One-dimensional arrangement of gold nanoparticles with tunable interparticle distance

Deterministic positioning of few aqueous colloidal quantum dots

Emerging quantum technologies that critically require the integration of quantum emitters on photonic platforms are hindered by the control over their position, quantity, and scalability. Herein, we describe a facile strategy to deposit aqueous silica-coated quantum dots (QDs) in a template of polymethyl methacrylate (PMMA) nanoholes that leverages saturated ethanol vapor drop-casting and subsequent lift-off of the template. Ethanol vapor incorporation into water droplets during the drying process reduces the meniscus contact angle, which increases capillary forces and enhances particle confinement within the pinning contact region. Furthermore, induced Marangoni flow controls the particle transport dynamics inside the droplets, making large-scale deposition possible. Controlling the hole diameter of the template demonstrates changes in the number of QDs per hole, which is consistent with the Poissonian distribution with the best results of ∼40% single-particle yield from an ∼80% total site occupancy. This method employs a simple setup, eliminating the need for intricate optimization, yet offers the potential for deterministic patterning within complex photonic platforms.

Efficient Focusing of Aerosol Particles in the Microchannel under Reverse External Force: A Numerical Simulation Study

Focusing aerosol particles efficiently is of great significance for high-precision aerosol jet printing and detection of the airborne target. A new method was proposed herein to achieve the efficient focusing of aerosol particles in the microchannel by using a reverse external force. Considering the slip at the interface between the gas and the aerosol particle, a numerical model of the particle movement in the microchannel was established and simulations were conducted on the gas-particle two-phase flow in the microchannel under the effect of the reverse external force. The results showed that a suitable reverse external force in a similar order of magnitude to the Stokes force can dramatically increase the velocity difference between the particle and the gas, which significantly enhances the Saffman lift force exerted on the aerosol particle. Eventually, the aerosol particle can be efficiently focused at the center of the microchannel in a short channel length. In addition, the influence of the channel geometry, the magnitude, and the direction of the external force on the particle focusing was also studied. This work is of great significance for the precise detection of aerosol particles and the design of nozzles for aerosol jet printing.

Linear One-Dimensional Assembly of Metal Nanostructures onto an Asymmetric Peptide Nanofiber with High Persistence Length

Self-assembled peptide fibrils have been used extensively to template the organization of metal nanoparticles in a one-dimensional (1D) array. It has been observed that the formation of the 1D arrays with a width of a single or few nanoparticles (viz. 20 nm diameter) is only possible if the templating fibers have comparable diameters (viz. ≤20 nm). Accordingly, until today, all the peptide-based templates enabling such 1D arrays have very low persistence lengths, a property that depends strongly on the diameter of the template, owing to the inherent flexibility of only a few nanometer-wide fibers. Here, we demonstrate the formation of high persistence length 1D arrays templated by a short self-assembling peptide fibril with an asymmetrically distributed charged surface. The asymmetric nature of the peptide fibril allows charge-dependent deposition of the nanoparticles only to the part of the fiber with complementary charges, and the rest of the fibril surface remains free of nanoparticles. Consequently, fibers with a much higher diameter, which will have a higher persistence length, are able to template single or few nanoparticle-wide 1D arrays. Detailed microscopy, molecular dynamics simulations, and crystal structure analysis provide molecular-level insights into fiber asymmetry and its interactions with diverse nanostructures such as gold and magnetic nanoparticles. This study will afford an alternative paradigm for high persistence length 1D array fabrication comparable to DNA nanotechnology and lithography but with tremendous cost-effectiveness.

Unexpected Coulomb Interactions in Nonpolar Solvent for Highly Efficient Nanoxerography of Perovskite Quantum Dots

In this work, an unexpected sign-dependent electrostatic assembly, also known as nanoxerography, of perovskite quantum dots was observed in nonpolar solutions. Electrical force microscope measurements showed that CsPbBr₃ quantum dots carry negative charges and tend to aggregate at the positively charged nanospots via Coulomb interactions despite that they are synthesized and dispersed in a neutral nonpolar solvent. The result was further confirmed by a statistical method developed in this work based on the Gibbs-Boltzmann distribution. More interestingly, we found that the existence of net charges is a common phenomenon for widely used oil-phase synthesized nanoparticles, including Au, Fe₂O₃, and CdSe/ZnS nanoparticles. This is contrary to the common belief and indicates the possibility of highly efficient nanoxerography for functional nanoparticles synthesized in nonpolar solvents.

