Electric‐Field‐Induced Alignment of Block Copolymer/Nanoparticle Blends

External electric fields readily align birefringent block‐copolymer mesophases. In this study the effect of gold nanoparticles on the electric‐field‐induced alignment of a lamellae‐forming polystyrene‐block‐poly(2‐vinylpyridine) copolymer is assessed. Nanoparticles are homogeneously dispersed in the styrenic phase and promote the quantitative alignment of lamellar domains by substantially lowering the critical field strength above which alignment proceeds. The results suggest that the electric‐field‐assisted alignment of nanostructured block copolymer/nanoparticle composites may offer a simple way to greatly mitigate structural and orientational defects of such films under benign experimental conditions.     

Palladium and gold nanoparticle array films formed by using self-assembly of block copolymer

The well-arrayed Pd and Au nanoparticle thin films were successfully prepared by making use of self-assembled PS-b-P4VP block copolymer (BCP) as a mask for the reduction of PdCl2 deposited on glass substrate. The films consisted of spherialcal nanoparticles with an average diameter of about 45 nm. For monitoring the size, shape and array formation of Pd nanopaticle films, this procedure was proved better to the conventional process in which PdCl2 impregnated in the channels of self assembled BCP film is reduced to form nanoparticle array. This observations of Pd nanoparticle array film formation is supported by the AFM and UV-VIS studies of Au nanoparticle array films formed by conventional method.     

Nanoparticle assemblies in thin films of supramolecular nanocomposites

We demonstrated a versatile approach to obtain layered nanoparticle sheets with in-plane hexagonal order and 3-D ordered arrays of single nanoparticle chains in thin films upon blending nanoparticles with block copolymer (BCP)-based supramolecules. Basic understanding on the thermodynamic and kinetic aspects of the assembly process paved a path to manipulate these assemblies to meet demands in nanoparticle-based device fabrication and understand structure-property correlations.     

Organization of nanoparticles on hard substrates using block copolymer films as templates

We present a technique for the organization of pre-synthesized nanoparticles on hard substrates, using block copolymer films as sacrificial templates. A thin block copolymer film is dip-coated on the substrate of interest and the sample is exposed to a solution containing nanoparticles. Spontaneous preferential adsorption of the nanoparticles on one phase of the block copolymer film results in their lateral organization. An oxygen plasma etch is used to remove the polymer film; the nanoparticles end up organized on the substrate. We demonstrate that this is a general approach for the patterning of inorganic nanoparticles on hard substrates, showing the organization of metal and semiconductor nanoparticles having different chemistries at the particle/solvent and solvent/polymer interfaces. The nanoparticle patterns that we present have typical periodicities in the nanometer scale. In some cases, microcontact printing is used to create a double length scale of organization, on the micrometer and on the nanometer level. The characteristic periodicity of the template is studied with respect to the nanoparticle size in order to optimize the organization. Finally, we describe how to extend this technique for the production of continuous gold nanowires on hard substrates. We expect that the flexibility of this approach and the degree of control that can be obtained over nanoparticle organization should make it a powerful tool for nanoscale fabrication.     

高致密度FeNi纳米颗粒薄膜制备与性能研究

传统磁控溅射装置制备的纳米颗粒薄膜粒径不均一并且实现粒径大小调控比较困难。本研究采用电场辅助沉积技术,在沉积平台施加5~30 kV的电场,以Si(100)为衬底制备了一系列纳米颗粒粒径均一的高致密度FeNi纳米颗粒薄膜材料。通过XRD、SEM以及VSM测量,研究了不同沉积电场下FeNi纳米颗粒薄膜的结构、形貌和磁性能。利用单端口短路微带线法对0.5~5.5 GHz范围内的微波磁谱进行表征。实验结果表明:沉积电场越大,纳米颗粒粒径均一且薄膜的致密度越高,越有利于薄膜综合磁性能的改善和饱和磁化强度增大。薄膜材料微波磁谱表明,电场辅助沉积技术制备的软磁薄膜材料能够在GHz频段得到广泛应用。     

A fluorescence-based method for determining the surface coverage and hybridization efficiency of thiol-capped oligonucleotides bound to gold thin films and nanoparticles

