Code: VASP

Level of theory: DFT

Exchange-correlation functional: PBE+SOC

K-point grid: 4x2x2 k-grid (primitive cells) and 4x1x1 for inorganic 2D model compounds

Basis set definition: PAW

Comment: Plane-wave basis set with energy cut-off of 500 eV

Code: VASP

Level of theory: DFT

Exchange-correlation functional: HSE+SOC

K-point grid: 4x2x2 k-grid (primitive cells) and 4x1x1 for inorganic 2D model compounds

Basis set definition: PAW

Comment: Plane-wave basis set with energy cut-off of 500 eV; non-local Fock exchange correlation contribution was increased to 73%

Starting materials: PbO (99.9%), methylammonium chloride, hydroiodic acid (HI, 57 wt % in H2O, distilled, stabilized, 99.95%), butylamine, hypophosphorous acid solution (H3PO2, 50 wt % in H2O)

Product: cherry red rectangular-shaped plates

Description: First, PbO powder (2232 mg, 10 mmol) was added to a mixture of 57% w/w aqueous HI solution (10.0 mL, 76 mmol) and 50% aqueous H3PO2 (1.7 mL, 15.5 mmol). The solution was heated to boiling and stirred, such that the PbO powder dissolved within 5 minutes. Solid CH3NH3Cl (338 mg, 5 mmol) was added, which caused a black powder to precipitate. The solution was rapidly mixed to redissolve the black powder precipitate. n-CH3(CH2)3NH2 (694 μL, 7 mmol) was neutralized with HI 57% w/w (5 mL, 38 mmol) in an ice bath. This solution was added to the initial solution, causing a black precipitate that soon redissolved under more stirring. The stirring ceased and the solution cooled to room temperature over ~2 hours.

Method: UV-vis absorption

Description: A Shimadzu UV-3600 UV-vis NIR spectrometer was used. BaSO4 was used as a reference of 100% reflectance for all measurements.

• Rayan C Duke University

Starting materials: PbO (99.9%), methylammonium chloride, hydroiodic acid (HI, 57 wt % in H2O, distilled, stabilized, 99.95%), butylamine, hypophosphorous acid solution (H3PO2, 50 wt % in H2O)

Product: cherry red rectangular-shaped plates

Description: First, PbO powder (2232 mg, 10 mmol) was added to a mixture of 57% w/w aqueous HI solution (10.0 mL, 76 mmol) and 50% aqueous H3PO2 (1.7 mL, 15.5 mmol). The solution was heated to boiling and stirred, such that the PbO powder dissolved within 5 minutes. Solid CH3NH3Cl (338 mg, 5 mmol) was added, which caused a black powder to precipitate. The solution was rapidly mixed to redissolve the black powder precipitate. n-CH3(CH2)3NH2 (694 μL, 7 mmol) was neutralized with HI 57% w/w (5 mL, 38 mmol) in an ice bath. This solution was added to the initial solution, causing a black precipitate that soon redissolved under more stirring. The stirring ceased and the solution cooled to room temperature over ~2 hours.

Method: UV-vis absorption

Description: A Shimadzu UV-3600 UV-vis NIR spectrometer was used. BaSO4 was used as a reference of 100% reflectance for all measurements.

• Rayan C Duke University

Starting materials: PbO (99.9%), methylammonium chloride, hydroiodic acid (HI, 57 wt % in H2O, distilled, stabilized, 99.95%), butylamine, hypophosphorous acid solution (H3PO2, 50 wt % in H2O)

Product: cherry red rectangular-shaped plates

Description: First, PbO powder (2232 mg, 10 mmol) was added to a mixture of 57% w/w aqueous HI solution (10.0 mL, 76 mmol) and 50% aqueous H3PO2 (1.7 mL, 15.5 mmol). The solution was heated to boiling and stirred, such that the PbO powder dissolved within 5 minutes. Solid CH3NH3Cl (338 mg, 5 mmol) was added, which caused a black powder to precipitate. The solution was rapidly mixed to redissolve the black powder precipitate. n-CH3(CH2)3NH2 (694 μL, 7 mmol) was neutralized with HI 57% w/w (5 mL, 38 mmol) in an ice bath. This solution was added to the initial solution, causing a black precipitate that soon redissolved under more stirring. The stirring ceased and the solution cooled to room temperature over ~2 hours.

