CsPbBr₃ Microplatelets: Synthesis & Applications

• CsPbBr₃ microplatelets are planar all‐inorganic perovskite particles exhibiting tunable optoelectronic, excitonic, and spin properties influenced by synthesis methods and strain engineering.
• Their fabrication via techniques like LARP, hot‐injection, and scaffold-directed growth enables precise control over crystal phase, defect density, and quantum confinement.
• Advanced surface passivation and integration strategies yield high photoluminescence quantum yields, robust spin coherence, and promise for applications in lasers, sensors, and scintillators.

CsPbBr$_3$ microplatelets are planar, monocrystalline or polycrystalline particles of cesium lead bromide with lateral dimensions ranging from hundreds of nanometers to several micrometers and thicknesses down to a few nanometers. As a prototypical all-inorganic metal halide perovskite, CsPbBr$_3$ has garnered significant attention as a model system for elucidating electronic structure, defect physics, exciton dynamics, and for deploying in high-performance optoelectronic and spintronic devices. The microplatelet geometry leads to unique photophysical, spin, and electronic phenomena driven by electronic dimensionality, quantum and dielectric confinement, strain engineering, surface physics, and strong many-body interactions. This entry synthesizes state-of-the-art understanding of CsPbBr$_3$ microplatelets in the context of synthesis, structure, electronic and optical properties, spin phenomena, defect and surface control, device integration, and future research directions.

1. Synthesis and Structural Engineering

CsPbBr$_3$ microplatelets can be fabricated by diverse methods including solution-processed growth, ligand-assisted reprecipitation (LARP), hot-injection, oriented self-assembly, and in situ conversion of precursor compounds in confined geometries. Parameters such as ligand chemistry, precursor concentration, growth temperature, and the presence of templating scaffolds (e.g. mesoporous SiO$_2$) strongly influence microplatelet size, crystal phase, defect density, and faceting (Dang et al., 2020, Romero-Perez et al., 13 Mar 2024, Cova et al., 19 Jun 2024). For instance, layer-by-layer oriented attachment promoted by ligand desorption leads to seamless fusion of nanoplatelets into microplatelets, though increased assembly time and elevated temperature favor the formation of Ruddlesden–Popper planar faults and mosaic domains mediated by CsBr bilayers at interfaces (Dang et al., 2020).

Key synthetic considerations:

Growth Method	Phase Control	Defect Density	Additional Remarks
LARP	Yes	Tunable	High scalability, rapid nucleation
Mosaic Self-Assembly	Yes	Modulated	Domain boundaries host R-P faults
Scaffold-Directed	Yes	Low–Moderate	Enables strong quantum confinement
Hot-injection	Yes	Low	Relies on precise temperature control

The control of microstrain and crystallographic orientation is crucial: nanoscale compressive strain acquired during film growth (e.g., substrate-induced) leads to bandgap narrowing and deteriorated stability, thus uniform strain minimization is essential for robust microplatelet optoelectronics (Li et al., 2018). Mosaicity—small-angle misorientation between crystalline domains—may also influence photophysical response and robustness.

2. Electronic, Optical, and Excitonic Properties

CsPbBr$_3$ microplatelets exhibit a direct bandgap in the green spectral region (typically 2.2–2.3 eV), with the precise value and optical absorption strength dependent on phase (cubic, orthorhombic), strain, and surface quality (Joshi et al., 2021). Density-functional theory with the modified Becke-Johnson potential yields a cubic-phase gap of 2.25 eV, closely matching experiment (Joshi et al., 2021).

Excitonic states exhibit large binding energies ($\sim$32.5 meV) with a Bohr radius of ~2.5 nm, and are well-described by a hydrogenic model with an effective dielectric constant $\varepsilon_\mathrm{eff}\approx 8.7$ (Yakovlev et al., 2023). Exciton-polaritonic effects—large longitudinal-transverse (LT) splitting ($\hbar\omega_{LT}\approx5.3$ meV)—arise from strong oscillator strength, supporting potential for polariton lasing and microcavity integration (Yakovlev et al., 2023). The reduced exciton mass, extracted experimentally, is $\mu=0.18m_0$, while the Landé $g$-factor for the exciton is $g_X=+2.35$.

Temperature- and pressure-dependent studies reveal a linear increase of the bandgap with temperature (slope $\sim$2.24%%%%13$g$14%%%% eV/K), governed by normal phonon coupling and volumetric thermal expansion with no evidence for anomalous electron–phonon contributions (Fasahat et al., 10 Sep 2024). The optical absorption of both bulk CsPbBr$_3$ and (001) surface microplatelets surpasses that of non-lead halide and hybrid alternatives, making these structures favorable for light-harvesting, emission, and guiding applications (Joshi et al., 2021).

