Synthesis and Applications of Halide Perovskite Nanocrystals in Optoelectronics

Round 1

Reviewer 1 Report (New Reviewer)

This is a nice summary of the perovskite NC applications in various fields, covering the applications, challenges, and suggestions in four optoelectronic fields. There are a few suggestions attached.

1.       Some figures have low quality. Would suggest improving it.

2.       If an outline can be provided, it is helpful to locate the interesting sections and obtain an overview of the work.

3.       Considering the application of perovskite nanocrystals in the visible photodetector part, I would like to know what the advantages of perovskite NCs are,  in comparison to the more stable 2D perovskite and classical II-VI group quantum dots photodetectors.

Author Response

Review Report (Reviewer 1)

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( ) I would like to sign my review report

English language and style

(x) Extensive editing of English language and style required

( ) Moderate English changes required

( ) English language and style are fine/minor spell check required

( ) I don't feel qualified to judge about the English language and style

Is the work a significant contribution to the field?        

Is the work well organized and comprehensively described?         

Is the work scientifically sound and not misleading?    

Are there appropriate and adequate references to related and previous work?   

Is the English used correct and readable?   

Comments and Suggestions for Authors

The manuscript "Synthesis and Applications of Halide Perovskite Nanocrystals in Optoelectronics" aims at drawing picture of the state of the art in the dynamic field of halide perovskiten naocrystals for various applications. The review is very basic, it mostly just lists some existing approaches on the synthesis and some applications with a few potential proposed improvements. The review critically lacks analysis of the current trends in such a rich and dynamic field as perovskite nanocrystals. Without this analysis it does not bring any novelty with respect to numerous reviews on very similar topics, which are regularly published in a variety of journals. In fact, each of the covered topics (synthesis, LEDs, solar cells, photodetectors) deserves a separate detailed review and I do not see a point in publishing an n-th work trying to englobe everything and simply listing existing technologies.

Reply： Thank you for the suggestions, and we provided new discussions in Conclusion and Outlook. (in lines 858-865 on page 36)

“However, from the results listed above for LED devices, we can observe that the progress of blue LEDs still lacks behind, especially in the violet or ultra-violet region. One of the reasons resulted from the incorporation of Cl⁻ anion, which will significantly degrade the stability of the perovskite structure. And ligand exchange might be effective in dealing with this problem. Perovskite NCs-based near-infrared photodetectors are also worth developing due to this region's lack of efficient absorption material. Wide bandgap perovskite NCs-based solar cells may be a future trend for commercialized Si tandem solar cells. For laser applications, perovskite NCs can be the new laser source.”

In addition, a critical analysis is lacking: what are the drawbacks of the existing synthesis methods and applications? What are future perspectives for each of the fields? No mention of the toxicity of the perovskite NCs is presented, no strategies to prepare and use Pb-free ones... Some very important papers are not cited

Reply： Thank you for the suggestions, and we provided new sections mainly about Pb-free perovskite nanocrystals.

(In lines 68-88 on pages 2-3)

 Introduction

“Toxic for human bodies and unfriendly to the environment are also huge concerns for the development of perovskite and perovskite NCs, and lead is the main reason for these concerns. Therefore, looking for new elements to replace lead is an essential step. Many research and theoretical simulations have recently focused on group IIA alkaline-earth metals, lanthanides, and other metals to substitute or partially replace lead. Group IIA alkaline-earth metals such as Mg²⁺[23], Ca²⁺[24,25], Sr²⁺[24,26], and Ba²⁺[24] might be some of the best candidates to substitute Pb²⁺ in perovskite or perovskite NCs. Moreover, they can further improve material properties or device performances. Qiang Hu et al. introduced Mg²⁺ doped CsPbCl₃ NCs, and PLQY increased from 1.3 % to 75.8 % [23]. Yuan Liu et al. introduced Sr²⁺ in CsPbBr₃ QD to form CsPb_1-xSr_xBr₃ QD and obtained an EQE of 13.8 % for sky-blue LED [26]. Lanthanides may be another candidate to substitute or replace Pb²⁺, and the element included Ce[27], Nd[28], Eu[29], and Yb[30]. Takayuki Chiba et al. introduced NdCl₃ in CsPbBr₃ NCs and achieved an extremely high PLQY of 97 % at an emission peak of 478 nm and an EQE of 2.7 % [28]. Jiajun Luo et al. introduced CsEuBr₃ perovskite, which is lead-free and achieved a PLQY of 69 % at a blue emission of 448 nm, and the EQE of the LED device reached 6.5 % [29]. Other metals such as Sn²⁺[31,32], Ni²⁺[33], Ti³⁺[34], Cu⁺[35], Al³⁺[36], Zn²⁺[37] and Mn²⁺[30] are also favorable to substitute Pb²⁺. Gencai Pan et al. introduced Ni²⁺ in CsPbCl_0.99Br_2.01 QD and achieved a high PLQY of 89 % and a blue emission at 470 nm [33]. Ti³⁺ and Cu⁺ ion-doped CsPbCl₃ NCs also provide significant PLQY enhancement compared to pristine ones [34,35]. Therefore, perovskite NCs with better material properties, are environmentally friendly, and less toxic can be continuously and gradually fulfilled simultaneously.”

