2.1. Hot Injection
The hot injection (HI) method has been previously developed to synthesize cadmium chalcogenides. The organometallic reagent was rapidly injected into a thermally co-ordinated solvent to generate discrete uniform nucleation
[55][37]. Hot injection leads to instantaneous nucleation, quenched by the rapid cooling of the reaction mixture, as supersaturation is relieved by a nucleation explosion. At lower temperatures, this causes further growth of the existing nuclei into mature NCs without causing new nucleation
[56][38]. In recent years, the HI method has been used to synthesize perovskite NCs. Protesescu et al. fabricated monodisperse CsPbX
3 NCs with cubic shape for the first time by the HI method
[37][39]. Typically, Cs
2CO
3 was added to a mixture of octadecene (ODE) and oleic acid (OA) and heated under nitrogen to form a Cs precursor. Subsequently, the cesium precursor was injected into a mixture of PbX
2, ODE, oleylamine (OAm), and OA at high temperatures (150–200 °C). The reaction system was then quenched in an ice bath to obtain NCs of different sizes, which could be adjusted by controlling the reaction conditions. Notably, the obtained NCs with different compositions displayed tunable emission spectra from 410 nm to 700 nm
[29,37] [29][39].
Interestingly, high temperatures and long reaction times could promote the generation of NWs. Yang et al. investigated the reaction kinetics of CsPbBr
3 NW synthesis at 150 °C. In the initial stage (t < 10 min), the reaction was dominated by the formation of nanocubes. After 10 min, thin nanoribbons with diameters of approximately 9 nm were observed. As time increased, the number of NCs decreased, and some square nanosheets were formed. At the later stage of the reaction (40–60 min), the nanosheets dissolved, and the main products were NWs with uniform diameters and lengths of up to 5 μm. However, the NWs disappeared over time, and the final products were large crystals
[16] [16].
Among all-inorganic halide perovskites, cubic-phase (α) CsPbI
3 has the narrowest band gap and shows great potential for use in many optoelectronic devices. Chen et al. proposed a new two-injection method at 60 °C and 120 °C to control the growth of α-CsPbI
3 NWs. α-CsPbI
3 NWs were synthesized via the reaction of cesium oleate with lead halides in a mixture of ODE, OA, and OAm. In the experiment, a low synthesis temperature of 60 °C was used to dissolve PbI
2. Two injections of the Cs precursor and an extended 30 min reaction time per injection allowed the NWs to grow fully under the protection of the ligand. Stable α-CsPbI
3 NWs were synthesized using this method and stored in an inert atmosphere for more than three months
[38][40].
Generally, long-chain alkyl carboxylic acids (OA) and alkyl amines (OAm) are used as ligands to synthesize perovskite NWs. Manna’s group fabricated different, thin CsPbBr
3 NWs using a modified HI method by tuning the concentration of short alkyl carboxylic acids (octanoic acid or hexanoic acid) and alkyl amines (octylamine and OAm). The widths of the thin NWs are 10, 5.1, and 3.4 nm, respectively
[39][41]. Semiconductor NWs with diameters smaller than the exciton Bohr radius have received considerable attention owing to their unique optical properties. Yang et al. developed highly uniform ultrathin CsPbBr
3 NWs with a diameter of 2.2 ± 0.2 nm, much lower than the exciton Bohr radius of CsPbBr
3. After purification and surface treatment, the ultrathin NWs exhibited bright emissions with a PL quantum yield (PLQY) of approximately 30%
[40][42].
2.2. Vapor Growth
Vapor-phase growth converts a crystalline material into a vapor phase through sublimation, evaporation, sputtering, or decomposition and then deposits it under appropriate conditions to realize the controllable atomic transfer of substances from the source material to the solid film
[57][43]. When compared to hot injection methods, the advantage of vapor-phase growth techniques is that the grown NWs tend to be of higher quality and have lower defect density. Vapor growth is typically performed in chemical vapor deposition (CVD) tubular reactors, which provide a controlled and scalable method for growing high-quality semiconductors
[58,59][44][45]. CVD has been widely used to fabricate halide perovskite NWs with excellent properties
[41,60,61][46][47][48]. Xing et al. prepared PbI
2 NWs synthesized on silicon oxide substrates by the CVD method and then converted them to MAPbI
3 NWs by a gas-solid heterophase reaction with MAI molecules. The original geometry of the NWs was maintained after the conversion. Furthermore, MAPbBr
3, MAPbI
xCl
3–x, MAPbI
xCl
3–x, MAPbIxBr
3–x, and MAPbBr
xCl
3–x NWs were prepared using the same method
[41][46].
