Tunable Emissive CsPbBr3/Cs4PbBr6 Quantum Dots Engineered by Discrete Phase Transformation for Enhanced Photogating in Field‐Effect Phototransistors

Abstract Precise control of quantum structures in hybrid nanocrystals requires advancements in scientific methodologies. Here, on the design of tunable CsPbBr3/Cs4PbBr6 quantum dots are reported by developing a unique discrete phase transformation approach in Cs4PbBr6 nanocrystals. Unlike conventional hybrid systems that emit solely in the green region, this current strategy produces adjustable luminescence in the blue (450 nm), cyan (480 nm), and green (510 nm) regions with high photoluminescence quantum yields up to 45%, 60%, and 85%, respectively. Concentration‐dependent studies reveal that phase transformation mechanisms and the factors that drive CsBr removal occur at lower dilutions while the dissolution–recrystallization process dominates at higher dilutions. When the polymer‐CsPbBr3/Cs4PbBr6 integrated into a field‐effected transistor the resulting phototransistors featured enhanced photosensitivity exceeding 105, being the highest reported for an n‐type phototransistor, while maintaining good transistor performances as compared to devices consisting of polymer‐CsPbBr3 NCs.


Introduction
Advancements in tunable light-emitting CsPbBr 3 perovskite quantum structures have made significant contributions to the field of optoelectronic devices, [1,2] driving major progress in energy harvesting, [3] lighting, [4] and photogating applications. [5]o achieve the desired quantization, the size dimensions must DOI: 10.1002/advs.202401973be below 7 nm, corresponding to atomiclevel nanostructures such as nanowires, nanoplatelets, or nanosheets. [6]However, challenges arise from rapid reaction rates, self-aggregation, and the loss of optical properties over time [7] Current methods for stabilizing perovskites such as core-shell structuration [8] or unique ligand functionalization [9] have not been successful in tailoring quantum structures.Therefore, the development of heterostructures capable of serving as better scaffolds for accommodating these perovskite quantum structures is crucial.Once such class of heterostructure materials is the mixed-multi-dimensional (0D/3D) CsPbBr 3 /Cs 4 PbBr 6 hybrid crystals which operate in the non-quantized regime according to previous reports in the literature. [10]The alignment compatibility between the two crystal systems CsPbBr 3 NC with the lattice of Cs 4 PbBr 6 results in enhanced passivation of the surfaces without inducing significant strain on the NC.This alignment has positive implications for the quality of a hybrid crystal and resulting optoelectronic properties. [11]Hence, the progression of CsPbBr 3 /Cs 4 PbBr 6 perovskites would benefit from the development of their quantumconfined architectures, an area that remains largely unexplored.
In this study, we present a simple post-synthetic transformation method to convert rhombohedral Cs 4 PbBr 6 nanocrystals (NCs) into tunable luminescent CsPbBr 3 /Cs 4 PbBr 6 hybrid QDs.Previous studies demonstrated a one-step phase transformation of Cs 4 PbBr 6 NCs to CsPbBr 3 NCs through chemical [12] and physical stimuli. [13,14]Additionally, the direct 0D to 3D phase transformation kinetics pose difficulties in understanding the mechanistic aspects. [15]10a,18] In contrast to these to these one-step phase transformation protocols, here, we introduce discrete phase transformation that allows seizure of a new class of intermediate states via well-defined surface modification, and explore alternative strategies for composition control.10a] Furthermore, detailed information such as transient optical properties, reaction feasibility with respect to precursor concentration, and compositional evaluation is crucial for a comprehensive understanding of this reaction process.So far, the traditional synthetic reports on non-quantized CsPbBr 3 /Cs 4 PbBr 6 hybrid structures have predominantly focused on generating green-emitting species, suggesting that the CsPbBr 3 NCs inclusions within the Cs 4 PbBr 6 matrix extend beyond the quantum-confined region. [19]Consequently, establishing compositions other than the green-emitting CsPbBr 3 /Cs 4 PbBr 6 NCs, tuning the sizes of CsPbBr 3 inclusions within the quantum confinement regimes in CsPbBr 3 /Cs 4 PbBr 6 QDs poses a significant challenge. [20]It is important to note that the pure Cs 4 PbBr 6 crystals, without halogen defects [21] or perovskite inclusions, [22] exhibit a non-luminescent or weak blue emission due to high excitation energy and domination of nonradiative recombination [10a,12a,23] Recent studies by Chen et al. have obtained tunable emissive properties in the blue, cyan, and green region in bulk Cs 4 PbBr 6 crystals by discreate crystallization process through thermodynamic control. [24]However, replicating such structures at the nanoscale remains challenging due to the lack of controlled synthetic methods.Overcoming these synthetic challenges not only holds the potential for constructing novel hybrid structures within the quantum-confined region but also expands the utility of this emerging class of hybrid perovskites, thereby pushing the boundaries of perovskite research.
