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[Muhammad Asharuddin](https://orcid.org/0009-0006-9435-3983), [Rahmat Hidayat](https://orcid.org/0000-0002-3081-4051), [Adhita Asma Nurunnizar](https://orcid.org/0009-0001-0153-4448), [Natalita Maulani Nursam](https://orcid.org/0000-0002-9004-6269), [Valdi Rizki Yandri](https://orcid.org/0000-0002-0730-7529), Waode Sukmawati Arsyad, [Joko Suwardy](https://orcid.org/0000-0002-2799-5023), [Efi Dwi Indari](https://orcid.org/0000-0002-5809-0083), [Yoshiyuki Yamashita](https://orcid.org/0000-0003-0994-8095)

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[The effect of surface ligands on the surface chemical states and photoluminescence characteristics in cesium lead bromide perovskite nanocrystals](https://mdr.nims.go.jp/datasets/85084b5a-6066-400f-aa70-fd2d9d52bdb1)

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Microsoft Word - Final_Draft_The_Effect_of_Surface_Ligands_acceptted.docx 1  The Effect of Surface Ligands on the Surface Chemical States and 1 Photoluminescence Characteristics in Cesium Lead Bromide Perovskite 2 Nanocrystals  3 Muhammad Asharuddina, Rahmat Hidayata*, Adhita Asma Nurunnizarb, Natalita Maulani 4 Nursamb, Valdi Rizki Yandric, Waode Sukmawati Arsyadd, Joko Suwardye, Efi Dwi Indarie, and 5 Yoshiyuki Yamashitaf,g* 6  7 a Physics of Magnetism and Photonics Research Division, Faculty of Mathematics and 8 Natural Sciences, Bandung Institute of Technology, Jl. Ganesha 10, Bandung 40132, West 9 Java, Indonesia 10 b Research Center of Electronics, National Research and Innovation Agency, Jl. Sangkuriang, 11 Bandung 40132, West Java, Indonesia 12 c Department of Electrical Engineering, Polytechnic State of Padang, Limau Manis Padang 13 25164, West Sumatra, Indonesia 14 d Physics Department, Faculty of Mathematics and Natural Sciences, Halu Oleo University, 15 Anduonohu, Kendari, South East Sulawesi, 93232, Indonesia 16 e Research Center for Quantum Physics, National Research and Innovation Agency (BRIN), 17 KST BJ Habibie Serpong, Banten, Indonesia 15314 18 f Nano Electronics Device Materials Group, Research Center for Electronic and Optical 19 Materials, National Institute of Materials Science, 305-0044 1-1 Namiki Tsukuba Ibaraki, 20 Japan 21 g Graduate School of Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-22 0395, Japan 23  24 *corresponding authors: rahmat@itb.ac.id, yamashita.yoshiyuki@nims.go.jp 25  26 Abstract. This paper presents the results of our study on the relationship between the surface 27 chemical states, which are influenced by ligands, and photoluminescence (PL) characteristics in cesium 28 lead halide perovskite nanocrystals (NCs). NCs were synthesized via the Ligand-Assisted 29 Reprecipitation (LARP) and Ultrasonic-Assisted (URSOA) methods, which were able to produce NCs 30 with and without ligands. Although both synthesis methods used similar precursor composition and 31 processing steps, the resulting crystal structures of NCs are different. The LARP method yielded 32 orthorhombic CsPbBr3 NCs, while the URSOA method yielded a mixture of hexagonal Cs4PbBr6 and 33 orthorhombic CsPbBr3 NCs with an approximate weight ratio of ~10:1. The X-ray diffraction data 34 indicated that both NCs with and without ligands have the same crystal structure. However, as 35 revealed from photoelectron spectroscopy analysis for NCs without ligands, the measured chemical 36 states were found to be different between the inner side and the surface of NCs, which could be 37 associated with surface defect species from the accumulation of Cs+ atoms, Pb atoms with zero 38 oxidation state, unbonded Br atoms, and Br vacancies near and at the surface of the NCs. The 39  2  difference appears to be correlated with the observed photoluminescence (PL) characteristics. 40 Although the photoelectron spectroscopies measured only the core level orbitals, the variation in the 41 measured chemical states may correlate with electronic structure alteration in valence orbitals, which 42 are involved in the photoexcitation and exciton relaxation mechanism. The PL of LARP NCs 43 (orthorhombic CsPbBr3) shows two components of PL decay, which are largely suppressed in NCs with 44 purification or NCs without ligands. However, for URSOA NCs (predominantly Cs4PbBr6 NCs), the PL 45 decays are almost similar for both with and without ligands. The present experimental data clearly 46 show that the variations in photoluminescence (PL) characteristics, in addition to the crystal structure 47 that determines the basic characteristics of the formed excitons, may also originate from the influence 48 of surface states or surface defect species that are affected by surface ligands. This insight may be 49 useful not only for the further development of passivation molecules in a general context but also for 50 the design of buffer layer molecules in perovskite heterojunction devices. 51  52 Keywords. CsPbBr3, Cs4PbBr6, halide perovskite nanocrystals, surface ligand, chemical states, 53 photoluminescence  54 1.  Introduction 55 Lead-halide perovskite nanocrystals (NCs) constitute a class of emerging materials of significant 56 interest because of the possibility of tailoring their electronic and optical properties, which results in 57 modifications of their bandgap energy, emission wavelength, and charge-carrier mobility. 1,2,3,4 58 Therefore, lead halide perovskite NCs have been much explored as promising nanomaterials for 59 application in optoelectronic devices such as solar cells5, photodetectors6, lasers7, and light-emitting 60 devices.8  Lead-halide perovskite materials can typically be classified into two categories based on the 61 type of their cations, i.e., organic lead halide perovskites (often also referred as hybrid lead halide 62 perovskites) and all inorganic lead halide perovskites. Organic lead halide perovskites incorporate 63 organic monocation, such as methylammonium (MA+/CH3NH3+), formamidinium (FA+/CH5N2+), or 64 other similar molecules.9 Other halide anions, particularly Bromide (Br−), have also been widely 65 reported, which showed a larger open circuit voltage and better chemical stability despite a smaller 66 power conversion efficiency compared to those with Iodide (I−).10 In contrast, instead of incorporating 67 organic molecular cations, inorganic lead halide perovskites contain inorganic cations of alkali metals, 68 the first group of the periodic table, such as rubidium (Rb)11 and cesium (Cs). 