Direct Assembly of Large Area Nanoparticle Arrays

Ultrahigh-resolution, high-fidelity quantum dot pixels patterned by dielectric electrophoretic deposition

The high pixel resolution is emerging as one of the key parameters for the next-generation displays. Despite the development of various quantum dot (QD) patterning techniques, achieving ultrahigh-resolution (>10,000 pixels per inch (PPI)) and high-fidelity QD patterns is still a tough challenge that needs to be addressed urgently. Here, we propose a novel and effective approach of orthogonal electric field-induced template-assisted dielectric electrophoretic deposition to successfully achieve one of the highest pixel resolutions of 23090 (PPI) with a high fidelity of up to 99%. Meanwhile, the proposed strategy is compatible with the preparation of QD pixels based on perovskite CsPbBr3 and conventional CdSe QDs, exhibiting a wide applicability for QD pixel fabrication. Notably, we further demonstrate the great value of our approach to achieve efficiently electroluminescent QD pixels with a peak external quantum efficiency of 16.5%. Consequently, this work provides a general approach for realizing ultrahigh-resolution and high-fidelity patterns based on various QDs and a novel method for fabricating QD-patterned devices with high performance.

Biomedical SERS - the current state and future trends

Surface enhanced Raman spectroscopy (SERS) is meeting the requirements in biomedical science being a highly sensitive and specific analytical tool. By employing portable Raman systems in combination with customized sample pre-treatment, point-of-care-testing (POCT) becomes feasible. Powerful SERS-active sensing surfaces with high stability and modification layers if required are available for testing and application in complex biological matrices such as body fluids, cells or tissues. This review summarizes the current state in sample collection and pretreatment in SERS detection protocols, SERS detection schemes, i.e. direct and indirect SERS as well as targeted and non-targeted SERS, and SERS-active sensing surfaces. Moreover, the recent developments and advances of SERS in biomedical application scenarios, such as infectious diseases, cancer diagnostics and therapeutic drug monitoring is given, which enables the readers to identify the sample collection and preparation protocols, SERS substrates and detection strategies that are best-suited for their specific applications in biomedicine.

Massive Electro-Microfluidic Particle Assembly Patterns in Droplet Array for Information Encoding

The assembly of colloidal particles into micro-patterns is essential in optics, informatics, and microelectronics. However, it is still a challenge to achieve quick, reversible, and precise assembly patterns within micro-scale spaces like droplets. Hereby, a method is presented that utilizes in-plane dielectrophoresis to precisely manipulate particle assemblies within microscale droplets. The electro-microfluidic particle assembly platform, equipped with ingenious electrode designs, enables the formation of diverse micro-patterns within a droplet array. The tunability, similarity, stability, and reversibility of this platform are demonstrated. The ability to assemble letters, numbers, and Morse code patterns within the droplet array underscores its potential for information encoding. Furthermore, using an example with four addressing electrodes beneath a droplet, 16 distinct pieces of information through electrical stimuli is successfully encoded. This unique capability facilitates the construction of a dynamic electronic token, indicating promising applications in anti-counterfeiting technologies.

Label-free plasmonic spectral profiling of serum DNA

Genetic and epigenetic modifications are linked to the activation of oncogenes and inactivation of tumor suppressor genes. Likewise, the associated molecular alternations can best inform precision medicine for personalized tumor treatment. Therefore, performing characterization of genetic and epigenetic alternations at the molecular level represents a crucial step in early diagnosis and/or therapeutics of cancer. However, the prevailing methods for DNA analysis involve a series of tedious and complicated steps, in which important genetic and epigenetic information could be lost or altered. To provide a potential approach for non-invasive, direct, and efficient DNA analysis, herein, we present a promising strategy for label-free molecular profiling of serum DNA in its pristine form by fusing surface-enhanced Raman spectroscopy with machine learning on a superior plasmonic nanostructured platform. Using DNA methylation and single-point mutation as two case studies, the presented strategy allows a well-balanced sensitive and specific detection of epigenetic and genetic changes at the single-nucleotide level in serum. We envision the presented label-free strategy could serve as a versatile tool for direct molecular profiling in pristine forms of a wide range of biological markers and aid biomedical diagnostics as well as therapeutics.

Nanocrystal Assemblies: Current Advances and Open Problems

We explore the potential of nanocrystals (a term used equivalently to nanoparticles) as building blocks for nanomaterials, and the current advances and open challenges for fundamental science developments and applications. Nanocrystal assemblies are inherently multiscale, and the generation of revolutionary material properties requires a precise understanding of the relationship between structure and function, the former being determined by classical effects and the latter often by quantum effects. With an emphasis on theory and computation, we discuss challenges that hamper current assembly strategies and to what extent nanocrystal assemblies represent thermodynamic equilibrium or kinetically trapped metastable states. We also examine dynamic effects and optimization of assembly protocols. Finally, we discuss promising material functions and examples of their realization with nanocrystal assemblies.

Spontaneous Particle Ordering, Sorting, and Assembly on Soap Films

Soap bubbles exhibit abundant fascinating phenomena throughout the entire life of evolution with different fundamental physics governing them. Nevertheless, the complicated dynamics of small objects in soap films are still unrevealed. Here, we report the first observation of spontaneous particle ordering in a complicated galaxy of soap films without any external energy. The balance of interfacial tension at two liquid-gas interfaces is theoretically predicted to govern belted wetted particles (BWPs) traveling along a specified path spontaneously. Such spontaneous particle path-finding is found to depend on the particle size and hydrophilic properties. Spontaneous particle sorting is directly realized via these discrete and distinctive paths for different particles. The deformation of the soap membrane facilitates 1D/2D particle organization along the path. This observation represents the discovery of a new spontaneous order phenomenon in soap film systems and provides a new energy-free approach for particle separation and soft colloidal crystal assembly.

