Quantum Dots and Nanohybrids and their Various Applications: A Review

Yogyata Chawre^a, Lakshita Dewangan^a, Ankita Beena Kujur^a, Indrapal Karbhal^a, Rekha Nagwanshi^b, Vishal Jain^c, Manmohan L. Satnami^a*

^a School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur, Chhattisgarh, India.

^b Department of Chemistry, Govt. Madhav P.G. Science College, Ujjain, Madhya Pradesh, India.

^c University Institute of Pharmacy, Pt. Ravishankar Shukla University, Raipur, Chhattisgarh, India.

 *Corresponding Author: manmohanchem@gmail.com

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 Abstract:

Organic/inorganic nanohybrids and quantum dots have attracted widespread interest due to their favorable   properties and promising applications. Great efforts have been made to design and fabricate versatile nanohybrids. Processing structure-properties-performance relationships are reviewed for compound quantum dots. In this review, various methods for synthesizing quantum dots as well as their resulting properties are discussed. This review focuses on the design, properties, sensing as well as energy applications of organic/inorganic nanohybrids as well as quantum dots. In this article, strategies for the fabrication, properties, functions, characterization techniques, various synthesis strategies and application of nanohybrids and quantum dots are briefly deliberated.

Keywords: nanohybrids; quantum dots; properties; sensing; energy applications.

1.     Introduction

 

Since time immemorial, nanomaterials are widely used for various purposes as they are the bridging materials between bulk and the molecular or atomic level. The major concern of the 21^st century is the growing pollution levels as well as to secure sustainability of energy that meets the global energy demands. In order to tackle these problems researchers all over the world have developed sensors in order to detect the toxicants and pollutants present in their immediate environment. Moreover, researchers are emphasizing on development of new catalytic and energy materials as well as investigating a clean and sustainable way to balance our long term dependence on fossil fuels and control the pollution levels. In this context, nanomaterials especially nanohybrids and quantum dots have emerged as a promising material because of their unique optical and electronic properties and the most important being tuning of their band gap.

Quantum dots have emerged as promising materials for nanosensors^1,2 , bioimaging ³ , catalytic degradation of organic pollutants⁴  photocatalysis⁵  and  photovoltaics ⁶ Many scientists and research groups have already  reported the use of QDs as sensors and for energy purposes. Zhang et al.⁷ have developed N-doped CQD for optical detection of Hg²⁺ ions. Zhang et al.⁸ have synthesized Graphene QDs which were used as dual fluorescent and electrochemical biosensors for glucose and H₂O₂ detection. A FRET between CQD and gold nanoparticle has been explored for the detection of organophosphorus and carbamate pesticides.⁹  Recently, Korram et al.¹⁰ have developed CdTe QDs based sensor for the detection of pesticides. Rehman et al.¹¹ have synthesized photoluminescent CdSe/ZnS QDs as electrochemical sensors for detecting pollutants. Xe et al.¹² have reported nitrogen doped carbon QDs as dual fluorescent and electrochemical biosensors for the detection of ascorbic acid. In the field of energy also QDs are widely used.Fernado et al.¹³ have shown the use of CQDs for photocatalytic energy conversions. Upon solar illumination on CQDs/QD NWs, photo-generated carriers were separated into photo-generated electrons (e⁻) and photogenerated holes (h⁺). The e⁻ and h⁺ react with adsorbed oxidants/reducers (O₂ /OH⁻ ) to produce photo-reactive oxygen radicals (¹O₂, ^•OH) which play the key role in the decomposition of the water for production of hydrogen fuel ¹⁴, decomposition of dyes and pollutants. ¹⁵ Recently, Mahala et al.¹⁶ developed CQD/ZnO heterostructure for enhanced photoanodic performance in photoelectrochemical water splitting. Recently, Mahala et al.¹⁷ reported ZnO nanosheets decorated with graphite like Carbon Nitride (g-C₃N₄) QDs as photoanodes for water splitting purpose. QDs are also used for modification of catalyst used for splitting. For example, Li et al.¹⁸ reported that CQDs enhanced the activity of ruthenium based electocatalyst. Metal oxide nanomaterials such as TiO₂ are considered as the best material for water splitting purposes. Na Mao has investigated the heterojunction between ZnO/Fe₂O₃ and g-C₃N₄ for enhanced photocatalytic activity ¹⁹  Sun et al. have synthesized  nanorods arrays of SnO₂ QDs interspersed multiphase TiO₂ heterojunctions for water splitting and self-rechargeable battery like applications. ²⁰

1.1.Nanohybrids

Nanohybrids are formed by the incorporation of two dissimilar components resulting in a single entity with either enhanced or completely new properties. These materials have attracted considerable attention due to unlimited possible combinations of the distinct properties of organic, inorganic or even bioactive components in a single material, with well-defined structures and functions.²¹They can be defined as the heteronanoparticles of organic and inorganic components consisting of discrete domains. They are known to combine not only the original functions of organic and inorganic parts, but also tend to produce new versatile properties due to the synergy of individual components, such as desirable optical, magnetic, electronic and electrical properties.²² The organic and the inorganic components mostly employed for synthesis of nanohybrids are shown in figure 1.

 

Figure 1: Organic and inorganic components employed in nanohybrids

 

Fabrications of organic and inorganic nanohybrids are usually done by three methods which basically include one-pot synthesis, wrapping and surface functionalization. In one-pot synthesis method the organic part does not usually participate in the reaction and acts as a capping agent. In this the inorganic parts are formed directly by one step reaction in the presence of organic parts which act as surface capping agents or templates. ²²  Wrapping is another method to fabricate organic/inorganic nanohybrids through noncovalent interaction between organic and inorganic components. Through precipitation or chemical reaction processes the organic components usually form nanoparticles (NPs) to encapsulate all or part of inorganic NPs. Similarly, self-assembled nanostructures of organic parts could also encapsulate the inorganic NPs which in turn can produce nanohybrids. Another method for fabrication purpose is the surface functionalization strategy which is the most widely used among the three because of the dual advantage of combining the organic and inorganic components as well as to tailor the properties of the nanohybrids. This basically includes “grafting from”, “grafting onto” as well as “self –assembly” approaches.²The schematic representation is shown in figure 2.