To address the fundamentals in phase transformation and crystal growth, we have developed a dilution-induced, kinetically controlled discrete phase transformation approach that enables the trapping of metastable species with adjustable luminescence across the blue, cyan, and green-emitting range (430-520 nm) in CsPbBr 3 /Cs 4 PbBr 6 QDs.This approach differs from the fabrication of conventional non-quantized hybrid systems that emit solely in the green region.The optical, spectroscopic, and electrical properties of these nanostructures were characterized to confirm the presence of CsPbBr 3 inclusions in Cs 4 PbBr 6 .To the best of our knowledge, such precise control of CsPbBr 3 quantum confinement in Cs 4 PbBr 6 NCs remains unique among this class of hybrid perovskites.The process begins with non-luminescent Cs 4 PbBr 6 NC solutions, which undergo controlled phase transfer at different dilutions upon heat-ing at 70 °C leading to the formation of CsPbBr 3 /Cs 4 PbBr 6 QDs.Next.10a,12c,18,25] At specific dilutions, two processes dominate, i) the controlled leaching of CsBr from the Cs 4 PbBr 6 crystal lattice leading to the formation of hybrid CsPbBr 3 /Cs 4 PbBr 6 QDs, and ii) the complete selfdissolution followed by recrystallization leading to formation of pure CsPbBr 3 QDs.In the present study, we observed the mechanistic path of "CsBr leaching" being dominant at higher concentrations and "self-dissolution" being favored at extreme dilutions of the Cs 4 PbBr 6 NCs.Based on this binary reciprocal trajectory, we postulate that the phase transformation process in 0D Cs 4 PbBr 6 NCs enables the precise control over the formation of either "hybrid CsPbBr 3 /Cs 4 PbBr 6 QDs" or "pure CsPbBr 3 QDs".The obtained CsPbBr 3 /Cs 4 PbBr 6 QDs were blended with organic semiconductors and utilized as an active layer into photo-field effect transistors (photo-FETs).Owing to the strong photogating capabilities of the CsPbBr 3 QDs and the insulating nature of Cs 4 PbBr 6 , the hybrid photoFETs featured high photosensitivity combined with robust field-effect transistor operation.

Results and Discussion
To investigate the kinetics of phase transformation in 0D Cs 4 PbBr 6 NCs and their transition to quantum-confined hybrid perovskite phases, a series of solution-based studies were conducted, coupled with real-time optical and microscopy tracking.The experimental conditions were systematically designed by varying the concentrations of colloidal Cs 4 PbBr 6 NCs in a suitable solvent (octadecane (ODE) or hexane) and the reaction temperature was gradually raised from room temperature (RT) to higher temperatures (70-140 °C) to achieve the desired goals.The influence of temperature on the NC's concentration was emphasized, providing crucial insights for designing optimized conditions to create precisely tunable emitting CsPbBr 3 /Cs 4 PbBr 6 QDs.
Throughout this phase transformation studies, monodisperse non-emissive Cs 4 PbBr 6 NCs with a size of 11.00 ± 0.04 nm were utilized (Figure S1, Supporting Information).10a] These NCs exhibit strong optical absorption at 314 nm (Figure S2, Supporting Information) attributed to the 1 S 0 → 3 P 1 transition of Pb 2+ centers of Cs 4 PbBr 6 NCs, [26] and exhibits higher absorption strength, significantly higher than its 3D perovskite phase. [27]The as synthesized NCs appears in a turbid white solution (inset in Figure S2, Supporting Information), which became transparent upon higher dilutions.The as-synthesized Cs 4 PbBr 6 NCs solution (original solution) without any dilution was labeled as "O".In contrast, the reaction mixture diluted two times or three times and so on, compared to the undiluted condition were labeled as "O 2d " or "O 3d ," respectively.The peak intensity at 314 nm in the absorption spectra recorded from different dilutions of Cs 4 PbBr 6 NCs reveals a variation in the molar absorption coefficient, which was decreased with the increasing NC dilution.This behavior differs from the traditional gold NCs, where the molar absorption coefficient is generally known to remain constant upon dilution (as shown in Figure S3, Supporting Information).These results strongly suggest that dilution induces surface-level alterations in Cs 4 PbBr 6 NCs, potentially affecting the composition, structure, or chemical properties that can impact the kinetics and mechanistic paths of phase transformation process, which will be further explored in subsequent sections.

Effect of Dilution on Transformation Temperature of Cs 4 PbBr 6 NCs
To explore the relationship between inter-NC interactions and the phase transition kinetics, a sequence of two reactions were carried out utilizing pristine Cs 4 PbBr 6 NCs at different dilution conditions labeled as "O" and "O 2d ."(Figure 1a).The controlled parameter was the temperature which was gradually increased from RT to the designated reaction temperature to observe a noteworthy phase change within the NCs.The phase transformation is evidenced by the color changes evolving from colorless to a faint yellow, or characteristic strong emissions from the CsPbBr 3 phase under UV light.Interestingly, the O 2d -Cs 4 PbBr 6 NC solu-tion exhibited a seamless shift from colorless to pale yellow upon reaching the temperature of 110 °C.In contrast, the O-Cs 4 PbBr 6 NC solution remained unchanged at this temperature.Furthermore, when exposed to UV light, solely the O 2d reaction shifted from non-emissive to cyan emitting solutions, while the other solution did not display any luminescence (photographs are compared in Figure 1b-i 1e-i).On the other hand, the heated O-Cs 4 PbBr 6 NCs did not exhibit any photoluminescence, similar to the pristine NCs (Figure 1e-ii,iii).12a] Majorly, this XRD, as well as TEM study, confirms that this cyan emission is originating from the hybrid structures, not from the individual CsPbBr 3 NCs.
