Hybridization of Single Nanocrystals of Cs4PbBr6 and CsPbBr3.

Nanocrystals of all-inorganic cesium lead halide perovskites (CsPbX3, X = Cl, Br, I) feature high absorption and efficient narrow-band emission which renders them promising for future generation of photovoltaic and optoelectronic devices. Colloidal ensembles of these nanocrystals can be conveniently prepared by chemical synthesis. However, in the case of CsPbBr3, its synthesis can also yield nanocrystals of Cs4PbBr6 and the properties of the two are easily confused. Here, we investigate in detail the optical characteristics of simultaneously synthesized green-emitting CsPbBr3 and insulating Cs4PbBr6 nanocrystals. We demonstrate that, in this case, the two materials inevitably hybridize, forming nanoparticles with a spherical shape. The actual amount of these Cs4PbBr6 nanocrystals and nanohybrids increases for synthesis at lower temperatures, i.e., the condition typically used for the development of perovskite CsPbBr3 nanocrystals with smaller sizes. We use state-of-the-art electron energy loss spectroscopy to characterize nanoparticles at the single object level. This method allows distinguishing between optical characteristics of a pure Cs4PbBr6 and CsPbBr3 nanocrystal and their nanohybrid. In this way, we resolve some of the recent misconceptions concerning possible visible absorption and emission of Cs4PbBr6. Our method provides detailed structural characterization, and combined with modeling, we conclusively identify the nanospheres as CsPbBr3/Cs4PbBr6 hybrids. We show that the two phases are independent of each other's presence and merge symbiotically. Herein, the optical characteristics of the parent materials are preserved, allowing for an increased absorption in the UV due to Cs4PbBr6, accompanied by the distinctive efficient green emission resulting from CsPbBr3.

Introduction

Semiconductor nanocrystals (NCs) are widely investigated due to their size-dependent energy structure.[1−3] As the NC size approaches the Bohr radius, the quantum confinement (QC) sets in modifying the wave function of the free electron and hole,[4−8] resulting in an increase of the band gap energy. On the other hand, perovskites (ABX3) are emerging as promising materials for (low-cost) solar cells.[9−17] More recently, all-inorganic cesium-lead perovskite (CsPbX3, X = Cl, Br, and I) NCs[18] have attracted a lot of attention as they combine the advantageous properties of perovskites and NCs. They exhibit high photoluminescence quantum yields (PL QYs, 50–90%) and narrow emission bands, and their emission color can be easily tuned over the whole visible range by changing the composition and NC size.[19−22] For CsPbBr3, next to the cubic-shaped NCs, also the formation of spherical NCs,[23] nanowires (NWs),[24−26] nanorods,[27] and nanoplatelets[28,29] has been reported by varying the reaction temperature and/or the ligands used in the synthesis. In fact, we have observed the formation of CsPbBr3 nanowires next to the nanocubes before.[30] CsPbX3 is known to crystallize in orthorhombic, tetragonal, and cubic symmetries where the cubic structure generally appears at high temperatures.[31−34] For instance, CsPbBr3 undergoes a phase transition at around 123 °C showing a cubic phase, whereas, at lower temperatures, its phase should be monoclinic.[31,35] It is also generally known that preparing pure CsPbBr3, in bulk, thin films, single crystals, or nanostructures, is challenging. The phase diagram of the stable CsX-PbX2 (X = Br, Cl) system shows the existence of two complex ternary compounds, the congruent melting (i.e., maintaining the same atomic composition) compound CsPbX3 and incongruent Cs4PbX6.[36,37] When preparing the CsPbBr3 (from PbBr2), the Cs4PbBr6 compound is expected to form as well if there is an excess of Cs, resulting in the CsBr:PbBr2 ratio being no longer exactly 1:1. Reversely, when growing Cs4PbBr6 crystals, it is difficult to prevent the formation of CsPbBr3 due to an unavoidable incongruent melting crystal growth process.[38,39] In this work, we study the coexistence of CsPbBr3 and Cs4PbBr6 NCs and their hybridized form, and characterize them structurally and optically. We make use of low-loss electron energy loss spectroscopy (EELS) in a state-of-the-art low-voltage monochromatic scanning transmission electron microscope (STEM) with a spatial resolution below 1.6 Å. This provides the unique insight in the absorption features and energy structure of a single nanoparticle, which is investigated in parallel with its structural parameters. In spite of recent contradicting reports on the emissive properties of Cs4PbBr6,[38,40−43] our single NC EELS measurements exclusively demonstrate that there can be no visible emission from pure Cs4PbBr6 NCs, which are insulating and have a large band gap energy (∼4 eV);[38,39,44] therefore, it is the CsPbBr3-phase within hybridized Cs4PbBr6/CsPbBr3 spherical NCs that is responsible for the green emission.

