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Layered metal iodides PbI₂ and BiI₃

PbI₂, BiI₃

Also called PbI₂, BiI₃, lead iodide, bismuth triiodide

van der Waals crystal semiconductor

Heavy atoms in a layered crystal: both compounds stop X-rays and gamma rays efficiently and have gaps wide enough to work as detectors at room temperature, without the cooling that germanium needs. PbI2 has a second life as the starting material for every lead halide perovskite, so the same sheets that make a detector are also the frame on which perovskite solar cells are built. Both peel, and monolayer PbI2 can be made by liquid exfoliation.

Crystal structure

  • Pb
  • I
Cell
Hexagonal, a = 4.55 Å
Atoms per cell
3
Pb–I bond
3.21 Å
Height
3.70 Å between the outer atom centres
The same octahedral layer as NiI2, with lead at the centre: the much larger atoms push the lattice out to 4.56 Å and the layer to 3.7 Å from iodine to iodine. PbI2 is a wide-gap semiconductor and the parent compound of the lead-halide perovskites – and, in this layered form, also what those perovskites decompose into. One layer of the 2H polytype of PbI2 (Wyckoff, Crystal Structures 1, 1963; COD 9009114): a = 4.56 Å, iodine planes 1.85 Å above and below the Pb plane.

Key properties

  • High photon stopping power from lead and bismuth, plus gaps wide enough for low leakage at room temperature – the combination radiation detectors need
  • PbI2 and mercuric iodide are the two classic direct converters for flat-panel X-ray imaging, where leakage current is the limiting problem
  • BiI3 detectors resolve 5.5 MeV alpha particles from thin crystals, but their energy resolution is still poor compared with established detectors
  • BiI3’s band gap was long disputed – careful optics resolve it into an indirect gap near 1.67 eV and a direct one near 1.96 eV
  • Liquid exfoliation gives monolayer PbI2 nanodisks; on graphene they align epitaxially and shift from the 1T to the 1H stacking
  • Vacancies in monolayer PbI2 migrate and heal themselves under the electron beam
  • PbI2 is the precursor for lead halide perovskites, and leftover PbI2 is a recurring feature of perovskite films

How it is made

  • Bulk crystals by vertical Bridgman growth or solution growth from iodide solutions
  • Thick films by physical vapour deposition onto imaging panels for X-ray detectors
  • Mechanical or liquid exfoliation for thin flakes and monolayer nanodisks

Uses, and how close they are

  • Room-temperature X-ray and gamma-ray detectorsprototype
  • Direct converters for flat-panel X-ray imaging (PbI2)prototype
  • Precursor for lead halide perovskite solar cells (PbI2)deployed
  • Photodetectors and ultraviolet-visible optoelectronicslab

Readiness runs lab → prototype → pilot → deployed.

Open problems

  1. Can crystal quality reach the level where BiI3 and PbI2 detectors deliver spectroscopic resolution, rather than only counting events?
  2. What sets the leakage current in thick detector films – bulk defects, electrodes, or grain boundaries?
  3. Does monolayer PbI2 behave like the bulk optically, and can it be grown rather than exfoliated into flakes large enough to measure?
  4. Can a lead-free heavy halide match PbI2’s stopping power without its toxicity?

Going deeper

Short notes for specialists. Choose a lens in the header and yours comes first.

For theoreticians · your lens

Simple layered halides with heavy cations: spin–orbit coupling shapes the conduction band, the layers are held by dispersion, and in BiI3 one third of the octahedral sites are vacant, which lowers the symmetry and makes the band structure harder to pin down than the chemistry suggests – its measured indirect and direct gaps differ by nearly 0.3 eV. Monolayer PbI2 adds a polytype question: the 1T and 1H stackings are close in energy, and a graphene substrate is enough to tip the balance.

For experimentalists · your lens

For detector work, report thickness, electrode metal, leakage current and resolution together – the literature is full of counts-only demonstrations that are not comparable. For optics, say which gap is quoted and how it was measured, since transmission and ellipsometry give different numbers for BiI3. Exfoliated PbI2 is beam-sensitive: vacancies form and migrate during imaging, so structural conclusions need dose control.

