Tip preparation

Figure Bl.19.32. AFM image of Blue Seript II plasmid (400 mu x 400 mu) in propanol, taken with super tip , prepared by earbon deposition on nomral tip in SEM, followed by ion milling. (Taken from [152], figure 1.)...

By far the most common method of STM tip preparation is electrochemical etching, which, in its simplest application, involves suspending a 0.5-1 mm diameter wire in a low-concentration electrolyte (0.3-3 m) such as NaOH or KOH and applying 1-10 V AC or DC between it and a counter electrode (Figure 3.13). Overall the etching reaction corresponds to... 

While the first STM studies of electrode surfaces were performed with self-built instruments, scanning tunneling microscopes for electrochemical use are nowadays commercially available at a price that hardly justifies the effort of homemade equipment. Nevertheless, new instrumental designs are now and then discussed in the literature, which are still worthwhile to be considered for special applications. There is, however, additional equipment required for the operation of an electrochemical STM, for which homemade designs may be advantageous over commercially available ones and hence is briefly mentioned here in terms of tip preparation and isolation, the electrochemical cell, and vibration damping. 

Other workers who have reported STM under solution include Itaya et. al. (59) and Fan et. al. (Fan, F-R.F. Bard, A.J. Anal. Chem.. submitted). Their instruments each employ three orthogonal piezoelectric elements as tip translators and both use glass tip insulation, though their tip preparation techniques differ. In the former case, a 10 pm Pt wire was sealed into a capillary and subsequently etched, while the latter workers used a 65 pm Pt wire which was sealed in soft glass, turned on a lathe, and then sonicated in concentrated... 

A tungsten tip, prepared by electrochemical etching, with a perfectly smooth end of very small radius observed by SEM or TEM, would not provide atomic resolution immediately. 

In this chapter, we describe various experimental methods of tip preparation and treatment. Most of them are found empirically. A thorough understanding of these procedures is still lacking. Some of the explanations are tentative. Tip preparation and characterization is one of the central experimental problems in STM, and we can certainly expect that a lot more development and understanding will be achieved in the near future. 

Schematically, the steps in this process are shown in Fig. 14.4. At the beginning, the local radius of the tip end is small. Field emission can be easily established. A high current though the tip end then causes local melting. The local curvature at the end of the tip suddenly decreases. The field emission current is then reduced dramatically. The tip end recrystallizes to have a relatively large radius. Feenstra et al. (1987a) observed that the tips prepared in this way always provide reproducible tunneling spectra, although atomic-resolution topographic images are generally not observed.

See Surface states Tersoff-Hamann approximation See s-wave-tip model Tetragonal symmetry 128 Tip annealing 286, 288 Tip preparation 281—285 cutting 282... 

The product is purified by the inverted filtration method. The apparatus used is shown in Fig. 3. A 2-I. round-bottomed flask is equipped with a reflux condenser and a bent glass tube, 8-10 mm. in diameter (Note 3). To the lower end of this tube is attached, by means of a cork, a 25-mm. tip prepared from a paper Soxhlet thimble and packed with glass wool. A 2-I. conical flask serves as a receiver for the hot filtrate. 

After a short introduction to tunneling in Sec. 2, special attention is given in Sec. 3 to operating conditions on semiconductors because these are not as trivial as for metals and may raise experimental problems. Questions related to in-situ spectroscopic characterization are addressed in the following section. Section 5 reviews in-situ as well as ex-situ studies (in UHV or in air after treatment of the surface in solution) according to the materials and electrochemical reactions involved. Silicon electrodes are treated separately, mostly in relation to electrochemical etching and por-pous layer formation. The two final sections outline perspectives and draw general conclusions. Details related to instrumentation and tip preparation are not discussed here unless they are specific to semiconductors. They are reviewed in [9]. Experimental aspects of in-situ AFM are not presented either, because the immersion of the surface in an electrolyte raises no specific problem. The theory and other applications of AFM are discussed elsewhere [3, 4]. 

In this chapter, several methods for the fabrication of different types of amperometric tips suitable for SECM are described. We have also suggested some methods for microelectrode fabrication, which have not yet been tested for SECM but may provide alternative ways for its tip preparation. Section II.A describes the techniques for the preparation of various metallic microelectrodes, including Pt, Ir-Pt, Au, Hg, and W. The manufacture of carbon microelectrodes is presented in Section II.B. Most of these tips are encapsulated in or supported with glass capillaries. Other coating materials and techniques are treated in Section II.C. 

---