Because they are easy to make and don’t cost much, halide perovskites show promise as materials for future energy production and conversion technologies. This is because their properties are naturally good and their visual properties can be tuned. The word “perovskite” refers to a solid shape that many different substances have, like calcium titanate (CaTiO3). Scientists and engineers are more interested in these materials now that solar technology has improved, making them more useful for light-emitting diodes, photonics, and photodetectors.
The ABX3 formula is shown by halide perovskites. The cations at the A and B sites have oxidation states of “+1” and “+2,” respectively. The anions, which are halides with X = Cl, Br, and I, have oxidation states of “−1.” The metal in the B-site is connected to six halides, creating a corner-sharing octahedra-type coordination [PbX6]. This leaves a space for the A-site cation to fit.
The creation of colloidal halide perovskite nanocrystals (PNCs) was a second big step forward, following the showing of their excellent solar performance. In 2014, Perez-Prieto and colleagues showed how to make hybrid halide perovskites called MAPbBr3 with methylammonium (MA) as the A-site cation. In 2015, Kovalenko and colleagues showed how to make all-inorganic CsPbX3 perovskites. These nanocrystals are even smaller than their excitonic Bohr radius because of surface coordination at the perovskite surface. This gives the PNCs, which are also called quantum dots (QDs), the quantum confinement effect.
Most of the topics in this chapter are talked about in terms of the qualities of lead halide PNCs because they are easy to understand, can be used in a lot of different situations, and are simple. The possible colloidal ways for making PNCs are talked about, and then the quantum size effects of size-tuneable PNCs that have already been made are looked at. The PNCs’ soft and changing crystal structure allows for quick halide swaps, and it is even talked about how post-synthetic treatments can make up for halide deficiencies in the PNCs. The creation of different shapes with the management of response parameters is another important physical effect of PNCs that is talked about below.
It is also talked about how to replace Pb with foreign metals at the B-site to get dopant emission and boost excitonic photoemissions. On the other hand, Pb-free PNCs are also interesting because they have the same physical qualities as their Pb-based counterparts.
Methodology
A method called colloidal QD synthesis is used to make PNCs. Ligands are added to the surface to make it stable and hydrophobic so that nonpolar solvents can be collected. To make PNCs, different colloidal synthesis methods are used, with bottom-up methods being the most common. People have also looked into top-down methods. More information about these methods is given in the next part.
Hot-injection (HI) Method
The hot-injection synthesis method (HI) is a simple way to make perovskite nanocrystals (PNCs). Precursors are introduced into a highly concentrated fluid at high temperatures. In Type I, a metal halide precursor solution is worked on in the reaction pot while an A-site cation precursor is hot-filled. In Type II, both A-site cation precursors and metal precursors are worked on in the reaction pot while halide precursors are injected.
Most of the time, APbX3 (A = Cs+; FA+, formamidinium) is synthesised for a lot of different uses. It has also been talked about how to use this manufacturing method to make Pb-free perovskites in NCs. Usually, to make HI, metal halide precursors are dissolved in the presence of the right ligand mixtures in the right stoichiometric ratio before the third element is injected hot. To make CsPbX3, PbX2 is added to the reaction pot along with oleic acid and oleylamine, which act as ligands. In an acid-base reaction, oleic acid gives off one proton (H+), which is then taken up by oleylamine to make oleylammonium oleate (RNH3+.RCOO−). When this species is present, PbX2 dissolves to make Pb-oleate and oleylammonium halide (RNH3X) at the right temperature (120–180 ∘C). This is known as the lead halide precursor solution.
Once all of the PbX2 has been broken down, the third element (an A-site cation like Cs-oleate) is quickly added at a certain temperature to help make PNCs of a certain size. The reaction is then quickly stopped by cooling them quickly with ice. At this point, fast cooling is needed because PNCs are growing so quickly.
