Chapter 1
Controllable Self-Assembly of One-Dimensional Nanocrystals

Shaoyi Zhang, Yang Yang and Zhihong Nie

University of Maryland, Department of Chemistry and Biochemistry, 8051 Regents Drive, College Park, MD, 20742, USA

1.1 Introduction

In the past decades, controlled assembly of nanocrystals (NCs) has been a topic of continuous interest. The tremendous interest in organizing NCs mainly stemmed from the potential applications of NCs in diverse areas, including chemical and biological sensing, energy storage and production, and optoelectronic devices. Recent advances at this frontier are also partially driven by the rapid development in the synthesis of relatively monodispersed NC building blocks with controlled size, shape, and composition [1, 2]. Compared to spherical NCs, one dimensional (1D) NCs, that is, nanorods (NRs) and nanowires (NWs), exhibit unique optical, electronic, and magnetic properties due to their shape anisotropy [3, 4]. Taking Au NRs as an example, they exhibit localized surface plasmon resonance (LSPR) and strong photothermal effect, which enable their broad applications in such areas as cancer imaging and therapy. The LSPR absorption of these NRs can be tailored in the range from visible to near-infrared wavelengths for specific applications by controlling the aspect ratio of NRs. To date, various methods have been developed for the organization of 1D NCs into functional structures. In particular, the bottom-up assembly approach offers a more robust, scalable, and cost-effective way to fabricate arrays of NCs in a controlled manner, compared to top-down techniques such as electron-beam and focused-ion beam lithography [5]. In this chapter, we have classified the current self-assembly methods into four major categories, namely templated assembly, field-driven assembly, assembly at interfaces, and ligand-guided assembly. The following section features the properties and applications of assemblies of 1D NCs. Finally, conclusion and outlook are presented in the last section.

1.2 Assembly Strategies

1.2.1 Templated Assembly

Templated assembly, as the name implies, is the assembly directed by a predesigned template, which governs the anchoring of NCs on (or within) the template. As a straightforward method, the geometry and patterning of templates or the distribution of functional groups on the template dictates the location, orientation, or alignment of NCs as well as the association state of NCs. Typically, the templating effect arises from physical confinement of the template, differential affinity of NCs toward the template surface, or chemical bonding between the ligands on NCs and templates. Recent progress in this frontier has allowed the assembly of 1D NCs into long-range-ordered structures with high yield and well-defined orientation of NCs at single-particle resolution. This largely fulfills the requirement of assemblies on substrates for applications including biosensors, optical devices, metamaterials, and so on [6]. In this section we discuss several categories of assembly systems, classified on the basis of the characteristics of templates.

1.2.1.1 Geometrically Patterned Template

Assembly based on geometrically patterned template utilizes the topographical patterns on a substrate to direct the interaction and shape-selective organization of NCs within the patterns. Representative topographical patterns include periodic polygonal grooves, discrete spherical poles, and parallel channels. These templates are usually fabricated by top-down approaches such as photolithography, chemical vapor deposition, inkjet printing, and focused beam (electrons, ions, laser, etc.) etching [7]. To achieve the desired assembly structures of NCs, the geometric parameters (e.g., size, depth, and geometry of patterns) have to be carefully tuned. By using this method, various types of assemblies have been produced, such as discrete clusters, 1D arrays, 2D monolayers, and 3D supercrystals.