(In lines 337-372 on pages 14-15)

“3.5 Optimizing nanocrystal perovskite device for lead-free

Pb ion is an indispensable material in perovskite, but the harm of lead to the environment and human body cannot be ignored. Therefore, the researcher has been devoted to using Sn (II), Sn (IV), Mn(II), Bi(III), Sb(III), and Cu(II) to replace lead in recent years. Several light-emitting elements of lead-free perovskite are introduced below. [56,57] A common element to replace Pb is Sn. The advantage of Sn is that it can reduce the lattice constant of the semiconductor, it will cause a wider band gap than the lead-containing perovskite, which can blue-shift the generated light and reduce the defects of Pb²⁺ ions to have a longer PL lifetime. The followings are some treatments in CsSnBr₃. First, Haichuan Mu et al. [58] showed the effect of annealing temperature on CsSnBr₃ film, which reduced the trap density from 3.5 × 10¹⁷ to 1.21 × 10¹⁷ cm ⁻³. Finally, they reached external quantum efficiency of 0.16 % and onset voltage of 5.5 V an electroluminescence (EL) peak at 674 nm can be obtained.

Jung-Min Heo et al. [59] point out that SnF₂ is often used as an additive in CsSnBr₃ optoelectronic devices to improve the oxidation susceptibility of CsSnBr₃. They also add TPBI inhibits the growth of perovskite to achieve uniformity and reduce the crystal size. Finally, adding SnF₂ obtains a brighter and more stable CsSnBr₃ PeLED (Figure 7a) (maximum brightness of about 160 cd m⁻²). However, the authors remind us that excessive addition of SnF₂ often leads to phase deviation, formation of nanosheet structures, etc. Although Sn is believed to have better environmental protection, the performance of Sn perovskite is not as good as that of Pb perovskite. In addition, because Sn is easily oxidizable, it also faces stability problems. Besides Sn, Cu is also a common ion to replace Pb. CsCuBr₂ and Cs₃Cu₂Br₅ were successfully synthesized by Tao Li et al. [60] (Figure 7b). They also produced a green light-emitting diode that can emit about 500-550nm and has stability for up to 5 months. (Figure 7c). Taehwan Jun et al. [61] used Cu instead of Pb to synthesize Cs₃Cu₂I₅ and achieve a high PLQY of 91.2 %.

 Compared with other ions that replace Pb, Cu has better stability. Although the misaligned energy levels cause EL performance not to be as good as traditional CsPbX ₃ LEDs, it still proved that a luminescent material that could obtain high PLQY is found. Besides, it provides another way to replace Pb without harming the environment. Sb is also an element that can replace Pb. Jian Zhang et al. [62] compared the quantum dots of Sb substituted for Pb to form Cs₃Sb₂Br_9. Perovskite containing Sb has a high absorption coefficient, small effective mass, and suitable properties close to the direct energy gap (2.36 eV). In the article, changing the anion in perovskite can adjust the wavelength of the light emitted from 370 nm to 560 nm (Table 5). Meanwhile, an improved ligand-assisted reprecipitation method (m-LARP) was used, making it possible to obtain crystal sizes with an average diameter of 3.07 ± 0.6 nm (Figure 7d), showing a significant development in the field of optoelectronics.”

