Three-dimensional crystalline and homogeneous metallic nanostructures using directed assembly of nanoparticles

Active Assembly of CsPbBr3 Nanorods into Microcolumns by Electric Field in Nonpolar Solvent

High-precision, controllable, mass-producible assembly of nanoparticles into complex structures or devices holds immense importance in the application across various fields but it remains challenging. Here a highly controllable and reversible active assembly of colloidal CsPbBr3 nanorods, driven by an external electric field is achieved. This approach enables the nanorods dynamically orient themselves, assemble into chains, aggregate into columns, and eventually form an ordered column array, with the electric field intensity varying from 0 to 50 V µm-1 at 100 kHz. The nanorods inside the columns align parallel to the electric field, leading to a well-ordered structure. With the analysis of the interactions among the nanorods, a quantitative interpretation of the assembly is proposed. Monte Carlo calculation is also introduced to simulate the assembly process and the results prove to be in great agreement with the experimental observations. This electric field-driven assembly presents an exciting opportunity to pave the way for next-generation sensors and photonic devices based on well-developed colloidal nanoparticles.

Electrophoretic Deposition of Single Nanoparticles

The controlled assembly of colloid particles on a solid substrate has always been a major challenge in colloid and surface science. Here we provide an overview of electrophoretic deposition (EPD) of single charge-stabilized nanoparticles. We demonstrate that surface templated EPD (STEPD) assembly, which combines EPD with top-down nanofabrication, allows a wide range of nanoparticles to be built up into arbitrary structures with high speed, scalability, and excellent fidelity. We will also discuss some of the current colloid chemical limitations and challenges in STEPD assembly for sub-10 nm nanoparticles and for the fabrication of densely packed single particle arrays.

Directed Assembly of Nanomaterials for Making Nanoscale Devices and Structures: Mechanisms and Applications

Nanofabrication has been utilized to manufacture one-, two-, and three-dimensional functional nanostructures for applications such as electronics, sensors, and photonic devices. Although conventional silicon-based nanofabrication (top-down approach) has developed into a technique with extremely high precision and integration density, nanofabrication based on directed assembly (bottom-up approach) is attracting more interest recently owing to its low cost and the advantages of additive manufacturing. Directed assembly is a process that utilizes external fields to directly interact with nanoelements (nanoparticles, 2D nanomaterials, nanotubes, nanowires, etc.) and drive the nanoelements to site-selectively assemble in patterned areas on substrates to form functional structures. Directed assembly processes can be divided into four different categories depending on the external fields: electric field-directed assembly, fluidic flow-directed assembly, magnetic field-directed assembly, and optical field-directed assembly. In this review, we summarize recent progress utilizing these four processes and address how these directed assembly processes harness the external fields, the underlying mechanism of how the external fields interact with the nanoelements, and the advantages and drawbacks of utilizing each method. Finally, we discuss applications made using directed assembly and provide a perspective on the future developments and challenges.

High-Rate Printing of Micro/Nanoscale Patterns Using Interfacial Convective Assembly

Printing of electronics has been receiving increasing attention from academia and industry over the recent years. However, commonly used printing techniques have limited resolution of micro- or sub-microscale. Here, a directed-assembly-based printing technique, interfacial convective assembly, is reported, which utilizes a substrate-heating-induced solutal Marangoni convective flow to drive particles toward patterned substrates and then uses van der Waals interactions as well as geometrical confinement to trap the particles in the pattern areas. The influence of various assembly parameters including type of mixing solvent, substrate temperature, particle concentration, and assembly time is investigated. The results show successful assembly of various nanoparticles in patterns of different shapes with a high resolution down to 25 nm. In addition, the assembly only takes a few minutes, which is two orders of magnitude faster than conventional convective assembly. Small-sized (diameter below 5 nm) nanoparticles tend to coalesce during the assembly process and form sintered structures. The fabricated silver nanorods show single-crystal structure with a low resistivity of 8.58 × 10^-5 Ω cm. With high versatility, high resolution, and high throughput, the interfacial convective assembly opens remarkable opportunities for printing next generation nanoelectronics and sensors.

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.

Size-Tunable Assembly of Gold Nanoparticles Using Competitive AC Electrokinetics

Alternating current (AC) electrokinetics is a facile way of patterning colloidal particles into advanced structures. We demonstrate the combined use of AC dielectrophoresis (AC-DEP) and AC electrohydrodynamics (AC-EHD) in a microwell electrode geometry for size-tunable assembly of gold nanoparticles (AuNPs) into one-dimensional microwires and two-dimensional films. The AC-DEP force scales with both particle size and field frequency, whereas the AC-EHD force depends only on the field frequency. So, a critical particle diameter ( d_c) exists, below which the EHD phenomenon becomes more important and beyond which the DEP force is dominating. We performed theoretical and experimental studies to determine " d_c" and how it gets affected by operating parameters like field frequency, voltage, particle number, electrolyte concentration, electrode size, and geometry. Our results show that the morphologies of the colloidal structures transition from films to microwires as the NP diameters vary from nanometers (< d_c) to microns (> d_c), and no assembly takes place at intermediate sizes (∼ d_c). While the film formation is governed purely by surface EHD flows, microwire synthesis is a result of EHD-assisted DEP phenomenon. Also, a minimum particle number, a low salt concentration, and an optimum frequency range is required to initiate assembly.