12 Compared to organic 69 lead halide perovskites, inorganic cesium lead halides exhibit better long-term stability under ambient 70 conditions.13  71  3  Lead-halide perovskite NCs are usually synthesized by wet chemical processes, where various 72 shapes of NCs can be obtained such as quantum dots (QDs), nanoplates, and nanowires .14 Various 73 methods, such as hot injection15, solvent-induced reprecipitation16, microwave-assisted (MA)17, 74 ultrasonic-assisted (URSOA)18, and ligand-assisted reprecipitation (LARP) methods, have been 75 developed to synthesize these perovskite NCs with relatively high products reproducibility and 76 homogeneity.19,20 The LARP method is the most widely utilized method because of high reproducibility 77 with the possibility of varying ligands, precursors and solvents.21 The ligands play important roles in 78 dissolving the precursors, crystal seed formation, controlling crystal growth, and crystal surface 79 encapsulation by the ligands.22,23 In LARP, however, the crystal seed formation must be initiated by 80 the addition of an antisolvent to the precursor solution.24 Lead-halide perovskite QDs with high 81 photoluminescence quantum yields are mostly synthesized using this LARP method.25 In addition, by 82 choosing appropriate ligands and their composition ratios, the synthesis can form NCs with a particular 83 crystal structure, such as monoclinic, orthorhombic26, tetragonal27 or cubic structures.27 Another 84 synthesis method is the URSOA method, which also use a similar precursors and ligands as used in the 85 LARP method. However, the URSOA method employ ultrasonic wave, which play important roles to 86 initiate the nucleation of crystal seed and control the crystal growth.18,28,29 This method does not 87 require the heating of precursors and could be implemented as a “one pot” synthesis process. 88 Molecular vibrations produced by ultrasonic waves cause local heating inside the precursor solution, 89 allowing a rapid formation of NCs in an entirely room temperature environment. 90 The PL characteristics and stability of these lead halide perovskites have recently become a major 91 focus of research. The PL properties of perovskite NCs have been assigned to be originated from 92 excitons with various characteristics, such as free-excitons, self-trapped excitons (STEs), and exciton 93 polarons.30–32 The PL characteristics are also determined by crystal structure. CsPbBr3, one of the most 94 investigated lead halide perovskites, shows different PL wavelength and intensity depending on the 95 crystal structure, namely orthorhombic26, tetragonal27, or cubic structures.27 Various synthesis 96 parameters, such as precursor concentrations, ligands, antisolvent polarity, etc., might significantly 97 affect the PL characteristics of the synthesis products despite the products showing the same crystal 98 structure.30 Moreover, although the synthesized lead halide QDs or NCs show a perfect crystal 99 structure, from the X-ray diffraction (XRD) measurement for instance, and can effectively absorb 100 photons, some samples may show weak PL, suggesting the occurrence of strong non-radiative 101 recombination pathways.33,34 This non-radiative sites perhaps not present inside the NCs, but on the 102 NCs surface.35,36. It is therefore important to understand the relationship between the PL 103 characteristics and the surface chemical states, which may be influenced by surface ligands, to verify 104 the cause of the observation of PL characteristic variation. However, only a limited number of studies 105  4  have been reported in detail on the effect of ligands on the PL characteristics, the crystal structures of 106 the NCs, and the surface chemical states at the NC surface.13,33 Here, we report our study on the effects 107 of ligands on the crystal structures of the cesium lead bromide perovskite NCs, the chemical states at 108 the surfaces, and the PL characteristics by performing several characterizations using X-ray diffraction 109 (XRD),  X-ray photoelectron spectroscopy (XPS), hard X-ray photoelectron spectroscopy (HAXPES), and 110 Fourier transform infrared spectroscopy (FTIR). This work establishes a direct correlation between 111 surface states and PL characteristics that has not been explicitly addressed in previous studies, 112 marking a distinct advance beyond conventional ligand engineering or passivation strategies. Among 113 the aforementioned synthesis methods, in this study, we chose the LARP and URSOA methods for 114 synthesizing cesium lead bromides because both LARP and URSOA can use the same composition and 115 concentrations of precursors and ligands. In addition, those methods can produce both NCs with and 116 without ligands, which are useful for achieving the aim of the present study.18,27,37 117  118 2.  Experimental Section 119 2.1.  Materials  120 Cesium bromide (CsBr, > 99.99%) and lead bromide (PbBr2, < 99,99%), purchased from Tokyo Chemical 121 Industry Co., Ltd., were used as the perovskite precursors. Linoleic acid (LA) and Oleylamine (OlAm) 122 (purchased from Tokyo Chemical Industry Co., Ltd.) were used as ligands. Dimethyl sulfoxide (DMSO) 123 and dimethylformamide (DMF) were used as precursor solvents to synthesize cesium lead bromide 124 NCs. Toluene was used as the antisolvent, and ethanol was used as the solvent for the purification 125 process. Analytical-grade solvents (Merck/Sigma Aldrich) were used without further purification. 126 2.2.  Synthesis of Perovskite NCs 127 LARP Method. Fig. 1 schematically shows the main steps of the LARP method. This method is different 128 from the hot injection method because the reaction takes place at room temperature (~28°C). 129 Therefore, the crystal growth is accelerated by the addition of anti-solvent at the end of the synthesis 130 process. In the present study, only the LA ligand was used in the synthesis. OlAm ligand, which is 131 commonly reported in the literature,37,38 was not used because OlAm needs a high temperature to be 132 completely removed form nanoparticle surfaces.39 High temperature treatment above 100°C may 133 change the crystal structure of the resulted cesium lead bromide perovskites.40 For preparing the 134 precursor solution, CsBr (0.5 mol) and PbBr2 (0.5 mol) were separately dissolved in a DMF:DMSO (7:3 135 v/v) mixed solvent. Subsequently, the CsBr solution was added dropwise to the PbBr2 solution while 136 stirred until a 1:1 volume ratio was achieved. The solution was then continuously stirred for 30 min. 137  5  The LA ligand was then added to the precursor solution with the volume ratio of the LA ligand and the 138 precursor solution of 1:2. The solution was stirred again for 15 min. Finally, toluene, as the antisolvent. 139 was added dropwise to the precursor solution to initiate the formation of the cesium lead bromide 140 NCs.27,37 The precipitated powder was dried in a vacuum chamber at 100°C until the remaining solvent 141 completely evaporated. The powder was then stored and labeled as an unpurified NCs, or in other 142 words, NCs with ligands (LARP-WOL). 