X-ray dose effects and strategies to mitigate beam damage in metal halide perovskites under high brilliance X-ray photon sources
Abstract
Metal halide perovskites (MHP) suffer from photo-structural-chemical instabilities whose intricacy requires state-of-the-art tools to investigate their properties under various conditions. This study addresses the damage caused by focused
Keywords
INTRODUCTION
Metal halide perovskite (MHP) materials have received enormous attention due to their photovoltaic and optoelectronic properties suitable for solar cells[1,2], photonic devices[3,4], and
This work uses a correlative multi-technique approach to investigate the effect of the focused
EXPERIMENTAL
Sample preparation
The CsFAMAPb(I,Br)3 perovskite with composition [(FAPbI3)0.87(MAPbBr3)0.13]0.92(CsPbI3)0.08 was deposited using the antisolvent method on polished Si substrates covered with 100 nm of Au. The substrates were cleaned with isopropanol and ultraviolet-ozone (UVO) treated for 30 min before the deposition. The perovskite solution was prepared using FAI (GreatCell Solar Materials), MABr (GreatCell Solar Materials), PbI2 (TCI America), PbBr2 (TCI America), and CsI (TCI America) with a concentration of 0.7 M regarding Pb component. The powders were dissolved in a mixture of anhydrous N, N-dimethylformamide (DMF, Sigma-Aldrich) and dimethyl sulfoxide (DMSO, Sigma-Aldrich) with a volume ratio of DMF:DMSO = 4:1 at room temperature. The solution was spin-coated at 2,000 rpm for 12 s followed by 5,000 rpm for 30 s. Additionally, 15 s before the end of the spin-coating, 150 µL of chlorobenzene was dropped on the film. After deposition, the substrates were thermal annealed for 60 min at 100 °C. The Cs0.05FA0.95PbI3 was deposited by the gas quenching method on the Mylar substrate[21]. The substrates were cleaned with UVO treated for 30 min before deposition. The perovskite precursor solution was prepared by solubilizing CsI (TCI America), FAI (GreatCell Solar Materials), PbI2 (TCI America), and 40 mol% of MACl, with an excess of 5% of the lead source in the mixture of solvents DMF (Sigma-Aldrich) and methylpyrrolidone (NMP, Sigma-Aldrich) (v:v = 4:1) with a concentration of 1.3 M. The solution was spin-coated at 4,000 rpm for
Synchrotron experiments
The sample CsFAMAPb(I, Br)3 was used for all the experiments except for the ptychography measurements performed on the Cs0.05FA0.95PbI3 sample. For the same environment condition [room temperature and air atmosphere (RT-Air), RT and nitrogen flow (RT-N2), or cryogenic temperature (152 K) and N2 flow
Ptychography measurements and analysis
An
In our experiments, 10100 positions were recorded in an area of 10 × 10 µm2 (step of 100 nm; probe size of (200 × 500) H × V nm2, overlap > 50%, including random variations in the probe positions to a perfect square grid)[27]. The scan was performed at the energy of 10 keV in fly-scan mode with a constant velocity of 10 µm/s, giving a total acquisition time of 172 s. The total dose of the scan was estimated to be 0.0015 GGy. For the reconstruction, 180 × 180 pixels (55 × 55 µm2) of the data PiMega area detector at the distance of
µ-FTIR
The
AFM
The atomic force microscopy (AFM) data were acquired at the In situ Growth Laboratory (LCIS) of the LNLS, which is part of the Brazilian Center for Research in Energy and Materials (CNPEM). The images were obtained using a Nanosurf Flex AFM. Scans covered an area of 10 × 10 µm2 with a step size of 20 nm at dynamic force contact mode with the cantilever type Tap 300-AI-G with 1.5 s per line. The data were analyzed with the Gwyddion software. The images here correspond to the morphology acquired in the
µ-PL
The micro-photoluminescence (µ-PL) data were acquired at the Brazilian Nanotechnology National Laboratory (LNNano), part of the CNPEM. The PL was obtained using a confocal Raman spectrometer Horiba XploRA plus. The damaged
Optical microscopy
The optical microscopy images were acquired at the Laboratory of Microscopic Samples (LAM) of the LNLS, part of the CNPEM. The images were obtained using a Nikon Eclipse LV100-UDM-POL microscope with a magnification of 100×. The colors in the image represent the real colors of the surface.
