Introduction to" Quantum Dots "

 C Content

 1. Introduction & brief overview 

 2. Density of States 

 3. Quantum confinement effect 

 4. Exciton

 5. Structure of the quantum dots

 6. Estimation of the size of the quantum dot

 7. Synthesis of the quantum dots

 8. Properties of the quantum dots 

 9. Conclusion 

 10. Reference   

1.Introduction : 

 Nanostructured semiconductors or Quantum Dots (QDs) are materials in continuous development that hold potential for a variety of new applications, including uses in fluorescent labels for biomedical science, photonic devices and sensor materials. In biomedical applications, several nano-diagnostic assays have been developed that use QDs. They have been applied to diagnostics, the treatment of diseases, bioimaging, drug delivery, engineered tissues and biomarkers. Carbon quantum dots (CQDs, C-dots or CDs), which are generally small carbon nanoparticles (less than 10 nm in size) with various unique properties, have found wide use in more and more fields during the last few years. Nanomaterials, as the name implies, is concerned with the study of materials whose at least one dimension is less than approximately 100 nm. Nanomaterials show unique optical, electrical and magnetic property. Some nanomaterials are found naturally, however for specific interest they are engineered to nanoscale from bulk materials. Engineered nanomaterials are designed at molecular levels and takes the advantage of small size and unique properties which is absent in their bulk counterpart. Nanomaterials belong to a group known as low dimensional structures (2D-0D) . The dimension is assigned based on number of directions in which carriers of the material acts as free. For example, 3-D structures are those in which carriers are free in all three directions. If we confine the movement of carrier in one direction and let it be free in other two directions then the 3-D structure will transform into a 2-D structure like quantum wells. Taking this confinement further and restricting carriers in two directions will give rise to 1-D structure such a nanowire. Finally, when confinement occurs in all three spatial directions and the carriers are not free to move in any direction then the structure will be 0-D i.e. quantum dots. In bulk semiconductor the electron exists in an energy level which is continuous. However, in a nanoscale level the energy level becomes discrete due to quantum confinement effect. On UV light illumination on QDs, the electrons move from the valence band to the conduction band across the bandgap of quantum dot leaving a positively charged hole in the valence band. This electron hole bound pair is referred as excitons. The average physical distance between the electron hole pair is known as exciton Bohr radius and it varies from material to material. When size of the semiconductor particles is smaller than the Bohr radius of exciton, the quantum effects will be notably observed. Under such condition we call the nanomaterials as quantum dots. It is a zero-dimensional structure in which its excitons are squeezed and are not allowed to move out of the dot. With excitons being confined in all three spatial directions, the quantum dots have properties intermediate between bulk materials and discrete atoms or molecules. In comparison to bulk materials the bandgap of the QDs are larger and its electronic states become discrete. When excited with energy greater than the bandgap, quantum dots emit photon releasing the absorbed energy. Due to quantum confinement effect the emission energy can be tuned by changing the size and composition of the quantum dots. The size dependent absorption and emission property of dots have potential application in biological imaging as they can easily be tuned from UV to NIR region of the spectrum. Quantum dots also play an important role in solar light harvesting by acting as a photo-absorbing material. Quantum dots sensitized solar cells (QDSSCs) have attracted great attention in last few years because of the recent popularity of the synthesis of well-defined QDs to construct cost effective photovoltaic solar cells. Multiple exciton generation and bandgap tunability in QDs are the properties which enhance the efficiency of QD based solar cells. The Fluorescence properties of quantum dots has been used as a sensing probe in optical sensors for detecting different metal ions, vitamins, proteins, pH and nitro aromatic compounds through energy transfer mechanism. Efforts are being made to synthesize QDs which is less toxic, water soluble, photo stable and cost effective. Carbon quantum dots which were accidently discovered in 2004 by Xu.et al. during purification of single wall carbon nanotubes, shows unique optical properties such as excitation dependent multicolor emission, tunable photoluminescence and unique photoinduced electron transfer. Due to low toxicity and easy route of synthesis, carbon quantum dots have extensively used in sensing and bioimaging applications. 