Figure 2: Construction of Inorganic/Organic Nanohybrids through Surface functionalization, Wrapping and     One-Pot Synthesis

 

As a result of which they are widely used in variety of fields such as biomaterials, optical materials, electronic materials, coating, energy storage, catalysis, and sensing purposes.²¹ Recently, Carbon-metal nanohybrids are gaining much popularity for their use in the energy- water-environment nexus.²³Apart from this Graphene oxide a precursor of carbon is also a promising candidate for the synthesis of nanohybrids as they are widely used for energy applications mnand as biosensors. ^24-26

Nanoparticles are materials which are of 1- 100 nm size range and they are categorized as two dimensional, one dimensional and zero dimensional. Under the zero dimensional comes the Quantum dot whose size ranges between 1-10nm. Quantum dots are defined as the nanostructured semiconductor materials. They behave differently from bulk solids due to quantum confinement effects. Moreover, limited number of electrons results in their discrete quantized energies in the density of states (DOS) for non-aggregated zero-dimensional structures. Here the dimensionality of the material depends on the fact that upon how many dimensions the charge carriers such as electrons and holes act as a free carrier.²⁷ Confinement of charge carriers in one direction usually results in two- dimensional structures known as quantum wells or films. If the confinement is in two directions then it produces quantum wires and if the confinement is in all the three dimensions then it results in production of quantum dots.²⁸ Quantum dots usually consists of few hundreds to few millions atoms but most of the charge carriers are confined that is only a small number of electrons or holes are free (<100) and this increase in the number of confined dimensions results in a stronger degree of electronic confinement and a wider range of tenability in the band gap.^29-31

Quantum dots are semiconductor nanostructure materials that confinethe motion of conduction band electrons, valence band holes or excitons in all the three spatial directions. The charge carriers such as electrons and holes may get mobilized in the presence of electric field to yield a current, but their lowest energy states are an electrostatically bound electron- hole pair known as exciton. This exciton is charge neutral and has a definite size within the crystal defined by the Bohr exciton diameter (a_B) but usually varies depending upon the size of    the materials. Relaxation of the excited electron back to the valence band annihilates the exciton and may be accompanied by the emission of a photon and this process is known as radiative recombination.^32-34 As emission spectrum of quantum dots are sensitive function of number of excitons it is shown that ‘‘engineering’’ of optical properties of quantum dots not only requires engineering of the single-particle levels but of many particle states and interactions as well.³⁵

1.2.General terms and aspects

Quantum size effect: Alteration of the electronic properties of the solids results if the dimensions of the relevant structural features interfered with delocalized nature of the electronic states. For semiconductor materials, this effect usually occurs between the size range of one to ten nanometers and results in the blue shift of the absorption and luminescence properties with decreasing size.³⁶

Two-photon action cross-section: This is the product of the two photon absorption cross section and the fluorescence quantum yield, which describes the probability of the simultaneous absorption of the two photons and transition of the fluorophore to an excited state by the energy of the two photons.

Quantum dots Surface Passivation: It is the modification of the surface of QD cores to improve properties such as fluorescence quantum yield. And it is achieved either by the deposition of a layer of inorganic, chemically inert materials or a layer of organic molecules or ligands.

Fluorescence Lifetime: It is the average time of an excited fluorophore that remains in the excited state before it emits a photon and decays to the ground state, which is measurable in the time or in the frequency domain.

Fluorescence Polarization (Emission Anisotropy): It is the measurement of the polarization of the emitted light upon excitation with linearly polarized light. The emission anisotropy basically shows the reflection freedom of the molecules in the excited state.

Stokes Shift: It is the difference between the spectral positions of the maxima of the lower energy absorption and luminescence arising from the same electronic transition.^36,

Molar absorption coefficient: It is the absorbance divided by the absorption path length of         the analyte or the species concentration.

Blinking: Continuously illuminated single quantum dots molecules emit detectable luminescence for limited time period. This is interrupted by dark periods during which no emission occurs.³⁷

Brightness: It is the product of the molar absorption coefficient at the excitation wavelength and the fluorescence quantum yield which is used to measure the intensity of the fluorescence signal obtained upon excitation at a specific wavelength or wavelength region.

Photoluminescence: It is the number of the emitted photons occurring per number of the absorbed photons.

 

2.     Structure of Quantum Dots

Structurally, QDs consist of a metalloid crystalline core and a “cap” or “shell” that shields the             core and renders the QD bioavailable. QD cores consist of a variety of metal complexes such as semiconductors, noble metals, and magnetic transition metals. Quantum Dots basically consists of core /shell structure and have dimensions in between those of the atomic and molecular levels with a band gap which depends upon a number of factors.³⁶ In order to improve its optical properties as well as to protect it the core structure is usually covered with other semiconductor material.³⁷ As mentioned earlier, tuning of band gap is the most important aspect of using quantum dots for a variety of applications. The most appropriate way of tuning the band gap is through alloying and doping of the quantum dots. ³⁸

Figure 3:    Core-Shell Structure of Quantum Dot.

