Journey of ZnO quantum dots from undoped to rare-earth and transition metal-doped and their applications

Currently, developments in the field of quantum dots (QDs) have attracted researchers worldwide. A large variety of QDs have been discovered in the few years, which have excellent optoelectronic, antibacterial, magnetic, and other properties. However, ZnO is the single known material that can exist in the quantum state and can hold all the above properties. There is a lot of work going on in this field and we will be shorthanded if we do not accommodate this treasure at one place. This manuscript will prove to be a milestone in this noble cause. Having a tremendous potential, there is a developing enthusiasm toward the application of ZnO QDs in diverse areas. Sol–gel method being the simplest is the widely-favored synthetic method. Synthesis via this method is largely affected by a number of factors such as the reaction temperature, duration of the reaction, type of solvent, pH of the solution, and the precipitating agent. Doping enhances the optical, magnetic, anti-bacterial, anti-microbial, and other properties of ZnO QDs. However, doping elements reside mostly on the surface of the QDs. The presence of doping elements inside the core is still a major challenge for doping techniques. In this review article, we have focused on pure, rare-earth, and transition metal-doped ZnO QD properties, and the various synthetic processes and applications. Quantum confinement effect is present in nearly every aspect of the QDs. The effect of quantum confinement has also been summarized in this manuscript. Furthermore, the doping of rare earth elements and transition metal, synthetic methods for different organic molecule-capped ZnO QDs, mechanisms for reactive oxygen species (ROS) generation, drug delivery system for cancer treatment, and many more application are discussed in this paper.


Introduction
From the past few years, efforts are being made for the development of stabilized colloidal systems. Different organic and inorganic (non-toxic) materials have been used to stabilize the colloidal systems, for example, Mesquite, 1 milk fat globule membrane, 2 Gum-chitosan mixture, xanthan gum, guar gum, and locust bean gum 3 for the formation of multiple emulsions and proteins for the formation of foams. 4 However, due to large industrial application of colloidal systems stabilized by solid particles, this is a hot area of research. Solid particles can be used in places where other stabilizers cannot be used, for example, at high temperatures. However, very little work has been done using solid particles as stabilizers. 5 Some researchers have used solid particles to stabilize foam as these are easily adsorbed on the gas-liquid interface. 6,7 Theoretically, QDs are crystals that have physical dimensions comparable to the exciton Bohr radius of the material of which they are made. These particles are conned in all the three dimensions. In case of ZnO QDs, it is found that the exciton Bohr radius is very small, which is nearly 0.9 nm. Thus, it is very difficult to synthesize ZnO QDs having radius less than the Bohr radius. Most of the ZnO QDs have a radius greater than the Bohr radius but comparable to it. These particles can be considered as quantum dots as they successfully show the effect of quantum connement. It has an optical band gap of 3.37 eV, which further increases with the decrease in the particle size and large exciton binding energy (60 meV). 8 Most of the semiconductor quantum dots are in the form of colloidal solution. Among other colloidal systems, the colloidal solution of ZnO QDs is in great demand as they have a large area of application, excellent safety, good biocompatibility, nontoxicity, and low cost. Also, its anti-bacterial activity, biocompatibility, reliable mechanical properties, and physicochemical stability makes it a desirable candidate for dental materials. It is a potential candidate as a disinfectant and an antibacterial agent. These properties depend upon the structural morphologies and defects, surface functionalization, and exposure conditions. An illustrative diagram (Fig. 1) is provided for detailed information about the synthesis, properties, and applications of ZnO QDs. Hence, ZnO shows a wide range of applications in material engineering, health science, 9 drug delivery 10,11 photocatalyst, gas sensors, 12 cancer therapy [13][14][15] photoelectric light-emitting diodes (LED), [16][17][18] solar cells, 19 and photo-detectors. 20 Quantum connement 21-23 is a major aspect of QD that keeps it at the next level compared to nanoparticles. It has a direct inuence over the optical properties of ZnO QDs. 24 Threedimensional quantum connement of the charge carriers increases the life-time of the carriers and the photoluminescence intensity, which nally enhances the optoelectronic properties of the QDs. The quantum connement effect can be easily understood as a "particle-in-a-box". This phenomenon is observed when the electron wave-function is inuenced by the size of the particles. For ZnO QDs having size less than 3.6 nm, a strong connement is observed. 25 To study the connement effect practically, we need a 3D model. For such cases, "particle-in-a-box" is replaced by the "particle-in-asphere" model. In such cases, the expression for exciton binding energy can be written as where E bg is the band gap energy of the particle with radius R, E bulk bg is the band gap energy for the bulk, m * e , m * h , 3, and 3 0 are the effective mass of the electron, effective mass of the hole, dielectric constant for the semiconductor, and vacuum permittivity, respectively. The last term is the stabilization correction term for exciton binding energy. 26 The theoretical study of exciton-phonon interaction and its effect on the ground state energy and oscillator strength shows a reduction with the particle size. R. T. Senger and K. K. Bajaj 27 found that the polaronic self-energy corrections of the exciton vanishes completely and the PB potential effectually transforms into a dynamically-screened coulomb potential. In the PB model, the connement effect on the exciton ground-state energy depends upon the exciton radius. The size of the exciton in very small dots depends on the boundary of the connement potential. Quantum-conned Stark effect comes into existence with the decrease in the particle size (size less than 8 nm). 28 In this effect, an electric eld develops inside the particle due to the applied negative potential, which breaks the symmetry and splits the energy inside the conduction band. Experimentally, we can see the quantum connement effect by studying the absorption and photoluminescence (PL) spectra ( Fig. 2e and f) of the QDs. Both these spectra show a blue shi with a decrease in the particle size. From the TEM images, we can clearly see the decrease in the particle size ( Fig. 2a-d). However, using ML, we can predict the different physical properties of the particles very accurately (Fig. 2g). 29 Quantum dots give excellent uorescence as compared to nanoparticles due to the availability of the electron-hole pair to interact with the surface states of the quantum dots. The ZnO QDs signicantly increase the uorescence of the material as compared to the ZnO nanoparticles ( Fig. 3e-g). ZnO QDs give yellow emission in the presence of UV light. The emission spectra may shi depending upon the precipitating agent used in the process, solvent, 30 induced defects, and the size of the particles. 31 The optical band gap of the QDs can also be tailored by the doping of different elements 32,33 and by using capping agents. From Fig. 3a and b, we can see that by capping QDs with SiO 2 , the UV spectra gives a blue shi in the range from 340 nm to 310 nm, whereas on capping nanoparticles with SiO 2 , there is a red shi. This is attributed to the change in the particle size, which decreases upon capping in the QDs and increases in the nanoparticles ( Fig. 3c and d). This is due to the restriction on agglomeration of ZnO QDs; the SiO 2 coating plays the role of restricting the agglomeration. Liu et al. 34 used Gd to reduce the size and increased the specic surface area of the ZnO QDs. Sun et al. 35 found that besides the reduction in size and increase in the vacancy defects, La 3+ also weakens the Zn-O bond, which give rise to more defects. However, excessive concentration of doping elements reduces the uorescence emission intensity and the quantum yield. 36 QDs also show the quenching of uorescence intensity with the addition of the metal ion. 37 The quenching of the uorescence intensity can also be possible by the passivation of surface defects by the organic ligands. It was found that the intensity of the photoluminescence (PL) spectra of the doped QDs rst increases with the concentration of the doping element and then starts decreasing with a further increase in the concentration of the doping elements. This is due to the weakening of the Zn-O bond with the increase in the doping concentration at the initial stage. This results in the production of more and more vacancy defects, which are responsible for the PL emission spectra. With a further increase in the concentration of the doping element, aer a certain limit, the Zn-O bond breaks and the PL intensity starts quenching. 38,39 Thus, ZnO QDs can be used as the uorescence probe for the detection of metal ion impurities in drinking water as the detection limit for several ions is very low. 40 ZnO QDs show strong anti-microbial properties at a specic pH 42 as ZnO can be easily dissolved into Zn 2+ . Protected (QDs with any capping agent) QDs release Zn 2+ ion at a specic location depending upon the capping agent. 43 Thus, these are helpful in targeted drug delivery systems 44 for the treatment of different diseases. Moreover, the concentration of the conduction band electron (e À ) and valence band holes (h + ) is very high even in the absence of UV (ultra-violet) light. 45 The presence of more electrons and holes in the conduction and valence band, respectively, as compared to the QDs nanoparticles, shows more ROS (reactive oxygen species) generation. Hence, QDs have a large capacity to degrade organic molecules, microbes, and bacteria. 46 The effect of the particle size on the Raman spectra of ZnO was also found. 47 In bulk ZnO, there are two phonon modes: longitudinal optical (LO) and transverse optical (TO). 48 These modes further split into A 1 and E 1 symmetries. There are two non-polar Raman-active phonon modes with E 2 symmetries also present. The low frequency E 2 mode is related to the vibration of the zinc ion lattice and the high frequency E 2 mode is related to the vibration of the oxygen ion lattice. The phonon peak shi that arises in the ZnO QDs is related to three main factors: phonon localization by defect creation, connement effect within the QD boundaries, and laser-induced heating effect. [49][50][51][52] Alim and his group 53 studied the resonant and nonresonant Raman spectra of ZnO. They found that the E 2 (high) peak shied by 3 cm À1 , which was due to the presence of defects. These defects are found to increase in the QDs and doped QDs in a large amount. They had also found that the resonant peak of the LO phonon mode shis linearly towards the lower frequency side with the increase in the UV laser power. From Fig. 4a and b, we can see that there is a small shi of 4 cm À1 in the LO phonon mode for bulk ZnO to ZnO QDs. A change in the laser power shows a large red shi (in tens of cm À1 ) as compared to the other two factors (in few cm À1 ).
Since the past decade, there has been a surge in the eld of ZnO QDs. This may be the outcome of congestion in the branch of nanoparticles, where researchers have not le any stone unturned. Yet we have not found any sizeable review article that can help to steer the research work forward. To overcome this drawback, we have tried to assemble all the material related to ZnO QDs in this review. In this review we have mainly focused on ZnO QDs. We have tried to cover most of the work done by researchers worldwide in this eld in the last few decades. The main parts of this review are the synthetic approaches, doped ZnO QDs, and applications. We have added the effect of quantum size on the different physical properties of ZnO in the introduction part itself. Aer introduction, we discuss the synthetic approaches adopted by different researchers for the fabrication of ZnO QDs. This part covers the various ZnO QD fabrication methods, which includes the method for water and ethanol stable QDs. We have also tried to cover different synthetic methods, which include the capping of ZnO QDs so that the QDs remain stable for a large duration. We have also studied the synthetic methods for the doping of different elements. In the next section, we have studied the effect of doping in the QDs. Here, our main focus was on rare earthdoped QDs, transition metal-doped QDs, and composite QDs. We have also covered the effect of doping on various physical properties. The last section is the application part. Here, we have studied the fabrication of some devices in brief. We have also focused on some other application besides the devices. The main content of this section is bio-sensors, photodetectors, light emitting diodes (LED), catalytic application, as well as anticancer and anti-bacterial applications.