                                                    2.Density of state

3.Quantum confinement:

Transition from classical laws to quantum mechanical laws of physics occurs when dimension of a particle approaches to atomic level. All phenomenon at microscopic level cannot be described without considering the concepts of basic quantum mechanics. The movement of electrons in crystals, chemical bonds and properties of atoms can be explained through quantum mechanics. When the material approaches towards nanoscale size its electronic and optical properties are completely different from those of the bulk materials.

The most popular term in the nano world is quantum confinement effect which is essentially due to changes in the atomic structure as a result of direct influence of ultra-small length scale on the energy band structure.

The length scale corresponds to the regime of quantum confinement ranges from 1 to 25 nm for typical semiconductor groups of IV, III-V and II-VI. In which the spatial extent of the electronic wave function is comparable with the particle size. As a result of these “geometrical” constraints, electrons “feel” the presence of the particle boundaries and respond to changes in particle size by adjusting their energy. This phenomenon is known as the quantum-size effect. A fundamental principle of wave particle duality states that all the matter (nuclei, electron, and photon) behaves as both wave and particle. The quantum effect of confinements becomes important when at least one of the dimensions of solid becomes comparable to the particles de Broglie wavelength which is given as 𝜆 = ℎ √2𝑚𝐸, where m is mass of the particle and E is energy.  

The quantum confinement effect (QCE) comes into the picture when the diameter of the particle approaches to de Broglie wavelength of electron wave function. When the confining dimension for a particle is large as compared to the wavelength associated with the particle then the particle behaves as it is free. The band gap of the material at this stage remains unchanged due to continuous energy levels, however at nanoscale the energy states becomes discrete and the energy bandgap changes from the original one. As a result, the bandgap of nanomaterials becomes shape and size dependent. On optical illumination there is blue shift in absorption spectra due to decrease in shape and size of the material. In semiconductor materials when a photon of sufficient energy strikes on the surface then electron from valence band moves to conduction band leaving a positively charged hole in the valence band. This electron hole pair is called excitons. When these excitons are squeezed into a dimension approaches to exciton Bohr radius then quantum confinement effect dominates. Due to three dimensional spatial confinements of excitons their movement is restricted in all directions and the dimension of the nanomaterials becomes zero dimensional. The exciton Bohr radius is given by Bohr exciton radius which makes materials properties size dependent. In general, the Bohr radius of a particle is defined as:  v   

where ε is the dielectric constant of the material, m* is the mass of the particle, m is the rest mass of the electron, and a₀ is the Bohr radius of the hydrogen atom. When the particle size approaches Bohr exciton radius, the quantum confinement effect causes increasing of the excitonic transition energy and blue shift in the absorption and luminescence band gap energy. In addition, quantum confinement leads to a collapse of the continuous energy bands of a bulk material into discrete, atomic like energy levels. The discrete structure of energy states leads to a discrete absorption spectrum, which is in contrast to the continuous absorption spectrum of a bulk semiconductor as shown in Fig.

A quantum confined structure is one in which the motion of the carriers (electron and hole) are confined in one or more directions by potential barriers. Based on the confinement direction, a quantum confined structure will be classified into three categories as quantum well, quantum wire and quantum dots or nanocrystals. 

Compared with bulk semiconductors, the quantum well has a higher density of electronic states near the edges of the conduction and valence bands, and therefore a higher concentration of carriers can contribute to the band-edge emission. As more numbers of the dimension is confined, more discrete energy levels can be found, in other words, carrier movement is strongly confined in a given dimension. Density of electron states in bulk, 2D, 1D and 0D semiconductor structure is shown in Fig:

In Nano Crystals, the Electronic energy levels are not continuous as in the bulk but are discrete (finite density of states), because of the confinement of the electronic Wave function to the physical dimensions of the particles. This phenomenon is called Quantum confinement and therefore Nano Crystals are also referred to as quantum dots (QDs).

2-D or Quantum Wells: The carriers act as free carriers in a plane. First observed in semiconductor systems

1-D or Quantum Wires: The carriers are free to move down the direction of the wire

0-D or Quantum Dots: Systems in which carriers are confined in all directions (no free carriers)

One of the most direct effects of reducing the size of materials to the nanometer range is the appearance of quantization effects due to the confinement of the movement of electrons. This leads to discrete energy levels depending on the size of the structure as it is known from the simple potential well treated in introductory quantum mechanics. Following these line artificial structures with properties different from those of the corresponding bulk materials can be created. Control over dimensions as well as composition of structures thus makes it possible to tailor material properties to specific applications. 