Method: Photoluminescence microscopy

Description: A Horiba LabRAM HR Evolution confocal Raman microscope was used with a 473 nm laser to excite samples at 50x magnification.

• Rayan C Duke University

Starting materials: PbO, FA acetate, HI, BA, H3PO2

Product: red plate-shaped crystals

Description: First, PbO (223 mg, 1 mmol) and FA acetate (52.1 mg, 0.5 mmol) were added to 1.5 mL of concentrated HI solution. These substances were dissolved under constant stirring and heating. Then, BA (89.1 μL, 0.9 mmol) was added to 0.25 mL of concentrated aqueous H3PO2 in a separate vial. This solution was stirred and was soon added to the initial solution. The temperature was lowered to 125º C until crystals began to form. The temperature was lowered again to 80ºC, the hot plate was turned off after 60 minutes, and after 30 minutes thereafter, the crystals were collected.

Method: Powder X-ray diffraction

Description: A Rigaku Miniflex600 powder X-ray diffractometer (Cu Kα graphite, λ = 1.5406 Å) was used. Operating settings included 40 kV/15 mA with Kβ foil filter.

Starting materials: PbO, FA acetate, HI, BA, H3PO2

Product: red plate-shaped crystals

Description: First, PbO (223 mg, 1 mmol) and FA acetate (52.1 mg, 0.5 mmol) were added to 1.5 mL of concentrated HI solution. These substances were dissolved under constant stirring and heating. Then, BA (89.1 μL, 0.9 mmol) was added to 0.25 mL of concentrated aqueous H3PO2 in a separate vial. This solution was stirred and was soon added to the initial solution. The temperature was lowered to 125º C until crystals began to form. The temperature was lowered again to 80ºC, the hot plate was turned off after 60 minutes, and after 30 minutes thereafter, the crystals were collected.

Method: Powder X-ray diffraction

Description: A Rigaku Miniflex600 powder X-ray diffractometer (Cu Kα graphite, λ = 1.5406 Å) was used. Operating settings included 40 kV/15 mA with Kβ foil filter.

Code: VASP

Level of theory: DFT

Exchange-correlation functional: PBE+SOC

K-point grid: 4x2x2 k-grid (primitive cells) and 4x1x1 for inorganic 2D model compounds

Basis set definition: PAW

Comment: Plane-wave basis set with energy cut-off of 500 eV

Code: VASP

Level of theory: DFT

Exchange-correlation functional: HSE+SOC

K-point grid: 4x2x2 k-grid (primitive cells) and 4x1x1 for inorganic 2D model compounds

Basis set definition: PAW

Comment: Plane-wave basis set with energy cut-off of 500 eV; non-local Fock exchange correlation contribution was increased to 73%

Code: VASP

Level of theory: DFT

Exchange-correlation functional: PBE+SOC

K-point grid: 4x2x2 k-grid (primitive cells) and 4x1x1 for inorganic 2D model compounds

Basis set definition: PAW

Comment: Plane-wave basis set with energy cut-off of 500 eV

Code: VASP

Level of theory: DFT

Exchange-correlation functional: HSE+SOC

K-point grid: 4x2x2 k-grid (primitive cells) and 4x1x1 for inorganic 2D model compounds

Basis set definition: PAW

Comment: Plane-wave basis set with energy cut-off of 500 eV; non-local Fock exchange correlation contribution was increased to 73%

Starting materials: PbO (99.9%), FA acetate, hydroiodic acid (HI, 57 wt % in H2O, distilled, stabilized, 99.95%), butylamine, hypophosphorous acid solution (H3PO2, 50 wt % in H2O)

Product: red plate-shaped crystals

Description: First, PbO (223 mg, 1 mmol) and FA acetate (52.1 mg, 0.5 mmol) were added to 1.5 mL of concentrated HI solution. These substances were dissolved under constant stirring and heating. Then, BA (89.1 μL, 0.9 mmol) was added to 0.25 mL of concentrated aqueous H3PO2 in a separate vial. This solution was stirred and was soon added to the initial solution. The temperature was lowered to 125º C until crystals began to form. The temperature was lowered again to 80ºC, the hot plate was turned off after 60 minutes, and after 30 minutes thereafter, the crystals were collected.

