A lot of research has been done on lead halide perovskite materials because they are so good at solar (PV) action. Recently, the nanosize of lead halide perovskite has grown into a new field of its own in the field of nanoscale materials. Perovskite is made up of the formula AMX3, which allows for a wide range of compositions, from blends of organic and inorganic materials to all-inorganic perovskites. These are usually called methylammonium (MA) lead halide (MAPbX3) or caesium lead halide (CsPbX3).
Chemically changing the perovskite is very important because it lets you tune the bandgap and change the qualities to fit a certain use. It also makes it possible to stay away from harmful substances and make fake paths better. Today, perovskite-based solar cells work more than 25% of the time using a mixed-cation perovskite makeup. This shows how important it is to chemically change perovskites.
At the subatomic level, mixed-halide systems are used instead of metal halide perovskite, which is still behind. Protesescue et al. wrote about perovskite NPs that had FA mixed with Cs and gave off near-infrared light. They also wrote about a mixed metal cation system called CsPbxMn1–xCl3, which gave us new ways to change the optical features of perovskite NPs.
The Goldschmidt tolerance factor (TF) is used to guess if a perovskite structure will form. This is because the electronic structure and structural flaws of NPs are what mostly change their visual qualities. The temperature and growing phase of the NPs have a big effect on their shape and size after they are injected.
NPs’ form and size are also greatly affected by liquid ligands. When making mixed organic–inorganic perovskite (OIP) nanoparticles, the solvent/anti-solvent mix is often used to start the process of making the NPs. Researchers Vybornyi et al. and Protesescu et al. found that MAPbX3 and FA lead bromide (PbBr2) cubic nanocrystals (NCs) could be made using a hot injection method, but this method needs a high temperature and an inert gas atmosphere, which makes it harder to use.
In hybrid OIP nanoparticles, alkylammonium cations stabilise the surface of the perovskite nanoparticles, especially on the (00n) faces. These are the sides that have negatively charged halides that are visible and attract the ammonium cations electrostatically.
Kinetics – how to control the size distribution
of Cs-based perovskite NPs
This chapter looks at how the length of growth affects the size distribution of perovskite nanoparticles (NPs). We made CsPbX3 NPs and studied them at four different growth times: 1, 4, 20, and 40 seconds. The study wants to look at how the distribution focusses and defocuses after the main growth stage, which lasts for about one to three seconds.
Synthesis of CsPbBr3 NPs
The study makes CsPbBr3 nanoparticles over four different growth times, showing that their absorption peak and photoluminescence peak are similar. Its absorption and PL are the same for CsPbBr3 NPs that grow over 4 seconds as they are for bulk CsPbBr3 that is put down in a single step. There isn’t much difference between the absorption of NPs and bulk. The NPs have slightly redshifted spectra because they are weakly quantum confined. It looks like the electron diffraction pattern of the CsPbBr3 NPs fits the cubic structure of CsPbBr3, and X-ray diffraction backs this up even more. It is the same as the cubic structure of CsPbBr3, and the XRD (X-ray diffraction) of the CsPbBr3 nanoparticles backs up this structure.
Tracking the size distribution of CsPbBr3 NPs
The research is mostly about the size range of cubic CsPbBr3 nanoparticles (NPs) that grow for 1 to 40 seconds at a time. The synthesis factors stayed the same, but the growing time changed. The NPs that were made had an average side length of 9.0 nm after 1 second of growth, 9.6 nm after 4 seconds, 8.4 nm after 20 seconds, and 9.3 nm after 40 seconds. To look at the NPs, high-resolution transmission electron microscopy (HR-TEM) readings were made. The findings showed that the FWHM of the size ranges got bigger as the growth time went up. This is a sign of a defocusing process called Ostwald ripening.
Synthesis of CsPbI3 NPs
The research is mostly about the size distribution of CsPbX3 nanoparticles (NPs) that were made over four different growth times: 1, 4, 20, and 40 seconds. The NPs have a similar absorbance peak and FWHM of 37 nm. Their PL-QY is between 75-77%. The PL peak is in the 695-705 nm range. Figures 4 and 5 show the NPs that were studied after a 4-s growth period. They show absorbance and PL readings, as well as HR-TEM, crystal fringes, and XRD. The ED and d-spacing support the needle-like shape of the NPs.
