**2. Physics behind the microwave heating**

Microwave is an electromagnetic wave having low energy with a wavelength and a frequency in the range of 1 m to 1 mm and 300 MHz to 300 GHz, respectively as shown in **Figure 1**. Mainly, the laboratory and household microwave oven operate at a frequency of 2.45 GHz, which corresponds to a wavelength of 11.2 cm. It can travel at the speed of light (~30 cm/nanosecond) like any other electromagnetic wave and consists of electric and magnetic fields oscillating in a direction perpendicular to each other. One can also define it as a Multiphysics phenomenon in which the heating arises due to interaction between matter and electromagnetic radiation.

**83**

*Doping of Semiconductors at Nanoscale with Microwave Heating (Overview)*

*Schematic representation of the electromagnetic spectrum in terms of wavelengths and frequencies.*

In contrast to other conventional methods of heating, here the medium itself gets self-heated as a result of the alignment of molecular dipoles present in it with respect to the field associated. The electric and magnetic components in microwave

*Schematic diagram of the interaction of an electric component of the microwave radiation with matter.*

The polar molecules are sensitive to an electric field, and thus as a result of force exerted by the field on the charged particles, they start to migrate or rotate in order to align along the field (**Figure 2**). Since the electric and magnetic components reverse direction rapidly with a frequency of 2.45 GHz, the electric dipoles have no time to orientate to the direction of electric filed. As a result, there occurs the angle between the orientation of the dipoles in space and the direction of the electric field and the energy loss by the dipoles occurs resulting in to rise of dielectric heating. Reflection, absorption, and transmission are the three modes by which the medium reacts to the electromagnetic waves, either in a single or combined fashion [18]. The effective dielectric loss factor for the dielectric heating can be expressed in terms of

> = + = ++ <sup>0</sup> *'' '' '' '' ''*

σ

ω

*ff polarisation dipolar interfacial dipolar* (1)

interact with matter in different manners as discussed below [1].

**2.1 Influence of electric field component**

dipolar polarization, ionic conduction as follows

ε

*DOI: http://dx.doi.org/10.5772/intechopen.95558*

**Figure 1.**

**Figure 2.**

*Doping of Semiconductors at Nanoscale with Microwave Heating (Overview) DOI: http://dx.doi.org/10.5772/intechopen.95558*

**Figure 1.**

*Microwave Heating - Electromagnetic Fields Causing Thermal and Non-Thermal Effects*

altering the kinetics and chemical reaction [9–11]. The rate accelerations caused by "specific microwave effect" as well as "non-thermal effects" have to be considered in the microwave heating mechanism. Baghbanzadeh *et al*. propose that microwave dielectric heating can be termed as "specific microwave effects" by which one can achieve rate accelerations that cannot be attained by the conventional methods [12]. In the case of "non-thermal microwave effects", the heating mechanism arises as a result of the direct interaction of microwaves with specific molecules or materials in the reaction medium [2, 12]. Jacob *et al.* report that the enhancement rate of reaction with microwave heating compared to conventional heating is mainly due to the thermal effects which arise due to three significant factors. Firstly, the localized heating effect is a consequence of superheating phenomena due to the abundant ions present in the medium. Secondly, the molecular agitation due to lag of dipole, in following the fast-moving EM wave. Thirdly, increase of diffusion rate

