Conformal PLD for 2D RP Perovskites

• Conformal Pulsed Laser Deposition is a method for solvent-free synthesis of 2D Ruddlesden-Popper metal halide films that offers precise control over stoichiometry and orientation.
• The technique utilizes a KrF excimer laser with specific parameters to achieve controlled, layer-by-layer growth and rapid nucleation at room temperature.
• It produces conformal, defect-free films with minimal roughness and robust optoelectronic performance, ideal for integration in advanced heterostructures.

Conformal Pulsed Laser Deposition (PLD) enables precise, solvent-free synthesis of two-dimensional (2D) Ruddlesden-Popper (RP) metal halide phases with uniform orientation and coverage. Distinguished from solution-based methods by direct control over stoichiometry, nucleation, and film growth at room temperature, PLD is particularly suited for fabricating (PEA)₂PbI₄ RP layers in complex heterostructures. The technique delivers conformal, oriented, and stable 2D perovskite films independently of substrate chemistry, with minimized secondary phase formation and robust optoelectronic performance (Solomon et al., 10 Mar 2025).

1. Deposition Parameters and Process Control

PLD for 2D RP metal halides utilizes a Coherent KrF excimer laser with wavelength λ = 248 nm, a fluence φ = 0.32 J/cm² focused on a 1.24 mm² spot, and a repetition rate f = 1 Hz. The target—ball-milled PbI₂ and PEAI in atomic ratio 1:8, pressed into a 20 mm diameter, 2.5 mm-thick disc—yields a stoichiometric plasma plume of inorganic (Pb, I) and organic (PEA⁺) species. Deposition is conducted at room temperature (RT ≈ 25 °C) with a high-purity Ar background (0.03 mbar) and a target-to-substrate distance of 55 mm. Film thickness is controlled via the number of laser pulses: 1 500 (≈20 nm), 3 000 (≈35 nm), and 6 000 (≈70 nm).

The growth rate per pulse, measured by SEM, is δ ≈ 1.3×10⁻² nm/pulse. This yields an instantaneous deposition rate at f = 1 Hz of $R \equiv dT/dt \approx 1.3 \times 10^{-2}$ nm/s. Pulse-by-pulse control over arriving flux ensures stoichiometric delivery, with net areal adatom density $N(t)$ scaling as $R = \frac{dN}{dt} \propto f \times S(\phi)$, where $S(\phi)$ reflects the fluence-dependent yield per pulse.

2. Mechanisms of Conformal Growth

Each 20–30 ns KrF pulse ablates the target, generating a quasi-isotropic plasma in Ar containing Pb, I, and PEA⁺ fragments. At RT, limited surface diffusion of bulky PEA⁺ cations promotes 2D sheet-by-sheet growth and inhibits island formation that plagues solution-processed analogues. In situ photoluminescence (PL), monitored between pulses, shows n = 1 (PEA)₂PbI₄ emission at λ_PL = 520 nm after N≈300 pulses (t≈300 s), increasing linearly with pulse count (I_PL∝N). This indicates rapid nucleation and lateral coalescence of 2D sheets from the earliest monolayers.

Suppression of secondary phases is achieved by the balance between arrival flux J and limited organic cation mobility, establishing uniform, conformal RP coverage.

3. Structural and Orientation Control

Specular θ–2θ X-ray diffraction (XRD) on films of 20, 35, and 70 nm thickness yields a strong (002) peak at 2θ=5.40° and a weaker (004) peak at 2θ≈11.0°, confirming the formation of the n = 1 phase. GIWAXS azimuthal integration reveals sharp diffraction at $q_z = 0.38$ Å⁻¹ and $0.76$ Å⁻¹ (for (002) and (004), respectively) with negligible $q_{xy}$ broadening, indicating highly oriented (001) texture and minimal mosaicity ($\Delta\varphi < 0.3^\circ$ at 20 nm, $\Delta\varphi \approx 0.8^\circ$ at 70 nm). Oriented growth is substrate-independent: identical texture is observed on amorphous SiOₓ/Si, epitaxial a-MAPbI₃/KCl, and MAPbI₃ (011) single crystals.

Van der Waals-type layered bonding intrinsic to the RP structure, not substrate registry, enforces robust (001) orientation.

4. Metrics of Conformality and Uniformity

Atomic-force microscopy (AFM) and confocal PL mapping across 100×100 µm² regions establish uniform, defect-free coverage of substrate terraces by the 2D sheets. For 70 nm films, n=1 phase PL at 520 nm (2.38 eV) exhibits <5% variation in integrated intensity. Root-mean-square roughness for 35 nm films stays below 2 nm, comparable to terrace step heights, and no pinholes or uncovered regions are detected. Isolated morphological defects yield minor n=2 phase PL at 568 nm (2.18 eV) localized within <1 µm² spots, amounting to <3% by volume. These negligible residues do not impact global conformality or continuity.

5. Heterostructure Stability and Cation Exchange Dynamics

Assessment of long-term stability utilizes 35 nm (PEA)₂PbI₄ films atop epitaxial a-MAPbI₃ (001) and strain-free MAPbI₃ (011) single crystals, with PL spectra tracked in N₂ for over 80 days. On epi-a-MAPbI₃, the n = 1 peak at 520 nm remains unchanged; on strain-free MAPbI₃, a new PL band at ~755 nm appears after ~7 days, increasing alongside higher-n RP emissions. This signifies PEA⁺ migration and formation of thicker RP phases, restricted to the top interface.

The behavior aligns with the Arrhenius equation for ionic diffusion:

$D(T) = D_0 \exp\bigl[-E_a/(k_B T)\bigr]$

Epitaxial compressive strain in a-MAPbI₃ elevates the activation energy $E_a$, suppressing cation exchange and stabilizing the n=1 phase under inert conditions.

6. Guidelines for PLD of 2D Metal-Halide Perovskites

PLD of 2D metal-halide perovskites should implement the following:

• Employ mixed-precursor targets with excess bulky-cation salt (e.g., PbI₂:PEAI=1:8) to favor n=1 sheets.
• Optimize fluence (φ=0.32 J/cm²) and repetition rate (1 Hz) for gentle, continuous flux supporting layer-by-layer growth at RT.
• Maintain moderate background pressure (0.03 mbar Ar) to moderate energetic species and enable conformal step coverage.
• Keep substrate at RT to restrain organic-cation mobility and inhibit phase segregation.
• Utilize substrate-induced strain (e.g., epitaxial films) to passivate ionic defects and diminish cation interdiffusion in heterostructures.

PLD’s species delivery mechanism and pulse-resolved nucleation control are generalizable to other layered perovskites (e.g., (BA)₂PbI₄, (FPEA)₂PbI₄) and complex architectures, affording a scalable, solvent-free pathway to orientation-controlled, conformal 2D films for advanced optoelectronic integration (Solomon et al., 10 Mar 2025).