Nanocrystal Quantum Dots

Victor I. Klimov

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351834520
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Nanotechnology & MEMS
• Index: Technology & Engineering

A review of recent advancements in colloidal nanocrystals and quantum-confined nanostructures, Nanocrystal Quantum Dots is the second edition of Semiconductor and Metal Nanocrystals: Synthesis and Electronic and Optical Properties, originally published in 2003. This new title reflects the book's altered focus on semiconductor nanocrystals.

Gathering contributions from leading researchers, this book contains new chapters on carrier multiplication (generation of multiexcitons by single photons), doping of semiconductor nanocrystals, and applications of nanocrystals in biology. Other updates include:

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• New insights regarding the underlying mechanisms supporting colloidal nanocrystal growth

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• A revised general overview of multiexciton phenomena, including spectral and dynamical signatures of multiexcitons in transient absorption and photoluminescence

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• Analysis of nanocrystal-specific features of multiexciton recombination

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• A review of the status of new field of carrier multiplication

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• Expanded coverage of theory, covering the regime of high-charge densities

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• New results on quantum dots of lead chalcogenides, with a focus studies of carrier multiplication and the latest results regarding Schottky junction solar cells

Presents useful examples to illustrate applications of nanocrystals in biological labeling, imaging, and diagnostics

The book also includes a review of recent progress made in biological applications of colloidal nanocrystals, as well as a comparative analysis of the advantages and limitations of techniques for preparing biocompatible quantum dots. The authors summarize the latest developments in the synthesis and understanding of magnetically doped semiconductor nanocrystals, and they present a detailed discussion of issues related to the synthesis, magneto-optics, and photoluminescence of doped colloidal nanocrystals as well. A valuable addition to the pantheon of literature in the field of nanoscience, this book presents pioneering research from experts whose work has led to the numerous advances of the past several years.

1 “Soft” Chemical Synthesis and Manipulation of Semiconductor Nanocrystals

Jennifer A. Hollingsworth and Victor I. Klimov

CONTENTS

1.1 Introduction

1.2 Colloidal Nanosynthesis

1.2.1 Tuning Particle Size and Maintaining Size Monodispersity

1.2.2 CdSe NQDs: The “Model” System

1.2.3 Optimizing Photoluminescence

1.2.4 Aqueous-Based Synthetic Routes and the Inverse-Micelle Approach

1.3 Inorganic Surface Modification

1.3.1 (Core)Shell NQDs

1.3.2 Giant-Shell NQDs

1.3.3 Quantum-Dot/Quantum-Well Structures

1.3.4 Type-II and Quasi-Type-II (Core)Shell NQDs

1.4 Shape Control

1.4.1 Kinetically Driven Growth of Anisotropic NQD Shapes: CdSe as the Model System

1.4.2 Shape Control Beyond CdSe

1.4.3 Focus on Heterostructured Rod and Tetrapod Morphologies

1.4.4 Solution–Liquid–Solid Nanowire Synthesis

1.5 Phase Transitions and Phase Control

1.5.1 NQDs under Pressure

1.5.2 NQD Growth Conditions Yield Access to Nonthermodynamic Phases

1.6 Nanocrystal Doping

1.7 Nanocrystal Assembly and Encapsulation

Acknowledgment

References

1.1 Introduction

An important parameter of a semiconductor material is the width of the energy gap that separates the conduction from the valence energy bands (Figure 1.1a, left). In semiconductors of macroscopic sizes, the width of this gap is a fixed parameter, which is determined by the material’s identity. However, the situation changes in the case of nanoscale semiconductor particles with sizes less than ~10 nm (Figure 1.1a, right). This size range corresponds to the regime of quantum confinement for which electronic excitations “feel” the presence of the particle boundaries and respond to changes in the particle size by adjusting their energy spectra. This phenomenon is known as the quantum size effect, whereas nanoscale particles that exhibit it are often referred to as quantum dots (QDs).

As the QD size decreases, the energy gap increases, leading, in particular, to a blue shift of the emission wavelength. In the first approximation, this effect can be described using a simple “quantum box” model. For a spherical QD with radius R, this model predicts that the size-dependent contribution to the energy gap is simply proportional to 1/R² (Figure 1.1b). In addition to increasing energy gap, quantum confinement leads to a collapse of the continuous energy bands of the bulk material into discrete, “atomic” energy levels. These well-separated QD states can be labeled using atomic-like notations (1S, 1P, 1D, etc.), as illustrated in Figure 1.1a. The discrete structure of energy states leads to the discrete absorption spectrum of QDs (schematically shown by vertical bars in Figure 1.1c), which is in contrast to the continuous absorption spectrum of a bulk semiconductor (Figure 1.1c).

Semiconductor QDs bridge the gap between cluster molecules and bulk materials. The boundaries between molecular, QD, and bulk regimes are not well defined and are strongly material dependent. However, a range from ~100 to ~10,000 atoms per particle can been considered as a crude estimate of sizes for which the nanocrystal regime occurs. The lower limit of this range is determined by the stability of the bulk crystalline structure with respect to isomerization into molecular structures. The upper limit corresponds to sizes for which the energy level spacing is approaching the thermal energy kT, meaning that carriers become mobile inside the QD.

