This part talks about mixed perovskite nanomaterials, which are different from all-inorganic perovskite production and have different features and stable behaviour.
Two-dimensional hybrid perovskite nanorods
This part talks about how 2D OIP nanorods (NRs) are made and what their features are. These NRs are made at a low temperature of about 80°C, and their chemical make-up is based on the formula (C8H17NH3)2(CH3NH3)2Pb3(IxBr1–x)10,0> x > 1. XRD, ED, and FFT analyses were used to describe the NRs.
The TEM images of the 2D perovskite NRs with the structure R2(MA)n−1MnX3n+1 show that the halide makeup did not change the NRs’ size or shape. NRs are 2.25 nm (plus or minus 0.3 nm) wide and 11.36 nm (plus or minus 2.4 nm) long on average. The structure of NRs changes when Br is added. The lattice goes from having tetragonal parameters to having a 2D perovskite structure mixed with a 3D tetragonal structure.
More details below 14° can be seen in the XRD pattern of these NRs. These details are typical of the 2D perovskite structure. This is where the peaks come from when the X-rays hit the (00n) faces of the 2D perovskite crystal and bent them. The different NR compositions have a lattice parameter “c” that is bigger than the tetragonal “c.” This proves that the NRs are two-dimensional.
On the 2D hybrid perovskite NRs, absorbance and PL readings were made. It was seen that the full iodide and full bromide NRs can only receive light up to 650 and 530 nm, respectively. The main reason for this difference in absorption is the use of octylammonium as a ligand in the creation of NRs, which is bigger than the MA. The crystal growth is limited by the octylammonium attaching to the NRs surface through its alkyl chain. This makes the perovskite grow in a 2D structure, which changes the absorbance and PL to shorter wavelengths (higher energies) and larger bandgaps as a result.
This second reason for the change in the optical features has to do with the halide exchange, which involves bromide ions entering the (OA)2(MA)2Pb3I10 NRs and iodide ions leaving them. It is the orbitals of Br(4p) and Pb(6s) that decide how much energy the absorption peak has. The PbI2 conduction band is mostly made up of Pb(6p) orbitals, and its VB is made up of Pb(6s) orbitals and I(5p) orbitals.
Tauc plot values were used to figure out the bandgap of different NR mixtures. The Eg numbers are shown in Figure 25(d) in red. The Eg of the material goes up as the Br/I ratio goes up. Because of the changes in size, the bandgap of these NRs is between 1.9 and 2.26 eV.
The PL-QY and bandgap energies of the perovskite NR compositions were measured and figured out using the Tauc plot. The two-dimensional hybrid perovskite nanorods have a PL-QY of almost 30%, which is a lot higher than the PL-QY of the bulk MAPbX3 perovskites.
Role of the ligands
The addition of organic molecules to the manufacturing process changes how nanorods (NRs) form. Some of the organic chemicals that were used in this process are MA iodide/bromide (MAI/MABr), OAI, and OAc. Because OAI can’t become part of the perovskite crystal, it sticks to certain spots on the surface of the perovskite, stopping growing in a certain direction. Another important thing that helps make these 2D NRs is OAc.
Several amounts of OAc to OAI were looked at to see how ligands affect the creation of 2D NRs. When 100% OAc was used, no QDs were seen, which means that the QDs alone can’t keep the structure stable. But when 100% OAI was used, QDs were made, which means that OAI is enough to keep their growth stable. Iodide in the OAI helps this ligand stick to the surface of the QDs/NRs. This process is known as “chemisorption.” When it comes to OAc, the relationship is physisorption, which means that the particles and OAc stick to each other physically.
It is possible for QDs and NRs to form when the ligand ratios are different, like when the ratio is 0.250 (more OAI than the standard) or 0.075 (more OAc than the standard). At a frequency of 0.075, it was rare for NRs to form. We can say that the OAI is connected to the surface of NRs and that the OAc plays a key role in turning these QDs into NRs. Because it fills the octahedral hole, the OAI sticks to the surface of NRs much better than the OAc.
