Self-assembly of charged CdTe nanoparticles

We show a method of the organization of charged CdTe nanoparticles which allows the generation of a self-assembled monolayer of above 10.000 μm² in a time of about 90 s. The analysis of adsorption kinetics of particles on a surface shows that it is well described by the Langmuir isotherm. We have found that thermal and electrical conductivity of a substrate play an important role. Nevertheless, deficiency of a substrate does not affect the adsorption kinetic. The structure of a formed monolayer essentially depends on pH and, as a consequence, on the charge of the particles. This method can be effective for the production of a CdTe nanoparticle monolayer with well controlled area and a degree of filling on a surface.     

Influence of substrate hardness transition on built-up of nanostructured WC-12Co by cold spraying

The ability of cold spray process to retain the feedstock microstructure into coating makes it possible to deposit nanostructured WC-Co coatings. In the present study, the deposition behavior of nanostructured WC-12Co coating was examined through the surface morphology and cross-sectional structure of the deposited single WC-12Co particle impacting on the substrates with different hardness using a nanostructured WC-12Co powder. Substrates included stainless steel, nickel-based self-fluxing alloy coatings with different hardness. It was observed from the top surface and cross-section of individual WC-12Co particles that the penetration leading to particle deposition depends on substrate hardness. When the substrate surface is covered by WC-12Co particles, the hardness of the newly formed substrate, i.e. the coating, increases greatly. The significant increase of the surface hardness leads to the rebounding off of impacting particles and erosion of the deposited particles, which prohibits effective built-up of coating. However, it was found that with spray jet fixed, a deposit with a thickness up to over 700 μm can be built-up. A model involving in substrate hardness transition during deposition is proposed to explain such phenomenon, which can be employed to optimize the conditions to build up a uniform nanostructured WC-12Co coating.     

Sensor and microelectronic elements based on nanoscale granular systems

Specific effects observed in small metal particles due to their distinction from bulk material as well as phenomena inherent to an ensemble of these particles coupled by electron tunneling have attracted considerable attention to nanosized systems. Electron and photon emission was revealed as a power was fed into the metal nanoparticle films deposited on insulating substrates either by passage of an electrical current in the film plane or by laser irradiation in the infrared and visible range. The electrical conductivity of metal nanoparticle films close to the percolation threshold is sensitive to temperature, substrate bending and adsorption of various gases. Besides, the current–voltage characteristics of the conduction current of a system consisting of a metal nanoparticle film and an adsorbate exhibit a voltage-controlled negative resistance region. These peculiar properties enable nanosized particle systems to be used for various applications. The present review deals with a variety of sensors for physical properties and microelectronics elements based on nanoparticle films. The mechanisms underlying the special properties are discussed. Some technological methods ensuring better parameter definition and long-term stability of sensors are also described.     

Laser ablation-based methods for nanostructuring of materials

An overview of laser-ablation-based methods for nanofabrication developed by our research group is presented. All methods imply laser-related ablation of material from a solid target and the production of nanoclusters, which are then either deposited on a substrate to form a nanostructured thin film or released to a liquid to form a colloidal nanoparticle solution. Particular attention is given on the absence or presence of the plasmarelated absorption of laser radiation, which provides different nano fabrication regimes. The methods lead to the production of nanomaterials, which are of importance for photonics and biosensing applications.     

Characterisation of silica nanoparticles prior to in vitro studies: from primary particles to agglomerates

