Colloidal growth and self-assembled structures of semiconductor nanorods

Abstract

Semiconductor nanorods (NRs), namely elongated NCs, are of particular interest in the field of nanomaterials and nanotechnology because of their unique properties including linearly polarized emission and effective charge transport along their long axis. To capitalize on shape- and structure-dependent properties of semiconductor NRs, high-precision control and exquisite design of their growth are desired. Nevertheless, the mechanism behind the axial growth of NR still remains unclear because of the difficulty in instant analysis of growth surfaces. To unveil the NR growth mechanism, we design colloidal dual-diameter semiconductor NRs (DDNRs), 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. Building on these findings, we report the synthesis of single-diameter CdSe/CdS core/shell NRs with CdSe cores of controllable position, which reveals strong structure-optical polarization relationship. The understanding on NR growth mechanism with controllable anisotropy will serve as a cornerstone for the exquisite design of more complex anisotropic nanostructures.

When NRs are introduced in application devices, asymmetry and directionality of the ensemble are closely related to collective properties. Since unique characteristics of NRs are manifested along the axial direction, NRs must be assembled into precise configurations suitable for target applications. For example, end-to-end network of NRs offers the most effective pathway for carrier transport, thereby serving as continuous electrical channels without impairing intrinsic properties of NRs such as quantum confinement effect. However, despite their intensive demands and potentials, end-to-end linking of colloidal NRs has been relatively underexplored because this assembly is commonly not favored due to strong attraction along side-by-side direction. In this study, we present a 2-dimensional (2-D) networking of colloidal CdSe nanorods (NRs) in monolayer thickness through end-to-end linking, which yields a single homogeneous cluster over millimeter-scale. The approach established here enables the creation of the homogeneous percolating network of NRs in ultra-long-range useful for optoelectronics and photovoltaics.

On the other hand, to display linearly polarized emission from NR ensemble, NRs should be assembled with both side-by-side and end-to-end alignments. There has been extensive researches on this kind of assemblies of NRs. However, although side-by-side bundle of NRs is favorably formed in concentrated NRs, the size of the domain, where constituent NRs are aligned in end-to-end direction, is very small. Besides, inhomogeneous and thick NR assembled structures are unsuitable for conventional film devices. In light of these problems, the most desired form of NR ensemble for its potential usefulness as polarizable material is long-range and uniform self-assembled monolayers with compact packing of NRs into smectic phase. Here, we present highly ordered smectic-phase self-assembled monolayers of CdSe/CdS NRs over large length scale at air/solution interface by introducing depletion force between the interface and NR. Unidirectional alignment in the smectic-phase monolayer is expected to exhibit strong photoluminescence anisotropy, enabling its use as emissive film in polarized light-emitting devices. Furthermore, this simple and scalable assembly approach can provide unique means to create ultra-long-range and ultra-thin self-assembled monolayers of nano-building blocks.