*3.1.4 Lanthanide ion doped lanthanum trifluoride (LaF3)*

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

## **3.2 Doping of carbon based nanomaterials**

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

**Figure 8.**

*(a) UCPL data for core, active shell, and inert shell nanoparticles under 980 nm continuous-wave excitation. (b) Energy diagram showing the corresponding transition of UCPL of (a) where the energy transfer pathways for the Yb3+ and Tm3+-codoped up conversion nanoparticles are depicted [58].*

#### **Figure 9.**

*Microwave-induced synthesis of Ni/graphitic-shell nanocrystals and graphitic hollow carbon Nano spheres [61].*

microwave-assisted approach to prepare metal/graphitic shell nanocrystals and CNT in a very short time using ordinary carbon precursor.

The microwave-assisted technique facilitates the growth of heteroatom-doped graphene with better catalytic activity as well. Nitrogen doping up to 8.1% on graphene was achieved by Kwang *et al* within a minute with the aid of the microwave radiation. The binding configuration of nitrogen over graphitic basal planes can be varied with the irradiation power of microwave. The conductivity enhancement upto 300 Scm−1 was obtained in this case in comparison to nitrogen-doped via arc discharge method, nitrogen plasma process, etc. showing a lesser conductivity [63]. The dielectric heating of MW induces a high energy state that helps the graphitic basal plane to accommodate the dopants in order to convert graphite to N-doped graphene. The selective dielectric heating, which arises due to the difference in the dielectric constants of solvent and reactant can enhance the efficiency of doping without the rise of a thermal gradient [64]. The solid phase microwaveassisted synthetic method is adopted for the large-scale production of N-doped carbon nanodots (CNDs) using different citric acid/urea (C/U) weight ratios, which result in size variation of CNDs as shown in **Figure 10** with the transmission electron microscope (TEM) images. The dopant ion concentration can be varied in a precise manner that results in N-doped graphene QDs and graphitic-carbon nitride quantum dots (g-CNQD). The doped material is found to exhibit a 38.7% quantum yield due to the presence of N and O rich edge groups resulting from the interaction of microwave on graphene [65].

**91**

dopant ions.

**Figure 10.**

than 102

data storage devices in future.

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

**4. Electrical and memristor property of Mn2+doped CdSe QDs**

rise in the electrical conductivity to the order of 104

*shows the corresponding selected-area diffraction pattern [65].*

Huge enhancement in the conductivity of microwave-assisted doped QDs has been reported in many pieces of literature. Microwave heating enables the tuning of electrical conductivity in a desired manner by proper incorporation of dopants into the desired locations of the host material. This is evidenced by the

*HR-TEM micrographs of N-doped carbon nanodots (CNDs) samples prepared using the SPMA method for different citric acid /urea (C/U) weight ratios of 3/1 (a), 2/1 (b), 1/1 (c), 1/1.5 (d), 1/2 (e), and 1/3 (f). Inset* 

over undoped one as shown in **Figure 11(a)** [17]. Here, the conduction mechanism is controlled by the electric field-assisted thermal ionization of trapped charge carriers in CdSe QDs as described in Poole–Frenkel effect as shown in **Figure 11(b)** [66]. The bandgap has no role in the conductivity and the observed colossal conductivity enhancement is solely due to the concentration of Mn2+

The STM study performed on a monolayer device of Mn2+ doped CdSe QDs synthesized via microwave method founds to exhibit excellent memory characteristics as described in **Figure 12** [17]. This memristor property is evident from **Figure 12(b)** where the doped CdSe QDs switched to a high conducting state at the bias of 2.5 V. It is also observed that the device switched back to its low conducting state when the tip swept towards 3 V and the ON/OFF ratio obtained was higher

. The reproducible nature of the resistive switching property over many cycles further confirms the reliability of the measurement. The threshold voltage at which the device switches to a high conducting state is found to be decreasing with an increase in the dopant concentration. Thus the notable electric bistability and the low threshold voltage of as synthesized doped CdSe QDs with the aid of simple and domestic microwave method promises its application in vivid area of future technologies which ensures minimum energy consumption per byte of the resistive

for 2% Mn2+ doped CdSe

*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*

**Figure 10.**

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

microwave-assisted approach to prepare metal/graphitic shell nanocrystals and

*Microwave-induced synthesis of Ni/graphitic-shell nanocrystals and graphitic hollow carbon Nano* 

*(a) UCPL data for core, active shell, and inert shell nanoparticles under 980 nm continuous-wave excitation. (b) Energy diagram showing the corresponding transition of UCPL of (a) where the energy transfer pathways* 

The microwave-assisted technique facilitates the growth of heteroatom-doped graphene with better catalytic activity as well. Nitrogen doping up to 8.1% on graphene was achieved by Kwang *et al* within a minute with the aid of the microwave radiation. The binding configuration of nitrogen over graphitic basal planes can be varied with the irradiation power of microwave. The conductivity enhancement upto 300 Scm−1 was obtained in this case in comparison to nitrogen-doped via arc discharge method, nitrogen plasma process, etc. showing a lesser conductivity [63]. The dielectric heating of MW induces a high energy state that helps the graphitic basal plane to accommodate the dopants in order to convert graphite to N-doped graphene. The selective dielectric heating, which arises due to the difference in the dielectric constants of solvent and reactant can enhance the efficiency of doping without the rise of a thermal gradient [64]. The solid phase microwaveassisted synthetic method is adopted for the large-scale production of N-doped carbon nanodots (CNDs) using different citric acid/urea (C/U) weight ratios, which result in size variation of CNDs as shown in **Figure 10** with the transmission electron microscope (TEM) images. The dopant ion concentration can be varied in a precise manner that results in N-doped graphene QDs and graphitic-carbon nitride quantum dots (g-CNQD). The doped material is found to exhibit a 38.7% quantum yield due to the presence of N and O rich edge groups resulting from the

CNT in a very short time using ordinary carbon precursor.

*for the Yb3+ and Tm3+-codoped up conversion nanoparticles are depicted [58].*

interaction of microwave on graphene [65].

**90**

**Figure 8.**

**Figure 9.**

*spheres [61].*

*HR-TEM micrographs of N-doped carbon nanodots (CNDs) samples prepared using the SPMA method for different citric acid /urea (C/U) weight ratios of 3/1 (a), 2/1 (b), 1/1 (c), 1/1.5 (d), 1/2 (e), and 1/3 (f). Inset shows the corresponding selected-area diffraction pattern [65].*
