Top-down quantum dot fabrication methods include laser fabrication,electrochemical methods,chemical oxidation,and ultrasonic methods.high-pulsed laser ablation(PLAL)is often used to prepare quantum dots by laser.the target material in the solution is irradiated by intense pulsed laser,and the surface of the target material is rapidly heated,melted and evaporated at High temperature.However,under the action of the solution,multiple cavitation bubbles will be formed,and the cooling effect of the liquid forces the bubbles to shrink and fuse to grow,thus forming quantum dots of different sizes and shapes^[16]。 laser preparation has the advantages of using less chemical precursors,reducing the generation of by-products,fast synthesis rate and high doping efficiency,and is a green,simple and inexpensive method for preparing nanocrystals.However,it also needs to strictly control the influence of Laser wavelength,pulse duration and repetition rate,energy density per pulse,solvent properties and other factors,which is mainly Used to prepare carbon,silicon carbide,graphene and other quantum dot materials.Kang et al.used PLAL technology to directly use ethanol and coal as raw materials to prepare graphene oxide quantum dots with excellent optical properties without purification^[17]。 These QDs have good biocompatibility and can be used for bioimaging applications.electrochemical method is a method to produce quantum dots by electrolytic cell under the action of voltage,which reduces unnecessary Electrochemical reactions,has unique chemical and physical properties and low manufacturing cost,and is a simple method to synthesize nanocrystals^[18]。 Li et al.Used the graphene membrane formed by filtration as the working electrode in the three-electrode set to obtain monodisperse graphene quantum dots with a size range of 3–5 nm,and integrated the quantum dots as a novel electron acceptor material into a solar cell based on P₃HT,which achieved a power conversion efficiency of 1.28%and successfully improved the performance of the device^[19]。 Chemical oxidation method is a method to prepare quantum dots by decomposing the precursor under the action of strong acid oxidants such as nitric acid,sulfuric acid and citric acid,which is often used to prepare carbon or silicon carbide quantum dots^[20]。 it has the advantages of simplicity,high efficiency and large-scale production,but due to the action of strong acid and strong oxidant,quantum dots will have surface defects and affect the performance.Wang et al.Used citric acid as raw material and mixed It with N-(γ-aminoethyl)-γ-aminopropylmethylmethanoxysilane(AEAPMS)at 240℃to synthesize amorphous carbon quantum dots with a photoluminescence quantum yield(PLQY)of 47%within 1 min^[21]。 the ultrasonic method uses continuous ultrasonic waves to first generate alternating pressure waves in the liquid,which makes the liquid flow and form small vacuum bubbles(cavitation nuclei),and then the repeated formation and collapse of these cavitation nuclei will lead to the high-speed impact movement of the liquid,forming a strong fluid shear force,so that the bulk material is cut into nano-quantum dots^[22]。 The process of nanocrystal preparation by ultrasonic method is simple and efficient,but it is difficult to obtain high-quality products,and it also needs to strictly control the influence of factors such as frequency,output power,sound intensity,sound pressure and so on on the process of nanocrystal preparation.Our group used melamine to synthesize g-C₃N₄powder by thermal condensation polymerization in air and N₂atmosphere for 2 H,and then ultrasonically exfoliated the g-C₃N₄powder in water or ethanol to prepare QDs,obtaining a quantum yield of 49.8%^[23]。

the top-down preparation process of quantum dots is mature,which has the advantages of simple operation,abundant precursors and economy,but at the same time,it faces the disadvantages of high energy consumption,difficult control of variables,low yield,uncontrollable shape and size,and high defects,as shown in Table 1。

The bottom-up quantum dot preparation methods mainly include thermal microwave method,thermal injection method and one-pot thermal method.According to the type of solvent,it can be divided into hydrothermal method and solvothermal method.The quantum dots synthesized by bottom-up method in organic phase have better crystallinity and relatively high PLQY,which is also the most commonly used quantum dot synthesis method.Therefore,the solvothermal-based thermal injection and one-pot thermal method for the preparation ofⅠ-Ⅲ-Ⅵ₂quantum dots are described in detail below 。

In the hot injection synthesis method ofⅠ-Ⅲ-Ⅵ₂family quantum dots,the cation precursor and ligand solvent react at high temperature,and then the anion precursor is rapidly injected into the hot solution,resulting in immediate supersaturation and aggregation-induced rapid nucleation.The obtained quantum dots have good dispersion and size uniformity,and the operation process is roughly shown in Figure 2A^[25]。 According to the principle of HSAB,the reactants can be divided into acid or base according to their tendency to accept or donate electrons.Hard acids tend to combine with hard bases,while soft acids tend to combine with soft bases,that is,"soft affinity"and"hard affinity",which is the key to the selection of ligand solvents in the synthesis of single-phase stable quantum dots.Group I and III metal ions are usually soft and hard acids,respectively,and the anion is usually a soft base.Because of this unbalanced reactivity,two or more ligands are usually selected to increase or decrease the reactivity,respectively,so that they can match each other and avoid the formation of binary quantum dots（Ag₂S,Cu₂S）^[14][26]。