 

4.EXCITONS

An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and in some liquids. An exciton can form when a photon is absorbed by a semiconductor. This excites an electron from the valence band into the conduction band. In turn, this leaves behind a positively charged electron hole. The electron in the conduction band is then effectively attracted to this localized hole by the repulsive Coulomb forces from large numbers of electrons surrounding the hole and excited electron. This attraction provides a stabilizing energy balance.

The excitons radius can be taken as an index of extent of confinement experienced by a nanoparticle. If the dimension of nanoparticles (d) is greater that radius of exciton (a_eff), it is the weak confinement region, and if d< a_eff, it is strong confinement region, and if d>>a_eff, no confinement. Under weak confinement conditions, the excitons can undergo unrestricted translational motion. But for strong confinement this translational motion is restricted. There is an increase in overlap of electron and hole wavefunctions with decrease in particle size and thereby enhanced electron hole interaction. An optical index of confinement is the blue shift (shift to higher energies) of optical absorption edge and exciton energy with decrease in nanoparticles size.

 5.Structure of the quantum dot

Quantum dots generally consist of an inorganic semiconductor core which is surrounded by a shell and a cap which is responsible for improved solubility in aqueous buffers.

Figure shows the basic structure of quantum dots. Inorganic core is responsible for optical and semiconducting properties. For sustaining quantum confinement in quantum dots encapsulation of the quantum dot within organic surfactant is carried out. During the synthesis of quantum dots the surfactant is grown and forms ligands on the surface of core. The surface to volume ratio of the quantum dots is high as a result there is a greater number of atoms on the surface. One of the major drawbacks is that all the surface atoms are not attached to ligands and some are bare. These dangling bonds are trap on the surface as a result electron or holes which are stuck on the trap do not show fluorescence quantum yield. In 1996 Hines and Guyot-Sionnest solved this problem by giving an idea that passivate the quantum dots core by another inorganic shell before capping it with ligands. This approach increases the photoluminescence quantum yield as compared to organically capped counter parts. The quantum yield enhancement is due to the increase in confinement of electrons and holes to the pair in the core and dangling bonds on the surface of quantum dots. Shell plays a crucial role in separating the core from surrounding medium by acting as a physical barrier. The shell also reduces the surface defects by eliminating trap sites by binding up all the dangling bonds present on the surface. It has been observed that as the thickness of the shell increases there is a red shift in the threshold energy, drastic increase in non-linear absorption coefficient and independent of dielectric environment, the refractive index changes. Similar kind of change has been observed in many cases when an impurity in core shell quantum dots is moved from the core to shell center. Mostly IIVI, IV-VI and III-V semiconductors are used for making core and shell with configuration such as (CdSe) CdS, (InAs) CdSe).

Based on bandgaps and relative position of valance band and conduction band of the semiconductors involved in core shell structure, the structure of quantum dots can be described in four types.

§  Type – 1

§  Inverse type – 1

§  Type – 2

§  Inverse type – 2

In type 1, the band gap of semiconductor core material is smaller than that of the shell material and the band gap of the core completely falls in the shells band gap. The excited electrons and holes are completely confined in core region of type 1 core shell structure.

In inverse type 1, the band gap of the core is greater than that of shell, which results the band gap of shell to lie within the band gap of core. The excited electrons and holes depending on the thickness of the shells are confined partially or completely within the shell.

In type 2, valence band of the core lies in the bandgap of the shell or conduction band of the shell lies in the bandgap of core. In type 2, one of the electron or hole is confined to the core while other one is confined to the shell.

In Inverse type 2, the valence band edge of the shell lies within the bandgap of the core whereas conduction band edge of the core lies with the bandgap of shell. In this type of structure one of the charge carrier electrons or holes is delocalized in the core shell structure while the other one is confined in core.

