2D nanosheets of layered double perovskites: synthesis, photostable bright orange emission and photoluminescence blinking

Lead (Pb)-free layered double perovskites (LDPs) with exciting optical properties and environmental stability have sparked attention in optoelectronics, but their high photoluminescence (PL) quantum yield and understanding of the PL blinking phenomenon at the single particle level are still elusive. Herein, we not only demonstrate a hot-injection route for the synthesis of two-dimensional (2D) ∼2–3 layer thick nanosheets (NSs) of LDP, Cs4CdBi2Cl12 (pristine), and its partially Mn-substituted analogue [i.e., Cs4Cd0.6Mn0.4Bi2Cl12 (Mn-substituted)], but also present a solvent-free mechanochemical synthesis of these samples as bulk powders. Bright and intense orange emission has been perceived for partially Mn-substituted 2D NSs with a relatively high PL quantum yield (PLQY) of ∼21%. The PL and lifetime measurements both at cryogenic (77 K) and room temperatures were employed to understand the de-excitation pathways of charge carriers. With the implementation of super-resolved fluorescence microscopy and time-resolved single particle tracking, we identified the occurrence of metastable non-radiative recombination channels in a single NS. In contrast to the rapid photo-bleaching that resulted in a PL blinking-like nature of the controlled pristine NS, the 2D NS of the Mn-substituted sample displayed negligible photo-bleaching with suppression of PL fluctuation under continuous illumination. The blinking-like nature in pristine NSs appeared due to a dynamic equilibrium flanked by the active and in-active states of metastable non-radiative channels. However, the partial substitution of Mn2+ stabilized the in-active state of the non-radiative channels, which increased the PLQY and suppressed PL fluctuation and photo-bleaching events in Mn-substituted NSs.


Introduction
Metal halide perovskite nanostructures have received signicant attention owing to their outstanding optoelectronic properties. 1 In spite of their unique properties, the foremost shortcoming is the presence of lead (Pb), which obstructs them from further applications. 2 Hence, the search for environmentally-friendly replacements that maintain the remarkable optical properties of Pb-based perovskite halides has become an increasingly signicant research area. Promising approaches included isovalent substitution of Pb 2+ by less-or non-toxic metal ions, like Sn 2+ and Ge 2+ . 3 However, facile oxidation of these cations under ambient conditions leads to the inferior stability of crystal structures and impedes their applications. Subsequently, strategies for employing cations with an oxidation state other than +2 in place of Pb 2+ in perovskites are being extensively explored. The lower dimensional A 3 M 2 III X 9 -type perovskites, 4 and paired monovalent-trivalent cation based double perovskites, A 2 M I M III X 6 [where M(I) and M(III) correspond to Ag + , Cu + , Na + , etc. and Bi 3+ , Sb 3+ , In 3+ , etc., respectively], 5 are being actively studied, and have shown promising properties.
Recently, dimensional reduction and heterovalent substitution resulted in layered double perovskite (LDP) halides, which can be reckoned as the two-dimensional (2D) variety of double perovskites or the double-metal version of layered perovskites. 6 Different from the conventional double perovskites, LDPs integrate the divalent and trivalent cations into 2D structures, which further led to the chemical and structural diversity. Additionally, the dimensional reduction can engender modications in the electronic structure. 6 Solis-Ibarra and co-workers initially synthesized a mixed-metal h111i-oriented layered double perovskite with the general formula of A n+1 [B(II)B ′ (III)] n X 3n+3 , where n > 2 (i.e., Cs 4 CuSb 2 Cl 12 ). 7 Subsequently, few LDPs are available, which are either computationally proposed or experimentally synthesized. 6a,8 Till now, the bulk form of LDPs (e.g., A 4 M II M III 2 X 12 ) has been synthesized mainly by acid precipitation methods. 6b Despite the great promise of this family of materials, very few reports have explored the synthesis of LDP nanostructures by the solution-based colloidal method. 9 Recently, colloidal synthesis of Cs 4 M(II)Bi 2 Cl 12 [M(II) = Cd, Mn] nanocrystals (NCs) was reported using the hot-injection method; however, a maximum photoluminescence quantum yield (PLQY) of only 4.6% was achieved for the Cs 4 (Cd 1−x Mn x ) Bi 2 Cl 12 NCs. 9b Therefore, facile synthesis strategies for both the bulk form and the nanostructures (e.g., 2D nanoplates or nanosheets) of such LDPs should be developed to improve the PLQY and to understand the PL properties at the single particle level.
