9.2.1.5 Limitations and continued development of STM

This chapter outlines limitations and continued development of scanning tunneling microscopy (STM). The most severe limitation of scanning tunneling microscopy is the present inability to prepare stable, atomically-sharp tips with known physical and chemical properties. Tips are prepared using recipes similar to those used for field-emission and field-ion microscope tips, but without the advantage of field-ion characterization except in rare instances. The unknown geometry and electronic properties of the tip are usually of the largest uncertainties encountered in attempting detailed interpretation of STM results. In addition, at room temperature surface diffusion of even tungsten on tungsten is significant, leading to tip instability and changing the atomic and electronic properties of the tip. At temperatures below 300K, tips are more stable due to the reduced surface diffusion, and thermal drift is nearly eliminated. Current efforts in STM are devoted to increasing the range of accessible temperatures and achieving higher speed measurements to further enhance its utility.