Synthetic approach
Among a number of techniques for the preparation of ZnO QDs (such as laser-induced fragmentation in liquid, 54 radiofrequency (RF) atmospheric pressure plasmas (APPs) 55 ) the sol-gel method is the most popular as it is relatively more efficient, simple, and inexpensive. This method was rst adopted by Bahnemann 56 for the preparation of ZnO QDs in the year 1987. They used 2-propanol as a solvent for the preparation of zinc acetate solution and NaOH was used for precipitation. The precipitate was nally suspended in water. Later, Spanhel 57 used ethanol in place of 2-propanol for the synthesis of colloidal QDs. Aer that, in the last decade, many researchers have successfully synthesized QDs of ZnO using different solvents such as 2-propanol, ethanol, and water. Different hydroxides such as KOH, NaOH, and LiOH have been used as the precipitating agent. Solvents such as water, ethanol, and n-hexane have been used for the resuspension of the precipitate. Many research articles have emerged since then involving growth kinetics during the process, gelation, particle size control, use of capping agents, etc. Recently many researchers have successfully doped different elements in ZnO QDs.

Sol-gel synthesis
In this paragraph, we will discuss the typical sol-gel method for the synthesis of ZnO QDs. Mostly, zinc acetate (ZnAc) is used as a precursor material in this method. A schematic diagram of ZnO QD synthesis is shown in Fig. 5. The quantity of ZnAc is kept in the range of hundreds of micromoles to few a millimoles. Ethanol is mostly used to prepare the ZnAc solution. Temperature is kept in the range of 50 to 90 C. A solution of the precipitating agent is also prepared simultaneously. For this solution, LiOH (lithium hydroxide), NaOH (sodium hydroxide), and KOH (potassium hydroxide) are also dissolved in ethanol. Finally, the solution of the precipitating agent is added in the ethanolic solution of ZnAc. A white precipitate is formed, which is then washed and collected by centrifugation. Generally, ethanol is used to wash the precipitate. The white precipitate is then resuspended in ethanol or water. The concentration of the zinc ion is kept low in the ethanolic solution; according to Ostwald theory, 58-60 it will constrain the particle to grow to a larger size.
The reaction temperature and time also affect the particle size. Chen et al. 61 studied the effect of reaction temperature and time. They used water-ethanol mixed solvent in the 2 : 1 ratio for the preparation of the ZnAc solution. Then, they mixed NaOH solution at 50, 60, 70, 80, and 90 C for 6, 7, 8, 9, and 10 h reaction time. From both these variations, they have found that the particle size increases with the reaction temperature and time. The PL (photoluminescence) intensity of the QDs aer 7 h of reaction was found to be the maximum. According to Chen,61 it was the time required to complete the reaction. Similarly, they found a higher emission peak for 60 C reaction temperature. Regonia 29 also studied the effect of reaction temperature and time on the optical properties of ZnO QDs. They found an increase in the particle size and decrease in the band gap. The increase in the particle size was conrmed by the TEM images of the QD grown at different temperatures. We can see that the particle size increases for synthesis at room temperatures, 40, 60, and 80 C. The UV-visible absorption and photoluminescence spectra of the samples conrmed the decrease in the band gap energy. This variation can be easily defended by the Ostwald ripening theory as discussed earlier. They also studied the effect of different synthetic conditions on the physical properties of the QDs. Machine learning has proven to be the best supporting tool for predicting different physical properties. Instead of using articial neural networks (ANN), they used Kernel ridge regression (KRR) and ridge regression (RR) as it needs very limited data sets for training the algorithm in comparison to ANN, which takes a large amount of data for training. In the eld of nanomaterials, it is very tedious to take such a large amount of data. From Fig. 7 we can see that the KRR and RR model performs much better than the ANN model. The predicted band gap is very near to the experimental value.   A modied sol-gel method known as E. Meulenkamp's method can be adopted for the synthesis of ZnO QDs. This produces uniform and mono-disperse ZnO QDs by the precise control of the water content. In this method, rst, a precursor solution of zinc salt (mostly zinc acetate) was formed in dehydrated ethanol and then to produce QDs from it, a calculated amount of water was added. Chen and his group 62 studied the effect of the water content on the growth kinetics of ZnO QDs. The focus of these studies was on the two main processes, i.e., oriented attachment (OA) and Ostwald ripening (OR). These result show that the water content affects both the processes. The TEM images of the QDs formed by this method are shown in Fig. 6. Fig. 6a shows the mono-disperse and uniform particles, and the SAED pattern in the inset. The crystal fringes shown in the inset of Fig. 6b conrm the 002 plane of ZnO (Table 1).