Furthermore, we explored the phase transformation temperature for the O-Cs 4 PbBr 6 NCs via raising the temperature gradually beyond 110 °C.A phase transition process on these viscous dispersions occurred at 140 °C, accompanied by a color change from turbid white to yellow green.The absorption spectra of the resulting sample resembled those of pure CsPbBr 3 NCs, exhibiting a band at 510 nm and the absence of the 314 nm peak associated with Cs 4 PbBr 6 (Figure 1d-iv).This sample also displayed a strong green emission at 512 nm (Figure 1d-iv).TEM images of this solution show the presence of polydisperse CsPbBr 3 NCs with cubic morphology of sizes ≈30-60 nm (Figure 1c-iv).The growth toward larger CsPbBr 3 NCs resulted from the oriented self-assembly of NCs governed by the phase transformation that occurred at higher temperature in these viscous conditions. [28]t is worth comparing this with a typical HI method where 110-170 °C is considered as an ideal temperature for producing nearly monodisperse CsPbBr 3 NCs of size ≈5-20 nm. [29]However, under the same concentration conditions as the HI method, here, the phase transformation from 0D to 3D CsPbBr 3 NCs occurs only beyond 135 °C.Based on our previous [10a] and other groups [12b,30] reports, it is known that the Cs 4 PbBr 6 phase could be kinetically stabilized from CsPbBr 3 by the extraction of PbBr 2 through the chelating molecules at RT.In our study, the colloidal Cs 4 PbBr 6 NCs were also readily stabilized due to the presence of higher concentrations of PbBr 2 -chelated molecules (ionic OA − and Olam + ).But for the reversible reaction, i.e., from Cs 4 PbBr 6 to CsPbBr 3 NCs, there are two possibilities; i) the release of CsBr into the polar environment, or ii) the incorporation of PbBr 2 into the 0D NC lattices.To trigger either of these processes to occur in our case, one should perturb the PbBr 2 -chelated molecules causing the release of perovskite formation precursors (Pb 2+ and Br − ).Consequently, the disruption of ionic species and the subsequent liberation of PbBr 2 precursors may demand a higher energy input in comparison to the conventional HI method.This observation paves the way for the generation of novel perovskite materials by facilitating controlled phase transformations through the interplay between the reaction temperature and concentration of NCs and chelated molecules.

Dilution-Based Kinetic Control: Tunable Emission in CsPbBr 3 /Cs 4 PbBr 6 QDs
Based on the previous experiments concerning phase transformation in Cs 4 PbBr 6 NCs, it has been observed that the transformation process of Cs 4 PbBr 6 NCs could be kinetically controlled through dilution rather than by thermodynamic control.As a result, we introduced the dilution-based kinetic control approach by adjusting the level of dilution in a desired reaction for designing a specific composition of hybrid CsPbBr 3 /Cs 4 PbBr 6 QDs.Toward this end, a series of phase transformation reactions conducted by changing the concentration of NCs by diluting the O-Cs 4 PbBr 6 NCs with n-hexane (O 3d , O 5d , and O 10d ) was followed by heating at 70 °C as can be seen in Figure 2a and Figures S5 and S6 (Supporting Information).
Interestingly, the initial non-luminescent Cs 4 PbBr 6 NCs (Figure 2a-iv) evolve into hybrid CsPbBr 3 /Cs 4 PbBr 6 QDs exhibiting a gradual shift in their emitting properties from blue, cyan, and green while diluting the reaction solutions (photographs in Figure 2a).Previous studies by Chen et al. have reported similar emission colors in bulk Cs 4 PbBr 6 crystals, ranging from blue to cyan and green, due to the presence of differently sized CsPbBr 3 inclusions. [24]So far, such controlled emission in these Cs 4 PbBr 6 NCs has not been established.These three (O 3d O 5d and O 10d ) phase transformation reactions, in their absorption spectra show the presence of Cs 4 PbBr 6 matrix from their characteristic 313 nm peak along with the other characteristic peaks at 430 and 480, and 500 nm, respectively, indicating the presence of hybrid structures (Figure 2c).The PL spectra of these QDs are Stokes-shifted with respect to the optical absorption spectra.The PL emissions of these reactions were located at 450, 485, and 510 nm (Figure 2c).The PLQY for the green, cyan, and blue-emitting hybrid QDs synthesized in this study were 85%, 60%, and 45%, respectively.The PLQY of the green-emitting hybrid systems was notably higher, yet still slightly lower than that of best values reported in literature for surface-treated hybrid structures.It is worth mentioning that the PLQY of CsPbBr 3 /Cs 4 PbBr 6 hybrid structures is influenced by the size of the host material, as indicated in the literature.19c,33] These QDs were precipitated by the addition of methyl acetate (MA) and used for further characterization or WLED fabrication.Figure 2d shows the powder XRD patterns of the resulting QDs obtained from these samples.The characteristic diffraction peaks of Cs 4 PbBr 6 NCs at ≈12°were found across all the samples, indicating that these samples contained Cs 4 PbBr 6 NCs.Next, we fabricated a WLED by using CsPbBr 3 /Cs 4 PbBr 6 QDs and K 2 SiF 6 :Mn 4+ as green and red-emitting phosphors, respectively, and a blue chip as the excitation source.The result of electroluminescence (EL) spectrum of the WLED device is shown in Figure 2e.The EL spectrum is composed of three emission bands: blue, green, and red, which belong to the blue LED chip, our samples phosphor, and KSF phosphor, respectively.The constructed WLED device exhibited the CIE color coordinates (0.3054,0.3043).Figure 2f, a color temperature of 8143 K, an efficiency 20.31 lm W −1 , the obtained color gamut of the assembled backlight unit is compared with the NTSC and Rec.2020 standards, this work color gamut can cover ≈120.0%and 89.6% of the NTSC and Rec.2020, respectively.We also tested the color stability of the prepared WLED devices under different driving currents, as shown in Figure S7 (Supporting Information).With the increase of drive current, the emission intensity of Cs 4 PbBr 6 increases, and the CIE of WLED device changes little.19b,24] This is due to the reduction in PLQY of such hybrid materials in solid state with decrease in their crystal sizes.In contrast to the pure CsPbBr 3 perovskites, where bulk crystal shows poor emission whereas bulk or micron-sized hybrid structures exhibit strong emission in their solid state. [27]igure 2b displays the TEM analysis of these hybrid NCs which are comparable to that of the pure Cs 4 PbBr 6 NCs exhibiting rhombohedral or pseudospherical shapes.The pure NCs possess a diameter of ≈10 nm, while the blue, cyan and green hybrid QDs exhibit diameters of ca. 13 nm, respectively.We have not found any CsPbBr 3 -related phases (nanocubes or nanowires) in this TEM analysis also indicating these species are of hybrid QDs.