Method: UV-vis absorption

Description: A Shimadzu UV-3600 UV-vis NIR spectrometer was used. BaSO4 was used as a reference of 100% reflectance for all measurements.

• Rayan C Duke University

Starting materials: PbO (99.9%), FA acetate, hydroiodic acid (HI, 57 wt % in H2O, distilled, stabilized, 99.95%), butylamine, hypophosphorous acid solution (H3PO2, 50 wt % in H2O)

Product: red plate-shaped crystals

Description: First, PbO (223 mg, 1 mmol) and FA acetate (52.1 mg, 0.5 mmol) were added to 1.5 mL of concentrated HI solution. These substances were dissolved under constant stirring and heating. Then, BA (89.1 μL, 0.9 mmol) was added to 0.25 mL of concentrated aqueous H3PO2 in a separate vial. This solution was stirred and was soon added to the initial solution. The temperature was lowered to 125º C until crystals began to form. The temperature was lowered again to 80ºC, the hot plate was turned off after 60 minutes, and after 30 minutes thereafter, the crystals were collected.

Method: Photoluminescence microscopy

Description: A Horiba LabRAM HR Evolution confocal Raman microscope was used with a 473 nm laser to excite samples at 50x magnification.

• Rayan C Duke University

Starting materials: PbO (99.9%), FA acetate, hydroiodic acid (HI, 57 wt % in H2O, distilled, stabilized, 99.95%), butylamine, hypophosphorous acid solution (H3PO2, 50 wt % in H2O)

Product: red plate-shaped crystals

Description: First, PbO (223 mg, 1 mmol) and FA acetate (52.1 mg, 0.5 mmol) were added to 1.5 mL of concentrated HI solution. These substances were dissolved under constant stirring and heating. Then, BA (89.1 μL, 0.9 mmol) was added to 0.25 mL of concentrated aqueous H3PO2 in a separate vial. This solution was stirred and was soon added to the initial solution. The temperature was lowered to 125º C until crystals began to form. The temperature was lowered again to 80ºC, the hot plate was turned off after 60 minutes, and after 30 minutes thereafter, the crystals were collected.

Method: Photoluminescence microscopy

Description: A Horiba LabRAM HR Evolution confocal Raman microscope was used with a 473 nm laser to excite samples at 50x magnification.

Starting materials: PbO, DMA chloride, HI, BA, H3PO2

Product: red plate-shaped crystals

Description: First, PbO (223 mg, 1 mmol) and DMA chloride acetate (40.8 mg, 0.5 mmol) were added to 1.75 mL of concentrated HI solution. These substances were dissolved under constant stirring and heating. Then, BA (49.6.1 μL, 0.5 mmol) was added to 0.25 mL of concentrated aqueous H3PO2 in a separate vial. This solution was stirred and was soon added to the initial solution. The temperature was lowered to 125º C until crystals began to form. The temperature was lowered again to 80ºC, the hot plate was turned off after 60 minutes, and after 30 minutes thereafter, the crystals were collected.

Method: Powder X-ray diffraction

Description: A Rigaku Miniflex600 powder X-ray diffractometer (Cu Kα graphite, λ = 1.5406 Å) was used. Operating settings included 40 kV/15 mA with Kβ foil filter.

Starting materials: PbO, DMA chloride, HI, BA, H3PO2

Product: red plate-shaped crystals

Description: First, PbO (223 mg, 1 mmol) and DMA chloride acetate (40.8 mg, 0.5 mmol) were added to 1.75 mL of concentrated HI solution. These substances were dissolved under constant stirring and heating. Then, BA (49.6.1 μL, 0.5 mmol) was added to 0.25 mL of concentrated aqueous H3PO2 in a separate vial. This solution was stirred and was soon added to the initial solution. The temperature was lowered to 125º C until crystals began to form. The temperature was lowered again to 80ºC, the hot plate was turned off after 60 minutes, and after 30 minutes thereafter, the crystals were collected.