The size, surface charge and agglomeration state of nanoparticles under physiological conditions are fundamental parameters to be determined prior to their application in toxicological studies. Although silica-based materials are among the most promising candidates for biomedical applications, more systematic studies concerning the characterisation before performing toxicological studies are necessary. This interest is based on the necessity to elucidate the mechanisms affecting its toxicity. We present here TEM, SAXS and SMPS as a combination of methods allowing an accurate determination of single nanoparticle sizes. For the commercial material, Ludox TM50 single particle sizes around 30 nm were found in solution. DLS measurements of single particles are rather affected by polydispersity and particles concentration but this technique is useful to monitor their agglomeration state. Here, the influence of nanoparticle concentration, ionic strength (IS), pH and bath sonication on the agglomeration behaviour of silica particles in solution has been systematically investigated. Moreover, the colloidal stability of silica particles in the presence of BSA has been investigated showing a correlation between silica and protein concentrations and the formation of agglomerates. Finally, the colloidal stability of silica particles in standard cell culture medium has been tested, concluding the necessity of surface modification in order to preserve silica as primary particles in the presence of serum. The results presented here have major implications on toxicity investigations because silica agglomeration will change the probability and uptake mechanisms and thereby may affect toxicity.     

Nanotitania-coated multi-walled carbon nanotube composite by facile colloidal processing route for photocatalytic applications

Multi-walled carbon nanotubes (MWCNTs) and titanium dioxide nanocomposites (MWCNTs/TiO₂) were fabricated by a simple novel colloidal processing route and tested as a photocatalyst for degradation of methylene blue under UV irradiation. The novel idea behind this work is to make MWCNTs and TiO₂ nanoparticle suspensions separately highly oppositely charged and utilize the electrostatic force of attraction between two entities to deposit nanotitania onto MWCNTs surface. Particle charge detector, scanning electron microscopy, transmission electron microscope, energy dispersive X-rays, X-rays diffraction (XRD), and Raman spectroscopy were used to characterize the composite. XRD and Raman spectroscopic analysis showed the crystalline structure of deposited TiO₂ over MWCNTs surface structure as anatase phase. It was found that MWCNTs/TiO₂ composite structure have much higher photocatalytic activity compared to TiO₂ nanoparticles. The composite material developed may find potential applications in the degradation of organic pollutants in aqueous medium under UV irradiation.     

Solid freeform fabrication of alumina using laser-assisted ESAVD

Electrostatic spray assisted vapour deposition (ESAVD) has been used successfully to produce films and coatings. A combination of laser-assisted (LA) deposition and ESAVD has been developed for the direct fabrication of alumina ceramics. This novel laser-assisted ESAVD technique involves spraying a liquid precursor onto a substrate whilst a laser beam is simultaneously used to heat the precursor and cause the decomposition and chemical reactions of the precursor in order to produce a solid deposit. A desired three-dimensional (3-D) object can be formed through layer-by-layer deposition using a computer-controlled platform. Dense or porous alumina ceramics can be deposited on the substrate by optimizing the processing parameters. This paper reports the fabrication of alumina parts by the laser-assisted ESAVD method. The effect of processing parameters including laser scanning speed, laser beam size and laser power on the microstructure will be presented. Scanning electron microscope (SEM) and X-ray diffraction (XRD) have been used to characterise the phase and structure of the deposited alumina parts.     

Hetero-Colloidal Metal Particle Multilayer Films Grown Using Electrostatic Interactions at the Air–water Interface

The formation of nanoparticle multilayer films by electrostatic immobilization of surface-modified colloidal particles at the air–water interface has been recently demonstrated by us. In this paper, we extend our study to show that multilayer assemblies consisting of metal particles of different chemical nature (hetero-colloidal particle superlattices) and size can be deposited by the versatile Langmuir–Blodgett technique. Multilayer films consisting of a different number of bilayers of gold and silver colloidal particles have been deposited and characterized using quartz crystal microgravimetry and UV–visible spectroscopy measurements. It is observed that while layer-by-layer deposition of the different colloidal particle assemblies is possible by this technique without a detectable variation in the cluster density in the different layers, a degree of post-deposition reorganization of the clusters occurs in the film. In addition to this aging behavior, the effect of different organic solvents on the reorganization process has also been studied.     

