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Sci Rep. 2017; 7: 45906.
Published online 2017 April 12. doi:  10.1038/srep45906
PMCID: PMC5388844

Luminescent manganese-doped CsPbCl3 perovskite quantum dots

Abstract

Nanocrystalline cesium lead halide perovskites (CsPbX3, X = Cl, Br, and I) form an exciting new class of semiconductor materials showing quantum confinement. The emission color can be tuned over the full visible spectral region making them promising for light‒emitting applications. Further control over the optical and magnetic properties of quantum dots (QDs) can be achieved through doping of transition metal (TM) ions such as Mn2+ or Co2+. Here we demonstrate how, following QD synthesis in the presence of a Mn‒precursor, dropwise addition of silicon tetrachloride (SiCl4) to the QDs in toluene results in the formation of Mn‒doped CsPbCl3 QDs showing bright orange Mn2+ emission around 600 nm. Evidence for successful doping is provided by excitation spectra of the Mn2+ emission, with all features of the CsPbCl3 QD absorption spectrum and a decrease of the 410 nm excitonic emission life time with increasing Mn‒concentration, giving evidence for enhanced exciton to Mn2+ energy transfer. As a doping mechanism we propose a combination of surface etching and reconstruction and diffusion doping. The presently reported approach provides a promising avenue for doping TM ions into perovskites QDs enabling a wider control over optical and magnetic properties for this new class of QDs.

Lead halide perovskites MPbX3 (where M = CH3NH3+, HC(NH2)2+, and Cs+; X = Cl, Br, and I) have recently gained interest as the result of their promising performance for a variety of optoelectronic applications, especially photovoltaics1,2,3, but also light‒emitting diodes (LEDs)4,5, lasing6,7, and photodetectors8,9. These materials have favorable intrinsic properties including broad absorption spectra, tunable optical spectra, high luminescence quantum yields, high carrier mobility, and long carrier diffusion length. In the past three years, a rapidly increasing number of studies were published focused on synthetic methods and morphology control of colloidal perovskites nanocrystals (NCs) aimed at improving their performance in practical applications. The hybrid organic‒inorganic perovskites materials, such as CH3NH3PbX3 (X = Cl, Br, and I), are outstanding solar harvesting materials in photovoltaic devices with 20% certified power conversion efficiencies10,11. Sargent et al. utilized perovskites shell‒capped colloidal QDs films of (PbS‒CH3NH3PbI3) to fabricate novel and efficient photovoltaic devices12.

The all inorganic analogue perovskites CsPbX3 QDs were soon after reported by Kovalenko et al.13. These QDs show efficient luminescence that can be tuned over the full visible spectrum (400‒700 nm) by tuning of size (through quantum confinement effects) and composition (anion exchange)13,14. Using a facile solution‒based synthesis approach, cubic CsPbX3 QDs with colorful narrow emission bands and high quantum yields of up to 90% were realized13. The highly efficient narrow band emission makes these QDs very useful for application in lighting and displays. For example, colloidal CsPbBr3 QDs have been reported with efficient narrow band emission (FWHM = 86 meV; quantum yield = 90%) and reduced emission blinking making them promising for the lighting and display industry15. Interestingly, it is not generally known that this class of CsPbX3 QDs has been known for almost 20 years. Emission and quantum confinement of CsPbX3 QDs that form in Pb‒doped CsX scintillator crystals was reported in the late 1990’s16. For the II‒VI QDs such as ZnO, ZnS, ZnSe, CdS, CdSe there has been a strong effort to dope the QDs with transition metal (TM) ions such as Mn2+ or Co2+ aimed at introducing new optical and/or magnetic functionalities17,18,19,20,21. A large number of publications have reported different categories of doped QDs22,23,24. For example, efficient Mn2+ emission in ZnSe QDs is promising for application in solar concentrators as a result of the large spectral shift of the Mn2+ emission which prevents undesired reabsorption25. Doping of magnetic ions in QDs also holds great promise in the field of spintronics with the possibility to dope a single magnetic ion in a single QD.

Doping can be straightforward by simply adding a dopant precursor in the reaction mixture, e.g. in ZnS:Mn2+, but more often doping of QDs proved to be challenging26. In the past decade new doping techniques have been developed allowing doping of systems that were previously considered not possible to dope. For example, doping CdSe with Mn2+ was considered impossible but was later realized using a single source precursor method. More recently with diffusion doping even concentrations up to 30% Mn2+ are now incorporated in CdSe27. For perovskite QDs doping has only recently been realized and is highly interesting as it allows the addition of optical or magnetic functionalities to this exciting new class of materials28,29.

