Semiconductor Nanoparticles for Environmental Applications PDF

Semiconductor nanoparticles for environmental applications Band structure A semiconductor type metal oxide comprises of band structure, where electrons and holes in bound state occupies the valence band while the conduction band is empty. The difference in the energies of the valence band...

Semiconductor nanoparticles for environmental applications Band structure A semiconductor type metal oxide comprises of band structure, where electrons and holes in bound state occupies the valence band while the conduction band is empty. The difference in the energies of the valence band and conduction band is called band gap, which corresponds to energies ranging between near UV and red region of the visible light. On photoexcitation of material with light of energy greater than or equal to the band gap of the materials, it leads to the transport of electrons to the conduction band and hole in the valence band. The generated electron-hole pair is bound by the Coulombic interaction into a quasi-particle referred to as exciton. The binding energy of exciton for nanoparticles of size R ≈ 1-2 nm is in the range of 50-200 meV. Because of such small binding energy of the excitonic pairs, irradiation of semiconductor type metal oxide nanoparticles with energies equivalent to the band gap or more leads to separation of electrons and holes where electrons undergo transition to conduction band and holes stay in the valence band. The particle size dependent optical and electronic property of metal oxide nanoparticles is related to size of concerned exciton. The radius of exciton can be determined from the planetary model of the Bohr hydrogen atom. This model was modified by introducing the dielectric constant of the metal oxide and reduced effective mass (m*), to give the Bohr radius of the exciton aB as ћ2 𝜀 1 1 𝑎𝐵 = (𝑚∗ 𝑚 + ∗ 𝑚 ) (1) 𝑒2 𝑒 𝑜 𝑚ℎ 𝑜 Where ћ is the reduced Planck’s constant; 𝑚𝑒∗ and 𝑚ℎ∗ are the effective masses of electrons and holes, respectively; ε corresponds to dielectric constant of metal oxide; mo is the rest mass of an electron and e corresponds to the charge of an electron. The correlation between the size and the band gap energy of semiconductor nanoparticles was explained by the Luis Brus by applying the particle-in-sphere-model and effective mass approximation to the bulk Wannier Hamiltonian. According to this approximation, the lowest eigen value of a quantum confined system is given as: ℎ2 1.8 𝑒 2 𝐸𝑔(𝑛𝑎𝑛𝑜) = 𝐸𝑔(𝑏𝑢𝑙𝑘) + − + 0.284𝐸𝑅 (2) 8 𝑚𝑜 𝑚∗ 𝑅 2 𝜀𝑅 Where Eg is the band gap energy, R is average crystallite size, m* corresponds to reduced mass of electron-hole pair, e the electron charge, ε is the bulk optical dielectric constant and ER Rydberg energy. In the above equation 2, the band gap energy of a metal oxide nanoparticle comprises of four energy terms. The first term (i.e., Eg) corresponds to ℎ2 band gap energy of the same material in bulk state. The second term, i.e., ( ) is the quantum localization term 8 𝑚𝑜 𝑚∗ 𝑅 2 1.8𝑒 2 (i.e. the kinetic energy term) and the third term, i.e., ( ) is the electrostatic energy due to coulomb attraction of the 𝜀𝑅 electron–hole pair and the last term is the size-independent Rydberg energy term and is usually small and ignored. On the basis of the size of the metal oxide semiconductor (R) and the delocalization region of the exciton (exciton radii, aB), there are two types of quantum confinement effects: 1) When R aB, known as weak confinement region, here the size of the nanoparticles are smaller than the exciton radius. Here, the columbic interaction between the electron and holes are significant in comparison to the confinement energies. The electronic band structure of the nanoparticles remains the same as in the bulk crystal, while the changes due to decrease in size treated as mere perturbations. Depending on the dimension of the confinement, nanoparticles can further be classified as quantum well (material is confined in one direction, exciton can move freely in two directions), quantum wire (material is reduced in two directions and the excitons can move freely in one direction) and quantum dot (material is reduced in all directions and exciton cannot move in any direction). Conduction band Ebulk Eg Valence band Bulk Decrease in particle size Quantum Confinement Fig.1.1. Schematic representation of quantum confinement effect. Energy bands structure of metal oxide split into discrete, quantized levels with the increase in the band gap with decreasing particle size It has been reported that indirect band gap show better photocatalytic activity as compare to direct band gap metal oxides. As in the case of anatase, rutile and brookite forms of TiO2, anatase has an indirect band gap of 3.2 eV, whereas rutile and brookite have direct band gap of 3.0 eV and 3.3 eV, respectively. For rutile and brookite, an photogenerated electron in conduction band can directly combine with the holes in the valence band by radiative recombination. However, in indirect band gap anatase the direct recombination of photogenerated electron with hole is not possible, thus increasing the electron-hole lifetime as compare to rutile and brookite. This results in more availability of photogenerated charge carriers for carrying out photocatalytic degradation, therefore resulting in better photocatalytic performance of anatase as compare to rutile and brookite. 