In type 1 core/shell structure the shell enhances the optical properties of the core by passivating the surface of the core. The shell actually acts as a physical barrier and separates the core from surrounding environment. Core/shell structure are stable for long terms against photo degradation. The growth of the shell reduces the surface dangling bonds which may acts as a trap state for electrons and holes and hence reduces fluorescence. The first published report on type 1 core shell structure is CdSe/ZnS [82]. The ZnS shell increases the fluorescence quantum yield and makes the structure stable against photo bleaching. With the growth of the shell the absorption peaks shift towards red (5–10 nm) in UV-Vis absorption spectrum in wavelength of photoluminescence. The other examples of type 1 structures are CdSe/CdS, CdS/ZnS.

In reverse type 1, material with smaller bandgap is grown on the core which has larger bandgap. With the increasing thickness of the shell a significant red shift in bandgap has been observed. By growing another shell of larger bandgap on reverse type 1 structure, the quantum yield can be improved. Some of examples of reverse type 1 are ZnSe/CdSe, CdSe/ZnTe, CdS/CdSe and CdS/HgS.

Type 2 shell structure is made with an aim to have a significant red shift in emission wavelength of the quantum dots. The band alignment in this case leads to effective narrow bandgap as compared to the individual core and shell materials. By manipulating the shell thickness the emission color can be changed. The focus is to shift emission color towards mare spectral range which is quite difficult to attain from other materials. Far near infra-red emission type 2 core shell quantum dots.

have been developed. CdTe/CdSe, CdSe/ZnSe, CdSe/ZnTe are some of the examples of type 2 core shell structures. Apart from traditional semiconductor quantum dots, carbon quantum dots (CQDs) are nanomaterials with prominent fluorescence properties. CQDs consist of a carbogenic core with surface groups attached to them. Most CQDs have an amorphous core with sp2 carbon, the lattice spacing of which is same as of graphite. Presence of oxygen on most of the CQDs surface enhances its solubility in aqueous solution and can also further be functionalized as compared from other nanomaterials of carbon such as graphene and carbon nanotubes.

Protection of the quantum dots structure and enhancement of its air stability

Quantum dots are generally stored in an inert atmosphere as they degrade when exposed to air. Such a strict condition is not feasible for its wider application in industries. In orders to protect the quantum dots structure and to enhance its air stability, a number of organic and inorganic coatings have been developed. Surface coating not only provides stability but also plays an important role in tuning the properties of quantum dots. The surface modification through organic molecules is carried out by modifying the surface with a suitable functional group. Generally, an organic molecule is adsorbed on the surface of quantum dots and acts as a capping agent. The essential chemical approachability far the quantum dot is provided by the capping agent by altering the terminal groups of the ligands to the outer environment. For example, quantum dots with hydrophobic capping agent cannot be utilized in applications where aqueous solubility is required. Some of the benefits of organic capping agents over the surface of quantum dot includes simultaneous achievement of colloidal suspension and capacity to bio- conjugated the quantum dots.

7.Synthesis of the Quantum Dots

There are two main way to synthesize the quantum dots one is called top down approach and other one is bottom up approach. The schematic representation of the preparative methods of the quantum dots.

 

 Bottom-up approach: Bottom-up, or self-assembly, approaches to nanofabrication use chemical or physical forces operating at the nanoscale to assemble basic units into larger structures. A number of different self-assembly techniques have been used to synthesize the Q- dots, and they may be broadly subdivided into wet-chemical and vapor-phase methods. Microemulsion, sol-gel, competitive reaction chemistry, hot-solution decomposition, and electrochemistry are generally placed in the category of wet-chemical methods. Self-assembly of nanostructures in material grown by molecular beam epitaxy (MBE), sputtering, liquid metal ion sources, or aggregation of gaseous monomers are generally categorized under vapor-phase methods.

