2D Materials-Based Quantum Dots: Gateway Towards Next-Generation Optical Devices

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This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi:

10.1002/adom.201700257.

Article type: Review

2D–Materials-Based Quantum Dots: Gateway Towards Next-Generation Optical Devices

Sathish Chander Dhanabalan, Balaji Dhanabalan, Joice Sophia Ponraj,* Qiaoliang Bao,* and Han Zhang*

S. C. Dhanabalan, H. Zhang

SZU-NUS Collaborative Innovation Center for Optoelectronic Science and Technology, and Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, Shenzhen University, Shenzhen, China, 518060

E-mail: [email protected] B. Dhanabalan

Sardar Vallabhbhai Patel International School of Textiles & Management, Coimbatore-641004, Tamilnadu, India.

B. Dhanabalan, J. S. Ponraj

Department of Nanoscience and Technology, Bharathiar University, Coimbatore-641046, Tamilnadu, India.

E-mail: [email protected] Qiaoliang Bao

Department of Materials Science and Engineering, Monash University, Clayton, Victoria 3800, Australia

E-mail: [email protected]

Keywords: two-dimensional materials, quantum dots, graphene, transition metal dichalcogenides, topological insulators, bioimaging

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photonic technology with emphasis on future research scope to make use of these materials. We also discuss the different applications of 2D QDs emphasizing the realization of fluorescent probes which are in great demand to well-establish these materials in the healthcare sector for the betterment of mankind. This study is a key priority and will bring a great impact in the advancement of simple yet challenging 2D QDs by bringing them towards the next level of applications point-of-view similar to that of graphene.

1. Introduction

Two-dimensional (2D) materials have gained much attraction in recent years due to their interesting properties and widespread applications in the field of photonic device fabrication.[1] Researchers combine these 2D materials with graphene thereby forming hybrid materials to realize devices with enhanced properties and also to overcome the gapless nature of graphene.[2] 2D materials can be classified in a broad category: graphene family, transition metal dichalcogenides (TMDs), topological insulators (TIs) and 2D oxides. From previous research works, it was found that

zero-dimensional graphene quantum dots (GQDs) exhibit strong quantum confinement and edge effects giving rise to new physical properties with respect to 2D graphene sheets which are applicable to other QDs in 2D family.[3] Graphene family includes

graphene, graphene oxide (GO), flurographene, hexagonal boron nitride (h-BN), hexagonal boron carbon nitride (h-BCN) graphitic carbon nitride and so on.

Transition metal dichalcogenides has the general form of MX2 in which M symbolizes

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diselenide (MoSe2), molybdenum diteluride (MoTe2), tunsgsten disulphide (WS2),

tungsten diselenide (WSe2) or tungsten diteluride (WTe2). Transition metal oxides,

namely van der Waals oxides include molybdenum oxide (MoOx), vanadium

pentoxide (V2O5), ruthenium oxide (RuO2), tungsten oxide (WO3), tin oxide (SnO2)

and so on. The materials such as bismuth selenide (Bi2Se3), bismuth telluride (Bi2Te3)

and antimony telluride (Sb2Te3)form topological insulators. The monolayer of

phosphorus is named as phosphorene.

Quantum confinement in quantum dot is realized if the radius of the semiconductor crystal is comparable to the exciton Bohr radius of bulk material.[7] The infinite exciton Bohr radius observed in graphene with quantum confinement is due to the zero effective mass of charge carriers at band edges.[8] The exciton Bohr radius, a0 is

given as,[9] 2 0 2 * * 1 1 ( ) e h a e m m    (1)

where me* and mh* represents the effective mass of electron and hole, respectively, e denotes the electron charge and ε is the dielectric constant of the semiconductor that varies as a function of size. Being the first two-dimensional nanomaterial, graphene attained great interest due to its single atom thickness, transparent, flexible and strongest.[8a,10] The advantage of forming quantum dots rather than graphene sheet is that the zero band gap graphene could be modified to graphene quantum dots (GQDs) with band gap as the edges of graphene are confined to two dimensions.[11] It was also reported that the GQDs have larger carrier lifetime than graphene sheet.[11b]

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This review starts with the classification of two-dimensional materials. The different techniques adopted for the synthesis of quantum dots based on 2D materials are explained in depth. The essential applications of two-dimensional QDs in the fields of optoelectronics and biophotonics including magneto-optical devices, optoelectronic devices, photocatalyst, photothermal agents, fluorescent probes in bioimaging, up/down conversion and saturable absorbers (SA) applications is presented in the final section. We have provided the perspectives for researchers who are interested in this group of materials by emphasizing the future demand of them. Figure 1 depicts the schematic illustration of the organization of present review. Overall, the present review is intended to impart the reader a deep knowledge of 2D QDs for their challenging demands compared to other 2D nanostructures of 2D materials to be in par with the current technology with emphasis on future research scope to make use of these materials. Therefore, this review will highlight the great complement for detailed knowledge in device physics of 2D materials’ QDs. It also grants contemporary ideas for the researchers who aim to favor them in widespread applications by moving forward similar to the direction of graphene.

2. Classification of 2D-Materials-Based Quantum Dots

In the process of classifying the quantum dots based on different two-dimensional materials, we would like to organize them in five different categories namely graphene family, TMDs, transition metal oxides, TIs, and phosphorene.

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Graphene family comprises of broad class of materials including GQDs, boron nitride (BN) QDs, boron carbon nitride (BCN) QDs and carbon QDs. Carbononics is a new term coined for the research based on graphene involving its electronics, photonics and spintronics by engineering the size, shape, strain, edge and layer number by realizing artificial materials made up of carbon atoms.[13] GQDs are one- or few-layered graphene sheets having lateral dimension smaller than 100 nm which is well known for their unique photoluminescent performance.[14] The single-crystalline GQDs have exciting optical properties including strong excitonic absorption bands covering the visible region, excellent excitonic fluorescence, higher molar extinction coefficients and long-term photostability.[15] Interestingly, the size of GQDs can be tuned by cage-opening of fullerene.[16] GQDs with appropriate surface

functionalization act as effective and low-cost acceptor material for organic

photovoltaic cells and organic light emitting diodes applications.[17] The size of GQDs are smaller than that of GO with enhanced photoluminescence (PL).[18] Their

properties such as highly tunable PL, enhanced photostability, atomically-thin structure, molecular size, biocompatibility and easy functionalization make them attractive candidates for optical sensing.[19] Different carbon precursors such as fullerene, glucose, graphite or graphene oxides, carbon nanotubes and carbon ﬁbres were used in the synthesis of GQDs.[16,20] The fact is that the fullerene, graphite, carbon nanotubes and carbon ﬁbres are expensive precursors. Interestingly, graphene-based double quantum dot system realized with single-electron transport of two lateral quantum dots coupled in series for integrated quantum circuits.[21] Graphene oxide is one atomic thick layer with 2D structure demonstrating chemically reactive oxygen-groups of epoxy, ether and hydroxyl oxygen-groups on the basal plane together with

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independent PL where reduction in size of QDs (from 8 nm to 1 nm) caused the change in color of luminescence from orange–red to blue.[23]

Carbon QDs (C-QDs or CQDs) are small carbon nanoparticles (<10 nm) became more attractive owing to their strong fluorescence, rapid electron transfer and electron reservoir properties along with different levels of surface passivation which can be used as nontoxic ﬂuorescence agents in the field of in-vitro and in-vivo

bioimaging.[24] C-QDs are less toxic, environmental friendly, low cost, biocompatible and chemically inert material exhibiting similar fluorescence properties of that

semiconductor QDs and can be a better substitute.[25] Compared to the conventional semiconductor QDs, C-QDs showed bandgap independent optical absorption and fluorescence emissions.[26] Additionally, there is no such theoretically defined fluorescence color and dot size relationships in C-QDs making them better material for ultracompact QD-like fluorescence probes.[27]