Deterministic Assembly of Single Sub-20 nm Functional Nanoparticles Using a Thermally Modified Template with a Scanning Nanoprobe

A deterministic assembly technique for single sub-20 nm functional nanoparticles is developed based on nanostructured templates fabricated by hot scanning nanoprobes. With this technique, single nanoparticles including quantum dots, polystyrene fluorescent nanobeads, and gold nanoparticles are successfully assembled into 2D arrays with high yields. Experimental and theoretical analyses show that the key for the high yields is the hot-probe-based template fabrication technique, which creates geometrical nanotraps and modifies their surface energy simultaneously. In addition to single nanoparticle patterning, further experiments demonstrate that this technique is also capable of building complex nanostructures, such as nanoparticle clusters with well-defined shapes and heterogeneously integrated nanostructures consisting of quantum dots and silver nanowires. It opens the door to many important applications.

Enzyme-like electrocatalysis from 2D gold nanograss-nanocube assemblies

Nanotechnology's rapid development of nanostructured materials with disruptive material properties has inspired research for their use as electrocatalysts to potentially substitute enzymes. Herein, a novel electrocatalytic nanomaterial was constructed by growing gold nanograss (AuNG) on 2D nanoassemblies of gold nanocubes (AuNC). The resulting structure (NG@NC) was used for the detection of H₂O₂via its electrochemical reduction. The NG@NC electrode displayed a large active surface area, resulting in improved electron transfer efficiency. On the nanoscale, AuNG maintained its structure, providing high stability and reproducibility of the sensing platform. Our nanostructured electrode showed excellent catalytic activity towards H₂O₂ at an applied potential of -0.5 V vs Ag/AgCl. This facilitated H₂O₂ detection with excellent selectivity in an environment like human urine, and a linear response from 50 µM to 30 mM, with a sensitivity of 100.66 ± 4.0 μA mM^-1 cm^-2. The NG@NC-based sensor hence shows great potential in nonenzymatic electrochemical sensing.

Multiplexed Assembly of Plasmonic Nanostructures Through Charge Inversion on Substrate for Surface Encoding

Plasmonic nanomaterials are excellent and promising building blocks for information encoding and decoding. However, the positioning of multiplexed nanomaterials into recognizable structures remains a major challenge in nanotechnology. Herein, we developed a novel method for fabricating diversified nanostructures through surface charge inversion from amino-modified substrates to carboxyl-modified ones, as well as the corresponding electrostatic-induced assembly of metal nanoparticles. Under optimal conditions, the selected gold nanospheres (NSs) and peanut-like gold nanorods were successively located into patterns of spaced lines on the same substrate. Due to their unique optical properties, these two types of designed nanoarrays exhibited distinct color contrast and spectrum difference under dark-field scattering microscopy. Furthermore, this general strategy can be extended to wide ranges of nanoparticles with different morphologies and compositions for other multifunctional and high-demanding encoding applications.

Directed Chemical Assembly of Single and Clustered Nanoparticles with Silanized Templates

The assembly of nanoscale materials into arbitrary, organized structures remains a major challenge in nanotechnology. Herein, we report a general method for creating 2D structures by combining top-down lithography with bottom-up chemical assembly. Under optimal conditions, the assembly of gold nanoparticles was achieved in less than 30 min. Single gold nanoparticles, from 10 to 100 nm, can be placed in predetermined patterns with high fidelity, and higher-order structures can be generated consisting of dimers or trimers. It is shown that the nanoparticle arrays can be transferred to, and embedded within, polymer films. This provides a new method for the large-scale fabrication of nanoparticle arrays onto diverse substrates using wet chemistry.

Tailoring two-dimensional nanoparticle arrays into various patterns

A simple and effective technique has been developed to fabricate patterns of nanoparticle arrays. Lithographically fabricated structures in resists serve as scissors to tailor two-dimensional nanoparticle arrays on a flat poly(dimethylsiloxane) (PDMS) stamp. The desired patterns of nanoparticle arrays remaining on the PDMS stamp after tailoring can be printed onto solid substrates. Various regular nanoparticle patterns, such as squares, triangles, disks, and pentagons, can be easily prepared using this technique. Arbitrary nanoparticle patterns as complex as Chinese characters have been successfully demonstrated. Moreover, nanoparticle stripes with width ranging from micrometers to quasi single nanoparticle diameter have also been achieved. Nanoparticle stripes have been integrated into electronic devices for transport measurements.

