Electrostatic funneling for precise nanoparticle placement: a route to wafer-scale integration

Effect of Ionic Strength, Nanoparticle Surface Charge Density, and Template Diameter on Self-Limiting Single-Particle Placement: A Numerical Study

For the bottom-up approach where functional materials are constructed out of nanoscale building blocks (e.g., nanoparticles), it is essential to have methods that are capable of placing the individual nanoscale building blocks onto exact substrate positions on a large scale and on a large area. One of the promising placement methods is the self-limiting single-particle placement (SPP), in which a single nanoparticle in a colloidal solution is electrostatically guided by electrostatic templates and exactly one single nanoparticle is placed on each target position in a self-limiting way. This paper presents a numerical study on SPP, where the effects of three key parameters, (1) ionic strength (IS), (2) nanoparticle surface charge density (σ_NP), and (3) circular template diameter (d), on SPP are investigated. For 40 different parameter sets of (IS, σ_NP, d), a 30 nm nanoparticle positioned at R⃗ above the substrate was modeled in two configurations (i) without and (ii) with the presence of a 30 nm nanoparticle at the center of a circular template. For each parameter set and each configuration, the electrostatic potentials were calculated by numerically solving the Poisson-Boltzmann equation, from which interaction forces and interaction free energies were subsequently calculated. These have identified realms of parameter sets that enable a successful SPP. A few exemplary parameter sets include (IS, σ_NP, d) = (0.5 mM, -1.5 μC/cm², 100 nm), (0.05 mM, -0.5 μC/cm², 100 nm), (0.5 mM, -1.5 μC/cm², 150 nm), and (0.05 mM, -0.8 μC/cm², 150 nm). This study provides clear guidance toward experimental realizations of large-scale and large-area SPPs, which could lead to bottom-up fabrications of novel electronic, photonic, plasmonic, and spintronic devices and sensors.

Writing chemical patterns using electrospun fibers as nanoscale inkpots for directed assembly of colloidal nanocrystals

Applications that range from electronics to biotechnology will greatly benefit from low-cost, scalable and multiplex fabrication of spatially defined arrays of colloidal inorganic nanocrystals. In this work, we present a novel additive patterning approach based on the use of electrospun nanofibers (NFs) as inkpots for end-functional polymers. The localized grafting of end-functional polymers from spatially defined nanofibers results in covalently bound chemical patterns. The main factors that determine the width of the nanopatterns are the diameter of the NF and the extent of spreading during the thermal annealing process. Lowering the surface energy of the substrates via silanization and a proper choice of the grafting conditions enable the fabrication of nanoscale patterns over centimeter length scales. The fabricated patterns of end-grafted polymers serve as the templates for spatially defined assembly of colloidal metal and metal oxide nanocrystals of varying sizes (15 to 100 nm), shapes (spherical, cube, rod), and compositions (Au, Ag, Pt, TiO2), as well as semiconductor quantum dots, including the assembly of semiconductor nanoplatelets.

Self-assembly of spherical and rod-shaped nanoparticles with full positional control

The controlled positioning of spherical gold nanoparticles and gold nanorods upon self-assembly on a substrate is of great interest for the fabrication of tailored plasmonic devices. Here, an electrostatic approach with a sequential two-step assembly protocol is presented as a cost-effective and high-yield alternative to previously presented, more complex proof of concepts. Three different geometries can be separately produced in large quantities relying on electrostatic attraction and repulsion of the charge-carrying building blocks: a single gold nanoparticle at the tip, the side or on top of a gold nanorod. DLVO theory is used to explain the electrostatic assembly strategy. The process is highly efficient and assembly yields between 79% (at the tip) and 94% (for the nanoparticle at the long side of the nanorod) are achieved.

Feedback control for shaping density distributions of colloidal particles in microfluidic devices

Directed self-assembly has great potential for the precise manufacture of structured materials at the micro/nano-scale. A local particle density often has to be controlled to make the assembly of complicated structures with no defects attainable. However, the control of spatial particle density distributions is challenged by the need for multiple actuators, kinetic trapping and the stochastic nature of self-assembly systems. In this paper, a novel feedback control approach for shaping spatial density distributions of colloidal particles is presented. The control objective is to maintain the ratio of the particle densities of two adjacent regions close to a desired value. A microfluidic device with a triple-parallel microelectrode is fabricated to provide multiple actuators for particle manipulation. The multiple-electrode actuators can be operated flexibly to either direct particles between two adjacent regions or to maintain particles within regions by preventing undesired particle movements. A feedback control scheme is implemented to control the density ratio over a broad range of tested set points. The method is generic and can be extended to include additional parallel electrodes for the control of density distributions at higher resolutions due to a modular design.