Method: Powder X-ray diffraction

Description: A Rigaku Miniflex600 powder X-ray diffractometer (Cu Kα graphite, λ = 1.5406 Å) was used. Operating settings included 40 kV/15 mA with Kβ foil filter.

Code: VASP

Level of theory: DFT

Exchange-correlation functional: PBE+SOC

K-point grid: 4x2x2 k-grid (primitive cells) and 4x1x1 for inorganic 2D model compounds

Basis set definition: PAW

Comment: Plane-wave basis set with energy cut-off of 500 eV

Code: VASP

Level of theory: DFT

Exchange-correlation functional: HSE+SOC

K-point grid: 4x2x2 k-grid (primitive cells) and 4x1x1 for inorganic 2D model compounds

Basis set definition: PAW

Comment: Plane-wave basis set with energy cut-off of 500 eV; non-local Fock exchange correlation contribution was increased to 73%

Code: VASP

Level of theory: DFT

Exchange-correlation functional: PBE+SOC

K-point grid: 4x2x2 k-grid (primitive cells) and 4x1x1 for inorganic 2D model compounds

Basis set definition: PAW

Comment: Plane-wave basis set with energy cut-off of 500 eV

Code: VASP

Level of theory: DFT

Exchange-correlation functional: HSE+SOC

K-point grid: 4x2x2 k-grid (primitive cells) and 4x1x1 for inorganic 2D model compounds

Basis set definition: PAW

Comment: Plane-wave basis set with energy cut-off of 500 eV; non-local Fock exchange correlation contribution was increased to 73%

Starting materials: PbO (99.9%), dimethylammonium chloride (99%), hydroiodic acid (HI, 57 wt % in H2O, distilled, stabilized, 99.95%), butylamine, hypophosphorous acid solution (H3PO2, 50 wt % in H2O)

Product: red plate-shaped crystals

Description: First, PbO (223 mg, 1 mmol) and DMA chloride acetate (40.8 mg, 0.5 mmol) were added to 1.75 mL of concentrated HI solution. These substances were dissolved under constant stirring and heating. Then, BA (49.6.1 μL, 0.5 mmol) was added to 0.25 mL of concentrated aqueous H3PO2 in a separate vial. This solution was stirred and was soon added to the initial solution. The temperature was lowered to 125º C until crystals began to form. The temperature was lowered again to 80ºC, the hot plate was turned off after 60 minutes, and after 30 minutes thereafter, the crystals were collected.

Method: UV-vis absorption

Description: A Shimadzu UV-3600 UV-vis NIR spectrometer (at 200-2500 nm region at 293 K) was used. BaSO4 was used as a reference of 100% reflectance for all measurements.

• Rayan C Duke University

Starting materials: PbO (99.9%), dimethylammonium chloride (99%), hydroiodic acid (HI, 57 wt % in H2O, distilled, stabilized, 99.95%), butylamine, hypophosphorous acid solution (H3PO2, 50 wt % in H2O)

Product: red plate-shaped crystals

Description: First, PbO (223 mg, 1 mmol) and DMA chloride acetate (40.8 mg, 0.5 mmol) were added to 1.75 mL of concentrated HI solution. These substances were dissolved under constant stirring and heating. Then, BA (49.6.1 μL, 0.5 mmol) was added to 0.25 mL of concentrated aqueous H3PO2 in a separate vial. This solution was stirred and was soon added to the initial solution. The temperature was lowered to 125º C until crystals began to form. The temperature was lowered again to 80ºC, the hot plate was turned off after 60 minutes, and after 30 minutes thereafter, the crystals were collected.

Method: UV-vis absorption

Description: A Shimadzu UV-3600 UV-vis NIR spectrometer (at 200-2500 nm region at 293 K) was used. BaSO4 was used as a reference of 100% reflectance for all measurements.