 Figure 7. (a) Luminance–voltage of CsSnBr₃ [58]. Copyright 2022, American Chemical Society (b)SEM image of Cs₃Cu₂Br₅ Micro rods [60]. (c) Electroluminescence of Cs₃Cu₂Br₅Micro rods [60]. (d) Size distribution analysis for colloidal Cs₃Sb₂Br₉ [62]. Copyright 2017, American Chemical Society

Table 5. Summary of PLQY and Emission Peaks for Colloidal Cs₃Sb₂X₉ IPQDs Copyright 2017, American Chemical Society

”

(In lines 523-538 on pages 22)

“4.4 Lead-Free Perovskite Nanocrystal Solar Cells

Although lead-based PSCs show outstanding performance with high conversion efficiency, they are assumed to be harmful to the environment due to lead toxicity. This concern hinders large-commercial development and photovoltaic field application [81]. On the other hand, despite lower efficiency, with suitable bandgap, lead-free perovskite offers a circumvent way to avoid environmental and human health damage of environment and human health.

Sn exhibits optoelectronic properties similar to Pb. Moreover, The characteristics of narrower bandgap and higher carrier mobilities make Sn an essential candidate for lead-free PSCs. In 2008, Wang et al. [82] produced CsSnI₃ NCs with high stability at room temperature. With antioxidant solvent additive (ASA) triphenyl phosphite (TPPi), the CsSnI₃ NCs can remain clear and stable at room temperature after 90 days. The champion device presented an efficiency of 5.03 %, the highest record for all-inorganic lead-free perovskite solar cells. The same year, Xu et al. synthesized MASnI_xBr_3-x NCs by integrating Br⁻ ions into the host crystal structure and tuning the bandgap (1.5~2.3 eV) of perovskite NCs with different I/Br ratios. Although the corresponding device showed low efficiency of 0.321 %, it came to 8.79 % when applied as a light harvester for mesoscopic solar cells. [83]”

Figure 11. (a) The scheme of the production process of CsSnI₃ QD solution and the corresponding device structure. [82]. Copyright 2018 Clearance Center. (b) The steady-state measurement of the solar cells is based on the CsSnI₃ QD film continuously measured under AM1.5 illumination and the J-V measurements for CsSnI₃ QD solar cell devices. [82]. Copyright 2018 Clearance Center. (c) Energy levels distribution of the device and characteristic J-V curves for MASnBr₂I QDs-sensitized solar cells before and after tailored with the trace of N719. [83]. Copyright 2018 Elsevier.

Lead-free perovskite has become a hotspot in photovoltaic applications for the environment and human health. But the research on lead-free perovskite NCs is much less than on lead-free perovskite. Despite the slow development, we can still see the potential in the photovoltaic field. On the other hand, there are some studies about the necessity of lead alternatives in the PV industry. They focused on cost, availability, sustainability, and eco-friendliness concepts, and the result showed that lead-based perovskite's advantages outweigh the risks connected with its manufacture [84]. Still, there are many concerns about lead-based perovskite solar cells, which is why we have to continue the study of lead-free perovskite.

”

... The definition of Bohr radius is not correct.

Reply： Sorry for the mistake. And I have changed the Bohr radius to exciton Bohr radius with the accurate description in lines 211-221 on pages 8-9 as follows：

“Because of the critical influence of size on colloidal perovskite nanoparticles, we discuss how size affects performance. Perovskite nanoparticles can be divided by the exciton Bohr radius in different perovskite. As perovskite nanoparticles are smaller than the double exciton Bohr radius, excitons will occur in quantum confinement.[48]

Perovskite nanoparticles of size less than the double exciton Bohr radius have high exciton binding energy and low exciton diffusion length resulting in high PLQY at room temperature.

The exciton Bohr radius is the preferred separation distance between the electron and hole probability distributions. The formula gives the exciton Bohr radius as:

ε_r is the dielectric constant (relative permittivity), m is the mass, μ is reduced mass, and a_b represents the Bohr radius.”

Finally, the level of English is not appropriate. Many terms are not used correctly and stylistically the text requires a very extensive editing and check.

Reply： We are sorry for the inappropriate English; we repolished it before resubmitting.

Submission Date

30 August 2022

Date of this review

05 Sep 2022 17:21:45

Author Response File: Author Response.docx

Reviewer 2 Report (New Reviewer)

In the manuscript titled “Synthesis and Applications of Halide Perovskite Nanocrystals in Optoelectronics”, the authors discussed the synthesis and applications of perovskite nanocrystals (NCs), which have more crucial benefits than traditional bulk materials. It is recommended for publication after the following concerns are addressed:

1.      In line 943 and 944, the authors said that “there are several typical nanocrystals: quantum dots, nanorods, nanowires, nanoplates, and nanosheets” and they described the application of nanowires for lasers. But in section 2, only the synthesis of quantum dots is mentioned. They should also include synthesis of other nanocrystals in section 2.

