Quasi-2D Tin Iodide Perovskites

• Quasi-2D TIPs are lead-free Sn–I perovskites with reduced 3D connectivity, forming diverse architectures such as layered ABX3 polymorphs, RP nanosheets, and quantum-well structures.
• They exhibit tunable optoelectronic properties including widened band gaps, flat-band features, broadband self-trapped exciton emission, and phase-dependent lasing behavior.
• Precursor coordination and solvent chemistry critically steer dimensional control, phase competition, and stability, providing pathways for tailored synthesis and device optimization.

Quasi-2D tin iodide perovskites (TIPs) are lead-free Sn–I halide perovskites in which the inorganic connectivity is reduced from a fully three-dimensional corner-sharing network to layered, finite-thickness, or otherwise low-dimensional motifs. In the literature, this category spans at least three structurally distinct cases: layered polymorphs of nominally $ABX_3$ tin iodides in which the three-dimensional SnI$_6$ network is broken along one dimension; conventional Ruddlesden–Popper (RP) phases such as $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$; and quasi-2D quantum-well compounds of the form $(5IPA3)_2(MA)_{n-1}Sn_nI_{3n+1}$, where $n$ denotes the inorganic slab thickness. Across these classes, quasi-2D TIP behavior is governed by unusually small energy differences between competing structures, strong sensitivity to precursor coordination chemistry, and optical responses ranging from flat-band electronic structures and widened band gaps to broadband self-trapped-exciton emission and room-temperature lasing in air (Huan et al., 2015, Ghimire et al., 2022, Schütt et al., 2023, Cho et al., 10 Jul 2025).

1. Structural classes and crystallographic motifs

The structural scope of quasi-2D TIPs is broader than a single homologous series. First-principles work on CH$_3$NH$_3$SnI$_3$, NH$_4$SnI$_3$, and HC(NH$_6$0)$_6$1SnI$_6$2 showed that, in addition to the commonly known motif in which corner-shared SnI$_6$3 octahedra form a three-dimensional network, these materials may also favor a two-dimensional layered motif formed by alternating layers of connected SnI$_6$4 octahedra and A-site cations. In that layered motif, the conventional 3D network is “completely broken along one dimension,” the 3D structures retain the topology of the ideal cubic perovskite in a pseudo-cubic geometry, and the octahedral layers are strongly shifted with respect to each other. Because the compositions remain $_6$5, these phases are best regarded as competing layered polymorphs rather than RP phases with altered stoichiometry (Huan et al., 2015).

A distinct quasi-2D realization is the RP nanosheet $_6$6, described as a 2D octylammonium tin iodide perovskite nanosheet in which inorganic tin iodide layers are separated by octylammonium organic layers. Its powder XRD pattern shows periodically spaced diffraction peaks below $_6$7 $_6$8, interpreted as oriented growth and periodic stacking of a 2D layered structure. A third realization is the 5IPA3-based layered series $_6$9, where $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$0 is 2-(3,5-dicarboxyphenoxy)ethan-1-aminium and $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$1 specifies the number of connected inorganic perovskite layers between organic spacer layers (Ghimire et al., 2022, Cho et al., 10 Jul 2025).

Class	Representative composition	Structural feature
Layered $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$2 polymorph	CH$(\mathrm{Octylammonium})_2\mathrm{SnI}_4$3NH$(\mathrm{Octylammonium})_2\mathrm{SnI}_4$4SnI$(\mathrm{Octylammonium})_2\mathrm{SnI}_4$5, NH$(\mathrm{Octylammonium})_2\mathrm{SnI}_4$6SnI$(\mathrm{Octylammonium})_2\mathrm{SnI}_4$7, HC(NH$(\mathrm{Octylammonium})_2\mathrm{SnI}_4$8)$(\mathrm{Octylammonium})_2\mathrm{SnI}_4$9SnI$(5IPA3)_2(MA)_{n-1}Sn_nI_{3n+1}$0	3D network broken along one dimension
RP 2D nanosheet	$(5IPA3)_2(MA)_{n-1}Sn_nI_{3n+1}$1	Tin iodide layers separated by octylammonium layers
Quasi-2D quantum-well series	$(5IPA3)_2(MA)_{n-1}Sn_nI_{3n+1}$2	Layered slabs with thickness indexed by $(5IPA3)_2(MA)_{n-1}Sn_nI_{3n+1}$3

This structural diversity is central to the field. A common misconception is that quasi-2D TIPs are restricted to classic RP stoichiometries. The $(5IPA3)_2(MA)_{n-1}Sn_nI_{3n+1}$4 polymorph study instead shows that low-dimensional Sn–I frameworks can emerge without changing nominal composition, whereas the octylammonium and 5IPA3 systems exemplify spacer-defined layered architectures.