Cationic precursors are mostly acetylacetonate metal salts,acetates,metal halides and organometallic compounds,and sulfur or selenium sources are used as anionic precursors.the cation precursor and ligand solvent are often dissolved in the non-coordinating solvent octadecene(ODE),which does not participate in the reaction,but can reduce the viscosity of the system,which is conducive to creating a reaction environment with uniform material concentration,making the particle size distribution of quantum dots more uniform^[27]。 Oleic acid(OA),oil ammonia(OLA),dodecyl mercaptan(DDT)and so on are the commonly used ligand solvents in the hot injection method.Oleic acid is an L-type ligand,which provides weak reduction and contributes to the formation of metal and metal chalcogenide quantum dots.Oleic acid has L-type and X-type characteristics,occasionally adoptsη²coordination,and binds more strongly to the QD surface,providing stronger coordination sites for metals.As a result,the quantum dots can be better passivated.These two types of ligands have the ability to remove surface trap States and enhance the optical properties of quantum dots.Among them,the addition of a large amount of oil ammonia can make the size of quantum dots larger,while oleic acid can make the size of quantum dots smaller^[28][29]。 Thiol can be used as the sulfur source of the anionic precursor,such as dodecyl thiol(DDT),which is not only a common surface ligand for the synthesis ofⅠ-Ⅲ-Ⅵ₂semiconductor nanocrystals,but also a sulfur source.Taking the synthesis of CuInS₂（CIS）quantum dots as an example,the synthesis of CIS quantum dots using DDT is divided into two stages:the first stage is a complexation reaction at about 100℃,in which metal cations are combined with DDT to form an intermediate sheet-like two-dimensional polymer;The second stage is nucleation and growth at about 230°C.The resulting ligand shell consists of a bilayer composed of DDT and dodecyl sulfide(DS)molecules combined in a 1:1 ratio in a head-to-tail manner(R-S-R),with DDT as the inner layer and DS as the outer layer,as shown in Figure 2B^[20,30]。 At the same time,the reaction balance of two cations can be controlled,the group I element and the group VI element are easy to form intermediate products,such as Ag₂S,Cu₂S and the like,and the use of DDT as a ligand solvent not only reduces the generation of by-products,Moreover,the proton transfer between different kinds of ligands is avoided,and the stability of the quantum dots is improved.The sulfur source is also commonly dissolved in a non-coordinating solvent ODE or a ligand solvent(such as OLA).Compared with DDT,using sulfur powder as a sulfur source can nucleate at a lower temperature.Deng et al.Used this sulfur source to inject and synthesize green emitting CuInS₂/ZnS quantum dots at a low temperature of 130°C,and Cu⁺and In³⁺reacted with the high concentration of H₂S released by S-OLA,resulting in a high concentration of CuInS₂nuclei,as shown in Figure 2B^[31]。 Li et al.Used AgNO₃and In(OAc)₃as cation precursors,OA and DDT as surface ligands,and injected sulfur powder dissolved In OLA to synthesize AgInS₂QDs.The emission wavelength was tuned by controlling the In/Ag ratio.When In/Ag=4,the maximum PLQY was 57%,and the quantum yield was increased to 72%by passivating the surface defects with ZnS shell^[32]。 The selenium source can dissolve selenium powder in a non-coordinating solvent ODE at high temperature,or a mixture of ligand solvents such as OLA and trioctylphosphonium(TOP).In the thermal injection synthesis of quantum dots,vacuum treatment is usually required to remove water and oxygen components,inert protective gases such as nitrogen and argon are introduced,the reaction system is heated to a certain temperature,and then anionic precursors(such as sulfur or selenium sources)are injected,and the metal precursors react rapidly with sulfur and selenium sources.In order to nucleate and grow quantum dots,if the outer shell(such as ZnS,ZnSe,GaS_x,InS_x,etc.)Is wrapped,it is often necessary to inject the prepared shell precursor solution at low temperature,grow at high temperature,and slowly drop it under high-speed stirring to obtain good crystallinity^[33]。 To sum up,the hot injection method has the characteristics of strong applicability,rapidity and high efficiency,and can prepare high-performance and size-uniform quantum dots by controlling the reaction time,reaction temperature and injection rate,but at the same time,it faces uncertainties such as difficult operation,need for atmosphere protection,and poor reproducibility。

One-pot thermal method is also commonly used to prepareⅠ-Ⅲ-Ⅵ₂quantum dots.Anionic and cationic precursors,surface ligand solvents(OA,OLA,DDT,etc.),and non-coordinating solvents(ODE)are usually added directly in a three-necked flask or autoclave.The anionic-cationic precursor and the ligand solvent are dissolved in the organic solvent at the same time,and the reaction system is heated to a proper temperature and stirred to promote the reaction.The metal salt is decomposed to nucleate and grow into quantum dots in the presence of the surface ligand solvent and the high boiling point organic solvent.The choice of ligand solvent can improve the dispersion and stability of quantum dots 。

Compared with the thermal injection method,the one-pot thermal method for preparing theⅠ-Ⅲ-Ⅵ₂family quantum dots has simpler operation steps and shorter reaction time,has the application prospect of large-scale production,However,the temperature,reaction time,stoichiometric ratio and core-shell structure variables need to be strictly controlled,which is very important to control the size and morphology of quantum dots and avoid the problems of wide dispersion and large size.Due to the simultaneous addition of anion and cation precursors,it is necessary to ensure the reaction balance of anions and cations,and the control of reaction conditions is more demanding.Mei et al.Used a one-pot thermal method to directly dissolve CuI,Ga(acac)₃,and Se powders into an organic solution of DDT,OLA,and ODE to synthesize CuGaSe₂/ZnSe core-shell QDs with a PLQY of 77.73%,by changing the amount of oil ammonia to achieve the particle size control of QDs from 4.20 to 5.73 nm,and introducing sulfide ions to allow anion alloying and changing the Cu/Ga precursor molar ratio,the spectral emission range can be tuned from 485 to 630 nm,as shown in Figure 3A^[29]。 Wei et al.Dissolved AgI,CuI,Ga(acac)₃,and Se powders in a mixed solvent of OLA,ODE,and DDT to synthesize Ag-Cu-Ga-Se multicomponent QDs by one-pot heating.Under ZnSe shell passivation,the PLQY was as high as 71.9%.The shift of the emission peak from 620 to 510 nm was tuned by controlling the molar ratio of(Ag+Cu)/Ga.It was also explored that the increase in the amount of oil ammonia would increase the size of QDs.Fig.3 B is the growth mechanism^[34]。