Large-scale fabrication of highly ordered sub-20 nm noble metal nanoparticles on silica substrates without metallic adhesion layers

Periodic noble metal nanoparticles offer a wide spectrum of applications including chemical and biological sensors, optical devices, and model catalysts due to their extraordinary properties. For sensing purposes and catalytic studies, substrates made of glass or fused-silica are normally required as supports, without the use of metallic adhesion layers. However, precise patterning of such uniform arrays of silica-supported noble metal nanoparticles, especially at sub-100 nm in diameter, is challenging without adhesion layers. In this paper, we report a robust method to large-scale fabricate highly ordered sub-20 nm noble metal nanoparticles, i.e., gold and platinum, supported on silica substrates without adhesion layers, combining displacement Talbot lithography (DTL) with dry-etching techniques. Periodic photoresist nanocolumns at diameters of ~110 nm are patterned on metal-coated oxidized silicon wafers using DTL, and subsequently transferred at a 1:1 ratio into anti-reflection layer coating (BARC) nanocolumns with the formation of nano-sharp tips, using nitrogen plasma etching. These BARC nanocolumns are then used as a mask for etching the deposited metal layer using inclined argon ion-beam etching. We find that increasing the etching time results in cone-shaped silica features with metal nanoparticles on the tips at diameters ranging from 100 nm to sub-30 nm, over large areas of 3×3 cm². Moreover, subsequent annealing these sub-30 nm metal nanoparticle arrays at high-temperature results in sub-20 nm metal nanoparticle arrays with ~10¹⁰ uniform particles.

When lithography meets self-assembly: a review of recent advances in the directed assembly of complex metal nanostructures on planar and textured surfaces

One of the foremost challenges in nanofabrication is the establishment of a processing science that integrates wafer-based materials, techniques, and devices with the extraordinary physicochemical properties accessible when materials are reduced to nanoscale dimensions. Such a merger would allow for exacting controls on nanostructure positioning, promote cooperative phenomenon between adjacent nanostructures and/or substrate materials, and allow for electrical contact to individual or groups of nanostructures. With neither self-assembly nor top-down lithographic processes being able to adequately meet this challenge, advancements have often relied on a hybrid strategy that utilizes lithographically-defined features to direct the assembly of nanostructures into organized patterns. While these so-called directed assembly techniques have proven viable, much of this effort has focused on the assembly of periodic arrays of spherical or near-spherical nanostructures comprised of a single element. Work directed toward the fabrication of more complex nanostructures, while still at a nascent stage, has nevertheless demonstrated the possibility of forming arrays of nanocubes, nanorods, nanoprisms, nanoshells, nanocages, nanoframes, core-shell structures, Janus structures, and various alloys on the substrate surface. In this topical review, we describe the progress made in the directed assembly of periodic arrays of these complex metal nanostructures on planar and textured substrates. The review is divided into three broad strategies reliant on: (i) the deterministic positioning of colloidal structures, (ii) the reorganization of deposited metal films at elevated temperatures, and (iii) liquid-phase chemistry practiced directly on the substrate surface. These strategies collectively utilize a broad range of techniques including capillary assembly, microcontact printing, chemical surface modulation, templated dewetting, nanoimprint lithography, and dip-pen nanolithography and employ a wide scope of chemical processes including redox reactions, alloying, dealloying, phase separation, galvanic replacement, preferential etching, template-mediated reactions, and facet-selective capping agents. Taken together, they highlight the diverse toolset available when fabricating organized surfaces of substrate-supported nanostructures.

Tailored Surfaces/Assemblies for Molecular Plasmonics and Plasmonic Molecular Electronics

Molecular plasmonics uses and explores molecule-plasmon interactions on metal nanostructures for spectroscopic, nanophotonic, and nanoelectronic devices. This review focuses on tailored surfaces/assemblies for molecular plasmonics and describes active molecular plasmonic devices in which functional molecules and polymers change their structural, electrical, and/or optical properties in response to external stimuli and that can dynamically tune the plasmonic properties. We also explore an emerging research field combining molecular plasmonics and molecular electronics.

Enhanced Photoresponse of Conductive Polymer Nanowires Embedded with Au Nanoparticles

A conductive polymer nanowire embedded with a 1D Au nanoparticle chain with defined size, shape, and interparticle distance is fabricated which demonstrates enhanced photoresponse behavior. The precise and controllable positioning of 1D Au nanoparticle chain in the conductive polymer nanowire plays a critical role in modulating the photoresponse behavior by excitation light wavelength or power due to the coupled-plasmon effect of 1D Au nanoparticle chain.

Highly Sensitive Electro-Plasmonic Switches Based on Fivefold Stellate Polyhedral Gold Nanoparticles

Electron-photon coupling in metal nanostructures has raised a new trend for active plasmonic switch devices in both fundamental understanding and technological applications. However, low sensitivity switches with an on/off ratio less than 5 have restricted applications. In this work, an electrically modulated plasmonic switch based on a surface-enhanced Raman spectroscopy (SERS) system with a single fivefold stellate polyhedral gold nanoparticle (FSPAuNP) is reported. The reversible switch of the SERS signal shows high sensitivity with an on/off ratio larger than 30. Such a high on/off ratio arises primarily from the plasmonic resonance shift of the FSPAuNP with the incident laser due to the altered free electron density on the nanoparticle under an applied electrochemical potential. This highly sensitive electro-plasmonic switch may enable further development of plasmonic devices.