7.3. Dynamics of charge carriers (electrons and holes) After photoexcitation, the electrons in the conduction band can either de-excite to valence band via several possible pathways or the photoexcited electrons diffuse to the surface of the metal oxide nanoparticles and can undergo inter-particle charge transfer process. In the case of de-excitation of electron from conduction band to the valence band, the photoexcited electrons can undergo radiative recombination process (path-1), non-radiative recombination (path 2); trapped at an intermediate energy level lying in between the band gap; recombination from the trapped state transition (path 3); non-radiative recombination from the trapped state (path-4) or diffusion to the nanoparticles surface to carry out redox reaction (path 5) as revealed in Fig. 1.3. Conduction band (5) Reduction Reaction h (4) O2 + (3) (2) (1) (4) (3) Oxidation Reaction Valence band (5) H2O + OH Fig. 1.3. Photochemical pathways available to the photoexcited charge carriers of metal oxide nanoparticles The trapped states often correspond to defect sites which can capture electrons or holes, and subsequently migrate to the surface of the nanoparticles. The migration of the charge carriers to the surface of the nanoparticles is attributed to diffusion process. There is a marked difference in the order of time factor in the diffusion time (diff) and the radiative recombination time ( rec) of the charge carriers. The separation of the charge carriers improve with the decrease in particle size. For TiO2 nanoparticles of size 10 nm the radiative recombination time ( rec) is 100 ns whereas the time for migration of an electron to the surface of nanoparticles (diff) is approximately 10 ps. Because of this difference in the time scale, the migration of electrons to the surface of TiO 2 nanoparticles is favoured over recombination with holes. Sensing Applications of Quantum Dots Interaction with Light Absorption of photons whose energy is equal to or greater than the band gap energy of semiconductor quantum dots would lead to optical transition due to excitation of electrons from valance band to conduction band (Figure 1.2). Absorption of photon with energy greater than the band gap leads to excitation above the conduction band edge and electrons can loss excess energy by non-radiative processes (Figure 1.2). Therefore, semiconductor quantum dots can absorb broad spectrum of light which gives rise to a broad absorption spectrum. Though band gap increases with decrease in the particle size of the quantum dots, but the absorption co-efficient increases.48 This suggests higher absorption coefficient for smaller sized quantum dots.49 The excitation of electrons to conduction band is an unstable state and hence the photoexcited electrons in conduction band relaxes in various pathways (as shown in Figure 1.2). It is found that the photoluminescence of semiconductor quantum dots depends on their particle size, energy and electronic states.50 Therefore, photoluminescence can be varied by engineering the size, shape of this particles and also depends on the building precursors.51 When quantum dots absorb photons with energies greater than band gap then the photoexcited electrons in higher energy state relaxes in a two-stage processes. In first step, photoexcited electrons from higher states of conduction band relaxes to the conduction band minima by non-radiative process. The holes may relax to the valence band maxima by phonon emission and then the charge carriers can relax across the band gap by radiative and non-radiative process. The photoluminescence phenomenon occurs due to radiative recombination, while quenching of the photoluminescence would occur when non-radiative process is dominant. Besides, presence of shallow and deep trap levels between the band gap produces red shift in the photoluminescence peak position. This shallow trap levels are present near the bands (Figure 1.2). While deep trap states are present in the mid of the forbidden energy gap and are not very relevant to photoluminescence.52 The trap levels originated from the surface defects and impurities in the materials complicate its photoluminescence property. These surface defects can be originated from dangling bonds of the surface atoms, crystal defects, impurities present. 