Tracking the size distribution of CsPbI3 NPs
All other manufactured factors are kept the same, and the study is only interested in how the growth time changes from 1 to 40 seconds. After the tests, cube-shaped CsPbI3 nanoparticles were made. Their average side length was 13.40 nm for growth of 1 second, 10.5 nm for growth of 4 seconds, 7.5 nm for growth of 20 seconds, and 13.46 nm for growth of 40 seconds. We looked at the size distribution using HR-TEM data, which showed the CsPbI3 NPs at different growth times and the size distribution histograms that went with them. The FWHM goes down over 4 and 20 s of growth and then back up over 40 s of growth, which means that the image defocuses, then focusses, then defocuses, and so on. This means that kinetics can be used to change the size distribution of Cs-based perovskite NPs.
Discussion
The sizes of CsPbBr3 and CsPbI3 are not all the same. In CsPbBr3, the size range gets wider between 1 and 40 seconds of growth time. In CsPbI3, it gets smaller between 1 and 20 seconds and bigger between 20 and 40 seconds. Smaller particles dissolve into the monomer pool, while bigger particles grow at a rate that depends on how big they are. The particles closest to the critical radius grow the fastest and “catch up” to the bigger particles. This brings more particles in the same size range, which narrows the distribution. Figure 7 shows the growth rate that changes with size. In a high monomer concentration environment, most particles are the same size after a burst of nucleation, a phase of distribution narrowing (focussing), and a phase of distribution spreading (defocusing). This is because the critical radius is inversely linked to the phase of distribution narrowing.
Ostwald ripening
When growth uses up all the monomers, the critical radius grows bigger than usual. This makes a lot of the population below the critical radius dissolve and add monomers to bigger particles that are still growing. When the lens is focused, most of the particles grow quickly enough to catch up to the bigger particles in size. When the lens is defocused, most of the particles shrink or disappear, but some grow, making the distribution wider. Kinetically, small particles are more likely to form, while thermodynamically, big particles are more likely to form. However, small particles have a higher surface-to-volume ratio than big particles. This means that molecules inside the particles are more stable than the small particles.
The focussing and defocusing processes are different for CsPbI3 and CsPbBr3. This is because of the different amounts of monomers. The percentage of monomers is inversely related to the critical radius, and most of the growth of CsPbX3 nanostructures happens in the first one to three seconds. The diffusion coefficient for Br is higher than that for I, and the temperature at which CsPbBr3 NPs are made is higher (170°C) than that for CsPbI3, which suggests that the molecules move through the system more quickly. After 4 seconds of growth, there are fewer monomers in CsPbBr3 than in CsPbI3. This makes the critical radius bigger, which causes the population to become less focused.
Summary
This chapter talks about the features of CsPbX3 nanoparticles (NPs) and how they move by watching how long they grow. TEM was used to look at the size distribution, which let the NP size distribution be focused and defocused. In CsPbI3, focussing happened after 20 seconds of growth time, and defocusing happened after more growth time. CsPbBr3, on the other hand, did not show focussing. This difference is mostly due to the fact that the monomer content drops faster in CsPbBr3 than in CsPbI3.
There are a lot of particles of the mode size in the reaction jar, and there are a lot of monomers. Small particles below the critical radius break apart, while particles of the mode size grow quickly, which causes them to focus. As the monomer content drops because of growth, the critical radius rises. This makes mode-sized particles dissolve while large particles keep growing, which causes defocusing. Particles below the critical radius break apart over time, while large particles get bigger, making the positive skew stand out even more.
For CsPbI3, the right growth time is needed to get a small size range. For CsPbBr3, a higher monomer content is probably a good thing. For future use in light uses, it is important to understand these processes.
Tuning the length and optical properties
of perovskite NWs
This chapter talks about how to make CsPbX3 NWs, where X can be Cl, Br, or I. The process shows that the visual features and length of NWs can be changed. Hydrohalic acids (HXs) were added during the process, which made NWs shorter and changed how they looked. The characterisation covers the molecular, visual, and physical qualities, and it also talks about how these NWs are made.