In general,hot injection and one-pot thermal methods are commonly used bottom-up methods to prepare high-qualityⅠ-Ⅲ-Ⅵ₂quantum dots.They can achieve precise control of the size and morphology of quantum dots,reduce the existence of defects,and obtain high-performance quantum dots,but also face the disadvantages of time-consuming,need for protective gas and special equipment,and difficult application;The preparation of quantum dots by microwave method has the characteristics of high efficiency,low energy consumption and high yield,but it is often difficult to obtain high-performance quantum dots,as shown in Table 2.These preparation methods provide a powerful basis for material preparation for further research and application.Future research can further optimize the preparation conditions and explore new preparation methods to obtain higher qualityⅠ-Ⅲ-Ⅵ₂family quantum dot materials 。

Ⅰ-Ⅲ-Ⅵ₂family quantum dots exhibit a broad absorption spectrum.Unlike binary quantum dots,which have obvious exciton absorption peaks in the visible and ultraviolet regions,ternary quantum dots exhibit a continuous absorption band,a broad emission spectrum,a high fluorescence lifetime,and a significant Stokes shift.Most of these properties are different from those of binary quantum dots due to different mechanisms.The long PL lifetime and larger Stokes shift are mostly due to defect levels.For example,the optical properties of copper-based multicomponent quantum dots are mainly due to copper-related defect levels.There are mainly three crystal structures ofⅠ-Ⅲ-Ⅵ₂family quantum dots:low-temperature stable phase tetragonal chalcopyrite(CP),high-temperature stable phase cubic sphalerite(or ZB sphalerite)and hexagonal wurtzite(WZ),as shown in Figure 4A,and the low-temperature tetragonal chalcopyrite crystal structure is mostly synthesized in the experiment^[35]。 In CuInS₂QDs,Cu⁺and In³⁺are ordered in the tetragonal unit cell,Each S²⁻is tetrahedrally coordinated by two Cu⁺and two In^3+[36]。 The conduction band minimum(CBM)and valence band maximum(VBM)ofⅠ-Ⅲ-Ⅵ₂quantum dots correspond to the same wave vector and are direct gap semiconductors.The energy band structure of theⅠ-Ⅲ-Ⅵ₂quantum dot and its derivatives is shown in Fig.4B.Taking the AgInS₂quantum dot as an example,the conduction band of the quantum dot is composed of the s orbital of I/VI elements and the p orbital of III elements,while the valence band is composed of the d orbital of I subgroup elements and the p orbital of VI main group elements.The band gap of copper-based chalcogenide quantum dots is larger than that of silver-based chalcogenide quantum dots,and the emission spectrum is also wider,which may be due to the fact that the d orbital of copper element contributes more to the valence band than that of silver element,so that the hybridization at the top of VB is easier to split into multiple subbands^[37]。

The luminescence mechanism ofⅠ-Ⅲ-Ⅵ₂quantum dots has been discussed for a long time in academia.Up to now,there are mainly DAP recombination theory and FTB recombination theory,as shown in Figure 4C;According to the DAP recombination theory,which is mainly related to the recombination of the valence band,the conduction band and the local donor state electron and acceptor state hole located between them,a remarkable feature of DAP emission is that the energy transfer is dependent on the excitation power,that is,with the increase of the excitation power density,The peak energy of the PL spectrum will shift to higher energy,and the DAP emission associated with sulfur source vacancy defects(V_S),copper ion vacancies(V_Cu)or cation replacement defects(In_Cu)may be one of the sources of PL emission^[38]; According to the FTB recombination theory,which means the recombination of delocalized conduction band(CB)electrons and holes located at defects(copper ions,silver ions),Zang et al.Found that thick-shell CIS/ZnS quantum dots show enhanced photostability by comparing CIS core,thin-shell CIS/ZnS and thick-shell CIS/Zn single particle measurements.the PL peak width at room temperature is as low as 60 meV,which is explained by introducing the FTB recombination theory,and the random positioning of the emission center in the QD can be one of the reasons for the change of PL energy exceeding 300 meV^[39]; DAP recombination is dominant compared to band-edge recombination。

For CP structure CuInS₂quantum dots,the optical properties of quantum dots are affected by the Cu ion related defect level due to the flexible non-stoichiometric elemental composition and the low formation enthalpy of defects.Kraatz et al.Used femtosecond transient absorption(fs-TA)spectroscopy to study the subpicosecond dynamics of quantum dots.The absorption excited state appears instantaneously under the action of the excitation laser,which means that the excited state absorption signal originates from the conduction band(CB )^[40]。 Some of the excited electrons undergo ultrafast nonradiative relaxation from the CB into a long-lived highly radiative donor state,and the optical transition comes from the high-lying band-gap donor state.Therefore,this radiative transition is considered to be a donor to VB transition between the band gaps;At the same time,many emission peaks are found in the spectra of CP CuInS₂quantum dots,which means that there are many radiative recombination channels,not simply bandgap luminescence,and the defect States located between the bandgaps are often V_S,V_Cuand CuIn substituted In_Cu.Among them,V_S,In_Cuact as donor States,while copper V_Cuas acceptor States,but the contribution of donor state In_iand acceptor state Cu_idefects to radiative recombination luminescence is neglected.In the ZB structure of CuInS₂quantum dots,the In_Cudefect does not participate in the radiative luminescence process,which reveals the existence of other donor state defects,and the radiative luminescence process of WZ structure of CuInS₂quantum dots is similar to that of CP structure.As shown in fig.4D,DAP recombination is one of the reasons for the broad half-peak width,long radiative lifetime,and large Stokes shift due to the diversity of donor and acceptor defect States,According to Zhang et al.,the short emission lifetime can be attributed to non-radiative recombination and emission related to surface and internal defects,while the long emission lifetime can be attributed to donor-acceptor pair recombination related to copper element^[41]。 When the Cu/In ratio exceeds a certain value,the emission intensity decreases exponentially until quenching due to excessive Cu-related intrinsic defects,which can be explained by the quenching effect caused by DAP interaction.The defect States of Cu⁺and Cu²⁺can be controlled by changing the stoichiometry of QDs.In CuInS₂QDs,both Cu⁺and Cu²⁺related defect States are luminescence centers,but the activation mechanism and radiative recombination luminescence are different.As shown in Figure 4E,the emission mechanism of Cu⁺is easily affected by both hole and electron capture,however,the mechanism of Cu²⁺defect is only affected by electron capture,in the presence of a large amount of V_Cu.The charge compensation contributes to the formation of Cu²⁺defects,the Cu²⁺can lead to exciton degradation,and the efficient recombination of electrons with excess holes and VB edge photogenerated holes leads to additional non-radiative recombination,resulting in the quenching effect^[30]。 In the absence of the Cu²⁺center,a single CuInS₂QD emits light after the photogenerated hole is rapidly localized to the Cu⁺center and radiatively recombines with the delocalized CB electron.The intrinsic stoichiometric CuInS₂QDs show luminescence quenching near both VB and CB,which implies that there are two defect States between the band gaps that inhibit the enhancement of luminescence efficiency in the case of abundant copper content,and the reduction of copper content or the formation of core-shell structure helps to eliminate these defect States,making the DAP recombination emission dominant and improving the luminescence efficiency 。

