Self-assembly Of Au Nanoparticles Research Articles

Self-assembled ferrite nanodots on multifunctional Au nanoparticles

The fabrication of magnetic oxide nanodots was studied without the use of conventional lithographic techniques and patterning masks. Self-assembled Au nanoparticles with an average size of approximately 17 nm were formed via simple sputtering and used as the underlayer for the magnetic oxide film. Subsequently, hexagonal ferrite, SrFe12O19, was sputtered on the Au nanoparticles, resulting in an SrFe12O19/Au sample. Self-assembled SrFe12O19 nanodots were obtained with an average size of 40–50 nm. The morphology of the Au nanoparticle underlayer acted as a template for the SrFe12O19 film, such that the self-assembled SrFe12O19 nanodots were formed. In addition, the fabrication of the SrFe12O19 film on the Au nanoparticles induced the down-sizing of the magnetic domain structures of SrFe12O19 to the nanoscale. Importantly, although the nanodots showed nanometric magnetic domains, a sufficient magnetization magnitude in the SrFe12O19 nanodots was revealed. Furthermore, the SrFe12O19 nanodot fabrication area was ∼8.5 cm2, thereby the current technique can be applied to the development of future functional magnetic nanodots.

In Situ Visualization of Self-Assembly of Charged Gold Nanoparticles

Self-assembly of Au nanoparticles (NPs) coated with positively charged cetyltrimethylammonium ions (CTA(+)) and negatively charged citrate ions in aqueous liquid cell was investigated by in situ transmission electron microscopy (TEM). Under electron illumination in TEM, the hydrated electrons will reduce the overall positive charges of the CTA(+) covered Au NPs and decrease the repulsive electrostatic forces among NPs, leading to assembly of individual NPs into one-dimensional structures. On the contrary, the negatively charged Au NPs coated with citrate ions are steady in liquid cell regardless of electron beam intensity.

Self-assembled Au nanoparticles on heated Corning glass by dc magnetron sputtering: size-dependent surface plasmon resonance tuning

We report on the growth of Au nanoparticles on Corning glass by direct current magnetron sputtering and on the optical absorption of the films. The substrate temperature was kept to relatively high temperatures of 100 or 450 °C. This lead to the growth of Au nanoparticles instead of smooth Au films as the surface energy of Au is much larger than the one of glass. The size of the particles depended on the substrate temperature and deposition time and was shown to follow a logarithmic normal distribution function. Both, the surface plasmon resonance position and bandwidth, were found to depend upon the average particle size. The surface plasmon resonance position showed a 75 nm continuous blue shift from 14 nm down to 2.5 nm average particle size. Thus, we have shown how to tune the nanoparticle size and surface plasmon resonance of Au by varying the substrate temperature and deposition time. The experimental results are reproduced reasonably using a method which is based on the size- and wavelength-dependent complex dielectric function of Au within the framework of the Mie theory for the optical properties of metallic nanospheres.

Tuning the properties of magnetic thin films by interaction with periodic nanostructures.

The most important limitation for a significant increase of the areal storage density in magnetic recording is the superparamagnetic effect. Below a critical grain size of the used CoCrPt exchange-decoupled granular films the information cannot be stored for a reasonable time (typically ten years) due to thermal fluctuations arbitrary flipping of the magnetization direction. An alternative approach that may provide higher storage densities is the use of so-called percolated media, in which defect structures are imprinted in an exchange-coupled magnetic film. Such percolated magnetic films are investigated in the present work. We employ preparation routes that are based on (i) self-assembly of Au nanoparticles and (ii) homogeneous size-reduction of self-assembled polystyrene particles. On such non-close-packed nanostructures thin Fe films or Co/Pt multilayers are grown with in-plane and out-of-plane easy axis of magnetization. The impact of the particles on the magnetic switching behavior is measured by both integral magnetometry and magnetic microscopy techniques. We observe enhanced coercive fields while the switching field distribution is broadened compared to thin-film reference samples. It appears possible to tailor the magnetic domain sizes down to the width of an unperturbed domain wall in a continuous film, and moreover, we observe pinning and nucleation at or close to the imprinted defect structures.