• Rayan C Duke University

Starting materials: PbO (99.9%), dimethylammonium chloride (99%), hydroiodic acid (HI, 57 wt % in H2O, distilled, stabilized, 99.95%), butylamine, hypophosphorous acid solution (H3PO2, 50 wt % in H2O)

Product: red plate-shaped crystals

Description: First, PbO (223 mg, 1 mmol) and DMA chloride acetate (40.8 mg, 0.5 mmol) were added to 1.75 mL of concentrated HI solution. These substances were dissolved under constant stirring and heating. Then, BA (49.6.1 μL, 0.5 mmol) was added to 0.25 mL of concentrated aqueous H3PO2 in a separate vial. This solution was stirred and was soon added to the initial solution. The temperature was lowered to 125º C until crystals began to form. The temperature was lowered again to 80ºC, the hot plate was turned off after 60 minutes, and after 30 minutes thereafter, the crystals were collected.

Method: Photoluminescence

Description: A Horiba LabRAM HR Evolution confocal Raman microscope was used with a 473 nm laser to excite samples at 50x magnification.

• Rayan C Duke University

Starting materials: PbO (99.9%), dimethylammonium chloride (99%), hydroiodic acid (HI, 57 wt % in H2O, distilled, stabilized, 99.95%), butylamine, hypophosphorous acid solution (H3PO2, 50 wt % in H2O)

Product: red plate-shaped crystals

Description: First, PbO (223 mg, 1 mmol) and DMA chloride acetate (40.8 mg, 0.5 mmol) were added to 1.75 mL of concentrated HI solution. These substances were dissolved under constant stirring and heating. Then, BA (49.6.1 μL, 0.5 mmol) was added to 0.25 mL of concentrated aqueous H3PO2 in a separate vial. This solution was stirred and was soon added to the initial solution. The temperature was lowered to 125º C until crystals began to form. The temperature was lowered again to 80ºC, the hot plate was turned off after 60 minutes, and after 30 minutes thereafter, the crystals were collected.

Method: Photoluminescence

Description: A Horiba LabRAM HR Evolution confocal Raman microscope was used with a 473 nm laser to excite samples at 50x magnification.

• Rayan C Duke University

Starting materials: PbO (99.9%), guanidinium chloride (99%), hydroiodic acid (HI, 57 wt % in H2O, distilled, stabilized, 99.95%), butylamine, hypophosphorous acid solution (H3PO2, 50 wt % in H2O)

Product: red plate-shaped crystals

Description: First, PbO (223 mg, 1 mmol) and GA chloride (47.8 mg, 0.5 mmol) were added to 1.5 mL of concentrated HI solution. These substances were dissolved under constant stirring and heating. Then, BA (49.6 μL, 0.5 mmol) was added to 0.25 mL of concentrated aqueous H3PO2 in a separate vial. This solution was stirred and was soon added to the initial solution. The temperature was lowered to 125º C until crystals began to form. The temperature was lowered again to 80ºC, the hot plate was turned off after 60 minutes, and after 30 minutes thereafter, the crystals were collected.

Method: Powder X-ray diffraction

Description: A Rigaku Miniflex600 powder X-ray diffractometer (Cu Kα graphite, λ = 1.5406 Å) was used. Operating settings included 40 kV/15 mA with Kβ foil filter.

• Rayan C Duke University

Starting materials: PbO, GA chloride, HI, BA, H3PO2

Product: red plate-shaped crystals

Description: First, PbO (223 mg, 1 mmol) and GA chloride (47.8 mg, 0.5 mmol) were added to 1.5 mL of concentrated HI solution. These substances were dissolved under constant stirring and heating. Then, BA (49.6 μL, 0.5 mmol) was added to 0.25 mL of concentrated aqueous H3PO2 in a separate vial. This solution was stirred and was soon added to the initial solution. The temperature was lowered to 125º C until crystals began to form. The temperature was lowered again to 80ºC, the hot plate was turned off after 60 minutes, and after 30 minutes thereafter, the crystals were collected.

Method: Powder X-ray diffraction

Description: A Rigaku Miniflex600 powder X-ray diffractometer (Cu Kα graphite, λ = 1.5406 Å) was used. Operating settings included 40 kV/15 mA with Kβ foil filter.