The research onⅠ-Ⅲ-Ⅵ₂quantum dots is mainly based on Cu-based S group and Ag-based S group.Compared with traditional II-VI quantum dots,ternary Cu-based chalcogenide or Ag-based chalcogenide semiconductor quantum dots can be considered that the divalent metal ions in II-VI quantum dots are replaced by monovalent first subgroup cations and trivalent main group cations,so their crystal structure is similar to that of II-VI quantum dot system 。

The optical properties ofⅠ-Ⅲ-Ⅵ₂family quantum dots can be optimized by composition control,core-shell structure,doping and alloying,surface ligands and ligand exchange.Because the nucleation process of quantum dots can be explained by LaMer nucleation theory,the growth process of quantum dots conforms to Ostwald theory^[46,47][48]。 The schematic diagram of LaMer theoretical growth mechanism is shown in Figure 5A,so controlling different reaction temperatures and holding times has a crucial impact on the size of quantum dots.The optical properties of QDs,such as emission wavelength and quantum yield,can be tuned by the quantum size effect.TheⅠ-Ⅲ-Ⅵ₂family of quantum dots also has a flexible chemical element composition,so that the regulation of the composition can also be used as one of the means for optimizing the optical performance.Different precursor ratios will result in different band gaps of synthesized quantum dots,which will lead to different optical properties of quantum dots,such as wavelength range and quantum yield^[49]。 The smaller the ratio of the amount of precursors of the first subgroup（Cu⁺,Ag⁺）and the third main group（In³⁺,Ga³⁺,Al³⁺）,the greater the blue shift of the emission wavelength of the synthesized QDs,as shown in Fig.5 B.At the same time,as the luminescent center,the amount of copper ions also affects the emission intensity,and excessive copper ion precursors will quench the optical properties of QDs.The optical properties of synthesized QDs are also closely related to surface ligands,which can not only enhance the chemical stability of QDs and prevent them from agglomeration,but also participate in photophysical processes through vibrational coupling.In this process,additional surface polarons are formed to suppress the surface defect States of quantum dots.Surface ligands such as oleylamine(OLA),oleic acid(OA),dodecanethiol(DDT)and trioctylphosphine(TOP)are widely used in the synthesis of quantum dots^[50]。 The surface ligands can be classified into L-type,X-type,and Z-type.Metal halide is a Z-shaped ligand.According to Young's acid-base theory,different surface ligands are selected according to different precursors.The role of each ligand has been introduced in the thermal injection method above.Using DDT as a surface ligand to synthesize CuInS₂quantum dots,the PLQY of CIS/ZnS quantum dots is more than 80%.A large number of dangling bonds and defects on the surface of CIS quantum dots will increase the probability of exciton non-radiative recombination,which will affect the optical properties of quantum dots,so it is necessary to design the structure of quantum dots.Encapsulating one or more shells on the surface ofⅠ-Ⅲ-Ⅵ₂QDs to form a core-shell structure can suppress the non-radiative recombination process of excitons and resonance energy transfer(FRET)to obtain QDs with high PLQY^[51]。 According to the energy offset between the highest occupied molecular orbital(HOMO)and the lowest unoccupied molecular orbital(LUMO)levels of two adjacent materials and the localization mechanism of different carriers observed after photoexcitation,the core-shell structures of quantum dots are mainly divided into three types:Ⅰ-type,Ⅱ-type,and anti-Ⅰ-type^[52]。 As shown in Figure 5C,in the I-type core-shell structure quantum dot,the band gap of the shell material is larger than that of the inner core,and after being excited,electrons and holes are mainly confined to the part of the inner core material with a narrower band gap,thus generating direct excitons.In the type-Ⅱcore-shell structure quantum dot,the staggered energy level arrangement leads to the spatial separation of electrons and holes on different sides of the heterojunction,and after excitation,the electrons and holes are separated in different regions of the core shell to form a spatial indirect exciton;In the inverse I-type core-shell structure quantum dots,the band gap of the shell semiconductor material is smaller than that of the core,and after excitation,some carriers are confined in one of the components,while the other carriers are dispersed in the whole quantum dot.ZnS,ZnSe,GaS_xand InS_xare commonly used as shell materials,and the band gap of ZnS is 3.54 eV^[53,54]。 As a common shell material,the matching degree with the CIS inner core is as high as 98%,and the quantum yield of the core-shell structure of CuInS₂/ZnS or AgInS₂/ZnS quantum dots as an I-type core-shell structure can be up to more than 80%.Wu et al.Synthesized CIZS/ZnS/ZnS multi-shell QDs by using the multilayer core-shell structure optimization idea,and the thick-shell CIZS/ZnSZnS core-shell QDs have high stability and high quantum yield of 77%^[55]。 the growth of ZnS shell on quantum dots involves the competition of several processes such as Cation exchange,epitaxial growth and internal diffusion.cation exchange leads to a reduction in the luminescent core,resulting in a blue shift in the spectrum,while internal diffusion tends to form an alloy shell^[30]。 ZnS and ZnSe shells are coated with Cd-based quantum dots by epitaxial deposition to form a II-type core-shell structure,and indirect carriers weaken the quantum confinement effect and show a red shift in the spectrum^[56,57]。 Due to the characteristics of non-stoichiometry and large lattice matching ofⅠ-Ⅲ-Ⅵ₂quantum dots,doping and alloying have become one of the important means to regulate the optical properties ofⅠ-Ⅲ-Ⅵ₂quantum dots.The doped elements are mostly Zn,Ga,Ag,Mn and other elements,the alloyed quantum dots with homogeneous structure are obtained by doping,and the band gap of the quantum dots is adjusted by doping the alloy under the condition of lattice matching,so that the quantum efficiency is improved,and the stability is enhanced.Yoon et al.Synthesized Zn-doped Zn-Cu-Ga-S(ZCGS)/ZnS multiple core-shell quantum dots by one-pot thermal method,and the photoluminescence quantum yield of blue light was as high as 80%under the passivation of surface defect States by multilayer ZnS shells^[58]。 Ag-Ga-Zn-S(AGZS)quantum dots(QDs)with narrow-band blue emission were synthesized By Xie et al.by controlling The molar ratio of Ag/Zn at 4∶1 and Ag/Ga at 1∶8 via a one-pot method.the AGZS QDs exhibited blue emission at 470 nm with a full width at half maximum(FWHM)of 48 nm^[59]。 The full width at half maximum is very narrow for multi-element quantum dots,and the narrow-band emission from band-edge recombination can be further explored by element doping and alloying ofⅠ-Ⅲ-Ⅵ₂quantum dots.Uematsu et al.Synthesized Ag-In-Ga-S/GaS_xmulti-element QDs by hot injection method,which exhibited strong green band-edge emission with a peak wavelength between 499 nm and 543 nm and a half-peak width of only 31 nm,which is equivalent to the half-peak width of Cd-based QDs.The narrow-band emission makesⅠ-Ⅲ-Ⅵ₂QDs have potential applications in the field of display^[26]。 Zhang et al.constructed an ultrathin indium sulfide shell based on Zn-Ag-In-Ga-S multiple quantum dots to improve the radiation recombination rate,so that the DAP emission was enhanced,the PLQY was significantly increased to 86.2%,and the QLED device(EQE)based on this was as high as 5.32%^[60]。 Therefore,doping and alloying has become one of the important means to explore the band-edge emission of multi-component quantum dots.Hoisang et al.explored surface modification on multi-alloying AIGS QDs,and performed surface ligand modification treatment on AIGS QDs by using metal halide（ZnCl₂）with Z-type ligands,which increased the PL quantum yield by 2–3 times,and the highest value reached 73.4%,as shown in Fig.5d^[28]。 The optical properties of QDs can also be further enhanced by surface ligand exchange^[61]。

