To capitalize on shape- and structure-dependent properties of semiconductor nanorods (NRs), high-precision control and exquisite design of their growth are desired. Cadmium chalcogenide (CdE; E = S or Se) NRs are the most studied class of such, whose growth exhibits axial anisotropy, i.e., different growth rates along the opposite directions of {0001} planes. However, the mechanism behind asymmetric axial growth of NRs remains unclear because of the difficulty in instant analysis of growth surfaces. Here, we design colloidal dual-diameter semiconductor NRs (DDNRs) under the quantum confinement regime, which have two sections along the long axis with different diameters. The segmentation of the DDNRs allows rigorous assessment of the kinetics of NR growth at a molecular level. The reactivity of a terminal facet passivated by an organic ligand is governed by monomer diffusivity through the surface ligand monolayer. Therefore, the growth rate in two polar directions can be finely tuned by controlling the strength of ligand-ligand attraction at end surfaces. Building on these findings, we report the synthesis of single-diameter CdSe/CdS core/shell NRs with CdSe cores of controllable position, which reveals a strong structure-optical polarization relationship. The understanding of the NR growth mechanism with controllable anisotropy will serve as a cornerstone for the exquisite design of more complex anisotropic nanostructures.

Colloidal semiconductor nanocrystals hold great promise in display technologies, as the tunable energy levels and narrow emission bandwidth allow for wide gamut in color space. Impetus for energy-efficient, high-color-quality display has driven the surge of interest in electrically driven quantum dot-based lightemitting diodes (QD-LEDs). While extensive efforts have led to synthesis of QDs with near-unity photoluminescence quantum yield and fabrication of QD-LEDs with external quantum efficiency reaching to the theoretical limit (∼20%), low out-coupling factor poses a challenge in the way of improving the device performance when spherical QDs are used. Geometrically anisotropic nanocrystals (NCs) such as nanorods or nanoplatelets represent a unique possible solution to enhancing light extraction efficiency. In this Perspective, we highlight important design principles of individual anisotropic NCs and their assembly in the context of LED applications.

In this study, we report the shape dependence of fluorescence polarization from colloidal CdSe nanoplatelets (NPLs). Despite the symmetry of their cubic unit cell structure, CdSe nanocrystals grow into two-dimensional platelets in the presence of acetate precursors, and the resulting NPLs exhibit polarized emission. The amount of acetate salts introduced during the synthesis plays a critical role in controlling the lateral aspect ratio of CdSe NPLs. Specifically, the more the acetate hydrate presents, the more squarelike face of NPLs emerges. As a result, we achieved CdSe NPLs with varying lateral aspect ratios ranging from 1.1 to 4.5 in our experimental conditions. At the same thickness, CdSe NPLs with higher lateral aspect ratios exhibit higher fluorescence polarization. We analyzed the shape dependence by preparing films of CdSe NPLs dropcast under an electric field and measuring emission and absorption polarizations from CdSe NPL films of known orientation parameters. The emission polarization stays nearly unchanged regardless of shape anisotropy of CdSe NPLs, while the absorption polarization is affected by the lateral aspect ratio. Now that these results allude to a likelihood that absorption polarization is responsible for the shape dependence of fluorescence polarization, we design a model to assess the correlation between the geometry of NPLs and the optical transition polarization by way of the local field effect. Theoretically estimated absorption polarization also shows shape dependence similar to experimental data, which suggests that the anisotropic local field effect is a primary denominator of shape-dependent fluorescence polarization in CdSe NPLs.

Highly-selective optical sensors that are capable of detecting complex stimuli have attracted significant interest in environmental, biomedical, and analytical chemistry applications. In this work, we report the development of novel and versatile platform for highly-efficient, colorimetric multi-functional sensors using block copolymer-integrated graphene quantum dots (bcp-GQDs). In particular, the multi-functional sensing behavior is successfully generated simply by grafting blue emitting, temperature-responsive block copolymers onto greenemitting, 10-nm size GQD with the GQD providing luminescent response to pH changes. Thus, the bcp-GQDs showed simultaneous, orthogonal sensing behavior to temperature and pH, as well as dose-dependent responses to different types of metal ions. In addition, the bcp-GQD sensor showed excellent reversibility and dispersion stability in pure water, indicating that our system is an ideal platform for environmental and biological applications. The detailed mechanism of the responsive behavior of the bcp-GQDs was elucidated by measurements of time-resolved fluorescence and dynamic light scattering.