As the hexagonal BN is an isoelectric analogue of graphite, it can be functionalized with lipophilic and hydrophilic amine molecules that motivate the exfoliation of layered structure leading to few-layered or mono-layered 2D sheets.[28]_BN

nanosheets are also called as “white graphene” comprises of few layers h-BN planes. Similar to that of G-QDs, the properties of BN QDs can also be tuned with the size, shape, edge and number of layers.[29] Density functional theory (DFT) calculations using QDs of graphene, BN and their hybrids performed with varying size, amount of substitution of GQD (BN QD) with BN-pairs in understanding the change in their properties showed the existence of border and edge carbon atoms to be the main cause of spin-dependent H-L gap in BN-partial-edge substitution-GQDs.[30]

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The quantum degree of freedom offers great challenges for TMD QDs in the field of quantum spintronics as information carrier by making use of their single electrons.[31]_MX

2 compounds (M=Mo, W, Nb, Ta; X= S, Se, Te) based on

monolayers consists of M atoms sandwiched between X-atoms. MX2 can result in two

crystal structures namely the trigonal prismatic (2H) phase and octahedral (1T) phase based on atomic stacking configurations. In a trigonal prismatic 2H phase, the

transition metal atom is coordinated by 6 chalcogen atoms whereas the transition metal atom demonstrates an octahedral chalcogen coordination in 1T phase.[32] The overall symmetry of TMDs can be hexagonal or rhombohedral with the metal atoms possessing octahedral or trigonal prismatic coordination.[33] In MoS2, band-gap tuning

from an indirect band gap in bulk form to a direct gap in semiconductor form can be achieved by tuning the numbers of layers and quantum confinement resulting from band gap transition in d-orbital-related interaction gives rise to photoluminescence (PL).[34] MoS2 can be an efficient catalyst when combine with semiconductors like

TiO2 and CdS.[35] It was reported that MoS2 QDs have the excitation dependent PL

with wavelengths in the range of 400–600 nm.[34c,36] The excitation dependent luminescence resulted from polydispersity of the MoS2 QD dispersions of 2 to 1000

nm in diameter and varying thickness. MoS2 QDs were dispersed in NMP where

NMP also had significant impact on the excitation dependent PL. MoS2 QDs (10–20

nm in size) have shown blue luminescent emission at 415 nm due to the quantum size effect irrespective of the excitation wavelength owing to their high homogeneity and water solubility.[37] The interesting fact is that the large-sized MoS2 (300 nm to 800

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conversion from indirect (1.2 eV in bulk) to direct (1.8 to 1.9 eV in monolayer) layer-dependent bandgap in MoS2 yield strong PL behavior (600 to 700 nm), strong

emission and giant enhancement (≈≈104_{) in PL quantum yield.[}33,38_{If we consider}

2H-MoS2, Mo atom is prismatically coordinated to six surrounding S atoms giving

rise to thermodynamically stable phase. On the other hand, six S atoms make a distorted octahedron around one Mo atom leading to metastable 1T-MoS2 phase.[32a]

Bulk WS2 has an indirect-bandgap (1.4 eV) whereas thin to monolayer has wide and

direct bandgap (2.1 eV).[40] WS2 QDs can be applied in optoelectronic devices,

semiconductor-based spintronics, conceptual valley-based electronics, bioimaging and quantum information technology.[41] Atomically thin gate defined QDs with 40

consecutive Coulomb diamonds were realized on WSe2 which helps in removing the

edge states caused by etching steps seen in gapless graphene QDs.[42]_{Srivastava et}

al.[43] reported the WSe2 QDs having energy value 20 to 100 meV lower than the two

dimensional excitonsalong with ≈≈1 meV zero-field splitting. WSe2 atomically thin

layers act as a host for quantum dot-like defects for better candidate in integrated solid-state quantum photonics, single quantum emitters and quantum information processing.[43–44]_{Gallium selenide (GaSe) with hexagonal layered structure possess}

weak van der Waals interaction between the sheets and it has three crystal structure (β, γ, and ε) determined by the way of Se-Ga-Ga-Se layer stacking on each other.[45]

2.3. Transition Metal Oxides

Molybdenum oxide (MoOx) is a n-type transition metal semiconductor with

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molybdenum disulfide (MoS2)[46] Different nanostructures such as nanosheets,[46e,47]

nanoflakes,[48] nanoribbons,[49] nanobelts,[50] nanoflowers,[51] nanorods,[46f,52] nanowires,[53] nanoplates,[46i,54] nanoparticles (NPs)[55] and quantum dots[56] were realized in molybdenum oxide. Vanadium pentoxide (V2O5) QDs–graphene hybrid

nanocomposite, ruthenium (IV) oxide (RuO2) QDs@V2O5 and TiO2 QDs–graphene

nanosheets were adopted in lithium batteries.[57] As compared to bulk V2O5, the

nanostructured V2O5 improves the performance characteristics such as capacity

fading, electrical conductivity and lithium (Li) ion diffusion in the application of Li battery cathode material.[57a,58] Due to their high photocatalytic activity and high durability against oxidative damage, semiconducting metal oxide QDs have

significant advantages. Tungsten oxide (WO3) finds application in photocatalysis, gas

sensors, batteries, field emission devices, supercapacitors, pH sensors and

electrochromic devices.[59] Tin oxide (SnO2) belongs to intrinsically n-type oxide

semiconductor having a wide band gap (Eg = 3.6 eV at 300 K). The interesting

characteristics such as tailored SnO2 surface structure and increased effective

Brunauer-Emmet-Teller (BET) surface area make SnO2 as an emerging candidate in

solar cells, oxidation catalysis, transparent conductors, rechargeable Li batteries, ﬁeld-effect transistors, ﬁeld emissions and chemical gas sensing.[60]

2.4. Topological Insulators

Bismuth chalcogenides (e.g., Bi2X3, X = S, Se, Te) are interesting candidates for

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of medicine and environment owing to its status of ‘green element’.[62] Bi2S3 QDs find

application in Quantum Dot Sensitized Solar cells (QDSSC) as it is a great competitor for cadmium free solar cells because of its optimum band gap.[63] Bi2Te3 is found to be

one of the best thermoelectric materials having highest figure of merit.[64]

2.5. Phosphorene

In black phosphorus, every phosphorus atom is covalently bonded to three neighboring atoms within the phosphorene layer composing a puckered honeycomb structure to motivate highly anisotropic thermal, electrical conductivities and optical responses.[65] Black phosphorus (BP) has thickness dependent tunable direct bandgap of 0.3-1.5 eV, on/off ratio (104_-105_{) and high}

carrier mobility (upto 104 cm2 V–1 s–1).[65c,66] Theoretical studies based on density functional theory showed that owing to quantum confinement effect, the diameter of black phosphorus quantum dots (BPQDs) could be inversely proportional to the electronic gap and the absorption gap.[65a] The anomalous size dependence (range of 0.8 to 1.8 nm) leads to blue-shift when the size of QDs increases and structural distortion resulting from the excited state relaxation caused huge Stokes shift in small BPQDs.

3. Fabrication of 2D QDs

It is necessary to realize a better synthesis method for the production of subnano-sized QDs in broad ranges of the target materials as the quantum size effect of

semiconducting metal oxide QDs become incomparably below 1 nm because of their relatively small Bohr exciton radius.[6a,67] Fabrication methods form the foundation stone for the device fabrication and therefore, they are very important to be

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dimensional quantum dots along with their application are presented in Table 1 and

Table 2.

3.1. Wet Chemical Synthesis Methods

The solvent based techniques include one-pot method,[56] liquid phase exfoliation (LPE),[5,38b,68] chemical exfoliation with ion intercalation,[29a,69] hydrothermal

method,[15,57a,b] solvothermal method,[59e,70] simple colloid process,[59f] microemulsion method,[71] high temperature inorganic synthesis,[72] electrochemical process,[14b,73] ultrasonication,[74] laser ablation,[75] microwave methods,[14b,76] pyrolysis and carbonization,[77] microfluidization,[78] photoreduction[79] and solution chemistry approach.[80]

3.1.1. One-Pot Method

The one-pot method is found to be less time-consuming and can be performed at room temperature (RT) without an external stimulus. GQDs were reported to be synthesized by one-pot combined with hydrothermal method.[81]_{Kuo et al.}[81a]_{produced hydrophilic}

N-doped GQDs and hydrophobic N-N-doped GQDs by simple, green route one-pot hydrothermal method. The first step was ultrasonication of graphite powder and pyridinium tribromine (PyBr3) contained aqueous solution for 1 h to attain exfoliated monolayer graphene sheets.