Large Area Directed Self-Assembly of Sub-10 nm Particles with Single Particle Positioning Resolution

Directed self-assembly of nanoparticles (DSA-n) holds great potential for device miniaturization in providing patterning resolution and throughput that exceed existing lithographic capabilities. Although nanoparticles excel at assembling into regular close-packed arrays, actual devices on the other hand are often laid out in sparse and complex configurations. Hence, the deterministic positioning of single or few particles at specific positions with low defect density is imperative. Here, we report an approach of DSA-n that satisfies these requirements with less than 1% defect density over micrometer-scale areas and at technologically relevant sub-10 nm dimensions. This technique involves a simple and robust process where a solvent film containing sub-10 nm gold nanoparticles climbs against gravity to coat a prepatterned template. Particles are placed individually into nanoscale cavities, or between nanoposts arranged in varying degrees of geometric complexity. Brownian dynamics simulations suggest a mechanism in which the particles are pushed into the template by a nanomeniscus at the drying front. This process enables particle-based self-assembly to access the sub-10 nm dimension, and for device fabrication to benefit from the wealth of chemically synthesized nanoparticles with unique material properties.

Water-Mediated Assembly of Gold Nanoparticles into Aligned One-Dimensional Superstructures

This Article shows that water in ethanol colloids of gold nanoparticles enhances the formation of linear clusters and, more important for applications in electronics, determines their assembly on surfaces. We show by dynamic light scattering that ethanol colloids contain mainly monomers and dimers and that wormlike superstructures are mostly absent, despite UV-vis evidence of aggregation. Water added to the colloid as a cosolvent was found to enhance the number of clusters as well as their average size, confirming its role in linear self-assembly, on the scale of a few particles. Water adsorbed from the atmosphere during coating was also found to be a powerful lever to tune self-assembly on surfaces. By varying the relative humidity, a sharp transition from branched to linear superstructures was observed, showing the importance of water as a cosolvent in the formation of cluster superstructures. We show that one-dimensional superstructures may form due to long-range mobility of precursor clusters on wet surfaces, allowing their rearrangement. The understanding of the phenomenon allows us to statistically align both clusters and resulting superstructures on patterned substrates, opening the way to rapid screening in molecular electronics.

Enabling low amounts of YAG:Ce(3+) to convert blue into white light with plasmonic Au nanoparticles

We report a new strategy to directly attach Au nanoparticles onto YAG:Ce(3+) phosphor via a chemical preparation method, which yields efficient and quality conversion of blue to yellow light in the presence of a low amount of phosphor. Photoluminescent intensity and quantum yield of YAG:Ce(3+) phosphor are significantly enhanced after Au nanoparticle modification, which can be attributed to the strongly enhanced local surface electromagnetic field of Au nanoparticles on the phosphor particle surface. The CIE color coordinates shifted from the blue light (0.23, 0.23) to the white light region (0.30, 0.33) with a CCT value of 6601 K and a good white light CRI value of 78, which indicates that Au nanoparticles greatly improve the conversion efficiency of low amounts of YAG:Ce(3+) in WLEDs.

On-demand and location selective particle assembly via electrophoretic deposition for fabricating structures with particle-to-particle precision

Programmable positioning of 2 μm polystyrene (PS) beads with single particle precision and location selective, "on-demand", particle deposition was demonstrated by utilizing patterned electrodes and electrophoretic deposition (EPD). An electrode with differently sized hole patterns, from 0.5 to 5 μm, was used to illustrate the discriminatory particle deposition events based on the voltage and particle-to-hole size ratio. With decreasing patterned hole size, a larger electric field was required for a particle deposition event to occur in that hole. For the 5 μm hole, particle deposition began to occur at 10 V/cm where as an electric field of 15 V/cm was required for particles to begin depositing in the 2 μm holes. The likelihood of particle depositions continued to increase for smaller sized holes as the electric field increased. Eventually, a monolayer of particles began to form at approximately 20 V/cm. In essence, a voltage threshold was found for each hole pattern of different sizes, allowing fine adjustments in pattern hole size and voltage to control when a particle deposition event took place, even with the patterns on the same electrode. This phenomenon opens a route toward controlled, multimaterial deposition and assembly onto substrates without repatterning of the electrode or complicated surface modification of the particles. An analytical approach using the theories for electrophoresis and dielectrophoresis found the former to be the dominating force for depositing a particle into a patterned hole. Ebeam lithography was used to pattern spherical holes in precise configurations onto electrode surfaces, where each hole accompanied a polystyrene (PS) particle placement and attachment during EPD. The versatility of e-beam lithography was utilized to create arbitrary pattern configurations to fabricate particle assemblies of limitless configurations, enabling fabrication of unique materials assemblies and interfaces.