1)     Hot Injection method

this method is first given by Lamer and Dinegar in which they show how the production of monodispesed colloids is dependent on rapid nucleation followed by controlled growth of the existing nuclei. Using this method Bawendi and co-workers first synthesized CdS, CdSe and CdTe QDs and termed this method as hot injection method. This method involves the formation of homogeneous nuclei by rapid injection of organometallic reagents into a hot solvent. The reaction solution also contains ligands or capping agents in order to prevent agglomeration in QDs. In a typical procedure, first of all surfactant like tri-octyl

phosphine (TOPO) is degassed and dries at temperature of 300 °C in vacuum. After this, a moisture free Cd precursor and tri-n-octylphosphine selenide is prepared and injected into the hot solution while rigorous stirring. This leads to homogeneous nucleation to form QDs with subsequent growth through ostwald ripening. In ostwald ripening, the

high free energy of smaller QDs lose its mass to large size which resists and slows down the growth of smaller dots. The co-ordinating ligands, capping agents and solvent plays an important role in preventing agglomeration. Reaction time and temperature is the main parameters for regulating the size. For different sizes of QDs aliquots are taken out from the flask at regular interval of time. This method has been used for the systems of II-IV and III-V quantum dots. This process suffers loss due to high temperatures and toxicity of some organometallic precursors.

2)     Physical vapour deposition:

In physical vapour deposition (PVD), from a solid or liquid phase material is vaporized in the form of atoms or molecules and transported through vacuum or low-pressure gases in the form of vapour i.e. taking plasma to that substrate where condensation occurs. The most commonly used PVD technique for industrial application is sputtering. In sputtering, a target material is bombarded with gaseous ions under high voltage acceleration. Due to momentum transfer from the incoming particle atoms and molecules are ejected from the target. For example SiGe, Si and CdSe/CdTe quantum dots prepared through physical vapour deposition.

3)     Sol-gel process

In this procedure a sol (colloidal solution) is prepared using a metal precursor in acidic or basic medium. The formation of sol is followed by the formation of gel which has both liquid and solid phase whose morphologies ranges from discrete particles to polymer chains. The steps involved in this procedure are hydrolysis, condensation (sol) and growth (gel formation). This method has been used for the synthesis of CdS, CdSe, CdTe, ZnO, TiO₂ quantum dots.

4)     Hydrothermal Method

This is a process of synthesizing single crystals, depending on solubility of aqueous solution in hot water at high temperature and pressure. The temperature should be monitored during crystallization process. At higher temperature, solvent dissolves whereas at lower cooler temperature nanoparticle growth occurs. The hydrothermal process occurs in special equipment: Hydrothermal Autoclave Reactor: It is a type of cylindrical vessel designed in such a way that it can withstand high temperature and pressure. Material of the autoclave needs to be resistant to solvent. Most commonly use autoclave is made up of Teflon which can withstand temperature of 250 °C easily. In order to avoid corrosive effect between solvent and interior material of autoclave, special protecting coatings are applied. This method has been used for preparation of many quantum dots such as carbon, graphene, CdSe, ZnS and MoS₂ etc.

Ø Top-down approach:

Top-down approach involves the breaking down of the bulk material into nanosized structures or particles. Top-down approaches are inherently simpler and depend either on removal or division of bulk material or on miniaturization of bulk fabrication processes to produce the desired structure with appropriate properties. Electron Beam Lithography, Laser Ablation, and Wet Chemical Etching are commonly used methods for the synthesis of quantum dots. In order to get a controlled shape and size of QDs, a series of experiments on quantum confinement effect is required. These methods of top-down approach suffer drawbacks of structural defects and incorporation of impurities during synthesis process.