Highly uniform large-scale assembly of nanoscale building blocks can enable unique collective properties for practical electronic and photonic devices. We present a two-dimensional (2-D), millimeter-scale network of colloidal CdSe nanorods (NRs) in monolayer thickness through end-to-end linking. The colloidal CdSe NRs are sterically stabilized with tetradecylphosphonic acid (TDPA), and their tips are partially etched in the presence of gold chloride (AuCl3) and didecyldimethylammonium bromide (DDAB), which make them unwetted in toluene. This change in surface wetting property leads to spontaneous adsorption at the 2-D air/toluene interface. Anisotropy in both the geometry and the surface property of the CdSe NRs causes deformation of the NR/toluene/air interface, which derives capillary attraction between tips of neighboring NRs inward. As a result, the NRs confined at the interface spontaneously form a 2-D network composed of end-to-end linkages. We employ a vertical-deposition approach to maintain a consistent rate of NR supply to the interface during the assembly. The rate control turns out to be pivotal in the preparation of a highly uniform large scale 2-D network without aggregation. In addition, unprecedented control of the NR density in the network was possible by adjusting either the lift-up speed of the immersed substrate or the relative concentration of AuCl3 to DDAB. Our findings provide important design criteria for 2-D assembly of anisotropic nanobuilding blocks.

Anisotropic microparticles are promising as a new class of colloidal or granular materials due to their advanced functionalities which are difficult to achieve with isotropic particles. However, synthesis of the anisotropic microparticles with a highly controlled size and shape still remains challenging, despite their intense demands. Here, we report a microfluidic approach to create uniform anisotropic microparticles using phase separation of polymer blends confined in emulsion drops. Two different polymers are homogeneously dissolved in organic solvent at low concentration, which is microfluidically emulsified to produce oil-in-water emulsion drops. As the organic solvent diffuses out, small domains are formed in the emulsion drops, which are then merged, forming only two distinct domains. After the drops are fully consolidated, uniform anisotropic microparticles with two compartments are created. The shape of the resulting microparticles is determined by combination of a pair of polymers and type of surfactant. Spherical microparticles with eccentric core and incomplete shell are prepared by consolidation of polystyrene (PS) and poly(lactic acid) (PLA), and microparticles with single crater are formed by consolidation of PS and poly(methyl methacrylate) (PMMA); both emulsions are stabilized with poly(vinyl alcohol) (PVA). With surfactants of triblock copolymer, acorn-shaped Janus microparticles are obtained by consolidating emulsion drops containing PS and PLA. This microfluidic production of anisotropic particles can be further extended to any combination of polymers and colloids to provide a variety of structural and chemical anisotropy.

Because of the large surface-to-volume ratio of colloidal nanocrystals (NCs), surfactant molecules grafted at the NC surface play an important role in NC growth, interparticle interaction, processing, and application. For this reason, much progress has been made in understanding the surface chemistry of NCs along with the organic ligand shell, particularly in terms of grafted polar groups. However, most explanations of aliphatic counterparts are based on spherical NCs that usually have a dilute ligand layer. In anisotropic NCs such as nanorods and nanoplatelets, the linearly extended dimension results in a highdensity aliphatic layer on the NC surface. Unlike spherical NCs, the compact organic shell could serve as a permeation membrane, effectively impeding a penetration of foreign molecules toward the NC surface. In this Perspective, we highlight the effects of ligand configuration on the properties of anisotropic NCs by exploring morphologies, assembled superstructures, and surface reaction of anisotropic NCs.

Relatively large faces of colloidal CdSe nanoplatelets (NPLs) drive the anisotropic nanomaterials into one-dimensional superstructures through stacking of NPLs when solvent evaporates. We observe that the assembly could result in the formation of twisted ribbon superstructures with varying pitch length depending on lateral dimensions of CdSe NPLs. Transmission electron microscopy images and simulated projection reveal that stacked NPLs are distorted. The estimation of the contact area between distorted NPLs suggests that this distortion leads to lower energy of overall nanoribbon superstructures. The average pitch length of superstructures on the lateral dimension of NPLs depends on the dimension of NPLs as it alters the distortion angle of NPLs and thus the rotation angle between NPLs. We investigate the energy transfer between NPLs in the context of the lateral dimension of NPLs and the geometric structure of their superstructures via transient photoluminescence decay measurements. Our analysis on the energy-transfer rate indicates that extinction coefficients, which are determined by lateral dimension of NPLs, are more responsible for the energy-transfer change than the rotation angle between CdSe NPLs within twisted ribbon superstructures. The dependence of the energy-transfer rate on the lateral dimension of NPLs highlights the importance of geometry of individual NPLs in the context of optical properties of NPLs in ensemble.