The formation of quantized hybrid QDs in this phase transformation process is further confirmed by the anion-exchange method, where iodide precursor was introduced into these systems and monitored their exchange kinetics by in situ PL changes (Figures S8 and S9, Supporting Information).A red shift in the PL emission stemming from the formation of CsPbBr 3-x I x from CsPbBr 3 inclusions in Cs 4 PbBr 6 is expected.Unlike in the case of the pure CsPbBr 3 NCs, the iodide exchange reaction does not occur in the blue-emitting CsPbBr 3 /Cs 4 PbBr 6 QDs, as the PL emission quenching upon the addition of iodide precursor, hence does not lead the formation of CsPbBr 3-x I x /Cs 4 PbBr 6 QDs.The iodide exchange in the case of cyan emitting CsPbBr 3 /Cs 4 PbBr 6 QDs underwent only a limited amount of iodide-exchange evidenced by the slight shift from 480 to 600 nm (Figure 2g-i).In addition, the intensity was drastically decreased, with no future shift in the PL position.The iodide exchange on the green hybrid structure too shows a drastic quenching in the PL emission while the PL wavelength shifted to 680 nm from 520 nm (Figure 2g-ii).The integrated intensities of the PL spectra of each exchanged QDs drastically quenched compared to parent samples which is in contrast to the pristine CsPbBr 3 NCs.At the similar exchange synthetic conditions, CsPbBr 3 NCs demonstrate more proficient exchange kinetics and substantially enhanced PL intensities. [34]he inefficiency of this Br − to I − exchange in hybrid structures is attributed to the fact that it occurs only at the outermost layers of the material due the presence of a protective shell around CsPbBr 3 NCs.
Figure 3a-c shows the evaluation of optical properties during the phase transformation from O 10d -Cs 4 PbBr 6 NC toward the hybridization in green emissive CsPbBr 3 /Cs 4 PbBr 6 QDs at 70 °C.The initial absorption spectra with a prominent band at 314 nm, corresponding to the Cs 4 PbBr 6 NCs remained consistent for the first 10 minutes at constant heating under ambient conditions.After ≈12 min of heating, a new absorption band emerged at 450 nm which coexisted with the original band at 314 nm, indicating the simultaneous presence of both phases.Under continuous heating for the next 30 min, a progressive redshift of the wavelength from 460 to 515 nm was observed.Simultaneously, the PL emission changed from a non-emissive state to green emissive species, with PL emissions occurring at various wavelengths: 460 nm (12 min), 490 nm (18 min), 500 nm (20 min), and 510 nm (25 min), ultimately reaching 520 nm.Interestingly, the intensity of the PL emission gradually increased during these wavelength changes due to the increased quantum yield.Subsequently, there were no further alterations in the absorption and emission positions upon continuous heating for the next hour.Similarly, the emergence of cyan CsPbBr 3 /Cs 4 PbBr 6 QDs from O 5d -NCs was investigated through the time-dependent absorption and PL and is presented in Figure S10 (Supporting Information).The absorption spectra at ≈10 min, a new absorption band appeared at 470 nm, alongside the initial absorption band at 314 nm.After 20 min of heating, the absorption band shifted to 485 nm, and the PL emission occurred at 490 nm.Remarkably, these values remained stable even after continuous heating for an additional 30 min, indicating the cyan emitting CsPbBr 3 /Cs 4 PbBr 6 QDs are kinetically stable at this dilution.