Synthesis of CsPbX3 NWs
To make CsPbX3 nanostructures (NWs), hot solutions of cesium-oleate and PbBr2 precursors had to be made. Using caesium carbonate, oleic acid, and 1-octadecene in a neutral environment, CsO3 was made, and PbBr2 was made in dimethylformamide (DMF). At room temperature, hot Cs-oleate was added to a bottle that already had ODE, OAc, oleylamine, and a different amount of HX acid in it. The bottle was filled with a PbBr2 precursor solution. After 10 seconds, acetone was added to stop the reaction, which created CsPbBr3 NWs that could stand on their own.
It was possible to make the base CsPbBr3 NWs without adding any acid. The TEM pictures showed nanoparticles that were micron-sized and not symmetrical. The width of these structures was about 3 nm. The NWs that were found were very thin, with an average width of about 3.3 nm. We saw the emission from CsPbBr3 NWs spread out in hexane under UV light. The PL of the CsPbBr3 NWs was around 475 nm, which is bluer than the emission peak of the cubic CsPbBr3 NPs, which was around 519 nm. For this blueshift to happen, the NPs need to be quantum-confined.
When the CsPbBr3 NWs were made, the XRD pattern showed that the crystal structure of CsPbX3 was orthorhombic, which was confirmed by the low-temperature synthesis. By making the crystallisation process stronger in the orthorhombic phase, the length and visual features of perovskite NWs could be tuned. The (004) plane had a strong signal and a sharp peak, which shows that NWs have an uneven shape.
Addition of hydrohalic acid (HX, X = Cl, Br, I)
The study is mostly about making CsPbX3 nanostructures by adding different amounts of hydrogen peroxide (HX) one after the other. The TEM pictures show that NWs kept their form and appearance as the amount of HX increased, and with almost every increase, the lengths got shorter. NWs, on the other hand, kept their width of about 3 nm.
There is a small difference between the three HX-based NWs in their tendency to shorten, which is probably because they are different amounts of acidic. When the environment is very acidic, OLA ligands become more positively charged and form oleyl-ammonium cations. These cations randomly bind to Cs+ ions in the lattice. This stops the growth of crystals, making a passivation layer that stops any more crystallisation.
An acid’s strength is based on its equilibrium constant, and HX acids are strong acids. When compared to HCl, HI and HBr are likely to shorten the chain bigger. But because different amounts of the acids were used and the shrinking effect of each acid was looked at separately, this assumption doesn’t always match what was seen.
Adding more HCl changes the shape of NWs, which makes sense because HCl is acidic. If it gets more acidic, it might change other parts of the NWS, making its form more like a plate. Figure 10 shows how the passivating action of oleyl-ammonium cations is very random.
When you add HBr, you get fewer side effects than when you add HCl. This might be because two identical Br− ions spontaneously swap halides. This swap of ions doesn’t damage the crystal structure as much as the ones with HCl and HI.
XRD of the new NWs shows that adding HCl and HI made the lengths of the NWs more evenly spread out.
The study looks into the visual features of perovskite nanostructures (NWs) that have been formed on highly orientated pyrolytic graphite (HOPG). There is a 3 nm height difference between the X-ray diffraction (XRD) patterns of CsPbBr3 NWs with different amounts of hydrohalic acids. These patterns include the orthorhombic standard pattern of CsPbBr3. It is thought that TEM pictures showed a single NW, but different scan areas showed several NWs of the same height.
If you look at the peaks of the orthorhombic CsPbBr3 crystal structure, you can see that HI-based NWs move to the left at higher angles, while HCl-based NWs move to the right at higher angles. This change can be explained by the size of the halide and the room it takes up in the lattice. When some Br− anions are switched out for Cl− anions, the lattice can get smaller. When Br− anions are switched out for I− anions, the lattice can get bigger.
As was already said, the XRD bands show an extra phase that is part of the Cs4PbBr6 phase. This phase got smaller when HCl was present and bigger when HI was present. This might be because the crystal structures of the swapped crystal and the Cs4PbBr6 phase didn’t match or match up.
To find out how tall the NWs were, atomic force microscope (AFM) measurements were made. Long structures measuring micrometres in length were seen when the AFM was scanned. The shortest structure had a uniform height of about 3 nm. Higher formations can be made up of groups of NWs or a single NW wrapped in organic matter.