Earlier, the synthesis of high-quality semiconducting quantum dots was very tough and the process of doping at this length scale makes it even more challenging. Erwin *et al.* reported that the difficulty in doping at nanoscale regime is due to the difference in mechanisms involved in doping at bulk and at the nanoscale, while other reports in literature claim the process of 'self-purification' as the leading cause for de-doping during the growth process [14]. The major daunting challenges arise mostly due to the lack of a comprehensive understanding of all the fundamental mechanisms associated with dopants incorporation and the absence of reliable synthetic procedures where the temperature-dependent dopant impurity atoms diffusion will be minimal [15]. Another challenge involved in doping at the nanoscale is the inherent statistical inhomogeneity of dopants among the nanocrystals. The doped nanomaterials always tend to exhibit a broad range of dopant populations per nanocrystal, which results in effective inhomogeneity in concentration of dopants among nanocrystals. Providing a uniform and instantaneous heating during the reaction process can minimize this problem to a great extent [16]. In this context, microwave heating became the suitable thermal energy source for doping the semiconducting nanocrystals, as it provides rapid and instantaneous heating. Short reaction time, faster reaction rate, uniform volumetric heating, cost-effective and eco-friendly method are the other remarkable features which make microwave heating a prime superior choice over other conventional methods of heating like a hot plate, oil bath, etc. [8]. Moreover, heating by means of conventional methods always results in 'a self-purification' mechanism where the dopants are diffused towards the surface of nanocrystals at the time of growth [14]. By adopting microwave-assisted techniques the aforementioned problems encountered while doping at the nanoscale can be eliminated to a great extent and the synthesized products are found to excel both in quality as well

Microwave is an electromagnetic wave having low energy with a wavelength and a frequency in the range of 1 m to 1 mm and 300 MHz to 300 GHz, respectively as shown in **Figure 1**. Mainly, the laboratory and household microwave oven operate at a frequency of 2.45 GHz, which corresponds to a wavelength of 11.2 cm. It can travel at the speed of light (~30 cm/nanosecond) like any other electromagnetic wave and consists of electric and magnetic fields oscillating in a direction perpendicular to each other. One can also define it as a Multiphysics phenomenon in which the heating arises due to interaction between matter and electromagnetic radiation.

**82**

as quantity [17].

**2. Physics behind the microwave heating**

of reactants [13].

*Schematic representation of the electromagnetic spectrum in terms of wavelengths and frequencies.*

#### **Figure 2.**

*Schematic diagram of the interaction of an electric component of the microwave radiation with matter.*

In contrast to other conventional methods of heating, here the medium itself gets self-heated as a result of the alignment of molecular dipoles present in it with respect to the field associated. The electric and magnetic components in microwave interact with matter in different manners as discussed below [1].

#### **2.1 Influence of electric field component**

The polar molecules are sensitive to an electric field, and thus as a result of force exerted by the field on the charged particles, they start to migrate or rotate in order to align along the field (**Figure 2**). Since the electric and magnetic components reverse direction rapidly with a frequency of 2.45 GHz, the electric dipoles have no time to orientate to the direction of electric filed. As a result, there occurs the angle between the orientation of the dipoles in space and the direction of the electric field and the energy loss by the dipoles occurs resulting in to rise of dielectric heating. Reflection, absorption, and transmission are the three modes by which the medium reacts to the electromagnetic waves, either in a single or combined fashion [18]. The effective dielectric loss factor for the dielectric heating can be expressed in terms of dipolar polarization, ionic conduction as follows

$$
\boldsymbol{\epsilon}\_{\boldsymbol{f}\boldsymbol{f}}^{\boldsymbol{\prime}} = \boldsymbol{\varepsilon}\_{\boldsymbol{polarisation}}^{\boldsymbol{\prime}} + \boldsymbol{\epsilon}\_{\boldsymbol{dipole}}^{\boldsymbol{\prime}} = \boldsymbol{\epsilon}\_{\boldsymbol{interfocal}}^{\boldsymbol{\prime}} + \frac{\boldsymbol{\sigma}}{\boldsymbol{\alpha}\boldsymbol{\epsilon}\_{\boldsymbol{\alpha}}} + \boldsymbol{\epsilon}\_{\boldsymbol{dipole}}^{\boldsymbol{\prime}} \tag{1}
$$

where **''** *polarisation* , '' *dipolar* , **''** *interfacial* , σ, ω, and <sup>0</sup> represent the polarization dielectric loss, dipolar dielectric loss, interfacial dielectric loss, electrical conductivity (S/m), angular frequency (Hz), and permittivity of free space (8.854 × 10−12 F/m), respectively [19].