Herein, we report a simple, solvent-free, scalable, and environment friendly all-solid-state mechanochemical synthesis of the bulk powders of LDPs, Cs 4 CdBi 2 Cl 12 (pristine) and a partially Mn-substituted analogue [Cs 4 Cd 0.6 Mn 0.4 Bi 2 Cl 12 ]. More importantly, we have synthesized the 2-3 layer thick 2D nanosheets (NSs) of the pristine and Mn-substituted analogue by the one-pot solution-based hot-injection method using benzoyl chloride as a halide precursor. These materials showed high thermal and environmental stability. The 2D NSs of the Mn-substituted analogue showed a relatively high PLQY of ∼21% at room temperature. A deeper understanding of the charge-carrier dynamics in these 2D NSs was achieved through time-resolved PL measurements both at room and cryogenic (77 K) temperatures. Super-resolved uorescence microscopy and time-resolved single particle tracking were employed to investigate the PL blinking event in 2D NSs of the pristine and Mn-substituted analogue at the single particle (herein, NS) level, which is a key process to control the emission efficiency. Photo-bleaching and temporal intermittency (blinking) behaviours have been perceived in controlled pristine NSs, whereas the single NS of the Mn-substituted analogue displayed relatively suppressed PL uctuation characteristics. An enriched PL intensity with a high photon count was evidenced in Mn-substituted NSs via structured illumination microscopy (SIM) accompanied by negligible photo-bleaching behaviour under continuous light illumination.

Results and discussion
Here, we rst discuss the solvent-free mechanochemical synthesis, stability, and optical properties of bulk polycrystalline powders, and subsequently, we present the colloidal synthesis of 2D nanosheets (NSs) of Cs 4 CdBi 2 Cl 12 (pristine), and Cs 4 Cd 0.6 Mn 0.4 Bi 2 Cl 12 (Mn-substituted analogue); and super-resolved uorescence microscopy results to understand the PL uctuation behaviour at the single NS level.
To synthesize the bulk powders of the pristine and Mn-substituted analogue, we have used CsCl, CdCl 2 , MnCl 2 , and BiCl 3 as precursors in the appropriate stoichiometric ratio. Typically, a stoichiometric amount of the metal chloride precursors was mixed and ground mechanically in a mortar and pestle for 2 hours in an ambient laboratory environment (Schemes 1 and 2). Previously, such bulk powders have been synthesized by the precipitation method using concentrated hydrochloric acid. 8a,10 In contrast, our all-solid state mechanochemical approach is solvent-free, and scalable to ∼1 g of pure crystalline product.
The powder X-ray diffraction (PXRD) pattern in Fig. 1b for the mechanochemically synthesized bulk powders was perfectly matched with the rhombohedral phase of pristine (space group R 3m), which indicates the phase purity of the obtained product. A right shi of XRD peaks in 2q was evidenced for the bulk Mn-substituted analogue, which corresponds to the smaller ionic radius of Mn 2+ (83 pm) as compared to that of Cd 2+ (95 pm). Cs 4 Cd 1−x Mn x Bi 2 Cl 12 (where, x = 0 and 0.4) exhibits a layered double perovskite (LDP) crystal structure, which can be derived from the conventional cubic ABX 3 Fig. 1a. 6a,11 The visual appearances of as-synthesized bulk powders are demonstrated in Fig. 1c and d. The Mn-substituted powders showed intense orange emission, whereas pristine powders displayed low intense purple colour under UV light. To investigate the environmental stability, the PXRD patterns of bulk pristine and Mn-substituted powders were monitored aer 30 days of exposure to the ambient conditions ( Fig. S1a and b, ESI †), which conrmed the absence of degradation. The thermal stabilities of these mechanochemically synthesized bulk powders were investigated by thermogravimetric analysis (TGA), as shown in Fig. S2a (ESI †). Both the samples showed stability up to 400°C, aer which approximately 19% and 23% weight losses occurred at ∼450°C for pristine and Mn-substituted analogue, respectively, and further decomposition has been observed thereaer. The characteristic Raman active vibrational modes arising from the different metal octahedra in pristine and Mn-substituted samples were ascertained from the implementation of Raman spectroscopy (Fig. S2b, ESI †). For the pristine sample, the Raman active vibrational bands at 118, 268, and 291 cm −1 appeared due to the Scheme 1 Scheme 2 breathing (T 2g ), asymmetric stretching (E g ) and symmetric stretching (A 1g ) modes of BiCl 6 octahedra, respectively, 12 whereas the bands at 147 and 243 cm −1 appeared from the Cd-Cl bending and stretching modes, respectively. 13 The shoulder observed in the 118 and 147 cm −1 bands of the doped samples is probably due to the partial Mn 2+ substitution. In addition, a small hump at 224 cm −1 is observed in the Raman spectra of the Mn-substituted sample, which might have appeared from the symmetric stretching of the Mn-Cl bond. 14 Moreover, the X-ray photoelectron spectroscopy (XPS) measurement conrmed the expected oxidation state of all elements (Cs, Cd, Mn, Bi and Cl) in the Mn-substituted sample ( Fig. S3a-e, ESI †).