Methods

EELS in STEM

A droplet of the as-synthesized NCs solution is drop-casted onto an Au quantifoil TEM grid with the coverage of a monolayer of graphene. The TEM grid is then dried in a vacuum chamber for further characterizations. STEM imaging and EELS experiments were performed on a JEOL ARM200 microscope equipped with a Schottky field emission gun, a JEOL double Wien filter monochromator, probe delta corrector, and a Gatan Quantum GIF spectrometer modified for low primary energy operation (15–60 keV) with high stability. Imaging and EELS collections are performed in high-energy resolution mode, where a slit of 0.5 μm is used for energy selection. In this condition, the electron probe has a zero loss peak with energy fwhm around 50 meV, a current of ∼8 pA, and a convergence semi-angle of 33 mrad. The EELS collection semi-angle was 11 mrad. The energy dispersion of the spectrometer was set to 10 meV/channel. A Gatan cryo-holder is used to cool down the sample at −110 °C with liquid nitrogen to reduce carbon redeposition and thermal excitation during all the imaging and EELS collection experiments. Dual EELS acquisition was applied where the unsaturated zero loss peak is used to align the simultaneously acquired high-loss spectrum (which has a much longer exposure time than the zero loss peak) to correct any energy drift during the acquisition. The probe size for the valence-loss EELS measurements was about 1.6 Å, providing an ultrahigh spatial resolution during the collection of the EEL spectra.[45,46] All data are raw, without filtering.

Optical Characterization

The optical density was measured in a LAMBDA 950 UV/Vis/NIR spectrophotometer (PerkinElmer). A combination of a tungsten-halogen and deuterium lamp is used together with a PMT and Peltier cooled PbS detector, to provide a detection range of Edet = 5.6–0.4 eV. The sample and solvent (toluene) measurements have been performed separately and afterward subtracted from each other to correct for the solvent absorption. The PL spectrum is recorded by a Jobin Yvon FluoroLog spectrofluorometer (Horiba) equipped with a 450 W xenon lamp (250–700 nm) coupled to a monochromator to provide a selective range of excitation wavelengths. The emission is collected in a right-angle geometry and corrected for spectral sensitivity.

X-ray Diffraction (XRD) Measurements

The powder XRD measurement was carried out on a benchtop MiniFlex II X-ray Diffractometer (Rigaku), from 5° to 50° with a turning speed of 2.5°/min.

Results

Colloidal NCs have been produced using the method described by Protesescu et al.[18] The synthesis was carried out at two different temperatures, 100 and 160 °C. It is well-known that the synthesis temperature plays an important role in determining the shape and size of CsPbX3 NCs.[28] For the synthesis at 160 °C, we observe cubic CsPbBr3 NCs with an average size of 8–10 nm, and also observed CsPbBr3 NWs and large nanoplatelets with high aspect ratio (see Figures S1 and S2 in the Supporting Information (SI)). We measure a PL QY of ∼80% for excitation at Eexc = 3 eV, indicating that the emission efficiency is not greatly affected. Additionally, larger nanoparticles (NPs, 10–20 nm) with a spherical shape and that appear hexagonal are formed next to the nanocubes (Figure a) in the same sample. Cs4PbX6 has at room temperature a trigonal crystal structure.[36,47] The Cs+ chains in Cs4PbX6 crystals, projected along the [0 0 1] plane, arrange in a hexagonal orientation.[47−49] Therefore, in (high-resolution) transmission electron microscopy (HR-TEM) images, the Cs4PbBr6 NCs will appear as hexagons (as of now referred to as nanohexagons), whereas the nanocubes are CsPbBr3 NCs. Lowering the synthesis temperature to 100 °C leads to the predominant formation of nanohexagons, while the nanocubes are the minority (Figure b, and see also Figure S3 in the SI). This indicates that the production of mostly either nanocubes or nanohexagons is synthesis temperature-controlled and is in this case not governed by the amount of reactants (all other conditions are kept equal). It also demonstrates that the chemical reaction (ratio of the relative amount of desired and undesired product)[50] is not 100% to the desired product—CsPbBr3 NCs. The coformation of different shaped CsPbBr3 NPs and Cs4PbBr6 NCs has been observed for any sample produced in the temperature range between 100 and 200 °C.

