Scanning tunnelling microscopy

Abstract

Scanning tunnolling microscop! (STM) is a rclativcly new tccbnique. originating as it does from pioneering work of Binnig and Rohrcr in the early 1980s ‘. The concept is relatively simple. J~tst place ;I slurp tungsten tip close to a conducting surl~lcc. so close that the W:ILC‘ functions of the closest tip atom and the surface atoms overlap. Apply ;I potential diffcrcncc between tip and surface and a tunnelling current will flow. By varying the potential and/‘or separation. the local density of states can be probed. By scanning the tip, a picture of the surface topography can be formed from variations in the tunnelhng current or from variations in tip height, necessary to maintain constant tunnelling current and thcrcforc constant tip’surl‘acc separation. One of ~hc first applications of STM, and certainly one that caused the most impact, was the unequivocal solution of the problem of the atomic arrangement of the rcconstructcd (7 x 7) surface of silicon( I I1 )‘. In die Tcw years since lhal pionccrins work, the uses of STM have burgeoned. As well as atomic resolution of topography. Mark function and local density of states. STM has been used. for cx~mplc, to obscrvc charge densit) WIVC~~. for magnetic imaging ‘. for observing spin polarized clcctrons’. for dctcrmining metallic udhcsion/friction”. for electron interferometry’ and for determining electrochemical reactivity’. I am sure 0lal I ha\e not listed aII variations as new methods seem to cmcrge each month. An important nc\v atom probe tcchniquc that has cmergcd from STM and is now reaching maturity is atomic force microscopy (AFM)“. This depends on mechanical interaction when ;I minute force is applied bctwccn ;I tip on a scnsitivc cantilckcr and a surface. The dcflcction of the cantilever is amplified by. for cxamplc. an optical Ic\,cr. and dctcctcd to build LIP an im;lgc of the surface potential. AFM is particularly LISCI‘UI I'OI insulating surfaces and for determining surface friction. Atomic resolution is possible. although AFM cannot quite achieve the rcsolulion of STM. In this reCec I \\ill only discuss STM : theory. instrumcntaCon and c.xamplcs of rcccnt rccults.