 

3.     Alloying of Quantum dots

Alloying is a promising way of tuning the band gap without changing the size of quantum dot.^39-44 By alloying with suitable materials the optoelectronic properties of the quantum dot change.⁴⁵  Mostly they are alloyed with multiple semiconductors which enhance the PL (photoluminescence) emission intensities as well narrow the full-width-half-maximum (FWHM) of PL by minimizing the surface and bulk defects.⁴⁶

 

4.     Doping of Quantum dots

Another method of tuning the band gap is through doping of the QD. Transition metals such as Cr,^47,48 Mn,^47,49-51 Fe, Co³² as well as inorganic elements  like P³⁸ and B³⁸ and many are used as dopants for a variety of technological applications.^52-55 These dopants generate local quantum states that lie within the band gaps thereby perturbing the band structures. They are called as activators because they are self- ionized and do not require any external source of energy, the credit of which goes to the quantum confinement effect. Auto ionization is possible only when quantum confinement energy exceeds the interaction energy between the charge carriers that is electrons and holes with the dopants. By changing the position⁵⁶ and amount ⁵⁷of dopant the optical properties of QD changes.

 

5.     Surface structure of Quantum dots

Electronic quantum states associated with the surface are called the surface states.⁵⁸ These surface states widely influence the optical properties of quantum dots because of high surface-to-volume ratio. ⁵⁹ Surface states are basically formed at the reconstructed surface by the presence of non-stoichiometry and voids resulting in the unsatisfied bonds. They mostly affect the luminescent intensity, quantum efficiency and  optical properties.⁶⁰  The surface states act as oxidizing and reducing agents as their energy lie in the band gap region⁶¹ of the quantum dot which traps the holes and electron carriers and thereby significantly affect the optical properties and in turn the effect the conductivity of QD. Surface passivation can further enhance the optical properties.

 

6.     Surface passivation

Surface passivation also known as capping of the surface is highly useful as defects in the surface may reduce the quantum yield and intensity. The surface passivation generally causes the saturation of the dangling bonds present at the surface and thereby helps in reducing the defects and enhancement of the properties. By depositing the organic^62-64 and the inorganic capping layers over the quantum dots surface modification can be achieved. In case of organic capping agents phosphenes (TOPO) and mercaptans (SH) are widely used. And in terms of inorganic capping agent material with wide band gap is frequently used.⁶⁵ The passivation layer can grow either epitaxially⁶⁶ or non-epitaxially⁶⁷ which greatly influences the properties. Surface passivation process is a very complicated process and needs to be done with utmost care as incomplete passivation results in less quantum yield, decrease of the intensity as various other effects resulting due to trapping of the charge carriers by the surface        --states.

 

7.     Properties of Quantum dots

7.1.Optical properties

The optical properties of quantum dots mostly depend upon their size. They have discrete energy levels and the energy gap between the two bands (conduction and valence band) will totally depend  upon their size. For a particle in one box stronger the confinement, more will be the separation of energy levels and more will be the distance between the band gaps. From this we can infer that as size decreases the emission of the light occurs at shorter wavelengths resulting in a blue shift. Conversely when the size increases the emission mostly gets shifted towards longer wavelength (red shift). The size of the QDs can be controlled by various synthesis procedures.⁶⁸

7.2.Quantum confinement effect

When the size of the quantum dots becomes sufficiently small such that its energy level spacing exceeds that of kT (Energy differences >kT, k- Boltzmann constant) it somewhat restricts the movement of electron and hole carriers. Two most important properties depend upon the size the first being as mentioned above that is the blue shift of the band gap energy. ⁴⁹ And the second being well separated and discrete energy states due to the presence of small atoms. These results in the electronic states of each energy level.⁶⁹  When the radius of the quantum dot becomes equal to or less than the Bohr radius (rB) the mobility of electrons and holes gets confined spatially within the dimensions thus leading to an  increase in the excitonic transition energy and results in the blue shift of the band gap energy. This is called the quantum confinement effect.

Figure 4: Illustration of quantum confinement effect

7.3.Luminescence properties

When the external energy is applied the electrons and holes get excited and undergo a transition from ground state to higher excited state. These energies are directly determined by the electronic properties of the material. The electron and hole may recombine and relax to the ground state.⁷⁰This relaxation and recombination process results in either radiative or non radiative processes. Radiative process generally emits photon whereas non radiative process emits either photon or Auger electrons.

8.     Radiative process

The radiative process of relaxation generally results in the spontaneous luminescence. These types of emission  result from band edge emission, defect emission or activator quantum state.

8.1.Band Edge Emission

Band edge emission occurs when the electrons in the conduction band recombine with the holes in the valence band and undergo relaxation process.⁷¹ The recombination of electrons and holes results in the formation of exciton which causes band edge emission at energies slightly lower than the energy of the band gap. The exciton state becomes the lowest energy state in the quantum dots. The band structure can easily be studied from the optical absorption spectra. ^72,73

Though quantum dot possesses several advantages over the dyes like tunable emission, better photo stability and so on but does show an intermittent luminescence process mentioned earlier called ‘blinking’. ^74,75 In the blinking process the quantum dots emit light followed by a period of dark during which no emission occurs. In other words, switching between an emitted and non-emitted states result in blinking. Blinking occurs when the electrons and holes get separated leading to charged quantum dots. This process is known as photo-induced ionized process and results in the lifetime of the ionized state to be dark. Apart from these proposed mechanism^76-79 there are others theories as well such as thermally activated ionization or resonant electron  tunnelling between the excited states of QDs and dark-trap. However the exact reason behind the process of blinking is still under suspicion.

8.2.Activator Emission

Extrinsic luminescence is the term used for luminescence those results from deliberately incorporated impurities that are the dopants. The main mechanism behind this type of radiative process is the electron hole recombination process. This can be observed via transition from conduction to acceptor band or from donor to acceptor band or donor to valence band. In some cases d-d transition is allowed in certain cases due to d-p mixing and where the orbitals get split into hyperfine structure that causes relaxation of the selection rule.⁸⁰ Therefore, making it possible for some transition elements to exhibit this type of luminescence. On similar terms f-f transition is also possible as the f orbitals remain shielded by   the s and p orbitals. f-f transition is exhibited by some rare earth elements.⁸¹ Though this kind of transition does occur but they are not very intense.