Paving Metal-Organic Frameworks with Upconversion Nanoparticles via Self-Assembly

The combination of metal-organic frameworks (MOFs) and luminescent nanomaterials with upconversion characteristics could enable the development of new nanomaterials and applications in information security, optical sensing, and theranostics. However, currently available methods are not ideally suitable for fabricating composites of MOF and upconversion nanomaterial, and incorporating upconversion nanomaterials with MOFs in a controllable manner remains challenging. Here, we demonstrate an in situ self-assembly route to the nanocomposites in which MOFs are homogeneously paved with upconversion nanoparticles. Without additional assistance, this strategy, mainly driven by electrostatic interactions, can be used to incorporate different upconversion nanoparticles with diverse MOFs. The as-synthesized composites can be further used to construct composites with unique structures, such as MOF@upconversion nanoparticles@MOF sandwiched nanocomposites, and would be useful for applications including luminescence-monitored drug delivery, anticounterfeiting, and photodynamic therapy. These findings should shed light on new avenues for fabricating multifunctional composites of MOF and upconversion nanomaterials for varied applications.

Arrays of highly complex noble metal nanostructures using nanoimprint lithography in combination with liquid-phase epitaxy

Current best-practice lithographic techniques are unable to meet the functional requirements needed to enable on-chip plasmonic devices capable of fully exploiting nanostructure properties reliant on a tailored nanostructure size, composition, architecture, crystallinity, and placement. As a consequence, numerous nanofabrication methods have emerged that address various weaknesses, but none have, as of yet, demonstrated a large-area processing route capable of defining organized surfaces of nanostructures with the architectural diversity and complexity that is routinely displayed in colloidal syntheses. Here, a hybrid fabrication strategy is demonstrated in which nanoimprint lithography is combined with templated dewetting and liquid-phase syntheses that is able to realize periodic arrays of complex noble metal nanostructures over square centimeter areas. The process is inexpensive, can be carried out on a benchtop, and requires modest levels of instrumentation. Demonstrated are three fabrication schemes yielding arrays of core-shell, core-void-shell, and core-void-nanoframe structures using liquid-phase syntheses involving heteroepitaxial deposition, galvanic replacement, and dealloying. With the field of nanotechnology being increasingly reliant on the engineering of desirable physicochemical responses through architectural control, the fabrication strategy provides a platform for advancing devices reliant on addressable arrays or the collective response from an ensemble of identical nanostructures.

Feedback control for defect-free alignment of colloidal particles

Precise alignment of small-scale building blocks into specific structural features is important for the manufacture of novel materials. Directed self-assembly is a promising route to align such small-scale building blocks with single-particle resolution. However, reliable alignment via directed self-assembly is challenging due to design uncertainty, randomness and potential disturbances acting on the system. This paper presents an integrated feedback control strategy to align colloidal particles reliably using directed self-assembly with electric field properties as manipulated variables in a microfluidic device. First, the particle density is controlled to make assembly of a defect-free structure attainable. Subsequently, a novel control method for particle alignment is implemented to self-assemble lines with single-particle resolution. The system's ergodicity is restricted systematically to assure that the density-control step at the higher hierarchy restricts the alignment-control step at the lower hierarchy. The method exploits several electrokinetic phenomena and all steps are fully automated. The approach is generic and can in principle be extended to include more density control steps to self-assemble more complicated structures.

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.