2.      Looking for new elements to replace lead is essential. The authors discussed about the lead-free perovskite NCs LEDs and solar cell in section 3.5 and 4.4. Do they consider combine the section 3.5 and 4.4 together and extend to other applications for setting up a new section of lead-free perovskite NCs devices?  

Author Response

Review Report (Reviewer 2)

Review Report Form

Open Review

( ) I would not like to sign my review report

(x) I would like to sign my review report

English language and style

(x) Extensive editing of English language and style required

( ) Moderate English changes required

( ) English language and style are fine/minor spell check required

( ) I don't feel qualified to judge about the English language and style

Is the work a significant contribution to the field?        

Is the work well organized and comprehensively described?         

Is the work scientifically sound and not misleading?    

Are there appropriate and adequate references to related and previous work?   

Is the English used correct and readable?   

Comments and Suggestions for Authors

Wang et al have reported a short review on “Synthesis and applications of perovskite nanocrystals in optoelectronics”. I have found many issues which need to be resolved before it could be considered for publication. Beside the language needs to be improved significantly for a better understanding of the readers. Overall the manuscript can be accepted if the necessary suggested changes are incorporated.

1. In section 2 they have reported the methods of synthesizing nanocrystals via different methods, impact of different surface modification treatments, ligands exchange, cation exchange methods for improved stabilities. Many abbreviations have been used for chemicals, solvent, and ligand names without giving their full names. Please try to provide the full names first and then the abbreviations: For example : DDAB-PbBr2,TOL, ODE, OA,

Reply： Thank you for the indication of our abbreviation problem. Here are our revisions. “oleylamine (OAm) (in line 95 on page 3), oleic acid (OA) (in line 95 on page 3), octadecene (ODE) (in line 113 on page 3), didodecyldimethylammonium bromide and lead bromide (DDAB-PbBr₂) (in line 140-141 on page 4), quaternary ammonium bromide (QABs) (in line 260 on page 11), dioctyldimethylammonium bromide (DQAB) (in line 262 on page 10), methyltrioctylammonium bromide (TrQAB) (in line 262 on page 10), and tetraoctylammonium bromide (TeQAB) (in line 263 on page 10), 4-fluoro-phenylmethylammonium iodide (4-F-PMAI) (in line 310 on page 13), 2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-MeOTAD) (in line 395-396 on page 16), methylammonium Iodide (MAI) (in line 403 on page 16), methyl acetate (MeOAc) (in line 438 on page 18), oleic acid (OA), oleylamine, (OLA), octanoic acid, and octylanmine(in line 460-461 on page 18).”

2. Avoid using any groups name instead use first author et al.

Reply：We corrected all of the groups name to first author et al, as “Brent A. Koscher et al.” (in the line 31 page 1), “Manoli et al.” (in the line 27 page 1), “Bodnarchuk et al.” (in the line 29 page 1), “Ya-Kun Wang et al.” (in the line 162 page 5), “Roo et al.” (in the line 128 page 4) , “Manoli et al.” (in the line 136 page 4) , “Bodnarchuk et al.” (in the line 146 page 4) , “Kim et al.” (in the line 145 page 5) , “Kojima et al.” (in the line 389 page 16) , “Im et al.” (in the line 392 page 16) , “Liu et al.” (in the line 435 page 17) , “Sanehira et al.” (in the line 445 page 18) , “Chen et al.” (in the line 454 page 18) , “Cha et al.” (in the line 477 page 19) , “Liu et al.” (in the line 441 page 18) , “Zai et al.” (in the line 494 page 20) , “Zheng et al.” (in the line 500 page 20) and “Yang et al.” (in the line 506 page 20).

3. The authors have mentioned “The purpose of using ligands is to control grain size.” It is not right to use the word grain for nanocrystals which suitable in case of bulk. It is better to use crystal size.

Reply： Thank you for the reminder. We corrected the grain size to “crystal” in line 102 on page 3.

4. “The colloidal perovskite nanocrystal is a kind of crystal with a size of about 2-20nm, which is divided into two types, nanocrystals or quantum dots.” It is not correct that nanocrystals and Quantum dots are the only two types in perovskite nano family! Please first define what are nanocrystals followed by how different shapes/morphology lead to different optical properties.