Tunable Single-Photon Emission with Wafer-Scale Plasmonic Array

Bright, scalable, and deterministic single-photon emission (SPE) is essential for quantum optics, nanophotonics, and optical information systems. Recently, SPE from hexagonal boron nitride (h-BN) has attracted intense interest because it is optically active and stable at room temperature. Here, we demonstrate a tunable quantum emitter array in h-BN at room temperature by integrating a wafer-scale plasmonic array. The transient voltage electrophoretic deposition (EPD) reaction is developed to effectively enhance the filling of single-crystal nanometals in the designed patterns without aggregation, which ensures the fabricated array for tunable performances of these single-photon emitters. An enhancement of ∼500% of the SPE intensity of the h-BN emitter array is observed with a radiative quantum efficiency of up to 20% and a saturated count rate of more than 4.5 × 106 counts/s. These results suggest the integrated h-BN-plasmonic array as a promising platform for scalable and controllable SPE photonics at room temperature.

Self-Confined Dewetting Mechanism in Wafer-Scale Patterning of Gold Nanoparticle Arrays with Strong Surface Lattice Resonance for Plasmonic Sensing

A self-confined solid-state dewetting mechanism is reported that can fundamentally reduce the use of sophisticated nanofabrication techniques, enabling efficient wafer-scale patterning of non-closely packed (ncp) gold nanoparticle arrays. When combined with a soft lithography process, this approach can address the reproducibility challenges associated with colloidal crystal self-assembly, allowing for the batch fabrication of ncp gold arrays with consistent ordering and even optical properties. The resulting dewetted ncp gold nanoparticle arrays exhibit strong surface lattice resonance properties when excited in inhomogeneous environments under normal white-light incidence. With these SLR properties, the sensitive plasmonic sensing of molecular interactions is achieved using a simple transmission setup. This study will advance the development of miniaturized and portable devices.

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.

High Q-Factor Plasmonic Surface Lattice Resonances in Colloidal Nanoparticle Arrays

Surface lattice resonances (SLRs) sustained by ordered metal arrays are characterized by their narrow spectral features, remarkable quality factors, and the ability to tune their spectral properties based on the periodicity of the array. However, the majority of these structures are fabricated using classical lithographic processes or require postannealing steps at high temperatures to enhance the quality of the metal. These limitations hinder the widespread utilization of these periodic metal arrays in various applications. In this work, we use the scalable technique of template-assisted assembly of metal colloids to produce plasmonic supercrystals over centimeter areas capable of sustaining SLRs with high Q factors reaching up to 270. Our approach obviates the need for any postprocessing, offering a streamlined and efficient fabrication route. Furthermore, our method enables extensive tunability across the entire visible and near-infrared spectral ranges, empowering the design of tailored plasmonic resonant structures for a wide range of applications.

Electric Field-Driven Self-Assembly of Gold Nanoparticle Monolayers on Silicon Substrates

Nanoparticles (NPs) bridge the gap between bulk materials and their equivalent molecular/atomic counterparts. The physical, optical, and electronic properties of individual NPs alter with the changes in their surrounding environment at the nanoscale. Similarly, the characteristics of thin films of NPs depend on their lateral and volumetric densities. Thus, attaining single monolayers of these NPs would play a vital role in the improved characteristics of semiconductor devices such as nanosensors, field effect transistors, and energy harvesting devices. Developing nanosensors, for instance, requires precise methods to fabricate a monolayer of NPs on selected substrates for sensing and other applications. Herein, we developed a physical fabrication method to form a monolayer of NPs on a planar silicon surface by creating an electric field of intensity 5.71 × 104 V/m between parallel plates of a capacitor, by applying a DC voltage. The physics of monolayer formation caused by an externally applied electric field on the gold NPs (Au-NPs) of size 20 nm in diameter and possesses a zeta potential of -250 to -290 mV, is further analyzed with the help of the finite element simulation. The enhanced electric field, in the order of 108 V/m, around the Au-NPs indicates a high surface charge density on the NPs, which results in a high electric force per unit area that guides them to settle uniformly on the surface of the silicon substrate.

Multi-wavelength unidirectional forward scattering properties of the arrow-shaped gallium phosphide nanoantenna

An arrow-shaped gallium phosphide nanoantenna exhibits both near-field electric field enhancement and far-field unidirectional scattering, and the interference conditions involve electric and magnetic quadrupoles as well as toroidal dipoles. By using long-wavelength approximation and exact multipole decomposition, the interference conditions required for far-field unidirectional transverse light scattering and backward near-zero scattering at multiple wavelengths are determined. The near-field properties are excellent, as exemplified by large Purcell factors of 4.5×109 for electric dipole source excitation, 464.68 for magnetic dipole source excitation, and 700 V/m for the field enhancement factor. The degree of enhancement of unidirectional scattering is affected by structural parameters such as the angle and thickness of the nanoantenna. The arrow-shaped nanoantenna is an efficient platform to enhance the electric field and achieve high directionality of light scattering. Moreover, the nanostructure enables flexible manipulation of light waves and materials, giving rise to superior near-field and far-field performances, which are of great importance pertaining to the practicability and application potential of optical antennas in applications such as spectroscopy, sensing, displays, and optoelectronic devices.

High-Resolution, Highly Transparent, and Efficient Quantum Dot Light-Emitting Diodes

Aiming at next-generation displays, high-resolution quantum dot light-emitting diodes (QLEDs) with high efficiency and transparency are highly desired. However, there is limited study involving the improvements of QLED pixel resolution, efficiency, and transparency simultaneously, which undoubtedly restricts the practical applications of QLED for next-generation displays. Here, the strategy of electrostatic force-induced deposition (EF-ID) is proposed by introducing alternating polyethyleneimine (PEI) and fluorosilane patterns to synergistically improve the pixel accuracy and transmittance of QD patterns. More importantly, the leakage current induced by the void spaces between pixels that is usually reported for high-resolution QLEDs is greatly suppressed by substrate-assisted insulating fluorosilane patterns. Finally, high-performance QLEDs with high resolution ranging from 1104 to 3031 pixels per inch (PPI) and a high efficiency of 15.6% are achieved, among the best performances of high resolution QLEDs. Notably, the high resolution QD pixels greatly enhance the transmittance of the QD patterns, thus prompting an impressive transmittance of 90.7% for the transparent QLEDs (2116 PPI), which represents the highest transmittance of transparent QLED devices. Consequently, this work contributes an effective and general approach for high-resolution QLEDs with high efficiency and transparency.

