InP is a III–V phosphide semiconductor used to make cadmium-free quantum dots. Quantum confinement of the exciton in a sphere of radius R sets the effective bandgap (a Brus-style relation, drawn here as a 2D disc cross-section through the dot's centre):
E_g(R) ≈ E_bulk + ħ²π² / (2 μ R²)
E_bulk(InP) = 1.35 eV, R in nm
Smaller cores blue-shift the emission; larger cores red-shift it — this is the size-tunable color effect used in QLED displays and biological imaging.
Unlike CdSe, a bare InP surface oxidizes readily and carries dangling-bond trap states that capture the exciton non-radiatively before it can emit a photon. Growing an epitaxial ZnS shell buries the core and exponentially suppresses tunneling of the exciton wavefunction to those surface traps:
k_nr(t) = k_floor + k_defect · exp(−2κt)
QY = k_r / (k_r + k_nr)
Here t is the shell thickness (in monolayers, 1 ML ZnS ≈ 0.31 nm), κ is an effective tunneling decay constant set by the ZnS confinement barrier, and k_defect scales with the native-oxide defect level on the core surface before shelling. Raising temperature adds a thermally-activated (Arrhenius) escape channel that also feeds non-radiative decay, so quantum yield falls at higher T even for a well-shelled dot.
- Core radius — sets the confinement energy and hence emission color (the disc's own glow color).
- Shell thickness — the physical fix real InP/ZnS synthesis uses: each extra monolayer exponentially cuts trap capture, visible as fewer red spark events on the rim.
- Native-oxide defect level — how poorly the P-precursor chemistry passivated the bare core before shell growth; scales the trap density the shell has to bury.
- Temperature — thermal quenching; watch QY fall as the core heats even with a thick shell.
- Scroll to zoom, drag to pan the cross-section; the bottom chart plots quantum yield against shell thickness for your current core/defect/temperature settings, with the live point marked.
This is the real reason commercial InP quantum dots (Samsung, Nanosys QLED TVs; InP/ZnS biolabels) always ship with a shell — a bare phosphide core alone typically reaches only a few percent QY.