Reply： We are sorry for misleading the shape information and deleting the sentences to the reviewer.

5. It is recommended the authors should provide a section on how different ligand affect the charge carrier transport in perovskite nanocrystal based optoelectronic devices.

Reply： We appreciate your opinions to fulfill our review paper, and we have added some contents in the introduction and a new table about the ligand types, carrier mobility, perovskite material, and PLQY in Table 1.

(In lines 44-53 on page 2)

“Despite improved PLQY and stability of perovskite NCs, the film's charge carrier transport between NCs still lacks exploration. Some research focused on short-chain ligands [15,16,17,18] to exchange long ligands on perovskite NCs used to synthesize NCs. Jin Woo Choi et al. introduced the post-ligand exchange process (PLEP) in the solid-state to achieve defect passivation and enhanced charge transporting properties. The CH₃NH₃PbBr₃ QD film thus showed a high PLQY value of 82 % and increased hole mobility of 6.2 × 10⁻³ cm²V⁻¹s⁻¹ with hexyl amine ligand. The hole mobility is one order more elevated than that of the pristine one without PLEP (2.0 × 10⁻⁴ cm²V⁻¹s⁻¹) [15]. As a result, the short-chain ligand exchange in the solid-state may be an effective method to resolve the drawbacks of perovskite NCs.”

(In lines 187-194 on page 7)

“Table 1. Result of perovskite film after ligand exchange. KI inorganic ligand increased the mobility from 9 × 10^–4 cm² V^–1 s^–1 to 6 × 10^–2 cm² V^–1 s⁻¹ [14]. Hexyl amine ligand and octyl amine ligand can both increase hole mobility. The value of hole mobility are 6.2 × 10⁻³ cm² V⁻¹ s⁻¹ and 1.3 × 10⁻³ cm² V⁻¹ s⁻¹, respectively [15]. 2-aminoethanethiol (AET) enhanced electron mobility from 8.1 cm² V⁻¹ s⁻¹ to 11.8 cm² V⁻¹ s⁻¹ [16]. The aromatic acid/amine (benzoic acid or 4-phenylbutylamine) can shorten the distance between QDs and reduce leakage current [17]. Both DPPA (3,3-Diphenylpropylamine) + TCA (Trans-cinnamic acid) and DPPA + TFCA (derivates of trans-cinnamic acid) can make charge transport faster [18].

”

6. Is there any special reasons that the authors exclude perovskite nanocrystals in lasers application?

Reply： We are sorry for the missing perovskite laser application in the review paper, and we added it as a new section in lines 687-825 on pages 29-34.

“6. Laser

6.1 Introduction of laser device

Laser is an element that amplifies light signals and generates light intensity by applying radiation, mainly consisting of an energy pumping source, gain medium, and optical resonator. Typical optical resonators include Fabry Perot, distributed feedback (DFB) gratings, whispering gallery cavities (WGM), and Vertical Cavity Surface-Emitting Lasers (VCSELS). [94,95] As an optoelectronic component, optical gain and amplified spontaneous emission (ASE) thresholds are the most critical laser indicator. In brief, optical gain requires the population inversion of ions generated by light excitation to carry out the light gain. Figure 18a shows the three-level system and four-level system. The three-level system uses a pump source to stimulate particles to jump from the lowest energy level to the third energy level, but the particles of the third level have a shorter lifetime; they will be reversed in a non-radiative manner. The four-level system also applies the same principle, but the characteristic of the four-level system is higher efficiency than the three-level system.

There are several typical nanocrystals: quantum dots, nanorods, nanowires, nanoplates, and nanosheets. [96] In quantum dots, lasing can be observed without an external optical resonator. Amplification was achieved by light scattering between quantum dots, and when the light randomly forms a closed loop, it results in random fluctuations in the laser mode. Nanorods, nanowires, and nanoplates have the characteristic that they can confine light in a resonant cavity with a uniform shape and smooth section. Due to the easy reflectivity caused by the different reflection index between perovskite and air, the output interface can be considered an optical reflector. According to the particularity mentioned above, nanorods, nanowires, and nanoplates have great potential for realizing highly coherent output and laser devices.