53 Therefore, the surface of the quantum dots must be well passivated in order to enhance the photoluminescence efficiency or quantum yield. Otherwise charge carriers could be trapped in the surface states and might relax by non- radiative recombination and hence diminish photoluminescence quantum yield. Again, there are direct band gap and indirect band gap semiconductor materials. It is noted that the photoluminescence is favorable in the case of direct band gap materials owing to spontaneous emission as the conduction band minima and valance band maxima are in the same momentum. In case of indirect band gap, the conduction band minima and valance band maxima are not in the same momentum. Therefore, during photon emission a phonon must be created to conserve the momentum and recombination often release the energy as phonons rather than photons. This process reduces the radiative recombination and dominates the non-radiative process. Thus, indirect band gap semiconductor material shows lower quantum efficiency. 54 In other words, it may be remarked that direct band gap semiconductor quantum dots are highly efficient optical materials in terms of photoluminescence property. (1) Excitation upon absorption of a photon (2) Carrier relaxation to form exciton (3) Radiative decay to emit a photon (4) Non-radiative decay (5) Both carriers trapped by surface traps (6) Emission from trap state Figure 1.2 Scheme of the basic processes involved in a photoexcited QDs Photoluminescence Quantum Efficiency and Lifetime The photoluminescence quantum yield (PLQY) and photoluminescence (PL) lifetime are important characteristics of a photoluminescent nanomaterial. PLQY is defined as the ratio of photons emitted to photon absorbed and this value is mostly higher for semiconductor quantum dots. Any decrease in the PLQY indicates increase in the non-radiative process or loss of charge carriers in trap sites. PL lifetime is the characteristic time that a photoexcited electron spends before returning to the ground state by emitting a photon. It gives an absolute (independent of concentration) measure and allows a dynamic picture of the photoluminescence to be obtained. The PL lifetime is also very important factor that accounts for the interaction of photoexcited electrons with the species at the outer surface of the quantum dots. Such interaction is non-radiative in nature and it corresponds to PL quenching. Surface of a semiconductor quantum dot plays important role in determining PLQY and PL lifetime. An increase in surface defects can reduce the PLQY and photoluminescence lifetime. Improvement in the PLQY and photoluminescence lifetime could be affected by the coating of the shells, surface capping organic molecules, synthetic condition, experimental condition such as temperature and pH. This PLQY and PL lifetime are also related to radiative and non-radiative lifetimes, as well as radiative and non-radiative decay rates. In addition, photobleaching is an important property, which is related to the stability of the quantum dots against exposure to light energy. The photobleaching phenomenon is severe in organic fluorophores due to which real time imaging using organic fluorophore is a challenge in monitoring biological activity. However, semiconductor quantum dots exhibit resistance to photobleaching as compared to organic fluorophore, which is an advantage in sensing applications. These excellent thermal and photostability of semiconductor quantum dots over organic dyes is mainly governed by the accessibility of the core surface. This depends on the well surface passivation of semiconductor quantum dots by ligand as well as shielding of their core materials with the inorganic surface layers. 1.1.2. Types of Semiconductor Quantum Dots They are broadly classified into following types: (a) Core-Type Quantum Dots Uniform, single-component materials which consists of a crystalline core coated with organic outside layer as capping agent. Example includes: Cadmium chalcogenides: The first colloidal material synthesized and thoroughly investigated was CdS quantum dots and most successful example of the cadmium chalcogenide material is CdTe and CdSe quantum dots due to its unique optical properties. Zinc chalcogenides: These are alternate to toxic cadmium-based semiconductor quantum dots. The most well studied materials are ZnSe and ZnS quantum dots. However, the optical properties are not as efficient as the cadmium counterparts. Though ZnTe are also being developed, but they exhibit poor photoluminescence quantum yield and are not quite stable in ambient condition. The other single component-based quantum dots are InP, InAS, PbS and PbSe which could be synthesized using colloidal and aqueous route. The Pb based quantum dots are not very demanding in sensing application owing to their very narrow band gap corresponding to NIR region. (b) Core-Shell Type Quantum Dots Two component system where combination of two different nanocrystals is brought together into a single nanostructure, where one of the nanocrystal acts as core and the other is a shell. Such an architecture of a nanostructure is named as core-shell quantum dots. Here, external shell improves the PLQY and stability of nanocrystals, as compared to its corresponding single component quantum dots as core. The core-shell quantum dots are categorized in three different forms: type-I, inverted type-I, type-II and quasi-type II depending on the positions of their valence band (VB), conduction band (CB) and