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synthesize MoS2 QDs/Graphene/TiO2 (MGT) composite under atmospheric pressure using

Na2MoO4·2H2O, thiocarbamide, P25 powder and graphene oxide as precursors. MoS2 QDs

were not formed in the composite without graphene indicating significance of graphene in the synthesis of MoS2 QDs. One-pot synthesis is also adopted for the green synthesis of carbon

quantum dots to incorporate the different steps of synthesis, surface passivation and functionalization into a single step with glycerol as carbon source with the help of microwave-assisted reaction.[83]

The advantages such as high stability, high reactivity, quantum size effect and improved surface effects attracts the synthesis of MoOx nanomaterials. The facile synthesis of MoOx

QDs can be done by one-pot method which is simple, rapid and environment friendly. Figure

2 A,B presents the steps involved in the one-pot synthesis of MoOx QDs from MoS2 powder

and hydrogen peroxide where hydrogen peroxide supplement excess oxygen for improving oxidation state of molybdenum. Xiao et al.[46c,56] reported the synthesis of highly photoluminescent MoOx via one-pot method by employing commercial molybdenum

disulfide (MoS2) powder and hydrogen peroxide (H2O2) as the precursor and oxidant,

respectively for two different applications. The role of H2O2 is to supplement excess oxygen

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photoluminescence intensity and Pi concentration within the range of 0.1–160.0 µM having the detection limit of 56 nM (3σ/k).[46c] The second application was aimed at constructing the accurate, reproducible and precise turn-off sensor for 2,4,6-Trinitrotoluene (TNT) detection in river water.[56]_{It was reported that the MoO}

x QDs-based sensor exhibited high selectivity

for TNT compared with its analogues in which TNT in river water samples might be detected readily without sample pretreatment processes. One can achieve MoOx QDs with absolute

fluorescence quantum yield of ≈2.55% and strong photoluminescence comparable to that of graphene quantum dots and MoS2 QDs.[77,85] MoOx QDs obtained by one-pot synthesis are

environment friendly and biocompatible which find its application as photoluminescent probe in various fields of chemi- and bio-sensors, environmental monitoring, biomedical and cell imaging.[46c,56]

3.1.2. Liquid Phase Exfoliation

The liquid phase exfoliation method mainly lies on the outcome of low-intensity ultrasonication with appropriate solvents, which involves the sonication of layered materials for few hours in the presence of solvents in an ultrasonic bath so that the respective nanosheets are separated by further centrifugation. This method has been widely employed in the synthesis of various 2D materials. One can achieve tunable PL in GQDs by means of edge functionality manipulation during the synthesis process with the parameters involved. Single- and multi-layered GQDs obtained simultaneously from XC-72 carbon black precursor via chemical oxidation in nitric acid were reported as best probes for cellular imaging (single-layer GQDs) with effective cell-penetrating efﬁciency without any bioconjugation and as optoelectronic devices (multi-layer GQDs).[86]_{The main advantages of chemical oxidation}

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involve easily available carbon source. Tyramine-functionalized GQDs using carbon black precursor for fluorometric sensing platform in order to detect a spectrum of metabolites and multiparametric blood analysis (glucose, cholesterol, L-lactate, and xanthine) was also reported.[19a]_{This metabolite profiling technique could be a great compliment in the}

diagnosis, study, and management of diabetes, obesity, lactic acidosis, gout, hypertension and so on. The synthetic scheme for liquid exfoliation of GQDs and GOQDs is given in Figure 3 (A) where the red dots correspond to oxygenous sites. Figure 3B–E portrays GQDs and GOQDs having diameter <4 nm demonstrating a highly crystalline monolayer structure with the lattice parameter value of ≈0.24 nm.[68c]

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conjugated GQDs were applied for speciﬁc labeling and dynamic tracking of insulin receptors in 3T3-L1 adipocytes in which apelin (signaling peptide in various cell types of adipocytes) improved the internalization and recycling of insulin receptors in adipocytes and it is an evident for the molecular mechanisms involved in controlling these cytokines in insulin sensitivity. The combination of ultrasonication and solvothermal method was followed for the synthesis of GQDs having tunable surface chemistry (increasing oxidation degree).[89] Moreover, degree of surface oxidation, fine solubility, high stability and applicable up-conversion PL could play significant role in tuning the fluorescence from blue to green.

Reports were found for the liquid synthesis of EDA-carbon dots (EDA is 2,2’-(ethylenedioxy)bis(ethylamine)) where the synthesis was performed by refluxing and centrifugation.[26,90] Meziani et al.[90] proposed photoinduced bactericidal functions i.e., visible/natural light bactericidal agents of EDA passivated C-QDs using Escherichia coli cells towards efficient bacteria destruction when illuminated with visible light even at ambient room lighting conditions can be found in Figure 3I,J. Atomic force microscopy (AFM) and TEM results showed the diameter of EDA- C-QDs to be 5 nm as depicted in Figure 3K. EDA passivated C-QDs doped with gold metal by photolysis of visible-light photoirradiation of the dots in HAuCl4 were also studied for ultracompact green ﬂuorescent probes in protein

analysis.[26] Štengl et al.[68b] synthesized BN QDs and BCN QDs from their respective nanosheets by refluxing in ethylene glycol at atmospheric pressure for a period of 48 hours. Ultrasonication forms an interesting route for the exfoliation of bulk layered materials down to their monolayer form in large-scale.

Recently, the wet grinding of pristine MoS2 in N-methyl-2-pyrrolidone (NMP) succeeded by

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solvents is reported in the synthesis of MoS2 QDs.[5] The significant role of grinding is that

the produced pure shear forces will be helpful for isolating multilayers of MoS2 whereas the

sonication process chisels down larger crystallites into smaller crystallites. Interestingly, the solvents NMP and DCB were found to be appropriate solvents in exfoliating together with the stable dispersion of 2D materials and better dispersion of TMDs could be achieved by solvents with either the surface tension of 40 mN m–1 to 50 mN m–1 or the surface energy of ≈70 mN m–1.[91] Gopalakrishnan et al.[34c] used the liquid exfoliation technique with a combination of bath sonication and probe sonication for the formation of heterodimensional nanostructures of MoS2 quantum dots interspersed in few-layered sheets of MoS2 which finds

application in hydrogen evolution reaction (HER) as electrodes. Figure 4A,B shows the schematic diagram of the growth process along with their TEM images. The final product will be easily separated from the solvent when it is post-treated by less polar organic solvent (chloroform) due to its precipitation. Here, the breakdown of bulk MoS2 into smaller particles

occurs because of the hydrodynamic forces emerging from elevated pressure and temperature at the time of the bath sonication.