The large blueshift in this case is due to the NWs being confined in a quantum way, as shown by optical studies. In some cases, two peaks can be seen in the PL spectrum. These peaks show two possible populations: one is NWs, and the other could be cubic-shaped CsPbBr3 NPs whose emission is redshifted.
Discussion
The study shows how hydrogen peroxide (HX) changes the length of nanowafers (NWs) in a perovskite crystal. With the addition of HX, the reaction solution becomes more acidic, which changes the OLA ligands into oleyl-ammonium cations that act like Cs+. In the process of CsPbX3 crystallisation, the OLA molecules fight with Cs+ to form a passivation layer on the rising surface of the NWs. As the NWs grow, competition takes place on their active surface, and oleyl-ammonium ligands form a passivation layer.
In a normal process, a polar liquid is added, and then Cs+, Pb2+, X−, and oleyl-ammonium cation crystallise into NW structures. This makes NWs precipitate. In this case, though, some of the Cs+ places on the growing surface are taken by oleyl-ammonium cations, which stops more growth in the way that the long oleic chain wants it to go. When Cs+ and oleyl-ammonium cations compete with each other and there is a lot of HX, random short NWs and length passivation effects happen.
Summary
It has been explained how to make Cs-based nanowires (NWs) and how adding hydrogen peroxide (HX) can change their length and visual qualities. TEM and AFM tests show that the synthesised NWs have a narrow size distribution of about 3 nm and two quantum-confined dimensions. In two ways, HXs change NWs: by adding more acid, they make them shorter, and through halide exchange reactions, they change the structure and visual features of the NWs. NWs’ traits depend on how acidic their HXs are because they can help amines lose their proton. Because the described NWs are not symmetrical, they can grow another semiconductor by epitaxially growing two or more semiconducting materials. This creates energy transport systems that take into account the visual and physical properties of these nanocomposites. This makes it possible to use NWs in optical settings.
Changing the A site in perovskite nanostructures
Because it is small, Cs+ is the most common artificial cation that can fit in the “A” spot of a perovskite structure. All-inorganic perovskites, on the other hand, can use a new element called Rb+. Rb+ is a little bit smaller than Cs+, so it can’t really form a perovskite structure. This part shows how Rb-based nanoparticles were made and what they are made of for the first time.
Rubidium lead chloride nanocrystals: synthesis and characterization
Two starting fluids were used to make rubidium lead halide nanoclusters (NCs): Rb2CO3, OAc, and ODE; and lead chloride (PbCl2), OAc, OLA, tri-n-octylphosphine (TOP), and ODE. The precursor solution for Rb-oleate was added to the precursor solution for PbCl2. An ice bath was then used to stop the process. The result was mixed with isopropanol twice to make a white residue, and then it was mixed again with hexane to learn more about it.
As seen in the HR-TEM pictures, the combination made NCs that look like squares. The stability of these NCs is very high; absorption spectra were taken on the day they were made and again after about 4 months of storage at room temperature. It is thought that these NCs have a bandgap of 4.05 eV.
The NCs that were obtained have light activity in the UV range. The density functional theory gives us 3.01 eV for the bulk crystal’s bandgap, while the pseudopotential self-interaction adjustment method gives us 4.13 eV. Based on more than 600 NCs, the size distribution graph shows that the average side length is 11.29 ± 0.04 nm.
The effective masses of holes and electrons, calculated from the band structure, are 0.53 and 0.19 of the electron mass (me), respectively. These masses are averaged over the crystal directions at the valence band top (VBT) and the conduction band minimum (CBM). Based on these numbers and the calculated dielectric function, which had a static value of ε(0)/ε vacc = 4.1, we found that the exciton Bohr radius is a0 = 1.584 nm and its binding energy is Eb = 0.111 eV. The numbers that were found for Rb6Pb5Cl16 and the numbers that were found for CsPbX3 compounds are very similar.