The optical properties were investigated for both pristine and Mn-substituted bulk powder samples by diffuse reectance and PL spectroscopic techniques in the solid state. Electronic absorption spectra of pristine and Mn-substituted samples are presented in Fig. 2a. The optical band gaps of both samples were estimated from the onset of the absorption edge by extrapolating the linear region, which appeared to be ∼372 nm (∼3.33 eV) nm and ∼408 nm (∼3.08 eV) for pristine and Mn-substituted analogue, respectively. 8a The absorption spectrum of the Mn-substituted sample involves two extra weaker peaks in the visible region at ∼490 and ∼430 nm, whereas these peaks are absent in the pristine sample (Fig. S4, ESI †). The additional peaks in the Mn-substituted sample can be assigned to the spin-forbidden 6 A 1 (S) / 4 T 1 (G) and 6 A 1 (S) / 4 T 2 (G) dd transitions in the high spin Mn 2+ ion, respectively. The solid-state room-temperature PL spectra of the bulk pristine sample displayed a PL band centred at 601 nm ( Fig. 2c), while the bulk Mn-substituted sample showed highly intense broad PL centred at 595 nm with an emission line-width of 60 nm (Fig. 2c). The representative orange PL emission of the Mn-substituted sample is ascribed to the spin and parity forbidden 4 T 1g (G) / 6 A 1g (S) transition within the 3d shell of the octahedrally coordinated Mn 2+ . 10 Moreover, the emission peak maxima in PL spectra of the Mn-substituted sample were found to be independent of the excitation wavelength as shown in Fig. 2d, which designates a single radiative decay pathway. 8a,10 Furthermore, the PL excitation (PLE) spectrum of both samples monitored at l max em matched well with the absorption spectrum (Fig. 2b). 8a However, the broadband and highly Stokes shied PL emission of the Mn-substituted sample could be due to the energy transfer from the intrinsic dark self-trap excitonic (STE) state to the Mn 2+ luminescent centre. 15 We have employed the solution-based hot-injection method for the colloidal synthesis of pristine and Mn-substituted nanostructured samples. In a typical synthesis of Cs 4 Cd x Mn 1−x Bi 2 Cl 12 , an appropriate proportion of Cs 2 CO 3 , Cd(OAc) 2 $2H 2 O and/or Mn(OAc) 2 and Bi(OAc) 3 was dissolved in 1-octadecene (ODE), following which oleic acid (OA) and oleylamine (OAm) with a ratio of 2 : 1 were added and the reaction was kept at 120°C under N 2 for 1 hour. Subsequently, the temperature was increased to 135°C and benzoyl chloride was injected. The reaction was then quenched in an ice water bath and the as-synthesized product was washed several times with isopropanol. The detailed synthesis strategy is presented in Scheme S1 (ESI †). The PXRD patterns of the as-synthesized nanostructures in Fig. 3a are well matched with the simulated pattern, which conrms the phase-purity. The shi of the PXRD peak to a slightly higher 2q angle for the Mn-substituted sample compared to that of the pristine sample is due to lattice contraction caused by incorporation of Mn 2+ at the Cd 2+ site of the pristine sample. The actual composition of the Mnsubstituted sample is further estimated from the ICP-AES analysis, which closely matched with the nominal stoichiometry (Table S1, ESI †). The detailed structural analysis was carried out  To obtain information on the presence of added ligands (OA and OAm) on the surface of nanostructures, Fourier transform infrared spectroscopy (FTIR) was employed. Both samples showed similar FTIR spectra, as shown in Fig. S7b (ESI †). The FTIR spectra conrmed the characteristic modes of the oleyl group [CH 3 (CH 2 ) 7 -CH]CH-(CH 2 ) 8− ], which exists in both OA and OAm. The peaks at ∼2854 and 2920 cm −1 correspond to the symmetric and asymmetric stretching modes in -CH 2 , respectively. The y c]c stretching mode is also evident as a less intense peak at 1635 cm −1 . As expected, the 1710 cm −1 peak appeared from the y c]o of the carboxylic group of OA. The 3175 cm −1 and 1517 cm −1 modes arose from the stretching and scissoring of NH 2 groups of OAm, respectively. Moreover, the distinct 1460 cm −1 and 1378 cm −1 peaks appeared from the C-H bending vibration of the hydrocarbon chain of OA/OAm. 1g Therefore, the FTIR study conrms the existence of the ligands on the surface of nanostructured samples.
