obtain oxides in surface modification of nanostructures gives the significance of these materials.     

Size and Interface Control of Novel Nanocrystalline Materials Using Pulsed Laser Deposition

We have developed a novel method based upon pulsed laser deposition to produce nanocrystalline materials with an accurate grain size and interface control. Using this method, the grain size in the case of Cu thin films was controlled by introducing a few monolayers of insoluble elements having high surface energy such as W, which increases interfacial energy and provides more nucleation sites. The grain size is determined by the thickness of Cu layer and the substrate temperature at which it transforms into islands (nanocrystalline grains) of fairly uniform size which we desgnate as self-assembling approach. Using this approach, the grain size was reduced from 160nm (Cu or Si (100) substrate) to 70–80nm for a simple W layer (Cu/W/Si (100)) to 4nm for a multilayer (Cu/W/Cu/W/Si (100)) thin film. The hardness of these films was evaluated using a nanoindentation technique, a significant increase in hardness from 2.0GPa for coarse-grained 180nm to 12.5GPa for 7nm films was observed. However, there is decrease in hardness below 7nm for copper nanocrystals. The increase in hardness with the decrease in grain size can be rationalized by Hall–Petch model. However, the decrease in slope and eventually the decrease in hardness below a certain grain size can be explained by a new model based upon grain-boundary deformation (sliding). We also used a similar materials processing approach to produce quantum dots in semiconductor heterostructures consisting of Ge and ZnO dots or nanocrystals in AlN or Al₂O₃ matrix. The latter composites exhibit novel optoelectronic properties with quantum confinement of phonons, electrons, holes and excitons. Similarly, we incorporated metal nanocrystals in ceramics to produce improved mechanical and optical properties.     

Polymer-like Behavior of Inorganic Nanoparticle Chain Aggregates

Studies of the behavior of nanoparticle chain aggregates (NCA) have shown properties similar to those of molecular polymers. Like polymer chains, NCA tend to gather up and become more compact when heated. Under tensile stress, folded chain segments pull out and the NCA elongates. When the tension is relaxed, the chains contract. The stretching of NCA may contribute to the ductility of compacts made from nanoparticles, a subject of current research interest. In a well established technological application, carbon black and pyrogenic silica NCA produce remarkable increases in elastic modulus and tensile strength when added to commercial rubber. This may be due to the mechanical interaction between the polymer chains and NCA. However, basic mechanisms of NCA elasticity differ from those of molecular polymers. The alignment of chain segments when the NCA are subjected to tension probably results from rotation and translation at grain boundaries between neighboring nanocrystals. The elastic properties depend on the van der Waals forces between segments of the chain that fold to minimize surface free energy. Under tension, these segments pull out, but tend to reform when the tension is relaxed. The processes that lead to NCA formation and control the strength of interparticle bonds are briefly reviewed.     

Metal Nanoparticle Photocatalysts: Synthesis,Characterization, and Application

Metal nanoparticles (NPs) have emerged as a kind of new photocatalyst to drive various chemical reactions by visible‐light irradiation. A distinct advantage of metal NP photocatalysts is that their light absorption is not limited to a certain wavelength but instead they are able to utilize a broad range of wavelengths, constituting a large fraction of the solar spectrum. Metal NPs like gold, silver, and copper NPs can strongly absorb visible light due to the localized surface plasmon resonance (LSPR) effect. Recent developments have shown that the light absorption properties strongly depend on the shape, size, and particle–particle interactions of NPs, which directly influence their photocatalytic activities. In this review, an overview of the preparation of metal NPs photocatalysts with various morphologies is given along with a brief discussion of the relationship between the morphology/composition and optical properties. The latest photocatalytic applications of these morphologies are also presented, and some of the challenges for the development of metal NPs photocatalysts are provided.     