Here we report doping of Mn2+ into perovskite CsPbCl3 QDs (CPC‒Mn) via a facile colloidal approach. A post synthesis treatment of CsPbCl3 QDs prepared in the presence of Mn‒stearate allows incorporation of Mn2+. Slowly dripping a small volume (5‒20 μl) of SiCl4 to the dispersion of QDs in toluene results in a slow incorporation of Mn2+ at room temperature. Evidence for successful doping is provided by a clear signature of efficient energy transfer from the CsPbCl3 QD exciton to Mn2+ ions. As Mn2+ incorporation proceeds, the excitonic emission intensity decreases and the bright orange Mn2+ emission increases. The excitation spectrum of the Mn2+ emission shows all the features of the CsPbCl3 QD exciton absorption. The life time of the exciton emission decreases as the Mn2+ concentration increases as a result of fast exciton to Mn2+ energy transfer. The proposed mechanism for incorporation involves surface etching and reconstruction and diffusion doping. Decomposition of SiCl4 by trace amounts of water in the toluene yields HCl. Surface etching by H+ and surface reconstruction enabled by chloride ions and Mn2+ form a surface layer and slowly drives Mn2+ into the QDs by ion diffusion. The decomposition of SiCl4 also gives rise to the formation of a protective SiO2 layer giving a relatively high stability to the QDs. Perovskites QDs are known to degrade and show a fast loss of emission intensity under ambient conditions. The PL of the CPC‒Mn QDs synthesized is relatively stable and retains over 25‒30% of the maximum intensity after stirring with SiCl4 for 40 days.

Results and Discussion

Synthesis and characterization

CsPbCl3 QDs were prepared using a similar procedure as reported by Kovalenko et al.13. Details on the QD synthesis and doping procedure can be found in the Methods section. In short, PbCl2 and the Mn‒precursor (Mn‒stearate) in 1‒octadecene (ODE) were dried at 120 °C after which dried oleylamine (OLAM) and oleic acid (OA) were added. The temperature was raised to 170 °C and a Cs‒oleate solution in ODE was injected. At this temperature the CsPbCl3 QDs form quickly. Next, the mixture was cooled and the NCs were extracted by centrifugation. To remove unreacted polar material the QD precipitate was redispersed in toluene and centrifuged. Precipitated particles were discarded. The final solution of CsPbCl3 QDs in toluene shows the characteristic violet QD emission but no Mn2+ emission, indicating that in this procedure Mn2+ is not incorporated. In the next step a small volume (5‒20 μl) of SiCl4 was dripped to 2 ml of the QD solution. During stirring a white precipitate forms. After different stirring times the precipitate was removed by centrifugation and a transparent QD solution was used for the spectroscopic measurements and characterization. As a control experiment SiCl4 was also added under vacuum conditions in which all water was removed from the toluene.

In Fig. 1a X‒ray diffraction (XRD) patterns of the various synthesis products are shown. Line 1 (blue) shows the typical XRD pattern of CsPbCl3 QDs made without Mn2+. The XRD pattern of the QDs synthesized in the presence of the Mn‒stearate precursor (green line) is similar. For the product formed after addition of SiCl4 and stirring for 1 h (red line) the formation of amorphous SiO2 is evident from the broad diffraction peak of amorphous SiO2 (PDF Card No. 82‒1576), observed clearly from 15° to 30°. Stirring for 10 days, the pure CsPbCl3 XRD patterns (black line) was still found when the precipitate (SiO2) was removed by centrifuging. The lines in the XRD patterns of the QDs match the powder diffraction pattern of cubic phase CsPbCl3 (Fig. S1a). The broadening of the diffraction peaks is because of the small size. The TEM image (Fig. 1b) shows the formation of monodisperse (13.1 ± 0.4 nm) side length CPC‒Mn QDs and a d‒spacing of 0.58 nm is clearly observed13,30. The particle size is similar to that for samples made without Mn‒precursor (Fig. S2). Figure S3 displays elemental analysis by energy dispersive X‒ray spectroscopy (EDX) to confirm that Mn is present in the sample after doping, although no Mn2+ emission is observed. Given the highly apolar nature of the Mn‒precursor, the Mn‒stearate can be expected to absorb in the apolar capping layer around the QDs.

Figure 1
Characterization of CPC‒Mn QDs.