Polymerization-induced phase separation as a one-step strategy to self-assemble alkanethiol-stabilized gold nanoparticles inside polystyrene domains dispersed in an epoxy matrix

The rational design of nanoparticle (NP)/polymer composites with advanced functional properties is based on controlling the distribution and self-assembly of NPs in the polymer matrix. In this study we report a new one-step strategy to produce the self-assembly of alkanethiol-stabilized Au NPs in one of the phases generated by polymerization-induced phase separation. The polymerization of a formulation composed of stoichiometric amounts of diglycidylether of bisphenol A (DGEBA) and m-xylylenediamine (mXDA), containing polystyrene (PS) and dodecanethiol-stabilized Au NPs as modifiers, produced the phase separation of PS and Au NPs into microdomains dispersed in the epoxy matrix. A subsequent phase separation and self-assembly of Au NPs took place inside the PS domains leading to an increase in their concentration in a region close to the interface as revealed by TEM images. SAXS spectra showed that NPs self-assembled as colloidal crystals with a body-centered cubic (bcc) structure. By an adequate selection of the amount of PS and the nature of the epoxy precursors, different morphologies of the final blend could be generated. This brings the possibility of controlling the dispersion and self-assembly of NPs in the final material.

Nanostructured Metal-Enhanced Photoluminescence of Micro-Sr2Si5N8:Eu2+ Phosphors

The enhancement of the photoluminescence of micro-Sr2Si5N8:Eu2+ phosphors on the surface of Ag and Au nanoparticles is studied. Ag nanoparticles cause a considerable enhancement in the photoluminescence intensity of microsized Sr2Si5N8:Eu2+ phosphors. We further investigate the enhanced emission of self-assembled wormlike Au nanoparticles from Sr2Si5N8:Eu2+ phosphors. A brightness photoluminescence intensity enhanced by about six times is observed, and is due to the stronger scattering and higher numbers of hot spots in the Au worms. The luminescence efficiency of the microparticles is higher than the nanoparticles, and it is very difficult to enhance light emission. Therefore, the realization of microphosphor photoluminescence enhancement is a great breakthrough.

Assembly of single-walled carbon nanotubes on patterns of Au nanoparticles

We report on the assembly of single-walled carbon nanotubes (SWNTs) and gold nanoparticles (NPs) hybrid structure without any surface modification of SWNTs on patterns of Au nanoparticles (NPs). Microscale Au NP patterns were created on composite self-assembled monolayer (SAM) templates of octadecanethiol (ODT) and octanedithiol (OD) through self-assembly of Au NPs via the thiol-Au chemical bond onto the OD region. On such templates, we observed extensive adhesion and strong affinity of SWNTs on the Au NPs and no SWNT on ODT. We also examined systematically the adhesion of SWNTs on ODT with varying coverage of vapour-deposited Au. We observed little SWNT attachment even when there are high-density of Au clusters on the ODT SAM. Extensive adhesion of SWNTs is observed only when the coverage of ODT by Au is almost complete. Dynamic contact angle measurements of dichlorobenzene on the ODT/Au substrates revealed a direct correlation between the surface wettability and the SWNT assembly on a molecular template.

Electrochemical synthesis using a self-assembled Au nanoparticle template of dendriticfilms with unusual wetting properties

This report describes the electrochemical preparation of dendritic silver films with unusualwetting properties coming from the use of a self-assembled gold nanoparticle (Au NP)template. It shows that the Au NP self-assembled monolayer on the highly orderedpyrolytic graphite (HOPG) surface is responsible for the formation of the dendriticmorphology, which is not observed for the same deposition conditions on a bareHOPG substrate. An interesting evolution of the wetting properties of these filmsduring the electrodeposition process is observed. Field emission scanning electronmicroscopy (FEGSEM), energy-dispersive spectrometry (EDS), x-ray photoelectronspectroscopy (XPS) and contact angle measurements are used to reveal the dendriticstructure of the deposited silver film at a later stage of the electrodeposition process.They also reveal surprising wetting properties in terms of hydrophobic surface.

