Study and characterization of diamond surface for biosensoring applications

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1.2– Diamond Surface Figure 1.9. Surface conductance of the hydrogenated (masked) and the de-hydrogenated (irradiated) part of a diamond (100) [41] in fig.1.9, both halves of the sample are in the low conductance state and remain so as long as they are kept in UHV. However, when they are brought up to air, the conductance of the masked and thus hydrogenated area rises by 4 orders of magnitude within the first twenty minutes of exposure and increases more slowly thereafter until it reaches 10 −5 A/V after three days. By contrast, the dehydrogenated part of the sample remains in its low con- ductance state with no sign of change whatsoever. This experiment clearly demonstrates that the hydrogenation of diamond is a necessary but not a sufficient condition for high surface conductivity. An additional ingredient, that is obviously coming from the air and that thermally desorbs in UHV above 400 ◦ C, is necessary. In order to act as an acceptor, the adsorbate must have its lowest unoc- cupied electronic level below the VBM of diamond. With an electron affinity χ C:H = -1.3 eV for hydrogenated diamond (fig.1.10), this requirement sets a lower limit for the electron affinity χ ad of the adsorbate: χ ad = E g - 1.3 eV = 4.2 eV, where E g = 5.5 eV is the band gap energy of diamond. Electron affinities of molecular atmospheric species lie below 2.5 eV and even for halo- gen atoms χ ad does not exceed 3.7 eV. Thus, direct electron transfer from the diamond into an atmospheric adsorbate appears to be impossible. However, a thin water layer, as it forms naturally on all surfaces exposed to atmosphere, provides an electron system which can act as a surface ac- ceptor for diamond. Electrons exchange from diamond to the water layer is governed by the redox reaction: 19

Anteprima della Tesi di Micaela Castellino

Anteprima della tesi: Study and characterization of diamond surface for biosensoring applications, Pagina 16

Tesi di Dottorato

Dipartimento: Dipartimento di Fisica Sperimentale

Autore: Micaela Castellino Contatta »

Composta da 180 pagine.

 

Questa tesi ha raggiunto 84 click dal 22/06/2011.

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