Colloidal nanocrystals (NCs, i.e., crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Today’s strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing in situ characterization and direct imaging techniques. New phenom...

Prospects of Nanoscience with Nanocrystals

Quantum dots are a unique class of emitters with size-tunable emission wavelengths, saturated emission colors, near-unity luminance efficiency, inherent photo- and thermal- stability and excellent solution processability. Quantum dots have been used as down-converters for back-lighting in liquid-crystal displays to improve color gamut, leading to the booming of quantum-dot televisions in consumer market. In the past few years, efficiency and lifetime of electroluminescence devices based on quantum dots achieved tremendous progress. These encouraging facts foreshadow the commercialization of quantum-dot light-emitting diodes (QLEDs), which promises an unprecedented generation of cost-effective, large-area, energy-saving, wide-color-gamut, ultra-thin and flexible displays. Here we provide a Progress Report, covering interdisciplinary aspects including material chemistry of quantum dots and charge-transporting layers, optimization and mechanism studies of prototype devices and processing techniques to produce large-area and high-resolution red-green-blue pixel arrays. We also identify a few key challenges facing the development of active-matrix QLED displays.

Quantum-Dot Light-Emitting Diodes for Large-Area Displays: Towards the Dawn of Commercialization.

We review the synthesis of semiconductor nanocrystals/colloidal quantum dots in organic solvents with special emphasis on earth-abundant and toxic heavy metal free compounds. Following the Introduction, section 2 defines the terms related to the toxicity of nanocrystals and gives a comprehensive overview on toxicity studies concerning all types of quantum dots. Section 3 aims at providing the reader with the basic concepts of nanocrystal synthesis. It starts with the concepts currently used to describe the nucleation and growth of monodisperse particles and next takes a closer look at the chemistry of the inorganic core and its interactions with surface ligands. Section 4 reviews in more detail the synthesis of different families of semiconductor nanocrystals, namely elemental group IV compounds (carbon nanodots, Si, Ge), III–V compounds (e.g., InP, InAs), and binary and multinary metal chalcogenides. Finally, the authors’ view on the perspectives in this field is given.

Synthesis of Semiconductor Nanocrystals, Focusing on Nontoxic and Earth-Abundant Materials.

Nanomedicine has offered new hope for cancer treatment.[1] Nanotherapeutics exhibit many advantages over small-molecule chemotherapeutics, including diminished systemic toxicity and improved circulation times. Unfortunately, non-uniformly leaky vasculature[2] and a dense interstitial structure[3] hinder their effective delivery to tumors.[4] These physiological abnormalities make transvascular transport—movement from vessels to the interstitium—and interstitial transport—movement through the interstitium to target cells—heterogeneous.[4a] Hence the tumor microenvironment limits the uniform penetration of nanotherapeutics by slowing or halting their transport through hydrodynamic and steric hindrance.[2a,3a,5] Overcoming these physiological barriers in tumors is an outstanding challenge for nanomedicine.

Fluorescent Nanorods and Nanospheres for Real-Time In Vivo Probing of Nanoparticle Shape-Dependent Tumor Penetration

This review summarizes traditional and recent nonconventional, bioinspired, methods for the aqueous synthesis of colloidal semiconductor quantum dots (QDs). The basic chemistry concepts are critically emphasized at the very beginning as these are strongly correlated with the selection of ligands and the optimal formation of aqueous QDs and their more sophisticated structures. The synergies of biomimetic and biosynthetic methods that can combine biospecific reactivity with the robust and strong optical responses of QDs have also resulted in new approaches to the synthesis of the nanoparticles themselves. A related new avenue is the recent extension of QD synthesis to form nanoparticles endowed with chiral optical properties. The optical characteristics of QD materials and their advanced forms such as core/shell heterostructures, alloys, and doped QDs are discussed: from the design considerations of optical band gap tuning, the control and reduction of the impact of surface traps, the consideration of charg...