143  144  145 To obtain cesium lead bromide NCs without ligands on their surface, the cesium lead bromide NC 146 products were purified to remove the ligands by using a mixture solution of ethanol and toluene (1:1 147 v/v). The NCs product was added to the solution and then sonicated at 80 W for 10 min using an 148 ultrasonic processor. A centrifuge was used to collect the cesium lead bromide NCs from the 149 dispersion solution. Finally, these purified NCs were also dried in a vacuum chamber at 100°C to 150 evaporate the remaining solvent. Herein, this purified NCs sample is referred to as NCs with 151 purification (LARP-WOL). The average crystal sizes of these NCs without and with purification were 152 estimated to be 29.64 nm for LARP-WL and 33.22 nm for LARP-WOL. These crystal sizes were 153 estimated from the FWHMs of the XRD peak by using the Debye-Scherrer equation.41 154 URSOA Method. Fig. 2 schematically shows the main steps of the URSOA method. The URSOA method 155 involves synthesis steps similar to those of the LARP method. However, in the URSOA method, instead 156 of the use of anti-solvent, ultrasonication is used to initiate the seed formation of NCs. The synthesis 157 process was initiated by mixing the precursors CsBr and PbBr2 in the same molar ratio, followed by 158 the addition of LA/OlAm ligands (LA:OlAm = 1:1) in a precursor-to-ligand ratio of 2:1 v/v. Subsequently, 159 the solution mixture was ultrasonicated at 90 W for 10 min using an ultrasonic processor. Similarly to 160 Fig. 1. Synthesis process of cesium lead bromide perovskite NCs via the rapid LARP method. Fig. 2. The URSOA synthesis process of cesium lead bromide NCs.  6  the final synthesis step of the LARP samples above, to obtain the NC products, the precipitate and 161 solvent were separated using a centrifuge and vacuum dryer at 100°C. The products were referred to 162 NCs with ligands (URSOA-WL). A similar synthesis process was also performed without ligands, where 163 the products referred to as NCs without ligands (URSOA-WOL). The average crystallite sizes were 164 estimated to be 34.64 nm for NCs with ligands (URSOA-WL) and 38.28 nm for NCs without ligands 165 (URSOA-WOL). The names of the synthesized samples and the differences in their synthesis processes 166 are summarized in Table 1. 167 Table 1. Sample names and their differences.  168 Sample name Synthesis Method LA ligand OlAm ligand An -solvent Purifica on Ultra-sonica on With ligands LARP-WL LARP   toluene    LARP-WOL LARP   toluene    URSOA-WL URSOA       URSOA-WOL URSOA       Note:  shows the treatment was not used, while  shows it was used 169  170 2.3.  Characterization methods 171 The XRD measurements were performed on dried powder for all samples of cesium lead bromide 172 NCs using the SmartLab X-ray diffractometer (Rigaku, Japan). The wavelength was 1.5406 Å (Cu-Kα 173 radiation). XPS and HAXPES measurements were performed using the Quantes (ULVAC-PHI). Al Kα 174 (hv:1486.6 eV) and Cr Kα (hv:5414.8 eV) sources were used for XPS and HAXPES measurements, 175 respectively. The total energy resolutions were 0.51 and 0.76 eV, respectively. The binding energies 176 were calibrated using the binding energy of the C 1s core-level of the C-H bond of organic molecules 177 (284.8 eV). To investigate the adsorbed states of the ligands of the NCs, Fourier Transform Infrared 178 (FTIR) measurements (FTIR Alpha II, Bruker) were carried out utilizing the attenuated total reflection 179 mode. The photoluminescence spectra of the cesium lead bromide NCs were measured using an 180 Ocean Optics USB 2000 spectrometer with a 406 nm light source laser. The PL decays were measured 181 using an experimental setup consisting of a pico-second laser (PicoQuant) at 420 nm (with 20 ps pulse 182 width, 50 mW light power, and 10 MHz repetition rate), a photon microdevice detector, and a data 183 acquisition interface (TimeHarp 260 from PicoQuant).42 184 3.  Results and Discussion 185  7  3.1.  Crystal structures, chemical states, and PL characteristics of Cesium Lead Bromide 186 NCs prepared by LARP method 187 3.1.1.  Crystal structures and chemical states 188  189 The XRD measurements were performed to identify the crystal structures of the NCs prepared by the 190 LARP method (LARP WOL and LARP-WL). The XRD patterns of both NCs, as shown in Fig. 3 (a), indicate 191 the prominent peaks at 2θ = 15.2°, 21.6°, 30.5°, and 30.8°, which could be assigned to the (101), (121), 192 (040), and (202) diffraction planes of orthorhombic CsPbBr3 by referring the powder diffraction file 193 PDF-01-072-7929 shown in Fig. 3Fig. 3 (a).43 The crystal structure of the orthorhombic CsPbBr3 is 194 shown in Fig. 3 (b). The lattice constants were estimated to be a = 8.24 Å, b = 11.74 Å, and c = 8.2 Å. 195 Because both NCs have the indistinguishable XRD patterns, the orthorhombic crystal structure 196 remains unchanged after the purification process. However, in general, XRD shows the entire volume 197 of the NCs, which is consequently insensitive to changes in the crystal surface.  198 Fig. 4 (a) and (b) show the Cs 3d5/2 core-level HAXPES spectra for the NCs without and with 199 purification (LARP-WL and LARP-WOL), respectively. Both NCs exhibit one peak at 724.2 eV, which is 200 attributed to the Cs atoms in the CsPbBr3 NCs.44,45 Fig. 4 (c) and (d) show the Pb 4f core-level HAXPES 201 spectra for the NCs without and with purification (LARP-WL and LARP-WOL), respectively, which show 202 two peaks at Pb 4f7/2 (138.2 eV) and Pb 4f5/2 (143.1 eV). These peaks are attributed to the Pb atoms in 203 CsPbBr3 NCs.46 Fig. 4 (e) and (f) show the Br 3d core-level HAXPES spectra for the NCs without and with 204 purification (LARP-WL and LARP-WOL). Both samples have one component (Br 3d5/2 (68.2 eV) and Br 205 3d3/2 (69.2 eV)), which is due to the Br atoms in CsPbBr3 NCs.27,47 Because HAXPES exhibits bulk 206 Fig. 3. (a) XRD pa erns of the NCs with purifica on (LARP-WOL) and without purifica on (LARP-WL) and the reference data of the orthorhombic CsPbBr3 (PDF-01-072-7929). (b) the top and the side views of crystal structure of orthorhombic CsPbBr3.  8  chemical information (the information depth of ~10 nm46), the chemical states of the inner side of the 207 NCs might not be affected by the surface ligands and the purification processes.  208  209  210 XPS measurements were performed to investigate the surface chemical states of the NCs.  Note 211 that XPS exhibits relatively surface sensitive method in which the information depth is ~ 3 nm.48,49 Fig. 212 5 (a) shows the survey XPS spectra of both NCs, showing the presence of Cs, Pb, Br, C, and O atoms in 213 the CsPbBr3 NCs. These NCs show significant differences between the peak intensities of the C 1s and 214 O 1s core-levels, where the NCs without purification (LARP-WL) show higher peak intensities of C 1s 215 and O 1s than the NCs with purification (LARP-WOL). Fig. 5 (b) shows the C 1s spectrum of the NCs 216 146 145 144 143 142 141 140 139 138 137 136 135Intensity (Arb. Units)Binding Energy (eV) Pb 4f7/2Pb 4f5/2HAXPES Pb 4fWithout PurificationLARP-WL(c) 146 145 144 143 142 141 140 139 138 137 136 135Intensity (Arb. Units)Binding Energy (eV) Pb 4f7/2Pb 4f5/2HAXPES Pb 4fWith PurificationLARP-WOL(d) 72 71 70 69 68 67 66 65LARP-WOLIntensity (Arb. Units)Binding Energy (eV)  Br 3d3/2Br 3d5/2HAXPES Br 3dWith Purification72 71 70 69 68 67 66 65LARP-WLIntensity (Arb. Units)Binding Energy (eV)Br 3d3/2Without PurificationBr 3d5/2HAXPES Br 3d (f) (e) 728 727 726 725 724 723 722 721 720Intensity (Arb. Units)Binding Energy (eV)  HAXPES Cs 3d5/2Without PurificationLARP-WL728 727 726 725 724 723 722 721 720Intensity (Arb. Units)Binding Energy (eV)  HAXPES Cs 3d5/2With PurificationLARP-WOL(a) (b) Fig. 4. HAXPES spectra of the NCs without (LARP-WL) and with purifica on (LARP-WOL) (a) and (b) for Cs 3d5/2; (c) and (d) for Pb 4f; and (e) and (f) for Br 3d, respec vely.  9  without purification (LARP-WL), which can be deconvoluted into three components. The peak at 284.8 217 eV is attributed to C-C and C-H bonds of the LA molecules, whereas the peaks at 285.3 eV and 289.3 218 eV are associated with C=C and O-C=O bonds of the LA molecules, respectively.50 For the O 1s 219 spectrum (Fig. 5 (c)), there are two components observed at 532.4 eV and 533.7 eV, which are 220 attributed to O-C and O-C=O bonds of the LA molecules, respectively.50,51 Fig. 5 (d) and (e) show the C 221 1s and O 1s core-level XPS spectra for the NCs with purification (LARP-WOL), showing a drastic 222 decrease in the areal peak intensities comparing to the case of the NCs without purification (LARP-223 WL). Therefore, the ligands at the CsPbBr3 NC surface are removed by the purification process. The 224 peaks at 284.4 eV and 532.3 eV for respective C 1s and O 1s core-levels may be attributed to the 225 surface contaminations formed by sample preparation process52, which are usually observed in XPS 226 spectra.53 227  228  229 Fig. 5. XPS spectra survey for the NCs without (LARP-WL) and with purifica on (LARP-WOL) (b) C 1s and (c) O 1s of the NCs without purifica on (LARP-WL). (d) C 1s and (e) O 1s of the NCs (a(d) (b) (c) (e)  10   230  231 Fig. 6 (a) shows the Cs 3d5/2 core-level XPS spectrum of the NCs without purification (LARP-WL). 