The surface science of titanium dioxide - Niser

78 U. Diebold / Surface Science Reports 48 (2003) 53±229
Fig. 11.
Society.
Non-contact AFM image of a TiO 2 (1 1 0)-(1 1) surface. From Fukui et al. [118]. # The American Physical
A de®nite interpretation of images as shown in Fig. 10 is rather dif®cult. For the sake of studying
surface structure and adsorbates, it is maybe better (and suf®cient) to focus on results obtained with the
`normal' tip state that can be obtained reproduciblyby`cleaning' with high voltage/high current pulses.
As mentioned above, onlyfew reports exist where satisfactoryimages have been obtained with
negative sample bias (®lled-state images) [77]. These were taken with a bias voltage that is too small to
bridge the 3 eV gap. It is likelythat a real `contrast reversal' would occur under the tunneling
conditions where the ®lled state of the VB are imaged, but, to this author's knowledge, such images
have not yet been reported.
Recently, non-contact AFM has been introduced as a complementary technique to study TiO 2
surfaces with atomic resolution. An image of the TiO 2 (1 1 0)-(1 1) surface, obtained byFukui et al.
[118], is reproduced in Fig. 11. Frequencymodulation described in [119] was used as the feed back
signal. While the contrast formation of atomicallyresolved AFM images using this technique is also
somewhat controversial [120], one would assume that the physical geometry should dominate in AFM.
The registryof (1 2) strands (see Section 2.2.2) in AFM images is consistent with the bright rows in
Fig. 11 being the protruding bridging oxygens. Consequently, the black spots in Fig. 11 were assigned
as oxygen vacancies in [118].
2.2.1.4. Surface defects. The abilityto control the amount of defects is one of the main attractions of TiO 2
as a `well-characterized' model system. Because imperfections such as vacancies introduce changes in the
electronic structure (in particular a band gap feature at 0.8 eV below E Fermi , and a shoulder in the XPS Ti2p
peak, see Section 3.2), theyhave been investigated with spectroscopic techniques for years. Much has been
learned about the structure of defects, mainlybecause of recent investigations with scanning probe
techniques. The following discussion considers steps; vacancies produced bythermal annealing,
sputtering, and electron bombardment; as well as common impurities such as Ca and H.
2.2.1.4.1. Step edges. Sputtering and annealing in UHV (at not too high temperatures) renders flat
(1 1) surfaces. As is expected for annealing of sputter-damaged surfaces, the terrace size increases with