**3. Form factor of perovskite nano-crystals**

#### **3.1 Crystal shape of perovskite nano structures**

As it is well known, semiconductor perovskite crystalline structure has a general formula of ABX3 where A is an organic or inorganic cation, organic methylammonium (MA+ ), formamidinium (FA+ ) or inorganic cesium (Cs+ ); B is a metal cation typically lead (Pb2+) or tin cations (Sn2+); and X is a halide anion. Cation A is slightly larger than centred cation B having 6 co-ordination number. There is a significant relationship, which is called "Goldschimidt Tolerance Factor" between the size of the ions and the formation and the shape of the crystal structure:

$$t' = \frac{r\_A + r\_X}{\sqrt{2} \left(r\_B + r\_X\right)}\tag{1}$$

Where, *Ar* and *Br* are cationic diameters of A and B respectively, and *Xr* is the anionic diameter of halide in Å. When cation A is too big or cation B is too small (>1), hexagonal perovskite crystal is formed. If cation A and has the ideal size (0.9 < *t* < 1), cubic perovskite crystal is formed. Finally, if cation is too small to fit into the space between metal cations (B) (0.7 < *t* < 0.9), orthorhombic perovskite structure is formed [42].

#### **3.2 Geometry of perovskite nano-crystals**

A typical "bulk" perovskite structure is called three-dimensional (3D) which is usually comply to the ABX3 formula completely [43–45]. In this situation, "3D" refers to the growth in every dimension without any confinement. X anions are combined through corner sharing to form a 3D network. Beside this, the cation A occupies the site in the middle of eight octahedra, and each element needs to owe the proper valence state to keep a whole charge balance [43]. To obtain nano crystalline perovskites, crystal growth must be restricted with at least one dimension by a capping agent or a matrix. Thus, It is possible to prepare various dimensioned PNSs such as zero, one or two dimension (0D, 1D or 2D respectively) (**Figure 6**) [1, 46].

#### **Figure 6.**

*Schematic illustration of low-dimensional perovskites and 3D perovskite crystal structures (reproduced with permission of Ref. [47]).*

**55**

*Perovskite Nanoparticles*

**Figure 7.**

*DOI: http://dx.doi.org/10.5772/intechopen.94588*

and photodetector [8] applications.

**4. Applications of perovskite nanoparticles**

**4.1 Optical properties of perovskite nanoparticles**

*0D PNSs* have been synthesized, by LARP [25], hot-injection [32] and template-assisted method [36] by different groups. Crystal growth is prevented in all dimensions by a ligand or a metal-oxide matrix to obtained quantum-dot-like NPs representing good Photoluminescence (PL). Prepared nanostructures have been used in many opto-electronic applications such as solar cells, LEDs and laser. Perovskite nanowires or nanorods are basically *1D PNSs* which represent outstanding anisotropic optical and electrochemical properties with very high PLQFs [41]. 1D PNSs have been proposed by Horvath et al. for the first time in the literature. Nanowires with 50–400 nm wide and 10 μm length has been prepared in DMF solution phase. Obtained NPSs have been used in Solar Cells [48], LED [4]

*(a) Schematic illustration of the perovskite nanopelets preparation. (b) Image of OA/MA perovskite* 

*suspensions in toluene under ambient light (reproduced with permission of Ref. [49]).*

Consequently, *2D PNSs* are nano pellet or nano sheet shaped materials which consist of several unit cells leading larger binding energy for the exciton and more intense PL. These types of NPs have similar networks with corresponded 3D perovskite, however the general formula of ABX3 is changed when the NP is very thin. Number of sheets in a nano pellet NPS can be determine by using absorption and emission spectroscopy. Differences of Absorption and emission maximum of nanosheets (n = 1, 2, 3 and 4) are very significant. Maximum absorption peak is red

In recent years, perovskite is a very important milestone in solar cell research, thanks to its perfect exciton and charge carrier properties. This excellent performance has allowed perovskite to be used as outstanding light emitters in Light Emitting Diodes (LEDs) and other optoelectronic applications [4, 7, 25, 50–56]. One of the most attractive features of perovskites is their emissions, which can be easily adjusted in the visible range compared to traditional III–V and II–VI groups. All inorganic-perovskite ABX3 emissions, including quantum dots and nanoplatelets,

shifted with the increasing of number of unit cell (nano- pellet) (**Figure 7**).

#### **Figure 7.**

*Perovskite and Piezoelectric Materials*

monium (MA+

structure is formed [42].