Electric field-induced assembly of Au-Ag alloy nanoparticles into nano-reticulation for ultrasensitive SERS

This paper discusses a method for assembling Au-Ag alloy nanoparticles (NPs) using direct current (DC) electric field to fabricate highly active SERS substrates. Different nanostructures could be obtained by regulating the intensity and action time of DC electric field. Under the condition of 5mA*10 min, we obtained Au-Ag alloy nano-reticulation (ANR) substrate with excellent SERS activity (Enhancement factor on order of magnitude of 10⁶). ANR substrate has excellent SERS performance due to the resonance matching between its LSPR mode and excitation wavelength. The uniformity of the Raman signal on ANR is greatly improved than bare ITO glass. ANR substrate also has the ability to detect multiple molecules: ANR substrate can respectively detect Rh6G and CV molecules with a concentration as low as 10^-10 M and 10^-9 M and the Raman spectral intensity of the probe molecules on the surface of the ANR substrate has good linear correlation with the molecular concentration (R²> 0.95). In addition, ANR substrate can detect both thiram and aspartame (APM) molecules far below (thiram for 0.0024 ppm and APM for 0.0625 g/L) the safety standard, which demonstrate its practical application potential.

Electrophoretic Deposition of Hybrid Calcium Alginate-Gold Nanoparticle Hydrogel Films via Catalyzed Electrooxidation of Hydroquinone

The electrophoretic deposition (EPD) of hybrid alginate (Alg)-Au nanoparticle (NP) films results from the localized pH drop at the electrode surface due to oxidation of hydroquinone (HQ) catalyzed by 4 and 15 nm diameter citrate-coated gold NPs (cit-Au NPs). The localized pH drop at the electrode leads to neutralization of both Alg and cit, leading to EPD of both Alg and cit-Au NPs simultaneously. Post-treatment of the film with Ca²⁺ solution leads to hybrid Ca-Alg-Au NP hydrogel films. The EPD of Alg in the presence of 4 nm cit-Au NPs occurs at ∼0.8 V (vs Ag/AgCl) as compared to ∼1.0 V in the presence of 15 nm cit-Au NPs and ∼1.4 V in the absence of cit-Au NPs. This is due to the higher catalytic activity of 4 nm cit-Au NPs compared to 15 nm cit-Au NPs for the oxidation of HQ. UV-vis spectra of Ca-Alg-Au NP hydrogel films show absorbance features for both Ca-Alg and Au NPs entrapped within the hydrogel. As the concentration of Au NPs in the EPD solution increases, the Ca-Alg absorbance and localized surface plasmon resonance (LSPR) peak of the Au NPs increases, confirming the role of the Au NPs as a catalyst for EPD of Alg. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra of the Ca-Alg-Au NP hydrogel films show characteristic peaks for Ca-Alg and protonated alginic acid groups. The hydrogel thickness is greater with cit-Au NPs compared to without cit-Au NPs at constant EPD potential and time. Forming Ca-Alg and hybrid Ca-Alg-Au NP hydrogel films at low potentials has potential applications in electrochemical and optical sensor development, catalysis, and biological studies.

Strategy for Patterning Single Colloidal Gold Nanoparticles on Two Photon Polymerized and Functionalized Ultrathin Nanostructures

Local and deterministic trapping of single nanoparticles (NPs) has always been a challenging topic due to the difficulties faced at such a small particle size. These difficulties are concerned with the stability, simplicity, robustness, and efficiency of the used trapping technique. Here, we used two-photon polymerization (TPP) of a prefunctionalized photopolymer to obtain a nanometric polymer layer and selectively attract single colloidal gold NPs (AuNPs). Thanks to a deep photochemical study of the threshold energy, we identified a photopolymerization regime allowing one to tune the polymer size and immobilize single gold nanoparticles. This method is promising for the fabrication of single photon sources.

Generalization of Self-Assembly Toward Differently Shaped Colloidal Nanoparticles for Plasmonic Superlattices

Periodic superlattices of noble metal nanoparticles  have demonstrated superior plasmonic properties compared to randomly distributed plasmonic arrangements due to near-field coupling and constructive far-field interference. Here, a chemically driven, templated self-assembly process of colloidal gold nanoparticles is investigated and optimized, and the technology is extended toward a generalized assembly process for variously shaped particles, such as spheres, rods, and triangles. The process yields periodic superlattices of homogenous  nanoparticle clusters on a centimeter scale. Electromagnetically simulated absorption spectra and corresponding experimental extinction measurements demonstrate excellent agreement in the far-field for all particle types and different lattice periods. The electromagnetic simulations reveal the specific nano-cluster near-field behavior, predicting the experimental findings provided by surface-enhanced Raman scattering measurements. It turns out that periodic arrays of spherical nanoparticles produce higher surface-enhanced Raman scattering enhancement factors than particles with less symmetry as a result of very well-defined strong hotspots.

Electric, magnetic, and shear field-directed assembly of inorganic nanoparticles

Ordered assemblies of inorganic nanoparticles (NPs) have shown tremendous potential for wide applications due to their unique collective properties, which differ from those of individual NPs. Various assembly methods, such as external field-directed assembly, interfacial assembly, template assembly, biomolecular recognition-mediated assembly, confined assembly, and others, have been employed to generate ordered inorganic NP assemblies with hierarchical structures. Among them, the external field-directed assembly method is particularly fascinating, as it can remotely assemble NPs into well-ordered superstructures. Moreover, external fields (e.g., electric, magnetic, and shear fields) can introduce a local and/or global field intensity gradient, resulting in an additional force on NPs to drive their rotation and/or translation. Therefore, the external field-directed assembly of NPs becomes a robust method to fabricate well-defined functional materials with the desired optical, electronic, and magnetic properties, which have various applications in catalysis, sensing, disease diagnosis, energy conversion/storage, photonics, nano-floating-gate memory, and others. In this review, the effects of an electric field, magnetic field, and shear field on the organization of inorganic NPs are highlighted. The methods for controlling the well-ordered organization of inorganic NPs at different scales and their advantages are reviewed. Finally, future challenges and perspectives in this field are discussed.

