Starting materials: phenethylammonium iodide (C8H12IN), lead iodide (PbI2)

Product: thin film

Description: (PEA)I, amd PbI2 made up the target solution in 1:1 DMSO/MEG by volume. They were mechanically mixed until visibly dissolved in solvent, taking about 5 min. In a growth chamber, the solution is cooled to -196°C under vacuum. When frozen, the top layer is removed using an Er:YAG laser (2.94 μm). The laser rasters across the surface to sublimate the MEG, causing the precursor material to be ejected onto the substrate (2 cm × 2 cm of SiO2 glass) spinning 7 cm above. The substrate temperature is approximately 10 °C while in the growth chamber. Deposit time was 4 h. Samples remained in a load lock under turbo vacuum (2 × 10–5 Torr) for an hour afterwards. The annealed films were additionally annealed for 10 min on a hot plate in an N2 environment at 110°.

Method: UV-vis absorption

Description: UV−vis absorption spectra were acquired using a Shimadzu UV-3600 spectrophotometer. Samples of films on glass substrates were measured. Samples were kept in ambient air conditions.

Starting materials: 200 mg (0.90 mmol) of PbO and 200 μL (1.59 mmol) of phenylethylammonium, fully dissolved in 4 mL of HI and 0.5 mL of H3PO2 solution.

Product: Exfoliated single crystal flakes of (PEA)2PbI4.

Description: 2D perovskite PEPI single crystals are synthesized based on previously reported slow-cooling method in Ref. https://dx.doi.org/10.1021/acsenergylett.8b01315. 200 mg (0.90 mmol) of PbO and 200 μL (1.59 mmol) of phenylethyl- ammonium are fully dissolved in 4 mL of HI and 0.5 mL of H3PO2 solution at 90 °C. The solution is then slowly cooled to room temperature at a rate of 2 °C h−1, giving orange sheet-like crystals. The crystals are then isolated from the parent solution by vacuum filtration, washed by a small amount of diethyl ether, and dried under vacuum. Thin crystals were exfoliated from the parent crystal using stiff heat release tape that serves as a handle. Sequential exfoliation steps with the tape yield successively thinner crystals. Many crystals were surveyed to select the best surface quality, flatness, and area.

Comment: Note that, while an XRD pattern was reported in this work, the XRD analysis and the space group were not reported and the space group listed here was taken from DOI: 10.1021/acs.inorgchem.7b01094.

Method: Reflection mode and transmission mode ellipsometry

Description: Transmittance was collected on a Cary 7000 UV-VIS-NIR spectrophotometer. Reflection ellipsometry was collected on a JA Woollam M2000DI at 45° to 75° using tape to suppress backside reflections. Transmission ellipsometry was collected on a JA Woollam M2000DI from −10° to 70°. The three data sets were processed as a multisample analysis in CompleteEASE. For bulk and cleaved crystals, reflection ellipsometry and reflection Mueller Matrix were collected using focus probes and either a JA Woollam M2000 or RC2, respectively.

Comment: Exciton energies were extracted from a uniaxial model of the ellipsometry data (2.385(5) eV in-plane and 2.419(7) eV out-of-plane). The exciton binding energy of 0.259 eV was calculated using ellipsometry dielectric parameters, an electron-hole image charge model and the experimental effective mass of DOI: 10.1021/acs.jpclett.0c03731.

Starting materials: 200 mg (0.90 mmol) of PbO and 200 μL (1.59 mmol) of phenylethylammonium, fully dissolved in 4 mL of HI and 0.5 mL of H3PO2 solution.

Product: Exfoliated single crystal flakes of (PEA)2PbI4.

Description: 2D perovskite PEPI single crystals are synthesized based on previously reported slow-cooling method in Ref. https://dx.doi.org/10.1021/acsenergylett.8b01315. 200 mg (0.90 mmol) of PbO and 200 μL (1.59 mmol) of phenylethyl- ammonium are fully dissolved in 4 mL of HI and 0.5 mL of H3PO2 solution at 90 °C. The solution is then slowly cooled to room temperature at a rate of 2 °C h−1, giving orange sheet-like crystals. The crystals are then isolated from the parent solution by vacuum filtration, washed by a small amount of diethyl ether, and dried under vacuum. Thin crystals were exfoliated from the parent crystal using stiff heat release tape that serves as a handle. Sequential exfoliation steps with the tape yield successively thinner crystals. Many crystals were surveyed to select the best surface quality, flatness, and area.

Comment: Note that, while an XRD pattern was reported in this work, the XRD analysis and the space group were not reported and the space group listed here was taken from DOI: 10.1021/acs.inorgchem.7b01094.

Method: Reflection mode and transmission mode ellipsometry

Description: Transmittance was collected on a Cary 7000 UV-VIS-NIR spectrophotometer. Reflection ellipsometry was collected on a JA Woollam M2000DI at 45° to 75° using tape to suppress backside reflections. Transmission ellipsometry was collected on a JA Woollam M2000DI from −10° to 70°. The three data sets were processed as a multisample analysis in CompleteEASE. For bulk and cleaved crystals, reflection ellipsometry and reflection Mueller Matrix were collected using focus probes and either a JA Woollam M2000 or RC2, respectively.

Comment: Exciton energies were extracted from a uniaxial model of the ellipsometry data (2.385(5) eV in-plane and 2.419(7) eV out-of-plane). The exciton binding energy of 0.259 eV was calculated using ellipsometry dielectric parameters, an electron-hole image charge model and the experimental effective mass of DOI: 10.1021/acs.jpclett.0c03731.

Starting materials: 200 mg (0.90 mmol) of PbO and 200 μL (1.59 mmol) of phenylethylammonium, fully dissolved in 4 mL of HI and 0.5 mL of H3PO2 solution.