Coated ZnO QDs
Luminescent centers of ZnO QDs obtained by the classical solgel route are rapidly destroyed by water. For the biomedical application of ZnO QDs, it is important that QDs are stable in water. Several strategies have been deployed by researchers worldwide to obtain water-stable QDs by protecting the QDs with SiO 2 , oleic acid, 86 etc. It also prevents QDs from agglomeration. 87 Zaiqian Yu 70 and his group have used N-isopropylacrylamide (NIPAM), N-tert-butyl acrylamide (TBAM), and (N-(3-aminopropyl)methacrylamidehydrochloride) (APM) for the preparation of water-stable ZnO QDs.
Geng et al. 65 used polyethylene glycol (PEG) for the preparation of water-stable monodisperse PEG capped ZnO QDs (shown in Fig. 8f). First, they prepared ZnO QDs in ethanol using ethyl acetate as the precipitating agent. Then, they added 4 mL PEG in 4 mL ZnO QDs and churned it at room temperature for 60 min. Finally, aer centrifugation and washing with ethanol and ultrapure water, the QDs were obtained by dispersing them in 10 mL ultrapure water. Similarly, Rizwan Khan 66 and his group used poly(vinylpyrrolidone) (PVP) for the photocatalytic application of ZnO QDs (shown in Fig. 8a-e). They dissolved 0.3 g ZnAC in an ethanolic solution of PVP (0.05 g PVP in 50 mL ethanol). Aer 5 min stirring at 70 C, they added 0.1 g NaOH for precipitation. This precipitate was then centrifuged at 3000 rpm for 5 min and washed with ethanol 3-4 times. Zwitterion-coated water stable ZnO QDs have also shown stability towards the dissolved salts. Zhang et al. 79 have prepared zwitterion-coated ZnO QDs and investigated the stability of 2 mg mL À1 QDs over a period of 30 days. They used saturated sodium chloride solution for testing. They did not nd any remarkable change in the absorbance for 30 days. The luminescence intensity decreased only 1% when the sample was kept at 4 C but at 37 C, the luminescence intensity decreased to 30% aer 1 day and remained 13% aer 30 days. Recently, an excellent application of SiO 2 -coated ZnO has been found for the fabrication of an ink-absorbing and UV-shielding lm. This lm is was made by the dispersion of SiO 2 -coated ZnO in the PVA matrix and then spin-coating this solution on to the PET substrate.
The schematic of the whole process is shown in Fig. 9, schematic 1. The UV-visible absorbance spectra is shown in Fig. 9a-d to show the UV absorbance property of the lm. Graphs were made for the various ratio of zinc acetate and NaOH and various reaction time as both these factors directly affect the size of the ZnO QD. The size difference ultimately affects the band gap of the particle. From this, we can clearly see that as the ratio increases, the absorbance peak shis towards the lower wavelength side, showing a decrease in the size of the ZnO QDs. 87

Ultrasonic method
The ultrasonic method has been adopted by some researchers for the synthesis of stable ZnO QDs. In this method, ultrasonication is used for the preparation of different precursor solutions. In this method, the dissolution, oxidation, reduction, decomposition, and hydrolysis reactions take place in the liquid phase under ultrasonication. 69 Weimin Yang and his group 68 used the ultrasonication method and the micro-reactor method for the preparation of ZnO QDs; they named this method as the ultrasonic microreactor method (experimental setup is shown in Fig. 10a). In this method, two solutions, namely, zinc acetate and PEG-400 (n(PEG-400) : n(Zn) ¼ 1 : 1) in 50 mL ethyl alcohol and LiOH in ethyl alcohol, were injected through separate syringes into a PTFE tube, which was immersed in an ultrasonic washer. Oleic acid was used to precipitate the QDs, which were then collected by centrifugation. In this process, ultrasonication creates bubbles in the reaction solution due to ultrasonic cavitation. These bubbles are shown in Fig. 10b. These bubbles divide the reaction solution into several parts and restrict the growth of the QDs on the surface of the bubbles as the tensile stress is more on the surface of the bubbles. The effect of the ow rate on the size of QDs is found to be negligible as both the QDs, prepared at 300 mL min À1 and 750 mL min À1 at 40 C and 180 W power, have nearly similar average size (shown in Fig. 10c and d). They thoroughly studied the effect of ultrasonic power, ow rate, and temperature on the synthesis and optical properties of the QDs. Fig. 10g shows the effect of reaction temperature on the emission and excitation wavelength. The corresponding graph of photoluminescence emission and excitation are shown in Fig. 10e and f, respectively. These two graphs are in close relation with Fig. 10g. Fig. 10h shows the variation in the quantum yield with temperature. Fig. 10k shows the effect of ultrasonic power on the emission and excitation wavelengths. The corresponding graph of photoluminescence emission and excitation are shown in Fig. 10i and j, respectively. Fig. 10l shows the variation in the quantum yield with ultrasonic power. Fig. 10o shows the effect of ow rate on the emission and excitation wavelength. The corresponding graphs of photoluminescence emission and excitation are shown in Fig. 10m and n, respectively. Fig. 10p shows the variation in the quantum yield with the ow rate. They successfully obtained a quantum yield of nearly 42%. 68

ZnO QD nanocomposite
Since the last few years, researchers have been working on the development of exible devices. To fabricate exible devices, a polymeric substrate is the basic unit due to its liquid processing nature and exibility for thin lm fabrication. The fabrication of composite materials with ZnO QDs is benecial for their long-term application. Graphene is an emerging candidate for QD composites. In recent years, graphene-based QD composites have attracted researchers worldwide. [88][89][90] Nanocomposites made up of QDs and graphene have tremendous benets over conventional materials. There are some features that make these materials superior over others, such as the strong quantum connement effect in the QDs, exciton dissociation, high photoconductive gain, and charge transfer at the interface of the heterojunctions. The decoration of graphene by ZnO QD results in the enhancement of electron emission from graphene, which reduces the work function and ionization potential, and increases the Fermi level of graphene. 90 The photo-responsivity and photoconductive gain of the ZnO QD/graphene heterojunction photodetectors can be enhanced to an extraordinary height if we are able to overcome the factors that hinder charge transfer at the interfaces. These factors are (1) contamination on the surface of the QDs and graphene; (2) layer of unreacted zinc acetate over ZnO QD, which creates a tunnel barrier; and (3) electron depletion layer due to adsorbed oxygen on the surface of the QD, which creates an interfacial barrier. 91 It was found that a layer of reduced graphene oxide could turn paramagnetic ZnO QD ferromagnetic. It is possible only due to the hybridization of the 3d orbitals of the Co 2+ -oxygen vacancy complex and the 2p z orbital of graphene. 89 During the synthesis of the ZnO-GO composites, GO QDs rst react with ZnO QDs. In this process, Zn-O-C and Zn-OOC hybrid bonding is formed by the reaction of hydroxyl group (Zn-OH) of ZnO QDs with the hydroxyl group (HO-C) and carboxylate group (HOOC) of GO QDs, respectively. The schematic synthesis diagram of these bonds is given in Fig. 11a-d. The PL spectra of the composite for different excitation wavelengths shows maximum intensity for wavelength l ex 350 nm having higher energy than the band gap energy (Fig. 11e). It is also interesting to note that the emission of violet-purple-blue emission is independent of the excitation wavelength (Fig. 11f). The energy level diagram in Fig. 11g is helpful in explaining the PL emission spectra of the composite. 16 Some researchers have fabricated the ZnO-silica composite using tetraethoxysilane (TEOS). Recently, Liang et al. 76 have mixed different amounts of ZnO QDs in the aqueous solution of TEOS. Patra and his group 67 have also prepared ZnO QDs using TEOS. In a simple sol-gel method, rst, they prepared an ethanolic solution of ZnAc. Then, they added KOH for maintaining the pH of the above solution at 10, 12, and 14. Then, they added TEOS to the above solution. The obtained colloidal solution was centrifuged and washed several times with methanol and water. Finally, the colloid was dispersed in water. Zain et al. 78 also used TEOS for the synthesis of ZnO QD-embedded silica nanoparticles. They found 55% to 80% luminescence emission at 100 C temperature and 40 g L À1 salinity. This increase in the stability was due to the hydrophobic properties of silica. You Liang and his group 77 have prepared kasugamycin (KAS)-conjugated ZnO QDs for a pH-responsive pesticide delivery system. It was found to be more effective against bacterial fruit blotch as compared to pure KAS or ammoniatreated ZnO QDs.
For the synthesis of KAS-conjugated ZnO QDs, the QDs were prepared rst, then (3-aminopropyl)trimethoxysilane (APTES) was modied on the surface of the QDs. The complete modication process is composed of three steps, which is shown in Fig. 12. From this gure, we can see that APTS-conjugated ZnO QDs then react with 4-formylbenzoic acid, followed by KAS to form the KAS-ZnO QDs.

