Insulators, STM

Figure 20. I U) curves for Cg-Au (left) and Gal-Au (right) in H2O as a function of pH (adjusted with phosphate buffer). The numbers 1—4 in the Gal-Au data identify voltage plateaus. Cartoons of the experimental arrangements for measuring curves of individual nanoclusters in solution are shown at the top of each data column. The insulated STM tip, ligand-capped Au nanocluster and an octanethiol-coated planar Au substrate are shown. Length and shapes are not to scale. (Reprinted with permission from Ref. [35], 1998, American Chemical Society.)...

Case II Reversible or Ouasi-Reversible Redox Species. If the tip-sample bias is sufficient to cause the electrolysis of solution species to occur, i.e., AEt > AEp, ev, the proximity of the STM tip to the substrate surface (d < 10 A) implies that the behavior of an insulated STM tip-substrate system may mimic that of a two-electrode thin-layer cell (TLC)(63). At the small interelectrode distances required for tunneling, a steady-state concentration gradient with respect to the oxidized (Ox) and and reduced (Red) electroactive species should be established between the tip and the substrate, and the resulting steady-state current will augment that present as a result of the convection of electroactive species from the bulk solution. In many cases, this steady state current is predicted to overwhelm the convective currents, so this situation is of concern when STM imaging under electrochemical conditions (64). 

REVOLUTION COUNTER INSULATION. STM EM) BY UO END BY n 100% 200% SPARE PACKING (BY MFGR ) j LANTERN RING in CENTER WITH CONN PLUGGED... 

INSULATION STM END BY UQ END BY LANTERN RING IN CENTER WITH CONN PLUGGED GPM... 

The insulated STM tip, ligand-capped Au nanoparticle and octanethiol-coated planar Au substrate are shown. Length and shapes are not to scale. Reprinted with permission from Ref [47] 1998, American Chemical Society. 

AFM measures the spatial distribution of the forces between an ultrafme tip and the sample. This distribution of these forces is also highly correlated with the atomic structure. STM is able to image many semiconductor and metal surfaces with atomic resolution. AFM is necessary for insulating materials, however, as electron conduction is required for STM in order to achieve tiumelling. Note that there are many modes of operation for these instruments, and many variations in use. In addition, there are other types of scaiming probe microscopies under development. 

The operation of the STM depends on the conduction of electrons between tip and sample. This means, of course, that insulating samples are, in general, not accessible to STM investigations. Nevertheless, a large body of work [32] dealing with STM characterization of thin organic films on conducting substrates is now in... 

Several designs for STM electrochemical cells have appeared in the literature [M]- hr addition to an airtight liquid cell and the tip insulation mentioned above, other desirable features include the incorporation of a reference electrode (e.g. Ag/AgCl in saturated KCl) and a bipotentiostat arrangement, which allows the independent control of the two working electrodes (i.e. tip and substrate) [ ] (figure BL19.11). 

Of partieular interest are those surfaees where AFM has provided eomplementary infonnation or revealed surfaee stnieture whieh eould not be obtained by STM. One obvious applieation is the imaging of insulators sueh as NaCl(OOl) [120]. In this ease it was possible to observe point defeets and themially aetivated atomie jump proeesses, although it was not possible to assign the observed maxima to anion or eation. 

Several striking examples demonstrating the atomically precise control exercised by the STM have been reported. A "quantum corral" of Fe atoms has been fabricated by placing 48 atoms in a circle on a flat Cu(lll) surface at 4K (Fig. 4) (94). Both STM (under ultrahigh vacuum) and atomic force microscopy (AFM, under ambient conditions) have been employed to fabricate nanoscale magnetic mounds of Fe, Co, Ni, and CoCr on metal and insulator substrates (95). The AFM has also been used to deposit organic material, such as octadecanethiol onto the surface of mica (96). New appHcations of this type of nanofabrication ate being reported at an ever-faster rate (97—99). 

Sample requirements STM—solid oonduaois and semiconductors, conduaive coating required for insulators SFM— solid conduaors, semiconductors and insulators... 

The STM uses this eflFect to obtain a measurement of the surface by raster scanning over the sample in a manner similar to AFM while measuring the tunneling current. The probe tip is typically a few tenths of a nanometer from the sample. Individual atoms and atomic-scale surface structure can be measured in a field size that is usually less than 1 pm x 1 pm, but field sizes of 10 pm x 10 pm can also be imaged. STM can provide better resolution than AFM. Conductive samples are required, but insulators can be analyzed if coated with a conductive layer. No other sample preparation is required. 

These materials are developed from the polyetherimides introduced by General Electric (see also Section 18.14.2). At the time of writing one grade, Ultem Siltem STM 1500, is being offered. It is of particular interest as a material for wire and cable insulation, as it not only has excellent flame resistance coupled with low smoke generation but also avoids possible toxic and corrosion hazards of halogenated polymers. This can be of importance where there are possible escape problems in the event of a fire, such as in tunnels, aircraft and marine (particularly submarine) vessels. 

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