Black cubic-phase CsPbI
3 has excellent photoelectric properties; however, it prefers phase transition at room temperature, thus limiting the development of devices
[31][31]. Fan et al. fabricated high-density vertical arrays of stable CsPbI
3 NWs in anodic aluminum membranes (AAMs). CsPbI
3 NWs were grown from lead metal nanocluster seeds using a simple vapor-phase growth method, and the NW arrays grown in AAMs had a stable cubic phase with excellent optical properties
[42][49]. The vapor-phase method can grow high-quality epitaxial nanostructures of inorganic perovskites owing to their thermal stability at high temperatures
[62,63][50][51]. Song et al. grew high-quality inorganic CsPbX
3 NWs and microwires with horizontal orientation on mica via vapor-phase epitaxy. By varying the growth time, single NWs, Y-shaped branches, and interconnected NW networks with six-fold symmetry were obtained. These well-connected CsPbBr
3 NWs showed great potential for optoelectronic applications
[43][52]. In addition to mica substrates, sapphire substrates can also be used to fabricate inorganic perovskite NWs. Joselevich et al. explored uniform CsPbBr
3 NWs with horizontal and directional orientations on flat and polyhedral sapphire surfaces, forming six-fold symmetric and two-fold symmetric arrays along specific directions of the sapphire substrates, respectively. They found a size-dependent PL emission shift well beyond the quantum range, and the emission peak redshifted from 514 nm to 528 nm as the diameter of the NWs changed from 45 nm to 1.5 μm
[44] [53].
Conventional solution- and vapor-phase methods are used to grow CsPbX
3 NCs through a noncatalytic and straightforward vapor-solid (VS) synthesis process rather than a catalytic vapor-liquid-solid (VLS) growth mechanism. However, the nanostructures obtained via the noncatalytic VS growth process are poor, particularly the uncontrolled radial growth along the sidewall of the NWs
[44][53]. The VLS mechanism represents an advantage of NW synthesis, which can accurately regulate the size, position, and surface characteristics of the NWs
[64,65,66,67,68,69,70][54][55][56][57][58][59][60].
Cahoon et al. first fabricated PbI
2 NWs autocatalytically using the VLS process and then converted the NWs to MAPbI
3 NWs via the low-temperature vapor-phase intercalation of MAI
[45][61]. Similarly, Shin et al. used a PbI
2 film to obtain highly uniform and dense arrays of oriented PbI
2 NWs by VLS growth and then transformed them into MAPbI
3 NWs through MAI in the vapor phase
[46][62]. In addition to organic-inorganic perovskite NWs, VLS growth has also been applied to fabricate all-inorganic perovskite NWs. Meng et al. developed a VLS growth method for a large vertical CsPbX
3 NW array with a uniform diameter. The Sn catalyst in the liquid state and supersaturated Cs, Pb, and X precursors induced the growth of CsPbX
3 NWs at the liquid-solid interface
[47][63].
2.3. Self-assembly
Self-assembly is a process of arranging individual components into ordered structures
[34][34]. Perovskite NCs exhibit more unstable ligand dynamics and defects than conventional semiconductor NCs do
[71,72,73][64][65][66]. Improving device performance by self-assembling NCs into NWs for surface defect passivation has been widely studied
[32,74][32][67]. Various external stimuli, such as solvent, temperature, and light, affect the self-assembly of NCs
[35,75][35][68].
2.3.1. Ligand-Assisted
Perovskite quantum dots (PQDs) have soft lattices, low formation energies, unstable ligand dynamics, and more common defects than those of conventional semiconductor QDs
[71,72][64][65]. Defects are critical for controlling the electrical and optical properties of perovskite materials
[76][69]. In addition, the presence of defects leads to poor storage stability. The passivation of surface defects can enhance the stability and PLQYs of luminescent nanoparticles. Zhong et al. applied a ligand-assisted reprecipitation method to synthesize MAPbX
3 QDs at room temperature
[77][70]. MAPbBr
3 NWs were fabricated by controlling the number of surfactant ligands
[48][71]. Compared with the self-assembly of organic-inorganic perovskite NWs, Pan et al. synthesized CsPbBr
3 PQDs (halide-vacancy-rich PQDs, HVPQDs) using a traditional HI method. The synthesized PQDs were self-assembled by the addition of OA and dodecyldimethylammonium sulfide (DDAS). The morphologies of the precipitates formed at different stages were observed by HRTEM. At the initial stage (t = 5 min), the particle size of the HVPQDs increased. The nanoparticles were fused in one direction, and a NW shape was observed at t = 60 min. Over time, the nanoparticles were transformed into NWs of approximately 20–60 nm in width and lengths of a few millimeters. Additional OA alters the surface ion equilibrium of the HVPQDs, and the S
2− group from DDAS occupies Br vacancies. Then, two adjacent HVPQD monomers prefer to attract owing to divalent sulfur, thereby inducing the selfassembly process
[49][72].
