Close-packed Nanoparticle Arrays Research Articles

Microsphere-Assisted Imaging of Periodic and Non-Periodic Structures

In this work, the difference in imaging small clusters of nanoparticles and hexagonally close packed nanoparticle arrays by microsphere-assisted microscopy is studied. For both high-contrast samples and low-contrast ones, compared with small cluster samples, a better image quality and a larger lateral magnification are observed when imaging periodic array samples. We propose that in imaging periodic samples, the evanescent waves carrying high-frequency information can be converted into propagating waves by shifting the incident wavevectors into different diffraction orders. Therefore, more high-frequency information of the samples can be collected by the imaging system, which results in an improved image quality and a larger magnification when observing periodic array samples.

Reliable SERS detection of pesticides with a large-scale self-assembled Au@4-MBA@Ag nanoparticle array

The fabrication of sensitive and reliable interfacial plasmonic platform for measuring chemical contaminants in various phases is an exciting topic in the food industry and for environment monitoring. In this study, a high-performance surface-enhanced Raman spectroscopy (SERS) analytic platform was developed through self-assembly of the gold@4-mercaptobenzoic acid@silver nanoparticles (Au@4-MBA@Ag NPs) at the cyclohexane/water interface. By addition of the inducer ethanol, the Au@4-MBA@Ag NPs in aqueous phase was effectively migrated to the biphasic interface, forming a large-scale close-packed nanoparticle array. The average gap between adjacent nanoparticles was smaller than 3 nm, where intensive SERS “hot spots” were created for high-sensitive detection. Furthermore, using the sandwiched 4-MBA molecule as the internal standard to correct the Raman signal fluctuations, the point-to-point and batch-to-batch reproducibility of Au@4-MBA@Ag array were improved with lower relative standard deviation (RSD) values of 8.84% and 14.97%, respectively, and pesticides (thiram and thiabendazole) analysis in both aqueous and organic phases were achieved with higher accuracy (R2 of 0.986 and 0.990) as compared with those without 4-MBA correction (R2 of 0.867 and 0.974). The high-throughput fabrication of the self-assembled nanoparticle array is a promising approach for development of a sensitive and reliable SERS platform for chemical contaminants monitoring in multiphase.

Non–close-packed nanoparticle arrays

Nanomaterials![Figure][1] Etching out one class of nanoparticles yields an open nanoarray. ILLUSTRATION: ELLA MARUSHCHENKO Films of colloidal nanoparticles usually form dense, close-packed lattices. If binary lattices could be made and one component removed, then a more open array could form, as long as the remaining nanoparticles could be stabilized. Udayabhaskararao et al. formed binary superlattices of gold and magnetite nanoparticles at an air-liquid interface that could then be transferred to carbon-coated surfaces (see the Perspective by Kotov). Selective etching of either of the nanoparticles created non–close-packed arrays with vacancies stabilized by the carbon surface. Science , this issue p. [514][2]; see also p. [448][3] [1]: pending:yes [2]: /lookup/doi/10.1126/science.aan6046 [3]: /lookup/volpage/358/448?iss=6362

Dimensionality-dependent charge transport in close-packed nanoparticle arrays: from 2D to 3D.

Charge transport properties in close-packed nanoparticle arrays with thickness crossing over from two dimensions to three dimensions have been studied. The dimensionality transition of nanoparticle arrays was realized by continually printing spatially well-defined nanoparticle monolayers on top of the device in situ. The evolution of charge transport properties depending on the dimensionality has been investigated in both the Efros-Shaklovskii variable-range-hopping (ES-VRH) (low temperature) regime and the sequential hopping (SH) (medium temperature) regime. We find that the energy barriers to transport decrease when the thickness of nanoparticle arrays increases from monolayer to multilayers, but start to level off at the thickness of 4–5 monolayers. The energy barriers are characterized by the coefficient βD at ES-VRH regime and the activation energy Ea at SH regime. Moreover, a turning point for the temperature coefficient of conductance was observed in multilayer nanoparticle arrays at high temperature, which is attributed to the increasing mobility with decreasing temperature of hopping transport in three dimensions.

Controllability of the Coulomb charging energy in close-packed nanoparticle arrays

We studied the electronic transport properties of metal nanoparticle arrays, particularly focused on the Coulomb charging energy. By comparison, we confirmed that it is more reasonable to estimate the Coulomb charging energy using the activation energy from the temperature-dependent zero-voltage conductance. Based on this, we systematically and comprehensively investigated the parameters that could be used to tune the Coulomb charging energy in nanoparticle arrays. We found that four parameters, including the particle core size, the inter-particle distance, the nearest neighboring number, and the dielectric constant of ligand molecules, could significantly tune the Coulomb charging energy.