An alternative method of sonication combined with solvothermal method was also employed to achieve MoS2/WS2 QDs for HER applications due to their enhanced exposed active edges

and in vitro cell imaging as they were less toxic with strong fluorescence using N,N-dimethylformamide (DMF), NMP, dimethylimidazolidinone (DMEU), DI, ethanol and acetone as organic solvents.[68a] Their experiments showed that DMF, NMP and DMEU were found to be good solvents and DI, ethanol and acetone were found to be poor solvents based on their role in the synthesis process. The strong fluorescence under ultraviolet (UV) light was observed in MoS2/WS2 QDs in contrary to the nanosheets and the strongly exfoliated

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≈2.5 nm, respectively in which different size obtained were attributed to the interior property of MoS2 and WS2 cells. HRTEM images demonstrated highly paralleled and ordered lattice

fringe confirming well-crystallized nature of both QDs. The d-spacing of MoS2 QDs of 0.23

nm corresponds to (103) faces of MoS2 crystals and d-spacing of WS2 QDs of 0.27 nm

correlates to (101) faces of WS2 crystals. The aureole found in selected area electron

diffraction (SAED) patterns affirms polycrystallinity of these QDs entirely different from that of bulk MoS2 and WS2. Stengl et al.[36] synthesized strongly luminescent MoS2 QDs from

natural mineral molybdenite using power ultrasound in a pressurized reactor assisted liquid exfoliation. The exposure of the liquid to ultrasonication cause the propagation of sound waves leading to alternating high-pressure and low pressure cycles based on the frequency of the electric generator. The bubbles formed during low-pressure cycle attain a critical size and collapse during the high-pressure cycle causing high pressures, high temperatures and fast liquid jets which is called as cavitation.[92] Therefore, the reaction conditions such as shape of sonotrode (ultrasonic probe, horn), density of solvent, ultrasound intensity, pressure and so on should be carefully selected for the success of the process to favor the process of delamination rather than grinding or milling.

Zhao et al.[93] reported the synthesis of WS2 nanodots (NDs) using surfactant-mediated

liquid-phase exfoliation with bath sonication technique for HER application. The various positions of the vessel will suffer distinct forces driving the production of nanodots with different sizes. The exfoliation or surface energy of WS2 estimated from computational

studies was greater than 250 mJ m–2 _{and if the surface energy of the solvent is relatively}

closer to the exfoliated nanodots, maximum dispersion of the exfoliated WS2 can be

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repulsive term to overcome the problem of sudden re-aggregation caused by the large surface energy. Moreover, the NDs can be balanced with the help of electrostatic repulsion as determined by zeta potential which vary from –7.3 to –6.6 mV after centrifugation symbolizing the charge suppression on the nanodot surface.[95]_{Liquid exfoliation can also be}

an eco-friendly method based on ultrasonication combined with hydrothermal using hexadecyltrimethylammonium bromide (CTAB) for the synthesis of WS2 QDs.[96] CTAB acts

as surfactant for reducing the exfoliation time thereby enhancing the efficiency of exfoliation in the formation of WS2 nanosheets. The resulting WS2 QDs exhibits narrow size, solubility

in water with stable fluorescence. Štengl et al.[97] exfoliated single-layer WS2 sheets by

ultrasonication which was further sonicated in ethylene glycol to achieve WS2 QDs. BPQDs

with lateral size of 2.6 nm were also achieved with the aid of liquid exfoliation method with the combination of probe and bath sonication which is depicted in Figure 4I–N along with their structural and morphological studies.[98] HRTEM studies depict the lattice fringes of 0.34 nm attributed to (021) plane of BP crystal. The statistical TEM and AFM analysis from 100 BPQDs showed average lateral size of 2.6±1.8 nm and average thickness of 1.5±0.6 nm indicating the presence of a stack of 2±1 quintuple layers of BP.

3.1.3. Chemical Exfoliation with Ion Intercalation

The production of GQDs by overcoming the technical difficulties of conventional methods such as high production yield, low oxidation and large scale synthesis could be achieved by means of chemical exfoliation with ion intercalation.[8a,11a,16,20d,80b,99] Multi-walled carbon nanotubes (MWCNT) and graphite flakes (GFs) were used in the synthesis of water–soluble luminescent GQDs by exfoliation and disintegration.[99]_{Figure 5A–Ddepicts the formation}

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intercalation compounds and thier UV/Vis absorption studies showed that newly- electronic band gaps (4.96–4.86 eV) were opened. The high reactivity of potassium–graphite intercalation compounds (K-GICs) was utilized in the GQDs synthesis by intercalating K atoms between the covalently-bonded graphene sheets in MWCNTs, thanks to the weak van der Waals force. K-GICs react with EtOH in inert atmosphere to generate hydrogen gas whereas exposure of K-GICs in air gives rise to violent combustion causing remarkable defects on the walls of graphene. Furthermore, EtOH–H2O reaction with K-GICs upon

ultrasonication exfoliates and disintegrates MWCNTs walls resulting in monolayered GQDs due to the breaking of C–C/C=C bonds.

Lin et al.[29a] fabricated monolayered BN QDs (lateral size of ≈10 nm) by exfoliation from h-BN flakes and these QDs could be applied as a non-toxic fluorescent label in the field of confocal microscopy of biological cells. Three main steps are associated with the synthesis of BN QDs: (i) formation of potassium intercalated hBN (K-hBN) where the K atoms were intercalated inbetween the covalently bonded BN nanosheets due to the presence of weak van der Waals force, (ii) exposure of K-hBN to air for a short period of time to achieve the de-intercalation reaction and (iii) reaction between K-hBN and EtOH-H2O by ultrasonication

resulting in exfoliation and disintegration of the hBN flakes edges. The above-mentioned research by Lin et al.[29a] showed that a direct bandgap (6.51 eV) of BN QDs greater than pure bulk BN was achieved owing to their quantum confinement effects. Figure 5E,F represents AFM and TEM studies of BN QDs that showed the size to be ≈10 nm, clear terminated edges with lattice fringe of 2.15 Å, B–N bond length of 1.443 Å and interlayer separations of 5 to 8 Å in hexagonal honeycomb BN structures confirming the formation of monolayered stacked BN QDs.

Chemical exfoliation with Li intercalation is the simple technique in the synthesis of MoS2

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monolayer quantum dots having relatively smaller size.[34a,69] It was reported that this method is adopted for the preparation of MoS2 QDs and the detailed schematic is given in Figure 5G

where the repeated exfoliation leads to fragile MoS2 and at the end of third exfoliation, large

quantity of monolayer MoS2 QDs were produced.[69] The toxic pyrophoric n-butyllithium

used in Li intercalation could be effectively replaced by lithium halides for better results.[100] Lin et al.[41a] synthesized monolayered WS2 QDs for nontoxic fluorescent label in bioimaging

by exfoliating in different steps as: (i) potassium (W) intercalation reaction carried out by intercalating K atoms between the WS2 sheets (K-WS2) using the weak van der Waals force,

(ii) deintercalation reaction by exposing K-WS2 to air for a limited period of time and (iii)

ultrasonication. The two important parameters namely large layer-layer distance (d = 6.18 Å) and feeble van der Waals force holding WS2 layers enhanced the process of intercalation.

TEM images in Figure 5H depicts good crystallinity, clear terminated edges (zigzag edges) as determined by fast Fourier transform (FFT), bond length of W-S (≈2.35 Å) of 2H-WS, the lattice fringe of (102) with ≈2.45 Å and hexagonal lattice structures. Both trilayered WS2

having layer distances of ≈0.62 nm (lattice fringe of 002) and stacked monolayered WS2

QDs having layer distances >1 nm were also demonstrated. The monolayered WS2 QDs

realized was abounded with the interesting properties such as large direct transition energy (3.16 eV) in comparison with the WS2 sheets (2.1 eV), improved PL (quantum yield ≈4%) in

the bluegreen specra at RT and giant spin-valley coupling (≈570 meV) large as compared to monolayered WS2 sheets (≈400 meV).[41a]

