A lot of study has been done on low-dimensional semiconductors, which include organic, artificial, and mixed types. This is because their molecular, physical, and electronic features are very complicated. The main goal in the field of halide perovskites is to make them more reliable in terms of performance and safety in different environments compared to their three-dimensional peers. These materials show promise for future optoelectronics because they are more stable, have better charge transport qualities, and have quantum confinement effects. This chapter talks about how to make zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D) halide perovskites, as well as their properties, uses, problems, and future possibilities.
Classification of Low-Dimensional Perovskites
The chapter talks about how to make and characterise low-dimensional perovskites. It stresses how important it is to spell out the names of the different types of perovskites because there isn’t a standard way to do this that is used everywhere in the literature.
Morphological Low-Dimensional Perovskites Through Size
Reduction (ABX3 Perovskites)
It is possible to make morphologically low-dimensional perovskites by making a 3D perovskite smaller. There are one or more building blocks that make up these low-dimensional perovskites, but their crystal structure is the same as that of 3D perovskites. They are made up of the same ABX3 building block as 3D perovskites, but they can only go in one or more ways. It’s like molecular low-dimensional perovskites with [PbX6] 4 building blocks in terms of how they are put together. It is possible to see a diagram of these structural low-dimensional perovskites.
Molecular Low-Dimensional Perovskites Through Structure
Tuning (Non-ABX3 Perovskites)
Molecular low-dimensional perovskites are made by adding organic spacer cations to ABX3 3D perovskites, which changes their chemical structure. These perovskites are made up of the same [PbX6] 4 inorganic octahedral building block as 3D perovskites, but the way they are put together relies on how organic cations are mixed in.
Synthesis and Characterization of Morphological
Low-Dimensional (ABX3) Halide Perovskites
Different synthesis and characterisation methods have been used by researchers to make and study morphological low-dimensional halide perovskites. There is more information about these perovskites in Chapters 2 and 3, but this section is mostly about some new ideas.
0D Quantum Dots
0D quantum dots are made by limiting the size of the ABX3 perovskite in all three dimensions down to the nanometre level. Dirin et al. showed in 2016 an easy template-assisted non-colloidal synthesis method for making 0D perovskite nanocrystals. In the first step, a mesoporous silica mould is mixed with a perovskite precursor solution. The extra solution is then taken off the pattern, and once the solution has dried, perovskite nanocrystals are made. The hole size of the template can be changed during this method of production to change the nanocrystals’ photoluminescence (PL) output, time-resolved PL, and PL quantum yield.
The hot drilling method has also been used to make 0D nanocrystals. Protesescu et al. made the first CsPbX3 quantum dots by adding lead halide (PbX2) and Cs-oleate to an organic solvent that was kept at a high temperature of 140–200 °C. This reaction starts quickly and grows quickly. The size of the nanocrystals can be changed by changing the temperature of the reaction. The fact that the nanocrystals were mixed into a poly(methylmethacrylate) (PMMA) polymer shows how strong and flexible the techniques used were.
1D Nanowires
You can make 1D halide perovskite nanowires and nanorods by limiting the ABX3 crystal’s size to the nanometre level in two dimensions. Zhang et al. reported in 2015 a catalyst-free solution-phase synthesis of single solid CsPbX3 nanowires. They mixed caesium oleate with lead halide in octadecene at 150–250 °C, along with oleic acid and oleylamine. By finding the best reaction time, temperature, and precursor content, you can change the nanowire’s shape, phase, and size. Covering a glass slide with PbAc2 and putting it in a CH3NH3X solution in isopropanol was another way to make the compound. Oener et al. made perovskite nanowires that can stand on their own using extrusion. A perovskite precursor solution is dropped onto an anodised aluminium oxide template that has holes on both sides. Changes in the template hole size and processing time can change the nanowire’s width and length. Spina et al. demonstrated a new way to make a 1D nanowire using open nanofluidic channels. This method lets the size, shape, aspect ratio, and direction of the nanowire be controlled, and could be used for making a lot of them.