For engineers · your lens

PbI2 already matters industrially as the precursor for perovskite photovoltaics. As detectors, both compounds are attractive on stopping power and cost but held back by crystal quality, leakage and, for PbI2, lead toxicity in a medical-imaging context. Neither is close to replacing CdZnTe.

In the research tracks

Growth methods

Recent news

The newest items tagged PbI2, BiI3, from the news feed updated 5 Oct 2026.

Preprintnot yet peer reviewed arXiv

Moiré Topology in Twisted Structures with Noncollinear Spin-Orbit Coupling

Moiré superlattices provide a powerful platform for flat bands and correlated topological phases, yet most established examples rely on valley-contrasting Berry curvature in hexagonal lattices. Here, we propose a different route to achieve topological moiré minibands based on noncollinear spin orbit coupling in…

TheoryPbI₂, BiI₃
Preprintnot yet peer reviewed arXiv

Breaking the Boundaries of the Goldschmidt Tolerance Factor with Ethylammonium Lead Iodide Perovskite Nanocrystals

We report the synthesis of ethylammonium lead iodide (EAPbI3) colloidal nanocrystals as another member of the lead halide perovskites family. The insertion of an unusually large A-cation (274 pm in diameter) in the perovskite structure, hitherto considered unlikely due to the unfavorable Goldschmidt tolerance factor…

ExperimentPbI₂, BiI₃
Preprintnot yet peer reviewed arXiv

Features of Surface Structuring of Direct and Indirect Band Gap Semiconductors by Femtosecond Laser

The impact of femtosecond (fs) laser radiation on semiconductors with direct (ZnSe, GaAs, CdZnTe) band gap, with the structurally induced direct-to-indirect band gap transition (PbI2, GaSe) and indirect band gap (Si) has been studied. The fs-laser treatment of semiconductors has been performed in the multi-pulse regime…

ExperimentEngineeringInSe, GaSePbI₂, BiI₃

All 8 items tagged PbI₂, BiI₃ in the news feed  ·  RSS feed for PbI₂, BiI₃

Key references

  1. Bismuth tri-iodide crystal for nuclear radiation detectorsMatsumoto et al. · IEEE Transactions on Nuclear Science 49, 2517 (2002)cited by 67doi:10.1109/TNS.2002.803883
  2. Band gap and structure of single crystal BiI3: Resolving discrepancies in literaturePodraza et al. · Journal of Applied Physics 114, 033110 (2013)cited by 148doi:10.1063/1.4813486
  3. Growth, fabrication, and testing of bismuth tri-iodide semiconductor radiation detectorsGokhale et al. · Radiation Measurements 74, 47 (2015)cited by 39doi:10.1016/j.radmeas.2015.01.015
  4. Atomic structure and defect dynamics of monolayer lead iodide nanodisks with epitaxial alignment on grapheneSinha et al. · Nature Communications 11, 823 (2020)cited by 61doi:10.1038/s41467-020-14481-z
Layered halides and perovskites 2D perovskites Layered halide perovskites van der Waals crystal
type
semiconductor
thickness
~0.6 nm per octahedral layer
band gap
1.6–2.4 eV
most mature
pilot
Passivation and capping layers in perovskite solar cells Mixed-anion layers BiOCl, BiOBr, BiOI Bismuth oxyhalides van der Waals crystal
type
semiconductor
thickness
~0.74 nm per BiOCl layer
band gap
1.8–3.2 eV
most mature
deployed
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type
semiconductor
thickness
~0.65 nm per Te–Bi–Br layer
most mature
lab
Spin–orbit spintronics, such as spin–charge conversion Mixed-anion layers SbSI, BiSI Chalcohalide chains SbSI and BiSI van der Waals crystal
type
semiconductor
thickness
Chains are ~1 nm across
band gap
1.6–2 eV
most mature
lab
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