3.1.4. Hydro/Solvothermal Method

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Kim et al.[101] studied the GQDs for their size-dependent shape/edge-state variations in which there was a strong correlation seen between the visible PL and size

dependency. The most common shapes for a given size (d) of GQDs in percentage (p) are given in Figure 6C where the average sizes (da) were estimated from HRTEM

images at each ‘d’ given in parentheses at the bottom of the figure. The produced circular and elliptical GQDs were of ≈5 and ≈12 nm average sized at d = 5 and 10 nm, respectively in which circular GQDs occupy more than 50% of the total number of GQDs at each “d”. There was a consistent strategy with the size and shape of GQDs as follows: (i) At d = 15 nm, circular GQDs disappeared leaving behind elliptical GQDs with ≈1/3 of them deformed, (ii) At d = 20 nm, majority of GQDs were hexagonal shaped with ≈1/4 of them deformed with curved sides, (iii) At d = 25 nm, the most of GQDs were hexagon-shaped with minor irregular shaped GQDs and (iv) At d = 35 nm, most of GQDs were parallelogram-type rectangles shaped with rounded vertices. GQDs can be obtained by cutting down graphene oxide via solvothermal method.[14b]_{Pan et al.}[20d,102]_{presented GQDs through hydrothermal}

cutting method which is organized in three steps (i) thermal reduction of GO sheets into chemically derived graphene sheets (GSs), (ii) oxidization of GSs in concentrated H2SO4 and HNO3 and (iii) hydrothermal deoxidization of oxidized GSs at low

temperature (200–300 °C). The carboxylic (COOH) groups at edge or hole sites together with epoxy (C–O–C) and carbonyl (C=O) groups at basal plane sites were generated during the process of oxidation leading to the formation of lines to enclose 3.6 nm sp2_{clusters. Hydrothermal reaction unzipped the linear chains which were}

removed later leaving behind the stable carboxylic groups to form GQDs

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1,4phenylenevinylene) polymer from GSs by hydrothermal approach which plays a major role in organic photovoltaics (OPV) and organic light emitting diodes (OLEDs) compared with GSs blended conjugated polymers.

A novel graphene nanostructure edge-terminated by a primary amine i.e., amino-functionalized GQDs (af-GQDs) was realized by Tetsuka et al.[103] to tune the optical properties thereby altering electronic structure in a standardized manner by orbital resonance of amine moieties with graphene core. The oxidized graphene sheets (OGSs) were used as precursors in amino-hydrothermal treatment (70 to 150 °C) using ammonia to attain af-GQDs having violet to yellow fluorescence by selecting sp2 domains mediated by a bond scission of surrounding oxygen groups. A primary amine was formed by the reaction of ammonia with epoxy groups and alcohols resulting from nucleophilic substitution leading to self-limited extraction of sp2

domains (ring-opening of epoxide) along with the direct bonding of primary amine to the graphene edge. Moreover, the color of PL can be tuned by the degree of amine functionalization which can be achieved with the variation of ammonia’s initial concentration and the change in reaction temperature. Low reaction temperature favors the amine functionalization and high temperature (>120 °C) cause dissociation of primary amine due to nucleophilic reactions thereby lowering the C/N ratio.[103] A different approach that combined laser fragmentation and post-hydrothermal

treatment to attain bright blue PL GQDs was realized in incorporating into

heterojunction hybrid solar cell to improve power conversion efficiency.[104]_Figure

6A,B shows the scanning electron microscopy (SEM) and HRTEM images of GQDs. The target solution was black in color and laser irradiation for few minutes converted it into light yellow color that ﬁnally changed to dark yellow. It describes the

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three layers which on further laser ablation (few minutes) were cut down to small fragments as seen in Figure 6A. TEM studies confirmed the diameters distribution of GQDs (3.5−6.0 nm) confirming the mono-dispersity with average diameter of 4.8 nm (35%) and highly crystallinity with lattice parameter of 0.319 nm where zigzag orientation dominated their edges.

Interestingly, hydrothermally synthesized GQDs using citric acid (CA) as carbon precursor and ethylene diamine as nitrogen source to obtain N-doped GQDs with the assistance of N-containing bases such as urea (U) exhibiting excitation-independent blue emission with single exponential lifetime decay was demonstrated.[105] The hydrothermal process encouraged the amide evolution among –NH2 and –COOH

along with the amine in which pyrrolic N in the graphene framework was formed by means of intramoleculur dehydrolysis taking place between neighboring amide and COOH groups. The synthesis of single-crystalline hydroxyl-functionalized GQDs (OH-GQDs) was carried out by inexpensive green hydrothermal reaction (water-phase molecular fusion) with nitration of pyrene and hydrothermal process in alkaline aqueous solutions for further size tuning and functionalization (–OH, –NH2, and –

NHNH).[15] The significance of this single-step method in correlation with

conventional multi-step organic-phase fusion is that it could be greener, simpler, cost-effective, milder and higher yield. Wang et al.[106] synthesized GQDs from reoxidized graphene oxide where fluorescence was quenched by Cu(II) in water to realize

ﬂuorescence sensor for the detection of Cu2+_{ions compared to other metal ions}

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doped GQDs (N:GQDs) from citric acid as the C source, urea as N source and thiourea as S source showed broad absorption band in the visible region due to the doping with nitrogen and sulfur.[107] N:GQDs and S, N:GQDs presented excitation wavelength independent PL behaviors under UV excitation with single exponential decays (τ = 7.6 (N:GQDs) and 12.8 ns ( S, N:GQDs)).

Solvothermally prepared biocompatible N:GQDs were obtained using

dimethylformamide as a solvent and nitrogen source exhibiting higher two-photon absorption cross-section of 48000 Göppert-Mayer units and penetration depth in tissue phantom can reach imaging depth of 1800 μm.[70d] GQDs obtained from solvothermal method reported PL quantum yield as high as 11.4% confirming the strong ﬂuorescence (green PL) and they could be more stable with high dissolution in most of the polar solvents without any chemical modiﬁcation.[70c] Liu et al.[108]

reported GQDs obtained by solvothermal method as better candidate for micro-supercapacitor. Zhu et al.[109] synthesized GQDs and tuned their luminescence from green to blue by modification or reduction process in which the chemical structure alters thereby suppressing non-radiative recombination of localized electron-hole pairs. Firstly, GQDs were prepared by a two-step solvothermal and separation method having high oxygen content with epoxy and -OH on molecular plane, –CON(CH3)2

and –COOH on edge. Secondly, these active chemical groups supplement them the ability to be modified with organic molecules or polymers for improving the surface chemistry in order to tune their properties. GQDs with controlled surface chemistry could be attained either by chemical modification (m-GQDs) with alkylamine coupling within a series of reactions or by NaBH4 reduction (r-GQDs) that caused

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The significant characteristics of C-QDs include photoinduced electron transfer, upconversion luminescence, high photocatalytic activity, structural stability under visible light and electron reservoir properties makes them perfect candidates kindling the researchers towards their synthesis by different means.[110]_{The solvothermal}

treatment of orange juice in ethanol was adopted to achieve carbon nanodots having thermal quenching of luminescence behavior where the quenching was tuned by annealing process.[111] The as-synthesized carbon nanodots presented positive thermal quenching and negative thermal quenching of emission was observed in annealed carbon nanodots at 200 °C with respect to increase in measurement temperature. Zhu et al.[112] synthesized carbon dots having particle diameters of 2 to 6 nm using

hydrothermal approach using citric acid and ethylenediamine. The C-QDs/MoS2

composite synthesized by Zhao et al.[24b] exhibited enhanced HER activity with Tafel slope of 45 mV/decade and long term stability in H2SO4 owing to its high charge

transfer efficiency. Ming et al.[73b] reported nanohybrid of TiO2/C-dots having

improved photocatalytic activity under visible-light. Figure 8A represents the

experimental-setup required for the synthesis of TiO2/C-dots where the anode graphite

rod corroded on continuous stirring for 120 h resulting in dark-yellow solution containing C-dots (See Figure 8B). The digital image of aqueous C-dots exhibited in Figure 8C showed weak brown color and dynamic light scattering (DLS) histogram given in Figure 8D confirmed well dispersed C-dots (3 to 6 nm). Figure 8E presents TEM image of C-dots explained the diameters of C-dots (≈4.5 nm) and HRTEM image in Figure 8F details the crystal lattice spacing of 0.321 nm which is in good agreement with graphitic carbon (002) lattice planes. XRD pattern of the nanohybrids represents anatase phase TiO2 where well-separated diffraction spots in SAED pattern

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structure. Ultraviolet photoelectron spectroscopy (UPS) spectra to understand the binding energy (BE) edges of different samples explained that the BE of TiO2/C-dots