Toward the end of the last century, Blaaderen et al. reported the crystallization of bulk colloidal crystals through the slow sedimentation of silica spheres onto a pole-patterned poly(methyl methacrylate) (PMMA) layer [8]. Xia et al. fabricated a series complex aggregates of polystyrene beads including polygonal or polyhedral clusters, linear or zigzag chains, and circular rings by combing physical templating and capillary forces [9]. Thanks to the developments in pattern design, great progress has been achieved in fabricating diverse arrays of 1D NCs on patterned substrates with high yield, good scalability, and superior morphology control [10]. Bach et al. produced free-standing arrays of hexagonal close-packed Au NRs on predefined locations using a patterned substrate containing square grooves of different wettability on the surface of the substrate [11]. Recently, Brugger et al. realized the capillary assembly of Au NRs into large-area ordered structures on substrates with predetermined surface patterns [12]. In a typical capillary assembly, the colloidal solution is confined between a patterned substrate and a sliding top plate. The receding meniscus directs the colloidal solution to move over the substrate in a controlled manner. Subsequently, NCs assemble from the three-phase contact line at predetermined assembly sites. In order to improve the accuracy and success rate in the placement of NCs, it is crucial to prevent the possible removal of NCs that are inserted in the traps. In this work, the precise control over the organization of Au NRs on the substrate was accomplished by the delicate design of the geometry of the traps. As shown in Figure 1.1, funneled traps with auxiliary sidewalls were fabricated to effectively prevent the removal of NRs after their insertion into the traps. With the introduction of sidewalls, the assembly yield goes up to 100%. The positional control of Au NRs goes down to the nanometer scale. As shown in Figure 1.1b, Au NRs can be selectively placed onto a substrate with arbitrary patterns by using this method.

Figure 1.1 Assembly of Au NRs on a patterned solid substrate. (a) Scheme of capillary assembly of Au NRs onto substrates with geometrical patterns. (b) SEM images of Au NRs patterns by topographically templated capillary assembly. Scale bar: 250 nm.

(Flauraud et al. 2016 [12]. Reproduced with permission of Nature Publishing Group.)

Apart from directly modulating the patterns on a substrate, confinement can be used to control the organization of NCs. Typically, a suspension of NCs is confined between a topologically patterned template at the top and a smooth substrate at the bottom. The subsequent evaporation of solvent in a controlled manner drives the formation of an orientated array of NCs. The above templates function as both regulating the solvent evaporation and controlling the deposition of NCs at given locations on the substrate. Typical templates such as the elastomeric poly(dimethyl siloxane) (PDMS) stamp and highly oriented pyrolytic graphite (HOPG) have been employed to provide confinement to facilitate the formation of closely packed arrays of 1D NCs [13]. In this case, the templates can be easily removed and recycled while the assemblies are left on flat substrates. As an example, Ahmed et al. reported the formation of perpendicular superlattices of hexagonally oriented CdS NRs using an HOPG template [14]. A dispersion of CdS NRs in toluene was trapped between a block of HOPG and a smooth silicon wafer. Upon slow evaporation of the solvent, a large-area (~2 µm2) monolayer of perpendicularly oriented NRs was formed on the substrate. It was found that the monodispersity and hexagonal facets along the c-axis of wurtzite NRs are crucial to the formation of highly ordered lattices. Also, the cleaved surface of the HOPG substrate efficiently trapped the NRs in a narrow capillary, facilitating the slow evaporation of solvents. Later, Liz-Marzan et al. reported a simple assembly method to produce large-area (up to millimeter size) supercrystal arrays of Au NRs using a patterned PDMS mold [15]. The supercrystals with tunable size, shape, and height exhibited homogeneous and intense electric field enhancement over the entire assemblies for effective surface-enhanced Raman spectroscopy (SERS) detection.

The assembly strategy using geometrically patterned templates allows the fabrication of large-area, predetermined assembly structures with controlled geometric parameters. These assemblies of NCs can be readily integrated with a device supported on a substrate and tailored for a broad range of applications such as sensors and optoelectronics by designing the templates. To facilitate the interaction between the template and NCs, this method often requires the surface modification of NCs with designed small or large molecules via methods such as ligand exchange.

1.2.1.2 Chemically Patterned Template

Chemically patterned templates such as carbon nanotubes (CNTs), polymer matrices, and peptide nanostructures have been extensively explored for guiding the assembly of 1D NCs. In this method, the organization of the NCs is determined by the spatial arrangement of the chemical functional groups that have strong affinity toward NCs. This approach enables the control of not only the position and orientation of NCs with single-particle precision but also the formation of NC arrays in a 3D space. We exclude assembly templated by DNA nanostructures in this section and leave this topic in the section on ligand-guided assembly.

In the simplest scenario, 1D nano- or micro-structures (e.g., CNT, nanofibers, etc.) can be used...