2.3.2. Light-Induced
Light can be delivered immediately to a precise location and closed system
[75][68]. Halide perovskites tend to change in response to light, such as photoinduction, which affects their phase transformation, nucleation growth, and ion migration
[78,79,80,81][73][74][75][76]. Light-induced synthesis and controlled photoinduced self-assembly are promising approaches for constructing novel structures and materials with potential applications in optical, sensing, and transmission systems
[82][77]. Liu et al. prepared defect-free NWs by assembling and fusing single cubic CsPbBr
3 NCs under visible light irradiation. After photoexcitation, the dissociated charged excitons diffuse to the perovskite NC surface and are trapped by the surface ligands. Light irradiation efficiently detached the surface ligands, and the NCs fused via bare surface contact. In order to elucidate the NW formation mechanism, the growth processes at different stages were monitored by HRTEM. During the initial phase (0–2 h), the NCs tended to align to form a linear structure. Subsequently, nanorods appeared after approximately 2–5 h. The NCs gradually disappeared over time, and the nanorods were continuously grafted into NWs. Adjacent NWs were attached and subsequently transformed into nanoribbons, even as longer and thicker NWs. The final product was a mixture of nanoribbons and NWs with diameters of hundreds of nanometers and chain lengths of 10–20 μm. They formed super-long but flexible chains of over 50 μm, equivalent to approximately 7000 tightly packed NCs
[50][78].
2.3.3. Polar Solvent Induction
Polar solvents greatly influence halide perovskites in terms of their luminescence properties, morphological characteristics, and surface defects
[83,84][79][80]. Sun et al. reported that polar solvent molecules can induce lattice distortion in CsPbI
3, triggering dipole moments and leading to the transformation of CsPbI
3 nanocubes into single-crystal NWs. The twisted nanocubes selfassembled and recrystallized into single-crystal NWs owing to the dipole–dipole force. In order to minimize the surface energy, the assembled NWs can be further self-assembled side-by-side into thick single-crystal NWs with diameters of up to submicrons
[51][81]. Similarly, Peng et al. obtained CsPbBr
3 NWs by using the room-temperature supersaturated recrystallization method. The precursor in N,N-dimethylformamide (DMF) solution was mixed with toluene and acetonitrile (ACN), and CsPbBr
3 NWs were rapidly generated with a cubic-to-orthorhombic phase transition
[52][82].
2.4. Solvothermal
Although various methods have been used to prepare ultra-thin CsPbX
3 NWs, the yields of these NWs is low
[40][42]. In addition, stability is another issue when the diameter of the NWs is too small
[39][41]. The solvothermal method involves the addition of a perovskite precursor and solvent to a high-pressure reaction kettle and reaction at a specific temperature for a particular time. When compared with existing synthetic methods, the solvothermal method involves simple steps, the precise control of the composition and morphology, and the high crystallinity of the synthesized products. Giri et al. added OA and OAm to a DMF solution containing MABr and PbBr
2, and the mixture was allowed to react in an autoclave for 30 min. They regulated MAPbBr
3 NCs with different morphologies by varying the reaction temperature (60 − 180 °C)
[85] [83]. Chen et al. also synthesized high-quality all-inorganic perovskite nanocrystals using a solvothermal method. When the precursor was heated directly without dissolution, the concentration of the precursor ions was relatively low. The precursor dissolved gradually with increasing temperature and a relatively small amount of CsPbX
3 NCs nucleated at a specific precursor concentration. With the help of the capping ligands, the nuclei eventually grew into nanocubes. In contrast, predissolving precursors can form a high concentration of precursor ions. More nuclei formed when the solution was heated, resulting in smaller nuclei. Finally, uniform NWs were formed by increasing the growth time instead of the nanocubes
[53][84].
2.5. Anion Exchange
Ion exchange reactions on colloidal nanocrystals provide pathways for fine-tuning the composition or other inaccessible materials and forms. Cation exchange is easy and common, whereas anion-exchange reactions are relatively difficult
[86][85]. To date, the anion exchange method has been successfully applied to perovskite NW fabrication, enabling a tunable emission range. The Kovalenko group prepared all-inorganic CsPbX
3 NCs by adding a halide source (lead halide or alkenyl ammonium halide) using an anion exchange method
[86][85]. Yang et al. used highly monodisperse CsPbBr
3 NWs as a template to control the composition of NWs via an anion-exchange reaction. Various alloy compositions were obtained, and their morphologies and crystal structures were maintained. The NWs exhibited high luminescence properties with PLQY ranging from 20% to 80%
[28][28]. Moreover, spatially resolved multicolor CsPbX
3 NW heterojunctions were synthesized using the anion exchange method, and the luminescence was adjustable throughout the visible spectrum
[87][86]. Polavarapu et al. also used synthesized CsPbBr
3 NWs with uniform widths as templates and achieved tunable bandgaps and a PL of NWs over the entire visible light range of 400–700 nm by anion exchange
[54] [87]. Metal halide perovskites are prone to chemical transformation through ion exchange, which is attributed to their “soft” crystal lattice, enabling rapid ion migration
[88,89][88][89].