Characterization of inhomogeneous colloidal layers using adapted coherence probe microscopy

Colloidal layers play an important role in environmental studies, for example in the movement of radionuclides in nuclear waste management. New characterization techniques are required for studying such complex, porous layers. The purpose of this work is to adapt coherence probe microscopy (CPM), which is typically used for measuring the surface roughness of single surfaces, to the analysis of thick inhomogeneous colloidal layers. Two types of layers, either composed of 80 nm or 400 nm alumina colloidal particles deposited on glass slides by decantation have been studied. One of the problems in performing routine roughness measurements of colloidal layers using CPM is the appearance of apparent pits below the level of the substrate surface. We demonstrate that this is due to partial detection of the buried colloid/substrate interface. Further, we have developed the “Z-scan” technique, which consists of building up an XYZ image stack by scanning the full depth of the sample. Any point in an XY image can then be investigated to study the local buried internal structure, layer thickness, and effective refractive index. Comparison of results with AFM and SEM confirm the structure found with CPM and the new “Z-scan” technique, which opens up new and useful applications.     

电解质浓度对胶体粒子表面有效电荷的影响

胶体粒子的表面有效电荷是决定胶体性质的重要物理量,但溶液环境(如电解质溶液浓度)是否影响其数值至今尚无统一认识,近年来的一些研究工作给出了存在争议的不同结果和假设.在直接实验测量方面,由于电解质离子和胶体表面吸附离子的置换,粒子表面基团的不完全电离和胶体粒子对离子吸附的共同作用,使得对这类粒子在不同溶液环境下的表面有效电荷的测量和变化机理的认识极为困难.针对该问题,本文测定了羧基和磺酸基修饰的聚苯乙烯胶体颗粒在不同粒子浓度和HCl浓度下的电导率,由于两种粒子与HCl电离产生的阳离子相同(均为H+),可根据电导率-数密度法(迁移法)得到胶体颗粒表面有效电荷数.通过实验结果分析,明确了HCl浓度以及粒子数密度对胶体粒子表面电荷的影响规律以及表面电荷随HCl浓度增大的原因.除此之外,羧基修饰颗粒比磺酸基修饰颗粒的表面电荷随HCl浓度变化更快;对于同一HCl浓度,磺酸基修饰胶体表面电荷不受粒子数密度影响,而羧基修饰胶体颗粒却与之相关.基于粒子表面电荷的理论模型,对这些问题都给出了相应的解释.     

Controlling the size and the activity of Fe particles for synthesis of carbon nanotubes

The properties of carbon nanotubes (CNTs) are controlled by their structure and morphology. Therefore, their selective synthesis, using catalytic chemical vapor deposition, requires precise control of a number of parameters including the size and activity of the catalyst nanoparticles. Previously, an environmental scanning transmission electron microscope (ESTEM) has been used to demonstrate that electron beam-induced decomposition (EBID) of Fe containing precursor molecules can be used to selectively deposit Fe catalyst nanoparticles that are active for CNT growth. We have extended these in situ ESTEM observations to further our understanding of the EBID parameters, such as deposition time, and substrate temperature, that control the size and placement of Fe catalyst particles for two precursors, namely diiron nonacarbonyl (Fe(2)(CO)(9)) and ferrocene (Fe(C(5)H(5))(2)). We found that the diameter of deposited particles increased with increasing deposition time. Electron energy-loss spectra, collected during deposition, show the incorporation of C in the Fe particles. The C content decreased as the substrate temperature was increased and was negligible at 100°C for Fe(2)(CO)(9). However, C and Fe were co-deposited at all temperatures (up to 450°C) when Fe(C(5)H(5))(2) was used as an iron source. After deposition, the substrate was heated to the CNT growth temperature in flowing hydrogen to remove the co-deposited C, which was an important step to activate the deposited Fe catalyst for the growth using acetylene. Our measurements revealed that the Fe nanoparticles fabricated from Fe(2)(CO)(9) had higher activity for CNT growth compared to the ones fabricated using Fe(C(5)H(5))(2). We also found that the co-deposited carbon could not be removed by heating in hydrogen in the case of Fe(C(5)H(5))(2). The particles deposited from Fe(C(5)H(5))(2) at 300°C to 450°C formed a core-shell structure with Fe surrounded by graphitic carbon. We speculate that the reduced activity for Fe(C(5)H(5))(2) is due to the C content in the deposit.     

