Electrostatically stabilised nanoparticles: self-organization and electron-beam patterning

The self-organisation of citrate- and magnesium oleate-stabilised gold nanoparticles on SiO2/Si substrates was investigated. In drop deposition, nucleation of citrate-stabilised gold nanoparticles was observed at the rim of the droplet, symmetric or multibranched dendroid gold structures were found in the area between the rim and the central part of the droplet, depending on the drying temperature. Homogeneous submonolayer nanoparticle coverage was obtained by immersion of amineterminated SiO2/Si surfaces into a citrate-stabilised colloidal gold acidic solution. Drop deposition of magnesium oleate-stabilised gold nanoparticles onto the SiO2/Si surfaces resulted in the formation of uniformly close-packed nanoparticle arrays. Under electron beam irradiation, no apparent changes were found for monolayer films of citrate-stabilized particles, but sintering of the nanoparticles was observed in multilayer films. In contrast, coalescence of magnesium oleate-stabilised gold nanoparticle occurred in monolayer films after electron irradiation.     

pH-dependent adsorption of Au nanoparticles on chemically modified Si3N4 MEMS devices

Microelectromechanical systems (MEMS) are devices that represent the integration of mechanical and electrical components in the micrometer regime. Self-assembled monolayers (SAMs) can be used to functionalise the surface of MEMS resonators in order to fabricate chemically specific mass sensing devices. The work carried out in this article uses atomic force microscopy (AFM) and X-ray photoemission spectroscopy (XPS) data to investigate the pH-dependent adsorption of citrate-passivated Au nanoparticles to amino-terminated Si₃N₄ surfaces. AFM, XPS and mass adsorption experiments, using ‘flap’ type resonators, show that the maximum adsorption of nanoparticles takes place at pH = 5. The mass adsorption data, obtained using amino functionalised ‘flap’ type MEMS resonators, shows maximum adsorption of the Au nanoparticles at pH = 5 which is in agreement with the AFM and XPS data, which demonstrates the potential of such a device as a pH responsive nanoparticle detector.     

Gold catalysts for the abatement of environmentally harmful materials: Modeling the structure dependency

FeO_x, TiO₂ and CeO_x layers were deposited by pulsed laser deposition (PLD) technique onto Au films or Au nanoparticles supported on SiO₂/Si(100). The samples were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS) and their reactivity was studied in catalytic CO oxidation. Comparison was made with reference samples of FeO_x/SiO₂/Si(100), TiO₂/SiO₂/Si(100), CeO_x/SiO₂/Si(100) and Au/SiO₂/Si(100) layers. The catalytic activity of the metal-oxide/Au/SiO₂/Si(100) samples must be attributed to active sites located on the metal-oxides overlayer modified by gold underneath, since no Au was exposed to the surface according to the XPS and SIMS. We found a promoting effect of gold on the catalytic activity of the FeO_x overlayer and an inhibiting effect of gold on the TiO₂ and CeO_x overlayers. These findings are discussed in terms of electronic interactions at the Au/metal oxide interface.     

Effect of additive ratio of mixed silane alkoxides on reactivity with TiO2 nanoparticle surface and their stability in organic solvents

To improve stability of TiO₂ nanoparticles in various organic solvents, their surface was modified using a mixture of silane coupling agents having hydrophobic and hydrophilic groups. Decyltrimethoxysilane (DTMS) and phenyltrimethoxysilane (PTMS) were used as the former and 3-aminopropyltrimethoxysilane (APTMS), as the latter. First, effects of mixing ratios of silane coupling agents on reactivity with TiO₂ nanoparticle surfaces were investigated. The addition of APTMS increased the pH value of TiO₂ nanoparticle suspension due to the presence of the amine group and largely affected to the reacted amount of hydrophobic silanes. Next, relationships between the reacted amounts of silanes and their dispersion stability in various organic solvents were then investigated. Surface-modified TiO₂ nanoparticles were re-dispersible into low-polar solvents such as hexane, toluene, and THF when the reacted amount of hydrophobic silane was relatively high; however, TiO₂ nanoparticles were re-dispersible in highly polar solvents when the reacted amount of hydrophilic silanes increased. By controlling the amount of hydrophilic and hydrophobic silanes added, it is possible to effectively improve the dispersion stability of TiO₂ nanoparticles in various organic solvents.     

Two-dimensional nanoparticle arrays formed by dewetting of thin gold films deposited on pre-patterned substrates

We demonstrate the formation of accurate 2D gold nanoparticle arrays via solid-state dewetting on a pre-patterned substrate. The annealing-induced dewetting of Au film on both flat and pre-patterned SiO₂ substrates is investigated. The pre-patterned structures affect clearly the formation of nanoparticles, and there is a depth effect of the pre-patterned grooves on the formation of nanoparticles during dewetting. Especially in pre-patterned areas with deep grid grooves (depth 150 nm) there is almost one single particle formed in the flat areas of every unit square, thus resulting in a very periodic 2D structure of gold nanoparticles.     