Transmission electron microscopy (TEM) revealed the 2D nanosheet (NS) morphology for both pristine and Mn-substituted samples (see Fig. 3b and c, respectively). High-resolution TEM (HRTEM) analysis revealed the d-spacing values from lattice fringes to be 0.385 nm and 0.277 nm (Fig. 3d and f), which correspond to the (110) and (119) crystal planes of pristine and Mn-substituted NSs, respectively. The selected area electron diffraction (SAED) patterns of the pristine and Mn-substituted NSs further conrm the single crystalline nature, with the diffraction spots corresponding to the [1-12] and [111] zone axis of the rhombohedral phase, respectively ( Fig. 3e and g). Additionally, the atomic force microscopy (AFM) analysis was executed to investigate the thickness of NSs (Fig. 3h and i), in which the height proles were found to be 7.33 and 11.35 nm in pristine and Mn-substituted NSs, respectively. These observations suggested that the NSs are two to three-layers thick (thickness of one layer corresponds to 3.35 nm). The lateral dimension of the NS ranges from 300 to 700 nm. The eld-emission scanning electron microscopy (FESEM) images and EDAX elemental mapping for the two samples further conrmed the uniform distribution of all elements (Cs, Cd and/or Mn, Bi, and Cl) in both samples ( Fig. S8 and S9, ESI †). Subsequently, the Mn-substituted NS sample was characterized by the electron paramagnetic resonance (EPR) measurement, as illustrated in Fig. S10 (ESI †). The single broad EPR resonance peak without any hyperne splitting might be ascribed to the stronger magnetic interactions. 10 The obtained g value of ∼1.99 suggests the presence of Mn 2+ (electronic conguration: [Ar]3d 5 , g ∼ 2.0) in the corresponding NS sample. 16 The optical properties of pristine and Mn-substituted NS samples were examined in the solution phase (dispersed in toluene). The UV-vis absorption spectra in Fig. S11a (ESI †) display a distinct and sharp absorption band maximum at ∼335 nm for both samples, which probably signies the presence of an excitonic peak, a representative of 2D layered materials. 17 The absorption maximum appeared due to the electronically spin-forbidden 1 S 0 / 3 P 1 transition of Bi 3+ ion. 9a,b While previous reports showed no noticeable PL emission at room temperature for Cs 4 CdBi 2 Cl 12 NCs, 9a,b we have obtained room temperature weak PL emission for the pristine NS sample, which is centred at 605 nm irrespective of the excitation wavelength (Fig. 4a). The peak possibly appears from the 3 E g / 1 A 1g electronic transition of the Cd 2+ ion in the [CdCl 6 ] 4− octahedral unit. 9b Conversely, an intense PL emission centred at 601 nm was observed for the Mn-substituted NS sample, which also remains unaffected by the different excitation wavelengths (Fig. 4a). This intense orange emission can be ascribed to the 4 T 1g (G) / 6 A 1g (S) electronic transition of Mn 2+ ions. 9b The PL emission of Mn-substituted NSs revealed a broad and large Stokes shied emission without any overlap between its excitation and emission spectra. The large Stokes shied broad PL emission of the pristine sample indicates that the emission originates from a characteristic STE state, whereas the broadband and Stokes-shied PL emission of Mn-substituted NSs appears because of the procient energy transfer from the STE state to the Mn 2+ centre. 15 Fig. S11b and c (ESI †) present the PLE spectra (monitored near PL peak emission) of both samples, which displayed nearly overlapped proles with the corresponding absorption spectra. The quantum yield (QY) of both NS samples (dispersed in toluene) was measured at room temperature. The QY of pristine NSs was found to be 0.27%, whereas a relatively high quantum yield of 20.56% was recorded for the Mn-substituted NS sample. To the best of our knowledge, this is the highest PLQY of any of the 2D NS samples of Pb-free LDPs. To examine the temperature stability of the NS sample, we performed temperature-dependent PL measurement of Mn-substituted NSs dispersed in toluene in the 25-85°C temperature range. The PL spectra at 85°C revealed a relative loss of only 26% of its initial intensity (Fig. S12a, ESI †), which can also be witnessed from the appearance of the sample under UV illumination (Fig. S12a, † inset). Furthermore, we added up to 120 ml of water into 3 ml of the NS sample in toluene and monitored the PL with gradual addition of water (Fig. S12b, ESI †). The PL spectra and visual appearance shown in Fig. S12b (ESI †) indicate a 57% relative loss of its initial intensity and a weak orange emission, respectively. These observations again lead to the relatively high temperature-and reasonable water stabilities of the as-synthesized Mn-substituted NS sample and show that this can be a promising candidate for optoelectronic application, comparable to the conventional Pb-based 3D perovskite NCs.