The Cs-precursor to PbX2 ratio, the hot-injection temperature, and the role of acid-base ligands are some of the reaction factors that need to be thought about. The Cs-to-PbX2 ratio should be 1:1.5 or less, because more of the Cs-precursor causes a Cs-rich phase to form, called Cs4PbX6, which can happen by itself or along with the intended product. To change the form and size of the PNCs, the reaction temperature is very important. It shouldn’t go above 200–210 ∘C to keep the reaction stable.
Organic acids and amines play a big part in the hot-injection synthesis of Perovskite Nanocrystals (PNCs). When oleylamine (RNH2) and oleic acid (RCOOH) react, they make RNH3+∅RCOO− species in 1-octadecene. These species are important for dissolved lead halides. As the temperature rises, the acid-base bond formation balance moves towards the reactive side. This means that 200–210 °C is usually the best temperature. Beyond 210 °C, the RNH3+⋅RCOO− species changes into free RNH2 and RCOOH, which separates the PbX2 from its binding.
Too much bromide in the reaction medium helps the oleylammonium cation get a proton and lowers the tendency for the proton to escape. Having oleylammonium bromide in the reaction medium also helps make PNCs smaller, which stops Ostwald ripening and lowers the surface energy. This method, called “instant capping,” makes PNCs smaller.
When annealing takes place at a high reaction temperature (250 ∘C), CsPbX3 changes into CsPb2X5. By adding oleylammonium halide salt, this change can be stopped. This stops phase transformation and keeps the average size of PNCs even after annealing for 30 to 45 minutes.
To make a metal oleate solution, another type II HI reaction method involves adding an A-site cation, lead compounds (but not halides), and oleic acid (RCOOH). The mixed reaction is steady up to 260 °C, and the halide precursor is added to the metal oleate solution when the temperature is right.
Ligand-assisted Reprecipitation (LARP) Method
Many people use the Ligand-assisted reprecipitation (LARP) process to make perovskite nanocrystals (PNCs). This approach uses antisolvent-mediated recrystallisation. The choice of solvents is very important for this process. There are two types: polar solvents (good solvents) and nonpolar solvents (bad solvents). You can make perovskite thin films using these processes all the time.
Before the recrystallisation process, ligands are added to the nonpolar solvent using the LARP method. These molecules work with the ions to make perovskites at the nanoscale level. This method can be used to make bulk perovskite products when chemicals are not available.
To make the precursor solution, AXE and PbX2 are usually mixed with a certain amount of any polar liquid (most often, DMF or DMSO). A certain amount of an antisolvent (usually hexane/octane or toluene) and ligands are added to another reaction pot while it is being stirred all the time. Fatty amines and long-chained fatty acids are used as ligands because they pair up with metals and halides, respectively.
The main factors that affect the LARP method are the temperature of the precipitation, the type of capping ligand used, the quantity of the precursor, and the amount of water in the precursor solution. The stoichiometry of the predecessors is very important for making different shapes. By changing the A-cation side/Pb molar ratio, different forms can be made, like nanoplatelets or nanocubes, which give the nanocrystals different qualities on their own.
LARP makes it possible to make PNCs on a scale of up to 50 times, getting the same high shape control and uniformity as the normal method.
Microwave-assisted Synthesis
The hydrogen ion (HI) method and microwave (MW)-assisted synthesis are both ways to make PNCs on a larger scale. It has benefits, like putting the ingredients in one pot at the start and faster reaction times. One problem is that air is present during the process, which is not good. It is possible to make PNCs by controlling the response factors in the same way that the HI method is used. In this case, using MW to help make mixed halide CsPbX3 PNCs leads to various shape-selective PNC preparations. Nanocubes are made at 160 °C, nanoplatelets at 80 °C, and nanorods when precursors are broken down before MW heating.
Ball-milling Process
To make perovskite nanocrystals (PNCs), which are good because they are ionic and have low crystallisation and lattice energy, mechanochemical methods are used. Precursors can be ground up in a mortar and pestle to make bulk perovskites, but high-energy ball milling is a potential way to make liquid nanocrystals. In this process, tiny hard balls made of zirconia, corundum, or stainless steel are ground. These balls hit each other, creating high pressure that helps the grinding process.
