1. Periodically-Ordered NMWs: NMWs are strictly periodic arrays with the same orientation and periodic wire gaps
1.1.Capillary Force-Assisted Growth: In this method, Normally, the Si model is employed to supply an external capillary force to guide liquid perovskite precursors to grow. For example, an asymmetric array consisting of periodic hydrophilic and hydrophobic wires is the source of the capillary force. The perovskite precursor gathers at the hydrophilic area and crystallizes along wires. Note that, crystallization will not occur in the hydrophobic area, so separate NWs are formed.
1.2.Space-Confined Growth: In this method, the perovskite is confined to grow between two adjacent substrates. Here, if the confined space is further reduced to a wire shape, perovskite crystals will grow along a confined linear direction. Normally, PDMS is usually chosen as the confined model due to its stable properties and ability to make tight contact with a solid substrate.
1.3.Film Post-Treatment: In this method, the fabricated perovskite film is etched into the desired morphology, which will not optimize the process of crystallization. Note that, in this method, we need both high pressure and high temperature to reshape the solid film.
2. Oriented-Ordered NMWs: NMWs have similar orientations but varied gaps.
2.1.AAO-Template-Assisted Growth: In this method, the AAO template can be obtained from a facile electrochemical etching process. Specifically, an Al foil is first polished in a polishing solution (such as HClO4 and alcohol). Then, The foil is anodized twice in an acidic solution (such as H2O, ethylene glycol, and H3PO4) under DC voltage and processed by wet etching in Phosphoric acid solution. The as-fabricated AAO template is a porous template with vertical and dense nanopores. Normally, the radius of nanopores is relatively small, making them difficult to fill with precursor solution. Considering this, a gas state precursor is more suitable due to the superior mobility of gas.
2.2.Surface-Guide Growth: In this method, during the growth of perovskite on a substrate, the surface morphology of the substrate will affect the resulting film. M-Plane Sapphire substrates are well known for their application in the CVD process, and they also adapted to the growth of perovskites.
2.3.Oriented-Force-Induced Growth: In this method, a directionally stable macroscopic force can also lead to oriented growth. Using gravity as an example, when the substrate inclines, the precursor solution will experience a force parallel to the inclination direction.
3. Disordered NMWs: NMWs have a random Orientation and distribution.
3.1.Dissolution-Recycllization: In this method, A fabricated perovskite film can be converted into an NW film by a dissolution-recrystallization process. For example, by spin-coating a drop of DMF/isopropanol mixture solution onto a MAPBI3 film, we can get disordered NWs.
3.2.Slow-Crystallization Growth: To form a dense film, the perovskite precursor is often heated at a high temperature, such as 150℃. In this process, perovskite will crystallize from the precursor very quickly, forming crystals grouped tightly together. However, if the annealing temperature is low there are no external forces, and the self-assembly property will dominate crystallization. Thus, the slow reaction between AX and BX2 has proven to be a successful method for growing single-crystalline perovskite NWs.
3.3.Self-Assembly: In addition to reactions between basic precursor elements, larger blocks such as nanocrystals can also assemble along a preferred direction. Preferred growth in solution is often determined by the surface energy of different crystal facets. By adjusting the precursor solvent or dissolution blocks, the desired facet will have minimum facet-free energy. In this situation, the perovskite block intends to grow along the facet with minimum free energy, which results in a wire shape.