2D Nanoplatelets
To make 2D nanoplatelets, nanosheets, and nanodisks, the size of 3D halide perovskites has to be limited in one direction. A process mixture with Cs-oleate, PbX2, OA, and OLA is used to make perovskite nanoplatelets at room temperature. Lipid-mediated reprecipitation is used to make square nanoplatelets that are big and well-defined. The form and size of the synthesised nanoplatelets can be changed even more by the choice of organic acid. The 2D nanosheets that Liu et al. made were made using a two-step manufacturing method. In the first step, a full PbI2 water solution is poured onto the substrate. This is followed by thermal heating at a high temperature, which creates PbI2 nanosheets. You can change the nanosheets’ shape and width by changing the melting temperature, the type of liquid used, and the material that is used.
Synthesis and Characterization of Molecular
Low-Dimensional (Non-ABX3) Halide Perovskites
Because they are made of ions, 3D perovskites have phase structure instability. Molecular low-dimensional perovskites can solve this problem by cutting along lines (100) or (110), which creates 2D or corrugated-2D structures. These structures can then be further cut to make 0D and 1D perovskites. These features—such as big Stokes shifts and high exciton binding energies—are very important for using them in optical devices. It is very important to understand how each dimensional step is different and unique.
0D
Compared to 3D perovskites, 0D perovskites need new ways to be made because they have the fewest dimensions and the tightest quantum containment. Because of the way they are structured, they have unique photophysical features. This has made people more interested in 0D perovskites because they are more stable and could be used in business. This part talks about new ways to make 0D perovskites, their special photophysical features, and how they might be used in optoelectronic devices.
Synthesis and Properties of 0D Perovskites
The study of 0D perovskites is still very new, but several ways to make them have been shown to work. There are three main groups that these methods can be put into: the thin film coating method, the liquid synthesis method, and the single crystal growth method.
To get a single crystal of 0D perovskites, one of the most popular ways is to cool the material down. Using the difference in how well the crystals dissolve at different temperatures, this method makes high-quality single crystals. For the first time, this method was used to make single crystals of 0D (CH3NH3)3Bi2I9.
Another common way to make single crystals of 0D perovskites is to use the seed crystal-assisted constant-temperature evaporation method. In this method, precursors are dissolved in a solution to make a seed crystal. This crystal is then added to another precursor solution that has been kept at a fixed temperature for a long time to make big, good crystals.
Another common way to make 0D perovskite crystals is to use the hydrothermal method. First, the precursors are mixed with water and put in a steriliser. For one day, the temperature is raised to 220 °C. The water-based precursor solution is then cooled to room temperature at a steady rate of 3 °C per hour.
The solvent evaporation crystallisation method makes it easier for the desired goods to form. Another way to make 0D perovskite crystals is through antisolvent vapour crystallisation. The vertical Bridgman method is a one-of-a-kind way to make chemicals. It involves putting ingredients into quartz ampoules and then releasing the pressure until it is 10−6 mbar. To get the crystals that are wanted, the quartz ampoules are heated to 250 C and then put into the Bridgman crystal growth oven.
Using colloidal synthesis methods is an easy and quick way to make perovskite nanocrystals. Hot-injection is a common way to make 0D perovskite. In this method, Cs-oleate precursors are shot into a PbX2 precursor solution that is very hot. The solution quickly cools down in an ice bath, which leads to the formation of 0D perovskite nanocrystals.
Room-temperature colloidal synthesis is easier to use because it doesn’t need a process that uses a lot of energy. To make Cs4PbBr5 nanocrystals, the reverse microemulsion method was used. In this method, a three-neck flask with light degassing and N2 purging is used to mix the Cs-oleate precursor, n-hexane, and OA. After mixing PbBr2 with DMF, HBr, OA, and OLA, it is poured into the flask and 0D perovskite nanocrystals form very quickly.
The supersaturated recrystallisation method is another way to make something at room temperature. First, all the precursors are dissolved in a DMF solution. Then, a lot of toluene is added to the precursor solution, which stops the synthesis process. Researchers have also come up with ways to make high-quality thin films of 0D perovskites that could be used in future devices. Most of the time, spin-coating and heating are used together to make 0D perovskite films.
It is proven by the crystal structure of 0D perovskites that the metal halide octahedra are completely separated from each other by protective organic ligands that circle the photoactive areas. In 0D perovskites, this means that there are no electrical bands between octahedra that are close to each other. You can also think of 0D perovskites as core-shell structures, which are easier to see with a space-filling model.