In the top-down approaches, a bulk semiconductor is thinned to form the Q-dots. Electron beam lithography, reactive-ion etching and/or wet chemical etching are commonly used to achieve Q-dots of diameter ~30 nm. Controlled shapes and sizes with the desired packing geometries are achievable for systematic experiments on quantum confinement effect. Alternatively, focused ion or laser beams have also been used to fabricate arrays of zero-dimension dots. Major drawbacks with these processes include incorporation of impurities into the Q-dots and structural imperfections by patterning. Etching, known for more than 20 years, plays a very important role in these nanofabrication processes. In dry etching, a reactive gas species is inserted into an etching chamber and a radio frequency voltage is applied to create a plasma which breaks down the gas molecules to more reactive fragments. These high kinetic energy species strike the surface and form a volatile reaction product to etch a patterned sample. When the energetic species are ions, this etching process is called reactive ion etching (RIE). With a masking pattern, selective etching of the substrate is achieved. Fabrication of GaAs/AlGaAs quantum structures as small as 40 nm has been reported using RIE with a mixture of boron trichloride and argon [183]. This RIE process has been used to produce close-packed arrays for testing of lasing in Q-dot semiconductors. Close packed arrays of ZnTe Q-dots with interdot distance of 180 nm to 360 nm were produced by RIE using CH₄ and H₂. Focused ion beam (FIB) techniques also offer the possibility of fabricating Q-dots with extremely high lateral precision. Highly focused beams from a molten metal source (e.g., Ga, Au/Si, Au/Si/Be, or Pd/As/B) may be used directly to sputter the surface of the semiconductor substrate. The shape, size and inter-particle distance of the Q-dots depend on the size of the ion beam but a minimum beam diameter of 8–20 nm has been reported for both lab and commercial systems, allowing etching of Q-dots to dimensions of <100 nm. The FIB technique can also be used to selectively deposit material from a precursor gas with a resolution of ~100 nm. Scanning ion beam images (analogous to scanning electron microscope images) can be developed by ion beam nanofabrication at the desired, predetermined locations with high resolution. However, this is a slow, low throughput process employing expensive equipment that leaves residual surface damage. Another method to achieve patterns with Q-dots dimensions is the use of electron beam lithography followed by etching or lift-off processes. This approach offers a high degree of flexibility in the design of nanostructured systems. Any shape of Q-dots, wires, or rings with precise separation and periodicity may be realized with this technique. This method was successfully employed for the synthesis of III-V and II-VI Q-dots with particle sizes as small as 30 nm.

1)     Electron Beam Lithography

In this method, a focused electron beam is scanned on a surface covered of electron sensitive film to design a desired shape. The electron sensitive film is called resist and generally made up of polymeric compound. The solubility of the resist film is changed by electron beam which results into selective removal of either masked or exposed region of resist by immersing it into a solvent. This whole process is termed as developing process. The purpose of the developing process is to create a small structure in the resist which can be further transferred to substrate material through etching. A high degree of flexibility in designing nanostructures such as quantum dots, quantum wells and nanorods can be made by this technique. As small as 30nm of particle size of III-V and II-VI group QDs can be employed by this technique.

2)     Etching Technique

In nanofabrication, etching process plays an important role. In dry etching, a reactive gas is taken in an etching chamber then with the help of controllable rf frequency voltage, a plasma is generated in the chamber. These plasma breaks down the molecules of the gas into more reactive fragments. These high energy species strike the patterned sample in order to etch the desired area. When ions are taken as energetic species, the etching process is termed as reactive ion etching (RIE). For selective etching of substrate masking is carried out. ZnTe quantum dots were produced by RIE using CH₄ and H₂ as reactive gases.

3)     Laser Ablation

When a beam of laser is focused on the surface of a solid target material in an ambient media, the temperature of irradiated portion gradually increases which results into vaporization of target material. The collision between evaporated species (atoms/clusters) and surrounding molecules generates electrons and holes forming laser induced plasma plumes. The size of the plasma plumes depends on the ambient media, pressure and laser intensity. In general, low pressure gas is preferable for creating large plumes, which generates smaller particles. The laser generated particles may easily form oxides and complexes with surrounding media, so to prevent it the ambient media should carefully be chosen. Coagulation of nanoparticles after formation is also critical issue which can be reduced by working in low pressure media. In order to acquire a desired shape and size of the quantum dots, laser source chosen plays an important role. The evaporation rate of the target material on which laser is irradiated is determined by the laser parameters such as laser source, wavelength, pulse and frequency.

8.Properties of the quantum dots

 

Ø Application of Quantum Dots in Bioimaging Applications

Currently, magnetic resonance imaging (MRI), optical imaging, and nuclear imaging are emerging as key imaging techniques in biological systems. They differ in terms of sensitivity, resolution, complexity, acquisition time and operational cost. However, these techniques are complementary to each other most of the times. Recently much focus is given on using quantum dots as optical imaging tools for biological systems. Some of the advantages of quantum dots over traditional organic dyes are as-

Organic dyes have narrow excitation spectrum it means that dyes can only be excited with a particular wavelength of light whereas quantum dots can be excited by a range of wavelength making them ideal for multiplexed imaging.

