We describe the close connection between novel chemical synthesis and optimized light emission by colloidal semiconductor quantum dots (qdots). We describe how new insights and systematic improvement in synthesis and characterization have led to highly luminescent qdots that are now used in three-color liquid-crystal displays in large televisions. We outline synthetic and structural issues that require further work to enable additional applications in solar concentrators, solid-state lighting, single-photon devices, optical computing, and in vivo infrared medical imaging. Chemical synthesis is the most creative and critical aspect of colloidal qdots.

Chemical Synthesis and Luminescence Applications of Colloidal Semiconductor Quantum Dots

Infrared technologies provide tremendous value to our modern-day society. The need for easy-to-fabricate, solution-processable, tunable infrared active optoelectronic materials has driven the development of infrared colloidal quantum dots, whose band gaps can readily be tuned by dimensional constraints due to the quantum confinement effect. In this Perspective, we summarize recent progress in the development of infrared quantum dots both as infrared light emitters (e.g., in light-emitting diodes, biological imaging, etc.) as well as infrared absorbers (e.g., in photovoltaics, solar fuels, photon up-conversion, etc.), focusing on how fundamental breakthroughs in synthesis, surface chemistry, and characterization techniques are facilitating the implementation of these nanostructures into exploratory device architectures as well as in emerging applications. We discuss the ongoing challenges and opportunities associated with infrared colloidal quantum dots.

Infrared Quantum Dots: Progress, Challenges, and Opportunities

The precise control in size/thickness, composition, crystal phases, doping, defects, and surface properties of two-dimensional (2D) layered transition metal chalcogenide (TMC) is important for the investigation of interwoven relationship between structures, functions, and practical applications. Of the multiple synthetic routes, solution-based top-down and bottom-up chemical methods have been uniquely important for their potential to control the size and composition at the molecular level in addition to their scalability, competitive production cost, and solution processability. Here, we introduce an overview of the recent advances in the solution-based preparation routes of 2D layered TMC nanostructures along with important scientific developments.

Recent Advances in the Solution-Based Preparation of Two-Dimensional Layered Transition Metal Chalcogenide Nanostructures.

While the surface termination of quasi-spherical metal chalcogenide nanocrystals or quantum dots has been widely investigated, it remains unclear whether the ensuing surface chemistry models apply to similar nanocrystals with anisotropic shapes. In this work, we report on the surface-chemistry of 2D CdSe nanoplatelets, where we make use of an improved synthesis strategy that yields stable and aggregation free nanoplatelet suspensions with a photoluminescence quantum yield as high as 55%. We confirm that such nanoplatelets are enriched in Cd and, by means of 1H nuclear magnetic resonance spectroscopy, we show that the Cd-rich surface is terminated by X-type carboxylate ligands. Not unlike CdSe quantum dots (QDs), entire cadmium carboxylate entities can be displaced by the addition of amines, and the desorption isotherm points toward a considerable binding site heterogeneity. Moreover, we find that even the slightest displacement of cadmium carboxylate ligands quenches the nanoplatelet photoluminescence. These experimental findings are further confirmed by density functional theory (DFT) calculations on a 5 monolayer model CdSe nanoplatelet. These simulations show that the most labile ligands are located in the vicinity of facet edges, and that the displacement of ligands from such edge sites creates midgap states that can account for the observed photoluminescence quenching. Next to extending surface chemistry insights from colloidal QDs to nanoplatelets, this work indicates that CdSe nanoplatelets constitute a unique nanocrystal model system to establish a comprehensive description of midgap trap states, which includes their structural, chemical, and electronic properties.

Colloidal CdSe Nanoplatelets, A Model for Surface Chemistry/Optoelectronic Property Relations in Semiconductor Nanocrystals.

A look back at LaMer’s 1950 model of particle formation is undertaken (LaMer, V. K.; Dinegar, R. H. Theory, Production and Mechanism of Formation of Monodispersed Hydrosols. J. Am. Chem. Soc. 1950,...

LaMer’s 1950 Model for Particle Formation ofInstantaneous Nucleation and Diffusion-Controlled Growth: A HistoricalLook at the Model’s Origins, Assumptions, Equations, and UnderlyingSulfur Sol Formation Kinetics Data

We report a tunable library of N,N,N′-trisubstituted selenourea precursors and their reaction with lead oleate at 60–150 °C to form carboxylate-terminated PbSe nanocrystals in quantitative yields. Single exponential conversion kinetics can be tailored over 4 orders of magnitude by adjusting the selenourea structure. The wide range of conversion reactivity allows the extent of nucleation ([nanocrystal] = 4.6–56.7 μM) and the size following complete precursor conversion (d = 1.7–6.6 nm) to be controlled. Narrow size distributions (σ = 0.5–2%) are obtained whose spectral line widths are dominated (73–83%) by the intrinsic single particle spectral broadening, as observed using spectral hole burning measurements. The intrinsic broadening decreases with increasing size (fwhm = 320–65 meV, d = 1.6–4.4 nm) that derives from exciton fine structure and exciton–phonon coupling rather than broadening caused by the size distribution.

Robert Andrew Swain

Papers

A Library of Selenourea Precursors to PbSe Nanocrystals with Size Distributions near the Homogeneous Limit