Fabrication of Centimeter-Scale Plasmonic Nanoparticle Arrays with Ultranarrow Surface Lattice Resonances

Plasmonic surface lattice resonances (SLRs) supported by metallic nanoparticle (NP) arrays show diverse applications including nanolasers, sensors, photocatalysis, and nonlinear optics. However, to rationally fabricate high-quality plasmonic NP arrays with ultranarrow SLR line widths over large areas remains challenging. This article describes a general approach for the efficient fabrication of centimeter-scale inorganic NP arrays with precisely controlled NP size, composition, position, and lattice geometry. This method combines the processes of solvent-assisted soft lithography and in situ site-specific NP growth to reproducibly create many replicates of NP arrays without utilizing cleanroom and specialized equipment. For demonstration, we show that Au NP arrays exhibit ultranarrow SLRs with a line width of 4 nm and a quality factor of 218 toward the theoretical limit.

Plasmonic/magnetic nanoarchitectures: From controllable design to biosensing and bioelectronic interfaces

Controllable design of the nanocrystal-assembled plasmonic/magnetic nanoarchitectures (P/MNAs) inspires abundant methodologies to enhance light-matter interactions and control magnetic-induced effects by means of fine-tuning the morphology and ordered packing of noble metallic or magnetic building blocks. The burgeoning development of multifunctional nanoarchitectures has opened up broad range of interdisciplinary applications including biosensing, in vitro diagnostic devices, point-of-care (POC) platforms, and soft bioelectronics. By taking advantage of their customizability and efficient conjugation with capping biomolecules, various nanoarchitectures have been integrated into high-performance biosensors with remarkable sensitivity and versatility, enabling key features that combined multiplexed detection, ease-of-use and miniaturization. In this review, we provide an overview of the representative developments of nanoarchitectures that being built by plasmonic and magnetic nanoparticles over recent decades. The design principles and key mechanisms for signal amplification and quantitative sensitivity have been explored. We highlight the structure-function programmability and prospects of addressing the main limitations for conventional biosensing strategies in terms of accurate selectivity, sensitivity, throughput, and optoelectronic integration. State-of-the-art strategies to achieve affordable and field-deployable POC devices for early multiplexed detection of infectious diseases such as COVID-19 has been covered in this review. Finally, we discuss the urgent yet challenging issues in nanoarchitectures design and related biosensing application, such as large-scale fabrication and integration with portable devices, and provide perspectives and suggestions on developing smart biosensors that connecting the materials science and biomedical engineering for personal health monitoring.

Site-Selective Assembly of Centimeter-Scale Arrays of Precisely Oriented Magnetic Nanoellipsoids

The precise organization and orientation of anisotropic nanoparticles (NPs) on substrates over a large area is key to the application of NP assemblies in functional optical, electronic, and magnetic devices, but achieving such high-precision NP assembly still remains challenging. Here, we demonstrate the site-selective assembly of magnetic nanoellipsoids into large-area precisely positioned, orientationally controlled arrays via a combination of chemical patterning and magnetic manipulation. Magnetic ellipsoidal NPs are selectively positioned on predetermined chemical patterns with high fidelity through electrostatic interactions and aligned uniformly in line with an applied magnetic field. The position, orientation, and interparticle spacing of the ellipsoids can be precisely tuned by controlling the chemical patterns and magnetic field. This approach is simple to implement and can generate centimeter-scale arrays in high yield (up to 99%). The arrays exhibit collective magnetic responses that are dependent on the orientation of the ellipsoids. This work offers a tool for the fabrication of precisely engineered arrays of anisotropic NPs for applications such as metasurface and artificial spin ice.

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.

Controlled assembly of gold nanoparticles in resonant gold nanoapertures for SERS applications

The controlled assembly of plasmonic nanoparticles is vital for realizing low-cost, high efficiency plasmonic substrates with tunable resonances. Here, we present a strategy to assemble gold nanoparticles (AuNPs) in resonant gold nanoapertures (NAs) to enable coupling-mediated near-field enhancement. The NAs templates are fabricated using shadow sphere lithography on polyelectrolyte (PE) coated substrates. Subsequently, AuNPs are assembled in the resonant NA templates via a simple immersion step. The PE layer, AuNP concentration, NaCl concentration, incubation time, and template thickness are used to control the particle number per aperture and the interparticle distance of the AuNP assemblies. The fabricated AuNP-NA substrates are evaluated for their SERS potential using 4-Mercaptobenzoic acid (MBA) as a Raman reporter molecule. The SERS intensity of the AuNP-NA templates can be enhanced by ten times by controlling the AuNP and NA template parameters as compared to the bare NA templates. Numerical simulations show that the coupling between the various plasmonic modes is crucial for this SERS enhancement. The proposed strategy can be used to fabricate hybrid AuNP-NA based SERS substrates with improved sensitivity.

Patch formation on diblock copolymer micelles confined in templates for inducing patch orientation and cyclic colloidal molecules

HYPOTHESIS

Chemically or physically distinct patches can be induced on the micelles of amphiphilic block copolymers, which facilitate directional binding for the creation of hierarchical structures. Hence, control over the direction of patches on the micelles is a crucial factor to attain the directionality on the interactions between the micelles, particularly for generating colloidal molecules mimicking the symmetry of molecular structures. We hypothesized that direction and combination of the patches could be controlled by physical confinement of the micelles.

EXPERIMENTS

We first confined spherical micelles of diblock copolymers in topographic templates fabricated from nanopatterns of block copolymers by adjusting the coating conditions. Then, patch formation was conducted on the confined micelles by exposing them with a core-favorable solvent. Microscopic techniques of SEM, TEM, and AFM were employed to investigate directions of patches and structures of combined micelles in the template.

FINDINGS

The orientation of the patches on the micelles was guided by the physical confinement of the micelles in linear trenches. In addition, by confining the micelles in a circular hole, we obtained a specific polygon arrangement of the micelles depending on the number of micelles in the hole, which enabled the formation of cyclic colloidal molecules consisting of micelles.

