Chemical Synthesis and Luminescence Applications of Colloidal

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Chemical Synthesis and Luminescence Applications of Colloidal Semiconductor Quantum Dots Jonathan Owen and Louis Brus* Chemistry Department, Columbia University, New York, New York 10027, United States ABSTRACT: 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, solidstate lighting, single-photon devices, optical computing, and in vivo infrared medical imaging. Chemical synthesis is the most creative and critical aspect of colloidal qdots.

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Figure 1. Archetypal quantum dot heterostructure. An emissive CdSe core is shelled in increasingly electrically insulating CdS and ZnS layers that improve the luminescence efficiency. Surface-bound ligands (e.g., carboxylates) are bound to a metal-enriched surface.

INTRODUCTION Colloidal quantum dots (qdots) form a new branch of chemistry; for example, a Scifinder search shows that 2357 review articles on qdots have been published since 2012. In this Perspective we offer our thoughts on a specific aspect of this work: light emission by colloidal inorganic qdots. Compared with molecules, inorganic phosphors, organic materials, and bulk semiconductors, qdots offer a number of powerful luminescence properties. Qdot chemical research now shows signs of approaching the same standards of rigor, characterization, and mechanistic understanding that underlie all of chemistry. In this work the importance of chemical synthesis cannot be overemphasized. A comprehensive multi-group review published in 2015 describes the broad range of present qdot research.1

These original qdots had poor crystalline quality, a wide size distribution, and weak luminescence. In the late 1980s, it was found that the qdot surfaces could be capped and stabilized by covalent bonding to organic species (i.e., phenyl-capped CdS qdots).3 The organic ligands provided stable colloidal dispersions and allowed the qdots to be isolated as a dry powder. Also, the idea of a core/shell structure was explored. By growing a high-band-gap insulating shell on a low-band-gap core (i.e., CdSe/ZnS core/shell), the surface of the CdSe core is passivated. This enormously increased the size-tunable core CdSe emission.4 Finally, it was discovered that high-quality crystalline qdots could be made by organometallic reactions in Lewis-basic solvents (for example, tri-n-butylphosphine oxide) under Ar at temperatures above 200 °C.5,6 The hightemperature Lewis base approach and the core/shell structure idea have both proven to be fruitful. The wide size distribution and poor quality of early colloids significantly broadened the optical spectra and thus obscured the true properties of individual crystalline qdots. Size broadening in the absorption spectra was demonstrated using laser hole-burning experiments at low temperature.7,8 Subsequently, the luminescence of individual CdSe/ZnS core/shell qdots at 23 °C under ambient conditions was observed using confocal methods.9,10 The quantum-confined luminescence of single qdots proved to be quite narrow, and thus qdots emit size-tunable vivid colors. This spectral purity opened the possibility of commercial application. However, the luminescence of single qdots typically “blinked”, turning “on” and “off”

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COLLOIDAL CHEMICAL SYNTHESIS AND OPTICAL CHARACTERIZATION Qdots are a widely tunable materials system that can be rationally designed for a specified property. Highly emissive qdots are typically heterostructures composed of a core and one or more shells with increasing electronic gap (Figure 1). The surfaces are bound by organic surfactants that mediate their crystallization and maintain the colloidal dispersion. Reproducibly achieving these structures depends entirely on synthetic control. Qdot chemical synthesis is a difficult problem, involving a number of mechanistic issues not encountered in other areas of chemistry. Yet now in 2017, after more than three decades of inventive effort, two important measures of high qdot quality have been achieved: near unity emission quantum yield at room temperature, and stability over billions of cycles of excitation and emission. Quantum size effects (i.e., increasing band gap with decreasing diameter) in liquid colloids were first noticed in aqueous suspensions of nanometer-size metal sulfide particles.2 © 2017 American Chemical Society

Received: May 22, 2017 Published: June 28, 2017 10939

DOI: 10.1021/jacs.7b05267 J. Am. Chem. Soc. 2017, 139, 10939−10943

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distinguished from ligands freely diffusing in solution.23 There is much fundamental science remaining to be explored in the binding affinity and exchange of surface ligands and its influence on electronic structure. These efforts are central to optimizing the luminous efficiency and stability of qdots. Like other heterogeneous materials, the atomic structure of qdots is complex and difficult to study. X-ray scattering and transmission electron microscopy (TEM) are readily used to study nanometer-scale structure. However, atomic length-scale structure, particularly the structure of defects and reconstructed surfaces, is more challenging and is key to understanding qdot emission characteristics, e.g., blinking. Borrowing methods from cryo-TEM protein structural determination, the use of direct electron detectors and the computational modeling of tomography data have made atomic resolution structures possible on single platinum particles (Figure 2A).24 Similarly,