In the next step a small volume of SiCl4 is added to the QDs in toluene. The role of this silica precursor is twofold: it will induce the growth of protective SiO2 around the QD, and, more importantly, it induces the incorporation of Mn2+ in the CsPbCl3 QDs. The role of SiO2 as a protective shell to protect the core material is well‒known. For example, the stability of CH3NH3PbBr3‒SiO231, CdSe/ZnS‒SiO232, and CdSe/CdS/ZnS‒SiO233, QDs were clearly improved in comparison to the uncoated QDs. The High Angle Annular Dark Field Scanning TEM (HAADF‒STEM) image after addition of SiCl4 and stirring for 10 days and removal of SiO2 by centrifugation is shown in Fig. 1c. The monodisperse cubic particles (7.7 ± 0.5 nm) are smaller than that for the QDs before SiCl4 addition. This indicates that SiCl4 addition causes etching of the CsPbCl3 QDs. In the HAADF‒STEM images also white dots are observed. Elemental mapping (Fig. S4) reveals that these dots are highly enriched in Pb.

In the HAADF‒STEM image of the sample before centrifugation both individual CPC‒Mn QDs and SiO2 nanoparticles were observed (Fig. 1d). This indicates that also a secondary nucleation of silica occurred and that silica was not only deposited on the QD surface in a seeded growth. EDX analysis for a region containing CPC‒Mn QDs (cyan square Fig. 1d) and a region containing mainly SiO2 (cyan circle Fig. 1d) are shown in Fig. 1e and f. In the EDX spectrum of the QD area (cyan square) signals of Cs, Pb, Cl, and Mn of the CPC‒Mn QDs are observed and signals corresponding to Si and O are weak. The elemental analysis of the cyan circle shows strong Si and O signals, indicating the presence of a SiO2 nanoparticles.

In a control experiment, SiCl4 was added to the QD dispersion and then stirred for 3 days in vacuum. TEM image and EDX analysis were recorded for the CPC‒Mn QDs made under vacuum are shown in Fig. S5. The CPC‒Mn QDs were covered by a SiO2 layer (Fig. S5a). The different SiO2 morphology is ascribed to a difference in water content which determines the HCl concentration following hydrolysis of SiCl434,35. The water present in toluene under ambient conditions (H2O content ~0.06%) induces faster decomposition of SiCl4 and silica formation31. In the resulting SiO2 covered CPC QD conglomerates all elemental signals of the CPC‒Mn QDs and SiO2 (Cs, Pb, Cl, Mn, Si, and O) were observed, as shown in Fig. S5b.

Optical spectroscopy

To monitor incorporation of luminescent dopants into QDs optical and time resolved spectroscopy are powerful tools. When a luminescent ion is inside of a QD, fast energy transfer from the QD exciton to the dopant is expected. Absorption of light by the QD is followed by exciton‒to‒dopant energy transfer and emission from the dopant ion. Monitoring emission and excitation spectra as well as changes in the time traces of the luminescence intensity of excitonic and dopant emission provides clear signatures for the successful incorporation of a dopant in a QD. In Fig. 2a,b absorption, excitation, and emission spectra are shown for three types of CsPbCl3 QDs: undoped QDs (CPC, no Mn added, green lines), CsPbCl3:5%Mn QDs (CPC:5%Mn, 5% Mn‒precursor added to reaction mixture, red lines) and CsPbCl3:5%Mn‒SiCl4 (CPC:5%Mn‒SiCl4, 5% Mn‒precursor added, followed by 10 μl SiCl4 and stirring for 1 day, blue lines). All three QDs show exciton emission around 400~410 nm. The emission spectra of the CPC and CPC:5%Mn samples are almost identical. For the CPC:5%Mn‒SiCl4 sample an extra emission band is observed in the red/orange spectral region around 600 nm. The excitonic emission shows a small (5 nm) blue shift and is slightly broader (halfwidth = 600 cm−1 versus 550 cm−1 for the CPC and CPC:5%Mn‒SiCl4 exciton emission).

Figure 2
Optical properties of the CsPbCl3:5%Mn2+ QDs.