The study is mostly about measuring rubidium lead chloride nanoparticles (NCs) and the black spots that go with them using XRD. As seen in the XRD pattern, the Rb6Pb5Cl16 tetragonal phase is not a perovskite phase. Monzel et al. found this phase while working to redetermine the phase diagram of RbCl/PbCl2. At room temperature, the perovskite phase RbPbCl3 can only exist as a mix of two phases that were not known before: Rb6Pb5Cl16 and RbPb2Cl5. RbPbCl3 forms a tetragonal phase when heated above 320 °C. When the temperature goes up, the RbPbCl3 perovskite changes into the cubic phase.
A repeated XRD test of the NCs found that the Rb6Pb5Cl16 NCs had 90.2% Rb6Pb5Cl16 phase and 9.8% Pb2O3 phase. The black spots in the NCs can be linked to the Pb2O3 phase. The difference in TEM pictures between grey and black areas shows elements with a lot of electrons, which is the amount of Pb in the NCs.
We used dark scanning transmission electron microscopy (STEM) and energydispersive X-ray spectroscopy (EDS) to confirm what the NCs are made of and what the black spots inside them are. The atoms Rb, Pb, Cl, and O were found in the area of interest by EDS testing. Through quantification research, it was found that the atoms made up 17.40, 28.89, 37.62, and 16.07 percent of the whole. The ratio of Rb to Pb is about 1:1, which means that the remaining Pb atoms may be linked to the O atoms that were found to make a lead oxide phase or, alternatively, metallic lead nanoparticles.
A black spot in the NC has a fast Fourier transform (FFT) that is linked to plain (111) of metallic Pb. The EDS study showed that there was more Pb, which supported the idea that the black Pb-rich particles inside the Rb6Pb5Cl16 NCs are either lead oxide or metallic Pb NPs. The Pb-terminated surface of NCs opens up an octahedral hole with Rb+ in the middle. This is a good place for the oxygen atom from OAc to bind, which ends the molecules with the H-C=O group instead of COOH. The crystalline results, which show that there is 9.8% Pb2O3 phase, agrees with this hypothesis.
It was used to learn more about the electronic features of these Rb6Pb5Cl16 NCs by measuring their valence band (VB) and surface photovoltage (SPV). We took the retrieved VB value, which was 0.46 eV below the Fermi level, and used the SPV method to find the work function of the NCs, which was 4.63 eV.
The role of ligands
The study looks at how ligands change the shape and formation of perovskite nanostructures (NCs). It does this by changing the volume ratio of OA:OLA between 1:2 and 1:3, but keeping the total molar amount of both ligands the same. It is well known that OAc and OLA pairs can be used to make perovskite NCs. It is possible for acid and base to balance out when both OAc and OLA are involved in the process. The OAc gives the OLA a proton, which changes it into an oleate anion. At the same time, the OLA ligand gains a proton, which changes it into an oleyl-ammonium cation.
HR-TEM pictures of 1:2 OAc:OLA NCs only show a few shapes, like big squares and long, thin NWs with black dots and a twisted square-like phase. NCs with OLA alone (without OA), on the other hand, don’t show up. This suggests that only a small amount of OA is needed to completely dissolve PbCl2.
When there is too much OLA, the HRTEM changes the form of the NCs. The new shapes may show that OLA binds to the surface chemically instead of physically. This means that different amounts of OLA ligands can change the direction of NCs’ growth by blocking certain crystallographic faces. When there is a lot more OLA than OA, the protonation process slows down. This means that there are fewer protonated OLA species, also known as oleyl-ammonium cations. These conditions might make it easier for the oleyl-ammonium cations to coordinately link with the Cl− anions in the lattice.
The XRD spectrum of the NCs with a 1:2 ratio of OA:OLA showed a main orthorhombic Pb(OH)Cl phase that matched the absorption of the nanostructures that were made. As there are a lot of unprotonated OLA ligands, they may interact with big areas on the surface of NCs. This could stop Rb-oleate species from helping NCs crystallise, creating a new Rb-free crystal phase of Pb(OH)Cl.
To learn more about this suggested process, theoretical models were run. In two of these reactions, the oxygen atom ends up on top of Rb from a hole in the form of a triangle made by three Pb surface atoms. It takes -2.88 eV of energy for the acid molecule to change its -OH ending into =O and -H, which are directly connected to the C atom that lost =O. The -OH group can also form the surface. The formation energy is -2.59 eV if it is placed between two Pb atoms.