Unlike the pure CsPbBr 3 NCs, these solutions of hybrid structures turned back to the initial Cs 4 PbBr 6 structures within a few hours while standing in ambient conditions (Figure 3d-g).This also indicates that these QDs are composed of hybrid perovskite structures and the continuous incorporation of CsBr or removal of PbBr 2 from the lattice leads the structures of the original nonemissive Cs 4 PbBr 6 NCs.At the same time intervals, the fully formed CsPbBr 3 NCs remained stable for several days without a change in their PL intensity.The reversible transformation reaction also followed a systematic backward trend, i.e., the green hybrid system first transformed to cyan, followed by blue, and finally to non-emissive species.The reversed non-emissive species again underwent forward transformation from non-emissive to emissive upon being subjected to heating (Figure 3f).We stud-ied such reversibility on green emissive samples for up to three cycles (Figure 3d,e).This kind of reversible transformation was also seen in the case of cyan and blue emitting CsPbBr 3/ Cs 4 PbBr 6 QDs (Figure S11, Supporting Information).However, the time required for reversible transformation of hybrid perovskites to non-emissive Cs 4 PbBr 6 NCs varied, and was in the following order, blue hybrid perovskites (1 h) < cyan hybrid perovskites (6 h) < green hybrid perovskites (15 h).This trend is directly proportional to the amount of CsBr required to remove from the hybrid perovskites to reach the pure 0D structures.This indicates that the quantitative removal of PbBr 2 or addition of CsBr in the hybrid perovskite lattices leads to the pure Cs 4 PbBr 6 NCs.

Dilution-Triggered Transformation of Cs 4 PbBr 6 NCs to CsPbBr 3 NCs: Thermodynamics and Kinetics
We found that the feasibility of the phase transformation in Cs 4 PbBr 6 NCs into respective CsPbBr 3 /Cs 4 PbBr 6 QDs without using any external chemical stimuli is highly influenced by the thermodynamic barrier.The above findings pointed out that this thermodynamic activation energy could be manipulated through the concentration of NCs which can be regulated manually and is inversely proportional to the NC's dilution.If this is the case, one can tune this transformation spontaneously at RT toward well-defined phase pure quantum confined CsPbBr 3 structures at optimized dilutions.To investigate this phenomenon, we conducted a range of progressive dilutions of NCs spanning from moderate to significantly higher dilutions (O 5d to O 1000d , corresponding to the concentrations of 5 to 0.025 μL mL −1 ) using three distinct and independent solvent systems, namely toluene, ODE, and hexane.Figure 4a shows the absorption spectra of these NC solutions at four different dilutions showing a systematic shift in the exciton peak position from 420, 450, 480, and 520 nm, respectively.Accordingly, the PL intensity of these NCs also shifted from 430 to 520 nm.The emergence of PL emis-sion as a function of dilution in all these solvents is portrayed in Figures S12-S14 (Supporting Information).The absorption peak intensity at 314 nm was only present for the blue emitting perovskite CsPbBr 3 sample (Figure 4a), whereas the corresponding peak is absent in the next samples showing the presence of pure CsPbBr 3 NCs.XRD data further confirms the presence of mixed phases (OD and 3D) in the blue emitting sample (Figure S15, Supporting Information) and the presence of pure orthorhom-bic CsPbBr 3 NCs in later cases (Figure S16, Supporting Information).Figure 4c,d presents a change in absorption peak intensities (absorption strength) at 314 nm and the evaluation of PL emission across 30 systematic dilutions of Cs 4 PbBr 6 NC in various solvents (toluene, ODE, and hexane) at RT (Figure S17, Supporting Information).Based on the absorption spectra, dilution versus and the absorption peak intensity displays nearly linear reduction in the absorption strength and finally reaches to near zero at a certain threshold ≈3 μL mL −1 (Figure 4c,d).Interestingly, this dilution leads toward a significant change in the optical properties, associated with the disappearance of absorption at 313 nm and the appearance of new bandgap energies as well as emergence of PL spectra readily tunable over the blue-green region in ca.30-60 s. (Figure 4b; Figure S18, Supporting Information).Notably, the raise of PL emission at the expense of absorption at 314 nm indicating the dissolution of Cs 4 PbBr 6 NCs and recrystallization into CsPbBr 3 quantum structures.The PL intensity profile shows a drastic enhancement at increased dilutions.The solutions under UV light irradiation also reflect similar results showing the transformation from non-luminescent Cs 4 PbBr 6 NCs to luminescent blue, cyan, and green-emitting single-phase CsPbBr 3 QDs (photographs presented in Figure 4e).These dilution-mediated changes in absorption and PL emissions are remained consistent across toluene, ODE, and hexane solvents, albeit with slight differences in intensities.For instance, hexane began to exhibit emissions in the blue region at higher dilutions, while toluene reached this stage at lower dilutions (represented by dotted lines in Figure 4e).The required NC's dilutions to initiate the spontaneous 0D to 3D transformations in colloidal solutions followed the order: hexane (O 15000d ) <ODE (O 3000d ) < toluene (O 1000d ).The polarity of these solvents also followed a similar order: toluene > ODE > hexane, indicating along the influence of dilution, there is a correlation between the phase transformation trend and solvent polarity. [35]n addition, the TEM analysis of this transformation in hexane at different dissolutions were studied, the trend showed that the Cs 4 PbBr 6 NCs dissolute to CsPbBr 3 nanoclusters followed by a systematic growth toward CsPbBr 3 nanoplatelets and nanocubes during this process (Figure 4f-h; Figure S19, Supporting Information).The intermediate phases composed of partially dissolute-hybrid structures, lamellar structures along with CsPbBr 3 nanoclusters, nanoplatelets, also noticed in TEM analysis (Figure S20, Supporting Information).