The 600 nm emission wavelength is typical of the 4T1  6A1 emission from Mn2+. Mn2+ can emit in the green to deep red spectral region depending on the local coordination. The Tanabe‒Sugano diagram for ions with the 3d5 configuration shows that the position of the emitting lowest energy excited state 4T1 shifts to lower energies as the crystal field splitting increases36,37. In CsPbCl3 the Mn2+ will substitute on the octahedrally coordinated Pb2+ site (the Cs+‒site is too large). Since Cl- ions are weak crystal field ligands in the spectrochemical series a green emission may be expected. However, octahedral coordination gives rise to a large crystal field splitting and even for weak ligands orange‒red emission is typically observed for Mn2+. For example, in CdCl2 and a variety of chloride perovskites where Mn2+ is in an octahedral Cl- coordination, the emission maximum of the 4T16A1 emission of Mn2+ has been observed at wavelengths varying between 570 nm and 630 nm36,37. The presently observed emission maximum at 600 nm is in excellent agreement with this range and provides further support for the assignment of the orange emission to 4T16A1 emission of Mn2+ in an octahedral Cl- coordination.

The absorption and excitation spectra of the different CPC QDs in Fig. 2a are all similar and show an exciton peak at 410 nm. Interestingly, the excitation spectrum of the red/orange Mn2+ emission for the CPC:5%Mn‒SiCl4 QDs (blue line) also shows the excitonic features of the QD absorption, indicating exciton‒to‒Mn2+ energy transfer, a signature of incorporation of Mn2+ in the CsPbCl3 QD. Careful inspection of the emission spectra for CPC:5%Mn before SiCl4 addition reveals a very weak emission feature in the orange spectral region (Fig. S6) that is not observed for the undoped CPC sample (Fig. 2b, green line). The weak Mn2+ emission disappears after washing with acetone (see Fig. S7) and also no Mn2+ emission was observed for acetone‒washed CPC:5%Mn after addition of SiCl4 under ambient conditions. This result indicates that the weak emission originates from Mn2+ precursor adsorbed in the QD surface layer. As the Mn2+ ions are not incorporated in the QD, energy transfer from the exciton to Mn2+ excited states is not efficient due to the weak coupling of the exciton to remote Mn2+ ions. Washing with acetone removes the surface absorbed Mn2+ precursor.

As shown above, the addition of a small volume of SiCl4 to the CPC QDs in toluene gives rise to Mn2+ emission after prolonged stirring at room temperature. To study the influence of the volume of SiCl4 added, emission spectra were recorded after addition of 5~20 μl SiCl4 into the CPC:5%Mn QDs toluene solution under ambient conditions. The emission spectra, shown in Fig. S8, show an enhanced CPC QD exciton emission as well as Mn2+ emission for all solutions after 12 h of stirring following SiCl4 addition. The increase in PL intensity is explained by better surface passivation of the QDs and will be discussed below. The intensity of the Mn2+ emission increases for SiCl4 volumes up to 10 μl and then decreases. For the highest volumes added (15 or 20 μl SiCl4) degradation of the CsPbCl3 QDs was observed. As the stability of the CPC QDs is compromised, also the Mn2+ emission intensity decreased for the highest volumes of SiCl4 added. For the reaction volume used (2 ml), the optimum SiCl4 amount equals 10 μl and this volume was used in subsequent experiments to follow the time evolution of Mn2+ incorporation.

To optimize the Mn‒concentration and to probe Mn‒incorporation over time, extensive spectroscopic characterization was done for CPC:x%Mn‒SiCl4 samples taken at different times after injection of SiCl4. In Fig. 3 excitation and emission spectra, luminescence decay curves of the excitonic and Mn2+ emission as well as the integrated emission intensity ratios of Mn/QD are shown for different times (1 hour to 10 days) after injection of 10 μl SiCl4 to a CPC:x%Mn QD dispersion prepared with 1, 3, 5, 7 and 10% of Mn‒precursor present in the initial reaction (concentrations in mole% relative to Pb2+). Figure 3a shows the ratio of the Mn2+ to QD emission intensity for samples taken after 1 h to 10 days. In Fig. 3b,c photographs are shown of the QD dispersions under UV (365 nm) illumination after 1 h and 10 days of stirring. In the photographs a clear change in emission color is observed between 1 h and 10 days. As a result of increased Mn‒incorporation the emission color shifts from violet to orange. Interestingly, the Mn/QD intensity ratios drastically increase after stirring for 1 day (green line). At all times, the highest Mn/QD ratio is found for a 5%Mn concentration. For higher nominal Mn-concentrations a drop in emission intensity is observed, possibly caused by concentration quenching. Concentration quenching is caused by energy transfer between dopant ions leading to migration to defects and quenching sites. Typically, this occurs at concentrations higher than 5% but as the distribution of the Mn2+ ions may not be homogeneous, a higher concentration of Mn2+ ions in the surface layer can explain a reduced luminescence intensity by concentration quenching through energy migration in a Mn-enriched surface layer. Based on this, it is concluded that the reaction conditions leading to CPC‒Mn QDs with the highest Mn‒emission intensity is CPC:5%Mn with 10 μl SiCl4 for the reaction volume (2 ml) and QD concentrations (20 mg/ml) used here. The changes in luminescence properties over time were followed for the CPC:5%Mn‒SiCl4 (10 μl) by recording excitation and emission spectra as well as luminescence decay curves of the exciton and Mn2+ emission for samples taken after 1 h, and 1, 3, 10 days. The results are shown in Fig. 3d‒g.