Figure 1

Cs4PbBr6 and CsPbBr NCs and their chemical and structural characterization. (a, b) ADF images of the samples prepared at 160 and 100 °C, respectively, where, for the latter, the nanohexagons dominate, where this is reversed for the higher synthesis temperature. (c) EDS spectra of the samples as indicated in (a) and (b). Mainly Cs, Pb, and Br are detected, where the Au signal comes from the reflection of the TEM grid. The ratio between Cs and Pb is found to be 1.2:1 and 4:1 for the samples synthesized at 160 and 100 °C, respectively (as determined from integrating the signal at the associated energy region). (d) X-ray diffraction pattern of the sample prepared at 100 °C, drop-casted on a glass substrate. The peaks related to the Cs4PbBr6 and CsPbBr3 structures are denoted as “a” and “b”, respectively. The broad peak around 20° is due to background noise, which is only amplified as the sample layer becomes thinner, as represented by the exemplary spectrum shown on top (red curve). (e) Atomic resolution STEM image of a Cs4PbBr6 NC. (f, g) Diffraction patterns of (e) obtained by taking a fast Fourier transform of the shown region. Two diffraction spots in this zone axis are identified to be the (−4 1 1) and (−2 1 0) planes, with an intersection angle of 19.5°, which corresponds to a lattice distance of 3.33 and 7.06 Å, respectively. (h) Modeled atomic arrangement using the experimentally determined zone axis, which agrees with the Cs4PbBr6 structure.

To confirm the elemental composition of the nanostructures in both samples, energy-dispersive X-ray spectroscopy (EDS) is performed. Figure c shows the corresponding EDS spectra, where mostly Cs, Pb, and Br are detected. The Au peak originates from the TEM grid. The ratio between Cs and Pb is determined at 4:1 for the 100 °C sample, corresponding to Cs4PbBr6. For the 160 °C sample, this is 1.2:1 (where one would expect 1:1), indicating the presence of excess Cs, which is due to Cs4PbBr6 NCs that were formed instead of CsPbBr3 NCs. The coexistence of both compounds has been reported before, in bulk and thin films as well as in nanometer-sized aggregates dispersed in a crystal lattice.[26,38,51] To confirm that they coexist in NC form in the sample synthesized at 100 °C, X-ray diffraction (XRD) was performed. The corresponding XRD pattern is shown in Figure d, with the peaks associated with Cs4PbBr6 and CsPbBr3 denoted by “a” and “b”, respectively, indicating the presence of both phases. An additional broad peak is observed at 20° which arises due to the background noise appearing for a thin sample layer. This is demonstrated by the top red curve, which corresponds to the same measurement performed on a thinner sample. A detailed structural analysis is performed on a Cs4PbBr6 NC, to identify its crystal symmetry. By taking a fast Fourier transform (FFT) of a selected region in an atomic resolution image, the lattice periodicity, i.e., the diffractogram, of a crystalline structure is revealed. Figure e shows an atomic resolution STEM image of a single Cs4PbBr6 NC. The lattice periodicity can be clearly seen. Although the individual atoms are not resolved due to the slight deviation from its zone axis, the lattice information can still be identified from the diffraction pattern (Figure f,g). Two diffraction spots are indicated which are identified as the (−4 1 1) and (−2 1 0) planes with an intersection angle of 19.5°, corresponding to a lattice distance of 3.33 and 7.06 Å, respectively. Using the experimentally obtained zone axis, the modeled atomic arrangement (Figure h) is shown to agree well with the experimental data: the “rows” of Cs atoms can be clearly seen in Figure a, identifying it as a pure Cs4PbBr6 NC (see also Figure S4).

