*3.1.1 Mn-doped CdSe quantum dots*

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 kinetics that eases the doping process.

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**).

#### *3.1.2 More examples on doped binary nanocrystals*

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

#### **Figure 6.**

*Electronic UV–vis (a) and photoluminescence (b) spectrum of Mn2+ doped CdSe QDs sample with and without microwave irradiation [17].*

#### **Figure 7.**

*(a) Survey scan of the X-ray photoelectron spectrum of 2% Mn2+-doped CdSe QDs. (b, c) high-resolution spectrum of Cd 3d electrons depicting doublet splitting with binding energies separated by 6.9 eV and Se 3d, respectively. (d) the high-resolution spectrum of Mn 2p core electrons showing doublet splitting with binding energy separated by 10.9 eV [17].*

UV light irradiation [44]. Here, the zinc blende crystal phase of ZnS was important for the efficient incorporation of Mn atoms. In the same year, Zhu *et al* reported the synthesis of of Mn-doped ZnS via green and rapid microwave-assisted approach and they also developed indapamide drug detector by recording room-temperature phosphorescence (RTP) with that doped material [45].

Later, Zhang *et al.* reported the aqueous synthesis of Mn and Cu doped ZnSe QDs by microwave radiation with higher quantum yields (QYs) and they have further extended this work to grow the white-light-emitting ZnSe/ZnS core/shell QDs via the co-doping of Mn and Cu [46]. Lead sulphide (PbS) QD are still emerging various applications in optoelectronics and its property was further tuned with a

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

silver (Ag) atom doping. It is also reported by Shkir *et al.* that the bandgap of PbS QDs was increased with Ag atom incorporation, which was predicted without mentioning the influence of size variation between the samples used [47]. Recently, another work reported on facile microwave synthesis of CdS quantum dots doped with Cr atoms as impurity doping and they have studied various properties like

Various metal oxides nanomaterials play a major role in the development of different novel daily life applications in the fields of display, sensors, medicine, biomedical devices, agriculture, information technology, optical, energy, electronics, and so on. Efforts are ongoing to tune their properties and applications further with incorporating impurity as dopants. For that reason, microwave heating based synthesis protocols is developing as a potential alternative to the conventional heating based growth process. Kar *et al.* developed a microwave synthesis of rare-earth element Eu3+ doped tin oxide (SnO2) to tune the optical and electrical properties of the host [49]. Jamatia *et al*. reported the microwave-assisted synthesis of Fe doped ZnO nanoparticles to show their application in polymer light-emitting diodes [50]. The wurtzite hexagonal crystal phase of ZnO nanoparticles and incorporation of the Fe dopant into the host ZnO crystal lattice was confirmed via X-ray diffraction analysis. This report claimed that the bandgap modification of ZnO via Fe doping is estimated from the Tauc plot without considering the influence of size. Similarly, many spinel structured metal oxides were also doped with different transition metal ions via microwave heating based synthesis technique with tunable structural, morphological, optical, vibrational, and magnetic properties and different potential applications like phosphor-based forensic testing and many more [51–54]. Interestingly, Er3+ doped α-Fe2O3 and Fe doped TiO2 nanoparticles were synthesized successfully with the help of microwave heating to study their crystal structure and optical properties [55, 56]. Recently, Yathisha *et al*. reported Zn2+ doped MgO nanoparticles utilizing microwave combustion route to study the influence on photovoltaic properties [57]. Therefore, microwave heating could explore further as a low-cost alternative synthesis protocol to design a new variant of nanomaterials.

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

*3.1.3 Doped oxide nanomaterials*

structural, opto-dielectric, electrical, and so on [48].

*3.1.4 Lanthanide ion doped lanthanum trifluoride (LaF3)*

**3.2 Doping of carbon based nanomaterials**

Lanthanum trifluoride (LaF3) is an ionic compound that is utilized as core-shellup conversion nanoparticles (UCNPs) for different filed of applications like sensing, biomedical, and solar cells. Tek *et al*. reported Yb3+ ion-doped (active) and undoped (inert) LaF3 shell coatings on a 20% Yb, 2% Tm codoped hexagonal phase LaF3 core with the help of microwave -assisted synthesis route [58]. They observed higher optical enhancement of inert shell compared with the active shell at all prominent emission peaks, which is explained with the energy band diagram indicating the

Carbon-based materials like graphene can also be doped via microwave (MW) heating. Since the nanocarbon materials are found to be sensitive to microwave radiation [59, 60], the technique of MW heating was employed in the modification of graphene materials. It is also reported that by the use of microwave heating, hollow carbon nanospheres can be synthesized within a short time which can be effectively used as a host material for doping [61, 62]. **Figure 9** shows the

energy transfer pathways for the Yb3+ and Tm3+ − co-doping (**Figure 8**).

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

silver (Ag) atom doping. It is also reported by Shkir *et al.* that the bandgap of PbS QDs was increased with Ag atom incorporation, which was predicted without mentioning the influence of size variation between the samples used [47]. Recently, another work reported on facile microwave synthesis of CdS quantum dots doped with Cr atoms as impurity doping and they have studied various properties like structural, opto-dielectric, electrical, and so on [48].