Influence of Hydrothermal Temperature on Structures and Photovoltaic Properties of SnO2 Nanoparticles

Two SnO₂ nanoparticles were synthesized by hydrothermal method at 170°C and 180°C, respectively. Transmission electron microscope observations reveal that the diameters of both the nanoparticles are around 6nm. At the same time, surface photovoltage spectroscopy measurements show that the nanoparticle synthesized at 180°C has more surface electronic states at 0.3eV below the conduction band than the one synthesized at 170°C. This means that the temperatures chosen in hydrothermal synthesis have significant influence on the surface electronic characteristics of resultant SnO² nanoparticles but the effect on their sizes is not obvious. However, after being calcined at 500°C for 2h, the diameter of the nanoparticle synthesized at 180°C increased to 23nm and that of the nanoparticle synthesized at 170°C increased to 32nm as calculated from X-ray diffraction pattern.     

Stability Investigation of Colloidal FePt Nanoparticle Systems by Spectrophotometer Analysis

FePt magnetic nanoparticle systems are an excellent candidate for ultrahigh-density magnetic recording. Monodisperse FePt nanoparticles are synthesized by superhydride reduction of FECl2·4H2O and Pt （acac）2 at 263℃ under N2 atmosphere. Transmission electron microscopy （TEM） images show monosize EePt nanoparticles with diameter of 4 nm and a standard deviation of about 10%. The average distance between monodispesre particles is nearly 3 nm, and oleic acid and oleylamine surround the nanoparticles as surfactants. Stability investigation of nanoparticle colloidal solution is done via speetrophotometery analysis. The results for FePt nanoparticles dispersed in hexane indicate that adding surfactants with concentration of 3 × 10^-3 part by volume for centrifugation stage increases the stability of FePt nanoparticles solution with concentration of 16 mg/mL, about 67%.     

金属纳米颗粒LSPR光纤生物传感DDA方法研究

研究了金属纳米颗粒的局部表面等离子体共振(LSPR)行为,并讨论了其在光纤生物传感领域的应用.采用离散偶极近似(DDA)的方法,从理论上分析了金属纳米颗粒的尺寸、形状对其传感灵敏度的影响.计算结果显示,金属纳米颗粒的等离子共振吸收峰同时受到颗粒尺寸和形状的影响,但形状对其传感灵敏度的影响最为明显,计算结果与实验数据能较好地吻合.     

Ballistic Deposition模型孔洞生长标度行为的数值模拟

为研究表面界面粗化生长中孔洞的标度行为,在对Ballistic Deposition(BD)模型进行数值模拟的基础上,对模型中孔洞的模拟生长情况进行统计和分析.结果表明,BD模型中孔洞数目随模型生长时间的变化从初始阶段的高于线性而渐趋近于线性,并对该规律进行理论分析.     

Experimental Study of a Nanoparticle Virtual Impactor

A nanoparticle virtual impactor was constructed and its performance under different operating conditions was investigated. Experimental evaluations showed that the nanoparticle virtual impactor has a 50% cutoff size ranging from 15 to 60nm. Further, the cutoff size of 60nm can be achieved at an impactor chamber pressure of 220torr when the nozzle upstream pressure is 760torr. This pressure level is much higher than that of thin-plate orifice nozzle impactors, which require 12torr to achieve the cutoff size of 66nm. Thus, the proposed virtual impactor can be operated with a small vacuum pump, which is more preferable for practical applications.In this study, the effects of design parameters on the impactor performance have also been experimentally investigated. The parameters include the separation distance between the collection probe and the acceleration nozzle, the pressure ratio of the upstream and downstream chambers, the diameter ratio of the collection probe and the nozzle, the flow ratio of the minor and total flows, total mass flow rates and the upstream pressure. The experimental data obtained were then scaled with the Stokes number defined by previous researchers. The performance of the proposed nanoparticle virtual impactors can therefore be estimated when the operating variables are given or measured. An important finding in this parametric study is that the optimal diameter ratio of collection probe to nozzle is around 1.8. It is different from the value of 1.4 recommended in previous studies with virtual impactors for submicron particle applications.     

Chemical Structures of the SiO2Si Interface

As a result of considerable progress in microfabrication technology for ultra-large scale integration (ULSI), it has become necessary to control oxide formation on an atomic scale in order to produce defect-free SiO₂/Si interfaces. However, the possibility of forming an atomically flat interface by oxidizing an atomically flat silicon surface without introducing structural defects is not yet clarified. In this article the present understanding of chemical structures of SiO₂/Si interfaces and initial stage of oxidation of silicon surfaces are reviewed.