To best distinguish between the Cs4PbBr6 and CsPbBr3 phases and their different electronic structure, single NCs of both compounds are investigated by low-loss EELS. The NPs are exposed to an electron beam, in which the electrons have a certain range of kinetic energies. The electrons (can) undergo inelastic scattering, and the loss of electrons is determined. In a low-loss EEL spectrum, a characteristic onset will appear if an electron in an NC is excited from the top of the valence band to the conduction band, followed by a steady increase of the signal. This directly corresponds to band-to-band absorption which starts at the band gap energy and whose magnitude grows as the density of states increases with energy.[52] Therefore, this can be regarded as an analogy to absorption spectroscopy. Figure a shows the EEL spectra for the probed NPs (solid lines) with the first derivative shown underneath (dotted lines) where its maximum determines the band gap energy (see the SI for further details). We find a value of 2.45 eV for the CsPbBr3 nanocube (red). In contrast, for the Cs4PbBr6 NC (blue spectrum), no characteristic step is revealed since Cs4PbBr6 is an insulator with a large and indirect band gap, hence the lack of an excitonic feature (see also Figure S5). Two distinctive absorption peaks at 4 and 5.5 eV are observed which are absent for the nanocube. More interestingly is the nanosphere, for which its EEL spectrum shows characteristics of both phases: an excitonic line as well as the two absorption bands in the UV (Figure b, green spectrum). Upon analysis of its crystal symmetry and structure, we confirm the nanospheres are formed by a hybridization of the Cs4PbBr6 and CsPbBr3 phase. This is further demonstrated by combining the experimentally obtained EEL spectra of the nanocube and the nanohexagon, hence obtaining a simulated spectrum (orange spectrum in Figure b) which equals the experimentally measured one. The preservation of the onset in the band structure and the characteristic absorption peaks in the UV indicate there is a phase segregation inside a single NC; i.e., the two phases are independent of each other’s presence but co-grow to form a single NC. It has been shown that Cs4PbBr6 NCs can be converted into cubic CsPbBr3 NCs by their reaction with excess PbBr2.[44] This indicates that the spherical NCs represent indeed an intermediate state between the Cs4PbBr6 and CsPbBr3 phases.

Figure 2

Energy band gap and structure. (a) Valence-loss EEL spectra representing the absorption of a CsPbBr3 nanocube (red spectrum) and a Cs4PbBr6 NC (blue), appearing in the same sample (160 °C). The band gap energy of the nanocube (2.45 eV) is determined from the peak of the first derivative of the EEL spectrum, which appears due to the abrupt onset in absorption (indicated by the dotted lines). No onset is observed for the Cs4PbBr6 NC which is an insulator with a large band gap energy of 4 eV. (b) Valence-loss EEL spectrum of a spherical nanocrystal (green) observed in the same sample, formed upon the hybridization of a nanocube and a nanohexagon. The latter process is represented by the simulated EEL spectrum (orange) which is obtained from adding the experimentally obtained spectra of the nanohexagon and nanocube as shown in (a). The hybrid has, as expected, a band gap energy similar to that of the nanocube.

The identified band structures of the investigated single NPs are compared with the ensemble optical characteristics. Figure shows the PL (solid lines) and normalized absorption (dotted lines) spectra of the samples synthesized at 100 °C (top) and 160 °C (bottom), excited at Eexc = 2.88 eV. The narrow PL bands centered around 2.45 and 2.53 eV for the two investigated (as-prepared) samples agree with CsPbBr3 NC emission, consistent with smaller and larger NC sizes for lower and higher synthesis temperatures, respectively.[18] An additional PL peak at 2.4 eV is observed for the sample synthesized at 100 °C, when diluted. This arises from disintegration and clustering of the NCs as ligands are removed upon dilution due to their dynamic bonding on the NCs surface,[53] which causes the PL to red shift toward the CsPbBr3 bulk value (2.33 eV). We do not observe emission from the Cs4PbBr6 NCs in the range from 3 to 4 eV which was recorded for Eexc = 4.13 eV (above the absorption band of Cs4PbBr6). In fact, we observe no significant change in the PL spectra when excited at 2.88 and 4.13 eV (see Figure S6) as they are almost identical. Therefore, the visible emission at 2.4–2.6 eV originates solely from the CsPbBr3 phase. We do observe a small red shift with decreasing excitation energy, which is explained by reabsorption and/or a residual energy transfer upon selective excitation of the NC ensemble, as a transfer always proceeds from small to large NCs.[54]

