Low-dimensional metal halide perovskites have emerged as promising materials for next-generation light-emitting devices owing to their exceptional photoluminescence quantum yields, narrow emission linewidths, and tunable emission through quantum and dielectric confinement. The structural dimensionality in these materials plays a critical role in governing their optoelectronic behaviour. While much attention has been given to their emission properties, a comprehensive understanding of their crystallographic evolution and orientation under various conditions remains lacking; yet, this structural knowledge is essential for achieving reproducible and efficient device integration.

In this work, we present an extensive grazing incidence wide-angle X-ray scattering (GIWAXS) study of 2D halide perovskite nanocrystals with varying monolayer thicknesses, alongside a bulkier-like nanocrystal, to systematically investigate the structural phase behaviour and orientation evolution. Two key experimental approaches were undertaken: in situ temperature-dependent GIWAXS and room-temperature comparative studies across different film processing routes. The in situ GIWAXS measurements, performed over a controlled temperature range, reveal a clear thickness-dependent phase behaviour.

While thinner 2D nanocrystals maintain a more cubic-like phase over a broader temperature window, thicker nanocrystals exhibit an earlier onset of the orthorhombic transition, resembling bulk-like behaviour. Intermediate thicknesses show a mixed or gradual transition, reflecting the subtle interplay between surface energy and lattice stability. These temperature-dependent studies also reveal how heating influences the preferential orientation of the crystallographic planes within the film, with notable reorientation occurring near phase transition temperatures. Complementary room-temperature GIWAXS measurements further explore how film-processing parameters influence the final film texture. By systematically varying the deposition strategies and precursor parameters, we found a significant effect on the preferential in-plane and out-of-plane orientations of the 2D nanocrystals. Such structural anisotropy can directly impact charge and energy transport in devices. Overall, this diffraction study establishes a robust structure–processing–property relationship in 2D halide perovskite, offering valuable insight into how nanostructure dimensionality and processing conditions govern structural phase stability and orientation. These findings not only deepen our understanding of perovskite crystallography at the nanoscale but also provide actionable strategies to tailor the alignment and phase behaviour for improved light-emitting performance