8.3.Defect Emission

These are the emission which arises because of the presence of localized impurity that is the defect states within the band itself. These states can act either as donors if they have excess of electrons or as acceptors if they have deficiency of electrons. A columbic attraction causes the electrons and holes to attract to this site and as a result gets trapped there.⁸² Further the defect states are categorized as shallow and deep levels. Out of which shallow levels exhibit radiative process and generally occur at lower temperature. The condition of lower temperature is important as external energy can cause the electrons to get excited. On the other side the deep levels show non-radiative transitions. Defect states are known to occur at the surface which reduces the optical and electrical properties mainly because of the trapping of the charge carriers.⁸³

9.     Non-Radiative process

As discussed above the deep level traps have the tendency to undergo non-radiative recombination and relaxation. It mainly consists of external conversion, internal conversion and Auger recombination. ⁸⁴ At surface states non-radiative recombination takes place. It is estimated that about 15–30% of atoms in QDs are present at the surface and signify defects due to unsaturated dangling bonds. These defects are dominant channels for nonradiative decay of carriers. Auger nonradiative process occurs mainly by carrier-to-carrier interaction. Auger process differs greatly between nano-systems and bulk systems of the same composition as the efficiency of Auger process depends on Coulomb electron-electron interaction. In atomic or nano-systems, electron-electron coupling is stronger than the electron-photon coupling. Therefore, the rate of Auger transition is higher compared to radiative transition.⁸⁵

10.  Quantum yield

Quantum yield determination is an important parameter for practical purposes which can be determined by making comparisons between emission intensity obtained from quantum dot to that of the standards.⁸⁶ The possibility of accurate determination of the quantum yield is fairly low due to instrumental errors, different methods for measuring quantum yield, change of slits for sample and standard during experiments and other unavoidable errors. The standard Quantum yield equation for Quantum Dots^87-90is as follows:

 X

Where, QY and 𝑄𝑌_𝑠𝑡𝑑 are quantum yields (St: Standards), A and ASt are absorbance values at the excitation wavelength, η and η_St are refractive indices of the solvents, and I and I_St are integrated emission areas for the QDs samples and the standards, respectively. Same excitation wavelength should be used for both the sample and standards.

11.  Functionalization of Quantum Dots

During the past few year experts have now come to the realization that surface area of quantum  dots are also of extreme importance for different types of applications. The physicochemical properties as well as the biological activity of the quantum dots can be tailored by the bioconjugation as well as interfacial chemistry.^37,91-95

11.1.       Bioconjugation

This includes two types which are (i) self-assembly or specific recognition and (ii) covalent coupling. These strategies help to couple biological molecules such as proteins, antibodies, enzymes and other with the quantum dots.^92,96  Bioconjugation is extremely important for bioanalysis and bio imaging purposes. In covalent conjugation method there is interaction taking place between biomolecule and the polymer coating of QD. Coupling of the amine- bearing biomolecules to carboxylated QDs results in amide bond formation. Also known as Short-gun method,⁹⁷ it is useful for a variety of applications but it also has some disadvantages that is with many proteins it is not able to provide a good control over the conjugation of number of proteins with the quantum dots. Further, when amine and carboxyl groups are present in large numbers on the surface of a protein their coupling results in the formation of cross linked aggregates.

11.2.       Interfacial Chemistry

Most of the application requires the use of core/shell quantum dot. Therefore it is the inorganic shell which is the first choice for modification purposes. QDs prepared by solvothermal methods are usually capped by hydrophobic surfactants and therefore require modification so that they can interact with the hydrophilic part of the biomolecules. For this ligand exchange method as well as encapsulation with an amphiphilic polymer is widely used. The important considerations that need to be taken care of during modification include net charge, colloidal stability (i.e., resistance to aggregation), long-term coating stability⁹⁸ (i.e., stable association between the organic coating and inorganic QD), compatibility with bioconjugate chemistries (i.e., for attaching biomolecules of interest), and resistance to the nonspecific adsorption of proteins and other biomolecules in a sample matrix (i.e., non- fouling). Ligand exchange method involves replacement of hydrophobic surfactants from QD synthesis with higher affinity hydrophilic ligands via mass action. Researchers are making efforts in order to refine the ligand exchange method.^99-101

 

Figure 5. General methods associated with the design and control the interface of core–shell structured fillers for dielectric capacitor application. The left column shows the range of filler types, the central column indicates the core–shell interfacial control methods and the right column shows the final nanocomposite structures.

 

The fillers can be considered at a range of dimensions, namely zero dimensional (0D ) nanofillers which include spherical nanoparticles, nanocubes and nanoparticles with irregular morphologies, one dimensional (1D) nanofillers which include nanowires, nanofibers, nanotubes and nanoribbons, and two dimensional (2D) nanofillers which include nanosheets and nanoplatelet, as shown in the left column of Fig. 3. The outersurface of the fillers can be coated with a range of materials, as seen in the center column of Fig.3, which can be used to tune the interface between filler and matrix; see right column of Fig. 3.