Vapor-Phase Nanopatterning of Aminosilanes with Electron Beam Lithography: Understanding and Minimizing Background Functionalization

Electron beam lithography (EBL) is a highly precise, serial method for patterning surfaces. Positive tone EBL resists enable patterned exposure of the underlying surface, which can be subsequently functionalized for the application of interest. In the case of widely used native oxide-capped silicon surfaces, coupling an activated silane with electron beam lithography would enable nanoscale chemical patterning of the exposed regions. Aminoalkoxysilanes are extremely useful due to their reactive amino functionality but have seen little attention for nanopatterning silicon surfaces with an EBL resist due to background contamination. In this work, we investigated three commercial positive tone EBL resists, PMMA (950k and 495k) and ZEP520A (57k), as templates for vapor-phase patterning of two commonly used aminoalkoxysilanes, 3-aminopropyltrimethoxysilane (APTMS) and 3-aminopropyldiisopropylethoxysilane (APDIPES). The PMMA resists were susceptible to significant background reaction within unpatterned areas, a problem that was particularly acute with APTMS. On the other hand, with both APTMS and APDIPES exposure, unpatterned regions of silicon covered by the ZEP520A resist emerged pristine, as shown both with SEM images of the surfaces of the underlying silicon and through the lack of electrostatically driven binding of negatively charged gold nanoparticles. The ZEP520A resist allowed for the highly selective deposition of these alkoxyaminosilanes in the exposed areas, leaving the unpatterned areas clean, a claim also supported by contact angle measurements with four probe liquids and X-ray photoelectron spectroscopy (XPS). We investigated the mechanistic reasons for the stark contrast between the PMMA resists and ZEP520A, and it was found that the efficacy of resist removal appeared to be the critical factor in reducing the background functionalization. Differences in the molecular weight of the PMMA resists and the resulting influence on APTMS diffusion through the resist films are unlikely to have a significant impact. Area-selective nanopatterning of 15 nm gold nanoparticles using the ZEP520A resist was demonstrated, with no observable background conjugation noted in the unexposed areas on the silicon surface by SEM.

Nanoparticle assembly enabled by EHD-printed monolayers

Augmenting existing devices and structures at the nanoscale with unique functionalities is an exciting prospect. So is the ability to eventually enable at the nanoscale, a version of rapid prototyping via additive nanomanufacturing. Achieving this requires a step-up in manufacturing for industrial use of these devices through fast, inexpensive prototyping with nanoscale precision. In this paper, we combine two very promising techniques-self-assembly and printing-to achieve additively nanomanufactured structures. We start by showing that monolayers can drive the assembly of nanoparticles into pre-defined patterns with single-particle resolution; then crucially we demonstrate for the first time that molecular monolayers can be printed using electrohydrodynamic (EHD)-jet printing. The functionality and resolution of such printed monolayers then drives the self-assembly of nanoparticles, demonstrating the integration of EHD with self-assembly. This shows that such process combinations can lead towards more integrated process flows in nanomanufacturing. Furthermore, in-process metrology is a key requirement for any large-scale nanomanufacturing, and we show that Dual-Harmonic Kelvin Probe Microscopy provides a robust metrology technique to characterising these patterned structures through the convolution of geometrical and environmental constraints. These represent a first step toward combining different additive nanomanufacturing techniques and metrology techniques that could in future provide additively nanomanufactured devices and structures.

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.

Plasmonic Nanolenses: Electrostatic Self-Assembly of Hierarchical Nanoparticle Trimers and Their Response to Optical and Electron Beam Stimuli

Asymmetric nanoparticle trimers composed of particles with increasing diameter act as "plasmonic lenses" and have been predicted to exhibit ultrahigh confinement of electromagnetic energy in the space between the two smallest particles. Here we present an electrostatic self-assembly approach for creating gold nanoparticle trimers with an assembly yield of over 60%. We demonstrate that the trimer assembly leads to characteristic red-shifts and show the localization of the relevant plasmon modes by means of cathodoluminescence and electron energy loss spectroscopy. The results are analyzed in terms of surface plasmon hybridization.

Use of Sacrificial Nanoparticles to Remove the Effects of Shot-noise in Contact Holes Fabricated by E-beam Lithography