Quantum dot illumination display devices are based on two technology routes:electroluminescence and photoluminescence.in the field of display,all kinds of display devices emerge In endlessly,among which the passive light-emitting technology of liquid crystal display(LCD)backlight and the self-light-emitting system of organic light-emitting diode(OLED)are the most representative^{[65⇓⇓~68]}。

LCDs based on twisted nematic(TN)and super-twisted nematic(STN)cell configurations have flourished with the development of thin film transistors(TFTs)^[69,70]。 the basic structure of LCD is backlight,light guide plate,diffusion film,polarizer,TFT substrate,liquid crystal,filter layer,polarizer,etc.the color emission of LCD is the result of the synergy of the backlight and the color filter.the early LCD backlight uses cold cathode fluorescent tubes(CCFL),and LEDs are widely used as the backlight in the future.Compared with LCD devices with CCFL as backlight,LEDs as backlight generally have the advantages of lightness,thinness,low energy consumption,wide color gamut,long service life and uniform brightness,which makes LCD devices with LEDs as backlight occupy a larger market scale.quantum dots can greatly improve the color quality and total lumen efficiency of LCD displays because of their narrow emission and high efficiency.as early as 2013,Sony introduced quantum dot TV(QD-LCD)based on quantum dot tube as backlight.Most of the QD-LCDs on the market are based on the excellent photoluminescence properties of quantum dots;the first idea is to use quantum dots to replace the traditional phosphor and blue LED chips as the backlight source.on-chip packaging,Side packaging and Surface packaging can be used.However,on-chip packaging tests the high temperature stability of quantum dots.side packaging will lead to uneven backlight.surface packaging has a large demand for quantum dots and leads to high cost.the second idea is to use quantum dot color enhancement film,but because of its limited absorption coefficient,quantum dot color film often needs thicker quantum dots to completely cut off blue light,which can be optimized by covering the absorption film^[71]。 as a self-luminous system,OLED is widely used As a high-end display with a small screen.OLED emits light through the transportation of electrons and holes through various functional layers and the recombination of electrons and holes in the organic light-emitting layer.Compared with LCD,OLED has a simpler structure.the liquid crystal layer in LCD is deflected by applying voltage.However,it is impossible to block all the light from the backlight source,resulting in light leakage,resulting in low contrast and black is not pure black.OLED has the advantages of almost unlimited contrast,low power consumption,fast response time and flexibility,but it also faces the disadvantages of stroboscopic and screen burning.the QLED structure is based on the electroluminescent performance of quantum dots,and the device structure is similar to the OLED structure,in which the organic light emitting layer is replaced by a quantum dot layer^[10,72]。 QLED device is a new type of display device with the rapid development of ZnO used in the electron transport layer.QLED has lower energy consumption and wider color gamut than OLED.However,electroluminescent QLED is still in the laboratory stage,and there is still a certain distance from commercialization^{[73⇓⇓~76]}。 Like the emerging Mini-LED,Micro-LED is also a self-luminous system similar to OLED.Mini-LED,also known as millimeter light-emitting diode,is an LED with a grain size of more than 100μm,while MicroLED is an LED miniaturization and matrix technology,which allows LED units to be less than 100μm and driven separately^[77⇓~79]。