Over the past few decades nanomaterials or more specifically quantum dots (QDs), have emerged as a new and a promising material for sensing and for catalysis purposes. This is mainly because of their unique optical and electronic properties, strong absorption coefficients, broad absorption spectra, narrow emission spectra, band gap can be easily tuned to absorb in a particular wavelength covering the entire solar spectrum and having high quantum yields. Due to the electro catalytic activity of metallic nanoparticles they were widely used as electrochemical biosensors but because of their toxicity and high prices they were not the best materials. In order to replace these metallic nanoparticles, new functional nanomaterials and quantum dots such as graphene quantum dots (GQDs), carbon quantum dot CQDs and semiconductor QDs are widely established as emerging materials.¹⁰² Similarly, QDs are also used for water splitting purposes by various techniques such as photoelectrochemical (PEC)¹⁰³, electrochemical (EC), photocatalytic, photobiological, radiolysis and thermal decomposition. The most important property of QDs that their band gap can be easily tuned up to absorb in a particular wavelength, making them the building blocks of photocatalysts and electrocatalysts. By incorporating quantum dots either by modifying the catalyst^104,105 or by developing the heterojunction¹⁰⁶, they increase the efficiency by overcoming the problems associated with water splitting and hence are widely useful for energy generation purposes. Apart from these they are also used for variety of applications such as in batteries¹⁰⁷ and supercapacitors for energy storage purposes and various photovoltaic applications. Many quantum dots such as CdTe, N-doped graphene oxide are used to develop a photocatalyst or a heterojunction for cost effective hydrogen production. QDs are highly useful for modifying the bandwidth materials will result in higher efficiency of energy transformation.

The new trend of research is slightly inclined to electrochemical and optical biosensing as well as in search of clean and sustainable fuel which can replace the nonrenewable resources. Due to simplicity, high sensitivity, and real time detection ability of optical and electrochemical biosensors have gained much attention by the researchers. These sensors are highly useful for various applications such as detection of various toxic substances or pollutants present in the environment, presence of metal ions, in medical diagnosis, food industry (for biomolecules) and monitoring of any disease progression. Another important emerging issue of this century is to meet the global energy demands. ¹⁰⁸ Researchers are emphasizing on development of new catalytic and energy materials as well as investigating a clean and sustainable way to balance our long-term dependence on fossil fuels and to control the pollution levels.

 

12.  Synthesis Processes

Several methods have been proposed for synthesis of quantum dots. The two most common methods are top down and bottom up approach. A schematic diagram is also shown below:

Figure 6: Systematic representation of the top-down and bottom-up method of for the synthesis of quantum dots.

 

12.1.       Top-down process

In this, the bulk materials are reduced in the nanoscale size range or it involves successive cutting of a bulk material to get nano sized particles or quantum materials. Etching known for the past few decades is a promising method for fabricating zero-dimensional dots. Reactive ion and wet chemical etching are more commonly used. Other than this dry etching is also used in which a reactive gas species is inserted into an etching chamber and a radio frequency voltage is applied to create plasma which breaks down the gas molecules into more reactive fragments. When these high kinetic energy species strike the surface it results in the formation of a volatile reaction product to etch a patterned sample. When the energetic species are ions, this etching process is called reactive ion etching (RIE).¹⁰⁹

Apart from these, for achieving high lateral precision focused ion beam (FIB) techniques¹¹⁰ are used. In this a molten metal source is used from which highly focused beams sputter the surface of the semiconductor substrate. The shape, size and inter-particle distance of the QDs depend on the size of the ion beam. This technique is also used to selectively deposit material from a precursor gas having a resolution of up to ~100 nm. Other method that is used is the electron beam lithography followed by etching or lift-off process which offers greater flexibility in the structure. Any shape of QDs, wires, or rings with precise separation and periodicity can be fabricated with this technique. ¹¹¹

12.2.       Bottom-Up approach

In this approach, the starting materials are developed by joining atoms or molecules to larger structures (nano). These are broadly classified into two types that is wet chemical method and      vapour phase method. Wet chemical method generally includes sol-gel, competitive reaction chemistry, hot-solution decomposition, electrochemistry and microemulsion whereas the vapour phase method includes sputtering, liquid metal ion sources, or aggregation of gaseous monomers and self-assembly of nanostructures in material grown by molecular beam epitaxy.

 

12.3.       Wet Chemical Method

This mainly involves precipitation, nucleation and growth process. Nucleation may be categorized as homogeneous, heterogeneous or secondary nucleation. In homogeneous nucleation solute atoms or molecules combine and reach a critical size without the assistance of a pre-existing solid interface. By varying factors, such as temperature, electrostatic double layer thickness, stabilizers or micelle formation, concentrations of precursors, ratios of anionic to cationic species and solvent, QDs of the desired size, shape and composition can be synthesized. ¹¹² This includes the following process:

a)     Sol Gel process:  In this process, a sol (nanoparticles dispersed in a solvent by Brownian motion) is prepared using a metal precursor (generally alkoxides, acetates or nitrates) in an acidic or basic medium. The three main steps in this process are hydrolysis, condensation (sol formation) and growth (gel formation). In this process, the sol (or solution) evolves gradually towards the formation of a gel-like network containing both a liquid phase and a solid phase.^40,69 In this the metal precursor hydrolyzes in the medium and condenses to form a sol, followed by polymerization to form a network (gel). Though this process was widely used because of its simplicity and cost effectiveness but with the development of more accurate method, this method is now used less as it includes a large number of defects and large size distribution.¹¹³

b)    

Microemulsions: This method includes two types which are normal microemulsion, i.e., oil-in-water, or as reverse microemulsions, i.e., water-in-oil. Out of both of them, reverse microemulsions are widely used where two immiscible liquids (polar water and nonpolar long-chain alkane) are thoroughly stirred and mixed to form emulsion. Surfactants such as, cetyltrimethyl- ammonium bromide (CTAB), sodium dodecyl sulphate (SDS) are used.¹¹⁸ Since the surfactants are terminated by hydrophilic and hydrophobic groups on opposite ends, several small droplets known as micelles are produced in the continuous oil medium. The growth is controlled by the molar ratio of water and surfactant (W). This is used to prepare core/shell QDs, such as CdS⁴², CdS:Mn/ZnS^114,88 ZnS/CdSe¹¹⁵, CdSe/ZnSe¹¹⁶, ZnSe⁷² and IV-VI QDs. ¹¹⁷  This method does have the disadvantage of low yield.