You can make APbX3 PNCs in two ways: first, mix an equal amount of an A-site cation halide (like CsX/methylammonium halide) and PbX2; second, use a bulk crystal of APbX3 as a starting material and grind it into small pieces while organic capping ligands are present.
The grinding time and the ball-to-material weight ratio need to be managed in order to get uniform PNCs. If the ball-to-material ratio is higher, the milling time needs to be shorter. If the optimal milling time is surpassed, nanocubes and nanoplatelets of different shapes form. The chemicals used are another thing that can be changed to change how uniform the PNCs are.
Quantum Confinement Effect
Nanocrystals are in the quantum confinement regime when their radius is about the same as the exciton Bohr radius. This phase changes the electron wave function, which is affected by the material’s crystalline structure and the boundary between the nanocrystal and the outside world. As particle size goes down, the bandgap dependent on nanocrystal size goes up. To describe an exciton’s Bohr width, we say how far apart its hole and electron are. After the PNCs get smaller than the excitonic Bohr circle of the semiconductor, the exciton binding energy stays the same. Excitonic Bohr diameter is less than the size of the PNCs. To make an electron-hole pair, the excitonic energy needs to be raised.
For II-VI and III-V silicon nanocrystals, the quantum confinement effect has been studied a lot. For APbX3 PNCs, the effect has been reported the most. Nanocubes, nanoplatelets, and nanowires are some of the shapes that CsPbX3 PNCs can take. We look at the dots’ diameter and compare it to the excitonic Bohr diameter. We also look at the nanocubes’ edge length. We talk about the synthetic control and excitonic energy shaping effects of CsPbBr3 NCs with different shapes.
Nanocubes
You can change the average size of perovskite nanocrystals (PNCs) by changing the X-to-Pb ratio while they are being made. When there are more halides in the fluid, the PNCs get smaller on average. Other metal bromide salts or oleylammonium bromide (OLAm-Br) salts can be used with the lead bromide precursor to make this happen. The growth of PNCs is controlled by the thermodynamic balance between the bromide ions in solution and the PNCs as they are. OLAm-Br adds too many bulky-oleylammonium ions to the surface of PNCs to replace the Cs+ ions, which stops them from growing any further. One example of how the quantum size of CsPbBr3 nanocubes changes things shows a hypsochromic shift as the edge length of the nanocubes gets shorter. Other halide cousins, like Cl and I, can also be used to control the size of molecules in the same way. The quantum size effect is hard to see in CsPbCl3 PNCs, though, because their excitonic Bohr radius is smaller. For CsPbI3 nanocubes, the size of the PNC and the quantum confinement regime can be changed by changing synthesis factors like the reaction temperature and the capping ligands.
Nanoplatelets
For improving the quantum confinement effect, the thickness of the CsPbX3 nanoplatelets is very important. The main thing that controls how thick these nanoplatelets are is the reaction temperature. For thickness-tuned nanoplatelets, a lower temperature diapason (90–130 ∘C) is best because it encourages uneven growth and creates a two-dimensional (2D) shape. When the reaction temperature is lowered, very thin CsPbBr3 nanoplatelets can form, even with just one layer of corner-shared [PbBr6] octahedra. The quantum confinement effect of CsPbBr3 nanoplatelets shows that as the nanoplatelet thickness goes down, the excitonic energy goes up very strongly. These nanocubes, nanoplatelets, and nanowires made of CsPbBr3 show a strong rise in excitonic energy as they get thinner in the absorption and PL spectra.
Nanowires
Like nanoplatelets, nanowires can achieve quantum confinement by adjusting their width so that it is less than the excitonic Bohr diameter. Figure 3.7c shows that the excitonic absorbance and emission spectra change to a bluer colour as the nanowire width gets smaller.
