Scanning tunneling microscopy layered materials

H. A. Mizes, S. Park and W. A. Harrison, Multiple-tip interpretation of anomalous scanning-tunneling-microscopy images of layered materials, Phys. Rev. B 36, 4491 (1987). 

SECM involves the measurement of the current through an ultramicroelectrode (UME) (an electrode with a radius, a, of the order of a few nm to 25 (zm) when it is held or moved in a solution in the vicinity of a substrate. Substrates, which can be solid surfaces of different types (e.g., glass, metal, polymer, biological material) or liquids (e.g., mercury, immiscible oil), perturb the electrochemical response of the tip, and this perturbation provides information about the nature and properties of the substrate. The development of SECM depended on previous work on the use of ultramicroelectrodes in electrochemistry and the application of piezoelectric elements to position a tip, as in scanning tunneling microscopy (STM). Certain aspects of SECM behavior also have analogies in electrochemical thin-layer cells and arrays of interdigitated electrodes. 

Borgschulte et al. [84] concluded from photoemission and scanning tunneling microscopy studies (STM/STS) that this critical cap layer thickness is related to inactivation of Pd by encapsulation of the Pd islands by yttrium oxide or hydroxide strong metal-support interaction (SMS I effect). The substitution of Pd by cheaper, preferably transparent, catalytic cap layers is desirable, especially for smart window devices. In this respect, the catalytic properties of transition metal oxides [85] for storage materials could also prove to be useful for smart windows. 

Wiesendanger, R. Anselmetti, D. STM on Layered Materials. In Scanning Tunneling Microscopy I, 2nd Ed. Giintherodt, H.-J., Wiesendanger. R.. Eds. Springer Series... 

It is not necessary to deal with these techniques in detail here, since there are several books and monographs on the subject. The fundamental theory and practice of electrochemical and spectroelectrochemical methods can be found in [1,2] and also in [3-5], where investigations of polymeric surface layers are emphasized. Excellent monographs on EQCM [6-9] and PBD [10] are also recommended for further studies. Infrared, Mdssbauer spectroscopy, ellipsometry, etc., are described in [I I], while electron spin resonance is discussed in [12], radiotracer in [13], scanning tunneling microscopy in [14], and scanning electrochemical microscopy in [15]. The fundamentals of electrochemical impedance spectroscopy are treated in [1,2,16] however, the different models elaborated for electrochemically active films and membranes can be found in various papers (see later), while the most important methods for analyzing impedance spectra, as reported before 1994, are well summarized in [3]. Nevertheless, the essential elements of these techniques are briefly discussed here, in order to help the reader to understand the experimental material presented in this book. 

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. 

The basical theories, equipments, measurement practices, analysis procedures and many results obtained by gas adsorption have been reviewed in different publications. For macropores, mercury porosimetry has been frequently applied. Identification of intrinsic pores, the interlayer space between hexagonal carbon layers in the case of carbon materials, can be carried out by X-ray dififaction (XRD). Recently, direct observation of extrinsic pores on the surface of carbon materials has been reported using microscopy techniques coupled with image processing techniques, namely scarming tunneling microscopy (STM) and atomic force microscopy (AFM) and transmission electron microscopy (TEM) for micropores and mesopores, and scanning electron microscopy (SEM) and optical microscopy for macropores [1-3],... 

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