Radio-frequency (RF)-based synthesis
Besides the above-mentioned solution routines, there are other different methods for the synthesis of ZnO QDs. Laser ablation, aerosol synthesis routes, chemical vapor synthesis (CVS), 92,93 and radio-frequency (RF) plasma-based synthesis are some of them. These are environment-friendly and continuous ow processes with high production rate, and produce highly crystalline QDs. Radio frequency-based synthesis can be used for high purity samples. Vollath and his coworkers were the pioneers who had started the production of ZrO 2 , TiO 2 , and Al 2 O 3 . Later in 2002, Kleinwechter 94 synthesized ZnO nanoparticles in the size range of 4 to 8 nm via aerosol synthesis routes using H 2 /O 2 /Ar plasma and O 2 /Ar plasma. In case of H 2 / O 2 /Ar plasma, the nucleation of ZnO nanoparticles starts outside the hot ame core, whereas Fe 2 O 3 SiO 2 forms inside the hot ame core. The particle size was in the range from 6.2 to 8.2 nm. In the case of O 2 /Ar microwave plasma synthesis method, the particles start nucleating in the plasma itself. The particle size was also found to be smaller than that of the above process; it was in the range of 4.2 to 5.9 nm. A change in the synthetic parameters not only affects the particle size but also affects different type of defects. It was found that with the increase in the hot wall reactor temperature, the stacking fault probability decreases for both (microwave plasma hot wall reactor and hot wall reactor) processes. However, the twin fault probability decreases only for the hot wall reactor process and increases slightly for the microwave plasma hot wall reactor process, whose twin fault probability is always lower than that of the hot wall reactor process. 96 Later in 2013, Felbier 95 and his coworkers synthesized ZnO QDs using radio-wave and nonthermal plasma instead of microwave. This process was largely scalable, ligand-free, and produces high quantum yield QDs. Recently Gunisha Jain 55 has developed a method for the production of highly ligand-free, defect-free, hydroxylterminated ZnO QDs using radio frequency atmospheric pressure micro plasma. The particle size was also nearly 1.9 nm. The luminescence stability was also more than eighteen months. They used zinc wire instead of diethylzinc, which was the main precursor of zinc to date.
In summary, there is only one method based on the precipitation of zinc salt (mostly zinc acetate) in alcoholic medium. Different researchers have used different precipitating agents such as KOH, NaOH, and LiOH. A variation in precipitation results in a variation in the different physical properties. It was   also found that at different pH values, we also get a variation in the band gap. Beside simple precipitating methods, there are some methods that are based on the ultrasonication of the precursor solution. Some methods have also been developed for more stable QDs, which are based on different capping agents such as SiO 2 , oleic acid, NIPAM, TBAM, APM, PEG, and PVP. The capping results are enthusiastic and produce more stable QDs as compared to pristine ZnO QDs. Conjugating organic compounds with QDs also results in stable QDs. Composites made in such a form are useful in a number of applications. The composites can be stable in salt water, which makes QDs applicable in saline conditions. Composites made up of graphene have tremendous potential in the near future (Fig. 13).

Doped ZnO QDs
Having unique properties in comparison to nanoparticles, QDs are very important in different application elds. 97 The smaller size of the ZnO QDs not only widens the band gap of the QDs but also increases the surface to volume ratio. In addition to these surface defects, huge charge density is also created at the surface of the QDs. These unique properties of the QDs can be further modied with the addition of doping techniques. The doping of different elements gives direction to the above factors in various specic directions, which include photocatalytic, optical, magnetic, anti-microbial, and other properties. The doping of transition metal results in a thermally unstable material and generates electron-hole recombination centers. 98

Rare earth metal-doped ZnO QDs
Rare earth elements, having more discrete energy levels, are very effective in photocatalytic and uorescence applications of QDs. Hence, most of the researchers have focused on the study of rare earth-doped ZnO QDs. Due to the lesser number of atoms as compared to the nanoparticles, it is not easy to dope QDs. Hong et al. 38 have found that most of the Pr 3+ ions were on the surface of the QDs. The ions that penetrated into the particles caused lattice imperfection. The particle size was also found to decrease with the increase in the concentration of the doping element. With the increase in the Pr 3+ concentration, the interaction between Pr 3+ and the ZnO crystal hinders the growth of the crystal. This reduction can be explained by the interaction in between the Pr 3+ ion and ZnO, which occurs on the surface of the crystal. Pr-Zn-O bonds form as a result of this interaction on the surfaces of the crystals. The XPS result conrms the formation of this bond. They thoroughly studied the XPS spectrum of pure, 0.5, 1, and 2 mol% Pr 3+ -doped ZnO QDs (shown in Fig. 14a) and found peaks at 531.5, 1044.9, and 1021.9 eV, which originate from O 1s, Zn 2p 1/2 , and Zn 2p 3/2 photoelectrons, respectively. Besides these peaks, they also found an extra peak at 933.4 eV in Pr 3+ -doped ZnO QDs. This peak shows an extra 0.2 eV shi from the standard peak, which originates from the photoelectrons of Pr 3d 5/2 in Pr 2 O 3 .
This result was due to the variation in the chemical bonding environment of Pr 3+ , which conrms the doping of Pr 3+ in the ZnO lattice. From the XPS spectra of the Pr 3d 5/2 peak (shown in Fig. 14e), we can observe an increase in the intensity with the increase in the doping concentration of Pr 3+ , which shows that the Pr 3+ concentration increases in the ZnO QD with concentration. The spectrum of Pr 3d 5/2 from 2 mol% Pr 3+ -doped ZnO QDs shows two peaks at 934.0 and 931.1 eV (shown in Fig. 14c)  and d). The HBEC of O 1s at 532.0 and 531.9 eV is due to the O-Zn bond surrounded by electron vacancies. The LBEC of O 1s at 530.5 eV is due to oxygen in the O-Zn bond. Both these spectra show a signicant increase in the FWHM of all the components of LBEC and HBEC. This broadening is due to the increase in the distortion of the electron cloud of oxygen in the Pr 3+ -doped samples. As most of the doping elements are doped at the surface of the ZnO crystal and the XPS studies are also limited to a few nm of the crystal surface, it is an effective method for the conrmation of doping. From the XPS full spectrum scanning, 36 we can see that the diffraction peaks of zinc and oxygen are only present before the doping of La. Sun et al. 35 found that the size of ZnO QDs decreases with the increase in the La content. Jakub Sowik 84 and his group extensively studied the effect of different rare earth elements in different proportions on the optical, structural, and photocatalytic properties of ZnO QDs. The PL spectra of different rare earth metal-doped ZnO QDs is given in Fig. 15a and b. Here also, the XPS spectra of different rare earth elements show the binding energy shi in Zn 3d and the valence band spectra, which conrms the doping of rare earth elements. Er-Doped ZnO QDs was found to be the most effective towards the decomposition of phenol solution under visible light. The activity toward phenol decomposition was nearly 90% in UV. The PL intensity of La-doped ZnO QDs shows the highest PL quantum yield (nearly 80%). Gd doping is not only found to increase the quantum yield from 31% to 94% but also produces a red shi in the absorption and emission spectra. 99 Similarly Huang et al. 36 synthesized La-doped ZnO QDs and studied the effect of the OH À ion and La concentration on the oxygen vacancy defect, which ultimately affects the uorescence performance of the ZnO QDs. The oxygen vacancy was maximum for the 1 : 1 OH À : Zn molar ratio and 7 mol% doping amount of La. The uorescence emission intensity was found to be in direct proportion to the oxygen vacancy concentration. The uorescence stability for 5 mol% doped QDs was nearly 5 months (shown in Fig. 15c). Rare earth doping will be very helpful in increasing the quantum efficiency of solar cells via down-conversion. Trivalent praseodymium could be an encouraging material as an activator for the host material. It was found that Pr 3+ doping effectively increases the PL intensity of some (578, 630, and 752 nm) defect-related emissions in the ZnO QDs. 32 This increase in the emission is mainly related to energy transfer from the Pr 3+ ion to the defect states.
From Fig. 15d, we can see that with the increase in the Pr concentration, the PL intensity also increases up to 1% doping concentration and then decreases for 2%. Thus, we can say that 1% is the doping limit for uorescence applications. This decrease in the PL intensity could be a result of the concentration quenching effect. This effect is a result of cross relaxation and energy migration when the concentration of doping elements exceeds certain limits.
Gd doped ZnO QDs. Alshahrie et al. 99 synthesized and studied Gd-doped ZnO QDs. It was found that Gd doping increases the crystallite size of the ZnO QDs while preventing their hexagonal structure. The XRD spectrum of Gd-doped ZnO QDs shows only a slight shi in the 101 peak, which may be due to the replacement of Zn by the Gd atom (shown in Fig. 16a). The TEM images show that the particle size increases from 3 to 18 nm with the increase in the concentration from 1 to 7% (Fig. 16b-e). The optical band gap and Stokes shi decrease with the concentration (Fig. 16f and g). The luminescence intensity increases six times due to the decrease in the crystal defects (Fig. 16h). However, FWHM decreases with the increase in the concentration (Fig. 16i). The quantum yield of luminescence also increases from 31 to 94% with doping (Fig. 16j). Fig. 16k shows the charge transfer mechanism in Gd-doped ZnO QDs. From this gure, we can see that Gd doping creates a band in the energy band gap of ZnO near the conduction band. Any excited electron goes to the conduction band of ZnO transfer to the conduction band of Gd via the relaxation nonradiative process. From this level, a very intense and coherent beam of light is produced.