Driving Chemical Reactions in Plasmonic Nanogaps with Electrohydrodynamic Flow

Nanoparticles from colloidal solution-with controlled composition, size, and shape-serve as excellent building blocks for plasmonic devices and metasurfaces. However, understanding hierarchical driving forces affecting the geometry of oligomers and interparticle gap spacings is still needed to fabricate high-density architectures over large areas. Here, electrohydrodynamic (EHD) flow is used as a long-range driving force to enable carbodiimide cross-linking between nanospheres and produces oligomers exhibiting sub-nanometer gap spacing over mm² areas. Anhydride linkers between nanospheres are observed via surface-enhanced Raman scattering (SERS) spectroscopy. The anhydride linkers are cleavable via nucleophilic substitution and enable placement of nucleophilic molecules in electromagnetic hotspots. Atomistic simulations elucidate that the transient attractive force provided by EHD flow is needed to provide a sufficient residence time for anhydride cross-linking to overcome slow reaction kinetics. This synergistic analysis shows assembly involves an interplay between long-range driving forces increasing nanoparticle-nanoparticle interactions and probability that ligands are in proximity to overcome activation energy barriers associated with short-range chemical reactions. Absorption spectroscopy and electromagnetic full-wave simulations show that variations in nanogap spacing have a greater influence on optical response than variations in close-packed oligomer geometry. The EHD flow-anhydride cross-linking assembly method enables close-packed oligomers with uniform gap spacings that produce uniform SERS enhancement factors. These results demonstrate the efficacy of colloidal driving forces to selectively enable chemical reactions leading to future assembly platforms for large-area nanodevices.

Directed assembly-based printing of homogeneous and hybrid nanorods using dielectrophoresis

Printing nano and microscale three-dimensional (3D) structures using directed assembly of nanoparticles has many potential applications in electronics, photonics and biotechnology. This paper presents a reproducible and scalable 3D dielectrophoresis assembly process for printing homogeneous silica and hybrid silica/gold nanorods from silica and gold nanoparticles. The nanoparticles are assembled into patterned vias under a dielectrophoretic force generated by an alternating current (AC) field, and then completely fused in situ to form nanorods. The assembly process is governed by the applied AC voltage amplitude and frequency, pattern geometry, and assembly time. Here, we find out that complete assembly of nanorods is not possible without applying both dielectrophoresis and electrophoresis. Therefore, a direct current offset voltage is used to add an additional electrophoretic force to the assembly process. The assembly can be precisely controlled to print silica nanorods with diameters from 20-200 nm and spacing from 500 nm to 2 μm. The assembled nanorods have good uniformity in diameter and height over a millimeter scale. Besides homogeneous silica nanorods, hybrid silica/gold nanorods are also assembled by sequentially assembling silica and gold nanoparticles. The precision of the assembly process is further demonstrated by assembling a single particle on top of each nanorod to demonstrate an additional level of functionalization. The assembled hybrid silica/gold nanorods have potential to be used for metamaterial applications that require nanoscale structures as well as for plasmonic sensors for biosensing applications.

Double Fano resonances in an individual metallic nanostructure for high sensing sensitivity

In this paper, we report on the design and observation of double Fano resonances (DFRs) in an individual symmetry-reduced nanostructure and the induced high sensing sensitivity. Such a plasmonic nanostructure consists of a partially overlapped double-metallic nanotriangles with unequal sizes fabricated by using fast and low-cost angle-resolved nanosphere lithography. Symmetry breaking generates two narrow quadrupolar dark modes, which further enhance the coupling with fundamental bright dipole modes within the same structure, manifesting the effect of DFRs. The resonance wavelength and line shape of DFRs can be tailored by changing the degree of asymmetry as well as the size of the designed nanostructure. Based on DFRs, a high sensitivity to dielectric environment with a maximum figure of merit of 35 is measured. Due to a fast manufacturing process with high reproducibility and high structural tunability, the fabricated individual metallic nanostructure provides an opportunity for significant potential applications in localized surface plasmon resonance based single or double-wavelength sensors in the near-infrared region.

High-Rate Assembly of Nanomaterials on Insulating Surfaces Using Electro-Fluidic Directed Assembly

Conductive or semiconducting nanomaterials-based applications such as electronics and sensors often require direct placement of such nanomaterials on insulating surfaces. Most fluidic-based directed assembly techniques on insulating surfaces utilize capillary force and evaporation but are diffusion limited and slow. Electrophoretic-based assembly, on the other hand, is fast but can only be utilized for assembly on a conductive surface. Here, we present a directed assembly technique that enables rapid assembly of nanomaterials on insulating surfaces. The approach leverages and combines fluidic and electrophoretic assembly by applying the electric field through an insulating surface via a conductive film underneath. The approach (called electro-fluidic) yields an assembly process that is 2 orders of magnitude faster compared to fluidic assembly. By understanding the forces on the assembly process, we have demonstrated the controlled assembly of various types of nanomaterials that are conducting, semiconducting, and insulating including nanoparticles and single-walled carbon nanotubes on insulating rigid and flexible substrates. The presented approach shows great promise for making practical devices in miniaturized sensors and flexible electronics.