c)     Hot Solution Decomposition Process: In 1993, Bawendi and co-workers reported the high temperature (~300 °C) pyrolysis of organometallic compound for the fabrication of Quantum dots. Precursors generally used are such as alkyl¹¹⁸, acetate¹¹⁹, carbonate and oxides ¹¹⁹of Group II elements. In this precursors are simultaneously injected along with TOPO (trioctyl-phosphine oxide) in a dry flask at 200– 350 ºC under vacuum that is accompanied with vigorous stirring at a temperature of ~300 ºC. This results in homogeneous nucleation to synthesize QDs, with the successive growth of QDs through ‘Ostwald ripening’. TOPO is used as a stabilizing solvent that stabilizes the QD dispersion, improves the passivation of the surface, and provides an adsorption barrier. This method has the advantage over other method that it is able to anneal the defects thereby resulting in single monodispersed layer. The growth is controlled by modulating the temperature. The major disadvantage includes the involvement of high temperatures, slow process and toxicity as well as poor water dispersion of precursors used.

d)     Vapour Phase Method: This process generally involves self-assembly of nanosized materials in which the layers are grown in an atom-by-atom process. In this the process of self-assembly takes place on substrate and the layered material grows as epitaxial layer which remains initially smooth and then followed sometimes by nucleation and growth of small islands.³⁶ Other than this, Molecular beam epitaxy (MBE) is also used for synthesis purposes. In this    the layers are deposited on the heated substrate. The basic principle of the MBE process is evaporation from an aperture source (Knudsen effusion cell) to form a beam of atoms or molecules. The beam is usually formed from solids or a combination of solid and gases.^120,121 Another way is the Layer growth by physical vapour deposition (PVD).¹²²The growth of layer occurs due to condensation of a solid from vapours produced by thermal evaporation or by sputtering process. Evaporation is achieved by arc-discharge, pulsed laser ablation or by electron beam heating. CVD (Cold Vapor Deposition) is another way for the formation of thin films from which the quantum dots can be self-assembled. In CVD, precursors are introduced in a chamber at a particular pressure and temperature and they diffuse to the heated substrate, react to form a film, followed by gas-phase by products desorbing from the substrate and being removed from the chamber.¹²² Though Vapour Phase Method is highly useful but fluctuation in the size of QDs often results in inhomogeneous optoelectronic properties.

12.4.       Other methods of Synthesis

12.4.1 Plasma synthesis: Plasma synthesis has evolved to be one of the most popular gas-phase approaches for the

production of quantum dots, especially those with covalent bonds.¹²³ For example; silicon (Si) and germanium (Ge) quantum dots have been synthesized by using nonthermal plasma. ¹²⁴ The size, shape, surface and composition of quantum dots can all be controlled in nonthermal plasma. Doping that seems quite challenging for quantum dots has also been realized in plasma synthesis.¹²⁵ Quantum dots synthesized  by plasma are usually in the form of powder, for which surface modification may be carried out.

 

12.4.2 Colloidal Synthesis: Colloidal synthesis generally involves arrested precipitation methods. This involves heating the solution at high temperature, so that the precursors get decomposed resulting in the formation of monomers which then nucleate and generate nanocrystals. Temperature is a critical factor in determining optimal conditions for the nanocrystal growth. It must be high enough to allow for rearrangement and annealing of atoms during the synthesis process while being low enough to promote crystal growth. The concentration of monomers is another critical factor that has to be stringently controlled during nanocrystal growth.¹²⁶ The growth process of nanocrystals can occur in two different regimes, "focusing" and "defocusing". At high monomer concentrations, the critical size (the size where nanocrystals neither grow nor shrink) is relatively small, resulting in growth of nearly all particles. In this regime, smaller particles grow faster than large ones (since larger crystals need more atoms to grow than small crystals) resulting in the size distribution focusing, yielding an improbable distribution of nearly monodispersed particles. The size focusing is optimal when the monomer concentration is kept such that the average nanocrystal size present is always slightly larger than the critical size. Over time, the monomer concentration diminishes; the critical size becomes larger than the average size present, and the distribution defocuses.

 

12.4.4 Electrochemical Assembly: Highly     ordered     arrays      of     quantum     dots may     also      be      self-assembled by electrochemical techniques. A template is created by causing an ionic reaction at an electrolyte-metal interface which results in the spontaneous assembly of nanostructures, including quantum dots, onto the metal which is then used as a mask for mesa-etching these nanostructures on a chosen substrate.¹²⁹

 12.4.5 Bulk Manufacture: Quantum dot manufacturing relies on a process called high temperature dual injection which has been scaled by multiple companies for commercial applications that require large quantities (hundreds of kilograms to tonnes) of quantum dots. This reproducible production method can be applied to a wide range of quantum dot sizes and compositions. The bonding in certain cadmium-free quantum dots, such as III-V-based quantum dots, is more covalent than that in II-VI materials, therefore it is more difficult to separate nanoparticle nucleation and growth via a high temperature dual injection synthesis. An alternative method of quantum dot synthesis, the molecular seeding process, provides a reproducible route to the production of high quality quantum dots in large volumes. The process utilises identical molecules of a molecular cluster compound as the nucleation sites for nanoparticle growth, thus avoiding the need for a high temperature injection step. Particle growth is maintained by the periodic addition of precursors at moderate temperatures until the desired particle size is reached. The molecular seeding process is not limited to the production of cadmium-free quantum dots; for example, the process can be used to synthesise  kilogram batches of high quality II-VI quantum dots in just a few hours. ¹²⁹

 

13.  Characterization of quantum dots

After the synthesis of quantum dots they are characterized by different techniques which are as follows:

13.1.       UV-Visible Spectroscopy:

The instrument used for this is UV-Visible Spectrophotometer. It is based on the principle of Beer-Lambert law. From this we get the information about the absorption spectra of the molecules. The absorption band obtained at a specific wavelength indicates the type of transition taking place. For example in case of Graphene quantum dot the absorption band obtained at 220nm indicates the π-π* electronic transitions due to sp² hybridized carbon atoms. The transition further indicates the atom or the functional group which is responsible for that transition.¹³⁰ From the equation,

                                                   A=ε cl

Where, A- absorbance, ε- molar extinction coefficient, c- concentration and l- path length. We can calculate the absorbance of the species.