Transition metal-doped ZnO QDs
Owing to the smaller size of transition metals, it is easier to dope them in ZnO QDs. There is more possibility of existence of doped transition elements inside the core as compared to rare earth elements. Radovanovic et al. 100 fabricated Co-doped ZnO QDs by the iso-crystalline core/shell (ICS) procedure. 101 As by the simple solution-based method, the doping elements reside mainly on the surface of the QDs. But by this method, his team successfully synthesized ZnO QDs with the doping elements in the core of the QDs. Transition metal-doping is also found to increase the photocatalytic and antibacterial properties of the ZnO QDs. Cadmium doping is found to be effective in increasing the production of ROS (reactive oxygen species), which plays a major role in the killing mechanism. 45 Similar results were observed by Lee et al. 102 for Al-doped ZnO QDs. The detection limit of mustard gas was 20 ppm. It was also found that these QDs show selectivity towards mustard gas.
Besides, rare earth transition metal-doped ZnO QDs have been found to show dilute magnetic properties. 103 As a consequence of their tremendous application in spin-electronic and spin-photonic devices, vigorous investigation has to be done in the eld of ferromagnetism in QDs. Magnetism in semiconductors arises due to the magnetic exchange interactions between the delocalized charge carriers and the localized magnetic impurities. These interactions result in large Zeeman and Faraday rotation effects. Mostly, in the early years, ligandto-metal charge transfer(LMCT) 104,105 transition was found to be the cause of ferromagnetism in doped ZnO QDs. 83 In nickel and cobalt-doped ZnO QDs, the LMCT transition was also found to be responsible for ferromagnetism. 106 This was also found to show a large Zeeman effect. 107,108 In both these cases, there was a sub band gap in the energy level due to this LMCT. Beside this sub band gap, unique midgap excited states have been found by Joseph W. May in cobalt-doped ZnO QDs. 109 Midgap transitions give rise to a number of properties such as magneto-electronic, magneto-optic, photocatalytic, and sensing properties. Stefan et al. 110 later found a colloidal analogue to bind the magnetic polaron (BMP) 111 responsible for magnetism in Mn 2+ -doped ZnO QDs. Yong 112 and his group, via DFT theory, 85,113 found that in the case of Mn 2+ -doped ZnO QDs, magnetism rises due to double exchange in the charge-transfer excited states. In case of p-type (N 2À doped) Mn-doped ZnO QDs, magnetic interaction between two Mn 2+ ions is mediated by the N 2À ion. This results in parallel alignment of the spin of both the Mn 2+ ion, which gives rise to ferromagnetism at room temperature. 114 Transition metal doping suppresses the growth of ZnO QDs and increases the various vacancy defects. Zhang et al. 33 have also found similar results for Cd-doped ZnO QDs. With the increase in the Cd concentration, there was a blue shi in the UV spectra due to the quantum connement effect. From Fig. 17a, a blue shi in the exciton absorption spectra was observed and from Fig. 17b, we can also see that there was a shi in the direct band gap for different concentrations of Cd. The PL intensity for the emission spectra was also found to increase with the increase in the doping concentration. Similarly, copper doping was also found to alter the energy band gap of the ZnO QDs. The energy band gap decreases with the concentration of Cu. From Fig. 17c, we can see that absorption increases with the doping concentration, which is mainly because of the substitution of Zn 2+ by Cu 2+ . The substitution of Zn 2+ results in an increase in the oxygen vacancies and electron concentration. 39 With Sn doping, it was found that when the doping concentration of Sn was less than 3%, the doping was interstitial, and when the doping concentration was greater than 5%, the doping was substitutional. 85 The UV-visible spectrum of Sn 2+ -doped ZnO QDs is shown in Fig. 9d and the corresponding change in the energy band gap is shown in Fig. 17e and f. From  Fig. 17f, we can clearly see that with the increase in the Sn 2+ concentration, initially, the band gap energy increases from 3.542 eV to 3.572 eV, then drops to 3.418 eV, and is nally increased to 3.480 eV. Similar results were shown by Sn 4+ -doped samples (shown in Fig. 17g-i). In case of the Sn 4+ -doped sample, the energy band gap rst increased from 3.542 eV to 3.613 eV, then dropped to 3.510 eV, and nally rose to 3.530 eV. The E gap reached the minimum value when the Sn 4+ concentration was 0.05. This not only affects the binding energy of the O 1s spectrum but also affects the concentration of different types of defects present in the QDs.
Mg-Doped ZnO QDs. Co-Doping in the ZnO QDs was rst introduced by Shi et al. 115 They fabricated Mg and Ce co-doped ZnO QDs via a simple facile low temperature route. The crystallite size was found to be decreasing with increasing Ce concentration. The TEM images also show that the particle size is decreasing with the increasing doping concentration (Fig. 18). The EPR spectra of all the samples shows a similar peak at g z 2.0035, which is due to the presence of oxygen vacancies (shown in Fig. 19a and b). These signals become stronger aer annealing the samples in a reducing atmosphere, which conrms that the EPR signals are from oxygen vacancies. The oxygen 1s XPS spectra (Fig. 19c-f) is composed of two peaks, the rst at $530 eV and the other at $532 eV, corresponding to the lattice oxygen (O L ) in ZnO and oxygen vacancies (O H ) in the ZnO matrix, respectively. The ratio of O L /O 1s decreases from 31% to 21% with the increase in the Ce concentration and the ratio of O H /O 1s increases from 68.99% to 78.45% with the increase in the Ce concentration. This conrms that the oxygen vacancy increases with the Ce concentration, which is in accordance with the EPR spectra.
In summary, doping in QDs is not as easy as in nanoparticles. Doping mainly results in the smaller size of QDs as it restricts the particle growth. Doping also alters the energy band gap of the QDs, which causes a blue shi. Larger doping elements mostly reside on the surface of the QDs. But there are some methods that help the doping elements to reside at the core of the QDs. Rare earth doping mostly results in the manipulation of the luminescence properties. It is also possible to use these as photocatalysts. Besides, rare-earth doping and transition metal doping is found to show magnetism in the QDs. Here, doping elements not only reside on the surface but also inside the core up to some limit. Magnetic interactions between the spins are mostly LMCT and double exchange interactions. These result in room temperature ferromagnetism as well as large Zeeman and Faraday rotation effects.

Application
ZnO is a wide band gap semiconductor having a direct band gap of 3.37 eV. This band gap can further increase with the decrease in the particle size, which is characteristic of ZnO QDs. Besides, the modulation in the band gap QDs also differs in a number of ways such as modied charge carrier density, 116 large surface-tovolume ratio, and the various energy levels that are distinct from those of nanoparticles. These mutations in the properties of the QDs are useful in some of the pioneer applications of the ZnO QDs, which are not found in nanoparticles. Impurity element detection is one of these applications, which is not applicable using nanoparticles. Apart from impurity detection, energy storage, 117 gas and chemical sensing, 118 optoelectronics, 20,119 anti-bacterial, 120 cancer treatment, 121,122 cellular imaging, 123,124 electronics, 125 and photocatalytic activity 126 are the other major application elds of QDs. However, these QDs will denitely augment the performance of these devices, which have based on nanoparticles to date. The surge in the application of QDs in different elds during the last few decades proves its golden future.