charge carrier delocalization (Figure 1.3). In case of type-I core-shell quantum dots, smaller band gap core materials are covered by a wider band gap shell material. Here the VB and CB of the core material lie in between the VB and CB of shell material (Figure 1.3a). Thus, both the charge carriers (electron and hole) are confined in the core structure and enhanced PLQY is achieved. Examples include CdS/ZnS, CdSe/ZnS, CdSe/CdS type-I core-shell quantum dots. For inverse type-I core-shell quantum dots, larger band gap core material is covered by a smaller band gap shell material and upon photoexcitation charge carriers are confined in the smaller band gap shell material (Figure 1.3b). This inverse type-I structure includes CdS/CdSe, ZnSe/CdSe, CdS/HgS, core-shell quantum dots In type-II core-shell quantum dots, both VB and CB energy level of core material lie above the VB and CB of shell material or vice versa (Figure 1.3c). As a result, one carrier is confined to core and another carrier confined to shell material. The recombination of the charge carriers occurs at the interface of the core and the shell. In this configuration the spatial charge separation is more favorable and electrons can easily be transferred to the surrounding environment for PL quenching. CdTe/CdSe, CdTe/CdS, CdSe/ZnTe are some examples of type-II core-shell quantum dots. In case of quasi type-II core-shell quantum dots, CB energy level of core material lies within the band gap of the shell or the VB energy level of shell is within the band gap of core (Figure 1.3d).64 InP/CdS or PbS/CdS are the examples of quasi type-II core-shell quantum dots. (a) (b) (c) (d) Figure 1.3 Schematic representation of types of core-shell QDs (a) type-I; (b) inverse type-I; (c) type-II; (d) quasi type-II (c) Alloyed Type Quantum Dots Multicomponent materials with gradient composition determine the optical and electronic property of quantum dots. In case of alloyed type quantum dots, band gap can be tuned by only varying the stoichiometric ratio of two binary quantum dots whereas the sizes of quantum dots are usually fixed. CdSxSe1-x, CdSe1-xTex, CdTexSy are some examples of alloyed type quantum dots. (d) Doped Quantum Dots Doping in semiconductor quantum dots plays an important role to improve the PLQY and stability. Doping can create the local quantum states within the band gap of quantum dots. Therefore, optical properties can be tuned by changing the amounts and positions of the dopants. Semiconductor quantum dots have been doped with different elements such as, Cr, Mn, Fe, Co, Cu, Ag, P, B, Na and Li for different applications. 1.1.3. Synthesis Methods of Semiconductor Quantum Dots Several methods have been developed for chemical synthesis of semiconductor quantum dots. A few well followed methods are discussed below: (a) Hot-Injection Organometallic Synthesis Hot injection method is one of the traditional and robust method which was developed for synthesizing quantum dots with high quantum yield and photostability. In this method specific organic solvents having high boiling point are heated and during this process organometallic precursor solution is injected. Among different types of organic solvents trioctylphosphine (TOP)/ trioctylphosphine oxide (TOPO) system are mostly used for synthesis of high quality quantum dots. But this route is extremely toxic, expensive and explosive. Therefore, inexpensive, less toxic method was developed where non-coordinating octadecane (ODE) solvent was used instead of coordinating solvent, e.g., TOPO. Paraffin liquid and oleic acid are alternately used as reaction medium instead of TOP/TOPO system. (b) Non-injection Organometallic Synthesis In hot-injection method a solution of organometallic precursors is injected rapidly into a hot mixture of organic solvents. However, maintaining the reaction temperature upon injection of precursor solution is very difficult which limits its production in large scale and shows poor reproducibility. Therefore, non-injection organometallic route is developed to produce quantum dots in large scale. In this method quantum dots are synthesized by pyrolysis of organometallic and chalcogen precursors, which can be protected by different organic capping layer such as TOP, TOPO, oleic acid, octadecyl amine and octadecane. (c) Aqueous Synthesis Quantum dots synthesized by organometallic route (hot-injection or non-injection approaches) exhibit high crystallinity, high PLQY and narrow size distribution. However, using the organic solvent in organometallic route is a serious environmental problem and the quantum dots synthesized by organometallic route require additional phase transfer steps to disperse the quantum dots in water for biological and clinical applications. Therefore, aqueous method of synthesizing quantum dots is applied as an alternative to organometallic synthesis method. Water is a much greener solvent as compared to other organic solvent and surface functionalization with water soluble ligands during synthesis is suitable for biological and environmental application. On