Controlling growth density and patterning of single crystalline silicon nanowires

This study examines the usage of well-patterned Au nanoparticles (NPs) as a catalyst for one-dimensional growth of single crystalline Si nanowires (NWs) through the vapor-liquid-solid (VLS) mechanism. The study reports the fabrication of monolayer Au NPs through the self-assembly of Au NPs on a 3-aminopropyltrimethoxysilane (APTMS)-modified silicon substrate. Results indicate that the spin coating time of Au NPs plays a crucial role in determining the density of Au NPs on the surface of the silicon substrate and the later catalysis growth of Si NWs. The experiments in this study employed optical lithography to pattern Au NPs, treating them as a catalyst for Si NW growth. The patterned Si NW structures easily produced and controlled Si NW density. This approach may be useful for further studies on single crystalline Si NW-based nanodevices and their properties.

Energy Transfer between Confined Dye and Surface Attached Au Nanoparticles of Mesoporous Silica

Nanoscale architectures have been designed by entrapping rhodamine 6G dye molecules into the channels of mesoporous silica and Au nanoparticles anchor onto the surface of the mesoporous matrix. The surface energy transfer between confined dye and Au nanoparticles has been studied by steady state and time-resolved spectroscopy. The appearance of second surface plasmon band at 680 nm with increasing the concentration of mesoporous silica indicates the formation of self-assembled structure of Au nanoparticles which is established by TEM and DLS studies. A mechanism for self-assembled Au nanoparticles is proposed. The PL quenching (76.3% to 27.4%) and energy transfer efficiency (51.8% to 17.4%) can be tuned with changing the arrangement of Au nanoparticles. Analysis reveals that the energy transfer from dye to Au nanoparticles is a surface energy transfer process and it follows 1/d4 distance dependence. This anisotropy decay reveals that the dye molecules are aligned inside the channels of mesoporous silica. Such energy transfer between confined dye and Au nanoparticles could pave the way for designing new optical based materials for the application in chemical sensing or light harvesting system.

Self-assembled Au nanoparticles in SiO2 by ion implantation and wet oxidation

Implantation, annealing, and oxidation processes have been used to form Au nanoparticles with a narrow size and depth distribution in a SiO2 layer. Different approaches have been attempted: in particular, the gettering of Au to fill preformed nanocavities (obtained by H-implantation and annealing) and thus overcome the broad particle size distribution that is normally associated with nanoparticles formed by implantation and annealing. The results suggest that nanocavities cannot be directly formed in SiO2 by H-implantation and a subsequent annealing due partly to the high mobility of H atoms in SiO2. However, cavities formed in Si are useful in obtaining a narrow size and depth distribution of Au precipitates: the Si substrate can then be oxidized to form Au nanoparticles in SiO2. Sequential wet oxidations of Si samples containing Au nanoparticles have revealed several interesting phenomena, namely, segregation of Au particles at a growing oxide interface, Au-enhanced oxidation, dissolution and reprecipitation of Au precipitates during oxidation, and preferential wetting of Au on the oxide layer. In particular, the Au dissolution and reprecipitation processes are Si interstitial mediated. By completely oxidizing the top Si layer, an array of Au precipitates can be confined at a precise depth within a SiO2 layer corresponding to the front interface of a buried oxide layer. The size distribution of the resulting Au precipitates in SiO2 is smallest when Au is first gettered to cavities and vacancies are subsequently introduced into the Si layer prior to oxidation.

Adsorption characteristics of Au nanoparticles onto poly(4-vinylpyridine) surface revealed by QCM, AFM, UV/vis, and Raman scattering spectroscopy

In this work, we report that the adsorption and aggregation processes of Au nanoparticles on a polymer surface can be monitored by means of surface-enhanced Raman scattering (SERS) spectroscopy. Specifically, we were able to analyze the adsorption process of citrate-stabilized Au nanoparticles onto a film of poly(4-vinylpyridine) (P4VP) by taking a series of SERS spectra, during the self-assembly of Au nanoparticles onto the polymer film. In order to better analyze the SERS spectra, we separately conducted quartz crystal microbalance (QCM), UV/vis spectroscopy, and atomic force microscope (AFM) measurements. The adsorption kinetics revealed by QCM under the in situ conditions was in fair agreement with that determined by the ex situ AFM measurement. The number of Au nanoparticles adsorbed on P4VP increased almost linearly with time: 265 Au nanoparticles per 1 μm 2 were adsorbed on the P4VP film after 6 h of immersion. The SERS signal measured in the ex situ condition showed a more rapid increase than that of QCM; however, its increasing pattern was quite similar to that of UV/vis absorbance at longer wavelengths, suggesting that Au nanoparticles actually became agglomerated on P4VP.