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 β_D at ES-VRH regime and the activation energy E_a 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.

Spatially confined assembly of nanoparticles

The ability to assemble NPs into ordered structures that are expected to yield collective physical or chemical properties has afforded new and exciting opportunities in the field of nanotechnology. Among the various configurations of nanoparticle assemblies, two-dimensional (2D) NP patterns and one-dimensional (1D) NP arrays on surfaces are regarded as the ideal assembly configurations for many technological devices, for example, solar cells, magnetic memory, switching devices, and sensing devices, due to their unique transport phenomena and the cooperative properties of NPs in assemblies. To realize the potential applications of NP assemblies, especially in nanodevice-related applications, certain key issues must still be resolved, for example, ordering and alignment, manipulating and positioning in nanodevices, and multicomponent or hierarchical structures of NP assemblies for device integration. Additionally, the assembly of NPs with high precision and high levels of integration and uniformity for devices with scaled-down dimensions has become a key and challenging issue. Two-dimensional NP patterns and 1D NP arrays are obtained using traditional lithography techniques (top-down strategies) or interfacial assembly techniques (bottom-up strategies). However, a formidable challenge that persists is the controllable assembly of NPs in desired locations over large areas with high precision and high levels of integration. The difficulty of this assembly is due to the low efficiency of small features over large areas in lithography techniques or the inevitable structural defects that occur during the assembly process. The combination of self-assembly strategies with existing nanofabrication techniques could potentially provide effective and distinctive solutions for fabricating NPs with precise position control and high resolution. Furthermore, the synergistic combination of spatially mediated interactions between nanoparticles and prestructures on surfaces may play an increasingly important role in the controllable assembly of NPs. In this Account, we summarize our approaches and progress in fabricating spatially confined assemblies of NPs that allow for the positioning of NPs with high resolution and considerable throughput. The spatially selective assembly of NPs at the desired location can be achieved by various mechanisms, such as, a controlled dewetting process, electrostatically mediated assembly of particles, and confined deposition and growth of NPs. Three nanofabrication techniques used to produce prepatterns on a substrate are summarized: the Langmuir-Blodgett (LB) patterning technique, e-beam lithography (EBL), and nanoimprint lithography (NPL). The particle density, particle size, or interparticle distance in NP assemblies strongly depends on the geometric parameters of the template structure due to spatial confinement. In addition, with smart design template structures, multiplexed NPs can be assembled into a defined structure, thus demonstrating the structural and functional complexity required for highly integrated and multifunction applications.

Plasmonic polymers unraveled through single particle spectroscopy

Plasmonic polymers are quasi one-dimensional assemblies of nanoparticles whose optical responses are governed by near-field coupling of localized surface plasmons. Through single particle extinction spectroscopy correlated with electron microscopy, we reveal the effect of the composition of the repeat unit, the chain length, and extent of disorder on the energies, intensities, and line shapes of the collective resonances of individual plasmonic polymers constructed from three different sizes of gold nanoparticles. Our combined experimental and theoretical analysis focuses on the superradiant plasmon mode, which results from the most attractive interactions along the nanoparticle chain and yields the lowest energy resonance in the spectrum. This superradiant mode redshifts with increasing chain length until an infinite chain limit, where additional increases in chain length cause negligible change in the energy of the superradiant mode. We find that, among plasmonic polymers of equal width comprising nanoparticles with different sizes, the onset of the infinite chain limit and its associated energy are dictated by the number of repeat units and not the overall length of the polymer. The intensities and linewidths of the superradiant mode relative to higher energy resonances, however, differ as the size and number of nanoparticles are varied in the plasmonic polymers studied here. These findings provide general guidelines for engineering the energies, intensities, and line shapes of the collective optical response of plasmonic polymers constructed from nanoparticles with sizes ranging from a few tens to one hundred nanometers.

Synergistic modulation of surface interaction to assemble metal nanoparticles into two-dimensional arrays with tunable plasmonic properties

A simple strategy based on the synergistic modulation of inter-particle and substrate-particle interaction is applied for the large-scale fabrication of two-dimensional (2D) Au and Ag nanoparticle arrays. The surface charge of the substrate is used to redistribute the double layer electric charges on the particles and to modulate the inter-particle distance within the 2D nanoparticle arrays on the substrate. The resultant arrays, with a wide range of inter-particle distances, display tunable plasmonic properties. It can be foreseen that such 2D nanoparticle arrays possess potential applications as multiplexed colorimetric sensors, integrated devices and antennas. Herein, it is demonstrated that these arrays can be employed as wavelength-selective substrates for multiplexed acquisition of surface-enhanced Raman scattering (SERS) spectra. This simple one step process provides an attractive and low cost strategy to produce high quality and large area 2D ordered arrays with tunable properties.