(17.24 eV) is higher than TiO2 (17.20 eV) and C-dots (17.22 eV). The possible

mechanism for the better photocatalytic performance of the TiO2, C-dots, TiO2/C-dots

and P25 for photodegradation is also described with schematic diagram. Report was also available related to the synthesis of C-QDs/Ag3PO4 composites having enhanced

photocatalytic activity.[110]

Owing to its low toxicity and two-photon fluorescence (TPF) ability, MoS2 QDs

synthesized with the combination of ultrasonication and ethanol-thermal treatment was successfully applied in TPF bio-imaging.[70a] These QDs exhibit bright blue fluorescence and the TPF mechanism arises from the two photons coming to QDs merge their energies to stimulate ground state MoS2 to an excited state advancing

further with the normal fluorescence-emission pathway. Schematic of hydrothermal formation process of MoS2 QDs/CdS nanoﬂower is presented in Figure 6 (F). The

high crystallinity, monodispersed and uniform nanorod structure having length of 100 nm and lateral size of 10 nm shown in Figure 6 (G) confirmed the formation of hierarchical structured nanosphere from collective nanorods of ≈300 nm in length and ≈20 nm in width.35a The water-soluble molybdenum disulfide quantum dots were obtained by hydrothermal method towards the realization of novel fluorescence sensor for Hyaluronidase (HAase).[113] The structural studies from TEM image given in Figure 7B showed the well dispersed MoS2 QDs with narrow lateral size distribution

ranging from 1.5 to 4.5 nm with average diameter of ≈2.8 nm. HRTEM image confirmed the highly paralleled and ordered lattice fringe with high crystallinity of MoS2 QDs (lattice spacing of ≈0.21 nm). AFM image explained the thickness of

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selective detection of 2,4,6-trinitrophenol (TNP) in water samples and test papers using PL quenching was realized from hydrothermally synthesized-MoS2 QDs with

sodium molybdate and cysteine as precursors.[85] The growth process and TEM images is depicted in Figure 6 (D, E). The statistical analysis of TEM from 100 MoS2

QDs showed highly uniform, monodisperse with narrow distribution of 2.15 ± 0.34 nm in diameter. MoS2 QDs-treated paper displayed strong blue PL which is the

foundation for making photoluminescent paper. The word “TNP” written with TNP solution as ink on the PL paper which exposed to 365 nm UV light gives rise to darker intensity with the increase in TNP concentration. Water-soluble monolayer MoS2 QDs having zigzag-terminated edge and hexagonal lattice structure was

achieved with ammonium molybdate, thiourea as precursors and N-acetyl-L-cysteine (NAC) as the capping agent in hydrothermal approach (See Figure 6 (H)).[114 The as-prepared MoS2 QDs exhibit better dispersity with more stability in aqueous

suspension and have direct bandgap of 3.96 eV in correlation with the monolayer MoS2 nanosheets which is 1.89 eV. Owing to the two successive transfers of energy

from the NIR absorption created by the NAC capping agent to the hexagonal structure of MoS2 QDs, the interesting upconversion photoluminescence was noticed which

makes them an effective fluorescent reagent for detecting tetracycline hydrochloride under UV and NIR irradiation.[114]

Recent studies on single-step solvothermal approach for obtaining MoS2 QDs

modified with different elements such as Fe, Mg and Li was reported and the results showed that Fe and Mg act as dopants while Li act as intercalant where FexMo1-xS2

QD was identified as the best multifunctional MoS2 QDs among others.[115] Reports

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method.[70b] The thickness-dependent bandgap plays an important role in broadband SAs with respect to different thickness of BP could operate on ﬁber lasers and solid-state lasers in the range of 1 to 3 μm. Our research group has stabilized the growth of BPQDs (2.1 ± 0.9) by facile solvothermal approach using NMP solution which could be a great candidate for saturable absorber in Er-doped fiber laser for the generation of 1.08 ps pulse duration centered at a wavelength of 1567.5 nm.[116] Figure 7A depicts the detailed growth procedure in which BP crystals were grinded into BP powders and added into a flask with saturated NaOH NMP solution under vigorous stirring for 6 h at 140 °C in nitrogen atmosphere. The use of NaOH NMP solution in the growth process helped to overcome the sensitivity of BP to water and oxygen with easy oxidation under visible light and hence, the reaction was also carried out in N2

atmosphere.

The environmental friendly materials realized by hydrothermal method have attracted attention in providing solutions for global energy related issues towards lithium-ion batteries. By combining RuO2 QDs and vanadium pentoxide, bowknot-like RuO2

QDs@V2O5 (named as RQDV) was synthesized by hydrothermal method.[57b] RuO2

QDs were reported to have high electronic conductivity, short electron transport pathways and high-speed Li+ permeation caused by its unique nanostructure as understood from the schematic shown in Figure 6I and the dispersion of RuO2 QDs in

V2O5 further improve the electronic conductivity.[117] Figure 6J details the HRTEM

image of annealed RQDV in which lattice spacing of 2.61 Å indexed to (301) planes of orthorhombic V2O5 and lattice spacing of 2.79 Å indexed to the (111) planes of the

RuO2. This RQDV cathode showed improved cycling stability (160 mA h g–1, 87%

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solution method was reported for the construction of V2O5 QD/graphene hybrid

(VQDG) nanocomposite from reduced graphene oxide (rGO) suspension and vanadium solution by controlled nucleation and growth process.[58] V2O5 QDs

supplement large number of active sites with short Li+ transfer distance and attached well on rGO to reduce the surface stress at the time of charging and discharging. The resulting VQDG demonstrated high capacity (245 mA h g–1) and stable cycling performance (300 cycles with 89% capacity retention). Epifani et al.[59e] adopted solvothermal method with tungsten chloroalkoxide and oleic acid as precursors to synthesize monoclinic WO3 QDs towards the realization of gas sensors with improved

response of oxidizing (nitrogen oxide) and reducing (ethanol) concentration from 1 to 5 ppm, 100 to 500 ppm at 100 °C and 200 °C. Oleic acid can also be replaced by oleylamine or n-dodecanol. Xu et al.[12] have grown SnO2 QDs with diameter of

≈≈0.5 to 2.5 nm by means of solvothermal method and generated a variety of porous structures with the help of the as-prepared SnO2 QDs as basic building blocks. Figure 7C–E gives the structural studies of SnO2 QDs where self-assembled 2D

patterns of SnO2 QDs as building blocks were made on silicon-wafer surfaces. The

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adjacent pores in single strings varied (D-string = 589 nm for distorted hexagonal pattern, 617 nm for square pattern).

3.1.5. Simple Colloid Process

Cong et al.[59f] reported the synthesis of WO3 QDs via simple colloid process

with tungsten aryloxide (W(OC6H5)6) as precursor which could be a promising

candidate for pseudocapacitor and electrochromic application. A surface layer of octylamine ligand was used to stabilize the QDs in which the exchange of it to pyridine supplements remarkable hydrophilicity and conductivity towards achieving high-performance electrochemical behavior. It was found that they exhibit fast ion transport in charging and discharging QDs electrodes having remarkable

electrochromic performance. Figure 7F–H presents the illustration of growth and characterization of tungsten oxide QDs with aliphatic amines anchoring on the

surface. The monodispersed nearly spherical shaped crystals with average diameter of 1.6 nm were confirmed and the dispersion exhibited a uniform pale-blue color. Figure 7H demonstrates clear fringes in each individual nanocrystal indicating

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by subtracting the band gap from VB maximum, yielding −0.51 and 0.5 eV for QDs and bulk materials, respectively. The ultrasmall dimensions of the QDs resulting in strong quantum confinement effect yield upward shit of CB minimum, downward shift of VB maximum and resultant widened band gap.

