Semiconductor surface core level shift

Similar effects can also occur in surface electronic structure when a moiety is weakly physisorbed onto the surface. The surface core-level shifts measured at the vacuum interface are reduced when atoms or molecules are physisorbed onto the surface. Changes may also occur in the valence electronic structure upon physisorption, such as the disappearance of intrinsic surface states on metals and semiconductors. 

Table 5.2-15 Surface core level shifts for elemental semiconductors ...

Grains of semiconductors of a few nanometers across emit blue-shifted light but the larger chunks of the same material do not [30, 31]. The band gap (Eq) of CdSe can be tuned from deep red (1.7 eV) to green (2.4 eV) by simply reducing the solid diameter from 20 to 2 nm [32]. The Eg [33, 34] and the core-level shift (AE ) [35] of nanosemiconductors increase whereas the dielectric susceptibiUty (x) decreases when the solid size is reduced. Without triggering the electron-phonon (e-p) interaction or electron-hole (e-h) production, STS/M revealed that at 4 K the Eg expands from 1.1 to 3.5 eV when the diameter of Si nanorod reduces from 7.0 to 1.3 nm associated with some 10 % of Si-Si surface-bond contraction [36]. 

In terms of BOLS perturbation to the Hamiltonian of an extended solid, one is able to reconcile the change of Eq, pl, pa, bandwidth, core-level shift, and the charge entrapment and polarization induced by crystal size reduction. Introducing the effect of CN imperfection in the surface skin to the convention of an extended solid evolves the entire band stmcture of a nanometric semiconductor. This approach allows one to discriminates the contribution from crystal binding from the effect of e-p coupling in determining the Eq expansion and PL blueshift. 

In semiconductors, core levels are shifted because of charge transfer between surface atoms. Table 5.2-15... 

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