### **2.2 Influence of magnetic field component**

Like an electric field, a magnetic field interacts too with matter and induces heat through magnetic loss, joule heating, and so on. However, sufficient studies apart from dielectric heating are still very rare. Meanwhile, Cheng *et al.* reported that magnetic loss contributes significantly to microwave heating compared to dielectric heating [20]. The necessary physical processes generating heat energy as a result of interaction between material medium and the magnetic field component are the eddy current loss, hysteresis loss, and magnetic resonance loss [21, 22]. The overall losses that constitute the effective magnetic permeability ( µ*effective* ′′ ) we may define as

$$
\mu^{\ddot{\phantom{\phantom{0}}}}\_{\text{effective}} = \mu^{\ddot{\phantom{0}}}\_{\text{hytereisi}} + \mu^{\ddot{\phantom{0}}}\_{\text{eddy current}} + \mu^{\ddot{\phantom{0}}}\_{\text{reidal}} \tag{2}
$$

where µ*hysteresis* ′′ , µ*eddy current* ′′ , and µ*residual* ′′ represent the hysteresis magnetic loss (H/m), eddy current magnetic loss (H/m), and residual magnetic loss (H/m), respectively [18].

## **3. Doping at the nanoscale and microwave heating**

Microwave heating has been the subject of interest for doping semiconductors at nanoscale owing to its ability to control the synthesis process explicitly. Apart from being cost-effective, the dielectric heating by microwave irradiation minimizes the dopant diffusion problem and provides quick reaction among precursors. The process of nucleation and growth of nanocrystals have been described in theories like LaMer burst nucleation [23], Watzky and Finke's slow nucleation followed by autocatalytic growth [24], and LSW theory, etc. [25, 26]. Nucleation is the process where nuclei act as a template for nanocrystal growth. Uniform formation of nuclei throughout the growth medium defined as 'homogeneous nucleation' can be easily and efficiently achieved by microwave irradiation in contrast to conventional methods of heating. Volumetric heating provided by microwave irradiation raises

#### **Figure 3.**

*Schematic illustration of main differences between the microwave heating (a) and traditional heating method (b).*

**85**

quency,

permittivity,

µ

nanoscale materials.

µ

*Doping of Semiconductors at Nanoscale with Microwave Heating (Overview)*

*d* α

ε

precursors plays a significant role in the formation of nanocrystals.

where α is the absorption coefficient of microwaves,

<sup>0</sup> is the permeability, *eff*

**3.1 Doping of semiconducting nanomaterials**

the internal temperature of the whole medium simultaneously and homogeneously as illustrated in **Figure 3**. This favors a quick nucleation process which results in solution supersaturation leading to homogeneous nucleation. Microwave-assisted technique aids to measure, manipulate, and thereby optimize the nucleation process and parameters that in turn influences the stability of the synthesized particles along with an added advantage of automatic data recording [12]. Efficient doping is determined by the surface morphology and shape of nanocrystals and the presence of surfactants in the reaction medium. Temperature plays a significant role in molding the aforementioned factors [27]. This demands the necessity for a proper thermal energy source like microwave heating while synthesizing nanocrystals. High penetration depth (*d*) offered by microwave heating is yet another factor that distinguishes it from the conventional methods of heating. It is defined as the distance at which the microwave power reduces to 1*/e* of its incident power. It has inverse proportionality with oscillating frequency, dielectric, and magnetic loss factor. The formula for determinate a penetration depth (*d*) may be written as

*eff*

1 2 (3)