Artificial Protein Cage with Unusual Geometry and Regularly Embedded Gold Nanoparticles

Artificial protein cages have great potential in a number of areas including cargo capture and delivery and as artificial vaccines. Here, we investigate an artificial protein cage whose assembly is triggered by gold nanoparticles. Using biochemical and biophysical methods we were able to determine both the mechanical properties and the gross compositional features of the cage which, combined with mathematical models and biophysical data, allowed the structure of the cage to be predicted. The accuracy of the overall geometrical prediction was confirmed by the cryo-EM structure determined to sub-5 Å resolution. This showed the cage to be nonregular but similar to a dodecahedron, being constructed from 12 11-membered rings. Surprisingly, the structure revealed that the cage also contained a single, small gold nanoparticle at each 3-fold axis meaning that each cage acts as a synthetic framework for regular arrangement of 20 gold nanoparticles in a three-dimensional lattice.

Artificial Intelligence Algorithms Enable Automated Characterization of the Positive and Negative Dielectrophoretic Ranges of Applied Frequency

The present work describes the phenomenological approach to automatically determine the frequency range for positive and negative dielectrophoresis (DEP)-an electrokinetic force that can be used for massively parallel micro- and nano-assembly. An experimental setup consists of the microfabricated chip with gold microelectrode array connected to a function generator capable of digitally controlling an AC signal of 1 V (peak-to-peak) and of various frequencies in the range between 10 kHz and 1 MHz. The suspension of latex microbeads (3-μm diameter) is either attracted or repelled from the microelectrodes under the influence of DEP force as a function of the applied frequency. The video of the bead movement is captured via a digital camera attached to the microscope. The OpenCV software package is used to digitally analyze the images and identify the beads. Positions of the identified beads are compared for successive frames via Artificial Intelligence (AI) algorithm that determines the cloud behavior of the microbeads and algorithmically determines if the beads experience attraction or repulsion from the electrodes. Based on the determined behavior of the beads, algorithm will either increase or decrease the applied frequency and implement the digital command of the function generator that is controlled by the computer. Thus, the operation of the study platform is fully automated. The AI-guided platform has determined that positive DEP (pDEP) is active below 500 kHz frequency, negative DEP (nDEP) is evidenced above 1 MHz frequency and the crossover frequency is between 500 kHz and 1 MHz. These results are in line with previously published experimentally determined frequency-dependent DEP behavior of the latex microbeads. The phenomenological approach assisted by live AI-guided feedback loop described in the present study will assist the active manipulation of the system towards the desired phenomenological outcome such as, for example, collection of the particles at the electrodes, even if, due to the complexity and plurality of the interactive forces, model-based predictions are not available.

Synthesis, Assembly, Optical Properties, and Sensing Applications of Plasmonic Gap Nanostructures

Plasmonic gap nanostructures (PGNs) have been extensively investigated mainly because of their strongly enhanced optical responses, which stem from the high intensity of the localized field in the nanogap. The recently developed methods for the preparation of versatile nanogap structures open new avenues for the exploration of unprecedented optical properties and development of sensing applications relying on the amplification of various optical signals. However, the reproducible and controlled preparation of highly uniform plasmonic nanogaps and the prediction, understanding, and control of their optical properties, especially for nanogaps in the nanometer or sub-nanometer range, remain challenging. This is because subtle changes in the nanogap significantly affect the plasmonic response and are of paramount importance to the desired optical performance and further applications. Here, recent advances in the synthesis, assembly, and fabrication strategies, prediction and control of optical properties, and sensing applications of PGNs are discussed, and perspectives toward addressing these challenging issues and the future research directions are presented.

Plasmonic alloy nanochains assembled via dielectrophoresis for ultrasensitive SERS

It is great challenge and interesting for researchers to fabricate substrates for enhanced Raman and sensor, and assemble some easy-to-synthesize metallic nanomaterials into controllable nanostructures with special morphologies and arrangements, via alternating current (AC) electric field. The Au-Ag alloy nanoparticles (Au-Ag alloy NPs) colloidal suspension with excellent dispersibility synthesized by wet chemical method, and the morphology of the assembly can be well controlled by regulating the frequency of the AC electric field. Au-Ag alloy nanochains array (Au-Ag ANCs) with dense plasmonic "hot spots" is formed when the AC electric field of 4V_pp-30kHz is applied, which is supported by the result of finite element method (FEM) numerical simulation. Experimental results demonstrate that Au-Ag ANCs show excellent SERS activity: Au-Ag ANCs can detect both Rhodamine 6G (Rh6G) and crystal violet (CV) in the magnitude order of 10^-10 M, and the Raman peaks intensity and analyte concentration has a strong linear correlation (R² is 0.99339 and 0.95916, respectively). Besides, the introduction of Au-Ag ANCs makes the Raman spectra intensity of thiram (a pesticide) with a concentration of 30 ppm on the surface of the blank ITO glass significantly enhanced, and it can detect thiram with a concentration as low as 0.03 ppm. In addition, Au-Ag ANCs substrate exhibits great uniformity and stability, so they have considerable application potential in the field of quantitative detection of trace substances.

One Strategy for Nanoparticle Assembly onto 1D, 2D, and 3D Polymer Micro and Nanostructures

The integration of nanoparticles (NPs) into photonic devices and plasmonic sensors requires selective patterning of these NPs with fine control of their size, shape, and spatial positioning. In this article, we report on a general strategy to pattern different types of NPs. This strategy involves the functionalization of photopolymers before their patterning by two-photon laser writing to fabricate micro- and nanostructures that selectively attract colloidal NPs with suitable ligands, allowing their precise immobilization and organization even within complex 3D structures. Monolayers of NPs without aggregations are obtained and the surface density of NPs on the polymer surface can be controlled by changing either the time of immersion in the colloidal solution or the type of amine molecule chemically grafted on the polymer surface. Different types of NPs (gold, silver, polystyrene, iron oxide, colloidal quantum dots, and nanodiamonds) of different sizes are introduced showing a potential toward nanophotonic applications. To validate the great potential of our method, we successfully demonstrate the integration of quantum dots within a gold nanocube with high spatial resolution and nanometer precision. The promise of this hybrid nanosource of light (plasmonic/polymer/QDs) as optical nanoswitch is illustrated through photoluminescence measurements under polarized exciting light.