232 There is one peak at 724.2 eV, which is attributed to the Cs atoms in CsPbBr3 NCs.44,45 Fig. 6 (b) shows 233 the Cs 3d5/2 core-level XPS spectrum of the NCs with purification (LARP-WOL). There are two 234 components at 724.2 and 722.9 eV. The higher binding energy component at 724.2 eV (green 235 component shown in Fig. 6 (b)) could be attributed to the Cs atom in CsPbBr3 NCs44,45, whereas the 236 lower-binding-energy component at 722.9 eV (purple component) might be due to Cs+ accumulation 237 at the surface.54,55 Fig. 6 (c) and (d) show the Pb 4f core-level spectra of the NCs without and with 238 purification. The NC without purification (LARP-WL) (Fig. 6 (c)) has one component (Pb 4f7/2 (138.2 eV) 239 728 727 726 725 724 723 722 721 720Without PurificationLARP-WLIntensity (Arb. Units)Binding Energy (eV)  XPS Cs 3d5/2(a) 728 727 726 725 724 723 722 721 720With PurificationLARP-WOLIntensity (Arb. Units)Binding Energy (eV)XPS Cs 3d5/2(b) (c) 146 145 144 143 142 141 140 139 138 137 136 135 134Intensity (Arb. Units)Binding Energy (eV)Pb 4f5/2Pb 4f7/2With PurificationLARP-WOLXPS Pb 4f(d) (e) (f) 146 145 144 143 142 141 140 139 138 137 136 135 134Intensity (Arb. Units)Binding Energy (eV)Pb 4f5/2Pb 4f7/2 Without PurificationLARP-WLXPS Pb 4fFig. 6. XPS spectra of the NCs without (LARP-WL) and with purifica on (LARP-WOL); (a) and (b) for Cs 3d5/2; (c) and (d) for Pb 4f; and (e) and (f) for Br 3d, respec vely.   11  and Pb 4f5/2 (143.1 eV)), which is attributed to the Pb atoms in CsPbBr3 NCs.46 In contrast, the NC with 240 purification (LARP-WOL) (Fig. 6 (d)) has two components. The higher binding energy component (Pb 241 4f7/2 (138 eV) and Pb 4f5/2 (142.9 eV)) is attributed to the Pb atom in CsPbBr3 NCs, whereas the lower 242 binding energy state (Pb 4f7/2 (136.8 eV) and Pb 4f5/2 (141.7 eV)), might be due to Pb atom with 243 oxidation state of zero (Pb0) which might not bond with the Br atoms and could be present at the NCs 244 surface.55–57 245 The Br 3d core-level XPS spectra for the NCs without and with purification (respective LARP-WL 246 and LARP-WOL) are shown in Fig. 6 (e) and (f). For the NCs without purification (LARP-WL) (Fig. 6 (e)), 247 the Br 3d core-level spectrum has one component (green color). The green component at 68.2 eV (Br 248 3d5/2) and 69.2 eV (Br 3d3/2) could be originated from the Br atoms in CsPbBr3 NCs.27,47 In contrast, for 249 the NCs with purification (LARP-WOL) (Fig. 6 (f)), the Br 3d XPS spectrum shows two components. The 250 component at 68.0 eV (Br 3d5/2) and 69.0 eV (Br 3d3/2) (green color) could be originating from the Br 251 atom in the CsPbBr3 NCs.27,47 The lowest binding energy component at 66.8 eV (Br 3d5/2) and 67.8 eV 252 (Br 3d3/2) (purple color) might be due to unbonded Br atoms present at the NC surface.58 All core-level 253 spectra were fitted using a Voigt profile function, and the detailed fitting parameters are provided in 254 the Supplementary Information (Table S1 and S2). 255 In order to investigate the chemical species of the ligands before and after purification, the FTIR 256 measurements were performed for those NCs prepared by LARP methods. Fig. 7 shows the FTIR 257 spectra of the NCs without purification (LARP-WL) and with purification. (LARP-WOL). For the NCs 258 without purification (LARP-WL), a strong vibration peak at 1712.9 cm-1 is attributed to the C=O 259 (carbonyl) stretching vibration mode of the LA molecules whereas the peak at 2850–2924 cm-1 is due 260 to symmetric and asymmetric vibrations of CH2 groups.38,59,60 The peak at 3012 cm-1 is originated from 261 C-H stretching in the C=C-H species.61 For the NCs with purification (LARP-WOL), although the 262 intensities of those vibration bands are drastically decreased, several vibration bands such as C=O (at 263 1710.5 cm-1) and C-H vibrations (at 2848.6 cm-1 and 2921.6 cm-1) are still observed, indicating that an 264 extremely small residue of the LA molecules remains after the purification. Further the details in the 265 vibration band assignments of these FTIR spectra are shown in the Supplementary Information (Table 266 S3). Note that these FTIR results are consistent with the C 1 s XPS results described above, where 267 significantly weak components of C 1s and O 1s remain after purification process (LARP-WL) as shown 268 in Fig. 5 (a).  269 From XRD, HAXPES, XPS, and FTIR results, the structural models before and after purification are 270 proposed in Figs. 8Fig. 8 (a) and (b). Before purification (Fig. 8 (a)), the LA ligands are attached to the 271 surface of the NCs. The carboxylate group of the ligands, which are negatively charged (COO-)62, might 272  12  bond to the positively charged Pb2+ and Cs+ at the NC surface. As a result, the ligands could passivate 273 the surface atoms, thus preventing the formation of defect species at the NC surface. After the 274 purification process (Fig. 8 (b)), the ligands are removed leading to random the crystal periodicity 275 termination at NC surface, forming the accumulation of Cs cations, Pb0, and unbonded Br. In addition, 276 the Br anions might be also detached from the NC surface after purification, resulting in the formation 277 of Br vacancies. Our results suggest that the ligands might effectively prevent the formation of defect 278 species at the NCs surface. 279  280 Fig. 7. FTIR spectra of the NCs without purifica on (LARP-WL) and with purifica on 281 (LARP-WOL). The FTIR spectrum of the LA ligand is also shown as a reference 282 in the vibra on component assignments. 283  284 Fig. 8. Possible structures of the NCs near the surface (a) before and (b) a er 285 purifica on. 286 4000 3500 3000 2500 2000 1500 1000 500CH2 sym C-CC-HCOO- C-OC=OC-HTransmittance (Arb. Units)Wavenumber (cm-1)Linoleic AcidWithout Purification (LARP-WL)With Purification (LARP-WOL)CH2 asym (a) (b)  13   287 3.1.2.  PL characteris cs 288 Fig. 9 (a) shows the PL spectra of the NCs without purification (LARP-WL) and with purification (LARP-289 WOL). The NCs without purification (LARP-WL) exhibit a shorter wavelength peak and higher PL 290 intensity compared to the NCs with purification (LARP-WOL). Weak PL intensity observed at the NCs 291 without purification (LARP-WL) might be attributed to the surface defect species63,64, such as the 292 accumulation of Cs+, Pb0, unbonded Br atoms, and Br vacancies at the NC surface. These defect species 293 might form trap states in the bandgap and act as non-radiative recombination centers, decreasing the 294 PL intensity.65 For the observed red-shift peak, Liu et al. have conducted computational studies on the 295 electronic structure of CsPbBr3 NCs and have shown that CsPbBr3 NCs with Br vacancies have a smaller 296 band gap in comparison to the defect free CsPbBr3 NCs.66 Thus, the observed red-shift for the NCs with 297 purification (LARP-WOL)  may be thus caused by the Br vacancies formed at the NC surface.67  298  299   300 (a)                                   (b) 301 Fig. 9. (a) PL spectra of the NCs without (LARP-WL) and with purifica on (LARP-WOL). 