Detection of metal ions
The development of chemosensors for the detection of metal ion impurities 127 in drinking water has become a hot area for intensive research. Many researchers are working in this eld for the development of selective detection of transition and heavy metal ion impurities. From the different available detection methods such as colorimetry, 128-130 mass spectrometry, 131-133 atomic absorption spectroscopy, 134-136 electrochemical 137-140 and uorescence 141-145 methods, the uorescence methods are the most reliable and effective because of their high sensitivity, high efficiency, and operative simplicity. Praethong Laopa and Tirayut Vilaivan 40 synthesized cationic polymer-coated ZnO QDs for the selective detection of the Fe(II) ion. The uorescence study of p(METAC38-PEOMEMA62)-ZnO, p(METAC100-PEOMEMA0)-ZnO, and p(METAC9-PEOMEMA91)-ZnO QDs was investigated in the presence and absence of 10.71 mM metal ions such as Fe 2+ , Fe 3+ , Hg 2+ , Co 2+ , Pb 2+ , Cd 2+ , Ni 2+ , Ag + , Cu 2+ , and Na + . From Fig. 20a and b, we can see that the variation in the uorescence intensity in the absence and presence of the ion (F 0 /F) for different ions is maximum for Fe 2+ , which shows selectivity towards the Fe 2+ ion. We can also see that the uorescence intensity for this ion decreases with the increase in the concentration of the ion. Similarly, Tong et al. 37 used a mixture of urea and ZnO QDs for the detection of the Cr 6+ ion. The detection limit of Cr 6+ was 19.53 nM. Fig. 20c and d shows (F 0 /F) and the uorescence intensity of different ions, respectively. This shows the selectivity towards the Cr 6+ ion. Daniel 146 and his team developed a hand-held device for the real time detection of the poisonous Hg 2+ ion. The detection limit of the device is 0.1 ppb and it has a linear range of detection from 0.1 ppb to 10 000 ppb. From Fig. 20e, we can see the uorescence spectra of ZnO QDs in the presence of different ions. The intensity for the Hg 2+ ion is the highest, which shows the selectivity of the instrument towards this ion. The bar diagram shown in Fig. 20f conrms this result. The presence of chlorine in water can also be detected by the ZnO QDs. 147 As the concentration of chlorine increases, it absorbs electrons from the oxygen vacancies. This results in a decline in the emission intensity of the QDs. From Fig. 20g, we can see the F 0 /F for different ions, which shows selectivity towards chlorine in the solution. Fig. 20h shows the uorescence spectra for the chlorine ion at different concentrations. Capping with different polymers or organic materials produces selectivity toward different polluting elements. These ZnO QD-based detecting solutions are very cheap and easy to handle. Thus, these QDs can become a useful tool for the detection of impurities (Fig. 21).

Sensors
The production of ionic oxygen species on the surface of the ZnO QDs is more favorable as compared to that on nanoparticles since there are more conduction band electrons on the surface of the QDs, resulting in the formation of a thick electron depletion layer. Thus, the electron requires a large amount of energy to migrate due to creation of a high potential barrier. On the adsorption of gas molecules, there is a drop in the potential barrier and resistance. These properties are favorable for the fabrication of gas-sensing devices. Ratan et al. 148 have fabricated an interdigitated metal-semiconductor-metal (MSM)-based hydrogen gas sensor. This sensor shows high selectivity towards H 2 with respect to sulfur dioxide, ammonia, and organic vapors such as methanol, chlorobenzene, acetone, and chloroform. Such gas sensing and selectivity towards a specic gas was also observed by Shouli Bai and his group. 149 They fabricated ZnO QD-based sensors, which show a sensitive response for NO 2 in comparison to CO and CH 4 . Zhang et al. 150 synthesized water-soluble Cd-doped ZnO QDs and utilized the uorescence-linked immunoassay (FLISA) method to inspect the level of bisphenol A (BPA) in water. Eu-Doped ZnO QDs are very helpful in the detection of Bacillus anthracis spores, which causes highly contagious diseases in human beings. 151 The sensing mechanism of the QD-based sensors is quite different as compared to nanoparticle-based sensors. The change in the electrical conductivity is the basis of the sensing property of the nanoparticles. [152][153][154] The red-emitting Eu(III) ion acts as a signal reporting unit via chelation with calcium dipicolinate (CaDPA), which is the biomarker of Bacillus anthracis spores.

Biosensors
The production of ionic oxygen species on the surface of the ZnO QDs is not the sole accountable mechanism for the sensing properties of the ZnO QDs. Chemical detection, bio-imaging, bioanalysis, and cold-illumination are the elds that are under the grip of chemiluminescence (CL) due to its unique light emission process and high luminescence efficiency. In the present era, various inorganic nanoparticles such as Au, Ag, and CdTe have shown eye-catching special CL properties, which have attracted considerable attention of the researchers worldwide. However, their practical applications have been restricted by the relatively low quantum yield as compared to other organic materials. ZnO is emerging as a potential candidate for bio-imaging because of its excellent photoluminescence efficiency. Thus, functionalizing ZnO QDs with different molecules has unlocked the door not only for target-specic drug delivery but also for the detection of some specic enzymes and organic molecules, which are undetectable by bare ZnO QDs. J. P. Stelmaszyk 155 and his team employed the uorescence property of ZnO QDs for the detection of proteinase 3 (PR3) by functionalizing it with a peptide probe (Tyr-Tyr-Abu-Asn-Thr-Pro-OH) and an organic quencher (BHQ). This peptide probe signicantly decreases the detection limit from 43 pmol to 1.3 pmol. This will be benecial in the detection of diseases related to neutrophil activation and the over-expression of PR3. Due to the accumulative near-eld effect of ZnO QDs, the enhanced uorescence of N-acetyl-b-D-glucosaminidase (NAGase) is useful for the detection of Streptococcus dysgalactia, which is responsible for the prominent inammatory disease, bovine mastitis (BM), in milk-producing animals. 41 Chen et al. 156 have used QDs as a biosensor for the detection of histone acetylation using acetyl coenzyme A (Ac-CoA) as the target molecule. This is an indirect method for the detection of histone acetyltransferase (HAT). The schematic diagram for the detection mechanism is given in Fig. 22a and b. In this process, CoA is produced as a by-product of acetylation of Ac-CoA, which can be easily detected by the photo-electrochemical biosensor due to the presence of phosphate and thiol groups in its structure. From Fig. 22c, we can see that with the increase in the HAT concentration, the photocurrent also increases gradually. Fig. 22d shows a linear relation between the logarithmic concentration of HAT and the photocurrent intensity. From its wide range of linear relation, we can predict its potential for the detection of HAT. It shows high selectivity towards the HAT molecule in comparison to other molecules (as shown in Fig. 22e). It has relatively similar cycles for detection (as shown in Fig. 22f), having nearly 0.92% calculated relative standard deviation (RSD). Suppressing the working of electroluminescence reagents is emerging as an effective tool for the detection of biomolecules. 157 b-Cyclodextrin (b-CD)-capped ZnO QDs decorated with pyridoxal 5 0 -phosphate (PLP) and pyridoxal (Py) is useful for the detection of histamine. 158

Bio-imaging
High uorescence, chemi-luminescence, and non-toxicity of ZnO QDs is helpful in bio-imaging, when combined with some suitable organic molecules. In the present era, various inorganic nanoparticles such as Au, Ag, and CdTe have shown special CL properties that have attracted considerable attention of researchers world-wide. However, their practical applications have been restricted by the relatively low quantum yield as compared to other organic materials. ZnO is emerging as a potential candidate for bio-imaging because of its excellent photoluminescence efficiency. Lei and his group 124 prepared the ZnO QD-HPEI composite using the reux method. The ethanolic solution of ZnO/HPEI nanocomposite, on characterizing with uorescence microscopy, gives blue emission (shown in Fig. 23a-d). The UV-visible and photoluminescence spectra of ZnO/HPEI are shown in Fig. 23e-j. These spectra conrm that heating and change in the concentration do not affect the UVvisible and photoluminescence results.
Similarly, Liu et al. 123 have used ZnO QDs for the bio-imaging of HeLa cells. They synthesized the SiO 2 -coated ZnO QDs. For chemo-luminescence bio-imaging, ZnO/SiO 2 was dissolved in ultrapure water and then it was used for staining the culturegrown HeLa cells. These samples were then added to CPPO, which was then cross-linked with F127 for improving the hydrophobicity of CPPO. Aer adding H 2 O 2 , this sample was ready for imaging. This whole process is shown in Fig. 24a.
With increasing H 2 O 2 concentration, the intensity of imaging was also found to be increase (shown in Fig. 24b and c), which indicates a direct relation between H 2 O 2 and the imaging intensity. However, the cell viability for different concentrations of ZnO/SiO 2 aer 24 h of incubation remains nearly constant (shown in Fig. 24d).