For better understanding of the charge carrier dynamics in these NS samples, PL measurement was performed at cryogenic temperature (77 K). Fig. 4b displays the low-temperature PL spectra for both the pristine and Mn-substituted NSs, respectively. An enhanced PL intensity and red-shied emission towards the near-infrared (NIR) region were observed for both the samples at 77 K in comparison to room temperature PL. Similar trends in PL with temperature were reported earlier for Cs 3 BiBr 6 NCs. 19 The enhanced PL intensity at 77 K for both samples is attributed to the suppression of the thermally activated non-radiative recombination pathways. 20 The red-shied PL events for Mn-substituted NSs were originated from the enhanced crystal eld strength owing to the lattice contraction at low temperature, which resulted in reduced energy of the 4 T 1g / 6 A 1g transition and a subsequent, red-shied energy peak. 10 The suppression of electron-phonon coupling at low temperature might be responsible for the emission line-width narrowing in the Mn-substituted sample. 9b In contrast, a broader PL signature was witnessed for pristine NSs at 77 K, which have been tted with a Gaussian line-shape function into two contributions (Fig. S13, ESI †). The low-energy PL contribution is ascribed to the additional recombination from intragap localized states (i.e., shallow-defect states) that cause the unusual spectral broadening. 21 Such observations were previously reported for Pb-based perovskites. 22 To obtain insights into the excitonic recombination dynamics, time-resolved PL studies were carried out for both NS samples at 298 K and 77 K, as depicted in Fig. 4c and d. The PL decays of pristine and Mn-substituted NS samples are tted with the biexponential function. The decay curve can be represented by using the following equation: where A denotes the emission intensity, and A 1 and A 2 are the tting parameters describing the relative amplitude for varied lifetime components, while s 1 and s 2 are the respective lifetime components. The details of the tted lifetime parameters with their relative proportions are summarized in Table S5 (ESI †), in which the lifetimes fall in the order of the microsecond time scale due to the forbidden nature of the involved transitions. Notably, the PL decay plots for both NS samples in Fig. 4c and d suggested slower decay dynamics and a prolonged average lifetime (s avg ) at 77 K than that at 298 K (Table S5, ESI †), which might be due to the higher probability of radiative recombination, and subsequent elimination of non-radiative recombination channels. 9a Furthermore, to verify the presence of the trap-state contribution at 77 K for pristine NSs, we have measured the lifetime by monitoring at two deconvoluted PL contributions, and a much shorter s avg was achieved when monitored at low energy PL contribution. 9a,22c Previously reported Cs 4 Mn x Cd 1−x Bi 2 Cl 12 NC samples also exhibited similar enhancement in the average lifetime at cryogenic temperature compared to that at 300 K. 9a,b In order to investigate the PL properties of individual single crystalline NSs, we acquired uorescence images via super-resolved structured illumination microscopy (SIM) aer depositing a diluted solution of NSs onto a glass coverslip. Fig. 5a and d show the bright-eld optical images of the NSs, where spatially separated NSs can be clearly observed on the glass coverslip. In order to acquire uorescence images from these NSs, a 405 nm laser was employed as the excitation source for the super-resolution SIM imaging study at the single-particle level. A band-pass lter collected the emission in an optical window of 575-650 nm. The uorescence images from the NS samples, obtained by structured illumination microscopy (SIM), are shown in Fig. 5b and e. The SIM image typically looked crisper with improved resolution (see Fig. S14 and S15, ESI †) as compared to a standard wide-eld image. It is evident from the SIM images that because of the high PLQY of the Mn-substituted NS, the spatially separated particles of the Mn-substituted analogue could be readily visualized with high contrast. In