in a seemingly random fashion under continuous illumination. Each qdot was slightly different in structure and thus somewhat different in optical and photophysical behavior. Some individual qdots had high quantum yields while others had low quantum yields (even zero). These problems motivated further work to improve the synthesis. In the 1990s, qdot research accelerated. In 1993, Murray, Norris, and Bawendi reported a high-temperature synthesis of crystalline qdots of Cd with S, Se, and Te.11 These colloids had unprecedented monodispersity and could be made in a wide range of chosen sizes that clearly illustrated the spectral tunability. This was a significant advance, providing a reliable synthesis for other workers to enter the field. A similarly reliable, high-temperature synthesis of CdSe/ZnS core/shell qdots was reported by Hines and Guyot-Sionnest in 1996.12 These qdots had a 50% ensemble quantum yield at 23 °C, making it clear that both spectral purity and high brightness were attainable. Qdot research then exploded in many different directions. Significant effort has been devoted to understanding qdot nucleation, size control, and degree of monodispersity. Mechanistic studies conclude that the rate of decomposition of molecular synthesis reagents into soluble units of the qdot material (e.g., CdSe) limits the crystal growth kinetics.13 In the Sugimoto model of nucleation and growth, this monomer supply rate controls the concentration of nuclei formed.14 For a given amount of semiconductor material, the number of nuclei controls the final qdot size: if the number of crystals is higher, the final size is smaller, and vice versa. Thus, the qdot size attained following complete precursor conversion can be determined by the reactivity of synthesis reagents during the nucleation phase.15 This insight led to development of reagent libraries of widely varying conversion rates.16,17 Chalcogenoureas in particular provide conversion reactivity that is orthogonal to the nucleation and growth processes.16,17 These precursors allow the monomer supply to be independently optimized at a desired temperature or in a surfactant medium that provides better crystallinity or narrower distribution of sizes. Orthogonal precursor reactivity also simplifies the qdot product because the conversion byproducts do not bind the qdot surface.18 In this manner, orthogonal reactivity can improve qdot luminescence properties while simplifying mechanistic studies of crystal growth and surface ligand binding. Designing syntheses with this principle in mind should help unravel the structure−function relationship behind photoluminescence blinking and quantum yield. Our basic understanding of ligand−qdot surface bonding and its influence on luminescence has markedly improved in the past decade.19−21 Surface ligands are critical to qdot solubility and processing for applications. Ligands also have a strong but poorly understood influence on electronic states at the surface, so-called “trap states” that can degrade the luminous efficiency. In the late 2000s, it was demonstrated that qdots are typically enriched in cationic metal or anionic chalcogenide ions (e.g., Cd2+ or Se2−) that balance charge with anionic surfactant ligands (e.g., carboxylates) or outer-sphere counterions (e.g., alkylammonium).22 This model displaced the earlier picture of a datively bound Lewis basic ligand shell. Ligand exchange and binding affinity are being investigated using solution-phase nuclear magnetic resonance (NMR) spectroscopy. The large size of the qdot slows tumbling and thus broadens the NMR spectroscopic features of ligands bound to the surface. In this way bound ligands can be

Figure 2. (A) Atomic-resolution tomographic reconstruction of a platinum nanoparticle obtained using TEM, enabled by direct electron detectors and a graphene sample cell.23 (B) Atomic structure solutions obtained from PDF simulation.25 The data (blue) match the structural model and PDF simulation (red).

pair distribution function (PDF) methods that analyze the Bragg peaks and the diffuse scattering of X-rays from powdered or soluble nanomaterials provide angstrom-scale structural information.25 A recent study of a qdots with atomically precise molecular formulas used PDF to determine their atomic structures (Figure 2).26 Single-crystal X-ray studies also yield complete atomic structures for precise small qdots.27 Nevertheless, the routine determination of qdot atomic structure as a guide to understanding and improving synthesis and luminescence remains a significant problem.