**3.2 Geometry of perovskite nano-crystals**

**3. Form factor of perovskite nano-crystals**

**3.1 Crystal shape of perovskite nano structures**

), formamidinium (FA+

As it is well known, semiconductor perovskite crystalline structure has a general

2( ) *A X B X*

Where, *Ar* and *Br* are cationic diameters of A and B respectively, and *Xr* is the anionic diameter of halide in Å. When cation A is too big or cation B is too small (>1), hexagonal perovskite crystal is formed. If cation A and has the ideal size (0.9 < *t* < 1), cubic perovskite crystal is formed. Finally, if cation is too small to fit into the space between metal cations (B) (0.7 < *t* < 0.9), orthorhombic perovskite

A typical "bulk" perovskite structure is called three-dimensional (3D) which is usually comply to the ABX3 formula completely [43–45]. In this situation, "3D" refers to the growth in every dimension without any confinement. X anions are combined through corner sharing to form a 3D network. Beside this, the cation A occupies the site in the middle of eight octahedra, and each element needs to owe the proper valence state to keep a whole charge balance [43]. To obtain nano crystalline perovskites, crystal growth must be restricted with at least one dimension by a capping agent or a matrix. Thus, It is possible to prepare various dimensioned PNSs such as zero, one or two dimension (0D, 1D or 2D respectively) (**Figure 6**) [1, 46].

*Schematic illustration of low-dimensional perovskites and 3D perovskite crystal structures (reproduced with* 

*r r*

*r r* ′ <sup>+</sup> <sup>=</sup> <sup>+</sup>

) or inorganic cesium (Cs+

); B is a metal

(1)

formula of ABX3 where A is an organic or inorganic cation, organic methylam-

*t*

cation typically lead (Pb2+) or tin cations (Sn2+); and X is a halide anion. Cation A is slightly larger than centred cation B having 6 co-ordination number. There is a significant relationship, which is called "Goldschimidt Tolerance Factor" between the size of the ions and the formation and the shape of the crystal structure:

**54**

**Figure 6.**

*permission of Ref. [47]).*

*(a) Schematic illustration of the perovskite nanopelets preparation. (b) Image of OA/MA perovskite suspensions in toluene under ambient light (reproduced with permission of Ref. [49]).*

*0D PNSs* have been synthesized, by LARP [25], hot-injection [32] and template-assisted method [36] by different groups. Crystal growth is prevented in all dimensions by a ligand or a metal-oxide matrix to obtained quantum-dot-like NPs representing good Photoluminescence (PL). Prepared nanostructures have been used in many opto-electronic applications such as solar cells, LEDs and laser.

Perovskite nanowires or nanorods are basically *1D PNSs* which represent outstanding anisotropic optical and electrochemical properties with very high PLQFs [41]. 1D PNSs have been proposed by Horvath et al. for the first time in the literature. Nanowires with 50–400 nm wide and 10 μm length has been prepared in DMF solution phase. Obtained NPSs have been used in Solar Cells [48], LED [4] and photodetector [8] applications.

Consequently, *2D PNSs* are nano pellet or nano sheet shaped materials which consist of several unit cells leading larger binding energy for the exciton and more intense PL. These types of NPs have similar networks with corresponded 3D perovskite, however the general formula of ABX3 is changed when the NP is very thin. Number of sheets in a nano pellet NPS can be determine by using absorption and emission spectroscopy. Differences of Absorption and emission maximum of nanosheets (n = 1, 2, 3 and 4) are very significant. Maximum absorption peak is red shifted with the increasing of number of unit cell (nano- pellet) (**Figure 7**).

## **4. Applications of perovskite nanoparticles**

#### **4.1 Optical properties of perovskite nanoparticles**

In recent years, perovskite is a very important milestone in solar cell research, thanks to its perfect exciton and charge carrier properties. This excellent performance has allowed perovskite to be used as outstanding light emitters in Light Emitting Diodes (LEDs) and other optoelectronic applications [4, 7, 25, 50–56]. One of the most attractive features of perovskites is their emissions, which can be easily adjusted in the visible range compared to traditional III–V and II–VI groups. All inorganic-perovskite ABX3 emissions, including quantum dots and nanoplatelets,

can cover the entire visible area, even close to the infrared or ultraviolet region, by substituting halide elements from chloride to iodine [26, 34, 51, 57, 58]. Another way to adjust the emission is to insert other organic molecules into it or replace anions/ cations.