Based on our observations and previous reports [10a,12c] we propose this whole process of transformation as shown in the Figure 5a-d.Note that the facets of the rhombohedral Cs 4 PbBr 6 NC phase is considered to be neutral because of the passivated oleate-ammonium complex could stabilize the neutral-charged surface composed of negatively charged PbBr 6 octahedra partially balanced by in-plane Cs + ions. [25]Hence, at higher concentrations, the Cs 4 PbBr 6 NCs in solutions tended to self-aggregate as evidenced by the visible appearance of turbid-like solutions in the as synthesized NC solution.Moreover, at this concentration the absorption peak position was slightly red-shifted to 317 nm (gray line in Figure 5b) with increased peak width along with substantially high scattering part compared to the isolated NCs (green line in Figure 5b).The scattering part and the peak width were gradually reduced upon additional dilution and the peak position was back original (314 nm).TEM analysis of the concen-trated colloidal solution revealed the presence of self-assembled structures of sizes in the range of few hundred nanometers (Figure 5a).In contrast, at dilute conditions, the TEM images showed separate individual NCs (Figure 5c).This indicates that the spontaneous assembly of the individual NCs to ordered structures occurred at higher concentrations, which lowers the free energy through the decrease in the surface energy.Thus, the highly-organized self-assembled NCs suspensions are thermodynamically stable.Furthermore, the close proximity of the NCs restricts the available reaction space, which prevents the mass transfer from individual NCs to the surrounding environment.As a result, the transformation process was kinetically hindered (Figure 5e-i).Dilution caused the NCs to separate from their selfassembled state, resulting in increased surface energy and allowing them to interact with the ligands (ionic oleylamine and oleic acid).Only upon heating, this condition favored the release of CsBr from the Cs 4 PbB 6 NCs leading to the formation of CsPbBr 3 nano-inclusions within the Cs 4 PbB 6 matrix.The extent of CsBr loss could influence the formation of CsPbBr 3 inclusions within the Cs 4 PbB 6 matrix, which was determined by local NCs concentrations. [30]In this scenario, the quantized emissions (blue, cyan, green) originated from the CsPbBr 3 /Cs 4 PbBr 6 QDs structures (Figure 5e-ii).
The situation of the NCs in solution upon gradual increase in dilution was studied by TEM and dynamic light scattering (DLS).

Hybrid Organic/Inorganic Field-Effect Phototransistors with CsPbBr 3 /Cs 4 PbBr 6 QDs
So far, research on perovskite-based photoFETs (3-terminal devices that can be both operated electrically with a gate electrode and optically by illumination of the channel) has primarily concentrated on enhancing device performance in terms of photosensitivity (P) (where P = I photocurrent /I dark with I photocurrent = I illumination − I dark ) and photoresponsivity. [36]In pursuit of optimizing higher P, previous studies have primarily leveraged gate control to induce a threshold voltage (V Th ) shift, resulting in exceptionally high P values at specific gate voltages (V GS ).While this approach has proven highly effective in reducing I dark and elevating I photocurrent , it presents several challenges when it comes to integrating these devices into practical electronics applications.
First, the operation of these devices frequently demands high drain voltages often exceeding 10 volts.and typically employ interdigitated architectures to obtain easily measurable drainsource current (I DS ) values.Furthermore, for photoFETs to be integrated into photoresponsive circuits such as photoFET-based inverter circuits, [37] high FET performances in terms of mobility and I on /I off and an adequate V Th are required. [37]As such, to achieve devices that can fully combine logic and photoresponse features, FET performances should not be sacrificed.Since these devices rely on the photogating effect, further improvements in terms of performance could be achieved by blending the perovskite and its derivatives into a semiconducting polymer layer. [38]However, this strategy would result in an increase in unwanted interfacial effects, further lowering the transistor performances.These potentially unwanted interfacial effects are two.First, the interface trapping of charges by the perovskite will result in a decrease in electrical performance.Second, it is energetically favorable for charges to go from the perovskite into the semiconductor.Since the photogating effect relies on charges trapped in the perovskite to generate the electric field leading to an increase in current, it could be negatively affected by the application of a strong gate voltage.This could lead to a very strong decrease in the current when both "gates" are operating simultane-ously.These effects can be mitigated by reducing the conductivity of the perovskite.Furthermore, a reduced conductivity of the perovskite would result in slightly reduced off currents at high concentrations of perovskites (and therefore slightly reduced dark currents), owing to the fact that pure perovskites pathways in the film would feature reduced charge transport.
To determine the conductivity and transistor operation of the green emitting CsPbBr 3 /Cs 4 PbBr 6 QDs, bottom-contact bottomgate field effect transistors (Figure 6a) were fabricated with a ≈1 μm drop casted layer of perovskites in the channel.The I DS current was measured as a function of the gate voltage and compared to CsPbBr 3 NCs (Figure 6b).While neither device featured gate control, the currents of the transistor containing the hybrid perovskite were found to be 2 orders of magnitude lower as compared to the CsPbBr 3 NCs containing device.The conductivity of the CsPbBr 3 NCs and CsPbBr 3 /Cs 4 PbBr 6 QDs films were found to be 4.2 × 10 −8 and 1.718 × 10 −10 S m −1 respectively.While both values were low, the conductivity of the CsPbBr 3 NCs could still result in increased I dark and reduced P, even in a thinner film.Illumination within the absorption range of the perovskites did not result in a major change in current (Figure S24, Supporting Information).Cyclic voltammetry measurements revealed no difference in energy levels between the two NCs (Figure S25a, Supporting Information) hence none of these perovskites should act as a trap for charges when blended with organic polymers, whether p-or n-type (Figure S25b,c, Supporting Information).
