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Chem120A+Notes+-+Charge+Separation+at+Interfaces

Chem120A+Notes+-+Charge+Separation+at+Interfaces - Charge...

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Charge Distribution at Interfaces
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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 (Friedel Oscillation).
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Spatial Extent of Surface Dipole Layer The 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 edge Fermi level Ionized donor x x=0 x=d Space charge region Region of space charge neutrality 0 2 the height of the effective potential energy at the interface relative to the bulk Fermi level the electron density in the bulk s bulk e s bulk e V d en V n εε = 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. Electrode Electrolyte - - - - - - - - + + + + + + + + + + + + + + - + - - - - - - + + + + + + + Helmholtz Layer Diffuse Layer Potential Distance to Electrode
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