Patterned Close‐Packed Nanoparticle Arrays with Controllable Dimensions and Precise Locations

Patterned close-packed nanoparticle arrays are fabricated using lithography and self-assembly. Microcontact printing is used to selectively transfer ordered nanoparticle monolayers, which are self-assembled at the air/water interface, onto relief structures, which are defined lithographically. The morphology and position of the nanoparticle arrays are determined by the relief structures, while the internal order of the arrays is achieved through the self-assembly process and is maintained during the transfer.

Fabrication of Discrete Polystyrene Nanoparticle Arrays with Controllable Their Structural Parameters

We have demonstrated the fabrication of two-dimensionally periodic non-close packed arrays of spherical polystyrene nanoparticles with controllable their structural parameters including diameter and interpartcile distance. The principle of this procedure relies on stepwise integration of spin-coat-assisted colloidal self-assembly of the single layer of close-packed polystyrene nanoparticle on a substrate, and subsequent etching of the particle under atmospheric pressure helium plasma. The plasma process converted the close-packed nanoparticle array into non-close-packed arrangement remaining with unchanged their original spherical shape and periodicity. Owing to the etching process underwent isotropically, the structural parameters could be controlled with nanometric accuracy by the treatment time. The etching rate strongly depended on the working pressure conditions, and the etching rate under 250 Torr was ca. 3 times faster than that of the 760 Torr. The effects of the working pressure indicated the neutral helium radicals and photons diffused from the plasma might be primarily responsible for the etching.

Fabrication of discrete array of metallodielectric nanoshells and their surface plasmonic properties

In this paper we describe the fabrication of two-dimensionally periodic non-close-packed nanoshell arrays, consisting of a spherical polystyrene core coated with a thin gold shell, as well as their surface plasmonic properties. The principle of this procedure relies on stepwise integration of spin-coat-assisted colloidal self-assembly of the single layer of close-packed polystyrene nanoparticle, atmospheric pressure plasma-induced isotropic etching, and deposition of gold thin film by thermal evaporation. The plasma process converted the close-packed nanoparticle array into non-close-packed arrangement without changing their original spherical shape and periodicity. Both experimental and theoretical studies revealed that the densely packed nanoshell array with a 160 nm inner core diameter and a 20 nm thick shell strongly scattered and absorbed near infrared light, due to the interaction between primitive plasmon modes associated with the surface of the nanoparticle. Furthermore, the resultant nanoshell array was utilized for near infrared light responsible localized surface plasmon resonance based sensor. The bulk refractive index sensitivity was 220 nm RIU − 1 .

Sequential tunneling and inelastic cotunneling in nanoparticle arrays

We investigate transport in weakly coupled metal nanoparticle arrays, focusing on the regime where tunneling is competing with strong single electron charging effects. This competition gives rise to an interplay between two types of charge transport. In sequential tunneling, transport is dominated by independent electron hops from a particle to its nearest neighbor along the current path. In inelastic cotunneling, transport is dominated by cooperative multielectron hops that each go to the nearest neighbor but are synchronized to move charge over distances of several particles. In order to test how the temperature-dependent cotunnel distance affects the current-voltage (I-V) characteristics, we perform a series of systematic experiments on highly ordered close-packed nanoparticle arrays. The arrays consist of ∼5.5 nm diameter gold nanocrystals with tight size dispersion, spaced ∼1.7 nm apart by interdigitating shells of dodecanethiol ligands. We present I-V data for monolayer, bilayer, trilayer, and tetralayer arrays. For stacks 2–4 layers thick we compare in-plane measurements with data for vertical transport perpendicular to the array plane. Our results support a picture whereby transport inside the Coulomb blockade regime occurs by inelastic cotunneling, while sequential tunneling takes over at large bias above the global Coulomb blockade threshold Vt(T) and at high temperatures. For the smallest measurable voltages, our data was fitted well by recent predictions for the temperature dependence zero-bias conductance due to multiple cotunneling. At finite but small bias, the cotunnel distance is predicted to set the curvature of the nonlinear I-V characteristics, in good agreement with our data. The absence of significant magnetic-field dependence up to 10 T in the measured I-V characteristics further supports the picture of inelastic cotunneling events where individual electrons hop no further than the nearest neighbor. At large bias, above the global Coulomb blockade threshold, the I-V characteristics follow power-law behavior with temperature-independent exponent close to two, predicted for sequential tunneling along branching paths that optimize the overall charging energy cost.