0D perovskites have special photophysical qualities because of their unique structure. As an example, (C4N2H14Br)4SnBr6 and (C9NH20)2SbCl5 have very large Stokes shifts and wide FWHMs. This suggests that the emissions come from lower energy states, like self-trapped excitons, rather than direct excited states and the recombination of free excitons. They can be used for many things, like solar cells, because they give off a wide range of emissions from different recombination paths.
0D Cesium Lead Halides
Because they are so different, 0D perovskites like caesium lead halides have gotten a lot of attention. These 0D perovskites are made up of separate octahedra that are held together by caesium ions instead of organic bonds. It’s hard to tell the difference between 0D Cs4PbX6 and 3D CsPbX3 because different studies have found different physical qualities for each.
0D Cs-based perovskites, on the other hand, have octahedra that are fully separated from each other, while 3D Cs-based perovskites have a network of octahedra that share halide anions at their corners. This makes for a rhombohedral phase with a system that is greatly deformed and compressed. The 0D phase has large bandgaps, high exciton binding energies, and different densities of states from the 3D versions. It also doesn’t have any orbital overlap between the different octahedra.
People have different opinions on the green absorption seen in what is thought to be pure Cs4PbBr6. Some people think that the presence of green photoluminescence (PL) in 0D Cs4PbBr6 is caused by the formation of defect states, which goes against the idea that halide perovskites can handle defects. Another reason is that CsPbBr3 nanocrystals form inside the 0D Cs4PbBr6 matrix, which helps explain the main PL from these materials. But this disagreement hasn’t been settled yet because the PL lifetime and other features don’t exactly match those of a different CsPbBr3 nanocrystal.
By making a 0D/3D hybrid, the better visual qualities of 3D CsPbX3 can be kept and stabilised by the 0D Cs4PbBr6 parts, this field is likely to see more progress in the future. Because it doesn’t carry electricity well and blocks energy, Cs4PbBr6 by itself isn’t thought to be useful for future device uses. Putting these materials together to work together seems like a good idea because they have a lot of tunability in the caesium lead halide species, which means they could be used to make effective optoelectronic devices.
1D
1D perovskites haven’t been studied as much as 0D and 2D perovskites because they have strong quantum confinement and self-trapped excitons, which cause broad photoluminescence (PL). This means that 1D perovskites might be useful for making devices that target white light output. The PL process in 1D perovskites is not fully controlled by self-trapped excitons, though, because the octahedra are not completely away from each other.
Yuan et al. did a full study on bulk and microscale solid 1D organic lead bromide perovskite in 2017. They discovered that when UV light hits the crystals, they give off strong blue-white light that has two separate spectral features. Because PL has a long lifetime, it’s likely that these broad emissions come from self-trapped exciton states. The two spectrum features show that free excitons and self-trapped exciton states exist together and cause different kinds of radiative recombination.
Overall, the 1D perovskite system is like the photophysical features of curved 2D perovskites because free exciton and self-trapped exciton states exist at the same time. However, the field doesn’t have a deep knowledge of this phase because not much work has been done on molecular 1D perovskites before. By looking into 1D perovskites, we can learn more about the basic ideas behind bulk quantum wire systems.
To sum up, because they have strong quantum confinement and self-trapped excitons, 1D perovskites could be used in devices that aim for white light outputs. But the structure and features of 1D perovskite are still being studied, and more work needs to be done to fully understand this exciting technology.
2D and Quasi-2D
To test fundamental qualities like conductivity, ferroelectric hysteresis, charge motion, and carrier diffusion length, it is important to have high-quality perovskite single crystals. This is a way to look into new physical processes, like exciton physics: interlayer van der Waals interactions in 2D and quasi-2D perovskite nanocrystals. Many people have worked hard to make high-quality 2D perovskite crystals. Because 2D perovskites have organic ligands and are ionic, solution-based manufacturing methods have been developed a lot.