Nano-patterns fabricated with extreme ultraviolet (EUV) or electron-beam (E-beam) lithography exhibit unexpected variations in size. This variation has been attributed to statistical fluctuations in the number of photons/electrons arriving at a given nano-region arising from shot-noise (SN). The SN varies inversely to the square root of a number of photons/electrons. For a fixed dosage, the SN is larger in EUV and E-beam lithographies than for traditional (193 nm) optical lithography. Bottom-up and top-down patterning approaches are combined to minimize the effects of shot noise in nano-hole patterning. Specifically, an amino-silane surfactant self-assembles on a silicon wafer that is subsequently spin-coated with a 100 nm film of a PMMA-based E-beam photoresist. Exposure to the E-beam and the subsequent development uncover the underlying surfactant film at the bottoms of the holes. Dipping the wafer in a suspension of negatively charged, citrate-capped, 20 nm gold nanoparticles (GNP) deposits one particle per hole. The exposed positively charged surfactant film in the hole electrostatically funnels the negatively charged nanoparticle to the center of an exposed hole, which permanently fixes the positional registry. Next, by heating near the glass transition temperature of the photoresist polymer, the photoresist film reflows and engulfs the nanoparticles. This process erases the holes affected by SN but leaves the deposited GNPs locked in place by strong electrostatic binding. Treatment with oxygen plasma exposes the GNPs by etching a thin layer of the photoresist. Wet-etching the exposed GNPs with a solution of I2/KI yields uniform holes located at the center of indentations patterned by E-beam lithography. The experiments presented show that the approach reduces the variation in the size of the holes caused by SN from 35% to below 10%. The method extends the patterning limits of transistor contact holes to below 20 nm.

Understanding How Charged Nanoparticles Electrostatically Assemble and Distribute in 1-D

The effects of increasing the driving forces for a 1-D assembly of nanoparticles onto a surface are investigated with experimental results and models. Modifications, which take into account not only the particle-particle interactions but also particle-surface interactions, to previously established extended random sequential adsorption simulations are tested and verified. Both data and model are compared against the heterogeneous random sequential adsorption simulations, and finally, a connection between the two models is suggested. The experiments and models show that increasing the particle-surface interaction leads to narrower particle distribution; this narrowing is attributed to the surface interactions compensating against the particle-particle interactions. The long-term advantage of this work is that the assembly of nanoparticles in solution is now understood as controlled not only by particle-particle interactions but also by particle-surface interactions. Both particle-particle and particle-surface interactions can be used to tune how nanoparticles distribute themselves on a surface.

Unclonable Plasmonic Security Labels Achieved by Shadow-Mask-Lithography-Assisted Self-Assembly

An unclonable plasmonic anti-counterfeiting strategy is demonstrated, which involves the use of molecule-embedded metal@silica core-shell nanoparticles as information carriers. A shadow-mask-lithography-assisted self-assembly is developed for the fabrication of the plasmonic security labels. The produced security labels show multiple sets of coding information that are highly unique, technically unreplicable, and can be robustly decoded by portable microscopes within seconds.

Micropatterned charge heterogeneities via vapor deposition of aminosilanes

Aminosilanes are routinely employed for charge reversal or to create coupling layers on oxide surfaces. We present a chemical vapor deposition method to pattern mica surfaces with regions of high-quality aminosilane (3-aminopropyltriethoxysilane, APTES) monolayers. The approach relies on the vapor deposition of an aminosilane through a patterned array of through-holes in a PDMS (poly(dimethylsiloxane)) membrane that acts as a mask. In aqueous solutions the surfaces have regular patterns of charge heterogeneities with minimal topographical variations over large areas. This versatile dry lift-off deposition method alleviates issues with multilayer formation and can be used to create charge patterns on curved surfaces. We identify the necessary steps to achieve high quality monolayers and charge reversal of the underlying mica surface: (1) hexane extraction to remove unreacted PDMS oligomers from the membrane that would otherwise deposit on and contaminate the substrate, (2) oxygen plasma treatment of the top of the membrane surfaces to generate a barrier layer that blocks APTES transport through the PDMS, and (3) low of the vapor pressure of APTES during deposition to minimize APTES condensation at the mica-membrane-vapor contact lines and to prevent multilayer formation. Under these conditions, AFM imaging shows that the monolayers have a height of 0.9 ± 0.2 nm with an increase in height up to 3 nm at the mica-membrane-vapor contact lines. Fluorescence imaging demonstrates pattern fidelity on both flat and curved surfaces, for feature sizes that vary between 6.5 and 40 μm. We verify charge reversal by measuring the double layer forces between a homogeneous (unpatterned) APTES monolayers and a mica surface in aqueous solution, and we characterize the surface potential of APTES monolayers by measuring the double-layer forces between identical APTES surfaces. We obtain a surface potential of +110 ± 6 mV at pH 4.0.