Solution-processed Halide Exchange
It is easier for substitutional halide exchange processes to start when halide ions are in perovskite nanocrystals (PNCs). When different halide predecessors are added, the halides quickly share with each other at the PNC surface. This creates mixed halide perovskites that give off light in a new spectral window. Adding different halide predecessors to a different PNCs solution makes it possible to make different glowing PNCs. Mixed halide perovskites show full PL features across the whole visible range of light. You can also do halide swaps by mixing different perovskite PNC solutions. For mixed halide perovskites to give off an emission, they need to have halides that come one after the other, like Cl-Br or Br-I. When you mix the Cl-I system, you never get any hybrid emission. This is because the swaps between Cl and I put stress on the structure of the perovskite lattice. For a halide swap to go well, you need a group of different halogen compounds.
Post-synthesis Defect Recovery
It is possible for perovskite nanocrystals (PNCs) to lose halides, which can damage the crystal’s structure and turn off its photoluminescence quantum yield (PLQY). Halide spaces happen during the making and/or cleaning step. CsPbI3 perovskite PNCs that give off red light almost keep their PLQY after the purification step, but CsPbBr3 PNCs lose it noticeably, and CsPbCl3 PNCs have poor PLQY even before the purification step, especially CsPbCl3.
CsPbCl3 has a low PLQY because it has deep trap states in its big bandgap. Other PNCs with Br or I have smaller bandgaps, which means they have shallow traps. These flaws help nonradiative recombination happen, which lowers PLQY. In contrast, a lack of halides in CsPbBr3 and CsPbI3 leads to the formation of shallow-defect states. In these states, electrons are only slightly localised and holes are spread out in the bandgap because their bandgap isn’t as wide.
Several post-synthesis processes are used to make up for the high number of halide flaws in PNCs in order to improve their photophysical properties. Metal halides or nonmetal halides are one way to treat this. When metal chlorides or nonmetal chlorides are added to CsPbCl3 PNCs, the PL intensity rises sharply up to several orders of magnitude. This fills up deep trap states, stops the nonradiative recombination channel, and speeds up the radiative recombination pathway.
As the nanoplatelet thickness (MLs) decreases in an experiment with quantum-confined CsPbBr3 nanoplatelets, PLQY goes down. This is made better by adding PbBr2 after the nanoplatelets have been made.
Different Shapes of the Nanocrystals
Different growth rates and reaction conditions can cause colloidal PNCs (PNCs) to crystallise in different forms. CsPbX3 PNCs usually take the shape of 3D cubes, 2D pebbles, 1D nanorods, and 0D circular dots. It is possible to make nanocrystals with different forms by changing things like binding ligands, B-site doping of metal ions, heat processing, and temperature tuning. These methods can cause nanocrystals to form in a variety of shapes.
Shape-controlling Reaction Parameters
Temperature
When using the HI method to make PNCs, temperature is very important. The reaction temperature for CsPbBr3 nanocubes is usually between 140 and 200 °C. The mean size of the nanocubes gets smaller when the temperature goes down, but nanoplatelets form when the growth is uneven. In the 90–130 °C range, nanoplatelets change their thickness instead of their length or width. Nanoplatelets and nanocubes can have their edge length and width changed by changing the temperature. It is also necessary to get 3D nanocubes and PNCs with less than three dimensions.
Annealing Time
An annealing process changes the physical features of an object by heating it to a high temperature. When you anneal the CsPbX3 NCs solution at 150–200 °C for 720–960 minutes, you get nanowires with a high aspect ratio. In this process, a Cs-precursor is injected into a Pb-halide solution, which quickly forms nanocubes. When the heat is applied again, smaller cubes stick together and high aspect ratio nanowires are made. Smaller nanocrystals get bigger through Ostwald ripening when they are heated. When oleylamine is used as a fluid to make nanowires, they can only grow in one direction.
Role of Capping-ligand
Creating PNCs colloidally involves using oleylamine and oleic acid as capping ligands to help nanoparticles crystallise. Making CsPbBr3 at lower temperatures lets you make different shapes, such as 0D dots, 2D nanoplatelets, and 2D nanosheets that lie flat. Changing the amount of capping ligands also helps shape modification while keeping other reaction parameters the same. When oleylamine is switched out for octylamine, the capping ligand’s chain length is shortened. This makes nanoplatelets instead of nanocubes.
