Self-Organization of Nanorods into Ultra-Long Range Two-Dimensional Monolayer End-to-End Network

Abstract

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.

Biaxial nematic phase stability and demixing behaviour in monolayers of rod–plate mixtures

“…when the mixture demixes in different phases usually possessing liquid-crystal ordering [8][9][10][11]. Finally many experiments on the adsorption of rod-like colloidal particles at the interfaces separating two immiscible fluids showed the propensity of these particles to self-assemble into clusters of different geometries [12][13][14]. The degree of adsorption of these particles at the interface and their relative orientation with respect to it strongly depend on their chemical compositions.…”

“…This in turn can modify their wetting properties and consequently the effective capillary forces acting between particles. The resulting effect is the existence of anisotropy in the pair interaction potential which forces the particles to self-assemble into clusters [13,15,16]. When colloids with very different chemical properties are adsorbed at the interface they usually phase separate into phases with different composition of species.…”

End‐to‐End Self‐Assembly of Semiconductor Nanorods in Water by Using an Amphiphilic Surface Design

“…[1,16] Thes elective binding of thiolated molecules to the (111) facet at both ends of Au NRs led to the development of various approaches for the end-to-end assembly of NRs through Au-thiol chemistry. [13,16,17] On the other hand, template-free self-directed end-to-end assemblies of semiconductor NRs are rather limited, [18,19] which could be as ar esult of strong van der Waals and dipole-dipole forces in al ateral direction preferentially leading to side-by-side assemblies. [20] Theh igher reactivity of the tips of wurtzite CdSe NRs afforded the selective deposition of gold nanoparticles on the ends of the NRs,b ridging two or more NRs together through welding of the gold tips to give end-to-end assemblies.…”

“…[18] Very recently,K im, Lee,a nd co-workers obtained at wo-dimensional end-to-end network of CdSe NRs at the air/toluene interface by means of tipselective etching in toluene. [19] Thee tching changed the surface property of the ends and brought NRs to the air/ toluene interface,a tw hich the capillary attraction force between the NR ends led to the formation of end-to-end assemblies.Thus the facet-selective binding of ligands is akey feature in the introduction of anisotropy at the NR surface. However,t he design of self-directed end-to-end self-assemblies in solution remains achallenge for semiconductor NRs.…”

“…This blue shift suggests the slight etching of CdSe NRs.The presence of halogen ions such as Cl À was reported to cause chemical etching selectively at NR tips. [19,25] It was difficult to detect electronic coupling [26] between NRs by UV/Vis absorption spectroscopy,a lthough TEM measurements showed that the solution aged for 3days contained oligomers such as dimer and trimers (Figure 1b). Optical interaction such as energy transfer was difficult to detect because of the weak emission of NRs.P owder X-ray diffraction (XRD) measurements did not produce any noticeable difference between TDPA-capped CdSe NRs and the assemblies of DMAET-capped NRs ( Figure S5).…”