Since the early Middle Ages, stained glass windows have adorned domestic buildings and churches across Europe. The making of colored glass was based on glass manufacturing—and was also an art (1, 2). We know now that the vivid colors in those windows are from the processing of metal salts, finely ground powders of metals, or metal oxides embedded in glass. Yellow-colored glasses, for example, were made in the Renaissance period using lead-based compounds, with lead later replaced by cadmium. In the late 19th century, a patent was filed on a procedure to prepare glasses with colors ranging from deep ruby to orange by mixing powders of cadmium sulfide (CdS) and cadmium selenide (CdSe) in different proportions (3). In 1926, a report was published on the shift in the absorption edge in glasses stained with CdS depending on the tempering (i.e., annealing) time (4). This report was the first to hypothesize a correlation between such a shift and the size of the CdS nanoparticles in the glass. Remarkably, processes similar to the ones described in recipes from the Middle Ages are used in the modern era to prepare colored glass and even optical filters, such as Schott glasses (5).
Alexei I. Ekimov. Image credit: Nexdot.
In 1979, Alexei Ekimov at the Vavilov Optical Institute in the former Soviet Union began studying the growth of colloidal particles in glasses, specifically various cadmium chalcogenides and copper halides. Similar to the medieval glass makers, he also relied on the rapid cooling of glass melts, but Ekimov aimed to understand how the nanoparticles formed inside the glass matrix. He discovered that the optical properties of glasses containing the metal salt CuCl not only showed excitonic absorption peaks resembling those of bulk CuCl but also that the spectral position of these peaks depended on the annealing temperature of the glass (6). Lower temperatures resulted in absorption wavelengths that were shifted to shorter wavelengths (a blue shift). With his team, Ekimov used small-angle X-ray scattering to determine that smaller colloid sizes exhibited larger blue shifts; these glasses were found to contain nanoparticles with sizes between 1.7 and 3.1 nanometer (7). With other members of the Institute [and drawing from theory developed by other Soviet scientists (8)], Ekimov correlated the size of the CuCl nanoparticles with the temperature and duration of annealing. Having established a nucleation and growth mechanism, his team could produce improved dispersity of nanoparticles made from other materials (CdSe) by rapidly heating the glasses at high temperatures to nucleate the nanoparticles and then quickly decreasing the temperature to prevent further nucleation, which allowed only slow growth of the nuclei already formed (9).
The blue shift in wavelength with decreasing nanoparticle size is due to quantum confinement, a representative example of a “particle in a box.” Quantum-size effects were not, however, new physics in the 1970s. Herbert Fröhlich had already postulated in 1937 that a gas of free electrons confined to a metal particle with sizes 10 nanometer or smaller should exhibit properties different from the bulk (10). Other scientists, including Landau in 1930 (11) and Lifshitz and Kosevich in 1955 (12), had also predicted various quantum confinement effects, and Sandomirskii predicted in 1963 that the localization of carriers in semiconductor films should lead to an increase in their electronic band gap (13). Years later, Alexander Efros, triggered by Ekimov’s experimental results and teaming up with him and additional co-workers, developed a general theory of the optical properties of semiconductor nanoparticles (14, 15); more elaborate aspects of the theory came soon after (16).
In other parts of the world, particularly in Europe and the United States, various groups were also working on nanoparticles, primarily as colloids in liquid solutions and for use in solar energy applications. Given the global shock of the 1973 oil crisis, research on alternative energy sources was of intense interest. The first scientist to report size-dependent optical properties in colloidal suspensions of CdS nanoparticles and to associate them with quantum effects was Louis Brus at Bell Laboratories (17). Independently, Brus also developed a theory to describe the quantum-confined properties of these nanoparticles and then generalized the study to a broad range of semiconductors, including metal chalcogenides and silver halides (18, 19). Then, new members joined his team and focused on identifying more reliable methods to control the sizes of the nanoparticles using reverse micelles, nanometer-sized droplets of water stabilized in a bulk organic solvent by a spherical monolayer of surfactant molecules. These nanoscale reactors were effective at controlling the growth of CdS and CdSe nanoparticles (ideally one particle per micelle) and avoiding particle–particle aggregation. To arrest the growth, the team used organometallic molecules containing a phenyl group that ended up coating the surfaces of the particles. These surface ligands enabled the extraction of the semiconductor nanoparticles from the micelles and their subsequent purification and postsynthesis treatment (20).
Louis E. Brus. Image credit: Eileen Barroso (Columbia University, New York, NY).