Room temperature single electron transistor with two-dimensional array of Au–SiO2 core–shell nanoparticles

The composite nanoparticles of gold core coated with SiO₂ shell have been fabricated into 2-dimensional array on a silicon surface by a simple self-assembly method combined with the technique of AFM (atomic force microscopy) nanolithography. The double-barrier-tunneling junction with AFM tip was also fabricated for the room-temperature single-electron tunneling study, by which the AFM tip was orientated on the surface of the SiO₂ coated gold composite nanoparticles. The 2D array shows well-pronounced Coulomb staircases with a period of 200 mV at room temperature, demonstrating single electron transistor behavior.     

Self-chemisorption of azurin on functionalized oxide surfaces for the implementation of biomolecular devices

In this work, we investigate the formation of redox protein Azurin (Az) monolayers on functionalized oxygen exposing surfaces. These metallo-proteins mediate electron transfer in the denitrifying chain of Pseudomonas bacteria and exhibit self-assembly properties, therefore they are good candidates for bio-electronic applications. Azurin monolayers are self-assembled onto silane functionalized surfaces and characterized by atomic force microscopy (AFM). We show also that a biomolecular field effect transistor (FET) in the solid state can be implemented by interconnecting an Azurin monolayer immobilized on SiO₂ with two gold nanoelectrodes. Transport experiments, carried out at room temperature and ambient pressure, show FET behavior with conduction modulated by the gate potential.     

Valence band and catalytic activity of Au nanoparticles in Fe2O3/SiO2/Si(100) environment

A 10-nm-thick gold film was evaporated onto a SiO₂/Si(100) substrate and was implanted by Ar⁺ ions at 40 keV and 10¹⁵ at/cm² dose creating island like Au nanoparticles. An 80-nm-thick gold film was also deposited the onto same substrate and considered as reference.Fe₂O₃ film was deposited onto the gold nanoparticles and the gold/oxide interface was modeled. The valence band and the structure were measured by means of photoelectron spectroscopy (XPS) and by transmission electron microscopy (TEM), respectively. The catalytic activity was detected by CO oxidation, which was higher after the deposition of Fe₂O₃ layer onto Au nanoparticles than that on a continuous Au film. This observation was correlated to the nanosize and the redistributed valence band density of states of gold in the Au/Fe₂O₃ interface.     

Utilization of Si atomic steps for Cu nanowire fabrication

Techniques for controlling atomic step position at low-temperature and selective growth of Cu nanowires along the atomic step edges have been studied. By immersing the Si(111) substrates with well-defined step/terrace surfaces in the Cu-contained water with the dissolved oxygen content of less than 1 ppb, selective growth of Cu nanowires along the step edges was successfully achieved. Total reflection X-ray fluorescence spectroscopy (TXRF) revealed that the fabricated nanowires were composed of mono-atomic Cu rows. For step position control, the characteristics of step-flow pinning effect of SiO₂ films were investigated. Fine SiO₂ line patterns drawn by anodic oxidation using AFM probes enable us to obtain the step-free Si areas predetermined by the patterns.     

Successive nucleation and a rodlike growth of SiOx nanoparticles into the pore bottoms of an anodic alumina template

Although the SiO_x nanoparticles were previously reported to electrochemically nucleate from the bottoms of the pore array formed in the AAO/Ti/Si system, new applications of anodic aluminum oxide templates, i.e., successive nucleation and a rodlike growth of SiO_x nanoparticles, were observed utilizing the subsequent annealing technique after the nanoparticle precipitation. Tailoring the pore bottom profile by doping type and level of wafers also critically affected to nucleate the SiO_x nanoparticles from the pore bottoms. By anodization, as previously reported, only one nanoparticle per each pore was generally precipitated from the pyramid-shaped Si-containing TiO_x nanopillars; however, subsequent annealing under a low pressure of hydrogen enabled the successive precipitation of nanoparticles. Annealing under atmospheric pressure with H₂ and N₂ resulted in the rodlike growth of a single nanoparticle without successive nanoparticle precipitation.     

