STM resolution

Fig. 1.23. Free-electron-metal model of STM resolution. The sample is modeled as a corrugated free-electron-metal surface. The tip is modeled as a curved free-electron-metal surface with radius r, at the closest approach to the sample surface. (After Stoll, 1984.)...

The effect of p,- or dangling bonds on STM resolution can be understood in the light of the reciprocity principle (Chen, 1988), which is the fundamental microscopic symmetry between the tip and the sample By... 

Various improved methods for making STM tips linked with SEM studies have been discussed in the literature, for example, Lemke (1990), Melmed (1991), and references therein. However, since the STM resolution does not have a direct correlation with the look of the tip under SEM, the simple dc dropoff method, as described here, is usually sufficient. From the experience of the author, two simple improvements can be helpful. The first is to install an insulating piece between the cathode and anode across the liquid surface to prevent the hydrogen bubbling on the cathode from perturbing the meniscus near the anode. The second is to save the dropped piece as the tip, which might be better than the upper one. 

The high spahal in situ STM resolution of biological macromolecules offers new theoretical challenges. Even novel charge transfer phenomena have been revealed as noted. We therefore first overview a few conceptual notions of the fundamental electrochemical ET process, with emphasis on ET phenomena in nanogap electrode systems and in situ STM. 

Electrochemically controlled SAMs of the alkanethiol class characterized to high voltammetric resolution and to molecular and sub-molecular structural in situ STM resolution have been reviewed recently [60, 151]. We note here first some issues of importance to functionalized alkanethiols as linker molecules for gentle immobilization of fully functional redox metalloprotein monolayers on singlecrystal Au(lll) electrode surfaces. We discuss next specifically the functionalized alkanethiols cysteine (Cys) and homocysteine (Hey). These two molecules represent a core protein building block and a core metabolite, respectively. The former has been used to display unique sub-molecular in situ STM resolution [152]. The latter shows a unique dual surface dynamics pattern that could be followed both by single-molecule in situ STM and by high-resolution capacitive voltammetry. 

The ability to control the position of a fine tip in order to scan surfaces with subatomic resolution has brought scanning probe microscopies to the forefront in surface imaging techniques. We discuss the two primary techniques, scanning tunneling microscopy (STM) and atomic force microscopy (AFM) the interested reader is referred to comprehensive reviews [9, 17, 18]. 

Figure Al.7.7. Atomic-resolution, empty-state STM image (100 A x 100 A) of the reconstmcted Si(l 11)-7 7 surface. The bright spots correspond to a top layer of adatoms, with 12 adatoms per unit cell (courtesy of Alison Baski).

More recently, studies employing STM have been able to address surface self-diffiision across a terrace [16, 17. 18 and 19], It is possible to image the same area on a surface as a fiinction of time, and watch the movement of individual atoms. These studies are limited only by the speed of the instrument. Note that the performance of STM instruments is constantly improving, and has now surpassed the 1 ps time resolution mark [20]. Not only has self-diflfiision of surface atoms been studied, but the diflfiision of vacancy defects on surfaces has also been observed with STM [18]. 

The most popular of the scanning probe tecimiques are STM and atomic force microscopy (AFM). STM and AFM provide images of the outemiost layer of a surface with atomic resolution. STM measures the spatial distribution of the surface electronic density by monitoring the tiumelling of electrons either from the sample to the tip or from the tip to the sample. This provides a map of the density of filled or empty electronic states, respectively. The variations in surface electron density are generally correlated with the atomic positions. 

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. 

Figure A3.10.14 STM image of 0.25 ML Aii vapour-deposited onto Ti02(l 10). Atomie resolution of the substrate is visible as parallel rows. The Au elusters are seen to nueleate preferentially at step edges.

Figure Bl.19.9. Plasmid DNA (pUClS) on mica imaged by STM at high resolution. The inset is a cut-out of a zoomed-in image taken inunediately after the overview. (Taken from [42], figure 2.)...

One of the most important advances in electrochemistry in the last decade was tlie application of STM and AFM to structural problems at the electrified solid/liquid interface [108. 109]. Sonnenfield and Hansma [110] were the first to use STM to study a surface innnersed in a liquid, thus extending STM beyond the gas/solid interfaces without a significant loss in resolution. In situ local-probe investigations at solid/liquid interfaces can be perfomied under electrochemical conditions if both phases are electronic and ionic conducting and this... 

Since then, STM has been established as an insttument fot foteftont research in surface physics. Atomic resolution work in ultrahigh vacuum includes studies of metals, semimetals and semiconductors. In particular, ultrahigh-vacuum STM has been used to elucidate the reconstructions that Si, as well as other semiconducting and metallic surfaces undergo when a submonolayer to a few monolayers of metals are adsorbed on the otherwise pristine surface. ... 

Because STM measures a quantum-mechanical tunneling current, the tip must be within a few A of a conducting surface. Therefore any surface oxide or other contaminant will complicate operation under ambient conditions. Nevertheless, a great deal of work has been done in air, liquid, or at low temperatures on inert surfaces. Studies of adsorbed molecules on these surfaces (for example, liquid crystals on highly oriented, pyrolytic graphite ) have shown that STM is capable of even atomic resolution on organic materials. 

The three-dimensional, quantitative nature of STM and SFM data permit in-depth statistical analysis of the surface that can include contributions from features 10 nm across or smaller. By contrast, optical and stylus profilometers average over areas a few hundred A across at best, and more typically a pm. Vertical resolution for SFM / STM is sub-A, better than that of other profilometers. STM and SFM are excellent high-resolution profilometers. 

For SFM, maintaining a constant separation between the tip and the sample means that the deflection of the cantilever must be measured accurately. The first SFM used an STM tip to tunnel to the back of the cantilever to measure its vertical deflection. However, this technique was sensitive to contaminants on the cantilever." Optical methods proved more reliable. The most common method for monitoring the defection is with an optical-lever or beam-bounce detection system. In this scheme, light from a laser diode is reflected from the back of the cantilever into a position-sensitive photodiode. A given cantilever deflection will then correspond to a specific position of the laser beam on the position-sensitive photodiode. Because the position-sensitive photodiode is very sensitive (about 0.1 A), the vertical resolution of SFM is sub-A. 

The three-dimensional, digital nature of SFM and STM data makes the instruments excellent high-resolution profilometers. Like traditional stylus or optical profilometers, scanning probe microscopes provide reliable height information. However, traditional profilometers scan in one dimension only and cannot match SPM s height and lateral resolution. 

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