Self-assembly of length-tunable gold nanoparticle chains in organic solvents

The one-dimensional coagulation of gold colloidal particles dispersed in organic solvent was investigated with transmission electron microscopy. The results indicate that the length of the nanoparticle chains can be modulated by changing the concentration of the solutions. It was also demonstrated that the wetting of the substrate surface hardly influenced the morphology of the nanoparticle chains, which revealed that the particle chains had been formed in the solution before deposition on the substrates. A general theoretical interpretation is provided to explain the linear coagulation of gold colloidal particles, on the basis of the asymmetrical distribution of the charges absorbed on the surface of the gold colloidal particles, as well as the action of the solvent molecules. Received: 8 April 2002 / Accepted: 1 July 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +86-025/361-9983, E-mail: jhliao@seu.edu.cn     

Uniform thin films of TiO2 nanoparticles deposited by matrix-assisted pulsed laser evaporation

We report morphological and optical properties of a colloidal TiO₂ nanoparticle film, deposited on a quartz substrate by using the Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique. Atomic Force Microscopy demonstrated that a good uniformity of the deposition can be obtained. The presence of agglomerates with dimensions of about 1 μm in size was noticed. Form UV-vis transmission spectra, recorded in the 200-800 nm range, the optical constants and the energy gap were determined besides the film thickness. The optical constants resulted in agreement with the values reported in literature for TiO₂ nanoparticle thin films.     

Electron dynamics in supported metal nanoparticles: relaxation and charge transfer studied by time-resolved photoemission

Resonant excitation of the plasmon polariton in supported nanoparticles leads to a strong enhancement of the multiphoton photoemission yield and consequently, the total yield is dominated by the emission from the nanoparticles although they cover only a minor fraction of the surface. This allows investigation of the electron dynamics in supported nanoparticles, directly in the time domain. Here, Ag nanoparticles grown on graphite are used to demonstrate that the transient shape of the photoemission spectrum in time-resolved two-color multiphoton photoemission spectroscopy, reveals the electron relaxation within the nanoparticle, and the dynamic charge transfer between substrate and nanoparticle. The photoemission spectra map the transient electron energy distribution and exhibit a transient shift that is attributed to a dynamic charging of the nanoparticle. The comparison with model calculations comprising the full relaxation cascade in the nanoparticle and substrate, shows that the dynamic charge transfer accounts for almost half of the total deposited energy in the nanoparticle. PACS 78.67.Bf; 73.22.-f; 67.40.Fd; 78.47.+p     

银氧铯光电阴极中银胶粒的作用

银氧铯阴极中银胶粒的作用是讨论此种阴极长波光电发射机理的关键。本文对文献中出现的对这种阴极中银胶粒的性质和作用的观点进行了分析和评述。评述是在“它的长波光电发射来自固溶小银胶粒”的观点上进行的,并认为这个观点是合理的。本文对Лахомов等人的实验作了介绍,并指出烘烤后红外灵敏度的增加是由于该阴极中因加热扩散而增加了小银胶粒,因为在固溶胶体系中用扩散办法来增加杂质原子浓度并不能使施主原子浓度增加(假定激活烘烤温度不变)。本文还对这种阴极的红外光吸收和红外光电发射之间的关系进行了讨论。 关键词：     

Electric double layer of anisotropic dielectric colloids under electric fields

Anisotropic colloidal particles constitute an important class of building blocks for self-assembly directed by electrical fields. The aggregation of these building blocks is driven by induced dipole moments, which arise from an interplay between dielectric effects and the electric double layer. For particles that are anisotropic in shape, charge distribution, and dielectric properties, calculation of the electric double layer requires coupling of the ionic dynamics to a Poisson solver. We apply recently proposed methods to solve this problem for experimentally employed colloids in static and time-dependent electric fields. This allows us to predict the effects of field strength and frequency on the colloidal properties.     

