Emerging Semiconductor Material's Values

Material	Structure	Bandgap $E_g$ (eV)	Type	Key Dependency	Notes
$\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ (MAPbI${}_3$)	Perovskite ($AB{X}_3$); tetragonal at 300 K, cubic $Pm\bar{3}m$ above $\approx 330\mathrm{K}$	$\approx 1.55\text{–}1.60$	Direct	Halide composition, phase (temperature)	The original benchmark hybrid halide perovskite for solar cells; degrades under heat and moisture, which drove the move toward FA- and Cs-based compositions below.
$\mathrm{CH}(\mathrm{N}{\mathrm{H}}_2{)}_2\mathrm{Pb}{\mathrm{I}}_3$ (FAPbI${}_3$)	Perovskite, trigonal/cubic $\alpha$-phase (photoactive) vs. hexagonal $\delta$-phase (non-photoactive)	$\approx 1.48$	Direct	Phase stability (α vs. δ), compositional additives	Lower bandgap than MAPbI${}_3$ (better single-junction match to the solar spectrum) but the desired $\alpha$-phase is metastable at room temperature without compositional engineering (e.g. Cs or Br incorporation).
$\mathrm{CsPb}{\mathrm{I}}_3$	Perovskite; cubic $Pm\bar{3}m$ ($\alpha$-phase, high-$T$) or orthorhombic (non-perovskite $\delta$-phase, RT-stable)	$\approx 1.73$ (α-phase)	Direct	Phase (α vs. δ), particle size/strain	All-inorganic perovskite, more thermally robust than the hybrid organic-inorganic compounds above; commonly used as the wide-gap top cell in perovskite tandem solar cells.
$\mathrm{CsPbB}{\mathrm{r}}_3$	Perovskite; orthorhombic at 300 K, cubic $Pm\bar{3}m$ above $\approx 403\mathrm{K}$	$\approx 2.3$	Direct	Halide composition, quantum confinement (nanocrystal size)	More environmentally stable than iodide perovskites; widely used in perovskite LEDs and as quantum-dot nanocrystals with size-tunable emission.
$\mathrm{Mo}{\mathrm{S}}_2$ (monolayer)	2D hexagonal (2H); monolayer point group $D_{3h}$ (no inversion center)	$\approx 1.8\text{–}1.9$	Direct	Layer number	Bulk 2H-$\mathrm{Mo}{\mathrm{S}}_2$ (space group $P{6}_3/mmc$) is indirect ($\approx 1.2\text{–}1.3\mathrm{eV}$); the transition to a direct gap as thickness is reduced to a single layer is the defining feature of this material class.
$\mathrm{W}{\mathrm{S}}_2$ (monolayer)	2D hexagonal (2H); monolayer point group $D_{3h}$	$\approx 2.0\text{–}2.1$	Direct	Layer number	Bulk indirect gap $\approx 1.3\mathrm{eV}$; larger spin-orbit splitting than $\mathrm{Mo}{\mathrm{S}}_2$ due to heavier W atom, relevant for valleytronics research.
$\mathrm{MoS}{\mathrm{e}}_2$ (monolayer)	2D hexagonal (2H); monolayer point group $D_{3h}$	$\approx 1.5\text{–}1.6$	Direct	Layer number	Bulk indirect gap $\approx 1.1\mathrm{eV}$.
$\mathrm{WS}{\mathrm{e}}_2$ (monolayer)	2D hexagonal (2H); monolayer point group $D_{3h}$	$\approx 1.6\text{–}1.7$	Direct	Layer number	Bulk indirect gap $\approx 1.2\mathrm{eV}$.
Graphene	2D hexagonal (honeycomb); point group $D_{6h}$ (monolayer)	$0$	Zero-gap (semimetal, linear/Dirac dispersion)	Number of layers, applied field (bilayer)	Not a semiconductor in the conventional sense — no bandgap exists to open without external intervention (e.g. a perpendicular field in bilayer graphene, or lateral confinement into nanoribbons). Included here as the reference point for the 2D materials family.
Black phosphorus (few-layer)	2D orthorhombic (puckered honeycomb); layer point group $C_{2h}$	$\approx 0.3\text{–}2.0$	Direct	Layer number, in-plane crystal axis (anisotropic)	Bandgap tunes continuously and strongly with thickness, from $\approx 0.3\mathrm{eV}$ (bulk) to $\approx 2.0\mathrm{eV}$ (monolayer, “phosphorene”) — a wider and more continuous range than the TMDs above. Highly anisotropic in-plane due to its puckered structure, and chemically unstable in ambient air without encapsulation.

Important Questions

Ques: Why don’t these materials get single, settled property values the way Si or GaAs do?

Every material in this table has at least one property that is not fixed by chemical composition alone. The halide perovskites are soft, ionic crystals whose structural phase (and therefore bandgap) shifts with temperature, humidity, strain, and even measurement history, and their halide/cation stoichiometry is often intentionally mixed (e.g. mixed-halide or mixed-cation perovskites) specifically to tune the gap — so there is no single “the” bandgap the way there is for a covalently bonded bulk crystal like Si. The TMDs and other 2D materials have properties that depend on layer count because reducing a van der Waals-bonded crystal to a monolayer removes interlayer coupling entirely, fundamentally changing the electronic structure (indirect bulk gap to direct monolayer gap) rather than just shifting it slightly. Values in this table should therefore be read as representative points for a specific, stated condition (a given phase, a given layer count) rather than fixed material constants. Reference: A. Kojima et al., “Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells,” J. Am. Chem. Soc. 131, 6050 (2009); K.F. Mak et al., “Atomically Thin MoS${}_2$: A New Direct-Gap Semiconductor,” Phys. Rev. Lett. 105, 136805 (2010).

Ques: Why is graphene included in a semiconductor materials table when it has no bandgap?

Graphene is the structural and conceptual parent of the entire 2D-materials family that includes the TMDs above, and it is frequently discussed alongside them in device research (as an electrode, a heterostructure component, or a comparison point), so it is included here as a reference despite not being a semiconductor itself. Its zero-gap, linear (“Dirac”) band dispersion is precisely what distinguishes it from the TMDs, which is why engineering any usable bandgap into graphene-based devices (via nanoribbons, bilayer fields, or chemical functionalization) has been a major research thread in its own right. Reference: K.S. Novoselov et al., “Electric Field Effect in Atomically Thin Carbon Films,” Science 306, 666 (2004).