Ramanery et al.[61a] reported the studies of Bi2S3 QDs with chitosan as the capping

ligands by means of “green” aqueous colloidal process at RT and ambient pressure. The chitosan polymer ligand helped in nucleating and stabilizing Bi2S3 QDs with the

following interesting reasons to be used widely: (i) exclusive chemical,

physicochemical and biological properties, (ii) bountiful raw materials available for production and (iii) non-toxic, biocompatible and eco-friendly.[118] It was also reported that Bi2S3 QDs can be functionalized with O-carboxymethyl chitosan

(O-CMC) as capping ligands using single-step aqueous method at RT for fluorescent core-shell nanoprobe application.[119] The layered Bi2Se3 with direct band gap of 0.35

eV can lead to stable colloidal QDs by dissolving Bi2Se3 powder in acetonitrile as the

bonding between nearby Bi2Se3 layers is weak van der Waals.[120] Therefore, the

solvent molecules are able to pass through the van-der-Waals layers resulting in intercalation compounds with the convenient organic molecules so that the crystal is broken up into small clusters.

3.1.6. High Temperature Inorganic Synthesis

This modified high temperature inorganic synthesis method was adopted for the synthesis of gallium telluride (GaTe) quantum dots using GaMe3, trioctyl phosphine

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prepared from 12.5 mL of TOP added with 1.579 g of selenium and

TOP/TOPO/TOPSe mixture was subjected to 278 °C which further stabilized at 266 to 268 °C. In the conventional synthesis, 45 small amounts of octyl phosphonic acid (OPA) were seen in the trioctyl phosphine which is a common impurity in TOP and TOPO. During the reaction process, the degradation of OPA leads to the formation of phosphonic anhydrides and the presence of OPA in considerable quantity will ligate the particle edges. Hence, the modified method employed TOP/TOPO mixtures without OPA causing the elimination of phosphonic anhydrides.

3.1.7. Electrochemical Process

The electrochemical method could be a reliable and reproducible process in the production of GQDs which could be stored for months in water after synthesis.[11a] N-doping in GQDs (N:GQDs) can efficiently tune the intrinsic properties. Therefore, N:GQDs with oxygen-rich functional groups realized with electrochemical approach were found to be electrocatalytically active and emit blue luminescence.[73c]_In

addition to this, N:GQDs supported by 2D graphene sheets could be a better metal-free electrocatalysts for oxygen reduction reaction than commercial Pt/C electrodes, N-doped CNTs, and N-doped graphene sheets. Ananthanarayanan et al.[121] presented high yield electrochemical exfoliation of GQDs from chemical vapor deposition (CVD) grown three dimensional (3D) graphene towards the sensitive detection of ferric ions in which the ionic liquid 1-butyl-3-methylimidazolium

hexafluorophosphate (BMIMPF6) electrolyte act as wide electrochemical potential

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incorporated in stem cells to understand the mechanism of their development and tissue regeneration.[122] Figure 8G presents the proposed mechanism of strong yellow luminescent GQDs which starts with the electrochemical oxidative cleavage of graphite anode followed by the reduction and modiﬁcation of graphene with

hydrazine. The O and OH radicals resulting from anodic oxidation of water acted as electrochemical ‘‘scissors’’ for cutting down carbon nanocrystals to form oxygenated groups. The hydrazine hydrate is involved in purification and filtration of GQDs to a certain size range. The interesting feature of this work is that stabilizer-free GQDs demonstrated strong photoluminescence, good photostability, easy penetration into stem cells and low cytotoxicity. Green luminescent size-tunable GQDs used electrochemical approach by making use of MWCNTs demonstrated quantum efficiency of 6.3% and 5.1% where PL could be tuned by changing the size with a systematic variation of diameter of CNT, electric field, temperature and electrolyte concentration.[20f] Figure 8H depicts two-step process for the electrochemical transformation of MWCNTs to GQDs and it occurs because of Li/propylene

carbonate complexes intercalation to yield size-tunable GQDs by means of exfoliation of oxidized MWCNTs.

The eco-friendly electrochemical approach is effective for the large-scale synthesis of pure C-nanodots for high up-conversion photoluminescence applications which is nothing but the emission at smaller wavelength compared to that of excitation

wavelength.[14b,73b_{The significance of this study is that it used only pure water for the}

green synthesis process rather than any other chemicals.[24b] Alkali-assisted

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processed C-QDs showed size-dependent PL with upconversion luminescence

towards the realization of photocatalysts from the nanocomposites of TiO2/CQDs and

SiO2/CQDs synthesized using sol-gel method.[123] Figure 8L demonstrates

characterization of C-QDs. TEM image showed the uniform monodisperse CQDs with diameters of 4 nm. Figure 8L1 depicts the corresponding fluorescent microscope images of CQDs: blue, green, yellow and brown where different colors i.e., different emission might be caused by graphite sheets of different size, symmetry, and defects. Figure 8L2–L7 presents HRTEM images of CQDs with different diameters

irrespective of similar lattice spacing (0.32 nm) in good agreement with the <002> spacing of graphitic carbon. The optical images of CQDs having four typical sizes illuminated under white light (left: daylight lamp) and UV light (right: 365 nm, center) in which bright blue, green, yellow and red PL was strong enough so that it can be observed with the naked eye. CQDs absorb visible light when TiO2/CQDs or

SiO2/CQDs nanocomposite photocatalyst is illuminated and consequently emit

shorter wavelength light (325 to 425 nm) due to upconversion so that TiO2 or SiO2 is

further excited resulting in electron/hole pairs. The active oxygen radicals formed from the reaction of electron/hole pairs and adsorbed oxidants/reducers (O2/OH) that

further cause degradation of the dyes. The relative position of the CQDs band edge of the attached CQDs to the surface of TiO2 or SiO2, allows the transfer of electrons

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Gopalakrishnan et al.[73a] reported a single-step electrochemical etching process for the synthesis of MoS2 QDs from bulk MoS2 pellets, 1- butyl-3-methylimidazolium

chloride [BMIm]Cl and lithium bis-trifluoromethylsulphonylimide (LiTFSI) placed in a two-electrode cell (1 cm diameter separated by 1 cm) in different concentrations (0.1, 1 and 5 wt.%) of aqueous LiTFSI or [BMIm]Cl to be realized for HER

application due to their excellent electrocatalytic activity. HRTEM images of MoS2

QDs as depicted in Figure 8M,N showed average particle size of 2.5 nm and 4.6 nm obtained from aq. LiTFSI electrolyte whereas larger particles of dimension 2.8 nm and 5.8 nm resulted from aq. [BMIm]Cl. The size distribution control could be tuned by the applied DC voltage and electrolyte composition. Therefore, this method is interesting as it uses environmentally benign ionic liquids for generating oxygen and hydroxyl free radicals in the production of MoS2 QDs exhibiting large number of

active edge sites to be utilized in quantum information systems, nanoelectronics and energy conversion.

Photo-Fenton reaction using UV irradiation was used for the etching of graphene oxide leading to the formation of GQD in which inclusion of Fe2+_{and H}

2O make the

over-fact reaction that can be slowed down by Fenton reagent.[125] The realization of covalently assembled GQDs/Au electrode enriched with periphery carboxylic groups via photo-Fenton reaction using cysteamine as a cross-linker for H2O2 detection

which is the most common representatives of reactive oxygen species in biological system could be made possible because of the high peroxidase-like activity and small lateral size of GQDs.[126] The obtained GQDs possess greater peroxidase activity compared to the micrometer-sized GO sheets.

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addition causing increasing oxidation activity along with the formation of powerful hydroxyl radical to be applied in treating pollutants in the environment.[127] Electro-Fenton reaction of MoS2 (or graphene oxide) nanosheets was performed in

electrolytic cell equipped with an electrochemical workstation connected by a three-electrode system.[3b] Schematic diagram and further studies of TEM, HRTEM, high-angle annular dark-field scanning TEM (HAADF-STEM) and AFM is presented in Figure 8J–L. The entire reaction was monitored by TEM and the reaction time was controlled by handling degree of etching and concomitant porosity to get MoS2

nanosheets and nanoporous MoS2 nanosheets. TEM image depicted uniform

distribution of MoS2 QDs without agglomeration and HRTEM image explained the

length and width of MoS2 QDs as 11.5 and 5.3 nm, respectively, confirming different

morphology of zero-dimensional MoS2-QDs with respect to other one-dimensional

nanomaterials (nanoribbon, nanotube, and nanowire). The electrolytic cell contains 50 mL mixed solution of 0.3 mg mL–1 exfoliated MoS2 (or GO) nanosheets, 0.05 M

FeSO4 with pH of 3, applied potential of –0.5 V and the solution was stirred

continuously (500 rpm) with O2 and Fe2+ as major pre-Fenton reagents. The main

process monitored from transmission electron microscopy showed that MoS2

nanosheets were etched due to hydroxyl radicals obtained from rapid homogeneous fracturing of nanosheets into QDs with the aid of transition of nanoporous

morphology after 60 min of electro-Fenton reaction.