In the field of lighting,WLEDs can also be packaged by using the photoluminescence and electroluminescence properties of quantum dots.In the early 1990s,Nakamura et al.Invented high-brightness blue LEDs,which was the key to the development of white LEDs.the invention of blue LEDs broke the limitation of monochromatic LEDs because it could be used to excite fluorescent materials to emit white light^[80]。 white LEDs on the market mostly use yellow emitting cerium:yttrium aluminum garnet(Ce:YAG)inorganic phosphor coated on indium gallium nitride(InGaN)blue chips or RGB tricolor LEDs to produce White light,but packaged WLEDs have low CRI values^[81]。 high CRI is generally a desirable characteristic for artificial lighting sources to produce high-quality white light,with WLEDs with CRI>80 considered good sources for artificial lighting,and WLEDs with CRI>90 considered optimal sources for artificial lighting systems.WLEDs obtained by quantum dot photoluminescence under the excitation of blue chips can obtain high color rendering index,high luminous efficiency and long lifetime^[82]。 Similarly,a quantum dot electroluminescent QLED structure can also be used to prepare an ultrathin high-quality white light device,and the device structure can be a white light device of a multi-system laminated quantum dot luminescent layer,a white light device of a single-layer mixed quantum dot luminescent layer,or a white WLED device of a single-system quantum dot luminous layer。

quantum dots have broad application prospects in the field of light-emitting displays due to their unique optical properties.Deng et al.Found that in green and blue QLEDs with Cd-based Quantum dots as the emitting layer(EML),the electron leakage effect is enhanced due to the energy disorder and size difference between the QD and the hole transport layer(HTL).the HTL material was modulated to reduce the LUMO energy level and reduce the energy disorder,and finally the conversion rate of charge carriers to excitons in QLEDs reached about 100%,and the EQE of green and blue QLEDs reached 28.7%and 21.9%^[83]。 Won et al.Etched the InP surface with hydrofluoric acid to facilitate the growth of ZnSe shell.the thickness of the shell can inhibit energy transfer and Auger recombination to obtain high luminous efficiency.InP/ZnSe/ZnS multi-shell quantum dots were prepared with perfect quantum yield(100%)and narrow spectrum(35 nm).They were used in the EML of QLEDs.the optimized QLEDs showed an EQE of 21.4%,which has the application potential of display devices^[13]。 However,due to the characteristics of wide spectrum,Ⅰ-Ⅲ-Ⅵ₂family quantum dots have a certain gap with the narrow-band emission required in the display field,and do not have a significant competitive advantage,but the multi-element quantum dots optimized by structural design still have the performance of narrow-band emission.It provides an idea for the next generation of environmentally friendly quantum dots in the field of display,and at the same time,the wide spectral characteristics makeⅠ-Ⅲ-Ⅵ₂quantum dots have great application prospects in the field of lighting,with the advantages of full spectral coverage,high color rendering index(CRI),large area planar light source and so on 。