Photoluminescence emission characteristics in microplatelets (or strongly confined analogs in heterostructures) can be engineered across the visible spectrum by controlling lateral size (quantum confinement) and surface chemistry: intense blue emission ($\sim$470–480 nm) is achieved with sub-3 nm crystals embedded in a Cs$_4$PbBr$_6$ matrix, as modeled by the Brus equation (Romero-Perez et al., 13 Mar 2024). In typical micrometer-scale platelets, green emission near 530 nm dominates, but bandgap and emission energy may show strain-driven redshifts (Li et al., 2018).

3. Defect Physics, Surface Engineering, and Passivation

Microplatelets are characterized by a lower surface-to-volume ratio compared to nanocrystals, yet their extended two-dimensional surfaces are vulnerable to bromine vacancies, surface traps, and structural reconstruction at the edges. Defect-tolerant behavior arises from strong polaronic screening: ARPES studies find that holes acquire an effective mass $\sim$50% heavier than bare DFT predictions ($m_h\sim0.26m_e$, $m_0=0.17m_e$), consistent with large polaron formation (Fröhlich coupling parameter $\alpha=1.82$) (Puppin et al., 2019).

Bromine vacancies, Pb dangling bonds, and interfacial disorder drive nonradiative recombination if not carefully managed. Several strategies have proven effective:

• Post-synthetic chemical passivation: GdBr$_3$-DMF treatment creates a tightly bound Gd$^{3+}$ solvation shell, freeing Br$^-$ ions that rapidly passivate surface vacancies. This increases the photoluminescence quantum yield (PL QY) from 35% to nearly 100% in solution, with 80% in films, for blue-emitting platelet derivatives (Wang et al., 2023).
• Sr$^{2+}$ doping: Partial substitution on the Pb$^{2+}$ site suppresses defect density and microstrain (confirmed by Williamson-Hall analysis), raising PL QY of green platelets from 84.7% (pristine) to 92.6% at 2% Sr$^{2+}$ doping, and maximizes luminous efficacy in WLEDs (Yuce et al., 2022).
• Graphene interfacing: Monolayer graphene placed atop perovskite microplatelets creates an interfacial electrostatic barrier (reducing carrier leakage), n-dopes the graphene to activate plasmon modes, and hybridizes with trap states to passivate defects. The plasmon–exciton resonance, with momentum transfer provided by interface corrugations (periodicity $\sim$1.8 nm), yields up to three orders-of-magnitude PL enhancement (Park et al., 23 Aug 2024).
• Polymer encapsulation: Poly(methyl methacrylate) (PMMA) composites provide dual effects: partial luminescence degradation of already high-quality platelets but defect passivation (notably on electron-poor Pb sites) in defect-rich systems, resulting in substantial increases in radioluminescence and reducing slow emission tails after irradiation (Cova et al., 19 Jun 2024).

Surface passivation and morphology control are essential in suppressing nonradiative traps, stabilizing emission, and enabling radiation hardness or operational robustness under environmental exposure.

4. Exciton, Spin, and Polaron Dynamics

Excitonic and polaronic phenomena underpin the optoelectronic function in CsPbBr$_3$ microplatelets. Large polarons formed by strong electron–phonon coupling lead to mass enhancement and transport properties marked by high mobility and suppressed nonradiative recombination (Puppin et al., 2019). Efficient anti-Stokes photoluminescence (ASPL)—up-conversion from sub-gap excitation via resonant absorption of multiple phonons—is explained by polaron-assisted Kramers–Heisenberg dynamics, yielding up-conversion efficiencies near unity (Zhang et al., 2023).

Spin physics is dominated by valence-band hole spin coherence and its strong hyperfine coupling to $^{207}$Pb nuclear spins. Time-resolved Faraday rotation in microplatelets reveals hole Larmor precession with a $g$-factor varying from $0.85$ (large, weakly confined platelets) to $1.5$ (small, highly confined platelets) (Meliakov et al., 2 Sep 2024). The precession frequency is $\omega_L = |g| \mu_B B/\hbar$. Damping times decrease from $\sim$1 ns at 5 K to $\sim$50 ps at 300 K, set by dephasing from hyperfine field fluctuations ($\Delta_E=2$–$5\,\mu$eV). The Merkulov–Efros–Rosen model fits both zero-field coherent oscillations and field-dependent dephasing, indicating robust spin phenomena even at room temperature and highlighting prospects for spintronic or quantum photonics with microplatelets.

