**1. Introduction**

Inorganic and/or hybrid perovskites have become prominent in solutionprocessed optoelectronics. Thousands of reports have been proposed about photovoltaics [1–3], light-emitting diodes (LEDs) [4–6], lasers [6, 7] and photodetectors [8–12] containing perovskite semiconductors per annum over the past decade. Halide perovskite nanostructures exhibit tremendous optic and electrical properties such as strong absorption and/or emission, higher photoluminescence quantum yields (PLQYs) [13–15], higher exciton binding energies and tuneable bandgaps than those of bulk perovskites and other nanomaterials. With these outstanding properties, they may offer many scopes for optoelectronics. Particularly, hybrid lead halide perovskite nanostructures (PNSs) offer unique opportunities for lightemitting diode (LED) applications. PNSs represent larger exciton binding energies and longer carrier decay times than those of bulk crystals. In addition to this, their narrow emission band makes them good candidates for LED [16] and laser [17]. Since Schmidt et al. reported the synthesis of first colloidal PNS by a simple procedure under ambient conditions in 2014 [18], many research groups have started working on these materials. In the synthesis of PNSs, it is possible to use organic ligands for capping to stop crystals` growth in the nanometer scale. Furthermore,

capping ligands can reduce surface defects in the same way of the traditional NC preparation. Thus, size and shape of PNSs can be tune finely from a single perovskite layer or below the exciton Bohr radius for using the effect to quantum confinement to multilayers which exhibits bulk-like properties. With this method, it is possible to prepare nanostructures like quantum dots (QDs), nanoplatelets (NPLs), nanosheets (NSs), and nanowires (NWs) [19, 20].

In a simple PNS growth process, the methylammonium cations are embedded in the voids of the corner-sharing PbX6 octahedra, the long alkyl chain cations only at the periphery of the octahedra with their chains hanging it. Therefore, long alkylammonium ions can be used as the capping agents to limit the growth of the PNSs in three-dimension (3D). There are five common methods proposed in the literature which exhibits good prospects for obtaining various uniform and defect free PNSs. These are named as solvent-induced precipitation, hot injection, template assisted, ligand assisted reprecipitation and emulsion methods [13, 14].

In order to determine the structural and optic properties of PNSs, some characterization techniques such as small angle x-ray scattering (SAXS), scanning electron microscope (SEM), absorption and emission spectroscopy have been used extensively. It is crucial to know the shape and optical response of the nanocrystals for the further opto-electronic application of them.

As it is mentioned above, well-defined PNSs were used in many applications such as optical lasing and LEDs. Some of those applications have been investigated intensively by many groups all around the world. Some milestone studies have been presented comparatively.