Crystal nucleation in colloidal rod suspensions: The effect of depletant size

In order to better control the assembly of nanorods, knowledge of the pathways by which they form ordered structures is desirable. In this paper, we characterize crystal nucleation in suspensions of spherocylindrical rods with aspect ratio L/D = 2.3 in the presence of both small and large polymer depletants. Using a combination of Langevin dynamics and Monte Carlo simulations, together with biased sampling techniques, we show that the preferred pathway always involves the formation of monolayer assemblies irrespective of the volume fraction of the initial isotropic phase and the diameter of the depletants. This includes the previously neglected case of nucleation from the colloidal liquid phase and shows that the presence of depletion attraction can alter nucleation pathways even when the initial phase is dense.

High-Resolution Nanopatterning of Free-Standing, Self-Supported Helical Polypeptide Rod Brushes via Electron Beam Lithography

In this study of nanopatterned helical poly(benzyl-l-glutamate) (PBLG) brushes, rod-type brush arrays were fabricated via an integrated process of high-resolution lithography and surface-initiated vapor deposition polymerization (SI-VDP). "Nanospikes" of polymer brushes with spacings of less than 100 nm were produced. The topology and areal behavior of the resulting patterned rod-like brushes were analyzed and compared with patterned coil-type brushes. A geometric study of these self-assembled "nanospikes" was carried out, and their cross sections were investigated via focused ion beam (FIB) and scanning electron microscopy (SEM). Furthermore, the presence of poly(N-isopropylacrylamide) (PNIPAM) brushes in unpatterned regions was shown to inhibit undesired "inter-spike" bridging of the PBLG brushes, resulting in more well-defined nanostructures. It was shown that rod-like polypeptide brushes are capable of self-segregation and become arranged vertically without any external support from their surroundings, to form a rod bundle end-point functional topography that could provide possible pathways for studies of model biological surfaces, directed assembly of nanoparticles, or binary mixed brush surfaces with dual properties.

Large scale self-assembly of plasmonic nanoparticles on deformed graphene templates

Hierarchical heterostructures of two-dimensional (2D) nanomaterials are versatile platforms for nanoscale optoelectronics. Further coupling of these 2D materials with plasmonic nanostructures, especially in non-close-packed morphologies, imparts new metastructural properties such as increased photosensitivity as well as spectral selectivity and range. However, the integration of plasmonic nanoparticles with 2D materials has largely been limited to lithographic patterning and/or undefined deposition of metallic structures. Here we show that colloidally synthesized zero-dimensional (0D) gold nanoparticles of various sizes can be deterministically self-assembled in highly-ordered, anisotropic, non-close-packed, multi-scale morphologies with templates designed from instability-driven, deformed 2D nanomaterials. The anisotropic plasmonic coupling of the particle arrays exhibits emergent polarization-dependent absorbance in the visible to near-IR regions. Additionally, controllable metasurface arrays of nanoparticles by functionalization with varying polymer brushes modulate the plasmonic coupling between polarization dependent and independent assemblies. This self-assembly method shows potential for bottom-up nanomanufacturing of diverse optoelectronic components and can potentially be adapted to a wide array of nanoscale 0D, 1D, and 2D materials.

Reversible Design of Dynamic Assemblies at Small Scales

Emerging bottom-up fabrication methods have enabled the assembly of synthetic colloids, microrobots, living cells, and organoids to create intricate structures with unique properties that transcend their individual components. This review provides an access point to the latest developments in externally driven assembly of synthetic and biological components. In particular, we emphasize reversibility, which enables the fabrication of multiscale systems that would not be possible under traditional techniques. Magnetic, acoustic, optical, and electric fields are the most promising methods for controlling the reversible assembly of biological and synthetic subunits since they can reprogram their assembly by switching on/off the external field or shaping these fields. We feature capabilities to dynamically actuate the assembly configuration by modulating the properties of the external stimuli, including frequency and amplitude. We describe the design principles which enable the assembly of reconfigurable structures. Finally, we foresee that the high degree of control capabilities offered by externally driven assembly will enable broad access to increasingly robust design principles towards building advanced dynamic intelligent systems.

M. M. de Almeida J, Pastoriza-Santos I, C. C. Coelho L. Advances in Plasmonic Sensing at the NIR-A Review

Surface plasmon resonance (SPR) and localized surface plasmon resonance (LSPR) are among the most common and powerful label-free refractive index-based biosensing techniques available nowadays. Focusing on LSPR sensors, their performance is highly dependent on the size, shape, and nature of the nanomaterial employed. Indeed, the tailoring of those parameters allows the development of LSPR sensors with a tunable wavelength range between the ultra-violet (UV) and near infra-red (NIR). Furthermore, dealing with LSPR along optical fiber technology, with their low attenuation coefficients at NIR, allow for the possibility to create ultra-sensitive and long-range sensing networks to be deployed in a variety of both biological and chemical sensors. This work provides a detailed review of the key science underpinning such systems as well as recent progress in the development of several LSPR-based biosensors in the NIR wavelengths, including an overview of the LSPR phenomena along recent developments in the field of nanomaterials and nanostructure development towards NIR sensing. The review ends with a consideration of key advances in terms of nanostructure characteristics for LSPR sensing and prospects for future research and advances in this field.

Fabrication of plasmonic arrays of nanodisks and nanotriangles by nanotip indentation lithography and their optical properties

Fabrication of plasmonic nanostructures in a precise and reliable manner is a topic of huge interest because their structural details significantly affect their plasmonic properties. Herein, we present nanotip indentation lithography (NTIL) based on atomic force microscopy (AFM) indentation for the patterning of plasmonic nanostructures with precisely controlled size and shape. The size of the nanostructures is controlled by varying the indentation force of AFM tips into the mask polymer; while their shapes are determined to be nanodisks (NDs) or nanotriangles (NTs) depending on the shapes of the AFM tip apex. The localized surface plasmon resonance of the NDs is tailored to cover most of the visible-wavelength regime by controlling their size. The NTs show distinct polarization-dependent plasmon modes consistent with full-wave optical simulations. For the demonstration of the light-matter interaction control capability of NTIL nanostructures, we show that photoluminescence enhancement from MoS2 layers can be deliberately controlled by tuning the size of the nanostructures. Our results pave the way for the AFM-indentation-based fabrication of plasmonic nanostructures with a highly precise size and shape controllability and reproducibility.