Blends of NCs and organic semiconductors with equal wt% were deposited as active layer for photoFETs.These devices were characterized in the dark and under illumination at 420 nm (within the absorption range of the NCs).Two polymers were chosen for this experiment:  ,5] thiadiazolo [3,4-c]  pyridine] (PCD-TPT), respectively n-and p-type semiconductors.[40] P(NDI2OD-T2) has been exploited as it is one of the most commonly used n-type polymers in previous studies on perovskite photogating. [39,40]Transfer curves of devices based on P(NDI2OD-T2) + CsPbBr Each of the 4 blends resulted in devices in which the photogating effect was observable with currents much higher under illumination, at specific gate voltages, leading to P values over 10 3 .However, a strong increase in I illuminated was observed in the case of the CsPbBr 3 /Cs 4 PbBr 6 QDs containing devices.38c,d,41,42] This can be explained by two factors.First, the blending strategy is highly beneficial for the photogating effect since the amount of polymer in proximity to the NCs is strongly increased.Second, a reduction of the NCs pathways occurs since the polymers act as charge-trapping elements leading to a reduced I dark .This effect is further compounded by the very low conductivity of the CsPbBr 3 /Cs 4 PbBr 6 QDs.Furthermore, in the case of the blends with hybrid QDs, the effects of the photogating and the bottom gate of the device are less affected by each other as compared to the case of the CsPbBr 3 NCs devices.The detrimental effect of the photogating on the transistor operation in the devices containing CsPbBr 3 NCs is very strong leading to a large decrease in mobility upon illumination both in the case of P(NDI2OD-T2) and PCD-TPT containing photoFETs.However, this effect is not observed in CsPbBr 3 /Cs 4 PbBr 6 QDs containing devices leading to a 13-fold higher mobility for P(NDI2OD-T2) containing photoFETs under illumination (Tables S1 and S2, Supporting Information).As compared to pure P(NDI2OD-T2) devices, the transistor characteristics are mostly conserved in the dark with similar I on /I off and mobility values reduced by one order of magnitude (Figure S27a,c, Supporting Information).PCD-TPT + CsPbBr 3 /Cs 4 PbBr 6 QDs photoFETs perform even better in this regard, with I on /I off and mobility values remaining similar to the pristine PCD-TPT transistors (Figure S27b,d, Supporting Information).These results provide evidence that this strategy is highly effective at both increasing the photosensitivity and maintaining good transistor operation even under illumination, paving the way for multifunctional optoelectronic devices.

Conclusion
In summary, this study has demonstrated a discrete phase transformation process in Cs 4 PbBr 6 NCs, leading to creation of tunable hybrid CsPbBr 3 /Cs 4 PbBr 6 QDs with adjustable luminescence across the blue, cyan, and green spectral regions achieved through the strategic kinetic trapping of the metastable phases.Furthermore, the research uncovered the unique chemical properties of the hybrid structures, including reversible interactions, iodide-ion exchanges, and the role of CsBr leaching and dissolution-recrystallization mechanisms.Furthermore, the photoFETs containing CsPbBr 3 /Cs 4 PbBr 6 QDs blended with n-type semiconducting polymers, exhibited increased performances in the dark and under illumination compared to phasepure CsPbBr 3 NCs blends.The blended devices using the hybrid QDs could better harness the photogating effect of CsPbBr 3 owing to its separation from the semiconducting polymer by the insulating Cs 4 PbBr 6 .Overall, our research highlighted the intricate interparticle interaction of colloidal Cs 4 PbBr 6 NCs in regulating phase transformation for creating adjustable emissions in hybrid structures, and offers promising potential for the future development of photofield-effect transistors.

Figure 1 .
Figure 1.Concentration controlled phase transformation process in Cs 4 PbBr 6 NCs.a) Schematic representation of the phase transformation process conducted at two different dilutions of Cs 4 PbBr 6 NCs: i) O 2d -Cs 4 PbBr 6 NCs obtained by heating pure Cs 4 PbBr 6 NCs at 110 °C, ii) O-Cs 4 PbBr 6 NCs, iii) O-Cs 4 PbBr 6 NCs heating at 110 °C, and iv) O-Cs 4 PbBr 6 NCs further heated at 140 °C.b) Respective photographs of the reaction solutions presented in scheme (a), collected at daylight and UV light.c) TEM images showcasing the morphology of the respective i) CsPbBr 3 /Cs 4 PbBr 6 QDs, ii,iii) Cs 4 PbBr 6 NCs, and iv) CsPbBr 3 NCs.The scale bars correspond to 50 nm.d,e) UV/vis absorption spectra (d) and photoluminescence spectra (e) of the corresponding solutions (i-iv).
,iii).The lack of visible changes in the Osolution indicates the importance of concentration which dictates the NC's surface for facilitating the transformation process.At 110 °C, the nature of these reactions, and their resulting crystal phase were studied by absorption spectra, photoluminescence (PL), X-ray diffraction pattern (XRD), and transmission electron microscopy (TEM).TEM images revealed the size of the heated O 2d -Cs 4 PbBr 6 NCs increased from 9 to 18 nm (Figure 1c-i,ii), while the heated O-Cs 4 PbBr 6 NCs remained the same size as the initial Cs 4 PbBr 6 NCs (Figure 1c-iii).The absorption spectra of the heated O-Cs 4 PbBr 6 NCs were similar to those of the pristine NCs, showing a single peak at 314 nm (Figure 1d-ii,iii).In contrast, the heated O 2d -Cs 4 PbBr 6 NCs exhibited a strong absorption peak at 430 nm, along with a faint band at 320 nm, indicating the presence of a heterostructure (Figure 1d,i).The photoluminescence spectra of O 2d -NCs displayed an emission peak at 450 nm, likely due to the embedded CsPbBr 3 QDs (Figure

Figure 2 .