It is necessary for both ligands to be present for NRs to form. The OAI sticks to the perovskite surface better than the OAc, which only sticks to the NRs’ surface physically. So, pure NRs were made in a certain ratio of OAI/OAc = 0.186. This was enough to keep the structure stable and direct the growth of NRs at the same time.
Summary
The study shows how to make 2D hybrid perovskite nanostructures (C8H17NH3)2(CH3NH3)2Pb3(IxBr1–x)10, where 0< x > 1 and antisolvents are used at low temperatures. Changing the halides can change the optical qualities. A good size distribution and strong PL were seen. The NRs have a flat shape because OAI is used as the ligand. The bigger bandgap seen in these NRs is because they are made up of two dimensions. XRD, ED, and FFT show that these NRs have a crystalline structure. It is looked into what role ligands play in making these NRs, with OAI being a key part of the process.
The effect of the alkylammonium ligands
length on OIP NPs
This study looks into how the length of the alkylammonium cations affects the visual and physical features of OIP nanoparticles (NPs). As partners, three alkylammonium cations were looked at in this study: octyl, dodecyl, and octadecyl ammonium. The same alkylammonium cations were used to compare the features of NPs and 2D perovskite films. These alkylammonium cations work as ligands for NPs, but as barrier molecules for 2D perovskite thin films.
Octylammonium (C8), dodecyl (C12), and octadecyl ammonium (C18) were used as ligands in this work. The three linear alkylammonium cations were used to make NPs that were based on both bromide and iodide. The OIP NPs were made in a normal atmosphere by adding lead halide (PbI2 or PbBr2) and methylammonium halide (MAI or MABr) solutions in DMF to a hot medium (80 °C) that had ODE, OAc, and alkylammonium halide (C8-I, C8-Br, C12-I, C12-Br, or C18-I, C18-Br) while stirring very quickly. NPs were made when chloroform was added, and when the dispersions were centrifuged, the heavy by-products settled to the bottom, leaving behind clear dispersions.
In Figures 27(b) and 28(a)–(f), the size of the NPs was found to depend on the length of the alkylammonium. It is clear that there is a link between the length of the alkylammonium and the average size of the NPs getting bigger. It’s also interesting to note that when iodie-C8 was used instead of bromide-C8, the shapes of the particles changed (NRs vs. nanocubes), which could be because the surfaces had different amounts of energy. Longer alkyl groups have stronger van der Waals (VDW) bonds than short ones. The surface energy of the NPs changes as the length of the ligand does. In this case, γC18 > γC12 > γC8. So, the NP surface-to-volume ratio is low when the surface energy is high. This is why NPs made from a long alkylammonium are bigger than those made from a short alkylammonium.
The absorption and PL spectra of bromide- and iodide-based NPs with various alkylammonium lengths are shown in Figure 29(a) and (d), and (b) and (e), respectively. Figure 29(c) and (f) show how the NPs glow under ultraviolet (UV) light.
What makes bromide and iodide NPs optically different is that their p orbitals are different (4p for bromide and 5p for iodide), which we’ve already talked about. Because these NPs are so small, they have a confinement effect that makes the absorption start and PL maximum move to higher energies than they would be in the main material [2]. Because of the strong ionic interaction between the ammonium group and the solid octahedra, the alkylammonium ligands have a big effect on the electronic structure of the OIP NPs. There is a big difference between the dielectric constants of the mineral part and the organic part (alkylammonium). This makes the bandgap energy of the perovskite go up. Changing the length of the alkylammonium atom also changed the optical properties (absorption onset and the PL maximum). This might be because of three things: (1) the changing dielectric constant of the ligands; (2) the angle between the ligand and the perovskite surface, which determines how much the crystalline structure is distorted; and (3) the strength of the VDW interactions between nearby alkylammonium ligands on the surface of NPs.