comparison, the pristine NS sample, although visible, appeared much dimmer compared to the Mn-substituted NS sample. The particle size histogram from SIM images (Fig. 5c) revealed a distribution of NSs with sizes ranging from 300-600 nm. The size distributions of the perovskite nanosheets (NSs), presented in Fig. 5c, have been determined by using the uorescence intensity proles of the NSs. Rapid scanning and large area sampling capability of uorescence microscopy helped in quick determination of the size distribution prole of the NSs. From the SIM images (Fig. S15, ESI †), we rst plotted the cross-sectional intensity prole of the individual NS. The full width at half maximum (FWHM) from the Gaussian t of this cross-sectional prole was reported as the size of the NSs. In an unbiased fashion, we have picked ∼40 particles from the imaging window and measured the FWHM for determining the size distribution of the NSs. In this respect it should be noted that the typical achievable lateral (xy) resolution of SIM is around 100-120 nm. On the other hand, from our image analysis we found that even the smaller size NSs were >200 nm (Fig. 5c). This indicates that the size of NSs was well above the resolution limit of the SIM methods of imaging. Therefore, we believe that in this case, SIM images are capable of providing near accurate size estimation and it is comparable to the data obtained from the TEM analysis ( Fig. 3b  and c). Importantly, given that the average size from uorescence images (∼400 nm) matches with that from other techniques (e.g., TEM), it ruled out the possibility of analysis being performed on aggregated NSs. In addition, a quantitative estimation from the uorescence intensity plot in Fig. 5f clearly reects a considerably higher average intensity from the Mn-substituted NSs compared to the pristine NSs. Moreover, we have also calculated the PL intensity vs. particle size data for a large number of NSs of the pristine and Mn-substituted samples. These data have been represented in a scatter plot in Fig S16, † which also indicated a considerably large uorescence intensity in the case of Mn-substituted NSs with similar size particles.
Finally, to acquire deeper insights into the PL characteristics of the pristine and Mn-substituted NSs, we investigated temporal instability of PL from individual single crystalline NSs under continuous excitation. In general, the nature of time-dependent PL intermittency (or "blinking") is dependent on the metastable non-radiative recombination sites. 23 This further results in a low-emissive dark (OFF) state or quenched PL state (active), whereas a smaller number of such traps provides an enhanced photo-stability in the intense high emissive (ON) state or bright PL state (in-active). 23,24 These low-emissive states in blinking events are a major setback in wide-ranging applications, such as in LEDs and biological settings for tracking single biomolecules. 24a Therefore, it is important to characterize the temporal instability of PL behaviour of the as-synthesized NSs under constant illumination. For the uorescence time-trace analysis, we continuously collected emission (video) over 100 s for pristine and Mn-substituted NS samples individually under the total internal reection uorescence (TIRF) setting (videos S1 and S2, respectively, ESI †). Fig. 6a and d depict the representative NSs that were considered for the temporal PL instability study in the  respective samples. The video snapshots from the respective samples under TIRF at an interval of 15 s are shown in Figs. S17a and b (ESI †). The pristine NS exhibited a rapid reduction in luminescence intensity in the PL versus time-trace plot; however, such behaviour was not observed in the Mn 2+ -substituted NS sample ( Fig. 6b and e, respectively). Aer an initial decrease in intensity due to the illumination induced photo-bleaching, the zoomed-in time trace plot in Fig. 6c reveals that the pristine NS tends to display a PL blinking-like behaviour with a number of OFF states aer 200 s exposure to continuous laser radiation. This blinking-like nature can be presumably ascribed to the presence of trap states that results in the OFF state. 25 This is further supported by the time-resolved PL decay at room temperature, where nearly 50% contribution in the life-time originates from the faster component (s 1 ), which resulted from the trap state contribution (Table S3, ESI †).