Using a fluorescence-based method, we have determined the number of thiol-derivatized single-stranded oligonucleotides bound to gold nanoparticles and their extent of hybridization with complementary oligonucleotides in solution. Oligonucleotide surface coverages of hexanethiol 12-mer oligonucleotides on gold nanoparticles (34 +/- 1 pmol/cm2) were significantly higher than on planar gold thin films (18 +/- 3 pmol/cm2), while the percentage of hybridizable strands on the gold nanoparticles (1.3 +/- 0.3 pmol/cm2, 4%) was lower than for gold thin films (6 +/- 2 pmol/cm2, 33%). A gradual increase in electrolyte concentration over the course of oligonucleotide deposition significantly increases surface coverage and consequently particle stability. In addition, oligonucleotide spacer sequences improve the hybridization efficiency of oligonucleotide-modified nanoparticles from approximately 4 to 44%. The surface coverage of recognition strands can be tailored using coadsorbed diluent oligonucleotides. This provides a means of indirectly controlling the average number of hybridized strands per nanoparticle. The work presented here has important implications with regard to understanding interactions between modified oligonucleotides and metal nanoparticles, as well as optimizing the sensitivity of gold nanoparticle-based oligonucleotide detection methods.     

Mesoporous block copolymer nanorods by swelling-induced morphology reconstruction

Engineering the topography of thin block copolymer (BCP) films by surface reconstruction associated with selective swelling of one of the blocks has been investigated intensively. Here we show that swelling-induced structural transitions in nanorods consisting of amphiphilic BCPs involve pronounced reshaping of the nonswollen glassy domains in the course of the transition from the equilibrium morphology of the molten BCP in cylindrical confinement to that of the BCP dissolved in the swelling agent. The reconstruction process can be quenched to retain intermediate nonequilibrium morphologies. The collapse of the swollen chains upon drying yields polymeric nanorods exhibiting complex nanoscopic architectures characterized by a variety of mesopore structures and surface topographies, including channels along the nanorods, bunches of partially interconnected strands, and strings of spheres. The complex BCP nanorods thus obtained can be used as soft templates for the rational arrangement of metal nanoparticles.     

Directed Self‐Assembly of Star‐Block Copolymers by Topographic Nanopatterns through Nucleation and Growth Mechanism

Exploring the ordering mechanism and dynamics of self‐assembled block copolymer (BCP) thin films under confined conditions are highly essential in the application of BCP lithography. In this study, it is aimed to examine the self‐assembling mechanism and kinetics of silicon‐containing 3‐arm star‐block copolymer composed of polystyrene (PS) and poly(dimethylsiloxane) blocks as nanostructured thin films with perpendicular cylinders and controlled lateral ordering by directed self‐assembly using topographically patterned substrates. The ordering process of the star‐block copolymer within fabricated topographic patterns with PS‐functionalized sidewall can be carried out through the type of secondary (i.e., heterogeneous) nucleation for microphase separation initiated from the edge and/or corner of the topographic patterns, and directed to grow as well‐ordered hexagonally packed perpendicular cylinders. The growth rate for the confined microphase separation is highly dependent upon the dimension and also the geometric texture of the preformed pattern. Fast self‐assembly for ordering of BCP thin film can be achieved by lowering the confinement dimension and also increasing the concern number of the preformed pattern, providing a new strategy for the design of BCP lithography from the integration of top‐down and bottom‐up approaches.     

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.     

The effect of perpendicular electric fields on the orientation of zinc oxide crystals during the oxidation of zinc thin films

The effect of direct current (d.c.) electric fields on the crystal orientation of zinc oxide thin films was investigated. Evaporated zinc thin films were oxidized in air under the application of perpendicular electric fields (detached electrodes) at different temperatures. The application of positive fields at 550 °C improved thec-axis orientations up to 1000 V cm^–1. Further increase in field strength caused thec-axis orientations to decline to their original values (no applied field). The application of negative normal fields at 550 °C deteriorated thec-axis orientations up to 800 V cm^–1. Thec-axis orientations improved to their original values by increasing the field strength to greater than 800 V cm^–1. The crystal size remained unchanged, but the surface morphology was affected by the application of the electric fields at temperatures above the melting point of zinc. No significant change in optical properties was detected for samples that were subjected to electric fields. Whenever thec-axis orientation improved, crystals on the film surface became rounded and a more ordered microstructure was observed. On the other hand, the deterioration ofc-axis orientation was manifested by the increase in the number of whisker-shaped crystal needles on the surface of the ZnO thin films.     

How to Control AFM Nanoxerography for the Templated Monolayered Assembly of 2 nm Colloidal Gold Nanoparticles

This paper reports on the directed monolayered assembly of 2 nm colloidal gold nanoparticles onto charge patterns written by atomic force microscopy (AFM) on poly(methylmethacrylate) thin films with a sub-100-nm spatial resolution. The impact of key experimental parameters (surface potential of charge patterns, immersion time in the colloidal solution, nanoparticle concentration, rinsing time) on the nanoparticle assembly was quantified for the first time. This study reveals that the high level of control of this so-called AFM nanoxerography process will allow one to construct promising colloid-based devices integrating localized nanoparticle monolayers of desired density.     