Doping in Perovskite Nanocrystals
In perovskites, doping means adding a new alien atom to replace an old element. We looked at it in three different halide perovskites sites (A, B, and X-site) to find out about new photovoltaic features, flaw healing, and phase stability. In A-site doping, alkali metal ions and organic alkylammonium or amidinium are used. In X-site doping, halide ions are replaced with thiocyanate (SCN), which is a pseudohalide. When dopant ions are added to an A- or X-site, they have less of an effect than when they are added to a B-site. It is talked about how to make B-site doped PNCs, how they affect optoelectronics and phase stability, and how to make them.
Mn2+ Doping
Mn doping in high band gap semiconductors happens when excitonic energy moves to the Mn(II) dopant state. This creates a broad PL with a millisecond-order lifetime and short-range emission that can be tuned. For forced Mn doping in perovskite semiconductors to work, the host semiconductor must have a high bandgap so that it can efficiently pass excitonic energy to the Mn(II) dopant. It is only possible for CsPbCl3, CsPb(Cl/Br)3, and quantum-confined CsPbBr3 NCs to give off Mn dopant properly. But adding Mn(II) can also be done successfully in bulk-CsPbBr3 and CsPbI3 NCs, which only offer longer stability and don’t give off any light linked to the Mn(II) dopant.
The Mn(II) dopant changes the PNCs by adding new light emission from the dopant centres, making the PNCs more stable, and reducing the average particle size. Mn(II) coordinates with halides octahedrally (MnX6) to form a lattice that is the same as PbX6 octahedra. When smaller [MnX6] is switched out for [PbX6], it affects how the grid shrinks and how small the average particle is. The amount of Mn2+ in the lattice affects how much of the dopant release can be tuned. The bathochromic change of dopant emission happens as the concentration of dopant ions rises. When there is too much dopant in CsPb(Mn)Cl3 PNCs, there is a small change in the excitonic emission that can be seen.
Lanthanide Doping
Lanthanides, which are like Mn-dopant, can give off a specific kind of dopant emission from Perovskite Nanocrystals (PNCs). The octahedral coordination (LnX6) that lanthanide halides can offer can also be used instead of the PbX6 octahedra. Lanthanide dopants transfer excitonic energy during the f–f transition, which is where the release comes from. Figure 3.13c,d shows the absorption and emission of a group of Ln3+ doped CsPbCl3 PNCs. The absorption comes only from the host excitonic transition, with no dopant-related absorption. The emission comes from both the host excitonic transition and the f–f transition caused by the lanthanide dopants. This finding shows that Ln3+ doped CsPbCl3 PNCs emit lanthanide dopants in a way that is sensitive to the host. Yb3+ is the best lanthanide dopant for the quantum cutting effect because it emits near 990 nm (absorption start 1.25 eV, more than twice the host exciton’s 2.95 eV) and gives close to 200% more PLQY.
Other B-site Dopants
It has been shown that dopant ions like Mn(II) and lanthanide can successfully dope the Pb-site, making CsPbI3 more stable and supporting the α-phase. CsPbBr3 is also more stable in normal settings and works better as a solar material. Adding Cu(II), Ni(II), and Cd(II) to the B-site has been shown to improve PQY, but it is still very low compared to the one that isn’t doped. This is another good way to get back flaws that are deep in high-bandgap CsPbCl3 NCs. Adding Sr(II) as a doping level can help lower Schottky flaws in the perovskite structure, making it easier for iodide vacancies to be filled and improving the dynamics of radiative recombination. To make doped perovskite PNCs, you can use standard manufacturing methods, which can be done in situ or after the fact.