Quantum yield of quantum dots is higher and its saturation intensity in aqueous conditions is larger as compared from organic fluorophores. This makes quantum dots much brighter probes for imaging.

Organic dyes are unstable on exposure to light which results short observation time in bioimaging whereas quantum dots are photostable even on continuous exposure of light hence can be utilized for long term monitoring.

Q-dots fluorescence-based bioimaging can be broadly classified into four types of modes: intensity, spectrum, lifetime and time-gated. All of these modes can be used at the same time for multimodality imaging. Generally, a high QY from Q-dots is required for intensity-based imaging. On the other hand, their narrow emitting spectra make Q-dots suitable for multiple colours imaging. The longer fluorescence lifetime of Q-dots compared with that of tissue avoid the noise from autofluorescence. Therefore, there is an advantage to use both lifetime and time-gated modes simultaneously. PL from Q-dots has been a widely used tool in biology.

The use of NIR photons is promising for biomedical imaging in living tissue due to longer attenuation distances and lack of autofluorescence in the IR region. This technology often requires exogenous contrast agents with combinations of hydrodynamic diameter, absorption, QY and stability that are not possible with conventional organic dye.

 

Ø Quantum dot LASER  

Quantum dot laser is a semiconductor laser that uses quantum dots as the active laser medium in its light emitting region. Due to the tight confinement of charge carriers in quantum dots, they exhibit an electronic structure similar to atoms. Improvements in modulation bandwidth, lasing threshold, relative intensity noise, linewidth enhancement factor and temperature insensitivity have all been observed. The quantum dot active region may also be engineered to operate at different wavelengths by varying dot size and composition. This allows quantum dot lasers to be fabricated to operate at wavelengths previously not possible using semiconductor laser technology.

 

Ø Quantum dots in photocatalytic application

quantum dot based composite catalysts are a good option for the production of photocatalytic hydrogen. The reason for considering quantum dot role on photocatalytic hydrogen production is due to its exceptional properties such as absorption of light in visible region, multiple exciton generation and better charge transport and separation properties. Photocatalytic applications of CdSe, CdTe, CdS and TiO₂ have been reported by many groups. Outstanding photocatalytic performance has been observed in visible light in active

CdTe/CdSe core shell quantum dots, Carbon quantum dots have also been extensively used in a photocatalysis because of its chemical stability, low toxicity and water solubility. The performance of the photocatalytic efficiency mainly depends on separated and transport efficiency of the photo-generated electrons and holes. However, recombination occurs due to various kinds of defects. It has been demonstrated that carbon quantum dots have large capability to store electrons, thus photo-excited electrons from photocatalysts or semiconductor can be shuttled in the network of CQD which shows declined recombination at function interface.

Ø Quantum dots Sensitized Solar Cells

The use of quantum dots as a photo absorbing material in solar cell fabrication is centre of attraction. The high absorption coefficients are known to decrease the dark current which overall enhances the efficiency of solar cells. By changing the size of quantum dots bandgap of it can be tailored. This tunability of bandgap can control their absorption range. The multiple exciton generation (MEG) which has been observed in some of the quantum dots increase the charge carriers which as a result also increase the efficiency of cell.

Ø Biosensor

CQDs have been used as biosensor carriers for their high solubility in water, flexibility in surface modification, nontoxicity, excitation-dependent multicolour emission, excellent biocompatibility, good cell permeability, and high photostability. The CQDs-based biosensors can be used for visual monitoring of glucose, cellular copper, phosphate, iron, Potassium, and nuclic acid.

 

9.Conclusion

Quantum dots due to its unique properties such as bandgap tunability, broad absorption, narrow emission, multiple exciton generation and high quantum yield, can be the best alternative to organic dyes used in photovoltaics, biosensing and biomedical applications. Various synthesis techniques such as Laser ablation, molecular beam epitaxy, chemical vapour deposition and chemical ablation have been used for synthesizing QDs. Carbon QDs shows fluorescent properties and can be used as a fluorescent probe in an optical sensor for detection of metal ions and other analytes. Enhancement in fluorescent properties of QDs by surface modification is the thrust area of research for its application in sensing and bio imaging.