Optoelectronics
Beside gas sensing and bio-imaging, ZnO QDs can also be utilized for photo-sensors and photodetectors. Buddha Deka Boruah and Abha Misra 159 fabricated a graphene-based ZnO QD photodetector with response and recovery time of nearly 0.29 seconds. Similarly, Gong et al. 91 also fabricated a ZnO QD-based photodetector using graphene. They printed QDs directly on the graphene eld effect transistor (GFET) channel. This utilizes the high mobility of graphene and the high electron density of ZnO QDs for the better performance of the device. The photoresponse of the ZnO QD-based photodetectors can also be increased with the aid of localized surface plasmon resonance (LSPR). [160][161][162] In this technique, novel metal nanoparticles are fabricated over the sensing material. In such a case, a nearsurface electric eld is formed due to the collective oscillation of electrons on the surface nanoparticles, which ultimately increases the absorption of light. Liu et al. 160 fabricated the ZnO QD thin lm decorated with Au antennas. This type of photodetector has shown nearly 9.1 and 4.9 times increase in the photo-responsivity and normalized detectivity, respectively. The QDs have also been found to be applicable for the fabrication of LED. 163 The excitation wavelengths of the UV range helps in developing ZnO-based visible light emitting QLEDs. The defectrelated emissions of green, yellow, and orange-red from the ZnO QDs can be completely restricted by reducing the oxygen vacancies. The hybridization of the Zn interstitials with the antibonding O-states of graphene oxide (GO) QDs thus help in reducing these emissions and increasing the violet-blue emission.
The electron transport layer (ETL) plays a major role in any optoelectronic device. It should be capable of quickly transporting electrons so as to avoid charge recombination. Having high electron mobility, ZnO plays this role very effectively in most optoelectronic devices. Fig. 25g and h shows the schematic diagram of a ZnO-GO-based QLED device and the energy level diagram, respectively. Fig. 25i shows the blue electroluminescence (EL) of the device. Fig. 25k shows an increase in the luminance and the current density with voltage. Fig. 25l shows the nearly constant luminous efficiency and the quantum efficiency with increasing voltage. Fig. 25m shows the practical 100 pixels 8 V display of the blue QLED. It has been found that the ETL (electron transport layer) made of ZnO QDs has more current efficiency in comparison to organic ETL. The energy level diagram of the ZnO QD-ETL based device has been shown in Fig. 25a, which shows that the electron can be easily transported through the ETL to the Al cathode. It also helps in blocking the hole movements. Fig. 25e and f shows the device structure of the QLED-based on organic ETL and ZnO ETL. It was due to better charge balance by the ZnO QDs. 164 The efficiency can be further increased with the help of doped ZnO QD-ETL. 165 Doping in the ETL lis the conduction band minima and reduces the electron mobility. The energy level diagram ( Fig. 25b) shows a shi in the conduction band minima with the doping concentration of Mg. The TRPL spectra of the device (shown in Fig. 25c) shows a reduction in exciton quenching at the interface. This is very essential for improving the device performance. Fig. 25d shows a reduction in the current density with increasing Mg concentration. This is helpful in increasing the charge balance. Doping also results in an increase in the external quantum efficiency. Shi et al. 115 fabricated Mg-and Cedoped ZnO QDs LED for white light emission. The EL spectrum of the device is shown in Fig. 25e and the device image is shown in the inset. At 3 V potential and 200 mA driven current, this LED-correlated color temperature (CCT) and the color rendering index (CRI) were 5733 K and 81, respectively. For any optoelectronic device, ETL is an important part. 166 Because of its low fabrication temperature and high electron mobility in comparison to TiO 2 , 167 ZnO is one of the best candidates for ETL. Even in perovskite-based devices, ZnO is preferred as the ETL. In perovskite-based solar cells, ZnO increases the V oc , which results in an increase in the ll factor and efficiency. 167 The high electron mobility of ZnO is useful in high electron extraction. It improves the band alignment with inorganic perovskite materials, improves the device stability, and reduces interfacial non-radiative recombination. 168 The introduction of ZnO in inorganic perovskite lms results in more compactly and uniformly distributed crystalline grains as it is evident that perfectly oriented grains considerably increase the power conversion efficiency. 169 The ETL of ZnO in this case also enhances the transport of photo-generated carriers from the perovskite lm to the electrodes, which improves the rise and fall time of perovskite-based photodetectors. 170 Transparent nanocomposites made from ZnO QDs, SiO 2 , and epoxy 171 for encapsulating LED 172 would be highly benecial in the near future. Besides encapsulating LED, nanocomposites are also applicable in other elds. Li et al. 173 prepared ZnO QDbased polymer composite using poly(methyl methacrylate) (PMMA) as the polymer for UV-shielding 87 material. This material can be used in a number of applications such as contact lenses, UV-shielding windows, or glasses. It was also found that the ZnO QDs can initiate photo-degradation of the host polymer matrix, which can be applied for recording materials. Georgia G. Goourey 81 and his team studied the effect of ZnO nanoparticles with size ranging from 5 to 30 nm on acrylate photopolymers. They found partial quenching of the yellow green uorescence with the incorporation of the QDs, whereas with the increase in the size to 30 nm, photodegradation decreases drastically. ZnO QDs were also found to degrade organic compounds, which are poisonous to human health. Fakhri et al. 82 prepared the ZnO QD/CuO composite for the degradation of the Tetanus toxin. They found 75% degradation under UV irradiation.

Photocatalysis
The smaller size of the ZnO QDs is benecial over nanoparticles in the eld of photocatalysis as the interplay between the accessible surface area and the ability to absorb incident photons results in higher quantum efficiency of photocatalysis. Researchers are working on a slow-growing process, in which the particles can interact with their surrounding particles or their own particles and grow at the later stages of the process into a larger particle or aggregate. Particles developed in such a way could be an effective tool for the effective removal of emerging water pollutants such as pesticides, herbicides, pharmaceuticals, and fragrances. Some of the covalent bonds can be easily broken down by UV light but some of them need a photocatalyst. Thus, here begins the role of ZnO QDs, which degrades the chemical bond of some complex organic molecules via the production of reactive oxygen species such as superoxide radicals O 2 c À , singlet oxygen 1 O 2 , hydroperoxyl radicals HO 2 c, hydroxyl radicals cOH, and hydrogen peroxide H 2 O 2 , which are generated at the metal oxide surface upon UV irradiation in aqueous solutions. The rate of charge generation in the ZnO QDs is much faster than that in other indirect semiconductors. In the presence of water molecules, the photogenerated holes generates the hydroxyl radical (cOH) (shown in Fig. 26a). The photocatalytic degradation of the dye is faster in the presence of QDs having a smaller size. As the size of the QDs, increases the rate of degradation decreases but the kinetics are found to show an increase at the initial stage. The kinetics then remain constant for whole the process (shown in Fig. 26b-d). The size of the QDs increases with time and its energy band gap decreases with time (shown in Fig. 26e-g). Taha Ahmed and Tomas Edvinsson 24 studied the photocatalytic application of ZnO QDs in colloidal suspensions, which shows a higher photocatalytic degradation of organic dyes for QDs between 3 and 6 nm.

Anti-microbial
Bacterial infection is a perpetual health problem, leading to complicated wound healing problems. Numerous efforts have been made by a number of researchers worldwide to overcome the contagious problem. Materials such as metal oxides, silver particles, and hydrogels are some of the antibacterial materials made by researchers. Recently, it has been found that the antibacterial property of metal oxide materials is more effective in comparison to other antibacterial materials. Among them, ZnO has been found to be the most promising because of its availability, biocompatibility, low-cost, negligible toxicity towards human cells, as well as antifungal and antibacterial properties. Liang et al. 43 synthesized graphene oxide-modied ZnO QDs that possess excellent killing effect for Staphylococcus aureus (Gram-positive) and Escherichia coli (Gramnegative). The size of the ZnO QDs also affects the antimicrobial activity. As the size of the QDs decreases, the density of the charged carrier defects also increases on the surface of the QDs, which generates more and more ROS. These ROS are responsible for the killing process. Fig. 26 (a) Schematic diagram for charge generation on the surface of the ZnO QDs and the energy level diagram, absorption spectra of the dye for the decolorization process with initial diameter (b) 3.2 nm, (c) 5.8 nm, (d) 6.7 nm, and band gap and size of the growing ZnO QDs for particles with starting diameters of (e) 3.2 nm, (f) 5.8 nm, (g) 6.7 nm. 24 Adapted with permission from ref. 24. Copyright (2020) American Chemical Society.
From Fig. 27, we can see the whole killing mechanism. In detail, the anionic membrane is rst attacked by the cationic ZnO QDs composite, which destroys the cell membrane of the bacteria. When the composite attacks the membrane, it releases Zn 2+ ions due to the lower pH level at that region. The absorption of Zn 2+ on the surface of the membrane not only inhibits the respiratory action of the enzymes but also produces reactive oxygen species (ROS). ROS damages the membrane, DNA, and mitochondria. GO sheets produce hyperthermia, which prevents infections in the wound. Abinit Saha and his group 174 found that QDs having particle size in the range of 3-5 nm have the maximum ability to destabilize the CRP (Cyclic AMP Receptor Protein) structure of the E. coli bacteria.