Recycling silver nanoparticle debris from laser ablation of silver nanowire in liquid media toward minimum material waste

As silver nanowires (Ag NWs) are usually manufactured by chemical synthesis, a patterning process is needed to use them as functional devices. Pulsed laser ablation is a promising Ag NW patterning process because it is a simple and inexpensive procedure. However, this process has a disadvantage in that target materials are wasted owing to the subtractive nature of the process involving the removal of unnecessary materials, and large quantities of raw materials are required. In this study, we report a minimum-waste laser patterning process utilizing silver nanoparticle (Ag NP) debris obtained through laser ablation of Ag NWs in liquid media. Since the generated Ag NPs can be used for several applications, wastage of Ag NWs, which is inevitable in conventional laser patterning processes, is dramatically reduced. In addition, electrophoretic deposition of the recycled Ag NPs onto non-ablated Ag NWs allows easy fabrication of junction-enhanced Ag NWs from the deposited Ag NPs. The unique advantage of this method lies in using recycled Ag NPs as building materials, eliminating the additional cost of junction welding Ag NWs. These fabricated Ag NW substrates could be utilized as transparent heaters and stretchable TCEs, thereby validating the effectiveness of the proposed process.

Template Dissolution Interfacial Patterning of Single Colloids for Nanoelectrochemistry and Nanosensing

Deterministic positioning and assembly of colloidal nanoparticles (NPs) onto substrates is a core requirement and a promising alternative to top-down lithography to create functional nanostructures and nanodevices with intriguing optical, electrical, and catalytic features. Capillary-assisted particle assembly (CAPA) has emerged as an attractive technique to this end, as it allows controlled and selective assembly of a wide variety of NPs onto predefined topographical templates using capillary forces. One critical issue with CAPA, however, lies in its final printing step, where high printing yields are possible only with the use of an adhesive polymer film. To address this problem, we have developed a template dissolution interfacial patterning (TDIP) technique to assemble and print single colloidal AuNP arrays onto various dielectric and conductive substrates in the absence of any adhesion layer, with printing yields higher than 98%. The TDIP approach grants direct access to the interface between the AuNP and the target surface, enabling the use of colloidal AuNPs as building blocks for practical applications. The versatile applicability of TDIP is demonstrated by the creation of direct electrical junctions for electro- and photoelectrochemistry and nanoparticle-on-mirror geometries for single-particle molecular sensing.

Structural Colors by Synergistic Birefringence and Surface Plasmon Resonance

One-dimensional nanomaterials including cellulose nanocrystals (CNCs) and gold nanorods (GNRs) are widely used in optical materials due to their respective inherent features: birefringence with accompanying light retardation and surface plasmon resonance (SPR). Herein, we successfully combine these properties of both nanorods to generate synergistic and readily tunable structural colors in hybrid composite polymer films. CNCs and GNRs are embedded either in the same or in separate films after unidirectional alignment in dynamic hydrogels. By synergistically leveraging CNCs and GNRs with diverse amounts in hybrid films or stacked separate films, wide-ranging structural colors are obtained, far beyond those from films solely with aligned CNCs or GNRs. Higher GNR contents enhance light absorption at 520 nm with promoted magenta colors, while more CNCs affect the overall phase retardation with light absorption between 400 and 700 nm between crossed polarizers. Moreover, adjusting the angles between films solely with CNCs or GNRs via a stacking/rotating technique successively manipulates colors with flexible film combinations. By rotating the films with aligned GNRs (0-180°), light absorption can traverse from ∼500 to 650 nm. Thus, tuning the adjustable synergism of birefringence of CNCs and SPR of GNRs provides great potential for structural colors, which enlightens inspirations for designing functional optical materials.

Depletion-Mediated Uniform Deposition of Nanorods with Patterned, Multiplexed Assembly

Device-scale, uniform, and controllable deposition of nanoparticles on various substrates is fundamentally important not only for the fabrication of thin-film devices but also for the large sample statistics of single-particle performances. However, it is challenging to obtain such predefined depositions using a simple and efficient method. Here, we present a novel strategy for obtaining the uniform and particle density/spacing-tunable deposition of nanorods on a linker-free substrate. The deposition is driven by the tailored particle-substrate depletion attraction owing to the size-matched design of the substrate roughness and the nanorod diameter. Both gold nanorods and upconversion nanorods were applied to demonstrate the generality of the method. The high particle density of more than 21 per μm² and correspondingly the small particle spacing of fewer than 0.3 μm were achieved on a scalable substrate template. On this basis, orientational ordering and pattern-selective deposition of nanorods were realized by controlling the liquid flow rate and employing the substrate with patterned roughness areas, respectively. With the roughness-directed density-tunable depositions of nanorods integrated onto a single platform, multiplexed gold nanorod assembly and programmable surface-enhanced Raman mapping were achieved, with a promising prospect in information encoding by using the Raman signals as the translation units. The thermal stability and related transition temperature of about 160 °C of gold nanorods were also revealed as an application of single-particle statistics. This practical method could be extended to wide ranges of potential applications in plasmonic coupling devices, cryptography, or single-particle performance statistics with the feature of the high-throughput, low-cost, and scalable fabrication.