Cover Picture: Crosslinked Nanoparticle Stripes and Hexagonal Networks Obtained via Selective Patterning of Block Copolymer Thin Films (Adv. Mater. 18/2005)

The construction and fixation of ordered nanoparticle arrays allow not only the exploitation of the array's collective physical properties but also the possibility of manipulating discrete aggregates within a larger‐scale assembly scheme. A simple approach utilizing block copolymer thin films as templates and coordination chemistry as a mild crosslinking mechanism is reported by Shenhar, Rotello, and co‐workers on p. 2206. The cover image (by Nicholas Fischer) shows terpyridine‐functionalized gold nanoparticles organized in stripes on top of a microphase‐separated thin film of polystyrene‐block‐poly(methyl methacrylate) and crosslinked thorough the formation of iron–bisterpyridine complexes.     

Fabrication and characterization of transparent superhydrophilic/superhydrophobic silica nanoparticulate thin films

The realization of transparent and superhydrophilic/superhydrophobic surfaces by silica nanoparticulate thin films was exploited in this work. An aqueous electrostatic layer-by-layer assembly process was utilized to fabricate nanoparticulate thin films with adhesion/body/top layer structure on glass substrates by using SiO₂ nanoparticles and polyelectrolytes. The effects of volume ratio of differently sized silica nanoparticle solutions for the body layer deposition on transmittance in visible light region and surface wettability of the nanoparticulate thin films were systematically studied. The experimental results revealed that both optical transparency and superhydrophobicity/superhydrophilicity can be achieved on the same SiO₂ nanoparticulate thin film by using appropriate volume ratios of differently sized silica nanoparticle solutions for body layer deposition, and with and without silane treatment in the fabrication process. The high contrast of wettability that can be achieved by this way suggests the possibility of the creation of superhydrophilic/superhydrophobic patterning and superhydrophilic-superhydrophobic gradient on the same surfaces.     

Tunable electronic interfaces between bulk semiconductors and ligand-stabilized nanoparticle assemblies

Interfaces between nanoscale and bulk electroactive materials are important for the design of electronic devices using solution-processed nanoparticles. We report that thin films of hexanethiolate-capped gold nanoparticles with a core diameter of 2.1+/-0.4 nm deposited onto n-InP wafers form Schottky contacts whose barrier height can be actively tuned from 0.27+/-0.03 to 1.11+/-0.09 eV by electrochemically adjusting the nanoparticle Fermi level. This result is remarkable because interfacial barriers at conventional metal-semiconductor contacts are largely insensitive to the initial Fermi level of the metal. Furthermore, it highlights two general features of solution-processed nanoparticle assemblies in comparison with traditional bulk electronic materials: (1) the ability of ions to permeate the nanoparticle assembly enables the electrochemical injection of charges and hence active control of the Fermi level, and (2) ligand passivation of nanoparticle surfaces prevents interfacial reactions with the semiconductor that could otherwise lead to strong Fermi-level pinning.     

Synthesis and characterization of polyimide-based hybrid thin films from a novel colloidal core-shell nanocomposite particles

In this study, poly(4,4-(hexafluoroisopropylidenediphthalic anhydride)-co-oxydianiline) (6FDA-ODA) and a novel core-shell nanoparticle consisting of a core (SnO2/TiO2) and a shell (ZrO2/Sb2O3) with the composition (SnO2:TiO2:ZrO2:Sb2O3 = 18:5:3:4) were used to prepare polyimide/nanoparticles hybrid thin films. The resultant hybrid thin films were investigated by FTIR, TGA, DSC, TEM, SEM, AFM, alpha-step, UV-Vis, and n&k analyses. The results show that the prepared hybrid thin films had a good thermal stability. The size of nanoparticles was effectively controlled in the range of 8-10 nm in the hybrid thin films. These nanoparticles were evenly distributed across the hybrid thin films and no phase separation occurred. In terms of the optical properties, the prepared hybrid thin films had good transparency in the range of visible light. The cutoff wavelength had a blue shift as the content of the nanoparticles increased. The refractive index of prepared hybrid thin films increased with corresponding increases in nanoparticle content. Moreover, the prepared polyimide/core-shell nanoparticle hybrid thin films displayed excellent film formability and planarity.     