The study ends with some interesting information about how alkylammonium cations change the visual and physical features of OIP nanoparticles. Understanding how chemicals interact with the perovskite will help researchers make NPs that work better and faster in a variety of situations.
Study of the VDW interactions
The study looks at the optical features of hybrid perovskite nanostructures made of 2D perovskite films with different lengths of alkylammonium ligands. Even though these films all have the same n-value, their absorption bands show a range of peak numbers and levels of dominance. 2D perovskite films have different visual qualities for n-values that are close to each other. This is because the lengths of the alkylammonium molecules change.
Because they combine with VDW, the alkylammonium cation changes how the perovskite layers are put together. The pXRD patterns of 2D perovskite films show that as the length of the alkylammonium rises, the d001 value grows. This makes the angle of the (001) diffraction peak move towards low angles. This relationship was seen when n = 1 and n = 3 layers of C8-Br were used. Weak 3D perovskite peaks were seen in layers with n > 1. These were at 14–15 and 28–29, where the artificial parts are spaced out and reflect X-ray photons.
The diffractogram shows a single 2D perovskite crystal structure, with a single set of (00n) peaks. These peaks are caused by X-rays reflecting off of integers in the (001) plane. But for n > 1, there were several sets of peaks instead of just one. These sets were valid for both halides (Br− and I−) and all lengths of alkylammonium (C8, C12, and C18). The short alkylammonium has these points that stand out more than the long alkylammonium.
When long alkylammonium cations are used, n = 1 perovskite structures tend to form. On the other hand, n > 1 perovskite structures are not wanted to form. These findings help us understand why the absorption start point of small NPs is moved towards shorter wavelengths. Short ligands make NPs with a lot of surface area to volume because of weak VDW interactions between ligands that are close to each other. Many interactions between alkylammonium and bromide have a big effect on the electronic structure and make the bandgap bigger.
Summary
The study shows how to make OIP nanoparticles using different lengths of alkylammonium and focusses on their visual, physical, and structural features. The research discovered that VDW interactions have a big effect on how nanoparticles form, their size, and their visual qualities. The researchers used the same alkylammonium ligands as building blocks to compare thin films and NPs. The main reason for the changes in the nanoparticles’ visual and physical features was their interactions with VDW. PL-QY of the nanoparticles ranged from 30% to 60%, based on the length of the halide and alkylammonium ligands. For future photonic uses, it is very important to know how the length of an alkylammonium molecule affects the qualities of nanoparticles.
Summary and outlook
This book talks about how to make all-inorganic and mixed perovskite nanomaterials, with a focus on how they grow quickly, how to change their shape and size, and how to change their chemical make-up. It shows how to make Cs-based nanoparticles that can be changed by exchanging halides. The second part shows how to use a smaller artificial cation (Rb+) in NPs to absorb UV light and change their optical qualities to work in the visible and near-infrared ranges. The bandgap can change when the size of the solid cation changes. This is because the Pb–X–Pb angle changes.
The third part talks about hybrid nanostructures, with a focus on making and studying NR-based perovskite with different unit cell thicknesses and good size distribution. The chemicals in hybrid perovskite nanoparticles have a big effect on how stable and what shape they have. A link was found between 2D perovskite films and hybrid perovskite NPs that used the same alkylammonium cations as ligands to control the NPs’ size and shape.
The semiconductor OIP opens up an interesting area in nanoscale materials. This is different from semiconductor nanoparticles, where manufacturing and optical perovskite are caused by the quantum size effect. The chemistry make-up determines the qualities, and making these nanoparticles is now easier and faster. Perovskite nanoparticles have a high PL-QY, which means they can be used in optoelectronic devices that work well. However, many basic features have not been studied yet, which puts these all-inorganic and mixed perovskite nanoparticles at the cutting edge of science.
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