the design strategy of QLED is to effectively recombine the electron and hole carriers injected into the two functional layers in the quantum dot light-emitting layer to produce the emission unique to the band gap or defect,so the charge mobility of each layer(hole,electron transport layer and light-emitting layer)should be improved or synchronized,and the energy barrier of hole and electron injection should be minimized^[10]。 to achieve high emission efficiency,which is required To ensure charge transfer in the preferred direction while minimizing short-circuit current losses,the typical structure of a QLED device includes a hole-transport material(HTM)layer and an electron-transport material(ETM)layer,with a quantum dot light-emitting layer sandwiched between them.the sequential deposition of multiple functional layers corrects the flow of carriers.Usually,the injection speed of hole carriers is slower than that of electrons.Improving the injection efficiency of holes can minimize the loss of carriers^[84]。 QLED devices require energy level matching between different electron and hole transport layers.In addition to energy level matching,the conductivity and charge transfer interaction of carriers within these layers also determine the overall efficiency of QLEDs.Li et Al.Used a one-pot thermal method to synthesize CuInS₂/ZnS quantum dots,and used the quantum dots as a light-emitting layer to prepare high-efficiency red QLED devices(ITO/PEDOT:PSS/Poly-TDP/QDs/ZnMgO/Al),and used dodecanethiol(DDT)and octanethiol(OTT)ligands to modify the surface of the quantum dots,respectively.Because the DDT-modified quantum dots have higher PLQY,the corresponding QLED devices have higher brightness at a given voltage.However,the chain length of DDT is longer than that of OTT,which contains more CH₂groups.While suppressing the interaction of quantum dots,the carriers need to overcome a higher injection barrier.The QLED based on OTT modified core-shell quantum dots shows an external quantum efficiency of 3.36%at a forward current of 2.8 V.With the increase of voltage,the relative intensity of the EL spectrum increases steadily,the FWHM(75 nm)remains almost unchanged,and the CuInS₂/ZnS quantum dots show a certain thermal stability,as shown in Figure 6a^[85]。 Kim et al.Synthesized three kinds of CuInS₂/ZnS core-shell quantum dots with different ZnS shell encapsulation time and used them as the light-emitting layer(EML)of QLED devices.By optimizing the core-shell structure or alloying of quantum dots,the performance of QLED devices was improved and the non-radiative processes such as Auger recombination(AR)and FRET of excitons were inhibited^[86]。 The shell thickness of quantum dots has a great influence on the device efficiency,and a properly thick shell makes the exciton wave function interaction between adjacent quantum dots weaker to minimize the FRET process between quantum dots.The QLED was fabricated by using the quantum dots wrapped in three layers for 10 H as the EML,TFB as the hole transport layer(HTL)and ZnO nanoparticles as the electron transport layer(ETL),showing an excellent current efficiency of 18.2 cd/A and an external quantum efficiency of 7.3%,as shown in Figure 6 B.Ye et al.Also synthesized multi-shell CuInS₂/ZnS/ZnS QDs through a high-temperature long strategy,and the PLQY was as high as 76%under multi-shell passivated surface defects.By introducing polyethyleneimine(PEI)between the EML of the QLED and the EML layer with ZnO as the carrier injection,the balance of carrier injection was improved.Compared with the structure without PEI,the device performance was greatly improved.The EQE of the QLED was increased from 0.82%to 1.56%,and the maximum brightness was 5825 cd/m^2[87]。 Zaiats et al.Fabricated QLEDs based on core-shell CuInS₂/ZnS quantum dots,and explored the effects of different hole transport layers of PVK and TPD on QLEDs.Although the HOMO-LUMO energy levels of these two hole transport materials are similar,the voltage dependence of QLEDs is quite different^[84]。 By monitoring the current-voltage characteristics,in order to probe the resistivity of the two HTM layers,PVK exhibits an almost logarithmic I-V behavior,while for TPD it shows a linear dependence between current and voltage.Yoon et Al.explored white QLEDs with high brightness and high color rendering index.in order to avoid the introduction of cd element,they synthesized Zn-Cu-Ga-S/ZnS core-shell quantum dots with efficient blue light emission peak at 475 nm,with PLQY as high as 80%.Compared with two different hole transport layers of PVK and TFB,the LUMO level of TFB is lower,which is−2.2 eV.PVK has a relatively high LUMO of−1.9 eV,so it can effectively act as an electron blocking layer to prevent the electron flow from the EML to the HTL.the ITO/PEDOT:PSS/PVK/QDs/ZnMgO/al structure was used to package the QLED,and the device achieved a maximum current efficiency of 11.8 Cd/A and a maximum EQE of 7.1%,as shown in Figure 6 C^[58]。 Wei et al.Synthesized AgInS₂quantum dots by stoichiometric thermal injection method with In/Ag=4 molar ratio,and the PLQY was increased from 57%to 72%when the quantum dots were coated with ZnS shell.The core-shell structure passivated surface defects and inhibited FRET,thus obtaining higher PLQY.The full width at half maximum(FWHM)and the peak value of PL were further narrowed and blue-shifted with the encapsulation of the shell.The device encapsulated with ITO/PEDOT:PSS/TPD/QDs/MgZnO/Al structure showed a maximum EQE of 1.25%and a maximum brightness of 1120 cd/m²by using AIS/ZnS QDs as the light-emitting layer^[32]。 Xie et al.Realized narrow-band blue Ag-Ga-Zn-S quantum dots by one-pot method.AGZS quantum dots showed blue emission at 470 nm,PL half-peak width was 48 nm,and band-edge emission was dominant over DAP recombination emission.The presence of a large number of Ag⁺vacancies in AGZS QDs enhances the radiative recombination from conduction band electrons to V_Agwith the weakening of surface defect States and DAP recombination,and the enhanced radiative recombination from conduction band to V_Agis the key reason for the narrow-band emission^[59]。 The alloyed AGZS QDs bandwidth and PLQY show a strong dependence on the Ag/Ga and Ag/Zn precursor ratios,and the QLED device based on the AGZS multi-element QDs exhibits a narrow electroluminescence of 53 nm and a brightness of more than 123.1 cd/m².The fabricated QLED device structure is ITO/PEDOT:PSS/TFB/QDs/ZnMgO/Al,as shown in Figure 6d.The multi-element silver-based chalcogenide QDs exhibit a narrower spectral emission than the copper-based,but the quantum efficiency is slightly.Motomura et al.Used amorphous GaS as the shell to wrap the AgInS₂core to form a core-shell quantum dot.The peak wavelength of the photoluminescence spectrum of the core-shell quantum dot is 560 nm,and the half-peak width is 45 nm.GaS as the shell can effectively suppress the surface defects of the quantum dot,making the band-edge emission dominant.They packaged the quantum dots with core-shell structure into a QLED with flip-chip structure,and the device structure was ITO/ZnO/QDs/TCTA/MoO₃/Al.The QLED device with only AIS core-shell quantum dots as EML showed a large number of broad-spectrum defect emission and sharp band-edge emission coexisted,while the ETL mixed in the quantum dots as EML could effectively suppress the defect emission and show narrow-band edge emission^[33]。 3TPYMB was introduced in the luminescent layer.The addition of 3TPYMB improves the electron injection in the quantum dot,makes the carrier recombination more balanced,and improves the probability of band-edge emission.It not only significantly improves the efficiency of QLED devices by optimizing the optical properties of quantum dots,but also improves the overall efficiency of devices by fine control of functional layers.Motomura et al.Used polybasic Ag-In-Ga-S/GaS_xcore-shell quantum dots with high color purity as the luminescent layer,and used compounds such as GaCl₃and Ga(DDTC)₃to improve the luminescent properties of AIGS/GaS_xpolybasic core-shell quantum dots to prepare QLEDs with ITO/ZnMgO/QDs/TCTA/MoO₃/Al structure,and the EL spectrum of the device had a half-peak width of 33 nm^[88]。 This is extremely sharp for low-toxicity QDs and reaches the color coordinates of green EL(0.260,0.695),as shown in Figure 6e.Silver-based chalcogenide quantum dots have great potential for development in the field of QLED display.Zhang et Al.Wrapped an ultrathin indium sulfide shell on the multi-element ZAIGS quantum dots to form an indium-rich double-layer structure and a sulfur-rich three-layer structure,and the defect emission was suppressed by the wrapping of the indium sulfide shell,so that the DAP emission was enhanced,and the PLQY was increased to 86.2%.as an EML QLED device(ITO/PEDOT:PSS/PTAA/QDs/TPBi/Al),the EQE reached 5.32%,as shown in Figure 6f^[60]。