Code: VASP

Level of theory: DFT

Exchange-correlation functional: PBE+SOC

K-point grid: 4x2x2 k-grid (primitive cells) and 4x1x1 for inorganic 2D model compounds

Basis set definition: PAW

Comment: Plane-wave basis set with energy cut-off of 500 eV

Code: VASP

Level of theory: DFT

Exchange-correlation functional: HSE+SOC

K-point grid: 4x2x2 k-grid (primitive cells) and 4x1x1 for inorganic 2D model compounds

Basis set definition: PAW

Comment: Plane-wave basis set with energy cut-off of 500 eV; non-local Fock exchange correlation contribution was increased to 73%

Code: VASP

Level of theory: DFT

Exchange-correlation functional: PBE+SOC

K-point grid: 4x2x2 k-grid (primitive cells) and 4x1x1 for inorganic 2D model compounds

Basis set definition: PAW

Comment: Plane-wave basis set with energy cut-off of 500 eV

Code: VASP

Level of theory: DFT

Exchange-correlation functional: HSE+SOC

K-point grid: 4x2x2 k-grid (primitive cells) and 4x1x1 for inorganic 2D model compounds

Basis set definition: PAW

Comment: Plane-wave basis set with energy cut-off of 500 eV; non-local Fock exchange correlation contribution was increased to 73%

Starting materials: PbO (99.9%), guanidinium chloride (99%), hydroiodic acid (HI, 57 wt % in H2O, distilled, stabilized, 99.95%), butylamine, hypophosphorous acid solution (H3PO2, 50 wt % in H2O)

Product: red plate-shaped crystals

Description: First, PbO (223 mg, 1 mmol) and GA chloride (47.8 mg, 0.5 mmol) were added to 1.5 mL of concentrated HI solution. These substances were dissolved under constant stirring and heating. Then, BA (49.6 μL, 0.5 mmol) was added to 0.25 mL of concentrated aqueous H3PO2 in a separate vial. This solution was stirred and was soon added to the initial solution. The temperature was lowered to 125º C until crystals began to form. The temperature was lowered again to 80ºC, the hot plate was turned off after 60 minutes, and after 30 minutes thereafter, the crystals were collected.

Method: UV-vis absorption

Description: A Shimadzu UV-3600 UV-vis NIR spectrometer was used. BaSO4 was used as a reference of 100% reflectance for all measurements.

• Rayan C Duke University

Starting materials: PbO, GA chloride, HI, BA, H3PO2

Product: red plate-shaped crystals

Description: First, PbO (223 mg, 1 mmol) and GA chloride (47.8 mg, 0.5 mmol) were added to 1.5 mL of concentrated HI solution. These substances were dissolved under constant stirring and heating. Then, BA (49.6 μL, 0.5 mmol) was added to 0.25 mL of concentrated aqueous H3PO2 in a separate vial. This solution was stirred and was soon added to the initial solution. The temperature was lowered to 125º C until crystals began to form. The temperature was lowered again to 80ºC, the hot plate was turned off after 60 minutes, and after 30 minutes thereafter, the crystals were collected.

Method: Kubelka-Munk Equation

Description: Reflectance vs. wavelength data was used with the Kubelka-Munk equation to estimate the band gap.

Starting materials: PbO (99.9%), guanidinium chloride (99%), hydroiodic acid (HI, 57 wt % in H2O, distilled, stabilized, 99.95%), butylamine, hypophosphorous acid solution (H3PO2, 50 wt % in H2O)

Product: red plate-shaped crystals

Description: First, PbO (223 mg, 1 mmol) and GA chloride (47.8 mg, 0.5 mmol) were added to 1.5 mL of concentrated HI solution. These substances were dissolved under constant stirring and heating. Then, BA (49.6 μL, 0.5 mmol) was added to 0.25 mL of concentrated aqueous H3PO2 in a separate vial. This solution was stirred and was soon added to the initial solution. The temperature was lowered to 125º C until crystals began to form. The temperature was lowered again to 80ºC, the hot plate was turned off after 60 minutes, and after 30 minutes thereafter, the crystals were collected.

Method: Photoluminescence microscopy

Description: A Horiba LabRAM HR Evolution confocal Raman microscope was used with a 473 nm laser to excite samples at 50x magnification.

• Rayan C Duke University