[Paper Review] How semiconductor nanoplatelets form

Andreas Riedinger, Florian D. Ott+7 more|arXiv (Cornell University)|May 20, 2016

Quantum Dots Synthesis And Properties46 references117 citations

TL;DR

The paper tests and rejects the templated-nucleation mechanism for zincblende nanoplatelets and shows they form due to an intrinsic growth-kinetics instability in isotropic melts, supported by CdSe and CdS1-xSex nanoplatelet syntheses and a kinetic-growth model.

ABSTRACT

Colloidal nanoplatelets - quasi-two-dimensional sheets of semiconductor exhibiting efficient, spectrally pure fluorescence - form when liquid-phase syntheses of spherical quantum dots are modified. Despite intense interest in their properties, the mechanism behind their anisotropic shape and precise atomic-scale thickness remains unclear, and even counterintuitive when their crystal structure is isotropic. One commonly accepted explanation is that nanoclusters nucleate within molecular templates and then fuse. Here, we test this mechanism for zincblende nanoplatelets and show that they form instead due to an intrinsic instability in growth kinetics. We synthesize CdSe and CdS1-xSex nanoplatelets in template- and solvent-free isotropic melts containing only cadmium carboxylate and chalcogen, a finding incompatible with previous explanations. Our model, based on theoretical results showing enhanced growth on narrow surface facets, rationalizes nanoplatelet formation and experimental dependencies on temperature, time, and carboxylate length. Such understanding should lead to improved syntheses, controlled growth on surfaces, and broader libraries of nanoplatelet materials.

Motivation & Objective

• Motivate understanding of why colloidal nanoplatelets adopt quasi-two-dimensional shapes with precise thickness.
• Test whether nanocluster templating and fusion explain nanoplatelet formation in zincblende systems.
• Develop and validate a kinetic-growth model that accounts for nanoplatelet formation and experimental conditions.

Proposed method

• Synthesize CdSe and CdS1-xSex nanoplatelets in template- and solvent-free isotropic melts containing only cadmium carboxylate and chalcogen.
• Evaluate compatibility of results with the nanocluster templating mechanism.
• Propose and apply a growth-kinetics model showing enhanced growth on narrow surface facets.
• Analyze how temperature, growth time, and carboxylate length affect nanoplatelet formation.

Experimental results

Research questions

• RQ1 Do nanoplatelets form via nucleation and fusion within molecular templates, or via an intrinsic growth-kinetics instability?
• RQ2 Can isotropic melts without templates reproduce nanoplatelet formation in Cd-based systems?
• RQ3 How do synthesis parameters (temperature, time, carboxylate length) influence nanoplatelet thickness and lateral dimensions?
• RQ4 What mechanisms explain enhanced growth on narrow facets leading to anisotropic, platelet-like shapes?

Key findings

• Nanoplatelets form in template- and solvent-free isotropic melts with only cadmium carboxylate and chalcogen.
• The observed formation is incompatible with the nanocluster templating mechanism.
• A kinetic-growth model explains formation via intrinsic instability and enhanced growth on narrow surface facets.
• Experimental dependencies on temperature, time, and carboxylate length are rationalized by the model.