Cu(In,Ga)Se2 thin film solar cells from nanoparticle precursors

A non-vacuum process for Cu(In,Ga)Se₂ (CIGS) thin film solar cells from nanoparticle precursors was described in this work. CIGS nanoparticle precursors was prepared by a low temperature colloidal route by reacting the starting materials (CuI, InI₃, GaI₃ and Na₂Se) in organic solvents, by which fine CIGS nanoparticles of about 15 nm in diameter were obtained. The nanoparticle precursors were then deposited onto Mo/glass substrate by the doctor blade technique. After heat treating the CIGS/Mo/glass layers in Se gas atmosphere, a complete solar cell structure was fabricated by depositing the other layers including CdS buffer layer, ZnO window layer and Al electrodes by conventional methods. The resultant solar cell showed a conversion efficiency of 0.5%.     

A particle-in-cell simulation of dust charging and shielding in lowpressure glow discharges

The transport of particles (“dust”) in low pressure electrical glow discharges is being studied in regard to its role in contaminating silicon wafers during plasma etching and deposition. Particles (10 s nm-μm) negatively charge in glow discharges and, to first order, appear to be massively large negative ions around which sheaths develop. The forces on particles in plasmas include electrostatic (drift of charged particles in electric fields) and viscous ion drag. The latter force is momentum transfer from ions to particles by either collisions or orbital motion. This force critically depends on the charge on the particle and the shape of the sheath surrounding the particle. In this work, we report on a pseudoparticle-in-cell (PIC) simulation of the transport of electrons and ions in the vicinity of dust particles in low pressure glow discharges. The simulation produces the electrical charge on the dust particle, the sheath structure around the dust particle and the orbital dynamics of the ions. A companion molecular dynamics simulation uses these parameters to produce ion-dust and electron-dust particle cross sections for momentum transfer and collection. Results will be discussed for charge, sheath thickness, cross sections and viscous ion drag forces on dust particles as a function of radius and plasma parameters     

Two-dimensional nanopatterning by PDMS relief structures of polymeric colloidal crystals

A new constructive method of fabricating a nanoparticle self-assembly on the patterned surface of a poly(dimethylsiloxane) (PDMS) relief nanostructure was demonstrated. Patterned PDMS templates with close-packed microwells were fabricated by molding against a self-assembled monolayer of polystyrene spheres. Alkanethiol-functionalized gold nanoparticles with an average particle size of 2.5 nm were selectively deposited onto a hydrophobic self-assembled monolayer printed on the substrate by the micro-contact printing (μCP) of the prepared PDMS microwell, in which the patterned gold nanoparticles consisted of close-packed hexagons with an average diameter of 370 nm. In addition, two-dimensional colloidal crystals derived from PMMA microspheres with a diameter of 380 nm and a negative surface charge were successfully formed on the hemispherical microwells by electrostatic force using positively charged PAH-coated PDMS as a template to produce multidimensional nanostructures.     

Effect of chromium interlayer deposition on periodic silver nanoparticle array structure fabricated by nanosphere lithography

Effect of chromium interlayer deposition on 2-dimensional, periodic silver nanoparticle array structure was systematically investigated. The silver nanoparticle array was fabricated by nanosphere lithography with assembled polystyrene nanospheres being as a deposition mask. The chromium interlayer was deposited by thermal evaporation either on the nanosphere mask or directly on the silicon substrate. The structures of the achieved silver nanoparticle arrays were characterized by scanning electron microscope and were compared with that of silver nanoparticle array without the interlayer. With analysis of the anomalies among the structures the critical role of the interlayer in the periodic nanoparticle array fabrication was revealed.