In situ synthesis of large-area single sub-10 nm nanoparticle arrays by polymer pen lithography

In order to take advantage of the unique properties of nanoparticles in integrated devices, it is desirable to position monodispersed nanoparticles on substrates with controlled placement. Herein, we utilize small molecules such as ethylene glycol (EG) or glycerol to facilitate the delivery of nanoparticle precursors to the substrates in the polymer pen lithography (PPL) process. Subsequently, large-area ordered single nanoparticle arrays, including sub-10 nm Ag nanoparticle, 30 nm Au nanoparticle and 80 nm Fe2O3 nanoparticle arrays have been synthesized in situ with controllable sizes and pitches.

Tunable assembly of gold nanoparticles on nanopatterned poly(ethylene glycol) brushes

The organization of metallic nanoparticles (NPs) into ordered arrays on nanopatterned surfaces is an enabling process to fabricate devices and study the properties of the particles. Tailoring the interaction between NPs and nanopatterns is a necessity to gain a high level of control in this process. Here, nanopatterned poly(ethylene glycol) (PEG) brushes are presented as a platform for the organization of Au NPs on surfaces. The binding of citrate-stabilized Au NPs to the PEG brushes depends on the size of the particles and molecular weight of the brushes: the density of NPs immobilized on the nanopatterns of PEG brushes increases with decreasing the diameter of the particles and increasing the chain length of the brushes. The key aspect of the process is to pattern PEG brushes with high resolution and chemical contrast to provide controllable and specific interaction between Au NPs and nanopatterns at a single particle resolution. The modulation of the number (0-4) of Au NPs (e.g., 30 nm) per patterned feature with a high level of accuracy and the generation of patterned heterostructures that consist of two different sizes (e.g., 40 and 20 nm) of particles constitute two examples showing the capabilities of the presented platform.

Synthesis of fivefold stellate polyhedral gold nanoparticles with {110}-facets via a seed-mediated growth method

New Type of Gold Nanoparticle: A new class of fivefold stellate polyhedral gold nanoparticles (FSPAuNPs) with {110} facets have been synthesized by a seed-mediated growth method without adding surfactant. The size of FSPAuNPs can be simply adjusted from nanoscale to microscale by varying the amount of seeds, which results in a shift of the surface plasmon resonance peak from the visible to the NIR range.

Guided assembly of gold colloidal nanoparticles on silicon substrates prepatterned by charged particle beams

Colloidal gold nanoparticles represent technological building blocks which are easy to fabricate while keeping full control of their shape and dimensions. Here, we report on a simple two-step maskless process to assemble gold nanoparticles from a water colloidal solution at specific sites of a silicon surface. First, the silicon substrate covered by native oxide is exposed to a charged particle beam (ions or electrons) and then immersed in a HF-modified solution of colloidal nanoparticles. The irradiation of the native oxide layer by a low-fluence charged particle beam causes changes in the type of surface-terminating groups, while the large fluences induce even more profound modification of surface composition. Hence, by a proper selection of the initial substrate termination, solution pH, and beam fluence, either positive or negative deposition of the colloidal nanoparticles can be achieved.

Formation of 1D and 2D gold nanoparticle arrays by divalent DNA-gold nanoparticle conjugates

Divalent DNA-AuNP (gold nanoparticle) conjugates comprising two DNA strands at diametrically opposed positions are prepared. Highly linear 1D and tetragonal lattice-like 2D AuNP arrays are constructed using the conjugates and DNA assemblies based on T- and double-crossover motifs and the Holliday junction.

Fabrication of single gold particle arrays with pattern directed electrochemical deposition

A simple and efficient method for fabricating gold nanoparticle (AuNP) arrays is developed. With this method, the AuNP arrays are fabricated by taking an electrochemical deposition (ECD) process on the ITO substrate, which is initially patterned with nanoimprint lithography (NIL). The stamp for NIL is fabricated by the cost-efficient nanosphere lithography (NSL). The size of the AuNPs can be adjusted by varying the potential and duration of ECD. In this work, the diameters of AuNPs are varied from 130 to 420 nm. The AuNP arrays can be readily extended to other conductive substrates, which may be applied for detecting and sensing.

Nanorods of Well-Defined Length and Monodisperse Cross-Section Obtained from Electrostatic Complexation of Nanoparticles with a Semiflexible Biopolymer

We show by combining small-angle X-ray scattering (SAXS) and cryo-transmission electron microscopy (cryo-TEM) that anionic silica nanoparticles (SiNPs) assemble into well-defined 1D cluster when mixed with a dilute solution of semiflexible chitosan polycation. The nanorods are stable in excess of SiNPs and composed of 10 SiNPs well-ordered into straight single strands with length L_rod ≈ 184.0 nm and radius R_rod = 9.2 nm = R_SiNPs. We point out that the ratio between the chitosan persistence length and the SiNP radius, which is here equal to 1, can be the determining condition to obtain such original objects.

