**3. Quantum size effects**

In quantum dots, the size of particle is smaller than the Bohr radius of the electron-hole pair distance, excitons, leading to quantum confinement. During this state, the energy levels become discrete and can be predicted by the particle in a box model. The possible discrete energy levels of the electrons in such quantum dots depend on their size and accordingly to their bands gap [5]. As well, optical properties of quantum dots with sizes smaller than the de Broglie wavelength naturally depend on their electrical properties.

As shown in **Figure 2**, quantum dots as excellent luminescent materials show broad excitation range with narrow and symmetric emission spectra after excitation. Their emissions are typically much narrower than emissions of common fluorophores or organic dye molecules. In fact, the absorption of the wavelength of light with the energy equal to the band gap energy promotes the electron from the valence band to the conduction band. Afterward, the exited electron relaxes directly from the conduction band to the valence band the and emits a photon [6].

Quantum dots are becoming, nowadays, one of the fast growing and most exciting research subjects. The unique physical and optical properties of quantum dots have made them attractive tools and vectors for research in molecular biology, material science, chemical analysis, etc.

Nowadays, nanocrystals are usually prepared with atoms from groups II–VI, III–V, or IV–VI in the periodic table and eventually become many different types of quantum dots (**Table 1**).

The quantum dots can also be created by combining these systems such as GaAs–ZnS, GaAs–ZnTe, InP–ZnS, InP–ZnSe, and InP–CdS [10].

**5**

**Figure 3.**

*Various methods for the synthesis of nanomaterials.*

*Introductory Chapter: Quantum Dots*

**4. Optical properties**

**Table 1.**

**5. Fabrication**

*DOI: http://dx.doi.org/10.5772/intechopen.92151*

**Type Quantum dots**

IV-VI PbSe, PbS IV C, Si, Ge

*Summary of different types of quantum dots [7–9].*

IB-VI Ag2S, ZnAgS, CuS, CuInS2, CuInSe2

IIIA–V GaAs, InGaAs, InP, InAs, InGaN

IIB–VI CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe

properties can be prepared for different purposes.

Particle size is one of the most important aspects that can affect the optical properties of quantum dots. Of course, various other factors such as the shape and composition of quantum dots, as well as the methods and materials used in the synthesis of these particles also affect the optical properties and frequency of fluorescent light emitted or absorbed by these nanocrystals [11]. Therefore, by manipulating each of the effective factors, special quantum dots with specific

Generally, there are two methods to synthesize nanomaterials including quantum dots, top-down and bottom-up (**Figure 3**). In the top-down synthesis approach, the bulk material is transferred to nanometer size using an electron beam or high-energy ions. The methods that fall into this category are electron beam lithography, reactive ion etching, focused beam lithography, and dip pen lithography [12]. However, these methods have limitations and disadvantages such as structural defects in the patterning and impurities in the quantum dots synthesized [13]. In the bottom-up methods, atoms or molecules are assembled step by step to produce nanomaterials. These synthesis approaches are highly diverse and generally

**Figure 2.** *Size-dependent fluorescence spectra of quantum dots.*

*Introductory Chapter: Quantum Dots DOI: http://dx.doi.org/10.5772/intechopen.92151*


**Table 1.**

*Quantum Dots - Fundamental and Applications*

confinement.

**3. Quantum size effects**

depend on their electrical properties.

material science, chemical analysis, etc.

*Size-dependent fluorescence spectra of quantum dots.*

quantum dots (**Table 1**).

of the particle can be delicately varied depending on its size. Therefore, only by controlling the size of quantum dots, various particles with different colors can be produced to emit or absorb specific wavelengths of light [4]. As shown in **Figure 1**, different sized quantum dots emit different color light due to quantum

rally occurring in atomic and nuclear physics of the quantum system.

The physics of quantum dots counterpart with a lot of the behaviors that natu-

In quantum dots, the size of particle is smaller than the Bohr radius of the electron-hole pair distance, excitons, leading to quantum confinement. During this state, the energy levels become discrete and can be predicted by the particle in a box model. The possible discrete energy levels of the electrons in such quantum dots depend on their size and accordingly to their bands gap [5]. As well, optical properties of quantum dots with sizes smaller than the de Broglie wavelength naturally

As shown in **Figure 2**, quantum dots as excellent luminescent materials show broad excitation range with narrow and symmetric emission spectra after excitation. Their emissions are typically much narrower than emissions of common fluorophores or organic dye molecules. In fact, the absorption of the wavelength of light with the energy equal to the band gap energy promotes the electron from the valence band to the conduction band. Afterward, the exited electron relaxes directly from the conduction band to the valence band the and emits a photon [6]. Quantum dots are becoming, nowadays, one of the fast growing and most exciting research subjects. The unique physical and optical properties of quantum dots have made them attractive tools and vectors for research in molecular biology,

Nowadays, nanocrystals are usually prepared with atoms from groups II–VI, III–V, or IV–VI in the periodic table and eventually become many different types of

The quantum dots can also be created by combining these systems such as

GaAs–ZnS, GaAs–ZnTe, InP–ZnS, InP–ZnSe, and InP–CdS [10].

**4**

**Figure 2.**

*Summary of different types of quantum dots [7–9].*
