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History and Introduction of QDs and QDLEDs

Semiconductor nanocrystals (NCs) are the most widely studied of the nanoscale semiconductors. In early 1981, Alexei Ekimov and Alexander Efros, working at the S.I. Vavilov State Optical Institute and A.F. Ioffe Institute, Russia, discovered nanocrystalline, semiconducting quantum dots (QDs) in a glass matrix and conducted pioneering studies of their electronic and optical properties. Simultaneously, in 1985, Louis Brus at Bell Laboratories in Murray Hill, NJ, discovered colloidal semiconductor NCs (QDs), for which he shared the 2008 Kavli Prize in Nanotechnology. Over the years, QDs have been established as a new type of semiconductor nanocrystalline material whose size is smaller than or close to the excitonic Bohr radius of its bulk material. Common semiconductor materials include Si, Ge, compounds of group II–VI (e.g. CdSe), and compounds of group III–V (e.g. indium phosphide [InP]). When the size of these bulk semiconductor materials is larger than their exciton Bohr radii, electrons and holes are able to move freely and independently in the bulk materials. However, when the size of QDs is smaller than their own exciton Bohr radius, after being excited by light, an electron in the valence band will leap to the conduction band, leaving a hole in the valence band, and the electron and hole form an exciton due to Coulomb effect, which is confined in a space smaller than the exciton Bohr radius, and the electron and hole will be quantized, which is called the “quantum size effect” of nanomaterials. This quantum size effect allows QDs to have discrete energy levels, thus giving them unique physicochemical properties [1]. Colloidal semiconductor NCs have size‐dependent particle properties, while their surface ligands make them solution‐processable, which gives them a “particle‐solution” duality.

Figure 1.1a shows the energy level diagrams of molecular, QD, and bulk semiconductor materials. The molecular orbital energy level diagram is composed of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), while the energy level diagram of QDs consists of some discrete energy levels, and the bulk semiconductor material consists of conduction and valence bands. Figure 1.1b illustrates the spatial extent of the confined domains of electrons and holes and the respective energy as a function of the density of electronic states for bulk semiconductor materials, two‐dimensional quantum sheets, one‐dimensional quantum wires, and zero‐dimensional QDs, depending on the size of the material. For a bulk semiconductor material, the dimensions in all three dimensions are larger than its own Bohr exciton radius, and electrons and holes are free to move independently in all three dimensions; while for a two‐dimensional quantum sheet, the dimensions in two dimensions are larger than its own Bohr exciton radius, and electrons and holes are free to move independently in two dimensions; and for a one‐dimensional quantum wire, the dimensions in one dimension are larger than its own Bohr exciton radius, while for a one‐dimensional quantum wire, the dimensions in one dimension are larger than its own Bohr exciton radius, and electrons and holes are free to move independently in two dimensions. For a one‐dimensional quantum wire, whose dimension in one dimension is larger than its own exciton radius, electrons and holes are free to move independently in one dimension; and for a zero‐dimensional QD, whose dimension in all three dimensions is smaller than its own exciton radius, electrons and holes are restricted from moving freely and independently in all dimensions. In general, QD is a collective term for a two‐dimensional quantum sheet, a one‐dimensional quantum wire, and a zero‐dimensional QD.

Figure 1.1 (a) Schematic diagram of energy levels of molecular, quantum dot, and bulk semiconductor materials; (b) spatial extent of the confined domain of electrons and holes and the respective energy as a function of the density of electronic states for bulk semiconductor materials, two‐dimensional quantum sheets, one‐dimensional quantum wires, and zero‐dimensional quantum dots depending on the material size.

Figure 1.2 Preparation pathways for quantum dots: the “top‐down” method and the “bottom‐up” method.

1.1 Preparation Route of Quantum Dots

There are two completely different ways to prepare QDs, namely the “top‐down method” and the “bottom‐up method”, as shown in Figure 1.2. The top‐down method is to prepare QDs by reducing the dimensionality and size of the bulk semiconductor material; the bottom‐up method is to combine atoms or molecules into QDs by chemical synthesis. The former approach is limited by the ultra‐fine processing technology, which cannot produce QDs below 10 nm at present, and the morphological regulation of QDs is also limited to some extent. The latter is mainly achieved through colloidal chemical synthesis, which can produce colloidal QDs of different sizes and shapes.

1.2 Light‐Emitting Characteristics of Quantum Dots

1.2.1 Particle Size and Emission Color

QDs are semiconductor particles having a few nanometers in size, their optical and electronic properties are quite different from those of larger particles as a result of quantum mechanics. When the QDs are illuminated by UV light, an electron in the QD can be excited from the transition of an electron valence band to the conductance band. The excited electron can drop back into the valence band releasing its energy as light. The color of that light depends on the energy difference between the conductance band and the valence band. The QD absorption and emission features correspond to transitions between discrete quantum mechanically allowed energy levels in the box, which are reminiscent of atomic spectra.

1.2.2 Quantum Dot Optical Property

The QDs are defined as the semiconductor NCs with the quantum confinement. Thus, the semiconductor nanoparticles with dimensions QDs have the following features:

1.2.2.1 Quantum Surface Effect

The surface effect refers to the fact that as the particle size of QDs decreases, most of the atoms are located on the surface of QDs, and the specific surface area of quantum dots increases with decreasing particle size. Due to the large specific surface area of QDs (nanoparticles), the increase in the number of atoms in the surface phase leads to the lack of coordination, unsaturated bonds, and suspension bonds of surface atoms. This makes these surface atoms highly reactive, extremely unstable, and easily bonded with other atoms. This surface effect will cause the large surface energy and high activity of nanoparticles. The activity of surface atoms not only causes changes in the surface atomic transport and structural type of nanoparticles but also causes changes in the surface electron spin conformation and electronic energy spectrum. This feature offers a route to manipulate QD interactions with their environment. QDs can be tethered to proteins, antibodies, or other biologic species and used as optically addressable bio‐labels. On the other hand, passivation of QD surface can improve the QD stability and increase the photoluminescent quantum efficiency. Surface defects lead to trapped electrons or holes, which in turn affect the luminescent properties of QDs and cause nonlinear optical effects. Metallic materials show various characteristic colors through light reflection. Due to the surface effect and size effect, the light reflection coefficient of nanoparticles decreases significantly, usually less than 1%, so nanoparticles are generally black in color, and the smaller the particle size, the darker the color, i.e. the stronger the light absorption ability of nanoparticles, showing a broadband strong absorption spectrum. Surface effect or ligand modification offers an additional tool for manipulating energy levels and electronic and optical properties.

1.2.2.2 Quantum Size Effect

Quantum size effect refers to the phenomenon that the electron energy levels near the Fermi energy level change from quasi‐continuous to discrete energy levels, that is, when the particle size drops to a certain value, the energy level splits or energy gap widens, in other words, the energy spectrum becomes discrete, and as a result, the bandgap becomes size‐dependent. When the change in energy level is greater than the change in thermal, optical, and electromagnetic energy, it leads to the magnetic, optical, acoustic, thermal, electrical and superconducting properties of nanoparticles being significantly different from those of conventional materials. This feature of QDs is that the energy gap changes with the increase in the grain size, the larger the grain size, the smaller the energy gap, and vice versa, the larger the energy gap. That is, the smaller the QD, the shorter the wavelength of light (blueshift), and the larger the QD, the longer the wavelength of light (redshift). According to the size effect of QDs, we can use the method of changing the size of the grain to regulate the tuning of the light spectrum of the material and no...