the other hand, this aqueous method of synthesis uses lower reaction temperature which is beneficial for large scale production of quantum dots. Extensively studied aqueous synthetic method includes hydrothermal synthesis, chemical precipitation, microwave irradiation, ultrasonic treatment etc. (d) Biosynthesis Another environment friendly route is to synthesize quantum dots by biosynthesis or biomanufacturing process using the microorganisms. Especially metal sulfide quantum dots such as CdS, ZnS and PbS can be possible to synthesize by this method. Biosynthesis of metal sulfide quantum dots contains two strategies. In one strategy, metal ions and sulfide ions can enter the cell cytoplasm and can convert into nanocrystal by the intracellular enzymes. The other strategy includes extracellular synthesis route where formation of quantum dots occurs by enzymes produced on the cell membrane. 1.1.4. Disadvantages of Using Semiconductor Quantum Dots Semiconductor quantum dots exhibit several advantages against organic dyes but it also exhibits several disadvantages. For example: (i) semiconductor quantum dots show blinking effect owing to presence of surface defects where electrons and holes can be trapped and consequently PLQY deteriorates. It limits the applications of the semiconductor quantum dots; (ii) semiconductor quantum dots when entered into live cells tends to aggregate. This interferes with cellular functioning and it sometimes kill the cells in the delivering process; (iii) The toxicity of semiconductor quantum dots to cells is a major issue. Even if the coatings of quantum dots are comprised, they have been found to be cytotoxic after oxidative or photocatalytic degradation of their core coatings. Several studies have shown that quantum dots can accumulate in the kidney, spleen and liver and it is still unknown whether such bioaccumulated quantum dots can be cleared from the body. The above concerns have given rise to the need of an alternative material to classical metal-based semiconductor quantum dots. Low toxicity, prominent biocompatibility, high photostability, good solubility, colorful photoluminescence, insignificant photo blinking, low cost enabled carbon based photoluminescent nanomaterial as strong competitors and potential alternatives to those toxic heavy metal-based semiconductor quantum dots that are currently in use. 1.2. Carbon Based Photoluminescent Nanomaterials Carbon is one of the essential and important chemical elements for all living organisms as it is the main building block of organic compounds. Carbon based nanomaterials are gaining significant scientific attention in the field of science and technology. These materials include carbon nanotubes (CNT), fullerenes, graphene and its derivatives graphene oxide (GO), reduced graphene oxide, nano diamonds and carbon dots (C-dots). Among these, C-dots are new rising star of carbon family with excellent photoluminescent property and have attracted much attention due to their wide range of applications as photoluminescent probes in chemical and biological sensors, catalysis, environmental monitoring and bioimaging. 1.2.1. Types of Carbon Dots Carbon dots (C-dots) are photoluminescent carbon nanomaterials with sizes less than 10 nm and have attracted the research community by their unique PL property, excellent photostability, high solubility, low toxicity, favorable biocompatibility, easily available and cheap precursors, high sensitivity and selectivity to target analytes. These nanomaterials can be classified into different categories. Though there are confusing nomenclature to identify the different category but lately these nanomaterials are systematically classified based on their structure, arrangement of carbons atoms, precursors used during synthesis and different synthesis approaches. Based on these criteria, the carbon based photoluminescent nanomaterials are named differently, e.g., graphene quantum dots (GQDs), carbon quantum dots (CQDs), carbon nanodots (CNDs) and polymer dots. The GQDs must have a π-conjugated nanosheets with sp2 carbons while CQDs contain crystalline core based on a mixture of sp2 and sp3 carbons. On the other hand, amorphous quasi-spherical nanodots with a disordered structural core are referred to in other papers as carbon nanoclusters, polymer dots and carbon nanodots (CNDs). C-dots were discovered accidentally during separation and purification of single walled carbon nanotube by Xu et al in 2004. Two years later in 2006, such carbonaceous fluorescence nanoparticles were given the name “carbon quantum dots’’ by Sun et al, who synthesized the CQDs via surface passivation of poly(propionylethyleneimine-co- ethyleneimine) and polyethylene glycol. 