It was also looked at what would happen if the ligand ratios were switched to 2:1 OA:OLA or 3:1 OA:OLA. When compared to the 1:1 OA:OLA NCs, a twisted form was seen. As shown in Figure 18(a), OAc ligands help keep the size more even and stable. There are no NCs (1:3 OAc:OLA ratio) visible in Figure 18(b). This could be because there aren’t enough OLA, which can make it very hard for the NCs to stay stable.
Summary
Making rubidium PbCl2 nanocrystals (NCs) is a new way to make a UV-active material with interesting properties and a complicated structure. HR-TEM showed NCs with a square shape, and XRD showed how symmetrical the tetragonal Rb6Pb5Cl16 phase was. Theoretical estimates of electric and structural features were in line with results from experiments. A main phase of Pb(OH)Cl was made when Rb cations and OLA competed for space on the surface. This phase had an optical redshift that was linked to square-shaped NCs. More OLA made a pure Pb(OH)Cl that didn’t have any Rb atoms in the crystal. On a nanometric level, Pb(OH)Cl material is appealing because it can work as an anode for lithium-ion batteries.
Near-ultraviolet to mid-visible bandgap tuning of
mixed-cation RbxCs1–xPbX3 (X = Cl or Br) perovskite NPs
The study aims to change the absorption of nanocomposite (NCs) to longer wavelengths by adding a mixed-cation makeup, which means that Rb+ and Cs+ will be mixed as the A-cation. To do this, the TF of a number of different perovskite compounds is found. It looks at the coordination number of the ions. Cubic perovskite has a coordination number of 6, and the cation has a coordination number of 12. Ionic radii that work well are Cs+ (1.74 Å), Rb+ (1.61 Å), Pb2+ (1.19 Å), Cl− (1.81 Å), and Br− (1.96 Å).
The researchers show that it is possible to use the hot injection method to add Rb+ cation and Cs+ cation to RbxCs1–xPbX3 NCs. A mix of Rb2CO3 and Cs2CO3 was put in a three-neck flask with Rb+ and Cs+-oleate precursors. OAc and ODE were also added. PbCl2 or PbBr2 were added to an extra three-neck flask along with OAc, OLA, and ODE to make the lead halide (PbX2) precursor. TOP was added to make the whole thing dissolve.
Different mixedcation perovskite NPs with chloride (RbxCs1–xPbCl3) and bromide (RbxCs1–xPbBr3) are shown along with their absorption and photoluminescence (PL) spectra. While the x values are 0, 0.2, 0.4, 0.6, and 0.8, the absorption of both chloride and bromide shifts to the blue. The absorption spectra show that when the Rb+ content is greater, the absorption starts at a lower wavelength and the PL peaks move in the same direction.
They also tried making mixed-cation Rb+/Cs+ NCs with iodide as the halide, but the end result doesn’t seem stable and doesn’t show any optical shift. Mixed-halide systems are often used for bandgap tuning, but mixed-cation systems can also change the bandgap. Because the halides have different p orbital energies, they change the energy level of the VB, which in turn changes the bandgap. Changing the A-cation has an effect on the bandgap that isn’t directly seen because it changes the structure.
In the end, the study shows that in all-inorganic perovskite nanoparticles, the VB’s energy level moves lower, which widens the bandgap and changes the absorption to shorter wavelengths.
The research is mostly about making Rb+-based nanocrystals (NCs) with Cl and Br, which led to a different phase of Rb6Pb5Cl16. The PL-QY values for NCs are pretty high, about the same as those found for CsPbX3 (X = Cl, Br) NCs. The PL-QY is very much affected by how the nanoparticles are cleaned and the conditions of each production.
These NCs have a perovskite crystal structure with low Rb+ amounts, as shown by powder XRD (pXRD). The values of x can be 0.2, 0.4, 0.6, 0.8, or 1 and X can be Cl or Br. In the event that x = 0, there are peaks that show a CsPbCl3 perovskite structure that is NOT square. The same CsPbCl3 perovskite peaks were seen when the Rb+ content was increased to x = 0.2 and x = 0.4. You can see that the perovskite peaks have moved a little, which is linked to small changes in the unit cell parameters when Rb is replaced with Cs. As the amount of Rb+ goes up, the perovskite peaks get smaller and new peaks show up.
