Scanning tunneling microscopy of insulators

Guokenberger R, Fleim M, Cevo G, Knapp FI F, Wiegrabe W and Flillebrand A 1994 Scanning tunnelling microscopy of insulators and biological specimens based on lateral conductivity of ultrathin water films Science 266 1538... 

Stolyarova E, Rim KT, Ryu S et al (2007) High-resolution scanning tunneling microscopy imaging of mesoscopic graphene sheets on an insulating surface. Proc Natl Acad Sci USA 104 9209-9212... 

The development of local probe techniques such as Scanning Tunneling Microscopy (STM) or Atomic Force Microscopy (AFM) and related methods during the past fifteen years (Nobel price for physics 1986 to H. Rohrer and G. Binning) has opened a new window to locally study of interface phenomena on solid state surfaces (metals, semiconductors, superconductors, polymers, ionic conductors, insulators etc.) at an atomic level. The in-situ application of local probe methods in different systems (UHV, gas, or electrochemical conditions) belongs to modem nanotechnology and has two different aspects. 

In constant current imaging, the piezoelectric actuator is part of an electronic feedback loop, which maintains a fixed set point tip current. A difficulty arises for mixed insulating and conducting substrates for which the feedback loop cannot simply withdraw the tip according to increases in current (conductors) or decreases in current (insulators). Constant current imaging, which is routine in scanning tunneling microscopy, is therefore harder to implement unless the conductivity of the sample is uniform. 

Wang, J., Martinez, T., Yaniv, D.R., and McCormick, L. (1990) Characterization of the microdistribution of conductive and insulating regions of carbon paste electrodes with scanning tunneling microscopy. /. Electroanal. Chem., 286, 265-272. 

With the advent of cryogenic scanning tunneling microscopy (STM) it became possible to achieve vacuum tunneling gap conditions between sample and counter electrode, and to achieve atomic resolution on the surfaces of these materials. Vacuum tunneling is considered to be the best experimental situation, because it can avoid chemical reactions on the interface between the specimen and the insulating layer. In parallel to the above advancement, various physical properties of these materials have been intensively investigated, and many unique features of HTSC have been revealed. 

Scanning tunneling microscopy (STM) is also a tool for surface morphological studies, which is widely used in situ It is based on the analysis of a tunneling current between a very sharp microscopic tip and the electrode surface caused by a bias potential applied between the two. This method is well established for the study of electrochemical systems. Its advantage over AFM is that it is technically much simpler to use for in situ studies of electrochemical studies, and it obtains better resolution. However, the application of STM to nonaqueous systems is impossible when the electrode surfaces are covered by surface species, which are electrically insulating. 

Figure 1 Time evolution of a fission track in an insulator and conductor. A typical PD MS experiment will have an insulator as the energy deposition medium. Protonated molecules from a biological matrix are probably formed during the last stage when material ablation from the surface takes place. The craters depicted here have been observed using scanning tunnelling microscopy.

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