2. Energetics, phase competition, and electronic consequences of layering

The energetic landscape of quasi-2D TIPs is unusually shallow. For CH$(5IPA3)_2(MA)_{n-1}Sn_nI_{3n+1}$5NH$(5IPA3)_2(MA)_{n-1}Sn_nI_{3n+1}$6SnI$(5IPA3)_2(MA)_{n-1}Sn_nI_{3n+1}$7, NH$(5IPA3)_2(MA)_{n-1}Sn_nI_{3n+1}$8SnI$(5IPA3)_2(MA)_{n-1}Sn_nI_{3n+1}$9, and HC(NH$n$0)$n$1SnI$n$2, the energy difference $n$3 at the LDA+SOC level is $n$4, $n$5, and $n$6 meV/atom, respectively. Cross-checks with LDA, PBE, HSE06, and vdW-DF2 preserve the same overall conclusion: CH$n$7NH$n$8SnI$n$9 slightly favors the 3D form, NH$_3$0SnI$_3$1 tends to favor the layered form, and HC(NH$_3$2)$_3$3SnI$_3$4 is essentially degenerate, with the sign depending on the functional. The authors therefore characterize the two motifs as “essentially comparable in energy” (Huan et al., 2015).

Phonon calculations performed with a $_3$5 supercell using phonopy show no meaningful instabilities beyond a numerical phonon error of about $_3$6 THz, roughly $_3$7 meV/atom, so all six selected structures correspond to local minima of the energy landscape. Within the harmonic approximation, the Helmholtz free energy was evaluated as

$_3$8

with the vibrational contribution sampled on a $_3$9 $_3$0-mesh. NH$_3$1SnI$_3$2 favors the 2D structure at low $_3$3 but becomes less stable than the 3D form at about $_3$4 K and above; CH$_3$5NH$_3$6SnI$_3$7 and HC(NH$_3$8)$_3$9SnI$_3$0 favor the 3D form over the temperature range examined. Even so, the paper emphasizes that the free-energy differences are comparable to thermal energy, stated as $_3$1 meV/atom at room temperature (Huan et al., 2015).

The 2D and 3D motifs are separated by low solid-state CI-NEB barriers of roughly $_3$2–$_3$3 meV/atom. The transformation involves breaking long out-of-plane Sn–I bonds, rotating in-plane Sn–I bonds and organic cations, and forming new Sn–I bonds. This suggests strong phase competition and a soft polymorphic landscape. A plausible implication is that synthesis conditions, interfaces, strain, and kinetic trapping can steer whether a nominally $_3$4 tin iodide manifests as a 3D network, a layered phase, or a mixture of both.

Layering also changes the electronic structure qualitatively. The 3D phases have high-curvature parabolic bands, whereas the 2D phases have many flat bands near the band edges and lifted CBM degeneracy. At the HSE06 level, the band gaps increase from $_3$5 to $_3$6 eV for CH$_3$7NH$_3$8SnI$_3$9, from $_4$0 to $_4$1 eV for NH$_4$2SnI$_4$3, and from $_4$4 to $_4$5 eV for HC(NH$_4$6)$_4$7SnI$_4$8 when going from 3D to 2D. The paper does not report effective masses, transport coefficients, or exciton binding energies for these polymorphs, but the flat edge bands imply a substantially more anisotropic low-dispersion electronic structure than in the 3D analogues (Huan et al., 2015).

3. Precursor coordination chemistry and the molecular origin of dimensional control

A precursor-level description of quasi-2D TIP formation emerges from the study of 14 tetracoordinated tin iodide solution complexes of the form $_4$9, where a $_3$0 unit is coordinated by four solvent molecules. The solvents include HMPA, DMPU, DEF, DMSO, DMI, DMAC, NMP, DMF, NMAC, 3MOx, GBL, PC, TMU, and ACN, classified by Gutmann donor number $_3$1. All considered complexes are energetically stable; the most stable complex is the HMPA adduct, the least stable is $_3$2, and formation energy becomes less favorable with decreasing donor number (Schütt et al., 2023).

