10.2 ZnO Nanocrystals Passivated with H, N, and O
211
10.2.2 Band Gap, Work Function, and Magnetism
Annealing the specimen up to 900 °C under the ambient pressure of 0.03 H 2 + 0.97
Ar (type I) and 0.21 O 2 + 0.79 N 2 (type II) for 24 h create two kinds of defected
ZnO [29]. Type I (H-induced) shows the PL energy at 2.46 eV and type II (O and
N induced) at 2.26 eV (Fig. 10.3c). Annealing under pure O 2 at ambient pressure
lowers the PL peak energy to an even lower value of 2.15 eV [29].
N and O passivation reduces the band gap of ZnO rather than enlarges it because
of the potential weakening by lone pair induced polarization. The band gap and the
CLS as well, is proportional to the bond energy. Dipoles formation narrows the band
gap by screening the crystal potential and band tail creation. For both I and II types,
the VB maximum moves down, and the VB expands slightly as the relative intensity
of the green emission to that of UV emission increases. However, H passivation
removes the dipoles of unpaired electrons by hydrogen termination, which removes
the band tails and widens the band gap.
The skin dipoles are critical to the dilute magnetism, electron emission, and
hydrophobicity of ZnO. The effects of under- and hetero-coordination do enhance
each other in modulating the band gap, work function, electroaffinity, and the density
of surface dipoles that determine surface hydrophobicity. These attributes make the
sharp edge of ZnO structures hydrophobic, magnetic, and easy to emit electrons and
sensitive in photocatalysis.
10.3 Scratched SrTiO 3 Skin: Defect States
An in situ XPS measurement confirmed the defect-enhanced entrapment and polarization at the SrTiO 3 skin shown in Fig. 10.4 [33]. The ZPS of two VB spectra
collected from the SrTiO 3 skin before and after 3-keV Ar
+ bombarding for 20 min creating the additional polarization at 1 eV and entrapment at 12 eV. O 1s band exhibits
a 0.6 eV entrapment without presence of polarization because of its low sensitivity. These observations confirmed the BOLS-NEP expectation that defect creation
by surface roughening enhances the entrapment and polarization. The valence ZPS
shows the features of polarization, electron-hole pair production, and the entrapment,
which is the same to the defected TiO 2 (see Fig. 10.4a).
Table 10.2 summarizes the irregular-coordination coupling effect on the electronic structures of ZnO and SrTiO 3 nanocrystals. The photoluminescence energy
correlates to the CLS with involvement of electron-phonon coupling as an addition [34, 35]. Atomic CN reduction by creating defect enhances the BOLS-NEP
effect. The chemical passivation and undercoordination enhances each other on the
charge entrapment and polarization.
Mechanical scratching of the SiO 2 surface [36] and O-plasma etching of the
HOPG [37] also induce the local charge entrapment for the same reason of underand hetero-coordination combination. Using the ZPS technique, one can readily
211
10.2.2 Band Gap, Work Function, and Magnetism
Annealing the specimen up to 900 °C under the ambient pressure of 0.03 H 2 + 0.97
Ar (type I) and 0.21 O 2 + 0.79 N 2 (type II) for 24 h create two kinds of defected
ZnO [29]. Type I (H-induced) shows the PL energy at 2.46 eV and type II (O and
N induced) at 2.26 eV (Fig. 10.3c). Annealing under pure O 2 at ambient pressure
lowers the PL peak energy to an even lower value of 2.15 eV [29].
N and O passivation reduces the band gap of ZnO rather than enlarges it because
of the potential weakening by lone pair induced polarization. The band gap and the
CLS as well, is proportional to the bond energy. Dipoles formation narrows the band
gap by screening the crystal potential and band tail creation. For both I and II types,
the VB maximum moves down, and the VB expands slightly as the relative intensity
of the green emission to that of UV emission increases. However, H passivation
removes the dipoles of unpaired electrons by hydrogen termination, which removes
the band tails and widens the band gap.
The skin dipoles are critical to the dilute magnetism, electron emission, and
hydrophobicity of ZnO. The effects of under- and hetero-coordination do enhance
each other in modulating the band gap, work function, electroaffinity, and the density
of surface dipoles that determine surface hydrophobicity. These attributes make the
sharp edge of ZnO structures hydrophobic, magnetic, and easy to emit electrons and
sensitive in photocatalysis.
10.3 Scratched SrTiO 3 Skin: Defect States
An in situ XPS measurement confirmed the defect-enhanced entrapment and polarization at the SrTiO 3 skin shown in Fig. 10.4 [33]. The ZPS of two VB spectra
collected from the SrTiO 3 skin before and after 3-keV Ar
+ bombarding for 20 min creating the additional polarization at 1 eV and entrapment at 12 eV. O 1s band exhibits
a 0.6 eV entrapment without presence of polarization because of its low sensitivity. These observations confirmed the BOLS-NEP expectation that defect creation
by surface roughening enhances the entrapment and polarization. The valence ZPS
shows the features of polarization, electron-hole pair production, and the entrapment,
which is the same to the defected TiO 2 (see Fig. 10.4a).
Table 10.2 summarizes the irregular-coordination coupling effect on the electronic structures of ZnO and SrTiO 3 nanocrystals. The photoluminescence energy
correlates to the CLS with involvement of electron-phonon coupling as an addition [34, 35]. Atomic CN reduction by creating defect enhances the BOLS-NEP
effect. The chemical passivation and undercoordination enhances each other on the
charge entrapment and polarization.
Mechanical scratching of the SiO 2 surface [36] and O-plasma etching of the
HOPG [37] also induce the local charge entrapment for the same reason of underand hetero-coordination combination. Using the ZPS technique, one can readily