Interestingly, from the time-dependent PL traces up to 120 s in Fig. 6e and the recorded video (video S2, ESI †) for Mn-substituted NSs, no signicant decrease in luminescence intensity from PL-versus-time traces was observed from a single particle. The PL intensity is found to slightly uctuate between the high intensity ON states, which is well-above the background intensity level. A zoomed in plot within 50 s regimes (aer the initial 200 s exposure) further conrms the absence of the non-zero OFF state and a distinctly suppressed PL uctuation nature (Fig. 6f). The PL intensity traces found at the single NS level established that the inclusion of Mn 2+ in pristine NSs (i.e., Mn-substituted analogue) can effectively arrest the dark OFF-state. Such a rare behaviour with suppressed PL uctuations without any photo-bleaching event was observed earlier for halide perovskite-semiconductor core/shell quantum dots, 24a CsPbI 3 NCs when NH 4 I was used as the precursor, 26 MAPbX 3 QDs (X = Br, I) using a bromide or iodide donor (MABr and MAI) as a ller, 27 and Yb 3+ -and Er 3+ -doped up-converting NaYF 4 NCs. 28 The weak PL emission in pristine NSs is induced by the dynamic equilibrium between active and in-active states of the metastable non-radiative recombination channels, causing PL uctuations or a blinking-like nature. However, in partially Mn-substituted NSs, the PLQY is much higher, which means the number of non-radiative centers is much smaller, but we still see some PL uctuation like events. Similar defects are probably still present in Mn-substituted NSs; however, their number is much less, and these non-radiative centers are mostly in the passive state. Thereby, partial Mn 2+ substitution seems to stabilize the in-active state of the non-radiative centers, which leads to an intense PL emission, and seldom and short-lived off states.

Conclusions
Facile solvent-free mechanochemical synthesis and optical property investigations have been demonstrated for the bulk powder of pristine and its partially Mn-substituted analogue. Importantly, a modied hot-injection route was implemented to synthesize the 2D NSs of pristine and Mn-substituted analogue. A weak PL emission is perceived for pristine NSs at room temperature with a QY of <1%. Conversely, the Mn-substituted NSs showed an intense orange emission at room temperature together with a QY of ∼21%. The steady-state and time-resolved PL measurements at cryogenic temperature (77 K) revealed the involvement of trap states in weak PL emission of the pristine sample, which is substantially reduced by the addition of Mn 2+ in partially Mn-substituted NSs, resulting in an intense PL. Finally, super resolution uorescence microscopy was implemented to understand the single nanosheet's PL properties in the pristine NSs and the Mn-substituted analogue. The pristine NS exhibited illumination induced photo-bleaching and PL blinking-like nature, whereas the Mn-substituted NS analogue demonstrated suppression of PL uctuations with negligible photo-bleaching behaviour. The PL intensity traces with time at the single NS level established that the introduction of Mn 2+ in pristine NSs can stabilize the in-active state of the metastable non-radiative channels. Characterized by an intense orange emission with suppressed PL uctuations and negligible photo-bleaching, the Mn-substituted NSs demonstrate their unique optical properties at the single NS level, which might be utilized for further optoelectronic applications in the future.

Data availability
All data are available in the manuscript and in the ESI. †

Author contributions
K. B. conceived the idea and designed the study. A. B., K. K., P. A., J. P. and K. B. carried out the synthesis, structural and optical measurements, and other analyses. R. S., S. K. and S. S. A. studied the photoluminescence blinking properties. K. K. wrote the rst dra and everyone contributed to editing the manuscript.

Conflicts of interest
The authors declare no conict of interest.