1.2.2. Synthesis Methods of Carbon Dots C-dots are prepared from various carbon precursors through top-down and bottom-up approaches. The top-down synthetic route includes arc discharge, laser ablation, electrochemical oxidation methods and these methods mainly break down larger sized carbon precursors (e.g., graphite, carbon nanotubes, carbon black, activated carbon and graphene oxide etc.) into smaller sized C-dots. In bottom-up approach, smaller precursors are used for synthesizing C-dots. This approach involves combustion process, pyrolytic processes, chemical oxidation approaches, microwave synthesis routes, heating, template methods, reverse micelle approaches, hydrothermal or solvothermal methods. These methods of synthesis can be used to form C-dots mostly from wide range of molecular precursors, e.g., citrate, carbohydrates, ascorbic acids, glucosamine, amino acids, saccharides, candle soot, ethylenediaminetetraacetic acid (EDTA), chitosan, etc. However, the formation of C-dots from these precursors comprises of four steps: dehydration, polymerization, carbonization and passivation. After the synthesis unreacted precursors, side products and large carbon particles exists along with the C-dots. Therefore, suitable purification process is required to get the C-dots with enhanced PL property. 1.2.3. Properties of Carbon Dots (a) Optical Properties C-dots prepared by different synthetic routes and from different sets of precursors exhibit different absorption behaviors in UV-vis region. Mostly, they show absorption in the range of 200-400 nm with a tail extending to the visible region (Figure 1.5). The corresponding absorption shoulders assigned to the π-π* transition and n-π* transition. Notably, the UV-vis absorption peaks of C-dots are attributed to surface functional groups and oxygen or nitrogen groups present in the core of the carbon structure. Wavelength (nm) Figure 1.5 UV-vis and PL spectra of the C-dots Absorption of photons in UV and visible region by C-dots leads to PL property. The intensity of their PL is quantitatively reflected from PLQY, which depends on precursor of carbon, synthetic approaches and post synthesis passivation. The C-dots prepared by “top-down” method exhibit a relatively lower quantum yield as compared to those prepared by the “bottom up” approaches. Besides, the surface state emission and intrinsic state emission are the two main transition routes responsible for the PL of C-dots (Figure 1.6). The surface state emission mainly depends on the carbon bonds and surface chemical groups which exhibit excitation wavelength dependent PL property. While the intrinsic state emission is associated to the carbon core which shows excitation independent PL property. Several other mechanisms are also involved to explain the complicated PL behaviors of C-dots which include surface defect state, quantum size effect, and presence of individual emitters (e.g., fluorophore molecules and emitter functional group) on the surface of C-dots. Each of the above can sometimes successfully explain the PL property of some C-dots and may fail to explain for other C-dots. For example, some C-dots do not exhibit size dependent property, excitation- wavelength dependent PL property and sometimes more than two mechanisms are required to explain the PL property. As C-dots are nanoparticles of carbons, it contains numerous surface defects. These surface defects are responsible for generation of PL for some C-dots. Surface functionalization and passivation have an impact to stabilize the surface defects which can facilitate the more effective radiative recombination of surface confined electrons and holes. Thus, brighter photoluminescence was observed. Figure 1.6 Various mechanisms involved to explain the photoluminescence behavior of carbon dots For some C-dots quantum confinement effect is successful to explain the PL property. PL of C-dots originates from the transition between lowest un-occupied molecular orbital (LUMO) to the highest occupied molecular orbital (HOMO). The HOMO-LUMO gap depends on the size of C-dots which is proven through theoretical calculation. When size of C-dots increases PL peak undergoes a red shift and this phenomenon is mostly observed for C-dots prepared from top-down method. However the size dependent property always not leads to a red shift in their PL property. In this case sp2 carbon core is taken into consideration and when the size of core is small, energy of PL is high. But C-dots with amorphous core is found to exhibit an inverse trend. PL of C-dots also originates due to electron- hole recombination in the π- π* energy levels of sp2 carbon clusters embedded within a sp3 matrix. Increase in the amount of sp2 domains led to increase the PL intensity of C-dots. Such large conjugated sp2 domains are mostly obtained for C-dots prepared from hydrothermal method. PL property of C-dots sometimes depend on the zigzag edge sites that are carbene-like with a triplet ground state. These edge structures of C-dots significantly affect their electronic structure. Here carbene centers were stabilized at zig-zag sites through σ-π coupling owing to localization of π electrons. Sometimes, PL of C-dots is observed from assembled individual emitters e.g., fluorophore molecule or emitter functionalized groups which are located on the C-dots surface. This resulted multiple emitting sites and tunable broad emission could be observed (Figure 1.6) Excitation Dependent PL Property Excitation dependent PL is one of the important properties of C-dots (Figure 1.7), which makes them distinct from other photoluminescent material. This excitation dependent PL behavior arises from particles of different sizes in the sample and presence of multiple emissive sites within one and same C-dot. This type of PL property depends on the reaction conditions and the precursors used for the synthesis. Excitation independent property was also observed for some C-dots due to presence of uniform sizes and emissive sites. Therefore, by altering their surfaces, shapes and carbonizing degree both excitation dependent and independent PL can be observed.  