3.1.8. Ultrasonication Process

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size-dependent and pH-dependent PL GQDs were synthesized by cup sonication method from different types of coal including anthracite (‘a’), bituminous coal (‘b’) and coke (‘c’) leading to the realization of unique coal structure exhibiting advantage over pure sp2_{-carbon allotropes could be applied in bioimaging, biomedicine,}

photovoltaics and optoelectronics. 128 Figure 9A–D describes the synthesis and characterization of GQDs from coal as source. SEM image in Figure 9B explains that ground bituminous coal and anthracite have irregular size and shape distributions compared to that of regular spherical shaped coke. TEM image depicts the uniformly distributed sizes and shapes (2.96 ± 0.96 nm in diameter). The hexagonal lattice noticed in FFT images confirms the crystalline hexagonal structures. AFM image explains the heights as 1.5 to 3 nm revealing the two to four layers of graphene oxide-like structures. GQDs were obtained from carbon fiber ultrasonicated in acidic media followed by the reaction for 12 h at 95 ± 5 °C and citric acid added was stirred for 1 h for introducing carboxyl groups on the surface of the GQDs.[129]

Jin et al.[130] realized amine-functionalized GQDs via two-step cutting process from GO where the PL of GQDs were shifted (red-shift) because of the following factors; (i) charge transfer between the functional groups and GQDs thereby tuning the bandgap (good agreement with DFT studies) and (ii) protonation or deprotonation of the functional groups caused by pH change. The epoxy chain were formed from the epoxy groups seen on the plane of GO which on oxidation lead to the formation of carbonyl group and further reduction process resulted in decrease in the size of oxidized GOs. Due to the removal of bridging O atoms found in the remaining epoxy chains of oxidized GOs, cutting down of graphene happened by the reduction of N2H4. The reaction of oxidized GOs with diamine terminated polyethylene glycol

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group-functionalized GQDs by means of ring-opening reaction of the epoxy groups on GOs. Zhuo et al.[74f] proposed ultrasonic method for the synthesis of GQDs having excitation-independent downconversion and upconversion PL.

Son et al.[74e]_{demonstrated white-light-emitting diode derived from the combination}

of QD with other emissive materials from the hybrid nanostructures of ZnO core wrapped in a shell of single-layer graphene (ZnO-GQDs). From Figure 9E–H,

graphite oxide was first prepared by surface treatment of natural graphite powder with H2SO4 and HNO3. Zinc acetate dihydrate and GO were then mixed in DMF resulting

in embryo ZnO QDs that grows further to ≈10.6 nm in diameter. Two chemical reactions occur at the outermost edges of embryo ZnO QDs. Furthermore, theoretical DFT calculations showed the existence of additional peaks emerged by the splitting of lowest unoccupied orbitals of the graphene into three orbitals with distinct energy levels. Two possible reaction that could occur at the outermost edges of the embryo ZnO QDs: (i) Zn2+ ions chemisorbed on the embryo ZnO QDs (Zn2+ (ZnO)) react with GO functional groups resulting in Zn–O–C bonding and (ii) Zn2+ ions bonded on GO (Zn2+_{(GO)) also form Zn–O bonding and then combine with embryo ZnO QDs}

that lead to chemical exfoliation process of GO. They have used the word ‘quasi’ as the ZnO QDs were not completely encircled by graphene. XRD patterns showed broad (002) and (100) diffraction peaks of graphene along with (100), (002), (101) and (102) peaks of ZnO confirming the ZnO–graphene quasi-core–shell QDs contain graphene and ZnO QDs. Raman spectra showed E2 (high) phonon frequency at

around 436 cm–1 for ZnO QDs. The peak at 475 cm–1 attributes to E2 (high) that was

shifted by 39 cm–1 than that of ZnO QDs and A1(1LO) phonon frequency was found

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Kumar et al.[29b] studied the exfoliation of BN QDs by breaking down h-BN

nanosheets in the form of thinning or delamination via mechanical pressure with time dependent high intensity sonication for stronger reduction of QDs size. The high intensity sonication of bulk h-BN powder break and rupture BN NSs owing to the mechanical pressure occur during sonication process inside the liquid H3PO4 medium.

The mechanical waves create low-pressure and high-pressure cycles repeatedly inside liquid medium. At the time of low pressure cycle, small vacuum bubbles or voids were formed in the acid containing bulk h-BN powder and attain a desired volume so that they cannot absorb any energy which collapse violently at the subsequent high pressure cycle. Mechanical pressure effect was reported to be prominent around the edges sites and surface of thick BN NSs. We can organize this process ion three different stages: (i) Stage I or primary stage- the mechanical pressure (bold blue arrows) gets induced around side edges of thick BN NSs that aids in isolate BN powder into smaller size thin BN NSs as they were held together by weak van der Waals forces. (ii) Stage II or secondary stage- mechanical pressure given on thin BN NSs surface (transformed in stage I) makes it unstable. (iii) Stage III- thin BN NSs cannot tolerate the mechanical pressure exerted on the surface which rupture violently to breaks down into smaller sized BN NSs resulting in BN QDs. From Figure 9I–L HRTEM images taken at different sonication time, structure and shape of BN QDs are influenced by the sonication time. Figure 9I indicates HRTEM image of BN QDs obtained after 12 h of high intensity sonication showed uniformly sized homogenous QDs with average lateral sizes of 3 to 6 nm without agglomeration. Inset of Figure 9I presents crystalline structure of BN QDs with interlayer separation of 2.1 Å

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BN QDs which were non-uniform too. Figure 9J,K,L shows the HRTEM of larger sized BN QDs sonicated for 6, 9 and 10 h, respectively.

MoS2 nanodots modiﬁed TiO2 (MoS2/P25) composite was synthesized by ultrasonic

mixing of MoS2 powder with 1-vinyl-2-pyrrolidone solution (NVP) to obtain MoS2

nanodots which is further sonicated with TiO2 nanopowder (P25) and it has shown

enhanced photocatalytic degradation activity under simulated sunlight with rhodamine B (RhB) and methylene blue (MB) as the target pollutants.[74a] Tetrabutylammonium (TBA)-assisted ultrasonication was adopted for the growth of MoS2 QDs to be applied

in the field of up/down conversion in cell bioimaging and singlet oxygen (1O2)

production in photodynamic therapy (PDT) where TBA is significant for intercalation to achieve improved exfoliation by cutting down the MoS2 QDs.[74b] Figure 9M

presents the schematic of the TBA-assisted synthesis process of MoS2 QDs. The

sulfuric acid-assisted ultrasonic method was also used for the fabrication of MoS2

QDs having excellent fluorescence properties employed in bioimaging and

nanovector for intracellular microRNA detection applications.[131] The sulfuric acid used in this method can exfoliate, intercalate and cut the micro-sized MoS2 to MoS2

flakes, monolayer MoS2 flake and finally into nano-sized MoS2 QDs. One can tune

the different nanostructures such as flake, nanoporous and QDs of MoS2 with the

controlled change in ultrasonic durations. The luminescent MoS2 nanocrystals was

achieved by inexpensive and impurity free sono-chemical exfoliation method followed by centrifugation.[132]_WS

2 QDs of average size around 2.4 nm were

successfully synthesized using ultrasonication process with WS2 powder and NMP

solution under vigorous stirring for 15 h at 29 °C.[133]