Energigaps

energigaps are energy gaps in a system's spectrum—the energy ranges where no quantum states exist. In crystalline solids, energigaps correspond to band gaps between the valence band, which is filled, and the conduction band, which is typically empty at zero temperature. The size of an energigap, usually measured in electronvolts (eV), helps determine whether a material behaves as a conductor (very small or zero gap), a semiconductor (about 0.1 to 3 eV), or an insulator (greater than about 3 eV). Direct energigaps allow electron transitions between bands with little momentum change, producing strong optical absorption and emission, whereas indirect gaps require phonon involvement and have weaker optical responses.

Energigaps are influenced by composition, crystal structure, and external conditions. Temperature can alter a gap, and pressure or strain can widen or narrow it. Quantum confinement in nanoscale structures tends to increase energigaps. Doping introduces impurity levels within the gap and can modify optical properties, but it does not erase the intrinsic gap of the host material.

Common examples include silicon around 1.12 eV at room temperature and gallium arsenide around 1.43 eV. Diamond has a large gap near 5.5 eV. In two-dimensional materials, monolayer MoS2 has a direct gap near 1.8 eV, while bulk MoS2 has an indirect gap. Graphene has an effectively zero gap, though external fields or symmetry breaking can open a gap.

Measuring energigaps can be done via optical absorption and photoluminescence spectroscopy, UV–vis spectroscopy, and scanning tunneling spectroscopy. Understanding energigaps is central to designing electronic and optoelectronic devices such as diodes, transistors, LEDs, and solar cells. See also band theory and direct versus indirect gaps.