13.2.       Fluorescence Spectroscopy:

The instrument used for this purpose is the Spectrofluorometer. In this the material whose fluorescence spectra is to be determined is taken into the sample chamber which is then excited by a specific wavelength of light. The material gets excited and the electron of the atom present in the valence band gets excited and goes to the conduction band. After remaining there for a certain period of time they return to their ground state thereby emitting light, which is called fluorescence. It basically gives fluorescence spectrum which gives the emission peak at a specific wavelength.¹³¹

13.3.       IR-FTIR:

The instrument used is IR-FTIR Spectrometer. This type of characterization technique is used to find out the functional groups or the types of bonds that are chemical bonding present in the synthesized quantum dot or nanohybrids. It basically gives information about the surface groups and the possible structure of the prepared species. For example- the FTIR spectra of S,N GQDs show an absorption peak at 3440 cm^-1 is related to C–OH, 1643 cm^-1 is attributed to stretching vibration of C=C bonds. Results demonstrated that product contains COOH and N and S-containing functional groups.¹³¹

13.4.       Scanning electron microscope:

The instrument used for this purpose is the Scanning Electron Microscope. It provides information regarding the surface morphology of the synthesized material. It scans the surface and determines the size of the quantum dot that is prepared.¹³⁰ In SEMs, samples are positioned at the bottom of the electron column, and the scattered electrons (back-scattered or secondary) are captured by electron detectors. It creates an image by detecting the reflected or knocked off electron. It basically provides us with facts regarding the sample surface, size and composition.

13.5.       Transmission electron microscope:

The instrument used for this is Transmission Electron Microscope. It also provides information regarding the size and the structure of the material synthesized.¹³¹ TEM uses transmitted electrons (electrons that are passing through the sample) to create an image. As a result, TEM offers valuable information on the inner structure of the sample, such as crystal structure, morphology and stress state information. This technique provides higher resolution and magnification compared to SEM.

13.6.       X-Ray diffraction:

The instrument used for this purpose is the Defractometer. It is used to for investigation of the phase structure and crystallinity. It is used to identify the crystalline phases present in a material and thereby reveal chemical composition information. It is performed by directing an x-ray beam at a sample and measuring the scattered intensity as a function of the outgoing direction. Once the beam is separated, the scatter, also called a diffraction pattern, indicates the sample’s crystalline structure. For example - in S, N-GQDs XRD pattern confirms presence of broad peaks at 2θ = 25° which is attributed to the amorphous nature and disordered carbons.¹³²These are the main characterization techniques which are used .Apart from all these NMR Spectroscopy,electrochemical techniques such as Cyclic voltammetry, Photoluminescence Spectroscopy and many more can also be employed for revealing the overall characteristics  of the synthesized material.

14.  Noteworthy contribution

In the last few years our research group have been investigating sensing of toxic metal ions, organophosphorus pesticides and biological molecules (glucose, cholesterols, biothioletc) using metallic nanoparticles, QDs and CQDs^{130, 133,134} Surface functionalized CdTe and Mn²⁺ doped CdTe@ZnS quantum dots based nanosensors have been developed for detection of arsenic and glucose.^134,135 The interaction of nanoparticles and QD with biomolecules has also been  investigated. Graphene oxide decorated with L-cysteine capped silver nanoparticle have been designed for the antibacterial activities. Moreover, biosensing assay have been developed for monitoring acetylcholinesterase inhibition and their reactivation.

15.  Applications:

Recently Quantum dots based Nanohybrid materials have received tremendous attention in several research applications. ¹³⁶ Bioinspired quantum dots nowadays are widely useful in cancer treatment. ¹³⁷ Moreover, carbon quantum dots are also useful as sensitizers in the photoanode of solar cells. ¹³⁸ Further various applications are shown in figure 7:      

Figure 7: Various applications of Quantum Dots as well as Quantum dots based Nanohybrid.

 

15.1.       Nanohybrids and quantum dots as sensors for detection of harmful substances

Nanohybrids and quantum dots are widely used as a sensor by many scientists for the detection of the harmful substances such as dyes, organic pollutants, harmful inorganic ions present in the environment.As QDs (CQD, QD) gives FL emission in visible region of spectrum. The presence of harmful substances will significantly quench their FL  (photoluminescent) intensity. The selectivity of the toxicant to be detected is done by adding different toxicants (pesticides)¹³⁹ of same concentration and observing the quenching effect. Once the toxicant gets detected the sensitivity is usually detected by adding different concentrations of the same pollutants and then analysing the result. Below is the table indicating the type of the material used and their use as sensors.