ω

is the oscillating fre-

<sup>0</sup> is the vacuum

ε

0 0

′′ is the dielectric loss factor,

′ is the magnetic loss factor, etc. [19]. In addition to the above-

The efficient absorption of the EM wave by the solvent is determined by its loss tangent factor. It is defined as the ability of a material to convert electromagnetic energy into heat energy at a given frequency and temperature [1]. A high value is desired for maximum absorption, however, heating aided by microwave radiation is achievable even in the presence of a low tan (*δ*) solvent provided there exists either a polar reactant or reagent such that the overall dielectric nature of the reaction medium favors the microwave heating. In the case of conventional heating methods, the transfer of heat is slow and inefficient, resulting in a huge temperature gradient owing to the different thermal conductivity of materials. However, in the case of microwave radiation, there is a direct coupling between the microwave energy and the molecules resulting in core volumetric heating. The most commonly used frequency of the microwave is 2.45 GHz, possessing an energy of 0.0016 eV, which is lower than that of Brownian motion and therefore insufficient to break the bonds. This property of microwaves makes them incapable of carrying out any unwanted reactions and thereby solely ensuring effective doping at

 ω µ µε ε

mentioned factors, the specific interaction of electromagnetic wave with the

Nanocrystals are broadly classified as nanoparticles and quantum dots. Generally, tiny particles of a dimension of 100 nm or below are termed as nanoparticles. However, quantum dots (QDs) are a class of nanomaterials with their charge carriers confined in all three dimensions of the length scale of exciton Bohr radius [28]. While doping the QDs, the dopants have a high tendency to come out of it due to the thermal diffusion because their size is in the nanometer range. This problem can be resolved greatly by having a comprehensive idea about the various mechanisms involved during doping and following a proper synthesis process [29]. However, various properties, including optical, magnetic, and electronic, of semiconducting quantum dots can be tailored in a desired fashion by the incorporation

′′ ′ = = <sup>2</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.95558*

*Doping of Semiconductors at Nanoscale with Microwave Heating (Overview) DOI: http://dx.doi.org/10.5772/intechopen.95558*

the internal temperature of the whole medium simultaneously and homogeneously as illustrated in **Figure 3**. This favors a quick nucleation process which results in solution supersaturation leading to homogeneous nucleation. Microwave-assisted technique aids to measure, manipulate, and thereby optimize the nucleation process and parameters that in turn influences the stability of the synthesized particles along with an added advantage of automatic data recording [12]. Efficient doping is determined by the surface morphology and shape of nanocrystals and the presence of surfactants in the reaction medium. Temperature plays a significant role in molding the aforementioned factors [27]. This demands the necessity for a proper thermal energy source like microwave heating while synthesizing nanocrystals. High penetration depth (*d*) offered by microwave heating is yet another factor that distinguishes it from the conventional methods of heating. It is defined as the distance at which the microwave power reduces to 1*/e* of its incident power. It has inverse proportionality with oscillating frequency, dielectric, and magnetic loss factor. The formula for determinate a penetration depth (*d*) may be written as

$$d = \frac{1}{\alpha} = \sqrt{\frac{2}{\alpha^2 \mu\_0 \mu' \varepsilon\_{\text{eff}}^"{} \varepsilon\_0}}\tag{3}$$

where α is the absorption coefficient of microwaves,ω is the oscillating frequency, µ<sup>0</sup> is the permeability, *eff* ε ′′ is the dielectric loss factor, ε <sup>0</sup> is the vacuum permittivity, µ′ is the magnetic loss factor, etc. [19]. In addition to the abovementioned factors, the specific interaction of electromagnetic wave with the precursors plays a significant role in the formation of nanocrystals.

The efficient absorption of the EM wave by the solvent is determined by its loss tangent factor. It is defined as the ability of a material to convert electromagnetic energy into heat energy at a given frequency and temperature [1]. A high value is desired for maximum absorption, however, heating aided by microwave radiation is achievable even in the presence of a low tan (*δ*) solvent provided there exists either a polar reactant or reagent such that the overall dielectric nature of the reaction medium favors the microwave heating. In the case of conventional heating methods, the transfer of heat is slow and inefficient, resulting in a huge temperature gradient owing to the different thermal conductivity of materials. However, in the case of microwave radiation, there is a direct coupling between the microwave energy and the molecules resulting in core volumetric heating. The most commonly used frequency of the microwave is 2.45 GHz, possessing an energy of 0.0016 eV, which is lower than that of Brownian motion and therefore insufficient to break the bonds. This property of microwaves makes them incapable of carrying out any unwanted reactions and thereby solely ensuring effective doping at nanoscale materials.