Synthesis of 2D and Quasi-2D Perovskites Single Crystal
The study is mostly about making and studying 2D perovskite single crystals using different solution-based approaches, such as the cooling down method, co-solvent evaporation, solvent evaporation, and the stacked solution method. Mixing and dissolving compounds in hot water, like divalent metal halide (MX2) and organic amine halide (RNH2 HX), is part of the slowing down method. Once it’s all broken down, a continuous spectrum of the bulk and microscale 1D perovskite crystals can be seen.
The photoluminescence fades in the bulk, and the size of the 1D perovskite crystals is recorded at 475 nm when the temperature is room temperature. The colour coordinates of the 1D perovskites and the curved 2D perovskite (EDBE)PbBr4 are shown in this work.
The cooling down method (CDM), co-solvent evaporation (CSE), solvent evaporation (SE), and layered solution method (LSM) are all ways to make 2D and quasi-2D perovskites. The rate of cooling stays the same, and crystal growth starts at a certain temperature. The growth stops when the solution reaches room temperature.
For instance, Mitzi was the first person to report making BA2PbI4 using the cooling down crystallisation method in 1996. To make the perovskite, divalent metal oxide (MO) is sometimes used instead of divalent metal halide (MX2).
The study gives useful information about the structure and physical features of 2D perovskite single crystals by using different solution-based approaches. The results show that making these materials can be done with different levels of success, based on the conditions and the qualities that are wanted.
Researchers have been interested in making Ruddlesden–Popper (BA)2(CH3NH3)n−1PbnI3n+1 (n = 1, 2, 3, and 4) perovskites (RPPs). It was possible to make the coloured sheets of crystals by mixing a neutralised base butylamine (BA = C4H9NH2) with HI, H3PO2, and PbO in a hot water solution. To get a good handle on the stoichiometry, BA was needed to lead the reaction.
In 2018, it was revealed that a temperature-controlled crystallisation method could make a group of 2D perovskites (BA)2(CH3NH3)n−1PbnI3n+1 (n = 1, 2, 3, and 4) RPPs. To make uniform solutions with this method, PbO, BAI, MAI, and HI (which contains H3PO2) were mixed in different amounts and heated while being stirred magnetically. Using a 3 °C h−1 cooling rate to go from 110 °C to room temperature made it possible to get big perovskite single crystals and remove them from the water solution. This method can be used to get perovskite single crystals up to 1 cm in size.
X-ray diffraction (XRD) images can be used to look at the crystal structure and phase uniformity of RPPs after they have been made. Aside from BA organic cations, 2D perovskites can also be made with big alkylammonium cations of different lengths. These cations mostly serve as structure guides to control the distance between the artificial layers. We made 2D hybrid halide perovskites out of phenylethylamine (PEA = C8H9NH2) that are 2–3 mm thick by controlling the temperature during the cooling down crystallisation process.
We were able to successfully make RPP single crystals, which lets us study how visual features and crystal structure are related. Chen et al. discovered that Rashba splitting and phonon scattering have an effect on the spin coherence lifetime. These two effects are related to the thickness of the artificial layer. Peng et al. also thought that adding the bigger organic cation PEA could lower the amount of self-doping and crystal sizes by stopping defects from forming crystals, compared to using BA cations.
While the water-based cooling method has worked well in the past, it can only be used with small ligands like BA and PEA. It is hard to grow and get a high-quality single crystal of high “n” number quasi-2D perovskites because they are easily dissolved and have a rigid structure. Another option is to use a polar chemical liquid, like DMF, to help grow perovskite crystals. DMF and water can be mixed together, or DMF alone can be used.
The study talks about how organic solvents can be used to make 2D and quasi-2D perovskites. Slowly evaporating acetone is used to get monolayers of perovskites (C6H13NH3)2PbI4, (C6H13NH3)2(CH3NH3)Pb2Br7, and (C6H13NH3)2(CH3NH3)Pb3Br10. Depending on the vapour pressure of the liquid, this process may take a long time. Adding another liquid makes a mixed solvent system, which speeds up the rate of evaporation. This speeds up the growth of the crystals. For instance, a mixture of acetone and nitromethane was used to make a big (C10H21NH3)2PbI4 crystal that was 4 × 10 × 0.1 mm3.