 

Table 1: QDs and Nanohybrids used as sensors

Material used	Sensors	Harmful substance   detected	Reference
Zinc oxide/graphitic carbon nitride Nanohybrid	Electrochemical Sensor	4-Nitrophenol	140
Titania/Electro-Reduced Graphene Oxide Nanohybrid	Electrochemical                    sensor	Allura Red dye	141
MXene and transition metal Dichalcogenide nanohybrid(Ti3C2Tx/WSe2 nanohybrids)	Gas sensor	Volatile organic        compounds	142
Cobalt oxide sulfide nanosheets nanohybrid	Electrochemical Sensor	Hydrogen Peroxide	143
Nitrogen enriched graphene quantum dots	Fluorescence optical sensor	Formaldehyde	144
CQD derived from Magniferaindica	optical sensor	Iron (Fe2+)	145
Graphene quantum dots (GQDs)	photoluminescence sensor	organophosphate pesticide	146
CQDs-NIPs (non-imprinted)                            polymers) Nanohybrid	optical sensor	Promethazine Hydrochloride	147
Au@Carbon quantumdots-MXene nanocomposite	Electrochemical Sensor	Nitrite	148
Graphene quantum dots, porous carbon and molecularity imprinted	Fluorescence sensor probe	Sulfadimethoxine	149
Black phosphorus quantum dots over carboxylated multiwalled carbon nanotubes as 0D/1D nanohybrid	Electrochemical Sensor	Ochratoxin a	150

 

15.2.       Quantum dots and nanohybrids for bio-imaging and bio-sensing purposes:

Due to unique optical properties of QDs as well as nanohybrids they are widely used for biological purposes. They are used for the bioimaging of the cancerous cells. They are also used as nanomedicines. Researchers are now aiming at using the unique optical properties of QDs in biological imaging. Due to higher extinction coefficients, higher QY, absorbance and emissions can be tuned with size, less photobleaching, generally broad excitation windows but narrow emission peaks are the important properties that make QDs suitable for biological applications. Below is the table indicating the various uses of QDs and nanohybrids in biomedical and biological fields.

Table 2: QDs and Nanohybrids used for biological purposes

Material used	Application	References
L-cysteine and poly-L-arginine grafted carboxymethylcellulose/Ag-In-S quantum dot fluorescent nanohybrids	in vitro bioimaging of brain cancer cells.	151
Strongly green-photoluminescent graphene quantum dots	bioimaging of MC3T3 cell	152
Ag@SiO2 nanohybrid	Photothermal therapy and bioimaging of HeLa cells	153
ZnO QD@PMAA-co-PDMAEMA	plasmid DNA delivery and bioimaging of COS-7 cells (African green monkey kidney cells)	154
GSH-Capped CuInS2 Quantum Dots	Bioimaging of MCF10CA1a breast tumor cell	155
Copper Indium  Sulfide Colloidal  Nanohybrids	CEACAM6 is a known tumor  associated antigen of pancreatic cancer.	156
CdS QD derived Using Tea Leaf Extract	A549 cells of lung cancer cells	157
Silicon quantum dots and copper nanoclusters	Visual assay of L-Cysteine	158
Hierarchical porous ZIF-8 nanohybrids	Detection of dopamine	159

 

15.3.       Quantum dots and nanohybrids for photovoltatic applications:

One of the most important and emerging issues of this century is to meet the global energy demands. Researchers are emphasizing on development of new catalytic and energy materials as well as investigating a clean and sustainable way to balance our long-term dependence on fossil fuels and to control the pollution levels. In this context, scientists are turning towards nanomaterials especially quantum dots because of their unique features such as low photobleaching, long lifetime, and high quantum yields. Semiconductor QDs such as CdS, CdSe and Si are promising materials for photovoltaic applications. By modifying these QDs with metal oxides nanoarrays or hybrids such as CQD substantially improve their power conversion efficiency (PCE) in solar cells. Various methods for their synthesis such as top- down, bottom-up, vapour-phase and wet chemical methods are already discussed above. By developing a modified and cost-effective quantum dot with one of the above techniques will highly improve PCE. A schematic diagram is represented below.

Figure 8: Charge transfer processes at photoanode/electrolyte interface.

Kaur at el. synthesized a nanocomposite between CdTe QDs and an NTU-9 MOF and when it was tested as a photoanode material in a QD-sensitized solar cell (QD-DSSC), its power conversion efficiency improved by approximately 1.5% relative to the raw QD form.¹⁶⁰ Shi at     el. developed a stable CsPbI₃ Perovskite Quantum Dots Enabled by in-situ Ytterbium doping. The solar cells adopting optimally Yb-doped CsPbI₃ QDs achieved the best power conversion efficiency (PCE) of 13.12% and displayed significantly improved storage stability under ambient conditions.¹⁶¹ Similarly, it was found that the solar cells based on these PbS QDs show excellent performance with stable efficiency in the range of 10% to 10.5%.¹⁶² In           addition to this, semiconductor quantum dot solids also showed promising results towards in photocatalytic and photovoltaic applications due to their optoelectronic properties.¹⁶³

15.4.       Quantum dots sensitized solar cells:

In the past few years, dyes have been used as sensitizers for harvesting energy. In the 1990s, the dye sensitized solar cell (DSC) was extensively developed by Graetzel .^164,165  In this, the light is absorbed by the dye followed by an electron transfer from an excited state of the dye molecule into the conduction band of wide band-gap semiconductors. ^162-166 such as TiO₂, ZnO, TiO₂, NbO₂ or Ta₂O₅. The hole on the dye is scavenged by a redox couple in solution. For better efficiency and charge separation, less than a monolayer of dye is required on the wide band-gap semiconductors. Usually, dyes have a strong absorption band in the visible region but very low absorption in the UV and NIR regions. In addition to this dyes suffer from photodegradation. Therefore, in order to overcome the above mentioned problems the solar cells are nowadays fabricated with tunable band-gap semiconductors quantum dots.

15.5.       Quantum dots and nanohybrids as photocatalyst for degradation of harmful aquatic pollutants:

The waste water that is discharged from industrial and domestic sources contains a large amount of harmful and toxic organic, inorganic and biological pollutants which may take several years for its degradation.Utilization of advanced oxidation processes such as photocatalysis has gained a widespread importance due to their capability of non-selective and complete degradation of organic/biological pollutants in industrial wastewater. Nanocomposite such as TiO₂/GQD was used for the degradation of dyes in aquatic environment. Materials like graphitic- C₃N₄, zinc oxide, iron oxide are widely used for hydrogen generation. By developing a quantum dot based catalyst or a