High-throughput fabrication of anti-counterfeiting colloid-based photoluminescent microtags using electrical nanoimprint lithography

This work demonstrates the excellent capability of the recently developed electrical nanoimprint lithography (e-NIL) technique for quick, high-throughput production of well-defined colloid assemblies on surfaces. This is shown by fabricating micron-sized photoluminescent quick response (QR) codes based on the electrostatic directed trapping (so called nanoxerography process) of 28 nm colloidal lanthanide-doped upconverting NaYF4 nanocrystals. Influencing experimental parameters have been optimized and the contribution of triboelectrification in e-NIL was evidenced. Under the chosen conditions, more than 300 000 nanocrystal-based QR codes were fabricated on a 4 inch silicon wafer, in less than 15 min. These microtags were then transferred to transparent flexible films, to be easily integrated onto desired products. Invisible to the naked eye, they can be decoded and authenticated using an optical microscopy image of their specific photoluminescence mapping. Beyond this very promising application for product tracking and the anti-counterfeiting strategies, e-NIL nanoxerography, potentially applicable to any types of charged and/or polarizable colloids and pattern geometries opens up tremendous opportunities for industrial scale production of various other kinds of colloid-based devices and sensors.

Tuning deposition of magnetic metallic nanoparticles from periodic pattern to thin film entrainment by dip coating method

In this work, we report on the self-assembly of bimetallic CoFe carbide magnetic nanoparticles (MNPs) stabilized by a mixture of long chain surfactants. A dedicated setup, coupling dip coating and sputtering chamber, enables control of the self-assembly of MNPs from regular stripe to continuous thin films under inert atmosphere. The effects of experimental parameters, MNP concentration, withdrawal speed, amount, and nature of surfactants, as well as the surface state of the substrates are discussed. Magnetic measurements revealed that the assembled particles were not oxidized, confirming the high potentiality of our approach for the controlled deposition of highly sensitive MNPs.

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.

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.

Design parameters for voltage-controllable directed assembly of single nanoparticles

Techniques to reliably pick-and-place single nanoparticles into functional assemblies are required to incorporate exotic nanoparticles into standard electronic circuits. In this paper we explore the use of electric fields to drive and direct the assembly process, which has the advantage of being able to control the nano-assembly process at the single nanoparticle level. To achieve this, we design an electrostatic gating system, thus enabling a voltage-controllable nanoparticle picking technique. Simulating this system with the nonlinear Poisson-Boltzmann equation, we can successfully characterize the parameters required for single particle placement, the key being single particle selectivity, in effect designing a system that can achieve this controllably. We then present the optimum design parameters required for successful single nanoparticle placement at ambient temperature, an important requirement for nanomanufacturing processes.

Density functional analysis of like-charged attraction between two similarly charged cylinder polyelectrolytes

A systematic theoretical investigation is performed for electrostatic potential of mean force (EPMF) between two similarly charged rods (modeling DNA) immersed in a primitive model electrolyte solution. Two scientific anomalies are disclosed: (i) although a like-charge attraction (LCA) generally becomes stronger with bulk electrolyte concentration, the opposite effect unexpectedly occurs if the two rod surfaces involved are sufficiently charged and (2) contrary to what is often asserted, that the presence of multivalent counterion is necessary to induce the LCA, it is found that the univalent counterion induces the LCA solely only if bulk electrolyte concentration rises sufficiently and the rod surface charge quantities are high. On the basis of the system energetics calculated first by a classical density functional theory in three-dimensional space, a hydrogen-bonding style mechanism is advanced to reveal the origin of the LCA, and by appealing to fairly common-sense concepts such as bond energy, bond length, number of hydrogen bonds formed, and counterion single-layer saturation adsorption capacity, the present mechanism successfully explains the scientific anomalies and effects of counterion and co-ion diameters in eliciting the LCA first investigated in this work. To add weight to the hydrogen-bonding style mechanism, a theoretical investigation is further performed regarding the effects of the rod surface charge density, co-ion valence, relative permittivity of the medium, temperature, nonelectrostatic interion interactions, and rod diameter in modifying the EPMF, and several novel phenomena are first confirmed, which is self-consistently explained by the present hydrogen-bonding style mechanism.