Silicon nanotips formed by self-assembled Au nanoparticle mask

The Au nanoparticle monolayer is formed by self-assembly technology on the Si substrates terminated with different functional groups. Silicon nanotips were fabricated by a self-assembled gold colloidal particle monolayer as an etch mask. The silicon nanotips with high density and uniformity in height and shape were obtained using reactive ion etching (RIE). The Si nanotips on the surface of the 3-aminopropyltrimethoxysilane (APTMS)-treated Si substrate are less-ordered array and uniformity than 3-mercaptopropyltrimethoxysilane (MPTMS)-treated Si substrate at the same etching conditions. The ordered array and uniformity of Si nanotips on the APTMS-modified Si substrate was improved through heat-treatment. This result is implied the different functional groups on the Si surfaces could affect the formation of the Si nanostructures during RIE process. The uniformly nanotip pattern with height of >20 nm is obtained on the etched nanoparticle-coated Si substrate. This method can be applied to patterning a wide variety of thin film materials into tip arrays.

Formation of Self-Assembled Au Nanoparticles and the Study of Their Optical Properties by Steady-State and Time-Resolved Spectroscopies

Here, we report the formation of self-assembled Au nanoparticles using different capping agents and study their optical properties by steady-state and time-resolved spectroscopies. The generation of a second surface plasmon band at 680 nm, with increasing concentration of the capping agent, reveals the formation of the self-assembled structure of Au nanoparticles, which is further confirmed by TEM and time-resolved anisotropy studies. A mechanism for self-assembled Au nanoparticles is also proposed. Quenching efficiency of isolated Au nanoparticles is different from assembled Au nanoparticles, which varies with capping agent concentrations. The calculated energy transfer efficiencies from Rhodamine 6G to Au nanoparticles are 35.6, 21.7, and 10.1%, and the measured distances between the dye and Au nanoparticles are 91.7, 108.9, and 136.5 Å for 50 μM, 75 μM, and 125 μM concentrations of 3-mercaptopropionic acid, respectively, which are very nicely matched with the thickness of molecular coverage on Au nanoparticles.

Acetanilide mediated reversible assembly and disassembly of Au nanoparticles

Herein we report the generation of Au nanoparticles (NPs) by sparingly soluble acetanilide in water. We also report the formation of linear chain-like superstructures of self-assembled Au NPs, in the presence of excess acetanilide. This was achieved in two different ways. In the first method, acetanilide was added, with increasing concentration, into aqueous HAuCl 4 to produce Au NPs as well as for the formation of assembly, which varied according to the concentration of acetanilide. The other route involved formation of spherical Au NPs at the lowest concentration of acetanilide, which was followed by the formation of assembly of various lengths upon further addition of variable amount of acetanilide. The assemblies were stable in aqueous solution for days with characteristic UV–vis absorption spectra consisting of two peaks. While the wavelength of the first peak remained the same, the position of the second peak changed to longer wavelength with increasing acetanilide concentration. Interestingly, the linear chain-like arrays could be broken into individual particles by first dilution of the solution concentration followed by treatment with ultrasonic waves. The individual Au NPs again formed linear chain-like arrays upon addition of excess acetanilide.