The structural trend is systematic. High-$_3$3 solvents produce shorter Sn–M distances, longer Sn–I distances, and stronger perturbation of the tin-iodide backbone. Typical Sn–M distances lie in the range $_3$4–$_3$5 Å with an average around $_3$6 Å. Important exceptions are explicitly reported: one GBL does not bind in $_3$7, two PC molecules do not bind in $_3$8, and $_3$9 undergoes extreme distortion, with one Sn–I separation of about $_6$00 Å and the other almost $_6$01 Å. The electronic structure is likewise asymmetric: the highest occupied orbitals are overwhelmingly localized on the $_6$02 unit, whereas the LUMO is strongly solvent dependent. Representative SnI$_6$03-centered LUMO fractions range from $_6$04 in the PC complex to $_6$05 and $_6$06 in the DEF and DMF complexes. TDDFT further shows that the first optical excitation is generally weak because occupied and unoccupied frontier states have only partial wave-function overlap; the spectral weight is nevertheless red-shifted by solvent coordination relative to isolated $_6$07 (Schütt et al., 2023).

These precursor results do not directly calculate layered solids, phase selection, or spacer-cation competition. Even so, they provide a molecular-scale basis for quasi-2D TIP solvent engineering. A plausible implication is that strongly coordinating solvents stabilize molecularly coordinated Sn–I units, delay condensation into extended iodostannate networks, and alter the rate at which spacer cations can replace solvent ligands during layered assembly. Conversely, weak donors should promote faster desolvation and crystallization. The same study explicitly warns against direct transfer of Pb-perovskite solvent intuition to Sn systems: Sn complexes are more asymmetric, more strongly coordinated by high-donor solvents, and more structurally labile than their Pb analogues (Schütt et al., 2023).

4. Structural reconstruction, self-trapping, and broadband emission in 2D nanosheets

Colloidal 2D octylammonium tin iodide nanosheets provide a chemically different route to quasi-2D TIP behavior. The parent material is the RP perovskite $_6$08, synthesized by a modified hot-injection method using tin(II) oleate in diphenyl ether, with sequential injection of octylamine, tri-$_6$09-butylphosphine, and 1,2-diiodoethane at $_6$10, followed by slow cooling at about $_6$11. The as-synthesized red nanosheets are transparent under room light, luminescent under UV, emit under green excitation, and show an absorption onset at $_6$12 nm, an excitonic absorption peak at $_6$13 nm, a band gap of $_6$14 eV, and narrow PL at $_6$15 nm with FWHM $_6$16 nm, Stokes shift $_6$17 nm, and absolute PLQY $_6$18 (Ghimire et al., 2022).

This red phase undergoes an irreversible structural reconstruction into white hexagonal nanosheets upon repeated washing with hexane, washing with toluene, or light exposure; weaker vacuum during precursor drying ($_6$19 mbar rather than $_6$20 mbar at $_6$21) also produces white samples directly. XRD provides the main structural evidence. The red nanosheets show low-angle periodicity at $_6$22, corresponding to $_6$23 nm; subtracting the assumed tin iodide octahedron thickness of $_6$24 nm gives a $_6$25 nm interlayer organic region. The white phase shifts to $_6$26 or approximately $_6$27–$_6$28, corresponding to $_6$29–$_6$30 nm and an interlayer organic region of $_6$31 nm. The authors therefore propose expulsion of oleic acid ligands from the interlayer region, followed by lattice rearrangement with octylamine/octylammonium (Ghimire et al., 2022).

The optical reconstruction is large. The white phase has an absorption onset at $_6$32 nm, dual excitonic bands at $_6$33 and $_6$34 nm, a band gap of $_6$35 eV, broad PL peaking at $_6$36 nm with FWHM $_6$37 nm, Stokes shift $_6$38 nm, and absolute PLQY $_6$39. Average PL lifetimes extracted from bi-exponential fits are $_6$40 for the red nanosheets and $_6$41 ns for the white nanosheets; the abstract summarizes the white-phase lifetime as “about $_6$42.” During partial reconstruction, mixed-phase behavior appears under excitation-wavelength-dependent PL, with broad emission spanning roughly $_6$43–$_6$44 nm under $_6$45 nm excitation and narrowing toward $_6$46 nm as excitation increases from $_6$47 to $_6$48 nm (Ghimire et al., 2022).

