1. Introduction
Metal halide perovskites have attracted considerable interest for optoelectronic applications [1]-[3]. These materials generally adopt the AMX3 structure, where A is a monovalent cation (e.g., Cs+), M is a divalent metal ion (e.g., Pb2+), and X represents a halide ion (e.g., Cl−, Br−) [4].
Cesium lead bromide (CsPbBr3) has emerged as a highly attractive perovskite for optoelectronic devices due to its direct bandgap, strong PL, and long carrier diffusion lengths -[5]. Its fully inorganic composition offers improved thermal and environmental stability over hybrid perovskites, making it well-suited for applications such as LEDs, photodetectors, and nanophotonic components [4] [6]-[13].
Controlled synthesis of CsPbX3 with tunable shapes has been widely explored through colloidal methods [4]- -[17]. Time-dependent growth strategies have enabled the transition from nanocubes to NWs and nanosheets, with intermediate stages yielding mixed morphologies . Room-temperature reprecipitation techniques have also enabled the formation of green-emitting nanoplatelets and nanosheets with controlled lateral dimensions [15] [16] . Additionally, temperature has been identified as a critical factor influencing anisotropic growth . These studies underscore the critical role of synthesis conditions in controlling the dimensionality and optical response of CsPbBr3 nanostructures.
In this work, we report a solution-phase synthesis of CsPbBr3 NWs and NCs, yielding mixed-dimensional structures and morphology. The synthesized nanostructures exhibit a highly crystalline orthorhombic phase, strong green emission with a narrow PL linewidth, and a ~6 nm Stokes shift, demonstrating their potential for high-efficiency optoelectronic applications.
2. Experimental
2.1. Materials and Methods
Cesium carbonate (Cs2CO3, 99.9%), 1-octadecene (ODE, 90%, technical grade), oleic acid (OA, 90%, technical grade), oleylamine (OAm, 70%), lead (II) bromide (PbBr2, 99.999%), and anhydrous toluene (99%) were purchased from Sigma-Aldrich. Tert-butanol (t-BuOH, 90%) was obtained from Loba Chemie. All chemicals were used as received without further purification.
2.2. Synthesis of CsPbBr3 Nanostructures
CsPbBr3 nanocrystals with green luminescence were synthesized following a hot-injection method reported in previous studies [17] [18]. The precursor amounts, ratios, and purification procedures are described in detail in the cited references (see Figure 1 for details). In short, cesium oleate was prepared by dissolving Cs2CO3 in ODE and OA at 120˚C under vacuum. Separately, PbBr2, OAm, OA, and ODE were heated under similar conditions; then, 0.4 mL of hot Cs-oleate was injected at 140˚C under N2, quenched after 15 seconds, and the product was centrifuged, washed (with Tert-butanol) and redispersed in toluene.
2.3. Characterization Techniques
The surface morphology and crystalline structure were examined using FESEM (JEOL JSM-7610F Plus) and HRTEM (Tecnai G2 20 S-TWIN (FEI, 200 kV)). The crystal structure was determined using a PANalytical X’Pert Pro diffractometer
Figure 1. Four-step synthesis of perovskite nanostructures: from cesium oleate preparation to stable colloidal solution.
with Cu-Kα radiation (λ = 1.5406 Å). Optical absorption spectra were recorded using an Agilent Cary 5000 UV-Vis-NIR spectrophotometer. PL measurements were carried out using a custom-built setup equipped with a 400 nm diode laser.
3. Results and Discussion
3.1. Surface Morphology Analysis
FESEM was employed to investigate the surface morphology of the synthesized CsPbBr3 nanostructures presented in Figure 2. Figure 2(a) presents a FESEM image showing a mixture of NWs and NCs, including micrometer-sized cubes and elongated structures. Figure 2(b) provides a high-resolution image of the NWs, NCs, and micron-sized rods and cubic crystals. Figure 2(c) displays NWs extending up to 15 μm in length with diameters of approximately 200 nm. Figure 2(d) shows a closer view of an individual NW, highlighting its uniform morphology and high aspect ratio. These observations confirm the successful synthesis of mixed-dimensional CsPbBr3 nanostructures with well-defined morphology. FESEM images reveal a mixture of NWs and NCs, with cubic particles and elongated nanowires observed across the sample (Figures 2(a)-(d)). NWs with diameters < 200 nm and L ~15 μm were confirmed by high-magnification imaging.
The formation of mixed-dimensional CsPbBr3 nanostructures, such as nanowires (NWs) and nanocubes (NCs), arises from the interplay between precursor availability, ligand coordination, and reaction kinetics during hot-injection synthesis [7] [18]-[20]. A moderate volume of octadecene (~10 mL) maintains optimal monomer concentration, enabling controlled nucleation [7] [19]. Oleylamine (OAm) and oleic acid (OA) act as dynamic ligands, selectively passivating crystal facets and directing anisotropic growth . At a reaction temperature of 140˚C, anisotropic growth is favored due to sufficient thermal energy overcoming nucleation barriers, consistent with the kinetic model described by Zhang et al. (2020) . Rapid quenching of the reaction in an ice bath effectively arrests further growth, preserving the elongated morphology of NWs . This approach yields a heterogeneous mixture of NWs (~15 μm) and NCs, in agreement with earlier reports [7] [19].
