Semiconductor Nanorod Self-Assembly at the Liquid/Air Interface Studied by in Situ GISAXS and ex Situ TEM

Abstract

ABSTRACT:We study the self-assembly of colloidal CdSe/CdS nanorods (NRs) at the liquid/air interface combining timeresolved in situ grazing-incidence small angle X-ray scattering (GISAXS) and ex situ transmission electron microscopy (TEM). Our study shows that NR superstructure formation occurs at the liquid/air interface. Short NRs self-assemble into micrometers long tracks of NRs lying side by side flat on the surface. In contrast, longer NRs align vertically into ordered superstructures. Systematic variation of the NR length and initial concentration of the NR dispersion allowed us to tune the orientation of the NRs in the final superstructure. With GISAXS, we were able to follow the dynamics of the self-assembly. We propose a model of hierarchical self-organization that provides a basis for the understanding of the length-dependent self-organization of NRs at the liquid/air interface. This opens the way to new materials based on NR membranes and anisotropic thin films. KEYWORDS: Nanocrystals, self-assembly, superlattice, grazing-incidence small-angle X-ray scattering T he ability to design the chemical and physical properties of colloidal nanocrystals (NCs) at the atomic level, combined with the possibilities of NC self-assembly is of great fundamental and applied interest. Self-assembly of NCs is a promising route toward the fabrication of new classes of materials in which collective properties may emerge as consequence of dipolar or quantum coupling between the NC building blocks.1,2 NC superlattices and NC "membranes" are increasingly exploited as tailored nanostructured materials in optoelectronic devices such as LEDs, 3−5 lasers 6 and solar cells.7−10 Direct formation of nanocrystal superlattices at the liquid/air interface is currently emerging as a promising method to fabricate functional ultrathin films. Previous works reported promising results on the formation of (mm 2 large) areas of thin films of binary and ternary superlattices composed of spherical NCs. These studies enhanced our understanding of the driving forces underlying NCs self-assembly. 11−17A new direction in this field is self-assembly of highly anisotropic nanocrystals, 18 such as nanorods (NRs). 19 The formation of vertically aligned and hexagonally packed superlattices was initially performed using an external electric field during the process of solvent evaporation.20,21 Subsequent efforts, however, have demonstrated that vertically aligned NR superlattices can be obtained by controlled solvent evaporation in the absence of any external forces.22−34 Self-assembly parameters like the aspect ratio of the NRs, evaporation temperature, and type of solvent were extensively explored. 35 However, only the final structures were studied. To our knowledge, no in situ investigation of the dynamics of the selfassembly process has been reported, yet.Here, we present a study of the mechanism of and the final structures formed by semiconductor NRs self-assembly. We observed that self-assembly occurs at the suspension/air interface and consists o...

Synthesis and X-ray Characterization of Cobalt Phosphide (Co2P) Nanorods for the Oxygen Reduction Reaction

“…The small X-ray scattering (SAXS) pattern in Figure 3a confirms the nanorod self-assembly seen in Figure 2a,b with a peak at q = 0.117 Å -1 , corresponding to the expected average center-to-center distance of 2.8 ± 0.9 nm for the hard core surrounded by two layers of OLAC in the soft ligand shell. 50 Grazing incidence small-angle X-ray scattering of Co 2 P showed that there is local hexagonal packing of the NRs in the films ( Figure S3). This is based on higher scattering intensities at azimuthal angles of integer multiples of π/3 seen in Figure 3b.…”

Quantifying Angular Correlations between the Atomic Lattice and the Superlattice of Nanocrystals Assembled with Directional Linking

“…1,2 Colloidal NCs stabilized by organic surfactants have been shown to pose excellent building blocks for the design of synthetic MCs with tailored structural properties which are conveniently obtained by self-assembly of NCs from solution on a solid or liquid substrate by exploiting ligand-ligand 3 interactions. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] Typically, the utilized ligands consist of wide-gap, bulky hydrocarbons which render the MCs insulating. [26][27][28][29][30][31][32][33] MCs obtained in this way exhibit average grain sizes of ~150 µm 2 , which enables a detailed characterization by electron and/or X-ray microscopy.…”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…Colloidal NCs can also selfassemble into three-dimensionally ordered colloidal superparticles [131]. The geometry and properties of these superstructures can be tailored by the size, shape, composition and surface chemistry of the NC or hetero-NC building blocks [129][130][131][132][133][134][135][136][137][138]. In particular, surface ligands have been shown to have a dramatic impact on the directionality of the self-organization process [135,[139][140][141][142][143], leading in some cases to atomically aligned NC superlattices [135,139,143].…”