RP phase perovskites are made by adding a single ammonium cation. To make 2D Dion-Jacobson (DJ) phase perovskites using the above methods, diammonium cations can also be added. Complex R′ is more likely to form 2D layers when diammonium cations are present, and diammonium cations can get rid of van der Waals gaps and connect the layers directly. For instance, the liquid evaporation method was used to make 2D diammonium NH3(CH2)nNH3PbI4 perovskites single crystals.
It is important to understand how 2D and quasi-2D perovskites crystals grow in order to get good grade single crystals. The idea behind both slowing down and liquid removal is to use a supersaturated solution to make perovskite crystals form. One stage of perovskite crystal growth in a supersaturated solution is the formation of the crystal nucleus. The second stage is the formation of a big single crystal by the continued growth of the crystal nucleus. It is not only the thermodynamic temperature but also the natural linear ratio of the content of the supersaturated solution that drives the crystallisation process.
The steady-state nucleation rate tells us how many micelles will form on the crystal surface at the microscopic level, and the crystal growth rate tells us how much the quality of the crystal changes at the macro level. Most people know that the quality of a crystal is better when the growth rate is low.
There are two steps in the growth of perovskite crystals: solute passage from the fluid to the crystal surface and solute formation on the crystal surface. The solute diffusion rate changes quickly as the temperature changes, and the temperature can be changed to control it. Making changes to the liquid deposition process is harder than making changes to the diffusion rate. Crystallisation kinetics shows that high-quality crystals can grow on already-formed crystals or seeds without too much nucleation when the concentration of the liquid is kept above the solubility curve.
The layered solution method (LSM) is another way that perovskite crystals can grow. For LSM to work, two reactants must be dissolved in two solutions that can dissolve each other but have different densities. When two solvents have different solubilities and densities, a clear interface forms between them. This causes a big single crystal to form at the interface. When Kamminga et al. used the stacked solution method to make single crystal perovskites at room temperature, they used four phenyl alkylammonium cations with different lengths of alkyl chains.
Because they are stable, cheap, and easy to use, these solution methods are often used to make different kinds of perovskites crystals. A lot of work has gone into making nonlead 2D perovskites, as well as lead 2D and quasi-2D perovskites. They made a 2D perovskite with Sn-based perovskite (BA)2(MA)Pb2I4 and (PEA)2GeI4 perovskite, which has a straight bandgap of 2.12 eV and is more stable than other perovskite materials.
To sum up, 2D and quasi-2D perovskites are big systems of materials because they have a lot of different molecules and crystal structures. At this point, only three types of crystal shapes have been found in 2D and almost-2D perovskites. How the growth is done depends on how easily the ligands dissolve, how polar the ligand chain is, and what the structure factors of the goal perovskites should be.
Synthesis of 2D and Quasi-2D Perovskites Nanocrystal
For many years, people have been interested in making atomically thin 2D halide perovskite nanoparticles. The main method used has been mechanical flaking, which is a way to get big single sheets. It’s not good for high-throughput production or integrating a lot of devices, though. Recently, Dou et al. used a solution synthesis method to make atomically thin 2D halide perovskite sheets that were grown directly on large areas using a ternary solvent mixture. The size, form, and make-up of the 2D halide perovskite crystals could be better controlled with this method.
By changing the type of solvent and the amount mixed in, a more organised study on solvent engineering during crystal growth was carried out. It was discovered that the growth of 2D halide perovskite crystals is controlled by how diffusion-dominated branched growth and c-axis inhibition work together. With this method, you can get crystals with sides that are up to about 40 μm wide. Recently, similar methods were used to make 2D halide perovskites with different cations, like PEA+. Based on these works, solution processing may be a better way to make high-quality perovskite 2D crystals than chemical vapour deposition (CVD) growth or mechanical peeling.
In most 2D halide perovskites, simple organic cations like BA or PEA are used. These cations only act as shielding walls in the 2D structure. It’s possible that semiconducting organic cations could be used to change the optical and electronic qualities more significantly. Gao et al. created a group of perovskites with mixed shapes. They called them “organic semiconductor-incorporated perovskites” or “OSiPs.” Using smart molecular design, a group of π-conjugated ligands with adjustable energy levels and band gaps have been made and successfully added to the 2D perovskite lattice.
