Small nanocrystals have a big impact

As is the case for semiconductors, our daily lives depend on material properties. For instance, we require conductive materials for electron transport, transparent materials for windows, and brightly colored materials for paintings. All these properties are material dependent. For instance, we use copper for conductive materials since copper is a good electron conductor, and we use silicon dioxide (SiO2) for windows since SiO2 as it is transparent. We use other materials that strongly interact with light for instance in paintings, like The Great Wave off Kanagawa by Hokusai or the the Starry Night, by Vincent van Gogh, in which Prussian blue (named after the Prussian army who used the blue pigment to dye their uniforms) (Fe4[Fe(CN)6]3) is used to represent dark, deep blue.3-4 In all the aforementioned cases, the material properties, such as e.g. conductivity, melting point, colour, etc., do not depend on the size or shape of the used material. However, when one starts to look at material properties at the nano-scale (1-100 nm, or 0.000000001 - 0.0000001 meter), in a range where the amount of atoms start to be limited, certain material properties start to become size dependent, and they can differ greatly from the properties of their bulk sized counterparts. This extra property tunability has driven the scientific community, especially the field of semiconductors, to go “nano”, as it would allow for an extra tunability of the material properties. For instance, one can envision modifying the nano-size of materials in order to alter the optical and conductive properties, rather than having to change the composition of a metal, or the molecular structure of an organic molecule.

These colloidal semiconductor NCs have been synthesized and studied for at least the last three decades. Now, in 2018, we have an excellent understanding of how the nanoscale influences semiconductors, and we have access to a plethora of different compositions with a very great size and shape control on the nanoscale. Furthermore, the field has moved to colloidal NCs, meaning that the NCs can be synthesized as small, stable, solid NCs in a solution. This does not only allow for more control over the size during the synthesis (the NCs can be precisely tuned to the size of an atom), but also has a large processability advantage, as colloidal NC solutions can be deposited using simple deposition techniques like inkjet printing or spray painting. All these advancements have pushed these NCs from being the subject of fundamental lab studies to being employed in several types of applications today, such as light emitters in TVs (as is shown in figure 1).5 Current colloidal semiconductor NCs also have the capacity to be used in a wide range of applications in the future, such as solar cells, LEDs, lasers, luminescent solar concentrators.6 They are also extremely important with regard to advancements in nano-biotechnology, and they can be used in drug delivery techniques, in vivo imaging and photothermal cancer therapy treatments.7-8

A brief history of colloidal nanocrystals

Although the field of colloidal semiconductor NCs started to gain attention in the 1980s, particles with properties that are tunable on the nanoscale, especially metals, have been used for thousands of years. One of the most reported and visual examples is stained glass, which dates back to the middle ages.5, 9 Medieval stained glass artists knew that baking their glass with gold and silver salts at different temperatures or for different amounts of times resulted in different colored stained glasses (even though the composition remained the same).5, 9-10 Using silver salt, they could for instance produce every primary color (blue, yellow and red). Similarly, they created red, green and orange by using gold salts. Therefore, mixing silver and gold gave them access to any color that they needed to stain their windows. In a similar way, ceramics with different bright colors were made in Italy during the Renaissance period (i.e. during the 15th and early 16th century), using only different copper and silver salts, resulting in a wide variety of colors.11-12 These ceramics, called Deruta ceramics (derived from the name of the small town Deruta, in the region of Umbria, Italy), were therefore a result of differently sized copper and silver NCs. Of course, at this time, the scientific reason behind the production of these different colors was not known. Nevertheless, this is a perfect example of the tunable properties of materials in NC form. In fact, the use of “nanoparticles” as single coloring agents can be traced back even earlier in history: lead sulfide NCs (about 5 nm) were used by the ancient Egyptians to dye their hair,13 and iron nanoparticles were used in the Bronze Age to dye pottery red.14 In retrospect, we now know that, in each of these cases, different reaction temperatures and times resulted in differently sized and shaped copper, silver, gold or iron NCs, all of which exhibited different colors due to their nano-size.10-12, 14

Although the use of NCs may go back centuries, the first reported scientific study on size dependent optical properties dates back to 1857, when Michael Faraday tried to study the relationship between gold (and other metal) layers of different thicknesses and light.15 In these experiments, he accidentally created different colored solutions while trying to mount gold leafs onto microscope slides. Based on these observations, he concluded that “known phenomena appeared to indicate that a mere variation in the size of its particles gave rise to a variety of resultant colors” and that “reflexion, refraction, absorption etc. depended upon such relations, there was reason to expect that these functions would change sensibly by the substitution of different sized particles”. Nowadays, we know that the different colors of metal particles stem from the so-called Localized Surface Plasmon Resonance (LSPR) effect, which, in turn, originates from the collective oscillation of electrons at the surface of the particles. This effect is highly sensitive to the surface of the NC, therefore it is highly size and shape dependent. The discovery that the electronic structure of NCs can change came a century later, when small nanometer-sized copper chloride and silver iodide, which will than were referred to as ‘crystallites’, were embedded into NaCl matrices in a very similar way as to how the medieval stained glass artists made their stain glass windows.16-17 For these crystallites, it was observed that the bandgap depended on their size, and, thus, on changes in the electronic structure of the NCs. One of the first theoretical models describing the relation between the electronic properties of semiconductors and size dates back to the 1960s, when Evens and Young correlated the changing optical properties of thin layered MoS2 with the changes in its electronic structure.18 Twenty years later, in the 1980s, the first models of small spherical semiconductor NCs, so called ‘Quantum dots (QDs)’, were published (figure 2a-c).19-21 One of the major breakthroughs with regard to linking the experimental data with theoretical models was achieved as a result of the quick pace at which advancements were made in the synthesis of colloidal PbS and CdSe NCs.22-23 One obstacle that was still limiting the linking of experimental data with theoretical models was the accurate determination of the NCs size (and shape), especially in the smaller size range of 1-10 nm. To this end, the discovery and development of the transmission electron microscope (TEM, which was first demonstrated by Max Knoll and Ernst Ruska in 1931, but it did not reach the required resolution until the 1970s-1980s, in 1986, Ernst Ruska was awarded the Nobel Prize in Physics “for his fundamental work in electron optics, and for the design of the first electron microscope.") gave researchers the final boost they needed to link the experimental data with theoretical models, as it directly gave “images” of NCs (figure 2d). Another major breakthrough was in 1993, when the Bawendi’s group reported a synthetic procedure that was able to produce (nearly) monodisperse colloidal CdSe NCs, reaching near atomic precision (figure 2e-g).24 Over the following decades, the research field of colloidal semiconductor NCs quickly expanded and matured into a large research community. Today, the synthesis field has expanded to encompass different materials, such as PbSe, CuInS2, InAs, Cu2-xS, ZnS and many other semiconductors, and it has achieved a large range of sizes and shapes.6 Furthermore, highly complex binary NCs, consisting of multiple semiconductor domains, can be synthesized resulting in an extreme high tunability of the NC’s properties.25-26 Moreover, the field has matured and started to move from fundamental research towards applications, with NCs being studied for use in a wide range of applications such as LEDs, lasers, solar cells and cancer treatments.6, 27

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