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QDOTS FOR DISPLAYS AND SOLID-STATE LIGHTING The GaN blue light-emitting diode (LED) won the 2014 Nobel Prize in physics for enabling efficient white-light-emitting light bulbs. GaN LED light bulbs are far more energy efficient than Edison-type filament light bulbs, and as their cost has come down they have been widely used. Blue LEDs have also transformed the television display industry. Color displays have red, green, and blue pixels; the more monochromatic each of these colors, the more vivid and life-like is the display. An ideal display converts electricity into these monochromatic colors as efficiently and inexpensively as possible. Many large-area TVs now use GaN LEDs to make blue 450 nm light. Part of this 10940

DOI: 10.1021/jacs.7b05267 J. Am. Chem. Soc. 2017, 139, 10939−10943

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Figure 3. Colloidal qdot research timeline illustrating evolution of nanostructure and luminescence performance. (A) Liquid colloidal quantum dots first described.2 (B) High-temperature synthesis of qdots with narrow size distributions and size-dependent photoluminescence.11 (C) Core/shell CdSe/ZnS qdots with 50% luminous efficiency.12 (D) Multi-shell architecture CdSe/CdS/Cd1‑xZnxS/ZnS.32 (E) Atomically thin nanoplatelets with narrow luminescence.37 (F) “Giant” shell CdSe/CdS qdot.30 (G) Shelling of nanoplatelets produces narrow band luminescence with 80% luminous efficiency.38 (H) Graded alloy CdSe/CdSe1‑xSx/CdS with high luminous efficiency of bi-excitons.37 (I) Gram-scale synthesis of spherical quantum well CdS/CdSe/CdS with low internal defects and thick shell provides near-unity luminous quantum efficiency from the ensemble.33

improve the performance of qdots in high-flux applications such as lasing and solid-state lighting.39 There is also interest in platelet geometries that are 2−5 or more atomic monolayers thick.40 Depending on the thickness, the emission color can be tuned across the visible. Spectra of platelets show essentially no size broadening at 23 °C, which is a consequence of their atomically precise thickness and much larger lateral dimensions. This geometry exhibits narrower emission spectra compared to those of the best commercial qdots. Platelets can be shelled in a similar manner developed for spherical qdots to increase their photoluminescence quantum efficiency.41 Many hundreds of patent applications on qdots and their displays have been filed by Samsung, Nanosys, 3M, and QD Vision among others. These companies conduct proprietary research and development, focusing largely on questions of low Cd qdots, qdots showing extreme resistance to oxidation under illumination, qdots that retain a high quantum yield under high light flux, and electrical excitation of qdots. Both InP-type and CdSe-type qdots are used commercially in displays. InP qdot synthesis has proven to be more difficult than CdSe synthesis, and the optical properties and chemical stability at present are not as good. In general, synthetic difficulty increases in the sequence II−VI, III−V, IV as the bonding becomes progressively more covalent.

blue light is absorbed by two different colloidal qdots, one designed to emit in the green at 525 nm and the other in the red at 625 nm. This hybrid LED−qdot technology takes advantage of the high electrical efficiency of GaN LEDs, the low cost of colloidal qdots, and the huge optimized manufacturing base for liquid-crystal display screens. Significant effort has been invested in optimizing core/shell qdots for red and green emission,28,29 as shown by the timeline in Figure 3. The basic idea of core/shell structure is to interpose an insulating region (without defects) between the core excited state and localized trap states on the outer qdot surface.30 This substantially lessens but does not eliminate the influence of surface trap states on luminescence. Conceptually, with a sharp interface between the ZnS shell and the CdSe core, the excited-state electron and hole wave functions are confined to the core. The emission wavelength is then determined by the core size. However, because of the lattice mismatch between ZnS and CdSe, the interface is under strain, and structural defects form that degrade core luminescence. It was found that if such qdots are either made or annealed at 300 °C, partial inter-diffusion between the ZnS and CdSe occurs.31 This interdiffusion tends to eliminate defects, with the effect of improving the luminescence quantum yield to near 100% and decreasing blinking. Similar improvements occur when CdS shells are grown on CdSe cores,32,33 which have a lower lattice mismatch, or in more complex multi-shell structures designed to reduce the interfacial strain.34,35 Controlled alloying of lower band gap cores and higher band gap shells can now be directly achieved by appropriately mixing organometallic reagents.36 These improvements allow qdots with multiple shells to be synthesized that provide high-quality green emission.37 With an increasing fraction of ZnS in the alloy core, a physically larger core can be made while maintaining the green emission. In this case both the core alloy composition and size determine the emission wavelength. When the interface between core and shell is gradual rather than abrupt, the non-radiative Auger recombination is decreased in the event that two excitons are simultaneously present in the qdota common situation at high excitation rates.38 Together, a graded alloy interface and a thick shell can