Figure 3
Spectral evolution of CsPbCl3:x%Mn2+ (x = 1, 3, 5, 7, and 10) QDs and CsPbCl3:5%Mn2+ QDs as function of reaction time.

Focusing on the CPC:5%Mn‒SiCl4 (10 μl) system, the results show a continued increase in the Mn2+ emission intensity relative to the excitonic emission (Fig. 3e) for reaction times up to 10 days. The excitation spectra of the Mn2+ emission show the characteristics of the CsPbCl3 QD absorption spectrum, giving evidence for energy transfer from the QD exciton state to the Mn2+ dopant. For longer reaction times a blue shift in the first excitonic peak is observed, both in emission and excitation (Fig. 3d‒e). The shift to shorter wavelengths can be explained by a decrease in the size of the QDs as a result of slow etching accompanying the Mn‒incorporation process. As shown in Fig. 1 the QD size decreases from 12 to 8 nm after 10 days of stirring and this can explain the observed blue shift. The highest Mn2+ emission intensity is observed after 10 days of stirring. At longer times the intensity starts to decreases but even after 40 days a clear Mn2+ emission is observed with ~30% of the maximum emission intensity.

To gain further insight in the energy transfer process and incorporation of Mn2+ in the CPC QDs, luminescence decay curves were recorded for both the exciton and Mn2+ emission. Table 1 summarizes the lifetime (for QD exciton and Mn2+ emission) of CPC:5%Mn‒SiCl4 (10 μl) for various reaction times. Energy transfer from the excitonic state to Mn2+ results in a faster exciton emission decay. The Mn2+ emission decay can be different for surface absorbed Mn2+ than for Mn2+ incorporated in the QD as energy transfer to defects or surface quenching sites is more prominent for surface Mn2+ resulting in shorter decay times for emission from surface Mn2+ ions. The luminescence decay dynamics were recorded for samples taken at several times during the incorporation of Mn2+ for the CPC:5%Mn‒SiCl4 (10 μl) system as this was shown to give the most efficient Mn2+ emission. In Fig. 3f, the PL decay curves of the excitonic QD emission are shown. Initially, i.e. for the decay curves recorded for samples taken after 1 h to 1 day, a lengthening of the decay time is observed. The decay curve for the exciton emission from the sample stirred for 1 day is close to single exponential with a ~16 ns decay time. The initial lengthening of the decay time can be explained by improved surface passivation as a result of the chloride formation following SiCl4 decomposition. Better surface passivation by the Cl- ions remove surface trap states resulting in a more efficient excitonic emission. The 16 ns decay time can be considered to be close to the pure radiative decay time of the exciton emission at room temperature. The results agree with the observation of higher absolute (exciton) emission intensities of CPC QDs in the initial reaction period (up to 1 day) as shown in Fig. S8a.

Table 1
Lifetimes of QD exciton and Mn2+ emission of the CPC:5%Mn‒SiCl4 (10 μl) for various reaction times based on a bi‒exponential fit of the luminescence decay curves.

After prolonged reaction times, the Mn2+ emission intensity strongly increases, while the excitonic emission intensity decreases. Concurrently, the exciton emission decay becomes shorter and also non‒exponential (blue and violet curves in Fig. 3f). After stirring for 10 days a τavg = 6.3 ns decay time is observed. The shortening of the decay time is explained by energy transfer from the QD exciton state to the Mn2+ dopants. The non‒exponential character reflects that transfer rates vary for differently doped QDs. Undoped QDs still decay with the radiative exciton decay time while QDs with one or more Mn2+ ions will decay with a decay rate that is the sum of the radiative decay rate and the transfer rate. The exciton‒to‒Mn2+ transfer rate will also depend on the location of the Mn2+ ion in the CPC‒Mn QD. Transfer to a centrally located Mn2+ ion is expected to be faster than to a Mn2+ ion in the outer shell of the QD where the overlap with excitonic wavefunction is smaller.