Covalent Coupling of Nanoparticles with Low-Density Functional Ligands to Surfaces via Click Chemistry

We demonstrate the application of the 1,3-dipolar cycloaddition (“click” reaction) to couple gold nanoparticles (Au NPs) functionalized with low densities of functional ligands. The ligand coverage on the citrate-stabilized Au NPs was adjusted by the ligand:Au surface atom ratio, while maintaining the colloidal stability of the Au NPs in aqueous solution. A procedure was developed to determine the driving forces governing the selectivity and reactivity of citrate-stabilized and ligand-functionalized Au NPs on patterned self-assembled monolayers. We observed selective and remarkably stable chemical bonding of the Au NPs to the complimentarily functionalized substrate areas, even when estimating that only 1–2 chemical bonds are formed between the particles and the substrate.

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.

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.

pH-responsive one-dimensional periodic relief grating of polymer brush-gold nanoassemblies on silicon surface

In this work, we focus on the fabrication of the nanoassemblies consisting of the poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) brushes and gold nanoparticles (AuNPs). The employed process involves grafting of the PDMAEMA chains on an underlying substrate in a brush conformation followed by the immobilization of surface functionalized AuNPs by means of physical interaction (electrostatic attraction, entanglement, and hydrogen bonding). Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and UV-vis spectroscopy have been employed to characterize the prepared PDMAEMA-AuNP nanoassemblies. Polymer brushes possessing various thicknesses have been found to suppress the nanoparticles' aggregation and, hence, facilitate the surface coverage. Furthermore, we patterned the PDMAEMA-AuNP nanoassemblies as an one-dimensional periodic relief grating (OPRG). The subwavelength structure of OPRG has the optical features including artificial refractive index, form birefringence and resonance and band gap effects. A mean refractive index of the PDMAEMA-AuNP nanoassemblies can be controlled by the filling factors of the OPRG structure, so that a desired distribution of refractive index of the polymer brushes-gold OPRG under various stimuli can be realized. The employed approach is simple and highly versatile for the modification of surfaces with a wide range of NPs.

Directed self-assembly of nanogold using a chemically modified nanopatterned surface

Electron-beam lithography (EBL) was used to define an aminosilane nanopatterned surface in order to electrostatically self-assemble gold nanoparticles (Au NPs). The chemically modified nanopatterned surfaces were immersed into a Au NP solution to allow the Au NPs to self-assemble. Equilibrium self-assembly was achieved in only 20 min. The number of Au NPs that self-assembled on an aminosilane dot was controlled by manipulating the diameters of both the Au NPs and the dots. Adding salt to the Au NP solution enabled the Au NPs to self-assemble in greater numbers on the same sized dot. However, the preparation of the Au NP solution containing salt was sensitive to spikes in the salt concentration. These spikes led to aggregation of the Au NPs and non-specific deposition of Au NPs on the substrate. The Au NP patterned surfaces were immersed in a sodium hydroxide solution in order to lift-off the patterned Au NPs, but no lift-off was observed without adequate physical agitation. The van der Waals forces are too strong to allow for lift-off despite the absence of electrostatic forces.

Plasmon-driven selective deposition of au bipyramidal nanoparticles

We demonstrate the plasmon-selective and driven deposition of (bipyramidal) Au nanoparticles on transparent substrates (glass coverslips) utilizing total internal reflection (TIR) illumination. Near-IR laser light undergoing TIR at a glass-water interface causes colloidal Au bipyramids to irreversibly deposit onto the glass surface. We demonstrate that the deposition process has particle (i.e., shape) selectivity that is associated with resonant plasmon excitation. Specifically, the deposition is selective for the bipyramids over spheroidal particles that are also present in solution due to the former's surface plasmon resonance in the near-IR region. Our measurements, finite difference time domain simulations, and the results of an analytical model show that the optical (i.e., scattering and gradient) forces that act on the particles are large and cause the observed acceleration and directed motion of the bipyramids. These directional forces play a major role in the spatial pattern of particle deposition that is observed. In addition, the resonant photothermal heating of the Au bipyramids causes an irreversible loss in colloidal stability, thus allowing them to adhere to the surface. Structural (i.e., scanning electron microscopy) characterization of the deposited bipyramids reveals a slight reduction in aspect ratio relative to the ensemble, consistent with the proposed (heating) mechanism. To our knowledge this is the first demonstration of the plasmon-selective deposition of metal nanoparticles from a heterogeneous mixture.