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INFRARED QDOT APPLICATIONS The ability to rationally design qdot optical properties is displayed in the potential use of qdots in solar concentrators for photovoltaics. A concentrator is a large-area, thin, transparent, planar waveguide with embedded qdots. Qdots absorb sunlight and emit band gap luminescence that is largely trapped by total internal reflection inside the waveguide. This light propagates to the waveguide edges where Si photovoltaic cells are mounted. A high emitted light flux on the solar cells (compared with sunlight) is possible if there is a large Stokes shift between absorption and fluorescence wavelengths, and thus vanishing qdot re-absorption of emitted light. This application also requires stability under irradiation and a near 100% luminescence quantum efficiency. Several groups have demon10941

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IR semiconductor lasers made by high-vacuum chemical vapor deposition (CVD) are the basic drivers of the worldwide fiber-optic Internet. Compact thin films of qdots, especially platelets, have shown tunable lasing under optical pumping; such films can be conformally deposited on patterned laser cavity substrates. Mode-locked lasers and amplifiers based upon qdots offer advantages in ultrafast optical networking;58 such devices have been made from Stranski−Krastanov selfassembled CVD-grown quantum dots at telecommunications wavelengths near 1400 nm.59 Efficient electrical injection leading to colloidal qdot lasing remains a major challenge.

strated large Stokes shifts with high quantum efficiency in core/ shell qdots with very thick shells.42−45 The ultimate light flux concentration with nearly perfect materials is extremely high, and is limited only by photon entropies in the second law of thermodynamics.46 One could imagine such photovoltaic concentrators as partially transparent surface layers on large windows in buildings. Inexpensive and non-toxic emitting qdots that absorb the entire solar spectrum and emit at the optimal wavelength (900 nm in the near-IR) for Si photocells need to be developed. This is a synthetic materials problem that would seem to be a significant opportunity.47 Qdots emitting further in the IR, near 1400−1600 nm, are valuable for non-invasive in vivo optical imaging with cellular resolution in animals.48 Excitation and emission in the IR significantly increase contrast, sensitivity, and penetration depth by reducing absorption and scattering by blood and other tissue, as compared with visible imaging. InAs core multi-shell qdots with phospholipid surface functionalization have shown quantum yields of ca. 20%, near 1300 nm, enabling real-time imaging of entire live mice. There is a significant opportunity here for non-heavy-metal IR-emitting qdots.

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FINAL THOUGHTS Chemical synthesis is amazingly powerful, having brought qdots to a level that is displacing well-established phosphor technologies in liquid-crystal displays. Applications in solidstate lighting, lasing, and light-emitting diodes are also promising, provided that stability and luminous efficiency continue to improve. There is much room for improvement in our understanding of qdot nucleation and growth and surface chemistry, which provide the keys to further improving qdot optical performance.

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POTENTIAL SINGLE QDOT DEVICES

Entirely different applications are possible for individual qdots. One qdot emitting single photons could be the basic light source in quantum optical information processing schemes.49 An ideal source is triggerable by some external influence, operates at room temperature, and has a very narrow emission line width. Also, charged qdots with an extra electron could serve as qbits in quantum computers, with photon emission and absorption being used to manipulate the qbit.50 Single qdots in optical cavities in principle could be made to lase, and such devices would test the basic physics of cavity quantum electrodynamics.51 At present there are no single-photon emitters that show the stability and reproducibility required by such devices. Current colloidal qdots, while better than single-molecule emitters, still need significant improvement. Even the best present qdots are not the simple two-level systems envisioned in these applications. Studies of the photon stream from the best current individual core/shell qdots under continuous excitation reveal complex and unpredictable internal photophysics that vary from one qdot to the next.52 This complexity is related to qdot structure in ways we do not understand. The fundamental nature of “blinking” remains an unsolved problem.53 These single emitter applications are clearly an opportunity for synthesis in further understanding, improvement, and control of qdot structure.

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Jonathan Owen: 0000-0001-5502-3267 Louis Brus: 0000-0002-5337-5776 Notes

The authors declare no competing financial interest.

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REFERENCES

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ELECTRICAL EXCITATION There are also possibilities for efficient and stable qdots in applications requiring electrical excitation of qdot diode emission and/or lasing.54−57 Large-area light-emitting diodes involving qdot emitters could be used for solid-state lighting to replace Hg-containing fluorescent tubes, and also for thin and flexible active matrix three-color displays. A qdot needs to capture both a moving electron and a hole in order to emit, and thus a less insulating shell is required as compared with a qdot designed for optical excitation. Such devices are being intensively pursued and can offer efficiencies that are now comparable to those of OLEDs. 10942

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