Figure 3g show the luminescence decay curves of the Mn2+ emission as a function of reaction time. Long decay times in the ms time regime are observed which is typical for the spin‒ and parity forbidden Mn2+ emission23,24. Initially the decay curves are non‒exponential and the faster initial decay reflects emission from Mn2+ at the QD surface where partial quenching by surface defect states introduces a fast non‒radiative decay channel. For long reaction times (3 and 10 days) the decay is close to single exponential with a long 1.4 ms decay time suggesting that Mn2+ is incorporated inside the CPC host and decays with a ms radiative decay rate typical for the spin‒ and parity forbidden Mn2+ emission.

Reaction mechanism

The synthesis procedure and characterization discussed above provide strong evidence that addition of SiCl4 to the CPC QDs in toluene in the presence of the Mn‒stearate precursor results in the slow incorporation of Mn2+ in the CsPbCl3 QDs. It is interesting to discuss potential incorporation mechanisms and the location of the Mn2+ ions in the CPC QDs. Control experiments help to provide insight in the mechanism. No incorporation of Mn2+ was observed after thorough washing of the CPC‒Mn QDs with acetone before the addition of SiCl4 (Fig. S7). This shows that the (apolar) Mn‒precursor is absorbed at the CPC QD surface after the initial QD synthesis procedure in the presence of the Mn‒stearate precursor. A second control experiment involves dripping of SiCl4 to the CPC QDs with Mn‒precursor in toluene in vacuum. Again, no Mn‒incorporation is observed under these conditions. Only the weak emission of Mn2+ ions around 590 nm and a faint blue emission, possibly from organic chromophores formed during ligand decomposition, were observed (Fig. S9). The absence of water prevents decomposition of SiCl4 and demonstrates that hydrolysis of SiCl4 is crucial in the reaction:

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Hydrolysis of SiCl4 gives rise to the formation of HCl that can react with the Mn‒stearate precursor (releasing Mn2+) and with the CPC QD surface to both etch the surface (H+) and reconstruct the QD surface (Cl-). The blue shift of the excitonic emission with reaction time and the TEM images showing a decrease in particle size are consistent with the occurrence of etching of the CPC QDs. The SiO2 formed during the decomposition reaction of SiCl4 and H2O gives rise to SiO2 spheres observed in TEM and possibly also form a thin protective SiO2 layer around the CPC QDs after prolonged reaction times. The increase of the QD exciton emission and lengthening of the exciton emission decay time in the initial hours after addition of SiCl4 signifies the positive role of the HCl in the passivation of surface defect states. Similar observations were made previously for hybrid perovskite thin films38. A remarkable increase in photoluminescence intensity and lengthening of the emission decay time was observed upon addition of phosphoric acid and explained by the removal of (surface) defect states38. In addition to surface reconstruction, in the presence of Mn2+ and Cl- a Mn‒Cl surface layer can form on the CPC QD. Etching and reconstruction of the CsPbCl3 surface can occur with the incorporation of Mn2+ on Pb2+ sites in the surface layer of the CPC QDs. Subsequent diffusion of the Mn2+ into the CPC QD brings Mn2+ inside the CPC QDs, similar to the Mn2+ doping of CdSe QDs through diffusion doping27. Fig. 4 illustrates the proposed reaction mechanism for the incorporation of Mn2+ and SiO2 formation. The reaction mechanism is consistent with the long reaction times of hours to days observed for diffusion doping and cation exchange reactions at room temperature18,39.

Figure 4
Schematic illustration of Mn2+ ions diffusion doping mechanism and SiO2 formation.

Further research is required to provide a more complete understanding of details on the incorporation mechanism. Based on the present experiments it cannot be excluded that the Mn2+ ions remain located in a surface layer of the CPC QDs. The observation of a transition from non‒exponential decay to a single exponential 1.4 ms decay of the Mn2+ emission is important as it gives evidence that the local surroundings for the Mn2+ ions changes during the reaction. Initially surface Mn2+ ions are present with a shorter life time because of the proximity of surface defect states. For longer reaction times the transition to a purely exponential decay indicates the transition to the same (octahedral) coordination by Cl- for all Mn2+ ions. This observation however does not prove that the Mn2+ ions are homogeneously distributed through the CsPbCl3 QD. The continued observation of exciton emission indicates that the transfer efficiency is limited and that the Mn2+ ions may be preferentially situated in the surface layer where the coupling with the excitonic wavefunction is weaker. Further research on incorporation of Mn2+ in other perovskite QDs is also interesting. For example, for CsPbBr3 QDs the exciton shifts to lower energies (compared to CsPbCl3) while, because of a smaller crystal field splitting for the weaker Br- ligands, the Mn2+ emission is expected to shift to higher energies. This may lead to a cross-over between the exciton and Mn2+ emission depending on the quantum dot size, similar to what has been observed for Mn2+-doped CdSe quantum dots.24