Semiconductor QDs have been prepared by a variety of “physical” and “chemical” methods. Some examples of physical processes, characterized by high energy input, include molecular-beam-epitaxy (MBE) and metalorganic-chemical-vapor-deposition (MOCVD) approaches to QDs,^1,2,3 and vapor-liquid-solid (VLS) approaches to quantum wires.^4,5 High-temperature methods have also been applied to chemical routes, including particle growth in glasses.^6,7 Here, however, the emphasis is on “soft” (low-energy-input) colloidal chemical synthesis of crystalline semiconductor nanoparticles that will be referred to as nanocrystal quantum dots (NQDs). NQDs comprise an inorganic core overcoated with a layer of organic ligand molecules. The organic capping provides electronic and chemical passivation of surface dangling bonds, prevents uncontrolled growth and agglomeration of the nanoparticles, and allows NQDs to be chemically manipulated like large molecules with solubility and reactivity determined by the identity of the surface ligand. In contrast to substrate-bound epitaxial QDs, NQDs are “freestanding.” This discussion concentrates on the most successful synthesis methods, where success is determined by high crystallinity, adequate surface passivation, solubility in nonpolar or polar solvents, and good size monodispersity. Size monodispersity permits the study and, ultimately, the use of materials-size-effects to define novel materials properties. Monodispersity in terms of colloidal nanoparticles (1–15 nm size range) requires a sample standard deviation of σ ≤ 5%, which corresponds to ± one lattice constant.⁸ Although colloidal monodispersity in this strict sense is increasingly common, preparations are also included in this chapter that achieve approximately σ ≤ 20%, in particular where other attributes, such as novel compositions or shape control, are relevant. In addition, “soft” approaches to NQD chemical and structural modification as well as to NQD assembly into artificial solids or artificial molecules are discussed.

FIGURE 1.1 (a) A bulk semiconductor has continuous conduction and valence energy bands separated by a fixed energy gap, E_g,0 (left), while a QD is characterized by discrete atomic-like states with energies that are determined by the QD radius R (right). (b) The expression for the size-dependent separation between the lowest electron [1S(e)] and hole [1S(h)] QD states (QD energy gap) obtained using the “quantum box” model [m_eh = m_em_h/(m_e + m_h), where m_e and m_h are effective masses of electrons and holes, respectively]. (c) A schematic representation of the continuous absorption spectrum of a bulk semiconductor (curved line), compared to the discrete absorption spectrum of a QD (vertical bars).

1.2 Colloidal Nanosynthesis

The most successful NQD preparations in terms of quality and monodispersity entail pyrolysis of metal-organic precursors in hot coordinating solvents (120°C–360°C). Generally understood in terms of La Mer and Dinegar’s studies of colloidal particle nucleation and growth,^8,9 these preparative routes involve a temporally discrete nucleation event followed by relatively rapid growth from solution-phase monomers and finally slower growth by Ostwald ripening (referred to as recrystallization or aging) (Figure 1.2). Nucleation is achieved by quick injection of precursor into the hot coordinating solvents, resulting in thermal decomposition of the precursor reagents and supersaturation of the formed “monomers” that is partially relieved by particle generation. Growth then proceeds by addition of monomer from solution to the NQD nuclei. Monomer concentrations are below the critical concentration for nucleation, and, thus, these species only add to existing particles, rather than form new nuclei.¹⁰ Once monomer concentrations are sufficiently depleted, growth can proceed by Ostwald ripening. Here, sacrificial dissolution of smaller (higher-surface-energy) particles results in growth of larger particles and, thereby, fewer particles in the system.⁸ Recently, a more precise understanding of the molecular-level mechanism of “precursor evolution” has been described for II-VI¹¹ and IV-VI¹² NQDs. Further, it has also been proposed that the traditional La Mer model is not valid for hot-injection synthesis schemes because nucleation, ripening, and growth may occur almost concurrently. Moreover, the presence of strongly coordinating ligands may also alter nucleation and growth processes, further complicating the simple interpretation of reaction events.¹³ Finally, a modification of the Ostwald ripening process has also been described wherein the particle concentration decreases substantially during the growth process. This process has been called “self-focusing.”^14,15

FIGURE 1.2 (a) Schematic illustrating La Mer’s model for the stages of nucleation and growth for monodisperse colloidal particles. (b) Representation of the synthetic apparatus employed in the preparation of monodisperse NQDs. (Reprinted with permission from Murray, C. B., C. R. Kagan, and M. G. Bawendi, Annu. Rev. Mater. Sci., 30, 545, 2000.)

Alternatively, supersaturation and nucleation can be triggered by a slow ramping of the reaction temperature. Precursors are mixed at low temperature and slowly brought to the temperature at which precursor reaction and decomposition occur sufficiently quickly to result in supersaturation.¹⁶ Supersaturation is again relieved by a “nucleation burst,” after which temperature is controlled to avoid additional nucleation events, allowing monomer addition to exi...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Table of Contents
• Preface to the Second Edition
• Preface to the First Edition
• Editor
• Contributors
• Chapter 1 “Soft” Chemical Synthesis and Manipulation of Semiconductor Nanocrystals
• Chapter 2 Electronic Structure in Semiconductor Nanocrystals: Optical Experiment
• Chapter 3 Fine Structure and Polarization Properties of Band-Edge Excitons in Semiconductor Nanocrystals
• Chapter 4 Intraband Spectroscopy and Dynamics of Colloidal Semiconductor Quantum Dots
• Chapter 5 Multiexciton Phenomena in Semiconductor Nanocrystals
• Chapter 6 Optical Dynamics in Single Semiconductor Quantum Dots
• Chapter 7 Electrical Properties of Semiconductor Nanocrystals
• Chapter 8 Optical and Tunneling Spectroscopy of Semiconductor Nanocrystal Quantum Dots
• Chapter 9 Quantum Dots and Quantum Dot Arrays: Synthesis, Optical Properties, Photogenerated Carrier Dynamics, Multiple Exciton Generation, and Applications to Solar Photon Conversion
• Chapter 10 Potential and Limitations of Luminescent Quantum Dots in Biology
• Chapter 11 Colloidal Transition-Metal-Doped Quantum Dots
• Index