Nanostructures made of only artificial perovskite are more like a tetragonal Rb6Pb5Cl16 phase than perovskite RbPbCl3. The previous part talked about how to make Rb6Pb5Cl16. This shows how the crystal structure changes when the Cs+ cation is swapped out for the smaller Rb+ cation in the lattice. So, we can say that increasing the amount of Rb+ while decreasing the amount of Cs+ will lead to changes in the shape of the perovskite, which will make it less stable based on the cation radii.
The orthorhombic CsPbBr3 perovskite structure is shown in Figure 21(b) when x = 0. Low amounts of Rb+ produced the same perovskite crystal structure, except for the situation where x = 0.2, which shows two strong peaks. It’s possible that these strong peaks are caused by impurities in the Cs4PbBr6 phase. This phase has peaks at angles 12.8 and 25.9, which can match the observed peaks.
The pXRD shows that the x = 0.6 and 0.8 products have a tetragonal Rb4PbBr6 structure. The security of the phase creation is mostly tied to how stable it is. We found that the RbPbX3 phase can only be kept stable by ns2-type A-cations, like In+ and Tl+.
Using a mixed-cation method, the study looks into how perovskite nanoparticles (NCs) are made. This study found that adding more Rb+ to the NCs causes more phase changes in both Cl- and Br-based NCs because of octahedral bending. As the octahedral angle gets bigger, a more stable crystal phase forms until the perovskite crystal structure is gone for good. When x = 0.8 was used for Br, the perovskite phase had already not formed.
ImageJ software was used to measure the size ranges of each object that had a different amount of Rb+. It was seen that the average size of the NCs gets smaller for both Cl and Br when the amount of Rb+ in them goes up. When you add more rubidium to a crystal instead of caesium, the d-spacing between the crystalline plains gets smaller because Rb+ is less dense than Cs+. This changes the overall size of the NCs as you add more Rb+.
If you change the monovalent cation, it will have different effects depending on how rigid the system is. When you change the original cation in a cubic structure, the lattice parameters will change, which will cause the lattice to either grow or shrink. If the structure is tetragonal or orthorhombic, on the other hand, A-cation replacement has two effects that are at odds with each other on the octahedra’s size and bending angles.
A control experiment was done to make sure that the changes in visual qualities are caused by adding Rb+. The absorption spectra of Cs0.2PbBr3, Cs0.4PbBr3, and Cs0.8PbBr3 that did not contain Rb+ were compared to those of the preparations that did contain Rb+.
As a whole, it was shown that adding Rb+ to the NCs makes the absorption spectra more blue than when Rb+ is not added. It is clear from this control experiment that the visual changes happen because Rb is in the NCs.
When you add more Rb+, the shape of NCs changes, which may have something to do with the crystal phase changing because the perovskite phase deforms badly into a mix of Cs+- and Rb+-based phases. There are black dots in all of the pictures. These are Pb0 seeds that form before the reaction starts in the PbX2 flask. Based on these results, using Rb+ in the same molar ratio as its Cs+ cousin is probably not enough to finish crystallisation. This is why more Pb0 seeds show up when Rb+ amounts are higher.
Summary
This study looks at what happens when a small amount of Rb+ cation is added to CsPbX3 nanocomposite nanostructures (NCs). It shows how adding the small amount of Rb+ cation changes the structural pressure on the artificial CsPbX3 perovskite NCs. The study gives a full description of changes that happen to A-cations at the nanoscale level, showing that the structures are very flexible. The NCs have high PL-QYs, which are similar to the original CsPbX3 NPs. The antibonding overlap of Pb2+ and X- orbitals is changed when the octahedral tilting adds the Rb+ cation over the Cs+ cation. NCs are square-shaped in TEM pictures, and visual features change in control preparations that don’t have Rb. It was thought that high Rb+ rates might cause unstable perovskites, but the results show that mixed-cation perovskite NPs are actually made, and they have qualities like known CsPbX3 NPs. Fine-tuning the optical properties of perovskite nanostructures by cation in the nanoscale is a step towards better understanding them and finding new uses for them in optoelectronics.
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