302 (b) The normalized intensity of PL as a func on of me measured for the LARP-303 WL and LARP-WOL NCs. The fi ng results are shown as solid lines. 304  305 Fig. 9(b) shows the PL intensity as a function of time (decay) for NCs with and without purification, 306 indicating that the PL decay of the NCs with purification (LARP-WOL) exhibits more rapid decay 307 compared to the case of the NCs without purification (LARP-WL). According to the previous studies, 308 the PL intensity as a function of time (decay) can be fitted with a bi-exponential function consisting of 309 the fast and the slow decay components, which is given by68,69 310 0 2 4 6 8 10 12 14 16 18 20 22 24 26 280.00.20.40.60.81.0Normalized PL Intensitytime (ns) LARP WL LARP WOL Fitting Result - LARP WL Fitting Result - LARP WOL(LARP-WOL) (LARP-WL)  14  𝐼 𝑡 𝐼 exp 𝐼 exp ,    (1) 311 where I(t) represents the PL intensity at the time t, I1 and I2 denote the initial intensities of the fast 312 and the slow decay components, and τ1 andτ2 denote the decay time constants of the fast and the 313 slow decays, respectively. The average decay time (τavg) can be calculated using the following 314 equation69 : 315 𝜏       (2) 316 The decay parameters obtained from fitting results are shown in Table 2. The fast decay component 317 could be ascribed to the radiative recombination of free excitons, whereas the slow decay could be 318 associated with the radiative recombination of trapped excitons.70,71 Longer decay time for the NCs 319 without purification (LARP-WL) might indicate that the radiative recombination of free excitons could 320 be dominant.72 In contrast, faster decays in the NCs with purification (LARP-WOL) may be explained 321 by non-radiative recombination via defect states.69,73 322 According to the previous report, the fast decay is originated from the free exciton recombination, 323 and the recombination occurs at the inner side of NCs70. On the other hand, the slow decay due to the 324 trapped exciton may occur mainly at the NC surface.71 In the case of the NCs with purification (LARP-325 WOL), the surface defect species such as accumulation of Cs, Pb0, the unbonded Br atoms, and the Br 326 vacancies could trap the excitons as non-radiative centers, resulting in shorter lifetime for radiative 327 recombination and dominance of non-radiative recombination. A similar trend was observed in the 328 average decay time, where NCs without purification (LARP-WL) is longer than NCs with purification 329 (LARP-WOL), indicating that ligands effectively suppress non-radiative recombination pathways and 330 prolong carrier lifetimes.    331 Table 2. The fi ng results obtained from curve fi ngs of PL decays in Fig. 9 using a bi-exponen al 332 decay func on. I1 and I2 are the normalized ini al intensi es. 333 Sample I1 τ1 (ns) I2 τ2 (ns) τ avg (ns) NCs without purifica on (LARP-WL) 0.58 1.77 0.50 14.12 12.55 NCs with purifica on (LARP-WOL) 0.56 0.98 0.43 5.02 4.20  334 3.2.  Crystal structures, chemical states and PL characteristics of Cesium Lead Bromide NCs 335 prepared by URSOA 336 3.2.1.  Crystal structures and chemical states in the NCs 337  15  The XRD patterns of the NCs without and with ligands prepared by the URSOA method (URSOA-338 WOL and URSOA-WL) are shown in Fig. 10 (a). These NCs show not only the indistinguishable XRD 339 patterns of the NCs prepared by the LARP method but also exhibit additional XRD peaks observed only 340 by the URSOA method. By comparison with the reference XRD data PDF- 01-077-8224, the diffraction 341 peaks at 12.7°, 12.9°, 20.1°, 22.5°, 25.5°, 27.7°, 28.6°, 30.3°, 30.9°, 34.2° and 38.9° could be assigned 342 to the crystal planes (012), (110), (113), (300), (024), (131), (214), (223), (006), (134), and (324) of 343 Cs4PbBr6 with hexagonal crystal structure.18,69 The lattice constant was estimated to be a=b=13.72 Å 344 and c=17.32 Å, with α=β=90° and γ=120°.74,75 In addition, in comparison with PDF-01-072-7929, the 345 peaks at 29°, 30.7°, 34.6°, and 43.7° can be assigned to the (122), (202), (103), and (242) planes of the 346 orthorhombic CsPbBr3 crystal structure.27,69 The lattice constant was estimated to be a = 8.24 Å, b = 347 11.74 Å, and c = 8.2 Å. These XRD patterns indicate the formation of both CsPbBr3 and Cs4PbBr6 crystal 348 phases in the NCs prepared by URSOA method. 349 It should be noted that CsPbBr3 exhibits a three-dimensional (3D) perovskite structure with 350 orthorhombic structure (Pnma space group) at room temperature. This phase compose of a 351 continuous network of corner-sharing [PbBr6]4- octahedra, where Cs⁺ ions occupy the inters al A-site 352 positions within the perovskite framework76, as shown in Fig. 10  (b). In contrast, Cs4PbBr6 has a 353 hexagonal crystal structure (R-3 c space group)18,77, like an array or cluster of isolated [PbBr6]4- 354 octahedra groups connected with Cs+ ions, which act as spacers to maintain the separation between 355 octahedra, as shown in Fig. 10 (c).18 Therefore, regardless of the presence of the ligands, two distinct 356 phases of the perovskite crystals are formed, namely orthorhombic CsPbBr3 and hexagonal Cs4PbBr6 357 crystal structures. By performing quantitative phase-analysis using the Rietveld method, the XRD 358 patterns can be analyzed to determine the proportion of each phase.78 The phase molar ratios of 359 Cs4PbBr6 to CsPbBr3 for the NCs with and without ligands (respective URSOA-WL and URSOA-WOL) 360 were estimated to be 10.1:1 and 11.5:1, respectively, which are relative good agreement with the 361 other studies.69,79,80 362 Fig. 11 (a) and (b) show the Cs 3d5/2 core-level HAXPES spectra for the NCs with and without ligands 363 (URSOA-WL and URSOA-WOL). These HAXPES spectra were fitted by referring to the phase molar 364 ratios of Cs4PbBr6:CsPbBr3, based on the XRD analysis results above, at 10.1:1 for URSOA-WOL and 365 11.5:1 for URSOA-WL. The component at 724 eV (green color peak) can be assign to the Cs atoms in 366 the CsPbBr3 NCs, whereas the lower binding energy component at 723.3 eV (cream color peak) can be 367 assigned to the Cs atoms in the Cs4PbBr6 NCs.44,45,81  Fig. 11 (c) and (d) show the Pb 4f core-level HAXPES 368 spectra for the NCs with and without ligands (URSOA-WL and URSOA-WOL), which also indicate that 369 the Pb atoms exhibit two different chemical states. The lower binding energy component at 137.4 eV 370 (Pb 4f7/2) and 142.3 eV (Pb 4f5/2)) is attributed to the Pb atoms in the Cs4PbBr6 NCs 82, whereas the 371  16  higher binding energy component at 138.0 eV (Pb 4f7/2) and 142.8 eV (Pb 4f5/2) could be due to the Pb 372 atom in the CsPbBr3 NCs.46 Fig. 11 (e) and (f) show the Br 3d core-level HAXPES spectra. The component 373 at 67.2 eV (Br 3d5/2) and 68.2 eV (Br 3d3/2) could be attributed to the Br atoms of the Cs4PbBr6 NCs 28,83, 374 whereas the higher binding energy component (green color) peaked at 68.2 eV (Br 3d5/2) and 69.2 eV 375 (Br 3d3/2) could be due to the Br atoms of the CsPbBr3 NCs.28 Therefore, the chemical states of the 376 inner side of the NCs are not affected by the presence and absence of the ligands.  