Fermi level equilibration

Metal islands or nanoparticles deposited on a semiconductor surface undergo Fermi level equilibration following charging with photogenerated electrons. The effect of Fermi level equilibration is predominantly seen when the metal deposits consist of small islands or small particles. Unlike the ohmic contact observed in bulk metals, the nanoparticles retain the charge before transferring them to the redox species. During extended UV-photolysis, electron-capture by metal islands of Ag, Au and Cu becomes inhibited as their Fermi level shifts close to the conduction band of the semiconductor. Pt on the other hand acts as an electron sink and fails to achieve Fermi level equilibration. 

By employing Cgo/Cgo as a probe-redox couple it is possible to estimate the apparent Fermi levels of Ti02 and TiOz/Au nanopartides [184]. The partide size of 

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Fermi level equilibration in quantum dot-metal nanojunctions, J. Phys. Chem. B 105 (2001) 8810-8815. 

V. Subraniam, E. Wolf, P.V. Kamat, Catalysis with Ti02/gold nanocomposites. Effect of metal particle size on the Fermi level equilibration, J. Am. Chem. Soc. 126 (2004) 4943-4950. 

Figure 9.18 Illustration of Fermi-level equilibration process upon irradiation of semiconductor-supported metal nanoparticles. (Reprinted with permission from [193]. Copyright 2004 American Chemical Society.)...

The interplay between energy levels associated with the semiconductor support and the metal nanoparticle dopant has been discussed recently in terms of metal particle size effect on Fermi-level equilibration (Figure 9.18) [193]. 

Subramanian, V., Wolf, E. E., Kamat, P. (2003). Green emission to probe photoinduced charge events in ZnO-Au nanoparticles, charge distribution and fermi-level equilibration. Journal of Physical Chemistry B, 107, 7479—7485. http //dx.doi.org/10.1021/jp0275037. 

The Fermi level in lead (which is a base metal) lies at an higher energy than in the silver particles. A mechanism has been proposed in which electron tunneling fi om lead to silver occurs in close encounters of the particles. Pb ions are then detached from the lead particles into solution. They are subsequently reduced on the surface of the negatively charged silver particles. The rate of Fermi level equilibration was found to be drastically increased by the presence of small concentrations of methyl viologen, MV . ... 

Figure 21. Relay mechanism of Fermi level equilibration between Ag and Pb particles in the presence of methylviologen. ...

Kongkan A, Kamat PV (2007) Electron storage in single wall carbon nanotubes. Fermi level equilibration in semiconductor—SWCNT suspensions. ACS Nano 1 13-21... 

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