#### **3.1 Doping of semiconducting nanomaterials**

Nanocrystals are broadly classified as nanoparticles and quantum dots. Generally, tiny particles of a dimension of 100 nm or below are termed as nanoparticles. However, quantum dots (QDs) are a class of nanomaterials with their charge carriers confined in all three dimensions of the length scale of exciton Bohr radius [28]. While doping the QDs, the dopants have a high tendency to come out of it due to the thermal diffusion because their size is in the nanometer range. This problem can be resolved greatly by having a comprehensive idea about the various mechanisms involved during doping and following a proper synthesis process [29]. However, various properties, including optical, magnetic, and electronic, of semiconducting quantum dots can be tailored in a desired fashion by the incorporation

*Microwave Heating - Electromagnetic Fields Causing Thermal and Non-Thermal Effects*

loss, dipolar dielectric loss, interfacial dielectric loss, electrical conductivity (S/m), angular frequency (Hz), and permittivity of free space (8.854 × 10−12 F/m),

Like an electric field, a magnetic field interacts too with matter and induces heat through magnetic loss, joule heating, and so on. However, sufficient studies apart from dielectric heating are still very rare. Meanwhile, Cheng *et al.* reported that magnetic loss contributes significantly to microwave heating compared to dielectric heating [20]. The necessary physical processes generating heat energy as a result of interaction between material medium and the magnetic field component are the eddy current loss, hysteresis loss, and magnetic resonance loss [21, 22]. The overall

*'' '' ''*

(H/m), eddy current magnetic loss (H/m), and residual magnetic loss (H/m),

*Schematic illustration of main differences between the microwave heating (a) and traditional heating* 

 µ

Microwave heating has been the subject of interest for doping semiconductors at nanoscale owing to its ability to control the synthesis process explicitly. Apart from being cost-effective, the dielectric heating by microwave irradiation minimizes the dopant diffusion problem and provides quick reaction among precursors. The process of nucleation and growth of nanocrystals have been described in theories like LaMer burst nucleation [23], Watzky and Finke's slow nucleation followed by autocatalytic growth [24], and LSW theory, etc. [25, 26]. Nucleation is the process where nuclei act as a template for nanocrystal growth. Uniform formation of nuclei throughout the growth medium defined as 'homogeneous nucleation' can be easily and efficiently achieved by microwave irradiation in contrast to conventional methods of heating. Volumetric heating provided by microwave irradiation raises

*interfacial* , σ, ω, and <sup>0</sup> represent the polarization dielectric

µ*effective*

 µ*effective hysteresis eddy current residual* =+ + (2)

′′ represent the hysteresis magnetic loss

′′ ) we may define as

where **''**

respectively [19].

where µ*hysteresis* ′′ , µ*eddy current* ′′ , and

respectively [18].

*polarisation* , ''