The effect of Au amount on size uniformity of self-assembled Au nanoparticles

The self-assembled fabrication of nanostructure, a dreaming approach in the area of fabrication engineering, is the ultimate goal of this research. A finding was proved through previous research that the size of the self-assembled gold nanoparticles could be controlled with the mole ratio between AuCl4- and thiol. In this study, the moles of Au were fixed, only the moles of thiol were adjusted. Five different mole ratios of Au/S with their effect on size uniformity were investigated. The mole ratios were 1:1/16, 1:1/8, 1:1, 1:8, 1:16, respectively. The size distributions of the gold nanoparticles were analyzed by Mac-View analysis software. HR-TEM was used to derive images of self-assembled gold nanoparticles. The result reached was also the higher the mole ratio between AuCl4- and thiol the bigger the self-assembled gold nanoparticles. Under the condition of moles of Au fixed, the most homogeneous nanoparticles in size distribution derived with the mole ratio of 1:1/8 between AuCl4- and thiol. The obtained nanoparticles could be used, for example, in uniform surface nanofabrication, leading to the fabrication of ordered array of quantum dots.

Spectroscopic observation of chemical change during molecular electronics experiments

A molecular electronics test platform is presented that combines spectroscopic and electrical characterizations of test molecules. Two-dimensional arrays of 12nm gold particles are formed between the gaps of addressable interdigitated electrodes on an oxidized silicon substrate. Molecules of interest are introduced onto the self-assembled Au nanoparticles by surface exchange in an ethanol solution. Conductivity changes of four to six orders of magnitude are observed. Molecular exchange is verified by the surface-enhanced Raman spectroscopy on the same test specimen. This technique also reveals real-time, irreversible voltage-induced chemical change during testing.

Electrical bistability by self-assembled gold nanoparticles in organic diodes

Electrical bistability is reported in metal-organic-metal diodes. The device consists of two Al electrodes separated by a layer of 2-amino-4,5-imidazoledicarbonitrile that contains embedded Au nanoparticles (NPs) supported by parylene nanopillars. Electrical characterization of the device shows two well-defined states with high (off) and low (on) impedances. The on/off ratio is about 104. This conspicuous memory effect is rationalized in terms of charge storage mediated by the NP states. The fabrication method is general and provides a good control on both the size uniformity and the self-assembly of Au NPs embedded in the organic materials.

Water-Dissolvable Sodium Sulfate Nanowires as a Versatile Template for the Fabrication of Polyelectrolyte- and Metal-Based Nanotubes

This study presents the synthesis of water-dissolvable sodium sulfate nanowires, where Na(2)SO(4) nanowires were produced by an easy reflux process in an organic solvent, N,N-dimethylformamide (DMF) and formed from the coexistence of AgNO(3), SnCl(2), dodecylsodium sulfate (SDS), and cetyltrimethylammonium bromide (CTAB). Na(2)SO(4) nanowires were derived from SDS, and the morphology control of the Na(2)SO(4) nanowires was established by the cooperative effects of Sn and NO(3)(-), while CTAB served as the template and led to homogeneous nanowires with a smooth surface. Since the as-synthesized sodium sulfate nanowires are readily dissolved in water, these nanowires can be treated as soft templates for the fabrication of nanotubes by removing the Na(2)SO(4) core. This process is therefore significantly better than other reported methodologies to remove the templates under harsh condition. We have demonstrated the preparation of biocompatible polyelectrolyte (PE) nanotubes using a layer-by-layer (LbL) method on the Na(2)SO(4) nanowires and the formation of Au nanotubes by the self-assembly of Au nanoparticles. In both nanotube synthesis processes, PEI (polyethylenimine), PAA (poly(acrylic acid)), and Au nanoparticles served as the building blocks on the Na(2)SO(4) templates, which were then rinsed with water to remove the core templates. This unique water-dissolvable template is anticipated to bring about versatile and flexible downstream applications.

The formation of supported monodisperse Au nanoparticles by UV/ozone oxidation process

The influence of surface preparation of GaAs (100)- and (111)B-oriented substrates on the self-assembly of Au nanoparticles has been investigated. It has been shown that an UV/ozone surface oxidation process, applied prior to the deposition of a thin Au film, enables the formation of well-distributed Au particles that are very uniform in size. Particles of 4.5±1.4nm diameter have been obtained by the appropriate heat treatment of a 1-nm-thick Au film. The general applicability of this approach has been demonstrated by reproducing the results on a Si (111) substrate. The process therefore yielded comparable results independent of the surface orientation. Possible operative mechanisms are discussed including Ostwald ripening, particle migration and coalescence, and Coulomb repulsion of the Au deposition.