Stability

For practical application of perovskite quantum dots stability is an important issue. Thermal and photo stability are often limited for the Pb‒halide perovskites and also sensitivity to moisture is serious issue that hampers the introduction of this new class of materials in products for the solar cell and lighting market40,41,42. To evaluate the stability of the emission of the SiCl4‒modified CPC‒Mn QDs solutions was followed over time. The emission intensity shows a slow decrease in intensity but retains 27% of the maximum intensity after 40 days in air, as shown in Fig. 5a. The temperature dependence of the photoluminescence was measured for CPC‒Mn QDs (exciton emission) and SiCl4‒modified CPC‒Mn QDs (Mn2+ emission) after incorporation in a poly methyl methacrylate (PMMA) matrix. The decrease of emission intensity with temperature is shown in Fig. 5b. At 100 °C the emission of CPC‒Mn QDs is completely quenched, whereas the SiCl4‒modified CPC‒Mn QDs maintained ~50% of their initial intensity.

Figure 5
Stability of the CsPbCl3:5%Mn2+‒SiCl4 (10 μl) system.

In conclusion, we have reported a new doping strategy for the successful incorporation of Mn2+ ions in perovskite CsPbCl3 QDs. Upon addition of SiCl4 to a CsPbCl3 QD/Mn‒precursor dispersion in toluene incorporation of Mn2+ occurs as demonstrated by the evolution of strong 600 nm Mn2+ emission (4T1  6A1) following excitation in the QD exciton states. Excitation, emission and luminescence life time measurements reveal the presence of energy transfer from the delocalized QD exciton state to the localized 3d5 states of Mn2+. The proposed incorporation mechanism involves surface etching and reconstruction by HCl formed through decomposition of SiCl4 in the presence of trace amounts of water in toluene. The presently reported synthesis approach is promising for exploring other transition‒metal or lanthanide ion doped perovskite QDs in order to gain a wider control over the optical and magnetic properties of doped QDs for applications in the fields of lighting and spintronics.

Methods

Preparation of Cs‒oleate

The synthesis approach has been reported by Kovalenko et al.13. In brief, Cs2CO3 (0.2035 g, Sigma Aldrich, 99.9%), oleic acid (0.625 ml, OA, Sigma Aldrich, 90%), and 1‒octadecene (10 ml, ODE, Sigma Aldrich, 90%) were loaded into a 50 ml 3‒neck flask and degassed under vacuum at 120 °C for 1 h. Subsequently, the solution was heated to 150 °C under N2 for all Cs2CO3 reacting with OA.

Preparation of CsPb1‒xCl3:x%Mn2+ NCs

PbCl2 (0.0497 g, Sigma Aldrich, 99.999%), C36H70MnO4 (x = 5; 0.0058 g, Chemos GmbH), and ODE (5 ml) were loaded into a 25 ml 3‒neck flask and degassed under vacuum at 120 °C for 1 h. Afterwards, dried oleylamine (0.5 ml, OLA, Sigma Aldrich, 70%) and OA (0.5 ml) were injected into the solution at 120 °C under N2 atmosphere. The temperature was raised to 170 °C and Cs‒oleate solution (0.4 ml) was quickly injected into the mixture. After the desired different reaction time, the mixture was cooled by the ice‒water bath. The NCs were extracted from the crude solution through centrifuging at 3500 r.p.m. for 10 min. First, the supernatant was discarded and the precipitated particles were redispersed in toluene forming the stable colloidal solution. Second, the colloidal solution was centrifuged again, and the precipitated particles were discarded and the supernatant was redispersed in toluene forming the final solution.

Addition of silicon tetrachloride (SiCl4)

Various amounts of SiCl4 (Sigma Aldrich, 99%) were introduced into a 20 ml sample bottle containing 2 ml of the colloidal CPC‒Mn QDs toluene solution (H2O content 0.0623%)31. After different stirring time, the white precipitates (SiO2) and the transparent solutions were separated through centrifuging at 10000 r.p.m. for 10 min. In a control experiment, 10 μl SiCl4 was mixed with CPC‒Mn QDs (nearly water free) in a vacuum three‒necked flask.