Reversible changes in the orientation of gold nanorod arrays on polymer brushes

Nanoparticles exhibit a number of unique properties such as localized surface plasmon resonance (LSPR). As this LSPR is sensitive to geometrical or spatial conditions, the arrangement of nanoparticles, in particular the active arrangement of plasmonic structures, is an important issue. In this study, gold nanorod (GNR) arrays were prepared by GNR attachment on anionic polymer (DNA) brushes via electrostatic interactions and their stimuli-responsive changes in orientation were investigated. As a result, the orientation of GNR arrays on DNA brushes reversibly changed by the modulation of electrostatic interactions between GNRs and polymers via changes in the solution pH. As these extensive GNR arrays are prepared via easy bottom-up processes, GNR surface properties are easily tuned by simple modification, and DNAs could be replaced with various synthetic polymers, we believe that this study will lead to the development of next-generation materials and devices with actively tunable structures.

Collector Droplet Behavior during Formation of Nanowire Junctions

Formation of nanowire networks is an appealing strategy for demonstrating novel phenomena at the nanoscale, e.g., detection of Majorana Fermions, as well as an essential step in realizing complex nanowire-based architectures. However, a detailed description of mechanisms taking place during growth of such complex structures is lacking. Here, the experimental observations of gold-catalyzed germanium nanowire junction formation are explained utilizing phase field modeling corroborated with real-time in situ scanning electron microscopy. When the two nanowires collide head on during the growth, we observe two scenarios. (i) Two catalytic droplets merge into one, and the growth continues as a single nanowire. (ii) The droplets merge and subsequently split again, giving rise to the growth of two daughter nanowires. Both the experiments and modeling indicate the critical importance of the liquid-solid growth interface anisotropy and the growth kinetics in facilitating the structural transition during the nanowire merging process.

Chemical Funneling of Colloidal Gold Nanoparticles on Printed Arrays of End-Grafted Polymers for Plasmonic Applications

Spatially defined assembly of colloidal metallic nanoparticles is necessary for fabrication of plasmonic devices. In this study, we demonstrate high-resolution additive jet printing of end-functional polymers to serve as templates for directed self-assembly of nanoparticles into architectures with substantial plasmonic activity. The intriguing aspect of this work is the ability to form patterns of end-grafted poly(ethylene glycol) through printing on a hydrophobic layer that consists of fluoroalkylsilanes. The simultaneous dewetting of the underlying hydrophobic layer together with grafting of the printed polymer during thermal annealing enables fabrication of spatially defined binding sites for assembly of nanoparticles. The employment of electrohydrodynamic jet printing and aqueous inks together with reduction of the feature size during thermal annealing are critically important in achieving high chemical contrast patterns as small as ∼250 nm. Gold nanospheres of varying diameters selectively bind and assemble into nanostructures with reduced interparticle distances on the hydrophilic patterns of poly(ethylene glycol) surrounded with a hydrophobic background. The resulting plasmonic arrays exhibit intense and pattern-specific signals in surface-enhanced Raman scattering (SERS) spectroscopy. The localized seed-mediated growth of metallic nanostructures over the patterned gold nanospheres presents further routes for expanding the composition of the plasmonic arrays. A representative application in SERS-based surface encoding is demonstrated through large-area patterning of plasmonic structures and multiplex deposition of taggant molecules, all enabled by printing.

Self-assembly of anisotropic nanoparticles into functional superstructures

Self-assembly of colloidal nanoparticles (NPs) into superstructures offers a flexible and promising pathway to manipulate the nanometer-sized particles and thus make full use of their unique properties. This bottom-up strategy builds a bridge between the NP regime and a new class of transformative materials across multiple length scales for technological applications. In this field, anisotropic NPs with size- and shape-dependent physical properties as self-assembly building blocks have long fascinated scientists. Self-assembly of anisotropic NPs not only opens up exciting opportunities to engineer a variety of intriguing and complex superlattice architectures, but also provides access to discover emergent collective properties that stem from their ordered arrangement. Thus, this has stimulated enormous research interests in both fundamental science and technological applications. This present review comprehensively summarizes the latest advances in this area, and highlights their rich packing behaviors from the viewpoint of NP shape. We provide the basics of the experimental techniques to produce NP superstructures and structural characterization tools, and detail the delicate assembled structures. Then the current understanding of the assembly dynamics is discussed with the assistance of in situ studies, followed by emergent collective properties from these NP assemblies. Finally, we end this article with the remaining challenges and outlook, hoping to encourage further research in this field.

Close-Packed Ultrasmooth Self-assembled Monolayer of CsPbBr3 Perovskite Nanocubes

The use of colloidal self-assembly to form the complex multiscale patterns in many optoelectronic devices has been a long-standing dream of the nanoscience community. While great progress has been made using charged colloids in polar solvents, controlled assembly from nonpolar solvents is much more challenging. The major challenge is colloidal clustering caused by strong van der Waals (vdW) attraction between long-chain surface capping ligands passivating the surface of nanocrystals. Such clustering degrades ordering in packing during the self-assembly process. While ligand exchange to provide colloidal stability in polar phases is often an option, this is not the case for the exciting new class of halide perovskites due to the material's solubility in essentially all polar solvents. Here, we report surface-functionalized self-assembly of luminescent CsPbBr₃ perovskite nanocubes by partially replacing long-chain oleyl groups (18 carbon chain) with short-chain thiocyanate (SCN^-). This enables the fabrication of ultrasmooth monolayer thin films of nanocubes with a root-mean-square (RMS) roughness of around 4 Å. This ultrasmooth large area self-assembled layer could act as high-efficiency optoelectronic devices like solar cells, light-emitting diodes (LEDs), transistors, etc. We correlate our experimental results with simulations, providing detailed predictions for lattice constants with chain conformations showing reduced free energy for cubes grafted with short-chain thiocyanate compared to long-chain oleyl groups, thus facilitating better self-assembly.

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.

Fabrication of Single-Nanocrystal Arrays

To realize the full potential of nanocrystals in nanotechnology, it is necessary to integrate single nanocrystals into addressable structures; for example, arrays and periodic lattices. The current methods for achieving this are reviewed. It is shown that a combination of top-down lithography techniques with directed assembly offers a platform for attaining this goal. The most promising of these directed assembly methods are reviewed: capillary force assembly, electrostatic assembly, optical printing, DNA-based assembly, and electrophoretic deposition. The last of these appears to offer a generic approach to fabrication of single-nanocrystal arrays.