Real-time template-assisted manipulation of nanoparticles in a multilayer nanofluidic chip

The ability to control dynamically the flow and placement of nanoscale particles and biomolecules in a biocompatible, aqueous environment will have profound impact in advancing the fields of nanoplasmonics, nanophotonics, and medicine. Here, an approach based on electrokinetic forces is demonstrated that enables dynamically controlled placement of nanoparticles into a predefined pattern. The technique uses an applied voltage to manipulate nanoparticles in a multilayer nanofluidic chip architecture. Simulations of the nanoparticles' motion in the nanofluidic chip validate the approach and are confirmed by experimental demonstration to produce uniform 200-nm-diameter spherical nanoparticle arrays. The results are important as they provide a new method that is capable of dynamically capturing and releasing nanoscale particles and biomolecules in an aqueous environment, which could lead to the creation of reconfigurable nanostructure patterns for nanoplasmonic, nanophotonic, biological sensing, and drug-delivery applications.

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.

Integration of nanowire devices in out-of-plane geometry

We report the fabrication of arrays of single and multiple out-of-plane nanowire devices on a single substrate, an important step for the fabrication of novel three-dimensional devices and the integration of individually addressable nanowires onto current Si planar technology platforms. Vertical nanowire device fabrication can greatly increase device densities; however integrating such devices into arrays with registry to the substrate requires precise control over the number and position of the nanowires. Here we report the directed assembly of gold nanoparticle seeds into patterned arrays for the growth of nanowires using chemical recognition and electrophoretic methods. Chemical recognition provides highly reproducible control of the position and number of nanoparticles per pattern element and is shown to be in good agreement with a simple electrostatic model. Individually addressed out-of-plane, vapor-liquid-solid grown Ge nanowires with single and multiple nanowires per element are fabricated and electrically characterized.

Modulating two-dimensional non-close-packed colloidal crystal arrays by deformable soft lithography

We report a simple method to fabricate two-dimensional (2D) periodic non-close-packed (ncp) arrays of colloidal microspheres with controllable lattice spacing, lattice structure, and pattern arrangement. This method combines soft lithography technique with controlled deformation of polydimethylsiloxane (PDMS) elastomer to convert 2D hexagonal close-packed (hcp) silica microsphere arrays into ncp ones. Self-assembled 2D hcp microsphere arrays were transferred onto the surface of PDMS stamps using the lift-up technique, and then their lattice spacing and lattice structure could be adjusted by solvent swelling or mechanical stretching of the PDMS stamps. Followed by a modified microcontact printing (microcp) technique, the as-prepared 2D ncp microsphere arrays were transferred onto a flat substrate coated with a thin film of poly(vinyl alcohol) (PVA). After removing the PVA film by calcination, the ncp arrays that fell on the substrate without being disturbed could be lifted up, deformed, and transferred again by another PDMS stamp; therefore, the lattice feature could be changed step by step. Combining isotropic solvent swelling and anisotropic mechanical stretching, it is possible to change hcp colloidal arrays into full dimensional ncp ones in all five 2D Bravais lattices. This deformable soft lithography-based lift-up process can also generate patterned ncp arrays of colloidal crystals, including one-dimensional (1D) microsphere arrays with designed structures. This method affords opportunities and spaces for fabrication of novel and complex structures of 1D and 2D ncp colloidal crystal arrays, and these as-prepared structures can be used as molds for colloidal lithography or prototype models for optical materials.

Precise Ge quantum dot placement for quantum tunneling devices

This study demonstrates the precise placement of Ge quantum dots (QDs) in an SiO2 or Si3N4 matrix in a self-organized manner by thermally oxidizing SiGe in nanostructures. The effectiveness of this method is shown by a variety of geometries including nanotrenches, nanorods and polygonal nanocavities. Modulating the structural geometry and peripheral spacer materials effectively places a single Ge QD in the center of an oxidized SiGe nanostructure or individual QDs at the corners (edges). This study also reports the fabrication of Ge QD single-electron devices that exhibit clear Coulomb staircases and differential conductance oscillations at room temperature.