*dipolar* , **''**

**2.2 Influence of magnetic field component**

losses that constitute the effective magnetic permeability (

µµ

**3. Doping at the nanoscale and microwave heating**

µ*residual*

*''*

**84**

**Figure 3.**

*method (b).*

of impurity dopant atoms [30]. Moreover, this can also generate some new physical properties, including spin-polarizable excitonic photoluminescence, exciton storage, excitonic magnetic polaron formation, and magnetic circular dichroism so on. The proper incorporation of impurity atoms into the semiconducting QDs is a tough job but can be identified by observing the following features like red-shifted PL emission and large Zeeman splitting of excitonic excited states that are a result of strong exchange coupling between dopant and the carrier [31, 32]. In the year 2000, Mikulec *et al*. reported the most significant result on QDs doping; in which they reported manganese (Mn) doped CdSe nanocrystals with the evidential result obtained from electron paramagnetic resonance (EPR) [33]. Later, a variety of doped semiconducting material were reported by tailoring both the host atoms such as ZnS, PbS, MgO, Al2O3, α-Fe2O3, CdS, ZnSe, etc. and dopant atoms such as Mn, Cu, Ag, Fe, Zn, Cr, Er, etc. [34, 35]. However, there is a limitation to select the host system and the respective dopant atoms. Suppose, incorporation of Mn into nanocrystals of CdS and ZnSe easy but not into CdSe even though the bulk solubility almost equal to 50% for all three [27, 36].

Depending on dopants' diffusivity, the dopant precursors are injected at different time intervals, suppose along with the host precursors or at the time of nucleation or growth as shown in **Figure 4** [37]. The major problem involved in doping at the nanoscale is that many dopants fail to be incorporated within the host lattice and instead get adsorbed on the surface [38]. High formation energy for defects renders the impurity atoms to be thermodynamically unstable, resulting in the expulsion of dopants from the host lattice, in turn leading to self-purification [14, 27, 39]. Apart from thermodynamics, kinetics also play a significant role in determining the stability of added impurities in solution phase synthesis. Chen *et al.* have reported a detailed study regarding all the elemental processes involved with doping, such as surface adsorption, lattice incorporation, lattice diffusion, and lattice ejection as represented schematically in **Figure 5** [40]. Maintenance of appropriate temperature is a crucial factor even in the phenomena mentioned above.

The high cost of commercially doped QDs is one reason that limits its wide range of applications. Therefore, cost-effective synthesis protocols need to be developed to produce high-quality doped QDs. This limitation and the ones mentioned above are lifted off using microwave heating for doping the QDs. It is also found to be an economical and eco-friendly method in line with green chemistry. Now let us discuss some semiconducting QDs systems where doping has been performed with microwave heating technique.

**87**

*Doping of Semiconductors at Nanoscale with Microwave Heating (Overview)*

*Schematic diagram showing temperature-dependent dopant lattice diffusion [37].*

CdSe QDs is n-type intrinsically, and a flagship candidate in nanoscale research history shows several novel properties as a member of the II-VI binary semiconductor group. It was attractive to the researchers to demonstrate various optoelectronic applications as its energy band overlaps nicely with the solar energy spectrum [41]. The fundamental properties of CdSe are enhanced via doping, which further increases its demand in the semiconductor industry. However, doping of CdSe by Mn2+ ions is challenging due to the self-purification effect, as reported by Erwin *et al.* [27, 29, 42]. The doping process is mainly governed by the surface kinetic effect. Microwave heating helps one to have exquisite control over this surface

Meladom *et al.* developed a robust synthesis protocol for efficient doping of Mn2+ into CdSe QDs in an aqueous medium with mild microwave heating as a final step [17]. A household microwave oven was used to heat the CdSe QDs solution for 60 seconds duration with the set point of 450 W (operational frequency 2.45 GHz). This heating step was repeated three times by giving 5 minutes intervals. The motivation was to tune the electrical conductivity of CdSe QDs thin film by varying doping concentration only as the size of QDs kept similar for all the samples. Microwave heating improves the quality of QDs in terms of optical properties, which was confirmed by recording UV–vis absorbance and photoluminescence both excitation and emission spectra, as shown in **Figure 6(a)** and **(b)**, respectively. In all the cases, peak intensities were enhanced and bandwidth reduced, which indicates the reduction of surface defects of QDs. The chemical composition of the doped CdSe QDs sample was confirmed with X-ray photoelectron spectroscopy (XPS), energydispersive X-ray spectroscopy (EDS), and inductively coupled plasma - atomic emission spectroscopy (ICP-AES) measurements data. XPS result confirmed the efficient

incorporation of Mn atoms as dopants inside the host CdSe QDs (**Figure 7**).