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Figure 2. (a) Low-magnification FESEM image of CsPbBr3 NWs and NCs, displaying a mixture of morphologies including microcrystals and nanorod structures; (b) higher magnification image revealing the coexistence of NWs, NCs, and micron-sized rods; (c) NWs with lengths up to ~15 μm and diameters around 200 nm; (d) close-view of a single NW.
3.2. Microstructural and Crystallographic Analysis
HRTEM was employed to further investigate the structural and crystallographic features of the CsPbBr3 NWs and NCs. Figure 3(a) shows a bundle of NWs with diameters below 200 nm and lengths of approximately 1 μm. Figure 3(b) presents the surface morphology of an individual NW. The high-resolution image in Figure 3(c) displays lattice fringes of average interplanar d-spacing ~0.30 nm of the CsPbBr3 NWs. This value is consistent with the d-spacing corresponding to the XRD peak at 2θ ≈ 29.02˚ (see Figure 4). The SAED pattern in Figure 3(d) displays discrete diffraction spots, confirming the single-crystalline nature of the NWs. SAED patterns (Figure 3(d)).
3.3. Crystal Structure (XRD) Analysis
The XRD pattern (Figure 4) exhibits prominent diffraction peaks at 2θ ~14.64˚, 14.79˚, 29.97˚, 30.28˚, 41.74˚, and 62.84˚ and confirms that the synthesized CsPbBr3 material exhibits a highly crystalline structure.
Figure 3. (a) HRTEM image showing a bundle of CsPbBr3 NWs with diameters < 200 nm and lengths ~1 μm; (b) surface morphology of a single NW; (c) high-resolution image of lattice fringes; (d) SAED pattern displaying distinct diffraction spots, confirming the single-crystalline nature of the NW.
These peaks are consistent with the reference orthorhombic phase from ICSD #97851 [12] [21]. The observed peak splitting at ~14.6˚ and ~30.0˚ is characteristic of the orthorhombic phase and confirms deviation from the higher-symmetry cubic structure. The sharpness and intensity of these peaks indicate good crystallinity, with estimated crystallite sizes in the range of 50 - 100 nm based on the Scherrer equation [22].
It is important to note that the Scherrer method reflects the size of coherently diffracting crystalline domains, which can be considerably smaller than the overall physical dimensions of the nanostructures observed via FESEM, such as the nanowires up to 15 μm in length [23].
The identification of the orthorhombic phase holds considerable importance for optoelectronic applications. Unlike the more symmetric cubic phase, the orthorhombic structure offers greater thermodynamic stability at ambient conditions and is often associated with a marginally larger band gap and stronger exciton binding. These characteristics can enhance the photostability and improve charge carrier behavior in device performance [24]-[26].
3.4. Optical Properties
Figure 5 shows the UV-Vis absorption and PL spectra for the NWs and NCs. The inset displays the sample under ambient light, appearing yellow, and under
Figure 4. XRD pattern of (b) CsPbBr3 showing sharp peaks consistent with the orthorhombic phase (ICSD #97851). Side figures (a) and (c) highlight the characteristic peak splitting near 2θ ~14.6˚ and ~30.0˚, respectively.
400 nm laser excitation, exhibiting bright green luminescence, indicative of efficient radiative recombination [6]. The UV-Vis spectrum reveals two absorption features with onsets near 396 nm and 520 nm, consistent with previously reported transitions in anisotropic and confined CsPbBr3 nanostructures [18]. PL measurements reveal a strong peak at ~526 nm with a FWHM of ~23 nm. The ~6 nm Stokes shift between the absorption edge and emission peak indicates efficient excitonic emission and trap-assisted recombination and confirms the presence, typical of high-quality perovskite nanostructures [27] [28]. Photographs under 400 nm excitation show strong green luminescence, confirming efficient radiative recombination, in line with the observed optical data. These results demonstrate that the CsPbBr3 nanostructures possess excellent light-emitting properties.
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Figure 5. UV-Vis absorption and PL spectra of CsPbBr3 NWs and NCs and an image of the sample (ambient light (yellow) and under 400 nm excitation (green luminescence)). The PL peak (526 nm, FWHM ~23 nm) indicates strong emission and high optical quality.
4. Conclusion
Morphology-controlled CsPbBr3 NWs and NCs were synthesized and characterized, exhibiting phase-pure orthorhombic structure and excellent luminescent properties. HRTEM and XRD analyses confirmed high crystallinity and structural integrity, while optical characterization revealed strong green emission with minimal Stokes shift. These findings demonstrate the potential of CsPbBr3 nanostructures for optoelectronic devices.
Acknowledgements
The authors gratefully acknowledge the Head, Department of Physics, University of Rajasthan, for providing access to the UV-Vis spectroscopy facility.