WLEDs for illumination are usually realized by both Electroluminescence and Photoluminescence.Photoluminescence uses blue chips to excite doped or alloyed quantum dots,and phosphors are often introduced to supplement the spectrum.electroluminescence is realized by single system white quantum dots,mixed RGB multi-system quantum dots,stacked multi-layer EML and RGB three-unit series structure^[89]。 Electroluminescence provides ideas for exploring ultra-thin large-area lighting devices,as shown in Figure 7.Kim et al.Prepared Mn-doped CGS:Mn/ZnS quantum dots,which exhibited two different PL bands of QD defect emission and Mn-doped emission,and the PL spectra of the two emissions were simultaneously competitive and could be easily adjusted by changing the Mn concentration.Undoped and doped QDs,which exhibited similar levels of 74%–76%in PLQY,were encapsulated on blue light-emitting diode(LED)chips to fabricate WLED devices,showing CRI and LEs of 95,34.5 lm/W at 60 mA and 84,20.2 lm/W at 250 mA,respectively^[90]。 Chen et al.Synthesized water-soluble yellow-orange emitting AIS/ZnS QDs with PLQY up to 45.7%by hydrothermal method^[91]。 By combining a blue InGaN LED chip with a mixture of AIS/ZnS quantum dots and Lu₃Al₅O₁₂：Ce³⁺phosphor,a bright white light was produced,with WLED showing high LEs of 77.98 lm/W and a CRI of 85.This makes QDs of great potential for optoelectronic applications.Li et al.Prepared CuInS₂/ZnS QDs with PLQY up to 85%by simple solvothermal synthesis,and the emission peak can be tuned from 651 to 775 nm.Using CuInS₂/ZnS QDs and(Ba,Sr)₂SiO₄：Eu²⁺phosphor as color converters,combined with blue LED chips,WLEDs with CRI up to 90 were prepared,as shown in Figure 7 a^[92]。 In order to avoid the spectral shift caused by agglomeration,the quantum dots and phosphors should be mixed with the binder at a high speed and the proportion should be strictly controlled.The mixture of quantum dot and phosphor provides a new idea for that development of prepare high-quality white light devices,and the characteristics of wide spectrum and easy tuning of the quantum dot are utilized to make up for the spectrum of the phosphor,thereby broadening the application field of the quantum dots.Su et al.Synthesized AgInS₂/ZnS quantum dots by microwave hydrothermal method,whose photoluminescence can be tuned from 540 nm to 622 nm,in aqueous solution containing AIS/ZnS quantum dots.The PAAm hydrogel synthesized by radical crosslinking polymerization of acrylamide monomer and BAAm crosslinker has high transparency,good shape flexibility and excellent ductility.By controlling the composition of AIS/ZnS quantum dots,uniform QD/PAAm hybrid films with different colors can be obtained.The films can be combined with blue LED chips to manufacture WLEDs^[93]。 the WLED based on the yellow composite film showed white emission with a CRI of 75.6,while the WLED fabricated By stacking the green film on the red film had a relatively high CRI of 87.5 and a CCT of 3669 K,as shown in Figure 7 B.by controlled annealing of CIGS quantum dots in the presence of zinc ions to form ZCIGS solid solutions with different distributions of substitution and doping elements,Gugula et al.Obtained quantum dots with quantum yields as high as 82%,with limited or even eliminated reabsorption,and color rendering indices as high as 88,enabling the production of high-quality WLEDs using a single color conversion layer^[94]。 Not only can quantum dots mixed with fluorescent powder be used as a white light source under the excitation of a blue light chip,but also quantum dots with different luminous wavelengths can be used,and the spectra are mutually complemented to form a white light spectrum,for example,Kim et al.Synthesized CuInS₂quantum dots with green emission and red emission.The PLQY is 85%and 83%,respectively.The two kinds of quantum dots are co-encapsulated in a blue LED chip to fabricate white WLED,and the device exhibits a balanced three-color white EL spectral distribution^[95]。 By changing the weight ratio of the two QDs,the prepared WLEDs have a CRI of 94~97 and a high Les of 43.1~68.8 lm/W,as shown in Figure 7 C.High-quality white light can also be obtained by using the QLED structure to directly inject carriers into the light emitting layer without relying on the blue chip excitation,which can be ultra-thin and flexible compared with the blue chip excitation.Kim et al.Synthesized CGS/ZnS quantum dots with a ratio of Cu/Ga=1/8,and tuned a single system white WLED device with a wide spectrum wavelength to obtain a high-quality light source,which had a high CRI of 83–88,and the device showed excellent performance of 1007 cd/m²in terms of brightness,with an EQE of 1.9%,showing the application prospect of flexible ultra-thin lighting materials,as shown in Fig.7d^[96]。 Zeng et al.Introduced In element doping In the above CGS/ZnS QDs to fill the weak red emission and enhance the stability of WLED devices,achieving the maximum brightness of 1402 cd/m²and EQE of 2.4%,as shown in Fig.7e.Single-system white light as EML avoids the non-radiative exciton quenching process and strict proportion control caused by multi-system mixed Auger recombination^[97]。 Yoon et Al.Synthesized Zn-Cu-Ga-S/ZnS and CuInS₂/ZnS core-shell QDs,and the spectral distribution of white EL could be conveniently adjusted by changing the content ratio of ZCGS and CIS in EML.The mixed QDs were used as the light-emitting layer to encapsulate the WLED electroluminescent device,and the device structure was ITO/PEDOT:PSS/PVK/QDs/ZnMgO/Al.The best white QLED produced a 2172 cd/m²brightness peak and a 4.6%EQE peak,showing a high color rendering of up to 82^[58]。 Randomly mixed quantum dot EML devices inevitably exhibit EL spectra that vary with applied bias,possibly due to electric-field-driven EML charge migration,with restricted hole injection into the EML and stronger electron injection than hole,This makes the charge imbalance in the EML inevitably promote the non-radiative exciton quenching process through Auger recombination.Under low voltage driving,the small band gap quantum dots contribute more to the EML,and with the increase of voltage,the gain effect of large band gap quantum dots on the EL spectrum increases^[98]。