Microwave-assisted synthesis has also been utilized by many research groups around the world to dope various other binary II-VI semiconductor-based nanocrystals. Molaei *et al.* reported the synthesis of copper (Cu) doped ZnSe nanocrystals in the aqueous medium to study the doping effect on the optical properties [43]. Synthesis of Mn2+ ion-doped ZnS quantum dots was reported by Joicy *et al.* using a rapid microwave irradiation step without any surfactants, which showed photocatalytic activity by observing photodegradation of methyl orange dye under

*3.1.2 More examples on doped binary nanocrystals*

*DOI: http://dx.doi.org/10.5772/intechopen.95558*

*3.1.1 Mn-doped CdSe quantum dots*

**Figure 5.**

kinetics that eases the doping process.

**Figure 4.** *General schematic model of the colloidal synthesis of doped quantum dots [37].*

*Doping of Semiconductors at Nanoscale with Microwave Heating (Overview) DOI: http://dx.doi.org/10.5772/intechopen.95558*

**Figure 5.**

*Microwave Heating - Electromagnetic Fields Causing Thermal and Non-Thermal Effects*

almost equal to 50% for all three [27, 36].

mentioned above.

microwave heating technique.

*General schematic model of the colloidal synthesis of doped quantum dots [37].*

of impurity dopant atoms [30]. Moreover, this can also generate some new physical properties, including spin-polarizable excitonic photoluminescence, exciton storage, excitonic magnetic polaron formation, and magnetic circular dichroism so on. The proper incorporation of impurity atoms into the semiconducting QDs is a tough job but can be identified by observing the following features like red-shifted PL emission and large Zeeman splitting of excitonic excited states that are a result of strong exchange coupling between dopant and the carrier [31, 32]. In the year 2000, Mikulec *et al*. reported the most significant result on QDs doping; in which they reported manganese (Mn) doped CdSe nanocrystals with the evidential result obtained from electron paramagnetic resonance (EPR) [33]. Later, a variety of doped semiconducting material were reported by tailoring both the host atoms such as ZnS, PbS, MgO, Al2O3, α-Fe2O3, CdS, ZnSe, etc. and dopant atoms such as Mn, Cu, Ag, Fe, Zn, Cr, Er, etc. [34, 35]. However, there is a limitation to select the host system and the respective dopant atoms. Suppose, incorporation of Mn into nanocrystals of CdS and ZnSe easy but not into CdSe even though the bulk solubility

Depending on dopants' diffusivity, the dopant precursors are injected at different time intervals, suppose along with the host precursors or at the time of nucleation or growth as shown in **Figure 4** [37]. The major problem involved in doping at the nanoscale is that many dopants fail to be incorporated within the host lattice and instead get adsorbed on the surface [38]. High formation energy for defects renders the impurity atoms to be thermodynamically unstable, resulting in the expulsion of dopants from the host lattice, in turn leading to self-purification [14, 27, 39]. Apart from thermodynamics, kinetics also play a significant role in determining the stability of added impurities in solution phase synthesis. Chen *et al.* have reported a detailed study regarding all the elemental processes involved with doping, such as surface adsorption, lattice incorporation, lattice diffusion, and lattice ejection as represented schematically in **Figure 5** [40]. Maintenance of appropriate temperature is a crucial factor even in the phenomena

The high cost of commercially doped QDs is one reason that limits its wide range of applications. Therefore, cost-effective synthesis protocols need to be developed to produce high-quality doped QDs. This limitation and the ones mentioned above are lifted off using microwave heating for doping the QDs. It is also found to be an economical and eco-friendly method in line with green chemistry. Now let us discuss some semiconducting QDs systems where doping has been performed with

**86**

**Figure 4.**

*Schematic diagram showing temperature-dependent dopant lattice diffusion [37].*
