The Surface Space Charge (Surface Dipole)• Free electrons spill out into vacuum and leave partial positive charge in the bulk to form a surface dipole layer.• For metals, the spatial extent is limited to the topmost layer of atoms at the interface.• For semiconductors or insulators, this dipole layer may be tens or hundreds of atomic layers into the bulk.• The electron density exhibits fluctuations on the bulk side (FriedelOscillation).
Spatial Extent of Surface Dipole LayerThe Debye length increases when the bulk electron density decreases.This is why the surface dipole layer extends much deeper into semiconductors or insulators than in metals. -------+++ ++ ++ + + +Surface Charge (-)Conduction band edgeFermi levelIonized donorxx=0x=dSpace charge regionRegion of space charge neutrality02the height of the effective potential energy at the interface relative to the bulk Fermi levelthe electron density in the bulksbulkesbulkeVdenVnεε=Debye length
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Surface Space Charge at Solid-Liquid Interface• The formation of Helmholtz layer affects the charge transfer at the solid-liquid interface.• The charged solid-liquid interfacial layer helps stabilize colloid systems.ElectrodeElectrolyte--------++++++++++++++-+------+++++++Helmholtz LayerDiffuse LayerPotentialDistance to Electrode