Postsynthesis Doping
Postsynthesis halide exchange of perovskite nanocrystals (PNCs) is a simple way to make a reaction happen because the PCNs are soft and have low formation energy. When a metal halide is added to PNCs that have already been made, halide swaps happen, and Pb is replaced with the matching foreign metal ions. This method, called halide exchange-driven cation exchange (HEDCE), lets preferred dopant metals be added after the fact. The halide exchange, which helps the metal ions be replaced with Pb, is what makes this post-synthesis doping possible. A lot of research has been done on this way to dope Mn(II) after the fact. In this method, MnCl2 is added to CsPbBr3, which emits green light. This changes the excitonic emission to blue light right away because a mixed halide perovskite forms so quickly. The Mn d–d release starts to show up after a few minutes to a couple of hours and gets stronger over time. Post-synthesis doping is not just looked at for Mn dopants; it is also looked at for Yb3+, Cd2+, and Ni2+. It’s not clear or well-studied whether this method can be used for all other metal halides to add to the B-site, and this is still an open question for the future.
Lead-free Perovskite Nanocrystals
Because of worries about the safety of Pb-based PNCs and the need for better options, the market for lead-free perovskite has grown. Some alternatives to Pb(II) are Sn(II), Sn(IV), Ge(II), Bi(III), In(III), Ag(I), Cu(I), and Te(IV). These can be used as low- or non-toxic B-site replacements, creating all-inorganic or mixed organic-inorganic metal halide perovskites. We are putting different perovskite and perovskite-inspired structures into groups based on how they are put together and how many dimensions they have. These groups include 3D ABX3, A2BB′X6, double-halide perovskite, 0D A2BX6, vacancy-ordered double perovskite, and A3B2X9.
Classifications According to the Structure and Compositions
Lead-free PNCs (LF-PNCs) with 3D and 0D shapes are shown in Figure 3.14. AsnX3 types have a straight bandgap and don’t emit much from one band to another. On the other hand, lead-free double halide perovskites and 0D materials emit a lot but come from excitons that are trapped within themselves. These materials are made using both HI and LARP methods. CsSnX3 PNCs need HI methods because the reaction conditions need to be changed. Trictylphosphene is a slightly reducing agent that needs to be used to dissolve SnX2 salts instead of long-chained acids and amines. Oleylamine and oleic acid are often used as capping ligands by other PNCs. Sn-based perovskites are unstable in normal air because they break down and oxidise quickly. When mixed with higher valence B-site metal ions, double halide perovskites and vacancy-ordered double halide perovskites are more stable in normal circumstances.
Challenges of the Lead-free Perovskites
It has been found that Sn(II)-based perovskites are unstable in normal settings, which limits how they can be used. Using ligand engineering with perfluorooctanoic acid (PFOA), which gives strong F- to combine with Sn2+ and pull electrons away, has helped solve part of the problem. The perovskite nanocrystals are more stable because the PFOA group blocks their surface.
Theoretical studies suggest that a group of lead-free materials will be screened with a mix of different A-site cations, B-site metals, and halides. However, some of these options can’t happen during PNC formation because the process for formation is hard to work with ligands present.
LF-PNCs have low PLQY compared to bulk perovskites. This is because the halide ions on the surface of the nanocrystals are not well aligned, which increases the amount of nonradiative recombinations. So, LF-PNCs are still a little behind LHP nanocrystals when it comes to optical uses.
Summary
This chapter talks about the steps used to make PNCs, focussing on their different physical traits, the science behind their creation, how surface flaws are made, and how they can be fixed. It shows how temperature affects the size and shape of PNCs and how important capping ligands and their ratios are for understanding morphology. Cleaning up colloidal PNCs gets rid of extra byproducts and unreacted ligands, but it also leaves halide gaps on the surface, which creates defect states and stops emission PNCs. A lot of people are interested in postsynthesis chemical treatment in this field. This chapter also talks about how to make different metal ions doped PNCs, with a focus on replacing Pb(II) with other dopant metals to improve optical properties and lower the amount of harmful Pb. Finally, a quick look at Pb-free perovskites is given. These are a hot topic in halide perovskite because they are seen as more environmentally friendly than Pb-based perovskites.
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