377  378 Fig. 10. XRD pa erns of the NCs without (blue line) and with (green line) ligands. The (★) symbol indicates the diffrac on peak from CsPbBr3 phase, whereas the (▲) symbol indicates the diffrac on peak from Cs4PbBr6 phase. Reference data for orthorhombic CsPbBr3 (PDF-01-072-7929) (black line) and hexagonal Cs4PbBr6 (PDF-01-077-8224) (red line) are also shown for comparison. (b) The orthorhombic crystal structure of CsPbBr3 (Pnma space group) and (c) the hexagonal crystal structure of Cs4PbBr6 (R-3 c  17  Fig. 12 (a) shows the survey XPS spectrum of the NCs with and without ligands (URSOA-WL and 379 URSOA-WOL), showing a substantial difference in the peak intensities of C 1s and O 1s; the NCs with 380 ligands (URSOA-WL) have much higher peak intensities of C 1s and O 1s than the NCs without ligands 381 (URSOA-WOL). Fig. 12 (b) shows the C 1s spectrum of the NCs with ligands (URSOA-WL), which can be 382 deconvoluted into three components. The peak at 284.8 eV is attributed to the C atoms of C-C and C-383 H bonds of the LA and the OlAm molecules, whereas the peaks at 285.4 eV and 288.1 eV are due to 384 the C atoms of C=C/C-N and O-C=O bonds of the LA and the OlAm molecules, respectively.50 For the O 385 1s XPS spectrum of the NCs with ligands (URSOA-WL) (Fig. 12 (c)), there are two components at 532.2 386 eV and 533.6 eV, which might be attributed to O-C and O-C=O bonds of the LA and the OlAm molecules, 387 respectively.50,51 The weak peak intensities of the C 1s and O1s core-levels of the NCs without ligands 388 (URSOA-WOL), as shown in Fig. 12 (d) and (e), might be originated from the contaminants formed by 389 sample preparation process.52  390  18   391  392 145 144 143 142 141 140 139 138 137 136 135 Intensity (Arb. Units)Binding Energy (eV)Without LigandURSOA-WOLHAXPES Pb 4f Cs4PbBr6CsPbBr3Pb 4f5/2Pb 4f7/2728 727 726 725 724 723 722 721 720Cs4PbBr6CsPbBr3Intensity (Arb. Units)Binding Energy (eV)With LigandURSOA-WLHAXPES Cs 3d5/2728 727 726 725 724 723 722 721 720Intensity (Arb. Units)Binding Energy (eV)Without LigandURSOA-WOLHAXPES Cs 3d5/2Cs4PbBr6CsPbBr3145 144 143 142 141 140 139 138 137 136 135 Intensity (Arb. Units)Binding Energy (eV)With LigandURSOA-WLHAXPES Pb 4fCs4PbBr6CsPbBr3Pb 4f5/2Pb 4f7/271 70 69 68 67 66 65Intensity (Arb. Units)Binding Energy (eV)With LigandURSOA-WLHAXPES Br 3d Cs4PbBr6CsPbBr3Br 3d3/2Br 3d5/271 70 69 68 67 66 65Intensity (Arb. Units)Binding Energy (eV)Without LigandURSOA-WOLHAXPES Br 3d Cs4PbBr6CsPbBr3Br 3d5/2Br 3d3/2(a) (b) (c) (d) (e) (f) Fig. 11. HAXPES spectra for the NCs with (URSOA-WL) and without ligand (URSOA-WL): (a) and (b) for Cs 3d;5/2 (c) and (d) for Pb 4f; and (e) and (f) for Br 3d, respec vely.  19   393  394 Fig. 13 (a) shows the Cs 3d5/2 core-level XPS spectrum for the NCs with the ligands (URSOA-WL). 395 The higher binding energy component at 724 eV is attributed to the Cs atom in the CsPbBr3 NCs (green 396 color)45, whereas the lower binding energy component at 723.3 eV is due to the Cs atom in Cs4PbBr6 397 NCs (cream color).28 In contrast, for the NCs without ligands (URSOA-WOL), there are three 398 components as shown in Fig. 13 (b). The highest and the second highest binding energy components 399 at 724 eV and 723.3 eV are attributed to the Cs atom in the CsPbBr3 and the Cs4PbBr6 NCs, 400 respectively.28,45 The lowest binding energy component at 722 eV (purple color) might be attributed 401 to the accumulation of Cs+ at the surface of the NCs.54,55 402  403  404 URSOA-WL URSOA-WL URSOA-WL URSOA-WOL (a) (d) URSOA-WOL (e) URSOA-WOL (b) (c) Fig. 12. (a) XPS survey spectra for the NCs with and without ligands. XPS spectra for (b) C 1s and (c) O 1s of NCs with ligands (URSOA-WL). XPS spectra for (d) C 1s and (e) O 1s of NCs without ligands (URSOA WOL) 20   405 Fig. 13 (c) and (d) show the Pb 4f XPS spectra for the NCs with and without ligands (URSOA-WL and 406 URSOA-WOL). For the NCs with the ligands (URSOA-WL), the highest and the second highest binding 407 energy peaks at 138.0 eV and 137.3 eV could be attributed to the Pb atoms in the CsPbBr3 and Cs4PbBr6 408 NCs, respectively.82 In contrast, for the NCs without the ligands (URSOA-WOL), the components at the 409 highest binding energy and the second highest binding energy can be attributed to the Pb atoms in 410 the CsPbBr3 and the Cs4PbBr6 NCs, respectively. The lowest binding energy component observed at 411 136.2 eV (purple color) might be attributed to Pb0.46,57 Fig. 13 (e) shows the Br 3d spectrum of the NCs 412 with ligands (URSOA-WL). The green color components are attributed to the Br atoms in CsPbBr3 NCs, 413 27,47,84  whereas the cream color components might be due to the Br atoms in Cs4PbBr6 NCs.28,84 On the 414 other hand, for Br 3d spectrum of the NCs without ligands (URSOA-WOL), there is one additional 415 (b) (d) (c) 145 144 143 142 141 140 139 138 137 136 135Intensity (Arb. Units)Binding Energy (eV) Pb 4f5/2 Without LigandURSOA-WOLXPS Pb 4f Pb 4f7/2Pb0Cs4PbBr6CsPbBr3145 144 143 142 141 140 139 138 137 136 135Intensity (Arb. Units)Binding Energy (eV)With LigandURSOA-WLXPS Pb 4fCs4PbBr6CsPbBr3Pb 4f5/2Pb 4f7/2(a) 727 726 725 724 723 722 721 720Intensity (Arb. Units)Binding Energy (eV)  Without LigandURSOA-WOLXPS Cs 3d5/2 Cs4PbBr6CsPbBr3Cs accumulation728 727 726 725 724 723 722 721 720Intensity (Arb. Units)Binding Energy (eV)With LigandURSOA-WLXPS Cs 3d5/2Cs4PbBr6CsPbBr3(f) (e) Fig. 13.  XPS spectra of the NCs with (URSOA-WL) and without ligands (URSOA-WOL); (a) and (b) Cs 3d5/2; (c) and (d) Pb 4f; and (e) and (f) Br 3d, respec vely.  21  component (purple color) observed at 66.7 eV (Br 3d5/2) (Fig. 13 (f)), which may be attributed to the 416 unbonded Br atoms present at the NCs surface.58  417 Fig. 14 shows the FTIR spectra of the NCs with and without ligands (URSOA-WL and URSOA-WOL). 418 The NCs with ligands (URSOA-WL) show similar vibrational peaks to the LA (green) and the OlAm 419 (magenta) molecules, indicating that the LA and the OlAm molecules could bond to the NC surface. 420 The bands at 2919 and 2849 cm-1 correspond to asymmetric and symmetric stretching vibration mode 421 of the –CH2– groups38,59,60 in long-chain alkyl groups of both LA and OlAm molecules. A weak shoulder 422 around 3006 cm-1 corresponds to C-H stretching mode.59 The vibration peak at 1731 cm-1 is attributed 423 to the C=O (carbonyl) stretching vibration mode of the LA molecules. The absorption at 1538 cm-1 is 424 attributed to the asymmetric stretching vibration mode of COO- of the carboxylate group, whereas 425 the band around ~1400 cm-1 corresponds to symmetric stretch mode of COO- of the carboxylate group, 426 indicating the deprotonated carboxylic acid of the LA molecules. Additionally, a band around 1500 cm-427 1 is assigned to N–H bending mode38,85, which is due to amine species of the OlAm molecule.60 The 428 peak at 1019 cm-1 is assigned to the C–N stretching vibration modes of the OlAm molecule.86 In 429 contrast, for the NCs without ligand (URSOA-WOL), the vibrational peaks corresponding to the LA and 430 the OlAm molecules are hardly ever observed, indicating the LA and the OlAm molecules could not 431 bond to the NC surface. The detail vibrational band assignments for the FTIR spectra of these samples 432 are provided in the Supplementary Information (Table S4). 