Novel Nanoprinting for Oral Delivery of Poorly Soluble Drugs

Many of the newly developed drugs for cancer, and some of those for cardiovascular disease, are poorly soluble in water and cannot be taken orally. This can be overcome by employing a new and effective delivery system utilizing nanotechnology. We present a new method for oral preparation of poorly soluble drugs that entails assembling (printing) drug-loaded polymeric micelles into sub-100 nm orally acceptable nanorods (NRs). Due to their small size, these NRs will have a high permeability through cells and thus should transport through the intestine to allow for drug delivery in the blood. These NRs drugs are expected to penetrate tumors more efficiently and much faster than individual nanoparticles and may also be useful for drug delivery to atherosclerotic plaque. This should lead to better bioavailability of the drug with reduced toxicity and side effects. Currently used micellar formulations are administered intravenously, which is invasive and could be toxic due to high doses and interaction with normal healthy tissues. Oral drug administration is the easiest and most desirable way to deliver most drugs, including those that are poorly soluble.

Surfactant-Assisted Voltage-Driven Silver Nanoparticle Chain Formation across Microelectrode Gaps in Air

Here we describe the electrodeposition of Ag in the presence of cetyltrimethylammonium bromide (CTAB) onto 5 μm gap Au interdigitated array (IDA) electrodes that are bare, thiol-functionalized, or thiol-functionalized and seeded with 4 nm diameter Au nanoparticles (NPs). After deposition, applying a voltage between 5 and 10 V in air for 0 to 1000 s resulted in one-dimensional (1D) Ag NP chains spanning across the IDA gap. The Ag NP chains form on IDAs functionalized with thiols and Au NP-seeded at about 5 V and at 10 V for the other nonseeded surfaces. Ag NP chains do not form at all up to 10 V when IDAs are treated with ozone or water soaking to remove possible CTA(+) ions from the surface, when Ag deposition takes place in the absence of CTAB, or when the voltage is applied under dry N2 (low humidity). Chain formation occurs by Ag moving from the positive to negative electrode. Coating the devices with a negatively charged surfactant, sodium dodecyl sulfate, also results in Ag NP chains by Ag moving from the positive to the negative electrodes, which confirms that the chains form by electrochemical oxidation at the positive electrode and deposition at the negative electrode. The surfactant ions and thin layer of water present in the humid environment facilitate this electrochemical process.

Hybridization in Three Dimensions: A Novel Route toward Plasmonic Metamolecules

Plasmonic metamolecules have received much interest in the last years because they can produce a wide spectrum of different hybrid optical resonances. Most of the configurations presented so far, however, considered planar resonators lying on a dielectric substrate. This typically yields high damping and radiative losses, which severely limit the performance of the system. Here we show that these limits can be overcome by considering a 3D arrangement made from slanted nanorod dimers extruding from a silver baseplate. This configuration mimics an out-of-plane split ring resonator capable of a strong near-field interaction at the terminations and a strong diffractive coupling with nearby nanostructures. Compared to the corresponding planar counterparts, higher values of electric and magnetic fields are found (about a factor 10 and a factor 3, respectively). High-quality-factor resonances (Q ≈ 390) are produced in the mid-IR as a result of the efficient excitation of collective modes in dimer arrays.

Cotton textile enabled, all-solid-state flexible supercapacitors

A hierarchical NiCo₂O₄@NiCo₂O₄ nanostructure was grown on flexible, cotton activated carbon textiles (ACTs) for flexible asymmetric supercapacitor electrode, which exhibited an exceptional combination of electrochemical and mechanical properties.

Hierarchical self-assembly of gold nanoparticles into patterned plasmonic nanostructures

The integration of nanoparticle superstructures into daily life applications faces major challenges including the simplification of the self-assembly process, reduced cost, and scalability. It is, however, often difficult to improve on one aspect without losing on another. We present in this paper a benchtop method that allows patterning a macroscopic substrate with gold nanoparticle supercrystals in a one-step process. The method allows parallelization, and patterned substrates can be made with high-throughput. The self-assembly of a variety of building blocks into crystalline superstructures takes place upon solvent evaporation, and their precise placement over millimeter scale areas is induced by confinement of the colloidal suspension in micron-sized cavities. We mainly focus on gold nanorods and demonstrate their hierarchical organization up to the device scale. The height of the formed nanorod supercrystals can be tuned by simply varying nanorod concentration, so that the topography of the substrate and the resulting optical properties can be readily modulated. The crystalline order of the nanorods results in homogeneous and high electric field enhancements over the assemblies, which is demonstrated by surface-enhanced Raman scattering spectroscopy.