Confine an electron in a box of size d along N of 3 spatial axes. Along each confined axis the momentum is quantized: E_n = n²ħ²π²/(2m*d²), n = 1,2,3…
3D bulk g(E) ∝ √E (smooth, continuous)
2D well g(E) ∝ Σₙ Θ(E−Eₙ) (staircase steps)
1D wire g(E) ∝ Σₙ 1/√(E−Eₙ) (Van Hove spikes)
0D dot g(E) ∝ Σₙ δ(E−Eₙ) (discrete lines)
- Dimensionality — how many axes are squeezed to nanoscale d; the rest stay macroscopic (free).
- Confined size d — smaller d pushes subband levels Eₙ further apart (E₀ ∝ 1/d²).
- Fermi filling — shading below the Fermi energy shows which states are occupied.
This is the mechanism behind quantum-dot LED colour, 2D-electron-gas Hall devices and nanowire transistors: squeezing a electron gas one dimension at a time reshapes its density of states from a smooth curve into sharp discrete structure.