433  Fig. 15 (a) shows the proposed surface structure of the NCs with ligands obtained from XRD, 434 HAXPES, XPS, and FTIR results. In the case of URSOA-WL, the LA and the OlAm molecules could interact 435 with the NC surface. The carboxylate groups of the LA molecules may form a bonding with the Cs and 436 the Pb cations at the outermost crystal sites, or they may also attach at the Br vacancies on the NC 437 surface.62 In the case of the OlAm molecules, the amine group (−NH2) might be positively charged due 438 to the formation of -NH3+87,88, which might bond to the Br anions at the NC surface. The amine group 439 enables the OlAm ligand to bond specifically to Br anion at the outermost NCs surface. Thus, these 440 ligands are able to passivate the surface defects and inhibit the presence of uncoordinated atoms. On 441 the other hand, the NCs without ligand (URSOA-WOL) exhibit the formation of surface defect species 442 such as the accumulation of Cs+, Pb0, unbonded Br atoms, and Br vacancies at the NC surface. 443 Therefore, the ligands effectively prevent the formation of defects near the surface of the NCs.  444  22   445 Fig. 14. FTIR spectra of the NCs without ligands (URSOA-WOL) and with ligands 446 (URSOA-WL). The FTIR spectra of LA and OlAm ligands are also shown as the 447 references. 448  449 3.2.2.  PL characteris cs 450 Fig. 16 (a) shows the PL spectra of the NCs with and without ligands (URSOA-WL and URSOA-WOL) 451 where the PL peaks for both NCs are observed at almost the same wavelengths of 523 and 525 nm, 452 4000 3500 3000 2500 2000 1500 1000 500CH2 asyma-COO-N-HC-HC-N C-HCOO-%TransmittanceWavenumber (cm-1)Without LigandWith LigandLinoleic AcidOleylamineC-H CH2 symC=OC-O(a) (b) Fig. 15. Structural models of the NC near the surface (a) with ligands (URSOA-WL) and (b) without ligands (URSOA-WOL). Cs4PbBr6 CsPbBr3 Cs4PbBr6   23  respectively. The NCs with ligands (URSOA-WL) exhibit a stronger PL intensity than the NCs without 453 ligands (URSOA-WOL). Based on the XPS results described above, defect species are not observed at 454 the NC surface in the case of the NCs with ligands (URSOA-WL). Under these circumstances, the 455 radiative recombination could be more predominant than the non-radiative recombination, leading 456 to intense PL intensity. In contrast, in the case of the NCs without ligands (URSOA-WOL), the NCs could 457 form the surface defect species which may form the trap states within the band gap.89 The excitons 458 trapped at the surface defect species could exhibit non-radiative recombination process. Thus, weaker 459 PL intensity observed for the NCs without ligands (URSOA-WOL) may be caused by the surface defect 460 species which might act as non-radiative decay center.  461  462  463  464  465  466  467  468 Fig. 16. (a) PL spectra of the NCs prepared by URSOA method: the NCs with (red line) 469 and without ligands (black line). (b) The normalized intensity of PL as a func on 470 of me measured for the URSOA-WL and URSOA-WOL NCs. The fi ng results 471 are also shown as solid lines. 472  473 The PL decays observed in the NCs prepared by the URSOA method (URSOA-WL and URSOA-WOL) 474 show almost similar decay characteristics, as seen in Fig. 16 (b). These PL decays were fitted using a 475 bi-exponential function with two distinct decay time constants, including their average decay time.70,71 476 The PL decays in Fig. 16 (b) and the fitting results (Table 3) indicate that the NCs prepared by the 477 URSOA method (URSOA-WL and URSOA-WOL) exhibit indistinguishable decay features. This fact 478 implies that the excitons in these NCs might not be affected by the ligands and the surface defect 479 species. According to the previous study, due to the zero-dimensional (0D) characteristics of Cs4PbBr6 480 crystal structure, strong exciton confinement might occur in the isolated individual [PbBr6]4- 481 octahedron.17 Although the NCs prepared by the URSOA method contain Cs4PbBr6 and CsPbBr3 phases 482 with the ratio of nearly equals to 10:1, the PL might be predominantly occurred on the Cs4PbBr6 NCs. 483 The contribution from CsPbBr3 NCs in the PL spectra is not seen because the PL peak of CsPbBr3 NCs 484 is slightly red-shifted to a wavelength longer than 525 nm, as described in 4.1.2. The PL characteristics 485 (a) With Ligand (URSOA-WL) Without Ligand (URSOA-WOL) (a) (b)  24  of these URSOA NCs indicate a higher exciton confinement in the Cs4PbBr6 NCs phase compared to 486 the CsPbBr3 NCs phase.18,90 Regarding the possibility of phase ratio changes, it should be noted that 487 such phase change can be disregarded by considering the XRD data of those URSOA NCs, which show 488 the similar phase composition (hexagonal Cs4PbBr6 and orthorhombic CsPbBr3) in URSOA NCs with 489 and without ligands (URSOA-WL and -WOL NCs). Therefore, a slight increase in the PL intensity of the 490 URSOA-WL is originated from the surface ligands, which act as passivation molecules.  491 Table 3. The fi ng results obtained from the curve fi ngs of PL decays in Fig. 16 (b) using a bi-492 exponen al func on. I1 and I2 are the normalized ini al intensi es. 493 Sample I1 τ1 (ns) I2 τ2 (ns) τ avg (ns) NCs with Ligand (URSOA-WL) 0.55 1.22 0.43 7.26 6.19 NCs without Ligand (URSOA-WOL) 0.55 1.06 0.42 7.23 6.14  494 3.3.  Comparison between NCs obtained from LARP and URSOA synthesis methods 495 First of all, we discuss the influence of synthesis methods on the crystal structures of the resulting NCs. 496 Although both LARP and URSOA methods incorporate similar synthesis steps and precursor 497 compositions, the resulting NCs have different crystal structures. The LARP method yielded 498 orthorhombic CsPbBr3 NCs. However, the URSOA method yielded a mixture of the orthorhombic 499 CsPbBr3 phase and the hexagonal Cs4PbBr6 NCs phase, where the phase molar ratio of 500 Cs4PbBr6:CsPbBr3 is nearly equal to 10:1. The LARP method facilitates a gradual crystallization process, 501 resulting in the high crystal symmetry CsPbBr3 NCs. In contrast, in the URSOA method, ultrasonic 502 waves may lead to cavitation effects and localized heating, characterized by intense atomic vibrations. 503 This condition may hinder the formation of highly symmetric crystal structures. Under these 504 conditions, ultrasonication tends to produce Cs4PbBr6 NCs more preferentially rather than CsPbBr3 505 NCs.29 506 Next, we discuss the relationship between the ligand and the surface chemical states. In the LARP 507 method, LA could dissociate into a linoleate anion (L−) and proton (H+), in which the L− anion tends to 508 act as Lewis acids. During the crystal (surface) formation, the L− anion might bond to the Cs+ and Pb2+ 509 cations by using carboxyl -COO− group  whereas H+ may bond to the Br atom (as shown in Fig. 8).51,62,91 510 Therefore, the LA molecules might prevent the formation of defect states at the NC surface. In the 511 absence of the LA ligand, since the Cs, Pb, and Br atoms at the NC surface might be unstable, these 512 atoms could be easily removed from their sites in the crystalline structure, forming the accumulations 513 of Cs+, Pb0, unbonded Br atoms, and Br vacancies at the NC surface. In the URSOA method, on the 514  25  other hand, two kinds of ligands, namely LA and OlAm were used. The LA molecules might dissociate 515 into L− and H+, where L− could bond to the Cs+ and Pb2+ cations and H+ bond to Br− anions at the NC 516 surface. In addition, H⁺ might interact with NH2 species of the OlAm molecules, forming OlAm+ or –517 NH3+ cations.14,91 These cations could bond to Br− anions at the NC surface.92 Several previous studies 518 have highlighted the significance of the interaction between Br− and ligands, particularly the formation 519 of Br–oleylammonium on the NC surface. 