The favored emission mechanism is not ordinary band-edge recombination and not dominant mid-gap defect emission. The white phase shows no additional absorption or PLE below band gap and no emission under below-band-gap excitation, so the authors argue against localized mid-gap emissive defect states. Instead, they attribute the emission to structural reconstruction, stereoactive Sn$_6$49 $_6$50 lone pairs, strong excited-state distortion, self-trapped excitons, and possible localization within tin iodide clusters formed during reconstruction. Their power-law analysis uses

$_6$51

with $_6$52 below 10 mW and $_6$53 above 10 mW for the red nanosheets, and $_6$54 below 8 mW for the white nanosheets. The resulting physical picture is effectively cluster-localized, lone-pair-assisted self-trapped-exciton emission in a reconstructed low-dimensional Sn–I matrix rather than emission intrinsic to an unmodified RP lattice (Ghimire et al., 2022).

5. Optical gain, microlasing, and ambient stability in quasi-2D TIP microcrystals

The most advanced photonic realization among the cited studies is the 5IPA3-based quasi-2D series $_6$55. Here $_6$56 denotes the inorganic slab thickness, and the quantum-well band-edge energies estimated from absorption-edge onset are $_6$57, $_6$58, $_6$59, and $_6$60 eV for $_6$61, respectively. Temperature-dependent PL fitted with

$_6$62

gives exciton binding energies of $_6$63 meV for $_6$64 and $_6$65 meV for $_6$66. The paper also states that for 5IPA3-$_6$67, the 3D Mott transition density for transparency exceeds $_6$68, with high-$_6$69 cases lying roughly between $_6$70 and $_6$71. This is the basis for the observed gain hierarchy: dielectric lasing occurs only for $_6$72, whereas plasmonic lasing occurs for $_6$73 and $_6$74 (Cho et al., 10 Jul 2025).

The structural design centers on the spacer 5IPA3, which contains two carboxyl groups and is proposed to form a robust, directional hydrogen-bonding network. For $_6$75, the organic sublattice thickness is $_6$76 Å, the inorganic perovskite layer thickness is $_6$77 Å, the dielectric permittivity is approximately $_6$78, and the refractive index is estimated as $_6$79. The authors further state that sub-nanometer-thick organic lattices are expected to allow sub-picosecond out-of-plane exciton transport via tunneling and out-of-plane exciton diffusion lengths of $_6$80 nm. Microplates were grown by fast recrystallization from aqueous solvent and then sonicated in the mother solution; for 5IPA3-$_6$81, the median length is $_6$82m, mean width $_6$83m, median thickness $_6$84 nm, and the thickness corresponds to 131 quantum wells (Cho et al., 10 Jul 2025).

On a dielectric Si/$_6$85 substrate with 200 nm thermal oxide, optically pumped $_6$86 microcrystals show broad spontaneous emission centered at $_6$87 nm with linewidth $_6$88 nm and narrowband single-mode lasing at $_6$89 nm above threshold. Under 532 nm, 5 ns pumping, the threshold is $_6$90, corresponding to $_6$91, the narrowest measured linewidth is $_6$92 nm, and the lasing $_6$93-factor is 2850 with $_6$94. The smallest lasing microcrystal has a longest dimension of approximately $_6$95m. FDTD identifies the mode as whispering-gallery-like in a square cavity; for a dielectric 4-$_6$96m particle, the simulated cold-cavity $_6$97 is 200, the mode volume is $_6$98, and the required gain coefficient for room-temperature lasing is $_6$99 (Cho et al., 10 Jul 2025).

On ultrasmooth gold, hybrid plasmonic-photonic modes extend lasing to thinner quasi-2D phases. Both $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$00 and $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$01 lase at room temperature in ambient air. Under nanosecond pumping, the threshold range is $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$02 to $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$03; for $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$04, Supplementary Table S3 gives $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$05, equivalent to $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$06, with emission at $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$07 nm, linewidth $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$08 nm, $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$09, and $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$10. FDTD gives a cold-cavity $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$11 of 50, a mode volume of $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$12, a Purcell factor of 2, and an intrinsic gain requirement of $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$13. Lifetime shortening on gold relative to oxide/silicon supports faster recombination and is interpreted as approximately twofold radiative-rate enhancement (Cho et al., 10 Jul 2025).

