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Scanning tunneling microscopy (STM) was applied to observe a surface oxide layer of titanium in air. When a sample bias voltage was set at -0.2 V, the STM tip penetrated into the surface oxide damaged the sample, while clear and stable images of the surface TiO2 layer were obtained under photoexcitation. The results were discussed in terms of enhanced electric conductivity of the oxide layer under photoexcitation.
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Sugimura et al. (1992) studied this question.
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