Metal-halide perovskites have been shown to be remarkable and promising optoelectronic materials. However, despite ongoing research from multiple perspectives, some fundamental questions regarding their optoelectronic properties remain controversial. One reason is the high-variance of data collected from, often unstable, polycrystalline thin films. Here we use ordered arrays of stable, single-crystal cesium lead bromide (CsPbBr₃) nanowires grown by surface-guided chemical vapor deposition to study fundamental properties of these semiconductors in a one-dimensional model system. Specifically, we uncover the origin of an unusually large size-dependent luminescence emission spectral blue-shift. Using multiple spatially resolved spectroscopy techniques, we establish that bandgap modulation causes the emission shift, and by correlation with state-of-the-art electron microscopy methods, we reveal its origin in substantial and uniform lattice rotations due to heteroepitaxial strain and lattice relaxation. Understanding strain and its effect on the optoelectronic properties of these dynamic materials, from the atomic scale up, is essential to evaluate their performance limits and fundamentals of charge carrier dynamics.

The interest in metal halide perovskites has grown as impressive results have been shown in solar cells, light emitting devices, and scintillators, but this class of materials have a complex crystal structure that is only partially understood. In particular, the dynamics of the nanoscale ferroelastic domains in metal halide perovskites remains difficult to study. An ideal in situ imaging method for ferroelastic domains requires a challenging combination of high spatial resolution and long penetration depth. Here, we demonstrate in situ temperature-dependent imaging of ferroelastic domains in a single nanowire of metal halide perovskite, CsPbBr₃. Scanning X-ray diffraction with a 60 nm beam was used to retrieve local structural properties for temperatures up to 140 °C. We observed a single Bragg peak at room temperature, but at 80 °C, four new Bragg peaks appeared, originating in different real-space domains. The domains were arranged in periodic stripes in the center and with a hatched pattern close to the edges. Reciprocal space mapping at 80 °C was used to quantify the local strain and lattice tilts, revealing the ferroelastic nature of the domains. The domains display a partial stability to further temperature changes. Our results show the dynamics of nanoscale ferroelastic domain formation within a single-crystal perovskite nanostructure, which is important both for the fundamental understanding of these materials and for the development of perovskite-based devices.

Significance: Surface-guided growth of planar nanowires is an attractive way of creating aligned arrays of nanowires to enable their large-scale integration into practical devices, but the kinetics and mechanism of planar vs. regular nonplanar growth are poorly controlled and understood. We present kinetic data for planar nanowire growth supported by a theoretical model. Planar vs. nonplanar nanowire growth rates show different power dependence on nanowire diameter attributed to the dimensionality of precursor material diffusion. Whereas the regular nonplanar growth is dominated by surface diffusion over the nanowire sidewalls, planar growth is found to be dominated by surface diffusion over the substrate. This knowledge enables much higher control over the diameter and length distribution of surface-guided nanowires in different material systems.

The surface-guided growth of horizontal nanowires (NWs) allows assembly and alignment of the NWs on the substrate during the synthesis, thus eliminating the need for additional processes after growth. One of the major advantages of guided growth over post-growth assembly is the control on the NWs direction, crystallographic orientation and position. In this study, we use the guided growth approach to synthesize high-quality, single-crystal, aligned horizontal ZnS NWs on flat and faceted sapphire surfaces, and show how the crystal planes of the different substrates affects the crystal structure and orientation of the NWs. We also show initial results of the effect of Cu doping on their photoluminescence. Such high-quality aligned ZnS NWs can potentially be assembled as key components in phosphorescent displays and markers due to their unique optical properties. The ZnS NWs have either wurtzite or zinc-blende structure depending on the substrate orientations and contain intrinsic point defects such as sulfur vacancies, which are common in this material. The crystallographic orientations are consistent with those of guided NWs from other semiconductor materials, demonstrating the generality of the guided growth phenomenon. The successfully grown ZnS NWs and the Cu doping are the first step toward fabrication of optoelectronic devices based on ZnS nanostructures.