Figure 18. (a)Schematic of the four-level system and three-level system [95]. Copyright 2019 The Authors. InfoMat published by John Wiley & Sons Australia, Ltd on behalf of UESTC (b) Experimentally determined PLQYs of the samples [97]. Copyright 2018, American Chemical Society (c) Pump-fluence-dependent emission spectra from a CsPbBr₃ NC film [99]. Copyright 2016, American Chemical Society

 Among all nanocrystals, quantum dots are the most common. Quantum dots have the advantage of localizing the charge carrier to avoid the occurrence of non-radiation recombination. However, it still exists two main problems. One is the phonon bottleneck effect, which will make exciton hard to relax to a low energy level. Another drawback of quantum dots laser devices is that a sizeable surface-to-volume ratio will cause a higher Auger recombination rate. Both of them result in an insufficient optical gain. A common way to solve the above problem is using a femtosecond laser for pumping, minimizing the impact of the Auger process with terse pulse times. Colloidal CsPbX₃ nanocrystals are widely studied perovskite due to the broad tuning of their light absorption and emission properties. In 2018, Yi Wang et al. [97] used the addition of PbBr₂, oleic acid (OA), and oleylamine (OLAM) to CsPbBr₃ colloidal perovskite made the grain size smaller (12 nm) and more uniformly. After the treatment, PLQY increased from 60 % to 95 % (Figure 18b) and obtained ASE as 1.2 μJ/cm².

    The advantages of two-photon-pumped lasers include avoiding unnecessary scattering and absorption losses, but low fluorescence quantum yields and fast Auger recombination cause high amplified spontaneous emission (ASE) thresholds [98]. This is harmful to the development of light-emitting elements. Hence, Yanqing Xu et al.’s report [99] combines CsPbBr₃ nanocrystals with glass microcapillary tubes to obtain an extremely low ASE of 0.8 m J /cm² and the emission wavelength at 525 to 535 nm. However, the device's performance has been poor due to the phase separation caused by CsPbBr₃. Hence, Loredana Protesescu et al. [100] used FAPbBr₃ as perovskite to overcome the lack of stability of CsPbBr₃. It also obtained smaller nanocrystals by controlling the reflection temperature and the amount of oleylammonium bromide (OAmBr) precursor. Finally, it can decrease ASE to 14 ± 2 m J /cm ² and acquire the emission wavelength at 530 to 535 nm.

6.2 Application of Perovskite NCs in Laser

Due to the long carrier lifetime and low non-radiation recombination rates, halide perovskites have outstanding performance in solar cells, and these properties are also ideal for lasers. Moreover, the easy processes and low cost make perovskite NCs more popular in the laser region. In 2015, Xing et al. [8] presented the vapor phase synthesis of high crystallinity MAPbI₃, MAPbBr_3, and MAPbI_xCl_3-x perovskite nanowires. These rectangular cross-sectional perovskite nanowires had excellent optical properties and long electron-hole diffusion lengths, representing good gain and efficient visual feedback. The optical-pumped room-temperature MAPbI₃ nanowire lasers showed a near-infrared wavelength of 777 nm, a low threshold of 11 J/cm², and a quality factor of 405. In the same year, Zhu et al. [9] demonstrated a room-temperature and widely wavelength-tunable laser from single-crystal MAPbX₃ (X=I, Br, Cl) perovskite nanowires (Figure 19a). The lasers achieved a high-quality factor of 3600 and low lasing thresholds of 220 nJ/cm² (Figure 19a).

Moreover, the kinetics analysis based on time-resolved fluorescence revealed that the estimated lasing quantum yields approached 100 % due to little charge carrier trapping in these single-crystal nanowires. These reports showed the promise of laser devices based on lead halide perovskites. However, organic-inorganic perovskite materials are known for their instability. All-inorganic perovskites, cesium lead halide perovskites, offer a solution with excellent emission tunability and simple synthesis. In 2016, Eaton et al. [101] reported the low-temperature, solution-phase growth of single-crystal CsPbBr₃ nanowires with low threshold lasing, high stability, and high quality of 1009. Pulsed excitation can be maintained for over 1 hour under constant lasing, equivalenting to 109 excitation cycles, and lasing persisted upon exposure to ambient atmosphere. The same year, Fu et al. [102] manufactured single-crystal CsPbX₃ nanowires and their alloys [CsPb(Br, Cl)₃] by facile solution growth. Moreover, they reported a low-temperature vapor-phase halide exchange method to covert CsPbBr₃ nanowires into [CsPb(Br, Cl)₃] alloys and metastable CsPbI₃ with well-preserved perovskite crystal lattice and nanowires morphology. The corresponding CsPbBr₃ nanowires laser devices showed stable lasing emission with no measurable degradation after at least 8 hours or 7.2 × 10⁹ laser shots under continuous illumination. Next year, Wang et al. [103] realized the first vertical cavity surface emitting lasers (VCSELs) based on the CsPbX₃ perovskite NCs, showing a low threshold of 9 μJ/cm², beam divergence of 3.6^o, and good stability.