Figure 2. Tunable CsPbBr 3 /Cs 4 PbBr 6 QDs through the discrete phase transformation process, anion exchange process, and exploring the WLED properties.a) Photographs of the dilution-based kinetically trapped i) blue, ii) cyan, and iii) green emitting CsPbBr 3 /Cs 4 PbBr 6 QDs, and iv) the thermodynamically stable O-Cs 4 PbBr 6 NCs.These photographs are recorded under UV light.b) TEM images of the respective solutions (i-iv).The scale bars correspond to 50 nm.c) UV/vis absorption spectra and PL spectra of the blue (i), (ii), and green (iii) emitting CsPbBr 3 /Cs 4 PbBr 6 QDs.d) XRD patterns of all the above-mentioned NCs (i-iv).e) Electroluminescence spectrum of the WLED device fabricated using green emitting CsPbBr 3 /Cs 4 PbBr 6 QDs, inset is the photograph of the device and its CIE coordinates displayed in (f).g) PL changes up on the Br − to I − exchange in cyan i) and green ii) spectra emitting CsPbBr 3 /Cs 4 PbBr 6 QDs.Arrow marks indicate the final PL emission from CsPbBr 3-x I x /Cs 4 PbBr 6 QDs after completion of Br − to I − exchange.

Figure 3 .
Figure 3. Controlled growth of non-luminescent Cs 4 PbBr 6 to CsPbBr 3 /Cs 4 PbBr 6 QDs and their reversibility.a) Photographs of the colloidal solutions collected under UV light, showing the progression of non-luminescent Cs 4 PbBr 6 NCs to green-emitting CsPbBr 3 /Cs 4 PbBr 6 QDs in hexane; schematic representation of this transformation is provided at the bottom of (a).b,c) Time-dependent changes in PL spectra (b) and UV/vis absorption spectra (c) during the transformation of Cs 4 PbBr 6 NCs to green-emitting CsPbBr 3 /Cs 4 PbBr 6 QDs.d-g) The formed green emitting CsPbBr 3 /Cs 4 PbBr 6 QDs turned back to non-luminescent Cs 4 PbBr 6 NCs and this process is reversible at two different temperatures (RT and 70 °C) (schematically represented in g).Reversible cycles (d) among QDs to NCs, and respective PL spectra (e) and photographs under UV light (f) are presented.

Figure 4 .
Figure 4. Spontaneous transformation of Cs 4 PbBr 6 NCs to CsPbBr 3 NCs in different solvents at extreme dilutions and mechanistic details. a,b) UV/vis absorption spectra (a) and PL spectra (b) of Cs 4 PbBr 6 NCs at various dilutions in hexane.c,d,e) The changes in the optical properties of Cs 4 PbBr 6 NCs as a function of different dilutions in the three different solvents.The change in the absorption strength at 314 nm (c), the change in the PL intensities (d), and respective photographs under UV light (e).The dotted lines represent the emergence of blue range of emission under UV light which varies depend on the type of solvent.NC solutions are at different concentrations in the three different solvents.f-h) TEM images collected at the intermediated stages of spontaneous transformation from Cs 4 PbBr 6 NCs to CsPbBr 3 NCs.The scale bar corresponds to 50 nm.
Figures S21-S23 (Supporting Information) show the separation of NCs from the rhombohedral-type self-aggregated crystals to the individual NCs via the hexagonal packing monolayered NCs.At increased dilution before the phase transformation the nanoparticles were smaller in size (8.7 ± 0.1 nm) compared to the original NCs (11 ± 0.1 nm) further confirming the surface and compositional modifications (Figure 5c,d).From these observations, at excessive dilutions (O 1000d or more), the dissociation of PbBr 2 -ligands and increased ligand influences acted on individual NCs, leading to excessive release of CsBr and subsequent loss of Cs 4 PbBr 6 NCs identity.This abrupt change in the system at higher dilutions ultimately resulted in the dissolution of Cs 4 PbBr 6 NCs and their recrystallization into quantum-confined CsPbBr 3 NCs through well-controlled crystal growth conditions at RT (Figure 5e-iii).

Figure 5 .
Figure 5. a) TEM image of Cs 4 PbBr 6 NCs at higher concentrations show the presence self-aggregated O-Cs 4 PbBr 6 NCs.The scale bar corresponds to 200 nm.b) The changes in the UV/vis absorption spectra of the colloidal O-Cs 4 PbBr 6 NCs as a function of dilution.c,d) TEM image and particle size distributions represents the reduction in the average size of Cs 4 PbBr 6 NCs at higher dilutions (O 10d in hexane).The scale bar corresponds to 50 nm.e) Schematic representation of the mechanistic insights into the discrete phase transformation process in Cs 4 PbBr 6 NCs, concluded based on various systematic concentration and temperature dependent studies.