Spin Parameter	Value (range)	Dependence
Hole $g$-factor	0.8–1.5	Increases with confinement/energy
Dephasing time $T_2^*$	1 ns – 50 ps	Decreases with temperature
Hyperfine energy $\Delta_E$	2–5 μeV	Dominant for hole spins

5. Photoluminescence, Recombination, and Temperature Effects

Photoluminescence properties of microplatelets are determined by quantum confinement, surface trap density, and carrier–phonon interactions. In small-diameter platelets or embedded nanocrystals, blue-shifted emission and narrower linewidths arise from increased confinement (as described by the Brus equation) (Romero-Perez et al., 13 Mar 2024), but as the lateral size increases and the surface-to-volume ratio drops, the influence of surface states diminishes, leading to higher PLQY, less pronounced thermal quenching, and reduced multi-exponential recombination (Kulebyakina et al., 2023).

Temperature-dependent PL studies show that the bandgap increases linearly with temperature; emission intensity declines primarily due to deep surface traps becoming thermally accessible. The photoluminescence quenching and multi-timescale decay observed in nanocrystals give way to more radiative, monoexponential decay behavior in microplatelets (Kulebyakina et al., 2023, Fasahat et al., 10 Sep 2024). The interplay of radiative and nonradiative rates and phonon coupling can be described by:

• PL linewidth broadening: $$\Gamma(T) = \Gamma_{\text{inh}} + (\gamma_{\text{ph}})/(e^{E_a/(k_BT)} - 1)$$,
• Trap-limited integrated intensity: $$I(T) \propto \exp\{-\frac{\pi D^2\sigma}{2}[1+\operatorname{erf}(\frac{k_BT\ln(\gamma_{\text{tr}}/\gamma_r) - \bar{E}_t}{\sqrt{2}\Gamma_t})]\}$$,

where $D$ is platelet diameter, $\sigma$ trap density, $\gamma_{tr}$ trap capture rate, and $E_t$ trap energy distribution parameters.

6. Device Integration and Applications

Monolithic integration of CsPbBr$_3$ microplatelet-like films as gain media has been realized in distributed feedback (DFB) lasers on silicon nitride waveguide platforms, demonstrating room-temperature lasing near 540 nm, low amplified spontaneous emission (ASE) thresholds ($8.9$–$14.5~\mu$J/cm$^2$), and lasing thresholds of $0.755$ mJ/cm$^2$ under 0.3 ns pumping (Fabrizi et al., 13 Dec 2024). The planar hot-press process (PHP-CsPbBr$_3$) enables high crystallinity and low surface roughness, critical for coupling into Si$_3$N$_4$ waveguides through first-order grating DFB cavities.

Additional applications include:

• Ultrasensitive gas sensors: Microcrystals grown on electrodes display detection limits down to 4 ppb (ozone) and 1 ppm (hydrogen), enabled by trap healing and high defect tolerance (Argyrou et al., 2020). Similar principles apply to microplatelet geometries.
• Radiation detectors (scintillators): Platelets synthesized by LARP and embedded in PMMA offer ultrafast radioluminescence (1.8 ns), suitable for fast radiation detection (Cova et al., 19 Jun 2024).
• Display technology: Heterostructures with Cs$_4$PbBr$_6$ and nanoscopically small CsPbBr$_3$ platelets achieve efficient and stable blue emission for full-color displays (Romero-Perez et al., 13 Mar 2024).

Defect passivation chemistry (GdBr$_3$-DMF, Sr$^{2+}$ doping, surface graphene capping) and domain engineering (mosaic morphologies, oriented attachment) enable optimization for each device's demands—leverage for robust, high-yield excitonic photonics and charge transport.

7. Outlook and Challenges

The combination of tunable quantum confinement, robust spin and optical coherence, and scalable, low-temperature processing cements CsPbBr$_3$ microplatelets as key materials in quantum, optoelectronic, and spintronic device architectures. Challenges ahead include further improving operational stability under ambient conditions, suppressing ionic migration and photodegradation, orchestrating interface and strain engineering at device-relevant scales, and integrating electrical injection and out-coupling in laser platforms. In-depth understanding of phonon–polaron–exciton coupled systems, hyperfine-induced spin decoherence, and defect-tolerant charge injection remains central for translating microplatelet science into next-generation functional devices.