Under picosecond pumping near the band edge, the same material shows its strongest operational stability. For $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$14 microcrystals on gold, 765 nm, 70 ps, 2.5 MHz pumping yields a typical threshold of $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$15 per pulse, about four times lower than for nanosecond pumping, with linewidth $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$16 nm at $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$17 threshold and $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$18. The authors present this work as the first air-stable, room-temperature lasing from quasi-2D TIP microcrystals under ambient conditions. Dark-air stability of 5IPA3-$(\mathrm{Octylammonium})_2\mathrm{SnI}_4$19 reaches up to $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$20 min, compared with rapid degradation of PEA-$(\mathrm{Octylammonium})_2\mathrm{SnI}_4$21; under 375 nm CW illumination at $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$22, the mean photostability lifetime is $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$23 min for 5IPA3-$(\mathrm{Octylammonium})_2\mathrm{SnI}_4$24 versus $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$25 min for PEA-$(\mathrm{Octylammonium})_2\mathrm{SnI}_4$26. Under picosecond lasing conditions, emission persists for over $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$27 pump pulses, and in one example for half a billion pulses, although nanosecond pumping still causes mode hopping after $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$28 shots and severe intensity loss after $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$29 shots (Cho et al., 10 Jul 2025).

6. Conceptual implications, misconceptions, and unresolved problems

Several general conclusions follow from these studies. First, quasi-2D TIPs are not a single structural family. They include same-stoichiometry layered polymorphs of $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$30 compounds, canonical spacer-separated RP phases, reconstructed low-dimensional derivatives, and finite-$(\mathrm{Octylammonium})_2\mathrm{SnI}_4$31 layered quantum wells. Treating all quasi-2D TIPs as interchangeable obscures major differences in octahedral connectivity, interlayer chemistry, and electronic structure (Huan et al., 2015, Ghimire et al., 2022, Cho et al., 10 Jul 2025).

Second, intense broadband emission in low-dimensional TIPs should not automatically be assigned to simple defect luminescence. In the reconstructed octylammonium nanosheets, the absence of below-band-gap absorption or PLE and the very large Stokes shift led the authors to reject dominant mid-gap emissive defect states in favor of self-trapped excitons promoted by Sn$(\mathrm{Octylammonium})_2\mathrm{SnI}_4$32 $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$33 lone pairs and possible tin iodide clusters (Ghimire et al., 2022).

Third, improved air stability does not imply complete elimination of instability. The 5IPA3 strategy substantially delays degradation and enables ambient-air, room-temperature lasing, but the microcrystals still eventually oxidize, nanosecond pumping still produces rapid degradation, and the paper explicitly lists open questions on electrically pumped lasing, the balance between excitonic and electron-hole-plasma gain, and the chemistry of the dark purple intermediate phase (Cho et al., 10 Jul 2025).

Fourth, the solvent in Sn-based quasi-2D processing is not a passive background. The $(\mathrm{Octylammonium})_2\mathrm{SnI}_4$34 study shows that donor number changes precursor geometry, Sn–I bond strength, frontier-state localization, and the first optical excitations. A plausible implication is that solvent identity helps determine not only solubility and viscosity but also dimensional selection, intermediate stability, and defect formation during film growth (Schütt et al., 2023).

Significant gaps remain. The layered-$(\mathrm{Octylammonium})_2\mathrm{SnI}_4$35 polymorph study does not provide transport, excitonic, or defect calculations and treats phonons and free energies in the harmonic approximation; the octylammonium reconstruction study does not solve the crystal structure of the white phase; and the microlaser study is optically pumped only. Together, these limitations indicate that quasi-2D TIP research has established structural plausibility, precursor sensitivity, emissive reconstruction pathways, and ambient photonic functionality, but not yet a unified predictive framework for phase selection, degradation chemistry, and electrically driven device operation (Huan et al., 2015, Ghimire et al., 2022, Schütt et al., 2023, Cho et al., 10 Jul 2025).