Cancer treatment
ROS generation gives power to ZnO QDs so that it can also kill the cancer cells. 175 For the efficacious operation of anticancer drugs, it is essential to deliver it at the tumor-specic region.  The difference in the pH level of the extracellular and intracellular environment of the tumor tissues is very useful for targeted drug delivery. Many researchers have applied the pH responsive property of ZnO QDs for the targeted drug delivery of anticancer medicines. Arivazhagan et al. 80 utilized the pH difference for target-specic drug delivery. They found that the microenvironment of MCF-7 and metastatic MDA-MB-231 human breast cancer tumor cells is more acidic than that of other normal healthy cells. For such a case, ZnO QDs are highly cytotoxic as compared to other nanostructures of ZnO. Faheem Muhammad and his group 42 obtained this property by conjugating folic acid on to the surface of the ZnO QDs. Doxorubicin (DOX) and ZnO QDs were simultaneously found to be more efficacious than alone. Fig. 28 shows the loading of DOX on folic acid-capped ZnO QDs. Next, we can see that these particles are preferentially accepted at the cell membrane of the cancer cell by the folate receptors. Here, due to the appropriate pH conditions, the DOX drug is released, which results in the killing of the cancer cell.
Lastly, we can say that ZnO can help in cancer treatment not only by its killing ability but also by acting as a drug carrier. It could play a major role in overcoming the challenges such as recurrence of disease, drug resistance, and side-effect of the drug. Drug resistance is found with many known anti-cancer drugs currently in use. Recurrence poses a challenge that is also related to these known drugs. ZnO will be very helpful in overcoming these challenges. Its drug delivery mechanism will improve the selectivity of the drug towards cancer cells, which nally increases its effectiveness. It will also reduce the possibility of drug side-effects as it will only act at the specic site.

Li-Ion batteries
Beside these widely-accepted applications, some miscellaneous applications of ZnO QDs also exist. These are not as popular as the above applications, probably because of their low feasibility. But research is still going on in these elds with the hope of developing an efficient tool that may remove the barrier and make it feasible. The use of ZnO QDs for the storage of lithium ions in lithium-based batteries is one such application. The Liion storage properties of different ZnO QD-based composite materials are shown in Table 2. It is found that the anode material made up of ZnO has a large theoretical capacity of nearly 987 mA h g À1 . However, a major challenge in the use of ZnO as the anode material is the huge volume change and low electron conductivity. To overcome this problem, an effective method is to use graphene or its derivatives in ZnO-based anode materials. RGO-based ZnO QD composites have found to show high electrochemical performance for Li-ion storage, with a high reversible capacity of 800 mA h g À1 even aer 200 cycles and stable reversible capacity of 668 mA h g À1 at a high current density of 1000 mA g À1 even aer 700 cycles. This is because of the conservation of the electrode structure, which allows the smooth ow of the Li-ion during charging and discharging. The anchoring of ZnO QDs on the supporting framework of RGO provides such an excellent stabilization. 117 Initial cathodic scanning shows a peak at 0.31 V, which is attributed to the reduction of Zn 2+ to Zn metal and the generation of a solid electrolyte interphase. The peaks at 0.69 and 0.37 V are due to the reduction of ZnO to Zn and the formation of the Li x Zn alloy with Li + , respectively. The dealloying of Li x Zn is clearly visible from the four peaks of the anodic scanning curve at 0.27, 0.36, 0.55, and 0.68 V. A large peak at 1.3 V in the anodic scanning curve is due to the formation of ZnO in the reaction of Zn and Li 2 O (shown in Fig. 29a). From Fig. 29b, we can see that the rst charging and discharging curve shows a voltage at 1.3 V and a large plateau at 0.3 V. The initial discharge and charge capacity were 1027 and 766 mA h g À1 , respectively, which gives an initial coulombic efficiency of 74.6%. The cyclic performance of the ZnO/RGO composite electrode and ZnO@RGO at a current density of 200 mA g À1 gives 766 mA h g À1 initial reversible capacity, which drops to 605 aer 20 cycles (shown in Fig. 29c). Aer 30 cycles, the coulombic efficiency stabilizes above 97% and aer 200 cycles, the reversible capacity reaches 800 mA h g À1 . Fig. 29d shows the average reversible capacities of 840, 670, 590, 515, 400, and 315 mA h g À1 at 0.1, 0.5, 1.0, 2.0, 5.0, and 10.0 A g À1 , respectively, for the ZnO/RGO composite electrode. Small ZnO size and the large and thin supporting structure of RGO are responsible for the excellent rate capability of the ZnO/RGO electrode. The cyclic performance of the ZnO/RGO electrode at a higher current density of 1000 mA g À1 is shown in Fig. 29e. In summary, the main applications of doped and undoped ZnO QDs are found to in antibacterial, antifungal, and anticancer medicines. Other applications are the sensing and detection of toxic materials. Quantum connement enhances the properties of the ZnO QDs, which opens the door for noble applications. pH-responsive drug delivery will be game changing in the treatment of cancer. The photocatalytic activity will revolutionize its application in different chemical industries and environment-related challenges. Graphene and polymer-based composites can widen the application eld of ZnO QDs. Graphene and reduced graphene-based ZnO composites are better candidates for storage applications in the future.

Conclusion
In recent years, a lot of work has been done by researchers worldwide in the eld of ZnO QDs. The quantum connement effect predominantly emerges nearly in all the properties of QDs. It has manipulated the whole character, as we have seen in the case of ZnO nanoparticles. The sol-gel method is the most suitable synthetic method for the preparation of ZnO QDs. With the variation in the used solvent and the precipitating agent, different physical properties can be altered. The important factor that always affects the dot formation is the pH of the solution, which should not be below 7 and should be higher than 10 as a pH less than 7 does not favor the process to start and a pH more than 10 pH results in the agglomeration of the particles. The essence of the whole manuscript is given in the following points.
(1) With a reduction in the particle size below 8 nm, strong quantum connement effect comes into play. Due to excitonphonon interaction, the ground state energy and oscillator strength reduces with the particle size of the QDs. Polaronic self-energy corrections of the exciton vanishes completely and the PB potential effectually transforms into a dynamicallyscreened Coulomb potential.
(2) Longitudinal optical (LO) and transverse optical (TO) modes of bulk ZnO splits into A 1 and E 1 symmetries in ZnO QDs. The phonon peak shi that arises in the ZnO QDs is related to three main factors: phonon localization by defect creation, connement effect within the QD boundaries, and the laser-induced heating effect.
(3) The uorescence intensity of the QDs increases with the doping of elements. But aer a certain limit, it starts decreasing. The uorescence intensity is quenched with the addition of any metal ion impurity or organic ligand. This property is useful for uorescence probes. The surface charge density also increases in the QDs.
(4) The sol-gel method is the most popular as it is a relatively more efficient, simple, and inexpensive method over others. With the help of this method, QDs stable in water and other alcohols (mostly ethanol) can be prepared. Composites made via SiO 2 , graphene, and PMMA are able to increase the stability of the QDs. Radio frequency-based synthesis is emerging as a new technique for QD synthesis.
(5) Most of the doping elements are found on the surface of the QDs. With the help of the core/shell (ICS) procedure, we can inject doping elements inside the core of the ZnO QDs. The doping of rare earth elements is difficult as compared to transition metal doping. Rare earth elements are useful for optical properties and transition metal doping induces magnetism in the QDs.
(6) Rare earth doping is mostly favorable for the variation in the photoluminescence-related properties, whereas transition metal doping mostly induces magnetic behavior in the QDs. Both types of doping introduces photocatalytic character in the QDs by producing different vacancies.
(7) The detection of metal ion impurities (Fe 2+ , Cr 6+ , Hg 2+ , Cl À , etc.) distinguishes it from nanoparticles. The interaction of graphene and QDs is helpful in optoelectronic device fabrication. The enhancement in ROS generation increases its antibacterial and anti-microbial activity. ROS generation also increases the cytotoxicity of the QDs. An effective drug delivery mechanism sharpens the QDs' cytotoxicity. Li-Ion storage can be achieved by hanging QDs in the framework of graphene or graphene-derived components. (8) Having such a wide eld of application, ZnO QDs have a very bright future. They can be used as an impurity-detecting tool. They can be used for the production of low-cost gas sensors. They can replace TiO 2 from the ETL in many optoelectronic devices. Apart from their commercial use in cosmetics, they can also be used in anti-microbial and antibacterial ointments. Their use in the treatment of cancer will denitely increase the effectiveness of the drug and will decrease the side-effects of the drug at the same time.

Conflicts of interest
There are no conicts to declare.