Scanning Tunneling Microscope
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Scanning Tunneling Microscope Fengzhou Fang1 and Bingfeng Ju2 1 The State Key Laboratory of Precision Measuring Technology & Instruments, Centre of MicroNano Manufacturing Technology, Tianjin University, Tianjin, China 2 Department of Mechanical Engineering, Zhejiang University, Hangzhou, China
Synonyms Scanning tunneling microscopy
Definition Scanning tunneling microscope (STM) is an instrument for imaging conductive solid surfaces with an atomic resolution based on the concept of quantum tunneling.
Theory and Application Introduction STM was originally developed in 1981 by Gerd Binnig and Heinrich Rohrer (Binnig and Rohrer 1982), who were awarded the 1986 Nobel Prize in Physics for this great invention. Over the years, the STM has been proved to be an extremely versatile and powerful technique for many
disciplines in material science (Yao and Wang 2004), precision engineering (Weckenmann and Hoffmann 2007; Hansen et al. 2006), physics, biology, and so on (Gao 2010). The STM can be used not only in ultrahigh vacuum but also in ambient of air, water, liquid, or gas and at temperatures ranging from near-zero Kelvin to a few hundred degrees Celsius. Apart from surface topograph imaging, since the quantum tunneling also depends on the chemical nature of sample and tip, the STM also serves for characterization of electronic properties of solid samples, atomic manipulation, and nanostructure fabrication. Physics Principle of Tunneling Tunneling phenomena have been studied for long time and can be well understood in terms of quantum theory. Considering an one-dimensional vacuum barrier between two electrodes (the sample and the tip) and assuming their work functions to be the same and thus the barrier height to be F, if a bias voltage of V is applied between the two electrodes with a barrier width d, according to quantum theory under first-order perturbation (Hansen et al. 2006), the tunneling current is I¼
2pe X f Em ½1 f ðEv þ eV Þ m, v jMmv j2 d Em Ev
# CIRP 2016 The International Academy for Production Engineering et al. (eds.), CIRP Encyclopedia of Production Engineering, DOI 10.1007/978-3-642-35950-7_6596-4
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Scanning Tunneling Microscope
where f(E) is the Fermi function, M m v is the tunneling matrix element between states c m and c n of the respective electrodes, and E m and E n are the energies of c m and c n. The tunneling matrix element Mmv can be expressed as ð
Mmv ¼
2 dS cm • ∇cv cv • ∇cm 2m
where the integral is over all the surfaces surrounding the barrier region. To estimate the magnitude of Mm v, the wave function of the sample c n can be expanded in the generalized planewave forma: cv ¼
Os 1=2
X
h 1=2 i aG exp k2 þ jkG 2 Þ z
G
expðikG xÞ where Os is the volume of the sample, k ¼ h 1 ð2møÞ1=2 is the decay rate, ø is the work function, kG ¼ k== þ G, k== is the surface component of Bloch vector, and G is the surface reciprocal vector. The above formulas mean that the tunneling current depends on the tunneling gap distance d very sensitively. In the typic
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