4:00pm - 5:00pm
Room 209 Havemeyer
New York, NY 10027
Abstract: In the colloidal growth of semiconductor nanocrystals, capping ligands have been known to confer three advantages: (i) passivation of surface dangling bonds; (ii) controlled NR growth during reaction; and (iii) solution processable products. In the case of nanorods (NRs), surface ligands serve not only as a protection layer but also result in anisotropic growth kinetics. In this presentation, I will discuss my group’s recent experimental results relevant to the role of ligands in growth and assembly of cadmium chalcogenide NRs.
We have investigated diffusion of active species monomers through ligand layers using CdSe NRs as a model system. Colloidal NRs are of special interest for optoelectronic applications because its shape anisotropy leads to unique optical and physical characteristics, expandable with morphological and structural deviation. Previous studies focused on the development of diverse NR structures. However, synthesis relied on empirical observations under specific conditions, and general NR growth process remained elusive. I present a new answer for detailed growth mechanism of colloidal semiconductor NRs. For this, we developed dual-diameter nanorod (DDNR) structure via colloidal synthesis, where two sections along the long axis in each NR have different diameters at a few nanometer scale. The vivid segmentation offers an ideal platform for monitoring the growth process of NRs, presenting important determinants in the reactivity of distinguishable NR facets. By controlling the discovered factors, single-diameter NRs with controllable core position also became available. I will put the findings in perspective by outlining the effect of diffusion of monomers and surface growth reactions.
In addition, we observed that density of surface ligands alters the colloidal interaction between nanorods, ultimately affecting the way NRs assemble during the film deposition process. Assembly of CdSe/CdS core/shell NRs results in various ordered structures in the presence of free oleic acid molecules. Electron microscopy and X-ray scattering data suggest that the assembly is initiated at the air-dispersion interface due to the preferential depletion attraction of NR sidewall to the interface. Interestingly, subsequent growth of superstructures depends heavily on the ligand density that determines the relative magnitude of interface-NR depletion attraction to inter-NR attraction. Highly ordered structures of NRs, especially for the monolayer smectic-phase, are promising as a polarizable emissive layer for optical devices. I will discuss the implication of the growth and assembly of NRs in the context of design of high-efficiency light emitting devices based on QDs.
Friday, February 15, 2019 at 4:00pm
Room 209 Havemeyer
Columbia University, Morningside Campus, Havemeyer Hall | 3000 Broadway, New York, NY 10027, USA | 212-854-2202 | http://chem.columbia.edu/