Drawbacks of nanoparticles grown in reverse micelles were their poor crystallinity, incomplete surface passivation, and size heterogeneity, which resulted in inefficient light emission. Moungi Bawendi, as part of Brus’ team, was tasked with identifying new ligand molecules that could improve the chemical and physical properties. Realizing that the room temperature of the synthesis in reverse micelles could be a key impediment in removing defects, Bawendi thermally annealed the nanoparticles in the presence of different molecules. He observed a substantial improvement in nanoparticle crystallinity as well as sharper optical absorption and emission peaks using specific molecules, especially tributyl phosphine (TBP) (21). He also serendipitously found that TBP combined with its oxidized form (TBP oxide, TBPO) produced the most desirable results. With better control over the highly crystalline inorganic nanoparticle cores coated by organic ligand shells, these materials became increasingly referred to as nanocrystals.
Moungi G. Bawendi. Image credit: Justin Knight (photographer).
Despite these developments, the process of growing nanocrystals of a specific size, with a narrow size distribution, and having a high photoluminescence quantum yield lacked a reproducible protocol. A breakthrough was achieved when Bawendi, after moving to MIT as an independent investigator, together with his graduate students Chris Murray and David Norris, developed the so-called hot injection synthesis in 1993 (22). This process consisted of injecting a cold “precursor” solution of dimethyl cadmium and a complex of trioctyl phosphine oxide (TOP) with a chalcogen precursor (both dissolved in TOP), into a flask containing trioctyl phosphine oxide (TOPO) heated at high temperatures (240 to 300 °C). The quick injection (lasting only a fraction of a second) of the cold mixture in the hot TOPO resulted in a rapid nucleation of ultrasmall cadmium chalcogenide nanoparticles. Since the reaction temperature also dropped after the fast injection, the nucleation events occurred only for a very limited time span, after which the nuclei were allowed to grow at a temperature that was set lower than the injection temperature. This nucleation and growth synthesis in solution mimicked Ekimov’s approach in solids of fast heating of the glass containing metal salts followed by rapid cooling as a way to constrain the nucleation events to a narrow time window.
The surfactants used in the hot-injection synthesis (TOP and TOPO) were of the same type as those tested by Bawendi to improve the quality of nanocrystals grown in reverse micelles (TBP and TBPO). For this synthesis, however, the surfactants were directly involved in controlling the growth of the nanocrystals and remained tightly bound to their surfaces once the reaction mixture was cooled to room temperature, which ensured high colloidal stability. This method produced highly crystalline nanoparticles, with increased monodispersity and well-defined optical features.
The easy exchange of ideas between chemists and physicists drove many of the next rapid advances in colloidal nanocrystals, including nomenclature. Since these nanocrystals could be synthesized with sizes in the quantum-confined range, they become known as “colloidal quantum dots.” Also, the hot-injection synthesis initially borrowed chemicals used to prepare semiconducting films by chemical vapor deposition that were later optimized for the solution-based reactions (23–25). Another advance was the preparation of “epitaxial” core-shell nanocrystals, quantum dots having a shell of a higher band gap material than that of the core (26). This shell both passivated the surface traps and helped better confine the carriers in the core, which not only boosted the efficiency of light emission but also made the dots robust against environmental agents, such as oxygen and moisture. Numerous groups could now prepare quantum dots of different materials (and sizes) that could emit over a broad range of wavelengths, from the deep ultraviolet to the near and mid-infrared (27, 28). The growing community quickly realized that quantum dots had several key advantages over emitters such as organic fluorophores, including robustness against photodegradation, simultaneous excitation of quantum dots with different emissive properties by the same light source, and purer emission colors. Moreover, since the surfaces of quantum dots could be functionalized with a wide range of molecules, including biomolecules, they could be used as labels in bioimaging (29, 30).
Quantum dots certainly exhibit a richness in physical properties, and many have wondered: Could the Nobel Prize have been awarded in physics? Although a case could be made, the primary reason why chemists were recognized is that they were the ones to invent methods to prepare the materials on which the physics (partly already known and anticipated) could be tested. Colloidal quantum dots are a beautiful example of interdisciplinary science and, in some sense, represent the beginnings of a new field launched over two decades ago: nanoscience. Exquisite control over their size, shape, and properties brought different communities together and have enabled diverse applications, from light-emitting diodes and television displays to solar devices and photodetectors to photocatalysts. We are excited about the next possibilities for quantum dots in areas as broad as energy conversion, quantum light sources, sensing, and imaging. The future of these small structures and nanoscience is bright—forged by an expanding and creative community.
Footnotes
This article is part of a series of articles in PNAS highlighting the discoveries and profiling recipients of the Nobel Prize.
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