Preparation of CsPbCl3:Mn2+ bulk material

The CPC‒Mn microcrystalline powder was prepared by melting starting material in an evacuated and sealed quartz ampoule43. High‒purity CsCl (Brunschwig Chemie B.V., 99.9%), PbCl2 (Sigma Aldrich, 99.999%), and MnCl2 (Sigma Aldrich, 99%) were mixed thoroughly and loaded into a quartz ampoule. The ampoule was evacuated and suspended with platinum wire in the high temperature region of a sintering furnace. The temperature was raised to 300 °C with a heating rate of 5 °C/min and kept for 2 h under continuous pumping to remove surface absorbed water. Then, the ampoule was sealed and the sample was sintered at 675 °C for 1 h, after which the sample was allowed to naturally cool to room temperature in the furnace.

Polymerization reaction of CPC:5%Mn‒SiCl4 + PMMA

PMMA (0.8 g) powder was dispersed homogeneously in toluene (2 ml) for stirring 1 day. Then CPC:5%Mn‒SiCl4 QDs (20 mg) were added into the PMMA‒toluene solution and stirred for 1 day. The transparent mixture was dripped into a glass container and a copper hold, as shown in the inset of Fig. 5b. The polymerization took place under a vacuuming procedure for 1 day.

X‒ray diffraction

XRD patterns were recorded using Cu Kα radiation (λ = 1.5418 Å) on a PW 1729 Philips diffractometer, operating at 40 kV and 20 mA. For XRD analysis, the samples were prepared by depositing the QDs colloidal solution on the silicon wafer and evaporating the solvent under vacuum atmosphere.

Optical Characterization

Absorption spectra were obtained using a Perkin‒Elmer Lambda 950 UV/VIS/IR spectrophotometer. Excitation and emission spectra were measured by an Edinburgh Instruments FLS920 spectrofluorometer equipped with a 450 W Xenon lamp. Visible emission (400‒850 nm) was detected by a Hamamatsu R928 photomultiplier tube (PMT).

Lifetime

Photoluminescence (PL) decay curves of the CsPbCl3 QDs were recorded by using an Edinburgh Instruments diode laser with an excitation wavelength of 376.8 nm (65 ps pulse width, 0.2‒20 MHz repetition rate) and a Hamamatsu H74220‒02 PMT in combination with the Edinburgh spectrofluorometer. PL decay measurements of the Mn2+ dopants were done using an optical parametric oscillator (OPO) system (Opotek HE 355 II) pumped by the third harmonic of a Nd:YAG laser as excitation source and a Hamamatsu R928 PMT as light detection. The samples for optical analysis were prepared by dissolving the crude QDs production in toluene in a quartz cuvette.

Electron Microcopy and Energy Dispersive X‒ray Spectroscopy

Scanning transmission electron microscopy (STEM) images and Energy dispersive X‒ray spectroscopy (EDX) measurements were performed using a TalosTM F200X transmission electron microscope (from FEI), equipped with a high‒brightness field emission gun (X‒FEG) and a Super‒X G2 EDX detector operated at 200 kV. TEM images were obtained using FEI TECNAI T12, operating at 120 kV. The samples for TEM imaging were prepared by depositing the QDs colloidal solution on the carbon‒coated copper TEM grid and evaporating the solvent under inert atmosphere.

Additional Information

How to cite this article: Lin, C.C. et al. Luminescent manganese-doped CsPbCl3 perovskite quantum dots. Sci. Rep. 7, 45906; doi: 10.1038/srep45906 (2017).

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Supplementary Material

Supplementary Information:

Acknowledgments

This work was supported by the Ministry of Science and Technology of Taiwan (Contract No. MOST 104–2917–I–564–060). We thank Prof. Alfons van Blaaderen for provide beneficial suggestions. We thank Chenghui Xia for help to measure some PL data. Da Wang acknowledges partial funding from the European Research Council under the European Union’s Seventh Framework Programme (FP–2007–2013)/ERC Advanced Grant Agreement 291667 HierarSACol.

Footnotes

The authors declare no competing financial interests.

Author Contributions C.C.L. conceived the idea of the synthesis method and performed the synthesis, measurement and characterization. K.Y.X. synthesized the CPC‒Mn powder and analysis. D.W. contributed to TEM measurement and analysis. A.M. revised the manuscript and supervised the project. C.C.L. wrote the first version of the manuscript and all authors discussed the results and contributed to the final version of the paper.

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