93,94 This interaction facilitates robust passivation, thereby 520 preventing the formation of Br vacancies and under-coordinated Pb²+.93,94 Thus, the LA and the OlAm 521 ligands could neutralize the surface defect species, including defect vacancies, and passivate the 522 surface defect states that could act as non-radiative recombination centers. In contrast to NCs without 523 those ligands, where the surface defect species, such as the accumulations of Cs+, Pb0, unbonded Br 524 atoms, and Br vacancies, were formed at the NC surface, the presence of ligands evidently prevents 525 the formation of those surface defect species.  526 Finally, we also need to discuss the difference in PL characteristics observed in the NCs prepared 527 by the LARP and URSOA methods in relation to their surface chemical states The PL of LARP NCs mainly 528 seems weaker than the URSOA NCs. In the LARP method, we did not use OlAm ligand during the 529 synthesis. In the URSOA method, however, OlAm ligand was used in addition to OA ligand. It has been 530 reported the formation of Br–oleylammonium provides strong passivation and can stabilize excitons 531 on the NC surface, which thus results in high photoluminescence efficiency. 93–95  The presence of only 532 Cs–oleate cannot provide effective passivation, resulting in a large number of trap states  and reducing 533 photoluminescence efficiency.93 Therefore, these facts strongly emphasize the effect of ligands on PL 534 characteristics by minimizing the non-radiative recombination sites on the surface of NCs.   535 In addition, it is also important to consider also the difference of PL characteristics in those LARP 536 and URSOA in relation to their crystal structures. The PL characteristics of the LARP CsPbBr3 NCs will 537 be related to its orthorhombic crystal structure, where the [PbBr6]4- octahedrons are continuously 538 connected through a corner-sharing arrangement to form a 3D network76, as illustrated in Fig. 10 (b). 539 This 3D network facilitates the formation of excitons with large Bohr radius and long diffusion 540 length.96,97 Certain excitons may diffuse towards the surfaces of the NCs, where they subsequently 541 emit PL. However, certain excitons may be trapped at the surface defect sites, which can undergo 542 nonradiative recombination or emit PL at a lower photon energy and intensity. Consequently, PL 543 quenching may occur and shorten the PL lifetime, as observed in the NCs without ligand (LARP-WOL). 544 Different situation occurs in the URSOA NCs consisting of the predominant Cs4PbBr6 NCs structure, 545 where each [PbBr6]4− octahedron is surrounded by the Cs atoms being isolated without sharing the 546 corner atoms with neighboring [PbBr6]4− octahedron (Fig. 10 (c)). In this arrangement, [PbBr6]4− 547 octahedron could be rather isolated, forming 0D arrangement.28,90 This 0D arrangement could confine 548  26  the excitons within the individual octahedra, showing the excitons with a smaller Bohr radius.90,98 As 549 a result, the peak position does not change and PL intensity is just slightly decreased.  550  551 4.  Conclusion 552 Cesium lead bromide perovskite NCs have been synthesized using the LARP and URSOA methods, 553 where those methods used similar processing steps and precursor solutions. However, the crystal 554 structures prepared by these methods were significantly different from each other. The LARP method 555 yielded the CsPbBr3 NCs with orthorhombic crystal structures, whereas the URSOA method yielded a 556 mixture of hexagonal Cs4PbBr6 and orthorhombic CsPbBr3 NCs. From HAXPES, it was found that the 557 chemical states of the interior or inner side of the NCs were not affected by the presence or absence 558 of the ligands. On the other hand, from XPS, the NCs without the ligand showed additional chemical 559 states originating from the accumulation of Cs cations, Pb0, unbonded Br atoms, and Br vacancies at 560 the NCs surface. These surface chemical states can be then associated with surface defects, which can 561 act as non-radiative recombination sites. The PL of the LARP NCs exhibits two distinct PL decay 562 components attributed to free and trapped excitons within the orthorhombic CsPbBr3 crystal structure. 563 This structure, characterized by its 3D nature, may facilitate exciton diffusion or migration to the 564 surface of the NCs. Therefore, excitons reaching the NCs without ligands may undergo non-radiative 565 recombination, resulting in a weak PL with a short PL lifetime. Here, excitons in CsPbBr3 NCs are 566 sensitive to surface states or surface defects. In contrast, despite the PL characteristics of the URSOA 567 NCs also show two PL decay components, also suggesting the formation of free excitons and trapped 568 excitons in the Cs4PbBr6 NCs, the PL characteristics of the NCs with and without ligands are not so 569 sensitive to the presence or absence of ligands. This characteristic may be attributed to the 0D 570 characteristics of the Cs4PbBr6 crystal structures, where excitons are more localized in the PbBr6 571 octahedron without long migration or diffusion to the NCs surface. Therefore, URSOA NCs are more 572 tolerant of the presence of surface defects compared to LARP NCs. The present experimental findings 573 in this study may provide new insight into the effect of ligands on the NCs surface structures, which is 574 seen from the formation of additional surface chemical states originating from surface defect species, 575 and their impact on PL characteristics of these lead halide perovskite materials. This insight may be 576 useful not only for the further development of passivation molecules for halide perovskites in general 577 but also for the design of buffer layer molecules in perovskite heterojunction devices.  578  579 Conflicts of interest 580 There are no conflicts of interest to declare. 581  27  Acknowledgements 582 This work was supported by Program Riset Internasional ITB of Bandung Institute of Technology 583 2022. This work was also partially supported by JSPS KAKENHI Grant number JP25K08480. MA 584 gratefully acknowledge the NIMS Internship Program 2023-2024 of National Institute of Materials 585 Science, Japan.  586 References 587 1 J. A. Sichert, Y. Tong, N. Mutz, M. Vollmer, S. Fischer, K. Z. Milowska, R. García Cortadella, B. Nickel, 588 C. Cardenas-Daw, J. K. Stolarczyk, A. S. Urban and J. Feldmann, Nano Le ., 2015, 15, 6521–6527. 589 2 H. Wang, X. Li, M. Yuan and X. Yang, Small, 2018, 14, 1703410. 590 3 C. Wehrenfennig, G. E. Eperon, M. B. Johnston, H. J. 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