Methodology
We deposited the perovskite CsFAMAPb(Br, I)3 using the traditional antisolvent method onto Si/Au substrates (see details in the experimental section). This substrate is well-suited for this study because it allows
RESULTS AND DISCUSSION
Figure 1A shows the nano-XRF map of the iodine emission of a 5 × 5 µm2 irradiated area acquired at the RT-Air condition. To irradiate and map the sample, we used the fly-scan snake mode with the scan starting on the left-bottom corner of the image and the beam spot going up and down while sliding to the right related to the sample. The
Figure 1.
The decreasing iodine XRF emission in the irradiated region is observed in Figure 1B. To inspect this irradiated region, we performed a larger scan of 10 × 10 µm2 centered at the same point as the previous
As the hybrid organic-inorganic perovskite comprises a mixture of MA and FA, which are not observable by XRF, the effect on the organic part was evaluated using µ-FTIR. Figure 1E shows a µ-FTIR map obtained by the integration of the C-N antisymmetric stretching vibration (1,700 cm-1) of the FA molecules[35]. The decreased intensity in the
Figure 2 shows the
Figure 2. Absorbed
The iodine consumption attenuates as the total deposited dose reduces to 1.9 and 0.7 GGy (Figure 2B and C, respectively). Comparing the ratio between the iodine fluorescence in a 1 × 1 µm2 area in the inner and the non-irradiated regions, we note that the highest dose leads to an iodine consumption of about 28%, attenuated to 16% at 1.9 GGy and 5% at 0.7 GGy [Figure 2D]. The last measurement is within the error bar and is barely seen on the XRF map.
Regarding the organic cations, the µ-FTIR map (Figure 2A-C, FTIR column) shows the expected tendency of diminishing the beam damage with a smaller amount of energy deposited on the sample. The dose effect is mitigated with an attenuation down to 0.7 GGy dose. The tendency is clear by evaluating the integrated profile at the 1,700 cm-1 resonance (C-N antisymmetric stretching) in the irradiated region, which gives about a 30% decrease for the two highest doses and is absent for the smallest one [Figure 2E]. The profile shows a reduction broader than in the irradiated area (pink column), which should come partially from the limited spatial resolution at 1,700 cm-1 due to the diffraction limit at this wavelength (5,882 nm). This broadening reduces at shorter wavelengths, as shown by the µ-FTIR images at the region of N-H stretching vibrations (3,100-3,400 cm-1) [Supplementary Figure 5], with a sharper profile more consistent with the irradiated area.
The decrease in iodine and organic cation FA corroborates with the excavation in the irradiated area. The total thickness of the perovskite was previously measured by a profilometer to be around 330 nm. Taking the depth observed in the AFM topographic images [Supplementary Figure 6], the ratio of the excavation to the film thickness is estimated to be 27% (90 nm) and 21% (70 nm) from the highest to the intermediated dose, respectively. These ratios are comparable to the iodine attenuation shown in Figure 2D. We should point out that error bars for these measurements are within 15%, as measured by the statistical fluctuations outside the irradiated area.
These experiments were performed using the same dose rate and changing the exposition time. One can inquire which parameter is relevant, dose rate or total dose. To verify if the damage depends on the dose rate, we measured the iodine XRF signal as a function of time at a fixed sample position for two rates (1.2 and 0.23 GGy/s) at the energy of 10 keV. Figure 3A shows that the higher dose rate causes the expected faster decay in the iodine content. However, when we present the iodine consumption not as a function of time but as a function of the total absorbed dose [Figure 3B], we note that the iodine decay for both dose rates is superimposed. The decaying signal fits with two exponential components for both dose rates and gives similar decay coefficients: 972 and 28 GGy-1 for 1.2 GGy/s and 916 and 36 GGy-1 for 0.23 GGy/s, suggesting that the process occurs independently of the dose rate. A reasonable hypothesis could be that the free surface excavation is faster initially. However, as more iodine and organic molecules are knocked out, the remaining atoms at the surface, Pb and Br, form a thin coating metallic layer (evidence from FTIR broad band formed and optical images), slowing the process. Nonetheless, additional experiments are necessary to confirm this hypothesis.
Figure 3. Effect of the absorbed dose rate on iodine signal with the
This experiment demonstrates that reducing the dose rate is ineffective in mitigating beam damage in the air. What matters is the integrated absorbed dose needed to extract reliable information from the
The atmosphere and temperature conditions were investigated as another strategy to mitigate the beam damage. The same dose conditions (2.9, 1.9, and 0.7 GGy) were applied in three environmental situations: RT-Air, RT-N2, and Cryo-N2. The RT-Air condition corresponds to previously discussed measurements
Figure 4. Effect temperature and atmosphere on damage from an
Figure 5 shows the nano-XRF and µ-FTIR of the regions measured with the smaller doses, 1.9 and 0.7 GGy, at RT-N2. Reducing the absorbed dose and combining it with an inert atmosphere effectively mitigate the damage. The inorganic elemental distribution is stable, even if the organic component still suffers a small reduction. The topographic look-up by AFM (not shown) did not present any observable excavation. Interestingly, the PL properties are almost unaltered at the RT-N2 condition, with a slight blueshift emission compared to the pristine sample (Figure 5C and D, column µ-PL).