Aqueous Based Semiconductor Nanocrystals

Emissive saturated colors are key components of new generations of lighting and display technologies. Quantum dots have evolved in the past two decades to fulfill many of the requirements of color purity, stability, and efficiency that are critical to transitioning these materials from the laboratory into these markets. A fundamental feature of quantum dots is the tunability of their emission color through precise control of their size and composition, giving access to UV, visible, and near-infrared wavelengths. Continuing improvements in engineering core–shell quantum dot structures, where a 1–10 nm binary, ternary, or alloyed semiconductor core particle is surrounded by a shell composed of one or more semiconductors of a wider bandgap, have resulted in materials with fluorescence quantum yields that approach unity, narrow symmetric spectral line shapes, and remarkable stabilities. In this article, we review progress in the development of highly luminescent core–shell quantum dots of different semiconductor families in view of their integration in light-emitting applications. CdSe-based quantum dots already fulfill many of the requirements of lighting and display applications in terms of fluorescence quantum yield, color purity, and stability.

Pure colors from core–shell quantum dots

Ternary metal chalcogenide nanocrystals (NCs) offer exciting opportunities as novel materials to be explored on the nanoscale showing optoelectronic properties tunable with size and composition. CuInS2 (CIS) NCs are the most widely studied representatives of this family as they can be easily prepared with good size control and in high yield by reacting the metal precursors (copper iodide and indium acetate) in dodecanethiol (DDT). Despite the widespread use of this synthesis method, both the reaction mechanism and the surface state of the obtained NCs remain elusive. Here, we perform in situ X-ray diffraction using synchrotron radiation to monitor the pre- and postnucleation stages of the formation of CIS NCs. SAXS measurements show that the reaction intermediate formed at 100 °C presents a periodic lamellar structure with a characteristic spacing of 34.9 A. WAXS measurements performed after nucleation of the CIS NCs at 230 °C demonstrate that their growth kinetics depend on the degree of precursor conver...

Growth Mechanism and Surface State of CuInS2 Nanocrystals Synthesized with Dodecanethiol.

Chemically synthesised In(Zn)P alloy nanocrystals are doped with Eu3+ ions using europium oleate as a molecular precursor and are subsequently covered with a ZnS shell. The presence of zinc in the synthesis of the InP core nanocrystals leads to the formation of an In(Zn)P alloy structure, making it possible to obtain stable fluorescence emission at 485 nm. We demonstrate by means of steady state and time resolved photoluminescence measurements that resonant energy transfer takes place from the In(Zn)P/ZnS host to the Eu3+ dopant ions. It results in the characteristic phosphorescence lines of Eu3+ originating from the transitions between the lowest-lying excited state 5D0 to the 7FJ (J = 1, 2, 3, 4) ground states. The maximum phosphorescence efficiency is obtained for an initially applied Eu3+ : In3+ molar ratio of 0.3 : 1, resulting in a final doping level of approximately 4%.

Europium doped In(Zn)P/ZnS colloidal quantum dots

Over the past two decades, colloidal semiconductor nanocrys-tals (NCs) have been the object of intense research. In order to fully exploit their unique size-dependent physical proper-ties, robust synthetic pathways are needed yielding NCs of con-trolled size, shape, composition, and surface chemistry. The largest progress in the chemical synthesis of NCs has been achieved so far with II–VI semiconductors such as CdSe and CdTe.

Axel Maurice

Papers

Pure colors from core–shell quantum dots

Growth Mechanism and Surface State of CuInS2 Nanocrystals Synthesized with Dodecanethiol.

Europium doped In(Zn)P/ZnS colloidal quantum dots

Synthesis of Colloidal Indium Antimonide Nanocrystals Using Stibine

Quantum Dots: Synthesis of Colloidal Indium Antimonide Nanocrystals Using Stibine (Part. Part. Syst. Charact. 10/2013)