Product: Exfoliated single crystal flakes of (PEA)2PbI4.

Description: 2D perovskite PEPI single crystals are synthesized based on previously reported slow-cooling method in Ref. https://dx.doi.org/10.1021/acsenergylett.8b01315. 200 mg (0.90 mmol) of PbO and 200 μL (1.59 mmol) of phenylethyl- ammonium are fully dissolved in 4 mL of HI and 0.5 mL of H3PO2 solution at 90 °C. The solution is then slowly cooled to room temperature at a rate of 2 °C h−1, giving orange sheet-like crystals. The crystals are then isolated from the parent solution by vacuum filtration, washed by a small amount of diethyl ether, and dried under vacuum. Thin crystals were exfoliated from the parent crystal using stiff heat release tape that serves as a handle. Sequential exfoliation steps with the tape yield successively thinner crystals. Many crystals were surveyed to select the best surface quality, flatness, and area.

Comment: Note that, while an XRD pattern was reported in this work, the XRD analysis and the space group were not reported and the space group listed here was taken from DOI: 10.1021/acs.inorgchem.7b01094.

Method: Reflection mode and transmission mode ellipsometry

Description: Transmittance was collected on a Cary 7000 UV-VIS-NIR spectrophotometer. Reflection ellipsometry was collected on a JA Woollam M2000DI at 45° to 75° using tape to suppress backside reflections. Transmission ellipsometry was collected on a JA Woollam M2000DI from −10° to 70°. The three data sets were processed as a multisample analysis in CompleteEASE. For bulk and cleaved crystals, reflection ellipsometry and reflection Mueller Matrix were collected using focus probes and either a JA Woollam M2000 or RC2, respectively.

Comment: Exciton energies were extracted from a uniaxial model of the ellipsometry data (2.385(5) eV in-plane and 2.419(7) eV out-of-plane). The value provide here is extracted from a cleaved crystal, in-plane.

Code: FHI-aims

Level of theory: Spin-orbit coupled hybrid DFT

Exchange-correlation functional: HSE06 functional; exchange mixing parameter: 0.25, screening parameter: 0.11 (Bohr radii)^(-1)

K-point grid: 3x7x7

Level of relativity: Spin-orbit coupling included as follows: Self-consistent scalar relativity (atomic zero-order regular approximation) with spin-orbit coupling applied non-selfconsistently in the energy band structure calculation.

Basis set definition: All-electron; "intermediate" numerical settings and basis sets.

Numerical accuracy: Note that DFT-computed energy band gap values, even at the level of DFT-HS06+SOC, are not intended to capture the experimentally correct fundamental gap with quantitative accuracy. Rather, they are collected be comparable to other computational band gaps at the same level of theory in order to capture trends between different sources.

Geometry used in the calculation

Comment: The geometry used was computationally optimized (unit cell and atomic positions) starting from the XRD-determined structure reported in https://doi.org/10.1021/acs.inorgchem.7b01094 . The level of theory used was DFT-PBE including the Tkatchenko-Scheffler van der Waals correction. The structure is available in the HybriD3 database as dataset number 745.

Starting materials: (PEA)2PbI4 crystals, DMF, glass substrates

Product: Thin film on glass

Description: Dissolve the 2D single crystals into DMF at a concentration 6%∼10% relative to the total weight. Spin-coat at 3000 rpm for 30 s on glass substrates. Anneal in air at 100 °C for 10 min before measurement.

Method: Photoluminescence

Description: The photoluminescence spectra were measured using a Horiba-Jobi-Yvon LabRAM ARAMIS system, with a 325 nm He-Cd laser excitation. The laser beam was collimated and focused through a 40X UV objective onto the sample surface at room temperature. Refer to figure 5.

• Rayan C Duke University

Starting materials: (PEA)2PbI4 crystals, DMF, glass substrates

Product: Thin film on glass

Description: Dissolve the 2D single crystals into DMF at a concentration 6%∼10% relative to the total weight. Spin-coat at 3000 rpm for 30 s on quartz substrates. Anneal in air at 100 °C for 10 min before measurement.

Method: Photoluminescence

Description: The photoluminescence spectra were measured using a Horiba-Jobi-Yvon LabRAM ARAMIS system, with a 325 nm He-Cd laser excitation. The laser beam was collimated and focused through a 40X UV objective onto the sample surface at room temperature. Refer to figure 5.

• Rayan C Duke University

Starting materials: PbI2, BiI3, C6H5C2H4NH3I

Product: dark, red, platelike crystals, with a bismuth ratio Pb/Bi of approx. 0.3%

Description: Under an argon atmosphere, lead iodide (PbI2), bismuth iodide (BiI3) and phenylethylammonium iodide (C6H5C2H4NH3I) were mixed at a molar ratio of 0.9:0.1:2 and heated at 333K for a duration of 30 min. As a solvent, nitromethane was added for crystallization.

Method: PLE-spectroscopy

Description: As a detector, a charge-coupled device (CCD) camera cooled by liquid nitrogen and equipped with a polychromator was utilized. PL spectra were measured with an excitation by Xe-lamp light at 2.48 eV. PLE spectra represent detected PL at 2.35 eV.

Comment: Note that the x axis of PLE spectroscopy is the excitation frequency, which is varied in this experiment, while the y axis is the PL intensity measured at fixed energy (here, 2.35 eV).

• Volker Blum Duke University

Starting materials: (PEA)2PbI4 (synthesized), DMF

Description: Thin films were prepared by dissolving (PEA)2PbI4 (0.50M) in DMF and was spin-coated onto glass substrates at 3000 rpm for 30 s. The films were then heated to 130ºC for 10 minutes.

Method: UV-vis absorption

Description: the spectrum was recorded using SHIMADZU UV-3600.

• Rayan C Duke University