Furthermore, the emission wavelength of lasers can be tuned across the red, green, and blue regions remaining comparable thresholds (Figure 19b). Although excellent stability of perovskite NCs has been achieved by surface modification or encapsulation in polymer and silica, they are not sufficiently refrained from the external environment due to the non-dense structures of these protective layers. Yuan et al. [104] reported a nanocrystallization strategy to directly grow CsPbBr₃ NCs on a specially designed TeO₂-based glass matrix. Due to the effective protecting of dense structural glass, NC-embedded glass showed bright green emission assigned to exciton recombination radiation and great photo/thermal stability (Figure 19c). In addition, after immersing NC-embedded glass in water for up to 120 hours, the emission intensity remained at 90%, showing an outstanding water resistance. In 2020, Tang et al. [105] proposed a solid-solid anion-diffusion process to construct a single CsPbCl_3-3xBr_3x perovskite alloy and a typical geometrical structure nanowire with the lowest lasing threshold of 11 μJ/cm² at 525 nm. With the different halide ratios, the CsPbCl_3-3xBr_3x perovskite can tune a widely tunable bandgap from 480 to 525 nm, achieving a broadly and continuously tunable laser.

Figure 19. (a) Integrated emission intensity and FWHM as a function of P of MAPbI₃ NWs, the optical image of single NW, and the widely tunable lasing emission wavelength form single-crystal MAPbX₃ NW lasers. [9]. Copyright 2015 Nature Publishing Group. (b) Blue and red lasing spectra of vertical cavity surface emitting lasers from CsPb(Br/Cl)₃ and CsPb(I/Br)₃ perovskite NCs under pump intensity of 38.2 and 30.5 μJ/cm² [103]. Copyright 2017 John Wiley & Sons, Inc. (c) Water resistance test by directly immersing QDs@glass/Colloidal QDs in aqueous solution, Two-dimensional excitation-emission mapping of QDs@glass, and Temperature-dependent PL intensities for QDs@glass and colloidal QDs via three heating/cooling cycles [104]. Copyright 2018 American Chemical Society.

6.3 Application of Perovskite NCs in Nonlinear optics

Nonlinear optics, the nonlinear relationship between the interaction of light and optical medium [106], can tunnel the wavelength of lasers. For many precise operations of processing and medical region, the single-wavelength laser cannot satisfy the demand. Still, nonlinear materials can change different output wavelength lasers with nonlinear optical frequency conversion. Therefore, nonlinear optics is an essential part of laser application. Halide perovskite NCs have been confirmed as potential materials of nonlinear optics due to the crystal structure and components of perovskite NCs. In 2015, Walters et al. [107] found the two-photon absorption of MAPbBr₃ perovskites. When MAPbBr₃ single crystals are pumped with 800 nm light, a band-to-band photoluminescence at 572 nm is observed (Figure 20a). The absorption coefficient of MAPbBr₃ perovskites was 8.6 cm/GW at 800 nm. (Figure 20b). The same year, Wang et al. [108] reported the multiphoton absorption and resultant photoluminescence of CsPbBr₃ NCs (Figure 20c) and found a large two-photon absorption cross-section of up to 1.2 × 10⁶ GM for nine nm-sized CsPbBr₃ NCs. They manufactured the three-photon pumped stimulated emission in green spectra range from colloidal NCs for the first time. Although this nonlinear process demonstrates the viability of halide perovskites as a convenient and low-cost nonlinear absorber for laser applications, their practical applications have been obstructed by the lack of materials holding both efficient two-photon absorption and ease of achieving population inversion. Xu et al. [99] tackled the problem by applying colloidal nanocrystals of CsPbBr₃ perovskite NCs and produced two-photon-pumped semiconductor perovskite NC lasers. They reported that a highly efficient two-photon absorption cross-section of 2.7 × 10⁶ GM in toluene solutions of CsPbBr3 NCs had an exciting significant optical gain of over 500 cm⁻¹. Next year, Amendola et al. [109] use a novel one-step synthesis based on pulsed-laser irradiation in a liquid environment (PLIL) to produce bromide perovskite NCs (Figure 20d). This PLIL procedure did not include any uses of high-boiling-point polar solvents or templating agents and ran at room temperature. They also showed the straightforward inclusion of laser-generated perovskite NCs in a polymeric matrix to form a nanocomposite with single- and two-photon luminescence properties. These findings advocated that perovskite NCs can be used as an excellent gain medium for high-performance frequency-up-conversion lasers for practical applications.