Figure 3

Photoluminescence and absorption spectra of the ensembles. PL (solid lines) and absorption (dotted lines) spectra of the samples synthesized at 100 °C (top panel) and 160 °C (bottom panel) with different concentrations, excited at Eexc = 2.88 eV. The PL peaks at 2.45 and 2.53 eV correspond to CsPbBr3 NC emission, for small and large NC sizes, respectively, which agree with the high and lower synthesis temperature. For the (diluted) samples synthesized at 100 °C, an additional PL peak around 2.4 eV is observed, which appears due to disintegration and clustering of the CsPbBr3 NCs due to removal of the ligands upon dilution, causing the PL to red shift toward the CsPbBr3 bulk value. For the 100 °C sample, a clear absorption peak is observed around 3.8 eV, which agrees with values reported for Cs4PbBr6.[36,38,39,55] No emission from the Cs4PbBr6 NCs is observed in the UV around 3–4 eV (for Eexc = 4.13 eV).

All the PL bands show, in the range between 2.4 and 2.5 eV, a small Stokes shift from their corresponding absorption spectra that exhibit (a) clear excitonic peak(s) at the respective onset(s), as is typically observed for CsPbBr3 NCs. In addition, a distinct absorption peak at ∼3.8 eV is observed for the sample containing mostly Cs4PbBr6 NCs, in agreement with our result from EELS and with what is reported for Cs4PbBr6 NCs, large single crystals, and thin films.[38,39,44,55]

Discussion

Since the spherical NCs appear in our STEM measurements often slightly cubic and/or as hexagons with rounded corners, their optical ensemble characteristics could be erroneously attributed to (pure) CsPbBr3 or Cs4PbBr6 NCs. Recent reports on the synthesis of single crystals assign the ∼520 nm (2.38 eV) and the ∼550 nm (2.25 eV) emission bands to Cs4PbBr6 and CsPbBr3, respectively,[40−42] although it is also reported that confined CsPbBr3 quantum dots could contribute.[40,43] On the other hand, Nikl et al.[38] also studied the emission characteristics of Cs4PbBr6 crystals and thin films and show absorption features at 520 nm and the same green emission around 545 nm, ascribing it to the CsPbBr3-like phase being present in the bulk Cs4PbBr6 sample. The optical transitions of both CsPbX3 and Cs4PbX6 crystals are explained as being related to transitions between states of the Pb2+ ion (which form regular octahedra that define the perovskite structure). Cs4PbBr6 itself is said to have absorption peaks at 220 nm (5.6 eV) and 310 nm (4 eV). These observations, specifically with respect to the green emission, are to some extent in contrast with each and clarification is needed. Preparing pure Cs4PbBr6 or CsPbBr3 NCs in ensembles proves challenging as the coformation of the two phases (i) requires extra care during synthesis, and (ii) can easily escape detection due to the large band gap of Cs4PbBr6. Hence, ensemble characterization cannot provide the ability to fully distinguish between their optical and at the same time structural properties. This, on the other hand, is fully accomplished by single particle EELS. We observe that the energy structure of a single pure Cs4PbBr6 NC has a large band gap featuring no absorption in the visible/near UV (∼2–3.5 eV). This excludes the possibility of emission in that energy range, conclusively resolving the current dispute.[38,40−44] Distinct absorption bands are observed at 4 and 5.5 eV—in full agreement with the work of Nikl et al. It is the CsPbBr3-phase within the hybridized Cs4PbBr6/CsPbBr3 spherical NCs that is responsible for the visible PL as an excitonic feature is clearly identified in its EEL spectrum. Furthermore, even if the ensemble is excited below the 4 eV absorption peak of Cs4PbBr6, the green emission is still observed.

Conclusion

In conclusion, we have analyzed single pure Cs4PbBr6 NCs and demonstrated that they only absorb in the UV, manifesting themselves as two distinct peaks at 4 and 5.5 eV. We have identified and characterized CsPbBr3/Cs4PbBr6 hybridized spherical NCs which preserve the optical characteristics of both parent materials: they show a strong green emission at 2.45 eV and the absorption bands in the UV, resulting from the CsPbBr3 and Cs4PbBr6 phases, respectively. In particular, our method demonstrates the advantage of particle-by-particle EELS analysis over macroscopic optical ensemble measurements, which is critical if a specimen is a composition of different NPs. The nanospheres together with CsPbBr3 nanocubes and Cs4PbBr6 nanohexagons are appearing during the commonly used colloidal synthesis of all-inorganic perovskite NCs. Lowering the synthesis temperature steers the reaction toward the formation of mostly Cs4PbBr6 NCs.