Enhancing Tungsten Oxide Gasochromism with Noble Metal Nanoparticles: The Importance of the Interface

Crystalline tungsten trioxide (WO₃) thin films covered by noble metal (gold and platinum) nanoparticles are synthesized via wet chemistry and used as optical sensors for gaseous hydrogen. Sensing performances are strongly influenced by the catalyst used, with platinum (Pt) resulting as best. Surprisingly, it is found that gold (Au) can provide remarkable sensing activity that tuned out to be strongly dependent on the nanoparticle size: devices sensitized with smaller nanoparticles display better H₂ sensing performance. Computational insight based on density functional theory calculations suggested that this can be related to processes occurring specifically at the Au nanoparticle-WO₃ interface (whose extent is in fact dependent on the nanoparticle size), where the hydrogen dissociative adsorption turns out to be possible. While both experiments and calculations single out Pt as better than Au for sensing, the present work reveals how an exquisitely nanoscopic effect can yield unexpected sensing performance for Au on WO₃, and how these performances can be tuned by controlling the nanoscale features of the system.     

Reversible Switching of Block Copolymer Nanopatterns by Orthogonal Electric Fields

It is demonstrated that the orientation of striped patterns can be reversibly switched between two perpendicular in‐plane orientations upon exposure to electric fields. The results on thin films of symmetric polystyrene‐block‐poly(2‐vinyl pyridine) polymer in the intermediate segregation regime disclose two types of reorientation mechanisms from perpendicular to parallel relative to the electric field orientation. Domains orient via grain rotation and via formation of defects such as stretched undulations and temporal phase transitions. The contribution of additional fields to the structural evolution is also addressed to elucidate the generality of the observed phenomena. In particular solvent effects are considered. This study reveals the stabilization of the meta‐stable in‐plane oriented lamella due to sequential swelling and quenching of the film. Further, the reorientation behavior of lamella domains blended with selective nanoparticles is addressed, which affect the interfacial tensions of the blocks and hence introduce another internal field to the studied system. Switching the orientation of aligned block copolymer patterns between two orthogonal directions may open new applications of nanomaterials as switchable electric nanowires or optical gratings.     

Positionally defined, binary semiconductor nanoparticles synthesized by scanning probe block copolymer lithography

We report the first method for synthesizing binary semiconductor materials by scanning probe block copolymer lithography (SPBCL) in desired locations on a surface. In this work, we utilize SPBCL to create polymer features containing a desired amount of Cd(2+), which is defined by the feature volume. When they are subsequently reacted in H(2)S in the vapor phase, a single CdS nanoparticle is formed in each block copolymer (BCP) feature. The CdS nanoparticles were shown to be both crystalline and luminescent. Importantly, the CdS nanoparticle sizes can be tuned since their diameters depend on the volume of the originally deposited BCP feature.     

Interferometric investigation and simulation of refractive index in glass matrixes containing nanoparticles of varying sizes

The relationship between refractive index and nanoparticle radii of cadmium selenide (CdSe) nanoparticles embedded within glass matrixes was investigated experimentally and by simulations. A homemade automated Michelson interferometer arrangement employing a rotating table and a He-Ne laser source at a wavelength of 632.8 nm determined the refractive index versus nanoparticle radii of embedded cadmium selenide (CdSe) nanoparticles. The refractive index was found to decrease linearly with nanoparticle radius increase. However, one sample showed a step increase in refractive index; on spectroscopic analysis, it was found that its resonant wavelength matched that of the He-Ne source wavelength. The simulations showed that two conditions caused the step increase in refractive index: low plasma frequency and matched sample and source resonances. This simple interferometer setup defines a new method of determining the radii of nanoparticles embedded in substrates and enables refractive index tailoring by modification of exact annealing conditions.     

Electron Transport Through Thiolized Gold Nanoparticles in Single-Electron Transistor

We propose an analytical parametric model for defining energy spectra of nanoparticles with a number of atoms of up to 3,300. This allows us to perform Monte-Carlo simulations for single-electron transistor (SET) based on gold nanoparticles with a size of up to 5.2 nm at temperatures from 0.1 to 300 K. At the first step, energy spectra were calculated for isomers of gold nanoparticles, consisting of up to 33 gold atoms using methods of quantum mechanics: density functional theory (DFT) with LANL2DZ basis set for “geometry” optimization; unrestricted Hartree–Fock method (UHF)x with SBKJC basis set to evaluate energy parameters of nanoobjects, which include gold atoms with many electrons. It was found that the general structure of the energy spectra changes unsignificantly if the number of atoms is greater than 27. Moreover, the size of the energy gap and the position of energy levels in it are linear functions of one parameter—the total electric charge of the nanoparticle. These features of energy spectra allowed us to perform calculations of the transport characteristics for a real SET using gold nanoparticle as a central conducting island.