Directing Close-Packing of Midnanosized Gold Nanoparticles at a Water/Hexane Interface

This paper reports a method for assembling midnanosized Au nanoparticles (25 < d < 100 nm) into close-packed 2D arrays with a high degree of local order. An in situ coating of Au nanoparticles with an alkanethiol surfactant can induce close-packed particle arrays when the nanoparticles are trapped at the interface. The film morphology was determined to be either monoparticulate or multiparticulate depending on the 1-dodecanethiol concentration. This paper reports on the optimum conditions for the formation of a close-packed 2D array by suggesting a phase diagram obtained from a plot of the 1-dodecanethiol concentration versus the nanoparticle diameter. When the diameter of Au nanoparticles was larger than ca. 40 nm, the full coverage of 1-dodecanethiols on the surface of nanoparticles was not enough to form close-packed nanoparticle arrays. The larger nanoparticles required the more 1-dodecanethiols to form a close-packed 2D array. This method also allows the nanoparticle surface to be coated asymmetrical...

Superhydrophobic Coatings for Microdevices

We have developed two simple techniques to impart superhydrophobic properties to the surfaces of microdevices. In the first approach, thin films of a fluoropolymer were spin-coated on the device surfaces followed by an oxygen plasma treatment. By varying the oxygen plasma treatment time, the water contact angles on device surface could be tuned from 120° to 169°. In the second approach, a nanoimprint process was used to create nanostructures on the devices. To fabricate nanoimprint stamps with various feature sizes, nanosphere lithography was employed to produce a monolayer of well-ordered close-packed nanoparticle array on the silicon surfaces. After oxygen plasma trimming, metal deposition and dry etching process, silicon stamps with different nanostructures were obtained. These stamps were used to imprint nanostructures on hydrophobic coatings, such as Teflon, over the device surfaces. The water contact angle as high as 167° was obtained by the second approach.

Assembly of Highly Ordered Nanoparticle Monolayers at a Water/Hexane Interface

This paper reports a methodology for preparing ordering hydrophilic metal nanoparticles into close-packed 2-dimensional arrays at a hexane-water interface with alkanethiol in the hexane layer. The destabilization of metal nanoparticles by the addition of alcohol caused the nanoparticles to adsorb to an interface where the surface of entrapped Au nanoparticle was in situ coated with the long-chain alkanethiols present in a hexane layer. The adsorption of alkanethiol to the nanoparticle surface caused the conversion of the electrostatic repulsive force to a van der Waals interaction, which is a key feature in forming highly ordered close-packed nanoparticle arrays.

Elastic membranes of close-packed nanoparticle arrays

Nanoparticle superlattices are hybrid materials composed of close-packed inorganic particles separated by short organic spacers. Most work so far has concentrated on the unique electronic, optical and magnetic behaviour of these systems. Here, we demonstrate that they also possess remarkable mechanical properties. We focus on two-dimensional arrays of close-packed nanoparticles and show that they can be stretched across micrometre-size holes. The resulting free-standing monolayer membranes extend over hundreds of particle diameters without crosslinking of the ligands or further embedding in polymer. To characterize the membranes we measured elastic properties with force microscopy and determined the array structure using transmission electron microscopy. For dodecanethiol-ligated 6-nm-diameter gold nanocrystal monolayers, we find a Young's modulus of the order of several GPa. This remarkable strength is coupled with high flexibility, enabling the membranes to bend easily while draping over edges. The arrays remain intact and able to withstand tensile stresses up to temperatures around 370 K. The purely elastic response of these ultrathin membranes, coupled with exceptional robustness and resilience at high temperatures should make them excellent candidates for a wide range of sensor applications.

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.

2D nanoparticle arrays by partial dissolution of ordered pore films

A low cost and simple route was presented to fabricate large-scale ordered nanoparticle arrays by partial dissolution of ordered pore films (or monolayer inverse opal) in a solution. By this method, we have fabricated Fe 2O 3 and In 2O 3 hexagonal close-packed nanoparticle arrays. This method is also applicable for synthesis of other material nanoparticle arrays. The particle size in the arrays can be controlled by the dissolution time, which is, additionally, beneficial to study size dependent optical, magnetic, electrochemical, thermodynamic, catalytic properties of nanoparticls.