Self-aligned nanolithography in a nanogap

A self-aligned nanolithography approach is reported to form a nanoscale via hole in a nanogap. Field emission between two opposite electrodes of a nanogap is used to expose the polymer resist within the nanogap region. A via hole pattern forms in the nanogap area after the exposure process. The via hole pattern is obtained by ablating the polymer resists within the nanogap between two nanoelectrodes upon applying a certain bias voltage. The diameter of the via holes can be controlled to have comparable dimension to the nanogap width. Single or array of via holes have been demonstrated with variable diameters from 20 nm to over 100 nm. The self-aligned via nanoholes can be used to deposit functional nanostructures that are precisely aligned with the nanogap electrodes to form electrode-nanoisland-electrode tunneling junction devices. Electrical characterization of such devices shows typical tunneling characteristics at room temperature. These studies demonstrate a general pathway to making electrical contacts to individual nanostructures for functional nanodevice engineering.

CMOS-compatible fabrication of room-temperature single-electron devices

Devices in which the transport and storage of single electrons are systematically controlled could lead to a new generation of nanoscale devices and sensors. The attractive features of these devices include operation at extremely low power, scalability to the sub-nanometre regime and extremely high charge sensitivity. However, the fabrication of single-electron devices requires nanoscale geometrical control, which has limited their fabrication to small numbers of devices at a time, significantly restricting their implementation in practical devices. Here we report the parallel fabrication of single-electron devices, which results in multiple, individually addressable, single-electron devices that operate at room temperature. This was made possible using CMOS fabrication technology and implementing self-alignment of the source and drain electrodes, which are vertically separated by thin dielectric films. We demonstrate clear Coulomb staircase/blockade and Coulomb oscillations at room temperature and also at low temperatures.

UV-light induced fabrication of CdCl2 nanotubes through CdSe/Te nanocrystals based on dimension and configuration control

Since the discovery of WS2 nanotubes in 1992 ( Nature 1992, 360, 444), there have been significant research efforts to synthesize nanotubes and fullerene-like hollow nanoparticles (HNPs) of inorganic materials ( Nat. Nanotechnol. 2006, 1, 103) due to their potential applications as solid lubrications ( J. Mater. Chem. 2005, 15, 1782), chemical sensing ( Adv. Funct. Mater. 2006, 16, 371), drug delivering ( J. Am. Chem. Soc. 2005, 127, 7316), catalysis ( Adv. Mater. 2006, 18, 2561), or quantum harvesting ( Acc. Chem. Res. 2006, 39, 239). Nanotubes can be produced either by rolling up directly from layer compounds ( Nature 2001, 410, 168) or through other mechanisms ( Adv. Mater. 2004, 16, 1497) such as template growth ( Nature 2003, 422, 599) and decomposition ( J. Am. Chem. Soc. 2001, 123, 4841). The Kirkendall effect, a classical phenomenon in metallurgy ( Trans. AIME 1947, 171, 130), was recently exploited to fabricate hollow 0-D nanocrystals ( Science 2004, 304, 711) as well as 1-D nanotubes ( Nat. Mater. 2006, 5, 627). Although the dimension of resulting hollow nanostructures depends on precursors, the hollow nanomaterials can also be organized into various dimensional nanostructures spontaneously or induced by an external field. In this letter, we report, for the first time, the UV-light induced fabrication of the ends-closed 1-D CdCl2 nanotubes from 0-D CdSe solid nanocrystals through the Kirkendall effect and the head-to-end assembled process. Our results demonstrate the possibility to control the dimension (0-D to 1-D) and the configuration (solid to hollow) of nanostructures simultaneously and have implications in fabricating hollow nano-objects from zero-dimensional to multidimensional.

Strategies for controlled placement of nanoscale building blocks

The capability of placing individual nanoscale building blocks on exact substrate locations in a controlled manner is one of the key requirements to realize future electronic, optical, and magnetic devices and sensors that are composed of such blocks. This article reviews some important advances in the strategies for controlled placement of nanoscale building blocks. In particular, we will overview template assisted placement that utilizes physical, molecular, or electrostatic templates, DNA-programmed assembly, placement using dielectrophoresis, approaches for non-close-packed assembly of spherical particles, and recent development of focused placement schemes including electrostatic funneling, focused placement via molecular gradient patterns, electrodynamic focusing of charged aerosols, and others.