Structural verification and optical characterization of SiO2–Au–Cu2O nanoparticles

In this paper, SiO₂–Au–Cu₂O core/shell/shell nanoparticles were synthesized by reducing gold chloride on 3-amino-propyl-triethoxysilane molecules attached silica nanoparticle cores for several stages. Cu₂O nanoparticles were synthesized readily with the size of 4–5 nm using a simple route of sol–gel method Then, they were clung to the surface of Au seeds. The morphology of the resultant particles was studied using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Transmission electron microscopy images demonstrate growth of monodispersed gold seeds and Cu₂O nanoparticles in narrow size up to 10 nm and 5 nm, respectively. The presence of gold and Cu₂O coating was confirmed by X-ray diffraction, Fourier transform infrared spectroscopy and UV–Vis spectroscopy. Absorption spectroscopy shows considerably 40 nm blue shift in absorption edge for SiO₂–Au–Cu₂O nanostructure rather than SiO₂–Au core/shell nanoparticles.     

Surface treatment for high-quality Al2O3 and HfO2 layers deposited on HF-dipped surface by atomic layer deposition

Al₂O₃ and HfO₂ thin layers were deposited on either 0.7-nm chemical SiO₂ surface layers, HF-dipped Si surfaces or on HF-dipped Si surfaces with an innovative Cl₂ surface treatment. This chemical treatment leads to the formation of one mono-layer of –OH groups on the silicon surface without any SiO _x growth. Thicknesses, composition, and structure of the high-k layers as well as the nature of their interfaces with silicon were studied using spectrometric ellipsometry, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS). While deposition on a HF-dipped Si surface led to a nucleation retardation and to a 3-dimensional growth mode, high-quality, uniform Al₂O₃ layers were obtained on a Cl₂-treated Si surface. XPS and ATR analyses showed a very small SiO _x regrowth, less than 0.26 nm during deposition.     

Kinetics of interfacial layer formation during deposition of HfO2 on silicon

Very thin HfO₂ films were deposited directly on Si substrates by the pulsed laser deposition technique in a wide range of substrate temperatures and oxygen pressures to investigate the kinetics of the interfacial layer formation. Angle-resolved X-ray photoelectron spectroscopy (XPS) investigations showed that the interfacial layer formed between the Si substrate and the deposited oxide contains a mixture of HfO₂ and SiO₂ without any strong evidence to support the formation of a silicate-type compound. X-Ray reflectivity measurements showed that the mass density of the interfacial layer is higher than that of pure SiO₂, while spectroscopic ellipsometry measurements showed that the refractive index is higher than that of pure SiO₂, therefore corroborating the XPS results.     

Enzyme biocatalyst route to superhydrophobic surfaces on microstructured poly(ethylene terephthalate) film

A tunable and green enzyme biocatalyst route to develop superhydrophobic surfaces on microstructured poly(ethylene terephthalate) (PET) films by the tailoring of the micro- and nano scale hierarchical structures is described. Upon the aminolysis of PET films with hexamethylenediamine, the primary amine groups are covalently attached onto the PET surfaces and microstructured pattern is formed. The binding of citrate-stabilized Au nanoparticles onto the PET surfaces via the covalent bond between the gold nanoparticles and the primary amine groups introduced on the PET surfaces was followed spectroscopically. The biocatalytic enlargement of the Au nanoparticles using the enzyme-generated H₂O₂ as reducing agent for the reduction of AuCl₄⁻ at the attached Au nanoparticle seeds on the PET surfaces was followed by spectroscopic means and atom force microscopy (AFM). The AFM experiments indicated that micro- and nano scale hierarchical structures were tailored by the enzyme biocatalyst route. Superhydrophobic surfaces with water contact angles as high as 158.6 ± 2.0° was achieved upon the chemisorption of 1-octadecanethiol as low surface energy material. This route can be potentially applicable to superhydrophobic PET-based microfluidic devices with reduced friction surfaces.     

In situ probe nanophase transition in nanocomposite using thermal AFM

Nanocomposites made from inorganic nanoparticles and polymers have many applications in optics, electronics and biomaterials. However, the glass transition temperature (T_g) of a nanocomposite is very difficult to measure accurately by conventional thermal analysis such as DSC or TMA when the concentration of the nanoparticle reaches a threshold of the percolation network. At this threshold stage, the phase transition in the nano domains of the matrix is too small to be detected by macroscale thermal analysis. We have developed a methodology basis on thermal atomic force microscope (AFM) to monitor the nanophase transition of the nanocomposite in situ upon heating. This method has demonstrated the capability in determining the T_g of a nanocomposite made by spherical SiO₂ nanoparticles dispersed in polyacrylate. The threshold of the percolation network of this nanocomposite is at 40 wt% of SiO₂ nanoparticles according to the results of refractive index, AFM, nanoindentation, DSC, TMA and TGA.     