Figure 5. Perovskite damage with reduced dose and inert atmosphere. Sample irradiated with (A) 1.9 GGy and (B) 0.7 GGy in RT-N2. From left to right, nano-XRF damage scan, nano-XRF probe scan and µ-FTIR (1,700 cm-1). (C and D) Respective µ-PL at RT-N2.
The high
Figure 6 presents a schematic model for the damage caused by the
The conservation of energy and momentum dictates that the electron must transfer energy to an atom when they interact in elastic scattering. If the transferred energy exceeds the displacement energy (Ed), the atom moves to an interstitial position, creating a defect. The Ed depends on the bond strength, crystal lattice, and atomic weight of an atom[39]. As the perovskite is composed of medium to heavy elements, an intense displacement in the bulk is not expected. However, if an atom is at the surface, the Ed is lower, and the atom can leave the specimen in a sputtering-like process[39]. One expects a high sputtering contribution from the organic molecules, independently of the atmosphere and temperature, because they are more volatiles. However, those from the inorganic elements (Pb, I, and Br) should be smaller unless they have high mobility and are more reactive, as is the case of the iodine ions exposed to the atmosphere of the air. However, even in this condition, the contribution of sputtering from inorganic elements should be smaller compared with organic molecules.
Hence, we hypothesize that the MHP damage mechanism combines radiolysis and sputtering, with the first making a larger contribution. Radiolysis produces reactive species that react with other elements, forming volatile species that leave the sample. In the air, the O2 and H2O molecules can react with perovskite in various ways, creating solid species such as PbO, Pb(OH)2, PbCO3, PbIOH, Pb, PbI2, and
To highlight the relevance of controlling the
Figure 7. Simultaneous
The intentional irradiation removed some iodine over an area of about 1 µm2 but depleted the organic molecules (seen by ptychography) over a larger area than the spot size. The possibility of measuring in fly-scan mode and under a controlled dose and N2 atmosphere was essential to mitigate eventual sample damages. The phase contrast using very low doses clearly brings new insights into the morphology and local heterogeneities of MHP compounds. Hu et al. reported an
CONCLUSIONS
Our results bring fundamental insights into the damage caused by
Using focused
As a clear demonstration of how powerful the use of sophisticated new techniques in synchrotron radiation facilities can be, we applied the optimized conditions below the harmful dose to demonstrate the feasibility of investigating MHPs with the
DECLARATIONS
Acknowledgments
This research used facilities of the Brazilian Synchrotron Light Laboratory (LNLS) and Brazilian Nanotechnology National Laboratory (LNNano), part of the Brazilian Center for Research in Energy and Materials (CNPEM), a private non-profit organization under the supervision of the Brazilian Ministry for Science, Technology, and Innovations (MCTI). The CARNAUBA, IMBUIA, LAM, and LCIS facilities’ staff of LNLS are acknowledged for their assistance during the beamtime and sample preparation. The facilities’ staff of LNNano is acknowledged for their assistance during the profilometer and PL measurements. da Silva FMC and Guaita MGD acknowledge CNPq scholarships. Szostak R acknowledges FAPESP (Grant 2021/01357-6). The authors gratefully acknowledge support from FAPESP (Grants 2017/11986-5), Shell, and the support given by ANP (Brazil’s National Oil, Natural Gas, and Biofuels Agency) through the R&D levy regulation.
Authors’ contributions
Conceptualization, methodology, synchrotron and complementary experiments, data analysis and interpretation, writing the draft of the manuscript and review: da Silva FMC, Szostak S
Synchrotron experiments, sample preparation: Guaita MGD
Methodology, data discussion, review: Teixeira VC
Funding acquisition and review: Nogueira AF
Funding acquisition, review, conceptualization, methodology, and writing-review: Tolentino HCN
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work was supported by FAPESP (2021/01357-6; 2017/11986-5).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2024.
Supplementary Materials
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da Silva, F. M. C.; Szostak, R.; Guaita, M. G. D.; Teixeira, V. C.; Nogueira, A. F.; Tolentino, H. C. N.
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