Pan et al. created an easy way to make big, free-standing nanosheets of different RP perovskites by crystallising them at the point where the precursor solution drops meet air and water. For this method, a drop of a warm, saturated water solution made from pure hydroiodic acid and perovskite precursors is put on a glass slide. As the temperature drops, the droplet becomes supersaturated, which starts the process of nucleation and crystal growth. Maintaining the right supersaturation is important for getting monolayer or few-layer layers while maximising horizontal growth. This is done by carefully setting the amounts of each precursor and the starting cooling temperatures.
This method can be used to make a lot of different types of 2D and quasi-2D perovskites nanocrystals, but it can only be used at the surfaces of water and air and not with organic solvents. So, this method needs to be improved in order to make perovskites nanocrystals with much bigger organic cations, like π-conjugated ligands.
Applications of Low-Dimensional Halide Perovskites
Light-emitting diodes (LEDs), solar cells, photodetectors, X-ray detectors, scintillators, luminescent solar concentrators, and electrochemical capacitors are some of the electrical devices that 0D halide perovskites are used to make. They have a narrow bandgap, high PLQY, adjustable PL, and high colour clarity, which makes them look good for future display technologies.
Low-dimensional halide perovskites have been used in solar cells that have a power conversion efficiency (PCE) of about 17.4%. In 2019, Liang et al. looked at the special features of 0D perovskites and how they can be used in photoelectrochemistry. On the other hand, the large bandgap and high exciton binding energy of 1D halide perovskites make them look like they could be useful in lighting uses.
Zhu et al. used perovskite nanowires to show wavelength-tunable lasing that works at room temperature with very low lasing limits and high quality factors. Chen et al. made a hybrid phototransistor with CsPbI3 nanorod bilayers and an organic semiconductor. The transistor has a high photoresponsivity, an ultrahigh photosensitivity, and great stability.
2D and quasi-2D halide perovskites have been used a lot to make solar cells and LEDs, just like their 1D and 0D cousins. The short circuit current density and PCE of 2D and quasi-2D halide perovskite-based solar cells go up as the “n” value or the number of artificial repeated layers goes up.
The use of Sn-based 2D halide perovskites with organic cations that are linked has shown promise in thermoelectric device uses. Li et al. looked into how light can help ions move through carbon nanotubes and 2D perovskite heterostructures. They found that these materials could also work as photomemory with a two-phase performance that could store multiple values and be erased.
2D halide perovskites allow both electronic and ionic transport, which means that resistive memory devices only need a small amount of power to work. Because of this, they work well with neuromorphic systems and circuits. 2D halide perovskites are a good candidate for gain media in lasers because they can be tuned to different colours and can be processed in solutions.
Current Challenges and Prospects of
Low-Dimensional Halide Perovskites
This chapter talks about the many studies that have been done on low-dimensional halide perovskites, which are more versatile in terms of their makeup than their 3D cousins. Choosing the right organic capping ligand can make perovskite more stable in its natural state. Using Sn instead of Pb in halide perovskites or Ag/Bi in double perovskites can make perovskites better for the environment. By looking at how changes in makeup affect the breakdown process, we can solve the stability and toxicity problems that are stopping the commercialisation of low-dimensional perovskites right now.
There are three types of connection modes in 2D halide perovskites: corner sharing, edge sharing, and face sharing. Changes in the composition can have a big effect on the crystal lattice as a whole. Learn how these crystal lattices change photophysical features like band gap, exciton binding energies, and charge transport to get the most out of low-dimensional perovskites.
More research is needed to fully understand how low-dimensional perovskites crystallise and how fast they do it. Zhou et al. looked into the crystal structure of quasi-2D halide perovskites that were made with different amounts of MA and FA organic cations. The pure MA-based halide perovskite has a lot more crystals than the MA/FA mixed cation perovskite. However, adding the FA cation made the charge carriers last longer and the PCE of the solar cells that were made better. A thorough study of how cations and anions affect the speed at which crystals form and the performance of devices will be a great way to control the design of smart materials.
Looking into compound systems is another new way to make halide perovskites more resistant to mechanical damage. Finkenauer et al. made a polymer-perovskite hybrid film that helped change the PL, carrier lifetime, and efficiency of the solar cell that they made. Looking into perovskite-polymer compounds in the same way in the low-dimensional world can help make low-dimensional photonic devices more reliable.
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