Figure 20. (a) Schematic showing two-photon absorption of 800 nm light and up-conversion to 572 nm photoluminescence and a linear optical absorbance and normalized two-photon-induced photoluminescence as a function of wavelength for a single MAPbBr₃ crystal. [107]. Copyright 2015 American Chemical Society. (b) The two-photon absorption coefficient of inverse transmission versus peak intensity. [107]. Copyright 2015 American Chemical Society. (c) The linear absorption spectrum and one-, two-, and three-photon excited PL spectra from the solution of CsPbBr₃ NCs and the excitation intensity-dependent PL showed the photograph of the resolution of CsPbBr₃ nanocrystals when an 800 nm laser beam passed through. [108]. Copyright 2015 American Chemical Society. (d) Schematic of perovskite NCs synthesis, schematic of optical absorption, PL and TPIF spectra of the PMMA/perovskite NCs nanocomposite, and schematic of TPIF intensity versus excitation power. [109]. Copyright 2016 John Wiley & Sons, Inc.”

Submission Date

30 August 2022

Date of this review

11 Sep 2022 22:53:52

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report (New Reviewer)

The revised version looks good to me

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

The manuscript "Synthesis and Applications of Halide Perovskite Nanocrystals in Optoelectronics" aims at drawing picture of the state of the art in the dynamic field of halide perovskiten naocrystals for various applications. The review is very basic, it mostly just lists some existing approaches on the synthesis and some applications with a few potential proposed improvements. The review critically lacks analysis of the current trends in such a rich and dynamic field as perovskite nanocrystals. Without this analysis it does not bring any novelty with respect to numerous reviews on very similar topics, which are regularly published in a variety of journals. In fact, each of the covered topics (synthesis, LEDs, solar cells, photodetectors) deserves a separate detailed review and I do not see a point in publishing an n-th work trying to englobe everything and simply listing existing technologies.

In addition, a critical analysis is lacking: what are the drawbacks of the existing synthesis methods and applications? What are future perspectives for each of the fields? No mention of the toxicity of the perovskite NCs is presented, no strategies to prepare and use Pb-free ones... Some very important papers are not cited... The definition of Bohr radius is not correct.

Finally, the level of English is not appropriate. Many terms are not used correctly and stylistically the text requires a very extensive editing and check.

Reviewer 2 Report

Wang et al have reported a short review on “Synthesis and applications of perovskite nanocrystals in optoelectronics”. I have found many issues which need to be resolved before it could be considered for publication. Beside the language needs to be improved significantly for a better understanding of the readers. Overall the manuscript can be accepted if the necessary suggested changes are incorporated.

1.       In section 2 they have reported the methods of synthesizing nanocrystals via different methods, impact of different surface modification treatments, ligands  exchange, cation exchange methods for improved stabilities. Many abbreviations have been used for chemicals, solvent, and ligand names without giving their full names. Please try to provide the full names first and then the abbreviations: For example : DDAB-PbBr2,TOL, ODE, OA,

2.       Avoid using any groups name instead use first author et al.

3.       The authors have mentioned “The purpose of using ligands is to control grain size.” It is not right to use the word grain for nanocrystals which suitable in case of bulk. It is better to use crystal size.

4.       “The colloidal perovskite nanocrystal is a kind of crystal with a size of about 2-20nm, which is divided into two types, nanocrystals or quantum dots.” It is not correct that nanocrystals and Quantum dots are the only two types in perovskite nano family! Please first define what are nanocrystals followed by how different shapes/morphology lead to different optical properties.

5.       It is recommended the authors should provide a section on how different ligand affect the charge carrier transport in perovskite nanocrystal based optoelectronic devices.

6.       Is there any special reasons that the authors exclude perovskite nanocrystals in lasers application?