Control over position, orientation, and spacing of arrays of gold nanorods using chemically nanopatterned surfaces and tailored particle-particle-surface interactions

The synergy of self- and directed-assembly processes and lithography provides intriguing avenues to fabricate translationally ordered nanoparticle arrangements, but currently lacks the robustness necessary to deliver complex spatial organization. Here, we demonstrate that interparticle spacing and local orientation of gold nanorods (AuNR) can be tuned by controlling the Debye length of AuNR in solution and the dimensions of a chemical contrast pattern. Electrostatic and hydrophobic selectivity for AuNR to absorb to patterned regions of poly(2-vinylpyridine) (P2VP) and polystyrene brushes and mats was demonstrated for AuNR functionalized with mercaptopropane sulfonate (MS) and poly(ethylene glycol), respectively. For P2VP patterns of stripes with widths comparable to the length of the AuNR, single- and double-column arrangements of AuNR oriented parallel and perpendicular to the P2VP line were obtained for MS-AuNR. Furthermore, the spacing of the assembled AuNR was uniform along the stripe and related to the ionic strength of the AuNR dispersion. The different AuNR arrangements are consistent with predictions based on maximization of packing of AuNR within the confined strip.

Site-selective assembly and reorganization of gold nanoparticles along aminosilane-covered nanolines prepared on indium-tin oxide

We have fabricated gold nanoparticle (AuNP) arrays on indium-tin oxide (ITO) substrates in a nearly one-dimensional fashion. AuNPs were site-selectively immobilized on ITO of which the surface had been patterned by a nanolithography process based on scanning probe microscopy. The fabricated nanoscale lines covered with aminosilane self-assembled monolayer served as chemisorption sites for citrate-stabilized AuNPs of 20 nm in diameter, accordingly, AuNP nanolines with a thickness of single nanoparticle diameter were spontaneously assembled on the lines. In this 1D array, the AuNPs were almost separated from each other due to the electrostatic repulsion between their negatively charged surface layers. Furthermore, a reorganization process of the immobilized AuNP arrays has been successfully demonstrated by replacing each AuNP's surface layer from citric acid to dodecanethiol. By this process, the AuNPs lost their electrostatic repulsion and became hydrophobic so as to be attracted to each other through hydrophobic interaction, resulting in reorganization of the AuNP array. By repeating the deposition and reorganization cycle, AuNPs were more densely packed. The optical absorption peak of the arrays due to their plasmonic resonance was found to shift from 526 to 590 nm in wavelength with repeating cycles, indicating that the resonance manner was changed from the single nanoparticle mode to the multiple particle mode with interparticle coupling.

Highly selective immobilization of Au nanoparticles onto isolated and dense nanopatterns of poly(2-vinyl pyridine) brushes down to single-particle resolution

Chemical patterns consisting of poly(2-vinyl pyridine) (P2VP) brushes in a background of a cross-linked polystyrene (PS) mat enabled the highly selective placement of citrate-stabilized Au nanoparticles (NPs) in arrays on surfaces. The cross-linked PS mat prevented the nonspecific binding of Au NPs, and the regions functionalized with P2VP brushes allowed the immobilization of the particles. Isolated chemical patterns of feature sizes from hundreds to tens of nanometers were prepared by standard lithographic techniques. The number of 13 nm Au NPs bound per feature increased linearly with increasing area of the patterns. This behavior is similar to previous reports using 40 nm particles or larger. Arrays of single NPs were obtained by reducing the dimensions of patterned P2VP brushes to below ~20 nm. To generate dense (center-to-center distance = 80 nm) linear chemical patterns for the placement of rows of single NPs, a block-copolymer (BCP)-assisted lithographic process was used. BCPs healed defects associated with the standard lithographic patterning of small dimensions at high densities and led to highly registered, linear, single NP arrays.

Scanning the potential energy surface for synthesis of dendrimer-wrapped gold clusters: design rules for true single-molecule nanostructures