The effect of the presence of Ag nanoparticles on the photocatalytic degradation of oxalic acid adsorbed on TiO2 nanoparticles monitored by ATR-FTIR

Photocatalytic degradation of oxalic acid adsorbed on the Ag/P25 TiO₂ composite nanoparticle films were investigated using ATR-FTIR technique under UV irradiation. Ag/P25 TiO₂ composite nanoparticle films with various Ag content were tested. Topography and chemical structure/composition of the composite nanoparticle films were analyzed by AFM and XPS respectively. It was found that in the degradation reaction of the oxalic acid, the presence of only 2% Ag nanoparticles leads to six times more oxalic acid degradation compared to that degraded in the presence of pure P25 TiO₂ nanoparticles. The degradation rate of the oxalic acid is three times higher in the case of Ag/TiO₂ composite nanoparticle film than in the case of pure TiO₂ nanoparticles. It was observed that both the rate of oxalic acid degradation and the degraded amount of the oxalic acid were significantly affected by Ag incorporation.     

Annealing induced phosphorus protrusion into thin-oxide films from heavily phosphorus-doped silicon (100)

Phosphorus (P) protrusion into thin-oxide film from heavily P-doped Si (100) upon annealing was investigated using X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). After annealing at 750 °C, the P was segregated at the SiO₂/Si interface and its concentration decayed exponentially toward both the thin-oxide film and the substrate. The chemical state of protruded-P in the oxide was nearly elemental. After annealing, the AFM images of the sample surface showed that some of the plateaus grew in height while maintaining their lateral shape. The total increment volume of the grown plateaus corresponded roughly to the volume of protruded-P in the oxide film estimated by XPS.     

Multifunctional iron and iron oxide nanoparticles in silica

Iron and iron oxide nanoparticles in silica layers deposited by sol–gel techniques on Si wafers were formed and studied. It was shown that multifunctional nanoparticles of different iron oxides possessing various physical properties can be fabricated by means of post-growth annealing of (SiO₂:Fe)/SiO₂/Si samples in various atmospheres. The hematite, maghemite, and iron nanoparticles were found to be dominant upon annealing the samples in air, argon, and hydrogen atmosphere, respectively. The physical properties of produced hybrid structures were studied by Raman and FT-IR spectroscopy, spectroscopic ellipsometry, AFM, and magnetic measurements. The sol–gel technique with subsequent annealing procedure is demonstrated to be an effective method for the formation of multifunctional hybrid structures composed of iron or iron oxide nanoparticles in silica matrix.     

Thin ZnO nanocomposite poly(styrene–acrylic acid) films on Si and SiO2 surfaces

The properties and formation of self-assembled ZnO nanoclusters using polystyrene-based diblock copolymers are reported. The polystyrene–polyacrylic acid copolymer consisting of a majority block (polystyrene) and a minority block (polyacrylic acid) with a block number average molecular weight ratio of 16,500/4500 and a block repeat unit ratio of 159/63 was used in order to obtain self-assembly due to microphase separation with spherical morphology. The self-assembly of the inorganic nanoparticles was achieved at room temperature in the liquid phase using a ZnCl₂ precursor dopant attached to the minority block, and both dry and wet chemical processing techniques compatible with semiconductor manufacturing were developed in order to convert the ZnCl₂ precursor into ZnO. The polymer films were applied by standard spin-on photolithographic techniques on Si wafers with and without thermally grown SiO₂ surface films. A study by X-ray photoelectron spectroscopy (XPS) confirmed the conversion of the ZnCl₂ dopant precursor into ZnO within the copolymer matrix, and atomic force microscopy (AFM) showed the spherical morphology of the resultant ZnO nanoclusters. Conversion of the precursor into ZnO was achieved both by a wet chemical treatment and by developing a new dry chemical treatment process based on ozone exposure. The study showed that the dry treatment has better stability and shorter conversion exposure times on the Si surfaces than the wet treatment approach, resulting in lateral size distribution between 250 and 350 nm and height distribution between 80 and 130 nm for the ZnO nanoclusters.     

Effect of monodispersed silica nanoparticles on DNA separation by micro-capillary electrophoresis

In this work, we investigate micro-capillary electrophoresis (μ-CE) using monodispersed SiO₂ nanoparticles added to an electrophoresis buffer solution to obtain high mobility and high separation of double-strand DNA. Various particle sizes of monodispersed SiO₂ nanoparticle solutions were mixed with conventional 0.7% hydroxyl propyl methyl cellulose (HPMC) buffer solution to achieve μ-CE. We achieved perfect separation of the DNA specimen (100 bp to 1.5 kbp) at a certain SiO₂ nanoparticle size. In addition, we discuss the effect of SiO₂ nanoparticles during electrophoresis.