Figure 1.7 Excitation dependent PL of C-dots135 Wavelength (nm) Photoluminescent Quantum Yield (PLQY) and PL Lifetime The PL properties of C-dots can be characterized by its quantum yield which is depend on the carbon precursors used and synthesis procedure. Hydrothermal and microwave method of synthesis mostly produced C-dots with high PLQY. Besides, C-dots show good resistance to photobleaching. PL lifetime of C-dots depends on the nature of photoluminescent sites and the environment. Graphitic core and surface traps are also responsible for the PL lifetime. 1.3. Sensing Applications of Photoluminescent Nanomaterials ` The ability to modify photoluminescent properties of semiconductor quantum dots and carbon dots by analyte of interest has been utilized for developing detection strategy. Here the concerned nanomaterials are called optical probe. The desired properties of an optical probe are: linear response over wide concentration range, high sensitivity, specific detection, quick response time, hydrophilic nature and response to real sample analysis. The small size, large surface area, various functionalized groups make them very reactive and sensitive to the surrounding environment which leads to enhancement (turn-on) and quenching (turn-off) of photoluminescence. Here enhancement or quenching of PL is utilized as the analytical response signal. The detection mechanism mainly includes dynamic quenching, static quenching, photoinduced electron transfer (PET), ligand displacement by analyte ions, cation exchange, aggregation, fluorescence resonance energy transfer (FRET) and inner filter effect (IFE). 1.3.1 Sensing Mechanism (a) Dynamic or Collisional Quenching and Static Quenching-Based Sensing Collisional quenching occurs when photoexcited photoluminescent material is deactivated upon contact with other species present in the solution. Such species are called quencher, which are in fact the analytes of interest. A wide variety of quenchers are available like heavy metal ions, electron deficient molecules etc. In addition, photoluminescent material produces non-fluorescent complexes with quenchers. This process is called static quenching since it occurs in the ground state and does not depend on diffusion or molecular quenching. (b) Surface Interaction-Based Sensing The nature of surface ligands has a tremendous effect on the type and selectivity of the detection. This includes: (i) Transfer of photoexcited electrons from excited state to the acceptor molecules i.e., analytes, which lead to PL quenching. This quenching mechanism is known as photoinduced electron transfer (PET) mechanism. (ii) The surface ligand on the QDs (say, capping agents) is displaced by the analyte due to stronger affinity between the analytes and ligand than the QD and ligand. This leads to agglomeration of QDs and consequently the PL is quenched. (iii) The PL of the QDs is first quenched by a PL quencher and “OFF’’ state is generated. After that when analyte is introduced then quencher molecule binds to the analyte ion and quenching is disabled and PL of the QDs is restored to an “ON’’ state again. (c) Inner Filter Effect (IFE)- Based Sensing Inner filter effect is a well-known phenomenon attributed to PL quenching. It results from the absorption of excitation and/or emission light by absorbers (mainly quencher) in the detection system. Here, absorption bands of the absorbers should possess overlap region with the excitation and emission band of the photoluminescent material (QDs or C-dots). The degree of PL quenching can vary by changing the absorbers concentration, making the IFE based methods applicable. (d) Fluorescence Resonance Energy Transfer-Based Sensing (FRET) FRET involves the non-radiative energy transfer between molecules, which happens due to long-range dipole–dipole interactions between a donor (D) molecule in the excited state and an acceptor (A) molecule in the ground state. This process involves the simultaneous quenching of the donor fluorescence and electronic excitation of the acceptor. According to Forster's theory, the rate of energy transfer depends upon the following factors: the extent of spectral overlap between the donor emission and the acceptor UV-vis absorption, the quantum yield of the donor, the relative orientation of the donor and acceptor transition dipoles, and the distance between the donor and acceptor molecules.