The formation of true single-molecule complexes between organic ligands and nanoparticles is challenging and requires careful design of molecules with size, shape, and chemical properties tailored for the specific nanoparticle. Here we use computer simulations to describe the atomic-scale structure, dynamics, and energetics of ligand-mediated synthesis and interlinking of 1 nm gold clusters. The models help explain recent experimental results and provide insight into how multidentate thioether dendrimers can be employed for synthesis of true single-ligand-nanoparticle complexes and also nanoparticle-molecule-nanoparticle "dumbbell" nanostructures. Electronic structure calculations reveal the individually weak thioether-gold bonds (325 ± 36 meV), which act collectively through the multivalent (multisite) anchoring to stabilize the ligand-nanoparticle complex (∼7 eV total binding energy) and offset the conformational and solvation penalties involved in this "wrapping" process. Molecular dynamics simulations show that the dendrimer is sufficiently flexible to tolerate the strained conformations and desolvation penalties involved in fully wrapping the particle, quantifying the subtle balance between covalent anchoring and noncovalent wrapping in the assembly of ligand-nanoparticle complexes. The computed preference for binding of a single dendrimer to the cluster reveals the prohibitively high dendrimer desolvation barrier (1.5 ± 0.5 eV) to form the alternative double-dendrimer structure. Finally, the models show formation of an additional electron transfer channel between nitrogen and gold for ligands with a central pyridine unit, which gives a stiff binding orientation and explains the recently measured larger interparticle distances for particles synthesized and interlinked using linear ligands with a central pyridine rather than a benzene moiety. The findings stress the importance of organic-inorganic interactions, the control of which is central to the rational engineering and eventual large-scale production of functional building blocks for nano(bio)electronics.

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.

Free-standing one-dimensional plasmonic nanostructures

The field of plasmonics has become one of the most interesting and active research areas in nanotechnology, enabling numerous fundamental studies and applications. The ability to tailor the size, shape, and environment of metal nanostructures is the key component for controlling the plasmonic properties of individual or aggregated nanostructures. In this feature article, a category of chemically nanofabricated, unique free-standing one-dimensional (1D) plasmonic nanostructures has been summarized. The dispersible plasmonic nanostructures were obtained in high yield with control over gap size and feature size. This ability was exploited to tune the emerging plasmonic properties overcoming the difficulties of other methods to do so, leading to applications in analytical detection, biological sensing, and nanoelectronics.

Patterning of plasmonic nanoparticles into multiplexed one-dimensional arrays based on spatially modulated electrostatic potential

We report a new strategy to pattern plasmonic nanoparticles into multiplexed one-dimensional arrays based on the spatially modulated electrostatic potential. The 32 nm Au nanoparticles can be simultaneously deposited on one chip with tunable interparticle distance by solely adjusting the width of the grooves. Furthermore, 32 and 13 nm Au nanoparticles can be selectively deposited in grooves of different widths on one chip. As a result, the surface plasmon absorption bands on the chip can be tuned depending on the interparticle distance or the particle size of multiplex 1D arrays, which could enhance the Raman scattering cross section of the adsorbed molecules and result in multiplex surface-enhanced Raman scattering (SERS) response on the chip. This strategy provides a general method to fabricate 1D multiplex arrays with different particle sizes and interparticle distances on one chip.

Patterned self-assembly of gold nanoparticles on chemical templates fabricated by soft UV nanoimprint lithography

Chemical templates for the patterned immobilization of gold nanoparticles were fabricated by soft UV nanoimprint lithography. The template structures were fabricated by means of the consecutively performed process steps of nanoimprint lithography, reactive ion etching, chemical functionalization with amino groups, and lift-off of imprint resist. These chemical templates were used for the defined assembly of 20 nm diameter citrate stabilized gold nanoparticles from aqueous solution. By reducing the ionic strength of the solution, one- and zero-dimensional particle assemblies were generated on sub-100-nm template structures. By this means, the pattern resolution predefined by the lithography process could be easily enhanced by dilution of the nanoparticle solution.

Scanning probe block copolymer lithography

Integration of individual nanoparticles into desired spatial arrangements over large areas is a prerequisite for exploiting their unique electrical, optical, and chemical properties. However, positioning single sub-10-nm nanoparticles in a specific location individually on a substrate remains challenging. Herein we have developed a unique approach, termed scanning probe block copolymer lithography, which enables one to control the growth and position of individual nanoparticles in situ. This technique relies on either dip-pen nanolithography (DPN) or polymer pen lithography (PPL) to transfer phase-separating block copolymer inks in the form of 100 or more nanometer features on an underlying substrate. Reduction of the metal ions via plasma results in the high-yield formation of single crystal nanoparticles per block copolymer feature. Because the size of each feature controls the number of metal atoms within it, the DPN or PPL step can be used to control precisely the size of each nanocrystal down to 4.8 ± 0.2 nm.

High-resolution, parallel patterning of nanoparticles via an ion-induced focusing mask

An ion-induced focusing mask under the simultaneous injection of ions and charged aerosols generates invisible electrostatic lenses around each opening, through which charged nanoparticles are convergently guided without depositing on the mask surface. The sizes of the created features become significantly smaller than those of the mask openings due to the focusing capability. It is not only demonstrated that material-independent nanoparticles including proteins can be patterned as an ordered array on any surface regardless of the conductive, nonconductive, or flexible nature of the substrate, but also that the array density can be increased. Highly sensitive gas sensors based on these focused nanoparticle patterns are fabricated via the concept.